• Charles Munyon, Katharine C Eakin, Jennifer A Sweet, and Jonathan P Miller.
• Department of Neurological Surgery, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, United States.
• Brain Res. 2014 Sep 25; 1582: 220-6.

Objectivemild 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.Methodssix 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.ResultsmTBI 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 12h (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).Conclusionmemory deficits after mTBI are associated with decreased intrinsic burst activity and impaired context-specific firing patterns in the hippocampus during object exploration.Copyright © 2014 Elsevier B.V. All rights reserved.

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