Effects Of Melatonin In The Dog's Erg

Abstract

Abstract: : Purpose: Evaluate the effects of oral melatonin administration on the photopic and scotopic electroretinograms of beagle dogs. Methods: Binocular, full–field ERGs were recorded from fully anaesthetised beagle dogs (N=6: 3males, 3 females) at two different occasions: one without melatonin (control) and one with melatonin administred at a dose of 90 mg/dog, per os. The photopic flicker retinal responses were evoked at 4 different temporal frequencies (6 Hz, 12 Hz, 20 Hz and 30 Hz). The flash retinal responses were evoked at 20 different increasing intensities presented in photopic and scotopic conditions in order to assess (from the derived luminance–response curves) both Vmax and log K parameters. Results: Although maximal amplitudes from the flicker were obtained at 20 Hz frequency, all animals showed a reduction in amplitude after melatonin. Similarly, the a– and b–wave amplitudes of the photopic ERG were reduced significantly 60 minutes after melatonin ingestion whereas the effect on the scotopic b–wave amplitude was not significant. The photopic Vmax calculated mean (SD) values were 259.3 µV (49.3 µV) and 118.6 µV (26.7 µV) for the control group and the treated group, respectively. The scotopic Vmax calculated mean (SD) values were 532.9 µV (66.1 µV) and 467.5 µV (50.2 µV) for the control group and the treated group, respectively. No change in log K was observed. Conclusions: Our results clearly indicate that melatonin given at 90 mg/dog can significantly impact the cone response. This inhibitory effect of melatonin on cone function is unclear. We propose that a melatonin inhibitory effect on dopamine production may alter cone function at the photoreceptor layer of the dog retina.

Keywords: melatonin • photoreceptors • electroretinography: clinical