The distribution of GAP-43 in normal rat spinal cord.

We have studied the distribution of the growth-associated protein GAP-43 in the spinal cord of adult rats by light and electron microscopy, using a new antiserum raised against GAP-43/beta-galactosidase fusion protein. We show that GAP-43 is present at all vertebral levels but is more concentrated in cervical and thoracic regions. In addition to heavy staining in the corticospinal tracts of the white matter, staining can be seen at the light microscopic level throughout the grey matter and is particularly heavy around the central canal and in the superficial dorsal horn. Electron microscopic examination revealed that GAP-43 immunostaining is confined to a subpopulation of axons and axon terminals. Staining occurs in small myelinated and unmyelinated fibres and in terminals which are mainly small and make single axodendritic or axosomatic synapses. Staining in such terminals occurs in the axoplasm but is heaviest immediately adjoining the axolema. Staining was not observed in dendrites, nor in large myelinated axons or large axon terminals. Our results indicate that GAP-43 is expressed in adult rat spinal cord in a subpopulation of small diameter fibres and axon terminals. The distribution and morphology of these terminals is consistent with several different possible origins including corticospinal projection neurons, small diameter primary afferent neurons, and descending raphe-spinal serotonin containing neurons.