Excitation of rat spinal ventral horn neurons by purinergic P2X and P2Y receptor activation

ATPγS, a nonhydrolyzable ATP analog, was found to dose-dependently generate an inward current at a holding potential of −70 mV (EC50 = 43 μM) in lamina IX neurons of rat spinal cord slices using the whole-cell patch-clamp technique. This inward current had an extrapolated reversal potential of −9 mV and was resistant to the Na+-channel blocker tetrodotoxin, glutamate-receptor antagonists or nominally Ca2+-free medium. ATPγS also increased the frequency and amplitude of glutamatergic spontaneous excitatory postsynaptic current (sEPSC); this action was dose-dependent and sensitive to tetrodotoxin. Unlike ATPγS, the P2X-receptor agonist, BzATP or α,β-methylene ATP, did not change holding currents, but the current response produced by ATPγS disappeared in the presence of the P2-receptor antagonist PPADS. The sEPSC frequency and amplitude increase was observed with α,β-methylene ATP, but not with the P2Y-receptor agonist, 2-methylthio ADP, UTP or UDP. The current response by ATPγS was suppressed by the addition of GDPβS into the patch-pipette solution. As for ATPγS, 2-methylthio ADP produced an inward current, while UTP and UDP had no effect on holding currents. The P2Y1-receptor antagonist MRS2179 inhibited the ATPγS-induced inward current, but did not affect the sEPSC frequency and amplitude increase produced by ATPγS. These data indicate that extracellular ATP increases the excitability of lamina IX neurons by membrane depolarization (probably through non-selective cation-channel activation) and spontaneous excitatory transmission enhancement, which may be mediated by P2Y1 and P2X receptors, respectively. This finding supports the idea that purinergic receptor antagonists provide a therapy for spinal cord injury.