Aspartate-induced changes in extracellular free calcium in 'in vitro' hippocampal slices of rats

Extracellular free calcium ([Ca2+]0) and neuronal activity were measured with double-barreled Ca2+-selective/reference microelectrodes in the CA1 field of rat hippocampal slices during stimulation of Schaffer collaterals and during iontophoretic application of the excitatory amino acid aspartate (Asp). Baseline [Ca2+]0 was 1.5 mM. Repetitive stimulation and aspartate application caused decreases in [Ca2+]0 (delta Ca), which were largest in the pyramidal cell layer. During iontophoretic application of GABA, aspartate often failed to induce significant alterations in [Ca2+]0. Mg2+ (9 mM) and Ni2+ (3mM) when added to the perfusion medium reduced aspartate-dependent delta Ca by 15 and 70% respectively. In tetrodotoxin (10(-6) M) containing Ringer the aspartate-induced delta Ca were about 10% smaller. These findings suggest that the decreases in [Ca2+]0 are mainly due to a postsynaptic Ca2+ entry through selective Ca2+ conductances.