Adenosine A_2A receptor up-regulates retinal wave frequency via starburst amacrine cells in the developing rat retina

Background: Developing retinas display retinal waves, the patterned spontaneous activity essential for circuit refinement. During the first postnatal week in rodents, retinal waves are mediated by synaptic transmission between starburst amacrine cells (SACs) and retinal ganglion cells (RGCs). The neuromodulator adenosine is essential for the generation of retinal waves. However, the cellular basis underlying adenosine's regulation of retinal waves remains elusive. Here, we investigated whether and how the adenosine A _2A receptor (A_2AR) regulates retinal waves and whether A_2AR regulation of retinal waves acts via presynaptic SACs. Methodology/Principal Findings: We showed that A_2AR was expressed in the inner plexiform layer and ganglion cell layer of the developing rat retina. Knockdown of A_2AR decreased the frequency of spontaneous Ca ²⁺ transients, suggesting that endogenous A_2AR may up-regulate wave frequency. To investigate whether A_2AR acts via presynaptic SACs, we targeted gene expression to SACs by the metabotropic glutamate receptor type II promoter. Ca²⁺ transient frequency was increased by expressing wild-type A_2AR (A_2AR-WT) in SACs, suggesting that A_2AR may up-regulate retinal waves via presynaptic SACs. Subsequent patch-clamp recordings on RGCs revealed that presynaptic A _2AR-WT increased the frequency of wave-associated postsynaptic currents (PSCs) or depolarizations compared to the control, without changing the RGC's excitability, membrane potentials, or PSC charge. These findings suggest that presynaptic A_2AR may not affect the membrane properties of postsynaptic RGCs. In contrast, by expressing the C-terminal truncated A _2AR mutant (A_2AR-ΔC) in SACs, the wave frequency was reduced compared to the A_2AR-WT, but was similar to the control, suggesting that the full-length A_2AR in SACs is required for A _2AR up-regulation of retinal waves. Conclusions/Significance: A _2AR up-regulates the frequency of retinal waves via presynaptic SACs, requiring its full-length protein structure. Thus, by coupling with the downstream intracellular signaling, A_2AR may have a great capacity to modulate patterned spontaneous activity during neural circuit refinement.