Allodynia is the technical term for severe pain evoked by normally non-painful stimulation such as that produced by stroking sunburnt skin. Many chronic pain patients suffer from allodynia, a devastating condition that adversely affects the ability to perform everyday activities such as dressing and cooking, to name a few. The scientific literature generally agrees that allodynia is due to an injury-induced change in connectivity of nerve fibers normally responsible for signaling gentle touch of the skin to the spinal cord. However, the specific identity of the fiber types involved remains unclear. Rapidly conducting, thickly myelinated (insulated) Abeta-fibers are generally understood to be the main touch fibers, and most studies on allodynia have focused on this fiber type. However, recent work has shown that slowly conducting unmyelinated C-tactile (CT) fibers may also play an important role in allodynia. Whereas Abeta-fibers respond to a wide range of touch stimuli, CT fibers respond specifically to slow stroking of the hairy skin. We have previously demonstrated that the intracolonic capsaicin model of visceral pain is characterized by referred pain on the abdomen. Capsaicin, the main ingredient in chili peppers, was introduced into the colon via a flexible cannula, evoking a brief hypersensitivity to gentle touch of the abdomen. Moreover, gently brushing the abdomen increases neuronal firing in pain processing regions of the spinal cord, as measured by immunohistochemical labeling of c-Fos. The same brushing of normal, uninjured mice has no effect on spinal c-Fos. Together, these findings demonstrate that touch fibers can activate pain-processing neurons in the spinal cord after injury. However, it is not clear which fiber type is responsible for this effect. In this project, we are performing experiments designed to clarify the relative contributions of Abeta- and/or CT-fibers in allodynia. Specifically, we will use different stimulus protocols to selectively activate either abdominal CT- or Abeta-fibers in mice with acute capsaicin-induced visceral pain in order to measure the level of c-Fos labeling in pain processing regions of the spinal cord. This study will allow us to identify the extent to which these fiber types contribute to allodynia and, consequently, lead to more specific targeting of therapeutics.
