The Changes Of Central Auditory System In D-galactose Induced Rats And The Relating Mechenism

PartⅠEstablishment of rat model with central auditory system mimetic aging incuced by D-galactoseObjective: To explore the establishment of the central presbycusis rat model induced by D-galactose.Methods: One group of rats was injected subcutaneously with 150 mg D-galactose daily for 8 weeks, while control animals received vehicle only. Using HE and Nissle staining to observe the morphological changes of auditory cortex (AC), inferior colliculus (IC) and cochlear nucleus (CN). A TaqMan real time PCR assay was used to quantify the mitochondrial DNA 4834 bp deletion.Results: In D-gal-treated rats, reduction of cytoplasmic Nissl bodies, shrunken neurons, cavitation, glial proliferation, and pathological capillary changes, including tortuosity, looping, stringing, and bundling, were observed. The number of neurons of AC, IC and CN (322.58±35.25, 344.08±40.50,207.33±22.01/mm2, respectively) reduced comparing to control group (278.13±41.67, 292.83±43.91, 145.41±25.07/μm2, respectively). Mitochondial DNA 4834 bp deletion rates in AC, IC and CN were significantly higher (16.22±1.29 %, 21.56±4.57 %, 13.96±2.47 %, respectively) than control group (11.24±1.09 %, 10.01±0.98 %, 8.78±0.94 %, respectively).Conclusion: The mimetic aging effect in central auditory system of the rat can be induced by D-galactose. PartⅡAge-related changes in central auditory system: Comparison of D-galactose-induced aging rats and naturally aging ratsObjective: To compare the changes of central auditory system between D-galactose-induced aging rats and naturally aging rats, and to investigate potential central mechanisms of presbycusis.Methods: Rats were divided into (1) D-gal group: Rats were daily subcutaneously injected with 150 mg/kg D-gal for 8 weeks; (2) control group: Rats were given the same volume of vehicle; (3) natural aging group: Rats aged 24 months which received neither D-gal nor other treatments. Central auditory function was evaluated by auditory brainstem response and middle latency response. A TaqMan real time PCR assay was used to quantify the mitochondrial DNA4834 bp deletion in auditory cortex (AC), inferior colliculus (IC) and cochlear nucleus (CN). Lipid peroxidation levels, pathological changes observed in the light and transmission electron microscope, and apoptosis were detected.Results: D-gal treated rats and naturally aged rats showed some similar changes: delayed auditory brainstem response (ABR) latencies (III, IV, V), middle latency response (MLR) Pa latency, and I-IV interpeak latency; increased mtDNA 4834 bp deletion rates and lipid peroxidation levels; and increased rate of neuronal apoptosis; and neurodegenerative changes in the AC, IC, and CN. However, the threshold of ABR in the D-gal group showed no significant change.Conclusion: Age-related central auditory dysfunction and pathological changes can be present in both aging rats and those induced with the D-gal mimetic aging model. Oxidative stress, large scale mtDNA 4834 bp deletion, and apoptosis are likely to be involved in the aging of the central auditory system. PartⅢIncreased mitochondrial DNA damage and decreased base excision repair in the central auditory system of D-galactose-induced aging ratsObjective: To investigate the mechanism of mitochondrial DNA (mtDNA) damage and the role of DNA repair enzymes in the central auditory system aging.Methods: 48 Sprague-Dawley rats were randomly divided into 4 groups: Aging rats were induced by subcutaneous administration of low (150mg/kg), medium (300mg/kg) and high (500mg/kg) dose of D-galactose (D-gal). Rats were given the same volume of vehicle as control group. Quantitative real-time PCR and Western blotting analyses were performed to identify the mtDNA 4834bp deletion and selected mitochondrial DNA repair enzymes, DNA polymeraseγ(polγ) and 8-oxoguanine-DNA-glycosylase (OGG1), in the auditory cortex (AC), inferior colliculus (IC) and cochlear nucleus (CN) . Mitochondrial membrane potential was detected using florescent probe.Results: MtDNA 4834 bp deletion rate were significantly increased and the expression of polγand OGG1 were remarkably down-regulated in AC, IC and CN of D-gal-treated rats compared to control rats. Mitochondrial membrane potential in AC, IC and CN were significant decreased compared to control group.Conclusion: During aging, increased mtDNA damage likely results from decreased DNA repair capacity in the central auditory system. DNA repair enzymes such as polγand OGG1 may provide novel pharmacological targets to promote DNA repair and rescue the central auditory system in patients with degenerative diseases.