Inhibition of C-fibre mediated sensory transmission in the rat following intraplantar formalin.

Using extracellular recordings from deep dorsal horn neurones in the anaesthetized rat, the effects of intraplantar (i.pl.) administration of formalin were examined. Phasic patterns of dorsal horn firing were observed which temporally concur with previously described behavioural responses following i.pl. formalin. However, unexpectedly, formalin abolished C-fibre mediated transmission, and significantly reduced responses to noxious mechanical stimulation, while electrically evoked A-fibre responses were unaffected. This suggests that whilst the first phase formalin response is mediated by direct afferent stimulation, the second phase behavioural response is dependant on a central hyperexcitability of the recipient second-order dorsal horn neurones, which may be maintained by peripheral input only from A-fibres. This study therefore provides further evidence for a centrally-mediated secondary component of the formalin model, and for the first time, describes loss of high-threshold C-fibre input to the spinal cord during this phase.