A sensitive test for studying the effects of opioids on a C-fibre reflex elicited by a wide range of stimulus intensities in the rat.

A C-fibre reflex elicited by electrical stimulation within the receptive field of the sural nerve was recorded from the ipsilateral biceps femoris muscle in anaesthetized rats. Recruitment curves were built by varying the stimulus intensity from 0 to 50 mA and temporal evolution was studied by using a constant level of stimulation. At a constant level of stimulation, intrathecal administration of morphine resulted in a depressive effect on the C-fibre reflex in the 0.18-0.75 microgram range (ED50 = 0.2 microgram). Study of the recruitment curves showed that, in the 0.18-0.375 microgram range, morphine had little effect on the threshold, but induced significant decreases in the slopes. At doses above 0.75 microgram, morphine modified both the threshold and the slope of the recruitment curves. Systemic naloxone totally reversed these effects. It is concluded that intrathecal morphine not only produces a shift in the encoding functions of the spinal cord but also reduces the gain of these functions. It is suggested that this method is reliable for the pharmacological study of the spinal transmission of nociceptive signals.