Mechanisms for altered reproductive function in female rats following neonatal administration of raloxifene.

OBJECTIVE: Raloxifene is a non-steroidal selective estrogen receptor modulator (SERM) that mimics estrogenic activity on bone density and blood lipid concentration without uterotropic actions. Previous data from our laboratory indicated that, as is the case for estrogen, neonatal administration of raloxifene disturbed normal differentiation of the hypothalamic circuitries governing the gonadotropic axis. In contrast, raloxifene did not act in the same way as estrogen does on the neuronal systems controlling sexual receptivity in the female rat. At present, however, the mechanisms for these organizing effects of raloxifene are not completely elucidated.

DESIGN AND METHODS: To analyze this phenomenon, female rats were injected daily with raloxifene (50, 100, 250 or 500 microg/rat per day) between days 1 and 5 of age. On day 23, hypothalamic gonadotropin-releasing hormone (LHRH) mRNA expression was assessed, and pituitary and plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels were measured in basal and LHRH-stimulated conditions. In addition, LH and FSH responses to ovariectomy were evaluated in raloxifene-treated females. Finally, we monitored the ability of neonatal administration of a potent LHRH agonist ([d-Ala(6),d-Gly(10)]-LHRH ethylamide; 0.01 microg/kg per 12 h on days 1-5) to counteract the effects of raloxifene.

RESULTS: Our analyses demonstrated that prepubertal rats (23-day-old females) treated neonatally with raloxifene showed decreased hypothalamic LHRH mRNA expression levels, reduced pituitary content of LH and FSH, reduced basal and LHRH-stimulated LH secretion in vivo and in vitro, and decreased response to ovariectomy. In addition, adult females treated neonatally with raloxifene showed anovulation and reduced serum LH levels; these effects were not prevented by the simultaneous administration of a LHRH agonist.

CONCLUSION: In conclusion, our data demonstrate that neonatal administration of raloxifene can disrupt the programming of hypothalamic-pituitary-ovarian axis function. Reduced LH secretion, under basal and LHRH-stimulated conditions and after ovariectomy, is probably related to decreased LHRH expression, reduced pituitary LH content and/or decreased pituitary responsiveness to hypothalamic LHRH.