Effects of morphine upon the lamina V type cells activities in the dorsal horn of the decerebrate cat.

The effects of morphine (2 mg/kg i.v.) upon the transmission of nociceptive messages at the spinal level have investigated in decerebrate cats by studying its effects on the activities of lamina V dorsal horn interneurons. In contrast to previous results obtained on the spinal cat, morphine had little or no effects on lamina V type cells in the decerebrate preparation. The mean values for spontaneous activity and responses to natural noxious stimulation were practically identical before and after morphine administration. Moreover, no significant depressive effect was found on responses induced by supramaximal transcutaneous stimulation. However, for this type of activity a depressive effect was revealed, if only the late component of units which presented bimodal responses were considered. We were unable to demonstrate after morphine administration an increase of the descending inhibitory effects induced on lamina V cells by stimulation of the central inferior nucleus of the raphe. Additional experiments using reversible spinalization (by cooling the cord at the thoracic level) suggest that the lack of effect of morphine on decerebrate animals could be explained by the fact that in this preparation, descending inhibitory influences are strongly exacerbated and thus may mask the depressive effects of this drug. These results indicate that the direct electrophysiological evidence of an increase of the descending control systems after morphine administration must be performed in the intact preparation in order to avoid the effects ot their exacerbation in the decerebrate state.