Contribution of noradrenergic neurons to 3,4-dihydroxyphenylacetic acid concentrations in the regions of the rat brain containing incertohypothalamic dopaminergic neurons.

The purpose of this study was to determine the source of 3,4-dihydroxyphenylacetic acid (DOPAC) in medial zona incerta (MZI) and dorsomedial nucleus (DMN), with the overall aim of ascertaining whether alterations in the concentration of this dopamine (DA) metabolite reflect the activity of incertohypothalamic dopaminergic neurons. In both the MZI and DMN, the concentration of norepinephrine (NE) exceeds that of DA, reflecting a higher density of noradrenergic vs. dopaminergic neurons in these brain regions. The ratio of DOPAC to DA was greater than the ratio of 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) to NE indicating that the activity of dopaminergic neurons in the MZI and DMN is greater than that of noradrenergic neurons. Destruction of noradrenergic neuronal axons in the ventral bundle following bilateral injections of the neurotoxin 5-amino-2,4-dihydroxy-alpha-methylphenylethylamine (5-ADMP) decreased NE concentrations in the MZI and DMN, but had no effect on the concentrations of DA or DOPAC, revealing that under basal conditions noradrenergic neurons contribute little to DOPAC concentrations in these brain regions. The DA receptor antagonist haloperidol increased, while the DA receptor agonist apomorphine decreased DOPAC concentrations in the MZI and DMN, indicating that alterations in the activity of incertohypothalamic dopaminergic neurons are accompanied by corresponding changes in the concentration of this DA metabolite. On the other hand, activation of noradrenergic neurons following administration of the alpha 2-adrenergic receptor antagonist idazoxan increased DOPAC concentrations in both the MZI and DMN in intact, but not in ventral bundle-lesioned rats.(ABSTRACT TRUNCATED AT 250 WORDS)