Relationships between the effects of the anti-estrogen, CI-628, on sexual behavior, uterine growth, and cell nuclear estrogen retention after estradiol-17 β-benzoate administration in the ovariectomized rat

Abstract The effects of a synthetic anti-estrogen, CI-628 were studied in adult ovariectomized rats for (1) estradiol-17 β-benzoate (EB)-stimulated sexual behavior, (2) EB-induced uterine growth, and (3) retention of radioactivity in neural and peripheral estrogen target tissues after tritiated estradiol-17 β-benzoate ([ 3 H]EB) injection. For behavioral tests, all animals received EB (2 μg/rat; s.c.) at H 0 and, 42 h later, progesterone (P, 500 μg/rat; s.c.). Behavioral testing with an intact male began at H + 47. A single injection of CI-628 (1 mg/rat; s.c.) given 15 min before EB (H 0) reduced lordosis quotients to 43% of control values. Significant inhibition, but to a lesser extent, also occurred when CI-628 was given as early as 24 h prior to (H — 24) or as late as 6 h after (H + 6) EB. EB-induced uterine growth was inhibited by CI-628 (1 mg/rat; s.c.) given at H — 24 or at H 0 (relative to EB controls). However, CI-628, given at H 0, either alone or with EB, had stimulatory effects on uterine growth, when compared to baseline controls (i.e. animals receiving only vehicle injections). Retention of radioactivity, subsequent to an s.c. injection of [ 3 H]EB (90 μ Ci= 0.7 μg) persisted for more than 24 h in whole homogenates and nuclear fractions of hypothalamic-preoptic area (HPOA), pituitary, and uterus (animals were killed 3, 12, or 24 h after [ 3 H]EB). A single s.c. injection of CI-628 at H 0 or H — 24 resulted in a long-lasting (greater than 24 h) significant inhibition of estrogen radioactivity retention in nuclear fractions, but not whole homogenates, of the HPOA. However, this inhibition rarely went below 30% of control values. In contrast, in the pituitary and the uterus, CI-628 inhibited estrogen radioactivity retention to a much greater extent, and did so for both whole homogenates and nuclear fractions. These HPOA vs. peripheral tissue differences in the inhibitory effects of CI-628 persisted over a range of doses from 1 to 8 mg. When given at H 0, s.c. injections of CI-628 caused a delayed onset of maximal inhibition of nuclear accumulation of radioactivity as compared to i.p. administered CI-628. This was particularly true in the HPOA, where no inhibition occurred at all until more than 3 h after an H 0 CI-628 (s.c.) injection. Inhibitory effects of i.p. injected CI-628 (1 mg/rat), both on behavior and nuclear estrogen uptake, were apparent even when the anti-estrogen was first given 21 h after EB. The inhibition of radioactivity retention in the nuclear fraction of HPOA as a result of CI-628 is consistent with the reduced lordosis quotients caused by similar injection procedures. Results emphasize: (1) possible mechanisms of action of CI-628 in terms of differences in the uterine vs. neurobehavioral data, (2) the possible importance of long-term presence of estrogen in neural tissues for the activation of lordosis behavior, (3) differences attributable to the use of [ 3 H]EB, as opposed to tritiated unesterified estradiol, for biochemical measurements of estrogen retention.