Synaptic Localization of P2X₇ Receptors in the Rat Retina

The distribution of P2X₇ receptor (P2X₇R) subunits was studied in the rat retina using a subunit-specific antiserum. Punctate immunofluorescence was observed in the inner and outer plexiform layers. Double labeling of P2X₇ and the horizontal cell marker, calbindin, revealed extensive colocalization in the outer plexiform layer (OPL). Significant colocalization of P2X₇R and kinesin, a marker of photoreceptor ribbons, was also observed, indicating that this receptor may be expressed at photoreceptor terminals. Furthermore, another band of P2X₇R puncta was identified below the level of the photoreceptor terminals, adjacent to the inner nuclear layer (INL). This band of P2X₇R puncta colocalized with the active-zone protein, bassoon, suggesting that "synapse-like" structures exist outside photoreceptor terminals. Preembedding immunoelectron microscopy demonstrated P2X₇R labeling of photo-receptor terminals adjacent to ribbons. In addition, some horizontal cell dendrites and putative "desmosome-like" junctions below cone pedicles were labeled. In the inner plexiform layer (IPL), P2X₇R puncta were observed surrounding terminals immunoreactive for protein kinase C-α, a marker of rod bipolar cells. Double labeling with bassoon in the IPL revealed extensive colocalization, indicating that P2X₇R is likely to be found at conventional cell synapses. This finding was confirmed at the ultrastructural level: only processes presynaptic to rod bipolar cells were found to be labeled for the P2X₇R, as well as other conventional synapses. These findings suggest that purines play a significant role in neurotransmission within the retina, and may modulate both photoreceptor and rod bipolar cell responses.