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J. Dübel, S. Haverkamp, T. Kuner, T. Euler; Chloride imaging in ON–type bipolar cells of a Clomeleon indicator mouse line . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1324.
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Purpose: Bipolar cells relay photoreceptor signals to the inner retina. In the outer plexiform layer their dendrites receive glutamatergic input from photoreceptors and supposedly GABAergic input from horizontal cells. Since horizontal cells are OFF cells – they depolarize at light offset – maximal GABA release occurs in darkness. In OFF bipolar cells this could help forming their antagonistic receptive field organization, while in ON bipolar cells GABAergic inhibition would diminish (or even abolish) the excitatory center response. Recent studies suggests that, due to differential expression of chloride transporters, ON bipolar cells could have a stable chloride concentration gradient between dendrites and axon terminals. Higher [Cl–] in the dendrites could shift the chloride reversal potential to values more positive than the resting potential, allowing GABA to mediate chloride efflux (depolarization) and hence provide ON bipolar cells with the "correct" surround input for an antagonistic receptive field. Methods: Retina explants were prepared from transgenic mice that express Clomeleon, a ratiometric biosensor for chloride, in a subset of ON bipolar cells. Using two–photon microscopy we recorded the intracellular [Cl–] optically under resting conditions and after application of GABA. Unlike electrophysiological recordings, optical measurements allow chloride signals in fine processes to be monitored without perturbing the intracellular ionic composition. Results: The somatic [Cl–] in ON bipolar cells varied largely (between 5 and 40 mM). In the majority of cells the dendritic [Cl–] was consistently higher than the somatic [Cl–], while the axonal [Cl–] was similar to or lower than the somatic [Cl–]. Depending on the actual intracellular [Cl–] (and thus the chloride reversal potential), application of GABA led in some cells to a decrease in [Cl–] and therefore presumably caused excitation, in other cells, however, GABA increased the [Cl–], presumably causing inhibition. The largest changes in [Cl–] were observed in the dendrites. Conclusions: Our findings suggest the presence of a sustained chloride gradient between the dendrites and the axon of ON bipolar cells.
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