Morphine and NMDA receptor antagonism reduce c-fos expression in spinal trigeminal nucleus produced by acute injury to the TMJ region

Intensity coding by TMJ-responsive neurons in superficial laminae of caudal medullary dorsal horn of the rat

Temporomandibular disorders (TMD) represent a family of recurrent conditions that often cause pain in the temporomandibular joint (TMJ) region and muscles of mastication. To determine if TMJ-responsive neurons encoded the intensity of pro-inflammatory chemical signals, dose-effect relationships were assessed after direct injection bradykinin into the joint space and compared with responses after injection of glutamate or saline. Neurons were recorded from superficial laminae of the trigeminal subnucleus caudalis/upper cervical cord junction region (Vc/C(2)) and identified by palpation of the TMJ region in barbiturate-anesthetized male rats. The majority (62 of 84) of units received convergent input from facial skin, while 26% were driven only by deep input from the TMJ region. Conduction-velocity based on the latency to firing after electrical stimulation of the TMJ region indicated 64% of units were driven by A-delta fiber input only. Bradykinin (0.1-10 microM) excited 69% of neurons tested, and 70% (19 of 27) of these units were activated by the lowest dose (0.1 microM). Glutamate (50-200 mM) excited 27% of units; however, when tested after bradykinin, 58% of units were activated by glutamate. Some TMJ units (17%) were excited by saline injection alone and not enhanced further by bradykinin or glutamate. Most (88%) TMJ units were activated by injection of the small fiber excitant, mustard oil (20% solution), into the TMJ region. Units responsive to bradykinin or glutamate were not restricted to particular classes [e.g., wide dynamic range (WDR), nociceptive specific (NS), deep only]. A small percentage of TMJ units (approximately 15%) were activated antidromically from the contralateral posterior thalamus. In parallel studies using c-fos immunocytochemistry, bradykinin (1 microM) injection into the TMJ region produced a greater number of Fos-positive neurons at the Vc/C(2) region than glutamate (200 mM) or saline. These results revealed two broad classes of TMJ units that encoded the intensity of pro-inflammatory chemical stimuli applied to the TMJ region, units that received convergent nociceptive input from facial skin (i.e., WDR and NS units) and units that responded only to deep input from the TMJ region. On the basis of encoding properties and efferent projection status, it is concluded that activation of TMJ units within the superficial laminae at the Vc/C(2) region contribute to the diffuse and spreading nature of TMD pain sensation.

Sex differences in brainstem neural activation after injury to the TMJ region

The basis for a higher prevalence of painful temporomandibular disorders (TMD) among women than men is not known. The present study used Fos-like immunoreactivity (Fos-LI) to quantify the pattern and magnitude of neural activation within the trigeminal brainstem complex of male and female rats caused by acute inflammatory injury to the temporomandibular joint (TMJ) region. Also, Fos-LI was assessed in animals given morphine, a preferential mu opioid receptor agonist, or U50,488H, a selective kappa opioid agonist, prior to TMJ injury to determine if opioid modulation of neural activation was similar in males and females. The general pattern of Fos-LI after TMJ injury was similar in males and females. This pattern was characterized by a high density of Fos-positive neurons in the dorsal paratrigeminal nucleus (dPa5), subnucleus interpolaris/caudalis transition region (Vi/Vc-vl), and in the superficial laminae at the subnucleus caudalis/upper cervical spinal cord (Vc/C2) junction ipsilateral to TMJ injury. In contrast to other regions the number of Fos-positive neurons produced at the Vc/C2 junction was proportional to the concentration of mustard oil injected into the TMJ region. In addition, proestrus females produced higher levels of Fos-LI at the Vc/C2 junction than diestrus females or males. Morphine caused a greater dose-related reduction in Fos-LI at the dPa5 and Vc/C2 junction in males than females. By contrast, U50,488H caused a dose-related reduction in Fos-LI only at the Vc/C2 junction of proestrus females. These results support the hypothesis that the Vc/C2 junction region plays a critical role in the integration of pain signals originating from the TMJ region and may underlie sex differences in sensory processing related to TMJ pain.

An in vitro temporomandibular joint-nerve preparation for pain study in rats

A novel in vitro TMJ-nerve preparation was developed to quantitatively study peripheral sensory mechanisms of temporomandibular joint (TMJ). The TMJ region on one side (including mandibular head, disc, retrodiscal tissue and mandibular fossa) of adult Wistar albino rats was excised together with the auriculo-temporal nerve. The block was preserved in a modified Krebs-Henseleit solution saturated with O(2)/CO(2) (95/5%) gas mixture. Using a calibrated von Frey type apparatus, mechanical noxious stimulation was applied directly to various sites within the TMJ region. In addition, thermal and chemical noxious stimuli were also attempted. Stable recordings of single unit activities from the auriculo-temporal nerve could be obtained for as long as 5 h, which was sufficient to analyze the response properties of the TMJ units to various stimuli. This new preparation would be useful for investigating TMJ peripheral sensory mechanisms, especially pain, and potentially makes it possible to reveal neural mechanisms of temporomandibular arthralgia, a syndrome that has recently shown an increased incidence in clinical dentistry.

Temporomandibular-evoked jaw muscle reflex: role of brain stem NMDA and non-NMDA receptors

This study investigated the possible involvement of brain stem excitatory amino acid receptor mechanisms and the trigeminal subnucleus caudalis (Vc) in temporomandibular joint (TMJ)-evoked reflex jaw muscle activity. Glutamate injected into the TMJ of anesthetized rats reflexly evoked activity in the jaw muscles. Application of lidocaine, but not saline, to the surface of the caudal brainstem overlying Vc significantly suppressed TMJ-evoked jaw muscle activity, while application of NMDA or non-NMDA receptor antagonists also significantly attenuated jaw muscle activity. These results provide evidence that Vc is a critical relay in the TMJ-evoked reflex activation of the jaw muscles, and that both NMDA and non-NMDA receptor mechanisms may contribute to these effects.

Effects of previous noxious stimulus applied to remote areas on noxious stimulus-evoked c-Fos expression in the rat trigeminal nucleus caudalis

Noxious stimulus-evoked c-Fos expression in the spinal dorsal horn is modulated by noxious stimuli applied previously to remote areas of the body. To confirm the existence of such modulation in c-Fos expression in the trigeminal system, changes in c-Fos expression in the trigeminal nucleus caudalis induced by formalin injection into the rat whisker pad were examined by previously injecting formalin into different areas (contralateral whisker pad, ipsilateral or contralateral forepaw) of the body. Formalin injection-evoked c-Fos expression in this nucleus was significantly reduced by previous formalin injection into the contralateral whisker pad or ipsilateral forepaw but not into the contralateral forepaw. The interval between the two injections of formalin that produced a maximal reduction of formalin injection-evoked c-Fos expression was 1 h, and the reduction of c-Fos expression was less when the interval of the two noxious stimuli was longer or shorter than 1 h. These results suggested that noxious stimulus-evoked c-Fos expression in the trigeminal nucleus caudalis is reduced by noxious stimulus applied previously to remote areas, and the reduction is dependent on the area of previous noxious stimulation and interval between the two noxious stimuli.