Calcium-permeable glutamate receptors in horizontal cells of the mammalian retina

Abstract

Mechanisms that mediate the calcium influx in mammalian horizontal cells were studied. Horizontal cells (HCs) enzymatically dissociated from the rabbit retina were recorded by the whole-cell configuration of the patch-clamp technique and by calcium image ratioespectrophotometry of Fura-2 loaded cells. AMPA-preferring glutamate receptors were shown to permeate Ca²⁺ in mammalian HCs by ionic substitution experiments. Furthermore, after blocking the L-type calcium current with nifedipine (100 μM), calcium current through the AMPA-preferring glutamate receptors was measured. Calcium image ratioespectrophotometry was performed on the dissociated HCs in order to determine the changes in the intracellular calcium ([Ca²⁺]_i). Fura-2 microspectrophotometry showed that in HCs, K⁺-induced cell depolarization promoted an increase in [Ca²⁺]_i, mediated by the L-type calcium channels, since it was abolished in the presence of nifedipine. The increase in [Ca²⁺]_i upon cell depolarization was observed throughout each cell; however, it was maximal at the cell soma. Activation of glutamate receptors in dissociated HCs by glutamate, AMPA or kainate promoted an increase in [Ca²⁺]_i. This increase in [Ca²⁺]_i was abolished in nominally Ca²⁺-free solution (0 mM Ca²⁺); in contrast, nifedipine decreased the glutamate-induced influx of calcium in ca. 50%. The present study demonstrates that calcium may permeate through glutamate receptors expressed in HCs of the rabbit retina.