Effects of single and repeated daily injections of morphine, clonidine, and l-nantradol on avoidance responding of rats.

Interruption of a photobeam by rats was maintained under a continuous shock avoidance schedule, and moderate response rates were maintained at low shock frequencies. Responding decreased, and shock frequency increased, in a dose-dependent manner after acute injections of the narcotic morphine, the antihypertensive l-nantradol clonidine, and the cannabinoid l-nantradol. Clonidine and l-nantradol were about 100 times more potent than morphine for decreasing overall responding, and l-nantradol was about 3 times more potent than clonidine for decreasing escape responding. When drugs were given repeatedly prior to daily experimental sessions, tolerance developed to response rate decreases of morphine and l-nantradol within seven to ten sessions, but tolerance did not develop to rate decreases of clonidine for up to 30 sessions. Continued decreased responding by clonidine was antagonized by yohimbine, but not by prazosin or naltrexone. These results extend observations for the acute effects of l-nantradol and clonidine to operant responding under a schedule of continuous shock avoidance. Different potencies for drugs in the present and previous experiments suggest important effects of response topography on dose effects.