Microinfusions of excitatory amino acid antagonists into the trigeminal sensory complex antagonize the jaw opening reflex in freely moving rats

Topiramate Inhibits Trigeminovascular Neurons in the Cat

To facilitate understanding the action of antimigraine preventives the effect of topiramate on trigeminocervical activation in the cat was examined. Animals ( n = 7) were anaesthetized and physiologically monitored. The superior sagittal sinus (SSS) was stimulated to produce a model of trigeminovascular nociceptive activation. Cumulative dose-response curves were constructed for the effect of topiramate at doses of 3, 5, 10, 30 and 50 mg/kg on SSS-evoked firing of trigeminocervical neurons. Topiramate reduced SSS evoked firing in a dose-dependent fashion. The maximum effect was seen over 30 min for the cohort taken together. At 3 mg/kg firing was reduced by 36 ± 13% (mean ± SEM) after 15 min. At 5 and 50 mg/kg firing was reduced by 59 ± 6% and 65 ± 14%, respectively, after 30 min. Inhibition of the trigeminocervical complex directly, or neurons that modulate sensory input, are plausible mechanisms for the action of preventives in migraine.

Meal duration as a measure of orofacial nociceptive responses in rodents

A lengthening in meal duration can be used to measure an increase in orofacial mechanical hyperalgesia having similarities to the guarding behavior of humans with orofacial pain. To measure meal duration unrestrained rats are continuously kept in sound attenuated, computerized feeding modules for days to weeks to record feeding behavior. These sound-attenuated chambers are equipped with chow pellet dispensers. The dispenser has a pellet trough with a photobeam placed at the bottom of the trough and when a rodent removes a pellet from the feeder trough this beam is no longer blocked, signaling the computer to drop another pellet. The computer records the date and time when the pellets were taken from the trough and from this data the experimenter can calculate the meal parameters. When calculating meal parameters a meal was defined based on previous work and was set at 10 min (in other words when the animal does not eat for 10 min that would be the end of the animal's meal) also the minimum meal size was set at 3 pellets. The meal duration, meal number, food intake, meal size and inter-meal interval can then be calculated by the software for any time period that the operator desires. Of the feeding parameters that can be calculated meal duration has been shown to be a continuous noninvasive biological marker of orofacial nociception in male rats and mice and female rats. Meal duration measurements are quantitative, require no training or animal manipulation, require cortical participation, and do not compete with other experimentally induced behaviors. These factors distinguish this assay from other operant or reflex methods for recording orofacial nociception.

A non-invasive model for measuring nociception after tooth pulp exposure

Temporomandibular arthritis will lengthen a rodent's meal duration. We hypothesized that meal duration would also lengthen after tooth pulp exposure, suggesting that this behavior could be used to measure tooth nociception. To test this hypothesis, we placed rats in feeding units and subjected 4 anterior mandibular molars to pulp exposure, with and without pre-treatment with the analgesic buprenorphine-HCl. In the first study, male Sprague-Dawley rats were placed in computerized sound-attenuated feeding modules, the pulp of 4 molars on the mandible were exposed, and meal duration was measured for 13 days. In a second study, rats were injected with either the analgesic buprenorphine-HCl or saline every 12 hrs; injections were started one day before pulp exposure. Meal duration was determined before and after treatment. In the first study, pulp exposure significantly increased daily meal duration for 8 days. In the second study, pulp exposure lengthened daily meal duration, but the group that was treated with buprenorphine-HCl showed no significant difference compared with control rats without pulp exposure. Evidence supports that a lengthening in meal duration is a response to tooth nociception and that this nociception can be measured for over a week.

Modulation of respiratory pattern and upper airway muscle activity by the pedunculopontine tegmentum: role of NMDA receptors

The pedunculopontine tegmental nucleus (PPT) is postulated to have important functions relevant to the regulation of rapid eye movement (REM) sleep and arousal, and various motor control systems including respiration. We have recently shown that pharmacologic activation of a neuronal subpopulation within the PPT, induced by micropipette injection of glutamate in nanoliter volumes, can produce respiratory rhythm disturbances and changes in genioglossus muscle activity in anesthetized rats. The aim of this study was to determine whether the respiratory pattern disturbance and increased genioglossus muscle tone induced by glutamate injection within the PPT are mediated by activation of N-methyl-D-aspartate (NMDA) receptors within the PPT. Experiments were performed in eight adult male spontaneously breathing Sprague-Dawley rats anesthetized using nembutal. Respiratory movements were monitored by piezoelectric strain gauge. Three-barrel glass pipettes were used to pressure inject glutamate (as a probe for respiratory modulating sites), ketamine (an NMDA channel blocker), and oil-red dye (to aid in histological verification of the injection sites) within the PPT. Electroencephalograms were recorded from the sensorimotor cortex, the hippocampus, and the pons, contralateral to the injection site. Electromyograms (EMGs) were recorded from the genioglossus muscle. The typical response to glutamate injection within the PPT respiratory-modulating region was immediate apnea followed by tachypnea and increased genioglossal tonic activity. The noncompetitive NMDA receptor channel-antagonist ketamine, injected at the same site and in the same volume as glutamate (5 nl), blocked respiratory dysrhythmia and genioglossal EMG responses to subsequent glutamate injections. For the first time, the present results suggest that respiratory rhythm and upper airway muscle tone are controlled by the activation of pedunculopontine tegmental nucleus NMDA receptors.

Peripheral NMDA receptor modulation of jaw muscle electromyographic activity induced by capsaicin injection into the temporomandibular joint of rats

We have previously documented that peripheral N-methyl-d-aspartate (NMDA) receptor mechanisms are involved in nociceptive reflex increases in jaw muscle activity to injection of mustard oil or glutamate into the rat temporomandibular joint (TMJ). The aim of the present study was to determine whether peripheral NMDA receptor mechanisms are also involved in the nociceptive reflex responses in the jaw muscles evoked by injection of the inflammatory irritant and algesic chemical capsaicin into the TMJ. The effects of peripheral injection of NMDA receptor antagonists, MK-801 and APV, on the increases in electromyographic (EMG) activities of digastric and masseter muscles reflexly evoked by capsaicin injection into the TMJ were tested in halothane-anesthetized male rats. The capsaicin injection following pre-injection of vehicle evoked significant increases in EMG activity in both digastric and masseter muscles whereas pre-injection of MK-801 or APV into the TMJ resulted in a significant concentration-related reduction in the magnitude of capsaicin-evoked digastric and masseter EMG activity (ANOVA-on-ranks, P < 0.05). This finding indicates that capsaicin-evoked digastric and masseter EMG activity can be attenuated by pre-injection into the TMJ of NMDA receptor antagonists, and that the activation of peripheral NMDA receptors may be important in the mechanisms whereby capsaicin evokes nociceptive trigeminal responses.

Hyperekplexia and stiff-man syndrome: abnormal brainstem reflexes suggest a physiological relationship

BACKGROUND AND OBJECTIVES

Hyperekplexia and the stiff-man syndrome (SMS) are both conditions with exaggerated startle suggesting abnormal brainstem function. Investigation of brainstem reflexes may provide insight into disturbed reflex excitation and inhibition underlying these movement disorders.

PATIENTS AND METHODS

Using four-channel EMG, we examined four trigeminal brainstem reflexes (monosynaptic masseter, masseter inhibitory, glabella, and orbicularis oculi blink reflexes) and their spread into pericranial muscles in five patients with familial hyperekplexia (FH), two with acquired hyperekplexia (AH), 10 with SMS, and 15 healthy control subjects.

RESULTS

Both FH/AH and SMS patients had abnormal propagation of brainstem reflexes into pericranial muscles. All patients with hyperekplexia showed an abnormal short-latency (15-20 ms) reflex in the trapezius muscle with a characteristic clinical appearance ("head retraction jerk") evoked by tactile or electrical stimulation of the trigeminal nerve, but normal monosynaptic masseter reflexes. Inhibitory brainstem reflexes were attenuated in some FH/AH patients. Four of 10 patients with SMS had similar short-latency reflexes in the neck muscles and frequently showed widespread enhancement of other excitatory reflexes, reflex spasms, and attenuation of inhibitory brainstem reflexes.

CONCLUSION

Reflex excitation is exaggerated and inhibition is attenuated in both stiff-man syndrome and familial or acquired hyperekplexia, indicating a physiological relationship. Reflex transmission in the brainstem appears biased towards excitation which may imply dysfunction of inhibitory glycinergic or GABAergic interneurons, or both.

Temporal and regional expression of NMDA receptor subunit NR3A in the mammalian brain

NR3A is a developmentally regulated N-methyl-D-aspartate receptor (NMDAR) subunit that was previously known as NMDAR-L or chi-1. Unlike other NMDAR subunits, NR3A inhibits the NMDAR-associated ion channel in a novel manner, and a role in synaptogenesis has been suggested for this subunit. Here, we report a comprehensive study to delineate the temporal and anatomic expression of NR3A protein in the mammalian brain by using a monoclonal anti-NR3A antibody. NR3A protein was found to peak at postnatal day (P) 8, and to decrease gradually from P12 to adulthood in the rat central nervous system. Moreover, NR3A protein was heavily expressed in all areas of the isocortex, portions of the amygdaloid nuclei, and selective cell layers and nuclei of the hippocampus, thalamus, hypothalamus, brainstem, and spinal cord. NR3A protein was also expressed in the cerebellar cortex, whereas only weak signal was detected in the previous in situ studies by using riboprobes. At an ultrastructural level, NR3A was associated specifically with asymmetrical synapses and localized to postsynaptic membranes. This information will facilitate future research on NMDARs by providing clues to possible inclusion of the NR3A subunit in NMDARs in many brain regions.

Use of the jaw opening reflex for assessing the effects of local anaesthetics in freely moving rats

INTRODUCTION

In order to characterize a nonbehavioral model for assessing local anaesthetic (LA) activity, the effects of different LA agents (articaine, bupivacaine, procaine, and tetracaine) were measured in the conscious rat using the jaw-opening reflex (JOR).

METHODS

One hundred sixty rats were chronically implanted with stimulating electrodes in the dental pulp of the low incisor. While animals were conscious and unrestrained, the JOR threshold was measured electrophysiologically via electrodes wrapped around the digastric muscle. Each LA was administered in the infratemporal area. The increase of the JOR threshold was assessed during a 3-h period following injection.

RESULTS

Statistical analysis of the data showed a dose-dependent response to the four drugs tested. When the highest dose of each drug (articaine and procaine: 24 mg kg(-1), bupivacaine: 6 mg kg(-1), tetracaine: 3 mg kg(-1)) was administered (i) an immediate effect was observed for tetracaine and bupivacaine, whereas a 5-min delay was needed for articaine and procaine to act on the JOR threshold and (ii) an increase (>60%) of the JOR threshold was observed. The effects lasted 90 min for articaine, 45 min for procaine and bupivacaine, and 15 min for tetracaine before a return to baseline values.

DISCUSSION

The rat JOR response combined with infratemporal injection of test drugs can be used for the pharmacological evaluation of LAs.

Qualitative light and electron microscope study of glutamate receptors in the caudal spinal trigeminal nucleus of the rat

Though ionotropic and metabotropic glutamate receptors have recently been recognized to play important roles in the transmission of orofacial nociceptive impulses, their detailed distribution in the spinal trigeminal nucleus has not been systematically investigated. There is also controversy regarding the electron microscope localization of metabotropic receptors. We therefore undertook this investigation to address the above-mentioned issues in the caudal part of the spinal trigeminal nucleus, using light and electron microscope immunocytochemistry, to provide baseline information for the development of agonists and antagonists of these receptors in the clinical treatment of orofacial pain. The results showed some moderately to strongly stained glutamate receptor 1 neurons, and many strongly stained glutamate receptor 2/3 neurons in lamina II of the nucleus, suggesting that the latter may play an important role in orofacial pain processing, with the former playing a minor role. The metabotropic glutamate receptor 1 immunoreactive product was localized mostly in dendrites, while most of the metabotropic glutamate receptors 2/3 immunoreactive product was deposited in axon terminals containing synaptic vesicles of different shapes, suggesting that glutamate receptors 2/3 may control the release of both excitatory and inhibitory neurotransmitters.

Glutamatergic transmission in the trigeminal nucleus assessed with local blood flow

Stimulation of the superior sagittal sinus in humans is pain-producing and in experimental animals leads to excitation of neurons in the caudal trigeminal nucleus and dorsal horns of the C(1/)C(2) cervical spinal cord: the trigeminocervical complex. Neuronal excitation is generally associated with an increase in local blood flow due to flow/metabolism coupling and we have used local blood flow in the trigeminocervical complex to examine the role of N-methyl-D-aspartate (NMDA)-mediated transmission in these neurons. Cats were anaesthetised with alpha-chloralose (60 mg/kg, ip; supplements 20 mg/kg iv) after surgical preparation under halothane (0.5-3%). Animals were paralysed with gallamine triethiodide to prevent possible movement artefact distorting the laser Doppler signals. The superior sagittal sinus was isolated for electrical stimulation (150 V; 250 microsec duration; 0.5, 1, 2, 5, 10 and 20 Hz) and the dorsal surface of the spinal cord exposed at the C(2) level. Blood flow was recorded from the region over the trigeminocervical complex by careful placement of a laser Doppler flow probe. Flow was recorded continuously by an online collection programme and NMDA-mediated transmission modulated by intravenous administration of MK-801 (0.4, 1 and 4 mg/kg, iv) at the stimulation frequency of 5 Hz. Stimulation of the superior sagittal sinus produced a stimulus-locked, frequency-dependent increase in blood flow in the region of the trigeminocervical complex. The mean maximum response was 39+/-4% at 20 Hz. MK-801 had no effect on the resting flow signal but markedly attenuated the SSS-evoked response in a dose-dependent manner. The mean maximum response after 4 mg/kg MK-801 was 13+/-2%. NMDA-mediated transmission is likely to be involved in nociceptive trigeminovascular transmission within the trigeminocervical complex and offers a possible target for both acute and preventative treatment of migraine.

Trigeminovascular nociceptive transmission involves N-methyl-D-aspartate and non-N-methyl-D-aspartate glutamate receptors

Interest in the fundamental mechanisms underlying headache, particularly the pathophysiology of migraine and cluster headache, has lead to the study of the physiology and pharmacology of the trigeminovascular system and its central ramifications. Cats were anaesthetized (60 mg/kg alpha-chloralose, i.p., along with halothane for all surgical procedures) and prepared for physiological monitoring. The animals were placed in a stereotaxic frame and ventilated. A midline craniotomy and C2 laminectomy were performed for access to the superior sagittal sinus and C2 dorsal horn, respectively. The sinus was isolated from the underlying cortex and stimulated electrically after the animals had been paralysed with gallamine (6 mg/kg, i.v.). Units linked to stimulation were recorded with a tungsten-in-glass microelectrode placed in the most caudal part of the trigeminal nucleus, the trigeminocervical complex. Signals from the neurons were amplified, filtered and passed to a microcomputer, where post-stimulus histograms were constructed on-line to analyse the responses to stimulation. Units responded to sagittal sinus stimulation with a typical latency of 8-10 ms. All units studied had a probability of firing of 0.6 or greater. Intravenous injection of the non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (4 mg/kg, i.v.), resulted in a substantial and prolonged blockade of firing of units in the trigeminocervical complex. Similarly, administration of the non-N-methyl-D-aspartate excitatory amino acid receptor blocker, GYKI 52466, lead to a dose-dependent inhibition of trigeminovascular-evoked responses in the trigeminocervical complex. These data demonstrate the participation of both N-methyl-D-aspartate- and non-N-methyl-D-aspartate-mediated mechanisms in transmission within the trigeminocervical complex, and suggest a clear preclinical role of glutamatergic mechanisms in primary headache syndromes, such as migraine and cluster headache.

Evidence that excitatory amino acid receptors within the temporomandibular joint region are involved in the reflex activation of the jaw muscles

We have previously shown that injection of the inflammatory irritant and small-fiber excitant mustard oil (MO) into the temporomandibular joint (TMJ) region can reflexively induce a prolonged increase in the activity of both digastric and masseter muscles in rats. It is possible that peripheral excitatory amino acid (EAA) receptors play a role in this effect, because MO-evoked increases in jaw muscle activity are attenuated by preapplication of the noncompetitive NMDA receptor antagonist MK-801 into the TMJ region. In the present study the EAA receptor agonists glutamate, NMDA, kainate, and AMPA were applied locally to the TMJ region. Jaw muscle responses similar to those evoked by MO application to the TMJ region were achieved with glutamate, NMDA, AMPA, and kainate. Repeated application of glutamate, NMDA, or AMPA at intervals of 30 min evoked responses in the ipsilateral jaw muscles that were of comparable magnitude. Co-application of the NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (0.5 micromol) significantly reduced the magnitude of the glutamate- and NMDA-evoked ipsilateral jaw muscle responses without affecting responses evoked by AMPA. In contrast, co-application of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (1 nmol) significantly reduced the magnitude of the glutamate- and AMPA-evoked ipsilateral jaw muscle responses without affecting responses evoked by NMDA. This evidence suggests that both NMDA and non-NMDA EAA receptor types are located within the TMJ region and may contribute to jaw muscle activity that can be reflexively evoked from the TMJ region.

Effects of RPR 100893, a potent NK1 antagonist, on the jaw-opening reflex in the guinea pig

RPR 100893 appears as a new potent NK1 selective non-peptide antagonist both in vitro and in vivo, and exhibits high affinity for guinea pig and human NK1 receptor [M. Tabart, J.-F. Peyronel, Synthesis of RPR 100893, prototype of a new series of potent and selective non-peptide NK1 antagonists: the triarylperhydroisoindolols, Bioorg. Med. Chem. Lett., 4 (1994) 673-676.]. Intra-oral administration of RPR 100893 (3, 15, 10, 30 mg/kg) was performed in freely moving guinea pigs during recording of the short- (6-10 ms) and long-latency (18-26 ms) jaw-opening reflex (JOR) elicited by electrical stimulation (0.5 Hz) of the lower incisor tooth pulp. RPR 100893 induced a noticeable and dose-dependent increase of the long-latency reflex thresholds (P<0. 001) but was ineffective on the short-latency responses (P=0.14). The results suggest that, in guinea pigs, the long-latency JOR requires activation of NK1 receptors, while the earlier reflex component, elicited by activation of periodontal afferents, does not. These NK1 receptors could be located either on JOR interneurons activated by tooth pulp afferents or on digastric motoneurons, receiving the tooth pulp input through a polysynaptic pathway.

Involvement of NMDA receptor mechanisms in jaw electromyographic activity and plasma extravasation induced by inflammatory irritant application to temporomandibular joint region of rats

The aim of this study was to examine the possible role of N-methyl-D-aspartate (NMDA) receptor mechanisms in responses induced by the small-fibre excitant and inflammatory irritant mustard oil injected into the temporomandibular joint (TMJ) region of rats. The effects of the non-competitive NMDA antagonist MK-801 were tested on the mustard oil-evoked increases in electromyographic (EMG) activity of the masseter and digastric muscles and Evans Blue plasma extravasation. Five minutes before the mustard oil injection, MK-801 or its vehicle was administered systemically (i.v.), into the third ventricle (i.c.v.), or locally into the TMJ region. Compared with control animals receiving vehicle, the rats receiving MK-801 at an i.v. dose of 0.5 mg/kg (n = 5) showed a significant reduction in the incidence and magnitude of EMG responses as well as in the plasma extravasation evoked by mustard oil; MK-801 at an i.v. dose of 0.1 mg/kg (n = 5) had no significant effect on plasma extravasation or on the incidence and magnitude of EMG responses but did significantly increase the latency of EMG responses. An i.c.v. dose of 0.1 mg/kg (n = 5) or 0.01 mg/kg (n = 5) had no significant effect on plasma extravasation or incidence of EMG responses but did significantly reduce the magnitudes of the masseter EMG response; the 0.01 mg/kg dose also significantly increased the latency of the digastric EMG response. The magnitudes of both the masseter and digastric EMG responses were also significantly reduced by MK-801 administered into the TMJ region at a dose of 0.1 mg/kg (n = 5) but not by 0.01 mg/kg (n = 5); neither dose significantly affected the incidence of EMG responses or the plasma extravasation. These data suggest that both central and peripheral NMDA receptor mechanisms may play an important role in EMG responses evoked by the small-fibre excitant and inflammatory irritant mustard oil, but that different neurochemical mechanisms may be involved in the plasma extravasation induced by mustard oil.

N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor antagonism reduces Fos-like immunoreactivity in central trigeminal neurons after corneal stimulation in the rat

The role of glutamate receptors in processing noxious sensory input from the cornea was assessed in barbiturate-anesthetized rats. Animals were treated with selective antagonists for N-methyl-D-aspartate or non-N-methyl-D-aspartate receptor subtypes prior to application of mustard oil to the corneal surface. Neural activation was estimated from the number of neurons that produced Fos, the protein product of the immediate early gene, c-fos, as detected by immunocytochemistry. Fos-positive neurons were found at two distinct regions of the spinal trigeminal nucleus: the subnucleus interpolaris/caudalis transition and the subnucleus caudalis/upper cervical cord transition. The number of Fos-positive neurons was reduced dose-dependently by the competitive N-methyl-D-aspartate receptor antagonist, 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (0.08-8 nmol, i.c.v.), or by the non-N-methyl-D-aspartate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (2.5-250 nmol, i.c.v.). The greatest reduction in Fos-positive cells was seen at the subnucleus caudalis/upper cervical cord transition after blockade of either receptor subtype. Combined blockade of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors did not cause a further reduction in the number of Fos-positive neurons than was seen after the highest dose of either antagonist alone. Peripheral or central administration of the nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester, had no effect on the number of Fos-positive neurons after corneal stimulation. These results suggest that corneal input to neurons at the subnucleus caudalis/upper cervical cord transition, and to a lesser extent, at the subnucleus interpolaris/subnucleus caudalis transition depends on excitatory amino acid transmission. Both N-methyl-D-aspartate and non-N-methyl-D-aspartate glutamate receptor subtypes, but not the formation of nitric oxide, contribute to the processing of acute corneal stimuli by central trigeminal neurons.

The NMDA receptor antagonist MK-801 reduces Fos-like immunoreactivity in central trigeminal neurons and blocks select endocrine and autonomic responses to corneal stimulation in the rat

The N-methyl-D-aspartate (NMDA) receptor is implicated in multiple aspects of pain processing by the central nervous system. However, the role of NMDA receptors in the endocrine and autonomic aspects of nociception remains uncertain. The present study examined the influence of the NMDA receptor antagonist, MK-801 (0.02-2.0 mg/kg, intracarotid), on the adrenal and autonomic responses to corneal stimulation (mustard oil, 20% sol.) in barbiturate-anesthetized rats. Fos-like immunoreactivity (Fos-LI) evoked by corneal stimulation was quantified within the spinal trigeminal nucleus (Vsp) of MK-801 pretreated animals to assess activation of central trigeminal neurons. Corneal stimulation-evoked increases in the plasma concentrations of adrenocorticotropin (ACTH), epinephrine and norepinephrine were reduced dose-dependently by MK-801. Plasma ACTH also increased after moderate hemorrhage, a response that was not affected by MK-801. MK-801 did not reduce the magnitude of corneal stimulation-evoked increases in arterial pressure and heart rate; however, prestimulus arterial pressure was reduced by drug treatment. Fos-LI was distributed bimodally within the ipsilateral caudal Vsp: one peak of Fos-LI in the subnucleus interpolaris/caudalis transition region and a second peak within the superficial laminae of the subnucleus caudalis/upper cervical cord transition region. The magnitude of both peaks of Fos-LI was reduced dose-dependently by MK-801. These results indicate a significant contribution from NMDA receptors in control of select endocrine and autonomic responses that accompany trigeminal nociception and in activation of central trigeminal neurons that process corneal nociceptive input.