Adenosine-evoked hyperpolarization of retinal ganglion cells is mediated by G-protein-coupled inwardly rectifying K⁺ and small conductance Ca²⁺-activated K⁺ channel activation

Adenosine is a neuromodulator that activates presynaptic receptors to regulate synaptic transmission and postsynaptic receptors to hyperpolarize neurons. Here, we report that adenosine-induced hyperpolarization of retinal ganglion cells is produced by the activation of A₁ receptors, which initiates a signaling cascade that activates G-protein-coupled inwardly rectifying K⁺ (GIRK) channels and small conductance Ca ²⁺-activated K⁺ (SK) channels. Rat retinal ganglion cells were stimulated by focal ejection of the adenosine receptor agonist 5′-N-ethylcarboxamidoadenosine (NECA) while cell activity was monitored with whole-cell patch recordings and Ca²⁺imaging. Focal ejections of NECA evoked outward currents in all cells tested and reduced light- and depolarization-induced spiking. The NECA-evoked current was abolished by the A₁ antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) but was unaffected by A_2a, A_2b, and A₃ antagonists, indicating that the response was mediated entirely by A₁ receptors. The GIRK channel blocker rTertiapin-Q diminished the NECA-evoked inhibitory current by 56 ± 12%, whereas the SK channel blocker apamin decreased the NECA-induced current by 42 ± 7%. The SK component of the NECA-evoked current coincided with an increase in intracellular Ca²⁺ and was blocked by IP₃ receptor antagonists and depletion of internal Ca ²⁺ stores, suggesting that A₁ receptor activation leads to an increase in IP₃ , which then elevates intracellular Ca ²⁺and activates SK channels. This A₁-mediated, prolonged SK channel activation has not been described previously. The coactivation of GIRK and SK channels represents a novel mechanism of adenosine-mediated neuromodulation that could contribute to the regulation of retinal ganglion cell activity.