Intracranial estradiol benzoate implants and lordosis behavior of ovariectomized guinea pigs

Adult ovariectomized Hartley guinea pigs were bilaterally implanted in the ventromedial-arcuate area, caudal anterior hypothalamus, basal anterior hypothalamic-preoptic area or midbrain with stainless steel cannulae with removable inserts. At 0 h, the animals received inserts containing crystalline estradiol benzoate or cholesterol followed 36 h later by a systemic injection of 0.6 mg progesterone. Estradiol benzoate implants in the ventromedial-arcuate area facilitated estrous behavior in 79% of animals. Median characteristics of estrous behavior, classified as ‘good-excellent’, shown by these animals were latency after progesterone injection= 3h and duration= 6h. Implants of estradiol benzoate in the basal anterior hypothalamic-preoptic area resulted in display of estrous behavior in 89% of animals. The behavior shown by these animals was categorized as ‘good’ and latency= 6h and duration= 6h. Implants of estradiol benzoate in caudal anterior hypothalamus (nucleus supra-opticus diffusus) resulted in heat in 50% of animals. Heat was classified as ‘poor’ in these animals and latency= 6h and duration= 4.5h. Maximum lordosis did not vary according to brain region and was about 6 sec for all animals facilitated by intracranial estradiol benzoate. No animals displayed estrous behavior after receiving midbrain implants of estradiol benzoate. None of the cholesterol implanted animals displayed heat. The data are consistent with the concept that there are discrete estradiol-sensitive areas in the basal hypothalamus and basal anterior hypothalamic-preoptic area with a region of relatively poor sensitivity in between.