NEUROCHEMICAL EVIDENCE THAT THE INHIBITORY EFFECT OF GALANIN ON TUBEROINFUNDIBULAR DOPAMINE NEURONS IS ACTIVITY-DEPENDENT

The purpose of the present study was to examine the effects of galanin on the basal and stimulated activity of tuberoinfundibular dopaminerg ic (TIDA) neurons in male and female rats. TIDA neuronal activity was estimated by measuring dopamine (DA) synthesis [accumulation of 3,4-di hydroxyphenylalanine (DOPA) after administration of a decarboxylase in hibitor] and metabolism [ratio of 3,4-dihydroxyphenylacetic acid (DOPA C) to DA concentrations] in terminals of these neurons in the median e minence. Central administration of galanin (2 mu g/rat, i.c.v.) produc ed a rapid (by 15 min) increase in plasma prolactin concentrations, bu t had no effect on the ratio of DOPAC to DA in the median eminence of either male or female rats. In contrast, galanin decreased the ratio o f DOPAC to DA in the median eminence of both male and female rats whos e TIDA neuronal activity was stimulated following administration of th e DA antagonist haloperidol (1 mg/kg, s.c., 12 h). The galanin recepto r antagonist galanin-(1-13)-bradykinin-(2-9)-amide had no effect on th e accumulation of DOPA in the median eminence of male rats per se, but blocked the inhibitory effects of either exogenous or endogenous gala nin on median eminence DOPA accumulation in haloperidol-treated rats. These results indicate that in both male and female rats, galanin-indu ced activation of prolactin secretion is not mediated by changes in to nic inhibition of hormone release by TIDA neurons, and that TIDA neuro ns are responsive to the inhibitory effects of galanin only under acti vated conditions.