Non-opiate [beta]-endorphin fragments and dopamine--V [gamma]-type endorphins and prolactin secretion in rats

Abstract

The effects on prolactin secretion of three peptide-derivatives of β-endorphin which show neuroleptic-like activities in rats were studied.

Intravenous administration of γ-endorphin (β-endorphin (βE) 1–17) enhanced plasma prolactin levels. γ-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 γ-endorphin could be prevented by co-incubation with the opiateantagonist naloxone.

In vivo studies with non-opiate-like fragments of γ-endorphin, des-Tyr1-γ-endorphin (DTγE, βE 2–17) and des-enkephalin-γ-endorphin (DEγE, βE 6–17), showed that these peptides suppressed the plasma prolactin levels, when prolactin release was slightly stimulated. However, DEγE dose-dependently enhanced plasma prolactin levels, when prolactin release was low. Subchronic treatment with DEγE for 4 days made prolactin release more sensitive to the inhibitory action of small doses of apomorphine. Neither DTγE nor DEγE 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 γ-endorphin increases prolactin secretion by interfering with dopaminergic systems in the pituitary and that DTγE and DEγE have a modulatory action on prolactin secretion.