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).