Alaproclate effects on voltage-dependent K + channels and NMDA receptors: Studies in cultured rat hippocampal neurons and fibroblast cells transformed with Kvl.2 K + channel cDNA

Abstract

The effects of alaproclate on voltage-dependent K + currents and N-methyl- d-aspartate (NMDA) and ψ -aminobutyric acid A (GABA A) receptor currents were investigated in cultured rat hippocampal neurons using whole-cell voltage clamp recording techniques. Alaproclate produced a concentration-dependent block of the sustained voltage-dependent K + current activated by depolarization from −60 to +40mV (IC 50, 6.9μM). At similar concentrations alaproclate also blocked the sustained voltage-dependent K + current in fibroblast cells transformed to stably express Kvl.2 K + channels. Analysis of tail currents and the voltage-dependence of the alaproclate block suggested an open-channel blocking mechanism. Alaproclate also produced a potent block of NMDA receptor currents in hippocampal neurons (IC 50, 1.1 μM), but did not affect GABA A receptor currents (concentrations up to 100μM). The alaproclate block of NMDA receptors occurred predominantly by an open-channel mechanism, although the drug was also able to block closed NMDA channels at a much slower rate. The interaction of alaproclate with NMDA receptors (activated by 10 μM NMDA) appeared to be governed by a first order binding reaction with forward and reverse rate constants of 6.7 × 10 3M −1, and 0.025 sec −1, respectively (at −60 mV). At depolarized potentials the alaproclate-induced block of the NMDA receptor current was strongly reduced, a result opposite to that seen with the voltage-activated K + currents, suggesting that the K + channel block may occur at a superficial internal site, whereas the NMDA receptor block occurs at a deep external site. ( + )-Alaproclate was a more potent blocker of K + currents than (-)-alaproclate, whereas a reversed stereoselectivity was observed for NMDA receptor current, supporting the view that alaproclate block of the two channel types occurs at structurally distinct binding sites.