In rats, acute morphine dependence results in antagonist-induced response suppression of intracranial self-stimulation.
BACKGROUND
Lower (0.001-1.0 mg/kg) doses of the opioid antagonist naltrexone produce few behavioral effects in otherwise drug-free rats responding for ICSS, but reduce response rates by up to 75% after a single dose of morphine.


OBJECTIVE
The present study represents an effort to verify that other opioid antagonists produce this acute opioid dependence effect, and to characterize their relative pharmacological profiles.


METHODS
We implanted bipolar electrodes in the lateral hypothalamus of adult male rats, and then trained them to lever-press on an "autotitration" ICSS schedule, where responding on a "reset" lever allows the rat to control the frequency of stimulation; performance stabilized at approximately 1.5 responses/s.


RESULTS
During twice-weekly test sessions, cumulative doses of five of seven opioid antagonists produced significant response rate decreases (30-80%) in saline-pretreated rats; nalorphine (ED25=15.6 mg/kg)> naltrexone (ED25=13.1 mg/kg)>naloxone (ED25=7.3 mg/kg)>levallorphan (ED25=13.96 mg/kg)>(-)cyclazocine (ED25=0.028 mg/kg). A single MOR pretreatment (10 mg/kg, 4 h) significantly enhanced the rate-decreasing effects of six of the seven agonists tested; by 10-fold (-) cyclazocine>13-fold (nalorphine)>93-fold (levallorphan)>972-fold (naloxone)>2190-fold (naltrexone). The pure non-selective antagonist diprenorphine potently decreased rates after MOR pretreatment (ED25= 0.01 mg/kg), but did not after saline pretreatment. The mixed opioid agonist-antagonist drug nalbuphine (1.0-30 mg/kg) did not affect responding after either saline or MOR.


CONCLUSIONS
Antagonists with a high affinity for, and a lack of intrinsic activity at, the micro-opioid receptor precipitate the greatest behavioral changes in rats acutely dependent on MOR.
Easterling
Keith W
KW
Department of Pharmacology, School of Medicine, Emory University, Atlanta, GA, USA. keaster@emory.edu
Holtzman
Stephen G
SG
D000208:Acute Disease; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects*; D001921:Brain / Q000187:drug effects* / Q000503:physiopathology; D004558:Electric Stimulation; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009021:Morphine Dependence / Q000503:physiopathology* / Q000523:psychology; D009292:Narcotic Antagonists / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000819:agonists* / Q000037:antagonists & inhibitors*; D012054:Reinforcement, Psychology
D009292:Narcotic Antagonists; D017450:Receptors, Opioid, mu; D009020:Morphine