Opioid inhibition of immunoreactive corticotropin-releasing factor secretion into the hypophysial-portal circulation of rats

Abstract

The role of the opioid peptides in the regulation of adrenocorticotropin (ACTH) secretion remains unclear. In rats, morphine and the enkephalins exert a stimulatory effect on the hypothalamic-pituitary-adrenal axis, while β-endorphin (β-E) and dynorphin (DYN) are reported to have stimulatory or inhibitory activity. Alternatively, data from human studies indicate a clear inhibitory role of opiates. In the present studies, secretion of immunoreactive corticotropin releasing factor (irCRF) into the hypophysial-portal circulation was directly measured before and after intracerebroventricular administration of β-E, DYN and naltrexone (NTX). Both β-E and DYN were equipotent in their dose-related inhibition of irCRF secretion. The inhibitory action of β-E was reversed by NTX, while the action of DYN was only partially blocked. Administration of NTX alone resulted in a significant elevation of spontaneous and stimulated irCRF secretion. Finally, injection (icv) of 1.0 nmol β-E or DYN blocked the nitroprusside-hypotension induced elevation of irCRF. These observations suggest, that under the conditions of these experiments, exogenous β-E acting primarily via μ opioid receptors and DYN acting via κ and μ receptors exert tonic inhibitory effects on the activity of CRF secreting cells. Furthermore, it appears that β-E and DYN are capable of modulating (inhibiting) stimulated secretion of irCRF and thus activity of the hypothalamic-pituitary-adrenal axis.