Contribution of presynaptic Ca2+ entry to stimulus induced decreases of extracellular free calcium in “in vitro” hippocampal slices of rats

Abstract

Reductions of extraeellular free calcium ~a 2+] may increase neuronalexeitabili~y, but depress synaptic transfer. Therefore, changes xn ~a "] and field potentials were measured with ion-selective electrodes in area Ca ] of the rat hippoeampal slice. Field potential responses to weak stimulation of Schaffer G~llaterals became multipeaked when the baseline of ~a ~ ] was lowered from 1,6 mM to 0,8 mM. During a further reduction to 0,2 mM evoked responses became smaller and finally disappeared. Trains of 10 see and 15 Hz induced often stepwxse decreases xn ~a by up to 0,5 mM which were largest in the stratum pyramldale. The phases of accelerated decay correlated with the appe~rence of additional population spikes. When baseline ~a ~ was lowered from 1,6 mM to 0,2 mM[Ca2~ksig nals reversibly decreased. However, significant reductions of 0,01 mM could still be recorded when the synaptic transfer was blocked, as judged from field potential and intraeellular recordings. These decreases were largest in the stratum radiatum. High intensity stimulation resulted in a partial recovery of synaptie transfer. This method is a good tool to investigate the action of transmitters and drugs on presynaptic calcium entry. Thus we were able to demonstrate that GABA and adenosine impaired presynaptic ~a~ entry.