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

Abstract

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.

Fig. 1.

Comparison of typical responses evoked in the jaw muscles as a result of local application of MO and EAA receptor agonists to the TMJ region. Data points indicate the mean EMG (n = 6) relative to preinjection baseline EMG activity. Error bars indicate SE. Solid arrows below each curve indicate the time of application. Application of MO to the TMJ region resulted in a characteristic increase in the electromyographic activity of jaw muscles both ipsilateral and contralateral to the injection site (Yu et al., 1995, 1996). Application of the EAAs to the TMJ region evoked activity in the ipsilateral jaw muscles that was similar to that evoked by MO. The above data suggest that activation of either NMDA or non-NMDA EAA receptors within the TMJ region may provide sufficient stimulation to evoke reflex jaw muscle activity. Ipsi. Dig., Ipsilateral digastric muscle; Ipsi. Mass., ipsilateral masseter muscle; Cont. Mass., contralateral masseter Muscle; Cont. Dig., contralateral digastric muscle.

Fig. 2.

Dose–response relationship for evoked activity in the digastric muscles by application of EAA receptor agonists to the TMJ region. Each data point on the dose–response curves represents a mean value (n = 6). Error bars indicate SE. Note that responses of magnitude similar to those evoked by MO in the ipsilateral digastric were achieved with 0.5 μmol doses of NMDA, AMPA, and kainate and 2.5 μmol doses of glutamate, respectively.

Fig. 3.

Dose–response relationship for evoked activity in the masseter muscles by application of EAA receptor agonists to the TMJ region. Each data point on the dose–response curves represents a mean value (n = 6). Error bars indicate SE. Note that responses of magnitude similar to those evoked by MO in the ipsilateral and contralateral masseter were only achieved for glutamate at a dose of 5.0 μmol, the highest dose applied for any of the EAA receptor agonists.

Fig. 4.

Digastric muscle activity evoked by repeated applications of EAA receptor agonists to the TMJ region.A, Line plots illustrate the activity evoked in the ipsilateral digastric muscle by three applications (solid arrow) each of glutamate (2.5 μmol), NMDA, AMPA, and kainate (0.5 μmol) in four individual experiments. Relative EMG activity was calculated by normalizing each EMG area bin to the mean of baseline EMG activity before the first injection. B, Bar graphs indicate the mean ± SE (n = 6) for each of the three trials. Note that for glutamate, NMDA, and AMPA, repeated application evoked activity of equal magnitude when compared with the first response (p > 0.05, repeated measures ANOVA). However, the magnitude of digastric muscle responses to repeated applications of kainate progressively decreased (p < 0.05, repeated measures ANOVA), suggesting that a process of desensitization may be occurring.

Fig. 5.

Effect of APV on the activity evoked in the ipsilateral digastric muscle by application of EAA receptor agonists to the TMJ region. Line plots illustrate the activity evoked in the ipsilateral digastric muscle by two applications (solid arrow) each of glutamate (2.5 μmol), NMDA (0.5 μmol), or AMPA (0.5 μmol) in three individual experiments. The first application contained only the indicated EAA receptor agonist, whereas in the second application both the EAA and APV (0.5 μmol;solid line) were applied together to the TMJ. Note that co-application of APV reduced the magnitude for glutamate- and NMDA-evoked digastric muscle responses without affecting AMPA-evoked digastric muscle responses, indicating that this dose of APV was selective for NMDA receptors.

Fig. 6.

Effect of CNQX (1 nmol; solid line) on the activity evoked in the ipsilateral digastric muscle by application of EAA receptor agonists to the TMJ region. Line plots illustrate the activity evoked in the ipsilateral digastric muscle as described in Figure 5. Note that co-application of CNQX reduced the magnitude for glutamate- and AMPA-evoked digastric muscle responses without affecting NMDA-evoked digastric muscle responses, indicating that this dose of CNQX was selective for non-NMDA receptors.

Fig. 7.

Effect of application of lidocaine to the subnucleus caudalis on the activity evoked in the ipsilateral jaw muscles by application of glutamate to the TMJ region. Line plots inA illustrate the activity evoked in the ipsilateral digastric and masseter muscles by three applications (solid arrow) of glutamate (2.5 μmol) to the TMJ region from two individual experiments. The second application of glutamate to the TMJ region was preceded by application of either normal saline or lidocaine over the subnucleus caudalis (solid line). Bar graphs indicate the mean ± SE (n = 5) for each application. Note that application of lidocaine, but not normal saline, to the brainstem reversibly blocked glutamate-evoked activity in the ipsilateral digastric and masseter muscles (asterisk, p < 0.05, Student’st test). This result indicates that application of glutamate to the TMJ region evokes activity in the jaw muscles by a reflex pathway through subnucleus caudalis.