CB1 Cannabinoid Receptor Stimulation Modulates Transient Receptor Potential Vanilloid Receptor 1 Activities in Calcium Influx and Substance P Release in Cultured Rat Dorsal Root Ganglion Cells

The cannabinoid agonist CB-13 produces peripherally mediated analgesia in mice but elicits tolerance and signs of central nervous system activity with repeated dosing

ABSTRACT

Activation of cannabinoid receptor type 1 (CB 1 ) produces analgesia in a variety of preclinical models of pain; however, engagement of central CB 1 receptors is accompanied by unwanted side effects, such as psychoactivity, tolerance, and dependence. Therefore, some efforts to develop novel analgesics have focused on targeting peripheral CB 1 receptors to circumvent central CB 1 -related side effects. In the present study, we evaluated the effects of acute and repeated dosing with the peripherally selective CB 1 -preferring agonist CB-13 on nociception and central CB 1 -related phenotypes in a model of inflammatory pain in mice. We also evaluated cellular mechanisms underlying CB-13-induced antinociception in vitro using cultured mouse dorsal root ganglion neurons. CB-13 reduced inflammation-induced mechanical allodynia in male and female mice in a peripheral CB 1 -receptor-dependent manner and relieved inflammatory thermal hyperalgesia. In cultured mouse dorsal root ganglion neurons, CB-13 reduced TRPV1 sensitization and neuronal hyperexcitability induced by the inflammatory mediator prostaglandin E 2 , providing potential mechanistic explanations for the analgesic actions of peripheral CB 1 receptor activation. With acute dosing, phenotypes associated with central CB 1 receptor activation occurred only at a dose of CB-13 approximately 10-fold the ED 50 for reducing allodynia. Strikingly, repeated dosing resulted in both analgesic tolerance and CB 1 receptor dependence, even at a dose that did not produce central CB 1 -receptor-mediated phenotypes on acute dosing. This suggests that repeated CB-13 dosing leads to increased CNS exposure and unwanted engagement of central CB 1 receptors. Thus, caution is warranted regarding therapeutic use of CB-13 with the goal of avoiding CNS side effects. Nonetheless, the clear analgesic effect of acute peripheral CB 1 receptor activation suggests that peripherally restricted cannabinoids are a viable target for novel analgesic development.

The interactive effects of verapamil and CB1 cannabinoid receptor antagonist/inverse agonist, AM251 on passive avoidance learning and memory in rat

There are reports regarding the effects of intracellular Ca2+ and synthesis and release of endocannabinoids. The secretion of endocannabinoids depends on the L-type calcium channel. The present study evaluated the involvement of the cannabinoid CB1 receptors in the effect of L-type calcium channel blocker verapamil on passive avoidance learning (PAL) in adult male rats. In this study, we examined the effects of an acute administration of the cannabinoid CB1 receptors antagonist/inverse agonist AM251 following a chronic administration of the Ca2+ channel blocker verapamil on PAL. Male Wistar rats were administered verapamil (10, 25 and 50 mg/kg) or saline intraperitoneally (i.p) daily for 13 days (n = 10/group). After this treatment period, a learning test (acquisition) was performed, and a retrieval test was performed the following day. The results indicated that chronic systemic administration of verapamil (in a dose-dependent manner) impaired memory acquisition and retrieval. Pre-training acute administration of a selective CB1 antagonist/inverse agonist, AM251 (5 mg/kg, i.p.) did not change memory acquisition and retrieval. Co-administration of the verapamil and AM251 significantly reversed verapamil-induced amnesia, suggesting a functional interaction between AM251 and verapamil. The results indicated the interactive effects of cannabinoid CB1 receptors and L-type calcium channel in passive avoidance learning and AM251 can counter the effects of verapamil on memory.

Topical application of cannabinoid-ligands ameliorates experimental dry-eye disease

PURPOSE

Dry eye disease (DED) is a multifactorial disease, with limitations regarding efficacy and tolerability of applied substances. Among several candidates, the endocannabinoid system with its receptors (CB1R and CB2R) were reported to modulate inflammation, wound healing and pain, which are also core DED pathomechanisms. This study is to investigate the therapeutic responses of Δ-9 tetrahydrocannabinol (a non-selective agonist) and two selective antagonists, SR141716A (CB1R antagonist) and SR144528 (CB2R antagonist), as a topical application using a DED mouse model.

METHOD

Experimental DED was induced in naïve C57BL/6 mice. Expression of CBR at the ocular surface of naïve and DED mice was determined by qPCR and in-situ hybridization. Either THC or CBR antagonists were compounded in an aqueous solution and dosed during the induction of DED. Tear production, cornea sensitivity, and cornea fluorescence staining were tested. At the end of each experiment, corneas were stained with β3-tubulin for analysis of corneal nerve morphology. Conjunctiva was analyzed for CD4⁺ and CD8⁺ infiltration.

RESULTS

CB1R and CB2R are present at the ocular surface, and desiccating stress increased CBR expressions (p < 0.05). After 10 days of DED induction, treated groups demonstrated a reduced CBR expression in the cornea, which was concurrent with improvements in the DED phenotype including fluorescence staining & inflammation. Applying THC protected corneal nerve morphology, thus maintained corneal sensitivity and reduced CD4⁺ T-cell infiltration. The CB1R antagonist maintained cornea sensitivity without changing nerve morphology.

CONCLUSIONS

Endocannabinoid receptor modulation presents a potential multi-functional therapeutic approach for DED.

On location for cannabinoid control of multimodal behavior

In this issue of Neuron, Soria-Gomez et al. (2021) investigate whether activation of the type 1 cannabinoid receptor at specific subcellular locations within a single neural circuit produces multimodal behavior. Their results demonstrate that location matters: striatonigral mitochondrial CB1 drives catalepsy while striatonigral plasma membrane CB1 receptors enable antinociception.

Mechanisms of Cannabinoids and Potential Applicability to Skin Diseases

The legalisation of cannabis in a growing number of jurisdictions has led to increasing interest in its potential therapeutic effects in a range of disorders, including cutaneous conditions. Cannabinoids have been used as natural medicines for centuries; however, their biological activity in the skin is a new area of study. Recent data suggest that cannabinoids are involved in neuro-immuno-endocrine modulation of skin functioning, yet their effect on the features of dermatologic conditions is unclear. This article sought to review the mechanisms by which cannabinoids regulate skin functioning through the lens of relevance to treatment of dermatologic diseases looking at the effects of cannabinoids on a range of cellular activities and dermatologic conditions both in vitro and in vivo. We identified studies demonstrating an inhibitory effect of cannabinoids on skin inflammation, proliferation, fibrosis, pain, and itch-biological mechanisms involved in the pathogenesis of many dermatologic conditions. Cannabinoids have the potential to expand the therapeutic repertoire of a wide spectrum of skin disorders. Given their widespread unregulated use by the general public, basic and clinical studies are required to elucidate the effectiveness and long-term effects of topical and systemic cannabinoids in cutaneous disorders.

Molecular mechanisms underlying the actions of arachidonic acid-derived prostaglandins on peripheral nociception

Arachidonic acid-derived prostaglandins not only contribute to the development of inflammation as intercellular pro-inflammatory mediators, but also promote the excitability of the peripheral somatosensory system, contributing to pain exacerbation. Peripheral tissues undergo many forms of diseases that are frequently accompanied by inflammation. The somatosensory nerves innervating the inflamed areas experience heightened excitability and generate and transmit pain signals. Extensive studies have been carried out to elucidate how prostaglandins play their roles for such signaling at the cellular and molecular levels. Here, we briefly summarize the roles of arachidonic acid-derived prostaglandins, focusing on four prostaglandins and one thromboxane, particularly in terms of their actions on afferent nociceptors. We discuss the biosynthesis of the prostaglandins, their specific action sites, the pathological alteration of the expression levels of related proteins, the neuronal outcomes of receptor stimulation, their correlation with behavioral nociception, and the pharmacological efficacy of their regulators. This overview will help to a better understanding of the pathological roles that prostaglandins play in the somatosensory system and to a finding of critical molecular contributors to normalizing pain.

Δ9-THC and related cannabinoids suppress substance P- induced neurokinin NK1-receptor-mediated vomiting via activation of cannabinoid CB1 receptor

Δ⁹-THC suppresses cisplatin-induced vomiting through activation of cannabinoid CB₁ receptors. Cisplatin-evoked emesis is predominantly due to release of serotonin and substance P (SP) in the gut and the brainstem which subsequently stimulate their corresponding 5-HT₃-and neurokinin NK₁-receptors to induce vomiting. Δ⁹-THC can inhibit vomiting caused either by the serotonin precursor 5-HTP, or the 5-HT₃ receptor selective agonist, 2-methyserotonin. In the current study, we explored whether Δ⁹-THC and related CB₁/CB₂ receptor agonists (WIN55,212-2 and CP55,940) inhibit vomiting evoked by SP (50 mg/kg, i.p.) or the NK₁ receptor selective agonist GR73632 (5 mg/kg, i.p.). Behavioral methods were employed to determine the antiemetic efficacy of cannabinoids in least shrews. Our results showed that administration of varying doses of Δ⁹-THC (i.p. or s.c.), WIN55,212-2 (i.p.), or CP55,940 (i.p.) caused significant suppression of SP-evoked vomiting in a dose-dependent manner. When tested against GR73632, Δ⁹-THC also dose-dependently reduced the evoked emesis. The antiemetic effect of Δ⁹-THC against SP-induced vomiting was prevented by low non-emetic doses of the CB₁ receptor inverse-agonist/antagonist SR141716A (<10 mg/kg). We also found that the NK₁ receptor antagonist netupitant can significantly suppress vomiting caused by a large emetic dose of SR141716A (20 mg/kg). In sum, Δ⁹-THC and related cannabinoids suppress vomiting evoked by the nonselective (SP) and selective (GR73632) neurokinin NK₁ receptor agonists via stimulation of cannabinoid CB₁ receptors.

Opportunities for cannabis in supportive care in cancer

Cannabis has the potential to modulate some of the most common and debilitating symptoms of cancer and its treatments, including nausea and vomiting, loss of appetite, and pain. However, the dearth of scientific evidence for the effectiveness of cannabis in treating these symptoms in patients with cancer poses a challenge to clinicians in discussing this option with their patients. A review was performed using keywords related to cannabis and important symptoms of cancer and its treatments. Literature was qualitatively reviewed from preclinical models to clinical trials in the fields of cancer, human immunodeficiency virus (HIV), multiple sclerosis, inflammatory bowel disease, post-traumatic stress disorder (PTSD), and others, to prudently inform the use of cannabis in supportive and palliative care in cancer. There is a reasonable amount of evidence to consider cannabis for nausea and vomiting, loss of appetite, and pain as a supplement to first-line treatments. There is promising evidence to treat chemotherapy-induced peripheral neuropathy, gastrointestinal distress, and sleep disorders, but the literature is thus far too limited to recommend cannabis for these symptoms. Scant, yet more controversial, evidence exists in regard to cannabis for cancer- and treatment-related cognitive impairment, anxiety, depression, and fatigue. Adverse effects of cannabis are documented but tend to be mild. Cannabis has multifaceted potential bioactive benefits that appear to outweigh its risks in many situations. Further research is required to elucidate its mechanisms of action and efficacy and to optimize cannabis preparations and doses for specific populations affected by cancer.

Some Prospective Alternatives for Treating Pain: The Endocannabinoid System and Its Putative Receptors GPR18 and GPR55

Background: Marijuana extracts (cannabinoids) have been used for several millennia for pain treatment. Regarding the site of action, cannabinoids are highly promiscuous molecules, but only two cannabinoid receptors (CB₁ and CB₂) have been deeply studied and classified. Thus, therapeutic actions, side effects and pharmacological targets for cannabinoids have been explained based on the pharmacology of cannabinoid CB₁/CB₂ receptors. However, the accumulation of confusing and sometimes contradictory results suggests the existence of other cannabinoid receptors. Different orphan proteins (e.g., GPR18, GPR55, GPR119, etc.) have been proposed as putative cannabinoid receptors. According to their expression, GPR18 and GPR55 could be involved in sensory transmission and pain integration.

Methods: This article reviews select relevant information about the potential role of GPR18 and GPR55 in the pathophysiology of pain.

Results: This work summarized novel data supporting that, besides cannabinoid CB₁ and CB₂ receptors, GPR18 and GPR55 may be useful for pain treatment.

Conclusion: There is evidence to support an antinociceptive role for GPR18 and GPR55.

Depolarizing Effectors of Bradykinin Signaling in Nociceptor Excitation in Pain Perception

Inflammation is one of the main causes of pathologic pain. Knowledge of the molecular links between inflammatory signals and pain-mediating neuronal signals is essential for understanding the mechanisms behind pain exacerbation. Some inflammatory mediators directly modulate the excitability of pain-mediating neurons by contacting the receptor molecules expressed in those neurons. For decades, many discoveries have accumulated regarding intraneuronal signals from receptor activation through electrical depolarization for bradykinin, a major inflammatory mediator that is able to both excite and sensitize pain-mediating nociceptor neurons. Here, we focus on the final effectors of depolarization, the neuronal ion channels, whose functionalities are specifically affected by bradykinin stimulation. Particular G-protein coupled signaling cascades specialized for each specific depolarizer ion channels are summarized. Some of these ion channels not only serve as downstream effectors but also play critical roles in relaying specific pain modalities such as thermal or mechanical pain. Accordingly, specific pain phenotypes altered by bradykinin stimulation are also discussed. Some members of the effector ion channels are both activated and sensitized by bradykinin-induced neuronal signaling, while others only sensitized or inhibited, which are also introduced. The present overview of the effect of bradykinin on nociceptor neuronal excitability at the molecular level may contribute to better understanding of an important aspect of inflammatory pain and help future design of further research on the components involved and pain modulating strategies.

The interactive role of CB1 receptors and L-type calcium channels in hippocampal long-term potentiation in rats

Long-term potentiation (LTP) of synaptic responses is a widely researched model of synaptic plasticity that occurs during learning and memory. The cannabinoid system is an endogenous system that modulate this kind of synaptic plasticity. In addition, voltage dependent calcium channels is essential for induction of LTP at some synapses in the hippocampus. However, there is currently debate over the interaction between L-type calcium channels and cannabinoid system on the synaptic plasticity. In this study, we examined the effects of an acute administration of the cannabinoid antagonist AM251 following a chronic administration of the Ca²⁺ channel blocker verapamil on LTP induction in the hippocampal dentate gyrus(DG) of rats. Male Wistar rats were administered verapamil(10,25,50mg/kg) or saline intraperitoneally(IP) daily for 13days(n=10/group). After this treatment period, animals were anesthetized with an IP injection of urethane; the recording and stimulating electrodes were positioned in the DG and the perforant pathway. After obtaining a steady state baseline response, a single IP injection of saline or AM251(1 or 5mg/kg) was administered. LTP was induced by high-frequency stimulation(HFS). The population spike(PS) amplitude and the slope of excitatory postsynaptic potentials(EPSP) were compared between the experimental groups. The acute administration of the CB1 antagonist AM251 increased LTP induction. The EPSP slopes and PS amplitude in the verapamil and AM251 groups differed after HFS, such that AM251 increased LTP, whereas verapamil decreased LTP induction. These findings suggest that there are functional interactions between the L-type calcium channels and cannabinoid system in this model of synaptic plasticity in the hippocampus.

Interaction Between the Cannabinoid and Vanilloid Systems on Anxiety in Male Rats

Introduction:

Previous studies have shown that the cannabinoid system is involved in anxiety. In addition, transient receptor potential vanilloid type-1 (TRPV1) channels are new targets for the development of anxiolytics. The present study investigated the possible interaction between the cannabinoid and vanilloid systems on anxiety-like behavior in rats.

Methods:

Four different groups of male Wistar rats received intraperitoneal (IP) injections of (1) vehicle (DMSO+saline), (2) cannabinoid receptor agonist WIN55212-2 (WIN) (1 mg/kg), (3) TRPV1 receptor antagonist capsazepine (CPZ) (5 mg/kg), or (4) combined WIN (1 mg/kg) and CPZ (5 mg/kg) treatment 30 minutes before testing in the elevated plus maze.

Results:

The results showed that compared to the control (vehicle), both WIN and CPZ increased the time spent and number of entries on the open arms. Co-administration of WIN and CPZ had a synergistic effect, i.e., the number of entries and time spent on the open arms was greater than that in the groups administered the two compounds alone. The total distance travelled by rats and total number of entries on to the arms did not significantly differ between groups.

Conclusion:

Acute neuropharmacological blockade of the TRPV1 receptor or stimulation of the CB1 receptor produced an anxiolytic effect. It seems that antagonism of the vanilloid system modulates cannabinoid gain that rises the anxiolytic effect. TRPV1 antagonism may amend generation of endocannabinoids, which in turn increases anxiolytic impact. These results suggest that two systems could act on or share a common signaling pathway affecting the expression of anxiety.

Tramadol and its metabolite m1 selectively suppress transient receptor potential ankyrin 1 activity, but not transient receptor potential vanilloid 1 activity

BACKGROUND

The transient receptor potential vanilloid 1 (TRPV1) and the transient receptor potential ankyrin 1 (TRPA1), which are expressed in sensory neurons, are polymodal nonselective cation channels that sense noxious stimuli. Recent reports showed that these channels play important roles in inflammatory, neuropathic, or cancer pain, suggesting that they may serve as attractive analgesic pharmacological targets. Tramadol is an effective analgesic that is widely used in clinical practice. Reportedly, tramadol and its metabolite (M1) bind to μ-opioid receptors and/or inhibit reuptake of monoamines in the central nervous system, resulting in the activation of the descending inhibitory system. However, the fundamental mechanisms of tramadol in pain control remain unclear. TRPV1 and TRPA1 may be targets of tramadol; however, they have not been studied extensively.

METHODS

We examined whether and how tramadol and M1 act on human embryonic kidney 293 (HEK293) cells expressing human TRPV1 (hTRPV1) or hTRPA1 by using a Ca imaging assay and whole-cell patch-clamp recording.

RESULTS

Tramadol and M1 (0.01-10 μM) alone did not increase in intracellular Ca concentration ([Ca]i) in HEK293 cells expressing hTRPV1 or hTRPA1 compared with capsaicin (a TRPV1 agonist) or the allyl isothiocyanate (AITC, a TRPA1 agonist), respectively. Furthermore, in HEK293 cells expressing hTRPV1, pretreatment with tramadol or M1 for 5 minutes did not change the increase in [Ca]i induced by capsaicin. Conversely, pretreatment with tramadol (0.1-10 μM) and M1 (1-10 μM) significantly suppressed the AITC-induced [Ca]i increases in HEK293 cells expressing hTRPA1. In addition, the patch-clamp study showed that pretreatment with tramadol and M1 (10 μM) decreased the inward currents induced by AITC.

CONCLUSIONS

These data indicate that tramadol and M1 selectively inhibit the function of hTRPA1, but not that of hTRPV1, and that hTRPA1 may play a role in the analgesic effects of these compounds.

Spinal cord injury induced neuropathic pain: Molecular targets and therapeutic approaches

Neuropathic pain, especially that resulting from spinal cord injury, is a tremendous clinical challenge. A myriad of biological changes have been implicated in producing these pain states including cellular interactions, extracellular proteins, ion channel expression, and epigenetic influences. Physiological consequences of these changes are varied and include functional deficits and pain responses. Developing therapies that effectively address the cause of these symptoms require a deeper knowledge of alterations in the molecular pathways. Matrix metalloproteinases and tissue inhibitors of metalloproteinases are two promising therapeutic targets. Matrix metalloproteinases interact with and influence many of the studied pain pathways. Gene expression of ion channels and inflammatory mediators clearly contributes to neuropathic pain. Localized and time dependent targeting of these proteins could alleviate and even prevent neuropathic pain from developing. Current therapeutic options for neuropathic pain are limited primarily to analgesics targeting the opioid pathway. Therapies directed at molecular targets are highly desirable and in early stages of development. These include transplantation of exogenously engineered cell populations and targeted gene manipulation. This review describes specific molecular targets amenable to therapeutic intervention using currently available delivery systems.

The interactive role of cannabinoid and vanilloid systems in hippocampal synaptic plasticity in rats

Long-term potentiation (LTP) has been most thoroughly studied in the hippocampus, which has a key role in learning and memory. Endocannabinoids are one of the endogenous systems that modulate this kind of synaptic plasticity. The activation of the vanillioid system has also been shown to mediate synaptic plasticity in the hippocampus. In addition, immunohistochemical studies have shown that cannabinoid receptor type 1 (CB1) and vanilloid receptor 1 (TRPV1) are closely located in the hippocampus. In this study, we examined the hippocampal effects of co-administrating WIN55-212-2 and capsaicin, which are CB1 and TRPV1 agonists, respectively, on the induction of LTP in the dentate gyrus (DG) of rats. LTP in the hippocampal area was induced by high-frequency stimulation (HFS). Our results indicated that the cannabinoid agonist reduced both field excitatory post-synaptic potential (fEPSP) slope and population spike (PS) amplitude after HFS with respect to the control group, whereas the vanilloid agonist increased these parameters along with the increased induction of LTP as compared to the control group. We also showed that the co-administration of cannabinoid and vanilloid agonists had different effects on fEPSP slope and PS amplitude. It seems that agonists of the vanilloid system modulate cannabinoid outputs that cause an increase in synaptic plastisity, while in contemporary consumption of two agonist, TRPV1 agonist can change production of endocannabinoid, which in turn result to enhancement of LTP induction. These findings suggest that the two systems may interact or share certain common signaling pathways in the hippocampus.

Presynaptic TRPV1 vanilloid receptor function is age- but not CB1 cannabinoid receptor-dependent in the rodent forebrain

Neocortical and striatal TRPV1 (vanilloid or capsaicin) receptors (TRPV1Rs) are excitatory ligand-gated ion channels, and are implicated in psychiatric disorders. However, the purported presynaptic neuromodulator role of TRPV1Rs in glutamatergic, serotonergic or dopaminergic terminals of the rodent forebrain remains little understood. With the help of patch-clamp electrophysiology and neurochemical approaches, we mapped the age-dependence of presynaptic TRPV1R function, and furthermore, we aimed at exploring whether the presence of CB1 cannabinoid receptors (CB1Rs) influences the function of the TRPV1Rs, as both receptor types share endogenous ligands. We found that the major factor which affects presynaptic TRPV1R function is age: by post-natal day 13, the amplitude of capsaicin-induced release of dopamine and glutamate is halved in the rat striatum, and two weeks later, capsaicin already loses its effect. However, TRPV1R receptor function is not enhanced by chemical or genetic ablation of the CB1Rs in dopaminergic, glutamatergic and serotonergic terminals of the mouse brain. Altogether, our data indicate a possible neurodevelopmental role for presynaptic TRPV1Rs in the rodent brain, but we found no cross-talk between TRPV1Rs and CB1Rs in the same nerve terminal.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Cannabinoid receptor antagonists AM251 and AM630 activate TRPA1 in sensory neurons

Cannabinoid receptor antagonists have been utilized extensively in vivo as well as in vitro, but their selectivity has not been fully examined. We investigated activation of sensory neurons by two cannabinoid antagonists - AM251 and AM630. AM251 and AM630 activated trigeminal (TG) sensory neurons in a concentration-dependent fashion (threshold 1 μM). AM251 and AM630 responses are mediated by the TRPA1 channel in a majority (90-95%) of small-to-medium TG sensory neurons. AM630 (1-100 μM), but not AM251, was a significantly more potent agonist in cells co-expressing both TRPA1 and TRPV1 channels. We next evaluated AM630 and AM251 effects on TRPV1- and TRPA1-mediated responses in TG neurons. Capsaicin (CAP) effects were inhibited by pre-treatment with AM630, but not AM251. Mustard oil (MO) and WIN55,212-2 (WIN) TRPA1 mediated responses were also inhibited by pre-treatment with AM630, but not AM251 (25 uM each). Co-treatment of neurons with WIN and either AM630 or AM251 had opposite effects: AM630 sensitized WIN responses, whereas AM251 inhibited WIN responses. WIN-induced inhibition of CAP responses in sensory neurons was reversed by AM630 pre-treatment and AM251 co-treatment (25 μM each), as these conditions inhibit WIN responses. Hindpaw injections of AM630 and AM251 did not produce nocifensive behaviors. However, both compounds modulated CAP-induced thermal hyperalgesia in wild-type mice and rats, but not TRPA1 null-mutant mice. AMs also partially regulate WIN inhibition of CAP-induced thermal hyperalgesia in a TRPA1-dependent fashion. In summary, these findings demonstrate alternative targets for the cannabinoid antagonists, AM251 and AM630, in peripheral antihyperalgesia which involve certain TRP channels.

A peripherally restricted cannabinoid receptor agonist produces robust anti-nociceptive effects in rodent models of inflammatory and neuropathic pain

Cannabinoids are analgesic in man, but their use is limited by their psychoactive properties. One way to avoid cannabinoid receptor subtype 1 (CB1R)-mediated central side-effects is to develop CB1R agonists with limited CNS penetration. Activation of peripheral CB1Rs has been proposed to be analgesic, but the relative contribution of peripheral CB1Rs to the analgesic effects of systemic cannabinoids remains unclear. Here we addressed this by exploring the analgesic properties and site of action of AZ11713908, a peripherally restricted CB1R agonist, in rodent pain models. Systemic administration of AZ11713908 produced robust efficacy in rat pain models, comparable to that produced by WIN 55, 212-2, a CNS-penetrant, mixed CB1R and CB2R agonist, but AZ11713908 generated fewer CNS side-effects than WIN 55, 212-in a rat Irwin test. Since AZ11713908 is also a CB2R inverse agonist in rat and a partial CB2R agonist in mouse, we tested the specificity of the effects in CB1R and CB2R knock-out (KO) mice. Analgesic effects produced by AZ11713908 in wild-type mice with Freund's complete adjuvant-induced inflammation of the tail were completely absent in CB1R KO mice, but fully preserved in CB2R KO mice. An in vivo electrophysiological assay showed that the major site of action of AZ11713908 was peripheral. Similarly, intraplantar AZ11713908 was also sufficient to induce robust analgesia. These results demonstrate that systemic administration of AZ11713908, produced robust analgesia in rodent pain models via peripheral CB1R. Peripherally restricted CB1R agonists provide an interesting novel approach to analgesic therapy for chronic pain.

The endogenous cannabinoid anandamide inhibits transient receptor potential vanilloid type 1 receptor-mediated currents in rat cultured primary sensory neurons

The activity of the transient receptor potential vanilloid type 1 ion channel (TRPV1) that is expressed by the great majority of polymodal nociceptors is pivotal for the development of inflammatory heat hyperalgesia. The responsiveness of TRPV1 is regulated by a series of intracellular signalling molecules including the cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA); increased or decreased PKA activity results in TRPV1 sensitisation or desensitisation, respectively. Activation of the cannabinoid 1 (CB1) receptor that is expressed by the majority of the TRPV1-expressing primary sensory neurons reduces PKA activity. Therefore, here we studied whether activation of the CB1 receptor resulted in reduced TRPV1-mediated responses in cultured rat primary sensory neurons. We found that TRPV1-mediated whole-cell currents were significantly reduced respectively, by 50% and 25% by 10 nM and 30 nM of the endogenous CB1 receptor agonist, anandamide. The PKA inhibitor, H89 (10 microM) also had a significant inhibitory effect on TRPV1-mediated currents ( approximately 70%). These findings suggest that activation of the CB1 receptor can reduce the activity of TRPV1 in primary sensory neurons, and that this inhibitory effect could be mediated through the reduction of PKA-mediated phosphorylation of TRPV1.

Cannabinoid CB(1) receptor activation stimulates neurite outgrowth and inhibits capsaicin-induced Ca(2+) influx in an in vitro model of diabetic neuropathy

Cannabinoid CB(1) receptors mediate, in part, the neuroprotectant properties of endocannabinoids, and altered signalling via the CB(1) receptor may contribute to the pathogenesis of diabetic neuropathy. We investigated CB(1) receptor function in PC12 cells differentiated into a neuronal phenotype with nerve growth factor (NGF, 50 ng/ml) in 5.5 and 50 mM concentrations of glucose. High glucose was associated with impaired NGF-induced neurite outgrowth (P < 0.01; n = 185-218) and reduced expression of CB(1) receptor mRNA (P < 0.01; n = 6) on day 6 of culture. Whilst treatment of hyperglycemic cells with HU210 (0.03-3 microM) increased neurite length in a concentration-dependent manner (P < 0.01; n = 136-218), CB(1) receptor expression was not significantly altered by chronic agonist stimulation (P = 0.32; n = 6 per group). Application of the CB(1) agonist HU210 (1 microM) inhibited capsaicin-induced calcium transients to a similar degree in cells cultured in high glucose (40%) versus normal (43%) (P < 0.05; n = 33-50). HU210-mediated rescue of neurite outgrowth and inhibition of calcium influx was blocked by the selective CB(1) antagonist AM251 (1 microM), but not by the selective CB(2) antagonist AM630 (1 microM), confirming the role of CB(1) receptors. High glucose treatment did not significantly elevate endocannabinoid levels. These results suggest that high glucose concentrations are associated with decreased expression, but preserved function of CB(1) receptors in nerve cells.

Sustained morphine treatment augments capsaicin-evoked calcitonin gene-related peptide release from primary sensory neurons in a protein kinase A- and Raf-1-dependent manner

Studies have shown that long-term (5alpha,6alpha)-7,8-didehydro-4,5-epoxy-17-methylmorphinan-3,6-diol (morphine) treatment increases the sensitivity to painful heat stimuli (thermal hyperalgesia). The cellular adaptations contributing to sustained morphine-mediated pain sensitization are not fully understood. It was shown previously (J Neurosci 22:6747-6755, 2002) that sustained morphine exposure augments pain neurotransmitter [such as calcitonin gene-related peptide (CGRP)] release in the dorsal horn of the spinal cord in response to the heat-sensing transient receptor potential vanilloid 1 receptor agonist 8-methyl-N-vanillyl-6-nonenamide (capsaicin). In the present study, we demonstrate that sustained morphine-mediated augmentation of CGRP release from isolated primary sensory dorsal root ganglion neurons is dependent on protein kinase A and Raf-1 kinase. Our data indicate that, in addition to neural system adaptations, sustained opioid agonist treatment also produces intracellular compensatory adaptations in primary sensory neurons, leading to augmentation of evoked pain neurotransmitter release from these cells.

Sensitization of capsaicin-sensitive lung vagal afferents by anandamide in rats: role of transient receptor potential vanilloid 1 receptors

Anandamide (AEA), an arachidonic acid derivative produced during inflammatory conditions, is an endogenous agonist of both transient receptor potential vanilloid 1 (TRPV1) receptors and cannabinoid CB1 receptors. Sensitization of capsaicin-sensitive lung vagal afferent (CSLVA) fibers by chemical mediators is important in the pathogenesis of hyperreactive airway diseases. We investigated the effect of the intravenous infusion of AEA (2 mg·kg−1·ml−1, 0.5 ml/min for 2 min) on the sensitivity of CSLVA fibers to chemical and mechanical stimulation in anesthetized rats. In artificially ventilated rats, AEA infusion only mildly elevated the baseline activity of CSLVA fibers. However, CSLVA fiber responses to right atrial injection of capsaicin, AEA, or adenosine and to lung inflation (tracheal pressure = 30 cmH2O) were all markedly potentiated during AEA infusion, which reverted 20 min after termination of the infusion. The potentiating effect on the sensitivity of CSLVA fibers to adenosine injection or lung inflation was completely blocked by pretreatment with capsazepine (a TRPV1 receptor antagonist) but was unaffected by pretreatment with AM281 (a CB1 receptor antagonist). In spontaneously breathing rats, right atrial injection of adenosine evoked an apneic response that is presumably mediated through CSLVA fibers. Similarly, the adenosine-evoked apneic response was potentiated during AEA infusion, and this potentiating effect was also completely prevented by pretreatment with capsazepine. These results suggest that AEA infusion at the dose tested produces a mild activation of TRPV1 receptors and this nonspecifically increases CSLVA fiber sensitivity to chemical and mechanical stimulation.

Release of arachidonic acid by 2-arachidonoyl glycerol and HU210 in PC12 cells; roles of Src, phospholipase C and cytosolic phospholipase A(2)alpha

The phospholipase A(2) (PLA(2))-prostanoid cascade is involved in cannabinoid receptor-mediated neuronal functions. We investigated the signaling mechanism for the release of arachidonic acid by cannabinoids, 2-arachidonoyl glycerol (2-AG) and HU210, in rat PC12 cells and in primary cultured cells from the mouse cerebellum. The effect of selective inhibitors for signaling pathways and/or enzymes (alpha type cytosolic PLA(2) (cPLA(2)alpha), G protein, Src kinases, phospholipase C, protein kinase C) was assessed. Methods included translocation of the chimeric protein GFP-cPLA(2)alpha, the activities of Src family kinases, Ca(2+)-dependent fluorescence and cyclic AMP accumulation. Treatment with 2-AG and HU210 at greater concentrations than 3 muM caused the release of arachidonic acid, and the response was inhibited by AM251 (an antagonist of cannabinoid CB(1) receptor) and by pyrrophenone (a selective inhibitor of cPLA(2)alpha) in PC12 cells. The cannabinoid treatment caused the intracellular translocation of cPLA(2)alpha and an increase in the intracellular Ca(2+) level. Treatment with HU210 caused tyrosine phosphorylation of Src and Fyn, and increased their kinase activities. Pretreatment with inhibitors of tyrosine kinases or phospholipase C abolished the cannabinoids-induced release of arachidonic acid and Ca(2+) response, and protein kinase C inhibitor reduced the release of arachidonic acid. 2-AG caused the release of arachidonic acid from cultured cells of the mouse cerebellum via similar mechanisms. These data reveal that cannabinoids activated cPLA(2)alpha in a Src-phospholipase C-protein kinase C-dependent manner probably via cannabinoid CB(1) receptor and/or CB(1)-like receptor in neuronal cells.

Activation of the neurokinin-1 receptor by substance P triggers the release of substance P from cultured adult rat dorsal root ganglion neurons

Background

Although substance P (SP) is an important primary afferent modulator in nociceptive processes, it is unclear whether SP regulates its own release from primary sensory neurons.

Results

Using a highly sensitive radioimmunoassay for SP, we have demonstrated that the activation of neurokinin-1 receptor by SP or GR73632 (a potent neurokinin-1 receptor agonist) triggered an increase of SP release from cultured adult rat dorsal root ganglion (DRG) neurons depending on the dose and exposure time within 60 min, and thereafter, the SP release level gradually decreased over 360 min. Accompanying the SP release, a significant reduction in the percentage of neurons expressing neurokinin-1 receptor on their membranes during exposure to SP (200 pg/dish) occurred time dependently (56 ± 5% and 32 ± 2% at 180 and 360 min, respectively). The GR73632-evoked (10 nM, 60 min) SP release was attenuated by several inhibitors for mitogen-activated protein kinase kinase, p38 mitogen-activated protein (MAP) kinase and cyclooxygenase-2 (COX-2), protein kinase C (PKC), respectively. In contrast, a c-Jun NH₂-terminal kinase inhibitor increased the GR73632-evoked SP release.

Conclusion

These results indicate that the neurokinin-1 receptor activation by its agonists regulates the SP release process involving the activation of MAP kinases, PKCs and COX-2 from cultured DRG neurons.

Signaling Pathways in Sensitization: Toward a Nociceptor Cell Biology

The electrophysiological properties of peripheral neurons activated by noxious stimuli, the primary afferent nociceptors, have been investigated intensively, and our knowledge about the molecular basis of transducers for noxious stimuli has increased greatly. In contrast, understanding of the intracellular signaling mechanisms regulating nociceptor sensitization downstream of ligand binding to the receptors is still at a relatively nascent stage. After outlining the initiated signaling cascades, we discuss the emerging plasticity within these cascades and the importance of subcellular compartmentalization. In addition, the recently realized importance of functional interactions with the extracellular matrix, cytoskeleton, intracellular organelles such as mitochondria, and sex hormones will be introduced. This burgeoning literature establishes new cellular features crucial for the function of nociceptive neurons and argues that additional focus should be placed on understanding the complex integration of cellular events that make up the "cell biology of pain."

Feedback mechanisms in the regulation of intracellular calcium ([Ca2+]i) in the peripheral nociceptive system: role of TRPV-1 and pain related receptors

Multimodal stimuli like heat, cold, bacterial or mechanical events are able to elicit pain, which is necessary to guarantee survival. However, the control of pain is of major clinical importance. The perception and transduction of pain is differentially modulated in the peripheral and central nervous system (CNS): while peripheral structures modulate these signals, the perception of pain occurs in the CNS. In recent years major advances have been made in the understanding of the processes which are involved in pain sensation. For the peripheral pain reception, the importance of specific pain receptors of the transition receptor pore (TRP)-family (e.g. the TRPV-1 receptor) has been analyzed. These receptors/channels are localized at the cell membrane of nociceptive neurones as well as in membranes of intracellular calcium stores like the endoplasmic reticulum. While the associated channel conducts different ions, a major proportion is calcium. Therefore, this review focuses on (1) the modulations of intracellular calcium ([Ca2+]i) initiated by the activation of pain receptors and (2) the consequences of [Ca2+]i changes for the processing of pain signals at the peripheral side. The possible interference of TRPV-1 induced [Ca2+]i modulations to the function of other membrane receptors and channels, like voltage gated calcium, sodium or potassium channels, or co-expressed CB1-receptors will be discussed. The latter interactions are of specific interest since the analgetic properties of endo- and exo-cannabinoids are mediated by CB1 receptors and their activation significantly modulates the calcium induced release of pain related transmitters. Furthermore, multiple cross links between different pain modulating intracellular pathways and their dependence on [Ca2+]i modulations will be illuminated. Overall, this review will summarize new insights resulting in the understanding of the prominent influence of [Ca2+]i for processes which are involved in pain sensation.

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.

Cannabinoids and pain

Convincing evidence from preclinical studies demonstrates that cannabinoids can reduce pain responses in a range of inflammatory and neuropathic pain models. The anatomical and functional data reveal cannabinoid receptor-mediated analgesic actions operating at sites concerned with the transmission and processing of nociceptive signals in brain, spinal cord and the periphery. The precise signalling mechanisms by which cannabinoids produce analgesic effects at these sites remain unclear; however, significant clues point to cannabinoid modulation of the functions of neurone and immune cells that mediate nociceptive and inflammatory responses. Intracellular signalling mechanisms engaged by cannabinoid receptors-like the inhibition of calcium transients and adenylate cyclase, and pre-synaptic modulation of transmitter release-have been demonstrated in some of these cell types and are predicted to play a role in the analgesic effects of cannabinoids. In contrast, the clinical effectiveness of cannabinoids as analgesics is less clear. Progress in this area requires the development of cannabinoids with a more favourable therapeutic index than those currently available for human use, and the testing of their efficacy and side-effects in high-quality clinical trials.

Olopatadine attenuates the enhancement of capsaicin-evoked substance P release by bradykinin from cultured dorsal root ganglion neurons

Olopatadine, a second-generation antihistamine, has recently been suggested to have an inhibitory effect on the tachykinin release from guinea-pig tracheobronchial smooth muscle preparation. In the present study, using a highly sensitive radioimmunoassay for substance P, we observed that olopatadine attenuated the enhancement of capsaicin-induced substance P release by bradykinin with an IC(50) value of 12.5 microM, without any inhibitory effect on the substance P release induced by capsaicin, potassium or bradykinin from cultured dorsal root ganglion neurons. These data suggest that olopatadine may therefore be involved in the bradykinin-induced sensitization of the transient receptor potential vanilloid 1 in cultured dorsal root ganglion neurons.

The mu opioid agonist morphine modulates potentiation of capsaicin-evoked TRPV1 responses through a cyclic AMP-dependent protein kinase A pathway

Background

The vanilloid receptor 1 (TRPV1) is critical in the development of inflammatory hyperalgesia. Several receptors including G-protein coupled prostaglandin receptors have been reported to functionally interact with the TRPV1 through a cAMP-dependent protein kinase A (PKA) pathway to potentiate TRPV1-mediated capsaicin responses. Such regulation may have significance in inflammatory pain. However, few functional receptor interactions that inhibit PKA-mediated potentiation of TRPV1 responses have been described.

Results

In the present studies we investigated the hypothesis that the μ opioid receptor (MOP) agonist morphine can modulate forskolin-potentiated capsaicin responses through a cAMP-dependent PKA pathway. HEK293 cells were stably transfected with TRPV1 and MOP, and calcium (Ca²⁺) responses to injection of the TRPV1 agonist capsaicin were monitored in Fluo-3-loaded cells. Pre-treatment with morphine did not inhibit unpotentiated capsaicin-induced Ca²⁺ responses but significantly altered capsaicin responses potentiated by forskolin. TRPV1-mediated Ca²⁺ responses potentiated by the direct PKA activator 8-Br-cAMP and the PKC activator Phorbol-12-myristate-13-acetatewere not modulated by morphine.

Immunohistochemical studies confirmed that the TRPV1 and MOP are co-expressed on cultured Dorsal Root Ganglion neurones, pointing towards the existence of a functional relationship between the G-protein coupled MOP and nociceptive TRPV1.

Conclusion

The results presented here indicate that the opioid receptor agonist morphine acts via inhibition of adenylate cyclase to inhibit PKA-potentiated TRPV1 responses. Targeting of peripheral opioid receptors may therefore have therapeutic potential as an intervention to prevent potentiation of TRPV1 responses through the PKA pathway in inflammation.

Lack of evidence for functional TRPV1 vanilloid receptors in rat hippocampal nerve terminals

Although TRPV(1) vanilloid receptors (TRPV(1)Rs) have been assumed to be present in the brain, their role is not well-defined. Here, we tested the widely used TRPV(1)R agonists (E)-capsaicin (0.1-100 microM) and resiniferatoxin (RTX, 0.1 microM) on resting and K(+)-evoked Ca(2+) entry and radiolabelled GABA release in rat hippocampal nerve terminals. (E)-capsaicin and RTX failed to evoke Ca(2+) transients or to trigger [(3)H]GABA outflow. Both (E)-capsaicin (EC(50), 40.4 microM) and its enantiomer (Z)-capsaicin (EC(50), 22.9 microM), which is inactive at the TRPV(1)R, inhibited the K(+)-evoked Ca(2+) entry, and to similar extent, the Ca(2+)-dependent K(+)-evoked [(3)H]GABA release. The TRPV(1)R enhancer/partial agonist 2-aminoethoxydiphenyl borate (1-300 microM) induced rapid Ca(2+) entry. None of the above-mentioned findings proved to be sensitive to the TRPV(1)R antagonists iodoresiniferatoxin (I-RTX; 3 microM) and SB366791 (3 microM). The CB(1) cannabinoid receptor antagonist AM251 (EC(50), 1.1 microM) and I-RTX (EC(50), 4.6 microM) also diminished the K(+)-evoked Ca(2+) entry per se. We observed competitive antagonism between I-RTX and AM251, indicating that the two molecules may act at the same site. In conclusion, there is a need to examine the discrepancy between ex vivo and in vitro data to understand the neurochemical and physiological functions of brain TRPV(1)Rs.

The long-term exposure of rat cultured dorsal root ganglion cells to bradykinin induced the release of prostaglandin E2 by the activation of cyclooxygenase-2

The effects of long-term exposure of primary cultured rat dorsal root ganglion (DRG) cells to bradykinin (BK), compared to short-term exposure, were investigated to establish whether BK could induce prostaglandin E2 (PGE2) release from DRG cells. Short-term exposure (30 min) resulted in a small but significant amount of PGE2 release which was mainly inhibited by a selective COX-1 inhibitor, SC-560 but only partially by a selective COX-2 inhibitor, NS-398, and did not induce COX-2 protein as determined by Western blotting. In contrast, long-term exposure (3 h) induced a large amount of PGE2 release, which was completely abolished by indomethacin or NS-398. The level of COX-2 mRNA began to be detected by ribonuclease protection assay after 30 min of 100 nM BK exposure, maintained maximal expression for 1 h, and subsequently declined to the basal level. The level of COX-2 protein was expressed to follow the time course of COX-2 mRNA induction by BK in a delayed but similar kinetic manner. The expression of COX-2 induced by BK in DRG cells was inhibited by a BK B2 receptor antagonist, HOE140, but not a B1 receptor antagonist, Lys-des-Arg9, (Leu8)-BK. Thus, BK has been shown to induce COX-2 protein by B2 receptor, which may cause prostanoid generation in rat DRG cells, which may play an important role in the pathogenesis of inflammatory pain and hyperalgesia around the primary sensory neurons.

Substance P release evoked by capsaicin or potassium from rat cultured dorsal root ganglion neurons is conversely modulated with bradykinin

To clarify the molecular mechanism of substance P (SP) release from dorsal root ganglion (DRG) neurons, we investigated the involvement of several intracellular effectors in the regulation of SP release evoked by capsaicin, potassium or/and bradykinin. Bradykinin-evoked SP release from cultured adult rat DRG neurons was attenuated by either the mitogen-activated protein kinase kinase (MEK) inhibitor (U0126) or cycloheximide. As the long-term exposure of DRG neurons to bradykinin (3 h) resulted in extracellular signal-regulated kinase (ERK) phosphorylation at an early stage and thereafter induced cyclooxygenase-2 (COX-2) protein expression, which both contribute to the SP release triggered by bradykinin B2 receptor. The long-term exposure of DRG neurons to bradykinin enhanced the SP release by capsaicin, but attenuated that by potassium. Interestingly, the inositol 1,4,5-triphosphate (IP3)-induced calcium release blocker [2-aminoethyl diphenylborinate (2-APB)] not only inhibited the potassium-evoked SP release, but also completely abolished the enhancement of capsaicin-induced SP release by bradykinin from cultured DRG neurons. Together, these findings suggest that the molecular mechanisms of SP release by bradykinin involve the activation of MEK, and also require the de novo protein synthesis of COX-2 in DRG neurons. The IP3-dependent calcium release could be involved in the processes of the regulation by bradykinin of capsaicin-triggered SP release.