Monoamines (noradrenaline, dopamine, serotonin) in the rat cochlear nuclei: endogenous levels and turnover

Noradrenaline (NA), dopamine (DA), serotonin (5-HT) and their metabolites, 3-methoxy,4-hydroxyphenylglycol (MHPG) and 5-hydroxyindoleacetic acid (5-HIAA), were determined using high-performance liquid chromatography with electrochemical detection in the rat anteroventral cochlear nucleus (AVCN), in the dorsal part of the nucleus including the dorsal cochlear nucleus (DCN) and the posteroventral cochlear nucleus (PVCN) and as a comparison, in the locus coeruleus (LC) and dorsal raphe nucleus (RD) which contain the corresponding noradrenergic and serotonergic cell bodies. In both cochlear nuclei (CN), the endogenous levels of NA, 5-HT and related metabolites were smaller than in LC or RD. NA turnover assessed from the ratio MHPG/NA or after treatment with alpha-methylparatyrosine was faster in the CN than in LC; in contrast, 5-HT turnover was lower in the CN than in RD as shown by the ratio 5-HIAA/5-HT. In agreement with previous histological findings, NA and 5-HT were more concentrated in AVCN than in DCN+PCVN; however, the turnover of both monoamines was faster in the dorsal nuclei. In addition, the CN contained small amounts of dopamine and DOPAC; both DA levels and the ratio DA/NA (0.10 vs. 0.04) were greater in the dorsal than in the ventral part suggesting the presence of non-precursor-specific DA pools. Our data suggest that the functional involvement of monoamines may be different in cochlear subnuclei.