Onset of Electrical Excitability During a Period of Circus Plasma Membrane Movements in Differentiating Xenopus Neurons

Living neurons are usually first identifiable in primary cultures at the time of neurite initiation, and studies of excitability have been restricted largely to the subsequent period. A morphological early marker is described that identifies neurons for whole-cell voltage-clamp recordings before neurite initiation. Video time-lapse recordings of cultured cells dissociated from neurectoderm of Xenopus neural plate stage embryos reveal cells demonstrating circus movements, in which blebs of plasma membrane propagate around the cell circumference within a period of several minutes. All neurons demonstrate circus movements before morphological differentiation; the fraction of cells exhibiting circus movements that differentiate morphologically depends on the substrate on which they are cultured. Blockade of circus activity with cytochalasin B does not prevent neuronal differentiation. Circus movements are not neurectoderm-specific because they similarly predict differentiation of myocytes developing in mesodermal cultures. Initially inexcitable, neurons develop voltage-dependent K+, Na+, and Ca2+ currents during the period of several hours in which they exhibit circus movements. The early development of depolarization-induced elevations of [Ca2+]i several hours before morphological differentiation corresponds to the previously described onset of functionally significant spontaneous elevations of [Ca2+]i in these neurons and demonstrates a role for early expression of voltage-dependent ion channels.