Formalin-induced increase in P2X(3) receptor expression in dorsal root ganglia: implications for nociception.

1. ATP-gated P2X receptors in nociceptive sensory neurons participate in the transmission of pain signals from the periphery to the spinal cord. The effect of formalin on the expression of P2X(3) receptors in dorsal root ganglia (DRG) was characterized using molecular and immunological approaches and the patch-clamp technique. 2. Adult Sprague-Dawley rats were injected with 100 microL of 5% formalin in the planar surface of the hindpaw and were killed 30 min and 1, 3, 6, 12, 24 and 48 h later for in vitro analyses. The expression and distribution of P2X(3) receptors in the lumbar spinal cord and in L5/L6 DRG were examined; 24 and 48 h after formalin injection, currents in neurons were examined using whole-cell patch-clamp recording. 3. Western blots showed that anti-P2X(3) antibody recognized a major monomer of approximately 64 kDa in DRG. Immunoreactivity for P2X(3) receptors was detected predominantly in the cytoplasm and plasma membrane of small (< 25 microm) and middle-sized (25-50 microm) DRG neurons. Expression of the P2X(3) transcript in the DRG was unchanged 30 min and 1 h after formalin injection, but increased after 12 h. There was no distinct change in P2X(3) immunostaining of the spinal cord lamina at 30 min or 1 h after injection, but after 24 h P2X(3) labelling increased. At 24 h after the formalin injection, currents in isolated small and middle-sized DRG neurons were increased by 1 micromol/L alpha,beta-methylene-ATP. These currents were completely inhibited by 1 micromol/L A-317491, a potent and selective P2X(3) receptor antagonist. 4. These data suggest that formalin injection leads to early upregulation of P2X(3) expression in the spinal cord and DRG and that this may be one of the mechanisms giving rise to nociception.