Chapter 2 - Oligodendrocyte metabolic impairments affect
2.2.2. An oligodendrocyte-specific metabolic defect impairs temporal
To understand if defects in temporal processing seen with dysmyelination are related purely to conduction velocity deficits or are also related to an axonal inability to respond to short variations on the stimulus timing, I used the same protocols for temporal assessment seen in Chapter 1 (gap-detection and click tracing), for the metabolic mutants.
In Figure 40, we can see the results of the gap-detection assessment in the MCT1 +/-mice. Individual examples of control and mutant animals (Figure 40-A) illustrate that mutant animals appear to have reduced spiking probability to small gaps compared to control animals. In addition, MCT1+/- mice had strongly reduced response amplitudes, especially when looking at the responses to broad-band noise (Figure 40-B). This was also reflected in the pre-gap sound, where I found a significant reduction in the response amplitude in mutant animals (ANOVA, F(1,252)=30.06, p<0.0001). This reduction was also maintained through all the gap lengths (p<0.001 for all gaps). Because of the amplitude difference between control
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and mutants, the comparison of post-gap windows between groups needs to take differences in amplitude into consideration. Indeed, the comparison of the PSTH at different gaps, showed a significant effect of group (ANOVA, F(1,1612)=167.29, p<0.0001) and a significant effect of gap (F(5,1612)=4.69, p=0.0003) but no interaction (F(5,1612)=1.87, p<0.096) (Figure 40-B). The reduction in amplitude seems to be a generalized feature of the cortical responses in MCT1+/- mice, since a tendency towards a lower amplitude was also observed in click responses (Figure 39).
Figure 40. Temporal acuity is not impaired in the auditory cortex of MCT1+/- mice.
A) An individual example of the gap-detection recordings of a control (upper, black) and mutant (lower, purple) mouse. These raster plots represent the spiking activity (individual dots) that occurs during sound presentation (gray patches) to the 10 stimulus repetitions. Above the raster plots, a representation of the sounds played is presented in black bars, for a sound without a gap (0 ms gap, left side) and a sound with a 2 ms gap (right side).
B) The average PSTH of all animals of control (black) and MCT1+/- mice (purple) with the S.E.M. represented as a shaded error bar following the same color code. Pre-gap and post-gap responses to the presentation of 0.5 to 5 ms are represented. Significant differences were seen between groups for the PSTH comparison throughout the gaps (ANOVA, p<0.0001). n=8 control and n=6 mutant mice. C) Quantification of the differences between baseline and post-gap activity (50 ms window). A dotted black line depicts the threshold of p=0.05 for significance. The longer the gap length, the higher the significance value. No significant differences were seen between groups (Kruskal-Wallis test, p=0. 4). The plot shows the median values for both groups, error bars represent the standard error of the median. The internal distribution plot in C) shows all the p-value distributions for the 3 ms gap condition in control (gray) and mutant (purple) mice. A vertical black dotted line depicts the p value corresponding to 0.05 significant. It is possible to see that most of the responses of both the control and mutant to the 3 ms gap are significantly different than baseline activity (left side of the vertical line) and have very similar distributions. n=13 recording sites from 8 control animals and n=8-9 recordings from 6 MCT1+/- mice.
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121 To overcome the confounding effect of the difference in amplitude when interpreting the post-gap responses, instead of comparing one group with another, I tested whether a significant evoked response was observed after each gap, comparing it to the baseline activity for individual recordings as in section 1.2.3. No differences in baseline activity were seen between groups (p=0.56). Interestingly, I did not observe an effect of gap in the reduction of p-values (Kruskal-Wallis test, χ2(9, n=219)=9.31, p=0.40), probably due to the strong responses of control animals to the 0.5 ms sound. MCT1+/- mice have normal temporal acuity.
Individual examples of a Hif1fl/fl;PLP-CreERT2wt and a Hif1fl/fl;PLP-CreERT2+ recording are shown in Figure 41-A, for a sound without a gap and one with a 2 ms gap. The post-gap PSTH at different gap lengths (Figure 41-B) shows that the amplitude of pre-gap responses in Hif1fl/fl;PLP-CreERT2+ is significantly different from control animals (ANOVA, F(1,189)=21.31, p<0.0001).
Because of this difference, the comparison between control and mutant animals is hard to interpret, as discussed with the MCT1 mice. In Figure 41-C, we can see the comparison of baseline activity with post-gap activity. As expected, the longer gaps elicited stronger spiking probability, both in control and mutant animals. I observed a significant effect of gap overall, (Kruskal-Wallis test, χ2(9, n=221)=35.25, p=5.3e-5), and a significant effect of group (Kruskal-Wallis test, χ2(1, n=221)=22.8, p=1.7e-6). There were no differences in baseline activity between groups (p=0.57). These results show that, similar to what was observed for MBPshi mice, the Hif1fl/fl;PLP-CreERT2+ mice have a strong reduction in spiking probability specially after the presentation of 1, 2 and 3 ms gaps (p=0.01, 0.058 and 0.013).
This reflects a strong impairment in temporal acuity of mutantmice with an oligodendrocyte metabolic impairment. Interestingly, the effect in these mutants, for which myelin thickness is only mildly impaired, is comparable to that seen in MBPshi mice with extensive dysmyelination.
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Figure 41. Temporal acuity is impaired in the auditory cortex of Hif1afl/f;PLP-creRT2+ mice.
A) An individual example of the gap-detection recordings of a control (upper, black) and mutant (lower, green) mouse. These raster plots represent the spiking activity (individual dots) that occurs during sound presentation (gray patches) to the 10 stimulus repetitions. Above the raster plots, a representation of the sounds played is presented in black bars, for a sound without a gap (0 ms gap, left side) and a sound with a 2 ms gap (right side).
B) The average PSTH of all animals shows the responses of control (black) and Hif1afl/f;PLP-creRT2+ mice (green) with the S.E.M. represented as a shaded error bar following the same color code. Pre-gap and post-gap responses to the presentation of 0.5 to 5 ms are showed. Significant differences were seen between groups throughout the gaps (ANOVA, p<0.0001). n=6 control and n=6 mutant mice. C) Quantification of the differences between baseline and post-gap activity (50 ms window). A dotted black line depicts the threshold of p=0.05 for significance. The longer the gap length, the higher the significance value. A significant difference was seen for gaps (ANOVA, p<0.0001) and groups (ANOVA, p<0.0001, green asterisk). The plot shows the median values for both groups, error bars represent the standard error of the median. The internal distribution plot in C) shows all the p-value distributions for the 3 ms gap condition in control (gray) and mutant (green) mice. A vertical black dotted line depicts the p value corresponding to 0.05 significance. Most of the responses of both the control and mutant to the 3 ms gap are significantly different than baseline activity (left side of the vertical line) and have very similar distributions. n=8-13 recording sites of 6 control and n= 10-12 recordings of 6 mutant mice.
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