Figure 5. Lateralized and layer-specific activation of CREB and downstream plasticity genes.

(A) A sample recording at one week after tetanus toxin injection shows interictal spikes confined to the left hemisphere. (B) At this time, most rats (n=5/7) show selective CREB phosphorylation in layer 2/3 of the spiking hemisphere relative to the contralateral, non-spiking hemisphere. This phosphorylation extends far beyond the injection site (solid arrow; slice approximately 1mm anterior to injection site). (C) A three-dimensional heatmap shows the interictal spike field size at two weeks after tetanus toxin that is associated with left-sided gene inductions shown in (D) within 1mm of the injection site (solid arrow). By two weeks after tetanus toxin injection, lateralized gene expression can be seen in 3 of 8 tetanus-injected rats, while in others it was seen bilaterally, suggesting further spread. Using radioactive in situ hybridizations, EGR1 shows robust signal throughout the cortex, but is especially strong in the superficial part of layer 2/3. NARP and BNDF, however, show relatively restricted expression in layer 2/3 of the spiking hemisphere (open arrows), while the non-spiking hemisphere shows comparatively sparse signal. The housekeeping gene GAPDH was used as a control and shows an even signal across both hemispheres in layers 2–6. This was also seen in surgery-naïve animals (n=3) for both GAPDH and all the other probes shown in this figure (data not shown). In situ hybridization images were inverted from dark-field images to improve visibility.