The function of gap junctions and chemical synapses in a simple neural circuit

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C. elegans locomotion neural circuit is an excellent model for decoding the circuit and gene bases of behaviors. Previous studies have produced a detailed wiring diagram of this circuit, and proposed that two pairs of premotor interneurons, AVA and AVB, play key roles in locomotion by activating A-type and B-type cholinergic motor neurons (A-MNs and B-MNs), respectively. However, how the various neurons interact at the synaptic level is largely unknown. Our recent success in voltage-clamping motor neurons and recording gap junction currents between neurons in worms makes it possible to look into properties of the circuit that are otherwise inaccessible. Our preliminary studies revealed several unexpected and novel properties of the locomotion circuit, including retrograde gap junction currents between the premotor interneurons and motor neurons, a putative degenerin/epithelial sodium channel (DEG/ENaC) serving as a stretch receptor in B-MNs, retrograde regulation of the backward circuit by GABAergic MNs (D-MNs), and electrical coupling between the left and right AVA interneurons. This proposal is to use a combination of electrophysiological, genetic, molecular, and behavioral approaches to test a new model with three specific aims. Aim 1 is to test the hypotheses that B-MNs activate AVB premotor interneurons through gap junctions, and that B-MN activity depends on a stretch receptor. We will determine molecular mechanisms of the gap junction rectification and the effects of deficient electrical coupling on AVB activity and locomotion behavior. We will identify the putative DEG/ENaC in B-MNs and determine its roles in B-MN activity and locomotion. Aim 2 is to investigate the mechanism of the rectifying coupling between AVA and A-MNs, and the physiological role of electrical coupling between the left and right AVA interneurons. We will determine whether the rectification is conferred by the N-terminal of UNC-7 innexin in AVA and whether disrupting the coupling between AVA interneurons alters locomotion behavior. Aim 3 is to test the hypotheses that D-MNs inhibit A-MNs retrogradely via AVA, and that AVB facilitates forward locomotion by inhibiting AVA. We will assess the effect of disrupting the novel D-MN inhibitory circuit on locomotion. We will identify the postsynaptic receptors mediating AVB inhibition on AVA, and analyze the effect that disrupting this receptor has on locomotion. This project may reshape our understanding of the C. elegans locomotion neural circuit, and help us approach the long-term goal of elucidating the circuit and gene bases of behaviors.