Organization of efferent projections from the internal segment of globus pallidus in primate as revealed by flourescence retrograde labeling method

The exact cellular origin and the degree of collaterlization of the major efferent projections from the internal segment of globus pallidus (GPi) in squirrel monkey (Saimiri sciureus) were studied using Evans blue (EB) and a mixture of DAPI-primuline (DP) as retrograde fluorescent tracers. After the conconmitant of EB in VA/VL thalamic nuclei and of DP in habenula on the same side, numerous EB-labeled cells were found in the central portion of GPi compared to a much smaller number of DP-labeled neurons mostly encountered at the periphery of GPi. Only very few double-labeled cells were visualized in these experiments indicating that the pallidohabenular and pallidothalamic pathways arise largely from two different cell populations, each having a preferential distribution in GPi. On the other hand, a multitude of both EB- and DP-labeled cells occurred in the central portion of GPi after the concomitant injection of EB in VA/VL nuclei and of DP in nucleus tegmenti pedunculopontinus of the midbrain tegmentum. Although the EB-labeled cells tend to be more abundant in the dorsolateral half, and the DP-labeled cells more numerous in the ventromedial half of GPi, about 70–75% of the cells in the core of GPi were double-labeled in such a case. This indicates that the pallidothalamic and pallidotegmental fibers arise largely from the same neurons in the core of GPi. A number of DP-labeled cells was also found in the contralateral GPi revealing that the pallidotegmental pathway is partly (15–20%) crossed. In addition, numerous DP-labeled cells (projecting to brain stem) occured in the medial two-thirds of the substantia nigra pars reticulata (SNr), whereas EB-labeled cells (projecting to the thalamus) abounded in the lateral third of SNr. A small number of double-labeled SNr cells were also encountered after thalamus-midbrain injection.These findings suggest that in regard to its output elements, the primate GPi is organized according to a complex pattern consisting of: (1) a central ‘motor’ zone where most neurons send axonal branches to both thalamus and midbrain; and (2) a peripheral ‘limbic’ zone which encroaches largely upon the lateral hypothalamus and whose cells project only to habenula. These two pallidal zones are furthermore embedded in a peripallidal neuronal network composed of large acetylcholinesterase-containing cells related to nucleus basalis and projecting diffusely to neocortex.     

Efferent projections of the subthalamic nucleus in the squirrel monkey as studied by the PHA-L anterograde tracing method

The organization of the efferent connections of the subthalamic nucleus was studied in the squirrel monkey (Saimiri sciureus) by using the lectin Phaseolus vulgaris-leucoagglutinin (PHA-L) as an anterograde tracer. At the level of the basal forebrain, anterogradely labeled fibers and axon terminals were mostly found in the striatopallidal complex and the substantia innominata. In cases in which the PHA-L injection sites were placed in the central or the lateral third of the subthalamic nucleus, numerous anterogradely labeled fibers were seen to arise from the injection loci and innervate massively the globus pallidus. At pallidal levels the fibers formed bands lying parallel and adjacent to the medullary laminae. The number and the complexity of the topographical organization of these bands varied with the size and the location of the PHA-L injection site. When examined at a higher magnification, the bands of subthalamopallidal fibers appeared as rich plexuses of short axon collaterals with small bulbous enlargements that closely surrounded the cell bodies and primary dendrites of pallidal cells. In contrast, PHA-L injection involving the medial tip of the subthalamic nucleus did not produce bandlike fiber patterns in the globus pallidus. Instead, the labeled fibers formed a diffuse plexus occupying the ventral part of the rostral pole of the globus pallidus as well as the subcommissural pallidal region. The substantia innominata contained a moderate number of labeled fibers and axon terminals following injection of PHA-L in the medial tip of the subthalamic nucleus. A small to moderate number of anterogradely labeled fibers were seen in the putamen after all PHA-L injections. These subthalamostriatal fibers were long, linear, and branched infrequently. At midbrain level the substantia nigra contained a significant number of anterogradely labeled fibers and axon terminals following PHA-L injection in the subthalamic nucleus. The subthalamonigral fibers descended along the ventromedial part of the cerebral peduncle and swept laterally to reach their target. Most of these fibers formed small plexuses along the base of the pars reticulata, whereas a few others ascended along the cell columns of the pars compacta that impinged deeply within the pars reticulata. More caudally in the brainstem, a small number of fibers occurred in the area of the pedunculopontine nucleus and in the periaqueductal gray. These findings indicate that besides its well-known connection with the pallidum, the subthalamic nucleus gives rise to widespread projections to other components of the basal ganglia in primates.     

Organization of the efferent projections of the medial superior olivary nucleus in the cat as revealed by HRP and autoradiographic tracing methods

Features of the organization of the efferent axonal projections from the medial superior olivary nucleus (MSO) in the cat were studied. In order to determine the origin and distribution of projections from MSO, the retrograde horseradish peroxidase (HRP) and autoradiographic tracing methods were used. The results showed that (1) in both HRP and autoradiographic studies the projection to the inferior colliculus was largely ipsilateral, although a contralateral component was present; (2) the projection field of MSO was confined to the ventral division of the central nucleus of the inferior colliculus, and within this field the labeling was heavier in the rostral and dorsolateral parts of the ventral division; (3) the projection to the inferior colliculus was topographic with ventral parts of MSO projecting ventrally and dorsal parts of MSO projecting dorsolaterally; (4) the projection field in the central nucleus formed successive laminae oriented from ventrolateral to dorsomedial; (5) the axonal course was via the medial or internal segment of the lateral lemniscus; and (6) some fibers in this course ended additionally within the dorsal nucleus of the lateral lemniscus. This latter projection was also topographically organized. These observations supported previously described features of lamination and tonotopic order for afferents of the inferior colliculus, as well as recent suggestions that functional segregation of afferent connections exists within the laminated portion of the central nucleus of the inferior colliculus.     

Pedunculopontine nucleus in the squirrel monkey: Projections to the basal ganglia as revealed by anterograde tract-tracing methods

The efferent projections of the pedunculopontine nucleus (PPN) to the ganglia have been studied in the squirrel monkey (Saimiri sciureus) with [³H]leucine and Phaseolus vulgaris-leucoagglutinin (PHA-L) as anterograde tracers. Following unilateral injections of [³H]leucine or PHA-L in the central portion of the PPN, numerous autoradiographic linear profiles or PHA-L-labeled fibers ascend to the forebrain, both ipsilaterally and contralaterally. These fibers form a compact bundle that courses in the central portion of the mesopontine tegmentum. At rostral mesencephalic levels, theis bundle splits into ventromedial and dorsolateral fascicles that arborize in basal ganglia and thalamic nuclei, respectively. The substantia nigra and the subthalamic nucleus are by far the most densely innervated structures of the basal ganglia. In these two nuclei, labeled fibers arborize profusely ipsilaterally and less abundantly contralaterally. The labeled fibers in the substantia nigra are thin and varicose and arborize almost exclusively in the pars compacta, where they closely surround the soma and proximal dendrites of dopaminergic neurons. In the subthalamic nucleus labeled fibers are also thin and appear to contact more than one neuron along their course. Numerous labeled fibers also occur in the pallidal complex, where they arborize most profusely in the internal segment. Several thick, labeled fibers oriented dorsolaterally in the pallidal complex give rise to thinner fibers that closley surround the soma and proximal dendrites of pallidal neurons. Some labeled fibers are also scattered in the striatum. These fibers abound in the peripallidal and ventral portions of the putamen, are more sparsely distributed in the remaining portion of the putamen as well as in the caudate nucleus, and are virtually absent in the ventral striatum. These results reveal that the PPN gives rise to a massive and highly ordered innervation of the basal ganglia in the squirrel monkey. This nucleus may thus act as an important relay in the basal ganglia circuitry in primates. © 1994 Wiley-Liss, Inc.     

Distribution of the efferent projections of the rat posterior thalamic nucleus as revealed by Phaseolus vulgaris immunohistochemistry.

Following iontophoretic injection of Phaseolus vulgaris in the rat posterior thalamic nucleus (PT) dense terminal fields were labeled in the ipsilateral red nucleus, zona incerta, intermediate and deep layers of the superior colliculus, pretectum, basilar pontine gray, and dorsal pole of the caudal division of the ventral lateral thalamic nucleus. Several other structures were more sparsely labeled; the reticular gigantocellular and pontine oral nuclei, nucleus of the mesencephalic tract of the trigeminal nerve, deep mesencephalic and posterior hypothalamic nuclei and the rostral part of the lateral posterior thalamic nucleus. The motor and, to a lesser extent, somatosensory cortices were labeled only following tracer injection in the rostral PT. The posterior thalamic nucleus thus projects to several centers primarily treating somatosensory and/or nociceptive messages, and to precerebellar and motor nuclei. Since a previous study has established that the PT also receives a major somatosensory and/or nociceptive input (Roger and Cadusseau, 1984), a non-specific, modulatory role of the PT is therefore proposed in the treatment of some aspects of somatosensory and/or nociceptive information.     

Distribution and ultrastructural features of the serotonin innervation in rat and squirrel monkey subthalamic nucleus

The main purpose of this light and electron microscopic immunocytochemical study was to characterize and compare the serotonin (5‐HT) innervation of the subthalamic nucleus (STN) in rats and squirrel monkeys (Saimiri sciureus) following labeling with an antibody against the 5‐HT transporter (SERT). Unbiased counts of SERT+ axon varicosities revealed an average density of 5‐HT innervation higher in monkeys (1.52 × 10⁶ varicosities/mm³) than rats (1.17 × 10⁶), particularly in the anterior half of the nucleus (1.70 × 10⁶). As measured by electron microscopy, SERT+ axon varicosity profiles in the STN of both species were smaller than unlabeled profiles. The number of SERT+ profiles displaying a synaptic junction indicated that, in both rat and monkey STN, approximately half of 5‐HT axon varicosities were asynaptic. In monkeys, all synaptic junctions made by SERT+ varicosities were asymmetrical, as opposed to only 77% in rats. Despite the higher density of 5‐HT innervation in the anterior half of monkey STN, the ultrastructural features of its SERT+ varicosities, including synaptic incidence, did not significantly differ from those in its posterior half. These findings suggest that, throughout the rat and monkey STN, 5‐HT afferents may exert their influence via both synaptic delivery and diffusion of 5‐HT, and that an ambient level of 5‐HT maintained in STN by these two modes of transmission might also modulate neuronal activity and influence motor behavior. A better understanding of the factors governing the complex interplay between these signaling processes would greatly improve our knowledge of the physiopathology of the STN.     

The efferent projections of the rat lateral habenular nucleus revealed by the PHA-L anterograde tracing method

The efferent connections of the rat lateral habenular nucleus (LHb) were demonstrated using anterograde transport of the lectin Phaseolus vulgaris leucoagglutinin (PHA-L). Following PHA-L injections into the LHb, neuronal somata located in the lateral two thirds of the LHb were labeled with PHA-L. Individual axonal fibers and terminal specializations were clearly visible. This permitted detailed mapping of both efferent fiber pathways and terminal distributions. Previous reports on fiber pathways were substantially confirmed and several new findings were revealed. (1) Major rostrally oriented fibers enter the medial forebrain bundle via 3 routes which initially branch from the fasciculus retroflexus: the mediodorsal thalamic nucleus and ventromedial thalamic nucleus; the zona incerta and fields of Forel; and the ventral tegmental area of Tsai. (2) A major decussation to the contralateral thalamic nuclei occurs in the central medial thalamic nucleus. (3) Caudally directed fibers follow two courses: one to the deep mesencephalic nucleus, central grey, dorsal raphe nucleus and the deep layers of the superior colliculus; and the other to the median raphe nucleus, oral pontine reticular formation and raphe pontis nucleus. The present results offer more detailed information concerning the dorsal diencephalic system.     

The magnocellular and parvocellular divisions of the monkey subthalamic nucleus as revealed by cluster analysis of neuronal sizes.

Cluster analysis of neuronal somal sizes in the subthalamic nucleus of rhesus monkeys from newborn to adult age allows the segregation of two territories with predominance of small and large cells, respectively. The topographic distribution of the 'parvocellular' and 'magnocellular' segments is similar when samples are obtained from coronal, horizontal and sagittal series of sections. The parvocellular component occupies the rostral pole, the entire rostrocaudal extent of the medial tip and dorsomedial border, and probably also the caudal cap. The magnocellular segment is in the central core extending to the ventrolateral border except for the medial tip. These findings and their correlation with the results of other morphologic and physiologic studies allow the following conclusions. (1) The monkey subthalamic nucleus contains at least two differentially distributed cell subpopulations. (2) The magnocellular division is more related to the pallido-subthalamic-pallidal loop involving the lateral pallidal segment. (3) The parvocellular division appears strategically located to control the pallidal output to diencephalic and mesencephalic targets. (4) Cluster analysis can reveal the existence of more than one neuronal population in a particular brain structure where an overall unimodal distribution of cell sizes may suggest the presence of a single type.     

Serotoninergic projections from the raphe nuclei to the preoptic area in sheep as revealed by immunohistochemistry and retrograde labeling.

A retrograde tracer, fluorogold, was injected into the sheep preoptic area in order to demonstrate the origin of the serotoninergic fibers observed in this area. Within the raphe nuclei, retrogradely fluorogold-labeled neurons were observed mainly in the median raphe nucleus (B8), groups B6/B5, and in the area lateral to the nucleus interpeduncularis (group S4), but not in the dorsal raphe nucleus. About 50% of these fluorogold-containing neurons were immunostained with a specific antiserum raised against serotonin. Double-labeled neurons (serotonin-immunoreactive and fluorogold containing neurons) represented less than 20% of the whole number of serotoninergic neurons. We concluded that a few serotoninergic neurons in the median raphe nucleus and in groups B5/B6 and S4 project to the preoptic area. Moreover, these nuclei contained non-serotoninergic neurons which project to the same area. These results give new information on the serotoninergic innervation of the preoptic area in the sheep.     

Spinal projections from the nucleus locus coeruleus and nucleus subcoeruleus in the cat and monkey as demonstrated by the retrograde transport of horseradish peroxidase

The rostral pons of the cat and rhesus monkey were examined for the presence of labeled cells following injections of horseradish peroxidase (HRP) into the lumbar spinal cord. Labeled cells were found in the ipsilateral dorsolateral pontine tegmentum and in the contralateral ventrolateral pontine reticular formation. In both the cat and monkey, labeled cells were located in the nucleus locus coeruleus, nucleus subcoeruleus, in or near the Kölliker-Fuse nucleus, and in the ventral part of the lateral parabrachial nucleus. There is a striking similarity between the distribution of HRP-labeled cells in the dorsolateral pontine tegmentum of the cat and monkey and that of catecholamine-containing cells observed in this area in previous studies.     

Afferent and efferent connections of the parabigeminal nucleus in cat revealed by retrograde axonal transport of horseradish peroxidase

Afferent and efferent connections of the parabigeminal nucleus (PBG) of the cat have been demonstrated by means of horseradish peroxidase (HRP) tracing technique. Following HRP injection in the PBG, labelled cells were observed mainly in the deep layers of the ipsilateral superior colliculus (SC). The other labelled structures were the prepositus hypoglossi complex (PH), the ventral nucleus of the lateral geniculate body (LGV), the locus coeruleus, the cuneiform nucleus, the periaqueductal gray and the dorsomedial hypothalamic area. Efferent projections of the PBG were investigated by HRP injection in SC, LGV, PH, hypothalamus and in some acoustic relays, i.e. medial geniculate body and inferior colliculus. Only the PBG-SC projection appeared to be well systematized. The positive labelling of the PBG following injection of LGV and hypothalamus is discussed in terms of the specificity of the injection. The absence of afferent and efferent connections of the PGB with any acoustic relay tends to exclude this nucleus from the auditory system in contrast to previous suggestions. On the basis of the close reciprocal PBG-SC connections a possible role of the PBG within visuomotor tectal function is proposed.     

Fiber projections from trigeminal nucleus caudalis in primate (squirrel monkey and baboon)

Small radiofrequency lesions were produced in the trigeminal nucleus caudalis and adjacent lateral reticular formation (LRF) in seven squirrel monkeys and four baboons. Frozen brain sections were stained for cellular morphology and for axonal and terminal degeneration. The lesions in pars marginalis and gelatinosa produced terminal degeneration in pars magnocellularis and LRF. Pars magnocellularis lesions produced degeneration into the ascending trigeminal intranuclear tract and the adjacent lateral reticular formation. Ascending thalamic and reticular projections emanated from LRF lesions. These observations suggest that elements of the LRF adjacent to the nucleus caudalis are an integral part of the trigeminal somesthetic fiber projection system and may subserve the perception of noxious stimuli.     

Afferents to the oculomotor nucleus in the goldfish (Carassius auratus) as revealed by retrograde labeling with horseradish peroxidase.

The goal of this work was to compare the distribution and morphology of neurons projecting to the oculomotor nucleus in goldfish with those previously described in other vertebrate groups. Afferent neurons were revealed by retrograde labeling with horseradish peroxidase. The tracer was electrophoretically injected into the oculomotor nucleus. The location of the injection site was determined by the antidromic field potential elicited in the oculomotor nucleus by electrical stimulation of the oculomotor nerve. Labeled axons whose trajectories could be reconstructed were restricted to the medial longitudinal fasciculus. In order of quantitative importance, the afferent areas to the oculomotor nucleus were: (1) the ipsilateral anterior nucleus and the contralateral tangential and descending nuclei of the octaval column. Furthermore, a few labeled cells were found dorsomedially to the caudal pole of the unlabeled anterior octaval nucleus; (2) the contralateral abducens nucleus. The labeled internuclear neurons were arranged in two groups within and 500 microns behind the caudal subdivision of the abducens nucleus; (3) a few labeled cells were observed in the rhombencephalic reticular formation near the abducens nucleus, most of which were contralateral to the injection site. Specifically, stained cells were found in the caudal pole of the superior reticular nucleus, throughout the medial reticular nucleus and in the rostral area of the inferior reticular nucleus; (4) eurydendroid cells of the cerebellum, located close to the contralateral eminentia granularis pars lateralis, were also labeled; and (5) a small and primarily ipsilateral group of labeled cells was located at the mesencephalic nucleus of the medial longitudinal fasciculus. The similarity in the structures projecting to the oculomotor nucleus in goldfish to those in other vertebrates suggests that the neural network involved in the oculomotor system is quite conservative throughout phylogeny. Nevertheless, in goldfish these projections appeared with some specific peculiarities, such as the cerebellar and mesencephalic afferents to the oculomotor nucleus.     

Parvalbumin and calbindin D-28k in the entopeduncular nucleus,subthalamic nucleus,and substantia nigra of the rat as revealed by double-immunohistochemical methods

The cellular localization of calbindin D-28k (CB) and parvalbumin (PV) was analyzed by means of double-immunohistochemical techniques applied to single sections in the entopeduncular nucleus (EP), the subthalamic nucleus (STh), and the substantia nigra (SN) of the rat. In EP, PV-positive cells abounded centrally, where CB immunostaining was minimal. The medial and ventral sectors of EP were markedly enriched with CB neurophil but devoid of PV-positive cells. CB-positive neurons abounded particularly in the rostral pole of EP. In STh, PV-positive neurons and neuropil were concentrated in the lateral two thirds of this nucleus. Only a few PV-positive cells were detected in sectors of STh devoid of PV-positive neuropil. The STh was completely devoid of CB immunostaining. In the rostral two thirds of SN, PV-positive neurons were largely confined to the lateral half of the pars reticulata (SNR), and occurred more ventrally and medially in the caudal third. Intense CB-immunoreactive neuropil was found in medial and dorsal parts of rostral SNR, and CB-positive cells were observed in the SN pars compacta and the ventral tegmental area. PV and CB cells were also observed in the pars lateralis of SN. The markedly heterogeneous pattern of distribution of PV and CB in EP, STh, and SN suggests that these two calcium-binding proteins may label distinct functional domains in each of these three components of the rat basal ganglia. Synapse 25:359–367, 1997. © 1997 Wiley-Liss, Inc.