Activation of Brainstem Pro-opiomelanocortin Neurons Produces Opioidergic Analgesia, Bradycardia and Bradypnoea

Vagus nerve stimulation rescues persistent pain following orthopedic surgery in adult mice

ABSTRACT

Postoperative pain is a major clinical problem imposing a significant burden on patients and society. In a survey 2 years after orthopedic surgery, 57% of patients reported persisting postoperative pain. However, only limited progress has been made in the development of safe and effective therapies to prevent the onset and chronification of pain after orthopedic surgery. We established a tibial fracture mouse model that recapitulates clinically relevant orthopedic trauma surgery, which causes changes in neuropeptide levels in dorsal root ganglia and sustained neuroinflammation in the spinal cord. Here, we monitored extended pain behavior in this model, observing chronic bilateral hindpaw mechanical allodynia in both male and female C57BL/6J mice that persisted for >3 months after surgery. We also tested the analgesic effects of a novel, minimally invasive, bioelectronic approach to percutaneously stimulate the vagus nerve (termed percutaneous vagus nerve stimulation [pVNS]). Weekly pVNS treatment for 30 minutes at 10 Hz for 3 weeks after the surgery strongly reduced pain behaviors compared with untreated controls. Percutaneous vagus nerve stimulation also improved locomotor coordination and accelerated bone healing. In the dorsal root ganglia, vagal stimulation inhibited the activation of glial fibrillary acidic protein-positive satellite cells but without affecting microglial activation. Overall, these data provide novel evidence supportive of the use of pVNS to prevent postoperative pain and inform translational studies to test antinociceptive effects of bioelectronic medicine in the clinic.

Human IPSC-Derived PreBötC-Like Neurons and Development of an Opiate Overdose and Recovery Model

Opioid overdose is the leading cause of drug overdose lethality, posing an urgent need for investigation. The key brain region for inspiratory rhythm regulation and opioid-induced respiratory depression (OIRD) is the preBötzinger Complex (preBötC) and current knowledge has mainly been obtained from animal systems. This study aims to establish a protocol to generate human preBötC neurons from induced pluripotent cells (iPSCs) and develop an opioid overdose and recovery model utilizing these iPSC-preBötC neurons. A de novo protocol to differentiate preBötC-like neurons from human iPSCs is established. These neurons express essential preBötC markers analyzed by immunocytochemistry and demonstrate expected electrophysiological responses to preBötC modulators analyzed by patch clamp electrophysiology. The correlation of the specific biomarkers and function analysis strongly suggests a preBötC-like phenotype. Moreover, the dose-dependent inhibition of these neurons' activity is demonstrated for four different opioids with identified IC50's comparable to the literature. Inhibition is rescued by naloxone in a concentration-dependent manner. This iPSC-preBötC mimic is crucial for investigating OIRD and combating the overdose crisis and a first step for the integration of a functional overdose model into microphysiological systems.

Supraspinal glycinergic neurotransmission in pain: A scoping review of current literature

The neurotransmitter glycine is an agonist at the strychnine-sensitive glycine receptors. In addition, it has recently been discovered to act at two new receptors, the excitatory glycine receptor and metabotropic glycine receptor. Glycine's neurotransmitter roles have been most extensively investigated in the spinal cord, where it is known to play essential roles in pain, itch, and motor function. In contrast, less is known about supraspinal glycinergic functions, and their contributions to pain circuits are largely unrecognized. As glycinergic neurons are absent from cortical regions, a clearer understanding of how supraspinal glycine modulates pain could reveal new pharmacological targets. This review aims to synthesize the published research on glycine's role in the adult brain, highlighting regions where glycine signaling may modulate pain responses. This was achieved through a scoping review methodology identifying several key regions of supraspinal pain circuitry where glycine signaling is involved. Therefore, this review unveils critical research gaps for supraspinal glycine's potential roles in pain and pain-associated responses, encouraging researchers to consider glycinergic neurotransmission more widely when investigating neural mechanisms of pain.

Impact of chemogenetic activation of dorsal vagal complex astrocytes in mice on adaptive glucoregulatory responses

The dorsal vagal complex (DVC) regulates diverse aspects of physiology including food intake and blood glucose homeostasis. Astrocytes play an active role in regulating DVC function and, by extension, physiological parameters. DVC astrocytes in ex vivo slices respond to low tissue glucose. The response of neurons to low glucose is conditional on intact astrocyte signalling in slice preparations, suggesting astrocytes are primary sensors of glucose deprivation (glucoprivation). Based on these published findings we hypothesised that in vivo DVC astrocyte manipulation with chemogenetics would be sufficient to alter physiological responses that control blood glucose. We found that 2-h after systemic 2-DG-induced glucoprivation there were no observable changes in morphology of glial fibrillary acidic protein (GFAP)-immunoreactive DVC cells, specifically those in the nucleus of the solitary tract (NTS). Chemogenetic activation of DVC astrocytes was sufficient to suppress nocturnal food intake by reducing both meal size and meal number and this manipulation also suppressed 2-DG-induced glucoprivic food intake. Chemogenetic activation of DVC astrocytes did not increase basal blood glucose nor protect against insulin-induced hypoglycaemia. In male mice, chemogenetic DVC astrocyte activation did not alter glucose tolerance. In female mice, the initial glucose excursion was reduced in a glucose tolerance test, suggesting enhanced glucose absorption. Based on our data and published work, we propose that DVC astrocytes may play an indispensable homeostatic role, that is, are necessary to maintain the function of glucoregulatory neuronal circuitry, but alone their bulk activation is not sufficient to result in adaptive glucoregulatory responses. It is possible that there are state-dependent effects and/or DVC astrocyte subsets that have this specialised role, but this was unresolvable using the experimental approaches employed here.

The integrated brain network that controls respiration

Respiration is a brain function on which our lives essentially depend. Control of respiration ensures that the frequency and depth of breathing adapt continuously to metabolic needs. In addition, the respiratory control network of the brain has to organize muscular synergies that integrate ventilation with posture and body movement. Finally, respiration is coupled to cardiovascular function and emotion. Here, we argue that the brain can handle this all by integrating a brainstem central pattern generator circuit in a larger network that also comprises the cerebellum. Although currently not generally recognized as a respiratory control center, the cerebellum is well known for its coordinating and modulating role in motor behavior, as well as for its role in the autonomic nervous system. In this review, we discuss the role of brain regions involved in the control of respiration, and their anatomical and functional interactions. We discuss how sensory feedback can result in adaptation of respiration, and how these mechanisms can be compromised by various neurological and psychological disorders. Finally, we demonstrate how the respiratory pattern generators are part of a larger and integrated network of respiratory brain regions.

Pro-Opiomelanocortin (POMC) neurons in the nucleus of the solitary tract mediate endorphinergic endogenous analgesia in mice

ABSTRACT

The nucleus of the solitary tract (NTS) contains pro-opiomelanocortin (POMC) neurons which are one of the two major sources of β-endorphin in the brain. The functional role of these NTS POMC neurons in nociceptive and cardiorespiratory function is debated. We have shown that NTS POMC optogenetic activation produces bradycardia and transient apnoea in a working heart brainstem preparation and chemogenetic activation with an engineered ion channel (PSAM) produced opioidergic analgesia in vivo . To better define the role of the NTS POMC neurons in behaving animals, we adopted in vivo optogenetics (ChrimsonR) and excitatory/inhibitory chemogenetic DREADD (hM3Dq/hM4Di) strategies in POMC-Cre mice. We show that optogenetic activation of NTS POMC neurons produces time-locked, graded, transient bradycardia and bradypnoea in anaesthetised mice which is naloxone sensitive (1 mg/kg, i.p) suggesting a role of β-endorphin. Both optogenetic and chemogenetic activation of NTS POMC neurons produces sustained thermal analgesia in behaving mice which can be blocked by naloxone. It also produced analgesia in inflammatory pain (carrageenan) but not in a neuropathic pain model (tibial nerve transection). Inhibiting NTS POMC neurons does not produce any effect on basal nociception but inhibits stress-induced analgesia (unlike inhibition of arcuate POMC neurons). Activation of NTS POMC neuronal populations in conscious mice did not cause respiratory depression, anxiety or locomotor deficit (in open field) nor affective preference. These findings indicate that NTS POMC neurons play a key role in the generation of endorphinergic endogenous analgesia and can also regulate cardiorespiratory function.

Functional anatomy of the vagus system: How does the polyvagal theory comply?

Due to its pivotal role in autonomic networks and interoception, the vagus attracts continued interest from both basic scientists and therapists of various clinical disciplines. In particular, the widespread use of heart rate variability as an index of autonomic cardiac control and a proposed central role of the vagus in biopsychological concepts, e.g., the polyvagal theory, provide a good opportunity to recall basic features of vagal anatomy. In addition to the "classical" vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents.

Nucleus Tractus Solitarius Neurons Activated by Hypercapnia and Hypoxia Lack Mu Opioid Receptor Expression

Impaired chemoreflex responses are a central feature of opioid-induced respiratory depression, however, the mechanism through which mu opioid receptor agonists lead to diminished chemoreflexes is not fully understood. One brainstem structure involved in opioid-induced impairment of chemoreflexes is the nucleus of the solitary tract (NTS), which contains a population of neurons that express mu opioid receptors. Here, we tested whether caudal NTS neurons activated during the chemoreflex challenge express mu opioid receptors and overlap with neurons activated by opioids. Using genetic labeling of mu opioid receptor-expressing neurons and cFos immunohistochemistry as a proxy for neuronal activation, we examined the distribution of activated NTS neurons following hypercapnia, hypoxia, and morphine administration. The main finding was that hypoxia and hypercapnia primarily activated NTS neurons that did not express mu opioid receptors. Furthermore, concurrent administration of morphine with hypercapnia induced cFos expression in non-overlapping populations of neurons. Together these results suggest an indirect effect of opioids within the NTS, which could be mediated through mu opioid receptors on afferents and/or inhibitory interneurons.

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.

Involvement of the Dorsal Vagal Complex in Alcohol-Related Behaviors

The neurobiological mechanisms that regulate the development and maintenance of alcohol use disorder (AUD) are complex and involve a wide variety of within and between systems neuroadaptations. While classic reward, preoccupation, and withdrawal neurocircuits have been heavily studied in terms of AUD, viable treatment targets from this established literature have not proven clinically effective as of yet. Therefore, examination of additional neurocircuitries not classically studied in the context of AUD may provide novel therapeutic targets. Recent studies demonstrate that various neuropeptides systems are important modulators of alcohol reward, seeking, and intake behaviors. This includes neurocircuitry within the dorsal vagal complex (DVC), which is involved in the control of the autonomic nervous system, control of intake of natural rewards like food, and acts as a relay of interoceptive sensory information via interactions of numerous gut-brain peptides and neurotransmitter systems with DVC projections to central and peripheral targets. DVC neuron subtypes produce a variety of neuropeptides and transmitters and project to target brain regions critical for reward such as the mesolimbic dopamine system as well as other limbic areas important for the negative reinforcing and aversive properties of alcohol withdrawal such as the extended amygdala. This suggests the DVC may play a role in the modulation of various aspects of AUD. This review summarizes the current literature on neurotransmitters and neuropeptides systems in the DVC (e.g., norepinephrine, glucagon-like peptide 1, neurotensin, cholecystokinin, thyrotropin-releasing hormone), and their potential relevance to alcohol-related behaviors in humans and rodent models for AUD research. A better understanding of the role of the DVC in modulating alcohol related behaviors may lead to the elucidation of novel therapeutic targets for drug development in AUD.

Asprosin, a novel therapeutic candidate for painful neuropathy: an experimental study in mice

Recent studies indicate presence of a strong link between adipokines and neuropathic pain. However, the effects of asprosin, a novel adipokine, on neuropathic pain have not been studied in animal models.Mouse models were employed to investigate the antinociceptive effectiveness of asprosin in the treatment of three types of neuropathic pain, with metabolic (streptozocin/STZ), toxic (oxaliplatin/OXA), and traumatic (sciatic nerve ligation/CCI [chronic constriction nerve injury]) etiologies, respectively. Changes in nociceptive behaviors were assessed relative to controls using thermal (the hot plate and cold plate tests, at 50 °C and 4 °C respectively) and mechanical pain (von Frey test) tests after intraperitoneal (i.p.) administration of asprosin (10 µg/kg) and gabapentin (50 mg/kg) in several times intervals. Besides, possible effect of asprosin on the motor coordination of mice was assessed with a rotarod test. Serum level of asprosin was quantified by ELISA.In neuropathic pain models (STZ, OXA, and CCI), asprosin administration significantly reduced both mechanical and thermal hypersensitivity, indicating that it exhibits a clear-cut antihypersensitivity effect in the analyzed neuropathic pain models. The most effective time of asprosin on pain threshold was observed 60 min after its injection. Also, asprosin displayed no notable effect on the motor activity. Asprosin levels were significantly lower in neuropathic pain compared to healthy group (p < 0.05).The results yielded by the present study suggest that asprosin exhibits an analgesic effect in the neuropathic pain models and may have clinical utility in alleviating chronic pain associated with disease and injury originating from peripheral structures.

Contribution of the caudal medullary raphe to opioid induced respiratory depression

BACKGROUND

Opioid-induced respiratory depression can be partially antagonized in the preBötzinger Complex and Parabrachial Nucleus/Kölliker-Fuse Complex. We hypothesized that additional opioid antagonism in the caudal medullary raphe completely reverses the opioid effect.

METHODS

In adult ventilated, vagotomized, decerebrate rabbits, we administrated remifentanil intravenously at "analgesic", "apneic", and "very high" doses and determined the reversal with sequential naloxone microinjections into the bilateral Parabrachial Nucleus/Kölliker-Fuse Complex, preBötzinger Complex, and caudal medullary raphe. In separate animals, we injected opioid antagonists into the raphe without intravenous remifentanil.

RESULTS

Sequential naloxone microinjections completely reversed respiratory rate depression from "analgesic" and "apneic" remifentanil, but not "very high" remifentanil concentrations. Antagonist injection into the caudal medullary raphe without remifentanil independently increased respiratory rate.

CONCLUSIONS

Opioid-induced respiratory depression results from a combined effect on the respiratory rhythm generator and respiratory drive. The effect in the caudal medullary raphe is complex as we also observed local antagonism of endogenous opioid receptor activation, which has not been described before.

Chemogenetics: drug-controlled gene therapies for neural circuit disorders

Many patients with nervous system disorders have considerable unmet clinical needs or suffer debilitating drug side effects. A major limitation of exiting treatment approaches is that traditional small molecule pharmacotherapy lacks sufficient specificity to effectively treat many neurological diseases. Chemogenetics is a new gene therapy technology that targets an engineered receptor to cell types involved in nervous system dysfunction, enabling highly selective drug-controlled neuromodulation. Here, we discuss chemogenetic platforms and considerations for their potential application as human nervous system therapies.

POMC neuronal heterogeneity in energy balance and beyond: an integrated view

Hypothalamic AgRP and POMC neurons are conventionally viewed as the yin and yang of the body's energy status, since they act in an opposite manner to modulate appetite and systemic energy metabolism. However, although AgRP neurons' functions are comparatively well understood, a unifying theory of how POMC neuronal cells operate has remained elusive, probably due to their high level of heterogeneity, which suggests that their physiological roles might be more complex than initially thought. In this Perspective, we propose a conceptual framework that integrates POMC neuronal heterogeneity with appetite regulation, whole-body metabolic physiology and the development of obesity. We highlight emerging evidence indicating that POMC neurons respond to distinct combinations of interoceptive signals and food-related cues to fine-tune divergent metabolic pathways and behaviours necessary for survival. The new framework we propose reflects the high degree of developmental plasticity of this neuronal population and may enable progress towards understanding of both the aetiology and treatment of metabolic disorders.

Astrocytes in the nucleus of the solitary tract: Contributions to neural circuits controlling physiology

The nucleus of the solitary tract (NTS) is the primary brainstem centre for the integration of physiological information from the periphery transmitted via the vagus nerve. In turn, the NTS feeds into downstream circuits regulating physiological parameters. Astrocytes are glial cells which have key roles in maintaining CNS tissue homeostasis and regulating neuronal communication. Recently an increasing number of studies have implicated astrocytes in the regulation of synaptic transmission and physiology. This review aims to highlight evidence for a role for astrocytes in the functions of the NTS. Astrocytes maintain and modulate NTS synaptic transmission contributing to the control of diverse physiological systems namely cardiovascular, respiratory, glucoregulatory, and gastrointestinal. In addition, it appears these cells may have a role in central control of feeding behaviour. As such these cells are a key component of signal processing and physiological control by the NTS.

Neurochemistry of the Kölliker-Fuse nucleus from a respiratory perspective

The Kölliker-Fuse nucleus (KF) is a functionally distinct component of the parabrachial complex, located in the dorsolateral pons of mammals. The KF has a major role in respiration and upper airway control. A comprehensive understanding of the KF and its contributions to respiratory function and dysfunction requires an appreciation for its neurochemical characteristics. The goal of this review is to summarize the diverse neurochemical composition of the KF, focusing on the neurotransmitters, neuromodulators, and neuropeptides present. We also include a description of the receptors expressed on KF neurons and transporters involved in each system, as well as their putative roles in respiratory physiology. Finally, we provide a short section reviewing the literature regarding neurochemical changes in the KF in the context of respiratory dysfunction observed in SIDS and Rett syndrome. By over-viewing the current literature on the neurochemical composition of the KF, this review will serve to aid a wide range of topics in the future research into the neural control of respiration in health and disease.

Neurochemical Characterization of Brainstem Pro-Opiomelanocortin Cells

Genetic research has revealed pro-opiomelanocortin (POMC) to be a fundamental regulator of energy balance and body weight in mammals. Within the brain, POMC is primarily expressed in the arcuate nucleus of the hypothalamus (ARC), while a smaller population exists in the brainstem nucleus of the solitary tract (POMCNTS). We performed a neurochemical characterization of this understudied population of POMC cells using transgenic mice expressing green fluorescent protein (eGFP) under the control of a POMC promoter/enhancer (PomceGFP). Expression of endogenous Pomc mRNA in the nucleus of the solitary tract (NTS) PomceGFP cells was confirmed using fluorescence-activating cell sorting (FACS) followed by quantitative PCR. In situ hybridization histochemistry of endogenous Pomc mRNA and immunohistochemical analysis of eGFP revealed that POMC is primarily localized within the caudal NTS. Neurochemical analysis indicated that POMCNTS is not co-expressed with tyrosine hydroxylase (TH), glucagon-like peptide 1 (GLP-1), cholecystokinin (CCK), brain-derived neurotrophic factor (BDNF), nesfatin, nitric oxide synthase 1 (nNOS), seipin, or choline acetyltransferase (ChAT) cells, whereas 100% of POMCNTS is co-expressed with transcription factor paired-like homeobox2b (Phox2b). We observed that 20% of POMCNTS cells express receptors for adipocyte hormone leptin (LepRbs) using a PomceGFP:LepRbCre:tdTOM double-reporter line. Elevations in endogenous or exogenous leptin levels increased the in vivo activity (c-FOS) of a small subset of POMCNTS cells. Using ex vivo slice electrophysiology, we observed that this effect of leptin on POMCNTS cell activity is postsynaptic. These findings reveal that a subset of POMCNTS cells are responsive to both changes in energy status and the adipocyte hormone leptin, findings of relevance to the neurobiology of obesity.

Regulation of food intake by astrocytes in the brainstem dorsal vagal complex

A role for glial cells in brain circuits controlling feeding has begun to be identified with hypothalamic astrocyte signaling implicated in regulating energy homeostasis. The nucleus of the solitary tract (NTS), within the brainstem dorsal vagal complex (DVC), integrates vagal afferent information from the viscera and plays a role in regulating food intake. We hypothesized that astrocytes in this nucleus respond to, and influence, food intake. Mice fed high‐fat chow for 12 hr during the dark phase showed NTS astrocyte activation, reflected in an increase in the number (65%) and morphological complexity of glial‐fibrillary acidic protein (GFAP)‐immunoreactive cells adjacent to the area postrema (AP), compared to control chow fed mice. To measure the impact of astrocyte activation on food intake, we delivered designer receptors exclusively activated by designer drugs (DREADDs) to DVC astrocytes (encompassing NTS, AP, and dorsal motor nucleus of the vagus) using an adeno‐associated viral (AAV) vector (AAV‐GFAP‐hM3Dq_mCherry). Chemogenetic activation with clozapine‐N‐oxide (0.3 mg/kg) produced in greater morphological complexity in astrocytes and reduced dark‐phase feeding by 84% at 4 hr postinjection compared with vehicle treatment. hM3Dq‐activation of DVC astrocytes also reduced refeeding after an overnight fast (71% lower, 4 hr postinjection) when compared to AAV‐GFAP‐mCherry expressing control mice. DREADD‐mediated astrocyte activation did not impact locomotion. hM3Dq activation of DVC astrocytes induced c‐FOS in neighboring neuronal feeding circuits (including in the parabrachial nucleus). This indicates that NTS astrocytes respond to acute nutritional excess, are involved in the integration of peripheral satiety signals, and can reduce food intake when activated.

Presynaptic Mechanisms and KCNQ Potassium Channels Modulate Opioid Depression of Respiratory Drive

Opioid-induced respiratory depression (OIRD) is the major cause of death associated with opioid analgesics and drugs of abuse, but the underlying cellular and molecular mechanisms remain poorly understood. We investigated opioid action in vivo in unanesthetized mice and in in vitro medullary slices containing the preBötzinger Complex (preBötC), a locus critical for breathing and inspiratory rhythm generation. Although hypothesized as a primary mechanism, we found that mu-opioid receptor (MOR1)-mediated GIRK activation contributed only modestly to OIRD. Instead, mEPSC recordings from genetically identified Dbx1-derived interneurons, essential for rhythmogenesis, revealed a prevalent presynaptic mode of action for OIRD. Consistent with MOR1-mediated suppression of presynaptic release as a major component of OIRD, Cacna1a KO slices lacking P/Q-type Ca2+ channels enhanced OIRD. Furthermore, OIRD was mimicked and reversed by KCNQ potassium channel activators and blockers, respectively. In vivo whole-body plethysmography combined with systemic delivery of GIRK- and KCNQ-specific potassium channel drugs largely recapitulated these in vitro results, and revealed state-dependent modulation of OIRD. We propose that respiratory failure from OIRD results from a general reduction of synaptic efficacy, leading to a state-dependent collapse of rhythmic network activity.

Hedonic drinking engages a supraspinal inhibition of thermal nociception in adult rats

The taste of sucrose is commonly used to provide pain relief in newborn humans and is innately analgesic to neonatal rodents. In adulthood, sucrose remains a strong motivator to feed, even in potentially hazardous circumstances (ie, threat of tissue damage). However, the neurobiological mechanisms of this endogenous reward-pain interaction are unclear. We have developed a simple model of sucrose drinking-induced analgesia in Sprague-Dawley rats (6-10 weeks old) and have undertaken a behavioral and pharmacological characterization using the Hargreaves' test of hind-paw thermal sensitivity. Our results reveal an acute, potent, and robust inhibitory effect of sucrose drinking on thermal nociceptive behaviour that unlike the phenomenon in neonates is independent of endogenous opioid signalling and does not seem to operate through classical descending inhibition of the spinal cord circuitry. Experience of sucrose drinking had a conditioning effect whereby the apparent expectancy of sucrose enabled water alone (in euvolemic animals) to elicit a short-lasting placebo-like analgesia. Sweet taste alone, however, was insufficient to elicit analgesia in adult rats intraorally perfused with sucrose. Instead, the sucrose analgesia phenomenon only appeared after conditioning by oral perfusion in chronically cannulated animals. This sucrose analgesia was completely prevented by systemic dosing of the endocannabinoid CB1 receptor antagonist rimonabant. These results indicate the presence of an endogenous supraspinal analgesic circuit that is recruited by the context of rewarding drinking and is dependent on endocannabinoid signalling. We propose that this hedonic sucrose-drinking model may be useful for further investigation of the supraspinal control of pain by appetite and reward.

POMC: The Physiological Power of Hormone Processing

Pro-opiomelanocortin (POMC) is the archetypal polypeptide precursor of hormones and neuropeptides. In this review, we examine the variability in the individual peptides produced in different tissues and the impact of the simultaneous presence of their precursors or fragments. We also discuss the problems inherent in accurately measuring which of the precursors and their derived peptides are present in biological samples. We address how not being able to measure all the combinations of precursors and fragments quantitatively has affected our understanding of the pathophysiology associated with POMC processing. To understand how different ratios of peptides arise, we describe the role of the pro-hormone convertases (PCs) and their tissue specificities and consider the cellular processing pathways which enable regulated secretion of different peptides that play crucial roles in integrating a range of vital physiological functions. In the pituitary, correct processing of POMC peptides is essential to maintain the hypothalamic-pituitary-adrenal axis, and this processing can be disrupted in POMC-expressing tumors. In hypothalamic neurons expressing POMC, abnormalities in processing critically impact on the regulation of appetite, energy homeostasis, and body composition. More work is needed to understand whether expression of the POMC gene in a tissue equates to release of bioactive peptides. We suggest that this comprehensive view of POMC processing, with a focus on gaining a better understanding of the combination of peptides produced and their relative bioactivity, is a necessity for all involved in studying this fascinating physiological regulatory phenomenon.

Chemogenetic Tools for Causal Cellular and Neuronal Biology

Chemogenetic technologies enable selective pharmacological control of specific cell populations. An increasing number of approaches have been developed that modulate different signaling pathways. Selective pharmacological control over G protein-coupled receptor signaling, ion channel conductances, protein association, protein stability, and small molecule targeting allows modulation of cellular processes in distinct cell types. Here, we review these chemogenetic technologies and instances of their applications in complex tissues in vivo and ex vivo.

Various transgenic mouse lines to study proopiomelanocortin cells in the brain stem label disparate populations of GABAergic and glutamatergic neurons

Products of the proopiomelanocortin (POMC) prohormone regulate aspects of analgesia, reward, and energy balance; thus, the neurons that produce POMC in the hypothalamus have received considerable attention. However, there are also cells in the nucleus of the solitary tract (NTS) that transcribe Pomc, although low levels of Pomc mRNA and relative lack of POMC peptide products in the adult mouse NTS have hindered the study of these cells. Therefore, studies of NTS POMC cells have largely relied on transgenic mouse lines. Here, we set out to determine the amino acid (AA) transmitter phenotype of NTS POMC neurons by using Pomc-Gfp transgenic mice to identify POMC cells. We found that cells expressing the green fluorescent protein (GFP) represent a mix of GABAergic and glutamatergic cells as indicated by Gad2 and vesicular Glut2 ( vGlut2) mRNA expression, respectively. We then examined the AA phenotype of POMC cells labeled by a Pomc-Cre transgene and found that these are also a mix of GABAergic and glutamatergic cells. However, the NTS cells labeled by the Gfp- and Cre-containing transgenes represented distinct populations of cells in three different Pomc-Cre mouse lines. Consistent with previous work, we were unable to reliably detect Pomc mRNA in the NTS despite clear expression in the hypothalamus. Thus, it was not possible to determine which transgenic tool most accurately identifies NTS cells that may express Pomc or release POMC peptides, although the results indicate the transgenic tools for study of these NTS neurons can label disparate populations of cells with varied AA phenotypes.

Progress in understanding mechanisms of opioid-induced gastrointestinal adverse effects and respiratory depression

Opioids evoke analgesia through activation of opioid receptors (predominantly the μ opioid receptor) in the central nervous system. Opioid receptors are abundant in multiple regions of the central nervous system and the peripheral nervous system including enteric neurons. Opioid-related adverse effects such as constipation, nausea, and vomiting pose challenges for compliance and continuation of the therapy for chronic pain management. In the post-operative setting opioid-induced depression of respiration can be fatal. These critical limitations warrant a better understanding of their underpinning cellular and molecular mechanisms to inform the design of novel opioid analgesic molecules that are devoid of these unwanted side-effects. Research efforts on opioid receptor signalling in the past decade suggest that differential signalling pathways and downstream molecules preferentially mediate distinct pharmacological effects. Additionally, interaction among opioid receptors and, between opioid receptor and non-opioid receptors to form signalling complexes shows that opioid-induced receptor signalling is potentially more complicated than previously thought. This complexity provides an opportunity to identify and probe relationships between selective signalling pathway specificity and in vivo production of opioid-related adverse effects. In this review, we focus on current knowledge of the mechanisms thought to transduce opioid-induced gastrointestinal adverse effects (constipation, nausea, vomiting) and respiratory depression.

δ-cells and β-cells are electrically coupled and regulate α-cell activity via somatostatin

Key points

We used a mouse expressing a light‐sensitive ion channel in β‐cells to understand how α‐cell activity is regulated by β‐cells.

Light activation of β‐cells triggered a suppression of α‐cell activity via gap junction‐dependent activation of δ‐cells.

Mathematical modelling of human islets suggests that 23% of the inhibitory effect of glucose on glucagon secretion is mediated by β‐cells via gap junction‐dependent activation of δ‐cells/somatostatin secretion.

Abstract

Glucagon, the body's principal hyperglycaemic hormone, is released from α‐cells of the pancreatic islet. Secretion of this hormone is dysregulated in type 2 diabetes mellitus but the mechanisms controlling secretion are not well understood. Regulation of glucagon secretion by factors secreted by neighbouring β‐ and δ‐cells (paracrine regulation) have been proposed to be important. In this study, we explored the importance of paracrine regulation by using an optogenetic strategy. Specific light‐induced activation of β‐cells in mouse islets expressing the light‐gated channelrhodopsin‐2 resulted in stimulation of electrical activity in δ‐cells but suppression of α‐cell activity. Activation of the δ‐cells was rapid and sensitive to the gap junction inhibitor carbenoxolone, whereas the effect on electrical activity in α‐cells was blocked by CYN 154806, an antagonist of the somatostatin‐2 receptor. These observations indicate that optogenetic activation of the β‐cells propagates to the δ‐cells via gap junctions, and the consequential stimulation of somatostatin secretion inhibits α‐cell electrical activity by a paracrine mechanism. To explore whether this pathway is important for regulating α‐cell activity and glucagon secretion in human islets, we constructed computational models of human islets. These models had detailed architectures based on human islets and consisted of a collection of >500 α‐, β‐ and δ‐cells. Simulations of these models revealed that this gap junctional/paracrine mechanism accounts for up to 23% of the suppression of glucagon secretion by high glucose.

We used a mouse expressing a light‐sensitive ion channel in β‐cells to understand how α‐cell activity is regulated by β‐cells.

Light activation of β‐cells triggered a suppression of α‐cell activity via gap junction‐dependent activation of δ‐cells.

Mathematical modelling of human islets suggests that 23% of the inhibitory effect of glucose on glucagon secretion is mediated by β‐cells via gap junction‐dependent activation of δ‐cells/somatostatin secretion.

Deep Brain Stimulation Improves the Symptoms and Sensory Signs of Persistent Central Neuropathic Pain from Spinal Cord Injury: A Case Report

Central neuropathic pain (CNP) is a significant problem after spinal cord injury (SCI). Pharmacological and non-pharmacological approaches may reduce the severity, but relief is rarely substantial. While deep brain stimulation (DBS) has been used to treat various chronic pain types, the technique has rarely been used to attenuate CNP after SCI. Here we present the case of a 54-year-old female with incomplete paraplegia who had severe CNP in the lower limbs and buttock areas since her injury 30 years prior. She was treated with bilateral DBS of the midbrain periaqueductal gray (PAG). The effects of this stimulation on CNP characteristics, severity and pain-related sensory function were evaluated using the International SCI Pain Basic Data Set (ISCIPBDS), Neuropathic Pain Symptom Inventory (NPSI), Multidimensional Pain Inventory and Quantitative Sensory Testing before and periodically after initiation of DBS. After starting DBS treatment, weekly CNP severity ratings rapidly decreased from severe to minimal, paralleled by a substantial reduction in size of the painful area, reduced pain impact and reversal of pain-related neurological abnormalities, i.e., dynamic-mechanical and cold allodynia. She discontinued pain medication on study week 24. The improvement has been consistent. The present study expands on previous findings by providing in-depth assessments of symptoms and signs associated with CNP. The results of this study suggest that activation of endogenous pain inhibitory systems linked to the PAG can eliminate CNP in some people with SCI. More research is needed to better-select appropriate candidates for this type of therapy. We discuss the implications of these findings for understanding the brainstem's control of chronic pain and for future progress in using analgesic DBS in the central gray.