Nociceptors Lacking TRPV1 and TRPV2 Have Normal Heat Responses (original) (raw)
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Journal of neurophysiology, 2015
Differential thermal nociception across inbred mouse strains has genetic determinants. Thermal nociception is largely attributed to the heat/capsaicin receptor TRPV1; however, the contribution of this channel to the genetics of thermal nociception has not been revealed. In this study we compared TRPV1 expression levels and electrophysiological properties in primary sensory neurons and thermal nociceptive behaviors between two (C57BL/6 and BALB/c) inbred mouse strains. Using immunofluorescence and patch-clamp physiology methods, we demonstrated that TRPV1 expression was significantly higher in isolectin B4 (IB4) -positive trigeminal sensory neurons of C57BL/6 relative to BALB/c; the expression in IB4-negative neurons was similar between the strains. Furthermore, using electrophysiological cell classification (current signature method), we showed differences between the two strains in capsaicin sensitivity in IB4-positive neuronal cell types 2 and 13, that were previously reported as ...
TRPV1 gene disruption results in a loss of capsaicin and proton responsiveness, but has minimal effects on heat-induced nocifensive behavior, suggesting that sensory transduction of heat is independent of TRPV1. TRPV3, another heat-activated ion channel but insensitive to capsaicin, was shown to be expressed in keratinocytes as well as in sensory neurons projecting to the skin. Recently, 2-aminoethoxydiphenyl borate was introduced as a TRPV3 agonist, but its selectivity was questioned by showing that it activated recombinant TRPV1 and TRPV2 as well. We used the isolated mouse skin-saphenous nerve preparation and whole-cell patch-clamping of cultured dorsal root ganglia neurons from TRPV1-/- and wildtype mice. We found no phenotypic differences between the heat responses of polymodal C-fibers, whereas cultured dorsal root ganglia neurons of TRPV1-/- hardly showed any heat-activated currents. Only C-fibers of wildtype but not TRPV1-/- mice were clearly sensitized to heat by 2-aminoethoxydiphenyl borate 10 and 100 microM; heat-activated current in wildtype neurons was only facilitated at 100 microM. Noxious heat-induced calcitonin gene-related peptide release showed clear deficits (<50%) in TRPV1 deficient skin, but the stimulated calcitonin gene-related peptide release from the isolated skull dura was unaffected. In both models, 2-aminoethoxydiphenyl borate was able to potentiate the heat response (46 degrees C, 5 min) in a concentration-dependent manner, again, only in wildtype but not TRPV1-/- mice, suggesting that TRPV2/3 are not involved in this sensitization to heat. The results further suggest that TRPV1 is not responsible for the normal heat response of native nociceptors but plays the essential role in thermal sensitization and a prominent one in controlling dermal calcitonin gene-related peptide release, i.e. neurogenic inflammation.
The Journal of Pain, 2008
In the present study, a murine ex-vivo somatosensory system preparation was used to determine the response characteristics of cutaneous sensory neurons staining positively for TRPV1 or TRPV2. TRPV1 immunostaining was found exclusively (11/11) in a specific set of mechanically insensitive unmyelinated (C) nociceptors that responded to heating of their receptive fields. No cutaneous Cfibers that responded to both mechanical and heat stimuli stained positively for TRPV1 (0/62). The relationship between TRPV2 and heat transduction characteristics was not as clear, as few unmyelinated or myelinated fibers that responded to heat contained TRPV2. TRPV2 was found most frequently in mechanically sensitive myelinated fibers, including both low threshold and high threshold mechanoreceptors (nociceptors). While TRPV2 was found in only 1 of 6 myelinated polymodal nociceptors, it was found in a majority (10/16) of myelinated mechanical nociceptors. Thus, while the in vivo role of TRPV1 as a heat sensitive channel in cutaneous sensory neurons is clearly defined, the role of TRPV2 in cutaneous neurons remains unknown. These results also suggest that TRPV1 may be essential for heat transduction in a specific subset of mechanically insensitive cutaneous nociceptors, and that this subset may constitute a discrete heat input pathway for inflammation-induced thermal pain.
Thermal nociception and TRPV1 function are attenuated in mice lacking the nucleotide receptor P2Y2
PAIN, 2008
Recent studies indicate that ATP and UTP act at G protein-coupled (P2Y) nucleotide receptors to excite nociceptive sensory neurons; nucleotides also potentiate signaling through the pro-nociceptive capsaicin receptor, TRPV1. We demonstrate here that P2Y 2 is the principal UTP receptor in somatosensory neurons: P2Y 2 is highly expressed in dorsal root ganglia and P2Y 2 (−/−) mice showed profound deficits in UTP-evoked calcium transients and potentiation of capsaicin responses. P2Y 2 (−/−) mice were also deficient in the detection of painful heat: baseline thermal response latencies were increased and mutant mice failed to develop thermal hypersensitivity in response to inflammatory injury (injection of complete Freund's adjuvant into the hindpaw). P2Y 2 was the only Gq-coupled P2Y receptor examined that showed an increase in DRG mRNA levels in response to inflammation. Surprisingly, TRPV1 function was also attenuated in P2Y 2 (−/−) mice, as measured by the frequency and magnitude of capsaicin responses in vitro and behavioral responses to capsaicin administration in vivo. However, TRPV1 mRNA levels and immunoreactivity were not reduced, and behavioral sensitivity to capsaicin could be largely restored in P2Y 2 (−/−) mice by pretreatment with bradykinin, suggesting that normal function of TRPV1 requires ongoing modulation by G proteincoupled receptors. These results indicate that nucleotide signaling through P2Y 2 plays a key role in thermal nociception.
ThermoTRP Channels in Nociceptors: Taking a Lead from Capsaicin Receptor TRPV1
Current Neuropharmacology, 2008
Nociceptors with peripheral and central projections express temperature sensitive transient receptor potential (TRP) ion channels, also called thermoTRP's. Chemosensitivity of thermoTRP's to certain natural compounds eliciting pain or exhibiting thermal properties has proven to be a good tool in characterizing these receptors. Capsaicin, a pungent chemical in hot peppers, has assisted in the cloning of the first thermoTRP, TRPV1. This discovery initiated the search for other receptors encoding the response to a wide range of temperatures encountered by the body. Of these, TRPV1 and TRPV2 encode unique modalities of thermal pain when exposed to noxious heat. The ability of TRPA1 to encode noxious cold is presently being debated. The role of TRPV1 in peripheral inflammatory pain and central sensitization during chronic pain is well known. In addition to endogenous agonists, a wide variety of chemical agonists and antagonists have been discovered to activate and inhibit TRPV1. Efforts are underway to determine conditions under which agonistmediated desensitization of TRPV1 or inhibition by antagonists can produce analgesia. Also, identification of specific second messenger molecules that regulate phosphorylation of TRPV1 has been the focus of intense research, to exploit a broader approach to pain treatment. The search for a role of TRPV2 in pain remains dormant due to the lack of suitable experimental models. However, progress into TRPA1's role in pain has received much attention recently. Another ther-moTRP, TRPM8, encoding for the cool sensation and also expressed in nociceptors, has recently been shown to reduce pain via a central mechanism, thus opening a novel strategy for achieving analgesia. The role of other thermoTRP's (TRPV3 and TRPV4) encoding for detection of warm temperatures and expressed in nociceptors cannot be excluded. This review will discuss current knowledge on the role of nociceptor thermoTRPs in pain and therapy and describes the activator and inhibitor molecules known to interact with them and modulate their activity.
Experimental Brain Research, 2009
The capsaicin receptor TRPV1 is a polymodal sensory transducer molecule in the pain pathway. TRPV1 integrates noxious heat, tissue acidosis and chemical stimuli which are all known to cause pain. Studies on TRPV1-deWcient mice suggest that TRPV1 is essential for acid sensing by nociceptors and for thermal hyperalgesia in inXammation of the skin, but not for transducing noxious heat. After TRPV1, other TRPV channels were cloned with polymodal properties and sensitivity to noxious heat, named TRPV2, TRPV3 and TRPV4. While TRPV3 and TRPV4 are predominantly warm sensors, TRPV2's threshold is in the noxious range (>52°C). However, mice deWcient of TRPV2 and TRPV1 or TRPV3 or TRPV4 show no major impairment of noxious heat sensing. Ruthenium red, a water soluble polycationic dye, was found to block the pore of the capsaicin-operated cation channel TRPV1 thus interfering with all polymodal ways of TRPV1 activation. Antagonistic eVects of the dye were subsequently described on many other TRP-channels, especially on the heat-sensitive ones of the vanilloid family, TRPV2, TRPV3 and TRPV4. In this study, we used the rat skin-nerve preparation to deWne the possible actions of ruthenium red on the proton, capsaicin and noxious heat activation of native polymodal nociceptors. Ruthenium red was found to suppress only the capsaicin-induced excitation and desensitization of these nerve endings. On the contrary, the proton and heatinduced discharge responses of the single Wbres were not inXuenced. Additionally, we found that the dye concentration dependently increases the excitability of the neurons resulting in ongoing activity and burstlike discharge. These diVerential results are discussed in the light of recent Wndings from transgenic mouse models, and they point once more to major (pharmacological) diVerences between cellular models of nociception, including spinal ganglion neuron and transfected cell lines, and the real native nerve endings.
Neurophysiology, 2011
Transient receptor potential channels (TRP) have been extensively investigated over the past few years. Recent findings in the field of pain have established a family of six thermoTRP channels (TRPA1, TRPM8, TRPV1, TRPV2, TRPV3, and TRPV4) that exhibits sensitivity to increases or decreases in temperature, as well as to chemical substances eliciting the respective hot or cold sensations. Such irritants include menthol, cinnamaldehyde, gingerol, mustard oil, capsaicin, camphor, eugenol, and others. In this study, we used behavioral and electrophysiological methods to investigate if mustard oil (allyl isothiocyanate, AITC) and capsaicin affect the sensitivity to thermal, innocuous cold, and mechanical stimuli in male rats. Unilateral intraplantar injections of AITC and capsaicin induced significant decreases in the latency for ipsilateral paw withdrawal from a noxious heat stimulus, i.e., heat hyperalgesia. These agents also significantly reduced the mechanical withdrawal thresholds of the injected paw, i.e., mechanical allodynia. Bilateral intraplantar injections of AITC resulted in a twophase effect on cold avoidance (thermal preference test). A low concentration of AITC (5%) did not change cold avoidance similarly to the vehicle control, while higher AITC concentrations (10 and 15%) significantly reduced cold avoidance, i.e., induced cold hypoalgesia. Capsaicin acted in almost the same manner. These results indicate that TRPA1 channels are clearly involved in pain reactions, and the TRPA1 agonist AITC enhances the heat pain sensitivity, possibly by indirectly modulating TRPV1 channels co-expressed in nociceptors with TRPA1s. In electrophysiological experiments, neuronal responses to electrical and graded mechanical and noxious thermal stimulations were tested before and after cutaneous application of AITC. Repetitive application of AITC initially increased the firing rate of spinal wide-dynamic range neurons; this was followed by rapid desensitization that persisted when AITC application was reapplied 30 min later. The responses to noxious thermal (but not to mechanical) stimuli were significantly enhanced irrespective of whether the neuron was directly activated by AITC. These findings indicate that AITC produced peripheral sensitization of heat nociceptors. Overall, our data support the role of hermosensitive TRPA1 and TRPV1 channels in pain modulation and show that these thermoTRP channels are promising targets for the development of a new group of analgesic drugs.