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. 2002 Sep 15;543(Pt 3):899-916.
doi: 10.1113/jphysiol.2002.019703.

The scotopic threshold response of the dark-adapted electroretinogram of the mouse

Shannon M Saszik  1 , John G Robson, Laura J Frishman
Affiliations

The scotopic threshold response of the dark-adapted electroretinogram of the mouse

Shannon M Saszik et al. J Physiol. .

Abstract

The most sensitive response in the dark-adapted electroretinogram (ERG), the scotopic threshold response (STR) which originates from the proximal retina, has been identified in several mammals including humans, but previously not in the mouse. The current study established the presence and assessed the nature of the mouse STR. ERGs were recorded from adult wild-type C57/BL6 mice anaesthetized with ketamine (70 mg kg(-1)) and xylazine (7 mg kg(-1)). Recordings were between DTL fibres placed under contact lenses on the two eyes. Monocular test stimuli were brief flashes (lambda(max) 462 nm; -6.1 to +1.8 log scotopic Troland seconds(sc td s)) under fully dark-adapted conditions and in the presence of steady adapting backgrounds (-3.2 to -1.7 log sc td). For the weakest test stimuli, ERGs consisted of a slow negative potential maximal approximately 200 ms after the flash, with a small positive potential preceding it. The negative wave resembled the STR of other species. As intensity was increased, the negative potential saturated but the positive potential (maximal approximately 110 ms) continued to grow as the b-wave. For stimuli that saturated the b-wave, the a-wave emerged. For stimulus strengths up to those at which the a-wave emerged, ERG amplitudes measured at fixed times after the flash (110 and 200 ms) were fitted with a model assuming an initially linear rise of response amplitude with intensity, followed by saturation of five components of declining sensitivity: a negative STR (nSTR), a positive STR (pSTR), a positive scotopic response (pSR), PII (the bipolar cell component) and PIII (the photoreceptor component). The nSTR and pSTR were approximately 3 times more sensitive than the pSR, which was approximately 7 times more sensitive than PII. The sensitive positive components dominated the b-wave up to > 5 % of its saturated amplitude. Pharmacological agents that suppress proximal retinal activity (e.g. GABA) minimized the pSTR, nSTR and pSR, essentially isolating PII which rose linearly with intensity before showing hyperbolic saturation. The nSTR, pSTR and pSR were desensitized by weaker backgrounds than those desensitizing PII. In conclusion, ERG components of proximal retinal origin that are more sensitive to test flashes and adapting backgrounds than PII provide the 'threshold' negative and positive (b-wave) responses of the mouse dark-adapted ERG. These results support the use of the mouse ERG in studies of proximal retinal function.

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Figures

Figure 1
Figure 1. ERG responses to brief full-field flashes of increasing energy, from bottom to top, for one dark-adapted subject over the range of stimulus energies that generally were tested
Here and in subsequent figures, a brief flash occurred at time zero, and all flashes were < 5 ms in duration. The inset shows an ERG response (in μV) on an expanded time axis (ms) to a stimulus of -0.57 log sc td s to better illustrate the oscillatory potentials that were present when high energy flashes were used (subject MM180).
Figure 2
Figure 2. ERG responses to brief full-field flashes of increasing energy for one dark-adapted subject for the nine weakest stimuli that were tested
Stimulus energy doubled from bottom to top. The two vertical lines indicate where the positive peak and negative trough of the scotopic ERG occurred, 110 and 200 ms, after the flash, respectively (subject MM150).
Figure 3
Figure 3. ERG amplitudes measured at fixed times after the stimulus for 20 subjects
A, ERG amplitudes measured at 110 ms after the flash plotted vs. stimulus energy (log sc td s), on the bottom axis and Rh* per rod on the top axis. The symbols represent the data and the continuous line represents the average five-component model fit to the data. The inset shows data from one subject (MM149) with a fitted hyperbolic function (eqn (1)) fitted to the saturation of the curve. B, ERG amplitudes measured at 200 ms after the flash. The format is as in A. The inset shows data from one subject (MM180), with a fitted hyperbolic function (data for high stimulus energies not shown).
Figure 4
Figure 4. ERG amplitudes measured at fixed times after the stimulus for four different subjects
ERG amplitudes measured at 110 (left) and 200 ms (right) after the flash (•) are plotted versus stimulus energy (log sc td s). Each (left-right) pair of panels shows data from the same animal, as marked in the left-hand plot. The continuous line in each plot is the five-component model fitted to the data (see text). A and B show the best and worst fits, by eye, for data measured at 110 ms, and C and D, at 200 ms after the flash. Subject numbers are marked in the left-hand panels.
Figure 5
Figure 5. ERG responses to brief full-field flashes of increasing energy for one dark-adapted subject before (left) and after (right) injection of GABA
The insets in the middle of the figure show energy-scaled responses for the indicated energies. The insets at the top of the figure show responses (μV) to a high-energy flash on an expanded time axis (ms) to illustrate the effect of GABA on the oscillatory potentials (subject MM207).
Figure 6
Figure 6. ERG amplitudes measured at 110 and 200 ms after the stimulus for the subject whose ERGs are illustrated Fig. 5 before (•) and after (○) injection of GABA
A, amplitudes measured at 110 ms with a hyperbolic function (eqn (1), line) fitted to the post-GABA data. B, ERG amplitudes measured at 200 ms with a hyperbolic function (eqn (1)) fitted to the post-GABA data for the same subject.
Figure 7
Figure 7. Model lines using average parameters from the 20 subjects illustrated in Fig. 3 for responses at 110 (A) and 200 ms (B) after the stimulus flash
For both times, the model included the five components: pSTR and nSTR (ramp with abrupt saturation), pSR (exponential saturation, eqn (2)), PII, (hyperbolic saturation, eqn (1)), and PIII (no saturation). The full model (black line) is decomposed into the five components (coloured lines). The insets have linear vertical axes (μV) so that the most sensitive model components of both polarities can be seen.
Figure 8
Figure 8. ERG responses under fully dark-adapted conditions in the presence of four steady backgrounds of increasing illumination from left to right in half log unit steps
Dark-adapted responses were recorded from one mouse (MM186) and responses to all backgrounds for a littermate (MM188) on the same day.
Figure 9
Figure 9. ERG amplitude at 110 (A) and 200 ms (B) for all subjects tested in the presence of steady backgrounds
• represent the data and the lines represent the model generated from average parameters for all of the subjects at each of the various backgrounds that were tested (10 subjects at -3.2, 11 subjects at -2.7, 8 subjects at -2.2 and 7 subjects at -1.7 log sc td).
Figure 10
Figure 10. Effect of background illumination,model lines using average parameters
Model lines based on average parameters under dark-adapted conditions (black line, from Fig. 3) and in the presence of steady backgrounds (colours) from Fig. 9 at 110 (A) and 200 ms (B).
Figure 11
Figure 11. Effect of background illumination on model components
Lower panel, plot of 1/average sensitivity of four modelled components, nSTR at 200 ms, pSTR at 110 ms, pSR at 110 ms and PII at 110 ms. Sensitivity was calculated from the average model fits to the dark-adapted ERGs (Fig. 3 and Table 1), and the ERGs in the presence of steady backgrounds (Fig. 9). The lines represent a curve, 1/S = K(I + ID) (eqn (4) in the text). The arrows mark the ID (dark light) values for the three most sensitive model components, nSTR, pSTR and pSR on the left, and PII on the right. In the inset all components have been normalized for dark-adapted sensitivity. The upper panel shows the average Vmaxversus background illumination for the model components.
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