Involvement of solitary tract nucleus in control of nociceptive transmission in cat spinal cord neurons

Measures of cardiovascular function suggest autonomic nervous system dysregulation after surgical induction of joint injury in the male Lewis rat

OBJECTIVES

Functional changes in the autonomic nervous system may help explain variability in the progression of knee osteoarthritis (OA). Thus, the objective of this study was to evaluate autonomic nervous system shifts, measured via heart rate response variables, in rat knee joint injury and OA models.

METHODS

Cardiovascular characteristics were measured at baseline and bi-weekly for 8 weeks after skin incision, medial collateral ligament transection (MCLT), or MCLT+medial meniscus transection (MCLT+MMT). Heart rate was also assessed during a mild stressor (elevated maze). At endpoint, cardiovascular responses to mechanical knee stimuli were evaluated, as well as responses to 1-phenylbiguanide, a 5HT_3A receptor agonist with reported ability to stimulate vagal responses.

RESULTS

During low activity, a slower heart rate occurred in MCLT (299 ± 10 bpm) and MCLT+MMT (310 ± 10 bpm) animals compared to controls (325 ± 10 bpm). Furthermore, patellar ligament mechanical stimuli produced an immediate decrease in heart rate and blood pressure in all groups. Finally, a larger drop in heart rate was observed in MCLT (252 ± 40 bpm) and MCLT+MMT (263 ± 49 bpm) following administration of 1-phenylbiguanide compared to skin incision (168 ± 45 bpm).

CONCLUSIONS

Acute mechanical stimulation of the patellar ligament produced drops in heart rate, suggesting a possible joint-brain connection that modulates autonomic responses. With both joint injury, cardiac vagal activation was altered in response to pharmacological stimulation, with chronic longitudinal heart rate reduction. These data provide some preliminary evidence of potential functional shifts in autonomic nervous system function in models of joint injury and OA.

Pancreas-Brain Crosstalk

The neural regulation of glucose homeostasis in normal and challenged conditions involves the modulation of pancreatic islet-cell function. Compromising the pancreas innervation causes islet autoimmunity in type 1 diabetes and islet cell dysfunction in type 2 diabetes. However, despite the richly innervated nature of the pancreas, islet innervation remains ill-defined. Here, we review the neuroanatomical and humoral basis of the cross-talk between the endocrine pancreas and autonomic and sensory neurons. Identifying the neurocircuitry and neurochemistry of the neuro-insular network would provide clues to neuromodulation-based approaches for the prevention and treatment of diabetes and obesity.

Functional involvement of nucleus tractus solitarii neurons projecting to the parabrachial nucleus in trigeminal neuropathic pain

Peripheral nerve injury can induce neuroplastic changes in the central nervous system and result in neuropathic pain. This study investigated functional involvement in dorsal paratrigeminal nucleus (dPa5) and nucleus tractus solitarii (NTS) neurons projecting to the parabrachial nucleus (PBN) after trigeminal nerve injury. Anatomical quantification was performed based on phosphorylated extracellular signal-regulated kinase (pERK) expression underlying orofacial neuropathic pain associated with infraorbital nerve chronic constriction injury (ION-CCI) in rats. ION-CCI rats exhibited heat and mechanical hypersensitivity in the ipsilateral upper lip. After injection of retrograde tracer fluorogold (FG) into the contralateral PBN, ION-CCI rats received capsaicin or noxious mechanical stimulation to the upper lip. The total number of FG-labeled neurons in dPa5 and NTS did not change after ION-CCI, and pERK expression in dPa5 did not differ between sham and ION-CCI rats. In the NTS contralateral to ION-CCI, the number of pERK-immunoreactive neurons and percentage of pERK-immunoreactive FG-labeled PBN projection neurons were increased after capsaicin stimulation in ION-CCI rats. The present findings suggest that enhanced noxious inputs from the NTS to the PBN after trigeminal nerve injury modulates PBN neuron activity, which accompanies the affective components of orofacial neuropathic pain.

Current Directions in the Auricular Vagus Nerve Stimulation I - A Physiological Perspective

Electrical stimulation of the auricular vagus nerve (aVNS) is an emerging technology in the field of bioelectronic medicine with applications in therapy. Modulation of the afferent vagus nerve affects a large number of physiological processes and bodily states associated with information transfer between the brain and body. These include disease mitigating effects and sustainable therapeutic applications ranging from chronic pain diseases, neurodegenerative and metabolic ailments to inflammatory and cardiovascular diseases. Given the current evidence from experimental research in animal and clinical studies we discuss basic aVNS mechanisms and their potential clinical effects. Collectively, we provide a focused review on the physiological role of the vagus nerve and formulate a biology-driven rationale for aVNS. For the first time, two international workshops on aVNS have been held in Warsaw and Vienna in 2017 within the framework of EU COST Action "European network for innovative uses of EMFs in biomedical applications (BM1309)." Both workshops focused critically on the driving physiological mechanisms of aVNS, its experimental and clinical studies in animals and humans, in silico aVNS studies, technological advancements, and regulatory barriers. The results of the workshops are covered in two reviews, covering physiological and engineering aspects. The present review summarizes on physiological aspects - a discussion of engineering aspects is provided by our accompanying article (Kaniusas et al., 2019). Both reviews build a reasonable bridge from the rationale of aVNS as a therapeutic tool to current research lines, all of them being highly relevant for the promising aVNS technology to reach the patient.

Visualizing the trigeminovagal complex in the human medulla by combining ex-vivo ultra-high resolution structural MRI and polarized light imaging microscopy

A trigeminovagal complex, as described in some animals, could help to explain the effect of vagus nerve stimulation as a treatment for headache disorders. However, the existence of a trigeminovagal complex in humans remains unclear. This study, therefore investigated the existence of the trigeminovagal complex in humans. One post-mortem human brainstem was scanned at 11.7T to obtain structural (T1-weighted) and diffusion magnetic resonance images ((d)MR images). Post-processing of dMRI data provided track density imaging (TDI) maps to investigate white matter at a smaller resolution than the imaging resolution. To evaluate the reconstructed tracts, the MR-scanned brainstem and three additional brainstems were sectioned for polarized light imaging (PLI) microscopy. T1-weighted images showed hyperintense vagus medullar striae, coursing towards the dorsomedial aspect of the medulla. dMRI-, TDI- and PLI-images showed these striae to intersect the trigeminal spinal tract (sp5) in the lateral medulla. In addition, PLI images showed that a minority of vagus fibers separated from the vagus trajectory and joined the trigeminal spinal nucleus (Sp5) and the sp5. The course of the vagus tract in the rostral medulla was demonstrated in this study. This study shows that the trigeminal- and vagus systems interconnect anatomically at the level of the rostral medulla where the vagus fibers intersect with the Sp5 and sp5. Physiological and clinical utility of this newly identified interconnection is a topic for further research.

Targeted Vagus Nerve Stimulation for Rehabilitation After Stroke

Stroke is a leading cause of disability worldwide, and in approximately 60% of individuals, upper limb deficits persist 6 months after stroke. These deficits adversely affect the functional use of the upper limb and restrict participation in day to day activities. An important goal of stroke rehabilitation is to improve the quality of life by enhancing functional independence and participation in activities. Since upper limb deficits are one of the best predictors of quality of life after stroke, effective interventions targeting these deficits may represent a means to improve quality of life. An increased understanding of the neurobiological processes underlying stroke recovery has led to the development of targeted approaches to improve motor deficits. One such targeted strategy uses brief bursts of Vagus Nerve Stimulation (VNS) paired with rehabilitation to enhance plasticity and support recovery of upper limb function after chronic stroke. Stimulation of the vagus nerve triggers release of plasticity promoting neuromodulators, such as acetylcholine and norepinephrine, throughout the cortex. Timed engagement of neuromodulators concurrent with motor training drives task-specific plasticity in the motor cortex to improve function and provides the basis for paired VNS therapy. A number of studies in preclinical models of ischemic stroke demonstrated that VNS paired with rehabilitative training significantly improved the recovery of forelimb motor function compared to rehabilitative training without VNS. The improvements were associated with synaptic reorganization of cortical motor networks and recruitment of residual motor neurons controlling the impaired forelimb, demonstrating the putative neurobiological mechanisms underlying recovery of motor function. These preclinical studies provided the basis for conducting two multi-site, randomized controlled pilot trials in individuals with moderate to severe upper limb weakness after chronic ischemic stroke. In both studies, VNS paired with rehabilitation improved motor deficits compared to rehabilitation alone. The trials provided support for a 120-patient pivotal study designed to evaluate the efficacy of paired VNS therapy in individuals with chronic ischemic stroke. This manuscript will discuss the neurobiological rationale for VNS therapy, provide an in-depth discussion of both animal and human studies of VNS therapy for stroke, and outline the challenges and opportunities for the future use of VNS therapy.

Evolution and physiology of neural oxygen sensing

Major evolutionary trends in animal physiology have been heavily influenced by atmospheric O2 levels. Amongst other important factors, the increase in atmospheric O2 which occurred in the Pre-Cambrian and the development of aerobic respiration beckoned the evolution of animal organ systems that were dedicated to the absorption and transportation of O2, e.g., the respiratory and cardiovascular systems of vertebrates. Global variations of O2 levels in post-Cambrian periods have also been correlated with evolutionary changes in animal physiology, especially cardiorespiratory function. Oxygen transportation systems are, in our view, ultimately controlled by the brain related mechanisms, which senses changes in O2 availability and regulates autonomic and respiratory responses that ensure the survival of the organism in the face of hypoxic challenges. In vertebrates, the major sensorial system for oxygen sensing and responding to hypoxia is the peripheral chemoreflex neuronal pathways, which includes the oxygen chemosensitive glomus cells and several brainstem regions involved in the autonomic regulation of the cardiovascular system and respiratory control. In this review we discuss the concept that regulating O2 homeostasis was one of the primordial roles of the nervous system. We also review the physiology of the peripheral chemoreflex, focusing on the integrative repercussions of chemoreflex activation and the evolutionary importance of this system, which is essential for the survival of complex organisms such as vertebrates. The contribution of hypoxia and peripheral chemoreflex for the development of diseases associated to the cardiovascular and respiratory systems is also discussed in an evolutionary context.

The medullary dorsal reticular nucleus enhances the responsiveness of spinal nociceptive neurons to peripheral stimulation in the rat

Single-unit spinal recordings combined with application of glutamate into the medullary dorsal reticular nucleus were used to assess the action of this nucleus upon deep dorsal horn neurons in rats. Injection of high glutamate concentrations (10 and 100 mm) induced a dramatic and long-lasting increase of the responses of wide-dynamic range neurons to electrical stimulation of the sciatic nerve in the noxious range, without affecting ongoing discharges. Post-stimulus time histograms revealed that this increase concerned the post-discharge, but not A- or C-fibre-mediated responses, which remained unchanged independently of the stimulation frequency applied. The onset of the glutamate-induced response enhancement occurred with a concentration-dependent time delay and developed slowly until its maximum. These data indicate that the medullary dorsal reticular nucleus exerts a facilitating action upon deep dorsal horn wide-dynamic range neurons by enhancing their capacity to respond to peripheral stimulation through prolongation of their discharge. This action is accompanied by the strengthening of wind-up of deep dorsal horn wide-dynamic range neurons, hence providing a plausible substrate for chronic pain states. These results are in agreement with previous behavioural studies suggesting a pronociceptive role for the dorsal reticular nucleus [Almeida et al. (1996) Brain Res. Bull., 39, 7-15; Almeida et al. (1999) Eur. J. Neurosci., 11, 110-122], and support the involvement of a reverberating circuit, previously described in morphological studies [Almeida et al. (1993) Neuroscience, 55, 1093-1106; Almeida et al. (2000) Eur. J. Pain, 4, 373-387], which probably operates only at a certain threshold of activation.

Projection sites of superficial and deep spinal dorsal horn cells in the nucleus tractus solitarii of the rat

By using anterograde transport of biotin dextran amine injected into the cervical spinal dorsal horn, we have shown that fibres from superficial and deep dorsal horn project to the nucleus tractus solitarii via two distinct pathways. Afferent fibres from the superficial lamina (I-III) were found to course in the dorsal funiculus and terminate bilaterally in the caudal zone of the nucleus tractus solitarii (NTS), mainly within the commissural subnucleus. In contrast, afferents from the deeper dorsal horn laminae (IV-V) were found to course in the dorsolateral fasciculus and terminate ipsilaterally, mostly in the lateral areas of the caudal nucleus tractus solitarii. Similar, but more extensive patterns of labelled fibres were produced by injections into the white matter of the dorsal funiculus and dorsolateral fasciculus, respectively. These observations suggest that the caudal NTS not only serves as a location of visceral afferent convergence and integration, but may also be a receptive area for monosynaptic projections from dorsal horn neurons receiving sensory afferent inputs. Such projections may represent pathways through which NTS neurons are influenced by nociceptive and non-nociceptive information from the dorsal horn and thereby can co-ordinate the appropriate autonomic response, including adjustments in cardiorespiratory reflex output.

ORL-1 and mu opioid receptor antisera label different fibers in areas involved in pain processing

Mu opioid receptors (MOR) mediate the analgesic effects of opioid drugs such as morphine. The opioid receptor-like (ORL-1) receptor is structurally related to opioid receptors and the ORL-1 receptor agonist, orphanin FQ/nociceptin, induces analgesia at the spinal level, but appears to recruit different circuitry than that used by mu opioids. When administered intracerebroventricularly, orphanin FQ/nociceptin produces hyperalgesia and/or reverses opioid analgesia. The functionally distinct actions elicited by MOR and ORL-1 receptors, which activate similar intracellular signaling systems and show similar regional distributions, could be explained by their differential cellular localization. By using double label immunohistochemistry and confocal microscopy, the present study investigates the distribution of MOR and ORL-1 receptors in regions of the rat nervous system that are involved with nociceptive processing. In general co-localization of MOR and ORL-1 receptor immunoreactivity was not observed in either perikarya or neuropil in the dorsal root ganglia, nor in the Lissauer's tract and superficial laminae of the spinal cord. Likewise, there was no evidence for co-localization of these receptors within the periaqueductal gray, the nucleus raphe magnus, the gigantocellular reticular nucleus, and the nucleus of the solitary tract. These observations indicate that MOR and ORL-1 receptors are expressed predominantly on different fiber systems in these regions. This differential distribution is consistent with the distinct pharmacology of ORL-1 and MOR receptor agonists and suggests that the antisera to MOR and ORL-1 receptors may provide useful markers for further investigations of analgesic and counteranalgesic pathways modulating pain perception.

Comparison of the effects of nucleus tractus solitarius and ventral medial medulla lesions on illness-induced and subcutaneous formalin-induced hyperalgesias

We have previously demonstrated that illness-inducing agents (lipopolysaccharide (LPS)) and inflammatory agents (subcutaneous (s.c.) formalin) induce hyperalgesia by similar pathways. The present series of experiments compared the effects of medullary lesions on these phenomena. These experiments demonstrate that s.c. formalin-induced hyperalgesia, like illness-induced hyperalgesia, is dependent on the nucleus raphe magnus (NRM) but independent of the nucleus reticularis paragigantocellularis (NRPgc). However, these two forms of hyperalgesia differ with regards to their dependence on the nucleus tractus solitarius (NTS). Illness-induced hyperalgesia is abolished by unilateral (left) NTS lesions, whereas formalin-induced hyperalgesia remains unaffected by this procedure. These data provide further evidence that hyperalgesias induced by illness agents and by inflammatory agents are mediated by similar but not identical pathways. They also illustrate that neural structures have the capacity for opposed actions, in that both the NTS and NRM are documented to underlie hyperalgesia as well as analgesia. This capacity for opposed action may prove to be characteristic of structures involved in pain modulation.

The organization and function of endogenous antinociceptive systems

Much progress has been made the understanding of endogenous pain-controlling systems. Recently, new concepts and ideas which are derived from neurobiology, chaos research and from research on learning and memory have been introduced into pain research and shed further light on the organization and function of endogenous antinociception. These most recent developments will be reviewed here. Three principles of endogenous antinociception have been identified, as follows. (1) Supraspinal descending inhibition: the patterns of neuronal activity in diencephalon, brainstem and spinal cord during antinociceptive stimulation in midbrain periaqueductal gray (PAG) or medullary nucleus raphe magnus have now been mapped on the cellular level, using the c-Fos technique. Results demonstrate that characteristic activity patterns result within and outside the PAG when stimulating at its various subdivisions. The descending systems may not only depress mean discharge rates of nociceptive spinal dorsal horn neurons, but also may modify harmonic oscillations and nonlinear dynamics (dimensionality) of discharges. (2) Propriospinal, heterosegmental inhibition: antinociceptive, heterosegmental interneurons exist which may be activated by noxious stimulation or by supraspinal descending pathways. (3) Segmental spinal inhibition: a robust long-term depression of primary afferent neurotransmission in A delta fibers has been identified in superficial spinal dorsal horn which may underlie long-lasting antinociception by afferent stimulation, e.g. by physical therapy or acupuncture.

Temporal modulation of antinociception by reciprocal connections between the dorsomedial medulla and parabrachial region

Microinjection of carbachol into the dorsal parabrachial regio (PBRd) of guinea pigs induces analgesia from the 5th to the 15th min postinjection, as evaluated by the reduction of the vocalization in response to an electric shock applied to one paw. When reversible blockade of the dorsomedial medulla or specifically of the nucleus tractus solitarius (NTS) is performed with xylocaine 5 min after microinjection of carbachol into the PBRd, the analgesic effect continues up to the 45th and to the 60th min, respectively. Blockade of the dorsomedial medulla is achieved by topical application of xylocaine to the area postrema (AP) or microinjection of the drug into the NTS. A prolongation of the duration of the analgesic effect also occurs after the inverse procedure, i.e., after reversible blockade of the PBRd 5 min after topical application of carbachol (1 microgram/microliter)to the AP or microinjection of carbachol into the NTS. In this case, the analgesic action, which lasted up to 30 min when carbachol was applied to the AP and 60 min when microinjected into the NTS, was prolonged up to 60 min and to 80 min, respectively, after reversible blockade of PBR. The present data suggest that the reciprocal connections between the different regions of the dorsomedial medulla and the PBR play an important role in the modulation of the duration of the analgesic effect, and that this fact may be of adaptive importance in the defensive analgesia that occurs in the confrontation between prey and predator.

Neurocircuitry of illness-induced hyperalgesia

We have previously demonstrated that illness-inducing agents such as lithium chloride (LiCl) and the bacterial cell wall endotoxin lipopolysaccharide (LPS) produce hyperalgesia on diverse pain measures. The present series of studies attempted to identify the neurocircuitry mediating these effects. These studies have demonstrated that illness-inducing agents produce hyperalgesia by activating: (a) peripheral nerves rather than by generating a blood-borne mediator (Expt. 1); (b) vagal afferents, specifically afferents within the hepatic branch of the vagus (Expt. 2); (c) as yet unidentified brain site(s) rostral to the mid-mesencephalon (Expt. 6); (d) a centrifugal pathway that arises from the nucleus raphe magnus, and not from the adjacent nucleus reticularis paragigantocellularis pars alpha (Expts. 4 and 5); (e) a centrifugal pathway in the dorsolateral funiculus of the spinal cord (Expt. 3); and (f) the same centrifugal pathways for diverse illness inducing agents (Expts. 3, 7 and 8). These data call for the re-evaluation of a number of assumptions inherent in previous studies of hyperalgesia.

Efferents of the opiocortin-containing region of the commissural nucleus tractus solitarius

Efferent projections of the commissural nucleus tractus solitarius (cNTS0 in the region containing opiocortin-immunoreactive (-IR) neurons were identified using Phaseolus vulgaris leucoagglutinin (PHA-L). Efferents were identified in the bed nucleus of the stria terminalis, preoptic area, amygdala, hypothalamus, periaqueductal gray, parabrachial nucleus, locus coeruleus, medullary catecholaminergic groups, and NTS. The PHA-L-IR varicosities in lateral parabrachial nucleus were identified in close association with CRF-IR and enkephalin-IR cells. These data on cNTS projections are consistent with our previous immunocytochemical and lesion studies on opiocortin connectivity and provide anatomical evidence that neurons in the cNTS may influence cardiovascular and sympathetic nervous system function via connectivity with nuclei in the lateral brain stem.

Structural types of spinal cord marginal (lamina I) neurons projecting to the nucleus of the tractus solitarius in the rat

The structural types of spinal cord marginal (lamina I) neurons projecting to the nucleus of the tractus solitarius (NTS) were studied. Upon injections of cholera toxin subunit B (CTb) into the caudal part of the NTS, including its lateral and medial portions, labeled cells occurred bilaterally in laminae I, IV-VII, and X, and the lateral spinal nucleus (LSN). After injections into the lateral portion alone, only a few cells were labeled in laminae V, VII, and X, and the LSN, and none in the superficial dorsal horn. Of 1882 labeled marginal cells, 38% belonged to the flattened type, 37% to the pyramidal type, and 25% to the fusiform type. Flattened and pyramidal cells were labeled in considerably greater numbers than those reported when other supraspinal targets of these cells were injected with CTb. Since cells in the NTS are known to be under marked gamma-aminobutyric acidergic (GABA-ergic) inhibition, it is possible that only strong input conveyed by great numbers of flattened and pyramidal cells is capable of overcoming that barrier. Fusiform cells were labeled in numbers similar to those observed previously after tracer injections into the two other targets of this neuronal type, the parabrachial nuclei and the lateral reticular nucleus. Considering that these regions, as well as the NTS, control cardiovascular and respiratory functions, it is suggested that fusiform cells transmit noxious input that will influence autonomic reflexes processed in the three nuclei.

Morphological characterization of marginal (lamina I) neurons immunoreactive for substance P, enkephalin, dynorphin and gamma-aminobutyric acid in the rat spinal cord

Neurons of the rat spinal cord were immunostained for substance P, enkephalin and dynorphin in colchicine-treated animals, and for gamma-aminobutyric acid (GABA). Lamina I stained cells were classified in the four neuronal groups of our previous morphological classification of marginal cells (See Lima and Coimbra, 1986), according to their configuration in the three main anatomical planes. Most lamina I cells exhibiting substance P-immunoreactivity belonged in the group of flattened neurons. Most enkephalinergic cells were pyramidal neurons, while GABA-immunoreactive cells included all multipolar stained neurons and some fusiform neurons. Dynorphin-immunoreactive cells could be fusiform, pyramidal or flattened. The different neurochemical nature and supraspinal projection patterns are suggestive of functional specificity for each group. It is likely that each immunocytochemical subset in each cell group includes tract cells acting at their projection target and intrinsic neurons with local functional roles.

Dorsal raphe nucleus efferents: termination in peptidergic fields

This study was performed to identify the efferents of the dorsal raphe nucleus (DRN), particularly in regard to nociception. Phaseolus vulgaris leucoagglutinin (PHA-L) was microiontophoresed into the DRN and visualized immunocytochemically; PHA-L-immunoreactive (-IR) fibers and terminals were identified in the forebrain, hypothalamus, midline and intralaminar thalamus, habenula, periaqueductal gray, locus coeruleus, parabrachial nucleus, medullary raphe and reticular nuclei, and nucleus tractus solitarius. Dual immunocytochemistry was used to identify corticotropin-releasing factor, neurotensin, and enkephalin neurons in DRN terminal fields, in some cases in close proximity to PHA-L-IR terminals. Terminal fields were identified in regions that influence nociception, and the neuroactive substances identified in these terminal fields may play modulatory roles in nociception.

Morphine or U-50,488 suppresses Fos protein-like immunoreactivity in the spinal cord and nucleus tractus solitarii evoked by a noxious visceral stimulus in the rat

Immunohistochemical visualization of Fos protein, the nuclear phosphoprotein product of the early-immediate gene c-fos, permits identification of populations of neurons that are activated in response to a variety of stimuli. This study examined the distribution of Fos-like immunoreactive (FLI) neurons in the spinal cord and the nucleus tractus solitarii (NTS) of the caudal medulla evoked by a noxious visceral stimulus in the unanesthetized rat. It also compared the inhibition of pain behavior and Fos expression by a mu-selective opioid agonist, morphine, and a kappa-selective opioid agonist, U-50,488. Intraperitoneal injection of 3.5% acetic acid in the unanesthetized rat evoked the expression of FLI in a discrete population of spinal cord neurons, the distribution of which closely mirrored the spinal terminations of visceral primary afferents. Specifically, FLI neurons were concentrated in laminae I, IIo, V, VII, and X. Large numbers of Fos-immunoreactive neurons were also present in the NTS of the caudal medulla, most likely as a result of spinosolitary tract and vaginal afferent input. The number of labeled neurons in both the spinal cord and the NTS was significantly correlated with the number of abdominal stretches, a pain behavior measure. Both morphine (1-10 mg/kg s.c.) and U-50,488 (3-30 mg/kg s.c.) produced a dose-dependent inhibition of the pain behavior in these animals and a dose-dependent suppression of the number of FLI neurons in both the spinal cord and in the NTS; complete suppression of FLI neurons was, however, not necessary for the production of antinociception. Furthermore, although equianalgesic doses of morphine and U-50,488 reduced the number of labelled neurons in the spinal cord to a comparable extent, morphine reduced the number of immunoreactive neurons in the NTS to a greater extent than did U-50,488. These results suggest that morphine and U-50,488 have comparable effects on the transmission of visceral nociceptive messages by spinal neurons, but differentially affect the autonomic response to noxious visceral stimuli.

Serotonin-, substance P- and tyrosine hydroxylase-like immunoreactive neurons projecting from the midbrain periaqueductal gray to the nucleus tractus solitarii in the rat

Serotonin-, substance P- and tyrosine hydroxylase-like immunoreactive neurons in the midbrain periaqueductal gray (PAG) were observed to send their axons to the nucleus tractus solitarii in the rat by the retrograde horseradish peroxidase tracing method combined with the immunocytochemical technique. These neurons were most frequently observed in the ventrolateral subnucleus and ventral portion of the medial subnucleus of the PAG at the entire rostrocaudal levels.

Nucleus tractus solitarii lesions alter the metabolic and hyperthermic response to central prostaglandin E1 in the rat

1. Given that the nucleus tractus solitarii (NTS) may regulate the ability of brown adipose tissue to evoke non-shivering thermogenesis and that brown fat may mediate the rise in whole-body metabolism observed following central pyrogen administration, we assessed whether interruption of baroreceptor afferents coursing though the NTS would interfere with the ability of prostaglandin E1 to evoke a normal fever response profile. 2. Infusion of 150-600 ng of prostaglandin E1 (PGE1) into a lateral cerebral ventricle of the conscious rat resulted in a rise in core temperature, and also an increase in whole-body metabolic rate, brown adipose tissue temperature, arterial blood pressure and heart rate. 3. Following bilateral electrolytic lesions to the NTS, resting core and brown fat temperatures, metabolic rates, blood pressures and heart rates in the NTS-lesioned animals were comparable to control rats. However, the PGE1-evoked increase in metabolic rate, along with the rise in core and brown adipose tissue temperatures and heart rate were attenuated. The pressor response was, however, enhanced, possibly due to the demonstrated interference by the lesions with normal baroreflex control. 4. The findings suggest that the nucleus tractus solitarii region of the rats' brain may be important in mediating the thermogenesis evoked by central PGE1.

[The nucleus tractus solitarius: neuroanatomic, neurochemical and functional aspects]

The nucleus tractus solitarii (NTS) has long been considered as the first central relay for gustatory and visceral afferent informations only. However, data obtained during the past ten years, with neuroanatomical, biochemical and electrophysiological techniques, clearly demonstrate that the NTS is a structure with a high degree of complexity, which plays, at the medullary level, a key role in several integrative processes. The NTS, located in the dorsomedial medulla, is a structure of small size containing a limited number of neurons scattered in a more or less dense fibrillar plexus. The distribution and the organization of both the cells and the fibrillar network are not homogeneous within the nucleus and the NTS has been divided cytoarchitectonically into various subnuclei, which are partly correlated with the areas of projection of peripheral afferent endings. At the ultrastructural level, the NTS shows several complex synaptic arrangements in form of glomeruli. These arrangements provide morphological substrates for complex mechanisms of intercellular communication within the NTS. The NTS is not only the site of vagal and glossopharyngeal afferent projections, it receives also endings from facial and trigeminal nerves as well as from some renal afferents. Gustatory and somatic afferents from the oropharyngeal region project with a crude somatotopy within the rostral part of the NTS and visceral afferents from cardiovascular, digestive, respiratory and renal systems terminate viscero-topically within its caudal part. Moreover the NTS is extensively connected with several central structures. It projects directly to multiple brain regions by means of short connections to bulbo-ponto-mesencephalic structures (parabrachial nucleus, motor nuclei of several cranial nerves, ventro-lateral reticular formation, raphe nuclei...) and long connections to the spinal cord and diencephalic and telencephalic structures, in particular the hypothalamus and some limbic structures. The NTS is also the recipient of several central afferent inputs. It is worth to note that most of the structures that receive a direct projection from the NTS project back to the nucleus. Direct projections from the cerebral cortex to the NTS have also been identified. These extensive connections indicate that the NTS is a key structure for autonomic and neuroendocrine functions as well as for integration of somatic and autonomic responses in certain behaviors. The NTS contains a great diversity of neuroactive substances. Indeed, most of the substances identified within the central nervous system have also been detected in the NTS and may act, at this level, as classical transmitters and/or neuromodulators.(ABSTRACT TRUNCATED AT 400 WORDS)

Nociception and pain

In recent years, progress in the treatment of pain has been strongly influenced by new insights into the mechanisms underlying pain and nociception. The following article briefly reviews some recent reports which make a significant contribution to our knowledge of the structure and function of nocisensors, the neuropeptides involved in the nociceptive system, and the spinal and supraspinal central mechanisms of nociception.

Antinociception produced by electrical stimulation of vagal afferents: independence of cervical and subdiaphragmatic branches

Expt. 1 showed that electrical stimulation of either the main dorsal or ventral branch of the subdiaphragmatic vagus could produce inhibition of the nociceptive tail-flick reflex in lightly anesthetized rats. The antinociception produced by electrical stimulation of the dorsal subdiaphragmatic vagus was eliminated by resection of the right cervical vagus, but relatively unaffected by resection of the left cervical vagus. The opposite effects for cervical vagal resection were obtained with electrical stimulation of the ventral branch of the subdiaphragmatic vagus. These results indicate that the antinociceptive effects of subdiaphragmatic vagal stimulation are mediated via uncrossed afferents traveling in the cervical vagus to activate an inhibitory spinopetal system. These findings are consistent with the established anatomy of vagal afferents. Expt. 2 showed that degeneration of the dorsal subdiaphragmatic vagus did not alter the threshold intensity of right cervical vagal stimulation necessary to produce inhibition of the tail-flick reflex. These results demonstrate that the antinociceptive effects of cervical vagal stimulation are primarily due to activation of the cardiopulmonary component of the nerve, rather than the subdiaphragmatic component. The second experiment also demonstrated that the subdiaphragmatic branch of the vagus can be selectively degenerated with ricin while leaving the cervical branch intact, even though the cell bodies of both sets of afferents are located within the nodose ganglion. These data are discussed in terms of vagal afferents and their role in the modulation of nociceptive transmission.