Controlled Cortical Impact Results in an Extensive Loss of Dendritic Spines that Is Not Mediated by Injury-Induced Amyloid-Beta Accumulation

The clinical manifestations that occur after traumatic brain injury (TBI) include a wide range of cognitive, emotional, and behavioral deficits. The loss of excitatory synapses could potentially explain why such diverse symptoms occur after TBI, and a recent preclinical study has demonstrated a loss of dendritic spines, the postsynaptic site of the excitatory synapse, after fluid percussion injury. The objective of this study was to determine if controlled cortical impact (CCI) also resulted in dendritic spine retraction and to probe the underlying mechanisms of this spine loss. We used a unilateral CCI and visualized neurons and dendtritic spines at 24 h post-injury using Golgi stain. We found that TBI caused a 32% reduction of dendritic spines in layer II/III of the ipsilateral cortex and a 20% reduction in the dendritic spines of the ipsilateral dentate gyrus. Spine loss was not restricted to the ipsilateral hemisphere, however, with similar reductions in spine numbers recorded in the contralateral cortex (25% reduction) and hippocampus (23% reduction). Amyloid-beta (A beta), a neurotoxic peptide commonly associated with Alzheimer disease, accumulates rapidly after TBI and is also known to cause synaptic loss. To determine if A beta contributes to spine loss after brain injury, we administered a gamma-secretase inhibitor LY450139 after TBI. We found that while LY450139 administration could attenuate the TBI-induced increase in A beta, it had no effect on dendritic spine loss after TBI. We conclude that the acute, global loss of dendritic spines after TBI is independent of gamma-secretase activity or TBI-induced A beta accumulation.