The estrogen receptor alpha (ERalpha) in the hypothalamus plays important roles in the regulation of reproductive development, physiology, and behavior. However, the expression of the ERalpha may change during aging or in response to varying estrogen levels. The present study measured changes in the numbers of ERalpha-expressing cells in specific hypothalamic and preoptic nuclei of ovariectomized female Sprague-Dawley rats at three ages (young [3-4 months], middle-aged [10-12 months], or old [24-26 months]) and with or without estrogen replacement. Numbers of ERalpha-immunoreactive neurons were quantified in four regions relevant to reproductive function: the anteroventral periventricular nucleus (AVPV), medial preoptic nucleus (MPN), arcuate nucleus (ARH), and ventromedial nucleus (VMN), using an unbiased stereologic approach. In the AVPV and VMN, significant age-related increases in the numbers of ERalpha-expressing cells from the middle-aged to the old group were detected, and no differences were observed in the MPN and ARH, indicating that ERalpha neuron number is maintained or even elevated during aging. No significant effects of estrogen on ERalpha cell number were detected in any of the four regions studied. Therefore, ERalpha cell number in the rat hypothalamus and preoptic area changes with aging in a region-specific manner.

Although the estrogen receptor beta (ER beta) is a major target for actions of estrogen on the brain, little is known about its neural expression during aging, when levels and the mode of estrogen release undergo substantial changes. Therefore, in the present study we examined effects of aging and estrogen treatment on the number of cells expressing the ER beta in female rats. Two regions relevant to reproductive function were analyzed: the anteroventral periventricular nucleus (AVPV) and the principal nucleus of the bed nucleus of the stria terminalis (pBST). The numbers of ER beta-expressing cells were quantified using an unbiased stereological approach. Female rats were used at three ages [young (3-4 months), middle-aged (10-12 months), and old (24-26 months)], with or without estrogen replacement. Because the estrogen milieu impacts the function of neurotransmitter receptors such as the N-methyl-D-aspartate receptor in the brain, we also investigated the colocalization of ER beta and the obligatory N-methyl-D-aspartate receptor subunit, NR1. We observed a significant age-related decrease in ER beta cell number in the AVPV, but not the pBST. No significant effect of estrogen on ER beta cell number was detected in either brain region at any age. Approximately 10% and 3% of cells expressing ER beta also coexpressed NR1 in AVPV and pBST, respectively, and this did not differ with age or treatment. Taken together, our results demonstrate 1) there are age-related changes in ER beta cell number that are region specific; 2) this expression is not altered by estrogen replacement; and 3) a subset of ER beta-positive cells coexpresses NR1.

Polychlorinated biphenyls (PCBs) can disrupt the reproductive axis, particularly when the exposure occurs during the vulnerable developmental periods. Some effects of environmental endocrine disruptors such as PCBs may be exerted through binding to estrogen receptors (ERs). In this study we examined the endocrine-disrupting effects of Aroclor 1221 (a commercial PCB mixture), focusing on its actions on the ER-ss, which has been implicated in mediating effects of endocrine-disrupting chemicals. A low, ecologically relevant dose of Aroclor 1221 or vehicle (ethanol) was administered three times each to rat dams, on gestational day 16 and on postpartum days 1 and 4, a developmental period during which steroid hormones have permanent effects on adult brain structure and function. Effects on ER-ss cell number in the anteroventral periventricular nucleus (AVPV) were quantified; this sexually dimorphic nucleus of the brain is essential to female reproductive function. For comparison, we quantified ER-ss cell number in another hypothalamic region, the supraoptic nucleus (SON). Using a stereologic approach, we found that Aroclor 1221 caused a highly significant down-regulation of the number of ER-ss-expressing cells in the AVPV, but had no effect in the SON. Thus, PCB exposure has consequences for neural ER expression, and these findings have implications for wildlife and humans that have been exposed to environmental estrogens, particularly during the susceptible periods of early development.

Effects of N-methyl-D-aspartate (NMDA) receptor (NMDAR) activation on neuroendocrine function can be modulated by the steroid hormone milieu. For example, the hypothalamic GnRH neurons, the primary cells regulating reproductive function, are stimulated by NMDAR agonists, and this is greatly potentiated by estrogen. We hypothesized that the actions of glutamate and estrogen may converge at target cells in the brain in which the NMDA and estrogen receptors (ERs) are coexpressed. To this end, we used quantitative stereological techniques to determine the colocalization of the obligatory NMDAR subunit, NR1, and the ERalpha, in the anteroventral periventricular nucleus and the medial preoptic nucleus, two critical regions for reproductive physiology and behavior. We observed extensive colocalization of ERalpha and NR1 in these brain regions (approximately 80%). In the anteroventral periventricular nucleus, treatment of ovariectomized rats with estrogen up-regulated the coexpression, whereas in the medial preoptic nucleus, estrogen had no effect, demonstrating a regional specificity to the estrogen sensitivity. The number of ERalpha cells that did not express NR1 was not altered by estrogen treatment in either brain region. Thus, we speculate that the extensive colocalization of ERalpha and the NMDAR provides an anatomical level at which estrogen and glutamate can act at target cells, and potentially synergize, to influence neuroendocrine and autonomic functions.