NGF BUT NOT NT-3 OR BDNF PREVENTS THE A-FIBER SPROUTING INTO LAMINA-II OF THE SPINAL-CORD THAT OCCURS FOLLOWING AXOTOMY

There is a stereotypical pattern of primary afferent terminations with in the mature spinal cord; however, this pattern is not immutable. Per ipheral axotomy causes A fibers to sprout into lamina II, a region fro m which they are normally excluded. We have investigated the role of n eurotrophins in this response. Rats which had undergone sciatic axotom y were treated intrathecally with NGF, BDNF, or NT-3. A fibers were vi sualized using transganglionic labeling with cholera toxin B subunit; small fibers were visualized using CGRP immunostaining. NGF (12 mu g/d ay for 2 weeks), but not NT-3 or BDNF, prevented both the axotomy-indu ced reduction in CGRP staining within lamina II and the sprouting of A fibers into this region. It is likely that the prevention of A fiber sprouting is a secondary consequence of NGF rescuing small fibers. Thi s effect of NGF on dorsal horn sprouting has implications both for our understanding of the maintenance of CNS connectivity and for the trea tment of neuropathic pain states.