Figure 2.

Experimental design. (A–C) Photomicrographs illustrating the location (arrows) of stimulating (A and C) and recording (B) sites. Calibration bar is 0.5 mm. (D) Bipolar stimulating (St.) electrodes were chronically implanted in the medial MS (left schematic drawing). As shown at the top-right diagram, animals were also implanted with stimulating and recording (Rec.) electrodes aimed to activate the CA3–CA1 synapses of the right hippocampus. Abbreviations: Cg: cingulate cortex; coll.: collaterals; CP: caudate-putamen; DG: dentate gyrus; gcc: genu of the corpus callosum; LSI: lateral septal nucleus, intermediate part; LV: lateral ventricle; NAcC: core of the accumbens nucleus; D, L, V: dorsal, lateral, and ventral. (E) In a first experimental step (test, T), we recorded the electrical activity of the hippocampal CA1 area, and input/output (i/o) curves and paired-pulse (pp) facilitation at the CA3–CA1 synapse in all of the animals. Animal's shaping session (shaping, Sh) consisted of: 1) a BL period 5 min long for recordings of fEPSPs evoked at the CA3–CA1 synapse with the animal located in a small box; 2) up to 10 shaping sessions (20 min each) in a Skinner box (SB), during which animals were presented with a train of stimuli to the MS followed 40 ms later by a single pulse applied to Schaffer collaterals every time the animals were located nearby the lever; and 3) a recovery (R) recording period 5 min long in the small box. Finally, the animals were allowed to self-stimulate (self-stimulation session, SS) when pressing the lever. For this, we used the same protocol as for animals' shaping, with a total of 7 self-stimulation sessions. Only 1 session (Sh or SS) per day was carried out. (F) A diagram summarizing all the experimental protocols.