Progesterone-induced release of luteinizing hormone (LH) and of follicle-stimulating hormone (FSH) was studied in immature female rats 3 days after an injection of estrogen or after the placement of an electrolytic lesion in the basal hypothalamus. Both types of sequential treatment elevated plasma concentrations of LH more than 20 -fold as compared with controls. Plasma FSH concentrations increased by a factor 3 after the sequence brain lesion-progesterone and by a factor 5 after the combined

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... ormonal treatment. Uterine weights, determined as a cumulative index of prior exposure to endogenous or exogenous estrogen, were similarly increased in both groups. It is concluded that this type of brain lesion is able to replace an injection of estrogen in priming an immature female rat for the subsequent progesterone-induced gonadotropin release and that a unilateral hypothalamic lesion placed, on day 23 of life, near the origin of the pituitary stalk, is the approximate equivalent of 10 (log estradiol benzoate injected subcutaneously at that time. These findings support our view that brain lesions advance sexual maturation in the female rat via the precocious activation of ovarian steroidogenesis rather than by removing neural restraints upon the reproductive system. The observation originally made by D ONOVAN and V AN D E R WE R FF TEN B O SC H (Ig!6) that sexual maturation in the female rat can be advanced by small electrolytic lesions placed in certain areas of the brain has been confirmed and extended by various investigators (see D ON O V A N and VA N D E R WE R FF TEN BoscH, Ig65 ; CRIT-CHI ,OW and B AR -SR L A, I g67 ; RA MIR $ Z , 1973 ; D AVID S ON , 1974 for review). In contrast, the mechanism by which such lesions are able to stimulate the reproductive system has remained obscure (DA VI DSON, 1974).