[Effect of met enkephalin, leu enkephalin and their retro analogs on the lactotropic function of the pituitary gland]

When administered subcutaneously to male rats, the novel peptides retromet-enkephalin and retro-leu-enkephalin stimulated prolactin (PRL) release less powerfully than morphine but somewhat greater than met-enkephalin. Leu-enkephalin did not affect PRL secretion in vivo. Neither morphine nor any of opioid peptides under study were able to stimulate basal or the dopamine-inhibited PRL release from cultured adenohypophyseal cells of male rats. It is concluded that stimulatory effects of opiates and opioid peptides on PRL secretion in rats are mediated via the hypothalamus or some other neural structures.