Mechanisms of effect of adrenocorticotropic hormone on the rat pain sensitivity

The mechanisms of effect of adrenocortricotropic hormone on pain sensitivity were studied in anaesthetized Sprague-Dawley male rats. Systemic ACTH administration increased the pain thresholds (from 3 min up to the 30 min after injection) in rats with normal production of hormones. The initial stage of ACTH analgesic effect was fully eliminated by blockade of opiate receptors, ACTH-induced reaction was observed only from 15 up to 30 min after injection. In rats with deficiency of glucocorticoids production, the duration of ACTH-induced analgesic effect decreased to 15 min. The analgesic effect was completely prevented by combination of blockade of glucocorticoids production and blockade of opiate receptors. Thus the ACTH-induced analgesic effect is provided at least by two mechanisms: 1) appearing during the first minutes after injection (from 3 min up to 15 min) and mediated by opiate receptors rather than glucocorticoids, and 2) appearing from 15 min up to 30 min after injection and mediated by glucocorticoids but not opiate receptors.