Ovariectomized adult rats were cannulated via the left carotid artery. Three days later they were heparinized and connected via the cannula to a peristaltic pump for continuous withdrawal of blood. Concentrations of LH in 60 μl samples of whole blood collected at approximately 5 min intervals were determined by radioimmunoassay. Untreated rats, ovariectomized at least one month earlier, exhibited considerable fluctuation in plasma LH concentrations over a 6 hr period. Treatment of such rats with subcutaneous injections of estradiol valerate and progesterone at the time of cannulation resulted in a reduction and stabilization of plasma LH levels when measured 3 days later. A rapid increase in plasma LH levels invariably occurred following the injection of neutralized crude acidic extracts of rat hypothalamic tissue into the aorta of steroid-treated castrates but did not occur following the injection of similarly prepared extracts of cerebral tissue. LH contamination of the hypothalamic extracts, determined by radioimmunoassay, accounted for a maximum of 1% of any observed response. Individual rats were able to respond to as many as 3 injections of hypothalamic extract with no apparent decrease in responsiveness. A linear relationship was observed between the log-dose of hypothalamic extract injected and the increase in plasma LH concentration in individual rats. Untreated anestrous ewes were cannulated via the carotid artery for removal of blood samples and injection of sheep hypothalamic tissue extracts. The unanesthetized sheep responded to repeated injections of hypothalamic extract with dramatic (8- to 40-fold) increases in serum LH, indicating a log-dose related response. (Endocrinology86: 1305, 1970)
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Gay et al. (1970) studied this question.