Discrete TrkB-expressing neurons of the dorsomedial hypothalamus regulate feeding and thermogenesis

Oxytocin-receptor-expressing neurons in the lateral parabrachial nucleus activate widespread brain regions predominantly involved in fluid satiation

Fluid satiation is an important signal and aspect of body fluid homeostasis. Oxytocin-receptor-expressing neurons (OxtrPBN) in the dorsolateral subdivision of the lateral parabrachial nucleus (dl LPBN) are key neurons which regulate fluid satiation. In the present study, we investigated brain regions activated by stimulation of OxtrPBN neurons in order to better characterise the fluid satiation neurocircuitry in mice. Chemogenetic activation of OxtrPBN neurons increased Fos expression (a proxy marker for neuronal activation) in known fluid-regulating brain nuclei, as well as other regions that have unclear links to fluid regulation and which are likely involved in regulating other functions such as arousal and stress relief. In addition, we analysed and compared Fos expression patterns between chemogenetically-activated fluid satiation and physiological-induced fluid satiation. Both models of fluid satiation activated similar brain regions, suggesting that the chemogenetic model of stimulating OxtrPBN neurons is a relevant model of physiological fluid satiation. A deeper understanding of this neural circuit may lead to novel molecular targets and creation of therapeutic agents to treat fluid-related disorders.

The Role of BDNF and TrkB in the Central Control of Energy and Glucose Balance: An Update

The global rise in obesity and related health issues, such as type 2 diabetes and cardiovascular disease, is alarming. Gaining a deeper insight into the central neural pathways and mechanisms that regulate energy and glucose homeostasis is crucial for developing effective interventions to combat this debilitating condition. A significant body of evidence from studies in humans and rodents indicates that brain-derived neurotrophic factor (BDNF) signaling plays a key role in regulating feeding, energy expenditure, and glycemic control. BDNF is a highly conserved neurotrophin that signals via the tropomyosin-related kinase B (TrkB) receptor to facilitate neuronal survival, differentiation, and synaptic plasticity and function. Recent studies have shed light on the mechanisms through which BDNF influences energy and glucose balance. This review will cover our current understanding of the brain regions, neural circuits, and cellular and molecular mechanisms underlying the metabolic actions of BDNF and TrkB.

Genomic signatures of climate adaptation in bank voles

Evidence for divergent selection and adaptive variation across the landscape can provide insight into a species' ability to adapt to different environments. However, despite recent advances in genomics, it remains difficult to detect the footprints of climate-mediated selection in natural populations. Here, we analysed ddRAD sequencing data (21,892 SNPs) in conjunction with geographic climate variation to search for signatures of adaptive differentiation in twelve populations of the bank vole (Clethrionomys glareolus) distributed across Europe. To identify the loci subject to selection associated with climate variation, we applied multiple genotype-environment association methods, two univariate and one multivariate, and controlled for the effect of population structure. In total, we identified 213 candidate loci for adaptation, 74 of which were located within genes. In particular, we identified signatures of selection in candidate genes with functions related to lipid metabolism and the immune system. Using the results of redundancy analysis, we demonstrated that population history and climate have joint effects on the genetic variation in the pan-European metapopulation. Furthermore, by examining only candidate loci, we found that annual mean temperature is an important factor shaping adaptive genetic variation in the bank vole. By combining landscape genomic approaches, our study sheds light on genome-wide adaptive differentiation and the spatial distribution of variants underlying adaptive variation influenced by local climate in bank voles.

From genomic spectrum of NTRK genes to adverse effects of its inhibitors, a comprehensive genome-based and real-world pharmacovigilance analysis

Introduction: The discovery of neurotrophic tyrosine receptor kinase (NTRK) gene fusions has facilitated the development of precision oncology. Two first-generation NTRK inhibitors (larotrectinib and entrectinib) are currently approved for the treatment of patients with solid tumors harboring NTRK gene fusions. Nevertheless, comprehensive NTRK profiling at the pan-cancer genomic level and real-world studies pertaining to the adverse events of NTRK inhibitors are lacking. Methods: We characterize the genome of NTRK at the pan-cancer level through multi-omics databases such as The Cancer Genome Atlas (TCGA). Through the FDA Adverse Event Reporting System (FAERS) database, we collect reports of entrectinib and larotrectinib-induced adverse events and perform a pharmacovigilance analysis using various disproportionality methods. Results: NTRK1/2/3 expression is lower in most tumor tissues, while they have higher methylation levels. NTRK gene expression has prognostic value in some cancer types, such as breast invasive carcinoma (BRCA). The cancer type with highest NTRK alteration frequency is skin cutaneous melanoma (SKCM) (31.98%). Thyroid carcinoma (THCA) has the largest number of NTRK fusion cases, and the most common fusion pair is ETV6-NTRK3. Adverse drug events (ADEs) obtained from the FAERS database for larotrectinib and entrectinib are 524 and 563, respectively. At the System Organ Class (SOC) level, both drugs have positive signal value for "nervous system disorder". Other positive signals for entrectinib include "cardiac disorders", "metabolism and nutrition disorders", while for larotrectinib, it is "hepatobiliary disorders". The unexpected signals are also listed in detail. ADEs of the two NTRK inhibitors mainly occur in the first month. The median onset time of ADEs for entrectinib and larotrectinib was 16 days (interquartile range [IQR] 6-86.5) and 44 days ([IQR] 7-136), respectively. Conclusion: Our analysis provides a broad molecular view of the NTRK family. The real-world adverse drug event analysis of entrectinib and larotrectinib contributes to more refined medication management.

Synaptic plasticity and the role of astrocytes in central metabolic circuits

Neural circuits in the brain, primarily in the hypothalamus, are paramount to the homeostatic control of feeding and energy utilization. They integrate hunger, satiety, and body adiposity cues from the periphery and mediate the appropriate behavioral and physiological responses to satisfy the energy demands of the animal. Notably, perturbations in central homeostatic circuits have been linked to the etiology of excessive feeding and obesity. Considering the ever-changing energy requirements of the animal and required adaptations, it is not surprising that brain-feeding circuits remain plastic in adulthood and are subject to changes in synaptic strength as a consequence of nutritional status. Indeed, synapse density, probability of presynaptic transmitter release, and postsynaptic responses in hypothalamic energy balance centers are tailored to behavioral and physiological responses required to sustain survival. Mounting evidence supports key roles of astrocytes facilitating some of this plasticity. Here we discuss these synaptic plasticity mechanisms and the emerging roles of astrocytes influencing energy and glucose balance control in health and disease. This article is categorized under: Cancer > Molecular and Cellular Physiology Neurological Diseases > Molecular and Cellular Physiology.

A parabrachial-hypothalamic parallel circuit governs cold defense in mice

Thermal homeostasis is vital for mammals and is controlled by brain neurocircuits. Yet, the neural pathways responsible for cold defense regulation are still unclear. Here, we found that a pathway from the lateral parabrachial nucleus (LPB) to the dorsomedial hypothalamus (DMH), which runs parallel to the canonical LPB to preoptic area (POA) pathway, is also crucial for cold defense. Together, these pathways make an equivalent and cumulative contribution, forming a parallel circuit. Specifically, activation of the LPB → DMH pathway induced strong cold-defense responses, including increases in thermogenesis of brown adipose tissue (BAT), muscle shivering, heart rate, and locomotion. Further, we identified somatostatin neurons in the LPB that target DMH to promote BAT thermogenesis. Therefore, we reveal a parallel circuit governing cold defense in mice, which enables resilience to hypothermia and provides a scalable and robust network in heat production, reshaping our understanding of neural circuit regulation of homeostatic behaviors.

Brain-derived neurotrophic factor: Its role in energy balance and cancer cachexia

Brain-derived neurotrophic factor (BDNF) plays an important role in the development of the central and peripheral nervous system during embryogenesis. In the mature central nervous system, BDNF is required for the maintenance and enhancement of synaptic transmissions and the survival of neurons. Particularly, it is involved in the modulation of neurocircuits that control energy balance through food intake, energy expenditure, and locomotion. Regulation of BDNF in the central nervous system is complex and environmental factors affect its expression in murine models which may reflect to phenotype dramatically. Furthermore, BDNF and its high-affinity receptor tropomyosin receptor kinase B (TrkB), as well as pan-neurotrophin receptor (p75NTR) is expressed in peripheral tissues in adulthood and their signaling is associated with regulation of energy balance. BDNF/TrkB signaling is exploited by cancer cells as well and BDNF expression is increased in tumors. Intriguingly, previously demonstrated roles of BDNF in regulation of food intake, adipose tissue and muscle overlap with derangements observed in cancer cachexia. However, data about the involvement of BDNF in cachectic cancer patients and murine models are scarce and inconclusive. In the future, knock-in and/or knock-out experiments with murine cancer models could be helpful to explore potential new roles for BDNF in the development of cancer cachexia.

Regulation of Satiety by Bdnf-e2-Expressing Neurons through TrkB Activation in Ventromedial Hypothalamus

The transcripts for Bdnf (brain-derived neurotrophic factor), driven by different promoters, are expressed in different brain regions to control different body functions. Specific promoter(s) that regulates energy balance remain unclear. We show that disruption of Bdnf promoters I and II but not IV and VI in mice (Bdnf-e1^-/-, Bdnf-e2^-/-) results in obesity. Whereas Bdnf-e1^-/- exhibited impaired thermogenesis, Bdnf-e2^-/- showed hyperphagia and reduced satiety before the onset of obesity. The Bdnf-e2 transcripts were primarily expressed in ventromedial hypothalamus (VMH), a nucleus known to regulate satiety. Re-expressing Bdnf-e2 transcript in VMH or chemogenetic activation of VMH neurons rescued the hyperphagia and obesity of Bdnf-e2^-/- mice. Deletion of BDNF receptor TrkB in VMH neurons in wildtype mice resulted in hyperphagia and obesity, and infusion of TrkB agonistic antibody into VMH of Bdnf-e2^-/- mice alleviated these phenotypes. Thus, Bdnf-e2-transcripts in VMH neurons play a key role in regulating energy intake and satiety through TrkB pathway.

Function of brain-derived neurotrophic factor in the hypothalamus: Implications for depression pathology

Depression is a prevalent mental health disorder and is the number one cause of disability worldwide. Risk factors for depression include genetic predisposition and stressful life events, and depression is twice as prevalent in women compared to men. Both clinical and preclinical research have implicated a critical role for brain-derived neurotrophic factor (BDNF) signaling in depression pathology as well as therapeutics. A preponderance of this research has focused on the role of BDNF and its primary receptor tropomyosin-related kinase B (TrkB) in the cortex and hippocampus. However, much of the symptomatology for depression is consistent with disruptions in functions of the hypothalamus including changes in weight, activity levels, responses to stress, and sociability. Here, we review evidence for the role of BDNF and TrkB signaling in the regions of the hypothalamus and their role in these autonomic and behavioral functions associated with depression. In addition, we identify areas for further research. Understanding the role of BDNF signaling in the hypothalamus will lead to valuable insights for sex- and stress-dependent neurobiological underpinnings of depression pathology.

Cooling-activated dorsomedial hypothalamic BDNF neurons control cold defense in mice

BDNF and its expressing neurons in the brain critically control feeding and energy expenditure (EE) in both rodents and humans. However, whether BDNF neurons would function in thermoregulation during temperature challenges is unclear. Here, we show that BDNF neurons in the dorsomedial hypothalamus (DMH^BDNF ) of mice are activated by afferent cooling signals. These cooling-activated BDNF neurons are mainly GABAergic. Activation of DMH^BDNF neurons or the GABAergic subpopulations is sufficient to increase body temperature, EE, and physical activity. Conversely, blocking DMH^BDNF neurons substantially impairs cold defense and reduces energy expenditure, physical activity, and UCP1 expression in BAT, which eventually results in bodyweight gain and glucose/insulin intolerance. Therefore, we identify a subset of DMH^BDNF neurons as a novel type of cooling-activated neurons to promote cold defense. Thus, we reveal a critical role of BDNF circuitry in thermoregulation, which deepens our understanding of BDNF in controlling energy homeostasis and obesity.

Rapid and Lasting Effects of Activating BDNF-Expressing PVH Neurons on Energy Balance

Brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin receptor kinase B (TrkB), are implicit in causing obesity. Mutations that reduce BDNF and TrkB expression are associated with obesity in humans and mice. Recently, it was reported that Bdnf gene deletion in the neurons of the paraventricular hypothalamus (PVH) caused positive energy balance and severe obesity in the form of hyperphagia, impaired adaptive thermogenesis, and decreased energy expenditure. Thus, we hypothesize that activation of these neurons will have the opposite effect and provide an opportunity for long-lasting obesity treatment. To specifically activate BDNF-expressing PVH (PVH^BDNF) neurons, we injected Cre-dependent adeno-associated virus (AAV) expressing the excitatory DREADD hM3Dq bilaterally into the PVH of Bdnf^2A-Cre/+ knock-in mice and then administered clozapine-N-oxide (CNO). Using this technique, we demonstrated that acute activation of these neurons rapidly decreased normal nocturnal feeding and fasting-induced feeding in male and female mice. At thermoneutral temperatures, acute activation also rapidly increased adaptive thermogenesis, increased core body temperature, increased locomotion, increased energy expenditure, and decreased respiratory exchange ratio (RER) in male and female mice. These observations indicate that acute stimulation of PVH^BDNF neurons promotes negative energy balance and weight loss. However, the rapid decrease in RER after activation of PVH^BDNF neurons was followed by a delayed and prolonged increase in RER that remained elevated for 3 d in female mice. Thus, although acute activation of PVH^BDNF neurons promotes negative energy balance in the short term, long-term effects of activation include sexually dimorphic overcompensatory mechanisms that may promote positive energy balance in female mice.

Role of Dorsomedial Hypothalamus GABAergic Neurons in Sleep-Wake States in Response to Changes in Ambient Temperature in Mice

Good sleep quality is essential for maintaining the body's attention during wakefulness, which is easily affected by external factors such as an ambient temperature. However, the mechanism by which an ambient temperature influences sleep-wake behaviors remains unclear. The dorsomedial hypothalamus (DMH) has been reported to be involved in thermoregulation. It also receives projection from the preoptic area, which is an important region for sleep and energy homeostasis and the suprachiasmatic nucleus-a main control area of the clock rhythm. Therefore, we hypothesized that the DMH plays an important role in the regulation of sleep related to ambient temperatures. In this study, we found that cold exposure (24/20/16/12 °C) increased wakefulness and decreased non-rapid eye movement (NREM) sleep, while warm exposure (32/36/40/44 °C) increased NREM sleep and decreased wakefulness compared to 28 °C conditions in wild-type mice. Then, using non-specific and specific apoptosis, we found that lesions of whole DMH neurons and DMH γ-aminobutyric acid (GABA)-ergic neurons induced by caspase-3 virus aggravated the fluctuation of core body temperature after warm exposure and attenuated the change in sleep-wake behaviors during cold and warm exposure. However, chemogenetic activation or inhibition of DMH GABAergic neurons did not affect the sleep-wake cycle. Collectively, our findings reveal an essential role of DMH GABAergic neurons in the regulation of sleep-wake behaviors elicited by a change in ambient temperature.

Central sympathetic network for thermoregulatory responses to psychological stress

In mammals, many types of psychological stressors elicit a variety of sympathoexcitatory responses paralleling the classic fight-or-flight response to a threat to survival, including increased body temperature via brown adipose tissue thermogenesis and cutaneous vasoconstriction, and increased skeletal muscle blood flow via tachycardia and visceral vasoconstriction. Although these responses are usually supportive for stress coping, aberrant sympathetic responses to stress can lead to clinical issues in psychosomatic medicine. Sympathetic stress responses are mediated mostly by sympathetic premotor drives from the rostral medullary raphe region (rMR) and partly by those from the rostral ventrolateral medulla (RVLM). Hypothalamomedullary descending pathways from the dorsomedial hypothalamus (DMH) to the rMR and RVLM mediate important, stress-driven sympathoexcitatory transmission to the premotor neurons to drive the thermal and cardiovascular responses. The DMH also likely sends an excitatory input to the paraventricular hypothalamic nucleus to stimulate stress hormone release. Neurons in the DMH receive a stress-related excitation from the dorsal peduncular cortex and dorsal tenia tecta (DP/DTT) in the ventromedial prefrontal cortex. By connecting the corticolimbic emotion circuit to the central sympathetic and somatic motor systems, the DP/DTT → DMH pathway plays as the primary mediator of the psychosomatic signaling that drives a variety of sympathetic and behavioral stress responses. These brain regions together with other stress-related regions constitute a central neural network for physiological stress responses. This network model is relevant to understanding the central mechanisms by which stress and emotions affect autonomic regulations of homeostasis and to developing new therapeutic strategies for various stress-related disorders.

The Pleiotropic Potential of BDNF beyond Neurons: Implication for a Healthy Mind in a Healthy Body

Brain-derived neurotrophic factor (BDNF) represents one of the most widely studied neurotrophins because of the many mechanisms in which it is involved. Among these, a growing body of evidence indicates BDNF as a pleiotropic signaling molecule and unveils non-negligible implications in the regulation of energy balance. BDNF and its receptor are extensively expressed in the hypothalamus, regions where peripheral signals, associated with feeding control and metabolism activation, and are integrated to elaborate anorexigenic and orexigenic effects. Thus, BDNF coordinates adaptive responses to fluctuations in energy intake and expenditure, connecting the central nervous system with peripheral tissues, including muscle, liver, and the adipose tissue in a complex operational network. This review discusses the latest literature dealing with the involvement of BDNF in the maintenance of energy balance. We have focused on the physiological and molecular mechanisms by which BDNF: (I) controls the mitochondrial function and dynamics; (II) influences thermogenesis and tissue differentiation; (III) mediates the effects of exercise on cognitive functions; and (IV) modulates insulin sensitivity and glucose transport at the cellular level. Deepening the understanding of the mechanisms exploited to maintain energy homeostasis will lay the groundwork for the development of novel therapeutical approaches to help people to maintain a healthy mind in a healthy body.

A hypothalamomedullary network for physiological responses to environmental stresses

Various environmental stressors, such as extreme temperatures (hot and cold), pathogens, predators and insufficient food, can threaten life. Remarkable progress has recently been made in understanding the central circuit mechanisms of physiological responses to such stressors. A hypothalamomedullary neural pathway from the dorsomedial hypothalamus (DMH) to the rostral medullary raphe region (rMR) regulates sympathetic outflows to effector organs for homeostasis. Thermal and infection stress inputs to the preoptic area dynamically alter the DMH → rMR transmission to elicit thermoregulatory, febrile and cardiovascular responses. Psychological stress signalling from a ventromedial prefrontal cortical area to the DMH drives sympathetic and behavioural responses for stress coping, representing a psychosomatic connection from the corticolimbic emotion circuit to the autonomic and somatic motor systems. Under starvation stress, medullary reticular neurons activated by hunger signalling from the hypothalamus suppress thermogenic drive from the rMR for energy saving and prime mastication to promote food intake. This Perspective presents a combined neural network for environmental stress responses, providing insights into the central circuit mechanism for the integrative regulation of systemic organs.

Metabolic homeostasis via BDNF and its receptors

Metabolic disorders result from dysregulation of central nervous system and peripheral metabolic energy homeostatic pathways. To maintain normal energy balance, neural circuits must integrate feedforward and feedback signals from the internal metabolic environment to orchestrate proper food intake and energy expenditure. These signals include conserved meal and adipocyte cues such as glucose and leptin, respectively, in addition to more novel players including brain-derived neurotrophic factor (BDNF). In particular, BDNF's two receptors, tropomyosin related kinase B (TrkB) and p75 neurotrophin receptor (p75NTR), are increasingly appreciated to be involved in whole body energy homeostasis. At times, these two receptors even seem to functionally oppose one another's actions, providing the framework for a potential neurotrophin mediated energy regulatory axis, which we explore further here.

Cocaine-induced Fos expression in the rat brain: Modulation by prior Δ9-tetrahydrocannabinol exposure during adolescence and sex-specific effects

It has been suggested that cannabis consumption during adolescence may be an initial step to cocaine use in adulthood. Indeed, previous preclinical data show that adolescent exposure to cannabinoids (both natural and synthetic) potentiates cocaine self-administration in rats. Here we aimed at gaining a deeper understanding of the cellular activation patterns induced by cocaine as revealed by Fos imaging and how these patterns may change due to adolescent exposure to THC. Male and female Wistar rats were administered every other day THC (3 mg/kg i.p.) or vehicle from postnatal day 28-44. At adulthood (PND90) they were given an injection of cocaine (20 mg/kg i.p.) or saline and sacrificed 90 min later. Cocaine-induced Fos activation was measured by immunohistochemistry as an index of cellular activation. We found that cocaine-induced activation in the motor cortex was stronger in THC-exposed rats. Moreover, there was significant sex-dependent interaction between cocaine and adolescent THC exposure in the dorsal hypothalamus, suggesting that cocaine induced a more robust cellular activation in THC-exposed females but not in THC-treated males. Other THC- and cocaine-induced effects were also evident. These results add to the previous literature suggesting that the behavioral, cellular, molecular, and brain-activating actions of cocaine are modulated by early experience with cannabinoids and provide additional knowledge that may explain the enhanced actions of cocaine in rats exposed to cannabinoids during their adolescence.