Neonatal capsaicin attenuates mechanical nociception in the rat

The contribution of chemosensitive neurons to mechanical nociception and hyperalgesia was studied by evaluating mechanical nociceptive threshold and the effect of three directly-acting hyperalgesic agents (prostaglandin E2, prostaglandin E1 and the A2-adenosine agonist, CGS21680) in rats treated neonatally with capsaicin. Mechanical nociceptive threshold was quantified by the Randall-Selitto paw-withdrawal method. The baseline mechanical paw-withdrawal threshold of the capsaicin-treated rats was 40% higher than that of the untreated rats. In the capsaicin-treated rats mechanical hyperalgesia was not induced by prostaglandin E2, prostaglandin E1 or CGS21680. These results are consistent with the suggestion that mechanical nociception and hyperalgesia induced by inflammatory substances is mediated by action on capsaicin-sensitive nociceptors.