Depending on the estrous/menstrual cycle stage in females, ovarian-derived estradiol (E₂) exerts either a negative or a positive effect on the hypothalamic–pituitary axis to regulate the synthesis and secretion of pituitary gonadotropins, LH, and FSH. To study the role of estrogen receptor-α (ERα) mediating these effects, we assessed the relevant parameters in adult wild-type (WT) and ERα-null (αERKO) female mice in vivo and in primary pituitary cell cultures. The αERKO mice exhibited significantly higher plasma and pituitary LH levels relative to WT females despite possessing markedly high levels of circulating E₂. In contrast, hypothalamic GnRH content and circulating FSH levels were comparable between genotypes. Ovariectomy led to increased plasma LH in WT females but no further increase in αERKO females, while plasma FSH levels increased in both genotypes. E₂ treatment suppressed the high plasma LH and pituitary Lhb mRNA expression in ovariectomized WT females but had no effect in αERKO. In contrast, E₂ treatments only partially suppressed plasma FSH in ovariectomized WT females, but this too was lacking in αERKO females. Therefore, negative feedback on FSH is partially E₂/ERα mediated but more dependent on ovarian-derived inhibin, which was increased threefold above normal in αERKO females. Together, these data indicate that E₂-mediated negative feedback is dependent on functional ERα and acts to primarily regulate LH synthesis and secretion. Studies in primary cultures of pituitary cells from WT females revealed that E₂ did not suppress basal or GnRH-induced LH secretion but instead enhanced the latter response, indicating that the positive influence of E₂ on gonadotropin secretion may occur at the level of the pituitary. Once again this effect was lacking in αERKO gonadotropes in culture. These data indicate that the aspects of negative and positive effects of E₂ on gonadotropin secretion are ERα dependent and occur at the level of the hypothalamus and pituitary respectively.

Disruption of the Esr1 gene encoding estrogen receptor α (ERα) by insertion of a neomycin resistance gene (neo) into exon 2 (αERKO mice) was shown previously to cause infertility in male mice. While full-length ERα protein was not expressed in αERKO mice, alternative splicing resulted in the low-level expression of a truncated form lacking the N-terminus A/B domain and containing the DNA- and ligand-binding domains. Thus, it was unclear whether the reproductive phenotype in αERKO males was only due to the lack of full-length ERα or was affected by the presence of the variant ERα isoform. The present study examined male mice with deletion of exon 3 of Esr1 gene, lacking the DNA-binding domain, and null for ERα (Ex3αERKO). Dilation of some seminiferous tubules was apparent in male Ex3αERKO mice as early as postnatal day 10 and was pronounced in all tubules from day 20 onward. At 6 weeks of age, sperm numbers and sperm motility were lower in Ex3αERKO mice than in wild-type (WT) mice, and the rete testis and efferent ductules were dilated. Mating studies determined that adult Ex3αERKO males were infertile and failed to produce copulatory plugs. Serum testosterone levels and Hsd17b3 and Cyp17a1 transcript levels were significantly higher, but serum estradiol, progesterone, LH, and FSH levels and Cyp19a1 transcript levels were not significantly different from those in WT mice. These results confirm and extend those seen in other studies on male mice with deletion of exon 3 of Esr1 gene. In addition, the reproductive phenotype of male Ex3αERKO mice recapitulated the phenotype of αERKO mice, strongly suggesting that the αERKO male infertility was not due to the presence of the DNA-binding domain in the truncated form of ERα and that full-length ERα is essential for maintenance of male fertility.