The possibility that a component of morphine-induced analgesia is contributed indirectly via the release of endogenous opioids

Abstract

The abdominal constriction test was used to explore the effects of cold stress (4°C) of varyiing duration on the antinociceptive effects of morphine in mice. It was found that after 15 min exposure to cold significant endogenous analgesia was observed, together with a significant potentiation of the antinociceptive effects of morphine. However, after more prolonged exposure to 4°C, the endogenous analgesia was no longer seen and, in addition the antinociceptive effects of morphine diminished progressively, and after 2 h or longer exposure it was significantly less than control values. It was shown that, in the absence of cold stress, pretreatment with corticosterone, dexamethasone or ACTH reduced the potency of morphine in a manner similar to that seen after 4 h cold stress. It was also shown that the potency of morphine in adrenalectomized mice was not affected by 3 h cold stress. It is concluded that adrenal corticoids, released in response to the stressor effect of exposure to cold, are responsible for the antagonism of the antinociceptive action of morphine seen in mice after extended exposure to cold. It is suggested that the initial potentiation of the antinociceptive potency of morphine is due to the release of endogenous opioids. With more prolonged stress, the stores of these substances may become progressively depleted, possibly as a result of raised corticosteroid levels, so that the component of analgesia contributed by them is steadily reduced and ultimately abolished.