Projections from the central cervical nucleus to the cerebellar nuclei in the rat, studied by anterograde axonal tracing

Routes of administration for adeno-associated viruses carrying gene therapies for brain diseases

Gene therapy is a powerful tool to treat various central nervous system (CNS) diseases ranging from monogenetic diseases to neurodegenerative disorders. Adeno-associated viruses (AAVs) have been widely used as the delivery vehicles for CNS gene therapies due to their safety, CNS tropism, and long-term therapeutic effect. However, several factors, including their ability to cross the blood-brain barrier, the efficiency of transduction, their immunotoxicity, loading capacity, the choice of serotype, and peripheral off-target effects should be carefully considered when designing an optimal AAV delivery strategy for a specific disease. In addition, distinct routes of administration may affect the efficiency and safety of AAV-delivered gene therapies. In this review, we summarize different administration routes of gene therapies delivered by AAVs to the brain in mice and rats. Updated knowledge regarding AAV-delivered gene therapies may facilitate the selection from various administration routes for specific disease models in future research.

No Medium-Term Spinocerebellar Input Plasticity in Deep Cerebellar Nuclear Neurons In Vivo?

The existence of input plasticity in the deep cerebellar nuclear (DCN) cells of the adult cerebellum could have profound implications for our understanding of cerebellar function. Whereas the existence of plastic changes in mossy fiber (mf) synaptic responses in DCN neurons has been demonstrated in juvenile slices, there has so far been no direct demonstration of this form of plasticity in the adult cerebellum in vivo. In the present paper, we recorded from neurons in the anterior interposed nucleus (AIN) and stimulated the spinocerebellar tracts (SCT) directly or via the skin to obtain mf activation and the inferior olive to activate climbing fibers (cfs) in the nonanesthetized, adult, decerebrated cat. We used three different types of protocols that theoretically could be expected to induce plasticity, each of which involved episodically intense afferent activation lasting for 10 min. These were conjunctive mf-cf activation, which effectively induces plasticity in cortical neurons; mf and cf activation in a pattern resembling the protocol for inducing classical conditioning; and conjunctive activation of two excitatory mf inputs. None of these protocols had any statistically significant effect on the evoked responses in the AIN neurons. We conclude that the input plasticity for excitatory mfs in the AIN cells of the adult cerebellum in vivo is likely to be less effective than that of parallel fiber synaptic inputs in cerebellar cortical cells, at least in the timespan of 1 h.

Cerebellar afferents originating from the medullary reticular formation that are different from mossy, climbing or monoaminergic fibers in the rat

Integration of cortical Purkinje cell inputs and brain stem inputs is essential in generating cerebellar outputs to the cerebellar nuclei (CN). Currently, collaterals of climbing and mossy fiber axons, noradrenergic, serotoninergic and cholinergic axons, and collaterals of rubrospinal axons are known to innervate the CN from the brain stem. We investigated whether other afferents to the CN from the medulla exist in the rat. Retrograde labeling revealed the presence of neurons that project to the CN but not to the cerebellar cortex in the median reticular formation in the rostrodorsal medulla (tentatively named 'caudal raphe interpositus area', CRI). Anterograde tracer injection into the CRI labeled abundant axonal terminals in the CN, mainly in the ventral parvocellular part of the posterior interposed and lateral nucleus. Axonal reconstruction showed that a single CRI axon projected to the CN with 170-1086 varicosities, more broadly and densely than collaterals of a mossy or climbing fiber axon. CRI axons had no or a few collaterals that projected to the granular and Purkinje cell layers of the cerebellar cortex with some small terminals, indicating that these axons are different from mossy fiber axons. CRI axons also had collaterals that projected to the medial vestibular nucleus and an ascending branch that was not reconstructed. The location of the CRI, electron microscopic observations, and immunostaining results all indicated that CRI axons are not monoaminergic. We conclude that CRI axons form a type of afferent projection to the CN that is different from mossy, climbing or monoaminergic fibers.

The anatomy of the cerebellum

Vertebrate cerebella occupy a position in the rostral roof of the 4th ventricle and share a common pattern in the structure of their cortex. They differ greatly in their external form, the disposition of the neurons of the cerebellar cortex and in the prominence of their afferent, intrinsic and efferent connections.

Precerebellar cell groups in the hindbrain of the mouse defined by retrograde tracing and correlated with cumulative Wnt1-cre genetic labeling

The precerebellar nuclei are hindbrain and spinal cord centers that send fibers to the cerebellum. The neurons of the major hindbrain precerebellar nuclei are derived from the rhombic lip. Wnt1, a developmentally important gene involved in intercellular signaling, is expressed in the developing rhombic lip. We sought to investigate the relationship between the cell clusters expressing Wnt1 and the precerebellar nuclei in the hindbrain. We therefore defined the hindbrain precerebellar nuclei by retrograde tracing, following cerebellar injections of HRP, and compared these results with the cell clusters expressing Wnt1 in newborn mice. We found that 39 distinct hindbrain nuclei project to the cerebellum. Of these nuclei, all but three (namely the oral pontine reticular nucleus, the caudal pontine reticular nucleus, and the subcoeruleus nucleus) contain neurons expressing Wnt1. This shows a high degree of overlap between the precerebellar nuclei and the nuclei that express Wnt1. However, it should be noted that neurons expressing Wnt1 are also found in the superior olivary complex, which is a basal plate derivative lacking cerebellar projections.

Projection patterns of single mossy fiber axons originating from the dorsal column nuclei mapped on the aldolase C compartments in the rat cerebellar cortex

Although cerebellar mossy fibers are the most abundant cerebellar afferents and are deeply involved in cerebellar function, the organization of their projection has remained obscure, particularly in relation to cerebellar compartmentalization. The dorsal column nuclei (DCN) are a major source of cerebellar mossy fibers and possess distinct somatotopic representations of specific somatosensory submodalities. We reconstructed individual dextran-labeled DCN axons completely from serial sections and mapped their terminals on the longitudinal cerebellar compartments that were visualized by aldolase C immunostaining to clarify their projection pattern. Individual axons branched and formed about 100 rosette terminals in the cerebellar cortex, but infrequently projected to the cerebellar nuclei (1 out of 15 axons). Cortical terminals were clustered in multiple areas in the vermis and pars intermedia mostly, but not exclusively, ipsilateral to the origin of the axon. The gracile, cuneate, and external cuneate nuclei (ECuN) mainly projected to the copula pyramidis and lobule V, paramedian and simple lobules, and lobules I-V and VIII-IX, respectively, although there was some overlap. The majority of terminals were located within aldolase C negative or lightly positive compartments. However, terminals of a single axon can be located on aldolase C-negative as well as on aldolase C-positive compartments. In particular, the rostral ECuN, which is responsive to shoulder movements, projected consistently to lobule IX, which were mostly aldolase C-positive. In sum, DCN-cerebellar axons project to multiple compartments with terminals clustered mainly in the conventional spinocerebellar region with a coarse topography, which shows some relationship to the cortical compartments defined by aldolase C.

Delivery of AAV-IGF-1 to the CNS extends survival in ALS mice through modification of aberrant glial cell activity

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease of the motor system. Recent work in rodent models of ALS has shown that insulin-like growth factor-1 (IGF-1) slows disease progression when delivered at disease onset. However, IGF-1's mechanism of action along the neuromuscular axis remains unclear. In this study, symptomatic ALS mice received IGF-1 through stereotaxic injection of an IGF-1-expressing viral vector to the deep cerebellar nuclei (DCN), a region of the cerebellum with extensive brain stem and spinal cord connections. We found that delivery of IGF-1 to the central nervous system (CNS) reduced ALS neuropathology, improved muscle strength, and significantly extended life span in ALS mice. To explore the mechanism of action of IGF-1, we used a newly developed in vitro model of ALS. We demonstrate that IGF-1 is potently neuroprotective and attenuates glial cell-mediated release of tumor necrosis factor-alpha (TNF-alpha) and nitric oxide (NO). Our results show that delivering IGF-1 to the CNS is sufficient to delay disease progression in a mouse model of familial ALS and demonstrate for the first time that IGF-1 attenuates the pathological activity of non-neuronal cells that contribute to disease progression. Our findings highlight an innovative approach for delivering IGF-1 to the CNS.

Cerebellar efferent neurons in teleost fish

In tetrapods, cerebellar efferent systems are mainly mediated via the cerebellar nuclei. In teleosts, the cerebellum lacks cerebellar nuclei. Instead, the cerebellar efferent neurons, termed eurydendroid cells, are arrayed within and below the ganglionic layer. Tracer injections outside of the cerebellum, which retrogradely label eurydendroid cells demonstrate that most eurydendroid cells possess two or more primary dendrites which extend broadly into the molecular layer. Some eurydendroid cells mostly situated in caudal portions of the cerebellum have only one primary dendrite. The eurydendroid cells receive inputs from the Purkinje cells and parallel fibers, but apparently do not receive inputs from the climbing fibers. Eurydendroid cells of the corpus cerebelli and medial valvula project to many brain regions, from the diencephalon to the caudal medulla. A few eurydendroid cells in the valvula project directly to the telencephalon. About half of the eurydendroid cells are aspartate immunopositive. Anti-GABA and anti-zebrin II antibodies that are known as markers for the Purkinje cells in mammals also recognize the Purkinje cells in the teleost cerebellum, but do not recognize the eurydendroid cells. These results suggest that the eurydendroid cells receive GABAergic inputs from the Purkinje cells. This relationship between the eurydendroid and Purkinje cells is similar to that between the cerebellar nuclei and Purkinje cells in mammals. The eurydendroid cells of teleost have both dissimilar as well as similar features compared to neurons of the cerebellar nuclei in tetrapods.

GABA-, glycine-, and glutamate-immunoreactive bouton profiles in apposition to neurons of the central cervical nucleus in the rat

The neurons of the central cervical nucleus (CCN) convey information about the position and movements of the head, and receive excitatory input from dorsal neck muscles and the labyrinth. Both of these afferent sources form glutamatergic synaptic contacts with CCN neurons. However, these sensory afferent sources can also inhibit CCN neurons. To further elucidate the synaptic organization, we made an electron microscopic investigation, identifying and evaluating the relative frequency of bouton profiles containing the inhibitory transmitters GABA and glycine in apposition to identified CCN neurons. In addition, labeling for glutamate was performed. The identification of the CCN neurons was made possible by injections of retrograde tracer substances into the cerebellum. These substances were made visible by preembedding immunocytochemistry or postembedding immunogold staining. Such staining was also used to detect the three amino acids that were found in boutons apposed to the identified neurons (cf. Ornung et al., J. Comp. Neurol. 1996;365:413-426; Lindå et al., J. Comp. Neurol. 2000;425:10-23). Due to the relatively poor transport of the tracer substances into dendrites of the CCN neurons, the analysis was restricted to the cell body and included bouton profiles in direct apposition to the soma membrane. Data from 10 CCN neurons revealed that about 50% of the apposing bouton profiles were immunoreactive for GABA, and about 34% for glycine. In four neurons, the degree of colocalization of GABA and glycine was determined to be close to 30%. Thus, the vast majority of glycine-labeled profiles also contained GABA, while a considerable fraction of the profiles were immunoreactive for only GABA. The values for glycine immunoreactive bouton profiles presented here may represent somewhat low estimates, depending on the method used. Data from four neurons showed that about 18% of the profiles were labeled for glutamate. The large fraction of purely GABA immunoreactive profiles, or at least a substantial group of them, is suggestive of their derivation from axons descending from the brainstem.

Uncrossed and crossed projections from the upper cervical spinal cord to the cerebellar nuclei in the rat, studied by anterograde axonal tracing

In the upper cervical spinal segments, neurons in the medial part of lamina VI give rise to uncrossed spinocerebellar axons, whereas the central cervical nucleus (CCN) and neurons in laminae VII and VIII give rise to crossed spinocerebellar axons. Using anterograde labeling with biotinylated dextran in the rat, we examined the projections of these neuronal groups to the cerebellar nuclei. Uncrossed and crossed projections were distinguished by cerebellar lesions placed on the side contralateral or ipsilateral to the tracer injections confined to the second and third cervical spinal segments (C2 and C3, respectively). Labeled terminals of uncrossed projections were seen in the middle, dorsal, and ventrolateral parts of the middle subdivision and in the ventral part of the caudomedial subdivision of the medial nucleus. In the anterior interpositus nucleus, terminals were seen in the middle of the mediolateral extent, whereas, in the posterior interpositus nucleus, they were seen in lateral and caudal parts. The terminals of crossed projections from the CCN were distributed ventrally in medial to ventrolateral parts of the middle subdivision of the medial nucleus. Some terminals were seen in the caudomedial subdivision of the medial nucleus. In the anterior interpositus nucleus, labeled terminals were seen mainly in rostromedial parts, whereas, in the posterior interpositus nucleus, they were seen in caudal and dorsal parts of the medial half. The present study suggests that the medial lamina VI group and the CCN in the upper cervical segments project to the different areas of the cerebellar nuclei and are concerned with different functions.

The entire trajectory of single climbing and mossy fibers in the cerebellar nuclei and cortex

The present study has revealed that OC axons gave rise to a number of thin collaterals. Due to the abundance of these non-CF thin collaterals, it seems better to make a distinction between the terms CFs and OC axons, as was done in the present paper. The present findings on the innervation of PC dendrites by CFs are basically similar to those in previous reports (Ramón y Cajal, 1911; Palay and Chan-Palay, 1974). The number of swellings on a single CF in the present study (n = 250) is comparable to a previously measured value in the rat (n = 288; Rossi et al., 1993) and larger than a value in the frog (n = about 100 beads; Llinás et al., 1969). The average number of CFs per OC axon in this study was close to the number (n = about 7) inferred in the rat by counting the total number of IO neurons and PCs (Schild, 1970). Contact of interneurons by some swellings of CFs in the molecular layer was emphasized by Scheibel and Scheibel (1954) in their study with Golgi staining. Despite the contact of CF terminals on interneurons, the formation of a synaptic structure between them has been excluded in an electron-microscopic study (Hámori and Szentàothai, 1980). On the other hand, electrophysiological studies have demonstrated a weak excitatory effect of CFs on some interneurons (Eccles et al., 1966). Terminals in the granular layer were originated either from thin collaterals of OC axons or from retrograde collaterals of CF terminal arborizations. The former was the main source of swellings in the granular layer. The morphology of the thin collaterals in the present study was consistent with "globose varicosities connected by a fine thread" as described in Golgi preparations and electron micrograms (Chan-Palay and Palay, 1971). Swellings of thin collaterals (about 1.7% of the total number of swellings per OC axon) were most abundant in the upper portion of the granular layer just underneath the PC layer, in which Golgi cells are usually located. Furthermore, some of these swellings were observed to touch presumed Golgi cells in the present study, which is consistent with electron-microscopic findings on the innervation of somata of Golgi cells by thin collaterals (Hámori and Szentàothai, 1980; Chan-Palay and Palay, 1971). Inferior olive stimulation has been shown electrophysiologically to have a weak direct excitatory effect on Golgi cells (Eccles et al., 1966). Ninety-one percent of the OC axons examined had nuclear collaterals; since the possibility of insufficient staining could not be excluded, this percentage may be an underestimation. The ratio of swellings in the cerebellar nuclei versus those of CF terminal arborizations was about 0.036 in individual OC axons in the present study. However, since the volume of the cerebellar nuclei is much smaller than that of the cerebellar cortex, and significant convergence of input from OC axons to cerebellar nucleus neurons is present (Sugihara et al., 1996), cerebellar nucleus projection of OC fibers can still be functionally important. Some swellings seemed to make contact with the soma and the proximal portions of dendrites of large neurons in the present study, which is consistent with the steep rising phase of postsynaptic excitatory potentials in cerebellar nucleus neurons following IO stimulation (Kitai et al., 1977; Shinoda et al., 1987). Although intracellular potentials were presumably recorded only from large output neurons in the cerebellar nuclei, the present study suggested that small neurons were also innervated by OC axons. The present study revealed that virtually all reconstructed LRN axons projected not only to the Cx as mossy fibers, but also to the DCN including the VN by their axon collaterals. None of the LRN neurons specifically projected to the DCN without projecting to the Cx, namely all axon terminals of LRN neurons in the DCN and VN belonged to axon collaterals of mossy fibers projecting to the Cx. (ABSTRACT TRUNCATED)

Projections from the upper lumbar cord to the cerebellar nuclei in the rat, studied by anterograde axonal tracing

The spinocerebellar tracts arising from the upper lumbar cord consist of the dorsal and the ventral spinocerebellar tracts (DSCT and VSCT), which ascend ipsilaterally and contralaterally, respectively. By using anterograde labeling with biotinylated dextran in the rat, this study examined whether the lumbar DSCT and the VSCT project to the cerebellar nuclei. Injections of the tracer were made unilaterally at levels between the L1 and L3 segments, with diffusion to either a rostral or a caudal segment. The injections resulted in bilateral labeling of axon terminals in the cerebellar nuclei. In the medial nucleus, labeled terminals were distributed in medial, ventral, and ventrolateral parts of the middle subdivision and ventral parts of the caudomedial subdivision. In the anterior interpositus nucleus, they were distributed in medial and dorsomedial parts throughout the rostrocaudal extent. Labeled terminals were seen within the dorsomedial crest region. In the posterior interpositus nucleus, labeled terminals were seen in the rostromedial extension, the caudomedial part, and the caudal pole. Labeled terminals were seen in the hilus and the ventral part of the lateral nucleus. Projections of the DSCT and the VSCT to these regions were confirmed after tracer injections preceded by sectioning of either tract. Both tracts projected bilaterally, but the DSCT projected mainly ipsilaterally. The present study suggests that the spinocerebellar tracts originating from the upper lumbar cord (the lumbar DSCT and the VSCT) project to specific areas of the cerebellar nuclei to transmit information about the peripheral and central events during the movement of hindlimbs.

Projection patterns of single mossy fibers originating from the lateral reticular nucleus in the rat cerebellar cortex and nuclei

Projection of neurons in the lateral reticular nucleus (LRN) to the cerebellar cortex (Cx) and the deep cerebellar nuclei (DCN) was studied in the rat by using the anterograde tracer biotinylated dextran amine (BDA). After injection of BDA into the LRN, labeled terminals were seen bilaterally in most cases in the vermis, intermediate zone, and hemisphere of the anterior lobe, and in various areas in the posterior lobe, except the flocculus, paraflocculus, and nodulus. Areas of dense terminal projection were often organized in multiple longitudinal zones. The entire axonal trajectory of single axons of labeled LRN neurons was reconstructed from serial sections. Stem axons entered the cerebellum through the inferior cerebellar peduncle (mostly ipsilateral), and ran transversely in the deep cerebellar white matter. They often entered the contralateral side across the midline. Along the way, primary collaterals were successively given off from the transversely running stem axons at almost right angles to the Cx and DCN, and individual primary collaterals had longitudinal arborizations that terminated as mossy fibers in multiple lobules of the Cx. These collaterals arising from single LRN axons terminated bilaterally or unilaterally in the vermis, intermediate area, and sometimes hemisphere, and in different cerebellar and vestibular nuclei simultaneously. The cortical terminals of single axons appeared to be distributed in multiple longitudinal zones that were arranged in a mediolateral direction. All of the LRN axons examined (n = 29) had axon collaterals to the DCN. All of the terminals observed in the DCN and vestibular nuclei belonged to axon collaterals of mossy fibers terminating in the Cx.

Connections between the cerebellar nucleus interpositus and the vestibular nuclei: an anatomical study in the rat

Interposito-vestibular connections were analysed, using the anterograde and retrograde tracer biotinylated dextran amine. The interposito-vestibular projections mainly arise from medial portions of the cerebellar nuclei interpositi anterior (NIA) and posterior (NIP), and reach each of the main vestibular nuclei, ipsilaterally. The highest density of projections is found throughout nucleus vestibularis lateralis. Fibres also reach the peripheral part of nucleus superior, the caudal part of nucleus inferior, and the lateral part of nucleus medialis. Some fibres also reach groups I, x and f. Contralaterally, few fibres reach zones of the vestibular nuclei symmetric to the ipsilateral projection. A small, reciprocal, vestibulo-interposed projection is sent from the vestibular nuclei onto NIA-NIP. Possible influences of the interposito-vestibular projections upon the major targets of the vestibular nuclei, spinal motoneurones and oculomotor neurones, are discussed.

Projections from the thoracic cord to the cerebellar nuclei in the rat, studied by anterograde axonal tracing

The dorsal spinocerebellar tract (DSCT) of the thoracic cord (the thoracic DSCT) consists of uncrossed ascending axons originating from Clarke's column, marginal neurons of Clarke's column, and lamina V neurons, and crossed ascending axons originating from neurons in lamina VIII and the ventromedial part of lamina VII. The present study has examined, by using anterograde labeling with biotinylated dextran, whether the thoracic DSCT projects to the cerebellar nuclei. The tracer was injected into the thoracic cord, for two to four segments at levels between the T4 and T9 segments in the rat. The distribution of anterogradely labeled axons and terminals was bilateral but predominantly ipsilateral to the injections. Labeled axons entered the medial nucleus from its rostromedial aspect and terminated widely in medial and ventral parts of the middle subdivision. Furthermore, they terminated in the medial and ventral part of the caudomedial subdivision. Labeled terminals were seen in rostromedial parts of the anterior interpositus nucleus and in medial to caudal parts of the posterior interpositus nucleus. A small number of labeled terminals were consistently seen in the ventral part of the lateral nucleus and the dorsolateral hump region. The present study shows that the thoracic DSCT projects bilaterally, but predominantly ipsilaterally, to the medial and the anterior and posterior interpositus nuclei and suggests that it conveys input related to posture and movement of the trunk and respiratory movement of the thorax.

Projections from the cervical enlargement to the cerebellar nuclei in the rat, studied by anterograde axonal tracing

Spinocerebellar projections from the cervical enlargement originate from neurons in the medial part of lamina VI and the central part of lamina VII. In the present study, the topographic projections of the cervical enlargement to the cerebellar nuclei were examined by anterograde tracing with biotinylated dextran in the rat. Following injections of the tracer into the spinal cord at levels between the C5 and T1 segments, anterogradely labeled axons and terminals were immunohistochemically demonstrated in the cerebellar nuclei. Unilateral injections revealed that projections are bilateral, but predominantly ipsilateral, to the cells of origin. Labeled axons entered the media nucleus from its rostrodorsal and rostromedial aspects. Labeled terminals were distributed to dorsal and medial parts of the middle subdivision at its rostral levels and to medial parts of the caudomedial subdivision of the medial nucleus. Most axons terminated in the middle subdivision. Single axons were seen to course rostrocaudally in the medial nucleus and give off terminal axons to both subdivisions. A few labeled terminals were seen in the dorsolateral protuberance of the medial nucleus, and in the anterior interpositus and the posterior interpositus nuclei. No labeled terminals were seen in the lateral cerebellar nucleus. The present study demonstrates that spinocerebellar neurons in laminae VI and VII of the cervical enlargement project to dorsomedial areas of the medial nucleus at rostral levels, bilaterally but predominantly ipsilaterally. It is suggested that these areas specifically receive cutaneous and muscular input related to the forelimb movement.

Spinovestibular projections in the rat, with particular reference to projections from the central cervical nucleus to the lateral vestibular nucleus

Projections from the spinal cord to the vestibular nuclei were examined following injections of Phaseolus vulgaris-leucoagglutinin, cholera toxin subunit B, or biotinylated dextran at various levels of the spinal cord in the rat. Labeled terminals were abundant after injections of the tracers into the C2 and C3 segments containing the central cervical nucleus. Labeled terminals were seen in the descending vestibular nucleus and the parvocellular, magnocellular, and caudal parts of the medial vestibular nucleus throughout its rostrocaudal extent. Labeled terminals were most numerous in the lateral vestibular nucleus throughout its rostrocaudal extent. The projections from the central cervical nucleus to the vestibular nuclei were exclusively contralateral to the cells of origin because the axons of the central cervical nucleus neurons cross in the spinal cord. Following tracer injections in the cervical enlargement, many labeled terminals were seen in the magnocellular part of the medial vestibular nucleus, but a few were seen in the lateral and the descending vestibular nucleus. Injections into more caudal segments resulted in sporadic terminal labeling in the magnocellular part of the medial vestibular nucleus, the descending vestibular nucleus, and the caudal part of the lateral vestibular nucleus. The results indicate that primary neck afferent input relayed at the central cervical nucleus is mediated directly to the contralateral vestibular nuclei. It is suggested that this projection serves as an important linkage from the upper cervical segments to the lateral vestibulospinal tract in the tonic neck reflex.