Screening for Drug Reinforcement Using Intravenous Self-Administration in the Rat

The reinforcing activity of 31 psychoactive drugs was evaluated. Drugs were offered by intravenous self-administration to groups of naive rats. Reinforcement is indicated by an injection rate greater than that for rats offered only saline. Two similar protocols were used. In the first protocol, rats were offered drug at a selected dose for 5 days, then the dose was reduced by one log unit (to 0.1 times the initial dose) for an additional 4 days. In the second protocol, saline only was offered the first 3 days to eliminate rats with high or low operant rates, and the dose was reduced 0.5 log unit (to 0.32 times the initial dose) instead of one log unit. An empirical score, based on the injection rates during the last 3 days of each period, describes the reinforcing activity. Replicate tests using both protocols gave similar scores. Literature data for reinforcing activity in monkeys was available for 27 of the drugs tested. Results for rats and monkeys agreed for 24 of the 27 drugs. Nalorphine and ethylketazocine were reinforcers only in rats, and ethanol was a reinforcer only in monkeys.