The effects of chronically administered corticosterone on the responses of the rat isolated anococcygeus muscle to agonists (acetylcholine, norepinephrine) were compared with those of morphine withdrawal in intact or metyrapone-pretreated rats and with those of a single dose of reserpine in intact or adrenalectomized rats. The possibility that the similar supersensitivity, which occurs in thyroidectomized rats, might be associated with elevated corticosterone levels was investigated by measuring fluorimetrically the plasma levels of corticosterone in thyroidectomized rats. Corticosterone, morphine withdrawal or a single dose of reserpine produced similar supersensitivities, characterized by increased maximum responses to both agonists (acetylcholine, norepinephrine) and an increased pD2 value for acetylcholine. This effect of morphine withdrawal together with the weight loss characteristic of this condition was absent in metyrapone-pretreated rats. A single dose of reserpine had no effect in adrenalectomized rats. Plasma corticosterone levels were elevated in thyroidectomized rats. These results suggest that the common factor underlying the identical supersensitivities produced by corticosterone, morphine withdrawal, a single dose of reserpine or thyroidectomy is the ability of each treatment to raise plasma corticosterone levels.
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