THE CLUSTERING OF AXONAL SODIUM-CHANNELS DURING DEVELOPMENT OF THE PERIPHERAL NERVOUS-SYSTEM

The distribution of Na+ channels in rat peripheral nerve was measured during development by using immunofluorescence. Small segments of scia tic nerve from postnatal day 0-13 (P0-P13) pups were labeled with an a ntibody raised against a well conserved region of the vertebrate Na+ c hannel. At day P0 axons contained almost no Na+ channel aggregates. Th e number of clusters increased dramatically throughout the first week. In almost all cases Na+ channels clustered in the vicinity of Schwann cell processes. At least four classes of aggregates were noted. Clust ers formed singly at Schwann cell edges, in pairs or in broad regions between neighboring Schwann cells, and in more focal zones at presumpt ive nodes. Almost all Na+ channel aggregates had reached the latter st age by the end of the first week. Histograms plotting the frequency of occurrence of each cluster type suggested a sequence of events in nod e formation involving the initiation of channel aggregation by Schwann cell processes. The requirement for Schwann cells during sodium chann el clustering was tested by blocking proliferation of these cells with the antimitotic agent mitomycin C. Na+ channel clustering was sharply reduced, whereas node formation was normal at a distal site along the same nerve. Immunocytochemical defection of myelin-associated glycopr otein (MAG) indicated that Schwann cells must begin to ensheathe axons before inducing Na+ channel clustering.