Absence of Large-Scale Dendritic Plasticity of Layer 5 Pyramidal Neurons in Peri-Infarct Cortex

When stroke or traumatic brain injury lead to cortical damage, how do surviving neurons rewire the brain to restore lost functionalities? Several Golgi studies have argued for de novo growth and branching of dendrites of pyramidal neurons in the spared hemisphere, but the results could not always be replicated. Functional brain imaging studies in humans and rodents suggest that significant neuronal plasticity occurs in areas surrounding the cortical lesion, but whether dendritic rearrangements occur there has been less well studied, especially after stroke. We used in vivo two-photon microscopy in adult mice expressing green fluorescent protein to monitor longitudinally the length and branch complexity of entire apical dendritic arbors from layer 5 pyramidal neurons distributed over a large peri-infarct cortex region after middle cerebral artery occlusion. We find no evidence of growth of dendrites or addition of new branches to their arbors over a period of 3 months after stroke. Instead, we observed a two-step pruning process: an initial decrease in dendritic length, followed by a loss of dendritic branches. Importantly, the shortening of branch tips reflected a general shrinkage in the dendritic apical tree, suggesting that mechanical forces attributable to the involution of the infarct contributed to the changes in dendritic length. These results help resolve a long-standing debate regarding the role of large-scale dendritic plasticity of pyramidal neurons in functional recovery after cortical injury.