Progressive and lasting amplification of accumbal nicotine-seeking neural signals.

Although neuroadaptations in the nucleus accumbens (NAc) are thought to contribute to nicotine addiction, little is known about the chronic effects of nicotine on NAc neuronal activity. In the present experiment, rats were exposed to a 23 d period of nicotine self-administration (SA), a 30 d abstinence period, and a 7 d period of reexposure to SA. Chronic electrophysiological procedures were used to record the activity of individual NAc neurons on the 3rd and 23rd days of initial SA and on the 1st, 3rd, and 7th days of reexposure. Between-session comparisons showed that NAc neurons exhibit two patterns of plasticity under the present experimental conditions. First, phasic-increase firing patterns time-locked to the nicotine-reinforced lever press do not change during initial SA, but then show increases in prevalence and amplitude after abstinence, which persist during reexposure. Second, for neurons that show no phasic response time-locked to the nicotine-reinforced lever press, average baseline and SA firing rates decrease during initial SA, return to normal during abstinence, and decrease again during reexposure. As a combined consequence of the two types of neurophysiological plasticity, average firing rate of NAc neurons at the time of nicotine-directed behavior undergoes a progressive and persistent net amplification, across the successive stages of SA, abstinence, and reexposure. This net increase in NAc firing at the time of nicotine-directed behavior occurs in association with an increase in animals' motivation to seek nicotine. The adaptations that occur in nicotine-exposed animals do not occur in animals exposed to sucrose. The NAc neurophysiological plasticity potentially contributes to compulsive tobacco use.