| Background and Aims Melatonin is a neuroendocrinal hormone secreted by pineal gland. It is widely distributed in the body and has many biological effects. It can affect on the rhythm of day and night, sexual maturation and reproduction, immune response, neoplasma, stress, aging and apoptosis. Studies have shown that the concentration of melatonin in the body decreases with the increasing of age which leads to senescence and many age-related diseases. So melatonin is regarded as a natural anti-senescence hormone. The aging process can be hastened by cutting off the pineal gland. The span of mice can be prolonged and much physiological process delayed by extrinsic melatonin or with : the transplantation of pineal gland in old mice from the young. In this way, melatonin may antagonize senescence andpresbycusis.Studies have shown that D-galactose can be oxidized and produce much free radical in the body so that several antioxidative enzymes are inhibited and peroxyl product accumulates. Galanol also accumulates in the cells and the metabolism is disturbed. In this way, the aging process can be accelerated and many age-related diseases occur. In this study, D-galactose was used to make the model of presbycusis and the method was modified so that an ideal model can be found for the further study. The protective effects and mechanism of melatonin on aging cochlea were investigated by observing the variety of morphology and function in presbycusis models.Methods Eighty healthy guinea pigs were chosen andtwenty of them were randomly divided into four groups: control group and three D-galactose groups with different doses (150 200 300mg/kg). Auditory brainstem responses were recorded and the concentration of superoxide dismutase (SOD) in hepatic tissue was measured. The frozen slides of cochlea and cochlear nuclei were made to investigate the variety of neurons and lipofuscin.The rest sixty animals were random divided into five groups: the normal control group (A), the presbycusis model group (B), the presbycusis model group treated withmelatonin (C: MT l.0mg/kg; D: MT 0. 3mg/kg), the presbycusis model group treated with VitC (E). D-galactose was used to induce presbycusis via intraperitoneal injection. Auditory brainstem responses were recorded before and after treatment. The concentration of glutathione peroxidase (GSH-Px) in cochlea was measured. Cochlea were strained by succinate dehydrogenase histochemical method (SDH) . The damage of hair cells on basilar membrane was observed.Results The latencies of I III waves in each D-galactose group delayed significantly in comparison with that in control group and the concentration of SOD decreased in different extent. The interpeak intervals of I-III waves and the threshold value increased significantly only in 300mg/kg group (P<0. 05). In control group, spiral ganglion cells and neurons in cochlear nuclei arranged thickly, strained evenly, Nissl bodies normal, and lipofuscin granules can' t be seen beside the spiral ganglion cells. In model group, the cells arranged thinly, swelling, Nissl bodies decreasing, endotheliocytes proliferating, and a large amount of lipofuscin granules can be seen.In group B, the latencies of I III waves and the interpeakintervals of I-III waves delayed and the threshold valuesignificantly increased (P<0. 05). Group C had no significantdifference in comparison with group A (P>0. 05). The findingsin group D were similar to that in group E. These two groups both had significant difference in comparison with that in group A and B (P<0. 05). The concentration of GSH-Px had no significant difference between group C and group A (P>0. 05). The result of GSH-Px in other groups decreased significantly compared with that in group A (P<0. 05). The observation of basilar membrane showed that three rows of outer hair cells in group C arranged in good order, well stained, and had no damage and defect as well as group A. Hair cells in other groups arranged irregularly, not in equal shape. The stained granules were ba...
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