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Sensitisation of the capsaicin receptor, transient receptor potential vanilloid type 1 (TRPV1) ion channel in nociceptive primary sensory neurons (PSN) underlies the development of inflammatory heat hyperalgesia. Removal of the negative-dominant splice variant of the TRPV1 molecule, TRPV1b from TRPV1/TRPV1b heterotetrameric channels, which should be associated with changes in the expression of TRPV1 and TRPV1b transcripts and proteins, has been suggested to contribute to that sensitisation. Respective reverse-transcriptase polymerase chain reaction (RT-PCR) and Western-blotting revealed that both TRPV1 and TRPV1b mRNA, and their encoded proteins are expressed in rat cultured PSN. Sequencing of the RT-PCR products showed that TRPV1b mRNA lacks the entire exon 7. Further, growing PSN for 2 days in the presence of 10μM bradykinin (BK) and 10μM prostaglandin E2 (PGE2) significantly increases TRPV1 responsiveness and TRPV1 mRNA expression, without producing any changes in TRPV1b mRNA, and TRPV1 and TRPV1b protein expression. These data challenge the hypothesis that alterations in the composition of the TRPV1 ion channel contributes to the sensitisation., (Copyright © 2014 Elsevier Ireland Ltd. All rights reserved.)

We have studied scalding-type burn injury-induced activation of extracellular signal-regulated kinase 1/2 (ERK1/2) in the spinal dorsal horn, which is a recognised marker for spinal nociceptive processing. At 5min after severe scalding injury to mouse hind-paw, a substantial number of phosphorylated ERK1/2 (pERK1/2) immunopositive neurons were found in the ipsilateral dorsal horn. At 1h post-injury, the number of pERK1/2-labelled neurons remained substantially the same. However, at 3h post-injury, a further increase in the number of labelled neurons was found on the ipsilateral side, while a remarkable increase in the number of labelled neurons on the contralateral side resulted in there being no significant difference between the extent of the labelling on both sides. By 6h post-injury, the number of labelled neurons was reduced on both sides without there being significant difference between the two sides. A similar pattern of severe scalding injury-induced activation of ERK1/2 in spinal dorsal horn neurons over the same time-course was found in mice which lacked the transient receptor potential type 1 receptor (TRPV1) except that the extent to which ERK1/2 was activated in the ipsilateral dorsal horn at 5 min post-injury was significantly greater in wild-type animals when compared to TRPV1 null animals. This difference in activation of ERK1/2 in spinal dorsal horn neurons was abolished within 1h after injury, demonstrating that TRPV1 is not essential for the maintenance of ongoing spinal nociceptive processing in inflammatory pain conditions in mouse resulting from at least certain types of severe burn injury., (Copyright © 2010 European Federation of International Association for the Study of Pain Chapters. Published by Elsevier Ltd. All rights reserved.)

Aims: Xenon provides effective analgesia in several pain states at sub-anaesthetic doses. Our aim was to examine whether xenon may mediate its analgesic effect, in part, through reducing the activity of transient receptor potential vanilloid type 1 (TRPV1), a receptor known to be involved in certain inflammatory pain conditions., Main Methods: We studied the effect of xenon on capsaicin-evoked cobalt uptake in rat cultured primary sensory neurons and in human TRPV1 (hTRPV1)-expressing human embryonic kidney 293 (HEK293) cells. We also examined xenon's effect on the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) in the rat spinal dorsal horn evoked by hind-paw injection of capsaicin., Key Findings: Xenon (75%) reduced the number of primary sensory neurons responding to the TRPV1 agonist, capsaicin (100 nM-1 μM) by ~25% to ~50%. Xenon reduced the number of heterologously-expressed hTRPV1 activated by 300 nM capsaicin by ~50%. Xenon (80%) reduced by ~40% the number of phosphorylated ERK1/2-expressing neurons in rat spinal dorsal horn resulting from hind-paw capsaicin injection., Significance: Xenon substantially reduces the activity of TRPV1 in response to noxious stimulation by the specific TRPV1 agonist, capsaicin, suggesting a possible role for xenon as an adjunct analgesic where hTRPV1 is an active contributor to the excitation of primary afferents which initiates the pain sensation., (Copyright © 2010 Elsevier Inc. All rights reserved.)

Aims: Activation of the cannabinoid 1 (CB1) receptor in cultured primary sensory neurons reduces responses mediated through the transient receptor potential vanilloid type 1 receptor (TRPV1), which plays a pivotal role in the development of heat hyperalgesia and visceral hyper-reflexia in inflammatory conditions. Here, we studied the effect of cannabinoid-evoked inhibitory effect on TRPV1 in inflammatory conditions., Main Methods: The effect of anandamide (1 nM-30 nM) and 1,1-dimethylheptyl-11-hydroxytetrahydrocannabinol (HU210; 1 microM-10 microM) was assessed on capsaicin (10 nM or 100 nM)-evoked cobalt uptake in rat cultured primary sensory neurons following the incubation of the cells in an "inflammatory environment" created by the major inflammatory mediators, bradykinin (5 microM), prostaglandin E(2) (5 microM) and nerve growth factor (100 ng/ml) for 10 min., Key Findings: 1 nM and 10 nM anandamide significantly reduced the 10 nM but not the 100 nM capsaicin-evoked responses. HU210 did not produce a significant change in responses evoked by capsaicin at either of its concentrations. The anandamide-induced inhibitory effect could not be reversed by the CB1 receptor antagonist, rimonabant (200 nM) or the membrane-permeable cAMP analogue, 8Br-cAMP (100 microM)., Significance: These findings suggest that anandamide may inhibit TRPV1-mediated responses in a non-CB1/non-cannabinoid 2 receptor-dependent manner in primary sensory neurons in inflammatory conditions., (Copyright 2010 Elsevier Inc. All rights reserved.)

Clinically relevant concentrations of isoflurane or sevoflurane sensitize transient receptor potential vanilloid type 1 to several of its activators, including capsaicin. It has, moreover, been suggested these volatile general anaesthetics may augment nociceptive signalling arising from surgical procedures and thereby contribute to post-operative pain. To investigate this suggestion, we have studied intraplantar capsaicin injection-induced phosphorylation of extracellular signal-regulated kinase 1/2 in spinal dorsal horn neurons (which is a recognized marker of spinal nociceptive processing) in rat during isoflurane or sevoflurane anaesthesia after 60 min under anaesthesia. Control animals were anaesthetized with pentobarbital (which of itself does not activate extracellular signal-regulated kinase 1/2 in spinal dorsal horn neurons). Unilateral intraplantar capsaicin injection in control animals evoked extracellular signal-regulated kinase 1/2 phosphorylation in a group of neurons in lamina I and lamina II of the ipsilateral spinal dorsal horn in a somatotopically appropriate area. In contrast, both anaesthetic gases (given for 60 min and without subsequent capsaicin injection) induced extracellular signal-regulated kinase 1/2 activation in a different group of mainly lamina I neurons bilaterally. The total number of spinal dorsal horn neurons labelled on the ipliateral side following capsaicin injection into the isoflurane-, or sevoflurane-, anaesthetized animals was significantly less than that produced by capsaicin alone. Further, capsaicin injection into isoflurane-, or sevoflurane-, anaesthetized animals reduced extracellular signal-regulated kinase 1/2 phosphorylation induced by the gases alone on both sides. These findings do not support the suggestion that isoflurane-, or sevoflurane-, induced sensitization of transient receptor potential vanilloid type 1 by capsaicin, or other agonist, is translated into induction of spinal nociceptive processing and consequential pain sensation., (Copyright (c) 2010 IBRO. Published by Elsevier Ltd. All rights reserved.)

Pain originating in inflammation is the most common pathologic pain condition encountered by the anesthesiologist whether in the context of surgery, its aftermath, or in the practice of pain medicine. Inflammatory agents, released as components of the body's response to peripheral tissue damage or disease, are now known to be collectively capable of activating transient receptor potential vanilloid type 1, transient receptor potential vanilloid type 4, transient receptor potential ankyrin type 1, and acid-sensing ion channels, whereas individual agents may activate only certain of these ion channels. These ionotropic receptors serve many physiologic functions-as, indeed, do many of the inflammagens released in the inflammatory process. Here, we introduce the reader to the role of these ionotropic receptors in mediating peripheral pain in response to inflammation.

The majority of polymodal nociceptors express the non-selective cationic channel, transient receptor potential vanilloid type 1 receptor, which plays a pivotal role in the development of inflammatory heat hyperalgesia and visceral hyper-reflexia. Thus, blocking transient receptor potential vanilloid type 1 receptor-mediated signalling in primary sensory neurons would provide significant pain relief and reduced visceral hyperactivity in inflammatory conditions. Here, we report that cannabinoids including the endogenous agent, anandamide (3-30 nM) and the synthetic compounds, arachidonyl-2-chloroethylamide (500 nM) and 1,1-dimethylheptyl-11-hydroxytetrahydrocannabinol (1 microM) significantly reduce cobalt influx that is mediated through the transient receptor potential vanilloid type 1 receptor in rat cultured primary sensory neurons. The cannabinoid-evoked inhibitory effect can be reversed by rimonabant (200 nM), an antagonist of the cannabinoid 1 receptor. While anandamide- and arachidonyl-2-chloroethylamide fail to evoke inhibitory effects on the transient receptor potential vanilloid type 1 receptor-mediated responses, the inhibitory effect of 1,1-dimethylheptyl-11-hydroxytetrahydrocannabinol is maintained, when the cannabinoids are applied together with the inflammatory mediators, prostaglandin E(2) (10 microM) and bradykinin (10 microM). These results indicate that activation of the cannabinoid 1 receptor can reduce the activity of the transient receptor potential vanilloid type 1 receptor in primary sensory neurons, though the inhibitory effect of agents, which activate both the cannabinoid 1 and the transient receptor potential vanilloid type 1 receptor could be reduced in inflammatory conditions.

Our previous finding, that the capsaicin- and KCl-induced Ca(2+)-dependent production of the intra- and intercellular signaling molecule N-arachidonoyl ethanolamine (anandamide) in cultured primary sensory neurons could be abolished and reduced by approximately 2/3 by capsaicin-induced degeneration of capsaicin-sensitive neurons, respectively suggests that a major sub-population of capsaicin-sensitive cells together with a group of non-capsaicin-sensitive cells should express enzymes involved in Ca(2+)-dependent anandamide synthesis. N-acyl phosphotidylethanolamine phospholipase D (NAPE-PLD) is known to be involved in Ca(2+)-dependent anandamide production. Hence, here, we used reverse transcriptase and quantitative real time polymerase chain reaction to study NAPE-PLD expression in dorsal root ganglia and to clarify the sub-population of cells expressing this enzyme. Cultures prepared from mouse dorsal root ganglia were grown either in the absence or presence of the neurotoxin, capsaicin (10 muM) overnight. We report, that NAPE-PLD is expressed both in dorsal root ganglia and cultures prepared from dorsal root ganglia and grown in the absence of capsaicin. Furthermore, we also report that capsaicin application downregulates the expression of NAPE-PLD as well as the capsaicin receptor, transient receptor potential vanilloid type 1 ion channel, by about 70% in the cultures prepared from dorsal root ganglia. These findings indicate that a major sub-population of capsaicin-sensitive primary sensory neurons expresses NAPE-PLD, and suggest that NAPE-PLD is expressed predominantly by capsaicin-sensitive neurons in dorsal root ganglia. These data also suggest that NAPE-PLD might be a target to control the activity and excitability of a major sub-population of nociceptive primary sensory neurons.

The recently identified splice variant of the transient receptor vanilloid type 1 (TRPV1) molecule, TRPV1b, produces a negative-dominant effect on the responsiveness of the TRPV1 channel, which is increased by peripheral inflammatory processes. Here, we studied, using real-time polymerase chain reaction, whether cyclophosphamide injection-evoked cystitis is associated with altered TRPV1/TRPV1b expression in the L5-L6 dorsal root ganglia, which innervate the urinary bladder. We found that while TRPV1 mRNA expression was unchanged, the amount of TRPV1b transcript was significantly reduced in L5-L6 dorsal root ganglia during cystitis. These data indicate that peripheral inflammatory events induce changes in TRPV1b expression in primary sensory neurons, which may result in increased responsiveness of the TRPV1 channel.