Canine S- and M/L- cone electroretinograms

Sammanfattning: Full-field flash electroretinography is an electrodiagnostic method used to examine the function of retinal cells. Light stimulation of the eye elicits electrical potentials in the retina. By aid of a corneal electrode and a reference electrode close to the back of the eye, the electrical potentials can be recorded and presented as an electroretinogram (ERG). ERGs driven by mainly one type of cones can be used to examine the function of a single cone class. In human beings, studies have shown the cone class sensitive to light of short wavelengths, the S-cones, to be more vulnerable to acquired damage than the other cone classes (Daley et al., 1987; Gouras et al., 1993; Greenstein et al., 1989). Dogs have two cone classes, S-cones, and M/L-cones (most sensitive to medium to long wavelengths). Hitherto, no guidelines or protocols for separating S- and M/L-cone driven ERGs in the dog have been published. Hence we know little about how each of these two cone classes are affected in different canine retinal diseases and there may be diseases still unrevealed due to the lack of proper diagnostics. The aim of this study was to separate and examine S- and M/L- cone driven canine ERGs. In this study, S- and M/L-cone driven ERGs were recorded using chromatic stimuli and selective chromatic adaptation in six healthy Beagle dogs. The method means that a light stimulus with a wavelength that maximally stimulates one cone class is presented on a bright chromatic background that saturates the rods and strongly supresses the responses from the cone type/types that are not of interest. We used a violet light stimulus (411 nm) on a steady, bright, red background light (627 nm) to elicit responses mainly driven by the canine S-cones. To examine the pure M/L-cone driven ERG, the violet light was instead used as background light whilst the red light was used as the stimulus. In addition, a second experiment was performed where we used the two stimuli on a steady, bright, blue background (470 nm) that would suppress the S- and M/L-cones almost equally, as well as bleach the rhodopsin completely. Selective chromatic adaptation provided non-univariant responses despite the use of equal relative light intensities for the stimuli, which suggests that we were stimulating two different cone mechanisms. The S-cone driven ERG had longer b-wave implicit times and lower b-wave amplitudes, which seemed to saturate at lower stimulus intensities compared to the M/L-cone driven ERG, which had shorter b-wave implicit times and higher b-wave amplitudes that increased over a larger range of stimulus intensities. A prominent d-wave (response to the cessation of light) was seen on the M/L-cone driven ERG, whilst this was absent, or at least not obvious, on the S-cone driven ERG. Our results are in agreement with the results from a study of feline cone ERGs (Zrenner & Gouras, 1979).