Thermally active TRPV1 tonically drives central spontaneous glutamate release

Effects of nasal allergens and environmental particulate matter on brainstem metabolites and the consequence of brain-spleen axis in allergic rhinitis

BACKGROUND

The growing consensus links exposure to fine particulate matter (PM2.5) with an increased risk of respiratory diseases. However, little is known about the additional effects of particulate matter on brainstem function in allergic rhinitis (AR). Furthermore, it is unknown to what extent the PM2.5-induced effects in the brainstem affect the inflammatory response in AR. This study aimed to determine the effects, mechanisms and consequences of brainstem neural activity altered by allergenic stimulation and PM2.5 exposure.

METHODS

Using an AR model of ovalbumin (OVA) elicitation and whole-body PM2.5 exposure, the metabolic profile of the brainstem post-allergen stimulation was characterized through in vivo proton magnetic resonance imaging (1H-MRS). Then, the transient receptor potential vanilloid-1 (TRPV1) neuronal expression and sensitivity in the trigeminal nerve in AR were investigated. The link between TRPV1 expression and brainstem differential metabolites was also determined. Finally, we evaluated the mediating effects of brainstem metabolites and the consequences in the brain-spleen axis in the inflammatory response of AR.

RESULTS

Exposure to allergens and PM2.5 led to changes in the metabolic profiles of the brainstem, particularly affecting levels of glutamine (Gln) and glutamate (Glu). This exposure also increased the expression and sensitivity of TRPV1+ neurons in the trigeminal nerve, with the levels of TRPV1 expression closely linked to the brainstem metabolism of Glu and Gln. Moreover, allergens increased the activity of p38, while PM2.5 led to the phosphorylation of p38 and ERK, resulting in the upregulation of TRPV1 expression. The brainstem metabolites Glu and Gln were found to partially mediate the impact of TRPV1 on AR inflammation, which was supported by the presence of pro-inflammatory changes in the brain-spleen axis.

CONCLUSION

Brainstem metabolites are altered under allergen stimulation and additional PM2.5 exposure in AR via sensitization of the trigeminal nerve, which exacerbates the inflammatory response via the brain-splenic axis.

Principles of synaptic encoding of brainstem circadian rhythms

Circadian regulation of autonomic tone and reflex pathways pairs physiological processes with the daily light cycle. However, the underlying mechanisms mediating these changes on autonomic neurocircuitry are only beginning to be understood. The brainstem nucleus of the solitary tract (NTS) and adjacent nuclei, including the area postrema and dorsal motor nucleus of the vagus, are key candidates for rhythmic control of some aspects of the autonomic nervous system. Recent findings have contributed to a working model of circadian regulation in the brainstem which manifests from the transcriptional, to synaptic, to circuit levels of organization. Vagal afferent neurons and the NTS possess rhythmic clock gene expression, rhythmic action potential firing, and our recent findings demonstrate rhythmic spontaneous glutamate release. In addition, postsynaptic conductances also vary across the day producing subtle changes in membrane depolarization which govern synaptic efficacy. Together these coordinated pre- and postsynaptic changes provide nuanced control of synaptic transmission across the day to tune the sensitivity of primary afferent input and likely govern reflex output. Further, given the important role for the brainstem in integrating cues such as feeding, cardiovascular function and temperature, it may also be an underappreciated locus in mediating the effects of such non-photic entraining cues. This short review focuses on the neurophysiological principles that govern NTS synaptic transmission and how circadian rhythms impacted them across the day.

TRPV1 enhances cholecystokinin signaling in primary vagal afferent neurons and mediates the central effects on spontaneous glutamate release in the NTS

The gut peptide cholecystokinin (CCK) is released during feeding and promotes satiation by increasing excitation of vagal afferent neurons that innervate the upper gastrointestinal tract. Vagal afferent neurons express CCK1 receptors (CCK1Rs) in the periphery and at central terminals in the nucleus of the solitary tract (NTS). While the effects of CCK have been studied for decades, CCK receptor signaling and coupling to membrane ion channels are not entirely understood. Previous findings have implicated L-type voltage-gated calcium channels as well as transient receptor potential (TRP) channels in mediating the effects of CCK, but the lack of selective pharmacology has made determining the contributions of these putative mediators difficult. The nonselective ion channel transient receptor potential vanilloid subtype 1 (TRPV1) is expressed throughout vagal afferent neurons and controls many forms of signaling, including spontaneous glutamate release onto NTS neurons. Here we tested the hypothesis that CCK1Rs couple directly to TRPV1 to mediate vagal signaling using fluorescent calcium imaging and brainstem electrophysiology. We found that CCK signaling at high concentrations (low-affinity binding) was potentiated in TRPV1-containing afferents and that TRPV1 itself mediated the enhanced CCK1R signaling. While competitive antagonism of TRPV1 failed to alter CCK1R signaling, TRPV1 pore blockade or genetic deletion (TRPV1 KO) significantly reduced the CCK response in cultured vagal afferents and eliminated its ability to increase spontaneous glutamate release in the NTS. Together, these results establish that TRPV1 mediates the low-affinity effects of CCK on vagal afferent activation and control of synaptic transmission in the brainstem.NEW & NOTEWORTHY Cholecystokinin (CCK) signaling via the vagus nerve reduces food intake and produces satiation, yet the signaling cascades mediating these effects remain unknown. Here we report that the capsaicin receptor transient receptor potential vanilloid subtype 1 (TRPV1) potentiates CCK signaling in the vagus and mediates the ability of CCK to control excitatory synaptic transmission in the nucleus of the solitary tract. These results may prove useful in the future development of CCK/TRPV1-based therapeutic interventions.

Transient receptor potential channels as an emerging therapeutic target for oropharyngeal dysphagia

Oropharyngeal dysphagia is a serious health concern in older adults and patients with neurological disorders. Current oropharyngeal dysphagia management largely relies on compensatory strategies with limited efficacy. A long-term goal in swallowing/dysphagia-related research is the identification of pharmacological treatment strategies for oropharyngeal dysphagia. In recent decades, several pre-clinical and clinical studies have investigated the use of transient receptor potential (TRP) channels as a therapeutic target to facilitate swallowing. Various TRP channels are present in regions involved in the swallowing process. Animal studies have shown that local activation of these channels by their pharmacological agonists initiates swallowing reflexes; the number of reflexes increases when the dose of the agonist reaches a particular level. Clinical studies, including randomized clinical trials involving patients with oropharyngeal dysphagia, have demonstrated improved swallowing efficacy, safety, and physiology when TRP agonists are mixed with the food bolus. Additionally, there is evidence of plasticity development in swallowing-related neuronal networks in the brain upon TRP channel activation in peripheral swallowing-related regions. Thus, TRP channels have emerged as a promising target for the development of pharmacological treatments for oropharyngeal dysphagia.

Circadian regulation of glutamate release pathways shapes synaptic throughput in the brainstem nucleus of the solitary tract (NTS)

Circadian regulation of autonomic reflex pathways pairs physiological function with the daily light cycle. The brainstem nucleus of the solitary tract (NTS) is a key candidate for rhythmic control of the autonomic nervous system. Here we investigated circadian regulation of NTS neurotransmission and synaptic throughput using patch-clamp electrophysiology in brainstem slices from mice. We found that spontaneous quantal glutamate release onto NTS neurons showed strong circadian rhythmicity, with the highest rate of release during the light phase and the lowest in the dark, that were sufficient to drive day/night differences in constitutive postsynaptic action potential firing. In contrast, afferent evoked action potential throughput was enhanced during the dark and diminished in the light. Afferent-driven synchronous release pathways showed a similar decrease in release probability that did not explain the enhanced synaptic throughput during the night. However, analysis of postsynaptic membrane properties revealed diurnal changes in conductance, which, when coupled with the circadian changes in glutamate release pathways, tuned synaptic throughput between the light and dark phases. These coordinated pre-/postsynaptic changes encode nuanced control over synaptic performance and pair NTS action potential firing and vagal throughput with time of day. KEY POINTS: Vagal afferent neurons relay information from peripheral organs to the brainstem nucleus of the solitary tract (NTS) to initiate autonomic reflex pathways as well as providing important controls of food intake, digestive function and energy balance. Vagally mediated reflexes and behaviours are under strong circadian regulation. Diurnal fluctuations in presynaptic vesicle release pathways and postsynaptic membrane conductances provide nuanced control over NTS action potential firing and vagal synaptic throughput. Coordinated pre-/postsynaptic changes represent a fundamental mechanism mediating daily changes in vagal afferent signalling and autonomic function.

Beneficial Effects of Capsaicin in Disorders of the Central Nervous System

Capsaicin is a natural compound found in chili peppers and is used in the diet of many countries. The important mechanism of action of capsaicin is its influence on TRPV1 channels in nociceptive sensory neurons. Furthermore, the beneficial effects of capsaicin in cardiovascular and oncological disorders have been described. Many recent publications show the positive effects of capsaicin in animal models of brain disorders. In Alzheimer’s disease, capsaicin reduces neurodegeneration and memory impairment. The beneficial effects of capsaicin in Parkinson’s disease and depression have also been described. It has been found that capsaicin reduces the area of infarction and improves neurological outcomes in animal models of stroke. However, both proepileptic and antiepileptic effects of capsaicin in animal models of epilepsy have been proposed. These contradictory results may be caused by the fact that capsaicin influences not only TRPV1 channels but also different molecular targets such as voltage-gated sodium channels. Human studies show that capsaicin may be helpful in treating stroke complications such as dysphagia. Additionally, this compound exerts pain-relieving effects in migraine and cluster headaches. The purpose of this review is to discuss the mechanisms of the beneficial effects of capsaicin in disorders of the central nervous system.

Cannabinoid and vanilloid pathways mediate opposing forms of synaptic plasticity in corticotropin-releasing hormone neurons

Activity-dependent release of retrograde signaling molecules form micro-feedback loops to regulate synaptic function in neural circuits. Single neurons can release multiple forms of these signaling molecules, including endocannabinoids and endovanilloids, which act via cannabinoid (CB) receptors and transient receptor potential vanilloid 1 (TRPV1) receptors. In hypothalamic corticotrophin-releasing hormone (CRH) neurons, endocannabinoids acting via CB1 receptors have been shown to play an important role in regulating excitability and hence stress hormone secretion. However, the importance of endovanilloid signaling in CRH neurons is currently unclear. Here, we show that, in response to postsynaptic depolarization, CRH neurons release endocannabinoid/endovanilloid molecules that can activate CB1 and TRPV1 receptors. Activation of CB1 receptors suppresses glutamate neurotransmission whereas activation of TRPV1 enhances spontaneous glutamate transmission. However, the excitatory effects of TRPV1 are normally masked by the inhibitory effects of CB1. When the degradation of the endocannabinoid 2-arachidonoylglycerol (2-AG) was inhibited, this revealed tonic activation of CB1 receptors, suggesting tonic endocannabinoid release. However, we found no evidence for tonic activation of TRPV1 receptors under similar conditions. These findings show that activation of CRH neurons can drive the release of signaling molecules that activate parallel endocannabinoid and endovanilloid receptor pathways to mediate opposing forms of synaptic plasticity.

Toxic Peptide From Palythoa caribaeorum Acting on the TRPV1 Channel Prevents Pentylenetetrazol-Induced Epilepsy in Zebrafish Larvae

PcActx peptide, identified from the transcriptome of zoantharian Palythoa caribaeorum, was clustered into the phylogeny of analgesic polypeptides from sea anemone Heteractis crispa (known as APHC peptides). APHC peptides were considered as inhibitors of transient receptor potential cation channel subfamily V member 1 (TRPV1). TRPV1 is a calcium-permeable channel expressed in epileptic brain areas, serving as a potential target for preventing epileptic seizures. Through in silico and in vitro analysis, PcActx peptide was shown to be a potential TRPV1 channel blocker. In vivo studies showed that the linear and oxidized PcActx peptides caused concentration-dependent increases in mortality of zebrafish larvae. However, monotreatment with PcActx peptides below the maximum tolerated doses (MTD) did not affect locomotor behavior. Moreover, PcActx peptides (both linear and oxidized forms) could effectively reverse pentylenetetrazol (PTZ)-induced seizure-related behavior in zebrafish larvae and prevent overexpression of c-fos and npas4a at the mRNA level. The excessive production of ROS induced by PTZ was markedly attenuated by both linear and oxidized PcActx peptides. It was also verified that the oxidized PcActx peptide was more effective than the linear one. In particular, oxidized PcActx peptide notably modulated the mRNA expression of genes involved in calcium signaling and γ-aminobutyric acid (GABA)ergic-glutamatergic signaling, including calb1, calb2, gabra1, grm1, gria1b, grin2b, gat1, slc1a2b, gad1b, and glsa. Taken together, PcActx peptide, as a novel neuroactive peptide, exhibits prominent anti-epileptic activity, probably through modulating calcium signaling and GABAergic-glutamatergic signaling, and is a promising candidate for epilepsy management.

Voltage-gated calcium channels contribute to spontaneous glutamate release directly via nanodomain coupling or indirectly via calmodulin

Neurotransmitter release occurs either synchronously with action potentials (evoked release) or spontaneously (spontaneous release). Whether the molecular mechanisms underlying evoked and spontaneous release are identical, especially whether voltage-gated calcium channels (VGCCs) can trigger spontaneous events, is still a matter of debate in glutamatergic synapses. To elucidate this issue, we characterized the VGCC dependence of miniature excitatory postsynaptic currents (mEPSCs) in various synapses with different coupling distances between VGCCs and synaptic vesicles, known as a critical factor in evoked release. We found that most of the extracellular calcium-dependent mEPSCs were attributable to VGCCs in cultured autaptic hippocampal neurons and the mature calyx of Held where VGCCs and vesicles were tightly coupled. Among loosely coupled synapses, mEPSCs were not VGCC-dependent at immature calyx of Held and CA1 pyramidal neuron synapses, whereas VGCCs contribution was significant at CA3 pyramidal neuron synapses. Interestingly, the contribution of VGCCs to spontaneous glutamate release in CA3 pyramidal neurons was abolished by a calmodulin antagonist, calmidazolium. These data suggest that coupling distance between VGCCs and vesicles determines VGCC dependence of spontaneous release at tightly coupled synapses, yet VGCC contribution can be achieved indirectly at loosely coupled synapses.

Alloxan as a better option than streptozotocin for studies involving painful diabetic neuropathy

Previous data indicate that the diabetogenic substance streptozotocin might act in nociceptive neurons changing the sensory signal, regardless of hyperglycemia. In the present article the effects of streptozotocin were compared with another diabetogenic drug, alloxan, for diabetes induction in rats. A possible direct effect of these drugs was tested by means of in vivo experiments and in vitro assays using cultured primary nociceptive neurons. Streptozotocin (17.5 and 35 mg/kg), alloxan (15 and 30 mg/kg) or vehicle were injected in adult male rats and the animal groups were separated according to glycemic levels. Body mass, glycemia and paw mechanical sensitivity were evaluated for 5 weeks. Streptozotocin caused an increase in mechanical sensitivity in both hyperglycemic and normoglycemic rats, while alloxan induced mechanical sensitization only in hyperglycemic animals. Injection of both substances induced local inflammation at rat paws; however, only streptozotocin caused significant mechanical sensitization when injected near to sensory neurons at the dorsal root ganglia. Also, streptozotocin treatment induced a reduction in intracellular calcium levels and inhibited capsaicin induced calcium transients and membrane depolarization. Alloxan did not affect calcium levels or membrane potential in primary nociceptive neurons. These findings suggest that alloxan might be a better option for animal studies regarding painful diabetic neuropathy as streptozotocin directly affects nociceptive neurons, probably by modulating TRPV1 channel activation.

The Heat Sensing Trpv1 Receptor Is Not a Viable Anticonvulsant Drug Target in the Scn1a +/- Mouse Model of Dravet Syndrome

Cannabidiol has been approved for the treatment of drug-resistant childhood epilepsies including Dravet syndrome (DS). Although the mechanism of anticonvulsant action of cannabidiol is unknown, emerging data suggests involvement of the transient receptor potential cation channel subfamily V member 1 (Trpv1). Pharmacological and genetic studies in conventional seizure models suggest Trpv1 is a novel anticonvulsant target. However, whether targeting Trpv1 is anticonvulsant in animal models of drug-resistant epilepsies is not known. Thus, we examined whether Trpv1 affects the epilepsy phenotype of the F1.Scn1a ^+/- mouse model of DS. We found that cortical Trpv1 mRNA expression was increased in seizure susceptible F1.Scn1a ^+/- mice with a hybrid genetic background compared to seizure resistant 129.Scn1a ^+/- mice isogenic on 129S6/SvEvTac background, suggesting Trpv1 could be a genetic modifier. Previous studies show functional loss of Trpv1 is anticonvulsant. However, Trpv1 selective antagonist SB-705498 did not affect hyperthermia-induced seizure threshold, frequency of spontaneous seizures or survival of F1.Scn1a ^+/- mice. Surprisingly, Trpv1 deletion had both pro- and anti-seizure effects. Trpv1 deletion did not affect hyperthermia-induced seizure temperature thresholds of F1.Scn1a ^+/- ; Trpv1 ^+/- at P14-16 but was proconvulsant at P18 as it reduced seizure temperature thresholds. Conversely, Trpv1 deletion did not alter the frequency of spontaneous seizures but reduced their severity. These results suggest that Trpv1 is a modest genetic modifier of spontaneous seizure severity in the F1.Scn1a ^+/- model of DS. However, the opposing pro- and anti-seizure effects of Trpv1 deletion and the lack of effects of Trpv1 inhibition suggest that Trpv1 is unlikely a viable anticonvulsant drug target in DS.

DMV extrasynaptic NMDA receptors regulate caloric intake in rats

Acute high-fat diet (aHFD) exposure induces a brief period of hyperphagia before caloric balance is restored. Previous studies have demonstrated that this period of regulation is associated with activation of synaptic N-methyl-D-aspartate (NMDA) receptors on dorsal motor nucleus of the vagus (DMV) neurons, which increases vagal control of gastric functions. Our aim was to test the hypothesis that activation of DMV synaptic NMDA receptors occurs subsequent to activation of extrasynaptic NMDA receptors. Sprague-Dawley rats were fed a control or high-fat diet for 3-5 days prior to experimentation. Whole-cell patch-clamp recordings from gastric-projecting DMV neurons; in vivo recordings of gastric motility, tone, compliance, and emptying; and food intake studies were used to assess the effects of NMDA receptor antagonism on caloric regulation. After aHFD exposure, inhibition of extrasynaptic NMDA receptors prevented the synaptic NMDA receptor-mediated increase in glutamatergic transmission to DMV neurons, as well as the increase in gastric tone and motility, while chronic extrasynaptic NMDA receptor inhibition attenuated the regulation of caloric intake. After aHFD exposure, the regulation of food intake involved synaptic NMDA receptor-mediated currents, which occurred in response to extrasynaptic NMDA receptor activation. Understanding these events may provide a mechanistic basis for hyperphagia and may identify novel therapeutic targets for the treatment of obesity.

TRPM3 expression and control of glutamate release from primary vagal afferent neurons

Vagal afferent fibers contact neurons in the nucleus of the solitary tract (NTS) and release glutamate via three distinct release pathways: synchronous, asynchronous, and spontaneous. The presence of TRPV1 in vagal afferents is predictive of activity-dependent asynchronous glutamate release along with temperature-sensitive spontaneous vesicle fusion. However, pharmacological blockade or genetic deletion of TRPV1 does not eliminate the asynchronous profile and only attenuates the temperature-dependent spontaneous release at high temperatures (>40°C), indicating additional temperature-sensitive calcium conductance(s) contributing to these release pathways. The transient receptor potential cation channel melastatin subtype 3 (TRPM3) is a calcium-selective channel that functions as a thermosensor (30-37°C) in somatic primary afferent neurons. We predict that TRPM3 is expressed in vagal afferent neurons and contributes to asynchronous and spontaneous glutamate release pathways. We investigated these hypotheses via measurements on cultured nodose neurons and in brainstem slice preparations containing vagal afferent to NTS synaptic contacts. We found histological and genetic evidence that TRPM3 is highly expressed in vagal afferent neurons. The TRPM3-selective agonist, pregnenolone sulfate, rapidly and reversibly activated the majority (∼70%) of nodose neurons; most of which also contained TRPV1. We confirmed the role of TRPM3 with pharmacological blockade and genetic deletion. In the brain, TRPM3 signaling strongly controlled both basal and temperature-driven spontaneous glutamate release. Surprisingly, genetic deletion of TRPM3 did not alter synchronous or asynchronous glutamate release. These results provide convergent evidence that vagal afferents express functional TRPM3 that serves as an additional temperature-sensitive calcium conductance involved in controlling spontaneous glutamate release onto neurons in the NTS.NEW & NOTEWORTHY Vagal afferent signaling coordinates autonomic reflex function and informs associated behaviors. Thermosensitive transient receptor potential (TRP) channels detect temperature and nociceptive stimuli in somatosensory afferent neurons, however their role in vagal signaling remains less well understood. We report that the TRPM3 ion channel provides a major thermosensitive point of control over vagal signaling and synaptic transmission. We conclude that TRPM3 translates physiological changes in temperature to neurophysiological outputs and can serve as a cellular integrator in vagal afferent signaling.

Corticosterone inhibits vagal afferent glutamate release in the nucleus of the solitary tract via retrograde endocannabinoid signaling

Circulating blood glucocorticoid levels are dynamic and responsive to stimuli that impact autonomic function. In the brain stem, vagal afferent terminals release the excitatory neurotransmitter glutamate to neurons in the nucleus of the solitary tract (NTS). Vagal afferents integrate direct visceral signals and circulating hormones with ongoing NTS activity to control autonomic function and behavior. Here, we investigated the effects of corticosterone (CORT) on glutamate signaling in the NTS using patch-clamp electrophysiology on brain stem slices containing the NTS and central afferent terminals from male C57BL/6 mice. We found that CORT rapidly decreased both action potential-evoked and spontaneous glutamate signaling. The effects of CORT were phenocopied by dexamethasone and blocked by mifepristone, consistent with glucocorticoid receptor (GR)-mediated signaling. While mRNA for GR was present in both the NTS and vagal afferent neurons, selective intracellular quenching of G protein signaling in postsynaptic NTS neurons eliminated the effects of CORT. We then investigated the contribution of retrograde endocannabinoid signaling, which has been reported to transduce nongenomic GR effects. Pharmacological or genetic elimination of the cannabinoid type 1 receptor signaling blocked CORT suppression of glutamate release. Together, our results detail a mechanism, whereby the NTS integrates endocrine CORT signals with fast neurotransmission to control autonomic reflex pathways.

Insights into Potential Targets for Therapeutic Intervention in Epilepsy

Epilepsy is a chronic brain disease that affects approximately 65 million people worldwide. However, despite the continuous development of antiepileptic drugs, over 30% patients with epilepsy progress to drug-resistant epilepsy. For this reason, it is a high priority objective in preclinical research to find novel therapeutic targets and to develop effective drugs that prevent or reverse the molecular mechanisms underlying epilepsy progression. Among these potential therapeutic targets, we highlight currently available information involving signaling pathways (Wnt/β-catenin, Mammalian Target of Rapamycin (mTOR) signaling and zinc signaling), enzymes (carbonic anhydrase), proteins (erythropoietin, copine 6 and complement system), channels (Transient Receptor Potential Vanilloid Type 1 (TRPV1) channel) and receptors (galanin and melatonin receptors). All of them have demonstrated a certain degree of efficacy not only in controlling seizures but also in displaying neuroprotective activity and in modifying the progression of epilepsy. Although some research with these specific targets has been done in relation with epilepsy, they have not been fully explored as potential therapeutic targets that could help address the unsolved issue of drug-resistant epilepsy and develop new antiseizure therapies for the treatment of epilepsy.

Targeting Chemosensory Ion Channels in Peripheral Swallowing-Related Regions for the Management of Oropharyngeal Dysphagia

Oropharyngeal dysphagia, or difficulty in swallowing, is a major health problem that can lead to serious complications, such as pulmonary aspiration, malnutrition, dehydration, and pneumonia. The current clinical management of oropharyngeal dysphagia mainly focuses on compensatory strategies and swallowing exercises/maneuvers; however, studies have suggested their limited effectiveness for recovering swallowing physiology and for promoting neuroplasticity in swallowing-related neuronal networks. Several new and innovative strategies based on neurostimulation in peripheral and cortical swallowing-related regions have been investigated, and appear promising for the management of oropharyngeal dysphagia. The peripheral chemical neurostimulation strategy is one of the innovative strategies, and targets chemosensory ion channels expressed in peripheral swallowing-related regions. A considerable number of animal and human studies, including randomized clinical trials in patients with oropharyngeal dysphagia, have reported improvements in the efficacy, safety, and physiology of swallowing using this strategy. There is also evidence that neuroplasticity is promoted in swallowing-related neuronal networks with this strategy. The targeting of chemosensory ion channels in peripheral swallowing-related regions may therefore be a promising pharmacological treatment strategy for the management of oropharyngeal dysphagia. In this review, we focus on this strategy, including its possible neurophysiological and molecular mechanisms.

Elevation of Transient Receptor Potential Vanilloid 1 Function in the Lateral Habenula Mediates Aversive Behaviors in Alcohol-withdrawn Rats

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

Chronic alcohol use and withdrawal leads to increased pain perception, anxiety, and depression. These aberrant behaviors are accompanied by increased excitatory glutamatergic transmission to, and activity of, the lateral habenula neurons.Vanilloid type 1, or TRPV1, channels are expressed in the habenula and they facilitate glutamatergic transmission. Whether TRPV1 channel plays a role in habenula hyperactivity is not clear.

WHAT THIS ARTICLE TELLS US THAT IS NEW

Glutamatergic transmission in the lateral habenula was inhibited by TRPV1 channel antagonists. In vivo, local administration of TRPV1 antagonists into the lateral habenula attenuated hyperalgesia, anxiety, and relapse-like drinking in rats who chronically consumed alcohol.The data suggest that enhanced TRPV1 channel function during withdrawal may contribute to aberrant behavior that promotes relapse alcohol consumption.

BACKGROUND

Recent rat studies indicate that alcohol withdrawal can trigger a negative emotional state including anxiety- and depression-like behaviors and hyperalgesia, as well as elevated glutamatergic transmission and activity in lateral habenula neurons. TRPV1, a vanilloid receptor expressed in the habenula, is involved in pain, alcohol dependence, and glutamatergic transmission. The authors therefore hypothesized that TRPV1 contributes to the changes in both the behavioral phenotypes and the habenula activity in alcohol-withdrawn rats.

METHODS

Adult male Long-Evans rats (n = 110 and 280 for electrophysiology and behaviors, respectively), randomly assigned into the alcohol and water (Naïve) groups, were trained to consume either alcohol or water-only using an intermittent-access procedure. Slice electrophysiology was used to measure spontaneous excitatory postsynaptic currents and firing of lateral habenula neurons. The primary outcome was the change in alcohol-related behaviors and lateral habenula activity induced by pharmacologic manipulation of TRPV1 activity.

RESULTS

The basal frequency of spontaneous excitatory postsynaptic currents and firing of lateral habenula neurons in alcohol-withdrawn rats was significantly increased. The TRPV1 antagonist capsazepine (10 µM) induced a stronger inhibition on spontaneous excitatory postsynaptic currents (mean ± SD; by 26.1 ± 27.9% [n = 11] vs. 6.7 ± 18.6% [n = 17], P = 0.027) and firing (by 23.4 ± 17.6% [n = 9] vs. 11.9 ± 16.3% [n = 12], P = 0.025) in Withdrawn rats than Naive rats. By contrast, the TRPV1 agonist capsaicin (3 μM) produced a weaker potentiation in Withdrawn than Naïve rats (spontaneous excitatory postsynaptic currents: by 203.6 ± 124.7% [n = 20] vs. 415.2 ± 424.3% [n = 15], P < 0.001; firing: 38.1 ± 14.7% [n = 11] vs. 73.9 ± 41.9% [n = 11], P < 0.001). Conversely, capsaicin's actions in Naïve but not in Withdrawn rats were significantly attenuated by the pretreatment of TRPV1 endogenous agonist N-Oleoyldopamine. In Withdrawn rats, intra-habenula infusion of TRPV1 antagonists attenuated hyperalgesia and anxiety-like behaviors, decreased alcohol consumption upon resuming drinking, and elicited a conditioned place preference.

CONCLUSIONS

Enhanced TRPV1 function may contribute to increased glutamatergic transmission and activity of lateral habenula neurons, resulting in the aberrant behaviors during ethanol withdrawal.

Cannabinoids, TRPV and nitric oxide: the three ring circus of neuronal excitability

Endocannabinoid system is considered a relevant player in the regulation of neuronal excitability, since it contributes to maintaining the balance of the synaptic ionic milieu. Perturbations to bioelectric conductances have been implicated in the pathophysiological processes leading to hyperexcitability and epileptic seizures. Cannabinoid influence on neurosignalling is exerted on classic receptor-mediated mechanisms or on further molecular targets. Among these, transient receptor potential vanilloid (TRPV) are ionic channels modulated by cannabinoids that are involved in the transduction of a plethora of stimuli and trigger fundamental downstream pathways in the post-synaptic site. In this review, we aim at providing a brief summary of the most recent data about the cross-talk between cannabinoid system and TRPV channels, drawing attention on their role on neuronal hyperexcitability. Then, we aim to unveil a plausible point of interaction between these neural signalling systems taking into consideration nitric oxide, a gaseous molecule inducing profound modifications to neural performances. From this novel perspective, we struggle to propose innovative cellular mechanisms in the regulation of hyperexcitability phenomena, with the goal of exploring plausible CB-related mechanisms underpinning epileptic seizures.

Understanding diverse TRPV1 signaling - an update

The transient receptor potential vanilloid 1 (TRPV1) is densely expressed in spinal sensory neurons as well as in cranial sensory neurons, including their central terminal endings. Recent work in the less familiar cranial sensory neurons, despite their many similarities with spinal sensory neurons, suggest that TRPV1 acts as a calcium channel to release a discrete population of synaptic vesicles. The modular and independent regulation of release offers new questions about nanodomain organization of release and selective actions of G protein–coupled receptors.

Distinct Calcium Sources Define Compartmentalized Synaptic Signaling Domains

Nervous system communication relies on neurotransmitter release for synaptic transmission between neurons. Neurotransmitter is contained within vesicles in presynaptic terminals and intraterminal calcium governs the fundamental step of their release into the synaptic cleft. Despite a common dependence on calcium, synaptic transmission and its modulation varies highly across the nervous system. The precise mechanisms that underlie this heterogeneity, however, remain unclear. The present review highlights recent data that reveal vesicles sourced from separate pools define discrete modes of release. A rich diversity of regulatory machinery may further distinguish the different forms of vesicle release, including presynaptic proteins involved in trafficking, alignment, and exocytosis. These multiple vesicle release mechanisms and vesicle pools likely depend on the arrangement of vesicles in relation to specific calcium entry pathways that create compartmentalized spheres of calcium influence (i.e., domains). This diversity permits release specialization. This review details examples of how individual neurons rely on multiple calcium sources and unique regulatory schemes to provide differential release and discrete modulation of neurotransmitter release from specific vesicle pools-as part of network signal integration.

5-HT3R-sourced calcium enhances glutamate release from a distinct vesicle pool

The serotonin 3 receptor (5-HT₃R) is a calcium-permeant channel heterogeneously expressed in solitary tract (ST) afferents. ST afferents synapse in the nucleus of the solitary tract (NTS) and rely on a mix of voltage-dependent calcium channels (CaVs) to control synchronous glutamate release (ST-EPSCs). CaV activation triggers additional, delayed release of glutamate (asynchronous EPSCs) that trails after the ST-EPSCs but only from afferents expressing the calcium-permeable, transient receptor potential vanilloid type 1 receptor (TRPV1). Most afferents express TRPV1 and have high rates of spontaneous glutamate release (sEPSCs) that is independent of CaVs. Here, we tested whether 5-HT₃R-sourced calcium contributes to these different forms of glutamate release in horizontal NTS slices from rats. The 5-HT₃R selective agonist, m-chlorophenyl biguanide hydrochloride (PBG), enhanced sEPSCs and/or delayed the arrival times of ST-EPSCs (i.e. increased latency). The specific 5-HT₃R antagonist, ondansetron, attenuated these effects consistent with direct activation of 5-HT₃Rs. PBG did not alter ST-EPSC amplitude or asynchronous EPSCs. These independent actions suggest two distinct 5-HT₃R locations; axonal expression that impedes conduction and terminal expression that mobilizes a spontaneous vesicle pool. Calcium chelation with EGTA-AM attenuated the frequency of 5-HT₃R-activated sEPSCs by half. The mixture of chelation-sensitive and resistant sEPSCs suggests that 5-HT₃R-activated vesicles span calcium diffusion distances that are both distal (micro-) and proximal (nanodomains) to the channel. Our results demonstrate that the calcium domains of 5-HT₃Rs do not overlap other calcium sources or their respective vesicle pools. 5-HT₃Rs add a unique calcium source on ST afferents as part of multiple independent synaptic signaling mechanisms.

Central nervous system circuits that control body temperature

Maintenance of mammalian core body temperature within a narrow range is a fundamental homeostatic process to optimize cellular and tissue function, and to improve survival in adverse thermal environments. Body temperature is maintained during a broad range of environmental and physiological challenges by central nervous system circuits that process thermal afferent inputs from the skin and the body core to control the activity of thermoeffectors. These include thermoregulatory behaviors, cutaneous vasomotion (vasoconstriction and, in humans, active vasodilation), thermogenesis (shivering and brown adipose tissue), evaporative heat loss (salivary spreading in rodents, and human sweating). This review provides an overview of the central nervous system circuits for thermoregulatory reflex regulation of thermoeffectors.

TRPV1 channels contribute to spontaneous glutamate release in nucleus tractus solitarii following chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) reduces afferent-evoked excitatory postsynaptic currents (EPSCs) but enhances basal spontaneous (s) and asynchronous (a) EPSCs in second-order neurons of nucleus tractus solitarii (nTS), a major area for cardiorespiratory control. The net result is an increase in synaptic transmission. The mechanisms by which this occurs are unknown. The N-type calcium channel and transient receptor potential cation channel TRPV1 play prominent roles in nTS sEPSCs and aEPSCs. The functional role of these channels in CIH-mediated afferent-evoked EPSC, sEPSC, and aEPSC was tested in rat nTS slices following antagonist inhibition and in mouse nTS slices that lack TRPV1. Block of N-type channels decreased aEPSCs in normoxic and, to a lesser extent, CIH-exposed rats. sEPSCs examined in the presence of TTX (miniature EPSCs) were also decreased by N-type block in normoxic but not CIH-exposed rats. Antagonist inhibition of TRPV1 reduced the normoxic and the CIH-mediated increase in sEPSCs, aEPSCs, and mEPSCs. As in rats, in TRPV1+/+ control mice, aEPSCs, sEPSCs, and mEPSCs were enhanced following CIH. However, none were enhanced in TRPV1-/- null mice. Normoxic tractus solitarii (TS)-evoked EPSC amplitude, and the decrease after CIH, were comparable in control and null mice. In rats, TRPV1 was localized in the nodose-petrosal ganglia (NPG) and their central branches. CIH did not alter TRPV1 mRNA but increased its protein in NPG consistent with an increased contribution of TRPV1. Together, our studies indicate TRPV1 contributes to the CIH increase in aEPSCs and mEPSCs, but the CIH reduction in TS-EPSC amplitude occurs via an alternative mechanism. NEW & NOTEWORTHY This study provides information on the underlying mechanisms responsible for the chronic intermittent hypoxia (CIH) increase in synaptic transmission that leads to exaggerated sympathetic nervous and respiratory activity at baseline and in response to low oxygen. We demonstrate that the CIH increase in asynchronous and spontaneous excitatory postsynaptic currents (EPSCs) and miniature EPSCs, but not decrease in afferent-driven EPSCs, is dependent on transient receptor potential vanilloid type 1 (TRPV1). Thus TRPV1 is important in controlling nucleus tractus solitarii synaptic activity during CIH.

Activation of TRPV1 and TRPM8 Channels in the Larynx and Associated Laryngopharyngeal Regions Facilitates the Swallowing Reflex

The larynx and associated laryngopharyngeal regions are innervated by the superior laryngeal nerve (SLN) and are highly reflexogenic. Transient receptor potential (TRP) channels have recently been detected in SLN innervated regions; however, their involvement in the swallowing reflex has not been fully elucidated. Here, we explore the contribution of two TRP channels, TRPV1 and TRPM8, located in SLN-innervated regions to the swallowing reflex. Immunohistochemistry identified TRPV1 and TRPM8 on cell bodies of SLN afferents located in the nodose-petrosal-jugular ganglionic complex. The majority of TRPV1 and TRPM8 immunoreactivity was located on unmyelinated neurons. Topical application of different concentrations of TRPV1 and TRPM8 agonists modulated SLN activity. Application of the agonists evoked a significantly greater number of swallowing reflexes compared with the number evoked by distilled water. The interval between the reflexes evoked by the agonists was shorter than that produced by distilled water. Prior topical application of respective TRPV1 or TRPM8 antagonists significantly reduced the number of agonist-evoked reflexes. The findings suggest that the activation of TRPV1 and TRPM8 channels present in the swallowing-related regions can facilitate the evoking of swallowing reflex. Targeting the TRP channels could be a potential therapeutic strategy for the management of dysphagia.

Excitatory and Inhibitory Neurons Utilize Different Ca2+ Sensors and Sources to Regulate Spontaneous Release

Spontaneous neurotransmitter release (mini) is an important form of Ca²⁺-dependent synaptic transmission that occurs in the absence of action potentials. A molecular understanding of this process requires an identification of the underlying Ca²⁺ sensors. Here, we address the roles of the relatively low- and high-affinity Ca²⁺ sensors, synapotagmin-1 (syt1) and Doc2α/β, respectively. We found that both syt1 and Doc2 regulate minis, but, surprisingly, their relative contributions depend on whether release was from excitatory or inhibitory neurons. Doc2α promoted glutamatergic minis, while Doc2β and syt1 both regulated GABAergic minis. We identified Ca²⁺ ligand mutations in Doc2 that either disrupted or constitutively activated the regulation of minis. Finally, Ca²⁺ entry via voltage-gated Ca²⁺ channels triggered miniature GABA release by activating syt1, but had no effect on Doc2-driven minis. This work reveals an unexpected divergence in the regulation of spontaneous excitatory and inhibitory transmission in terms of both Ca²⁺ sensors and sources.

Activation of TRPV1 in nucleus tractus solitarius reduces brown adipose tissue thermogenesis, arterial pressure, and heart rate

The sympathetic nerve activity (SNA) to brown adipose tissue (BAT) regulates BAT thermogenesis to defend body temperature in cold environments or to produce fever during immune responses. The vagus nerve contains afferents that inhibit the BAT SNA and BAT thermogenesis evoked by skin cooling. We sought to determine whether activation of transient receptor potential vanilloid 1 (TRPV1) channels in the nucleus tractus solitarius (NTS), which are prominently expressed in unmyelinated vagal afferents, would affect cold-evoked BAT thermogenesis, cardiovascular parameters, or their vagal afferent-evoked responses. In urethane-chloralose-anesthetized rats, during skin cooling, nanoinjection of the TRPV1-agonist resiniferatoxin in NTS decreased BAT SNA (from 695 ± 195% of baseline during cooling to 103 ± 8% of baseline after resiniferatoxin), BAT temperature (-0.8 ± 0.1°C), expired CO₂ (-0.3 ± 0.04%), mean arterial pressure (MAP; -20 ± 5 mmHg), and heart rate (-44 ± 11 beats/min). Pretreatment of NTS with the TRPV1 antagonist capsazepine prevented these resiniferatoxin-mediated effects. Intravenous injection of the TRPV1 agonist dihydrocapsaicin also decreased all the measured variables (except MAP). Bilateral cervical or subdiaphragmatic vagotomy attenuated the decreases in BAT SNA and thermogenesis evoked by nanoinjection of resiniferatoxin in NTS but did not prevent the decreases in BAT SNA and BAT thermogenesis evoked by intravenous dihydrocapsaicin. We conclude that activation of TRPV1 channels in the NTS of vagus nerve intact rats inhibits BAT SNA and decreases BAT metabolism, blood pressure, and heart rate. In contrast, the inhibition of BAT thermogenesis following systemic administration of dihydrocapsaicin does not require vagal afferent activity, consistent with a nonvagal pathway through which systemic TRPV1 agonists can inhibit BAT thermogenesis.

Activation of astrocytic PAR1 receptors in the rat nucleus of the solitary tract regulates breathing through modulation of presynaptic TRPV1

KEY POINTS

In the rat nucleus of the solitary tract (NTS), activation of astrocytic proteinase-activated receptor 1 (PAR1) receptors leads to potentiation of neuronal synaptic activity by two mechanisms, one TRPV1-dependent and one TRPV1-independent. PAR1-dependent activation of presynaptic TRPV1 receptors facilitates glutamate release onto NTS neurons. The TRPV1-dependent mechanism appears to rely on astrocytic release of endovanilloid-like molecules. A subset of NTS neurons excited by PAR1 directly project to the rostral ventral respiratory group. The PAR1 initiated, TRPV1-dependent modulation of synaptic transmission in the NTS contributes to regulation of breathing.

ABSTRACT

Many of the cellular and molecular mechanisms underlying astrocytic modulation of synaptic function remain poorly understood. Recent studies show that G-protein coupled receptor-mediated astrocyte activation modulates synaptic transmission in the nucleus of the solitary tract (NTS), a brainstem nucleus that regulates crucial physiological processes including cardiorespiratory activity. By using calcium imaging and patch clamp recordings in acute brain slices of wild-type and TRPV1^-/- rats, we show that activation of proteinase-activated receptor 1 (PAR1) in NTS astrocytes potentiates presynaptic glutamate release on NTS neurons. This potentiation is mediated by both a TRPV1-dependent and a TRPV1-independent mechanism. The TRPV1-dependent mechanism appears to require release of endovanilloid-like molecules from astrocytes, which leads to subsequent potentiation of presynaptic glutamate release via activation of presynaptic TRPV1 channels. Activation of NTS astrocytic PAR1 receptors elicits cFOS expression in neurons that project to respiratory premotor neurons and inhibits respiratory activity in control, but not in TRPV1^-/- rats. Thus, activation of astrocytic PAR1 receptor in the NTS leads to a TRPV1-dependent excitation of NTS neurons causing a potent modulation of respiratory motor output.

Efferent neural pathways for the control of brown adipose tissue thermogenesis and shivering

The fundamental central neural circuits for thermoregulation orchestrate behavioral and autonomic repertoires that maintain body core temperature during thermal challenges that arise from either the ambient or the internal environment. This review summarizes our understanding of the neural pathways within the fundamental thermoregulatory reflex circuitry that comprise the efferent (i.e., beyond thermosensory) control of brown adipose tissue (BAT) and shivering thermogenesis: the motor neuron systems consisting of the BAT sympathetic preganglionic neurons and BAT sympathetic ganglion cells, and the alpha- and gamma-motoneurons; the premotor neurons in the region of the rostral raphe pallidus, and the thermogenesis-promoting neurons in the dorsomedial hypothalamus/dorsal hypothalamic area. Also included are inputs to, and neurochemical modulators of, these efferent neuronal populations that could influence their activity during thermoregulatory responses. Signals of metabolic status can be particularly significant for the energy-hungry thermoeffectors for heat production.

Capsaicin Enhances Glutamatergic Synaptic Transmission to Neonatal Rat Hypoglossal Motor Neurons via a TRPV1-Independent Mechanism

We investigated whether capsaicin modulated synaptic transmission to hypoglossal motor neurons (HMNs) by acting on transient receptor potential vanilloid type 1 (TRPV1) receptors. Using whole-cell patch clamp recording from neonatal rat HMNs, we found that capsaicin increased spontaneous excitatory post-synaptic current (sEPSC) frequency and amplitude. Interestingly, the only effect of capsaicin on spontaneous inhibitory post-synaptic currents (sIPSCs) was a significant decrease in sIPSC amplitude without altering frequency, indicating a post-synaptic mechanism of action. The frequency of miniature excitatory post-synaptic currents (mEPSCs), recorded in the presence of tetrodotoxin (TTX), was also increased by capsaicin, but capsaicin did not alter mEPSC amplitude, consistent with a pre-synaptic mechanism of action. A negative shift in membrane current (I_holding) was elicited by capsaicin under both recording conditions. The effect of capsaicin on excitatory synaptic transmission remained unchanged in the presence of the TRPV1 antagonists, capsazepine or SB366791, suggesting that capsaicin acts to modulate EPSCs via a mechanism which does not require TRPV1 activation. Capsaicin, however, did not alter evoked excitatory post-synaptic currents (eEPSCs) or the paired-pulse ratio (PPR) of eEPSCs. Repetitive action potential (AP) firing in HMNs was also unaltered by capsaicin, indicating that capsaicin does not change HMN intrinsic excitability. We have demonstrated that capsaicin modulates glutamatergic excitatory, as well as glycinergic inhibitory, synaptic transmission in HMNs by differing pre- and post-synaptic mechanisms. These results expand our understanding regarding the extent to which capsaicin can modulate synaptic transmission to central neurons.

Viscerosensory input drives angiotensin II type 1A receptor-expressing neurons in the solitary tract nucleus

Homeostatic regulation of visceral organ function requires integrated processing of neural and neurohormonal sensory signals. The nucleus of the solitary tract (NTS) is the primary sensory nucleus for cranial visceral sensory afferents. Angiotensin II (ANG II) is known to modulate peripheral visceral reflexes, in part, by activating ANG II type 1A receptors (AT_1AR) in the NTS. AT_1AR-expressing NTS neurons occur throughout the NTS with a defined subnuclear distribution, and most of these neurons are depolarized by ANG II. In this study we determined whether AT_1AR-expressing NTS neurons receive direct visceral sensory input, and whether this input is modulated by ANG II. Using AT_1AR-GFP mice to make targeted whole cell recordings from AT_1AR-expressing NTS neurons, we demonstrate that two-thirds (37 of 56) of AT_1AR-expressing neurons receive direct excitatory, visceral sensory input. In half of the neurons tested (4 of 8) the excitatory visceral sensory input was significantly reduced by application of the transient receptor potential vallinoid type 1 receptor agonist, capsaicin, indicating AT_1AR-expressing neurons can receive either C- or A-fiber-mediated input. Application of ANG II to a subset of second-order AT_1AR-expressing neurons did not affect spontaneous, evoked, or asynchronous glutamate release from visceral sensory afferents. Thus it is unlikely that AT_1AR-expressing viscerosensory neurons terminate on AT_1AR-expressing NTS neurons. Our data suggest that ANG II is likely to modulate multiple visceral sensory modalities by altering the excitability of second-order AT_1AR-expressing NTS neurons.

Distinct Calcium Sources Support Multiple Modes of Synaptic Release from Cranial Sensory Afferents

Most craniosensory afferents have unmyelinated axons expressing TRP Vanilloid 1 (TRPV1) receptors in synaptic terminals at the solitary tract nucleus (NTS). Neurotransmission from these synapses is characterized by substantial asynchronous EPSCs following action potential-synched EPSCs and high spontaneous rates that are thermally sensitive. The present studies blocked voltage-activated calcium channels (CaV) using the nonselective CaV blocker Cd(2+) or the specific N-type blocker ω-conotoxin GVIA to examine the calcium dependence of the synchronous, asynchronous, spontaneous, and thermally gated modes of release. In rat brainstem slices containing caudal NTS, shocks to the solitary tract (ST) triggered synchronous ST-EPSCs and trailing asynchronous EPSCs. Cd(2+) or GVIA efficiently reduced both synchronous and asynchronous EPSCs without altering spontaneous or thermal-evoked transmission. Activation of TRPV1 with either the selective agonist resiniferatoxin (150 pm) or temperature augmented basal sEPSC rates but failed to alter the synchronous or asynchronous modes of release. These data indicate that calcium sourced through TRPV1 has no access to the synchronous or asynchronous release mechanism(s) and conversely that CaV-sourced calcium does not interact with the thermally evoked mode of release. Buffering intracellular calcium with EGTA-AM or BAPTA-AM reduced asynchronous EPSC rates earlier and to a greater extent than synchronous ST-EPSC amplitudes without altering sEPSCs or thermal sensitivity. Buffering therefore distinguishes asynchronous vesicles as possessing a highly sensitive calcium sensor located perhaps more distant from CaV than synchronous vesicles or thermally evoked vesicles from TRPV1. Together, our findings suggest separate mechanisms of release for spontaneous, asynchronous and synchronous vesicles that likely reside in unique, spatially separated vesicle domains.

SIGNIFICANCE STATEMENT

Most craniosensory fibers release glutamate using calcium entry from two sources: CaVs and TRPV1. We demonstrate that calcium segregation distinguishes three vesicle release mechanisms. Most surprisingly, asynchronous release is associated with CaV and not TRPV1 calcium entry. This reveals that asynchronous release is an additional and separate phenotypic marker of unmyelinated afferents rather than operated by TRPV1. The functional independence of the two calcium sources expands the regulatory repertoire of transmission and imbues these inputs with additional modulation targets for synaptic release not present at conventional CaV synapses. Peptides and lipid mediators may target one or both of these calcium sources at afferent terminals within the solitary tract nucleus to independently modify release from distinct, functionally segregated vesicle pools.

Direct Anandamide Activation of TRPV1 Produces Divergent Calcium and Current Responses

In the brainstem nucleus of the solitary tract (NTS), primary vagal afferent neurons express the transient receptor potential vanilloid subfamily member 1 (TRPV1) at their central terminals where it contributes to quantal forms of glutamate release. The endogenous membrane lipid anandamide (AEA) is a putative TRPV1 agonist in the brain, yet the extent to which AEA activation of TRPV1 has a neurophysiological consequence is not well established. We investigated the ability of AEA to activate TRPV1 in vagal afferent neurons in comparison to capsaicin (CAP). Using ratiometric calcium imaging and whole-cell patch clamp recordings we confirmed that AEA excitatory activity requires TRPV1, binds competitively at the CAP binding site, and has low relative affinity. While AEA-induced increases in peak cytosolic calcium were similar to CAP, AEA-induced membrane currents were significantly smaller. Removal of bath calcium increased the AEA current with no change in peak CAP currents revealing a calcium sensitive difference in specific ligand activation of TRPV1. Both CAP- and AEA-activated TRPV1 currents maintained identical reversal potentials, arguing against a major difference in ion selectivity to resolve the AEA differences in signaling. In contrast with CAP, AEA did not alter spontaneous glutamate release at NTS synapses. We conclude: (1) AEA activation of TRPV1 is markedly different from CAP and produces different magnitudes of calcium influx from whole-cell current; and (2) exogenous AEA does not alter spontaneous glutamate release onto NTS neurons. As such, AEA may convey modulatory changes to calcium-dependent processes, but does not directly facilitate glutamate release.

Ethyl Vanillin Activates TRPA1

The nonselective cation channel transient receptor potential ankryn subtype family 1 (TRPA1) is expressed in neurons of dorsal root ganglia and trigeminal ganglia and also in vagal afferent neurons that innervate the lungs and gastrointestinal tract. Many TRPA1 agonists are reactive electrophilic compounds that form covalent adducts with TRPA1. Allyl isothiocyanate (AITC), the common agonist used to identify TRPA1, contains an electrophilic group that covalently binds with cysteine residues of TRPA1 and confers a structural change on the channel. There is scientific motivation to identify additional compounds that can activate TRPA1 with different mechanisms of channel gating. We provide evidence that ethyl vanillin (EVA) is a TRPA1 agonist. Using fluorescent calcium imaging and whole-cell patch-clamp electrophysiology on dissociated rat vagal afferent neurons and TRPA1-transfected COS-7 cells, we discovered that EVA activates cells also activated by AITC. Both agonists display similar current profiles and conductances. Pretreatment with A967079, a selective TRPA1 antagonist, blocks the EVA response as well as the AITC response. Furthermore, EVA does not activate vagal afferent neurons from TRPA1 knockout mice, showing selectivity for TRPA1 in this tissue. Interestingly, EVA appears to be pharmacologically different from AITC as a TRPA1 agonist. When AITC is applied before EVA, the EVA response is occluded. However, they both require intracellular oxidation to activate TRPA1. These findings suggest that EVA activates TRPA1 but via a distinct mechanism that may provide greater ease for study in native systems compared with AITC and may shed light on differential modes of TRPA1 gating by ligand types.

CaMKIIα may modulate fentanyl-induced hyperalgesia via a CeLC-PAG-RVM-spinal cord descending facilitative pain pathway in rats

Each of the lateral capsular division of central nucleus of amygdala(CeLC), periaqueductal gray (PAG), rostral ventromedial medulla(RVM) and spinal cord has been proved to contribute to the development of opioid-induced hyperalgesia(OIH). Especially, Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα) in CeLC and spinal cord seems to play a key role in OIH modulation. However, the pain pathway through which CaMKIIα modulates OIH is not clear. The pathway from CeLC to spinal cord for this modulation was explored in the present study. Mechanical and thermal hyperalgesia were tested by von Frey test or Hargreaves test, respectively. CaMKIIα activity (phospho-CaMKIIα, p-CaMKIIα) was evaluated by western blot analysis. CaMKIIα antagonist (KN93) was micro-infused into CeLC, spinal cord or PAG, respectively, to evaluate its effect on behavioral hyperalgesia and p-CaMKIIα expression in CeLC, PAG, RVM and spinal cord. Then the underlying synaptic mechanism was explored by recording miniature excitatory postsynaptic currents (mEPSCs) on PAG slices using whole-cell voltage-clamp methods. Results showed that inhibition of CeLC, PAG or spinal CaMKIIα activity respectively by KN93, reversed both mechanical and thermal hyperalgesia. Microinjection of KN93 into CeLC decreased p-CaMKIIα expression in CeLC, PAG, RVM and spinal cord; while intrathecal KN93 can only block spinal but not CeLC CaMKIIα activity. KN93 injected into PAG just decreased p-CaMKIIα expression in PAG, RVM and spinal cord, but not in the CeLC. Similarly, whole-cell voltage-clamp recording found the frequency and amplitude of mEPSCs in PAG cells were decreased by KN93 added in PAG slice or micro-infused into CeLC in vivo. These results together with previous findings suggest that CaMKIIα may modulate OIH via a CeLC-PAG-RVM-spinal cord descending facilitative pain pathway.

TRPA1 expression and its functional activation in rodent cortex

TRPA1 is a non-selective cation channel involved in pain sensation and neurogenic inflammation. Although TRPA1 is well established in a number of organs including the nervous system, its presence and function in the mammalian cortex remains unclear. Here, we demonstrate the expression of TRPA1 in rodent somatosensory cortex through immunostaining and investigate its functional activation by whole-cell electrophysiology, Ca²⁺ imaging and two-photon photoswitching. Application of TRPA1 agonist (AITC) and antagonist (HC-030031) produced significant modulation of activity in layer 5 (L5) pyramidal neurons in both rats and mice; AITC increased intracellular Ca²⁺ concentrations and depolarized neurons, and both effects were blocked by HC-030031. These modulations were absent in the TRPA1 knockout mice. Next, we used optovin, a reversible photoactive molecule, to activate TRPA1 in individual L5 neurons of rat cortex. Optical control of activity was established by applying a tightly focused femtosecond-pulsed laser to optovin-loaded neurons. Light application depolarized neurons (n = 17) with the maximal effect observed at λ = 720 nm. Involvement of TRPA1 was further confirmed by repeating the experiment in the presence of HC-030031, which diminished the light modulation. These results demonstrate the presence of TRPA1 in L5 pyramidal neurons and introduce a highly specific approach to further understand its functional significance.

Dynasore blocks evoked release while augmenting spontaneous synaptic transmission from primary visceral afferents

The recycling of vesicle membrane fused during exocytosis is essential to maintaining neurotransmission. The GTPase dynamin is involved in pinching off membrane to complete endocytosis and can be inhibited by dynasore resulting in activity-dependent depletion of release-competent synaptic vesicles. In rat brainstem slices, we examined the effects of dynasore on three different modes of glutamate release–spontaneous, evoked, and asynchronous release–at solitary tract (ST) inputs to neurons in the nucleus of the solitary tract (NTS). Intermittent bursts of stimuli to the ST interspersed with pauses in stimulation allowed examination of these three modes in each neuron continuously. Application of 100 μM dynasore rapidly increased the spontaneous EPSC (sEPSC) frequency which was followed by inhibition of both ST-evoked EPSCs (ST-EPSC) as well as asynchronous EPSCs. The onset of ST-EPSC failures was not accompanied by amplitude reduction–a pattern more consistent with conduction block than reduced probability of vesicle release. Neither result suggested that dynasore interrupted endocytosis. The dynasore response profile resembled intense presynaptic TRPV1 activation. The TRPV1 antagonist capsazepine failed to prevent dynasore increases in sEPSC frequency but did prevent the block of the ST-EPSC. In contrast, the TRPV1 antagonist JNJ 17203212 prevented both actions of dynasore in neurons with TRPV1-expressing ST inputs. In a neuron lacking TRPV1-expressing ST inputs, however, dynasore promptly increased sEPSC rate followed by block of ST-evoked EPSCs. Together our results suggest that dynasore actions on ST-NTS transmission are TRPV1-independent and changes in glutamatergic transmission are not consistent with changes in vesicle recycling and endocytosis.

Medicinal Chemistry, Pharmacology, and Clinical Implications of TRPV1 Receptor Antagonists

Transient receptor potential vanilloid 1 (TRPV1) is an ion channel expressed on sensory neurons triggering an influx of cations. TRPV1 receptors function as homotetramers responsive to heat, proinflammatory substances, lipoxygenase products, resiniferatoxin, endocannabinoids, protons, and peptide toxins. Its phosphorylation increases sensitivity to both chemical and thermal stimuli, while desensitization involves a calcium-dependent mechanism resulting in receptor dephosphorylation. TRPV1 functions as a sensor of noxious stimuli and may represent a target to avoid pain and injury. TRPV1 activation has been associated to chronic inflammatory pain and peripheral neuropathy. Its expression is also detected in nonneuronal areas such as bladder, lungs, and cochlea where TRPV1 activation is responsible for pathology development of cystitis, asthma, and hearing loss. This review offers a comprehensive overview about TRPV1 receptor in the pathophysiology of chronic pain, epilepsy, cough, bladder disorders, diabetes, obesity, and hearing loss, highlighting how drug development targeting this channel could have a clinical therapeutic potential. Furthermore, it summarizes the advances of medicinal chemistry research leading to the identification of highly selective TRPV1 antagonists and their analysis of structure-activity relationships (SARs) focusing on new strategies to target this channel.

Inhibition of CaMKIIα in the Central Nucleus of Amygdala Attenuates Fentanyl-Induced Hyperalgesia in Rats

Opioid-induced hyperalgesia (OIH) is a less-studied phenomenon that has been reported in both preclinical and clinical studies. Although the underlying cause is not entirely understood, OIH is a real-life problem that affects millions of patients on a daily basis. Research has implicated the important contribution of Ca(2+)/calmodulin-dependent protein kinase IIα (CaMKIIα) to OIH at the level of spinal nociceptors. To expand our understanding of the entire brain circuitry driving OIH, in this study we investigated the role of CaMKIIα in the laterocapcular division of the central amygdala (CeLC), the conjunctive point between the spinal cord and rostro-ventral medulla. OIH was produced by repeated fentanyl administration in the rat. Correlating with the development of mechanical allodynia and thermal hyperalgesia, CaMKIIα activity was significantly elevated in the CeLC in OIH. In addition, the frequency and amplitude of spontaneous miniature excitatory postsynaptic currents (mEPSCs) in CeLC neurons were significantly increased in OIH. 2-[N-(2-hidroxyethyl)-N-(4-methoxy-benzenesulfonyl)]-amino-N-(4-chlorocinnamyl)-N-methylbenzylamine, a CaMKIIα inhibitor, dose dependently reversed sensory hypersensitivity, activation of CeLC CaMKIIα, and mEPSCs in OIH. Taken together, our data for the first time implicate a critical role of CeLC CaMKIIα in OIH.

TRPV1 Channel: A Potential Drug Target for Treating Epilepsy

Epilepsy has 2-3% incidence worldwide. However, present antiepileptic drugs provide only partial control of seizures. Calcium ion accumulation in hippocampal neurons has long been known as a major contributor to the etiology of epilepsy. TRPV1 is a calcium-permeable channel and mediator of epilepsy in the hippocampus. TRPV1 is expressed in epileptic brain areas such as CA1 area and dentate gyrus of the hippocampus. Here the author reviews the patent literature on novel molecules targeting TRPV1 that are currently being investigated in the laboratory and are candidates for future clinical evaluation in the management of epilepsy. A limited number of recent reports have implicated TRPV1 in the induction or treatment of epilepsy suggesting that this may be new area for potential drugs targeting this debilitating disease. Thus activation of TRPV1 by oxidative stress, resiniferatoxin, cannabinoid receptor (CB1) activators (i.e. anandamide) or capsaicin induced epileptic effects, and these effects could be reduced by appropriate inhibitors, including capsazepine (CPZ), 5'-iodoresiniferatoxin (IRTX), resolvins, and CB1 antagonists. It has been also reported that CPZ and IRTX reduced spontaneous excitatory synaptic transmission through modulation of glutaminergic systems and desensitization of TRPV1 channels in the hippocampus of rats. Immunocytochemical studies indicated that TRPV1 channel expression increased in the hippocampus of mice and patients with temporal lobe epilepsy. Taken together, findings in the current literature support a role for calcium ion accumulation through TRPV1 channels in the etiology of epileptic seizures, indicating that inhibition of TRPV1 in the hippocampus may possibly be a novel target for prevention of epileptic seizures.

Mechanisms, pools, and sites of spontaneous vesicle release at synapses of rod and cone photoreceptors

Photoreceptors have depolarized resting potentials that stimulate calcium-dependent release continuously from a large vesicle pool but neurons can also release vesicles without stimulation. We characterized the Ca(2+) dependence, vesicle pools, and release sites involved in spontaneous release at photoreceptor ribbon synapses. In whole-cell recordings from light-adapted horizontal cells (HCs) of tiger salamander retina, we detected miniature excitatory post-synaptic currents (mEPSCs) when no stimulation was applied to promote exocytosis. Blocking Ca(2+) influx by lowering extracellular Ca(2+) , by application of Cd(2+) and other agents reduced the frequency of mEPSCs but did not eliminate them, indicating that mEPSCs can occur independently of Ca(2+) . We also measured release presynaptically from rods and cones by examining quantal glutamate transporter anion currents. Presynaptic quantal event frequency was reduced by Cd(2+) or by increased intracellular Ca(2+) buffering in rods, but not in cones, that were voltage clamped at -70 mV. By inhibiting the vesicle cycle with bafilomycin, we found the frequency of mEPSCs declined more rapidly than the amplitude of evoked excitatory post-synaptic currents (EPSCs) suggesting a possible separation between vesicle pools in evoked and spontaneous exocytosis. We mapped sites of Ca(2+) -independent release using total internal reflectance fluorescence (TIRF) microscopy to visualize fusion of individual vesicles loaded with dextran-conjugated pHrodo. Spontaneous release in rods occurred more frequently at non-ribbon sites than evoked release events. The function of Ca(2+) -independent spontaneous release at continuously active photoreceptor synapses remains unclear, but the low frequency of spontaneous quanta limits their impact on noise.

Central control of body temperature

Central neural circuits orchestrate the behavioral and autonomic repertoire that maintains body temperature during environmental temperature challenges and alters body temperature during the inflammatory response and behavioral states and in response to declining energy homeostasis. This review summarizes the central nervous system circuit mechanisms controlling the principal thermoeffectors for body temperature regulation: cutaneous vasoconstriction regulating heat loss and shivering and brown adipose tissue for thermogenesis. The activation of these thermoeffectors is regulated by parallel but distinct efferent pathways within the central nervous system that share a common peripheral thermal sensory input. The model for the neural circuit mechanism underlying central thermoregulatory control provides a useful platform for further understanding of the functional organization of central thermoregulation, for elucidating the hypothalamic circuitry and neurotransmitters involved in body temperature regulation, and for the discovery of novel therapeutic approaches to modulating body temperature and energy homeostasis.

Thermosensing mechanisms and their impairment by high-fat diet in orexin neurons

In homeotherms, the hypothalamus controls thermoregulatory and adaptive mechanisms in energy balance, sleep-wake and locomotor activity to maintain optimal body temperature. Orexin neurons may be involved in these functions as they promote thermogenesis, food intake and behavioral arousal, and are sensitive to temperature and metabolic status. How thermal and energy balance signals are integrated in these neurons is unknown. Thus, we investigated the cellular mechanisms of thermosensing in orexin neurons and their response to a change in energy status using whole-cell patch clamp on rat brain slices. We found that warming induced an increase in miniature excitatory postsynaptic current (EPSC) frequency, which was blocked by the transient receptor potential vanilloid-1 (TRPV1) receptor antagonist AMG9810 and mimicked by its agonist capsaicin, suggesting that the synaptic effect is mediated by heat-sensitive TRPV1 channels. Furthermore, warming inhibits orexin neurons by activating ATP-sensitive potassium (KATP) channels, an effect regulated by uncoupling protein 2 (UCP2), as the UCP2 inhibitor genipin abolished this response. These properties are unique to orexin neurons in the lateral hypothalamus, as neighboring melanin-concentrating hormone neurons showed no response to warming within the physiological temperature range. Interestingly, in rats fed with western diet for 1 or 11weeks, orexin neurons had impaired synaptic and KATP response to warming. In summary, this study reveals several mechanisms underlying thermosensing in orexin neurons and their attenuation by western diet. Overeating induced by western diet may in part be due to impaired orexin thermosensing, as post-prandial thermogenesis may promote satiety and lethargy by inhibiting orexin neurons.

Spinal cord thermosensitivity: An afferent phenomenon?

We review the evidence for thermoregulatory temperature sensors in the mammalian spinal cord and reach the following conclusions. 1) Spinal cord temperature contributes physiologically to temperature regulation. 2) Parallel anterolateral ascending pathways transmit signals from spinal cooling and spinal warming: they overlap with the respective axon pathways of the dorsal horn neurons that are driven by peripheral cold- and warm-sensitive afferents. 3) We hypothesize that these ‘cold’ and ‘warm’ ascending pathways transmit all extracranial thermosensory information to the brain. 4) Cutaneous cold afferents can be activated not only by cooling the skin but also by cooling sites along their axons: we consider that this is functionally insignificant in vivo. 5) By a presynaptic action on their central terminals, local spinal cooling enhances neurotransmission from incoming ‘cold’ afferent action potentials to second order neurons in the dorsal horn; this effect disappears when the spinal cord is warm. 6) Spinal warm sensitivity is due to warm-sensitive miniature vesicular transmitter release from afferent terminals in the dorsal horn: this effect is powerful enough to excite second order neurons in the ‘warm’ pathway independently of any incoming sensory traffic. 7) Distinct but related presynaptic mechanisms at cold- and warm-sensitive afferent terminals can thus account for the thermoregulatory actions of spinal cord temperature.

Genetic and pharmacological evidence for low-abundance TRPV3 expression in primary vagal afferent neurons

Primary vagal afferent neurons express a multitude of thermosensitive ion channels. Within this family of ion channels, the heat-sensitive capsaicin receptor (TRPV1) greatly influences vagal afferent signaling by determining the threshold for action-potential initiation at the peripheral endings, while controlling temperature-sensitive forms of glutamate release at central vagal terminals. Genetic deletion of TRPV1 does not completely eliminate these temperature-dependent effects, suggesting involvement of additional thermosensitive ion channels. The warm-sensitive, calcium-permeable, ion channel TRPV3 is commonly expressed with TRPV1; however, the extent to which TRPV3 is found in vagal afferent neurons is unknown. Here, we begin to characterize the genetic and functional expression of TRPV3 in vagal afferent neurons using molecular biology (RT-PCR and RT-quantitative PCR) in whole nodose and isolated neurons and fluorescent calcium imaging on primary cultures of nodose ganglia neurons. We confirmed low-level TRPV3 expression in vagal afferent neurons and observed direct activation with putative TRPV3 agonists eugenol, ethyl vanillin (EVA), and farnesyl pyrophosphate (FPP). Agonist activation stimulated neurons also containing TRPV1 and was blocked by ruthenium red. FPP sensitivity overlapped with EVA and eugenol but represented the smallest percentage of vagal afferent neurons, and it was the only agonist that did not stimulate neurons from TRPV3(-/-1) mice, suggesting FPP has the highest selectivity. Further, FPP was predictive of enhanced responses to capsaicin, EVA, and eugenol in rats. From our results, we conclude TRPV3 is expressed in a discrete subpopulation of vagal afferent neurons and may contribute to vagal afferent signaling either directly or in combination with TRPV1.

Perspectives of TRPV1 Function on the Neurogenesis and Neural Plasticity

The development of new strategies to renew and repair neuronal networks using neural plasticity induced by stem cell graft could enable new therapies to cure diseases that were considered lethal until now. In adequate microenvironment a neuronal progenitor must receive molecular signal of a specific cellular context to determine fate, differentiation, and location. TRPV1, a nonselective calcium channel, is expressed in neurogenic regions of the brain like the subgranular zone of the hippocampal dentate gyrus and the telencephalic subventricular zone, being valuable for neural differentiation and neural plasticity. Current data show that TRPV1 is involved in several neuronal functions as cytoskeleton dynamics, cell migration, survival, and regeneration of injured neurons, incorporating several stimuli in neurogenesis and network integration. The function of TRPV1 in the brain is under intensive investigation, due to multiple places where it has been detected and its sensitivity for different chemical and physical agonists, and a new role of TRPV1 in brain function is now emerging as a molecular tool for survival and control of neural stem cells.

Localization of TRPV1 and P2X3 in unmyelinated and myelinated vagal afferents in the rat

The vagus nerve is dominated by afferent fibers that convey sensory information from the viscera to the brain. Most vagal afferents are unmyelinated, slow-conducting C-fibers, while a smaller portion are myelinated, fast-conducting A-fibers. Vagal afferents terminate in the nucleus tractus solitarius (NTS) in the dorsal brainstem and regulate autonomic and respiratory reflexes, as well as ascending pathways throughout the brain. Vagal afferents form glutamatergic excitatory synapses with postsynaptic NTS neurons that are modulated by a variety of channels. The organization of vagal afferents with regard to fiber type and channels is not well understood. In the present study, we used tract tracing methods to identify distinct populations of vagal afferents to determine if key channels are selectively localized to specific groups of afferent fibers. Vagal afferents were labeled with isolectin B4 (IB4) or cholera toxin B (CTb) to detect unmyelinated and myelinated afferents, respectively. We find that TRPV1 channels are preferentially found in unmyelinated vagal afferents identified with IB4, with almost half of all IB4 fibers showing co-localization with TRPV1. These results agree with prior electrophysiological findings. In contrast, we found that the ATP-sensitive channel P2X3 is found in a subset of both myelinated and unmyelinated vagal afferent fibers. Specifically, 18% of IB4 and 23% of CTb afferents contained P2X3. The majority of CTb-ir vagal afferents contained neither channel. Since neither channel was found in all vagal afferents, there are likely further degrees of heterogeneity in the modulation of vagal afferent sensory input to the NTS beyond fiber type.

Molecular underpinnings of synaptic vesicle pool heterogeneity

Neuronal communication relies on chemical synaptic transmission for information transfer and processing. Chemical neurotransmission is initiated by synaptic vesicle fusion with the presynaptic active zone resulting in release of neurotransmitters. Classical models have assumed that all synaptic vesicles within a synapse have the same potential to fuse under different functional contexts. In this model, functional differences among synaptic vesicle populations are ascribed to their spatial distribution in the synapse with respect to the active zone. Emerging evidence suggests, however, that synaptic vesicles are not a homogenous population of organelles, and they possess intrinsic molecular differences and differential interaction partners. Recent studies have reported a diverse array of synaptic molecules that selectively regulate synaptic vesicles' ability to fuse synchronously and asynchronously in response to action potentials or spontaneously irrespective of action potentials. Here we discuss these molecular mediators of vesicle pool heterogeneity that are found on the synaptic vesicle membrane, on the presynaptic plasma membrane, or within the cytosol and consider some of the functional consequences of this diversity. This emerging molecular framework presents novel avenues to probe synaptic function and uncover how synaptic vesicle pools impact neuronal signaling.