The medial zona incerta (MZI) and dorsomedial nucleus of the hypothalamus (DMN), which contain cell bodies and terminals of incertohypothalamic dopaminergic (DA) neurons, are densely innervated by both noradrenergic (NE) and 5-hydroxytryptaminergic (5-HT) neurons. In view of emerging anatomical and pharmacological evidence suggesting possible interactions between 5-HT and catecholaminergic neurons, the effects of experimental procedures that inhibit or disrupt 5-HT neurons on the activities of catecholaminergic neurons terminating in these regions were examined in the present study. Catecholaminergic neuronal activity was estimated by measuring catecholamine synthesis (accumulation of 3,4-dihydroxyphenylalanine [DOPA] after administration of a decarboxylase inhibitor) and metabolism (concentrations of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) and the norepinephrine metabolite 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG)) in the MZI and DMN of both male and female rats. Inhibition of 5-HT neurons following administration of the 5-HT1A autoreceptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) increased the accumulation of DOPA in the DMN and the concentrations of DOPAC in the MZI and DMN, indicating an activation of catecholaminergic neurons in these regions. Concentrations of MHPG were increased in the MZI and DMN by 8-OH-DPAT or 5,7-dihydroxytryptamine-induced lesions of 5-HT neurons, revealing that NE neurons terminating in these regions were activated following procedures that decrease 5-HT neuronal function. Following destruction of NE neurons projecting to the MZI and DMN, 8-OH-DPAT no longer increased DOPAC concentrations in these brain regions.(ABSTRACT TRUNCATED AT 250 WORDS)