DIFFERENTIAL EFFECTS OF HYPOPHYSECTOMY UPON ANALGESIA INDUCED BY 2 GLUCOPRIVIC STRESSORS AND MORPHINE

Pain threshold elevations induced in rats after acute exposure to stressful cold-water swims and to inescapable foot shocks are significantly attenuated by hypophysectomy. The effects of hypophysectomy on the dose-dependent and time-dependent analgesia induced by morphine and by the glucoprivic agents, 2-deoxy-D-glucose (2-DG) and insulin. Two reflex pain tests, the tail-pinch and the flinch-jump were employed. In normal rats, insulin induced prolonged (180 min) analgesia at doses of 16 U[units]/kg on the tail-pinch test and 256 U/kg on the flinch-jump test. The same agents induced small and brief pain threshold elevations in hypophysectomized animals. 2-DG increased both measures in both groups but its effects were more marked in hypophysectomized rats. Hypophysectomized rats exhibited a potentiated analgesic effect on both tests after high doses of morphine. Low doses of morphine transiently increased tail-pinch thresholds in normal, but not hypophysectomized subjects. Apparently the pituitary is involved in the multiple pain-inhibitory mechanisms.