Tramadol—induced physical dependence and its effects on behavioral sensitization to methamphetamine in mice

Tramadol is a widely used and non-controlled analgesic,which stimulates both centrally opiatergic and monoaminergic systems.The epidemiological data indicate that tramadol possesses relatively high poly-drug abuse potential.Therefore,the present study was designed to assess the physical dependence of tramadol and investigate the effects of tramadol on behavioral sensitization to methamphetamine (MA) and its toxicity.Mice were made acute dependence on tramadol by injection (sc) of tramadol.After 3h,naloxone was given (ip) to precipitate withdrawal symptoms.The results showed that tramadol displayed marked naloxoneprecipitated withdrawal symptoms.5-HTP,a precursor of 5-HT,attenuated tramadol withdrawal symptoms,but parachlorophenylalanine,a tryptophan hydroxylase inhibitor,aggravated them.In the open field test,tramadol enhanced the development and expression of behavioral sensitization to MA in mice.In addition,co-administration of tramadol (120mg·kg^-1,a non-lethal dose) and MA reduced the LD50 for MA from 63.4mg·kg^-1 to 32.3mg·kg^-1.Our findings suggest that tramadol produce physical dependence in itself and potentiate MA-induced addictive behavior and toxicity.