Projections from the pericruciate cortex to the nucleus of Darkschewitsch and other structures at the mesodiencephalic junction in the cat

Input and output organization of the mesodiencephalic junction for cerebro-cerebellar communication

Most studies investigating the impact of the cerebral cortex (CC) onto the cerebellum highlight the role of the pons, which provides the mossy fibers to the cerebellum. However, cerebro‐cerebellar communication may also be mediated by the nuclei of the mesodiencephalic junction (MDJ) that project to the inferior olive (IO), which in turn provides the climbing fibers to the molecular layer. Here, we uncover the precise topographic relations of the inputs and outputs of the MDJ using multiple, classical, and transneuronal tracing methods as well as analyses of mesoscale cortical injections from Allen Mouse Brain. We show that the caudal parts of the CC predominantly project to the principal olive via the rostral MDJ and that the rostral parts of the CC predominantly project to the rostral medial accessory olive via the caudal MDJ. Moreover, using triple viral tracing technology, we show that the cerebellar nuclei directly innervate the neurons in the MDJ that receive input from CC and project to the IO. By unraveling these topographic and prominent, mono‐ and disynaptic projections through the MDJ, this work establishes that cerebro‐cerebellar communication is not only mediated by the pontine mossy fiber system, but also by the climbing fiber system.

The parafascicular nucleus relays spinal inputs to the striatum: an electron microscope study in the rat

A disynaptic projection from the spinal cord to the striatum was observed in the rat light and electron microscopically. An anterograde tracer, wheat germ agglutinin conjugated to horseradish peroxidase was injected into the ventral gray matter of the upper cervical spinal cord, and a retrograde tracer, biotinylated dextran amine was injected into the striatum of a rat. Then the parafascicular nucleus was examined. Some anterogradely labeled axon terminals originating in the spinal cord were observed to synapse with retrogradely labeled dendrites of parafascicular nucleus neurons which sent axons to the striatum. We concluded that information from the spinal cord was transmitted to the striatum, being relayed by parafascicular nucleus neurons.

Efferents from the lateral frontal cortex to spinomedullary target areas, trigeminal nuclei, and spinally projecting brainstem regions in the hedgehog tenrec

This study was done in the Madagascan lesser hedgehog tenrec, an insectivore with a very poorly differentiated neocortex. The cortical region, known to give rise to spinal projections, was injected with tracer, and the cortical efferents to brainstem and spinal cord were analyzed. Bulbar reticular fields, in addition, were identified according to their cells of origin and the laterality of their spinal projections after injection of tracer. Only few cortical fibers could be traced from the bulbar pyramid into the ipsilateral spinal cord, particularly to the lateral funiculus. The projections to the dorsal column nuclei and the classical spinally projecting brainstem regions were also weak. Faint projections were demonstrated to the nucleus of the posterior commissure and the nucleus of Darkschewitsch. In comparison to other mammals, there was no evidence that the contralateral cortico-bulbo-spinal pathway was strengthened, substituting for the almost non-existent contralateral corticospinal projection. Unlike the sensorimotor apparatus controlling limb and body movements, the brainstem regions controlling the head and neck received prominent cortical projections. Direct corticotrigeminal projections and indirect pathways were well represented. The projections to the trigeminal nuclei and the lateral reticular fields were clearly bilateral; those to the superior colliculus were predominantly ipsilateral. The corticobulbar fibers left the pyramid along its entire extent; the principal trigeminal nucleus and the dorsolateral pontine tegmentum were supplied by additional fibers of the corticotegmental tract. The lateral frontal cortex also projected densely to the dorsolateral hypothalamus, the periaqueductal gray, and the adjacent mesencephalic tegmentum, components of the emotional motor system.

A projection linking motor cortex with the LM-suprageniculate nuclear complex through the periaqueductal gray area which surrounds the nucleus of Darkschewitsch in the cat

Whereas a previous study by one of us (Hicks et al., 1986) suggested that periaqueductal gray (PAG) neurons projecting to the lateralis medialis-suprageniculate (LM-SG) complex might mediate transmission of affective-related nociceptive information, our present work suggests instead, a function in processes related to movement. Cells of the nucleus of Darkschewitsch (ND) are known to have reciprocal projections with the motor cortex (MX), in particular with the hand area of MX, and also to project to the rostral medial accessory olivary (MAO) nucleus (Onodera and Hicks, 1995a). That the ND might be related to saccadic oculomotor function, as well as to the control of hand movements through its connections via the olivo-cerebellar circuit, is indicated by the fact that ND receives a strong projection from the substantia nigra pars reticulata and zona incerta (SNR/ZI) and projects directly and/or indirectly to eye movement nuclei (Onodera and Hicks, 1995b). Thus, ND may function in permitting integration of eye-hand motor coordination. This study focussed on the area of PAG surrounding ND. WGA-HRP was injected into MX and many labelled terminals and large neurones were in ND, with lesser numbers being observed in the area of the PAG surrounding ND. After injections into ND and closely adjacent areas, labelled terminals were observed sparsely distributed with a restricted area of the LM-SG complex. After injections into LM-SG area, small neuronal somata were seen in the area of the PAG surrounding ND, but no labelled somata were detected in ND. Thus if the cells of this PAG area, like those of ND, have similar functions owing to their common reciprocal connections with MX, then the small neurones in PAG projecting to LM-SG may constitute an important link in the circuitry subserving visual processing and/or the regulation of orienting movements of the hand, head and eye.

Patterns of transmitter labelling and connectivity of the cat's nucleus of Darkschewitsch: a wheat germ agglutinin-horseradish peroxidase and immunocytochemical study at light and electron microscopical levels

Immunocytochemical studies using antibodies raised against a number of probable synaptic transmitters of the mesodiencephalic area, and fibre-tracing studies using wheat germ agglutinin-horseradish peroxidase (WGA-HRP), have been performed in adult cats. Glutamate and aspartate immunoreactivity produced a strong labelling of many cell bodies and terminals in the nucleus of Darkschewitsch (ND). gamma-Aminobutyrate (GABA) immunoreactivity in the ND appeared as a moderate label in some small neurones, and as a strong label in a few glial-like cells, in addition to being present in high levels to produce strong labelling in many GABA-immunopositive terminals that possessed pleomorphic vesicles. Some choline acetyltransferase-positive terminals and dendrites and a few substance P-positive fine fibres possessing varicosities also were observed in the ND. Following WGA-HRP injection in the ND, dense terminal labelling was seen ipsilaterally in the rostral half of the medial accessory olive, suggesting that there may be a certain degree of mediolateral and dorsoventral topographic correspondance within the ND-olive projection. In the same cases, many cell bodies containing HRP reaction product also were found 1) ipsilaterally in the motor cortex, anterior pretectal nucleus, and a restricted area of the caudal part of the substantia nigra pars reticulata; 2) contralaterally in the anterior and posterior interposed cerebellar nuclei as well as in a portion of the lateral cerebellar nucleus; and 3) bilaterally in the zona incerta, the posterior pretectal nucleus, the pedunculopontine tegmental nuclei, the spinal trigeminal nucleus, the dorsal column nuclei, and the spinal cord. Details of the interrelationships and functional considerations amongst the ND, adjacent nuclei, and longitudinal zones of the cerebellum are discussed.

Postnatal development of crossed and uncrossed corticorubral projections in kitten: a PHA-L study

Morphological changes in individual corticorubral fibers and the pattern of crossed and uncrossed corticorubral projections were studied during the postnatal development of cats in order to understand cellular mechanisms for restriction of corticorubral projections with development. The anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into restricted areas of the pericruciate cortex in kittens and PHA-L-labeled axons in the red nucleus were examined at postnatal days (PND) 7-73. In accordance with our previous study (Murakami and Higashi, Brain Res. 1988; 447:98-108), a crossed corticorubral projection was observed in addition to the uncrossed one in every experimental animal. During the early period of development (PND 7-8), swellings of irregular shape were observed along the entire course of the axons and they were often interconnected with extremely fine axonal segments. These axons bifurcated only infrequently and often ended as growth cones. These features were common to both uncrossed and crossed corticorubral axons. At later stages of development (PND 28 or later), the total number of swellings decreased and axonal swellings with smooth contours became dominant. A quantitative examination of axonal branches indicated that axons on the ipsilateral side branch occurred more frequently at later stages of development. However, there was no substantial change in branching frequency for the crossed corticorubral fibers during development. In parallel with morphological changes in individual axons, the crossed projection that was initially relatively abundant was reduced during development. Since a PHA-L injection can be confined to a small region of cortex, topographic projections can easily be detected. At PND 7-8 there was no well-defined topographic order in the ipsilateral corticorubral projection. Adult-like topography was first discernible at PND 13. These observations suggest that the unilateral uncrossed corticorubral projection in the adult cat is achieved at least in part by the formation of axonal arbors in the uncrossed projection. This was accompanied by the failure of crossed fibers to form complex arbors. It is possible that a similar mechanism also operates in the formation of topographic maps.

A comparison of the distribution of the cerebellar and cortical connections of the nucleus of Darkschewitsch (ND) in the cat: a study using anterograde and retrograde HRP tracing techniques

Bidirectional transport of lectin conjugated horseradish peroxidase was employed to investigate the relative distribution of the cerebellar and cortical connections of the nucleus of Darkschewitsch in the cat. Injection of horseradish peroxidase into the deep cerebellar nuclei produced terminal labeling which extended throughout the length of the contralateral nucleus of Darkschewitsch and into the perifascicular region. Injection of horseradish peroxidase into the pericruciate cortex produced both ipsilateral terminal labeling which extended throughout the length of the nucleus of Darkschewitsch and into the perifascicular region, and ipsilateral retrograde neuronal labeling. Labeled neurons displayed a variety of shapes and sizes, were more numerous in sections cut at rostral levels of the nucleus of Darkschewitsch, and were located both within and outside fields of terminal labeling. Comparison of the distribution of labeling following cerebellar and cortical injections indicates that convergence and overlap of input from these two sources occur in the nucleus of Darkschewitsch. These findings provide the morphological basis for integration of cerebellar and cortical information in this nucleus which may, in turn, influence output from neurons which project to the cortex or to the inferior olivary nucleus.

The cerebral and cerebellar connections of pretecto-thalamic and pretecto-olivary neurons in the anterior pretectal nucleus of the cat

The neuronal connections of the anterior pretectal nucleus (PTA) were investigated in the cat. For the light microscopy, the retrograde double-labeling technique by means of Fluoro-Gold (FG) and horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) was used. Following injections of one tracer into the central lateral nucleus of the thalamus (CL) and the other into the dorsal accessory olivary nucleus (DAO), distributions of labeled neurons in the PTA were observed. Most of the labeled neurons were single-labeled either with FG or with WGA-HRP. The result indicated that pretecto-thalamic projection neurons were distributed throughout the whole extent of the PTA, whereas pretecto-olivary projection neurons were located in a restricted area of the ventral part of the PTA. Only a very small number of double-labeled neurons were found in the PTA. These two efferent projections thus seemed to be derived from different populations of PTA neurons. For the electron microscopy, a combination of retrograde transport of horseradish peroxidase (HRP) and anterograde degeneration technique was used. After HRP injections into the CL combined with lesions either in the motor cortex (MCx) or in the anterior interpositus nucleus of the cerebellum (Cbl), some degenerating axon terminals originating from the cerebrum or cerebellum were found to synapse with retrogradely labeled pretecto-thalamic projection neurons. We have already observed direct cerebral and cerebellar projections to the pretecto-olivary projection neurons (J. Comp. Neurol., 259 (1987) 348-363). We conclude that the two different populations of PTA neurons comprise two different kinds of neuronal circuitries, i.e. MCx-PTA-CL-MCx and Cbl-PTA-DAO-Cbl, and that these two circuitries might interrelate with each other in the PTA at the cellular level.

Presence of crossed corticorubral fibers and increase of crossed projections after unilateral lesions of the cerebral cortex of the kitten: a demonstration using anterograde transport of Phaseolus vulgaris leucoagglutinin

Brain lesions made during early developmental stages produce more prominent remodeling of synaptic organization than those made in adults. This difference in the extent of neuronal or synaptic plasticity between immature and mature animals may be due to difference in the capacity for axonal elongation. Alternatively, it could be due to the prevention of retraction of exuberant projections present only in the early developmental stages. Aberrant crossed corticorubral projections seen after neonatal hemispherectomy have been ascribed to collateral sprouting. To determine whether these results from the prevention of retraction of crossed fibers, we studied the corticorubral pathway in normal kittens and compared it with that observed after unilateral cortical lesion, using the plant lectin Phaseolus vulgaris leucoagglutinin (PHA-L). One to two weeks after injection of PHA-L, many immunocytochemically labelled fibers were observed in the red nucleus (RN) ipsilateral to the cortical injection. Although very few, labelled fibers were also seen in the RN contralateral to the injection in normal kittens. By contrast, many labelled fibers were seen in the RN contralateral to the injection in lesioned animals. Many growth-cone like axonal endings were also observed. The abundant crossed corticorubral fibers seen in lesioned animals may be ascribed to the increase in the number of fibers crossing the midline towards the contralateral RN or they could be due to increased branching of pre-existing crossed fibers.

An ultrastructural analysis of afferent terminals to the anterior pretectal nucleus in the cat

The synaptic organization of three kinds of afferent projections in the feline anterior pretectal nucleus (PTA) was analyzed by a combination of the degeneration and retrograde transport of horseradish peroxidase (HRP) techniques, along with that of the degeneration and anterograde transport of HRP techniques. Retrograde labeling of PTA neurons was performed by injections of HRP in the dorsal accessory olivary nucleus (DAO). Three kinds of afferent sources of the PTA--the cerebral motor cortex, the anterior interpositus nucleus of the cerebellum, and the gracile nucleus--were subjected to electrolysis, suction, or injection of kainic acid or HRP for identification of axon terminals of each system. Axon terminals of these different afferent sources identified by degeneration or anterograde HRP transport techniques showed similar morphological features: They were relatively large (1-6.5 microns in diameter), contained round or ovoid synaptic vesicles, and made asymmetrical synaptic contacts. When the degeneration study was combined with the retrograde HRP transport technique, some degenerating terminals from the motor cortex, anterior interpositus, or gracile nuclei were found to synapse directly with HRP-labeled dendrites or somata of the PTA neurons projecting to the DAO. Each combination of the degeneration and anterograde HRP transport techniques revealed the fact that neither degenerating nor HRP-labeled terminals were found to synapse with the same neuronal structure. These observations indicate that the PTA neurons relay afferent inputs from three different sources directly to the DAO, and that there is a possibility of parallel processing rather than convergence of three different afferent systems via the PTA to the DAO.

Anatomical organization of cortico-mesencephalo-olivary pathways in the cat as demonstrated by axonal transport techniques

Cortical projections arising from areas 4 and 6 and terminating in midbrain cell groups known to project to the inferior olive (IO) have been studied in the cat. Injections of the bidirectional tracers horseradish peroxidase (HRP) and wheat germ agglutinin (WGA) conjugated to HRP were made into the midbrain. All cases of lateralized midbrain injections resulted in virtually ipsilateral labelling of lamina V cortical neurons. Retrogradely labelled neurons in cortical areas 4 and 6 were found after injections located in the interstitial nucleus of Cajal (INC), nucleus of Darkschewitsch (ND), and in the caudal parafascicular (Pf) and subparafascicular (sPf) nuclei (perifascicular region, PF). Injections that were more caudal and within the parvi- and magnocellular red nucleus (RNp and RNm) labelled cells not only in areas 4 and 6 but also in portions of adjacent areas 3a, 3b, 5a, and 7. These midbrain injections also resulted in the anterograde labelling of projections observed to terminate in the ipsilateral IO. The distribution in the midbrain of projections arising from cortical areas 4 and 6, and portions of areas 3a and 3b, was studied with autoradiographic methods. After injections of tritium-labelled amino acids in those cortical areas, a pattern of largely ipsilateral terminations was revealed. Whereas all cortical areas studied labelled the PF, differential grain distributions in central mesencephalic nuclei were apparent after injections in various portions of the motor and adjacent somatosensory cortex. Injections involving the frontal eye fields (FEF) labelled the INC bilaterally, but ipsilateral terminations were densest. These cases also labelled the region of the fields of Forel. When the neck region of the cortex was involved in the injections, the more caudal aspects of the INC (INCc) and the RN were labelled. The cortical areas related to the upper limb gave rise to terminations in the ND and the RNp. Contributions to both ND and RNp inputs from injections in the FEF and neck regions were also occasionally but not consistently noted. A relatively discrete injection in the vibrissae field weakly labelled ND. Additional components of the motor cortical projections to the superior colliculus (SC) and pretectal nuclei were also analysed since those regions also project to the IO. Cortical regions involving the representation of the neck musculature were shown to project principally ipsilaterally to lamina IV of the SC as well as to the anterior pretectal nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

The nucleus of Darkschewitsch in the cat: a Nissl, Golgi, and electron microscope analysis

A light and electron microscope study was carried out to elucidate the cytoarchitectural organization of the nucleus of Darkschewitsch (ND) in the cat. From the anatomical staining methods, including Nissl and Golgi-Cox, it appears that the ND shows a clear heterogeneity of shape and size of the neuronal population. The small or medium-sized neurons show a high nuclear/cytoplasmic ratio and a modest basophilia. Spiny extrusions are present on many of the neurons, arranged either as varicosities giving a rosary feature or clumped in small groups over the dendritic processes; these are absent at the level of the soma. From the electron microscope analysis it appears that the neuropil is not very extensive because the neuronal bodies are numerous and compact. The synaptic complex is extensive both at the level of the nerve cell bodies and at the level of the neuropil. Since many of the synapses display the features typical of the inhibitory synapses, it is possible that they represent the anatomical basis of an inhibitory integrative function.

The termination of spinomesencephalic fibers in cat. An experimental anatomical study

The projections to the midbrain from the spinal cord have been investigated in the cat with the degeneration technique and by using horseradish peroxidase (HRP) as an anterograde tracer. Two types of spinal cord lesions were performed: 1) Cordotomies at cervical or thoracic levels transecting the ventral and lateral funiculi. 2) Transections of the ventral, ventrolateral, dorsolateral or dorsal funiculus, respectively, at cervical levels. In the anterograde tracing experiments HRP was injected into the spinal cord at cervical, lumbar or sacral levels. The results show large projections to the lateral and ventrolateral parts of the periaqueductal gray (PAG1), the posterior pretectal nucleus (PP) and the nucleus of Darkschewitsch (D). More moderate projections go to the medial division of the periaqueductal gray (PAGm), the cuneiform nucleus (CF), the mesencephalic reticular formation (MRF), lateral part of the deep layer of the superior colliculus (SP) and magnocellular medial geniculate nucleus (GMmc), while scattered spinal fibers are present in the dorsal part of the periaqueductal gray (PAGd), the external inferior collicular nucleus (IX), the intermediate layer of the superior colliculus (SI), the lateral part of the red nucleus (NR) and in the Edinger-Westphal portion of the oculomotor nucleus (3). In addition a few fibers are present in the interstitial nucleus of Cajal (CA) and anterior pretectal nucleus (PAc). The results indicate that at midcervical levels most of the spinomesencephalic fibers ascend in the ventral funiculus, with only a moderate fraction ascending in the ventral half of the lateral funiculus. Almost no fibers ascend in the dorso-lateral funiculus and none appear to pass in the dorsal funiculus. No distinct somatotopic pattern was found in the spinomesencephalic projections, but more fibers from cervicobrachial segments terminate in the rostral than in the caudal parts of the terminal fields in PAG, CF, SP and IX, while the lumbar fibers were more numberous in the caudal parts. PP seems to receive spinal fibers mainly from the caudal half of the cord.

Topographical arrangements of feline motor cortical projections onto the pretectum

The projections from motor cortex to pretectum were traced autoradiographically in 15 cats aimed at revealing a topographical arrangement. The anterior sigmoid and rostral coronal gyrus projected amply onto the ventrolateral part of the nucleus pretectalis anterior and to a large part of the ventrocaudal part of the nucleus pretectalis posterior. The posterior sigmoid gyrus rostral to the postcruciate dimple and the caudal part of the coronal gyrus also projected to the nucleus pretectalis anterior. In the posterior pretectal nucleus their target areas were smaller and located more ventromedially. The strongest projections originated from the coronal gyrus.

Cortical projections to nuclei adjacent to the oculomotor complex in the medial dien-mesencephalic tegmentum in the monkey

Cortical projections to cell groups surrounding the oculomotor complex were studied by using the retrograde and anterograde capabilities of the horseradish peroxidase (HRP) technique in old and new world monkeys. Fluid HRP injections or transcannular solid polyacrylamide HRP gel implants were made into the oculomotor nucleus (OMN) and adjacent nuclei to label retrogradely corticofugal neurons that project to this region, and cortical HRP gel implants were made in various areas of the frontal lobe to label anterogradely the trajectories and terminations of cortico-paraoculomotor projections and thus to confirm the retrograde findings. Projections to the paraoculomotor cell groups in the medial dien-mesencephalic tegmentum originate almost exclusively from the frontal lobe. Both retrograde and anterograde studies confirmed that the prearcuate cortex in the concavity of the arcuate sulcus, including the frontal eye field, and, to a lesser extent, suprarcuate rostral dorsal area 6 cortex and the dorsomedial convexity (area 9), project to the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) in the dorsal region of the prerubral field, nucleus of Darkschewitsch (ND), medial accessory nucleus of Bechterew (NB) and dorsomedial parvocellular red nucleus (dmPRN). The premotor area 6 and motor area 4 cortex, on the other hand, give rise to projections that target a larger portion of the parvocellular red nucleus, extending rostrally into the ventral region of the prerubral field, and a rather intense projection to the ND. The interstitial nucleus of Cajal (IC) was distinguished more by its light, or lack of, projections from the frontal cortex. The inferior parietal lobule (IPL, area 7) which has certain common physiological properties with the frontal eye field (FEF area 8) related to the oculomotor system, lacked retrogradely labeled neurons in all cases where transcannular gel implants into the OMN eliminated the possibility of HRP uptake in the corpus callosum or other structures traversed in needle injections, suggesting that the IPL affects eye movement primarily through its rostrally directed corticocortical associational connections with the FEF. In additional cases, the ND-NB-dmPRN configuration of cells that receives FEF input is shown to project to the inferior olivary complex (i.e., is pre-olivo-cerebellar), whereas riMLF and IC give rise to descending projections in the MLF, which target extraocular muscle motor nuclei, vestibular complex, and spinal cord. The results are discussed in terms of the potential role of the cerebral cortex in eye movement mechanisms.

An electron microscope study of the cortico-pretecto-olivary projection in the cat by a combined degeneration and horseradish peroxidase tracing technique

The anterior pretectal nucleus (PTA) of the cat was observed electron microscopically after motor cortical ablation and horseradish peroxidase (HRP) injection into the inferior olive of the same animal. Direct synaptic connections were found between degenerated cortico-pretectal axon terminals and dendrites of pretecto-olivary projection neurons retrogradely labeled with HRP in the ventrolateral part of the PTA. Therefore, this combined method revealed that the PTA is a relay station of the cortico-olivary projection.

Cortico-Darkschewitsch-olivary projection in the cat: an electron microscope study with the aid of horseradish peroxidase tracing technique

The nucleus of Darkschewitsch (ND) of the cat was observed electron microscopically after surgical ablation of the motor cortex and horseradish peroxidase (HRP) injection into the inferior olive of the same animals. Degenerated axon terminals containing pleomorphic synaptic vesicles were observed to synapse chiefly with medium-sized or small dendritic processes, some of which were labeled with HRP retrogradely. Therefore, a cortico-olivary projection which was relayed at the ND was revealed at an ultrastructural level.