Neuronal galvanotropism is independent of external Ca2+ entry or internal Ca2+ gradients

The mechanism by which growing neurites sense and respond to small applied electrical fields is not known, but there is some evidence that the entry o f Ca2+ from the external medium, with the subsequent formation of intracell ular Ca2+ gradients, is important in this process. We have employed two app roaches to test this idea. Xenopus spinal neurites were exposed to electric al fields in a culture medium in which Ca2+ was chelated to very low levels compared to the normal extracellular concentration of 2 mM. In other exper iments, Loading the neurites with the calcium buffer, 1,2-bis(o-aminophenox y)ethane-N,N,N',N'-tetraacetic acid (BAPTA), disrupted the putative interna l Ca2+ gradients, and the effects on the electrical response were determine d. Fields of 100 mV/mm were applied for 12 h, and no difference was detecte d in the cathodal turning response between the treated neurites and the unt reated controls. Using the Differential Growth Index (DGI), an asymmetry in dex, to quantitate the turning response, we recorded DGIs of -0.64, -0.65, and -0.62 for control cells, cells in Ca2+-free medium, and cells preloaded with BAPTA, respectively. Furthermore, we detected an increase in neurite length for those neurons cultured in Ca2+-free medium; they were 1.5-1.7 ti mes as long as neurites from neurons cultured in normal Ca2+ medium. Likewi se, we found that BAPTA-loaded neurites were longer than control neurites, Our data indicate that neuronal galvanotropism is independent of the entry of external Ca2+ or of internal Ca2+ gradients. Both cell-permeant agonisti c and antagonistic analogs of cyclic 3',5'-adenosine monophosphate (cAMP) i ncreased the response to applied electrical fields. (C) 2000 John Wiley & S ons. Inc.