THE DIRECTION OF NEURITE GROWTH IN A WEAK DC ELECTRIC-FIELD DEPENDS ON THE SUBSTRATUM - CONTRIBUTIONS OF ADHESIVITY AND NET SURFACE-CHARGE

We investigated the influence of the growth surface on the direction o f Xenopus spinal neurite growth in the presence of a de electric field of physiological magnitude. The direction of galvanotropism was deter mined by the substratum; neurites grew toward the negative electrode ( cathode) on untreated Falcon tissue culture plastic or on laminin subs trata, which are negatively charged, but neurites growing on polylysin e, which is positively charged, turned toward the positive electrode ( anode). Growth was oriented randomly on all substrata without an elect ric field. We tested the hypothesis that the charge of the growth surf ace was responsible for reversed galvanotropism on polylysine by growi ng neurons on tissue culture dishes with different net surface charges . Although neurites grew cathodally on both Plastek substrata, the fre quency of anodal turning was greater on dishes with a net positive cha rge (Plastek C) than on those with a net negative charge (Plastek M). The charge of the growth surface therefore influenced the frequency of anodal galvanotropism but a reversal in surface charge was insufficie nt to reverse galvanotropism completely, possibly because of differenc es in the relative magnitude of the substratum charge densities. The i nfluence of substratum adhesion on galvanotropism was considered by gr owing neurites on a range of polylysine concentrations. Growth cone to substratum adhesivity was measured using a blasting assay. Adhesivity and the frequency of anodal turning were graded over the range of pol ylysine concentrations (0 = 0.1 < 1 < 10 = 100 mu g/ml). The direction of neurite growth in an electric field is therefore influenced by bot h substratum charge and growth cone-to-substratum adhesivity. These da ta are consistent with the idea that spatial or temporal variation in the expression of adhesion molecules in embryos may interact with natu rally occurring electric fields to enhance growth cone pathfinding. (C ) 1998 Academic Press.