Immunocytochemical distribution of the protein kinase C substrate B-50 (GAP43) in developing rat pyramidal tract.

The neuron-specific phosphoprotein B-50 is a major substrate of kinase C in fetal nerve growth cones, neonatal neural and synaptosomal plasma membranes. B-50 is identical to a growth-associated protein GAP43. Similarly, increases in B-50 occur during rat brain development, neuronal differentiation and axon regeneration. To document the relation between the expression of B-50 and the outgrowth of central axons, we studied B-50 in the developing pyramidal tract in rats at postnatal days 2, 7 and 90 (P2, P7 and P90), at the third cervical spinal segment C3, using affinity-purified antibodies to B-50. At P2 and P7, when outgrowth of pyramidal tract fibers is occurring, B-50 immunoreactivity (BIR) is intense in these fibers. BIR is reduced from P2 to P7 in the ascending fiber tracts of the cuneatus and the gracilis, which develop earlier. At P90 when most of the dorsal funiculus fibers have reached their targets and many are myelinated, BIR is dramatically reduced. In agreement, a 10-fold decrease in B-50 content was measured at P90, as compared to P7. Therefore, our results indicate that B-50 is only expressed relatively abundant in axons of the funiculus posterior during outgrowth. By inference, B-50 may be a differentiating marker to detect elongating fibers.