Endogenous opioid peptide system in male brown bullhead catfish, Ictalurus nebulosus lesueur: Characterization of naloxone binding and the response to naloxone during the annual reproductive cycle

Abstract

AbstractThe endogenous opioid peptide system was investigated in male brown bullhead catfish. 3H‐naloxone binding was assessed in 20,000g membrane preparations from catfish brains. 3H‐naloxone bound to a high‐affinity, low‐capacity receptor (Kd = 0.27nM; Bmax = 119.52 fmol/mg protein) and a low‐affinity, high‐capacity receptor (Kd = 5.77 nM; Bmax = 323.06 fmol/mg protein). Binding was ligand and tissue specific. 3H‐naloxone binding was regionally distributed, with highest levels in telencephalon:preoptic area, and lower levels in remaining diencephalon, mesencephalon, and cerebellum:brain stem. A seasonal cycle in receptor number was observed, with highest numbers occurring during testicular recrudescence in association with elevated plasma androgens. In vivo naloxone treatment resulted in decreased levels of testosterone and 11‐ketotestosterone during testicular recrudescence but had no effect during the resting phase. These data indicate the presence of an endogenous opioid peptide system in brown bullhead catfish which is involved in the regulation of reproductive function in this species.