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Mean (+SE) paw-lick latencies of PD 60 male and female rats (n = 8–9/sex). Rats were pre-treated with fentanyl (10 or 100 μg/kg) or saline from PD 4 to PD 9 and injected with morphine (0, 1.25, 2.5, 5.0, or 10 mg/kg, sc) on PD 60. *Significantly different from 0 mg/kg morphine. #Significantly different from saline-pre-treated rats in the same morphine drug condition.
