Monoamines (norepinephrine, dopamine, serotonin) in the rat medial vestibular nucleus: Endogenous levels and turnover

Monoamine (norepinephrine, dopamine, serotonin) and metabolite endogenous levels were determined in the rat medial vestibular nucleus (MVN) using HPLC with electrochemical detection. As a comparison, the locus coeruleus (LC) and dorsal raphe nucleus (RD) which contain the cell bodies of MVN noradrenergic and serotoninergic neurons respectively were also analyzed. Norepinephrine (NE) and serotonin (5-HT) basal levels of MVN were high (33.8 and 39.2 pmol/mg protein respectively) but lesser than in LC or RD. Great amounts of MHPG and 5-HIAA were also present in the MVN. The turnover of NE assessed both from the ratio MHPG/NE and by the decrease in the NE content after treatment with alpha-methylparatyrosine was faster in the MVN (half-life: 1.5h) than in LC (half-life: 3.6h). On the other hand, the ratio 5-HIAA/5-HT was lower in the MVN (0.58) than in the RD (0.85) indicating a smaller 5-HT turnover in the MVN. In addition, like LC and RD, the MVN contained meaningful amounts of dopamine (DA) and DOPAC. The high ratio DA!NE (0.27) suggests the presence of non precursor specific dopaminergic pools. However, individualized dopaminergic neurons have not yet been demonstrated. The data are discussed in line with the possible neurotransmitter function of monoamines in the MVN.