Animal model with painful scar: pain-related behavior and immunohistochemical study on the spinal dorsal horn and peripheral tissue

[Abstract] Sustained limb pain or back pain after surgery or injury occasionally shifts to chronic pain. In some cases, scar formed in damaged tissues can evoke chronic pain. However, there is a general lack of animal models to study chronic scar-related pain state. To develop a rat model with painful scar, a stick (3mm of diameter) was inserted into the planta of unilateral hind-paw in anesthetized rats, followed by exfoliation of cutaneous and tendon tissues. Withdrawal responses were measured using von Frey filaments at different areas in the scar for four weeks. The test results for withdrawal responses suggested that a painful scar in a rat's planta resulted in reliable and quantifiable mechanical hyperalgesia lasting for four weeks. In the immunohistochemical study, increased expression of calcitonin gene-related peptide (CGRP) was observed in dorsal root ganglion (DRG) and in the spinal dorsal horn, and these changes are partly related to pathological pain state on the scar side. This model should contribute towards understanding the sensitization mechanism of painful scar and to develop new treatments for painful scar in humans.