Behavioural sensitization after repeated exposure to Delta(9)-tetrahydrocannabinol and cross-sensitization with morphine

Rationale: Repeated exposure to several drugs of abuse has been reported to induce behavioural sensitization. So far no evidence has been provided tha t such a phenomenon also applies to cannabinoids. Objectives: In this study we investigated if repeated exposure to Delta (9)-tetrahydrocannabinol (De lta (9)-THC) induces behavioural sensitization. In addition we tested the p ossibility of cross-sensitization between Delta (9)-THC and morphine. Metho ds: Male Sprague-Dawley rats were administered for 3 days, twice daily, wit h increasing doses of Delta (9)-tetrahydrocannabinol (2, 4 and 8 mg/kg i.p. ) or increasing doses of morphine (10, 20 and 40 mg/kg s.c.) or vehicle. Af ter a washout of 14 days the animals were challenged with Delta (9)-THC (75 and 150 mug/kg i.v.), with a synthetic cannabinoid agonist WIN55212-2 (75 and 150 mug/kg i.v.) or with morphine (0.5 mg/kg i.v.), through a catheter inserted into the left femoral vein 24 h before, and the behaviour recorded . Results: Rats previously administered with Delta (9)-THC showed a greater behavioural activation compared to controls in response to challenge with Delta (9)-THC (150 mug/kg i.v.) and to challenge with morphine (0.5 mg/kg i .v.). Similar to that observed after repeated opiates, this behavioural sen sitization was characterized by stereotyped activity. Animals administered with a schedule of morphine that induces behavioural sensitization to morph ine also showed a behavioural sensitization to challenge with cannabinoids (Delta (9)-THC and WIN55212-2, 75 and 150 mug/kg i.v.). The effect of the c hallenge with Delta (9)-THC was prevented by the administration of the CB1 antagonist SR141716A (1 mg/kg i.p.), 40 min beforehand. Conclusions: The re sults of the present study demonstrate that repeated exposure to Delta (9)- THC induces behavioural sensitization not only to cannabinoids but also to opiates. This cross-sensitization was symmetrical since rats behaviourally sensitized to morphine were also sensitized to cannabinoids. These observat ions further support the evidence of an interaction between the opioid and the cannabinoid system and might provide a neurobiological basis for a rela tionship between cannabis use and opiate abuse.