Studies involving VTA DA neurons were performed at 32-34°C, with borosilicate (3-5 MΩ) electrodes containing (in mM): 140 K-gluconate, 2 MgCl2, 1.1 EGTA, 5 HEPES, 2 Na-ATP, 0.3 Na3GTP, and 5 phosphocreatine, pH 7.4. For sIPSC measurements, KCl replaced K-gluconate in the pipette solution. Upon achieving whole-cell access, the current response to a 1-s voltage-ramp (−60 to −120 mV) was measured to assess Ih presence and size. Following a 5-min stabilization period, spontaneous action potential frequency and duration were measured in whole-cell current clamp mode (I=0). Cells exhibiting relatively large capacitance (>50 pF), Ih current (>100 pA at −120 mV), moderate and stable spontaneous activity (1-5 Hz), and long action potential durations (>2.5 s) were considered to be putative DA neurons. Indeed, these properties correlate well with expression of tyrosine hydroxylase and Pitx3, selective markers of DA neurons (Grace and Onn 1989; Johnson and North 1992; Momiyama et al. 1996; Cameron et al. 1997; Klink et al. 2001; Neuhoff et al. 2002; Mathon et al. 2005; Labouebe et al. 2007).
