Changes in the cochlear dopaminergic system of the aged rat

The levels of dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic a cid (DOPAC) and homovanillic acid (HVA) have been quantified in cochleae of male and female rats aged 3, 6, 9, 12, 19 and 24 months. Animals were expo sed for 1 h, under general anesthesia, to: (1) silence (basal conditions) o r (2) white noise at 90 dB SPL. Afterwards, the concentrations of DA, DOPAC and HVA were determined by HPLC with electrochemical detection in homogena tes of individual cochleae. In basal conditions, the cochlear concentration s of DA, DOPAC and HVA in aged females were higher than in adult ones. The concentrations of DA and DOPAC were also higher in aged males with respect to adult ones. A decrease in DA and an increase in DOPAC and HVA concentrat ions, with respect to silence, were detected when adult animals were expose d to noise. Meanwhile, aged animals showed either a noise-induced increase or no modification of DA and DOPAC with respect to basal levels. Present re sults suggest ace-related failures in DA release and metabolizing mechanism s within the cochlea, together with a compensatory DA synthesis increase. H owever, the possibility of an initial damage in the primary auditory neuron s which could also stimulate the synthesis of DA must not be excluded. Pres ent age-related changes could indicate that the cochlear dopaminergic inner vation is affected during the aging process. Since this innervation plays a n important role in both the modulation and the protection of the primary a uditory neurons, its metabolic alteration could profoundly modify the audit ory process. (C) 2001 Published by Elsevier Science B.V.