Further evidence for the involvement of the diencephalo-dopaminergic system in pain modulation: a neurochemical study on the effect of morphine in the arthritic rat.

Numerous studies from the past two decades suggest that the bulbospinal serotonergic and noradrenergic systems are involved in pain modulation at the spinal level. More recently the occurrence of a diencephalo-dopaminergic system has been demonstrated and there is evidence that this pathway may also participate in pain modulation. Several works have been devoted to the effect of morphine on the activity of the dopaminergic system in supraspinal areas but it is not at present known if opiates modify the activity of this system in the spinal cord sensory areas. In the present study the effect of morphine on the metabolism of dopamine (DA), 5-hydroxytryptamine (5-HT) and norepinephrine (NE) in different laminae of the lumbar spinal cord of arthritic rats was investigated. A relatively small dose of morphine (2.5 mg/kg s.c.), which produces marked analgesia in these animals, induces a naloxone-reversible increase of DA metabolism in the different laminae of the spinal cord that contain neurons which respond to the activation of articular nociceptors. In contrast, the same dose of morphine does not significantly affect 5-HT and NE metabolism in the same areas. These observations suggest that the diencephalo-dopaminergic system is probably of equal importance to (or more important than) the two other monoaminergic systems in pain modulation at spinal level.