Voltage-gated sodium channels amplify both light and dark adapted electroretinograms in mice

Abstract

Purpose: To evaluate the contribution of voltage-gated sodium (Na_v) channels to the mouse electroretinogram (ERG).

Methods: Ganzfeld ERGs were recorded using DTL electrodes from adult C57BL/6 mice anesthetized with Avertin. Stimuli were brief xenon flashes. The following pharmacological agents were injected intravitreally 1,2,5,6-Tetrahydropyridin-4-yl methylphosphinic acid (TPMPA 500 μM) to block GABAc receptors, tetrodotoxin (TTX, 3,6, or 20 µM) to block Na_v channels, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX 200 µM) to block ionotropic glutamate receptors.

Results: : TTX (6-20 μM predicted concentration at the retina) sufficient to block retinal output, as measured with cortical visual evoked potentials, caused a large (~40% of control) decrease in the dark-adapted ERG b-wave response to high flash energies (1.0 log cd * s/m2). In addition, the b-wave was reduced by ~20% even at flash energies that should isolate the primary rod pathway (-2.2 log cd *s/m2). Isolating the photoreceptor to ON bipolar cell circuit with CNQX reduced the effect of TTX to ~15% at the highest flash energies used and eliminated the effect at low flash energies . This indicates that 1) Na_v channels in ON cone bipolar cells contribute a relatively small component of the overall effect of TTX on the dark adapted b-wave and 2) that the large decrease in b-wave amplitude caused by TTX in dark adapted conditions is dependent on ionotropic glutamate receptors and is not located on the rod bipolar cells themselves. We found that the remainder of the large reduction of the dark-adapted b-wave caused by TTX was due to reduced GABA input to GABAc receptors on rod bipolar cells.

Conclusions: We investigated the contribution of Na_v channels to the mouse electroretinogram using varied concentrations of TTX and measuring cortical responses to ensure the complete blockade of Na_v channels. Na_v channel block reduced the ERG b-wave over a range of light-adapting backgrounds and in dark-adapted conditions. We provide a mechanism showing that TTX acts mainly on two contributors to the ERG in a range of lighting conditions, reducing rod bipolar cell light responses by blocking GABAc receptor mediated GABAergic input and by directly blocking Na_v channels in ON cone bipolar cells.

Keywords: 510 electroretinography: non-clinical • 569 ion channels • 559 inhibitory neurotransmitters