• addiction;
  • associative learning;
  • conditioning;
  • electrophysiology;
  • reward


Environmental stimuli predictive of appetitive events can elicit Pavlovian approach responses that enhance an organism's ability to track and secure natural rewards, but may also contribute to the compulsive nature of drug addiction. Here, we examined the activity of individual nucleus accumbens (NAc) neurons during an autoshaping paradigm. One conditioned stimulus (CS+, a retractable lever presented for 10 s) was immediately followed by the delivery of a 45-mg sucrose pellet to a food receptacle, while another stimulus (CS–, a separate retractable lever presented for 10 s) was never followed by sucrose. Approach responses directed at the CS+ and CS– were recorded as lever presses and had no experimental consequence. Rats (n = 9) selectively approached the CS+ on more than 80% of trials and were surgically prepared for electrophysiological recording. Of 76 NAc neurons, 57 cells (75%) exhibited increases and/or decreases in firing rate (i.e. termed ‘phasically active’) during the CS+ presentation and corresponding approach response. Forty-seven percent of phasically active cells (27 out of 57) were characterized by time-locked but transient increases in cell firing, while 53% (30 out of 57) showed a significant reduction in firing for the duration of the CS+. In contrast, the same excitatory subpopulation exhibited smaller increases in activity relative to CS– onset, while the inhibitory subpopulation showed no change in firing during the CS– period. The magnitude and prevalence of cue-related neural responses reported here indicates that the NAc encodes biologically significant, repetitive approach responses that may model the compulsive nature of drug addiction in humans.