Vagal afferent modulation of a nociceptive reflex in rats: involvement of spinal opioid and monoamine receptors

Update on Neuromodulation for Migraine and Other Primary Headache Disorders: Recent Advances and New Indications

PURPOSE OF REVIEW

Neuromodulation techniques currently available for headache management are reviewed in this article, with a focus on recent advances in non-invasive devices for migraine and trigeminal autonomic cephalalgias.

RECENT FINDINGS

The currently available FDA-cleared non-invasive devices for migraine include transcutaneous supraorbital and supratrochlear nerve stimulation, single-pulse transcranial magnetic stimulation (sTMS), external concurrent occipital and trigeminal neurostimulation (eCOT-NS), remote electrical neuromodulation (REN), and non-invasive vagal nerve stimulation (nVNS) with indications for migraine and trigeminal autonomic cephalalgias. Emerging non-invasive techniques being explored for use in migraine include transcranial direct current stimulation (tDCS), kinetic oscillation stimulation (KOS), and auricular transcutaneous vagal nerve stimulation (at-VNS). In addition to primary headache, non-invasive neuromodulation is being investigated for comorbid conditions such as depression. Non-invasive neuromodulation devices remain a safe, well-tolerated, and effective therapy for patients with primarily migraine and trigeminal autonomic cephalalgias. Ongoing research is needed to determine efficacy in other headache disorders and comorbid conditions.

Role of Vagus Nerve Stimulation in the Treatment of Chronic Pain

Vagus nerve stimulation (VNS) can modulate vagal activity and neuro-immune communication. Human and animal studies have provided growing evidence that VNS can produce analgesic effects in addition to alleviating refractory epilepsy and depression. The vagus nerve (VN) projects to many brain regions related to pain processing, which can be affected by VNS. In addition to neural regulation, the anti-inflammatory property of VNS may also contribute to its pain-inhibitory effects. To date, both invasive and noninvasive VNS devices have been developed, with noninvasive devices including transcutaneous stimulation of auricular VN or carotid VN that are undergoing many clinical trials for chronic pain treatment. This review aimed to provide an update on both preclinical and clinical studies of VNS in the management for chronic pain, including fibromyalgia, abdominal pain, and headaches. We further discuss potential underlying mechanisms for VNS to inhibit chronic pain.

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.

Transcutaneous auricular VNS applied to experimental pain: A paired behavioral and EEG study using thermonociceptive CO2 laser

Background

Transcutaneous auricular Vagal Nerve Stimulation (taVNS) is a non-invasive neurostimulation technique with potential analgesic effects. Several studies based on subjective behavioral responses suggest that taVNS modulates nociception differently with either pro-nociceptive or anti-nociceptive effects.

Objective

This study aimed to characterize how taVNS alters pain perception, by investigating its effects on event-related potentials (ERPs) elicited by different types of spinothalamic and lemniscal somatosensory stimuli, combined with quantitative sensory testing (detection threshold and intensity ratings).

Methods

We performed 3 experiments designed to study the time-dependent effects of taVNS and compare with standard cervical VNS (cVNS). In Experiment 1, we assessed the effects of taVNS after 3 hours of stimulation. In Experiment 2, we focused on the immediate effects of the duty cycle (OFF vs. ON phases). Experiments 1 and 2 included 22 and 15 healthy participants respectively. Both experiments consisted of a 2-day cross-over protocol, in which subjects received taVNS and sham stimulation sequentially. In addition, subjects received a set of nociceptive (thermonociceptive CO₂ laser, mechanical pinprick) and non-nociceptive (vibrotactile, cool) stimuli, for which we recorded detection thresholds, intensity of perception and ERPs. Finally, in Experiment 3, we tested 13 epileptic patients with an implanted cVNS by comparing OFF vs. ON cycles, using a similar experimental procedure.

Results

Neither taVNS nor cVNS appeared to modulate the cerebral and behavioral aspects of somatosensory perception.

Conclusion

The potential effect of taVNS on nociception requires a cautious interpretation, as we found no objective change in behavioral and cerebral responses to spinothalamic and lemniscal somatosensory stimulations.

Contribution of Baroreceptor Function to Pain Perception and Perioperative Outcomes

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain homeostasis by coordinating physiologic responses to external and internal stimuli. While it is recognized that carotid and cardiopulmonary baroreceptor reflexes modulate autonomic output to mitigate excessive fluctuations in arterial blood pressure and to maintain intravascular volume, increasing evidence suggests that baroreflex pathways also project to key regions of the central nervous system that regulate somatosensory, somatomotor, and central nervous system arousal. In addition to maintaining autonomic homeostasis, baroreceptor activity modulates the perception of pain, as well as neuroimmune, neuroendocrine, and cognitive responses to physical and psychologic stressors. This review summarizes the role that baroreceptor pathways play in modulating acute and chronic pain perception. The contribution of baroreceptor function to postoperative outcomes is also presented. Finally, methods that enhance baroreceptor function, which hold promise in improving postoperative and pain management outcomes, are presented.

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.

Motivational Non-directive Resonance Breathing as a Treatment for Chronic Widespread Pain

Chronic widespread pain (CWP) is one of the most difficult pain conditions to treat due to an unknown etiology and a lack of innovative treatment design and effectiveness. Based upon preliminary findings within the fields of motivational psychology, integrative neuroscience, diaphragmatic breathing, and vagal nerve stimulation, we propose a new treatment intervention, motivational non-directive (ND) resonance breathing, as a means of reducing pain and suffering in patients with CWP. Motivational ND resonance breathing provides patients with a noninvasive means of potentially modulating five psychophysiological mechanisms imperative for endogenously treating pain and increasing overall quality of life.

Brain Stimulation Differentially Modulates Nociception and Inflammation in Aversive and Non-aversive Behavioral Conditions

Inflammation and pain are major clinical burdens contributing to multiple disorders and limiting the quality of life of patients. We previously reported that brain electrical stimulation can attenuate joint inflammation in experimental arthritis. Here, we report that non-aversive electrical stimulation of the locus coeruleus (LC), the paraventricular hypothalamic nucleus (PVN) or the ventrolateral column of the periaqueductal gray matter (vlPAG) decreases thermal pain sensitivity, knee inflammation and synovial neutrophilic infiltration in rats with intra-articular zymosan. We also analyzed the modulation of pain and inflammation during aversive neuronal stimulation, which produces defensive behavioral responses such as freezing immobility to avoid predator detection. Electrical stimulation with higher intensity to induce freezing immobility in rats further reduces pain but not inflammation. However, tonic immobility further reduces pain, knee inflammation and synovial neutrophilic infiltration in guinea pigs. The duration of the tonic immobility increases the control of pain and inflammation. These results reveal survival behavioral and neuromodulatory mechanisms conserved in different species to control pain and inflammation in aversive life-threatening conditions. Our results also suggest that activation of the LC, PVN, or vlPAG by non-invasive methods, such as physical exercise, meditation, psychological interventions or placebo treatments may reduce pain and joint inflammation in arthritis without inducing motor or behavioral alterations.

A pilot study on skin potential recordings as a measure of nociception in pain-free dogs and humans, and in dogs with persistent pain

The Pain Trace™ device can detect changes in the skin's electrical potentials claimed to be associated with pain related alterations in the sympathetic and parasympathetic nervous system activity. Positive voltages represent the absence of major pain, whereas negative voltages represent moderate to severe pain. Unlike in humans and horses, no baseline skin potential recordings have been reported in dogs. In study Part 1 baseline skin potentials were recorded in healthy dogs and compared to readings obtained in human volunteers. In dogs, data were recorded with electrodes placed at three separate sites: neck, axilla, and thorax. In humans, data were collected from the palms. Readings over a 90-second period were averaged and comparisons between groups were performed using the Kruskal-Wallis test. All voltage recordings were positive. Readings in dogs had greater variability. Recordings from the thorax were more homogeneous, this being the reason why this site was chosen for study Part 2. No significant differences in recordings were noted between pain-free dogs and humans. The main hypothesis was that shifting from positive to negative skin potential voltages serves as an indicator of canine patients sensing moderate to severe pain. Therefore, we obtained preoperative readings from dogs with cranial cruciate ligament disease that were experiencing associated persistent pain, and compared these data with readings from pain-free dogs (thorax). In dogs undergoing surgery, all pre-surgery voltage readings were positive and thus no consistent relationship between skin potential recordings and pain perception could be established. Further investigation is needed to confirm any relationship between skin potential and pain severity in dogs.

Do the psychological effects of vagus nerve stimulation partially mediate vagal pain modulation?

There is preclinical and clinical evidence that vagus nerve stimulation modulates both pain and mood state. Mechanistic studies show brainstem circuitry involved in pain modulation by vagus nerve stimulation, but little is known about possible indirect descending effects of altered mood state on pain perception. This possibility is important, since previous studies have shown that mood state affects pain, particularly the affective dimension (pain unpleasantness). To date, human studies investigating the effects of vagus nerve stimulation on pain perception have not reliably measured psychological factors to determine their role in altered pain perception elicited by vagus nerve stimulation. Thus, it remains unclear how much of a role psychological factors play in vagal pain modulation. Here, we present a rationale for including psychological measures in future vagus nerve stimulation studies on pain.

Analgesic and anti-edematogenic effects of oral trypsin were abolished after subdiaphragmatic vagotomy and spinal monoaminergic inhibition in rats

AIMS

Rheumatoid arthritis brings great burdens to the patients. In addition to the highly expensive treatment, they are commonly associated with severe side effects. In such context, the research for safe and affordable treatments is needed.

MAIN METHODS

Arthritis was induced by CFA (0.5mg/mL) in female wistar rats. Trypsin was given p.o. (2.95mg/kg; 2mL) 24h after the intra-articular CFA injection. Articular incapacitation was measured daily by counting the paw elevation time (PET; s) during 1-min periods of stimulated walk, throughout the 7-days after intra-articular CFA injection. Articular diameter (AD) was accessed just after each PET measurement, taken the difference between naïve and diseased knee-joint diameter (cm).

KEY FINDINGS

The present study showed that orally administered trypsin was able to reduce nociception and edema, effects that could be observed throughout the evaluation period. These effect, however, were not observed in animals underwent subdiaphragmatic vagotomy, suggesting a vagal mediation for trypsin effects. Likewise, these effects were blocked in rats which received intrathecal injection of the neurotoxins 5,7-dihydroxytryptamine or 6-hydroxydopamine, suggesting the involvement of spinal amines from axon terminals.

SIGNIFICANCE

The present study proposes that oral trypsin may cause vagal activation, followed by the activation of descending inhibitory pathways and such mechanism may lead to a novel approach for the treatment of arthritis.

Activation of Brainstem Pro-opiomelanocortin Neurons Produces Opioidergic Analgesia, Bradycardia and Bradypnoea

Opioids are widely used medicinally as analgesics and abused for hedonic effects, actions that are each complicated by substantial risks such as cardiorespiratory depression. These drugs mimic peptides such as β-endorphin, which has a key role in endogenous analgesia. The β-endorphin in the central nervous system originates from pro-opiomelanocortin (POMC) neurons in the arcuate nucleus and nucleus of the solitary tract (NTS). Relatively little is known about the NTSPOMC neurons but their position within the sensory nucleus of the vagus led us to test the hypothesis that they play a role in modulation of cardiorespiratory and nociceptive control. The NTSPOMC neurons were targeted using viral vectors in a POMC-Cre mouse line to express either opto-genetic (channelrhodopsin-2) or chemo-genetic (Pharmacologically Selective Actuator Modules). Opto-genetic activation of the NTSPOMC neurons in the working heart brainstem preparation (n = 21) evoked a reliable, titratable and time-locked respiratory inhibition (120% increase in inter-breath interval) with a bradycardia (125±26 beats per minute) and augmented respiratory sinus arrhythmia (58% increase). Chemo-genetic activation of NTSPOMC neurons in vivo was anti-nociceptive in the tail flick assay (latency increased by 126±65%, p<0.001; n = 8). All effects of NTSPOMC activation were blocked by systemic naloxone (opioid antagonist) but not by SHU9119 (melanocortin receptor antagonist). The NTSPOMC neurons were found to project to key brainstem structures involved in cardiorespiratory control (nucleus ambiguus and ventral respiratory group) and endogenous analgesia (periaqueductal gray and midline raphe). Thus the NTSPOMC neurons may be capable of tuning behaviour by an opioidergic modulation of nociceptive, respiratory and cardiac control.

Subdiaphragmatic vagotomy promotes nociceptive sensitivity of deep tissue in rats

To verify whether vagal dysfunction is associated with chronic pain, we evaluated the effects of subdiaphragmatic vagotomy (vgx) on the sensitivity toward noxious stimuli in rats. Vgx rats showed sustained hyperalgesia in the gastrocnemius muscle without tissue damage (no increase in vgx-induced plasma creatine phosphokinase or lactose dehydrogenase levels) accompanied by hypersensitivity to colonic distension. We found a dramatic increase in the levels of metabotropic glutamate receptor 5, protein kinase C (PKC) gamma and phosphorylated-PKCgamma within the spinal cord dorsal horn in vgx rats, which suggests that vgx may evoke sensory nerve plasticity. Morphine produced a dose-dependent increase in the withdrawal threshold in both vgx and sham-operated rats, but the effect of a lower dose in vgx rats was weaker than that in sham-operated rats. Muscle hyperalgesia in vgx rats was also attenuated by gabapentin and amitriptyline, but was not affected by diclofenac, dexamethasone or diazepam. These findings indicate that subdiaphragmatic vagal dysfunction caused chronic muscle hyperalgesia accompanied by visceral pain and both gabapentin and amitriptyline were effective for subdiaphragmatic vagotomy-induced pain, which are partially similar to fibromyalgia syndrome. Furthermore, this chronic muscle pain may result from nociceptive neuroplasticity of the spinal cord dorsal horn.

Subdiaphragmatic vagal afferent nerves modulate visceral pain

Activation of the vagal afferents by noxious gastrointestinal stimuli suggests that vagal afferents may play a complex role in visceral pain processes. The contribution of the vagus nerve to visceral pain remains unresolved. Previous studies reported that patients following chronic vagotomy have lower pain thresholds. The patient with irritable bowel syndrome has been shown alteration of vagal function. We hypothesize that vagal afferent nerves modulate visceral pain. Visceromotor responses (VMR) to graded colorectal distension (CRD) were recorded from the abdominal muscles in conscious rats. Chronic subdiaphragmatic vagus nerve sections induced 470, 106, 51, and 54% increases in VMR to CRD at 20, 40, 60 and 80 mmHg, respectively. Similarly, at light level of anesthesia, topical application of lidocaine to the subdiaphragmatic vagus nerve in rats increased VMR to CRD. Vagal afferent neuronal responses to low or high-intensity electrical vagal stimulation (EVS) of vagal afferent Adelta or C fibers were distinguished by calculating their conduction velocity. Low-intensity EVS of Adelta fibers (40 microA, 20 Hz, 0.5 ms for 30 s) reduced VMR to CRD at 40, 60, and 80 mmHg by 41, 52, and 58%, respectively. In contrast, high-intensity EVS of C fibers (400 microA, 1 Hz, 0.5 ms for 30 s) had no effect on VMR to CRD. In conclusion, we demonstrated that vagal afferent nerves modulate visceral pain. Low-intensity EVS that activates vagal afferent Adelta fibers reduced visceral pain. Thus EVS may potentially have a role in the treatment of chronic visceral pain.

Increased Pain Sensitivity in Low Blood Pressure

There is broad evidence for a functional interaction between the cardiovascular and pain regulatory systems. One result of this interaction is the reduced sensitivity to acute pain in individuals with elevated blood pressure, which has been established in numerous studies. In contrast to this, possible alterations in pain perception related to the lower range of blood pressure have not yet been investigated. In the present study pain sensitivity was assessed in 30 hypotensive women (mean blood pressure 95/56 mmHg) and 30 normotensive control persons (mean blood pressure 119/77 mmHg) based on a cold pressor test. Possible effects on pain perception of hypotension-related impairment of subjective state were controlled for by including a mood-scale. The hypotensive as compared to the normotensive group displayed lower pain threshold and pain tolerance levels, as well as increased sensory and affective experiences of pain. Moreover, a slight negative correlation was found, both in hypotensive and control persons, between pain sensitivity and the degree of blood pressure increase during the execution of the cold pressor test. In accordance with the previous findings on hypertension-related hypoalgesia, the present results suggest an inverse relationship between blood pressure and pain sensitivity across the total blood pressure spectrum. Different degrees of pain attenuation through afferent input from the arterial baroreceptor system are discussed as a physiological mechanism mediating this relationship.

Vagal Nerve Stimulation: Overview and Implications for Anesthesiologists

Vagal nerve stimulation is an important adjunctive therapy for medically refractory epilepsy and major depression. Additionally, it may prove effective in treating obesity, Alzheimer's disease, and some neuropsychiatic disorders. As the number of approved indications increases, more patients are becoming eligible for surgical placement of a commercial vagal nerve stimulator (VNS). Initial VNS placement typically requires general anesthesia, and patients with previously implanted devices may present for other surgical procedures requiring anesthetic management. In this review, we will focus on the indications for vagal nerve stimulation (both approved and experimental), proposed therapeutic mechanisms for vagal nerve stimulation, and potential perioperative complications during initial VNS placement. Anesthetic considerations during initial device placement, as well as anesthetic management issues for patients with a preexisting VNS, are reviewed.

The effects of Valsalva manoeuvres on venepuncture pain

BACKGROUND AND OBJECTIVE

Stimulating the vagus nerve reduces pain. The purpose of this study was to evaluate the effect of a Valsalva manoeuvre, which stimulates the vagus nerve, on perception of pain during peripheral venous cannulation in adult patients.

METHODS

This was a prospective randomized clinical trial. One hundred and ten patients scheduled for elective surgery were randomly divided into two groups. Half of the patients, Group A, underwent venepuncture during a Valsalva manoeuvre and the other half of the patients, Group B, underwent venepuncture without performing a Valsalva manoeuvre. Patients made a pain assessment using a 0-10 point numerical rating scale.

RESULTS

The numerical rating scale score was 1.5+/-1.2 for Group A and 3.1+/-1.9 for Group B, the difference being statistically significant (P<0.0001).

CONCLUSIONS

On the basis of data from this study, the Valsalva manoeuvre may be of the value before venous cannulation as a simple and practical method to reduce pain from venous cannulation.

Vagus nerve stimulation suppresses pain but has limited effects on neurogenic inflammation in humans

Left vagus nerve stimulation reduces pain perception in humans. In animal studies it has been shown that beyond the inhibitory effect, which the vagus nerve exerts via its widespread central connections, there might be also a peripheral effect on nociceptors. In humans, the exact mechanisms of VNS-mediated analgesia are still unclear. To test whether VNS also affects activation of primary nociceptive afferents in humans, we investigated 11 patients before and after implantation of a vagus nerve stimulator by using tonic pressure as pain stimulus. Vasodilator axon reflexes ("neurogenic" inflammation) were quantified by laser-Doppler-imaging and served as surrogates for primary afferent activation. Pain was measured on a visual analogue scale (VAS). The squeezing experiment was performed three times at 15 min intervals in each session. As controls 9 healthy age- and gender-matched subjects were studied. As shown in our previous study, VNS significantly reduces pain to tonic pressure. Likewise, there was a moderate reduction of the blood flow within the area of the axon reflex, which indicates a possible but limited inhibitory effect of VNS on peripheral nociceptors. Our data suggests that VNS might affect peripheral nociceptor function in humans. Since VNS has been shown to be more effective in experimental procedures in which pain magnitude is amplified by central processing, further studies are warranted to elucidate whether the central or peripheral effect is most important for VNS-mediated analgesia.

Interactions between the cardiovascular and pain regulatory systems: an updated review of mechanisms and possible alterations in chronic pain

Endogenous pain regulatory system dysfunction appears to play a role in the maintenance of chronic pain. An important component of the pain regulatory process is the functional interaction between the cardiovascular and pain regulatory systems, which results in an association between elevated resting blood pressure (BP) and diminished acute pain sensitivity. This BP/pain sensitivity relationship is proposed to reflect a homeostatic feedback loop helping restore arousal levels in the presence of painful stimuli. Evidence is emerging that this normally adaptive BP/pain sensitivity relationship is significantly altered in chronic pain conditions, affecting responsiveness to both acute and chronic pain stimuli. Several mechanisms that may underlie this adaptive relationship in healthy individuals are overviewed, including endogenous opioid, noradrenergic, and baroreceptor-related mechanisms. Theoretical models are presented regarding how chronic pain-related alterations in the mechanisms above and increased pain facilatory system activity (central sensitization) may contribute to altered BP/pain sensitivity interactions in chronic pain. Clinical implications are discussed.

Brain stem convergence of pelvic viscerosomatic inputs via spinal and vagal afferents

Single medullary reticular formation (MRF) neurons receive ascending spinal inputs from multiple somatic and pelvic visceral territories. MRF neurons were examined for responses to both pelvic (PN) and vagus (abdominal branches: VAG-abd) nerve stimulation, which dually innervate certain pelvic viscera. Recordings in 12 urethane-anesthetized male rats were performed. Of 121 PN-responsive MRF neurons, 50% responded to VAG-abd. Twenty-seven (22%) responded to colonic distention. All 121 neurons responded to noxious stimulation of somatic territories, including many areas outside the perigenital region (including the hindpaws, ears, face). These data demonstrate input originating from different spinal and cranial nerves. The functional significance of this viscerosomatic convergence to MRF is unknown, but could relate to sensory/autonomic integration for coordinating multiple bodily functions, including reproductive and eliminative events.

Vagus nerve stimulation attenuates heat- and formalin-induced pain in rats

The analgesic effect of vagus nerve stimulation (VNS) has not yet been demonstrated in animals with the devices used in the clinic. We studied in awake rats the effects of two VNS protocols on the hind paw hot water test and compared the results with those previously obtained in the oro-facial formalin test. A stringent duty cycle (20 s on/18 s off) increased heat pain tolerance in both hind paws (average 188%) after 2 h of stimulation. VNS with parameters used in epilepsy (30 s on/5 min off) decreased heat tolerance after 2 h, but produced a significant antinociceptive effect after days of stimulation. VNS may thus be useful in pain disorders, even with the less stringent protocol.

The effect of vagus nerve stimulation on migraines

Vagus nerve stimulation (VNS) inhibits nociceptive behavior in animals. VNS might reduce pain in patients with VNS device implanted for intractable seizures. One case report described possible benefits on migraines. We contacted all patients who received VNS therapy for intractable epilepsy between 1993 and 1999 at Southern Illinois University, Springfield, Illinois. Patients who had concomitant chronic pain were subsequently interviewed. Pain intensity before and after VNS implantation was rated by the patient as average, worst, and least and on numeric rating scale from 1 to 10. Current pain measurements were compared to preimplantation by using Global Pain Relief Rating Scale. Of 62 patients who received VNS, 27 patients were interviewed; 4 patients had common migraine, and no other chronic pain syndromes were identified. All patients with migraine reported reductions in headache frequency and numeric rating scale score for average and least headache intensity. One patient reported complete relief of headaches. Improvement was reported to start 1 to 3 months after initiation of therapy. On Global Pain Relief Rating Scale, 1 patient reported complete pain relief, 2 reported a lot of pain relief, and 1 reported slight pain relief. Concomitant antiepileptic drugs were decreased in 3 patients and slightly increased in 1. VNS might be beneficial for prophylactic therapy of migraine.

Convergence of cervical and trigeminal sensory afferents

Cranial nociceptive perception shows a distinct topographic distribution, with the trigeminal nerve receiving sensory information from the anterior portions of the head, the greater occipital nerve, and branches of the upper cervical roots in the posterior regions. However, this distribution is not respected during headache attacks, even if the etiology of the headache is specific for only one nerve. Nociceptive information from the trigeminal and cervical territories activates the neurons in the trigeminal nucleus caudalis that extend to the C2 spinal segment and lateral cervical nucleus in the dorsolateral cervical area. These neurons are classified as multimodal because they receive sensory information from more than one afferent type. Clinically, trigeminal activation produces symptoms in the trigeminal and cervical territory and cervical activation produces symptoms in the cervical and trigeminal territory. The overlap between the trigeminal nerve and cervical is known as a convergence mechanism. For some time, convergence mechanisms were thought to be secondary to clinical observations. However, animal studies and clinical evidence have expanded our knowledge of convergence mechanisms. In this paper, the role of convergence mechanisms in nociceptive physiology, physiopathology of the headaches, clinical diagnosis, and therapeutic conduct are reviewed.

Vagal modulation of spinal nicotine-induced inhibition of the inflammatory response mediated by descending antinociceptive controls

Noxious stimuli activate neuroendocrine axes, inhibiting inflammation, an effect that is powerfully attenuated by ongoing activity in subdiaphragmatic vagal afferents. To evaluate whether this inhibitory effect of vagal afferent activity is mediated by descending antinociceptive control, we tested whether antagonizing descending antinociceptive controls: (i) enhances the inhibition of inflammation produced by spinal nicotine (which stimulates central terminals of nociceptors) and (ii) occludes the enhancing effect of subdiaphragmatic vagotomy, in the rat. Spinal intrathecal co-administration of the alpha-adrenergic receptor antagonist phentolamine and the non-selective opioid receptor antagonist naloxone, and acute subdiaphragmatic vagotomy each produced enhancement, with similar magnitude, of nicotine-induced inhibition of plasma extravasation, produced by the potent inflammatory mediator, bradykinin. The combination of subdiaphragmatic vagotomy and intrathecal receptor antagonists, however, produced no further enhancement compared to each treatment alone. These findings support the suggestion that activity in descending antinociceptive controls modulates noxious stimulus-induced inhibition of inflammation and the vagal modulation of noxious stimulus-induced inhibition of inflammation is mediated by descending antinociceptive controls.

Vagus nerve stimulation in awake rats reduces formalin-induced nociceptive behaviour and fos-immunoreactivity in trigeminal nucleus caudalis

Besides its well-established efficacy in epilepsy, vagus nerve stimulation (VNS) may be of potential interest in pain treatment. It has, however, not yet been assessed in animal pain models with the devices and stimulation protocols used in humans. We have therefore studied in awake rats the effects of left cervical VNS on trigeminal nociception using an implantable electrode and stimulator (NCP-Cyberonics). VNS was applied for 24h at 2 mA intensity, 20 Hz frequency, 0.5 ms pulse width and a duty cycle of 20s ON/18s OFF. As a nociceptive stimulus, we injected formalin into the left mystacial vibrissae, assessed behaviour for 45 min and sacrificed the animals 45 min later. Fos-immunoreactive (Fos-Ir) neurons were counted in laminae I-II of trigeminal nucleus caudalis (TNC) on both sides. We used three groups of control animals: VNS without formalin, formalin without VNS and sham VNS (implanted without stimulation or formalin). Whereas sham VNS had no significant effect, VNS alone increased Fos expression in ipsilateral TNC in addition to the expected increase in nucleus tractus solitarius. It also significantly attenuated the increase of Fos-Ir neurons observed in ipsilateral TNC laminae I-II after formalin injection. If the proper VNS effect on Fos-expression was subtracted, the reduction of formalin-induced nociceptor activation was 55%. VNS also reduced nociceptive behaviour on average by 96.1% during the early phase (0-6 min) and by 60.7% during the late phase (6-45 min) after the formalin injection. These results suggest that VNS applied with a device used in human therapy may have in awake rats a significant antinociceptive effect in a model of trigeminal pain.

Subdiaphragmatic vagal afferent innervation in activation of an opioidergic antinociceptive system in response to colorectal distension in rats

Abstract In a number of different experimental paradigms of somatic pain, there is evidence for a vagally mediated antinociceptive system. This pathway probably involves opioid mechanisms. However, whether this pathway is activated in visceral pain or if it involves subdiaphragmatic vagal afferents is unclear. The aim of the present study was to determine whether subdiaphragmatic vagal afferents mediate antinociception in response to a visceral stimulus and whether this involves an opioid pathway. Colorectal distension was performed in fasted, conscious male Sprague-Dawley rats using a balloon catheter connected to an electronic distension device. The number of abdominal contractions (visceromotor response) in response to a tonic colorectal distension (60 mmHg for 10 min) was recorded. Experiments were performed in sham or subdiaphragmatically vagotomized, perineural vehicle- or capsaicin-treated rats (to functionally denervate vagal afferents) before and after administration of naloxone (25 mg kg(-1) bodyweight intraperitoneally). Vagotomy, capsaicin and naloxone pretreatments all significantly enhanced the visceromotor response to colorectal distension. The effect of naloxone in capsaicin-treated rats did not appear to be additive. These results suggest that activation of subdiaphragmatic afferents, which can be blocked by capsaicin, may play a role in opioid-dependent antinociceptive pathways activated by a noxious visceral stimulus.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Facilitation and attenuation of a visceral nociceptive reflex from the rostroventral medulla in the rat

BACKGROUND & AIMS

Noxious inputs from somatic tissue are subject to biphasic descending modulation from the rostroventral medulla (RVM). In the present study, we investigated descending facilitatory and inhibitory influences from the RVM on a visceral nociceptive reflex.

METHODS

The visceromotor response (VMR), a contraction of peritoneal musculature during noxious colorectal distention (80 mm Hg, 20 seconds), was quantified as the integrated electromyogram.

RESULTS

At 22 sites in the RVM, electrical stimulation produced biphasic effects, facilitating the VMR at low (5, 10, and 25 microA) and inhibiting it at greater (>50 microA) intensities of stimulation. Electrical stimulation at all intensities tested (5-200 microA) in other sites in the RVM only inhibited (30 sites) or only facilitated (12 sites) the VMR to colorectal distention. Activation of glutamatergic receptors in the RVM replicated the effects of electrical stimulation. Reversible blockage (intraspinal lidocaine injection) or irreversible transection of spinal funiculi revealed that descending facilitatory influences from the RVM were conveyed in the ventrolateral/ventral funiculus, whereas descending inhibitory influences were contained in the dorsolateral funiculi.

CONCLUSIONS

Spinal visceral nociceptive reflexes are subject to facilitatory modulation from the RVM, providing the basis for a mechanism by which visceral sensations can be enhanced from supraspinal sites.

Vagotomy decreases excitability in primary vagal afferent somata

Standard patch-clamp and intracellular recording techniques were used to monitor membrane excitability changes in adult inferior vagal ganglion neurons (nodose ganglion neurons, NGNs) 5 days following section of the vagus nerve (vagotomy). NGNs were maintained in vivo for 5 days following vagotomy, and then in vitro for 2-9 h prior to recording. Vagotomy increased action potential (AP) threshold by over 200% (264 +/- 19 pA, mean +/- SE, n = 66) compared with control values (81 +/- 20 pA, n = 68; P < 0.001). The number of APs evoked by a 3 times threshold 750-ms depolarizing current decreased by >70% (from 8.3 to 2.3 APs, P < 0.001) and the number of APs evoked by a standardized series of (0.1-0.9 nA, 750 ms) depolarizing current steps decreased by over 80% (from 16.9 APs to 2.6 APs, P < 0.001) in vagotomized NGNs. Similar decreases in excitability were observed in vagotomized NGNs in intact ganglia in vitro studied with "sharp" microelectrode techniques. Baseline electrophysiological properties and changes following vagotomy were similar in right and left NGNs. A "sham" vagotomy procedure had no effect on NGN properties at 5 days, indicating that changes were due to severing the vagus nerve itself, not surrounding tissue damage. NGNs isolated after being maintained 17 h in vivo following vagotomy revealed no differences in excitability, suggesting that vagotomy-induced changes occur some time from 1-5 days after injury. Decreased excitability was still observed in NGNs isolated after 20-21 days in vivo following vagotomy. These data indicate that, in contrast to many primary sensory neurons that are thought to become hyperexcitable following section of their axons, NGNs undergo a marked decrease in electrical excitability following vagotomy.

An excitatory role for 5-HT in spinal inflammatory nociceptive transmission; state-dependent actions via dorsal horn 5-HT(3) receptors in the anaesthetized rat

The involvement of 5-HT(3) receptor mediated modulation of formalin and carrageenan induced inflammatory transmission was investigated. The effects of the selective 5-HT(3) receptor antagonist ondansetron on the electrically evoked responses of dorsal horn neurones in normal animals were compared to those following carrageenan. The effect of pre-treatment on the formalin response was also studied. Ondansetron had no significant effect on the electrically evoked responses of dorsal horn neurones in normal animals or following carrageenan induced inflammation, but significantly inhibited both phases of the formalin response. Our results suggest that 5-HT(3) receptors in the spinal cord have no significant role under normal conditions. However, during formalin (but not carrageenan) induced inflammation this system is activated, maintaining the response of nociceptive spinal neurones to peripheral formalin.

Low intensity vagal nerve stimulation lowers human thermal pain thresholds

The effect of vagal nerve stimulation (VNS) on thermal pain sensation was studied in eight subjects who had vagal nerve stimulators surgically implanted for purposes of seizure control. Prior to their involvement in the study, all subjects had the intensity of their VNS (30 Hz, 0.5 ms, 1.0-2.75 mA) adjusted upwards until achieving their desired clinical effect of reduced seizures. Thermal pain thresholds were determined using a Medoc TSA-2001 with a thermode applied to the skin of the forearm. During VNS at settings 100% of those used clinically to control their seizures, subjects showed a statistically significant decrease in their thermal pain threshold of 1.1+/-0.4 degrees C. Acute effects of graded VNS on thermal pain thresholds were determined in seven of the subjects after cessation of chronic VNS. Two thermal threshold measurements were obtained while the subject received sham stimulation (0 mA intensity), during tactile control stimulation and during 30 s of VNS at intensities approximately 33, 66 and 100% of the settings utilized to control their seizures. Tactile control stimulation was provided by electrical stimulation of the skin of the ankle with the intensity adjusted by the patient to match the intensity of any sensations felt in the neck during VNS. Subjects were not aware of the settings employed. Their stimulator was adjusted with each trial and an ascending/descending ordering of intensity was utilized with an inter-trial interval of 2 min. Thermal pain thresholds were significantly decreased in relation to tactile control stimulation at all intensities of VNS tested with the greatest effect occurring at the 66% level. Subjects were also monitored non-invasively and hemodynamic responses to VNS were determined. No significant alterations in hemodynamic variables were observed. The findings of this human study are consistent with experiments in non-human animals which demonstrate a pro-nociceptive effect of low intensity VNS.

Role of vagal afferents and the rostral ventral medulla in intravenous serotonin-induced changes in nociception and arterial blood pressure

Intravenous administration of serotonin inhibits the nociceptive tail-flick (TF) reflex, partially through activation of vagal afferents. The present study examined the role of the rostral ventral medulla (RVM) in i.v. serotonin-produced inhibition of the TF reflex. In Experiment 1, the effects of anesthetic blockade of the RVM on serotonin-produced inhibition of the TF were determined. Lidocaine attenuated the serotonin-produced inhibition of the TF reflex, but had no effect on the cardiovascular effects of serotonin. In Experiment 2, the effects of i.v. serotonin on neural activity in the RVM in intact and cardiopulmonary deafferented rats were determined. Neurons in the RVM were classified as ON and OFF cells, where ON cells were excited by noxious heat, and OFF cells were inhibited. The effects of i.v. serotonin on TF latency, blood pressure, and ON or OFF cell activity were then determined. In intact rats, serotonin produced a dose-dependent increase in TF latency, triphasic changes in blood pressure, and bi- or triphasic changes in ON or OFF cell activity. The changes in blood pressure included an initial sharp decrease in blood pressure (Bezold-Jarisch reflex), followed by a brief pressor response, followed by a delay depressor response. ON cells were generally excited, although there was a period during which the excitation decreased. OFF cells were initially excited, followed by a period of inhibition, followed by a second period of excitation. Bilateral cervical vagotomy attenuated the increase in TF latency, the Bezold-Jarisch reflex, and the excitation of OFF cells, and potentiated the excitation of ON cells and the pressor response. Bilateral sinoaortic deafferentation attenuated the Bezold-Jarisch reflex and potentiated the pressor response. These findings indicate that i.v. serotonin inhibits the TF reflex through at least two distinct mechanisms, one of which requires the RVM. In addition, serotonin produces a vagally mediated excitation of OFF cells and inhibition of ON cells that may mediate some of the antinociception.

Endogenous mechanisms of sensory modulation

We provide evidence supporting the idea that the relationship between tissue damage, or the threat of tissue damage, and the response to such stimuli is variant and dependent on neuronal networks by which attentional, emotional and cognitive components of pain experience activate endogenous descending modulatory systems. Most previous studies have focused on responses to transient noxious stimuli with little information on the influence of descending modulation on behavioral responses to persistent pain and hyperalgesia after tissue or nerve injury. Utilizing correlative behavioral and neuronal studies we have demonstrated that (1) behavioral context modulates neuronal activity in nociceptive and non-nociceptive somatosensory pathways, supporting the hypothesis that responses in these pathways are not immutable; (2) descending modulation influences behavior and neuronal activity at spinal cord levels after inflammation and persistent pain; and (3) there are descending facilitatory as well as inhibitory influences on behavior and spinal cord neuronal activity that may impact on persistent pain particularly of deep muscle and visceral origin. Cortical as well as subcortical pathways are available by which dorsal horn activity can be modulated by attentional, motivational and cognitive factors. It appears that the same neuronal mechanisms in the forebrain and brain stem are available for behavioral modulation in a learned task involving the threat of tissue damage (transient noxious stimuli) as are available in the development and amplification of persistent pain produced by inflammation. These parallel brain mechanisms emphasize the saliency of pain experience as an important learned behavior for the survival of the organism, similar to sequential goal-directed behaviors in an operant task.

Suppressive effect of vagal afferents on the activity of the trigeminal spinal neurons related to the jaw-opening reflex in rats: involvement of the endogenous opioid system

The purpose of the present study is to test the hypothesis that via the endogenous pain control system, vagal afferent input modulates the activity of the trigeminal spinal nucleus oralis (TSNO) related to the tooth pulp (TP)-evoked jaw-opening reflex (JOR). Extracellular single-unit recordings were made from 36 TSNO units responding to TP electrical stimulation with a constant temporal relationship to a digastric electromyogram (dEMG) signal in 26 pentobarbital-anesthetized rats. The activity of 36 TSNO neurons and the amplitude of the dEMG increased proportionally during 1.0-3.5 times the threshold for JOR. Some of these neurons (4 out of 5) were also excited by chemical stimulation (bradykinin, 1-2 microl, 1 mM) of TP. In 31 out of 36 TSNO neurons (86%), their activities during tooth pulp stimulation were suppressed by conditioning stimulation of the right vagus nerve. The suppressive effect of vagal afferent stimulation occurred at conditioning-test intervals of 20-150 ms after the onset of the stimulation, and its maximal suppressive effect occurred at approximately 50 ms. The mean time course of this suppressive effect paralleled that of the dEMG. After administration of naloxone (0.5 and 1.0 mg/kg, i.v.), an opiate receptor blocker, the suppressive effect on the activity of TSNO neurons (6 out of 8) was significantly attenuated at the conditioning-test interval of 50 ms compared to the control (p < 0.01). These results suggested that vagal afferent input inhibits nociceptive transmission in the TSNO related to TP-evoked JOR and this inhibitory effect may occur via the endogenous opioid system in rats.

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.

Effects of cardiac vagal afferent electrostimulation on the responses of trigeminal and trigeminothalamic neurons to noxious orofacial stimulation

We have previously reported that electrical stimulation of cardiac vagal afferents produces an inhibition of the feline's digastric reflex evoked by tooth-pulp stimulation. In the present study, we evaluated whether cardiac vagal afferent stimulation (CVAS) alters the responses of trigeminal sensory neurons to noxious orofacial stimulation in alpha-chloralose-anesthetized cats. A total of 37 trigeminal and trigeminothalamic neurons were recorded from trigeminal nucleus caudalis and trigeminal nucleus oralis. Thirty-five of these 37 neurons were classified as wide-dynamic-range (WDR) neurons because they had cutaneous receptive fields and responded to both noxious heat and non-noxious tactile stimuli. The effects of continuous CVAS (5 Hz, 3 msec, 2 mA) on heat-evoked responses (6 sec 50 degrees C heat pulse) were examined on 32 WDR neurons. CVAS inhibited (21 of 32 neurons), facilitated (5 of 32 neurons) or did not affect neuronal responses (6 of 32 neurons) to noxious heat. The effects of CVAS on heat-evoked responses of trigeminal and trigeminothalamic neurons were equivalent. The effects of intermittent CVAS (7 pulses at 333 Hz, 5 mA, delivered 200 msec prior to the test stimulus) on the responses to electrical test stimuli delivered to the center of a neuron's cutaneous receptive field or to the tooth pulp were also examined. Intermittent CVAS inhibited (15 of 24 neurons), facilitated (4 of 24 neurons) or had no effect (5 of 24 neurons) on A delta-mediated responses evoked by the electrical stimulation of facial skin. Intermittent CVAS either inhibited (8 of 12 neurons) or had no effect (4 of 12 neurons) on C-fiber-mediated responses evoked by electrical stimulation of the facial skin. Eight cells were recorded that received tooth-pulp input. Six of these 8 cells also received afferent input from facial skin, the remaining 2 cells responded only to tooth-pulp stimulation. Intermittent CVAS either inhibited (7 of 8 neurons) or had no effect (1 of 8 neurons) on A delta-mediated responses evoked by electrical stimulation of the tooth pulp. The modulatory actions of intermittent CVAS on trigeminal and trigeminothalamic neuronal responses to convergent afferent input from both skin and tooth pulp were equivalent. The outcomes of this study provide additional evidence that cardiopulmonary vagal afferent stimulation modulates neuronal responses to noxious stimulation and suggest that alterations in cardiopulmonary dynamics may modulate nociception.

Brain-mediated responses to vaginocervical stimulation in spinal cord-transected rats: role of the vagus nerves

The present study was designed to ascertain whether the vagus nerves convey functional sensory activity from the reproductive tract in rats. Previously, vaginocervical mechanostimulation (VS) was shown to increase pupil diameter (PD) and the threshold of vocalization to tail shock (Voc-T). These responses were attenuated but not abolished by combined bilateral transection of the 'genito-spinal' nerves (i.e. pelvic, hypogastric and pudendal). Subsequent bilateral vagotomy further reduced or abolished the residual responses. In the present study, spinal cord transection above the known level of entry of the genito-spinal nerves was combined with bilateral vagotomy. In ovariectomized rats, after spinal cord transection at thoracic 7 (T7X), lumbar 5 (L5X) levels, or sham surgery (Sh), responses to VS were measured, the vagus nerves were then transected bilaterally, and responses to VS were again measured. VS significantly increased Voc-T and PD after sham procedure or spinal cord transection at either level. Subsequent bilateral vagotomy abolished the VS-induced increase in PD in the T7X group. Due to low survival rate, the effect of vagotomy on Voc-T could not be determined. Consequently, we performed a second experiment. In non-ovariectomized rats, VS significantly increased PD but reduced Voc-T in the T7X group compared to the Sh group, and subsequent bilateral vagotomy abolished both responses. These findings provide evidence that, in the rat, the vagus nerves provide a functional sensory pathway from the reproductive tract directly to the medulla oblongata of the brain, bypassing the spinal cord.

Projections of ankle joint afferents to the spinal cord and brainstem of the chicken (Gallus g. domesticus)

The projections of the ankle joint capsule afferents were studied by transganglionic transport of horseradish peroxidase injected directly into the ankle joint. The number and size of the labelled dorsal root ganglion cells were measured from synsacral nerves 2-9. In the dorsal root ganglia, all sizes of sensory neurones were labelled, and the largest number of labelled cells was in ganglia 5-7. The extensive sympathetic innervation of the ankle joint was identified by the large number of cell bodies labelled in the sympathetic ganglia of the paravertebral chain. Labelled afferent fibres projected to the spinal cord from the 2nd to the 8th synsacral nerves, with the rostral projection mainly via Lissauer's tract and the dorsal funiculus. Terminal labelling in the dorsal horn was identified in laminae I-III and VI, with a slight projection to V. Two areas of dense labelling, which did not correspond with the largest number of labelled dorsal root ganglion cells, were identified. A rostral area with the highest density of label was observed at the level of synsacral nerves 3-4 and a second slightly less dense area between synsacral nerves 7-8. In the caudal medulla, diffuse terminal labelling was observed in the nucleus gracilis et cuneatus, nucleus of the tractus solitarius, and the nucleus cuneatus externus. These results are discussed in a comparative context to identify similarities and differences between different primary afferent projections in birds and mammals and to highlight the possible functional significance of the avian articular afferent projection.

Responses of on and off cells in the rostral ventral medulla to stimulation of vagal afferents and changes in mean arterial blood pressure in intact and cardiopulmonary deafferented rats

The relationships between mean arterial blood pressure (MAP) and the activity of putative pain modulatory neurons of the rostroventral medulla (ON and OFF cells) were determined in intact and cardiopulmonary deafferented rats. A total of 173 neurons were recorded from 97 rats as follows: 32 ON cells and 25 OFF cells from 39 intact rats; 32 ON cells and 20 OFF cells from 24 rats with bilateral sino-aortic deafferentation (SAD); 12 ON cells and 20 OFF cells from 19 rats with bilateral cervical vagotomy (CVAG); and 20 ON cells and 12 OFF cells from 15 rats with both SAD and CVAG. ON and OFF cells showed spontaneous fluctuations in activity such that ON cell activity was negatively correlated with MAP whereas OFF cell activity was positively correlated with MAP under conditions of no applied stimuli. These correlations were present in both intact and cardiopulmonary deafferented rats. Further, experimentally induced increases in MAP decreased ON cell activity and increased OFF cell activity in intact rats, but not in rats with SAD, CVAG, or the combination of SAD and CVAG. Experimentally induced decreases in MAP decreased OFF cell activity in intact rats and rats with CVAG, but not in rats with SAD or the combination of SAD and CVAG. These findings indicate that ON and OFF cells are modulated by baroreceptor activity, but baroreceptor input is not necessary for the spontaneous fluctuations in ON and OFF cell activity. Electrical stimulation of vagal afferents (VAS) inhibited 60% of the OFF cells studied, excited 4%, and produced biphasic effects consisting of excitation at low intensities and inhibition at greater intensities in 28% of all OFF cells. In general, VAS excited the majority of the ON cells studied, although there were significant differences between effects in intact and cardiopulmonary deafferented rats. Greater intensities of VAS that inhibited OFF cells and excited ON cells also inhibited the tail flick. Thus, inhibition of OFF cells and excitation of ON cells was correlated with antinociception. The effects of intravenous (i.v.) administration of 1.0 mg/kg morphine on neuronal activity did not differ between intact and cardiopulmonary deafferented rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Review article: the hypersensitive gut--peripheral kappa agonists as a new pharmacological approach

Hypersensitivity to pain is a common component of functional bowel disorders. Hyperalgesia may be induced by various stimuli which produce a cocktail of inflammatory mediators that decrease the pain threshold. Drugs able to block these peripheral events within the gut may offer a new pharmacological approach for treating functional bowel disorders. Kappa opioids have been shown to inhibit somatic pain through a peripheral mechanism of action, acting directly on receptors located on peripheral sensory endings. They can block both the nociceptive messages as well as the release of sensory peptides. This paper reviews the effects of opioid agonists on gut visceral pain and motility anomalies induced by visceral pain. Kappa opioids have strong effects on all models tested, with a peripheral mechanism of action allowing the design of drugs acting only in the periphery and having no central nervous system side-effects. This contrasts with mu agonists which are centrally active on pain and worsen the subsequent transit and motility anomalies.

Effects of vagal afferent nerve stimulation on noxious heat-evoked Fos-like immunoreactivity in the rat lumbar spinal cord

Electrophysiological and behavioral studies have described modulation of nociception by vagal afferent fibers. The objectives of this study were to 1) use Fos-like immunoreactivity as a marker for neuronal activity to examine populations of neurons in the spinal cord that are activated by a noxious heat stimulus, 2) determine whether heat-evoked Fos-like immunoreactivity can be modulated by vagal afferent stimulation, and 3) determine whether vagally-mediated effect on heat-evoked Fos-like immunoreactivity can be blocked by intrathecally administered serotoninergic receptor and alpha-adrenergic receptor antagonists. Neurons demonstrating Fos-like immunoreactivity were located in the ipsilateral superficial and deep dorsal horn laminae extending from the caudal L3 through the rostral L6 region of the spinal cord. Stimulation of the right cervical vagus nerve attenuated significantly (42%) heat-evoked Fos-like immunoreactivity in the superficial laminae. The reduction in Fos-like immunoreactivity by vagal stimulation was abolished by intrathecal administration of methysergide, a nonselective serotoninergic receptor antagonist, but not by phentolamine, a nonselective alpha-adrenoceptor antagonist. These results suggest that vagal afferent modulation of spinal nociceptive transmission is mediated, at least in part, by serotonin receptors.

Subcutaneous formalin produces centrifugal hyperalgesia at a non-injected site via the NMDA-nitric oxide cascade

Previous work has demonstrated that pain facilitation can occur following injection of subcutaneous irritants, such as formalin. Such studies have focused on apparent pain facilitation induced at the site of irritant injection. Changes in processing of incoming pain information have typically been assumed to result from activation of neurocircuitry intrinsic to the spinal cord. The present series of studies have examined hyperalgesia exhibited at a site distant from the site of irritant injection and have begun to define the neurocircuitry and neuropharmacology underlying this pain enhancement. This work demonstrates that s.c. formalin injected into the dorsum of one hindpaw in rats produces prolonged hyperalgesia as measured by the tailflick test. Hyperalgesia is not mediated solely by circuitry intrinsic to the spinal cord, but rather involves activation of centrifugal pathways originating within the brain and descending to the spinal cord via pathway(s) outside of the dorsolateral funiculus. At the level of the spinal cord, this hyperalgesic state is mediated by an NMDA-nitric oxide cascade, since hyperalgesia can be abolished by administration of either an NMDA antagonist (APV) or a nitric oxide synthesis inhibitor (L-NAME).

Role of vagal afferents in the antinociception produced by morphine and U-50,488H in the colonic pain reflex in rats

The mechanisms underlying the antinociception induced by morphine or U-50,488H (trans-(+-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]- cyclohexyl)benzeneacetamide) against painful colonic distension were examined in anaesthetized rats. The respective ED50 values for morphine and U-50,488H were 0.34 and 0.35 mg/kg for the i.v. route, and 1.68 and 167 micrograms/rat for the i.c.v. route. Morphine was active by the intrathecal route (ED50 = 7.8 micrograms) whereas U-50,488H had no effect at doses up to 100 micrograms/rat. The morphine response was selectively antagonized by naloxone (30 micrograms/kg i.v.) whereas that of U-50,488H was blocked by nor-binaltorphimine (10 mg/kg s.c.). Bilateral vagotomy abolished the response to morphine at 0.35 mg/kg i.v. and reduced by 41.3% that to 1 mg/kg morphine, but had no effect on that to U-50,488H or i.c.v. morphine (10 micrograms/rat). It is concluded that peripheral mu- and kappa-opioid receptors may produce antinociception for colonic pain and that vagal integrity is required for mu-opioid but not kappa-opioid peripheral antinociception.