Iwanir, Shachar et al. (2015) International Worm Meeting "Serotonin promotes exploitation in complex environments by accelerating decision-making.."

Brown, Adam, Biron, David, Iwanir, Shachar, Najjar, Dana, Nagy, Stanislav

[International Worm Meeting, 2015]

When resources are not uniformly available, fast responses can provide a competitive advantage. The neurotransmitter serotonin (5-HT) was shown affect C. elegans locomotion on food in steady-state and following a period of food deprivation. We identified a novel role of 5-HT signaling in C. elegans: mediating a rapid response required for efficient exploitation in complex environments. Deficiency in 5-HT synthes­is, as well as genetic ablation of serotonergic neurons, resulted in gradual responses and defective exploitation. Both the ADF and NSM serotonergic neuronal types were implicated in these responses, albeit in different ways. Collectively, physiological imaging, genetic silencing, functional rescues, and optogenetic activation revealed that NSM responses initiated upon encounter and were key to rapid decision-making. In contrast, the onset of ADF responses preceded the physical encounter with the food. The relevance of responding rapidly was demonstrated in patchy environments, where the absence of 5-HT signaling significantly reduced exploitation. Collectively, our results suggest a novel role for 5-HT in decision-making, demonstrate its fitness consequences, and show that NSM and ADF act in concert to modulate locomotion in complex environments. .

Iwanir, Shachar et al. (2013) International Worm Meeting "A calcium-rich breakfast: physiological activity in C. elegans serotonergic neurons during the enhanced slowing response and upon emergence from lethargus."

Palmer, Meagan, Fitzgerald, Ivy, Brown, Adam, Iwanir, Shachar, Najjar, Dana, Biron, David

[International Worm Meeting, 2013]

Serotonin has been implicated in the assessment of resource availability and in the modulation of sleep in vertebrates. Extracellular serotonin levels in the hypothalamus are increased by the presence of food. Additionally, the firing of serotonergic cells in the brainstem raphe nuclei is tonic during wakefulness and significantly reduced during sleep states. In the nematode Caenorhabditis elegans, serotonin signaling has been shown to mediate the enhancing effect of starvation on the slowing response to encountering a food source. However, the understanding of the cellular mechanisms and physiological dynamics underlying this response is incomplete. We combined behavioral studies with physiological imaging of calcium dynamics in the serotonergic NSM and ADF neurons of freely-behaving animals in order to compare the responses to reintroduction of food in two situations: following a period of food deprivation and at the emergence from lethargus, a sleep-like state that involves a cessation of feeding. We identified similarities in the patterns of locomotion and physiological activity of NSM between these two circumstances. The enhanced slowing response was characterized by two phases: (i) preceding a spatial encounter with food, physiological activity in ADF neurons increased (likely via a chemosensory mechanism), concomitant with a gradual decrease in the rate of locomotion, and (ii) upon encountering food, NSM exhibited calcium transients lasting 1-2 minutes, accompanied by a sharp decrease in the rate of locomotion. Emergence from lethargus was associated with a period of extensive feeding and reduced locomotion, accompanied by a transient increased activity in NSM (but not ADF). Our results may point to a common origin of the modulation of serotonin signaling by arousal and feeding states, and suggest roles for both chemosensation and metabolic cues in the regulation of the enhanced slowing response.

Lee, Kyung Suk et al. (2015) International Worm Meeting "Serotonin-dependent Pulse-Width-Modulation control of food uptake."

Lee, Kyung Suk, Iwanir, Shachar, Biron, David, Levine, Erel, Kopito, Ronen

[International Worm Meeting, 2015]

Animals integrate physiological and environmental signals to regulate food uptake by modulating feeding behavior. Failure to regulate feeding may have devastating results, including obesity and diabetes, underscoring the importance of understanding its underlying mechanisms. The nematode C. elegans, whose food uptake consists simply of pumping bacteria from the environment into the gut, provides excellent opportunities for discovering principles of conserved regulatory mechanisms. Here we take an approach rooted in control theory, and measure at high resolution the dynamics of feeding in response to food availability using a custom microfluidic device. We show that worms implement a graded response by balancing two discrete modes of feeding, a control mechanism known as Pulse-Width-Modulation (PWM). The bioamine serotonin, which has a conserved role in regulating feeding, is shown to be essential for the recurring transitions into the active feeding mode. Two types of serotonergic neurons are implicated in this mechanism, and two families of serotonin receptors are shown to have distinct roles in tuning its properties. We propose that PWM is a conserved mechanism of behavior and motor control.

Katz, Menachem et al. (2015) International Worm Meeting "Glia control locomotion and sleep in C. elegans."

Iwanir, Shachar, Shaham, Shai, Biron, David, Katz, Menachem, Dragomir, Elena, Corson, Francis

[International Worm Meeting, 2015]

Glia are abundant components of the nervous system, yet their contributions to neural circuits and animal behavior are poorly characterized. To study these contributions, we ablated the astrocyte-like CEPsh glia that ensheath synapses in the C. elegans nerve ring. We find that ablation results in enhanced locomotory quiescence preceding molting-associated lethargy (a sleep-like state), and molting-independent short-duration locomotory pausing. Synapses between the neuron ALA, which governs sleep-like behavior, and the locomotory interneuron AVE are ensheathed by CEPsh glia. Loss of ALA function suppress both precocious lethargus and locomotory pausing, suggesting that CEPsh blocks a locomotion-inhibiting signal from ALA. Several observations suggest this inhibition occurs at the ALA-AVE synapses. Inactivation of AVE promotes locomotory pausing independently of ALA or CEPsh glia states, suggesting it acts downstream of these two cells. In addition, CEPsh glia ablation results in prolonged calcium transients in AVE, which are rescued by ALA inactivation. Finally, AVE calcium levels normally correlate with backwards locomotion; in CEPsh-ablated animals, calcium induction in AVE is uncoupled from movement, and coupling is restored by ALA inactivation. These results suggest that CEPsh glia are essential for the functions of ALA-AVE synapses. To understand how CEPsh glia control synaptic activity, we developed a robust method for isolating these cells from larvae, and generated a list of CEPsh glia-enriched genes using RNAseq. RNAi studies have uncovered potential roles for some enriched genes in locomotion control. CEPsh glia also ensheath the outer aspect of the nerve ring, where they are in close proximity to glutamatergic synapses. Studies in other labs showed that glutamatergic signaling levels correlate with reversal frequency. We found that CEPsh-disrupted animals have an increased reversal frequency, suggesting accumulation of glutamate at synapses. Analysis of the CEPsh transcriptome revealed enriched expression of glt-1, a homolog of the astrocytic glutamate clearance transporter (EAAT2). glt-1 mutants were previously shown to have increased reversal frequency, and we found that expression of glt-1 in CEPsh glia rescues this defect. Together, our studies establish functional and molecular homology between CEPsh glia and vertebrate astrocytes, and identify exciting roles for these cells in locomotory control in C. elegans.

Ruach, Rotem et al. (2019) International Worm Meeting "Irrational behavior in C. elegans arises due to asymmetric modulatory effects between options."

Eyal, Itskovits, Iwanir, Shachar, Ruach, Rotem, Pritz, Christian, Bokman, Eduard, Zaslaver, Alon

[International Worm Meeting, 2019]

Animals, like humans, repeatedly violate rational-choice paradigms, yet the underlying reasons remain debatable. This is primarily because population variability, past experience, current state, and future expectations affect decision-making cognitive processes, thus precluding decisive conclusions. Here, we established C. elegans nematodes as a powerful model for studying rationality of an innate behavior - chemotaxis, thus overcoming many of the above confounding effects. Moreover, innate behaviors presumably evolved to comply with rational economic axioms to maximize fitness. Surprisingly, we found that worms' chemotaxis behavior robustly violates key rationality paradigms of transitivity, independence of irrelevant alternatives and regularity. These violations arise due to asymmetric modulatory effects between the presented options. Functional analysis of the entire chemosensory system at a single-neuron resolution, coupled with analyses of mutants, defective in individual neurons, reveals that these asymmetric effects originate in specific sensory neurons. Thus, asymmetric modulations between options' representations may provide a simple explanation for irrational behavior.

Katz, Menachem et al. (2013) International Worm Meeting "CEPsh glia modulate a sleep-related neuronal circuit in C. elegans."

Dragomir, Elena, Biron, David, Iwanir, Shachar, Corson, Francis, Shaham, Shai, Katz, Menachem

[International Worm Meeting, 2013]

Glia are essential components of the nervous system. However, their precise roles in the regulation of neuronal circuit activities and animal behavior are poorly understood. In 1895, Ramon y Cajal postulated that astrocytes regulate the transition between sleep and waking states. Remarkably, recent studies in mice and flies hint that Cajal's ideas may be more than theoretical musings. Yet, the mechanisms by which glia influence sleep are not fully resolved. Sleep in C. elegans is defined by behavioral lethargy that coincides with molting, linking development and locomotion. The CEPsh glia, which envelope the C. elegans nerve ring, are reminiscent of astrocytes, and extend processes that abut specific nerve-ring synapses. L1 ablation of CEPsh glia results in molting-independent short duration locomotory pausing, episodes of longer immobility immediately preceding molting, and developmental delay. The ALA neuron was previously implicated in sleep control. Interestingly, synapses between ALA and the backward command interneuron AVE are ensheathed by CEPsh glia, suggesting functional roles for these glia in modulating ALA-AVE synaptic activity. Indeed, loss of ALA function suppresses the locomotory defects, developmental delay, and precocious lethargus of CEPsh glia-ablated animals. Furthermore, inactivation of AVE in adults induces pausing reminiscent of CEPsh ablation. In animals lacking CEPsh glia, the duration of spontaneous calcium transients in AVE is prolonged and calcium spikes are decorrelated from backward locomotion. Correlation is restored upon ALA inactivation. These results as well as our functional epistasis studies of ALA, AVE and CEPsh glia are consistent with a model in which glia attenuate a sleep-promoting inhibitory connection between ALA and AVE. We propose that glia play key roles in C. elegans sleep control and that this role may be conserved.

Nagy, Stanislav et al. (2015) International Worm Meeting "Homeostasis in C. elegans sleep is characterized by two behaviorally and genetically distinct mechanisms."

Sanders, Jarred, Nagy, Stanislav, Iwanir, Shachar, Tramm, Nora, Shirley, Ian, Levine, Erel, Biron, David

[International Worm Meeting, 2015]

Biological homeostasis invokes modulatory responses aimed at stabilizing internal conditions. Using tunable photo- and mechano-stimulation, we identified two distinct categories of homeostatic responses during the sleep-like state of C. elegans (lethargus). In the presence of weak or no stimuli, extended motion caused a subsequent extension of quiescence. The neuropeptide Y receptor homolog, NPR-1, and an inhibitory neuropeptide known to activate it, FLP-18, were required for this process. In the presence of strong stimuli, the correlations between motion and quiescence were disrupted for several minutes but homeostasis manifested as an overall elevation of the time spent in quiescence. This response to strong stimuli required the function of the DAF-16/FOXO transcription factor in neurons, but not that of NPR-1. Conversely, response to weak stimuli did not require the function of DAF-16/FOXO. These findings suggest that routine homeostatic stabilization of sleep may be distinct from homeostatic compensation following a strong disturbance.

Iwanir S et al. (2013) Sleep "The microarchitecture of C. elegans behavior during lethargus: homeostatic bout dynamics, a typical body posture, and regulation by a central neuron."

Biron D, Tramm N, Ish D, Nagy S, Iwanir S, Wright C

[Sleep, 2013]

STUDY OBJECTIVES: The nematode C. elegans develops through four larval stages before it reaches adulthood. At the transition between stages and before it sheds its cuticle, it exhibits a sleep-like behavior during a stage termed lethargus. The objectives of this study were to characterize in detail behavioral patterns and physiological activity of a command interneuron during lethargus. MEASUREMENTS AND RESULTS: We found that lethargus behavior was composed of bouts of quiescence and motion. The duration of individual bouts ranged from 2 to 100 seconds, and their dynamics exhibited local homeostasis: the duration of bouts of quiescence positively correlated with the duration of bouts of motion that immediately preceded them in a cAMP-dependent manner. In addition, we identified a characteristic body posture during lethargus: the average curvature along the body of L4 lethargus larvae was lower than that of L4 larvae prior to lethargus, and the positions of body bends were distributed non-uniformly along the bodies of quiescent animals. Finally, we found that the AVA interneurons, a pair of backward command neurons, mediated locomotion patterns during L4 lethargus in similar fashion to their function in L4 larvae prior to lethargus. Interestingly, in both developmental stages backward locomotion was initiated and terminated asymmetrically with respect to AVA intraneuronal calcium concentration. CONCLUSIONS: The complex behavioral patterns during lethargus can be dissected to quantifiable elements, which exhibit rich temporal dynamics and are actively regulated by the nervous system. Our findings support the identification of lethargus as a sleep-like state. CITATION: Iwanir S; Tramm N; Nagy S; Wright C; Ish D; Biron D. The microarchitecture of C. elegans behavior during lethargus: homeostatic bout dynamics, a typical body posture, and regulation by a central neuron. SLEEP 2013;36(3):385-395.