Effect of apomorphine plus 5-hydroxytryptophan on plasma prolactin levels in male rats

The relative potency of dopaminergic inhibition and serotonergic stimulation of prolactin secretion in male rats was compared. 5-Hydroxytryptophan (5-HTP), 100 mg/kg, i.p., the precursor of serotonin, produced a 6-11 fold increase in plasma prolactin. Apomorphine, a dopamine agonist, at doses of 1-10 mg/kg, i.p., had no significant effect on plasma prolactin. However, when apomorphine was given with or before 5-HTP, it nearly completely blocked the increase in prolactin produced by 5-htp. these results indicate that inhibition of prolactin secretion by dopaminergic stimulation can overcome the prolactin releasing effect of serotonin. The results are consistent with the hypothesis that prolactin secretion is ordinarily under a weak serotonergic stimulation and a profound dopaminergic inhibition. It is also possible that apomorphine affects plasma prolactin levels by increasing prolactin clearance.