Morphine and shuttlebox self-stimulation in the rat: tolerance studies

Rats were trained to turn lateral hypothalamic electrical brain stimulation on and off by crossing back and forth in a shuttlebox. Injections of 10 mg/kg morphine doubled the amount of time animals left the stimulation ON without altering OFF times. Tolerance did not develop to this action during 5 daily trials. Injections of 20 mg/kg produced a 4- to 5-fold increase in average ON times together with a large increase in average OFF times. Tolerance did not develop to the ON time increase over 5 daily injections, but did partially develop to the OFF time increase. The ON time increase appears to be based on a mechanism separate from the analgesic action of morphine and from the OFF time increase. Differentiable neurological structures and receptor systems may mediate these actions.