Electric Low-Frequency Stimulation of the Tongue Induces Long-Term Depression of the Jaw-Opening Reflex in Anesthetized Mice

Effect of local administration of eicosapentaenoic acid on the jaw-opening reflex in rats

Although eicosapentaenoic acid (EPA) application in vitro inhibits voltage-gated Na⁺ (Nav) channels in excitable tissues, the acute local effect of EPA on the jaw-opening reflex in vivo remains unknown. The aim of the present study was to determine whether local administration of EPA to adult male Wistar rats could attenuate the excitability of the jaw-opening reflex in vivo, including nociception. The jaw-opening reflex evoked by electrical stimulation of the tongue was recorded by a digastric muscle electromyogram (dEMG) in pentobarbital-anesthetized rats. The amplitude of the dEMG response was significantly increased in proportion to the electrical stimulation intensity (1×-5× threshold). At 3×, local administration of EPA dose-dependently inhibited the dEMG response, lasting 60 min, with maximum inhibition observed within approximately 10 min. The mean magnitude of dEMG signal inhibition by EPA was almost equal to that observed with a local anesthetic, 1% lidocaine, and with a half dose of lidocaine plus a half dose of EPA. These findings suggest that EPA attenuates the jaw-opening reflex, possibly by blocking Nav channels of primary nerve terminals, and strongly support the idea that EPA is a potential therapeutic agent and complementary alternative medicine for the prevention of acute trigeminal nociception.

Inhibition of cortical somatosensory processing during and after low frequency peripheral nerve stimulation in humans

OBJECTIVE

Transcutaneous low-frequency stimulation (LFS) elicits long-term depression-like effects on human pain perception. However, the neural mechanisms underlying LFS are poorly understood. We investigated cortical activation changes occurring during LFS and if changes were associated with reduced nociceptive processing and increased amplitude of spontaneous cortical oscillations post-treatment.

METHODS

LFS was applied to the radial nerve of 25 healthy volunteers over two sessions using active (1 Hz) or sham (0.02 Hz) frequencies. Changes in resting electroencephalography (EEG) and laser-evoked potentials (LEPs) were investigated before and after LFS. Somatosensory-evoked potentials were recorded during LFS and source analysis was carried out.

RESULTS

Ipsilateral midcingulate and operculo-insular cortex source activity declined linearly during LFS. Active LFS was associated with attenuated long-latency LEP amplitude in ipsilateral frontocentral electrodes and increased resting alpha (8-12 Hz) and beta (16-24 Hz) band power in electrodes overlying operculo-insular, sensorimotor and frontal cortical regions. Reduced ipsilateral operculo-insular cortex source activity during LFS correlated with a smaller post-treatment alpha-band power increase.

CONCLUSIONS

LFS attenuated somatosensory processing both during and after stimulation.

SIGNIFICANCE

Results further our understanding of the attenuation of somatosensory processing both during and after LFS.

Cortical stimulation in pharmacoresistant focal epilepsies

Pharmacoresistance and adverse drug events designate a considerable group of patients with focal epilepsies that require alternative treatments such as neurosurgical intervention and neurostimulation. Electrical or magnetic stimulations of cortical brain areas for the treatment of pharmacoresistant focal epilepsies emerged from preclinical studies and experience through intraoperative neurophysiological monitoring in patients. Direct neurostimulation of seizure onset zones in neocortical brain areas may specifically affect neuronal networks involved in epileptiform activity without remarkable adverse influence on physiological cortical processing in immediate vicinity. Noninvasive low-frequency transcranial magnetic stimulation and cathodal transcranial direct current stimulation are suggested to be anticonvulsant; however, potential effects are ephemeral and require effect maintenance by ongoing stimulation. Invasive responsive neurostimulation, chronic subthreshold cortical stimulation, and epicranial cortical stimulation cover a broad range of different emerging technologies with intracranial and epicranial approaches that still have limited market access partly due to ongoing clinical development. Despite significant differences, the present bioelectronic technologies share common mode of actions with acute seizure termination by high-frequency stimulation and long-term depression induced by low-frequency magnetic or electrical stimulation or transcranial direct current stimulation.

Systemic administration of the dietary constituent resveratrol inhibits the nociceptive jaw-opening reflex in rats via the endogenous opioid system

The aim of the present study was to investigate whether, under in vivo conditions, systemic administration of resveratrol could attenuate the rat nociceptive jaw-opening reflex (JOR) via the endogenous opioid system. The JOR evoked by electrical stimulation of the tongue was recorded as digastric muscle electromyograms (dEMG) in pentobarbital-anesthetized rats. The amplitude of the dEMG increased significantly in proportion to the intensity of electrical stimulation (from 1× to 5 × threshold for the JOR). dEMG amplitude in response to 3× threshold electrical stimulation of the tongue was dose-dependently inhibited by intravenous administration of resveratrol (0.5-2mg/kg). Maximum inhibition of dEMG amplitude was seen within approximately 10min. These inhibitory effects were reversible, with dEMG responses returning to control levels after approximately 20min. Pretreatment of rats with naloxone resulted in significant, dose-dependent attenuation of the inhibitory effects of resveratrol on dEMG amplitude compared with control. These findings suggest that resveratrol inhibits the nociceptive JOR via the endogenous opioid system. Further, the findings of the present study strongly support the idea that resveratrol, which is not known to have any toxic side effects, combined with an opioid could be a potential therapeutic agent for the prevention of acute trigeminal nociception.

Laser-Evoked Cortical Potentials in Cluster Headache

Craniofacial nociceptive processing in patients ( n = 25) suffering from unilateral cluster headache was assessed by laser-evoked cortical potentials (LEPs). Latencies and amplitudes of late (N2, P2) and middle-latency (N1) LEPs were measured in chronic (CCH, n = 9) and episodic cluster headache (ECH, n = 17). In CCH patients on headache side N1c occurred later and P2 amplitude was smaller than on the healthy control side. In active periods of ECH patients P2 latency was shorter on the headache side. In remission periods of ECH patients the N2P2 ratio was lower on the headache side. In 19 out of 26 examinations in 25 headache patients LEP deviated from normative data in healthy controls ( n = 10) without any specific pattern of altered parameters. LEPs document pathological changes in craniofacial nociception in cluster headache. However, there seems to be no pathognomonic deviation pattern that enables reliable diagnosis of cluster headache and application of LEPs in further studies of pathophysiological mechanisms.

Treatment of chronic migraine with transcutaneous stimulation of the auricular branch of the vagal nerve (auricular t-VNS): a randomized, monocentric clinical trial

Background

Aim of the study was assessment of efficacy and safety of transcutaneous stimulation of the auricular branch of the vagal nerve (t-VNS) in the treatment of chronic migraine.

Methods

A monocentric, randomized, controlled, double-blind study was conducted. After one month of baseline, chronic migraine patients were randomized to receive 25 Hz or 1 Hz stimulation of the sensory vagal area at the left ear by a handhold battery driven stimulator for 4 h/day during 3 months. Headache days per 28 days were compared between baseline and the last month of treatment and the number of days with acute medication was recorded The Headache Impact Test (HIT-6) and the Migraine Disability Assessment (MIDAS) questionnaires were used to assess headache-related disability.

Results

Of 46 randomized patients, 40 finished the study (per protocol). In the per protocol analysis, patients in the 1 Hz group had a significantly larger reduction in headache days per 28 days than patients in the 25 Hz group (−7.0 ± 4.6 vs. −3.3 ± 5.4 days, p = 0.035). 29.4 % of the patients in the 1 Hz group had a ≥50 % reduction in headache days vs. 13.3 % in the 25 Hz group. HIT-6 and MIDAS scores were significantly improved in both groups, without group differences. There were no serious treatment-related adverse events.

Conclusion

Treatment of chronic migraine by t-VNS at 1 Hz was safe and effective. The mean reduction of headache days after 12 weeks of treatment exceeded that reported for other nerve stimulating procedures.

Neuronal nitric oxide synthase is involved in the induction of nerve growth factor-induced neck muscle nociception

BACKGROUND

Neck muscle nociception mediated by nitric oxide may play a role in the pathophysiology of tension-type headache.

OBJECTIVE

The present study addresses the involvement of neuronal nitric oxide synthase (nNOS) in the facilitation of neck muscle nociception after local application of nerve growth factor (NGF).

METHODS

After administration of NGF into semispinal neck muscles, the impact of neck muscle noxious input on brainstem processing was monitored by the jaw-opening reflex in anesthetized mice. The modulatory effect of preceding and subsequent administration of an inhibitor of neuronal nitric oxide synthase on central facilitation was addressed in a controlled study.

RESULTS

With preceding i.p. application of saline or 0.096 mg/kg of the specific nNOS inhibitor Nω-propyl-L-arginine (NPLA), NGF induced a sustained reflex facilitation within 60 minutes. Preceding injection of 0.96 mg/kg or 1.92 mg/kg NPLA completely prevented the potentially facilitatory effect of NGF. Subsequent administration of 0.96 mg/kg NPLA did not affect established NGF-evoked reflex facilitation. Thus, NPLA prevents facilitation of brainstem processing by noxious myofascial input from neck muscles in a dose-dependent manner.

CONCLUSION

These findings suggest that nNOS is involved in the induction but not the maintenance of NGF-evoked facilitation of nociception in the brainstem. These results from an experimental animal model may support the idea of NOS and nNOS as potential targets for pharmacological treatment of tension-type headache.

Inhibition of nNOS prevents and inhibition of iNOS reverses α,β-meATP-induced facilitation of neck muscle nociception in mice

Infusion of α,β-methylene ATP (α,β-meATP) into murine neck muscle facilitates brainstem nociception. Unspecific nitric oxide synthase (NOS) inhibition prevents and reverses this sensitization. It is unclear whether neuronal (nNOS), inducible (iNOS) or endothelial NOS isoenzymes are involved in this α,β-meATP effect. Hypothesized involvement of nNOS isoenzyme was addressed by preceding (0.5, 1, and 2 mg/kg) and subsequent (2 mg/kg) intraperitoneal injection of the nNOS-inhibitor NPLA. iNOS involvement was addressed by subsequent, intraperitoneal administration of the iNOS-inhibitor 1400 W (2 mg/kg). Brainstem nociception was monitored by the jaw-opening reflex elicited via electrical tongue stimulation in 45 anesthetized mice. Preceding NPLA dose-dependently prevented α,β-meATP-induced reflex facilitation. Whereas subsequent inhibition of nNOS showed no effect, iNOS inhibition by 1400 W significantly reversed reflex facilitation. Data provide evidence that nNOS plays a major role in induction and iNOS in maintenance of facilitation in neck muscle nociception. Divergent roles of NOS isoenzymes may promote research on target specific treatment for headache and neck muscle pain.

Inhibition of nitric oxide synthases prevents and reverses alpha,beta-meATP-induced neck muscle nociception in mice

INTRODUCTION

Tension-type headache (TTH) is associated with noxious input from neck muscles. Intravenous administration of the unspecific nitric oxide synthase inhibitor L-NMMA in chronic TTH patients caused analgesia and reduction of neck muscle tenderness.

METHODS

The unspecific nitric oxide synthase inhibitor L-NMMA was applied in an experimental model for neck muscle nociception in anesthetized mice (N = 25).

RESULTS

Local injection of α,β-meATP into semispinal neck muscles induced sustained facilitation of brainstem nociception as monitored by the jaw-opening reflex. Preceding intraperitoneal administration of L-NMMA (0.05, 0.1, 1 mg/kg) prevented reflex facilitation evoked by α,β-meATP in a dose-dependent manner. Intraperitoneal injection of L-NMMA subsequent to intramuscular α,β-meATP application reversed established brainstem reflex facilitation back to baseline values.

DISCUSSION

Both experiments with preceding and subsequent L-NMMA indicate the involvement of nitric oxide synthases in the induction and maintenance of facilitation. However, future experiments will have to address the involvement of various isoenzymes in order to provide for new therapeutic concepts in TTH.

Low-frequency electrical stimulation induces long-term depression in patients with chronic tension-type headache

Repetitive low-frequency electrical stimulation (LFS) induces pain inhibition in healthy volunteers and in animals, but it is unknown whether it has an analgesic effect in patients with headache. The aim of this study was to investigate if LFS could induce prolonged pain inhibition, called long-term depression (LTD), in patients with chronic tension-type headache (CTTH). Twenty CTTH patients and 20 healthy volunteers were exposed to 20 min LFS (1 Hz) to the forehead. LTD was measured as a decrease in pain response to electrical stimulation in a 1-h post-LFS period following LFS. The LFS induced a significant and stable inhibition of pain (LTD) both in patients with CTTH (post-LFS average decrease in pain rating: 19.6 +/- 3.9%, all P < 0.005, Holm-Sidak) and in healthy controls (30.1 +/- 5.0%, all P < 0.001, Holm-Sidak). During the LFS period, the pain ratings decreased consistently in both groups. In conclusion, a significant and stable pain inhibition (LTD) can be induced in CTTH patients by LFS.

Nerve growth factor and ATP excite different neck muscle nociceptors in anaesthetized mice

Neck muscle nociception probably plays a major role in the pathophysiology of tension-type headache. Recent studies have demonstrated sustained facilitation of brainstem nociception due to noxious neck muscle input evoked by nerve growth factor (NGF) or alpha,beta-methylene ATP (ATP) in mice. Hypothesized different afferent pathways in NGF and ATP models were addressed by local application of tetrodotoxin (TTX) in neck muscles. Brainstem nociception was monitored in 55 anaesthetized mice by the jaw-opening reflex elicited by electrical tongue stimulation. Sole administration of 100 nmol/l ATP or 0.8 micromol/l NGF evoked sustained reflex facilitation for at least 95 min. Preceding TTX administration prevented ATP-induced facilitation, but was without effect on NGF. Subsequent administration of 100 nmol/l TTX reversed ATP-evoked facilitation, but was ineffective on NGF. Divergent effects of TTX suggest preferential excitation of group III muscle afferents by ATP and group IV by NGF. Thus, both models address different pathways in pericranial pain.

ATP induces sustained facilitation of craniofacial nociception through P2X receptors on neck muscle nociceptors in mice

Noxious input from neck muscles probably plays a key role in tension-type headache pathophysiology. ATP selectively excites group III and IV muscle afferents in vitro. Accordingly, ATP infusion into trapezius muscle induces strong pain and local tenderness in healthy man. The present study addresses the impact of ATP on neck muscle nociception in anaesthetized mice. Craniofacial nociceptive processing was tested by the jaw-opening reflex via noxious electrical tongue stimulation. Within 2 h after injection of 100 nmol/l or 1 micromol/l ATP into semispinal neck muscles, reflex integrals significantly increased by 114% or 328%, respectively. Preceding intramuscular administration of the P2X receptor antagonist PPADS (3-100 nmol/l) suppressed the ATP effect. Subsequent application of PPADS (100 nmol/l) caused a total recovery of facilitated reflex to baseline values. ATP induces sustained facilitation of craniofacial nociception by prolonged excitation of P2X receptors in neck muscles.

Impaired thermal perception in cluster headache

Cluster headache is characterized by attacks of severe periorbital pain. Repetitive burst activity in afferent fibers may induce plastic alterations in somatosensory synaptic processing as a prerequisite for recurring and chronic pain. This psychophysical study addressed hypothesized dysfunctions in craniofacial somatosensory processing in cluster headache disease. Thermal and mechanical sensory functions in the periorbital region were assessed by quantitative sensory testing (QST) in 25 cluster headache patients and 60 healthy volunteers. Perception of warmth (p<0.01), cold (p<0.000001), and pressure pain (p<0.05) was reduced on the cluster side as compared with the contralateral asymptomatic side. In contrast to healthy volunteers, warm detection threshold (WDT) and thermal sensory limen (TSL) on one side did not positively correlate with the other side. WDT and TSL negatively correlated with the elapsed time since last attack. All patients showed QST abnormalities on the headache side in comparison to healthy controls. Loss of sensory functions strongly preponderated gain. Several lines of evidence indicate a pivotal role of the hypothalamus in cluster headache pathophysiology. The impairment of warm and cold perception in patients may be based upon a dysfunction of the hypothalamus which is strongly involved in thermosensory control.

Heterosynaptic long-term depression of craniofacial nociception: divergent effects on pain perception and blink reflex in man

Noxious low-frequency stimulation (LFS) of presynaptic nerve fibers induces long-term depression (LTD) of synaptic transmission. In vitro studies suggest a sole homosynaptic effect. Consequently, the present study addressed the hypothesis that LTD of craniofacial nociception in man is mediated by a homosynaptic mechanism. Nociceptive supraorbital afferents were excited by electric pulses via a concentric electrode in ten healthy volunteers. The electrically evoked bilateral blink reflex (BR) was recorded from both orbicularis oculi muscles by surface electrodes. The BR was evoked in blocks of ten electric stimuli each (0.1 Hz) with an interblock interval of 8 min. Conditioning noxious LFS (1 Hz, 20 min) was applied via concentric electrode either to the same site as BR test stimuli (ipsilateral) or to the corresponding contralateral forehead area (contralateral). LFS and test stimulus intensities corresponded to about threefold the pain threshold. After three baseline stimulus blocks, either conditioning ipsilateral or contralateral LFS were applied or stimulation was interrupted for 20 min as a control task. Afterwards, test stimulation blocks were continued for 40 min. Each volunteer participated in all three sessions on different days. Noxious LFS induced LTD of the BR independently from the side of conditioning stimulation. Pain perception decreased after ipsilateral LFS but not after contralateral LFS. The bilateral effect of noxious LFS on the BR provides evidence for heterosynaptic LTD based on bilateral projections of supraorbital nerve afferents onto spinal trigeminal nuclei. The divergent effect on pain perception may be due to a preferential contralateral projection of nociceptive afferents onto reflex interneurons but not onto trigeminothalamic projection neurons.

Unilateral application of an inflammatory irritant to the rat temporomandibular joint region produces bilateral modulation of the jaw-opening reflex

The aim of this study was to determine the effect of unilateral acute inflammation of craniofacial deep tissues on the ipsilateral and contralateral jaw-opening reflex (JOR). The effects of mustard oil (MO), injected into the temporomandibular joint region, were tested on the JOR recorded in the digastric muscle and evoked by low-intensity electrical stimulation of the ipsilateral and contralateral inferior alveolar nerve in anesthetized rats. The MO injection induced a long-lasting suppression of the amplitude of both ipsilaterally and contralaterally evoked JOR, although the latency and duration of the JOR were unaffected. The suppressive effect was more prominent for the contralaterally evoked JOR, and observed even when background activity in the digastric muscle was increased by the MO injection. The results indicate that changes in the JOR amplitude following MO injection do not simply reflect alterations in motoneuronal excitability, and suggest that inflammation of deep craniofacial tissues modulates low-threshold sensory transmission to the motoneurons.

Neurophysiological monitoring of alveolar nerve function during sensor-controlled Er:YAG laser corticotomy in rabbits

BACKGROUND AND OBJECTIVES

The sensor-controlled Er:YAG laser system may be a potent tool for tissue specific cutting in surgery. In order to investigate the impact of the laser on neural tissues, inferior alveolar nerve (IAN) function was monitored by the jaw-opening reflex (JOR) during specific corticotomy in rabbits.

STUDY DESIGN/MATERIALS AND METHODS

Laser jaw corticotomy was performed in 13 anesthetized rabbits. During and after specific corticotomy the JOR was evoked by electric intraoral stimulation to monitor effects on IAN function.

RESULTS

The JOR permanently abolished in one case and transiently failed immediately after surgery but largely recovered within days in another rabbit. In one experiment JOR threshold increased 7 days after corticotomy. Histology did not prove any objective nerve pathology.

CONCLUSIONS

Monitoring IAN function by the JOR demonstrated the relatively low risk of nerve damaging during sensor-controlled laser corticotomy.

Dopamine D2-like receptor activation antagonizes long-term depression of orofacial sensorimotor processing in anesthetized mice

Long-term depression (LTD) of orofacial sensorimotor processing recently has been demonstrated in anesthetized mice. Due to the remarkable role of dopamine in central nervous system LTD, the influence of dopamine D2 receptor activation on LTD of the jaw-opening reflex (JOR) was investigated. Electric low-frequency stimulation (LFS, 1 Hz) of the tongue suppressed the JOR integral by 43% for at least 1 h. After systemic administration of the dopamine D2-like receptor agonist quinpirole, LTD was significantly attenuated to 14%. JOR decreased for only about 15 min after LFS according to a short-term depression. Under systemic application of the dopamine D2-like receptor antagonist sulpiride, LTD significantly increased to 64%, again for at least 1 h. Thus, D2-like receptor activation prevented LTD, and D2-like receptor blockade amplified LTD of the reflex. The time course of inhibition may be due to a dopaminergic D2-like receptor mechanism that antagonizes the transfer from short-term into long-term depression. Considering a putative mediation of LTD by the endogenous pain control system, the results correspond to the known inhibitory control of this system by a D2-like receptor mechanism.