Effects of Intraventricular Infusion of Serotonin, Norepinephrine, and Dopamine on Spontaneous LH Release in Castrate Male Sheep1

Various dosages of serotonin (5-HT), norepinephrine (NE), and dopamine (DA) were infused into the third ventricle of the hypothalamus of unanesthetized castrate male sheep. Sequential blood samples were collected at 10-min intervals from indwelling vascular cannulae for 2 h prior to infusion and for 4 h following infusion. Plasma LH was quantified by specific double-antibody radioimmunoassay. Infusion of isotonic saline (0.2 ml) into the third ventricle had no statistically significant effect on plasma LH. Therefore, the effects of all monoamine infusions were compared statistically against pretreatment concentrations in the appropriate trials. 5-HT infusion (100µ g dosage) caused a significant (p < 0.01) and sustained depression of spontaneous LH release. NE infusion at dosages of 100 and 150 µg also produced highly significant (p < 0.01) decreases in plasma LH. DA infusions at five different dosages ranging from 2.5 to 150 µg failed to consistently alter plasma LH concentrations. These results implicate 5-HT and NE, but not DA, as synaptic transmitters involved in regulating LH release in castrate sheep.