Autoradiographic localization of beta-endorphin binding in the pancreas

Autoradiographic localization of 125I-labeled beta-endorphin binding in the rabbit pancreas demonstrated specific binding in the pancreatic islet cells. Binding was inhibited by (1) nonradioactive beta-endorphin, (2) the opioid antagonist naloxone, (3) the mu receptor agonists morphine and [D-Ala2, (Me)Phe4, Gly(ol)5]enkephalin, (4) the delta receptor agonist [D-penicillamine2, D-penicillamine5]-enkephalin, (5) the mu and delta agonist met-enkephalin and (6) the delta and kappa agonist dynorphin. Specific binding was not clearly demonstrable in the acinar portion of the rabbit pancreas. The binding characteristics of 125I-beta-endorphin in the pancreatic islets were comparable with those of mu and delta opioid receptors in the rabbit brain. In the pancreas, beta-endorphin binding appeared to be concentrated in discrete areas in the islets. Combined immunohistochemistry and autoradiography demonstrated that beta-endorphin binding was primarily concentrated in the glucagon-containing alpha and somatostatin-containing delta cells, but was also found in the insulin-containing beta cells to a lesser extent. Given the intraislet location of the opioid binding sites, and our previous finding of immunoreactive beta-endorphin in the pancreatic beta cells and the inhibitory effect of beta-endorphin on insulin secretion, it appears that beta-endorphin may serve a paracrine or autocrine function in the regulation of pancreatic hormone secretion.