Comparison of the binding of optically pure (-)- and (+)- [3H]nicotine to rat brain membranes

A comparison of the binding of (-)- and (+)-[3H]nicotine to rat brain membranes revealed that only the (-)-enantiomer showed high affinity binding; while the (+)-enantiomer was at least 1/10 as effective as the (-)-enantiomer when in competition with (-)-[3H]nicotine as the ligand. Positive cooperativity, which is observed with (-)-[3H]nicotine in the presence of low concentrations of (+)-nicotine, may account for the seeming paradox.