Abstract
The retina converts the image into the neural signal and, in this process, the individual photoreceptors work as pixels. But the conversion of the image is not simply pixel by pixel. An important function of the retina is to enhance the contrast of the image by lateral inhibition. As a result, a neuron in the early visual system has a concentric receptive field with a center-surround antagonism. Many vision scientists agree now that horizontal cells (HCs) contribute to the formation of the center-surround receptive field. HCs have a large receptive field due to electrical coupling. They have AMPA receptors, and a tonic glutamate release from photoreceptors keeps HCs depolarized in the dark. During surround illumination HCs are hyperpolarized. We recently found that pH of the invaginating synaptic cleft of the cone terminal is related to the membrane voltage of HCs (Hirasawa & Kaneko, 2003). It is kept acidic in the dark and is alkalinized by surround illumination. The pH change we found in the retinal slices of the newt disappeared when the slice was superfused with a solution with enriched pH buffering capacity. We concluded that the surround illumination enhances the amount of L-glutamate release from the alkalinized cone terminal (the effect opposite to spot illumination), resulting in the formation of the center-surround receptive field of the second- and higher-order neurons in the visual system. By an imaging technique, we measured the pH of the extracellular space of an HC isolated from the carp retina and found that depolarization of HC acidifies the immediate surrounding of the HC. The mechanisms of pH change are still under study. [Jpn J Physiol 55 Suppl:S4 (2005)]
