The effects of chronic morphine administration upon a modified Sidman avoidance schedule that utilizes response-independent shock.

Six male squirrel monkeys were subjected to a Sidman nondiscriminated avoidance schedule (R-S, S-S-20 sec) that superimposed 3-min conditioned stimuli (CS) unavoidable shock pairings upon the ongoing avoidance behavior. Five of the 6 animals demonstrated facilitated avoidance response rates during the CS, while one animal demonstrated suppressed rates during the CS. Morphine sulfate (0.5, 1.0, 2.0, 3.0, 4.0 mg/kg) altered these patterns, causing the suppressor to facilitate, while the facilitators demonstrated reduced levels of response elevation during the aversive CS. Morphine also led to a reliable reduction in overall response rate and an increase in the number of shocks received. No consistent drug effects were noted with regard to general motor activity. These results were interpreted to suggest that a potent analgesic agent, such as morphine, was able to reduce the level of fear motivation normally generated by the aversive CS. Since changes in relative rate during the aversive CS were quite reliable both within and between animals, it was suggested that this behavioral schedule might prove useful in assaying the fear-reducing qualities of a variety of drugs. A cautionary note, however, indicated that other explanations, most notably, a rate dependent hypothesis, could account for the data without assuming the level of fear was altered.