抄録
Inhibitory action of extracellular H+ on neural activity occurs through pH-sensitive channels, such as voltage-gated Ca 2+ channels and NMDA receptors. In order to interpret the formation of receptive field surrounds in the retinal neurons, it has been suggested that GABA mediates the feedback signal from horizontal cells (HCs) to cones. Hirasawa and Kaneko (2003) proposed an H+-mediated feedback hypothesis to substitute for the GABA hypothesis since picrotoxin, a GABA-receptor antagonist, could not suppress the feedback signal. To verify the idea that the depolarized HCs release H+ we measured, by a fluorescence ratio imaging technique, the pH of the immediate external surface (pHo) of HCs isolated from carp or goldfish retina. When HCs stained by 5-hexadecanoylaminofluorescein, a pH-sensitive lipophilic dye, were depolarized by application of kainate or by high extracellular K+, pHo decreased. The amount of pHo reduction was monotonically dependent on the amount of depolarization, as much as 0.21 ± 0.05 pH unit by 100 mM K+. Reduction of pHo was suppressed by 0.4 μM bafilomycin A1, a specific inhibitor of V-ATPase (proton pump), suggesting that the HC depolarization enhanced an outward H+ flux by voltage-sensitive H+ pump. Immunohistochemical analysis using an anti V-ATPase antibody revealed the existence of V-ATPase on the HC cytoplasmic membrane. Double labelling of V-ATPase and GAD revealed that the V-ATPase was essentially localized at the surface of HC which would affect the cone terminals. [J Physiol Sci. 2007;57 Suppl:S50]
