1. ¹Department of Psychology, University of Pennsylvania, Philadelphia, PA, USA
2. ²Neuroscience Graduate Group, University of Pennsylvania Medical School, Philadelphia, PA, USA
3. ³Department of Psychiatry, University of Pennsylvania Medical School, Philadelphia, PA, USA
4. ⁴Department of Physiology, University of California, San Francisco, CA, USA

Correspondence: Dr LL Peoples, Department of Psychology, University of Pennsylvania, 3720 Walnut Street, Philadelphia, PA 19106, USA. Tel: +1 215 898 4253; Fax: +1 215 898 7301; E-mail: lpeoples@psych.upenn.edu

Received 17 March 2006; Revised 6 July 2006; Accepted 27 July 2006; Published online 4 October 2006.

Abstract

Hypoactivity of the accumbens is induced by repeated cocaine exposure and is hypothesized to play a role in cocaine addiction. However, it is difficult to understand how a general hypoactivity of the accumbens, which facilitates multiple types of motivated behaviors, could contribute to the selective increase in drug-directed behavior that defines addiction. Electrophysiological recordings, made during sessions in which rats self-administer cocaine, show that most accumbal neurons that encode events related to drug-directed behavior achieve and maintain higher firing rates during the period of cocaine exposure (Task-Activated neurons) than do other accumbal neurons (Task-Non-Activated neurons). We have hypothesized that this difference in activity makes the neurons that facilitate drug-directed behavior less susceptible than other neurons to the chronic inhibitory effects of cocaine. A sparing of neurons that facilitate drug-directed behavior from chronic hypoactivity might lead to a relative increase in the transmission of neuronal signals that facilitate drug-directed behavior through accumbal circuits and thereby contribute to changes in behavior that characterize addiction (ie differential inhibition hypothesis). A prediction of the hypothesis is that neurons that are activated in relation to task events during cocaine self-administration sessions will show less of a decrease in firing across repeated self-administration sessions than will other neurons. To test this prediction, rats were exposed to 30 daily (6 h/day) cocaine self-administration sessions. Chronic extracellular recordings of single accumbal neurons were made during the second to third session and the 30th session. Between-session comparisons showed that decreases in firing were exhibited by Task-Non-Activated, but not by Task-Activated, neurons. During the day 30 session, the magnitude of the difference in firing rate between the two groups of neurons was positively related to the propensity of animals to seek and take cocaine. The findings of the present study are consistent with a basic prediction of the differential inhibition hypothesis and may be relevant to understanding cocaine addiction.