Orexin-A enhances feeding in male rats by activating hindbrain catecholamine neurons

CCK-sensitive C fibers activate NTS leptin receptor-expressing neurons via NMDA receptors

The hormone leptin reduces food intake through actions in the peripheral and central nervous systems, including in the hindbrain nucleus of the solitary tract (NTS). The NTS receives viscerosensory information via vagal afferents, including information from the gastrointestinal tract, which is then relayed to other central nervous system (CNS) sites critical for control of food intake. Leptin receptors (lepRs) are expressed by a subpopulation of NTS neurons, and knockdown of these receptors increases both food intake and body weight. Recently, we demonstrated that leptin increases vagal activation of lepR-expressing neurons via increased NMDA receptor (NMDAR) currents, thereby potentiating vagally evoked firing. Furthermore, chemogenetic activation of these neurons was recently shown to inhibit food intake. However, the vagal inputs these neurons receive had not been characterized. Here we performed whole cell recordings in brain slices taken from lepRCre × floxedTdTomato mice and found that lepR neurons of the NTS are directly activated by monosynaptic inputs from C-type afferents sensitive to the transient receptor potential vanilloid type 1 (TRPV1) agonist capsaicin. CCK administered onto NTS slices stimulated spontaneous glutamate release onto lepR neurons and induced action potential firing, an effect mediated by CCKR1. Interestingly, NMDAR activation contributed to the current carried by spontaneous excitatory postsynaptic currents (EPSCs) and enhanced CCK-induced firing. Peripheral CCK also increased c-fos expression in these neurons, suggesting they are activated by CCK-sensitive vagal afferents in vivo. Our results indicate that the majority of NTS lepR neurons receive direct inputs from CCK-sensitive C vagal-type afferents, with both peripheral and central CCK capable of activating these neurons and NMDARs able to potentiate these effects.

Lesion of NPY Receptor-expressing Neurons in Perifornical Lateral Hypothalamus Attenuates Glucoprivic Feeding

Glucoprivic feeding is one of several counterregulatory responses (CRRs) that facilitates restoration of euglycemia following acute glucose deficit (glucoprivation). Our previous work established that glucoprivic feeding requires ventrolateral medullary (VLM) catecholamine (CA) neurons that coexpress neuropeptide Y (NPY). However, the connections by which VLM CA/NPY neurons trigger increased feeding are uncertain. We have previously shown that glucoprivation, induced by an anti-glycolygic agent 2-deoxy-D-glucose (2DG), activates perifornical lateral hypothalamus (PeFLH) neurons and that expression of NPY in the VLM CA/NPY neurons is required for glucoprivic feeding. We therefore hypothesized that glucoprivic feeding and possibly other CRRs require NPY-sensitive PeFLH neurons. To test this, we used the ribosomal toxin conjugate NPY-saporin (NPY-SAP) to selectively lesion NPY receptor-expressing neurons in the PeFLH of male rats. We found that NPY-SAP destroyed a significant number of PeFLH neurons, including those expressing orexin, but not those expressing melanin-concentrating hormone. The PeFLH NPY-SAP lesions attenuated 2DG-induced feeding but did not affect 2DG-induced increase in locomotor activity, sympathoadrenal hyperglycemia, or corticosterone release. The 2DG-induced feeding response was also significantly attenuated in NPY-SAP-treated female rats. Interestingly, PeFLH NPY-SAP lesioned male rats had reduced body weights and decreased dark cycle feeding, but this effect was not seen in female rats. We conclude that a NPY projection to the PeFLH is necessary for glucoprivic feeding, but not locomotor activity, hyperglycemia, or corticosterone release, in both male and female rats.

Chemogenetic activation of ventral medullary astrocytes enhances feeding and corticosterone release in response to mild glucoprivation

To investigate the role of glial cells in the regulation of glucoprivic responses in rats, a chemogenetic approach was used to activate astrocytes neighboring catecholamine (CA) neurons in the ventromedial medulla (VLM) where A1 and C1 CA cell groups overlap (A1/C1). Previous results indicate that activation of CA neurons in this region is necessary and sufficient for feeding and corticosterone release in response to glucoprivation. However, it is not known whether astrocyte neighbors of CA neurons contribute to glucoregulatory responses. Hence, we made nanoinjections of AAV5-GFAP-hM3D(Gq)-mCherry to selectively transfect astrocytes in the A1/C1 region with the excitatory designer receptor exclusively activated by designer drugs (DREADDs), hM3D(Gq). After allowing time for DREADD expression, we evaluated the rats for increased food intake and corticosterone release in response to low systemic doses of the antiglycolytic agent, 2-deoxy-d-glucose (2DG), alone and in combination with the hM3D(Gq) activator clozapine-n-oxide (CNO). We found that DREADD-transfected rats ate significantly more food when 2DG and CNO were coadministered than when either 2DG or CNO was injected alone. We also found that CNO significantly enhanced 2DG-induced FOS expression in the A1/C1 CA neurons, and that corticosterone release also was enhanced when CNO and 2DG were administered together. Importantly, CNO-induced activation of astrocytes in the absence of 2DG did not trigger food intake or corticosterone release. Our results indicate that during glucoprivation, activation of VLM astrocytes cells markedly increases the sensitivity or responsiveness of neighboring A1/C1 CA neurons to glucose deficit, suggesting a potentially important role for VLM astrocytes in glucoregulation.

Physiological Role of Orexinergic System for Health

Orexins, or hypocretins, are excitatory neuropeptides involved in the regulation of feeding behavior and the sleep and wakefulness states. Since their discovery, several lines of evidence have highlighted that orexin neurons regulate a great range of physiological functions, giving it the definition of a multitasking system. In the present review, we firstly describe the mechanisms underlining the orexin system and their interactions with the central nervous system (CNS). Then, the system's involvement in goal-directed behaviors, sleep/wakefulness state regulation, feeding behavior and energy homeostasis, reward system, and aging and neurodegenerative diseases are described. Advanced evidence suggests that the orexin system is crucial for regulating many physiological functions and could represent a promising target for therapeutical approaches to obesity, drug addiction, and emotional stress.

Orexins (hypocretins): The intersection between homeostatic and hedonic feeding

Orexins are hypothalamic neuropeptides originally discovered to play a role in the regulation of feeding behaviour. The broad connections of orexin neurons to mesocorticolimbic circuitry suggest they may play a role in mediating reward-related behaviour beyond homeostatic feeding. Here, we review the role of orexin in a variety of eating-related behaviour, with a focus on reward and motivation, and the neural circuits driving these effects. One emerging finding is the involvement of orexins in hedonic and appetitive behaviour towards palatable food, in addition to their role in homeostatic feeding. This review discusses the brain circuitry and possible mechanisms underlying the role of orexins in these behaviours. Overall, there is a marked bias in the literature towards studies involving male subjects. As such, future work needs to be done to involve female subjects. In summary, orexins play an important role in driving motivation for high salient rewards such as highly palatable food and may serve as the intersection between homeostatic and hedonic feeding.

Repeated Pharmacogenetic Catecholamine Neuron Activation in the Ventrolateral Medulla Attenuates Subsequent Glucoregulatory Responses

Hindbrain catecholamine (CA) neurons are essential for elicitation of protective counterregulatory responses (CRRs) to glucose deficit, including increased feeding and elevation of circulating corticosterone, epinephrine, and glucose. Severe or repeated antecedent glucoprivation results in attenuation of these CRRs and failure to correct glucose deficit, constituting a potentially lethal condition known as hypoglycemia-associated autonomic failure (HAAF) that may occur in patients with diabetes on insulin therapy. Recently, we demonstrated that selective pharmacogenetic activation of CA neuron subpopulations in the ventrolateral medulla during normoglycemia elicits these CRRs in a site-specific manner. In the present experiment, we examined the effect of repeated pharmacogenetic activation of CA neurons in the A1/C1 cell group on subsequent elicitation of feeding, corticosterone secretion, and respiratory quotient. We found that this prior treatment attenuated these responses to subsequent pharmacogenetic stimulation, similar to attenuation of these CRRs following repeated antecedent glucoprivation. This suggests that functional impairment of A1/C1 CA neurons resulting from antecedent glucoprivation may account, at least in part, for impairment of specific CRRs critical for restoration of normoglycemia in response to glucose deficit. Thus, a pharmacogenetic approach to selective activation of key neural circuits could provide a means of identifying neuropathogenic mechanisms contributing to HAAF.

The dynamic regulation of appetitive behavior through lateral hypothalamic orexin and melanin concentrating hormone expressing cells

The lateral hypothalamic area (LHA) is a heterogeneous brain structure extensively studied for its potent role in regulating energy balance. The anatomical and molecular diversity of the LHA permits the orchestration of responses to energy sensing cues from the brain and periphery. Two of the primary cell populations within the LHA associated with integration of this information are Orexin (ORX) and Melanin Concentrating Hormone (MCH). While both of these non-overlapping populations exhibit orexigenic properties, the activities of these two systems support feeding behavior through contrasting mechanisms. We describe the anatomical and functional properties as well as interaction with other neuropeptides and brain reward and hedonic systems. Specific outputs relating to arousal, food seeking, feeding, and metabolism are coordinated through these mechanisms. We then discuss how both the ORX and MCH systems harmonize in a divergent yet overall cooperative manner to orchestrate feeding behavior through transitions between various appetitive states, and thus offer novel insights into LHA allostatic control of appetite.

Mechanisms of Memory Impairment Induced by Orexin-A via Orexin 1 and Orexin 2 Receptors in Post-traumatic Stress Disorder Rats

Post-traumatic stress disorder (PTSD) patients exhibit abnormal learning and memory. Axons from orexin neurons in the lateral hypothalamus innervate the hippocampus, modulating learning and memory via the orexin 1 and 2 receptors (OX1R and OX2R). However, the role of the orexin system in the learning and memory dysfunction observed in PTSD is unknown. This was investigated in the present study using PTSD animal model-single prolonged stress (SPS) rats. Spatial learning and memory in the rats were evaluated with the Morris water maze (MWM) test; changes in body weight and food intake were recorded to assess changes in appetite; and the expression of orexin-A and its receptors in the hypothalamus and hippocampus was examined and quantified by immunohistochemistry, western blotting and real-time PCR. The results showed that spatial memory was impaired and food intake was decreased in SPS rats; this was accompanied by downregulation of orexin-A in the hypothalamus and upregulation of OX1R and OX2R in the hippocampus and of OX1R in the hypothalamus. Intracerebroventricular administration of orexin-A improved spatial memory and enhanced appetite in SPS rats and partly reversed the increases in OX1R and OX2R levels in the hippocampus and hypothalamus. These results suggest that the orexin system plays a critical role in the memory and appetite dysfunction observed in PTSD.

Hindbrain glucoregulatory mechanisms: Critical role of catecholamine neurons in the ventrolateral medulla

Glucose is the required metabolic substrate for the brain. Yet the brain stores very little glucose. Therefore, the brain continuously monitors glucose availability to detect hypoglycemia and to mobilize system-wide responses to protect and restore euglycemia. Catecholamine (CA) neurons in the hindbrain are critical elements of the brain's glucoregulatory mechanisms. They project widely throughout the brain and spinal cord, innervating sites controlling behavioral, endocrine and visceral responses. Hence, CA neurons are capable of triggering a rapid, coordinated and multifaceted response to glucose challenge. This article reviews experimental data that has begun to elucidate the importance of CA neurons for glucoregulation, the functions of specific CA subpopulations in the ventrolateral medulla, and the extended circuitry through which they engage other levels of the nervous system to accomplish their essential glucoregulatory task. Hopefully, this review also suggests the vast amount of work yet to be done in this area and the justification for engaging in that effort.

Activation of catecholamine neurons in the ventral medulla reduces CCK-induced hypophagia and c-Fos activation in dorsal medullary catecholamine neurons

Catecholamine (CA) neurons within the A1 and C1 cell groups in the ventrolateral medulla (VLM) potently increase food intake when activated by glucose deficit. In contrast, CA neurons in the A2 cell group of the dorsomedial medulla are required for reduction of food intake by cholecystokinin (CCK), a peptide that promotes satiation. Thus dorsal and ventral medullary CA neurons are activated by divergent metabolic conditions and mediate opposing behavioral responses. Acute glucose deficit is a life-threatening condition, and increased feeding is a key response that facilitates survival of this emergency. Thus, during glucose deficit, responses to satiation signals, like CCK, must be suppressed to ensure glucorestoration. Here we test the hypothesis that activation of VLM CA neurons inhibits dorsomedial CA neurons that participate in satiation. We found that glucose deficit produced by the antiglycolytic glucose analog, 2-deoxy-d-glucose, attenuated reduction of food intake by CCK. Moreover, glucose deficit increased c-Fos expression by A1 and C1 neurons while reducing CCK-induced c-Fos expression in A2 neurons. We also selectively activated A1/C1 neurons in TH-Cre⁺ transgenic rats in which A1/C1 neurons were transfected with a Cre-dependent designer receptor exclusively activated by a designer drug (DREADD). Selective activation of A1/C1 neurons using the DREADD agonist, clozapine- N-oxide, attenuated reduction of food intake by CCK and prevented CCK-induced c-Fos expression in A2 CA neurons, even under normoglycemic conditions. Results support the hypothesis that activation of ventral CA neurons attenuates satiety by inhibiting dorsal medullary A2 CA neurons. This mechanism may ensure that satiation does not terminate feeding before restoration of normoglycemia.

Chronopathophysiological implications of orexin in sleep disturbances and lifestyle-related disorders

Sleep, a mysterious behavior, has recently been recognized as a crucial factor for health and longevity. The daily sleep/wake cycle provides the basis of biorhythms controlling whole-body homeostasis and homeodynamics; therefore, disruption of sleep causes several physical and psychological disorders, including cardiovascular disease, obesity, diabetes, cancer, anxiety, depression, and cognitive dysfunction. However, the mechanism linking sleep disturbances and sleep-related disorders remains unknown. Orexin (also known as hypocretin) is a neuropeptide produced in the hypothalamus. Central levels of orexin oscillate with the daily rhythm and peak at the awake phase. Orexin plays a major role in stabilizing the wakefulness state. Orexin deficiency causes sleep/wake-state instability, resulting in narcolepsy. Hyper-activation of the orexin system also causes sleep disturbances, such as insomnia, and hence, suvorexant, an orexin receptor antagonist, has been clinically used to treat insomnia. Importantly, central actions of orexin regulate motivated behaviors, stress response, and energy/glucose metabolism by coordinating the central-autonomic nervous systems and endocrine systems. These multiple actions of orexin maintain survival. However, it remains unknown whether chronopharmacological interventions targeting the orexin system ameliorate sleep-related disorders as well as sleep in humans. To understand the significance of adequate orexin action for prevention of these disorders, this review summarizes the physiological functions of daily orexin action and pathological implications of its mistimed or reduced action in sleep disturbances and sleep-related disorders (lifestyle-related physical and neurological disorders in particular). Timed administration of drugs targeting the orexin system may prevent lifestyle-related diseases by improving the quality of life in patients with sleep disturbances.

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

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

Selective Pharmacogenetic Activation of Catecholamine Subgroups in the Ventrolateral Medulla Elicits Key Glucoregulatory Responses

Catecholamine (CA) neurons in the ventrolateral medulla (VLM) contribute importantly to glucoregulation during glucose deficit. However, it is not known which CA neurons elicit different glucoregulatory responses or whether selective activation of CA neurons is sufficient to elicit these responses. Therefore, to selectively activate CA subpopulations, we injected male or female Th-Cre+ transgenic rats with the Cre-dependent DREADD construct, AAV2-DIO-hSyn-hM3D(Gq)-mCherry, at one of four rostrocaudal levels of the VLM: rostral C1 (C1r), middle C1 (C1m), the area of A1 and C1 overlap (A1/C1), and A1. Transfection was highly selective for CA neurons at each site. Systemic injection of the Designer Receptor Exclusively Activated by Designer Drugs (DREADD) receptor agonist, clozapine-N-oxide (CNO), stimulated feeding in rats transfected at C1r, C1m, or A1/C1 but not A1. CNO increased corticosterone secretion in rats transfected at C1m or A1/C1 but not A1. In contrast, CNO did not increase blood glucose or induce c-Fos expression in the spinal cord or adrenal medulla after transfection of any single VLM site but required dual transfection of both C1m and C1r, possibly indicating that CA neurons mediating blood glucose responses are more sparsely distributed in C1r and C1m than those mediating feeding and corticosterone secretion. These results show that selective activation of C1 CA neurons is sufficient to increase feeding, blood glucose levels, and corticosterone secretion and suggest that each of these responses is mediated by CA neurons concentrated at different levels of the C1 cell group.

RNA-seq expression profiling of rat MCAO model following reperfusion Orexin-A

Orexin-A is a neuropeptide with potent neuroprotective activity towards cerebral ischemia-reperfusion (I/R) injury, but few studies have attempted to elucidate the mechanism. Herein, we performed global gene expression profiling of the hippocampus following reperfusion with Orexin-A using RNA sequencing (RNA-seq). RNA-seq identified 649 differentially expressed genes (DEGs) in the Orexin-A group compared with saline controls (I/R group), of which 149 were up-regulated and 500 were down-regulated. DEGs were confirmed using qRT-PCR, their molecular functions, biological processes and molecular components were explored using Gene Ontology (GO) analysis and 206 KEGG pathways were associated with Orexin-A treatment. MAPK, chemokine and calcium signalling pathways were mainly responsible for the neuroprotective effects of Orexin-A. Hspb1, Igf2 and Ptk2b were selected for functional interaction analysis by GeneMANIA. The results suggest that Orexin-A modifies gene expression in the hippocampus, leading to neuroprotection from I/R injury. The study provides a basis for future elucidation of the molecular mechanisms underlying Orexin-A.

Impaired chemosensory control of breathing after depletion of bulbospinal catecholaminergic neurons in rats

Bulbospinal catecholaminergic neurons located in the rostral aspect of the ventrolateral medulla (C1 neurons) or within the ventrolateral pons (A5 neurons) are involved in the regulation of blood pressure and sympathetic outflow. A stimulus that commonly activates the C1 or A5 neurons is hypoxia, which is also involved in breathing activation. Although pharmacological and optogenetic evidence suggests that catecholaminergic neurons also regulate breathing, a specific contribution of the bulbospinal neurons to respiratory control has not been demonstrated. Therefore, in the present study, we evaluated whether the loss of bulbospinal catecholaminergic C1 and A5 cells affects cardiorespiratory control during resting, hypoxic (8% O₂), and hypercapnic (7% CO₂) conditions in unanesthetized rats. Thoracic spinal cord (T4-T8) injections of the immunotoxin anti-dopamine β-hydroxylase-saporin (anti-DβH-SAP-2.4 ng/100 nl) and the retrograde tracer Fluor-Gold or ventrolateral pontine injections of 6-OHDA were performed in adult male Wistar rats (250-280 g, N = 7-9/group). Anti-DβH-SAP or 6-OHDA eliminated most bulbospinal C1 and A5 neurons or A5 neurons, respectively. Serotonergic neurons and astrocytes were spared. Depletion of the bulbospinal catecholaminergic cells did not change cardiorespiratory variables under resting condition, but it did affect the response to hypoxia and hypercapnia. Specifically, the increase in the ventilation, the number of sighs, and the tachycardia were reduced, but the MAP increased during hypoxia in anti-DβH-SAP-treated rats. Our data reveal that the bulbospinal catecholaminergic neurons (A5 and C1) facilitate the ventilatory reflex to hypoxia and hypercapnia.

Genetically and functionally defined NTS to PBN brain circuits mediating anorexia

The central nervous system controls food consumption to maintain metabolic homoeostasis. In response to a meal, visceral signals from the gut activate neurons in the nucleus of the solitary tract (NTS) via the vagus nerve. These NTS neurons then excite brain regions known to mediate feeding behaviour, such as the lateral parabrachial nucleus (PBN). We previously described a neural circuit for appetite suppression involving calcitonin gene-related protein (CGRP)-expressing PBN (CGRP^PBN) neurons; however, the molecular identity of the inputs to these neurons was not established. Here we identify cholecystokinin (CCK) and noradrenergic, dopamine β-hydroxylase (DBH)-expressing NTS neurons as two separate populations that directly excite CGRP^PBN neurons. When these NTS neurons are activated using optogenetic or chemogenetic methods, food intake decreases and with chronic stimulation mice lose body weight. Our optogenetic results reveal that CCK and DBH neurons in the NTS directly engage CGRP^PBN neurons to promote anorexia.

Neurons in the nucleus of the solitary tract (NTS) are known to receive visceral signals from the gut during feeding. Here, the authors identify two populations of CCK- and DBH-expressing NTS neurons that work to suppress food intake when activated via opto- or chemogenetic stimulation.

Afferent and efferent connections of C1 cells with spinal cord or hypothalamic projections in mice

The axonal projections and synaptic input of the C1 adrenergic neurons of the rostral ventrolateral medulla (VLM) were examined using transgenic dopamine-beta hydroxylase Cre mice and modified rabies virus. Cre-dependent viral vectors expressing TVA (receptor for envelopeA) and rabies glycoprotein were injected into the left VLM. EnvelopeA-pseudotyped rabies-EGFP glycoprotein-deficient virus (rabies-EGFP) was injected 4-6 weeks later in either thoracic spinal cord (SC) or hypothalamus. TVA immunoreactivity was detected almost exclusively (95 %) in VLM C1 neurons. In mice with SC injections of rabies-EGFP, starter cells (expressing TVA + EGFP) were found at the rostral end of the VLM; in mice with hypothalamic injections starter C1 cells were located more caudally. C1 neurons innervating SC or hypothalamus had other terminal fields in common (e.g., dorsal vagal complex, locus coeruleus, raphe pallidus and periaqueductal gray matter). Putative inputs to C1 cells with SC or hypothalamic projections originated from the same brain regions, especially the lower brainstem reticular core from spinomedullary border to rostral pons. Putative input neurons to C1 cells were also observed in the nucleus of the solitary tract, caudal VLM, caudal spinal trigeminal nucleus, cerebellum, periaqueductal gray matter and inferior and superior colliculi. In sum, regardless of whether they innervate SC or hypothalamus, VLM C1 neurons receive input from the same general brain regions. One interpretation is that many types of somatic or internal stimuli recruit these neurons en bloc to produce a stereotyped acute stress response with sympathetic, parasympathetic, vigilance and neuroendocrine components.