Molecular underpinnings of motor pattern generation: differential targeting of shal and shaker in the pyloric motor system

The patterned activity generated by the pyloric circuit in the stomatogastr ic ganglion of the spiny lobster, Panulirus interruptus, results not only f rom the synaptic connectivity between the 14 component neurons but also fro m differences in the intrinsic properties of the neurons. Presumably, diffe rences in the complement and distribution of expressed ion channels endow t hese neurons with many of their distinct attributes. Each pyloric cell type possesses a unique, modulatable transient potassium current, or A-current (I-A), that is instrumental in determining the output of the network. Two g enes encode A-channels in this system, shaker and shal. We examined the hyp othesis that cell-specific differences in shaker and shal channel distribut ion contribute to diversity among pyloric neurons. We found a stereotypic d istribution of channels in the cells, such that each channel type could con tribute to different aspects of the firing properties of a cell. Shal is pr edominantly found in the somatodendritic compartment in which it influences oscillatory behavior and spike frequency. Shaker channels are exclusively localized to the membranes of the distal axonal compartments and most likel y affect distal spike propagation. Neither channel is detectably inserted i nto the preaxonal or proximal portions of the axonal membrane. Both channel types are targeted to synaptic contacts at the neuromuscular junction. We conclude that the differential targeting of shaker and shal to different co mpartments is conserved among all the pyloric neurons and that the channels most likely subserve different functions in the neuron.