THE EFFECT OF LYSINE VASOPRESSIN AND HYPOTHALAMIC EXTRACTS ON THE RATE OF CORTICOSTERONE SECRETION IN RATS TREATED WITH DEXAMETHASONE AND PENTOBARBITONE

SUMMARY The effects of lysine vasopressin (LVP) on the rate of secretion of corticosterone and on its concentration in the peripheral plasma have been compared. A dose of 80 m-u. LVP was required to increase the peripheral plasma concentration of corticosterone, whereas as little as 80 μu. produced a significant increase in the rate of corticosterone secretion. A log—dose response curve has been constructed between 80 and 800 μu. LVP using the secretion rate assay. The effect of hypothalamic extracts on corticosterone secretion in the intact rat was readily detected and a diurnal difference of hypothalamic activity in adrenalectomized rats was found. Extracts of rat cerebral cortex were ineffective in increasing the secretion rate of corticosterone in the dexamethasone—pentobarbitone-treated rat; hypothalamic extracts from intact rats and LVP in doses of 0·1 and 0·8 m-u. showed no corticotrophin(ACTH)-like activity as demonstrated by the measurement of corticosterone secretion in the hypophysectomized rat. Although the measurement of corticosterone secretion in rats pretreated with dexamethasone and pentobarbitone does not fulfil all the criteria for a bioassay for ACTH-releasing factor, it is a very sensitive technique for the detection of the effects of LVP on the activation of the pituitary-adrenal system and can be used to study changes of ACTH-releasing activity in hypothalamic extracts.