Acquisition and performance effects of scopolamine and of treatment withdrawal in avoidance situations

Abstract

Two experiments were carried out in order to compare the effects of scopolamine on acquisition with those of changes of state in either direction (no drug to drug and vice versa). Two avoidance tasks the acquisition of which was known to be facilitated by small doses of tertiary antimuscarinics were used. The first study on continuous discriminated lever press avoidance, with either light onset or light offset as warning signal, showed a performance enhancement after the treatment of pretrained animals, and a performance decrement in animals trained in the drug state and then shifted to placebo. The data, however, also showed a partial carry-over of the scopolamine facilitation to the no drug condition, and a partial carry-over of the no drug retardation to the drug condition. The facilitation of acquisition (reduction of shock rate, enhancement of total response rate and of discriminated responses) and all other phenomena indicated above were more marked in the light off condition, which depressed acquisition and performance in untreated animals. The second experiment on discrete-trial two-way avoidance with light onset (non directional) as CS used a 2 × 2 × 2 × 2 design, combining the features of previous studies which had shown contrasting effects of changes of state, depending on various experimental conditions. A performance decrement after treatment withdrawal was again observed in all instances. However, this experiment confirmed that the performance effects of drug administration to pretrained animals depend both on the distribution of previous practice (single 250-trial session versus 5 sessions of 50 trials each) and on the interval between the end of training and the retest (1 day versus 14 days). These results and various literature data on antimuscarinics indicate that the contrasting effects of state changes cannot be explained by simple general models. In particular, these data exclude important state dependence phenomena in the case of these agents.