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.     

Age but not sex affects motor recovery after unilateral sensorimotor cortex suction-ablation in the rat

Both age and sex can influence recovery after brain injury. To determine the impact of these variables on motor recovery, young (2 month old) and older (5-6 months old) male and female rats were first trained to traverse a narrow elevated beam. Rats then underwent suction-ablation of right sensorimotor cortex or sham operation. Motor recovery was measured by repeated testing on the beam over 3 weeks. Shamoperated rats performed perfectly regardless of age or sex throughout testing. There was no difference in beam-walking scores among the groups of lesioned rats on the first trial 24 hrs. after injury (Kruskal-Wallis H = 0.18, p = 0.98). There was a significant effect of age (two-way ANOVA F1,32 = 29.58, p < 0.0001) but not sex (ANOVA F1,32 = 0.78, p = 0.38) on subsequent recovery. These data show that motor recovery after unilateral injury to the sensorimotor cortex varies with age, but not sex.     

One day of motor training with amphetamine impairs motor recovery following spinal cord injury

It has previously been reported that a single dose of amphetamine paired with training on a beam walking task can enhance locomotor recovery following brain injury (Feeney et al., 1982). Here, we investigated whether this same drug/training regimen could enhance functional recovery following either thoracic (T9) or cervical (C5) spinal cord injury. Different groups of female Sprague-Dawley rats were trained on a beam walking task, and in a straight alley for assessment of hindlimb locomotor recovery using the BBB locomotor scale. For rats that received C5 hemisections, forelimb grip strength was assessed using a grip strength meter. Three separate experiments assessed the consequences of training rats on the beam walking task 24 h following a thoracic lateral hemisection with administration of either amphetamine or saline. Beginning 1 h following drug administration, rats either received additional testing/retraining on the beam hourly for 6 h, or they were returned to their home cages without further testing/retraining. Rats with thoracic spinal cord injuries that received amphetamine in conjunction with testing/retraining on the beam at 1 day post injury (DPI) exhibited significantly impaired recovery on the beam walking task and BBB. Rats with cervical spinal cord injuries that received training with amphetamine also exhibited significant impairments in beam walking and locomotion, as well as impairments in gripping and reaching abilities. Even when administered at 14 DPI, the drug/training regimen significantly impaired reaching ability in cervical spinal cord injured rats. Impairments were not seen in rats that received amphetamine without training. Histological analyses revealed that rats that received training with amphetamine had significantly larger lesions than saline controls. These data indicate that an amphetamine/training regimen that improves recovery after cortical injury has the opposite effect of impairing recovery following spinal cord injury because early training with amphetamine increases lesion severity.     

Scopolamine facilitates recovery of function following unilateral electrolytic sensorimotor cortex lesions in the rat

Following brain injury there is an excessive release of excitatory neurotransmitters that may lead to secondary cell death. Although much research has focused on glutamate-NMDA receptor interactions, acetylcholine-muscarinic receptor interactions may also prove to be important for an understanding of the pathophysiological events that lead to secondary degeneration after brain damage. Previous experiments have shown that the muscarinic receptor antagonist scopolamine facilitates recovery from very transient (1 h-10 days) behavioral deficits after fluid percussion injury. The present study extends these findings by investigating whether scopolamine can facilitate recovery from the more enduring behavioral deficits (14-60 days) that follow electrolytic lesions of the rat somatic sensorimotor cortex (SMC). Rats received unilateral lesions of the SMC and a regimen of scopolamine (1 mg/kg) or saline beginning 15 min after surgery. Following SMC lesions rats exhibited an impairment in placing the forelimb contralateral to the lesion as well as an ipsilateral somatosensory asymmetry on a bilateral tactile stimulation test. Rats treated with scopolamine showed a reduction in the initial magnitude of the contralateral placing deficit and an accelerated rate of recovery compared with saline-treated control rats. In contrast, scopolamine had no effect on recovery from the ipsilateral somatosensory asymmetry. These data are consistent with the idea that muscarinic receptor stimulation plays a role in the production of secondary brain damage, that blockade of this receptor leads to a facilitation of recovery on some behavioral tasks, and that electrolytic lesions may trigger some of the same posttraumatic events described in other models of neural trauma.     

Post-lesion administration of the NMDA receptor antagonist MK-801 does not impair motor recovery after unilateral sensorimotor cortex injury in the rat

Although treatment with N-methyl-D-aspartate (NMDA) receptor antagonists reduce neuronal loss after cerebral infarction and brain trauma in laboratory animals, there is little data concerning the effects of these drugs on behavioral recovery. Because NMDA receptor antagonists impede certain kinds of learning, and because motor recovery after sensorimotor cortex injury in the rat is dependent on post-lesion experience, we hypothesized that treatment with MK-801 after focal brain injury would be detrimental. Groups of rats were first trained to traverse a narrow elevated beam and then subjected a right sensorimotor cortex suction-ablation lesion. In the first experiment, 24 h later, each rat received a single dose of either saline or the NMDA receptor antagonist MK-801 (0.5, 1.0, or 2.0 mg/kg). Beam-walking recovery was measured over the next 12 days. In a second experiment, rats were given 3 doses of MK-801 (0.5 mg/kg) at 24 h intervals beginning 24 h after cortex injury. In a third experiment, lesioned and sham-operated rats were allowed to recover for 12 days and then given MK-801 (0.5 mg/kg). Despite obvious behavioral effects of the drug, there was no overall difference in beam-walking performances among the treatment groups in any of the experiments. If 're-learning' is involved in motor recovery after cortex injury, the present results suggest that the process is not susceptible to permanent disruption by the early or late administration of an NMDA receptor antagonist.     

Norepinephrine depletion facilitates recovery of function after focal ischemia in the rat

Previous studies have suggested that increased norepinephrine plays an important role in recovery of function after brain injury; however, the majority of these studies used drugs that are known to also affect other monoamines to increase or decrease norepinephrine. The purpose of the present study was to determine if norepinephrine is required to promote recovery after ischemia. A form of enriched rehabilitation was used to rehabilitate animals after ischemia and the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine was used to selectively destroy norepinephrine projections from the locus coeruleus. Three sensorimotor tests were used to evaluate the recovery of the animals. Depletion of norepinephrine improved sensorimotor recovery in standard-housed animals and did not impede recovery in the rehabilitation groups. Dopamine beta hydroxylase staining was used to confirm N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine-depleted terminal norepinephrine levels. The amount of norepinephrine terminal staining negatively correlated with recovery of function in the staircase test after ischemia. In addition, enriched rehabilitation increased, but depletion of norepinephrine had no effect on, brain-derived neurotrophic factor protein levels, which have also been linked to improved recovery of function. Together the above findings question the previously postulated role of norepinephrine in recovery of function after stroke.     

'Disengage' sensorimotor deficit following apparent recovery from unilateral dopamine depletion

Sensorimotor behavior in Long-Evans rats was evaluated acutely and chronically after unilateral dopamine depletion caused by infusions of 6-hydroxydopamine into the nigrostriatal system. In each rat, control infusions were delivered to the opposite hemisphere and a noradrenaline uptake blocker was used as a pretreatment to help protect noradrenaline cells. During the first few postoperative weeks, head movement reactions to repetitive tactile-perioral stimulation contralateral to the dopamine-depleted hemisphere were delayed but not eliminated. With recovery, facilitated by special training, the rats were able to respond quickly. However, two lasting abnormalities were observed. First, the types of head-orienting movements directed toward contralateral stimulation were different from that directed toward ipsilateral stimulation. Second, when the animals were engaged in eating behavior there was a complete failure to orient to contralateral stimulation, whereas they instantly disengaged from eating to orient to ipsilateral stimulation. When not eating, orienting was rapid and reliable to stimulation of either side of the body. These data may have implications for the role of the striatum and connected structures in the organization and integration of sensorimotor and ingestive behavior.     

Cerebellar norepinephrine infusions facilitate recovery after sensorimotor cortex injury

This study reports the effects of norepinephrine infusions into cerebellum after unilateral sensorimotor cortex injury. The results demonstrate an immediate and permanent acceleration in motor recovery in awake rats infused with 150 micrograms norepinephrine into the cerebellum contralateral to a right sensorimotor cortex ablation. A vehicle infusion or infusion of norepinephrine into the ipsilateral cerebellum produced no beneficial effects on functional recovery.     

Course of motor recovery following ventrolateral spinal cord injury in the rat

The purpose of this study was to determine the importance of the pathways running in the ventrolateral spinal funiculus for overground locomotion in adult, freely behaving rats. Left-sided ventrolateral cervical spinal cord injury was performed in adult female Long-Evans rats. The behavioural abilities of these animals were analyzed at 2 days, and weekly for up to 5.5 weeks following spinal cord injury. Behavioural testing consisted of Von Frey filament testing, ladder walking, a paw usage task, and the assessment of ground reaction forces during unrestrained trotting. Animals with injury to the left ventrolateral cervical spinal cord did not develop enhanced sensitivity to pedal mechanical stimulation. At 2 days following injury, animals had impaired skilled locomotion as indicated by increased number of footslips during ladder walking. At 2 days, these animals also used both limbs together more often for support while rearing, while using the forelimb ipsilateral to the injury less than did uninjured animals. Ground reaction force determination revealed that animals tend to bear less weight on the forelimb and hindlimb ipsilateral to the spinal cord injury 2 days after injury. All animals recovered normal or near normal sensorimotor abilities although subtle asymmetries in ground reaction forces were detectable at 5.5 weeks following spinal cord injury. These results suggest that axons in the ventrolateral spinal funiculi contribute to limb movements during exploration and locomotion but their roles can be served by other pathways after ventrolateral spinal injury.     

Norepinephrine depletion impairs neuroendocrine responses to fear but not novel environmental stimuli in the rat

Emotional stimuli suppress vasopressin secretion and potentiate oxytocin and prolactin secretion by the pituitary in the rat. We studied effects of central norepinephrine depletion on these hormonal responses to novel environmental or fear stimuli. Male Wistar rats were injected intracerebroventricularly with 5-amino-2,4-dihydroxy-α-methylphenylethylamine, a selective neurotoxin to noradrenergic fibers. The neurotoxin treatment reduced the hypothalamic content of norepinephrine by 71% but did not significantly affect the dopamine content. Novel environmental stimuli suppressed vasopressin secretion and augmented secretion of oxytocin and prolactin in the vehicle-injected rats. The neurotoxin did not block the neuroendocrine responses. Intermittently applied electric footshocks also induced the similar neuroendocrine responses in the vehicle-injected rats. The neurotoxin significantly reduced the neuroendocrine responses. The drug, however, did not significantly alter vasopressin release after continuously applied footshocks. Environmental stimuli previously paired with footshocks (conditioned fear stimuli) suppressed vasopressin secretion and augmented secretion of oxytocin and prolactin in the vehicle-injected animals. Motor activity was suppressed during the conditioned fear stimuli. The neurotoxin impaired the neuroendocrine and behavioral responses whether the drug was injected before or after the conditioning. These data demonstrate the distinction between the neural mechanisms underlying the neuroendocrine responses to fear and to novel stimuli, suggesting that noradrenergic neurons are selectively involved in the hypothalamo-hypophysial responses to fear stimuli.     

Selective long‐term reorganization of the corticospinal projection from the supplementary motor cortex following recovery from lateral motor cortex injury

Brain injury affecting the frontal motor cortex or its descending axons often causes contralateral upper extremity paresis. Although recovery is variable, the underlying mechanisms supporting favorable motor recovery remain unclear. Because the medial wall of the cerebral hemisphere is often spared following brain injury and recent functional neuroimaging studies in patients indicate a potential role for this brain region in the recovery process, we investigated the long‐term effects of isolated lateral frontal motor cortical injury on the corticospinal projection (CSP) from intact, ipsilesional supplementary motor cortex (M2). After injury to the arm region of the primary motor (M1) and lateral premotor (LPMC) cortices, upper extremity recovery is accompanied by terminal axon plasticity in the contralateral CSP but not the ipsilateral CSP from M2. Furthermore, significant contralateral plasticity occurs only in lamina VII and dorsally within lamina IX. Thus, selective intraspinal sprouting transpires in regions containing interneurons, flexor‐related motor neurons, and motor neurons supplying intrinsic hand muscles, which all play important roles in mediating reaching and digit movements. After recovery, subsequent injury of M2 leads to reemergence of hand motor deficits. Considering the importance of the CSP in humans and the common occurrence of lateral frontal cortex injury, these findings suggest that spared supplementary motor cortex may serve as an important therapeutic target that should be considered when designing acute and long‐term postinjury patient intervention strategies aimed to enhance the motor recovery process following lateral cortical trauma. J. Comp. Neurol. 518:586–621, 2010. © 2009 Wiley‐Liss, Inc.     

Adaptation in the motor cortex following cervical spinal cord injury

BACKGROUND: The nature of the adaptive changes that occur in the cerebral cortex following injury to the cervical spinal cord are largely unknown. OBJECTIVE: To investigate these adaptive changes by examining the relationship between the motor cortical representation of the paretic right upper extremity compared with that of the tongue. The tongue was selected because the spinal cord injury (SCI) does not affect its movement and the cortical representation of the tongue is adjacent to that of the paretic upper extremity. METHODS: FMRI was used to map cortical representations associated with simple motor tasks of the right upper extremity and tongue in 14 control subjects and 9 patients with remote (>5.5 months) cervical SCI. RESULTS: The mean value for the site of maximum cortical activation during upper limb movement was identical between the two groups. The site of maximum left hemispheric cortical activation during tongue movement was 12.8 mm (p < 0.01) medial and superior to that of control subjects, indicating the presence of a shift in cortical activation. CONCLUSION: The findings indicate that the adult motor cortex does indeed adapt following cervical SCI. The nature of the adaptation and the underlying biological mechanisms responsible for this change require further investigation.     

Forebrain dopamine depletion impairs motor behavior in lamprey

The structure of the basal ganglia appears to be conserved throughout vertebrate evolution, with characteristic cellular and transmitter components in each area, and the same types of afferent input. As described in rodents and primates, depletion of the striatal dopamine results in characteristic motor deficits. To explore if this role of the basal ganglia in modulating motor function was present early in vertebrate evolution, we investigated here the effects of striatal dopamine depletion in the lamprey, a cyclostome, which diverged from the main vertebrate line around 560 million years ago. The lamprey striatum contains the same cellular elements as found in mammals, and receives the same types of input, including a prominent dopamine innervation. We show here that MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine; 100 mg/kg i.p.), a neurotoxin, depletes forebrain and striatal dopamine levels in lamprey to 15% of control values, and has profound effects on motor performance. Twenty-four and 48 h after MPTP injection, lampreys demonstrated marked reductions in spontaneous swimming and the duration of each swimming episode. Impairments in the ability to initiate movements were shown by a decreased rate of initiation. Furthermore, the initiation and maintenance of locomotion induced by olfactory mucosa stimulation was severely impaired, as was the coordination of different motor tasks. These deficits were ameliorated by the dopamine agonist apomorphine. The motor deficits arising after striatal dopamine depletion are thus qualitatively similar in cyclostomes and mammals. The role of the dopamine innervation of the striatum thus appears to be conserved throughout vertebrate evolution.     

Both compensation and recovery of skilled reaching following small photothrombotic stroke to motor cortex in the rat

Large lesions produced by stroke to the forelimb region of motor cortex of the rat feature post-stroke improvement that in the main is due to compensation. The present study describes both recovery and compensation of forelimb use in a reach-to-eat (skilled reaching) task following small photothrombotic stroke. The rats were pretrained before stroke, and then assessed using endpoint measures and biometric movement analysis during rehabilitation in the acute and chronic post-stroke periods. Histological and MRI analysis indicated that the stroke consisted of a small lesion surrounded by cortex featuring scattered cell loss, likely of the large pyramidal cells that characterize the forelimb region of motor cortex. The stroke reduced reaching success, especially on the most demanding measure of success on first reach attempts, in the acute period, but with rehabilitation, performance returned to pre-stroke levels. Reach movements as assessed by biometric measures were severely impaired acutely but displayed significant recovery chronically although this recovery was not complete. The results suggest that not only do rats show post-stroke compensation in skilled reaching but they can also display functional recovery. It is suggested that recovery is mediated by the spared neurons in the peri-infarct region of forelimb motor cortex. The results demonstrate the utility of a small lesion model for studying post-stroke neural and behavioral change and support the view that optimal post-stroke treatment should be directed toward limiting tissue loss.     

Right vs. left sensorimotor cortex suction-ablation in the rat: no difference in beam-walking recovery

The ability of rats to traverse a narrow elevated beam has been used to quantitate recovery of hindlimb motor function after unilateral injury to the sensorimotor cortex. We tested the hypothesis that the rate of spontaneous beam-walking recovery varies with the side of the cortex lesion. Groups of rats that were trained at the beam-walking task underwent suctoin-ablation of either the right or left hindlimb sensorimotor cortex. There was no difference in hindlimb motor function between the groups on the first post-operative beam-walking trial carried out the day after cortex ablation and no difference between the groups in overall recovery rates over the next two weeks. Subsequent analyses of lesion surface parameters showed no differences in lesion size or extent. Regardless of the side of the lesion, there were also no differences between the right and left hemispheres in norepinephrine content of the lesioned or contralateral cortex. We conclude that the side of sensorimotor cortex ablation injury does not differentially affect the rate of spontaneous motor recovery as measured with the beam-walking task.     

Acute unilateral sensorimotor cortex injury in the rat blocks d-amphetamine induced norepinephrine release in cerebellum

Pharmacologic studies have implicated norepinephrine in amphetamine-facilitated motor recovery following sensorimotor cortex injury in rats. We studied the acute effects of unilateral sensorimotor cortex ablation on the release of norepinephrine in cerebellum with in vivo dialysis. In rats without a cortex lesion, the administration of a single dose of d-amphetamine (2.6 mg/kg base weight, i.p.) resulted in a substantial increase in dialyzable norepinephrine in cerebellum reaching its peak 30-40 min later (～ 14 pg/μl, not corrected for recovery). The administration of the same dose of d-amphetamine to rats 60 min following a suction-ablation lesion of the right sensorimotor cortex did not result in norepinephrine release into the cerebellar dialysates. These data provide evidence for an acute remote effect of sensorimotor cortex injury on amphetamine-induced norepinephrine release in the cerebellum (diaschisis).     

Remodeling of synaptic structures in the motor cortex following spinal cord injury

After spinal cord injury (SCI), structural reorganization occurs at multiple levels of the motor system including the motor cortex, and this remodeling may underlie recovery of motor function. The present study determined whether SCI leads to a remodeling of synaptic structures in the motor cortex. Dendritic spines in the rat motor cortex were visualized by confocal microscopy in fixed slices, and their density and morphology were analyzed after an overhemisection injury at C4 level. Spine density decreased at 7 days and partially recovered by 28 days. Spine head diameter significantly increased in a layer-specific manner. SCI led to a higher proportion of longer spines especially at 28 days, resulting in a roughly 10% increase in mean spine length. In addition, filopodium-like long dendritic protrusions were more frequently observed after SCI, suggesting an increase in synaptogenic events. This spine remodeling was accompanied by increased expression of polysialylated neural cell adhesion molecule, which attenuates adhesion between the pre- and postsynaptic membranes, in the motor cortex from as early as 3 days to 2 weeks after injury, suggesting a decrease in synaptic adhesion during the remodeling process. These results demonstrate time-dependent changes in spine density and morphology in the motor cortex following SCI. This synaptic remodeling seems to proceed with a time scale ranging from days to weeks. Elongation of dendritic spines may indicate a more immature and modifiable pattern of synaptic connectivity in the motor cortex being reorganized following SCI.     

Hemicerebellectomy impairs the modulation of cutaneomuscular reflexes by the motor cortex following repetitive somatosensory stimulation

We examined the cutaneomuscular reflex of the plantaris muscle of rats in response to cutaneous stimulation in isolation and in conjunction with subthreshold high-frequency trains of stimuli applied on the motor cortex, prior to and following repetitive peripheral stimulation. The cutaneomuscular reflex was also investigated under the same paradigm following hemicerebellectomy. The enhancement of cutaneomuscular responses associated with subthreshold high-frequency trains of stimulation following repetitive peripheral stimulation was prevented by hemicerebellectomy. Our results suggest that the pathways passing through the cerebellum are involved in the calibration of cutaneomuscular responses.     

Task-specific compensation and recovery following focal motor cortex lesion in stressed rats

One reason for the difficulty to develop effective therapies for stroke is that intrinsic factors, such as stress, may critically influence pathological mechanisms and recovery. In cognitive tasks, stress can both exaggerate and alleviate functional loss after focal ischemia in rodents. Using a comprehensive motor assessment in rats, this study examined if chronic stress and corticosterone treatment affect skill recovery and compensation in a task-specific manner. Groups of rats received daily restraint stress or oral corticosterone supplementation for two weeks prior to a focal motor cortex lesion. After lesion, stress and corticosterone treatments continued for three weeks. Motor performance was assessed in two skilled reaching tasks, skilled walking, forelimb inhibition, forelimb asymmetry and open field behavior. The results revealed that persistent stress and elevated corticosterone levels mainly limit motor recovery. Treated animals dropped larger amounts of food in successful reaches and showed exaggerated loss of forelimb inhibition early after lesion. Stress also caused a moderate, but non-significant increase in infarct size. By contrast, stress and corticosterone treatments promoted reaching success and other quantitative measures in the tray reaching task. Comparative analysis revealed that improvements are due to task-specific development of compensatory strategies. These findings suggest that stress and stress hormones may partially facilitate task-specific and adaptive compensatory movement strategies. The observations support the notion that hypothalamic-pituitary-adrenal axis activation may be a key determinant of recovery and motor system plasticity after ischemic stroke.