Abstract
The Erythrina alkaloids erysodine and dihydro-β-erythroidine (DHβE) are potent and selective competitive inhibitors of α4β2 nicotinic acetylcholine receptors (nAChRs) but little is known about the molecular determinants of sensitivity of this receptor subtype to inhibition by this class of antagonists. We addressed this issue by examining the effects of DHβE and a range of aromatic Erythrina alkaloids on [3H]cytisine binding and receptor function in conjunction with homology models of the α4β2 nAChR, mutagenesis and functional assays. The lactone group of DHβE and a hydroxyl group at position C16 in aromatic Erythrina alkaloids were identified as major determinants of potency, which was decreased when the conserved residue Y126 in loop A of the α4 subunit was substituted by alanine. Sensitivity to inhibition was also decreased by substituting the conserved aromatic residues α4W182 (loop B), α4Y230 (loop C) and β2W82 (loop D) and the non-conserved β2T84, however only α4W182 was predicted to contact bound antagonist, suggesting α4Y230, β2W82 and β2T84 contribute allosterically to the closed state elicited by bound antagonist. Additionally, homology modelling predicted strong ionic interactions between the ammonium centre of the Erythrina alkaloids and β2D196, leading to uncapping of loop C. Consistent with this, β2D196A abolished sensitivity to inhibition by DHβE or erysodine but not by epierythratidine, which is not predicted to form ionic bonds with β2D196. This residue is not conserved in subunits that comprise nAChRs with low sensitivity to inhibition by DHβE or erysodine, which highlights β2D196 as a major determinant of the receptor selectivity of Erythrina alkaloids.
- Dopamine
- Nicotinic cholinergic
- Biogenic Amine
- Structure-activity relationships and modeling
- Nuclear Magnetic Resonance
- Receptor binding studies
- Monoamine oxidases
- Alcohol
Footnotes
- Received April 9, 2010.
- Revision received June 9, 2010.
- Accepted June 14, 2010.
- The American Society for Pharmacology and Experimental Therapeutics