Elsevier

Brain Research Reviews

Volume 37, Issues 1–3, November 2001, Pages 235-248
Brain Research Reviews

Review
Gonadotropin-releasing hormone neurons, NMDA receptors, and their regulation by steroid hormones across the reproductive life cycle

https://doi.org/10.1016/S0165-0173(01)00121-7Get rights and content

Abstract

The effects of ovarian steroid hormones on gonadotropin-releasing hormone (GnRH) neurons have been studied for many years. In addition to their regulation by sex steroids, GnRH neurons are affected by inputs from neurotransmitters such as glutamate, acting via the NMDA receptor (NMDAR). Moreover, the NMDAR itself is subject to estrogen regulation. Thus, effects of ovarian steroids on GnRH neurons and the NMDAR, and their interactions, are under intense investigation. Messenger RNA and protein levels of GnRH and NMDAR subunits were measured in neuroendocrine brain regions in response to estrogen treatment, or across the reproductive cycle. Stimulatory effects of ovarian steroids on GnRH gene expression occur during the preovulatory LH surge in young adult rats, and this is abolished in middle-aged rats that have an attenuated LH surge. Effects of estrogen on GnRH neurons have also been studied in the ovariectomized, estrogen-primed rat, and while results vary between laboratories, there appear to be age-related changes in the sensitivity of GnRH neurons to estrogen. Estrogen also has effects on NMDAR mRNA levels. In intact rats, mRNA levels of NMDAR decrease during reproductive aging in the preoptic area, the site of GnRH perikarya, while in the medial basal hypothalamus-median eminence, the site of GnRH neuroterminals, levels of NMDAR subunit mRNAs increase with aging. Thus, glutamatergic inputs to GnRH perikarya and neuroterminals and other neuroendocrine cells may change during reproductive aging in intact rats. In ovariectomized rats, NMDAR subunit mRNA levels also undergo age-related changes, and respond to estrogen replacement in a subunit- and age-specific manner. Notably, there are major differences in NMDAR gene expression during aging between intact and ovariectomized rats, suggesting that ovarian factors other than estrogen play a role in the regulation of this receptor.

Introduction

The effects of estrogen on gonadotropin-releasing hormone (GnRH) neurons have been studied for many years (reviewed in Refs. [42], [54]). However, there is still considerable controversy about this topic. For example, estrogen replacement has been shown to increase [50], [66], [75], [77], decrease [99], [101], or have no effect [36] on GnRH gene expression (Table 1) and release (reviewed in Refs. [48], [54], [73]). Differences in results may arise from the use of different techniques to measure GnRH gene expression or peptide levels, differences in species or strains, and the age of the animal. In addition, estrogen exerts both negative and positive feedback effects on GnRH neurons, and therefore it is possible that discrepancies in results may also be due to the ability of estrogen to stimulate or inhibit the neuroendocrine axis depending upon the experimental model. Another relevant controversy is whether GnRH neurons express the estrogen receptor (α and/or β) [18], [43], [45], [53], [81], [83], or if estrogen feedback onto GnRH neurons is mediated by interneurons expressing the estrogen receptor, although there is far more evidence for the latter.

GnRH neurons are regulated by numerous neurotransmitters, neuropeptides, and neurotrophic factors. Together with steroid hormone feedback, these multiple inputs to GnRH neurons enable these crucial neuroendocrine cells to integrate information about the internal and external environment in order to ensure that the timing of reproductive function occurs appropriately. One important neurotransmitter regulating GnRH neurons is glutamate, acting through N-methyl-d-aspartate (NMDA) receptors (NMDARs), as well as non-NMDARs [14], [15], [65]. Activation of the NMDA receptor stimulates GnRH neurons at the onset of puberty [21], [31], [37], [92], [93], is involved in the preovulatory LH surge [16] and the maintenance of pulsatile GnRH/LH release [7], [28], and stimulates GnRH gene expression [35], [38], [56], [70] (Table 2). During reproductive aging, the stimulatory effects of NMDAR activation on GnRH neurons become diminished [5], [12], [38], [103], and this may be involved in the transition to acyclicity that occurs at middle age.

The effects of NMDAR activation on GnRH neurons are dependent upon the steroid hormone environment. The stimulation of GnRH or LH release by NMDAR agonists is augmented in the presence of estrogen in ovariectomized animals [6], [20], or under higher estrogen levels in intact females [20]. The steroid-induced LH surge is blocked by an NMDAR antagonist, indicating an interaction of NMDARs and estrogen in reproductive processes [16]. In addition, glutamate levels in the POA are higher in estrogen-treated than control-treated peripubertal female rats [19]. However, NMDAR binding does not appear to be affected by estrogen [17], [23], suggesting another mechanism by which the estrogen environment alters NMDA’s effects on GnRH neurons.

The present review summarizes the current literature, together with results of research from my laboratory, on the regulation of GnRH neurons by estrogen during the reproductive life cycle. In addition, because GnRH neurons are regulated by glutamatergic input mediated by the NMDAR, and this receptor is subject to estrogen regulation, information on the effects of ovarian steroids on NMDAR subunit protein and mRNA levels in neuroendocrine brain regions will also be summarized. Overall, it is clear that the effects of estrogen on both GnRH and NMDAR protein and mRNA levels are differentially regulated by reproductive status and age.

Section snippets

Changes in GnRH gene expression during the reproductive cycle

Alterations in GnRH gene expression in female rats during the estrous cycle have been measured by several laboratories (reviewed in Refs. [42], [54]). Presumably, measurements of GnRH gene expression indicate the potential for an increase in biosynthesis of the decapeptide, with primary transcript demonstrating changes at a transcriptional level, and mRNA at a transcriptional or post-transcriptional level (e.g., mRNA stability). The results of most of these studies indicate that GnRH gene

Neuroanatomical changes in GnRH neurons during reproductive aging

It has been reported that the number of GnRH neurons does not change [44], [79], [98], or undergoes small decreases [29], [60], during reproductive aging. Our laboratory has quantitated and mapped the number and distribution of GnRH perikarya in the organum vasculosum of the lamina terminalis (OVLT)/preoptic area (POA) of female rats during reproductive aging. Studies were conducted on rats at four stages of reproductive function: young, regularly cycling proestrous rats (Y); middle-aged,

Regulation of GnRH gene expression by estrogen in ovariectomized rats

The ovariectomized (OVX), steroid-primed rat has proven to be an important model for understanding the regulation of GnRH neurons during the LH surge. Many groups have administered estrogen (with or without progesterone) to OVX rats and measured its effects on GnRH gene expression. As stated above, the results from different laboratories have varied considerably, probably due to differences in the experimental models, with some laboratories showing stimulatory, others inhibitory, and still

Changes in hypothalamic NMDA receptor (NMDAR) mRNA levels in intact rats during the reproductive life cycle

Age-related changes in the NMDAR system have been reported in many central nervous system regions. In the hippocampus, NR1 and NR2b subunit mRNA levels decrease with aging in male rats [59]. Our laboratory has also measured changes in NMDAR mRNA levels in the hippocampus and found that NR1 and NR2b mRNA levels increase during aging in female rats [3], [4]. Thus, the literature on changes in NMDARs with aging in the hippocampus is quite controversial and further studies are necessary to resolve

Changes in expression of NMDA receptor (NMDAR) levels in GnRH neurons of intact rats during the reproductive life cycle

The issue of whether GnRH neurons express the NMDAR has been under considerable debate. Several groups could not conclusively demonstrate the presence of substantial amounts of the NR1 subunit on GnRH neurons, using dual in situ hybridization, or in situ hybridization in combination with immunohistochemistry [1], [27], [91]. Using dual-label immunocytochemistry, our laboratory demonstrated that approximately 20% of GnRH neurons express the NR1 subunit in adult rat OVLT/POA [37], [38], and the

Regulation of hypothalamic NMDAR gene expression by estrogen in ovariectomized rats

In neuroendocrine and other brain regions, effects of NMDAR activation are dependent upon the estrogen environment. With respect to GnRH neurons, the stimulation of GnRH gene expression and release is enhanced in the presence of estrogen in ovariectomized animals [6], [20]. Regulation of the NMDAR by estrogen has been demonstrated in a subregion of the POA, the anteroventral periventricular nucleus [40]. The colocalization of the NMDAR with the estrogen receptor has not been studied

Summary

The regulation of neuroendocrine molecules by estrogen involves an interaction of the ovarian status of the animal (intact or ovariectomized), reproductive cycle stage (if the animal has ovaries), age, and the region of the brain studied (POA-AH or MBH-ME). Thus, GnRH gene expression in intact animals increases during the preovulatory LH surge, but in ovariectomized animals given estrogen, this regulation is more complex and varies depending upon the experimental model. This latter finding

Acknowledgements

I would like to acknowledgment the excellent technical assistance of Twethida Oung, Glendy Yeung, William G.M. Janssen and Brooke H. Miller, and Andrew P. Leonard for graphics. This report was supported by funding from the NIH (PO1-AG16765-02) and the Brookdale Foundation.

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