A study of forelimb movements evoked by intracortical microstimulation after hemicerebellectomy in newborn, young and adult rats

A Working Hypothesis for the Role of the Cerebellum in Impulsivity and Compulsivity

Growing evidence associates cerebellar abnormalities with several neuropsychiatric disorders in which compulsive symptomatology and impulsivity are part of the disease pattern. Symptomatology of autism, addiction, obsessive-compulsive (OCD), and attention deficit/hyperactivity (ADHD) disorders transcends the sphere of motor dysfunction and essentially entails integrative processes under control of prefrontal-thalamic-cerebellar loops. Patients with brain lesions affecting the cortico-striatum thalamic circuitry and the cerebellum indeed exhibit compulsive symptoms. Specifically, lesions of the posterior cerebellar vermis cause affective dysregulation and deficits in executive function. These deficits may be due to impairment of one of the main functions of the cerebellum, implementation of forward internal models of the environment. Actions that are independent of internal models may not be guided by predictive relationships or a mental representation of the goal. In this review article, we explain how this deficit might affect executive functions. Additionally, regionalized cerebellar lesions have been demonstrated to impair other brain functions such as the emergence of habits and behavioral inhibition, which are also altered in compulsive disorders. Similar to the infralimbic cortex, clinical studies and research in animal models suggest that the cerebellum is not required for learning goal-directed behaviors, but it is critical for habit formation. Despite this accumulating data, the role of the cerebellum in compulsive symptomatology and impulsivity is still a matter of discussion. Overall, findings point to a modulatory function of the cerebellum in terminating or initiating actions through regulation of the prefrontal cortices. Specifically, the cerebellum may be crucial for restraining ongoing actions when environmental conditions change by adjusting prefrontal activity in response to the new external and internal stimuli, thereby promoting flexible behavioral control. We elaborate on this explanatory framework and propose a working hypothesis for the involvement of the cerebellum in compulsive and impulsive endophenotypes.

Behavioral effects of neonatal lesions on the cerebellar system

Several rodent models with spontaneous mutations causing cerebellar pathology are impaired in motor functions during the neonatal period, including Grid2(Lc), Rora(sg), Dab1(scm), Girk2(Wv), Lmx1a(dr-sst), Myo5a(dn), Inpp4a(wbl), and Cacna1a(rol) mice as well as shaker and dystonic rats. Deficits are also evident in murine null mutants such as Zic1, Fgfr1/FgFr2, and Xpa/Ercc8. Behavioral deficits are time-dependent following X-irradiated- or aspiration-induced lesions of the cerebellum in rats. In addition, motor functions are deficient after lesions in cerebellar-related pathways. As in animal subjects, sensorimotor disturbances have been described in children with cerebellar lesions. These results underline the importance of the cerebellum and its connections in the development of motor functions.

Binge ethanol prior to traumatic brain injury worsens sensorimotor functional recovery in rats

A significant number of patients suffering from traumatic brain injury (TBI) have a high blood alcohol level at the time of injury. Furthermore, drinking alcohol in a binge-like pattern is now recognized as a national problem, leading to a greater likelihood of being injured. Our objective was to determine the consequences of a binge paradigm of alcohol intoxication at the time of TBI on long-term functional outcome using a sensitive test of sensorimotor function. We trained adult, male, Sprague Dawley rats on the skilled forelimb reaching task and then administered a single binge dose of ethanol (2g/kg, i.p.) or saline for three consecutive days (for a total of 3 doses). One hour after the final ethanol dose, rats underwent a TBI to the sensorimotor cortex corresponding to the preferred reaching forelimb. Animals were then tested for seven weeks on the skilled forelimb reaching task to assess the profile of recovery. We found that the group given ethanol prior to TBI displayed a slower recovery curve with a lower recovery plateau as compared to the control group. Therefore, even a relatively short (3 day) episode of binge alcohol exposure can negatively impact long-term recovery from a TBI, underscoring this significant public health problem.

Motor cortex bilateral motor representation depends on subcortical and interhemispheric interactions

The corticospinal tract is a predominantly crossed pathway. Nevertheless, the primary motor cortex (M1) is activated bilaterally during unilateral movements and several animal studies showed that M1 has a bilateral motor representation. A better understanding of the uncrossed corticospinal system is especially important for elucidating its role in recovery of limb control after unilateral injury. We used intracortical microstimulation (ICMS) to determine the representation of contralateral and ipsilateral forelimb joints at single M1 sites in the rat. Most sites representing an ipsilateral joint corepresented the same joint contralaterally. We next determined whether ipsilateral responses evoked in one hemisphere depended on the function of M1 in the opposite hemisphere using reversible inactivation and pyramidal tract lesion. Ipsilateral responses were eliminated when the homotopic forelimb area of M1 in the opposite hemisphere was inactivated or when the pyramidal tract on the nonstimulated side was sectioned. To determine the role of transfer between M1 in each hemisphere we sectioned the corpus callosum, which produced a 33% increase in ipsilateral ICMS thresholds. Neither M1 inactivation nor callosal section changed contralateral response thresholds, indicating the absence of tonic excitatory or inhibitory drive to the opposite M1. Finally, ipsilateral responses following M1 inactivation and pyramidal tract lesion could be evoked after systemic administration of the K(+) channel blocker 4-aminopyridine, suggesting the presence of latent connections. Our findings show important interactions between the corticospinal systems from each side, especially at the spinal level. This has important implications for recruiting the ipsilateral corticospinal system after injury.

Functional recovery of children and adolescents after cerebellar tumour resection

This study examined whether lesions to the cerebellum obtained in early childhood are better compensated than lesions in middle childhood or adolescence. Since cerebellar lesions might affect motor as well a cognitive performance, posture, upper limb and working memory function were assessed in 22 patients after resection of a cerebellar tumour (age at surgery 1-17 years, minimum 3 years post-surgery). Working memory was only impaired in those patients who had received chemo- or radiation therapy. Postural sway was enhanced in 64% of the patients during dynamic posturography conditions, which relied heavily on vestibular input for equilibrium control. Upper limb function was generally less impaired, but 54% of the patients revealed prolonged deceleration times in an arm pointing task, which probably does not reflect a genuine cerebellar deficit but rather the patients' adopted strategy to avoid overshooting. Age at surgery, time since surgery or lesion volume were poor predictors of motor or cognitive recovery. Brain imaging analysis revealed that lesions of all eight patients with abnormal posture who did not receive chemo- and/or radiation therapy included the fastigial and interposed nuclei (NF and NI). In patients with normal posture, NI and NF were spared. In 11 out of 12 patients with abnormal deceleration time, the region with the highest overlap included the NI and NF and dorsomedial portions of the dentate nuclei in 10 out of 12 patients. We conclude that cerebellar damage inflicted at a young age is not necessarily better compensated. The lesion site is critical for motor recovery, and lesions affecting the deep cerebellar nuclei are not fully compensated at any developmental age in humans.

An electron microscopic examination of the corticospinal projection to the cervical spinal cord in the rat: lack of evidence for cortico-motoneuronal synapses

We investigated whether direct, cortico-motoneuronal connections are present in the rat, using both light microscopic and electron microscopic techniques. Corticospinal fibres were labelled using the anterograde tracer, biotinylated dextran-amine (BDA), which was injected into forelimb sensorimotor cortex. Motoneurons were retrogradely labelled after injection of cholera toxin subunit B (CTB) into forelimb muscles, contralateral to the injected hemisphere. Terminals of peripheral afferent fibres, which were also labelled by CTB, were easily distinguishable from, and much larger than, BDA-labelled corticospinal terminals. At the light microscope level, corticospinal terminals were found in all laminae contralateral to the injection site, most extensively in laminae VI and VII of cervical segments C5-C8. Although labelling in the ventral horn (lamina IX) was present, it was extremely sparse. A total of 47 corticospinal synapses were studied at the electron microscope level; most of these were in lamina VII and the majority (35/47; 74%) made axo-dendritic contacts with asymmetrical synapses; one made an axo-somatic synapse, and in the remaining 11 cases no postsynaptic structure could be identified. All corticospinal terminals contained spherical boutons. Serial sectioning of eight BDA-labelled corticospinal boutons in lamina IX revealed that most (seven out of eight) did not make synaptic contacts with any neuronal structure, and none made any contact with adjacent dendrites of CTB-labelled motoneurons. Thus these results provide no positive ultrastructural evidence for direct cortico-motoneuronal synaptic connections within lamina IX between corticospinal axon boutons and the proximal dendrites of forelimb motoneurons. The results confirm other lines of evidence suggesting that such connections are not present in the rat.

Long-Evans rats have a larger cortical topographic representation of movement than Fischer-344 rats: a microstimulation study of motor cortex in naïve and skilled reaching-trained rats

Intracortical microstimulation of the frontal cortex evokes movements in the contralateral limbs, paws, and digits of placental mammals including the laboratory rat. The topographic representation of movement in the rat consists of a rostral forelimb area (RFA), a caudal forelimb area (CFA), and a hind limb area (HLA). The size of these representations can vary between individual animals and the proportional representation of the body parts within regions can also change as a function of experience. To date, there have been no investigations of strain differences in the cortical map of rats, and this was the objective of the present investigation. The effect of cortical stimulation was compared in young male Long-Evans rats and Fischer-344 rats. The overall size of the motor cortex representation was greater in Long-Evans rats compared to Fischer-344 rats and the threshold required to elicit a movement was higher in the Fischer-344 rats. An additional set of animals were trained in a skilled reaching task to rule out the possibility that experiential differences in the groups could account for the result and to examine the relationship between the differences in topography of cortical movement representations and motor performance. The Long-Evans rats were quantitatively and qualitatively better in skilled reaching than the Fischer-344 rats. Also, Long-Evans rats exhibited a relatively larger area of the topographic representation and lower thresholds for eliciting movement in the contralateral forelimb. This is the first study to describe pronounced strain-related differences in the microstimulation-topographic map of the motor cortex. The results are discussed in relation to using strain differences as a way of examining the behavioral, the physiological, and the anatomical organization of the motor system.

Consequences of cerebellar lesions at early and later ages: clinical relevance of animal experiments

Animal experiments demonstrated that reactions of the brain after early lesions differ from those after lesions at adult age. Detailed knowledge on the neuroanatomical and neurophysiological consequences of brain lesions was obtained in humans and will be gained from lesion experiments in animals. Prerequisites for extrapolating animal data to the clinical situation are discussed: knowledge on the maturational stage at which the lesion occurs and the behavioral expression of the damaged neural system. The extensive remodelling after early unilateral cerebellar hemispherectomy and its consequences for behavioural development in the rat are presented and discussed.

Hemicerebellectomy and motor behaviour in rats. II. Effects of cerebellar lesion performed at different developmental stages

Rats with a right hemicerebellectomy (HCb) performed in adulthood or at weaning were compared behaviourally to rats with a similar lesion performed on the first postnatal day. The age at which animals received cerebellar lesions made a significant difference with respect to the behavioural outcome in adulthood. Posture, locomotion and motor behaviour were analysed by a battery of sensorimotor tests. Behavioural measurements showed a clear relationship between age at surgery and behavioural effects; rats with neonatal cerebellar lesions showed a slight extensor hypotonia contralateral to the lesion side and efficient locomotor activity, while the adult operated group exhibited a severe extensor hypotonia ipsilateral to the lesion side and hampered locomotion characterized by a wide base and ataxia. Weanling operated rats displayed a symptomatology similar to that observed in adult operates, although less severe. In the postural dynamic adjustments which the sensorimotor tests required, the youngest operated animals obtained higher scores in comparison to the other two experimental groups, except for the lack of hindlimb usage in the suspension on a wire test. These results, which show the importance of the age-at-lesion factor for the recovery of motor function after HCb in the rat, are discussed in the light of the widespread anatomical reorganization already demonstrated following neonatal HCb in rats.

Sparing of skilled forelimb reaching and corticospinal projections after neonatal motor cortex removal or hemidecortication in the rat: support for the Kennard doctrine

Skilled forelimb use in reaching for food was studied in rats with variously sized and placed unilateral cortical lesions given in adulthood or on the day of birth. Fluorescent retrograde labelling was used to document changes in corticospinal tracts. In free choice tests, preferential use of the limb ipsilateral to damage was induced by adult motor, but not parietal or occipital, cortex damage. Similar preference for the ipsilateral limb was induced by neonatal motor and parietal, but not occipital, cortex damage. In both adult and neonate groups success with the preferred limb decreased in proportion to the increase in lesion size. To force use of the non-preferred limb, a bracelet, which prevented reaching but not other movements, was attached to the forearm of the preferred forelimb. Success with the non-preferred limb was poorer than with the preferred limb and success again decreased in proportion to the increase in lesion size. Adult and neonatal rats were divided into 4 groups according to the extent of motor cortex damage. Across all lesion sizes the neonatal operates were significantly more successful than the adult operates and their reaching movements appeared more normal. Surprisingly, some rats with large adult motor cortex lesions or hemidecortications were able to reach. Slow-motion video analysis of reaching impairments in both adult and neonate groups showed that limb extension and food grasping were less impaired than limb retraction and adduction of the limb to the mouth. The results show that the integrity of a neocortical hemisphere is not essential for contralateral limb use in reaching, but contributes to successful use of the limb. Following neonatal lesions, facilitation may be promoted by the ipsilateral neocortex through an augmented ipsilateral corticofugal pathway.

Forelimb motor cortical projections in normal rats and after neonatal hemicerebellectomy: an anatomical study based upon the axonal transport of WGA/HRP

Cerebral cortical projections from the forelimb motor cortex, as defined by intracortical microstimulation where movements were evoked at low current intensities (less than 15 microA), were examined in normal rats and in adult rats that sustained neonatal hemicerebellectomy. The distribution pattern of cortical efferent projections in normal rats generally appeared more restricted than previously described. This restricted distribution is attributed to the use of WGA/HRP as the axonal tracing method and to the electrophysiological definition of the injection site as the motor cortex. The observed remodeling of the corticobulbar projections, seen after cerebellar lesions in the young, largely confirmed previous reports. Moreover, no alterations in the laterality of distribution in corticospinal projection were found. Aberrant corticospinal projections were sought in an effort to provide an anatomical basis to a previous description of abnormally low-threshold ipsilateral forelimb responses evoked from the motor cortex in adult rats after neonatal cerebellar lesions. This apparent absence of corticospinal tract remodeling after neonatal hemicerebellectomy suggests that the abnormal responses are mediated by the normal corticospinal pathways. This possibility is discussed in terms of an alteration in the spinal circuits that may change the responsiveness of spinal motoneurons to a given pyramidal discharge.