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Abstract:

Animals interact with a constantly changing environment and have to be able to adapt their behavior accordingly. At the molecular level, external and internal stimuli are processed by a plethora of signaling molecules that modulate neuronal circuits and their behavioral output. A diverse group of small messengers is represented by neuropeptides that mostly act through binding of G protein-coupled receptors. Besides modulatory actions in the nervous system, neuropeptides can have diverse effects on animal physiology, and are implicated in a broad repertoire of biological processes such as reproduction, feeding, learning and memory. In this work, neuropeptidergic signaling was studied in the genetically tractable model Caenorhabditis elegans, taking advantage of the nematode's extensive genetic tool box and well-defined nervous system that contains only 302 neurons. Despite this apparent simplicity, C. elegans displays many complex behaviors including adaptation, learning and memory. By a combined bioinformatic and reverse pharmacology approach, we identified three novel neuropeptidergic systems in C. elegans related to mammalian vasopressin/oxytocin signaling, and myoinhibiting peptide and short neuropeptide F signaling in insects. We further investigated the function of C. elegans vasopressin/oxytocin-like signaling to provide evolutionary and mechanistic insights into the actions of this neuropeptide family.Vasopressin and oxytocin are related neuropeptides of the mammalian hypothalamus. Acting as hormones, they are important regulators of vertebrate physiology controlling water balance and reproduction. In addition, vasopressin and oxytocin influence social behavior, learning, and memory by neuromodulatory actions in the brain. Related neuropeptides with conserved physiological effects are present in invertebrate lineages as well. However, the functional knowledge on invertebrate orthologs is generally limited. Putative neuromodulatory actions remain unexplored, and few insights have been obtained into the mechanisms underlying physiological functions.The C. elegans vasopressin/oxytocin-related signaling system identified in this study was named the nematocin system. Like most invertebrates, C. elegans has a single vasopressin/oxytocin-like peptide that activates the nematocin receptor NTR-1. In vivo expression analysis revealed that the nematocin precursor is exclusively expressed in the C. elegans nervous system, which is also the main expression site for its target receptor. The expression of NTR-1 in several chemosensory neurons suggests that nematocin acts as a neuromodulator of the C. elegans sensory circuit. Guided by previously described functions of NTR-1 expressing neurons, we investigated the effects of impaired nematocin signaling on salt chemotaxis behavior of C. elegans. Nematocin is not required for salt chemotaxis under normal conditions, but is important for modulating this behavior in light of previous experience. Worms are normally attracted to low salt concentrations, but quickly learn to avoid salt if they experience it in the absence of food. This behavioral change is referred to as gustatory plasticity, and represents a type of associative learning. Mutants with impaired nematocin signaling are inefficient in gustatory plasticity. Insights from cell-specific rescue experiments indicate that nematocin targets a salt-sensory neuron in the gustatory plasticity circuit, and originates from a pair of interneurons, the AVK cells. Molecular players that interact with nematocin signaling in the gustatory circuit were identified by the behavioral analysis of double mutants and supplementation studies, and include the aminergic neurotransmitters serotonin and dopamine. The molecular-genetic study of the C. elegans nematocin system indicates that vasopressin/oxytocin-related peptides have modulatory effects on invertebrate neuronal circuits governing associative learning. This function is similar to the described actions of vasopressin and oxytocin neuropeptides on learning and memory in mammals, and suggests ancient roots for the neuropeptidergic control of cognitive processes. Functional conservation in C. elegans enables future exploitation of this model to characterize the basic mechanisms of central vasopressin/oxytocin signaling. Insights into the molecular and cellular network presented in this work provide a foundation for unraveling the precise neuromodulatory mechanisms of C. elegans nematocin signaling.