抄録
In the hippocampus, it has been reported that perforant path-granule cell synapses show frequency-dependent and reversible synaptic depression; however the precise mechanism has not been elucidated. Here, we analyzed this frequency-dependent depression electrophysiologically in terms of the release probability and the pool size of synaptic vesicles. We recorded field excitatory postsynaptic potentials (fEPSPs) from medial and lateral perforant path (MPP and LPP) synapses as well as from Schaffer collateral synapses (CA1 synapses) of mouse hippocampal slices. When the stimulus frequency was increased from 0.033 to 1 Hz, fEPSPs decreased at MPP and LPP synapses, whereas it increased at CA1 synapses. Thus, both perforant path synapses exhibited clear frequency-dependent depression. During the depression, the paired-pulse ratio increased at MPP synapses, while it decreased at LPP and CA1 synapses, suggesting that the release probability decreased at MPP synapses, whereas it increased at LPP and CA1 synapses. Higher-frequency stimulation (20 Hz, 10 stimuli) further decreased fEPSPs at MPP and LPP synapses, and in this condition, CA1 synapses also exhibited depression. We also estimated the size of the readily releasable pool of these synapses and found that the vesicle pools of MPP and LPP synapses are much smaller than those of CA1 synapses. These results suggest that the frequency-dependent depression at perforant path synapses is caused mainly by the depletion of synaptic vesicles in the pool. [J Physiol Sci. 2008;58 Suppl:S127]
