Transient receptor potential vanilloid 1 channels control acetylcholine/2-arachidonoylglicerol coupling in the striatum

The neurotransmitter acetylcholine (Ach) controls both excitatory and inhibitory synaptic transmission in the striatum. Here, we investigated the involvement of the endocannabinoid system in Ach-mediated inhibition of striatal GABA transmission, and the potential role of transient receptor potential vanilloid 1 (TRPV1) channels in the control of Ach-endocannabinoid coupling. We found that inhibition of Ach degradation and direct pharmacological stimulation of muscarinic M1 receptors reduced striatal inhibitory postsynaptic currents (IPSCs) through the stimulation of 2-arachidonoylglicerol (2AG) synthesis and the activation of cannabinoid CB1 receptors. The effects of M1 receptor activation on IPSCs were occlusive with those of metabotropic glutamate receptor 5 stimulation, and were prevented in the presence of capsaicin, agonist of TRPV1 channels. Elevation of anandamide (AEA) tone with URB597, a blocker of fatty acid amide hydrolase, mimicked the effects of capsaicin, indicating that endogenous AEA acts as an endovanilloid substance in the control of M1-dependent 2AG-mediated synaptic effects in the striatum. Accordingly, both capsaicin and URB597 effects were absent in mice lacking TRPV1 channels. Pharmacological interventions targeting AEA metabolism and TRPV1 channels might be considered alternative therapeutic routes in disorders of striatal cholinergic or endocannabinoid neurotransmission.     

Persistent synaptic activity produces long-lasting enhancement of endocannabinoid modulation and alters long-term synaptic plasticity

Learning and memory are thought to involve activity-dependent changes in synaptic efficacy such as long-term potentiation (LTP) and long-term depression (LTD). Recent studies have indicated that endocannabinoid-dependent modulation of inhibitory transmission facilitates induction of hippocampal LTP and that endocannabinoids play a key role in certain forms of LTD. Here, we show that repetitive low-frequency synaptic stimulation (LFS) produces persistent up-regulation of endocannabinoid signaling at hippocampal CA1 GABAergic synapses. This LFS also produces LTD of inhibitory synapses and facilitates LTP at excitatory, glutamatergic synapses. These endocannabinoid-mediated plastic changes could contribute to information storage within the brain.     

TRPV1 activation by endogenous anandamide triggers postsynaptic long-term depression in dentate gyrus

The transient receptor potential TRPV1 is a nonselective cation channel that mediates pain sensations and is commonly activated by a wide variety of exogenous and endogenous, physical and chemical stimuli. Although TRPV1 receptors are mainly found in nociceptive neurons of the peripheral nervous system, these receptors have also been found in the brain, where their role is far less understood. Activation of TRPV1 reportedly regulates neurotransmitter release at several central synapses. However, we found that TRPV1 suppressed excitatory transmission in rat and mouse dentate gyrus by regulating postsynaptic function in an input-specific manner. This suppression was a result of Ca(2+)-calcineurin and clathrin-dependent internalization of AMPA receptors. Moreover, synaptic activation of TRPV1 triggered a form of long-term depression (TRPV1-LTD) mediated by the endocannabinoid anandamide in a type 1 cannabinoid receptor-independent manner. Thus, our findings reveal a previously unknown form of endocannabinoid- and TRPV1-mediated regulation of synaptic strength at central synapses.     

Presynaptic CB1 receptors regulate synaptic plasticity at cerebellar parallel fiber synapses

Endocannabinoids are potent regulators of synaptic strength. They are generally thought to modify neurotransmitter release through retrograde activation of presynaptic type 1 cannabinoid receptors (CB1Rs). In the cerebellar cortex, CB1Rs regulate several forms of synaptic plasticity at synapses onto Purkinje cells, including presynaptically expressed short-term plasticity and, somewhat paradoxically, a postsynaptic form of long-term depression (LTD). Here we have generated mice in which CB1Rs were selectively eliminated from cerebellar granule cells, whose axons form parallel fibers. We find that in these mice, endocannabinoid-dependent short-term plasticity is eliminated at parallel fiber, but not inhibitory interneuron, synapses onto Purkinje cells. Further, parallel fiber LTD is not observed in these mice, indicating that presynaptic CB1Rs regulate long-term plasticity at this synapse.     

Postsynaptic endocannabinoid release is critical to long-term depression in the striatum

The striatum functions critically in movement control and habit formation. The development and function of cortical input to the striatum are thought to be regulated by activity-dependent plasticity of corticostriatal glutamatergic synapses. Here we show that the induction of a form of striatal synaptic plasticity, long-term depression (LTD), is dependent on activation of the CB1 cannabinoid receptor. LTD was facilitated by blocking cellular endocannabinoid uptake, and postsynaptic loading of anandamide (AEA) produced presynaptic depression. The endocannabinoid necessary for striatal LTD is thus likely to be released postsynaptically as a retrograde messenger. These findings demonstrate a new role for endocannabinoids in the induction of long-term synaptic plasticity in a circuit necessary for habit formation and motor control.     

Endocannabinoid- and mGluR5-dependent short-term synaptic depression in an isolated neuron/bouton preparation from the hippocampal CA1 region

Endocannabinoids released from the postsynaptic neuronal membrane can activate presynaptic CB1 receptors and inhibit neurotransmitter release. In hippocampal slices, depolarization of the CA1 pyramidal neurons elicits an endocannabinoid-mediated inhibition of gamma-aminobutyric acid release known as depolarization-induced suppression of inhibition (DSI). Using the highly reduced neuron/synaptic bouton preparation from the CA1 region of hippocampus, we have begun to examine endocannabinoid-dependent short-term depression (STD) of inhibitory synaptic transmission under well-controlled physiological and pharmacological conditions in an environment free of other cells. Application of the CB1 synthetic agonist WIN55212-2 and endogenous cannabinoids 2-AG and anandamide produced a decrease in spontaneous inhibitory postsynaptic current (sIPSC) frequency and amplitude, indicating the presence of CB1 receptors at synapses in this preparation. Endocannabinoid-dependent STD is different from DSI found in hippocampal slices and the neuron/bouton preparation from basolateral amygdala (BLA) since depolarization alone was not sufficient to induce suppression of sIPSCs. However, concurrent application of the metabotropic glutamate receptor (mGluR) agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) and postsynaptic depolarization resulted in a transient (30-50 s) decrease in sIPSC frequency and amplitude. Application of DHPG alone had no effect on sIPSCs. The depolarization/DHPG-induced STD was blocked by the CB1 antagonist SR141716A and the mGluR5 antagonist MPEP and was sensitive to intracellular calcium concentration. Comparing the present findings with earlier work in hippocampal slices and BLA, it appears that endocannabinoid release is less robust in isolated hippocampal neurons.     

N-Arachidonyl dopamine sensitizes rat capsaicin-sensitive lung vagal afferents via activation of TRPV1 receptors

We investigated the effect of N-arachidonyl dopamine (NADA), an endogenous agonist of both transient receptor potential vanilloid 1 (TRPV1) and cannabinoid CB1 receptors, on the sensitivity of rat capsaicin-sensitive lung vagal afferent (CSLVA) fibers. In artificially ventilated rats, an intravenous infusion of NADA (400 μg/kg/ml, 0.5 ml/min for 2 min) mildly elevated the baseline CSLVA fiber activity, whereas it markedly potentiated CSLVA fiber responses to a right atrial injection of capsaicin or adenosine, and to lung inflation. The potentiating effect on CSLVA fiber sensitivity to an adenosine injection or lung inflation was blocked by capsazepine pretreatment (a TRPV1 receptor antagonist), but was unaffected by AM251 pretreatment (a CB1 receptor antagonist). In spontaneously breathing rats, a NADA infusion similarly potentiated the CSLVA fiber-mediated apneic response evoked by an adenosine injection, and this potentiating effect was also prevented by capsazepine pretreatment. We concluded that NADA at the dose tested non-specifically increases CSLVA fiber sensitivity to chemical and mechanical stimulation via activation of TRPV1 receptors.     

Effects of anandamide and noxious heat on intracellular calcium concentration in nociceptive drg neurons of rats

As an endogenous agonist at the cannabinoid receptor CB1 and the capsaicin-receptor TRPV1, anandamide may exert both anti- and pronociceptive actions. Therefore we studied the effects of anandamide and other activators of both receptors on changes in free cytosolic calcium ([Ca(2+)](i)) in acutely dissociated small dorsal root ganglion neurons (diameter: < or =30 microm). Anandamide (10 microM) increased [Ca(2+)](i) in 76% of the neurons. The EC(50) was 7.41 microM, the Hill slope was 2.15 +/- 0.43 (mean +/- SE). This increase was blocked by the competitive TRPV1-antagonist capsazepine (10 microM) and in Ca(2+)-free extracellular solution. Neither exclusion of voltage-gated sodium channels nor additional blockade of voltage-gated calcium channels of the L-, N-, and/or T-type, significantly reduced the anandamide-induced [Ca(2+)](i) increase or capsaicin-induced [Ca(2+)](i) transients (0.2 microM). The CB1-agonist HU210 (10 microM) inhibited the anandamide-induced rise in [Ca(2+)](i). Conversely, the CB1-antagonist AM251 (3 microM) induced a leftward shift of the concentration-response relationship by approximately 4 microM (P < 0.001; Hill slope, 2.17 +/- 0.75). Intracellular calcium transients in response to noxious heat (47 degrees C for 10 s) were highly correlated with the anandamide-induced [Ca(2+)](i) increases (r = 0.84, P < 0.001). Heat-induced [Ca(2+)](i) transients were facilitated by preincubation with subthreshold concentrations of anandamide (3 microM), an effect that was further enhanced by 3 microM AM251. Although anandamide acts on both TRPV1 and CB1 receptors in the same nociceptive DRG neurons, its pronociceptive effects dominate. Anandamide triggers an influx of calcium through TRPV1 but no intracellular store depletion. It facilitates the heat responsiveness of TRPV1 in a calcium-independent manner. These effects of anandamide differ from those of the classical exogenous TRPV1-agonist capsaicin and suggest a primarily modulatory mode of action of anandamide.     

Calcineurin is required for TRPV1-induced long-term depression of hippocampal interneurons

Transient receptor potential vanilloid 1 (TRPV1) mediates a novel form of presynaptic long-term depression (LTD) in hippocampal interneurons. To date, while TRPV1 is currently being heavily studied in the PNS for its anti-nociceptive and anti-inflammatory properties, much less is known regarding TRPV1 signaling and function in the CNS, including the mechanism mediating hippocampal interneuron LTD. Here we performed whole-cell voltage clamp electrophysiology experiments on CA1 hippocampal interneurons from Sprague-Dawley male rats to identify this signaling mechanism. Because calcineurin is linked to multiple synaptic plasticity types, we investigated whether TRPV1 activates presynaptic calcineurin, which in turn induces LTD. To do so we employed calcineurin inhibitors cyclosporin A or FK-506. To determine the location of the calcineurin involved we either bath applied calcineurin antagonists, blocking calcineurin activity ubiquitously in the slice, presynaptically and postsynaptically, or applied antagonists to the internal solution to block calcineurin postsynaptically. Both calcineurin antagonists applied to the bath blocked TRPV1-dependent LTD, indicating calcineurin involvement in LTD. Because calcineurin antagonist applied to the internal solution did not block TRPV1-LTD, it suggests presynaptic calcineurin is required for LTD. Finally, because high frequency stimulus used to induce LTD could potentially activate receptors besides TRPV1, we confirmed that bath, but not intracellularly applied cyclosporin A, also blocked TRPV1 agonist-induced depression of CA1 interneurons. In conclusion, these data illustrate that presynaptic calcineurin activity is required for both TRPV1-induced LTD and TRPV1 agonist-induced depression. This finding is the first to demonstrate the TRPV1-induced signaling mechanism in CA1 hippocampus.     

Effect of resiniferatoxin on glutamatergic spontaneous excitatory synaptic transmission in substantia gelatinosa neurons of the adult rat spinal cord

The transient receptor potential (TRP) vanilloid type 1 (TRPV1) agonist, capsaicin, enhances glutamatergic spontaneous excitatory synaptic transmission in CNS neurons. Resiniferatoxin (RTX) has a much higher affinity for TRPV1 than capsaicin, but its ability to modulate excitatory transmission is unclear. We examined the effect of RTX on excitatory transmission using the whole-cell patch-clamp technique in substantia gelatinosa (SG) neurons of adult rat spinal cord slices. Bath-applied RTX dose-dependently increased the frequency, but not the amplitude, of spontaneous excitatory postsynaptic current (sEPSC), independent of its application time. In about a half of the neurons tested, this effect was accompanied by an inward current at −70 mV that was sensitive to glutamate-receptor antagonists. Repeated application of RTX did not affect excitatory transmission. RTX was more potent than capsaicin but showed similar efficacy. RTX activity could be blocked by capsazepine or SB-366791, a TRPV1 antagonist, but not tetrodotoxin, a Na⁺-channel blocker, and could be inhibited by pretreatment with capsaicin but not the TRPA1 agonist, allyl isothiocyanate. RTX enhances the spontaneous release of l-glutamate from nerve terminals with similar efficacy as capsaicin and produces a membrane depolarization by activating TRPV1 in the SG, with fast desensitization and slow recovery from desensitization. These results indicate a mechanism by which RTX can modulate excitatory transmission in SG neurons to regulate nociceptive transmission.     

Opposing effects of cannabinoids and vanilloids on evoked quantal release at the frog neuromuscular junction

Cannabinoids and vanilloids are two distinct groups of substances that share some pharmacological targets. Here we report that two cannabinoid type 1 receptor (CB1) agonists, WIN 55212-2 (WIN) and arachidonyl-2′-chloroethylamide (ACEA) have opposing effects on evoked quantal acetylcholine release – WIN decreased quantal content while ACEA increased quantal content. The decrease in quantal content by WIN was blocked by the CB1 antagonist AM 251. The increase in quantal content by ACEA was not blocked by AM 251, indicating it acts through a receptor other than CB1. As ACEA is also an agonist for the vanilloid receptor (TRPV1) we tested the effect of vanilloids on quantal content. Similar to ACEA, the vanilloid agonist capsaicin increased quantal content, and this effect was blocked by capsazepine, a TRPV1 antagonist. Capsazepine also blocked the increase in quantal content by ACEA. Together these data show an inhibitory effect of CB1 activation on evoked acetylcholine release and the first evidence for the presence of a vanilloid receptor at the neuromuscular junction.     

Functional type I vanilloid receptor expression by substantia gelatinosa neurons of trigeminal subnucleus caudalis in mice

The aim of this study was to investigate the existence of functional TRPV1 receptor by substantia gelatinosa (SG) neurons of the trigeminal subnucleus caudalis (Vc), which is implicated in the processing of nociceptive information from orofacial regions. The direct membrane effects of a TRPV1 receptor agonist, capsaicin, were examined by gramicidin-perforated patch clamp recording using a trigeminal brainstem slice preparation containing Vc from immature mice. Capsaicin (1–2 μM) induced a membrane depolarization in 58 out of 71 SG neurons tested (82%). Capsaicin-induced depolarization was maintained in 20 out of 32 (63%) SG neurons in the presence of amino acid and voltage-dependent sodium channel blockers (10 μM CNQX, 20 μM AP-5, 0.5 μM TTX, 50 μM picrotoxin and 2 μM strychnine). In addition, capsaicin-induced depolarization was maintained in the presence of L-732,138 (1 μM), an NK1 receptor antagonist, in 14 out of 17 neurons (82%) tested. The capsaicin-induced depolarizing effects were blocked by a TRPV1 receptor antagonist, capsazepine (10 μM). These results indicate that a sub-population of SG neurons in the Vc express functional TRPV1 receptors, and that capsaicin can directly activate the TRPV1 receptor on the postsynaptic membrane of SG neurons.     

Polymodal activation of the endocannabinoid system in the extended amygdala

The reason why neurons synthesize more than one endocannabinoid (eCB) and how this is involved in the regulation of synaptic plasticity in a single neuron is not known. We found that 2-arachidonoylglycerol (2-AG) and anandamide mediate different forms of plasticity in the extended amygdala of rats. Dendritic L-type Ca(2+) channels and the subsequent release of 2-AG acting on presynaptic CB1 receptors triggered retrograde short-term depression. Long-term depression was mediated by postsynaptic mGluR5-dependent release of anandamide acting on postsynaptic TRPV1 receptors. In contrast, 2-AG/CB1R-mediated retrograde signaling mediated both forms of plasticity in the striatum. These data illustrate how the eCB system can function as a polymodal signal integrator to allow the diversification of synaptic plasticity in a single neuron.     

Postsynaptic TRPV1 triggers cell type-specific long-term depression in the nucleus accumbens

Synaptic modifications in the nucleus accumbens (NAc) are important for adaptive and pathological reward-dependent learning. Medium spiny neurons (MSNs), the major cell type in the NAc, participate in two parallel circuits that subserve distinct behavioral functions, yet little is known about differences in their electrophysiological and synaptic properties. Using bacterial artificial chromosome transgenic mice, we found that synaptic activation of group I metabotropic glutamate receptors in NAc MSNs in the indirect, but not direct, pathway led to the production of endocannabinoids, which activated presynaptic CB1 receptors to trigger endocannabinoid-mediated long-term depression (eCB-LTD) as well as postsynaptic transient receptor potential vanilloid 1 (TRPV1) channels to trigger a form of LTD resulting from endocytosis of AMPA receptors. These results reveal a previously unknown action of TRPV1 channels and indicate that the postsynaptic generation of endocannabinoids can modulate synaptic strength in a cell type-specific fashion by activating distinct pre- and postsynaptic targets.     

Rapid redistribution of glutamate receptors contributes to long-term depression in hippocampal cultures.

Synaptic strength can be altered by a variety of pre- or postsynaptic modifications. Here we test the hypothesis that long-term depression (LTD) involves a decrease in the number of glutamate receptors that are clustered at individual synapses in primary cultures of hippocampal neurons. Similar to a prominent form of LTD observed in hippocampal slices, LTD in hippocampal cultures required NMDA receptor activation and was accompanied by a decrease in the amplitude and frequency of miniature excitatory postsynaptic currents. Immunocytochemical analysis revealed that induction of LTD caused a concurrent decrease in the number of AMPA receptors clustered at synapses but had no effect on synaptic NMDA receptor clusters. These results suggest that a subtype-specific redistribution of synaptic glutamate receptors contributes to NMDA receptor-dependent LTD.     

Epileptiform activity in rat hippocampus strengthens excitatory synapses

Although epileptic seizures are characterized by excessive excitation, the role of excitatory synaptic transmission in the induction and expression of epilepsy remains unclear. Here, we show that epileptiform activity strengthens excitatory hippocampal synapses by increasing the number of functional (RS)-α-amino-3hydroxy-5methyl-4-isoxadepropionate (AMPA)-type glutamate receptors in CA3–CA1 synapses. This form of synaptic strengthening occludes long-term potentiation (LTP) and enhances long-term depression (LTD), processes involved in learning and memory. These changes in synaptic transmission and plasticity, which are fully blocked with N -methyl-D-aspartate (NMDA) receptor antagonists, may underlie epilepsy induction and seizure-associated memory deficits.     

Endocannabinoid-mediated short-term suppression of excitatory synaptic transmission to medium spiny neurons in the striatum

Medium spiny neurons in the dorsal striatum receive glutamatergic excitatory synaptic inputs from the cerebral cortex. These synapses undergo long-term depression that requires release of endocannabinoids from medium spiny neurons and activation of cannabinoid CB1 receptors. However, it remains unclear how cortico-striatal synapses exhibit endocannabinoid-mediated short-term suppression, which has been found in various brain regions including the hippocampus and cerebellum. Endocannabinoids are released from postsynaptic neurons by strong depolarization and resultant Ca2+ elevation or activation of postsynaptic Gq/11-coupled receptors such as group I metabotropic glutamate receptors (mGluRs) and M1/M3 muscarinic acetylcholine receptors. Moreover, endocannabioids are effectively released when weak depolarization is combined with Gq/11-coupled receptor activation. We found that muscarinic activation induced transient suppression of excitatory synaptic transmission to medium spiny neurons, which was independent of retrograde endocannabinoid signaling but was mediated directly by presynaptic muscarinic receptors. Neither postsynaptic depolarization alone nor depolarization and muscarinic activation caused suppression of cortico-striatal synapses. In contrast, activation of group I mGluRs readily suppressed cortico-striatal excitatory synaptic transmission. Furthermore, postsynaptic depolarization induced clear suppression when combined with group I mGluR activation. These results indicate that group I mGluRs but not muscarinic receptors contribute to endocannabinoid-mediated short-term suppression of cortico-striatal excitatory synaptic transmission.     

Voltage-controlled plasticity at GluR2-deficient synapses onto hippocampal interneurons

High-frequency stimulation of pyramidal cell inputs to developing (P9-12) hippocampal stratum radiatum interneurons expressing GluR2-lacking, Ca(2+)-permeable AMPA receptors produces long-term depression of synaptic transmission, if N-methyl-d-aspartate (NMDA) receptors are blocked. Here we show that these same synapses display a remarkably versatile signal integration if postsynaptic NMDA receptors are activated. At synapses expressing GluR2-deficient AMPA receptors, tetanic stimulation that activates NMDA receptors triggered long-term potentiation or depression (LTP or LTD) depending on membrane potential. LTP was elicited at most synapses when the interneuron was held at -30 mV during the stimulus train but was typically prevented by postsynaptic hyperpolarization to -70 mV, by strong depolarization to 0 mV, by d-2-amino-5-phosphonovaleric acid, or by postsynaptic injection of the Ca2+ chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid. At synapses with predominantly GluR2-containing AMPA receptors, repetitive stimulation did not change synaptic strength regardless of whether NMDA receptors were activated. The interactions among GluR2 expression, NMDA receptor expression, and membrane potential thus confer on hippocampal interneurons a distinctive means for differential decoding of high-frequency inputs, resulting in enhanced or depressed transmission depending on the functional state of the interneuron.     

Modification of AMPA receptor clustering regulates cerebellar synaptic plasticity

Cerebellar long-term depression (LTD) induced at parallel fiber-Purkinje neuron synapses is proposed to underlie certain types of motor learning. alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors, which mediate chemical transmission in these synapses, are clustered on the postsynaptic membrane. By increasing local density of the receptors, clustering is believed to increase synaptic efficacy. This article focuses on molecular mechanisms regulating the synaptic AMPA receptor clustering in Purkinje cells, which could underlie the expression of cerebellar LTD. Synaptic AMPA receptor clusters in dendritic spines of Purkinje cells are disrupted upon protein kinase C (PKC)-mediated phosphorylation of serine 880 in the C-terminal domain of GluR2. Phosphorylation of this residue causes significant reduction in the affinity of GluR2 C-terminal tail for glutamate receptor interacting protein (GRIP), a molecule known to be crucial for AMPA receptor clustering. Consequently, AMPA receptors on the synaptic membrane are destabilized and internalized by endocytosis. Based on these findings, a model for the expression of cerebellar LTD is proposed, in which a decrease in the number of postsynaptic AMPA receptors, initiated by phosphorylation of GluR2 serine 880, is the major mechanism underlying cerebellar LTD.     

Cloning and pharmacological characterization of mouse TRPV1

The Transient Receptor Potential cation channel V1 (TRPV1) is expressed in peripheral nociceptive neurons and is subject to polymodal activation via various agents including capsaicin, noxious heat, low extracellular pH, and direct phosphorylation by protein kinase C (PKC). We have cloned and heterologously expressed mouse TRPV1 (mTRPV1) and characterized its function utilizing FLIPR-based calcium imaging to measure functional responses to various small molecule agonists, low pH and direct phosphorylation via PKC. The various TRPV1 agonists activated mTRPV1 with a rank order of agonist potency of (resiniferatoxin (RTX) = arvanil > capsaicin = olvanil > OLDA > PPAHV) (EC50 values of 0.15+/-0.04 nM, 0.27+/-0.07 nM, 9.1+/-1.2 nM, 3.7+/-0.3 nM, 258+/-105 nM, and 667+/-151 nM, respectively). Additionally, mTRPV1 was activated by either low pH or with addition of the PKC activator phorbol 12-myristate 13-acetate (PMA). The TRPV1 antagonists iodinated-resiniferatoxin (I-RTX) or BCTC were both able to block capsaicin, pH and PKC-induced responses of mTRPV1 (IC50 (I-RTX) = 0.35+/-0.12 nM, 1.9+/-0.7 nM, and 0.80+/-0.68 nM, IC50 (BCTC) = 1.3+/-0.36 nM, 0.59+/-0.16 nM, and 0.37+/-0.15 nM, respectively). However, the antagonist capsazepine was only able to inhibit a capsaicin-evoked response of mTRPV1 with an IC50 of 1426+/-316 nM. Comparable results were achieved with rat TRPV1, while capsazepine blocked all modes of human TRPV1 activation. Thus, the mTRPV1 cation channel has a molecular pharmacological profile more akin to rat TRPV1 than either human or guinea pig TRPV1 and the molecular pharmacology suggests that capsazepine may be an ineffective TRPV1 antagonist for in vivo models of inflammatory pain in the mouse.