It has been well-documented that a subset of NAc neurons (approximately 30%) exhibit patterned discharges (i.e., increases and/or decreases in cell firing) relative to the lever press response for intravenous cocaine (Carelli, 2000). An important issue addressed in the present study was to determine whether locations at which phasic changes in dopamine release occur are the same locations at which NAc neurons exhibit patterned discharges. To this end, rapid dopamine release and NAc cell firing was simultaneously measured in another set of animals (n = 23) at discrete locations within the NAc core and shell during cocaine self-administration. In these experiments, the carbon-fiber electrode was inserted into the desired region of the NAc, and its position was adjusted until extracellular activity of a single neuron was detected. At each of these locations electrical stimulation of the VTA was applied before the session to ensure the electrode was in the midst of dopamine terminals. In these experiments, FSCV had 200 ms resolution to allow sufficient time for single unit collection.
