Local Skin Lesions in the Rat after Subcutaneous Deposition of Capsaicin

Abstract

Capsaicin is used to investigate the role of peripheral sensory nerve fibers. In previous studies of rats treated by injection of capsaicin into the skin of the neck, ‘spontaneous’ lesions in the head and neck region were observed. In this study, the course of development over time, the regional distribution and the innervation of capsaicin-induced dermal lesions were assessed in young male Sprague-Dawley rats. In one experiment, capsaicin was administered subcutaneously by injection in the skin of the neck. In a second experiment, capsaicin was injected in the back by a long needle that tunneled under the skin and allowed the capsaicin to be deposited in the subcutaneous fat of the neck. The density and the distribution of dermal nerve fibers were investigated by immunohistochemistry, using antisera against a panneuronal marker, protein gene product 9.5 (PGP), and calcitonin gene-related peptide (CGRP). In the first experiment, rats developed lesions in the neck area 11 days after injection. In the second experiment, lesions appeared in the skin of the back and occasionally in the neck area 10 days after injection. Development of lesions in the afflicted areas was paralleled by local reduction in the density of CGRP-immunoreactive (IR) nerve fibers, 80% in the first experiment and 72% in the second. The number of PGP-IR fibers was likewise reduced, by 39 and 41%, respectively. The density of the CGRP-IR fibers in the wound area was the same as in the adjacent, nonlesioned skin. The healing of the capsaicin-induced lesions was slow compared with surgical wounds in control animals. The wounds healed with hypertrophic scars. The healing process in the skin of the back was associated with the proliferation of CGRP-IR fibers. The study shows cutaneous lesions to appear in the region of the subcutaneous deposition of capsaicin. A uniform depletion of capsaicin-sensitive nerve fibers in the area of deposition suggests that an additional factor is needed to induce lesions. Possibly, impaired nociception in the afflicted area results in more vigorous grooming behavior and this, in turn, in a local skin damage.