声预处理对强噪声引起听力损伤保护作用的研究
作者：刘亚光 何延军 李道德 郑素贤 牛聪敏
单位：航天医学与医学工程
关键词：声预处理；听力保护；听阀；毛细胞；噪声损伤；过氧化脂质类
航天医学与医学工程000501摘要： 目的 探讨低频声预处理对强噪声及中频强噪声引起听力损伤的保护作用。 方法 两组声预处理组豚鼠在中心频率0.5 kHz 85 dB倍频带噪声作用下，连续暴露4 d(6 h/d)后，在无噪声情况下恢复3 d。随后分别在中心频率0.5 kHz和1 kHz 110 dB的倍频带噪声下，暴露1 h。两组对照组豚鼠不经声预处理，分别直接暴露在中心频率 0.5 KHz和1 KHz 110 dB的信频带噪声下，暴露1 h。然后观察各组在强噪声暴露后，听力恢复情况、红细胞膜丙二醛(MDA)含量以及耳蜗外毛细胞铺片检查。结果 强噪声暴露后，经过声预处理的两组动物，听阈偏移程度、红细胞膜MDA含量、耳蜗外毛细胞受损情况明显好于对照组(P<0.05)。 结论 声预处理不仅可以减轻随后同一频率段的强噪声(0.5 kHz、110 dB)引起的听力损伤，还可减轻随后稍高频率段的强噪声(1 kHz、110 dB)引起的听力损伤，声预处理的保护作用可能是通过激活细胞抗氧化酶系统，抑制或减少毒性自由基产生，从而减轻毒性自由基对细胞膜的损伤。
, http://www.100md.com
中图分类号：R852.24 文献标识码：A 文章编号：1002-0837(2000)05-0313-05
Effects of Sound Preconditioning on Hearing Loss from Low or Middle-Frequency Noise Exposure
LIU Ya-guang,HE Yan-jun,LI Dao-de,ZHENG Su-xian,NIU Cong-min
(Institute of Space Medico-Engineering, Beijing 100094)
Abstract: Objective To explore prior noise exposures or sound conditioning as a moderator of hearing loss produced by traumatic exposure to low or middle-frequency noise. Method Two experimental groups of guinea-pigs were conditioned using a 0.5 kHz octave band noise (OBN) at 85 dB,6 h/d for 4 d.The subjects were allowed to recover for 3 d after conditioning.Then the first group was exposed to a 0.5 kHz OBN at 110 dB for 1 h,the second group was exposed to 1 kHz OBN at 110 dB for 1 h.Two control groups received 0.5 kHz and 1 kHz OBN respectively at 110 dB for 1 h without prior sound conditioning. Result Hearing threshold shifts recorded at 48 h after the high-level noise exposure in conditioned groups and control groups demonstrated that conditioning provided significant protection on hearing threshold shift from low or middle frequency noise exposure.Histological examination revealed significantly less hair cell loss in the conditioned than in the control groups.In addition,malondialdehyde(MDA) levels in red blood cell(RBC) of conditioned groups were significantly lower as compared with that of the control groups. Conclusion Low-frequency conditioning provided significant protection not only on hearing threshold shift caused by noise of the same frequency,but also on that caused by middle frequency noise.
, 百拇医药
Key words:sound preconditioning;hearing conservation;auditory thresholds;hair cells;noise injures;lipid peroxides
Susceptibility of the ear to acoustic trauma can be reduced by a variety of experimental treatments.For instance,significant reductions in noise-induced loss of auditory sensitivity and cochlea damage have been achieved by inducing-hypothermia[1],decreasing physical exercise[2],activating the acoustic reflex[3],as well as stimulating the efferent olivocochlear bundle acoustically
, 百拇医药
Recieved date:1999-12-21
and/or electrically[4].Several investigators have observed a progressive reduction in the hearing threshold shifts with repeated exposures to the same noise separated by periodic rest[5,6].So,it is possible that significant protection from the damaging effects of high-level noise could be achieved by ‘pre-exposing' the subjects to acoustic stimulus at a lower and non-traumatic level.The prophylactic tone or noise exposures that increase the resistance to acoustic trauma are referred to as ‘training' or ‘conditioning' exposures,typically they provide 10～25 dB protection from permanent threshold shifts(PTS)[7,8].
, 百拇医药
Though these are interesting results,certain limitations in their methodology call for a replication of these experiments under more realistic conditions.The purpose of this experiment is to make clear (1) What is the relationship between the conditioning exposure and the resultant changes in vulnerability to noise induced hear loss (NIHL).(2) whether low-frequency conditioning provides protection from middle-frequency noise-induced threshold shifts and (3) What is the biological basis of the conditioning effect:does the antioxidant system become more effective or is there a fundamental change in the cochlear?
, http://www.100md.com
Method
Subjects 56 young (250～300 g) guinea-pigs served as the subjects,The animals were randomly divided into four groups.Hearing thresholds of the animals were determined with auditory brainstem response (ABR) prior to the experiment and after the noise exposures.
Audiometry Hearing thresholds of all the animals were measured at least five times using evoked potential recording.The recordings were made in awake animals held in a light restraint to minimize artifacts due to movements.The average of these five measures consist the pre-exposure thresholds.In addition,recordings were made just before and immediately after conditioning (85 dB) exposure,as well as 48 h after the high level exposure (110 dB).Hearing thresholds shifts were calculated with reference to the pre-exposure baseline and test stimuli were presented in acoustic system using evoked response recorder (NIHON KOHDEN).
, http://www.100md.com
Noise exposure The noise used for both prior conditioning and high level exposures was an OBN centered at 0.5 kHz or 1 kHz.The noise was digitally generated with a D/A converter and routed through an attenuater (Digitech MEQ28) and a power amplifier (CROWN 2400) to an acoustic horn (JBL).Noise levels were measured using a sound level meter (BK4165,BK2231).The microphone was positioned in the cage at the level of the animal's head.
Animals (n=14) in the two experimental groups were preconditioned with a 0.5 kHz OBN at 85 dB for 4 consecutive days(6 h/d),and then were allowed to recover for 3 d.The recovery period was chosen based on previous observations that hearing returns to normal by the end of the period[8].Following recovery,animals in the first conditioned group were exposed to the same spectrum at 110dB for 1 h,while those in the second conditioned group were exposed to a 1 kHz OBN at 110 dB for 1 h.Control animals (n=14) didn't receive any preconditioning and were only exposed to a 0.5 kHz or 1 kHz OBN at 110 dB for 1 h.
, 百拇医药
Histochemistry RBC MDA was determined in all the 4 groups to investigate the degree of oxidative damage.Blood samples for MDA determination were collected into heparinized glass tubes and processed within 1 h.Separation of RBC from plasma was obtained by centrifugation at 1500 G for 5 min.RBC were then washed twice in normal saline and suspended into paired volumes of distilled water.Samples were then frozen and stored for 2 d at -20℃ before analysis.
Histology Following the measurement of threshold shift(TS),each animal was decapitated.The right cochlea was removed and perfused through the round window membrane with 2.5% gluteraldehyde in veronal acetate buffer at pH=7.4.It was then perfused with 1% osmium tetroxide in veronal acetate buffer 24 h later,and rised in the same buffer,dissected in 70% ETOH and evaluated using a phase-contrast microscope.Outer hair populations were assessed to determine if the hair cell were present or not.
, http://www.100md.com
Statistics TS values were treated with analysis of variance (ANOVA) to determine if differences between conditioned and unconditioned animals were statistically significant.In addition,t-test was performed for RBC MDA and Four Fold Table Chi-Square Test was performed for the hair cell loss data of conditioned and uncomditioned animals with subsequent comparisons between hair cell loss values.
Result
Hearing thresholds before conditioning The average hearing threshold of the animals in the four groups before conditioning were given(table 1).Data analysis showed that there were no significant differences among the four groups.Therefore any difference between groups following noise exposure can be attributed to the experimental treatment (conditioning).
, http://www.100md.com
Hearing threshold shifts during and after conditioning(0.5 kHz OBN 85 dB) The average TS recorded during conditioning with the 0.5 kHz OBN at 85 dB were showed(table 1).Thresholds of the animals in the two conditioned groups were significantly elevated during conditioning.After 3 d of recovery in a quiet animal colony,mean thresholds of the two conditioned groups returned to normal (table 1).Paired t-tests confirmed that there were no significant difference between thresholds before conditioning and that measured 3 d after conditioning.
, 百拇医药
Hearing threshold shifts after high-level noise exposure When tested 48 h after the high-level exposure,all groups exhibited significant threshold elevations (compared to pre-exposure level).There were no significant difference in TS between the two conditioning groups.As compared to the conditioned animals,the control group exhibited significantly higher TS.
RBC MDA After high-level noise exposure,RBC MDA concentration in the conditioned animals was significantly higher than that in control groups (table 2).While no significant difference was found between conditioned groups and/or the two control groups.
, 百拇医药
Table 1 Mean hearing thresholds and threshold shifts(TS) in all groups after high level noise exposure(HLNE) group
Hearing Threshold (dB)
TS(dB)
Before CD
During CD
Before HLNE
After HLNE
After HLNE
0.5 kHz-CD
, 百拇医药
9.4±4.2
13.8±2.8#
11.3±1.9
14.4±3.2#*
5.0±2.7*
0.5 kHz control
11.9±2.6----
22.5±5.3#
10.6±5.0
1 kHz-CD
12.5±2.7
, http://www.100md.com
16.3±2.8#
13.1±3.1
16.9±2.6#*
4.4±3.2*
1 kHz control
10.0±3.8----
24.4±5.6#
14.4±5.6
Note:CD: conditioning;HLNE:high-level noise expesure;*P<0.05,as compared with control;#P<0.05,as compared with pre-exposureTable 2 RBC MDA Level in animals of the various groups (nmol/L) Item
, 百拇医药
0.5 kHz Conditioned
0.5 kHz control
1 kHz conditioned
1 kHz control
MDA(nmol/L)
22.1±2.3*
28.1±1.6
21.7±2.0*
29.1±2.3
Note:*P<0.05，as compared with control
, 百拇医药
Fig.1 Surface preparations of organ of Corti for (a) affected region in animal.(b) same region in conditioned animal
Histology changes The amount of hair loss in individual animals corresponded well with hearing loss.5 or 6 of 8 in two control groups showed extensive hair loss across the cochlea.On the other hand, 4 or 5 of 7 in two conditioned groups showed only little hair cell loss.The difference in outer hair cell (OHC) loss between the conditioned and control groups was statistically significant (Four Fold Table Chi-Square Test).In the control animals(Fig.1a),most of the OHCs in row 2,3 were missing,while only fewer in row 1 OHCs were missing.A surface preparation from the corresponding region in the conditioned animals showed that most of the OHCs in all three rows were almost intact.(Fig.1b)
, 百拇医药
Discussion
The conditioning exposures used in this experiment produced a consistent decrease in TS.After 3 d rest period following the conditioning exposure,hearing sensitivity returned to baseline levels in all animals.The results were consistent with findings by other investigators[9,10].The decrease in TS with repeated interrupted exposures suggested that the auditory system was becoming increasingly resistant to the effects of noise.More interestingly,conditioning with 0.5 kHz OBN provided protection of hearing loss and hair cell loss not only from high level noise exposure of the same frequency,but also from that of higher frequency (1 kHz).Hearing threshold shift observed after exposures to 0.5 kHz and 1kHz noise were minimal in pattern and magnitude.
, 百拇医药
The results of this investigation were in consistancy with the findings by Canlon[7] and Henderson[11].That is,prior noise exposures in same way altered the response of the auditory system and made the ear more resistant to damage from subsequent high-level noise exposures.In explaining the mechanisms of damage from high level noise exposure,previous studies showed PTS and associated hair cell loss.Basically,such observations suggested a strong possibility that the damage was caused by mechanical failure within the cochlea.It is presently not yet clear whether the protection effect seen in this study occurs as a result of improved repair processes in the cochlea or an actual change of the mechanical operation of the cochlea.
, http://www.100md.com
In this study,RBC MDA levels in conditioned groups decreased significantly as compared with those in control groups.RBC MDA is the product of lipid peroxidation reaction,and RBC MDA content indicates the capacity of antioxidant system.Thus,we assumed that periods of stasis induced by noise exposure result in the release of free oxygen radicals (FORS) in the affected tissues.FORS are typically generated by reperfusion which is evident in reduced cochlear sensitivity after noise exposure.The conditioning phenomenon could trigger the antioxidant system that inhibit FORS formation or prevent FORS-induced lipid peroxidation to reduce ischemia induced damage.
, http://www.100md.com
Other possible mechanisms of conditioning might include fundamental changes in the cells of the organ of Corti or perhaps the generation of preventive agents such as heat shock proteins[12].It needs more further studies to clarify the biological basis of conditioning effect.
Noise exposure is a well-known cause of acquired sensory hearing loss in adults,as noise exposure cannot be avoided in actual life.Though the correct use of well-designed and properly fitted traditional hearing protection devices (HPDs) can reduce or prevent cochlear damage,there are,however,a number of industrial and military occupations and situations in which HPDs are either not available or are impractical or impossible to wear.In these cases,conditioning could provide a means of preventing or reducing the hazards from noise exposure.
, 百拇医药
Obviously,much more needs to be known about the acoustic boundary conditions that produce cochlear protection and about the biological bases for the protective effect before clinical applications can be seriously considered.Nevertheless,it is encouraging to think that the inherent plasticity of the cochlea may be exploited clinically in protecting the hearing of individuals in noisy environments.
作者地址：刘亚光.航天医学工程研究所，北京 100094
[References]
, 百拇医药
[1] Henry KR,Chole RA.Hypothermia protects the cochlea from noise damage[J].Hear Res,1984,16:225～230
[2] Colletti V,Fiorino FG.Physical exercise and active protection from temporary threshold shifts[M].In:Noise-Induced Hearing Loss.St louis:Mosby Year Book,1992:500～510
[3] Borg E,Nilsson R.Acoustic reflex in industrial noise[M].In:S.Silman,eds.The Acoustic Reflex:Scientific Aspects and Clinical Application.New York:Academic Press,1984:413～440
, 百拇医药
[4] Rajan R.Protective functions of the efferent pathways to the mammalian cochlea: a review[M].In:Noise-Induced Hearing Loss.St Louis:Mosby Year-Book,1992:429～444
[5] Miller JD,Watson CS.Deafening effects of noise on the cat[J].Acta Otolaryngol,1963,176(suppl):1～91
[6] Clark WW,Bohne BA.Effect of periodic rest on hearing loss and cochlear damage following exposure to noise[J].J Acoust Soc Am,1987,82:1253～1264
, 百拇医药
[7] Canlon B,Borg E,Flock A.Protection against noise trauma by pre-exposure to a low level acoustic stimulus[J].Hear Res,1988,34:197～200
[8] Pierre Campo,Malini subramaniam,Donald Henderson.The effect of conditioning exposures on hearing loss from traumaic exposure[J].Hear Res,1991,55:195～200
[9] Kryter KD,Ward WD.Hazardous exposure to intermittent and steady-state noise[J].J Acoust Soc Am,1966,39:451～464
, 百拇医药
[10] Subramaniam M,Henderson D.The effect of exposure level on the development of progressive resistance to noise[J].Hear Res,1992,52:181～188
[11] Henderson D,Campo P.Development of resistance to noise[M].In:Noise-Induced Hearing Loss.St Louis:Mosby Year-Book,1992:476～488
[12] Wenthold RJ,Kim HN,Dechesne CJ.Putative biochemical processes in NIHL[C].In:Ⅳ International Conferece on the Effects of Noise on the Auditory System.France:Beaune,1990,May:28～30, 百拇医药
单位：航天医学与医学工程
关键词：声预处理；听力保护；听阀；毛细胞；噪声损伤；过氧化脂质类
航天医学与医学工程000501摘要： 目的 探讨低频声预处理对强噪声及中频强噪声引起听力损伤的保护作用。 方法 两组声预处理组豚鼠在中心频率0.5 kHz 85 dB倍频带噪声作用下，连续暴露4 d(6 h/d)后，在无噪声情况下恢复3 d。随后分别在中心频率0.5 kHz和1 kHz 110 dB的倍频带噪声下，暴露1 h。两组对照组豚鼠不经声预处理，分别直接暴露在中心频率 0.5 KHz和1 KHz 110 dB的信频带噪声下，暴露1 h。然后观察各组在强噪声暴露后，听力恢复情况、红细胞膜丙二醛(MDA)含量以及耳蜗外毛细胞铺片检查。结果 强噪声暴露后，经过声预处理的两组动物，听阈偏移程度、红细胞膜MDA含量、耳蜗外毛细胞受损情况明显好于对照组(P<0.05)。 结论 声预处理不仅可以减轻随后同一频率段的强噪声(0.5 kHz、110 dB)引起的听力损伤，还可减轻随后稍高频率段的强噪声(1 kHz、110 dB)引起的听力损伤，声预处理的保护作用可能是通过激活细胞抗氧化酶系统，抑制或减少毒性自由基产生，从而减轻毒性自由基对细胞膜的损伤。
, http://www.100md.com
中图分类号：R852.24 文献标识码：A 文章编号：1002-0837(2000)05-0313-05
Effects of Sound Preconditioning on Hearing Loss from Low or Middle-Frequency Noise Exposure
LIU Ya-guang,HE Yan-jun,LI Dao-de,ZHENG Su-xian,NIU Cong-min
(Institute of Space Medico-Engineering, Beijing 100094)
Abstract: Objective To explore prior noise exposures or sound conditioning as a moderator of hearing loss produced by traumatic exposure to low or middle-frequency noise. Method Two experimental groups of guinea-pigs were conditioned using a 0.5 kHz octave band noise (OBN) at 85 dB,6 h/d for 4 d.The subjects were allowed to recover for 3 d after conditioning.Then the first group was exposed to a 0.5 kHz OBN at 110 dB for 1 h,the second group was exposed to 1 kHz OBN at 110 dB for 1 h.Two control groups received 0.5 kHz and 1 kHz OBN respectively at 110 dB for 1 h without prior sound conditioning. Result Hearing threshold shifts recorded at 48 h after the high-level noise exposure in conditioned groups and control groups demonstrated that conditioning provided significant protection on hearing threshold shift from low or middle frequency noise exposure.Histological examination revealed significantly less hair cell loss in the conditioned than in the control groups.In addition,malondialdehyde(MDA) levels in red blood cell(RBC) of conditioned groups were significantly lower as compared with that of the control groups. Conclusion Low-frequency conditioning provided significant protection not only on hearing threshold shift caused by noise of the same frequency,but also on that caused by middle frequency noise.
, 百拇医药
Key words:sound preconditioning;hearing conservation;auditory thresholds;hair cells;noise injures;lipid peroxides
Susceptibility of the ear to acoustic trauma can be reduced by a variety of experimental treatments.For instance,significant reductions in noise-induced loss of auditory sensitivity and cochlea damage have been achieved by inducing-hypothermia[1],decreasing physical exercise[2],activating the acoustic reflex[3],as well as stimulating the efferent olivocochlear bundle acoustically
, 百拇医药
Recieved date:1999-12-21
and/or electrically[4].Several investigators have observed a progressive reduction in the hearing threshold shifts with repeated exposures to the same noise separated by periodic rest[5,6].So,it is possible that significant protection from the damaging effects of high-level noise could be achieved by ‘pre-exposing' the subjects to acoustic stimulus at a lower and non-traumatic level.The prophylactic tone or noise exposures that increase the resistance to acoustic trauma are referred to as ‘training' or ‘conditioning' exposures,typically they provide 10～25 dB protection from permanent threshold shifts(PTS)[7,8].
, 百拇医药
Though these are interesting results,certain limitations in their methodology call for a replication of these experiments under more realistic conditions.The purpose of this experiment is to make clear (1) What is the relationship between the conditioning exposure and the resultant changes in vulnerability to noise induced hear loss (NIHL).(2) whether low-frequency conditioning provides protection from middle-frequency noise-induced threshold shifts and (3) What is the biological basis of the conditioning effect:does the antioxidant system become more effective or is there a fundamental change in the cochlear?
, http://www.100md.com
Method
Subjects 56 young (250～300 g) guinea-pigs served as the subjects,The animals were randomly divided into four groups.Hearing thresholds of the animals were determined with auditory brainstem response (ABR) prior to the experiment and after the noise exposures.
Audiometry Hearing thresholds of all the animals were measured at least five times using evoked potential recording.The recordings were made in awake animals held in a light restraint to minimize artifacts due to movements.The average of these five measures consist the pre-exposure thresholds.In addition,recordings were made just before and immediately after conditioning (85 dB) exposure,as well as 48 h after the high level exposure (110 dB).Hearing thresholds shifts were calculated with reference to the pre-exposure baseline and test stimuli were presented in acoustic system using evoked response recorder (NIHON KOHDEN).
, http://www.100md.com
Noise exposure The noise used for both prior conditioning and high level exposures was an OBN centered at 0.5 kHz or 1 kHz.The noise was digitally generated with a D/A converter and routed through an attenuater (Digitech MEQ28) and a power amplifier (CROWN 2400) to an acoustic horn (JBL).Noise levels were measured using a sound level meter (BK4165,BK2231).The microphone was positioned in the cage at the level of the animal's head.
Animals (n=14) in the two experimental groups were preconditioned with a 0.5 kHz OBN at 85 dB for 4 consecutive days(6 h/d),and then were allowed to recover for 3 d.The recovery period was chosen based on previous observations that hearing returns to normal by the end of the period[8].Following recovery,animals in the first conditioned group were exposed to the same spectrum at 110dB for 1 h,while those in the second conditioned group were exposed to a 1 kHz OBN at 110 dB for 1 h.Control animals (n=14) didn't receive any preconditioning and were only exposed to a 0.5 kHz or 1 kHz OBN at 110 dB for 1 h.
, 百拇医药
Histochemistry RBC MDA was determined in all the 4 groups to investigate the degree of oxidative damage.Blood samples for MDA determination were collected into heparinized glass tubes and processed within 1 h.Separation of RBC from plasma was obtained by centrifugation at 1500 G for 5 min.RBC were then washed twice in normal saline and suspended into paired volumes of distilled water.Samples were then frozen and stored for 2 d at -20℃ before analysis.
Histology Following the measurement of threshold shift(TS),each animal was decapitated.The right cochlea was removed and perfused through the round window membrane with 2.5% gluteraldehyde in veronal acetate buffer at pH=7.4.It was then perfused with 1% osmium tetroxide in veronal acetate buffer 24 h later,and rised in the same buffer,dissected in 70% ETOH and evaluated using a phase-contrast microscope.Outer hair populations were assessed to determine if the hair cell were present or not.
, http://www.100md.com
Statistics TS values were treated with analysis of variance (ANOVA) to determine if differences between conditioned and unconditioned animals were statistically significant.In addition,t-test was performed for RBC MDA and Four Fold Table Chi-Square Test was performed for the hair cell loss data of conditioned and uncomditioned animals with subsequent comparisons between hair cell loss values.
Result
Hearing thresholds before conditioning The average hearing threshold of the animals in the four groups before conditioning were given(table 1).Data analysis showed that there were no significant differences among the four groups.Therefore any difference between groups following noise exposure can be attributed to the experimental treatment (conditioning).
, http://www.100md.com
Hearing threshold shifts during and after conditioning(0.5 kHz OBN 85 dB) The average TS recorded during conditioning with the 0.5 kHz OBN at 85 dB were showed(table 1).Thresholds of the animals in the two conditioned groups were significantly elevated during conditioning.After 3 d of recovery in a quiet animal colony,mean thresholds of the two conditioned groups returned to normal (table 1).Paired t-tests confirmed that there were no significant difference between thresholds before conditioning and that measured 3 d after conditioning.
, 百拇医药
Hearing threshold shifts after high-level noise exposure When tested 48 h after the high-level exposure,all groups exhibited significant threshold elevations (compared to pre-exposure level).There were no significant difference in TS between the two conditioning groups.As compared to the conditioned animals,the control group exhibited significantly higher TS.
RBC MDA After high-level noise exposure,RBC MDA concentration in the conditioned animals was significantly higher than that in control groups (table 2).While no significant difference was found between conditioned groups and/or the two control groups.
, 百拇医药
Table 1 Mean hearing thresholds and threshold shifts(TS) in all groups after high level noise exposure(HLNE) group
Hearing Threshold (dB)
TS(dB)
Before CD
During CD
Before HLNE
After HLNE
After HLNE
0.5 kHz-CD
, 百拇医药
9.4±4.2
13.8±2.8#
11.3±1.9
14.4±3.2#*
5.0±2.7*
0.5 kHz control
11.9±2.6----
22.5±5.3#
10.6±5.0
1 kHz-CD
12.5±2.7
, http://www.100md.com
16.3±2.8#
13.1±3.1
16.9±2.6#*
4.4±3.2*
1 kHz control
10.0±3.8----
24.4±5.6#
14.4±5.6
Note:CD: conditioning;HLNE:high-level noise expesure;*P<0.05,as compared with control;#P<0.05,as compared with pre-exposureTable 2 RBC MDA Level in animals of the various groups (nmol/L) Item
, 百拇医药
0.5 kHz Conditioned
0.5 kHz control
1 kHz conditioned
1 kHz control
MDA(nmol/L)
22.1±2.3*
28.1±1.6
21.7±2.0*
29.1±2.3
Note:*P<0.05，as compared with control
, 百拇医药
Fig.1 Surface preparations of organ of Corti for (a) affected region in animal.(b) same region in conditioned animal
Histology changes The amount of hair loss in individual animals corresponded well with hearing loss.5 or 6 of 8 in two control groups showed extensive hair loss across the cochlea.On the other hand, 4 or 5 of 7 in two conditioned groups showed only little hair cell loss.The difference in outer hair cell (OHC) loss between the conditioned and control groups was statistically significant (Four Fold Table Chi-Square Test).In the control animals(Fig.1a),most of the OHCs in row 2,3 were missing,while only fewer in row 1 OHCs were missing.A surface preparation from the corresponding region in the conditioned animals showed that most of the OHCs in all three rows were almost intact.(Fig.1b)
, 百拇医药
Discussion
The conditioning exposures used in this experiment produced a consistent decrease in TS.After 3 d rest period following the conditioning exposure,hearing sensitivity returned to baseline levels in all animals.The results were consistent with findings by other investigators[9,10].The decrease in TS with repeated interrupted exposures suggested that the auditory system was becoming increasingly resistant to the effects of noise.More interestingly,conditioning with 0.5 kHz OBN provided protection of hearing loss and hair cell loss not only from high level noise exposure of the same frequency,but also from that of higher frequency (1 kHz).Hearing threshold shift observed after exposures to 0.5 kHz and 1kHz noise were minimal in pattern and magnitude.
, 百拇医药
The results of this investigation were in consistancy with the findings by Canlon[7] and Henderson[11].That is,prior noise exposures in same way altered the response of the auditory system and made the ear more resistant to damage from subsequent high-level noise exposures.In explaining the mechanisms of damage from high level noise exposure,previous studies showed PTS and associated hair cell loss.Basically,such observations suggested a strong possibility that the damage was caused by mechanical failure within the cochlea.It is presently not yet clear whether the protection effect seen in this study occurs as a result of improved repair processes in the cochlea or an actual change of the mechanical operation of the cochlea.
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In this study,RBC MDA levels in conditioned groups decreased significantly as compared with those in control groups.RBC MDA is the product of lipid peroxidation reaction,and RBC MDA content indicates the capacity of antioxidant system.Thus,we assumed that periods of stasis induced by noise exposure result in the release of free oxygen radicals (FORS) in the affected tissues.FORS are typically generated by reperfusion which is evident in reduced cochlear sensitivity after noise exposure.The conditioning phenomenon could trigger the antioxidant system that inhibit FORS formation or prevent FORS-induced lipid peroxidation to reduce ischemia induced damage.
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Other possible mechanisms of conditioning might include fundamental changes in the cells of the organ of Corti or perhaps the generation of preventive agents such as heat shock proteins[12].It needs more further studies to clarify the biological basis of conditioning effect.
Noise exposure is a well-known cause of acquired sensory hearing loss in adults,as noise exposure cannot be avoided in actual life.Though the correct use of well-designed and properly fitted traditional hearing protection devices (HPDs) can reduce or prevent cochlear damage,there are,however,a number of industrial and military occupations and situations in which HPDs are either not available or are impractical or impossible to wear.In these cases,conditioning could provide a means of preventing or reducing the hazards from noise exposure.
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Obviously,much more needs to be known about the acoustic boundary conditions that produce cochlear protection and about the biological bases for the protective effect before clinical applications can be seriously considered.Nevertheless,it is encouraging to think that the inherent plasticity of the cochlea may be exploited clinically in protecting the hearing of individuals in noisy environments.
作者地址：刘亚光.航天医学工程研究所，北京 100094
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