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The electroretinogram
As described in previous chapters, the electroretinogram (ERG) recorded from the corneal surface of the eye represents the massed response of the retina to light stimulation. The ERG is easily recorded from the mouse eye with minor modifications of the general methodologies that have been employed for humans for many years. The basic methodology for recording the ERG in the mouse has been described in detail elsewhere.34 The solid curve shown in figure 81.1 is an example of an ERG recording from a normal C57BL/6J mouse in response to a bright flash of light. The major components of the response are the a-wave, which is the first negative corneal potential, and the b-wave, which is the first positive corneal potential. ERGs to bright flashes presented in the dark also contain a high-frequency oscillatory component on the ascending limb of the b-wave, collectively called the oscillatory potentials (OPs). An example record of dark-adapted OPs obtained from a normal C57BL/6J mouse are shown in the lower left panel of figure 81.2. After the onset of steady illumination, the relatively fast a- and b-waves are followed by a slower positive-going c-wave (not shown in figure 81.1). Other components of the ERG not shown in figure 81.1 will be described below.
Figure 81.1.
Representative ERG response to a bright flash obtained from a normal adult C57BL6 mouse. (See text for details.)
Figure 81.2.
Rod (upper left panel) and cone (upper right panel) ERGs obtained from a normal C57BL6/J mouse. Each trace displays the response to increasing light intensities. Upper left inset, Peak-to-peak amplitude versus retinal illuminance fitted with a Naka-Rushton function to obtain the maximum saturated b-wave amplitude, Vmax, the semisaturation intensity, k, and the rod ERG threshold. Right inset, Cone b-wave amplitude versus intensity series fitted with a linear regression to derive the cone ERG threshold intensity. Lower left, A single ERG recording filtered to illustrate the major oscillatory potentials. Lower right, Representative a-wave ERG recordings to a range of flash intensities for a normal mouse. The smooth dotted curves are the fit of a rod model (see text for details) from which estimates for RmP3, the maximum saturated photoreceptor response, and S, a sensitivity parameter, were derived. (See text for details.)
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