Patch clamp characterization of sodium channels expressed from rat brain cDNA

Sodium currents, INa, were recorded from Xenopus laevis oocytes which had been injected with mRNA synthesized by in vitro transcription of the rat brain sodium channel II cDNA (Noda et al. 1986 a, b). Patch pipettes were used to apply depolarizing voltage steps and to record macroscopic sodium currents of between 50 and 750 pA from cell-attached patches of the oocyte membrane. With a combination of whole-cell and patch clamp recording, the properties of the implanted sodium channels could be studied in detail. They were analyzed according to the model of Hodgkin and Huxley (1952 a) assuming three activation gates. The activation of the sodium currents is characterized by an equilibrium potential of -29 mV and an apparent gating charge of 8.7 e0. At -64 mV half of the sodium currents were inactivated. From single-channel current recordings, an elementary sodium channel conductance of 19 pS and an average open time of 0.43 ms were obtained at -32 mV membrane potential and 16 degrees C. The single-channel and activation properties of rat brain sodium channel II are therefore comparable to those found in peripheral nerve and skeletal muscle, but inactivation occurs at less negative potentials. This could be a specific property of the brain sodium channels and may underlie the maintained inward sodium currents reported in brain neurones (French and Gage 1985).