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.
D000818:Animals; D004361:Drug Tolerance; D004558:Electric Stimulation; D005260:Female; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D010424:Pentobarbital / Q000494:pharmacology; D051381:Rats; D012653:Self Stimulation / Q000187:drug effects*; D013997:Time Factors
D009020:Morphine; D010424:Pentobarbital