Do Rats Generate a Photopic a–Wave?

Abstract

Abstract: : Purpose: Unlike other species, rodent photopic ERGs do not normally include a noticable a–wave. What are the optimal recording conditions to elicit a photopic a–wave from rats? Methods: Photopic ERGs were obtained from adult albino Sprague–Dawley (SD; n=6) and pigmented Long Evans (LE; n=6) rats. Flash intensity was set at Vmax intensity (0.9 log cd.sec.m^–2 ) and delivered at an inter–stimulus interval (ISI) of 960 and 9600 msec. In order to enhance the a–wave, 1– the intensity of the rod desensitizing background (BG) was progressively reduced from 100 cd.m^–2 to 0 cd.m^–2 and, 2– intravitreal injections of the ON– pathway blocker L–AP–4 (5 µl; 4mM) was performed. Results: Irrespective of the ISI used, we could not evidence an a–wave for recordings obtained against rod desensitizing backgrounds brighter than 10 cd.m^–2. Use of dimmer (≤10 cd.m^–2) backgrounds did however generate an a–wave. SD rats, ISI 960 (BG 0 = 7.4 ± 12.4 µV), ISI: 9600 (BG 10 = 13.4 ± 14.8 µV; BG 5 = 29.2 ± 15.5 µV; BG 1 = 48.9 ± 28.8 µV; BG 0 cd.m^–2 = 76.6 ± 29.7 µV). LE rats, ISI: 960 (BG 1 = 25.9 ± 21.3 µV; BG 0 = 42.4 ± 14.3 µV), ISI 9600 (BG 5 = 28.3 ± 22.4 µV; BG 1 = 61.6 ± 23.4 µV. BG 0 = 116.4 ± 27.0 µV). L–AP–4 failed to evidence an a–wave in responses normally devoid of one. Conclusions: The photopic (cone) mediated ERG of rats does not appear to include an a–wave. Large and reproducible a–waves were only obtained in conditions (low background and slow ISI) that could have favored a rod intrusion. Furthermore, the fact that we could not evidence an a–wave following L–AP–4 clearly rules out a possible b–wave masking effect. The strain difference (LE a–waves > SD a–waves) is interesting and might suggest other investigative avenues in trying to understand why the photopic ERG of rodents do not normally include an a–wave. Supported by CIHR and Réseau Vision du FRSQ.

Keywords: electroretinography: non-clinical • retina • drug toxicity/drug effects