Electric field-guided growth of cultured frog nerves : role of surface charge, second messengers and other nerve guidance cues
I have studied the orientation, rate of growth and branching behaviour of embryonic Xenopus neuritse exposed to raised external cations, to polyanions, to applied dc electric fields and to combinations of these treatments. Interactions between electric fields and two other nerve growth cone guidance cues, CSPGs and neurotransmitters, also were examined. The principle findings were as follows. 1. In small dc electric fields, neurites turned cathodally, grew faster and branched more than controls in a field strength-dependent manner. 2. Polycations totally inhibited field-induced cathodal orientation, enhanced rates of growth and enhanced and polarised branching. The failure of neurites to orient and branch asymmetrically can be explained by a mechanism involving modulation of membrane surface charge. Such a mechanism can not explain the modulations to growth rates, which may involve inhibition of Ca2+ influx and of phosphoinositide metabolism. 3. The lyotropic anions, thiocyanate and perchlorate had no effect on field-induced cathodal turning or branching, whereas incubation with sulphate was inhibitory. Increasing surface charge with lyotropic anions does not therefore enhance galvanotropic orientation, or underlie the inhibitory effects of sulphate which may occur due to changes in the inositolphospholipid second messenger system. 4. Two PGs, differing in the sulphation pattern of their GAG side chains, respectively enhanced and inhibited the guidance effects of a small applied electric field. 5. d- Tubocurarine inhibited field-induced cathodal-turning and branching whereas, atropine and suramin markedly enhanced galvanotropism. This implicates activation of growth cone nicotinic AChRs in the induction of neurite orientation and additionally, provides the first evidence that co-activation of neurotransmitter receptors may regulate directed nerve growth.