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 condition s that often cause pain in the temporomandibular joint (TMJ) region and mus cles of mastication. To determine if TMJ-responsive neurons encoded the int ensity of pro-inflammatory chemical signals, dose-effect relationships were assessed after direct injection bradykinin into the joint space and compar ed with responses after injection of glutamate or saline. Neurons were reco rded from superficial laminae of the trigeminal subnucleus caudalis/upper c ervical cord junction region (Vc/C-2) and identified by palpation of the TM J region in barbiturate-anesthetized male rats. The majority (62 of 84) of units received convergent input from facial skin, while 26% were driven onl y by deep input from the TMJ region. Conduction-velocity based on the laten cy to firing after electrical stimulation of the TMJ region indicated 64% o f units were driven by A-delta fiber input only. Bradykinin (0.1-10 muM) ex cited 69% of neurons tested, and 70% (19 of 27) of these units were activat ed by the lowest dose (0.1 muM). Glutamate (50-200 mM) excited 27% of units ; however, when tested after bradykinin, 58% of units were activated by glu tamate. Some TMJ units (17%) were excited by saline injection alone and not enhanced further by bradykinin or glutamate. Most (88%) TMJ units were act ivated 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), nocicept ive specific (NS), deep only]. A small percentage of TMJ units (similar to 15%) were activated antidromically from the contralateral posterior thalamu s. In parallel studies using c-fos immunocytochemistry, bradykinin (1 muM) injection into the TMJ region produced a greater number of Fos-positive neu rons 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-i nflammatory 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 regio n contribute to the diffuse and spreading nature of TMD pain sensation.