Decreased bursting and novel object-specific cell firing in the hippocampus after mild traumatic brain injury

• Memory deficits after mild TBI are seen even in the absence of significant hippocampal cell loss.
• Mild TBI is associated with reduced bursting in the hippocampus during exploration.
• Novel object recognition task performance correlates with object specificity at the cellular level.

Objective: mild traumatic brain injury (mTBI) can produce lasting memory deficits even in the absence of cell loss. We investigated changes in hippocampal firing patterns during exploration and during a novel object recognition (NOR) task. Methods: six male Sprague-Dawley rats were subjected to mTBI via fluid percussion injury and were compared with sham-operated rats. Microelectrodes were implanted into CA1 and CA3 and multiple units were recorded from the pyramidal cell layer. Spontaneous “burst” characteristics were analyzed and temporal firing patterns were correlated with object encounters to establish object-specific firing patterns. Results: mTBI was associated with significantly less hippocampal bursting (p<0.05) with a trend toward longer bursts and lower interburst spike frequency. mTBI was also associated with no preference for a novel object at 12 h (p<0.05). During the NOR task, a subset of pyramidal cells were identified which consistently demonstrated a transiently increased firing rate upon encounter of a specific object (“object-specific” cell). Across both groups, there was a significant (p<0.05) correlation between preference for object novelty and the difference between the total number of novel object-specific cells and familiar object-specific cells. The proportion of object-specific cells that responded to the unexpected (novel) object compared to those responding to the familiar object was significantly smaller in rats that had been exposed to mTBI (p<0.05). Conclusion: memory deficits after mTBI are associated with decreased intrinsic burst activity and impaired context-specific firing patterns in the hippocampus during object exploration.