AGING EFFECTS ON MONOAMINES IN RAT MEDIAL VESTIBULAR AND COCHLEAR NUCLEI

Noradrenaline (NA), dopamine (DA), serotonin (5-HT) and their metaboli tes-3-methoxy, 4-hydroxyphenylglycol (MHPG), 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA)-were determined using HPLC in medial vestibular nucleus (MVN), anteroventral cochlear nucleus (AVCN), dorsal+posteroventral cochlear nucleus (DCN+PVCN), loc us coeruleus (LC) and raphe dorsalis of Dark Agouti-Hanovre (DA-HAN) r ats aged 4, 21 and 24 months. In older rats, the main noradrenergic ch anges were a decrease of NA content with an increase of the MHPG/NA ra tio in MVN and a selective NA increase in AVCN. 5-HT and 5-HIAA levels were increased in all the brainstem nuclei except raphe dorsalis. DA and DOPAC remained unchanged. These data show that noradrenergic neuro ns in sensory nuclei are differently affected by aging whereas seroton ergic activation occurs in most of them possibly as a compensatory res ponse to dysfunction of sensory input and processing. The increase of NA stores in the AVCN of aged rats is in line with the elevated audito ry brainstem threshold reported in old rats and could improve the sign al-to-noise ratio. Noradrenergic neurons in the MVN seem to be more se nsitive to age effect than cochlear nuclei; however, even if neuronal loss occurs, NA synthesis and/or metabolism increase to ensure normal or increased noradrenergic activity.