Spinal projections from the medullary reticular formation of the North American opossum: heterogeneity

Injections of Algesic Solutions into Muscle Activate the Lateral Reticular Formation: A Nociceptive Relay of the Spinoreticulothalamic Tract

Although musculoskeletal pain disorders are common clinically, the central processing of muscle pain is little understood. The present study reports on central neurons activated by injections of algesic solutions into the gastrocnemius muscle of the rat, and their subsequent localization by c-Fos immunohistochemistry in the spinal cord and brainstem. An injection (300μl) of an algesic solution (6% hypertonic saline, pH 4.0 acetate buffer, or 0.05% capsaicin) was made into the gastrocnemius muscle and the distribution of immunolabeled neurons compared to that obtained after control injections of phosphate buffered saline [pH 7.0]. Most labeled neurons in the spinal cord were found in laminae IV-V, VI, VII and X, comparing favorably with other studies, with fewer labeled neurons in laminae I and II. This finding is consistent with the diffuse pain perception due to noxious stimuli to muscles mediated by sensory fibers to deep spinal neurons as compared to more restricted pain localization during noxious stimuli to skin mediated by sensory fibers to superficial laminae. Numerous neurons were immunolabeled in the brainstem, predominantly in the lateral reticular formation (LRF). Labeled neurons were found bilaterally in the caudalmost ventrolateral medulla, where neurons responsive to noxious stimulation of cutaneous and visceral structures lie. Immunolabeled neurons in the LRF continued rostrally and dorsally along the intermediate reticular nucleus in the medulla, including the subnucleus reticularis dorsalis caudally and the parvicellular reticular nucleus more rostrally, and through the pons medial and lateral to the motor trigeminal nucleus, including the subcoerulear network. Immunolabeled neurons, many of them catecholaminergic, were found bilaterally in the nucleus tractus solitarii, the gracile nucleus, the A1 area, the CVLM and RVLM, the superior salivatory nucleus, the nucleus locus coeruleus, the A5 area, and the nucleus raphe magnus in the pons. The external lateral and superior lateral subnuclei of the parabrachial nuclear complex were consistently labeled in experimental data, but they also were labeled in many control cases. The internal lateral subnucleus of the parabrachial complex was labeled moderately. Few immunolabeled neurons were found in the medial reticular formation, however, but the rostroventromedial medulla was labeled consistently. These data are discussed in terms of an interoceptive, multisynaptic spinoreticulothalamic path, with its large receptive fields and role in the motivational-affective components of pain perceptions.

Reticulospinal neurons in the pontomedullary reticular formation of the monkey (Macaca fascicularis)

Recent neurophysiological studies indicate a role for reticulospinal neurons of the pontomedullary reticular formation (PMRF) in motor preparation and goal-directed reaching in the monkey. Although the macaque monkey is an important model for such investigations, little is known regarding the organization of the PMRF in the monkey. In the present study, we investigated the distribution of reticulospinal neurons in the macaque. Bilateral injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) were made into the cervical spinal cord. A wide band of retrogradely labeled cells was found in the gigantocellular reticular nucleus (Gi) and labeled cells continued rostrally into the caudal pontine reticular nucleus (PnC) and into the oral pontine reticular nucleus (PnO). Additional retrograde tracing studies following unilateral cervical spinal cord injections of cholera toxin subunit B revealed that there were more ipsilateral (60%) than contralateral (40%) projecting cells in Gi, while an approximately 50:50 ratio contralateral to ipsilateral split was found in PnC and more contralateral projections arose from PnO. Reticulospinal neurons in PMRF ranged widely in size from over 50 microm to under 25 microm across the major somatic axis. Labeled giant cells (soma diameters greater than 50 microm) comprised a small percentage of the neurons and were found in Gi, PnC and PnO. The present results define the origins of the reticulospinal system in the monkey and provide an important foundation for future investigations of the anatomy and physiology of this system in primates.

Spinoreticular neurons that receive group III input are inhibited by MLR stimulation

In decerebrate paralyzed cats, we examined the responses of 18 spinoreticular neurons to electrical stimulation of the mesencephalic locomotor region. The activity of each of the spinoreticular neurons was recorded extracellularly from laminae IV through VI of the L(7) and S(1) spinal cord. In addition, each of the 18 spinoreticular neurons received group III afferent input from the tibial nerve. Spinoreticular projections were established for each of 18 neurons by antidromic invasion of the ventro lateral medulla at the P11 though P14 levels. The onset latencies and current thresholds for antidromic invasion from the ventro lateral medulla averaged 15.0 +/- 3.8 ms and 117 +/- 11 microA, respectively. Electrical stimulation of the mesencephalic locomotor region attenuated the spontaneous activity or the responses of each of the spinoreticular neurons to tibial nerve stimulation at currents that recruited group III afferents. Our data support the notion that thin-fiber muscle afferent input to the ventrolateral medulla is gated by a central command to exercise.

Immunohistochemical distribution of enkephalin, substance P, and somatostatin in the brainstem of the leopard frog, Rana pipiens

The brainstems of frogs contain many of the neurochemicals that are found in mammals. However, the clustering of nuclei near the ventricles makes it difficult to distinguish individual cell groups. We addressed this problem by combining immunohistochemistry with tract tracing and an analysis of cell morphology to localize neuropeptides within the brainstem of Rana pipiens. We injected a retrograde tracer, Fluoro-Gold, into the spinal cord, and, in the same frog, processed adjacent sections for immunohistochemical location of antibodies to the neuropeptides enkephalin (ENK), substance P (SP), and somatostatin (SOM). SOM+ cells were more widespread than cells containing immunoreactivity (ir) to the other substances. Most reticular nuclei in frog brainstem contained ir to at least one of these chemicals. Cells with SOM ir were found in nucleus (n.) reticularis pontis oralis, n. reticularis magnocellularis, n. reticularis paragigantocellularis, n. reticularis dorsalis, the optic tectum, n. interpeduncularis, and n. solitarius. ENK-containing cell bodies were found in n. reticularis pontis oralis, n. reticularis dorsalis, the nucleus of the solitary tract, and the tectum. The midbrain contained most of the SP+ cells. Six nonreticular nuclei (griseum centrale rhombencephali, n. isthmi, n. profundus mesencephali, n. interpeduncularis, torus semicircularis laminaris, and the tectum) contained ir to one or more of the substances but did not project to the spinal cord. The descending tract of V, and the rubrospinal, reticulospinal, and solitary tracts contained all three peptides as did the n. profundus mesencephali, n. isthmi, and specific tectal layers. Because the distribution of neurochemicals within the frog brainstem is similar to that of amniotes, our results emphasize the large amount of conservation of structure, biochemistry, and possibly function that has occurred in the brainstem, and especially in the phylogenetically old reticular formation.

Pathological study of the diffuse myelin pallor in the anterolateral columns of the spinal cord in amyotrophic lateral sclerosis

An immunohistochemical study using a monoclonal antibody against macrophage (Ki-M1p) was performed to examine which fiber tracts are affected in the spinal cords and brainstems of ALS patients. In 21 out of 30 ALS patients, various degrees of macrophage infiltration were observed diffusely in the anterolateral columns beyond the corticospinal tracts. On the other hand, a few macrophages were scattered in 20 non-ALS patients in the anterolateral columns outside the corticospinal tracts. In ALS brainstems, the macrophages were mainly localized in the corticospinal tracts. The result suggests that the diffuse myelin pallor in the anterolateral columns beyond the corticospinal tracts may be derived from intrinsic spinal cord lesions. Quantitative investigation using a monoclonal antibody against phosphorylated neurofilaments (SMI-31) revealed that the decrease in the numbers of small fibers would induce the diffuse myelin pallor in anterolateral columns of ALS patients. From these findings, we propose that the propriospinal bundles are candidates for the degenerating fibers in the anterolateral columns of ALS.

The organisation of spinal projecting brainstem neurons in an animal model of muscular dystrophy. A retrograde tracing study on mdx mutant mice

Previous studies we performed on the mdx mouse demonstrated marked central nervous system alterations in this model of human Duchenne muscular dystrophy, such as reduction in number and pathological changes of cortico-spinal neurons. Prompted by these findings we extended the survey of the mdx brain to the major brainstem-descending pathways: the rubro-, vestibulo-, reticulo-, and raphe-spinal projections. Horseradish peroxidase microinjections were performed in the cervical spinal cord of mdx and control mice. The rubro-spinal neurons were found to be significantly reduced in mutants compared to controls. The vestibulo-spinal, reticulo-spinal, and raphe-spinal cell populations, though less numerous in mdx than in control mice, were instead substantially spared. Our data further unveil the selective nature of mdx brain damage indicating a marked and selective involvement of the highest centers for motor control.

Ascending projections from the area around the spinal cord central canal: A Phaseolus vulgaris leucoagglutinin study in rats

A single small iontophoretic injection of Phaseolus vulgaris leucoagglutinin labels projections from the area surrounding the spinal cord central canal at midthoracic (T6-T9) or lumbosacral (L6-S1) segments of the spinal cord. The projections from the midthoracic or lumbosacral level of the medial spinal cord are found: 1) ascending ipsilaterally in the dorsal column near the dorsal intermediate septum or the midline of the gracile fasciculus, respectively; 2) terminating primarily in the dorsal, lateral rim of the gracile nucleus and the medial rim of the cuneate nucleus or the dorsomedial rim of the gracile nucleus, respectively; and 3) ascending bilaterally with slight contralateral predominance in the ventrolateral quadrant of the spinal cord and terminating in the ventral and medial medullary reticular formation. Other less dense projections are to the pons, midbrain, thalamus, hypothalamus, and other forebrain structures. Projections arising from the lumbosacral level are also found in Barrington's nucleus. The results of the present study support previous retrograde tract tracing and physiological studies from our group demonstrating that the neurons in the area adjacent to the central canal of the midthoracic or lumbosacral level of the spinal cord send long ascending projections to the dorsal column nucleus that are important in the transmission of second-order afferent information for visceral nociception. Thus, the axonal projections through both the dorsal and the ventrolateral white matter from the CC region terminate in many regions of the brain providing spinal input for sensory integration, autonomic regulation, motor and emotional responses, and limbic activation.

Brainstem neurons with descending projections to the spinal cord of two elasmobranch fishes: thornback guitarfish, Platyrhinoidis triseriata, and horn shark, Heterodontus francisci

We studied two cartilaginous fishes and described their brainstem supraspinal projections because most nuclei in the reticular formation can be identified that way. A retrogradely transported tracer, horseradish peroxidase or Fluoro-Gold, was injected into the spinal cord of Platyrhinoidis triseriata (thornback guitarfish) or Heterodontus fransisci (horn shark). We described labeled reticular cells by their position, morpohology, somatic orientation, dendritic processes, and laterality of spinal projections. Nineteen reticular nuclei have spinal projections: reticularis (r.) dorsalis, r. ventralis pars alpha and beta, r. gigantocellularis, r. magnocellularis, r. parvocellularis, r. paragigantocellularis lateralis and dorsalis, r. pontis caudalis pars alpha and beta, r. pontis oralis pars medialis and lateralis, r. subcuneiformis, r. peduncularis pars compacta, r. subcoeruleus pars alpha, raphe obscurus, raphe pallidus, raphe magnus, and locus coeruleus. Twenty nonreticular nuclei have spinal projections: descending trigeminal, retroambiguus, solitarius, posterior octaval, descending octaval, magnocellular octaval, ruber, Edinger-Westphal, nucleus of the medial longitudinal fasciculus, interstitial nucleus of Cajal, latral mesencephalic complex, periventricularis pretectalis pars dorsalis, central pretectal, ventromedial thalamic, posterior central thalamic, posterior dorsal thalamic, the posterior tuberculum, and nuclei B, F, and J. The large number of distinct reticular nuclei with spinal projections corroborates the hypothesis that the reticular formation of elasmobranches is complexly organized into many of the same nuclei that are found in frogs, reptiles, birds, and mammals.

Effects of neonatal hypoxia on the medulla-spinal cord descending neurons

Hypoxic changes in the medulla-spinal cord descending neurons were studied morphologically using a retrograde neurotracer, choleratoxin B subunit (CTb). On postnatal day 7, Sprague-Dawley rats were subjected to a hypoxic load of 8% oxygen for 5 hours. In the rats that survived, CTb was injected into the lumbar enlargement at postnatal day 26, and they were killed at postnatal day 28 for histologic analysis. Retrograde transported CTb was visualized by immunohistochemistry. The results were compared with those obtained from control rats. In the control rats, CTb-positive cells were observed in the nucleus reticularis gigantocellularis, nucleus reticularis magnocellularis, nucleus raphe magnus, nucleus raphe obscurus, and nucleus raphe pallidus. In the hypoxic rats, although CTb-positive cells were detected in the same areas as the control rats, there was a noteworthy decrease in the number of CTb-positive cells in all areas, and there were many cells with hypoxic degeneration. In all of the nuclei a marked decrease in the number of CTb-positive cells was observed. Because medulla-spinal cord descending neurons have important roles in the regulation of postural muscle tone, these results may account for the pathophysiology of abnormal muscle tonus accompanying hypoxic brain damage.

Descending projections of the posterior nucleus of the hypothalamus: Phaseolus vulgaris leucoagglutinin analysis in the rat

No previous report in any species has systematically examined the descending projections of the posterior nucleus of the hypothalamus (PH). The present report describes the descending projections of the PH in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris leucoagglutinin. PH fibers mainly descend to the brainstem through two routes: dorsally, within the central tegmental tract, and ventromedially, within the mammillo-tegmental tract and its caudal extension, ventral reticulo-tegmental tracts. PH fibers were found to distribute densely to several nuclei of the brainstem. They are (from rostral to caudal) 1) lateral/ ventrolateral regions of the diencephalo-mesopontine periaqueductal gray (PAG); 2) the peripeduncular nucleus; 3) discrete nuclei of pontomesencephalic central gray (dorsal raphe nucleus, laterodorsal tegmental nucleus, and Barrington's nucleus); 4) the longitudinal extent of the central core of the mesencephalic through meduallary reticular formation (RF); 5) the ventromedial medulla (nucleus gigantocellularis pars alpha, nucleus raphe magnus, and nucleus raphe pallidus); 6) the ventrolateral medulla (nucleus reticularis parvocellularis and the rostral ventrolateral medullary region); and 7) the inferior olivary nucleus. PH fibers originating from the caudal PH distribute much more heavily than those from the rostral PH to the lower brainstem. The PH has been linked to the control of several important functions, including respiration, cardiovascular activity, locomotion, antinociception, and arousal/wakefulness. It is likely that descending PH projections, particularly those to the PAG, the pontomesencephalic RF, Barrington's nucleus, and parts of the ventromedial and ventrolateral medulla, serve a role in a PH modulation of complex behaviors involving integration of respiratory, visceromotor, and somatomotor activity.

Descending antinociceptive pathway from the rostral ventrolateral medulla: a correlative anatomical and physiological study

Microinjections of the excitatory amino acid L-glutamate were made into the rostral ventrolateral medulla (RVLM) of anesthetised cats, to map the sites at which selective stimulation of cell bodies elicited a significant antinociceptive response (> or = 15% inhibition of the increase in L7 ventral root activity reflexly evoked by stimulation of C-fiber afferents). Antinociceptive sites were largely confined to the RVLM subregion ventromedial to the retrofacial nucleus, extending from the caudal pole of the facial nucleus to the level approximately 2.5 mm more caudal. Increases in arterial pressure were also elicited from some sites in the RVLM, but these were mainly lateral to the antinociceptive sites. In a second series of experiments, rhodamine labeled microspheres or cholera toxin B-gold (CTB-gold) were injected into the dorsal horn of the L7 segment. In three of these experiments in which the injection sites were restricted to the dorsal horn, retrogradely labeled cells in the caudal pons and medulla were virtually all within either the nucleus raphe magnus or the RVLM. Furthermore, the labeled cells in the RVLM were virtually confined to a discrete group located just ventromedial to the retrofacial nucleus, i.e. within the antinociceptive region as mapped by glutamate microinjection. The results of the present study indicate that antinociceptive effects are elicited by stimulation of a subregion in the RVLM, which is located medial to the pressor region. Further, the antinociceptive effects may be mediated, at least in part, by cells projecting directly to the dorsal horn in the spinal cord.

Origin of serotoninergic afferents to the hypoglossal nucleus in the rat

The hypoglossal nucleus contains serotonin and several different serotonin receptors, and serotonin is present in fibers and terminals contacting hypoglossal motoneurons. Serotonin alters the excitability of hypoglossal motoneurons, and may influence hypoglossal motoneuron activity in a variety of physiological processes. Since the hypoglossal nucleus contains no serotoninergic somata, the present study sought to identify the sources of serotoninergic afferents to the hypoglossal nucleus. Fluorogold was injected into the hypoglossal nucleus and serotoninergic immunofluorescence was utilized in a dual-fluorescence technique to identify the sources of serotoninergic afferents to the hypoglossal nucleus. The results demonstrate that most serotoninergic afferents to the hypoglossal nucleus originate from the nuclei raphe pallidus and obscurus, while fewer originate from the nucleus raphe magnus and the parapyramidal region. Other regions of the medial tegmental field and the pons that contain both serotoninergic neurons and neuronal afferents to the hypoglossal nucleus contain no double-labeled neurons.

Raphespinal and reticulospinal axon collaterals to the hypoglossal nucleus in the rat

Neurons in the medial tegmental field project directly to spinal somatic motoneurons and to cranial motoneuron pools such as the hypoglossal nucleus. The axons of these neurons may be highly collateralized, projecting to multiple levels of the spinal cord and to many diverse regions at different levels of the neuraxis. We employed a double fluorescent retrograde tracer technique to examine whether medial tegmental neurons that project to the spinal cord also project to the hypoglossal nucleus. Injections of Diamidino Yellow into the hypoglossal nucleus and Fast Blue into the spinal cord produced large numbers of double labeled neurons in the medial tegmental field, particularly in the caudal raphe nuclei and adjacent ventromedial reticular formation. In these structures the number of neurons projecting to both the hypoglossal nucleus and the spinal cord was equivalent to the number of neurons projecting to multiple levels of the spinal cord observed in control animals. Fewer neurons projecting to both the hypoglossal nucleus and the spinal cord were observed in several other nuclei and subregions of the medial tegmental field, while almost no such neurons were observed in the lateral tegmental field or other pontomedullary structures. These results demonstrate that neurons of the caudal raphe nuclei and adjacent ventromedial reticular formation project to both the spinal cord and the hypoglossal nucleus, and support the concept that the diffuse projections to motoneuron pools from the medial tegmental field globally modulate both spinal and cranial somatic motoneuron excitability.

Funicular organization of avian brainstem-spinal projections

The purpose of this study was to determine which reticulospinal projections need to be preserved to allow voluntary walking and to differentiate between those pathways descending within the ventrolateral funiculus versus the ventromedial funiculus. Retrogradely transported tracers (True Blue, Fast Blue, Diamidino Yellow dihydrochloride, fluorescein-conjugated dextran-amines) were used alone as discrete funicular injections (4-5 microliters) into the lumbar cord (L1), or in conjunction with a more rostral subtotal lesion of the low thoracic cord, to determine the trajectories of brainstem-spinal projections in adult ducks and geese. No difference was found between the species. The major components of the ventromedial funiculus include projections from the medullary reticular formation, pontine reticular formation, raphe obscurus and pallidus, lateral vestibular nucleus, and interstitial nucleus, and to a minor extent from the locus coeruleus, lateral hypothalamus, and nucleus periventricularis hypothalami. The components of the ventrolateral funiculus (VLF) include projections from the nucleus of the solitary tract, nucleus alatus, pontomedullary reticular formation, raphe pallidus, raphe magnus, locus coeruleus, subcoeruleus, lateral vestibular, and descending vestibular nuclei. The principal descending projections within the dorsolateral funiculus (DLF) arose from the red nucleus, the paraventricular nucleus, locus coeruleus, subcoeruleus, dorsal division of the caudal medullary reticular formation, and raphe magnus. The functional implications of the distribution of these descending pathways are discussed with regard to locomotion. Since birds were able to walk despite bilateral lesion of the DLF or VMF but were unable to walk following a bilateral lesion of the VLF, this suggests that medullary reticulospinal pathways coursing within the VLF are essential for the provision of locomotor drive.

Evidence of a Monosynaptic Pathway Between Cells of the Ventromedial Medulla and the Motoneuron Pool of the Thoracic Spinal Cord in Rat: Electron Microscopic Analysis of Synaptic Contacts

Previous electrophysiological and anatomical data have suggested the existence of a descending pathway from the ventromedial medulla into the thoracic motoneuron pool. However, systematic light and electron microscopic analysis have not yet been done to reveal such a projection. In the present study, the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into several discrete regions of the medioventral medulla and descending PHA-L-labelled axons were investigated in the thoracic ventral horn using both light and electron microscopy. Light microscopic analysis of descending projections from 20 distinct areas of the medioventral medulla showed that neurons that project predominantly to the intermediate and ventral regions of the thoracic spinal grey matter are located caudal to the facial nucleus. Monosynaptic contacts were found between axons originating from five distinct regions of the medioventral medulla (containing raphé and/or gigantocellular reticular neurons) and cells in the thoracic motoneuron pool. PHA-L-labelled boutons formed synaptic contacts with large calibre dendrites and with somata. Seventy-two per cent of the investigated 32 boutons appeared to have symmetrical synaptic membrane specializations. The majority of the boutons contained only small, pleomorphic vesicles. Our findings show the existence of a direct monosynaptic pathway between the neurons of the ventromedial medulla and thoracic motor nuclei, providing anatomical support for previous physiological data.

Descending brainstem projections of the pedunculopontine tegmental nucleus in the rat

Descending brainstem projections from the pedunculopontine tegmental nucleus (PPN) were studied in the rat by use of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) and the retrograde tracer lectin-conjugated horseradish peroxidase (HRP-WGA). Results of these experiments demonstrated prominent bilateral projections to the pontomedullary reticular nuclei, but direct connections to the motor and sensory nuclei of the cranial nerves could not be ascertained. The PPN fibers terminated mainly in the pontine reticular nuclei oralis and caudalis and in ventromedial portions (pars alpha and pars ventralis) of the gigantocellular reticular nucleus. A smaller number of labeled fibers distributed to more dorsal regions of the gigantocellular nucleus, lateral para-gigantocellular, ventral reticular nucleus of the medulla and lateral reticular nucleus. Although a significant number of PHA-L labeled fibers was seen in two cases in the contralateral medial portion of the facial nucleus, and all cases exhibited a sparse predominantly ipsilateral projection to the lateral facial motor neurons, the retrograde tracing experiments have revealed that these facial afferents originated in the nuclei surrounding the PPN. The results are discussed in the context of PPN involvement in motor functions. It is suggested that the PPN may participate in a complex network involved in the orienting reflex.

Supraspinal projections to the ventromedial lumbar spinal cord in adult male rats

In the present study, the fluorescent tract tracing compound Fluorogold was used to study the afferents of the SNB (spinal nucleus of the bulbocavernosus), which is found in the ventromedial spinal grey and innervates penile muscles of the male rat. Fluorogold was iontophoretically injected into the SNB, which was located by recording antidromic activation of the motoneurons after stimulating the bulbocavernosus muscle. Retrogradely labeled cells were found in laminae I, V-IX, and area X of the lumbar spinal cord, suggesting segmental input to the SNB. Supraspinally, the greatest number of labeled cells were in the medulla oblongata, particularly in the lateral vestibular nucleus, gigantocellular reticular nucleus, and ventral and alpha divisions of the gigantocellular reticular nucleus. Labeled cells were also observed in the medullary raphe nuclei, the ventral medullary nucleus, and the spinal vestibular nucleus. In the pons, labeled cells were observed in the nucleus locus coeruleus, nucleus subcoeruleus, and caudal pontine reticular nucleus. No labeled cells were present in the cerebellum, rostral pons, mesencephalon, and cerebral cortex. The most rostral occurrence of labeled cells was in the medial parvicellular division of the hypothalamic paraventricular nucleus. These potential afferents to the SNB identified in male rats imply that the inputs to motoneurons that innervate sex-specific muscles involved in male reproductive behavior may be similar to the inputs to lumbar motoneurons described in the female rat that innervate muscles involved in female sexual behavior.

Origins and terminations of bulbospinal axons that contain serotonin and either enkephalin or substance-P in the North American opossum

We have shown previously that some enkephalin, substance-P, and serotoninergic neurons in the medullary raphe and adjacent reticular formation project to the spinal cord in the opossum. In the present study we have combined the retrograde transport of True Blue and immunofluorescence histochemistry to determine whether methionine enkephalin or substance-P containing bulbospinal neurons are serotoninergic. Furthermore, we have used the same immunofluorescence protocol to determine whether spinal axons contain the same substances. Neurons that immunostained for both enkephalin and serotonin were observed in many brainstem nuclei. However, those that projected to the spinal cord were limited to the nuclei raphe magnus and obscurus, and the ventral part of nucleus reticularis gigantocellularis, pars ventralis. Neurons that immunostained for both substance P and serotonin were fewer in number, but some of the ones in the above nuclei and within the nucleus raphe pallidus, projected to the spinal cord. Spinal axons exhibiting both enkephalin- and serotonin-like immunoreactivity were observed in the superficial laminae of the dorsal horn, lamina X, and the intermediolateral cell column, whereas those showing both substance-P and serotonin-like immunoreactivity were seen primarily in lamina X, the intermediolateral cell column, and the ventral horn. Some of the axons in the ventral horn were in close apposition to presumed motoneurons. Comparison of the above results with those obtained from previous studies of bulbospinal projections has allowed us to infer the origins of axons that innervate different spinal targets.

Quantitative re-evaluation of descending serotonergic and non-serotonergic projections from the medulla of the rodent: evidence for extensive co-existence of serotonin and peptides in the same spinally projecting neurons, but not from the nucleus raphe magnus

A quantitative analysis of serotonin (5-HT) and spinally-projecting neurons was re-evaluated in the rodent. The findings indicate that most (nearly 90%) of the medullary 5-HT neurons projected to the lumbar spinal cord, and most (up to 85%) of the spinally projecting neurons within the distribution of the serotonergic neurons contained 5-HT immunoreactivity. However, in nucleus raphe magnus (NRM) only about two-thirds of the projection cells were 5-HT immunoreactive. These data support two general conclusions: (1) the raphe-spinal system consists primarily of an extensive 5-HT pathway with neuronal subsets containing the co-localized peptides. Only the NRM contains a major non-5-HT projection. (2) As most of the medullary 5-HT neurons project to the caudal spinal segments of the rodent, collateralization of individual 5-HT neurons is extensive and widespread, existing to different spinal cord levels, as well as to other medullary nuclei, e.g., cranial nerve and inferior olivary nuclei. These findings argue that although differences are present within the 5-HT distributions, the raphe-spinal system as a whole should be considered to be a relatively homogeneous pathway containing 5-HT as the common element rather than as separate populations containing major projections of 5-HT alone, 5-HT co-localized with peptides and peptides without 5-HT.

Medullary neurons with projections to lamina X of the rat as demonstrated by retrograde labeling after HRP microelectrophoresis

Brainstem neurons were retrogradely labeled with microelectrophoresis of HRP or WGA-HRP into lamina X of the cervical or lumbar cord of rats. The results reveal that lamina X of the lumbar cord receives bulbar projections originating mainly within the nucleus raphe magnus and the nucleus reticularis paragigantocellularis (including the medial or alpha-ventral part and lateral part) and that lamina X of the cervical cord receives projections from similar but more extensive regions in the lower brainstem. These findings provide a neuroanatomical substrate for medullary descending modulation of nociceptive transmission in lamina X.

Developmental plasticity of the rubrospinal tract in the North American opossum

We have shown previously that rubral axons can grow caudal to a lesion of their pathway at thoracic levels of the spinal cord in the developing opossum, Didelphis virginiana. In the present report we expand on that observation and present evidence which suggests that the critical period for plasticity of the rubrospinal tract ends earlier at cervical than at thoracic levels. In addition, we show that most rubrospinal neurons die as a result of axotomy during early stages of the critical period. The opossum was chosen for study because the development of its rubrospinal tract occurs after birth. In one set of experiments the area containing the rubrospinal tract was lesioned at cervical or thoracic levels and after 30 days or more, retrograde transport techniques were used to determine if rubral axons had grown caudal to the lesion. When the lesions were made at rostral cervical levels between estimated postnatal day 26 and maturity, neurons could not be labeled in the contralateral red nucleus by injections of retrograde markers ipsilateral to the lesion and caudal to it. We were not able to obtain adequate survival after cervical lesions made prior to estimated postnatal day 26. When the lesions were made at mid to caudal thoracic levels between estimated postnatal days 19 and 26, neurons could be labeled in the contralateral red nucleus. When comparable lesions were made at estimated postnatal day 40, there was usually a decrease in the number of labeled neurons, and when they were made at estimated postnatal day 54, none was labeled. In selected cases, operated at estimated postnatal day 19, cell counts provided evidence for loss of neurons in the red nucleus contralateral to the lesion. In orthograde transport experiments performed on animals with thoracic lesions of the rubrospinal tract made between estimated postnatal days 18 and 33, rubral axons could be labeled caudal to the lesion, and they seemed to take the most direct route around it. Although they sometimes assumed abnormal positions caudal to the lesion, rubral axons appeared to reach areas of the gray matter appropriate to them. When lesions were made at estimated postnatal day 54 or in older animals, labeled axons could be traced to the lesion site but not caudal to it.(ABSTRACT TRUNCATED AT 400 WORDS)

Descending pathways to the spinal cord: a comparative study of 22 mammals

In order to estimate the qualitative commonalities and range of variation among major descending spinal pathways relevant to mankind's ancestral lineage, the supraspinal cell groups originating fibers descending directly to the spinal cord were examined in 22 mammalian species. In a standardized retrograde tract-tracing procedure, flakes of raw HRP were applied directly to the freshly cut fibers of the spinal cord after it had been hemisected at the C1-C2 junction. After a 72-hour survival period, brain and spinal cord tissues were processed by conventional HRP-processing techniques. This procedure was performed on 94 individual animals. Of this total, 41 individual cases were eliminated by a rigorous culling procedure. The results are based on 53 individuals representing 15 species selected for their successive kinship with mankind and seven species in two other lineages selected for the convergence of their visual or sensorimotor systems with anthropoids. The 22 species represent 19 genera, 14 families, eight orders, and two subclasses of Mammalia. The results show that at least 27 supraspinal cell groups, each containing intensely labeled cells, can be readily identified in each of the species. Despite vast quantitative differences in cell number and cell size, this qualitative uniformity among the relatively large number of diverse taxa suggests that the same pathways were probably present in the extinct ancestors throughout mankind's mammalian lineage and are probably still present in extant viviparous mammals as well. If so, these pathways are as old in phylogenetic history as the last common ancestor of marsupial and placental mammals--dating from the late Jurassic to early Cretaceous, perhaps 145-120 million years ago. Further comparison of the results with similar experimental findings in members of other vertebrate classes supports the notion that several of these same pathways can be traced to even more remote ancestry, with some possibly as old as the entire vertebrate subphylum--dating from the early Devonian or before, perhaps 430 million years ago. Within mankind's ancestral lineage, from the appearance of vertebrates to the appearance of mammals, there seems to have been an irregular stepwise augmentation of the set of descending pathways until the full mammalian complement was finally attained with the appearance of the corticospinal tract.

Ascending and descending projections of the nucleus reticularis gigantocellularis in the cat demonstrated by the anterograde neural tracer, Phaseolus vulgaris leucoagglutinin (PHA-L)

Ascending and descending projections of the nucleus reticularis gigantocellularis (NRGc) were studied in the cat by the anterograde tracer, Phaseolus vulgaris leucoagglutinin. Ascending fibers from the left or the right NRGc coursed through the bilateral medial reticular formation and some of them reached the diencephalon. In the brainstem, PHA-L-labeled fibers and their terminals were observed in the medial reticular formation, the cranial motor nuclei (III, IV, V, VI, VII, XII), the vestibular complex, the LC complex, the raphe nuclei, the periaqueductal gray, the red nucleus, the Edinger-Westphal nucleus and the interstitial nucleus of Cajal. In the diencephalon, they were observed in the dorsal thalamus and the hypothalamic regions. In the caudal medulla, labeled fibers and their terminals were observed in the nucleus prepositus hypoglossi, the nucleus intercalatus and the inferior olive. Descending axons from the NRGc coursed bilaterally through the ventral and ventrolateral funiculi as far caudal as the upper thoracic cord. Single axon collaterals arising from the descending axons gave off terminal fibers to the left or the right gray matter. Their terminals were located in laminae V-X, mainly in laminae VII and VIII. In lamina IX, they were distributed mainly in the medial portion. A few fibers originating from the descending axons ipsilateral to the PHA-L injection side coursed through the anterior or posterior commissure, and ended in laminae VI, VII and VIII. The functional implications of these findings are discussed in relation to the behavioral state control and the generalized motor inhibition.

Origins of brainstem-spinal projections in the duck and goose

The purpose of this study was to determine the location of neuronal cell bodies with projections to the cervical or lumbar spinal cord in the adult duck and goose. Bilateral or unilateral injections (5-10 microliter) of the retrograde tracer dye True Blue (TB:5%) were made into the high cervical or high lumbar levels of the spinal cord. Similar results were obtained in both species. First, we found no evidence of retrogradely labelled cells in the telencephalon. In the brainstem, the distribution of TB cells was similar to those previously reported for the pigeon; however, the present study now demonstrates that some of these descending pathways project as far as the lumbar cord. We also discovered that there is a topographical representation of spinal projecting neurons within the avian medullary-pontine reticular formation.

Anatomical and functional connections of neurons of the rostral medullary raphe of the rabbit

Single cell recordings were made from neurons in the rostral medullary raphe (RMR) of the rabbit. The recording sites were ones that had been shown to yield pressor responses from electrical stimulation and by pressure injections of glutamate. Electrical stimulation of the intermediolateral (IML) region of the spinal cord led to antidromic activation of 12 of the 100 cells studied. Eleven of these cells were located in raphe pallidus or raphe magnus, and one cell was located in raphe obscurus. These findings were consistent with the results of horseradish peroxidase (HRP) histochemistry experiments. Injections of HRP into the IML led to heavy cell body labeling in raphe pallidus and raphe magnus, but sparse labeling in raphe obscurus. Cells in the RMR could be orthodromically activated by electrical stimulation of the putative defense area of the periaqueductal (PAG) but not by stimulation of putative defense areas in the hypothalamus. Most of these cells were located in raphe pallidus or raphe magnus. Similarly, HRP injections into raphe pallidus and raphe magnus led to heavy cell body labeling in the PAG but not the hypothalamus; no cell body labeling was found in the PAG when injections were made into raphe obscurus.

Anatomical evidence for a strong ventral parabrachial projection to nucleus raphe magnus and adjacent tegmental field

Injections with [3H]leucine in the ventral parabrachial nuclei and nucleus Kölliker-Fuse of the cat revealed strong projections to the nucleus raphe magnus (NRM) and adjacent tegmentum, while similar injections in the adjacent nucleus subcoeruleus produced diffuse projections to large parts of the tegmentum, but not specifically to the NRM. Horseradish peroxidase (HRP) injections in the area of the NRM and adjacent tegmentum demonstrated many labeled neurons in the ventral parabrachial nuclei, nucleus Kölliker-Fuse and nucleus subcoeruleus. These results suggest that the inhibition of nociception induced by stimulation in the ventral parabrachial nuclei may be based on the projections of this area to NRM and adjacent tegmentum.

Evidence for developmental plasticity of the rubrospinal tract. Studies using the North American opossum

Using axonal tracing techniques we have shown that rubral axons are capable of growing around lesions of the rubrospinal tract during early stages of development in the North American opossum and that a critical period for such growth exists. The opossum was employed for study because it is born in a very immature state 12-13 days after conception and the entire development of its rubrospinal tract occurs postnatally.

The origin of projections from the medullary reticular formation to the spinal cord, the diencephalon and the cerebellum at different stages of development in the North American opossum: studies using single and double labeling techniques

We have employed the retrograde transport of horseradish peroxidase alone or conjugated to wheat germ agglutinin, to label neurons within the medullary reticular formation which project to the spinal cord, the diencephalon and the cerebellum at different stages of development in the North American opossum. At selected ages, the fluorescent markers Fast Blue and Diamidino Yellow were also used in double-labeling experiments to determine if single neurons innervate both the spinal cord and diencephalon or the spinal cord and cerebellum, presumably via axonal collaterals. The opossum was employed because it is born in a very immature state, 12 days after conception, and is thus available for injections at early stages of development. At all ages studied, the location of retrograde labeling within the medullary reticular formation after spinal, diencephalic or cerebellar placements of horseradish peroxidase or its conjugate appeared similar to that obtained in the adult animal. Such results suggest that the origin of projections from the medullary reticular formation to the areas injected is specified early in development. At some ages, however, the labeling density appeared greater than in the adult animal. When either Fast Blue or Diamidino Yellow was injected into the spinal cord and the other marker was placed into the diencephalon at such ages, relatively few neurons of the medullary reticular formation were double-labeled. When one marker was injected into the spinal cord and the other was placed within the cerebellum, no double-labeled neurons were found. These results indicate that at the ages studied, relatively few neurons of the medullary reticular formation provide collateral innervation to either the spinal cord and diencephalon or the spinal cord and cerebellum. Similar conclusions have been reached previously for the adult opossum. We have interpreted our results to suggest that the organization of reticular projections, at least to the areas injected, may not be shaped by the selective elimination of axonal collaterals as in certain other areas of the brain.

Descending projections from the gigantocellular tegmental field in the cat: cells of origin and their brainstem and spinal cord trajectories

The trajectories and the cells of origin of the pontobulbar gigantocellular tegmental field descending pathways were studied in the cat using anterograde WGA-HRP and retrograde HRP techniques. Four main descending pathways and cells of origin were delineated: (1) Predominantly large neurons in the pontine gigantocellular tegmental field (average soma diameter = 43.4 microns) and rostral bulbar gigantocellular tegmental field (41.3 microns) gave rise to reticulospinal fibers descending in the ipsilateral medial longitudinal fasciculus and ventral funiculus and distributed in laminae V-X with an ipsilateral predominance. These were primarily large-diameter fibers. (2) Predominantly large neurons (46.9 microns) in the bulbar gigantocellular tegmental field gave rise to reticulospinal fibers descending in the contralateral medial longitudinal fasciculus and ventral funiculus. These were mainly large-diameter fibers. (3) Neurons of predominantly medium size (29.5 microns) in the pontine gigantocellular tegmental field gave rise to reticuloreticular fibers descending directly to and distributed bilaterally in the bulbar reticular formation. These were small-diameter fibers. (4) Neurons of predominantly medium size (28.9 microns) in the bulbar gigantocellular tegmental field gave rise to reticulospinal fibers descending in the ipsilateral reticular formation and lateral funiculus. These were small-diameter fibers.

The axons of raphespinal sympathoinhibitory neurons branch in the cervical spinal cord

This study shows that the axons of some raphespinal sympathoinhibitory neurons projecting to the third thoracic spinal segment emit branches in the third-fourth cervical spinal segments of cats. This was demonstrated by using time-controlled collision of neuronal action potentials initiated by stimuli applied at these spinal levels. Antidromic mapping revealed that the cervical branches of these neurons likely terminated in Rexed's lamina VII. In contrast to these raphe neurons, the axons of ventrolateral medullospinal sympathoexcitatory neurons did not emit cervical branches.

Substance P-containing medullary projections to the intermediolateral cell column: identification with retrogradely transported rhodamine-labeled latex microspheres and immunohistochemistry

Substance P (SP)-containing medullary neurons that project to the intermediolateral cell column (IML) of the rat were studied. Neurons were retrogradely labeled with rhodamine-labeled latex microspheres (RITC-M) injected into the T-3 IML, and SP-immunoreactive neurons were identified with immunocytochemistry. RITC-M labeled cells occurred in the nucleus reticularis paragigantocellular lateralis (RPgcl), adjacent and lateral to the pyramidal tract at the level of the rostral inferior olivary nucleus and extended to the mid-facial nucleus in the medulla. Cells were also labeled caudal to the RPgcl, in the nucleus reticularis ventralis, pars alpha (RVa), rostral to the RVa, in the nucleus reticularis gigantocellularis (RGc), and in the raphe nuclei. SP immunoreactivity (SP-IR) was seen in cells that were also retrogradely labeled. These double-labeled cells were observed in the RPgcl, RVa and the raphe pallidus. These data show that the IML receives SP-neuronal projections from multiple locations in the medulla. The SP-neuronal projections from the RPgcl of the ventral medulla to the IML likely represent one component of the ventral medullary region that influences cardiovascular functions.

Neurochemical identification of afferents onto spinomedullary neurons in the rat spinal cord central gray matter

The synaptic relationship between spinal cord central gray projection neurons and immunocytochemically identified afferents in the rat were examined at the light microscopic level using the combined techniques of retrogradely transported True blue and serotonin (5-HT), enkephalin (ENK), and substance P (SP) immunocytochemistry. At L4-L6, numerous retrogradely labeled neurons could be identified around the central canal after large bulbar injections of True blue. Of these projection neurons, 75% were apposted by 5-HT varicosities, 57% by ENK varicosities and 58% by SP varicosities. Hemisection of the spinal cord produced a marked reduction in the amount of 5-HT immunoreactivity and the number of putative 5-HT contacts observed on neurons of the spinal cord central gray. A small decrease in SP immunoreactivity and putative contacts was seen after dorsal rhizotomy. Neither rhizotomy nor hemisection produced discernable changes in ENK immunofluorescence. Based on the distributions of 5-HT, ENK and SP in the spinal cord, we suggest that a more precise delineation of lamina X in the rat can be made according to immunocytochemical rather than strictly morphological criteria.

An autoradiographic analysis of ascending projections from the medullary reticular formation in the rat

Ascending projections from the several nuclei of the medullary reticular formation were examined using the autoradiographic method. The majority of fibers labeled after injections of [3H]leucine into nucleus gigantocellularis ascended within Forel's tractus fasciculorum tegmenti which is located ventrolateral to the medial longitudinal fasciculus. Nucleus gigantocellularis injections produced heavy labeling in the pontomesencephalic reticular formation, the intermediate layers of the superior colliculus, the pontine and midbrain central gray, the anterior pretectal nucleus, the ventral midbrain tegmentum including the retrorubral area, the centromedian-parafascicular complex, the fields of Forel/zona incerta, the rostral intralaminar nuclei and the lateral hypothalamic area. Nucleus gigantocellularis projections to the rostral forebrain were sparse. Labeled fibers from nucleus reticularis ventralis, like those from nucleus gigantocellularis, ascended largely in the tracts of Forel and distributed to the pontomedullary reticular core, the facial and trigeminal motor nuclei, the pontine nuclei and the dorsolateral pontine tegmentum including the locus coeruleus and the parabrachial complex. Although projections from nucleus reticularis ventralis diminished significantly rostral to the pons, labeling was still demonstrable in several mesodiencephalic nuclei including the cuneiform-pedunculopontine area, the mesencephalic gray, the superior colliculus, the anterior pretectal nucleus, the zona incerta and the paraventricular and intralaminar thalamic nuclei. The main bundle of fibers labeled by nucleus gigantocellularis-pars alpha injections ascended ventromedially through the brainstem, just dorsal to the pyramidal tracts, and joined Forel's tegmental tract in the midbrain. With the brainstem, labeled fibers distributed to the pontomedullary reticular formation, the locus coeruleus, the raphe pontis, the pontine nuclei, and the dorsolateral tegmental nucleus and adjacent regions of the pontine gray. At mesodiencephalic levels, labeling was present in the rostral raphe nuclei (dorsal, median and linearis), the mesencephalic gray, the deep and intermediate layers of the superior colliculus, the medial and anterior pretectal nuclei, the ventral tegmental area, zona incerta as well as the mediodorsal and reticular nuclei of the thalamus. Injections of the parvocellular reticular nucleus labeled axons which coursed through the lateral medullary tegmentum to heavily innervate lateral regions of the medullary and caudal pontine reticular formation, cranial motor nuclei (hypoglossal, facial and trigeminal) and the parabrachial complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Collateralization of descending pathways from the brainstem to the spinal cord in a lizard, Varanus exanthematicus

With the multiple fluorescent retrograde tracer technique, the collateralization in the spinal cord of descending supraspinal pathways was studied in a lizard, Varanus exanthematicus. Fast Blue (FB) gels were implanted unilaterally in the spinal gray matter of the cervical enlargement and Nuclear Yellow (NY) gels were implanted ipsilaterally in two series of experiments in all spinal funiculi of the lumbar enlargement or in midthoracic spinal segments, respectively. All brainstem nuclei known to project to the spinal cord in reptiles were found to give rise to branching axons that may influence widely separate levels of the spinal cord. The number of double-labeled FB-NY cells varied in these brainstem nuclei from none to half the number of neurons projecting to the cervical enlargement. Highly collateralizing projections (expressed as the percentage of double-labeled neurons, DL) were found to arise from the nucleus raphes inferior, the contralateral nucleus reticularis superior pars lateralis, the contralateral nuclei vestibulares ventromedialis and descendens, and the ipsilateral nucleus reticularis inferior pars ventralis. A lower percentage of DL neurons was noted for the contralateral nucleus ruber and bilaterally for the nucleus reticularis medius and nucleus reticularis inferior. Extensive brainstem projections directed to cervical and high thoracic spinal levels originate from the area lateralis hypothalami, the nucleus of the fasciculus longitudinalis medialis, the contralateral nucleus cerebellaris medialis, and from the nucleus tractus solitarii. Projections preferentially directed to midthoracic or lower levels of the spinal cord were found to arise from the ipsilateral locus coeruleus, the contralateral nucleus reticularis superior pars lateralis, the nucleus reticularis inferior pars ventralis, the nucleus reticularis inferior, and the nucleus raphes inferior. In contrast to findings in mammals, in Varanus exanthematicus the red nucleus, the nucleus vestibularis ventrolateralis, and certain parts of the reticular formation did not display a clear-cut somatotopic organization. In general two different patterns of collateralization can grossly be discerned: a gradual decrease of spinal collaterals caudalward, which can be interpreted as a certain specificity of such projections; and a constant number of collateral nerve fibers throughout the spinal cord that can be interpreted as either a nonspecific or, in contrast, a highly specific system, focussed exclusively on the cervical and lumbar enlargements.

Analgesia and the cardiovascular changes evoked by stimulating neurones in the ventrolateral medulla in rats

In rats anaesthetised with Saffan, selective excitation of neurones in the ventrolateral medulla in the region of nucleus paragigantocellularis lateralis (PGL) by microinjection of the synaptic excitant, D,L-homocysteic acid, produced an increase in the latency of the tail flick response to noxious heat usually accompanied by an increase in blood pressure and heart rate. These findings are discussed in relation to the role of neurones in PGL in generating both tonic descending inhibition in the dorsal horn and sympathetic vasomotor tone as well as their involvement in a descending pathway that mediates the hypoalgesia which is a feature of certain stress-induced hypertensive states.

Projections from the nucleus tractus solitarii to the rostral ventrolateral medulla

Projections from the nucleus tractus solitarii (NTS) to autonomic control regions of the ventrolateral medulla, particularly the nucleus reticularis rostroventrolateralis (RVL), which serves as a tonic vasomotor center, were analyzed in rat by anterograde, retrograde, and combined axonal transport techniques. Autonomic portions of the NTS, including its commissural, dorsal, intermediate, interstitial, ventral, and ventrolateral subnuclei directly project to RVL as well as to other regions of the ventrolateral medulla. The projections are organized topographically. Rostrally, a small cluster of neurons in the intermediate third of NTS, the subnucleus centralis, and neurons in proximity to the solitary tract selectively innervate neurons in the retrofacial nucleus and nucleus ambiguus. Neurons generally located in more caudal and lateral sites in the NTS innervate the caudal ventrolateral medulla (CVL). The RVL, CVL, and nucleus retroambiguus are interconnected. A combined retrograde and anterograde transport technique was developed so as to prove that projections from the NTS to the ventrolateral medulla specifically innervate the region of RVL containing neurons projecting to the thoracic spinal cord or the region of the nucleus containing vagal preganglionic neurons. When the retrograde tracer, fast blue, was injected into the thoracic spinal cord, and wheat germ agglutinin-conjugate horseradish peroxidase (HRP) was injected into the NTS, anterogradely labeled terminals from the NTS surrounded the retrogradely labeled neurons in the RVL and in the nucleus retroambiguus in the caudal medulla. Among the bulbospinal neurons in the RVL innervated by the NTS were adrenaline-synthesizing neurons of the C1 group. When fast blue was applied to the cervical vagus, and HRP was injected into the NTS, anterogradely labeled terminals from the NTS surrounded retrogradely labeled neurons in the rostral dorsal motor nucleus of the vagus, the region of the nucleus ambiguus, the retrofacial nucleus, and the dorsal portion of the RVL, a region previously shown to contain cardiac vagal preganglionic neurons. This combined anterograde and retrograde transport technique provides a useful method for tracing disynaptic connections in the brain. These data suggest that the RVL is part of a complex of visceral output regions in the ventrolateral medulla, all of which receive afferent projections from autonomic portions of the NTS. Bulbospinal neurons in the RVL, in particular the C1 adrenaline neurons, may provide a portion of the anatomic substrate of the baroreceptor and other visceral reflexes.

The efferent projections from the reticular formation and the locus coeruleus studied by anterograde and retrograde axonal transport in the rat

Following injections of [3H]leucine into the formatio reticularis gigantocellularis (Rgc), reticularis pontis caudalis (Rpc), reticularis pontis oralis (Rpo), reticularis mesencephali (Rmes), or the locus coeruleus (LC) of the rat, autoradiographic study revealed prominent reticuloreticular projections from all areas and secondary projections onto cranial nerve motor nuclei from most areas within the brain stem. Common long descending projections extended the full length of the spinal cord terminating in the ventromedial ventral horn and intermediate zone and more sparsely in the base of the dorsal horn and (particularly from Rgc) the region of the motoneurons. Common long ascending projections extended into the forebrain via Forel's tegmental fascicles. A dorsal branch of fibers innervated the intralaminar and midline nuclei of the thalamus. The major fiber system continued forward through Forel's fields and ascended into the pallidum from Rpo, Rmes, and LC and into the neostriatum from Rmes and LC. Fascicles from all areas also ascended in the medial forebrain bundle through the lateral hypothalamus to the lateral preoptic area, substantia innominata, and nuclei of the diagonal band. From Rpo, Rmes, and LC, fibers continued forward to reach the cerebral cortex, where the innervation was sparse and discrete from Rpo and Rmes but moderate and ubiquitous from LC. Retrograde transport of true blue and/or nuclear yellow revealed inverse gradients along the brain stem longitudinal axis of interdigitated cells respectively projecting caudally into the spinal cord (with the greatest number of cells in Rgc, Rpc, and Rpo) and rostrally into the diencephalon (with the greatest number of cells in Rmes and LC), with very few cells projecting both to the spinal cord and the diencephalon. From the basal forebrain, a large number of reticular and LC cells were retrogradely labelled, whereas from the frontal cortex, a much smaller number of reticular cells was labelled. These results document the widespread efferent projections from the reticular formation and overlapping, yet more extensive, projections from the LC.

Funicular trajectories of brainstem neurons projecting to the lumbar spinal cord in the monkey (Macaca fascicularis): a retrograde labeling study

Brainstem nuclei projecting to the lumbar spinal cord in the monkey were identified by using horseradish peroxidase and the fluorescent dye granular blue. These retrogradely transported tracers were used in fluid and/or gel forms to determine the funicular trajectories of the brainstem-spinal projections. The major descending components of the dorsal funiculus arose from the n. gracilis, n. cuneatus, and the n. of the solitary tract. Major components of the dorsolateral funiculus (DLF) came from the raphe complex, medullary and pontine reticular formation, locus coeruleus, Edinger-Westphal n., and red n. Other nuclei giving rise to minor contributions to the DLF included n. gracilis, n. cuneatus, n. of the solitary tract, medial and spinal vestibular n., subcoeruleus, periaqueductal gray, interstitial n. of Cajal, n. of Darkschewitsch, and the anteromedian n. The major components of ventral cord paths (ventrolateral and ventral funiculi) arose from the raphe complex, the medullary and pontine reticular formation, lateral and spinal vestibular n., and the coerulean complex. Minor contributions to the ventral paths descended from the dorsal motor n. of X, n. of the solitary tract, medial vestibular n., paralemniscal reticular formation, dorsal parabrachial n., n. cuneiformis, periaqueductal gray, Kölliker-Fuse n., and red n. The possible functional implications of the funicular distribution of these descending pathways are discussed from the perspective of descending inhibition and pain modulation.

The spinal terminations of single, physiologically characterized axons originating in the pontomedullary raphe of the cat

Single myelinated axons were recorded in the dorsolateral funiculus of the cat and physiologically characterized as descending from the midline medulla or midline pons. Following further physiological characterization (e.g., conduction velocity, adequate stimulus, receptive field, activation by stimulation of periaqueductal gray), the axons were labeled with horseradish peroxidase that was iontophoretically ejected from the recording micropipette. Histochemical reaction allowed visualization of the stained axons and their arborizations in the spinal gray matter. The conduction velocities of the sampled axons ranged from 7.3 to 117.2 m/second with a mean of 35.5 m/second. However, unmyelinated axons could not be sampled with the technique employed here. Descending axons could be divided into two groups: (1) those which terminated in laminae I, II, V, and X, and (2) those which terminated in laminae V, VII, and X. Axons from both groups had myelinated parent axons, were activated by periaqueductal gray stimulation, and responded to noxious pinch of their receptive field. Terminal collaterals from both groups of axons were generally transversely oriented. These results suggest heterogeneous functions for these descending axons which may include modulation of nociceptive input to higher centers.

Spinal projections of the gigantocellular reticular formation in the rat. Evidence for projections from different areas to laminae I and II and lamina IX

We have used the autoradiographic method to study the organization of spinal projections from the gigantocellular reticular nucleus in the rat. Of particular note was the evidence obtained for projections to laminae I, II and IX. Reticular projections to laminae I and II arise more rostrally in Gi than those to lamina IX. Projections to laminae III-VIII and X as well as to autonomic nuclei have also been documented. Our results suggest that the gigantocellular reticular nucleus of the rat can be subdivided on connectional grounds.

Postsynaptic dorsal column pathway of the rat. II. Evidence against an important role in nociception

The response characteristics of neurons at the origin of the postsynaptic dorsal column (PSDC) pathway were determined in unanesthetized, decerebrated, spinalized rats. Sixty-four percent of PSDC neurons responded only to innocuous mechanical stimuli. Thirty-six percent responded to innocuous stimuli but were more powerfully activated by noxious pinch. Ninety-three percent of the tested PSDC neurons were not activated by any of several intensities of sustained, repeated noxious heating of their receptive fields. The failure of pinch-responsive PSDC cells to respond to thermal stimulation, even in sensitized skin, suggests that they do not receive a functionally significant input from C polymodal nociceptors, heat nociceptors, or mechanical-heat nociceptors. We conclude, therefore, that the postsynaptic dorsal column pathway is not importantly involved in nociception in the rat.

Efflux of 5-hydroxytryptamine and noradrenaline into spinal cord superfusates during stimulation of the rat medulla

High pressure liquid chromatography with electrochemical detection was used to quantify the efflux, in the same sample, of endogenous 5-hydroxytryptamine (5-HT), noradrenaline (NA), and 5-hydroxyindoleacetic acid (5-HIAA) into superfusates of the rat spinal cord in vivo. The efflux of these three agents was measured prior to, and during, electrical stimulation of the nucleus raphe magnus (n.r.m.) and nucleus reticularis paragigantocellularis (n.r.p.g.), two medullary nuclei implicated in antinociception. In untreated rats, basal efflux of 5-HT and NA was 0.21 and 0.12 ng/ml of superfusate respectively; the basal efflux of 5-HIAA was 18.17 ng/ml. Stimulation of the n.r.m. and n.r.p.g. in these animals increased the efflux of 5-HT and 5-HIAA, but did not alter the efflux of NA. 60 min after administration of fluoxetine (10 mg/kg, I.P.), a 5-HT uptake inhibitor, basal efflux of 5-HT and NA was unaltered, but the basal efflux of 5-HIAA was decreased. In these rats, stimulation of the n.r.m. and n.r.p.g. increased the efflux of 5-HT and of NA. The efflux of 5-HIAA was not altered. In rats pre-treated with both fluoxetine and desipramine (10 mg/kg, I.P.), the basal efflux of NA was increased while that of 5-HIAA was decreased; the basal efflux of 5-HT was not affected. The efflux of NA, but not of 5-HT, was increased in these animals during stimulation of the n.r.m. and n.r.p.g. The efflux of 5-HIAA was not changed by stimulation. Addition of fluoxetine alone or with desipramine to the superfusate in high concentrations greatly increased basal efflux of 5-HT. Failure of stimulation of the ventromedial medulla to increase the efflux of 5-HT in these animals may be related to feed-back inhibition of release by the high concentration of 5-HT initially present in the superfusate. These results indicate that electrical stimulation of the n.r.m. and n.r.p.g. increases the efflux of endogenous 5-HT and NA from the spinal cord. These stimulation sites are coincident with brain-stem sites at which stimulation produces antinociception by activation of spinal serotonergic and noradrenergic receptors. Thus, the ability of stimulation at these sites to evoke the spinal release of the probable neurotransmitters further supports the hypothesis that the antinociceptive effect is mediated by activation of serotonergic and noradrenergic neurones projecting to the spinal cord.