Timing-dependent effects of whisker trimming in thalamocortical slices including the mouse barrel cortex

Whisker trimming produces depression of cortical responses in the barrel cortex. However, it is unclear how the developmental timing modifies the effects of whisker trimming. We investigated cortical responses in thalamocortical slices that included the mouse barrel cortex using flavoprotein fluorescence imaging. A topological relationship was observed between the thalamic stimulated sites and cortical areas showing fluorescence changes. By adjusting the position of the thalamic stimulated sites and the cortical windows in which amplitudes of the fluorescence changes were measured, we succeeded to reduce the variability of cortical responses between slices. We then investigated the effects of whisker trimming in the thalamocortical slices. Whisker trimming from 4 weeks to 8 weeks (at 4-8 weeks) of age significantly reduced cortical responses at 8 weeks. However, whisker trimming started before 4 weeks produced only slight depression or no significant effect on the thalamocortical responses. As sensory deprivation during a critical developmental period is known to prevent elimination of synapses, the presence of aberrant synapses may compensate the cortical depression induced by whisker trimming started before 4 weeks. To test this possibility, whisker trimming performed at 0-6 or 0-7 weeks of age was followed by regrowth of whiskers for 1-2 weeks. Clear and significant potentiation of cortical responses was observed in these mice at 8 weeks when compared with those of naive mice of the same age. Overall, these data suggest that whisker trimming, producing depression of thalamocortical responses, prevents elimination of aberrant synapses during a critical developmental period before 4 weeks in the mouse barrel cortex.