In vitro effects of a synethetic enkephalin analogue (D-Met2,Pro5)-enkephalinamide on the hypothalamus-pituitary--testis function in rats.

The effects of (D-Met2,Pro5)-enkephalinamide, a synthetic analogue of the natural opioid pentapeptides, were investigated on the hypothalamus-pituitary-testis axis using in vitro tissue and isolated cell incubation systems. (D-Met2,Pro5)-enkephalinamide in a concentration range 10(-7)--10(-5) M did not alter the testosterone production and the troph-hormone sensitivity of the isolated Leydig cells, or the gonadotroph hormone release and the luteinizing-hormone--releasing hormone (LH-RH) sensitivity of the isolated anterior pituitary cells. However 10(-5)--10(-5) M synthetic enkephalin analogue induced a dose-dependent inhibition of LH-RH content and release of the mediobasal hypothalamus, which was prevented by equimolar concentrations of naloxone.