Evidence for sexual dimorphism of estrogen receptors in hypothalamus and thymus of neonatal and immature Wistar rats

Abstract

It is hypothesized that the rat thymus is sexually differentiated. To begin testing this, we used the 5-day-old rat, whose hypothalamus is sexually differentiated during this period. Cytosolic estrogen receptors (ER) were measured in cytosols prepared from the brain, thymus and uterus of Wistar rats at 5, 18 and 30 days post-partum. ER concentrations were significantly higher in hypothalamus in cytosols of female 5-day-old rats, and this difference had disappeared by day 18. The pattern in thymus was identical to that observed in hypothalamus, suggesting the presence in the thymus of the aromatase system that converts androgen to estrogen, and that estrogen-mediated sexual differentiation of thymus might be proceeding at 5 days. Unlike the case for hypothalamus, no experimental model exists at present for testing functional sexual differentiation in thymus. Therefore we tested the effects of aromatase inhibitors on estrogen receptor activity in thymus well after the five-day period, and before atrophy of the thymus has commenced. Male and female rats were implanted at 15 days of age with SILASTIC implants containing 5 mg of estradiol or with 25 mg of the aromatase inhibitor 4-hydroxyandrostenedione (4-OHA) and cytosolic ER prepared at 30 days and activity measured. Administration of estradiol resulted in failure to detect available receptor, suggesting that the binding components measured were ER. After 4-OHA administration, ER concentrations were significantly increased in cytosols from male but not female hypothalamus and thymus. There is therefore a basis for exploring further the hypothesis that rat thymus is sexually differentiated.

Introduction

There is much anecdotal evidence of sexual dimorphism of immune function in health and disease [1], but to the authors’ knowledge there is no direct evidence of sexual differentiation of any tissue or organ of the immune system. Both rodent and primate brains are sexually differentiated, and the process appears to be mediated by locally produced estrogen [2]. The brain, thymus and uterus are target organs for the estrogenic hormones that mediate uterine growth, sexual function and behavior, and which in pharmacological and physiological doses have profound atrophic effects on the developing and adult thymus [3]. Physiological plasma levels of estradiol-17β enhance the responsiveness of lymphocytes to in vitro mitogenic challenge [4]. There is much evidence that the hormone heightens autoimmune responsiveness in disease such as systemic lupus erythematosus, both in humans and mice [5]. Thymus atrophy commences with the onset of puberty in many species, and there is evidence that sex hormones may mediate, at least in part, this atrophic process. There is therefore a strong case for the investigation of the effects of estrogens on immune tissues in both health and disease.

In previous studies, we found that the aging rat thymus was regenerated after orchidectomy, and that regeneration was blocked by testosterone [6]. Estradiol was far more potent than was the androgen in this regard [7], and the atrophic actions of testosterone could be blocked by concomitant administration of substances that inhibited the aromatization of androgens to estrogens [8]. Aromatase inhibitors alone could induce thymus regeneration in aging male rats [9], and this prompted us to ask whether the same mechanisms might be operating in the developing thymus, and whether estrogen receptors might be involved. A role for estrogen in thymus is supported by the observation that an estrogen antagonist, tamoxifen, reversed thymus involution in adult rats [10]. In the present study, we have measured estrogen receptors in the neonatal rat thymus and the effects of estradiol and an aromatase inhibitor on brain, thymus and uterus of developing male and female rats.