Young mice (20–26 days old) were injected i.p. either with 10 mg/kg of morphine or saline. The brains were dissected out 20–30 h later and cut into slices, and whole-cell patch-clamp recordings of VTA dopamine neurons were conducted as previously described in [17]. In short, the horizontal slices containing the VTA were prepared in cutting solution containing (in mM): 60 NaCl, 2 KCl, 8 MgCl2, 0.3 CaCl2, 30 NaHCO3, 1.25 NaH2PO4, 140 sucrose, 10 D-glucose. Recordings were made in carbogen-bubbled artificial cerebrospinal fluid (ACSF, containing (in mM): 126 NaCl, 1.6 KCl, 1.2 NaH2PO4, 1.2 MgCl2, 2.5 CaCl2, 18 NaHCO3, and 11 D-glucose. ACSF was perfused over the slice at the rate of 2 ml/min. The drugs used in the recordings were diluted to the ACSF. Electrodes had a resistance of 3–5 MΩ when filled with internal solution containing (in mM): 130 cesium methanesulfonic acid, 10 HEPES, 0.5 EGTA, 8 NaCl, 5 QX314, 4 MgATP, 0.3 MgGTP, 10 BAPTA (pH 7.2–7.25), osmolarity 278±5 mOsm.
