SPONTANEOUS CA2+ TRANSIENTS IN DEVELOPING HIPPOCAMPAL PYRAMIDAL CELLS

To determine the spatiotemporal pattern of hippocampal pyramidal cell activity during development, we examined cytosolic Ca2+ dynamics in ti ssue slices derived from early postnatal rats. After a brief(12-60 h) culture period, slices were stained with a calcium-sensitive dye, Fluo -3. Fluorescence imaging of the Fluo-3-stained slices with a scanning laser confocal microscope afforded simultaneous observation of many ce lls at high spatial resolution. Time-lapse imaging revealed spontaneou s Ca2+ transients in the somata and dendrites of many pyramidal cells in areas CA1 and CA3. For the most part, Ca2+ activity in neighboring pyramidal cells appeared to be uncorrelated, although we occasionally observed synchronous Ca2+ transients in adjacent cells. The transients were blocked by both tetrodotoxin (l mu M) and a mixture of the gluta mate receptor antagonists, APV (50 mu M) + CNQX (10 mu M). Thus, spont aneous Ca2+ transients appear to be a consequence of activity-dependen t release of glutamate acting postsynaptically through ion-otropic glu tamate receptors. Although gamma-aminobutyric acid (GABA) is thought t o be an excitatory neurotransmitter during hippocampal development (Ch erubini et al., 1991, Trends Neurosci. 14(12):515-519), the spontaneou s Ca2+ transients were not blocked by the GABA, receptor antagonist pi crotoxin(100 mu M). Furthermore, application of GABA (50 mu M) abolish ed the spontaneous Ca2+ events, possibly via GABA, receptor-mediated i nhibition of postsynaptic cells. The present results join other recent observations suggesting that isolated neural tissues support spontane ous activity; although the patterns and mechanisms of the activity rep orted here appear to differ from those of previous studies. Difference s in the patterns of spontaneous activity during development mag contr ibute to variations in the functional organization of different region s of CNS tissue. (C) 1994 John Wiley and Sons, Inc.