The actions of serotonin, norepinephrine, and epinephrine on hypothalamic processes leading to adenohypophyseal luteinizing hormone release

Abstract

The effects of third ventricle (3V) infusion of exogenous monoamines on the secretion of LH were investigated in Nembutal-blocked proestrous rats after electrochemical stimulation (ECS) of the medial preoptic area (MPOA). Two microcannulae were anatomically oriented through the ventral surface of the brain to establish a push-pull flow system in the 3V at the rate of 3.9 microliter/min for 160 min. Serotonin perfusions (2.6 x 10(-3) M for 160 min at 3.9 microgram/min) significantly depressed the MPOA-ECS-induced rise in plasma LH at 160 and 200 min, but did not diminish the secretion of LH after an intraatrial injection of 250 ng LRH. Seemingly, the reduced plasma LH which follows MPOA-ECS in serotonin-treated rats is the consequence of decreased hypothalamic LRH secretion. Desipramine (3.6 x 10(-4) M), a norepinephrine (NE) uptake inhibitor, did not prevent serotonin inhibition. In contrast, the specific serotonin uptake inhibitor. Lilly 110140 (3.2 X 10(-4) M), and cyproheptadine (2.9 x 10(-4) M) blocked the inhibitory effects of serotonin. Serotonin was also tested in proestrous rats anesthetized with Nembutal within 1 h after the beginning of the light cycle (morning rats) and in proestrous rats at least 15 min after the start of the dark cycle (night rats). Serotonin (2.6 x 10(-3) M) failed to diminish the rise in plasma LH after ECS in morning rats, but depressed the ECS-induced increase of LH in night rats. Perfusions of 3 X 10(-8) M NE at 3.9 microliter/min for 160 min augmented the plasma LH increase after MPOA-ECS but only at 160 and 200 min, while its response to LRH was not tested. At higher concentrations, 3 x 10(-6) M NE markedly suppressed the rise in plasma LH after ECS and also depressed the secretion of LH after an intraatrial injection of LRH. In contrast, 3 x 10(-6) M epinephrine which inhibited the response to LRH, potentiated the MPOA-ECS-induced rise in plasma LH. Higher concentrations of epinephrine (3 x 10(-4) M), however, inhibited both the ECS and LRH-induced secretion of LH.