THE OPTIC TRACT AND TECTAL ABLATION INFLUENCE THE COMPOSITION OF NEUROFILAMENTS IN REGENERATING OPTIC AXONS OF XENOPUS-LAEVIS

Neurofilaments have been proposed to regulate axonal stability and dia meter through changes in number and subunit composition. We have found that pathway and target innervation directly influence the molecular composition of neurofilaments within regenerating optic axons of Xenop us laevis. immunocytochemistry was used to examine neurofilaments with in two abnormal visual pathways. The first was an aberrant, transient retinoretinal projection, which formed when some axons entered the con tralateral optic nerve at the chiasm. The second was formed by regener ating axons deprived of their normal targets by surgical ablation of b oth optic tecta. Distal to an orbital nerve crush, the neurofilament p roteins NF-L, NF-M, NF-H, and XNIF disappear from degenerating fibers. In normally regenerating axons, these neurofilament proteins emerge i n a progression reminiscent of development. In the aberrant retinoreti nal projection, levels of XNIF, NF-L, and -M remained lower than in no rmally regenerating axons, whereas NF-H and a phosphorylated form of N F-M were undetectable for at least 35 d after nerve crush. Normally, t hese two latter forms reappear between 15 and 21 d after surgery. Thus , this transient, incorrect axonal projection expressed neurofilaments in a very different pattern from correctly regenerating axons. In tec ta-ablated frogs, staining of phosphorylation independent epitopes of XNIF, NF-L, and -M increased normally after axons entered the tract, b ut that of NF-H and phosphorylated NF-M remained low for at least 42 d after axotomy. Thus, separate parts of the visual pathway influence t he complexity of neurofilaments.