Peripheral adenosine 5'-triphosphate enhances nociception in the formalin test via activation of a purinergic p2X receptor.

The pronociceptive effects of adenosine 5'-triphosphate (ATP) were examined in the low concentration formalin model (0.5%) by coadministration of ATP, ATP analogs (alpha,beta-methylene-ATP and 2-methylthio-ATP) and antagonists (suramin, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid) with formalin and determining effects on the expression of flinching behaviours. Coadministration of ATP (5-500 nmol) with formalin enhanced phase 2 (12-60 min after injection) but not phase 1 (0-10 min after injection) responses. alpha,beta-methylene-ATP (0.5-50 nmol) but not 2-methylthio-ATP (50-500 nmol) produced a similar enhancement of activity, generating an order of potency of alpha,beta-methylene-ATP, ATP >> 2-methylthio-ATP. This enhancement was primarily expressed in the latter part of phase 2, 30-60 min after injection. Coadministration of suramin 50-500 nmol, a non-selective P2X and P2Y purinoceptor antagonist and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid 5-500 nmol, a selective P2X purinoceptor antagonist, dose-dependently inhibited the augmentation of the formalin response by ATP 50 nmol, but did not reduce the response to formalin itself. Pretreatment for 30 min with higher doses of suramin inhibited the response to formalin (0.5%, 1.5%) and this appeared to be by a systemically mediated action as it was seen following administration into the contralateral paw. The results of this study provide evidence in support of a P2X purinoceptor mediated augmentation of the pain signal by ATP. The delayed time-course of the effect suggests that it may occur in concert with other mediators that are recruited by the inflammatory process, rather than reflecting a direct depolarization of sensory nerves. Other behavioural paradigms may be required to examine the fast onset, direct effect. Suramin appears to exert both local and systemic effects on the expression of pain behaviours in response to formalin.