Abstract
The existence of the opioid-mediated pain inhibiting system has been well demonstrated. Facilitation of this system by administration of analgesic drugs, such as morphine or by electrical stimulation provides relief of excess pain but not of deafferented pain (thalamic or suprathalamic pain or phantom limb pain). This experiment clearly demonstrates that the suppressive effect of this opioidmediated system is exerted only on the nociceptive neurons in the subnucleus caudalis of the spinal trigeminal nucleus (STNcd), but not on the sustained neuronal hyperactivity provoked by the deafferentation of the peripheral trigeminal nerve.
Preliminary studies using 16 adult cats ascertained STNcd located from obex to 2 mm caudally, and an electrophysiological study elicited nociceptive neurons distributed 3-4 mm lateral to the midline. Gasserian ganglion of the left side was totally coagulated through a subtemporal extradural approach, and it was found that a definitive neuronal hyperactivity could be observed from about 10 days after the denervation.
The experiment was performed using another 13 adult cats, which were unilaterally (left side) denervated. The longest survival period of the cats was 63 days after the denervation. Forty-three neurons were identified in the STNcd of the intact (non-denervated) side, the majority of which (40 neurons) being wide dynamic range neurons. Twenty-two neurons were identified in the STNcd of the denervated side, 12 of which (55%) showing sustained high amplitude firings with no correlation to noxious stimuli (denervation hyperactivity). Such spontaneously remarkable hyperactive neurons were never detected in neurons in the intact side. Intraventricular injection of morphine or enkephalinamide, or electrical stimulation of periaqueductal gray apparently and significantly suppressed the nociceptive neuronal firing of the intact side. By contrast denervation hyperactivity was not affected.
These results are compatible with the clinical experience that only excess pain but not deafferented pain is relieved by various procedures to facilitate the opioid-mediated system. From these experimental confirmations, it can be postulated that there is a close relationship between the deafferentation hyperactivity provoked in this experiment and the so-called “deafferented pain”.
