Occurrence of beta-endorphin binding sites in the pituitary of the frog Rana esculenta: effect of beta-endorphin on luteinizing hormone secretion

The possible effect of proopiomelanocortin-derived peptide, beta-endorphin on frog gonadotrope cells was investigated. Binding and internalization of beta-endorphin to pituitary pars distalis cultured cells were visualized by immunofluorescence and analyzed by means of confocal laser scanning microscopy. Using biotinylated endorphin, the time-course of beta-binding showed that this opioid was internalized through receptor-mediated endocytosis, the mechanism in which actin and clathrin were involved; then, the lysosomal degradation program occurred at later stages. The beta-endorphin binding was well antagonized by Naloxone, the opiate receptor antagonist, and up-regulated since more rapid response was obtained in the previously primed cells. The double immunostaining reaction for beta-endorphin and LH beta-subunit revealed that half the beta-endorphin labeled cell population was positively immunostained for LH beta-subunit, and beta-endorphin was able to induce an increasing trend of LH secretion in cultured pars distalis cells. Therefore, it seems that beta-endorphin acts directly on pituitary pars distalis and influences gonadotropin secretion through the interaction with its own receptor.