The infusion of 50 µ g norepinephrine (NE) into the third ventricle of the brain was found to be highly effective in stimulating the release of an ovulatory surge of LH from the pituitary gland in estrogen-primed rabbits. Intraventricular dopamine (D), in dosages of 5 and 50 µ g, was not only completely ineffective in inducing ovulation, but the higher dosage actually blocked the ovulatory effect of NE, often for a month or more after infusion. In experimentts in which LH was measured by radioimmunoassay, intraventricular NE stimulated a marked rise in LH starting within minutes, peaking at 2 h and returning to pre-infusion levels by 6 h. No rise in serum LH was observed following the intraventricular infusion of D. Both dosages of D were quite effective in blocking LH output in response to NE infused 2 h later, and the 50-µ g dosage of D generally prevented any rise in serum LH concentration for some weeks after the infusion of intraventricular NE. The results are consistent with the hypothesis that a noradrenergic synapse is functional in hypothalamic mechanisms activating the release of pituitary LH.

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