Traumatic brain injury is a leading cause of death and disability in the United States. The injury is often composed of two processes: the primary injury, which can involve irreversible loss of tissue, and the secondary injury, which involves a cascade of reactive processes such as excitotoxicity that occur in the hours and days after the initial insult. Excitotoxic stimulation of neuronal circuits can lead to cellular dysfunction and modulation of neuronal sensitivity. One mechanism of dysfunction involves the calcium-regulated phosphatase, calcineurin. Calcineurin has been shown to be involved in the modulation of the neuronal post-synaptic structures known as dendritic spines. One means by which CaN regulates spine structure is through the dephosphorylation of the down-stream effector proteins such as, cofilin. This study tracks the changes in CaN activity levels as well as the phosphorylation state of cofilin in the cortex and hippocampus in each hemisphere of the laterally injured brain. We report that the lateral brain injury causes an increase in CaN activity in the hippocampus with a corresponding dephosphorylation of cofilin. Trauma-induced changes in CaN follow a slightly different time course in cortical tissue, as there is a biphasic modulation of cofilin that begins with an increased phosphorylation which is followed by an extended dephosphorylation. This dephosphorylation is partially prevented by a single post-injury injection of FK506, a calcineurin inhibitor. Since dephosphorylation of cofilin is a rate-limiting step in dendritic spine collapse, the results of this study demonstrate a potential cellular mechanism through which traumatic brain injury results in altered neuronal function.

Traumatic brain injury (TBI) induces a robust cellular proliferative response among neural stem/progenitor cells (NS/NPCs) in the dentate gyrus of the hippocampus. This proliferative effect is thought to contribute to the innate cognitive recovery observed following TBI. Inhibition of hippocampal neurogenesis impairs cognitive function. Furthermore, enhancement of injury-induced hippocampal neurogenesis via intraventricular administration of basic fibroblast growth factor (bFGF) improves cognitive function in animals following TBI. In this experiment, we investigated the direct association between injury-induced hippocampal neurogenesis and cognitive recovery utilizing an antimitotic agent, arabinofuranosyl cytidine (Ara-C). In this study, adult rats received a moderate lateral fluid percussion injury (LFPI). Immediately following injury, Ara-C with or without bFGF was infused into the lateral ventricle via an osmotic mini-pump for 7 days. To label dividing cells animals received daily single injections of 5-bromo-2'-deoxyuridine (BrdU) at 2-7 days post-injury. To examine the effect of Ara-C on cell proliferation, a group of animals was sacrificed at 1 week following injury. Brain sections were immunostained for BrdU and cell type specific markers, and the number of BrdU+ cells in the hippocampus was assessed by stereology. To examine the effect of inhibition of injury-induced cell proliferation on cognitive recovery, animals were assessed on Morris water maze tasks (MWM) either at 21 to 25 days or 56-60 days post-injury. We found that post-injury Ara-C treatment significantly reduces injury-induced cell proliferation in the DG and abolishes the innate cognitive recovery on MWM performance at 56-60 days post-injury. Additionally, Ara-C diminishes bFGF enhanced cell proliferation in the DG and cognitive recovery following TBI. These results support the causal relationship between injury-induced hippocampal neurogenesis and cognitive functional recovery. Our studies suggest that the post-TBI neurogenic response is an endogenous repair mechanism that contributes to the restoration of hippocampal function post-injury.

Traumatic brain injury (TBI), a leading cause of death and disability in the United States, causes potentially preventable damage in part through the dysregulation of neural calcium levels. This dysregulation likely affects the activity of the calcium-sensitive phosphatase, calcineurin, with serious implications for neural function. To test this possibility, the present study characterized the role of calcineurin in a rat model of brain trauma, the lateral fluid percussion injury model. Golgi-Cox histochemistry revealed an acute post-TBI loss and delayed overgrowth of dendritic spines on principal cortical cells. The spine loss appeared to require calcineurin activity, since administering a calcineurin inhibitor, FK506, 1 hour after TBI prevented the spine loss. Additional experiments showed how calcineurin activity might be related to the spine loss. Specifically, Western blots and enzyme activity assays revealed an acute increase in the cortical activity of calcineurin and its downstream effector, the actin-depolymerizing protein, cofilin. The cofilin activation was blocked by the same FK506 treatment that prevented spine loss, suggesting a relationship between cofilin activation and spine loss. To investigate long-term consequences of calcineurin activation after TBI, rats were administered FK506 (Tacrolimus) 1 hour after TBI and then monitored for spontaneous seizure activity months later. Acute post-TBI treatment with FK506 reduced the frequency of late non-convulsive seizures but did not prevent late convulsive seizures, cortical atrophy, or thalamic damage. The results of the present study implicate calcineurin in the acute dendritic remodeling and late non-convulsive seizures that occur after TBI. Importantly, these findings reveal calcineurin as a potential therapeutic target in the treatment of TBI and its sequalae.