Enhancement of Ca spikes in nerve cells of adult mammals during neurite growth in tissue culture

Abstract

GENERATION of Na and Ca spikes in various types of vertebrate nerve cells^1–6 indicates that both Na and Ca channels are generally involved in inducing action potentials in their cell bodies. Although it has been suggested that these ionic channels may change their configuration in the plasma membrane, in particular during degeneration of nerve cells^7,8 or during growth of axons associated with ontogenetic differentiation^9–11, no systematic or quantitative study has been reported. We have recently established a technique to obtain primary culture of nerve cells of mature mammals^12,13, and have also developed a technique to compare membrane capability of carrying inward Na and Ca currents among nerve cells of various ages in the cell culture. We report here that the capability of membranes to carry Ca currents, as represented by the maximum rate of rise (MRR) of a Ca spike, is enhanced during a particular period of the cell culture, whereas there is no significant change in Na currents. This may be explained as an increase in number of the Ca channels or as a change in conformation of the Ca channels, associated, in part, with neurite outgrowth. As we used nerve cells which had fully differentiated in the host bodies, the possibility of an effect due to ontogenetic differentiation in the tissue culture condition^14–19 can be excluded. The present finding also demonstrates another aspect of nerve cell membrane that ionic channel molecules, especially the Ca channels, can alter their features rather flexibly in the plasma membrane.