Scotopic threshold response of the electroretinogram of dogs

To study characteristics of the scotopic threshold response (STR) in dogs.

We recorded the electroretinogram (ERG) in anesthetized Beagles under several recording conditions to examine characteristics of the STR.

Seven clinically normal Beagles.

Beagles were paralyzed by continuous i.v. administration of muscle relaxant and were artificially ventilated with a gas mixture of nitrous oxide and oxygen during experiments. Corneal ERG were recorded, using full-field white stimuli, under several recording conditions.

The canine STR was a simple corneal negative response elicited by dim stimuli below the b-wave threshold. Maximal amplitude of the canine STR was approximately 50 to 120 microV, much larger than those of other animal species, such as human beings, monkeys, and cats. Peak latency of the maximal STR was approximately 75 to 96 milliseconds. The spectral sensitivity of the canine STR corresponded to that of the canine rod visual pigment. The canine STR was completely eliminated by dim background light, which had no suppressive effect on the a- and b-waves. By using such dim background light, we could record the photoreceptor-derived a-wave in the ERG intensity series.

In Beagles, the STR strongly influences the ERG waveforms recorded under the dark-adapted condition. Special attention may be necessary in using the dark-adapted ERG to assess the retinal photoreceptor function. Canine STR is expected to be a good indicator of assessment of the proximal retinal function in dogs, and Beagles might be a good experimental animal for the study of the STR.