Does β-endorphin modulate basal and dopamine-inhibited prolactin release by an action at the anterior pituitary?¹

Previous studies performed to elucidate an anterior pituitary site of action for β-endorphin (B-EP) in the regulation of prolactin (PRL) release have been inconclusive. The purpose of the present study was to further investigate if B-EP could stimulate PRL release by a direct action at the anterior pituitary. Anterior pituitaries from ovariectomized rats were dispersed, and the cells were plated in Ham's F-10 medium. Following a 48-hour preincubation period, the effects of B-EP on basal PRL release and on the dopamine-inhibited PRL release were studied over a 24-hour period. Overall, B-EP at concentrations ranging from 10^-8 to 10^-6 M had no effect on basal release of PRL at 1,3,5 and 24 h, although a small but significant increase of 39% was observed with 10^-6 M B-EP at 1 h of incubation only. B-EP at 10^-6 M, but not at lower concentrations, significantly suppressed the inhibitory effects of 10^-6 M dopamine on PRL release at I h but not at longer periods of incubation. Pretreatment of the cells for 3 h with 10^-6 M naloxone completely antagonized the suppressive effect of B-EP on the dopamine-mediated inhibition of PRL release. Naloxone pretreatment of the cells had no effect on basal nor dopamine-inhibited release of PRL. Because of the high concentration of B-EP required to suppress the dopamine inhibition and the transitory nature of this B-EP effect, it is unlikely that the primary site of action for B-EP in regulating PRL release is at the anterior pituitary. However, high concentrations of B-EP may antagonize the inhibitory effect of dopamine on PRL release at the anterior pituitary through a specific opiate-mediated mechanism.