Cocaine discrimination in a choice self-administration procedure in rhesus monkeys

The discrimination of cocaine in a choice intravenous self-administration procedure in six rhesus monkeys was investigated using several techniques. The first technique allowed the monkey a choice of saline and cocaine simultaneously. The only cue available to differentiate one solution from the other was the introceptive stimuli provided by cocaine. In this preparation, no monkeys discriminated one solution from the other. The second technique allowed six monkeys access to both solutions successively in 15 minute alternating periods both with and without secondary light cues. One of the monkeys could make a weak discrimination without light cues, but the other two in this condition did not develop a discrimination. The three monkeys with light cues all developed and maintained a strong discrimination and chose cocaine over saline. All monkeys were then returned to the simultaneous availability of both solutions with each group retaining their discriminative cue conditions. Monkeys without light cues could not develop or maintain discrimination. Monkeys with secondary light cues maintained and improved their discrimination. When the light cues were removed from the schedule, discrimination was disrupted and not regained indicating the monkeys were largely if not completely dependent upon the light for developing the discrimination. The cocaine infusion dosage was doubled for one group of animals to determine the effect of cocaine dosage on discrimination. This group had cocaine and saline available simultaneously without secondary discriminative cues. This manipulation did not aid in discrimination but rather resulted in approximately a 50% drop in infusions for both solutions. Finally, three monkeys were exposed to a simultaneous presentation of both drugs with intermittent light cues. This was an attempt to sensitize the monkey to the dis- criminable properties of the cocaine by gradually decreasing the duration of the light interval. This manipulation did not aid in discrimination. When the three animals were retested in the original condition of simultaneous availability without light cues, discrimination did not develop. These results indicate that rhesus monkeys can and will discriminate between two solutions simultaneously available for self-administration if the animal has external cues to aid in discrimination. This technique offers new advantages to self-administration investigation. This procedure represents a model which more closely approximates human drug abuse.