Evidence for a kappa-opioid receptor on pituitary astrocytes: an autoradiographic study.

Specific binding of the opioid ligand [3H]etorphine was localized in cryostat sections of the rat pituitary. The binding was concentrated over the pars nervosa. Competition with partially selective unlabelled displacers indicated that the greater part of this binding was of the kappa-subtype, sensitive to inhibition by dynorphin and bremazocine. The kappa nature of the binding was confirmed using the prototype kappa ligand [3H]ethylketocyclazocine in the presence of excess unlabelled mu and delta opioid receptor ligands. Sections of the pituitary stalk caused a loss of the hypothalamic projection to the pars nervosa and a marked gliosis. Concurrent with these changes there was a significant increase in the density of kappa opioid binding. These observations suggest that the majority of opioid receptors in the pars nervosa are of the kappa subtype and are located on the pituicytes.