Role of TRPV1 and ASIC3 in formalin-induced secondary allodynia and hyperalgesia.

BACKGROUND: In the present study we determined the role of transient receptor potential V1 channel (TRPV1) and acid-sensing ion channel 3 (ASIC3) in chronic nociception. METHODS: 1% formalin was used to produce long-lasting secondary allodynia and hyperalgesia in rats. Western blot was used to determine TRPV1 and ASIC3 expression in dorsal root ganglia. RESULTS: Peripheral ipsilateral, but not contralateral, pre-treatment (-10min) with the TRPV1 receptor antagonists capsazepine (0.03-0.3µM/paw) and A-784168 (0.01-1µM/paw) prevented 1% formalin-induced secondary mechanical allodynia and hyperalgesia in the ipsilateral and contralateral paws. Likewise, peripheral ipsilateral, but not contralateral, pre-treatment with the non-selective and selective ASIC3 blocker benzamil (0.1-10µM/paw) and APETx2 (0.02-2µM/paw), respectively, prevented 1% formalin-induced secondary mechanical allodynia and hyperalgesia in both paws. Peripheral ipsilateral post-treatment (day 6 after formalin injection) with capsazepine (0.03-0.3µM/paw) and A-784168 (0.01-1µM/paw) reversed 1% formalin-induced secondary mechanical allodynia and hyperalgesia in both paws. In addition, peripheral ipsilateral post-treatment with benzamil (0.1-10µM/paw) and APETx2 (0.02-2µM/paw), respectively, reversed 1% formalin-induced secondary mechanical allodynia and hyperalgesia in both paws. TRPV1 and ASIC3 proteins were expressed in dorsal root ganglion in normal conditions, and 1% formalin injection increased expression of both proteins in this location at 1 and 6 days compared to naive rats. CONCLUSIONS: Data suggest that TRPV1 and ASIC3 participate in the development and maintenance of long-lasting secondary allodynia and hyperalgesia induced by formalin in rats. The use of TRPV1 and ASIC3 antagonists by peripheral administration could prove useful to treat chronic pain.