A sleep- and locomotion stop neuron with compartmentalized Ca2+ dynamics as a CPG regulator?

• Applicant: Professor Dr. Alexander Gottschalk
• Subject Area: Cognitive, Systems and Behavioural Neurobiology ; Sensory and Behavioural Biology ; Molecular Biology and Physiology of Neurons and Glial Cells
• Term: since 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 323383487

Animals need to be able to actively end locomotion, in order to await certain events, or to re-initiate locomotion in a different direction, e.g. during navigation. Animals also need to stop locomotion when they enter sleep, and typically this occurs by different mechanisms. However, a single neuron of the nematode Caenorhabditis elegans, RIS, is involved in orchestrating developmentally timed sleep in larval stages, and, as we could show, also acts as a locomotion stop neuron in adult animals. RIS activity occurs right before slowing, and is further involved in instructing reversals. RIS shows compartmentalized Ca2+ activity along its axon: While the terminal part becomes active whenever slowing occurs, a ventral branch of the axon shows activity only when the slowing event is followed by a reversal. By optogenetic stimulation we showed that RIS stops locomotion by releasing GABA and FLP-11 neuropeptides. The latter is involved in silencing rhythmic / synchronized activity of motor neurons, as part of pattern generators for locomotion. However, this aspect is not clarified in detail yet. Furthermore, we do not know the exact interplay of RIS with other neurons in the C. elegans nervous system, that cooperate to initiate and regulate locomotion and reversals. In this extension of our previous proposal, we want to address the function of RIS in concert with other neurons. Preliminary data, where we concomitantly imaged Ca2+ activity in RIS and RIM neurons (the latter are involved in orchestrating reversal behavior) in moving animals, shows an active interplay of these cells, where RIS activity, surprisingly, precedes RIM. Furthermore, by voltage imaging, we show active gap junction connections of RIS to RIM and AVJ neurons, which have anatomical connections to the branch of the RIS axon. We will analyze these interactions in detail. We will also address interactions of RIM with PVC forward command interneurons, that chemically synapse onto the RIS branch, and which may inhibit RIS in order to prevent reversals, as indicated by reduced activity preceding such events. Using voltage imaging, we found that RIM and RIS neurons show tightly coupled, but reciprocal membrane potential changes, possibly mediated by rectifying gap junctions. We will analyze these interactions and their dependence on distinct molecular players (gap junction subunits, neuropeptide receptors). Also, we will assess electrical events in motor neurons along the ventral nerve cord, following RIS photostimulation. We expect this to influence different groups of motor neurons differently, enabling us to uncover specific effects in pattern generation.Analyzing the functions of RIS in sleep and locomotion stop, and how it uncouples pattern generators, will help understanding how these fundamental functions, present in one neuron in the compact C. elegans nervous system, may have evolved and distributed to distinct brain systems in higher animals.

• DFG Programme: Research Grants