Experimental study on mechanism of pain control in hypothalamo-brain-stem system

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The mechanism of pain control in hypothalamo-brain stem system was investigated in rats by way of the microinjection method and a hot-plate analgesia meter, being with special reference to the role of the in morphine analgesia.1. An injection cannula was stereotactically inserted into the paraventricular nucleus(PVN) of hypothalamus of rats. On the 5-7th postoperative day, morphine(7mug) was injected and the pain threshold(paw lick latency on the hot-plate, 52.0(〕SY.+-.〔)0.1ﾟC) was measured. Morphine microinjected rats disclosed marked diminution of pain susceptibity. PVN is a definite site of morphine action.2. The effect of naloxone was also studied as for the mechanism of pain control of PVN.Rats which was administered intraperitoneally naloxone, showed moderate decrease of the analgesic effect of morphine administration into the PVN.3. Both effects of morphine and naloxone were also measured in the rats morphine microinjected into the periaqueductal gray matter. These rats showed the similar patern of analgesia of PVN groups.4. These findings suggested that PVN appeared to be the important site in control of pain in hypothalamo-brain stem system. And PVN also appears to be the initial site of descending pain control system in the brain stem down to the spinal cord.