Assays for Corticotropin-Releasing Factor (CRF) Using Rats Treated with Morphine, Chlorpromazine, Dexamethasone and Nembutal

Rats treated with morphine and Nembutal (M-N) or chlorpromazine, morphine and Nembutal (CPZ-M-N) or dexamethasone, morphine and Nembutal (Dex-M-N) were used as assay systems for corticotropin-releasing factor (CRF). These 3 groups were compared for reliability and ease of performance of the assay. A high percentage of the rats treated with M-N died during the assay due to respiratory difficulties, and animals in this group often showed a very high plasma concentration of corticosterone. The mortality rate of CPZ-M-N rats was much lower than that of M-N preparations. CPZ-M-N rats showed consistently greater responses than Dex-M-N rats when given the following CRF preparations: the extracts of rat stalk median eminence (SME), β-CRF, α-CRF and hypothalamic CRF. This suggests a direct suppression of the CRF activity by dexamethasone. Dex-M-N completely suppressed ACTH release caused by 50 mU of vasopressin and partially blocked ACTH secretion following 100 mU of vasopressin. In CPZ-M-N preparations, both 50 and 100 raU of vasopressin caused a similar but moderate rise in plasma corticosterone level. Comparable increases in plasma corticosterone were observed in M-N rats after the same doses of vasopressin, but the relatively high control values and large variation in the responses combined to prevent these increases from reaching statistical significance. Dose-response relationship for arginine vasopressin was absent in the dose-range up to 100 mU in every experiment using CPZ-M-N rats. On the other hand, a clear-cut dose-response relationship for β-CRF and SME extracts was obtained from CPZ-M-N rats. However, the dose-response curves for these 2 test substances were not parallel. Adrenocortical response to ACTH was essentially the same in CPZ-M-N or Dex-M-N treated rats and in rats hypophysectomized 24 hr previously. The results of these experiments indicate that CPZ-M-N treated rats have many advantages as a new assay preparation for CRF and can be used with greater ease and reliability. (Endocrinology81: 235,1967)