Fractional vesamicol receptor occupancy and acetylcholine active transport inhibition in synaptic vesicles

Vesamicol [(-)-(trans)-2-(4-phenylpiperidino)cyclohexanol] receptor binding and inhibition of acetylcholine (AcCh) active transport by cholinergic synaptic vesicles that were isolated from Torpedo electric organ were studied for 23 vesamicol enantiomers, analogues, and other drugs. Use of trace [3H]vesamicol and [14C]AcCh allowed simultaneous determination of the concentrations of enantiomer, analogue, or drug required to half-saturate the vesamicol receptor (Ki) and to half-inhibit transport (IC50), respectively. Throughout a wide range of potencies for different compounds, the Ki/IC50 ratios varied from 1.5 to 24. Compounds representative of the diverse structures studied, namely deoxyvesamicol, chloroquine, and levorphanol, were competitive inhibitors of vesamicol binding. It is concluded that many drugs can bind to the vesamicol receptor and binding to only a small fraction of the receptors can result in AcCh active transport inhibition. Possible mechanisms for this effect are discussed.