Unpredictable stress delays recovery from exercise-induced muscle pain: contribution of the sympathoadrenal axis

INTRODUCTION: Although stress is a well-establish risk factor for the development of chronic musculoskeletal pain, the underlying mechanisms, specifically the contribution of neuroendocrine stress axes, remain poorly understood. OBJECTIVE: To evaluate the hypothesis that psychological stress-induced activation of the sympathoadrenal stress axis prolongs the muscle pain observed after strenuous exercise. METHODS: Adult male Sprague-Dawley rats were exposed to unpredictable sound stress and eccentric exercise. The involvement of the sympathoadrenal stress axis was evaluated by means of surgical interventions, systemic administration of epinephrine, and intrathecal β(2)-adrenergic receptor antisense. RESULTS: Although sound stress alone did not modify nociceptive threshold, it prolonged eccentric exercise-induced mechanical hyperalgesia. Adrenal medullectomy (ADMdX) attenuated, and administration of stress levels of epinephrine to ADMdX rats mimicked this effect of sound stress. Knockdown of β(2)-adrenergic receptors by intrathecal antisense also attenuated sound stress-induced prolongation of eccentric exercise-induced hyperalgesia. CONCLUSION: Together, these results indicate that sympathoadrenal activation, by unpredictable sound stress, disrupts the capacity of nociceptors to sense recovery from eccentric exercise, leading to the prolongation of muscle hyperalgesia. This prolonged recovery from ergonomic pain is due, at least in part, to the activation of β(2)-adrenergic receptors on muscle nociceptors.