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 (IATP) 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. IATP could be induced with ATP, ADP, and α,β-mATP but not with adenosine. It was antagonized by suramin. The current–voltage relationship ofIATP showed marked inward rectification. Dose–response analysis yielded an EC50 of 14.5 μm, with a Hill coefficient of 0.9. Noise analysis ofIATP suggested a mean single channel conductance of 2.3 pS. Retinal P2X purinoceptor channels exhibited a high permeability for Ca2+. PCa/PCs obtained from reversal potentials of IATP under bi-ionic conditions amounted to 2.2 ± 0.7. In the majority of cells, the decay ofIATP 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.
