Evaluation of vagal afferent modulation of the digastric reflex in cats

Baroreceptor Modulation of the Cardiovascular System, Pain, Consciousness, and Cognition

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain cardiovascular homeostasis by coordinating the responses to external and internal environmental stressors. While it is well known that carotid and cardiopulmonary baroreceptors modulate sympathetic vasomotor and parasympathetic cardiac neural autonomic drive, to avoid excessive fluctuations in vascular tone and maintain intravascular volume, there is increasing recognition that baroreceptors also modulate a wide range of non-cardiovascular physiological responses via projections from the nucleus of the solitary tract to regions of the central nervous system, including the spinal cord. These projections regulate pain perception, sleep, consciousness, and cognition. In this article, we summarize the physiology of baroreceptor pathways and responses to baroreceptor activation with an emphasis on the mechanisms influencing cardiovascular function, pain perception, consciousness, and cognition. Understanding baroreceptor-mediated effects on cardiac and extra-cardiac autonomic activities will further our understanding of the pathophysiology of multiple common clinical conditions, such as chronic pain, disorders of consciousness (e.g., abnormalities in sleep-wake), and cognitive impairment, which may result in the identification and implementation of novel treatment modalities. © 2021 American Physiological Society. Compr Physiol 11:1373-1423, 2021.

Pain modulation induced by respiration: phase and frequency effects

The voluntary control of respiration is used as a common means to regulate pain and emotions and is fundamental to various relaxation and meditation techniques. The aim of the present study was to examine how breathing frequency and phase affect pain perception, spinal nociceptive activity (RIII-reflex) and brain activity (scalp somatosensory-evoked potentials - SEP's). In 20 healthy volunteers, painful electric shocks individually adjusted to 120% of the RIII-reflex threshold were delivered to the sural nerve near the end of inspiration or expiration phases, during three cued-breathing conditions: (1) slow breathing (0.1 Hz) with slow (4s) inspiration (0.1Hz-SlowIns), (2) slow breathing (0.1 Hz) with fast (2s) inspiration (0.1 Hz-FastIns), and (3) normal breathing (0.2 Hz) with fast (2s) inspiration (0.2 Hz). Pain ratings were not affected by breathing patterns (p=0.3), but were significantly lower during inspiration compared with expiration (p=0.02). This phase effect was also observed on the N100 component of SEP's, but only in the 0.1-Hz-FastIns condition (p=0.03). In contrast, RIII-reflex amplitude was greater during inspiration compared with expiration (p=0.02). It was also decreased in the 0.1-Hz-SlowIns compared with the 0.2-Hz condition (p=0.01). Slow breathing also increased the amplitude of respiratory sinus arrhythmia (RSA), although these changes were not significantly associated with changes in pain responses. In conclusion, this study shows that pain and pain-related brain activity may be reduced during inspiration but these changes are dissociated from spinal nociceptive transmission. The small amplitude of these effects suggests that factors other than respiration contribute to the analgesic effects of relaxation and meditation techniques.

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.

Responses of the hypothalamic-pituitary-adrenal axis and pain threshold changes in the orofacial region upon cold pressor stimulation in normal volunteers

AIM

This study focused on the relationship between the HPA axis function and the heat pain threshold in the orofacial region upon cold pressor (CP) stimulation.

METHODS

Ten healthy male individuals participated in this study. CP stimulation was applied to each participant, and their peripheral blood was collected 5 min before, during and 5, 15, 30, 45, 60 min after receiving CP. In addition, 5 of those 10 participants were selected at random and they experienced a mock CP trial on different days. The heat pain thresholds on the facial skin about 10mm anterior to the right external auditory canal (trigeminal V2 region) in each subject were simultaneously recorded 5 min before and 5, 30, 60 min after CP stimulation. The blood pressure and heart rate were continuously monitored throughout the course of the CP and mock trials using an electric blood pressure meter.

RESULTS

Significant increases in the plasma concentration of cortisol, beta-endorphin and ACTH were induced by CP stimulation, while no significant increases were observed under the mock trial conditions. The blood pressure and heart rate showed concomitant increases during CP stimulation. In addition, the heat pain threshold in the orofacial region significantly increased after receiving CP stimulation. These results suggest that CP stimulation activated the HPA axis thereby increasing the heat pain threshold in the orofacial region in healthy individuals.

CONCLUSIONS

This observed pain threshold increase might be due to the activation of an endogenous opioid system, such as increase in the circulating beta-endorphin levels.

Digastric muscle activities in anoxic infant rats

The digastric muscle acts for both feeding (including mastication and swallowing) and respiration. In this study, we examined whether or not the muscle activity is detectable during anoxia in developing rats. Rats at 4 different ages, days 5, 10, 16, and 24, were exposed to 100% N2 under pentobarbital or ketamine–xylazine anesthesia, and the electromyograms of digastric muscles (dEMG) and the diaphragm (diaEMG) were examined simultaneously. Prior to the anoxic exposure, at all ages, the dEMG was similar to or less apparent than the diaEMG, which was detected at each inspiratory movement. In anoxia, we first observed dEMG activity, mostly sporadic (days 5 and 10) or mostly tonic (days 16 and 24), when diaEMG activity was temporarily suppressed (we termed it Phase 1). Second, synchronous phasic or tonic dEMG and phasic diaEMG were recorded temporarily before terminal apnea (we termed it Phase 2). These phenomena were also obtained in vagotomized rats (all ages) or in rats injected with the N-methyl-D-aspartate receptor antagonist MK-801 (dizocilpine (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate) (days 16 and 24). In conclusion, our results suggest that in anoxia, dEMG activity is detectable during diaEMG suppression in early anoxia, irrespective of the developmental age, the anesthetic (pentobarbital or ketamine–xylazine), vagotomy, or MK-801 injections.Key words: newborn, hypoxia, apnea, swallowing, trigeminal nerve.

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.

Use of noninvasive dental dolorimetry to evaluate analgesic effects of intravenous and intrathecal administration of morphine in anesthetized dogs

OBJECTIVE

To determine whether changes in amplitude of the reflex-evoked muscle action potential (REMP) elicited by noninvasive dental dolorimetry (electrical stimulation of the tooth pulp) in anesthetized dogs may be used to objectively evaluate the effectiveness of IV and intrathecal (IT) administration of morphine.

ANIMALS

6 male Beagles that were 2 to 6 years old.

PROCEDURE

Dogs were used in a crossover design with at least a 5-day washout period between treatments. Each dog received morphine, saline (0.9% NaCl) solution, and oxytocin via the IV and IT routes of administration; however, only results for morphine and saline treatments were reported here. Dogs were anesthetized and prepared for noninvasive dental dolorimetry. After IV or IT administration, electrical stimulation was applied to a tooth, and REMPs of the digastricus muscle were recorded at 5-minute intervals for 60 minutes. To determine differences in REMP amplitude between treatments, a linear regression line was fitted for each dog-treatment combination.

RESULTS

The IV administration of morphine significantly inhibited REMP amplitude, compared with IV administration of saline solution. Intrathecal administration of morphine significantly inhibited REMP amplitude, compared with IT administration of saline solution.

CONCLUSIONS AND CLINICAL RELEVANCE

Noninvasive dental dolorimetry in anesthetized dogs has promise as a technique for use in evaluating the analgesic potential of drugs administered IV and IT through evaluation of their effect on REMP amplitude recorded for the digastricus muscle.

Effect of intrathecal and intravenous administration of oxytocin on amplitude of the reflex-evoked muscle action potential after electrical stimulation of the tooth pulp in anesthetized dogs

OBJECTIVE

To determine whether intrathecal (IT) or IV administration of oxytocin will diminish amplitude of the reflex-evoked muscle action potential (REMP) in the digastricus muscle during electrical stimulation of the tooth pulp in anesthetized dogs, thus suggesting an analgesic effect for oxytocin.

ANIMALS

6 male Beagles that were 2 to 6 years old.

PROCEDURE

Dogs were used in a crossover design with at least a 5-day washout period between treatments. Each dog received morphine, saline (0.9% NaCl) solution, and oxytocin by both the IT and IV routes of administration. Noninvasive dental dolorimetry was used to assess changes in pain threshold following administration of treatments. Effectiveness of analgesia was determined on the basis of change in REMP amplitude in the digastricus muscle.

RESULTS

Morphine administered IV significantly inhibited REMP amplitude, compared with IV administration of saline solution or oxytocin. There was not a significant change in REMP amplitude between saline solution and oxytocin administered IV. Intrathecal administration of morphine significantly inhibited REMP amplitude, compared with IT administration of saline solution or oxytocin. Intrathecal administration of oxytocin significantly increased REMP amplitude, compared with IT administration of saline solution or morphine.

CONCLUSIONS AND CLINICAL RELEVANCE

Although IV administration of oxytocin did not have an effect on REMP amplitude, compared with IV administration of saline solution, IT administration of oxytocin had the opposite effect of morphine and increased REMP amplitude of the digastricus muscle. These data do not support the use of oxytocin as an analgesic agent in dogs.

Volume expansion suppresses the tooth-pulp evoked jaw-opening reflex related activity of trigeminal neurons in rats

The aim of the present study is to clarify whether physiological stimulation of vagal afferents modulates the activity of the trigeminal spinal nucleus oralis (TSNO) neurons related to the tooth-pulp (TP)-evoked jaw-opening reflex (JOR) in pentobarbital-anesthetized rats. The activity of TSNO neurons and the amplitude of digastric electromyogram (dEMG) increased proportionally during 1.0-3.5 times the threshold for JOR. The amplitude of the dEMG of 14 out of 17 rats was suppressed by physiological stimulation of vagal afferents after intravenous infusion of Ficoll. Out of 23, 18 TSNO unit activities in 14 rats were also suppressed by Ficoll infusion. This suppressive effect of unit and dEMG activities returned to the control level within 25 min. After administration of naloxone (0.5 and 1.0 mg/kg, i.v.) the suppressive effect of Ficoll infusion on the activity of TSNO neurons (5/7) was significantly attenuated compared to the control (p < 0.01). The inhibition TSNO neuronal and dEMG activities by Ficoll infusion was volume-dependent in a range of 5-10% of total blood volume. Furthermore, right vagus nerve ligation greatly inhibited the suppressive effect of Ficoll-induced TSNO activity. These results therefore suggest that low-pressure cardiopulmonary baroreceptors whose afferents travel in the vagus nerve inhibit the pulpal nociceptive transmission.

Group differences in pain modulation: pain-free women compared to pain-free men and to women with TMD

Previously reported differences in sensitivity to experimental pain stimuli between the sexes, as well as between temporomandibular disorder (TMD) patients and healthy control subjects, may be attributable in part to group differences in two pain modulatory mechanisms: the baroreceptor reflex arc and the endogenous opioid system. Twenty-two pain-free (PF) men, 20 PF women and 20 women with TMD underwent two testing sessions in which heat pain and ischemic arm pain threshold and tolerance were measured during both sessions, but followed relaxation during one session and laboratory stress tasks during the other. Blood pressure (BP) and plasma -endorphin (E) concentration were measured during a baseline rest and during the stress or relaxation periods. PF men's threshold and tolerance for heat pain, but not for ischemic pain, exceeded that of PF women's during both sessions. PF women and TMD women did not differ in sensitivity to either pain modality; however, significantly lower ischemic pain threshold (IPTh) was linked to oral contraceptive use in PF women but not TMD patients. In the men alone, higher baseline systolic BP (SBP) was correlated with higher heat pain threshold on both days and heat pain tolerance on the stress day. Conversely, in TMD women, higher baseline SBP was correlated with lower ischemic pain tolerance (IPTol) on both days; BP and pain sensitivity were not related in PF women. In men, but not in PF or TMD women, stress systolic and diastolic BP were positively correlated with heat pain threshold and tolerance and higher diastolic reactivity to stress were correlated with higher heat pain and IPTh and tolerance. On the stress day, higher baseline E level was strongly associated with higher IPTol in PF women but marginally associated with lower IPTol in TMD women. Thus, it appears that a BP-related analgesic mechanism (probably baroreceptor-mediated) predominates in PF men, while an endogenous opioid mechanism predominates in PF women. Stress enhances the expression of these central mechanisms. Female TMDs appear unable to effectively engage normal pain-inhibitory systems; opioid receptor desensitization and/or downregulation are probably implicated, because TMDs' production of E appears normal.

Vagotomy prevents morphine-induced reduction in Fos-like immunoreactivity in trigeminal spinal nucleus produced after TMJ injury in a sex-dependent manner

Acute injury to the temporomandibular joint (TMJ) region activates neurons in multiple, but spatially discrete, areas of the trigeminal spinal nucleus as seen by an increase in Fos-like immunoreactive neurons (Fos-LI). Pretreatment with morphine greatly reduces Fos-LI produced in the dorsal paratrigeminal area (dPa5), ventrolateral pole of the subnucleus interpolaris/caudalis (Vi/Vc-vl) transition region, and laminae I-II at the subnucleus caudalis/upper cervical cord junction (Vc/C2) suggesting a role for these areas in processing pain signals from the TMJ region. To determine if vagal afferents contribute to neural activation after TMJ injury or reduction of activity after morphine, Fos-LI was quantified in the lower brainstem and upper cervical spinal cord of intact and vagotomized male and female rats under barbiturate anesthesia. Bilateral cervical vagotomy (VgX) did not affect Fos-LI produced by TMJ injury in males or females in the absence of morphine. By contrast, morphine-induced reduction in Fos-LI produced at the Vi/Vc-vl transition region was prevented by prior VgX in males and diestrus females, but not in proestrus females. Morphine inhibition of Fos-LI produced in laminae I-II at the Vc/C2 junction region was diminished in vagotomized males compared to intact animals, but not affected in females. In an autonomic control area, the caudal ventrolateral medulla (CVLM), VgX reversed the morphine-induced reduction in Fos-LI in males and females similarly compared to their respective intact controls. These results were consistent with the hypothesis that the Vi/Vc-vl transition region plays a unique role in deep craniofacial pain processing and may integrate autonomic and opioid-related modulatory signals in a manner dependent on sex hormone status.

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.

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.

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.

A parametric analysis of the effects of cardiopulmonary vagal electrostimulation on the digastric reflex in cats

The thoracic trunk and the cardiac branch of the vagus were stimulated electrically in chloralose-anesthetized cats. The experiments were conducted to determine the parameters of vagal afferent stimulation (VAS) capable of producing an inhibition of the digastric reflex (DGR), to assess the duration of this inhibition, and to test whether endogenous opioids mediate the inhibitory effects. In experiments using intermittent trains of pulses, the effects of pulse number (1, 2, 7 or 35 pulses), frequency (13, 66 or 333 Hz), intensity (0.1, 0.5, 1, 2, 3, 4 or 5 mA), and duration (1 or 3 ms) were evaluated. A 7 pulse train (3 mA) was sufficient to produce maximal inhibition (77 +/- 7%) of the tooth-pulp stimulation-evoked DGR regardless of the pulse duration or frequency. These effects were mediated by vagal afferents since stimulation of the central end produced as much inhibition as stimulation of the intact nerve. VAS also significantly reduced the DGR when elicited by tooth-pulp intensities at 1x -5x threshold. In experiments using 90 s of continuous VAS, 16 combinations of frequency and intensity yielded a threshold intensity for DGR inhibition between 0.1 and 0.5 mA and a threshold frequency at 2 Hz. Maximal DGR inhibition was produced at 5 Hz-0.5 mA by VAS. Reflex inhibition occurred within 10 s and outlasted VAS for longer than 60 s. Opiate-receptor blockade did not alter VAS inhibition of the DGR and, thus, opioids are not likely to mediate VAS-induced digastric inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

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.