Effects of PREGS, a putative neurosteroid, on the transmission of perforant path-granule cell synapses were investigated by the optical recording technique in the rat hippocampal slices stained voltage-sensitive dyes. Application of PREGS to the bath solution resulted in an acute augmentation of postsynaptic EPSP in a dose-dependent manner. The PREGS effect was dependent on extracellular Ca²⁺ concentration ([Ca²⁺]_o), whereas was independent of NMDA receptor activation. PREGS also potentiated paired-pulse facilitation (PPF), implying that PREGS positively modulates presynaptic neurotransmitter releases. Direct support for this mechanism was that PREGS augmented the synaptically induced glial depolarization (SIGD) that reflects the activity of electrogenic glutamate transporters in glial cells during the uptake of released glutamate. The selective 7nAChR antagonist -BGT or MLA prevented the PREGS-effect on SIGD. Furthermore DMXB, a selective 7nAChR agonist, mimicked the PREGS-effect on SIGD and antagonized the effect of PREGS. The presynaptic effect of PREGS was partially attenuated by the L-type Ca²⁺ channel (VGCC) blocker, nifedipine. Taken together we propose a novel mechanism underlying facilitated synaptic transmission by PREGS; this neurosteroid sensitizes presynaptic 7nAChR followed by an activation of L-type VGCC to increase glutamate release.