BACKGROUND:Drug addiction involves two main central nervous systems,namely the dopamine and noradrenaline systems.These systems are primarily distributed in five brain regions:the ventrai tegmental area,the nucleus accumbens,the prefrontal coaex,the hippocampus,and the locus coeruleus.OBJECTIVE:To investigate regional changes of guanine nucleotide binding protein-inhabitant 2(Gi2)in dopaminergic and noradrenergic neurons in brains of morphine-tolerant and-dependent rats.DESIGN,TIME,AND SETTING:A randomized centrel study was performed at the Department of Neurobiology in the Second Military Medical University of Chinese PLA(Shanghai,China) between September 2002 and March 2004.MATERIALS:Thirty-six,healthy, male, Sprague-Dawley (SD) rats were used to establish morphine-dependent models.Morphine hydrochloride was a product of Shenyang First Pharmaceutical Factory (China);naloxone hydrochloride was a product of Beijing Four-Ring Pharmaceutical Factory (China);and α subunit of Gt2 antibody was offered by Santa Cruz Biotechnology,Inc(USA).METHODS:Thirty-six SD rats were randomly divided into six groups(n=6):(1)acute morphine-dependent group,(2)acute abstinent group,(3)acute control group,(4)chronic morphine-dependent group,(5)chronic abstinent group,and(6)chronic control group.Rats in the acute morphine-dependent and the acute groups were injected with morphine(5 mg/kg),one injection every two hours,for a total of eight injections.In the acute and chronic morphine-dependent rat models,morphine withdrawal syndrome was precipitated by an injection of naloxone (5 mg/kg).Rats in the acute control group were given a peritoneal iniection of physiological saline at the same administration time as the above two groups.Rats in the chronic morphine-dependent and chronic abstinent groups were injected with morphine three times per day.The administration dose on day 1 was initially 5 mg/kg at 20:00,which increased by 5 mh/kg at 8:00,12:00,and 20:00 until day 7.On day 13,the dose continuously increased bv 10 mg/kg until a chronic morphine-dependent rat model was successfully induced.Aflerwards.the rats presented with withdrawal syndromes on naloxone (5 mg/kg)at 8:00 on the same day.Rats in the chronic centrel group were injected with physiological saline at the same time of the two chronic groups.MAIN OUTCOME MEASURES:The concentration of Gl2 protein in the five brain regions(ventral tegmental area,nucleus accumbens,prefrontal cortex,locus coeruleus,and hippocampus)was detected by immunohistochemistry.RESULTS:In the acute morphine-dependent and acute abstinent groups,Gl2 protein concentration was significantly decreased in the nucleus accumbens,compared to the acute control group(P<0.01),while no obvious changes were detected in other brain regions.In the chronic morphine-dependent and chronic abstinent groups,Gl2 protein concentration was significantly decreased in the nucleus accumbeas,but significantly jncreased in the Iocus coeruleus(P<0.01)compared to the chronic centrel group.CONCLUSION:Morphine dependence and tolerance may induce obvious reductions of Gi2 protein levels in the nucleus accumbens of rats.Chronic morphine dependence desensitizes the homologous neurons.