Figure 4.

The OD shift following brief MD is expressed in the isolated thalamocortical component of the VEP. (A) Raster plot of a representative unit recorded before and after muscimol+ infusion demonstrating that cortical spiking was completely eliminated. (B) Single unit firing rate is dramatically reduced following muscimol+ infusion (n = 29, black bars represent medians (4.4 and 0 for baseline and drug respectively); p < 0.0001, Mann-Whitney U test). Inset shows waveform width analysis of the units: filled gray circles correspond to the gray bar and the narrowest spike width. Data are plotted in 0.25 ms bins. Units were sampled in all layers: 11 in superficial layers, 8 in layer 4, and 9 in deep layers. Units unresponsive to muscimol+ were axons in layer 4. (C) Experimental design for induction of c-fos expression after muscimol+ infusion. 24 hour dark exposure of mice with cannulated mice was followed by infusion of 0.5 μl of muscimol+ and an hour of subsequent light exposure. Mice were then perfused and stained for c-fos. Black boxes on the diagram represent the regions that were analyzed for c-fos expression. (D) c-fos expression is normal in the uninfused hemisphere (left) and completely abolished in the infused hemisphere (right). Scale: 100 μm. Insets represent enlarged areas from the control and infused cortices to clearly illustrate the absence of stained nuclei in the hemisphere treated with muscimol+. Scale: 15 μM. (E) A representative example of the change in VEP amplitude to binocular stimulation before and after infusion of muscimol+. Grey circles represent baseline VEP amplitude and black circles represent VEP amplitude following muscimol+ infusion. (F) Representative VEP traces from (E) evoked by 1 Hz visual stimulation (arrows indicate time of stimulus reversal). Grey rectangle highlights the baseline VEP trace. Scale bar: 0.1 mV, 0.1 s. (G) Traces indicate representative FPs evoked by contrast reversing grating stimuli during baseline (left), after infusion of muscimol+ (middle), and after infusion of CNQX (right). Scale: 50 μV, 50 ms. The baseline VEP is significantly reduced by infusion of muscimol+ and is completely eliminated by the infusion of CNQX. The grey line represents the limit of detection (see Figure 3). (H) Traces indicate representative FPs evoked by contrast reversing grating stimuli during baseline (left), after infusion of muscimol+ into the cortex (middle), and after injection of muscimol into the ipsilateral LGN (right). Scale: 50 μV, 50ms. The baseline VEP is significantly reduced by infusion of muscimol+ and is completely eliminated by the injection of muscimol into LGN. The grey line represents the limit of detection. (I) Application of muscimol+ significantly reduces VEP amplitude (n = 7, paired t-test, p = 0.0004), but has no significant effect on the C/I ratio. (J) The decrease in the C/I ratio following MD persists following acute muscimol+ infusion (day 3 baseline is not significantly different from the muscimol+ values (Wilcoxon Signed Rank test). * indicates a level of significance of p < 0.05 as compared to day 0.