Induction of the Luteinizing Hormone Surge by Intrahypothalamic Application of Estrogen in the Rhesus Monkey

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Abstract
The site of action of estrogen in inducing the LH [luteinizing hormone] surge in the rhesus monkey was investigated by intracranial implantation of crystalline estradiol-17.beta.. Platforms were placed upon the skulls of intact female monkeys, which permitted placement of stainless steel cannulae filled with estradiol-17.beta. into various neural sites or the pituitary. When estradiol was placed into the medial basal hypothalamus at the level of the arcuate nucleus (probably in the 3rd ventricle) on Day 3 of the menstrual cycle, 8 of 9 monkeys responded with an LH surge, with peak LH levels occurring 48-72 h later. No LH surges followed placement of estradiol in the anterior hypothalamus or preoptic area, the premammillary region, the pituitary, 2 mm lateral to midline in the basal hypothalamus or following placement of empty cannulae in the medial basal hypothalamus. Thus, it seems unlikely that estrogen induces the LH surge in the rhesus monkey by an action in the rostral hypothalamus. Because of the possibility of diffusion of estrogen into the hypophyseal portal vascular system, estrogen implants in the medial basal hypothalamus may induce the LH surge by acting there, in the pituitary or in both sites.