Evaluation of the discriminative stimulus and reinforcing effects of dihydroetorphine

These experiments examined whether dihydroetorphine (DHE) could serve as a reinforcer in rhesus monkeys and evoke the discriminative stimulus effects of heroin (HER) in rats, two procedures useful in predicting the overall abuse potential of compounds. Rhesus monkeys were trained to self-administer i.v. HER (10 micrograms/kg for monkeys M-BA, M-NI, and M-HO; 3 micrograms/kg for monkey M-PO) during daily; 2-h experimental sessions under FR 10 Timeout 4 min schedules. VEH and doses of HER (1-30 micrograms/kg), codeine (COD; 30-1000 micrograms/kg), and DHE (1-100 ng/kg) were then substituted for the HER maintenance doses. Results indicated that DHE served as a reinforcer. The dose of DHE that maintained peak numbers of infusions was 171 and 8571 times smaller than those maintaining peak numbers of infusions of heroin and codeine, respectively. Additionally, male Sprague-Dawley rats were trained to discriminate 0.3 mg/kg HER s.c. from vehicle (VEH) in an FR 10 (fixed-ratio 10), food-reinforced, operant procedure. During tests, HER, morphine (MOR), and DHE dose-dependently evoked heroin-lever responding with ED50s of 0.055, 0.74, and 0.00033 mg/kg, respectively. These results indicate that DHE is self-administered by rhesus monkeys, and potently produces the discriminative stimulus effects of HER in rats, and suggest that DHE would have a substantial potential for abuse.