Biological Role of Pituitary Estrogen Receptors ERα and ERβ on Progesterone Receptor Expression and Action and on Gonadotropin and Prolactin Secretion in the Rat

Abstract

Estrogen (E) is a key regulator of the synthesis and secretion of pituitary reproductive hormones [luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (PRL)]. Until recently, it was thought that all biological actions of E at the pituitary were manifested through a single E receptor (R). The pituitary, like many other reproductive tissues, expresses two isoforms of ER, α and β, both activated by E. The relative contribution of α and β forms in E regulatory actions is largely unknown. To this end, 2-week-old ovariectomized (OVX) rats were injected over 3 days with 25 µg estradiol benzoate (EB), 1.5 mg of propylpyrazole triol (PPT), a selective ERα agonist, 1.5 mg of the selective ERβ agonist diarylpropionitrile (DPN) or a combination of PPT and DPN. Controls were injected with 0.2 ml oil. At 10:00 h on the day after treatment, trunk blood was collected to determine serum concentration of LH, FSH and PRL, and pituitaries were processed for RT-PCR analysis of total (A+B) progesterone receptor (PR) mRNA, immunocytochemistry of PR and incubation. Pituitaries from each of the five groups were incubated in DMEM, with or without 20 nM of the antiprogestin at the receptor ZK299, for 3 h with: 10^–8M 17β-estradiol, 10^–6M PPT, 10^–6M DPN, PPT+DPN or medium alone, respectively, to determine LH, FSH and PRL secretion, and, when challenged with two pulses of 15 min 1 h apart of 10^–8M gonadotropin-releasing hormone (GnRH) (GnRH self-priming). EB, PPT and PPT+DPN treatments increased PR mRNA and the number and intensity of nuclei immunoreactive (IR) for PR in gonadotropes, and reduced the number of gonadectomy cells. Like E, PPT alone or in combination with DPN stimulated PRL secretion, increased basal and GnRH-stimulated LH and FSH secretion and induced GnRH self-priming in the absence of ZK299 in the incubation medium. DPN alone had only a significant E-like effect on gonadectomy cells and IR-PR, but not on GnRH self-priming. In addition, while DPN lacked an agonistic action on peripheral tissue and serum pituitary reproductive hormones concentration, EB, PPT and PPT+DPN induced similar uterine ballooning and vaginal cornification, and increased and decreased, respectively, serum concentrations of PRL and gonadotropins. Overall, these results indicate that most of these E actions on the pituitary are exerted through the ERα isoform. The finding that activation of ERβ with its selective DPN agonist had an estrogenic effect on IR-PR nuclei, but not on GnRH self-priming, a characteristic ERα-mediated effect of E, suggests that the biological action of E at the pituitary may involve both isoforms of ER.