Effect of CNS peptides on hypothalamic regulation of pituitary secretion

A variety of neuropeptides, such as TRH, somatostatin, VIP, Substance P, neurotensin, CCK, gastrin, and opioid peptides, alter secretion of GH and PRL from the pituitary. These actions differ according to the route of administration or with experimental conditions, especially anesthesia. Among these peptides, the most consistent results have been obtained with opioid peptides, which stimulate GH and PRL release. Both beta-endorphin and enkephalins are capable of stimulating GH and PRL release in anesthetized and unanesthetized, freely moving rats. The effect is blocked by naloxone, an opiate receptor antagonist. GH secretion induced by opioid peptides seems to be mediated by an alpha-adrenergic mechanism, since treatment with DDC and fusaric acid, which are dopamine-beta-hydroxylase inhibitors, reserpine, and phenoxybenzamine which is an alpha-adrenergic blocking agent, blunted GH secretion. However, pimozide, a dopamine receptor antagonist, and propranolol, a beta-adrenergic blocking agent, were without effect. On the other hand, basal PRL secretion was augmented by pimozide, suggesting the possible involvement of dopamine. It is also possible that serotonin is involved in the GH and PRL release induced by opioid peptides. The physiological significance of opioid peptides in regulating GH and PRL secretion is still unclear. Contradictory results (12,25) have been obtained concerning the effect of naloxone on basal or stimulated GH and PRL secretion in rats, monkeys and humans when tested by the continuous blood sampling method, which rules out the erroneous evaluation of results caused by episodicity of plasma hormone levels. Further studies should clarify the physiological role of opioid peptides in regulating pituitary function.