Decoupling of horizontal cells in carp and turtle retinae by intracellular injection of cyclic AMP.

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Abstract
Horizontal cells are electrically coupled through gap junctions. This is a disadvantage in elucidating the membrane properties of the cells. In order to block gap junctions, adenosine 3'',5''-cyclic monophosphate (cyclic AMP) or its analogues, dibutyryl cyclic AMP and 8-bromo cyclic AMP, were ionophoretically injected into horizontal cells of the carp or turtle retina. Before injection of the chemicals the input resistance of the cell was so low as to be unmeasurable, because the applied current leaked through gap junctions. After injection, however, the input resistance was significantly increased. After the injection dye-coupling bteween horizontal cells was not observed when examined by intracellular injection of Lucifer Yellow dye, supporting the idea that high concentrations of intracellular cyclic AMP block gap junctions. In this situation responses to light delivered to the receptive field centre were increased in amplitude, while responses to light delivered to the receptive field surround were greatly diminished. After injection horizontal cells were readily polarized by conventional intracellular current injection. The hyperpolarizing light responses in carp and turtle luminosity-type cells (H1 cells) could be reversed by depolarizing the horizontal cells, and the reversal potentials were estimated to be about 0 mV. In addition, the resistance increase which accompanied the hyperpolarizing light responses could be detected. In turtle biphasic chromaticity-type horizontal cells (H2 cells), hyperpolarizing light responses to shorter wavelengths and depolarizing ones to longer wavelengths could be reversed by depolarizing the horizontal cells. Both responses have almost the same reversal potential at about 0 mV. The membrane resistance changes associated with light responses were also detected; the resistance increased during the hyperpolarizing response, while it decrased during the depolarizing response. These observations suggest that the ionic mechanisms of both responses are probably the same, irrespective of their polarities.