Basal and scopolamine-evoked release of hippocampal acetylcholine following traumatic brain injury in rats

Evaluating the Efficacy of Chronic Galantamine on Sustained Attention and Cholinergic Neurotransmission in A Pre-Clinical Model of Traumatic Brain Injury

Cholinergic disruptions underlie attentional deficits following traumatic brain injury (TBI). Yet, drugs specifically targeting acetylcholinesterase (AChE) inhibition have yielded mixed outcomes. Therefore, we hypothesized that galantamine (GAL), a dual-action competitive AChE inhibitor and α7 nicotinic acetylcholine receptor (nAChR) positive allosteric modulator, provided chronically after injury, will attenuate TBI-induced deficits of sustained attention and enhance ACh efflux in the medial prefrontal cortex (mPFC), as assessed by in vivo microdialysis. In Experiment 1, adult male rats (n = 10-15/group) trained in the 3-choice serial reaction time (3-CSRT) test were randomly assigned to controlled cortical impact (CCI) or sham surgery and administered GAL (0.5, 2.0, or 5.0 mg/kg; i.p.) or saline vehicle (VEH; 1 mL/kg; i.p) beginning 24-h post-surgery and once daily thereafter for 27 days. Measures of sustained attention and distractibility were assessed on post-operative days 21-25 in the 3-CSRT, following which cortical lesion volume and basal forebrain cholinergic cells were quantified on day 27. In Experiment 2, adult male rats (n = 3-4/group) received a CCI and 24 h later administered (i.p.) one of the three doses of GAL or VEH for 21 days to quantify the dose-dependent effect of GAL on in vivo ACh efflux in the mPFC. Two weeks after the CCI, a guide cannula was implanted in the right mPFC. On post-surgery day 21, baseline and post-injection dialysate samples were collected in a temporally matched manner with the cohort undergoing behavior. ACh levels were analyzed using reverse phase high-performance liquid chromatography (HPLC) coupled to an electrochemical detector. Cortical lesion volume was quantified on day 22. The data were subjected to ANOVA, with repeated measures where appropriate, followed by Newman-Keuls post hoc analyses. All TBI groups displayed impaired sustained attention versus the pooled SHAM controls (p's < 0.05). Moreover, the highest dose of GAL (5.0 mg/kg) exacerbated attentional deficits relative to VEH and the two lower doses of GAL (p's < 0.05). TBI significantly reduced cholinergic cells in the right basal forebrain, regardless of treatment condition, versus SHAM (p < 0.05). In vivo microdialysis revealed no differences in basal ACh in the mPFC; however, GAL (5.0 mg/kg) significantly increased ACh efflux 30 min following injection compared to the VEH and the other GAL (0.5 and 2.0 mg/kg) treated groups (p's < 0.05). In both experiments, there were no differences in cortical lesion volume across treatment groups (p's > 0.05). In summary, albeit the higher dose of GAL increased ACh release, it did not improve measures of sustained attention or histopathological markers, thereby partially supporting the hypothesis and providing the impetus for further investigations into alternative cholinergic pharmacotherapies such as nAChR positive allosteric modulators.

Cannabidiol's neuroprotective properties and potential treatment of traumatic brain injuries

Cannabidiol (CBD) has numerous pharmacological targets that initiate anti-inflammatory, antioxidative, and antiepileptic properties. These neuroprotective benefits have generated interest in CBD's therapeutic potential against the secondary injury cascade from traumatic brain injury (TBI). There are currently no effective broad treatment strategies for combating the damaging mechanisms that follow the primary injury and lead to lasting neurological consequences or death. However, CBD's effects on different neurotransmitter systems, the blood brain barrier, oxidative stress mechanisms, and the inflammatory response provides mechanistic support for CBD's clinical utility in TBI. This review describes the cascades of damage caused by TBI and CBD's neuroprotective mechanisms to counter them. We also present challenges in the clinical treatment of TBI and discuss important future clinical research directions for integrating CBD in treatment protocols. The mechanistic evidence provided by pre-clinical research shows great potential for CBD as a much-needed improvement in the clinical treatment of TBI. Upcoming clinical trials sponsored by major professional sport leagues are the first attempts to test the efficacy of CBD in head injury treatment protocols and highlight the need for further clinical research.

Influence of Sex and Muscarinic Activity on Memory Retrieval in Mouse Model of Traumatic Brain Injury

Traumatic brain injury (TBI) is a serious global risk factor leading to the onset of cognitive impairment and neurodegenerative diseases. Cognitive and memory impairment following a TBI is associated with the dysregulation of cholinergic neurotransmission in the brains of subjects. The extent of memory impairment following a TBI is linked with the sex of the subject. This study aimed to identify the sex-dimorphic role of muscarinic cholinergic modulation in neurological functioning and episodic memory retrieval in a mouse model of TBI. Balb/c mice were divided into four groups of males and four groups of females (i.e., Sham, TBI, TBI + Scopolamine 1 mg/kg, and TBI + Donepezil 1 mg/kg). After training with the Morris water maze test and fear conditioning, all groups were subjected to brain injury (7.84 × 10^-5 J impact force) except for the Sham mice. Following brain injury, scopolamine or donepezil was administered to the respective groups for 5 days. Acute scopolamine immediately after brain trauma showed a neuroprotective effect in the males only, while subchronic donepezil significantly impaired neurological functioning in both sexes. Subchronic scopolamine and donepezil treatment reversed the TBI-induced retrograde amnesia for spatial memory in male mice. Contextual fear memory retrieval was not affected by the TBI and treatments in both sexes. Thus, we concluded that the sex-dimorphic response of the muscarinic receptors in TBI-induced memory impairment depends on the type of memory. This study highlights the potential for therapeutic modalities in TBI subjects.

Approaches to Monitor Circuit Disruption after Traumatic Brain Injury: Frontiers in Preclinical Research

Mild traumatic brain injury (TBI) often results in pathophysiological damage that can manifest as both acute and chronic neurological deficits. In an attempt to repair and reconnect disrupted circuits to compensate for loss of afferent and efferent connections, maladaptive circuitry is created and contributes to neurological deficits, including post-concussive symptoms. The TBI-induced pathology physically and metabolically changes the structure and function of neurons associated with behaviorally relevant circuit function. Complex neurological processing is governed, in part, by circuitry mediated by primary and modulatory neurotransmitter systems, where signaling is disrupted acutely and chronically after injury, and therefore serves as a primary target for treatment. Monitoring of neurotransmitter signaling in experimental models with technology empowered with improved temporal and spatial resolution is capable of recording in vivo extracellular neurotransmitter signaling in behaviorally relevant circuits. Here, we review preclinical evidence in TBI literature that implicates the role of neurotransmitter changes mediating circuit function that contributes to neurological deficits in the post-acute and chronic phases and methods developed for in vivo neurochemical monitoring. Coupling TBI models demonstrating chronic behavioral deficits with in vivo technologies capable of real-time monitoring of neurotransmitters provides an innovative approach to directly quantify and characterize neurotransmitter signaling as a universal consequence of TBI and the direct influence of pharmacological approaches on both behavior and signaling.

(-)-Phenserine Ameliorates Contusion Volume, Neuroinflammation, and Behavioral Impairments Induced by Traumatic Brain Injury in Mice

Traumatic brain injury (TBI), a major cause of mortality and morbidity, affects 10 million people worldwide, with limited treatment options. We have previously shown that (-)-phenserine (Phen), an acetylcholinesterase inhibitor originally designed and tested in clinical phase III trials for Alzheimer's disease, can reduce neurodegeneration after TBI and reduce cognitive impairments induced by mild TBI. In this study, we used a mouse model of moderate to severe TBI by controlled cortical impact to assess the effects of Phen on post-trauma histochemical and behavioral changes. Animals were treated with Phen (2.5 mg/kg, IP, BID) for 5 days started on the day of injury and the effects were evaluated by behavioral and histological examinations at 1 and 2 weeks after injury. Phen significantly attenuated TBI-induced contusion volume, enlargement of the lateral ventricle, and behavioral impairments in motor asymmetry, sensorimotor functions, motor coordination, and balance functions. The morphology of microglia was shifted to an active from a resting form after TBI, and Phen dramatically reduced the ratio of activated to resting microglia, suggesting that Phen also mitigates neuroinflammation after TBI. While Phen has potent anti-acetylcholinesterase activity, its (+) isomer Posiphen shares many neuroprotective properties but is almost completely devoid of anti-acetylcholinesterase activity. We evaluated Posiphen at a similar dose to Phen and found similar mitigation in lateral ventricular size increase, motor asymmetry, motor coordination, and balance function, suggesting the improvement of these histological and behavioral tests by Phen treatment occur via pathways other than anti-acetylcholinesterase inhibition. However, the reduction of lesion size and improvement of sensorimotor function by Posiphen were much smaller than with equivalent doses of Phen. Taken together, these results show that post-injury treatment with Phen over 5 days significantly ameliorates severity of TBI. These data suggest a potential development of this compound for clinical use in TBI therapy.

Post-Injury Administration of Galantamine Reduces Traumatic Brain Injury Pathology and Improves Outcome

Acetylcholine is an excitatory neurotransmitter in the central nervous system that plays a key role in cognitive function, including learning and memory. Previous studies have shown that experimental traumatic brain injury (TBI) reduces cholinergic neurotransmission, decreases evoked release of acetylcholine, and alters cholinergic receptor levels. Galantamine (U.S. Food and Drug Administration approved for the treatment of vascular dementia and Alzheimer's disease) has been shown to inhibit acetylcholinesterase activity and allosterically potentiate nicotinic receptor signaling. We investigated whether acute administration of galantamine can reduce TBI pathology and improve cognitive function tested days after the termination of the drug treatment. Post-injury administration of galantamine was found to decrease TBI-triggered blood-brain barrier (BBB) permeability (tested 24 h post-injury), attenuate the loss of both GABAergic and newborn neurons in the ipsilateral hippocampus, and improve hippocampal function (tested 10 days after termination of the drug treatment). Specifically, significant improvements in the Morris water maze, novel object recognition, and context-specific fear memory tasks were observed in injured animals treated with galantamine. Although messenger RNAs for both M1 (Nos2, TLR4, and IL-12ß) and M2 (Arg1, CCL17, and Mcr1) microglial phenotypes were elevated post-TBI, galantamine treatment did not alter microglial polarization tested 24 h and 6 days post-injury. Taken together, these findings support the further investigation of galantamine as a treatment for TBI.

Lateral Fluid Percussion Injury Impairs Hippocampal Synaptic Soluble N-Ethylmaleimide Sensitive Factor Attachment Protein Receptor Complex Formation

Traumatic brain injury (TBI) and the activation of secondary injury mechanisms have been linked to impaired cognitive function, which, as observed in TBI patients and animal models, can persist for months and years following the initial injury. Impairments in neurotransmission have been well documented in experimental models of TBI, but the mechanisms underlying this dysfunction are poorly understood. Formation of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex facilitates vesicular docking and neurotransmitter release in the synaptic cleft. Published studies highlight a direct link between reduced SNARE complex formation and impairments in neurotransmitter release. While alterations in the SNARE complex have been described following severe focal TBI, it is not known if deficits in SNARE complex formation manifest in a model with reduced severity. We hypothesized that lateral fluid percussion injury (lFPI) reduces the abundance of SNARE proteins, impairs SNARE complex formation, and contributes to impaired neurobehavioral function. To this end, rats were subjected to lFPI or sham injury and tested for acute motor performance and cognitive function at 3 weeks post-injury. lFPI resulted in motor impairment between 1 and 5 days post-injury. Spatial acquisition and spatial memory, as assessed by the Morris water maze, were significantly impaired at 3 weeks after lFPI. To examine the effect of lFPI on synaptic SNARE complex formation in the injured hippocampus, a separate cohort of rats was generated and brains processed to evaluate hippocampal synaptosomal-enriched lysates at 1 week post-injury. lFPI resulted in a significant reduction in multiple monomeric SNARE proteins, including VAMP2, and α-synuclein, and SNARE complex abundance. The findings in this study are consistent with our previously published observations suggesting that impairments in hippocampal SNARE complex formation may contribute to neurobehavioral dysfunction associated with TBI.

Lithium increases hippocampal SNARE protein abundance after traumatic brain injury

Rodent models of traumatic brain injury (TBI) reproduce secondary injury sequela and cognitive impairments observed in patients afflicted by a TBI. Impaired neurotransmission has been reported in the weeks following experimental TBI, and may be a contributor to behavioral dysfunction. The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, the machinery facilitating vesicular docking and fusion, is a highly-conserved mechanism important for neurotransmission. Following TBI, there is a reduction in both the formation of the SNARE complex and the abundance of multiple SNARE proteins, including the chaperone protein cysteine string protein α (CSPα). Treatment with lithium in naïve rats reportedly increases the expression of CSPα. In the context of TBI, brain-injured rats treated with lithium exhibit improved outcome in published reports, but the mechanisms underlying the improvement are poorly understood. The current study evaluated the effect of lithium administration on the abundance of SNARE proteins and SNARE complex formation, hemispheric tissue loss, and neurobehavioral performance following controlled cortical impact (CCI). Sprague Dawley rats were subjected to CCI or sham injury, and treated daily with lithium chloride or vehicle for up to 14days. Administration of lithium after TBI modestly improved spatial memory at 14days post-injury. Semi-quantitative immunoblot analysis of hippocampal lysates revealed that treatment with lithium attenuated reductions in key SNARE proteins and SNARE complex formation at multiple time points post-injury. These findings highlight that treatment with lithium increased the abundance of synaptic proteins that facilitate neurotransmission and may contribute to improved cognitive function after TBI.

Septohippocampal Neuromodulation Improves Cognition after Traumatic Brain Injury

Traumatic brain injury (TBI) often results in persistent attention and memory deficits that are associated with hippocampal dysfunction. Although deep brain stimulation (DBS) is used to treat neurological disorders related to motor dysfunction, the effectiveness of stimulation to treat cognition remains largely unknown. In this study, adult male Harlan Sprague-Dawley rats underwent a lateral fluid percussion or sham injury followed by implantation of bipolar electrodes in the medial septal nucleus (MSN) and ipsilateral hippocampus. In the first week after injury, there was a significant decrease in hippocampal theta oscillations that correlated with decreased object exploration and impaired performance in the Barnes maze spatial learning task. Continuous 7.7 Hz theta stimulation of the medial septum significantly increased hippocampal theta oscillations, restored normal object exploration, and improved spatial learning in injured animals. There were no benefits with 100 Hz gamma stimulation, and stimulation of sham animals at either frequency did not enhance performance. We conclude, therefore, that there was a theta frequency-specific benefit of DBS that restored cognitive function in brain-injured rats. These data suggest that septal theta stimulation may be an effective and novel neuromodulatory therapy for treatment of persistent cognitive deficits following TBI.

Traumatic Brain Injury Impairs Soluble N-Ethylmaleimide-Sensitive Factor Attachment Protein Receptor Complex Formation and Alters Synaptic Vesicle Distribution in the Hippocampus

Traumatic brain injury (TBI) impairs neuronal function and can culminate in lasting cognitive impairment. While impaired neurotransmitter release has been well established after experimental TBI, little is understood about the mechanisms underlying this consequence. In the synapse, vesicular docking and neurotransmitter release requires the formation of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex. Impairments in vesicle docking, and alterations in SNARE complex formation are associated with impaired neurotransmitter release. We hypothesized that TBI reduces SNARE complex formation and disrupts synaptic vesicle distribution in the hippocampus. To examine the effect of TBI on the SNARE complex, rats were subjected to controlled cortical impact (CCI) or sham injury, and the brains were assessed at 6 h, 1 d, one week, two weeks, or four weeks post-injury. Immunoblotting of hippocampal homogenates revealed significantly reduced SNARE complex formation at one week and two weeks post-injury. To assess synaptic vesicles distribution, rats received CCI or sham injury and the brains were processed for transmission electron microscopy at one week post-injury. Synapses in the hippocampus were imaged at 100k magnification, and vesicle distribution was assessed in pre-synaptic terminals at the active zone. CCI resulted in a significant reduction in vesicle number within 150 nm of the active zone. These findings provide the first evidence of TBI-induced impairments in synaptic vesicle docking, and suggest that reductions in the pool of readily releasable vesicles and impaired SNARE complex formation are two novel mechanisms contributing to impaired neurotransmission after TBI.

Alterations in Cholinergic Pathways and Therapeutic Strategies Targeting Cholinergic System after Traumatic Brain Injury

Traumatic brain injury (TBI) results in varying degrees of disability in a significant number of persons annually. The mechanisms of cognitive dysfunction after TBI have been explored in both animal models and human clinical studies for decades. Dopaminergic, serotonergic, and noradrenergic dysfunction has been described in many previous reports. In addition, cholinergic dysfunction has also been a familiar topic among TBI researchers for many years. Although pharmacological agents that modulate cholinergic neurotransmission have been used with varying degrees of success in previous studies, improving their function and maximizing cognitive recovery is an ongoing process. In this article, we review the previous findings on the biological mechanism of cholinergic dysfunction after TBI. In addition, we describe studies that use both older agents and newly developed agents as candidates for targeting cholinergic neurotransmission in future studies.

Decoding hippocampal signaling deficits after traumatic brain injury

There are more than 3.17 million people coping with long-term disabilities due to traumatic brain injury (TBI) in the United States. The majority of TBI research is focused on developing acute neuroprotective treatments to prevent or minimize these long-term disabilities. Therefore, chronic TBI survivors represent a large, underserved population that could significantly benefit from a therapy that capitalizes on the endogenous recovery mechanisms occurring during the weeks to months following brain trauma. Previous studies have found that the hippocampus is highly vulnerable to brain injury, in both experimental models of TBI and during human TBI. Although often not directly mechanically injured by the head injury, in the weeks to months following TBI, the hippocampus undergoes atrophy and exhibits deficits in long-term potentiation (LTP), a persistent increase in synaptic strength that is considered to be a model of learning and memory. Decoding the chronic hippocampal LTP and cell signaling deficits after brain trauma will provide new insights into the molecular mechanisms of hippocampal-dependent learning impairments caused by TBI and facilitate the development of effective therapeutic strategies to improve hippocampal-dependent learning for chronic survivors of TBI.

A Rehabilomics focused perspective on molecular mechanisms underlying neurological injury, complications, and recovery after severe TBI

The molecular mechanisms underlying TBI pathophysiology and recovery are both complex and varied. Further, the pathology underlying many of the clinical sequelae observed in this population evolve over the acute injury period and encompass the subacute and chronic phases of recovery, supporting the contemporary concept that TBI is a chronic disease rather than a static insult from which limited recovery occurs. TBI related complications can also span from acute care to the very chronic stages of recovery that occur years after the initial trauma. Despite ongoing neurodegeneration, the TBI recovery period is also characterized by a propensity for neuroplasticity and rewiring through multiple mechanisms. This review summarizes key elements of acute pathophysiology, how they link to structural damage and ongoing degeneration, and how this process coincides with a permissive neuroplastic environment. The pathophysiology of selected TBI related complications is also discussed. Each of these concepts is studied through the lens of Rehabilomics, wherein an emphasis is placed on biomarker studies characterizing these pathophysiological mechanisms, and biomarker profiles are assessed in relation to multi-modal outcomes and susceptibility to rehabilitation relevant complications. In reviewing these concepts, implications for future research and theranostic principles for patient care are presented.

Regional specific alterations in brain acetylcholinesterase activity after repeated blast exposures in mice

Acetylcholinesterase (AChE) which catalyzes the hydrolysis of the neurotransmitter acetylcholine has been recognized as one of the major regulators of stress responses after traumatic brain injury (TBI). Repeated blast exposure induces TBI (blast TBI) with a variable neuropathology at different brain regions. Since AChE inhibitors are being used as a line of treatment for TBI, we sought to determine the time course of AChE activity in the blood and different brain regions after repeated blast exposures using modified Ellman assay. Our data showed that repeated blast exposures significantly reduced AChE activity in the whole-blood and erythrocytes by 3-6h, while plasma AChE activity was significantly increased by 3h post-blast. In the brain, significant increase in AChE activity was observed at 6h in the frontal cortex, while hind cortex and hippocampus showed a significant decrease at 6h post-blast, which returned to normal levels by 7 days. AChE activity in the cerebellum and mid brain showed a decrease at 6h, followed by significant increase at 3 days and that was decreased significantly at 14 days post-blast. Medulla region showed decreased AChE activity at 24h post-blast, which was significantly increased at 14 days. These results suggest that there are brain regional and time-related changes in AChE activity after tightly coupled repeated blast exposures in mice. In summary, acute and chronic regional specific changes in the AChE activity after repeated blast exposures warrant systematic evaluation of the possibility of AChE inhibitor therapeutics against blast TBI.

Deficits in ERK and CREB activation in the hippocampus after traumatic brain injury

Traumatic brain injury (TBI) activates several protein kinase signaling pathways in the hippocampus that are critical for hippocampal-dependent memory formation. In particular, extracellular signal-regulated kinase (ERK), a protein kinase activated during and necessary for hippocampal-dependent learning, is transiently activated after TBI. However, TBI patients experience hippocampal-dependent cognitive deficits that occur for several months to years after the initial injury. Although basal activation levels of ERK return to sham levels within hours after TBI, we hypothesized that activation of ERK may be impaired after TBI. Adult male Sprague-Dawley rats received either sham surgery or moderate parasagittal fluid-percussion brain injury. At 2, 8, or 12 weeks after surgery, the ipsilateral hippocampi of sham surgery and TBI animals were sectioned into transverse slices. After 2h of recovery in oxygenated artificial cerebrospinal fluid, the hippocampal slices were stimulated with glutamate or KCl depolarization, then analyzed by western blotting for phosphorylated, activated ERK and one of its downstream effectors, the transcription factor cAMP response element-binding protein (CREB). We found that activation of ERK (p<0.05) and CREB (p<0.05) after 30s of glutamate stimulation or KCl depolarization was decreased in hippocampal slices from animals at 2, 8, or 12 weeks after TBI as compared to sham animals. Basal levels of phosphorylated or total ERK were not significantly altered at 2, 8, or 12 weeks after TBI, although basal levels of phosphorylated CREB were decreased 12 weeks post-trauma. These results suggest that TBI results in chronic signaling deficits through the ERK-CREB pathway in the hippocampus.

Dietary choline supplementation improves behavioral, histological, and neurochemical outcomes in a rat model of traumatic brain injury

Novel pharmacological approaches that safely and effectively lessen the degree of neurological impairment following traumatic brain injury (TBI) are sorely needed. Non-invasive approaches that could be used over an extended periods of time might be particularly useful. Previous studies from our lab have hypothesized that TBI-induced decreases in hippocampal and cortical alpha7 neuronal nicotinic cholinergic receptor (nAChR) expression might contribute to cognitive impairment that follows brain injury. The purpose of this study was to determine whether the low-potency, but selective alpha7 nAChR agonist choline might be a useful treatment for improvement of neurological outcome in a rat model of TBI. Male Sprague-Dawley rats were exposed to control or choline-supplemented diets for 2 weeks prior to experimental brain injury (1.5-mm cortical contusion injury) and throughout the recovery phase. Dietary choline supplementation resulted in a modest degree of improvement in spatial memory as assessed in the Morris water maze test. In addition, choline treatment resulted in significant cortical tissue sparing, reduced brain inflammation, and normalized some TBI-induced deficits in nAChR expression. The results of this study suggest that alpha7 nAChR agonists may be useful drugs to enhance recovery following brain injury.

Applications of the P50 evoked response to the evaluation of cognitive impairments after traumatic brain injury

This article reviews the applications of the P50 evoked response to paired auditory stimuli (P50 ERP) in the study and evaluation of cognitive impairments after traumatic brain injury (TBI). The cholinergic hypothesis of cognitive impairment after TBI and the relationship of impaired auditory sensory gating to that hypothesis are presented. The neurobiology of impaired sensory gating, the relationship of that neurobiology to the P50 ERP, and the principles of P50 ERP recording are discussed. Studies of the P50 ERP among patients with persistent cognitive complaints after TBI are reviewed. Finally, possible clinical applications and limitations of the P50 ERP in the study, evaluation, and treatment of patients with cognitive impairments after TBI are offered.

The cholinergic hypothesis of cognitive impairment caused by traumatic brain injury

Cognitive impairments are among the most common neuropsychiatric sequelae of traumatic brain injury at all levels of severity. Cerebral cholinergic neurons and their ascending projections are particularly vulnerable to acute and chronic traumatically mediated dysfunction. In light of the important role of acetylcholine in arousal, attention, memory, and other aspects of cognition, cerebral cholinergic systems contribute to and may also be a target for pharmacologic remediation among individuals with post-traumatic cognitive impairments. This article will review the evidence in support of this hypothesis. Evidence of relatively selective damage to cholinergic injury, the development of persistent anticholinergic sensitivity, and the effects of cholinergic augmentation on memory performance are presented first. Thereafter, neuropathologic, electrophysiologic, and pharmacologic evidence of cholinergic dysfunction after traumatic brain injury in humans is reviewed. Finally, future directions for investigation of the cholinergic hypothesis and possible clinical applications of this information are discussed.

Lipid peroxidation and aluminium effects on the cholinergic system in nerve terminals

In the present study, we analyzed how aluminium and oxidative stress induced by ascorbate/Fe(2+) affect the mechanisms related with the cholinergic system in a crude synaptosomal fraction isolated from rat brain. [(3)H]Choline uptake, [(3)H]acetylcholine release, membrane potential and Na(+)/K(+)-ATPase activity were determined in the presence or in the absence of aluminium in control conditions and in the presence of ascorbate (0.8 mM)/Fe(2+) (2.5 micro M). The extent of lipid peroxidation was measured by quantifying thiobarbituric acid reactive substances (TBARS). Under oxidizing conditions aluminium increased the formation of TBARS by about 30%, but was without effect when the synaptosomal preparation was incubated in the absence of oxidants. Additionally, aluminium potentiated the inhibition of the high-affinity [(3)H]choline uptake observed following lipid peroxidation and had the same effect on the Na(+)/K(+)-ATPase activity. [(3)H]Acetylcholine release induced by 4-aminopyridine, and membrane potential were not significantly affected under oxidizing conditions, either in the absence or in the presence of aluminium. We can conclude that aluminium, by potentiating lipid peroxidation, affects the uptake of choline in nerve endings. This effect, occurring during brain oxidative injury, might contribute to the cholinergic dysfunction and neuronal cell degeneration known to occur in Alzheimer's disease.

Nefiracetam improves Morris water maze performance following traumatic brain injury in rats

Nefiracetam, a pyrrolidone derivative, is a nootropic agent that has facilitated cognitive function in a wide variety of animal models of cognitive dysfunction. The purpose of this study was to investigate the efficacy of the chronic postinjury administration of nefiracetam (DM-9384) in improving cognitive performance following central fluid percussion brain injury in rats. Twenty-four hours following surgical preparation, a sham injury or a moderate fluid percussive injury (2.1 atm) was delivered. Nefiracetam was administered chronically (0 or 9 mg/kg, po, for sham animals and 0, 3, or 9 mg/kg for injured animals) on postinjury days 1-15. Cognitive performance was assessed using the Morris water maze (MWM) on postinjury days 11-15. Chronic administration of 3 and 9 mg/kg nefiracetam attenuated MWM deficits produced by central fluid percussive brain injury. Importantly, the MWM performance of the injured animals treated with 9 mg/kg did not significantly differ from uninjured, sham animals. The 9-mg/kg dose of nefiracetam did not have a positive or negative effect on MWM performance of uninjured animals. The results of the present experiment suggest that a nootropic such as nefiracetam may be an appropriate treatment for trauma-induced cognitive dysfunction.

A prospective study of serum pseudocholinesterase levels in patients with chronic spinal pain: a preliminary study

STUDY DESIGN

One-dimensional polyacrylamide gel electrophoresis was used to study serum esterase enzymatic activity in three groups of patients and one group of normal volunteers.

OBJECTIVES

To determine whether there is a statistically significant correlation between variations of serum pseudocholinesterase and the perception of pain in patients with chronic spinal pain.

SUMMARY OF BACKGROUND DATA

Changes in levels of cholinesterase in the extracellular space of the brain and in the cerebral spinal fluid have been found to be associated in animal pain experimentation.

METHODS

Ninety-three surgical patients with chronic spinal pain, six surgical control subjects operated for conditions not associated with pain, 21 normal control volunteers, and nine disabled patients receiving monetary benefits were studied. The patients were analyzed for a period of time by rating the perception of their pain with a visual assessment score at the time venous blood was drawn. Serum samples were prepared, serum pseudocholinesterase was monitored, separated, and quantified according to Allen et al.5 Paired sample t tests were used to statistically evaluate the data.

RESULTS

A trend of correlation was noted between preoperative serum pseudocholinesterase levels and visual assessment score: serum pseudocholinesterase levels increased as visual assessment score increased. The mean preoperative serum pseudocholinesterase level of chronic spinal pain patients (1313; SE = 26), which was significantly higher than the mean levels of the normal control volunteers (941; SE = 24; P<0.001) and that of surgical control subjects (1018; SE = 63; P <0.01), decreased significantly with anesthesia (P<0.005). The mean preoperative serum pseudocholinesterase level of the surgical controls, however, remained unchanged with anesthesia. A correlation demonstrated between visual assessment score and serum pseudocholinesterase in chronic spinal pain patients was not observed in six of nine patients receiving disability payments for more than a year.

CONCLUSIONS

Measurements of quantitative alterations of serum pseudocholinesterase levels may be useful in the treatment of patients with chronic spinal pain.

Activating the posttraumatic cholinergic system for the treatment of cognitive impairment following traumatic brain injury

Cognitive impairment after traumatic brain injury (TBI) is correlated with decreased cholinergic markers of neuronal viability. The purpose of this experiment was to test the hypothesis that pharmacological activation of the muscarinic cholinergic system during the recovery period after TBI will improve cognitive performance. LU 25-109-T is a partial muscarinic M1 agonist that also acts as an antagonist at presynaptic M2 autoreceptors (thus increasing ACh release). Injured rats were injected subcutaneously daily for 15 days with either 0.0, 3.6, or 15 mumol/kg of LU 25-109-T beginning 24 h after a receiving a moderate (2.1 +/- 0.1 atm) level of central fluid percussion brain injury. Cognitive performance was assessed on days 11-15 postinjury in a Morris water maze (MWM). Injured rats treated with 15 mumol/kg, but not those treated with 3.6 mumol/kg, showed a significant improvement (p < 0.01) in MWM performance as compared with injured vehicle-treated rats. This result supports the hypotheses that a decrease in posttraumatic cholinergic neurotransmission contributes to TBI-induced cognitive deficits and that increasing cholinergic tone during the recovery period following TBI will improve cognitive performance.

Reduced evoked release of acetylcholine in the rodent neocortex following traumatic brain injury

Neocortical acetylcholine (ACh) release was examined in awake, freely-moving rats at 14 days following lateral controlled cortical impact. Extracellular ACh was measured prior to and after an intraperitoneal administration of scopolamine, which evokes ACh release by blocking autoreceptors. At 14 days post-injury there was a significant reduction in scopolamine-evoked ACh release. The data suggest that neocortical cholinergic neurotransmission is chronically compromised, and may contribute to post-traumatic memory deficits.