Voltage-clamp analysis of sodium channels in wild-type and mutant Drosophila neurons.

In this study we describe a preparation in which we examined directly, using tight-seal whole-cell recording, sodium currents from embryonic Drosophila neurons maintained in culture. Sodium currents were expressed in approximately 65% of the neurons prepared from wild-type Drosophila embryos when examined at room temperature, 24 hr after plating. While current density was low, other features of the sodium current in wild-type neurons, including the voltage sensitivity, steady-state inactivation, macroscopic time course, and TTX sensitivity were similar to those found in other excitable cells. Physiological and biochemical evidence has led to the suggestion that mutations in the nap, seizure, and tip-E loci of Drosophila may affect voltage-dependent sodium channels. There was no significant difference in the percentage of neurons expressing sodium currents in cultures prepared from embryos with mutations at the nap, sei or tip-E loci compared with wild-type cultures. Sodium currents recorded from napts appeared similar in all of the properties examined to those in wild-type cells. However, neuronal sodium current density was 40-60% lower in cultures prepared from both tip-E and seits1 embryos. The voltage dependence and gating properties of these sodium channels, as well as the TTX sensitivity, appear similar to wild type. These results indicate that both the tip-E and sei loci are important in regulation of sodium current density in embryonic neurons.