Coordination of rhythmic locomotion depends upon a precisely balanced interplay between central and peripheral control mechanisms. Although poorly understood, peripheral proprioceptive mechanosensory input is thought to provide information about body position for moment-to-moment modifications of central mechanisms mediating rhythmic motor output. Pickpocket1 (PPK1) is a Drosophila subunit of the epithelial sodium channel (ENaC) family displaying limited expression in multiple dendritic (md) sensory neurons tiling the larval body wall and a small number of bipolar neurons in the upper brain. ppk1 null mutant larvae had normal external touch sensation and md neuron morphology but displayed striking alterations in crawling behavior. Loss of PPK1 function caused an increase in crawling speed and an unusual straight path with decreased stops and turns relative to wild-type. This enhanced locomotion resulted from sustained peristaltic contraction wave cycling at higher frequency with a significant decrease in pause period between contraction cycles. The mutant phenotype was rescued by a wild-type PPK1 transgene and duplicated by expressing a ppk1RNAi transgene or a dominant-negative PPK1 isoform. These results demonstrate that the PPK1 channel plays an essential role in controlling rhythmic locomotion and provide a powerful genetic model system for further analysis of central and peripheral control mechanisms and their role in movement disorders.

The ability to detect salt is critical for the survival of terrestrial animals. Based on amiloride-dependent inhibition, the receptors that detect salt have been postulated to be DEG/ENaC channels. We found the Drosophila DEG/ENaC genes Pickpocket11 (ppk11) and Pickpocket19 (ppk19) expressed in the larval taste-sensing terminal organ and in adults on the taste bristles of the labelum, the legs, and the wing margins. When we disrupted PPK11 or PPK19 function, larvae lost their ability to discriminate low concentrations of Na(+) or K(+) from water, and the electrophysiologic responses to low salt concentrations were attenuated. In both larvae and adults, disrupting PPK11 or PPK19 affected the behavioral response to high salt concentrations. In contrast, the response of larvae to sucrose, pH 3, and several odors remained intact. These results indicate that the DEG/ENaC channels PPK11 and PPK19 play a key role in detecting Na(+) and K(+) salts.

Drosophila melanogaster has proven to be a good model for understanding the physiology of ion channels. We identified two novel Drosophila DEG/ ENaC proteins, Pickpocket (PPK) and Ripped Pocket (RPK). Both appear to be ion channel subunits. Expression of RPK generated multimeric Na+ channels that were dominantly activated by a mutation associated with neurodegeneration. Amiloride and gadolinium, which block mechanosensation in vivo, inhibited RPK channels. Although PPK did not form channels on its own, it associated with and reduced the current generated by a related human brain Na+ channel. RPK transcripts were abundant in early stage embryos, suggesting a role in development. In contrast, PPK was found in sensory dendrites of a subset of peripheral neurons in late stage embryos and early larvae. In insects, such multiple dendritic neurons play key roles in touch sensation and proprioception and their morphology resembles human mechanosensory free nerve endings. These results suggest that PPK may be a channel subunit involved in mechanosensation.

Although the ability to sense temperature is critical for many organisms, the underlying mechanisms are poorly understood. Using the calcium reporter yellow cameleon 2.1 and electrophysiological recordings, we identified thermosensitive neurons and examined their physiologic response in Drosophila melanogaster larvae. In the head, terminal sensory organ neurons showed increased activity in response to cooling by < or =1 degrees C, heating reduced their basal activity, and different units showed distinct response patterns. Neither cooling nor heating affected dorsal organ neurons. Body wall neurons showed a variety of distinct response patterns to both heating and cooling; the diverse thermal responses were strikingly similar to those described in mammals. These data establish a functional map of thermoresponsive neurons in Drosophila larvae and provide a foundation for understanding mechanisms of thermoreception in both insects and mammals.