Age-related changes in hypothalamic gonadotropin-releasing hormone and N-methyl-D-aspartate receptor gene expression, and their regulation by oestrogen, in the female rat

Cardiovascular evaluation of female rats with 6-OHDA-induced parkinsonism: Possible protection by ovarian hormones and participation of nitric oxide

AIMS

Parkinson's disease (PD) is a neurological disorder caused by environmental and genetic factors, characterized by the death of dopaminergic neurons of the substantia nigra pars compacta (SNpc), leading to a decrease of dopamine in the striatum. In addition to motor symptoms, PD has several abnormalities, among which are cardiovascular changes, such as orthostatic and postprandial hypotension, and blood pressure lability. Studies demonstrate gender differences in PD pathogenesis, indicating that female hormones have a protective role against disease development. However, no studies examining cardiovascular changes in a female rat model of parkinsonism exist.

MAIN METHODS

Wistar female rats were subjected to ovariectomy (OVX) or sham surgery. After seven days, these animals were subjected to bilateral infusion of 6-hydroxydopamine (6-OHDA) or vehicle solution in their SNpc. On the 14th experimental day, a femoral artery catheterization was performed to record cardiovascular parameters after 24 h in conscious state. Analyses of cardiovascular variability and spontaneous baroreflex were performed. The nitrite (NO) concentration in the heart, thoracic aorta, abdominal aorta, and plasma was measured.

KEY FINDINGS

The sham-6-OHDA group had no decrease in the mean arterial pressure compared to sham-saline group, whereas the OVX-6-OHDA group presented a baseline decrease in comparison to sham-6-OHDA. The OVX-6-OHDA group showed an NO increase in the heart and abdominal aorta, whereas the sham-6-OHDA group did not. The very low frequency variability component decreased in the sham-6-OHDA but not in the OVX-6-OHDA group.

SIGNIFICANCE

We suggest a cardiovascular protection by ovarian hormones in PD with a possible NO involvement.

Calcium Signaling During Brain Aging and Its Influence on the Hippocampal Synaptic Plasticity

Calcium (Ca²⁺) ions are highly versatile intracellular signaling molecules and are universal second messenger for regulating a variety of cellular and physiological functions including synaptic plasticity. Ca²⁺ homeostasis in the central nervous system endures subtle dysregulation with advancing age. Research has provided abundant evidence that brain aging is associated with altered neuronal Ca²⁺ regulation and synaptic plasticity mechanisms. Much of the work has focused on the hippocampus, a brain region critically involved in learning and memory, which is particularly susceptible to dysfunction during aging. The current chapter takes a specific perspective, assessing various Ca²⁺ sources and the influence of aging on Ca²⁺ sources and synaptic plasticity in the hippocampus. Integrating the knowledge of the complexity of age-related alterations in neuronal Ca²⁺ signaling and synaptic plasticity mechanisms will positively shape the development of highly effective therapeutics to treat brain disorders including cognitive impairment associated with aging and neurodegenerative disease.

From Healthy Aging to Frailty: In Search of the Underlying Mechanisms

Population aging is accelerating rapidly worldwide, from 461 million people older than 65 years in 2004 to an estimated 2 billion people by 2050, leading to critical implications for the planning and delivery of health and social care. The most problematic expression of population aging is the clinical condition of frailty, which is a state of increased vulnerability that develops as a consequence of the accumulation of microscopic damages in many physiological systems that lead to a striking and disproportionate change in health state, even after an apparently small insult. Since little is known about the biology of frailty, an important perspective to understand this phenomenon is to establish how the alterations that physiologically occur during a condition of healthy aging may instead promote cumulative decline with subsequent depletion of homoeostatic reserve and increase the vulnerability also after minor stressor events. In this context, the present review aims to provide a description of the molecular mechanisms that, by having a critical impact on behavior and neuronal function in aging, might be relevant for the development of frailty. Moreover, since these biological systems are also involved in the coping strategies set in motion to respond to environmental challenges, we propose a role for lifestyle stress as an important player to drive frailty in aging.

Effects of Inducible Nitric Oxide Synthase Inhibition on Cardiovascular Risk of Adult Endotoxemic Female Rats: Role of Estrogen

Aim: Autonomic modulation responds to ovarian hormones and estrogen increases nitric oxide bioavailability. Also, females have minor susceptibility to sepsis and a higher survival rate. However, few studies have evaluated the role of estrogen in cardiovascular, autonomic, and oxidative parameters during initial endotoxemia and under inducible nitric oxide synthase (iNOS) inhibition in female rats.

Methods: Female wistar rats were subjected to ovariectomy and divided into three groups: OVX (ovariectomized), OVX+E (OVX plus daily estradiol) and SHAM (false surgery). After 8 weeks, mean arterial pressure (MAP) and heart rate (HR) were recorded in non-anesthetized catheterized rats, before and after intravenous LPS injection, preceded by S-methylisothiourea sulfate (SMT) injection, or sterile saline. Cardiovascular recordings underwent spectral analysis for evaluation of autonomic modulation. Two hours after LPS, plasma was collected to assess total radical-trapping antioxidant (TRAP), nitrite levels (NO2), lipoperoxidation (LOOH), and paraoxonase 1 (PON1) activity.

Results: Two hours after LPS, females treated with SMT presented a decrease of MAP, when compared to saline-LPS groups. At this same time, all SMT+LPS groups presented an increase of sympathetic and a decrease of parasympathetic modulation of HR. Two hours after saline+LPS, OVX presented decreased total radical-trapping antioxidant (TRAP) compared to SHAM. When treated with SMT+LPS, OVX did not altered TRAP, while estradiol reduced LOOH levels.

Conclusion: iNOS would be responsible for sympathetic inhibition and consumption of antioxidant reserves of females during endotoxemia, since iNOS is inhibited, treatment with estradiol could be protective in inflammatory challenges.

Effects of puerarin on estrogen-regulated gene expression in gonadotropin-releasing hormone pulse generator of ovariectomized rats

Effects of puerarin on the hypothalamic gonadotropin-releasing hormone (GnRH) pulse generator function is investigated, for the first time, in ovariectomized rats at the level of mRNA expression of estrogen-responsive genes, e.g., estrogen receptor (ER), GnRH and its receptor (GnRHR). Rats were treated orally for 90 days either with a soy-free diet containing two different doses of puerarin (low dose of 600 mg/kg and high dose of 3000 mg/kg) or estradiol benzoate (E2B) at either low dose (4.3 mg/kg) or high dose (17.3 mg/kg). Levels of mRNA expression in the medial preoptic area/anterior hypothalamus (MPOA/AH), mediobasal hypothalamus/median eminence (MBH/ME) and adenohypophysis were measured by quantitative TaqMan® real-time RT-PCR. Plasma levels of luteinizing hormone (LH) and prolactin (PRL) were measured by radioimmunoassay. In the MPOA/AH, both puerarin and E2B decreased ERα mRNA levels without any significant changes in ERβ and GnRH mRNA levels. Both puerarin and E2B did not significantly alter the expression levels of ERα, ERβ and GnRHR in the MBH/ME. E2B exerted significant effects on the down-regulation of adenohypophyseal GnRHR mRNA transcripts and serum LH levels. Puerarin did not cause significant changes in pituitary GnRHR mRNA transcripts and serum LH and PRL levels. This is the first study to demonstrate that in ovariectomized rat models of ovarian hormone deprivation, puerarin acted as a weak estrogen-active compound in the hypothalamic GnRH pulse generator through the downregulation of MPOA/AH ERα mRNA expression.

Regulation of Gonadotropin-Releasing Hormone-(1-5) Signaling Genes by Estradiol Is Age Dependent

Gonadotropin-releasing hormone (GnRH) is a key regulatory molecule of the hypothalamus-pituitary (PIT)-gonadal (HPG) axis that ultimately leads to the downstream release of estradiol (E2) and progesterone (P). These gonadal steroids feed back to the hypothalamus and PIT to regulate reproductive function and behavior. While GnRH is thought to be the master regulator of reproduction, its metabolic product GnRH-(1-5) is also biologically active. Thimet oligopeptidase 1 (also known as EP24.15) cleaves GnRH to form GnRH-(1-5). GnRH-(1-5) is involved in regulation of the HPG axis, exerting its actions through a pair of orphan G protein-coupled receptors, GPR101 and GPR173. The physiological importance of GnRH-(1-5) signaling has been studied in several contexts, but its potential role during reproductive senescence is poorly understood. We used an ovariectomized (OVX) rat model of reproductive senescence to assess whether and how GnRH-(1-5) signaling genes in hypothalamic subnuclei change in response to aging and/or different estradiol replacement regimens designed to model clinical hormone replacement in women. We found that Gpr101 and Gpr173 mRNA expression was increased with age in the arcuate nucleus, while expression of Gpr173 and EP24.15 increased with age in the medial preoptic area. Treatment with E2 in younger OVX animals increased expression of Gpr101, Gpr173, and EP24.15. However, older animals treated with E2 showed decreased expression of these GnRH-(1-5) signaling genes, displaying an age-related decline in responsiveness to E2. To our knowledge, this is the first study to systematically assess the effects of age and different clinically relevant regimens of E2 replacement on GnRH-(1-5) signaling genes.

The Role of Kisspeptin in Female Reproduction

CONTEXT

Kisspeptin (KISS1), a recently discovered neuropeptide that acts upstream of gonadotropin-releasing hormone (GnRH) neurons, is critical for maturation and function of the reproductive axis. This review aimed at providing comprehensive and up-to-date information on Kisspeptin and its role in female reproduction.

EVIDENCE ACQUISITION

A literature review was performed using PubMed for all English language articles published between 1999 and 2016.

RESULTS

The kisspeptin system (KISS1/G protein-coupled receptor-54,GPR54) has recently been addressed as an essential gatekeeper of puberty onset and gonadotropin secretion. Compelling evidence has documented that hypothalamic Kisspeptin mediates steroid feedback and metabolic cues at different developmental stages throughout lifespan. Furthermore, in pre/postnatally androgenized animal models, which exhibit many of the characteristics of Polycystic Ovarian Syndrome (PCOS), the hypothalamic expression of KISS1 and GnRH is abnormal, which might lead to multiple tissue abnormalities observed in this disorder.

CONCLUSIONS

Kisspeptin, a principal activator of GnRH neurons and the target of endocrine and metabolic cues, is a prerequisite for the onset of puberty and maintenance of normal reproductive function, as abnormal KISS1/GPR54 system has been reported in both animal models and patients with certain forms of infertility, e.g. Idiopathic Hypogonadotropic hypogonadism (IHH) and PCOS. The information suggests that kisspeptin or its receptor represents a potential therapeutic target in the treatment of patients with fertility disorders.

Aging-Related Changes in Ovarian Hormones, Their Receptors, and Neuroendocrine Function

Ovarian steroid hormones exert a broad range of effects on the body and brain. In the nervous system, estrogen and progesterone have crucial feedback actions on the hypothalamic neurons that drive the reproductive axis. In addition, hormones exert a variety of actions on other traditionally nonreproductive functions such as cognition, learning and memory, neuroprotection, mood and affective behavior, and locomotor activity. The actions of hormones on the hypothalamus are largely mediated by their nuclear hormone receptors, the two estrogen receptors, ERα and ERβ, and the two progesterone receptor isoforms, PR-A and PR-B. Thus, changes in the circulating concentrations of estrogens and progestins during the life cycle can result in differential activation of their receptors. Furthermore, changes in the numbers, activity, and distribution of hypothalamic ERs and PRs can occur as a function of developmental age. The purpose of this article is to review the literature on the causes and consequences of alterations in steroid hormones, their neural receptors, and their interactions on reproductive senescence. We have also discussed several important experimental design considerations, focusing on rodent models in current use for understanding the mechanisms of menopause in women.

NMDA Receptor Function During Senescence: Implication on Cognitive Performance

N-methyl-D-aspartate (NMDA) receptors, a family of L-glutamate receptors, play an important role in learning and memory, and are critical for spatial memory. These receptors are tetrameric ion channels composed of a family of related subunits. One of the hallmarks of the aging human population is a decline in cognitive function; studies in the past couple of years have demonstrated deterioration in NMDA receptor subunit expression and function with advancing age. However, a direct relationship between impaired memory function and a decline in NMDA receptors is still ambiguous. Recent studies indicate a link between an age-associated NMDA receptor hypofunction and memory impairment and provide evidence that age-associated enhanced oxidative stress might be contributing to the alterations associated with senescence. However, clear evidence is still deficient in demonstrating the underlying mechanisms and a relationship between age-associated impaired cognitive faculties and NMDA receptor hypofunction. The current review intends to present an overview of the research findings regarding changes in expression of various NMDA receptor subunits and deficits in NMDA receptor function during senescence and its implication in age-associated impaired hippocampal-dependent memory function.

Expression of Vesicular Glutamate Transporter 2 (vGluT2) on Large Dense-Core Vesicles within GnRH Neuroterminals of Aging Female Rats

The pulsatile release of GnRH is crucial for normal reproductive physiology across the life cycle, a process that is regulated by hypothalamic neurotransmitters. GnRH terminals co-express the vesicular glutamate transporter 2 (vGluT2) as a marker of a glutamatergic phenotype. The current study sought to elucidate the relationship between glutamate and GnRH nerve terminals in the median eminence—the site of GnRH release into the portal capillary vasculature. We also determined whether this co-expression may change during reproductive senescence, and if steroid hormones, which affect responsiveness of GnRH neurons to glutamate, may alter the co-expression pattern. Female Sprague-Dawley rats were ovariectomized at young adult, middle-aged and old ages (~4, 11, and 22 months, respectively) and treated four weeks later with sequential vehicle + vehicle (VEH + VEH), estradiol + vehicle (E₂ + VEH), or estradiol + progesterone (E₂+P₄). Rats were perfused 24 hours after the second hormone treatment. Confocal microscopy was used to determine colocalization of GnRH and vGluT2 immunofluorescence in the median eminence. Post-embedding immunogold labeling of GnRH and vGluT2, and a serial electron microscopy (EM) technique were used to determine the cellular interaction between GnRH terminals and glutamate signaling. Confocal analysis showed that GnRH and vGluT2 immunofluorescent puncta were extensively colocalized in the median eminence and that their density declined with age but was unaffected by short-term hormone treatment. EM results showed that vGluT2 immunoreactivity was extensively associated with large dense-core vesicles, suggesting a unique glutamatergic signaling pathway in GnRH terminals. Our results provide novel subcellular information about the intimate relationship between GnRH terminals and glutamate in the median eminence.

Aging-induced changes in sex-steroidogenic enzymes and sex-steroid receptors in the cortex, hypothalamus and cerebellum

We investigated age-induced changes in mRNA expression profiles of sex-steroidogenic enzymes and sex-steroid receptors in 3-, 12-, and 24-month-old male rat brain subregions [cerebral cortex (CC), hypothalamus (Hy) and cerebellum (CL)]. In many cases, the expression levels of mRNA decreased with age for androgen synthesis enzyme systems, including Cyp17a1, Hsd17b and Srd5a in the CC and CL, but not in the Hy. Estradiol synthase Cyp19a1 did not show age-induced decline in the Hy, and nearly no expression of Cyp19a1 was observed in the CC and CL over 3-24 m. Androgen receptor Ar increased in the Hy but decreased in the CC with age. Estrogen receptor Esr1 increased in the CC and Hy, and did not change in the CL with age. Esr2 did not change in the CC and Hy, but decreased in the CL with age. As a comparison, age-induced changes of brain-derived neurotrophic factor mRNA were also investigated.

Insulin-like growth factor-I regulates LH release by modulation of kisspeptin and NMDA-mediated neurotransmission in young and middle-aged female rats

This study investigated potential mechanisms by which age and IGF-I receptor (IGF-Ir) signaling in the neuroendocrine hypothalamus affect estradiol-positive feedback effects on GnRH neuronal activation and on kisspeptin and N-methyl-D-aspartate (NMDA)-induced LH release and on the abundance of NMDA receptor subunits Nr1 and Nr2b and Kiss1r transcript and protein in the hypothalamus of young and middle-aged female rats. We infused vehicle, IGF-I, or JB-1, a selective antagonist of IGF-Ir, into the third ventricle of ovariectomized female rats primed with estradiol or vehicle and injected with vehicle, kisspeptin (3 or 30 nmol/kg), or NMDA (15 or 30 mg/kg). Regardless of dose, NMDA and kisspeptin resulted in significantly more LH release, GnRH/c-Fos colabeling, and c-Fos immunoreative cells in young than in middle-aged females. Estradiol priming significantly increased Kiss1r, Nr1, and Nr2b receptor transcript and protein abundance in young but not middle-aged female hypothalamus. JB-1 attenuated kisspeptin and NMDA-induced LH release, numbers of GnRH/c-Fos and c-Fos cells, and Kiss1r, Nr1, and Nr2b transcript and protein abundance in young females to levels observed in middle-aged females. IGF-I significantly enhanced NMDA and kisspeptin-induced LH release in middle-aged females without increasing numbers of GnRH/c-Fos or c-Fos immunoreactive cells. IGF-I infusion in middle-aged females also increased Kiss1r, Nr1, and Nr2b protein and transcript to levels that were equivalent to young estradiol-primed females. These findings indicate that age-related changes in estradiol-regulated responsiveness to excitatory input from glutamate and kisspeptin reflect reduced IGF-Ir signaling.

Disruption of reproductive aging in female and male rats by gestational exposure to estrogenic endocrine disruptors

Polychlorinated biphenyls (PCBs) are industrial contaminants and known endocrine-disrupting chemicals. Previous work has shown that gestational exposure to PCBs cause changes in reproductive neuroendocrine processes. Here we extended work farther down the life spectrum and tested the hypothesis that early life exposure to Aroclor 1221 (A1221), a mixture of primarily estrogenic PCBs, results in sexually dimorphic aging-associated alterations to reproductive parameters in rats, and gene expression changes in hypothalamic nuclei that regulate reproductive function. Pregnant Sprague Dawley rats were injected on gestational days 16 and 18 with vehicle (dimethylsulfoxide), A1221 (1 mg/kg), or estradiol benzoate (50 μg/kg). Developmental parameters, estrous cyclicity (females), and timing of reproductive senescence were monitored in the offspring through 9 months of age. Expression of 48 genes was measured in 3 hypothalamic nuclei: the anteroventral periventricular nucleus (AVPV), arcuate nucleus (ARC), and median eminence (females only) by real-time RT-PCR. Serum LH, testosterone, and estradiol were assayed in the same animals. In males, A1221 had no effects; however, prenatal estradiol benzoate increased serum estradiol, gene expression in the AVPV (1 gene), and ARC (2 genes) compared with controls. In females, estrous cycles were longer in the A1221-exposed females throughout the life cycle. Gene expression was not affected in the AVPV, but significant changes were caused by A1221 in the ARC and median eminence as a function of cycling status. Bionetwork analysis demonstrated fundamental differences in physiology and gene expression between cycling and acyclic females independent of treatment. Thus, gestational exposure to biologically relevant levels of estrogenic endocrine-disrupting chemicals has sexually dimorphic effects, with an altered transition to reproductive aging in female rats but relatively little effect in males.

Middle-aged female rats retain sensitivity to the anorexigenic effect of exogenous estradiol

It is well established that estradiol (E2) decreases food intake and body weight in young female rats. However, it is not clear if female rats retain responsiveness to the anorexigenic effect of E2 during middle age. Because middle-aged females exhibit reduced responsiveness to E2, manifesting as a delayed and attenuated luteinizing hormone surge, it is plausible that middle-aged rats are less responsive to the anorexigenic effect of E2. To test this we monitored food intake in ovariohysterectomized young and middle-aged rats following E2 treatment. E2 decreased food intake and body weight to a similar degree in both young and middle-aged rats. Next, we investigated whether genes that mediate the estrogenic inhibition of food intake are similarly responsive to E2 by measuring gene expression of the anorexigenic genes corticotropin-releasing hormone (CRH), proopiomelanocortin (POMC), the long form of the leptin receptor (Lepr) and serotonin 2C receptors (5HT2CR) and the orexigenic genes agouti-related peptide (AgRP), neuropeptide Y (NPY), prepromelanin-concentrating hormone (pMCH) and orexin in the hypothalamus of young and middle-aged OVX rats treated with E2. As expected, E2 increased expression of all anorexigenic genes while decreasing expression of all orexigenic genes in young rats. Although CRH, 5HT2CR, Lepr, AgRP, NPY and orexin were also sensitive to E2 treatment in middle-aged rats, POMC and pMCH expression were not influenced by E2 in middle-aged rats. These data demonstrate that young and middle-aged rats are similarly sensitive to the anorexigenic effect of E2 and that most, but not all feeding-related genes retain sensitivity to E2.

Accelerated ovarian failure: a novel, chemically induced animal model of menopause

Current rodent models of menopause fail to adequately recapitulate the menopause transition. The intact aging model fails to achieve very low estrogen levels, and the ovariectomy model lacks a perimenopause phase. A new rodent model of accelerated ovarian failure (AOF) successfully replicates human perimenopause and postmenopause, including estrous acyclicity and fluctuating, followed by undetectable, estrogen levels, and allows for the dissociation of the effects of hormone levels from the effects of aging. In this model, an ovotoxic chemical, 4-vinylcyclohexene diepoxide (VCD), selective for primary and primordial follicles, is injected intraperitonelly in animals for 15 days. As the mature follicle population is depleted through natural cycling, ovarian failure follows increasing periods of acyclity. Administered at low doses, VCD specifically causes apoptotic cell death of primordial follicles but does not affect other peripheral tissues, including the liver and spleen, nor does it affect brain inflammation markers. In addition to reducing confounds associated with genetic and surgical manipulations, the AOF model maintains the presence of ovarian tissue which importantly parallels to the menopause transition in humans. The VCD injection procedure can be applied to studies using transgenic or knockout mice strains, or in other disease-state models (e.g., ischemia, atherosclerosis, or diabetes). This AOF model of menopause will generate new insights into women's health particularly in determining the critical periods (i.e., a window of opportunity) during perimenopause for restoring ovarian hormones for the most efficacious effect on memory and mood disorders as well as other menopausal symptoms.

Age- and hormone-regulation of opioid peptides and synaptic proteins in the rat dorsal hippocampal formation

Circulating estrogen levels and hippocampal-dependent cognitive functions decline with aging. Moreover, the responses of hippocampal synaptic structure to estrogens differ between aged and young rats. We recently reported that estrogens increase levels of post-synaptic proteins, including PSD-95, and opioid peptides leu-enkephalin and dynorphin in the hippocampus of young animals. However, the influence of ovarian hormones on synaptic protein and opioid peptide levels in the aging hippocampus is understudied. Here, young (3- to 5-month-old), middle-aged (9- to 12-month-old), and aged (about 22-month-old) female rats were ovariectomized and then, 4 weeks later, subcutaneously implanted with a silastic capsule containing vehicle or 17β-estradiol. After 48 h, rats were subcutaneously injected with progesterone or vehicle and sacrificed 1 day later. Coronal sections through the dorsal hippocampus were processed for quantitative peroxidase immunohistochemistry of leu-enkephalin, dynorphin, synaptophysin, and PSD-95. With age, females showed opposing changes in leu-enkephalin and dynorphin levels in the mossy fiber pathway, particularly within the hilus, and regionally specific changes in synaptic protein levels. 17β-estradiol, with or without progesterone, altered leu-enkephalin levels in the dentate gyrus and synaptophysin levels in the CA1 of young but not middle-aged or aged females. Additionally, 17β-estradiol decreased synaptophysin levels in the CA3 of middle-aged females. Our results support and extend previous findings indicating 17β-estradiol modulation of hippocampal opioid peptides and synaptic proteins while demonstrating regional and age-specific effects. Moreover, they lend credence to the "window of opportunity" hypothesis during which hormone replacement can modulate hippocampal structure and circuitry to improve cognitive outcomes.

Transcriptional effects of prenatal and neonatal exposure to PFOS in developing rat brain

Perfluorooctane sulfonate (PFOS), a persistent and bioaccumulative compound, is widely distributed in the environment. To explore the molecular mechanism of neonatal neurotoxic effects, we evaluated the transcriptional effects of prenatal and neonatal exposure to PFOS in developing rat brain by performing gene expression profiling in the cerebral cortex. Dams received 3.2 mg/kg PFOS in their feed from gestational day 1 (GD1) to weaning (PND 21). Pups then had free access to treated feed until PND 35. Six Illumina RatRef-12 Expression BeadChips were used to identify gene expression changes on postnatal days (PNDs) 1, 7, and 35. Significantly affected genes (P < 0.05) were involved in neuroactive ligand-receptor interaction, calcium signaling pathways, cell communication, long-term potentiation/depression, the cell cycle, and peroxisome proliferator-activated receptor (PPAR) signaling. To compare prenatal and lactational exposure contributions to transcriptional effects, a subset of altered genes obtained from the gene-profile study that represented neurobiological functions was analyzed using RT-PCR in a follow-up cross-foster study lasting from PND1 to 21. Prenatal and postnatal exposure to PFOS caused potential neurotoxicity as demonstrated by developmentally different global transcriptional changes. Prenatal exposure was more effective in altering expression of several genes. Also, transcriptional effects of PFOS exposure on neurodevelopment occurred primarily by disrupting the neuroendocrine system.

Three-dimensional properties of GnRH neuroterminals in the median eminence of young and old rats

The decapeptide gonadotropin-releasing hormone (GnRH), which regulates reproduction in all vertebrates, is stored in, and secreted from, large dense-core secretory vesicles in nerve terminals in the median eminence. GnRH is released from these terminals with biological rhythms that are critical for the maintenance of normal reproduction. During reproductive aging in female rats, there is a loss of GnRH pulses and a diminution of the GnRH surge. However, information about the specific role of GnRH nerve terminals is lacking, particularly in the context of aging. We sought to gain novel ultrastructural information about GnRH neuroterminals by performing three-dimensional (3D) reconstructions of GnRH neuroterminals and their surrounding microenvironment in the median eminence of young (4-5 months) and old (22-24 months) ovariectomized Sprague-Dawley female rats. Median eminence tissues were freeze-plunge embedded and serial ultrathin sections were collected on slot grids for immunogold labeling of GnRH immunoreactivity. Sequential images were used to create 3D models of GnRH terminals. These reconstructions provided novel perspectives into the morphological properties of GnRH terminals and their neural and glial environment. We also noted that the cytoarchitectural features of the median eminence became disorganized with aging. Quantitative measures showed a significant decrease in the apposition between GnRH terminal membranes and glial cells. Our data suggest reproductive aging in rats is characterized by structural organizational changes to the GnRH terminal microenvironment in the median eminence.

Gonadotropin-releasing hormone neuroterminals and their microenvironment in the median eminence: effects of aging and estradiol treatment

The GnRH decapeptide controls reproductive function through its release from neuroendocrine terminals in the median eminence, a site where there is a convergence of numerous nerve terminals and glial cells. Previous work showed dynamic changes in the GnRH-glial-capillary network in the median eminence under different physiological conditions. Because aging in rats is associated with a diminution of GnRH release and responsiveness to estradiol feedback, we examined effects of age and estradiol treatment on these anatomical interactions. Rats were ovariectomized at young (4 months), middle-aged (11 months), or old (22-23 months) ages, allowed 4 wk to recover, and then treated with vehicle or estradiol for 72 h followed by perfusion. Immunofluorescence of GnRH was measured, and immunogold electron microscopic analyses were performed to study the ultrastructural properties of GnRH neuroterminals and their microenvironment. Although the GnRH immunofluorescent signal showed no significant changes with age and estradiol treatment, we found that the median eminence underwent both qualitative and quantitative structural changes with age, including a disorganization of cytoarchitecture with aging and a decrease in the apposition of GnRH neuroterminals to glia with age and estradiol treatment. Thus, although GnRH neurons can continue to synthesize and transport peptide, changes in the GnRH neuroterminal-glial-capillary machinery occur during reproductive senescence in a manner consistent with a disconnection of these elements and a potential dysregulation of GnRH neurosecretion.

Susceptibility to Calcium Dysregulation during Brain Aging

Calcium (Ca(2+)) is a highly versatile intracellular signaling molecule that is essential for regulating a variety of cellular and physiological processes ranging from fertilization to programmed cell death. Research has provided ample evidence that brain aging is associated with altered Ca(2+) homeostasis. Much of the work has focused on the hippocampus, a brain region critically involved in learning and memory, which is particularly susceptible to dysfunction during senescence. The current review takes a broader perspective, assessing age-related changes in Ca(2+) sources, Ca(2+) sequestration, and Ca(2+) binding proteins throughout the nervous system. The nature of altered Ca(2+) homeostasis is cell specific and may represent a deficit or a compensatory mechanism, producing complex patterns of impaired cellular function. Incorporating the knowledge of the complexity of age-related alterations in Ca(2+) homeostasis will positively shape the development of highly effective therapeutics to treat brain disorders.

Age- and hormone-regulation of N-methyl-D-aspartate receptor subunit NR2b in the anteroventral periventricular nucleus of the female rat: implications for reproductive senescence

Glutamate, acting through its N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the hypothalamus, regulates reproductive neuroendocrine functions via direct and indirect actions upon gonadotrophin-releasing hormone (GnRH) neurones. Previous studies indicate that the NMDA receptor subunit NR2b undergoes changes in protein and gene expression in the hypothalamus in general, and on GnRH neurones in particular, during reproductive ageing. In the present study, we examined whether the NR2b-expressing cell population, both alone and in association with the NR1 subunit (i.e. the latter subunit is necessary for a functional NMDA receptor), is altered as a function of age and ⁄ or steroid hormone treatment. Studies focused on the anteroventral periventricular (AVPV) nucleus of the hypothalamus, a region critically involved in the control of reproduction. Young (3-5 months), middle-aged (9-12 months), and aged (approximately 22 months) female rats were ovariectomised and, 1 month later, they were treated sequentially with oestradiol plus progesterone, oestradiol plus vehicle, or vehicle plus vehicle, then perfused. Quantitative stereologic analysis of NR2b-immunoreactive cell numbers in the AVPV showed an age-associated decrease in the density of NR2b-immunoreactive cells, but no effect of hormone treatment. In a second study, immunofluorescent double labelling of NR2b and NR1 was analysed by confocal microscopy of fraction volume, a semi-quantitative measure of fluorescence intensity. No effect of ageing was detected for immunofluorescent NR1 or NR2b alone, whereas the NR2b fraction volume increased in the oestradiol plus vehicle group. With ageing, the fraction volume of the NR2b/NR1-colocalised subunits increased. Together with the stereology results, this suggests that, although fewer cells express the NR2b subunit in the ageing AVPV, a greater percentage of these subunits are co-expressed with NR1. Our results suggest that the subunit composition of NMDA receptors in the AVPV undergo both age- and hormonal-regulation, which may be related to previous observations of changes in functional responses of reproductive neuroendocrine systems to NMDA receptor modulators with ageing.

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.

Expression and activation of N-methyl-D-aspartate receptor subunit-1 receptor subunits in gonadotrophin-releasing hormone neurones of young and middle-aged mice during the luteinising hormone surge

Glutamate is an important excitatory neurotransmitter that stimulates the release of gonadotrophin-releasing hormone (GnRH) and participates in the generation of the luteinising hormone (LH) surge. To determine the mechanisms of action of glutamate and possible changes in the glutamatergic input to GnRH neurones during reproductive ageing, we measured the expression and activation of the mandatory N-methyl-D-aspartate receptor subunit-1 (NMDAR1) in GnRH neurones of young and middle-aged mice prior to and during a steroid-induced LH surge. The results show that, in young animals, approximately 55% of all GnRH neurones contain immunoreactive NMDAR1 protein and this percentage does not change during the day of the LH surge. In approximately 10% of the GnRH neurones, NMDA receptor protein is phosphorylated at Ser 890 prior to the surge, whereas, in approximately 55% of the GnRH neurones, NMDAR1 subunits are phosphorylated during the LH surge. Activation of NMDAR1 receptor subunits stimulates the calcium-calmodulin-kinase IV (CaMK IV). pathway, which leads to the translocation of CaMK IV into the nucleus where this enzyme can phosphorylate the cAMP response element-binding protein (CREB) and CREB-binding protein. We show that, in young animals, approximately 20% of the GnRH neurones contain CaMK IV in their nuclei 7 h prior to the LH surge; this percentage increases to 60% at the beginning of the surge and decreases to approximately 40% some 2 h into the LH surge. In middle-aged animals, approximately 25% of the GnRH neurones contain NMDAR1 protein and only 10% of the GnRH neurones contain phosphorylated NMDAR1 protein prior to and during the surge; however 2 h after the peak of the surge, 20% of the GnRH neurones contain phosphorylated NMDAR1 subunits. Similarly, 20% of GnRH neurones contain nuclear CaMK IV and this percentage does not change during the day of the LH surge. The results suggest that, in the young animal, glutamatergic innervation of GnRH neurones during the LH surge causes the activation and phosphorylation of NMDAR1 receptor subunits which results in the translocation of CaMK IV into the nucleus. However, both, the expression and activation of NMDAR1 receptor subunits are greatly reduced in the middle-aged animals, which could result in the absence of LH surges.

Sex differences in angiotensin signaling in bulbospinal neurons in the rat rostral ventrolateral medulla

Sex differences may play a significant role in determining the risk of hypertension. Bulbospinal neurons in the rostral ventrolateral medulla (RVLM) are involved in the tonic regulation of arterial pressure and participate in the central mechanisms of hypertension. Angiotensin II (ANG II) acting on angiotensin type 1 (AT(1)) receptors in RVLM neurons is implicated in the development of hypertension by activating NADPH oxidase and producing reactive oxygen species (ROS). Therefore, we analyzed RVLM bulbospinal neurons to determine whether there are sex differences in: 1) immunolabeling for AT(1) receptors and the key NADPH oxidase subunit p47 using dual-label immunoelectron microscopy, and 2) the effects of ANG II on ROS production and Ca(2+) currents using, respectively, hydroethidine fluoromicrography and patch-clamping. In tyrosine hydroxylase-positive RVLM neurons, female rats displayed significantly more AT(1) receptor immunoreactivity and less p47 immunoreactivity than male rats (P < 0.05). Although ANG II (100 nM) induced comparable ROS production in dissociated RVLM bulbospinal neurons of female and male rats (P > 0.05), an effect mediated by AT(1) receptors and NADPH oxidase, it triggered significantly larger dihydropyridine-sensitive long-lasting (L-type) Ca(2+) currents in female RVLM neurons (P < 0.05). These observations suggest that an increase in AT(1) receptors in female RVLM neurons is counterbalanced by a reduction in p47 levels, such that ANG II-induced ROS production does not differ between females and males. Since the Ca(2+) current activator Bay K 8644 induced larger Ca(2+) currents in females than in male RVLM neurons, increased ANG II-induced L-type Ca(2+) currents in females may result from sex differences in calcium channel densities or dynamics.

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.

Effects of perinatal polychlorinated biphenyls on adult female rat reproduction: development, reproductive physiology, and second generational effects

Perinatal exposures to endocrine-disrupting chemicals, such as polychlorinated biphenyls (PCBs), can cause latent effects on reproductive function. Here, we tested whether PCBs administered during late pregnancy would compromise reproductive physiology in both the fetally exposed female offspring (F1 generation), as well as in their female offspring (F2 generation). Pregnant Sprague-Dawley rats were treated with the PCB mixture, Aroclor 1221 (A1221; 0, 0.1, 1, or 10 mg/kg), on Embryonic Days 16 and 18. Somatic and reproductive development of F1 and their F2 female offspring were monitored, including ages of eye opening, pubertal landmarks, and serum reproductive hormones. The results showed that low doses of A1221 given during this critical period of neuroendocrine development caused differential effects of A1221 on F1 and F2 female rats. In both generations, litter sex ratio was skewed toward females. In the F1 generation, additional effects were found, including a significant alteration of serum LH in the 1 mg/kg A1221 group. The F2 generation showed more profound alterations, particularly with respect to fluctuations in hormones and reproductive tract tissues across the estrous cycle. On proestrus, the day of the preovulatory GnRH/gonadotropin surge, F2 females whose mothers had been exposed perinatally to A1221 exhibited substantially suppressed LH and progesterone concentrations, and correspondingly smaller uterine and ovarian weights on estrus, compared with F2 descendants of control rats. These latter changes suggest a dysregulation of reproductive physiology. Thus, low levels of exposure to PCBs during late fetal development cause significant effects on the maturation and physiology of two generations of female offspring. These findings have implications for reproductive health and fertility of wildlife and humans.

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.

Postpubertal decrease in hippocampal dendritic spines of female rats

Hippocampal dendritic spine and synapse numbers in female rats vary across the estrous cycle and following experimental manipulation of hormone levels in adulthood. Based on behavioral studies demonstrating that learning patterns are altered following puberty, we hypothesized that dendritic spine number in rat hippocampal CA1 region would change postpubertally. Female Sprague-Dawley rats were divided into prepubertal (postnatal day (P) 22), peripubertal (P35) and postpubertal (P49) groups, with the progression of puberty evaluated by vaginal opening, and estrous cyclicity subsequently assessed by daily vaginal smears. Spinophilin immunoreactivity in dendritic spines was used as an index of spinogenesis in area CA1 stratum radiatum (CA1sr) of hippocampus. First, electron microscopy analyses confirmed the presence of spinophilin specifically in dendritic spines of CA1sr, supporting spinophilin as a reliable marker of hippocampal spines in young female rats. Second, stereologic analysis was performed to assess the total number of spinophilin-immunoreactive puncta (i.e. spines) and CA1sr volume in developing rats. Our results indicated that the number of spinophilin-immunoreactive spines in CA1sr was decreased 46% in the postpubertal group compared to the two younger groups, whereas the volume of the hippocampus underwent an overall increase during this same developmental time frame. Third, to determine a potential role of estradiol in this process, an additional group of rats was ovariectomized (OVX) prepubertally at P22, then treated with estradiol or vehicle at P35, and spinophilin quantified as above in rats perfused on P49. No difference in spinophilin puncta number was found in OVX rats between the two hormone groups, suggesting that this developmental decrease is independent of peripheral estradiol. These changes in spine density coincident with puberty may be related to altered hippocampal plasticity and synaptic consolidation at this phase of maturity.

Age-Related Structural and Metabolic Changes in the Pelvic Reproductive End Organs

In this work, we provide preliminary data and a review of the literature regarding normal structural and functional changes that occur in the aging uterus, ovary, testicle, and prostate gland. It is expected that such knowledge will help physicians to distinguish physiologic changes from pathologic changes at an early stage. We retrospectively reviewed pelvic magnetic resonance imaging (MRI) scans of 131 female and 79 male subjects ages 13 to 86 years to determine changes in volume of the uterus, ovary, and prostate gland with age. Scrotal ultrasound examinations of 150 male subjects ages 0 to 96 years also were analyzed retrospectively to determine changes in testicular volume with age. In addition, (18)F-fluorodeoxyglucose positron emission tomography ((18)F-FDG-PET) scans of 145 male subjects ages 11 to 90 years were analyzed retrospectively to assess for changes in maximum standardized uptake value (SUV(max)) of the testicles with age. The uterus had a mean volume of 38.55 +/- 3.68 cm(3) at 17 to 19 years of age, increased to a peak volume of 71.76 +/- 19.81 cm(3) between 35 to 40 years, and then declined to 24.02 +/- 8.11 cm(3) by the eighth decade of life. The maximal ovarian volume per subject maintained a relatively stable size in early life, measuring 9.46 +/- 3.25 cm(3) during the second decade of life, 8.46 +/- 3.32 cm(3) in the mid-fourth decade of life, and 7.46 +/- 3.33 cm(3) at 45 years of age, after which it declined to 4.44 +/- 2.02 cm(3) by the late fifth decade of life. The ovaries were not identifiable on MRI in subjects beyond the sixth decade of life. The volume of the prostate increased from 23.45 +/- 6.20 cm(3) during the second decade of life to 47.5 +/- 41.59 cm(3) by the late eighth decade of life; the central gland of the prostate increased from 9.96 +/- 3.99 cm(3) to 29.49 +/- 28.88 cm(3) during the same age range. Mean testicular volume was 11.2 +/- 5.9 cm(3). Testicular volume increased with age from birth to 25 years. After age 25, there was a significant decline in the testicular volume. The mean SUV(max) for the testicles was 1.9 +/- 0.5. Testicular metabolic activity demonstrated an increasing trend until the age of 35 years. A plateau in SUV(max) was observed after the age of 35 years until the age of 65 years. A slight decrease in SUV(max) was observed after the age of 65 years. The pelvic structures of men and women change both structurally and functionally over the lifespan, and such changes can be quantified using ultrasound, MRI, and (18)F-FDG-PET.

Panic-prone state induced in rats with GABA dysfunction in the dorsomedial hypothalamus is mediated by NMDA receptors

Rats with chronic inhibition of GABA synthesis and consequently enhanced glutamatergic excitation in the dorsomedial hypothalamus (DMH) develop panic-like responses, defined as tachycardia, tachypnea, hypertension, and increased anxiety as measured by a social interaction (SI) test, after intravenous sodium lactate infusions, a phenomenon similar to patients with panic disorder. Therefore, the present studies tested the role of the postsynaptic NMDA and AMPA type glutamatergic receptors in the lactate-induced panic-like responses in these rats. Rats were fit with femoral arterial and venous catheters and Alzet pumps [filled with the GABA synthesis inhibitor L-allylglycine (L-AG; 3.5 nmol/0.5 microl per hour) or its inactive isomer D-AG] into the DMH. After 4-5 d of recovery only those rats with L-AG pumps exhibited panic-like responses to lactate infusions. Using double immunocytochemistry, we found that rats exhibiting panic-like responses (e.g., L-AG plus lactate) had increased c-Fos immunoreactivity in DMH neurons expressing the NMDA receptor 1 (NR1) subunit, but not those expressing the glutamate receptor 2 and 3 subunits of the AMPA receptors. To confirm this pharmacologically, we tested another group of rats implanted with l-AG pumps with intravenous lactate infusions preceded by injections of either NMDA [aminophosphonopentanoic acid (AP-5) or (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine maleate (MK-801)] or non-NMDA [CNQX or 4-(8-methyl-9H-1,3-dioxolo[4,5-h][2,3]benzodazepin-5-yl)-benzenamine dihydrochloride (GYKI52466)] antagonists into the DMH. Injections of NMDA, but not non-NMDA, antagonists into the DMH resulted in dose-dependent blockade of the tachycardia, tachypnea, hypertension, and SI responses after lactate infusions. These results suggest that NMDA, and not non-NMDA, type glutamate receptors regulate lactate-induced panic-like responses in rats with GABA dysfunction in the DMH.

Neuroendocrine control of reproductive aging: roles of GnRH neurons

The process of reproductive senescence in many female mammals, including humans, is characterized by a gradual transition from regular reproductive cycles to irregular cycles to eventual acyclicity, and ultimately a loss of fertility. In the present review, the role of the hypothalamic gonadotropin-releasing hormone (GnRH) neurons is considered in this context. GnRH neurons provide the primary driving force upon the other levels of the reproductive axis. With respect to aging, GnRH cells undergo changes in biosynthesis, processing and release of the GnRH decapeptide. GnRH neurons also exhibit morphologic and ultrastructural alterations that appear to underlie these biosynthetic properties. Thus, functional and morphologic changes in the GnRH neurosecretory system may play causal roles in the transition to acyclicity. In addition, GnRH neurons are regulated by numerous inputs from neurotransmitters, neuromodulators and glia. The relationship among GnRH cells and their inputs at the cell body (thereby affecting GnRH biosynthesis) and the neuroterminal (thereby affecting GnRH neurosecretion) is crucial to the function of the GnRH system, with age-related changes in these relationships contributing to the reproductive senescent process. Therefore, the aging hypothalamus is characterized by changes intrinsic to the GnRH cell, as well as its regulatory inputs, which summate to contribute to a loss of reproductive competence in aging females.

Perinatal androgenization prevents age-related neuron loss in the sexually dimorphic nucleus of the preoptic area in female rats

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.

Effects of Bak Foong Pills and Menoease Pills on white blood cell distribution in old age female rats

This study examined the effects of Bak Foong Pills (BFP) and the new BFP-derived post-menopause formula, Menoease Pills (MBFP), on the distribution of peripheral white blood cells (WBC) between BFP/MBFP-treated and non-treated rats. Eighteen months old female SD rats were used to mimic post-menopausal and old age animal models. The percentage distribution of lymphocytes, monocytes and granulocytes were measured using flow cytometry with and without treatments of BFP or MBFP. Results showed that WBC distribution in old age rats were significantly different from that of adult rats, suggesting that as the animal aged, their WBC distributions were altered. Old age rats were observed to have much lower percentages of lymphocytes, but higher percentages of granulocytes when compared to the adult rats, indicating possible attenuated immunity. Following treatment with BFP or MBFP, WBC populations were found to be redistributed back into the ranges observed in adult animals. Furthermore, MBFP, was found to alter WBC distribution in a dose-dependent manner. When compared to estrogen (E(2)), a well documented regulator of immune function, results showed that MBFP was able to show significantly greater effects on WBC redistribution compared to E(2). However, in ovariectomised (ovx) old age rats, neither MBFP nor E(2) treated groups showed any changes in WBC redistribution. These results indicate that MBFP may share similarities to E(2). Indeed, the effect of MBFP and E(2) seems to require intact ovaries, which are believed to be necessary for the modulation of WBC distributions and immune functions. Overall, our findings suggest that BFP and MBFP may be able to regulate WBC population in old age female rats, and thus, indicate their potential role on improving the attenuated immunity evident in post-menopausal and elderly women.

Menopausal increases in pulsatile gonadotropin-releasing hormone release in a nonhuman primate (Macaca mulatta)

Reproductive function in all vertebrates is controlled by the circhoral release of the neuropeptide, GnRH, into the portal capillary system leading to the anterior pituitary. Despite its primary role in sexual maturation and the maintenance of adult reproductive function, changes in the concentrations and pattern of GnRH release have not yet been reported in any primate species during the menopausal transition and postmenopause. Such knowledge is essential for ascertaining both the mechanisms for, and consequences of, the menopausal process. Here we used a push-pull perfusion method to measure and compare the parameters of pulsatile GnRH release in adult rhesus monkeys at 8.4 +/- 1.5 yr (young adult females, early follicular phase, n = 6) and 28.8 +/- 0.3 yr (aged females, n = 4, of which two monkeys were in the menopausal transition, and two were postmenopausal). Our results demonstrate that: 1) GnRH release is pulsatile in both young and aged monkeys; 2) mean concentrations of GnRH increase during reproductive aging; and 3) GnRH pulse frequency does not differ between aged monkeys and young monkeys in the early follicular phase. We conclude that not only do GnRH neurons have the continued capacity to release GnRH in a pulsatile manner but also they can do so with enhanced GnRH levels in aged primates. To our knowledge, this is the first direct demonstration of elevated pulsatile GnRH concentrations in a primate species during reproductive senescence, a result that may have implications for menopausal symptoms.

Stereologic analysis of estrogen receptor alpha (ER alpha) expression in rat hypothalamus and its regulation by aging and estrogen

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.

Direct and indirect regulation of gonadotropin-releasing hormone neurons by estradiol

Estrogen signaling to GnRH neurons is critical for coordinating the preovulatory surge release of LH with follicular maturation. Until recently it was thought that estrogen signaled GnRH neurons only indirectly through numerous afferent systems. This minireview presents new evidence indicating that GnRH neurons are directly regulated by estradiol (E2), primarily through estrogen receptor (ER)-beta, and indirectly through E2-sensitive neurons in the anteroventral periventricular (AVPV) region. The data described suggest that E2 generally represses GnRH gene expression but that this repression is transiently overcome by indirect E2-dependent signals relayed by AVPV neurons. We also present evidence that the AVPV neurons responsible for relaying E2 signals to GnRH neurons are multifunctional gamma aminobutyric acid-ergic/glutamatergic/neuropeptidergic neurons.

Age-related changes in estrogen receptor beta in rat hypothalamus: a quantitative analysis

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.

Functional analysis of the rat N-methyl-D-aspartate receptor 2A promoter: multiple transcription starts points, positive regulation by Sp factors, and translational regulation

N-Methyl-d-aspartate (NMDA) receptor subunit 2A (NR2A) is an important modulatory component of the NMDA subtype of glutamate receptors. To investigate the transcription mechanism of the NR2A gene, we cloned the 5'-flanking sequence from a rat genomic library. RNA mapping with rat brain RNA revealed two sets of major and several minor transcription start points in a single exon of 1140 bp. Reporter gene and mutation studies indicated that core promoter activity resided in exon 1, whereas the 5'-flanking sequence up to 1.5 kb showed no significant impact on promoter activity. Fragments containing minor transcription start points were able to drive a reporter gene in transfected cells and produce nascent RNAs in an in vitro transcription system. All fragments tested showed more promoter activity in dissociated neurons of the rat embryonic cerebrocortex and cell lines expressing NR2A mRNA than that in glial cultures and non-neuronal cells. Within exon 1 there are three GC-box elements that displayed distinct binding affinity to both Sp1- and Sp4-like factors. Overexpression of Sp1 or Sp4, but not Sp3, significantly increased the activity of the promoter containing these elements. Inclusion of exon 2 and 3 sequences, which contain five short open-reading frames, attenuated promoter-driven reporter activity more than 3-fold but attenuated the level of reporter mRNA less than 1.4-fold. Our results suggest that the core promoter of the rat NR2A gene requires exon 1, that Sp factors positively regulate this core promoter, and that a post-transcriptional mechanism may negatively regulate expression of the gene.

Role of Na+ and Ca2+ channels in the preoptic LH surge generating mechanism in proestrous rats

We studied whether Na+ and Ca2+ channels are involved in the neural mechanism responsible for the surge of gonadotropin-releasing hormone (GnRH) in proestrous rats. In experiment 1, female rats in proestrus were i.p. injected at 1345 h with pentobarbital sodium (35 mg/kg) to block spontaneous surge of LH and electrical stimulation was applied between 1400 and 1600 h to the preoptic area (POA) together with POA injection of 0.5 microl saline containing the Na+ channel blocker tetrodotoxin (TTX) at a concentration of 1 microM, 2 microM, or 5 microM. Since 5 microM TTX completely blocked the increase in serum LH concentrations evoked by the POA stimulation, we used this concentration in experiment 2 to observe the TTX effect on the spontaneous LH surge. In experiment 2, bilateral injections of 1.5 microl of 5 microM TTX at 1430 h in the POA in proestrous rats postponed the peak time and reduced the peak level of the LH surge. In experiment 3, bilateral injections of 1.5 microl of 5 microM L-type Ca2+ channel blocker nifedipine at 1430 h in the POA completely blocked the LH surge. Since the cell bodies of GnRH neurons are primarily concentrated in the POA in rats, these results suggest that both voltage-sensitive Na+ channels and Ca2+ channels contribute to the generation of action potentials at GnRH cell bodies for the surge release of GnRH.

Colocalization and hormone regulation of estrogen receptor alpha and N-methyl-D-aspartate receptor in the hypothalamus of female rats

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.