CHARACTERIZATION OF CEMBRANOID INTERACTION WITH THE NICOTINIC ACETYLCHOLINE-RECEPTOR

The class of diterpenoids with a 14-carbon cembrane ring, the cembrano ids, includes both competitive and noncompetitive inhibitors of the ni cotinic acetylcholine receptor (AChR). All 20 coelenterate-derived cem branoids studied in this report inhibited [piperidyl-3,4-H-3]-phencycl idine ([H-3]-PCP) binding to its high-affinity site on the electric or gan AChR, with IC(50)s ranging from 0.9 mu M for methylpseudoplexaurat e to 372 mu M for lophotoxin. Inhibition was complete with all cembran oids but lophotoxin and most Hill coefficients were close to 1. Methyl pseudoplexaurate and [H-3]-PCP binding was competitive. Methylpseudopl exaurate and the fourth most potent cembranoid, eunicin, competed with each other for [H-3]-PCP displacement, indicating that there exist on e or more cembranoid sites on the AChR. Cembranoid affinity for the AC hR correlated with hydrophobicity, but was also dependent on other fea tures. Methylpseudoplexaurate and n-octanol also competed with each ot her for [H-3]-PCP displacement, indicating that the cembranoid site is linked to the n-octanol site on the AChR. Unlike lophotoxin. the five cembranoids tested did not inhibit [I-125]Tyr(54)-alpha-bungarotoxin binding to the AChR agonist sites. All seven cembranoids tested on ooc yte-expressed electric organ AChR reversibly blocked acetylcholine-ind uced currents, although the inhibitor concentration curves were shallo w and the inhibition was incomplete.