Abstract
Abstract: :
Purpose:Studies of photosensitivity in melanopsin knockout mice (Ruby et al., 2002, Science 298, 2211; Panda et al., 2002, Science 298, 2213; Lucas et al., 2003, Science 299, 245) have previously assumed that rod & cone pathways are unaffected by melanopsin loss. Here, this assumption was tested using electroretinography. Methods:Adult melanopsin knockout mice (OPN4–/–, Hattar et al., 2002, Science 295, 1065), housed under a 12 h light: 12 h dark cycle, were tested in the middle of their light phase. Two full–field strobe flash ERG protocols were conducted. Flash ERGs (0.5Hz stimulus frequency, 40 repetitions averaged for analysis) were performed on overnight dark adapted animals (n=3), whereas flicker ERGs (20 – 30 Hz stimulus frequency, 40 repetitions averaged for analysis) were recorded following short–term (10min) dark adaptation (n=5). All records were collected at 10 kHz and differentially filtered with a high pass of 0.5 Hz and low pass 200 Hz. Animals were anaesthetised with a solution of ketamine (70mg/kg) and xylazine (7mg/kg). Results:Scotopic, rod–dominated ERGs were recorded using dim blue flashes (0.5 Hz) and evoked b–waves (stimulus 1.2 log units above threshold response, mean amplitude was 117.08 + 26.10 µV and implicit time 103.94 + 7.58 ms, mean + S.E.M., n=3). Mixed rod and cone ERGs using bright white photopic flashes (0.5 Hz) showed a normal waveform including a negative a–wave followed by a positive–going b–wave. Overlaid on this waveform was the multiple wavelet oscillatory potential (OP) response complex. The a–wave had an amplitude of 75.26 + 23.73 µV (mean + S.E.M., n=3) with an implicit time of 14.38 + 2.97 ms. The b–wave amplitude was 321.63 + 55.34 µV, with an implicit time of 32.66 + 3.85 ms. Post hoc digital filtering was used to isolate the OP, the sum of the amplitudes of the initial four OP wavelets was 333.42 + 71.05 µV and the latency from the stimulus onset to the first wavelet peak was 20.70 + 2.52 ms. The cone dominated flicker ERG (20 Hz) showed a sinusoidal waveform of expected period and with mean amplitude of 20.26 + 2.93 µV (mean + S.E.M., n=5). Conclusions:Full–field strobe flash ERG revealed that both rod & cone pathways are functional in mice lacking the melanopsin gene.
Keywords: electroretinography: non–clinical • retina • opsins