Leumorphin, a novel opioid peptide, promotes lordosis in female rats.

The actions of leumorphin, a recently characterized endogenous opioid peptide, oppose of most opioid peptides in facilitating lordosis reflex, a major component of female sexual behavior in the rat. Maximal lordosis appeared promptly after infusion of 1 nmol leumorphin into the ventromedial hypothalamus of ovariectomized estrogen-primed rats. This facilitation lasted for as long as 5 h, unless interrupted by midbrain infusion of an antiserum to prolactin. The result is a discovery of a novel substance of remarkable strength in facilitating lordosis, an effect presumably mediated by midbrain release of prolactin.