Effect of cyclothiazide on spontaneous miniature excitatory postsynaptic currents in rat hippocampal pyramidal cells

Spontaneous miniature excitatory postsynaptic currents (mEPSCs) were recorded from the CA1 region of slices using the whole-cell patch-clamp technique. Cyclothiazide (0.1 mM), a complete blocker of desensitization of (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) channels, was applied to determine the changes in amplitude and kinetics of mEPSCs occurring with complete suppression of desensitization. The amplitude of mEPSC (A) was not affected significantly by cyclothiazide, but both the rise (tau(r)) and the decay time (tau(d)) were consistently increased (from 2.3 to 6.5 ms and from 9.9 to 22.2 ms respectively). The amplitude dependence of both tau(d) and tau(r) became much greater, but there was no upward shift of the best-fitting lines. The slopes of the control best-fitting lines were (+/-SD; ms/pA; n=5) 0.39+/-0.05 for tau(d):A and 0.12+/-0.07 for tau(r):A, but, in the presence of cyclothiazide, the corresponding slopes were much steeper (2.1+/-0.60 and 0.68+/-0.21; holding potential was -50 mV and temperature 32 degrees C). These changes, which were slow to develop, suggest that cyclothiazide blocks AMPA receptor channel desensitization, whilst having no effect on the closing rate of AMPA channels. Judging by the extent of change, the speed of diffusion of glutamate in the synaptic cleft is probably similar to that in water. In conclusion, this study provides evidence that: (1) under control conditions, desensitization of AMPA channels plays a major role in shaping the time course of synaptic currents in CA1; and (2) cyclothiazide prolongs their time course solely by abolishing desensitization.