Abstract
It is widely accepted that horizontal cells (HCs) contribute to the formation of the center-surround receptive field, which is important to enhance the contrast of the image by lateral inhibition. HCs have a large receptive field due to electrical coupling. HCs are depolarized in the dark by a tonic glutamate release from photoreceptors (PRs). During surround illumination HCs are hyperpolarized. Three mechanisms have been proposed as responsible to the formation of receptive field surround. (1) GABA-mediated feedback from HC to PRs, (2) the electric field produced by the current flowing into HCs, and (3) proton extruded from HC into the invaginating cleft of the cone terminal. Tatsukawa et al (2005) have shown that GABAergic inputs from horizontal cells to cone is certainly present, but their contribution is very limited. Both the ephaptic hypothesis and proton hypothesis propose that shifts in ICa activation are responsible. In the ephaptic hypothesis current flowing into the invaginating cone synapse during changes in HC membrane potential through connexin 26 hemigap junctions at the tips of HC dendrites produce the requisite current sink (Fahrenfort et al. 2004; Kamermans et al. 2001). The proton hypothesis suggests that the modulation of ICa in cone PR is related to the membrane voltage of HCs (Hirasawa & Kaneko, 2003). It is kept acidic in the dark and is alkalinized by surround illumination. Recent studies by Vessey et al (2005) and by Cadetti and Thoreson (2006) favor the proton hypothesis, and our group succeeded to demonstrate that polarization of HC causes pH changes of the immediate surrounding extracellular surface of HCs. [J Physiol Sci. 2007;57 Suppl:S18]
