Harris E. Hill

Reduction of Pain-Conditioned Anxiety by Analgesic Doses of Morphine in Rats

Summary 1. Rats, maintained at 70% of satiation weight, were conditioned to press a bar at a rapid and constant rate in a modified Skinner Box. After about 15 days of training when this behavior had been thoroughly established, a method for producing conditioned anxiety was introduced. Shortly after each animal began the daily bar-pressing session a 60-cycle tone, which sounded for 4 minutes, was terminated by the application of a strong: electrical shock. After several days of conditioning this procedure produced almost complete cessation of bar-pressing during the tone period. In testing the effect of a known analgesic the administration of graded doses of morphine (4-11 mg/kg) produced proportional restoration of the inhibited bar-pressing. 2. The reduction or elimination of inhibition by morphine was considered to be a reduction of anxiety associated with anticipation of noxious stimuli. The results parallel in all essential details previous methodological work on man, and strongly suggest that the present procedure may be useful as a technic for the screening of possible analgesic drugs.