Experimental study on mechanism of pain control in hypothalamo-pituitary system

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The mechanism of pain control in hypothalamo-pituitary 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 paraventricular nucieus(PVN) of hypothalamus in morphine analgesia.1. An injection cannula was stereotactically inserted into the PVN of 34 normal and hypophysectomized rats of 6 groups. On the 5-7th postoperative day, morphine (7mug) was injected and the pain threshold (paw lick latency on the hot-plate, 52.0(]SY.+-. p)0.1ﾟC) was measured. Morphine microinjected 16 rats disclosed marked diminution of pain susceptibility. PVN is a definite site of morphine action.2. The effect of hypophysectomy was also studied in the role of pain control of PVN. Morphine administration in the hypophysectomized rats showed mild reinforces the analgesic effect.3. Corticotropin releasing factor (CRF) and saline solution were also micro-injected into the PVN of 10 rats, respectively In these experiments the pain threshold was measured in the following 4 groups, namely in the CRF or saline injected groups with hypophysectomy and non-hypophysectomy. Although PVN appears to be the important site in control of pain relief in hypothalamo-pituitary system, the microinjection of CRF into the PVN with or without hypophysectomy didn't produce the diminution in pain.In summary, these findings suggested that PVN appeared to be the important site in control of pain in hypothalamopituitary system. However, CRF and pituitary gland did not play the important role in the mechanism of this type of pain control.