Central cholinergic blockade and two-way avoidance acquisition: The role of response disinhibition

Abstract

The effects of scopolamine hydrobromide on two-way avoidance learning by rats were studied in various experimental conditions, selected so as to vary the extent to which response-suppressing tendencies interfere with active avoidance acquisition. Previous experiments had repeatedly shown an avoidance facilitation by central antimuscarinics in the following conditions: discrete-trial two-way test with non-directional light as CS, 5-sec CS-US intervals, 1.5 mA shocks, 30-sec intertrial intervals (ITI's), intertrial responses (ITR's) punished, absence of partition or barrier in the shuttle-box, and six separate 50-trial sessions at about 24-hr intervals. In the present series of tests, the control performance was markedly enhanced, and the scopolamine facilitation correspondingly attenuated, when ITR's were not punished and a central partition still absent, but not when ITR's were not punished and partition present. The drug effect was reversed, i.e. in the direction of a retardation of learning, when ITI's of 70 sec or more were used, and when all training (with 30-sec ITI's) was given in a single 300-trial session. Two experiments with different shock intensities, however, did not give reliable evidence of a drug × intensity interaction. These results give additional support to the hypothesis that response disinhibition after central cholinergic blockade can account for most, if not all the data on antimuscarinics and active avoidance acquisition, i.e. for the data obtained in experiments without change of state at some stage of training or testing.