Cellular mechanisms mediating activity-dependent extracellular space shrinkage in the retina

Volume transmission plays an essential role in CNS function, with neurotransmitters released from synapses diffusing through the extracellular space (ECS) to distant sites. Changes in the ECS volume fraction (α) will influence the diffusion and the concentration of transmitters within the ECS. We have recently shown that neuronal activity evoked by physiological photic stimuli results in rapid decreases in ECS α as large as 10% in the retina. We now characterize the cellular mechanisms responsible for this ECS shrinkage. We find that block of inwardly rectifying K⁺ channels with Ba²⁺ , inhibition of the Na⁺ /K⁺ /2Cl^- cotransporter with bumetanide, or block of AQP4 water channels with TGN-020 do not diminish the light-evoked ECS decrease. Inhibition of the Na⁺ /HCO₃ ^- cotransporter by removing HCO₃ ^- from the superfusate, in contrast, reduces the light-evoked ECS decrease by 95.6%. Inhibition of the monocarboxylate transporter with alpha-cyano-4-hydroxycinnamate (4-CIN) also reduces the ECS shrinkage, but only by 32.5%. We tested whether the swelling of Müller cells, the principal glial cells of the retina, is responsible for the light-evoked ECS shrinkage. Light stimulation evoked a 6.3% increase in the volume of the fine processes of Müller cells. This volume increase was reduced by 97.1% when HCO₃ ^- was removed from the superfusate. We conclude that a large fraction of the activity-dependent decrease in ECS α is generated by the activation of the Na⁺ /HCO₃ ^- cotransporter in Müller cells. The monocarboxylate transporter may also contribute to the response.