Glutamate receptor subtypes in human retinal horizontal cells

Glutamate receptor currents were examined in horizontal cells from cultured human retina using whole-cell recording procedures. Horizontal cells possess both AMPA and kainate receptors and both produce significant sustained currents. The kainate-induced current did not show significant desensitization and was not enhanced by concanavalin A. The sustained AMPA current was smaller than the kainate current, but the difference was almost entirely due to pronounced desensitization. The horizontal cell AMPA current was enhanced by cyclothiazide but not by PEPA, indicating the presence of the flip receptor variant. GYKI-52466 blocked the AMPA response (IC₅₀ = 5 μM against 100 μM AMPA) but also blocked the kainate response (IC₅₀ = 45 μM against 100 μM kainate). The diversity of glutamate receptors in human horizontal cells suggests that synaptic input to these neurons may be multiplexed through both kainate and AMPA channels.