Morphine analgesia and the bulbospinal serotonergic system: increase in concentration of 5-hydroxyindoleacetic acid in the rat spinal cord with analgesics.

Administration of an analgesic dose of morphine increased the concentration of the metabolite of 5-hydroxytryptamine (5HT), 5-hydroxyindoleacetic acid (5HIAA), in the spinal cord of rats. The time course of the change in 5HIAA concentration in the dorsal half of the spinal cord corresponded to that of the morphine analgesia, while such was not the case in the ventral half. When the spinal cord was transected at C1, the effect of morphine on 5HIAA concentration disappeared. After transection at the inter-collicular level, this effect remained. The possible relationship between morphine analgesia and descending serotonergic neurons in the spinal cord is discussed.