Constitutive expression of GAP-43 correlates with rapid, but not slow regrowth of injured dorsal root axons in the adult rat

It has been postulated that the neuronal growth associated protein GAP-43 p lays an essential role in axon elongation. Although termination of developm ental axon growth is generally accompanied by a decline in expression of GA P-43, a subpopulation of dorsal root ganglion (DRG) neurons retains constit utive expression of GAP-43 throughout adulthood. Peripheral nerve regenerat ion occurring subsequent to injury of the peripheral axon branches of adult DRG neurons is accompanied by renewed elevation of GAP-43 expression. Lesi ons of DRG central axon branches in the dorsal roots are also followed by S ome regenerative growth, but little or no increase in GAP-43 expression abo ve the constitutive level is observed. To determine whether dorsal root axo n regeneration occurs only from neurons which constitutively express GAP-43 , we have used retrograde fluorescent labeling to identify those DRG neuron s which extend axons beyond a crush lesion of the dorsal root. Only GAP-43 immunoreactive neurons supported axon regrowth of 7 mm or greater within th e first week. At later times, axon regrowth is seen to occur from neurons b oth with and without GAP-43 immunoreactivity. We conclude that regeneration of injured axons within the dorsal root is not absolutely dependent on the presence of GAP-43, but that expression of GAP-43 is correlated with a cap acity for rapid growth. (c) 1999 Academic Press.