Emergence of disinhibition-induced synchrony in the CA3 region of the guinea pig hippocampus in vitro
1 INSERM U739, CHU Pité-Salpêtrière, UPMC, 105 bd de l'Hpital, 75013 Paris, France
Abstract
Suppressing inhibition mediated by GABAA receptors induces rhythmic bursts of synchronous firing in the CA3 region of the hippocampus. Extracellular and intracellular records were made from guinea pig hippocampal slices to examine the emergence of this synchrony. We found that application of GABAA receptor antagonists initiated a sequence of changes in the activity of the CA3 neuronal population. First, the frequency of firing detected in multiunit records increased. Then, firing began to oscillate with increases followed by decreases in firing that occurred at intervals of 0.5–2 s. The coherence of the rhythmic activity at a single site increased with time, and discharges at distant sites in the CA3 region became correlated. Fluctuations in firing were associated with extracellular field potentials. Finally, epileptiform events associated with large field potentials began to recur at intervals of 5–10 s. The onset of fully synchronous events was sudden and correlated with a large increase in the amplitude of the field potential. Thus the CA3 population can express states of partial population synchrony preceding the onset of epileptiform discharges. A similar activity was induced and maintained by applying low doses of GABAA receptor antagonists. Intracellular records suggest that inhibitory signalling mediated by GABAB receptors contributes to the emergence of this activity. States of partial synchrony in the CA3 region exposed to GABAA receptor antagonists therefore depend on alternating periods of firing, presumably dependent on excitatory synaptic mechanisms, and silence, mediated in part by the activation of GABAB receptors.
, 百拇医药
Introduction
Synchronous discharges are generated by neuronal populations in most brain regions and are often associated with specific behavioural states. Precisely timed simultaneous discharges of spinal motoneurones are required for motor activities (Grillner & Wallen, 1999). The simultaneous firing of hypothalamic neurones underlies the liberation of hormones (Lincoln & Wakerley, 1975). Synchronous activities have also been associated with olfactory perception (Laurent, 1999) and more generally with the treatment of sensory information (Engel et al. 2001). Neuronal synchrony is observed during perinatal development, in many brain structures including the hippocampus (Ben-Ari et al. 1989), retina (Wong, 1999) and medulla (Fortin et al. 1995). Pathological neuronal synchronization is associated with several disease states including Parkinsons (Levy et al. 2000) and the epilepsies (Schwartzkroin, 1994).
, 百拇医药
While this list shows that synchrony is ubiquitous in the nervous system, the term is not well defined. It derives from measurements on two different types of signal. Synchronous brain activity was first deduced (Berger, 1929) from observations of rhythmic waves in electroencephalographic (EEG) records (Pedley & Traub, 1990). Oscillating EEG signals of distinct frequencies are generated in most brain regions and reflect, at least in part, the occurrence of synchronous synaptic events. However, while an EEG signal is a continuous function of time, the term ‘synchrony’ has also been applied to the generation of action potentials, which is a discrete process. Action potential synchrony has usually been quantified by cross-correlation measures on isolated single-unit discharges (Bedenbaugh & Gerstein, 1997; Brecht et al. 1999) which determines the temporal relations between neuronal firing in cell pairs. Few studies have compared the synchrony of action potential generation in a neuronal population to synchrony as defined from EEG or field potential measurements.
, 百拇医药
We examined this question for the transition to synchronous firing that occurs when GABAA-mediated synaptic transmission is suppressed in slices of the hippocampus maintained in vitro (Schwartzkroin & Prince, 1978; Miles & Wong, 1987). Disinhibition leads to the emergence of rhythmic population bursts at intervals of 5–10 s. The initiation of these events depends in part on excitatory synaptic interactions between CA3 pyramidal cells. The duration of individual synchronous bursts is thought to be limited by transmitter availability at these connections (Staley et al. 1998) and the interval between bursts may depend on the dynamics of replenishment of releasable transmitter (Staley et al. 2001). However the emergence of this population behaviour has not been studied in detail.
, http://www.100md.com
In this study we used intracellullar and multiple extracellular recordings to examine the transition to synchronous activity induced by disinhibition. We found that synchrony emerged gradually rather than in an all-or-none fashion. CA3 cell activity first increased, and then cyclically recurring increases and decreases in population firing began to occur coherently throughout the CA3 region before the emergence of epileptiform population events. These oscillations reflect a state of partially synchronous CA3 cell firing and are accompanied by biphasic field potentials. Intracellularly, field potentials correspond to a barrage of synaptic excitation which is terminated by an inhibition mediated by activation of GABAB receptors. Thus patterned fluctuations in CA3 cell firing that precede disinhibition-induced epileptiform synchrony are partly shaped by GABAergic signalling.
, 百拇医药
Methods
Slice preparation
Hippocampal slices were prepared according to local INSERM guidelines from guinea-pigs of weight 100–200 g and age 6–10 weeks, which were anaesthetized by intraperitoneal injection of ketamine (200 mg kg–1) and chloral hydrate (800 mg kg–1). Animals were perfused intracardially with a cold (3–5°C) solution consisting of 248 mM sucrose, 26 mM NaHCO3, 1 mM KCl, 10 mM MgCl2, 1 mM CaCl2 and 10 mM glucose, equilibrated with 5% CO2 in 95% O2. They were then killed by decerebration, both hippocampi were dissected free and several slices of thickness 400 μm were cut in the same sucrose-containing solution using a vibratome. Slices were then transferred to an interface recording chamber where they were maintained at 35°C, perfused with a solution containing 124 mM NaCl, 26 mM NaHCO3, 4 mM KCl, 2 mM MgCl2, 2 mM CaCl2 and 10 mM glucose, and equilibrated with 5% CO2 in 95% O2.
, http://www.100md.com
Drugs
We used picrotoxin (50 μM), bicuculline (10 μM) or gabazine (SR-95531, 5 μM) to suppress GABAA receptor mediated synaptic signalling. The effects of lower concentrations of GABAA receptor antagonists were examined in some experiments. Synaptic events mediated via activation of GABAB receptors were blocked by CGP 52432 (2–4 μM). Drugs were obtained from Tocris Cookson (Bristol, UK).
Recordings
, http://www.100md.com
Electrodes used for extracellular recording were made from tungsten wire of 50 μm diameter. Their tips were etched to a diameter of 2–6 μm under a dissecting microscope, by passing an alternating current (1–5 V for 1–2 min) between the tungsten wire and a carbon electrode both immersed in 5 M KNO2. Electrodes were insulated by applying a coat of varnish to within about 200 μm of their tip. Their resistance in physiological solution was 500–1000 k. Up to four metal electrodes were mounted on holders controlled by separate manipulators. Potential differences between tungsten electrodes and a reference Ag–AgCl electrode were amplified 1000x and filtered with a pass-band of 0.1 Hz to 10 kHz (AM systems, 1700). Intracellular records were made with glass electrodes filled with 2 M potassium acetate and bevelled to final resistances of 40–60 M. Intracellular records were amplified with an Axoclamp 2B amplifier (Axon Instruments, Union City, CA, USA) and filtered with a high cut-off at 3 kHz. CA3 inhibitory cells records were distinguished from those of pyramidal cells by several criteria. They included a shorter duration action potential, a larger action potential afterhyperpolarization and the discharge of weakly accommodating action potentials in response to depolarizing current injections rather than the burst firing pattern typical of CA3 pyramidal cells (Miles et al. 1996). Signals were stored on a modified videotape recorder (Neurodata, Cygnus-Technology, Delaware Water Gap, PA, USA), digitized at 20 kHz using a 12-bit, A–D converter (Digidata 1200A, Axon Instruments) and monitored during experiments with the programme Axoscope (Axon Instruments).
, http://www.100md.com
Experimental procedures
Multiple tungsten electrodes were placed in the stratum pyramidale of the CA3 region to record extracellular signals. Such electrodes record action potentials generated by neurones within a radius of about 80 μm (Cohen & Miles, 2000). We therefore placed electrodes with separations of about 500 μm so that they would detect the activity of distinct CA3 cell populations. In some experiments these records were compared with field potentials recorded by another extracellular electrode. The field potential electrode was placed in the CA3 stratum radiatum, where we reasoned that synaptically generated field potentials would be maximal due to the recurrent synapses between CA3 pyramidal cells. Records were made at distances of at least 200 μm from str. pyramidale to minimize contributions of spikes generated by back-propagating action potentials (Cohen & Miles, 2000). Extracellular action potentials, possibly generated by the sparse interneurones with somata in this region, were occasionally recorded by these electrodes.
, 百拇医药
Data treatment
Signals derived from extracellular recordings were processed using routines written in the LabVIEW programming environment (National Instruments, Austin, TX, USA). Several indices were derived from each recording (Fig. 1). Distinct frequency components were separated by digitally implemented Bessel filters. Filtering with a pass band of 300–5000 Hz permitted the isolation of action potentials (Fig. 1B). Extracellular spikes were detected using a previously described ‘up-only’ algorithm (Cohen & Miles, 2000) and their timing and amplitude was measured. Since extracellular spike amplitude depends largely on the arbitrary spatial relations between an electrode and the spike generating element, we represented all detected spikes as vertical lines of the same height in some plots (Fig. 1C). To estimate the frequency of all spikes recorded by an electrode, time sequences of the occurrence of detected action potentials were convolved with a gaussian function of S.D. 5–5000 ms (Szucs, 1998). The resulting traces correspond to a frequency function for firing in a population of several hundred CA3 cells (Fig. 1D). A similar continuous measure of average amplitude for spikes detected by a single electrode was obtained by convolving the amplitude of each action potential with a gaussian function of S.D. 20 ms and dividing at each time by the instantaneous frequency. Field potentials were derived from the same extracellular recordings using a band pass of 0.1–100 Hz (Fig. 1A). Extracellular field potential events were detected using the same ‘up-only’ algorithm together with a user-defined threshold
, http://www.100md.com
Extracellular records made with one electrode in stratum pyramidale (SP) and another in stratum radiatum (SR) in the presence of functional synaptic inhibition. A, low frequency signals derived from the SP and SR electrodes, with a 0.5–100 Hz pass band. B, multiunit activity detected with a pass band of 100–3000 Hz from SP and SR electrodes. C, each detected spike from the SP signal is displayed as a vertical bar. D, continuous index of spike frequency derived by convolving spike timing with a gaussian function of S.D. 20 ms (black trace) or 1000 ms (red trace).
, 百拇医药
We wished to quantify fluctuations in multiunit activty recorded from a single electrode. This was done by considering rapid fluctuations in the frequency of detected spikes (as in Fig. 1D) while correcting for the slow increase in activity due to the suppression of synaptic inhibition (Cohen & Miles, 2000). The correction was made using two estimates of frequency obtained by convolving spike trains with a gaussian of S.D. 1000 ms, to capture slow changes in population firing (F), and with a gaussian of S.D. 50 ms, to detect transient fluctuations of activity (f). A sliding variance index, VAR(t), was computed to measure the time course of fast frequency fluctuations:
, 百拇医药
We constructed sliding covariance (COV) and correlation (COR) indices to measure the coherence of changes in the frequency of all spikes generated by different cell populations recorded by pairs of extracellular electrodes. The indices, again corrected for the increased firing due to disinhibition as in calculations of variance, were constructed according to the equations:
F and f correspond to frequency functions convolved with gaussian functions to reveal slow (S.D. 1000 ms) and fast fluctuations (S.D. 50 ms) in the frequency of spikes detected by one electrode. G and g are the equivalent parameters estimated for activity detected by a second electrode. VAR is computed as above and the mean is calculated over a sliding window centred at time t. COR, the correlation index can vary between +1, indicating a perfect coherence between the two signals, and –1, corresponding to changes in frequency in opposing directions in signals derived from the two recordings. The correlation index was qualitatively insensitive to changes in the range of 50–200% for temporal resolution. Programming details are available from icohen@bcm.tmc.edu and all tools described in this article are available as part of the software package Spikoscope distributed under the GNU Public License at http://glab.bcm.tmc.edu.
, 百拇医药
Results
Spontaneous action potentials were always detected in extracellular records from the CA3 stratum pyramidale in control conditions. Their frequency fluctuated but recurring temporal patterns were rarely evident. Field potentials recorded from the stratum radiatum showed occasional small deflections (Fig. 1). The application of antagonists at GABAA receptors (picrotoxin, 50 μM, n= 5; bicuculline, 10 μM, n= 6; or gabazine (SR-95531), 5 μM, n= 3 slices) induced an epileptiform population activity. Population bursts were generated in recurring fashion at intervals of 3–10 s. Each burst was followed by a silent period and then an increase in activity before the next burst.
, 百拇医药
Epileptiform activity did not emerge in an all-or-none fashion but rather after a transition period of increasingly synchronous firing (Fig. 2A–E). Disinhibition first induced an increase in mean firing frequency (Fig. 2D, red trace) in records from single extracellular electrodes in the CA3 stratum pyramidale. Then patterned fluctuations in population firing emerged (Fig. 2B and D black trace) with little further increase in mean frequency. They consisted of transient increases followed by decreases in firing and recurred at intervals in the range 0.5–2 s. The amplitude and coherence of these frequency fluctuations increased with time until fully synchronous epileptiform bursts emerged, usually abruptly (Fig. 2C and D). Full synchrony emerged with a large increase in field potential amplitude and an increase in the interval between events, which were used to define the timing of the transition. The mean time elapsing between the start of perfusion and the emergence of full synchrony was 6.5 min (range 4.5 min to 9.7 min). The mean action potential frequency increased by 31% from 47 ± 24 spikes s–1 in control conditions to 62 ± 25 spikes s–1 just before the onset of epileptiform activity (mean ±S.D., n= 13 recording sites in 5 animals). Peak firing frequencies for fully synchronous population events were much higher than those during partial synchrony, even though multiunit activity may be underestimated in this condition.
, 百拇医药
A–C, multiunit activity recorded by an electrode in the CA3 stratum pyramidale at 2, 6 and 7.5 min after perfusion with 10 μM bicucculline. D–F, evolution of indices over the full transition. Timing of traces shown in A–C are indicated below graph F. D, frequency of all detected spikes convolved with gaussian functions of 100 ms (black) and 1600 ms (red) reveal fast and slow variations during the transition to synchrony. Slow variations reach a plateau frequency before the first epileptiform discharge (vertical dotted line). Fast oscillations increase in amplitude before the onset of fully synchronous firing. E, a sliding variance index reveals a progressive increase in coherence of fast fluctuations around the mean frequency. F, the mean amplitude of detected action potentials increased only after the onset of epileptiform activity. This implies that spike superpositions within 1–2 ms do not occur during the transition period. Black and red traces correspond to smoothing gaussians of 100 and 1600 ms, respectively.
, 百拇医药
We used the variance index described in Methods to assess changes in fluctuations in spike frequency at a single site during the transition to epileptiform discharges (Fig. 2E). In 13 records from single sites, its value increased from 965 ± 645 Hz2 in control conditions to 5334 ± 2425 Hz2 just before the onset of epileptiform discharges (mean ±S.D., n= 13, average over 30 s time windows). This more than fivefold increase in variance is much larger than that expected (70%) with a scaling of firing in independent neurones and the observed 30% increase in frequency. Thus epileptiform synchrony is preceded by increasingly coherent variations in the firing of a local neuronal population.
, 百拇医药
During this increase in activity, close collisions (Ankri & Korn, 1999) might lead to underestimates of firing frequency. We assessed the prevalence of spike collisions by measuring the mean amplitude of detected action potentials. Since the amplitudes of superimposed spikes sum, an increase in mean amplitude would suggest an increased number of collisions. However, mean spike amplitude was constant until the onset of epileptiform discharges (Fig. 2F). Thus spikes generated by different cells superimpose rarely, within the 1–2 ms resolution of our detection algorithm (Cohen & Miles, 2000), before the transition to epileptiform activity.
, 百拇医药
We next compared the evolution of population activity in records from different sites in the CA3 region (Fig. 3A–C). Recording electrodes were separated by at least 500 μm, in order to sample activity from different neuronal populations. On exposure to GABAA receptor antagonists, firing frequency (Fig. 3D and E) and variance (Fig. 3F) appeared to increase in parallel in records from different sites. We quantified relations between activity from distinct electrodes (Fig. 3H) using the correlation index described in Methods. This index could take values between –1 for antiphase changes in frequency and 1 for in-phase changes in frequency at two sites. In control conditions the index fluctuated with time by 0.08 units (S.D.) with a mean value from 11 pairs of extracellular recordings of 0.07 ± 0.03 units (mean ±S.E.M.). The mean correlation index increased to 0.74 ± 0.03 units (mean ±S.E.M.) during the 30 s before the onset of fully synchronous epileptiform discharges. Thus, the coherence of population activities recorded from separate sites increased together with the increase in fluctuations of activity at a single site. However, these two changes always followed the initial increase in neuronal activity induced by disinhibition (cf. Fig. 3D–H).
, 百拇医药
A–C, multiunit activity recorded by two electrodes separated by 500 μm in CA3 stratum pyramidale at 5, 8 and 8.5 min after perfusion with 50 μM picrotoxin. D–H, evolution of indices over the full transition. Timing of traces shown in A–C are indicated below graph H. D and E, fast (black trace, convolved with a gaussian of S.D. 20 ms) and slow (red trace, gaussian of S.D. 1000 ms) fluctuations in spike frequency from the two sites. The vertical dotted line indicates the first epileptiform discharge. F, evolution with time of variance indices calculated for activity at each site. G and H, changes in covariance and correlation indices for activity recorded from the two electrodes. The recording started at the addition of picrotoxin. Traces shown in A correspond to the initial increase in firing. Activity at the two sites becomes progressively more synchronized in the traces for B and C. The value of the correlation index increased to 0.8 just before full synchrony.
, 百拇医药
We next compared changes in the frequency of multiunit activity with field potentials generated in the stratum radiatum during the transition to epileptiform synchrony (Fig. 4). In control conditions, small field potentials were evident. As synaptic inhibition was suppressed the timing of action potentials generated at separate sites in the stratum pyramidale became increasingly rhythmic. The increasing coherence of firing of distinct CA3 cell populations was associated with the emergence and growth of periodic field potentials (Fig. 4A–C). Changes in the frequency and amplitude of field potential events (Fig. 4D), defined by application of the ‘up-only’ algorithm, are shown in Fig. 4E and F, while Fig. 4G shows the correlation index. The frequency and amplitude of field potentials increased in parallel with the increasing coherence of firing at distinct sites.
, 百拇医药
A–D, frequency signals, convolved with a gaussian of S.D. 160 ms, derived from three multiunit records made from the CA3 stratum pyramidale (SP1, SP2 and SP3) together with the timing of spikes detected at each site and the field potential recorded by an electrode placed in stratum radiatum (SR). A, baseline activity before perfusion. B, at 6.5 min after the onset of perfusion with picrotoxin (50 μM) a poorly organized rhythmicity is evident in the frequency signals. C, at 7.5 min field potentials emerge and the rhythmicity in the frequency traces, SP1, 2 and 3 is enhanced. D, at 8.5 min, rhythmic activity is evident in both firing frequency traces and in the SR field. E–H, evolution over a full transition. E, SR field potential recording during the transition. Bicuculline perfusion started 6.5 min before the first full synchrony discharge (vertical dashed line). Full synchrony potentials are clipped. F and G, frequency and average amplitude of field potentials derived by convolving their timing and amplitude with a gaussian of S.D. 20 s. H, multiunit correlation index time course derived from activity at SP1 and SP2.
, http://www.100md.com
Both field potential amplitude and the interval between population bursts increased abruptly at the transition to fully synchronous discharges (Fig. 5A). The amplitude of the largest field potential associated with partial synchrony burst was 81 ± 51 μV, while that associated with the first full synchrony burst was 2250 ± 1300 μV (mean ±S.D., n= 4 animals). This suggests that many more cells were recruited to fire during the first epileptiform burst. Even so, partially synchronous events and fully synchronous discharges exhibited some common features (Fig. 5B and C). They were both preceeded by an accelerating frequency of multiunit discharges and a slow field potential. Field potentials were generated in both stratum pyramidale and radiatum during fully synchronous epileptiform bursts and were much larger than those associated with partially synchronous events. Firing reached a higher peak frequency and was of longer duration for fully synchronous events. Field potentials corresponding to partially synchronous bursts consisted of a fast negative deflection of duration 20–50 ms followed by a slower positive wave of duration 150–300 ms. The fast deflection was correlated with the peak increase in extracellular firing. The slower positive wave occurred simultaneously with an absence in extracellular firing and was larger in amplitude in records from stratum radiatum than in stratum pyramidale (Fig. 5B).
, http://www.100md.com
A, multiunit records from 3 electrodes located in stratum pyramidale, the timing of detected spikes (upper traces) and field potential recorded in stratum radiatum (lower trace) during the transition to epileptiform synchrony. The transition was abrupt with a large increase in field potential amplitude and a prolongation of the interval between population bursts. B, partial synchrony discharge. The timing of spikes recorded from three sites in stratum pyramidale and field potentials from stratum pyramidale and stratum radiatum. Each event consisted of an acceleration of firing followed by a silent period. Field potentials were larger in stratum radiatum and were terminated by a slow positive wave of duration 100–200 ms and an absence of firing. Note that the polarity of the field potential in stratum pyramidale was opposed to that in stratum radiatum. C, epileptiform discharges involve more multiunit firing and larger field potentials than partially synchronous events. The SR field fluctuates before the discharge while the SP field potential is restricted to the spike burst.
, http://www.100md.com
We attempted to induce a stable partially synchronous activity so that we could examine the mechanisms involved in its generation. As shown in the traces of action potential timing in Fig. 6A, lower concentrations of antagonists at GABAA receptors (bicuculline, 2–5 μM, n= 8; picrotoxin 5–15 μM, n= 6 slices) could induce a stable, maintained activity that did not become fully synchronous. In these experimental conditions full synchrony did not emerge for the duration of the experiment (up to 6 h). Intracellular records were made from CA3 pyramidal cells (n= 12) and inhibitory cells (n= 4) during these experiments. Eight of 12 pyramidal cells discharged with some but not all partially synchronous events (Fig. 6B). In contrast inhibitory cells recorded near to stratum pyramidale (n= 4) fired at high frequency during each event (Fig. 6C). In pyramidal cell recordings the cellular correlate of the wave component of the field was a hyperpolarizing potential. The hyperpolarization was evident in records from pyramidal cells that did not discharge during population events suggesting that it originated synaptically (Fig. 6B). In four cells it reversed at 102 ± 7 mV (Fig. 6D and E) and it was suppressed by the GABAB receptor antagonist CGP52432 (5 μM, n= 4 cells). These data suggest that high frequencies of interneurone firing during partially synchronous events generate a synaptic potential mediated by GABAB receptors which acts to terminate population firing.
, 百拇医药
Stable partially synchronous activity that did not progress to epileptiform synchrony was produced by low doses of GABAA antagonists. A, timing of action potentials recorded from three electrodes in stratum pyramidale in the presence of 3 μM bicuculline. Traces obtained at an interval of 30 min show the stability of the activity. B and C, records during stable partially synchronous discharges are from the top: intracellular potential, extracellular record (filtered with a pass-band of 0.1–10 kHz), extracellular record (filtered at 0.5–10 kHz) and a histogram of the timing of the first intracellular action potential with respect to the mid-point of the field event. B, two traces of pyramidal cell activity, one with and the other without firing. Multiunit activity was associated with a barrage of EPSPs which could induce firing, while the slow wave component of the extracellular field was associated with an intracellular hyperpolarization. C, inhibitory cells discharged at high frequency simultaneously with the extracellular action potentials. D, reversal of the hyperpolarizing event in a pyramidal cell. Three membrane potential traces are shown at each of four membrane potentials. Intracellular traces were triggered from the peak of firing in the multiunit recording. E, plot of the reversal of the hyperpolarizing event. Membrane potential was plotted against the difference between potential at 200 ms before extracellular firing and a time corresponding to the maximal hyperpolarization at –52 mV (dotted line in D). Linear regression analysis indicated that the inhibitory event reversed at –105 mV.
, http://www.100md.com
If inhibitory signalling mediated by GABAB receptors shapes partially synchronous population firing, then antagonists at these receptors should disrupt it. We tested this point by applying the antagonist CGP52432 (5 μM) during the expression of partially synchronous firing induced by low concentrations of GABAA receptor antagonists. In four slices, rhythmicity was abolished, and in six slices population events were much prolonged and consisted of an oscillating series of EPSPs accompanied by a maintained population firing while their frequency was reduced (Fig. 7A).
, 百拇医药
A, CGP52432 (2 μM) disrupted partially synchronous activity. Bic shows intracellular and extracellular records obtained during the expression of a stable partially synchronous activity (4 μM bicuculline). CGP52432 suppressed intracellular hyperpolarizations. The duration of population bursts increased and their frequency was reduced but a slower thythmicity remained. B, prior application of CGP52432 (2 μM) abolished the expression of partially synchronous oscillations, but did not suppress epileptiform bursts (bicuculline, 20 μM). Multiunit frequency derived from all spikes detected by an extracellular electrode. Ba and b, extracellular records from two sites show the absence of partially synchronous firing after applying bicuculline in the presence of CGP52432. Bc, the first and second synchronous bursts were prolonged.
, 百拇医药
These data suggest that signals mediated by GABAB receptors are involved in the emergence of partially synchronous rhythmic activity observed when GABAA mediated inhibition is suppressed. We next asked whether this activity is a necessary precursor for the emergence of epileptiform bursting. In five experiments (Fig. 7B) the GABAB antagonist CGP52432 (5 μM) was applied before fast GABAergic synaptic signalling was suppressed by bicuculline (10 μM). The GABAB antagonist alone changed neither the frequency nor the pattern of CA3 population firing (n= 5 slices). Subsequent application of bicucculine induced an epileptiform activity in all slices that was not preceded by partially synchronous activity (Fig. 7Bb). The first two to five synchronous bursts were of longer duration than when GABAB mediated signalling was functional (Fig. 7Bc).
, http://www.100md.com
Discussion
When fast synaptic inhibition is functional, discharges of the CA3 hippocampal cell population in vitro are asynchronous and largely unpatterned. When it is suppressed, all cells of the network discharge in epileptiform synchrony. Here we show that during the transition between these states, CA3 cells discharge in a partially synchronous fashion (Figs 2–4). Some, but not all, pyramidal cells fire simultaneously in events that recur at intervals of 0.5–2.0 s and are associated with local field potentials (Figs 4 and 5). Partially synchronous population activity may be induced and maintained by low doses of GABAA receptor antagonists (Fig. 5). The patterning of this activity depends in part on IPSPs mediated by GABAB receptors (Figs 6 and 7) which act to terminate the partial population discharges.
, http://www.100md.com
Changes in population activity during the transition to epileptiform synchrony
These data reveal a sequence of changes in CA3 population activity initiated by the application of GABAA receptor antagonists, and ultimately leading to the sudden emergence of epileptiform bursting. First firing, measured as the summed frequency of multiunit activity, increased (Fig. 2D). This initial increase in CA3 population activity presumably results from the reduction in the inhibitory control of pyramidal cell firing (Miles et al. 1996; Hausser & Clark, 1997; Cohen & Miles, 2000). It seems probable that both inhibitory and pyramidal cell firing increased during this period, although interneurone activity was not explicitly measured.
, 百拇医药
Then oscillations in CA3 cell firing were detected at single sites (Fig. 2D). A loss of inhibitory control has been shown to facilitate the transmission of firing via recurrent excitatory circuits involving CA3 pyramidal cells (Miles & Wong, 1987). Data presented here suggest that inhibitory cells are also excited and generate GABAB-mediated IPSPs which act to curtail the spread of firing via recurrent circuits (Fig. 6C). It seems that high frequency inhibitory cell firing is needed to liberate sufficient inhibitory transmitter to activate postsynaptic GABAB receptors (Alger, 1984; Benardo, 1997; Kim et al. 1997; Thomson & Destexhe, 1999) which are largely expressed at extra-synaptic sites (Scanziani, 2000; Kulik et al. 2003). The resulting sequences of summed EPSPs followed by summed GABAB-mediated IPSPs (Fig. 7A) are similar to events described by Scanziani et al. (1991). They generate a characteristic extracellular signal sequence consisting of a fast field potential spike corresponding to multiunit firing followed by a slower field potential wave correlated with the GABAB-mediated IPSP (Fig. 6).
, http://www.100md.com
Records from several sites showed that partially synchronous activity was expressed in coherent fashion throughout the CA3 region (Fig. 3G and H). If the population activity depends in part on synaptic events mediated via recurrent collaterals of CA3 pyramidal cells then its spatial coherence should depend on their projection patterns (Li et al. 1994). These collaterals ramify throughout the CA3 region in overlapping fashion and so seem unlikely to favour the formation of distinct clusters of rhythmically active cells. The axons of interneurones generating the inhibitory synaptic events that terminate partially synchronous bursts are also likely to diverge widely throughout the CA3 region (Gulyas et al. 1993; Buhl et al. 1994).
, http://www.100md.com
The increase in coherence of CA3 population firing corresponds to an increase in the synchronization of single cell discharges. Neurones will tend to be recruited to the synchronously firing ensemble both via the excitation mediated by recurrent connections between pyramidal cells, and paradoxically by the increase in amplitude of GABAB-mediated signals. Pyramidal cells have been shown to fire on the rebound after a synaptic inhibition (Cobb et al. 1995) and the divergence of inhibitory cell axons should assist a synchronous rebound firing. As for rhythmically discharging single cells (Gorman et al. 1982), both excitatory and inhibitory influences may help to shape rhythmic population activities.
, http://www.100md.com
The oscillations recurred as events which became increasingly coherent with time before epileptiform firing was initiated with an abrupt increase in the extracellular field potential amplitude (Fig. 5A). This increase in amplitude presumably corresponds to the sudden recruitment of many CA3 cells to the population discharges. The cellular or synaptic mechanisms for this transition are not clear. Since partial synchrony bursts were terminated by a large, mostly GABAB receptor mediated, inhibitory potential (Fig. 7A), the transition to prolonged full synchrony bursts could occur when the strength of AMPA- and NMDA-mediated excitatory synaptic events overcome the braking effect of the GABAB-mediated inhibition. The contribution of the residual GABAA receptor-mediated inhibition might act more on global tissue activity level than as a burst brake since such an effect was not observed after GABAB blockade in stable partial synchrony (Fig. 7A). At the transition to epileptiform synchrony there was an increase in the mean amplitude of extracellular spikes reflecting collisions of action potentials generated by distinct cells (Fig. 2F). This increase in synchrony of cellular firing in the range of 1–3 ms may indicate an enhanced ephaptic coupling (Haas & Jefferys, 1984; Faber & Korn, 1989) or in the efficacy of electrotonically mediated signalling (MacVicar & Dudek, 1981; Nimmrich et al. 2005).
, 百拇医药
Extracellular measures of synchrony
Neuronal synchrony is not a well defined term since it can refer to the simultaneous occurrence of either action potential discharges or synaptic events over a range of time scales. In this study we examined a form of neuronal synchrony that increased in coherence with time after application of GABAA receptor antagonists. Field potential measures of synchrony were compared (Fig. 4) with those derived from the frequency of action potentials generated by a neuronal population of several hundreds of CA3 cells (Cohen & Miles, 2000). Field potentials above baseline were always associated with peaks of multiunit activity, whereas early in the transition some bursts of spikes were not associated with above baseline field potentials (Fig. 4). This observation suggests that indices based on frequency could be at least as sensitive as the field potential measures in detecting the emergence of weak, partial synchrony in which, initially at least, only a small fraction of the CA3 population fired simultaneously within a time interval of 20–100 ms. We note that since CA3 cells fire spontaneously in vitro, the spike frequency index is non-null in control conditions and so can be both positively and negatively modulated. This may enhance its sensitivity to small changes in population activity.
, 百拇医药
Functional significance
Partially synchronous neuronal activities similar to those described here have been observed in hippocampal slices exposed to GABAA receptor antagonists at doses that do not completely suppress fast synaptic inhibition (Schneiderman, 1986; Scanziani et al. 1991). Do they have a counterpart in the range of activities generated by the hippocampus in vivo The most comparable activity is sharp wave bursts observed in passive behaviours and slow wave sleep (Suzuki & Smith, 1985; Buzsaki, 1986). Sharp waves are also generated in the CA3 region by activity in a small proportion of CA3 pyramidal cells and interneurones (Csicsvari et al. 2000). If the analogy is precise, our data suggest that a temporary reduction in the inhibitory control of CA3 population activity might result in the emergence of sharp waves. It remains to be determined what changes in the modulating transmitter environment of the CA3 region could underly a transient disinhibition in the intact animal.
, 百拇医药
Disinhibited hippocampal slices have been used for some time as a chronic model of epilepsy based on observations that drugs interfering with GABAergic function are convulsants and that reinforcing GABAergic signalling may have antiepileptic effects. More recent work suggests that GABAergic systems are perturbed in epilepsies of the human temporal lobe, but are due to changes in postsynaptic chloride homeostasis, rather than transmitter release or receptor function (Cohen et al. 2002). Even so our data may reveal mechanisms operating during the emergence of epileptiform synchrony. Changes in the dynamics of human EEG signals have been detected before seizure onset (Martinerie et al. 1998). Our data suggest precursor oscillations depend on inhibitory signalling, mediated by GABAB receptors. Even so, we note that the prior suppression of GABAB receptor-mediated signalling did not prevent the expression of synchrony (Fig. 7B). Rather synchrony was induced with no transition through states of partial synchrony and consisted of events of longer duration occurring at a lower frequency.
, 百拇医药
References
Alger BE (1984). Characteristics of a slow hyperpolarizing synaptic potential in rat hippocampal pyramidal cells in vitro. J Neurophysiol 52, 892–910.
Ankri N & Korn H (1999). A statistical method for correcting distortions of amplitude distribution histograms due to collisions of synaptic events. J Neurosci Meth 91, 83–99.
Bedenbaugh P & Gerstein GL (1997). Multiunit cross correlation differs from the average single unit normalised correlation. Neural Computation 9, 1265–1275.
, 百拇医药
Benardo LS (1997). Recruitment of GABAergic inhibition and synchronization of inhibitory interneurons in rat neocortex. J Neurophysiol 77, 3134–3144.
Ben-Ari Y, Cherubini E, Corradetti R & Gaiarsa JL (1989). Giant synaptic potentials in immature rat CA3 hippocampal neurones. J Physiol 416, 303–325.
Berger H (1929). Uber das Elektrenkephalogramm des Menschen. 1st report. Arch Psychiat Nervenkr 87, 527–570.
, http://www.100md.com
Brecht M, Singer W & Engel AK (1999). Patterns of synchronisation in the superior colliculus of anesthetised rats. J Neurosci 19, 3567–3579.
Buhl EH, Halasy K & Somogyi P (1994). Diverse sources of hippocampal unitary inhibitory postsynaptic potentials and the number of synaptic release sites. Nature 368, 823–828.
Buszaki G (1986). Hippocampal sharp waves: their origin and significance. Brain Res 398, 242–252.
, 百拇医药
Cobb SR, Buhl EH, Halasy K, Paulsen O & Somogyi P (1995). Synchronization of neuronal activity in hippocampus by individual GABAergic interneurons. Nature 378, 75–78.
Cohen I & Miles R (2000). Contributions of intrinsic and synaptic activities to the generation of neuronal discharges in in vitro hippocampus. J Physiol 524, 485–502.
Cohen I, Navarro V, Clemenceau S, Baulac M & Miles R (2002). On the origin of interictal activity in human temporal lobe epilepsy in vitro. Science 298, 1418–1421.
, 百拇医药
Csicsvari J, Hirase H, Mamiya A & Buzsaki G (2000). Ensemble patterns of hippocampal CA3-CA1 neurons during sharp wave-associated population events. Neuron 28, 585–594.
Engel AK, Fries P & Singer W (2001). Dynamic predictions: oscillations and synchrony in top-down processing. Nat Rev Neurosci 2, 704–716.
Faber DS & Korn H (1989). Electrical field effects: their relevance in central neural networks. Physiol Rev 69, 821–863.
, 百拇医药
Fortin G, Kato F, Lumsden A & Champagnat J (1995). Rhythm generation in the segmented hindbrain of chick embryos. J Physiol 486, 735–744.
Gorman AL, Hermann A & Thomas MV (1982). Ionic requirements for membrane oscillations and their dependence on the calcium concentration in a molluscan pace-maker neurone. J Physiol 327, 185–217.
Grillner S & Wallen P (1999). On the cellular bases of vertebrate locomotion. Prog Brain Res 123, 297–309.
, http://www.100md.com
Gulyas AI, Miles R, Hajos N & Freund TF (1993). Precision and variability in postsynaptic target selection of inhibitory cells in the hippocampal CA3 region. Eur J Neurosci 5, 1729–1751.
Haas HL & Jefferys JG (1984). Low-calcium field burst discharges of CA1 pyramidal neurones in rat hippocampal slices. J Physiol 354, 185–201.
Hausser M & Clark BA (1997). Tonic synaptic inhibition modulates neuronal output pattern and spatiotemporal synaptic integration. Neuron 19, 665–678.
, http://www.100md.com
Kim U, Sanchez-Vives MV & McCormick DA (1997). Functional dynamics of GABAergic inhibition in the thalamus. Science 278, 130–134.
Kulik A, Vida I, Lujan R, Haas CA, Lopez-Bendito G, Shigemoto R & Frotscher M (2003). Subcellular localization of metabotropic GABAB (B) receptor subunits GABA (B1a/b) and GABA (B2) in the rat hippocampus. J Neurosci 23, 11026–11035.
Laurent G (1999). A systems perspective on early olfactory coding. Science 286, 723–728.
, 百拇医药
Levy R, Hutchison WD, Lozano AM & Dostrovsky JO (2000). High-frequency synchronization of neuronal activity in the subthalamic nucleus of parkinsonian patients with limb tremor. J Neurosci 20, 7766–7775.
Li XG, Somogyi P, Ylinen A & Buzsaki G (1994). The hippocampal CA3 network: an in vivo intracellular labeling study. J Comp Neurol 339, 181–208.
Lincoln DW & Wakerley JB (1975). Factors governing the periodic activation of supraoptic and paraventricular neurosecretory cells during suckling in the rat. J Physiol 250, 443–461.
, http://www.100md.com
MacVicar BA & Dudek FE (1981). Electrotonic coupling between pyramidal cells: a direct demonstration in rat hippocampal slices. Science 213, 782–785.
Martinerie J, Adam C, Le Van Quyen M, Baulac M, Clemenceau S, Renault B & Varela FJ (1998). Epileptic seizures can be anticipated by non-linear analysis. Nat Med 4, 1173–1176.
Miles R, Toth K, Gulyas AI, Hajos N & Freund TF (1996). Differences between somatic and dendritic inhibition in the hippocampus. Neuron 16, 815–823.
, http://www.100md.com
Miles R & Wong RKS (1987). Inhibitory control of local excitatory circuits in the guinea-pig hippocampus. J Physiol 388, 611–629.
Nimmrich V, Maier N, Schmitz D & Draguhn A (2005). Induced sharp wave-ripple complexes in the absence of synaptic inhibition in mouse hippocampal slices. J Physiol 563, 663–670.
Pedley TA & Traub RD (1990). Physiological Basis of the EEG, 2nd edn, ed. Daly DD & Pedley TA, pp. 107–137. Raven Press, New York.
, 百拇医药
Scanziani M (2000). GABA spillover activates postsynaptic GABAB receptors to control rhythmic hippocampal activity. Neuron 25, 673–681.
Scanziani M, Gahwiler BH & Thompson SM (1991). Paroxysmal inhibitory potentials mediated by GABAB receptors in partially disinhibited rat hippocampal slice cultures. J Physiol 444, 375–396.
Schneiderman JH (1986). Low concentrations of penicillin reveal rhythmic, synchronous synaptic potentials in hippocampal slice. Brain Res 398, 231–241.
, 百拇医药
Schwartzkroin PA (1994). Cellular electrophysiology of human epilepsy. Epilepsy Res 17, 185–192.
Schwartzkroin PA & Prince DA (1978). Cellular and field potential properties of epileptogenic hippocampal slices. Brain Res 147, 117–130.
Staley KJ, Bains JS, Yee A, Hellier J & Longacher JM (2001). Statistical model relating CA3 burst probability to recovery from burst-induced depression at recurrent collateral synapses. J Neurophysiol 86, 2736–2747.
, 百拇医药
Staley KJ, Longacher M, Bains JS & Yee A (1998). Presynaptic modulation of CA3 network activity. Nat Neurosci 1, 201–209.
Suzuki SS & Smith GK (1985). Single-cell activity and synchronous bursting in the rat hippocampus during waking behavior and sleep. Exp Neurol 89, 71–89.
Szucs A (1998). Applications of the spike density function in analysis of neuronal firing patterns. J Neurosci Meth 81, 159–167.
Thomson AM & Destexhe A (1999). Dual intracellular recordings and computational models of slow inhibitory postsynaptic potentials in rat neocortical and hippocampal slices. Neuroscience 92, 1193–1215.
Wong RO (1999). Retinal waves and visual system development. Ann Rev Neurosci 22, 29–47., 百拇医药(Ivan Cohen, Gilles Huberfeld and Richard)
Abstract
Suppressing inhibition mediated by GABAA receptors induces rhythmic bursts of synchronous firing in the CA3 region of the hippocampus. Extracellular and intracellular records were made from guinea pig hippocampal slices to examine the emergence of this synchrony. We found that application of GABAA receptor antagonists initiated a sequence of changes in the activity of the CA3 neuronal population. First, the frequency of firing detected in multiunit records increased. Then, firing began to oscillate with increases followed by decreases in firing that occurred at intervals of 0.5–2 s. The coherence of the rhythmic activity at a single site increased with time, and discharges at distant sites in the CA3 region became correlated. Fluctuations in firing were associated with extracellular field potentials. Finally, epileptiform events associated with large field potentials began to recur at intervals of 5–10 s. The onset of fully synchronous events was sudden and correlated with a large increase in the amplitude of the field potential. Thus the CA3 population can express states of partial population synchrony preceding the onset of epileptiform discharges. A similar activity was induced and maintained by applying low doses of GABAA receptor antagonists. Intracellular records suggest that inhibitory signalling mediated by GABAB receptors contributes to the emergence of this activity. States of partial synchrony in the CA3 region exposed to GABAA receptor antagonists therefore depend on alternating periods of firing, presumably dependent on excitatory synaptic mechanisms, and silence, mediated in part by the activation of GABAB receptors.
, 百拇医药
Introduction
Synchronous discharges are generated by neuronal populations in most brain regions and are often associated with specific behavioural states. Precisely timed simultaneous discharges of spinal motoneurones are required for motor activities (Grillner & Wallen, 1999). The simultaneous firing of hypothalamic neurones underlies the liberation of hormones (Lincoln & Wakerley, 1975). Synchronous activities have also been associated with olfactory perception (Laurent, 1999) and more generally with the treatment of sensory information (Engel et al. 2001). Neuronal synchrony is observed during perinatal development, in many brain structures including the hippocampus (Ben-Ari et al. 1989), retina (Wong, 1999) and medulla (Fortin et al. 1995). Pathological neuronal synchronization is associated with several disease states including Parkinsons (Levy et al. 2000) and the epilepsies (Schwartzkroin, 1994).
, 百拇医药
While this list shows that synchrony is ubiquitous in the nervous system, the term is not well defined. It derives from measurements on two different types of signal. Synchronous brain activity was first deduced (Berger, 1929) from observations of rhythmic waves in electroencephalographic (EEG) records (Pedley & Traub, 1990). Oscillating EEG signals of distinct frequencies are generated in most brain regions and reflect, at least in part, the occurrence of synchronous synaptic events. However, while an EEG signal is a continuous function of time, the term ‘synchrony’ has also been applied to the generation of action potentials, which is a discrete process. Action potential synchrony has usually been quantified by cross-correlation measures on isolated single-unit discharges (Bedenbaugh & Gerstein, 1997; Brecht et al. 1999) which determines the temporal relations between neuronal firing in cell pairs. Few studies have compared the synchrony of action potential generation in a neuronal population to synchrony as defined from EEG or field potential measurements.
, 百拇医药
We examined this question for the transition to synchronous firing that occurs when GABAA-mediated synaptic transmission is suppressed in slices of the hippocampus maintained in vitro (Schwartzkroin & Prince, 1978; Miles & Wong, 1987). Disinhibition leads to the emergence of rhythmic population bursts at intervals of 5–10 s. The initiation of these events depends in part on excitatory synaptic interactions between CA3 pyramidal cells. The duration of individual synchronous bursts is thought to be limited by transmitter availability at these connections (Staley et al. 1998) and the interval between bursts may depend on the dynamics of replenishment of releasable transmitter (Staley et al. 2001). However the emergence of this population behaviour has not been studied in detail.
, http://www.100md.com
In this study we used intracellullar and multiple extracellular recordings to examine the transition to synchronous activity induced by disinhibition. We found that synchrony emerged gradually rather than in an all-or-none fashion. CA3 cell activity first increased, and then cyclically recurring increases and decreases in population firing began to occur coherently throughout the CA3 region before the emergence of epileptiform population events. These oscillations reflect a state of partially synchronous CA3 cell firing and are accompanied by biphasic field potentials. Intracellularly, field potentials correspond to a barrage of synaptic excitation which is terminated by an inhibition mediated by activation of GABAB receptors. Thus patterned fluctuations in CA3 cell firing that precede disinhibition-induced epileptiform synchrony are partly shaped by GABAergic signalling.
, 百拇医药
Methods
Slice preparation
Hippocampal slices were prepared according to local INSERM guidelines from guinea-pigs of weight 100–200 g and age 6–10 weeks, which were anaesthetized by intraperitoneal injection of ketamine (200 mg kg–1) and chloral hydrate (800 mg kg–1). Animals were perfused intracardially with a cold (3–5°C) solution consisting of 248 mM sucrose, 26 mM NaHCO3, 1 mM KCl, 10 mM MgCl2, 1 mM CaCl2 and 10 mM glucose, equilibrated with 5% CO2 in 95% O2. They were then killed by decerebration, both hippocampi were dissected free and several slices of thickness 400 μm were cut in the same sucrose-containing solution using a vibratome. Slices were then transferred to an interface recording chamber where they were maintained at 35°C, perfused with a solution containing 124 mM NaCl, 26 mM NaHCO3, 4 mM KCl, 2 mM MgCl2, 2 mM CaCl2 and 10 mM glucose, and equilibrated with 5% CO2 in 95% O2.
, http://www.100md.com
Drugs
We used picrotoxin (50 μM), bicuculline (10 μM) or gabazine (SR-95531, 5 μM) to suppress GABAA receptor mediated synaptic signalling. The effects of lower concentrations of GABAA receptor antagonists were examined in some experiments. Synaptic events mediated via activation of GABAB receptors were blocked by CGP 52432 (2–4 μM). Drugs were obtained from Tocris Cookson (Bristol, UK).
Recordings
, http://www.100md.com
Electrodes used for extracellular recording were made from tungsten wire of 50 μm diameter. Their tips were etched to a diameter of 2–6 μm under a dissecting microscope, by passing an alternating current (1–5 V for 1–2 min) between the tungsten wire and a carbon electrode both immersed in 5 M KNO2. Electrodes were insulated by applying a coat of varnish to within about 200 μm of their tip. Their resistance in physiological solution was 500–1000 k. Up to four metal electrodes were mounted on holders controlled by separate manipulators. Potential differences between tungsten electrodes and a reference Ag–AgCl electrode were amplified 1000x and filtered with a pass-band of 0.1 Hz to 10 kHz (AM systems, 1700). Intracellular records were made with glass electrodes filled with 2 M potassium acetate and bevelled to final resistances of 40–60 M. Intracellular records were amplified with an Axoclamp 2B amplifier (Axon Instruments, Union City, CA, USA) and filtered with a high cut-off at 3 kHz. CA3 inhibitory cells records were distinguished from those of pyramidal cells by several criteria. They included a shorter duration action potential, a larger action potential afterhyperpolarization and the discharge of weakly accommodating action potentials in response to depolarizing current injections rather than the burst firing pattern typical of CA3 pyramidal cells (Miles et al. 1996). Signals were stored on a modified videotape recorder (Neurodata, Cygnus-Technology, Delaware Water Gap, PA, USA), digitized at 20 kHz using a 12-bit, A–D converter (Digidata 1200A, Axon Instruments) and monitored during experiments with the programme Axoscope (Axon Instruments).
, http://www.100md.com
Experimental procedures
Multiple tungsten electrodes were placed in the stratum pyramidale of the CA3 region to record extracellular signals. Such electrodes record action potentials generated by neurones within a radius of about 80 μm (Cohen & Miles, 2000). We therefore placed electrodes with separations of about 500 μm so that they would detect the activity of distinct CA3 cell populations. In some experiments these records were compared with field potentials recorded by another extracellular electrode. The field potential electrode was placed in the CA3 stratum radiatum, where we reasoned that synaptically generated field potentials would be maximal due to the recurrent synapses between CA3 pyramidal cells. Records were made at distances of at least 200 μm from str. pyramidale to minimize contributions of spikes generated by back-propagating action potentials (Cohen & Miles, 2000). Extracellular action potentials, possibly generated by the sparse interneurones with somata in this region, were occasionally recorded by these electrodes.
, 百拇医药
Data treatment
Signals derived from extracellular recordings were processed using routines written in the LabVIEW programming environment (National Instruments, Austin, TX, USA). Several indices were derived from each recording (Fig. 1). Distinct frequency components were separated by digitally implemented Bessel filters. Filtering with a pass band of 300–5000 Hz permitted the isolation of action potentials (Fig. 1B). Extracellular spikes were detected using a previously described ‘up-only’ algorithm (Cohen & Miles, 2000) and their timing and amplitude was measured. Since extracellular spike amplitude depends largely on the arbitrary spatial relations between an electrode and the spike generating element, we represented all detected spikes as vertical lines of the same height in some plots (Fig. 1C). To estimate the frequency of all spikes recorded by an electrode, time sequences of the occurrence of detected action potentials were convolved with a gaussian function of S.D. 5–5000 ms (Szucs, 1998). The resulting traces correspond to a frequency function for firing in a population of several hundred CA3 cells (Fig. 1D). A similar continuous measure of average amplitude for spikes detected by a single electrode was obtained by convolving the amplitude of each action potential with a gaussian function of S.D. 20 ms and dividing at each time by the instantaneous frequency. Field potentials were derived from the same extracellular recordings using a band pass of 0.1–100 Hz (Fig. 1A). Extracellular field potential events were detected using the same ‘up-only’ algorithm together with a user-defined threshold
, http://www.100md.com
Extracellular records made with one electrode in stratum pyramidale (SP) and another in stratum radiatum (SR) in the presence of functional synaptic inhibition. A, low frequency signals derived from the SP and SR electrodes, with a 0.5–100 Hz pass band. B, multiunit activity detected with a pass band of 100–3000 Hz from SP and SR electrodes. C, each detected spike from the SP signal is displayed as a vertical bar. D, continuous index of spike frequency derived by convolving spike timing with a gaussian function of S.D. 20 ms (black trace) or 1000 ms (red trace).
, 百拇医药
We wished to quantify fluctuations in multiunit activty recorded from a single electrode. This was done by considering rapid fluctuations in the frequency of detected spikes (as in Fig. 1D) while correcting for the slow increase in activity due to the suppression of synaptic inhibition (Cohen & Miles, 2000). The correction was made using two estimates of frequency obtained by convolving spike trains with a gaussian of S.D. 1000 ms, to capture slow changes in population firing (F), and with a gaussian of S.D. 50 ms, to detect transient fluctuations of activity (f). A sliding variance index, VAR(t), was computed to measure the time course of fast frequency fluctuations:
, 百拇医药
We constructed sliding covariance (COV) and correlation (COR) indices to measure the coherence of changes in the frequency of all spikes generated by different cell populations recorded by pairs of extracellular electrodes. The indices, again corrected for the increased firing due to disinhibition as in calculations of variance, were constructed according to the equations:
F and f correspond to frequency functions convolved with gaussian functions to reveal slow (S.D. 1000 ms) and fast fluctuations (S.D. 50 ms) in the frequency of spikes detected by one electrode. G and g are the equivalent parameters estimated for activity detected by a second electrode. VAR is computed as above and the mean is calculated over a sliding window centred at time t. COR, the correlation index can vary between +1, indicating a perfect coherence between the two signals, and –1, corresponding to changes in frequency in opposing directions in signals derived from the two recordings. The correlation index was qualitatively insensitive to changes in the range of 50–200% for temporal resolution. Programming details are available from icohen@bcm.tmc.edu and all tools described in this article are available as part of the software package Spikoscope distributed under the GNU Public License at http://glab.bcm.tmc.edu.
, 百拇医药
Results
Spontaneous action potentials were always detected in extracellular records from the CA3 stratum pyramidale in control conditions. Their frequency fluctuated but recurring temporal patterns were rarely evident. Field potentials recorded from the stratum radiatum showed occasional small deflections (Fig. 1). The application of antagonists at GABAA receptors (picrotoxin, 50 μM, n= 5; bicuculline, 10 μM, n= 6; or gabazine (SR-95531), 5 μM, n= 3 slices) induced an epileptiform population activity. Population bursts were generated in recurring fashion at intervals of 3–10 s. Each burst was followed by a silent period and then an increase in activity before the next burst.
, 百拇医药
Epileptiform activity did not emerge in an all-or-none fashion but rather after a transition period of increasingly synchronous firing (Fig. 2A–E). Disinhibition first induced an increase in mean firing frequency (Fig. 2D, red trace) in records from single extracellular electrodes in the CA3 stratum pyramidale. Then patterned fluctuations in population firing emerged (Fig. 2B and D black trace) with little further increase in mean frequency. They consisted of transient increases followed by decreases in firing and recurred at intervals in the range 0.5–2 s. The amplitude and coherence of these frequency fluctuations increased with time until fully synchronous epileptiform bursts emerged, usually abruptly (Fig. 2C and D). Full synchrony emerged with a large increase in field potential amplitude and an increase in the interval between events, which were used to define the timing of the transition. The mean time elapsing between the start of perfusion and the emergence of full synchrony was 6.5 min (range 4.5 min to 9.7 min). The mean action potential frequency increased by 31% from 47 ± 24 spikes s–1 in control conditions to 62 ± 25 spikes s–1 just before the onset of epileptiform activity (mean ±S.D., n= 13 recording sites in 5 animals). Peak firing frequencies for fully synchronous population events were much higher than those during partial synchrony, even though multiunit activity may be underestimated in this condition.
, 百拇医药
A–C, multiunit activity recorded by an electrode in the CA3 stratum pyramidale at 2, 6 and 7.5 min after perfusion with 10 μM bicucculline. D–F, evolution of indices over the full transition. Timing of traces shown in A–C are indicated below graph F. D, frequency of all detected spikes convolved with gaussian functions of 100 ms (black) and 1600 ms (red) reveal fast and slow variations during the transition to synchrony. Slow variations reach a plateau frequency before the first epileptiform discharge (vertical dotted line). Fast oscillations increase in amplitude before the onset of fully synchronous firing. E, a sliding variance index reveals a progressive increase in coherence of fast fluctuations around the mean frequency. F, the mean amplitude of detected action potentials increased only after the onset of epileptiform activity. This implies that spike superpositions within 1–2 ms do not occur during the transition period. Black and red traces correspond to smoothing gaussians of 100 and 1600 ms, respectively.
, 百拇医药
We used the variance index described in Methods to assess changes in fluctuations in spike frequency at a single site during the transition to epileptiform discharges (Fig. 2E). In 13 records from single sites, its value increased from 965 ± 645 Hz2 in control conditions to 5334 ± 2425 Hz2 just before the onset of epileptiform discharges (mean ±S.D., n= 13, average over 30 s time windows). This more than fivefold increase in variance is much larger than that expected (70%) with a scaling of firing in independent neurones and the observed 30% increase in frequency. Thus epileptiform synchrony is preceded by increasingly coherent variations in the firing of a local neuronal population.
, 百拇医药
During this increase in activity, close collisions (Ankri & Korn, 1999) might lead to underestimates of firing frequency. We assessed the prevalence of spike collisions by measuring the mean amplitude of detected action potentials. Since the amplitudes of superimposed spikes sum, an increase in mean amplitude would suggest an increased number of collisions. However, mean spike amplitude was constant until the onset of epileptiform discharges (Fig. 2F). Thus spikes generated by different cells superimpose rarely, within the 1–2 ms resolution of our detection algorithm (Cohen & Miles, 2000), before the transition to epileptiform activity.
, 百拇医药
We next compared the evolution of population activity in records from different sites in the CA3 region (Fig. 3A–C). Recording electrodes were separated by at least 500 μm, in order to sample activity from different neuronal populations. On exposure to GABAA receptor antagonists, firing frequency (Fig. 3D and E) and variance (Fig. 3F) appeared to increase in parallel in records from different sites. We quantified relations between activity from distinct electrodes (Fig. 3H) using the correlation index described in Methods. This index could take values between –1 for antiphase changes in frequency and 1 for in-phase changes in frequency at two sites. In control conditions the index fluctuated with time by 0.08 units (S.D.) with a mean value from 11 pairs of extracellular recordings of 0.07 ± 0.03 units (mean ±S.E.M.). The mean correlation index increased to 0.74 ± 0.03 units (mean ±S.E.M.) during the 30 s before the onset of fully synchronous epileptiform discharges. Thus, the coherence of population activities recorded from separate sites increased together with the increase in fluctuations of activity at a single site. However, these two changes always followed the initial increase in neuronal activity induced by disinhibition (cf. Fig. 3D–H).
, 百拇医药
A–C, multiunit activity recorded by two electrodes separated by 500 μm in CA3 stratum pyramidale at 5, 8 and 8.5 min after perfusion with 50 μM picrotoxin. D–H, evolution of indices over the full transition. Timing of traces shown in A–C are indicated below graph H. D and E, fast (black trace, convolved with a gaussian of S.D. 20 ms) and slow (red trace, gaussian of S.D. 1000 ms) fluctuations in spike frequency from the two sites. The vertical dotted line indicates the first epileptiform discharge. F, evolution with time of variance indices calculated for activity at each site. G and H, changes in covariance and correlation indices for activity recorded from the two electrodes. The recording started at the addition of picrotoxin. Traces shown in A correspond to the initial increase in firing. Activity at the two sites becomes progressively more synchronized in the traces for B and C. The value of the correlation index increased to 0.8 just before full synchrony.
, 百拇医药
We next compared changes in the frequency of multiunit activity with field potentials generated in the stratum radiatum during the transition to epileptiform synchrony (Fig. 4). In control conditions, small field potentials were evident. As synaptic inhibition was suppressed the timing of action potentials generated at separate sites in the stratum pyramidale became increasingly rhythmic. The increasing coherence of firing of distinct CA3 cell populations was associated with the emergence and growth of periodic field potentials (Fig. 4A–C). Changes in the frequency and amplitude of field potential events (Fig. 4D), defined by application of the ‘up-only’ algorithm, are shown in Fig. 4E and F, while Fig. 4G shows the correlation index. The frequency and amplitude of field potentials increased in parallel with the increasing coherence of firing at distinct sites.
, 百拇医药
A–D, frequency signals, convolved with a gaussian of S.D. 160 ms, derived from three multiunit records made from the CA3 stratum pyramidale (SP1, SP2 and SP3) together with the timing of spikes detected at each site and the field potential recorded by an electrode placed in stratum radiatum (SR). A, baseline activity before perfusion. B, at 6.5 min after the onset of perfusion with picrotoxin (50 μM) a poorly organized rhythmicity is evident in the frequency signals. C, at 7.5 min field potentials emerge and the rhythmicity in the frequency traces, SP1, 2 and 3 is enhanced. D, at 8.5 min, rhythmic activity is evident in both firing frequency traces and in the SR field. E–H, evolution over a full transition. E, SR field potential recording during the transition. Bicuculline perfusion started 6.5 min before the first full synchrony discharge (vertical dashed line). Full synchrony potentials are clipped. F and G, frequency and average amplitude of field potentials derived by convolving their timing and amplitude with a gaussian of S.D. 20 s. H, multiunit correlation index time course derived from activity at SP1 and SP2.
, http://www.100md.com
Both field potential amplitude and the interval between population bursts increased abruptly at the transition to fully synchronous discharges (Fig. 5A). The amplitude of the largest field potential associated with partial synchrony burst was 81 ± 51 μV, while that associated with the first full synchrony burst was 2250 ± 1300 μV (mean ±S.D., n= 4 animals). This suggests that many more cells were recruited to fire during the first epileptiform burst. Even so, partially synchronous events and fully synchronous discharges exhibited some common features (Fig. 5B and C). They were both preceeded by an accelerating frequency of multiunit discharges and a slow field potential. Field potentials were generated in both stratum pyramidale and radiatum during fully synchronous epileptiform bursts and were much larger than those associated with partially synchronous events. Firing reached a higher peak frequency and was of longer duration for fully synchronous events. Field potentials corresponding to partially synchronous bursts consisted of a fast negative deflection of duration 20–50 ms followed by a slower positive wave of duration 150–300 ms. The fast deflection was correlated with the peak increase in extracellular firing. The slower positive wave occurred simultaneously with an absence in extracellular firing and was larger in amplitude in records from stratum radiatum than in stratum pyramidale (Fig. 5B).
, http://www.100md.com
A, multiunit records from 3 electrodes located in stratum pyramidale, the timing of detected spikes (upper traces) and field potential recorded in stratum radiatum (lower trace) during the transition to epileptiform synchrony. The transition was abrupt with a large increase in field potential amplitude and a prolongation of the interval between population bursts. B, partial synchrony discharge. The timing of spikes recorded from three sites in stratum pyramidale and field potentials from stratum pyramidale and stratum radiatum. Each event consisted of an acceleration of firing followed by a silent period. Field potentials were larger in stratum radiatum and were terminated by a slow positive wave of duration 100–200 ms and an absence of firing. Note that the polarity of the field potential in stratum pyramidale was opposed to that in stratum radiatum. C, epileptiform discharges involve more multiunit firing and larger field potentials than partially synchronous events. The SR field fluctuates before the discharge while the SP field potential is restricted to the spike burst.
, http://www.100md.com
We attempted to induce a stable partially synchronous activity so that we could examine the mechanisms involved in its generation. As shown in the traces of action potential timing in Fig. 6A, lower concentrations of antagonists at GABAA receptors (bicuculline, 2–5 μM, n= 8; picrotoxin 5–15 μM, n= 6 slices) could induce a stable, maintained activity that did not become fully synchronous. In these experimental conditions full synchrony did not emerge for the duration of the experiment (up to 6 h). Intracellular records were made from CA3 pyramidal cells (n= 12) and inhibitory cells (n= 4) during these experiments. Eight of 12 pyramidal cells discharged with some but not all partially synchronous events (Fig. 6B). In contrast inhibitory cells recorded near to stratum pyramidale (n= 4) fired at high frequency during each event (Fig. 6C). In pyramidal cell recordings the cellular correlate of the wave component of the field was a hyperpolarizing potential. The hyperpolarization was evident in records from pyramidal cells that did not discharge during population events suggesting that it originated synaptically (Fig. 6B). In four cells it reversed at 102 ± 7 mV (Fig. 6D and E) and it was suppressed by the GABAB receptor antagonist CGP52432 (5 μM, n= 4 cells). These data suggest that high frequencies of interneurone firing during partially synchronous events generate a synaptic potential mediated by GABAB receptors which acts to terminate population firing.
, 百拇医药
Stable partially synchronous activity that did not progress to epileptiform synchrony was produced by low doses of GABAA antagonists. A, timing of action potentials recorded from three electrodes in stratum pyramidale in the presence of 3 μM bicuculline. Traces obtained at an interval of 30 min show the stability of the activity. B and C, records during stable partially synchronous discharges are from the top: intracellular potential, extracellular record (filtered with a pass-band of 0.1–10 kHz), extracellular record (filtered at 0.5–10 kHz) and a histogram of the timing of the first intracellular action potential with respect to the mid-point of the field event. B, two traces of pyramidal cell activity, one with and the other without firing. Multiunit activity was associated with a barrage of EPSPs which could induce firing, while the slow wave component of the extracellular field was associated with an intracellular hyperpolarization. C, inhibitory cells discharged at high frequency simultaneously with the extracellular action potentials. D, reversal of the hyperpolarizing event in a pyramidal cell. Three membrane potential traces are shown at each of four membrane potentials. Intracellular traces were triggered from the peak of firing in the multiunit recording. E, plot of the reversal of the hyperpolarizing event. Membrane potential was plotted against the difference between potential at 200 ms before extracellular firing and a time corresponding to the maximal hyperpolarization at –52 mV (dotted line in D). Linear regression analysis indicated that the inhibitory event reversed at –105 mV.
, http://www.100md.com
If inhibitory signalling mediated by GABAB receptors shapes partially synchronous population firing, then antagonists at these receptors should disrupt it. We tested this point by applying the antagonist CGP52432 (5 μM) during the expression of partially synchronous firing induced by low concentrations of GABAA receptor antagonists. In four slices, rhythmicity was abolished, and in six slices population events were much prolonged and consisted of an oscillating series of EPSPs accompanied by a maintained population firing while their frequency was reduced (Fig. 7A).
, 百拇医药
A, CGP52432 (2 μM) disrupted partially synchronous activity. Bic shows intracellular and extracellular records obtained during the expression of a stable partially synchronous activity (4 μM bicuculline). CGP52432 suppressed intracellular hyperpolarizations. The duration of population bursts increased and their frequency was reduced but a slower thythmicity remained. B, prior application of CGP52432 (2 μM) abolished the expression of partially synchronous oscillations, but did not suppress epileptiform bursts (bicuculline, 20 μM). Multiunit frequency derived from all spikes detected by an extracellular electrode. Ba and b, extracellular records from two sites show the absence of partially synchronous firing after applying bicuculline in the presence of CGP52432. Bc, the first and second synchronous bursts were prolonged.
, 百拇医药
These data suggest that signals mediated by GABAB receptors are involved in the emergence of partially synchronous rhythmic activity observed when GABAA mediated inhibition is suppressed. We next asked whether this activity is a necessary precursor for the emergence of epileptiform bursting. In five experiments (Fig. 7B) the GABAB antagonist CGP52432 (5 μM) was applied before fast GABAergic synaptic signalling was suppressed by bicuculline (10 μM). The GABAB antagonist alone changed neither the frequency nor the pattern of CA3 population firing (n= 5 slices). Subsequent application of bicucculine induced an epileptiform activity in all slices that was not preceded by partially synchronous activity (Fig. 7Bb). The first two to five synchronous bursts were of longer duration than when GABAB mediated signalling was functional (Fig. 7Bc).
, http://www.100md.com
Discussion
When fast synaptic inhibition is functional, discharges of the CA3 hippocampal cell population in vitro are asynchronous and largely unpatterned. When it is suppressed, all cells of the network discharge in epileptiform synchrony. Here we show that during the transition between these states, CA3 cells discharge in a partially synchronous fashion (Figs 2–4). Some, but not all, pyramidal cells fire simultaneously in events that recur at intervals of 0.5–2.0 s and are associated with local field potentials (Figs 4 and 5). Partially synchronous population activity may be induced and maintained by low doses of GABAA receptor antagonists (Fig. 5). The patterning of this activity depends in part on IPSPs mediated by GABAB receptors (Figs 6 and 7) which act to terminate the partial population discharges.
, http://www.100md.com
Changes in population activity during the transition to epileptiform synchrony
These data reveal a sequence of changes in CA3 population activity initiated by the application of GABAA receptor antagonists, and ultimately leading to the sudden emergence of epileptiform bursting. First firing, measured as the summed frequency of multiunit activity, increased (Fig. 2D). This initial increase in CA3 population activity presumably results from the reduction in the inhibitory control of pyramidal cell firing (Miles et al. 1996; Hausser & Clark, 1997; Cohen & Miles, 2000). It seems probable that both inhibitory and pyramidal cell firing increased during this period, although interneurone activity was not explicitly measured.
, 百拇医药
Then oscillations in CA3 cell firing were detected at single sites (Fig. 2D). A loss of inhibitory control has been shown to facilitate the transmission of firing via recurrent excitatory circuits involving CA3 pyramidal cells (Miles & Wong, 1987). Data presented here suggest that inhibitory cells are also excited and generate GABAB-mediated IPSPs which act to curtail the spread of firing via recurrent circuits (Fig. 6C). It seems that high frequency inhibitory cell firing is needed to liberate sufficient inhibitory transmitter to activate postsynaptic GABAB receptors (Alger, 1984; Benardo, 1997; Kim et al. 1997; Thomson & Destexhe, 1999) which are largely expressed at extra-synaptic sites (Scanziani, 2000; Kulik et al. 2003). The resulting sequences of summed EPSPs followed by summed GABAB-mediated IPSPs (Fig. 7A) are similar to events described by Scanziani et al. (1991). They generate a characteristic extracellular signal sequence consisting of a fast field potential spike corresponding to multiunit firing followed by a slower field potential wave correlated with the GABAB-mediated IPSP (Fig. 6).
, http://www.100md.com
Records from several sites showed that partially synchronous activity was expressed in coherent fashion throughout the CA3 region (Fig. 3G and H). If the population activity depends in part on synaptic events mediated via recurrent collaterals of CA3 pyramidal cells then its spatial coherence should depend on their projection patterns (Li et al. 1994). These collaterals ramify throughout the CA3 region in overlapping fashion and so seem unlikely to favour the formation of distinct clusters of rhythmically active cells. The axons of interneurones generating the inhibitory synaptic events that terminate partially synchronous bursts are also likely to diverge widely throughout the CA3 region (Gulyas et al. 1993; Buhl et al. 1994).
, http://www.100md.com
The increase in coherence of CA3 population firing corresponds to an increase in the synchronization of single cell discharges. Neurones will tend to be recruited to the synchronously firing ensemble both via the excitation mediated by recurrent connections between pyramidal cells, and paradoxically by the increase in amplitude of GABAB-mediated signals. Pyramidal cells have been shown to fire on the rebound after a synaptic inhibition (Cobb et al. 1995) and the divergence of inhibitory cell axons should assist a synchronous rebound firing. As for rhythmically discharging single cells (Gorman et al. 1982), both excitatory and inhibitory influences may help to shape rhythmic population activities.
, http://www.100md.com
The oscillations recurred as events which became increasingly coherent with time before epileptiform firing was initiated with an abrupt increase in the extracellular field potential amplitude (Fig. 5A). This increase in amplitude presumably corresponds to the sudden recruitment of many CA3 cells to the population discharges. The cellular or synaptic mechanisms for this transition are not clear. Since partial synchrony bursts were terminated by a large, mostly GABAB receptor mediated, inhibitory potential (Fig. 7A), the transition to prolonged full synchrony bursts could occur when the strength of AMPA- and NMDA-mediated excitatory synaptic events overcome the braking effect of the GABAB-mediated inhibition. The contribution of the residual GABAA receptor-mediated inhibition might act more on global tissue activity level than as a burst brake since such an effect was not observed after GABAB blockade in stable partial synchrony (Fig. 7A). At the transition to epileptiform synchrony there was an increase in the mean amplitude of extracellular spikes reflecting collisions of action potentials generated by distinct cells (Fig. 2F). This increase in synchrony of cellular firing in the range of 1–3 ms may indicate an enhanced ephaptic coupling (Haas & Jefferys, 1984; Faber & Korn, 1989) or in the efficacy of electrotonically mediated signalling (MacVicar & Dudek, 1981; Nimmrich et al. 2005).
, 百拇医药
Extracellular measures of synchrony
Neuronal synchrony is not a well defined term since it can refer to the simultaneous occurrence of either action potential discharges or synaptic events over a range of time scales. In this study we examined a form of neuronal synchrony that increased in coherence with time after application of GABAA receptor antagonists. Field potential measures of synchrony were compared (Fig. 4) with those derived from the frequency of action potentials generated by a neuronal population of several hundreds of CA3 cells (Cohen & Miles, 2000). Field potentials above baseline were always associated with peaks of multiunit activity, whereas early in the transition some bursts of spikes were not associated with above baseline field potentials (Fig. 4). This observation suggests that indices based on frequency could be at least as sensitive as the field potential measures in detecting the emergence of weak, partial synchrony in which, initially at least, only a small fraction of the CA3 population fired simultaneously within a time interval of 20–100 ms. We note that since CA3 cells fire spontaneously in vitro, the spike frequency index is non-null in control conditions and so can be both positively and negatively modulated. This may enhance its sensitivity to small changes in population activity.
, 百拇医药
Functional significance
Partially synchronous neuronal activities similar to those described here have been observed in hippocampal slices exposed to GABAA receptor antagonists at doses that do not completely suppress fast synaptic inhibition (Schneiderman, 1986; Scanziani et al. 1991). Do they have a counterpart in the range of activities generated by the hippocampus in vivo The most comparable activity is sharp wave bursts observed in passive behaviours and slow wave sleep (Suzuki & Smith, 1985; Buzsaki, 1986). Sharp waves are also generated in the CA3 region by activity in a small proportion of CA3 pyramidal cells and interneurones (Csicsvari et al. 2000). If the analogy is precise, our data suggest that a temporary reduction in the inhibitory control of CA3 population activity might result in the emergence of sharp waves. It remains to be determined what changes in the modulating transmitter environment of the CA3 region could underly a transient disinhibition in the intact animal.
, 百拇医药
Disinhibited hippocampal slices have been used for some time as a chronic model of epilepsy based on observations that drugs interfering with GABAergic function are convulsants and that reinforcing GABAergic signalling may have antiepileptic effects. More recent work suggests that GABAergic systems are perturbed in epilepsies of the human temporal lobe, but are due to changes in postsynaptic chloride homeostasis, rather than transmitter release or receptor function (Cohen et al. 2002). Even so our data may reveal mechanisms operating during the emergence of epileptiform synchrony. Changes in the dynamics of human EEG signals have been detected before seizure onset (Martinerie et al. 1998). Our data suggest precursor oscillations depend on inhibitory signalling, mediated by GABAB receptors. Even so, we note that the prior suppression of GABAB receptor-mediated signalling did not prevent the expression of synchrony (Fig. 7B). Rather synchrony was induced with no transition through states of partial synchrony and consisted of events of longer duration occurring at a lower frequency.
, 百拇医药
References
Alger BE (1984). Characteristics of a slow hyperpolarizing synaptic potential in rat hippocampal pyramidal cells in vitro. J Neurophysiol 52, 892–910.
Ankri N & Korn H (1999). A statistical method for correcting distortions of amplitude distribution histograms due to collisions of synaptic events. J Neurosci Meth 91, 83–99.
Bedenbaugh P & Gerstein GL (1997). Multiunit cross correlation differs from the average single unit normalised correlation. Neural Computation 9, 1265–1275.
, 百拇医药
Benardo LS (1997). Recruitment of GABAergic inhibition and synchronization of inhibitory interneurons in rat neocortex. J Neurophysiol 77, 3134–3144.
Ben-Ari Y, Cherubini E, Corradetti R & Gaiarsa JL (1989). Giant synaptic potentials in immature rat CA3 hippocampal neurones. J Physiol 416, 303–325.
Berger H (1929). Uber das Elektrenkephalogramm des Menschen. 1st report. Arch Psychiat Nervenkr 87, 527–570.
, http://www.100md.com
Brecht M, Singer W & Engel AK (1999). Patterns of synchronisation in the superior colliculus of anesthetised rats. J Neurosci 19, 3567–3579.
Buhl EH, Halasy K & Somogyi P (1994). Diverse sources of hippocampal unitary inhibitory postsynaptic potentials and the number of synaptic release sites. Nature 368, 823–828.
Buszaki G (1986). Hippocampal sharp waves: their origin and significance. Brain Res 398, 242–252.
, 百拇医药
Cobb SR, Buhl EH, Halasy K, Paulsen O & Somogyi P (1995). Synchronization of neuronal activity in hippocampus by individual GABAergic interneurons. Nature 378, 75–78.
Cohen I & Miles R (2000). Contributions of intrinsic and synaptic activities to the generation of neuronal discharges in in vitro hippocampus. J Physiol 524, 485–502.
Cohen I, Navarro V, Clemenceau S, Baulac M & Miles R (2002). On the origin of interictal activity in human temporal lobe epilepsy in vitro. Science 298, 1418–1421.
, 百拇医药
Csicsvari J, Hirase H, Mamiya A & Buzsaki G (2000). Ensemble patterns of hippocampal CA3-CA1 neurons during sharp wave-associated population events. Neuron 28, 585–594.
Engel AK, Fries P & Singer W (2001). Dynamic predictions: oscillations and synchrony in top-down processing. Nat Rev Neurosci 2, 704–716.
Faber DS & Korn H (1989). Electrical field effects: their relevance in central neural networks. Physiol Rev 69, 821–863.
, 百拇医药
Fortin G, Kato F, Lumsden A & Champagnat J (1995). Rhythm generation in the segmented hindbrain of chick embryos. J Physiol 486, 735–744.
Gorman AL, Hermann A & Thomas MV (1982). Ionic requirements for membrane oscillations and their dependence on the calcium concentration in a molluscan pace-maker neurone. J Physiol 327, 185–217.
Grillner S & Wallen P (1999). On the cellular bases of vertebrate locomotion. Prog Brain Res 123, 297–309.
, http://www.100md.com
Gulyas AI, Miles R, Hajos N & Freund TF (1993). Precision and variability in postsynaptic target selection of inhibitory cells in the hippocampal CA3 region. Eur J Neurosci 5, 1729–1751.
Haas HL & Jefferys JG (1984). Low-calcium field burst discharges of CA1 pyramidal neurones in rat hippocampal slices. J Physiol 354, 185–201.
Hausser M & Clark BA (1997). Tonic synaptic inhibition modulates neuronal output pattern and spatiotemporal synaptic integration. Neuron 19, 665–678.
, http://www.100md.com
Kim U, Sanchez-Vives MV & McCormick DA (1997). Functional dynamics of GABAergic inhibition in the thalamus. Science 278, 130–134.
Kulik A, Vida I, Lujan R, Haas CA, Lopez-Bendito G, Shigemoto R & Frotscher M (2003). Subcellular localization of metabotropic GABAB (B) receptor subunits GABA (B1a/b) and GABA (B2) in the rat hippocampus. J Neurosci 23, 11026–11035.
Laurent G (1999). A systems perspective on early olfactory coding. Science 286, 723–728.
, 百拇医药
Levy R, Hutchison WD, Lozano AM & Dostrovsky JO (2000). High-frequency synchronization of neuronal activity in the subthalamic nucleus of parkinsonian patients with limb tremor. J Neurosci 20, 7766–7775.
Li XG, Somogyi P, Ylinen A & Buzsaki G (1994). The hippocampal CA3 network: an in vivo intracellular labeling study. J Comp Neurol 339, 181–208.
Lincoln DW & Wakerley JB (1975). Factors governing the periodic activation of supraoptic and paraventricular neurosecretory cells during suckling in the rat. J Physiol 250, 443–461.
, http://www.100md.com
MacVicar BA & Dudek FE (1981). Electrotonic coupling between pyramidal cells: a direct demonstration in rat hippocampal slices. Science 213, 782–785.
Martinerie J, Adam C, Le Van Quyen M, Baulac M, Clemenceau S, Renault B & Varela FJ (1998). Epileptic seizures can be anticipated by non-linear analysis. Nat Med 4, 1173–1176.
Miles R, Toth K, Gulyas AI, Hajos N & Freund TF (1996). Differences between somatic and dendritic inhibition in the hippocampus. Neuron 16, 815–823.
, http://www.100md.com
Miles R & Wong RKS (1987). Inhibitory control of local excitatory circuits in the guinea-pig hippocampus. J Physiol 388, 611–629.
Nimmrich V, Maier N, Schmitz D & Draguhn A (2005). Induced sharp wave-ripple complexes in the absence of synaptic inhibition in mouse hippocampal slices. J Physiol 563, 663–670.
Pedley TA & Traub RD (1990). Physiological Basis of the EEG, 2nd edn, ed. Daly DD & Pedley TA, pp. 107–137. Raven Press, New York.
, 百拇医药
Scanziani M (2000). GABA spillover activates postsynaptic GABAB receptors to control rhythmic hippocampal activity. Neuron 25, 673–681.
Scanziani M, Gahwiler BH & Thompson SM (1991). Paroxysmal inhibitory potentials mediated by GABAB receptors in partially disinhibited rat hippocampal slice cultures. J Physiol 444, 375–396.
Schneiderman JH (1986). Low concentrations of penicillin reveal rhythmic, synchronous synaptic potentials in hippocampal slice. Brain Res 398, 231–241.
, 百拇医药
Schwartzkroin PA (1994). Cellular electrophysiology of human epilepsy. Epilepsy Res 17, 185–192.
Schwartzkroin PA & Prince DA (1978). Cellular and field potential properties of epileptogenic hippocampal slices. Brain Res 147, 117–130.
Staley KJ, Bains JS, Yee A, Hellier J & Longacher JM (2001). Statistical model relating CA3 burst probability to recovery from burst-induced depression at recurrent collateral synapses. J Neurophysiol 86, 2736–2747.
, 百拇医药
Staley KJ, Longacher M, Bains JS & Yee A (1998). Presynaptic modulation of CA3 network activity. Nat Neurosci 1, 201–209.
Suzuki SS & Smith GK (1985). Single-cell activity and synchronous bursting in the rat hippocampus during waking behavior and sleep. Exp Neurol 89, 71–89.
Szucs A (1998). Applications of the spike density function in analysis of neuronal firing patterns. J Neurosci Meth 81, 159–167.
Thomson AM & Destexhe A (1999). Dual intracellular recordings and computational models of slow inhibitory postsynaptic potentials in rat neocortical and hippocampal slices. Neuroscience 92, 1193–1215.
Wong RO (1999). Retinal waves and visual system development. Ann Rev Neurosci 22, 29–47., 百拇医药(Ivan Cohen, Gilles Huberfeld and Richard)