[Effects of morphine on the activity of various dorsal horn neurons of the spinal cord involved in nociception].

In spinal preparation, morphine exerts a specific direct inhibitory action on the activities of dorsal horn neurones induced by noxious stimuli. The effect of morphine is preferential for the responses evoked by A delta and C fibre afferents and its specificity of action has been demonstrated pharmacologically in terms of isomerism, dose dependency and reversal of the inhibitions by opiate antagonists. These results are in good agreement with recent data related to the localization at the spinal level of opiate receptors, and terminal rich in Enkephalin and substance P. Numerous behavioural and pharmacological investigations suggest that morphine is also acting at the level of the brainstem by reinforcing the activity of descending control systems which modulate the transmission of noxious inputs at the spinal level. However this second modality of action remains extremely difficult to demonstrate from an electrophysiological point of view.