Temperature Dependence of Pyrethroid Modification of Single Sodium Channels in Rat Hippocampal Neurons

Pyrethroid modulation of sodium channels is unique in the sense that it is highly dependent on temperature, the potency being augmented by lowering the temperature. To elucidate the mechanisms underlying the negative temperature dependence of pyrethroid action, single sodium channel currents were recorded from cultured rat hippocampal neurons using the inside-out configuration of patch-clamp technique, and the effects of the pyrethroid tetramethrin were compared at 22 and 12°C. Tetramethrin-modified sodium channels opened with short closures and/or transitions to subconductance levels at 22 and 12°C. The time constants of the burst length histograms for tetramethrin-modified channels upon depolarization to −60 mV were 7.69 and 14.46 msec at 22 and 12°C, respectively (Q₁₀= 0.53). Tetramethrin at 10 μm modified 17 and 23% of channels at 22 and 12°C, respectively, indicating that the sensitivity of the sodium channel of rat hippocampal neurons to tetramethrin was almost the same as that of tetrodotoxin-sensitive sodium channels of rat dorsal root ganglion neurons and rat cerebellar Purkinje neurons. The time constants for burst length in tetramethrin-modified sodium channels upon repolarization to −100 mV from −30 mV were 8.26 and 68.80 msec at 22 and 12°C (Q₁₀= 0.12), respectively. The prolongation of tetramethrin-modified whole-cell sodium tail currents upon repolarization at lower temperature was ascribed to a prolongation of opening of each channel. Simple state models were introduced to interpret behaviors of tetramethrin-modified sodium channels. The Q₁₀ values for transition rate constants upon repolarization were extremely large, indicating that temperature had a profound effect on tetramethrin-modified sodium channels. Received: 31 January 2000/Revised: 18 May 2000