The failure of morphine to interact with serotonin on nucleus raphe magnus descending inhibition or morphine-induced suppression of cat spinal cord multireceptive neurones.

Morphine sulphate, administered in three cumulative doses (0.5, 1.0, and 2.0 mg/kg, i.v.) to alpha-chloralose anaesthetized cats, reduced the nociceptive activity of deep dorsal horn multireceptive neurones but failed to alter the descending nucleus raphe magnus (NRM) phasic inhibition of these neurones. Morphine was also administered to fluoxetine (6.0 mg/kg, i.v.) pretreated animals. Fluoxetine is a selective 5-hydroxytryptamine (5-HT) uptake blocker, which should enhance 5-HT synaptic transmission. In these animals, morphine suppressed the neuronal nociceptive activity to the same extent as seen with morphine alone and did not affect the NRM inhibition. These results do not support the notion that morphine activates a descending serotonergic inhibition from the NRM or that serotonin mediates morphine inhibition of spinal nociceptive transmission in cats.