The effects of hypothalamic implants of ovarian steroids on oestrous behaviour in rabbits

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Abstract
The brains of ovariectomized female rabbits which were anoestrous in behavior when tested with vigorous males were implanted either bilaterally or unilaterally with 10% oestradiol benzoate in cholestroL Rabbits with implants in the ventromedial-premammillary hypothalamus ca. 1 mm lateral to the mid line became highly oestrous in behavior within 1-2 days of implantation and mated 100% of the times tested. Implants of blank tubing, cholesterol or progesterone in this area, or oestrogen implants elsewhere in the hypothalamus had no effect on sexual behavior. The uteri of all females implanted with diluted oestrogen were atrophic and similar to those of castrate controls, indicating that the effects of hypothalamic implants on sexual behavior were direct local effects of oestrogen on the CNS [central nervous system]. Rabbits implanted bilaterally with undiluted oestradiol benzoate showed some slight systemic spread of the implanted oestrogen. Oestrous behavior was evoked within an average of 3.4 days by implantation into a large hypothalamic area, presumably as a result of oestrogen diffusing from the site of implantation to the critcal ventromedial-premammillary region. Injections of progesterone in dosages known to block oestrous behavior in intact or oestrogen-primed rabbits, failed to block this behavior in females implanted with pure oestrogen in the ventromedial-premammillary hypothalamus. Implantation of progestins into the ventromedial-premammillary area in ovariectomized rabbits did not usually inhibit oestrous behavior induced by small threshold doses of systemically administrated oestrogen. The possibilities that oestrogen and progesterone (1) interact in different areas of the CNS or (2) compete for "steroid receptors" in the ventromedial-premammillary hypothalamus are discussed.