- Serum prolactin, growth hormone and cortisol levels were examined in normal volunteers following administration of carbidopa, a peripheral decarboxylase inhibitor. 24-hour urinary tryptamine levels dropped markedly indicating that inhibition of decarboxylase was effective. Prolactin levels rose while growth hormone and cortisol remained unchanged. Since the tuberoinfundibular dopamine nerve terminals lie outside the blood brain barrier, this study suggests that these neurons are involved in prolactin but not in growth hormone or cortisol regulation. Findings are compatible with two alternate hypotheses--either that dopamine is a physiologic prolactin inhibiting factor (PIF) or that tuberoinfundibular dopamine neurons regulate the release of PIF.