Galanin Interacts with Serotonin in Stimulating Prolactin Secretion in the Rat

The interaction of galanin (GAL) with serotonin (5-HT) in the regulation of prolactin (PAL) secretion was investigated in urethaneanesthetized male rats. Intracerebroventricular administration of 5-HT (1 and 10 μ g) and GAL (1 μ g) caused an increase in plasma PRL levels, but co-administration of GAL did not show any additive effect on 5-HT-induced PRL secretion. Pretreatment with methysergide (0.25 mg/kg), a nonselective 5-HT1 and 5-HT2 receptor antagonist, partially inhibited the PRL increase induced by GAL. On the other hand, neither ketanserin (0.25 mg/kg), a selective 5-HT2 receptor antagonist, nor ICS 205–930 (0.25 mg/kg), a selective 5-HT3 receptor blocker, had any effect on GAL-induced increase in PRL secretion. Parachlorophenylalanine (300 mg/kg), a 5-HT synthesis inhibitor, however, caused a marked enhancement of PRL release induced by GAL, which was partially inhibited by a 5-HT neurotoxin, 5, 6-dihydroxytryptamine. Parachlorophenylalanine also caused a potentiation of 5-HT-induced PRL release, possibly by sensitizing 5-HT receptors. These findings suggest that 5-HT receptors are, at least partly, involved in GAL-induced PRL release in the rat.