Phenotyping of nNOS neurons in the postnatal and adult female mouse hypothalamus

Applying the area fraction fractionator (AFF) probe for total volume estimations of somatic, dendritic and axonal domains of the nigrostriatal dopaminergic system in a murine model

BACKGROUND

The Cavalieri estimator is used for volume measurement of brain and brain regions. Derived from this estimator is the Area Fraction Fractionator (AFF), used for efficient area and number estimations of small 2D elements, such as axons in cross-sectioned nerves. However, to our knowledge, the AFF has not been combined with serial sectioning analysis to measure the volume of small-size nervous structures.

NEW METHOD

Using the nigrostriatal dopaminergic system as an illustrative case, we describe a protocol based on Cavalieri's principle and AFF to estimate the volume of its somatic, nuclear, dendritic, axonal and axon terminal cellular compartments in the adult mouse. The protocol consists of (1) systematic random sampling of sites within and across sections in regions of interest (substantia nigra, the nigrostriatal tract, caudate-putamen), (2) confocal image acquisition of sites, (3) marking of cellular domains using Cavalieri's 2D point-counting grids, and 4) determination of compartments' total volume using the estimated area of each compartment, and between-sections distance.

RESULTS

The volume of the nigrostriatal system per hemisphere is ∼0.38 mm3, with ∼5 % corresponding to perikarya and cell nuclei, ∼10 % to neuropil/dendrites, and ∼85 % to axons and varicosities.

COMPARISON WITH EXISTING METHODS

In contrast to other methods to measure volume of discrete objects, such as the optical nucleator or 3D reconstructions, it stands out for its versatility and ease of use.

CONCLUSIONS

The use of a simple quantitative, unbiased approach to assess the global state of a system may allow quantification of compartment-specific changes that may accompany neurodegenerative processes.

Estrogen receptor-α signaling in tanycytes lies at the crossroads of fertility and metabolism

BACKGROUND

Estrogen secretion by the ovaries regulates the hypothalamic-pituitary-gonadal axis during the reproductive cycle, influencing gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) secretion, and also plays a role in regulating metabolism. Here, we establish that hypothalamic tanycytes-specialized glia lining the floor and walls of the third ventricle-integrate estrogenic feedback signals from the gonads and couple reproduction with metabolism by relaying this information to orexigenic neuropeptide Y (NPY) neurons.

METHODS

Using mouse models, including mice floxed for Esr1 (encoding estrogen receptor alpha, ERα) and those with Cre-dependent expression of designer receptors exclusively activated by designer drugs (DREADDs), along with viral-mediated, pharmacological and indirect calorimetric approaches, we evaluated the role of tanycytes and tanycytic estrogen signaling in pulsatile LH secretion, cFos expression in NPY neurons, estrous cyclicity, body-weight changes and metabolic parameters in adult females.

RESULTS

In ovariectomized mice, chemogenetic activation of tanycytes significantly reduced LH pulsatile release, mimicking the effects of direct NPY neuron activation. In intact mice, tanycytes were crucial for the estrogen-mediated control of GnRH/LH release, with tanycytic ERα activation suppressing fasting-induced NPY neuron activation. Selective knockout of Esr1 in tanycytes altered estrous cyclicity and fertility in female mice and affected estrogen's ability to inhibit refeeding in fasting mice. The absence of ERα signaling in tanycytes increased Npy transcripts and body weight in intact mice and prevented the estrogen-mediated decrease in food intake as well as increase in energy expenditure and fatty acid oxidation in ovariectomized mice.

CONCLUSIONS

Our findings underscore the pivotal role of tanycytes in the neuroendocrine coupling of reproduction and metabolism, with potential implications for its age-related deregulation after menopause.

SIGNIFICANCE STATEMENT

Our investigation reveals that tanycytes, specialized glial cells in the brain, are key interpreters of estrogen signals for orexigenic NPY neurons in the hypothalamus. Disrupting tanycytic estrogen receptors not only alters fertility in female mice but also impairs the ability of estrogens to suppress appetite. This work thus sheds light on the critical role played by tanycytes in bridging the hormonal regulation of cyclic reproductive function and appetite/feeding behavior. This understanding may have potential implications for age-related metabolic deregulation after menopause.

Stress during pubertal development affects female sociosexual behavior in mice

Puberty is a crucial phase for the development of female sexual behavior. Growing evidence suggests that stress during this period may interfere with the development of sexual behavior. However, the neural circuits involved in this alteration remain elusive. Here, we demonstrated in mice that pubertal stress permanently disrupted sexual performance without affecting sexual preference. This was associated with a reduced expression and activation of neuronal nitric oxide synthase (nNOS) in the ventrolateral part of the ventromedial hypothalamus (VMHvl). Fiber photometry revealed that VMHvl nNOS neurons are strongly responsive to male olfactory cues with this activation being substantially reduced in pubertally stressed females. Finally, treatment with a NO donor partially restored sexual performance in pubertally stressed females. This study provides insights into the involvement of VMHvl nNOS in the processing of olfactory cues important for the expression of female sexual behavior. In addition, exposure to stress during puberty disrupts the integration of male olfactory cues leading to reduced sexual behavior.

Characterization of Sonic Hedgehog transcripts in the adult mouse brain: co-expression with neuronal and oligodendroglial markers

In the adult mammalian brain, astrocytes are proposed to be the major Sonic Hedgehog (Shh)-responsive cells. However, the sources of the Shh molecule mediating activation of the pathway are still poorly characterized. The present work investigates the distribution and phenotype of cells expressing Shh mRNA in the adult mouse brain. Using single-molecule fluorescent in situ hybridization (smfISH), we report much broader expression of Shh transcripts in almost all brain regions than originally reported. We identify Shh mRNA in HuC/D+ neuronal populations, including GABAergic (glutamic acid decarboxylase 67, Gad67), cholinergic (choline acetyltransferase, ChAT), dopaminergic (tyrosine hydroxylase, TH), nitrergic (neuronal nitric oxide synthase, nNOS), and in a small population of oligodendroglial cells expressing Sox10 and Olig2 mRNA transcription factors. Further analysis of Shh mRNA in cerebral cortical and hypothalamic neurons suggests that Shh is also expressed by glutamatergic neurons. Interestingly, we did not observe substantial Desert Hedgehog and Indian Hedgehog mRNA signals, nor Shh signals in S100β+ astrocytes and Iba1+ microglial cells. Collectively, the present work provides the most robust central map of Shh-expressing cells to date and underscores the importance of nitrergic neurons in regulating Shh availability to brain cells. Thus, our study provides a framework for future experiments aimed at better understanding of the functions of Shh signaling in the brain in normal and pathological states, and the characterization of novel regulatory mechanisms of the signaling pathway.

Metabolic control of puberty: 60 years in the footsteps of Kennedy and Mitra's seminal work

An individual's nutritional status has a powerful effect on sexual maturation. Puberty onset is delayed in response to chronic energy insufficiency and is advanced under energy abundance. The consequences of altered pubertal timing for human health are profound. Late puberty increases the chances of cardiometabolic, musculoskeletal and neurocognitive disorders, whereas early puberty is associated with increased risks of adult obesity, type 2 diabetes mellitus, cardiovascular diseases and various cancers, such as breast, endometrial and prostate cancer. Kennedy and Mitra's trailblazing studies, published in 1963 and using experimental models, were the first to demonstrate that nutrition is a key factor in puberty onset. Building on this work, the field has advanced substantially in the past decade, which is largely due to the impressive development of molecular tools for experimentation and population genetics. In this Review, we discuss the latest advances in basic and translational sciences underlying the nutritional and metabolic control of pubertal development, with a focus on perspectives and future directions.

Ghrelin-responsive mediobasal hypothalamic neurons mediate exercise-associated food intake and exercise endurance

Previous studies have implicated the orexigenic hormone ghrelin as a mediator of exercise endurance and the feeding response postexercise. Specifically, plasma ghrelin levels nearly double in mice when they are subjected to an hour-long bout of high-intensity interval exercise (HIIE) using treadmills. Also, growth hormone secretagogue receptor-null (GHSR-null) mice exhibit decreased food intake following HIIE and diminished running distance (time until exhaustion) during a longer, stepwise exercise endurance protocol. To investigate whether ghrelin-responsive mediobasal hypothalamus (MBH) neurons mediate these effects, we stereotaxically delivered the inhibitory designer receptor exclusively activated by designer drugs virus AAV2-hSyn-DIO-hM4(Gi)-mCherry to the MBH of Ghsr-IRES-Cre mice, which express Cre recombinase directed by the Ghsr promoter. We found that chemogenetic inhibition of GHSR-expressing MBH neurons (upon delivery of clozapine-N-oxide) 1) suppressed food intake following HIIE, 2) reduced maximum running distance and raised blood glucose and blood lactate levels during an exercise endurance protocol, 3) reduced food intake following ghrelin administration, and 4) did not affect glucose tolerance. Further, HIIE increased MBH Ghsr expression. These results indicate that activation of ghrelin-responsive MBH neurons is required for the normal feeding response to HIIE and the usual amount of running exhibited during an exercise endurance protocol.

New Horizons: Gonadotropin-Releasing Hormone and Cognition

Pulsatile secretion of gonadotropin-releasing hormone (GnRH) is essential for activating and maintaining the function of the hypothalamic-pituitary-gonadal axis, which controls the onset of puberty and fertility. Two recent studies suggest that, in addition to controlling reproduction, the neurons in the brain that produce GnRH are also involved in the control of postnatal brain maturation, odor discrimination, and adult cognition. This review will summarize the development and establishment of the GnRH system, with particular attention to the importance of its first postnatal activation, a phenomenon known as minipuberty, for later reproductive and nonreproductive functions. In addition, we will discuss the beneficial effects of restoring physiological (ie, pulsatile) GnRH levels on olfactory and cognitive alterations in preclinical Down syndrome and Alzheimer disease models, as well as the potential risks associated with long-term continuous (ie, nonphysiological) GnRH administration in certain disorders. Finally, this review addresses the intriguing possibility that pulsatile GnRH therapy may hold therapeutic potential for the management of some neurodevelopmental cognitive disorders and pathological aging in elderly people.

Changes of Signaling Pathways in Hypothalamic Neurons with Aging

The hypothalamus is an important regulator of autonomic and endocrine functions also involved in aging regulation. The aging process in the hypothalamus is accompanied by disturbed intracellular signaling including insulin/insulin-like growth factor-1 (IGF-1)/growth hormone (GH), phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/protein kinase B (AKT)/the mammalian target of rapamycin (mTOR), mitogen activated protein kinase (MAPK), janus kinase (JAK)/signal transducer and activator of transcription (STAT), AMP-activated protein kinase (AMPK), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-ĸB), and nitric oxide (NO). In the current review, I have summarized the current understanding of the changes in the above-mentioned pathways in aging with a focus on hypothalamic alterations.

A role for GnRH in olfaction and cognition: Implications for veterinary medicine

Pulsatile secretion of gonadotropin-releasing hormone (GnRH) is essential for the activation and maintenance of the function of the hypothalamic-pituitary-gonadal (HPG) axis, which controls the onset of puberty and fertility. Two provocative recent studies suggest that, in addition to control reproduction, the neurons in the brain that produce GnRH are also involved in the control postnatal brain maturation, odour discrimination and adult cognition. Long-acting GnRH antagonists and agonists are commonly used to control fertility and behaviour in veterinary medicine, primarily in males. This review puts into perspective the potential risks of these androgen deprivation therapies and immunization on olfactory and cognitive performances and well-aging in domestic animals, including pets. We will also discuss the results reporting beneficial effects of pharmacological interventions restoring physiological GnRH levels on olfactory and cognitive alterations in preclinical models of Alzheimer's disease, which shares many pathophysiological and behavioural hallmarks with canine cognitive dysfunction. These novel findings raise the intriguing possibility that pulsatile GnRH therapy holds therapeutic potential for the management of this behavioural syndrome affecting older dogs.

Sex-dimorphic hindbrain lactate regulation of ventromedial hypothalamic nucleus glucoregulatory neuron 5'-AMP-activated protein kinase activity and transmitter marker protein expression

BACKGROUND

The oxidizable glycolytic end-product L-lactate is a gauge of nerve cell metabolic fuel stability that metabolic-sensory hindbrain A2 noradrenergic neurons impart to the brain glucose-regulatory network. Current research investigated the premise that hindbrain lactate deficiency exerts sex-specific control of energy sensor and transmitter marker protein responses to hypoglycemia in ventromedial hypothalamic nucleus (VMN) glucose-regulatory nitrergic and γ-aminobutyric acid (GABA) neurons.

METHODS

Nitric oxide synthase (nNOS)- or glutamate decarboxylase65/67 (GAD)-immunoreactive neurons were laser-catapult-microdissected from male and female rat VMN after subcutaneous insulin injection and caudal fourth ventricular L-lactate or vehicle infusion for Western blot protein analysis.

RESULTS

Hindbrain lactate repletion reversed hypoglycemia-associated augmentation (males) or inhibition (females) of nitrergic neuron nNOS expression, and prevented up-regulation of phosphorylated AMPK 5'-AMP-activated protein kinase (pAMPK) expression in those neurons. Hypoglycemic suppression of GABAergic neuron GAD protein was averted by exogenous lactate over the rostro-caudal length of the male VMN and in the middle region of the female VMN. Lactate normalized GABA neuron pAMPK profiles in hypoglycemic male (caudal VMN) and female (all VMN segments) rats. Hypoglycemic patterns of norepinephrine (NE) signaling were lactate-dependent throughout the male VMN, but confined to the rostral and middle female VMN.

CONCLUSIONS

Results document, in each sex, regional VMN glucose-regulatory transmitter responses to hypoglycemia that are controlled by hindbrain lactate status. Hindbrain metabolic-sensory regulation of hypoglycemia-correlated nitric oxide or GABA release may entail AMPK-dependent mechanisms in specific VMN rostro-caudal segments in each sex. Additional effort is required to examine the role of hindbrain lactoprivic-sensitive VMN neurotransmitters in lactate-mediated attenuation of hypoglycemic hyperglucagonemia and hypercorticosteronemia in male and female rats.

A critical update on the leptin-melanocortin system

The discovery of leptin in 1994 was an "eureka moment" in the field of neurometabolism that provided new opportunities to better understand the central control of energy balance and glucose metabolism. Rapidly, a prevalent model in the field emerged that pro-opiomelanocortin (POMC) neurons were key in promoting leptin's anorexigenic effects and that the arcuate nucleus of the hypothalamus (ARC) was a key region for the regulation of energy homeostasis. While this model inspired many important discoveries, a growing body of literature indicates that this model is now outdated. In this review, we re-evaluate the hypothalamic leptin-melanocortin model in light of recent advances that directly tackle previous assumptions, with a particular focus on the ARC. We discuss how segregated and heterogeneous these neurons are, and examine how the development of modern approaches allowing spatiotemporal, intersectional, and chemogenetic manipulations of melanocortin neurons has allowed a better definition of the complexity of the leptin-melanocortin system. We review the importance of leptin in regulating glucose homeostasis, but not food intake, through direct actions on ARC POMC neurons. We further highlight how non-POMC, GABAergic neurons mediate leptin's direct effects on energy balance and influence POMC neurons.

Male minipuberty involves the gonad-independent activation of preoptic nNOS neurons

BACKGROUND

The maturation of the hypothalamic-pituitary-gonadal (HPG) axis is crucial for the establishment of reproductive function. In female mice, neuronal nitric oxide synthase (nNOS) activity appears to be key for the first postnatal activation of the neural network promoting the release of gonadotropin-releasing hormone (GnRH), i.e. minipuberty. However, in males, the profile of minipuberty as well as the role of nNOS-expressing neurons remain unexplored.

METHODS

nNOS-deficient and wild-type mice were studied during postnatal development. The expression of androgen (AR) and estrogen receptor alpha (ERα) as well as nNOS phosphorylation were evaluated by immunohistochemistry in nNOS neurons in the median preoptic nucleus (MePO), where most GnRH neuronal cell bodies reside, and the hormonal profile of nNOS-deficient male mice was assessed using previously established radioimmunoassay and ELISA methods. Gonadectomy and pharmacological manipulation of ERα were used to elucidate the mechanism of minipubertal nNOS activation and the maturation of the HPG axis.

RESULTS

In male mice, minipubertal FSH release occurred at P23, preceding the LH surge at P30, when balanopreputial separation occurs. Progesterone and testosterone remained low during minipuberty, increasing around puberty, whereas estrogen levels were high throughout postnatal development. nNOS neurons showed a sharp increase in Ser¹⁴¹² phosphorylation of nNOS at P23, a phenomenon that occurred even in the absence of the gonads. In male mice, nNOS neurons did not appear to express AR, but abundantly expressed ERα throughout postnatal development. Selective pharmacological blockade of ERα during the infantile period blunted Ser¹⁴¹² phosphorylation of nNOS at P23.

CONCLUSIONS

Our results show that the timing of minipuberty differs in male mice when compared to females, but as in the latter, nNOS activity in the preoptic region plays a role in this process. Additionally, akin to male non-human primates, the profile of minipuberty in male mice is shaped by sex-independent mechanisms, and possibly involves extragonadal estrogen sources.

NOS1 mutations cause hypogonadotropic hypogonadism with sensory and cognitive deficits that can be reversed in infantile mice

The nitric oxide (NO) signaling pathway in hypothalamic neurons plays a key role in the regulation of the secretion of gonadotropin-releasing hormone (GnRH), which is crucial for reproduction. We hypothesized that a disruption of neuronal NO synthase (NOS1) activity underlies some forms of hypogonadotropic hypogonadism. Whole-exome sequencing was performed on a cohort of 341 probands with congenital hypogonadotropic hypogonadism to identify ultrarare variants in NOS1. The activity of the identified NOS1 mutant proteins was assessed by their ability to promote nitrite and cGMP production in vitro. In addition, physiological and pharmacological characterization was carried out in a Nos1-deficient mouse model. We identified five heterozygous NOS1 loss-of-function mutations in six probands with congenital hypogonadotropic hypogonadism (2%), who displayed additional phenotypes including anosmia, hearing loss, and intellectual disability. NOS1 was found to be transiently expressed by GnRH neurons in the nose of both humans and mice, and Nos1 deficiency in mice resulted in dose-dependent defects in sexual maturation as well as in olfaction, hearing, and cognition. The pharmacological inhibition of NO production in postnatal mice revealed a critical time window during which Nos1 activity shaped minipuberty and sexual maturation. Inhaled NO treatment at minipuberty rescued both reproductive and behavioral phenotypes in Nos1-deficient mice. In summary, lack of NOS1 activity led to GnRH deficiency associated with sensory and intellectual comorbidities in humans and mice. NO treatment during minipuberty reversed deficits in sexual maturation, olfaction, and cognition in Nos1 mutant mice, suggesting a potential therapy for humans with NO deficiency.

Precocious puberty in narcolepsy type 1: Orexin loss and/or neuroinflammation, which is to blame?

Narcolepsy type 1 (NT1) is a rare neurological sleep disorder triggered by postnatal loss of the orexin/hypocretin neuropeptides. Overweight/obesity and precocious puberty are highly prevalent comorbidities of NT1, with a close temporal correlation with disease onset, suggesting a common origin. However, the underlying mechanisms remain unknown and merit further investigation. The main question we address in this review is whether the occurrence of precocious puberty in NT1 is due to the lack of orexin/hypocretin or rather to a wider hypothalamic dysfunction in the context of neuroinflammation, which is likely to accompany the disease given its autoimmune origins. Our analysis suggests that the suspected generalized neuroinflammation of the hypothalamus in NT1 would tend to delay puberty rather than hastening it. In contrast, that the brutal loss of orexin/hypocretin would favor an early reactivation of gonadotropin-releasing hormone (GnRH) secretion during the prepubertal period in vulnerable children, leading to early puberty onset. Orexin/hypocretin replacement could thus be envisaged as a potential treatment for precocious puberty in NT1. Additionally, we put forward an alternative hypothesis regarding the concomitant occurrence of sleepiness, weight gain and early puberty in NT1.

Female sexual behavior is disrupted in a preclinical mouse model of PCOS via an attenuated hypothalamic nitric oxide pathway

Polycystic ovary syndrome (PCOS) is a major endocrine disorder leading to female infertility worldwide. Patients with PCOS also often experience sexual dysfunction; however, the developmental and central mechanisms mediating this behavioral derangement are unclear. Here, we show that prenatal excess of anti-Müllerian hormone triggers PCOS-like impairment in female sexual behavior in mice. Sexual dysfunction in PCOS-like mice is associated with decreased expression of progesterone-sensitive neuronal nitric oxide synthase (nNOS) neurons in the hypothalamus. Chemogenetic inhibition of nNOS neuronal activity in the ventromedial nucleus of the hypothalamus recapitulates PCOS-like sexual dysfunction. Of clinical relevance, administration of nitric oxide donor rescues normal sexual behavior in PCOS-like mice.

Women with polycystic ovary syndrome (PCOS) frequently experience decreased sexual arousal, desire, and sexual satisfaction. While the hypothalamus is known to regulate sexual behavior, the specific neuronal pathways affected in patients with PCOS are not known. To dissect the underlying neural circuitry, we capitalized on a robust preclinical animal model that reliably recapitulates all cardinal PCOS features. We discovered that female mice prenatally treated with anti-Müllerian hormone (PAMH) display impaired sexual behavior and sexual partner preference over the reproductive age. Blunted female sexual behavior was associated with increased sexual rejection and independent of sex steroid hormone status. Structurally, sexual dysfunction was associated with a substantial loss of neuronal nitric oxide synthase (nNOS)-expressing neurons in the ventromedial nucleus of the hypothalamus (VMH) and other areas of hypothalamic nuclei involved in social behaviors. Using in vivo chemogenetic manipulation, we show that nNOS^VMH neurons are required for the display of normal sexual behavior in female mice and that pharmacological replenishment of nitric oxide restores normal sexual performance in PAMH mice. Our data provide a framework to investigate facets of hypothalamic nNOS neuron biology with implications for sexual disturbances in PCOS.

Quantitative relationship between cerebrovascular network and neuronal cell types in mice

The cerebrovasculature and its mural cells must meet brain regional energy demands, but how their spatial relationship with different neuronal cell types varies across the brain remains largely unknown. Here we apply brain-wide mapping methods to comprehensively define the quantitative relationships between the cerebrovasculature, capillary pericytes, and glutamatergic and GABAergic neurons, including neuronal nitric oxide synthase-positive (nNOS+) neurons and their subtypes in adult mice. Our results show high densities of vasculature with high fluid conductance and capillary pericytes in primary motor sensory cortices compared with association cortices that show significant positive and negative correlations with energy-demanding parvalbumin+ and vasomotor nNOS+ neurons, respectively. Thalamo-striatal areas that are connected to primary motor sensory cortices also show high densities of vasculature and pericytes, suggesting dense energy support for motor sensory processing areas. Our cellular-resolution resource offers opportunities to examine spatial relationships between the cerebrovascular network and neuronal cell composition in largely understudied subcortical areas.

Polycystic Ovary Syndrome and the Neuroendocrine Consequences of Androgen Excess

Polycystic ovary syndrome (PCOS) is a major endocrine disorder strongly associated with androgen excess and frequently leading to female infertility. Although classically considered an ovarian disease, altered neuroendocrine control of gonadotropin-releasing hormone (GnRH) neurons in the brain and abnormal gonadotropin secretion may underpin PCOS presentation. Defective regulation of GnRH pulse generation in PCOS promotes high luteinizing hormone (LH) pulsatile secretion, which in turn overstimulates ovarian androgen production. Early and emerging evidence from preclinical models suggests that maternal androgen excess programs abnormalities in developing neuroendocrine circuits that are associated with PCOS pathology, and that these abnormalities are sustained by postpubertal elevation of endogenous androgen levels. This article will discuss experimental evidence, from the clinic and in preclinical animal models, that has significantly contributed to our understanding of how androgen excess influences the assembly and maintenance of neuroendocrine impairments in the female brain. Abnormal central gamma-aminobutyric acid (GABA) signaling has been identified in both patients and preclinical models as a possible link between androgen excess and elevated GnRH/LH secretion. Enhanced GABAergic innervation and drive to GnRH neurons is suspected to contribute to the pathogenesis and early manifestation of neuroendocrine derangement in PCOS. Accordingly, this article also provides an overview of GABA regulation of GnRH neuron function from prenatal development to adulthood to discuss possible avenues for future discovery research and therapeutic interventions. © 2022 American Physiological Society. Compr Physiol 12:3347-3369, 2022.

Neuromeric Distribution of Nicotinamide Adenine Dinucleotide Phosphate-Diaphorase Activity in the Adult Lamprey Brain

This study reports for the first time the distribution and morphological characterization of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d; a reliable marker of nitric oxide synthase activity) positive elements in the central nervous system of the adult river lamprey (Lampetra fluviatilis) on the framework of the neuromeric model and compares their cytoarchitectonic organization with that of gnathostomes. Both NADPH-d exhibiting cells and fibers were observed in all major divisions of the lamprey brain as well as in the spinal cord. In the secondary prosencephalon, NADPH-d positive cells were observed in the mitral cell layer of the olfactory bulb, evaginated pallium, amygdala, dorsal striatum, septum, lateral preoptic nucleus, caudal paraventricular area, posterior entopeduncular nucleus, nucleus of the stria medullaris, hypothalamic periventricular organ and mamillary region sensu lato. In the lamprey diencephalon, NADPH-d labeled cells were observed in several nuclei of the prethalamus, epithalamus, pretectum, and the basal plate. Especially remarkable was the staining observed in the right habenula and several pretectal nuclei. NADPH-d positive cells were also observed in the following mesencephalic areas: optic tectum (two populations), torus semicircularis, nucleus M5 of Schöber, and a ventral tegmental periventricular nucleus. Five different cell populations were observed in the isthmic region, whereas the large sensory dorsal cells, some cells located in the interpeduncular nucleus, the motor nuclei of most cranial nerves, the solitary tract nucleus, some cells of the reticular nuclei, and small cerebrospinal fluid-contacting (CSF-c) cells were the most evident stained cells of the rhombencephalon proper. Finally, several NADPH-d positive cells were observed in the rostral part of the spinal cord, including the large sensory dorsal cells, numerous CSF-c cells, and some dorsal and lateral interneurons. NADPH-d positive fibers were observed in the olfactory pathways (primary olfactory fibers and stria medullaris), the fasciculus retroflexus, and the dorsal column tract. Our results on the distribution of NADPH-d positive elements in the brain of the adult lamprey L. fluviatilis are significantly different from those previously reported in larval lampreys and demonstrated that these animals possess a complex nitrergic system readily comparable to those of other vertebrates, although important specific differences also exist.

GnRH neurons recruit astrocytes in infancy to facilitate network integration and sexual maturation

Neurons that produce gonadotropin-releasing hormone (GnRH), which control fertility, complete their nose-to-brain migration by birth. However, their function depends on integration within a complex neuroglial network during postnatal development. Here, we show that rodent GnRH neurons use a prostaglandin D₂ receptor DP1 signaling mechanism during infancy to recruit newborn astrocytes that 'escort' them into adulthood, and that the impairment of postnatal hypothalamic gliogenesis markedly alters sexual maturation by preventing this recruitment, a process mimicked by the endocrine disruptor bisphenol A. Inhibition of DP1 signaling in the infantile preoptic region, where GnRH cell bodies reside, disrupts the correct wiring and firing of GnRH neurons, alters minipuberty or the first activation of the hypothalamic-pituitary-gonadal axis during infancy, and delays the timely acquisition of reproductive capacity. These findings uncover a previously unknown neuron-to-neural-progenitor communication pathway and demonstrate that postnatal astrogenesis is a basic component of a complex set of mechanisms used by the neuroendocrine brain to control sexual maturation.

Kisspeptin signaling and nNOS neurons in the VMHvl modulate lordosis behavior but not mate preference in female mice

It was recently shown that kisspeptin neurons in the anteroventral periventricular area (AVPV) orchestrate female sexual behavior, including lordosis behavior and mate preference. A potential target of AVPV kisspeptin signaling could be neurons expressing the neuronal form of nitric oxide synthase (nNOS) in the ventrolateral part of the ventromedial hypothalamus (VMHvl). Therefore, in the present study, we further refined the role of the VHMvl in female sexual behavior. Adult female mice received a bilateral cannula aimed at the VMHvl. A single injection with kisspeptin (Kp-10) or SNAP/BAY, a nitric oxide donor, significantly increased lordosis, whereas the nNOS inhibitor l-NAME decreased it. None of these drugs affected mate preference. Interestingly, administration of GnRH into the VMHvl had no effect on lordosis or mate preference. To determine whether the stimulatory effect of Kp-10 on lordosis was specific to the VMHvl, an additional group of females received Kp-10 directly into the paraventricular nucleus (PVN). No effect was found on lordosis and mate preference. These results suggest that kisspeptin most likely modulates lordosis behavior through nNOS neurons in the VMHvl whereas mate preference is modulated by kisspeptin through a separate neuronal circuit not including the VMHvl.

Ventromedial Nucleus of the Hypothalamus Neurons Under the Magnifying Glass

The ventromedial nucleus of the hypothalamus (VMH) is a complex brain structure that is integral to many neuroendocrine functions, including glucose regulation, thermogenesis, and appetitive, social, and sexual behaviors. As such, it is of little surprise that the nucleus is under intensive investigation to decipher the mechanisms which underlie these diverse roles. Developments in genetic and investigative tools, for example the targeting of steroidogenic factor-1-expressing neurons, have allowed us to take a closer look at the VMH, its connections, and how it affects competing behaviors. In the current review, we aim to integrate recent findings into the literature and contemplate the conclusions that can be drawn.

The KiNG of reproduction: Kisspeptin/ nNOS interactions shaping hypothalamic GnRH release

Gonadotropin-releasing hormone (GnRH) is the master regulator of the hypothalamic-pituitary-gonadal (HPG) axis, and therefore of fertility and reproduction. The release pattern of GnRH by the hypothalamus includes both pulses and surges. However, despite a considerable body of evidence in support of a determinant role for kisspeptin, the mechanisms regulating a GnRH pulse and surge remain a topic of debate. In this review we challenge the view of kisspeptin as an absolute "monarch", and instead present the idea of a Kisspeptin-nNOS-GnRH or "KiNG" network that is responsible for generating the "GnRH pulse" and "GnRH surge". In particular, the neuromodulator nitric oxide (NO) has opposite effects to kisspeptin on GnRH secretion in many respects, acting as the Yin to kisspeptin's Yang and creating a dynamic system in which kisspeptin provides the "ON" signal, promoting GnRH release, while NO mediates the "OFF" signal, acting as a tonic brake on GnRH secretion. This interplay between an activator and an inhibitor, which is in turn fine-tuned by the gonadal steroid environment, thus leads to the generation of GnRH pulses and surges and is crucial for the proper development and function of the reproductive axis.

Acupuncture Attenuates Blood Pressure via Inducing the Expression of nNOS

Background

Sympathetic activation leads to elevated blood pressure. Neuronal nitric oxide synthase (nNOS) inhibits sympathetic nervous system activity, thereby decreasing blood pressure (BP). nNOS is highly expressed in the arcuate nucleus (ARC) and ventrolateral periaqueductal gray (vlPAG), which play essential roles in the regulation of the cardiovascular and sympathetic nervous systems.

Objective

This study was designed to verify the hypothesis that acupuncture exerts an antihypertensive effect via increasing the expression of nNOS in ARC and vlPAG of spontaneously hypertensive (SHR) rats.

Methods

Rats without anesthesia were subject to daily acupuncture for 2 weeks. BP was monitored by the tail-cuff method. nNOS expressions in the ARC and vlPAG were detected by western blot and immunofluorescence. BP was measured after 7-Nitroindazole (7-NI), a specific nNOS inhibitor, was microinjected into ARC or vlPAG in SHR rats treated with acupuncture.

Results

Acupuncture for 14 days significantly attenuated BP, and the Taichong (LR3) acupoint was superior to Zusanli (ST36) and Fengchi (GB20) in lowering BP. In addition, acupuncture at Taichong (LR3) induced an increase of nNOS expression in ARC and vlPAG, whereas microinjection of 7-NI into ARC or vlPAG reversed the antihypertensive effect of acupuncture.

Conclusions

This study indicates that acupuncture at Taichong (LR3) induces a better antihypertensive effect than at Zusanli (ST36) or at Fengchi (GB20) in SHR rats, and enhancement of nNOS in ARC and vlPAG probably contributes to the antihypertensive effect of acupuncture.

Nitric oxide resets kisspeptin-excited GnRH neurons via PIP2 replenishment

Fertility relies upon pulsatile release of gonadotropin-releasing hormone (GnRH) that drives pulsatile luteinizing hormone secretion. Kisspeptin (KP) neurons in the arcuate nucleus are at the center of the GnRH pulse generation and the steroid feedback control of GnRH secretion. However, KP evokes a long-lasting response in GnRH neurons that is hard to reconcile with periodic GnRH activity required to drive GnRH pulses. Using calcium imaging, we show that 1) the tetrodotoxin-insensitive calcium response evoked by KP relies upon the ongoing activity of canonical transient receptor potential channels maintaining voltage-gated calcium channels in an activated state, 2) the duration of the calcium response is determined by the rate of resynthesis of phosphatidylinositol 4,5-bisphosphate (PIP₂), and 3) nitric oxide terminates the calcium response by facilitating the resynthesis of PIP₂ via the canonical pathway guanylyl cyclase/3',5'-cyclic guanosine monophosphate/protein kinase G. In addition, our data indicate that exposure to nitric oxide after KP facilitates the calcium response to a subsequent KP application. This effect was replicated using electrophysiology on GnRH neurons in acute brain slices. The interplay between KP and nitric oxide signaling provides a mechanism for modulation of the refractory period of GnRH neurons after KP exposure and places nitric oxide as an important component for tonic GnRH neuronal pulses.

The median preoptic nucleus: A major regulator of fluid, temperature, sleep, and cardiovascular homeostasis

Located in the midline lamina terminalis of the anterior wall of the third ventricle, the median preoptic nucleus is a thin elongated nucleus stretching around the rostral border of the anterior commissure. Its neuronal elements, composed of various types of excitatory glutamatergic and inhibitory GABAergic neurons, receive afferent neural signals from (1) neighboring subfornical organ and organum vasculosum of the lamina terminalis related to plasma osmolality and hormone concentrations, e.g., angiotensin II; (2) from peripheral sensors such as arterial baroreceptors and cutaneous thermosensors. Different sets of these MnPO glutamatergic and GABAergic neurons relay output signals to hypothalamic, midbrain, and medullary regions that drive homeostatic effector responses. Included in the effector responses are (1) thirst, antidiuretic hormone secretion and renal sodium excretion that subserve osmoregulation and body fluid homeostasis; (2) vasoconstriction or dilatation of skin blood vessels, and shivering and brown adipose tissue thermogenesis for core temperature homeostasis; (3) inhibition of hypothalamic and midbrain nuclei that stimulate wakefulness and arousal, thereby promoting both REM and non-REM sleep; and (4) activation of sympathetic pathways that drive vasoconstriction and heart rate to maintain arterial pressure and the perfusion of vital organs. The small size of MnPO belies its massive homeostatic significance.

Nitric oxide signalling in the brain and its control of bodily functions

Nitric oxide (NO) is a versatile molecule that plays key roles in the development and survival of mammalian species by endowing brain neuronal networks with the ability to make continual adjustments to function in response to moment-to-moment changes in physiological input. Here, we summarize the progress in the field and argue that NO-synthetizing neurons and NO signalling in the brain provide a core hub for integrating sensory- and homeostatic-related cues, control key bodily functions, and provide a potential target for new therapeutic opportunities against several neuroendocrine and behavioural abnormalities.

Investigating the NPY/AgRP/GABA to GnRH Neuron Circuit in Prenatally Androgenized PCOS-Like Mice

Polycystic ovary syndrome (PCOS), the most common form of anovulatory infertility, is associated with altered signaling within the hormone-sensitive neuronal network that regulates gonadotropin-releasing hormone (GnRH) neurons, leading to a pathological increase in GnRH secretion. Circuit remodeling is evident between GABAergic neurons in the arcuate nucleus (ARN) and GnRH neurons in a murine model of PCOS. One-third of ARN GABA neurons co-express neuropeptide Y (NPY), which has a known yet complex role in regulating GnRH neurons and reproductive function. Here, we investigated whether the NPY-expressing subpopulation (NPY^ARN) of ARN GABA neurons (GABA^ARN) is also affected in prenatally androgenized (PNA) PCOS-like NPY^ARN reporter mice [Agouti-related protein (AgRP)-Cre;τGFP]. PCOS-like mice and controls were generated by exposure to di-hydrotestosterone or vehicle (VEH) in late gestation. τGFP-expressing NPY^ARN neuron fiber appositions with GnRH neurons and gonadal steroid hormone receptor expression in τGFP-expressing NPY^ARN neurons were assessed using confocal microscopy. Although GnRH neurons received abundant close contacts from τGFP-expressing NPY^ARN neuron fibers, the number and density of putative inputs was not affected by prenatal androgen excess. NPY^ARN neurons did not co-express progesterone receptor or estrogen receptor α in either PNA or VEH mice. However, the proportion of NPY^ARN neurons co-expressing the androgen receptor was significantly elevated in PNA mice. Therefore, NPY^ARN neurons are not remodeled by prenatal androgen excess like the wider GABA^ARN population, indicating GABA-to-GnRH neuron circuit remodeling occurs in a presently unidentified non-NPY/AgRP population of GABA^ARN neurons. NPY^ARN neurons do, however, show independent changes in the form of elevated androgen sensitivity.

Evidence that Nitric Oxide Is Critical for LH Surge Generation in Female Sheep

Elevated and sustained estradiol concentrations cause a gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) surge that is necessary for ovulation. In sheep, several different neural systems have been implicated in this stimulatory action of estradiol and this study focused on somatostatin (SST) neurons in the ventral lateral region of the ventral medial nucleus (vlVMN) which express c-Fos during the surge. First, we determined if increased activity of SST neurons could be related to elevated GnRH secretion by assessing SST synapses onto GnRH neurons and neurons coexpressing kisspeptin, neurokinin B, dynorphin (KNDy). We found that the percentage of preoptic area GnRH neurons that receive SST input increased during the surge compared with other phases of the cycle. However, since SST is generally inhibitory, and pharmacological manipulation of SST signaling did not alter the LH surge in sheep, we hypothesized that nitric oxide (NO) was also produced by these neurons to account for their activation during the surge. In support of this hypothesis we found that (1) the majority of SST cells in the vlVMN (>80%) contained neuronal nitric oxide synthase (nNOS); (2) the expression of c-Fos in dual-labeled SST-nNOS cells, but not in single-labeled cells, increased during the surge compared with other phases of the cycle; and (3) intracerebroventricular (ICV) infusion of the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester, completely blocked the estrogen-induced LH surge. These data support the hypothesis that the population of SST-nNOS cells in the vlVMN are a source of NO that is critical for the LH surge, and we propose that they are an important site of estradiol positive feedback in sheep.

Role of Nitric Oxide in Insulin Secretion and Glucose Metabolism

Nitric oxide (NO) contributes to carbohydrate metabolism and decreased NO bioavailability is involved in the development of type 2 diabetes mellitus (T2DM). NO donors may improve insulin signaling and glucose homeostasis in T2DM and insulin resistance (IR), suggesting the potential clinical importance of NO-based interventions. In this review, site-specific roles of the NO synthase (NOS)-NO pathway in carbohydrate metabolism are discussed. In addition, the metabolic effects of physiological low levels of NO produced by constitutive NOS (cNOS) versus pathological high levels of NO produced by inducible NOS (iNOS) in pancreatic β-cells, adipocytes, hepatocytes, and skeletal muscle cells are summarized. A better understanding of the NOS-NO system in the regulation of glucose homeostasis can hopefully facilitate the development of new treatments for T2DM.

Ventromedial hypothalamus glucose-inhibited neurones: A role in glucose and energy homeostasis?

The ventromedial hypothalamus (VMH) plays a complex role in glucose and energy homeostasis. The VMH is necessary for the counter-regulatory response to hypoglycaemia (CRR) that increases hepatic gluconeogenesis to restore euglycaemia. On the other hand, the VMH also restrains hepatic glucose production during euglycaemia and stimulates peripheral glucose uptake. The VMH is also important for the ability of oestrogen to increase energy expenditure. This latter function is mediated by VMH modulation of the lateral/perifornical hypothalamic area (lateral/perifornical hypothalamus) orexin neurones. Activation of VMH AMP-activated protein kinase (AMPK) is necessary for the CRR. By contrast, VMH AMPK inhibition favours decreased basal glucose levels and is required for oestrogen to increase energy expenditure. Specialised VMH glucose-sensing neurones confer the ability to sense and respond to changes in blood glucose levels. Glucose-excited (GE) neurones increase and glucose-inhibited (GI) neurones decrease their activity as glucose levels rise. VMH GI neurones, in particular, appear to be important in the CRR, although a role for GE neurones cannot be discounted. AMPK mediates glucose sensing in VMH GI neurones suggesting that, although activation of these neurones is important for the CRR, it is necessary to silence them to lower basal glucose levels and enable oestrogen to increase energy expenditure. In support of this, we found that oestrogen reduces activation of VMH GI neurones in low glucose by inhibiting AMPK. In this review, we present the evidence underlying the role of the VMH in glucose and energy homeostasis. We then discuss the role of VMH glucose-sensing neurones in mediating these effects, with a strong emphasis on oestrogenic regulation of glucose sensing and how this may affect glucose and energy homeostasis.

Activation of arcuate nucleus GABA neurons promotes luteinizing hormone secretion and reproductive dysfunction: Implications for polycystic ovary syndrome

Background

Enhanced GABA activity in the brain and a hyperactive hypothalamic-pituitary-gonadal axis are associated with polycystic ovary syndrome (PCOS), the most common form of anovulatory infertility. Women with PCOS exhibit elevated cerebrospinal fluid GABA levels and preclinical models of PCOS exhibit increased GABAergic input to GnRH neurons, the central regulators of reproduction. The arcuate nucleus (ARN) is postulated as the anatomical origin of elevated GABAergic innervation; however, the functional role of this circuit is undefined.

Methods

We employed a combination of targeted optogenetic and chemogenetic approaches to assess the impact of acute and chronic ARN GABA neuron activation. Selective acute activation of ARN GABA neurons and their fiber projections was coupled with serial blood sampling for luteinizing hormone secretion in anesthetized male, female and prenatally androgenised (PNA) mice modelling PCOS. In addition, GnRH neuron responses to ARN GABA fiber stimulation were recorded in ex vivo brain slices. Chronic activation of ARN GABA neurons in healthy female mice was coupled with reproductive phenotyping for PCOS-like features.

Findings

Acute stimulation of ARN GABA fibers adjacent to GnRH neurons resulted in a significant and long-lasting increase in LH secretion in male and female mice. The amplitude of this response was blunted in PNA mice, which also exhibited a blunted LH response to GnRH administration. Infrequent and variable GABA_A-dependent changes in GnRH neuron firing were observed in brain slices. Chronic activation of ARN GABA neurons in healthy females impaired estrous cyclicity, decreased corpora lutea number and increased circulating testosterone levels.

Interpretation

ARN GABA neurons can stimulate the hypothalamic-pituitary axis and chronic activation of ARN GABA neurons can mimic the reproductive deficits of PCOS in healthy females. Unexpectedly blunted HPG axis responses in PNA mice may reflect a history of high frequency GnRH/LH secretion and reduced LH stores, but also raise questions about impaired function within the ARN GABA population and the involvement of other circuits.

Mapping neuronal inputs to Kiss1 neurons in the arcuate nucleus of the mouse

The normal function of the mammalian reproductive axis is strongly influenced by physiological, metabolic and environmental factors. Kisspeptin neuropeptides, encoded by the Kiss1 gene, are potent regulators of the mammalian reproductive axis by stimulating gonadodropin releasing hormone secretion from the hypothalamus. To understand how the reproductive axis is modulated by higher order neuronal inputs we have mapped the afferent circuits into arcuate (ARC) Kiss1 neurons. We used a transgenic mouse that expresses the CRE recombinase in Kiss1 neurons for conditional viral tracing with genetically modified viruses. CRE-mediated activation of these viruses in Kiss1 neurons allows the virus to move transynaptically to label neurons with primary or secondary afferent inputs into the Kiss1 neurons. Several regions of the brain showed synaptic connectivity to arcuate Kiss1 neurons including proopiomelanocortin neurons in the ARC itself, kisspeptin neurons in the anteroventral periventricular nucleus, vasopressin neurons in the supraoptic and suprachiasmatic nuclei, thyrotropin releasing neurons in the paraventricular nucleus and unidentified neurons in other regions including the subfornical organ, amygdala, interpeduncular nucleus, ventral premammilary nucleus, basal nucleus of stria terminalis and the visual, somatosensory and piriform regions of the cortex. These data provide an insight into how the activity of Kiss1 neurons may be regulated by metabolic signals and provide a detailed neuroanatomical map for future functional studies.

Modulation of Gonadotropin-Releasing Hormone Neuron Activity and Secretion in Mice by Non-peptide Neurotransmitters, Gasotransmitters, and Gliotransmitters

Gonadotropin-releasing hormone (GnRH) neuron activity and GnRH secretion are essential for fertility in mammals. Here, I review findings from mouse studies on the direct modulation of GnRH neuron activity and GnRH secretion by non-peptide neurotransmitters (GABA, glutamate, dopamine, serotonin, norepinephrine, epinephrine, histamine, ATP, adenosine, and acetylcholine), gasotransmitters (nitric oxide and carbon monoxide), and gliotransmitters (prostaglandin E2 and possibly GABA, glutamate, and ATP). These neurotransmitters, gasotransmitters, and gliotransmitters have been shown to directly modulate activity and/or GnRH secretion in GnRH neurons in vivo or ex vivo (brain slices), from postnatal through adult mice, or in embryonic or immortalized mouse GnRH neurons. However, except for GABA, nitric oxide, and prostaglandin E2, which appear to be essential for normal GnRH neuron activity, GnRH secretion, and fertility in males and/or females, the biological significance of their direct modulation of GnRH neuron activity and/or GnRH secretion in the central regulation of reproduction remains largely unknown and requires further exploration.

Distinct Neuronal Projections From the Hypothalamic Ventromedial Nucleus Mediate Glycemic and Behavioral Effects

The hypothalamic ventromedial nucleus (VMN) is implicated both in autonomic control of blood glucose and in behaviors including fear and aggression, but whether these divergent effects involve the same or distinct neuronal subsets and their projections is unknown. To address this question, we used an optogenetic approach to selectively activate the subset of VMN neurons that express neuronal nitric oxide synthase 1 (VMN^NOS1 neurons) implicated in glucose counterregulation. We found that photoactivation of these neurons elicits 1) robust hyperglycemia achieved by activation of counterregulatory responses usually reserved for the physiological response to hypoglycemia and 2) defensive immobility behavior. Moreover, we show that the glucagon, but not corticosterone, response to insulin-induced hypoglycemia is blunted by photoinhibition of the same neurons. To investigate the neurocircuitry by which VMN^NOS1 neurons mediate these effects, and to determine whether these diverse effects are dissociable from one another, we activated downstream VMN^NOS1 projections in either the anterior bed nucleus of the stria terminalis (aBNST) or the periaqueductal gray (PAG). Whereas glycemic responses are fully recapitulated by activation of VMN^NOS1 projections to the aBNST, freezing immobility occurred only upon activation of VMN^NOS1 terminals in the PAG. These findings support previous evidence of a VMN→aBNST neurocircuit involved in glucose counterregulation and demonstrate that activation of VMN^NOS1 neuronal projections supplying the PAG robustly elicits defensive behaviors.

The Versatile Tanycyte: A Hypothalamic Integrator of Reproduction and Energy Metabolism

The fertility and survival of an individual rely on the ability of the periphery to promptly, effectively, and reproducibly communicate with brain neural networks that control reproduction, food intake, and energy homeostasis. Tanycytes, a specialized glial cell type lining the wall of the third ventricle in the median eminence of the hypothalamus, appear to act as the linchpin of these processes by dynamically controlling the secretion of neuropeptides into the portal vasculature by hypothalamic neurons and regulating blood-brain and blood-cerebrospinal fluid exchanges, both processes that depend on the ability of these cells to adapt their morphology to the physiological state of the individual. In addition to their barrier properties, tanycytes possess the ability to sense blood glucose levels, and play a fundamental and active role in shuttling circulating metabolic signals to hypothalamic neurons that control food intake. Moreover, accumulating data suggest that, in keeping with their putative descent from radial glial cells, tanycytes are endowed with neural stem cell properties and may respond to dietary or reproductive cues by modulating hypothalamic neurogenesis. Tanycytes could thus constitute the missing link in the loop connecting behavior, hormonal changes, signal transduction, central neuronal activation and, finally, behavior again. In this article, we will examine these recent advances in the understanding of tanycytic plasticity and function in the hypothalamus and the underlying molecular mechanisms. We will also discuss the putative involvement and therapeutic potential of hypothalamic tanycytes in metabolic and fertility disorders.

Hypoglycemia: Role of Hypothalamic Glucose-Inhibited (GI) Neurons in Detection and Correction

Hypoglycemia is a profound threat to the brain since glucose is its primary fuel. As a result, glucose sensors are widely located in the central nervous system and periphery. In this perspective we will focus on the role of hypothalamic glucose-inhibited (GI) neurons in sensing and correcting hypoglycemia. In particular, we will discuss GI neurons in the ventromedial hypothalamus (VMH) which express neuronal nitric oxide synthase (nNOS) and in the perifornical hypothalamus (PFH) which express orexin. The ability of VMH nNOS-GI neurons to depolarize in low glucose closely parallels the hormonal response to hypoglycemia which stimulates gluconeogenesis. We have found that nitric oxide (NO) production in low glucose is dependent on oxidative status. In this perspective we will discuss the potential relevance of our work showing that enhancing the glutathione antioxidant system prevents hypoglycemia associated autonomic failure (HAAF) in non-diabetic rats whereas VMH overexpression of the thioredoxin antioxidant system restores hypoglycemia counterregulation in rats with type 1 diabetes.We will also address the potential role of the orexin-GI neurons in the arousal response needed for hypoglycemia awareness which leads to behavioral correction (e.g., food intake, glucose administration). The potential relationship between the hypothalamic sensors and the neurocircuitry in the hindbrain and portal mesenteric vein which is critical for hypoglycemia correction will then be discussed.

Female sexual behavior in mice is controlled by kisspeptin neurons

Sexual behavior is essential for the survival of many species. In female rodents, mate preference and copulatory behavior depend on pheromones and are synchronized with ovulation to ensure reproductive success. The neural circuits driving this orchestration in the brain have, however, remained elusive. Here, we demonstrate that neurons controlling ovulation in the mammalian brain are at the core of a branching neural circuit governing both mate preference and copulatory behavior. We show that male odors detected in the vomeronasal organ activate kisspeptin neurons in female mice. Classical kisspeptin/Kiss1R signaling subsequently triggers olfactory-driven mate preference. In contrast, copulatory behavior is elicited by kisspeptin neurons in a parallel circuit independent of Kiss1R involving nitric oxide signaling. Consistent with this, we find that kisspeptin neurons impinge onto nitric oxide-synthesizing neurons in the ventromedial hypothalamus. Our data establish kisspeptin neurons as a central regulatory hub orchestrating sexual behavior in the female mouse brain.

Mate preference and copulatory behavior in female rodents are coordinated with the ovulation cycles of the animal. This study shows that hypothalamic kisspeptin neurons control both mate choice and copulation, and therefore, that sexual behavior and ovulation may be synchronized by the same neuropeptide.

Kisspeptin, gonadotrophin-releasing hormone and oestrogen receptor α colocalise with neuronal nitric oxide synthase neurones in prepubertal female sheep

Puberty is a process that integrates multiple inputs ultimately resulting in an increase in gonadotrophin-releasing hormone (GnRH) secretion. Although kisspeptin neurones play an integral role in GnRH secretion and puberty onset, other systems are also likely important. One potential component is nitric oxide (NO), a gaseous neurotransmitter synthesised by nitric oxide synthase (NOS). The present study aimed to neuroanatomically characterise neuronal NOS (nNOS) in prepubertal female sheep and determine whether oestradiol exerts effects on this system. Luteinising hormone secretion was reduced by oestradiol treatment in prepubertal ovariectomised ewes. Neurones immunoreactive for nNOS were identified in several areas, with the greatest number present in the ventrolateral portion of the ventromedial hypothalamus, followed by the ventromedial hypothalamus, preoptic area (POA) and arcuate nucleus (ARC). Next, we determined whether nNOS neurones contained oestrogen receptor (ER)α and could potentially communicate oestradiol (E₂ ) feedback to GnRH neurones. Neuronal NOS neurones contained ERα with the percentage of coexpression (12%-40%) depending upon the area analysed. We next investigated whether a neuroanatomical relationship existed between nNOS and kisspeptin or nNOS and GnRH neurones. A high percentage of kisspeptin neurones in the POA (79%) and ARC (98%) colocalised with nNOS. Kisspeptin close contacts were also associated with nNOS neurones. A greater number of close contacts were observed in the ARC than the POA. A high percentage of POA GnRH neurones (79%) also expressed nNOS, although no GnRH close contacts were observed onto nNOS neurones. Neither the numbers of nNOS neurones in the POA or hypothalamus, nor the percentage of nNOS coexpression with GnRH, kisspeptin or ERα were influenced by oestradiol. These experiments reveal that a neuroanatomical relationship exists between both nNOS and kisspeptin and nNOS and GnRH in prepubertal ewes. Therefore, nNOS may act both directly and indirectly to influence GnRH secretion in prepubertal sheep.

The gentle art of saying NO: how nitric oxide gets things done in the hypothalamus

The chemical signalling molecule nitric oxide (NO), which freely diffuses through aqueous and lipid environments, subserves an array of functions in the mammalian central nervous system, such as the regulation of synaptic plasticity, blood flow and neurohormone secretion. In this Review, we consider the cellular and molecular mechanisms by which NO evokes short-term and long-term changes in neuronal activity. We also highlight recent studies showing that discrete populations of neurons that synthesize NO in the hypothalamus constitute integrative systems that support life by relaying metabolic and gonadal signals to the neuroendocrine brain, and thus gate the onset of puberty and adult fertility. The putative involvement and therapeutic potential of NO in the pathophysiology of brain diseases, for which hormonal imbalances during postnatal development could be risk factors, is also discussed.