PHARMACOLOGICAL COMPARISON OF 2 CORTICOTROPIN-RELEASING FACTOR ANTAGONISTS - IN-VIVO AND IN-VITRO STUDIES

The present study compared the effects of two analogs of corticotropin -releasing factor (CRF), [D-Phe(12), Nle(21,38) C(alpha)MeLeU(37)]CRF( 12-41) (D-PheCRF(12-41)) and alpha helical CRF(9-41), as antagonists o f CRF in in vivo and in vitro assays. In halothane-anesthetized rats, intracerebroventricular (i.c.v.) administration of both analogs inhibi ted the activation of locus coeruleus (LC) neuronal discharge produced by CRF (3.0 mu g, i.c.v.). LC activation by hypotensive stress elicit ed by intravenous (i.v.) infusion of nitroprusside was antagonized by the same doses of the CRF antagonists that were effective in antagoniz ing CRF, suggesting that the receptors involved in LC activation by CR F and by hypotensive stress are similar. However, D-PheCRF(12-41) was approximately 100 times more potent than a helical CRF(9-41) when admi nistered i.c.v. The IC50 values for D-PheCRF(12-41) as an antagonist o f CRF and of nitroprusside were 0.16 and 0.14 mu g, i.c.v., respective ly. The IC50 values for a helical CRF(9-41) as an antagonist of CRF an d of nitroprusside were 18 and 27 mu g, i.c.v., respectively. In contr ast, D-PheCRF(12-41) was Only slightly more potent than alpha helical CRF(9-41) in antagonizing CRF-stimulated cyclic AMP production in rat brain homogenates, with IC(50)s of 78 +/- 15 and 260 +/- 30 nM for D-P heCRF(12-41) and alpha helical CRF(9-41), respectively. Moreover, the antagonists had similar affinities for CRF binding sites in rat brain homogenates, with K(i)s of 15.5 +/- 4 nM and 10.3 +/- 6 nM for D-PheCR F(12-41) and alpha helical CRF(9-41), respectively. The results suppor t previous studies suggesting that CRF serves as a neurotransmitter to activate the LC during hypotensive stress. The relatively lower poten cy of alpha helical CRF(9-41) in vivo, but not in vitro, may be due to a differential distribution of the antagonists after i.c.v. administr ation or to possible affinity differences at a recently characterized nonreceptor CRF-binding protein.