Orofacial deep and cutaneous tissue inflammation and trigeminal neuronal activation - Implications for persistent temporomandibular pain

A rat model has been developed to characterize the responses of brainstem t rigeminal neurons to orofacial deep and cutaneous tissue inflammation and h yperalgesia. Complete Freund's adjuvant (CFA) was injected unilaterally int o the rat temporomandibular joint (TMJ) or perioral (PO) skin to produce in flammation in deep or cutaneous tissues, respectively. The TMJ and PO infla mmation resulted in orofacial behavioral hyperalgesia and allodynia that pe aked within 4-24 h and persisted for at least 2 weeks. Compared to cutaneou s CFA injection, the injection of CFA into the TMJ produced a significantly stronger inflammation associated with a selective upregulation of preprody norphin mRNA in the trigeminal spinal complex, an enhanced medullary dorsal horn hyperexcitability, and a greater trigeminal Fos protein expression, a marker of neuronal activation. The Fos-Li induced by TMJ inflammation pers isted longer, was more intense, particularly in the superficial laminae, an d more widespread rostrocaudally. Thus, the inflammatory irritant produces a stronger effect in deep than in cutaneous orofacial tissue. As there is h eavy innervation of the TMJ by unmyelinated nerve endings, a strong nocicep tive primary afferent barrage is expected following inflammation. An increa se in TMJ C-fiber input after inflammation and strong central neuronal acti vation may initiate central hyperexcitability and contribute to persistent pain associated with temporomandibular disorders. Since deep inputs may be more effective in inducing central neuronal excitation than cutaneous input s, greater sensory disturbances may occur in pain conditions involving deep tissues than in those involving cutaneous tissues. Copyright (C) 2001 S. K arger AG, Basel.