P2X₂ receptors on ganglion and amacrine cells in cone pathways of the rat retina

Extracellular ATP is known to mediate fast, excitatory neurotransmission through activation of ionotropic P2X receptors. In this study, the localization of the P2X₂ receptor (P2X₂R) subunit was studied in rat retina by using immunofluorescence immunohistochemistry and preembedding immunoelectron microscopy. The P2X₂R was observed in large ganglion cells as well as in a subset of amacrine cells. Double labeling revealed that 96% of all P2X₂R-immunoreactive amacrine cells showed γ-aminobutyric acid (GABA) immunoreactivity. Subsets of P2X ₂R-immunoreactive amacrine cells expressed nitric oxide synthase and substance P; however, no colocalization was observed with choline acetyltransferase, vasoactive intestinal peptide, or tyrosine hydroxylase. Nearest-neighbor analysis confirmed that P2X₂Rs were expressed by a heterogeneous population of amacrine cells. The synaptic connectivity of P2X₂R amacrine cells was also investigated. It was interesting that P2X₂R-immunoreactive amacrine cell dendrites stratified in the sublaminae of the inner plexiform layer occupied by cone, but not rod bipolar cell axon terminals. Immunoelectron microscopy revealed that P2X ₂-immunoreactive amacrine cell processes were associated with cone bipolar cell axon terminals as well as other conventional synapses in the inner plexiform layer. Taken together, these data provide further evidence for the involvement of extracellular ATP in neuronal signaling in the retina, particularly within cone pathways.