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Compartmentalization of calcium entry pathways in mouse rods

Photoreceptor metabolism, gene expression and synaptic transmission take place in a highly polarized structure consisting of the ellipsoid, subellipsoid, cell body and synaptic terminal regions. Although calcium, a key second messenger, regulates cellular functions throughout the photoreceptor, the molecular mechanisms underlying local region‐specific action of Ca2+ in photoreceptors are poorly understood. I have investigated the compartmentalization of voltage‐dependent and independent Ca2+ channels in mouse photoreceptors. Transient receptor potential channels isoform 6 (TRPC6), a putative store‐operated Ca2+ channel, was selectively localized to the cell body of rods. By contrast, voltage‐operated Ca2+ channels were expressed in the synaptic terminal and in the ellipsoid/subellipsoid regions. Likewise, Ca2+ store transporters and channels were strongly associated with the subellipsoid region. A moderate TRPC6 signal was observed in cell bodies of bipolar, amacrine and ganglion cells, but was absent from both plexiform layers. These results suggest that Ca2+ entry mechanisms in mammalian photoreceptors and bipolar cells are highly compartmentalized, consistent with local, region‐specific activation of Ca2+‐dependent processes.