Immediate release of prolactin and biphasic effects on growth hormone release following electrical stimulation of the median eminence.

The median eminence (ME) region of the hypothalamus was electrically stimulated through permanently implanted electrodes in 1 unanesthetized sheep. Plasma concentrations of GH and prolactin (PRL) were measured at 10-min intervals before, during and after 30-min periods of electrical stimulation or sham procedures. The onset of ME stimulation decreased plasma GH in all 17 cases in which prestimulation GH levels were 1.0 ng/ml or higher. In all 27 cases of ME stimulation, plasma GH never increased during the 30-min stimulation period. However, spontaneous GH increases were noted during sham procedures in 9 of 31 cases. The termination of ME stimulation was followed within 10--20 min by markedly increased GH levels in 24 of 27 cases. All 4 ewes responded to electrical stimulation with comparable biphasic effects on GH release even though electrode locations varied slightly. These results indicate localization and stimulation-induced release of endogenous neurohormones in the ME region with activities inhibiting the release of GH (GHIF). The secondary increase in plasma GH following the end of ME stimulation indicates a reversal of conditions unfavorable to GH release. This reversal may represent a rebound from the inhibitory action of GHIF or the post-inhibitory action of a hypothalamic GH-releasing factor or both. In 3 ewes with electrodes implanted in the region of the anterior ME, the onset of stimulation increased plasma PRL within 10--20 min in 17 out of 20 cases. The electrode in the fourth animal was located in the posterior ME, and when all cases were considered, stimulation through this electrode did not significantly increase or decrease plasma PRL. However, in 5 out of 7 cases stimulation of the posterior ME decreased plasma PRL. In summary, these results indicate the presence in the anterior ME of andogenous neurohormones with PRL-releasing activity (PRF). Conclusions about the posterior ME are equivocal, but this region may contain a PRL-release inhibiting compound in combination with PRF.