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The principal nucleus of the bed nucleus of the stria terminalis (BNSTp) is a sexually dimorphic nucleus, and the male BNSTp is larger and has more neurons than the female BNSTp. To assess the roles of neuroestrogen synthesized from testicular androgen by brain aromatase in masculinization of the BNSTp, we performed morphometrical analyses of the adult BNSTp in aromatase knockout (ArKO), estrogen receptor-α knockout (αERKO), and estrogen receptor-β knockout (βERKO) mice and their respective wild-type littermates. In wild-type littermates, the BNSTp of males had a larger volume and greater numbers of neuronal and glial cells than did that of females. The volume and neuron number of the BNSTp in ArKO and αERKO males and glial cell number of the BNSTp in αERKO males were significantly smaller than those of wild-type male littermates, and they were not significantly different from those in female mice with either gene knockout. In contrast, there was no significant morphological difference in the BNSTp between βERKO and wild-type mice. Next, we examined the BNSTp of ArKO males subcutaneously injected with estradiol benzoate (EB) on postnatal days 1, 2, and 3 (1.5 µg/day). EB-treated ArKO males had a significantly greater number of BNSTp neurons than did oil-treated ArKO males. The number of BNSTp neurons in EB-treated ArKO males was comparable to that in wild-type males. These findings suggested that masculinization of the BNSTp in mice involves the actions of neuroestrogen that was synthesized by aromatase and that this estrogen mostly binds to ERα during the postnatal period.

Background: The densely staining sexually dimorphic nucleus of the preoptic area (SDNPOA) is profoundly affected by gonadal steroids during perinatal development. Methods: We tested whether the SDNPOA in rats also remains responsive to gonadal hormones in adulthood. Results: Castration of 60-day-old male rats led to a reduction of soma size in SDNPOA neurons 28 days later, but not 14 days later. In contrast, the SDNPOA volume in males was unaffected by adult castration, increasing somewhat between 60 and 88 days of age in both castrated males and sham controls. For female rats ovariectomized at 60 days of age, testosterone treatment resulted in a significantly larger SDNPOA soma size after either 14 or 28 days of treatment, compared to blank-treated controls. Testosterone-treated ovariectomized females also had a modestly larger SDNPOA volume after 28 days, but not 14 days, of treatment compared to blank-treated ovariectomized animals. A second experiment revealed that these effects in females were due to ovariectomy: both SDNPOA soma size and regional volume shrank in females that were ovariectomized compared to females subjected to sham surgery. The effects of ovariectomy were blocked by testosterone treatment. The cessation of testosterone treatment in females returned SDNPOA soma size and regional volume to that of control-treated ovariectomized females. Conclusion: These results indicate considerable plasticity of the adult rat SDNPOA, and that (1) circulating androgens are required to maintain soma size, but not regional volume in males, and (2) ovarian steroids maintain both soma size and regional volume of the SDNPOA in females, an effect that can be mimicked with testosterone treatment and is fully reversible in adulthood.

Resveratrol (RES) is a phytoestrogen that has the ability to bind to estrogen receptors (ERs) and evoke biological effects that parallel those exerted by endogenous and synthetic estrogens. We have shown in previous studies that adult female rats acutely exposed to RES exhibit estrous cycle irregularity, ovarian hypertrophy, and alterations in sociosexual behavior. The present experiment characterizes the prolonged effects of maternal RES exposure throughout the lactational period on subsequent behavior, reproductive tissues, and brain morphology of the adult offspring. During adulthood, female offspring exposed to RES throughout nursing exhibited reduced body weight and increased ovarian weight, but exhibited normal estrous cyclicity and sociosexual behavior, without changes in the volume of the sexually dimorphic nucleus of the preoptic area or the anteroventral periventricular nucleus of the hypothalamus. During adulthood, males exposed to RES throughout nursing exhibited decreased body weight and plasma testosterone concentration, increased testicular weight, and reduced sociosexual behavior. These males also had significantly smaller sexually dimorphic nucleus of the preoptic area volumes and larger anteroventral periventricular nucleus volumes compared to male controls. These data suggest that postnatal exposure to RES may affect estrogenic activity in specific peripheral tissues (e.g., the gonads), while inducing antiestrogenic effects in the brain. Thus, the present study supports recent in vitro and in vivo findings that RES differs from most other phytoestrogens by acting as a possible mixed ER agonist/antagonist, depending on the tissue-specific availability of ER subtypes that are preferentially localized in specific brain regions and throughout the reproductive tract. More importantly these data indicate that maternal consumption of phytoestrogens during lactation can have lasting effects on the offspring that may not become apparent until they reach adulthood.

To investigate the effect of perinatal testosterone exposure, which simulates the endogenous testosterone peak, on neuron loss during aging, nuclear morphology was evaluated in male and female rats as well as in female rats treated with testosterone perinatally followed by ovariectomy (TE/Ovx). Additionally, neuronal apoptosis, which occurred primarily at postnatal day 8 (PND8), was identified by in situ TUNEL staining. Neuronal density, nuclear volume, total neuronal number and pyknotic ratio were estimated after HE stain at PND8, middle age and old age. The results showed that age-related decrease in neuronal nuclear volume and total neuron number in the sexually dimorphic nucleus of the preoptic area (SDN-POA) of female rats was significantly diminished by TE/Ovx. The pyknotic ratio in the SDN-POA of female rats at PND8 was significantly higher than that of males, and neuronal death was reversed by testosterone exposure, while no significant difference of pyknotic ratios was observed among male, female and TE/Ovx female rats at both middle and old age. Moreover, the high apoptotic incidence of female rats at PND8 was significantly diminished by testosterone exposure. These results suggest that neuron loss in the SDN-POA during aging may be predominantly determined by perinatal testosterone through modulation of postnatal neuronal apoptosis.

Sexual dimorphism has been found in the preoptic area of the hypothalamus (POA), a major site of glutamate actions via N-methyl-D-aspartate (NMDA) receptors. The sexually dimorphic nucleus of the preoptic area (SDN-POA) of male rats exhibits about seven-fold greater nuclear volume than that of females. A naturally occurring neonatal neuronal apoptosis, that can be prevented by testosterone, may contribute to this sexual difference in SDN-POA nuclear volume. Since activation of NMDA receptors in the POA induces GnRH secretion, it may be involved in both elevation of serum testosterone and prevention of neuronal death in the SDN-POA. In the present study, protein expression of NMDA receptors in the POA of male and female fetuses was quantified on the day preceding the fetal testosterone peak (embryonic day 16; ED 16). Rats were then distributed in four groups: (1) untreated males, (2) untreated females, (3) males pretreated with MK-801 (a noncompetitive NMDA receptor antagonist), and (4) females pretreated with MK-801. Serum levels of testosterone were estimated on the afternoon of ED 18. Expression of Bcl-2 and Bax, as well as neuronal apoptosis in SDN-POA, were observed on postnatal day 8. The results showed that (1) expression of NMDA receptors in the POA of male fetuses was higher than that of females on ED 16; (2) levels of testosterone were lower in MK-801 pretreated male fetuses than in intact males on ED 18; (3) expression of Bcl-2 in the POA of MK-801 pretreated male rats was significantly less than that of control males; (4) the apoptotic incidence in the SDN-POA of MK-801 pretreated male rats was significantly greater than in control males, while there was no significant difference in apoptotic incidence in the SDN-POA between MK-801 pretreated and intact females. These results suggest that the NMDA receptor is highly expressed in prenatal male fetuses, and that it might play an important role in the elevation of testosterone levels. Moreover, activation of NMDA receptors may protect SDN-POA neurons from naturally occurring neuronal death, by modulating testosterone and/or Bcl-2 expression.

The neurotoxic effect of monosodium L-glutamate (MSG) on the morphologies in the darkly stained sexually dimorphic nucleus of the preoptic area (SDN-POA) and the lighter-staining surrounding area (non-SDN-POA) within the medial preoptic nucleus (MPN) was evaluated. Male and female Long-Evans rats were used. MSG (4 mg/g of body weight) was administered subcutaneously to pups on days 1 and 3 postnatally. Normal saline was used as the vehicle. At the age of 6 months, the rats were sacrificed and the brain tissues were fixed for histological examination. The morphological changes, i.e., total volume, density, total neuron number, neuronal nuclear volume (NNV) and ratio of pyknosis, of the SDN-POA and non-SDN-POA within the MPN, were estimated using the AMS VIDS III semiautomatic image-analytic system. The results indicate that neonatal MSG treatment caused significant neuronal loss and decreases in total volume of the SDN-POA and non-SDN-POA of male and female rats. However, only the SDN-POA of MSG-treated male rats showed a significant increase of pyknosis and decrease of neuronal density. A significant enlargement of NNV in the SDN-POA and non-SDN-POA was observed in the MSG-treated male rats. These results indicate that the MPN shows sex-specific and area-specific changes after neonatal neurotoxicity due to MSG.

The spinal nucleus of the bulbocavernosus (SNB) is a sexually dimorphic nucleus of about 200 androgen-accumulating motor neurons in lumbar segments 5 and 6 of the rat spinal cord. In the present work we have used immunohistochemistry to investigate the change in calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) in these motor neurons induced by long-term castration. CGRP-LI was found in the cell bodies of the SNB motor neurons and in thin varicose fibers coursing through the nucleus. In the intact animals 48.4 +/- 3.9% (n = 8) of the SNB neurons were CGRP-LI positive while in the castrates this ratio increased to 80.0 +/- 1.7% (n = 9, p less than 0.001). Animals castrated for 4 weeks and then implanted with testosterone-containing Silastic capsules for an additional 4 weeks had the same percentage of CGRP-LI-containing SNB motor neurons as intact animals. These results demonstrate that a reduction in circulating testosterone levels increases the number of SNB motor neurons that express CGRP-LI implying that androgen stimulation inhibits the expression of CGRP-LI in SNB motor neurons.

The volume of the sexually dimorphic nucleus in the preoptic area (SDN-POA) of the rat brain is several fold larger in adult male rats that in adult females. This sex difference in brain structure was previously shown to develop under the influence of androgenic and estrogenic hormones during the perinatal period. We here report that treatment of newborn male and female rats with the estrogen antagonist tamoxifen significantly inhibited growth and differentiation of the SDN-POA in both sexes and it resulted in permanent anovulatory sterility in females. The findings suggest (a) that testicular androgens exert their growth promoting activity on SDN-POA development only after being converted into estrogens, and (b) that also in the female rat structural and possibly functional brain differentiation is under estrogenic control.