Dose and temporal parameters of morphine-induced hyperalgesia in domestic fowl.

Recent research in this laboratory has identified a biological model in which morphine produced a hyperalgesic response to a noxious thermal stimulus. Morphine effects, however, were examined at only one injection-to-test interval (10 min). Because a single injection-to-test interval is relatively uninformative, the present research was designed to more fully characterize this morphine hyperalgesic effect. In Experiment 1, 15-day-old White Leghorn cockerels were placed on a hot plate (59 degrees C) in 10 min after injection of morphine (1.25, 2.5, 5.0, 10.0 mg/ml/kg) or the distilled water vehicle (1 ml/kg). Latency to perform a jump response or attainment of a 90-sec no-jump criterion were recorded. Experiment 2 examined morphine effects (2.5 mg/ml/kg) on hot plate jump latencies at various injection-to-test intervals (10, 30, 60, and 240 min). Morphine produced a dose-dependent hyperalgesic response. Temporal characteristics of morphine effects were evident as a U-shaped function. The dose and temporal characteristics of morphine-induced hyperalgesia in White Leghorn cockerels are similar to the dose and temporal characteristics of morphine-induced analgesia typically seen in other species.