Presynaptic calcium transients evoked by paired-pulse stimulation in the hippocampal slice.

The fluorescent dye Calcium Green and optical recording techniques were used to record intracellular Ca2+ transients resulting from paired-pulse stimulation in stratum moleculare of area CA1 in guinea-pig hippocampal slices. Presumed presynaptic calcium transients were recorded while glutamatergic synaptic transmission was blocked by kynurenic acid. Peak responses to paired-pulses (10-160 ms interval) were higher than responses to single pulses of same stimulation strength (42-23% increase). The isolated response to the second pulse, however, was of smaller magnitude in comparison to the first one; the difference in magnitude depended on the interstimulus interval. Thus, the residual presynaptic free calcium concentration may be responsible for paired-pulse facilitation of synaptic transmission in hippocampus. At the same time, a use-dependent inactivation of presynaptic calcium channels may occur.