A slice was transferred into the submerged recording chamber and superfused with ACSF at a constant rate of 2.5 mL/min at 32°C. Recording electrodes made of borosilicate glass capillaries (1B150F, World Precision Instruments, Sarasota, FL) were filled with ACSF (resistance ~0.5 MΩ). Monopolar stimulating electrodes were made of Pt/Ir wires of diameter 25.4 μm (PTT0110, World Precision Instruments, Sarasota, FL) and had 100-μm-long exposed tips. Both the stimulating and recording electrodes were inserted under visual control perpendicular to the slice surface into the middle molecular layer of dentate gyrus (DG) at a distance 250–300 μm from each other. The initial slope of the field excitatory postsynaptic potentials (fEPSP) was measured at latencies 0.1 – 0.9 ms. Testing stimuli (duration 100 μs, current 80 μA) evoked field responses with amplitudes of 70–80% of maximum. Long-term potentiation (LTP) was induced by tetanization with three trains of stimuli (1 sec at 100 Hz; 5 min between the trains). Short-term potentiation (STP) and LTP were evaluated at 1–15 min and 30–60 min after the tetanus respectively. Fluoxetine hydrochloride (Sigma-Aldrich, cat. # F-132) was dissolved
