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).