The effects of scopolamine and traumatic brain injury on central cholinergic neurons

Anticholinergics: A potential option for preventing posttraumatic epilepsy

Interest in the use of anticholinergics to prevent the development of epilepsy after traumatic brain injury (TBI) has grown since recent basic studies have shown their effectiveness in modifying the epileptogenic process. These studies demonstrated that treatment with anticholinergics, in the acute phase after brain injury, decreases seizure frequency, and severity, and the number of spontaneous recurrent seizures (SRS). Therefore, anticholinergics may reduce the risk of developing posttraumatic epilepsy (PTE). In this brief review, we summarize the role of the cholinergic system in epilepsy and the key findings from using anticholinergic drugs to prevent PTE in animal models and new clinical trial protocols. Furthermore, we discuss why treatment with anticholinergics is more likely to prevent PTE than treatment for other epilepsies.

[The cholinergic deficiency syndrome in patients with depressed consciousness after severe brain injury]

AIM

To determine the clinical and electrophysiological (EEG) signs of cholinergic deficiency in the process of recovery of consciousness in patients with severe brain injury.

MATERIAL AND METHODS

Thirty-seven people (24 men and 13 women, mean age 32±14 years) were studied. A comprehensive study included assessment of neurological status, mental activity, and EEG.

RESULTS AND CONCLUSION

A set of neurological symptoms, including reduced muscle tone, autonomic disorders (dry mucous membranes and skin, tachycardia, hypotension, gastrointestinal tract), eye movement disorders, that were,in accordance with the literature, characteristicof the cholinergic deficiency syndrome was found. This syndrome was detected against the background of a comatose state, akinetic mutism and mutism with understanding of speech, disintegration of speech, disorientation and amnestic decline. EEG revealed stable over time (months) characteristic changes: slowing and asymmetric alpha activity, equivalent dipole sources of hippocampal and stem localization, persistent strengthening of intra-hemispheric coherent connections, especially on the left side. The regression of the cholinergic deficiency syndrome was accompanied by an increase of regularity, capacity and frequency of alpha-activity (from 7-8 to 9-10 Hz), prevalence of equivalent dipole sources in the hippocampus with their appearance in the occipital cortex, normalization of connections with right-brain coherence with the preservation of their pathologically high values on the left side.

Influence of Different Atropine Therapy Strategies on Fenthion-Induced Organ Dysfunction in Rats

We studied the influence of dose and timing of atropine therapy on fenthion-induced organ dysfunction. Thirty-six rats were randomized into six groups. All rats in the five groups except the control group were intoxicated with fenthion. The high-dose atropine group received 2 mg/kg of atropine, whereas the low-dose group received 100 microg/kg of atropine every hour for 24 hr. One group received 2 mg/kg of atropine in the first 4 hr of intoxication while the other group received 2 mg/kg of atropine in the last 4 hr before killed, which for all rats was 24 hr after intoxication. Pseudocholinesterase and aspartate aminotransferase and alanine aminotransferase levels and histopathological markers of lung, brain and liver were studied. None of our atropine therapy strategies in this study totally prevented harm on the three organs. Although the high dose of atropine administered for 24 hr had the least harmful markers for lung, it also had the most harmful markers for brain and liver. We did not succeed in finding a unique therapy strategy in our models beneficial for all studied organs in fenthion intoxication in rats. Atropine administration strategy should be oriented for the most affected organ pathology in fenthion intoxication.

Animal models of head trauma

Animal models of traumatic brain injury (TBI) are used to elucidate primary and secondary sequelae underlying human head injury in an effort to identify potential neuroprotective therapies for developing and adult brains. The choice of experimental model depends upon both the research goal and underlying objectives. The intrinsic ability to study injury-induced changes in behavior, physiology, metabolism, the blood/tissue interface, the blood brain barrier, and/or inflammatory- and immune-mediated responses, makes in vivo TBI models essential for neurotrauma research. Whereas human TBI is a highly complex multifactorial disorder, animal trauma models tend to replicate only single factors involved in the pathobiology of head injury using genetically well-defined inbred animals of a single sex. Although such an experimental approach is helpful to delineate key injury mechanisms, the simplicity and hence inability of animal models to reflect the complexity of clinical head injury may underlie the discrepancy between preclinical and clinical trials of neuroprotective therapeutics. Thus, a search continues for new animal models, which would more closely mimic the highly heterogeneous nature of human TBI, and address key factors in treatment optimization.

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.

Cognitive Impairment Following Traumatic Brain Injury

Cognitive impairments due to traumatic brain injury (TBI) are substantial sources of morbidity for affected individuals, their family members, and society. Disturbances of attention, memory, and executive functioning are the most common neurocognitive consequences of TBI at all levels of severity. Disturbances of attention and memory are particularly problematic, as disruption of these relatively basic cognitive functions may cause or exacerbate additional disturbances in executive function, communication, and other relatively more complex cognitive functions. Because of the high rate of other physical, neurologic, and psychiatric syndromes following TBI, a thorough neuropsychiatric assessment of the patient is a prerequisite to the prescription of any treatment for impaired cognition. Psychostimulants and other dopaminergically active agents (eg, methylphenidate, dextroamphetamine, amantadine, levodopa/carbidopa, bromocriptine) may modestly improve arousal and speed of information processing, reduce distractibility, and improve some aspects of executive function. Cautious dosing (start-low and go-slow), frequent standardized assessment of effects and side effects, and monitoring for drug-drug interactions are recommended. Cognitive rehabilitation is useful for the treatment of memory impairments following TBI. Cognitive rehabilitation may also be useful for the treatment of impaired attention, interpersonal communication skills, and executive function following TBI. This form of treatment is most useful for patients with mild to moderate cognitive impairments, and may be particularly useful for those who are still relatively functionally independent and motivated to engage in and rehearse these strategies. Psychotherapy (eg, supportive, individual, cognitive-behavioral, group, and family) is an important component of treatment. For patients with medication- and rehabilitation-refractory cognitive impairments, psychotherapy may be needed to assist both patients and families with adjustment to permanent disability.

Neurotrauma care in the new millennium

The next millennium will see an explosion of neuromonitoring technology that will provide a more detailed understanding of brain-injured patients. This understanding will allow an individualized and intelligent application of the wide range of therapies that will become available. The measure of success for all of these endeavors will be individual patients and physicians' ability to return them to their normal lives.

Attention and memory dysfunction after traumatic brain injury: cholinergic mechanisms, sensory gating, and a hypothesis for further investigation

Traumatic brain injury (TBI) is a common occurrence, with a rate of nearly 400,000 new injuries per year. Cognitive and emotional disturbances may become persistent and disabling for many injured persons, and frequently involve symptomatic impairment in attention and memory. Impairments in attention and memory have been well characterized in TBI, and are likely related to disruption of cholinergic functioning in the hippocampus. Additionally, disturbances in this neurotransmitter system may also account for disturbances in sensory gating and discriminative attention in this population. The electroencephalographic P50 waveform of the evoked response to paired auditory stimuli may provide a physiologic market of impaired sensory gating among TBI survivors. The first application of this recording assessment to the TBI population is reported. Preliminary findings in three cases are presented, and the interpretation of impaired sensory gating in this population is discussed. Given the impact of TBI on cholinergic systems, the effects of cholinergic augmentation on attention and memory impairment, and the availability of an electrophysiologic marker of cholinergic dysfunction responsive to cholinergic agents, a testable cholinergic hypothesis for investigation and treatment of these patients is proposed.

Novel pharmacologic strategies in the treatment of experimental traumatic brain injury: 1998

The mechanisms underlying secondary or delayed cell death following traumatic brain injury are poorly understood. Recent evidence from experimental models suggests that widespread neuronal loss is progressive and continues in selectively vulnerable brain regions for months to years after the initial insult. The mechanisms underlying delayed cell death are believed to result, in part, from the release or activation of endogenous "autodestructive" pathways induced by the traumatic injury. The development of sophisticated neurochemical, histopathological and molecular techniques to study animal models of TBI have enabled researchers to begin to explore the cellular and genomic pathways that mediate cell damage and death. This new knowledge has stimulated the development of novel therapeutic agents designed to modify gene expression, synthesis, release, receptor or functional activity of these pathological factors with subsequent attenuation of cellular damage and improvement in behavioral function. This article represents a compendium of recent studies suggesting that modification of post-traumatic neurochemical and cellular events with targeted pharmacotherapy can promote functional recovery following traumatic injury to the central nervous system.

Cerebro-protective effects of ENA713, a novel acetylcholinesterase inhibitor, in closed head injury in the rat

Focal ischemic brain damage and diffuse brain swelling occur in severe cases of traumatic head injury. Ischemia decreases brain acetylcholine (ACh) levels and head trauma upregulates acetylcholinesterase (AChE) in experimental animal models. The present study determined whether a brain-selective AChE inhibitor, ENA713, given once, up to 2 h after closed head injury (CHI) could reduce the vasogenic edema and accelerate recovery from neurological deficits induced by the injury in rats. ENA713 1-5 mg/kg produced a dose-related inhibition of AChE ranging from 40-85% in the cortex and hippocampus. Doses of 1, 2 and 5 mg/kg, significantly reduced the motor and neurological deficits and speeded recovery, as indicated by measurements made 7 and 14 days after injury. The two larger doses were still effective when injected 1 or 2 h after CHI. The acceleration by ENA713 of recovery of motor function was independent of its reduction in body temperature and was prevented by the simultaneous injection of mecamylamine (2.5 mg/kg), but not by scopolamine (0.2 or 1 mg/kg). Edema in the contused hemisphere (24 h after injury) and disruption of the blood brain barrier (4 h after injury) were significantly reduced (about 50%) by doses of 2 and 5 mg/kg, but not by 1 mg/kg. The data support the hypothesis that ENA713 exerts a neuroprotective effect in brain injury by preventing the decrease in cholinergic activity in cerebral vessels and in neurones.

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.

Novel pharmacologic therapies in the treatment of experimental traumatic brain injury: a review

Delayed or secondary neuronal damage following traumatic injury to the central nervous system (CNS) may result from pathologic changes in the brain's endogenous neurochemical systems. Although the precise mechanisms mediating secondary damage are poorly understood, posttraumatic neurochemical changes may include overactivation of neurotransmitter release or re-uptake, changes in presynaptic or postsynaptic receptor binding, or the pathologic release or synthesis of endogenous "autodestructive" factors. The identification and characterization of these factors and the timing of the neurochemical cascade after CNS injury provides a window of opportunity for treatment with pharmacologic agents that modify synthesis, release, receptor binding, or physiologic activity with subsequent attenuation of neuronal damage and improvement in outcome. Over the past decade, a number of studies have suggested that modification of postinjury events through pharmacologic intervention can promote functional recovery in both a variety of animal models and clinical CNS injury. This article summarizes recent work suggesting that pharmacologic manipulation of endogenous systems by such diverse pharmacologic agents as anticholinergics, excitatory amino acid antagonists, endogenous opioid antagonists, catecholamines, serotonin antagonists, modulators of arachidonic acid, antioxidants and free radical scavengers, steroid and lipid peroxidation inhibitors, platelet activating factor antagonists, anion exchange inhibitors, magnesium, gangliosides, and calcium channel antagonists may improve functional outcome after brain injury.

The effect of acetylcholine depletion on behavior following traumatic brain injury

Rats were injected with either saline; A-4 (40 mg/kg, i.p.), a bis tertiary amine derivative of hemicholinium-3; or A-5 (50 micrograms/kg, i.p.), a bis quaternary amine derivative of hemicholinium-3, 1 h prior to moderate fluid percussion brain injury. A variety of reflexes and responses were measured up to 60 min following injury, and body weight and several neurological measures were taken daily up to 10 days following injury. Pretreatment with either A-4 or A-5 significantly attenuated components of transient behavioral suppression, as well as more enduring deficits in body weight and beam walk and beam balance performance. A-4 administered prior to fluid percussion was found to reduce striatal, but not pontine, acetylcholine content. A-5 did not significantly reduce acetylcholine content in either area. Both A-4 and A-5 pretreatment prevented a significant increase in acetylcholine content in the cerebrospinal fluid following fluid percussion injury; however, only A-5 significantly reduced plasma acetylcholine content. These results confirm cholinergic involvement in the production of both transient and longer-lasting behavioral deficits following traumatic brain injury. Furthermore, traumatic brain injury may allow plasma constituents to gain access to the central nervous system.

The effects of morphine and traumatic brain injury on central cholinergic neurons

This study examined the effects of morphine and fluid percussion traumatic brain injury (TBI) on the activity of cholinergic neurons in specific areas of the rat brain 12 min after injury. Acetylcholine (ACh) turnover, used as an index of cholinergic neuronal activity, was determined using gas chromatography-mass spectrometry. Although morphine administration alone in general did not significantly affect ACh content and turnover in specific brain areas, morphine administered prior to TBI either prevented injury-induced changes in ACh turnover (dorsal pontine tegmentum) or actually reduced the rate constant for ACh utilization (kACh) and the turnover rate of ACh (TRACh) following injury (thalamus, amygdala, cingulate/frontal cortex, and hippocampus). Thus, the protective effects of morphine against enduring behavioral deficits following TBI may involve the inhibition of central cholinergic neurons.