Sagittal slices of the NAc shell and core (240 μm) were prepared as described previously (Thomas et al., 2001 (link)) from psychostimulant- and saline-treated mice (7-10 weeks of age). Shell recordings were made from medial shell slices that did not contain any dorsal striatal tissue (~0.44 - 0.52 mm lateral), while core recordings were made from slices between ~0.72 and 0.96 mm (see Figure 1B for additional details; (Paxinos and B. J. Franklin, 2001 )). Slices recovered in a holding chamber for at least 1 h before use. During recording they were superfused with ACSF (22–23°C) saturated with 95% O2 /5% CO2 and containing (in mM) 119 NaCl, 2.5 KCl, 1.0 NaH2PO4, 1.3 MgSO4, 2.5 CaCl2, 26.2 NaHCO3 and 11 glucose. Picrotoxin (100 μM) was added to block GABAA receptor-mediated IPSCs. Either a combination of CNQX (10 μM) and D-AP5 (50 μM) or kynurenic acid (2 mM) was used to block glutamatergic transmission during recording. Cells were visualized using infrared-differential interference contrast optics. Medium spiny neurons were identified by morphology and the presence of a hyperpolarized resting membrane potential (−75 to −85 mV). In a small number of cases (15 out of 477 cells), these identification characteristics yielded cells that showed the clear electrophysiological signature of fast-spiking interneurons (no inward rectification, short duration spikes and a fast, irregular firing pattern (Belleau and Warren, 2000 (link))). These cells were excluded from further investigation in this study.
To quantify firing properties, whole-cell current-clamp recordings were performed with electrodes (3–5 MΩ) containing 120 K-gluconate, 20 KCl, 10 HEPES, 0.2 EGTA, 2 MgCl2, 4 Na2ATP, 0.3 Tris–GTP. Data were filtered at 5 kHz, digitized at 10 kHz, and collected and analyzed using custom software (Igor Pro; Wavemetrics, Lake Oswego, OR). Membrane potentials were held at approximately −80 mV. Series resistances ranged from 10–18 MΩ and input resistances (Ri) were monitored on-line with a 40 pA, 150 ms current injection given before every 800 ms current injection stimulus. Only cells with a stable Ri(Δ < 10%) for the duration of the recording were kept for analysis. To measure “steady-state” voltage responses for each subthreshold pulse in a series of current injections, the voltage values were taken 780 ms following the onset of each current injection. Spike measurements for a given cell are the mean values measured from 1 to 3 cycles of current steps (800 msec duration at 0.1 Hz, −160 to +260 pA range with a 20 pA step increment). To quantify firing patterns, we measured the first action potential latency, the train duration and the mean inter-spike interval at three representative current injection values (160, 200 and 240 pA), except as noted in the text.
To quantify firing properties, whole-cell current-clamp recordings were performed with electrodes (3–5 MΩ) containing 120 K-gluconate, 20 KCl, 10 HEPES, 0.2 EGTA, 2 MgCl2, 4 Na2ATP, 0.3 Tris–GTP. Data were filtered at 5 kHz, digitized at 10 kHz, and collected and analyzed using custom software (Igor Pro; Wavemetrics, Lake Oswego, OR). Membrane potentials were held at approximately −80 mV. Series resistances ranged from 10–18 MΩ and input resistances (Ri) were monitored on-line with a 40 pA, 150 ms current injection given before every 800 ms current injection stimulus. Only cells with a stable Ri(Δ < 10%) for the duration of the recording were kept for analysis. To measure “steady-state” voltage responses for each subthreshold pulse in a series of current injections, the voltage values were taken 780 ms following the onset of each current injection. Spike measurements for a given cell are the mean values measured from 1 to 3 cycles of current steps (800 msec duration at 0.1 Hz, −160 to +260 pA range with a 20 pA step increment). To quantify firing patterns, we measured the first action potential latency, the train duration and the mean inter-spike interval at three representative current injection values (160, 200 and 240 pA), except as noted in the text.
