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.