Opposite effects of ammonia and carbon dioxide on dye coupling between horizontal cells in the carp retina

Effects of ammonia (NH3) and carbon dioxide (CO2) on the membrane potential of horizontal cells and on dye coupling between the cells in isolated retinas of the carp (Cyprinus carpio) were investigated. Ammonia (less than 300 ppm NH3 in air) initially depolarized and subsequently hyperpolarized, while CO2 (10% in air) hyperpolarized the membrane potential of horizontal cells, accompanied by a diminution of both center and surround responses to spot and annular light stimuli. During the course of amplitude diminution, the center response consistently became smaller with NH3 and larger with CO2 than the surround response. In the presence of intravitreally applied DA (50 microM) or amphetamine (100 microM), a fluorescent dye Lucifer Yellow CH (LY) was found to be restricted to single injected horizontal cells. The presence of intravitreal haloperidol (100 microM) for 20-25 min or an exposure of the retina to NH3 for 5-10 min diffused the restricted LY from single injected cells to numerous neighboring cells. On the other hand, CO2 was found to restrict the injected dye to single cells, an effect similar to that of DA and opposite to that of NH3 and haloperidol. The results suggest that NH3 appears to act as a coupler while CO2 acts as an uncoupler on gap junctions between horizontal cells in the carp retina, presumably by changing the intracellular pH. In addition, a brief exposure of cells, marked with LY in the presence of DA, to the exciting light 426 nm was found to prevent the NH3-induced dye diffusion from single cells to their neighbors; the reason is unknown.