Localized Behavioral Effects of Tritiated Estradiol Implants in the Ventromedial Hypothalamus of Female Rats*

Abstract

In an attempt to localize estrogen-sensitive brain sites sufficient to prime feminine sexual behavior, 30-gauge cannulae containing approximately 9 ng high specific activity [³H]-estradiol were implanted unilaterally or bilaterally into the ventromedial region of the hypothalamus of ovariectomized rats. Tests for lordosis behavior were conducted on days 3 and 6 postimplantation, 4–6 h after progesterone treatment. Brain, pituitary, and uterine tissue were then sampled for radioactivity. Cannulae delivered, on the average, 3 ng [³H]estradiol during 8 days. Radioactivity was localized almost exclusively to the hypothalamus or hemihypothalamus in which the implant was placed. This was true regardless of whether tissue radioactivity or radioactivity in cell nuclei was analyzed. In the bilateral implant experiments, average hypothalamic [³H]estradiol radioactivity amounted to 56 pg/rat in tissue and 220 fg in purified cell nuclei. The latter is 4% of the estimated estrogen eceptor capacity of cell nuclei of the hypothalamus. In the unilateral implant experiments, 10 of 24 rats responded at least once; the mean lordosis quotients were 38 for the first test (number responding, 7) and 61 for the second test (number responding, 8). Of 6 rats which received blank cannulae, none exhibited lordosis behavior after progesterone treatment. In the bilateral implant experiments, 6 of 11 rats responded at least once with a mean lordosis quotient for responders of 60 on the first test (n = 4) and 52 on the second test (n = 6). The results indicate that estrogenic stimulation of a subset of hypothalamic cells, representing a small percentage of the estradiol-concentrating capacity of the hypothalamus, can prime behavioral responsiveness.