The role of the paraventricular nucleus and pituitary gland in morphine analgesia.

The authors investigated the analgesic effects of small morphine doses injected into the paraventricular nucleus (PVN) of normal rats and hypophysectomized (Hx) rats. An injection cannula was stereotactically inserted into the PVN or third ventricle. On the 5-7th postoperative day, morphine (7 micrograms) was injected and the pain threshold (paw lick latency: PLL) was measured using a hot plate analgesia meter (52.0 +/- 0.1 degrees C). PVN morphine injection caused significantly longer PLL than the control (physiological saline) in both normal and Hx rats. Ventricular morphine injection did not increase PLL over the control. PVN is a site of morphine action. The analgesia induced by PVN morphine injection was not affected by hypophysectomy, or induced by leaking of morphine into the third ventricle.