Comparison of the adrenocorticotrophic effect of a synthetic depot corticotrophin in normal, dexamethasone-blocked, and hypophysectomised rats.

The ACTH stimulation test for depot preparations was carried out with a synthetic depot corticotrophin (Synacthen Depot®) in normal, dexamethasone-blocked, and hypophysectomised rats. It was found that the response curves (plasma and adrenal corticosterone) were very similar in all three groups, the peak values being somewhat higher in the normal and block dexamethasone-blocked animals. Towards the end of the stimulation test, the normal animals showed a marked deviation from the nycterohemeral rhythm. This change, which complicates the determination of the duration of action of the preparation used, is indicative of a disturbance in the hypothalamo-pituitaryadrenal feedback system. In dexamethasone-blocked animals, the effects of ACTH on the adrenals and thymus cannot be studied in isolation but only in combination with those of dexamethasone. Hence, despite all the technical difficulties involved, the test with hypophysectomised rats is still the method that yields the most easily assessable responses.