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

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Abstract
Twelve rats, maintained at 70% of satiation weight to insure strong hunger motivation were trained to press a bar at a constant rate in a modified Skinner box with aperiodic reinforcement by small food pellets. Later, superimposition of classical conditioning by introduction of a 4 minute steady tone terminated by a brief electric shock through the grid floor of the box resulted in inhibition of bar-pressing only for the duration of the conditional stimulus. Subcutaneous injections of morphine in doses of 4-14 mg/kg restored bar-pressing during the exhibition of the tone from a mean of 13.4% of previous rate under control conditions to 22-97% of previous rate under drug conditions. The effect was proportional to the dose of morphine up to 11 mg/kg. With higher doses, results were erratic, though a median value of 80% restoration was obtained. It is concluded that in the 4-11 mg/kg dose range, pain-anxiety is specifically reduced by morphine, and that the method may prove useful in the screening of analgesic drugs.

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