Naturally occurring clavines: Antagonism/partial agonism at 5-HT(2A) receptors and antagonism at α1-adrenoceptors in blood vessels

Article abstract of DOI:10.1055/s-2006-957922

The interaction of eight typical representatives of naturally occurring clavines (agroclavine, costaclavine, dihydrolysergol-I, elymoclavine, festuclavine, lysergene, lysergol, and pyroclavine) with 5-HT(2A) receptors and α₁-adrenoceptors was studied in rat tail artery and aorta, respectively. Clavines antagonized 5-HT-induced contractions with calculated pK(B) values (pK(p) values for partial agonists) of 4.84 - 7.81 and (R)-phenylephrine-induced contractions with calculated pK(B) values of 5.34 - 7.09. Specificity of clavines at 5-HT(2A) receptors relative to α₁-adrenoceptors was rather low. Low affinity for costaclavine at both 5-HT(2A) receptors (pK(p) = 4.84 ± 0.06) and α₁-adrenoceptors (pK(B) = 5.34 ± 0.05) indicates that the trans-junction of ring C and D of the ergoline pharmacophore is crucial for the binding of ergolines to these sites. Lysergol, lysergene, and costaclavine produced non-parallel displacements of the 5-HT concentration-response curve in the rat tail artery and caused small contractions by themselves. Lysergol contracted the rat tail artery with a pEC₅₀ of 6.36 ± 0.04 and an intrinsic activity of 0.18 ± 0.03 with respect to 5-HT. Lysergol-induced contractile responses were surmountably antagonized by ketanserin (10 nM) with a pK(B) of 9.1 which is consistent with an interaction of lysergol with 5-HT(2A) receptors. The pK(p) for the lysergol-5-HT(2A) receptor complex calculated from concentration-response curves to lysergol was 6.88 ± 0.07 and did not match the pK(p) of 7.66 ± 0.02 calculated from antagonism by lysergol of the contractile response to 5-HT. This suggests that lysergol and 5-HT possibly bind in two slightly different orientations at the 5-HT₂(A) receptor. It is concluded that partial agonism and pure antagonism at 5-HT₂(A) receptors on the one side and antagonism at α₁-adrenoceptors on the other side may contribute to the noxious effects of naturally occurring clavines.