Retrograde axonal transport of horseradish peroxidase from spinal cord to brain stem cell groups in the cat

Muscle denervation increases thyrotropin-releasing hormone (TRH) biosynthesis in the rat medullary raphe

To determine whether thyrotropin-releasing hormone (TRH) could exert a trophic role in ventral horn motor neurons, we examined the effect of muscle denervation with botulinum toxin A on TRH mRNA in the rat medullary raphe by in situ hybridization histochemistry. Compared to controls, denervated rats showed a significant increase in the number and silver grain density of hybridized medullary raphe neurons. Increased proTRH gene expression in the medullary raphe in response to motor unit perturbation indicates that TRH may be trophic to lower motor neurons.

Projections of the tectospinal tract to the upper cervical spinal cord of the cat: a study with the anterograde tracer PHA-L

The goal of the present experiments was to re-examine the spinal projections of neurons in the superior colliculus (SC) of the cat by taking advantage of the high sensitivity of the anterograde tracer, phaseolus vulgaris leucoagglutinin (PHA-L). In seven experiments, multiple injections of PHA-L into different regions of the SC labelled a total of 172 axons in the predorsal bundle; yet only 11 tectospinal tract (TST) axons were found in the upper cervical spinal cord. Collaterals emerging from these axons were rare and arose exclusively from TST axons with a diameter of less than 1 micron. Individual collaterals had different termination zones: some terminated in the lateral part of lamina V and VI after taking a dorsolateral course through lamina VII and VIII; others terminated in the medial part of lamina VII. One collateral terminated within lamina IX and the ventral part of lamina VIII. The combined termination of all collaterals was densest in lamina VII and dorsal lamina VIII. A small number of boutons were also found in the lateral parts of laminae V and VI, and in lamina IX and immediately adjacent regions in lamina VIII. Compared to axons belonging to other spinal descending systems, individual TST axons give rise to much simpler intraspinal collaterals with relatively few boutons. This feature, together with the relative paucity of TST axons, suggests that direct connections from the SC to neurons in the upper cervical spinal cord are sparse. Furthermore, our results are consistent with electrophysiological studies that show that few, if any, neck motoneurons receive monosynaptic connections from TST neurons. Projections to neck motoneurons must therefore involve a relay, either through other descending pathways, such as the reticulospinal system, or via local segmental interneurons.

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.

Catecholamine projections to the spinal cord in the rat and their relationship to central cardiovascular neurons

The organization of the neuroanatomical substrate which provides the supraspinal catecholaminergic innervation of the upper thoracic spinal cord in the rat was studied by means of retrograde labelling of neurons with primuline and other dyes, combined with simultaneous catecholamine fluorescence (FAGLU-method). It was shown that spinally projecting catecholaminergic neurons lie predominantly in the A6 and A11 groups, while a minority of these neurons are also located in the A1/C1 and A5 groups. The number of catecholamine-containing neurons in the A11 group in the rat was estimated to be 173 +/- 4. Most of them (up to 86%) were retrogradely labelled with primuline. The possible functional role of dopaminergic diencephalospinal, noradrenergic pontospinal, and adrenergic bulbospinal neuronal systems in cardiovascular control is discussed.

The projection of locus coeruleus neurons to the spinal cord in the rat determined by anterograde tracing combined with immunocytochemistry

Pontospinal noradrenergic neurons located in the A5, A7 and locus coeruleus/subcoeruleus (LC/SC) nuclei are the major source of the noradrenergic innervation of the spinal cord. However, the specific terminations of spinally-projecting noradrenergic neurons located in these nuclei have not been clearly defined. The purpose of the experiments described in this report was to more precisely define the spinal terminations of neurons located in the LC/SC using the anterograde tracer phaseolus vulgaris-leucoagglutinin in combination with dopamine-beta-hydroxylase (D beta H) immunocytochemistry. In addition, the spinal cord regions in which LC/SC neurons terminate was assessed by measuring the reduction in the density of D beta H-immunoreactive axon terminals in specific spinal cord regions after a unilateral electrolytic lesion that included LC/SC neurons. The results of these experiments indicate that the axons of LC neurons are located primarily in the ipsilateral ventral funiculus and terminate most heavily in the medial part of laminae VII and VIII, the motoneuron pool of lamina IX, and lamina X. LC neurons provide a moderately dense innervation of the ventral part of the dorsal horn, but only a very sparse innervation of the superficial dorsal horn. The SC projects ipsilaterally in the ventrolateral funiculus and terminates diffusely in the intermediate and ventral laminae of the spinal cord.

Locus coeruleus control of spinal motor output

Using electrophysiological techniques, we investigated the functional properties of the coeruleospinal system for regulating the somatomotor outflow at lumbar cord levels. Many of the fast-conducting, antidromically activated coeruleospinal units were shown to exhibit the alpha 2-receptor response common to noradrenergic locus coeruleus (LC) neurons. Electrically activating the coeruleospinal system potentiated the lumbar monosynaptic reflex and depolarized hindlimb flexor and extensor motoneurons via an alpha 1-receptor mechanism. The latter synaptically induced membrane depolarization was mimicked by norepinephrine applied iontophoretically to motoneurons. That LC inhibited Renshaw cell activity and induced a positive dorsal root potential at the lumbar cord also reinforced LC's action on motor excitation. We conclude that LC augments the somatomotor output, at least in part, via an alpha 1-adrenoceptor-mediated excitation of ventral horn motoneurons. Such process is being strengthened by LC's suppression of the recurrent inhibition pathway as well as by its presynaptic facilitation of afferent impulse transmission at the spinal cord level.

Effect of environmental stress on release of norepinephrine in posterior nucleus of the hypothalamus in awake rats: role of sinoaortic nerves

Norepinephrine(NE) release in posterior nucleus(PH) of the hypothalamus was examined before and during acute shaker (oscillation) stress in sinoaortic denervated(SAD) and sham-operated(SO) rats. NE in PH extracellular fluid of freely moving rats was collected by microdialysis and measured by sensitive radioenzymatic assay. Three days after SAD or SO operation, mean arterial pressure(MAP) and heart rate(HR) were significantly higher in SAD rats than SO rats. Baseline levels of NE in PH dialysate were also significantly elevated in SAD rats. Although five minutes of shaker stress elicited pressor and tachycardic responses coupled with increased NE release in PH of both groups, the increases in MAP and dialysate NE were larger in SAD than SO rats. These findings indicate that noradrenergic neurons in the PH respond to stress-induced stimuli and receive tonic input from baroreflex pathways.

Pontospinal transmitters and their distribution

The dorsolateral pontine tegmentum of the cat is known to contain a large population of catecholaminergic neurons. Additionally, several studies have also shown the presence of other neurochemicals (acetylcholine, enkephalin, neuropeptide Y, serotonin, somatostatin and substance P). In this study, we have employed retrograde transport of horseradish peroxidase in combination with immunocytochemistry to determine the locations of pontospinal neurons which contain catecholamine, enkephalin, neuropeptide Y, and serotonin. Furthermore, we have combined the retrograde transport of Fast Blue and immunofluorescence histochemistry to determine whether enkephalin-containing neurons are catecholaminergic. All pontospinal neurons, irrespective of the neurochemical content, were observed in the ventral and lateral parts of the dorsolateral pontine tegmentum at coronal levels P1.8-P4.0. These neurons were located in the nuclei locus coeruleus alpha and subcoeruleus and the Kölliker-Fuse nucleus. A high concentration of these neurons was evident in the Kölliker-Fuse nucleus when compared to the nuclei locus coeruleus alpha and subcoeruleus. Quantitative data have revealed that enkephalin is contained in a large proportion of the pontospinal catecholaminergic neurons (75%). The observations suggest that catecholaminergic neurons may contain one or more putative peptide neurotransmitters.

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.

The projections of locus coeruleus neurons to the spinal cord

Spinally projecting noradrenergic neurons located in the locus coeruleus/subcoeruleus (LC/SC) are a major source of the noradrenergic innervation of the spinal cord. However, the specific terminations of these neurons have not been clearly defined. The purpose of this chapter is to describe the results of experiments that used the anterograde tracer Phaseolus vulgaris leucoagglutinin in combination with dopamine-beta-hydroxylase immunocytochemistry to more precisely determine the spinal cord terminations of neurons located in the LC/SC. The results of these experiments indicate that the axons of LC neurons are located primarily in the ipsilateral ventral funiculus and terminate most heavily in the medial part of laminae VII and VIII, the motoneuron pool of lamina IX, and lamina X. LC neurons provide a moderately dense innervation of the ventral part of the dorsal horn, but only a very sparse innervation of the superficial dorsal horn. The SC projects ipsilaterally in the ventrolateral funiculus and terminates diffusely in the intermediate and ventral laminae of the spinal cord. Finally, the results of preliminary experiments indicate that different rat substrains may have LC neurons that exhibit qualitatively different termination patterns in the spinal cord. More specifically, LC neurons in some rat substrains innervate the dorsal horn, while those in other substrains primarily innervate the ventral horn and intermediate zone.

The spinotrigeminal pathway and its spatial relationship to the origin of trigeminospinal projections in the rat

The anterograde transport of horseradish peroxidase and tritiated amino acids was used to examine the distribution and morphology of spinal afferent fibers terminating in the rat spinal trigeminal complex. The results confirm the existence of a direct, ipsilateral projection from the spinal cord which is distributed exclusively to the deepest layers of the medullary dorsal horn narrow regions subjacent to the spinal trigeminal tract in trigeminal nucleus interpolaris, trigeminal nucleus oralis and the trigeminal main sensory nucleus. Spinal inputs also terminated in the insular trigeminal-cuneatus lateralis nucleus which is a distinct component of the interstitial system of the spinal trigeminal tract. The spinal afferent fibers which terminated in the dorsolateral parts of the spinal trigeminal complex arose from the dorsal column funiculi, while those that terminated in ventral parts of the complex arose from both the dorsal column and lateral funiculi. The tritiated amino acid experiments indicate that at least part of the spinotrigeminal pathway originates from cells located in the cervical spinal dorsal horn. The present findings also document a complex spatial relationship between the spinotrigeminal and trigeminospinal pathways which includes an extensive overlap between spinotrigeminal fibers and spinal projecting neurons in each of the lateralmost regions of the complex. This spatial overlap supports the existence of anatomical substrates which may underlie functional reciprocal loops between the spinal trigeminal complex and cervical spinal cord. Since these regions are primarily concerned with the processing of sensory information from lateral and posterior parts of the face, it follows that the spinotrigeminal pathway may be primarily concerned with the integration of head and neck functions. In addition, the spatial convergence of spinal inputs and the distribution of other trigeminal efferent neurons suggests that part of the spinotrigeminal pathway may be involved in spino-trigemino-thalamic and spino-trigemino-cerebellar pathways in parallel with other spinobulbar pathways in the medulla.

Descending noradrenergic influences on pain

Multiple separate and distinct supraspinally organized descending inhibitory systems have been identified which are capable of powerfully modulating spinal nociceptive transmission. Until recently, brainstem sites known to be involved in the centrifugal modulation of spinal nociceptive transmission were few in number, being limited to midline structures in the midbrain and medulla (e.g., periaqueductal gray and nucleus raphe magnus). However, with continued investigation, that number has increased and brainstem sites previously thought to be primarily involved in cardiovascular function and autonomic regulation (e.g., nucleus tractus solitarius; locus coeruleus/subcoeruleus (LC/SC); A5 cell group; lateral reticular nucleus) also have been demonstrated to play a role in the modulation of spinal nociceptive transmission. Spinal monoamines (norepinephrine (NE) and serotonin) have been shown to mediate stimulation-produced descending inhibition of nociceptive transmission from these brainstem sites. The majority of NE-containing fibers and terminations in the spinal cord arise from supraspinal sources; thus, the LC/SC, the parabrachial nuclei, the Kölliker-Fuse nucleus and the A5 cell group have all been suggested as possible sources of the spinal noradrenergic (NA) innervation involved in the centrifugal modulation of spinal nociceptive transmission. Several lines of evidence suggest that the LC/SC plays a significant role in a functionally important descending inhibitory NA system. Focal electrical stimulation in the LC produces an antinociception and increases significantly the spinal content of NA metabolites. The inhibition of the nociceptive tail-flick withdrawal reflex produced by electrical stimulation in the LC/SC has been demonstrated to be mediated by postsynaptic alpha 2-adrenoceptors in the lumbar spinal cord. Similarly, electrical or chemical stimulation given in the LC/SC inhibits noxious-evoked dorsal horn neuronal activity. Thus, results reported in electrophysiological experiments confirm those reported in functional studies and the NA coeruleospinal system appears to play a significant role in spinal nociceptive processing.

Topographically organized projections from the nucleus subceruleus to the hypoglossal nucleus in the rat: a light and electron microscopic study with complementary axonal transport techniques

Projections from the nucleus subceruleus (nSC) to the hypoglossal nucleus (XII) were investigated with complementary retrograde and anterograde axonal transport techniques at the light and electron microscopic level in the rat. Injections of WGA-HRP into XII resulted in labeling of neurons in and around the nSC. Labeled nSC neurons were few in number (less than 4 per 40-60 microns sections) and variable in size and shape. Most labeled nSC neurons were medium-sized (mean = 16.89 microns), fusiform, triangular, or oval, with 3-4 dendrites typically oriented dorsomedially and ventrolaterally. These neurons were found throughout the rostrocaudal extent of the nSC but were most numerous medial, dorsomedial, and ventromedial to the motor trigeminal nucleus. Others were observed rostral to the motor trigeminal nucleus and ventral to the parabrachial nuclear complex. Confirmation of retrograde results was obtained following injections of tritiated amino acids or WGA-HRP into the nSC. This resulted in labeling throughout the rostrocaudal extent of XII mainly ipsilaterally. Labeled fibers descended the brainstem in the dorsolateral and, to a lesser extent, in the ventromedial component of Probst's tract. Fibers entered XII mainly rostrally along the lateral border of the nucleus. All regions of XII were recipients of nSC afferents, but the caudoventromedial quadrant contained the greatest density of terminal labeling. Electron microscopic evaluation confirmed that nSC afferents synapsed on motoneurons in XII. Axon terminals containing WGA-HRP reaction product were found contacting dendrites and somata, but primarily the former (81.3% versus 10.6%). Axodendritic terminals synapsed mainly on medium-to-small sized dendrites (less than 3 microns in diameter). The majority of labeled axodendritic terminals (90.1%) contained small, round, and clear synaptic vesicles (S-type: 20-50 nm) and were associated with an asymmetric (60.6%), symmetric (11.4%), or no (18%) postsynaptic specialization. By contrast, most axosomatic terminals contained flattened vesicles (F-type) and formed a symmetric or no postsynaptic specialization (75%). Large dense core vesicles (55-90 nm) were observed within a small proportion of all labeled axon terminals (1.3%). The results from this study demonstrate that the nSC projects to XII, preferentially targets a specific subgrouping of protrusor motoneurons, and synapses on both somata and dendrites, although mainly on the latter. The implications of these data are discussed relative to tongue control.

Contribution of brainstem GABAergic circuitry to descending antinociceptive controls: I. GABA-immunoreactive projection neurons in the periaqueductal gray and nucleus raphe magnus

The fact that GABA receptor agonists and antagonists influence nociceptive thresholds when microinjected into the rostroventral medulla or in the spinal cord may reflect the involvement of GABAergic neuronal elements in endogenous antinociceptive pathways. In the present study we used immunocytochemistry and retrograde tract tracing to investigate the contribution of GABAergic projection neurons to the antinociceptive network linking the midbrain periaqueductal gray matter (PAG), the nucleus raphe magnus (NRM), and the spinal cord dorsal horn. The tracer, WGAapoHRP-Au was injected into either the NRM or the spinal cord and the distribution of labeled neurons in sections of the PAG and medulla, respectively, was studied. The same sections were immunostained to demonstrate GABA-immunoreactive neurons. Although GABA-immunoreactive neurons were abundant in the PAG, only 1.5% were retrogradely labeled from the NRM. Similarly, very few GABA-immunoreactive neurons within the cytoarchitectural boundaries of the NRM were retrogradely labeled from the spinal cord. A much higher proportion of GABA-immunoreactive neurons in the region lateral to the NRM, however, were retrogradely labeled from the spinal cord. Eighteen percent of GABA-immunoreactive neurons were retrogradely labeled in the nucleus reticularis paragigantocellularis; conversely, 15% of the retrogradely labeled neurons in this region were GABA-immunoreactive. These results indicate that GABAergic projections constitute a very minor component of the PAG-NRM-spinal cord pathway; however, there is a significant contribution of GABAergic neurons to the spinal projections that originate lateral to the NRM. The majority of GABAergic neurons in the PAG and NRM are presumed to be inhibitory interneurons that directly or indirectly regulate activity in efferent pathways from these regions.

Simultaneous labeling of vagal innervation of the gut and afferent projections from the visceral forebrain with dil injected into the dorsal vagal complex in the rat

The vagal innervation of the different layers of the rat gastrointestinal wall was identified with the fluorescent carbocyanine dye Dil, injected into the dorsal motor nucleus of the vagus (dmnX). Multiple, bilateral injections were used to label all dmnX preganglionic motoneurons, and as a consequence, most of the vagal primary afferents that terminate in the adjacent nucleus of the solitary tract (nts) were also retrogradely and transganglionically labeled. With Fluorogold used to label the enteric nervous system completely and specifically, the Dil-labeled vagal profiles could be visualized and quantified in their anatomical relation to the neurons of the myenteric and submucous ganglia. In the myenteric plexus, vagal fibers and terminals were found throughout the gastrointestinal tract as far caudal as the descending colon, but there was a general decreasing proximodistal gradient in the density of vagal innervation. All parts of the gastric myenteric plexus (fundus, corpus, antrum), as well as the proximal duodenum, were extremely densely innervated, with vagal fibers and terminals in virtually every ganglion and connective. Further caudally, both the percentage of innervated myenteric ganglia and the average density of label within the ganglia rapidly decreased, with the exception of the cecum and proximal colon, where up to 65% of the ganglia were innervated. In the gastric and duodenal submucosa very few and in the mucosa no vagal fibers and terminals were found. With both normal epifluorescence and laser scanning confocal microscopy, highly varicose or beaded terminal structures of various size and geometry could be identified. The Dil injections, which impregnated the dmnX as well as the adjacent nts, resulted in retrograde and anterograde labeling of all the previously reported forebrain connections with the dorsal vagal complex. We conclude that the myenteric plexus is the primary target of vagal innervation throughout the gastrointestinal tract, and that its innervation is more complete than previously assumed. In contrast, vagal afferent (and efferent) innervation of mucosa and submucosa seems conspicuously sparse or absent. Furthermore, the use of more focal injections of Dil offers the prospect to simultaneously identify specific subsets of vagal preganglionics and their central nervous inputs.

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.

Direct catecholaminergic innervation of primate spinothalamic tract neurons

Catecholaminergic axonal varicosities identified by immunocytochemical staining for dopamine-beta-hydroxylase were observed at the light microscopic level apposing the somata of retrogradely labeled spinothalamic tract neurons in the monkey spinal cord. Three retrogradely labeled and two intracellularly labeled spinothalamic neurons were serially sectioned and examined at selected intervals at the electron microscopic level. Electron microscopic study revealed that axonal boutons directly contacted the somata and/or dendrites of lamina I, IV, and V spinothalamic tract neurons. All of the profiles apposing one of the retrogradely labeled lamina I spinothalamic tract neurons were categorized from eight planes of section spaced at 1-micron intervals. Of the 305 profiles counted that were adjacent to this soma, 17 (5.6%) stained positively for dopamine-beta-hydroxylase. Of these 17 appositions, three were followed in serial sections to confirm that they had synaptic thickenings and alignment of vesicles along the membrane contacting the spinothalamic tract soma. Catecholaminergic boutons were observed apposing the somata and dendrites of intracellularly filled STT cells characterized as high threshold and wide dynamic range neurons. These observations clearly indicate a direct innervation of spinothalamic tract neurons by catecholaminergic neurons, providing anatomical data to support previous physiological findings demonstrating that catecholamines modulate nociceptive transmission.

Descending projections from the dorsolateral pontine tegmentum to the paramedian reticular nucleus of the caudal medulla in the cat

We examined whether the dorsolateral pontine cholinergic cells project to the paramedian reticular nucleus (PRN) of the caudal medulla. In 3 cats, wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) was injected into the PRN and we noted cells in the dorsolateral pons that contained the HRP reaction product, cells that were immunolabeled for choline acetyltransferase (ChAT), and cells that contained the HRP reaction product and were ChAT positive. We found cholinergic projections from the pedunculopontine tegmental and laterodorsal tegmental nuclei to the PRN. This finding is consistent with studies indicating a cholinoceptive region in the medial medulla mediating suppression of muscle tone. Our results demonstrate that this medullary region has monosynaptic input from pontine neurons implicated in generating the atonia of rapid eye movement sleep.

Effect of aging on NADPH-diaphorase neurons in laterodorsal tegmental nucleus and striatum of mice

Age-related changes of reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d)-containing neurons were examined quantitatively in the laterodorsal tegmental nucleus (TLD) and the caudate-putamen of mice. Six 2-month-old and six 25- to 30-month-old DDD mice were studied using computer-assisted image analysis. Although no age-related changes in neuronal counts were found in the TLD, the cell size in this nucleus showed a statistically significant reduction with aging. In addition, the degree of the age-related neuronal shrinkage differed within the TLD; the most significant occurring in the rostral, less in the caudal third and no significant alteration being found in the middle third portion of TLD. In contrast, NADPH-d-positive neurons in the striatum did not show distinct age-related changes. NADPH-d-containing neurons in the TLD correspond to cholinergic cells, which project to the forebrain. Thus, the age-related shrinkage of NADPH-d neurons in the TLD may be related to the cholinergic dysfunctions seen in the forebrain of senescent mice.

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.

Localization of enkephalinergic neurons in the dorsolateral pontine tegmentum projecting to the spinal cord of the cat

The dorsolateral pontine tegmentum of the cat is known to contain enkephalinergic neurons, with most of the enkephalin co-contained in the catecholaminergic neurons; however, enkephalinergic cells projecting to the spinal cord have not been identified. This study employs retrograde transport of horseradish peroxidase in combination with methionine-enkephalin or tyrosine hydroxylase immunocytochemistry to 1) determine the locations of pontospinal enkephalinergic neurons and 2) compare these with the locations of pontospinal catecholaminergic neurons. Pontospinal enkephalinergic neurons were observed in the nuclei locus coeruleus and subcoeruleus and the Kölliker-Fuse nucleus. A high concentration of these neurons was evident in the Kölliker-Fuse nucleus when compared to the nuclei locus coeruleus and subcoeruleus (P less than .01). Both the enkephalinergic and catecholaminergic neurons projecting to the spinal cord were located in the same general areas of the dorsolateral pontine tegmentum and there was no significant difference in the mean diameters of these two neuronal types (P greater than .05). Quantitative data concerning the pontospinal enkephalinergic neurons correlated well with previous data on pontospinal catecholaminergic neurons (Reddy et al., Brain Res. 491:144-149, '89). A majority of the descending neurons from the dorsolateral pontine tegmentum contain enkephalin (72-80%) and catecholamine (80-87%). The observations suggest that enkephalin is contained in many of the pontospinal catecholaminergic neurons.

Ascending unmyelinated primary afferent fibers in the dorsal funiculus

The primary purpose of the present study is to obtain evidence as to the destination of the recently discovered unmyelinated primary afferent fibers in the mammalian dorsal funiculus. To do this rat dorsal roots were transected unilaterally from segments T8 or T9 caudally, and the numbers of axons were determined in the C3 fasciculus gracilis in normal animals and from both sides of the rhizotomied animals. In addition, C3 fasciculus gracilis counts were done in animals that had complete T6 or T10 spinal transections. The data indicate that there is an 80% loss of unmyelinated axons ipsilaterally and a 60% loss contralaterally in the fasciculus gracilis of the rhizotomied animals. These findings are interpreted as indicating that a significant fraction of the unmyelinated fibers in the fasciculus gracilis ascend, presumably to the nucleus gracilis in the brain stem, and also that a significant number of these fibers branch. We also provide evidence for contralateral myelinated primary afferent fiber projection in the fasciculus gracilis and show that the myelinated primary afferent fibers seem to be a more diverse population than the unmyelinated primary afferent fibers in the C3 fasciculus gracilis.

Hypothalamic innervation of the pituitary in the catfish, Clarias batrachus (L.): a retrograde horseradish peroxidase study

Intrahypophysial administration of horseradish peroxidase (HRP) resulted in extensive labelling of the cells of nucleus preopticus and nucleus lateralis tuberis. Besides, isolated retrogradely labelled neurons were observed in the nucleus preopticus periventricularis in the preoptic area, nucleus of the horizontal commissure, nucleus hypothalamicus ventralis in the rostral tuberal area, and nucleus arcuatus hypothalamicus in the caudal tuberal area. A few labelled cells were also observed in the mamillary region.

PNMT-containing neurons in the rostral medulla oblongata (C1, C3 groups) are transneuronally labelled after injection of herpes simplex virus type 1 into the adrenal gland

Herpes simplex virus type 1 (HSV1) was injected into the rat adrenal gland. After 3 days the rat CNS was processed immunohistochemically to demonstrate viral antigen. In the lower thoracic spinal cord viral antigen was found in neurons in the intermediolateral column. In the medulla oblongata HSV1-positive neurons were found in the raphe pallidus and in the C1 and C3 regions of the rostral medulla. Approximately 50% of HSV1-positive neurons in the C1 and all the HSV1-positive neurons in the C3 area also contained phenylethanolamine N-methyltransferase (PNMT) and were thus identified as C1 and C3 cells. The HSV1-positive neurons in the C1 region which did not contain PNMT were also negative for tyrosine hydroxylase and were therefore not catecholamine-synthesizing neurons. The HSV1-positive neurons in the medulla oblongata were presumably transsynaptically labelled from the adrenal gland and our study therefore provides neuroanatomical evidence supporting the view that some C1 neurons are involved in controlling the function of the adrenal gland.

Coexistence of varying combinations of neuropeptides with 5-hydroxytryptamine in neurons of the raphe pallidus et obscurus projecting to the spinal cord

The coexistence of varying combinations of substance P (SP), somatostatin (SOM), thyrotropin-releasing hormone (TRH) and met-enkephalin-Arg-Gly-Leu (ENK) with 5-hydroxytryptamine (5-HT) as semiquantitatively revealed by immunocytochemistry in neuronal perikarya of the raphe pallidus et obscurus in the guinea-pig was analyzed. SOM coexisted most frequently with 5-HT, followed by SP, ENK and TRH. Many 5-HT neurons were immunoreactive to 2 or more peptides such as SP/SOM, SOM/ENK, SP/ENK, SOM/TRH, SP/TRH or SOM/SP/ENK. Most of these neurons were shown to project to the spinal cord by retrograde HRP labeling combined with immunocytochemistry. After hemisection of the cervical spinal cord at the C5 level, ENK and 5-HT immunoreactive nerve terminals in the ipsilateral intermediolateral nucleus of the thoracic spinal cord were decreased in number. The results indicate that neurons in the raphe pallidus et obscurus projecting to the spinal cord can be classified into subpopulations according to which peptides coexist with 5-HT, and may have different functions.

Development of the spinal projections of the nucleus paraventricularis hypothalami of the rat: an intra-uterine WGA-HRP study

The development of the spinal projections of the nucleus paraventricularis hypothalami (PVH) was studied by injecting wheatgerm-agglutinated horseradish peroxidase into the upper thoracic spinal cord of fetal rats at developmental ages ranging from embryonic day 16 (E16) to E21, and into the thoracic spinal cord of rat pups and adult rats. At E18 the first retrogradely labeled cells are seen in the PVH. These cells are located dorsally in the PVH, adjacent to the ventricular matrix. At E20 a more ventrally located group of retrogradely labeled cells appears. This ventral group joins caudally with the dorsal group of labeled cells. The dorsal group of labeled cells is closely packed, while the ventral group of labeled cells is intermingled with unlabeled cells. After E20 this spatial configuration remains essentially constant. Therefore descending projections from all spinal projecting cell groups in the PVH have reached lower cervical spinal cord levels at E20.

Diencephalic syndrome following cervical spinal cord trauma

The authors discuss about five cases of diabetes insipidus observed in patients affected by traumatic cervical spine fractures and/or dislocations, without either evident lesions of the cerebral structures at CT scan examination, or important craniocerebral trauma. In all patients polyuria and hyperthermia arose some days after the traumatic accident and regressed spontaneously or after exogeneous vasopressin administration. Vasopressin urinary levels confirmed the presence of a true diabetes insipidus, the origin of which is in largely obscure. A central medullary vasopressin mediated pathway, demonstrated only in experimental animals, may be responsible for such a finding.

Descending pathways to the spinal cord: II. Quantitative study of the tectospinal tract in 23 mammals

To study the early evolution of the mammalian motor systems, we have collected quantitative data on the nuclear origins of tracts descending into the spinal cord in 99 individuals representing 23 species of mammals and one species of reptile. In each individual, the spinal cord was hemisected at the C1-C2 junction and raw HRP immediately applied to the cut fibers. After a 3-day survival period, brain and spinal cord sections were treated with conventional tetramethylbenzidine procedures. In every case, this procedure resulted in heavy retrograde labeling of neural somata throughout the neuraxis from coccygeal cord to cerebral neocortex. Many thousands of supraspinal neurons were vividly labeled within at least 27 discrete cell groups in every mammal (Nudo and Masterton, '88). Despite the vast number and wide diversity of heavily labeled neurons, however, relatively few labeled somata were found in the superior colliculus. The total number of labeled cells in the tectum contralateral to the hemisection was highest in the cat (909) and second highest in the raccoon (628). In the remaining animals, the number was considerably less--averaging only 243 in the 23 mammalian species, 193 in the 21 noncarnivores, and 95 in the iguana. In 7 species of primates the average was 220, and in 3 species of Old World monkeys the average was 142. This wide variation in the number of tectospinal neurons is not related to body size, brain size, or absolute and relative tectum size. Arranging the animals in order of their kinship or recency-of-last-common-ancestor with Man, the average number of labeled tectal cells tends to decrease slightly, whereas arranging the same animals in order of their kinship with the cat or raccoon shows a marked and statistically reliable increase. Neither the evolutionary increase in the tectospinal tract along the Carnivora lineage nor the slight decrease along Man's lineage is altered by mathematical corrections for allometric or scaling factors. Of an array of morphological, visual, motor, and ecological traits tested statistically as a possible source of the variation in size of the tectospinal tract, only a primarily carnivorous feeding preference was found to be reliably related. The relatively small number of tectospinal fibers in most mammals in our sample, including the primates, suggests that the tectospinal tract in Man may be quite small, perhaps far too small to warrant continuing description as a "major descending tract."

Brainstem projections to cats' upper lumbar spinal cord: implications for abdominal muscle control

Unilateral L1-L2 gray matter HRP injections labeled neurons bilaterally in nucleus retroambiguus (expiratory neuron region of caudal ventral respiratory group) and ventromedial reticular formation. Minor labeling occurred in raphe, vestibular, and medial parabrachial nuclei, lateral reticular formation, and C1-C2. Midsagittal lesions between C1 and obex prevented nucleus retroambiguus labeling (except 1 contralateral cell adjacent an incomplete lesion), indicating that these neurons decussate between C1 and obex and have collaterals that recross the spinal cord.

Collateral projections of predorsal bundle cells of the superior colliculus in the rat

The deep layers of the superior colliculus contain cells which are premotor in the sense that they respond prior to the onset of shifts in gaze and send axons, by way of a pathway called the predorsal bundle, to the contralateral brainstem gaze centers and cervical spinal cord. Previous studies have suggested that these cells also contribute to other efferent pathways which arise in the deep layers. The present study examines the contributions of the cells of origin of the predorsal bundle to these additional pathways as a step toward understanding their roles in gaze mechanisms. In one series of experiments, retrograde tracers were used to compare the laminar distribution of predorsal bundle cells with the distributions of the cells of origin of three other pathways: those that project to the intralaminar region of the dorsal thalamus, those that project to the contralateral superior colliculus, and those that project to the ipsilateral brainstem tegmentum. Predorsal bundle cells were found primarily in stratum griseum intermedium sublayer b. This distribution overlaps extensively with the distribution of colliculus cells that project to the intralaminar region of the thalamus. In contrast, the majority of the colliculus cells that project to either the contralateral superior colliculus or the ipsilateral brainstem tegmentum do not overlap extensively with the predorsal bundle cells; instead, they are primarily located dorsal or ventral to sublayer b of stratum griseum intermedium. In a second series of experiments, two regions were injected with different retrograde fluorescent traces in single animals in order to study the collateral projections of the cells of origin of these pathways. The results indicate that many predorsal bundle cells project to the intralaminar region of the dorsal thalamus but that only a few contribute to the tectotectal pathway. The results also indicate that few tectotectal cells contribute to the ipsilateral tectobulbar pathway.

Projections from the rostral mesencephalic reticular formation to the spinal cord. An HRP and autoradiographical tracing study in the cat

Eye and head movements are strongly interconnected, because they both play an important role in accurately determining the direction of the visual field. The rostral brainstem includes two areas which contain neurons that participate in the control of both movement and position of the head and eyes. These regions are the caudal third of Field H of Forel, including the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) and the interstitial nucleus of Cajal with adjacent reticular formation (INC-RF). Lesions in the caudal Field H of Forel in monkey and man result in vertical gaze paralysis. Head tilt to the opposite side and inability to maintain vertical eye position follow lesions in the INC-RF in cat and monkey. Projections from these areas to extraocular motoneurons has previously been observed. We reported a study of the location of neurons in Field H of Forel and INC-RF that project to spinal cord in cat. The distribution of these fiber projections to the spinal cord are described. The results indicate that: 1. Unlike the neurons projecting to the extra-ocular muscle motoneurons, the major portion of the spinally projecting neurons are not located in the riMLF or INC proper but in adjacent areas, i.e. the ventral and lateral parts of the caudal third of the Field H of Forel and in the INC-RF. A few neurons were also found in the nucleus of the posterior commissure and ventrally adjoining reticular formation. 2. Neurons in caudal Field H of Forel project, via the ventral part of the ventral funiculus, to the lateral part of the upper cervical ventral horn. This area includes the laterally located motoneuronal cell groups, innervating cleidomastoid, clavotrapezius and splenius motoneurons. At lower cervical levels labeled fibers are distributed to the medial part of the ventral horn. Projections from the caudal Field H of Forel to thoracic or more caudal spinal levels are sparse. 3. Neurons in the INC-RF, together with a few neurons in the area of the nucleus of the posterior commissure, project bilaterally to the medial part of the upper cervical ventral horn, via the dorsal part of the ventral funiculus. This area includes motoneurons innervating prevertebral flexor muscles and some of the motoneurons of the biventer cervicis and complexus muscles. Further caudally, labeled fibers are distributed to the medial part of the ventral horn (laminae VIII and adjoining VII) similar to the projections of Field H of Forel. A few INC-RF projections were observed to low thoracic and lumbosacral levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Origins of the descending spinal projections in petromyzontid and myxinoid agnathans

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

Transneuronal transfer of herpes virus from peripheral nerves to cortex and brainstem

The transneuronal transfer of neurotropic viruses may represent an effective tool for tracing chains of connected neurons because replication of virus in the recipient neurons after transfer amplifies the "tracer signal." Herpes simplex virus type 1 was transferred transneuronally from forelimb and hindlimb nerves of rats to the cortical and brainstem neurons that project to the spinal enlargements to which the nerves receiving injections are connected. This transneuronal transfer of herpes simplex virus type 1 from peripheral nerves has the potential to be used to identify neurons in the brain that are related transsynaptically to different nerves and muscles.

Interconnections between hypothalamus and cerebellum

The cerebellum and hypothalamus are interconnected through a multitude of direct (monosynaptic) and indirect (polysynaptic) pathways. Direct hypothalamocerebellar fibres are mainly uncrossed and reach all parts of the cerebellar cortex and nuclei. They are neither mossy fibres nor climbing fibres, but appear to terminate in all layers of the cerebellar cortex as multilayered fibres. At least some of the hypothalamocerebellar fibres are histaminergic, and it appears that a small proportion of the hypothalamocerebellar neurons contain GABA. Indirect hypothalamocerebellar connections may be relayed through various brain stem nuclei. The hypothalamo-ponto-cerebellar pathway, which has a contralateral predominance, appears to be the quantitatively most important of these. The direct cerebellohypothalamic projection originates from the cerebellar nuclei and terminates in the posterior hypothalamus, in the same regions where the direct hypothalamocerebellar pathway has its main origin. Indirect cerebellohypothalamic connections with brain stem relays have also been demonstrated. The functions of hypothalamocerebellar circuits are so far unknown. However, these pathways are probably involved in the coordination and integration of somatic as well as non-somatic responses to a given set of inputs.

Neuronal and neurochemical mechanisms of hypothalamic inhibition of the nociceptive reflex

Using cats anesthetized with chloralose-Nembutal we have studied the effect of high frequency stimulation of the medial and lateral structures of the posterior, tuberal, and anterior hypothalamus, and also of the central gray matter of the midbrain on the nociceptive jaw-opening reflex induced by tooth pulp stimulation. We recorded the EMG response of the digastric muscle as the index of the nociceptive reflex. We have shown that the EMG response of the nociceptive reflex is effectively suppressed on stimulation of all hypothalamic structures and the central gray matter, the threshold of the suppressive action being lower on stimulation of the central gray matter. The effects of the hypothalamic suppression of the nociceptive reflex were eliminated almost completely after naloxone administration, with the exception of the central gray matter, which is slightly more resistant to the action of this agent. After bilateral electrolytic destruction of the central gray matter the antinociceptive effect of the hypothalamus was retained, decreasing only insignificantly, The effect of complete suppression of the amplitude of the EMG response, similar to the effect of stimulation of the hypothalamus and the central gray matter, was also observed after intravenous administration of phenapidine, an opiate agonist with a marked central analgesic action. The neuronal and neurochemical mechanisms of hypothalamic suppression of the nociceptive jaw-opening reflex are discussed.

Parallel suppression of extensor muscle tone and respiration by stimulation of pontine dorsal tegmentum in decerebrate cat

This paper describes the pontine brainstem area responsible for the suppression of postural muscle tone as well as of respiration in acute precollicular-postmammillary decerebrate (mesencephalic) cats. Stimulation of the dorsal part of the pontine tegmentum (DTF) along the midline (P4-P7, H-5 to H-6) decreased the bilateral tone of the hindlimb extensor muscles and the diaphragmatic activity. Tonic discharges of the extensor muscles were suppressed by DTF stimulation and the suppression of muscle activity continued for more than 5 min after termination of the stimulation. In contrast, the suppression of the diaphragmatic activity, which resulted in apnea in some of the animals tested, resumed in spite of the continuation of the stimulation. However, the rebound augmentation of the diaphragmatic activity appeared immediately after the termination of the stimulation. The existence of such a rebound phenomenon suggested that the suppressive effects on the diaphragmatic activity persisted during the entire period of the stimulation. The recovery of respiratory movements during the stimulation led us to suggest that the strong respiratory drives emerge to overcome the exerted DTF-elicited suppressive effects on respiration. In the paralyzed and vagotomized animal, the DTF-elicited suppressive effects on phrenic neural discharges were minimal when the end-tidal pCO2 was set at a higher level than during spontaneous breathing.

The nuclei of origin of monoaminergic, peptidergic, and cholinergic afferents to the cat nucleus reticularis magnocellularis: a double-labeling study with cholera toxin as a retrograde tracer

Using a sensitive double-immunostaining technique with nonconjugated cholera toxin B subunit (CT) as a retrograde tracer, we examined the cells of origin and the histochemical nature of afferents to the cat nucleus reticularis magnocellularis (Mc) of the medulla oblongata. After injections of CT confined to the Mc, we found that the major afferents to the Mc arise from: (1) the lateral part of the bed nucleus of the stria terminalis, the nucleus of the anterior commissure, the preoptic area, the central nucleus of the amygdala, the posterior hypothalamus, and the nucleus of the fields of Forel; (2) the Edinger-Westphal nucleus, the mesencephalic reticular formation, and the ventrolateral part of the periaqueductal grey; (3) the nuclei locus coeruleus alpha (LC alpha), peri-LC alpha, locus subcoeruleus, and reticularis pontis oralis and caudalis; (4) the caudal raphe nuclei; and (5) the nucleus reticularis ventralis of the medulla.(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.

Afferent and efferent connections of brainstem locomotor regions: study by means of horseradish peroxidase transport technique

Afferent and efferent connections of the hypothalamic and mesencephalic locomotor regions and also the bulbar locomotor strip were studied in cat using retrograde (horseradish peroxidase) transport technique. To study the sources of afferent projections, the enzyme microinjections were performed exactly into the same brain sites eliciting treadmill locomotion by means of electrical stimulation. When studying efferent projections horseradish peroxidase labeled neurons were revealed within locomotor regions after enzyme microinjections into different brain structures. Experimental data have shown that the hypothalamic and mesencephalic locomotor regions have mutual afferent and efferent projections with numerous brain areas including interconnections. Apart from the entopeduncular nucleus, the great number of different sensory nuclei are noted: among the sources of afferent projections are the nucleus tractus spinalis nervi trigemini, nucleus cuneatus, nucleus tractus solitarius and vestibular nuclei. In addition, after horseradish peroxidase injection into the mesencephalic locomotor region labeled neurons were found in the cochlear nuclei. Direct descending neuronal projections of the hypothalamic and mesencephalic locomotor regions are distributed mainly in the ipsilateral brainstem. Only a few of them reach the lumbar spinal cord. The most marked efferent projections of given regions are those to the brainstem reticular formation. After horseradish peroxidase injection into a functionally identified bulbar locomotor strip, labeled neurons were revealed in different stem regions mainly caudal to the enzyme injection site. The existence of a locomotor strip as an independent structural formation is called into question. When studying the locomotor region connections, the structural heterogeneity of these regions is revealed. Transitory fibers of ascending tracts are presumably within their limits side by side with neurons. The role of these fibers in stepping initiation by electrical stimulation of locomotor regions remains uncertain.

Morphological and electrophysiological identification of gigantocellular tegmental field neurons with descending projections in the cat: II. Bulb

There are three different descending projections from the bulbar gigantocellular tegmental field (BFTG) in the cat, as defined by intracellular recording and intracellular horseradish peroxidase (HRP) techniques. The first pathway arises from neurons which send axons to the contralateral medial longitudinal fasciculus (cMLF neurons); cMLF neurons show excitatory postsynaptic potentials (EPSPs) after stimulation of the ipsilateral pontine gigantocellular tegmental field (PFTG). Most cMLF neurons have large ellipsoid-polygonal somata (mean, 56.8 micron), thick axons (average diameter, 3.09 microns), mostly non-spiny dendrites and dendritic fields flattened in the anteroposterior direction. No cMLF neurons with axon collaterals in the BFTG are present in the data of this study. The second pathway arises from neurons which send axons to the ipsilateral MLF (iMLF neurons); iMLF neurons show EPSPs after stimulation of the ipsilateral PFTG. Most iMLF neurons have large ellipsoid-polygonal somata (mean, 60.2 microns), thick axons (average diameter, 3.00 microns), mostly non-spiny dendrites and dendritic fields that are only slightly flattened in the anteroposterior direction. As with cMLF neurons, no iMLF neurons with axon collaterals in the BFTG are present in the data of this study. The third pathway arises from neurons that send axons directly into the ipsilateral caudal bulbar reticular formation (iBRF neurons). Most iBRF neurons have smaller ellipsoid-polygonal somata (mean, 38.6 microns), thinner axons (average diameter, 1.84 microns), mostly nonspiny dendrites, and dendritic fields that are flattened in the anteroposterior direction. In contrast to cMLF and iMLF neurons, axon collaterals are present in 73% of iBRF neurons. About half of iBRF neurons have bifurcated axon collaterals with both anterior and posterior projections, and in these neurons antidromic spike potentials are elicited by stimulation of the ipsilateral PFTG.

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.