Selective action of morphine on reflex expression to nociceptive stimulation in the rat: a contribution to the assessment of analgesia.

In two experiments startle reflexes to both loud noises and electric shocks were elicited in rats. The two stimuli were paired so that the inhibitory effect of each stimulus on the response to the other could be assessed. In the first study it was shown that morphine (0-16 mg/kg) had a selective and dose related depressive effect on the response to shock. The response to a leading tone and inhibition produced by the shock on the response to a following tone were minimally affected. In the second study, morphine (10 mg/kg) again depressed the reaction to shock but not to tone, and its effect was antagonized by naloxone (10 mg/kg). The selective effect on responses to shock, leaving responses to tone relatively unaffected, reveals that reflex depression should not be attributed to a loss in motor functions. Further, that morphine had little effect on reflex inhibition produced by the shock suggests that the nociceptive properties of shock were affected rather than simple sensory processes. It is proposed that the method described here is useful for assessing changes in nociception in laboratory animals, and for discriminating between nociceptive, afferent, and efferent processes.