[Intracellular messengers and their roles in retinal gap junctions].

Intracellular injection of cAMP or cGMP into retinal horizontal cells blocked the gap junctions between the cells. Similar results were obtained when L-arginine was injected into the cells. L-Arginine is a substrate of nitric oxide (NO) which is believed to activate soluble guanylate cyclase to produce cGMP. The endothelium-derived relaxing factor (EDRF) in the blood vessels has been identified as NO. With respect to the nervous systems, production of NO and its synthase have been found in the brain, and NO has been discussed in relation to such phenomena as synaptic plasticity, long-term potentiation, and development. The decoupling effect of L-arginine suggests the presence of the L-arginine: NO: cGMP pathway in the retina as well. Before injection of cAMP, cGMP or L-arginine, the applied current leaked through the gap junctions. After the injection, the horizontal cells could be easily polarized by intracellular current injection, and the synaptic mechanisms were analyzed by measuring I-V curves. In luminosity-type (H1) horizontal cells, the reversal potential of light responses was estimated at about 0 mV. In addition, conductance decreases were detected during illumination. These findings support the widely accepted hypothesis that glutamate is released from the photoreceptors in darkness. In chromaticity-type cells (H2 and H3 cells), the reversal potentials of light responses were about 0 mV, suggesting that the ionic mechanisms of synaptic transmission are common among horizontal cell types.