TRPV1 in the central nervous system: synaptic plasticity, function, and pharmacological implications

Activation of TRPV1 receptor facilitates myelin repair following demyelination via the regulation of microglial function

The transient receptor potential vanilloid 1 (TRPV1) is a non-selective cation channel that is activated by capsaicin (CAP), the main component of chili pepper. Despite studies in several neurological diseases, the role of TRPV1 in demyelinating diseases remains unknown. Herein, we reported that TRPV1 expression was increased within the corpus callosum during demyelination in a cuprizone (CPZ)-induced demyelination mouse model. TRPV1 deficiency exacerbated motor coordinative dysfunction and demyelination in CPZ-treated mice, whereas the TRPV1 agonist CAP improved the behavioral performance and facilitated remyelination. TRPV1 was predominantly expressed in Iba1+ microglia/macrophages in human brain sections of multiple sclerosis patients and mouse corpus callosum under demyelinating conditions. TRPV1 deficiency decreased microglial recruitment to the corpus callosum, with an associated increase in the accumulation of myelin debris. Conversely, the activation of TRPV1 by CAP enhanced the recruitment of microglia to the corpus callosum and potentiated myelin debris clearance. Using real-time live imaging we confirmed an increased phagocytic function of microglia following CAP treatment. In addition, the expression of the scavenger receptor CD36 was increased, and that of the glycolysis regulators Hif1a and Hk2 was decreased. We conclude that TRPV1 is an important regulator of microglial function in the context of demyelination and may serve as a promising therapeutic target for demyelinating diseases such as multiple sclerosis.

New Insights into TRP Ion Channels in Stem Cells

Transient receptor potential (TRP) ion channels are cationic permeable proteins located on the plasma membrane. TRPs are cellular sensors for perceiving diverse physical and/or chemical stimuli; thus, serving various critical physiological functions, including chemo-sensation, hearing, homeostasis, mechano-sensation, pain, taste, thermoregulation, vision, and even carcinogenesis. Dysregulated TRPs are found to be linked to many human hereditary diseases. Recent studies indicate that TRP ion channels are not only involved in sensory functions but are also implicated in regulating the biological characteristics of stem cells. In the present review, we summarize the expressions and functions of TRP ion channels in stem cells, including cancer stem cells. It offers an overview of the current understanding of TRP ion channels in stem cells.

Transient Receptor Potential Vanilloid 1 Function at Central Synapses in Health and Disease

The transient receptor potential vanilloid 1 (TRPV1), a ligand-gated nonselective cation channel, is well known for mediating heat and pain sensation in the periphery. Increasing evidence suggests that TRPV1 is also expressed at various central synapses, where it plays a role in different types of activity-dependent synaptic changes. Although its precise localizations remain a matter of debate, TRPV1 has been shown to modulate both neurotransmitter release at presynaptic terminals and synaptic efficacy in postsynaptic compartments. In addition to being required in these forms of synaptic plasticity, TRPV1 can also modify the inducibility of other types of plasticity. Here, we highlight current evidence of the potential roles for TRPV1 in regulating synaptic function in various brain regions, with an emphasis on principal mechanisms underlying TRPV1-mediated synaptic plasticity and metaplasticity. Finally, we discuss the putative contributions of TRPV1 in diverse brain disorders in order to expedite the development of next-generation therapeutic treatments.

Serotonin mediates stress-like effects on responses to non-nociceptive stimuli in Hirudo

Noxious stimuli can elicit stress in animals that produce a variety of adaptations including changes in responses to nociceptive and non-nociceptive sensory input. One example is stress-induced analgesia that may be mediated, in part, by the endocannabinoid system. However, endocannabinoids can also have pro-nociceptive effects. In this study, the effects of electroshock, one experimental approach for producing acute stress, were examined on responses to non-nociceptive mechanical stimuli and nociceptive thermal stimuli in the medicinal leech (Hirudo verbana). The electroshock stimuli did not alter the leeches’ responses to nociceptive stimuli, but did cause sensitization to non-nociceptive stimuli, characterized by a reduction in response threshold. These experiments were repeated with drugs that either blocked synthesis of the endocannabinoid transmitter 2-arachidonoylglycerol (2-AG) or transient receptor potential vanilloid (TRPV) channel, which is known to act as an endocannabinoid receptor. Surprisingly, neither treatment had any effect on responses following electroshock. However, the electroshock stimuli reliably increased serotonin (5-hydroxytryptamine or 5HT) levels in the H. verbana CNS. Injection of 5HT mimicked the effects of the electroshocks, sensitizing responses to non-nociceptive stimuli and having no effect on responses to nociceptive stimuli. Injections of the 5HT receptor antagonist methysergide reduced the sensitization effect to non-nociceptive stimuli after electroshock treatment. These results indicate that electroshocks enhance response to non-nociceptive stimuli but do not alter responses to nociceptive stimuli. Furthermore, while 5HT appears to play a critical role in this shock-induced sensitizing effect, the endocannabinoid system seems to have no effect.

Summary: The role of serotonin and endocannabinoids in mediating the effects of potentially stress-inducing stimuli on Hirudo verbana’s response to nociceptive and non-nociceptive input.

Role of TRPV1 in High Temperature-Induced Mitochondrial Biogenesis in Skeletal Muscle: A Mini Review

Transient receptor potential vanilloid 1 (TRPV1) is a protein that is susceptible to cell environment temperature. High temperatures of 40–45°C can activate the TRPV1 channel. TRPV1 is highly expressed in skeletal muscle and located on the sarcoplasmic reticulum (SR). Therefore, TRPV1 activated by high-temperature stress releases Ca²⁺ from the SR to the cytoplasm. Cellular Ca²⁺ accumulation is a key event that enhances TRPV1 activity by directly binding to the N-terminus and C-terminus. Moreover, Ca²⁺ is the key messenger involved in regulating mitochondrial biogenesis in skeletal muscle. Long-term activation of TRPV1 may promote mitochondrial biogenesis in skeletal muscle through the Ca²⁺-CaMKII-p38 MAPK-PGC-1α signaling axis. The discovery of the TRPV1 channel highlights the potential mechanism for high-temperature stress improving muscle mitochondrial biogenesis. The appropriate hot stimulus in thermal environments might be beneficial to the muscular mitochondrial adaptation for aerobic capacity. However, the investigation of TRPV1 on mitochondrial biogenesis is at an early stage. Further investigations need to examine the role of TRPV1 in response to mitochondrial biogenesis in skeletal muscle induced by different thermal environments.

Activity-Dependent Modulation of Tonic GABA Currents by Endocannabinoids in Hirudo verbana

Endocannabinoids are lipid neuromodulators that are synthesized on demand and primarily signal in a retrograde manner to elicit depression of excitatory and inhibitory synapses. Despite the considerable interest in their potential analgesic effects, there is evidence that endocannabinoids can have both pro-nociceptive and anti-nociceptive effects. The mechanisms contributing to the opposing effects of endocannabinoids in nociception need to be better understood before cannabinoid-based therapies can be effectively utilized to treat pain. Using the medicinal leech, Hirudo verbana, this work investigates whether endocannabinoids modulate tonic inhibition onto non-nociceptive afferents. In voltage clamp recordings, we analyzed changes in the tonic inhibition in pressure-sensitive (P) cells following pre-treatment with endocannabinoids, 2-arachidonoylglycerol (2-AG) or anandamide (AEA). We also tested whether high frequency stimulation (HFS) of nociceptive (N) cells could also modulate tonic inhibition. Both endocannabinoid application and N cell HFS depressed tonic inhibition in the P cell. Depression of tonic inhibition by N cell HFS was blocked by SB 366791 (a TRPV1 inhibitor). SB 366791 also prevented 2-AG-and AEA-induced depression of tonic inhibition. HFS-induced depression was not blocked by tetrahydrolipstatin (THL), which prevents 2-AG synthesis, nor AM 251 (a CB1 receptor inverse agonist). These results illustrate a novel activity-dependent modulation of tonic GABA currents that is mediated by endocannabinoid signaling and is likely to play an important role in sensitization of non-nociceptive afferent pathways.

Increased TRPV1 Channels and FosB Protein Expression Are Associated with Chronic Epileptic Seizures and Anxiogenic-like Behaviors in a Preclinical Model of Temporal Lobe Epilepsy

Epilepsies are neurological disorders characterized by chronic seizures and their related neuropsychiatric comorbidities, such as anxiety. The Transient Receptor Potential Vanilloid type-1 (TRPV1) channel has been implicated in the modulation of seizures and anxiety-like behaviors in preclinical models. Here, we investigated the impact of chronic epileptic seizures in anxiety-like behavior and TRPV1 channels expression in a genetic model of epilepsy, the Wistar Audiogenic Rat (WAR) strain. WARs were submitted to audiogenic kindling (AK), a preclinical model of temporal lobe epilepsy (TLE) and behavioral tests were performed in the open-field (OF), and light-dark box (LDB) tests 24 h after AK. WARs displayed increased anxiety-like behavior and TRPV1R expression in the hippocampal CA1 area and basolateral amygdala nucleus (BLA) when compared to control Wistar rats. Chronic seizures increased anxiety-like behaviors and TRPV1 and FosB expression in limbic and brainstem structures involved with epilepsy and anxiety comorbidity, such as the hippocampus, superior colliculus, and periaqueductal gray matter. Therefore, these results highlight previously unrecognized alterations in TRPV1 expression in brain structures involved with TLE and anxiogenic-like behaviors in a genetic model of epilepsy, the WAR strain, supporting an important role of TRPV1 in the modulation of neurological disorders and associated neuropsychiatric comorbidities.

TRPV1 Antagonist Prevents Neonatal Sevoflurane-Induced Synaptic Abnormality and Cognitive Impairment in Mice Through Regulating the Src/Cofilin Signaling Pathway

Long-term neurodevelopmental disorders following neonatal anesthesia have been reported both in young animals and in children. The activation of transient receptor potential vanilloid 1 (TRPV1) channels in hippocampus adversely affects neurodevelopment. The current study explored the underlying mechanism of TRPV1 channels on long-lasting cognitive dysfunction induced by anesthetic exposure to the developing brain. we demonstrated that TRPV1 expression was increased after sevoflurane exposure both in vitro and in vivo. Sevoflurane exposure to hippocampal neurons decreased the synaptic density and the surface GluA1 expression, as well as increased co-localization of internalized AMPAR in early and recycling endosomes. Sevoflurane exposure to newborn mice impaired learning and memory in adulthood, and reduced AMPAR subunit GluA1, 2 and 3 expressions in the crude synaptosomal fractions from mouse hippocampus. The inhibition of TRPV1 reversed the phenotypic changes induced by sevoflurane. Moreover, sevoflurane exposure increased Src phosphorylation at tyrosine 416 site thereby reducing cofilin phosphorylation. TRPV1 blockade reversed these suppressive effects of sevoflurane. Our data suggested that TRPV1 antagonist may protect against synaptic damage and cognitive dysfunction induced by sevoflurane exposure during the brain developing stage.

Increased Incidence of Stress-related Tic Habit Cough in Children During the Recent Greek Financial Crisis

BACKGROUND/AIM

Tic habit cough (THC) is not easily recognized and requires a high index of suspicion. Since there is evidence for stress-related etiology, the study presents our experience during the eight-year period of the recent Greek financial crisis.

PATIENTS AND METHODS

This is a descriptive prospective cross-sectional clinical study using a consistent approach to diagnosis, possible triggers and treatment options.

RESULTS

Fifty-one children (26 girls, age 9.5±3.1y) from a total of 3,793 new referrals for chronic cough fulfilled the criteria of THC (1.34%). Incidence rates per 1,000 person-years were 1.34 (95%CI=0.34-5.40), 3.91 (95%CI=1.86-8.19), 4.03 (95%CI=2.01-8.06) and 14.60 (95%CI=10.43-20.44) for every two years. Poisson analysis showed an increased trend during the four time periods with an incidence rate ratio equal to 2.21 (95%CI=1.58-3.09, p<0.001). THC was mainly attributed to learning difficulties, as well as stress-related impact of peer or family bullying, family financial problems and child anxiety disorders.

CONCLUSION

The incidence of THC is associated with the negative aspects of stressors during a period of financial crisis. The potential underlining psycho-physiological mechanisms are discussed as well as the fact that prompt diagnosis may result in successful interventions.

TRPV1 sustains microglial metabolic reprogramming in Alzheimer's disease

As the brain-resident innate immune cells, reactive microglia are a major pathological feature of Alzheimer's disease (AD). However, the exact role of microglia is still unclear in AD pathogenesis. Here, using metabolic profiling, we show that microglia energy metabolism is significantly suppressed during chronic Aβ-tolerant processes including oxidative phosphorylation and aerobic glycolysis via the mTOR-AKT-HIF-1α pathway. Pharmacological activation of TRPV1 rescues Aβ-tolerant microglial dysfunction, the AKT/mTOR pathway activity, and metabolic impairments and restores the immune responses including phagocytic activity and autophagy function. Amyloid pathology and memory impairment are accelerated in microglia-specific TRPV1-knockout APP/PS1 mice. Finally, we showed that metabolic boosting with TRPV1 agonist decreases amyloid pathology and reverses memory deficits in AD mice model. These results indicate that TRPV1 is an important target regulating metabolic reprogramming for microglial functions in AD treatment.

Activation of V1a vasopressin receptors excite subicular pyramidal neurons by activating TRPV1 and depressing GIRK channels

Arginine vasopressin (AVP) is a nonapeptide that serves as a neuromodulator in the brain and a hormone in the periphery that regulates water homeostasis and vasoconstriction. The subiculum is the major output region of the hippocampus and an integral component in the networks that processes sensory and motor cues to form a cognitive map encoding spatial, contextual, and emotional information. Whereas the subiculum expresses high densities of AVP-binding sites and AVP has been shown to increase the synaptic excitability of subicular pyramidal neurons, the underlying cellular and molecular mechanisms have not been determined. We found that activation of V_1a receptors increased the excitability of subicular pyramidal neurons via activation of TRPV1 channels and depression of the GIRK channels. V_1a receptor-induced excitation of subicular pyramidal neurons required the function of phospholipase Cβ, but was independent of intracellular Ca²⁺ release. Protein kinase C was responsible for AVP-mediated depression of GIRK channels, whereas degradation of phosphatidylinositol 4,5-bisphosphate was involved in V_1a receptor-elicited activation of TRPV1 channels. Our results may provide one of the cellular and molecular mechanisms to explain the physiological functions of AVP in the brain.

Roles of Cannabidiol in the treatment and prevention of Alzheimer's disease by multi-target actions

Alzheimer's disease (AD) is one of the most common neurodegenerative diseases with chronic, progressive, and irreversible characteristics, affecting nearly 50 million older adults worldwide. The pathogenesis of AD includes the formation of senile plaques, the abnormal aggregation of tau protein and the gradual degeneration and death of cerebral cortical cells. The main symptoms are memory loss, cognitive decline and behavioral disorders. Studies indicate that cannabidiol(CBD) possesses various pharmacological activities including anti-inflammatory, anti-oxidation and neuroprotective activities. It has been suggested as a potential multi-target medicine for treatment of AD. In this review, we aim to summarize the underlying mechanisms and protective effects of CBD on signaling pathways and central receptors involved in the pathogenesis of AD, including the endocannabinoid system（eCBs）, the Transient receptor potential vanilloid type 1(TRPV1) receptor, and the Peroxisome proliferator-activated receptor (PPAR) receptor.

Selenium and Neurological Diseases: Focus on Peripheral Pain and TRP Channels

Pain is a complex physiological process that includes many components. Growing evidence supports the idea that oxidative stress and Ca2+ signaling pathways participate in pain detection by neurons. The main source of endogenous reactive oxygen species (ROS) is mitochondrial dysfunction induced by membrane depolarization, which is in turn caused by Ca2+ influx into the cytosol of neurons. ROS are controlled by antioxidants, including selenium. Selenium plays an important role in the nervous system, including the brain, where it acts as a cofactor for glutathione peroxidase and is incorporated into selenoproteins involved in antioxidant defenses. It has neuroprotective effects through modulation of excessive ROS production, inflammation, and Ca2+ overload in several diseases, including inflammatory pain, hypersensitivity, allodynia, diabetic neuropathic pain, and nociceptive pain. Ca2+ entry across membranes is mediated by different channels, including transient receptor potential (TRP) channels, some of which (e.g., TRPA1, TRPM2, TRPV1, and TRPV4) can be activated by oxidative stress and have a role in the induction of peripheral pain. The results of recent studies indicate the modulator roles of selenium in peripheral pain through inhibition of TRP channels in the dorsal root ganglia of experimental animals. This review summarizes the protective role of selenium in TRP channel regulation, Ca2+ signaling, apoptosis, and mitochondrial oxidative stress in peripheral pain induction.

Beyond Neuronal Heat Sensing: Diversity of TRPV1 Heat-Capsaicin Receptor-Channel Functions

Transient receptor potential vanilloid 1 (TRPV1) is a calcium-permeable ion channel best known for its ability to be gated by the pungent constituent of red chili pepper, capsaicin, and related chemicals from the group of vanilloids as well as by noxious heat. As such, it is mostly expressed in sensory neurons to act as a detector of painful stimuli produced by pungent chemicals and high temperatures. Its activation is also sensitized by the numerous endogenous inflammatory mediators and second messengers, making it an important determinant of nociceptive signaling. Except for such signaling, though, neuronal TRPV1 activation may influence various organ functions by promoting the release of bioactive neuropeptides from sensory fiber innervation organs. However, TRPV1 is also found outside the sensory nervous system in which its activation and function is not that straightforward. Thus, TRPV1 expression is detected in skeletal muscle; in some types of smooth muscle; in epithelial and immune cells; and in adipocytes, where it can be activated by the combination of dietary vanilloids, endovanilloids, and pro-inflammatory factors while the intracellular calcium signaling that this initiates can regulate processes as diverse as muscle constriction, cell differentiation, and carcinogenesis. The purpose of the present review is to provide a clear-cut distinction between neurogenic TRPV1 effects in various tissues consequent to its activation in sensory nerve endings and non-neurogenic TRPV1 effects due to its expression in cell types other than sensory neurons.

Possible Receptor Mechanisms Underlying Cannabidiol Effects on Addictive-like Behaviors in Experimental Animals

Substance use disorder (SUD) is a serious public health problem worldwide for which available treatments show limited effectiveness. Since the legalization of cannabis and the approval of cannabidiol (CBD) by the US Food and Drug Administration, therapeutic potential of CBD for the treatment of SUDs and other diseases has been widely explored. In this mini-review article, we first review the history and evidence supporting CBD as a potential pharmacotherapeutic. We then focus on recent progress in preclinical research regarding the pharmacological efficacy of CBD and the underlying receptor mechanisms on addictive-like behavior. Growing evidence indicates that CBD has therapeutic potential in reducing drug reward, as assessed in intravenous drug self-administration, conditioned place preference and intracranial brain-stimulation reward paradigms. In addition, CBD is effective in reducing relapse in experimental animals. Both in vivo and in vitro receptor mechanism studies indicate that CBD may act as a negative allosteric modulator of type 1 cannabinoid (CB1) receptor and an agonist of type 2 cannabinoid (CB2), transient receptor potential vanilloid 1 (TRPV1), and serotonin 5-HT_1A receptors. Through these multiple-receptor mechanisms, CBD is believed to modulate brain dopamine in response to drugs of abuse, leading to attenuation of drug-taking and drug-seeking behavior. While these findings suggest that CBD is a promising therapeutic candidate, further investigation is required to verify its safety, pharmacological efficacy and the underlying receptor mechanisms in both experimental animals and humans.

Activation of Oxytocin Receptors Excites Subicular Neurons by Multiple Signaling and Ionic Mechanisms

Oxytocin (OXT) is a nonapeptide that serves as a neuromodulator in the brain and a hormone participating in parturition and lactation in the periphery. The subiculum is the major output region of the hippocampus and an integral component in the networks that process sensory and motor cues to form a cognitive map encoding spatial, contextual, and emotional information. Whilst the subiculum expresses the highest OXT-binding sites and is the first brain region to be activated by peripheral application of OXT, the precise actions of OXT in the subiculum have not been determined. Our results demonstrate that application of the selective OXT receptor (OXTR) agonist, [Thr4,Gly7]-oxytocin (TGOT), excited subicular neurons via activation of TRPV1 channels, and depression of K+ channels. The OXTR-mediated excitation of subicular neurons required the functions of phospholipase Cβ, protein kinase C, and degradation of phosphatidylinositol 4,5-bisphosphate (PIP2). OXTR-elicited excitation of subicular neurons enhanced long-term potentiation via activation of TRPV1 channels. Our results provide a cellular and molecular mechanism to explain the physiological functions of OXT in the brain.

Capsaicin consumption reduces brain amyloid-beta generation and attenuates Alzheimer's disease-type pathology and cognitive deficits in APP/PS1 mice

Alzheimer's disease (AD) is the most common cause of age-related dementia and is currently incurable. The failures of current clinical trials and the establishment of modifiable risk factors have shifted the AD intervention from treatment to prevention in the at-risk population. Previous studies suggest that there is a geographic overlap between AD incidence and spicy food consumption. We previously reported that capsaicin-rich diet consumption was associated with better cognition and lower serum Amyloid-beta (Aβ) levels in people aged 40 years and over. In the present study, we found that intake of capsaicin, the pungent ingredient in chili peppers, reduced brain Aβ burden and rescued cognitive decline in APP/PS1 mice. Our in vivo and in vitro studies revealed that capsaicin shifted Amyloid precursor protein (APP) processing towards α-cleavage and precluded Aβ generation by promoting the maturation of a disintegrin and metalloproteinase 10 (ADAM10). We also found that capsaicin alleviated other AD-type pathologies, such as tau hyperphosphorylation, neuroinflammation and neurodegeneration. The present study suggests that capsaicin is a potential therapeutic candidate for AD and warrants clinical trials on chili peppers or capsaicin as dietary supplementation for the prevention and treatment of AD.

Cannabidiol attenuates the rewarding effects of cocaine in rats by CB2, 5-HT1A and TRPV1 receptor mechanisms

Cocaine abuse continues to be a serious health problem worldwide. Despite intense research there is currently no FDA-approved medication to treat cocaine use disorder. The recent search has been focused on agents targeting primarily the dopamine system, while limited success has been achieved at the clinical level. Cannabidiol (CBD) is a U.S. FDA-approved cannabinoid for the treatment of epilepsy and recently was reported to have therapeutic potential for other disorders. Here we systemically evaluated its potential utility for the treatment of cocaine use disorder and explored the underlying receptor mechanisms in experimental animals. Systemic administration (10-40 mg/kg) of CBD dose-dependently inhibited cocaine self-administration, shifted a cocaine dose-response curve downward, and lowered break-points for cocaine self-administration under a progressive-ratio schedule of reinforcement. CBD inhibited cocaine self-administration maintained by low, but not high, doses of cocaine. In addition, CBD (3-20 mg/kg) dose-dependently attenuated cocaine-enhanced brain-stimulation reward (BSR) in rats. Strikingly, this reduction in both cocaine self-administration and BSR was blocked by AM630 (a cannabinoid CB2 receptor antagonist), WAY100135 (a 5-HT1A receptor antagonist), or capsazepine (a TRPV1 channel blocker), but not by AM251 (a CB1 receptor antagonist), CID16020046 (a GPR55 antagonist), or naloxone (an opioid receptor antagonist), suggesting the involvement of CB2, 5-HT1A, and TRPV1 receptors in CBD action. In vivo microdialysis indicated that pretreatment with CBD (10-20 mg/kg) attenuated cocaine-induced increases in extracellular dopamine (DA) in the nucleus accumbens, while CBD alone failed to alter extracellular DA. These findings suggest that CBD may have certain therapeutic utility by blunting the acute rewarding effects of cocaine via a DA-dependent mechanism.

TRPV1 Contributes to the Neuroprotective Effect of Dexmedetomidine in Pilocarpine-Induced Status Epilepticus Juvenile Rats

To investigate the antiepileptic and neuroprotective effects of dexmedetomidine (Dex) in pilocarpine- (Pilo-) induced status epilepticus (SE) juvenile rats, rats were randomly assigned to the following six groups (n = 20): normal, normal+Dex, SE, SE+Cap, SE+Dex, and SE+Dex+Cap. The rats were treated with either diazepam (i.p., an antiepileptic drug) or Dex after the onset of SE. The Morris water maze was used to assess rat cognitive behavior. Flow cytometry was used to detect the concentrations of Ca²⁺, mitochondrial membrane potential, and reactive oxygen species. Transmission electron microscopy was performed to evaluate specimens of brain tissue. The levels of caspase 3 and TRPV1 were examined by western blot and immunohistochemistry (IHC). Treatment with Dex significantly decreased the escape latency of the SE rats (P < 0.05). Capsaicin, a TRPV1 agonist, delivery aggravated the performance of SE rats. Pathological changes in SE rat were attenuated by Dex and deteriorated by capsaicin. Swollen mitochondria and abnormal endoplasmic reticulum were found in SE rats and were then aggravated by capsaicin and reversed by Dex. Moreover, our data showed that Dex significantly restrained calcium overload, ROS production, and mitochondrial membrane potential loss, all of which were induced by Pilo and capsaicin (P < 0.05). Dex decreased the apoptotic rate in the Model SE group (P < 0.05) and TRPV1 and caspase 3 expression in the Dex treatment group (P < 0.05). Interestingly, all these effects of Dex were partially counteracted by the TRPV1 agonist, capsaicin (P < 0.05). Our study showed that Dex exerted a neuroprotective effect in Pilo-induced SE rats by inhibiting TRPV1 expression and provided information for therapy to SE patients.

Comparative studies of endocannabinoid modulation of pain

Cannabinoid-based therapies have long been used to treat pain, but there remain questions about their actual mechanisms and efficacy. From an evolutionary perspective, the cannabinoid system would appear to be highly conserved given that the most prevalent endogenous cannabinoid (endocannabinoid) transmitters, 2-arachidonyl glycerol and anandamide, have been found throughout the animal kingdom, at least in the species that have been analysed to date. This review will first examine recent findings regarding the potential conservation across invertebrates and chordates of the enzymes responsible for endocannabinoid synthesis and degradation and the receptors that these transmitters act on. Next, comparisons of how endocannabinoids modulate nociception will be examined for commonalities between vertebrates and invertebrates, with a focus on the medicinal leech Hirudo verbana. Evidence is presented that there are distinct, evolutionarily conserved anti-nociceptive and pro-nociceptive effects. The combined studies across various animal phyla demonstrate the utility of using comparative approaches to understand conserved mechanisms for modulating nociception. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Capsaicin-Induced Impairment of Functional Network Dynamics in Mouse Hippocampus via a TrpV1 Receptor-Independent Pathway: Putative Involvement of Na+/K+-ATPase

The vanilloid compound capsaicin (Cp) is best known to bind to and activate the transient receptor potential vanilloid receptor-1 (TrpV1). A growing number of studies use capsaicin as a tool to study the role of TrpV1 in the central nervous system (CNS). Although most of capsaicin’s CNS effects have been reported to be mediated by TrpV1 activation, evidence exists that capsaicin can also trigger functional changes in hippocampal activity independently of TrpV1. Recently, we have reported that capsaicin induces impairment in hippocampal gamma oscillations via a TrpV1-independent pathway. Here, we dissect the underlying mechanisms of capsaicin-induced alterations to functional network dynamics. We found that capsaicin induces a reduction in action potential (AP) firing rate and a subsequent loss of synchronicity in pyramidal cell (PC) spiking activity in hippocampus. Moreover, capsaicin induces alterations in PC spike-timing since increased first-spike latency was observed after capsaicin treatment. First-spike latency can be regulated by the voltage-dependent potassium current D (I_D) or Na⁺/K⁺-ATPase. Selective inhibition of I_D via low 4-AP concentration and Na⁺/K⁺-ATPase using its blocker ouabain, we found that capsaicin effects on AP spike timing were completely inhibited by ouabain but not with 4-AP. In conclusion, our study shows that capsaicin in a TrpV1-independent manner and possibly involving Na⁺/K⁺-ATPase activity can impair cognition-relevant functional network dynamics such as gamma oscillations and provides important data regarding the use of capsaicin as a tool to study TrpV1 function in the CNS.

Activation of TRPV1 Contributes to Recurrent Febrile Seizures via Inhibiting the Microglial M2 Phenotype in the Immature Brain

Transient receptor potential vanilloid type 1 (TRPV1) is a nonselective cation channel implicated in the nervous system as a key component of several inflammatory diseases. A massive amount of evidence has demonstrated that TRPV1 is extensively expressed in the central nervous system (CNS) and there might be a close relationship between TRPV1 and neuroinflammation, which is a crucial pathogenic factor in seizure generation, although it’s signaling mechanism has been less well characterized. Herein, we identified that TRPV1 is functionally expressed in the primary cultured mouse microglia and the membrane expression of TRPV1 is upregulated in rFS mice brain and specifically in activated microglia. Stimulation of TRPV1 promoted microglia activation and indirectly enhanced seizure susceptibility by inhibiting the neuroprotective effects of microglial transforming growth factor-beta1 (TGF-β1) via interaction with Toll-like receptor 4 (TLR4) in mice. Conversely, genetic deletion of TRPV1 alleviated hyperthermia or LPS-induced abnormal microglial activation and restored a balanced inflammatory microenvironment in the brain. Taken together, these findings show that microglial TRPV1, as a potential pro-inflammatory mediator, and participate in neuroinflammatory response, which will provide a novel therapeutic strategy for controlling the neuroinflammation-induced seizure.

Multifunctional TRPV1 Ion Channels in Physiology and Pathology with Focus on the Brain, Vasculature, and Some Visceral Systems

TRPV1 has been originally cloned as the heat and capsaicin receptor implicated in acute pain signalling, while further research has shifted the focus to its importance in chronic pain caused by inflammation and associated with this TRPV1 sensitization. However, accumulating evidence suggests that, apart from pain signalling, TRPV1 subserves many other unrelated to nociception functions in the nervous system. In the brain, TRPV1 can modulate synaptic transmission via both pre- and postsynaptic mechanisms and there is a functional crosstalk between GABA receptors and TRPV1. Other fundamental processes include TRPV1 role in plasticity, microglia-to-neuron communication, and brain development. Moreover, TRPV1 is widely expressed in the peripheral tissues, including the vasculature, gastrointestinal tract, urinary bladder, epithelial cells, and the cells of the immune system. TRPV1 can be activated by a large array of physical (heat, mechanical stimuli) and chemical factors (e.g., protons, capsaicin, resiniferatoxin, and endogenous ligands, such as endovanilloids). This causes two general cell effects, membrane depolarization and calcium influx, thus triggering depending on the cell-type diverse functional responses ranging from neuronal excitation to secretion and smooth muscle contraction. Here, we review recent research on the diverse TRPV1 functions with focus on the brain, vasculature, and some visceral systems as the basis of our better understanding of TRPV1 role in different human disorders.

TRPV1 Inhibits the Ventilatory Response to Hypoxia in Adult Rats, but Not the CO₂-Drive to Breathe

Receptors of the transient receptor potential (TRP) channels superfamily are expressed in many tissues and have different physiological functions. However, there are few studies investigating the role of these channels in cardiorespiratory control in mammals. We assessed the role of central and peripheral TRPV1 receptors in the cardiorespiratory responses to hypoxia (10% O₂) and hypercapnia (7% CO₂) by measuring pulmonary ventilation (V˙E), heart rate (HR), mean arterial pressure (MAP) and body temperature (Tb) of male Wistar rats before and after intraperitoneal (AMG9810 [2.85 µg/kg, 1 mL/kg]) or intracebroventricular (AMG9810 [2.85 µg/kg, 1 µL] or AMG7905 [28.5 μg/kg, 1 µL]) injections of TRPV1 antagonists. Central or peripheral injection of TRPV1 antagonists did not change cardiorespiratory parameters or Tb during room air and hypercapnic conditions. However, the hypoxic ventilatory response was exaggerated by both central and peripheral injection of AMG9810. In addition, the peripheral antagonist blunted the drop in Tb induced by hypoxia. Therefore, the current data provide evidence that TRPV1 channels exert an inhibitory modulation on the hypoxic drive to breathe and stimulate the Tb reduction during hypoxia.

TrpV1 receptor activation rescues neuronal function and network gamma oscillations from Aβ-induced impairment in mouse hippocampus in vitro

Amyloid-β peptide (Aβ) forms plaques in Alzheimer’s disease (AD) and is responsible for early cognitive deficits in AD patients. Advancing cognitive decline is accompanied by progressive impairment of cognition-relevant EEG patterns such as gamma oscillations. The endocannabinoid anandamide, a TrpV1-receptor agonist, reverses hippocampal damage and memory impairment in rodents and protects neurons from Aβ-induced cytotoxic effects. Here, we investigate a restorative role of TrpV1-receptor activation against Aβ-induced degradation of hippocampal neuron function and gamma oscillations. We found that the TrpV1-receptor agonist capsaicin rescues Aβ-induced degradation of hippocampal gamma oscillations by reversing both the desynchronization of AP firing in CA3 pyramidal cells and the shift in excitatory/inhibitory current balance. This rescue effect is TrpV1-receptor-dependent since it was absent in TrpV1 knockout mice or in the presence of the TrpV1-receptor antagonist capsazepine. Our findings provide novel insight into the network mechanisms underlying cognitive decline in AD and suggest TrpV1 activation as a novel therapeutic target.

Are the persistent effects of "gate control" stimulation on nociception a form of generalization of habituation that is endocannabinoid-dependent?

Repetitive activation of non-nociceptive afferents is known to attenuate nociceptive signaling. However, the functional details of how this modulatory process operates are not understood and this has been a barrier in using such stimuli to effectively treat chronic pain. The present study tests the hypothesis that the ability of repeated non-nociceptive stimuli to reduce nociception is a form of generalized habituation from the non-nociceptive stimulus-response pathway to the nociceptive pathway. Habituation training, using non-nociceptive mechanosensory stimuli, did reduce responses to nociceptive thermal stimulation. This generalization of habituation to nociceptive stimuli required endocannabinoid-mediated neuromodulation, although disrupting of endocannabinoid signaling did not affect "direct" habituation of to the non-nociceptive stimulus. Surprisingly, the reduced response to nociceptive stimuli following habituation training was very long-lasting (3-8 days). This long-term habituation required endocannabinoid signaling during the training/acquisition phase, but endocannabinoids were not required for post-training retention phase. The implications of these results are that applying principles of habituation learning could potentially improve anti-nociceptive therapies utilizing repeated non-nociceptive stimulation such as transcutaneous nerve stimulation (TENS), spinal cord stimulation (SCS), or electro-acupuncture.

Sensory neuronal sensitisation occurs through HMGB-1-RAGE and TRPV1 in high-glucose conditions

Many potential causes for painful diabetic neuropathy have been proposed including actions of cytokines and growth factors. High mobility group protein B1 (HMGB1) is a RAGE (also known as AGER) agonist whose levels are increased in diabetes and that contributes to pain by modulating peripheral inflammatory responses. HMGB1 enhances nociceptive behaviour in naïve animals through an unknown mechanism. We tested the hypothesis that HMGB1 causes pain through direct neuronal activation of RAGE and alteration of nociceptive neuronal responsiveness. HMGB1 and RAGE expression were increased in skin and primary sensory (dorsal root ganglion, DRG) neurons of diabetic rats at times when pain behaviour was enhanced. Agonist-evoked TRPV1-mediated Ca2+ responses increased in cultured DRG neurons from diabetic rats and in neurons from naïve rats exposed to high glucose concentrations. HMGB1-mediated increases in TRPV1-evoked Ca2+ responses in DRG neurons were RAGE- and PKC-dependent, and this was blocked by co-administration of the growth factor splice variant VEGF-A165b. Pain behaviour and the DRG RAGE expression increases were blocked by VEGF-A165b treatment of diabetic rats in vivo Hence, we conclude that HMGB1-RAGE activation sensitises DRG neurons in vitro, and that VEGF-A165b blocks HMGB-1-RAGE DRG activation, which may contribute to its analgesic properties in vivo.

Endocannabinoid-mediated potentiation of nonnociceptive synapses contributes to behavioral sensitization

Endocannabinoids, such as 2-arachidonoyl glycerol (2-AG) and anandamide, can elicit long-term depression of both excitatory and inhibitory synapses. This latter effect will result in disinhibition and would therefore be expected to produce an increase in neural circuit output. However, there have been no examples directly linking endocannabinoid-mediated disinhibition to a change in a functional neurobehavioral circuit. The present study uses the well-characterized central nervous system of the medicinal leech, Hirudo verbana, to examine the functional/behavioral relevance of endocannabinoid modulation of an identified afferent synapse. Bath application of 2-AG potentiates synaptic transmission by pressure-sensitive sensory neurons (P cells) as well as the magnitude of the defensive shortening reflex elicited by P-cell stimulation. This potentiation requires activation of TRPV-like channels. Endocannabinoid/TRPV signaling was found to produce sensitization of the shortening reflex elicited by either direct stimulation of nearby nociceptive afferents (N cells) or noxious stimulation applied to skin several segments away. In both cases, heterosynaptic potentiation of P-cell synapses was observed in parallel with an increase in the magnitude of elicited shortening and both synaptic and behavioral effects were blocked by pharmacological inhibition of 2-AG synthesis or TRPV-like channel activation. Serotonin (5-HT) is known to play a critical role in sensitization in Hirudo and other animals, and the 5-HT2 receptor antagonist ritanserin also blocked behavioral sensitization and the accompanying synaptic potentiation. These findings suggest a novel, endocannabinoid-mediated contribution to behavioral sensitization that may interact with known 5-HT-dependent modulatory processes. NEW & NOTEWORTHY There is considerable interest in the analgesic potential of cannabinoids. However, there is evidence that the cannabinoid system can have both pro- and antinociceptive effects. This study examines how an endogenous cannabinoid transmitter can potentiate nonnociceptive synapses and enhance their capacity to elicit a nocifensive behavioral response.

Involvement of TRPV1 Channels in Energy Homeostasis

The ion channel TRPV1 is involved in a wide range of processes including nociception, thermosensation and, more recently discovered, energy homeostasis. Tightly controlling energy homeostasis is important to maintain a healthy body weight, or to aid in weight loss by expending more energy than energy intake. TRPV1 may be involved in energy homeostasis, both in the control of food intake and energy expenditure. In the periphery, it is possible that TRPV1 can impact on appetite through control of appetite hormone levels or via modulation of gastrointestinal vagal afferent signaling. Further, TRPV1 may increase energy expenditure via heat production. Dietary supplementation with TRPV1 agonists, such as capsaicin, has yielded conflicting results with some studies indicating a reduction in food intake and increase in energy expenditure, and other studies indicating the converse. Nonetheless, it is increasingly apparent that TRPV1 may be dysregulated in obesity and contributing to the development of this disease. The mechanisms behind this dysregulation are currently unknown but interactions with other systems, such as the endocannabinoid systems, could be altered and therefore play a role in this dysregulation. Further, TRPV1 channels appear to be involved in pancreatic insulin secretion. Therefore, given its plausible involvement in regulation of energy and glucose homeostasis and its dysregulation in obesity, TRPV1 may be a target for weight loss therapy and diabetes. However, further research is required too fully elucidate TRPV1s role in these processes. The review provides an overview of current knowledge in this field and potential areas for development.

A putative lysophosphatidylinositol receptor GPR55 modulates hippocampal synaptic plasticity

GPR55, an orphan G-protein coupled receptor, is activated by lysophosphatidylinositol (LPI) and the endocannabinoid anandamide, as well as by other compounds including THC. LPI is a potent endogenous ligand of GPR55 and neither GPR55 nor LPIs' functions in the brain are well understood. While endocannabinoids are well known to modulate brain synaptic plasticity, the potential role LPI could have on brain plasticity has never been demonstrated. Therefore, we examined not only GPR55 expression, but also the role its endogenous ligand could play in long-term potentiation, a common form of synaptic plasticity. Using quantitative RT-PCR, electrophysiology, and behavioral assays, we examined hippocampal GPR55 expression and function. qRT-PCR results indicate that GPR55 is expressed in hippocampi of both rats and mice. Immunohistochemistry and single cell PCR demonstrates GPR55 protein in pyramidal cells of CA1 and CA3 layers in the hippocampus. Application of the GPR55 endogenous agonist LPI to hippocampal slices of GPR55^+/+ mice significantly enhanced CA1 LTP. This effect was absent in GPR55^-/- mice, and blocked by the GPR55 antagonist CID 16020046. We also examined paired-pulse ratios of GPR55^-/- and GPR55^+/+ mice with or without LPI and noted significant enhancement in paired-pulse ratios by LPI in GPR55^+/+ mice. Behaviorally, GPR55^-/- and GPR55^+/+ mice did not differ in memory tasks including novel object recognition, radial arm maze, or Morris water maze. However, performance on radial arm maze and elevated plus maze task suggests GPR55^-/- mice have a higher frequency of immobile behavior. This is the first demonstration of LPI involvement in hippocampal synaptic plasticity.

TRPV1 channels are critical brain inflammation detectors and neuropathic pain biomarkers in mice

The capsaicin receptor TRPV1 has been widely characterized in the sensory system as a key component of pain and inflammation. A large amount of evidence shows that TRPV1 is also functional in the brain although its role is still debated. Here we report that TRPV1 is highly expressed in microglial cells rather than neurons of the anterior cingulate cortex and other brain areas. We found that stimulation of microglial TRPV1 controls cortical microglia activation per se and indirectly enhances glutamatergic transmission in neurons by promoting extracellular microglial microvesicles shedding. Conversely, in the cortex of mice suffering from neuropathic pain, TRPV1 is also present in neurons affecting their intrinsic electrical properties and synaptic strength. Altogether, these findings identify brain TRPV1 as potential detector of harmful stimuli and a key player of microglia to neuron communication.

Comparative biology of pain: What invertebrates can tell us about how nociception works

The inability to adequately treat chronic pain is a worldwide health care crisis. Pain has both an emotional and a sensory component, and this latter component, nociception, refers specifically to the detection of damaging or potentially damaging stimuli. Nociception represents a critical interaction between an animal and its environment and exhibits considerable evolutionary conservation across species. Using comparative approaches to understand the basic biology of nociception could promote the development of novel therapeutic strategies to treat pain, and studies of nociception in invertebrates can provide especially useful insights toward this goal. Both vertebrates and invertebrates exhibit segregated sensory pathways for nociceptive and nonnociceptive information, injury-induced sensitization to nociceptive and nonnociceptive stimuli, and even similar antinociceptive modulatory processes. In a number of invertebrate species, the central nervous system is understood in considerable detail, and it is often possible to record from and/or manipulate single identifiable neurons through either molecular genetic or physiological approaches. Invertebrates also provide an opportunity to study nociception in an ethologically relevant context that can provide novel insights into the nature of how injury-inducing stimuli produce persistent changes in behavior. Despite these advantages, invertebrates have been underutilized in nociception research. In this review, findings from invertebrate nociception studies are summarized, and proposals for how research using invertebrates can address questions about the fundamental mechanisms of nociception are presented.

Effects of a TRPV1 agonist capsaicin on respiratory rhythm generation in brainstem-spinal cord preparation from newborn rats

The heat-sensitive transient receptor potential vanilloid 1 (TRPV1) channels are expressed in the peripheral and central nervous systems. However, there is no report on how the activation of TRPV1 causes the modulation of neuronal activity in the medullary respiratory center. We examined effects of capsaicin, a specific agonist of TRPV1 channels, on respiratory rhythm generation in brainstem-spinal cord preparation from newborn rats. Capsaicin induced a biphasic response in the respiratory rhythm (a transient decrease followed by an increase in the C4 rate). The second-phase excitatory effect (but not the initial inhibitory effect) in the biphasic response was partly blocked by capsazepine or AMG9810 (TRPV1 antagonists). Capsaicin caused strong desensitization. After its washout, the strength of C4 burst inspiratory activity was augmented once per four to five respiratory cycles. The preinspiratory and inspiratory neurons showed tonic firings due to membrane depolarization during the initial inhibitory phase. In the presence of TTX, capsaicin increased the fluctuation of the membrane potential of the CO₂-sensitive preinspiratory neurons in the parafacial respiratory group (pFRG), accompanied by slight depolarization. The C4 inspiratory activity did not stop, even 60-90 min after the application of 50/100 μM capsaicin. Voltage-sensitive dye imaging demonstrated that the spatiotemporal pattern of the respiratory rhythm generating networks after application of capsaicin (50 μM, 70-90 min) was highly similar to the control. A histochemical analysis using TRPV1 channel protein antibodies and mRNA demonstrated that the TRPV1 channel-positive cells were widely distributed in the reticular formation of the medulla, including the pFRG. Our results showed that the application of capsaicin in the medulla has various influences on the respiratory center: transient inhibitory and subsequent excitatory effects on the respiratory rhythm and periodical augmentation of the inspiratory burst pattern. The effects of capsaicin were partially blocked by TRPV1 antagonists but could be also induced at least partially via the non-specific action. Our results also suggested a minor contribution of the TRPV1 channels to central chemoreception.

The pharmacology of palmitoylethanolamide and first data on the therapeutic efficacy of some of its new formulations

Palmitoylethanolamide (PEA) has emerged as a potential nutraceutical, because this compound is naturally produced in many plant and animal food sources, as well as in cells and tissues of mammals, and endowed with important neuroprotective, anti-inflammatory and analgesic actions. Several efforts have been made to identify the molecular mechanism of action of PEA and explain its multiple effects both in the central and the peripheral nervous system. Here, we provide an overview of the pharmacology, efficacy and safety of PEA in neurodegenerative disorders, pain perception and inflammatory diseases. The current knowledge of new formulations of PEA with smaller particle size (i.e. micronized and ultra-micronized) when given alone or in combination with antioxidant flavonoids (i.e. luteolin) and stilbenes (i.e. polydatin) is also reviewed.

LINKED ARTICLES

This article is part of a themed section on Principles of Pharmacological Research of Nutraceuticals. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.11/issuetoc.

Differential Activation of TRP Channels in the Adult Rat Spinal Substantia Gelatinosa by Stereoisomers of Plant-Derived Chemicals

Activation of TRPV1, TRPA1 or TRPM8 channel expressed in the central terminal of dorsal root ganglion (DRG) neuron increases the spontaneous release of l-glutamate onto spinal dorsal horn lamina II (substantia gelatinosa; SG) neurons which play a pivotal role in regulating nociceptive transmission. The TRP channels are activated by various plant-derived chemicals. Although stereoisomers activate or modulate ion channels in a distinct manner, this phenomenon is not fully addressed for TRP channels. By applying the whole-cell patch-clamp technique to SG neurons of adult rat spinal cord slices, we found out that all of plant-derived chemicals, carvacrol, thymol, carvone and cineole, increase the frequency of spontaneous excitatory postsynaptic current, a measure of the spontaneous release of l-glutamate from nerve terminals, by activating TRP channels. The presynaptic activities were different between stereoisomers (carvacrol and thymol; (-)-carvone and (+)-carvone; 1,8-cineole and 1,4-cineole) in the extent or the types of TRP channels activated, indicating that TRP channels in the SG are activated by stereoisomers in a distinct manner. This result could serve to know the properties of the central terminal TRP channels that are targets of drugs for alleviating pain.

Psychiatric Disorders and TRP Channels: Focus on Psychotropic Drugs

Psychiatric and neurological disorders are mostly associated with the changes in neural calcium ion signaling pathways required for activity-triggered cellular events. One calcium channel family is the TRP cation channel family, which contains seven subfamilies. Results of recent papers have discovered that calcium ion influx through TRP channels is important. We discuss the latest advances in calcium ion influx through TRP channels in the etiology of psychiatric disorders. Activation of TRPC4, TRPC5, and TRPV1 cation channels in the etiology of psychiatric disorders such as anxiety, fear-associated responses, and depression modulate calcium ion influx. Evidence substantiates that anandamide and its analog (methanandamide) induce an anxiolytic-like effect via CB1 receptors and TRPV1 channels. Intracellular calcium influx induced by oxidative stress has an significant role in the etiology of bipolar disorders (BDs), and studies recently reported the important role of TRP channels such as TRPC3, TRPM2, and TRPV1 in converting oxidant or nitrogen radical signaling to cytosolic calcium ion homeostasis in BDs. The TRPV1 channel also plays a function in morphine tolerance and hyperalgesia. Among psychotropic drugs, amitriptyline and capsazepine seem to have protective effects on psychiatric disorders via the TRP channels. Some drugs such as cocaine and methamphetamine also seem to have an important role in alcohol addiction and substance abuse via activation of the TRPV1 channel. Thus, we explore the relationships between the etiology of psychiatric disorders and TRP channel-regulated mechanisms. Investigation of the TRP channels in psychiatric disorders holds the promise of the development of new drug treatments.

Differences in chloride gradients allow for three distinct types of synaptic modulation by endocannabinoids

Endocannabinoids can elicit persistent depression of excitatory and inhibitory synapses, reducing or enhancing (disinhibiting) neural circuit output, respectively. In this study, we examined whether differences in Cl(-) gradients can regulate which synapses undergo endocannabinoid-mediated synaptic depression vs. disinhibition using the well-characterized central nervous system (CNS) of the medicinal leech, Hirudo verbana Exogenous application of endocannabinoids or capsaicin elicits potentiation of pressure (P) cell synapses and depression of both polymodal (Npoly) and mechanical (Nmech) nociceptive synapses. In P synapses, blocking Cl(-) export prevented endocannabinoid-mediated potentiation, consistent with a disinhibition process that has been indicated by previous experiments. In Nmech neurons, which are depolarized by GABA due to an elevated Cl(-) equilibrium potentials (ECl), endocannabinoid-mediated depression was prevented by blocking Cl(-) import, indicating that this decrease in synaptic signaling was due to depression of excitatory GABAergic input (disexcitation). Npoly neurons are also depolarized by GABA, but endocannabinoids elicit depression in these synapses directly and were only weakly affected by disruption of Cl(-) import. Consequently, the primary role of elevated ECl may be to protect Npoly synapses from disinhibition. All forms of endocannabinoid-mediated plasticity required activation of transient potential receptor vanilloid (TRPV) channels. Endocannabinoid/TRPV-dependent synaptic plasticity could also be elicited by distinct patterns of afferent stimulation with low-frequency stimulation (LFS) eliciting endocannabinoid-mediated depression of Npoly synapses and high-frequency stimulus (HFS) eliciting endocannabinoid-mediated potentiation of P synapses and depression of Nmech synapses. These findings demonstrate a critical role of differences in Cl(-) gradients between neurons in determining the sign, potentiation vs. depression, of synaptic modulation under normal physiological conditions.

Ventral tegmental area dopamine and GABA neurons: Physiological properties and expression of mRNA for endocannabinoid biosynthetic elements

The ventral tegmental area (VTA) is involved in adaptive reward and motivation processing and is composed of dopamine (DA) and GABA neurons. Defining the elements regulating activity and synaptic plasticity of these cells is critical to understanding mechanisms of reward and addiction. While endocannabinoids (eCBs) that potentially contribute to addiction are known to be involved in synaptic plasticity mechanisms in the VTA, where they are produced is poorly understood. In this study, DA and GABAergic cells were identified using electrophysiology, cellular markers, and a transgenic mouse model that specifically labels GABA cells. Using single-cell RT-qPCR and immunohistochemistry, we investigated mRNA and proteins involved in eCB signaling such as diacylglycerol lipase α, N-acyl-phosphatidylethanolamine-specific phospholipase D, and 12-lipoxygenase, as well as type I metabotropic glutamate receptors (mGluRs). Our results demonstrate the first molecular evidence of colocalization of eCB biosynthetic enzyme and type I mGluR mRNA in VTA neurons. Further, these data reveal higher expression of mGluR1 in DA neurons, suggesting potential differences in eCB synthesis between DA and GABA neurons. These data collectively suggest that VTA GABAergic and DAergic cells have the potential to produce various eCBs implicated in altering neuronal activity or plasticity in adaptive motivational reward or addiction.

Current status and future directions of botulinum neurotoxins for targeting pain processing

Current evidence suggests that botulinum neurotoxins (BoNTs) A1 and B1, given locally into peripheral tissues such as skin, muscles, and joints, alter nociceptive processing otherwise initiated by inflammation or nerve injury in animal models and humans. Recent data indicate that such locally delivered BoNTs exert not only local action on sensory afferent terminals but undergo transport to central afferent cell bodies (dorsal root ganglia) and spinal dorsal horn terminals, where they cleave SNAREs and block transmitter release. Increasing evidence supports the possibility of a trans-synaptic movement to alter postsynaptic function in neuronal and possibly non-neuronal (glial) cells. The vast majority of these studies have been conducted on BoNT/A1 and BoNT/B1, the only two pharmaceutically developed variants. However, now over 40 different subtypes of botulinum neurotoxins (BoNTs) have been identified. By combining our existing and rapidly growing understanding of BoNT/A1 and /B1 in altering nociceptive processing with explorations of the specific characteristics of the various toxins from this family, we may be able to discover or design novel, effective, and long-lasting pain therapeutics. This review will focus on our current understanding of the molecular mechanisms whereby BoNTs alter pain processing, and future directions in the development of these agents as pain therapeutics.

Hyaluronan modulates TRPV1 channel opening, reducing peripheral nociceptor activity and pain

Hyaluronan (HA) is present in the extracellular matrix of all body tissues, including synovial fluid in joints, in which it behaves as a filter that buffers transmission of mechanical forces to nociceptor nerve endings thereby reducing pain. Using recombinant systems, mouse-cultured dorsal root ganglia (DRG) neurons and in vivo experiments, we found that HA also modulates polymodal transient receptor potential vanilloid subtype 1 (TRPV1) channels. HA diminishes heat, pH and capsaicin (CAP) responses, thus reducing the opening probability of the channel by stabilizing its closed state. Accordingly, in DRG neurons, HA decreases TRPV1-mediated impulse firing and channel sensitization by bradykinin. Moreover, subcutaneous HA injection in mice reduces heat and capsaicin nocifensive responses, whereas the intra-articular injection of HA in rats decreases capsaicin joint nociceptor fibres discharge. Collectively, these results indicate that extracellular HA reduces the excitability of the ubiquitous TRPV1 channel, thereby lowering impulse activity in the peripheral nociceptor endings underlying pain.

Hyaluronan is a major component of the extracellular matrix, and is used to treat joint pain in osteoarthritis. In this study, Caires et al. show hyaluronan achieves its analgesic effects by targeting TRPV1 and stabilising the channel in its closed state.