Evidence of silent nociceptor activation during the formalin test in rats

Background: Abnormal ongoing activity in C-mechano-insensitive (C-Mi) nociceptors is seen in patients with chronic pain. These 'silent' nociceptors are often under-reported in preclinical pain models - precisely because they aren't mechanically sensitive. The formalin test is widely used as a preclinical model of persistent sensitisation characterised by two behavioural phases, where phase 2 is considered to reflect central sensitisation and the role of peripheral afferent drive in its maintenance is disputed. Increased activity in C-mechanosensitive (C-M) nociceptors has been reported during both phases, but the behaviour of C-Mi nociceptors has not been assessed. Methods: Multi-contact microneurography was conducted from the saphenous nerve of male and female rats. An activity dependent slowing protocol was used to segregate C-fibres, and responses to mechanical stimuli were assessed. Formalin- (2.5%) or saline-evoked activity was then assessed during the two test phases. Results: 168 A- and C-fibres were identified with latency tracking, and classified using activity dependent slowing into C-M and C-Mi nociceptors. ADS-classified C-Mi nociceptors were active in both phase 1 and phase 2 (33.3% and 31.0%, n=18 and n=16, respectively). We find no evidence that formalin mechanically sensitises C-Mi nociceptors or alters the ADS profiles of C-fibres. There was no sex specific difference in C-fibre classes or the effect of formalin. Conclusions: These results indicate the presence of ongoing activity in a substantial population of C-Mi nociceptors in a commonly used persistent pain assay that may play a role in the sensitisation that is seen in phase 2 of the behavioural response.