Brainstem mossy fiber projections to lobules VIa, VIb,c, VII and VIII of the cerebellar vermis in the rat

Dopaminergic regulation of vestibulo-cerebellar circuits through unipolar brush cells

While multiple monoamines modulate cerebellar output, the mechanistic details of dopaminergic signaling in the cerebellum remain poorly understood. We show that dopamine type 1 receptors (Drd1) are expressed in unipolar brush cells (UBCs) of the mouse cerebellar vermis. Drd1 activation increases UBC firing rate and post-synaptic NMDAR -mediated currents. Using anatomical tracing and in situ hybridization, we test three hypotheses about the source of cerebellar dopamine. We exclude midbrain dopaminergic nuclei and tyrosine hydroxylase-positive Purkinje (Pkj) cells as potential sources, supporting the possibility of dopaminergic co-release from locus coeruleus (LC) axons. Using an optical dopamine sensor GRABDA2h, electrical stimulation, and optogenetic activation of LC fibers in the acute slice, we find evidence for monoamine release onto Drd1-expressing UBCs. Altogether, we propose that the LC regulates cerebellar cortex activity by co-releasing dopamine onto UBCs to modulate their response to cerebellar inputs. Pkj cells directly inhibit these Drd1-positive UBCs, forming a dopamine-sensitive recurrent vestibulo-cerebellar circuit.

Disynaptic cerebrocerebellar pathways originating from multiple functionally distinct cortical areas

The cerebral cortex and cerebellum both play important roles in sensorimotor processing, however, precise connections between these major brain structures remain elusive. Using anterograde mono-trans-synaptic tracing, we elucidate cerebrocerebellar pathways originating from primary motor, sensory, and association cortex. We confirm a highly organized topography of corticopontine projections in mice; however, we found no corticopontine projections originating from primary auditory cortex and detail several potential extra-pontine cerebrocerebellar pathways. The cerebellar hemispheres were the major target of resulting disynaptic mossy fiber terminals, but we also found at least sparse cerebrocerebellar projections to every lobule of the cerebellum. Notably, projections originating from association cortex resulted in less laterality than primary sensory/motor cortices. Within molecularly defined cerebellar modules we found spatial overlap of mossy fiber terminals, originating from functionally distinct cortical areas, within crus I, paraflocculus, and vermal regions IV/V and VI - highlighting these regions as potential hubs for multimodal cortical influence.

Modular output circuits of the fastigial nucleus for diverse motor and nonmotor functions of the cerebellar vermis

The cerebellar vermis, long associated with axial motor control, has been implicated in a surprising range of neuropsychiatric disorders and cognitive and affective functions. Remarkably little is known, however, about the specific cell types and neural circuits responsible for these diverse functions. Here, using single-cell gene expression profiling and anatomical circuit analyses of vermis output neurons in the mouse fastigial (medial cerebellar) nucleus, we identify five major classes of glutamatergic projection neurons distinguished by gene expression, morphology, distribution, and input-output connectivity. Each fastigial cell type is connected with a specific set of Purkinje cells and inferior olive neurons and in turn innervates a distinct collection of downstream targets. Transsynaptic tracing indicates extensive disynaptic links with cognitive, affective, and motor forebrain circuits. These results indicate that diverse cerebellar vermis functions could be mediated by modular synaptic connections of distinct fastigial cell types with posturomotor, oromotor, positional-autonomic, orienting, and vigilance circuits.

Coexpression of VGLUT1 and VGLUT2 in precerebellar neurons in the lateral reticular nucleus of the rat

Vesicular glutamate transporter (VGLUT) 1 and VGLUT2 have been reported to distribute complementally in most brain regions and have been assumed to define distinct functional elements. Previous studies have shown the expression of VGLUT1 mRNA and VGLUT2 mRNA in the lateral reticular nucleus (LRN), a key precerebellar nucleus sending mossy fibers to the cerebellum. In the present study, we firstly examined the coexpression of VGLUT1 and VGLUT2 mRNA in the LRN of the rat by dual-fluorescence in situ hybridization. About 81.89 % of glutamatergic LRN neurons coexpressed VGLUT1 and VGLUT2 mRNA, and the others expressed either VGLUT1 or VGLUT2 mRNA. We then injected the retrograde tracer Fluogold (FG) into the vermal cortex of cerebellum, and observed that 95.01 % and 86.80 % of FG-labeled LRN neurons expressed VGLUT1 or VGLUT2 mRNA respectively. We further injected the anterograde tracer biotinylated dextran amine (BDA) into the LRN, and found about 82.6 % of BDA labeled axon terminals in the granular layer of cerebellar cortex showed both VGLUT1- and VGLUT2-immunoreactivities. Afterwards, we observed under electron microscopy that anterogradely labeled axon terminals showing immunoreactivity for VGLUT1 or VGLUT2 made asymmetric synapses with dendritic profiles of cerebellar neurons. Finally, we selectively down-regulated the expression of VGLUT1 mRNA or VGLUT2 mRNA by using viral vector mediated siRNA transfection and detected that the fine movements of the forelimb of rats were disturbed. These results indicated that LRN neurons coexpressing VGLUT1 and VGLUT2 project to the cerebellar cortex and these neurons might be critical in mediating the forelimb movements.

Sensorimotor Coding of Vermal Granule Neurons in the Developing Mammalian Cerebellum

The vermal cerebellum is a hub of sensorimotor integration critical for postural control and locomotion, but the nature and developmental organization of afferent information to this region have remained poorly understood in vivo Here, we use in vivo two-photon calcium imaging of the vermal cerebellum in awake behaving male and female mice to record granule neuron responses to diverse sensorimotor cues targeting visual, auditory, somatosensory, and motor domains. Use of an activity-independent marker revealed that approximately half (54%) of vermal granule neurons were activated during these recordings. A multikernel linear model distinguished the relative influences of external stimuli and co-occurring movements on neural responses, indicating that, among the subset of activated granule neurons, locomotion (44%-56%) and facial air puffs (50%) were more commonly and reliably encoded than visual (31%-32%) and auditory (19%-28%) stimuli. Strikingly, we also uncover populations of granule neurons that respond differentially to voluntary and forced locomotion, whereas other granule neurons in the same region respond similarly to locomotion in both conditions. Finally, by combining two-photon calcium imaging with birth date labeling of granule neurons via in vivo electroporation, we find that early- and late-born granule neurons convey similarly diverse sensorimotor information to spatially distinct regions of the molecular layer. Collectively, our findings elucidate the nature and developmental organization of sensorimotor information in vermal granule neurons of the developing mammalian brain.SIGNIFICANCE STATEMENT Cerebellar granule neurons comprise over half the neurons in the brain, and their coding properties have been the subject of theoretical and experimental interest for over a half-century. In this study, we directly test long-held theories about encoding of sensorimotor stimuli in the cerebellum and compare the in vivo coding properties of early- and late-born granule neurons. Strikingly, we identify populations of granule neurons that differentially encode voluntary and forced locomotion and find that, although the birth order of granule neurons specifies the positioning of their parallel fiber axons, both early- and late-born granule neurons convey a functionally diverse sensorimotor code. These findings constitute important conceptual advances in understanding the principles underlying cerebellar circuit development and function.

Purkinje Cells Directly Inhibit Granule Cells in Specialized Regions of the Cerebellar Cortex

Inhibition of granule cells plays a key role in gating the flow of signals into the cerebellum, and it is thought that Golgi cells are the only interneurons that inhibit granule cells. Here we show that Purkinje cells, the sole output neurons of the cerebellar cortex, also directly inhibit granule cells via their axon collaterals. Anatomical and optogenetic studies indicate that this non-canonical feedback is region specific: it is most prominent in lobules that regulate eye movement and process vestibular information. Collaterals provide fast, slow, and tonic inhibition to granule cells, and thus allow Purkinje cells to regulate granule cell excitability on multiple timescales. We propose that this feedback mechanism could regulate excitability of the input layer, contribute to sparse coding, and mediate temporal integration.

How the cerebellum may monitor sensory information for spatial representation

The cerebellum has already been shown to participate in the navigation function. We propose here that this structure is involved in maintaining a sense of direction and location during self-motion by monitoring sensory information and interacting with navigation circuits to update the mental representation of space. To better understand the processing performed by the cerebellum in the navigation function, we have reviewed: the anatomical pathways that convey self-motion information to the cerebellum; the computational algorithm(s) thought to be performed by the cerebellum from these multi-source inputs; the cerebellar outputs directed toward navigation circuits and the influence of self-motion information on space-modulated cells receiving cerebellar outputs. This review highlights that the cerebellum is adequately wired to combine the diversity of sensory signals to be monitored during self-motion and fuel the navigation circuits. The direct anatomical projections of the cerebellum toward the head-direction cell system and the parietal cortex make those structures possible relays of the cerebellum influence on the hippocampal spatial map. We describe computational models of the cerebellar function showing that the cerebellum can filter out the components of the sensory signals that are predictable, and provides a novelty output. We finally speculate that this novelty output is taken into account by the navigation structures, which implement an update over time of position and stabilize perception during navigation.

Noradrenergic control of neuronal firing in cerebellar nuclei: modulation of GABA responses

The effects of noradrenaline (NA) on inhibitory responses to gamma aminobutyric acid (GABA) in neurones of the deep cerebellar nuclei were studied in vivo in rats, using extracellular single-unit recordings and microiontophoretic drug application. NA application altered GABA-evoked responses in 95 % of the neurones tested, but the effects differed between nuclei. Application of NA depressed GABA responses in the medial (MN) and posterior interpositus (PIN) nuclei, but enhanced GABA responses in the anterior interpositus nucleus (AIN). Comparable proportions of enhancing (57 %) and depressive (43 %) effects were found in the lateral nucleus (LN). The alpha2 noradrenergic receptor agonist clonidine mimicked the depressive effect of NA on GABA responses in MN and PIN and its enhancing effects in AIN and LN, while the alpha2 antagonist yohimbine partially blocked these effects. The beta-adrenergic agonist isoproterenol and antagonist timolol respectively induced and partially blocked enhancements of GABA responses in all nuclei except for LN, where isoproterenol had a weak depressive effect. It is concluded that NA modulates GABA responses by acting on both alpha2 and beta receptors. Activation of these receptors appears to be synergistic in the AIN and opposite in the remaining deep nuclei. These results support the hypothesis that the noradrenergic system participates in all the regulatory functions involving the cerebellum in a specific and differential manner, and suggest that any change in NA content, as commonly observed in ageing or stress, could influence cerebellar activity.

Development of the serotonergic cells in murine raphe nuclei and their relations with rhombomeric domains

The raphe nuclei represent the origin of central serotonergic projections. The literature distinguishes seven nuclei grouped into rostral and caudal clusters relative to the pons. The boundaries of these nuclei have not been defined precisely enough, particularly with regard to developmental units, notably hindbrain rhombomeres. We hold that a developmental point of view considering rhombomeres may explain observed differences in connectivity and function. There are twelve rhombomeres characterized by particular genetic profiles, and each develops between one and four distinct serotonergic populations. We have studied the distribution of the conventional seven raphe nuclei among these twelve units. To this aim, we correlated 5-HT-immunoreacted neurons with rhombomeric boundary landmarks in sagittal mouse brain sections at different developmental stages. Furthermore, we performed a partial genoarchitectonic analysis of the developing raphe nuclei, mapping all known serotonergic differentiation markers, and compared these results, jointly with others found in the literature, with our map of serotonin-containing populations, in order to examine regional variations in correspondence. Examples of regionally selective gene patterns were identified. As a result, we produced a rhombomeric classification of some 45 serotonergic populations, and suggested a corresponding modified terminology. Only a minor rostral part of the dorsal raphe nucleus lies in the midbrain. Some serotonergic neurons were found in rhombomere 4, contrary to the conventional assumption that it lacks such neurons. We expect that our reclassification of raphe nuclei may be useful for causal analysis of their differential molecular specification, as well as for studies of differential connectivity and function.

An internal model architecture for novelty detection: implications for cerebellar and collicular roles in sensory processing

The cerebellum is thought to implement internal models for sensory prediction, but details of the underlying circuitry are currently obscure. We therefore investigated a specific example of internal-model based sensory prediction, namely detection of whisker contacts during whisking. Inputs from the vibrissae in rats can be affected by signals generated by whisker movement, a phenomenon also observable in whisking robots. Robot novelty-detection can be improved by adaptive noise-cancellation, in which an adaptive filter learns a forward model of the whisker plant that allows the sensory effects of whisking to be predicted and thus subtracted from the noisy sensory input. However, the forward model only uses information from an efference copy of the whisking commands. Here we show that the addition of sensory information from the whiskers allows the adaptive filter to learn a more complex internal model that performs more robustly than the forward model, particularly when the whisking-induced interference has a periodic structure. We then propose a neural equivalent of the circuitry required for adaptive novelty-detection in the robot, in which the role of the adaptive filter is carried out by the cerebellum, with the comparison of its output (an estimate of the self-induced interference) and the original vibrissal signal occurring in the superior colliculus, a structure noted for its central role in novelty detection. This proposal makes a specific prediction concerning the whisker-related functions of a region in cerebellar cortical zone A₂ that in rats receives climbing fibre input from the superior colliculus (via the inferior olive). This region has not been observed in non-whisking animals such as cats and primates, and its functional role in vibrissal processing has hitherto remained mysterious. Further investigation of this system may throw light on how cerebellar-based internal models could be used in broader sensory, motor and cognitive contexts.

Dorsal column nuclei projection to the cerebellar caudal vermis in the rabbit revealed by the fluorescent double-labeling method

The organization of the projection from the dorsal column nuclei (DCN) to the lobules of the cerebellar caudal vermis was studied in the rabbit. Following unilateral injections of the retrograde fluorescent tracers fast blue (FB) and diamidino yellow (DY) into the pyramis (Pr) and uvula (Uv), respectively, a great number of single FB- (40%) and DY-labeled (60%) neurons were observed in the ipsilateral (79%) and contralateral (21%) DCN subdivisions. These neurons, as parents for the DCN-Pr and DCN-Uv projections, were numerous in the lateral cuneate nucleus (CuL; 84 and 74%, respectively) and in the complex of the gracile (Gr) and medial cuneate nuclei (CuM; Gr+CuM; 14 and 25%, respectively). A small percentage of the Pr projecting neurons was found in the CuM and Gr nuclei (2% in total). As regards the Uv, a rare and only ipsilateral projection arose from the CuM (1%), and no connection originated from the Gr. The distribution pattern of labeled neurons within individual subnuclei indicates that there are both separate regions and, to a great extent, common regions of the DCN-Pr and DCN-Uv projections. In these common regions, a small population of double FB+DY-labeled neurons (1.2%) was identified. Such neurons, present exclusively in the ipsilateral CuL and Gr+CuM, were the source of projection by way of axonal collaterals to the Pr and Uv simultaneously. It is suggested that the described connections may play a role in coordination of the axial and proximal forelimb muscles.

Spatial localization and projection densities of brainstem mossy fibre afferents to the forelimb C1 zone of the rat cerebellum

The present study uses a double retrograde tracer technique in rats to examine the spatial localization and pattern of axonal branching in mossy fibres arising from three major sources in the medulla-the external cuneate nucleus, the sensory trigeminal nucleus and the reticular formation, to two electrophysiologically-identified parts of the cerebellar cortex that are linked by common climbing fibre input - the forelimb-receiving parts of the C1 zone in lobulus simplex and the paramedian lobule. In each experiment a small injection of rhodamine-tagged beads was injected into one cortical region and an injection of fluorescein-tagged beads was injected into the other region. The main findings were: (i) the proportion of double-labelled cells in each of the three precerebeller sources of mossy fibres was positively correlated with those in the inferior olive; and (ii) the C1 zone in lobulus simplex was found to receive a greater density of projections from all three sources of mossy fibres than the C1 zone in the paramedian lobule. These data suggest that two rostrocaudally separated but somatotopically corresponding parts of the C1 zone receive common mossy fibre and climbing fibre inputs. However, the differences in projection densities also suggest that the two parts of the zone differ in the extent to which they receive mossy fibre signals arising from the same precerebellar nuclei. This implies differences in function between somatotopically corresponding parts of the same cortical zone, and could enable a higher degree of parallel processing and integration of information within them.

Patterns of fos activation in rat raphe nuclei during feeding behavior

To analyze the differential recruitment of the raphe nuclei during different phases of feeding behavior, rats were subjected to a food restriction schedule (food for 2 h/day, during 15 days). The animals were submitted to different feeding conditions, constituting the experimental groups: search for food (MFS), food ingestion (MFI), satiety (MFSa) and food restriction control (MFC). A baseline condition (BC) group was included as further control. The MFI and MFC groups, which presented greater autonomic and somatic activation, had more FOS-immunoreactive (FOS-IR) neurons. The MFI group presented more labeled cells in the linear (LRN) and dorsal (DRN) nuclei; the MFC group showed more labeling in the median (MRN), pontine (PRN), magnus (NRM) and obscurus (NRO) nuclei; and the MFSa group had more labeled cells in the pallidus (NRP). The BC exhibited the lowest number of reactive cells. The PRN presented the highest percentage of activation in the raphe while the DRN the lowest. Additional experiments revealed few double-labeled (FOS-IR+5-HT-IR) cells within the raphe nuclei in the MFI group, suggesting little serotonergic activation in the raphe during food ingestion. These findings suggest a differential recruitment of raphe nuclei during various phases of feeding behavior. Such findings may reflect changes in behavioral state (e.g., food-induced arousal versus sleep) that lead to greater motor activation, and consequently increased FOS expression. While these data are consistent with the idea that the raphe system acts as gain setter for autonomic and somatic activities, the functional complexity of the raphe is not completely understood.

Oculomotor cerebellum

The anatomical, physiological, and behavioral evidence for the involvement of three regions of the cerebellum in oculomotor behavior is reviewed here: (1) the oculomotor vermis and paravermis of lobules V, IV, and VII; (2) the uvula and nodulus; (3) flocculus and ventral paraflocculus. No region of the cerebellum controls eye movements exclusively, but each receives sensory information relevant for the control of multiple systems. An analysis of the microcircuitry suggests how sagittal climbing fiber zones bring visual information to the oculomotor vermis; convey vestibular information to the uvula and nodulus, while optokinetic space is represented in the flocculus. The mossy fiber projections are more heterogeneous. The importance of the inferior olive in modulating Purkinje cell responses is discussed.

Axonal ramification of neurons in the nucleus reticularis tegmenti pontis projecting to the paramedian lobule in the rabbit cerebellum

Projections of the nucleus reticularis tegmenti pontis (NRTP) to the cerebellar paramedian lobule were examined in the rabbit by means of the double fluorescent retrograde tract-tracing method. The rabbit NRTP is composed of a medial, large part comprising zones A (dorsomedial), B (central) and C (lateral), and of a lateral, small part (the processus tegmentosus lateralis; PTL). Following unilateral injections of Fast Blue (FB) into the rostral part of the paramedian lobule (rPML) and of Diamidino Yellow (DY) into the caudal part (cPML), known to receive spinal inputs from forelimb and hindlimb, respectively, substantial numbers of single labeled neurons were found in all bilateral NRTP divisions, apart from the zone C. Most projection neurons to the PML were located in the medial and medioventral regions of the zone B. Smaller numbers of projection neurons were located in the PTL, zone A and outside the zone B among fibers of the medial lemniscus. The pattern of FB and DY labeling suggested that neurons projecting to the rPML and cPML originated in common rather than separate regions within the NRTP. In addition, a small percentage (mean 1.3%) of double FB+DY labeled neurons were detected with a clear contralateral preponderance, among single labeled FB or DY cells. In spite of the rarity, all the NRTP neurons giving rise to intralobular collateral projections can be regarded as potential sources of simultaneous modulating influences upon two functional different forelimb (rPML) and hindlimb (cPML) regions. The findings have been discussed in relation to earlier studies in other species and commented on with respect to the possible functional meaning of these projections.

Immunocytochemical localization of the insulin-responsive glucose transporter 4 (Glut4) in the rat central nervous system

We have previously reported that the insulin-responsive glucose transporter GLUT4 is strongly expressed by discrete areas of the rat brain (Leloup et al. [1996] Molec. Brain Res. 38:45-53). In the present study, a sensitive immunocytochemical technique has been used to analyze extensively the anatomical and ultrastructural localizations of GLUT4 in the rat central nervous system in order to gain insight into the physiological role of this transporter. We confirm that GLUT4 is expressed by numerous neurons of the brain and spinal cord, whereas glial cells are more scarcely labeled. In both light and electron microscopy, we observe that the immunoreactivity for GLUT4 is localized mainly in the somatodendritic portion of neurons, where some cisterns of rough endoplasmic reticulum, ribosomal rosettes, certain Golgi saccules, and some intracytoplasmic vesicles are labeled. In contrast, axons and nerve terminals are only occasionally immunostained in certain brain regions such as the neocortex and the ventricular surfaces for example. The GLUT4-immunoreactive structures appear concentrated and most prominently immunostained in motor areas, such as the sensorimotor cortex, most basal ganglia and related nuclei, the cerebellum and deep cerebellar nuclei, a number of reticular fields, motor nuclei of cranial nerves, and motor neurons of the ventral horn of the spinal cord. The labeled regions, which also include some sensory nuclei, are often those in which Vissing et al. ([1996] J. Cerebral Blood Flow Metab. 16:729-736) have shown that exercise stimulates local cerebral glucose utilization, so that GLUT4 might be involved in this effect. On the other hand, the fact that the anatomical localizations of GLUT4 reported here generally agree with the distribution of insulin- or insulin-receptor- related receptors is important since it indicates that the translocation of GLUT4 might also be regulated by insulin in the central nervous system.