Assessment of endogenous CRF secretion by CRF release from the rat hypothalamus in vitro.

The literature on the control mechanism of corticotrophin releasing factor (CRF) secretion is reviewed and the limitations of CRF assays are discussed. A new specific in vitro CRF assay is described which does not respond to vasopressin because the pituitary tissue is not preincubated. The technique does not require surgical or pharmacological pretreatment since endogenous CRF release due to experimentation is prevented by handling the donor rats until 'adaptation' to handling occurs (after two weeks). The results show that the assay gives reliable estimates of the CRF potency of hypothalamic extracts. Although endogenous CRF release is usually assessed by changes in hypothalamic CRF content, this is not always a reliable index of CRF release. There is also considerable confusion about the effects of various pretreatments on CRF content. It was therefore decided to test whether the initial release of CRF from isolated hypothalami in vitro was able to reflect the changes in CRF release in vivo under various conditions. The results demonstrate that in vitro CRF release during two fifteen minute incubation periods immediately after killing is able to reflect the changes in endogenous CRF secretion (assessed by plasma corticosterone levels) under all the conditions tested (i.e. acute ether stress, during the circadian cycle and after chronic betamethasone and reserpine treatments in basal and acute stress conditions) and is therefore a valuable tool for assessing the effects of various treatments on acute CRF release. These results also provide further evidence for control of pituitary/adrenocortical activity by changes in CRF secretion. The circadian rise in plasma corticosterone is preceded by a threefold rise in CRF secretion. Chronic betamethasone treatment blocks basal and stress-induced CRF release, whereas long-term reserpine treatment only causes adaptation to vehicle-injection stress. Acute CRF release was unimpaired in aged rats. A corticotrophin inhibiting factor may be released from the hypothalamus under certain conditions.