Differential effects of hypophysectomy upon analgesia induced by two glucoprivic stressors and morphine

Pain threshold elevations induced in rats following acute exposure to stressful cold-water swims and to inescapable foot shocks are significantly attenuated by hypophysectomy. The present study investigated the effects of hypophysectomy upon 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/kg on the tail-pinch test and 256 U/kg on the flinch-jump test. However, the same agents induced small and brief pain threshold elevations in hypophysectomized animals. By contrast, though 2-DG increased both measures in both groups, its effects were more marked in hypophysectomized rats. Hypophysectomized rats also exhibited a potentiated analgesic effect on both tests following high doses of morphine. On the other hand, low doses of morphine transiently increased tail-pinch thresholds in normal, but not hypophysectomized subjects. These data provide further evidence of multiple pain-inhibitory mechanisms in which the pituitary plays a complex, but integral part.