Synaptic inputs to turtle horizontal cells analyzed after blocking of gap junctions by intracellular injection of cyclic nucleotides.

Intracellular injection of cAMP or cGMP into turtle horizontal cells significantly increased the input resistances, and the cells could thus be easily polarized by current injection, suggesting that the cyclic nucleotides blocked gap junctions between cells. Then, synaptic inputs onto triphasic chromaticity-type cells were analyzed. Hyperpolarizing and depolarizing light responses were all reduced with depolarizing current, and their polarities were reversed by further depolarization. Their reversal potentials coincided at around 0 mV. This level was the same as observed in luminosity-type and biphasic chromaticity-type cells, suggesting that the ionic mechanisms of synaptic transmission are common among horizontal cell types.