Microinjection of morphine within nucleus raphe magnus and dorsal horn neurone activities related to nociception in the rat.

The hypothesis of an increase by morphine of descending inhibitory controls acting upon the transmission of painful messages at the spinal level has been directly investigated in intact anaesthetized rats. The analgesic efficacy of morphine microinjections (5 micrograms in 0.2 microliter saline) applied within the nucleus raphe magnus (NRM) was examined using the threshold for vocalization after electric shock to the tail as a test: a mean threshold increase of 57% was observed. A few days later, the effects of similar microinjections upon dorsal horn cell activities were studied in acute experiments in the same animals. The response of dorsal horn convergent units induced by the activation of large myelinated (Aa) afferent fibres were unaffected by the microinjection of morphine within the NRM. In the case of the responses of convergent units induced by the activation of unmyelinated (C) afferent fibres, two different results were obtained after microinjection of morphine within the NRM: 8/14 units were not affected and 6/14 were clearly excited. A transient reversal of the excitatory effects was observed after the systemic administration of the opiate antagonist naloxone. The responses of marginal layer cells (lamina 1) were unaffected by the microinjection of morphine within the NRM. These unexpected results are discussed in view of the fact that they conflict with current concepts regarding morphine analgesia.