Repeated infusions of 6-hydroxydopamine (6-OHDA) into the third ventricle of the rabbit brain in dosages shown to depress hypothalamic norepinephrine (NE) by more than 80% failed to block the copulation-induced ovulatory surge of LH release from the adenohypophysis in estrogen-primed, multiparous New Zealand White does. Only when infusion of the neurotoxin produced a basal hypothalamic lesion did it intercept the coital stimulus and prevent LH release. In 5 rabbits the initial infusion of 6-OHDA stimulated an LH surge presumably by activating NE release from noradrenergic nerve endings. The failure of chronic depletion of hypothalamic NE to prevent coitally-induced LH release suggests that the transmitter may normally play only a modulatory role in hypothalamopituitary activation in the reflex ovulator, as has also been proposed for its function in cyclic ovulation in the rat. The results suggest that alternative systems may be brought into play to subserve reproductive mechanisms when hypothalamic NE is chronically depleted.
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