Abstract
It has been shown that pH of the invaginating synaptic cleft of the cone terminal is controlled by the membrane voltage of hirozontal cells, low in the dark and high during surround illumination (Hirasawa & Kaneko, 2003). High pH enhances the amount of L-glutamate released from the cone terminal, resulting in the formation of the antagonistic surround component of receptive field of bipolar cells and the higher-order neurons in the visual system. By an imaging technique, we measured the pH of the immediate surrounding space of a horizontal cell (pHo) isolated from the carp retina to study the mechanisms producing the pH change. Horizontal cells were stained with pH-sensitive dye, 5-hexadecanoylaminofluorescein (H110), and the ratio of the fluorescent emission of 530 nm excited by 490 nm to that excited by 440 nm light was calculated. Cells were superfused with a Ringer solution buffered with 28 mM NaHCO3. Depolarization of the horizontal cell by bath-application of 100 mM K+ solution ( [nearly equal] 100 mV depolarization) lowered pHo by 0.21±0.05 pH unit. The amount of pH change was related monotonically to the amount of depolarization. Since the cleft of the invaginating synapse of cone terminals is a space where communication to the open extracellular space is much restricted, the pH change of the cleft will be larger than the open space. It will probably result in suppression of transmitter release from the cone terminals. [Jpn J Physiol 55 Suppl:S164 (2005)]
