Paired-pulse ratio of synaptically induced transporter currents at hippocampal CA1 synapses is not related to release probability

When a synapse is stimulated in rapid succession, the second post-synaptic response can be larger than the first and termed paired-pulse facilitation. It has been reported that the paired-pulse ratio (PPR), which is the ratio of the amplitude of the second response to that of the first, depends on the probability of vesicular release at the synapse, and PPR has been used as an easy measure of the release probability. To re-examine the relation of PPR with transmitter release probability, we made whole-cell recordings from astrocytes and pyramidal neurons in the CA1 area of rat hippocampal slices, and studied responses evoked by paired-pulse stimulus of the Schaffer collaterals. In a control condition in which blockers for ionotropic glutamate receptors were added to the artificial cerebrospinal fluid, synaptically induced transporter currents (STCs) recorded from astrocytes showed PPF with similar dependency on stimulus interval as the AMPA-receptor-mediated excitatory post-synaptic currents (AMPA-EPSCs) recorded from pyramidal neurons. When the transmitter release was enhanced by raising Ca2+ concentration in the bathing medium or by applying 8-CPT, an adenosine A1 receptor antagonist, the PPR of the neuronal AMPA-EPSCs decreased significantly. In the same condition, although the amplitude of STCs was significantly increased, the PPR of STCs did not show significant change. The PPR of AMPA-EPSCs, however, recovered by lowering the stimulus intensity or by applying low concentration of NBQX, a competitive antagonist for AMPA-receptor. These results imply that the PPR of transmitter release at Schaffer collateral synapses stays constant as the release probability was altered.