Time-sensitive enhancement of motor learning with the less-affected forelimb after unilateral sensorimotor cortex lesions in rats

Unilateral damage to the forelimb region of the sensorimotor cortex (FLsmc) results in time-dependent changes in neuronal activity, structure and connectivity in the contralateral motor cortex of adult rats. These changes have been linked to facilitation of motor skill learning in the less-affected/ipsilesional forelimb, which is likely to promote its use in the development of behavioral compensation. The goal of this study was to determine whether an early post-lesion-sensitive time period exists for this enhanced learning and whether it is linked to synaptogenesis in the contralesional motor cortex. Rats were trained for 21 days on a skilled reaching task with the ipsilesional forelimb beginning 4 or 25 days after unilateral ischemic (endothelin-1-induced) FLsmc lesions or sham operations. As found previously, reaching performance was significantly enhanced in rats trained early post-lesion compared with sham-operates. In rats trained later post-lesion, performance was neither significantly different from time-matched sham-operates nor strikingly different from animals trained earlier post-lesion. In layer V of the contralesional motor cortex, stereological methods for light and electron microscopy revealed significantly more total, multisynaptic bouton and perforated synapses per neuron compared with sham-operates, but there were no significant differences between early- and late-trained lesion groups. Thus, there appears to be a sensitive time window for the maximal expression of the enhanced learning capacity of the less-affected forelimb but this window is broadly, rather than sharply, defined. These results indicate that relatively long-lasting lesion-induced neuronal changes are likely to underlie the facilitation of learning with the less-affected forelimb.     

Unilateral ischemic sensorimotor cortical damage in female rats: forelimb behavioral effects and dendritic structural plasticity in the contralateral homotopic cortex

Previous studies in male rats with unilateral sensorimotor cortical (SMC) damage have demonstrated dendritic structural plasticity in the contralateral homotopic cortex and an enhancement of skilled reaching performance in the forelimb ipsilateral to the lesion compared to sham-operated rats. The purpose of this study was to determine if these findings could be replicated in an ischemic lesion model in female rats. Female rats were given sham operations or unilateral ischemic (endothelin-1 induced) damage in the forelimb representation area of the SMC opposite their preferred forelimb. Animals then received either 20 consecutive days of training on a skilled reaching task with the non-preferred/unimpaired forelimb or no-training control procedures. The surface density of dendrites immunoreactive (IR) for microtubule-associated protein 2 (MAP2) was then measured in the motor cortex opposite the trained limb and/or lesion. Female rats with sufficiently large, but not very small, lesions performed better with the unimpaired forelimb than sham-operated rats on the reaching task. The post-lesion reaching performance was not found to be significantly dependent upon estrous stage at the time of surgery, in agreement with previous studies that failed to find sex or sex-hormone effects after other types of SMC damage. Additionally, there were major laminar-dependent increases in the surface density of MAP2 IR dendrites in the cortex opposite lesions and trained limbs. These findings in female rats are consistent with the dendritic and behavioral changes previously found in male rats. They extend these previous findings by indicating that lesion size is an important variable in the enhancement of reaching performance.     

Experience-dependent structural plasticity in cortex heterotopic to focal sensorimotor cortical damage

Structural plasticity following focal neocortical damage in adult rats has recently been found to be sensitive to postinjury rehabilitative training. Experience on a complex motor skills task, the acrobatic task, after unilateral lesions of the forelimb representation region of the sensorimotor cortex (FLsmc) enhanced synaptic structural changes in the cortex contralateral and homotopic to the lesions. Using tissue from this previous study, the present study examined whether a heterotopic region of the sensorimotor cortex of either hemisphere, the hindlimb representation area (HLsmc), would undergo structural changes following unilateral FLsmc lesions and whether these changes would also be sensitive to postinjury training on the acrobatic task. Stereological methods for light and electron microscopy were used to assess structural changes in lesion or sham-operated rats following 28 days of postoperative acrobatic training or simple repetitive exercise (motor controls). In the HLsmc contralateral to the lesions of rats receiving acrobatic training, there was a subtle, but significant, increase in cortical volume and in layer II/III neuropil and dendritic volume per neuron in comparison to shams. In rats receiving simple exercise after the lesions, these changes were not significantly different from shams. Acrobatic training also prevented a loss of cortical volume in the HLsmc adjacent to the lesion in comparison to shams. These data suggest that behavioral training following cortical injury facilitates structural plasticity in behaviorally relevant areas of the neocortex other than the homotopic cortex contralateral to the lesion. This structural plasticity might be relevant to the development of behavioral compensation after cortical injury.     

Abnormalities in skilled reaching movements are improved by peripheral anesthetization of the less-affected forelimb after sensorimotor cortical infarcts in rats

Unilateral damage to sensorimotor cortical (SMC) regions can profoundly impair skilled reaching function in the contralesional forelimb. Such damage also results in impairments and compensatory changes in the less-affected/ipsilesional forelimb, but these effects remain poorly understood. Furthermore, anesthetization of the ipsilesional hand in humans with cerebral infarcts has been reported to produce transient functional improvements in the paretic hand [Floel A, Nagorsen U, Werhahn KJ, Ravindran S, Birbaumer N, Knecht S, et al. Influence of somatosensory input on motor function in patients with chronic stroke. Ann Neurol 2004;56:206-12; Voller B, Floel A, Werhahn KJ, Ravindran S, Wu CW, Cohen LG. Contralateral hand anesthesia transiently improves poststroke sensory deficits. Ann Neurol 2006;59:385-8]. One aim of this study was to sensitively assay the bilateral effects of unilateral ischemic SMC damage on performance of a unimanual skilled reaching task (the single pellet retrieval task) that rats had acquired pre-operatively with each forelimb. The second aim was to determine whether partially recovered contralesional reaching function is influenced by anesthetization of the ipsilesional forelimb. Unilateral SMC lesions were found to result in transient ipsilesional impairments in reaching success and significant ipsilesional abnormalities in reaching movements compared with sham-operates. There were major contralesional reaching impairments which improved during a 4 week training period, but movements remained significantly abnormal. Anesthetization of the ipsilesional forelimb with lidocaine at this time attenuated the contralesional movement abnormalities. These findings indicate that unilateral ischemic SMC lesions impair skilled reaching behavior in both forelimbs. Furthermore, after partial recovery in the contralesional forelimb, additional improvements can be induced by transient anesthetization of the ipsilesional forelimb. This is consistent with the effects of unilateral anesthetization in humans which have been attributed to the modulation of competitive interhemispheric interactions. The present findings suggest that such interactions are also likely to influence skilled reaching function in rats.     

Evidence for bilateral control of skilled movements: ipsilateral skilled forelimb reaching deficits and functional recovery in rats follow motor cortex and lateral frontal cortex lesions

Unilateral damage to cortical areas in the frontal cortex produces sensorimotor deficits on the side contralateral to the lesion. Although there are anecdotal reports of bilateral deficits after stroke in humans and in experimental animals, little is known of the effects of unilateral lesions on the same side of the body. The objective of the present study was to make a systematic examination of the motor skills of the ipsilateral forelimb after frontal cortex lesions to either the motor cortex by devascularization of the surface blood vessels (pial stroke), or to the lateral cortex by electrocoagulation of the distal branches of the middle cerebral artery (MCA stroke). Plastic processes in the intact hemisphere were documented using Golgi-Cox dendritic analysis and by intracortical microstimulation analysis. Although tests of reflexive responses in forelimb placing identified a contralateral motor impairment following both cortical lesions, quantitative and qualitative measures of skilled reaching identified a severe ipsilateral impairment from which recovery was substantial but incomplete. Golgi-impregnated pyramidal cells in the forelimb area showed an increase in dendritic length and branching. Electrophysiological mapping showed normal size forelimb representations in the lesioned rats relative to control animals. The finding of an enduring ipsilateral impairment in skilled movement is consistent with a large but more anecdotal literature in rats, nonhuman primates and humans, and suggests that plastic changes in the intact hemisphere are related to that hemisphere's contribution to skilled movement.     

Partial recovery of skilled forelimb reaching after transplantation of fetal cortical tissue in adult rats with motor cortex lesion - anatomical and functional aspects

We have studied the effect of transplantation of embryonic frontal cortex on the motor deficit resulting from motor cortex lesion in the adult rat. Twenty-four 2-month-old rats were first trained in a food reaching task with right and left forelimbs. Then, at 4 months of age, the subjects were divided into two equal groups. In the lesion group, the animals sustained a lesion of the left motor cortex, whereas in the graft group the animals received a fetal cell suspension of embryonic (E16) frontal cortical tissue three days after the lesion. Postoperative reaching ability was assessed every week during eight weeks and then every two months until the age of one year. The results indicate that the deficit resulting from the lesion is bilateral but mainly affects the limb contralateral to the lesion. During the first 3 weeks of postoperative testing, both groups displayed comparable evolution of performance with contralateral forelimb, characterized by an initial large drop followed by progressive recovery. But, whereas in the lesion group performance did not increase after the fourth postoperative week, in the graft group the reaching scores further improved, without recovering, however, preoperative levels. This improvement was still observed eight months after transplantation. However, no improvement appeared using the limb ipsilateral to the transplant. An anatomical study of the volumes of transplant and/or lesion revealed that the importance of the recovery or deficit varied as a function of the sizes of the transplant and/or lesion within the rostral part of the motor cortex, approximately corresponding to the rostral forelimb area of Neafsey et al. [37]. It is therefore suggested that in adult rats, some components of the motor deficit resulting from a lesion of the motor cortex can be partially reduced by transplantation of homotopic embryonic cortex.     

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.     

Minor neuronal damage and recovered cellular proliferation in the hippocampus after continuous unilateral forelimb restraint in normal rats

Constraint-induced movement therapy (CIMT) involves the restraint of an intact limb to force the dominant use of an affected limb, in an attempt to enhance use-dependent plasticity and reduce dysfunction. To investigate whether forced disuse of an intact forelimb with CIMT causes a loss of limb function and degenerative damage in the brain, a staircase test and a horizontal ladder test were carried out in control rats and forelimb-restrained rats, and then Argyrophil III silver staining, which is capable of detecting subtle neuronal damage, was used to examine histological alterations associated with restraint. No significant changes in forelimb function were observed in restrained rats. However, atypical weak argyrophilic neurons, an indicator of minor neural damage, were found in the bilateral hippocampus of restrained rats. This damage was not found in the cortex, striatum, or spinal cord. Investigation of neurogenesis in the subventricular zone (SVZ) and subgranular zone (SGZ) revealed a clear reduction in the number of bromodeoxyuridine-positive cells in bilateral SGZ, but not in the SVZ, in restrained rats compared with controls. This reduction was accompanied by reduced mRNA expression of vascular endothelial growth factor and glial-derived neurotrophic factor. However, reduced cellular proliferation and decreased gene expression were recovered after the removal of the restraint. Our results suggest that forced disuse of the intact forelimb has no significant effect on skilled forelimb function but has a minor effect on neurogenesis in SGZ, suggesting that mild stress may be caused by the restraint.     

Escalating low‐dose Δ9‐tetrahydrocannabinol exposure during adolescence induces differential behavioral and neurochemical effects in male and female adult rats

Cannabinoid administration during adolescence affects various physiological processes, such as motor and affective response, cognitive‐related functions and modulates neurotransmitter activity. Literature remains scant concerning the parallel examination of the effects of adolescent escalating low‐dose Δ⁹‐tetrahydrocannabinol (Δ⁹‐THC) on the behavioral and plasticity profile of adult rats in both sexes. Herein, we investigated the long‐term behavioral, neurochemical and neurobiological effects of adolescent escalating low Δ⁹‐THC doses in adult male and female rats. In adult males, adolescent low‐dose Δ⁹‐THC exposure led to increased spontaneous locomotor activity, impaired behavioral motor habituation and defective short‐term spatial memory, paralleled with decreased BDNF protein levels in the prefrontal cortex. In this brain area, serotonergic activity was increased, as depicted by the increased serotonin turnover rate, while the opposite effect was observed in the hippocampus, a region where SERT levels were enhanced by Δ⁹‐THC, compared with vehicle. In adult females, adolescent Δ⁹‐THC treatment led to decreased spontaneous vertical activity and impaired short‐term spatial memory, accompanied by increased BDNF protein levels in the prefrontal cortex. Present findings emphasize the key role of adolescent escalating low Δ⁹‐THC exposure in the long‐term regulation of motor response, spatial‐related cognitive functions and neuroplasticity indices in adulthood. In this framework, these changes could, at a translational level, contribute to clinical issues suggesting the development of psychopathology in a sex‐differentiated manner following Δ⁹‐THC exposure during adolescence.     

Large cortical lesions produce enduring forelimb placing deficits in un-treated rats and treatment with NMDA antagonists or anti-oxidant drugs induces behavioral recovery

Previous studies have utilized a lesion model of cortical injury that produces transient behavioral impairments to investigate the recovery of function process. To better understand the recovery process, it would be beneficial to use a lesion model that produces more severe, enduring, behavioral impairments. The purpose of experiment 1 was to validate whether large lesions of the sensorimotor cortex (SMC), which included the rostral forelimb and caudal forelimb regions, produced enduring behavioral deficits. Rats were given large unilateral electrolytic lesions of the SMC, administered either the N-methyl-D-aspartate (NMDA) antagonist, MK-801 or saline 16 h after injury, and tested on a battery of behavioral tests. Enduring behavioral deficits were observed, for at least 6 months, on two tests of forelimb placing while transient deficits were observed on the foot-fault and somatosensory neutralization tests. Administration of MK-801 facilitated recovery on the somatosensory neutralization test; however, it did not induce recovery on either forelimb placing test. A second experiment was performed to determine if earlier administration of MK-801, the NMDA antagonist magnesium chloride (MgCl(2)), or the anti-oxidant N-tert-butyl-alpha-phenylnitrone (PBN) could induce behavioral recovery in this chronic model. Treatment with these drugs induced behavioral recovery on the forelimb placing tests, whereas, the saline-treated rats did not show any signs of behavioral recovery for at least 3 months. Anatomical analysis of the striatum showed that MK-801 and MgCl(2) but not PBN reduced the extent of lesion-induced striatal atrophy. These results suggest that administration of MK-801, MgCl(2), or PBN shortly after cortical injury can induce recovery of function when recovery is otherwise not expected in un-treated rats.     

Contralesional neural plasticity and functional changes in the less-affected forelimb after large and small cortical infarcts in rats

Some studies have found that unilateral cerebral damage produces significant deficits in the ipsilesional, "less-affected", body side. Other studies have found that such damage results in a paradoxical hyperfunctionality of the ipsilesional body side and a facilitation of learning-induced neuroplastic changes in the contralesional motor cortex. The purpose of this study was to determine whether these effects co-exist and/or vary with lesion severity. After small or large unilateral ischemic lesions of the sensorimotor cortex (SMC) or sham operations, adult male rats were trained for 20 days to acquire a motor task, skilled reaching for food, for the first time with the ipsilesional forelimb. Analyses of movement patterns indicated lesion-size-dependent ipsilesional abnormalities in grasping, retrieving and releasing food pellets. Despite these impairments, success rates were significantly increased and aiming errors reduced in lesion groups compared with sham operates. Performance was best in rats with small lesions that had more minor ipsilesional impairments. In the motor cortex contralateral to the lesion and trained limb, there were significant increases in the density of dendrites immunoreactive for microtubule-associated protein-2 (MAP2) and of N-methyl-D-aspartate receptor subunit 1 (NMDAR1) immunoreactivity compared with sham operates. These effects were correlated with reaching performance. Therefore, enhanced motor skill learning in the "less-affected" forelimb and contralesional neuroplastic changes are muted after larger lesions and co-exist with ipsilesional impairments. These effects may be related to a denervation-induced neural restructuring of the contralesional cortex that both disrupts pre-existing motor engrams and facilitates the establishment of new ones.     

Quantitative analyses of thalamic and cortical origins of neurons projecting to the rostral and caudal forelimb motor areas in the cerebral cortex of rats

On the basis of studies using intracortical microstimulation, the existence of rostrocaudally separate two forelimb representation areas has been inferred in the motor cortex of rats. They are termed caudal and rostral forelimb areas (CFA and RFA). In this study, it was confirmed first that RFA and CFA are located in cytoarchitectonically distinct areas (medial and lateral parts of agranular cortex (AGm and AGl), respectively). In the second part of this study, the distribution of thalamic and cortical neurons projecting to RFA and CFA was quantitatively compared by injections of small and relatively constant amounts of retrograde fluorescent dyes (diamidino yellow and fast blue) into these areas. It was observed that (1) main inputs to RFA originated from AGl, namely CFA (2) CFA received dominant inputs from AGm including RFA and caudally adjacent granular cortex and (3) wider cortical areas and larger number of neurons projected to CFA than to RFA. As for the thalamocortical projections, both RFA and CFA received inputs from various thalamic nuclei, such as VL, VM, Po, PC, PF, CL, but cells projecting to RFA and CFA were differentially located in each nucleus. It was found that labeled cell number and/or density in VM, PC, CL, CM and MD after RFA injections were significantly larger than those after CFA injections. On the other hand, labeled cell number and/or density in VPL and VL were significantly higher after CFA injections than after RFA injections. In comparison with findings in primates, the results suggest that the cortical motor areas of rats may be specialized for different aspects of motor control.     

Oxytocin injected into the ventral subiculum or the posteromedial cortical nucleus of the amygdala induces penile erection and increases extracellular dopamine levels in the nucleus accumbens of male rats

Oxytocin (20–100 ng) was found to be able to induce penile erection when injected unilaterally into the ventral subiculum or the posteromedial cortical nucleus of the amygdala of male rats. The pro‐erectile effect started mostly 30 min after treatment and was abolished by the prior injection of d(CH₂)₅Tyr(Me)²‐Orn⁸‐vasotocin (1–2 μg), an oxytocin receptor antagonist, into the ventral subiculum or posteromedial cortical nucleus. Oxytocin‐induced penile erection occurred 15 min after the increase in the concentration of extracellular dopamine and its metabolite 3,4‐dihydroxyphenylacetic acid in the dialysate obtained from the nucleus accumbens, which was also abolished by d(CH₂)₅Tyr(Me)²‐Orn⁸‐vasotocin. The pro‐erectile effect of oxytocin was also reduced by cis‐flupentixol (2 and 5 μg), a dopamine receptor antagonist, injected into the nucleus accumbens, and by (+)MK‐801 (5 μg), a noncompetitive N‐methyl‐d ‐aspartate receptor antagonist, injected into the ventral tegmental area, but not into the nucleus accumbens. Together with studies showing that glutamatergic efferents from the ventral subiculum/posteromedial cortical nucleus of the amygdala to other areas of the limbic system modulate the activity of mesolimbic dopaminergic neurons, these findings suggest that oxytocin injected into these areas increases glutamatergic neurotransmission in the ventral tegmental area. This, in turn, activates mesolimbic dopaminergic neurons, leading to penile erection. These results provide evidence that the ventral subiculum and the posteromedial cortical nucleus of the amygdala participate in a neural circuit that controls not only the consummatory aspects of sexual behaviour (e.g. penile erection and copulatory performance), but also its motivational/reward aspects, confirming a key role of oxytocin and dopamine in these processes.     

β(1–40) Amyloid peptide injection into the nucleus basalis of rats induces microglia reaction and enhances cortical γ-aminobutyric acid release in vivo

The nucleus basalis of adult rats was injected with β(1–40) amyloid peptide. A marked increase in basal and K⁺-evoked GABA release in the ipsilateral cortex and a significant decrease in GAD activity in the injected NB were found 30 days after injection. An intense activation of microglial cells that surrounded and infiltrated the deposit was observed. These data demonstrate that a local injection of β(1–40) peptide into the NB induces glia activation and affects GABAergic neurons.     

Dual effects of intermittent or continuous L-DOPA administration on gene expression in the globus pallidus and subthalamic nucleus of adult rats with a unilateral 6-OHDA lesion

Intermittent oral doses of levodopa (L-DOPA) are routinely used to treat Parkinson's disease, but with prolonged use can result in adverse motor complications, such as dyskinesia. Continuous administration of L-DOPA achieves therapeutic efficacy without producing this effect, yet the molecular mechanisms are unclear. This study examined, by in situ hybridization histochemistry, the effects of continuous or intermittent L-DOPA administration on gene expression in the globus pallidus and subthalamic nucleus of adult rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal pathway. Results were compared to 6-OHDA-treated rats receiving vehicle. Our results provide original evidence that continuous L-DOPA normalizes the 6-OHDA-lesion-induced increase in mRNA levels encoding for the 67 kDa isoform of glutamate decarboxylase in neurons of the globus pallidus and cytochrome oxidase subunit I mRNA levels in the subthalamic nucleus. The extent of normalization did not differ between the continuous and intermittent groups. In addition, intermittent L-DOPA induced an increase in the mRNA levels encoding for the 65 kDa isoform of glutamate decarboxylase in globus pallidus neurons ipsilateral to the lesion and a bilateral increase in c-fos mRNA expression in the subthalamic nucleus. These results suggest that continuous L-DOPA tends to normalize the 6-OHDA-lesion-induced alterations in cell signaling in the pallido-subthalamic loop. On the other hand, we propose that chronic intermittent L-DOPA exerts a dual effect by normalizing cell signaling in a subpopulation of neurons in the globus pallidus and subthalamic nucleus while inducing abnormal signaling in another subpopulation.