Photoreceptor calcium channels: Insight from night blindness

The genetic locus for incomplete congenital stationary night blindness (CSNB2) has been identified as the CACNA1f gene, encoding the α_1F calcium channel subunit, a member of the L-type family of calcium channels. The electroretinogram associated with CSNB2 implicates α_1F in synaptic transmission between retinal photoreceptors and bipolar cells. Using a recently developed monoclonal antibody to α_1F, we localize the channel to ribbon active zones in rod photoreceptor terminals of the mouse retina, supporting a role for α_1F in mediating glutamate release from rods. Detergent extraction experiments indicate that α_1F is part of a detergent-resistant active zone complex, which also includes the synaptic ribbons. Comparison of native mouse rod calcium currents with recombinant α_1F currents reveals that the current-voltage relationship for the native current is shifted approximately 30 mV to more hyperpolarized potentials than for the recombinant α_1F current, suggesting modulation of the native channel by intracellular factors. Lastly, we present evidence for L-type α_1D calcium channel subunits in cone terminals of the mouse retina. The presence of α_1D channels in cones may explain the residual visual abilities of individuals with CSNB2.