Interactions of GYKI 52466 and NBQX with cyclothiazide at AMPA receptors: Experiments with outside-out patches and EPSCs in hippocampal neurones

In outside-out patches from cultured hippocampal neurones, glutamate (1 mM) applied for 1 ms evoked currents which rose rapidly (τ(on) 451±31 μs) to a peak and then deactivated with slower kinetics (1.95±0.13 ms). Offset time constants were significantly slower with longer application durations (τ(off) 3.10±0.19, 3.82±0.25, 4.80±0.65 and 7.56±0.65 ms with 10, 20, 100 and 500 ms applications respectively). Desensitization was complete within 100 ms with a similar rate for all application durations (4.74±0.34 ms with 100 ms applications). GYKI 52466 reduced inward peak currents with an IC₅₀ of 11.7±0.6 μM and had similar potency on steady-state currents to longer glutamate applications. GYKI 52466 had no significant effect on desensitization or deactivation time constants but caused a modest and significant prolongation of onset kinetics at higher concentrations. Cyclothiazide (100 μM) potentiated steady-state currents 25-fold at 100 ms and caused a modest but significant slowing in onset kinetics (601±49 μs with 1 ms applications) but a more pronounced prolongation of deactivation time constants (5.55±0.66 ms with 1 ms applications). In 50% of neuronal patches cyclothiazide completely eliminated desensitization. In those patches with residual desensitization, the rate was not significantly different to control (5.36±0.43 ms with 100 ms applications). Following 100 ms applications of glutamate, GYKI 52466 had IC₅₀s of 11.7±1.1 μM and 75.1±7.0 μM in the absence and presence of cyclothiazide (100 μM) respectively. Onset kinetics were slowed from 400±20 μs to 490±30 μs by cyclothiazide (100 μM) and then further prolonged by GYKI 52466 (100 μM) to a double exponential function (τ(on1) 1.12±0.13 ms and τ(on2) 171.5±36.5 ms). GYKI 52466 did not re-introduce desensitization but concentration-dependently weakened cyclothiazide's prolongation of deactivation time constants (1 ms applications: 5.01±0.71, 4.47±0.80 and 2.28±0.64 ms with GYKI 52466 30, 100 and 300 μM respectively). NBQX reduced peak current responses with an IC₅₀ of 28.2±1.3 nM. Paradoxically, steady-state currents with 500 ms applications of glutamate were potentiated from 3.3±1.2 pA to 29.4±6.4 pA by NBQX (1 nM). Higher concentrations of NBQX then antagonized this potentiated response. The potency of NBQX in antagonizing steady-state currents to 500 ms applications of glutamate (IC₅₀ 120.9±30.2 nM) was 2-fold less than following 100 ms applications (IC₅₀ 67.7±2.6 nM). NBQX had no effect on rapid onset, desensitization or deactivation time constants. However, a slow relaxation of inhibition was seen with longer applications. NBQX was 2-5-fold less potent against inward currents in the presence of cyclothiazide (100 μM) depending on the application duration but had no effect on the rapid onset, desensitization or deactivation time constants. The same relaxation of inhibition was seen as with NBQX alone. NBQX (1 μM) reduced AMPA receptor-mediated EPSC amplitude to 7±1% of control with no effect on kinetics. Cyclothiazide (330 μM) caused a 2.8-fold prolongation of the decay time constant (control 26.6±2.2 ms, cyclothiazide 74.2±7.6 ms, n=9). Additional application of NBQX (1 μM) partly reversed this prolongation to 1.9 fold (47.7±2.5 ms, n=5).These results support previous findings that cyclothiazide also allosterically influences AMPA receptor agonist/antagonist recognition sites. There were no interactions between NBQX and cyclothiazide on desensitization or deactivation time constants of glutamate-induced currents but clear interactions on EPSC deactivation kinetics. This raises the possibility that the interactions of NBQX, GYKI 52466 and cyclothiazide on AMPA-receptor-mediated EPSC kinetics observed are due to modulation of glutamate-release at presynaptic AMPA receptors. Copyright (C) 1998 Elsevier Science Ltd.