GLP-1 neurons form a local synaptic circuit within the rodent nucleus of the solitary tract

The protective role of GLP-1 in neuro-ophthalmology

Despite recent advancements in the field of neuro-ophthalmology, the rising rates of neurological and ophthalmological conditions, mismatches between supply and demand of clinicians, and an aging population underscore the urgent need to explore new therapeutic approaches within the field. Glucagon-like peptide 1 receptor agonists (GLP-1RAs), traditionally used in the treatment of type 2 diabetes, are becoming increasingly appreciated for their diverse applications. Recently, GLP-1RAs have been approved for the treatment of obesity and recognized for their cardioprotective effects. Emerging evidence indicates some GLP-1RAs can cross the blood-brain barrier and may have neuroprotective effects. Therefore, this article aims to review the literature on the neurologic and neuro-ophthalmic role of glucagon-like peptide 1 (GLP-1). This article describes GLP-1 peptide characteristics and the mechanisms mediating its known role in increasing insulin, decreasing glucagon, delaying gastric emptying, and promoting satiety. This article identifies the sources and targets of GLP-1 in the brain and review the mechanisms which mediate its neuroprotective effects, as well as implications for Alzheimer's disease (AD) and Parkinson's disease (PD). Furthermore, the preclinical works which unravel the effects of GLP-1 in ocular dynamics and the preclinical literature regarding GLP-1RA use in the management of several neuro-ophthalmic conditions, including diabetic retinopathy (DR), glaucoma, and idiopathic intracranial hypertension (IIH) are discussed.

Analysis of the distribution of vagal afferent projections from different peripheral organs to the nucleus of the solitary tract in rats

Anatomical tracing studies examining the vagal system can conflate details of sensory afferent and motor efferent neurons. Here, we used a serotype of adeno-associated virus that transports retrogradely and exhibits selective tropism for vagal afferents, to map their soma location and central termination sites within the nucleus of the solitary tract (NTS). We examined the vagal sensory afferents innervating the trachea, duodenum, stomach, or heart, and in some animals, from two organs concurrently. We observed no obvious somatotopy in the somata distribution within the nodose ganglion. The central termination patterns of afferents from different organs within the NTS overlap substantially. Convergence of vagal afferent inputs from different organs onto single NTS neurons is observed. Abdominal and thoracic afferents terminate throughout the NTS, including in the rostral NTS, where the 7th cranial nerve inputs are known to synapse. To address whether the axonal labeling produced by viral transduction is so widespread because it fills axons traveling to their targets, and not just terminal fields, we labeled pre and postsynaptic elements of vagal afferents in the NTS . Vagal afferents form multiple putative synapses as they course through the NTS, with each vagal afferent neuron distributing sensory signals to multiple second-order NTS neurons. We observe little selectivity between vagal afferents from different visceral targets and NTS neurons with common neurochemical phenotypes, with afferents from different organs making close appositions with the same NTS neuron. We conclude that specific viscerosensory information is distributed widely within the NTS and that the coding of this input is probably determined by the intrinsic properties and projections of the second-order neuron.

Anti-Inflammatory Effects of GLP-1 Receptor Activation in the Brain in Neurodegenerative Diseases

The glucagon-like peptide-1 (GLP-1) is a pleiotropic hormone well known for its incretin effect in the glucose-dependent stimulation of insulin secretion. However, GLP-1 is also produced in the brain and displays a critical role in neuroprotection and inflammation by activating the GLP-1 receptor signaling pathways. Several studies in vivo and in vitro using preclinical models of neurodegenerative diseases show that GLP-1R activation has anti-inflammatory properties. This review explores the molecular mechanistic action of GLP-1 RAS in relation to inflammation in the brain. These findings update our knowledge of the potential benefits of GLP-1RAS actions in reducing the inflammatory response. These molecules emerge as a potential therapeutic tool in treating neurodegenerative diseases and neuroinflammatory pathologies.

The ins and outs of the caudal nucleus of the solitary tract: An overview of cellular populations and anatomical connections

The body and brain are in constant two-way communication. Driving this communication is a region in the lower brainstem: the dorsal vagal complex. Within the dorsal vagal complex, the caudal nucleus of the solitary tract (cNTS) is a major first stop for incoming information from the body to the brain carried by the vagus nerve. The anatomy of this region makes it ideally positioned to respond to signals of change in both emotional and bodily states. In turn, the cNTS controls the activity of regions throughout the brain that are involved in the control of both behaviour and physiology. This review is intended to help anyone with an interest in the cNTS. First, I provide an overview of the architecture of the cNTS and outline the wide range of neurotransmitters expressed in subsets of neurons in the cNTS. Next, in detail, I discuss the known inputs and outputs of the cNTS and briefly highlight what is known regarding the neurochemical makeup and function of those connections. Then, I discuss one group of cNTS neurons: glucagon-like peptide-1 (GLP-1)-expressing neurons. GLP-1 neurons serve as a good example of a group of cNTS neurons, which receive input from varied sources and have the ability to modulate both behaviour and physiology. Finally, I consider what we might learn about other cNTS neurons from our study of GLP-1 neurons and why it is important to remember that the manipulation of molecularly defined subsets of cNTS neurons is likely to affect physiology and behaviours beyond those monitored in individual experiments.

Linagliptin, a Selective Dipeptidyl Peptidase-4 Inhibitor, Reduces Physical and Behavioral Effects of Morphine Withdrawal

(1) Background: Recent data indicate that receptors for GLP-1 peptide are involved in the activity of the mesolimbic system. Thus, the purpose of the present study was to examine the effect of the selective dipeptidyl peptidase-4 (DPP-4) inhibitor, linagliptin, on morphine dependence in mice. (2) Methods: Morphine dependence in mice was obtained by administration of increasing doses of morphine for eight consecutive days, twice a day. On the 9th day of the experiment, the naloxone-induced (2 mg/kg, ip) morphine withdrawal signs (jumping) were assessed. Moreover, behavioral effects of short-term (60 h after morphine discontinuation) and long-term (14 days after morphine discontinuation) morphine withdrawal were observed. In terms of behavioral effects, the depressive effect in the forced swim test and anxiety in the elevated plus maze test were investigated. Locomotor activity of mice was also studied. (3) Results: The administration of linagliptin (10 and 20 mg/kg, ip) for 8 consecutive days before morphine injections significantly diminished the number of naloxone-induced morphine withdrawal signs (jumping) in mice. In addition, the cessation of morphine administration induced depressive behavior in mice which were observed during short- and long-term morphine withdrawal. Linagliptin administered during morphine withdrawal significantly reduced the depressive behavior in studied mice. Furthermore, the short-term morphine withdrawal evoked anxiety which also was reduced by linagliptin in mice. (4) Conclusions: The present study reveals that GLP-1 receptors are involved in morphine dependence. What is more, linagliptin might be a valuable drug in attenuating the physical symptoms of morphine dependence. It might be also a useful drug in reducing emotional disturbances which may develop during the morphine withdrawal period.

[Prolactin-releasing peptide increases rat anxiety]

BACKGROUND

Prolactin-releasing peptide(Prl-RP), in addition to stimulating the production of prolactin, interacts with various parts of the central nervous system, participating in the implementation of many functions that are reflected in behavior.

AIM

The effect of Prl-RP on the anxiety of white Wistar rats was studied since there were no data in the literature on the relationship between Prl-RP and anxiety.

MATERIALS AND METHODS

Anxiety was assessed in two tests. In the elevated plus-maze (EPM), the time spent in the open arms and the number of edge reactions were recorded. In the social preference test, the time spent near a stranger, near a familiar individual, and in neutral territory were recorded.

RESULTS

The administration of Prl-RP at a dose of 10-10 M with a volume of 10 µl in each nostril reduced the time spent by the animals in the open arms of the EPM, and the number of edge reactions. For testing the social interaction, animals were pre-selected for high or low levels of anxiety in the EPM. In rats with initially low levels of anxiety, Prl-RP reduced the time spent near a stranger, indicating an increase in anxiety levels. The behavior of rats with initially high levels of anxiety did not change after application of the Prl-RP.

CONCLUSION

The results of our experiments indicate that the intranasal administration of Prl-RP increases the anxiety of rats.

Brain GLP-1 and the regulation of food intake: GLP-1 action in the brain and its implications for GLP-1 receptor agonists in obesity treatment

This review considers the similarities and differences between the physiological systems regulated by gut-derived and neuronally produced glucagon-like peptide 1 (GLP-1). It addresses the questions of whether peripheral and central GLP-1 sources constitute separate, linked or redundant systems and whether the brain GLP-1 system consists of disparate sections or is a homogenous entity. This review also explores the implications of the answers to these questions for the use of GLP-1 receptor agonists as anti-obesity drugs.

Reappraising the role of the vagus nerve in GLP-1-mediated regulation of eating

Here, we provide a focused review of the evidence for the roles of the vagus nerve in mediating the regulatory effects of peripherally and centrally produced GLP-1 on eating behaviour and energy balance. We particularly focus on recent studies which have used selective genetic, viral, and transcriptomic approaches to provide important insights into the anatomical and functional organisation of GLP-1-mediated gut-brain signalling pathways. A number of these studies have challenged canonical ideas of how GLP-1 acts in the periphery and the brain to regulate eating behaviour, with important implications for the development of pharmacological treatments for obesity.

The role of nucleus of the solitary tract glucagon-like peptide-1 and prolactin-releasing peptide neurons in stress: anatomy, physiology and cellular interactions

Neuroendocrine, behavioural and autonomic responses to stressful stimuli are orchestrated by complex neural circuits. The caudal nucleus of the solitary tract (cNTS) in the dorsomedial hindbrain is uniquely positioned to integrate signals of both interoceptive and psychogenic stress. Within the cNTS, glucagon‐like peptide‐1 (GLP‐1) and prolactin‐releasing peptide (PrRP) neurons play crucial roles in organising neural responses to a broad range of stressors. In this review we discuss the anatomical and functional overlap between PrRP and GLP‐1 neurons. We outline their co‐activation in response to stressful stimuli and their importance as mediators of behavioural and physiological stress responses. Finally, we review evidence that PrRP neurons are downstream of GLP‐1 neurons and outline unexplored areas of the research field. Based on the current state‐of‐knowledge, PrRP and GLP‐1 neurons may be compelling targets in the treatment of stress‐related disorders.

The diverse effects of brain glucagon-like peptide 1 receptors on ingestive behaviour

Glucagon-like peptide 1 (GLP-1) is well known as a gut hormone and also acts as a neuropeptide, produced in a discrete population of caudal brainstem neurons that project widely throughout the brain. GLP-1 receptors are expressed in many brain areas of relevance to energy balance, and stimulation of these receptors at many of these sites potently suppresses food intake. This review surveys the current evidence for effects mediated by GLP-1 receptors on feeding behaviour at a wide array of brain sites and discusses behavioural and neurophysiological mechanisms for the effects identified thus far. Taken together, it is clear that GLP-1 receptor activity in the brain can influence feeding by diverse means, including mediation of gastrointestinal satiation and/or satiety signalling, suppression of motivation for food reward, induction of nausea and mediation of restraint stress-induced hypophagia, but many questions about the organization of this system remain.

Central and peripheral GLP-1 systems independently suppress eating

The anorexigenic peptide glucagon-like peptide-1 (GLP-1) is secreted from gut enteroendocrine cells and brain preproglucagon (PPG) neurons, which, respectively, define the peripheral and central GLP-1 systems. PPG neurons in the nucleus tractus solitarii (NTS) are widely assumed to link the peripheral and central GLP-1 systems in a unified gut-brain satiation circuit. However, direct evidence for this hypothesis is lacking, and the necessary circuitry remains to be demonstrated. Here we show that PPGNTS neurons encode satiation in mice, consistent with vagal signalling of gastrointestinal distension. However, PPGNTS neurons predominantly receive vagal input from oxytocin-receptor-expressing vagal neurons, rather than those expressing GLP-1 receptors. PPGNTS neurons are not necessary for eating suppression by GLP-1 receptor agonists, and concurrent PPGNTS neuron activation suppresses eating more potently than semaglutide alone. We conclude that central and peripheral GLP-1 systems suppress eating via independent gut-brain circuits, providing a rationale for pharmacological activation of PPGNTS neurons in combination with GLP-1 receptor agonists as an obesity treatment strategy.

Resveratrol Modulates the Gut-Brain Axis: Focus on Glucagon-Like Peptide-1, 5-HT, and Gut Microbiota

Resveratrol is a natural polyphenol that has anti-aging and anti-inflammatory properties against stress condition. It is reported that resveratrol has beneficial functions in various metabolic and central nervous system (CNS) diseases, such as obesity, diabetes, depression, and dementia. Recently, many researchers have emphasized the connection between the brain and gut, called the gut-brain axis, for treating both CNS neuropathologies and gastrointestinal diseases. Based on previous findings, resveratrol is involved in glucagon-like peptide 1 (GLP-1) secreted by intestine L cells, the patterns of microbiome in the intestine, the 5-hydroxytryptamine (5-HT) level, and CNS inflammation. Here, we review recent evidences concerning the relevance and regulatory function of resveratrol in the gut-brain axis from various perspectives. Here, we highlight the necessity for further study on resveratrol's specific mechanism in the gut-brain axis. We present the potential of resveratrol as a natural therapeutic substance for treating both neuropathology and gastrointestinal dysfunction.

Leptin suppresses development of GLP-1 inputs to the paraventricular nucleus of the hypothalamus

The nucleus of the solitary tract (NTS) is critical for the central integration of signals from visceral organs and contains preproglucagon (PPG) neurons, which express leptin receptors in the mouse and send direct projections to the paraventricular nucleus of the hypothalamus (PVH). Here, we visualized projections of PPG neurons in leptin-deficient Lep^ob/ob mice and found that projections from PPG neurons are elevated compared with controls, and PPG projections were normalized by targeted rescue of leptin receptors in LepRb^TB/TB mice, which lack functional neuronal leptin receptors. Moreover, Lep^ob/ob and LepRb^TB/TB mice displayed increased levels of neuronal activation in the PVH following vagal stimulation, and whole-cell patch recordings of GLP-1 receptor-expressing PVH neurons revealed enhanced excitatory neurotransmission, suggesting that leptin acts cell autonomously to suppress representation of excitatory afferents from PPG neurons, thereby diminishing the impact of visceral sensory information on GLP-1 receptor-expressing neurons in the PVH.

Leptin Sensitizes NTS Neurons to Vagal Input by Increasing Postsynaptic NMDA Receptor Currents

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 Effects of Mutual Interaction of Orexin-A and Glucagon-Like Peptide-1 on Reflex Swallowing Induced by SLN Afferents in Rats

(1) Background: Our previous studies revealed that orexin-A, an appetite-increasing peptide, suppressed reflex swallowing via the commissural part of the nucleus tractus solitarius (cNTS), and that glucagon-like peptide-1 (GLP-1), an appetite-reducing peptide, also suppressed reflex swallowing via the medial nucleus of the NTS (mNTS). In this study, we examined the mutual interaction between orexin-A and GLP-1 in reflex swallowing. (2) Methods: Sprague–Dawley rats under urethane–chloralose anesthesia were used. Swallowing was induced by electrical stimulation of the superior laryngeal nerve (SLN) and was identified by the electromyographic (EMG) signals obtained from the mylohyoid muscle. (3) Results: The injection of GLP-1 (20 pmol) into the mNTS reduced the swallowing frequency and extended the latency of the first swallow. These suppressive effects of GLP-1 were not observed after the fourth ventricular administration of orexin-A. After the injection of an orexin-1 receptor antagonist (SB334867) into the cNTS, an ineffective dose of GLP-1 (6 pmol) into the mNTS suppressed reflex swallowing. Similarly, the suppressive effects of orexin-A (1 nmol) were not observed after the injection of GLP-1 (6 pmol) into the mNTS. After the administration of a GLP-1 receptor antagonist (exendin-4(5-39)), an ineffective dose of orexin-A (0.3 nmol) suppressed reflex swallowing. (4) Conclusions: The presence of reciprocal inhibitory connections between GLP-1 receptive neurons and orexin-A receptive neurons in the NTS was strongly suggested.

GABA neurons in the nucleus tractus solitarius express GLP-1 receptors and mediate anorectic effects of liraglutide in rats

The glucagon-like peptide-1 receptor (GLP-1R) agonist liraglutide is approved for the treatment of obesity; however, there is still much to be learned regarding the neuronal sites of action that underlie its suppressive effects on food intake and body weight. Peripherally administered liraglutide in rats acts in part through central GLP-1Rs in both the hypothalamus and the hindbrain. Here, we extend findings supporting a role for hindbrain GLP-1Rs in mediating the anorectic effects of liraglutide in male rats. To dissociate the contribution of GLP-1Rs in the area postrema (AP) and the nucleus tractus solitarius (NTS), we examined the effects of liraglutide in both NTS AAV-shRNA-driven Glp1r knockdown and AP-lesioned animals. Knockdown of NTS GLP-1Rs, but not surgical lesioning of the AP, attenuated the anorectic and body weight-reducing effects of acutely delivered liraglutide. In addition, NTS c-Fos responses were maintained in AP-lesioned animals. Moreover, NTS Glp1r knockdown was sufficient to attenuate the intake- and body weight-reducing effects of chronic daily administered liraglutide over 3 weeks. Development of improved obesity pharmacotherapies requires an understanding of the cellular phenotypes targeted by GLP-1R agonists. Fluorescence in situ hybridization identified Glp1r transcripts in NTS GABAergic neurons, which when inhibited using chemogenetics, attenuated the food intake- and body weight-reducing effects of liraglutide. This work demonstrates the contribution of NTS GLP-1Rs to the anorectic potential of liraglutide and highlights a phenotypically distinct (GABAergic) population of neurons within the NTS that express the GLP-1R and are involved in the mediation of liraglutide signaling.

The role of linagliptin, a selective dipeptidyl peptidase-4 inhibitor, in the morphine rewarding effects in rats

Linagliptin is a selective dipeptidyl peptidase-4 (DPP-4) inhibitor which suppresses the rapid degradation of endogenous glucagon-like peptide-1 (GLP-1). In clinical practice, it is used as an antidiabetic drug, but recent studies have confirmed its role in the activity of the central nervous system (CNS). The reported study focused on the role of linagliptin (10 and 20 mg/kg, ip) in the morphine rewarding effect, analyzing how the agent had influenced the conditioned place preference (CPP) in rats via the expression, acquisition, extinction and reinstatement of the morphine rewarding effect. The obtained results clearly demonstrated linagliptin to inhibit the expression and acquisition, to accelerate the extinction and, eventually, to reduce the reinstatement of morphine-induced CPP. The undertaken experiments significantly extended our knowledge on the mechanisms behind the morphine rewarding effect.

Synaptic Inputs to the Mouse Dorsal Vagal Complex and Its Resident Preproglucagon Neurons

Stress responses are coordinated by widespread neural circuits. Homeostatic and psychogenic stressors activate preproglucagon (PPG) neurons in the caudal nucleus of the solitary tract (cNTS) that produce glucagon-like peptide-1; published work in rodents indicates that these neurons play a crucial role in stress responses. While the axonal targets of PPG neurons are well established, their afferent inputs are unknown.

Stress responses are coordinated by widespread neural circuits. Homeostatic and psychogenic stressors activate preproglucagon (PPG) neurons in the caudal nucleus of the solitary tract (cNTS) that produce glucagon-like peptide-1; published work in rodents indicates that these neurons play a crucial role in stress responses. While the axonal targets of PPG neurons are well established, their afferent inputs are unknown. Here we use retrograde tracing with cholera toxin subunit b to show that the cNTS in male and female mice receives axonal inputs similar to those reported in rats. Monosynaptic and polysynaptic inputs specific to cNTS PPG neurons were revealed using Cre-conditional pseudorabies and rabies viruses. The most prominent sources of PPG monosynaptic input include the lateral (LH) and paraventricular (PVN) nuclei of the hypothalamus, parasubthalamic nucleus, lateral division of the central amygdala, and Barrington's nucleus (Bar). Additionally, PPG neurons receive monosynaptic vagal sensory input from the nodose ganglia and spinal sensory input from the dorsal horn. Sources of polysynaptic input to cNTS PPG neurons include the hippocampal formation, paraventricular thalamus, and prefrontal cortex. Finally, cNTS-projecting neurons within PVN, LH, and Bar express the activation marker cFOS in mice after restraint stress, identifying them as potential sources of neurogenic stress-induced recruitment of PPG neurons. In summary, cNTS PPG neurons in mice receive widespread monosynaptic and polysynaptic input from brain regions implicated in coordinating behavioral and physiological stress responses, as well as from vagal and spinal sensory neurons. Thus, PPG neurons are optimally positioned to integrate signals of homeostatic and psychogenic stress.

SIGNIFICANCE STATEMENT Recent research has indicated a crucial role for glucagon-like peptide-1-producing preproglucagon (PPG) neurons in regulating both appetite and behavioral and autonomic responses to acute stress. Intriguingly, the central glucagon-like peptide-1 system defined in rodents is conserved in humans, highlighting the translational importance of understanding its anatomical organization. Findings reported here indicate that PPG neurons receive significant monosynaptic and polysynaptic input from brain regions implicated in autonomic and behavioral responses to stress, as well as direct input from vagal and spinal sensory neurons. Improved understanding of the neural pathways underlying the recruitment of PPG neurons may facilitate the development of novel therapies for the treatment of stress-related disorders.

Prolactin-Releasing Peptide: Physiological and Pharmacological Properties

Prolactin-releasing peptide (PrRP) belongs to the large RF-amide neuropeptide family with a conserved Arg-Phe-amide motif at the C-terminus. PrRP plays a main role in the regulation of food intake and energy expenditure. This review focuses not only on the physiological functions of PrRP, but also on its pharmacological properties and the actions of its G-protein coupled receptor, GPR10. Special attention is paid to structure-activity relationship studies on PrRP and its analogs as well as to their effect on different physiological functions, mainly their anorexigenic and neuroprotective features and the regulation of the cardiovascular system, pain, and stress. Additionally, the therapeutic potential of this peptide and its analogs is explored.

GLP-1: Molecular mechanisms and outcomes of a complex signaling system

Meal ingestion provokes the release of hormones and transmitters, which in turn regulate energy homeostasis and feeding behavior. One such hormone, glucagon-like peptide-1 (GLP-1), has received significant attention in the treatment of obesity and diabetes due to its potent incretin effect. In addition to the peripheral actions of GLP-1, this hormone is able to alter behavior through the modulation of multiple neural circuits. Recent work that focused on elucidating the mechanisms and outcomes of GLP-1 neuromodulation led to the discovery of an impressive array of GLP-1 actions. Here, we summarize the many levels at which the GLP-1 signal adapts to different systems, with the goal being to provide a background against which to guide future research.

Glucagon-like peptide-1 receptors within the nucleus of the solitary tract regulate alcohol-mediated behaviors in rodents

The ability of glucagon-like peptide-1 (GLP-1) to reduce food intake involves activation of GLP-1 receptors (GLP-1R) in the nucleus of the solitary tract (NTS). It has also been demonstrated that systemic administration of GLP-1R agonists attenuates alcohol-mediated behaviors via, to date, unknown mechanisms. Therefore, we evaluated the effects of NTS-GLP-1R activation by exendin-4 (Ex4) on alcohol-induced locomotor stimulation, accumbal dopamine release and memory of alcohol reward in the conditioned place preference (CPP) model in mice. Moreover, the ability of Ex4 infusion into the NTS on alcohol intake was explored in rats. Ex4 into the NTS inhibits the acute effects of alcohol as measured by alcohol-induced locomotor stimulation, accumbal dopamine release and the memory consolidation of alcohol reward in the CPP paradigm. In addition, NTS-Ex4 dose-dependently decreases alcohol intake in rats consuming alcohol for 12 weeks. Pharmacological suppression of GLP-1R in the NTS prevents the ability of systemic Ex4 to block the alcohol-induced locomotor stimulation in mice. These data add a functional role of GLP-1R within the NTS, involving alcohol-related behaviors. In addition, they may provide insight into the GLP-1R containing brain areas that modulate the ability of GLP-1R agonists to reduce alcohol reinforcement. Collectively, this further supports GLP-1R as potential treatment targets for alcohol use disorder.

Central control of gastrointestinal motility

PURPOSE OF REVIEW

This review summarizes the organization and structure of vagal neurocircuits controlling the upper gastrointestinal tract, and more recent studies investigating their role in the regulation of gastric motility under physiological, as well as pathophysiological, conditions.

RECENT FINDINGS

Vagal neurocircuits regulating gastric functions are highly plastic, and open to modulation by a variety of inputs, both peripheral and central. Recent research in the fields of obesity, development, stress, and neurological disorders highlight the importance of central inputs onto these brainstem neurocircuits in the regulation of gastric motility.

SUMMARY

Recognition of the pivotal role that the central nervous system exerts in the regulation, integration, and modulation of gastric motility should serve to encourage research into central mechanisms regulating peripheral motility disorders.

GLP-1 neurons form a local synaptic circuit within the rodent nucleus of the solitary tract

Glutamatergic neurons that express pre-proglucagon (PPG) and are immunopositive (+) for glucagon-like peptide-1 (i.e., GLP-1+ neurons) are located within the caudal nucleus of the solitary tract (cNTS) and medullary reticular formation in rats and mice. GLP-1 neurons give rise to an extensive central network in which GLP-1 receptor (GLP-1R) signaling suppresses food intake, attenuates rewarding, increases avoidance, and stimulates stress responses, partly via GLP-1R signaling within the cNTS. In mice, noradrenergic (A2) cNTS neurons express GLP-1R, whereas PPG neurons do not. In this study, confocal microscopy in rats confirmed that prolactin-releasing peptide (PrRP)+ A2 neurons are closely apposed by GLP-1+ axonal varicosities. Surprisingly, GLP-1+ appositions were also observed on dendrites of PPG/GLP-1+ neurons in both species, and electron microscopy in rats revealed that GLP-1+ boutons form asymmetric synaptic contacts with GLP-1+ dendrites. However, RNAscope confirmed that rat GLP-1 neurons do not express GLP-1R mRNA. Similarly, Ca2+ imaging of somatic and dendritic responses in mouse ex vivo slices confirmed that PPG neurons do not respond directly to GLP-1, and a mouse crossbreeding strategy revealed that <1% of PPG neurons co-express GLP-1R. Collectively, these data suggest that GLP-1R signaling pathways modulate the activity of PrRP+ A2 neurons, and also reveal a local "feed-forward" synaptic network among GLP-1 neurons that apparently does not use GLP-1R signaling. This local GLP-1 network may instead use glutamatergic signaling to facilitate dynamic and potentially selective recruitment of GLP-1 neural populations that shape behavioral and physiological responses to internal and external challenges.