On the experimental day, the reference electrode was connected to ground and a fresh carbon-fiber microelectrode was lowered into the NAc with a micromanipulator based on a previous design (Rebec et al., 1993; Garris et al., 1997). The micromanipulator was modified (Robinson, 2007) to fit tightly into the locking guide cannula; one rotation of the device moves the electrode 300 μm. Wires from the reference and stimulating electrodes and a head-stage amplifier connected to the carbon-fiber electrode were routed to a swivel (Med-Associates, St Albans, VT) which allowed free movement of the animal, and then to custom-built amplifiers (Chemistry Department Electronics Facility, UNC Chapel Hill, Chapel Hill, NC) and into a computer interface. The carbon-fiber electrode was lowered in 75 μm increments until it was in a site where electrical stimulation (125 μA biphasic pulses, 2 ms per phase, 60 Hz, 24 pulses) evoked dopamine release, establishing that the electrode was near functional dopaminergic terminals. Once the behavioral session started, data was collected in 60 s files.
