Effect of Midazolam on Propofol-induced Positive Affective State Assessed by Place Conditioning in Rats 

BACKGROUND: The effect of either midazolam or the combination of midazolam and propofol on the affective state was assessed in rats at subanesthetic doses and at recovery from anesthesia. METHODS: The putative drug(s)-induced affective states were repeatedly paired with one of two distinguishable compartments of an experimental cage, whereas the vehicle(s)-induced effect was repeatedly paired with the other compartment. During a subsequent choice test for one compartment over the other, the rats' preference for the drug(s)-paired compartment over the vehicle(s)-paired compartment is indicative of a pleasant state induced by the drug(s). In experiment 1, rats were conditioned with different doses of midazolam either at subanesthetic states or at recovery from anesthesia. In experiment 2, groups of rats were conditioned with different combinations of midazolam and propofol either at subanesthetic states or at recovery from anesthesia induced jointly by midazolam (10 mg/kg) and propofol (60 mg/kg). Experiment 3 was conducted in the same way as experiment 2, except that midazolam was paired with both compartments. In addition, these groups were tested not only in an undrugged state but also in a drugged (with midazolam) state. RESULTS: In experiment 1, rats exhibited a place preference for the environment previously associated with midazolam, at subanesthetic and anesthetic doses. Experiment 2 showed that a propofol-induced place preference was found to be dose-dependently suppressed by midazolam. Experiment 3 replicated the findings of experiment 2 and extended them to the mechanism by which midazolam blocked a propofol-induced place preference. CONCLUSIONS: Midazolam administered before propofol blocked the expression of a propofol-induced pleasant state.