Raphespinal projections in the North American opossum: evidence for connectional heterogeneity

Imaging Serotonergic Fibers in the Mouse Spinal Cord Using the CLARITY/CUBIC Technique

Long descending fibers to the spinal cord are essential for locomotion, pain perception, and other behaviors. The fiber termination pattern in the spinal cord of the majority of these fiber systems have not been thoroughly investigated in any species. Serotonergic fibers, which project to the spinal cord, have been studied in rats and opossums on histological sections and their functional significance has been deduced based on their fiber termination pattern in the spinal cord. With the development of CLARITY and CUBIC techniques, it is possible to investigate this fiber system and its distribution in the spinal cord, which is likely to reveal previously unknown features of serotonergic supraspinal pathways. Here, we provide a detailed protocol for imaging the serotonergic fibers in the mouse spinal cord using the combined CLARITY and CUBIC techniques. The method involves perfusion of a mouse with a hydrogel solution and clarification of the tissue with a combination of clearing reagents. Spinal cord tissue was cleared in just under two weeks, and the subsequent immunofluorescent staining against serotonin was completed in less than ten days. With a multi-photon fluorescent microscope, the tissue was scanned and a 3D image was reconstructed using Osirix software.

Distribution of raphespinal fibers in the mouse spinal cord

Background

Serotonergic raphespinal neurons and their fibers have been mapped in large mammals, but the non-serotonergic ones have not been studied, especially in the mouse. The present study aimed to investigate the termination pattern of fibers arising from the hindbrain raphe and reticular nuclei which also have serotonergic neurons by injecting the anterograde tracer BDA into them.

Results

We found that raphespinal fibers terminate in both the dorsal and ventral horns in addition to lamina 10. There is a shift of the fibers in the ventral horn towards the dorsal and lateral part of the gray matter. Considerable variation in the termination pattern also exists between raphe nuclei with raphe magnus having more fibers terminating in the dorsal horn. Fibers from the adjacent gigantocellular reticular nucleus show similar termination pattern as those from the raphe nuclei with slight difference. Immunofluorescence staining showed that raphespinal fibers were heterogeneous and serotoninergic fibers were present in all laminae but mainly in laminae 1, 2, medial lamina 8, laminae 9 and 10. Surprisingly, immunofluorescence staining on clarified spinal cord tissue revealed that serotoninergic fibers formed bundles regularly in a short distance along the rostrocaudal axis in the medial part of the ventral horn and they extended towards the lateral motor neuron column area.

Conclusion

Serotonergic and non-serotonergic fibers arising from the hindbrain raphe and reticular nuclei had similar termination pattern in the mouse spinal cord with subtle difference. The present study provides anatomical foundation for the multiple roles raphe and adjacent reticular nuclei play.

Electronic supplementary material

The online version of this article (doi:10.1186/s12990-015-0046-x) contains supplementary material, which is available to authorized users.

Mechanisms for the Anti-nociceptive Actions of the Descending Noradrenergic and Serotonergic Systems in the Spinal Cord

The sensation of pain plays a critical role as an alert and as a protection system against tissue damage from mechanical, chemical, and thermal stimuli. Despite the protective role of pain, the severity of pain sensation is markedly attenuated by the endogenous pain inhibitory systems that predominantly originate at the brain stem. Both behavioral and in vivo extracellular recording studies have sought the loci producing analgesia and clarification of the anti-nociceptive actions. Among those loci, the main descending systems to the spinal dorsal horn are noradrenergic and serotonergic. Although, in vivo studies have provided basic knowledge of these systems, the precise synaptic mechanisms underlying the analgesic actions have not yet been elucidated until recently. The newly developed in vitro slice and in vivo patch-clamp recordings have disclosed the synaptic mechanisms of the noradrenergic and serotonergic effects at the level of spinal dorsal horn. This paper reviews the anti-nociceptive action of these systems, while particularly focusing on the electrophysiological aspects of the systems at the single neuron level in the spinal dorsal horn as well as their origins and responsible receptor subtypes.

Deconstructing endogenous pain modulations

A pathway from the midbrain periaqueductal gray (PAG) through the ventromedial medulla (VMM) to the dorsal horn constitutes a putative endogenous nociceptive modulatory system. Yet activation of neurons in both PAG and VMM changes the responses of dorsal horn cells to non-noxious stimuli and elicits motor and autonomic reactions that are not directly related to nociception. Activation of mu-opioid receptors in VMM and PAG also modifies processes in addition to nociceptive transmission. The descending projections of VMM neurons are not specific to nociception as VMM projects to the spinal superficial dorsal horn where thermoreceptors as well as nociceptors terminate. In addition, experiments with pseudorabies virus demonstrate multi-synaptic pathways from VMM to sympathetic and parasympathetic target organs. VMM neurons respond to both noxious and unexpected innocuous stimuli of multiple modalities, and change their discharge during behaviors unrelated to pain such as micturition/continence and sleep/wake. In conclusion, all available evidence argues against the idea that PAG and VMM target nociception alone. Instead these brain stem sites may effect homeostatic adjustments made necessary by salient situations including but not limited to injury.

Potentiation, activation and blockade of GABAA receptors by etomidate in the rat sacral dorsal commissural neurons

Etomidate (ET), an imidazole general anesthetic, has been medically widely used. Recent evidence suggests that the inhibitory neurotransmitter GABA receptor may be one of the important molecular target(s) of general anesthetics. Up to date, little attention has been directed toward the sacral dorsal commissural nucleus (SDCN), which serves as a relay of sensory information from the pelvic viscera in the spinal cord. Therefore, the effect of ET on GABA(A) receptor function in neurons acutely dissociated from the SDCN was investigated using the nystatin-perforated patch-recording configuration under voltage-clamp conditions. At a holding potential of -40 mV, ET (above 10 microM) induced an inward ET-activated current (I(ET)) with the EC(50) value of 33 +/- 3 microM, which was reversibly blocked by bicuculline and picrotoxin. The reversal potential of I(ET) was close to the Cl(-) equilibrium potential. ET also displayed a biphasic modulatory effect on GABA responses. At lower concentrations (0.1-100 microM), ET reversibly potentiated GABA (1 microM)-activated Cl(-) currents in a bell-shaped manner, with the maximal facilitative effect at 10 microM, whereas at concentrations >100 microM, the peak of the ET-induced current was suppressed in the absence or presence of GABA (1 microM). These results suggest that in SDCN, in addition to the potentiation of GABA(A) receptor-mediated responses at low concentrations and the direct activation of GABA(A) receptors at moderate concentrations as expected, ET produced a fast blocking action at high concentrations. The general anesthetic-induced effects in SDCN, at least the potentiation of GABA responses, may significantly contribute to anesthesia of pelvic viscera during the general anesthesia.

Acute nociceptive somatic stimulus sensitizes neurones in the spinal cord to colonic distension in the rat

The common co-existence of fibromyalgia and chronic abdominal pain could be due to sensitization of spinal neurones (SNs), as a result of viscero-somatic convergence. The objective of this study is to explore the influence of acute nociceptive somatic stimulation in the form of acid injections, into the ipsilateral somatic receptive field of neurones responsive to colorectal distension (CRD), and the potential role of ionotropic glutamate receptors on sensitization. Action potentials of CRD-sensitive SNs were recorded extracellularly from the lumbar (L(2)-L(5)) spinal cord. Stimulus-response functions (SRFs) to graded CRD (10-80 mmHg, 30 s) were constructed before and 30 min after ipsilateral injection of low pH (4.0, 100 microl) saline into the somatic receptive fields. In some experiments, cervical (C(1)-C(2)) spinalization was performed to eliminate supraspinal influence. The selective NMDA receptor antagonist CGS 19755 and AMPA receptor antagonist NBQX were injected (25 micromol kg(-1), i.v.) to examine their influence on sensitization. Three types of neurones were characterized as short-latency abrupt (SLA, n = 24), short latency sustained (SLS, n = 12), and long-latency (LL, n = 6) to CRD. Ipsilateral injection of low pH (4.0) in the somatic receptive field, but not the contralateral gastrocnemius (GN) or front leg muscles, sensitized responses of these neurones to CRD. Spinalization had no influence on the development of low pH-induced sensitization. Both CGS 19755 and NBQX significantly attenuated the sensitized response to CRD in intact and spinalized animals. Acute nociceptive somatic stimulus sensitizes CRD-sensitive SNs receiving viscero-somatic convergence. The sensitization occurs at the spinal level and is independent of supraspinal influence. Ionotropic glutamate receptors in the spinal cord are involved in sensitization.

The rostral ventromedial medulla mediates the facilitatory effect of microinjected orphanin FQ in the periaqueductal gray on spinal nociceptive transmission in rats

Single unit extracellular recordings from spinal dorsal horn neurons were obtained with glass micropipettes in pentobarbital-anesthetized rats. A total of 50 wide dynamic range (WDR) neurons were studied in 25 rats. Microinjection of orphanin FQ (OFQ, 0.1 microg/0.1 microl) (a potent endogenous ligand of the opioid receptor-like receptor (ORL-1)) into the ipsilateral ventrolateral parts of periaqueductal gray (vlPAG) significantly increased C-response and post-discharge activity in most of the WDR neurons. Pre-microinjection of lidocaine (4%) into the nucleus raphe magnus (NRM) (0.5 microl), ipsilateral nucleus reticularis gigantocellularis (NGC) (0.6 microl), or nucleus gigantocellularis pars alpha (NGCalpha) and nucleus reticularis paragigantocellularis lateralis (NPGL) (0.5 microl) markedly reduced intra-vlPAG microinjection of OFQ-induced facilitatory effects on nociceptive responses of WDR neurons. Furthermore, if the NRM and ipsilateral NGC were simultaneously pre-microinjected with lidocaine, the intra-vlPAG microinjection of OFQ-induced facilitation on nociceptive responses of WDR neurons was eliminated. Also, a similar effect was observed when all the NRM, ipsilateral NGC, NGCalpha and NPGL were blocked with lidocaine. No significant effect on nociceptive responses of WDR neurons per se was found after blocking the NRM, ipsilateral NGC, NGCalpha/NPGL, or all the NRM, ipsilateral NGC, and NGCalpha/NPGL with lidocaine. These results indicate that (1) the facilitatory effect evoked by microinjection of OFQ into the vlPAG on nociceptive responses of WDR neurons in the spinal dorsal horn is primarily mediated by the NRM and ipsilateral NGC; (2) the NRM, ipsilateral NGC, and NGCalpha/NPGL do not mediate tonic descending inhibition of the spinal dorsal horn neurons.

Correlation between electrophysiology and morphology of three groups of neuron in the dorsal commissural nucleus of lumbosacral spinal cord of mature rats studied in vitro

The dorsal commissural nucleus (DCN) in the lumbosacral spinal cord receives afferent inputs from the pelvic organs via pudendal and pelvic nerves. Electrophysiological and morphological properties of neurons in the DCN of L6-S1 were examined using whole-cell recordings with biocytin-filled electrodes in transverse slices of mature rat spinal cord. Neurons were categorized into three groups according to their discharge in response to suprathreshold depolarizing pulses; neurons with tonic (19/42) and phasic (13/42) firing patterns, and neurons (10/42) that fired in bursts arising from a Ca(2+)-dependent hump. The predominantly fusiform somata of neurons labeled during recording (n = 31) had on average 3.1 primary dendrites, 7.5 terminating dendritic branches, 3.1 axon collaterals, and 14.2 axon terminations per neuron. The groups were morphologically distinct on the basis of their dendritic branching patterns. Phasic neurons (n = 10) had the most elaborate dendritic branching and the largest numbers of axon collaterals. All tonic neurons (n = 11) had axons/collaterals projecting to the intermediolateral area but none to the funiculi, suggesting that they function as interneurons in local autonomic reflexes. Many axons/collaterals of all phasic neurons lay within the DCN, suggesting that they integrate segmental and descending inputs. Seven of 10 neurons with Ca(2+)-dependent humps had axons/collaterals extending into one of the funiculi, suggesting that they project intersegmentally or to the brain. Ca(2+) hump neurons also had more axons/collaterals within the DCN and fewer in the intermediolateral area than tonic neurons. This correlation between firing pattern and morphology is an important step toward defining the cellular pathways regulating pelvic function.

The distribution of trigeminovascular afferents in the nonhuman primate brain Macaca nemestrina: a c-fos immunocytochemical study

An understanding of migraine must be based on data concerning the anatomy and physiology of the painsensitive intracranial structures. Stimulation of the superior sagittal sinus produces changes in brain blood flow and changes in neuropeptide levels similar to those seen in humans during migraine. To better understand the anatomy of the central ramifications of pain-sensitive intracranial structures we have examined the distribution of c-fos immunoreactivity in the monkey when the sinus is stimulated. Six adult Macaca nemestrina monkeys were anaesthetised. The superior sagittal sinus was isolated after a midline craniotomy and a paraffin well created. At 24 h after completion of the surgery the sinus was stimulated electrically for 1 h and the brain subsequently removed and processed for c-fos. In control animals in which the sinus was isolated but not stimulated there was a small amount of c-fos expression in the caudal brainstem and upper cervical spinal cord. Stimulation of the superior sagittal sinus evoked expression of c-fos in the caudal superfical laminae of the trigeminal nucleus and in superficial laminae of the dorsal horn of the C1 level of the upper cervical spinal cord. A lesser amount of c-fos was seen at C2 while no significant labelling above control was observed at C3. These data, while largely confirming the results from the cat concerning the central distribution trigeminovascular afferents, underscore a possibly unique specialisation of trigeminovascular afferents at the C1 level. Given the close evolutionary relationship of the monkey to man it is likely that the cells described in this study represent for primates the nucleus that mediates the pain of migraine.

Dorsal border periaqueductal gray neurons project to the area directly adjacent to the central canal ependyma of the C4-T8 spinal cord in the cat

In a previous study horseradish peroxidase (HRP) injections in the upper thoracic and cervical spinal cord revealed some faintly labeled small neurons at the dorsal border of the periaqueductal gray (PAG). The present light microscopic and electronmicroscopic tracing study describes the precise location of these dorsal border PAG-spinal neurons and their terminal organization. Wheat germ agglutinin-conjugated HRP (WGA-HRP) injections into cervical and upper thoracic spinal segments resulted in several hundreds of small retrogradely labeled neurons at the dorsal border of the ipsilateral caudal PAG. These neurons were not found after injections in more caudal segments. WGA-HRP injections in the dorsal border PAG region surprisingly resulted in anterogradely labeled fibers terminating in the area dorsally and laterally adjoining the central canal ependyma of the C4-T8 spinal cord. No anterogradely labeled fibers were found more caudal in the spinal cord. The labeled fibers found in the upper cervical cord were not located in the area immediately adjoining the ependymal layer of the central canal, but in the lateral part of laminae VI, VII and VIII and in area X bilaterally. Electronmicroscopic results of one case show that the dorsal border PAG-spinal neurons terminate in the neuropil of the subependymal area and in the vicinity of the basal membranes of capillaries located laterally to the central canal. The terminal profiles contain electron-lucent and densecored vesicles, suggesting a heterogeneity of possible transmitters. A striking observation was the lack of synaptic contacts, suggesting nonsynaptic release from the profiles. The function of the dorsal border PAG-spinal projection is unknown, but considering the termination pattern of the dorsal border PAG neurons on the capillaries the intriguing similarity between this projection system and the hypothalamohypophysial system is discussed.

Immunocytochemical distribution of ionotropic glutamate receptor subunits in the spinal cord of the rabbit

Several histochemical and physiological studies in the literature suggest that ionotropic glutamate receptors are involved in various sensory and motor control mechanisms at the spinal level. The present immunocytochemical study used three specific antibodies to GluR2,4, GluR5,6,7 and to NMDAR1 to differentiate between the regional distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), kainate and N-methyl-D-aspartate (NMDA) subtypes of glutamate receptors throughout the rabbit spinal cord. All of these immunoreactivities were prominent in the superficial dorsal horn and motor column. Each antibody gave rise to regionally specific immunostaining patterns but which were similar at all spinal levels. Numerous small neurons in superficial laminae were immunostained with GluR2,4 antibody while only neuropilar elements were immunostained with the two other antibodies. Cell bodies of the intermediate zone and fibres in the motor column were particularly densely immunostained with GluR5-7. Such an immunostaining pattern, which was particularly abundant with the GluR5-7 antibody, suggests the presence, at the spinal level, of an extensive population of neurons exhibiting a high density of kainate receptors. Immunostaining with NMDAR1 antibody was less dense in comparison with the two others and especially in the motoneuron area. The present results provide the first immunohistochemical comparison between the respective regional distributions of the three types of ionotropic glutamate receptors in the spinal cord. Their parallel distributions throughout the spinal cord support the concept of a tight functional cooperation between NMDA and non-NMDA receptors which has been extensively described for spinal events.

Effect of (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide on spinal motor systems in anesthetized intact and spinalized rats

In the present study, we examined the effect of the 5-HT1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT), on the mono- and polysynaptic reflexes in intact and spinalized rats. 8-OH-DPAT (10 micrograms/kg i.v.) significantly potentiated the amplitude of the monosynaptic reflex in intact rats. In contrast, 8-OH-DPAT (30 and 100 micrograms/kg i.v.) produced a significant dose-related inhibition of the amplitude of the monosynaptic reflex in spinalized rats. These results suggest that 8-OH-DPAT predominantly excites spinal motor systems at the supraspinal site, and inhibits such systems at a spinal cord site.

The anterolateral funiculus in the spinal cord in amyotrophic lateral sclerosis

We carried out a morphometric study on the myelinated fibers in the anterolateral funiculus (ALF) and lateral corticospinal tract (LCS) in the cervical segment of the spinal cord of 13 patients with classic amyotrophic lateral sclerosis (ALS), 6 of whom had been on a respirator; 5 age-matched subjects were used as controls. The results obtained revealed that: (1) the fiber-size distributions of the myelinated fibers in the ALF and LCS of the control subjects had peaks at 2 microns; (2) there were marked and significant losses of large myelinated fibers in the ALF and LCS of ALS patients; (3) the patients who required respirator support showed more severe degeneration in the ALF than those who required none; and (4) the degree of myelinated fiber loss in the LCS did not correlate with either the illness duration or the history of respirator use.

Brainstem network controlling descending drive to phrenic motoneurons in rat

Contraction of the diaphragm is controlled by phrenic motoneurons that receive input from sources that are not fully established. Bulbospinal (second-order) neurons projecting to phrenic motoneurons and propriobulbar (third-order) neurons projecting to these bulbspinal neurons were investigated in rat by transsynaptic transport of the neuroinvasive pseudorabies virus. Bulbospinal neurons were located predominantly in the medullary lateral tegmental field in two functionally described regions, the ventral respiratory group and Bötzinger complex. An intervening region, the pre-Bötzinger complex, contained essentially no phrenic premotoneurons. Bulbospinal neurons were also located in ventral, interstitial, and ventrolateral subnuclei of the solitary tract, and gigantocellular, Kölliker-Fuse, parabrachial, and medullary raphe nuclei. A monosynaptic pathway to phrenic motoneurons from the nucleus of the solitary tract was confirmed; monosynaptic pathways from upper cervical spinal cord, spinal trigeminal nucleus, medical and lateral vestibular nuclei, and medial pontine tegmentum were not verified. Locations of third-order neurons were consistent with described projections to the ventral respiratory group, from contralateral ventral respiratory group, Bötzinger complex, A5 noradrenergic cell group, and the following nuclei; solitary, raphe, Kölliker-Fuse, parabrachial, retrotrapezoid, and paragigantocellular. Novel findings included a projection from locus coeruleus to respiratory premotoneurons and the lack of previously described pathways from area postrema and spinal trigeminal nucleus. These second- and third-order neurons from the output network for diphragm motor control which includes numerous behaviors (e.g., respiration, phonation, defecation). Of the premotoneurons, the rostral ventral respiratory group is the primary population controlling phrenic motoneurons.

Expression of c-Fos-like immunoreactivity in the caudal medulla and upper cervical spinal cord following stimulation of the superior sagittal sinus in the cat

Migraine is an episodic vascular headache with a well-recognized clinical picture but a poorly understood pathogenesis. Stimulation of a pain-sensitive trigeminally innervated intracranial structure, the superior sagittal sinus (SSS), was undertaken to map the higher-order neurons potentially involved in the processing of vascular head pain. The animals were prepared for stimulation by exposure of the sinus and then maintained under alpha-chloralose anaesthesia for 24 h before SSS stimulation, perfusion and immunohistochemical processing for the detection of Fos protein. Examination of the medulla and upper cervical cord revealed marked increases in Fos-like immunoreactivity in laminae I and IIo of the trigeminal nucleus caudalis and the dorsal horn of the upper cervical spinal cord. In addition, Fos-like immunoreactivity was observed in lamina X of the upper cervical spinal cord, in the commissural and medial nuclei of the solitary tract and in the nucleus retroambigualis. The use of immunohistochemical detection of Fos has allowed visualization of several populations of neurons likely to be involved in the central neural processing of vascular headache syndromes, particularly migraine.

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.

Nitric oxide synthase is found in some spinothalamic neurons and in neuronal processes that appose spinal neurons that express Fos induced by noxious stimulation

To determine if nitric oxide (NO) and Fos immunoreactivity induced by noxious stimulation were colocalized in spinothalamic neurons, double-staining immunocytochemical techniques were combined with retrograde neuroanatomical tracing procedures. Initial studies on three rats demonstrated that Fos and nitric oxide synthase (NOS), the synthesizing enzyme for nitric oxide, did not coexist in spinothalamic tract neurons. However, some spinothalamic neurons were found to contain NOS and some NOS immunoreactive processes were found to appose Fos containing neurons. Thus the remainder of the study: (1) analyzed the relationship of NOS positive neuronal processes with Fos stained neurons using a Fos immunocytochemical technique in combination with either NOS immunofluorescence or NADPH-diaphorase histochemistry; and (2) quantitated the number of NOS containing cells that project to the thalamus using a combined immunofluorescent-retrograde tracing procedure. Both NOS-like immunoreactive (NOS IR) neuronal processes and NADPH-diaphorase positive neuronal processes in the dorsal horn of the lumbar spinal cord were found to appose Fos positive neurons located in laminae I and II of the dorsal horn. Approximately 40% of Fos-labeled cells in these superficial laminae were found to be in apposition to or in close proximity to NOS labeled neuronal processes. Examination of spinal cord sections for NOS-containing spinothalamic tract neurons revealed that lamina X was the only spinal cord region containing such double-labeled neurons. Further quantification revealed that approximately 10% of NOS positive neurons in lamina X were double-labeled with Fluorogold. These findings support the hypothesis that nitric oxide is involved in nociceptive events occurring in the spinal cord in response to a peripheral noxious stimulus and further indicate that nitric oxide may contribute to the central transmission of spinothalamic information.

5-HT2/5-HT1C receptor-mediated facilitatory action on unit activity of ventral horn cells in rat spinal cord slices

5-Methoxy-N,N-dimethyltryptamine (5-MeODMT) and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) facilitate motoneuron excitability through 5-HT1C/5-HT2 receptors in rats. Using spinal cord slices prepared from adult rats, we recorded unitary cell discharges, evoked by local stimulation of the adjacent site, extracellularly in the motor nuclei of the ventral horn. 5-MeODMT, DOI, 5-hydroxytryptamine (5-HT), 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) and tandospirone facilitated the probability of firing in the motor nuclei, with 5-MeODMT and DOI being the most potent. The effect of 5-MeODMT was significantly suppressed by ketanserin (a 5-HT2 receptor-selective antagonist), spiperone (a 5-HT1A/5-HT2 receptor antagonist) and cyproheptadine (a 5-HT1C/5-HT2 receptor antagonist), but not by 3-tropanyl-3,5-dichlorobenzoate (MDL 72222, a 5-HT3 receptor-selective antagonist) or pindolol (a 5-HT1A/5-HT1B receptor antagonist). This suggests that 5-HT2 and/or 5-HT1C receptors are involved in the facilitatory effects of 5-HT receptor agonists on the synaptic activity of ventral horn cells.

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.

Characterization of descending inhibition and facilitation from the nuclei reticularis gigantocellularis and gigantocellularis pars alpha in the rat

Descending influences on the spinal nociceptive tail-flick (TF) reflex produced by focal electrical stimulation and glutamate microinjection in the nuclei reticularis gigantocellularis (NGC) and gigantocellularis pars alpha (NGC alpha) were examined and characterized in rats lightly anesthetized with pentobarbital. Both inhibition and facilitation of the TF reflex were produced by electrical stimulation at identical sites in the NGC/NGC alpha; glutamate microinjection only inhibited the TF reflex. The chronaxie of stimulation for inhibition of the TF reflex was 169 +/- 28 microseconds. Inhibition of the TF reflex by stimulation was produced throughout the NGC and NGC alpha; intensities of stimulation for inhibition were least in the ventral NGC and in the NGC alpha. At threshold intensities of stimulation, inhibition of the TF reflex did not outlast the period of stimulation. Facilitation of the TF reflex was produced at many of the same sites at which stimulation inhibited the TF reflex, but always at lesser intensities of stimulation (mean, 10 microA vs. 43 microA for inhibition, n = 25). Stimulation in the NGC/NGC alpha at threshold intensities for facilitation or inhibition of the TF reflex did not significantly affect blood pressure. Strength-duration characterization of electrical stimulation and microinjection of glutamate into identical sites in the NGC and NGC alpha suggest that descending inhibition of the TF reflex results from activation of cell bodies in the NGC and NGC alpha.

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.

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.

Bulbospinal serotoninergic pathways in the frog Rana pipiens

We combined retrograde fluorescent tracing with rhodamine immunofluorescence to identify the origin of serotoninergic neurons with descending projections to the spinal cord of frogs. After injections of Fluoro-gold into the spinal cord, retrogradely labeled immunoreactive serotoninergic neurons were detected in the caudal part of the brainstem from the level of the obex through the level of the VIII nerve. These doubly labeled cells were distributed along the midline throughout the rostrocaudal extent of the dorsal portion of the raphe nuclear region. Doubly labeled neurons were more numerous in the rostral than in the caudal part of the raphe area. The fluorescent tracer 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate (DiI) was then placed in and around the middle and rostral raphe nuclear region. Anterogradely labeled fibers could be traced bilaterally in the lateral portion of the dorsal funiculus and the lateral and ventral funiculi. These fibers were seen terminating in the dorsal and ventral horns, as well as in the intermediate grey matter. After placement of DiI in the caudal raphe area, labeled fibers were found only in the intermediate grey and ventral horn. These findings suggest that the organization of bulbospinal serotoninergic pathways in the frog is similar to that of mammals, and that an isolated amphibian spinal cord preparation could be a useful model for pharmacological and physiological studies of the action of serotonin (5HT) in the spinal cord.

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

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

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

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

A possible pain control system in a non-mammalian vertebrate (a lizard, Gekko gecko)

In a lizard (Gekko gecko) the anterograde tracer PHA-L was microiontophoretically applied to the predominantly serotonergic nucleus raphes inferior. Extensive spinal projections from the rostral magnocellular part of this nucleus were demonstrated to the superficial layers of the dorsal horn and to the intermediate zone, more sparsely to the ventral horn. But, in addition, retrogradely labeled neurons were found in and just below a periventricular cell group in tegmentum mesencephali, i.e. the laminar nucleus of the torus semicircularis, a cell group which receives spinal afferents and projects to the spinal cord as the mammalian periaqueductal gray. These data suggest the presence of a three-tiered pain control system in a lizard composed of projections from the laminar nucleus of the torus semicircularis to the rostral part of the inferior raphe nucleus which in its turn projects to the superficial layers of the dorsal horn of the spinal cord.

Differential effects of p-chlorophenylalanine on indoleamines in brainstem nuclei and spinal cord of rats. II. Identification of immunohistochemically stained structures using computer-assisted image enhancement techniques

Following intraperitoneal injection of p-chlorophenylalanine (PCPA, 400 and 600 mg/kg) on 3 consecutive days, the brainstem and lumbar cord of rats were removed, frozen-sectioned and immunohistochemically stained (PAP method) for serotonin (5-HT). Using computer-assisted image analysis, the density of 5-HT staining in control, 400 and 600 mg/kg PCPA groups was determined. The mean number of pixels (representing 5-HT staining) was determined in 6 areas in the brainstem containing 5-HT cell bodies (nuclei raphe pallidus, raphe obscurus, rostral and caudal raphe magnus, raphe dorsalis and paragigantocellularis lateralis) and in the dorsal and ventral spinal cord. The results suggest a differential depletion of 5-HT within brainstem nuclei following PCPA treatment in that the most marked dose-related reductions were observed in nucleus raphe obscurus and caudal nucleus raphe magnus. Furthermore, a computer program designed to isolate terminal structures in the spinal cord identified a differential depletion of 5-HT terminals in the dorsal horn versus the ventral horn. The present study describes 3 analytical approaches combining immunohistochemistry with the computer-assisted image analysis technique and allows comparison between groups of animals which received the same or different drug treatments.

Differential effects of p-chlorophenylalanine on indoleamines in brainstem nuclei and spinal cord of rats. I. Biochemical and behavioral analysis

The involvement of endogenous serotonergic pathways in the mediation of antinociception has been indicated by electrophysiological, pharmacological and behavioral experiments. However, manipulation of the indole pathway, either by lesioning of raphe nuclei or drug intervention, often produces disparate results. In particular, serotonin (5-HT) synthesis inhibition with p-chlorophenylalanine (PCPA) has been reported to produce either hyperalgesia or analgesia, depending upon the type of pain measurement examined. In the present study, we sought to evaluate the effects of PCPA on (1) behavioral responses to noxious stimulation, and (2) levels of serotonin, tryptophan and 5-hydroxyindoleacetic acid (5-HIAA) in raphe nuclei (pallidus, obscurus, magnus and dorsalis) and spinal cord regions by HPLC with electrochemical detection. Treatment of rats with 400 or 600 mg/kg of PCPA for 3 consecutive days resulted in significant elevations in pain thresholds assessed by tail withdrawal from radiant heat as well as vocalization to electric shock of the tail. The effect of PCPA on vocalization threshold was particularly striking, for the majority of animals showed a nociceptive-specific attenuation of this response. Although the PCPA induced changes in indole content of the various raphe nuclei were not unequivocally dose-dependent, differential reductions of serotonin and 5-HIAA were clearly detected in the various raphe regions. Nuclei raphe pallidus and obscurus were depleted of 5-HT and 5-HIAA to the greatest extent, whereas levels detected in nuclei raphe magnus and dorsalis were reduced by 30-40% from control values. Metabolism of 5-HT and 5-HIAA appeared unaffected by PCPA in all regions examined except the dorsal portion of the spinal cord. These findings collectively suggest that the effects of PCPA are not uniform throughout the central nervous system and raise the possibility that discrepancies in the behavior literature may be attributed to drug-induced changes in some, but not all serotonergic pathways.

Serotonin- and substance P-containing projections to the nucleus tractus solitarii of the rat

The objective of the present study was to determine the location of the neurons that give rise to serotonin- and substance P-containing terminals in the nucleus tractus solitarii. This was done by injecting rhodamine-filled latex microspheres into the nucleus tractus solitarii of rats to retrogradely label neuronal cell bodies and by processing sections from the brains of these animals to determine whether the labelled neurons contained serotonin or substance P immunoreactivity. Serotonin-immunoreactive neurons that projected to the nucleus tractus solitarii were found in the nucleus raphe magnus, nucleus raphe obscurus, nucleus raphe pallidus, and in the ventral medulla, lateral to the pyramidal tract. Substance P-immunoreactive neurons that projected to the nucleus tractus solitarii were found in similar areas but were proportionately less numerous in the nucleus raphe magnus and proportionately more numerous in the nucleus raphe pallidus. It is concluded that neurons in the medullary raphe nuclei, some of which presumably utilize serotonin or substance P as a neurotransmitter, could regulate autonomic function via direct projections to the nucleus tractus solitarii.

The distribution of substance P-, enkephalin- and dynorphin-immunoreactive neurons in the medulla of the rat and their contribution to bulbospinal pathways

This study examined the medullary distribution of peptide-containing neurons at the origin of bulbospinal pathways in the rat. Antisera directed against substance P, methionine-enkephalin-arg-gly-leu and dynorphin B were used on sections in which spinally projecting brainstem neurons had been identified by the retrograde transport of a protein-gold complex that was injected into the spinal cord. Both the relative numbers and distribution of the different peptide-immunoreactive spinally projecting neurons differed. Methionine-enkephalin-immunoreactive neurons were twice as numerous as the substance P-immunoreactive cells and seven times more numerous than the dynorphin B-positive neurons. The methionine-enkephalin cells were found in all medullary raphé nuclei, and in the ventromedial and ventrolateral medullary reticular formation. Caudally, the methionine-enkephalin cells were concentrated laterally; more rostrally they were located more medially. Three major loci of methionine-enkephalin-immunoreactive cells were found: (1) the nucleus reticularis paragigantocellularis lateralis, at levels caudal to the facial nucleus, (2) the B3 cell group (nucleus raphé magnus and the nucleus reticularis magnocellularis, pars alpha) and the most rostral part of the B1 and B2 cell groups (nuclei raphé pallidus and obscurus), (3) a dense cluster of cells that flanks the dorsal surface of the dorsal accessory olive (referred to as the nucleus interfascicularis hypoglossi, pars dorsalis). Substance P-like cells were seen in all raphé nuclei except for the most anterior portion of the B3 cell group. Substance P-immunoreactive cells were also seen in both the ventromedial (nuclei reticularis ventralis and magnocellularis) and ventrolateral medulla (nucleus reticularis paragigantocellularis lateralis). Finally there was a dense concentration of substance P neurons in the nucleus interfascicularis hypoglossi, pars ventralis. The distribution of dynorphin-immunoreactive neurons differed significantly from that of methionine-enkephalin and substance P. Dynorphin cells were almost exclusively found in the ventrolateral medulla (nucleus reticularis paragigantocellularis lateralis), at all levels between the lateral reticular nucleus and the caudal pole of the facial nucleus. The proportion of each of these peptidergic-immunoreactive cells at the origin of bulbospinal pathways differed considerably. Substance P spinally projecting neurons were more numerous than methionine-enkephalin spinally projecting neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructure of the central gray region (lamina X) in cat spinal cord

The central gray region (lamina X) of the lumbar spinal cord in cat was examined by electron microscopy. This region consisted of three morphological zones. Medially, the first zone was comprised of ependyma which surrounded the central canal. The ependyma in the cat spinal cord was similar to most vertebrate spinal ependyma. Secondly, a subependymal zone consisted of glial processes arranged parallel to the long axis of the spinal cord. This glial zone was widest lateral to the central canal and extended approximately 75 microns. The lateral edge of the glial zone intermingled with a neuropil zone, the third zone. The components of the neuropil zone consisted of dendrites, myelinated and unmyelinated axons, synaptic terminals, astrocytes and neurons. The dendrites and neurons generally were oriented parallel with the long axis of the spinal cord. Three synaptic terminal types were categorized according to vesicular morphology, i.e. small round vesicles, flattened vesicles and dense core vesicles. The central gray region has been implicated in nociception and has been shown to receive both primary afferent and supraspinal input. The results from this study are consistent with the central gray region being an area of multiple synaptic inputs which may form the morphological basis of nociceptive processing that ascends to brainstem nuclei.

The intermediolateral nucleus: an 'open' or 'closed' nucleus?

The sympathetic preganglionic neurons located in the intermediolateral nucleus (IML) that project to the superior cervical ganglion of the rabbit were observed to have two major dendritic orientations after retrograde labeling with horseradish peroxidase. One projection extends longitudinally within IML. The second projection courses medially and presents a triangular shape in horizontal sections. The labeled processes that project medially arise from cells in IML and project through the intercalated nucleus towards the central autonomic area and follow the contour of the central canal. Medially oriented dendrites intruding into other areas of the intermediate grey matter show that IML is an 'open' rather than a 'closed' nucleus as has been recently suggested. The location and distribution of the sympathetic preganglionic neurons projecting to the superior cervical ganglion in the rabbit are compared with those reported for other species.

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

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

A light and electron microscopic analysis of the sacral parasympathetic nucleus after labelling primary afferent and efferent elements with HRP

Primary afferent input to the cat sacral parasympathetic nucleus (SPN) has been examined by injury filling sacral dorsal roots, ventral roots, or both with horseradish peroxidase (HRP). Appropriate spinal segments were processed for the demonstration of HRP with diaminobenzidine and prepared for sequential light (LM) and electron (EM) microscopy. At the LM level, a large fascicle of primary afferent fibers was observed passing ventrally along the lateral edge of the dorsal horn into the region of the SPN. Varicosities were seen throughout the course of the axons but were particularly abundant within the SPN. Injury filling of the ventral roots with HRP resulted in a Golgi-like labelling of preganglionic neurons and their dendritic arbors, as well as ventral root afferent fibers. Swellings on both dorsal and ventral root afferent axons were observed in close apposition to labelled preganglionic neurons and their dendrites. At the ultrastructural level, afferent terminals were found to contain clear spherical vesicles; 66% of these terminals also contained at least one dense-cored vesicle. Of particular interest was the presence of labelled dorsal and ventral root afferent terminals synapsing on labelled preganglionic neurons. Preganglionic neurons were also postsynaptic to unlabelled terminals containing clear spherical (79.7%) or pleomorphic vesicles (20.3%). These data indicate that preganglionic neurons receive direct input from several sources, and provide the first demonstration of direct input to these cells from sensory fibers in the dorsal and ventral roots. The connections described in the present study provide interesting and, as yet, unexplored possibilities for sensory and autonomic reflex integration.

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

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

Immunohistochemistry of synaptic input and functional characterizations of neurons near the spinal central canal

Neurons surrounding the central canal in sacral spinal segments were functionally characterized on the basis of somatic and/or visceral afferent input, then intracellularly marked with horseradish peroxidase (HRP). Tissue sections containing portions of HRP-stained neurons were subsequently immunohistochemically examined for the presence of contacts made by axonal enlargements containing vasoactive intestinal polypeptide (VIP), substance P (SP), somatostatin (SS), Leu-enkephalin (ENK), or serotonin (5-HT). ENK-and 5-HT-containing enlargements were found to contact all neurons examined. SP and SS terminals contacted fewer neurons, and were not associated with specific functional classes. On the other hand, VIP-containing fibers contacted only those neurons receiving visceral afferent input, thus supporting the contention that VIP is contained in a population of visceral afferent fibers projecting to the gray matter surrounding the central canal at sacral levels.

Functional and morphological features of neurons in the midline region of the caudal spinal cord of the cat

Neurons surrounding and dorsal to the central canal in caudal segments of the cat spinal cord were functionally and morphologically characterized. From electrophysiologically obtained responses these neurons were categorized into 3 functional groups based on excitation by somatic afferent stimulation. Eighteen of 36 units were activated by both low threshold and high threshold primary afferent inputs. Of the remaining 18 units, 9 responded only to innocuous intensities of stimulation and the other 9 were excited selectively by noxious peripheral stimulation. Neurons intracellularly marked with horseradish peroxidase formed a heterogeneous population with respect to perikaryal size, dendritic orientation and dendritic extent, and no evident correlations between functional categories and morphological features were observed in light microscopic analyses. Neurons immediately surrounding the central canal were functionally similar to neurons located in the dorsally adjacent gray matter.

Distribution of serotonin in the brain stem and spinal cord of the lizard Varanus exanthematicus: an immunohistochemical study

The distribution of serotonin-containing nerve cell bodies, fibers and terminals in the lizard Varanus exanthematicus was studied with the indirect immunofluorescence technique, using antibodies to serotonin. Most of the serotonin-containing cell bodies were found in the midline, in both of the raphe nuclei, i.e. the nuclei raphes superior and inferior. A considerable number of more laterally shifted serotonergic neurons was found particularly at three levels of the brain stem, viz. in the caudal mesencephalic tegmentum, at the isthmic level, and over a long distance in the medulla oblongata. These laterally situated serotonin-positive neurons were partly found within the confines of the substantia nigra, the nucleus reticularis superior and the lateral part of the nucleus reticularis medius and ventrolateral part of the nucleus reticularis inferior, respectively. No serotonergic cell bodies were found in the spinal cord. In the brain stem a dense serotonergic innervation was observed in all of the motor nuclei of the cranial nerves, in two layers of the tectum mesencephali, in the nucleus interpeduncularis pars ventralis, the nucleus profundus mesencephali pars rostralis, the periventricular grey, the nucleus parabrachialis, the vestibular nuclear complex, the nucleus descendens nervi trigemini, the nucleus raphes inferior, and parts of the nucleus tractus solitarii. Descending serotonergic pathways could be traced into the spinal cord via the dorsolateral, ventral and ventromedial funiculi, and were found to innervate mainly three parts of the spinal grey throughout the spinal cord, i.e. the dorsal part of the dorsal horn, the motoneuron area in the ventral horn, and the intermediate zone just lateral to the central canal. The results obtained in the present study suggest a close resemblance of the organization of the serotonergic system in reptiles and mammals, especially as to the serotonergic innervation of the spinal cord.

Distribution of catecholamines in the brain stem and spinal cord of the lizard Varanus exanthematicus: an immunohistochemical study based on the use of antibodies to tyrosine hydroxylase

Antibodies to tyrosine hydroxylase were used to study the distribution of nerve cells, fibers and terminals, containing catecholamines, in the lizard Varanus exanthematicus, by means of the indirect immunofluorescence technique. Tyrosine hydroxylase-containing cell bodies occurred in the hypothalamus, the ventral and dorsal tegmentum mesencephali, the substantia nigra, the isthmic reticular formation, in and ventrolaterally to the locus coeruleus, in the nucleus tractus solitarii and in a lateral part of the nucleus reticularis inferior. In addition tyrosine hydroxylase-containing cell bodies were found throughout the spinal cord, ventral to the central canal. Tyrosine hydroxylase-immunoreactive terminal areas in the brain stem were seen in the nucleus interstitialis of the fasciculus longitudinalis medialis, the nucleus raphes superior, the locus coeruleus, several parts of the reticular formation and the nucleus descendens nervi trigemini. Ascending catecholaminergic pathways could be traced from the ventral mesencephalic tegmentum as well as from the dorsal isthmic tegmentum rostralwards, through the lateral hypothalamus. These pathways correspond to the mesostriatal and isthmocortical projections respectively, as described in mammals. Furthermore, ascending catecholaminergic fibers could be traced from the catecholaminergic cell groups in the medulla oblongata to the isthmus, where they intermingle with the locus coeruleus neurons. These pathways correspond to the medullohypothalamic projection and to the dorsal periventricular system in mammals. Descending catecholaminergic fibers to the spinal cord pass via the dorsomedial part of the lateral funiculus, and mainly terminate in the dorsal horn. The results obtained in the present study have been placed in a comparative perspective, which illustrates the constancy of catecholaminergic innervation throughout phylogeny.

Developmental sequence in the origin of descending spinal pathways. Studies using retrograde transport techniques in the North American opossum (Didelphis virginiana)

The origin of descending pathways to thoracic and cervical levels of the spinal cord has been investigated with retrograde tracing techniques in a series of pouch young and adult opossums. The opossum was chosen because it is born in a very immature state, 12-13 days after conception, and has a protracted development in an external pouch. A few neurons in the pontine reticular formation and nucleus coeruleus were labeled by horseradish peroxidase (HRP) injections of the thoracic cord as early as postnatal day (PND) 3. By PND 5, similar injections labeled neurons in the same areas as well as in the medullary reticular formation, the raphe nuclei of the caudal pons and medulla, the spinal trigeminal nuclei, the vestibular complex, the accessory oculomotor nuclei and the interstitial nucleus of Cajal. When Nuclear Yellow (NY) was employed, neurons were also labeled in the red nucleus, the hypothalamus and possibly in the nucleus of the solitary tract. Regardless of the technique employed, neurons in the dorsal column nuclei were not labeled by thoracic injections until at least PND 14. Axons from the nucleus ambiguus, the fastigial and interposed nuclei of the cerebellum as well as the intermediate and deep layers of the superior colliculus reach cervical levels of the cord, where they are specifically targeted, by at least PND 17. They do not significantly overgrow those levels during development. Corticospinal axons are the last of the major descending pathways to innervate the spinal cord. Cortical neurons cannot be labeled by cervical injections of either HRP or NY until at least PND 30. Evidence for transient brainstem-spinal and corticospinal projections was obtained.

The periaqueductal gray projections to the rat spinal trigeminal, raphe magnus, gigantocellular pars alpha and paragigantocellular nuclei arise from separate neurons

Possible collateral branches of periaqueductal gray axons which distribute to the nucleus raphe magnus, nucleus reticularis paragigantocellularis, nucleus reticularis gigantocellularis pars alpha and the spinal trigeminal nucleus were analyzed with the double fluorescent retrograde tracer technique. With the exception of a small number of double-labeled neurons observed in the periaqueductal gray following injections of fluorescent dyes into the nuclei reticularis paragigantocellularis and gigantocellularis pars alpha, no double-labeled cells were found in this midbrain region following injections of tracers into various combinations of the above 4 nuclear groups. The results of this investigation indicate that these 4 brainstem nuclei are innervated predominantly by separate neuronal populations within the periaqueductal gray.

Anatomical and physiological studies of the gray matter surrounding the spinal cord central canal

Recent histochemical evidence suggests that neurons in the gray matter surrounding the central canal may play a role in nociception. We attempted to evaluate this possibility by studying the response properties and ascending projections of these cells in the rat. In the first series of experiments, the ascending projections of neurons around the central canal were studied by the method of retrograde transport of horseradish peroxidase (HRP). Predominantly contralateral labeling of neurons around the central canal appeared after HRP injections into the paramedian medullary or pontine reticular formation in intact or cerebellectomized animals. Far fewer cells were labeled by injections into the lateral medulla and/or pons. A small number of cells was retrogradely labeled by HRP injections into the vermal and intermediate regions of the cerebellum or the periaqueductal gray matter. Injections into other brainstem areas outside of the reticular formation also failed to label large numbers of neurons around the centra canal. In a second set of experiments, we recorded extracellular unitary activity from the lumbar enlargement in spinalized, decerebrate, unanesthetized rats. Thirteen units were functionally characterized and histologically localized within 300 micrometers of the central canal. All of the units identified responded exclusively to noxious stimuli applied within highly circumscribed receptive fields. Thus, neurons around the central canal contribute strongly to long ascending spinal cord projections. Physiologically, neurons found within this region are reminiscent of the noxious-specific cells in the outer most layers of the dorsal horn.