Complex alterations in inflammatory pain and analgesic sensitivity in young and ageing female rats: involvement of ASIC3 and Nav1.8 in primary sensory neurons.

OBJECTIVE AND DESIGN: Our aim was to determine an age-dependent role of Nav1.8 and ASIC3 in dorsal root ganglion (DRG) neurons in a rat pre-clinical model of long-term inflammatory pain.

METHODS: We compared 6 and 24 months-old female Wistar rats after cutaneous inflammation. We used behavioral pain assessments over time, qPCR, quantitative immunohistochemistry, selective pharmacological manipulation, ELISA and in vitro treatment with cytokines.

RESULTS: Older rats exhibited delayed recovery from mechanical allodynia and earlier onset of spontaneous pain than younger rats after inflammation. Moreover, the expression patterns of Nav1.8 and ASIC3 were time and age-dependent and ASIC3 levels remained elevated only in aged rats. In vivo, selective blockade of Nav1.8 with A803467 or of ASIC3 with APETx2 alleviated mechanical and cold allodynia and also spontaneous pain in both age groups with slightly different potency. Furthermore, in vitro IL-1β up-regulated Nav1.8 expression in DRG neurons cultured from young but not old rats. We also found that while TNF-α up-regulated ASIC3 expression in both age groups, IL-6 and IL-1β had this effect only on young and aged neurons, respectively.

CONCLUSION: Inflammation-associated mechanical allodynia and spontaneous pain in the elderly can be more effectively treated by inhibiting ASIC3 than Nav1.8.