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.     

Organization of forebrain projections from the medullary reticular formation in the North American opossum. Evidence for connectional heterogeneity

We have employed axonal transport techniques to study the organization of projections from the medullary reticular formation (RF) to the forebrain of the North American opossum. The results of retrograde transport studies using large injections of horseradish peroxidase (HRP), wheat germ agglutinin conjugated to HRP (WGA-HRP), and fluorescent markers suggest that all nuclei of the medullary RF project to the forebrain although the parvocellular reticular nucleus makes only a very small contribution. When injections of 3H-leucine or WGA-HRP are centered within areas shown by the retrograde transport studies to innervate the forebrain, characteristic patterns of orthograde labeling are produced. In most cases labeled axons form a major pathway which splits into dorsal and ventral divisions. The dorsal division innervates the parafascicular and central nuclei of the thalamus as well as the pretectum. In contrast, the ventral division projects to the lateral hypothalamus, the zona incerta, the ventral and dorsal lateral geniculate nuclei, the lateral part of the ventrobasal nucleus of the thalamus, the lateral preoptic area, the septal-diagonal band region, and the cerebral cortex. When injections are centered within specific ventrolateral areas of the medullary RF, ventral division labeling is also found within the dorsomedial, paraventricular and supraoptic nuclei of the hypothalamus. Relatively small injections of HRP or WGA-HRP into specific areas of the forebrain produce labeling which suggests that some areas receive projections from all nuclei of the medullary RF, whereas others do not. Our results suggest that forebrain projections of the medullary RF, like those to the spinal cord, are connectionally heterogeneous.     

Reappraisal of projection levels of the corticospinal fibers in the cat, with special reference to the fibers descending through the dorsal funiculus: a WGA-HRP study

The precise course and termination levels of the corticospinal tract (CST) in the cat were studied using the anterograde transport of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Following injection of WGA-HRP into the pericruciate (sensorimotor) cortex on one side, we observed the precise caudal termination levels of the CST fibers in the lateral and ventral funiculi. Simultaneously, the bilateral CSTs descending through the dorsal funiculus of the spinal cord were identified. Anterogradely labeled CST fibers within the lateral and ventral funiculi were observed bilaterally to reach the level of the third sacral (S3) spinal segment, which is lower than that ever described. The lowest level of the CST fibers within the dorsal funiculus, however, reached the level of the first sacral (S1) spinal segment. In conclusion, this study demonstrates that, in the cat, there exist 6 different CSTs (crossed and uncrossed lateral, ventral, dorsal) and that the termination levels of the lateral and ventral CSTs are much lower than those described in previous reports.     

Trigeminal and dorsal column nuclei projections to the anterior pretectal nucleus in the rat

The projections of the trigeminal (V) sensory nuclei (VSN) and the dorsal column nuclei (DCN) to the anterior pretectal nucleus (APT) of the rat were investigated by the use of anterograde and retrograde transport of wheat-germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Injections of WGA-HRP into the APT retrogradely labeled neurons in the contralateral VSN and DCN. The labeled neurons in the VSN were most concentrated in the rostral V subnucleus interpolaris (Vi), but were also found in caudal V subnucleus oralis (Vo). No labeled neurons were seen in V subnucleus caudalis. In the DCN, retrogradely labeled neurons were observed in rostral portions of both the cuneate (Cu) and gracile (Gr) nuclei. Injections of WGA-HRP into the rostral Vi or caudal Vo resulted in dense anterograde terminal labeling in the ventral two-thirds of the APT; the labeling was maximal in the ventromedial part of the caudal half of the APT and did not extend into its most rostral portion. Labeling resulting from injections of tracer into Cu or Gr was located primarily in the ventral half of the APT, was maximal in the mid-levels of the nucleus and extended into its rostral portions. These results indicate the existence of prominent somatosensory projections to the APT and are consistent with recent findings suggesting a role for the APT in sensorimotor integration.     

Cingulate gyrus of the cat receives projection fibers from the thalamic region ventral to the ventral border of the ventrobasal complex

Direct projections to the cingulate gyrus from the thalamic region lying just ventrally to the ventral border of the ventrobasal complex (VB) were found in the cat by two sets of experiments that used WGA-HRP (wheat germ agglutinin-horseradish peroxidase conjugate). In the first set of experiments, WGA-HRP was injected into the thalamic region around the ventral border of the VB. When the site of injection involved the thalamic region lying ventrally to the ventral border of the VB at the levels of the caudal two thirds of the VB, the cerebral cortex in the rostral part of the cingulate gyrus ipsilateral to the WGA-HRP injection contained fine HRP-positive granules, which indicated anterograde labeling of axon terminals. These labeled presumed axon terminals were mainly distributed to the superficial part of layer I, deep part of layer II, layer IV, and the most superficial part of layer V in the cingulate cortex. In the second set of experiments, WGA-HRP was injected into the cerebral cortex of the rostral part of the cingulate gyrus. When the site of injection involved the region of the cingulate gyrus, where presumed axon terminals had been labeled in the first set of experiments, the thalamic region just ventral to the ventral margin of the caudal two-thirds of the VB ipsilateral to the WGA-HRP injection contained neuronal cell bodies labeled retrogradely. The results indicate that some neurons that are located in the thalamic region just ventral to the ventral border of the caudal two-thirds of the VB send their axons to the cerebral cortex in the rostral part of the cingulate gyrus. The possible significance of the thalamocingulate projection found in the present study is discussed with relation to nociceptive behavior and function.     

Somatosensory projection to the mesencephalon: an anatomical study in the monkey

The terminal areas and cells of origin of the somatosensory projection to the mesencephalon in the monkey were investigated by the intraaxonal transport method. Following injection of wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) into the spinal enlargements, the lateral cervical nucleus (LCN), the dorsal column nuclei (DCN), or the spinal trigeminal nucleus, anterograde labeling was observed in several regions of the mid-brain. (1) Injection of tracer into the spinal enlargements resulted in dense terminal labeling in the parabrachial nucleus (PBN) and the periaqueductal gray matter (PAG); moderate termination was observed in the intercollicular nucleus (Inc), the intermediate and deep gray layers of the superior colliculus (SGI, SGP), the posterior pretectal nucleus (PTP), and the nucleus of Darkschewitsch (D); and scattered terminal fibers were seen in the cuneiform nucleus (CNF) and the pars compacta of the anterior pretectal nucleus (PTAc). The projections from the cervical enlargement to PAG, Inc, and the superior colliculus terminated more rostrally than those from the lumbar segments, indicating a somatotopic organization. (2) Terminal labeling after injection of tracer into LCN was found mainly in Inc, SGI, and SGP, but sparse labeling was also observed in the nucleus of the brachium of the inferior colliculus (BIN), PAG, PBN, PTP, and D. (3) The projection from DCN terminated densely in the external and pericentral nuclei of the inferior colliculus (ICX, ICP), Inc, SGI, SGP, PTP, PTAc, the nucleus ruber, and D, and weak terminal labeling was seen in BIN, PAG, and PBN. Comparisons of the anterograde labeling following injections involving both the gracile nucleus and the cuneate nucleus with that after injection restricted to the gracile nucleus alone suggested a somatotopic termination pattern in Inc, the superior colliculus, and the pretectal nuclei. (4) The patterns of projection from the laminar and alaminar parts of the spinal trigeminal nucleus differed: injection of tracer into the caudal part of the alaminar spinal trigeminal nucleus (nucleus interpolaris) resulted in dense anterograde labeling in SGI and SGP, moderate termination in Inc, and minor projections to PBN, PAG, and PTP, whereas after tracer injection into the laminar trigeminal nucleus (nucleus caudalis) terminal labeling was present only in PBN and PAG. Following injection of tracer into the midbrain terminal areas retrogradely labeled neurons were found in the spinal cord, LCN, DCN, and the spinal trigeminal nucleus, with the majority of labeled cells situated on the side contralateral to the injection site.(ABSTRACT TRUNCATED AT 400 WORDS)     

Corticospinal development in the North-American opossum: evidence for a sequence in the growth of cortical axons in the spinal cord and for transient projections

The course and distribution of corticospinal axons have been traced in a series of pouch young and adult opossums using wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Cortical axons enter the spinal cord approx. 28 days after birth (40 days after conception) and, at that time, are limited to those portions of the dorsal and lateral funiculi which contain them in the adult animal. Shortly thereafter, cortical axons are also found in regions of presumptive white matter where they are not seen in older pouch young or adult opossums. Those in the dorsal and lateral funiculi reach their caudal extent, the fourth thoracic segment, approx. 38 days after birth and do not significantly overgrow that level during development. Cortical axons grow into the gray matter exclusively from the dorsal and lateral funiculi and first innervate adjacent portions of laminae IV and V. They subsequently extend into laminae III and VI, then VII, VIII and X and finally, at the cervical enlargement, the medial edge of laminae I-II and lamina IX. There is a subsequent period of development during which the density of cortical innervation in all spinal laminae, particularly in the ventral horn, appears to exceed that in the adult opossum.     

Somatosensory and auditory relay nucleus in the rostral part of the ventrolateral medulla: a morphological study in the cat

A nucleus that possibly relays both somatosensory and auditory information was identified in the well-known autonomic control region in the rostral part of the ventrolateral medulla (RVL) of the cat by four sets of experiments using the WGA-HRP (wheat germ agglutinin-horseradish peroxidase conjugate) method. First, after injecting WGA-HRP into the dorsal column nuclei (DCN), anterograde and retrograde labeling was found bilaterally within and around a small cluster of medium-sized neurons in the RVL; more labeled neuronal cell bodies were seen in the cluster ipsilateral to the injection than in the contralateral cluster, whereas labeled axon terminals were distributed more densely on the contralateral side than on the ipsilateral side. The neuronal cluster in the RVL was located close to the ventrolateral surface of the medulla oblongata, constituting a short, slender column extending from a caudal level of the facial nucleus to the level of the rostral one-third of the inferior olive. This cluster of neurons was named the ventrolateral medullary nucleus (VLMN). In the second set of experiments, WGA-HRP was injected into the VLMN. Labeled neuronal cell bodies were seen in the reticular zone of the DCN bilaterally, with a slight dominance on the side contralateral to the injection, and further in the anteroventral division of the cochlear nuclei (CN) bilaterally, with a predominantly contralateral distribution. Labeled presumed axon terminals were seen bilaterally not only in the DCN and granular layer of the CN but also in the intercollicular region (IcR), lateral division of the posterior group of the thalamus (Pol), and medial geniculate nuclei (MG). Labeled terminals in the DCN were more numerous on the side ipsilateral to the injection than on the contralateral side, whereas those in other regions were distributed with a clear-cut contralateral dominance. In the third set of experiments, WGA-HRP injection into the CN resulted in anterograde and retrograde labeling in the VLMN. The labeling was bilateral, but more marked in the VLMN contralateral to the injection. In the fourth set of experiments, after WGA-HRP injection into the IcR, Pol, or MG, labeled neuronal cell bodies were located in the VLMN bilaterally with a dominant contralateral distribution. The results indicate that the VLMN possibly relays somatosensory and auditory information from the reticular zone of the DCN and anteroventral division of the CN to the IcR, Pol, and MG.     

The use of wheat germ agglutinin--horseradish peroxidase conjugates for studies of anterograde axonal transport

Injection of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), into the hemisected spinal cord of the rat, cat and monkey consistently resulted in the intense anterograde labeling of ascending spinal projections such as the spinothalamic tract and spinocerebellar tracts and their terminal fields. Injections of WGA-HRP in the dorsal column nuclei resulted in the anterograde labeling of the medial lemniscus and its terminal fields in the thalamus. Injection of similar amounts of horseradish peroxidase alone (HRP) in hemisected animals or the dorsal column nuclei resulted in little anterograde labeling. The rate of the anterograde transport of WGA-HRP in cut axons appears to be greater than 200 mm/day. Small amounts of transneuronal labeling appeared to occur after injection of WGA-HRP in both cut axons and undamaged cell bodies. These results suggest that the amount of anterograde labeling observed after injection of WGA-HRP into both cut axons and undamaged cell bodies is significantly greater than the anterograde labeling observed after injections of HRP alone. Therefore, in the central nervous system WGA-HRP appears to be a far more effective anterograde tracer than HRP alone.     

Subcortical projections to the centromedian and parafascicular thalamic nuclei in the cat

The primary objective of this study is to identify the totality of input to the centromedian and parafascicular (CM-Pf) thalamic nuclear complex. The subcortical projections upon the CM-Pf complex were studied in the cat with three different retrograde tracers. The tracers used were unconjugated horseradish peroxidase (HRP), horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP), and rhodamine-labeled fluorescent latex microspheres (RFM). Numerous subcortical structures or substructures contained labeled neurons with all three tracing techniques. These labeled structures included the central nucleus of the amygdala; the entopeduncular nucleus; the globus pallidus; the reticular and ventral lateral geniculate nuclei of the thalamus; parts of the hypothalamus including the dorsal, lateral, and posterior hypothalamic areas and the ventromedial and parvicellular nuclei; the zona incerta and fields of Forel; parts of the substantia nigra including the pars reticularis and pars lateralis, and the retrorubral area; the pretectum; the intermediate and deep layers of the superior colliculus; the periaqueductal gray; the dorsal nucleus of the raphe; portions of the reticular formation, including the mesencephalic, pontis oralis, pontis caudalis, gigantocellularis, ventralis, and lateralis reticular nuclei; the nucleus cuneiformis; the marginal nucleus of the brachium conjunctivum; the locus coeruleus; portions of the trigeminal complex, including the principal sensory and spinal nuclei; portions of the vestibular complex, including the lateral division of the superior nucleus and the medial nucleus; deep cerebellar nuclei, including the medial and lateral cerebellar nuclei; and lamina VII of the cervical spinal cord. Moreover, the WGA-HRP and rhodamine methods (known to be more sensitive than the HRP method) revealed several afferent sources not shown by HRP: the anterior hypothalamic area, ventral tegmental area, lateral division of the superior vestibular nucleus, nucleus interpositus, and the nucleus praepositus hypoglossi. Also, the rhodamine method revealed labeled neurons in laminae V and VI of the cervical spinal cord.     

Collaterals of spinothalamic tract cells to the periaqueductal gray: a fluorescent double-labeling study in the rat

Spinothalamic tract (STT) cells were investigated in the rat to determine the distribution of STT cells with terminals in both the ventrobasal (VB) thalamus and the lateral periaqueductal gray (PAG). Two retrogradely transported fluorescent dyes (Diamidino yellow and Granular blue) were injected into each animal. The distribution of single- and double-labeled cells was mapped in the cervical, thoracic, lumbar and sacral spinal cord. An average of 1.4% of all STT cells and 6.2% of PAG cells projected to both VB thalamus and PAG. Double-labeled cells were observed only in cervical and lumbar levels of the spinal cord, with the greatest number found in the cervical enlargement. The double-labeled cells were located in laminae I and V and also in the lateral spinal nucleus (LSN). The number of double-labeled cells found in each of these 3 areas varied depending on the spinal cord level. This population of neurons exhibiting collaterals provide an anatomical mechanism by which noxious stimuli activate neurons not only in the thalamus but also in the PAG, which is an area involved in stimulation-produced analgesia (SPA).     

Projection of nodose ganglion cells to the upper cervical spinal cord in the rat

Afferent fibers mediating pain from myocardial ischemia classically are believed to travel in sympathetic nerves to enter the thoracic spinal cord. After sympathectomies, angina pectoris still may radiate to the neck and inferior jaw. Sensory fibers from those regions are thought to enter the central nervous system through upper spinal cord segments. We postulated that axons from nodose ganglion cells might project to cervical cord segments. The purpose of this study was to determine the density and pathway of vagal afferent innervation to the upper cervical spinal cord. Following an injection of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the upper cervical spinal cord, approximately 5.8% of cells in the nodose ganglion contained reaction product. Cervical vagotomy did not diminish the density of WGA-HRP labeled cells in the nodose ganglion. However, a spinal cord hemisection cranial to the injection site eliminated labeling of nodose cells. These data indicate that a portion of vagal afferent neurons project from the nodose ganglion to the upper cervical spinal cord. In addition, vagal afferent fibers reach the spinal cord via a central route rather than through dorsal root ganglia.     

Localization of Barrington's nucleus in the pontine dorsolateral tegmentum of the rabbit.

The localization of Barrington's nucleus in the dorsolateral pons of the rabbit and its projections to the sacral spinal cord were examined by using retrograde and anterograde labeling methods combined with immunohistochemistry. After injection of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) or a fluorescence tracer, tetramethylrhodamine-dextran amine (TMR), into the sacral spinal cord segments, a cluster of neurons labeled with WGA-HRP or TMR were seen in the pontine dorsolateral tegmentum. To identify whether the retrogradely labeled neurons were situated within the locus coeruleus, the sections containing TMR-labeled neurons through the pons were incubated with anti-tyrosine hydroxylase (TH) antibody and observed under epifluorescence microscope. It was shown that the cluster of TMR-labeled neurons in the dorsolateral tegmentum were surrounded by TH-positive neurons, but they were negatively immunostained with TH-like immunoreactivity. In anterograde experiment, injection of WGA-HRP into the dorsolateral tegmentum resulted in many anterogradely labeled nerve fibers and terminals in the sacral spinal cord, including the sacral parasympathetic nucleus. The present results suggest that the cluster of neurons in the dorsolateral tegmentum of the rabbit may correspond to Barrington's nucleus revealed in the rat and cat, and thus may be involved in micturtion reflex of the rabbit.     

Evidence for dorsal root projection to somatostatin-immunoreactive structures in laminae I-II of the spinal dorsal horn.

In order to determine if somatostatin (SOM)-immunoreactive (I) cell bodies and processes in lamina (L) II of the rat spinal cord receive dorsal root input, the latter were anterogradely labeled by wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). SOM-I structures were demonstrated by immunohistochemistry. Cell bodies labeled transscellularly or transsynaptically by WGA-HRP and immunohistochemically for SOM were present in L II. In addition, a L I cell was double labeled. These results suggest that some dorsal root axons innervate SOM-I neurons in L I-II of the spinal cord. In addition to confirming immunohistochemical observations in published reports, we have revealed SOM-I central terminals in the type II glomerulus. Further, a SOM-I CI-terminal, presumed to be of primary afferent origin, contacted a SOM-I dendrite in L II. Since SOM has been implicated in nociceptive function in the dorsal horn, it is possible that some of the SOM-I structures identified are involved in nociceptive processing.     

Spinal and medullary input to the lateral cervical nucleus

The distributions of spinal and medullary cells projecting to the lateral cervical nucleus (LCN) have been investigated in young cats and dogs using the retrograde horseradish peroxidase (HRP) technique. Labeled spinal cells, whose axons contribute to the spinocervical tract (SCT), were found at all levels of the spinal cord ipsilateral to the injection sites. No significant differences were found between cat and dog, nor between cases with single injections at different levels of the LCN. SCT cells were found predominantly, in not exclusively, within lamina IV, with some extension into medial lamina V. No apparent mediolateral or dorsoventral density gradient was observed within lamina IV; cells of all sizes were labeled. Cells in cervical laminae I and V-VII were occasionally labeled; these, however, were considered to be propriospinal, supplying afferent fibers to the C_1–2 dorsal horn. Cells of origin of spinocerebellar fibers consistently remained unlabeled in cases with restricted HRP injections and minimal fiber damage in the dorsolateral funiculus (DLF) around the injection sites. These results, therefore, corroborate and refine the findings of electrophysiological studies of the SCT and the LCN. Labeled medullary cells were located in the caudoventral and rostral portions of the dorsal column nuclei (DCN; stellate and fusiform cells), the underlying n. medullae oblongatae centralis, subnucleus dorsalis (parvicellular medullary reticular formation), the marginal and magnocellular layers (both large and small cells) of the n. trigeminalis spinalis pars caudalis and also in pars interpolaris; a cluster of cells was also consistently labeled in the lateral reticular formation just ventral to pars caudalis. The projection from the DCN to the LCN was confirmed with the anterograde Nauta technique. Fiber degeneration was observed in the entire ipsilateral LCN, although it was less abundant than that observed in the adjacent C_1–2 dorsal horn. These results indicate that neurons in the rostral portions of the DCN not only may affect the input to the LCN (at the level of the dorsal horn), but also the output of the LCN itself. These data also suggest the possibility of both noxious and non-noxious facial input to the LCN.     

Electrophysiological evidence for a role of the anterolateral quadrant of the spinal cord in the transmission of noxious messages to the thalamic ventrobasal complex in the rat

Responses to noxious mechanical and thermal stimulation applied to the hindpaws were recorded extracellularly from the same neurons of the ventrobasal complex of the rat thalamus (VB) before and after lesions of various areas of the cervical cord in order to determine the pathways carrying the afferent messages. It was demonstrated that lesions of the dorsal and dorsolateral portions of the cord failed to eliminate the VB neuronal responses to noxious stimulation. By contrast, lesion of one antelateral quadrant eliminated the responses to a noxious stimulation applied to the hindpaw contralateral to the lesion. This occurred whether the lesion was ipsilateral or contralateral to the recording site. From the present study and the data in the literature, it is concluded that the fibers of the spino-thalamic tract which are completely crossed in the spinal cord, travel in the anterolateral quadrant and project directly onto the VB, are involved in the transmission of noxious messages from the cord to the VB neurons. This conclusion indicates that the VB neuronal responses to noxious stimulation of the hindpaw ipsilateral to the recording site depend on the spinothalamic projection to the opposite ventrobasal complex. This therefore suggests that some noxious messages which reach a particular VB neuron are conveyed via the opposite VB and the existence of a thalamo-cortico-thalamic loop is discussed.     

Fetal homotypic transplant in the excitotoxically neuron-depleted thalamus: light microscopy

One month after an in situ injection of kainic acid into the ventrobasal thalamic complex (VB), the lesioned area is totally depleted of neurons. The present study has been undertaken to determine the cytoarchitecture and connectivity of the nucleus constructed by fetal thalamic neurons implanted into the excitotoxically lesioned area. Adult rats received an injection of kainic acid inducing a total neuronal depletion of the right lateral thalamus (including both the nucleus reticularis thalami and the lateral portion of the ventrobasal complex). One month later, homotypic neurons were taken from the dorsal thalamic primordium of rat embryos (gestational age 15-16 days), dissociated, and injected into the lesioned area as a cell suspension. After 2-4-month survival, the cytoarchitecture of the neonucleus formed by the grafted neurons within the previously neuron-depleted area was analyzed. Additionally, connectivity was analyzed in seven rats in which dorsal column nuclei and/or cortical projections to the area were labeled anterogradely with either 3H-leucine or wheat-germ agglutinin conjugated to HRP, and the animals were perfused and processed following various histological procedures (Nissl staining, autoradiographic processing, and histochemistry for visualization of peroxidase). Fetal neurons grew, differentiated, and progressively occupied the previously neuron-depleted area of the adult host CNS. They organized themselves into a neonucleus with particular cytoarchitectural features including 1) the existence of two concentric zones--a central zone containing neurons and glial cells and a marginal zone only filled with a band of glial cells, 2) an increase in cellular density compared to the intact thalamus, 3) the grouping of neurons in spherical clusters, and 4) apparent polymorphism of neuronal somata. Lemniscal and corticothalamic afferents originating from the host were observed in the neonucleus when the fetal neurons had been implanted correctly into the lesioned area but not when they had been misplaced into either normal thalamic tissue or the internal capsule. The afferents labeled from either the dorsal column nuclei or the somatosensory cortex were, however, less dense in the neonucleus than in the normal thalamus. These results are discussed with regard to the normal cytoarchitecture and connectivity of the ventrobasal complex of the rat thalamus.     

Cortical projections to superficial laminae of the dorsal horn and to the ventral horn of the spinal cord in the North American opossum. Studies using the orthograde transport of WGA-HRP

The orthograde transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) has been used to study the distribution of corticospinal axons in adult and pouch-young opossums. As predicted from the results of degeneration and autoradiographic experiments, injections of WGA-HRP into limb areas of somatic motor-sensory cortex labeled axons in the dorsal and lateral funiculi of the cervical and upper thoracic spinal cord which could be traced to dense terminal zones in laminae III–VI. In addition, we obtained evidence for the presence of a few cortical axons in the ventral white matter and for innervation of the medial part of laminae I and II, laminae VII and VIII and lamina X. A few cortical axons are even present in lamina IX.     

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.     

Reciprocal parabrachial-cortical connections in the rat

The projection from the parabrachial nucleus (PB) to the cerbral cortex in the rat was studied in detail using the autoradiographic method for tracing anterograde axonal transport and the wheat germ agglutinin-horseradish peroxidase (WGA-HRP) method for both anterograde and retrograde tracing. PB innervates layers I, V and VI of a continuous sheet of cortex extending from the posterior insular cortex caudally, through the dorsal agranular and the granular anterior insular cortex and on rostrally into the lateral prefrontal cortex. Within the prefrontal area, PB fibers innervate primarily layer V of the ventrolateral cortex caudally, but more rostrally the innervated region includes progressively more dorsal portions of the prefrontal area, until by the frontal pole the entire lateral half of the hemisphere is innervated. This projection originates for the most part in a cluster of neurons in the caudal ventral part of the medial PB subdivision, although a few neurons in the adjacent parts of the PB, the Kolliker-Fuse nucleus and the subcoeruleus region also participate.After injection of WGA-HRP into the PB region, retrogradely labeled neurons were found in layer V of the same cortical areas which receive PB inputs. The importance of this monosynaptic reciprocal brainstem-cortical projection as a possible anatomical substrate for the regulation of cortical arousal is discussed.