Finding
For morphine to be effective in man it is probably acting at both levels.
Paper
Recent Advances in Understanding Pain
Citations: 3
Full textAbstract
input to the spinal cord arriving in small nerve fibres could be partially or completely inhibited, it came as a surprise when Reynolds (1969) reported that he had been operating upon rats while he stimulated a region of the mid-brain electrically: the rats behaved as though they felt no pain; and they showed none of the psysiological correlates of having pain nor the reflex effects associated with noxious stimulation. Even if he burned the animal? paws or performed abdominal operations, the animals appeared to be unconcerned. This technique of stimulating a parti cular region of the periaqueductal grey of the mid-brain to prevent pain became known as stimulation-produced analgesia. There are many sites, it is now known, from which stimulation produced analgesia can be obtained. They extend from the medulla oblongata, the grey matter sur rounding the aqueduct, to the posterior part of the hypothalamusurrounding the3rd ventricle. Itis probable that the sites above the medulla send fibres down to the medullary raphe nuclei and excite them into activity. From the lower raphe nuclei a descend ing inhibitory tract runs to the posterior horns, preventing the noxious input from getting into the central nervous system. Patients with chronic pain are now being treated by electrodes implanted in this system; they can stimulate this region of their own brains when they have pain. Owing to the effectiveness of raphe-spinal tract inhibition in stopping pain, the question arose whether the opiate drugs worked by activating the raphe nuclei. This was found to be so, though they also act at spinal level directly, for they induce the same inhibition in a spinal animal. For morphine to be effective in man it is probably acting at both levels. The similarity of the action of morphine and stimu lation-produced analgesia is shown by the fact that tolerance develops to stimulation-produced anal gesia just as it does to morphine. Moreover, in experiments on animals there is cross-tolerance: an animalwithimplantedelectrodes withinthissystem needs far larger doses of morphine to obtain anal gesia; and vice versa, animals made tolerant to morphine by frequent doses need more stimulation to get analgesia from electric stimulation. It occurred to many investigators to ask the question: if opiates form stereospecific complexes with the membrane of certain cells of the body, is this
Authors
P. Nathan
Journal
British Journal of Psychiatry