Convergence of cortical and cerebellar projections on single basilar pontine neurons: a light and electron microscopic study in the rat

Paradoxical kinesia may no longer be a paradox waiting for 100 years to be unraveled

Parkinson's disease (PD) is a progressive neurodegenerative disorder mainly characterized by bradykinesia and akinesia. Interestingly, these motor disabilities can depend on the patient emotional state. Disabled PD patients remain able to produce normal motor responses in the context of urgent or externally driven situations or even when exposed to appetitive cues such as music. To describe this phenomenon Souques coined the term "paradoxical kinesia" a century ago. Since then, the mechanisms underlying paradoxical kinesia are still unknown due to a paucity of valid animal models that replicate this phenomenon. To overcome this limitation, we established two animal models of paradoxical kinesia. Using these models, we investigated the neural mechanisms of paradoxical kinesia, with the results pointing to the inferior colliculus (IC) as a key structure. Intracollicular electrical deep brain stimulation, glutamatergic and GABAergic mechanisms may be involved in the elaboration of paradoxical kinesia. Since paradoxical kinesia might work by activation of some alternative pathway bypassing basal ganglia, we suggest the IC as a candidate to be part of this pathway.

Mesodiencephalic junction GABAergic inputs are processed separately from motor cortical inputs in the basilar pons

The basilar pontine nuclei (bPN) are known to receive excitatory input from the entire neocortex and constitute the main source of mossy fibers to the cerebellum. Various potential inhibitory afferents have been described, but their origin, synaptic plasticity, and network function have remained elusive. Here we identify the mesodiencephalic junction (MDJ) as a prominent source of monosynaptic GABAergic inputs to the bPN. We found no evidence that these inputs converge with motor cortex (M1) inputs at the single neuron or at the local network level. Tracing the inputs to GABAergic MDJ neurons revealed inputs to these neurons from neocortical areas. Additionally, we observed little short-term synaptic facilitation or depression in afferents from the MDJ, enabling MDJ inputs to carry sign-inversed neocortical inputs. Thus, our results show a prominent source of GABAergic inhibition to the bPN that could enrich input to the cerebellar granule cell layer.

Corollary discharge in precerebellar nuclei of sleeping infant rats

In week-old rats, somatosensory input arises predominantly from external stimuli or from sensory feedback (reafference) associated with myoclonic twitches during active sleep. A previous study suggested that the brainstem motor structures that produce twitches also send motor copies (or corollary discharge, CD) to the cerebellum. We tested this possibility by recording from two precerebellar nuclei—the inferior olive (IO) and lateral reticular nucleus (LRN). In most IO and LRN neurons, twitch-related activity peaked sharply around twitch onset, consistent with CD. Next, we identified twitch-production areas in the midbrain that project independently to the IO and LRN. Finally, we blocked calcium-activated slow potassium (SK) channels in the IO to explain how broadly tuned brainstem motor signals can be transformed into precise CD signals. We conclude that the precerebellar nuclei convey a diversity of sleep-related neural activity to the developing cerebellum to enable processing of convergent input from CD and reafferent signals.

Non-competitive antagonists of NMDA and AMPA receptors decrease seizure-induced c-fos protein expression in the cerebellum and protect against seizure symptoms in adult rats

The aim of the present study was to examine the role of ionotropic glutamate receptors in the cerebellum during generalized seizures. Epileptic neuronal activation was evaluated through the immunohistochemical detection of c-fos protein in the cerebellar cortex. Generalized seizures were precipitated by the intraperitoneal injection of 4-aminopyridine. The animals were pretreated with the NMDA receptor antagonists MK-801 (2 mg/kg), amantadine (50 mg/kg), and the AMPA receptor antagonist GYKI 52466 hydrochloride (50 mg/kg). Two hours after 4-aminopyridine injection, the number of c-fos immunostained cell nuclei was counted in serial immunohistochemical sections of the cerebellar vermis. The number of c-fos immunostained cell nuclei in the granular layer decreased significantly in animals pretreated with the glutamate receptor antagonists compared to the untreated animals having convulsion. We can conclude that mossy fiber stimulation exerts its seizure-generating action mainly through the ionotropic glutamate receptors of the mossy fiber synapses. Both NMDA and AMPA receptor antagonists are effective in reducing glutamate-mediated postsynaptic effects in the cerebellar cortex.

Chronic In Vivo Imaging of Ponto-Cerebellar Mossy Fibers Reveals Morphological Stability during Whisker Sensory Manipulation in the Adult Rat

The cerebellum receives extensive disynaptic input from the neocortex via the basal pontine nuclei, the neurons of which send mossy fiber (MF) axons to the granule cell layer of the contralateral cerebellar hemisphere. Although this cortico-cerebellar circuit has been implicated in tasks such as sensory discrimination and motor learning, little is known about the potential role of MF morphological plasticity in the function of the cerebellar granule cell layer. To address this issue, we labeled MFs with EGFP via viral infection of the basal pons in adult rats and performed in vivo two-photon imaging of MFs in Crus I/II of the cerebellar hemisphere over a period of several weeks. Following the acquisition of baseline images, animals were housed in control, enriched, or deprived sensory environments. Morphological dynamics were assessed by tracing MF axons and their terminals, and by tracking the stability of filopodia arising from MF terminal rosettes. MF axons and terminals were found to be remarkably stable. Parameters derived neither from measurements of axonal arbor geometry nor from the morphology of individual rosettes and their filopodial extensions significantly changed under control conditions over 4 weeks of imaging. Increasing whisker stimulation by manipulating the sensory environment or decreasing such stimulation by whisker trimming also failed to alter MF structure. Our studies indicate that pontine MF axons projecting to Crus I/II in adult rats do not undergo significant structural rearrangements over the course of weeks, and that this stability is not altered by the sustained manipulation of whisker sensorimotor experience.

Amygdala conditioning modulates sensory input to the cerebellum

Localization of emotional learning in the amygdala and discrete motor learning in the cerebellum provides empirical means to study the mechanisms mediating the interaction between fast emotional and slow motor learning. Behavioral studies have demonstrated that fear conditioning facilitates the motor conditioning. The present study tests the hypothesis that the amygdala output induces this facilitation by increasing the salience of the conditioned stimulus (CS) representation in the pontine nucleus (PN) input to the cerebellum. Paired trials of CS-US (unconditioned stimulus) were applied to anesthetized rats, a condition that allows for amygdala-based fear conditioning but not cerebellar-based motor conditioning. Multiple unit recordings in the PN served to assess the salience of the CS. Results showed that CS-US conditioning increased the PN-reactivity to the CS. Lidocaine-induced reversible inactivation of the amygdala prevented the facilitatory effect of conditioning on the PN-reactivity to the CS. These findings suggest that the amygdala-based conditioned responses reach the PN and increase the salience of the CS signal there, perhaps facilitating cerebellar conditioning. This facilitatory effect of the amygdala may be conceptualized under the 'two-stage theory of learning', which predicts that emotional learning in the first stage accelerates the motor learning in the second stage. We hereby demonstrate the physiological mechanism through which fast emotional learning in the first stage facilitates slow cerebellar learning in the second stage.

Topography of the complete corticopontine projection: from experiments to principal Maps

The mammalian brain is characterized by orderly spatial distribution of its cellular components, commonly referred to as topographical organization. The topography of cortical and subcortical maps is thought to represent functional or computational properties. In the present investigation, we have studied map transformations and organizing principles in the projections from the cerebral cortex to the pontine nuclei, with emphasis on the mapping of the cortex as a whole onto the pontine nuclei. Following single or multiple axonal tracer injections into different cortical regions, three-dimensional (3-D) distributions of anterogradely labeled axons in the pontine nuclei were mapped. All 3-D reconstructed data sets were normalized to a standardized local coordinate system for the pontine nuclei and uploaded in a database application (FACCS, Functional Anatomy of the Cerebro-Cerebellar System, available via The Rodent Brain Workbench, http://www.rbwb.org). The database application allowed flexible use of the data in novel combinations, and use of a previously published data sets. Visualization of different combinations of data was used to explore alternative principles of organization. As a result of these analyses, a principal map of the topography of corticopontine projections was developed. This map followed the organization of early spatiotemporal gradients present in the cerebral cortex and the pontine nuclei. With the principal map for corticopontine projections, a fairly accurate prediction of pontine target area can be made for any site of origin in the cerebral cortex. The map and the underlying shared data sets represent a basis for modeling of topographical organization and structure-function relationships in this system.

Focal projections of cat auditory cortex to the pontine nuclei

The pontine nuclei (PN) receive projections from the auditory cortex (AC) and they are a major source of mossy fibers to the cerebellum. However, they have not been studied in detail using sensitive neuroanatomical tracers, and whether all AC areas contribute to the corticopontine (CP) system is unknown. We characterized the projection patterns of 11 AC areas with WGA-HRP. We also compared them with their corticothalamic and corticocollicular counterparts. A third objective was to analyze the structure of the CP axons and their terminals with BDA. Both tracers confirm that all AC areas projected to lateral, central, and medial ipsilateral pontine divisions. The strongest CP projections were from nontonotopic and polymodal association areas. Preterminal fibers formed single terminal fields having many boutons en passant as well as terminal endings, and there was a specific morphological pattern for each pontine target, irrespective of their areal origin. Thus, axons in the medial division had a simpler terminal architecture (type 1 terminal plexus); both the central and lateral pons received more complex endings (type 2 terminal plexus). Auditory CP topographical distribution resembled visual and somatosensory CP projections, which preserve retinotopy and somatotopy in the pons, respectively. However, the absence of pontine tonotopy suggests that the AC projection topography is unrelated to tonotopy. CP input to the medial and central pons coincides with the somatosensory and visual cortical inputs, respectively, and such overlap might subserve convergence in the cerebellum. In contrast, lateral pontine input may be exclusively auditory.

Three-dimensional topography of corticopontine projections from rat barrel cortex: correlations with corticostriatal organization

Subcortical re-entrant projection systems connecting cerebral cortical areas with the basal ganglia and cerebellum are topographically specific and therefore considered to be parallel circuits or "closed loops." The precision of projections within these circuits, however, has not been characterized sufficiently to indicate whether cortical signals are integrated within or among presumed compartments. To address this issue, we studied the first link of the rat cortico-ponto-cerebellar pathway with anterograde axonal tracing from physiologically defined, individual whisker "barrels" of the primary somatosensory cortex (SI). The labeled axons in the pontine nuclei formed several, sharply delineated clusters. Dual tracer injections into different SI whisker barrels gave rise to partly overlapping, paired clusters, indicating somatotopic specificity. Three-dimensional reconstructions revealed that the clusters were components of concentrically organized lamellar subspaces. Whisker barrels in the same row projected to different pontine lamellae (side by side), the somatotopic representation of which followed an inside-out sequence. By contrast, whisker barrels from separate rows projected to clusters located within the same lamellar subspace (end to end). In the neostriatum, this lamellar topography was the opposite, with barrels in the same row contacting different parts of the same lamellar subspace (end to end). The degree of overlap among pontine clusters varied as a function of the proximity of the cortical injections. Furthermore, corticopontine overlap was higher among projections from barrels in the same row than among projections from different whisker barrel rows. This anisotropy was the same in the corticostriatal projection. These findings have important implications for understanding convergence and local integration in somatosensory-related subcortical circuits.

Rat somatosensory cerebropontocerebellar pathways: spatial relationships of the somatotopic map of the primary somatosensory cortex are preserved in a three-dimensional clustered pontine map

In the primary somatosensory cortex (SI), the body surface is mapped in a relatively continuous fashion, with adjacent body regions represented in adjacent cortical domains. In contrast, somatosensory maps found in regions of the cerebellar hemispheres, which are influenced by the SI through a monosynaptic link in the pontine nuclei, are discontinuous ("fractured") in organization. To elucidate this map transformation, the authors studied the organization of the first link in the SI-cerebellar pathway, the SI-pontine projection. After injecting anterograde axonal tracers into electrophysiologically defined parts of the SI, three-dimensional reconstruction and computer-graphic visualization techniques were used to analyze the spatial distribution of labeled fibers. Several target regions in the pontine nuclei were identified for each major body representation. The labeled axons formed sharply delineated clusters that were distributed in an inside-out, shell-like fashion. Upper lip and other perioral representations were located in a central core, whereas extremity and trunk representations were found more externally. The multiple clusters suggest that the pontine nuclei contain several representations of the SI map. Within each representation, the spatial relationships of the SI map are largely preserved. This corticopontine projection pattern is compatible with recently proposed principles for the establishment of subcortical topographic patterns during development. The largely preserved spatial relationships in the pontine somatotopic map also suggest that the transformation from an organized topography in SI to a fractured map in the cerebellum takes place primarily in the mossy fiber pontocerebellar projection.

Binding of signals relevant for action: towards a hypothesis of the functional role of the pontine nuclei

If numbers matter, the projection that connects the cerebral cortex to the cerebellum is probably one of the most-important pathways through the CNS. Its extensive development as one ascends the phylogenetic scale parallels that of the cerebral hemispheres and the cerebellum, and it accompanies improvements in motor skills, suggesting that this system might have a decisive role in the generation of skilled movement. This article focuses on the pontine nuclei (PN), which are intercalated in the cerebro-cerebellar pathway, a large nuclear complex in the ventral brainstem of mammals, whose raison d'être has as yet not been examined. By considering recent morphological and electrophysiological findings, this article argues that the PN are an interface that is needed to accommodate the grossly different computational principles governing the cerebral cortex and the cerebellum.

Electrophysiological properties of rat pontine nuclei neurons In vitro II. Postsynaptic potentials

We investigated the postsynaptic responses of neurons of the rat pontine nuclei (PN) by performing intracellular recordings in parasagittal slices of the pontine brain stem. Postsynaptic potentials (PSPs) were evoked by brief (0.1 ms) negative current pulses (10-250 microA) applied to either the cerebral peduncle or the pontine tegmentum. First, excitatory postsynaptic potentials (EPSPs) could be evoked readily from peduncular stimulation sites. These EPSPs exhibited short latencies, a nonlinear increment in response to increased stimulation currents, and an unconventional dependency on the somatic membrane potential. Pharmacological blockade of the synaptic transmission using 6,7-dinitroquinoxaline-2, 3-dione and ,-2-amino-5-phosphonovaleric acid, selective antagonists of the alpha-amino-3-hydroxy-5-methyl-4-isoxazilepropionate- (AMPA) and the N-methyl--aspartate (NMDA)-type glutamate receptors, showed that these EPSPs were mediated exclusively by excitatory amino acids via both AMPA and NMDA receptors. Moreover, the pharmacological experiments indicated the existence of voltage-sensitive but NMDA receptor-independent amplification of EPSPs. Second, stimulations at peduncular and tegmental sites also elicited inhibitory postsynaptic potentials (IPSPs) in a substantial proportion of pontine neurons. The short latencies of all IPSPs argued against the participation of inhibitory interneurons. Their sensitivity to bicuculline and reversal potentials around -70 mV suggested that they were mediated by gamma-aminobutyric acid-A (GABAA) receptors. In addition to single PSPs, sequences consisting of two to four distinct EPSPs could be recorded after stimulation of the cerebral peduncle. Most remarkably, the onset latencies of the following EPSPs were multiples of the first one indicating the involvement of intercalated synapses. Finally, we used the classic paired-pulse paradigm to study whether the temporal structure of inputs influences the synaptic transmission onto pontine neurons. Pairs of electrical stimuli applied to the cerebral peduncle resulted in a marked enhancement of the amplitude of the second EPSP for interstimulus intervals of 10-100 ms. Delays >200 ms left the EPSP amplitude unaltered. These data provide evidence for a complex synaptic integration and an intrinsic connectivity within the PN too elaborate to support the previous notion that the PN are simply a relay station.

Comparison of projection neurons in the pontine nuclei and the nucleus reticularis tegmenti pontis of the rat

Dendritic features of identified projection neurons in two precerebellar nuclei, the pontine nuclei (PN) and the nucleus reticularis tegmenti pontis (NRTP) were established by using a combination of retrograde tracing (injection of fluorogold or rhodamine labelled latex micro-spheres into the cerebellum) with subsequent intracellular filling (lucifer yellow) in fixed slices of pontine brainstem. A multivariate analysis revealed that parameters selected to characterize the dendritic tree such as size of dendritic field, number of branching points, and length of terminal dendrites did not deviate significantly between different regions of the PN and the NRTP. On the other hand, projection neurons in ventral regions of the PN were characterized by an irregular coverage of their distal dendrites by appendages while those in the dorsal PN and the NRTP were virtually devoid of them. The NRTP, dorsal, and medial PN tended to display larger somata and more primary dendrites than ventral regions of the PN. These differences, however, do not allow the differentiation of projection neurons within the PN from those in the NRTP. They rather reflect a dorso-ventral gradient ignoring the border between the nuclei. Accordingly, a cluster analysis did not differentiate distinct types of projection neurons within the total sample. In both nuclei, multiple linear regression analysis revealed that the size of dendritic fields was strongly correlated with the length of terminal dendrites while it did not depend on other parameters of the dendritic field. Thus, larger dendritic fields seem not to be accompanied by a higher complexity but rather may be used to extend the reach of a projection neuron within the arrangement of afferent terminals. We suggest that these similarities within dendritic properties in PN and NRTP projection neurons reflect similar processing of afferent information in both precerebellar nuclei.

Cerebellar and brainstem circuits involved in classical eyeblink conditioning

Model systems are one useful strategy for the investigation of the mechanisms of learning. Whereas mammalian model systems generally do not offer the ease of identifying circuitry and exploring cellular mechanisms of learning that is realized with invertebrate preparations /37,97/, research involving the rabbit classical eyeblink conditioning paradigm has now reached the state at which much of the basic conditioning neural circuit appears to have been identified /9,65,66,85,89,91/. Despite a dispute as to precisely where in the circuitry convergence of the associated stimuli may occur, there is substantial evidence identifying the stimulus input pathways and motor output pathway. The present summary of this research details these paths. In addition, the proposed sites of convergence of the conditioning stimuli are discussed. Finally, a hypothesized neural circuit responsible for classical eyeblink conditioning is presented along with some suggestions for future research directions.

Differential labeling of converging afferent pathways using biotinylated dextran amine and cholera toxin subunit B

We report a new technique for 2-tracer anterograde labeling that permits unequivocal identification of the differentially labeled projections in the same section. One pathway is labeled with biotinylated dextran amine and is visualized as a black to dark gray diaminobenzidine (DAB)-cobalt precipitate by an avidin-biotinylated peroxidase reaction. The other pathway is labeled with cholera toxin subunit B and is visualized as a reddish-brown reaction product using DAB without cobalt as the substrate for peroxidase immunohistochemistry. To maintain serial order, sections can be processed mounted on slides without any loss of sensitivity for either tracer.

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.