Mechanism of immobilization-induced hypersensitivity associated with changes in peripheral tissues

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This study examined mechanism of mechanical and thermal hypersensitivity in the rat hind paw during ankle cast immobilization of the hind limbs using rats. Two weeks post immobilization, mechanical and thermal sensitivities increased significantly in all rats. CGRP expression in the spinal dorsal horn increased in the rats immobilized for 8 weeks but not in those immobilized for 4 weeks. These results also highlight the possibility that prolonged immobilization induces central sensitization in the spinal cord. In the plantar skin of immobilized rats, peripheral nerve profiles were increased. NGF, TRPV1 and P2X3 expression levels in the epidermis (keratinocyte) were also increased. The time course of the increase in peripheral nerve fibers and in the expression of TRPV1 and P2X3 paralleled the development of hypersensitivity, which suggests that histological changes of the skin following cast immobilization may have some relation to the resulting hypersensitivity.