Multiple opioid receptor subtypes in the pituitary-adrenal axis: a cross-species study

This study examines the multiple opioid receptor subtypes present in the pituitary-adrenal axis of several species using in vitro autoradiography. The monkey neural lobe was found to contain all three subtypes with a preponderance of kappa sites, while the rat and bovine neurohypophyses contained only kappa sites. In the adrenal, opioid receptor sites were found exclusively in the medulla and varied with species. Monkey adrenal contained predominantly delta sites with a relatively small number of kappa sites present, while bovine adrenal medulla had predominantly kappa sites and only a relatively small proportion of delta receptor sites. No specific opioid receptor binding could be detected in the rat adrenal. These results emphasize the importance of species-differences in understanding the role of opioids in neuroendocrine regulation.