The electrode was placed above the VTA from a range of 0.48 to 0.55 mm lateral to the midline and from 3.08 to 3.3 mm caudal to the bregma, and lowered down into the VTA from 3.9 mm to 5 mm ventral to the cortical surface using a microdrive (model 650; David Kopf Instruments, Tujunga, CA).