Extracellular recordings
Extracellular recordings were used to determine the synaptic properties of the MPP – DG synapse or the LPP – DG synapse acquired from the supra-pyramidal blade of the DG (see schematic representation in Figure 1a). The MPP field excitatory postsynaptic potentials (MPP fEPSPs) or the LPP fEPSPs were evoked with a bipolar nichrome electrode placed in the middle one-third or outer one-third section of the DG’s molecular layer, respectively. The evoked fEPSPs were recorded with a borosilicate pipette (1-2 MΩ of tip resistance, when filled with NaCl solution, 3M) and placed 200-300 µM from the stimulation electrode. The current pulses were delivered via a high-voltage isolation unit (A365D; World Precision Instruments, Sarasota, FL, USA), commanded with a Master-8 pulse generator (AMPI, Jerusalem, Israel). Responses were amplified with a Dagan BVC-700A amplifier (Minneapolis, MN, USA) connected with a 100x gain headstage (Dagan, model 8024) and high-pass filtered at 0.3 Hz. Electrical noise suppression was accomplished with a Humbug noise eliminator (Quest Scientific Instruments; North Vancouver, BC, Canada). The evoked fEPSPs were displayed on a computer-based oscilloscope and digitalized for storage and offline analysis with LabVIEW 7.1 software (National Instruments, Austin, TX, USA). The A/D converter device used was the BNC-2110 (National Instrument, Austin, TX).