Does systemic morphine increase descending inhibitory controls of dorsal horn neurones involved in nociception?

Abstract

The direct spinal depressive effect of morphine and related compounds has been demonstrated in numerous electrophysiological studies in spinal animals. Recently, research has focussed on those dorsal horn neurones activated by a large range of noxious inputs, the activities of which are strongly modulated by both segmental and supraspinal inhibitory mechanisms (see refs. in ref. 3). In the rat, for instance, the response of dorsal horn cells to C fibre is depressed in a naloxone-reversible and dosedependent fashion by morphine; in direct contrast, responses to Aa fibre stimulation is poorly affected 11. In addition to a direct spinal action of morphine, an indirect action with the strenthening of inhibitory controls from supraspinal origin has been proposed 16 and extensively reviewed3,6,1z, 13. The direct conclusion of both hypotheses is that the depressive effects of systemic morphine would be greater in intact than in spinal preparations. Considering the contradictory reports in this area 4,s,10,14, we have studied the action of systemic morphine on the activities of dorsal horn cells involved in the transmission of nociceptive messages in intact rats. A comparison of results in spinal 11 and intact animals could provide a basis for the discussion of the initial hypotheses. Forty-two male Sprague-Dawley rats weighing 250-300 g were used. Surgical preparation, monitoring and recording were performed as previously described 11. Recordings were carried out under volatile anaesthesia, i.e. 0.5 ~ halothane in a nitrous oxide-oxygen mixture (2/3-1/3), during which the EEG showed monomorphic waves (80-100 #V), often rhythmic, in the range of theta (4--5 c/sec) accompanied by few 12--14 c/sec spindles. The recording of dorsal horn cells began only when the anaesthetic state was stable, generally 45 min after the discontinuation of the deeper anaesthesia employed during the surgical procedure. Under these conditions, noxious, visual or auditory stimulation did not induce the arousal reaction or peripheral vasomotor modifications usually observed during suffering or stress reactions.