An Increase in Single Unit Activity of the Medial Basal Hypothalamus Occurs during the Progesterone-Induced Luteinizing Hormone Surge in the Female Rhesus Monkey*

Abstract

Accumulated evidence indicated that a positive feedback effect of progesterone (P) occurred at the hypothalamic level. The present study in female rhesus monkeys [M. mulatta] examined the effects of P on single unit activity of neurons in the hypothalamus and on LH [luteinizing hormone] release. Single unit activity was recorded by inserting a flexible stainless steel electrode into the hypothalamus of the monkey, which was restrained in a chair under light ketamine sedation. The firing rate of the single unit activity of the ventral hypothalamus (1.5 .+-. 0.2 spikes/s; n = 57) in ovariectomized estrogen-primed monkeys was slow and was slower in the ventral hypothalamus than in the dorsal hypothalamus (6.2 .+-. 0.8 spikes/s; n = 80). P injection resulted in a dramatic increase in unit activity of the ventral hypothalamus (4.7 .+-. 0.6 spikes/s; n = 111), but not of the dorsal hypothalamus (5.3 .+-. 0.7 spikes/s; n = 72), and induced a concomitant release of LH. Both increases in circulating LH and unit activity of the ventral hypothalamus were significantly correlated over time (P < 0.02). In contrast, oil injections induced neither change. In the rheus monkey, P seems to activate neural substrates in the ventral hypothalamus to promote the release of LHRH and, and subsequently, LH.