Characterization of Voltage Gated Sodium Currents of Starburst Amacrine Cells in the Mouse Retina

Abstract

Purpose: : Starburst amacrine cells (SACs) are a major interneuron in the retina which play an important role in computing motion direction. The nature of SAC directionality is controversial and has been suggested to include asymmetric activation of voltage gated ion channels (Euler et al., 2002) Previous studies of SAC sodium currents have yielded conflicting data regarding their presence and role in information processing. We recently localized a tetrodotoxin resistant (TTX-R) voltage-gated sodium channel, Na_v1.8, expressed in SACs (O'Brien et al., 2008), which exhibits unusual activation and inactivation kinetics. We have therefore undertaken a biophysical characterization of voltage-gated sodium currents in mouse SACs.

Methods: : Starburst amacrine cells (n = 26) were recorded in retinal wholemounts in the whole-cell configuration, held at -70mV, and currents were elicited by 80ms voltage steps (-90mV to +30mV). Voltage-gated sodium currents (VGSCs) were isolated by bath application of 100µM CdCl₂ and 20mM TEA-Cl. Addition of TTX (1µM) to the cocktail of channel blockers was used to determine whether TTX -R sodium currents were present. SACs were morphologically identified by inclusion of Lucifer yellow (0.5%) and Neurobiotin (0.05%) in the CsMeSO₄ based internal solution

Results: : We observed the presence of voltage gated sodium currents in every SAC recorded. These inward currents had peak currents of -206.5±45.0mV (n=21). SAC sodium currents had 50% activation kinetic values at -30.8±4.8mV (n=20) and 50% inactivation kinetic values at -55.7±6.2mV (n=17). Recovery from inactivation for sodium currents was 1.92±0.6ms (n=16). Application of TTX led to blockade of all remaining inward currents.

Conclusions: : In contrast to previous studies of isolated cholinergic amacrine cells (Kaneda et al., 2007), our data demonstrate that mouse SACs do express VGSCs. Interestingly, while Na_v1.8 channels have been localized on SACs we did not observe TTX-R currents. However, the presence of TTX-sensitive currents in SACs may contribute to the generation of directional selectivity.

Keywords: amacrine cells • ion channels • retina