Mild traumatic brain injury leads to decreased inhibition and a differential response of calretinin positive interneurons in the injured cortex

It is clear that even mild forms of traumatic brain injury (TBI) can have lasting cognitive effects; however, the specific cellular changes responsible for the functional deficits remain poorly understood. Previous studies suggest that not all neurons respond in the same way and that changes to neuronal architecture may be subtype specific. The current study aimed to characterize the response of interneurons to TBI. To model TBI in vitro, the neurites of primary cortical neurons were transected at 15 days in vitro. In response, calretinin⁺ interneurons underwent significant neurite remodeling around the injury site. By examining the response of pyramidal neurons, GAD67-GFP⁺ interneurons, and calretinin⁺ interneurons to the injury, we found that this response was specific to the calretinin⁺ cells. To determine whether calretinin⁺ interneurons respond in this way to a clinically relevant in vivo model of mild diffuse and focal injury, we subjected mice to the lateral fluid percussion injury model. We found that calretinin⁺ interneuron density was unaltered by this mild injury, but consistent with our in vitro data, these neurons underwent morphological alterations in their dendrites. These alterations evolved over a 28-day period, and calretinin⁺ interneurons in the injured mice had a reduction in mean dendrite length and reduced number of secondary dendrites than those in the sham-injured controls by 7 days post-injury. Further, these structural alterations were accompanied by a reduction in the frequency of miniature inhibitory post-synaptic currents in layer V pyramidal neurons. These data suggest that even a mild TBI can lead to an overall change in the excitatory/inhibitory balance of the cortex that may play an important role in the longer-term behavioral pathology associated with mild TBI.