Generalization of the discriminative stimulus properties of x0394; 9 in rats

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Abstract
Rats were trained in a water maze to discriminate between IP injections of 3 mg/kg Δ9- (Δ9(11)-THC) and its vehicle. Both Δ8- and Δ9(11) were generalized to the training drug. In contrast to our observations in rhesus monkeys, where Δ9-THC is at least 100 times less potent than Δ9-THC, Δ9(11) was found to be only seven times less potent in the rat. Relative potencies, expressed as the dosage at which 50% of the animals gave drug responses (ED50) were 1.8 mg/kg and 12.2 mg/kg for Δ9-THC and Δ9(11) respectively. Twenty-four hours after receiving 7×ED50=12 mg/kg Δ9 the tests showed intermediate results when conducted with the training dosage; 4×ED50=50 mg/kg Δ9-THC 48 h prior to the training dosage of 3 mg/kg Δ9-THC completely blocked drug-appropriate responses. Coinjection of ED50 dosages of Δ9- and Δ9(11)-THC led to 90% drug responses, demonstrating the additivity of the cannabis-like effect of both cannabinoids. Differences in the individual sensitivity of the rats to the tested cannabinoids were observed. Findings are interpreted in terms of the receptor mechanism for cannabis-like activity.