Influence of tramadol on morphine discriminative behavior in rats.
OBJECTIVE
To study the potential of the psychological dependence of tramadol.


METHODS
Rats were trained to discriminate 4.0 mg/kg morphine, and to discriminate 0.5 mg/kg methamphetamine (MA) from saline under a fixed-ratio (FR10) schedule of reinforcement. After they acquired the discrimination, different doses of tramadol were used to substitute for training dose of morphine and MA. Naltrexone was injected concomitantly with tramadol.


RESULTS
Tramadol fully substituted morphine at a dose of 32 mg/kg or higher. The ED50 value of the discriminative effects of tramadol was 20.94 mg/kg, higher than that of morphine (2.04 mg/kg, P < 0.01). MA failed to generalize to tramadol at the doses tested. Naltrexone antagonized the discriminative response of tramadol.


CONCLUSIONS
Tramadol can substitute for morphine in morphine discriminative rats. The discriminative stimulus effects of tramadol are mediated by a mu opioid mechanism.
Ren
Y H
YH
National Institute on Drug Dependence, Beijing Medical University, Beijing 100083, China.
Zheng
J W
JW
D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D004192:Discrimination, Psychological / Q000187:drug effects*; D008297:Male; D009021:Morphine Dependence; D051381:Rats; D017207:Rats, Sprague-Dawley; D014147:Tramadol / Q000494:pharmacology*
D000701:Analgesics, Opioid; D014147:Tramadol