The Shift of ERG B-Wave Induced by Hours' Dark Exposure in Rodents

<div><p>Purpose</p><p>Dark adaptation can induce a rapid functional shift in the retina, and after that, the retinal function is believed to remain stable during the continuous dark exposure. However, we found that electroretinograms (ERG) b-waves gradually shifted during 24 hours’ dark exposure in rodents. Detailed experiments were designed to explore this non-classical dark adaptation.</p><p>Methods</p><p>In vivo ERG recording in adult and developing rodents after light manipulations.</p><p>Results</p><p>We revealed a five-fold decrease in ERG b-waves in adult rats that were dark exposed for 24 hours. The ERG b-waves significantly increased within the first hour’s dark exposure, but after that decreased continuously and finally attained steady state after 1 day’s dark exposure. After 3 repetitive, 10 minutes’ light exposure, the dark exposed rats fully recovered. This recovery effect was eye-specific, and light exposure to one eye could not restore the ERGs in the non-exposed eye. The prolonged dark exposure-induced functional shift was also reflected in the down-regulation on the amplitude of intensity-ERG response curve, but the dynamic range of the responsive light intensity remained largely stable. Furthermore, the ERG b-wave shifts occurred in and beyond classical critical period, and in both rats and mice. Importantly, when ERG b-wave greatly shifted, the amplitude of ERG a-wave did not change significantly after the prolonged dark exposure.</p><p>Conclusions</p><p>This rapid age-independent ERG change demonstrates a generally existing functional shift in the retina, which is at the entry level of visual system.</p></div

Prolonged dark exposure induced shift in ERG response and its restoration by subsequent light exposure.

<p>(A) ERG amplitude and peak latency measurements. (B) The preparation prior to ERG recording of three groups of rats. (C) A comparison of b-wave responses to a -1.14 log cd*s/m2 flash between normally reared (blue, n = 8) and dark prolonged rats (red, n = 8) showed a significant b-wave suppression. The mean response is represented by the solid line, and SEM is shown as the shadow. Dotted line shows the peak latency in normally reared rats. (D) A comparison of b-wave responses response to a 50 ms flash (-1.14log cd*s/m²) between normally reared rats (blue, n = 8) and prolonged dark exposed plus 3 repetitive, brief (10 minutes) light exposed rats (green, n = 8). Symbols are identical to those in C. (E) The b-wave amplitudes of the three groups (normally reared rats, n = 8; Prolonged DE rats, n = 8; Prolonged DE+3 LE rats, n = 8). (F) The statistics of the b-wave peak latencies of the three groups, samples, and symbols are identical to those in E (*** p<0.001, t test, error bars, ±SEM).</p

The shift of light intensity-ERG response curves after light manipulations.

<p>(A) Mean ERG b-waves of the 24 hours dark exposed (DE) rats (n = 8, red) and normally reared rats (n = 6, blue), SEM is shown as the shadow. (B) Light intensity-ERG amplitude curves to the 8 light intensities for the 24 hours dark exposed (prolonged DE) (n = 8, red) and normally reared rats (n = 6, blue). Mean (dark trace) and individual (light trace) were shown. * p<0.05, *** p<0.001, ANOVA. (C) Light intensity-peak latency curves of the b-waves in the 24 hours dark exposed rats (n = 8, red) and the normally reared rat eyes (n = 6, blue). (D) Normalized light intensity-ERG amplitude curves of the 24 hours dark exposed (n = 8) and normally reared rats (n = 6). (E) Normalized light intensity- ERG amplitude curves of the 24 hours dark exposed rats (n = 8) before (red squares) and after the 10 minutes light exposure (green circles). (F) Mean ERGs of the 24 hours dark exposed rats obtained before the 10 minutes light exposure (n = 8, red) and 130 minutes after the exposure (n = 8, green). (G) Light intensity-ERG amplitude curves of 24 hours dark exposed rats (n = 8) before (red squares) and after the 10 minutes light exposure (green circles). * p<0.05, *** p<0.001, paired t test. (H) Light intensity-peak latency curves of 24 hours dark exposed rats (n = 8) before (red squares) and after the 10 minutes light exposure (green circles). Vertical short bars denote SEM in B-D and F-H.</p

ERG amplitude and peak latency gradually decreased after the continuous dark exposure.

<p>(A) ERGs evoked by a 50 ms flash (-1.14log cd*s/m²) recorded during continuous dark exposure for 7 hours. Time 0 was set as 30 minutes after the onset of dark exposure. Gray lines are the peak latency at time 0. The solid blue lines are mean responses (n = 8), and the gray shadows represent SEM. (B) The statistics of the b-wave amplitudes at each recording time (n = 8, error bars, ± SEM). (C) The b-wave peak latencies recorded at different time points during continuous dark exposure. (D) The mean b-wave amplitudes of ERG response to a 50 ms flash (-1.14log cd*s/m²) of rats dark exposed for 0, 12, 24 and 48 hours (n = 8 for each group). The mean values of each group are illustrated by black squares (* p<0.05, *** p<0.001, t test). (E) The b-wave peak latency recordings at the identical time points in D. (* p<0.05, *** p<0.001. t test). (F)The b-wave amplitudes of the dark exposed rats (n = 10) to a 50 ms flash (-1.14log cd*s/m²) at day (6:00–18:00) and night (18:00–6:00) time period. (G)The ERG b-wave peak latencies in the above-mentioned two groups. The short vertical solid bars denote SEM in B-G.</p

Classical dark adaptation in ERG responses.

<p>Normally reared rats were put into a dark environment, and scotopic ERGs response to a 50 ms flash (-1.14log cd*s/m²) were recorded within the initial 50 minutes dark adaptation. (A) ERGs across time from one rat. (B)The Statistics of the b-wave amplitudes at each recording time (n = 4, error bars, ± SEM). (C) The scotopic ERG amplitudes at 3 time points (0 minutes, 30 minutes, 50 minutes) after the onset of dark exposure (n = 4, error bars, ± SEM).</p

Prolonged dark exposure induced suppression was restored by a subsequent light exposure with eye specificity.

<p>(A-C) Three typical cases showing that ERG b-wave amplitudes of the prolonged dark exposed (Prolonged DE) rats after the 10 minutes light exposure (LE). Typical ERG curves before and after light exposure were presented on top of each panel. (D) The ERGs of a Prolonged DE rat after an acoustic stimulation and subsequent 10 minutes light exposure. The acoustic white noise (100dB) lasted for 10 minutes (arrow). (E) The ERGs of a prolonged DE rat after paw pinches and subsequent 10 minutes light exposure. The rat’s paws were pinched by forceps (arrows). (F) Incremental effects of monocular light exposure on the ERGs of dark exposed rats. (G) Binocular ERG b-waves recorded before and after 10 minutes monocular light exposure in dark exposed rats. The mean values of each group are illustrated by black squares, and the gray squares show individual cases (* p<0.05, paired t test). The short vertical solid bars denote SEM. The ERG in this figure were all evoked by a 50 ms white flash (-1.14log cd*s/m²).</p

ERG shifts in and beyond the classical critical period and in both rats and mice.

<p>(A) ERG b-waves response to a 50 ms flash (-1.14log cd*s/m²) recorded from one animal in each group before and after 10 minutes light exposure. (B) Restoration effects in ERG b-wave amplitudes from the three groups before and after 10 minutes of light exposure (* p<0.05, all paired t test). Short vertical solid bars denote SEM.</p

Brain areas revealed by the factorial model.

<p>(STS  =  superior temporal sulcus, IFG  =  inferior frontal gyrus, M1 =  primary motor area, SMA  =  supplementary motor area).</p>*<p>Since the “Chew > NoChew” contrast revealed very strong activations covering large parts of the brain, and since this contrast is probably most susceptible to motion artifact, we used a more stringent threshold, i.e., <i>p</i><0.05 (FWE) at peak voxel containing more than 30 contiguous voxels.</p

Experienced stress before the presentation of noise (SVAS-5, left) and after the presentation of noise (SVAS-20, right) as a function of noise presentation and gum chewing in the fMRI session.

<p>* <i>p</i><0.05.</p

Brain regions revealed by the factorial and parametric models.

<p>(A) Brain regions sensitive to noise and noise-induced stress (“Noise > NoNoise”). (B) Brain regions in which the activation level positively correlates with the ratings of the subjectively experienced level of stress. (C) and (D) The time course of BOLD signal change in the left STS and the left AI reflecting the effect of noise (“Noise > NoNoise”) in the Chew and NoChew conditions. Error bars indicate the standard error of percent signal change (±SEM). To see more clearly the activations in insula, regions illustrated here used a voxel level threshold of <i>p</i><0.005 (uncorrected) and a extent threshold of 200 contiguous voxels.</p