The origins of descending spinal projections in lepidosirenid lungfishes

The origins of descending spinal projections in the lepidosirenid lungfishes were identified by retrograde transport of horseradish peroxidase (HRP) introduced into the rostral spinal cords of juvenile African (Protopterus annectans and Protopterus amphibians) and South American (Lepidosiren paradoxa) lungfishes. Standard HRP histochemistry revealed retrogradely labeled neurons in the nucleus of the medial longitudinal fasciculus, midbrain tegmentum, red nucleus, optic tectum, mesencephalic trigeminal nucleus, granule cell layer of the cerebellum, superior, middle, and inferior medullary reticular nuclei, magnocellular and descending octaval nuclei, region of the descending trigeminal tract, solitary complex, and the margins of the spinal gray matter anterior to the spinal HRP implant. A small number of retrogradely labeled neurons were also present in the ventral thalamus of Protopterus. A descending spinal projection from the forebrain was not evident in either genus of lepidosirenid lungfishes. The presence of projections to the spinal cord from the diencephalon, medial reticular formation of the midbrain and medulla, octaval (vestibular) nuclei, solitary complex, and probable nucleus of the descendin trigeminal tract in lungfishes and their overall similarity to comparable projections in other vertebrates suggest that these pathways are among those representative of the primitive pattern of descending spinal projections in vertebrates.     

Ascending projections of the brain stem reticular formation in a nonmammalian vertebrate (the lizard Varanus exanthematicus), with notes on the afferent connections of the forebrain

In the present study an attempt has been made to analyze the ascending reticular projections in the lizard Varanus exanthematicus by means of the horseradish peroxidase (HRP) technique. Reticular projections ascending to the telencephalon were found to arise in the mesencephalon, but not caudal to the mesorhombencephalic border. HRP injections into the dorsal thalamus have demonstrated retrogradely labeled cells in the mesencephalic reticular formation, particularly at the level of the oculomotor nerve and in the medial magnocellular zone of the rhombencephalic reticular formation, predominantly rostrally. HRP infiltrations at the mesodiencephalic border damaged most of the fibers passing beyond this junction, resulting in the uptake of HRP by the damaged axons and subsequent labeling of the cell bodies or origin of ascending reticular projections to the diencephalon and telencephalon. From a comparison of cell-labeling patterns in cases of HRP injections of, respectively, the dorsal thalamus and the mesodiencephalic border, it seems likely that the nucleus reticularis medius and more sparsely the nucleus reticularis inferior project to ventral diencephalic structures (ventral thalamus and hypothalamus), whereas the midbrain reticular formation and the rostral parts of the rhombencephalic reticular formation (nuclei reticulares isthmi and superior) project to both the dorsal thalamus and more ventral diencephalic structures. Projections arising throughout the rhombencephalic reticular formation, but predominantly in the nucleus reticularis inferior, were found to ascend to the midbrain reticular formation. The present experimental data in the lizard Varanus exanthematicus are comparable to the findings in mammals, with the exception of the reticulo-oculomotor pathways which have not been analyzed so far in reptiles. In addition to the aforementioned ascending reticular projections, the present study has demonstrated projections ascending from monoamine cell groups, various diencephalics structures, as well as from neuronal groups involved in somatosensory, auditory, and gustatory systems. Projections were found from the locus coeruleus and the nucleus raphes superior to the telencephalon, as well as from the substantia nigra and the presumable reptilian homologue of the mammalian ventral tegmental area to the basal forebrain and the dorsal thalamus. Bilateral projections were demonstrated from the principal trigeminal nucleus to the telencephalon, reminiscent of the quintofrontal tract of birds. Ascending projections to the diencephalon were found to originate bilaterally in the descending trigeminal nucleus and the dorsal funicular nucleus. Auditory projections to the midbrain arise bilaterally in the superior olivary complex and in the cochlear nuclear complex. Finally, the ascending gustatory pathway arising in the nucleus of the solitary tract was found to project to the “parabrachial region”, which in its turn has extensive projections to the forebrain.     

Tectal connections in Python reticulatus

The origins of the axons terminating in the mesencephalic tectum in Python reticulatus were examined by unilateral tectal injections of horseradish peroxidase. Kutrogradely labeled cells were observed bilaterally throughout the spinal cord in all subdivisions of the trigeminal system, with the exception of nucleus principalis, which showed labeled cells only on the ipsilateral side. Labeling of the reticular formation occurred bilaterally in nucleus reticular is interiormagnocellularis, nucleus reticularis lateralis, nucleus reticularis medius and the mesencephalic reticular formation. The tectum also receives bilateral projections from the dorsal tegmentaJ field, the nucleus of the lateral lemniscus and nucleus isthmi, and ipsilateral projections from nucleus profundus mesencephali. A few labeled cells were found ipsilaterally in the locus coeruleus and in nuclei vestibulares ventrolateralis and centromedialis. In the diencephalon labeled cells were observed ipsilaterally in nucleus ventrolateralis thalami, nucleus ventromedialis thalami, nucleus suprapeduncularis, and in the dorsal and ventral lateral geniculate nuclei. Bilateral labeling was observed in nucleus periventricularis hypo-thalami. Furthermore, labeling was ipsilaterally present in the ventral telen-cephalic areas. The tectum in Python reticulatus receives a wide variety of afferent connections which confirm the role of the tectum as an integration center of visual and exteroceptive information.     

Projections from the red nucleus and surrounding areas to the brainstem and spinal cord in the cat. An HRP and autoradiographical tracing study

HRP injections at the C2, T1 and S1 spinal levels and in the medullary lateral tegmental field revealed that the contralaterally projecting rubro-bulbospinal neurons are located not only in the caudal but also to a certain extent in the rostral red nucleus (RN). These RN projections are somatotopically organized. Neurons projecting to the sacral cord are located in the ventrolateral RN, those projecting to the upper part of the spinal cord lie in the dorsomedial RN and those projecting to the medullary lateral tegmentum were found in the dorsal portions of the RN. These last neurons are smaller than many of the other RN neurons. The HRP results also revealed that the RN does not project to the caudal raphe nuclei. The autoradiographical results confirmed the HRP findings. They further indicated that the contralateral RN projections to the caudal brainstem precerebellar nuclei (nucleus corporis pontobulbaris, lateral reticular nucleus, lateral cuneate nucleus) and the dorsal column nuclei are also somatotopically organized. This was also true for the RN projections to the dorsomedial and intermediate facial subnuclei and the caudal pontine and medullary lateral tegmental field. These areas receive afferents from mainly the dorsal portions of the RN. Regarding the RN projections to the spinal cord, the autoradiographical tracing results revealed somatotopically organized contralateral RN projections to laminae V, VI and VII. Moreover, a small but distinct RN projection to a dorsolaterally located group of motoneurons at the C8-T1 level was demonstrated. Ipsilaterally a minor projection to the cervical and upper thoracic lateral intermediate zone was observed. Finally, strong ipsilateral projections from the rostral mesencephalon to the inferior olive were seen. These projections were derived from various rostral mesencephalic areas, including the nucleus of Darkschewitsch, the nucleus accessorius medialis of Bechterew, the interstitial nucleus of Cajal and the area of the rostral interstitial nucleus of the medial longitudinal fasciculus. In the cat it was difficult to define which of the mesencephalic areas projecting to the inferior olive represented the parvocellular RN. A new subdivision of the RN is proposed based on its projections and not on the size of its cells. In this concept the first group is formed by the RN neurons projecting contralaterally to the caudal brainstem and spinal cord. The second group consists of RN neurons projecting to the inferior olive.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cells of origin of pathways descending to the spinal cord in two chondrichthyans,the shark Scyliorhinus canicula and the ray Raja clavata

The cells of origin of pathways descending to the spinal cord in the shark Scyliorhinus canicula and in the ray Raja clavata have been demonstrated by using the horseradish peroxidase (HRP) technique. Following HRP injections in the spinal cord of Scyliorhinus (fourth to sixth segment) and of Raja (15th to 20th segment) labeled neurons could be identified in the rhombencephalon, the mesencephalon, and in the diencephalon. Cells of origin of diencephalic nuclei, which project to the spinal cord, were observed in the nucleus periventricularis hypothalami and in the thalamus ventralis pars medialis which can in this respect be considered hypothalamic. Descending pathways from mesencephalic structures originate from the interstitial nucleus of the fasciculus longitudinalis medialis, the tectum mesencephali, the nucleus intercollicularis, the tectotegmental junction zone, and from diffusely arranged tegmental neurons. A contralateral rubrospinal pathway could be recognized in Raja, but not in Scyliorhinus. Rhombencephalic cells of origin of pathways descending to the spinal cord were found in all parts of the reticular formation, i.e., the nucleus raphes inferior, the nucleus reticularis inferior, medius, superior, and isthmi, in two vestibular nuclei, and in three nuclei, which have been tentatively indicated as nucleus B, F, and G. Furthermore cells of origin of descending pathways have been found in the nucleus tractus descendens nervi trigemini, in the nucleus funiculi lateralis, and in the nucleus tractus solitarii. The descending pathways of the two species studied have been compared with those of other vertebrates. It is concluded that the basic pattern in the organization of descending pathways to the spinal cord, as proposed by ten Donkelaar ('76) for terrestrial vertebrates, also holds for cartilaginous fishes.     

Localization of neurons afferent to the optic tectum in longnose gars

Afferent pathways to the optic tectum in the longnose gar were determined by unilateral tectal injections of HRP. Retrogradely labeled cells were observed in the ipsilateral caudal portion of the rostral entopeduncular nucleus and bilaterally in the rostral half of the lateral zone of area dorsalis of the telencephalon. The following diencephalic cell groups were also labeled following tectal injections: the ipsilateral anterior, ventrolateral, and ventromedial thalamic nuclei, the periventricular pretectal nucleus, and the central pretectal nucleus (bilaterally); the ventromedial thalamic and central pretectal nuclei revealed the largest number of labeled cells. At midbrain levels, retrogradely labeled cells were seen in the ipsilateral torus longitudinalis, nucleus isthmi, and accessory optic nucleus; cells were labeled bilaterally in the torus semicircularis and a rostral tegmental nucleus. Only a few cells were labeled in the contralateral optic tectum, suggesting that few of the fibers of the intertectal commissure are actually commissural to the tectum. At hindbrain levels, retrogradely labeled cells were seen bilaterally in the locus coeruleus, the superior, medial, and inferior reticular formations, the eurydendroid cells of the cerebellum, and the nucleus of the descending trigeminal tract; the contralateral dorsal funicular nucleus also exhibited labeling. Clearly, the tectum in gars receives a substantial number of nonvisual afferents from all major brain areas, most of which have been reported in other vertebrates. The functional significance of these afferent sources and their probable homologues in other vertebrate groups are discussed.     

Brain stem afferents of hypoglossal neurons in the rat

The origin of afferent connections of the hypoglossal nucleus in rats was investigated using horseradish peroxidase (HRP) as a retrograde tracer. Pressure injections (0.15–0.17 μl) of 15% HRP were introduced into the rostral, middle and caudal portions of the nucleus. Projections to the hypoglossal nucleus originated from 3 regions of the brainstem: the reticular formation, the spinal V complex and the nucleus of the solitary tract. Bilateral projections with ipsilateral predominance came from the lateral reticular formation: the dorsal aspect of the nucleus reticularis parvocellularis and its caudal continuation, the nucleus reticularis dorsalis. Fewer projections emerged from two nuclei of the medial reticular formation. The dorsal part of the nucleus reticularis ventralis at the spinomedullary junction contributed bilateral with mainly contralateral input to hypoglossal neurons. A few labeled neurons were situated bilaterally in the nucleus reticularis gigantocellularis of the rostral medulla. The input from the spinal V complex originated from the dorsal aspect along most of its length but particularly from the pars interpolaris and oralis subdivisions. Labeled neurons were located primarily in the posterior portion of the nucleus of the solitary tract. Projections from the spinal V complex and the solitary nucleus exhibited ipsilateral predominance. These results suggest that somatic and visceral centers of the rat brainstem play an important role in the control of the activity of hypoglossal motoneurons.     

Identification of the midbrain locomotor region and its relation to descending locomotor pathways in the Atlantic stingray, Dasyatis sabina.

The midbrain locomotor region (MLR) in the Atlantic stingray, Dasyatis sabina, was identified and characterized. Stimulation (50-100 microA, 60 Hz) of the midbrain in decerebrated, paralyzed animals was used to elicit locomotion monitored as alternating activity in nerves innervating an antagonist pair of elevator and depressor muscles. Effective sites for evoking locomotion in the midbrain included parts of several nuclei: the caudal portion of the interstitial nucleus of the medial longitudinal fasciculus and the caudomedial parts of the cuneiform and subcuneiform nuclei. This region did not include the red nucleus, any parts of the optic tectum or the medial or lateral mesencephalic nuclei. Electrical stimulation in the MLR evokes locomotion in either the ipsi- or contralateral pectoral fin, whereas stimulation in the medullary reticular formation evokes locomotion only in the contralateral fin. Lesion experiments were performed to identify the location of descending pathways from the midbrain to the medullary reticular formation. To abolish locomotion evoked by electrical stimulation in the MLR, the medial reticular formation in the rostral medulla had to be lesioned bilaterally, or the ipsilateral medial medullary reticular formation and fibers projecting from the MLR to the contralateral midbrain had to be disrupted. Injections of HRP into the magnocellular/gigantocellular reticular formation confirmed that this area received bilateral projections from the MLR. The MLR of the Atlantic stingray appears to be similar to the lateral component of the mammalian MLR and to the MLR in other non-mammalian vertebrates.     

Lower brainstem afferents to the cat posterior hypothalamus: a double-labeling study

Using a double-immunostaining technique with cholera toxin (CT) as a retrograde tracer, the authors examined the cells of origin and the histochemical nature of lower brainstem afferents to the cat posterior hypothalamus. The posterior hypothalamus, in particular the lateral hypothalamic area, receives substantial afferent projections from: substantia nigra, peripeduncular nucleus, ventral tegmental area, periaqueductal grey, mesencephalic reticular formation, peribrachial region including the locus coeruleus complex, rostral raphe nuclei and the rostral part of the nucleus magnus. In addition, a moderate number of retrogradely labeled neurons was found in: Edinger-Westphal nucleus, nucleus reticularis pontis oralis, nucleus reticularis magnocellularis, caudal lateral bulbar reticular formation around the nucleus ambiguus and lateral reticular nucleus and the nucleus of the solitary tract. The posterior hypothalamus receives: 1) dopaminergic inputs from A8, A9 and A10 cell groups; 2) noradrenergic inputs from A6 and A7 pontine, as well as A1 and A2 bulbar cell groups; 3) adrenergic inputs from C1 cell group in the caudal medulla; 4) serotoninergic inputs from the rostral raphe nuclei (B6, B7 and B8 cell groups); 5) cholinergic inputs from the peribrachial region of the dorsal pontine tegmentum as well as from the nucleus reticularis magnocellularis of the medulla; 6) peptidergic inputs such as methionine-enkephalin, substance P, corticotropin-releasing factor and galanin that originate mainly in the mesencephalic periaqueductal grey, the dorsal raphe nucleus and the peribrachial region of the dorsal pontine tegmentum.     

Afferent connections of the optic tectum in lampreys: an experimental study

Tectal afferents were studied in adult lampreys of three species (Ichthyomyzon unicuspis, Lampetra fluviatilis, and Petromyzon marinus) following unilateral BDA injections into the optic tectum (OT). In the secondary prosencephalon, neurons projecting to the OT were observed in the pallium, the subhipoccampal lobe, the striatum, the preoptic area and the hypothalamus. Following tectal injections, backfilled diencephalic cells were found bilaterally in: prethalamic eminence, ventral geniculate nucleus, periventricular prethalamic nucleus, periventricular pretectal nucleus, precommissural nucleus, magnocellular and parvocellular nuclei of the posterior commissure and pretectal nucleus; and ipsilaterally in: nucleus of Bellonci, periventricular thalamic nucleus, nucleus of the tuberculum posterior, and the subpretectal tegmentum, as well as in the pineal organ. At midbrain levels, retrogradely labeled cells were seen in the ipsilateral torus semicircularis, the contralateral OT, and bilaterally in the mesencephalic reticular formation and inside the limits of the retinopetal nuclei. In the hindbrain, tectal projecting cells were also bilaterally labeled in the dorsal and lateral isthmic nuclei, the octavolateral area, the sensory nucleus of the descending trigeminal tract, the dorsal column nucleus and the reticular formation. The rostral spinal cord also exhibited a few labeled cells. These results demonstrate a complex pattern of connections in the lamprey OT, most of which have been reported in other vertebrates. Hence, the lamprey OT receives a large number of nonvisual afferents from all major brain areas, and so is involved in information processing from different somatic sensory modalities.     

Spinal projections from the mesencephalon in the toad

Mesencephalic cell groups projecting to the spinal cord have been identified by means of the retrograde axonal transport of the enzyme horseradish peroxidase (HRP). The injections were made either in the cervical or lumbar enlargements of the toad spinal cord. Following injections in the cervical cord, labeled cells are located in the isthmus region, in the ipsilateral laminated nucleus posteroventralis tegmenti mesencephali (Potter). At more rostral levels the labeled cells are in the nucleus of the fasciculus longitudinalis medialis, in the nucleus interstitialis of the fasciculus longitudinalis medialis, in the contralateral red nucleus, in lamina six of the contralateral optic tectum, bilaterally in the nucleus of the posterior commissure and in the mesencephalic nucleus in the Vth nerve. Injections in the lumbar cord label neurons of the nucleus posteroventralis tegmenti mesencephali (Potter) and nucleus interstitialis of the fasciculus longitudinalis medialis. Nuclei that had not been previously identified in anurans but which were labeled after HRP spinal injections (i.e., the nucleus interstitialis of the fasciculus longitudinalis medialis, the nucleus of the posterior commissure and the red nucleus) have been delimited in normal material in Nissl-stained transverse sections. The spinal pathways from the mesencephalon can be classified into four projections: reticulospinal, rubrospinal, tectospinal and trigeminospinal. A comparison of these descending fiber systems with homologous pathways in other vertebrate species has been made.     

Mesencephalic and diencephalic afferents to the superior colliculus and periaqueductal gray substance demonstrated by retrograde axonal transport of horseradish peroxidase in the cat

The mesencephalic and diencephalic afferent connections to the superior colliculus and the central gray substance in the cat were examined by means of the retrograde transport of horseradish peroxidase (HRP). After deep collicular injections numerous labeled cells were consistently found in the parabigeminal nucleus, the mesencephalic reticular formation, substantia nigra pars reticulata, the nucleus of posterior commissure, the pretectal area, zona incerta, and the ventral nucleus of the lateral geniculate body. A smaller number of cells was found in the inferior colluculus, the nucleus of the lateral lemniscus, the central gray substance, nucleus reticularis thalami, the anterior hypothalamic area, and, in some cases, in the contralateral superior colliculus, Forel's field, and the ventromedial hypothalamic nucleus. Only the parabigeminal nucleus and the pretectal area showed labeled cells following injections in the superficial layers of the superior colliculus. In the cats submitted to injections in the central gray substance, labeled cells were consistently found in the contralateral superior colliculus, the mesencephalic reticular formation, substantia nigra parts reticulata, zona incerta and various hypothalamic areas, especially the ventromedial nucleus. In some cases, HRP-positive cells were seen in the nucleus of posterior commissure, the pretectal area, Forel's field, and nucleus reticularis thalami. A large injection in the mediodorsal part of the caudal mesencephalic reticular formation, which included the superior colliculus and the central gray substance, resulted in numerous labeled cells in nucleus reticularis thalami. The findings are discussed with respect to the suggested functional division of the superior colliculus into deep and superficial layers. Furthermore, the possible implications of labeled cells in zona incerta and the reticular thalamic nucleus are briefly discussed.     

Descending connections from the brainstem to the spinal cord in the electric fish Eigenmannia. Quantitative description based on retrograde horseradish peroxidase and fluorescent-dye transport

The descending connections from the brainstem to the spinal cord in Eigenmannia sp. were demonstrated using the horseradish peroxidase (HRP) technique. The spinal cord was transected and HRP crystals were deposited in the cut. The point of transection was located at varying distances from the head in different specimens. In all experiments, cells were labeled in both the rhombencephalic and mesencephalic tegmentum. No labeled cells were found in the cerebellum, the lateral-line lobes, the torus semicircularis, the tectum mesencephali, the hypothalamus, the diencephalon or the telencephalon. Labeled neurons were found in the ventrolateral column, nucleus formation reticularis (NFR) inferior, NFR medius, NFR superior pars superior and pars suprema, NFR tegmenti mesencephali lateralis, nucleus vestibularis magnocellularis and nucleus fasciculi longitudinalis medialis. Furthermore, the Mauthner cells and the neurons of the pacemaker nucleus were filled with HRP granules. The neurons labeled were predominantly the large ones of more than 25 microns in diameter which are very conspicuous along the brainstem. The number of these neurons in the different nuclei varied from animal to animal, however, the number of labeled neurons increased monotonically at a similar rate in all brainstem nuclei with more rostrally located transection sites. In a second series, the number of neurons terminating in a small number of segments independent of absolute position along the body axis was assessed using two different fluorescent dyes. Within tolerable statistical limits, this number was found to be constant, corroborating the data obtained with HRP. A possible interpretation of the data is placed in the context of physiological data previously presented.     

Afferents to brain stem nuclei (brain stem raphe, nucleus reticularis pontis caudalis and nucleus gigantocellularis) in the rat as demonstrated by microiontophoretically applied horseradish peroxidase

Using a retrograde tracer technique with microiontophoretically applied horseradish peroxidase (HRP), afferent projections to the brain stem raphe nuclei (BR, raphe magnus, pallidus and obscurus) and to two adjacent reticular nuclei, nucleus reticularis pontis caudalis (nRPC) and nucleus gigantocellularis (nGC) were identified. The most striking difference between the afferent projections to the BR and the adjacent nuclei as determined by this method is that afferents to the BR originate primarily from structures rostral to the pons, especially the mesencephalic central gray and the dorsal and ventral tegmentum. In contrast, the two reticular nuclei studied (nGC and nRPC) received afferent projections within or caudal to the pons-medulla. For example, the nGC receives prominent afferent projections from the gray matter of the spinal cord. In addition, evidence for interconnections between all of the adjacent nuclei (BR, nGC and nRPC) was found. Such afferent projections are compatible with the notion that the brain stem raphe nuclei may serve as connections within the brain stem for a descending system, while the nGC may be a relay in a feedback loop between the spinal cord and the reticular formation.     

Origins of the descending spinal projections in petromyzontid and myxinoid agnathans

The origins of the descending spinal pathways in sea lampreys (Petromyzon marinus), silver lampreys (Ichthyomyzon unicuspis), and Pacific hagfish (Eptatretus stouti) were identified by the retrograde transport of horseradish peroxidase (HRP) placed in the rostral spinal cord. In lampreys, the majority of HRP-labeled cells were located along the length of the brainstem reticular formation in the inferior, middle, and superior reticular nuclei of the medulla, mesencephalic tegmentum, and nucleus of the medial longitudinal fasciculus. Labeled reticular cells included the Mauthner and Müller cells. Horseradish-peroxidase-filled cells were also present in the descending trigeminal tract, intermediate and posterior octavomotor nuclei, and a diencephalic cell group, the nucleus of the posterior tubercle. As in lampreys, the reticular formation of the Pacific hagfish was the largest source of descending afferents to the spinal cord. Labeled cells were found in the dorsolateral and ventromedial reticular nuclei, the dorsal tegmentum at the juncture of the medulla and midbrain, and the nucleus of the medial longitudinal fasciculus. Additional medullary cells projecting to the cord were located in the perivagal nucleus, the central gray, and the anterior and posterior magnocellular octavolateralis nuclei. The existence of reticulospinal and possible vestibulo-, trigemino-, and solitary spinal projections in lampreys and hagfishes and the wide distribution of these pathways in jawed vertebrates suggest that they evolved in the common ancestor of gnathostomes and both groups of jawless fishes. However, descending spinal pathways from the cerebellum, red nucleus, and telencephalon appear to be gnathostome characters.     

The origin of reticulospinal fibers in the rat: a HRP study

The distribution as well as morphological characteristics of brain stem reticular neurons projecting to spinal cord both of aminergic and non-aminergic natures in the rat was investigated by means of the retrograde horseradish peroxidase (HRP) method. For the identification of aminergic neurons, a combination of HRP technique and monoamine-oxidase staining as well as a pretreatment of 6-hydroxydopamine and 5,6-dihydroxytryptamine (5,6-DHT) was applied. Following the injection of HRP to the cervical, thoracic and lumbar cord, a remarkable number of neurons in the nuclei reticularis ventralis (rv), reticularis lateralis (RI; Meesen & Olszewski), reticularis gigantocellularis, reticularis pontis caudalis, reticularis pontis oralis (rpo), and a small number of cells in the nucleus reticularis dorsalis dorsalis as well as the mesencephalic reticular nuclei were found to be labeled. In the case of cervical injection, HRP labeled cells were also found in the nucleus reticularis parvocellularis, adjacent to the nucleus tractus spinalis n. trigemini oralis, in which labeled neurons were observed. Within Rl, ventral division of rv and lateral part of rpo, the labeled noradrenaline neurons in A 1, 3 and 7, respectively, were found intermingled with the non-aminergic labeled neurons. Many neurons in the nucleus raphe magnus (ram), obscurus (rao) and pallidus (rap) were labeled. From the fact that there was a marked decrease in the number of the labeled cells in rao, rap, and a slight decrease in ram after 5,6-DHT treatment, it was suggested that the majority of labeled cells in rao, rap and a partial number of labeled cells in ram are serotonergic.     

Topographic organization of the brainstem afferents to the mediodorsal thalamic nucleus

The afferent projections from the brainstem to the mediodorsal thalamic nucleus (MD) were studied in the cat, by means of retrograde transport of horseradish peroxidase. A topographical arrangement of these projections is described. The medial part of MD is the area of the nucleus which receives fewer afferents from the brainstem. After injections in this part, labeled neurons were observed mainly in the interpeduncular nucleus, the ventral tegmental area and the substantia nigra. After injections of HRP in the intermediate part of the MD, labeled cells were seen mainly in the interpeduncular nucleus, substantia nigra, dorsal and centralis superior raphe nuclei, dorsal tegmental nucleus, and coeruleus complex. Less conspicuous was the number of labeled cells in the central gray and the dorsolateral portion of the tegmentum of the mesencephalon and pons. After injections in the lateral part of MD, labeled neurons were observed mainly in the deep layers of the superior colliculus, central gray, the oral paramedian pontine reticular tegmentum, and the interpeduncular nucleus. Labeled cells were also observed in the substantia nigra, locus coeruleus, dorsal tegmental nucleus, cuneiform area, and the mesencephalic reticular formation. These findings show the MD as a thalamic link of three different groups of brainstem structures projecting to different cortical areas with different functional significance.     

Origins of cerebellar mossy and climbing fibers immunoreactive for corticotropin-releasing factor in the rabbit

Corticotropin-releasing factor (CRF) has been implicated by both anatomical and physiological techniques as a potential cerebellar transmitter or modulator. In the present experiment, with the aid of immunohistochemistry, we have described specific cerebellar afferent pathways in the rabbit in which CRF is located. CRF-immunoreactive climbing fibers were present in the molecular layer throughout the cerebellum, but especially in lobules 8–9a. All inferior olivary neurons were CRF-immunoreactive. In lobules 8–9a, CRF-immunoreactive mossy fibers were organized in sagittal bands. The highest density of CRF-immunoreactive mossy fiber terminals was observed in the granule cell layer of lobules 8–9a and the flocculus. No CRF-immunoreactive perikarya were located in rabbit cerebellum. The brainstem origin of CRF-immunoreactive mossy fiber terminals was suggested by numerous CRF-immunoreactive perikarya located in the medial, lateral and descending vestibular nuclei, nucleus prepositus hypoglossi, nucleus x, paramedian reticular nucleus, gigantocellular reticular nucleus, lateral reticular nucleus, and raphe nuclei. Using double label experiments, we investigated the specific CRF afferent projection to the flocculus and posterior vermis. Horseradish peroxidase (HRP) injections into the posterior vermis double labeled CRF-immunoreactive neurons in the caudal medial and descending vestibular nuclei and nucleus prepositus hypoglossi. HRP injections into the flocculus double labeled more CRF-immunoreactive neurons in the nucleus prepositus hypoglossi than in the vestibular nuclei. HRP injections into either the posterior vermis or flocculus double labeled CRF-immunoreactive neurons in the paramedian reticular nucleus, nucleus reticularis gigantocellularis, and raphe nuclei. These data suggest that CRF may play an important role in vestibularly related functions of the cerebellum. © 1993 Wiley-Liss, Inc.     

The origin of descending pathways in the dorsolateral funiculus of the spinal cord of the cat and rat: further studies on the anatomy of pain modulation.

There is considerable evidence that the dorsolateral funiculus (DLF) of the spinal cord contains descending pathways critical for both opiate and brainstem stimulation-produced analgesia. To obtain a comprehensive map of brainstem neurons projecting to the spinal cord via the DLF, large injections of horseradish peroxidase (HRP) were made into the lumbosacral spinal cord of cat and rat. These injections were made caudal to midthoracic lesions which spared only a single DLF or ventral quadrant (VQ); thus only those neurons whose axons descended in the spared funiculus would be labelled. Cells with descending axons in the VQ were concentrated in the medullary nucleus raphe pallidus and obscurus, nucleus retroambiguus and in various subregions of the reticular formation including the nucleus reticularis ventralis, gigantocellularis, magnocellularis, pontis caudalis and pontis oralis. Significant numbers of neurons were also found in medial and lateral vestibular nuclei and in several presumed catecholamine-containing neurons of the dorsolateral pons. In the rat, but not in the cat, considerable numbers of cells are present in the mesencephalic reticular formation just lateral to the periaqueductal gray. In both species, some cells were found in the paraventricular nucleus of the hypothalamus. Brainstem cells projecting in the DLF were concentrated in the nucleus raphe magnus and in the adjacent nucleus reticularis magnocellularis, ipsilateral to the spared funiculus. Significant numbers of cells were found in the dorsolateral pons, differing somewhat in their distribution from those projecting in the VQ. DLF-projecting cells were also present in the ipsilateral Edinger-Westphal nucleus and periaqueductal grey contralateral red nucleus of the midbrain and in the ipsilateral hypothalamus. Smaller projections from other sites are described. These results are discussed in terms of the differential contribution of several brainstem neuronal groups, including the serotonergic nucleus, raphe magnus, the ventromedial reticular formation of the medulla, and various catecholamine-containing neurons of the dorsolateral pontine tegmentum to the analgesia produced by opiates and electrical brain stimulation.     

Telencephalic ascending acousticolateral system in a teleost (Sebastiscus marmoratus), with special reference to the fiber connections of the nucleus preglomerulosus

Acousticolateral systems were examined by means of the horseradish peroxidase tracing method in a teleost (Sebastiscus marmoratus). The torus semicircularis projected bilaterally to the optic tectum, nucleus ventromedialis thalami of Schnitzlein ('62), and reticular formation; contralaterally to the torus semicircularis; and ipsilaterally to the nucleus preglomerulosus of Schnitzlein ('62) and the inferior olive. No topographic organization was detected between the torus semicircularis and the nucleus preglomerulosus. Ipsilateral inputs to the torus were from dorsal telencephalic areas (pars centralis, Dc; pars dorsalis, Dd; and the dorsal part of pars medialis, dDm) and the optic tectum. Contralateral inputs to the torus were from the torus semicircularis, a caudal part of the cerebellum, and a portion of the trigeminal complex. The torus also received bilateral input from the nucleus ventromedialis thalami, nucleus of lemniscus lateralis, nucleus medialis, anterior octaval nucleus, descending octaval nucleus, and the reticular formation. Retrogradely labeled cells in the octaval nuclei were seen predominantly subsequent to HRP injections in the medial torus, while cells in the nucleus medialis were retrogradely labeled following injections into the lateral torus. HRP injections into the nucleus preglomerulosus labeled cells in the superficial region of the torus, while injections into the nucleus ventromedialis thalami labeled cells in the deep region. The nucleus preglomerulosus received inputs bilaterally from the nucleus of the lemniscus lateralis and reticular formation and ipsilaterally from the dorsal telencephalic areas (Dc, Dd, and dDm) and the torus semicircularis. In turn the nucleus preglomerulosus projected to Dd and Dm. Fibers arising in the nucleus ventromedialis thalami ended in Dc, Dd, Dm, and area ventralis pars supracommissuralis (Vs). Homology between the nucleus preglomerulosus and the central thalamic nucleus in amphibians, the nucleus reuniens in reptiles, the nucleus ovoidalis in birds, and the medial geniculate body in mammals is discussed.