ORALLY SELF-ADMINISTERED COCAINE IN RHESUS-MONKEYS - TRANSITION FROM NEGATIVE OR NEUTRAL BEHAVIORAL-EFFECTS TO POSITIVE REINFORCING EFFECTS

The establishment of orally delivered cocaine as a reinforcer was exam ined with nine rhesus monkeys. A 2% ethanol solution served as a reinf orcer for all nine monkeys, for it maintained substantially higher res ponse rates than did the concurrently available water vehicle. A test was initially conducted to determine whether cocaine would function as a reinforcer when substituted for 2% ethanol. When an intermediate co caine concentration (0.2 mg/ml) was substituted for the ethanol soluti on, the drug maintained behavior at rates less than (seven monkeys), e qual to (one monkey), or greater than (one monkey) those maintained by water. Thus, for eight of nine monkeys simple substitution of cocaine for ethanol was not sufficient to establish orally delivered cocaine as a reinforcer. In the next phase a stimulus-fading procedure was use d. Blocks of training and testing sessions alternated. Across blocks o f training sessions, increasing amounts of cocaine (0.0125, 0.025, 0.0 5, 0.1 mg/ml) were added to the 2% ethanol solution and subsequently t he ethanol concentration was gradually decreased until only the 0.1 mg /ml cocaine solution remained; water was always concurrently available . Between each block of training sessions, a block of test sessions wa s inserted. Test sessions compared relative rates of responding mainta ined by two concurrently available drug solutions: (1) a solution cont aining the combination of ethanol and cocaine used in the prior traini ng block and (2) a solution containing the same concentration of ethan ol but with no cocaine. Thus, differences in rates of behavior maintai ned by the two solutions could be attributed to the presence of cocain e and the existence and degree of any such differences could be monito red at each step in the acquisition procedure. The outcome of the trai ning procedure was that cocaine came to function as a reinforcer for s ix of the eight monkeys tested (the ninth monkey was not put through t he fading procedure, having shown higher cocaine than vehicle rates du ring the initial substitution procedure). During the phase when ethano l was faded from the drug solution, differences between the combinatio n cocaine-ethanol solution and the ethanol-only solution emerged: for the six monkeys that developed cocaine reinforced behavior, the combin ation solution maintained higher rates of responding than the ethanol solution alone. The opposite results were obtained with the remaining two monkeys. That cocaine had been established as a reinforcer was con firmed by persistent and orderly responding when dose and fixed-ratio size were subsequently varied.