Different Shaker family alpha-subunit genes generate distinct voltage-dependent K+ currents when expressed in heterologous expression systems. Thus it generally is believed that diverse neuronal K+ current phenotypes arise, in part, from differences in Shaker family gene expression among neurons. It is difficult to evaluate the extent to which differential Shaker family gene expression contributes to endogenous K+ current diversity, because the specific Shaker family gene or genes responsible for a given K+ current are still unknown for nearly all adult neurons. In this paper we explore the role of differential Shaker family gene expression in creating transient K+ current (IA) diversity in the 14-neuron pyloric network of the spiny lobster, Panulirus interruptus. We used two-electrode voltage clamp to characterize the somatic IA in each of the six different cell types of the pyloric network. The size, voltage-dependent properties, and kinetic properties of the somatic IA vary significantly among pyloric neurons such that the somatic IA is unique in each pyloric cell type. Comparing these currents with the IAs obtained from oocytes injected with Panulirus shaker and shal cRNA (lobster Ishaker and lobster Ishal, respectively) reveals that the pyloric cell IAs more closely resemble lobster Ishal than lobster Ishaker. Using a novel, quantitative single-cell-reverse transcription-PCR method to count the number of shal transcripts in individual identified pyloric neurons, we found that the size of the somatic IA varies linearly with the number of endogenous shal transcripts. These data suggest that the shal gene contributes substantially to the peak somatic IA in all neurons of the pyloric network.

The transient potassium (K+) current, or A-current (IA), plays an essential role in shaping the firing properties of identified neurons in the 14-cell pyloric network in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus. The different cells in the pyloric network have distinct IAs. To begin to understand the molecular basis for IA heterogeneity, we examined the relationship between the Panulirus shal current, the IAs in the lateral pyloric (LP) and pyloric dilator (PY) cells, and the Drosophila shal current. After isolating a complete open reading frame for lobster shal 1, which shows significant sequence homology to the fly, mouse, and rat shal homologs, we used a single-cell reverse transcription polymerase chain reaction method to demonstrate that the shal 1 gene was expressed in the LP and PY cells. Next, we compared the lobster shal 1 current generated in a Xenopus oocyte expression system to the IAs in the LP and PY neurons as well as to the Drosophila shal current in Xenopus oocytes. While the transient K+ lobster shal 1 current was similar to the IAs in pyloric neurons, a detailed comparison shows that they are not identical and differ in kinetic and voltage-dependent parameters. The highly homologous lobster and fly shal genes also produce currents with some significant similarities and differences in an oocyte expression system.

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