Characterization of Opioid Binding Sites in the Neural and Intermediate Lobe of the Rat Pituitary Gland by Quantitative Receptor Autoradiography

Previous studies have suggested an involvement of enkephalins in regulation of oxytocin (OXT) and vasopressin (AVP) release, which seems to disagree with the very low affinities of Met- and Leu-enkephalin for the kappa opioid receptor. As opioid receptors in the neural lobe exclusively exist of kappa receptors, we studied the binding characteristics of larger pro-enkephalin derived peptides for opioid binding sites in the neural lobe by means of light microscopic receptor autoradiography. In addition, the pharmacological characteristics of opioid binding sites in the neural lobe were compared with those in other parts of the pituitary. In the neural as well as the intermediate lobe both high and low affinity ³H-bremazocine binding sites were present. Binding to these sites was completely displaceable by both naloxone and nor-binaltorphimine, suggesting that these sites represent kappa opioid receptors. Also with regard to selectivity and affinity characteristics to other ligands, opioid binding sites in the neural and intermediate lobe were quite similar. In the anterior lobe a very low level of bremazocine binding was present, which could not be displaced by nor-binaltorphimine. Displacement studies with pro-enkephalin and pro-dynorphin derived peptides showed that both groups of peptides could bind to opioid binding sites in the neural and intermediate lobe. Especially the relatively large pro-dynorphin and pro-enkephalin derived peptides, such as dynorphin 1–17 and BAM22, appeared to be very potent ligands for these opioid binding sites and were much more potent than smaller fragments, such as dynorphin 1–8, and Met- and Leu-enkephalin. These results contradict the existence of a mismatch in the neural (and intermediate) lobe with regard to the local type of opioid peptides and receptors present.