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

Using extracellular recordings from deep dorsal horn neurones in the anaest hetized rat, the effects of intraplantar (i.pl.) administration of formalin were examined. Phasic patterns of dorsal horn firing were observed which t emporally concur with previously described behavioural responses following Lpl. formalin. However, unexpectedly, formalin abolished C-fibre mediated t ransmission, and significantly reduced responses to noxious mechanical stim ulation, while electrically evoked A-fibre responses were unaffected. This suggests that whilst the first phase formalin response is mediated by direc t afferent stimulation, the second phase behavioural response is dependant on a central hyperexcitability of the recipient second-order dorsal horn ne urones, which may be maintained by peripheral input only from A-fibres. Thi s study therefore provides further evidence for a centrally-mediated second ary 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. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.