Beta-Endorphin (Beta-EP) receptor-mediated-endocytosis in frog Rana esculenta pars distalis luteinizing hormone (LH) and prolactin (PRL) containing cells

It is known that in mammals the opioid peptide beta-EP has a physiological role in regulating pituitary hormone secretion including gonadotropins. The control of reproductive function by the proopiomelanocortin (POMC)-derived peptides in anurans is still under investigation. We have examined beta-EP specific binding sites endocytosis and recycling processes by confocal laser scanning microscopy in primary cultures of Rana esculenta pars distalis. A time course showed that at 10 min of incubation the beta-EP labeling was found at the the plasmamembrane level; after 1 h the labeling was more evident in vesicle-like cytoplasmic structures that, at 2 h of incubation, were localized strictly around the nucleus. The labeling disappeared 2 h later but, after rinsing, a subsequent re-incubation of the same cells generally showed a much higher degree of labeling than described above supporting the idea of a mechanism of cellular memory. To further investigate the nature of the vesicles, we examined the distribution of F-actin and clathrin in order to test the hypothesis that beta-EP is internalized through receptor-mediated-endocytosis wich involves the actin-clathrin interaction. Our study showed that some beta-EP positive cells (50%) did not display LH labeling and that beta-EP negative cells were LH secreting cells. Moreover, the majority of beta-EP positive cells (80%) were also found to be PRL-containing cells