Time course of the reaction of glial fibers in the somatosensory thalamus after lesions in the dorsal column nuclei

These experiments were designed to examine the relationship of glial hypertrophy to the time course of reactive synaptogenesis in the ventral posterolateral nucleus of the rat thalamus after lesions in the dorsal column nuclei. Because synaptogenesis is delayed for 30 days following lesions of the dorsal column nuclei, the initial hypertrophy of the glial processes in response to degeneration can be separated temporally from synaptogenesis. Glial hypertrophy was determined by measuring the relative area of neuropil occupied by profiles of glial processes on electron micrographs. The initial glial hypertrophy reached its peak 2 days after the lesion. However, at the time when synaptogenesis began, the area of neuropil occupied by glial processes was less than normal. When synaptogenesis was complete, the area of glial profiles also returned to normal. The role of glia in synaptogenesis was clearly different from its role in response to degeneration. In those systems such as the hippocampus, in which reactive synaptogenesis starts early in the recovery sequence, the relationship of glia to synaptogenesis may be masked by the glial response to degeneration. Hypertrophy of glial processes after lesions of other afferent pathways to the ventral posterolateral nucleus was compared to the hypertrophy following lesions of the dorsal column nuclei in order to see if there was a special relationship between glia and the lemniscal afferents to the ventral posterolateral nucleus. Lesions were placed in the medial lemniscus, somatosensory cortex, and the mesencephalon in addition to the dorsal column nuclei. The area of neuropil occupied by the glial processes expanded markedly after each of the lesions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nuclei of the lateral lemniscus project directly to the thalamic auditory nuclei in the pigeon

Non-mesencephalic origins of ascending afferents to the thalamic auditory nuclei, nuclei ovoidalis (Ov) and semilunaris parovoidalis (SPO), were identified in the pigeon in retrograde tracing experiments. These origins comprise dorsal and ventral lateral lemniscal nuclei. Injections of anterograde tracers into these nuclei produced terminal labelling in SPO in particular. These experiments show for the first time that the inferior colliculus is not an obligatory relay to the auditory thalamus.     

Lesions of the dorsal column nuclei or medial lemniscus of the cat: effect on motor performance

Motor performance in cats was evaluated by means of a beam-walking task after bilateral lesions were made in dorsal column nuclei (DCN) or medial lemniscus (ML) near its entrance to thalamus. Coordinated motor activity was not significantly impaired by ML lesions or by DCN lesions limited to the main nuclei, but was impaired by larger lesions to DCN that also involved the external cuneate nuclei.     

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.     

An input-output analysis of the dorsal column nuclei

The input-output (IO) relation of a nucleus is defined here as the total spike activity leaving the nucleus as a function of the total activity entering the nucleus measured over an appropriate time interval. The purpose of this study was to elucidate the mechanisms underlying the shape of this function. Parallel studies were conducted on chloralose-anesthetized cats to provide IO functions based on both evoked potential and single cell measures of output. Two sets of bipolar electrodes applied to the superficial radial nerve were used to stimulate and record input activity of various intensities. A bipolar electrode placed in the contralateral medial lemniscus provided evoked potential measures of output in one series of experiments. Glass micropipet electrodes placed in the cuneate nucleus provided single cell measures of output in the other series. We found good agreement between the average IO curves generated from the evoked potential and pooled single cell measurements. The single cell measurements showed further that the steep rise in the normalized IO relationship is due primarily to the recruitment of additional output cells by the increasing input. The saturation of output, which appears at 50 to 60% of the maximum input magnitude, reflects a limiting of both the number of cells active and the number of impulses evoked per active cell. The input-output relationships are well described by the function y = 1 ? e^?kx, derived from a simple saturation model of the nucleus. This model and our experimental data are consistent with the concept that single afferent fibers are capable of activating several output cells and, conversely, that each output cell receives input from several such afferent fibers.     

Brainstem and spinal projections of the deep cerebellar nuclei in the monkey, with observations on the brainstem projections of the dorsal column nuclei

The cytoarchitecture of the ventral lateral region of the primate thalamus has been appraised in the frontal, parasagittal and horizontal planes. A morphologically distinct region, possessing a sparse and diffuse distribution of large and small neurons is identified. The region includes several nuclei previously separately named by Olszewski⁴⁵. These are nuclei VPLo, VLc, X, VLps, and some cellular extensions into the VLo nucleus. The whole zone is continuous, and it is shown that no clear separation exists between any of the previously identified sub-nuclei. Connectional grounds are given for suggesting that this region should be considered as a common cerebellar relay nucleus to motor cortex.Morphological criteria for distinguishing the cellsparse nucleus from adjacent nuclei are given. These cytological criteria provide a basis for the experimental analysis of cortical and subcortical connectivity of the ventral lateral thalamic region. Close attention was paid to the border between the VPLo nucleus and the VPLc nucleus. VPLc is separated from VPLo by a clear border, and no transitional zone can be detected in the parasagittal or horizontal planes. Previous ambiguities in the delineation of the VPLo-VPLc border probably stem from analysis in the frontal plane, in which the border is not clear.     

The intercollicular region in the cat: a possible relay in the parallel somatosensory pathways from the dorsal column nuclei to the posterior complex of the thalamus

Neuronal connections of the intercollicular region were studied in the cat by the anterograde and retrograde WGA-HRP and HRP methods. The results indicate that some neurons in the intercollicular region, which comprises the intercollicular nucleus, external and pericentral nuclei of the inferior colliculus, and nucleus of the brachium of the inferior colliculus, receive afferent fibers from the dorsal column nuclei, bilaterally with a contralateral dominance, and send their axons to the lateral division of the posterior complex of the thalamus, bilaterally with an ipsilateral predominance.     

Distribution of the postsynaptic dorsal column projection in the cuneate nucleus of monkeys

Cells in the spinal cord that are postsynaptic to primary afferent fibers project to the dorsal column nuclei in the postsynaptic dorsal column pathway. The projection of cells in the cervical spinal cord of monkeys to the cuneate nucleus has been reported to avoid pars rotunda of that nucleus, the part that contains the somatotopic representation of the ipsilateral hand. We used the sensitive anterograde tracer Phaseolus vularis leucoagglutinin to reexamine this projection. We made multiple iontophoretic injections into the cervical enlargements of three monkeys (two Macaca fascicularis and one Macaca mulatta). Control injections were made in the contralateral dorsal columns of one of these and in the dorsal roots of a fourth animal (M. fascicularis) to test for transport by fibers of passage. After 28–39 days, the animals were deeply anesthetized and perfused, and the tissue was processed for immunohistochemical detection of the label. In all cases (excluding control injections), labeled fibers and varicosities were distributed widely in the ipsilateral cuneate and external cuneate nuclei, including pars rotunda. The dorsal column nuclei ipsilateral to control injections contained no label or only very few poorly labeled fibers, indicating that labeling through fibers of passage did not contribute importantly to the results. This study indicates that the postsynaptic projection to the cuneate nucleus is widespread and includes pars rotunda. Such projections may contribute to transmission of information originating in nociceptors through the dorsal column-medial lemniscal system to the ventrobasal thalamus. © Wiley-Liss, Inc.     

Ascending projections of the primary cochlear nuclei and nucleus laminaris in the pigeon

Auditory projections were studied, by the Nauta method, from medullary to mesencephalic levels following lesions in nuclei magnocellularis, angularis and laminaris and transection of the dorsal cochlear decussation and trapezoid body in the midline of the medulla. Fragmented axons project bilaterally to nucleus laminaris from the medial part of nucleus magnocellularis. Degenerated fibers from the lateral part of nucleus magnocellularis, medial part of nucleus angularis. and nucleus laminaris projects to the homolateral superior olivary nucleus. cross the raphé in the trapezoid body, ascend in the contralateral lateral lemniscus, distribute to the ventral and lateroventral nuclei of the lateral lemniscus and, at least third order axons from nucleus laminaris. terminate in nucleus mesencephali lateralis pars dorsalis. No ascending auditory neurons project, even following midventral section of the trapezoid body, to nucleus isthmi, nucleus semilunaris nor. with certainty, to the dorsal nucleus of the lateral lemniscus. This study supports the homology of the avian nucleus mesencephali lateralis pars dorsalis and nucleus laminaris with the mammalian central nucleus of the inferior colliculus and medial superior olivary nucleus respectively. Furthermore, on the basis of fiber projections and cellular organization. nucleus magnocellularis of the pigeon appears to correspond to the anterior ventral cochlear of higher mammals and the medial parts of nucleus angularis to the posterior ventral cochlear nucleus.     

Efferent projections of reuniens and rhomboid nuclei of the thalamus in the rat

The nucleus reuniens (RE) is the largest of the midline nuclei of the thalamus and exerts strong excitatory actions on the hippocampus and medial prefrontal cortex. Although RE projections to the hippocampus have been well documented, no study using modern tracers has examined the totality of RE projections. With the anterograde anatomical tracer Phaseolus vulgaris leuccoagglutinin, we examined the efferent projections of RE as well as those of the rhomboid nucleus (RH) located dorsal to RE. Control injections were made in the central medial nucleus (CEM) of the thalamus. We showed that the output of RE is almost entirely directed to the hippocampus and "limbic" cortical structures. Specifically, RE projects strongly to the medial frontal polar, anterior piriform, medial and ventral orbital, anterior cingulate, prelimbic, infralimbic, insular, perirhinal, and entorhinal cortices as well as to CA1, dorsal and ventral subiculum, and parasubiculum of the hippocampus. RH distributes more widely than RE, that is, to several RE targets but also significantly to regions of motor, somatosensory, posterior parietal, retrosplenial, temporal, and occipital cortices; to nucleus accumbens; and to the basolateral nucleus of amygdala. The ventral midline thalamus is positioned to exert significant control over fairly widespread regions of the cortex (limbic, sensory, motor), hippocampus, dorsal and ventral striatum, and basal nuclei of the amygdala, possibly to coordinate limbic and sensorimotor functions. We suggest that RE/RH may represent an important conduit in the exchange of information between subcortical-cortical and cortical-cortical limbic structures potentially involved in the selection of appropriate responses to specific and changing sets of environmental conditions.     

GABAergic neurons are present in the dorsal column nuclei but not in the ventroposterior complex of rats

Neurons containing glutamatic acid decarboxylase (GAD) are known to exist in the spinal dorsal horn, dorsal column nuclei (DCN), n. ventralis posterior (VP), and somatosensory cortex of cats. Recent work suggested that species differences exist concerning the presence and/or density of GAD-positive neurons in VP. The present experiments demonstrate that, in contrast with carnivores and primates, the rat's VP contains virtually no GAD-positive neurons and that virtually all neurons in it project to the cortex. This conclusion is supported by the failure to find, in Golgi-impregnated material, neurons with characteristics commonly attributed to Golgi type II neurons in VP of cats. The lack of GAD-positive neurons in VP of rats contrasts also with the presence of such neurons in the DCN in the same species. As in cats, about one third of the neurons in the cuneate n. are GAD-positive; these have mostly small perikarya and they are present throughout the nucleus. It is likely that these are intrinsic neurons, i.e. non-projecting beyond the limits of the DCN since a comparable percentage of neurons are unlabeled by simultaneous injections of horseradish peroxidase in multiple targets of the DCN. Like GAD-positive neurons, neurons unlabeled by the retrograde transport of HRP have, for the most part, small perikarya. It is possible that inhibitory mechanisms necessary for basic transfer functions in VP of rats are sustained through projections to this nucleus from the n. reticularis thalami. Extrinsic source of GABAergic input to the DCN seem to be absent or very weak. From this and previous evidence it may be proposed that intrinsic inhibitory interneurons have gradually developed in VP of rabbits, carnivores, and primates in parallel with more elaborate levels of thalamic integration of somatosensation.     

Interactions of ventral tract and dorsal column inputs into the cat cuneate nucleus

In decerebrate- decerebellate cats with spinal lesions separating the dorsal column (DC) from the spinocervicothalamic and ventral tract (VT) pathways, conditioning VT stimulation activated a few, but inhibited most, of the cuneate neurons discharging to test DC stimulation. This VT input into the cuneate nucleus is mediated through the brain stem.     

Vestibular projections to the thalamus of the pigeon: an anatomical study

In a series of retrograde tracing studies involving the injection of WGA-HRP into the thalamus of the pigeon, labeled neurons were consistently observed in anterior regions of the vestibular nuclei. Following small dorsal thalamic injections, labeled neurons were located predominantly in rostroventrolateral regions of the superior vestibular nucleus, less numerously within the ventral part of the lateral vestibular nucleus, and least numerously within the medial vestibular nucleus. Following large dorsal thalamic injections, many more vestibular neurons were labeled, and these were distributed more extensively throughout anterior parts of the superior, lateral, and medial nuclei. No labeled neurons were found in the descending nucleus. Injections of tritiated amino acids into vestibular nuclei revealed a terminal field within the dorsal thalamic nucleus: dorsolateralis posterior, pars rostralis. The location of this field between auditory, somatosensory, and paleostriatally and neostriatally projecting nuclei suggests a general similarity to the organization of vestibulothalamic projections in mammals.     

Laminar patterns of termination of cortico-cortical afferents in the somatosensory system

Two distinct patterns of terminal labeling were seen after injections of [3H]amino acids into the second somatosensory area (S2) and the retroinsular area (Ri). The first pattern, seen both in S2 after an injection in Ri and in the granular or dysgranular insular fields after an injection in S2, is characterized by heavy labeling in layers IIIb and IV. By contrast, the second pattern, seen in the first somatosensory area and Ri after an injection in S2, is distinguished by heavy labeling of layer I and no labeling in layer IV. A reciprocal relationship between these two laminar patterns has now been seen in the somatosensory, visual and auditory systems.     

Distribution of nigrothalamic projections in the dog.

The distribution of nigrothalamic projections was studied in the dog by using the autoradiographic tracing method. On the basis of a systematic series of tritiated amino acid injections into different portions of the substantia nigra, we found that the rostral three-fourths of the substantia nigra pars reticulata (SNr) gives rise to widespread thalamic projections. The nigrothalamic label occupied a longitudinal band extending from rostral ventral anterior nucleus (VA) through to caudal mediodorsal nucleus (MD). In the dog, VA is histochemically identified as an acetylthiocholinesterase (AChE)-negative region and is distinct from the adjacent ventral lateral nuclei which stain positively for AChE. In rostral thalamus, the dense autoradiographic label was observed in rostral VA within the AchE-negative region lying alongside the mammillothalamic tract (MT). The adjacent AChE-positive ventral lateral nuclei did not contain autoradiographic label. In addition, homogeneously distributed silver grains were observed throughout the ventromedial nucleus (VM) bilaterally. More caudally, as the ventral lateral thalamic compartment emerges, label was also observed within the internal medullary lamina region, including the paralaminar portion of VA, the central lateral nucleus (CL), and the paralaminar portion of MD. Finally, in caudal thalamus, the central portion of MD, as well as the parafascicular nucleus (Pf), contained autoradiographic label. The overall nigrothalamic distribution observed in the dog was similar to the distribution of nigrothalamic projections in monkeys with one exception. Unlike that of primates, the distribution of nigral efferents is to the whole extent of VM in the dog as in other non-primate species. Overall, we found that nigral efferents primarily project to three main thalamic targets: the VA/MD region, VM, and the internal medullary lamina region, which includes dorsal CL and paralaminar VA and MD. We propose that these three nigrothalamic territories may constitute critical links subserving different functional channels.     

Development of the precerebellar nuclei in the rat: III. The posterior precerebellar extramural migratory stream and the lateral reticular and external cuneate nuclei

Sequential thymidine radiograms from rats injected on day E15 and killed thereafter at daily intervals up to day E22 were analyzed to trace the migratory routes and settling patterns of neurons of the lateral reticular nucleus and the external cuneate nucleus. The neurons of the lateral reticular and external cuneate nuclei originate in the primary precerebellar neuroepithelium at the same site as the inferior olivary neurons but follow a different migratory route. The labeled young neurons that are produced on day E15 (the last one-third of the total) join the posterior precerebellar extramural migratory stream. The cells move circumferentially over the wall of the medulla in a ventral direction and by day E17 reach the midline and cross it beneath the inferior olive. The crossing cells apparently continue to migrate circumferentially on the opposite side. One complement of these cells begins to form a ventrolateral extramural condensation on day E19. By day E20 some cells begin to penetrate the parenchyma and settle as neurons of the lateral reticular nucleus. The settling of the lateral reticular neurons continues on the following day, and by day E22 all the cells destined for the lateral reticular nucleus have penetrated the parenchyma. A dorsomedial-to-ventrolateral neurogenetic gradient is indicated for the settling lateral reticular neurons. Another complement of migrating cells continues dorsally and forms a condensation on day E19 that we interpret as the external cuneate component of the crossed stream. These cells begin to penetrate the parenchyma on day E20, and by days E21 and E22 two components of the external cuneate nucleus are identifiable-the dorsal and ventral external cuneate nuclei. The neurons of the lateral reticular and external cuneate nuclei differ from neurons of all the other precerebellar nuclei in that their cerebellar projection is predominantly ipsilateral. We speculate that the axons of all precerebellar neurons are genetically specified to cross the midline ventrally to provide a contralateral efferent projection, but this is modified in the case of the ipsilaterally projecting lateral reticular and external cuneate neurons by the cell bodies following their neurites to the opposite side.     

Spatial relationships between the terminations of somatic sensory motor pathways in the rostral brainstem of cats and monkeys. II. Cerebellar projections compared with those of the ascending somatic sensory pathways in lateral diencephalon

In order to classify the presynaptic elements contacting the principle class of globus pallidus neurons, electron microscopic examination of serial sections made from a medially located large globus pallidus neuron, labeled with intracellular horseradish peroxidase, was undertaken. In addition, the use of labeled and light microscopically reconstructed material allowed us to quantitatively determine the distribution of each bouton type along the soma and dendrites. Six types of presynaptic terminals contacting the labeled cell have been recognized. Type 1 endings, the most numerous (84%), make symmetrical contacts on all portions of the cell, except spines, contain large pleomorphic, and a few large dense-core vesicles. Type 2 endings are filled with small spherical-to-ellipsoidal synaptic vesicles. They make asymmetrical contacts only with higher-order dendrites and account for 12% of synaptic contacts onto the labeled neuron. Type 3 endings are large, contain sparsely distributed large pleomorphic vesicles, and make two symmetrical synapses per bouton, one onto a spine head and the other onto the underlying dendritic shaft. They are infrequent (0.2%), being found only in association with dendritic spines. Type 4 endings contain large pleomorphic synaptic vesicles and no dense-core vesicles. They make symmetrical contacts with the short primary dendrites. Type 5 endings contain a mixture of small clear pleomorphic vesicles and numerous large dense-core vesicles. They contact only the cell body and the short primary dendrites, making up 20% of somatic synaptic contacts but less than 1% of contacts onto dendrites. Type 6 boutons contain oval and flattened synaptic vesicles and establish symmetrical contacts with higher-order dendritic branches and the cell body.     

Time course of reactive synaptogenesis in the subcortical somatosensory system

These experiments were designed to determine when synaptogenesis begins in the adult rat ventral posterolateral nucleus of the thalamus following lesions of the dorsal column nuclei. Given the relatively uncomplicated structure of the neuropil in the ventral posterolateral nucleus of the rat, the specificity of reactive synaptogenesis of the lemniscal input and the effect of the loss of lemniscal terminals on terminals from other sources could be determined. By use of morphometric analysis of electron micrographs, the numerical density of the 3 terminal types in the neuropil was determined at a series of postlesion survival times ranging from 12 hours to 50 days. Synaptogenesis began about 30 days after the lesions of the dorsal column nuclei and was complete by 50 days. The slow onset of synaptogenesis was in response to a loss of the lemniscal terminals, which account for only 3% of the total number of synapses in the ventral posterolateral nucleus. The low level of synaptogenesis early in the recovery process differs from the recovery seen in other central nervous system sites, which show an early rapid increase in synapses in response to much greater denervation. The loss of lemniscal terminals has relatively little effect on the numerical density or distribution of the terminals of other types. The new terminals that are formed come both from axons that originate from the undamaged portion of the dorsal column nuclei and from axons originating in the spinal cord.     

Functional and anatomical organization of cardiovascular pressor and depressor sites in the lateral hypothalamic area. II. Ascending projections

Microinjections of L-glutamate or D, L-homocysteic acid were used to stimulate cell bodies in the region of the lateral hypothalamic area (LHA) selectively. Subsequent iontophoretic injections of Phaseolus vulgaris-leucoagglutinin or pressure injections of wheat germ agglutininhorseradish peroxidase were made into regions containing identified pressor and depressor sites and their connections with the forebrain and cerebral cortex were traced. The results indicate that decreases in blood pressure (10–45 mm Hg) and heart rate (20–70 bpm) could be elicited from tuberal (LHAt) and posterior (LHAp) sites in the LHA and that these regions have ascending projections to the insular cortex, the ventral forebrain including the septal-diagonal band of Broca complex, the ventral palladium, substantia innominata, amygdala, and the lateral preoptic area. In contrast, increases in blood pressure (10–40 mm Hg) and heart rate (20–70 bpm) were elicited primarily from neurons located adjacent to the fornix in the perifornical area (PFA). Injections of tract tracers into this region produced terminal labeling that differed markedly from the pattern seen following injections of tracer into depressor sites in the LHA. In addition, the pattern of anterograde labeling seen following injections of tracer into the anterior PFA differed from that seen following injections of tracer into posterior PFA. Injections of tracer into the anterior PFA resulted in dense terminal labeling in the medial preoptic area and the parvicellular paraventricular nucleus of the hypothalamus whereas injections into the posterior PFA resulted in dense terminal labeling in the lateral septal nucleus, nucleus accumbens, bed nucleus of the stria terminalis, as well as the medial preoptic area and the parvocellular paraventricular nucleus of the hypothalamus. The results demonstrate that the posterolateral hypothalamus of the rat contains two regions with specific cardiovascular function and highly organized connections with diencephalic, forebrain, and cortical structures.