Individual differences in the reinforcing and punishing effects of nicotine in rhesus monkeys

Rationale The relatively weak reinforcing effects of nicotine in experimental studies have been attributed to possible aversive effects or the need to space nicotine administrations over time to expose reinforcing effects. Objective This study was designed to determine if the response-maintaining effects of nicotine are increased when availability is spaced through time, and whether nicotine is an effective punisher of remifentanil-maintained responding. Methods Compared to a cocaine reference dose, nicotine dose and timeout (TO) value were varied in eight rhesus monkeys responding for intravenous (i.v.) nicotine on varying fixed-ratio (FR) schedules of reinforcement. The aversive effects of nicotine were evaluated in four animals choosing between a standard dose of remifentanil alone or in combination with one of several doses of nicotine. Results In three of eight self-administration monkeys, 0.01 mg/kg/inj nicotine did not maintain responding at any FR value. In the other five animals, nicotine-maintained response rates increased with either FR or TO values to a certain point, and then slowed. Maximum nicotine-maintained response rates were much slower than those maintained by cocaine, and demand for nicotine was less than demand for cocaine. Nicotine was an effective punisher of remifentanil-maintained responding at doses ranging from 0.01 to 0.3 mg/kg/inj. Lower punishing dose seemed to be related to the absence of reinforcing effects within subject. Conclusion There are an order of magnitude individual differences in sensitivity to both the reinforcing and punishing effects of nicotine, and this drug may be unique in being a weak positive reinforcer in small doses and aversive in large doses.