Ionotropic purinergic receptors P2X in frog and turtle retina: Glial and neuronal localization

Purinergic signaling is represented in both the peripheral and central nervous system (CNS), and in particular in the retina, which may be regarded as a part of the CNS. While purigenic signaling is relatively well studied in mammalian retinas, little is known about it in retinas of lower vertebrates. The aim of present study was to investigate, using immunocytochemistry, the distribution of purinoreceptors P2X in retinas of frog and turtle, which are appropriate models of the brain neuron-to-glia interactions. The results showed widespread expression of all seven ionotropic purinoreceptors (P2X1–P2X7) in both frog and turtle retinas. They were predominantly expressed in Müller cells, the principal glial cells in the retina. All structures typical of Müller cells: the outer and the inner limiting membranes, the cells bodies in the inner nuclear layer, the radial processes in the inner plexiform layer (IPL), and the so called endfeet (frog) or the orthogonal arrays of particles (turtle) in the ganglion cells layer were immunostained. Colocalizations between P2X1–P2X7 and the glial cell marker Vimentin proved that the immunostaining was in the Müller cells. In addition to the glial staining, neuronal staining was also seen as fine puncta in the inner plexiform layer and by small dots and patches in the outer plexiform layer. Some cell bodies of horizontal, amacrine and ganglion cells were also stained. The results obtained imply that the purinergic P2X receptors may significantly contribute to the neuron-to-glia signaling in retinas of the lower vertebrates.