Ca2+-permeable P2X receptor channels in cultured rat retinal ganglion cells

Abstract

ATP has been identified as an excitatory neurotransmitter in both the CNS and peripheral nervous system; however, little is known about the functional properties of ATP-gated channels in central neurons. Here we used a culture preparation of the postnatal rat retina to test the responsiveness of identified retinal ganglion cells (RGCs) and putative amacrines to exogenous ATP and other purinoceptor agonists. Rapidly activating ATP- induced currents (I(ATP)) were exclusively generated in a subpopulation (~65%) of RGCs. The latter were identified by Thy1.1 immunostaining, repetitive firing patterns, and activation of glutamatergic autaptic currents. None of the putative amacrine cells was ATP-sensitive. I(ATP) could be induced with ATP, ADP, and α,β-mATP but not with adenosine. It was antagonized by suramin. The current-voltage relationship of I(ATP) showed marked inward rectification. Dose-response analysis yielded an EC50 of 14.5 μM, with a Hill coefficient of 0.9. Noise analysis of I(ATP) suggested a mean single channel conductance of 2.3 pS. Retinal P2X purinoceptor channels exhibited a high permeability for Ca2+ · P(Ca)/P(Cs) obtained from reversal potentials of I(ATP) under bi-ionic conditions amounted to 2.2 ± 0.7. In the majority of cells, the decay of I(ATP) was biphasic. The degree of current inactivation during the first 2 sec of agonist application was highly variable. Heterogeneity was also found with respect to the sensitivity to ADP and α, β-mATP and the blocking action of suramin, suggesting expression of multiple P2X receptor subtypes. Our results indicate that activation of P2X receptor channels represents an important pathway for Ca2+ influx in postnatal RGCs.