DIFFERENTIAL-EFFECTS OF HISTAMINE ON THE ACTIVITY OF HYPOTHALAMIC DOPAMINERGIC-NEURONS IN THE RAT

The effect of intracerebroventricular administration of histamine on h ypothalamic dopaminergic neuronal activity was estimated in male rats by measuring concentrations of dopamine and its metabolite 3,4-dihydro xyphenylacetic acid (DOPAC) in brain regions containing terminals or p erikarya of these neurons. Three distinct, regionally specific neuroch emical responses were apparent. In the median eminence and intermediat e lobe of the pituitary, histamine affected neither DOPAC nor dopamine concentrations, suggesting no effect on tuberoinfundibular or periven tricular-hypophysial dopaminergic neuronal activity. In the medial zon a incerta and in the dorsomedial, rostral periventricular and medial p reoptic hypothalamic nuclei, histamine effected a dose- and time-relat ed increase in both DOPAC and dopamine concentrations; these effects w ere blocked by destruction of noradrenergic neurons projecting to thes e regions, suggesting that these changes are attributable to noradrene rgic neuronal activation, and that histamine does not affect the activ ity of incertohypothalamic or periventricular-preoptic dopaminergic ne urons located in these brain regions. In the suprachiasmatic, caudal p eriventricular and paraventricular hypothalamic nuclei, histamine effe cted a dose- and time-related increase in DOPAC, but not dopamine, con centrations; these effects were blocked by the H-1 antagonist mepyrami ne, but not the H-2 antagonist, zolantidine. Destruction of noradrener gic neurons projecting to these regions did not prevent the histamine- induced increases in DOPAC concentrations. These data indicate that hi stamine increases the activity of dopaminergic neurons projecting to t he suprachiasmatic, caudal periventricular and paraventricular nuclei via an action at H-1 receptors. Overall, these results reveal that i.c .v. administration of histamine differentially affects the activity of the various dopaminergic neuronal systems of the rat hypothalamus.