Mild Traumatic Brain Injury Decreases Spatial Information Content and Reduces Place Field Stability of Hippocampal CA1 Neurons.

Both clinical and experimental studies have reported that mild traumatic brain injury (mTBI) can result in cognitive impairments in the absence of overt brain damage. Whether these impairments result from neuronal dysfunction/altered plasticity is an area that has received limited attention. In this study, we recorded activity of neurons in the cornu Ammonis (CA)1 subfield of the hippocampus in sham and mild lateral fluid percussion injured (mFPI) rats while these animals were performing an object location task. Electrophysiology results showed that the number of excitatory neurons encoding spatial information (i.e., place cells) was reduced in mFPI rats, and that these cells had broader and less stable place fields. Additionally, the in-field firing rate of place cells in sham operated, but not in mFPI, animals increased when objects within the testing arena were moved. Immunostaining indicated no visible damage or overall neuronal loss in mFPI brain sections. However, a reduction in the number of parvalbumin-positive inhibitory neurons in the CA1 subfield of mFPI animals was observed, suggesting that this reduction could have influenced place cell physiology. Alterations in spatial information content, place cell stability, and activity in mFPI rats coincided with poor performance in the object location task. These results indicate that altered place cell physiology may underlie the hippocampus-dependent cognitive impairments that result from mTBI.

Endoplasmic Reticulum Stress Contributes to the Loss of Newborn Hippocampal Neurons after Traumatic Brain Injury.

Adult hippocampal neurogenesis has been shown to be required for certain types of cognitive function. For example, studies have shown that these neurons are critical for pattern separation, the ability to store similar experiences as distinct memories. Although traumatic brain injury (TBI) has been shown to cause the loss of newborn hippocampal neurons, the signaling pathway(s) that triggers their death is unknown. Endoplasmic reticulum (ER) stress activates the PERK-eIF2α pathway that acts to restore ER function and improve cell survival. However, unresolved/intense ER stress activates C/EBP homologous protein (CHOP), leading to cell death. We show that TBI causes the death of hippocampal newborn neurons via CHOP. Using CHOP KO mice, we show that loss of CHOP markedly reduces newborn neuron loss after TBI. Injured CHOP mice performed significantly better in a context fear discrimination task compared with injured wild-type mice. In contrast, the PERK inhibitor GSK2606414 exacerbated doublecortin cell loss and worsened contextual discrimination. Administration of guanabenz (which reduces ER stress) to injured male rats reduced the loss of newborn neurons and improved one-trial contextual fear memory. Interestingly, we also found that the surviving newborn neurons in brain-injured animals had dendritic loss, which was not observed in injured CHOP KO mice or in animals treated with guanabenz. These results indicate that ER stress plays a key role in the death of newborn neurons after TBI. Further, these findings indicate that ER stress can alter dendritic arbors, suggesting a role for ER stress in neuroplasticity and dendritic pathologies.SIGNIFICANCE STATEMENT The hippocampus, a structure in the temporal lobe, is critical for learning and memory. The hippocampus is one of only two areas in which neurons are generated in the adult brain. These newborn neurons are required for certain types of memory, and are particularly vulnerable to traumatic brain injury (TBI). However, the mechanism(s) that causes the loss of these cells after TBI is poorly understood. We show that endoplasmic reticulum (ER) stress pathways are activated in newborn neurons after TBI, and that manipulation of the CHOP cascade improves newborn neuron survival and cognitive outcome. These results suggest that treatments that prevent/resolve ER stress may be beneficial in treating TBI-triggered memory dysfunction.

Mild Traumatic Brain Injury Reduces Spine Density of Projection Neurons in the Medial Prefrontal Cortex and Impairs Extinction of Contextual Fear Memory.

Epidemiology studies have found that a comorbidity exists between traumatic brain injury (TBI) and stress-related disorders. However, the anatomical and cellular bases for this association is poorly understood. An inability to extinguish the memory of a traumatic event lies at the core of many stress-related disorders. Experimental studies have shown that the medial pre-frontal cortex (mPFC), especially the infralimbic (IL) cortex, is required for extinction and for storing the memory of extinction. The output from the central nucleus of amygdala projects to the lateral hypothalamus, paraventricular nucleus, and central gray to regulate heart rate, stress hormone release, and freezing behavior, respectively. Projection neurons of the IL (layers II/III pyramidal neurons) are thought to stimulate GABAergic neurons in the amygdala, which, in turn, inhibit central amygdala output and reduce fear expression. Thus, loss and/or altered morphology of projection neurons of IL as a result of a mild TBI (mTBI) can compromise their ability to effectively inhibit the central amygdala, allowing the original fear memory to drive behavior. Using lateral mild fluid percussion injury (mFPI) in rats, we found that mFPI did not reduce neuronal numbers in the IL, but caused a significant reduction in overall dendritic spine density of both basal and apical dendrites on layer II/III pyramidal neurons. Spine numbers on layer V/VI pyramidal neurons were not significantly changed as a result of mFPI. The reduction in spine density on layer II/III pyramidal neurons we observed may diminish the efficacy of these neurons to inhibit the output of the central amygdala, thereby reducing the ability of the IL to suppress fear responses after extinction training. Consistent with this, mFPI rats display enhanced freezing behavior during and after extinction training as compared to sham-operated controls, although the ability to form contextual fear memories was not impaired. These results may have implications in stress-related disorders associated with mTBI.

Behavioral and histopathological alterations resulting from mild fluid percussion injury.

The majority of people who sustain a traumatic brain injury (TBI) have an injury that can be classified as mild (often referred to as concussion). Although head CT scans for most subjects who have sustained a mild TBI (mTBI) are negative, these persons may still suffer from neurocognitive and neurobehavioral deficits. In order to expedite pre-clinical research and develop therapies, there is a need for well-characterized animal models of mTBI that reflect the neurological, neurocognitive, and pathological changes seen in human patients. In the present study, we examined the motor, cognitive, and histopathological changes resulting from 1.0 and 1.5 atmosphere (atm) overpressure fluid percussion injury (FPI). Both 1.0 and 1.5 atm FPI injury caused transient suppression of acute neurological functions, but did not result in visible brain contusion. Animals injured with 1.0 atm FPI did not show significant motor, vestibulomotor, or learning and memory deficits. In contrast, 1.5 atm injury caused transient motor disturbances, and resulted in a significant impairment of spatial learning and short-term memory. In addition, 1.5 atm FPI caused a marked reduction in cerebral perfusion at the site of injury that lasted for several hours. Consistent with previous studies, 1.5 atm FPI did not cause visible neuronal loss in the hippocampus or in the neocortex. However, a robust inflammatory response (as indicated by enhanced GFAP and Iba1 immunoreactivity) in the corpus callosum and the thalamus was observed. Examination of fractional anisotropy color maps after diffusion tensor imaging (DTI) revealed a significant decrease of FA values in the cingulum, an area found to have increased silver impregnation, suggesting axonal injury. Increased silver impregnation was also observed in the corpus callosum, and internal and external capsules. These findings are consistent with the deficits and pathologies associated with mild TBI in humans, and support the use of mild FPI as a model to evaluate putative therapeutic options.