LHRH neurons in the female C57BL/6J mouse brain during reproductive aging: no loss up to middle age

Disruption of Intranasal GnRH Neuronal Migration Route into the Brain Induced by Proinflammatory Cytokine IL-6: Ex Vivo and In Vivo Rodent Models

Maternal immune activation results in altered levels of cytokines in the maternal-fetal system, which has a negative impact on fetal development, including the gonadotropin-releasing hormone (GnRH) system, which is crucial for the reproduction. Suppression of GnRH-neuron migration may be associated with cytokine imbalances, and primarily with proinflammatory cytokine interleukin (IL)-6. This study aimed to determine the effects of IL-6 and monoclonal antibody to IL-6 or IL-6R or polyclonal IgG on the formation of migration route of GnRH-neurons in ex vivo and in vivo rodent models on day 11.5 of embryonic development. The increased level of IL-6 in mouse nasal explants suppressed peripherin-positive fiber outgrowth, while this led to an increase in the number of GnRH-neurons in the nose and olfactory bulbs and a decrease in their number in the fetal brain. This effect is likely to be realized via IL-6 receptors along the olfactory nerves. The suppressive effect of IL-6 was diminished by monoclonal antibodies to IL-6 or its receptors and by IgG.

The initiation and maintenance of gonadotropin-releasing hormone neuron identity in congenital hypogonadotropic hypogonadism

Neurons that secrete gonadotropin-releasing hormone (GnRH) drive vertebrate reproduction. Genetic lesions that disrupt these neurons in humans lead to congenital hypogonadotropic hypogonadism (CHH) and reproductive failure. Studies on CHH have largely focused on the disruption of prenatal GnRH neuronal migration and postnatal GnRH secretory activity. However, recent evidence suggests a need to also focus on how GnRH neurons initiate and maintain their identity during prenatal and postnatal periods. This review will provide a brief overview of what is known about these processes and several gaps in our knowledge, with an emphasis on how disruption of GnRH neuronal identity can lead to CHH phenotypes.

Highlights of neuroanatomical discoveries of the mammalian gonadotropin-releasing hormone system

The anatomy and morphology of gonadotropin-releasing hormone (GnRH) neurons makes them both a joy and a challenge to investigate. They are a highly unique population of neurons given their developmental migration into the brain from the olfactory placode, their relatively small number, their largely scattered distribution within the rostral forebrain, and, in some species, their highly varied individual anatomical characteristics. These unique features have posed technological hurdles to overcome and promoted fertile ground for the establishment and use of creative approaches. Historical and more contemporary discoveries defining GnRH neuron anatomy remain critical in shaping and challenging our views of GnRH neuron function in the regulation of reproductive function. We begin this review with a historical overview of anatomical discoveries and developing methodologies that have shaped our understanding of the reproductive axis. We then highlight significant discoveries across specific groups of mammalian species to address some of the important comparative aspects of GnRH neuroanatomy. Lastly, we touch on unresolved questions and opportunities for future neuroanatomical research on this fascinating and important population of neurons.

Lactate-Dependent Cross-Talk Between Astrocyte and GnRH-I Neurons in Hypothalamus of Aged Brain: Decreased GnRH-I Transcription

GnRH-I produced by hypothalamic neurosecretory cells is considered a master regulator of mammalian reproduction. Although GnRH-I transcription is well studied, the effect of ageing on transcriptional regulation of GnRH-I has not yet been explored. Here, we elucidate the effects of ageing on the metabolic environment like lactate level and TNF-α and how these affect GnRH-I transcription. Using pathway analysis of transcriptomic data, we found that lactate is upregulated in ageing astrocytes due to the downregulation of cellular respiration pathways possibly resulting in greater pyruvate concentration for lactate production. This lactate could then be shuttled into neurons where it would affect GnRH-I transcription. We showed that supra-physiological level of lactate in young mouse brain can mimic metabolic disturbances in the old brain and cause downregulation in GnRH-I transcription at a young age. In particular, we found upregulation of GnRH-I repressors in the young brain treated with high levels of lactate similar to old brain. Hence, this confirmed that aged metabolic environment can affect GnRH-I transcription even in the young brain. Further downstream analysis using the TRUST database showed NF-Kb signalling which lies downstream of both lactate and TNF-α as being capable of upregulating GnRH-I repressors. Since NF-Kb signalling has been shown in our study as well as others to be induced by TNF-α during ageing, it is likely that GnRH-I transcriptional regulation is mediated through these pathways. Thus, we formed a model for explaining the downregulation of GnRH-I transcription during ageing through differential expression of its TFs in an aged metabolic environment.

Gene expression profiling of puberty-associated genes reveals abundant tissue and sex-specific changes across postnatal development

The timing of human puberty is highly variable, sexually dimorphic, and associated with adverse health outcomes. Over 20 genes carrying rare mutations have been identified in known pubertal disorders, many of which encode critical components of the hypothalamic-pituitary-gonadal (HPG) axis. Recent genome-wide association studies (GWAS) have identified more than 100 candidate genes at loci associated with age at menarche or voice breaking in males. We know little about the spatial, temporal or postnatal expression patterns of the majority of these puberty-associated genes. Using a high-throughput and sensitive microfluidic quantitative PCR strategy, we profiled the gene expression patterns of the mouse orthologs of 178 puberty-associated genes in male and female mouse HPG axis tissues, the pineal gland, and the liver at five postnatal ages spanning the pubertal transition. The most dynamic gene expression changes were observed prior to puberty in all tissues. We detected known and novel tissue-enhanced gene expression patterns, with the hypothalamus expressing the largest number of the puberty-associated genes. Notably, over 40 puberty-associated genes in the pituitary gland showed sex-biased gene expression, most of which occurred peri-puberty. These sex-biased genes included the orthologs of candidate genes at GWAS loci that show sex-discordant effects on pubertal timing. Our findings provide new insight into the expression of puberty-associated genes and support the possibility that the pituitary plays a role in determining sex differences in the timing of puberty.

Progress and Challenges in the Search for the Mechanisms of Pulsatile Gonadotropin-Releasing Hormone Secretion

Fertility relies on the proper functioning of the hypothalamic-pituitary-gonadal axis. The hormonal cascade begins with hypothalamic neurons secreting gonadotropin-releasing hormone (GnRH) into the hypophyseal portal system. In turn, the GnRH-activated gonadotrophs in the anterior pituitary release gonadotropins, which then act on the gonads to regulate gametogenesis and sex steroidogenesis. Finally, sex steroids close this axis by feeding back to the hypothalamus. Despite this seeming straightforwardness, the axis is orchestrated by a complex neuronal network in the central nervous system. For reproductive success, GnRH neurons, the final output of this network, must integrate and translate a wide range of cues, both environmental and physiological, to the gonadotrophs via pulsatile GnRH secretion. This secretory profile is critical for gonadotropic function, yet the mechanisms underlying these pulses remain unknown. Literature supports both intrinsically and extrinsically driven GnRH neuronal activity. However, the caveat of the techniques supporting either one of the two hypotheses is the gap between events recorded at a single-cell level and GnRH secretion measured at the population level. This review aims to compile data about GnRH neuronal activity focusing on the physiological output, GnRH secretion.

Prenatal programming of neuroendocrine reproductive function

It is now well recognized that the gestational environment can have long-lasting effects not only on the life span and health span of an individual but also, through potential epigenetic changes, on future generations. This article reviews the "prenatal programming" of the neuroendocrine systems that regulate reproduction, with a specific focus on the lessons learned using ovine models. The review examines the critical roles played by steroids in normal reproductive development before considering the effects of prenatal exposure to exogenous steroid hormones including androgens and estrogens, the effects of maternal nutrition and stress during gestation, and the effects of exogenous chemicals such as alcohol and environment chemicals. In so doing, it becomes evident that, to maximize fitness, the regulation of reproduction has evolved to be responsive to many different internal and external cues and that the GnRH neurosecretory system expresses a degree of plasticity throughout life. During fetal life, however, the system is particularly sensitive to change and at this time, the GnRH neurosecretory system can be "shaped" both to achieve normal sexually differentiated function but also in ways that may adversely affect or even prevent "normal function". The exact mechanisms through which these programmed changes are brought about remain largely uncharacterized but are likely to differ depending on the factor, the timing of exposure to that factor, and the species. It would appear, however, that some afferent systems to the GnRH neurons such as kisspeptin, may be critical in this regard as it would appear to be sensitive to a wide variety of factors that can program reproductive function. Finally, it has been noted that the prenatal programming of neuroendocrine reproductive function can be associated with epigenetic changes, which would suggest that in addition to direct effects on the exposed offspring, prenatal programming could have transgenerational effects on reproductive potential.

The menopause and aging, a comparative perspective

The neuroendocrinology of menopause is reviewed from a comparative perspective, with emphasis on laboratory rodent models. These changes are compared by the 2011 STRAW criteria (Stages of Reproductive Aging Workshop). Ovarian cell loss begins prenatally in all mammals studied, with exponential depletion of primary follicles and oocytes in association with loss of fecundity by midlife. Rodents and humans also share progressively increasing irregularity in ovulatory cycles and increasing fetal aneuploidy as oocyte depletion become imminent. Hypothalamic impairments of the estrogen-induced surge of pituitary gonadotrophins (luteinizing hormone, LH; follicle stimulating hormone, FSH) are prominent in middle-aged rodents, but sporadic in peri-menopausal women. In aging rodents, hypothalamic impairments of the LH surge have been experimentally associated with prolonged phases of sustained estradiol (E2) and very low progesterone (P4) ('unopposed estradiol'). Although peri-menopausal women also show hyper-estrogenic cycles, there is no indication for irreversible hypothalamic desensitization by E2. Ongoing cognitive assessments in clinical trials of estrogen therapy with and without P4 or other progestins may further inform about possible persisting effects of unopposed estrogens.This article is part of a Special Issue entitled 'Menopause'.

GnRH neurons of young and aged female rhesus monkeys co-express GPER but are unaffected by long-term hormone replacement

Menopause is caused by changes in the function of the hypothalamic-pituitary-gonadal axis that controls reproduction. Hypophysiotropic gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus orchestrate the activity of this axis and are regulated by hormonal feedback loops. The mechanisms by which GnRH responds to the primary regulatory sex steroid hormone, estradiol (E2), are still poorly understood in the context of menopause. Our goal was to determine whether the G protein-coupled estrogen receptor (GPER) is co-expressed in adult primate GnRH neurons and whether this changes with aging and/or E2 treatment. We used immunofluorescence double-labeling to characterize the co-expression of GPER in GnRH perikarya and terminals in the hypothalamus. Young and aged rhesus macaques were ovariectomized and given long-term (~2-year) hormone treatments (E2, E2 + progesterone, or vehicle) selected to mimic currently prescribed hormone replacement therapies used for the alleviation of menopausal symptoms in women. We found that about half of GnRH perikarya co-expressed GPER, while only about 12% of GnRH processes and terminals in the median eminence (ME) were double-labeled. Additionally, many GPER-labeled processes were in direct contact with GnRH neurons, often wrapped around the perikarya and processes and in close proximity in the ME. These results extend prior work by showing robust co-localization of GPER in GnRH in a clinically relevant model, and they support the possibility that GPER-mediated E2 regulation of GnRH occurs both in the soma and terminals in nonhuman primates.

Hypothalamic IGF-I gene therapy prolongs estrous cyclicity and protects ovarian structure in middle-aged female rats

There is substantial evidence that age-related ovarian failure in rats is preceded by abnormal responsiveness of the neuroendocrine axis to estrogen positive feedback. Because IGF-I seems to act as a permissive factor for proper GnRH neuronal response to estrogen positive feedback and considering that the hypothalamic content of IGF-I declines in middle-aged (M-A) rats, we assessed the effectiveness of long-term IGF-I gene therapy in the mediobasal hypothalamus (MBH) of M-A female rats to extend regular cyclicity and preserve ovarian structure. We used 3 groups of M-A rats: 1 group of intact animals and 2 groups injected, at 36.2 weeks of age, in the MBH with either a bicistronic recombinant adeno-associated virus (rAAV) harboring the genes for IGF-I and the red fluorescent protein DsRed2, or a control rAAV expressing only DsRed2. Daily vaginal smears were taken throughout the study, which ended at 49.5 weeks of age. We measured serum levels of reproductive hormones and assessed ovarian histology at the end of the study. Although most of the rats injected with the IGF-I rAAV had, on the average, well-preserved estrous cyclicity as well as a generally normal ovarian histology, the intact and control rAAV groups showed a high percentage of acyclic rats at the end of the study and ovaries with numerous enlarged cysts and scarce corpora lutea. Serum LH was higher and hyperprolactinemia lower in the treated animals. These results suggest that overexpression of IGF-I in the MBH prolongs normal ovarian function in M-A female rats.

Age increase of estrogen receptor-α (ERα) in cortical astrocytes impairs neurotrophic support in male and female rats

Rodent models show decreased neuronal responses to estradiol (E2) during aging (E2-desensitization) in association with reduced neuronal estrogen receptor (ER)-α, but little is known about age changes of E2-dependent astrocytic neurotrophic support. Because elevated expression of astrocyte glial fibrillary acidic protein (GFAP) is associated with impaired neurotrophic activity and because the GFAP promoter responds to ERα, we investigated the role of astrocytic ERα and ERβ in impaired astrocyte neurotrophic activity during aging. In vivo and in vitro, ERα was increased greater than 50% with age in astrocytes from the cerebral cortex of male rats (24 vs 3 months), whereas ERβ did not change. In astrocytes from 3-month-old males, experimentally increasing the ERα to ERβ ratio induced the aging phenotype of elevated GFAP and impaired E2-dependent neurite outgrowth. In 24-month-old male astrocytes, lowering ERα reversed the age elevation of GFAP and partially restored E2-dependent neurite outgrowth. Mixed glia (astrocytes to microglia, 3:1) of both sexes also showed these age changes. In a model of perimenopause, mixed glia from 9- to 15-month rats showed E2 desensitization: 9-month regular cyclers retained young-like ERα to ERβ ratios and neurotrophic activity, whereas 9-month noncyclers had elevated ERα and GFAP but low E2-dependent neurotrophic activity. In vivo, ERα levels in cortical astrocytes were also elevated. The persisting effects of ovarian acyclicity in vitro are hypothesized to arise from steroidal perturbations during ovarian senescence. These findings suggest that increased astrocyte ERα expression during aging contributes to the E2 desensitization of the neuronal responses in both sexes.

Opposite-sex housing reactivates the declining GnRH system in aged transgenic male mice with FGF signaling deficiency

The continued presence of gonadotropin-releasing hormone (GnRH) neurons is required for a healthy reproductive lifespan, but factors that maintain postnatal GnRH neurons have not been identified. To begin to understand these factors, we investigated whether 1) fibroblast growth factor (FGF) signaling and 2) interactions with the opposite sex are involved in the maintenance of the postnatal GnRH system. A transgenic mouse model (dnFGFR mouse) with the targeted expression of a dominant-negative FGF receptor (dnFGFR) in GnRH neurons was used to examine the consequence of FGF signaling deficiency on postnatal GnRH neurons. Male dnFGFR mice suffered a significant loss of postnatal GnRH neurons within the first 100 days of life. Interestingly, this loss was reversed after cohabitation with female, but not male, mice for 300-550 days. Along with a rescue in GnRH neuron numbers, opposite-sex housing in dnFGFR males also increased hypothalamic GnRH peptide levels, promoted a more mature GnRH neuronal morphology, facilitated litter production, and enhanced testicular morphology. Last, mice hypomorphic for FGFR3 exhibited a similar pattern of postnatal GnRH neuronal loss as dnFGFR males, suggesting FGF signaling acts, in part, through FGFR3 to enhance the maintenance of the postnatal GnRH system. In summary, we have shown that FGF signaling is required for the continued presence of postnatal GnRH neurons. However, this requirement is not absolute, since sexual interactions can compensate for defects in FGFR signaling, thereby rescuing the declining GnRH system. This suggests the postnatal GnRH system is highly plastic and capable of responding to environmental stimuli throughout adult life.

Culturing embryonic nasal explants for developmental and physiological study

Primary cultures obtained from embryonic nasal placodes can maintain olfactory neurons, olfactory ensheathing cells, and large numbers of gonadotropin releasing hormone-1 (GnRH) neurons. Depending on the age of the starting material, one can examine cell interactions important for placode formation or neuronal migration and axonal outgrowth. When generated at E11.5 in mouse, neuronal migration and axon outgrowth away from the main tissue mass occurs. This area of the explant, the periphery, is only a few cells thick. This characteristic offers the opportunity to image single cells and axons and allows pharmacological and molecular manipulations as well as physiological recordings to be performed. Here, we describe a system for culturing nasal explants used in our laboratory. This model system provides a method for obtaining physiological cellular responses with post hoc immunohistochemistry, or gene expression studies, on cells arising from the nasal placode.

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.

Physiology of the gonadotrophin-releasing hormone (GnRH) neurone: studies from embryonic GnRH neurones

Gonadotrophin-releasing hormone (GnRH)-secreting neurones are the final output of the central nervous system driving fertility in all mammals. Although it has been known for decades that the efficiency of communication between the hypothalamus and the pituitary depends on the pulsatile profile of GnRH secretion, how GnRH neuronal activity is patterned to generate pulses at the median eminence is unknown. To date, the scattered distribution of the GnRH cell bodies remains the main limitation to assessing the cellular events that could lead to pulsatile GnRH secretion. Taking advantage of the unique developmental feature of GnRH neurones, the nasal explant model allows primary GnRH neurones to be maintained within a micro-network where pulsatile secretion is preserved and where individual cellular activity can be monitored simultaneously across the cell population. This review summarises the data obtained from work using this in vitro model, and brings some insights into GnRH cellular physiology.

Fibroblast growth factor signaling in the developing neuroendocrine hypothalamus

Fibroblast growth factor (FGF) signaling is pivotal to the formation of numerous central regions. Increasing evidence suggests FGF signaling also directs the development of the neuroendocrine hypothalamus, a collection of neuroendocrine neurons originating primarily within the nose and the ventricular zone of the diencephalon. This review outlines evidence for a role of FGF signaling in the prenatal and postnatal development of several hypothalamic neuroendocrine systems. The emphasis is placed on the nasally derived gonadotropin-releasing hormone neurons, which depend on neurotrophic cues from FGF signaling throughout the neurons' lifetime. Although less is known about neuroendocrine neurons derived from the diencephalon, recent studies suggest they also exhibit variable levels of dependence on FGF signaling. Overall, FGF signaling provides a broad spectrum of cues that ranges from genesis, cell survival/death, migration, morphological changes, to hormone synthesis in the neuroendocrine hypothalamus. Abnormal FGF signaling will deleteriously impact multiple hypothalamic neuroendocrine systems, resulting in the disruption of diverse physiological functions.

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.

Role of fibroblast growth factor signaling in gonadotropin-releasing hormone neuronal system development

There is growing evidence demonstrating that fibroblast growth factor (FGF) signaling is important for the development of the gonadotropin-releasing hormone (GnRH) neuronal system. In humans, loss-of-function mutations in FGF receptor 1 (Fgfr1) and Fgf8 lead to hypogonadotropic hypogonadism (HH) with or without anosmia. Insights into how FGF signaling deficiency disrupts the GnRH system in humans are beginning to emerge from studies using transgenic mouse models. In this review, we summarize GnRH system defects in several lines of FGF signaling-deficient mice. We showed that FGF signaling is critically required for olfactory placode induction, differentiation, and GnRH neuronal fate specification and postnatal maintenance. Extrapolating from these transgenic mouse data, we suggest that idiopathic HH in patients harboring loss-of-function Fgfr1 and/or Fgf8 mutations is not merely a result of defective GnRH neuronal migration, but also insults accumulated in the GnRH system during fate specification and postnatal development.

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.

Age affects spontaneous activity and depolarizing afterpotentials in isolated gonadotropin-releasing hormone neurons

Neuronal activity underlying the pulsatile secretion of GnRH remains poorly understood, as does the endogenous generation of such activity. It is clear that changes at the level of the hypothalamus are taking place during reproductive aging, yet virtually nothing is known about GnRH neuronal physiology in aging and postreproductive animals. In these studies, we performed cell-attached and whole-cell recordings in GnRH-enhanced green fluorescent protein neurons dissociated from young (3 months), middle-aged (10 months), and old (15-18 months) female mice. All mice were ovariectomized; half were estradiol replaced. Neurons from all ages fired spontaneously, most in a short-burst pattern that is characteristic of GnRH neuronal firing. Membrane characteristics were not affected by age. However, firing frequency was significantly reduced in neurons from old animals, as was spike patterning. The amplitude of the depolarizing afterpotential, evoked by a 200-msec current pulse, was significantly smaller in aged animals. In addition, inward whole-cell currents were reduced in estradiol-treated animals, although they were not significantly affected by age. Because depolarizing afterpotentials have been shown to contribute to prolonged discharges of activity after a very brief excitatory input, a decreased depolarizing afterpotential could lead to attenuated pulses in older animals. In addition, decreases in frequency and pattern generation could lead to improper information coding. Therefore, changes in the GnRH neuron during aging could lead to dysregulated activity, potentially resulting in the attenuated LH pulses observed in the transition to reproductive senescence.

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.

Modulation of estrogen receptors during development inhibits neurogenesis of precursors to GnRH-1 neurones: in vitro studies with explants of ovine olfactory placode

The aim of the present study was to explore the putative effects of agonists and antagonists of the estradiol receptor on the early phase of GnRH-1 neuron development. To address this question we used an in vitro model of GnRH-1 neurons using cultured olfactory placode from sheep embryos on day 26 of gestation. Previous studies on this model have shown that in vitro the development of GnRH-1 neurons mimics in vivo development up to the start of pulsatile GnRH-1 secretion, To address the effects of modulating the estrogen receptor, cultures were treated with the endogenous and synthetic ligands of estradiol receptors: 17beta-estradiol, 17alpha-estradiol and tamoxifen. Neurogenesis was measured by incorporation of [(3)H]-thymidine. Morphometric parameters were evaluated by image analysis. The main results are that antagonism of estradiol receptors induced an important decrease in neurogenesis but had little effect on morphometric parameters, suggesting that during this early phase of development, maternal estrogens are important to achieve correct development of the GnRH-1 neuronal network.

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.

The aging reproductive neuroendocrine axis

It is well known that the reproductive system is one of the first biological systems to show age-related decline. While depletion of ovarian follicles clearly relates to the end of reproductive function in females, evidence is accumulating that a hypothalamic defect is critical in the transition from cyclicity to acyclicity. This minireview attempts to present a concise review on aging of the female reproductive neuroendocrine axis and provide thought-provoking analysis and insights into potential future directions for this field. Evidence will be reviewed, which shows that a defect in pulsatile and surge gonadotropin hormone-releasing hormone (GnRH) secretion exists in normal cycling middle-aged female rats, which is thought to explain the significantly attenuated pulsatile and surge luteinizing hormone (LH) secretion at middle-age. Evidence is also presented, which supports the age-related defect in GnRH secretion as being due to a reduced activation of GnRH neurons. Along these lines, stimulation of GnRH secretion by the major excitatory transmitter glutamate is shown to be significantly attenuated in middle-aged proestrous rats. Corresponding age-related defects in other major excitatory regulatory factors, such as catecholamines, neuropeptide Y, and astrocytes, have also been demonstrated. Age-related changes in hypothalamic concentrations of neurotransmitter receptors, steroid receptors, and circulating steroid hormone levels are also reviewed, and discussion is presented on the complex interrelationships of the hypothalamus-pituitary-ovarian (HPO) axis during aging, with attention to how a defect in one level of the axis can induce defects in other levels, and thereby potentiate the dysfunction of the entire HPO axis.

In situ GABAergic modulation of synchronous gonadotropin releasing hormone-1 neuronal activity

Evidence indicates that gonadotropin releasing hormone-1 [GnRH-1, also known as luteinizing hormone releasing hormone (LHRH)] neurons can exhibit synchronized neuroendocrine secretory activity before entrance into the CNS. In this study, we used calcium imaging to evaluate patterns of activity in individual, embryonic, GnRH-1 neurons as well as population dynamics of GnRH-1 neurons in mouse nasal explants maintained for 1 versus 3 weeks. Independent of age, GnRH-1 neurons displayed significant calcium peaks that synchronized at an interval of approximately 20 min across multiple GnRH-1 cells within an explant. Acute tetrodotoxin treatment decreased the amplitude of calcium peaks in individual GnRH-1 neurons and the duration but not the frequency of synchronized activity in the population of GnRH-1 neurons. Acute GABA(B) receptor antagonism increased the frequency of synchronized neuronal activity at both ages, whereas acute GABA(A) receptor antagonism decreased calcium oscillations in individual GNRH-1 cells as well as synchronization of the calcium pulses within the GnRH-1 population at the 1 week time point to background non-GNRH-1 cell levels. These results indicate that developing GnRH-1 neurons rely heavily on GABAergic signaling to initiate synchronized bouts of activity but thereafter, possess an innate capacity for synchronized activity patterns that are modulated by, but not completely dependent on GABAergic signaling.

Aging and increased hypothalamic glial fibrillary acid protein (GFAP) mRNA in F344 female rats. Dissociation of GFAP inducibility from the luteinizing hormone surge

During reproductive aging, female rodents show impaired inducibility of the estradiol (E2)-induced (preovulatory) surge of luteinizing hormone (LH), which is associated with hypothalamic neuronal impairments of aging. To evaluate if astrocytes show comparable age changes, we analyzed the regulation of glial fibrillary acidic protein (GFAP) mRNA which is transiently increased in the arcuate nucleus in association with the proestrus LH surge in young rats. In aging (18-month-old) F344 rats in persistent estrus, the loss of the E2-induced LH surge was paralleled by the lack of increased GFAP mRNA in the arcuate nucleus (in situ hybridization with X-ray film). We then tested the hypothesis that restoration of the LH surge by chronic ovariectomy (OVX) in aging rats would restore or block GFAP mRNA induction, respectively. Despite restoration of the inducible LH surge in aging rats by chronic OVX, there was no induction of GFAP mRNA in the arcuate nucleus. Moreover, young rats given chronic E2 implants for 6 weeks as a model for persistent estrus, retained induction of arcuate nucleus GFAP mRNA, despite loss of the induced LH surge. The aging rats were highly sensitive to gross pituitary enlargement from chronic E2 with 50% mortality; thus, the F344 genotype is not optimum for studies of aging-E2 interactions that require prolonged E2 treatments. More detailed cellular level analysis of GFAP mRNA is needed to define the relationship of GFAP expression to synaptic reorganization during the LH surge. The aging rats also showed higher levels of GFAP mRNA in the arcuate nucleus and ventromedial nucleus, consistent with the general trend for elevated GFAP mRNA during aging in other brain regions and in both sexes.

Parallel declines in Fos activation of the medial anteroventral periventricular nucleus and LHRH neurons in middle-aged rats

The middle-age decline in reproductive function is manifested by reduced LHRH release, resulting in a decreased magnitude and delay of onset of the LH surge. Earlier studies suggested that the reductions in LHRH neural activation in middle-aged rats resulted from deficits in the afferent drive to the LHRH neurons. One critical afferent to the LHRH neurons lies in the anteroventral periventricular preoptic area (AVPv) nucleus. The neurons of the medial AVPv are synchronously activated to express Fos with LHRH neurons at the time of an LH surge in young adult animals. The present study examined whether, in middle age, reductions in the activation of AVPv neurons accompany the reduction in Fos activation in LHRH neurons. Young (3- to 4-month-old) and middle-aged (10- to 12-month-old) spontaneously cycling and ovariectomized steroid-replaced rats were killed during peak and early descending phase of the LH surge, and their brains were examined for Fos in LHRH and AVPv neurons. Young animals had a characteristic increase in Fos expression in both LHRH and AVPv neurons. In middle-aged rats, the proportion of LHRH neurons expressing Fos at the time of an LH surge was reduced by approximately 50%, irrespective of whether surges were spontaneous or induced by exogenous steroids. A similar reduction in the number of Fos+ cells (by approximately 50%) was noted in the medial AVPv. Linear regression analysis of the relationship between the extent of Fos activation in LHRH and AVPv neurons revealed a strong positive correlation (r(2) = 0.66; P < 0.01), suggesting that changes in the AVPv's drive to LHRH neurons underlie the decrease in LHRH activity in middle age. A second series of experiments examined whether decreased input from the AVPv could account for reduced Fos activation in LHRH neurons seen in middle-aged animals. When the medial AVPv was lesioned, LHRH neurons failed to express Fos on the side ipsilateral to the lesion. Animals with lesioned medial AVPv also had significantly lower LH values than animals with an intact medial AVPv. Taken together, these data suggest that a principal deficit in middle-aged rats is the ability of the medial AVPv to stimulate LHRH neurons.

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

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.

Luteinizing hormone-releasing hormone (LHRH) biosynthesis and secretion in embryonic LHRH

Evidence indicates that LH-releasing hormone (LHRH) neurons can exhibit neuroendocrine secretory properties before entrance into the central nervous system. In this study, we evaluated LHRH biosynthesis and secretion in embryonic LHRH neurons maintained in nasal explants. Using ELISA and calcium imaging techniques, peptide content and single neuron activities were examined. LHRH neurons maintained for 7-10 days in vitro were found to possess a similar amount of LHRH/cell as the equivalent aged LHRH cells in vivo (postnatal day 1). LHRH peptide was measured in the medium of these relatively young cultures, and 40 mM KCl stimulated a 4-fold increase in LHRH secretion. KCl enhanced medium also resulted in a significant increase in LHRH content per culture (24.5 pg vs. 32.3). A similar effect was observed after muscimol-enhanced media (32.2 pg). Both agents also stimulated a substantial rise in intracellular calcium. Pretreatment of cultures with tetrodotoxin partially blocked the affects of muscimol on both peptide content and calcium activity, but not KCl. Calcium-depleted medium blocked the effects of KCl yet only attenuated the effects of muscimol. Treatment of cultures with cycloheximide blocked the effects of both muscimol and KCl. These results indicate that developing LHRH neurons are capable of synthesizing, secreting, and rapidly replenishing stores of LHRH peptide.

Neuroendocrine aging in the female rat: the changing relationship of hypothalamic gonadotropin-releasing hormone neurons and N-methyl-D-aspartate receptors

The reproductive axis undergoes alterations during aging, resulting in acyclicity and the loss of reproductive function. In the hypothalamus, changes intrinsic to GnRH neurons may play a critical role in this process, as may changes in inputs to GnRH neurons from neurotransmitters such as glutamate. We investigated the effects of age and reproductive status on neuroendocrine glutamatergic NMDA receptors (NRs), their regulation of GnRH neurons, and their expression on GnRH neurons, in female rats. First, we quantified NR subunit messenger RNAs (mRNAs) in preoptic area-anterior hypothalamus (POA-AH) and medial basal hypothalamus (MBH), the sites of GnRH perikarya and neuroterminals, respectively. In POA-AH, NR1 mRNA levels varied little with age or reproductive status. NR2a and NR2b mRNA levels decreased significantly between cycling and acyclic rats. In MBH, NR mRNAs all increased with aging, particularly in acyclic animals. Second, we tested the effects of N-methyl-D,L-aspartate (NMA) on GnRH mRNA levels in POA-AH of aging rats. NMA elevated GnRH mRNA levels in young rats, but decreased them in middle-aged rats. Third, we quantified expression of the NR1 subunit on GnRH perikarya in aging rats using double label immunocytochemistry. NR1 expression on GnRH cell bodies varied with age and reproductive status, with 30%, 19%, and 46% of GnRH somata double labeled with NR1 in young proestrous, middle-aged proestrous, and middle-aged persistent estrous rats, respectively. Thus, 1) the expression of hypothalamic NR subunit mRNAs correlates with reproductive status; 2) changes in NR subunit mRNA levels, if reflected by changes in protein levels, may result in alterations in the stoichiometry of the NR during aging, with possible physiological consequences; 3) the effects of NR activation on GnRH mRNA switches from stimulatory to inhibitory during reproductive aging; and 4) expression of the NR1 subunit on GnRH perikarya changes with reproductive status. These molecular, physiological, and cellular neuroendocrine changes are proposed to be involved in the transition to acyclicity in aging female rats.

Neuroendocrine mechanisms for reproductive senescence in the female rat: gonadotropin-releasing hormone neurons

Reproductive aging in female rats is characterized by profound alterations in the neuroendocrine axis. The preovulatory luteinizing hormone (LH) surge is attenuated, and preovulatory expression of the immediate early gene fos in gonadotropin-releasing hormone (GnRH) neurons is substantially reduced in middle-aged compared with young rats. We tested the hypothesis that alterations in GnRH gene expression may be correlated with the attenuation of the LH surge and may be a possible mechanism involved in neuroendocrine senescent changes. Sprague-Dawley rats ages 4 to 5 mo (young), 12-14 mo (middle-aged), or 25 to 26 mo (old) were killed at 10:00 AM or 3:00 PM on proestrus, the day of the LH surge, or diestrus I in cycling rats, and on persistent estrus or persistent diestrus in acyclic rats. RNase protection assays of GnRH mRNA and GnRH primary transcript were performed. GnRH mRNA levels increased significantly with age, whereas GnRH primary transcript levels, an index of GnRH gene transcription, decreased in old compared to young and middle-aged rats. This latter result suggests that an age-related change in GnRH mRNA levels occurs independently of a change in gene transcription, indicating a potential posttranscriptional mechanism. On proestrus, GnRH mRNA levels increased significantly from 10:00 AM to 3:00 PM in young rats. This was in contrast to proestrous middle-aged rats, in which this afternoon increase in GnRH mRNA levels was not observed. Thus, the normal afternoon increase in GnRH mRNA levels on proestrus is disrupted by middle age and may represent a substrate for the attenuation of the preovulatory GnRH/LH surge that occurs in rats of this age, prior to reproductive failure.

Electrical activity of the pulse generator of gonadotropin-releasing hormone in 26-month-old female rats

To know whether age-related changes occur in the activity of the pulse generator of gonadotropin-releasing hormone (GnRH), old (26 months) and young (3 months) female rats were examined by recording multiunit activity (MUA) in the median eminence region of the hypothalamus, concurrently with blood samplings through an intra-atrial cannula at 6-min intervals to determine serum luteinizing hormone (LH) concentrations. We have regarded the MUA showing intermittent increases (volleys) at 20-30 min intervals, followed by LH pulses, as the electrical activity of the GnRH pulse generator. We were successful in recording MUA in 18 (26%) of 69 old ovariectomized rats and in 8 (32%) of 25 young ovariectomized rats. The overall mean (+/-SE) of the interval between MUA volleys in old ovariectomized rats was 35.1 +/- 2.0 min (n = 18), which was significantly longer than that of 22.5 +/- 1.5 min (n = 8) in young ovariectomized rats. The mean interval between LH pulses in old ovariectomized rats was 32.2 +/- 3.6 (n = 10), also being significantly longer than that of 23.3 +/- 1.0 (n = 8) in young ovariectomized rats. Further, the LH pulse amplitude in old rats (0. 95 +/- 0.07 ng/ml) was significantly smaller than in young rats (3. 40 +/- 0.36 ng/ml). The present study also confirmed that the increase in serum LH after intravenous injection of 50 ng GnRH was much smaller in old ovariectomized rats. These results show that the electrical activity of the GnRH pulse generator is certainly reduced with age. Taken together with findings suggesting an age-dependent decrease in stimulated transmitter release, attenuation in both frequency and amplitude of GnRH pulses as well as in pituitary responsiveness to GnRH pulses may account for the decreased pulsatile LH secretion observed in aging rats.

Neuroendocrine modulation of the "menopause": insights into the aging brain

The menopause marks the permanent end of fertility in women. It was once thought that this dramatic physiological change could be explained simply by the exhaustion of the reservoir of ovarian follicles. New data from studies performed in women and animal models make us reassess this assumption. An increasing body of evidence suggests that there are multiple pacemakers that contribute to the transition to irregular cycles, decreasing fertility, and the timing of the menopause. We will present evidence that lends credence to the possibility that a dampening and desynchronization of the precisely orchestrated neural signals lead to miscommunication between the brain and the pituitary-ovarian axis, and that this constellation of hypothalamic-pituitary-ovarian events leads to the deterioration of regular cyclicity and heralds menopausal transition.

Changes in neurotensin mRNA by estrogen in the female rat preoptic area during aging: an in situ hybridization histochemistry study

The present study examined changes in the response of neurotensin mRNA to estrogen during aging at the single cell level. Ten days after ovariectomy, 3-, 10-, and 15-month-old female rats were implanted with estrogen or cholesterol and sacrificed 4 days later. An in situ hybridization study revealed that estrogen significantly increased the number of cells expressing neurotensin mRNA in the preoptic area of all age groups. Furthermore, frequency analysis indicated that estrogen significantly increased the proportion of heavily labeled cells in older rats but not in younger rats. Distributions of the grains/cells between cholesterol- and estrogen-treated rats suggested that older rats were at least as responsive to estrogen as young rats and possibly even more responsive. The result suggests that, at least as reflected by neurotensin mRNA, reproductive senescence in rats is not due to a general decrease in sensitivity to estrogen. Indeed, there is evidence of an increased responsiveness to estrogen with age.

The effects of estradiol on gonadotropin-releasing hormone neurons in the developing mouse brain

The hypothalamic-pituitary-gonadal (HPG) axis plays a critical role in the control of reproduction. Two key hormonal components of the HPG axis are gonadal steroids and gonadotropin-releasing hormone (GnRH). Gonadal steroids are known to organize the development of neural substrates which control adult reproductive behavior; GnRH is required for normal reproductive structure and function. The possibility that gonadal steroids may produce organizational changes in the pattern of GnRH staining observed in the brain is investigated through the use of injections of estradiol to neonatal mice and subsequent GnRH immunocytochemistry at 2 months of age. Our results indicate that the number of GnRH-immunoreactive (GnRH-ir) cells is normally lower in females than males. Estradiol did not affect the number of GnRH-ir cells in females, but significantly increased the number of GnRH-ir cells in males, suggesting that early exposure to estradiol results in masculinization of the GnRH axis of males.

Estrogen, the ovary, and neutotransmitters: factors associated with aging

Our studies in the C57BL/6J mouse have been designed to examine the interactions of aging and the ovary, and their mutual effects on neuroendocrine function. In the pituitary, ovarian status and not age determines responsiveness to gonadotropin hormone releasing hormone (GnRH), but estrogen (E2) is an important mediator in CNS changes, and removal of the ovary (OVX) is deleterious to the neuroendocrine hypothalamus. OVX for just six days in young animals results in synaptic loss between noradrenergic terminals and gonadotropin hormone releasing hormone (GnRH) neurons. Long-term OVX, hypothesized to protect against neuroendocrine aging, fails to guard against any studied age-related changes. Some age-related changes occur as early as midlife. Although neuron number remains constant at middle age, opiatergic neurons undergo significant functional changes by producing opiate antagonist peptides. This change appears to be caused by alterations in the prohormone convertases, which cleave propeptide to peptide. Altered peptides may trigger the loss of reproductive capacity. The midlife shift in opiate peptide production is a component of natural developmental processes that begin in the neonate and continue through old age. In the cholinergic system, E2 mediates numbers of cholinergic receptors, cholinergic neurons, and cholinergic-modulated memory systems in both young and old animals. Regardless of age, ovarian steroids, if present at physiologic levels, are beneficial to the neuroendocrine CNS, and long-term deprivation from ovarian-produced factors is deleterious in the systems we have examined. Our studies have shown that deprivation from ovarian steroid hormones in the female appears to be a major factor in the health of the CNS and in events associated with aging.

Male sexual behavior is associated with LHRH neuron number in middle-aged rats

LHRH administration is reported to facilitate male sexual behavior. The aim of the present study was to investigate whether male sexual behavior is associated with the number of LHRH neurons in the forebrain in middle-aged rats. Male Long-Evans rats (18-19 months) were assigned to three groups on the basis of sexual performance: (1) group MEI consisted of rats showing complete copulatory patterns, including mounts, intromissions and ejaculations, (2) group MI was composed of rats showing mounts and intromissions, but no ejaculation and (3) group NC were non-copulators, i.e. they did not show any copulatory behavior. Young adult rats (4-5 months), displaying sexual behavior, were used as controls. Following the sexual behavior tests, the number of LHRH neurons in the medial septum (MS), organum vasculosum of the lamina terminalis (OVLT), preoptic area (POA) and anterior hypothalamus (AH) was determined by immunocytochemistry. No difference was seen in the total number of LHRH neurons in these combined brain areas between group MIE and young controls. In the three middle-aged groups, the total number of LHRH neurons was greatest in group MIE, less in group MI, and lowest in group NC. In general, a similar trend was seen separately in the MS, OVLT and POA. These results suggest that changes in the number of LHRH neurons in the forebrain, in most cases, are age-related, at least in the middle-aged rats, but they also seem to be associated with male sexual performance.

The number of luteinizing hormone-releasing hormone immunoreactive neurons is significantly decreased in the forebrain of old-aged female rats

To see changes in the number of luteinizing hormone-releasing hormone (LHRH) neurons during aging, young (3 months old)- and old (26 months old)-aged female rats, four of each, were used 4 weeks after ovariectomy for immunocytochemistry. The estimated total number (mean +/- SEM) of LHRH immunoreactive (ir) neurons in the forebrain of old-aged rats (353 +/- 34) was significantly smaller than that in young-aged rats (672 +/- 51). In the forebrain, the decrease in the number of LHRH-ir neurons with age in rostral parts was greater than that in caudal parts, suggesting heterogeneous impairment of LHRH neurons in the rat forebrain during aging.

Chronic estradiol administration did not cause loss of hypothalamic LHRH or TIDA neurons in young or middle-aged C57BL/6J mice

Age-related decline in estrous cycle frequency and impaired pre-ovulatory gonadotropin surges at mid-life are modelled in young C57BL/6J mice by chronic (3 months) oral administration of estradiol (E2). However, the cellular events that induce damage to the neuroendocrine center that regulate gonadotropins with age or following E2 treatment are unclear. To address this issue, possible neuron loss was examined in relation to the loss of estrous cyclicity in E2-treated mice, in particular neurons of the hypothalamic luteinizing hormone releasing hormone (LHRH) and/or tuberoinfundibular dopaminergic (TIDA) systems. By immunocytochemical methods, there was no change in the number of LHRH or TIDA neurons in mice that have become acyclic due to age or E2 treatment. We conclude that the onset of acyclicity at middle-age or following chronic E2 treatment is not associated with loss of LHRH or TIDA neurons and that other neuroendocrine changes must be considered for the cause of acyclicity, particularly those involved in the synaptic regulation of LHRH secretion.

Loss of LH-RH neurons in the rostral forebrain of old female C57BL/6J mice

This study reports the quantitative immunohistochemical distribution of luteinizing hormone-releasing hormone (LH-RH) neurons within the rostral forebrain of young (5-6 month) and old (26-28 month) C57BL/6J mouse. Old mice demonstrated a significant (18%) loss of LH-RH-containing neurons (p less than 0.001, ANOVA). The most striking losses involved the preoptic area (24%) and more rostral regions (26%). The presence of pituitary or mesenteric tumors in older mice did not affect the density of LH-RH neurons. These observations indicate that LH-RH neurons comprise part of the neuronal population previously reported to be reduced in the preoptic area of older mice (4).

Hypothalamic-pituitary axis during reproductive aging in mice

We correlated the content of hypothalamic (HT) GnRH and pituitary (PT) GnRH receptor sites with PT and plasma gonadotropin levels throughout aging in C57BL/6J mice. Female mice of 4-6 months (young), 10-12 months (middle-age) or 15-18 months (old) of age were studied either intact or 15 days post-ovariectomy (OVX) with or without E2 therapy. In intact mice, HT GnRH content increased twofold during aging while GnRH receptor sites in PT remained unchanged. PT content of both FSH and LH gradually rose during aging while plasma concentration rose even more. OVX resulted in a significant decrease in both HT GnRH content and PT receptor sites and no age difference was observed. OVX also resulted in a significant increase in both PT content and plasma levels of gonadotropin in young and middle-age mice while old mice showed a blunted response. After E2 therapy for 7 days, HT GnRH content and PT GnRH receptor sites returned to normal levels in all age groups. By contrast, E2 therapy resulted in no change in PT content of FSH:LH in any age group. Whereas plasma FSH:LH levels returned to intact levels in young mice, they remained elevated to OVX levels in middle-age and old ones. Our results demonstrate an age related dichotomy in the PT production of FSH:LH unrelated to changes in either HT GnRH content or its PT receptor sites, thus suggesting cellular defects in post-receptor binding events within the pituitary.

Evidence that cells expressing luteinizing hormone-releasing hormone mRNA in the mouse are derived from progenitor cells in the olfactory placode

In situ hybridization histochemistry and immunocytochemistry were used to study the prenatal expression of luteinizing hormone-releasing hormone (LHRH) cells in the mouse. Cells expressing LHRH mRNA and peptide product were first detected on embryonic day 11.5 (E11.5) in the olfactory pit. On E12.5, the majority of LHRH cells were located on "tracks" extending from the olfactory pit to the base of the telencephalon. From E12.5 to E15.5, LHRH cells were detected in a rostral-to-caudal gradient in forebrain areas. Prior to E12.5, cells expressing LHRH mRNA were not detected in forebrain areas known to contain LHRH cells in postnatal animals. Quantitation of cells expressing LHRH mRNA showed that the number of labeled cells on E12.5 (approximately 800) equaled the number of LHRH cells in postnatal animals, but more than 90% of these cells were located in nasal regions. Between E12.5 and E15.5, the location of LHRH cells shifted. The number of LHRH cells in the forebrain increased, while the number of LHRH cells in nasal regions decreased over this same period. These findings establish that cells first found in the olfactory pit and thereafter in forebrain areas express the LHRH gene and correspond to the position of LHRH immunopositive cells found at these developmental times. To further examine the ontogeny of the LHRH system, immunocytochemistry in combination with [3H]thymidine autoradiography was used to determine when LHRH cells left the mitotic cycle. We show that LHRH neurons exhibit a discrete time of birth, suggesting that they arise as a single neuronal population between E10.0 and E11.0. Postnatal LHRH neurons were "birth-dated" shortly after differentiation of the olfactory placode and before LHRH mRNA was expressed in cells in the olfactory pit. Taken together, these studies support the hypothesis that all LHRH cells in the central nervous system arise from a discrete group of progenitor cells in the olfactory placode and that a subpopulation of these cells migrate into forebrain areas where they subsequently establish an adult-like distribution.

Spatiotemporal cell expression of luteinizing hormone-releasing hormone in the prenatal mouse: evidence for an embryonic origin in the olfactory placode

The spatiotemporal cell expression of luteinizing hormone-releasing hormone (LHRH) was investigated in mice during prenatal development using light microscopic immunocytochemistry. LHRH immunoreactive cells were first detected in the epithelium of the olfactory pit on gestational day 11 1/2 (E11.5). Some LHRH cells were just outside the olfactory epithelium (OE), clustered on short 'tracks' which extended dorsocaudally. Immunopositive LHRH cells were not observed in any other regions. At E12.5, immunopositive cells were still detected in the OE, but now many LHRH cells were clustered on 'tracks' within olfactory areas. These 'tracks' started at the OE, bilaterally, and extended towards and abutted the basal telencephalic hemispheres. Some immunopositive cells were detected in the rostral telencephalon but not caudal to the telencephalon. The LHRH cells outside the OE were unipolar or bipolar, morphologically resembled mature LHRH neurons and appeared to contact neighboring LHRH cells. The distribution of LHRH cells at E13.5 was similar to E12.5, with the exception that immunopositive cells now extended from the telencephalon to the rostral diencephalon. From E14.5 to E17.5 the majority of LHRH cells were located within the forebrain; extending throughout the diencephalon. Immunopositive cells were not detected in the OE, but were scattered rostrocaudally in olfactory septal areas. By E16.5, LHRH cells within the brain were distributed in a pattern similar to that of the adult mouse. These findings illustrate that LHRH cells express their peptide phenotype early in ontogeny and before their distribution in the forebrain is detected. These data are consistent with the hypothesis that LHRH cells are derived from the olfactory placode and migrate into the forebrain during prenatal development.