Identification of the A3 adenosine receptor in rat retinal ganglion cells.

PURPOSE Adenosine can protect retinal ganglion cells from the death that accompanies a general ischemic challenge as well as excitotoxic death. In other tissues, both A1 and A3 adenosine receptor subtypes can mediate protection. While a role for the A1 adenosine receptor in ganglion cell protection has been established, a potential for the A3 receptor has only recently been proposed. Although the pharmacology is promising, the molecular identity of the responsible receptor is unclear as previous studies were unable to detect message for the A3 receptor in retinal ganglion cells. We combined laser capture microdisection (LCM) and immunopurification with traditional and real-time PCR to unequivocally demonstrate the presence of the A3 receptor message in rat retinal ganglion cells. METHODS Retinal ganglion cells of Long-Evans rat pups were retrograde labeled with aminostilbamidine. Eyeballs were enucleated, embedded, frozen, sectioned, and fluorescent cells in the ganglion cell layer were collected with LCM. Purified ganglion cells were also isolated with a two-step panning procedure. cDNA for the A3 receptor obtained from the microdissected ganglion cell layer, immunopurified ganglion cells, whole retina and testis was amplified using RT-PCR, confirmed by DNA sequencing and compared with published sequences. A3 receptor message was also amplified using real-time PCR. Ca2+ levels in immunopanned ganglion cells were measured ratiometrically with fura-2. RESULTS RNA from immunopurified ganglion cells and from dye-loaded cells in the ganglion cell layer contained message for the A3 receptor when amplified with either traditional RT-PCR or real-time PCR. The entire encoding region was sequenced and found to be 99% identical to the published code. The sequence closely resembled the consensus form of the gene, with other sequences deviating from this default code. Molecular identification was functionally confirmed in purified ganglion cells as the A3 receptor agonist Cl-IB-MECA prevented the excessive Ca2+ rise triggered by P2X7 agonist BzATP. CONCLUSIONS Retinal ganglion cells express A3 adenosine receptor mRNA. Stimulation of this receptor can reduce the Ca2+ overload following excessive activation of P2X7 receptors.