ATPgammaS, a nonhydrolyzable
ATP analog, was found to dose-dependently generate an inward current at a holding potential of -70 mV (EC(50)=43 microM) 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 Ca(2+)-free medium.
ATP gamma S also increased the frequency and amplitude of glutamatergic spontaneous excitatory postsynaptic current (sEPSC); this action was dose-dependent and sensitive to
tetrodotoxin. Unlike
ATP gamma S, the P2X-receptor agonist,
BzATP or
alpha,beta-methylene ATP, did not change holding currents, but the current response produced by
ATP gamma S disappeared in the presence of the P2-receptor antagonist
PPADS. The sEPSC frequency and amplitude increase was observed with
alpha,beta-methylene ATP, but not with the P2Y-receptor agonist, 2-methylthio
ADP,
UTP or
UDP. The current response by
ATP gamma S was suppressed by the addition of
GDP beta S into the patch-pipette
solution. As for
ATP gamma S, 2-methylthio
ADP produced an inward current, while
UTP and
UDP had no effect on holding currents. The P2Y(1)-receptor antagonist
MRS2179 inhibited the
ATP gamma S-induced inward current, but did not affect the sEPSC frequency and amplitude increase produced by
ATP gamma S. 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 P2Y(1) and P2X receptors, respectively. This finding supports the idea that
purinergic receptor antagonists provide a
therapy for
spinal cord injury.