Arcuate neuropeptide Y inhibits sympathetic nerve activity via multiple neuropathways

Angiotensin in the Arcuate: Mechanisms Integrating Cardiometabolic Control: The 2022 COH Mid-Career Award for Research Excellence

The American Heart Association has identified obesity as a primary impediment to ongoing improvements in cardiovascular diseases, including hypertension. Although drugs, exercise, diets, and surgeries can each cause weight loss, few subjects maintain a reduced weight over the long term. Dysfunctional integrative control (ie, adaptation) of resting metabolic rate (RMR) appears to underlie this failed weight maintenance, yet the neurobiology of physiological and pathophysiological RMR control is poorly understood. Here, we review recent insights into the cellular and molecular control of RMR by Ang-II (angiotensin II) signaling within the arcuate nucleus of the hypothalamus. Within a unique subset of agouti-related peptide neurons, AT1R (Ang-II type 1 receptors) are implicated in the integrative control of RMR. Furthermore, a spontaneous G protein signal switch of AT1R within this neuron type appears to underlie the pathogenesis of RMR adaptation by qualitatively changing the cellular response to AT1R activation from a β-arrestin-1/Gαi (heterotrimeric G protein, α i subtype)-mediated inhibitory response to a Gαq (heterotrimeric G protein, α q subtype)-mediated stimulatory response. We conclude that therapeutic approaches to obesity are likely hampered by the plasticity of the signaling mechanisms that mediate the normal integrative control of energy balance. The same stimulus that would increase RMR in the normal physiological state may decrease RMR during obesity due to qualitative changes in second-messenger coupling. Understanding the mechanisms that regulate interactions between receptors such as AT1R and its various second messenger signaling cascades will provide novel insights into the pathogenesis of RMR adaptation and potentially point toward new therapeutic approaches for obesity and hypertension.

Activity of the hypothalamic neuropeptide Y increases in adult and decreases in old rats

Middle-aged obesity and aging anorexia with muscle loss (sarcopenia) of old people present public health burden. These alterations may appear both in humans and rodents suggesting the role for regulatory alterations. Previously, we demonstrated that biphasic changes in the weight-reducing (catabolic) effects of neuropeptides of the hypothalamus-adipose tissue axis (e.g. leptin) may contribute to both trends. With regard to the anabolic effects of the hypothalamic neuropeptide Y (NPY) inhibited by leptin, we hypothesized non-linear age-related changes with shifts in the opposite directions. We investigated the orexigenic and hypometabolic effects of intracerebroventricularly administered NPY (hyperphagia induced by NPY injection or changes in food intake, body weight, heart rate, body temperature, locomotor activity during a 7-day NPY infusion), the immunoreactivity and gene expression of NPY in the hypothalamic arcuate nucleus of male Wistar rats of five age groups from young to old. The orexigenic/hypometabolic efficacy and the immunoreactivity of NPY increased in middle-aged animals preceding the peak of adiposity observed in aging rats, then decreased preceding anorexia and weight loss in old rats. These shifts may contribute to the development of both age-related obesity and aging anorexia, sarcopenia, and should be considered in future drug development targeting the NPY system.

Hypothalamic Gliosis is Associated With Multiple Cardiovascular Disease Risk Factors

Background

Hypothalamic gliosis is mechanistically linked to obesity and insulin resistance in rodent models. We tested cross-sectional associations between radiologic measures of hypothalamic gliosis in humans and clinically relevant cardiovascular disease risk factors, as well as prevalent coronary heart disease.

Methods

Using brain MRI images from Framingham Heart Study participants (N=867; mean age, 55 years; 55% females), T2 signal intensities were extracted bilaterally from the region of interest in the mediobasal hypothalamus (MBH) and reference regions in the amygdala (AMY) and putamen (PUT). T2 signal ratios were created in which greater relative T2 signal intensity suggests gliosis. The primary measure compared MBH to AMY (MBH/AMY); a positive control ratio (MBH/PUT) also assessed MBH whereas a negative control (PUT/AMY) did not. Outcomes were BMI, HDL-C, LDL-C, fasting triglycerides, and the presence of hypertension (n=449), diabetes mellitus (n=66), metabolic syndrome (n=254), or coronary heart disease (n=25). Dietary risk factors for gliosis were assessed in a prospective analysis. Statistical testing was performed using linear or logistic regression.

Results

Greater MBH/AMY T2 signal ratios were associated with higher BMI (β = 21.5 [95% CI, 15.4-27.6]; P<0.001), higher fasting triglycerides (β = 1.1 [95% CI, 0.6-1.7]; P<0.001), lower HDL-C (β = -20.8 [95% CI, -40.0 to -1.6]; P=0.034), and presence of hypertension (odds ratio, 1.2 [95% CI, 1.1-1.4]; P=0.0088), and the latter two were independent of BMI. Findings for diabetes mellitus were mixed and attenuated by adjusting for BMI. Metabolic syndrome was associated with MBH/AMY T2 signal ratios (odds ratio, 1.3 [95% CI, 1.1-1.6]; P<0.001). Model results were almost uniformly confirmed by the positive control ratios, whereas negative control ratios that did not test the MBH were unrelated to any outcomes (all P≥0.05). T2 signal ratios were not associated with prevalent coronary heart disease (all P>0.05), but confidence intervals were wide. Self-reported percentages of macronutrient intake were not consistently related to future T2 signal ratios.

Conclusions

Using a well-established study of cardiovascular disease development, we found evidence linking hypothalamic gliosis to multiple cardiovascular disease risk factors, even independent of adiposity. Our results highlight the need to consider neurologic mechanisms to understand and improve cardiometabolic health.

Modulating reward and aversion: Insights into addiction from the paraventricular nucleus

BACKGROUND

Drug addiction, characterized by compulsive drug use and high relapse rates, arises from complex interactions between reward and aversion systems in the brain. The paraventricular nucleus (PVN), located in the anterior hypothalamus, serves as a neuroendocrine center and is a key component of the hypothalamic-pituitary-adrenal axis.

OBJECTIVE

This review aimed to explore how the PVN impacts reward and aversion in drug addiction through stress responses and emotional regulation and to evaluate the potential of PVN as a therapeutic target for drug addiction.

METHODS

We review the current literature, focusing on three main neuron types in the PVN-corticotropin-releasing factor, oxytocin, and arginine vasopressin neurons-as well as other related neurons, to understand their roles in modulating addiction.

RESULTS

Existing studies highlight the PVN as a key mediator in addiction, playing a dual role in reward and aversion systems. These findings are crucial for understanding addiction mechanisms and developing targeted therapies.

CONCLUSION

The role of PVN in stress response and emotional regulation suggests its potential as a therapeutic target in drug addiction, offering new insights for addiction treatment.

Neural Circuits Underlying Reciprocal Cardiometabolic Crosstalk: 2023 Arthur C. Corcoran Memorial Lecture

The interplay of various body systems, encompassing those that govern cardiovascular and metabolic functions, has evolved alongside the development of multicellular organisms. This evolutionary process is essential for the coordination and maintenance of homeostasis and overall health by facilitating the adaptation of the organism to internal and external cues. Disruption of these complex interactions contributes to the development and progression of pathologies that involve multiple organs. Obesity-associated cardiovascular risks, such as hypertension, highlight the significant influence that metabolic processes exert on the cardiovascular system. This cardiometabolic communication is reciprocal, as indicated by substantial evidence pointing to the ability of the cardiovascular system to affect metabolic processes, with pathophysiological implications in disease conditions. In this review, I outline the bidirectional nature of the cardiometabolic interaction, with special emphasis on the impact that metabolic organs have on the cardiovascular system. I also discuss the contribution of the neural circuits and autonomic nervous system in mediating the crosstalk between cardiovascular and metabolic functions in health and disease, along with the molecular mechanisms involved.

Sympathetic NPY controls glucose homeostasis, cold tolerance, and cardiovascular functions in mice

Neuropeptide Y (NPY) is best known for its effects in the brain as an orexigenic and anxiolytic agent and in reducing energy expenditure. NPY is also co-expressed with norepinephrine (NE) in sympathetic neurons. Although NPY is generally considered to modulate noradrenergic responses, its specific roles in autonomic physiology remain under-appreciated. Here, we show that sympathetic-derived NPY is essential for metabolic and cardiovascular regulation in mice. NPY and NE are co-expressed in 90% of prevertebral sympathetic neurons and only 43% of paravertebral neurons. NPY-expressing neurons primarily innervate blood vessels in peripheral organs. Sympathetic-specific NPY deletion elicits pronounced metabolic and cardiovascular defects in mice, including reductions in insulin secretion, glucose tolerance, cold tolerance, and pupil size and elevated heart rate, while notably, however, basal blood pressure was unchanged. These findings provide insight into target tissue-specific functions of NPY derived from sympathetic neurons and imply its potential involvement in metabolic and cardiovascular diseases.

GIRK2 potassium channels expressed by the AgRP neurons decrease adiposity and body weight in mice

It is well known that the neuropeptide Y (NPY)/agouti-related peptide (AgRP) neurons increase appetite and decrease thermogenesis. Previous studies demonstrated that optogenetic and/or chemogenetic manipulations of NPY/AgRP neuronal activity alter food intake and/or energy expenditure (EE). However, little is known about intrinsic molecules regulating NPY/AgRP neuronal excitability to affect long-term metabolic function. Here, we found that the G protein-gated inwardly rectifying K+ (GIRK) channels are key to stabilize NPY/AgRP neurons and that NPY/AgRP neuron-selective deletion of the GIRK2 subunit results in a persistently increased excitability of the NPY/AgRP neurons. Interestingly, increased body weight and adiposity observed in the NPY/AgRP neuron-selective GIRK2 knockout mice were due to decreased sympathetic activity and EE, while food intake remained unchanged. The conditional knockout mice also showed compromised adaptation to coldness. In summary, our study identified GIRK2 as a key determinant of NPY/AgRP neuronal excitability and driver of EE in physiological and stress conditions.

Sympathetic NPY controls glucose homeostasis, cold tolerance, and cardiovascular functions in mice

Neuropeptide Y (NPY) is best known for its effects in the brain as an orexigenic and anxiolytic agent and in reducing energy expenditure. NPY is also co-expressed with Norepinephrine (NE) in sympathetic neurons. Although NPY is generally considered to modulate noradrenergic responses, its specific roles in autonomic physiology remain under-appreciated. Here, we show that sympathetic-derived NPY is essential for metabolic and cardiovascular regulation in mice. NPY and NE are co-expressed in 90% of prevertebral sympathetic neurons and only 43% of paravertebral neurons. NPY-expressing neurons primarily innervate blood vessels in peripheral organs. Sympathetic-specific deletion of NPY elicits pronounced metabolic and cardiovascular defects in mice, including reductions in insulin secretion, glucose tolerance, cold tolerance, pupil size, and an elevation in heart rate, while notably, however, basal blood pressure was unchanged. These findings provide new knowledge about target tissue-specific functions of NPY derived from sympathetic neurons and imply its potential involvement in metabolic and cardiovascular diseases.

Cell-specific transcriptome changes in the hypothalamic arcuate nucleus in a mouse deoxycorticosterone acetate-salt model of hypertension

A common preclinical model of hypertension characterized by low circulating renin is the "deoxycorticosterone acetate (DOCA)-salt" model, which influences blood pressure and metabolism through mechanisms involving the angiotensin II type 1 receptor (AT1R) in the brain. More specifically, AT1R within Agouti-related peptide (AgRP) neurons of the arcuate nucleus of the hypothalamus (ARC) has been implicated in selected effects of DOCA-salt. In addition, microglia have been implicated in the cerebrovascular effects of DOCA-salt and angiotensin II. To characterize DOCA-salt effects upon the transcriptomes of individual cell types within the ARC, we used single-nucleus RNA sequencing (snRNAseq) to examine this region from male C57BL/6J mice that underwent sham or DOCA-salt treatment. Thirty-two unique primary cell type clusters were identified. Sub-clustering of neuropeptide-related clusters resulted in identification of three distinct AgRP subclusters. DOCA-salt treatment caused subtype-specific changes in gene expression patterns associated with AT1R and G protein signaling, neurotransmitter uptake, synapse functions, and hormone secretion. In addition, two primary cell type clusters were identified as resting versus activated microglia, and multiple distinct subtypes of activated microglia were suggested by sub-cluster analysis. While DOCA-salt had no overall effect on total microglial density within the ARC, DOCA-salt appeared to cause a redistribution of the relative abundance of activated microglia subtypes. These data provide novel insights into cell-specific molecular changes occurring within the ARC during DOCA-salt treatment, and prompt increased investigation of the physiological and pathophysiological significance of distinct subtypes of neuronal and glial cell types.

Leptin Increases: Physiological Roles in the Control of Sympathetic Nerve Activity, Energy Balance, and the Hypothalamic-Pituitary-Thyroid Axis

It is well established that decreases in plasma leptin levels, as with fasting, signal starvation and elicit appropriate physiological responses, such as increasing the drive to eat and decreasing energy expenditure. These responses are mediated largely by suppression of the actions of leptin in the hypothalamus, most notably on arcuate nucleus (ArcN) orexigenic neuropeptide Y neurons and anorexic pro-opiomelanocortin neurons. However, the question addressed in this review is whether the effects of increased leptin levels are also significant on the long-term control of energy balance, despite conventional wisdom to the contrary. We focus on leptin's actions (in both lean and obese individuals) to decrease food intake, increase sympathetic nerve activity, and support the hypothalamic-pituitary-thyroid axis, with particular attention to sex differences. We also elaborate on obesity-induced inflammation and its role in the altered actions of leptin during obesity.

Melanocortin MC4R receptor is required for energy expenditure but not blood pressure effects of angiotensin II within the mouse brain

The brain renin-angiotensin system (RAS) is implicated in control of blood pressure (BP), fluid intake, and energy expenditure (EE). Angiotensin II (ANG II) within the arcuate nucleus of the hypothalamus contributes to control of resting metabolic rate (RMR) and thereby EE through its actions on Agouti-related peptide (AgRP) neurons, which also contribute to EE control by leptin. First, we determined that although leptin stimulates EE in control littermates, mice with transgenic activation of the brain RAS (sRA) exhibit increased EE and leptin has no additive effect to exaggerate EE in these mice. These findings led us to hypothesize that leptin and ANG II in the brain stimulate EE through a shared mechanism. Because AgRP signaling to the melanocortin MC4R receptor contributes to the metabolic effects of leptin, we performed a series of studies examining RMR, fluid intake, and BP responses to ANG II in mice rendered deficient for expression of MC4R via a transcriptional block (Mc4r-TB). These mice were resistant to stimulation of RMR in response to activation of the endogenous brain RAS via chronic deoxycorticosterone acetate (DOCA)-salt treatment, whereas fluid and electrolyte effects remained intact. These mice were also resistant to stimulation of RMR via acute intracerebroventricular (ICV) injection of ANG II, whereas BP responses to ICV ANG II remained intact. Collectively, these data demonstrate that the effects of ANG II within the brain to control RMR and EE are dependent on MC4R signaling, whereas fluid homeostasis and BP responses are independent of MC4R signaling.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Involvement of Neuropeptide Y within Paraventricular Nucleus in Electroacupuncture Inhibiting Sympathetic Activities in Hypertensive Rats

Although electroacupuncture (EA) has been used to decrease the blood pressure (BP) clinically, the underlying mechanisms are not clearly clarified. This study aimed to assess the hypothesis that EA treatment exerts a hypotensive action via suppressing sympathetic activities and modulating neuropeptide Y (NPY) function within the paraventricular nucleus (PVN) of hypertensive rats. Male Sprague-Dawley rats were selected for the experiment, and the hypertensive models were established by the two-kidney, one-clip (2K1C) method. Then, the rats were randomly assigned to the sham group, 2K1C group, 2K1C plus EA group, and 2K1C plus sham EA group. EA treatment at the acupoints ST36 and ST40 overlying the peroneal nerves was given once a day for 30 days. The radiotelemetry system was applied to collect the arterial BP recordings. Power spectral analyses of BP variability, BP responses to ganglionic blockade, and plasma levels of norepinephrine and epinephrine were performed to assess the changes in sympathetic nerve activity. Real-time PCR and Western blots were carried out to examine the expression of NPY system in the PVN. The responses of PVN microinjection with NPY Y1R antagonist BIBO3304 were detected to check the endogenous NPY tone. The results showed that the enhanced arterial BP and sympathetic activities were effectively reduced by 30 days of EA treatment, and baroreflex sensitivity was improved in 2K1C hypertensive rats. The level of NPY mRNA and protein expression in the PVN was markedly upregulated by EA treatment in 2K1C rats. In addition, the pressor responses of PVN microinjection with NPY Y1R antagonist BIBO3304 in 2K1C models were remarkably augmented by the EA stimulation. Our results indicate that the increased NPY expression and function in the PVN induced by EA treatment contribute to antihypertensive and sympathetic suppression on hypertensive rats. The findings may elucidate the underlying mechanisms of the acupuncture to be a potential therapeutic strategy against hypertension.

The Role of Neuropeptide Y in Adipocyte-Macrophage Crosstalk during High Fat Diet-Induced Adipose Inflammation and Liver Steatosis

Obesity is associated with an increased risk of non-alcoholic fatty liver disease (NAFLD), which is initiated by adipocyte-macrophage crosstalk. Among the possible molecules regulating this crosstalk, we focused on neuropeptide Y (NPY), which is known to be involved in hypothalamic appetite and adipose tissue inflammation and metabolism. In this study, the NPY^−/− mice showed a marked decrease in body weight and adiposity, and lower free fatty acid and adipose inflammation without food intake alteration during a high fat diet (HFD). Moreover, NPY deficiency increased the thermogenic genes expression in brown adipose tissue. Notably, NPY-mRNA expression was upregulated in macrophages from the HFD mice compared to that from the mice on a standard diet. The NPY-mRNA expression also positively correlated with the liver mass/body weight ratio. NPY deletion alleviated HFD-induced adipose inflammation and liver steatosis. Hence, our findings point toward a novel intracellular mechanism of NPY in the regulation of adipocyte-macrophage crosstalk and highlight NPY antagonism as a promising target for therapeutic approaches against obesity and NAFLD.

Rolling out physical exercise and energy homeostasis: Focus on hypothalamic circuitries

Energy balance is the fine regulation of energy expenditure and energy intake. Negative energy balance causes body weight loss, while positive energy balance promotes weight gain. Modern societies offer a maladapted way of life, where easy access to palatable foods and the lack of opportunities to perform physical activity are considered the roots of the obesity pandemic. Physical exercise increases energy expenditure and, consequently, is supposed to promote weight loss. Paradoxically, physical exercise acutely drives anorexigenic-like effects, but the mechanisms are still poorly understood. Using an evolutionary background, this review aims to highlight the potential involvement of the melanocortin system and other hypothalamic neural circuitries regulating energy balance during and after physical exercise. The physiological significance of these changes will be explored, and possible signalling agents will be addressed. The knowledge discussed here might be important for clarifying obesity aetiology as well as new therapeutic approaches for body weight loss.

Skeleton interoception regulates bone and fat metabolism through hypothalamic neuroendocrine NPY

The central nervous system regulates activity of peripheral organs through interoception. In our previous study, we have demonstrated that PGE2/EP4 skeleton interception regulate bone homeostasis. Here, we show that ascending skeleton interoceptive signaling downregulates expression of hypothalamic neuropeptide Y (NPY) and induce lipolysis of adipose tissue for osteoblastic bone formation. Specifically, the ascending skeleton interoceptive signaling induces expression of small heterodimer partner-interacting leucine zipper protein (SMILE) in the hypothalamus. SMILE binds to pCREB as a transcriptional heterodimer on Npy promoters to inhibit NPY expression. Knockout of EP4 in sensory nerve increases expression of NPY causing bone catabolism and fat anabolism. Importantly, inhibition of NPY Y1 receptor (Y1R) accelerated oxidation of free fatty acids in osteoblasts and rescued bone loss in AvilCre:Ptger4fl/fl mice. Thus, downregulation of hypothalamic NPY expression lipolyzes free fatty acids for anabolic bone formation through a neuroendocrine descending interoceptive regulation.

Neuropeptide Y suppresses thermogenic and cardiovascular sympathetic nerve activity via Y1 receptors in the paraventricular nucleus and dorsomedial hypothalamus

In hungry animals, neuropeptide Y (NPY) neurones in the arcuate nucleus (ArcN) are activated to suppress energy expenditure, in part by decreasing brown adipose tissue sympathetic nerve activity (BAT SNA); however, the NPY receptor subtype and brain neurocircuitry are unclear. In the present study, we investigated the inhibition of BAT SNA by exogenous and endogenous NPY via binding to Y1 receptors (NPY1R) in the hypothalamic paraventricular nucleus (PVN) and dorsomedial hypothalamus (DMH), in anaesthetised male rats. Downstream projections of PVN/DMH NPY1R-expressing neurones were identified using male Npy1r-cre mice and localised unilateral DMH or PVN injections of an adeno-associated virus, which allows for the cre-dependent expression of a fluorescent protein (mCherry) in the cell bodies, axon fibres and nerve terminals of NPY1R-containing neurones. Nanoinjections of NPY into the DMH of cooled rats decreased BAT SNA, as well as mean arterial pressure (MAP) and heart rate (HR), and these responses were reversed by subsequent injection of the selective NPY1R antagonist, BIBO3304. In warmed rats, with little to no BAT SNA, bilateral nanoinjections of BIBO3304 into the DMH or PVN increased BAT SNA, MAP and HR. DMH NPY1R-expressing neurones projected heavily to the raphe pallidus (RPa), which houses BAT presympathetic neurones, as well as the PVN. In anaesthetised mice, DMH BIBO3304 increased splanchnic SNA, MAP and HR, all of which were reversed by nonselective blockade of the PVN with muscimol, suggesting that DMH-to-PVN connections are involved in this DMH BIBO3304 disinhibition. PVN Y1R expressing neurones also projected to the RPa, as well as to the nucleus tractus solitarius. We conclude that NPY tonically released in the DMH and PVN suppresses BAT SNA, MAP and HR via Y1R. Downstream neuropathways for BAT SNA may utilise direct projections to the RPa. Release of tonic NPY inhibition of BAT SNA may contribute to feeding- and diet-induced thermogenesis.

Bone-to-Brain: A Round Trip in the Adaptation to Mechanical Stimuli

Besides the classical ones (support/protection, hematopoiesis, storage for calcium, and phosphate) multiple roles emerged for bone tissue, definitively making it an organ. Particularly, the endocrine function, and in more general terms, the capability to sense and integrate different stimuli and to send signals to other tissues, has highlighted the importance of bone in homeostasis. Bone is highly innervated and hosts all nervous system branches; bone cells are sensitive to most of neurotransmitters, neuropeptides, and neurohormones that directly affect their metabolic activity and sensitivity to mechanical stimuli. Indeed, bone is the principal mechanosensitive organ. Thanks to the mechanosensing resident cells, and particularly osteocytes, mechanical stimulation induces metabolic responses in bone forming (osteoblasts) and bone resorbing (osteoclasts) cells that allow the adaptation of the affected bony segment to the changing environment. Once stimulated, bone cells express and secrete, or liberate from the entrapping matrix, several mediators (osteokines) that induce responses on distant targets. Brain is a target of some of these mediator [e.g., osteocalcin, lipocalin2, sclerostin, Dickkopf-related protein 1 (Dkk1), and fibroblast growth factor 23], as most of them can cross the blood-brain barrier. For others, a role in brain has been hypothesized, but not yet demonstrated. As exercise effectively modifies the release and the circulating levels of these osteokines, it has been hypothesized that some of the beneficial effects of exercise on brain functions may be associated to such a bone-to-brain communication. This hypothesis hides an interesting clinical clue: may well-addressed physical activities support the treatment of neurodegenerative diseases, such as Alzheimer’s and Parkinson’s diseases?

Restriction of food intake by PPP1R17-expressing neurons in the DMH

We set out to identify novel neuronal populations controlling feeding behaviors. Therefore, we focused on identifying novel neuronal populations modulating food intake and body weight. In this article, we report that DMH^Ppp1r17 neurons are activated by increased food intake and that activating them results in decreased food intake and body weight, while inhibiting them leads to increased body weight and food intake. These data suggest that DMH^Ppp1r17 neurons restrict binges of eating. In addition to its basic science importance, these findings could have therapeutic applications, as they suggest that pharmacologic activation of PPP1R17 neurons could potentially reduce weight in settings of obesity and binge-like eating.

Leptin-deficient ob/ob mice eat voraciously, and their food intake is markedly reduced by leptin treatment. In order to identify potentially novel sites of leptin action, we used PhosphoTRAP to molecularly profile leptin-responsive neurons in the hypothalamus and brainstem. In addition to identifying several known leptin responsive populations, we found that neurons in the dorsomedial hypothalamus (DMH) of ob/ob mice expressing protein phosphatase 1 regulatory subunit 17 (PPP1R17) constitutively express cFos and that this is suppressed by leptin treatment. Because ob mice are hyperphagic, we hypothesized that activating PPP1R17 neurons would increase food intake. However, chemogenetic activation of PPP1R17 neurons decreased food intake and body weight of ob/ob mice while inhibition of PPP1R17 neurons increased them. Similarly, in a scheduled feeding protocol that elicits increased consumption, mice also ate more when PPP1R17 neurons were inhibited and ate less when they were activated. Finally, we found that pair-feeding of ob mice reduced cFos expression to a similar extent as leptin and that reducing the amount of food available during scheduled feeding in DMH^Ppp1r17 neurons also decreased cFos in DMH^Ppp1r17 neurons. Finally, these neurons do not express the leptin receptor, suggesting that the effect of leptin on these neurons is indirect and secondary to reduced food intake. In aggregate, these results show that PPP1R17 neurons in the DMH are activated by increased food intake and in turn restrict intake to limit overconsumption, suggesting that they function to constrain binges of eating.

Does chronic systemic injection of the DREADD agonists clozapine-N-oxide or Compound 21 change behavior relevant to locomotion, exploration, anxiety, and depression in male non-DREADD-expressing mice?

Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) are chemogenetic tools commonly-used to manipulate brain activity. The most widely-used synthetic DREADD ligand, clozapine-N-oxide (CNO), is back-metabolized to clozapine which can itself activate endogenous receptors. Studies in non-DREADD-expressing rodents suggest CNO or a DREADD agonist that lacks active metabolites, such as Compound 21 (C21), change rodent behavior (e.g. decrease locomotion), but chronic injection of CNO does not change locomotion. However, it is unknown if chronic CNO changes behaviors relevant to locomotion, exploration, anxiety, and depression, or if chronic C21 changes any aspect of mouse behavior. Here non-DREADD-expressing mice received i.p. Vehicle (Veh), CNO, or C21 (1 mg/kg) 5 days/week for 16 weeks and behaviors were assessed over time. Veh, CNO, and C21 mice had similar weight gain over the 16-week-experiment. During the 3rd injection week, CNO and C21 mice explored more than Veh mice in a novel context and had more open field center entries; however, groups were similar in other measures of locomotion and anxiety. During the 14th-16th injection weeks, Veh, CNO, and C21 mice had similar locomotion and anxiety-like behaviors. We interpret these data as showing chronic Veh, CNO, and C21 injections given to male non-DREADD-expressing mice largely lack behavioral effects. These data may be helpful for behavioral neuroscientists when study design requires repeated injection of these DREADD agonists.

Neuropeptide Y Is an Immunomodulatory Factor: Direct and Indirect

Neuropeptide Y (NPY), which is widely distributed in the nervous system, is involved in regulating a variety of biological processes, including food intake, energy metabolism, and emotional expression. However, emerging evidence points to NPY also as a critical transmitter between the nervous system and immune system, as well as a mediator produced and released by immune cells. In vivo and in vitro studies based on gene-editing techniques and specific NPY receptor agonists and antagonists have demonstrated that NPY is responsible for multifarious direct modulations on immune cells by acting on NPY receptors. Moreover, via the central or peripheral nervous system, NPY is closely connected to body temperature regulation, obesity development, glucose metabolism, and emotional expression, which are all immunomodulatory factors for the immune system. In this review, we focus on the direct role of NPY in immune cells and particularly discuss its indirect impact on the immune response.

Activation of hypothalamic AgRP and POMC neurons evokes disparate sympathetic and cardiovascular responses

The arcuate nucleus of the hypothalamus (ARC) plays a key role in linking peripheral metabolic status to the brain melanocortin system, which influences a wide range of physiological processes including the sympathetic nervous system and blood pressure. The importance of the activity of agouti-related peptide (AgRP)- and proopiomelanocortin (POMC)-expressing neurons, two molecularly distinct populations of ARC neurons, for metabolic regulation is well established, but their relevance for sympathetic and cardiovascular control remains unclear. We used designer receptors exclusively activated by designer drug (DREADD) technology to study how activation of AgRP and POMC neurons affect renal sympathetic nerve traffic and blood pressure. In addition to the drastic feeding-stimulatory effect, DREADD-mediated activation of AgRP, but not POMC neurons, induced an acute reduction in renal sympathetic nerve activity in conscious mice. Paradoxically, however, DREADD-mediated chronic activation of AgRP neurons caused a significant increase in blood pressure specifically in the inactive light phase. On the other hand, chronic activation of POMC neurons led to a significant reduction in blood pressure. These results bring new insights to a previously unappreciated role of ARC AgRP and POMC neuronal activity in autonomic and cardiovascular regulation.NEW & NOTEWORTHY Agouti-related peptide (AgRP)- and proopiomelanocortin (POMC)-expressing neurons of the arcuate nucleus are essential components of the brain melanocortin system that controls various physiological processes. Here, we tested the metabolic and cardiovascular effects of direct activation of these two populations of neurons. Our findings show that, in addition to stimulation of food intake, chemogenetic mediated activation of hypothalamic arcuate nucleus AgRP, but not POMC, neurons reduce renal sympathetic traffic. Despite this, chronic activation of AgRP neurons increased blood pressure. However, chronic activation of POMC neurons led to a significant reduction in blood pressure. Our findings highlight the importance of arcuate nucleus AgRP and POMC neuronal activity in autonomic and cardiovascular regulation.

Leptin signalling on arcuate NPY neurones controls adiposity independent of energy balance or diet composition

Central action of the adipocyte hormone leptin via the neuropeptide Y (NPY) system is considered critical for energy homeostatic control. However, the precise mechanisms for this control are still not clear. To specifically investigate how leptin signalling on the NPY neurone contributes to the control of energy homeostasis, we generated an inducible adult-onset NPY neurone-specific leptin receptor (Lepr) knockout model and performed a comprehensive metabolic phenotyping study. Here, we show that the NPY neurone subpopulation that is directly responsive to leptin is not required for the inhibition of fasting-induced hyperphagia by leptin, although it is essential for the regulation of adiposity independent of changes in energy balance or diet composition. Furthermore, under obesogenic conditions such as a high-fat diet, a lack of Lepr signalling on NPY neurones results in significant increases in food intake and concomitant reductions in energy expenditure, leading to accelerated accumulation of fat mass. Collectively, these findings support the notion that Lepr-expressing NPY neurones act as the key relay point where peripheral adipose storage information is sensed, and corresponding responses are initiated to protect adipose reserves.

Crosstalk of Brain and Bone-Clinical Observations and Their Molecular Bases

As brain and bone disorders represent major health issues worldwide, substantial clinical investigations demonstrated a bidirectional crosstalk on several levels, mechanistically linking both apparently unrelated organs. While multiple stress, mood and neurodegenerative brain disorders are associated with osteoporosis, rare genetic skeletal diseases display impaired brain development and function. Along with brain and bone pathologies, particularly trauma events highlight the strong interaction of both organs. This review summarizes clinical and experimental observations reported for the crosstalk of brain and bone, followed by a detailed overview of their molecular bases. While brain-derived molecules affecting bone include central regulators, transmitters of the sympathetic, parasympathetic and sensory nervous system, bone-derived mediators altering brain function are released from bone cells and the bone marrow. Although the main pathways of the brain-bone crosstalk remain ‘efferent’, signaling from brain to bone, this review emphasizes the emergence of bone as a crucial ‘afferent’ regulator of cerebral development, function and pathophysiology. Therefore, unraveling the physiological and pathological bases of brain-bone interactions revealed promising pharmacologic targets and novel treatment strategies promoting concurrent brain and bone recovery.

Neuronal Networks in Hypertension: Recent Advances

Neurogenic hypertension is associated with excessive sympathetic nerve activity to the kidneys and portions of the cardiovascular system. Here we examine the brain regions that cause heightened sympathetic nerve activity in animal models of neurogenic hypertension, and we discuss the triggers responsible for the changes in neuronal activity within these regions. We highlight the limitations of the evidence and, whenever possible, we briefly address the pertinence of the findings to human hypertension. The arterial baroreflex reduces arterial blood pressure variability and contributes to the arterial blood pressure set point. This set point can also be elevated by a newly described cerebral blood flow-dependent and astrocyte-mediated sympathetic reflex. Both reflexes converge on the presympathetic neurons of the rostral medulla oblongata, and both are plausible causes of neurogenic hypertension. Sensory afferent dysfunction (reduced baroreceptor activity, increased renal, or carotid body afferent) contributes to many forms of neurogenic hypertension. Neurogenic hypertension can also result from activation of brain nuclei or sensory afferents by excess circulating hormones (leptin, insulin, Ang II [angiotensin II]) or sodium. Leptin raises blood vessel sympathetic nerve activity by activating the carotid bodies and subsets of arcuate neurons. Ang II works in the lamina terminalis and probably throughout the brain stem and hypothalamus. Sodium is sensed primarily in the lamina terminalis. Regardless of its cause, the excess sympathetic nerve activity is mediated to some extent by activation of presympathetic neurons located in the rostral ventrolateral medulla or the paraventricular nucleus of the hypothalamus. Increased activity of the orexinergic neurons also contributes to hypertension in selected models.

Obesity: sex and sympathetics

Obesity increases sympathetic nerve activity (SNA) in men, but not women. Here, we review current evidence suggesting that sexually dimorphic sympathoexcitatory responses to leptin and insulin may contribute. More specifically, while insulin increases SNA similarly in lean males and females, this response is markedly amplified in obese males, but is abolished in obese females. In lean female rats, leptin increases a subset of sympathetic nerves only during the high estrogen proestrus reproductive phase; thus, in obese females, because reproductive cycling can become impaired, the sporadic nature of leptin-induced sympathoexcitaton could minimize its action, despite elevated leptin levels. In contrast, in males, obesity preserves or enhances the central sympathoexcitatory response to leptin, and current evidence favors leptin’s contribution to the well-established increases in SNA induced by obesity in men. Leptin and insulin increase SNA via receptor binding in the hypothalamic arcuate nucleus and a neuropathway that includes arcuate neuropeptide Y (NPY) and proopiomelanocortin (POMC) projections to the paraventricular nucleus. These metabolic hormones normally suppress sympathoinhibitory NPY neurons and activate sympathoexcitatory POMC neurons. However, obesity appears to alter the ongoing activity and responsiveness of arcuate NPY and POMC neurons in a sexually dimorphic way, such that SNA increases in males but not females. We propose hypotheses to explain these sex differences and suggest areas of future research.

Sites and sources of sympathoexcitation in obese male rats: role of brain insulin

In males, obesity increases sympathetic nerve activity (SNA), but the mechanisms are unclear. Here, we investigate insulin, via an action in the arcuate nucleus (ArcN), and downstream neuropathways, including melanocortin receptor 3/4 (MC3/4R) in the hypothalamic paraventricular nucleus (PVN) and dorsal medial hypothalamus (DMH). We studied conscious and α-chloralose-anesthetized Sprague-Dawley rats fed a high-fat diet, which causes obesity prone (OP) rats to accrue excess fat and obesity-resistant (OR) rats to maintain fat content, similar to rats fed a standard control (CON) diet. Nonspecific blockade of the ArcN with muscimol and specific blockade of ArcN insulin receptors (InsR) decreased lumbar SNA (LSNA), heart rate (HR), and mean arterial pressure (MAP) in OP, but not OR or CON, rats, indicating that insulin supports LSNA in obese males. In conscious rats, intracerebroventricular infusion of insulin increased MAP only in OP rats and also improved HR baroreflex function from subnormal to supranormal. The brain sensitization to insulin may elucidate how insulin can drive central SNA pathways when transport of insulin across the blood-brain barrier may be impaired. Blockade of PVN, but not DMH, MC3/4R with SHU9119 decreased LSNA, HR, and, MAP in OP, but not OR or CON, rats. Interestingly, nanoinjection of the MC3/4R agonist melanotan II (MTII) into the PVN increased LSNA only in OP rats, similar to PVN MTII-induced increases in LSNA in CON rats after blockade of sympathoinhibitory neuropeptide Y Y1 receptors. ArcN InsR expression was not increased in OP rats. Collectively, these data indicate that obesity increases SNA, in part via increased InsR signaling and downstream PVN MC3/4R.

Adaptations in autonomic nervous system regulation in normal and hypertensive pregnancy

There is an increase in basal sympathetic nerve activity (SNA) during normal pregnancy; this counteracts profound primary vasodilation. However, pregnancy also impairs baroreflex control of heart rate and SNA, contributing to increased mortality secondary to peripartum hemorrhage. Pregnancy-induced hypertensive disorders evoke even greater elevations in SNA, which likely contribute to the hypertension. Information concerning mechanisms is limited. In normal pregnancy, increased angiotensin II acts centrally to support elevated SNA. Hypothalamic sites, including the subfornical organ, paraventricular nucleus, and arcuate nucleus, are likely (but unproven) targets. Moreover, no definitive mechanisms for exaggerated sympathoexcitation in hypertensive pregnancy have been identified. In addition, normal pregnancy increases gamma aminobutyric acid inhibition of the rostral ventrolateral medulla (RVLM), a key brainstem site that transmits excitatory inputs to spinal sympathetic preganglionic neurons. Accumulated evidence supports a major role for locally increased production and actions of the neurosteroid allopregnanolone as one mechanism. A consequence is suppression of baroreflex function, but increased basal SNA indicates that excitatory influences predominate in the RVLM. However, many questions remain regarding other sites and factors that support increased SNA during normal pregnancy and, more importantly, the mechanisms underlying excessive sympathoexcitation in life-threatening hypertensive pregnancy disorders such as preeclampsia.

Xiexin Tang ameliorates dyslipidemia in high-fat diet-induced obese rats via elevating gut microbiota-derived short chain fatty acids production and adjusting energy metabolism

ETHNOPHARMACOLOGICAL RELEVANCE

Traditional herbal medicine has been taken as a new and effective approach to treat many chronic diseases. Xiexin Tang (XXT), a compound recipe composed of Dahuang (Rheum palmatum L.), Huangqin (Scutellaria baicalensis Georgi) and Huanglian (Coptis chinensis Franch.), has been reported to have hypoglycemic and hypolipidemic effects, but its mechanism remains unclear. Our previous study found that Xiexin Tang markedly ameliorated the composition of the gut microbiota, especially for some short chain fatty acids (SCFAs) producing bacteria, and then notably increased SCFAs production. However, the mechanism of XXT on the fermentation of gut bacteria and further improvement of obesity is not yet clear.

AIM OF THE STUDY

This study aimed to unravel the molecular mechanism of XXT on the amelioration of obesity.

MATERIALS AND METHODS

Here, high-fat diet-induced obese rat model was established to investigate the intervention efficacy following oral administration of XXT. Additionally, the expressions of key enzymes of gut microbe-derived SCFAs biosynthesis and key targets in the signaling pathway of energy metabolism were investigated by ELISA and qPCR analysis.

RESULTS

Results showed that XXT could notably correct lipid metabolism disorders, alleviate systematic inflammation, improve insulin sensitivity and reduce fat accumulation. Additionally, XXT could increase gut microbiota-derived SCFAs-producing capacity by enhancing mRNA levels and activities of SCFA-synthetic key enzymes such as acetate kinase (ACK), methylmalonyl-CoA decarboxylase (MMD), butyryl-CoA: acetate CoA transferase (BUT) and butyrate kinase (BUK), which markedly decreased the adenosine triphosphate (ATP) contents, elevated adenosine diphosphate (ADP) and adenosine monophosphate (AMP) levels and further lowered the energy charge (EC) in obese rats via activating peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α)/uncoupling protein-2 (UCP-2) signaling pathway. What's more, XXT could notably ameliorate dyslipidemia via increasing the gene expression of 5'-AMP-activated protein kinase (AMPK) and blocking mammalian target of rapamycin (mTOR) signaling pathway.

CONCLUSIONS

Taken together, our data provided a novel insight into the role of XXT in losing weight from energy metabolism regulation, which unraveled the molecular mechanism of XXT on the alleviation of dyslipidemia and fat heterotopic accumulation. The study provided useful information for XXT in clinical application to treat obesity.

Agrp-Specific Ablation of Scly Protects against Diet-Induced Obesity and Leptin Resistance

Selenium, an essential trace element known mainly for its antioxidant properties, is critical for proper brain function and regulation of energy metabolism. Whole-body knockout of the selenium recycling enzyme, selenocysteine lyase (Scly), increases susceptibility to metabolic syndrome and diet-induced obesity in mice. Scly knockout mice also have decreased selenoprotein expression levels in the hypothalamus, a key regulator of energy homeostasis. This study investigated the role of selenium in whole-body metabolism regulation using a mouse model with hypothalamic knockout of Scly. Agouti-related peptide (Agrp) promoter-driven Scly knockout resulted in reduced weight gain and adiposity while on a high-fat diet (HFD). Scly-Agrp knockout mice had reduced Agrp expression in the hypothalamus, as measured by Western blot and immunohistochemistry (IHC). IHC also revealed that while control mice developed HFD-induced leptin resistance in the arcuate nucleus, Scly-Agrp knockout mice maintained leptin sensitivity. Brown adipose tissue from Scly-Agrp knockout mice had reduced lipid deposition and increased expression of the thermogenic marker uncoupled protein-1. This study sheds light on the important role of selenium utilization in energy homeostasis, provides new information on the interplay between the central nervous system and whole-body metabolism, and may help identify key targets of interest for therapeutic treatment of metabolic disorders.

Transcriptional Basis for Rhythmic Control of Hunger and Metabolism within the AgRP Neuron

The alignment of fasting and feeding with the sleep/wake cycle is coordinated by hypothalamic neurons, though the underlying molecular programs remain incompletely understood. Here, we demonstrate that the clock transcription pathway maximizes eating during wakefulness and glucose production during sleep through autonomous circadian regulation of NPY/AgRP neurons. Tandem profiling of whole-cell and ribosome-bound mRNAs in morning and evening under dynamic fasting and fed conditions identified temporal control of activity-dependent gene repertoires in AgRP neurons central to synaptogenesis, bioenergetics, and neurotransmitter and peptidergic signaling. Synaptic and circadian pathways were specific to whole-cell RNA analyses, while bioenergetic pathways were selectively enriched in the ribosome-bound transcriptome. Finally, we demonstrate that the AgRP clock mediates the transcriptional response to leptin. Our results reveal that time-of-day restriction in transcriptional control of energy-sensing neurons underlies the alignment of hunger and food acquisition with the sleep/wake state.

Regulation of substrate utilization and adiposity by Agrp neurons

The type of nutrient utilized by the organism at any given time—substrate utilization—is a critical component of energy metabolism. The neuronal mechanisms involved in the regulation of substrate utilization in mammals are largely unknown. Here, we found that activation of hypothalamic Agrp neurons rapidly altered whole-body substrate utilization, increasing carbohydrate utilization, while decreasing fat utilization. These metabolic changes occurred even in the absence of caloric ingestion and were coupled to increased lipogenesis. Accordingly, inhibition of fatty acid synthase—a key enzyme that mediates lipogenesis—blunted the effects of Agrp neuron activation on substrate utilization. In pair-fed conditions during positive energy balance, activation of Agrp neurons improved metabolic efficiency, and increased weight gain and adiposity. Conversely, ablation of Agrp neurons impaired fat mass accumulation. These results suggest Agrp neurons regulate substrate utilization, contributing to lipogenesis and fat mass accumulation during positive energy balance.

Agouti-related peptide (AgRP) producing neurons regulate food intake and metabolic processes in peripheral organs. Here, the authors show that hypothalamic AgRP neurons alter whole body substrate utilization to favour carbohydrate usage and lipid storage.

Next-Generation Tools to Study Autonomic Regulation In Vivo

The recent development of tools to decipher the intricacies of neural networks has improved our understanding of brain function. Optogenetics allows one to assess the direct outcome of activating a genetically-distinct population of neurons. Neurons are tagged with light-sensitive channels followed by photo-activation with an appropriate wavelength of light to functionally activate or silence them, resulting in quantifiable changes in the periphery. Capturing and manipulating activated neuron ensembles, is a recently-designed technique to permanently label activated neurons responsible for a physiological function and manipulate them. On the other hand, neurons can be transfected with genetically-encoded Ca²⁺ indicators to capture the interplay between them that modulates autonomic end-points or somatic behavior. These techniques work with millisecond temporal precision. In addition, neurons can be manipulated chronically to simulate physiological aberrations by transfecting designer G-protein-coupled receptors exclusively activated by designer drugs. In this review, we elaborate on the fundamental concepts and applications of these techniques in research.

Melanocortin-3 Receptors Expressed on Agouti-Related Peptide Neurons Inhibit Feeding Behavior in Female Mice

OBJECTIVE

Activation of hypothalamic agouti-related peptide expressing (AgRP)^+ve neurons during energy deficit is a negative valence signal, rapidly activating food-seeking behaviors. This study examined the roles of melanocortin-3 receptors (MC3Rs) coexpressed in a subpopulation of AgRP^+ve neurons.

METHODS

AgRP-MC3R mice expressing MC3Rs selectively in AgRP^+ve neurons were generated by crossing AgRP-IRES-Cre mice with LoxTBMc3r mice containing a "loxP-STOP-loxP" sequence in the 5' untranslated region. Body weight, body composition, and feeding behavior were assessed during ad libitum and time-restricted feeding conditions.

RESULTS

In females, food intake of AgRP-IRES-Cre^+ve (n = 7) or AgRP-IRES-Cre^-ve (n = 9) mice was not significantly different; these mice were therefore pooled to form the "control" group. Female AgRP-MC3R mice exhibited lower food intake (25.4 ± 2.4 kJ/12 h; n = 6) compared with controls (35.3 ± 1.8 kJ/12 h; n = 16) and LoxTBMc3r mice (32.1 ± 2.1 kJ/12 h; n = 9) in the active phase during the dark period. Food intake during the rest phase (lights on) when mice consume less food (9-10 kJ) was normal between genotypes. Body weight and composition of AgRP-MC3R and LoxTBMc3r mice were similar, suggesting compensatory mechanisms for reduced calorie intake. Remarkably, AgRP-MC3R mice continued to consume less food during refeeding after fasting and time-restricted feeding.

CONCLUSIONS

MC3Rs expressed on AgRP^+ve neurons appear to exert a strong inhibitory signal on hypothalamic networks governing feeding behavior.

The increased expression of GABA receptors within the arcuate nucleus is associated with high intraocular pressure

PURPOSE

To investigate the relationship between intraocular pressure (IOP) and GABA receptors within the arcuate nucleus (ARC).

METHODS

In the chronic high IOP rat model, ibotenic acid (IBO) was injected to induce impairment of the ARC, and IOP was measured at the 0, 1, 2, 3, and 4 week time points with a Tono-Pen. To assess the expression of GABA-A/B receptors within the ARC under persistent high IOP, we performed immunofluorescence (IF) and immunohistochemical (IHC) staining at 2 weeks and 4 weeks. Furthermore, we treated the ARC with GABA-A/B receptor antagonists separately, and IOP was evaluated, as well as retinal ganglion cell apoptosis in the chronic high IOP rat model. In the following induced high IOP animal model, the expression of GABA-A/B receptors within the ARC was evaluated in DBA/2J mice which developed progressive eye abnormalities spontaneously that closely mimic human hereditary glaucoma.

RESULTS

Compared with the control group, statistically significant downregulation of IOP was noted due to the IBO injection into the ARC at the 2, 3, and 4 week time points (p<0.05). Persistent high IOP elicited increased expression of the GABA-A/B receptors in the ARC compared with the control group (p<0.01). In addition, treatment with GABA-A/B receptor antagonists separately caused a decrease in the IOP, along with reduced retinal ganglion cell apoptosis (p<0.01). In the DBA/2J mice, the expression of the GABA receptors was statistically significantly increased (p<0.01).

CONCLUSIONS

GABA-A/B receptors in the ARC may be involved in regulation of IOP, and pathologically high IOP affects the expression of GABA-A/B receptors in the ARC.

DREADD Agonist 21 Is an Effective Agonist for Muscarinic-Based DREADDs in Vitro and in Vivo

Chemogenetic tools such as designer receptors exclusively activated by designer drugs (DREADDs) are routinely used to modulate neuronal and non-neuronal signaling and activity in a relatively noninvasive manner. The first generation of DREADDs were templated from the human muscarinic acetylcholine receptor family and are relatively insensitive to the endogenous agonist acetylcholine but instead are activated by clozapine-N-oxide (CNO). Despite the undisputed success of CNO as an activator of muscarinic DREADDs, it has been known for some time that CNO is subject to a low rate of metabolic conversion to clozapine, raising the need for alternative chemical actuators of muscarinic-based DREADDs. Here we show that DREADD agonist 21 (C21) (11-(1-piperazinyl)-5H-dibenzo[b,e][1,4]diazepine) is a potent and selective agonist at both excitatory (hM3Dq) and inhibitory (hM4Di) DREADDs and has excellent bioavailability, pharmacokinetic properties, and brain penetrability. We also show that C21-induced activation of hM3Dq and hM4Di in vivo can modulate bidirectional feeding in defined circuits in mice. These results indicate that C21 represents an alternative to CNO for in vivo studies where metabolic conversion of CNO to clozapine is a concern.

Carnitine acetyltransferase (Crat) in hunger-sensing AgRP neurons permits adaptation to calorie restriction

Hunger-sensing agouti-related peptide (AgRP) neurons ensure survival by adapting metabolism and behavior to low caloric environments. This adaption is accomplished by consolidating food intake, suppressing energy expenditure, and maximizing fat storage (nutrient partitioning) for energy preservation. The intracellular mechanisms responsible are unknown. Here we report that AgRP carnitine acetyltransferase (Crat) knockout (KO) mice exhibited increased fatty acid utilization and greater fat loss after 9 d of calorie restriction (CR). No differences were seen in mice with ad libitum food intake. Eleven days ad libitum feeding after CR resulted in greater food intake, rebound weight gain, and adiposity in AgRP Crat KO mice compared with wild-type controls, as KO mice act to restore pre-CR fat mass. Collectively, this study highlights the importance of Crat in AgRP neurons to regulate nutrient partitioning and fat mass during chronically reduced caloric intake. The increased food intake, body weight gain, and adiposity in KO mice after CR also highlights the detrimental and persistent metabolic consequence of impaired substrate utilization associated with CR. This finding may have significant implications for postdieting weight management in patients with metabolic diseases.-Reichenbach, A., Stark, R., Mequinion, M., Lockie, S. H., Lemus, M. B., Mynatt, R. L., Luquet, S., Andrews, Z. B. Carnitine acetyltransferase (Crat) in hunger-sensing AgRP neurons permits adaptation to calorie restriction.

Glucagon-like peptide-1 regulates brown adipose tissue thermogenesis via the gut-brain axis in rats

Endogenous intestinal glucagon-like peptide-1 (GLP-1) controls satiation and glucose metabolism via vagal afferent neurons (VANs). Recently, VANs have received increasing attention for their role in brown adipose tissue (BAT) thermogenesis. It is, however, unclear whether VAN GLP-1 receptor (GLP-1R) signaling affects BAT thermogenesis and energy expenditure (EE) and whether this VAN mechanism contributes to energy balance. First, we tested the effect of the GLP-1R agonist exendin-4 (Ex4, 0.3 μg/kg ip) on EE and BAT thermogenesis and whether these effects require VAN GLP-1R signaling using a rat model with a selective Glp1r knockdown (kd) in VANs. Second, we examined the role of VAN GLP-1R in energy balance during chronic high-fat diet (HFD) feeding in VAN Glp1r kd rats. Finally, we used viral transsynaptic tracers to identify the possible neuronal substrates of such a gut-BAT interaction. VAN Glp1r kd attenuated the acute suppressive effects of Ex4 on EE and BAT thermogenesis. Consistent with this finding, the VAN Glp1r kd increased EE and BAT activity, diminished body weight gain, and improved insulin sensitivity compared with HFD-fed controls. Anterograde transsynaptic viral tracing of VANs infected major hypothalamic and hindbrain areas involved in BAT sympathetic regulation. Moreover, retrograde tracing from BAT combined with laser capture microdissection revealed that a population of VANs expressing Glp1r is synaptically connected to the BAT. Our findings reveal a novel role of VAN GLP-1R signaling in the regulation of EE and BAT thermogenesis and imply that through this gut-brain-BAT connection, intestinal GLP-1 plays a role in HFD-induced metabolic syndrome.

Protective role of AgRP neuron's PDK1 against salt-induced hypertension

In the hypothalamic arcuate nucleus (ARC), orexigenic agouti-related peptide (AgRP) neurons regulate feeding behavior and energy homeostasis. The 3-phosphoinositide-dependent protein kinase-1 (PDK1) in AgRP neurons serves as a major signaling molecule for leptin and insulin, the hormones regulating feeding behavior, energy homeostasis and circulation. However, it is unclear whether PDK1 in AGRP neurons is also involved in regulation of blood pressure. This study explored it by generating and analyzing AgRP neuron-specific PDK1 knockout (Agrp-Pdk1^flox/flox) mice and effect of high salt diet on blood pressure in KO and WT mice was analyzed. Under high salt diet feeding, systolic blood pressure (SBP) of Agrp-Pdk1^flox/flox mice was significantly elevated compared to Agrp-Cre mice. When the high salt diet was switched to control low salt diet, SBP of Agrp-Pdk1^flox/flox mice returned to the basal level observed in Agrp-Cre mice within 1 week. In Agrp-Pdk1^flox/flox mice, urinary noradrenalin excretion and NUCB2 mRNA expression in hypothalamic paraventricular nucleus (PVN) were markedly upregulated. Moreover, silencing of NUCB2 in the PVN counteracted the rises in urinary noradrenalin excretions and SBP. These results demonstrate a novel role of PDK1 in AgRP neurons to counteract the high salt diet-induced hypertension by preventing hyperactivation of PVN nesfatin-1 neurons.

Control of Energy Expenditure by AgRP Neurons of the Arcuate Nucleus: Neurocircuitry, Signaling Pathways, and Angiotensin

PURPOSE OF REVIEW

Here, we review the current understanding of the functional neuroanatomy of neurons expressing Agouti-related peptide (AgRP) and the angiotensin 1A receptor (AT_1A) within the arcuate nucleus (ARC) in the control of energy balance.

RECENT FINDINGS

The development and maintenance of obesity involves suppression of resting metabolic rate (RMR). RMR control is integrated via AgRP and proopiomelanocortin neurons within the ARC. Their projections to other hypothalamic and extrahypothalamic nuclei contribute to RMR control, though relatively little is known about the contributions of individual projections and the neurotransmitters involved. Recent studies highlight a role for AT_1A, localized to AgRP neurons, but the specific function of AT_1A within these cells remains unclear. AT_1A functions within AgRP neurons to control RMR, but additional work is required to clarify its role within subpopulations of AgRP neurons projecting to distinct second-order nuclei, and the molecular mediators of its signaling within these cells.