Tamoxifen but Not Other Selective Estrogen Receptor Modulators Antagonizes Estrogen Actions on Luteinizing Hormone Secretion while Inducing Gonadotropin-Releasing Hormone Self-Priming in the Rat

Abstract

Selective estrogen receptor modulators (SERMs) are compounds which may function as agonists or antagonists depending upon the target tissue. This study compares the actions of different SERMs on luteinizing hormone (LH) secretion, and on gonadotropin-releasing hormone (GnRH) self-priming in the rat. To do this, 4-day cyclic rats were injected twice, on day 2 (metestrus) and day 3 of the estrous cycle, with one of the following SERMs: 0.25 mg ICI 182,780, 3 mg tamoxifen (TX), LY139481-HCl or LY117018-HCl, or 0.5 mg RU58668. Control rats were given subcutaneous injections of 0.2 ml oil. On the morning of day 4 (proestrus in controls), rats from each group were either injected intraperitoneally with pentobarbital (40 mg/kg) for in vivo study or decapitated and their pituitaries collected for incubation (in vitro study). Additionally, pituitaries taken on each day of the estrous cycle from control rats as well as on day 4 from SERM-treated rats were processed for immunohistochemical determination of the estrogen receptor-α (ERα) gonadotrope. The plasma concentration or accumulation of LH in the medium was determined after 1 h (basal secretion). Thereafter, an intravenous bolus of GnRH (50 ng/0.5 ml/100 g BW) or 10^–8M GnRH was injected or added to the medium, respectively. After 1 h of GnRH exposure, blood or medium were taken, and another challenge of GnRH was made. At the end of the 3rd h of the experiment, blood or medium samples were taken again and the LH plasma concentration or accumulation in the medium were determined. All SERM treatments reduced uterus weight and decreased basal and stimulated LH secretion. Also, on day 4, rats treated with any SERM other than TX showed vaginal smears infiltrated by leukocytes and a reduction in GnRH self-priming. TX-treated rats exhibited cornified vaginal smears and an estrogenic effect on GnRH self-priming. Moreover, 15-min exposure to two consecutive GnRH (10^–8M) challenges 1 h apart in incubated pituitaries with estradiol (E₂, 10^–8M), TX (10^–7M), E₂ + TX, or medium alone form ovariectomized rats injected for 3 days with estradiol benzoate (25 µg), TX (3 mg), estradiol benzoate + TX, or 0.2 ml oil, respectively, showed that TX increased GnRH self-priming, as did E₂, whereas it reduced the E₂-sensitizing effect on GnRH-stimulated LH secretion and cancelled the E₂-dependent GnRH self-priming. All SERMs prevented the physiological nucleocytoplasmic shuttling of ERα exhibited during proestrus in control rats, and TX, in addition, induced a significantly larger number of gonadotropes displaying strong cytosolic immunosignals corresponding to ERα than the rest of the experimental groups. Overall, data from this study indicated that, in contrast to the general antagonistic effect of the tested SERMs, TX seemed to display both selective agonist and antagonist activity at the gonadotrope level and on GnRH self-priming of LH secretion respectively.

Gonadotrope oestrogen receptor-α and -β and progesterone receptor immunoreactivity after ovariectomy and exposure to oestradiol benzoate, tamoxifen or raloxifene in the rat: correlation with LH secretion

“…In the presence of oestrogen, TX reduces basal and stimulated LH and PRL secretion (González et al 2000). However, in the absence of the cognate ligand, TX stimulates PRL secretion (Sánchez-Criado et al 2002, Bellido et al 2003 and induces both gonadotrope PR expression and ligand-independent activation of Fig. 1 legend for details of treatments.…”

“…Incubation of pituitaries was carried out as previously described (Sánchez-Criado et al 2002. Briefly, halves of anterior pituitaries were incubated at 37 C for 180 min, after 60 min of preincubation, with constant shaking (60 cycles/min) in an atmosphere of 95% O 2 /5% CO 2 .…”

“…Tamoxifen (TX) is a prototypical SERM (Cosman & Lindsay 1999) that displays selective biological activities (Jordan & Morrow 1999). At the rat pituitary gonadotrope level, TX reduces the GnRHstimulated luteinizing hormone (LH) secretion elicited by oestrogens, whereas, in the absence of the cognate ligand, it induces progesterone receptor (PR) expression (Bellido et al 2003) and GnRH self-priming (Sánchez-Criado et al 2002, Bellido et al 2003. The mechanism through which these selective effects of TX are achieved is unknown.…”

Estradiol and its membrane-impermeable conjugate estradiol–BSA inhibit tamoxifen-stimulated prolactin secretion in incubated rat pituitaries

“…At 0900 h on the first day after treatment, rats from all three experiments were decapitated, the neural lobe discarded and anterior pituitary glands dissected out, divided in halves, and incubated. Doses employed of EB and TX came from previous publications by this laboratory (Bellido et al 2003, Sánchez-Criado et al 2002, 2004.…”

“…At pituitary level, TX displays mixed agonist/antagonist activities. Whereas in the presence of the cognate ligand, TX antagonizes E action on luteinizing hormone (LH) (Tebar et al 1994, Sánchez-Criado et al 2002 and PRL (Lieberman et al 1983, Spritzer et al 1996 secretion, in the absence of the cognate ligand, TX induces GnRH selfpriming without affecting basal or GnRH-stimulated LH release in incubated pituitaries (Sánchez-Criado et al 2002), and stimulates PRL secretion (González et al 2000, Bellido et al 2003. Incubated pituitaries from two-week OVX rats injected daily over three days with 3 mg TX exhibit increased PRL release into the medium, as do pituitaries from OVX rats injected with the cognate ligand (Bellido et al 2003).…”