Background

Electroretinograms (ERGs) are used by researchers and clinicians to test retinal function by measuring the electrical response of retinal cells to a light stimulus. The ERG is a useful tool for measuring retinal responses in mice due to its high level of sensitivity and noninvasive nature, and can be utilized to assess eye disease and retinal degeneration (Duncan et al., 2003; Phillips et al., 2010; Zhao et al., 2011; Vollrath et al., 2015). In mouse genetic retinal disease models, ERGs can be used to assess retinal degeneration at multiple time points as the disease progresses (Duncan et al., 2003). For studies evaluating the effect of drug treatment on the mouse eye, retinal function can be assessed before and after treatment in the same eye (Zhao et al., 2011). In the following protocol we describe a method for measuring the functional response of photoreceptors and downstream retinal cells in mice that builds on a previously published approach (Phillips et al., 2010). The protocol can be readily applied to animals three weeks of age and older.

ERGs can be used to measure the electrical response to light flashes in either dark-adapted (scotopic) or light-adapted (photopic) mice. In scotopic ERGs, mice are presented with low intensity light flashes to induce rod activation, so the function of rod photoreceptor and downstream retinal cells can be examined (Fu, 2010). A prolonged period of dark-adaptation is critical to achieve maximal rod sensitivity, and to keep cone stimulation minimal (Pepperberg, 2003). In photopic ERGs, mice are presented with high intensity light flashes after a period of light stimulation. Under these conditions, there is high cone activation and the rod response is suppressed. Therefore, photopic ERGs can be used to measure the function of cone photoreceptors and downstream retinal cells (Fu, 2010).

This method of performing ERGs allows for the calculation of amplitude and time-to-peak of two major waves, the a-wave and b-wave. The a-wave is a measure of the initial response of photoreceptors to a brief flash of light (Brown, 1968; Perlman, 2015). The b-wave is a measure of the response of downstream retinal neurons, including bipolar cells, to photoreceptor stimulation (Brown, 1968; Perlman, 2015). Loss of amplitude in either the a-wave or b-wave may be attributed to a number of retinal dystrophies (Creel, 2015), while ‘supernormal’ waves with increased amplitude have been attributed to cone dystrophies (Phillips et al., 2010).