Antagonism between neuropeptides and monoamines in a distributed circuit for pathogen avoidance.

Pathogenic infection elicits behaviors that promote recovery and survival of the host. After exposure to the pathogenic bacterium Pseudomonas aeruginosa PA14, the nematode Caenorhabditis elegans modifies its sensory preferences to avoid the pathogen. Here, we identify antagonistic neuromodulators that shape this acquired avoidance behavior. Using an unbiased cell-directed neuropeptide screen, we show that AVK neurons upregulate and release RF/RYamide FLP-1 neuropeptides during infection to drive pathogen avoidance. Manipulations that increase or decrease AVK activity accelerate or delay pathogen avoidance, respectively, implicating AVK in the dynamics of avoidance behavior. FLP-1 neuropeptides drive pathogen avoidance through the G protein-coupled receptor DMSR-7, as well as other receptors. DMSR-7 in turn acts in multiple neurons, including tyraminergic/octopaminergic neurons that receive convergent avoidance signals from the cytokine DAF-7/transforming growth factor ß. Neuromodulators shape pathogen avoidance through multiple mechanisms and targets, in agreement with the distributed neuromodulatory connectome of C. elegans.

Specific configurations of electrical synapses filter sensory information to drive choices in behavior.

Synaptic configurations in precisely wired circuits underpin how sensory information is processed by the nervous system, and the emerging animal behavior. This is best understood for chemical synapses, but far less is known about how electrical synaptic configurations modulate, in vivo and in specific neurons, sensory information processing and context-specific behaviors. We discovered that INX-1, a gap junction protein that forms electrical synapses, is required to deploy context-specific behavioral strategies during C. elegans thermotaxis behavior. INX-1 couples two bilaterally symmetric interneurons, and this configuration is required for the integration of sensory information during migration of animals across temperature gradients. In inx-1 mutants, uncoupled interneurons display increased excitability and responses to subthreshold temperature stimuli, resulting in abnormally longer run durations and context-irrelevant tracking of isotherms. Our study uncovers a conserved configuration of electrical synapses that, by increasing neuronal capacitance, enables differential processing of sensory information and the deployment of context-specific behavioral strategies.

A genetically encoded tool for reconstituting synthetic modulatory neurotransmission and reconnect neural circuits in vivo.

Chemogenetic and optogenetic tools have transformed the field of neuroscience by facilitating the examination and manipulation of existing circuits. Yet, the field lacks tools that enable rational rewiring of circuits via the creation or modification of synaptic relationships. Here we report the development of HySyn, a system designed to reconnect neural circuits in vivo by reconstituting synthetic modulatory neurotransmission. We demonstrate that genetically targeted expression of the two HySyn components, a Hydra-derived neuropeptide and its receptor, creates de novo neuromodulatory transmission in a mammalian neuronal tissue culture model and functionally rewires a behavioral circuit in vivo in the nematode Caenorhabditis elegans. HySyn can interface with existing optogenetic, chemogenetic and pharmacological approaches to functionally probe synaptic transmission, dissect neuropeptide signaling, or achieve targeted modulation of specific neural circuits and behaviors.

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.

Integration of Plasticity Mechanisms within a Single Sensory Neuron of C. elegans Actuates a Memory.

Neural plasticity, the ability of neurons to change their properties in response to experiences, underpins the nervous system's capacity to form memories and actuate behaviors. How different plasticity mechanisms act together in vivo and at a cellular level to transform sensory information into behavior is not well understood. We show that in Caenorhabditis elegans two plasticity mechanisms-sensory adaptation and presynaptic plasticity-act within a single cell to encode thermosensory information and actuate a temperature preference memory. Sensory adaptation adjusts the temperature range of the sensory neuron (called AFD) to optimize detection of temperature fluctuations associated with migration. Presynaptic plasticity in AFD is regulated by the conserved kinase nPKCε and transforms thermosensory information into a behavioral preference. Bypassing AFD presynaptic plasticity predictably changes learned behavioral preferences without affecting sensory responses. Our findings indicate that two distinct neuroplasticity mechanisms function together through a single-cell logic system to enact thermotactic behavior. VIDEO ABSTRACT.

Bidirectional thermotaxis in Caenorhabditis elegans is mediated by distinct sensorimotor strategies driven by the AFD thermosensory neurons.

The nematode Caenorhabditis elegans navigates toward a preferred temperature setpoint (Ts) determined by long-term temperature exposure. During thermotaxis, the worm migrates down temperature gradients at temperatures above Ts (negative thermotaxis) and performs isothermal tracking near Ts. Under some conditions, the worm migrates up temperature gradients below Ts (positive thermotaxis). Here, we analyze positive and negative thermotaxis toward Ts to study the role of specific neurons that have been proposed to be involved in thermotaxis using genetic ablation, behavioral tracking, and calcium imaging. We find differences in the strategies for positive and negative thermotaxis. Negative thermotaxis is achieved through biasing the frequency of reorientation maneuvers (turns and reversal turns) and biasing the direction of reorientation maneuvers toward colder temperatures. Positive thermotaxis, in contrast, biases only the direction of reorientation maneuvers toward warmer temperatures. We find that the AFD thermosensory neuron drives both positive and negative thermotaxis. The AIY interneuron, which is postsynaptic to AFD, may mediate the switch from negative to positive thermotaxis below Ts. We propose that multiple thermotactic behaviors, each defined by a distinct set of sensorimotor transformations, emanate from the AFD thermosensory neurons. AFD learns and stores the memory of preferred temperatures, detects temperature gradients, and drives the appropriate thermotactic behavior in each temperature regime by the flexible use of downstream circuits.

The role of NR4A transcription factors in memory formation.

In various physiological contexts, Nr4a genes are transcribed in response to external stimuli as part of an immediate early response that initiates a cascade of gene expression ultimately leading to distinct physiological outcomes in each of these contexts. The signaling pathway that initiates Nr4a gene expression in most of these contexts consists of elevated intracellular cAMP activating PKA, which in turn leads to phosphorylation of CREB and new gene synthesis. This cAMP-PKA-CREB pathway is a central molecular pathway in the formation of a long-term memory. Indeed, learning induces Nr4a family gene expression, and long-term memory formation requires at least two waves of transcription after learning, suggesting that NR4A nuclear receptors may contribute to the second of these waves of gene expression. In this article, we review insights gained in other physiological contexts regarding Nr4a function and regulation, and highlight how these lessons can be applied to the study of memory formation.

Sleep-dependent plasticity requires cortical activity.

Recent findings in humans and animals suggest that sleep promotes synaptic plasticity, but the underlying mechanisms have not been identified. We have demonstrated recently an important role for sleep in ocular dominance (OD) plasticity, a classic form of in vivo cortical remodeling triggered by monocular deprivation (MD) during a critical period of development. The mechanisms responsible for the effects of sleep on OD plasticity are unknown but may depend on neuronal activity in the sleeping brain. We investigated the role of cortical activity in sleep-dependent plasticity by reversibly inactivating the sleeping visual cortex (V1) after a period of MD. Critical period cats were bilaterally implanted with cannulas in V1 and standard EEG/EMG electrodes for polysomnographic recording. After a period of MD, visual cortices were infused with the sodium channel blocker lidocaine in vehicle or vehicle only during sleep. A third group of cats served as sham controls and were infused with lidocaine outside of V1 (into the CSF). Both optical imaging of intrinsic cortical signals and microelectrode recordings showed that OD plasticity was significantly reduced in cats whose visual cortices were reversibly silenced during sleep. These findings demonstrate that the mechanisms governing this form of sleep-dependent plasticity require cortical activity. They provide an important insight into how sleep modifies synaptic circuitry by narrowing the range of possible candidate mechanisms to those that are activity dependent.