The role of TRPA1 and TRPV1 channels in trigeminal pain: data from animal models

Experimental and clinical observations pointed out a critical involvement of transient receptor potential (TRP) channels, particularly TRPA1 and TRPV1, in trigeminal pain and associated symptoms, including hyperalgesia and allodynia. In this study the role of TRP channels was investigate in two animal models of diseases related to the trigeminal system: migraine and trigeminal neuropathic pain (TNP). TRPA1 and TRPV1 antagonists (ADM_12 and AMG9810 respectively) were used in the nitroglycerin (NTG)-induced hyperalgesia at the trigeminal level induced by means of the orofacial formalin test, a well validated animal model of migraine. The behavioral effects of AMG9810 gave inconclusive results, probably because its effect was confounded by the vehicle used. Nonetheless, it appears that TRPV1 channels are somehow involved in NTG-induced trigeminal hyperalgesia, since the TRPV1 mRNA levels were found to be strongly increased after NTG injection in medulla, cervical spinal cord and trigeminal ganglia (TG). NTG also up-regulated the mRNA levels of TRPA1, c-fos, calcitonin gene-related peptide (CGRP) and Substance P (SP) in the same areas. Those transcripts, but TRPV1, were reduced after ADM_12 treatment, which abolished the NTG-induced trigeminal hyperalgesia. The increased availability of nitric oxide after NTG promotes the formation of pro-inflammatory agents which can activate and/or sensitize nociceptors by means of TRPA1 and TRPV1 channels, causing the release of CGRP and SP. Although no differences in CGRP and SP protein expression were found at nucleus trigeminalis caudalis (NTC) level, the increased transcripts may reflect compensatory mechanisms aimed at reintegrating CGRP and SP stores depleted after NTG administration. It is possible that ADM_12 caused a reduction of Ca2+ influx through TRPA1 channels, which in turn interfered with the cascade of second-messenger molecules and with the Ca2+-interacting proteins, ultimately preventing NTG-induced inflammatory pathways. For TNP, the role of TRPA1 channels (by means of ADM_12) was investigated by evaluating mechanical allodynia in a model of chronic constriction injury of the infraorbital nerve (IoN-CCI). The IoN-CCI rats showed a hyperresponsiveness (4 weeks after surgery) at the ipsilateral side that reflects a condition of mechanical allodynia, and a significant increase in TRPA1, TRPV1, CGRP and SP mRNA expression levels. Although a tendency towards a decrease was seen in the ipsilateral compared to the contralateral side in the IoN-CCI rats, no significant differences in CGRP and SP protein expression at the NTC level were seen. However, their transcripts were highly increased in the central areas containing the NTC, as well as the TG ipsilateral to the IoN ligation. Both the allodynic response and the increased mRNA levels of operated rats were abolished after ADM_12 treatment. Probably, the blockade of TRPA1 channels located on the trigeminal afferents prevented neuropeptides release thus resulting in a reduced neurogenic inflammation and the nociceptors sensitization. Contrary to the migraine model, ADM_12 reduced transcript levels of both TRPs in IoN-CCI rats. Thus, ADM_12 appears to be a specific antagonist for TRPA1 in migraine pain, but in TNP it seems to act also on TRPV1. Probably, the damage induced by the nerve injury lead to a re-organization in expression and nature of the channels that made ADM_12 able to block TRPV1 channels. Since TRPA1 and TRPV1 are functionally linked, ADM_12 could have a direct effect on TRPA1 and an indirect effect on TRPV1 channels. In conclusion, TRPA1 blockade might be useful in the treatment of these trigeminal pain disorders. Moreover, our data suggest an important role also for TRPV1 channels, which could be differently involved depending on the type of pain. Further exploration on the mechanisms underlying the antinociceptive effects of these TRPs should improve our understanding of trigeminal pain processing.