EM-autoradiography of cerebellar nucleocortical terminals in the cat

A novel inhibitory nucleo-cortical circuit controls cerebellar Golgi cell activity

The cerebellum, a crucial center for motor coordination, is composed of a cortex and several nuclei. The main mode of interaction between these two parts is considered to be formed by the inhibitory control of the nuclei by cortical Purkinje neurons. We now amend this view by showing that inhibitory GABA-glycinergic neurons of the cerebellar nuclei (CN) project profusely into the cerebellar cortex, where they make synaptic contacts on a GABAergic subpopulation of cerebellar Golgi cells. These spontaneously firing Golgi cells are inhibited by optogenetic activation of the inhibitory nucleo-cortical fibers both in vitro and in vivo. Our data suggest that the CN may contribute to the functional recruitment of the cerebellar cortex by decreasing Golgi cell inhibition onto granule cells.

Cerebellar Premotor Output Neurons Collateralize to Innervate the Cerebellar Cortex

Motor commands computed by the cerebellum are hypothesized to use corollary discharge, or copies of outgoing commands, to accelerate motor corrections. Identifying sources of corollary discharge, therefore, is critical for testing this hypothesis. Here we verified that the pathway from the cerebellar nuclei to the cerebellar cortex in mice includes collaterals of cerebellar premotor output neurons, mapped this collateral pathway, and identified its postsynaptic targets. Following bidirectional tracer injections into a distal target of the cerebellar nuclei, the ventrolateral thalamus, we observed retrogradely labeled somata in the cerebellar nuclei and mossy fiber terminals in the cerebellar granule layer, consistent with collateral branching. Corroborating these observations, bidirectional tracer injections into the cerebellar cortex retrogradely labeled somata in the cerebellar nuclei and boutons in the ventrolateral thalamus. To test whether nuclear output neurons projecting to the red nucleus also collateralize to the cerebellar cortex, we used a Cre-dependent viral approach, avoiding potential confounds of direct red nucleus-to-cerebellum projections. Injections of a Cre-dependent GFP-expressing virus into Ntsr1-Cre mice, which express Cre selectively in the cerebellar nuclei, retrogradely labeled somata in the interposed nucleus, and putative collateral branches terminating as mossy fibers in the cerebellar cortex. Postsynaptic targets of all labeled mossy fiber terminals were identified using immunohistochemical Golgi cell markers and electron microscopic profiles of granule cells, indicating that the collaterals of nuclear output neurons contact both Golgi and granule cells. These results clarify the organization of a subset of nucleocortical projections that constitute an experimentally accessible corollary discharge pathway within the cerebellum.

Cerebellar loops: a review of the nucleocortical pathway

Feedback pathways are a common circuit motif in vertebrate brains. Reciprocal interconnectivity is seen between the cerebral cortex and thalamus as well as between basal ganglia structures, for example. Here, we review the literature on the nucleocortical pathway, a feedback pathway from the cerebellar nuclei to the cerebellar cortex, which has been studied anatomically but has remained somewhat obscure. This review covers the work examining this pathway on a number of levels, ranging from its existence in numerous species, its organization within cerebellar circuits, its cellular composition, and a discussion of its potential roles in motor control. Recent interest in cerebellar modular organization raises the profile of this neglected cerebellar pathway, and it is hoped that this review will consolidate knowledge gained over several decades of research into a useful format, spurring new investigations into this evolutionarily conserved pathway.

Somatotopic nucleocortical projections to the multiple somatosensory cerebellar maps

The cerebellum is organized in a series of parasagittal compartments: in C1-C3 and C2 compartments Purkinje cells receive climbing fibre afferents from the rostral part of the accessory olives, and project their axon to the nucleus interpositus anterior and posterior, respectively. Within these compartments electrophysiological studies have shown that the cutaneous input carried by climbing fibre afferents is topographically organized so as to design a map of peripheral body districts. The body map is replicated over the anterior lobe-pars intermedia and the paramedian lobule, and anatomical studies have indicated that the replication is partly due to the axonal branching of olivocerebellar neurons. The aim of this study was to analyse the presence of a somatotopic organization and of a branching pattern in the nucleocortical projections, in relation to the replicated body maps within C1-C3 and C2 compartments. By using double retrograde neuronal tracing we explored, in the cat, the topographic distribution of single- and double-labelled cells in the interposed nuclear subdivisions, after tracer injections into forelimb or hindlimb regions of the anterior lobe-pars intermedia, paramedian lobule and hemisphere (medial crus II). Most of the nucleocortical neurons were found in ipsilateral nucleus interpositus posterior, with smaller numbers in the ipsilateral nucleus interpositus anterior. Nucleocortical neurons projecting to forelimb- or hindlimb-related areas are completely segregated, the forelimb neurons being located laterally and the hindlimb neurons medially in the nucleus interpositus posterior. Within their respective domains both the forelimb and hindlimb populations projecting to the anterior lobe-pars intermedia are partly segregated from those projecting to the paramedian lobule, in that the two populations are slightly shifted along the dorsoventral axis of the nucleus. Although mostly different, some of the cells are common to the two forelimb populations, since they send axonal branches to the homologous areas of the anterior lobe and paramedian lobule. Contralateral fastigial or interposed nucleocortical projections are restricted to the anterior lobe-pars intermedia, and their neurons of origin are different from those that project to the ipsilateral cerebellar cortex: i.e. they are not a bilateral, but a separate contralateral component.

Synaptic connections of Purkinje cell axons with nucleocortical neurones in the cerebellar medial nucleus of the rat

The cerebellar nucleocortical neurones may be part of a cortico-nucleocortical loop. It has not yet been demonstrated, however, whether they are directly afferented by Purkinje cell axons. This question has been addressed by using electron microscopic methods. WGA-HRP injections into the cerebellar vermis anterogradely labelled Purkinje cell terminals and retrogradely labelled nucleocortical neurones of the nucleus medialis. Postembedding GABA immunolabelling was used to double-labelled PC terminals and identified the GABA-immunoreactive nuclear neurones. Of the identified nucleocortical neurones, the majority were immunonegative, but a few were GABA-immunoreactive. Both types were in synaptic contact with identified Purkinje cell terminals.

A combined retrograde tracer and GABA-immunocytochemical study of the projection from nucleus interpositus posterior to the posterior lobe C2 zone of the cat cerebellum

The extent to which the cells of origin of the cerebellar nucleocortical pathway are immunopositive for gamma-aminobutyric acid (GABA) was investigated in four cats using retrograde labelling of nucleocortical neurons in combination with immunocytochemistry. Neurons were retrogradely labelled by injection of fluorescent (coumarin)-tagged latex microspheres into the c2 zone in the rostral part of the paramedian lobule. The zone was identified electrophysiologically by the characteristics of the climbing fibre responses evoked on the cerebellar surface by percutaneous stimulation applied to the left and right forepaws in pentobarbitone-anaesthetized animals. Sections of the cerebellum containing the retrogradely labelled neurons were processed for GABA immunocytochemistry using a fluorescent (rhodamine)-tagged immunoglobulin. When viewed with epifluorescence microscopy and appropriate filter blocks the retrogradely labelled nucleocortical neurons could be visualized in the same sections as the GABA-immunopositive neurons. Almost all of a total of 254 labelled nucleocortical neurons were located in nucleus interpositus posterior, where a total of 711 GABAergic neurons were also found. None of these cells contained coumarin-tagged beads and displayed immunoreactivity for GABA (i.e. none was double-labelled). When compared by area of their cell body, the nucleocortical and GABA-immunopositive neurons appeared to form two partially overlapping populations. The mean cell area of the nucleocortical neurons was 620 +/- 233 microns2 (SD), whereas the GABA-immunopositive neurons were much smaller, with a mean cell area of 220 +/- 115 microns2. The results suggest that GABA does not play a major role in the nucleocortical pathway to the c2 zone of the rostral paramedian lobule of the cat cerebellum.

Cerebellar nuclei and the nucleocortical projections in the rat: Retrograde tracing coupled to GABA and glutamate immunohistochemistry

The amino acids GABA and glutamate (Glu) are thought to be the principal substances in the central nervous system responsible for neuronal inhibition and excitation. Their distributions among the different neurons in a defined pathway may thus be indicative of the contributions of the cells to pathway function. Examples of such neurons are those of the cerebellar nuclei which, while regulating output from the Purkinje cells of the cerebellar cortex, are also found to project back to the cerebellar cortex. Immunohistochemical experiments were done to identify GABA and glutamate (Glu) containing cells in the adult rat cerebellar nuclei. Consecutive semithin and serial vibratome sections were incubated with antisera raised in rabbit against GABA and Glu. In semithin sections, only small neurons were intensely GABA immunoreactive (GABA-IR) (31.7%), and the majority (80.5%) were Glu immunoreactive (Glu-IR) of different sizes. Consistent with Glu being a metabolic precursor for GABA, 75.4% of the GABA-IR population colocalized Glu. In vibratome sections GABA-IR neurons showed some local differences in number, whereas the Glu-IR were uniformly distributed in the three nuclei studied. Measured mean diameters for these neurons showed a distinct size difference for the GABA- and Glu-IR with little overlap. Cerebellar nuclei neurons projecting to the cortex (nucleocortical neurons, NCN) were identified by locally preinjecting the retrograde transported WGA-apoHRP-colloidal gold complex in the cerebellar cortex. Vibratome sections of these cerebellar were silver intensified for the retrograde tracer and double labeled for GABA and Glu. Of the total number of identified NCN, 8.7% were GABA-IR (10 animals) and 47.7% Glu-IR (5 animals). Many retrograde labeled NCN in the core of the thick sections were immunonegative for both amino acids due to poor antibody penetration, thus underestimating the proportions of cells containing GABA and Glu. The size distributions for the GABA-IR and Glu-IR NCN were similar to those measured in non-retrograde labeled nuclei in thick sections. The conclusions reached are that GABA-IR neurons of the cerebellar nuclei, including the NCN, use GABA as the presumed inhibitory neurotransmitter and that Glu-IR neurons may use Glu or another excitatory neurotransmitter.

Topographic organization of the cerebellar nucleocortical projection in the albino rat: an autoradiographic orthograde study

The topography of the cerebellar nucleocortical projection was investigated in the albino rat by experiments employing an autoradiographic orthograde tracing method. The present results indicate that neurons in the deep cerebellar nuclei project to the granule cell layer of cerebellar cortex as mossy fiber terminals in an orderly way. Thus, the medial cerebellar nucleus projects mainly to the bilateral vermis with ipsilateral dominance. The interpositus and lateral cerebellar nuclei project mainly to the intermediate and lateral zones of the anterior and posterior lobes of the cortex, respectively. The paraflocculus and flocculus receive the nucleocortical projection from the caudal and ventral parts of the interpositus nuclei and the dentate nucleus. A mediolateral topography within each subdivision of the cerebellar nuclear complex was observed; the medial and lateral parts of the subdivision project to the more medial and lateral portions of the primary cortical targets of the subdivision, respectively.

Anatomical mapping of the cerebellar nucleocortical projections in the rat: a retrograde labeling study

An analysis of the cerebellar nucleocortical projections was made by means of retrograde cellular labeling with wheat germ agglutinin-horseradish peroxidase conjugate. Each of the main nuclear subregions appears to give rise to nucleocortical projections. The cortical distribution of the projections is referred to here in term of sagittal zones. Zones A, B, and C conform to the recent description in the rat (Buisseret-Delmas, '88a,b) on the basis of their olivocortical and corticonuclear projections. A corresponding description of zone D is given here. According to their distribution, three types of nucleocortical projections have been distinguished: 1) ipsilateral, reciprocal; 2) nonreciprocal; and 3) contralateral, symmetrical to the corticonuclear afferent. Reciprocal projections are strictly arranged in the sagittal direction, with the following zonal distribution. Zone A is subdivided into two subzones. Medial A zone receives its nuclear afferents from the medial aspect of the nucleus medialis (NM). The lateral A zone of the anterior lobe and lobule VI and that of the posterior lobe receive their reciprocal nuclear afferents from the ventrolateral NM and the dorsolateral protuberance, respectively. Zone B does not seem to receive nucleocortical projections. Zone C has three subzones in the rat. C1 is supplied from the medial third of the anterior and posterior subdivisions of the nucleus interpositus (NIA and NIP, respectively). C2 is supplied from the central third of the NIA and NIP. Rostrocaudally, the anterior lobe and lobule VIII are connected to the NIA, and lobules VI and VII to the NIP. C3 appears to be connected to the lateral third of NIA. Zone D contains three subzones mediolaterally in the rat. D0, not previously described, is defined on the basis of both its olivary afferent from the medial half of the ventral lamella of the principal olive and its corticonuclear projections onto the dorsolateral hump of Goodman et al. ('63). It receives a reciprocal nucleocortical afferent from the dorsolateral hump. D1 receives its olivary afferent from the dorsal lamella of the principal olive. It is reciprocally connected with the lateral, magnocellular part of the nucleus lateralis (NL). D2 is the most lateral subzone of the hemisphere. Its olivary afferent comes from the lateral half of the ventral lamella of the principal olive. D2 is reciprocally connected with the ventral, parvicellular subdivision of NL. The main cortical recipients for the nonreciprocal projections are the lateral A zone, the C3, and the D1 subzones.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatotopically organized transient projections from the primary somatosensory cortex to the cerebellar cortex

The organization of transient projections from the primary somatosensory cortex (S-I) to the cerebellar cortex in neonatal kittens was examined using orthograde intraaxonal labeling techniques. Tritiated amino acid injections into face, forelimb and hindlimb areas of representation in S-I labeled mossy fiber-like terminals of cerebrocerebellar axons in different areas of the cerebellar cortex bilaterally. The hindlimb area of S-I projected to lobules I-IV in the anterior lobe and to ventral folia of the paramedian lobule (PML). Injections into forelimb areas of S-I labeled terminals in lobules IV and V and in intermediate and dorsal folia of the PML. The face area of S-I projected to the lobules V and VI, to medial folia in the ansiform and simplex lobules and to dorsal PML folia. Labeled terminals were more numerous in the cerebellar cortex contralateral to the S-I injections, except in lobules I and II and the ventral PML where the density of hindlimb input was approximately the same on both sides. These observations were supplemented by findings that small wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections into the dorsal or ventral PML resulted in retrogradely labeled layer V pyramidal neurons in lateral (face and forelimb) and medial (hindlimb) areas of S-I respectively. The somatotopic organization of transient S-I cerebrocerebellar projections is very similar to the topography of cerebellar somatosensory afferent pathways in adult cats.

Quantitative evaluation of crossed and uncrossed projections from basal ganglia and cerebellum to the cat thalamus

Quantitative and qualitative analysis of crossed vs uncrossed projections from the substantia nigra, entopeduncular nucleus and individual cerebellar nuclei to the thalamus was undertaken in nine adult cats using retrograde labeling with horseradish peroxidase and fluorescent dyes. The results indicate that about 90% of entopeduncular nucleus neurons and 50% of substantia nigra neurons give rise to ipsilateral projections to the thalamus whereas the contralateral component of these projections originates from about 10 and 7% neurons of entopeduncular nucleus and substantia nigra, respectively. Some of the fibers constituting the contralateral component are represented by branching axon collaterals of the neurons projecting ipsilaterally. In the basal ganglia thalamic projection, its minor component (contralateral) targets the ventral anterior and ventral medial nuclei the same as its major component (ipsilateral). However, some preferential distribution of the contralateral projections to the ventral medial nucleus appears to exist. In regard to the cerebellothalamic projections it was found that about 90% of neurons located in the dentate and interpositus nuclei and 50% of neurons in the fastigial nucleus project to the contralateral thalamus while 16% of dentate nucleus neurons and 40% of fastigial nucleus neurons give rise to the ipsilateral cerebellothalamic projections. A considerable number of ipsilateral cerebellothalamic fibers are represented by divergent axon collaterals of the same neurons projecting to the contralateral thalamus. The cerebellothalamic projections from all cerebellar nuclei including the fastigial nucleus are targeted primarily to the ventral lateral nucleus both contra- and ipsilaterally. The ventral medial nucleus receives bilateral input from the fastigial nucleus which originates from about one quarter of the thalamus projecting neurons in this nucleus. Of all other cerebellar nuclei only the dentate nucleus projects to the ventral medial nucleus and this projection is exclusively contralateral.

Cerebellar nucleocortical projection with a survey of factors affecting the transport of radioactive tracers

The nucleocortical projection has been studied with the method of anterograde transport of tritiated amino acids and autoradiography. It was observed that this projection is made up of a small number of fibers. Counts of silver grain aggregates in projection sites of the granular layer were compared with counts from material on the cuneocerebellar projection. As it had been concluded in previous studies that the projection is quantitatively important, several modifications of the experimental conditions were tested. Radioactive leucine of high or low specific activity as well as a mixture of amino acids were used. It was observed that labeling is always more intense with a high specific activity tracer. Variations of the aspect and density of the projection were observed with survival periods lasting from a few hours to several days. With survival times between 9 and 15 hours, round aggregates of silver grains measuring from 10 to 20 microns were observed in the granular layer of labeled regions. At longer survival periods, the terminal structures in the granular layer appeared as coarse linear aggregates. It is suggested that the latter represent preterminal labeled fragments while the round accumulations correspond to terminal rosettes. Maximum density of the labeling in the granular layer occurred between 9 and 15 hours. At 2 days, the density was lowest, and at 5 or 6 days it was somewhat higher. Even under optimal conditions, the density of the nucleocortical terminals was low.

Projection systems and terminal localization of dorsal column afferents: an autoradiographic and horseradish peroxidase study in the rat

Projection systems from the gracile nucleus and the cuneate nuclear complex to their terminal sites in the mesencephalon, diencephalon, and cerebellum were examined by means of anterograde autoradiography and retrograde horseradish peroxidase methods. Three projection systems emerge from the dorsal column nuclei, decussate via internal arcuate fibers, and form the contralateral medial lemniscus (ML). At the obex, some fibers split off the ML and course dorsolaterally, forming an ascending lateral system which fits the "lemniscal adjunct channel" (LAC) concept of Graybiel ('72). The ML continues rostrally as the "main lemniscal line channel" (MLLC). At the inferior colliculus, some LAC fibers terminate in the pontine nuclei, parabrachial, dorsal reticular nuclei, and the external and ventral medial part of the central nucleus of the inferior colliculus. More rostrally at the level of the superior colliculus, terminal fields are found in the medial nucleus of the medial geniculate body, the suprageniculate, pretectal, and mesencephalic reticular nuclei, marking the end of the LAC. In the diencephalon, gracile fibers leave the MLLC and form a crescentlike terminal field along the extreme lateral border of the ventral posterior lateral nucleus (VPL) of the thalamus. Cuneate MLLC fibers terminate in a bandlike formation in the VPL medial to the gracile termination. The third fiber system, the cuneocerebellar projection, emerges from the cuneate, the external cuneate nuclei, and the "cellular bridge" and immediately enters the ipsilateral inferior cerebellar peduncle. Upon entering the cerebellum, the major fiber component remains ipsilateral and terminates as vertical bands in vermal and paravermal lobules, and lobules I through IVa. The posterior cerebellar lobe contains terminal bands in lobules VII-IX, the copula pyramidis, and the paramedian lobule. It is concluded that the dorsolateral fiber system conforms to Graybiel's LAC. It is more divergent and probably less modality specific, whereas the medial lemniscal system conforms to the MLLC, which is said to be modality specific, less divergent, and locked to specific sensory-motor response characteristics. The topography of cerebellar terminal bands indicates that there is sensory-motor representation from all parts of the body to all parts of the cerebellum, at least in the rat.

Selective retrograde labelling of the rat olivocerebellar climbing fiber system with D-[3H]aspartate

Selective retrograde labelling of the olivocerebellar climbing fiber system with D-[3H]aspartate has been observed in the rat, and the results have implications for the identification of a transmitter candidate as well as the neuroanatomical understanding of these cerebellar afferents. Microinjections of D-[3H]aspartate (50 nl, ca 10-2 M) were made into various parts of cerebellar cortex. Survival times were 6, 12 or 24 h. Pronounced diffusion of the tracer resulted in large injection sites. Within the zone of injection, glial elements were labelled over background. Most granule cells exposed to the tracer were unlabelled; the small numbers demonstrating labelling were believed to have been injured by the micropipette penetration. Beneath injection sites, large numbers of well-labelled nerve fibers appeared in the white matter and could be followed through the brainstem to the contralateral inferior olive, where labelled perikarya were found. After the inferior olivary neurons had been effectively destroyed with 3-acetylpyridine, evidence of cerebellar afferent labelling with D-[3H]aspartate was missing. Retrograde labelling of the olivocerebellar system was also observed after superfusion of the vermis with D-[3H]aspartate at concentrations in the range of Km for high affinity uptake (10(-5) or 10(-4) M, for 2 h). Mossy fiber or monoaminergic afferents to the cerebellum were never labelled with D-[3H]aspartate. The distribution of labelled cells in the olivary subnuclei after injections in different cerebellar areas was in line with the olivocerebellar organization previously described in the cat. Moreover, it was demonstrated that fibers from the different subnuclei follow different routes through the brainstem towards the cerebellum. Labelling of climbing fiber collaterals in uninjected parts of cerebellum indicated that some of the retrogradely migrating D-[3H]aspartate was directed in anterograde direction at axonal branching points. Collaterals were demonstrated in all deep cerebellar and Deiters' nuclei, and the results of intranuclear injections suggested that virtually every olivary neuron sends collaterals to these nuclei. Intracortical collaterals were organized in sagittal zones. Midline injections into the anterior lobe and VI lobule labelled collaterals in several zones of the posterior lobe spinal area and uninjected parts of the anterior lobe vermis. Hemispheral injection into copula pyramidis labelled collaterals in two prominent bundles in the anterior lobe.(ABSTRACT TRUNCATED AT 400 WORDS)

The cerebellar nucleo-cortical projection in the rat studied by the retrograde fluorescent double-labelling method

Injection of true blue into rat cerebellar cortex and nuclear yellow into either the superior cerebellar peduncle or the ventral thalamus produced double-labelled neurones in the cerebellar deep nuclei. This suggests that the nucleo-cortical projection arises from collaterals of cerebellofugal fibres. The topography of this projection, and differences in collateralization between thalamic and other cerebellofugal fibres are discussed.

Ultrastructural evidence for compensatory sprouting of climbing and mossy afferents to the cerebellar hemisphere after ipsilateral pedunculotomy in the newborn rat

Unilateral section of the inferior and middle cerebellar peduncles was performed in rats at postnatal days 1 or 2. The ultrastructure of the cerebellar hemispheric cortex ipsilateral to the lesion was examined 3 months later. The absence of contralateral inferior olive and of ipsilateral middle peduncle, together with a marked regression of the contralateral pontine gray, were indicative of successful pedunculotomy. In spite of a relative atrophy of the hemisphere, its cytological structure was qualitatively normal. Mossy and climbing fibers were present and their terminal varicosities disclosed normal features. The density of climbing fiber terminals was reduced compared to control cerebellum, whereas the density of mossy terminals seemed unchanged. subsequent to the reduction of climbing afferents two subclasses, or types, of Purkinje cells were present: A "normal" type characterized by its climbing fiber innervation and a "hyperspiny" type devoid of climbing fiber. In some of the adult rats pedunculotomized at birth, section of the contralateral peduncles was performed 24 hours before fixation. Terminal degeneration of climbing and mossy fibers was observed in the neonatally deprived hemisphere, providing the proof that these fibers result from a compensatory transcommissural sprouting of afferents destined to the contralateral hemicerebellum. These results demonstrate that the cerebellar cortex neonatally deprive of its main afferents can be innervated by climbing and mossy fibers through a process of transcommissural sprouting. Although the newly formed synapses maintain their target specificity, a functional reorganization must occur because of the altered distribution of both systems of afferents.