Micromolar Ca super(2+) transients in dendritic spines of hippocampal pyramidal neurons in brain slice

The magnitude and dynamics of [Ca super(2+)] changes in spines and dendrites of hippocampal CA1 pyramidal neurons have been characterized using a low affinity fluorescent indicator, mag-Fura 5, that is sensitive to Ca super(2+) in the micromolar range. During tetanic stimulation (1 s), we observed progressive [Ca super(2+)] increases in distal CA1 spines to as much as 20-40 mu M, both in organotypic slice culture and acute slice. Similar accumulations were reached during continuous depolarization (+10 mV, 1 s) when K super(+) channels had been blocked, but not with spike trains driven by postsynaptic current injection. The large [Ca super(2+)] increases due to tetanic stimulation were blocked by APV, indicating that NMDA receptor-dependent influx was critical for the large responses. These findings have significant implications for low affinity Ca super(2+)-dependent biochemical processes and show a new upper limit for [Ca super(2+)] changes measured in these neurons during stimulation.