Recording Field Excitatory Post-synaptic Potentials (fEPSP)

Field excitatory postsynaptic potentials (fEPSPs) were recorded through an extracellular microelectrode (2–8 MΩ resistance, filled with a 4 M NaCl solution) placed in stratum radiatum of Cornus Ammonis 1 (CA1). Stimulation (rectangular 0.1 milliseconds pulses), was delivered once every 15 s through a bipolar concentric wire electrode positioned in the Schaffer collaterals-commissural fibers, in the stratum radiatum near the CA3–CA1 border. The intensity of stimulus (80–200 μA intensity) was adjusted to obtain a large fEPSP with a minimum population spike contamination. To avoid supramaximal stimulation, the stimulus intensity was also adjusted to obtain a fEPSP slope within 50–80% of its maximum value under supramaximally stimulating conditions. When using inhibitors (modifier drug) of the NOS/sGC/PKG pathway, whenever the effect of the inhibitor alone produced an increase of the fEPSP slope higher than 80% of its maximum value, after the effect of the antagonist stabilized and prior to the second application of CPA (test drug), the intensity of the stimulus was also adjusted to obtain a baseline roughly identical to that obtained before the first application of CPA. Extracellular recordings were obtained with an Axoclamp 2B amplifier and digitized (using a National Instruments BNC 2120 interface at a sample interval of 50 μs (20 kHz)). Individual fEPSPs were monitored, and averages of eight consecutive responses were recorded and analyzed through the LTP 230d software (Anderson and Collingridge, 2001). Responses were quantified as the slope of the initial phase of the averaged fEPSPs, since slope measures are considered a more accurate measure of fEPSP magnitude than the amplitude, due to eventual contamination by the population spike.