One of the central functions of the nervous system is to enable survival. Arousal is the sustained state of hyper-vigilance that produces increased locomotor activity and sensory responsiveness. Research in invertebrates has demonstrated the ubiquity of arousal and revealed important conceptual insights. Despite these advances, the mechanistic processes underlying arousal remain unclear. We are interested in how neuromodulators control arousal in C. elegans. We found that the neuropeptide FLP-20 is required for locomotor arousal - the sustained increase in speed following a noxious stimulus- as flp-20 mutants show a defective arousal response when subjected to a mechanical "tap" stimulus. This defect can be rescued by re-expression of flp-20 specifically in the gentle touch neurons, indicating that flp-20-encoded peptides are released from the touch neurons to facilitate arousal. Through a candidate screen, we found that FLP-20 binds the G-protein coupled receptor FRPR-3 in an in vitro binding assay. Interestingly, frpr-3 mutants are also defective in arousal. We are currently identifying FRPR-3-positive neurons to determine which responding cells are required to bind FLP-20 and drive locomotor arousal after a tap stimulus. We also found that the corticotrophin releasing factor (CRF) signalling pathway in C. elegans acts to modulate arousal. We recently identified the ligand for the CRF receptor orthologue, SEB-3, to be encoded by a peptide precursor gene we have named crf-1. A gain-of-function mutation in seb-3, eg696, leads to increased arousal, and this is suppressed in mutants containing a nonsense allele for crf-1. Conversely, CRF-1 over-expression leads to an enhanced tap arousal phenotype that is lost in the absence of the SEB-3 receptor. CRF-1 is expressed in a small number of head neurons, including the AVK interneuron and SMB motorneurons. We found that transgenic over-expression of crf-1 solely from AVK is sufficient to drive enhanced arousal, but this is not the case for the nearby SMBs. The SEB-3 receptor is widely expressed throughout the nervous system, including in chemosensory and mechanosensory neurons, command interneurons, and cholinergic motor neurons in the head and ventral cord. We are currently investigating which of these SEB-3-expressing cells are required to respond to high levels of CRF-1 and promote an aroused behavioural state. Interestingly, CRF in humans, which is released from the hypothalamus in response to stress, acts to promote arousal and anxiety. Therefore, the SEB-3/CRF-1 system in C. elegans could represent a conserved stress pathway, and presents an attractive model to investigate CRF-mediated stress responses in a compact nervous system.

Affiliations:
- Cell Biology Division, MRC Laboratory of Molecular Biology, Cambridge, Cambridgeshire, UK
- Functional Genomics and Proteomics Group, Division of Animal Physiology and Neurobiology, KU Leuven, Belgium
- Neurobiology Division, MRC Laboratory of Molecular Biology, Cambridge, Cambridgeshire, UK