Non-opiate beta-endorphin fragments and dopamine-V. gamma-Type endorphins and prolactin secretion in rats

The effects on prolactin secretion of three peptide-derivatives of beta-endorphin which show neuroleptic-like activities in rats were studied. Intravenous administration of gamma-endorphin (beta-endorphin (beta E) 1-17) enhanced plasma prolactin levels. Gamma-Endorphin did not affect the prolactin secretion by hemipituitary glands incubated in vitro and by cultured pituitary cells, but it reversed, in a dose-dependent way, the dopamine-induced inhibition of prolactin release. This effect of gamma-endorphin could be prevented by co-incubation with the opiate-antagonist naloxone. In vivo studies with non-opiate-like fragments of gamma-endorphin, des-Tyr1-gamma-endorphin (DT gamma E, beta E 2-17) and des-enkephalin-gamma-endorphin (DE gamma E, beta E 6-17), showed that these peptides suppressed the plasma prolactin levels, when prolactin release was slightly stimulated. However, DE gamma E dose-dependently enhanced plasma prolactin levels, when prolactin release was low. Subchronic treatment with DE gamma E for 4 days made prolactin release more sensitive to the inhibitory action of small doses of apomorphine. Neither DT gamma E nor DE gamma E affected prolactin release by hemipituitary glands and cultured pituitary cells. They did also not affect the dopamine-induced inhibition of prolactin release in vitro. It is suggested that gamma-endorphin increases prolactin secretion by interfering with dopaminergic systems in the pituitary and that DT gamma E and DE gamma E have a modulatory action on prolactin secretion.