Temporal and spatial differences in intracellular Ca++ changes elicited by K+ and glutamate in single cultured neocortical neurons

Changes as a function of time in the intracellular Ca++ concentration ([Ca++]i) in cultured cerebral cortical neurons were monitored after exposure of the cells to either 55 mM KCl or 100 microM glutamate using the fluorescent Ca++ chelator fura-2. The changes in [Ca++]i were followed in both cell bodies and neurites. Depolarization with K+ led to an immediate increase in [Ca++]i in neurites followed by a slower rise in the cell bodies. In contrast, glutamate elicited a slow increase in [Ca++]i in both neurites and cell bodies, and this increase showed a plateau rather than a peak as that seen after exposure to K+. The Ca++ channel blockers verapamil and nifedipine affecting N- and L-type channels, respectively had differential effects on K+ stimulated increases in [Ca++]i. Nifedipine only affected the increase marginally whereas verapamil inhibited the response by 50-60% both in cell bodies and neurites. The glutamate-induced increase in [Ca++]i was inhibited by nifedipine by 60% in neurites whereas no effect was observed in cell bodies. The results show that depolarization elicited by K+ and glutamate has different effects in different parts of the neurons and that the pharmacological characteristics of voltage gated Ca++ channels are dramatically different in cell bodies and neurites. Moreover, the distribution of L-type channels activated by glutamate differs in cell bodies and neurites. Such differences in the spatial distribution of Ca++ channels are likely to be of major importance for the functional consequences of depolarization coupled increases in [Ca++]i such as transmitter release and neurotoxicity.