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

Serotonergic and dopaminergic neurons in the dorsal raphe are differentially altered in a mouse model for parkinsonism

Parkinson's disease (PD) is characterized by motor impairments caused by degeneration of dopamine neurons in the substantia nigra pars compacta. In addition to these symptoms, PD patients often suffer from non-motor comorbidities including sleep and psychiatric disturbances, which are thought to depend on concomitant alterations of serotonergic and noradrenergic transmission. A primary locus of serotonergic neurons is the dorsal raphe nucleus (DRN), providing brain-wide serotonergic input. Here, we identified electrophysiological and morphological parameters to classify serotonergic and dopaminergic neurons in the murine DRN under control conditions and in a PD model, following striatal injection of the catecholamine toxin, 6-hydroxydopamine (6-OHDA). Electrical and morphological properties of both neuronal populations were altered by 6-OHDA. In serotonergic neurons, most changes were reversed when 6-OHDA was injected in combination with desipramine, a noradrenaline (NA) reuptake inhibitor, protecting the noradrenergic terminals. Our results show that the depletion of both NA and dopamine in the 6-OHDA mouse model causes changes in the DRN neural circuitry.

The 'in's and out's' of descending pain modulation from the rostral ventromedial medulla

The descending-pain modulating circuit controls the experience of pain by modulating transmission of sensory signals through the dorsal horn. This circuit's key output node, the rostral ventromedial medulla (RVM), integrates 'top-down' and 'bottom-up' inputs that regulate functionally defined RVM cell types, 'OFF-cells' and 'ON-cells', which respectively suppress or facilitate pain-related sensory processing. While recent advances have sought molecular definition of RVM cell types, conflicting behavioral findings highlight challenges involved in aligning functional and molecularly defined types. This review summarizes current understanding, derived primarily from rodent studies but with corroborating evidence from human imaging, of the role of RVM populations in pain modulation and persistent pain states and explores recent advances outlining inputs to, and outputs from, RVM pain-modulating neurons.

Electrical stimulation affects the differentiation of transplanted regionally specific human spinal neural progenitor cells (sNPCs) after chronic spinal cord injury

BACKGROUND

There are currently no effective clinical therapies to ameliorate the loss of function that occurs after spinal cord injury. Electrical stimulation of the rat spinal cord through the rat tail has previously been described by our laboratory. We propose combinatorial treatment with human induced pluripotent stem cell-derived spinal neural progenitor cells (sNPCs) along with tail nerve electrical stimulation (TANES). The purpose of this study was to examine the influence of TANES on the differentiation of sNPCs with the hypothesis that the addition of TANES would affect incorporation of sNPCs into the injured spinal cord, which is our ultimate goal.

METHODS

Chronically injured athymic nude rats were allocated to one of three treatment groups: injury only, sNPC only, or sNPC + TANES. Rats were sacrificed at 16 weeks post-transplantation, and tissue was processed and analyzed utilizing standard histological and tissue clearing techniques. Functional testing was performed. All quantitative data were presented as mean ± standard error of the mean. Statistics were conducted using GraphPad Prism.

RESULTS

We found that sNPCs were multi-potent and retained the ability to differentiate into mainly neurons or oligodendrocytes after this transplantation paradigm. The addition of TANES resulted in more transplanted cells differentiating into oligodendrocytes compared with no TANES treatment, and more myelin was found. TANES not only promoted significantly higher numbers of sNPCs migrating away from the site of injection but also influenced long-distance axonal/dendritic projections especially in the rostral direction. Further, we observed localization of synaptophysin on SC121-positive cells, suggesting integration with host or surrounding neurons, and this finding was enhanced when TANES was applied. Also, rats that were transplanted with sNPCs in combination with TANES resulted in an increase in serotonergic fibers in the lumbar region. This suggests that TANES contributes to integration of sNPCs, as well as activity-dependent oligodendrocyte and myelin remodeling of the chronically injured spinal cord.

CONCLUSIONS

Together, the data suggest that the added electrical stimulation promoted cellular integration and influenced the fate of human induced pluripotent stem cell-derived sNPCs transplanted into the injured spinal cord.

Intrathecally administered substance P activated the spinal defecation center and enhanced colorectal motility in anesthetized rats

Noxious stimuli on the colorectum cause colorectal contractions through activation of descending monoaminergic pathways projecting from the supraspinal defecation center to the spinal defecation center. Since it is known that substance P is involved in the response to peripheral noxious stimuli in the spinal cord, we investigated the effects of intrathecally administered substance P at L6-S1 levels on colorectal motility in rats that were anesthetized with α-chloralose and ketamine. Intrathecally administered substance P enhanced colorectal motility, even after transection of the thoracic spinal cord at the T4 level. Severing the pelvic nerves, but not the colonic nerves, abolished substance P enhanced colorectal motility. In the spinal cord at L6-S1 levels, expression of mRNA coding neurokinin (NK) 1-3 receptors was detected by RT-PCR. Immunohistological experiments revealed that preganglionic neurons of the pelvic nerves express NK1 receptors, whereas expression of NK2 receptors was not found. In addition, substance P-containing fibers densely innervated around the preganglionic neurons expressing NK1 receptors. An intrathecally administered NK1 receptor antagonist (spantide) attenuated capsaicin-induced colorectal contractions. These results suggest that the colokinetic action of substance P is mediated by the NK1 receptor in the spinal defecation center. Our findings indicate that substance P may function as a neurotransmitter in the spinal defecation center.NEW & NOTEWORTHY We found that intrathecally administered substance P enhanced colorectal motility in anesthetized rats. Neurokinin (NK) 1 receptors, but not NK2 receptors, were detected in preganglionic neurons of the pelvic nerves. Blockade of NK1 receptors in the spinal cord attenuated the enhanced colorectal motility in response to intracolonic noxious stimuli. The findings indicate that substance P may function as a neurotransmitter in the spinal reflex pathway controlling defecation.

Role of Descending Serotonergic Fibers in the Development of Pathophysiology after Spinal Cord Injury (SCI): Contribution to Chronic Pain, Spasticity, and Autonomic Dysreflexia

As the nervous system develops, nerve fibers from the brain form descending tracts that regulate the execution of motor behavior within the spinal cord, incoming sensory signals, and capacity to change (plasticity). How these fibers affect function depends upon the transmitter released, the receptor system engaged, and the pattern of neural innervation. The current review focuses upon the neurotransmitter serotonin (5-HT) and its capacity to dampen (inhibit) neural excitation. A brief review of key anatomical details, receptor types, and pharmacology is provided. The paper then considers how damage to descending serotonergic fibers contributes to pathophysiology after spinal cord injury (SCI). The loss of serotonergic fibers removes an inhibitory brake that enables plasticity and neural excitation. In this state, noxious stimulation can induce a form of over-excitation that sensitizes pain (nociceptive) circuits, a modification that can contribute to the development of chronic pain. Over time, the loss of serotonergic fibers allows prolonged motor drive (spasticity) to develop and removes a regulatory brake on autonomic function, which enables bouts of unregulated sympathetic activity (autonomic dysreflexia). Recent research has shown that the loss of descending serotonergic activity is accompanied by a shift in how the neurotransmitter GABA affects neural activity, reducing its inhibitory effect. Treatments that target the loss of inhibition could have therapeutic benefit.

Serotonin discovery and stepwise disclosure of 5-HT receptor complexity over four decades. Part II. Some contributions of Manfred Göthert

About 40% of the papers within the scientific oeuvre of Manfred Göthert (1939-2019) were dedicated to serotonin (5-hydroxytryptamine, 5-HT). He was not only the witness of the gradual definition of the fourteen 5-HT receptor subtypes but also was involved directly by identifying 5-HT_1B, 5-HT_1D and 5-HT₃ receptors. Moreover, he identified presynaptic 5-HT receptors on central and/or peripheral serotoninergic, noradrenergic and/or cholinergic neurones. Two inhibitory (5-HT_1B, 5-HT_1D) and two facilitatory (5-HT₃, 5-HT₄) receptors were found, the 5-HT_1B receptor representing a possible target for antidepressant drugs. Ten years earlier than electrophysiologists, he identified ligand-gated receptors like the 5-HT₃ and the nicotinic acetylcholine (nACh) receptor as targets of halothane. Simultaneously with, but independent of, other authors he found that ethanol allosterically inhibits N-methyl-D-aspartate (NMDA) receptors, which are affected at an even lower concentration than 5-HT₃ and nACh receptors. The latter two receptors were shown to be subject to allosteric inhibition also by cannabinoids via a mechanism unrelated to cannabinoid CB₁ or CB₂ receptors; cannabinoid inhibition of 5-HT₃ receptors may represent a new target for the treatment of neuropathic pain.

Expression of RING Finger Protein 38 in Serotonergic Neurons in the Brain of Nile Tilapia, Oreochromis niloticus

Serotonin (5-hydroxytryptamine, 5-HT) is one of the major neurotransmitters, modulating diverse behaviours and physiological functions. Really interesting new gene (RING) finger protein 38 (RNF38) is an E3 ubiquitin ligase whose function remains unclear. A recent study has shown a possible regulatory relationship between RNF38 and the 5-HT system. Therefore, to gain insight into the role of RNF38 in the central 5-HT system, we identified the neuroanatomical location of 5-HT positive cells and investigated the relationship between RNF38 and the 5-HT system in the brain of the Nile tilapia, Oreochromis niloticus. Immunocytochemistry revealed three neuronal populations of 5-HT in the brain of tilapia; the paraventricular organ (PVO), the dorsal and ventral periventricular pretectal nuclei (PPd and PPv), and, the superior and inferior raphe (SR and IR). The 5-HT neuronal number was highest in the raphe (90.4 in SR, 284.6 in IR), followed by the pretectal area (22.3 in PPd, 209.8 in PPv). Double-label immunocytochemistry showed that the majority of 5-HT neurons express RNF38 nuclear proteins (66.5% in PPd; 77.9% in PPv; 35.7% in SR; 49.1% in IR). These findings suggest that RNF38 could be involved in E3 ubiquitination in the central 5-HT system.

Remodeling of lumbar motor circuitry remote to a thoracic spinal cord injury promotes locomotor recovery

Retrogradely-transported neurotrophin signaling plays an important role in regulating neural circuit specificity. Here we investigated whether targeted delivery of neurotrophin-3 (NT-3) to lumbar motoneurons (MNs) caudal to a thoracic (T10) contusive spinal cord injury (SCI) could modulate dendritic patterning and synapse formation of the lumbar MNs. In vitro, Adeno-associated virus serotype two overexpressing NT-3 (AAV-NT-3) induced NT-3 expression and neurite outgrowth in cultured spinal cord neurons. In vivo, targeted delivery of AAV-NT-3 into transiently demyelinated adult mouse sciatic nerves led to the retrograde transportation of NT-3 to the lumbar MNs, significantly attenuating SCI-induced lumbar MN dendritic atrophy. NT-3 enhanced sprouting and synaptic formation of descending serotonergic, dopaminergic, and propriospinal axons on lumbar MNs, parallel to improved behavioral recovery. Thus, retrogradely transported NT-3 stimulated remodeling of lumbar neural circuitry and synaptic connectivity remote to a thoracic SCI, supporting a role for retrograde transport of NT-3 as a potential therapeutic strategy for SCI.

Opposing effects of cervical spinal cold block on spinal itch and pain transmission

Inactivation of descending pathways enhanced responses of spinal dorsal horn neurons to noxious stimuli, but little is known regarding tonic descending modulation of spinal itch transmission. To study effects of cervical spinal cold block on responses of dorsal horn neurons to itch-evoking and pain-evoking stimuli, single-unit recordings were made from superficial dorsal horn wide dynamic range and nociceptive-specific-type neurons in pentobarbital-anesthetized mice. Intradermal histamine excited 17 units. Cold block starting 1 minute after intradermal injection of histamine caused a marked decrease in firing. The histamine-evoked response during and following cold block was significantly lower compared with control histamine-evoked responses in the absence of cold block. A similar but weaker depressant effect of cold block was observed for dorsal horn unit responses to chloroquine. Twenty-six units responded to mustard oil allyl isothiocyanate (AITC), with a further significant increase in firing during the 1-minute period of cold block beginning 1 minute after AITC application. Activity during cold block was significantly greater compared with the same time period of control responses to AITC in the absence of cold block. Ten units' responses to noxious heat were significantly enhanced during cold block, while 6 units' responses were reduced and 18 unaffected. Cold block had no effect on mechanically evoked responses. These results indicate that spinal chemonociceptive transmission is under tonic descending inhibitory modulation, while spinal pruriceptive transmission is under an opposing, tonic descending facilitatory modulation.

Neuropathic Pain After Spinal Cord Injury: Challenges and Research Perspectives

Neuropathic pain is a debilitating consequence of spinal cord injury (SCI) that remains difficult to treat because underlying mechanisms are not yet fully understood. In part, this is due to limitations of evaluating neuropathic pain in animal models in general, and SCI rodents in particular. Though pain in patients is primarily spontaneous, with relatively few patients experiencing evoked pains, animal models of SCI pain have primarily relied upon evoked withdrawals. Greater use of operant tasks for evaluation of the affective dimension of pain in rodents is needed, but these tests have their own limitations such that additional studies of the relationship between evoked withdrawals and operant outcomes are recommended. In preclinical SCI models, enhanced reflex withdrawal or pain responses can arise from pathological changes that occur at any point along the sensory neuraxis. Use of quantitative sensory testing for identification of optimal treatment approach may yield improved identification of treatment options and clinical trial design. Additionally, a better understanding of the differences between mechanisms contributing to at- versus below-level neuropathic pain and neuropathic pain versus spasticity may shed insights into novel treatment options. Finally, the role of patient characteristics such as age and sex in pathogenesis of neuropathic SCI pain remains to be addressed.

Regular physical activity prevents development of chronic muscle pain through modulation of supraspinal opioid and serotonergic mechanisms

The current study shows that blockade of opioid receptors systemically in the periaqueductal gray and the rostral ventromedial medulla prevents analgesia by 8 weeks of wheel running in a chronic muscle pain model. We further show increases in serotonin transporter expression and reversal of hyperalgesia with a selective reuptake inhibitor in the rostral ventromedial medulla in the chronic muscle pain model, and exercise normalizes serotonin transporter expression.

Introduction:

It is generally believed that exercise produces its effects by activating central opioid receptors; there are little data that support this claim. The periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) are key nuclei in opioid-induced analgesia, and opioids interact with serotonin to produce analgesia.

Objectives:

The purpose was to examine central inhibitory mechanisms involved in analgesia produced by wheel running.

Methods:

C57/Black6 mice were given access to running wheels in their home cages before induction of chronic muscle hyperalgesia and compared with those without running wheels. Systemic, intra-PAG, and intra-RVM naloxone tested the role of central opioid receptors in the antinociceptive effects of wheel running in animals with muscle insult. Immunohistochemistry for the serotonin transporter (SERT) in the spinal cord and RVM, and pharmacological blockade of SERT, tested whether the serotonin system was modulated by muscle insult and wheel running.

Results:

Wheel running prevented the development of muscle hyperalgesia. Systemic naloxone, intra-PAG naloxone, and intra-RVM naloxone reversed the antinociceptive effect of wheel running in animals that had received muscle insult. Induction of chronic muscle hyperalgesia increased SERT in the RVM, and blockade of SERT reversed the hyperalgesia in sedentary animals. Wheel running reduced SERT expression in animals with muscle insult. The serotonin transporter in the superficial dorsal horn of the spinal cord was unchanged after muscle insult, but increased after wheel running.

Conclusion:

These data support the hypothesis that wheel running produced analgesia through central inhibitory mechanisms involving opioidergic and serotonergic systems.

Activation of serotonergic neurons in the medullary caudal raphe shortens the laryngeal chemoreflex in anaesthetized neonatal rats

NEW FINDINGS

What is the central question of this study? Does activation of serotonergic neurons in the caudal medullary raphe, some of which project to the nucleus of the solitary tract, shorten the laryngeal chemoreflex? What is the main finding and its importance? We found that serotonin originating from neurons in the caudal raphe acts through a 5-HT₃ receptor located in the nucleus of the solitary tract to terminate reflex apnoea. Failure or deficiency of this arousal-related process is likely to be relevant to the pathogenesis of sudden infant death syndrome. Failure to terminate apnoea and arouse is likely to contribute to sudden infant death syndrome (SIDS). Serotonin is deficient in the brainstems of babies who have died of SIDS. We tested the hypothesis that activation of serotoninergic neurons in the caudal medullary raphe, some of which project to the nucleus of the solitary tract (NTS), would shorten the laryngeal chemoreflex (LCR). We studied anaesthetized neonatal rat pups between postnatal days 9 and 17. We injected 5-40 μl of water into the larynx to elicit the LCR and measured the duration of respiratory disruption. Microinjection of 50 nl of 100 μm AMPA into the caudal medullary raphe shortened the apnoeas (P < 0.001) and respiratory inhibition (P < 0.005) associated with the LCR. When 50 nl of 30 mm ondansetron, a 5-HT₃ antagonist, was microinjected bilaterally into the NTS, AMPA microinjected into the caudal raphe no longer shortened the LCR. After bilateral microinjection of vehicle into the NTS, AMPA microinjection into the caudal raphe significantly shortened the LCR. AMPA, a glutamate receptor agonist, may activate many neurons within the caudal raphe, but blocking the 5-HT₃ receptor-dependent responses in the NTS prevented the shortening of the LCR associated with AMPA microinjections into the caudal raphe. Thus, serotonin originating from neurons in the caudal raphe acts through a 5-HT₃ receptor located in the NTS to terminate or shorten the LCR. Serotonin is deficient in the brainstems of babies who have died of SIDS, and deficient serotonergic termination of apnoea is likely to be relevant to the pathogenesis of SIDS.

Plasticity of intact rubral projections mediates spontaneous recovery of function after corticospinal tract injury

Axons in the adult CNS fail to regenerate after injury, and therefore recovery from spinal cord injury (SCI) is limited. Although full recovery is rare, a modest degree of spontaneous recovery is observed consistently in a broad range of clinical and nonclinical situations. To define the mechanisms mediating spontaneous recovery of function after incomplete SCI, we created bilaterally complete medullary corticospinal tract lesions in adult mice, eliminating a crucial pathway for voluntary skilled movement. Anatomic and pharmacogenetic tools were used to identify the pathways driving spontaneous functional recovery in wild-type and plasticity-sensitized mice lacking Nogo receptor 1. We found that plasticity-sensitized mice recovered 50% of normal skilled locomotor function within 5 weeks of lesion. This significant, yet incomplete, spontaneous recovery was accompanied by extensive sprouting of intact rubrofugal and rubrospinal projections with the emergence of a de novo circuit between the red nucleus and the nucleus raphe magnus. Transient silencing of this rubro-raphe circuit in vivo via activation of the inhibitory DREADD (designer receptor exclusively activated by designer drugs) receptor hM4di abrogated spontaneous functional recovery. These data highlight the pivotal role of uninjured motor circuit plasticity in supporting functional recovery after trauma, and support a focus of experimental strategies on enhancing intact circuit rearrangement to promote functional recovery after SCI.

Whole-brain mapping of the direct inputs and axonal projections of POMC and AgRP neurons

Pro-opiomelanocortin (POMC) neurons in the arcuate nucleus (ARC) of the hypothalamus and nucleus tractus solitarius (NTS) of the brainstem play important roles in suppressing food intake and maintaining energy homeostasis. Previous tract-tracing studies have revealed the axonal connection patterns of these two brain areas, but the intermingling of POMC neurons with other neuron types has made it challenging to precisely identify the inputs and outputs of POMC neurons. In this study, we used the modified rabies virus to map the brain areas that provide direct inputs to the POMC neurons in the ARC and NTS as well as the inputs to the ARC AgRP neurons for comparison. ARC POMC neurons receive inputs from dozens of discrete structures throughout the forebrain and brainstem. The brain areas containing the presynaptic partners of ARC POMC neurons largely overlap with those of ARC AgRP neurons, although POMC neurons receive relatively broader, denser inputs. Furthermore, POMC neurons in the NTS receive direct inputs predominantly from the brainstem and show very different innervation patterns for POMC neurons in the ARC. By selectively expressing fluorescent markers in the ARC and NTS POMC neurons, we found that almost all of their major presynaptic partners are innervated by POMC neurons in the two areas, suggesting that there are strong reciprocal projections among the major POMC neural pathways. By comprehensively chartering the whole-brain connections of the central melanocortin system in a cell-type-specific manner, this study lays the foundation for dissecting the roles and underlying circuit mechanisms of specific neural pathways in regulating energy homeostasis.

mRNA expression profile of serotonin receptor subtypes and distribution of serotonergic terminations in marmoset brain

To better understand serotonin function in the primate brain, we examined the mRNA expression patterns of all the 13 members of the serotonin receptor (5HTR) family, by in situ hybridization (ISH) and the distribution of serotonergic terminations by serotonin transporter (SERT) protein immunohistochemical analysis. Ten of the 13 5HTRs showed significant mRNA expressions in the marmoset brain. Our study shows several new features of the organization of serotonergic systems in the marmoset brain. (1) The thalamus expressed only a limited number of receptor subtypes compared with the cortex, hippocampus, and other subcortical regions. (2) In the cortex, there are layer-selective and area-selective mRNA expressions of 5HTRs. (3) Highly localized mRNA expressions of 5HT1F and 5HT3A were observed. (4) There was a conspicuous overlap of the mRNA expressions of receptor subtypes known to have somatodendritic localization of receptor proteins with dense serotonergic terminations in the visual cortex, the central lateral (CL) nucleus of the thalamus, the presubiculum, and the medial mammillary nucleus of the hypothalamus. This suggests a high correlation between serotonin availability and receptor expression at these locations. (5) The 5HTRs show differences in mRNA expression pattern between the marmoset and mouse cortices whereas the patterns of both the species were much similar in the hippocampus. We discuss the possible roles of 5HTRs in the marmoset brain revealed by the analysis of their overall mRNA expression patterns.

Serotonin 1A receptors alter expression of movement representations

Serotonin has a myriad of central functions involving mood, appetite, sleep, and memory and while its release within the spinal cord is particularly important for generating movement, the corresponding role on cortical movement representations (motor maps) is unknown. Using adult rats we determined that pharmacological depletion of serotonin (5-HT) via intracerebroventricular administration of 5,7 dihydroxytryptamine resulted in altered movements of the forelimb in a skilled reaching task as well as higher movement thresholds and smaller maps derived using high-resolution intracortical microstimulation (ICMS). We ruled out the possibility that reduced spinal cord excitability could account for the serotonin depletion-induced changes as we observed an enhanced Hoffman reflex (H-reflex), indicating a hyperexcitable spinal cord. Motor maps derived in 5-HT1A receptor knock-out mice also showed higher movement thresholds and smaller maps compared with wild-type controls. Direct cortical application of the 5-HT1A/7 agonist 8-OH-DPAT lowered movement thresholds in vivo and increased map size in 5-HT-depleted rats. In rats, electrical stimulation of the dorsal raphe lowered movement thresholds and this effect could be blocked by direct cortical application of the 5-HT1A antagonist WAY-100135, indicating that serotonin is primarily acting through the 5-HT1A receptor. Next we developed a novel in vitro ICMS preparation that allowed us to track layer V pyramidal cell excitability. Bath application of WAY-100135 raised the ICMS current intensity to induce action potential firing whereas the agonist 8-OH-DPAT had the opposite effect. Together our results demonstrate that serotonin, acting through 5-HT1A receptors, plays an excitatory role in forelimb motor map expression.

Behavioral response and transmitter release during atonia elicited by medial medullary stimulation

Activation of the medial medulla is responsible for rapid eye movement (REM) sleep atonia and cataplexy. Dysfunction can cause REM sleep behavior disorder and other motor pathologies. Here we report the behavioral effects of stimulation of the nucleus gigantocellularis (NGC) and nucleus magnocellularis (NMC) in unrestrained cats. In waking, 62% of the medial medullary stimulation sites suppressed muscle tone. In contrast, stimulation at all sites, including sites where stimulation produced no change or increased muscle tone in waking, produced decreased muscle tone during slow-wave sleep. In the decerebrate cat electrical stimulation of the NGC increased glycine and decreased norepinephrine (NE) release in the lumbar ventral horn, with no change in γ-aminobutyric acid (GABA) or serotonin (5-HT) release. Stimulation of the NMC increased both glycine and GABA release and also decreased both NE and 5-HT release in the ventral horn. Glutamate levels in the ventral horn were not changed by either NGC or NMC stimulation. We conclude that NGC and NMC play neurochemically distinct but synergistic roles in the modulation of motor activity across the sleep-wake cycle via a combination of increased release of glycine and GABA and decreased release of 5-HT and NE. Stimulation of the medial medulla that elicited muscle tone suppression also triggered rapid eye movements, but never produced the phasic twitches that characterize REM sleep, indicating that the twitching and rapid eye movement generators of REM sleep have separate brain stem substrates.

Evolving concepts of sleep cycle generation: From brain centers to neuronal populations

As neurophysiological investigations of sleep cycle control have provided an increasingly detailed picture of events at the cellular level, the concept that the sleep cycle is generated by the interaction of multiple, anatomically distributed sets of neurons has gradually replaced the hypothesis that sleep is generated by a single, highly localized neuronal oscillator.

Cell groups that discharge during rapid-eye-movement (REM) sleep (REM-on) and neurons that slow or cease firing during REM sleep (REM-off) have long been thought to comprise at least two neurochemically distinct populations. The fact that putatively cholinoceptive and/or cholinergic (REM-on) and putatively aminergic (REM-off) cell populations discharge reciprocally over the sleep cycle suggests a causal interdependence.

In some brain stem areas these cell groups are not anatomically segregated and may instead be neurochemically mixed (interpenetrated). This finding raises important theoretical and practical issues not anticipated in the original reciprocal-interaction model. The electrophysiological evidence concerning the REM-on and REM-off cell groups suggests a gradient of sleep-dependent membrane excitability changes that may be a function of the connectivity strength within an anatomically distributed neuronal network. The connectivity strength may be influenced by the degree of neurochemical interpenetration between the REM-on and REM-offcells. Recognition of these complexities forces us to revise the reciprocal-interaction model and to seek new methods to test its tenets.

Cholinergic microinjection experiments indicate that some populations of REM-on cells can execute specific portions of the REM sleep syndrome or block the generation of REM sleep. This observation suggests that the order of activation within the anatomically distributed generator populations may be critical in determining behavioral outcome. Support for the cholinergic tenets of the reciprocal-interaction model has been reinforced by observations from sleep-disorders medicine.

Specific predictions of the reciprocal-interaction model and suggestions for testing these predictions are enumerated for future experimental programs that aim to understand the cellular and molecular basis of the mammalian sleep cycle.

Rostral ventral medulla cholinergic mechanism in pain-induced analgesia

The ascending nociceptive control (ANC), a novel spinostriatal pain modulation pathway, mediates a form of pain-induced analgesia referred to as noxious stimulus-induced antinociception (NSIA). ANC includes specific spinal cord mechanisms as well as circuitry in nucleus accumbens, a major component of the ventral striatum. Here, using the trigeminal jaw-opening reflex (JOR) in the rat as a nociceptive assay, we show that microinjection of the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine into the rostral ventral medulla (RVM) blocks NSIA, implicating RVM as a potentially important link between ANC and the PAG-RVM-spinal descending pain modulation system. A circuit connecting nucleus accumbens to the RVM is proposed as a novel striato-RVM pathway.

The Origin of Brainstem Noradrenergic and Serotonergic Projections to the Spinal Cord Dorsal Horn in the Rat

Although it has been proposed that the locus coeruleus is the predominant, if not exclusive, brainstem origin of the noradrenergic innervation of the spinal dorsal horn, pharmacological studies argue otherwise. In this study we made localized injections of the retrograde tracer wheatgerm agglutinin conjugated to apo-horseradish peroxidase gold (WGA:apoHRP-Au), in conjunction with immunocytochemical labeling for tyrosine hydroxylase (TH) or serotonin (5-HT), to identify the brainstem source of the noradrenaline (NA) and 5-HT innervation of the dorsal horn of the rat. Our studies were concentrated in the C5 spinal segment. The pattern of labeling was only studied in animals in which the tracer injection was restricted to the dorsal horn. In these rats, TH-immunoreactive neurons in widespread regions of the brainstem, including the locus coeruleus, subcoeruleus, A5, and A7 cell groups, were found to project to the dorsal horn. In terms of absolute numbers of double-labeled cells, no one noradrenergic cell group predominated. As expected, dorsal-horn-projecting 5-HT-immunoreactive neurons were found within the 5-HT populations of the rostroventromedial medulla and caudal pons, including the nucleus raphe magnus, nucleus paragigantocellularis (PGi), and ventral portions of the nucleus gigantocellularis (Gi). The majority of retrogradely labeled 5-HT-immunoreactive cells were, however, located off the midline, in the ipsilateral PGi and ventral Gi. Finally, a large number of retrogradely labeled, non-5-HT cells were found intermingled among the 5-HT cells of this region. Our results provide evidence that the noradrenergic regulation of nociceptive transmission at the spinal cord level arises from direct spinal projections of several brainstem noradrenergic cell groups.

Nociceptive behavior in animal models for peripheral neuropathy: spinal and supraspinal mechanisms

Since the initial description by Wall [Wall, P.D., 1967. The laminar organization of dorsal horn and effects of descending impulses. J. Neurophysiol. 188, 403-423] of tonic descending inhibitory control of dorsal horn neurons, several studies have aimed to characterize the role of various brain centers in the control of nociceptive input to the spinal cord. The role of brainstem centers in pain inhibition has been well documented over the past four decades. Lesion to peripheral nerves results in hypersensitivity to mild tactile or cold stimuli (allodynia) and exaggerated response to nociceptive stimuli (hyperalgesia), both considered as cardinal signs of neuropathic pain. The increased interest in animal models for peripheral neuropathy has raised several questions concerning the rostral conduction of the neuropathic manifestations and the role of supraspinal centers, especially brainstem, in the inhibitory control or in the abnormal contribution to the maintenance and facilitation of neuropathic-like behavior. This review aims to summarize the data on the ascending and descending modulation of neuropathic manifestations and discusses the recent experimental data on the role of supraspinal centers in the control of neuropathic pain. In particular, the review emphasizes the importance of the reciprocal interconnections between the analgesic areas of the brainstem and the pain-related areas of the forebrain. The latter includes the cerebral limbic areas, the prefrontal cortex, the intralaminar thalamus and the hypothalamus and play a critical role in the control of pain considered as part of an integrated behavior related to emotions and various homeostatic regulations. We finally speculate that neuropathic pain, like extrapyramidal motor syndromes, reflects a disorder in the processing of somatosensory information.

The activation of nicotinic acetylcholine receptors enhances the inhibitory synaptic transmission in the deep dorsal horn neurons of the adult rat spinal cord

Somatosensory information can be modulated by nicotinic acetylcholine receptors (nAChRs) in the superficial dorsal horn of the spinal cord. Nonetheless, the functional significance of nAChRs in the deep dorsal horn of adult animals remains unclear. Using whole-cell patch-clamp recordings from lamina V neurons in the adult rat spinal cord, we investigated whether the activation of nAChRs could modulate the inhibitory synaptic transmission in the deep dorsal horn. In the presence of CNQX and APV to block excitatory glutamatergic synaptic transmission, bath applications of nicotine (100 microM) significantly increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in almost all neurons tested. The effect of nicotine was mimicked by N-methyl-4-(3-pyridinyl)-3-butene-1-amine (RJR-2403, 100 microM), an alpha 4 beta 2-nAChR agonist, and was also mimicked by choline (10 mM), an alpha 7-nAChR agonist. The effect of nicotine was completely blocked by the nAChR antagonist mecamylamine (5 microM). In the presence of tetrodotoxin (0.5 microM), nicotine (100 microM) significantly increased the miniature IPSC frequency. On the other hand, RJR-2403 (100 microM) or choline (10 mM) did not affect miniature IPSCs. The application of nicotine (100 microM) also evoked a large inward current in all lamina V neurons tested when cells were held at -60 mV. Similarly, RJR-2403 (100 microM) induced inward currents in the majority of lamina V neurons examined. On the other hand, choline (10 mM) did not elicit any detectable whole-cell currents. These results suggest that several nAChR subtypes are expressed on the presynaptic terminals, preterminals, and neuronal cell bodies within lamina V and that these nAChRs are involved in the modulation of inhibitory synaptic activity in the deep dorsal horn of the spinal cord.

Effects of electrical stimulation of the raphe area on the micturition reflex in cats

The raphe nucleus has a variety of physiological functions, including emotion, regulation of skeletal muscle motoneurons, spinal transmission of nociceptive signals, sleep, respiration, gastric motility, and cardiovascular function. Recent evidence has shown that centrally administered serotonin has modulatory effects on micturition function, and that decreased brain serotonin might underlie depression and an overactive bladder. We applied high-frequency stimulation (HFS; 0.2-ms duration, 100 Hz) in the raphe nucleus and the adjacent midline area in 20 supracollicular decerebrate cats, which mostly elicited inhibition of the micturition reflex. The effective amplitude of the electrical stimulation for evoking inhibitory responses was less than 50 muA. We also examined single neuronal activities in the raphe nucleus in response to isovolumetric spontaneous micturition reflexes. In total, 79 neurons were recorded in the raphe nucleus that were related to urinary storage/micturition cycles. Of the neurons recorded, the most common were tonic storage neurons (48%), followed by tonic micturition neurons (28%), phasic storage neurons (18%), and phasic micturition neurons (6%). In addition to the tonic/phasic as well as storage/micturition classification, the neurons showed diverse discharge patterns: augmenting, constant and decrementing, with the constant discharge pattern being most common. Among neurons in the raphe nucleus, the neurons with a decrementing discharge pattern were concentrated in the rostral portion, whereas the augmenting and constant neurons existed diffusely. The storage and micturition neurons were intermingled in the rostral portion, whereas they were separate in the caudal portion. In conclusion, the results of the present study indicate that HFS of the raphe area inhibits the micturition reflex and that there are micturition-related neuronal firings in the raphe area in cats, suggesting that the raphe nucleus is involved in neural control of micturition.

5-Hydroxytryptamine2C receptors on pudendal motoneurons innervating the external anal sphincter

The aim of this study was to determine the localization of 5-hydroxytryptamine2C (5-HT2C) receptors on the motoneurons innervating the external anal sphincter (EAS) of male rats. Motoneurons were retrogradely labeled after percutaneous intramuscular injection of Fluorogold (FG) into the EAS. Using fluorescent immunohistochemistry, FG-positive EAS motoneurons that were immunoreactive for the 5-HT2C receptor (5-HT2C-IR) were targeted for specific examination with widefield microscopy or confocal laser scanning microscopy with spectral separation. Widefield microscopy revealed distributions of FG-positive EAS motoneurons in the L5-S1 gray matter corresponding to the dorsomedial cell group. 5-HT2C-IR positive cells were distributed in the intermediolateral cell column and the ventral horn. Ventral horn 5-HT2C-IR labeling included the dorsomedial cell group as well as the dorsolateral, ventromedial and ventrolateral areas. Confocal analysis of FG-positive EAS motoneurons and 5-HT2C-IR positive motoneuron profiles adjacent to EAS motoneurons that were not labeled with FG but presumably innervate the bulbospongiosus muscle confirmed that EAS motoneurons were immunopositive for the 5-HT2C receptor. These data suggest that previously identified descending serotonergic immunopositive fibers observed terminating on EAS motoneurons might mediate their input through the activation of 5-HT2C receptors.

Electrophysiological heterogeneity of spinally projecting serotonergic and nonserotonergic neurons in the rostral ventromedial medulla

This study examined the passive membrane and action potential properties of serotonergic and nonserotonergic neurons in the rostral ventromedial medulla (RVM) of the rat using whole cell patch-clamp recording techniques in the slice. Serotonergic neurons were identified by immunoreactivity for tryptophan hydroxylase (TrpH). Spinally projecting neurons were retrogradely labeled with 1'-dioactadecyl-3,3,3',3'-tetramethylindocarbodyanine perchlorate (DiI). Three types of neurons were identified within both spinally projecting serotonergic and nonserotonergic populations. Type 1 neurons exhibited irregular or sporadic spontaneous activity interspersed with periods of quiescence. Type 2 neurons were not spontaneously active and were additionally discriminated by a more negative resting membrane potential and a larger-amplitude action potential. Type 3 neurons fired repetitively without pause. Serotonergic neurons had a higher membrane resistance and greater action potential half-width than their nonserotonergic counterparts and rarely exhibited a fast afterhyperpolarization. Serotonergic type 3 neurons also fired more slowly and regularly than nonserotonergic type 3 neurons. Comparison of electrophysiological and immunohistochemical characteristics suggested that the smallest type 3 serotonergic neurons had an increased risk of immunohistochemical "misclassification" due to failure to detect TrpH, possibly due to more complete dialysis of intracellular contents during lengthy recordings. This risk was minimal for type 1 or 2 serotonergic neurons. The three different types of spinally projecting serotonergic neurons also differed markedly in their responsiveness to the mu opioid receptor agonist D-Ala2, NMePhe4, Gly5-ol]enkephalin. These results provide important new electrophysiological and pharmacological evidence for a significant heterogeneity among spinally projecting serotonergic RVM neurons. They may also provide a basis for resolving the controversy concerning the role of serotonergic RVM neurons in opioid analgesia.

Regulation of thermogenesis by the central melanocortin system

Adaptive thermogenesis represents one of the important homeostatic mechanisms by which the body maintains appropriate levels of stored energy and its core temperature. Dysregulation of adaptive thermogenesis promotes obesity. The central melanocortin system, in particular the melanocortin 4 receptor (MC4R) signaling pathway, influences the regulation of every aspect of energy balance, including thermogenesis, and plays a critical role in energy homeostasis in both rodent and man. This review will outline our current understanding of adaptive thermogenesis, focusing on the role of the central melanocortin pathway in the regulation of thermogenesis.

Stimulation of the lateral hypothalamus produces antinociception mediated by 5-HT1A, 5-HT1B and 5-HT3 receptors in the rat spinal cord dorsal horn

The lateral hypothalamus is part of an efferent system that modifies pain at the spinal cord dorsal horn, but the mechanisms by which lateral hypothalamus-induced antinociception occur are not fully understood. Previous work has shown that antinociception produced from electrical stimulation of the lateral hypothalamus is mediated in part by spinally projecting 5-hydroxytryptamine (5-HT) neurons in the ventromedial medulla. To further examine the role of the lateral hypothalamus in antinociception, the cholinergic agonist carbamylcholine chloride (125 nmol) was microinjected into the lateral hypothalamus of female Sprague-Dawley rats and nociceptive responses measured on the tail-flick and foot-withdrawal tests. Intrathecal injections of the selective 5-HT1A, 5-HT1B, 5-HT3 receptor antagonists, WAY 100135, SB-224289, and tropisetron, respectively, and the non-specific antagonist methysergide, were given. Lateral hypothalamus stimulation with carbamylcholine chloride produced significant antinociception that was blocked by WAY 100135, tropisetron, and SB-224289 on both the tail-flick and foot-withdrawal tests. Methysergide was not different from controls on the tail flick test, but increased foot-withdrawal latencies compared with controls. These results suggest that the lateral hypothalamus modifies nociception in part by activating spinally projecting serotonin neurons that act at 5-HT1A, 5-HT1B, and 5-HT3 receptors in the dorsal horn.

Serotonergic fiber sprouting to external anal sphincter motoneurons after spinal cord contusion

The present study analyzed the anatomical plasticity of serotonergic immunoreactive projections to external anal sphincter (EAS) motoneurons, and the behavioral plasticity of EAS reflexes, penile erection, and locomotion in rats with spinal contusion injury (SCI) or complete spinal cord transection (TX). Electromyographic activity of the EAS, penile erection latency, and BBB locomotor score exhibited parallel recovery over the 6-week recovery period after contusion SCI. This pattern of recovery was not observed in TX animals. While locomotor scores demonstrated a small increase after TX, erectile and anorectal function remained at abnormal levels established immediately after injury. Serotonergic immunofluorescent (5-HT-IF) staining at the lesion site identified a small number of fibers spared after SCI that may provide a substrate for functional recovery. Pixel density measurements of 5-HT-IF in the vicinity of retrogradely labeled EAS and unlabeled pudendal motoneurons necessary for penile erection provide indirect evidence of serotonergic sprouting that parallels the observed functional recovery in animals with SCI. No 5-HT-IF was detected caudal to the injury site in TX animals. These studies indicate: (1) lumbosacral eliminative and reproductive reflexes provide a valid means of studying the mechanisms of post-SCI plasticity; (2) the similar recovery curves suggest similar return of descending control, perhaps through sprouting of descending serotonergic fibers; (3) the observed deficits after TX likely represent the permanent removal of descending inhibition and reflect reorganization of segmental circuitry.

Changes in the substance P-containing innervation of the lumbosacral spinal cord in male Wistar rats as a consequence of ageing

Quantitative image analysis was used to determine age-related changes in the substance P-containing innervation of autonomic and somatic nuclei in the lumbosacral spinal cord, which are associated with the control of micturition and sexual reflexes. In the upper lumbar segments (L1-L2), significant declines in the distribution density of substance P-containing processes were observed in the dorsal grey commissure, the intermediolateral cell column and the ventral horn. More caudally, at levels corresponding to L5 through S1, significant reductions were seen in the dorsal grey commissure and within the sacral parasympathetic nucleus. In contrast to these observations, the substance P-immunoreactive innervation of the dorsolateral nucleus remained robust in aged animals and was not significantly different from young adults. It is possible that these distinct age-related patterns of change in substance P-containing innervation, are reflected in the urinary/sexual dysfunction's in aged animals.

Enkephalinergic inhibition of raphe pallidus inputs to rat hypoglossal motoneurones in vitro

Hypoglossal motoneurones play a major role in maintaining the patency of the upper airways and in determining airways resistance. These neurones receive inputs from many different regions of the neuroaxis including the caudal raphe nuclei. Whilst we have previously shown that glutamate is utilised in projections from one of these caudal raphe nuclei, the raphe pallidus, to hypoglossal motoneurones, these raphe pallidus-hypoglossal projections also contain multiple co-localised neuropeptides, including a population that are immunopositive for enkephalin. The role of enkephalin in the control of hypoglossal motoneurones is unknown. Therefore the aim of these studies was to determine whether enkephalins modulate caudal raphe glutamatergic inputs to hypoglossal motoneurones. Whole cell recordings were made from rat hypoglossal motoneurones in vitro, with glutamate-mediated excitatory postsynaptic currents (EPSCs) evoked in these neurones following electrical stimulation within the raphe pallidus. Superfusion of enkephalin significantly decreased the amplitude of these raphe pallidus evoked EPSCs (56.1+/-29% of control, P<0.001), an action that was mirrored by the tau-opioid receptor agonist, [D-Ala, N-Me-Phe, Gly-ol]-enkephalin acetate (DAMGO;53.8+/-26%, P<0.01), but not by the delta-opioid receptor agonist, [D-Pen]-enkephalin (DPDPE). Enkephalin also increased the amplitude ratio (1.57+/-0.36 vs. 1.14+/-0.27, P<0.01) of pairs of evoked EPSCs (paired pulse ratio), decreased the frequency (P<0.0001) but not the amplitude of miniature EPSCs, whilst having no effect on the inward current evoked by glutamate applied directly to the postsynaptic cell (97.8+/-2.2% of control, P=n.s.). Likewise, DAMGO also increased the paired pulse ratio (1.62+/-0.35 vs. 1.31+/-0.14, P<0.05) and decreased the frequency of miniature EPSCs (P<0.0001). Together, these data suggest that enkephalin acts at tau-opioid receptors located on the presynaptic terminals of raphe pallidus inputs to hypoglossal motoneurones to significantly decrease glutamate release from these projections.

The functional anatomy of neuropathic pain

The generation of neuropathic pain is a complex phenomenon involving a process of peripheral and central sensitization producing enhanced transmission of nociceptive inputs to the brain associated with the loss of discriminatory processing of noxious and innocuous stimuli. This increased flow of abnormally processed nociceptive inputs to the brain may overcome the ability of descending modulatory pathways to produce analgesia, causing further worsening of the pain. Several crucial locations involved in the physiologic generation of pain inputs (eg, peripheral nociceptors, dorsal horns, thalamus, cortex) show evidence of functional reorganization and altered nociceptive processing in association with chronic pain. These locations present the best targets for therapeutic intervention, including systemic administration of drugs able to counteract the chemical storm induced by neural injuries in the nociceptive afferents and dorsal horns, or for more focused intervention, such as neuroablative procedures; intrathecal drug delivery; and spinal cord, deep brain, or motor cortex stimulation.

Separate populations of neurons in the rostral ventromedial medulla project to the spinal cord and to the dorsolateral pons in the rat

Activation of neurons in the rostral ventromedial medulla (RVM) directly modulates spinal nociceptive transmission by projections to the spinal cord dorsal horn and indirectly by projections to neurons in the dorsolateral pons (DLP) that project to the spinal cord dorsal horn. However, it is not known whether the same neurons in the RVM produce both direct and indirect modulation of nociception. Deposits of the retrograde tracers Fluoro-Gold (FG) in the spinal cord dorsal horn and DiI in the DLP were used to determine whether the same RVM neurons project to both of these regions. Only 0.9+/-0.1% of RVM neurons retrogradely labeled with Fluoro-Gold from the spinal cord were also labeled with DiI placed in the DLP. In addition, spinally projecting RVM neurons were significantly larger than RVM neurons that project to the DLP. Finally, spinally projecting neurons were found predominantly on the midline and within the RVM; neurons that project to the DLP had a wider distribution and were present both within and outside of the RVM. Thus, separate and morphologically distinct populations of RVM neurons appear to modulate nociception by direct and indirect descending pathways.

Spinal muscarinic receptors are activated during low or high frequency TENS-induced antihyperalgesia in rats

Transcutaneous electrical nerve stimulation (TENS) is a non-pharmacological modality used clinically to relieve pain. Central involvement of serotonin and endogenous opioids are implicated in TENS-induced analgesia. Activation of spinal cholinergic receptors is antinociceptive and these receptors interact with opioid and serotonin receptors. In the current study, the possible involvement of spinal cholinergic receptors in TENS analgesia was investigated in rats. Hyperalgesia was induced by inflaming one knee joint with 3% kaolin-carrageenan and assessed by measuring paw withdrawal latency (PWL) to heat before and 4 h after injection. The non-selective nicotinic antagonist mecamylamine (50 microg), non-selective muscarinic antagonist atropine (30 microg) or one of the muscarinic subtype antagonists: pirenzepine (M1, 10 microg), methoctramine (M2, 10 microg), 4-DAMP (M3, 10 microg), or saline was administered intrathecally just prior to TENS treatment. Low or high frequency TENS was then applied to the inflamed knee and PWL was determined again. Atropine, pirenzepine and 4-DAMP significantly attenuated the antihyperalgesic effects of low and high frequency TENS while mecamylamine and methoctramine had no effects, compared to saline control. The results show that TENS-induced antihyperalgesia is mediated partially by activation of spinal muscarinic receptors but not spinal nicotinic receptors. Further, the results also indicate that spinal M1 and M3 muscarinic receptor subtypes mediate the muscarinic component of TENS antihyperalgesia.

Does a single intravenous injection of the 5HT3 receptor antagonist ondansetron have an analgesic effect in neuropathic pain? A double-blinded, placebo-controlled cross-over study

Neurokinin-1-expressing neurones in lamina I to III of the spinal cord are intimately involved in the regulation of ascending and spino-bulbal pathways that regulate excitatory transmission. In experimental animals, ablation of these neurones reduces the responses to a variety of nociceptive stimuli. Furthermore, in animals, spinal application of the selective 5HT3 receptor antagonist ondansetron mimics these effects, indicating that 5HT3 receptors play a pronociceptive role and mediate descending excitatory controls that allow spinal neurones to fully code peripheral stimuli. In this study, we examined the potential analgesic effect of a single IV injection of ondansetron in humans with chronic neuropathic pain. Each consenting subject received a single IV injection of 8 mg ondansetron and placebo in varying order at least 1 wk apart with pain scores being recorded for the 48 h preceding and after each injection. Pain scores were significantly reduced 2 h after ondansetron injection (but at no other time point). This suggests that ondansetron can have an analgesic effect in neuropathic pain. Side effects were minor and infrequent.

IMPLICATIONS

The selective 5HT3 receptor antagonist ondansetron, currently used as an antiemetic, may also have analgesic properties. Side effects with a single IV injection are infrequent and usually mild.

Superficial NK1-expressing neurons control spinal excitability through activation of descending pathways

The increase in pain sensitivity that follows injury is regulated by superficially located projection neurons in the dorsal horn of the spinal cord that express the neurokinin-1 (NK1) receptor. After selective ablation of these neurons in rats, we identified changes in receptive field size, mechanical and thermal coding and central sensitization of deeper dorsal horn neurons that are important for both pain sensations and reflexes. We were able to reproduce these changes by pharmacological block of descending serotonergic facilitatory pathways. Using Fos histochemistry, we found changes in the activation of serotonergic neurons in the brainstem as well as evidence for a loss of descending control of spinal excitability. We conclude that NK1-positive spinal projection neurons, activated by primary afferent input, project to higher brain areas that control spinal excitability--and therefore pain sensitivity--primarily through descending pathways from the brainstem.

Neurochemistry of nerve fibers apposing sympathetic preganglionic neurons activated by sustained hypotension

Sympathetic preganglionic neurons (SPN) in rat spinal cord were activated by the reflex stimulation of bulbospinal sympathetic neuronal pathways after a nitroprusside-induced hypotension. Hypotension-sensitive SPN, identified by immunoreactivity (IR) to the product of the immediate early gene c-fos and to choline acetyltransferase, were localized in the intermediolateral cell column of thoracic and upper lumbar cord, particularly middle to lower thoracic cord. Putative neurotransmitters, or their markers, in varicose fiber networks around SPN were identified. Nearly all hypotension-sensitive (Fos-IR) SPN were apposed by varicose fibers immunoreactive for tyrosine hydroxylase, serotonin, substance P, or enkephalin. Neuropeptide Y (NPY)- or phenylethanolamine-N-methyl transferase (PNMT)-IR varicose fibers apposed Fos-IR SPN in the upper and middle thoracic spinal cord, but in lower thoracic segments some Fos-IR SPN lacked these appositions. In thoracic segment 12, 51% +/- 5% of Fos-IR SPN (n = 9 rats) lacked PNMT contacts and 25% +/- 3% of Fos-IR SPN (n = 8 rats) lacked NPY contacts. In contrast to other chemically defined afferents, galanin-IR varicose fibers apposed fewer than half of the Fos-IR SPN in the middle to lower thoracic cord. Neurotransmitters/neuromodulators that might influence the activity of SPN acting in the baroreflex-mediated control of blood pressure have been identified. Uniformity in the neurochemistry of some fibers making connections with Fos-IR SPN, regardless of their segmental origin, suggests that common sets of neurons provide convergent inputs to all hypotension-sensitive SPN. Other fibers show topographic differences in their contacts with Fos-IR SPN, suggesting that subgroups of hypotension-sensitive SPN are targeted by particular neuron groups.

Calcitonin Gene-related Peptide (CGRP)-like Immunoreactivity and CGRP mRNA in Rat Spinal Cord Motoneurons after Different Types of Lesions

By use of the indirect immunofluorescence (IF) technique, radioimmunoassay (RIA) and in situ hybridization (ISH) histochemistry, the staining pattern, content and expression of calcitonin gene-related peptide (CGRP) in lumbar motoneurons of normal rats and rats subjected to sciatic nerve transection (SNT), ventral root transection (VRT), low thoracic spinal cord transection (SCT) alone or in combination with a subsequent SNT, as well as rats subjected to chemical lesioning of 5-hydroxytryptamine (5-HT) neurons by 5,7-dihydroxytryptamine (5,7-DHT), were studied. We here confirm that a large number of the lumbar motoneurons normally contain CGRP-like immunoreactivity (LI) and CGRP mRNA. SNT induced a transient increase in CGRP-LI, with a peak at days 2 - 5 after lesion, and normalized levels again after approximately 2 - 3 weeks. Comparable results were obtained with IF and RIA. This increase is probably a consequence of increased CGRP synthesis, since a parallel up-regulation of CGRP mRNA levels was seen. A normalization of CGRP mRNA did not occur during the period studied, despite an apparent normalization of peptide levels after 2 weeks, and this may in turn be due to an increased turnover and/or release of CGRP. The up-regulation of CGRP is probably caused by the axon injury itself, since a similar cellular reaction with respect to CGRP was observed in motoneurons subjected to VRT. However, SNT, which also lesions dorsal root afferents and causes a decline in CGRP-LI in the dorsal horn, induced an increase in CGRP-LI in motoneurons on the contralateral side also. Thus, it may be that severance of dorsal root afferents and/or changes in reflex activity may also influence the production of CGRP in motoneurons. SCT, which severs all descending synaptic input to the motor nucleus and causes a paralysis of muscles innervated by motoneurons below the lesion, resulted in a marked decline in both content of CGRP-LI (IF and RIA) and expression of CGRP mRNA. However, treatment with 5,7-DHT, which lesions 5-HT neurons, including those giving rise to the bulbospinal serotoninergic pathway, did not cause any dramatic changes in motor behaviour but induced an increase in both motoneuron content of CGRP-LI and expression of CGRP mRNA. In rats first subjected to SCT, which depresses CGRP, followed 2 weeks later by SNT, we found a marked increase in both content of CGRP-LI (IF and RIA) and expression of mRNA coding for CGRP. In summary our results show that the cellular production of the CGRP peptide, normally expressed in motoneurons, is influenced in a complex way by motoneuron injury as well as changes in the afferent input. There also appear to be important differences in the expression of CGRP in small (gamma) and large (alpha) motoneurons as well as between motoneurons of different nuclei, in normal as well as axotomized rats.

Ondansetron inhibits the analgesic effects of tramadol: a possible 5-HT(3) spinal receptor involvement in acute pain in humans

To investigate a possible antinociceptive role of serotonin receptor subtype 3 (5-HT(3)), we evaluated the effects of a coadministration of ondansetron, a 5-HT(3) selective antagonist, and tramadol, a central analgesic dependent on enhanced serotonergic transmission. Fifty-nine patients undergoing ear, throat, and nose surgery, using tramadol for 24-h postoperative patient-controlled analgesia (bolus = 30 mg; lockout interval = 10 min) were randomly allocated either to a group receiving ondansetron continuous infusion (1 mg. mL(-1). h(-1)) for postoperative nausea and vomiting (Group O) or to a control group receiving saline (Group T). Pain and vomiting scores and tramadol consumption were evaluated at 4, 8, 12, and 24 h. Pain scores were never >4, according to a 0-10 numerical rating scale, in both groups. Group O required significantly larger doses of tramadol at 4 h (213 versus 71 mg, P < 0.001), 8 h (285 versus 128 mg, P < 0.002), and 12 h (406 versus 190 mg, P < 0.002). Vomiting scores were higher in Group O at 4 h (P < 0.05) and 8 h (P = 0.05). We conclude that ondansetron reduced the overall analgesic effect of tramadol, probably blocking spinal 5-HT(3) receptors.

IMPLICATIONS

Serotonin is an important neurotransmitter of the descending pathways that down-modulate spinal nociception. In postoperative pain, ondansetron, a selective 5-HT(3) receptor antagonist, increased the analgesic dose of tramadol. We suggest that, when antagonized for antiemetic purpose, 5-HT(3) receptors foster nociception, because of their site-dependent action.