Calcium-dependent changes of paired-pulse modulation at single GABAergic synapses

Whole-cell patch-clamp and local electrical stimulation were used for measuring of monosynaptic GABAergic currents from rat hippocampal neurons in culture. Under control conditions (normal extracellular calcium, 2 mM) paired-pulse depression with 150 ms interpulse interval was observed. The mean current amplitude for both 1st and 2nd IPSCs displayed bell-shaped dependency from the stimulus amplitude. When extracellular Ca was either decreased to 0.5 mM or increased to 5 mM both mean amplitude of IPSCs and mean release probability decreased/increased correspondingly yet their dependences from the stimulus strength remained bell-shaped. Mean paired-pulse ratio was also affected—we observed facilitation of second IPSC in low-calcium whereas the latter was depressed in high-calcium solution. Our results suggest that calcium concentrations not only regulate the strength of paired-pulse plasticity but also can invert its direction.