Behavioral Testing of the Effects of Thermosensitive TRP Channel Agonists on Touch, Temperature, and Pain Sensations

Role of thermo TRPA1 and TRPV1 channels in heat, cold, and mechanical nociception of rats

A sensitive response of the nervous system to changes in temperature is of predominant importance for homeotherms to maintain a stable body temperature. A number of temperature-sensitive transient receptor potential (TRP) ion channels have been studied as nociceptors that respond to extreme temperatures and harmful chemicals. Recent findings in the field of pain have established a family of six thermo-TRP channels (TRPA1, TRPM8, TRPV1, TRPV2, TRPV3, and TRPV4) that exhibit sensitivity to increases or decreases in temperature, as well as to chemical substances eliciting the respective hot or cold sensations. In this study, we used behavioral methods to investigate whether mustard oil (allyl isothiocyanate) and capsaicin affect the sensitivity to heat, innocuous and noxious cold, and mechanical stimuli in male rats. The results obtained indicate that TRPA1 and TRPV1 channels are clearly involved in pain reactions, and the TRPA1 agonist allyl isothiocyanate enhances the heat pain sensitivity, possibly by indirectly modulating TRPV1 channels coexpressed in nociceptors with TRPA1. Overall, our data support the role of thermosensitive TRPA1 and TRPV1 channels in pain modulation and show that these two thermoreceptor channels are in a synergistic and/or conditional relationship with noxious heat and cold cutaneous stimulation.

NSAIDs attenuate hyperalgesia induced by TRP channel activation

Transient receptor potential (TRP) cation channels have been extensively investigated as targets for analgesic drug discovery. Because some non-steroidal anti-inflammatory drugs (NSAIDs) are structural analogs of prostaglandins (mediators of inflammation) and NSAIDs attenuate heat nociception and mechanical allodynia in models of inflammatory and neuropathic pain, we examined three widely used NSAIDs (diclofenac, ketorolac, and xefocam) on the activation of TRPA1 and TRPV1 channels using thermal paw withdrawal (Hargreaves) test and mechanical paw withdrawal (von Frey) test in male rats. Thermal withdrawal latencies and mechanical thresholds for both hind paws were obtained with 5, 15, 30, 45, 60, and 120 min intraplantar post-injection of TRPA1 agonizts, allyl isothiocyanate (AITC) (a natural compound of mustard oil) and cinnamaldehyde (CA), and TRPV1 agonist capsaicin or vehicle. Twenty minutes prior to the start of the experiment with TRP agonizts, diclofenac, ketorolac or xefocam were pre-injected in the same hindpaw and animals were examined by these two tests. After pretreatment of all three NSAIDs in the ipsilateral (injected) hindpaw that produced strong antinociceptive effects, AITC, CA, and capsaicin caused significant decreases in latency of the thermal withdrawal reflex compared with vehicle or the contralateral hindpaw. The same findings were observed for the paw withdrawal threshold. In approximately 30 min the effects of CA, AITC, and capsaicin returned to baseline. The data are different from our previous evidence, where TRPA1 agonizts AITC and CA and TRPV1 agonist capsaicin produced hyperalgesia for nearly 2 h and resulted in facilitation of these withdrawal reflexes (Tsagareli et al., 2010, 2013). Thus, our data showing that NSAIDs suppress thermal and mechanical hyperalgesia following TRP activation could presumably due to inactivation or desensitization of TRPA1 and TRPV1 channels by NSAIDs.