Neuronal coupling in the central nervous system: lessons from the retina.

The retina is a model system for studying gap junctional intercellular communication in the CNS. The cellular coupling can be graphically visualized in retinal whole mounts by injecting small cationic tracers into microscopically identified neurons; the pattern of tracer coupling shown by each type of retina neuron is highly stereotyped, with many types of amacrine cells and ganglion cells showing complex patterns of both homologous and heterologous coupling. Parallel physiological studies have demonstrated that the gap junctions can be modulated dynamically by neurotransmitters and by the level of ambient illumination. Taken together, the numerous structural and functional studies on gap junctions in the retina provide powerful support for the concept that electrical synapses are complex components of neuronal circuits, having many of the attributes normally ascribed to chemical synapses.