FUNCTIONAL-RELATIONSHIPS, PREVIOUS HISTORY AND THE DISCRIMINATION OF A DRUG MIXTURE IN RATS

These experiments aim to probe the role of different functional relati onships between drugs, responses and reinforcers in studies on the dis crimination of drug mixtures. Two-lever discriminations based on mixtu res of (+)-amphetamine (0.4 mg/kg) plus pentobarbitone (10 mg/kg) have been compared in three groups of rats (n = 8) trained to discriminate : (i) the mixture from saline, called the AND-discrimination; (ii) eit her the mixture, amphetamine or pentobarbitone from saline, the OR-dis crimination; (iii) the mixture from either amphetamine or pentobarbito ne, the AND-OR-discrimination. The rats were trained in a two-bar oper ant conditioning procedure with a tandem schedule of food reinforcemen t. The rats trained on the AND procedure acquired the discrimination m ore rapidly than the rats trained on the other two procedures, but aft er 60 training sessions, all discriminations were performed with simil ar (90-94%) accuracy. In rats trained under the AND and the OR procedu res, there was full generalization from the mixture to the largest dos es used of either amphetamine or pentobarbitone. In contrast, in rats trained under the AND-OR procedure, there was no generalization from t he mixture to any dose of either drug separately. The training procedu res for the OR and the AND-OR groups were changed to AND-discriminatio n training; the effects of the different previous histories were found to persist for a prolonged period, despite the fact that training con ditions were now the same in all groups. The effects of the drugs on r ates of responding were also influenced by the previous training histo ry. Thus, using different training procedures to alter functional rela tionships can substantially and persistently influence the major chara cteristics of a discrimination based on a mixture of drugs.