Autoradiographic Localization of β-Endorphin Binding in the Pancreas

Abstract

Autoradiographic localization of 125I-labeled β-endorphin binding in the rabbit pancreas demonstrated specific binding in the pancreatic islet cells. Binding was inhibited by (1) nonradioactive β-endorphin, (2) the opioid antagonist naloxone, (3) the μ receptor agonists morphine and [d-Ala2, (Me)Phe4, Gly(ol)5]enkephalin, (4) the δ receptor agonist [d-penicillamine2, d -penicillamine5]-enkephalin, (5) the μ and δ agonist met-enkephalin and (6) the δ and κ agonist dynorphin. Specific binding was not clearly demonstrable in the acinar portion of the rabbit pancreas. The binding characteristics of 125 I-β-endorphin in the pancreatic islets were comparable with those of μ and δ opioid receptors in the rabbit brain. In the pancreas, β-endorphin binding appeared to be concentrated in discrete areas in the islets. Combined immunohistochemistry and autoradiography demonstrated that β-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 β-endorphin in the pancreatic beta cells and the inhibitory effect of β-endorphin on insulin secretion, it appears that β-endorphin may serve a paracrine or autocrine function in the regulation of pancreatic hormone secretion.