Previous studies of human traumatic brain injury (TBI) have shown diffuse axonal injury as varicosities or spheroids in white matter (WM) bundles when using immunoperoxidase‐ABC staining with 22C11, a mouse monoclonal antibody against amyloid precursor protein (APP). These findings have been interpreted as TBI‐induced axonal pathology. In a mouse model of TBI however, when we used immunofluorescent staining with 22C11, as opposed to immunoperoxidase staining, we did not observe varicosities or spheroids. To explore this discrepancy, we performed immunofluorescent staining with Y188, an APP knockout‐validated rabbit monoclonal that shows baseline immunoreactivity in neurons and oligodendrocytes of non‐injured mice, with some arranged‐like varicosities. In gray matter after injury, Y188 intensely stained axonal blebs. In WM, we encountered large patches of heavily stained puncta, heterogeneous in size. Scattered axonal blebs were also identified among these Y188‐stained puncta. To assess the neuronal origin of Y188 staining after TBI we made use of transgenic mice with fluorescently labeled neurons and axons. A close correlation was observed between Y188‐stained axonal blebs and fluorescently labeled neuronal cell bodies/axons. By contrast, no correlation was observed between Y188‐stained puncta and fluorescent axons in WM, suggesting that these puncta in WM did not originate from axons, and casting further doubt on the nature of previous reports with 22C11. As such, we strongly recommend Y188 as a biomarker for detecting damaged neurons and axons after TBI. With Y188, stained axonal blebs likely represent acute axonal truncations that may lead to death of the parent neurons. Y188‐stained puncta in WM may indicate damaged oligodendrocytes, whose death and clearance can result in secondary demyelination and Wallerian degeneration of axons. We also provide evidence suggesting that 22C11‐stained varicosities or spheroids previously reported in TBI patients might be showing damaged oligodendrocytes, due to a cross‐reaction between the ABC kit and upregulated endogenous biotin.

Cognitive impairment caused by traumatic brain injury (TBI) can lead to devastating consequences for both patients and their families. The underlying neurological basis for TBI-induced cognitive dysfunction remains unknown. However, many lines of research have implicated the hippocampus in the pathophysiology of TBI. In particular, past research has found that theta oscillations, long thought to be the electrophysiological basis of learning and memory, are decreased in the hippocampus post-TBI. Here, we recorded in vivo electrophysiological activity in the hippocampi of 16 mice, 8 of which had previously undergone a TBI. Consistent with previous data, we found that theta power in the hippocampus was decreased in TBI animals compared to sham controls; however, this effect was driven by changes in broadband power and not theta oscillations. This result suggests that broadband fluctuations in the hippocampal local field potential can be used as an electrophysiological surrogate of abnormal neurological activity post-TBI.

Mild traumatic brain injury (mTBI) disrupts hippocampal function and can lead to long-lasting episodic memory impairments. The encoding of episodic memories relies on spatial information processing within the hippocampus. As the primary entry point for spatial information into the hippocampus, the dentate gyrus is thought to function as a physiological gate, or filter, of afferent excitation before reaching downstream area Cornu Ammonis (CA3). Although injury has previously been shown to alter dentate gyrus network excitability, it is unknown whether mTBI affects dentate gyrus output to area CA3. In this study, we assessed hippocampal function, specifically the interaction between the dentate gyrus and CA3, using behavioral and electrophysiological techniques in ex vivo brain slices 1 week following mild lateral fluid percussion injury (LFPI). Behaviorally, LFPI mice were found to be impaired in an object-place recognition task, indicating that spatial information processing in the hippocampus is disrupted. Extracellular recordings and voltage-sensitive dye imaging demonstrated that perforant path activation leads to the aberrant spread of excitation from the dentate gyrus into area CA3 along the mossy fiber pathway. These results suggest that after mTBI, the dentate gyrus has a diminished capacity to regulate cortical input into the hippocampus, leading to increased CA3 network excitability. The loss of the dentate filtering efficacy reveals a potential mechanism by which hippocampal-dependent spatial information processing is disrupted, and may contribute to memory dysfunction after mTBI.

Traumatic brain injury (TBI) is associated with aberrant network hyperexcitability in the dentate gyrus. GABA A ergic parvalbumin-expressing interneurons (PV-INs) in the dentate gyrus regulate network excitability with strong, perisomatic inhibition, though the post-traumatic effects on PV-IN function after TBI are not well understood. In this study, we investigated physiological alterations in PV-INs one week after mild lateral fluid percussion injury (LFPI) in mice. PV-IN cell loss was observed in the dentate hilus after LFPI, with surviving PV-INs showing no change in intrinsic membrane properties. Whole-cell voltage clamp recordings in PV-INs revealed alterations in both excitatory and inhibitory postsynaptic currents (EPSCs/IPSCs). Evoked EPSCs in PV-INs from perforant path electrical stimulation were diminished after injury but could be recovered with application of a GABA A-receptor antagonist. Furthermore, currentclamp recordings using minimal perforant path stimulation demonstrated a decrease in evoked PV-IN action potentials after LFPI, which could be restored by blocking GABA A ergic inhibition. Together, these findings suggest that injury alters synaptic input onto PV-INs, resulting in a net inhibitory effect that reduces feedforward PV-IN activation in the dentate gyrus. Decreased PV-IN activation suggests a potential mechanism of dentate gyrus network hyperexcitability contributing to hippocampal dysfunction after TBI. Significance Statement Traumatic brain injury (TBI) damages the hippocampus and causes long-lasting memory deficits. After TBI, the dentate gyrus, a crucial regulator of cortical input to the hippocampus, undergoes a dysfunctional net increase in excitation, though the circuit mechanisms underlying this network excitatory-inhibitory (E/I) imbalance are unclear. In this study, we found that TBI alters synaptic inputs onto an inhibitory interneuron population (PV-INs) in the dentate gyrus which results in 3 the decreased firing activity of these neurons due to a net inhibitory influence. The inhibition of PV-INs demonstrates a potential mechanism contributing to dentate gyrus network hyperexcitability and hippocampal dysfunction after TBI.