Chlorpromazine hyperalgesia antagonizes clonidine analgesia, but enhances morphine analgesia in rats tested in a hot-water tail-flick paradigm

Seventy-six male Sprague-Dawley rats were tested in a hot-water (55°±0.5° C) tail-flick paradigm. Tail-flick latencies (TFL) were obtained at 30 and 15 min before intraperitoneal injection of either morphine (2.5, 5.0 and 10.0 mg/kg) clonidine (25, 50, 100 and 200 µg/kg), chlorpromazine (CPZ, 2.5 and 5.0 mg/kg), dual injections of these drug combinations, or a saline control injection. Further TFL measures were taken immediately following drug administration and thereafter at 15 min intervals. The mean of the pre-drug TFL's served as each rat's baseline. All other TFL's were calculated as percentage changes from that baseline. Mean changes were determined for each treatment group and differences between groups, at each test time, were analysed.