Evidence for direct bulbospinal projections to laminae IX, X and the intermediolateral cell column. Studies using axonal transport techniques in the North American opossum

Spinal projecting neurons in rostral ventromedial medulla co-regulate motor and sympathetic tone

Many behaviors require the coordinated actions of somatic and autonomic functions. However, the underlying mechanisms remain elusive. By opto-stimulating different populations of descending spinal projecting neurons (SPNs) in anesthetized mice, we show that stimulation of excitatory SPNs in the rostral ventromedial medulla (rVMM) resulted in a simultaneous increase in somatomotor and sympathetic activities. Conversely, opto-stimulation of rVMM inhibitory SPNs decreased both activities. Anatomically, these SPNs innervate both sympathetic preganglionic neurons and motor-related regions in the spinal cord. Fiber-photometry recording indicated that the activities of rVMM SPNs correlate with different levels of muscle and sympathetic tone during distinct arousal states. Inhibiting rVMM excitatory SPNs reduced basal muscle and sympathetic tone, impairing locomotion initiation and high-speed performance. In contrast, silencing the inhibitory population abolished muscle atonia and sympathetic hypoactivity during rapid eye movement (REM) sleep. Together, these results identify rVMM SPNs as descending spinal projecting pathways controlling the tone of both the somatomotor and sympathetic systems.

Organization of brain somatomotor-sympathetic circuits

Numerous physiological and emotionally motivated behaviors require concomitant activation of somatomotor and sympathetic efferents. Likewise, adaptive and maladaptive responses to stress are often characterized by simultaneous recruitment of these efferent systems. This review describes recent literature that outlines the organization of somatomotor-sympathetic circuitry in the rat. These circuits were delineated by employing recombinant pseudorabies (PRV) viral vectors as retrograde trans-synaptic tract tracers. In these studies PRV-152, a strain that expresses enhanced green fluorescent protein, was injected into sympathectomized hindlimb muscle, while PRV-BaBlu, which expresses beta-galactosidase, was injected into the adrenal gland in the same animals. Immunofluorescent methods were then used to determine the presence of putative dual-function neurons that were infected with both viral strains. These somatomotor-sympathetic neurons (SMSNs) were detected in a number of brain regions. However, the most prominent nodes in this circuitry included the paraventricular, dorsomedial, and lateral nuclei of the hypothalamus, ventrolateral periaqueductal grey and ventromedial medulla. Phenotypic studies revealed subsets of SMSNs to be capable of synthesizing serotonin, or to contain neuroactive peptides vasopressin, oxytocin, orexins, or melanin-concentrating hormone. Based on these data and the results of studies employing monosynaptic tracers a central somatomotor-sympathetic circuit is proposed. This circuitry is likely recruited in diverse situations, including stress responses, cold defense, exercise and sleep. Furthermore, activation of specific classes of SMSNs likely shapes distinct stress-coping strategies. Dysregulation in the organization and function of this circuit may also contribute to the expression of physical symptoms of affective disorders, such as major depression, anxiety and panic.

Relationship of presympathetic-premotor neurons to the serotonergic transmitter system in the rat brainstem

Numerous physiological conditions and emotionally motivated behaviors require concomitant activation of somatomotor and sympathetic efferents. Using a virally mediated retrograde transsynaptic tract-tracing approach, we have previously determined locations of presympathetic-premotor neurons (PSPMNs) in the rat brainstem. These putative dual-function neurons send projections to somatomotor and sympathetic targets and likely participate in sympatho-somatomotor integration. A significant portion of these neurons is found within brainstem areas known to contain serotonergic neurons. Thus, we hypothesized that some of the PSPMNs utilize serotonin as their neurotransmitter. To test this hypothesis we first produced an antibody against TPH2, a brain-specific isoform of tryptophan hydroxylase (serotonin synthetic enzyme). We identified PSPMNs by using recombinant strains of the pseudorabies virus (PRV) for transsynaptic tract-tracing. PRV-152, a strain that expresses enhanced green fluorescent protein, was injected into sympathectomized gastrocnemius muscle, while PRV-BaBlu, which expresses beta-galactosidase, was injected into the adrenal gland in the same animals. Using immunofluorescent methods we determined whether coinfected neurons expressed TPH2. Our findings demonstrate that TPH2-positive PSPMNs are present at different rostrocaudal levels of the brainstem. Just over half of them are found at the pontomedullary junction within raphe obscurus, raphe magnus, and gigantocellular nucleus pars alpha. These cells may play a role in mediating responses to acute pain stimuli and/or participate in the central control of exercise. Overactivity of these serotonergic sympatho-somatomotor circuits may also play a role in the pathophysiology of serotonin syndrome.

Brainstem substrates of sympatho-motor circuitry identified using trans-synaptic tracing with pseudorabies virus recombinants

Previous physiological investigations have suggested the existence of a neural circuit that coordinates activation of motor and autonomic efferents before or at the onset of exercise. Traditionally these circuits have been postulated to involve forebrain areas. However, overlapping populations of medullary reticular formation neurons that participate in motor or autonomic control have been described previously, suggesting that individual pontomedullary reticular formation neurons may coordinate both motor and autonomic responses. We tested this hypothesis by conducting transneuronal retrograde tracing of motor and sympathetic nervous system pathways in rats using recombinant strains of pseudorabies virus (PRV). A PRV strain expressing the green fluorescent protein (PRV-152) was injected into the left gastrocnemius muscle, which was surgically sympathectomized, whereas another recombinant (PRV-BaBlu) was injected into the left adrenal gland. Immunofluorescence methods using monospecific antisera and distinct fluorophores identified neurons infected with one or both of the recombinants. Brainstem neurons coinfected with both PRV recombinants, which presumably had collateralized projections to both adrenal sympathetic preganglionic neurons and gastrocnemius motoneurons, were observed in several areas of the pontomedullary reticular formation. The largest number of such neurons was located in the rostral ventromedial medulla within the ventral gigantocellular nucleus, gigantocellular nucleus pars alpha, raphe obscurus, and raphe magnus. These neurons are candidates for relaying central command signals to the spinal cord.

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.

Origin of serotoninergic afferents to the hypoglossal nucleus in the rat

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

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

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

[125I]vasoactive intestinal polypeptide binding sites: quantitative autoradiographic distribution in the rat spinal cord

The quantitative autoradiographic distribution of [125I]vasoactive intestinal polypeptide (VIP) receptor binding sites was investigated in the rat spinal cord. [125I]VIP binding sites are discretely distributed, with a rostro-caudal gradient, along the longitudinal length of the cord; highest densities of sites being observed in its lumbar and sacral segments. In transverse sections, highest levels of [125I]VIP sites are present in laminae I and II, around the central canal, and in the parasympathetic lateral horn of the sacral segment. Moderate densities are seen along the medial border of the dorsal horn and the sympathetic lateral horn of the thoracic cord. Low amounts of labeling are observed in most structures of the ventral horn while white matter areas are apparently devoid of specific [125I]VIP binding. Thus, the distribution of spinal [125I]VIP receptor sites correlates well with that of VIP-like immunoreactive materials and support possible roles for this peptide in sensory neurotransmission and in the control of autonomic functions.

Ultrastructural localization of substance P, met-enkephalin, and somatostatin immunoreactivity in lamina X of the primate spinal cord

The ultrastructural localization of substance P (SP), met-enkephalin (MENK), and somatostatin (SS) in the lamina X area surrounding the central canal of the macaque monkey was examined by the indirect peroxidase-antiperoxidase method. The most common synaptic terminals in lamina X were simple terminals (S) with small rounded or pleomorphic clear vesicles; one to two dense-core vesicles were occasionally also present. These were found on soma, dendrites, and dendritic spines, in all regions of lamina X. A second class of terminal with round or oval clear vesicles was glomerular (G) in shape, with scalloped edges, and contained many mitochondria. These large terminals had several synaptic contacts onto dendrites, spines, and small terminals and were found mainly in the lateral region. The third class (L) contained small clear vesicles and several vesicles with large, dense cores (100-125 nm), and also contacted dendrites, mainly lateral to the canal. The fourth class of terminal (D) contained small clear vesicles and several vesicles with small, dense cores (75-100 nm); these contacted dendrites and somata in all areas. Very few terminals with flat vesicles were identified. There was an unequal distribution of immunoreactivity among the several terminal classes identified in lamina X. Most SP terminals were S terminals, but SP L terminals were also common; few were D terminals. MENK terminals were usually either S terminals or D terminals; L terminals were rarely MENK positive. SS terminals were commonly D terminals or S terminals; L terminals were also rarely SS positive. Only SP terminals were identified as G terminals. Synaptic targets of SP, MENK, and SS terminals were most commonly dendrites. In addition to unlabelled neurons, peptidergic neurons and their processes were also synaptic targets of terminals containing the same peptide. The distributions of these peptides in primate lamina X differ from that of the same peptides in primate superficial dorsal horn. These differences are important, in consideration of some of the parallels that may be drawn between the lamina X area and the superficial dorsal horn; both areas have high concentrations of the same peptides, receive nociceptive primary afferents, and contain spinothalamic and other projection neurons. Nevertheless, comparison of the distribution of immunoreactivity among terminal classes indicates that neurochemical organization at the ultrastructural level is quite distinct in each of the two areas. This may also reflect other roles of the lamina X area, including its involvement in visceral functions, although it would be expected that this element might be less prominent at the cervical levels we investigated.

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.

Termination patterns of serotoninergic medullary raphespinal fibers in the rat lumbar spinal cord: an anterograde immunohistochemical study

Electrical and chemical stimulation given in the ventral medullary raphe nuclei inhibits spinal nociceptive reflexes and spinal nociceptive transmission; serotoninergic receptors have been demonstrated to partially mediate that inhibition. In the present study, the termination patterns of raphespinal fibers in the rat lumbar spinal cord demonstrating serotonin-like immunoreactivity were examined by using the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in combination with immunohistochemistry. Fibers and terminations from the ventral medullary raphe nuclei (raphe magnus and raphe pallidus) demonstrating both PHA-L- and serotonin-like immunoreactivity were identified in all laminae of the dorsal horn and the ventral horn. Networks of large fibers, characterized by large boutons, and which did not demonstrate serotonin-like immunoreactivity, were identified in deeper laminae of the dorsal horn. The heterogeneous morphology of raphespinal fibers identified in the dorsal horn suggests that these fibers also may be heterogeneous in neurochemistry and function. Medial medullary sites outside the raphe nuclei were found to innervate the ventral horn and all laminae of the dorsal horn, with the exception of lamina I. Descending fibers and terminations also demonstrating serotonin-like immunoreactivity were identified in deep laminae (III, IV, V, VI) of the dorsal horn and in the ventral horn. Similarly, large fiber networks were identified which did not demonstrate serotonin-like immunoreactivity.

Cardiovascular responses to electrical stimulation of the ventrolateral medulla of the spontaneously hypertensive rat

Cardiovascular responses to electrical microstimulation of the ventrolateral medulla were investigated in both Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) under pentobarbital anesthesia. The threshold intensity required to elicit a change in blood pressure (BP) and the cardiovascular responses in these two groups of rats upon electrical stimulation were compared. It was found that the region with the lowest threshold intensities was located in the rostral ventrolateral medulla (RVL) and the threshold intensities were much lower in SHR than in WKY. Electrical stimulation of this brain region also resulted in a greater increase in BP during stimulation in SHR, compared to control. In SHR, upon termination of stimulation, the BP dropped to a level above the pre-stimulation level and this was followed by a prolonged, sustained elevation in BP before returning to the control level, whereas in WKY, the BP showed an initial drop to below the pre-stimulation level and then returned to the control level. These results suggest an enhanced responsiveness to electrical stimulation in SHR. Although the heart rate (HR) increased to a similar extent during electrical stimulation in both groups of rats, upon termination of stimulation WKY exhibited bradycardia followed by tachycardia before the HR returned to the pre-stimulation level, whereas SHR exhibited tachycardia which was maintained for a substantial period of time before returning to the pre-stimulation level. The results suggest a lower baroreceptor sensitivity in SHR. The change in HR/change in BP was smaller in SHR than in WKY, suggesting that the increase in HR may not contribute to the increase in BP during stimulation as much in SHR as in WKY.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular and muscle tone changes produced by microinjection of cholinergic and glutamatergic agonists in dorsolateral pons and medial medulla

Cardiovascular and muscle responses to L-glutamic acid (Glut) and cholinergic agonists injected into the dorsolateral pontine tegmentum and medial medullary reticular formation (MMRF) were examined in unanesthetized, decerebrated cats. Glut, or cholinergic agonists acetylcholine (ACh) or carbachol (Carb), were injected into pons and MMRF at sites from which electrical stimulation produced bilateral suppression of muscle tone. Glut injection in MMRF produced hypotension without change in heart rate at doses as low as 1 mM. At higher doses (0.1-0.4 M), Glut induced hypotension with bradycardia in 23 out of 40 injections in both pons and MMRF. High concentrations of microinjected Glut decreased muscle tone or produced complete atonia in pons and rostral MMRF. Both N-methyl-D-aspartic acid (NMDA) and non-NMDA receptor blockers attenuated or completely blocked the cardiovascular response, while only non-NMDA antagonists blocked muscle inhibition to Glut injection. Microinjection of cholinergic agonists produced consistent hypotension in all of the injections in pons and MMRF, however, the heart rate response was variable with increase (27/42), decrease (2/42), or no change (13/42) in rate seen. Cholinergic injection produced muscle atonia in pons and caudal MMRF but not in rostral MMRF. Both muscle and cardiovascular responses were blocked by atropine but not by hexamethonium. The time course of muscle atonia and cardiovascular change differed in most of the experiments. We conclude that muscle tone suppression and cardiovascular response to Glut or cholinergic agonists use different receptor mechanisms and possibly different neurons. However, the co-localization of these mechanisms suggests that neuronal networks in the medial medulla and dorsolateral pons coordinate motor and cardiovascular responses.

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

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

Intraspinal course of descending serotoninergic pathways innervating the rodent dorsal horn and lamina X

Although numerous studies have discussed serotoninergic pathways projecting from the brainstem to the spinal cord, the intraspinal course of these descending axons remains a matter of debate. One of the difficulties with traditional immunohistochemical methods is that the interpretation of results has been subjective. We have therefore employed a computer-assisted image-processing system to provide objective measurements of serotonin immuno-reactivity within the rodent dorsal horn and lamina X caudal to unilateral thoracic cordotomy in order to establish the intraspinal course of descending serotoninergic fibers. Lesion analysis combined with computer-determined optical density measurements, high-power microscopic examination, and camera lucida drawings demonstrated that the predominant serotoninergic pathway terminating within the dorsal horn courses unilaterally within the dorsal portion of the dorsolateral funiculus. The thoracic dorsal horn may receive a slightly greater contribution from the contralateral side than the lumbar dorsal horn. Lamina X receives serotoninergic innervation both from fibers descending within the ipsilateral dorsolateral funiculus and from more ventrally located ipsilateral pathways, and it also contains serotonin derived from other sources.

Electrophysiological identification of spinally projecting neurons in the lateral reticular nucleus of the rat

Eighty-four neurons in the caudal ventrolateral medullary reticular formation were antidromically activated by the stimulation of the dorsolateral funiculus in 49 urethane-anesthetized rats. Of 76 neurons, 37 had no spontaneous discharge. Of the neurons that had spontaneous discharges, 80% had firing rates between 0.1 and 15 Hz. The average conduction velocity, determined among 70 neurons, was 15.20 +/- 1.23 m/s, and 87% had conduction velocities within the range of 2-30 m/s. This study further confirms the existence of spinally-projecting neurons in the lateral reticular nucleus (LRN) of the caudal medulla, and some of them are probably responsible for the descending controls of nociception from the LRN.

Lamina X of primate spinal cord: distribution of five neuropeptides and serotonin

The distribution of substance P, somatostatin, cholecystokinin, vasoactive intestinal polypeptide, enkephalin and serotonin in axons, terminals and neurons was compared in the area surrounding the central canal (lamina X) at five representative levels of the monkey spinal cord, using peroxidase-antiperoxidase immunocytochemistry. Immunoreactive neurons containing each of the neurochemicals were identified. At the cervical, thoracic and lumbar levels the area lateral to the canal had dense terminal fields immunoreactive for each neurochemical. The dorsal commissural region, the pericanal area, and the ventral commissural area were supplied by some but not all of the substances. In the lower thoracic cord innervation extended into the dorsal midline area and into the ventromedial commissural region. In contrast, in the sacral cord, the dorsal commissural region could be subdivided on the basis of innervation, and the lateral region was densely supplied by only cholecystokinin and serotonin, while the sacral ventral commissure and the pericanal area were supplied by all six neurochemicals. The immunocytochemical mappings were compared with published maps of functional classes of neurons and with the distribution of primary afferents and descending fibers in lamina X. The dense peptidergic and serotonergic innervation in the lateral area and the dorsal commissural area corresponded particularly with the location of projection neurons and primary afferents described in other studies.

Bulbospinal thyrotropin-releasing hormone projections to the intermediolateral cell column: a double fluorescence immunohistochemical-retrograde tracing study in the rat

Whereas the neurochemistry of the peripheral autonomic nervous system has been well characterized, less is known concerning the neurotransmitters utilized by medullary projections onto sympathetic preganglionic neurons residing in the thoracolumbar spinal intermediolateral cell column. Retrograde transport of rhodamine-labeled fluorescent microspheres following discrete microinjection into the thoracic intermediolateral cell column was combined with immunohistochemistry to determine neuroanatomic location of thyrotropin-releasing hormone-immunoreactive neurons which project to the intermediolateral cell column in the rat. The ventromedial group of raphe nuclei including the nucleus raphe pallidus, obscurus, and magnus possessed the greatest number of medullary thyrotropin-releasing hormone-immunoreactive neurons which also contained rhodamine-labeled microspheres. High numbers of intermediolateral cell column-projecting thyrotropin-releasing hormone-immunoreactive neurons were also observed in nucleus reticularis paragigantocellularis lateralis and magnocellularis, the lateral reticular nucleus, and the superficial ventral (arcuate) medullary surface. Despite the observations that nucleus reticularis gigantocellularis, paramedianus, and ventralis pars beta project to the intermediolateral cell column, double-labeled cells were not observed in these nuclei. Furthermore, whereas the nucleus reticularis magnocellularis and gigantocellularis, and the lateral reticular nucleus displayed strong ipsilateral predominance in projecting to the intermediolateral cell column, other medullary reticular and raphe nuclei displayed bilateral projections. The present findings support the hypothesis that thyrotropin-releasing hormone-containing neurons in the ventral medulla project to the intermediolateral cell column, and may influence sympathetic preganglionic neurons.

Tonic GABAergic and cholinergic influences on pain control and cardiovascular control neurones in nucleus paragigantocellularis lateralis in the rat

In rats anaesthetised with alphaxalone/alphadolone (Saffan), bilateral microinjections of the GABA antagonist, bicuculline, into a restricted region of nucleus paragigantocellularis lateralis (PGL), ventromedial to the caudal pole of the facial nucleus, produced an increase in the latency of the tail flick response to noxious heat. The analgesia was always accompanied by a rise in mean arterial blood pressure but the time course of the cardiovascular and antinociceptive changes was different. Guanethidine (7 mg/kg i.v.) blocked the pressor response but had no effect on the magnitude or time course of the analgesia. In contrast, microinjection of physostigmine into PGL produced a pressor response but no change in the latency of the tail flick response. It is concluded that there are functionally distinct pools of neurones within PGL which respectively produce antinociception and changes in vasomotor activity. Ongoing activity in both types of neurone is regulated by a tonic inhibitory GABAergic influence. In addition, the cardiovascular neurones receive a tonic excitatory cholinergic input.

Differential control of cardiac and vasomotor activity by neurones in nucleus paragigantocellularis lateralis in the cat

1. In cats anaesthetized with chloralose, neuronal cell bodies were excited by micro-injection of DL-homocysteic acid (DLH). 2. Injections made into the ventrolateral medulla in the region of nucleus paragigantocellularis lateralis (p.g.l.) produced a rise in blood pressure accompanied by either an increase or decrease in heart rate according to the precise site of the injection. The tachycardia was blocked by propranolol (0.15-0.5 mg kg-1 I.V.) whilst bradycardia was abolished by vagotomy or atropine (0.2-1.5 mg kg-1 I.V.). 3. Recordings were made of blood flow to the hind-limb, renal and mesenteric vascular beds in order to study the haemodynamic changes separately in each region. 4. Vasoconstriction was seen in the renal and mesenteric beds and either vasodilatation or vasoconstriction could be evoked in hind-limb muscle. The vasodilatation in hind-limb muscle was resistant to atropine but significantly reduced by propranolol (0.15-1.5 mg kg-1 I.V.) and therefore was probably mediated mainly by circulating adrenaline. 5. The relative contribution by each vascular bed to the rise in peripheral resistance produced by micro-injection of DLH at any one site varied according to the site of the injection and there was a degree of topographical organization within p.g.l. The neuronal pools which elicit tachycardia, renal vasoconstriction and hind-limb dilatation were located at the rostral end of the nucleus whilst those which produced vasoconstriction in the hind-limb muscle and mesenteric vascular beds were represented further caudally.

Some anatomical observations on the projections from the hypothalamus to brainstem and spinal cord: an HRP and autoradiographic tracing study in the cat

The hypothalamus is closely involved in a wide variety of behavioral, autonomic, visceral, and endocrine functions. To find out which descending pathways are involved in these functions, we investigated them by horseradish peroxidase (HRP) and autoradiographic tracing techniques. HRP injections at various levels of the spinal cord resulted in a nearly uniform distribution of HRP-labeled neurons in most areas of the hypothalamus except for the anterior part. After HRP injections in the raphe magnus (NRM) and adjoining tegmentum the distribution of labeled neurons was again uniform, but many were found in the anterior hypothalamus as well. Injections of 3H-leucine in the hypothalamus demonstrated that: The anterior hypothalamic area sent many fibers through the medial forebrain bundle (MFB) to terminate in the ventral tegmental area of Tsai (VTA), the rostral raphe nuclei, the nucleus Edinger-Westphal, the dorsal part of the substantia nigra, the periaqueductal gray (PAG), and the interpeduncular nuclei. Further caudally a lateral fiber stream (mainly derived from the lateral parts of the anterior hypothalamic area) distributed fibers to the parabrachial nuclei, nucleus subcoeruleus, locus coeruleus, the micturition-coordinating region, the caudal brainstem lateral tegmentum, and the solitary and dorsal vagal nucleus. Furthermore, a medial fiber stream (mainly derived from the medial parts of the anterior hypothalamic area) distributed fibers to the superior central and dorsal raphe nucleus and to the NRM, nucleus raphe pallidus (NRP), and adjoining tegmentum. The medial and posterior hypothalamic area including the paraventricular hypothalamic nucleus (PVN) sent fibers to approximately the same mesencephalic structures as the anterior hypothalamic area. Further caudally two different fiber bundles were observed. A medial stream distributed labeled fibers to the NRM, rostral NRP, the upper thoracic intermediolateral cell group, and spinal lamina X. A second and well-defined fiber stream, probably derived from the PVN, distributed many fibers to specific parts of the lateral tegmental field, to the solitary and dorsal vagal nuclei, and, in the spinal cord, to lamina I and X, to the thoracolumbar and sacral intermediolateral cell column, and to the nucleus of Onuf. The lateral hypothalamic area sent many labeled fibers to the lateral part of the brainstem and many terminated in the caudal brainstem lateral tegmentum, including the parabrachial nuclei, locus coeruleus, nucleus subcoeruleus, and the solitary and dorsal vagal nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Enkephalin-like immunoreactive axon terminals make synapses with alpha-motoneurons in the chicken

Methionine (Met)-enkephalin (Enk)-like immunoreactive axon terminals on HRP-labeled alpha-motoneurons were studied using the indirect antibody peroxidase--anti-peroxidase technique in the chicken. Electron microscopically, Met-Enk-like immunoreactive axon terminals containing dense-cored vesicles and clear vesicles made axosomatic and axodendritic synapses with PLD alpha-motoneurons. Dense-cored vesicles were sometimes observed to be adjacent to the presynaptic membrane accompanied by the postsynaptic density. These results suggest morphologically that alpha-motoneurons may be directly modulated by enkephalins.

Immunohistochemical distribution of substance P, serotonin, and methionine enkephalin in sexually dimorphic nuclei of the rat lumbar spinal cord

The purpose of the present study was to identify chemically some potential inputs to lumbar motoneurons of the rat in the spinal nucleus of the bulbocavernosus, ventral motor pool, dorsolateral nucleus, and retrodorsolateral nucleus. Substance P-like immunoreactivity and serotonin-like immunoreactivity were found in all four motor nuclei, with dense immunoreactive profiles surrounding motoneurons and their processes. Enkephalin-like immunoreactivity was restricted to the sexually dimorphic nuclei, the spinal nucleus of the bulbocavernosus, and the dorsolateral nucleus. Within the spinal nucleus of the bulbocavernosus, enkephalin-like immunoreactive profiles were apposed to the processes of motoneurons but not their somata. In contrast, enkephalin-like immunoreactivity surrounded motoneuron somata in the medial part but not the lateral part of the dorsolateral nucleus, in the location of motoneurons projecting to the ischiocavernosus muscle. Moreover, the density of serotonin-like immunoreactivity was also greater in the medial part of the dorsolateral nucleus. On the basis of the chemo-architecture and the connections of the dorsolateral nucleus, we suggest the division of this motor column into a medial part composed of ischiocavernosus motoneurons surrounded by enkephalin- and serotonin-like immunoreactivity and a lateral part that contains neurons that project to the sphincter urethrae muscle. Total spinal transection severely depleted both serotonin-like and substance P-like material in the lumbar ventral horn. No changes in the distribution of enkephalin-like immunoreactivity were observed following this lesion. It is therefore suggested that in the ventral horn, substance P- and serotonin-like material are derived from supraspinal tracts, whereas enkephalin-like material is derived from intrinsic nerve cell bodies of the spinal cord.

Cardiovascular regulation and lesions of the central nervous system

The central nervous system has an important role in the second-to-second regulation of cardiac activity and vasomotor tone. Central lesions that lead to a disturbance in autonomic activity tend to cause electrocardiographic and pathological evidence of myocardial damage, cardiac arrhythmias, and disturbances of arterial blood pressure regulation. To a great extent such cardiovascular disturbances result from alterations in sympathetic activity. Similar alterations in sympathetic activity can occur under conditions of emotional stress and precipitate cardiac arrhythmias that can themselves lead to the syndrome of sudden death. Experimental and clinical evidence suggests that central neural mechanisms may be involved in this important human syndrome, but no central lesion has yet been identified to account for it. Recent experimental evidence, derived from hypertension research, suggests that chemical disturbances in the central nervous system, without accompanying structural lesions, may be found to explain cardiovascular disturbances such as sudden death and hypertension.

Topographic principles in the spinal projections of serotonergic and non-serotonergic brainstem neurons in the rat

The spinal projections from the raphe-associated brainstem areas containing serotonergic neurons were studied with aldehyde-induced fluorescence in combination with the retrograde fluorescent tracer True Blue in the rat. This technique makes it possible to determine simultaneously the projections of individual neurons and to detect whether serotonin is present in the same neurons. After tracer injections into the spinal cord retrogradely labeled serotonergic and non-serotonergic neurons were found in the medullary raphe nuclei and adjacent regions and to a lesser extent in association with the dorsal and median raphe nuclei in the mesencephalon. Large True Blue injections that covered one side of the spinal cord at mid-cervical level labeled about 60% of the ipsilaterally situated serotonergic neurons in the medullary raphe regions while the corresponding figure contralaterally was about 25%. On both sides a larger number of labeled non-serotonergic neurons were found; these were sometimes located dorsal to, but often intermingled with, the serotonergic cells. While the serotonergic projection from the mesencephalon could not be labeled from injections below cervical levels, the labeling in more caudal brainstem regions exhibited only minor variations depending on the rostrocaudal level of the spinal segment injected. Furthermore, quantitative data from injections at different levels indicate that the majority of the spinal-projecting neurons traverse most of the length of the cord. Summarizing the results obtained from small injections restricted to subregions of the cord we feel that it is possible to distinguish three fairly distinct pathways for spinal projections from the medullary raphe and adjacent regions: The dorsal pathway originates mainly from cells in the caudal pons and rostral medulla oblongata (rostral part of nucleus raphe magnus, nucleus raphe magnus proper, nucleus reticularis gigantocellularis pars alpha and nucleus paragigantocellularis). This pathway, which contains a large non-serotonergic component, descends through the dorsal part of the lateral funiculus and terminates mainly in the dorsal horn at all spinal cord levels. The intermediate pathway is largely serotonergic with its cell bodies located within the arcuate cell group (situated just ventral and lateral to the pyramids very close to the ventral surface of the brainstem) and in the nucleus raphe obscurus and pallidus and terminates in the intermediate grey at thoracolumbar and upper sacral levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Vasodilator and vasoconstrictor neurones of the ventrolateral medulla in the cat

Microinjections of D,L-homocysteic acid into the ventrolateral medulla, in the region of nucleus paragigantocellularis lateralis (PGL) which lies caudal to the facial nucleus and adjacent to the rostral third of the inferior olive, evoke a rise in arterial blood pressure and vasoconstriction in hindlimb muscle. Activation of a group of neurones located in a more rostral strip of tissue ventral to the facial nucleus produces vasodilatation in the hindlimb but no significant change in blood pressure. It appears that the ventrolateral medulla contains several subpopulations of neurones which can alter resistance to blood flow and hence distribution of flow and level of blood pressure by selective control of individual vascular beds.

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

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

Projections from the diencephalon and mesencephalon to nucleus paragigantocellularis lateralis in the cat

The distribution of labelled cells in the diencephalon and mesencephalon has been mapped following injections of horseradish peroxidase into nucleus paragigantocellularis lateralis in the cat. Most of the labelled cells were found ipsilateral to the injection site. A group of small and medium-sized labelled perikarya (11-40 microns in diameter) was present in the caudal part of the periaqueductal grey matter (A3-P2) and in the adjacent tegmentum. Small, round or fusiform cells (8-25 microns were labelled in the tuberal region of the hypothalamus in the dorsomedial hypothalamus and in the lateral hypothalamic area. It is suggested that the cardiovascular responses which can be elicited by stimulation in these regions of the periaqueductal grey and hypothalamus are mediated via a relay on to spinally projecting neurones in nucleus paragigantocellularis lateralis which synapse on sympathetic preganglionic neurones in the intermediolateral cell column.

Effects of spinal lesions on substance P levels in the rat sympathetic preganglionic cell column: evidence for local spinal regulation

Substance P has been localized to the neuropil of sympathetic preganglionic neurons in light and electron microscopic studies. Two recent reports have suggested that the majority of substance P in the rat intermediolateral cell column was contained in synaptic terminals of bulbospinal axons. However, previous investigations in our laboratory indicated the presence of major substance P spinal-sympathetic preganglionic neuron circuitry in pigeon. The present study used radioimmunoassay and immunohistochemistry to examine substance P levels in rat intermediolateral cell column following various spinal lesions in order to assess the relative contributions of bulbospinal and intraspinal substance P neurons to the substance P content of the intermediolateral cell column. The results from these experiments support the existence of both bulbospinal and intraspinal substance P-containing projections to the rat intermediolateral cell column. In addition, characterization of spinal cord substance P-like immunoreactivity by combined high performance liquid chromatography and radioimmunoassay, revealed that substance P in rat intermediolateral cell column was indistinguishable from synthetic substance P. Following transection of thoracic spinal cord, substance P-immunoreactive staining was still evident in the intermediolateral cell column caudal to the lesion. These substance P-positive fibers were studded with bouton-like swellings and appeared normal. Following high cervical hemisection, depletion of substance P (radioimmunoassay measurements) was bilateral and equal in the intermediolateral cell column: 25% depletion was observed after 7 days and 35% depletion after 14 days. However, rats which were hemisected at low cervical and/or mid-thoracic levels contained normal or elevated amounts of substance P in the intermediolateral cell column. Since substance P remains in the intermediolateral cell column following total transection, substance P spinal-sympathetic preganglionic neuron circuitry must exist. Additionally, depletion of substance P following high cervical hemisection suggests the existence of a substance P-containing, bilateral bulbospinal pathway to the intermediolateral cell column. The observation that substance P levels were normal or elevated following low cervical lesions raises the possibility that intraspinal substance P neurons can compensate for loss of substance P in the spinal cord. Sprouting or altered substance P metabolism and/or release by intraspinal substance P neurons could be responsible, suggesting an important homeostatic mechanism for maintaining substance P content within the intermediolateral cell column.

Collateralization of brainstem pathways in the spinal ventral horn in rat as demonstrated with the retrograde fluorescent double-labeling technique

The collateralization of brainstem pathways to the spinal ventral horn was studied in rat by means of injections of True Blue (TB) and Diamidino Yellow Dihydrochloride (DY .2HCl) at different levels in the spinal cord. TB (or DY .2HCl) was injected in the cervical gray and DY .2HCl (or TB) was injected ipsilaterally either at mid-thoracic or at lumbar levels. The retrogradely single- and double-labeled neurons were studied in the interstitial nucleus of Cajal, the lateral vestibular nucleus of Deiters, the nucleus (sub) coeruleus and the nucleus raphe pallidus, including the adjoining ventral medullary reticular formation. In all those brainstem nuclei many double-labeled neurons were present after both mid-thoracic and lumbar injections. This indicates that these brainstem spinal pathways to the ventral horn probably give off many collaterals along their trajectory in the spinal cord.

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.

Identification of spinally projecting neurones in the A1 catecholamine cell group of the ventrolateral medulla

In anaesthetized rats, neurones were found in the ventrolateral medulla that responded to antidromic stimulation of their axons in the thoracic spinal cord. These neurones were identified as being antidromic, according to various established electrophysiological criteria. A total of 44 antidromically identified neurones were found, 23 had conduction velocities below 2.0 m/s. Many (70%) had ongoing activity with a slow firing rate (0.2-5.0 Hz). Catecholamine-containing cells were visualized in this ventrolateral region using a modified Mg+-catalyzed glyoxylic acid technique and revealed multipolar, small diameter cells (17-20 microns) which were diffusely scattered (as demonstrated in previous histofluorescence studies). The Pontamine sky blue-marked recording sites of 7 units (0.3-1.1 m/s) showed close apposition to a CA-fluorescent cell whilst a further 4 (2.0-2.7 m/s) could not be correlated with the presence of a fluorescent cell. The results are discussed in the light of recent data in the rat, suggesting that the spinal cord catecholamine innervation arises from brainstem cell groups other than A1.

Brainstem projections to the facial nucleus of the opossum. A study using axonal transport techniques

The horseradish peroxidase and autoradiographic techniques have been used to determine the origin and intranuclear termination of brainstem axons projecting to the facial nucleus of the opossum and to define networks which could be utilized in some oral-facial behaviors. Two regions of the midbrain have dense projections to the facial nucleus. One region is the ventral periaqueductal gray and adjacent interstitial nucleus of the medial longitudinal fasciculus which project bilaterally to those areas of the facial nucleus supplying auricular and cervical musculature. A second is the paralemniscal zone of the caudolateral midbrain which innervates the same areas of the contralateral facial nucleus. The red nucleus and/or the adjacent tegmentum send a less dense projection to those regions of the contralateral facial nucleus which innervate buccolabial and zygomatic muscles. The dorsolateral pons (the parabrachial complex, the nucleus locus coeruleus, pars alpha, and the nucleus sensorius n. trigemini, pars dorsalis) projects densely to those areas of the ipsilateral facial nucleus which innervate buccolabial and zygomatic musculature. In contrast, the nucleus reticularis pontis, pars ventralis, projects bilaterally to parts of the facial nucleus supplying auricular and cervical muscles. There was evidence of some rostral to caudal organization in the latter projection. Neurons in medial parts of the lateral reticular formation project bilaterally to the facial nucleus. Those within the nucleus reticularis parvocellularis and the rostral nucleus reticularis medullae oblongatae ventralis innervate areas supplying buccolabial and zygomatic muscles. Neurons in the nucleus reticularis medullae oblongatae ventralis located caudal to the obex favor regions of the facial nuclei which supply auricular and cervical muscles. Neurons in the nucleus reticularis medullae oblongatae dorsalis and lamina V of the medullary and spinal dorsal horns project ipsilaterally to the facial nucleus in a manner suggesting that information from specific cutaneous areas reaches neurons supplying the muscles deep to them. The brainstem-facial connections are discussed in relation to the functionally diverse roles served by the facial nucleus in oral-facial behavior.

The sources of supraspinal afferents to the spinal cord in a variety of limbed reptiles. I. Reticulospinal systems

Horseradish peroxidase was injected into various levels of the spinal cord of turtles (Pseudemys and Chrysemys), lizards (Tupinambis, Iquana, Gekko, Sauromelus, and Gerrhonotus), and a crocodilian (Caiman). The results suggest that brainstem reticulospinal projections in limbed reptiles rival mammalian reticulospinal systems in complexity. The reptilian myelencephalic reticular formation can be divided into four distinct reticulospinal nuclei. Reticularis inferior pars dorsalis (RID) contains multipolar neurons which project bilaterally to the spinal cord. Reticularis inferior pars ventralis (RIV), which is only found in lizards and crocodilians, contains fusiform neurons with horizontally running dendrites and it projects ipsilaterally to the spinal cord. Reticularis ventrolateralis (RVL), which is found only in field lizards, contains triangular neurons whose dendrites parallel the ventrolateral edge of the brainstem and it projects ipsilaterally to the spinal cord. The myelencephalic raphe (RaI) varies considerably. RaI of turtles contains large reticulospinal neurons which form a continuous population with more laterally situated RID cells. RaI of lizards contains a few small reticulospinal neurons. RaI of the crocodilian Caiman contains giant reticulospinal neurons with laterally directed dendrites. The caudal metencephalic reticular formation of reptiles can be divided into two distinct reticulospinal nuclei. Reticularis medius (RM) contains large neurons with long, ventrally directed dendrites; it projects ipsilaterally to the spinal cord. Reticularis medius pars lateralis (RML) contains small neurons with laterally directed dendrites; it projects contralaterally to the spinal cord. The rostral mesencephalic and caudal mesencephalic reticular formation of reptiles can be divided into three distinct reticulospinal nuclei. Reticularis superior pars medialis (RSM) consists mostly of small, spindle-shaped neurons which project bilaterally to the spinal cord. In the lizard Tupinambis, however, large multipolar, ipsilaterally projecting neurons are occasionally seen in RSM. Reticularis superior pars lateralis (RSL) contains large, ipsilaterally projecting neurons with long, ventrolaterally directed dendrites. SRL in lizards can be divided into a dorsomedial portion, which projects ipsilaterally to the spinal cord, and a ventrolateral portion which projects contralaterally. The locus ceruleus-subceruleus field (LC-SC) contains small spindle-shaped neurons which project bilaterally to the spinal cord. Labelled reticulospinal neurons were also observed in the rostral metencephalic raphe (RaS) of the turtle brainstem. These cells are small, spindle-shaped neurons which resemble the small cells of the adjacent RSM field.

Antagonism of the excitatory effects of 5-hydroxytryptamine on sympathetic preganglionic neurones and neurones activated by visceral afferents

The responses of electrophysiologically identified sympathetic preganglionic neurones (SPGN) and interneurones activated by sympathetic visceral afferents (VA), to iontophoretic application of 5-hydroxytryptamine (serotonin) and serotonin receptor antagonists: methysergide and cinanserin, were tested in the T3 segment of the spinal cord of the cat. Serotonin had a definite and dose-dependent excitatory action on sympathetic preganglionic neurones and neurones activated by visceral afferents. This excitatory response to serotonin was characterized by the rapid development of tachyphylaxis. Methysergide antagonized the effects of serotonin in preganglionic neurones and neurones activated by visceral afferents. Cinanserin was less effective than methysergide. Preliminary data on the effect of inhibitors of serotonin uptake: chloroimipramine and fluoxetine are also reported. These findings support the idea that the sympathetic output and the activity of neurones activated by sympathetic viscero-afferents is modulated by an excitatory serotoninergic supraspinal input.

Projections of the superior colliculus to the supraspinal nucleus and the cervical spinal cord gray of the cat

Anterograde transport experiments reveal two novel findings regarding the distribution of descending tectofugal axons. First, such axons project to the supraspinal nucleus of the caudal medulla; this nucleus is known to project to the upper cervical spinal cord gray. Second, some tectospinal axons ramify within Rexed's lamina IX of the first 5 cervical spinal cord segments. This zone contains motoneurons which innervate neck musculature. Retrograde data reveal that tectospinal neurons occur in clusters within the intermediate and deep gray layers. A close relationship between the clusters of tectospinal neurons and a modular type of connectional organization of the intermediate and deep gray layers is suggested.

Efferent projections of the A1 catecholamine cell group in the rat: an autoradiographic study

Efferent connections of the region of the A1 catecholamine cell group were investigated by the anterograde autoradiographic method in rats, some of which had been pretreated with intraventricular injections of 6-hydroxydopamine (6-OHDA). Spinal projections were further studied by combining histofluorescence and horseradish peroxidase staining in the same sections. Projections from the A1 region ascend through the lateral hypothalamus to the bed nucleus of the stria terminalis, the medial preoptic area, and several hypothalamic nuclei: the dorsomedial nucleus, the dorsal hypothalamic area, the paraventricular and supraoptic nuclei, and the median eminence. These projections are predominantly ipsilateral. Since they are sensitive to 6-OHDA, they presumably arise from catecholamine cells of the A1 group. Other pathways are not noticeably affected by 6-OHDA. These include projections through the reticular formation to the contralateral nucleus ambiguus, the ipsilateral facial nucleus, and the Kölliker-Fuse nuclei, the parabrachial nuclei and the periaqueductal gray on both sides, as well as the intralaminar nuclei of the thalamus. These pathways probably do not arise from the A1 group, although a minor noradrenergic component cannot be ruled out. Spinal projections extend to the intermediolateral cell column and the ventral horn, and especially to the phrenic motor nucleus. However, these projections arise almost exclusively from non-catecholamine cells. Only a small minority of the fluorescent A1 cells are retrogradely labeled after injections of HRP into the upper thoracic spinal cord. They lie at the level of the pyramidal decussation. Likewise only a few fluorescent cells of the A2 group are labeled. Although the A1 and A2 groups were long thought to be principal sources of spinal norepinephrine, a review of the literature shows that this belief was based on equivocal evidence.

Spinal cord tracts mediating voluntary movement of hindlimb in dog

A transverse section of the dorso-lateral white column and the white matter ventrally to the tip of the anterior horn at T-8 completely eliminated the hindlimb flexion response of the dog to an auditory command for a period of 2-5 years. The severed white matter was believed to contain the corticospinal, rubrospinal, lateral reticulospinal, and propriospinal tracts.

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.