Effects of different opiate agonists on melanocyte-stimulating hormone release: in vivo and in vitro studies.

The effects of Leu-enkephalin, Met-enkephalin, and beta-endorphin on melanocyte-stimulating hormone (MSH) secretion were studied in vivo and in vitro. The three opioid peptides release MSH. In vitro this reslease is dose dependent for Met-enkephalin between 10 and 1000 ng/mL and for Leu-enkephalin between 10 and 100 ng/mL. beta-Endorphin releases MSH at the low concentration of 1 ng/mL and the effect is dose dependent between 1 and 100 ng/mL. Naloxone reverses this effect. In vivo the three petptides release MSH.