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 dopaminergic (TIDA) neurons in male and female rats. TIDA neuronal activity was estimated by measuring dopamine (DA) synthesis [accumulation of 3,4-dihydroxyphenylalanine (DOPA) after administration of a decarboxylase inhibitor] and metabolism [ratio of 3,4-dihydroxyphenylacetic acid (DOPAC) to DA concentrations] in terminals of these neurons in the median eminence. Central administration of galanin (2 µg/rat, i.c.v.) produced a rapid (by 15 min) increase in plasma prolactin concentrations, but 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 of DOPAC to DA in the median eminence of both male and female rats whose TIDA neuronal activity was stimulated following administration of the DA antagonist haloperidol (1 mg/kg, s.c, 12 h). The galanin receptor antagonist galanin-(1–13)-bradykinin-(2-9)-amide had no effect on the accumulation of DOPA in the median eminence of male rats per se, but blocked the inhibitory effects of either exogenous or endogenous galanin on median eminence DOPA accumulation in haloperidol-treated rats. These results indicate that in both male and female rats, galanin-induced activation of prolactin secretion is not mediated by changes in tonic inhibition of hormone release by TIDA neurons, and that TIDA neurons are responsive to the inhibitory effects of galanin only under activated conditions.