Physiological studies on ion channels and synaptic mechanisms of retinal neurons

Akimichi Kaneko

The vertebrate retina consists of three layers of cells. The first layer contains rods and cones in which the incident light is converted into a neural activity. The last layer is made of ganglion cells, the axon of which makes the optic nerve that carries the retinal output to the brain. Bipolar cells, located in the middle layer, connect photoreceptors and ganglion cells. Two types of neurons in the middle layer, horizontal and amacrine cells, provide lateral signal pathways, which are thought to mediate lateral inhibition between the neighboring radial pathways consisting of photoreceptor ? bipolar cell ? ganglion cell. In the past decades, signal flow in the radial pathways has been relatively well understood. Nevertheless, the properties and functional role of lateral pathways, particularly those of amacrine cells remain open. We recently studied amacrine cells in culture. They are identified by an amacrine cell-specific antibody (HPC-1/Syntaxin) and by an anti-GABA antibody (Fig. 1). To examine the generation and propagation of action potentials in the dendrites, we applied two patch clamp pipettes on the same cell, one on the soma and another on the dendrite. By using "the action potential clamp method", we have demonstrated that the action potential generated in the soma can propagate to the dendrite, and the propagation is mediated by the TTX-sensitive, voltage-dependent Na+ current. Furthermore, the amplitude and propagation of the dendritic action potential is modulated by locally applied GABA. These results suggest that the signal in amacrine cells is processed locally in dendrites, not as a whole in the soma.

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Morphology of rat amacrine cells cultured for 10 days. A-C: Immunohistological identification of amacrine cells. A: Bright field image of cultured amacrine cells. Large multipolar cells (a and b) and small cells (c and d) are seen. B: Cells a and b were immunoreactive to HPC-1/Syntaxin antibody (red). Cells c and d were negative. C: Cells a and b were also immunopositive for GABA (green), while cells c and d were negative. Scale bar = 10 um. D-E: An amacrine cell recorded by the patch clamp technique in the whole cell configuration with a pipette containing 0.1 % Lucifer yellow. D: Photomicrography taken by the Nomarski optics. The diameter of the dendritic field was approximately 250 um. Scale bar = 50 um. E: The same cell as D observed under the fluorescent microscopy.