In diabetic patients, hypoglycemia-associated autonomic failure (HAAF) is a potentially lethal condition that results in attenuation of critical protective responses to low blood glucose, following repeated prior hypoglycemic episodes. The underlying mechanism(s) of HAAF is not fully understood. We previously demonstrated that activation of catecholamine (CA) neurons in the ventrolateral medulla (VLM) is both required and sufficient to elicit key protective responses to glucose deficit. Here we test the hypothesis that repeated selective activation of VLM CA neurons is sufficient to induce HAAF, even in the absence of glucose deficit. Using stereotaxic injections of an adeno-associated virus in female TH-Cre transgenic rats, we transfected rostral C1 CA neurons to express hM3D(Gq). In these rats, we found that a single clozapine-N-oxide (CNO) injection evoked robust hyperglycemia, with a magnitude and time course similar to those evoked by the glucoprivic agent, 2-deoxy-D-glucose (2DG). However, following repeated CNO injections in these same rats, the hyperglycemic response evoked by either CNO or 2DG were significantly attenuated. Moreover, plasma epinephrine levels, and Fos expression in VLM CA neurons and in the adrenal medulla, also were attenuated following repeated CNO injections. This constellation of effects is similar to those that define HAAF. Taken together our results suggest that repeated stimulation of VLM CA neurons mimics and could be a cause of the impairment of counterregulatory responses associated with pathogenesis of HAAF.

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.

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.

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 CCKR 1 . 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.

Background and Purpose 'Food addiction' is the subject of intense public and research interest. However, this nosology based on neurobehavioural similarities among obese individuals, patients with eating disorders and those with substance use disorders (drug addiction) remains controversial. We thus sought to determine which aspects of disordered eating are causally linked to preclinical models of drug addiction. We hypothesized that extensive drug histories, known to cause addiction-like brain changes and drug motivation in rats, would also cause addiction-like food motivation. Experimental Approach Rats underwent extensive cocaine, alcohol, caffeine or obesogenic diet histories and were subsequently tested for punishment-resistant food self-administration or 'compulsive appetite', as a measure of addiction-like food motivation. Key Results Extensive cocaine and alcohol (but not caffeine) histories caused compulsive appetite that persisted long after the last drug exposure. Extensive obesogenic diet histories also caused compulsive appetite, although neither cocaine nor alcohol histories caused excess calorie intake and bodyweight during abstinence. Hence, compulsive appetite and obesity appear to be dissociable, with the former sharing common mechanisms with preclinical drug addiction models. Conclusion and Implications Compulsive appetite, as seen in subsets of obese individuals and patients with binge-eating disorder and bulimia nervosa (eating disorders that do not necessarily result in obesity), appears to epitomize 'food addiction'. Because different drug and obesogenic diet histories caused compulsive appetite, overlapping dysregulations in the reward circuits, which control drug and food motivation independently of energy homeostasis, may offer common therapeutic targets for treating addictive behaviours across drug addiction, eating disorders and obesity.

Comparative Anatomy of Medullary Vagal Nerve Nuclei (T.E. Finger). Viscerotopic Representation of the Alimentary Tract in the Dorsal and Ventral Vagal Complexes in the Rat (S.M. Altschuler, L. Rinamam, and R.R. Miselis). The Dorsal Vagal Complex Forms A Sensory-Motor Lattice: The Circuitry of Gastrointestinal Reflexes (T.L. Powley, H.R. Berthoud, E.A. Fox, and W. Laughton). Central Connections of the Nuclei of the Vagus Nerve (R.A. Leslie, D.J.M. Reynolds, and I.N.C. Lawes). Central Regulation of Brainstem Gastric Vago-Vagal Control Circuits (R.C. Rogers and G.E. Hermann). Vagal Afferent Innervation of the Enteric Nervous System (C. Sternini). Vagal Receptor Transport (T.H. Moran and P.R. McHugh). Vagal Afferent Mechanisms of Mechano- and Chemoreception (D. Grundy). Vagal Afferent Innervation and the Regulation of Gastric Motor Function (H.E. Raybould). Participation of Vagal Sensory Neurons in Putative Satiety Signals From the Upper Gastrointestinal Tract (R.C. Ritter, L. Brenner, and D.P. Yox). Cooperation of Vagal and Central Neural Systems in Monitoring Metabolic Events Controlling Feeding Behavior (S. Ritter, N.Y. Calingasan, B. Hutton, and T.T. Dinh). A Protective Role for Vagal Afferents: An Hypothesis (P.L.R. Andrews and I.N.C. Lawes). Index.

Leptin signaling within the nucleus of the solitary tract (NTS) contributes to the control of food intake, and injections of leptin into the NTS reduce meal size and increase the efficacy of vagus-mediated satiation signals. Leptin receptors (LepRs) are expressed by vagal afferents as well as by a population of NTS neurons. However, the electrophysiological properties of LepR-expressing NTS neurons have not been well characterized, and it is unclear how leptin might act on these neurons to reduce food intake. To address this question, we recorded from LepR-expressing neurons in horizontal brain slices containing the NTS from male and female LepR-Cre X Rosa-tdTomato mice. We found that the vast majority of NTS LepR neurons received monosynaptic innervation from vagal afferent fibers and LepR neurons exhibited large synaptic NMDA receptor (NMDAR)-mediated currents compared with non-LepR neurons. During high-frequency stimulation of vagal afferents, leptin increased the size of NMDAR-mediated currents, but not AMPAR-mediated currents. Leptin also increased the size of evoked EPSPs and the ability of low-intensity solitary tract stimulation to evoke action potentials in LepR neurons. These effects of leptin were blocked by bath applying a competitive NMDAR antagonist (DCPP-ene) or by an NMDAR channel blocker applied through the recording pipette (MK-801). Last, feeding studies using male rats demonstrate that intra-NTS injections of DCPP-ene attenuate reduction of overnight food intake following intra-NTS leptin injection. Our results suggest that leptin acts in the NTS to reduce food intake by increasing NMDAR-mediated currents, thus enhancing NTS sensitivity to vagal inputs. SIGNIFICANCE STATEMENT Leptin is a hormone that critically impacts food intake and energy homeostasis. The nucleus of the solitary tract (NTS) is activated by vagal afferents from the gastrointestinal tract, which promotes termination of a meal. Injection of leptin into the NTS inhibits food intake, while knockdown of leptin receptors (LepRs) in NTS neurons increases food intake. However, little was known about how leptin acts in the NTS neurons to inhibit food intake. We found that leptin increases the sensitivity of LepR-expressing neurons to vagal inputs by increasing NMDA receptor-mediated synaptic currents and that NTS NMDAR activation contributes to leptin-induced reduction of food intake. These findings suggest a novel mechanism by which leptin, acting in the NTS, could potentiate gastrointestinal satiation signals.

The stomach, upper small intestine, lower small intestine, the liver and the colon, have all been suggested as exercising control over food intake. This chapter focuses on vagal participation in the control of food intake by the stomach and small intestine. Recordings from gastric mechanoresponsive fibers in the vagus were among the first data supporting the vagal role in gastrointestinal sensation. Clearly, gastrointestinal vagal innervation detects and responds to mechanical and chemical properties of ingesta during each meal. It would be surprising, however, if changes in food intake were mediated exclusively by signals from this innervation. In fact we can say with certainty that they are not. Nevertheless, sensory neurons of the vagus nerve respond to a variety of stimuli appropriate as putative satiety signals and it is probable that vagal sensory mechanisms make an important contribution to the process of satiation by relaying information to the brain on chemical and mechanical qualities of ingesta.

Neuropeptide Y (NPY), peptide YY (PYY), and their cognate receptors (YR) are expressed by subpopulations of central and peripheral nervous system neurons. Intracerebroventricular injections of NPY or PYY increase food intake, and intrahypothalamic NPY1 or NPY5 receptor agonist injections also increase food intake. In contrast, injection of PYY in the periphery reduces food intake, apparently by activating peripheral Y2R. The dorsal vagal complex (DVC) of the hindbrain is the site where vagal afferents relay gut satiation signals to the brain. While contributions of the DVC are increasingly investigated, a role for DVC YR in control of food intake has not been examined systematically. We used in situ hybridization to confirm expression of Y1R and Y2R, but not Y5R, in the DVC and vagal afferent neurons. We found that nanoinjections of a Y2R agonist, PYY-(3-36), into the DVC significantly increased food intake over a 4-h period in satiated male rats. PYY-(3-36)-evoked food intake was prevented by injection of a selective Y2R antagonist. Injection of a Y1R/Y5R-preferring agonist into the DVC failed to increase food intake at doses reported to increase food intake following hypothalamic injection. Finally, injection of PYY-(3-36) into the DVC prevented reduction of 30-min food intake following intraperitoneal injection of cholecystokinin (CCK). Our results indicate that activation of DVC Y2R, unlike hypothalamic or peripheral Y2R, increases food intake. Furthermore, in the context of available electrophysiological observations, our results are consistent with the hypothesis that DVC Y2R control food intake by dampening vagally mediated satiation signals in the DVC.

We introduce the concept of trajectory-specific sensory manifolds. They are the unique multidimensional and time-varying combinations of afferent signals that obligatorily emerge during a limb movement. We use the example of muscle spindles (i.e., the muscle's proprioceptors for length and velocity) that arise during movements of an arm (a planar 2-DOF 6-muscle model) during the production of straight, curved and oscillatory hand movements. Through the use of parallel coordinates, we visualize the high-dimensional evolution of the afferent signaling across muscles and tasks. We demonstrate that a given movement gives rise to a distinct sensory manifold embedded in the 12-D space of spindle information that is largely independent of the choice of muscle coordination strategy. Given that muscle lengths and velocities are fully determined by joint kinematics, such manifolds provide a rich set of information to use in its control.

Key points Hindbrain NMDA receptors play important roles in reflexive and behavioural responses to vagal activation. NMDA receptors have also been shown to contribute to the synaptic responses of neurons in the nucleus of the solitary tract (NTS), but their exact role remains unclear. In this study we used whole cell patch‐clamping techniques in rat horizontal brain slice to investigate the role of NMDA receptors in the fidelity of transmission across solitary tract afferent–NTS neuron synapses. Results show that NMDA receptors contribute up to 70% of the charge transferred across the synapse at high (>5 Hz) firing rates, but have little contribution at lower firing frequencies. Results also show that NMDA receptors critically contribute to the fidelity of transmission across these synapses during high frequency (>5 Hz) afferent discharge rates. This novel role of NMDA receptors may explain in part how primary visceral afferents, including vagal afferents, can maintain fidelity of transmission across a broad range of firing frequencies. AbstractNeurons within the nucleus of the solitary tract (NTS) receive vagal afferent innervations that initiate gastrointestinal and cardiovascular reflexes. Glutamate is the fast excitatory neurotransmitter released in the NTS by vagal afferents, which arrive there via the solitary tract (ST). ST stimulation elicits excitatory postsynaptic currents (EPSCs) in NTS neurons mediated by both AMPA‐ and NMDA‐type glutamate receptors (‐Rs). Vagal afferents exhibit a high probability of vesicle release and exhibit robust frequency‐dependent depression due to presynaptic vesicle depletion. Nonetheless, synaptic throughput is maintained even at high frequencies of afferent activation. Here we test the hypothesis that postsynaptic NMDA‐Rs are essential in maintaining throughput across ST–NTS synapses. Using patch clamp electrophysiology in horizontal brainstem slices, we found that NMDA‐Rs, including NR2B subtypes, carry up to 70% of the charge transferred across the synapse during high frequency stimulations (>5 Hz). In contrast, their relative contribution to the ST‐EPSC is much less during low (<2 Hz) frequency stimulations. Afferent‐driven activation of NMDA‐Rs produces a sustained depolarization during high, but not low, frequencies of stimulation as a result of relatively slow decay kinetics. Hence, NMDA‐Rs are critical for maintaining action potential generation at high firing rates. These results demonstrate a novel role for NMDA‐Rs enabling a high probability of release synapse to maintain the fidelity of synaptic transmission during high frequency firing when glutamate release and AMPA‐R responses are reduced. They also suggest why NMDA‐Rs are critical for responses that may depend on high rates of afferent discharge.

We developed a novel Ag nanoparticles (NPs) supported on N-doped graphene (AgNPs/N-G) hybrid material that was used as an electrochemical sensor for H2O2 detection. AgNPs/N-G was synthesized by an environmentally friendly strategy with urea as the N-dopant and vitamin C as the reducing agent. Electrochemical measurements revealed a lower electrical resistance and higher electrocatalytic activity of AgNPs/N-G than undoped counterparts (AgNPs/reduced graphene oxide (rGO)). The AgNPs/N-G based sensor exhibited a fast response time (<2 s), a wide linear range (0.1-126.4 mM) and a low detection limit (1.2 μM). Moreover, the proposed sensor exhibited satisfying selectivity, reproducibility, and stability. These results indicated that AgNPs/N-G modified electrode could act as a promising nonenzymatic sensor for H2O2 detection.

Injection of the melanocortin-3/4 receptor agonist melanotan-II (MTII) into the nucleus of the solitary tract (NTS) produces rapid and sustained reduction of food intake. Melanocortin-4 receptors (MC4Rs) are expressed by vagal afferent endings in the NTS, but it is not known whether these endings participate in MTII-induced reduction of food intake. In experiments described here, we evaluated the contribution of central vagal afferent endings in MTII-induced reduction of food intake. Examination of rat hindbrain sections revealed that neuronal processes expressing immunoreactivity for the endogenous MC4R agonist α-melanoctyte-stimulating hormone course parallel and wrap around anterogradely labeled vagal afferent endings in the NTS and thus are aptly positioned to activate vagal afferent MC4Rs. Furthermore, MTII and endogenous MC4R agonists increased protein kinase A (PKA)-catalyzed phosphorylation of synapsin I in vagal afferent endings, an effect known to increase synaptic strength by enhancing neurotransmitter release in other neural systems. Hindbrain injection of a PKA inhibitor, KT5720, significantly attenuated MTII-induced reduction of food intake and the increase in synapsin I phosphorylation. Finally, unilateral nodose ganglion removal, resulting in degeneration of vagal afferent endings in the ipsilateral NTS, abolished MTII-induced synapsin I phosphorylation ipsilateral to nodose ganglion removal. Moreover, reduction of food intake following MTII injection into the NTS ipsilateral to nodose ganglion removal was significantly attenuated, whereas the response to MTII was not diminished when injected into the contralateral NTS. Altogether, our results suggest that reduction of food intake following hindbrain MC4R activation is mediated by central vagal afferent endings.

Hindbrain injection of a melanocortin-3/4 receptor agonist, MTII, reduces food intake primarily by reducing meal size. Our previously reported results indicate that N-methyl-d-aspartate-type glutamate receptors (NMDAR) in the nucleus of the solitary tract (NTS) play an important role in the control of meal size and food intake. Therefore, we hypothesized that activation of NTS NMDARs contribute to reduction of food intake in response to fourth ventricle or NTS injection of MTII. We found that coinjection of a competitive NMDAR antagonist (d-CPP-ene) with MTII into the fourth ventricle or directly into the NTS of adult male rats attenuated MTII-induced reduction of food intake. Hindbrain NMDAR antagonism also attenuated MTII-induced ERK1/2 phosphorylation in NTS neurons and prevented synapsin I phosphorylation in central vagal afferent endings, both of which are cellular mechanisms previously shown to participate in hindbrain melanocortinergic reduction of food intake. Together, our results indicate that NMDAR activation significantly contributes to reduction of food intake following hindbrain melanocortin receptor activation, and it participates in melanocortinergic signaling in NTS neural circuits that mediate reduction of food intake.

Vagal afferent nerve fibers transmit gastrointestinal satiation signals to the brain via synapses in the nucleus of the solitary tract (NTS). Despite their pivotal role in energy homeostasis, little is known about the cellular mechanisms enabling fleeting synaptic events at vagal sensory endings to sustain behavioral changes lasting minutes to hours. Previous reports suggest that the reduction of food intake by the satiation peptide, cholecystokinin (CCK), requires activation of N-methyl-D-aspartate-type glutamate receptors (NMDAR) in the NTS, with subsequent phosphorylation of ERK1/2 (pERK1/2) in NTS vagal afferent terminals. The synaptic vesicle protein synapsin I is phosphorylated by pERK1/2 at serines 62 and 67. This pERK1/2-catalyzed phosphorylation increases synaptic strength by increasing the readily releasable pool of the neurotransmitter. Conversely, dephosphorylation of serines 62 and 67 by calcineurin reduces the size of the readily releasable transmitter pool. Hence, the balance of synapsin I phosphorylation and dephosphorylation can modulate synaptic strength. We postulated that CCK-evoked activation of vagal afferent NMDARs results in pERK1/2-catalyzed phosphorylation of synapsin I in vagal afferent terminals, leading to the suppression of food intake. We found that CCK injection increased the phosphorylation of synapsin I in the NTS and that this increase is abolished after surgical or chemical ablation of vagal afferent fibers. Furthermore, fourth ventricle injection of an NMDAR antagonist or the mitogen-activated ERK kinase inhibitor blocked CCK-induced synapsin I phosphorylation, indicating that synapsin phosphorylation in vagal afferent terminals depends on NMDAR activation and ERK1/2 phosphorylation. Finally, hindbrain inhibition of calcineurin enhanced and prolonged synapsin I phosphorylation and potentiated reduction of food intake by CCK. Our findings are consistent with a mechanism in which NMDAR-dependent phosphorylation of ERK1/2 modulates satiation signals via synapsin I phosphorylation in vagal afferent endings.