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We sequenced the complementary DNA (cDNA) encoding guinea pig LHRH from an expression library derived from the preoptic area-anterior hypothalamus. Data from in situ hybridization and RNase protection assays verified that the cloned cDNA was complementary to guinea pig LHRH messenger RNA. The architecture of the deduced precursor resembles that of LHRH precursors identified in other species. In contrast, the predicted sequence of the decapeptide differs from mammalian LHRH by two amino acid substitutions in positions 2 and 7. This is a novel finding, because the amino acid sequence that comprises LHRH decapeptide is identical in all mammals studied to date. Moreover, the predicted substitution in amino acid position 2 is unique among vertebrates. A second observation of potential significance is the existence of two subspecies of LHRH messenger RNA differing only in the length of their 3' untranslated regions. These two transcripts were verified by sequence analysis of positive clones from the cDNA library and by RNase protection analysis of preoptic area-anterior hypothalamus extracts, and their presence is consistent with the two polyadenylation signals identified in the untranslated regions of the LHRH gene. Future studies will examine LHRH gene expression in guinea pigs, which like primates but unlike rats, have a true luteal phase as a component of their reproductive cycle.

The putative neural target sites of progesterone (P) action for the facilitation of estrous behavior in female rats were reexamined using the hormone implant technique. Subjects were ovariectomized, estrogen-primed Long-Evans females. All were outfitted with bilateral double barreled cannulae assemblies consisting of 23-gauge guide cannulae and 28-gauge inserts. Subjects in Exp 1 received sc injections of individually determined threshold priming doses of estradiol benzoate. Estrogenic priming for subjects in Exp 2 was provided by sc placed Silastic capsules (5 mm) filled with 17 beta-estradiol-cholesterol (1:20). Each subject was tested for estrous behavior with a male after P-filled cannulae and after blank inserts were lowered into the brain. Behavioral tests were conducted 1 and 4.5 h postcannulae placement. Thirty-five of 40 females with P in the ventromedial hypothalamus exhibited high levels of lordosis behavior during the 4.5-h test. Moreover, 29 of the 35 exhibited solicitation behavior as well. Estrous behavior was not exhibited by these animals during the 1-h test. Implants in other regions of the brain (i.e. mesencephalic reticular formation, preoptic area, central grey, caudate putamen, and hippocampus; n = 80) did not consistently facilitate estrous responsiveness. In Exp 3, levels of estrous responsiveness were similar before and after adrenalectomy (n = 10); thus, facilitation of estrous behavior by P-filled implants in the ventromedial hypothalamus was not dependent upon activation of the hypothalamic-pituitary-adrenal axis.

In an attempt to obtain a more precise localization of the neural target site(s) of estrogen-priming action in the activation of estrous behavior, 30-gauge cannulae containing a preparation of 17 beta-estradiol diluted 1:250 with cholesterol were implanted bilaterally into the brains of ovariectomized rats. Subjects were tested for sexual behavior with intact males at 4-day intervals for 3 weeks beginning 2-3 days after stereotaxic surgery. Animals received a systemic injection of 0.5 mg progesterone 4-6 h before all but 1 test. After this treatment, estrous behavior was observed in 19 of the 20 animals judged to have both cannulae resting in or within 0.25 mm of the ventromedial nucleus of the hypothalamus (VMN). Virtually no estrous behavior was observed without the administration of progesterone. Behavioral scores decreased as a function of the distance of the cannulae tips from the VMN. Cholesterol implants in the same region (n = 8) were without effect. Implants in other regions of the brain (e.g. preoptic area, diagonal band of Broca, lateral habenula, amygdala, or cortex; n = 52) were ineffective. These results indicate that estrogenic stimulation of the region of the VMN alone is sufficient to prime the activation of estrous behavior in the ovariectomized rat.