The Actions of Serotonin, Norepinephrine, and Epinephrine on Hypothalamic Processes Leading to Adenohypophyseal Luteinizing Hormone Release*

Publisher Website
Google Scholar
Abstract
The effects of 3rd ventricle (3V) infusion of exogenous monoamines on the secretion of LH [luteinizing hormone] 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 .mu.l/min for 160 min. Serotonin perfusions (2.6 .times. 10-3 M for 160 min at 3.9 .mu.g/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 [luteinizing hormone releasing hormone]. Seemingly, the reduced plasma LH which follows MPOA-ECS in serotonin-treated rats is the consequence of decreased hypothalamic LRH secretion. Desipramine (3.6 .times. 10-4 M), a norepinephrine (NE) uptake inhibitor, did not prevent serotonin inhibition. In contrast, the specific serotonin uptake inhibitor, Lilly 110140 [fluoxetine] (3.2 .times. 10-4 M), and cyproheptadine (2.9 .times. 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 .times. 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 .times. 10-8 M NE at 3.9 .mu.l/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 .times. 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 .times. 10-6 M epinephrine which inhibited the response to LRH, potentiated the MPOA-ECS-induced rise in plasma LH. Higher concentrations of epinephrine (3 .times. 10-4 M) inhibited both the ECS and LRH-induced secretion of LH.