Receptor-mediated calcium entry in retinal amacrine cells

In the vertebrate retina, multiple cell types express g protein-coupled receptors linked to phospholipase C. The signaling pathway engendered by activation of this enzyme can involve Ca2+-permeable transient receptor potential (TRP) channels. To begin to understand the role of these channels in the retina, we undertake an immunocytochemical localization of two TRPC channel subunits, TRPC1 and TRPC4. TRPC1 expression was observed in amacrine cells and their process in the chicken retina. TRPC4 expression was much more widespread with some degree of labeling found in all layers of the retina, and was shown to be expressed in Müller glial cells. Thus, the distributions of these two subunits indicate that different retinal cell types express TRPC channels containing different subunits. Recently, several sphingolipids have been demonstrated to play key roles in Ca2+ mobilization in neurons. Sphingosine-1-phosphate is a sphingolipid metabolite that has been shown to activate a class of g protein-coupled receptors (S1PRs) in other cell types. In the present study, we examine the signaling properties of S1P in retinal amacrine cells. S1P produced a noisy, inward cation current in amacrine cells that occurred through activation of S1P1R and S1P3R. The S1P-induced current was PLC-sensitive and was eliminated with La3+ and Gd2+, suggesting activation of TRPCs. S1P also elicited cytosolic Ca2+ elevations. The S1P-induced Ca2+ increase was mediated by S1P1R and S1P3R and was a result of both release of Ca2+ from internal stores and Ca2+ influx. Single-cell PCR amplification of TRPC channel subunits 1, 4, and 5 confirmed expression of these subunits in amacrine cells, suggesting that S1P is capable of activating TRPC-mediated Ca2+ entry in retinal amacrine cells through a novel lipid signaling pathway