After placing the animal into a stereotaxic frame under pentobarbital anesthesia (50 mg/kg, i.p.), the retrograde tracer Fluorogold (0.1 µl) was injected bilaterally to diffuse into the VTA/RMTg region (coordinates: anterior, −6.7 mm; lateral, 0.6 mm; deep, 8 mm) using a 5 μl Hamilton syringe controlled by a syringe pump at a rate of 0.1 μl/3 min. At least 4 days were allowed for the dye to diffuse.