Antagonistic Effect Of Melatonin On Morphine Dependence In Mice And Its Relationship With CAMP Level In Brain Regions

Objective: To establish a stable model of naloxone-precipitated withdrawalsymptoms in morphine-dependent mice. To confirm the antagonistic effect ofmelatonin on morphine dependence upon the established stable model,undertaken to supply the experimental foundations for the potential clinicalapplication of melatonin as a hopeful drug in the treatment or assistant treatmentof addiction. To elucidate the relationship between antagonism of melatoninon morphine dependence and cAMP level of some potential relatedmorphine-dependent nuclei or brain region, in order to clarify the possiblemechanism for antagonism of melatonin on morphine dependence and providethe theoretical foundation on clinical appliance of melatonin and its analoguesand make further recognition of neurobiological and molecular mechanisms ofaddiction.Methods: ⑴Evaluate effects of four factors on naloxone-precipitatedwithdrawal jumping and body weight loss for stabilizing the model, includingcumulated doses of morphine, frequencies of morphine administration,increasing manner of morphine dose day by day, and morphine administered ornot closely before precipitation. Then establish a stable model ofnaloxone-precipitated withdrawal symptoms in morphine-dependent ICR miceby following protocol: mice were administered subcutaneously (s.c.) repeatedlyt.i.d for five consecutive days with morphine 10,20,40,80, 100 mg/kg,respectively. On the testing day (Day 6), mice were injected s.c. with the finalmorphine administration (100 mg/kg), followed by naloxone injection (20mg/kg, s.c.) 2 hours later. After naloxone given, withdrawal jumps were scoredfor 30min and body weight loss was measured immediately before and 2h afterthe administration of naloxone by observers blind to treatment. Followingstudies were based on this model. ⑵Examine antagonistic effect ofmelatonin on development of morphine-dependence in ICR mice andwithdrawal syndrome in morphine-dependent ICR mice by different melatonintreatment: administration during the induction of morphine dependence oradministration closely before naloxone precipitation. ⑶After the ethologicalanalysis of action of melatonin on development of morphine-dependence inmice, mice of control, model and high melatonin treatment groups wereexecuted, and separated cerebral cortex, hippocampus, striatum anddiencephalon from the brain, then determined the cAMP content of differentregions by radioimmuoassay.Results: ①For a stable model of naloxone-precipitated symptoms inmorphine-dependent mice, cumulate doses of morphine should be enough, t.i.dof morphine administration is better than b.i.d , double-dose manner ofmorphine dose schedule day by day precedes escalating-dose mannersignificantly, morphine administered closely before precipitation excels not, ICRmice overmatches Kunming mice. As regards the established model in thisstudy, model mice get 100% jump rate, much more significant jumps and bodyweigh loss versus control mice, and rather small variation, indicating that themodel is stable and reliable for the research of morphine dependence. ②Micein melatonin treatment groups were injected i.p. with melatonin q.d. for the last3 consecutive days of induction of morphine dependence, with melatonin dose45, 90 mg/kg, respectively. It's shown that jumps and body weight loss inmelatonin treatment groups were decreased significantly versus those in modelmice and taken out dose dependence. This result suggests that melatonin canantagonize the development of morphine dependence in mice. Mice in MT70and MT140 groups were injected i.p. with melatonin 70 mg/kg and140 mg/kg,respectively, 10 minutes prior to naloxone injection. It's also shown that jumpsand body weight loss in melatonin treatment groups were decreasedsignificantly versus those in model mice. The result clues that melatonin canweaken withdrawal syndrome in morphine-dependent mice. ③cAMP contentof each brain region of melatonin treatment group has the declined tendencyversus that of model group, and there is significant in hippocampus,diencephalons and striatum between melatonin treatment group and modelgroup. It is implied that antagonism of melatonin on development of morphinedependence may be related with reduction of cAMP level of hippocampus,diencephalon and striatum.Conclusions: 1. Establish the stable and reliable model of naloxone-precipitatedwithdrawal symptoms in morphine-dependent mice. 2. Melatonin has theantagonistic effect on morphine dependence. 3. Antagonism of melatonin ondevelopment of morphine dependence maybe related with the reduction ofcAMP level of hippocampus, diencephalon and striatum.