Influence of naloxone on shock-induced freezing and analgesia.

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Abstract
Six experiments were designed to examine whether mild shock activates an opiate analgesia in rats. The first 3 experiments explored whether naloxone potentiates shock-induced freezing by blocking an opiate analgesia. In Experiment 1, subjects treated with either a low or a high dose of naloxone froze more following mild shock. Experiment 2 revealed that both dose levels of the drug increase pain reactivity. Experiment 3 suggested that a naloxone-induced increase in pain reactivity accounts for the drug''s effect on freezing. The last 3 experiments investigated the nature of the analgesia induced by mild shock. In Experiment 4, mild shock induced a profound analgesia, as measured by the tail-flick test. Experiment 5 demonstrated that mild shock elicits a transient naloxone-insensitive analgesia, which rapidly dissipates to reveal an analgesia that is reversed by a high dose of naloxone. Mild shock activates both the nonopiate and the opiate form of analgesia. A low dose of naloxone potentiates shock-induced analgesia. This potentiated analgesia is attenuated by a high dose of naloxone.