Acetylcholinesterase Inhibition Interacts with Training To Reverse Spatial Learning Deficits after Cortical Impact Injury

Enhancing cognitive function in chronic TBI: The Role of α7 nicotinic acetylcholine receptor modulation

Traumatic brain injury (TBI) results in several pathological changes within the hippocampus that result in adverse effects on learning and memory. Therapeutic strategies to enhance learning and memory after TBI are still in the early stages of clinical development. One strategy is to target the α7 nicotinic acetylcholine receptor (nAChR), which is highly expressed in the hippocampus and contributes to the formation of long-term memory. In our previous study, we found that AVL-3288, a positive allosteric modulator of the α7 nAChR, improved cognitive recovery in rats after moderate fluid-percussion injury (FPI). However, whether AVL-3288 improved cognitive recovery specifically through the α7 nAChR was not definitively determined. In this study we utilized Chrna7 knockout mice and compared their recovery to wild-type mice treated with AVL-3288 after TBI. We hypothesized that AVL-3288 treatment would improve learning and memory in wild-type mice, but not Chrna7-/- mice after TBI. Adult male C57BL/6 wild-type and Chrna7-/- mice received sham surgery or moderate controlled cortical impact (CCI) and recovered for 3 months. Mice were then treated with vehicle or AVL-3288 at 30 min prior to contextual fear conditioning. At 3 months after CCI, expression of α7 nAChR, choline acetyltransferase (ChAT), high-affinity choline transporter (ChT), and vesicular acetylcholine transporter (VAChT) were found to be significantly decreased in the hippocampus. Treatment of wild-type mice at 3 months after CCI with AVL-3288 significantly improved cue and contextual fear conditioning, whereas no beneficial effects were observed in Chrna7-/- mice. Parietal cortex and hippocampal atrophy were not improved with AVL-3288 treatment in either wild-type or Chrna7-/- mice. Our results indicate that AVL-3288 improves cognition during the chronic recovery phase of TBI through modulation of the α7 nAChR.

Positive allosteric modulation of the α7 nicotinic acetylcholine receptor as a treatment for cognitive deficits after traumatic brain injury

Cognitive impairments are a common consequence of traumatic brain injury (TBI). The hippocampus is a subcortical structure that plays a key role in the formation of declarative memories and is highly vulnerable to TBI. The α7 nicotinic acetylcholine receptor (nAChR) is highly expressed in the hippocampus and reduced expression and function of this receptor are linked with cognitive impairments in Alzheimer's disease and schizophrenia. Positive allosteric modulation of α7 nAChRs with AVL-3288 enhances receptor currents and improves cognitive functioning in naïve animals and healthy human subjects. Therefore, we hypothesized that targeting the α7 nAChR with the positive allosteric modulator AVL-3288 would enhance cognitive functioning in the chronic recovery period of TBI. To test this hypothesis, adult male Sprague Dawley rats received moderate parasagittal fluid-percussion brain injury or sham surgery. At 3 months after recovery, animals were treated with vehicle or AVL-3288 at 30 min prior to cue and contextual fear conditioning and the water maze task. Treatment of TBI animals with AVL-3288 rescued learning and memory deficits in water maze retention and working memory. AVL-3288 treatment also improved cue and contextual fear memory when tested at 24 hr and 1 month after training, when TBI animals were treated acutely just during fear conditioning at 3 months post-TBI. Hippocampal atrophy but not cortical atrophy was reduced with AVL-3288 treatment in the chronic recovery phase of TBI. AVL-3288 application to acute hippocampal slices from animals at 3 months after TBI rescued basal synaptic transmission deficits and long-term potentiation (LTP) in area CA1. Our results demonstrate that AVL-3288 improves hippocampal synaptic plasticity, and learning and memory performance after TBI in the chronic recovery period. Enhancing cholinergic transmission through positive allosteric modulation of the α7 nAChR may be a novel therapeutic to improve cognition after TBI.

Positive Allosteric Modulation of Cholinergic Receptors Improves Spatial Learning after Cortical Contusion Injury in Mice

We examined benzyl quinolone carboxylic acid (BQCA), a novel M1 muscarinic-positive allosteric modulator, for improving memory and motor dysfunction after cerebral cortical contusion injury (CCI). Adult mice received unilateral motorsensory cortical CCI or sham injury. Benzyl quinolone carboxylic acid (BQCA; 5, 10, and 20 mg/kg, intraperitoneally [i.p.] × 2/day × 3-4 weeks) or vehicle (Veh) were administered, and weekly evaluations were undertaken using a battery of motor tests, as well as the Morris water maze. Thereafter, cerebral metabolic activation was investigated in awake animals during walking with [¹⁴C]-2-deoxygIucose autoradiography, comparing CCI mice previously treated with BQCA (20 mg/kg) or vehicle. Relative changes in local cerebral glucose uptake (rCGU) were evaluated in three-dimensional-reconstructed brains using statistical parametric mapping. CCI resulted in mild hyperactivity in the open field, and modest significant motor deficits, as well as significantly decreased spatial learning at 3 weeks. BQCA in CCI mice resulted in significantly improved spatial recall during the third week, with minimal effects on motor outcomes. CCI significantly decreased rCGU in the ipsilesional basal ganglia-thalamocortical circuit and in somatosensory regions, with relative increases noted contralaterally, as well as in the cerebellum. Significant decreases in rCGU were noted in subregions of the ipsilesional hippocampal formation, with significant increases noted contralesionally. BQCA compared to vehicle-treated mice showed modest, though significantly increased, rCGU in motor regions, as well as a partial reversal of lesion-related rCGU findings in subregions of the hippocampal formation. rCGU in ipsilesional posterior CA1 demonstrated a significant inverse correlation with latency to find the submerged platform. BQCA at 20 mg/kg had no significant effect on general motor activity, body weight, or acute motor, secretory, or respiratory symptoms. Results suggest that BQCA is a candidate compound to improve learning and memory function after brain trauma and may not suffer the associated central nervous system side effects typically associated with even modest doses of other cholinergic enhancers.

Systemic administration of donepezil attenuates the efficacy of environmental enrichment on neurobehavioral outcome after experimental traumatic brain injury

BACKGROUND

The acetylcholinesterase inhibitor (AChEI) donepezil (DON) is recommended as a potential treatment for cognition after clinical traumatic brain injury (TBI) and therefore may be prescribed as an adjunct therapy during rehabilitation. However, a dose-response study evaluating DON after a controlled cortical impact (CCI) injury in rats did not reveal cognitive benefits.

OBJECTIVE

The aim of this study was to determine the effect of DON on behavioral and histological outcome when combined with environmental enrichment (EE), a preclinical model of neurorehabilitation. It was hypothesized that the combined treatments would produce a synergistic effect yielding improved recovery over neurorehabilitation alone.

METHODS

Isoflurane-anesthetized adult male rats received a CCI or sham injury and then were randomly assigned to EE or standard (STD) housing plus systemic injections of DON (0.25 mg/kg) or vehicle (VEH; 1.0 mL/kg saline) once daily for 19 days beginning 24 hr after injury. Function was assessed by established motor and cognitive tests on post-injury days 1-5 and 14-19, respectively. Cortical lesion volume was quantified on day 19.

RESULTS

DON was ineffective when administered alone. In contrast, EE conferred significant motor and cognitive benefits, and reduced cortical lesion volume vs. STD (p < 0.05). Combining the therapies weakened the efficacy of rehabilitation as revealed by diminished motor and cognitive recovery in the TBI+EE+DON group vs. the TBI+EE+VEH group (p < 0.05).

CONCLUSION

These data replicate previous findings showing that EE is beneficial and DON is ineffective after CCI and add to the literature a novel and unpredicted finding that supports neither the hypothesis nor the use of DON for TBI. Investigation of other AChEIs after CCI injury is necessary to gain further insight into the value of this therapeutic strategy.

Icariin Improves Cognitive Impairment after Traumatic Brain Injury by Enhancing Hippocampal Acetylation

OBJECTIVE

To examine the effect of icariin (ICA) on the cognitive impairment induced by traumatic brain injury (TBI) in mice and the underlying mechanisms related to changes in hippocampal acetylation level.

METHODS

The modifified free-fall method was used to establish the TBI mouse model. Mice with post-TBI cognitive impairment were randomly divided into 3 groups using the randomised block method (n=7): TBI (vehicle-treated), low-dose (75 mg/kg) and high-dose (150 mg/kg) of ICA groups. An additional sham-operated group (vehicle-treated) was employed. The vehicle or ICA was administrated by gavage for 28 consecutive days. The Morris water maze (MWM) test was conducted. Acetylcholine (ACh) content, mRNA and protein levels of choline acetyltransferase (ChAT), and protein levels of acetylated H3 (Ac-H3) and Ac-H4 were detected in the hippocampus.

RESULTS

Compared with the sham-operated group, the MWM performance, hippocampal ACh content, mRNA and protein levels of ChAT, and protein levels of Ac-H3 and Ac-H4 were signifificantly decreased in the TBI group (P<0.05). High-dose of ICA signifificantly ameliorated the TBI-induced weak MWM performance, increased hippocampal ACh content, and mRNA and protein levels of ChAT, as well as Ac-H3 protein level compared with the TBI group (P<0.05).

CONCLUSION

ICA improved post-TBI cognitive impairment in mice by enhancing hippocampal acetylation, which improved hippocampal cholinergic function and ultimately improved cognition.

Galantamine and Environmental Enrichment Enhance Cognitive Recovery after Experimental Traumatic Brain Injury But Do Not Confer Additional Benefits When Combined

Environmental enrichment (EE) enhances cognition after traumatic brain injury (TBI). Galantamine (GAL) is an acetylcholinesterase inhibitor that also may promote benefits. Hence, the aims of this study were to assess the efficacy of GAL alone (standard [STD] housing) and in combination with EE in adult male rats after TBI. The hypothesis was that both therapies would confer motor, cognitive, and histological benefits when provided singly, but that their combination would be more efficacious. Anesthetized rats received a controlled cortical impact or sham injury, then were randomly assigned to receive GAL (1, 2, or 3 mg/kg; intraperitoneally [i.p.]) or saline vehicle (VEH; 1 mL/kg; i.p.) beginning 24 h after surgery and once daily for 21 days (experiment 1). Motor (beam-balance/walk) and cognitive (Morris water maze [MWM]) assessments were conducted on post-operative Days 1-5 and 14-19, respectively. Cortical lesion volumes were quantified on Day 21. Sham controls were better versus all TBI groups. No differences in motor function or lesion volumes were observed among the TBI groups (p > 0.05). In contrast, GAL (2 mg/kg) enhanced MWM performance versus VEH and GAL (1 and 3 mg/kg; p < 0.05). In experiment 2, GAL (2 mg/kg) or VEH was combined with EE and the data were compared with the STD-housed groups from experiment 1. EE alone enhanced motor performance over the VEH-treated and GAL-treated (2 mg/kg) STD-housed groups (p < 0.05). Moreover, both EE groups (VEH or GAL) facilitated spatial learning and reduced lesion size versus STD + VEH controls (p < 0.05). No additional benefits were observed with the combination paradigm, which does not support the hypothesis. Overall, the data demonstrate that EE and once daily GAL (2 mg/kg) promote cognitive recovery after TBI. Importantly, the combined therapies did not negatively affect outcome and thus this therapeutic protocol may have clinical utility.

Cognitive Impairments Induced by Concussive Mild Traumatic Brain Injury in Mouse Are Ameliorated by Treatment with Phenserine via Multiple Non-Cholinergic and Cholinergic Mechanisms

Traumatic brain injury (TBI), often caused by a concussive impact to the head, affects an estimated 1.7 million Americans annually. With no approved drugs, its pharmacological treatment represents a significant and currently unmet medical need. In our prior development of the anti-cholinesterase compound phenserine for the treatment of neurodegenerative disorders, we recognized that it also possesses non-cholinergic actions with clinical potential. Here, we demonstrate neuroprotective actions of phenserine in neuronal cultures challenged with oxidative stress and glutamate excitotoxicity, two insults of relevance to TBI. These actions translated into amelioration of spatial and visual memory impairments in a mouse model of closed head mild TBI (mTBI) two days following cessation of clinically translatable dosing with phenserine (2.5 and 5.0 mg/kg BID x 5 days initiated post mTBI) in the absence of anti-cholinesterase activity. mTBI elevated levels of thiobarbituric acid reactive substances (TBARS), a marker of oxidative stress. Phenserine counteracted this by augmenting homeostatic mechanisms to mitigate oxidative stress, including superoxide dismutase [SOD] 1 and 2, and glutathione peroxidase [GPx], the activity and protein levels of which were measured by specific assays. Microarray analysis of hippocampal gene expression established that large numbers of genes were exclusively regulated by each individual treatment with a substantial number of them co-regulated between groups. Molecular pathways associated with lipid peroxidation were found to be regulated by mTBI, and treatment of mTBI animals with phenserine effectively reversed injury-induced regulations in the ‘Blalock Alzheimer’s Disease Up’ pathway. Together these data suggest that multiple phenserine-associated actions underpin this compound’s ability to ameliorate cognitive deficits caused by mTBI, and support the further evaluation of the compound as a therapeutic for 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.

Remote brain network changes after unilateral cortical impact injury and their modulation by acetylcholinesterase inhibition

We explored whether cerebral cortical impact injury (CCI) effects extend beyond direct lesion sites to affect remote brain networks, and whether acetylcholinesterase (AChE) inhibition elicits discrete changes in functional activation of motor circuits following CCI. Adult male rats underwent unilateral motor-sensory CCI or sham injury. Physostigmine (AChE inhibitor) or saline were administered subcutaneously continuously via implanted minipumps (1.6 micromoles/kg/day) for 3 weeks, followed by cerebral perfusion mapping during treadmill walking using [(14)C]-iodoantipyrine. Quantitative autoradiographs were analyzed by statistical parametric mapping and functional connectivity (FC) analysis. CCI resulted in functional deficits in the ipsilesional basal ganglia, with increased activation contralesionally. Recruitment was also observed, especially contralesionally, of the red nucleus, superior colliculus, pedunculopontine tegmental nucleus, thalamus (ventrolateral n., central medial n.), cerebellum, and sensory cortex. FC decreased significantly within ipsi- and contralesional motor circuits and between hemispheres, but increased between midline cerebellum and select regions of the basal ganglia within each hemisphere. Physostigmine significantly increased functional brain activation in the cerebellar thalamocortical pathway (midline cerebellum→ventrolateral thalamus→motor cortex), subthalamic nucleus/zona incerta, and red nucleus and bilateral sensory cortex. In conclusion, CCI resulted in increased functional recruitment of contralesional motor cortex and bilateral subcortical motor regions, as well as recruitment of the cerebellar-thalamocortical circuit and contralesional sensory cortex. This phenomenon, augmented by physostigmine, may partially compensate motor deficits. FC decreased inter-hemispherically and in negative, but not positive, intra-hemispherical FC, and it was not affected by physostigmine. Circuit-based approaches into functional brain reorganization may inform future behavioral or molecular strategies to augment targeted neurorehabilitation.

Donepezil is ineffective in promoting motor and cognitive benefits after controlled cortical impact injury in male rats

The acetylcholinesterase (AChE) inhibitor donepezil is used as a therapy for Alzheimer's disease and has been recommended as a treatment for enhancing attention and memory after traumatic brain injury (TBI). Although select clinical case studies support the use of donepezil for enhancing cognition, there is a paucity of experimental TBI studies assessing the potential efficacy of this pharmacotherapy. Hence, the aim of this pre-clinical study was to evaluate several doses of donepezil to determine its effect on functional outcome after TBI. Ninety anesthetized adult male rats received a controlled cortical impact (CCI; 2.8 mm cortical depth at 4 m/sec) or sham injury, and then were randomly assigned to six TBI and six sham groups (donepezil 0.25, 0.5, 1.0, 2.0, or 3.0 mg/kg, and saline vehicle 1.0 mL/kg). Treatments began 24 h after surgery and were administered i.p. once daily for 19 days. Function was assessed by motor (beam balance/walk) and cognitive (Morris water maze) tests on days 1-5 and 14-19, respectively. No significant differences were observed among the sham control groups in any evaluation, regardless of dose, and therefore the data were pooled. Furthermore, no significant differences were revealed among the TBI groups in acute neurological assessments (e.g., righting reflex), suggesting that all groups received the same level of injury severity. None of the five doses of donepezil improved motor or cognitive function relative to vehicle-treated controls. Moreover, the two highest doses significantly impaired beam-balance (3.0 mg/kg), beam-walk (2.0 mg/kg and 3.0 mg/kg), and cognitive performance (3.0 mg/kg) versus vehicle. These data indicate that chronic administration of donepezil is not only ineffective in promoting functional improvement after moderate CCI injury, but depending on the dose is actually detrimental to the recovery process. Further work is necessary to determine if other AChE inhibitors exert similar effects after TBI.