Chronic corticosterone treatment potentiates deficits following traumatic brain injury in rats: implications for aging

Brain Trauma, Glucocorticoids and Neuroinflammation: Dangerous Liaisons for the Hippocampus

Glucocorticoid-dependent mechanisms of inflammation-mediated distant hippocampal damage are discussed with a focus on the consequences of traumatic brain injury. The effects of glucocorticoids on specific neuronal populations in the hippocampus depend on their concentration, duration of exposure and cell type. Previous stress and elevated level of glucocorticoids prior to pro-inflammatory impact, as well as long-term though moderate elevation of glucocorticoids, may inflate pro-inflammatory effects. Glucocorticoid-mediated long-lasting neuronal circuit changes in the hippocampus after brain trauma are involved in late post-traumatic pathology development, such as epilepsy, depression and cognitive impairment. Complex and diverse actions of the hypothalamic–pituitary–adrenal axis on neuroinflammation may be essential for late post-traumatic pathology. These mechanisms are applicable to remote hippocampal damage occurring after other types of focal brain damage (stroke, epilepsy) or central nervous system diseases without obvious focal injury. Thus, the liaisons of excessive glucocorticoids/dysfunctional hypothalamic–pituitary–adrenal axis with neuroinflammation, dangerous to the hippocampus, may be crucial to distant hippocampal damage in many brain diseases. Taking into account that the hippocampus controls both the cognitive functions and the emotional state, further research on potential links between glucocorticoid signaling and inflammatory processes in the brain and respective mechanisms is vital.

Overnight Caloric Restriction Prior to Cardiac Arrest and Resuscitation Leads to Improved Survival and Neurological Outcome in a Rodent Model

While interest toward caloric restriction (CR) in various models of brain injury has increased in recent decades, studies have predominantly focused on the benefits of chronic or intermittent CR. The effects of ultra-short, including overnight, CR on acute ischemic brain injury are not well studied. Here, we show that overnight caloric restriction (75% over 14 h) prior to asphyxial cardiac arrest and resuscitation (CA) improves survival and neurological recovery as measured by, behavioral testing on neurological deficit scores, faster recovery of quantitative electroencephalography (EEG) burst suppression ratio, and complete prevention of neurodegeneration in multiple regions of the brain. We also show that overnight CR normalizes stress-induced hyperglycemia, while significantly decreasing insulin and glucagon production and increasing corticosterone and ketone body production. The benefits seen with ultra-short CR appear independent of Sirtuin 1 (SIRT-1) and brain-derived neurotrophic factor (BDNF) expression, which have been strongly linked to neuroprotective benefits seen in chronic CR. Mechanisms underlying neuroprotective effects remain to be defined, and may reveal targets for providing protection pre-CA or therapeutic interventions post-CA. These findings are also of high importance to basic sciences research as we demonstrate that minor, often-overlooked alterations to pre-experimental dietary procedures can significantly affect results, and by extension, research homogeneity and reproducibility, especially in acute ischemic brain injury models.

Rehabilitation modality and onset differentially influence whisker sensory hypersensitivity after diffuse traumatic brain injury in the rat

BACKGROUND

As rehabilitation strategies advance as therapeutic interventions, the modality and onset of rehabilitation after traumatic brain injury (TBI) are critical to optimize treatment. Our laboratory has detected and characterized a late-onset, long-lasting sensory hypersensitivity to whisker stimulation in diffuse brain-injured rats; a deficit that is comparable to visual or auditory sensory hypersensitivity in humans with an acquired brain injury.

OBJECTIVE

We hypothesize that the modality and onset of rehabilitation therapies will differentially influence sensory hypersensitivity in response to the Whisker Nuisance Task (WNT) as well as WNT-induced corticosterone (CORT) stress response in diffuse brain-injured rats and shams.

METHODS

After midline fluid percussion brain injury (FPI) or sham surgery, rats were assigned to one of four rehabilitative interventions: (1) whisker sensory deprivation during week one or (2) week two or (3) whisker stimulation during week one or (4) week two. At 28 days following FPI and sham procedures, sensory hypersensitivity was assessed using the WNT. Plasma CORT was evaluated immediately following the WNT (aggravated levels) and prior to the pre-determined endpoint 24 hours later (non-aggravated levels).

RESULTS

Deprivation therapy during week two elicited significantly greater sensory hypersensitivity to the WNT compared to week one (p < 0.05), and aggravated CORT levels in FPI rats were significantly lower than sham levels. Stimulation therapy during week one resulted in low levels of sensory hypersensitivity to the WNT, similar to deprivation therapy and naïve controls, however, non-aggravated CORT levels in FPI rats were significantly higher than sham.

CONCLUSION

These data indicate that modality and onset of sensory rehabilitation can differentially influence FPI and sham rats, having a lasting impact on behavioral and stress responses to the WNT, emphasizing the necessity for continued evaluation of modality and onset of rehabilitation after TBI.

Elucidating opportunities and pitfalls in the treatment of experimental traumatic brain injury to optimize and facilitate clinical translation

The aim of this review is to discuss the research presented in a symposium entitled "Current progress in characterizing therapeutic strategies and challenges in experimental CNS injury" which was presented at the 2016 International Behavioral Neuroscience Society annual meeting. Herein we discuss diffuse and focal traumatic brain injury (TBI) and ensuing chronic behavioral deficits as well as potential rehabilitative approaches. We also discuss the effects of stress on executive function after TBI as well as the response of the endocrine system and regulatory feedback mechanisms. The role of the endocannabinoids after CNS injury is also discussed. Finally, we conclude with a discussion of antipsychotic and antiepileptic drugs, which are provided to control TBI-induced agitation and seizures, respectively. The review consists predominantly of published data.

Triggering Receptor Expressed on Myeloid Cells 2 Deficiency Alters Acute Macrophage Distribution and Improves Recovery after Traumatic Brain Injury

Traumatic brain injury (TBI) affects 1.7 million persons annually in the United States (Centers for Disease Control and Prevention). There is increasing evidence that persons exposed to TBI have increased risk of the development of multiple neurodegenerative conditions, including Alzheimer disease (AD). TBI triggers a strong neuroinflammatory response characterized by astrogliosis, activation of microglia, and infiltration of peripheral monocytes. Recent evidence suggests that alterations in innate immunity promote neurodegeneration. This includes genetic studies demonstrating that mutations in triggering receptor expressed on myeloid cells 2 (TREM2) is associated with a higher risk for not only AD but also multiple neurodegenerative diseases. To examine whether TREM2 deficiency affects pathological outcomes of TBI, Trem2 knockout (Trem2^-/-) and C57BL/6J (B6) mice were given a lateral fluid percussion injury (FPI) and sacrificed at 3 and 120 days post-injury (DPI) to look at both acute and chronic consequences of TREM2 deficiency. Notably, at 3 DPI, B6 mice exposed to TBI exhibited increased expression of TREM2 in the brain. Further, Trem2^-/- mice exposed to TBI exhibited enhanced macrophage activation near the lesion, but significantly less macrophage activation away from the lesion when compared with B6 mice exposed to TBI. In addition, at 120 DPI, Trem2^-/- mice exposed to TBI demonstrated reduced hippocampal atrophy and rescue of TBI-induced behavioral changes when compared with B6 mice exposed to TBI. Taken together, this study suggests that TREM2 deficiency influences both acute and chronic responses to TBI, leading to an altered macrophage response at early time points, and improved pathological and functional outcomes at later time points.

Influence of age on brain edema formation, secondary brain damage and inflammatory response after brain trauma in mice

After traumatic brain injury (TBI) elderly patients suffer from higher mortality rate and worse functional outcome compared to young patients. However, experimental TBI research is primarily performed in young animals. Aim of the present study was to clarify whether age affects functional outcome, neuroinflammation and secondary brain damage after brain trauma in mice. Young (2 months) and old (21 months) male C57Bl6N mice were anesthetized and subjected to a controlled cortical impact injury (CCI) on the right parietal cortex. Animals of both ages were randomly assigned to 15 min, 24 h, and 72 h survival. At the end of the observation periods, contusion volume, brain water content, neurologic function, cerebral and systemic inflammation (CD3+ T cell migration, inflammatory cytokine expression in brain and lung, blood differential cell count) were determined. Old animals showed worse neurological function 72 h after CCI and a high mortality rate (19.2%) compared to young (0%). This did not correlate with histopathological damage, as contusion volumes were equal in both age groups. Although a more pronounced brain edema formation was detected in old mice 24 hours after TBI, lack of correlation between brain water content and neurological deficit indicated that brain edema formation is not solely responsible for age-dependent differences in neurological outcome. Brains of old naïve mice were about 8% smaller compared to young naïve brains, suggesting age-related brain atrophy with possible decline in plasticity. Onset of cerebral inflammation started earlier and primarily ipsilateral to damage in old mice, whereas in young mice inflammation was delayed and present in both hemispheres with a characteristic T cell migration pattern. Pulmonary interleukin 1β expression was up-regulated after cerebral injury only in young, not aged mice. The results therefore indicate that old animals are prone to functional deficits and strong ipsilateral cerebral inflammation without major differences in morphological brain damage compared to young.

Psychological and biological correlates of fatigue after mild-to-moderate traumatic brain injury

Relationships between chronic perceived stress, cortisol response (area under the curve) and posttraumatic brain injury fatigue were examined with persons from outpatient settings. Seventy-five injured persons with traumatic brain injury and their relatives/significant others participated in this cross-sectional study. Using interviews and self-reported data from the Neurofunctional Behavioral Inventory, the Perceived Stress Scale, the Profile of Mood States-Fatigue subscale, the McGill Pain Scale, as well as self-collection of salivary cortisol over a 12-hour period (N = 50), we found that perceived chronic stress explained 40% of the variance in fatigue until depressive symptoms and pain were in the model. Hypocortisolemia was evident. Somatic symptom frequency and perceived chronic stress represented 50% of the variability in post-TBI fatigue. Fatigue and stress management interventions, as suggested in the Centers for Disease Control Acute Concussion guidelines, may be beneficial in reducing this common symptom.

Functional Status, Chronic Stress, and Cortisol Response After Mild-to-Moderate Traumatic Brain Injury

It is well known that individuals experience difficulties with depressive symptoms and functional status after traumatic brain injury. However, it is unclear what the relationship is between these 2 phenomena and whether there is a biological explanation for. In this secondary analysis, we examined whether depressive symptoms explained postinjury functional status and whether chronic stress and salivary cortisol influenced this relationship. Participants included 75 persons within 24 months of their injury dates who were evaluated or treated in specialty clinics. All participants and a family member or significant other completed survey data, and 50 of the participants provided cortisol data. Results indicated that chronic stress, measured using Cohen's Perceived Stress Scale, completely mediated the relationship between depressive symptoms, measured using the Center for Epidemiological Studies Depression Scale, and psychological functioning, measured using the Patient Competency Rating Scale. Furthermore, those who provided cortisol data displayed hypocortisolemia in their 12-hr profile. Results from this analysis suggest that interventions focused on addressing the chronic stress experience may be important in limiting depressive symptoms and improving psychological functioning. Longitudinal study of this phenomenon is required to understand the progression of chronic stress after traumatic brain injury.

Exacerbated glial response in the aged mouse hippocampus following controlled cortical impact injury

Old age is associated with enhanced susceptibility to and poor recovery from brain injury. An exacerbated microglial and astrocyte response to brain injury might be involved in poor outcomes observed in the elderly. The present study was therefore designed to quantitate the expression of markers of microglia and astrocyte activation using real-time RT-PCR, immunoblot and immunohistochemical analysis in aging brain in response to brain injury. We examined the hippocampus, a region that undergoes secondary neuron death, in aged (21-24 months) and adult (5-6 months) mice following controlled cortical impact (CCI) injury to the sensorimotor cortex. Basal mRNA expression of CD11b and Iba1, markers of activated microglia, was higher in aged hippocampus as compared to the adult. The mRNA expression of microglial markers increased and reached maximum 3 days post-injury in both adult and aged mice, but was higher in the aged mice at all time points studied, and in the aged mice the return to baseline levels was delayed. Basal mRNA expression of GFAP and S100B, markers of activated astrocytes, was higher in aged mice. Both markers increased and reached maximum 7 days post-injury. The mRNA expression of astrocyte markers returned to near basal levels rapidly after injury in the adult mice, whereas again in the aged mice return to baseline was delayed. Immunochemical analysis using Iba1 and GFAP antibodies indicated accentuated glial responses in the aged hippocampus after injury. The pronounced and prolonged activation of microglia and astrocytes in hippocampus may contribute to worse cognitive outcomes in the elderly following TBI.

Middle age increases tissue vulnerability and impairs sensorimotor and cognitive recovery following traumatic brain injury in the rat

With increasing age comes an increased risk for sustaining traumatic brain injuries (TBI). However, the effect of age is rarely studied in animal models of TBI. The present study evaluated the effect of increased age on recovery of function following bilateral medial frontal cortex injury. Groups of young (3 months) and middle-aged (14 months) rats received bilateral frontal cortex contusions or sham injuries. The rats were tested on a variety of tests to measure sensorimotor performance (bilateral tactile adhesive removal test), skilled forelimb use (staircase test), and the acquisition of reference and working memory in the Morris water maze. Results indicated that injury produced significant impairments on all behavioral tests compared to sham controls. Middle-aged rats that received cortical contusions were significantly impaired on the bilateral tactile adhesive removal test, acquisition of a reference memory task, and working memory compared to young-injured rats. Histological analysis showed that middle-aged rats developed significantly larger lesion cavities but did not show an increase in the number of glial fibrillary acidic protein (GFAP+) cells compared to young-injured rats. Age alone also significantly impaired function on the bilateral adhesive tactile removal test, skilled forelimb use, the acquisition of a reference memory task, and also increased the number of GFAP+ cells compared to young rats. These results indicate that middle-aged rats respond to brain injury differently than young rats and that age is an important factor to consider in pre-clinical efficacy studies.

The hypothalamo-pituitary-adrenal axis response to experimental traumatic brain injury

Alterations in the hypothalamo-pituitary-adrenal (HPA) axis following traumatic brain injury have not been documented in detail. We used fluid percussion injury (FPI) to evaluate the early changes in components of the HPA axis following experimental traumatic brain injury. Wistar rats were sacrificed at 2 or 4 h following sham or FPI surgery. In situ hybridization histochemistry was used to determine the expression of mRNAs of corticotrophin releasing hormone (CRH) and arginine vasopressin (AVP) in the hypothalamus and pro-opiomelanocortin (POMC) in the pituitary. A group of animals undergoing no surgery were used as control. Repeated blood sampling from an indwelling catheter demonstrated that plasma corticosterone (CORT) levels peaked 30 min following surgery in sham and FPI animals but there was no significant difference in CORT concentration between these groups at any time. Pituitary POMC expression was increased following sham and FPI surgery (compared with control non-operated animals) but with no significant difference between the two groups undergoing surgery. Hypothalamic CRH mRNA expression was significantly higher in animals undergoing FPI compared with sham surgery. Hypothalamic AVP mRNA expression was not significantly increased when compared with control nonoperated animals. These data indicate that the anaesthesia and/or surgery associated with FPI or sham surgery induces a generalised activation of the HPA axis. The selective increase in CRH mRNA in animals undergoing FPI may be due to specific effects of traumatic brain injury rather than a general stress response and may suggest an additional neurotransmitter role for CRH following head injury. The absence of an AVP response suggests that the effects of FPI may be mediated through the CRH-alone-containing subpopulation of neurons.

Paradoxical effects of chronic corticosterone on forced swim behaviours in aged male and female rats

The effects of chronically administered corticosterone on forced swim test and open field test behaviours were explored in aged male and female rats. Though corticosterone has typically been associated with depressive behaviours, recent data have suggested a putative antidepressive effect of corticosterone. The current study used the forced swim test as a model of antidepressant efficacy in order to explore this. Aged male and female rats received either corticosterone (20 mg/kg) or the vehicle for 10 days before testing in the forced swim test, then for an additional 3 days before testing in the open field test. On day 11, each animal was individually tested on the duration of swimming, immobile, and struggling behaviours, and on day 14, for the display of rearing and line crossing behaviours. Results revealed that corticosterone significantly increased swimming and decreased immobility behaviour in females, but failed to do so in males. Additionally, there was a main effect of corticosterone on struggling behaviour such that it decreased it in males. There were no effects of corticosterone or sex on open field test behaviours, suggesting that the present findings are not accounted for by a general effect of corticosterone on motor behaviour. Overall, the data suggest that chronically administered corticosterone possesses effects that are sex-specific, and that it may exert mildly antidepressive effects in females, but the opposite effects in males. These data are consistent with emerging evidence that corticosterone may play a paradoxical antidepressive effect.

Traumatic brain injury reduces hippocampal alpha7 nicotinic cholinergic receptor binding

Changes in the expression of central nervous system (CNS) neurotransmitter receptors may contribute to behavioral and physiological deficits that occur following traumatic brain injury (TBI). Studies investigating the neurochemical basis for the protracted cognitive dysfunction that follows TBI have focused in part on cholinergic mechanisms. The present study compared the effects of mild and moderate cortical contusion injury (CCI) on the density of cholinergic receptor subtypes, NMDA-type glutamate receptors, and calcium channel expression. Quantitative autoradiography was used to determine the effects of CCI on receptor expression, 48 h following injury. The most robust and consistent change in receptor binding was in the density of alpha7 nicotinic receptors as determined by alpha-[125I]-bungarotoxin (BTX) binding. Bilateral deficits in BTX binding were present following both mild and moderate levels of injury. In contrast, changes in the density of alpha3/alpha4 nAChr's, muscarinic AChr's, NMDA-type glutamate receptors, and L-type calcium channel expression were more regionally restricted and lower in magnitude, as compared to changes in BTX binding. The high calcium permeability of the alpha7 nAChr may be related to the extensive decrease in BTX binding that occurs following TBI.

Future therapeutic targets in mood disorders: the glucocorticoid receptor

BACKGROUND

The hypercortisolaemia and dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis associated with mood disorders have been attributed to a breakdown in the glucocorticoid-receptor-mediated negative feedback mechanism regulating HPA activity. Reinstating normal feedback may be therapeutic in mood disorders.

AIMS

To review the evidence for the involvement of the glucocorticoid receptor in the pathogenesis and treatment of mood disorders.

METHOD

Medline and hand searches were carried out, selecting literature relevant to psychiatrists and psychopharmacologists.

RESULTS

A dysfunction in glucocorticoid receptors is integral to the HPA abnormalities of mood disorders. Antidepressant and mood-stabilising drugs can up-regulate glucocorticoid receptors, restoring glucocorticoid function. Preliminary clinical studies targeting the glucocorticoid receptor are encouraging.

CONCLUSIONS

Drugs designed specifically to up-regulate glucocorticoid receptors may be integral to future strategies in treating mood disorders.

Evaluation of learning and memory dysfunction and histological findings in rats with chronic stage contusion and diffuse axonal injury

We previously reported a modified fluid percussion device capable of consistently producing experimental cortical contusion (CC) and diffuse axonal injury (DAI) in separate groups of rats by lateral and midline fluid percussion, respectively. The purpose of the present study was to compare the differences in learning acquisition and memory retention impairments between these two types of injured rats in the chronic stage using the Morris water maze technique. We also compared the histological differences between these two different types of traumatic brain injury. The results showed a statistically significant difference in learning acquisition impairment between the sham and CC rats and also between the sham and DAI rats. However, a significant difference in memory retention impairment was observed only between the sham and DAI rats. Histologically, the neuronal cell loss of CA3 pyramidal cells in the hippocampus was observed on the ipsilateral side in the CC and bilaterally in DAI. The neuronal cell loss was seen in bilateral entorhinal cortex layer II in DAI, but it was not seen in CC. From these results, we speculate that the marked cell loss in the hippocampus CA3 region in both CC and DAI rats was related to the impairment of spatial learning acquisition. The marked cell loss in entorhinal cortex layer II in DAI rats may be one of the important factors in the impairment of spatial memory retention.

The effect of the adrenocortical axis upon recovery from closed head injury

Fragments and analogs of the hormone ACTH were previously shown to have beneficial effect on the outcome of head injury, while elevated levels of corticosterone (CS) exacerbate it. In the present study we investigated the role of the hypothalamo-pituitary-adrenal (HPA) axis in the pathophysiology of closed head injury (CHI). CHI was produced in ether-anesthetized rats by a calibrated weight-drop device. After evaluating the functional status according to a set of criteria, at 1 and 24 h, the rats were sacrificed and cortical tissue was removed to determine its water content. CHI was also produced in rats that underwent surgical procedures to remove their adrenal gland (ADEX) or the pituitary (HypoX), thus altering the levels of their circulating HPA hormones. Given after CHI, to rats with intact HPA axis, ACTH reduced edema and improved recovery. ADEX rats (6 days postsurgery) had 10-fold higher levels of plasma ACTH. ADEX rats subjected to CHI showed improved functional outcome (p = 0.008) and reduced edema (p = 0.02). We then produced CHI in three groups of rats: HypoX (15 days postsurgery), HypoX treated with ACTH, and controls. In HypoX rats, CHI resulted in increased mortality (35% vs 0) and edema in the surviving rats, and a slower recovery, as compared with the control. Mortality was prevented, edema slightly reduced, and recovery significantly improved after administration of 1-24-ACTH to HypoX rats with CHI. Our results suggest that ACTH has a cerebroprotective effect on the outcome of CHI.

Anabolic-androgenic steroid and adrenal steroid effects on hippocampal plasticity

Anabolic-androgenic steroids (AAS) are synthetic androgen-like compounds which are taken in high doses by athletes with the intention of enhancing muscular appearance, strength and/or athletic performance. Recent research indicates that high doses of AAS may influence the functions of the hippocampus. This evidence led us to explore the extent to which chronic AAS treatments influence spatial memory and the integrity of the hippocampus in the rat. Gonadally intact adult male Long-Evans rats were treated with either the AAS methandrostenolone, a steroid 'cocktail' (TNB; testosterone cypionate, boldenone undecylenate and nandrolone decanoate), or the oil vehicle daily for 12 weeks. A group of male rats treated with corticosterone (CORT; 10 mg/day) was also examined. Spatial memory was assessed in the Morris water maze after 10 weeks of hormone treatment. At 12 weeks, the animals were sacrificed, blood collected and the brain sectioned to assess hippocampal cell number. There were no impairments in the acquisition or retention of the Morris water maze in any hormone treatment group. Although serum testosterone levels were elevated in rats treated with TNB relative to the oil controls, neither the TNB or methandrostenolone treatments produced changes in hippocampal cell number. Serum CORT levels were significantly elevated in the rats treated with CORT and cell loss (15%) was detectable in the CA3b subfield in this group of animals. These results indicate that the AAS administered in the present study were not detrimental to hippocampal spatial memory or cell survival and that, while chronic CORT may produce mild hippocampal cell loss, this loss is not accompanied by deficits on a spatial memory task.

The effects of long-term corticosterone administration on hippocampal morphology and cognitive performance of middle-aged rats

The main objective of this research was to study the relationship between glucocorticoids, aging and the deterioration of cognitive functions. Towards this end, an attempt was made to develop an animal model which will enable the investigation of such interactions, using subcutaneously implanted sustained-release corticosterone pellets. The goal was to achieve moderately high concentrations of corticosterone in plasma, comparable to the peak basal levels or to those found under mild stress. Middle-aged (12 months old) Fischer-344 rats, used in this study, were divided before the prolonged hormonal treatment into cognitively 'impaired' and 'non-impaired' groups using the Morris water maze. The cognitive impairment, which was induced by the long-term corticosterone administration, was exhibited during acquisition of the 8-arm radial maze only in the 'non-impaired' group. Behavioral scores for drug-treated 'impaired' rats were not statistically different from those of the placebo-treated 'impaired' group. The morphological deterioration in hippocampal areas of the brain was quantified and revealed high correlation with the behavioral data. This animal model may become extremely useful in testing projected prophylactic therapy against the brain damage and cognitive deficits induced by the high corticosteroid-aging combination.

Increased anticholinergic sensitivity following closed skull impact and controlled cortical impact traumatic brain injury in the rat

Evidence suggests that prolonged memory deficits in several neurodegenerative diseases are attributable to deficits in central cholinergic neurotransmission. In traumatic brain injury (TBI), such cholinergic deficits also may contribute to prolonged memory disturbances. This study determined whether moderate magnitudes of TBI produced by controlled cortical impact and mild magnitudes of experimental TBI produced by a new closed head impact technique in rats would produce an enhanced vulnerability to the memory disruptive effects of scopolamine, a muscarinic cholinergic receptor antagonist. Water maze performance was used to determine changes in cholinergic hippocampal function following TBI. In the first experiment, rats received a moderate level of TBI by means of a controlled cortical impact. A Morris water maze task assessed spatial memory function on days 30-34 postinjury. During the 5 day assessment period, statistical analyses showed a group main effect for swim latency. Subsequent post hoc analyses indicated that injured rats had significantly longer latencies on days 30 and 31 (p < 0.05, injury vs sham controls). By days 32-35, injured rats showed no statistically significant deficits in spatial memory performance. On day 35, scopolamine (1 mg/kg, IP) was injected into injured rats and sham-injured rats 15 min prior to being retested in the maze. Results showed that although the scopolamine had no effects on the performance of the sham-injured rats, the same dose significantly (p < 0.05) increased the latency to find the hidden platform in the injured group. In the second experiment, rats received a mild concussive closed head impact. Water maze performance was assessed on days 8-12 postinjury. No significant water maze performance deficits were observed. On day 13, injured and uninjured rats were pharmacologically challenged with scopolamine (1 mg/kg) and retested. Similar to the first experiment, injured rats manifested a significantly greater (p < 0.05) sensitivity to scopolamine than sham controls. The results from both experiments suggest that concussive and more severe levels of TBI can produce an enhanced vulnerability to disruption of cholinergically mediated memory function, even when memory function appears normal in the absence of secondary challenges. These data demonstrate that covert deficits can persist after the recovery of normal function. These deficits may be attributable to a decrease in the ability of cholinergic neurons to function properly. These data also provide important insights into features of receptor-coupled disturbances that could contribute to the maintenance of enduring cognitive deficits following TBI.