Effect of enkephalin analogues Tyr-D-Ala-Gly-Phe-Cys(Et) and Tyr-D-Ala-Gly-Phe-Cys(Bu) on prolactin and growth hormone release in rats.

The effect of natural Met-enkephalin and its analogues Tyr-D-Ala-Gly-Phe-Cys(Et) (I) and Tyr-D-Ala-Gly-Phe-Cys(Bu) (II) on serum rat prolactin and rat growth hormone levels in male rats has been studied. Met-enkephalin and both analogues increased serum growth hormone and prolactin concentration, analogue I being the most potent. Enkephalin analogue I, with isosteric isomer of methionine in Met-enkephalin, and analogue II with butyl residue showed a lower stimulatory effect than natural Met-enkephalin. Stimulatory effect of analogues I and II on prolactin and growth hormone release was found to be reversed by a simultaneous treatment with the opiate antagonist naloxone.