Abstract
Abstract: :
Purpose:To determine the contributions of postreceptoral neurons to the photopic ERG of nob mice, a murine model for human complete–type congenital stationary night blindness (CSNB1) that lacks a b–wave from depolarizing bipolar cells. Methods: Ganzfeld ERGs were recorded from adult control mice, and nob mice anesthetized with ketamine and xylazine. Recordings were made between DTL fiber electrodes placed under contact lenses on the two eyes. Stimuli were sinusoidally modulated LEDs (max 513nm; mean luminance 2.55 log sc Td; contrast 100%; 3 to 36 Hz) and brief flashes (0.4–2.5 log sc Td s) on a rod suppressing background of 2.55 log sc Td to which mice were adapted for 30 minutes before recording. ERGs also were recorded from nob mice after PDA (cis–2, 3–piperidine–dicarboxylic acid, 3–5 mM) was injected intravitreally to block transmission to hyperpolarizing 2nd order and all 3rd order neurons. Results: With sinusoidal stimulation, the fundamental (F1) amplitude of the ERG of control mice was largest at 9 Hz (∼60 µv), while for nob mice the maximum amplitude was >20 µV at 6 Hz (before PDA) and at 3 Hz (after PDA). F1 responses could be recorded up to 21 Hz in control and nob eyes and 15 Hz in nob eyes after PDA. At 3 Hz – 6 Hz, F1 phase was about 210°–170° more delayed in nob than in control mice; phase was not altered appreciably by PDA. In control mice, the second harmonic (F2) response showed two peaks of ∼10 µV amplitude at 3 Hz and 13 Hz,. Both before and after PDA nob mice showed almost no F2 response. The brief flash response of nob mice was a long–lasting negative wave with a positive–going intrusion that started about 50 ms after the stimulus and peaked at about 120 ms. For the strongest stimulus the maximum initial negativity (a–wave) was about 80 µV and the peak of the positive–going intrusion returned at least 1/3, and in some cases almost all, the way to baseline. PDA completely eliminated the positive–going wave. Conclusions: Postreceptoral neurons make only a small contribution at the fundamental frequency to the nob photopic ERG response to sinusoidal stimulation. Postreceptoral neurons make a substantial positive–going contribution to the flash response of nob mice under light–adapted conditions.
Keywords: electroretinography: non-clinical • retina: distal (photoreceptors, horizontal cells, bipolar cells) • transgenics/knock-outs