Epinephrine-induced excitation and sensitization of rat C-fiber nociceptors

Although epinephrine (EPI) has been suggested to contribute to the pain and hyperalgesia associated with inflammation and nerve injury, there have been very few in vivo electrophysiologic studies of the effects of EPI on nociceptors. We found with the single-unit recording technique that the intradermal administration of EPI resulted in excitation of a group of C fibers and a decrease in the mechanical activation threshold in a non-overlapping group. Unexpectedly, the fibers that were neither excited nor demonstrated a decrease in threshold demonstrated as a group a significant increase in response to sustained suprathreshold mechanical stimuli, an effect not observed in the other 2 groups of C fibers. This identifies a novel response of C-fiber nociceptors to an inflammatory mediator and suggests it is present in a class of C fibers previously considered unresponsive to hyperalgesic inflammatory mediators. Perspective: Our study provides support for the suggestion that EPI, a neuroendocrine stress hormone as well as an inflammatory mediator, might contribute to pain syndromes, especially in the setting of chronic stress. (c) 2005 by the American Pain Society.