Middle-aged female rats lack the dynamic changes in GAD(67) mRNA levels observed in young females on the day of a luteinising hormone surge

GABA System Modifications During Periods of Hormonal Flux Across the Female Lifespan

The female lifespan is marked by periods of dramatic hormonal fluctuation. Changes in the ovarian hormones estradiol and progesterone, in addition to the progesterone metabolite allopregnanolone, are among the most significant and have been shown to have widespread effects on the brain. This review summarizes current understanding of alterations that occur within the GABA system during the major hormonal transition periods of puberty, the ovarian cycle, pregnancy and the postpartum period, as well as reproductive aging. The functional impacts of altered inhibitory activity during these times are also discussed. Lastly, avenues for future research are identified, which, if pursued, can broaden understanding of the GABA system in the female brain and potentially lead to better treatments for women experiencing changes in brain function at each of these hormonal transition periods.

Neuroendocrine control of the transition to reproductive senescence: lessons learned from the female rodent model

The natural transition to reproductive senescence is an important physiological process that occurs with aging, resulting in menopause in women and diminished or lost fertility in most mammalian species. This review focuses on how rodent models have informed our knowledge of age-related changes in gonadotropin-releasing hormone (GnRH) neurosecretory function and the subsequent loss of reproductive capacity. Studies in rats and mice have shown molecular, morphological and functional changes in GnRH cells. Furthermore, during reproductive aging altered sex steroid feedback to the hypothalamus contributes to a decrease of stimulatory signaling and increase in inhibitory tone onto GnRH neurons. At the site of the GnRH terminals where the peptide is released into the portal vasculature, the cytoarchitecture of the median eminence becomes disorganized with aging, and mechanisms of glial-GnRH neuronal communication may be disrupted. These changes can result in the dysregulation of GnRH secretion with reproductive decline. Interestingly, reproductive aging effects on the GnRH circuitry are observed in middle age even prior to any obvious physiological changes in cyclicity. We speculate that the hypothalamus may play a critical role in this mid-life transition. Because there are substantial species differences in these aging processes, we also compare and contrast rodent aging to that in primates. Work discussed herein shows that in order to understand neuroendocrine mechanisms of reproductive senescence, further research needs to be conducted in ovarian-intact models.

The neurobiology of preovulatory and estradiol-induced gonadotropin-releasing hormone surges

Ovarian steroids normally exert homeostatic negative feedback on GnRH release. During sustained exposure to elevated estradiol in the late follicular phase of the reproductive cycle, however, the feedback action of estradiol switches to positive, inducing a surge of GnRH release from the brain, which signals the pituitary LH surge that triggers ovulation. In rodents, this switch appears dependent on a circadian signal that times the surge to a specific time of day (e.g., late afternoon in nocturnal species). Although the precise nature of this daily signal and the mechanism of the switch from negative to positive feedback have remained elusive, work in the past decade has provided much insight into the role of circadian/diurnal and estradiol-dependent signals in GnRH/LH surge regulation and timing. Here we review the current knowledge of the neurobiology of the GnRH surge, in particular the actions of estradiol on GnRH neurons and their synaptic afferents, the regulation of GnRH neurons by fast synaptic transmission mediated by the neurotransmitters gamma-aminobutyric acid and glutamate, and the host of excitatory and inhibitory neuromodulators including kisspeptin, vasoactive intestinal polypeptide, catecholamines, neurokinin B, and RFamide-related peptides, that appear essential for GnRH surge regulation, and ultimately ovulation and fertility.

The neuroendocrine physiology of female reproductive aging: An update

The transition into menopause is a complex process that affects fertility and increases the risk for a number of health problems in aging women that include, but are not limited to osteoporosis, heart disease, diabetes mellitus and cognitive dysfunction. Improved nutrition and enhanced access to medical care have increased the average lifespan for women in developed countries, and many will spend more than one-third of their life in a post-menopausal state. Epidemiological studies indicate that a delayed natural menopause confers longevity and decelerates the appearance of much age-related morbidity, suggesting that developing treatments to delay menopause would significantly improve quality of life for women. Although menopause is ultimately defined by ovarian follicular exhaustion, several lines of scientific evidence in humans and animals now suggest that dysregulation of estradiol feedback mechanisms and hypothalamic-pituitary dysfunction contributes to the onset and progression of reproductive senescence, independent of ovarian failure. This article provides a brief update on our current understanding of the role of the hypothalamic-pituitary axis in the onset of and transition into female reproductive senescence.

Lack of functional GABA(B) receptors alters GnRH physiology and sexual dimorphic expression of GnRH and GAD-67 in the brain

GABA, the main inhibitory neurotransmitter, acts through GABA(A/C) and GABA(B) receptors (GABA(B)Rs); it is critical for gonadotropin regulation. We studied whether the lack of functional GABA(B)Rs in GABA(B1) knockout (GABA(B1)KO) mice affected the gonadotropin axis physiology. Adult male and female GABA(B1)KO and wild-type (WT) mice were killed to collect blood and tissue samples. Gonadotropin-releasing hormone (GnRH) content in whole hypothalami (HT), olfactory bulbs (OB), and frontoparietal cortexes (CT) were determined (RIA). GnRH expression by quantitative real-time PCR (qRT-PCR) was evaluated in preoptic area-anterior hypothalamus (POA-AH), medial basal-posterior hypothalamus (MBH-PH), OB, and CT. Pulsatile GnRH secretion from hypothalamic explants was measured by RIA. GABA, glutamate, and taurine contents in HT and CT were determined by HPLC. Glutamic acid decarboxylase-67 (GAD-67) mRNA was measured by qRT-PCR in POA-AH, MBH-PH, and CT. Gonadotropin content, serum levels, and secretion from adenohypophyseal cell cultures (ACC) were measured by RIA. GnRH mRNA expression was increased in POA-AH of WT males compared with females; this pattern of expression was inversed in GABA(B1)KO mice. MBH-PH, OB, and CT did not follow this pattern. In GABA(B1)KO females, GnRH pulse frequency was increased and GABA and glutamate contents were augmented. POA-AH GAD-67 mRNA showed the same expression pattern as GnRH mRNA in this area. Gonadotropin pituitary contents and serum levels showed no differences between genotypes. Increased basal LH secretion and decreased GnRH-stimulated gonadotropin response were observed in GABA(B1)KO female ACCs. These results support the hypothesis that the absence of functional GABA(B)Rs alters GnRH physiology and critically affects sexual dimorphic expression of GnRH and GAD-67 in POA-AH.

The excitatory peptide kisspeptin restores the luteinizing hormone surge and modulates amino acid neurotransmission in the medial preoptic area of middle-aged rats

Reproductive success depends on a robust and appropriately timed preovulatory LH surge. The LH surge, in turn, requires ovarian steroid modulation of GnRH neuron activation by the neuropeptide kisspeptin and glutamate and gamma-aminobutyric acid (GABA) neurotransmission in the medial preoptic area (mPOA). Middle-aged females exhibit reduced excitation of GnRH neurons and attenuated LH surges under estrogen-positive feedback conditions, in part, due to increased GABA and decreased glutamate neurotransmission in the mPOA. This study tested the hypothesis that altered kisspeptin regulation by ovarian steroids plays a role in age-related LH surge dysfunction. We demonstrate that middle-aged rats exhibiting delayed and attenuated LH surges have reduced levels of Kiss1 mRNA in the anterior hypothalamus under estrogen-positive feedback conditions. Kisspeptin application directly into the mPOA rescues total LH release and the LH surge amplitude in middle-aged rats and increases glutamate and decreases GABA release to levels seen in the mPOA of young females. Moreover, the N-methyl-D-aspartate receptor antagonist MK801 blocks kisspeptin reinstatement of the LH surge. These observations suggest that age-related LH surge dysfunction results, in part, from reduced kisspeptin drive under estrogen-positive feedback conditions and that kisspeptin regulates GnRH/LH release, in part, through modulation of mPOA glutamate and GABA release.

The role of the brain in female reproductive aging

In middle-aged women, follicular depletion is a critical factor mediating the menopausal transition; however, all levels of the hypothalamic-pituitary-gonadal (HPG) axis contribute to the age-related decline in reproductive function. To help elucidate the complex interactions between the ovary and brain during middle-age that lead to the onset of the menopause, we utilize animal models which share striking similarities in reproductive physiology. Our results show that during middle-age, prior to any overt irregularities in estrous cyclicity, the ability of 17beta-estradiol (E(2)) to modulate the cascade of neurochemical events required for preovulatory gonadotropin-releasing hormone (GnRH) release and a luteinizing hormone (LH) surge is diminished. Middle-aged female rats experience a delay in and an attenuation of LH release in response to E(2). Additionally, although we do not observe a decrease in GnRH neuron number until a very advanced age, E(2)-mediated GnRH neuronal activation declines during the earliest stages of age-related reproductive decline. Numerous hypothalamic neuropeptides and neurochemical stimulatory inputs (i.e., glutamate, norepinephrine (NE), and vasoactive intestinal peptide (VIP)) that drive the E(2)-mediated GnRH/LH surge appear to dampen with age or lack the precise temporal coordination required for a specific pattern of GnRH secretion, while inhibitory signals such as gamma-aminobutyric acid (GABA) and opioid peptides remain unchanged or elevated during the afternoon of proestrus. These changes, occurring at the level of the hypothalamus, lead to irregular estrous cycles and, ultimately, the cessation of reproductive function. Taken together, our studies indicate that the hypothalamus is an important contributor to age-related female reproductive decline.

Chapter 2: hypothalamic neural systems controlling the female reproductive life cycle gonadotropin-releasing hormone, glutamate, and GABA

The hypothalamic-pituitary-gonadal (HPG) axis undergoes a number of changes throughout the reproductive life cycle that are responsible for the development, puberty, adulthood, and senescence of reproductive systems. This natural progression is dictated by the neural network controlling the hypothalamus including the cells that synthesize and release gonadotropin-releasing hormone (GnRH) and their regulatory neurotransmitters. Glutamate and GABA are the primary excitatory and inhibitory neurotransmitters in the central nervous system, and as such contribute a great deal to modulating this axis throughout the lifetime via their actions on receptors in the hypothalamus, both directly on GnRH neurons as well as indirectly through other hypothalamic neural networks. Interactions among GnRH neurons, glutamate, and GABA, including the regulation of GnRH gene and protein expression, hormone release, and modulation by estrogen, are critical to age-appropriate changes in reproductive function. Here, we present evidence for the modulation of GnRH neurosecretory cells by the balance of glutamate and GABA in the hypothalamus, and the functional consequences of these interactions on reproductive physiology across the life cycle.

Restoration of the luteinizing hormone surge in middle-aged female rats by altering the balance of GABA and glutamate transmission in the medial preoptic area

Hypothalamic glutamate and gamma-aminobutyric acid (GABA) neurotransmission are involved in the ovarian hormone-induced GnRH-LH surge in rodents. We previously reported that middle-aged rats have significantly less glutamate release in the medial preoptic area than young rats on the day of the LH surge. The present study tested the hypothesis that the delayed and attenuated LH surge in ovariohysterectomized middle-aged rats primed with ovarian steroids results from reduced hypothalamic glutamate and increased GABA(A) neurotransmission. Microdialysis results show that middle-aged rats with attenuated LH surges had reduced extracellular glutamate and increased extracellular GABA levels in the medial preoptic area compared with young rats. Blocking GABA(A) receptors with bicuculline or inhibiting synaptic glutamate reuptake with L-trans-pyrrolidine-2,4-dicarboxylic acid increased extracellular Glu in the medial preoptic area and partially restored LH surge amplitude in middle-aged rats without altering LH surge onset. Complete recovery of LH surge amplitude was observed in middle-aged rats treated with the combination of bicuculline and L-trans-pyrrolidine-2,4-dicarboxylic acid. This treatment also restored the extracellular glutamate:GABA ratio in the medial preoptic area of middle-aged rats to the level of young rats. Immunoblot analysis revealed that estradiol and progesterone treatment reduced SLC32A1(formerly known as vesicular GABA transporter) levels and increased SLC17A6 (formerly known as vesicular glutamate transporter 2) levels in the anterior hypothalamus of ovariohysterectomized young but not middle-aged rats. These data suggest that both reduced availability of glutamate and increased activation of GABA(A) receptors under estrogen-positive feedback conditions contribute to the age-related delay in onset and attenuated amplitude of the LH surge.

NMDA receptor subunit NR2b: effects on LH release and GnRH gene expression in young and middle-aged female rats, with modulation by estradiol

BACKGROUND/AIMS

The loss of reproductive capacity during aging involves changes in the neural regulation of the hypothalamic gonadotropin-releasing hormone (GnRH) neurons controlling reproduction. This neuronal circuitry includes glutamate receptors on GnRH neurons. Previously, we reported an increase in the expression of the NR2b subunit protein of the NMDA receptor on GnRH neurons in middle-aged compared to young female rats. Here, we examined the functional implications of the NR2b subunit on the onset of reproductive aging, using an NR2b-specific antagonist ifenprodil.

METHODS

Young (3-5 months) and middle-aged (10-13 months) female rats were ovariectomized (OVX), 17beta-estradiol (E2) or vehicle (cholesterol) treated, and implanted with a jugular catheter. Serial blood sampling was undertaken every 10 min for 4 h, with ifenprodil (10 mg/kg) or vehicle injected (i.p.) after 1 h of baseline sampling. The pulsatile release of pituitary LH and levels of GnRH mRNA in hypothalamus were quantified as indices of the reproductive axis.

RESULTS

Our results showed effects of ifenprodil on both endpoints. In OVX rats given cholesterol, neither age nor ifenprodil had any effects on LH release. In E2-treated rats, aging was associated with significant decreases in pulsatile LH release. Additionally, ifenprodil stimulated parameters of pulsatile LH release in both young and middle-aged animals. Ifenprodil had few effects on GnRH mRNA; the only significant effect of ifenprodil was found in the middle-aged, cholesterol group.

CONCLUSION

Together, these findings support a role for the NR2b subunit of the NMDAR in GnRH/LH regulation. Because most of these effects were exhibited on pituitary LH release in the absence of a concomitant change in GnRH gene expression, it is likely that NMDA receptors containing the NR2b subunit play a role in GnRH-induced LH release, independent of de novo GnRH gene expression.