Neocerebellar control of the motor activity: experimental analysis in the rat. Comparative aspects

Functional Alteration of Cerebello-Cerebral Coupling in an Experimental Mouse Model of Parkinson's Disease

In Parkinson's disease, the degeneration of the midbrain dopaminergic neurons is consistently associated with modified metabolic activity in the cerebellum. Here we examined the functional reorganization taking place in the cerebello-cerebral circuit in a murine model of Parkinson's disease with 6-OHDA lesion of midbrain dopaminergic neurons. Cerebellar optogenetic stimulations evoked similar movements in control and lesioned mice, suggesting a normal coupling of cerebellum to the motor effectors after the lesion. In freely moving animals, the firing rate in the primary motor cortex was decreased after the lesion, while cerebellar nuclei neurons showed an increased firing rate. This increase may result from reduced inhibitory Purkinje cells inputs, since a population of slow and irregular Purkinje cells was observed in the cerebellar hemispheres of lesioned animals. Moreover, cerebellar stimulations generated smaller electrocortical responses in the motor cortex of lesioned animals suggesting a weaker cerebello-cerebral coupling. Overall these results indicate the presence of functional changes in the cerebello-cerebral circuit, but their ability to correct cortical dysfunction may be limited due to functional uncoupling between the cerebellum and cerebral cortex.

Circuits in the rodent brainstem that control whisking in concert with other orofacial motor actions

The world view of rodents is largely determined by sensation on two length scales. One is within the animal's peri-personal space; sensorimotor control on this scale involves active movements of the nose, tongue, head, and vibrissa, along with sniffing to determine olfactory clues. The second scale involves the detection of more distant space through vision and audition; these detection processes also impact repositioning of the head, eyes, and ears. Here we focus on orofacial motor actions, primarily vibrissa-based touch but including nose twitching, head bobbing, and licking, that control sensation at short, peri-personal distances. The orofacial nuclei for control of the motor plants, as well as primary and secondary sensory nuclei associated with these motor actions, lie within the hindbrain. The current data support three themes: First, the position of the sensors is determined by the summation of two drive signals, i.e., a fast rhythmic component and an evolving orienting component. Second, the rhythmic component is coordinated across all orofacial motor actions and is phase-locked to sniffing as the animal explores. Reverse engineering reveals that the preBötzinger inspiratory complex provides the reset to the relevant premotor oscillators. Third, direct feedback from somatosensory trigeminal nuclei can rapidly alter motion of the sensors. This feedback is disynaptic and can be tuned by high-level inputs. A holistic model for the coordination of orofacial motor actions into behaviors will encompass feedback pathways through the midbrain and forebrain, as well as hindbrain areas.

RNAi prevents and reverses phenotypes induced by mutant human ataxin-1

OBJECTIVE

Spinocerebellar ataxia type 1 is an autosomal dominant fatal neurodegenerative disease caused by a polyglutamine expansion in the coding region of ATXN1. We showed previously that partial suppression of mutant ataxin-1 (ATXN1) expression, using virally expressed RNAi triggers, could prevent disease symptoms in a transgenic mouse model and a knockin mouse model of the disease, using a single dose of virus. Here, we set out to test whether RNAi triggers targeting ATXN1 could not only prevent, but also reverse disease readouts when delivered after symptom onset.

METHODS

We administered recombinant adeno-associated virus (rAAV) expressing miS1, an artificial miRNA targeting human ATXN1 mRNA (rAAV.miS1), to a mouse model of spinocerebellar ataxia type 1 (SCA1; B05 mice). Viruses were delivered prior to or after symptom onset at multiple doses. Control B05 mice were treated with rAAVs expressing a control artificial miRNA, or with saline. Animal behavior, molecular phenotypes, neuropathology, and magnetic resonance spectroscopy were done on all groups, and data were compared to wild-type littermates.

RESULTS

We found that SCA1 phenotypes could be reversed by partial suppression of human mutant ATXN1 mRNA by rAAV.miS1 when delivered after symptom onset. We also identified the therapeutic range of rAAV.miS1 that could prevent or reverse disease readouts.

INTERPRETATION

SCA1 disease may be reversible by RNAi therapy, and the doses required for advancing this therapy to humans are delineated. Ann Neurol 2016;80:754-765.

On-beam synchrony in the cerebellum as the mechanism for the timing and coordination of movement

In trained reaching rats, we recorded simple spikes of pairs of Purkinje cells that, with respect to each other, were either aligned on a beam of shared parallel fibers or instead were located off beam. Rates of simple spike firing in both on-beam and off-beam Purkinje cell pairs commonly showed great variety in depth of modulation during reaching behavior. But with respect to timing, on-beam Purkinje cell pairs had simple spikes that were tightly time-locked to each other (either delayed or simultaneous) and to movement, despite the variability in rate. By contrast, off-beam Purkinje cell pairs had simple spikes that were not time-locked to each other, neither delayed nor simultaneous. We discuss the implications of these observations for the cerebellar role in timing and coordinating movement.

Stimulation of the superior cerebellar peduncle during the development of amygdaloid kindling in rats

Cerebellar manipulations have been used successfully in some intractable epileptic patients, however, their intrinsic mechanisms are not fully understood. To further clarify the cerebellar participation in epilepsy, we stimulated 10 rats with 100 Hz, 20 microA at the superior cerebellar peduncle (SCP) during amygdaloid kindling. Results were compared to 10 rats with an electrode placed at the SCP without stimulation and 10 rats without electrodes at the SCP used as control. We found that SCP stimulation increased the theta and alpha rhythms at the contralateral motor cortex. Such a stimulation produced hypertonicity of the forelimbs and tremor of the head. In this condition, we found that each of the behavioral stages during amygdaloid kindling in the SCP stimulated rats was reached earlier, while the amygdaloid electrographic afterdischarges (ADs) were longer during the first and shorter in the final trials as compared to controls. Moreover, amygdaloid ADs recorded exclusively during the behavioral stage-5 were significantly shorter than those recorded in the control conditions. We suggest that SCP stimulation could change the customary electrographic and convulsive expression of amygdala kindling in such a manner as to initially facilitate the limbic seizures and impede the secondary generalized seizures.

Role of the cerebellum in the control and adaptation of gait in health and disease

In humans, inability to stand and walk is the most limiting of motor disabilities. In humans, upright stance and gait is the most sensitive indicator of cerebellar disease. From animal and human studies, much has been learned about how the cerebellum coordinates normal movement, and how it may play roles in normal motor adaptation and learning. Much of this work suggests that different parts of the cerebellum control stance and gait in different ways, and differently located lesions cause different deficits. What is not known is whether the cerebellum can compensate for stance and gait disorders caused by lesions in other parts of the nervous system, or whether one part of the cerebellum can compensate for deficits caused by lesion of another part. These issues have become increasingly important in rehabilitation research and practice.

Tactile responses in the granule cell layer of cerebellar folium crus IIa of freely behaving rats

We recorded activity from the granule cell layer (GCL) of cerebellar folium Crus IIa as freely moving rats engaged in a variety of natural behaviors, including grooming, eating, and free tactile exploration. Multiunit responses in the 1000-4500 Hz range were found to be strongly correlated with tactile stimulation of lip and whisker (perioral) regions. These responses occurred regardless of whether the stimulus was externally or self-generated and during both active and passive touch. In contrast, perioral movements that did not tactually stimulate this region of the face (e.g., chewing) produced no detectable increases in GCL activity. In addition, GCL responses were not correlated with movement extremes. When rats used their lips actively for palpation and exploration, the tactile responses in the GCL were not detectably modulated by ongoing jaw movements. However, active palpation and exploratory behaviors did result in the largest and most continuous bursts of GCL activity: responses were on average 10% larger and 50% longer during palpation and exploration than during grooming or passive stimulation. Although activity levels differed between behaviors, the position and spatial extent of the peripheral receptive field was similar over all behaviors that resulted in tactile input. Overall, our data suggest that the 1000-4500 Hz multiunit responses in the Crus IIa GCL of awake rats are correlated with tactile input rather than with movement or any movement parameter and that these responses are likely to be of particular importance during the acquisition of sensory information by perioral structures.

Arborisation and termination of single motor thalamocortical axons in the rat

The aim of this study was to examine the arborisations and terminations of individual thalamocortical axons in the motor system of the rat. Small, extracellular injections of an anterograde tracer (dextran-biotin) were made into the ventrolateral (VL) or ventral posterolateral (VPL) thalamic nuclei to label thalamocortical projections. Eleven motor axons and one somatosensory axon were reconstructed through serial sections just rostral from the injection site to their terminations in sensorimotor cortex. The smallest arbor arising from a single motor axon extended approximately 0.9 mm rostrocaudal and 0.9 mm mediolateral, the largest extended 3.9 mm rostrocaudal and 1.0 mm mediolateral. In some cases, two distinct plexuses of terminals were formed by an axon. In addition, motor axons formed terminals in cortical layer V only or in layers I, III, and V. By contrast (and in keeping with previous reports), the somatosensory axon formed a single plexus of terminals in layer IV of the cortex that extended approximately 0.3 mm rostrocaudal and 0.4 mm mediolateral. It is concluded that individual motor thalamocortical neurones are in a position to influence much more widespread cortical regions than somatosensory thalamocortical neurones.

The neurobiological basis of movement initiation

Simple reaction time (RT) is defined as the elapsed time between presentation of a single stimulus and onset of movement. In choice RT, there are at least two stimuli, requiring two distinct responses. The neurobiological basis of RT in humans has mostly been evaluated in patients with Parkinson's disease or cerebellar disease. Lesion studies in animals have assessed the different contributions of various subregions of the basal ganglia and the cerebellum. There is a prolongation of simple RT and in some cases of choice RT in Parkinson's disease. Both simple and choice RT are susceptible to modulation by brain dopamine levels. However, such is not invariably the case, attesting to the contribution of non-dopaminergic neurons in the sensori-motor slowing found in Parkinson's disease. An increase in simple RT and in choice RT are found in patients with cerebellar atrophy. The initiation of fast ballistic movements is associated with the dentate efferent system. This system is modulated by dopaminergic and glutamatergic pathways to the striatum.

The projections of the lateral reticular nucleus to the deep cerebellar nuclei. An experimental analysis in the rat

The projections of the lateral reticular nucleus (LRN) to the cerebellar nuclei were studied using the retrograde axonal transport of tetramethyl rhodamine dextran amine (10% solution in 0.01 M neutral phosphate buffer) in 19 adult Wistar strain rats. The cerebellar nuclei receive topographically organized projections from the LRN. The projections are bilateral with an ipsilateral predominance and they are symmetrical. The contralateral component is progressively larger for projections to the nuclei interpositalis, to the nucleus lateralis and to the nucleus medialis. The projections to the various cerebellar nuclei arise from rostrocaudally oriented columns of neurons located in different (partly overlapping) areas of the magnocellular division of the LRN. The nucleus lateralis receives terminals from the dorsomedial area (mainly from the rostral level of the LRN), the nuclei interpositalis from the dorsolateral area (mainly from the central level) and the nucleus medialis from the intermedioventral area (mainly from the caudal level). Afferent fibres from the small subtrigeminal division were traced to the three cerebellar nuclei and from the parvocellular division to the nuclei interpositalis and medialis. The density of the projections from the LRN to the nuclei interpositalis increases progressively with the shift of the terminal field from the rostrolateral to the caudomedial part of the nucleus. The projections to the nucleus lateralis reach principally the dorsolateral hump, whereas only a few neurons project to the other divisions (parvo- and magnocellular). The projections to the various regions of the nucleus medialis show different densities. The highest density was found for projections to the caudal part, in particular to the dorsolateral protuberance and to the ventrolateral area of the middle division. Conversely, a low density of projections was found for the other areas of the middle division. The regions of the magnocellular division of the LRN which project to the nuclei lateralis (and are thus related to the cerebral cortex), interpositalis (related to the red nucleus) and medialis (related to the spinal cord) also receive afferent terminals from the cerebral cortex, the red nucleus and the spinal cord respectively, in addition to various afferent inputs. Thus, each of these areas is apparently concerned with integrating some spinal and supraspinal information in reverberating circuits.

On the cerebellum, cutaneomuscular reflexes, movement control and the elusive engrams of memory

This review focuses on the role of the cerebellum in regulating cutaneomuscular reflexes and provides a hypothesis regarding the way in which this action contributes to the coordination of goal-directed movements of the extremities. Specific attention is directed towards the cerebellum's role in conditioned and unconditioned eyeblink reflexes and limb withdrawal reflexes as models of its interactions with the cutaneomuscular reflex systems. The implications regarding the cerebellum as a storage site for motor engrams also is discussed in the context of these two behaviors. The proposed hypothesis suggests that the cerebellum regulates important features of the cutaneomuscular reflex circuits including the integration of their activity with descending pathways in a manner that implements these fundamental reflex circuits in the organization and control of goal-directed movements of the extremities.

Effects of muscimol inactivation of the cerebellar interposed-dentate nuclear complex on the performance of the nictitating membrane response in the rabbit

Intracranial microinjections of the GABAA agonist muscimol were used to assess the involvement of the dentato-interposed cerebellar nuclear complex in the performance of the conditioned (CR) and unconditioned (UR) nictitating membrane responses in the rabbit. Specifically, the experiments test the hypothesis that the cerebellar nuclei are involved in the performance of both the CRs and URs. The experiments employed temporary nuclear lesions to disrupt the CRs in order to examine parallel effects on URs. Animals were conditioned in a standard delay conditioning paradigm. Injection sites at which the muscimol application disrupted execution of the CRs were identified in each rabbit. Once these sites were found, the effects of muscimol and saline injections were evaluated while alternating paired trials with unpaired trials in which only the unconditioned stimuli were applied. There are two main findings in the present study. First, the activation of the GABAA receptors in the dentato-interposed cerebellar nuclear region reduced the amplitude and increased the latency of the UR. This change in the UR closely paralleled the disruption of the CR. This observation is consistent with the notion that the cerebellum is involved in the regulation of defensive flexion reflexes. Second, cerebellar nuclear inactivation did not eliminate the tone-induced enhancement of the UR. This finding suggests the presence of cerebellum-independent circuits subserving the intermodal interaction between the conditioned and unconditioned stimuli.

Cerebellar influences on accessory oculomotor nuclei of the rat: a neuroanatomical, immunohistochemical, and electrophysiological study

With the aim to evaluate a possible neocerebellar control on eye movements, the projections from the cerebellar lateral nucleus (LN) to the accessory oculomotor nuclei (i.e., the nucleus of posterior commissure, the nucleus of Darkschewitsch, and the interstitial nucleus of Cajal), the putative neurotransmitters subserving this pathway, and the nature of the synaptic influences exerted by these projections were studied in adult rats. We used the orthograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) to identify the mesencephalic areas where cerebellofugal fibers terminate, and retrograde labeling with the fluorescent dye fluoro-gold to estimate the incidence of cerebellar neurons projecting to the accessory oculomotor nuclei. Orthograde labeling showed that only a small contingent of cerebellofugal fibers reaches the contralateral accessory oculomotor nuclei. The retrogradely labeled cells were located primarily in the small-celled part of LN. By immunohistochemistry, we observed that all the cells retrogradely labeled from the accessory oculomotor area were also stained by using glutamate or aspartate antisera, but none of them were double-stained with a GABA antiserum. Electrical stimulation of the contralateral LN elicited changes in firing rate of a significant fraction of cells belonging to the accessory oculomotor nuclei (36.4% in the nucleus of posterior commissure, 47.1% in the nucleus of Darkschewitsch, and 44.6% in the interstitial nucleus of Cajal). In 57.8% of the cases, the responses were excitations, most of which had latencies and response characteristics compatible with a monosynaptic linkage. The remaining 42.2% of the cases were inhibitions with latencies ranging between 5 and 22 ms. Extracellular field potential recordings within the contralateral accessory oculomotor nuclei were interpreted as arising from impulses propagating along excitatory axons projecting in a bundle from the cerebellum. Stimulation of LN area in rats following intranuclear injection of kainic acid was not capable of evoking short latency excitations, so these responses can be considered to depend on the activation of LN efferents. The LN projection on accessory oculomotor nuclei could be part of the final precise control exerted by the neocerebellum on those brain structures concerned with movements of the eyes.

Afferent connections of the parvocellular reticular formation: a horseradish peroxidase study in the rat

The afferent connections of the parvocellular reticular formation were systematically investigated in the rat with the aid of retrograde and anterograde horseradish peroxidase tracer techniques. The results indicate that the parvocellular reticular formation receives its main input from several territories of the cerebral cortex (namely the first motor, primary somatosensory and granular insular areas), districts of the reticular formation (including its contralateral counterpart, the intermediate reticular nucleus, the nucleus of Probst's bundle, the dorsal paragigantocellular nucleus, the alpha part of the gigantocellular reticular nucleus, the dorsal and ventral reticular nuclei of the medulla, and the mesencephalic reticular formation), the supratrigeminal nucleus and the deep cerebellar nuclei. Moderate to substantial input to the parvocellular reticular formation appears to come from the central amygdaloid nucleus, the parvocellular division of the red nucleus, and the orofacial and gustatory sensory cell groups (comprising the mesencephalic, principal and spinal trigeminal nuclei, and the rostral part of the nucleus of the solitary tract), whereas many other structures, including the substantia innominata, the field H2 of Forel, hypothalamic nuclei, the superior colliculus, the substantia nigra pars reticulata, the retrorubral field and the parabrachial complex, seem to represent relatively modest additional input sources. Some of these projections appear to be topographically distributed within the parvocellular reticular formation. From the present results it appears that the parvocellular reticular formation receives afferents from a restricted group of sensory structures. This finding calls into question the traditional characterization of the parvocellular reticular formation as an intermediate link between the sensory nuclei of the cranial nerves and the medial magnocellular reticular districts, identified as the effector components of the reticular apparatus. Some of the possible physiological correlates of the fiber connections of the parvocellular reticular formation in the context of oral motor behaviors, autonomic regulations, respiratory phenomena and sleep-waking mechanisms are briefly discussed.

Functional organization of the direct and indirect projection via the reticularis thalami nuclear complex from the motor cortex to the thalamic nucleus ventralis lateralis

The projection systems which arise from the motor cortex to reach the nucleus ventralis lateralis (VL) were investigated in the rat. They included a direct as well as an indirect projection via the reticularis thalami nuclear complex (RT). The investigation was performed in two steps: i) the former concerned the projection to the VL as well as to the RT from individual cortical foci electrophysiologically identified by the motor effects evoked by electrical stimulation; the second step concerned the projection from the RT to functionally defined regions of the VL. The direct projection from the motor cortex to the VL is somatotopically arranged. The projection reciprocates the fiber system directed from the VL to the motor cortex. Thus cortical zones controlling the motor activity of the proximal segments of the limbs project onto the regions of the VL that project back to these same cortical areas. With regard to cortical zones controlling the motor activity of the distal segments of the limbs, they not only project to the region of the VL specifically related to them, but also to the region of the VL associated with the cortical areas responsible for movements of the proximal parts of the same limb. In that case fiber terminals were more dense in the VL region controlling the proximal segment than in the region controlling the distal segment of the same limb. This organization suggests that proximal adjustments may be automatically provided by the motor activity of the distal segments of the same limb. The motor cortex projects to the rostral region of the RT with a precise topographical organization. In particular, the projection shows a dorsoventral organization in the RT in relation to the caudorostral body representation in the motor cortex. The projection which arises from the rostral region of the RT also reaches the VL with a topographical arrangement. It discloses a rostrocaudal organization in the VL in relation to a dorsoventral displacement in the RT. Comparing the projection from the motor cortex to the RT and that from this nuclear complex to the VL it was shown that the regions of the VL and their receptive cortical areas were associated with the same regions of the RT. It was therefore concluded that the motor cortical projection to the VL relayed by the RT is somatotopically organized. In both direct and relayed pathways the projections from "hind-" and "forelimb" motor area are segregated, whereas the "head" projection overlaps, at least partially, the "forelimb" terminal field.(ABSTRACT TRUNCATED AT 400 WORDS)

Sagittal organisation of the olivocerebellonuclear pathway in the rat. III. Connections with the nucleus dentatus

The organization of olivary afferents and nuclear efferents of the D-zone of the rat cerebellum was studied by means of tracing with wheat-germ agglutinin-coupled peroxidase using tetramethylbenzidine as a chromagen. The tracer was injected iontophoretically within the cerebellar cortex. This allowed us to study both afferent and efferent pathways of the cerebellar lobules concerned with retrograde and anterograde tracing, respectively. Retrograde cellular labelling in the inferior olive was restricted to the principal olive (PO). Anterograde terminal labelling was found only within the various subdivisions of the nucleus lateralis or dentatus (ND). For any one of our small cortical injections there was a corresponding sagittal band of retrogradely labelled cells in the contralateral PO, and a sagittal band of terminal labelling through the ND. Based on both their olivary and nuclear connections, 3 sagittal subzones can be distinguished within the D-zone of the rat. From medial to lateral, we call them D0, D1 and D2. The 3 subzones run through part of the anterior and posterior lobes. D1 and D2 run continuously from their rostral to their caudal extents whereas D0 is discontinuous. It is interrupted through lobule VIc (crus I). The olivary projections to D0 arise within the medial half of the ventral lamella of the PO, including the dorsomedial cell column. Those to D1 arise within the dorsal lamella of the PO. Those to D2 arise within the lateral half of the ventral lamella of the PO. Rostrocaudally, widely distant cells of the same subdivision of the PO project to the same cerebellar lobule. This indicates extensive convergence of the olivary afferents within each of the 3 hemispheric compartments, D0, D1 and D2. Each of the 3 hemispheric subzones specifically projects to one of the 3 subdivisions distinguished within the ND of the rat, without apparent mediolateral overlapping. The medialmost D0 projects onto the dorsolateral hump; D1 projects more laterally onto the main, magnocellular part of the ND, and D2 projects ventrally onto the parvicellular subdivision of the ND. Thus the sagittal partition of the hemispheric cortex is reflected at the nuclear level. In contrast, Purkinje cell axons from individual lobules appear to branch extensively in the rostrocaudal direction. Therefore, within each of the 3 compartments D0, D1 as well as D2, the nuclear projection of the anterior lobe and the posterior lobe are largely coextensive.