The primary symptom and most debilitating aspect of fibromyalgia (FM) and other generalized pain syndromes is generalized musculoskeletal tenderness and pain. The pathogenic mechanism of these pain syndromes is, however, still poorly understood, and currently available analgesic therapies have proven to be only partially, and unpredictably, effective. Progress has been seriously hindered by the lack of an adequate experimental model of chronic generalized pain. During the current grant period we determined that interruption of activity in abdominal vagal afferents results in a state of enhanced mechanical hyperalgesia. This vagotomy-induced generalized hyperalgesia depends on adrenal medulla function, a characteristic highly relevant to fibromyalgia in which chronic stress is thought to make a major contribution. In further probing the role of stress in the pathophysiological mechanism of generalized hyperalgesia, we found that rats exposed to non-habituating sound stress exhibited a marked increase in hyperalgesia evoked by injection of the pronociceptive inflammatory mediator prostaglandin E2 (PGE2) or epinephrine in the skin. This enhancement, which developed more than a week after exposure to stress, required concerted action of glucocorticoids and catecholamines at the glucocorticoid and 22-adrenergic receptor, respectively, located in the peripheral terminals of sensory afferents. The altered response to pronociceptive mediators involved a switch in coupling of their receptors from predominantly stimulatory to inhibitory G-proteins (Gs to Gi) and for PGE2, in particular, emergence of novel dependence on protein kinase C5 signaling. Thus, an important mechanism in generalized pain syndromes may be stress-induced co-activation of hypothalamic-pituitary adrenal and sympathoadrenal axes, causing a long-lasting alteration in intracellular signaling pathways in the primary afferent nociceptor, enabling normally innocuous levels of immune mediators to produce chronic hyperalgesia.
The aims of this competing grant renewal application are to: 1) analyze the characteristics of stressors that induce enhancement of cytokine hyperalgesia in the musculoskeletal system;2) examine the role of interactions between the sympathoadrenal and hypothalamic-pituitary adrenal neuroendocrine stress axes, and their effector mediators, glucocorticoids and catecholamines, in stress-induced enhancement of cytokine hyperalgesia;3) evaluate the relevance of our model of fibromyalgia-like generalized pain in relation to an important co-morbid symptom, visceral hyperalgesia;and, 4) elucidate the mechanisms underlying nociceptor plasticity produced by neuroendocrine stress hormones and the role of changes in the primary afferent nociceptor in the development of enhanced cytokine hyperalgesia. The proposed studies are designed to provide new insights into the pathophysiology of generalized pain syndromes like fibromyalgia. In this regard, the proposed studies have the potential to achieve important progress toward a clinical goal that until now has been frustrated by the absence of workable animal models.
Fibromyalgia is characterized by widespread musculoskeletal pain and tenderness, disturbances in sleep, and co-occurrence of other generalized pain syndromes, such as irritable bowel syndrome and post-traumatic stress disorder. It affects 2-4% of the population and has a considerable impact on everyday life. Unfortunately, there is no treatment reliably successful in most patients. The mechanisms that underlie this pain condition, especially those contributing to the diffuse pain, have been elusive. Stress is extremely important in fibromyalgia;indeed, daily stresses commonly have a significant negative impact. While stress may play an important role in fibromyalgia, the mechanism underlying the associations between stress and pain in fibromyalgia has not been elucidated. Evidence that stress alone may not be sufficient to induce fibromyalgia has led investigators to look for additional causative factors. The observation of a much higher incidence of fibromyalgia in patients with inflammatory and infectious diseases, which also activate the bodies stress systems, suggest that inflammatory mediators might be an important additional factor. In the experiments in this grant proposal we will address the interaction between stress and inflammation in fibromyalgia and other generalized pain syndromes. The ultimate goal of the present research proposal is to provide a rational basis for the design of novel strategies to treat pain in patients with fibromyalgia. The findings should help tailor specific therapies that may prevent or at least ameliorate their significant suffering.
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