How do tranquilizing agents selectively inhibit conditioned avoidance responding?

A number of tranquilizing agents have been shown to inhibit conditioned avoidance responses (CAR) at doses that do not interfere with escape responses (ER). To test the hypothesis that this selective action may be due to differential response strengths of the two responses, rats were trained to press retractable levers in an operant chamber either to avoid a 0.5 mA shock during a 5.0-s warning period or to escape from a low-intensity shock within 5.0 s. The intensity of the latter shock was adjusted for each animal so that CAR and ER were comparable in terms of probability of occurrence and latency. While doses of chlorpromazine, clonidine, diazepam, and morphine that reduced CAR by 30%-50% did not significantly affect high-shock ER, i.e., ER on CAR trials where no CAR occurred, they interfered with low-shock ER to the same degree as CAR. These and other results suggest that the selective blockade of the CAR by these drugs in the CAR paradigm is primarily due to differential strengths of the CAR and ER. They also support studies concluding that tranquilizing drugs reduce avoidance because of a deficit in the ability to initiate motor responses, rather than interfering with associative processes or reducing situation-induced emotional reactions. However, the finding of a small differential effect, at least with chlorpromazine, on CAR and low-shock ER across trials within sessions indicated that different mechanisms may be involved in the suppression of these two responses.