Neuronal activity in nuclei pontis and reticularis tegmenti pontis related to forelimb movements of the monkey

The indirect corticopontine pathway relays perioral sensory signals to the cerebellum via the mesodiencephalic junction

In the cerebro-cerebellar loop, outputs from the cerebral cortex are thought to be transmitted via monosynaptic corticopontine gray (PG) pathways and subsequently relayed to the cerebellum. However, it is unclear whether this pathway is used constitutively for cerebro-cerebellar transduction. We examined perioral sensory pathways by unit recording from Purkinje cells in ketamine/xylazine-anesthetized mice. Infraorbital nerve stimulations enhanced simple spikes (SSs) with short and long latencies (first and second peaks), followed by SS inhibition. The second peak and SS inhibition were suppressed by muscimol (a GABAA agonist) injections into not only the PG but also the mesodiencephalic junction (MDJ). The pathway from the secondary somatosensory area (SII) to the MDJ, but not the cortico-PG pathway, transmitted the second peak signals. SS inhibition was processed in the SII and primary motor area. Thus, the indirect cortico-PG pathway, via the MDJ, is recruited for perioral sensory transduction.

Nucleus reticularis tegmenti pontis: a bridge between the basal ganglia and cerebellum for movement control

Neural processing in the basal ganglia is critical for normal movement. Diseases of the basal ganglia, such as Parkinson's disease, produce a variety of movement disorders including akinesia and bradykinesia. Many believe that the basal ganglia influence movement via thalamic projections to motor areas of the cerebral cortex and through projections to the cerebellum, which also projects to the motor cortex via the thalamus. However, lesions that interrupt these thalamic pathways to the cortex have little effect on many movements, including limb movements. Yet, limb movements are severely impaired by basal ganglia disease or damage to the cerebellum. We can explain this impairment as well as the mild effects of thalamic lesions if basal ganglia and cerebellar output reach brainstem motor regions without passing through the thalamus. In this report, we describe several brainstem pathways that connect basal ganglia output to the cerebellum via nucleus reticularis tegmenti pontis (NRTP). Additionally, we propose that widespread afferent and efferent connections of NRTP with the cerebellum could integrate processing across cerebellar regions. The basal ganglia could then alter movements via descending projections of the cerebellum. Pathways through NRTP are important for the control of normal movement and may underlie deficits associated with basal ganglia disease.

A Brainstem reticulotegmental neural ensemble drives acoustic startle reflexes

The reticulotegmental nucleus (RtTg) has long been recognized as a crucial component of brainstem reticular formation (RF). However, the function of RtTg and its related circuits remain elusive. Here, we report a role of the RtTg in startle reflex, a highly conserved innate defensive behaviour. Optogenetic activation of RtTg neurons evokes robust startle responses in mice. The glutamatergic neurons in the RtTg are significantly activated during acoustic startle reflexes (ASR). Chemogenetic inhibition of the RtTg glutamatergic neurons decreases the ASR amplitudes. Viral tracing reveals an ASR neural circuit that the cochlear nucleus carrying auditory information sends direct excitatory innervations to the RtTg glutamatergic neurons, which in turn project to spinal motor neurons. Together, our findings describe a functional role of RtTg and its related neural circuit in startle reflexes, and demonstrate how the RF connects auditory system with motor functions.

Disrupting cortico-cerebellar communication impairs dexterity

To control reaching, the nervous system must generate large changes in muscle activation to drive the limb toward the target, and must also make smaller adjustments for precise and accurate behavior. Motor cortex controls the arm through projections to diverse targets across the central nervous system, but it has been challenging to identify the roles of cortical projections to specific targets. Here, we selectively disrupt cortico-cerebellar communication in the mouse by optogenetically stimulating the pontine nuclei in a cued reaching task. This perturbation did not typically block movement initiation, but degraded the precision, accuracy, duration, or success rate of the movement. Correspondingly, cerebellar and cortical activity during movement were largely preserved, but differences in hand velocity between control and stimulation conditions predicted from neural activity were correlated with observed velocity differences. These results suggest that while the total output of motor cortex drives reaching, the cortico-cerebellar loop makes small adjustments that contribute to the successful execution of this dexterous movement.

The role of the monkey dorsal pontine nuclei in goal-directed eye and hand movements

Prevailing concepts on the control of goal-directed hand movements (HM) have focused on a network of cortical areas whose early parieto-occipital stages are thought to extract and integrate information on target and hand location, involving subsequent stages in frontal cortex forming and executing movement plans. The substantial experimental results supporting this "cortical network" concept for hand movements notwithstanding, the concept clearly needs refinement to account for the surprisingly mild HM disturbances resulting from disconnecting the parieto-occipital from the frontal stages of the network. Clinical observations have suggested the cerebropontocerebellar projection as an alternative pathway for the sensory guidance of HM. As a first step in assessing its role, we explored the pontine nuclei (PN) of rhesus monkeys, trained to make goal-directed hand and eye movements guided by spatial memory. We were indeed able to delineate a distinct cluster of neurons in the rostrodorsal PN, activated by the preparation and the execution of hand reaches, close to but distinct from the region in which saccade-related neurons may be observed. The movement-related activity of HM-related neurons starts earlier than that of saccade-related neurons and both neuron types are usually effector specific, i.e., they respond only to the movement of the preferred effector. This is also the case when motor synergies involving both effectors are executed. Our findings support the notion of a distinct precerebellar, pontine visuomotor channel for hand reaches that is anatomically and functionally largely separated from the one serving eye movements.

Primate area MST-l is involved in the generation of goal-directed eye and hand movements

The contributions of the middle superior temporal area (MST) in the posterior parietal cortex of rhesus monkeys to the generation of smooth-pursuit eye movements as well as the contributions to motion perception are well established. Here, we present the first experimental evidence that this area also contributes to the generation of goal-directed hand movements toward a moving target. This evidence is based on the outcome of intracortical microstimulation experiments and transient lesions by small injections of muscimol at identified sites within the lateral part of area MST (MST-l). When microstimulation was applied during the execution of smooth-pursuit eye movements, postsaccadic eye velocity in the direction of the preferred direction of the stimulated site increased significantly (in 93 of 136 sites tested). When microstimulation was applied during a hand movement trial, the hand movement was displaced significantly in the same direction (in 28 of 39 sites tested). When we lesioned area MST-l transiently by injections of muscimol, steady-state eye velocity was exclusively reduced for ipsiversive smooth-pursuit eye movements. In contrast, hand movements were displaced toward the contralateral side, irrespective of the direction of the moving target. Our results provide evidence that area MST-l is involved in the processing of moving targets and plays a role in the execution of smooth-pursuit eye movements as well as visually guided hand movements.

Motor projections to the basis pontis in rhesus monkey

Motor corticopontine studies suggest that the pons is topographically organized, but details remain unresolved. We used physiological mapping in rhesus monkey to define subregions in precentral motor cortex (M1), injected isotope tracers into M1 and the supplementary motor area (SMA), and studied projections to the basis pontis. Labeled fibers descend in the internal capsule (SMA in anterior limb and genu; M1 in posterior limb) and traverse the midsection of the cerebral peduncle, where SMA fibers are medial, and face, arm, and leg fibers are progressively lateral. Each motor region has unique terminations in the ipsilateral basis pontis and nucleus reticularis tegmenti pontis. Projections are topographically organized, preferentially in the caudal half of the pons, situated in close proximity to traversing corticofugal fibers. In nuclei that receive multiple inputs, terminations appear to interdigitate. Projections from the SMA-face region are most medial and include the median pontine nucleus. M1-face projections are also medial but are lateral to those from SMA-face. Hand projections are in medially placed curved lamellae in mid- and caudal pons. Dorsal trunk projections are in medial and ventral locations. Ventral trunk/hip projections encircle the peduncle in the caudal pons. Foot projections are heaviest caudally in laterally placed, curved lamellae. These results have relevance for anatomical clinical correlations in the human basis pontis. Furthermore, the dichotomy of motor-predominant caudal pons projections to cerebellar anterior lobe, contrasted with associative-predominant rostral pons projections to cerebellar posterior lobe, is consistent with new hypotheses regarding the cerebellar contribution to motor activity and cognitive processing.

Movement-related discharge in the cerebellar nuclei persists after local injections of GABA(A) antagonists

Limb movement-related neurons in the cerebellar nuclei (CN) typically produce bursts of discharge in association with movement. Consequently, given the inhibitory nature of the Purkinje cell (PC) projection to CN, it is puzzling that only a minority of movement-related PCs pause; the majority burst. Some of the movement-related CN activity may be the result of excitation from collaterals of mossy and climbing fiber projections to the cerebellar cortex. The only other input to CN is diffuse and neuromodulatory, from locus ceruleus and raphe nuclei. To investigate the role of the excitatory mossy fiber input, single units in CN were recorded in macaque monkeys during the performance of reaching and manipulation tasks, before and after blocking the PC input with local microinjections of GABA(A) antagonists (bicuculline or SR95531). After these injections, the movement-related modulation of CN discharge was greater and began earlier, compared with the modulation in the preinjection group of neurons. These observations indicate that an important excitatory drive is provided by extracerebellar inputs to CN, most likely from collaterals of mossy fibers. PCs may serve primarily to regulate this activity, by either pausing or bursting as necessary.

Effects of treadmill running on brain activation and the corticotropin-releasing hormone system

The present study was conducted to investigate the effects of treadmill running on the corticotropin-releasing hormone (CRH), CRH receptor type 1 (CRH-R1) and CRH-binding protein (CRH-BP) in the brain of rats that were killed either at rest, immediately after 60 min of treadmill running, or 180 min following a 60-min session of intensive exercise. The expression of the neuronal activity marker c-FOS was also determined in the three conditions of this study. The levels of c-FOS mRNA immediately following running were high in the cortex, caudate-putamen, lateral septum, bed nucleus of the stria terminalis, dorsal and medial thalamus, hypothalamus, pontine nuclei, locus coeruleus and hypoglossal nucleus. In most brain regions investigated, excluding the locus coeruleus and the cingulate cortex, c-FOS mRNA expression returned to control levels after 2 h of recovery. The highest concentration of cells co-expressing the protein Fos and CRH mRNA neurons was found in the parvocellular part of the paraventricular nucleus, which also expressed CRH heteronuclear RNA and CRH-R1 mRNA. The medial preoptic area (MPOA), the medial mammillary nucleus and the posterior hypothalamic as well as the somatosensory cortex, the medial geniculate nucleus, the reticulotegmental nucleus, and Barrington's nucleus also co-expressed Fos and CRH mRNA. The expression of CRH-BP gene was induced in the MPOA following running. In summary, the present study demonstrates that treadmill running leads to a strong expression of c-FOS mRNA that is widely distributed throughout the brain. c-FOS mRNA was found in structures of the somatosensory and somatomotor systems, indicating that these regions were activated during exercise. The pattern of distribution of c-FOS mRNA showed similarities with that triggered by neurogenic and systemic stresses. The present results also indicate that treadmill running can strongly activate the hypophysiotropic CRH system, which suggests, in agreement with the pattern of c-FOS mRNA distribution, that treadmill running has a strong stress component.

Ultrastructural study of the GABAergic and cerebellar input to the nucleus reticularis tegmenti pontis

The nucleus reticularis tegmenti pontis is an intermediate of the cerebrocerebellar pathway and serves as a relay centre for sensorimotor and visual information. The central nuclei of the cerebellum provide a dense projection to the nucleus reticularis tegmenti pontis, but it is not known to what extent this projection is excitatory or inhibitory, and whether the terminals of this projection contact the neurons in the nucleus reticularis tegmenti pontis that give rise to the mossy fibre collaterals innervating the cerebellar nuclei. In the present study the nucleus reticularis tegmenti pontis of the cat was investigated at the ultrastructural level following anterograde and retrograde transport of wheat germ agglutinin coupled to horseradish peroxidase (WGA-HRP) from the cerebellar nuclei combined with postembedding GABA immunocytochemistry. The neuropil of this nucleus was found to contain many WGA-HRP labeled terminals, cell bodies and dendrites, but none of these pre- or postsynaptic structures was double labeled with GABA. The vast majority of the WGA-HRP labeled terminals contained clear spherical vesicles, showed asymmetric synapses, and contacted intermediate or distal dendrites. Many of the postsynaptic elements of the cerebellar afferents in the nucleus reticularis tegmenti pontis were retrogradely labeled with WGA-HRP, while relatively few were GABAergic. We conclude that all cerebellar terminals in the nucleus reticularis tegmenti pontis of the cat are nonGABAergic and excitatory, and that they contact predominantly neurons that project back to the cerebellum. Thus, the reciprocal circuit between the cerebellar nuclei and the nucleus reticularis tegmenti pontis appears to be well designed to function as an excitatory reverberating loop.

Cerebellar nuclear projections from the basilar pontine nuclei and nucleus reticularis tegmenti pontis as demonstrated with PHA-L tracing in the rat

Small iontophoretic placements of the orthogradely transported axonal tracer Phaseolus vulgaris leucoagglutinin (PHA-L) were made in portions of the basilar pontine nuclei (BPN) or nucleus reticularis tegmenti pontis (NRTP) to determine if these cell groups provide projections to the cerebellar nuclei (CN) in the rat and if so, to visualize the morphology of the axons and terminals and illustrate any topographical organization in this system. Axons that originated from BPN or NRTP neurons and contained PHA-L were visualized by an immunohistochemical procedure that involved sequential incubation of tissue sections with goat anti-PHA-L antibody, biotinylated rabbit anti-goat immunoglobulin, and a biotin-avidin-peroxidase conjugate. Following injections of PHA-L restricted to ventral and medial portions of the BPN, labeled fibers were observed in the brachium pontis, the white matter dorsal to the CN, and to a lesser extent, in the white matter of the parafloccular stalk. Labeled preterminal axons entered the CN and gave rise to beaded axons that arborized primarily within dorsal portions of the lateral, interposed, and medial cerebellar nuclei. Injections of PHA-L restricted to either lateral portions of the BPN or ventrolateral regions of NRTP produced labeled fibers in the cerebellum that most frequently involved the parafloccular stalk and ventral portions of the CN. In contrast, dorsomedial NRTP injections resulted in the presence of labeled fibers both in the dorsal cerebellar white matter and the parafloccular stalk as well as dorsal and ventral portions of the CN. With the exception of the rostral and medial territory of interpositus anterior which received very sparse input, all portions of each CN subdivision seemed to exhibit some degree of terminal labeling. The density of labeled axon terminals in the CN appeared to be somewhat greater in the NRTP-injected cases compared to BPN-injected animals. These observations indicate that in the rat, both the BPN and NRTP contain neurons whose axons distribute to the CN. It is likely that most of the axons which project to the CN are collaterals of fibers that continue into the cerebellar cortex as mossy fibers but confirmation of this point must await further investigation. In light of the extensive projections from the cerebral cortex to the BPN and NRTP, this axonal system provides the cerebral cortex with a relatively direct route of access to the CN via one synapse in the BPN or NRTP.

Distributed motor commands in the limb premotor network

Neuroanatomical studies have demonstrated extensive interconnections between the motor cortex, red nucleus and cerebellum, forming a premotor network for controlling limb movement. Single-unit studies indicate that command signals for limb movements are distributed broadly throughout this network. Cellular studies have demonstrated multiple recurrent loops in this network, and the presence of excitatory and inhibitory amino acid neurotransmitters. A recent model suggests that movement commands are initiated by sensory inputs to these loops, and that positive feedback, regulated by inhibition from cerebellar Purkinje cells, distributes commands throughout the limb premotor network. This model offers a new framework for exploring relationships between basic neural mechanisms and concepts of motor performance that derive from experimental psychology.