Neurotransmitter modulation of extracellular H⁺ fluxes from isolated retinal horizontal cells of the skate

Self-referencing H⁺-selective microelectrodes were used to measure extracellular H⁺ fluxes from horizontal cells isolated from the skate retina. A standing H+ flux was detected from quiescent cells, indicating a higher concentration of free hydrogen ions near the extracellular surface of the cell as compared to the surrounding solution. The standing H⁺ flux was reduced by removal of extracellular sodium or application of 5-(N-ethyl-N-isopropyl) amiloride (EIPA), suggesting activity of a Na⁺-H⁺ exchanger. Glutamate decreased H⁺ flux, lowering the concentration of free hydrogen ions around the cell. AMPA/kainate receptor agonists mimicked the response, and the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) eliminatedthe effects of glutamate and kainate. Metabotropic glutamate agonists were without effect. Glutamate-induced alterations in H⁺ flux required extracellular calcium, and were abolished when cells were bathed in an alkaline Ringer solution. Increasing intracellular calcium by photolysis of the caged calcium compound NP-EGTA also altered extracellular H⁺ flux. Immunocytochemical localization of the plasmalemma Ca²⁺-H⁺-ATPase (PMCA pump) revealed intense labelling within the outer plexiform layer and on isolated horizontal cells. Our results suggest that glutamate modulation of H⁺ flux arises from calcium entry into cells with subsequent activation of the plasmalemma Ca²⁺-H⁺-ATPase. These neurotransmitter-induced changes in extracellular pH have the potential to play a modulatory role in synaptic processing in the outer retina. However, our findings argue against the hypothesis that hydrogen ions released by horizontal cells normally act as the inhibitory feedback neurotransmitter onto photoreceptor synaptic terminals to create the surround portion of the centre-surround receptive fields of retinal neurones.