Objective To make a quantitative analysis of damaging hair cells,and investigate the progression pattern of cochlear pathology and the primary death pathway of hair cells in cochleae of aging Fischer 344 rats.Methods Thirty-two Fischer 344 rats were used in this experiment.The animals were assigned to two groups:rats in young group (n=13) were 3-4 months old,and in aging group (n=19) were 20-27 months old.Rats in aging group were further divided into two subgroups based on the animals' age:20-23-month subgroup (n=13) and 24-27-month subgroup (n=7).The auditory brainstem response (ABR) thresholds of both ears elicited with tone bursts at 5,10,20 and 40 kHz were measured in both young,and aging Fischer 344 rats.Upon completion of the auditory test,animals were decapitated,and both left and right bullae were exposed.Following fixation,whole specimens comprising the basilar membrane with Corti organ were separated from the modiolus.Propidium iodide (PI),a popular DNA intercalating fluorescent probe was used to trace the morphological changes in hair cell nuclei,and a terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay was used to detect nuclear DNA fragmentation in the aging cochleae.Filamentous actin (F-actin),an important structural protein of outer hair cells (OHCs),was stained with FITC-labeled phalloidin to illustrate the morphologic viability of cuticular plate and the stereocilia for affirming the missed OHCs.Each Corti organ was thoroughly inspected from the apical to the basal turns of the cochlea under a fluorescence microscope.The numbers of damaging OHCs including missed nuclei,condensed nuclei and swollen nuclei were documented,and a cochleogram was drawn.Results There were significant thresholds at all tested frequencies between the young and aging rats (P

Objective Apoptosis and necrosis are two forms of hair cell death in cochlea following exposure to intense impulse noise.The present study was designed to examine the activity of caspase-3,caspase-8 and caspase-9 in the basilar membrane of cochlea,and investigate the cellular signal pathway associated with noise-induced hair cell death in cochlea.Methods Twenty-seven adult chinchillas were used in present study.The animals were randomly assigned into test group(no.15) and control group(no.12).The animals in test group were exposed to noise for the detection of the activity of caspase-3,caspase-8 and caspase-9,respectively,each with 5 animals.Animals in control group were not exposed to noise.Chinchillas were exposed to impulse noise at 155 dB peak sound pressure level(pSPL) for 75 times.Two hours after noise exposure,both cochleae were perfused with approximately 30?l of freshly prepared solution containing one of the three caspases(caspase-3,-8 and-9) respectively using a micro-syringe.One hour after perfusion,the animals were sacrificed and the cochleae were harvested.The Corti organs were double stained with propidium iodide(PI),and a DNA intercalating fluorescent probe was used to visualize the morphologic viability of hair cell nuclei.All the specimens were observed with a fluorescence microscope.Results The normal cochlea did not exhibit green fluorescence for any of the three caspases in Corti organ.Strong caspase-3,-8 and-9 green fluorescence appeared in the outer hair cells with condensed or fragmented nuclei,a sign of apoptosis,whereas no fluorescence was observed for any of the three caspases in the swollen outer hair cell nuclei,a sign of necrosis,in the cochleae exposed to intense impulse noise.Conclusion It is indicated that caspase-9 is involved in the intrinsic and caspase-8 involved in the extrinsic apoptotic pathway which occur simultaneously in apoptotic outer hair cells.No caspase activation occurs in necrotic outer hair cell in the cochlea following exposure to intense impulse noise.

Objective To compare the prevalence of apoptosis and necrosis, and investigate the primary death pathway of outer hair cells of rat cochlea following styrene exposure. Methods Fourteen adult Long Evans rats were used in the present study. The animals were randomly assigned into test group (n=8) and control group (n=6). Animals in test group were exposed to styrene by gavage at 400 mg/kg (2g styrene was mixed with 1ml olive oil). Treatment was performed once a day, 5 days per week for 3 weeks. Animals in control group were fed by gavage the same volume of olive oil on an identical time schedule used for the test group. The auditory brainstem response (ABR) thresholds of both ears elicited with clicks were measured before and at the end of the 3-week styrene or olive oil treatment. After hearing was re-assessed, animals were sacrificed and cochleae were quickly removed from the skull. Following fixation, whole specimens comprising the basilar membrane with Corti's organ were separated from the modiolus. Apoptotic, necrotic and missing outer hair cells (OHCs) were distinguished by combined assays of nuclear staining with propidium iodide (PI), TUNEL assay and filamentous actin(F-actin)staining with FITC-phalloidin. Each Corti's organ was thoroughly examined by fluorescence microscopy. The numbers of damaged OHCs (apoptotic, necrotic and missing OHCs) were documented. Results Neither threshold shift of ABR nor sign of hair cell (HC) damage was found in the cochlea of control animals. The animals of test group showed both physiological and pathological changes in the cochleae following the 3-week styrene treatment. ABR testing revealed an average of 15 dB of threshold shifts. F-actin staining exhibited the maximal level of OHCs damage in the middle portion of Corti's organ. The major damage occurred in the third row of OHCs, followed by the second and first rows of OHCs. Three types of morphological changes in damaged OHC nuclei were revealed by PI labeling: nuclear condensation, nuclear swelling and nuclear missing. Strong TUNEL green fluorescence appeared in the OHCs with condensed nuclei. Quantitative analysis showed that the average number of apoptotic OHCs was approximately three times greater than the number of necrotic OHCs (P=0.01). Conclusion It is indicated that apoptosis is the primary death pathway of OHCs leading to generation of the cochlear lesion following styrene exposure.

Objective To examine the role of mitochondrial energy-conversion efficiency in regulating the initiation and execution of the apoptotic process in outer hair cells(OHCs)of cochlea following exposure to intense noise.Methods Seventeen adult chinchillas were used in present study.The animals were randomly assigned to one of the two groups,12 animals were exposed to noise and the remaining 5 animals without exposure to noise served as normal control.For all the animals,3-nitropropionic acid solution(3-NP,50mmol/L),an irreversible inhibitor of succinate dehydrogenase(SDH),was dropped onto the round window of the cochlea of the right ear using a micro-syringe to inhibit the mitochondrial energy production,to serve as the test ear.Artificial perilymph solution(AP)was dropped onto the round window of the cochlea of the left ear to serve as the control ear.16h after application of 3-NP and AP,the animals were exposed to 75 pairs of impulse noise at 155 dB pSPL.The cochleae were harvested 2h after the noise exposure.The cochlear basilar membrane was stained with propidium iodide(PI),a DNA intercalating fluorescent probe used to visualize the morphologic viability of hair cell nuclei.All the specimens were examined with a fluorescence microscope.Results In the 3-NP-treated cochlea,medium degree of nuclear condensation of OHCs appeared to be the dominant nuclear pathology,and only a few OHCs showed nuclear fragmentation in the damaged area of the cochlea.In contrast,the AP-treated control ear exhibited a large quantity of nuclear fragments,and a small quantity of medium degree of nuclear condensation.Conclusion The present study indicates that mitochondrial energy-conversion function plays an important role in regulating the apoptotic process.Disruption of the mitochondrial energy can not deter the apoptotic process from initiation,but can slow down the pace of apoptotic progression in outer hair cells of the cochlea following exposure to intense noise.

Objective To quantitatively analyze the occurrence of apoptotic and necrotic outer hair cells (OHCs) and to evaluate the changes of these parameters with time after noise exposure. Methods Chinchillas were exposed to a narrow band noise at either 104 or 108 dB SPL for 1 hour. The animals were executed and dissected at either 1, 4 and 30 days after the noise exposure and the cochleas were collected for detection of OHC death mode. The apoptotic and necrotic OHCs were distinguished by examining the OHC nuclear morphology and confirmed by staining for caspase 3 activity or TUNEL assay. ABR thresholds for click stimuli were used to monitor changes in auditory function. Results The number of apoptotic cells were significantly greater than those of necrotic cells shortly after the noise exposure at day 1 for the 104 dB group, day1 and day4 for 108 dB group ( P =0 01, P =0 03, P =0 01) and the difference between the number of apoptotic cells and necrotic cells became statistically insignificant at day 4 and day 30 for the 104 dB group and day 30 for 108 dB group ( P =0 67, P =0 29, P =0 52). By day 30, apoptotic and necrotic pathologies continued, although in small quantity in both 104 dB group and 108 dB group. Conclusions The results of the study indicated that the early expansion of cochlear lesion is contributed primarily by apoptosis, whereas the later stage of lesion expansion is likely contributed equally by apoptosis and necrosis. The death of OHCs not only takes place during a noise exposure, but also continues for at least 30 days after noise exposure

BACKGROUNDOur previous studies have shown that both apoptosis and necrosis are involved in hair cell (HC) pathogenesis in aging cochleae. To better understand the biological mechanisms responsible for the regulation of HC death, we examined the activity of succinate dehydrogenase (SDH), a mitochondrial bioenergetic enzyme, in the HCs of aging cochleae.

METHODSThe auditory brainstem response thresholds elicited by tone bursts at 4, 10 and 20 kHz were measured in both young (2-3 months) and aging (22-23 months) Wistar rats. SDH activity was evaluated with a colorimetric assay using nitroblue tetrazolium monosodium salt. The SDH-labeled organs of Corti were double stained with propidium iodide, a DNA intercalating fluorescent probe for illustration of HC nuclei. All the specimens were examined with fluorescence microscopy and confocal microscopy.

RESULTSAging rats exhibited a significant elevation of ABR thresholds with threshold shifts being 34 dB at 20 kHz, 28 dB at 10 kHz, and 25 dB at 4 kHz. Consistent with the reduction in the cochlear function, aging cochleae exhibited the reduction of SDH staining intensity in the apical and the basal ends of the cochleae, where a large number of apoptotic, necrotic, and missing HCs were evident. The reduction in SDH staining appeared in a cell-death-mode dependent fashion. Specifically, SDH labeling remained in apoptotic HCs. In contrast, SDH staining was markedly reduced or absent in necrotic HCs.

CONCLUSIONSIn the aging cochlea, SDH activity is preserved in HCs undergoing apoptosis, but is substantially reduced in necrosis. These results suggest that mitochondrial energetic function is involved in the regulation of cell death pathways in the pathogenesis of aging cochleae.

Aging ; metabolism ; Animals ; Apoptosis ; physiology ; Cochlea ; cytology ; enzymology ; Female ; Hair Cells, Auditory ; enzymology ; Male ; Necrosis ; physiopathology ; Rats ; Rats, Wistar ; Succinate Dehydrogenase ; genetics ; metabolism

OBJECTIVE: To observe the effects of N-acetyl-L-cysteine (L-NAC) protect hair cells in the rat cochlea from injury of exposure to styrene. METHOD: Seventeen adult Long Evans rats were used in present study. The animals were randomly assigned into test group (n=9) and control group (n=8). The animals were exposed to styrene by gavage at 400 mg/kg (2 g styrene was mixed with 1 ml olive oil). Test group animals received styrene exposure plus L-NAC 325 mg/kg (L-NAC was dissolved in physiological saline solution) by intraperitoneal injection. Treatment was performed once a day, 5 days per week for 3 weeks. Control group animals received the same volume of saline injection on an identical time schedule used for the test group. The auditory brainstem response (ABR) thresholds of both ears elicited with clicks were measured before and at the end of the 3-week styrene or styrene plus L-NAC treatment. After hearing was re-assessed, animals were sacrificed and cochleae were quickly removed from the skull. Following fixation, whole specimens comprising the basilar membrane with Corti's organ were separated from the modiolus. The organs of Corti were stained with propidium iodide (PI) and the TUNEL assay to visualize the morphologic viability of hair cell nuclei, FITC-labeled phalloidin, a F-actin intercalating fluorescent probe used to visualize the morphologic viability of cuticular plate and the stereocilia in the hair cells. Each organ of Corti was thoroughly examined using fluorescence microscopy. The numbers of damaged OHCs (apoptotic, necrotic and missing OHCs) were documented. RESULT: There was a statistically significant decrease in ABR threshold shift (P<0.05) in the styrene-plus-L-NAC treated animals. The average percentage of damaged OHCs in the styrene-treated animals was 28.3%. In contrast, the average percentage of OHC damage in the styrene-plus-L-NAC treated group was only 10.6% (P<0.01). The percentage of reduction in the number of apoptotic cells in styrene-plus-L-NAC treated group was 78% (P<0.01). However, the mean reduction of necrotic cells was only 23% (P>0.05). CONCLUSION The results indicate that the treatment with L-NAC may effectively protect against the styrene-induced hair cells damage and preferably reduce the number of apoptotic OHCs.
Acetylcysteine ; analogs & derivatives ; pharmacology ; Animals ; Antioxidants ; pharmacology ; Cochlea ; cytology ; drug effects ; Evoked Potentials, Auditory, Brain Stem ; Hair Cells, Auditory ; drug effects ; pathology ; Lysine ; analogs & derivatives ; pharmacology ; Rats ; Rats, Long-Evans ; Styrene ; adverse effects