Ultrashort-loop positive feedback of corticotropin (ACTH)-releasing factor to enhance ACTH release in stress

Previous experiments have shown that intraventricular injection of ovine corticotropin (ACTH)-releasing factor (oCRF) in doses too low to elevate plasma ACTH by direct action on the pituitary does not lower plasma ACTH, suggesting that the peptide lacks a negative ultrashort-loop feedback action to suppress its own release under resting conditions. The present study was performed to determine whether oCRF has any action to alter CRF release in stress. The peptide was injected into the third ventricle or external jugular vein of freely moving ovariectomized female rats 5 min prior to application of ether stress. When oCRF was injected into the third ventricle in doses of 500 pg (0.1 pmol) or less, there was no significant alteration in plasma ACTH prior to ether stress; however, there was a significantly enhanced increase in plasma ACTH 2 and 5 min after ether stress applied 5 min after intraventricular injection of oCRF at doses of 50 (0.01 pmol) or 150 pg (0.03 pmol). These results suggest that the peptide acts on structures adjacent to the third ventricle to augment stress-induced CRF release. To rule out the possibility that the sensitivity of the pituitary itself to CRF increases dramatically following stress, 10 or 100 ng of oCRF was injected i.v. These doses produced a significant dose-related increase in plasma ACTH at 2, 5, or 15 min. In other groups receiving the same doses of oCRF and ether stressed 5 min later, plasma ACTH was significantly higher 2 or 5 min after ether stress when compared with plasma ACTH in ether-stressed saline-injected animals. However, in contrast with the results of intraventricular injection of oCRF, the release of ACTH was no greater than that obtained by summing the independent effects of exogenous oCRF and the CRF released by stress. We conclude that CRF may have a positive ultrashort-loop feedback action to enhance stress-induced ACTH release and that this enhancement is not due to increased sensitivity of anterior pituitary corticotrophs to CRF.