Alpha-melanocyte stimulating hormone discloses a stimulatory effect of beta-endorphin on somatostatin release

The influence of alpha-melanocyte stimulating hormone (alpha-MSH) and beta-endorphin (beta-END) on the secretion of somatostatin (SRIF) from the median eminence (ME) was studied using an in vitro incubation system. The MEs from adult male rats were first preincubated at 37 degrees C for 30 min with constant shaking in 0.4 ml of Krebs-Ringer bicarbonate-glucose buffer (pH 7.4) containing bacitracin in an atmosphere of 95% O2/5% CO2. Medium was discarded and replaced by medium containing different doses of alpha-MSH, beta-END, or a fixed dose of alpha-MSH (10(-7) M or 10(-9) M) plus beta-END at various concentrations. By themselves alpha-MSH and beta-END did not alter basal SRIF release, but in the presence of alpha-MSH (10(-7) M) beta-END stimulated somatostatin release. This effect was significant at concentrations of beta-END of 10(-8) M and higher. The permissive effect of alpha-MSH was observed at a concentration as low as 10(-9) M, but in this case the stimulatory effect of beta-END became evident only at higher doses tested (10(-7) M). It is suggested that alpha-MSH and beta-END participate in the modulation of SRIF release. By themselves beta-END and alpha-MSH did not affect basal release of SRIF but in the presence of alpha-MSH, beta-END had a stimulatory effect on SRIF release. The mechanism for this interaction is unknown. The results are consistent with the possibility that beta-END neurons have stimulatory and inhibitory effects on SRIF release and that alpha-MSH, by blocking the inhibitory components, discloses the stimulatory effect of beta-END on SRIF release.