Characterization of Extracellular H+ Fluxes from Isolated Glia and Slices of the Vertebrate Retina

Active glia-neuron interactions are crucial for the proper development and function of the nervous system. Bidirectional communication between neurons and glial cells has been shown to have modulatory effects on neuronal signaling. The exact mechanisms which glial cells employ to influence the behavior of neurons have not been fully understood. In the vertebrate retina, extracellular pH can act as a modulator of neuronal signaling by altering the activity of calcium channels, particularly on photoreceptor terminals. Experiments have demonstrated that when the pH of the extracellular medium is lowered to more acidic values, the calcium flux into photoreceptors is lessened. The exact cell type, which mediates these changes in extracellular pH, is still debatable with mounting evidence pointing to the role of horizontal cells in this process. This work presents evidence that retina Müller glial cells of tiger salamander are capable of mediating changes in extracellular proton concentration. Müller glia respond to extracellular ATP with a robust extracellular acidification measured with ion-selective microelectrodes. The mechanism through which these cells change the extracellular acidity involves the activation of membrane ATP receptors and the mobilization of intracellular signaling molecules with resulting intracellular calcium increases. The ATP-induced sustained acidifications are observed from isolated Müller glia as well as in retinal slices. Interestingly, Müller cells of the retinae of many other vertebrates tested, including Macaques and human, also respond to extracellular ATP through acidifications of the extracellular space, suggesting that this is a conserved and potentially physiologically important response. Müller cells respond to extracellular glutamate as well, but the changes in extracellular acidity are opposite of these evoked by ATP: glutamate induces Müller cell-mediated extracellular transient alkalinization, which seems to be due to the activity of membrane glutamate transporters. In slices, the initial transient alkalinization induced by glutamate is followed by a pronounced, sustained acidification, whose mechanism is independent of the glutamate transporters’ activity. This data suggest Müller glia could potentially play a role in extracellular proton modulation of neuronal activity in the vertebrate retina