GAP-43 immunoreactivity is enhanced after UV irradiation in the peripheral nervous system of the rat

Abstract

UV irradiation evoked thermal and mechanical hypersensitivity in the glabrous skin of the rat hind paw. During the period of hyperalgesia skin samples were taken from irradiated areas of the skin and the corresponding segments of the spinal cord, and the expression of growth-associated protein 43 (GAP-43) was quantitatively measured. Although GAP-43 was present in a small number of fibers in the skin of control animals, the total length of GAP-43 positive fibers per unit area was significantly increased within 3 days after irradiation in both ipsi- and contralateral hind limbs. The time course of GAP-43 expression corresponded to that of mechanical hyperalgesia. GAP-43 immunopositivity was at its maximum on day 3 after UV irradiation and declined to control level at day 14. Increased GAP-43 immunoreactivity was similar in both hind paws except on day 3 when it was significantly more on the ipsilateral side. Most GAP-43-positive fibers were arborizing branches in the skin, but strong positivity was found around blood vessels and around sweat glands. Glyoxylic acid-induced fluorescent-catecholamine marking was associated with blood vessels without extravascular sprouting in control or UV-irradiated skin. At 3 and 5 days after UV irradiation GAP-43-positive profiles were present in the spinal L4-5 segments. GAP-43 immunoreactivity was detected on both sides in the dorsal horn with greater intensity on the ipsilateral side. We conclude that the increased expression of GAP-43 in neuronal processes of the ipsi- and contralateral paw after UV irradiation is most unlikely to be directly associated with nerve sprouting. GAP-43 expression occurred mainly in sensory fibers.