Neuron-specific distribution of P2X7 purinergic receptors in the monkey retina

Extracellular ATP is a signaling molecule, working through P2X purinoceptors in the nervous system. P2X7 is a major subtype of the purinoceptors in the brain, where it is expressed mostly in glia cells and considered to work as a trigger of cytolysis. In the rodent retina, however, P2X7 is expressed in several classes of neurons including ganglion cells. In the present study we identified cells immunopositive for P2X7 by double immunolabeling. Immunoreactivity for P2X7 was observed in the inner nuclear layer (INL), the inner plexiform layer (IPL), and the ganglion cell layer (GCL). In the INL, strongly immunopositive cells corresponded to the subpopulation of horizontal cells. In the IPL, fine processes were immunopositive. In the GCL, most of the ganglion cells showed P2X7 immunoreactivity. At the ultrastructural level, immunoreactivity was confirmed in the cytoplasm of ganglion cells. No P2X7 immunoreactivity was found in non-neural cells, i.e., Müller cells or microglia. The immunohistochemical distribution of other purinoceptor subtypes (P2X1, P2X2, and P2X4) was also examined in the monkey retina. Immunoreactivity for P2X1 was strongly detected in a band, in sublamina a of the IPL. The band existed at almost the same level as tyrosine hydroxylase immunoreactivity, but did not seem to actually overlap. P2X2 was not expressed in the retina, and P2X4 was only faintly expressed at the scleral margin of the INL. Because P2X7 in the primate retina is expressed exclusively in neurons, it may in this location be involved in neural transmission rather than in cytolysis, as found for glia cells.