Alteration of spontaneous firing rate of primary myelinated afferents by ATP in adjuvant-induced inflamed rats.

The action of adenosine 5'-triphosphate (ATP) on primary myelinated mechanosensitive afferents was investigated in adjuvant-induced inflamed rats. ATP injected intravenously increased the spontaneous firing rate in most of the A fibers from sciatic nerve innervating inflamed hindpaw, whereas it affected only a few units in normal rats. The effects of ATP were blocked or attenuated by competitive P2 receptor antagonist suramin or reactive blue 2 in most of the fibers tested. Degranulation of mast cells with compound 48/80 did not change the response rate of afferents to ATP in inflamed rats while chemical sympathectomy with 6-hydroxydopamine decreased it to some extent, suggesting an involvement of sympathetic efferents in mediating ATP effects. The results support the view that ATP could change the activity of large mechanosensitive afferents via P2 receptors under state of inflammation, which may be related to touch-evoked pain.