Projections from the brain stem reticular formation to laminae I and II of the spinal cord. Studies using light and electron microscopic techniques in the North American opossum

Descending neural projections to the spinal cord in the channel catfish, Ictalurus punctatus

Retrograde transport of horseradish peroxidase was used to determine the descending projections to the spinal cord in an otophysan fish, the channel catfish, Ictalurus punctatus. The majority of cells projecting to the spinal cord are located in the reticular formation, which is organized into rhombomeric segments. Vestibulospinal neurons are located in the descending, magnocellular, and tangential octaval nuclei, as well as in the medial octavolateralis nucleus of the lateral line system. Cells in the facial lobe project to the spinal cord. Additionally, axons of cells of the trigeminal system and the nucleus of the lateral lemniscus project caudally into the spinal cord. In the midbrain, descending spinal projections arise from cells of the medial longitudinal fasciculus and the red nucleus. More rostrally, cells of the ventrolateral thalamus, dorsal periventricular hypothalamus, central pretectal and magnocellular preoptic nuclei also project to the cord. The results of this study indicate that there are a number of homologies in the descending systems of bony fishes and other vertebrate taxa, including tetrapods. We also provide further evidence that a red nucleus is present in the brains of bony fishes and is therefore a primitive vertebrate character antedating the evolution of tetrapods.

Serotoninergic and nonserotoninergic neurons in the medullary raphe system have axon collateral projections to autonomic and somatic cell groups in the medulla and spinal cord

Fluorescent double retrograde-tracing studies combined with fluorescent immunostaining for serotonin were carried out to determine the potential patterns of divergence in axonal projections to autonomic and somatic motor sites from medullary raphe and parapyramidal neurons. Injections (20-60 nl) of combinations of fluorescent retrograde tracers (Fast Blue, fluoro-gold, green latex microspheres, Diamidino Yellow) were made into the intermediolateral cell column (IML) of the spinal cord and the brainstem lateral tegmental field or ventral horn of the lumbar spinal cord of male Wistar rats. The animals were perfused after a 7-10-day survival period, and the brains were removed, sectioned (50 microns), and immunostained for serotonin. Following injections of different retrograde-tracer substances into the IML of the thoracic cord and the ventral horn of the lumbar cord, 36% of the neurons with axon collateral projections to the IML and the lumbar ventral horn were serotoninergic. Following injections of different retrograde-tracer substances into the IML and the lateral tegmental field, 26% of the neurons with axon collateral projections to the IML and the lateral tegmental field were serotoninergic. Many of the medullary neurons with projections to the lateral tegmental field and the lumbar cord were located dorsal and lateral to those neurons with projections to the IML. The results indicate that serotoninergic and nonserotoninergic neurons of the midline raphe system and parapyramidal region have axon collateral branches to the IML and the lateral tegmental field or the IML and the lumbar ventral horn. These projection neurons may form the anatomical substrate for the integration of autonomic and somatic motor activity.

Spinal cord afferent systems containing the nerve terminal protein NT75

In the adult spinal cord, immunocytochemical staining for NT75 is concentrated in nerve terminals in the superficial laminae of the dorsal horn. Deeper laminae of the dorsal horn contain moderate immunocytochemical labeling, but the ventral horn is only sparsely stained. The origin of spinal nerve terminals containing NT75 was investigated with lesion techniques, colchicine treatment, and retrograde tracing in combination with immunocytochemical staining. Primary afferent neurons express NT75 immunoreactivity and account for most of the dense staining in the superficial dorsal horn and part of the labeling in the deeper laminae. It was found that corticospinal and virtually all brainstem neurons with descending projections to the spinal cord also express NT75 immunoreactivity, including those terminating in the ventral horn. Colchicine treatment of the spinal cord also resulted in NT75 staining in most, if not all, spinal neurons. It appears that neurons in all three major sources of spinal afferents (primary sensory, descending, and intrinsic systems) can express NT75 immunoreactivity, but that some neurons normally contain higher levels of the protein in their nerve terminals. Previous analysis of developing spinal cord has shown widespread, dense NT75 labeling throughout the spinal gray in the early postnatal period, which later becomes restricted to the adult pattern. These studies support the hypothesis that many spinal pathways express high levels of NT75 immunoreactivity during development, but that only certain pathways maintain high levels in the adult.

Ultrastructural aspects of the coeruleo-spinal projection

Few studies have focussed on the ultrastructure of the coeruleo-spinal projection. In rat the projections from the area of the locus coeruleus (LC) and subcoeruleus (SC) to lumbar motoneuronal cell groups exhibited two different types of terminals: E-type terminals, containing many very small vesicles and S-type terminals, containing many spherical vesicles and an occasional dense-cored vesicle. These findings are in agreement with data indicating the existence of a noradrenergic (NA) and a non-NA projection from the area of the LC and SC to the spinal cord. A study on dopamine-beta-hydroxylase (D beta H)-immunoreactive terminals in lumbar motoneuronal cell groups showed that they contained several granular vesicles, which were not found in the E- and S-type terminals. Only a few immunoreactive terminals exhibited a synaptic specialization in a single, thin section. A low incidence of synaptic junctions was also found for the E-type terminals, but not for the S-type. Based on this and other data, it is suggested that the E-type terminal is NA, while the S-type may contain a non-NA transmitter, possibly acetylcholine. A low incidence of synaptic junctions in single, thin sections may indicate the presence of non-synaptic NA terminals, but direct evidence from serial-section analysis is not available. In the superficial dorsal horn, terminals derived from the area of the LC and SC were identified at the ultrastructural level in two studies, one using the anterograde degeneration technique in opossum, the other (presented in this chapter) using WGA-HRP anterograde tracing in rat. It was found in both studies that most of the labeled structures were small axons (mostly unmyelinated), while few terminals were labeled. They contained mostly spherical vesicles and, according to the degeneration study, a variable number of dense-cored vesicles. The labeled terminals appeared to make regular synaptic contacts mostly with small dendrites and occasionally with spines. They were not present in glomeruli or engaged in presynaptic arrangements. A study on NA terminals showed similar results, although large granular vesicles were not observed and fewer synapses were seen. On the few data available at present it is concluded that in the spinal superficial dorsal horn, most terminals derived from the area of the LC and SC are NA and establish conventional synapses. However, a non-NA component cannot be excluded.

Demonstration of two separate descending noradrenergic pathways to the rat spinal cord: evidence for an intragriseal trajectory of locus coeruleus axons in the superficial layers of the dorsal horn

The rat spinal cord receives noradrenergic (NA) projections from the locus coeruleus (LC) and the A5 and A7 groups. In contradiction to previous statements about the distribution of descending NA axons, we have recently proposed that in the rat LC neurons project primarily to the dorsal horn and intermediate zone, whereas A5 and A7 neurons project to somatic motoneurons and the intermediolateral cell column. The aim of the present study was to determine the funicular course and terminal distribution of descending NA axons from the LC and from the A5 and A7 groups. The organization of the coeruleospinal projection was analyzed by using the anterograde tracer Phaseolus vulgaris leucoagglutinin in combination with dopamine-beta-hydroxylase immunohistochemistry. The trajectory of A5 and A7 axons was studied in spinal cord sections of rats following ablation of the coeruleospinal projection with the neurotoxin DSP-4. To assess the relative contribution of the LC and the A5 and A7 groups to the NA innervation of the spinal cord, unilateral injections of the retrograde tracer True Blue were made at cervical, thoracic, and lumbar levels, and retrogradely labeled NA neurons were identified by dopamine-beta-hydroxylase immunofluorescence. The results of the anterograde tracing experiments confirm our previous findings that LC neurons project most heavily to the dorsal horn and intermediate zone. Analysis of horizontal sections revealed that LC axons descend the length of the spinal cord within layers I and II. In contrast to the intragriseal course of LC fibers, A5 and A7 axons travel in the ventral and dorsolateral funiculi and terminate in the ventral horn and the intermediolateral cell column. Retrograde transport studies indicate that the contribution of the A5 and A7 groups to the NA projection to the spinal cord is greater than that of the LC. We conclude that descending axons of the LC and A5 and A7 groups differ in their course and distribution within the spinal cord. The documentation of a definite topographic order in the bulbospinal NA projections suggests that the LC and the A5 and A7 groups have different functional capacities. The LC is in a position to influence the processing of sensory inputs, in particular nociceptive inputs, whereas A5 and A7 neurons are likely to influence motoneurons.

Evidence for projections from the rostral medullary raphe onto medullary catecholamine neurons in the rat

Following the iontophoretic deposition of Phaseolus vulgaris leucoagglutinin (PHA-L) into the rostral medullary raphe, which included portions of the caudal nucleus raphe magnus, rostral nucleus raphe pallidus, rostral nucleus raphe obscurus and rostral nucleus reticularis paragigantocellularis, two-color immunoperoxidase staining was employed to demonstrate contiguity between PHA-L-immunoreactive (PHA-LI) varicose fibers and boutons and medullary catecholamine (CA) cells. Raphe projections were contiguous with phenylethanolamine N-methyltransferase-immunoreactive (PNMTI) neurons in the C1, C2 and C3 cell groups and with tyrosine hydroxylase-immunoreactive (THI) neurons in the A1 and A2 cell groups. Contiguity between PHA-LI processes and medullary CA cells was observed most frequently in the C1 cell group. Preliminary findings of this study have been presented previously.

Substance P-immunoreactive elements in laminae I and II of the chicken spinal cord: a light- and electron-microscopic study

Substance P (SP)-immunoreactive (IR) elements were studied in laminae I and II of the chicken spinal cord in conjunction with an anti-SP monoclonal antibody at light- and electron-microscopic levels by means of the indirect antibody peroxidase-antiperoxidase technique. At the light-microscopic level, SP-IR elements were most intensely observed in the dorsolateral portion of the dorsal horn, laminae I and II. Electron-microscopically, SP-IR boutons contained large spherical dense-cored vesicles (diameter range: 60-125 nm) and spherical clear vesicles. They were subdivided into two groups: large SP-IR boutons, which were the central terminals in synaptic glomeruli, and small SP-IR boutons. In the synaptic glomerulus, two kinds of non-IR presynaptic profiles made axo-axonic synapses with the SP-IR central terminal: one was the presynaptic profile containing pleomorphic clear vesicles and the other was the presynaptic profile containing large dense-cored vesicles. A 'septate junction'-like structure was observed between large SP-IR boutons in synaptic glomeruli. The present results suggest that SP-containing primary afferents are modulated presynaptically by two different neurotransmitter or modulator systems.

Arborization of single axons of the spinal dorsolateral funiculus to the contralateral superficial dorsal horn

This study provides an anatomical basis for the observation that a unilateral lesion of the spinal dorsolateral funiculus (DLF) can reduce the inhibitory effect of electrical stimulation of the nucleus raphe magnus (NRM) on dorsal horn nociceptive neurons located caudal and contralateral to the lesion. We injected the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) into the NRM and traced the arborization of single DLF axons in the spinal gray matter. Although the majority of DLF axons arborized in the dorsal horn ipsilaterally, we found some axons which entered the spinal gray matter, traversed the gray matter to the central canal and then abruptly changed direction and coursed to the contralateral superficial dorsal horn. Very few branches were given off en route. These data indicate that some raphe-spinal axons may selectively influence the firing of neurons of the superficial dorsal horn, contralateral to the DLF in which they descend the spinal cord.

Cortical and brain stem projections to the spinal cord of the hedgehog (Erinaceus europaeus). A horseradish peroxidase study

Cortical and brain stem neurons projecting to the spinal cord in the hedgehog were studied by means of the horseradish peroxidase (HRP) tracing method. HRP injections were placed in the first cervical segments, in the cervical enlargement (C5-T3) and in the lumbar enlargement. Following injections in the first cervical segments and in the cervical enlargement labelled neurons were observed in the somatic motor and somatic sensory cortices, the paraventricular and the dorsomedial hypothalamic nucleus, the lateral hypothalamic area, the nuclei of field H of Forel, the red nucleus, the mesencephalic reticular formation, the deep layers of the superior colliculus, the Edinger-Westphal nucleus, the periaqueductal grey, the mesencephalic trigeminal nucleus, the loci coeruleus and subcoeruleus, the nuclei raphe dorsalis, centralis superior, raphe magnus, raphe pallidus, and raphe obscurus, the rhombencephalic reticular formation, the lateral, medial and caudal vestibular nuclei, the nucleus ambiguus, the nucleus of the solitary tract and the gracile nucleus. After HRP injections in the lumbar enlargement, labelled neurons were not found in the cortex, the dorsomedial hypothalamic nucleus, the nuclei of field H of Forel, the superior colliculus and the mesencephalic trigeminal nucleus. These results show that cortical and brain stem projections to the spinal cord are comparable to those described in other species.

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.

Bulbospinal projections in the primate: a light and electron microscopic study of a pain modulating system

The projections of the nucleus raphe magnus (NRM) and the immediately adjacent reticular formation were studied in the macaque monkey following injections of the rostroventral medulla with 3H-leucine and examination of the resultant labeled axons and terminals by light and electron microscopic autoradiography. Five monkeys had accurately placed injections, which resulted in fiber pathway labeling that coursed caudally, laterally, and dorsally to project to laminae I, II, and V of subnucleus caudalis of the trigeminal and then traveled in the dorsolateral funiculus of the cord and terminated in similar laminae of the spinal dorsal horn at cervical levels. The pathway was only lightly labeled caudal to the cervical enlargement and could not be readily discerned above background in the thoracic or lumbar cord. Electron microscopy revealed that axons and terminals serving this system constitute a heterogeneous population. Large-diameter myelinated axons (3-6-micron diameter), small myelinated axons (0.75-3-micron diameter), and clusters of nonmyelinated axons were labeled. Terminals in laminae I, II, and V contained mixtures of clear round and granular vesicles or clear pleomorphic and granular vesicles or formed the central element in synaptic glomeruli. The labeled profiles formed asymmetrical or symmetrical synapses on medium and small dendrites; labeled axosomatic synapses were not observed. In rare instances there were contacts between labeled profiles and vesicle-containing structures, which were probably dendritic, but whether the NRM axon was pre- or postsynaptic to such structures could not be determined. It was concluded that the NRM in the monkey is organized in a manner quite similar to that previously described in the cat. The wide variety of fiber types and synaptic terminals serving this system suggests that different classes of neurons participate in it, probably using several transmitter substances that result in varying postsynaptic effects on neurons located in the trigeminal complex and dorsal horn.

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.

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.

The ultrastructure and synaptic connections of the spinal terminations from single, physiologically characterized axons descending in the dorsolateral funiculus from the midline, pontomedullary region

Single axons descending in the spinal, dorsolateral funiculus which were directly activated by stimulating in or near nucleus raphe magnus (nRM) in the rostral medulla were iontophoretically injected with horseradish peroxidase (HRP). Labeled axons and terminal arborizations in the spinal cord were demonstrated by diaminobenzidine histochemistry following fixation. Sections were processed for electron microscopy and embedded between coverslips. Following examination with the light microscope, selected boutons were sectioned in ultrathin series and examined with the electron microscope. Two arborization patterns were observed with the light microscope. One terminated dorsally in laminae I, II, V, and X, while the other terminated ventrally in laminae V, VII, and X. At the ultrastructural level, boutons predominantly contacted dendrites and occasionally contacted cell somas. Ventrally projecting axons had boutons characterizable as Gray's type I. Some dorsally projecting axons also had Gray's type I boutons, while other axons had Gray's type II boutons. Thus, these descending axons may be heterogeneous in function. No differences in physiological characteristics were found between axons with Gray's type I boutons versus axons with Gray's type II boutons.

Collateral innervation of the inferior colliculus in the North American opossum: a study using fluorescent markers in a double-labeling paradigm

The innervation of the opossum inferior colliculus was investigated using the retrograde transport of fluorescent markers in a double-labeling paradigm. True Blue was injected into one inferior colliculus while Nuclear Yellow was placed into the other. Many single-labeled neurons were found in all of the brainstem and cortical areas previously labeled by comparable injections of HRP. When the two injections of fluorescent markers were bilaterally symmetrical within the inferior colliculi, double-labeled neurons were numerous in the medial and lateral superior olivary nuclei. Only a few double-labeled neurons were found in the auditory cortex (AC), the dorsal nucleus of the lateral lemniscus (DNLL), the nucleus reticularis gigantocellularis pars ventralis (RGcv), and the dorsal column nuclei (DColN) and the spinal trigeminal complex (TrS). These data suggest that two patterns of innervation are seen for afferent fibers from regions with bilateral connections to the IC. The first pattern, typified by projections from AC, DNLL, RGcv, the DColN and TrS, primarily consists of neurons with unilaterally directed axons, extending to either the ipsilateral or contralateral IC. The second pattern, typified by projections from LSO and MSO, displays a great number of neurons whose axons apparently innervate both inferior colliculi, presumably through a process of axon collateralization. The bilateral projection of axons from individual neurons in these two nuclei possibly reflects their involvement in the processing of sensory information from the two cochleas and may represent at least one pathway whereby binaural information is relayed to both inferior colliculi.

Ascending and descending projections from nucleus reticularis magnocellularis and nucleus reticularis gigantocellularis: an autoradiographic and horseradish peroxidase study in the rat

The projections of the rostral medulla were studied using retrograde and orthograde transport techniques in the rat. The present horseradish peroxidase (HRP) studies indicate that the ventral portion of nucleus reticularis gigantocellularis (NGC) and nucleus reticularis magnocellularis (NMC) project to both rostral and caudal levels of the spinal cord, while dorsal NGC projects only to the rostral cord. A differential density distribution of labeled cells was observed, with the greatest density of NGC-spinal neurons located rostral to the level of the inferior olive; and the greatest density of NMC-spinal neurons located caudally. This differential density distribution, when coupled with microiontophoretic application of [3H]amino acids allowed relatively independent labeling of the adjacent NGC- and NMC-spinal systems. On the basis of the HRP and autoradiographic studies 3 separate regions were delineated: dorsal NGC, ventral NGC and NMC. Descending projections from NGC were observed to the lateral vestibular nucleus, facial nucleus, hypoglossal nucleus and nucleus cuneatus. At cervical levels NGC fibers projected through the ventral and ventrolateral columns. Terminal fields were observed in laminae VII, VIII and to a lesser extent in IX. Labeled NGC fibers became difficult to follow by thoracic levels, which is consistent with the present HRP results. A continuum of descending NGC projections was observed with dorsally located NGC neurons projecting bilaterally through the ventral columns, and ventrally located NGC cells projecting through the ipsilateral ventrolateral columns. Ascending projections from NGC to the motor nucleus of V, trochlear nucleus, oculomotor nucleus, Edinger-Westphal nucleus, the ventral aspect of the periaqueductal gray, the deep and intermediate layers of the superior colliculus, nucleus parafasicularis and centromedianus, the Fields of Forel and the dorsal and lateral hypothalamic nuclei were observed. Descending projections from NMC to the dorsal nucleus of the vagus, hypoglossal nucleus, nucleus commissuralis and intercalatus were observed. At cervical levels, fibers project through the ipsilateral lateral columns, particularly its dorsal aspect. Terminal fields are located ipsilaterally in laminae IV, V and VI, and bilaterally in VII, VIII and X. NMC projections continue through caudal levels of the spinal cord including a projection to the ipsilateral intermediolateral columns. Ascending NMC projections are limited to the ventral pontine reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Descending serotonergic, peptidergic and cholinergic pathways from the raphe nuclei: a multiple transmitter complex

The localization of serotonergic, various peptidergic and possibly cholinergic neurons in the medullary raphe nuclei that project to the lumbosacral spinal cord have been studied using a retrograde transport method combined with immunocytochemical and histochemical techniques. Spinally projecting neurons stained for serotonin-like, substance P-like, enkephalin-like and thyrotropin-releasing hormone-like immunoreactivity and for the histochemical marker acetylcholinesterase were all observed in each of the raphe nuclei of the medulla, as well as in the adjacent ventrolateral reticular formation. The similar distributions of the descending serotonergic and peptidergic neurons in the raphe nuclei as well as quantitative data on their relative numbers suggest that a large fraction of raphe-spinal neurons contain serotonin co-existing with one or more peptides in the same cell.

Transmitters of the raphe-spinal complex: immunocytochemical studies

The localization of serotonergic and various peptidergic neurons in the medullary raphe nuclei that project to the lumbosacral spinal cord have been studied using a retrograde transport method combined with immunocytochemistry. Spinally projecting neurons stained for serotonin-like, substance P-like, enkephalin-like and thyrotropin-releasing hormone-like immunoreactivity were all observed in the raphe nuclei of the medulla, as well as in the adjacent ventrolateral reticular formation. The distribution of the descending serotonergic and peptidergic neurons in the raphe nuclei as well as quantitative data on their relative numbers suggest that a large fraction of raphe-spinal neurons contain serotonin co-existing with one or more peptides in the same cell.

Primary afferent distribution pattern in the marginal zone (lamina 1) of adult monkey and cat lumbosacral spinal cord

Using Golgi preparations from adult cat and monkey the present study describes the general arborization pattern of primary afferent collaterals to the marginal layer (lamina I) of the dorsal horn. The dorsal root origin of the golgi-impregnated afferents was verified in the cat by matching the structural properties of these fibers with horseradish peroxidase (HRP)-stained primary afferents which were filled with HRP via anterograde transport through dorsal rootlets. The marginal afferents, in both the cat and the monkey, are relatively fine-caliber fibers which emanate from the dorsolateral fasciculus of Lissauer. Superficially they form a transverse plexus of fibers which runs across the surface of the dorsal horn and is especially dense in the lateral portion of the dorsal aspect of the marginal layer where lamina I is thickest. In deeper regions of the marginal layer, marginal afferents are longitudinally oriented and have collaterals within the outer zone of lamina II. In both superficial and deep regions of the marginal layer the afferents run parallel with large numbers of marginal cell dendrites. Most marginal afferents give rise to collaterals with widely spaced, round to oval "boutons en passant" and "boutons terminaux" which are 1.5-2.0 micrometers in diameter. Structural correlation with previous physiological and anatomical studies suggest that most of these fibers are finely myelinated, A-delta, high-threshold mechanoreceptive fibers.

The reticulocerebellar projection in the cat as studied with retrograde transport of horseradish peroxidase

Injections of horseradish peroxidase into the various parts of the cerebellar cortex and the cerebellar nuclei in the cat result in labelled cells within the reticular formation proper. All the reticular nuclei (with the exception of the reticular formation of the mesencephalon) send fibres to the cerebellum. The highest number of labelled neurons after cerebellar injections is found in the caudal reticular formation, especially within nucleus reticularis ventralis, nucleus reticularis lateralis and nucleus reticularis gigantocellularis. Another region for an accumulation of labelled cells is the rostral part of nucleus reticularis pontis caudalis. Except for the paraflocculus, all cerebellar cortical areas and all cerebellar nuclei receive afferents from one or more of the nuclei within the reticular formation proper, but the largest number of labelled neurons is observed in cases with injections including the intermediate-lateral part of lobulus simplex and the adjacent areas of the anterior lobe and crus I. the projection is bilateral with an ipsilateral preponderance (the cerebellar nuclei appear to receive a higher number of fibres from the contralateral side). Cells of all sizes are labelled, but labelled giant cells are found only after large cortical injections.