Developmental Down-regulation of GAP-43 Expression and Timing of Target Contact in Rat Corticospinal Neurons

During early nervous system development axons grow toward the target tissue that they will innervate. As axons invade target tissue, growth slows and ceases. Neurons express high levels of the growth-associated protein GAP-43 during developmental axon growth, declining with maturation. It has been suggested that target contact provides a signal which down-regulates GAP-43 expression. To study this issue in more detail, we used in situ hybridization to quantify relative changes in GAP-43 mRNA in corticospinal tract neurons identified by Fast Blue retrograde labeling. We also used anterograde transport of biotinylated dextran amine to study the invasion of target by corticospinal axons. We find that GAP-43 mRNA is high during the first postnatal week and then declines in two phases. Approximately half of the initial level of GAP-43 expression in corticospinal neurons is lost by P12; then expression remains at a plateau until P21. Between P21 and P28, GAP-43 expression again declines by half and then remains steady at the adult level (one fourth of initial level). Corticospinal axons initially invade spinal gray matter during the first 2 postnatal weeks, in a rostrocaudal gradient. Varicosities suggestive of terminal boutons become numerous during the third and fourth week, and the morphology of corticospinal axon terminals achieves the mature form at the end of the fourth week. These data suggest that the first phase of down-regulation of GAP-43 in corticospinal neurons is coincident with initial target contact and that the second phase is coincident with final maturation of terminal arborization.