Ionic channel expression in primary afferent Aβ fibers:analysis of regulatory mechanism and therapeutic application to chronic pain

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Altered ionic channel expression and sympathetic nerve sprouting have been thought to underlie a condition of chronic pain, such as complex regional pain syndrome. Following a few weeks after injury, postganglionic sympathetic nerve endings sprout and surround cell bodies of dorsal root ganglion (DRG) neurons, and make abnormal synaptic contacts. Aberrant interactions between sensory and sympathetic neurons are also located in the peripheral sites including neuroma at a transected nerve and intradermal region. Despite extensive studies, however, little is known about the molecules that promote the outgrowth of sympathetic nerve. To identify chronic pain-associated proteins, we screened a subtractive cDNA library enriched for mRNAs increased in the proximal stump of transected rat sciatic nerve. Among the identified proteins, collagen type VI was induced prominently in the proximal stump. Collagen type VI-coated plastic plate allowed robust neurite outgrowth of dorsal root and superior cervical ganglion neurons dissociated from adult rats, NSC34 spinal motor neuron-like cells, and PC12 pheochromocytoma cells. The neurite-promoting effect was more long-lasting than those in collagen type I-, laminin-, poly-D-ornithine-, and poly-L-lysine-coated plates. In addition, coating with collagen type VI prevented death of PC12 and NSC34 cells to the extent comparable to laminin. Immunohistochemical analysis of damaged sciatic nerve revealed that intense immunoreactivity for collagen type VI was localized in nonmyelinating Schwann cells, macrophages and fibroblasts, and along vessels and tyrosine hydroxylase (TM-positive nerve fibers. These results suggest that collagen type VI promotes not only regeneration of motor and somatosensory nerves, but also sprouting of sympathetic nerve. We are planning to examine whether somatosensory and /or sympathetic neurons in contact with collagen type VI alter expression levels of ionic channels.