Assessment of anaesthetic action of morphine and fentanyl in rats.

In 150 Sprague-Dawley rats, morphine and fentanyl dose-effect curves were determined for the following three end points--prevention of purposeful movement response to a noxious stimulus (PM), loss of righting reflex (RR), and prevention of heart rate increase to a noxious stimulus (HR). Accordingly, for each agent, three series of experiments were performed with intravenous administration of the following doses: morphine--3-10 mg X kg-1 for PM, 3-10 mg X kg-1 for HR, 35-55 mg X kg-1 for RR; fentanyl - 5-15 micrograms X kg-1 for PM, 18-30 micrograms X kg-1 for RR, 200-400 micrograms X kg-1 for HR. Dose-effect curves were calculated with the use of probit procedure and potency ratios were determined on the bases of ED50 values. It was found that potency ratios of morphine and fentanyl are different for the studied end points. The ratios of RR ED50 to PM ED50 were 7.8 for morphine vs 2.4 for fentanyl (p less than 0.001), the ratios of HR ED50 to PM ED50 were 1 and 33, respectively (p less than 0.001). These results suggest that blockade of movement response to noxious stimulation (which is usually regarded as an index for analgesic action of opioids) and blockade of heart rate increase to noxious stimulation (which is one of the goals of anaesthesia) is not necessarily induced by intravenous narcotic anaesthetics through the same mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)