Increased naloxone reversibility in fentanyl dose-dose discrimination.

Two groups of rats were trained to discriminate 0.04 mg/kg of fentanyl from saline (drug-saline discrimination) or from 0.02 mg/kg of fentanyl (dose-dose discrimination). The ED50 of naloxone for antagonizing the discriminative effects of 0.04 mg/kg fentanyl was 0.05 and 0.0062 mg/kg, respectively. This 8.1 fold difference in naloxone reversibility suggests that dose-dose discrimination yields higher pharmacological specificity than the commonly used drug-saline discrimination.