Facilitation of lordosis in ovariectomized rats by intracerebral progesterone implants.

Sexual receptivity in female mammals occurs following appropriate hormonal stimulation ~°. In many species, the combined action of estrogen and progesterone is more effective than estrogen alone 6,16,41. A variety of behavioral, neuroanatomical and electrophysiological investigations suggest that these ovarian hormones affect sexual behavior by acting on the central nervous system 30 although effects on peripheral tissues might also be important. Estrogen may act primarily in the medial preoptic area (MPOA) to stimulate receptivity because chronic estradiol implants most effectively reinstate the lordosis reflex in ovariectomized females e9 when placed in this region. This is consistent with the selective accumulation of estradiol by cells in the MPOA although other sites within the central nervous system also show affinity for this hormonel,S, s2. The quantities of estradiol secreted by rats displaying 4-day estrous cycles are not sufficient to induce behavioral estrus; the subsequent action of progresterone is essential 41. The brain sites at which progesterone might act to facilitate receptivity have until recently remained unexplored. One early study demonstrated that bilateral implants of estradiol on one side and a progesterone pellet of over I mm diameter on the other induced female sexual behavior when placed either in the preopticanterior hypothalamic region or in the middle hypothalamus al. Because considerable diffusion of hormone may have occurred due to the large surface area of the progesterone pellet, it was not possible to specify regions of high progesterone sensitivity. Early studies of progesterone uptake by brain tissue indicated that selective accumulation of progesterone by subregions within the hypothalamus did not occur 4s,49. More recently the mesencephalic reticular formation has been implicated as a critical site for progesterone action because hormone implants in that region were effective in facilitating sexual receptivity whereas similar implants in various hypothalamic areas were not 4a. Recent biochemical investigations demonstrating considerable uptake of radioactivity in the rat midbrain following injection of [3H]progesterone are consistent with these behavioral resultsSS,SL