Rats that acquire a THC discrimination more rapidly are more sensitive to THC and faster in reaching operant criteria

Male Sprague-Dawley rats were trained to discriminate delta-9-tetrahydrocannabinol (THC) from saline in a two-lever operant task using successive training criteria. Untreated animals were first shaped to barpress for a milk reward with one lever available. As each animal reached criterion the second lever was installed, the first lever was removed, and the animal was treated with 3.0 mg/kg THC 30 min prior to barpress training. When criterion on the second lever was reached the rats were trained to discriminate THC from vehicle injections with both levers available. Following acquisition of the discrimination, test doses of THC at 0.00, 0.375, 0.75, 1.5 and 3.0 mg/kg revealed that the half of the 24 rats who reached criterion (STC) more rapidly exhibited significantly greater sensitivity to THC at the 0.75 mg/kg test dose than did the 12 slow-learner rats; the former group generated an ED50 of 0.77 mg/kg, whereas the ED50 for the later group was 1.63 mg/kg. The fast learners acquired both the initial barpress response and the discrimination more rapidly than did slow-learners. Results suggest that some animals are inherently more sensitive to THC and faster in meeting learning criteria.