A soy supplement and tamoxifen inhibit sexual behavior in female rats

Abstract

In addition to displaying proceptive (hopping and darting) and receptive (lordosis) behaviors during a sexual encounter with a male, female rodents will regulate the timing of the encounter by engaging in a series of approaches and withdrawals from the male, a behavior termed paced mating behavior. Proceptive, receptive, and paced mating behaviors are all regulated by, and sensitive to, estrogen and progesterone, suggesting that compounds capable of disrupting these critical hormones may also perturb the display of female sexual behavior. The present experiments examined the impact of the selective estrogen receptor modulator (SERM) tamoxifen and a popular soy phytoestrogen dietary supplement on female sexual behavior in rats. Ovariectomized female rats were given either tamoxifen (TAMOX) by implant or the soy supplement through the diet then injected with estradiol benzoate (EB, 10 μg) or oil followed 48 h later with an injection of progesterone (P, 500 μg). Animals were then tested for sexual behavior 4 h after the P injection. Neither compound had any effect on sexual behavior when administered in conjunction with P alone; however, both significantly diminished receptive behavior, as measured by the lordosis quotient (LQ), in animals primed with both EB and P. Similarly, the hopping and darting rate was also significantly depressed in both the soy- and TAMOX-treated animals, compared to the EB- and P-treated controls, with the soy-treated animals showing significantly less proceptive behavior than the TAMOX-treated animals. Finally, soy but not TAMOX significantly attenuated paced mating behavior in animals compared to the EB- and P-treated controls. These results demonstrate that both the soy supplement and TAMOX act as estrogen antagonists on both proceptive and receptive behavior in female rats.

Introduction

Soy and soy-based foods including tempeh, soy flour, soy infant formula, and tofu contain significant quantities of phytoestrogens, which are plant-produced, nonsteroidal, estrogen-like compounds. Because consumption of these compounds has been associated with a myriad of health benefits Adlercreutz et al., 1995, Patisaul and Whitten, 1999, they are becoming increasingly popular as food supplements and are frequently advertised as a natural alternative to hormone replacement therapy (HRT). While the health benefits of phytoestrogens, including a reduction of hot flushes (Murkies et al., 1995) and decreased hepatic cholesterol production Anderson et al., 1999, Herrington, 2000, are well documented, little are known about their neuroendocrine effects or their effects on behavior, particularly sexual motivation and function. Although, as their name implies, the phytoestrogens have been classified as weak estrogens, their action in both the periphery and the brain appear to be tissue and region dependent (Whitten et al., 2002), suggesting that they may behave more like the breast cancer drug tamoxifen and other selective estrogen receptor modulators (SERMs).

Tamoxifen (TAMOX) is widely prescribed for the treatment and prevention of breast cancer because it acts as an estrogen receptor antagonist in the breast (Gajdos and Jordan, 2002). However, through a mechanism that remains largely enigmatic, TAMOX acts as a weak estrogen in the uterus, resulting in an appreciable risk of uterine cancer in patients taking the drug Cosman and Lindsay, 1999, Goldstein, 2001. TAMOX also has mixed estrogenic/antiestrogenic effects in the brain. For example, TAMOX has an estrogen-like effect on hypothalamic differentiation in female neonatal rats (Pinilla et al., 2002) and the regulation of NMDA and AMPA receptors in the cortex, hippocampus, and striatum (Cyr et al., 2001). However, TAMOX inhibits estrogen-dependent GnRH self-priming (Sanchez-Criado et al., 2002) and antagonizes female behavior in rodents including estrogen-dependent reproductive Etgan and Shamamian, 1986, McKenna et al., 1992 and maternal behavior (Ahdieh et al., 1987). In humans, a small but growing literature has associated TAMOX treatment in postmenopausal women with increased incidence of hot flashes, elevated production of vaginal discharge, and decreased sexual function Day, 2001, Mourits et al., 2001, as well as the possibility of an increased risk for the onset of depression and emotional instability (Pluss and DiBella, 1984), suggesting that TAMOX action in the brain is both consequential and complex.

Although the mechanisms governing the tissue-specific effects of both the phytoestrogens and the SERMs are likely to be complex, the differential expression of the two major isoforms of the estrogen receptor (ERα and ERβ) likely plays a significant role. ERα and ERβ are differentially expressed in the brain Li et al., 1997, Shughrue et al., 1997, and the ligand binding region of ERβ is only 55% homologous to the ligand binding region of ERα (Kuiper et al., 1996), suggesting that the two receptors may have differential effects on gene expression and behavior. In general, phytoestrogens have a higher binding affinity for ERβ than ERα (Kuiper et al., 1998), suggesting that they may be more bioactive through ERβ than ERα, but we have found that a variety of phytoestrogens, including the supplement used in the this study, are capable of acting through both ERα and ERβ Patisaul et al., 2001, Whitten et al., 2002. We have also found that TAMOX influences estrogen-dependent gene expression through ERα and ERβ to a similar extent as the phytoestrogens. Most notably, both the phytoestrogens and TAMOX have an opposite effect as 17β-estradiol on ERβ mRNA expression in the PVN, and attenuate the upregulation of oxytocin receptors (OR) in the VMN by estrogen (Patisaul et al., 2003). These effects may at least partially explain why a phytoestrogen supplement diminished sexual receptivity in hormone-primed female rats (Patisaul et al., 2001). Here we hypothesize that the antiestrogenic actions of both this soy supplement and TAMOX may lead to an appreciable diminution of both sexually receptive and proceptive behaviors.

In female rats, sexual behavior consists of both the receptive lordosis posture, and a collection of proceptive actions including hopping, darting, and ear-wiggling (Beach, 1976). Both proceptive and receptive behavior in female rats is dependent upon the synergistic actions of estrogen and progesterone in the brain (Pfaff, 1999). It has already been shown that both TAMOX and the phytoestrogen supplement impair the lordosis response in rats Etgan and Shamamian, 1986, McKenna et al., 1992, Patisaul et al., 2001; however, the effects of TAMOX and soy on proceptivity are not known. This paper examines whether these compounds can impair proceptive behaviors as well as the receptive lordosis posture.

Given the opportunity, females will control the interval between intromissions through a series of approaches and withdrawals collectively known as paced mating behavior Bermant, 1961, Erskine, 1989. Females prefer a longer interval between sexual contacts than males and their latency to return to the male increases with the intensity of sexual contact (Bermant, 1961). Thus, the contact-return latency is longer after an intromission than a mount and longest after an ejaculation Erskine, 1989, Erskine et al., 1989. Similarly, a female is more likely to withdraw from a male after an intromission than a mount, and is most likely to withdraw after an ejaculation (Bermant, 1961). Both the contact-return latency and the number of exits can readily be observed and quantified in the laboratory using a double-sided area in which the female can readily enter both sides while the male is restricted to only one side Erskine, 1989, Mermelstein and Becker, 1995, Paredes and Vazquez, 1999. Here we examine the effects of a dietary soy supplement and the subcutaneous administration of TAMOX, in both the presence and absence of estradiol, on proceptive and receptive sexual behaviors in female rats as measured by pacing, hopping and darting, and the lordosis response.