2.13 Electrophysiological recordings
NAc slices were prepared from C57BL/6J mice as previously reports with some modifications(M. Li et al., 2018). Briefly, brains were rapidly removed and Sagittal plane brain slices (400 um thickness) containing NAc were cut using a vibrating blade microtome in ice-cold artificial cerebrospinal fluid (ACSF) containing (mM) 119 NaCl, 3.5 KCl, 1.3 MgSO4, 2.5 CaCl2, 1 NaH2PO4, 26.2 NaHCO3 and 11 glucose that was bubbled continuously with 95%O2–5%CO2 to adjust pH to 7.4. After 2 h of recovery at 28°C, an individual slice was transferred to a submerged recording chamber superfused with oxygenated ACSF at 30°C at a rate of 3–4 ml·min−1 . Prefrontal cortex-accumbal afferents were stimulated by delivering stimuli through a bipolar stimulating electrode implanted into the PFC near the PFC–NAc border 0.5–3 mm dorsal to the recording electrode placed in the NAc, as we reported(M. Li et al., 2018). Field excitatory postsynaptic potentials (fEPSPs) were recorded by a microelectrode filled with 3 M NaCl and the test frequency to evoke fEPSPs was 0.03 Hz. Stimulation intensities were chosen to produce a fEPSP with a slope that was 60% of that obtained with maximal stimulation. Input-output (I/O) relationship for synaptic transmission was recorded by varying the intensity of the single-pulse stimulation. Paired stimuli (25, 50, 75, 100 ms interval) were delivered, and the paired-pulse ratio (PPR) was calculated as the ratio between the mean slope of the second fEPSPs over the first fEPSPs. The initial slope of the fEPSPs was measured and expressed as a percentage change from the baseline fEPSPs level, calculated from the average of the last 20 min of the baseline recording period. LTD was induced by the following protocol: NMDAR-dependent LTD in the NAc was induced by a stimulating protocol that consisted of one train of stimulus at 1 Hz (15 min) after 10 min of stable baseline recording. The magnitude of NMDAR-dependent LTD was calculated from fEPSPs recorded after 40 min of LTD induction as the percentage of the baseline EPSP slopes(B. Zhang et al., 2017).