The organization of the descending ventral pallidal projections in the monkey

We studied the distribution and light- and electron-microscopic morphology of neurons in the hippocampal formation containing nitric oxide synthase (NOS), and thus likely to release nitric oxide, a freely diffusible neuromediator implicated in long-term potentiation. Only a small fraction of hippocampal neurons contained NOS or its marker, NADPH diaphorase. Most of the positive neurons were in the pyramidal layer of the subiculum, stratum radiatum of Ammon's horn, and subgranular zone of the dentate gyrus. Positive neurons were also conspicuous in the molecular layer of the dentate gyrus and in the pyramidal layer of CA3, sparse in the pyramidal layer of CA2 and CA1, and almost absent from presubiculum and parasubiculum. Numerous positive fibers were seen, especially in stratum radiatum and stratum lacunosum-moleculare of Ammon's horn. Double staining experiments demonstrated that nearly all NADPH diaphorase-positive neurons in the hippocampus also contained γ-aminobutyric acid. On the basis of their morphology, distribution, and inhibitory neurotransmitter content, most NOS-positive cells in the hippocampus are probably local circuit neurons. These data suggest that nitric oxide in CA1 may function as a paracrine agent, rather than a spatially precise messenger, in long-term potentiation. © 1993 Wiley-Liss, Inc.     

Distribution of cerebellothalamic and nigrothalamic projections in the dog: A double anterograde tracing study

The distribution of nigrothalamic and cerebellothalamic projections was investigated in the dog by a double labeling strategy combining the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and tritiated amino acids. Following tritiated amino acid injections into the substantia nigra pars reticulata (SNr) and WGA-HRP injections into the contralateral cerebellar nuclei, we found that the nigrothalamic and cerebellothalamic afferents distribute to three main targets: the central portion of the ventral anterior nucleus (VA) and the ventral lateral nucleus (VL), the internal medullary lamina (IML) region, which includes the paralaminar VA, the mediodorsal nucleus (MD) and the central lateral nucleus (CL), and finally the ventromedial nucleus (VM). We observed three distribution patterns of labeled fibers: (a) Dense single label was observed in the central portion of VA following the SNr injections and in VL following the cerebellar nuclei injections. (b) A complementary pattern consisting of alternating foci of nigral and cerebellar label was found in the IML region. This pattern was also observed in the caudal intralaminar nuclei where cerebellar label predominated in the centrum medianum (CM), while the parafascicular nucleus (Pf) primarily contained nigral label. (c) An overlapping pattern of autoradiographic and WGA-HRP label was found in the lateral half of the VM. Overall, the distribution of nigrothalamic and cerebellothalamic projections was widespread throughout much of rostrocaudal thalamus. However, the pattern of projections varied along a continuum from lateral to medial thalamus. In lateral thalamus, nigral and cerebellar projections distributed to separate nuclei while in medial thalamus, the projection pattern changed to focal and complementary in the IML and overlapping in VM. Taken together, these thalamic projections may constitute crucial links in different functional channels involved in alerting and orienting mechanisms associated with motor behavior. Our findings also suggest that the organization of motor thalamic afferents in the dog shares similarities with the segregated and parallel circuitry characteristic of primates as well as with the overlapping and converging circuits of rodents and other carnivores.     

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.     

Overlapping corticostriatal projections from the supplementary motor area and the primary motor cortex in the macaque monkey: An anterograde double labeling study

The purpose of the present study was to determine if the cortical efferents from homologous body regions of the supplementary motor area (SMA) and the primary motor cortex (MI) project to separate or to overlapping regions in the striatum. In order to investigate the dual corticostriatal projections, we employed an anterograde double labeling paradigm in which two tracers could be simultaneously detected in the same histological section. Prior to the injections, the forelimb representation in the two cortical motor areas was identified by using intracortical microstimulation in four Japanese monkeys (Macaca fuscata). Multiple injections of biotinylated dextran amine (BDA) were made into the forelimb regions of MI and wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) was injected into the arm region of the SMA. In additional animals, the tracers were reversed such that BDA was injected into the SMA and WGA-HRP was injected into the MI. The tissue was processed sequentially using different chromogens in order to visualize both tracers in a single section. We analyzed the distribution of the ipsilateral anterograde label. The striatal labeling from each cortical area basically consisted of a wide band of patchy dense labeling interrupted by lighter labeling. The SMA striatal projections were located mainly within the putamen, distributing from the level of the anterior commissure to the most posterior extent of the putamen. At an intermediate level, the label spread obliquely from the ventrolateral edge of the putamen dorsomedially as far as the lateral edge of the caudate nucleus. The label from the MI was observed in comparable portions of the putamen, although the SMA projections were shifted more anterior and dorsomedial to the MI projections and the heaviest projections from the SMA and the MI were separately located. On the basis of the double anterograde labeling technique, we found considerable overlap mainly in the central portion of the putamen from the SMA and MI forelimb representation. These results suggest that the homologous body regions of the SMA and MI send widespread, and substantially overlapping projections, to portions of the striatum. © 1996 Wiley-Liss, Inc.     

The glutamate-enriched cortical and thalamic input to neurons in the subthalamic nucleus of the rat: Convergence with GABA-positive terminals

Neurons of the subthalamic nucleus play a key role in the normal physiology and the pathophysiology of the basal ganglia. In order to understand better how the activity of subthalamic neurons and hence the output of the basal ganglia are controlled, we have reexamined the topography and examined in detail the synaptology and neurochemical nature of the two major excitatory projections to the subthalamic nucleus, that from the cortex and from the parafascicular nucleus of the thalamus. The approach was to use anterograde neuronal tracing and postembedding immunocytochemistry for amino acid transmitters. In confirmation of previous findings the cortical and thalamic projections were topographically organized, although the topography was more finely organized, and the projections more extensive, than previously demonstrated. Cortical and thalamic terminals made asymmetrical synaptic contacts with the dendrites and spines of subthalamic neurons. The thalamic terminals contacted larger postsynaptic targets, and therefore presumably more proximal regions of subthalamic neurons, than did the cortical terminals. Quantitative analysis of the postembedding immunolabelled sections revealed that the cortical and thalamic terminals were significantly enriched in glutamate-immunoreactivity when compared to identified γ-aminobutyric acid (GABA)-positive terminals, supporting physiological studies that suggest that these projections use glutamate as their neurotransmitter. In addition a small population of nonanterogradely labelled terminals that formed asymmetrical synapses and were immunopositive for GABA were identified. A larger population of terminals that formed symmetrical synapses were also immunbpositive for GABA and were probably derived from the globus pallidus. The latter type of terminal was found to make convergent synaptic input with cortical or thalamic terminals on this dendrites and spines of subthalamic neurons, indicating that the “indirect pathways” by which information flows through the basal ganglia converge at the level of individual neurons in the subthalamic nucleus. © 1995 Wiley-Liss, Inc.     

Topographic organization of the ventral striatal efferent projections in the rhesus monkey: An anterograde tracing study

The ventral striatum is considered to be that portion of the striatum associated with the limbic system by virtue of its afferent connections from allocortical and mesolimbic areas as well as from the amygdala. The efferent projections from this striatal region in the primate were traced by using 3H aminoacids and Phaseolus vulgaris-leucoagglutinin (PHA-L). Particular attention was paid to the topographic organization of terminal fields in the globus pallidus and substantia nigra, the projections to non-extrapyramidal areas, the relationship between projections from the nucleus accumbens and the other parts of the ventral striatum, and the comparison between ventral and dorsal striatal projections. This study demonstrates that in monkeys a circumscribed region of the globus pallidus receives topographically organized efferent fibers from the ventral striatum. The ventral striatal fibers terminate in the ventral pallidum, the subcommissural part of the globus pallidus, the rostral pole of the external segment, and the rostromedial portion of the internal segment. The more central and caudal portions of the globus pallidus do not receive this input. This striatal output appears to remain segregated from the dorsal striatal efferent projections to pallidal structures. Fibers from the ventral striatum projecting to the substantia nigra are not as confined to a specific region as those projecting to the globus pallidus. Although the densest terminal fields occur in the medial portion, numerous fibers also extend laterally to innervate the dorsal stratum of dopaminergic neurons of the substantia nigra and the retrorubral area. Furthermore, they project throughout the rostral-caudal extent of the substantia nigra. Projections from the medial part of the ventral striatum reach the more caudally located pedunculopontine tegmental nucleus. Thus unlike the above described terminals in the globus pallidus, the ventral striatum project widely throughout the substantia nigra, a fact that indicates that they may contribute to the integration between limbic and other output systems of the striatum. Finally, the ventral striatum projects to non-extrapyramidal regions including the bed nucleus of the stria terminals, the nucleus basalis magnocellularis, the lateral hypothalamus, and the medial thalamus.     

The overlap of spinothalamic and dorsal column nuclei projections in the ventrobasal complex of the rat thalamus: a double anterograde labeling study using light microscopy analysis

Projections from the spinal cord and the dorsal column nuclei (DCN) to the ventrobasal complex of the thalamus (VB) were studied in the rat by using double anterograde labeling strategy. This strategy was based on the injection of 3H-leucine into the DCN and of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the spinal cord and their subsequent transport. Adjacent 30-micron-thick sections were then processed differentially for autoradiography or for HRP by using tetramethyl benzidine (TMB) as a chromogen. Similar areas of the ventrobasal complex were labeled, in adjacent sections, after a large injection of 3H-leucine into the DCN and when wheat germ agglutinin-HRP had been injected in any part of the spinal cord. If, however, a small injection of the radioactive tracer was centered in the gracile nucleus and compared with an injection of WGA-HRP placed in the lumbar enlargement of the cord, the rostral and dorsal portions of the lateral VB were labeled from both sources. On the other hand, if tritiated leucine was injected into the cuneate nucleus, and WGA-HRP placed in the cervical enlargement, then the caudal and ventral portions of the lateral VB demonstrated overlap of both labels. The present results show that, in the rat, areas of termination of both the spinothalamic tract and the lemniscal pathway originating from the DCN overlap in the lateral VB. This overlap is somatotopically organized, thus indicating that the same area of the VB receives somatic inputs from one particular part of the body through both pathways. These results are discussed in comparison to those of comparable studies performed in the cat and in the monkey and with reference to the electrophysiological data that have demonstrated that, in the rat VB, neurons responding to noxious stimulation are intermingled with neurons exclusively responding to non-noxious stimulation.     

Impact of chronic subthalamic high-frequency stimulation on metabolic basal ganglia activity: a 2-deoxyglucose uptake and cytochrome oxidase mRNA study in a macaque model of Parkinson's disease

The mechanisms of action of high-frequency stimulation (HFS) of the subthalamic nucleus (STN) remain only partially understood. Hitherto, experimental studies have suggested that STN-HFS reduces the activity of STN neurons. However, some recent reports have challenged this view, showing that STN-HFS might also increase the activity of globus pallidus internalis (GPi) neurons that are under strong excitatory drive of the STN. In addition, most results emanate from studies applying acute STN-HFS, while parkinsonian patients receive chronic stimulation. Thus, the present study was designed to assess the effect of chronic (10 days) STN-HFS in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated nonhuman primate. For this purpose, 2-deoxyglucose (2-DG) uptake, a measure of global synaptic activity, was assessed in the basal ganglia and the motor thalamus after chronic unilateral STN-HFS. Cytochrome oxidase subunit 1 (COI) mRNA expression, a marker of efferent metabolic activity, was additionally assessed in the globus pallidus. Chronic STN-HFS (i) reversed abnormally decreased 2-DG uptake in the STN of parkinsonian nonhuman primates, (ii) reversed abnormally increased 2-DG accumulation in the GPi while COI mRNA expression was increased, suggesting global activation of GPi neurons, and (iii) reversed abnormally increased 2-DG uptake in the ventrolateral motor thalamus nucleus. The simultaneous decrease in 2-DG uptake and increase in COI mRNA expression are difficult to reconcile with the current model of basal ganglia function and suggest that the mechanisms by which STN-HFS exerts its clinical benefits are more complex than a simple reversal of abnormal activity in the STN and its targets.     

Spatial distribution of thalamic projections to the supplementary motor area and the primary motor cortex: A retrograde multiple labeling study in the macaque monkey

The exact knowledge on spatial organization of information sources from the thalamus to the supplementary motor area (SMA) and to the primary motor cortex (MI) has not been established. We investigated the distribution of thalamocortical neurons projecting to forelimb representations of the SMA and the MI using a multiple retrograde labeling technique in the monkey. The forelimb area of the SMA, and the distal and proximal forelimb areas of the MI were identified by electrophysiological techniques of intracortical microstimulation and single neuron recording. Injections were made into these three representations with three different dyes in the same animal (horseradish peroxidase conjugated to wheat germ agglutinin, diamidino yellow, and fast blue), and the thalamic neurons were retrogradely labeled. Injections into the SMA densely labeled thalamic neurons in nuclei ventralis lateralis pars oralis (VLo), ventralis lateralis pars medialis (VLm) and ventralis lateralis pars caudalis (VLc), but not in nucleus ventralis posterior lateralis pars oralis (VPLo). Injections into the MI labeled thalamic neurons primarily in VLo, VLc, and VPLo. We found that the distribution of projection neurons to the three areas was largely separate in the thalamus. However, in the middle part of VLo, and in a limited portion of VLc, thalamic neurons projecting to the SMA partially overlapped with those to the distal forelimb area of the MI. They overlapped little with those to the proximal forelimb area of the MI. We noted no overlap between the distributions of thalamic projection neurons to the distal and proximal forelimb areas of the MI. These findings suggest that the SMA and MI receive separate information from the thalamus, while sharing minor sources of common inputs. © 1995 Wiley-Liss, Inc.     

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.     

Immunohistochemical and biochemical studies on Lys8-Asn9-neurotensin8-13 (LANT6)-related peptides in the basal ganglia of pigeons, turtles, and hamsters

The distribution of the neurotensin-related hexapeptide LANT6 within the basal ganglia and its projection targets was studied in turtles, pigeons, and hamsters by using immunohistochemical techniques, radioimmunoassay (RIA), gel chromatography, and high performance liquid chromatography (HPLC). The results in turtles and pigeons were fundamentally similar. Within the basal ganglia, LANT6-like immunoreactivity (LLI) was observed in a population of large striatal neurons (comprising 1-5% of the total number of striatal neurons) and in essentially all of the medium-large pallidal neurons. In addition, LLI was observed in neurons of such other "striatal" and "pallidal" cell groups as the olfactory tubercle and ventral pallidum, respectively. Within the dopaminergic cell fields of the tegmentum, to which the pallidal cell groups project, LLI-containing fibers were abundant. Knife-cut studies confirmed that the majority of these LLI-containing fibers arise from telencephalic levels. Biochemical studies with RIA and HPLC showed large amounts of immunoreactive LANT6 (iLANT6) in the basal telencephalon (477 pmol/g) and tegmentum of pigeons (259 pmol/g), and this material was indistinguishable from the synthetic peptide. Lower levels of iLANT6 were demonstrated in the basal telencephalon (82 pmol/g) and tegmentum (156 pmol/g) of turtles, and the majority of this activity appeared to be associated with larger molecular forms of LANT6 or a peptide related to LANT6. In addition, one or more substances resembling Neuromedin N (NMN), a mammalian counterpart to LANT6, were detected in the turtle nervous system. The labeling patterns in hamsters were similar to those in pigeons and turtles, except that in hamsters fewer neurons were labeled and the labeling was generally lighter. The lighter level of labeling may reflect a difference between the LANT6-like material present in hamster nervous system and authentic LANT6. Biochemical studies revealed that a Neuromedin N-like substance, as well as high molecular weight forms of a LANT6-like substance, are present in hamster brain. In hamsters, neurons within globus pallidus, the entopeduncular nucleus, the ventral pallidum, and the polymorph layer of the olfactory tubercle were labeled for the presence of LANT6. Fiber labeling for LANT6 in the dopaminergic tegmental cell groups that receive pallidal input was, however, light. Thus, the present results establish that LANT6 in pigeons and LANT6-related peptides in turtles and hamsters are present within many pallidal neurons. In pigeons and turtles, these pallidal neurons give rise to a major LLI-containing projection to the dopaminergic cell groups of the tegmentum.(ABSTRACT TRUNCATED AT 400 WORDS)     

Bilateral corticospinal projections arise from each motor cortex in the macaque monkey: a quantitative study

The corticospinal projection is considered to influence fine motor function through nearly exclusively contralateral projections from the cortex in primates. However, unilateral lesions to this system in various species are frequently followed by significant functional improvement, raising the possibility that bilateral projections of this pathway may exist or emerge after injury. To examine the detailed anatomy and projections of the corticospinal motor neurons in rhesus monkeys (n = 4), we injected the high-resolution anterograde tracer biotinylated dextran amine (BDA) into 126 sites centered about the right lower extremity (LE) primary motor cortex. Projection and termination patterns were quantified at lumbar levels L1, L4, and L7 and mapped by using serial-section reconstructions. Notably, a mean of 10.1 +/- 0.6% (+/- SEM) of corticospinal tract (CST) axons descended in the lateral CST ipsilateral to the cortical BDA injection, and 87.9 +/- 1.0% of total CST axons projected in the contralateral lateral CST. The ipsilateral ventral CST contained only 1.0 +/- 0% of all projecting CST axons, whereas the contralateral ventral CST contained 0.3 +/- 0.2% of all axons. In addition, a minor dorsal column CST projection was identified. Measurement of BDA-labeled terminals in the spinal cord gray matter revealed that 11.2 +/- 2.2% of CST axons terminated ipsilateral to the side of cortical injection, and the remainder terminated contralaterally. As previously reported, most CST axons terminated in spinal cord laminae V-VIII, as well as the laterodorsal motoneuronal group of lamina IX (which innervates distal extremity muscles). Notably, many CST axons crossed the spinal cord midline (mean 19.9 +/- 4.9 axons per 40-microm-thick section). Detailed single-axon reconstructions revealed that most ipsilaterally projecting lateral CST axons terminated in ipsilateral gray matter. Notably, we found that the bouton-like swellings of many ipsilateral CST axons descending in the dorsolateral tract were located within Rexed's lamina IX, in close proximity to motoneuronal somata. Thus, bilateral projections of corticospinal axons originating from a single motor cortex could contribute to bilateral control of spinal motor neurons and to the highly evolved degree of fine motor control in primates. Furthermore, bilateral CST projections from a single motor cortex could represent a potential source of plasticity after injury, as well as a target of therapeutic effort in neural regeneration strategies.     

Effects of intrapallidal cannabinoids on rotational behavior in rats: Interactions with the dopaminergic system

The effect of unilateral intrapallidal cannabinoid receptor stimulation on rotational behavior in rats was explored. The potent cannabinoid agonist CP55,940 (5 μg/0.5 μl) induced ipsilateral turning when microinjected unilaterally into the globus pallidus. The D₂ dopamine agonist quinpirole reversed this ipsilateral rotation but failed to affect motor behavior on its own. Finally, the D₁ dopamine agonist SKF 82958 inhibited movement when administered into the globus pallidus, and this effect was not additive with CP55,940. Synapse 28:27–32, 1998. © 1998 Wiley-Liss, Inc.     

Ultrastructural localization of immunoreactive calbindin-D28k in the rat and monkey basal ganglia, including subcellular distribution with colloidal gold labeling

Normal cellular function depends on the controlled flux of Ca++ within intracellular compartments and across the plasma membrane. Proteins that bind Ca++ are thought to contribute to the regulation of intracellular Ca++ and, perhaps more importantly, signal functional changes in cell activity. In the brain, calbindin-D28k is among a class of calcium-binding proteins that are widely and heterogeneously distributed in select populations of neurons, among them neostriatal cells, but whose function is largely unknown. In this study of the monkey and rat neostriatum and globus pallidus, calbindin-D28k was localized with immunoperoxidase and immunogold methods in order to identify striatal cell populations that contain this protein and the subcellular compartments in which it is likely to function. Light and electron microscopy showed intense and extensive labeling of immunoreactive calbindin-D28k in the cell bodies, dendrites, and spines of medium-sized neostriatal spiny neurons and in their axon terminals which end in the globus pallidus. More discrete labeling with a gold-conjugated second antibody showed that the predominant site of calbindin-D28k was the matrix of the cytoplasm. Gold label was also associated with the karyoplasm of spiny cells and with the neurofilaments and axoplasmic matrix of striatopallidal axons and terminals, respectively. Membranes were either sparsely labeled (endoplasmic reticulum, mitochondria) or devoid of gold particles (nuclear envelope and plasmalemma). Radioimmunoassays of striatal subcellular fractions supported the anatomical findings by indicating that the soluble fractions of neostriatal tissue homogenates contained most of the calbindin-D28k immunoreactivity and that washes from forebrain synaptosomes treated with Triton X-100 yielded high levels of immunoreactive calbindin-D28k. These findings show that immunoreactive calbindin-D28k is localized to spiny neurons of the striatopallidal pathway and are consistent with previous observations on subcellular localization in nonneuronal tissues. If, as recently speculated, calbindin-D28k regulates calcium concentrations in neostriatal spiny neurons, this feature may be particularly involved with the high density of glutamatergic inputs to these cells. More work is needed to determine whether calbindin-D28k, when complexed to Ca++ in neostriatal spiny cells, signals the activation of protein kinases, phosphorylation, and/or neurotransmitter release, as has been shown for other Ca++-binding proteins in mammalian tissues.     

Axonal branching of basal forebrain projections to the neocortex: a double-labeling study in the cat

Double-labeling of basal forebrain neurons by retrograde axonal transport demonstrates divergent collateralization among undecussated axonal projections to the neocortex. These branched fibers originate from a considerable complement of large polymorphic cell bodies located mainly in the basal nucleus of Meynert. They terminate in multiple neocortical sites including the precruciate, postcruciate and/or cingulate gyri. This extensive intra- and intergyral axonal branching indicates that neurons in the basal forebrain of the cat have extensive axonal fields innervating adjacent neocortical gyri.     

The ventral nucleus of the lateral lemniscus of the gerbil (Meriones unguiculatus): organization of connections with the cochlear nucleus and the inferior colliculus

The spatial organization of projections from the ventral cochlear nucleus (VCN) to the ventral nucleus of the lateral lemniscus (VNLL) and from the VNLL to the central nucleus of the inferior colliculus (CNIC) was investigated by using neuroanatomical tracing methods in the gerbil. In order to label cells in the VNLL that project to the CNIC, focal injections of biotinylated dextran amine (BDA) were made into different CNIC regions. Retrogradely labeled cells were distributed throughout the dorsal-to-ventral axis of the VNLL in all cases. In contrast, the distribution of labeled cells across the lateral-to-medial dimension of the VNLL was related to the location of the injection site along the dorsolateral to ventromedial (frequency) axis of the CNIC. Cells projecting to dorsolateral (low-frequency) regions of the CNIC were located peripherally in the VNLL, mainly laterally and caudally, whereas those projecting to ventromedial (high-frequency) regions of the CNIC tended to be clustered centrally. Projections to the VNLL were labeled anterogradely following injections of BDA in the VCN. The distribution of terminal fields in the VNLL closely paralleled the topographic arrangement of cells projecting to the CNIC; projections from ventrolateral (low-frequency) areas of the VCN terminated mainly along the lateral and caudal borders of the VNLL, whereas projections from dorsomedial (high-frequency) areas terminated in more central regions. The results demonstrate a topographic organization of the major afferent and efferent connections of the gerbil VNLL.     

Synaptic innervation of neurons in the internal pallidal segment by the subthalamic nucleus and the external pallidum in monkeys

In order to better understand the way by which the subthalamic nucleus interacts with the globus pallidus to control the output of the basal ganglia, we carried out a series of experiments to investigate the pattern of synaptic innervation of the pallidal neurons by the subthalamic terminals in the squirrel monkey. To address this problem we used the anterograde transport of biocytin. Following injections of biocytin in the subthalamic nucleus, rich plexuses of labelled fibres and varicosities formed bands that lay along the medullary lamina in both segments of the ipsilateral pallidum. At the electron microscopic level, two populations of bioctyin-containing terminals were identified in the internal pallidum (GPi). A first group of small to medium-sized terminals (type 1) mean cross-sectional area ±S. D. = 0.41 ± 0.04 μm² contained round vesicles and formed asymmetric synapses with dendritic shafts (95%) of mixed sizes (maximum diameter ranging from 0.3 to 4.0 μm) and spine-like structures (5%). This second group of terminals (type 2) contained pleiomorphic vesicles, had a larger cross-sectional area (mean ± S. D. = 0.9 ± 0.4 μm²) and formed symmetric synapses predominantly with perikarya (41%) and large dendrites (57%). In some cases, the two types of terminals converged at the level of single GPi neurons. Postembedding immunogold method revealed that the type 2 terminals displayed gamma-aminobutyric acid (GABA) immunoreactivity, whereas the type 1 terminals did not. In the external pallidum (GPe), injections in the subthalamic nucleus labelled both type 1 or type 2 terminals. However, the labelled type 2 boutons were much less abundant in GPe than in GPi. The presence of biocytin-labelled perikarya in GPe and the fact that the type 2 terminals displayed GABA immunoreactivity led us to suspect that these terminals were derived from axons of GPe neurons. In agreement with this hypothesis, injctions of Phaseolus vulgaris-leucoagglutinin (PHA-L) in GPe labelled terminals in GPi that displayed the morphological features and a pattern of synaptic organization similar to the type 2 terminals. In conclusion, the results of our study demonstrate that the subthalamopallidal terminals form asymmetric synapses that are distributed along the dendritic tree of GPe and Gpi neurons. In contrast, the GPe projection to GPi give rise to large GABA-containing terminals that form symmetric synapses predominantly with the proximal region of pallidal neurons. Because the GABAergic axon terminals from GPe form synapses onto the perikarya and proximal dendrites of GPi neurons, the Gpe input is in a strategic position to reduce the excitatory influence generated more distally on the dendritic tree by the subthalamic nucleus. © 1994 Wiley-Liss, Inc.     

Branched projections of pallidal and peripallidal neurons to neocortex and neostriatum: a double-labeling study in the cat

Double-labeling of basal forebrain neurons by retrograde axonal transport of different markers demonstrated afferents shared by the neocortex and neostriatum. A considerable double-labeled complement of neurons located in the globus pallidus (lateral pallidal segment) and the adjacent interdigitating basal nucleus of Meynert (peripallidal region) had branched axonal collaterals projecting to the precruciate, cingulate and prorean gyri as well as to the head of the caudate nucleus.     

Nonintralaminar thalamostriatal projections in the gray squirrel (Sciurus carolinensis) and tree shrew (Tupaia glis)

In mammals, the corpus striatum receives prominent projections from the neocortex and from the intralaminar nuclei of the dorsal thalamus. The present study provides evidence based on anterograde degeneration and axonal transport that the corpus striatum also receives input from two nonintralaminar thalamic nuclei, the pulvinar and the medial geniculate body. Each of these nuclei projects to a separate region of the corpus striatum. Moreover, the same regions of the corpus striatum that receive projections from the pulvinar and medial geniculate body also receive projections from the cortical targets of these nuclei.     

Topographical projection from the hippocampal formation to the amygdala: a combined anterograde and retrograde tracing study in the rat

The hippocampal formation and amygdala are responsible for regulating emotion, learning, and behavior. The hippocampal projection to the amygdala has been demonstrated to originate in the subiculum and adjacent portion of field CA1 of the Ammon's horn (Sub/CA1) in the rat; however, the topographical organization of this pathway is still understudied. To make it clear, we performed anterograde and retrograde tracing with biotinylated dextran amine (BDA) and cholera toxin B subunit (CTb), respectively, in the rat. A series of BDA experiments revealed that the temporal-to-septal axis of origin determined a medial-to-lateral axis of termination in the amygdala. Briefly, the temporal region of the Sub/CA1 projects preferentially to the medial amygdaloid region including the medial, intercalated, and basomedial nuclei and the amygdalohippocampal transition area, and progressively more septal portions of the Sub/CA1 distribute their efferents in more lateral regions of the amygdala. Sub/CA1 fibers distributed in the central amygdaloid nucleus were relatively few. Retrograde tracing with CTb confirmed this topography and revealed little hippocampal innervation of the central nucleus of the amygdala. These observations suggest that distinct Sub/CA1 regions arranged along the longitudinal hippocampal axis may influence distinct modalities of the amygdala function.