Aminophylline differentiates between the depressant effects of morphine on the spinal nociceptive reflex and on the spinal ascending activity evoked from afferent C fibres.

The action of aminophylline on anti-nociceptive effects of morphine in rats was tested on the tail-flick response to noxious heat and on the activity evoked in ascending axons of the spinal cord by stimulation of nociceptive afferents. The depression of the tail-flick response produced by an intraperitoneal (i.p.) injection of morphine 2 mg/kg in intact and spinal rats was abolished by an i.p. injection of aminophylline 25 mg/kg. The activity evoked in ascending axons of spinal rats by electrical stimulation of afferent C fibres of the sural nerve was depressed by an intravenous (i.v.) injection of morphine 2 mg/kg. Aminophylline 25 mg/kg injected i.v. after morphine produced a slight and transient increase in the ascending activity immediately after its administration but did not abolish the depressant effect of morphine. Naloxone 0.2 mg/kg administered after aminophylline antagonized the depressant effect of morphine on the ascending activity. It is suggested that morphine exerts its depressant effect on the two nociceptive responses (the motor and the sensory response) by different mechanisms, one being sensitive to aminophylline, the other being relatively resistant to the action of the purine derivative.