Direct evidence of primary afferent sprouting in distant segments following spinal cord injury in the rat: colocalization of GAP-43 and CGRP.

Mechanical and thermal allodynia develops after spinal cord injury in three areas relative to the lesion: below level, at level, and above level. The present study tests colocalization of CGRP, associated with nociceptive neurons, with growth-associated protein (GAP-43), expressed in growing neurites, to test for neurite sprouting as a mechanism for reorganization of pain pathways at the level of the lesion and distant segments. Male Sprague-Dawley rats were divided into three groups: sham control (N = 10), hemisected at T13 and sacrificed at 3 days (N = 5) and at 30 days (N = 5) following surgery, the spinal cord tissue was prepared for standard fluorescent immunocytochemistry using mouse monoclonal anti-GAP-43 (1:200) and/or rabbit polyclonal anti-CGRP (1:200), density of immunoreaction product (IR) was quantified using the Bioquant software and values from the hemisected group were compared to similar regions from the sham control. We report significant increases at C8 and L5, in CGRP-IR in lamina III compared to control tissue (P < 0.05). We report significant bilateral increases in GAP-43-IR at C8, T13, and L5 segments in lamina I through IV, at 3 days post hemisection, compared to control tissue (P < 0.05), some of which is colocalized with alpha-CGRP. The increased area and density of GAP-43-IR is consistent with neurite sprouting, and the colocalization with alpha-CGRP indicates that some of the sprouting neurites are nociceptive primary afferents. These data are consistent with endogenous regenerative neurite growth mechanisms that occur near and several segments from a spinal lesion, that provide one of many substrates for the development and maintenance of the dysfunctional state of allodynia after spinal cord injury.