Morphine (0.01-10 mg/kg) promoted rapid autonomic learning of discriminative, Pavlovian conditioned heart rate decelerations to tone signals in male and female rabbits, and the higher doses (1-10 mg/kg) promoted decelerative heart rate orienting reflexes to novel tones. Morphine dose dependently reduced heart rate acceleration to signaled shock but had no effect on heart rate acceleration to unsignaled shock. Morphine did not impair retention of cardiac conditioned reflexes, and its U-shaped dose effect, increasing conditioned heart rate discrimination early in training, reappeared in extinction. The authors propose that morphine promotes autonomic learning of preparatory, compensatory reflexes to signaled stressors that reduce their stressful effects. This action may mimic the normal, adaptive function of an endogenous messenger released by the Pavlovian contingency.

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