Postsynaptic dorsal column pathway of the rat. III. Distribution of ascending afferent fibers

The distribution in the dorsal column nuclei (DCn) of post-synaptic dorsal column (PSDC) fibers was examined in rats following injections of Phaseolus vulgaris leucoagglutinin (PHA-L) in the spinal cord. Lemniscal neurons in the DCn were retrogradely labeled in the same animals by injecting the thalamus with Fluoro-Gold. In some experiments, primary afferent fibers were also labeled by injecting dorsal root ganglia with choleragenoid-conjugated HRP. Injections of PHA-L into the cervical enlargement labeled many fibers and varicosities throughout most of the ipsilateral cuneate nucleus. Labeled fibers were also present in the external cuneate and internal basilar nuclei. Injections of PHA-L into thoracic cord labeled fibers and varicosities in the medial cuneate and lateral gracile nuclei, as well as the external cuneate nucleus. Injections into the lumbar enlargement labeled fibers and varicosities throughout most of the gracile nucleus. Injections in sacral cord labeled fibers in the most medial part of the gracile nucleus. Dense labeling of PSDC fibers was found in areas with high densities of retrogradely labeled lemniscal neurons and areas with high densities of primary afferent fibers. In all regions of the DCn and in the external cuneate nucleus, fibers and varicosities labeled for PHA-L were seen in apposition to retrogradely labeled lemniscal cells. The distribution of postsynaptic afferent fibers in the DCn of the rat and its relationship to lemniscal neurons and primary afferent fibers contrast sharply with these features in cats.     

Central projections of cervical primary afferent fibers in the guinea pig: an HRP and WGA/HRP tracer study

The central course and the projections of the first and the second cervical dorsal root ganglia and of suboccipital muscle primary afferent fibers in the guinea pig were studied by means of anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA/HRP) or aqueous solution of horseradish peroxidase (HRP). Injections of WGA/HRP into the second cervical dorsal root ganglion produced labeling in the dorsal and ventral horns. Within the spinal cord, the largest amount of HRP reaction product was found within the lateral third of the substantia gelatinosa and within the central cervical nucleus. The main area of termination in the medulla was the external cuneate nucleus. However, HRP reaction product was also found within the medial and inferior vestibular nuclei, cell group x, the perihypoglossal nuclei, the nucleus of the solitary tract, and the nucleus of the spinal trigeminal tract. Descending fibers could be detected as caudal as spinal segment T5. Injections of WGA/HRP into the first cervical dorsal root ganglion produced heavy terminal label within the central cervical nucleus but not within the substantia gelatinosa. Again, the external cuneate nucleus was the main area of termination within the medulla. Label could not be observed within the vestibular nuclear complex or within the spinal trigeminal nucleus. Injections of aqueous HRP into the suboccipital muscles produced heavy transganglionic label within the central cervical nucleus, whereas the substantia gelatinosa totally lacked terminal label. Ascending proprioceptive fibers reached the external cuneate nucleus and group x. Scanty projections could be detected within the vestibular nuclei as well as within the perihypoglossal nuclei except for the nucleus prepositus hypoglossi. Label was absent in the spinal trigeminal nucleus.     

Central distribution of cervical primary afferents in the rat, with emphasis on proprioceptive projections to vestibular, perihypoglossal, and upper thoracic spinal nuclei

The projections of primary afferents from rostral cervical segments to the brainstem and the spinal cord of the rat were investigated by using anterograde and transganglionic transport techniques. Projections from whole spinal ganglia were compared with those from single nerves carrying only exteroceptive or proprioceptive fibers. Injections of horseradish peroxidase (HRP) or wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) were performed into dorsal root ganglia C2, C3, and C4. Free HRP was applied to the cut dorsal rami C2 and C3, greater occipital nerve, sternomastoid nerve, and to the C1/2 anastomosis, which contains afferents from suboccipital muscles and the atlanto-occipital joint. WGA-HRP injections into ganglia C7 and L5 were performed for comparative purposes. Injections of WGA-HRP or free HRP into rostral cervical dorsal root ganglia and HRP application to C2 and C3 dorsal rami produced labeling in dorsal and ventral horns at the level of entrance, the central cervical nucleus, and in external and main cuneate nuclei. From axons ascending to pontine and descending to upper thoracic spinal levels, medial collaterals were distributed to medial and descending vestibular, perihypoglossal and solitary nuclei, and the intermediate zone and Clarke's nucleus dorsalis in the spinal cord. Lateral collaterals projected mainly to the trigeminal subnucleus interpolaris and to lateral spinal laminae IV and V. Results from HRP application to single peripheral nerves indicated that medial collaterals were almost exclusively proprioceptive, whereas lateral collaterals were largely exteroceptive with a contribution from suboccipital proprioceptive fibers. WGA-HRP injections into dorsal root ganglia C7 and L5 failed to produce significant labeling within vestibular and periphypoglossal nuclei, although they demonstrated classical projection sites within the brainstem and spinal cord. The consistent collateralisation pattern of rostral cervical afferents along their whole rostrocaudal course enables them to contact a variety of precerebellar, vestibulospinal, and preoculomotor neurons. These connections reflect the well-known significance of proprioceptive neck afferents for the control of posture, head position, and eye movements.     

Central distribution of trigeminal and upper cervical primary afferents in the rat studied by anterograde transport of horseradish peroxidase conjugated to wheat germ agglutinin

Injections of WGA-HRP were made in the rat trigeminal ganglion and C1-3 dorsal root ganglia (DRGs) to study the central projection patterns and their relations to each other. Trigeminal ganglion injections resulted in heavy terminal labeling in all trigeminal sensory nuclei. Prominent labeling was also observed in the solitary tract nucleus and in the medial parts of the dorsal horn at C1-3 levels, but labeling could be followed caudally to the C7 segment. Contralateral trigeminal projections were found in the nucleus caudalis and in the dorsal horn at C1-3 levels. The C1 DRG was found to be inconstant in the rat. When it was present, small amounts of terminal labeling were found in the external cuneate nucleus (ECN) and the central cervical nucleus (CCN). No dorsal horn projections were seen from the C1 DRG. Injections in the C2 DRG resulted in heavy labeling in the ECN, nucleus X, CCN, and dorsal horn, where it was mainly located in lateral areas. Labeling could be followed caudally to the Th 7 segment. C2 DRG projections also appeared in the cuneate nucleus (Cun), in all the trigeminal sensory nuclei, and in the spinal, medial, and lateral vestibular nuclei. A small C2 DRG projection was observed in the ventral cochlear nucleus. C3 DRG injections resulted in heavy labeling in both medial middle and lateral parts of the dorsal horn, in the ECN, and in nucleus X, whereas the labeling in the CCN was somewhat weaker. Smaller projections were seen to trigeminal nuclei, Cun, and the column of Clarke. Comparisons of the central projection fields of trigeminal and upper cervical primary afferents indicated a somatotopic organization but with a certain degree of overlap.     

Projections of the second cervical dorsal root ganglion to the cochlear nucleus in rats

Physiological, anatomical, and clinical data have demonstrated interactions between somatosensory and auditory brainstem structures. Spinal nerve projections influence auditory responses, although the nature of the pathway(s) is not known. To address this issue, we injected biotinylated dextran amine into the cochlear nucleus or dorsal root ganglion (DRG) at the second cervical segment (C2). Cochlear nucleus injections retrogradely labeled small ganglion cells in C2 DRG. C2 DRG injections produced anterograde labeling in the external cuneate nucleus, cuneate nucleus, nucleus X, central cervical nucleus, dorsal horn of upper cervical spinal segments, and cochlear nucleus. The terminal field in the cochlear nucleus was concentrated in the subpeduncular corner and lamina of the granule cell domain, where endings of various size and shapes appeared. Examination under an electron microscope revealed that the C2 DRG terminals contained numerous round synaptic vesicles and formed asymmetric synapses, implying depolarizing influences on the target cell. Labeled endings synapsed with the stalk of the primary dendrite of unipolar brush cells, distal dendrites of presumptive granule cells, and endings containing pleomorphic synaptic vesicles. These primary somatosensory projections contribute to circuits that are hypothesized to mediate integrative functions of hearing.     

Transneuronal labeling of spinal interneurons and sympathetic preganglionic neurons after pseudorabies virus injections in the rat medial gastrocnemius muscle.

The distribution of retrogradely and transneuronally labeled neurons was studied in CNS of rats 4 days after injections of the Bartha strain of pseudorabies virus (PRV) into the medial gastrocnemius (MG) muscle. Tissue sections were processed for immunohistochemical detection of PRV. Retrogradely labeled cells were identified in the ipsilateral MG motor column in the caudal L4 and the L5 spinal segments. In order to evaluate the efficacy of PRV retrograde cell body labeling, the number of PRV retrogradely labeled neurons in the MG motor column was compared to the number labeled with two conventional retrograde cell body markers--Fluoro-Gold and cholera toxin-HRP. A ratio of 1:3 representing medium-sized (less than 30 microns) versus large neurons (greater than 30 microns) was found in the Fluoro-Gold dye experiments; a 1:2 ratio was seen in the PRV experiments. In contrast, when cholera toxin-HRP was used as a retrograde marker, mainly large neurons were labeled; the medium-to-large cell body ratio was 1:10 suggesting cholera toxin-HRP may have a greater affinity for the terminals of alpha-motoneurons as opposed to gamma-motoneurons. Transneuronally labeled cells were identified in the L1-L6 spinal gray matter, intermediolateral cell column (T11-L2), lateral spinal nucleus and medial part of lamina VII in C4 and C5 spinal segments, brainstem (caudal raphe nuclei, rostral ventrolateral medulla, A5 cell group, paralemniscal nucleus, locus coeruleus, subcoeruleus nucleus, red nucleus) and paraventricular hypothalamic nucleus. In the L5 spinal cord, transneuronally labeled neurons were seen in the ipsilateral spinal laminae I and II and bilaterally in spinal laminae IV-VIII, and X. Similar results were obtained in rats that had chronic unilateral L3-L6 dorsal rhizotomies indicating most of the labeling was due to retrograde transneuronal cell body labeling. In order to determine whether PRV was transported into the spinal cord by the dorsal root axons, the ipsilateral dorsal root ganglia (DRGs) were examined for PRV immunoreactivity; none was found. However, using the polymerase chain reaction, viral DNA was shown to be present in the ipsilateral DRGs indicating that some of spinal cord cell body labeling may have resulted from anterograde transneuronal labeling, as well.     

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.     

Afferent fibers in the hypoglossal nerve: a horseradish peroxidase study in the cat

The existence of afferent fibers in the cat hypoglossal nerve was studied by transganglionic transport of horseradish peroxidase (HRP). Injections of wheat germ agglutinin-conjugated HRP (WGA-HRP) into the hypoglossal nerve resulted in some retrograde labeling of cell bodies within the superior ganglia of the ipsilateral glossopharyngeal and vagal nerves. A few labeled cell bodies were also present ipsilaterally within the inferior ganglion of the vagal nerve and the spinal ganglion of the C1 segment. Some of the labeled glossopharyngeal and vagal fibers reached the nucleus of the solitary tract by crossing the dorsal portion of the spinal trigeminal tract. Others distributed to the spinal trigeminal nucleus pars interpolaris and to the ventrolateral part of the medial cuneate nucleus by descending through the dorsal portion of the spinal trigeminal tract. In the spinal cord these descending fibers, intermingling with labeled dorsal root fibers, distributed to laminae I, IV-V and VII-VIII of the C1 and C2 segments. Additional HRP experiments revealed that the fibers in laminae VII-VIII originate mainly from dorsal root of the C1 segment.     

Spinal and trigeminal dorsal horn projections to the parabrachial nucleus in the rat

We studied afferents to the parabrachial nucleus (PB) from the spinal cord and the spinal trigeminal nucleus pars caudalis (SNVc) in the rat by using the anterograde and retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Injections of WGA-HRP into medial PB retrogradely labeled neurons in the promontorium and in lamina I of the dorsal rostral SNVc, while injections into lateral PB and the K?lliker-Fuse nucleus retrogradely labeled neurons in these areas as well as in lamina I throughout the caudal SNVc and spinal dorsal horn. Injections of WGA-HRP into the caudal SNVc and dorsal horn of the spinal cord resulted in terminal labeling in the dorsal, central, and external lateral subnuclei of PB and the K?lliker-Fuse nucleus, all of which are known to receive cardiovascular and respiratory afferent information. Injections of WGA-HRP into the promontorium and dorsal rostral SNVc resulted in terminal labeling in the same PB subnuclei, as well as in the medial and the ventral lateral PB subnuclei, which are sites of relay for gustatory information ascending from the medulla to the forebrain. The spinal and trigeminal projection to PB may mediate the convergence of pain, chemosensory, and temperature sensibilities with gustatory and cardiorespiratory systems in PB.     

Evidence for corticotropin-releasing hormone projections from Barrington's nucleus to the periaqueductal gray and dorsal motor nucleus of the vagus in the rat

The present study used anterograde and retrograde tract tracing and immunohistochemistry to determine the efferent projections of corticotropin-releasing hormone (CRH) neurons of Barrington's nucleus in the rat. Injections of Phaseolus vulgaris-leucoagglutinin into Barrington's nucleus resulted in anterograde labeling in the dorsal motor nucleus of the vagus, periaqueductal gray, medial thalamic nuclei, lateral hypothalamus, paraventricular nucleus of the hypothalamus, lateral preoptic area, and lateral septum. The retrograde tract tracer, fluorogold, injected into the lumbosacral spinal cord labeled many, but not all, CRH-immunoreactive neurons in Barrington's nucleus. Moreover, some Barrington's neurons that were retrogradely labeled from the spinal cord were not CRH-immunoreactive. Several CRH-immunoreactive Barrington's neurons were retrogradely labeled by fluorogold injections into the periaqueductal gray, and these were located predominantly in the dorsal part of the nucleus. Additionally, some CRH-immunoreactive Barrington's neurons were retrogradely labeled from fluorogold injections into the dorsal motor nucleus of the vagus. In contrast, fluorogold injections into the lateral hypothalamus, lateral preoptic area, or lateral septum did not result in double labeling of CRH-immunoreactive neurons in Barrington's nucleus. These results suggest that many, but not all, CRH-containing neurons of Barrington's nucleus project to the lumbosacral spinal cord. In addition to their previously documented projections to the spinal cord, these neurons may be a source of CRH in the periaqueductal gray and dorsal motor nucleus of the vagus. CRH projections of Barrington's nucleus may play a role in behavioral or autonomic aspects of stress responses, in addition to their proposed role in micturition. © 1995 Wiley-Liss, Inc.     

Vagal and gastric connections to the central nervous system determined by the transport of horseradish peroxidase

Horseradish peroxidase (HRP, Sigma Type VI) crystals were encased in a parafilm envelope and applied to the transected central ends of the left and right cervical vagus nerves and the anterior and posterior esophageal vagus nerves of adult male hooded rats. Injections of 30% HRP were made into the muscle wall of the fundus and antrum regions of the stomach. After 48 hr survival time, animals were perfused intracardially with a phosphate buffer plus sucrose wash followed by glutaraldehyde and paraformaldehyde fixative. The brain stem, spinal cord and corresponding dorsal root ganglia, superior cervical sympathetic ganglion, and the nodose ganglion were removed and cut into 50 micron sections. All tissue was processed with tetramethylbenzidine (TMB) for the blue reaction according to Mesulum and counterstained with neutral red. Sequential sections were examined under a microscope. Labeled neurons and nerve terminals were identified using bright and dark field condensers and polarized light. In tissue from animals that had HRP applied to the cervical vagus nerves, retrogradely labeled neurons were identified ipsilaterally in the medulla located in the dorsal motor nucleus of the vagus (DMN) and the nucleus ambiguus (NA). Labeled cells extended from the DMN into the spinal cord in ventral-medial and laminae X regions C1 and C2 of cervical segments. Many neurons were labeled in the nodose ganglion. Anterogradely labeled terminals were observed throughout and adjacent to the solitary nucleus (NTS) dorsal to the DMN and intermixed among labeled neurons located in the DMN. In tissue from animals that had HRP applied to the esophageal vagus nerves, similar labeling was observed. However, fewer neurons were identified in the NA, the nodose ganglion, and only in laminae X of the cervical spinal cord segments C1 and C2. Also, very little terminal labeling was observed in and adjacent to the NTS. Labeled neurons in tissue from animals that had HRP injected into the stomach wall were observed bilaterally in the DMN, nodose ganglion, and only in laminae X at the C1 and C2 levels of the spinal cord. Labeled neurons also were observed in the dorsal root ganglia of the thoracic cord. These data indicate that cervical cord and NA neurons are important in the supradiaphragmatic motor innervation by the vagus. Also, many afferents to the NTS originate above the diaphragm. In addition, some afferents from the stomach enter the central nervous system via the thoracic spinal cord.     

Brainstem projections to the rat cuneate nucleus

Neurons in the pontomedullary tegmentum have been proposed as a final common pathway subserving descending inhibition in the dorsal column nuclei. To investigate the anatomical substrate for these descending effects, brainstem projections to the cuneate nucleus of rats were studied with injections of lectin-conjugated horseradish peroxidase. In rats with iontophoretic tracer injections in this nucleus, many labeled neurons were detected near the injection site, especially ventral and caudal to it. Intrinsic reciprocal projections were observed after injections in caudal, middle, or rostral levels of the cuneate nucleus. Neurons were labeled in the red nucleus, in agreement with previous anatomical studies, and also in the trigeminal, vestibular, and cochlear nuclei. An ipsilateral dorsomedial group of neurons was labeled in the upper cervical segments and scattered neurons were also labeled bilaterally near the central canal. Sparse retrograde labeling in the tegmentum was focused in the lateral paragigantocellular nucleus and caudal raphe. Consistent with the retrograde experiments, anterograde labeling after pressure injections of lectin-conjugated horseradish peroxidase in the pontomedullary tegmentum was very sparse within the dorsal column nuclei; labeling was dense, however, in the region immediately ventral to these nuclei. These results confirm previous work indicating that the activity of cuneate neurons is modulated by brainstem sensory nuclei. However, it appears that direct projections to the cuneate nucleus from pontine and rostral medullary regions are sparser than previously suggested. The last link of a polysynaptic descending inhibitory pathway may include GABAergic neurons immediately adjacent to the dorsal column nuclei and/or intrinsic to these nuclei.     

Projections to the spinal cord from medullary somatosensory relay nuclei.

Descending projections to the spinal card from the dorsal column nuclei were studied in the cat, rat and monkey with the retrograde horseradish peroxidase (HRP) technique, and in the cat with the autoradiographic anterograde axonal transport technique. Retrogradely labeled neurons were seen in the dorsal column nuclei after HRP injections at all levels of the spinal cord and additionally in the magnocellular division of the spinal caudalis nucleus of the trigeminal nerve after injections into cervical spinal segments in all three species. HRP-positive neurons were predominantly located along the middle of the rostro-caudal axis of the dorsal column nuclei and amongst the fusiform, triangular and polygonal cells that surround, especially ventrally, the cell nest zone containing thalamic relay neurons. The labeled neurons are densely concentrated in those portions of the dorsal column nuclei where most corticofugal and non-primary afferent projections terminate and where the terminal distribution of primary afferent fibers is overlapping and diffuse. Previous studies have shown that most neurons in this middle and ventral region do not project to the thalamus or cerebellum. The majority of the cells in the dorsal column nuclei with descending axons or axon collaterals project by way of the ipsilateral dorsal columns, but some fibers project into the dorsolateral funiculus; the descending trigeminal fibers course in the dorsolateral funiculus. The terminal fields for these fibers in the cervical spinal cord include the lateral cervical nucleus, laminae IV and V, and possibly lamina I. These results indicate that the dorsal column nuclei may contribute to a feedback mechanism regulating the flow of sensory information ascending along other somatosensory spinal pathways.     

Synaptic connections between spinal motoneurons and dorsal root ganglion cells in the cat

Light- and electron-microscopical horseradish peroxidase (HRP) studies have been employed in conjunction with a degeneration study in order to clarify the origin and axonal passage of afferent synaptic terminals in cat dorsal root ganglia. After injection of HRP into ganglia (C3) without involvement of the ventral roots and spinal nerves, a few ipsilateral spinal ventral horn neurons (C3) were retrogradely labeled with HRP. The labeled neurons were localized in the dorsomedial and the ventromedial nuclei. Following ventral rhizotomy of C3, the afferent terminals in the ganglia (C3) anterogradely degenerated and contained accumulated and disintegrated neurofilaments, depleted, aggregated and enlarged synaptic vesicles. Subsequent to an HRP and wheat germ agglutinin (WGA)-HRP-mixture injection into the dorsal neck or suboccipital muscles, many spinal motoneurons (C3) were labeled retrogradely with an HRP mixture. On the other hand, the afferent synaptic terminals in ganglia contained the membrane-bound and electron-dense bodies which were anterogradely labeled with an HRP mixture in addition to the normal synaptic elements. The present findings strongly suggest that some spinal motoneurons send their axon collaterals to the dorsal root ganglia, in which the terminals of the axon collaterals directly synapse with the dorsal root ganglion cells.     

Cervical primary afferent input to vestibulospinal neurons projecting to the cervical dorsal horn: An anterograde and retrograde tracing study in the cat

Vestibulospinal neurons in the caudal half of the medial and descending vestibular nuclei terminate in the cervical spinal cord, not only in the ventral horn and intermediate zone but also in the dorsal horn. The purpose of the present study was to examine whether the areas containing these vestibulospinal neurons are reached by cervical primary afferents. In one group of experiments, wheat germ agglutinin-horseradish peroxidase conjugate and horseradish peroxidase were pressure injected into spinal ganglia C₂-C₈ and revealed anterogradely labeled fibers and boutons in the caudal part (caudal to the dorsal cochlear nucleus) of the ipsilateral medial and descending vestibular nuclei. This projection was verified in experiments in which wheat germ agglutinin-horseradish peroxidase conjugate was microiontophoretically injected into the caudal half of either the medial or the descending vestibular nuclei and revealed retrogradely labeled cells only in ipsilateral spina ganglia C₂-C₇, with a maximum of cells in C₃. In another group of experiments, after microiontophoretic injections of Phaseolus vulgaris leucoagglutinin or Biocytin into either the medial or the descending vestibular nuclei, anterogradely labeled fibers and boutons were present in the cervical spinal cord, mainly bilaterally in the dorsal horn (laminae I–VI) but also, to a lesser extent, in the ventral horn and intermediate zone. The existence of a loop that relays cervical primary afferent information to vestibulospinal neurons projecting to the cervical spinal cord, in particular the dorsal horn, may have implications for vestibular control over local information processing in the cervical dorsal horn. © 1995 Wiley-Liss, Inc.     

Spinal cord neuron inputs to the cuneate nucleus that partially survive dorsal column lesions: A pathway that could contribute to recovery after spinal cord injury

Dorsal column lesions at a high cervical level deprive the cuneate nucleus and much of the somatosensory system of its major cutaneous inputs. Over weeks of recovery, much of the hand representations in the contralateral cortex are reactivated. One possibility for such cortical reactivation by hand afferents is that preserved second‐order spinal cord neurons reach the cuneate nucleus through pathways that circumvent the dorsal column lesions, contributing to cortical reactivation in an increasingly effective manner over time. To evaluate this possibility, we first injected anatomical tracers into the cuneate nucleus and plotted the distributions of labeled spinal cord neurons and fibers in control monkeys. Large numbers of neurons in the dorsal horn of the cervical spinal cord were labeled, especially ipsilaterally in lamina IV. Labeled fibers were distributed in the cuneate fasciculus and lateral funiculus. In three other squirrel monkeys, unilateral dorsal column lesions were placed at the cervical segment 4 level and tracers were injected into the ipsilateral cuneate nucleus. Two weeks later, a largely unresponsive hand representation in contralateral somatosensory cortex confirmed the effectiveness of the dorsal column lesion. However, tracer injections in the cuneate nucleus labeled only about 5% of the normal number of dorsal horn neurons, mainly in lamina IV, below the level of lesions. Our results revealed a small second‐order pathway to the cuneate nucleus that survives high cervical dorsal column lesions by traveling in the lateral funiculus. This could be important for cortical reactivation by hand afferents, and recovery of hand use. J. Comp. Neurol. 523:2138–2160, 2015. © 2015 Wiley Periodicals, Inc.     

Calbindin-D28k immunoreactivity in the spinal cord of Xenopus laevis and its participation in ascending and descending projections

Immunohistochemistry for calbindin-D28k (CB) revealed that the spinal cord of Xenopus laevis possess a large number of CB-containing neurons widely distributed in both the dorsal and ventral horns, including areas which possess long ascending projections to supraspinal structures. In addition, the presence of CB-immunoreactive axons in the spinal funiculi suggested that descending projections containing this calcium binding protein may originate in different brainstem nuclei. Apart from mapping CB-containing elements in the spinal cord, a double labeling approach was used that combined the retrograde transport of dextran amines with CB immunohistochemistry. Thus, dextran amine injections into the lateral reticular region of the rhombencephalon, the parabrachial region, the mesencephalon and the dorsal thalamus revealed many retrogradely labeled cells in the spinal cord, a few number of which were double labeled for CB and found in the superficial dorsal horn and in the ventral medial region of the ventral horn. Their axons passed mainly via the lateral funiculus. Tracer application into the cervical spinal cord, combined with CB immunohistochemistry, resulted in retrogradely labeled cells throughout the brain, five groups of which showed CB immunoreactivity: (1) the mesencephalic trigeminal nucleus, (2) the laterodorsal tegmental nucleus, (3) the raphe nucleus, (4) the middle reticular nucleus and (5) the inferior reticular nucleus. The presence of CB in spinal pathways suggests that CB may play a role in controlling spinal cells, mainly subserving visceroceptive and nociceptive information to supraspinal levels, and might also modulate reticulospinal pathways.     

The origin of brainstem-spinal pathways in the north american opossum (didelphis virginiana). Studies using the horseradish peroxidase method

Brainstem neurons which project to lumbar, thoracic and cervical spinal levels have been identified in the North American opossum by the horseradish peroxidase (HRP) technique. Neurons which relay to all of the levels studied are located within the medullary and pontine reticular formation as well as within the nucleus cuneatus, the nucleus of the tractus solitarius, the lateral reticular nucleus, the medullary and caudal pontine raphe nuclei, the lateral, medial and inferior vestibular nuclei, the nucleus “F,” the nucleus coeruleus, and the nucleus coeruleus, para α, the red nucleus, and the interstitial nucleus of Cajal. The lateral vestibulospinal and rubrospinal systems are topographically organized, although neurons projecting to different cord levels show considerable intermingling. Our material also provides evidence that raphe-spinal and reticulospinal connections are organized to some degree. Neurons which backfill after cervical and thoracic placements, but not after lumbar injections, are distributed within the caudal spinal trigeminal nucleus, the nucleus intercalatus, the dorsal vagal nucleus, the cuneiform area of Castaldi, the fields of Forel, and the nucleus of Darkschewitsch. Reactive neurons are present within the lateral, dorsal and posterior hypothalamic areas as well as within the periventricular and paraventricular nuclei after thoracic placements and within the superior colliculus after injections within the cervical cord. Additionally, neurons are reactive in the nucleus ambiguus, the interpolar division of the spinal trigeminal nucleus and the rostral division of the oculomotor nucleus (Oswaldo-Cruz and Rocha-Miranda, '68) after HRP placements into the third and fourth cervical segments.     

Selective retrograde transport of D-aspartate in spinal interneurons and cortical neurons of rats

Retrograde labeling of neuronal elements in the brain and spinal cord has been investigated by autoradiographic techniques following injections of D-[3H]aspartate (asp), [3H] gamma-aminobutyric acid (GABA) or horseradish peroxidase (HRP) in the medulla and spinal cord of rats. Twenty-four hours after D-[3H]asp injections focused upon the cuneate nucleus, autoradiographic labeling is present over fibers in the pyramidal tract, internal capsule and over layer V pyramids in the forelimb representation of the sensorimotor cortex. After [3H]GABA injections in the same nucleus no labeling attributable to retrograde translocation can be detected in spinal segments, brain stem or cortex. Conversely, injections of 30% HRP in the cuneate nucleus label neurons in several brain stem nuclei, in spinal gray and in layer V of the sensorimotor cortex. These observations give further support to the proposed existence of a selective retrograde transport of D-[3H]asp and are consistent with the available evidence which indicates that the corticodorsal column nuclei path use glutamate and/or aspartate as neurotransmitter(s). D-[3H]Asp injections focused on the dorsal horn at cervical segments label a fraction of perikarya of the substantia gelatinosa and a sparser population of larger neurons in laminae IV to VI for a distance of 3-5 segments above and below the injection point. No brain stem neuronal perikarya appear labeled following spinal injections of D-[3H]asp although autoradiographic grains overlie pyramidal tract fibers on the side contralateral to the injection. This labeling however has not been observed rostral to lower pontine levels nor over cortical neurons at any of the survival times used in the present experiments (6-72 h). As in cases with cuneate injections this pattern of labeling contracts with that obtained after spinal injections of either [3H]GABA or HRP. Although labeling of neocortical neurons has not been observed after spinal injections of D [3H]asp, possibly as a result of the length of corticospinal axons, retrograde labeling of these elements for at least some distance may be taken as suggestive of a special affinity of their terminals for glutamate and/or aspartate.