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Stress has been shown to affect brain activity and promote long-term changes in multiple neural systems. A variety of environmental and/or stressful stimuli have been shown to produce analgesia, a phenomenon often referred to as stress-induced analgesia (SIA). However, acute and chronic stresses also produce hyperalgesia in various behavioral tests. There are now several animal models in which stress enhances nociceptive responses. The dysfunction of the hypothalamo-pituitary-adrenocortical axis (HPA axis) and multiple neurotransmitter systems in the central nervous system (CNS), including endogenous opioid, serotonergic and noradrenergic systems, has been reported. These stress-induced hyperalgesia models may contribute to a better understanding of chronic pain and provide a more rational basis for drug therapies in a variety of pain syndromes.

This study was designed to systematically examine the effects of persistent orofacial tissue injury on prolonged neuronal activation in the trigeminal nociceptive pathways by directly comparing the effects of orofacial deep vs. cutaneous tissue inflammation on brainstem Fos protein expression, a marker of neuronal activation. Complete Freund's adjuvant (CFA) was injected unilaterally into the rat temporomandibular joint (TMJ) or perioral (PO) skin to produce inflammation in deep or cutaneous tissues, respectively. Rats were perfused 2 hours, 24 hours, 3 days, or 10 days following CFA injection. The TMJ and PO inflammation‐induced Fos expression paralleled the intensity and course of inflammation over the 10‐day observation period, suggesting that the increase in intensities and persistence of Fos protein expression may be associated with a maintained increase in peripheral input. Compared to PO CFA injection, the injection of CFA into the TMJ produced a significantly stronger inflammation associated with a greater Fos expression. In TMJ‐ but not in PO‐inflamed rats, Fos‐like immunoreactivity (LI) spread from superficial to deep upper cervical dorsal horn as the inflammation persisted and there was a dominant ipsilateral Fos‐labeling in the paratrigeminal nucleus. Common to TMJ and PO inflammation, Fos‐LI was induced in the trigeminal subnuclei interpolaris and caudalis, C1–2 dorsal horn, and other medullary nuclei. Substantial bilateral Fos‐LI was found in the interpolaris‐caudalis trigeminal transition zone. Further analysis revealed that Fos‐LI in the ventral transition zone was equivalent bilaterally, whereas Fos‐LI in the dorsal transition zone was predominantly ipsilateral to the inflammation. The differential induction of Fos expression suggests that an increase in TMJ C‐fiber input after inflammation and robust central neuronal hyperexcitability contribute to persistent pain associated with temporomandibular disorders. J. Comp. Neurol. 412:276–291, 1999. © 1999 Wiley‐Liss, Inc.

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