Intracellular release of caged calcium in skate horizontal cells using fine optical fibers.

Abstract

Horizontal cells are second order retinal neurons that receive direct input from photoreceptors and are involved in establishing a number of key features of visual perception. These cells mediate the formation of the inhibitory surround portion of the classic center-surround receptive fi elds of retinal neurons (1). The centersurround receptive fi elds are important for enhancing the contrast of visual objects and are also involved in color perception. The molecular mechanisms by which horizontal cells send lateral inhibitory signals to photoreceptors and bipolar cells are still under debate, but protons released from horizontal cells have been hypothesized to alter the fl ow of visual information within the outer retina (2). Indeed, small changes in extracellular pH can dramatically alter neural signals within the retina, in part because photoreceptor calcium channels are highly sensitive to protons. When protons bind to photoreceptor calcium channels, the voltage activation range of the channels shifts to more depolarized potentials and the overall conductance of the cell to calcium is reduced, which signifi cantly reduces neurotransmitter release (3). Our previous work has shown that glutamate, the neurotransmitter released by photoreceptors onto horizontal cells, modulates the fl ux of hydrogen ions from skate retinal horizontal cells (4). Glutamateinduced changes in H fl ux depend on the presence of extracellular calcium and likely refl ect the activation of plasma membrane calcium/H ATPases. These transporters extrude intracellular cal cium in exchange for extracellular hydrogen ions, decreasing the concentration of protons at the extracellular face of the horizontal cells (5).