Falling asleep after a big meal: Neuronal regulation of satiety

Sleep is required to consolidate odor memory and remodel olfactory synapses

Animals with complex nervous systems demand sleep for memory consolidation and synaptic remodeling. Here, we show that, although the Caenorhabditis elegans nervous system has a limited number of neurons, sleep is necessary for both processes. In addition, it is unclear if, in any system, sleep collaborates with experience to alter synapses between specific neurons and whether this ultimately affects behavior. C. elegans neurons have defined connections and well-described contributions to behavior. We show that spaced odor-training and post-training sleep induce long-term memory. Memory consolidation, but not acquisition, requires a pair of interneurons, the AIYs, which play a role in odor-seeking behavior. In worms that consolidate memory, both sleep and odor conditioning are required to diminish inhibitory synaptic connections between the AWC chemosensory neurons and the AIYs. Thus, we demonstrate in a living organism that sleep is required for events immediately after training that drive memory consolidation and alter synaptic structures.

Sleep Analysis in Adult C. elegans Reveals State-Dependent Alteration of Neural and Behavioral Responses

Sleep, a state of quiescence associated with growth and restorative processes, is conserved across species. Invertebrates including the nematode Caenorhabditis elegans exhibit sleep-like states during development, satiety, and stress. Here, we describe behavior and neural activity during sleep and awake states in adult C. elegans hermaphrodites using new microfluidic methods. We observed effects of fluid flow, oxygen, feeding, odors, and genetic perturbations on long-term sleep behavior over 12 h. We developed a closed-loop sleep detection system to automatically deliver chemical stimuli to assess sleep-dependent changes to evoked neural responses in individual animals. Sleep increased the arousal threshold to aversive stimulation, yet the associated sensory neuron and first-layer interneuron responses were unchanged. This localizes adult sleep-dependent neuromodulation within interneurons presynaptic to the premotor interneurons, rather than afferent sensory circuits. However, sleep prolonged responses in appetitive chemosensory neurons, suggesting that sleep modulates responsiveness specifically across sensory systems rather than broadly damping global circuit activity.

SIGNIFICANCE STATEMENT Much is known about molecular mechanisms that facilitate sleep control. However, it is unclear how these pathways modulate neural circuit-level sensory processing or how misregulation of neural activity contributes to sleep disorders. The nematode Caenorhabditis elegans provides the ability to study neural circuitry with single-neuron resolution, and recent studies examined sleep states between developmental stages and when stressed. Here, we examine an additional form of spontaneous sleep in adult C. elegans at the behavioral and neural activity levels. Using a closed-loop system, we show that delayed behavioral responses to aversive chemical stimulation during sleep arise from sleep-dependent sensorimotor modulation localized presynaptic to the premotor circuit, rather than early sensory circuits.

Feeding, hunger, satiety and serotonin in invertebrates

The serotonergic modulation of feeding behaviour has been intensively studied in several invertebrate groups, including Arthropoda, Annelida, Nematoda and Mollusca. These studies offer comparative information on feeding regulation across divergent phyla and also provide general insights into the neural control of feeding. Specifically, model invertebrates are ideal for parsing feeding behaviour into component parts and examining the underlying mechanisms at the levels of biochemical pathways, single cells and identified neural circuitry. Research has found that serotonin is crucial during certain phases of feeding behaviour, especially movements directly underlying food intake, but inessential during other phases. In addition, while the serotonin system can be manipulated systemically in many animals, invertebrate model organisms also allow manipulations at the level of single cells and molecules, revealing limited and precise serotonergic actions. The latter highlight the importance of local versus global modulatory effects of serotonin, a potentially significant consideration for drug and pesticide design.

Post-feeding behavior in crayfish (Procambarus clarkii): Description of an invertebrate behavioral satiety sequence

Previous studies have demonstrated that food consumption induces the behavioral satiety sequence (BSS) in some animals, a characteristic series of activities which include exploration, grooming, and resting. The BSS, while valuable in assessing the effects of drugs on food intake, has not been widely studied in non-mammalian species. Our experiment examined post-feeding behavior in crayfish using continuous recording of five behaviors: feeding, walking, grooming, leg wave, and quiescence. We found that food intake, but not sham feeding, significantly decreased feeding behavior, increased leg wave, and increased quiescence. Walking and grooming were not significantly altered by food intake. These findings indicate that post-feeding behaviors in crayfish share some characteristics with the mammalian BSS. Detailed descriptions of post-ingestive behavior in invertebrates may contribute to the development of drugs targeting feeding behaviors in medically and economically important species.

Transcriptional control of satiety in Caenorhabditis elegans

Obesity is an enormous worldwide health concern. Chronic illnesses associated with obesity include type-2 diabetes, hypertension, atherosclerosis and certain cancers. Communication between fat storage organs and the brain is essential for regulating feeding, metabolism and organismal activity—and hence obesity control. Model organism research provides opportunities to decipher conserved molecular mechanisms that regulate fat storage and activity levels, which is fundamental to understanding this disorder. We recently identified a transcription factor (ETS-5) that acts in specific neurons of the nematode Caenorhabditis elegans to control intestinal fat levels. Furthermore, we discovered a feedback mechanism where intestinal fat controls feeding and motor programs, similar to humans, where a sated stomach can inhibit feeding and induce lethargy. The precise molecular signals and neuronal circuitry underpinning brain-intestinal communication in C. elegans are however yet to be discovered. As most animals store surplus energy as fat, communication mechanisms that relay external information regarding food availability and quality, and internal energy reserves are likely conserved. Therefore, our identification of a neuronally-expressed transcriptional regulator that controls intestinal fat levels opens up new avenues of investigation for the control of metabolic disease and obesity.

The ETS-5 transcription factor regulates activity states in Caenorhabditis elegans by controlling satiety

Animal behavior is shaped through interplay among genes, the environment, and previous experience. As in mammals, satiety signals induce quiescence in Caenorhabditis elegans Here we report that the C. elegans transcription factor ETS-5, an ortholog of mammalian FEV/Pet1, controls satiety-induced quiescence. Nutritional status has a major influence on C. elegans behavior. When foraging, food availability controls behavioral state switching between active (roaming) and sedentary (dwelling) states; however, when provided with high-quality food, C. elegans become sated and enter quiescence. We show that ETS-5 acts to promote roaming and inhibit quiescence by setting the internal "satiety quotient" through fat regulation. Acting from the ASG and BAG sensory neurons, we show that ETS-5 functions in a complex network with serotonergic and neuropeptide signaling pathways to control food-regulated behavioral state switching. Taken together, our results identify a neuronal mechanism for controlling intestinal fat stores and organismal behavioral states in C. elegans, and establish a paradigm for the elucidation of obesity-relevant mechanisms.