PACAP Neurons in the Ventromedial Hypothalamic Nucleus Are Glucose Inhibited and Their Selective Activation Induces Hyperglycaemia

Neuronal glucose sensing mechanisms and circuits in the control of insulin and glucagon secretion

Glucose homeostasis is mainly under the control of the pancreatic islet hormones insulin and glucagon, which, respectively, stimulate glucose uptake and utilization by liver, fat, and muscle and glucose production by the liver. The balance between the secretions of these hormones is under the control of blood glucose concentrations. Indeed, pancreatic islet β-cells and α-cells can sense variations in glycemia and respond by an appropriate secretory response. However, the secretory activity of these cells is also under multiple additional metabolic, hormonal, and neuronal signals that combine to ensure the perfect control of glycemia over a lifetime. The central nervous system (CNS), which has an almost absolute requirement for glucose as a source of metabolic energy and thus a vital interest in ensuring that glycemic levels never fall below ∼5 mM, is equipped with populations of neurons responsive to changes in glucose concentrations. These neurons control pancreatic islet cell secretion activity in multiple ways: through both branches of the autonomic nervous system, through the hypothalamic-pituitary-adrenal axis, and by secreting vasopressin (AVP) in the blood at the level of the posterior pituitary. Here, we present the autonomic innervation of the pancreatic islets; the mechanisms of neuron activation by a rise or a fall in glucose concentration; how current viral tracing, chemogenetic, and optogenetic techniques allow integration of specific glucose sensing neurons in defined neuronal circuits that control endocrine pancreas function; and, finally, how genetic screens in mice can untangle the diversity of the hypothalamic mechanisms controlling the response to hypoglycemia.

Dynamic, sex- and diet-specific pleiotropism in the PAC1 receptor-mediated regulation of arcuate proopiomelanocortin and Neuropeptide Y/Agouti related peptide neuronal excitability by anorexigenic ventromedial nucleus PACAP neurons

This study furthers the investigation of how pituitary adenylate cyclase activating polypeptide (PACAP) and the PAC1 receptor (PAC1R) regulate the homeostatic energy balance circuitry. We hypothesized that apoptotic ablation of PACAP neurones in the hypothalamic ventromedial nucleus (VMN) would affect both energy intake and energy expenditure. We also hypothesized that selective PAC1R knockdown would impair the PACAP-induced excitation in anorexigenic proopiomelanocortin (POMC) neurones and inhibition of orexigenic neuropeptide Y (NPY)/agouti-related peptide (AgRP) neurones in the hypothalamic arcuate nucleus (ARC). The results show CASPASE-3-induced ablation of VMN PACAP neurones leads to increased energy intake and meal frequency as well as decreased energy expenditure in lean animals. The effects were more robust in obese males, whereas we saw the opposite effects in obese females. We then utilized visualized whole-cell patch clamp recordings in hypothalamic slices. PAC1R knockdown in POMC neurones diminishes the PACAP-induced depolarization, increase in firing, decreases in energy intake and meal size, as well as increases in CO2 production and O2 consumption. Similarly, the lack of expression of the PAC1R in NPY/AgRP neurones greatly attenuates the PACAP-induced hyperpolarization, suppression of firing, decreases in energy intake and meal frequency, as well as increases in energy expenditure. The PACAP response in NPY/AgRP neurones switched from predominantly inhibitory to excitatory in fasted animals. Finally, the anorexigenic effect of PACAP was potentiated when oestradiol was injected into the ARC in ovariectomized females. This study demonstrates the critical role of anorexigenic VMN PACAP neurones and the PAC1R in exciting POMC and inhibiting NPY/AgRP neurons to control homeostatic feeding.

PAC1 receptor modulation of freezing and flight behavior in periaqueductal gray

The midbrain periaqueductal gray (PAG) region is a critical anatomical regulator of fear-related species-specific defensive reactions (SSDRs). Pituitary adenylate-cyclase-activating polypeptide (PACAP), and its main receptor PAC1, play an important role in fear-related behavior and anxiety disorders. However, the function of the PACAP-PAC1 system within the PAG with regards to SSDRs has received little attention. To address this gap, we used transgenic PAC1flox/flox mice to examine both conditional and unconditional defensive reactions. We performed conditional PAC1 gene deletion within the ventrolateral(vl)PAG of PAC1flox/flox mice using an adeno-associated virus (AAV) coding for Cre recombinase. Following viral expression, we used a white noise fear conditioning preparation that produces both an unconditional activity burst to the onset of noise that is followed by conditional freezing. On Day 1, mice received five white noise foot-shock pairings, whereas on Day 2, they were exposed to white noise five times without shock and we scored the activity burst and freezing to the white noise. Following behavioral testing, histology for immunofluorescent analysis was conducted in order to identify PACAP positive cells and stress-induced c-fos activity respectively. We found that PAC1 deletion in vlPAG increased the unconditional activity burst response but disrupted conditional freezing. PAC1 deletion was accompanied by higher c-fos activity following the behavioral experiments. Furthermore, a significant portion of PACAP-EGFP positive cells showed overlapping expression with VGAT, indicating their association with inhibitory neurons. The findings suggested that intact PACAP-PAC1 mechanisms are essential for SSDRs in vlPAG. Therefore, midbrain PACAP contributes to the underlying molecular mechanisms regulating fear responses.

Constitutive and conditional deletion reveals distinct phenotypes driven by developmental versus neurotransmitter actions of the neuropeptide PACAP

Neuropeptides may exert trophic effects during development, and then neurotransmitter roles in the developed nervous system. One way to associate peptide-deficiency phenotypes with either role is first to assess potential phenotypes in so-called constitutive knockout mice, and then proceed to specify, regionally and temporally, where and when neuropeptide expression is required to prevent these phenotypes. We have previously demonstrated that the well-known constellation of behavioral and metabolic phenotypes associated with constitutive pituitary adenylate cyclase-activating peptide (PACAP) knockout mice are accompanied by transcriptomic alterations of two types: those that distinguish the PACAP-null phenotype from wild-type (WT) in otherwise quiescent mice (cPRGs), and gene induction that occurs in response to acute environmental perturbation in WT mice that do not occur in knockout mice (aPRGs). Comparing constitutive PACAP knockout mice to a variety of temporally and regionally specific PACAP knockouts, we show that the prominent hyperlocomotor phenotype is a consequence of early loss of PACAP expression, is associated with Fos overexpression in hippocampus and basal ganglia, and that a thermoregulatory effect previously shown to be mediated by PACAP-expressing neurons of medial preoptic hypothalamus is independent of PACAP expression in those neurons in adult mice. In contrast, PACAP dependence of weight loss/hypophagia triggered by restraint stress, seen in constitutive PACAP knockout mice, is phenocopied in mice in which PACAP is deleted after neuronal differentiation. Our results imply that PACAP has a prominent role as a trophic factor early in development determining global central nervous system characteristics, and in addition a second, discrete set of functions as a neurotransmitter in the fully developed nervous system that support physiological and psychological responses to stress.

Identification of Hypothalamic Glucoregulatory Neurons That Sense and Respond to Changes in Glycemia

To investigate whether glucoregulatory neurons in the hypothalamus can sense and respond to physiological variation in the blood glucose (BG) level, we combined continuous arterial glucose monitoring with continuous measures of the activity of a specific subset of neurons located in the hypothalamic ventromedial nucleus that express pituitary adenylate cyclase activating peptide (VMNPACAP neurons) obtained using fiber photometry. Data were collected in conscious, free-living mice during a 1-h baseline monitoring period and a subsequent 2-h intervention period during which the BG level was raised either by consuming a chow or a high-sucrose meal or by intraperitoneal glucose injection. Cross-correlation analysis revealed that, following a 60- to 90-s delay, interventions that raise the BG level reliably associate with reduced VMNPACAP neuron activity (P < 0.01). In addition, a strong positive correlation between BG and spontaneous VMNPACAP neuron activity was observed under basal conditions but with a much longer (∼25 min) temporal offset, consistent with published evidence that VMNPACAP neuron activation raises the BG level. Together, these findings are suggestive of a closed-loop system whereby VMNPACAP neuron activation increases the BG level; detection of a rising BG level, in turn, feeds back to inhibit these neurons. To our knowledge, these findings constitute the first evidence of a role in glucose homeostasis for glucoregulatory neurocircuits that, like pancreatic β-cells, sense and respond to physiological variation in glycemia.

ARTICLE HIGHLIGHTS

By combining continuous arterial glucose monitoring with fiber photometry, studies investigated whether neurons in the murine ventromedial nucleus that express pituitary adenylate cyclase activating peptide (VMNPACAP neurons) detect and respond to changes in glycemia in vivo. VMNPACAP neuron activity rapidly decreases (within <2 min) when the blood glucose level is raised by either food consumption or glucose administration. Spontaneous VMNPACAP neuron activity also correlates positively with glycemia, but with a longer temporal offset, consistent with reports that hyperglycemia is induced by experimental activation of these neurons. Like pancreatic β-cells, neurons in the hypothalamic ventromedial nucleus appear to sense and respond to physiological variation in glycemia.

mGluR5 in Astrocytes in the Ventromedial Hypothalamus Regulates Pituitary Adenylate Cyclase-Activating Polypeptide Neurons and Glucose Homeostasis

The ventromedial hypothalamus (VMH) is a functionally heterogeneous nucleus critical for systemic energy, glucose, and lipid balance. We showed previously that the metabotropic glutamate receptor 5 (mGluR5) plays essential roles regulating excitatory and inhibitory transmission in SF1+ neurons of the VMH and facilitating glucose and lipid homeostasis in female mice. Although mGluR5 is also highly expressed in VMH astrocytes in the mature brain, its role there influencing central metabolic circuits is unknown. In contrast to the glucose intolerance observed only in female mice lacking mGluR5 in VMH SF1 neurons, selective depletion of mGluR5 in VMH astrocytes enhanced glucose tolerance without affecting food intake or body weight in both adult female and male mice. The improved glucose tolerance was associated with elevated glucose-stimulated insulin release. Astrocytic mGluR5 male and female mutants also exhibited reduced adipocyte size and increased sympathetic tone in gonadal white adipose tissue. Diminished excitatory drive and synaptic inputs onto VMH Pituitary adenylate cyclase-activating polypeptide (PACAP+) neurons and reduced activity of these cells during acute hyperglycemia underlie the observed changes in glycemic control. These studies reveal an essential role of astrocytic mGluR5 in the VMH regulating the excitatory drive onto PACAP+ neurons and activity of these cells facilitating glucose homeostasis in male and female mice.SIGNIFICANCE STATEMENT Neuronal circuits within the VMH play chief roles in the regulation of whole-body metabolic homeostasis. It remains unclear how astrocytes influence neurotransmission in this region to facilitate energy and glucose balance control. Here, we explored the role of the metabotropic glutamate receptor, mGluR5, using a mouse model with selective depletion of mGluR5 from VMH astrocytes. We show that astrocytic mGluR5 critically regulates the excitatory drive and activity of PACAP-expressing neurons in the VMH to control glucose homeostasis in both female and male mice. Furthermore, mGluR5 in VMH astrocytes influences adipocyte size and sympathetic tone in white adipose tissue. These studies provide novel insight toward the importance of hypothalamic astrocytes participating in central circuits regulating peripheral metabolism.

Anoctamin 4 channel currents activate glucose-inhibited neurons in the mouse ventromedial hypothalamus during hypoglycemia

Glucose is the basic fuel essential for maintenance of viability and functionality of all cells. However, some neurons - namely, glucose-inhibited (GI) neurons - paradoxically increase their firing activity in low-glucose conditions and decrease that activity in high-glucose conditions. The ionic mechanisms mediating electric responses of GI neurons to glucose fluctuations remain unclear. Here, we showed that currents mediated by the anoctamin 4 (Ano4) channel are only detected in GI neurons in the ventromedial hypothalamic nucleus (VMH) and are functionally required for their activation in response to low glucose. Genetic disruption of the Ano4 gene in VMH neurons reduced blood glucose and impaired counterregulatory responses during hypoglycemia in mice. Activation of VMHAno4 neurons increased food intake and blood glucose, while chronic inhibition of VMHAno4 neurons ameliorated hyperglycemia in a type 1 diabetic mouse model. Finally, we showed that VMHAno4 neurons represent a unique orexigenic VMH population and transmit a positive valence, while stimulation of neurons that do not express Ano4 in the VMH (VMHnon-Ano4) suppress feeding and transmit a negative valence. Together, our results indicate that the Ano4 channel and VMHAno4 neurons are potential therapeutic targets for human diseases with abnormal feeding behavior or glucose imbalance.

Neural Progenitor Cells and the Hypothalamus

Neural progenitor cells (NPCs) are multipotent neural stem cells (NSCs) capable of self-renewing and differentiating into neurons, astrocytes and oligodendrocytes. In the postnatal/adult brain, NPCs are primarily located in the subventricular zone (SVZ) of the lateral ventricles (LVs) and subgranular zone (SGZ) of the hippocampal dentate gyrus (DG). There is evidence that NPCs are also present in the postnatal/adult hypothalamus, a highly conserved brain region involved in the regulation of core homeostatic processes, such as feeding, metabolism, reproduction, neuroendocrine integration and autonomic output. In the rodent postnatal/adult hypothalamus, NPCs mainly comprise different subtypes of tanycytes lining the wall of the 3^rd ventricle. In the postnatal/adult human hypothalamus, the neurogenic niche is constituted by tanycytes at the floor of the 3^rd ventricle, ependymal cells and ribbon cells (showing a gap-and-ribbon organization similar to that in the SVZ), as well as suprachiasmatic cells. We speculate that in the postnatal/adult human hypothalamus, neurogenesis occurs in a highly complex, exquisitely sophisticated neurogenic niche consisting of at least four subniches; this structure has a key role in the regulation of extrahypothalamic neurogenesis, and hypothalamic and extrahypothalamic neural circuits, partly through the release of neurotransmitters, neuropeptides, extracellular vesicles (EVs) and non-coding RNAs (ncRNAs).

Sustained remission of type 2 diabetes in rodents by centrally administered fibroblast growth factor 4

Type 2 diabetes (T2D) is a major health and economic burden worldwide. Despite the availability of multiple drugs for short-term management, sustained remission of T2D is currently not achievable pharmacologically. Intracerebroventricular administration of fibroblast growth factor 1 (icvFGF1) induces sustained remission in T2D rodents, propelling intense research efforts to understand its mechanism of action. Whether other FGFs possess similar therapeutic benefits is currently unknown. Here, we show that icvFGF4 also elicits a sustained antidiabetic effect in both male db/db mice and diet-induced obese mice by activating FGF receptor 1 (FGFR1) expressed in glucose-sensing neurons within the mediobasal hypothalamus. Specifically, FGF4 excites glucose-excited (GE) neurons while inhibiting glucose-inhibited (GI) neurons. Moreover, icvFGF4 restores the percentage of GI neurons in db/db mice. Importantly, intranasal delivery of FGF4 alleviates hyperglycemia in db/db mice, paving the way for non-invasive therapy. We conclude that icvFGF4 holds significant therapeutic potential for achieving sustained remission of T2D.

Altered glucose-dependent secretion of glucagon and ACTH is associated with insulin resistance, assessed by population analysis

This study aimed to characterize how the dysregulation of counter-regulatory hormones can contribute to insulin resistance and potentially to diabetes. Therefore, we investigated the association between insulin sensitivity and the glucose- and insulin-dependent secretion of glucagon, adrenocorticotropic hormone (ACTH), and cortisol in non-diabetic individuals using a population model analysis. Data, from hyperinsulinemic-hypoglycemic clamps, were pooled for analysis, including 52 individuals with a wide range of insulin resistance (reflected by glucose infusion rate 20-60 min; GIR20-60min). Glucagon secretion was suppressed by glucose and, to a lesser extent, insulin. The GIR20-60min and BMI were identified as predictors of the insulin effect on glucagon. At normoglycemia (5 mmol/L), a 90% suppression of glucagon was achieved at insulin concentrations of 16.3 and 43.4 µU/mL in individuals belonging to the highest and lowest quantiles of insulin sensitivity, respectively. Insulin resistance of glucagon secretion explained the elevated fasting glucagon for individuals with a low GIR20-60min. ACTH secretion was suppressed by glucose and not affected by insulin. The GIR20-60min was superior to other measures as a predictor of glucose-dependent ACTH secretion, with 90% suppression of ACTH secretion by glucose at 3.1 and 3.5 mmol/L for insulin-sensitive and insulin-resistant individuals, respectively. This difference may appear small but shifts the suppression range into normoglycemia for individuals with insulin resistance, thus, leading to earlier and greater ACTH/cortisol response when the glucose falls. Based on modeling of pooled glucose-clamp data, insulin resistance was associated with generally elevated glucagon and a potentiated cortisol-axis response to hypoglycemia, and over time both hormonal pathways may therefore contribute to dysglycemia and possibly type 2 diabetes.

Neuropeptides and small-molecule amine transmitters: cooperative signaling in the nervous system

Neuropeptides are expressed in cell-specific patterns throughout mammalian brain. Neuropeptide gene expression has been useful for clustering neurons by phenotype, based on single-cell transcriptomics, and for defining specific functional circuits throughout the brain. How neuropeptides function as first messengers in inter-neuronal communication, in cooperation with classical small-molecule amine transmitters (SMATs) is a current topic of systems neurobiology. Questions include how neuropeptides and SMATs cooperate in neurotransmission at the molecular, cellular and circuit levels; whether neuropeptides and SMATs always co-exist in neurons; where neuropeptides and SMATs are stored in the neuron, released from the neuron and acting, and at which receptors, after release; and how neuropeptides affect 'classical' transmitter function, both directly upon co-release, and indirectly, via long-term regulation of gene transcription and neuronal plasticity. Here, we review an extensive body of data about the distribution of neuropeptides and their receptors, their actions after neuronal release, and their function based on pharmacological and genetic loss- and gain-of-function experiments, that addresses these questions, fundamental to understanding brain function, and development of neuropeptide-based, and potentially combinatorial peptide/SMAT-based, neurotherapeutics.

The ventromedial hypothalamic nucleus: watchdog of whole-body glucose homeostasis

The brain, particularly the ventromedial hypothalamic nucleus (VMH), has been long known for its involvement in glucose sensing and whole-body glucose homeostasis. However, it is still not fully understood how the brain detects and responds to the changes in the circulating glucose levels, as well as brain-body coordinated control of glucose homeostasis. In this review, we address the growing evidence implicating the brain in glucose homeostasis, especially in the contexts of hypoglycemia and diabetes. In addition to neurons, we emphasize the potential roles played by non-neuronal cells, as well as extracellular matrix in the hypothalamus in whole-body glucose homeostasis. Further, we review the ionic mechanisms by which glucose-sensing neurons sense fluctuations of ambient glucose levels. We also introduce the significant implications of heterogeneous neurons in the VMH upon glucose sensing and whole-body glucose homeostasis, in which sex difference is also addressed. Meanwhile, research gaps have also been identified, which necessities further mechanistic studies in future.

Kir6.2-deficient mice develop somatosensory dysfunction and axonal loss in the peripheral nerves

Glucose-responsive ATP-sensitive potassium channels (K_ATP) are expressed in a variety of tissues including nervous systems. The depolarization of the membrane potential induced by glucose may lead to hyperexcitability of neurons and induce excitotoxicity. However, the roles of K_ATP in the peripheral nervous system (PNS) are poorly understood. Here, we determine the roles of K_ATP in the PNS using K_ATP-deficient (Kir6.2-deficient) mice. We demonstrate that neurite outgrowth of dorsal root ganglion (DRG) neurons was reduced by channel closers sulfonylureas. However, a channel opener diazoxide elongated the neurite. K_ATP subunits were expressed in mouse DRG, and expression of certain subunits including Kir6.2 was increased in diabetic mice. In Kir6.2-deficient mice, the current perception threshold, thermal perception threshold, and sensory nerve conduction velocity were impaired. Electron microscopy revealed a reduction of unmyelinated and small myelinated fibers in the sural nerves. In conclusion, K_ATP may contribute to the development of peripheral neuropathy.

Central Nervous System Control of Glucose Homeostasis: A Therapeutic Target for Type 2 Diabetes?

Historically, pancreatic islet beta cells have been viewed as principal regulators of glycemia, with type 2 diabetes (T2D) resulting when insulin secretion fails to compensate for peripheral tissue insulin resistance. However, glycemia is also regulated by insulin-independent mechanisms that are dysregulated in T2D. Based on evidence supporting its role both in adaptive coupling of insulin secretion to changes in insulin sensitivity and in the regulation of insulin-independent glucose disposal, the central nervous system (CNS) has emerged as a fundamental player in glucose homeostasis. Here, we review and expand upon an integrative model wherein the CNS, together with the islet, establishes and maintains the defended level of glycemia. We discuss the implications of this model for understanding both normal glucose homeostasis and T2D pathogenesis and highlight centrally targeted therapeutic approaches with the potential to restore normoglycemia to patients with T2D.

Pituitary Adenylate Cyclase Activating Polypeptide Inhibits A10 Dopamine Neurons and Suppresses the Binge-like Consumption of Palatable Food

Pituitary adenylate cyclase-activating polypeptide (PACAP) binds to PACAP-specific (PAC1) receptors in multiple hypothalamic areas, especially those regulating energy balance. PACAP neurons in the ventromedial nucleus (VMN) exert anorexigenic effects within the homeostatic energy balance circuitry. Since PACAP can also reduce the consumption of palatable food, we tested the hypothesis that VMN PACAP neurons project to the ventral tegmental area (VTA) to inhibit A₁₀ dopamine neurons via PAC1 receptors and K_ATP channels, and thereby suppress binge-like consumption. We performed electrophysiological recordings in mesencephalic slices from male PACAP-Cre and tyrosine hydroxylase (TH)-Cre mice. Initially, we injected PACAP (30 pmol) into the VTA, where it suppressed binge intake in wildtype male but not female mice. Subsequent tract tracing studies uncovered projections of VMN PACAP neurons to the VTA. Optogenetic stimulation of VMN PACAP neurons in voltage clamp induced an outward current and increase in conductance in VTA neurons, and a hyperpolarization and decrease in firing in current clamp. These effects were markedly attenuated by the K_ATP channel blocker tolbutamide (100 μM) and PAC1 receptor antagonist PACAP_6-38 (200 nM). In recordings from A₁₀ dopamine neurons in TH-Cre mice, we replicated the outward current by perfusing PACAP_1-38 (100 nM). This response was again completely blocked by tolbutamide and PACAP_6-38, and associated with a hyperpolarization and decrease in firing. These findings demonstrate that PACAP activates PAC1 receptors and K_ATP channels to inhibit A₁₀ dopamine neurons and sex-dependently suppress binge-like consumption. Accordingly, they advance our understanding of how PACAP regulates energy homeostasis via the hedonic energy balance circuitry.

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 Role of Mediobasal Hypothalamic PACAP in the Control of Body Weight and Metabolism

Body energy homeostasis results from balancing energy intake and energy expenditure. Central nervous system administration of pituitary adenylate cyclase activating polypeptide (PACAP) dramatically alters metabolic function, but the physiologic mechanism of this neuropeptide remains poorly defined. PACAP is expressed in the mediobasal hypothalamus (MBH), a brain area essential for energy balance. Ventromedial hypothalamic nucleus (VMN) neurons contain, by far, the largest and most dense population of PACAP in the medial hypothalamus. This region is involved in coordinating the sympathetic nervous system in response to metabolic cues in order to re-establish energy homeostasis. Additionally, the metabolic cue of leptin signaling in the VMN regulates PACAP expression. We hypothesized that PACAP may play a role in the various effector systems of energy homeostasis, and tested its role by using VMN-directed, but MBH encompassing, adeno-associated virus (AAVCre) injections to ablate Adcyap1 (gene coding for PACAP) in mice (Adcyap1MBHKO mice). Adcyap1MBHKO mice rapidly gained body weight and adiposity, becoming hyperinsulinemic and hyperglycemic. Adcyap1MBHKO mice exhibited decreased oxygen consumption (VO2), without changes in activity. These effects appear to be due at least in part to brown adipose tissue (BAT) dysfunction, and we show that PACAP-expressing cells in the MBH can stimulate BAT thermogenesis. While we observed disruption of glucose clearance during hyperinsulinemic/euglycemic clamp studies in obese Adcyap1MBHKO mice, these parameters were normal prior to the onset of obesity. Thus, MBH PACAP plays important roles in the regulation of metabolic rate and energy balance through multiple effector systems on multiple time scales, which highlight the diverse set of functions for PACAP in overall energy homeostasis.

Adaptive Changes in the Central Control of Energy Homeostasis Occur in Response to Variations in Energy Status

Energy homeostasis is regulated in coordinate fashion by the brain-gut axis, the homeostatic energy balance circuitry in the hypothalamus and the hedonic energy balance circuitry comprising the mesolimbcortical A10 dopamine pathway. Collectively, these systems convey and integrate information regarding nutrient status and the rewarding properties of ingested food, and formulate it into a behavioral response that attempts to balance fluctuations in consumption and food-seeking behavior. In this review we start with a functional overview of the homeostatic and hedonic energy balance circuitries; identifying the salient neural, hormonal and humoral components involved. We then delve into how the function of these circuits differs in males and females. Finally, we turn our attention to the ever-emerging roles of nociceptin/orphanin FQ (N/OFQ) and pituitary adenylate cyclase-activating polypeptide (PACAP)-two neuropeptides that have garnered increased recognition for their regulatory impact in energy homeostasis-to further probe how the imposed regulation of energy balance circuitry by these peptides is affected by sex and altered under positive (e.g., obesity) and negative (e.g., fasting) energy balance states. It is hoped that this work will impart a newfound appreciation for the intricate regulatory processes that govern energy homeostasis, as well as how recent insights into the N/OFQ and PACAP systems can be leveraged in the treatment of conditions ranging from obesity to anorexia.

Circadian Rhythms of the Hypothalamus: From Function to Physiology

The nearly ubiquitous expression of endogenous 24 h oscillations known as circadian rhythms regulate the timing of physiological functions in the body. These intrinsic rhythms are sensitive to external cues, known as zeitgebers, which entrain the internal biological processes to the daily environmental changes in light, temperature, and food availability. Light directly entrains the master clock, the suprachiasmatic nucleus (SCN) which lies in the hypothalamus of the brain and is responsible for synchronizing internal rhythms. However, recent evidence underscores the importance of other hypothalamic nuclei in regulating several essential rhythmic biological functions. These extra-SCN hypothalamic nuclei also express circadian rhythms, suggesting distinct regions that oscillate either semi-autonomously or independent of SCN innervation. Concurrently, the extra-SCN hypothalamic nuclei are also sensitized to fluctuations in nutrient and hormonal signals. Thus, food intake acts as another powerful entrainer for the hypothalamic oscillators' mediation of energy homeostasis. Ablation studies and genetic mouse models with perturbed extra-SCN hypothalamic nuclei function reveal their critical downstream involvement in an array of functions including metabolism, thermogenesis, food consumption, thirst, mood and sleep. Large epidemiological studies of individuals whose internal circadian cycle is chronically disrupted reveal that disruption of our internal clock is associated with an increased risk of obesity and several neurological diseases and disorders. In this review, we discuss the profound role of the extra-SCN hypothalamic nuclei in rhythmically regulating and coordinating body wide functions.

Cellular and systemic mechanisms for glucose sensing and homeostasis

Glucose is a major source of energy in animals. Maintaining blood glucose levels within a physiological range is important for facilitating glucose uptake by cells, as required for optimal functioning. Glucose homeostasis relies on multiple glucose-sensing cells in the body that constantly monitor blood glucose levels and respond accordingly to adjust its glycemia. These include not only pancreatic β-cells and α-cells that secrete insulin and glucagon, but also central and peripheral neurons regulating pancreatic endocrine function. Different types of cells respond distinctively to changes in blood glucose levels, and the mechanisms involved in glucose sensing are diverse. Notably, recent studies have challenged the currently held views regarding glucose-sensing mechanisms. Furthermore, peripheral and central glucose-sensing cells appear to work in concert to control blood glucose level and maintain glucose and energy homeostasis in organisms. In this review, we summarize the established concepts and recent advances in the understanding of cellular and systemic mechanisms that regulate glucose sensing and its homeostasis.

Changes in neuronal activity across the mouse ventromedial nucleus of the hypothalamus in response to low glucose: Evaluation using an extracellular multi-electrode array approach

The hypothalamic ventromedial nucleus (VMN) is involved in maintaining systemic glucose homeostasis. Neurophysiological studies in rodent brain slices have identified populations of VMN glucose-sensing neurones: glucose-excited (GE) neurones, cells which increased their firing rate in response to increases in glucose concentration, and glucose-inhibited (GI) neurones, which show a reduced firing frequency in response to increasing glucose concentrations. To date, most slice electrophysiological studies characterising VMN glucose-sensing neurones in rodents have utilised the patch clamp technique. Multi-electrode arrays (MEAs) are a state-of-the-art electrophysiological tool enabling the electrical activity of many cells to be recorded across multiple electrode sites (channels) simultaneously. We used a perforated MEA (pMEA) system to evaluate electrical activity changes across the dorsal-ventral extent of the mouse VMN region in response to alterations in glucose concentration. Because intrinsic (ie, direct postsynaptic sensing) and extrinsic (ie, presynaptically modulated) glucosensation were not discriminated, we use the terminology 'GE/presynaptically excited by an increase (PER)' and 'GI/presynaptically excited by a decrease (PED)' in the present study to describe responsiveness to changes in extracellular glucose across the mouse VMN. We observed that 15%-60% of channels were GE/PER, whereas 2%-7% were GI/PED channels. Within the dorsomedial portion of the VMN (DM-VMN), significantly more channels were GE/PER compared to the ventrolateral portion of the VMN (VL-VMN). However, GE/PER channels within the VL-VMN showed a significantly higher basal firing rate in 2.5 mmol l^-1 glucose than DM-VMN GE/PER channels. No significant difference in the distribution of GI/PED channels was observed between the VMN subregions. The results of the present study demonstrate the utility of the pMEA approach for evaluating glucose responsivity across the mouse VMN. pMEA studies could be used to refine our understanding of other neuroendocrine systems by examining population level changes in electrical activity across brain nuclei, thus providing key functional neuroanatomical information to complement and inform the design of single-cell neurophysiological studies.

Selective sexual differentiation of neurone populations may contribute to sex-specific outputs of the ventromedial nucleus of the hypothalamus

Sex differences among neurones in the ventrolateral region of the ventromedial hypothalamic nucleus (VMHvl) allow for the display of a diversity of sex-typical behaviours and physiological responses, ranging from mating behaviour to metabolism. Here, we review recent studies that interrogate the relationship between sex-typical responses and changes in cellular phenotypes. We discuss technologies that increase the resolution of molecular profiling or targeting of cell populations, including single-cell transcriptional profiling and conditional viral genetic approaches to manipulate neurone survival or activity. Overall, emerging studies indicate that sex-typical functions of the VMH may be mediated by phenotypically distinct and sexually differentiated neurone populations within the VMHvl. Future studies in this and other brain regions could exploit cell-type-specific tools to reveal the cell populations and molecular mediators that modulate sex-typical responses. Furthermore, cell-type-specific analyses of the effects of sexually differentiating factors, including sex hormones, can test the hypothesis that distinct cell types within a single brain region vary with respect to sexual differentiation.