Alkylene tether-length dependent γ-aminobutyric acid type A receptor competitive antagonism by tacrine dimers

Bis(7)-tacrine was previously demonstrated as an antagonist of γ-aminobutyric acid type A (GABAA) receptors. In this study, the effects of a series of alkylene-linked tacrine dimers on GABAAreceptors were examined. In radioligand binding assay, the analogues differed in binding affinity for GABAAreceptors, and potency monotonically increased as the tether was shortened from nine to two methylenes. Bis(2)-tacrine, the shortest tacrine dimer, could displace [3H]muscimol from rat brain membranes with an IC50of 0.48 μM, which was 11, 13 and 525 times more potent than the GABAAreceptor antagonist (+)-bicuculline, bis(7)-tacrine and tacrine, respectively. In whole-cell patch-clamp recordings, these dimeric tacrine analogues competitively antagonized GABA-induced inward current with a rank order of potency of bis(2)-tacrine > bicuculline > bis(7)-tacrine > bis(9)-tacrine > tacrine, and the potency of bis(2)-tacrine was 11, 18 and 487 times higher than that of (+)-bicuculline, bis(7)-tacrine and tacrine, respectively. Bis(2)-tacrine shifted the GABA concentration-response curve to the right in a parallel manner, and the inhibition was voltage-independent between -80 and +20 mV. It can be concluded that the shorter the alkylene linkage in tacrine dimers the stronger the binding affinity and higher the antagonistic effect on the GABAAreceptor will be.