Morphine--caffeine analgesic interaction in rats

The ability of caffeine to modify the effect of morphine on motor response to noxious stimulation was studied in 195 rat experiments. Motor reaction responses to noxious stimuli were studied in three series of experiments with three different techniques of mechanical tail stimulation. In each series of experiments, dose-response curves for morphine (probit analysis) were determined with and without the addition of caffeine (30 mg X kg-1). It was found that caffeine decreased morphine ED50 values in all three series of experiments, from 1.1 to 0.6 mg X kg-1 (P less than 0.002), from 3.2 to 2.5 mg X kg-1 (P less than 0.01), and from 13.2 to 6.2 mg X kg-1 (P less than 0.002). When caffeine was used alone in a dose of 30 mg X kg-1, there were no significant changes in motor reaction responses with any of the three methods applied for the assessment of morphine-caffeine combinations. These data indicate that caffeine potentiates the inhibitory effect of morphine on motor response to noxious stimulation in rats. It has been suggested that the effect of morphine on the motor response to somatic noxious stimulation results primarily from activation of inhibitory control systems concerned with this response. Caffeine may modulate the antinociceptive effect of morphine by stimulating one of these systems.