Drug sensitivity of individual rats determines degree of drug discrimination

Rats were observed to learn to discriminate between the stimulus properties of intraperitoneal 0.16 mg/kg apomorphine and saline, in a two-lever operant task, at different rates. Half of the 12 rats reached criterion performance in a mean of 22.5 session, whereas the other half reached criterion in a mean of 44.2 sessions. These two groups, i.e., the early and later learners, were tested with a range of apomorphine doses and the former group had an ED50 of 0.01 mg/kg, whereas the later group generated an ED50 of 0.07 mg/kg apomorphine. These results suggest that the early learners were significantly more sensitive to apomorphine than the later learners and this may explain the discrepancies in the drug-discrimination literature regarding different ED50's generated at the same drug training dose.