Influence of naloxone on shock-induced freezing and analgesia.

Six experiments were designed to examine whether mild shock activates an opiate analgesia in rats. The first three 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. The results of Experiment 3 suggested that a naloxone-induced increase in pain reactivity accounts for the drug's effect on freezing. The last three 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. This suggests that mild shock activates both the nonopiate and the opiate form of analgesia. Experiment 5 also showed that a low dose of naloxone potentiates shock-induced analgesia. Experiment 6 revealed that this potentiated analgesia is attenuated by a high dose of naloxone. Implications of the results are discussed.