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      Protooncogene c-fos Involvement in the Molecular Mechanism of Rat Brain Sexual Differentiation

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          Brain sexual differentiation is mediated through testosterone, which acts during the perinatal period in the form of both 5α-dihydrotestosterone and estradiol. In order to gain insight into the molecular mechanisms involved, we studied induction of c-fos, an index of functional neuronal activation, in the 2-day-old female rat brain after injection of a masculinizing dose of testosterone. Administration of testosterone resulted in induction of c-fos gene expression in the hypothalamus, as determined by Northern analysis. Following immunocytochemistry, we demonstrated an increase in the number of Fos-positive nuclei in the median and medial preoptic nucleus, the medial preoptic area extending to the lateral preoptic area, and the peri- and paraventricular area. In an effort to see whether testosterone acted as 5α-dihydrotestosterone or as estradiol, we injected either steroid and looked at fos induction. Estradiol mimicked the effect of testosterone, while 5α-dihydrotestosterone was without effect. Furthermore, injection of an estrogen receptor blocker, clomiphene, together with testosterone, abolished the testosterone-induced increase in Fos-positive nuclei, thus confirming the finding that testosterone induces c-fos by acting through estrogen receptors. Electrophoretic mobility shift assays showed that nuclear extracts from 2-day-old female hypothalamus contain a protein, most probably the estrogen receptor, which binds specifically to oligodeoxynucleotides with the sequence of either vitERE, the consensus estrogen-responsive element (ERE) found in the vittelogenin gene, or fosERE, the ERE found in the 3′-untranslated region of the mouse c-fos gene. This suggests that the effect of testosterone-derived estradiol on c-fos expression is a direct one, mediated by binding of estrogen receptors to an ERE in the c-fos gene-regulatory regions.

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          Most cited references 12

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          Evidence for a morphological sex difference within the medial preoptic area of the rat brain.

          The present report demonstrates the existence of a marked sexual difference in the volume of an intensely staining cellular component of the medial preoptic nucleus (MPON) of the rat. Moreover, this sexual dimorphism is shown to be independent of several specific hormonal conditions in the adult, but significantly influenced, perhaps determined, by the perinatal hormone environment. Adult rats were gonadectomized and sacrificed 2 or 5-6 weeks later, or sacrificed after gonadectomy and priming with estradiol benzoate (2 microgram/day x 3) and 500 microgram progesterone, or testosterone propionate (TP, 500 microgram/day x 14), or the ingestion of propylthiouracil (0.15% of the diet) for one month, or following water deprivation for 24 h. These treatments did not affect the sexual dimorphism in the MPON and, in all groups, nuclear volume in the male animals was significantly greater than that of females whether nuclear volume was expressed in absolute terms or relative to brain weight. On the other hand, the volume of the MPON of the adult male castrated neonatally was significantly reduced when compared to that of the male castrated at the time of weaning, i.e. after the period of sexual differentiation of the brain. Consistent with the view that this nuclear region undergoes sexual differentiation is the fact that the volume of the MPON was significantly greater in female rats injected with 1 mg TP on day 4 of life than in oil-treated females. More subtle sex differences in the volume of the suprachiasmatic nucleus were also detected, as were several treatment effects. Although these differences may fall within the error of the analytical procedure, it is possible that hormone- or sex-dependent morphological differences exist elsewhere in the brain. Nevertheless, the gross sexual dimorphism in the MPON clearly demonstrates a possible morphological basis for the sexual differentiation of brain function.
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            The distribution of monoaminergic cells and fibers in a periventricular preoptic nucleus involved in the control of gonadotropin release: immunohistochemical evidence for a dopaminergic sexual dimorphism.

            A small, discrete nucleus at the rostral end of the third ventricle, the anteroventral periventricular nucleus (AVPv), has been reported to be involved in the control of gonadotropin release. Since monoaminergic neurotransmitter systems have also been implicated in this function we used an indirect immunohistochemical approach to examine the distribution of 3 monoaminergic neurotransmitter systems in this nucleus. Sections through the AVPv of both colchicine and non-colchicine-treated adult male and female Sprague-Dawley rats were processed for immunohistofluorescence with antisera directed against tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), or serotonin (5-HT), and were subsequently counterstained with the fluorescent Nissl stain ethidium bromide. The distributions of TH-, DBH- and 5-HT-immunoreactive neural elements within the AVPv were evaluated and a comparison was made between males and females. In both sexes, few 5-HT-stained fibers were seen within the borders of the AVPv, in contrast to the relatively high 5-HT-stained fiber density of the surrounding region. A dramatic sexual dimorphism was found in the distribution of TH-immunoreactive fibers and cell bodies. Compared to males, the AVPv in the female contained 3-4 times as many TH-stained perikarya, and a 2- to 3-fold greater density of TH-stained fibers. A low to moderate density of DBH-immunoreactive fibers, and no DBH-stained cell bodies, were seen in the nucleus. A clear sex difference was not found in the density of DBH-stained fibers in the AVPv, indicating that the sexual dimorphism in TH-immunoreactive neural elements in this nucleus is due to a greater density of dopaminergic fibers and a greater number of dopaminergic cell bodies in the female. These results suggest that dopamine may participate in the control of gonadotropin secretion at the level of the AVPv.
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              SDN-POA volume, sexual behavior, and partner preference of male rats affected by perinatal treatment with ATD.

              The present study investigated 1) the importance of the aromatization process during the perinatal period for the development of the sexually dimorphic nucleus in the preoptic area of the hypothalamus (SDN-POA) of male rats, and 2) the relationship between SDN-POA volume and parameters of masculinization in male rats that were treated perinatally with the aromatase-inhibitor ATD. Males were treated with ATD either prenatally or pre- and neonatally, or with the vehicle. Masculine sexual behavior and partner preference were investigated in adulthood. Thereafter, animals were sacrificed and SDN-POA volume was measured. The SDN-POA volume was reduced in both the prenatally and the pre- and neonatally treated group, with a larger reduction in the latter than in the former group. Combined pre- and neonatal ATD treatment resulted in reduced frequency of mounts, intromissions, and ejaculations, as well as a reduced preference for a female over a male. The SDN-POA size was significantly and positively correlated with frequency of masculine sexual behavior, as well as preference for a female over a male.

                Author and article information

                S. Karger AG
                June 2001
                13 June 2001
                : 73
                : 6
                : 387-396
                Laboratory of Biology-Biochemistry, Faculty of Nursing, University of Athens, Greece
                54657 Neuroendocrinology 2001;73:387–396
                © 2001 S. Karger AG, Basel

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                Page count
                Figures: 3, References: 60, Pages: 10
                Brain Effects of Gonadal Hormones


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