Contribution of voltage-gated sodium channels to b- and d-waves of frog electroretinogram under different conditions of light adaptation

The effect of blockade of voltage-gated sodium (Nav) channels by tetrodotoxin (TTX) on the V-log I function of the ERG b- and d-waves was investigated in light and dark adapted frog eyecups. TTX diminished the b- and d-wave amplitude under both conditions of adaptation at all stimulus intensities except for the middle intensity range in dark adapted eyes, where it had no effect on the b-wave amplitude. TTX delayed the time course of the responses, obtained with low intensity stimuli and widened the dynamic range of the b-wave. The inhibitory effect of TTX on the cone-mediated, but not rod-mediated b- and d-wave amplitude persisted after the blockade of proximal retinal activity by NMDA, indicating that it may be due to a blockade of Nav channels on the ON and OFF bipolar cells.