Is There a Contribution of Both Cav1.4 and Cav1.3 L-Type Calcium Channels to Retinal Synaptic Transmission?

Abstract

Cav1.3 and Cav1.4 L-type calcium channels were previously both shown to be expressed in the retina. Whereas Cav1.4 channels are predominantly expressed in the outer plexiform layer (OPL) at photoreceptor ribbon synapses, reports on the distribution pattern of Cav1.3 channels in the retina are controversial. One study reported the uniform expression across all retinal cell layers, and others showed accumulation in photoreceptor inner segments or the OPL or the inner nuclear layer or the ganglion cell (GC) layer of the retina. Mutations in the pore-forming α1-subunit, found in patients diagnosed with Congenital Stationary Night Blindness type 2 (CSNB2), result in impaired signaling between photoreceptor cells and second-order neurons. Exemplary, we report the functional consequences of the novel CaV1.4 mutation GV found in an Austrian family. Biophysical analysis of GV channels in whole-cell patch-clamp experiments revealed a reduced current density ([pA/pF]: wt: 12.8±1.4, n=18; GV: 3.7±1.0, n=7; p<0.001, 15 mM Ca2+ used as charge carrier) due to decreased surface expression of functional channels, expected to lead to impaired retinal signaling. In contrast, the contribution of Cav1.3 channels to synaptic transmission is rather ambiguous. We therefore performed multi electrode (MEA) analyses of light-dark evoked GC activity in Cav1.3-/- mice retinas. Retinas were excised from adult wildtype (wt) and Cav1.3-/- mice (red-light, carbogen-equilibrated Ames) and mounted ganglion-cell-side-down (via nitrocellulose-menbrane) on a MEA array. Preliminary data showed a prolongation of GC response latencies in Cav1.3-/- compared to wt supporting the idea that Cav1.3 channels also contribute to synaptic transmission. However qRT-PCR analysis of Cav1.4-/- retinas showed a significant upregulation of Cav1.3-mRNA, thus an alternative role for signal transduction might also be suggested. Support: Austrian Science Funds (FWF), P22526:AK; SFB F44 (F44020): JS, AK, (F44060): GJO, Medical University Vienna.