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

Pro-Opiomelanocortin (POMC) neurons in the nucleus of the solitary tract mediate endorphinergic endogenous analgesia in mice

ABSTRACT

The nucleus of the solitary tract (NTS) contains pro-opiomelanocortin (POMC) neurons which are one of the two major sources of β-endorphin in the brain. The functional role of these NTS POMC neurons in nociceptive and cardiorespiratory function is debated. We have shown that NTS POMC optogenetic activation produces bradycardia and transient apnoea in a working heart brainstem preparation and chemogenetic activation with an engineered ion channel (PSAM) produced opioidergic analgesia in vivo . To better define the role of the NTS POMC neurons in behaving animals, we adopted in vivo optogenetics (ChrimsonR) and excitatory/inhibitory chemogenetic DREADD (hM3Dq/hM4Di) strategies in POMC-Cre mice. We show that optogenetic activation of NTS POMC neurons produces time-locked, graded, transient bradycardia and bradypnoea in anaesthetised mice which is naloxone sensitive (1 mg/kg, i.p) suggesting a role of β-endorphin. Both optogenetic and chemogenetic activation of NTS POMC neurons produces sustained thermal analgesia in behaving mice which can be blocked by naloxone. It also produced analgesia in inflammatory pain (carrageenan) but not in a neuropathic pain model (tibial nerve transection). Inhibiting NTS POMC neurons does not produce any effect on basal nociception but inhibits stress-induced analgesia (unlike inhibition of arcuate POMC neurons). Activation of NTS POMC neuronal populations in conscious mice did not cause respiratory depression, anxiety or locomotor deficit (in open field) nor affective preference. These findings indicate that NTS POMC neurons play a key role in the generation of endorphinergic endogenous analgesia and can also regulate cardiorespiratory function.

Vagus nerve stimulation for primary headache disorders: An anatomical review to explain a clinical phenomenon

BACKGROUND

Non-invasive stimulation of the vagus nerve has been proposed as a new neuromodulation therapy to treat primary headache disorders, as the vagus nerve is hypothesized to modulate the headache pain pathways in the brain. Vagus nerve stimulation can be performed by placing an electrode on the ear to stimulate the tragus nerve, which contains about 1% of the vagus fibers. Non-invasive vagus nerve stimulation (nVNS) conventionally refers to stimulation of the cervical branch of the vagus nerve, which is made up entirely of vagal nerve fibers. While used interchangeably, most of the research to date has been performed with nVNS or an implanted vagus nerve stimulation device. However, the exact mechanism of action of nVNS remains hypothetical and no clear overview of the effectiveness of nVNS in primary headache disorders is available.

METHODS

In the present study, the clinical trials that investigated the effectiveness, tolerability and safety of nVNS in primary headache disorders were systematically reviewed. The second part of this study reviewed the central connections of the vagus nerve. Papers on the clinical use of nVNS and the anatomical investigations were included based on predefined criteria, evaluated, and results were reported in a narrative way.

RESULTS

The first part of this review shows that nVNS in primary headache disorders is moderately effective, safe and well-tolerated. Regarding the anatomical review, it was reported that fibers from the vagus nerve intertwine with fibers from the trigeminal, facial, glossopharyngeal and hypoglossal nerves, mostly in the trigeminal spinal tract. Second, the four nuclei of the vagus nerve (nuclei of the solitary tract, nucleus ambiguus, spinal nucleus of the trigeminal nerve and dorsal motor nucleus (DMX)) show extensive interconnections. Third, the efferents from the vagal nuclei that receive sensory and visceral input (i.e. nuclei of the solitary tract and spinal nucleus of the trigeminal nerve) mainly course towards the main parts of the neural pain matrix directly or indirectly via other vagal nuclei.

CONCLUSION

The moderate effectiveness of nVNS in treating primary headache disorders can possibly be linked to the connections between the trigeminal and vagal systems as described in animals.

Proof of concept: short-term non-invasive cervical vagus nerve stimulation in patients with drug-refractory gastroparesis

BACKGROUND

Gastric electric stimulation (GES) is a treatment approach to refractory gastroparesis, possibly acting centrally via afferent vagus nerve stimulation (VNS). Non-invasive VNS (nVNS) is a potential alternative to GES that could eliminate the safety risks of or identify likely responders to implantable neurostimulators.

OBJECTIVE

This open-label proof-of-concept study assessed the effects of nVNS in patients with severe drug-refractory gastroparesis.

METHODS

Patients used the Gastroparesis Cardinal Symptom Index (GCSI) to grade symptoms in diaries daily for 2 weeks before treatment (baseline) and during ≥3 weeks of nVNS therapy. Adverse events (AEs) were also diarised. Treatment was self-administered using an nVNS device (gammaCore, electroCore) and consisted of 120 s stimulations to the vagus nerve in the neck (two stimulations to each side three times daily during weeks 1 and 2; three stimulations to each side three times daily during week 3 and beyond). Response was defined as a ≥1 point decrease from baseline in GCSI score.

RESULTS

Thirty-five patients enrolled; 23 were compliant with study procedures and were included in the analysis; 7 continued treatment beyond 3 weeks. Response rates were 35% (8/23) at 3 weeks and 43% (10/23) for the duration of therapy (3-6 weeks). For the entire cohort and the 10 responders, improvements from baseline were noted for mean total GCSI and GCSI subscale scores (nausea/vomiting, postprandial fullness/early satiety, bloating). No serious AEs were reported.

CONCLUSIONS

These preliminary results provide a signal that nVNS may be useful for treating refractory gastroparesis. Larger controlled studies are warranted.

Localization of TRPV1 and P2X3 in unmyelinated and myelinated vagal afferents in the rat

The vagus nerve is dominated by afferent fibers that convey sensory information from the viscera to the brain. Most vagal afferents are unmyelinated, slow-conducting C-fibers, while a smaller portion are myelinated, fast-conducting A-fibers. Vagal afferents terminate in the nucleus tractus solitarius (NTS) in the dorsal brainstem and regulate autonomic and respiratory reflexes, as well as ascending pathways throughout the brain. Vagal afferents form glutamatergic excitatory synapses with postsynaptic NTS neurons that are modulated by a variety of channels. The organization of vagal afferents with regard to fiber type and channels is not well understood. In the present study, we used tract tracing methods to identify distinct populations of vagal afferents to determine if key channels are selectively localized to specific groups of afferent fibers. Vagal afferents were labeled with isolectin B4 (IB4) or cholera toxin B (CTb) to detect unmyelinated and myelinated afferents, respectively. We find that TRPV1 channels are preferentially found in unmyelinated vagal afferents identified with IB4, with almost half of all IB4 fibers showing co-localization with TRPV1. These results agree with prior electrophysiological findings. In contrast, we found that the ATP-sensitive channel P2X3 is found in a subset of both myelinated and unmyelinated vagal afferent fibers. Specifically, 18% of IB4 and 23% of CTb afferents contained P2X3. The majority of CTb-ir vagal afferents contained neither channel. Since neither channel was found in all vagal afferents, there are likely further degrees of heterogeneity in the modulation of vagal afferent sensory input to the NTS beyond fiber type.

Neuromodulation of chronic headaches: position statement from the European Headache Federation

The medical treatment of patients with chronic primary headache syndromes (chronic migraine, chronic tension-type headache, chronic cluster headache, hemicrania continua) is challenging as serious side effects frequently complicate the course of medical treatment and some patients may be even medically intractable. When a definitive lack of responsiveness to conservative treatments is ascertained and medication overuse headache is excluded, neuromodulation options can be considered in selected cases. Here, the various invasive and non-invasive approaches, such as hypothalamic deep brain stimulation, occipital nerve stimulation, stimulation of sphenopalatine ganglion, cervical spinal cord stimulation, vagus nerve stimulation, transcranial direct current stimulation, repetitive transcranial magnetic stimulation, and transcutaneous electrical nerve stimulation are extensively published although proper RCT-based evidence is limited. The European Headache Federation herewith provides a consensus statement on the clinical use of neuromodulation in headache, based on theoretical background, clinical data, and side effect of each method. This international consensus further gives recommendations for future studies on these new approaches. In spite of a growing field of stimulation devices in headaches treatment, further controlled studies to validate, strengthen and disseminate the use of neurostimulation are clearly warranted. Consequently, until these data are available any neurostimulation device should only be used in patients with medically intractable syndromes from tertiary headache centers either as part of a valid study or have shown to be effective in such controlled studies with an acceptable side effect profile.

Vagal afferent modulation of spinal trigeminal neuronal responses to dural electrical stimulation in rats

Vagus nerve stimulation (VNS) is an approved antiepileptic and antidepressant treatment, which has recently shown promise as a therapy for drug-resistant primary headaches. Specific neurobiological mechanisms underlying its anticephalgic action are not elucidated, partly because of the deficiency of research-related findings. The spinal trigeminal nucleus (STN) plays a prominent role in pathophysiology of headaches by modulating pain transmission from intracranial structures to higher centers of the brain. To determine whether vagal stimulation may affect trigeminovascular nociception, we investigated the effects of VNS on the STN neuronal activity in the animal model of headache. In anesthetized rats the spike activity of the STN neurons with convergent orofacial and meningeal inputs was monitored, and the changes in neuronal responses to electrical stimulation of the dura mater under preconditioning or under continuous electrical stimulation of the left cervical vagus nerve were studied. Preconditioning vagal afferent stimulation (200-ms train of pulses at 30 Hz applied before each dural stimulus) did not produce substantial changes in the STN spike activity. However, continuous VNS with frequency of 10 Hz in 48% of cases significantly suppressed trigeminal neuronal responses to dural electrical stimulation. In line with the decrease in evoked activity, the VNS-induced depression of ongoing neuronal firing was observed. Although the inhibitory effect was prevailing, 29.5% of STN neurons were facilitated by VNS, whereas 22.5% were unresponsive to the stimulation. These results provide an evidence of VNS-induced modulation of trigeminovascular nociception, and therefore contribute to a deeper understanding of neurophysiological mechanisms underlying effects of vagal stimulation in chronic drug-resistant headaches.

Ingestion of amniotic fluid enhances the facilitative effect of VTA morphine on the onset of maternal behavior in virgin rats

Previous research has shown that injection of morphine into the ventral tegmental area (VTA) facilitates the onset of maternal behavior in virgin female rats, and injection of the opioid antagonist naltrexone into the VTA disrupts the onset of maternal behavior in parturient rats. Placentophagia -- ingestion of placenta and amniotic fluid, usually at parturition -- modifies central opioid processes. Ingestion of the active substance in placenta and amniotic fluid, Placental Opioid-Enhancing Factor (POEF), enhances the hypoalgesic effect of centrally administered morphine, and more specifically, enhances delta- and kappa-opioid-receptor-mediated hypoalgesia and attenuates mu-opioid-receptor-mediated hypoalgesia. POEF (in placenta or amniotic fluid) ingestion does not, by itself, produce hypoalgesia. In the present study, we tested the hypothesis that ingestion of amniotic fluid enhances the facilitative effect of opioid activity (unilateral morphine injection) in the VTA on the rate of onset of maternal behavior. Virgin female Long-Evans rats were given one intra-VTA injection of morphine sulfate (0.0, 0.01, or 0.03 microg, in saline) and an orogastric infusion of 0.25 ml amniotic fluid or saline once each day of the first three days of the 10-day testing period. Subjects were continuously exposed to foster pups that were replaced every 12 h; replacement of pups was followed by a 15-min observation period. Maternal behavior latency was determined by the first of two consecutive tests wherein the subject displayed pup retrieval, pup licking in the nest, and crouching over all foster pups, during the 15-min observation. We confirmed the previous finding that the VTA injection, alone, of 0.03 microg morphine shortened the latency to show maternal behavior and that 0.0 microg and 0.01 microg morphine did not. Ingestion of amniotic fluid (and therefore POEF) facilitated the onset of maternal behavior in rats receiving an intra-VTA microinjection of an otherwise subthreshold dose of morphine (0.01 microg).

Pelvic nerve input mediates descending modulation of homovisceral processing in the thoracolumbar spinal cord of the rat

BACKGROUND & AIMS

Colonic afferents project to the lumbosacral and thoracolumbar spinal cord via the pelvic and hypogastric/lumbar colonic nerves, respectively. Both spinal regions process inflammatory colonic stimuli. The role of thoracolumbar segments in processing acute colorectal pain is questionable, however, because the lumbosacral spinal cord appears sufficient to process reflex responses to acute pain. Here, we show that activity in pelvic nerve colonic afferents actively modulates thoracolumbar dorsal horn neuron processing of the same colonic stimulus through a supraspinal loop: homovisceral descending modulation.

METHODS

Dorsal horn neurons were recorded in the rat thoracolumbar spinal cord after acute or chronic pelvic neurectomy and cervical cold block.

RESULTS

Acute pelvic neurectomy or lidocaine inhibition of lumbosacral dorsal roots facilitated the excitatory response of thoracolumbar dorsal horn neurons to colorectal distention (CRD) and decreased the percentage of neurons inhibited by CRD, suggesting colonic input over the pelvic nerve inhibits thoracolumbar processing of the same stimulus. Ectopic activity developed in the proximal pelvic nerve after chronic neurectomy reactivating the inhibitory circuit, inhibiting thoracolumbar neurons. Cervical cold block alleviated the inhibition in intact or chronic neurectomized rats. However, the facilitated response after acute pelvic neurectomy was inhibited by cervical cold block, exposing an underlying descending facilitation. Inhibiting pelvic nerve input after cervical cold block had minimal effect.

CONCLUSIONS

These data demonstrate that input over the pelvic nerve modulates the response of thoracolumbar spinal neurons to CRD by a supraspinal loop and that increasing thoracolumbar processing increases visceral hyperalgesia.

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.

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.

Role of NK2 receptors in gastric barosensitivity and in experimental ileus in rats

This study was performed to evaluate the role of tachykinin NK2 receptors in gastric barosensitivity and in postsurgical intestinal atony, using a selective NK2 antagonist (MEN 11420). Gastric distensions were performed in rats equipped with a gastric balloon and electrodes implanted in the neck muscles. Ileus was produced by laparotomy and caecum palpation in rats previously prepared with electrodes implanted on the proximal jejunum. Fifteen minutes before gastric distension or laparotomy, the animals received MEN 11420 (10, 100 or 200 microg kg-1 intravenously) or saline. The first distending pressure to increase the integrated neck electromyogram > 100% was considered the pain threshold. MEN 11420 (100 microg kg-1) increased significantly pain threshold (20.5 +/- 1.2 vs. 17.0 +/- 0.8 mm Hg) but did not modify gastric volumes at the three doses tested. Abdominal surgery was followed by a total inhibition of jejunal spiking activity lasting 80.4 +/- 18.7 min. MEN 11420 (10 and 100 microg kg-1) shortened the duration of motor inhibition by 36 and 39%, and induced a premature recovery of the phase III of migrating myoelectric complex at the lowest dose tested (130 +/- 32 vs. 192 +/- 28 min). We conclude that NK2 receptors, probably located on afferent fibres, are involved in gastric barosensitivity and in postsurgical intestinal atony.

Electrical stimulation of the subdiaphragmatic vagus in rats: inhibition of heat-evoked responses of spinal dorsal horn neurons and central substrates mediating inhibition of the nociceptive tail flick reflex

Electrical stimulation of the subdiaphragmatic branch of the vagus nerve (SDVAS) inhibits the nociceptive tail flick (TF) reflex. The present experiments examined (1) the central substrates mediating SDVAS-produced inhibition of the TF reflex and (2) the effects of SDVAS on either background or noxious heat-evoked responses of spinal dorsal horn neurons. Microinjections of ibotenic acid in the ipsilateral nucleus tractus solitarius (NTS), nucleus raphe magnus (NRM), or bilateral locus coeruleus/subcoeruleus (LC/SC) significantly increased the intensity of SDVAS required to inhibit the TF reflex in lightly anesthetized rats. In studies of class-2 spinal dorsal horn neurons, SDVAS produced significant intensity-dependent inhibition of noxious heat-evoked responses of 17/25 (68%) units, facilitation of 4/25 (16%) units, and no effect on 4/25 (16%) units. In studies of class-3 spinal dorsal horn neurons, SDVAS produced significant intensity-dependent inhibition of noxious heat-evoked responses of 8/9 (89%) units. Noxious heat-evoked responses of 1/9 (11%) unit were facilitated by SDVAS. In general, the background activity of either class-2 or class-3 units was not significantly affected by SDVAS. SDVAS produced a significant rightward, parallel shift in the stimulus response function (SRF) of class-2 neurons to noxious, graded heat stimuli ranging from 40 to 52 degrees C, while SDVAS produced a significant increase in the threshold and a significant reduction in the slope of the SRF of class-3 neurons. These data indicate that SDVAS generally inhibits noxious heat-evoked responses of lumbosacral spinal dorsal horn neurons in the rat, but does not significantly affect background activity of the same neurons. Furthermore, the inhibition of the TF reflex produced by SDVAS depends on central relays in the ipsilateral NTS, NRM, and bilateral LC/SC.

Vagal afferent modulation of nociception

Chemical, electrical or physiological activation of cardiopulmonary vagal (cervical, thoracic or cardiac), diaphragmatic vagal (DVAG) or subdiaphragmatic vagal (SDVAG) afferents can result in either facilitation or inhibition of nociception in some species. In the rat, these effects depend upon vagal afferent input to the NTS and subsequent CNS relays, primarily in the NRM and ventral LC/SC, although specific relay nuclei vary as a function of the vagal challenge stimulus. Spinal pathways and neurotransmitters have been identified for vagally mediated effects on nociception and consistently implicate the involvement of descending 5-HT and noradrenergic systems, as well as intrinsic spinal opioid receptors. Species differences may exist with respect to both the effects of DVAG and SDVAG afferents on nociception and the efficacy of vagal afferents to modulate nociception. However, it is also possible that such differences reflect the modality of noxious input (e.g., visceral versus cutaneous), the type of neuronal activity investigated (e.g., resting versus noxious-evoked), spinal location of recording (e.g., thoracic versus lumbosacral) and/or parameters of stimulation. It is also possible that activation of some vagal afferents is aversive, but whether this contributes to changes in nociception produced by vagal activation has not clearly been established. Finally, the vagal-nociceptive networks described in this review provide a fertile area for future study. These networks can provide an understanding of physiological and pathophysiological peripheral events that affect nociception.

Evaluation of vagal afferent modulation of the digastric reflex in cats

In the present study, we have examined the relative ability of cervical, thoracic, cardiac and diaphragmatic vagal stimulation to modulate the digastric reflex produced by tooth-pulp stimulation in anesthetized cats. The right maxillary tooth pulp was stimulated and the digastric reflex was recorded from the right digastric muscle. Cervical vagal stimulation produced a biphasic effect on the digastric reflex. The reflex was facilitated at conditioning test intervals less than 20 ms and inhibited at conditioning test intervals between 100 ms and 500 ms. Cardiac and thoracic vagal stimulation did not significantly facilitate the digastric reflex but inhibited the reflex at conditioning test intervals between 50 ms and 500 ms with maximum inhibition observed at 200 ms. In contrast, diaphragmatic vagal stimulation produced a weaker inhibition of the digastric reflex. The relative ability of different vagal segments to inhibit the digastric reflex was: thoracic = cardiac = cervical greater than diaphragmatic. The inhibitory effects were not related to cardiovascular responses to vagal afferent stimulation. These findings suggest cardiopulmonary vagal afferents represent an important source of vagal afferents which modulate the digastric reflex in the cat.

Antinociception and cardiovascular responses produced by intravenous morphine: the role of vagal afferents

The mechanisms of the antinociceptive, depressor and bradycardic responses produced by intravenous (i.v.) administration of morphine were examined in rats lightly anesthetized with pentobarbital sodium. Intravenous administration of 0.1, 0.25, 0.5, 1.0 or 2.5 mg/kg of morphine produced dose-dependent inhibition of the nociceptive tail flick (TF) reflex, hypotension, and bradycardia. Bilateral cervical vagotomy (CVAG) significantly attenuated the antinociception produced by i.v. morphine and the degree of attenuation was inversely related to drug dose. CVAG had no effect on the depressor response produced by lesser doses of morphine (0.1 or 0.5 mg/kg), but at greater doses converted the depressor response into either a pressor response (1.0 mg/kg) or an initial pressor response followed by a depressor response (2.5 mg/kg). Morphine-induced bradycardia was blocked by CVAG at all drug doses tested (0.1, 0.5, 1.0 and 2.5 mg/kg). In selective tests of either 0.5 or 2.5 mg/kg of i.v. morphine, prior administration of the peripherally acting opioid receptor antagonist naloxone methobromide (NMB) attenuated the antinociception to the same degree as CVAG. NMB also completely blocked the depressor and bradycardic responses of these doses of morphine. Bilateral subdiaphragmatic vagotomy (SDVAG) resulted in a marginal attenuation of antinociception at 0.5 mg/kg but not 2.5 mg/kg of morphine, and the attenuation produced by SDVAG was delayed in onset following morphine administration relative to that produced by CVAG. Bilateral sino-aortic deafferentation (SAD) had no significant effect on the antinociception in tests with 0.5 mg/kg of morphine. SDVAG and SAD had little effect on cardiovascular responses produced by these doses of morphine. The spinal antinociceptive systems activated by vagal afferents following i.v. morphine administration were characterized with the 0.5 mg/kg dose. Spinal cold block significantly antagonized the antinociception, hypotension and bradycardia produced by this dose of morphine. Intrathecal administration of naloxone (1.5, 15 or 30 micrograms) significantly antagonized the antinociception compared to saline controls, whereas intrathecal administration of methysergide (30 micrograms), phentolamine (30 micrograms), or the combination of methysergide with phentolamine (30 micrograms each) had no significant effect on the antinociception. These intrathecal doses of naloxone also antagonized the depressor and bradycardic responses produced by morphine. However, the antagonism produced by 1.5 micrograms of intrathecal naloxone was not due to spread to the systemic circulation, since i.v. administration of 1.5 micrograms of naloxone did not significantly affect either the antinociceptive or cardiovascular responses produced by morphine. These findings indicate that vagal afferents play a significant role in the antinociception produced by i.v. administration of morphine.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative characterization and spinal substrates of antinociception produced by electrical stimulation of the subdiaphragmatic vagus in rats

Physiological, chemical or electrical activation of vagal afferents produces antinociception in rats. The present study examined the effects of electrical stimulation of the subdiaphragmatic vagus (SDV) on the nociceptive tail-flick reflex, arterial blood pressure, and heart rate in rats lightly anesthetized with pentobarbital sodium. The intensity of SDV stimulation (20 Hz, 2 msec) necessary to inhibit the tail-flick reflex to a 10 sec cut-off latency for 3 consecutive trials was defined as the threshold to produce antinociception. Electrical stimulation of the SDV suppressed the tail-flick reflex in a linear, intensity-dependent manner. In addition, the threshold varied as a function of either the frequency or the pulse width of SDV stimulation, such that decreases in either frequency or pulse width, from 20 Hz and 2 msec, respectively, systematically increased the threshold current necessary to produce antinociception. SDV stimulation also produced modest decreases in heart rate (HR), but arterial blood pressure (ABP) responses were highly variable in both the magnitude and direction of change. Intrathecal administration of 30 micrograms of the serotonergic receptor antagonist methysergide significantly increased the threshold for antinociception produced by SDV stimulation from 80 to 938.8 microA, whereas 15 micrograms of methysergide had no significant effect. Intrathecal administration of saline, 30 micrograms of naloxone, or 30 micrograms of phentolamine had no significant effect on the threshold of SDV stimulation required to produce antinociception. Systemic administration of naloxone (4 mg/kg i.p. or i.v.) also had no effect on the antinociceptive threshold. Intrathecal administration of these receptor antagonists had no significant effect on the ABP and HR responses produced by electrical stimulation of the SDV at the threshold intensity producing antinociception.(ABSTRACT TRUNCATED AT 250 WORDS)