The possible effect of proopiomelanocortin-derived peptide, β-endorphin on frog gonadotrope cells was investigated. Binding and internalization of β-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 β-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 β-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 β-endorphin and LH β-subunit revealed that half the β-endorphin labeled cell population was positively immunostained for LH β-subunit, and β-endorphin was able to induce an increasing trend of LH secretion in cultured pars distalis cells. Therefore, it seems that β-endorphin acts directly on pituitary pars distalis and influences gonadotropin secretion through the interaction with its own receptor.
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