Peptide neuromodulation in invertebrate model systems

The Hypothalamic Medial Preoptic Area-Paraventricular Nucleus Circuit Modulates Depressive-Like Behaviors in a Mouse Model of Postpartum Depression

Estrogen fluctuations have been implicated in various mood disorders, including perimenopausal and postpartum depression (PPD), likely through complex neural networks. γ-aminobutyric acid-ergic (GABAergic) neurons in the medial preoptic area (MPOA) that express estrogen receptor 1 (ESR1) are essential for the development and expression of depressive-like behaviors in ovarian hormone withdrawal (HW) mice. However, the precise circuit mechanisms through which MPOA GABAergic neurons influence behavior remain incompletely understood. Here, we identified robust projections from MPOA GABAergic neurons to the paraventricular nucleus of the hypothalamus (PVN). In HW mice, chemogenetic activation of MPOA GABAergic neurons targeting PVN attenuated depressive-like behaviors. Conversely, in nonhormone withdrawal (NHW) control mice (which received continuous estrogen), suppression of MPOA GABAergic projections to PVN exacerbated depressive-like behaviors. Further analyses using quantitative polymerase chain reaction and immunostaining identified arginine vasopressin (AVP) as a key neuropeptide in this pathway in the HW mouse model. Chemogenetic inhibition of PVNAVP neurons significantly alleviated depressive-like behaviors in HW mice, while their activation in NHW mice worsened depressive-like behaviors. These behaviors were dependent on AVP expression in PVNAVP neurons. Moreover, in HW mice, chemogenetic inhibition of PVNAVP neurons receiving MPOA input mitigated depressive-like behaviors. Conversely, in NHW mice, activation of these neurons exacerbated depressive-like behaviors. Electrophysiological recordings demonstrated that MPOA GABAergic neurons directly inhibit PVNAVP neurons. Thus, our findings suggest that PVNAVP neurons serve as downstream effectors of MPOA GABAergic neurons via monosynaptic inhibitory signaling to regulate depressive-like behaviors. Targeting this circuit may offer a novel therapeutic strategy for PPD.

Decoding Neuropeptide Complexity: Advancing Neurobiological Insights from Invertebrates to Vertebrates through Evolutionary Perspectives

Neuropeptides are vital signaling molecules involved in neural communication, hormonal regulation, and stress response across diverse taxa. Despite their critical roles, neuropeptide research remains challenging due to their low abundance, complex post-translational modifications (PTMs), and dynamic expression patterns. Mass spectrometry (MS)-based neuropeptidomics has revolutionized peptide identification and quantification, enabling the high-throughput characterization of neuropeptides and their PTMs. However, the complexity of vertebrate neural networks poses significant challenges for functional studies. Invertebrate models, such as Cancer borealis, Drosophila melanogaster, and Caenorhabditis elegans, offer simplified neural circuits, well-characterized systems, and experimental tools for elucidating the functional roles of neuropeptides. These models have revealed conserved neuropeptide families, including allatostatins, RFamides, and tachykinin-related peptides, whose vertebrate homologues regulate analogous physiological functions. Recent advancements in MS techniques, including ion mobility spectrometry and MALDI MS imaging, have further enhanced the spatial and temporal resolution of neuropeptide analysis, allowing for insights into peptide signaling systems. Invertebrate neuropeptide research not only expands our understanding of conserved neuropeptide functions but also informs translational applications including the development of peptide-based therapeutics. This review highlights the utility of invertebrate models in neuropeptide discovery, emphasizing their contributions to uncovering fundamental biological principles and their relevance to vertebrate systems.

Circadian rhythms are more resilient to pacemaker neuron disruption in female Drosophila

The circadian system regulates the timing of multiple molecular, physiological, metabolic, and behavioral phenomena. In Drosophila, as in other species, most of the research on how the timekeeping system in the brain controls the timing of behavioral outputs has been conducted in males, or sex has not been included as a biological variable. A critical set of circadian pacemaker neurons in Drosophila release the neuropeptide pigment-dispersing factor (PDF), which functions as a key output factor in the network with complex effects on other clock neurons. Lack of Pdf or its receptor, PdfR, results in most flies displaying arrhythmicity in activity-rest cycles under constant conditions. However, our results show that female circadian rhythms are less affected by mutations in both Pdf and PdfR. CRISPR-Cas9-mediated mutagenesis of Pdf, specifically in ventral lateral neurons (LNvs), also has a greater effect on male rhythms. We tested the influence of M-cells on the circadian network and showed that speeding up the molecular clock specifically in M-cells led to sexually dimorphic phenotypes, with a more pronounced effect on male rhythmic behavior. Our results suggest that the female circadian system is more resilient to manipulations of M-cells and the PDF pathway, suggesting that circadian timekeeping is more distributed across the clock neuron network in females.

Convergent effects of peptides on the initiation of feeding motor programs in the mollusk Aplysia

Neuropeptides configure the feeding network of Aplysia. For example, egestive activity is promoted by small cardioactive peptide (SCP), and ingestive activity is promoted by a combination of feeding circuit activating peptide (FCAP) and cerebral peptide 2 (CP-2). In addition, SCP and FCAP/CP-2 have a common network effect that does not contribute to motor program specification. They increase the excitability of an interneuron, B63. In this report, we further characterized the effects of peptides on B63. We performed voltage-clamp experiments and used a step protocol to look at steady-state currents. We found that SCP and FCAP/CP-2 both induced an inward current that was virtually absent in low-sodium saline. Previous work has established that B63 is unusual in the feeding circuit in that subthreshold depolarizations are autonomously generated that can trigger motor programs. Here, we show that this autonomous activity is more frequent in the presence of peptides. Previous studies have also shown that activity of the feeding central pattern generator (CPG) can be initiated by neurons that excite B63, e.g., by cerebral buccal interneuron 2 (CBI-2), a projection neuron that triggers biting-like motor programs. Here, we show that the latency of CBI-2-induced activity is decreased by stimulation of the esophageal nerve (EN) (which releases endogenous SCP). These results, taken together with previous results, indicate that peptides that act divergently to configure network activity additionally act convergently to promote motor program induction. We present data that suggest that this arrangement facilitates brief switches between ingestive and egestive motor activity.NEW & NOTEWORTHY The activity of most networks is affected by multiple neuromodulators. Studies that have sought to determine why this is the case have focused on how the effects of one modulator differ from those of another (how modulators uniquely determine motor output). This study differs in that we ask why a convergent (common) network modification is important. We show that it can promote program induction and present data that suggest this may have consequences for task switching.

Discovery and functional characterization of a bombesin-type neuropeptide signaling system in an invertebrate

Neuropeptide signaling systems are key regulators of physiological and behavioral processes in animals. However, the evolutionary history of some neuropeptides originally discovered in vertebrates is unknown. The peptide bombesin (BN) was first isolated from the skin of the toad Bombina bombina and subsequently BN-related neuropeptides have been identified in other chordates, including gastrin-releasing peptide (GRP) and neuromedin B (NMB) in mammals, and a GRP-like peptide in the cephalochordate Branchiostoma japonicum. However, BN-type neuropeptides have hitherto not been identified in any nonchordate animals. Here, we report the discovery and functional characterization of a BN-type neuropeptide signaling system in an echinoderm-the starfish Asterias rubens. BN-type precursor proteins were identified in several echinoderm species based on their amino acid sequences and gene structures, and the mature structure of the A. rubens BN-type neuropeptide ArBN was determined using mass spectrometry. A protein related to vertebrate GRP/NMB-type G protein-coupled receptors was identified experimentally as the receptor for ArBN in A. rubens. Analysis of the distribution of the ArBN precursor in A. rubens using mRNA in situ hybridization and immunohistochemistry revealed a widespread pattern of expression in the central nervous system, digestive system, and locomotory organs. Moreover, effects of ArBN in A. rubens included contraction and retraction of the evertible stomach and inhibition of feeding behavior. Our findings show that the evolutionary history of BN-type neuropeptide signaling can be traced back to the deuterostome common ancestor of echinoderms and chordates. Furthermore, an ancient role of BN-type neuropeptides as regulators of feeding behavior has been revealed.

What Drosophila can tell us about state-dependent peptidergic signaling in insects

Plasticity in animal behavior and physiology is largely due to modulatory and regulatory signaling with neuropeptides and peptide hormones (collectively abbreviated NPHs). The NPHs constitute a very large and versatile group of signaling substances that partake at different regulatory levels in most daily activities of an organism. This review summarizes key principles in NPH actions in the brain and in interorgan signaling, with focus on Drosophila. NPHs are produced by neurons, neurosecretory cells (NSCs) and other endocrine cells in NPH-specific and stereotypic patterns. Most of the NPHs have multiple (pleiotropic) functions and target several different neuronal circuits and/or peripheral tissues. Such divergent NPH signaling ensures orchestration of behavior and physiology in state-dependent manners. Conversely, many neurons, circuits, NSCs, or other cells, are targeted by multiple NPHs. This convergent signaling commonly conveys various signals reporting changes in the external and internal environment to central neurons/circuits. As an example of wider functional convergence, 26 different Drosophila NPHs act at many different levels to regulate food search and feeding. Convergence is also seen in hormonal regulation of peripheral functions. For instance, multiple NPHs target renal tubules to ensure osmotic homeostasis. Interestingly, several of the same osmoregulatory NPHs also regulate feeding, metabolism and stress. However, for some NPHs the cellular distribution and functions suggests multiple unrelated functions that are restricted to specific circuits. Thus, NPH signaling follows distinct patterns for each specific NPH, but taken together they form overlapping networks that modulate behavior and physiology.

Neuroendocrine Control of Synaptic Transmission by PHAC-1 in C. elegans

A dynamic interplay between fast synaptic signals and slower neuromodulatory signals controls the excitatory/inhibitory (E/I) balance within neuronal circuits. The mechanisms by which neuropeptide signaling is regulated to maintain E/I balance remain uncertain. We designed a genetic screen to isolate genes involved in the peptidergic maintenance of the E/I balance in the C. elegans motor circuit. This screen identified the C. elegans orthologs of the presynaptic phosphoprotein synapsin (snn-1) and the protein phosphatase 1 (PP1) regulatory subunit PHACTR1 (phac-1). We demonstrate that both phac-1 and snn-1 alter the motor behavior of C. elegans, and genetic interactions suggest that SNN-1 contributes to PP1-PHAC-1 holoenzyme signaling. De novo variants of human PHACTR1, associated with early-onset epilepsies [developmental and epileptic encephalopathy 70 (DEE70)], when expressed in C. elegans resulted in constitutive PP1-PHAC-1 holoenzyme activity. Unregulated PP1-PHAC-1 signaling alters the synapsin and actin cytoskeleton and increases neuropeptide release by cholinergic motor neurons, which secondarily affects the presynaptic vesicle cycle. Together, these results clarify the dominant mechanisms of action of the DEE70 alleles and suggest that altered neuropeptide release may alter E/I balance in DEE70.

Insulin control in fruit flies

Investigating how the production of insulin is regulated in fruit flies reveals surprising insights that may help to better understand how this process unfolds in humans.

Aminergic and peptidergic modulation of insulin-producing cells in Drosophila

Insulin plays a critical role in maintaining metabolic homeostasis. Since metabolic demands are highly dynamic, insulin release needs to be constantly adjusted. These adjustments are mediated by different pathways, most prominently the blood glucose level, but also by feedforward signals from motor circuits and different neuromodulatory systems. Here, we analyze how neuromodulatory inputs control the activity of the main source of insulin in Drosophila - a population of insulin-producing cells (IPCs) located in the brain. IPCs are functionally analogous to mammalian pancreatic beta cells, but their location makes them accessible for in vivo recordings in intact animals. We characterized functional inputs to IPCs using single-nucleus RNA sequencing analysis, anatomical receptor expression mapping, connectomics, and an optogenetics-based 'intrinsic pharmacology' approach. Our results show that the IPC population expresses a variety of receptors for neuromodulators and classical neurotransmitters. Interestingly, IPCs exhibit heterogeneous receptor profiles, suggesting that the IPC population can be modulated differentially. This is supported by electrophysiological recordings from IPCs, which we performed while activating different populations of modulatory neurons. Our analysis revealed that some modulatory inputs have heterogeneous effects on the IPC activity, such that they inhibit one subset of IPCs, while exciting another. Monitoring calcium activity across the IPC population uncovered that these heterogeneous responses occur simultaneously. Certain neuromodulatory populations shifted the IPC population activity towards an excited state, while others shifted it towards inhibition. Taken together, we provide a comprehensive, multi-level analysis of neuromodulation in the insulinergic system of Drosophila.

Pigment-dispersing factor neuropeptides act as multifunctional hormones and modulators in tardigrades

Pigment-dispersing factors (PDFs) are neuropeptides that play key roles in controlling the circadian rhythms in various insects, whereas their function remains elusive in other protostomes including tardigrades (water bears). Here we show that the three PDFs of the tardigrade Hypsibius exemplaris are co-localized in two pairs of inner lobe cells in the brain, whereas only one PDF occurs in four additional cerebral and two extracerebral cells. The axons of the inner lobe cells pass through the contralateral brain hemisphere, descend to the ventral nerve cord and terminate in two pairs of potential release sites in the posteriormost trunk ganglion. Using in vitro assays, we demonstrate that all three PDFs and their deorphanized receptor (PDFR) are functional. Widespread localization of PDFR suggests that tardigrade PDFs may act as multifunctional hormones and neuromodulators that control major functions including light detection, neural processing, locomotion, feeding, digestion, osmoregulation, growth, embryonic development and oogenesis/reproduction.

Synaptic Targets of Circadian Clock Neurons Influence Core Clock Parameters

Neuronal connectivity in the circadian clock network is essential for robust endogenous timekeeping. In the Drosophila circadian clock network, four pairs of small ventral lateral neurons (sLNvs) serve as critical pacemakers. Peptidergic communication via sLNv release of the key output neuropeptide Pigment Dispersing Factor (PDF) has been well characterized. In contrast, little is known about the role of the synaptic connections that sLNvs form with downstream neurons. Connectomic analyses revealed that the sLNvs form strong synaptic connections with a group of previously uncharacterized neurons, SLP316. Here, we show that silencing synaptic output in the SLP316 neurons via tetanus toxin (TNT) expression shortens the free-running period, whereas hyper-exciting them by expressing the Na[+] channel NaChBac results in period lengthening. Under light-dark cycles, silencing SLP316 neurons also causes lower daytime activity and higher daytime sleep. Our results revealed that the main postsynaptic partners of the Drosophila pacemaker neurons are a non-clock neuronal cell type that regulates the timing of sleep and activity.

Neural substrates of cold nociception in Drosophila larva

Metazoans detect and differentiate between innocuous (non-painful) and/or noxious (harmful) environmental cues using primary sensory neurons, which serve as the first node in a neural network that computes stimulus specific behaviors to either navigate away from injury-causing conditions or to perform protective behaviors that mitigate extensive injury. The ability of an animal to detect and respond to various sensory stimuli depends upon molecular diversity in the primary sensors and the underlying neural circuitry responsible for the relevant behavioral action selection. Recent studies in Drosophila larvae have revealed that somatosensory class III multidendritic (CIII md) neurons function as multimodal sensors regulating distinct behavioral responses to innocuous mechanical and nociceptive thermal stimuli. Recent advances in circuit bases of behavior have identified and functionally validated Drosophila larval somatosensory circuitry involved in innocuous (mechanical) and noxious (heat and mechanical) cues. However, central processing of cold nociceptive cues remained unexplored. We implicate multisensory integrators (Basins), premotor (Down-and-Back) and projection (A09e and TePns) neurons as neural substrates required for cold-evoked behavioral and calcium responses. Neural silencing of cell types downstream of CIII md neurons led to significant reductions in cold-evoked behaviors and neural co-activation of CIII md neurons plus additional cell types facilitated larval contraction (CT) responses. Further, we demonstrate that optogenetic activation of CIII md neurons evokes calcium increases in these neurons. Finally, we characterize the premotor to motor neuron network underlying cold-evoked CT and delineate the muscular basis of CT response. Collectively, we demonstrate how Drosophila larvae process cold stimuli through functionally diverse somatosensory circuitry responsible for generating stimulus-specific behaviors.

Loss of neuropeptide signalling alters temporal expression of mouse suprachiasmatic neuronal state and excitability

Individual neurons of the hypothalamic suprachiasmatic nuclei (SCN) contain an intracellular molecular clock that drives these neurons to exhibit day-night variation in excitability. The neuropeptide vasoactive intestinal polypeptide (VIP) and its cognate receptor, VPAC2, are synthesized by SCN neurons and this intercellular VIP-VPAC2 receptor signal facilitates coordination of SCN neuronal activity and timekeeping. How the loss of VPAC2 receptor signalling affects the electrophysiological properties and states of SCN neurons as well as their responses to excitatory inputs is unclear. Here we used patch-clamp electrophysiology and made recordings of SCN neurons in brain slices prepared from transgenic animals that do not express VPAC2 receptors (Vipr2-/- mice) as well as animals that do (Vipr2+/+ mice). We report that while Vipr2+/+ neurons exhibit coordinated day-night variation in their electrical state, Vipr2-/- neurons lack this and instead manifest a range of states during both day and night. Further, at the population level, Vipr2+/+ neurons vary the membrane threshold potential at which they start to fire action potentials from day to night, while Vipr2-/- neurons do not. We provide evidence that Vipr2-/- neurons lack a component of voltage-gated sodium currents that contribute to SCN neuronal excitability. Moreover, we determine that this aberrant temporal control of neuronal state and excitability alters neuronal responses to a neurochemical mimic of the light-input pathway to the SCN. These results highlight the critical role VIP-VPAC2 receptor signalling plays in the temporal expression of individual neuronal states as well as appropriate ensemble activity and input gating of the SCN neural network.

Neuropeptide signaling network of Caenorhabditis elegans: from structure to behavior

Neuropeptides are abundant signaling molecules that control neuronal activity and behavior in all animals. Owing in part to its well-defined and compact nervous system, Caenorhabditis elegans has been one of the primary model organisms used to investigate how neuropeptide signaling networks are organized and how these neurochemicals regulate behavior. We here review recent work that has expanded our understanding of the neuropeptidergic signaling network in C. elegans by mapping the evolutionary conservation, the molecular expression, the receptor-ligand interactions, and the system-wide organization of neuropeptide pathways in the C. elegans nervous system. We also describe general insights into neuropeptidergic circuit motifs and the spatiotemporal range of peptidergic transmission that have emerged from in vivo studies on neuropeptide signaling. With efforts ongoing to chart peptide signaling networks in other organisms, the C. elegans neuropeptidergic connectome can serve as a prototype to further understand the organization and the signaling dynamics of these networks at organismal level.

Understanding the neurobiology of social behavior through exploring brain-wide dynamics of neural activity

Social behavior is highly complex and adaptable. It can be divided into multiple temporal stages: detection, approach, and consummatory behavior. Each stage can be further divided into several cognitive and behavioral processes, such as perceiving social cues, evaluating the social and non-social contexts, and recognizing the internal/emotional state of others. Recent studies have identified numerous brain-wide circuits implicated in social behavior and suggested the existence of partially overlapping functional brain networks underlying various types of social and non-social behavior. However, understanding the brain-wide dynamics underlying social behavior remains challenging, and several brain-scale dynamics (macro-, meso-, and micro-scale levels) need to be integrated. Here, we suggest leveraging new tools and concepts to explore social brain networks and integrate those different levels. These include studying the expression of immediate-early genes throughout the entire brain to impartially define the structure of the neuronal networks involved in a given social behavior. Then, network dynamics could be investigated using electrode arrays or multi-channel fiber photometry. Finally, tools like high-density silicon probes and miniscopes can probe neural activity in specific areas and across neuronal populations at the single-cell level.

The Drosophila circadian clock gene cycle controls the development of clock neurons

Daily behavioral and physiological rhythms are controlled by the brain's circadian timekeeping system, a synchronized network of neurons that maintains endogenous molecular oscillations. These oscillations are based on transcriptional feedback loops of clock genes, which in Drosophila include the transcriptional activators Clock (Clk) and cycle (cyc). While the mechanisms underlying this molecular clock are very well characterized, the roles that the core clock genes play in neuronal physiology and development are much less understood. The Drosophila timekeeping center is composed of ~150 clock neurons, among which the four small ventral lateral neurons (sLNvs) are the most dominant pacemakers under constant conditions. Here, we show that downregulating the clock gene cyc specifically in the Pdf-expressing neurons leads to decreased fasciculation both in larval and adult brains. This effect is due to a developmental role of cyc, as both knocking down cyc or expressing a dominant negative form of cyc exclusively during development lead to defasciculation phenotypes in adult clock neurons. Clk downregulation also leads to developmental effects on sLNv morphology. Our results reveal a non-circadian role for cyc, shedding light on the additional functions of circadian clock genes in the development of the nervous system.

Targeted single cell expression profiling identifies integrators of sleep and metabolic state

Animals modulate sleep in accordance with their internal and external environments. Metabolic cues are particularly potent regulators of sleep, allowing animals to alter their sleep timing and amount depending on food availability and foraging duration. The fruit fly, Drosophila melanogaster, suppresses sleep in response to acute food deprivation, presumably to forage for food. This process is dependent on a single pair of Lateral Horn Leucokinin (LHLK) neurons, that secrete the neuropeptide Leucokinin. These neurons signal to insulin producing cells and suppress sleep under periods of starvation. The identification of individual neurons that modulate sleep-metabolism interactions provides the opportunity to examine the cellular changes associated with sleep modulation. Here, we use single-cell sequencing of LHLK neurons to examine the transcriptional responses to starvation. We validate that a Patch-seq approach selectively isolates RNA from individual LHLK neurons. Single-cell CEL-Seq comparisons of LHLK neurons between fed and 24-hr starved flies identified 24 genes that are differentially expressed in accordance with starvation state. In total, 12 upregulated genes and 12 downregulated genes were identified. Gene-ontology analysis showed an enrichment for Attacins, a family of anti-microbial peptides, along with several transcripts with diverse roles in regulating cellular function. Targeted knockdown of differentially expressed genes identified multiple genes that function within LHLK neurons to regulate sleep-metabolism interactions. Functionally validated genes include an essential role for the E3 ubiquitin Ligase insomniac, the sorbitol dehydrogenase Sodh1, as well as AttacinC and AttacinB in starvation-induced sleep suppression. Taken together, these findings provide a pipeline for identifying novel regulators of sleep-metabolism interactions within individual neurons.

The Circadian Neuropeptide PDF has Sexually Dimorphic Effects on Activity Rhythms

The circadian system regulates the timing of multiple molecular, physiological, metabolic, and behavioral phenomena. In Drosophila as in other species, most of the research on how the timekeeping system in the brain controls timing of behavioral outputs has been conducted in males, or sex was not included as a biological variable. The main circadian pacemaker neurons in Drosophila release the neuropeptide Pigment Dispersing Factor (PDF), which functions as a key synchronizing factor in the network with complex effects on other clock neurons. Lack of Pdf or its receptor, PdfR, results in most flies displaying arrhythmicity in activity-rest cycles under constant conditions. However, our results show that female circadian rhythms are less affected by mutations in both Pdf and PdfR. Crispr-Cas9 mutagenesis of Pdf specifically in the ventral lateral neurons (LNvs) also has a greater effect on male rhythms. We tested the influence of the M-cells over the circadian network and show that speeding up the molecular clock specifically in the M-cells leads to sexually dimorphic phenotypes, with a more pronounced effect on male rhythmic behavior. Our results suggest that the female circadian system is more resilient to manipulations of the PDF pathway and that circadian timekeeping is more distributed across the clock neuron network in females.

Egg-laying hormone expression in identified neurons across developmental stages and reproductive states of the nudibranch Berghia stephanieae

Neuropeptides play essential roles in coordinating reproduction. Egg-laying hormone (ELH) is conserved in genetic sequence and behavioral function across molluscs, where neuronal clusters secrete ELH to modulate and induce egg-laying. Here we investigated ELH in the nudibranch mollusc, Berghia stephanieae. ELH preprohormone gene orthologs, which showed clade-specific differences at the C-terminus of the predicted bioactive peptide, were identified in brain transcriptomes across several nudipleuran species, including B. stephanieae. ELH shares deep homology with the corticotropin-releasing hormone gene family, which has roles broadly in stress response. Injection of synthesized B. stephanieae ELH peptide into mature individuals induced egg-laying. ELH gene expression in the brain and body was mapped using in-situ hybridization chain reaction. Across the adult brain, 300-400 neurons expressed ELH. Twenty-one different cell types were identified in adults, three of which were located unilaterally on the right side, which corresponds to the location of the reproductive organs. Ten cell types were present in pre-reproductive juvenile stages. An asymmetric cluster of approximately 100 small neurons appeared in the right pedal ganglion of late-stage juveniles. Additional neurons in the pleural and pedal ganglia expressed ELH only in adults that were actively laying eggs and sub-adults that were on the verge of doing so, implicating their direct role in reproduction. Outside the brain, ELH was expressed on sensory appendages, including in presumptive sensory neurons. Its widespread expression in the nudibranch B. stephanieae suggests that ELH plays a role beyond reproduction in gastropod molluscs.

Two C-terminal isoforms of Aplysia tachykinin-related peptide receptors exhibit phosphorylation-dependent and phosphorylation-independent desensitization mechanisms

Diversity, a hallmark of G protein-coupled receptor (GPCR) signaling, partly stems from alternative splicing of a single gene generating more than one isoform for a receptor. Additionally, receptor responses to ligands can be attenuated by desensitization upon prolonged or repeated ligand exposure. Both phenomena have been demonstrated and exemplified by the deuterostome tachykinin signaling system, although the role of phosphorylation in desensitization remains a subject of debate. Here, we describe the signaling system for tachykinin-related peptides (TKRPs) in a protostome, mollusk Aplysia. We cloned the Aplysia TKRP precursor, which encodes three TKRPs (apTKRP-1, apTKRP-2a, and apTKRP-2b) containing the FXGXR-amide motif. In situ hybridization and immunohistochemistry showed predominant expression of TKRP mRNA and peptide in the cerebral ganglia. TKRPs and their posttranslational modifications were observed in extracts of central nervous system ganglia using mass spectrometry. We identified two Aplysia TKRP receptors (apTKRPRs), named apTKRPR-A and apTKRPR-B. These receptors are two isoforms generated through alternative splicing of the same gene and differ only in their intracellular C termini. Structure-activity relationship analysis of apTKRP-2b revealed that both C-terminal amidation and conserved residues of the ligand are critical for receptor activation. C-terminal truncates and mutants of apTKRPRs suggested that there is a C-terminal phosphorylation-independent desensitization for both receptors. Moreover, apTKRPR-B also exhibits phosphorylation-dependent desensitization through the phosphorylation of C-terminal Ser/Thr residues. This comprehensive characterization of the Aplysia TKRP signaling system underscores the evolutionary conservation of the TKRP and TK signaling systems, while highlighting the intricacies of receptor regulation through alternative splicing and differential desensitization mechanisms.

The Caenorhabditis elegans neuroendocrine system and their modulators: An overview

In this review we seek to systematically bring what has been published in the literature about the nervous system, endocrine system, neuroendocrine relationships, neuroendocrine modulations and endocrine disruptors in the alternative model Caenorhabditis elegans. The serotonergic, dopaminergic, GABAergic and glutamatergic neurotransmitters are related to the modulation of the neuroendocrine axis, leading to the activation or inhibition of several processes that occur in the worm through distinct and interconnected pathways. Furthermore, this review addresses the gut-neuronal axis as it has been revealed in recent years that gut microbiota impacts on neuronal functions. This review also approaches xenobiotics that can positively or negatively impact the neuroendocrine system in C. elegans as in mammals, which allows the application of this nematode to screen new drugs and to identify toxicants that are endocrine disruptors.

Neuromodulation and the toolkit for behavioural evolution: can ecdysis shed light on an old problem?

The geneticist Thomas Dobzhansky famously declared: 'Nothing in biology makes sense except in the light of evolution'. A key evolutionary adaptation of Metazoa is directed movement, which has been elaborated into a spectacularly varied number of behaviours in animal clades. The mechanisms by which animal behaviours have evolved, however, remain unresolved. This is due, in part, to the indirect control of behaviour by the genome, which provides the components for both building and operating the brain circuits that generate behaviour. These brain circuits are adapted to respond flexibly to environmental contingencies and physiological needs and can change as a function of experience. The resulting plasticity of behavioural expression makes it difficult to characterize homologous elements of behaviour and to track their evolution. Here, we evaluate progress in identifying the genetic substrates of behavioural evolution and suggest that examining adaptive changes in neuromodulatory signalling may be a particularly productive focus for future studies. We propose that the behavioural sequences used by ecdysozoans to moult are an attractive model for studying the role of neuromodulation in behavioural evolution.

Network Statistics of the Whole-Brain Connectome of Drosophila

Brains comprise complex networks of neurons and connections. Network analysis applied to the wiring diagrams of brains can offer insights into how brains support computations and regulate information flow. The completion of the first whole-brain connectome of an adult Drosophila, the largest connectome to date, containing 130,000 neurons and millions of connections, offers an unprecedented opportunity to analyze its network properties and topological features. To gain insights into local connectivity, we computed the prevalence of two- and three-node network motifs, examined their strengths and neurotransmitter compositions, and compared these topological metrics with wiring diagrams of other animals. We discovered that the network of the fly brain displays rich club organization, with a large population (30% percent of the connectome) of highly connected neurons. We identified subsets of rich club neurons that may serve as integrators or broadcasters of signals. Finally, we examined subnetworks based on 78 anatomically defined brain regions or neuropils. These data products are shared within the FlyWire Codex and will serve as a foundation for models and experiments exploring the relationship between neural activity and anatomical structure.

Cilia Provide a Platform for the Generation, Regulated Secretion, and Reception of Peptidergic Signals

Cilia are microtubule-based cellular projections that act as motile, sensory, and secretory organelles. These structures receive information from the environment and transmit downstream signals to the cell body. Cilia also release vesicular ectosomes that bud from the ciliary membrane and carry an array of bioactive enzymes and peptide products. Peptidergic signals represent an ancient mode of intercellular communication, and in metazoans are involved in the maintenance of cellular homeostasis and various other physiological processes and responses. Numerous peptide receptors, subtilisin-like proteases, the peptide-amidating enzyme, and bioactive amidated peptide products have been localized to these organelles. In this review, we detail how cilia serve as specialized signaling organelles and act as a platform for the regulated processing and secretion of peptidergic signals. We especially focus on the processing and trafficking pathways by which a peptide precursor from the green alga Chlamydomonas reinhardtii is converted into an amidated bioactive product-a chemotactic modulator-and released from cilia in ectosomes. Biochemical dissection of this complex ciliary secretory pathway provides a paradigm for understanding cilia-based peptidergic signaling in mammals and other eukaryotes.

Sensory neurons couple arousal and foraging decisions in Caenorhabditis elegans

Foraging animals optimize feeding decisions by adjusting both common and rare behavioral patterns. Here, we characterize the relationship between an animal's arousal state and a rare decision to leave a patch of bacterial food. Using long-term tracking and behavioral state classification, we find that food leaving decisions in Caenorhabditis elegans are coupled to arousal states across multiple timescales. Leaving emerges probabilistically over minutes from the high arousal roaming state, but is suppressed during the low arousal dwelling state. Immediately before leaving, animals have a brief acceleration in speed that appears as a characteristic signature of this behavioral motif. Neuromodulatory mutants and optogenetic manipulations that increase roaming have a coupled increase in leaving rates, and similarly acute manipulations that inhibit feeding induce both roaming and leaving. By contrast, inactivating a set of chemosensory neurons that depend on the cGMP-gated transduction channel TAX-4 uncouples roaming and leaving dynamics. In addition, tax-4-expressing sensory neurons promote lawn-leaving behaviors that are elicited by feeding inhibition. Our results indicate that sensory neurons responsive to both internal and external cues play an integrative role in arousal and foraging decisions.

Optimized design and in vivo application of optogenetically functionalized Drosophila dopamine receptors

Neuromodulatory signaling via G protein-coupled receptors (GPCRs) plays a pivotal role in regulating neural network function and animal behavior. The recent development of optogenetic tools to induce G protein-mediated signaling provides the promise of acute and cell type-specific manipulation of neuromodulatory signals. However, designing and deploying optogenetically functionalized GPCRs (optoXRs) with accurate specificity and activity to mimic endogenous signaling in vivo remains challenging. Here we optimize the design of optoXRs by considering evolutionary conserved GPCR-G protein interactions and demonstrate the feasibility of this approach using two Drosophila Dopamine receptors (optoDopRs). These optoDopRs exhibit high signaling specificity and light sensitivity in vitro. In vivo, we show receptor and cell type-specific effects of dopaminergic signaling in various behaviors, including the ability of optoDopRs to rescue the loss of the endogenous receptors. This work demonstrates that optoXRs can enable optical control of neuromodulatory receptor-specific signaling in functional and behavioral studies.

Thermosensation and Temperature Preference: From Molecules to Neuronal Circuits in Drosophila

Temperature has a significant effect on all physiological processes of animals. Suitable temperatures promote responsiveness, movement, metabolism, growth, and reproduction in animals, whereas extreme temperatures can cause injury or even death. Thus, thermosensation is important for survival in all animals. However, mechanisms regulating thermosensation remain unexplored, mostly because of the complexity of mammalian neural circuits. The fruit fly Drosophila melanogaster achieves a desirable body temperature through ambient temperature fluctuations, sunlight exposure, and behavioral strategies. The availability of extensive genetic tools and resources for studying Drosophila have enabled scientists to unravel the mechanisms underlying their temperature preference. Over the past 20 years, Drosophila has become an ideal model for studying temperature-related genes and circuits. This review provides a comprehensive overview of our current understanding of thermosensation and temperature preference in Drosophila. It encompasses various aspects, such as the mechanisms by which flies sense temperature, the effects of internal and external factors on temperature preference, and the adaptive strategies employed by flies in extreme-temperature environments. Understanding the regulating mechanisms of thermosensation and temperature preference in Drosophila can provide fundamental insights into the underlying molecular and neural mechanisms that control body temperature and temperature-related behavioral changes in other animals.

Chemical cognition: chemoconnectomics and convergent evolution of integrative systems in animals

Neurons underpin cognition in animals. However, the roots of animal cognition are elusive from both mechanistic and evolutionary standpoints. Two conceptual frameworks both highlight and promise to address these challenges. First, we discuss evidence that animal neural and other integrative systems evolved more than once (convergent evolution) within basal metazoan lineages, giving us unique experiments by Nature for future studies. The most remarkable examples are neural systems in ctenophores and neuroid-like systems in placozoans and sponges. Second, in addition to classical synaptic wiring, a chemical connectome mediated by hundreds of signal molecules operates in tandem with neurons and is the most information-rich source of emerging properties and adaptability. The major gap-dynamic, multifunctional chemical micro-environments in nervous systems-is not understood well. Thus, novel tools and information are needed to establish mechanistic links between orchestrated, yet cell-specific, volume transmission and behaviors. Uniting what we call chemoconnectomics and analyses of the cellular bases of behavior in basal metazoan lineages arguably would form the foundation for deciphering the origins and early evolution of elementary cognition and intelligence.

System-wide mapping of peptide-GPCR interactions in C. elegans

Neuropeptides and peptide hormones are ancient, widespread signaling molecules that underpin almost all brain functions. They constitute a broad ligand-receptor network, mainly by binding to G protein-coupled receptors (GPCRs). However, the organization of the peptidergic network and roles of many peptides remain elusive, as our insight into peptide-receptor interactions is limited and many peptide GPCRs are still orphan receptors. Here we report a genome-wide peptide-GPCR interaction map in Caenorhabditis elegans. By reverse pharmacology screening of over 55,384 possible interactions, we identify 461 cognate peptide-GPCR couples that uncover a broad signaling network with specific and complex combinatorial interactions encoded across and within single peptidergic genes. These interactions provide insights into peptide functions and evolution. Combining our dataset with phylogenetic analysis supports peptide-receptor co-evolution and conservation of at least 14 bilaterian peptidergic systems in C. elegans. This resource lays a foundation for system-wide analysis of the peptidergic network.

Heterogeneous receptor expression underlies non-uniform peptidergic modulation of olfaction in Drosophila

Sensory systems are dynamically adjusted according to the animal's ongoing needs by neuromodulators, such as neuropeptides. Neuropeptides are often widely-distributed throughout sensory networks, but it is unclear whether such neuropeptides uniformly modulate network activity. Here, we leverage the Drosophila antennal lobe (AL) to resolve whether myoinhibitory peptide (MIP) uniformly modulates AL processing. Despite being uniformly distributed across the AL, MIP decreases olfactory input to some glomeruli, while increasing olfactory input to other glomeruli. We reveal that a heterogeneous ensemble of local interneurons (LNs) are the sole source of AL MIP, and show that differential expression of the inhibitory MIP receptor across glomeruli allows MIP to act on distinct intraglomerular substrates. Our findings demonstrate how even a seemingly simple case of modulation can have complex consequences on network processing by acting non-uniformly within different components of the overall network.

Molecular and Neural Mechanisms of Temperature Preference Rhythm in Drosophila melanogaster

Temperature influences animal physiology and behavior. Animals must set an appropriate body temperature to maintain homeostasis and maximize survival. Mammals set their body temperatures using metabolic and behavioral strategies. The daily fluctuation in body temperature is called the body temperature rhythm (BTR). For example, human body temperature increases during wakefulness and decreases during sleep. BTR is controlled by the circadian clock, is closely linked with metabolism and sleep, and entrains peripheral clocks located in the liver and lungs. However, the underlying mechanisms of BTR are largely unclear. In contrast to mammals, small ectotherms, such as Drosophila, control their body temperatures by choosing appropriate environmental temperatures. The preferred temperature of Drosophila increases during the day and decreases at night; this pattern is referred to as the temperature preference rhythm (TPR). As flies are small ectotherms, their body temperature is close to that of the surrounding environment. Thus, Drosophila TPR produces BTR, which exhibits a pattern similar to that of human BTR. In this review, we summarize the regulatory mechanisms of TPR, including recent studies that describe neuronal circuits relaying ambient temperature information to dorsal neurons (DNs). The neuropeptide diuretic hormone 31 (DH31) and its receptor (DH31R) regulate TPR, and a mammalian homolog of DH31R, the calcitonin receptor (CALCR), also plays an important role in mouse BTR regulation. In addition, both fly TPR and mammalian BTR are separately regulated from another clock output, locomotor activity rhythms. These findings suggest that the fundamental mechanisms of BTR regulation may be conserved between mammals and flies. Furthermore, we discuss the relationships between TPR and other physiological functions, such as sleep. The dissection of the regulatory mechanisms of Drosophila TPR could facilitate an understanding of mammalian BTR and the interaction between BTR and sleep regulation.

The head mesodermal cell couples FMRFamide neuropeptide signaling with rhythmic muscle contraction in C. elegans

FMRFamides are evolutionarily conserved neuropeptides that play critical roles in behavior, energy balance, and reproduction. Here, we show that FMRFamide signaling from the nervous system is critical for the rhythmic activation of a single cell of previously unknown function, the head mesodermal cell (hmc) in C. elegans. Behavioral, calcium imaging, and genetic studies reveal that release of the FLP-22 neuropeptide from the AVL neuron in response to pacemaker signaling activates hmc every 50 s through an frpr-17 G protein-coupled receptor (GPCR) and a protein kinase A signaling cascade in hmc. hmc activation results in muscle contraction through coupling by gap junctions composed of UNC-9/Innexin. hmc activation is inhibited by the neuronal release of a second FMRFamide-like neuropeptide, FLP-9, which functions through its GPCR, frpr-21, in hmc. This study reveals a function for two opposing FMRFamide signaling pathways in controlling the rhythmic activation of a target cell through volume transmission.

Belly roll, a GPI-anchored Ly6 protein, regulates Drosophila melanogaster escape behaviors by modulating the excitability of nociceptive peptidergic interneurons

Appropriate modulation of escape behaviors in response to potentially damaging stimuli is essential for survival. Although nociceptive circuitry has been studied, it is poorly understood how genetic contexts affect relevant escape responses. Using an unbiased genome-wide association analysis, we identified an Ly6/α-neurotoxin family protein, Belly roll (Bero), which negatively regulates Drosophila nociceptive escape behavior. We show that Bero is expressed in abdominal leucokinin-producing neurons (ABLK neurons) and bero knockdown in ABLK neurons resulted in enhanced escape behavior. Furthermore, we demonstrated that ABLK neurons responded to activation of nociceptors and initiated the behavior. Notably, bero knockdown reduced persistent neuronal activity and increased evoked nociceptive responses in ABLK neurons. Our findings reveal that Bero modulates an escape response by regulating distinct neuronal activities in ABLK neurons.

Identification of three elevenin receptors and roles of elevenin disulfide bond and residues in receptor activation in Aplysia californica

Neuropeptides are ubiquitous intercellular signaling molecules in the CNS and play diverse roles in modulating physiological functions by acting on specific G-protein coupled receptors (GPCRs). Among them, the elevenin signaling system is now believed to be present primarily in protostomes. Although elevenin was first identified from the L11 neuron of the abdominal ganglion in mollusc Aplysia californica, no receptors have been described in Aplysia, nor in any other molluscs. Here, using two elevenin receptors in annelid Platynereis dumerilii, we found three putative elevenin GPCRs in Aplysia. We cloned the three receptors and tentatively named them apElevR1, apElevR2, and apElevR3. Using an inositol monophosphate (IP1) accumulation assay, we demonstrated that Aplysia elevenin with the disulfide bond activated the three putative receptors with low EC50 values (ranging from 1.2 to 25 nM), supporting that they are true receptors for elevenin. In contrast, elevenin without the disulfide bond could not activate the receptors, indicating that the disulfide bond is required for receptor activity. Using alanine substitution of individual conserved residues other than the two cysteines, we showed that these residues appear to be critical to receptor activity, and the three different receptors had different sensitivities to the single residue substitution. Finally, we examined the roles of those residues outside the disulfide bond ring by removing these residues and found that they also appeared to be important to receptor activity. Thus, our study provides an important basis for further study of the functions of elevenin and its receptors in Aplysia and other molluscs.

Heart-brain Rhythmic Synchronization during Meditation: A Nonlinear Signal Analysis

Background

Heart-brain synchronization is the integration of mind, body, and spirit. It occurs when the electrical activity of the heart and brain is synchronized. In recent years, there has been mounting curiosity to investigate the effects of meditation on heart-brain synchronization with respect to mental and emotional health and well-being. The current investigation aims to explore the rhythmic synchronicity between the brain and the heart during heartfulness meditation (HM) practice.

Materials and Methods

The study was performed on 45 healthy volunteers who were categorized into three equal groups: long-term meditators (LTMs), short-term meditators (STMs), and nonmeditators (NMs). The electroencephalogram (EEG) signals were recorded to measure the prefrontal activity, and electrocardiogram (ECG) signals were recorded to measure the cardiac activity. The data were recorded in four states: baseline, meditation, transmission, and posttransmission. The detrended fluctuation analysis (DFA) method was used for the analysis of EEG and ECG signals.

Results

The result indicates that DFA values of EEG and ECG declined during meditation and transmission states as compared to pre- and postmeditation states. Significant results were obtained for the LTM group in all the states. A positive correlation was also observed between DFA of the heart and brain for the LTM group and no significant correlations were observed for the STM and NM groups.

Conclusion

The shreds of evidence suggest that heart-brain synchronization facilitates mental and emotional stability. HM practice has the potential to regulate the fluctuation of the mind. Regular meditation practice may result in physiological synchrony between cardiac and neural behavior, which can be considered a quality index for meditation practice.

Characterization of an Aplysia vasotocin signaling system and actions of posttranslational modifications and individual residues of the ligand on receptor activity

The vasopressin/oxytocin signaling system is present in both protostomes and deuterostomes and plays various physiological roles. Although there were reports for both vasopressin-like peptides and receptors in mollusc Lymnaea and Octopus, no precursor or receptors have been described in mollusc Aplysia. Here, through bioinformatics, molecular and cellular biology, we identified both the precursor and two receptors for Aplysia vasopressin-like peptide, which we named Aplysia vasotocin (apVT). The precursor provides evidence for the exact sequence of apVT, which is identical to conopressin G from cone snail venom, and contains 9 amino acids, with two cysteines at position 1 and 6, similar to nearly all vasopressin-like peptides. Through inositol monophosphate (IP1) accumulation assay, we demonstrated that two of the three putative receptors we cloned from Aplysia cDNA are true receptors for apVT. We named the two receptors as apVTR1 and apVTR2. We then determined the roles of post-translational modifications (PTMs) of apVT, i.e., the disulfide bond between two cysteines and the C-terminal amidation on receptor activity. Both the disulfide bond and amidation were critical for the activation of the two receptors. Cross-activity with conopressin S, annetocin from an annelid, and vertebrate oxytocin showed that although all three ligands can activate both receptors, the potency of these peptides differed depending on their residue variations from apVT. We, therefore, tested the roles of each residue through alanine substitution and found that each substitution could reduce the potency of the peptide analog, and substitution of the residues within the disulfide bond tended to have a larger impact on receptor activity than the substitution of those outside the bond. Moreover, the two receptors had different sensitivities to the PTMs and single residue substitutions. Thus, we have characterized the Aplysia vasotocin signaling system and showed how the PTMs and individual residues in the ligand contributed to receptor activity.

G protein-coupled receptor kinase-2 (GRK-2) controls exploration through neuropeptide signaling in Caenorhabditis elegans

Animals alter their behavior in manners that depend on environmental conditions as well as their developmental and metabolic states. For example, C. elegans is quiescent during larval molts or during conditions of satiety. By contrast, worms enter an exploration state when removed from food. Sensory perception influences movement quiescence (defined as a lack of body movement), as well as the expression of additional locomotor states in C. elegans that are associated with increased or reduced locomotion activity, such as roaming (exploration behavior) and dwelling (local search). Here we find that movement quiescence is enhanced, and exploration behavior is reduced in G protein-coupled receptor kinase grk-2 mutant animals. grk-2 was previously shown to act in chemosensation, locomotion, and egg-laying behaviors. Using neuron-specific rescuing experiments, we show that GRK-2 acts in multiple ciliated chemosensory neurons to control exploration behavior. grk-2 acts in opposite ways from the cGMP-dependent protein kinase gene egl-4 to control movement quiescence and exploration behavior. Analysis of mutants with defects in ciliated sensory neurons indicates that grk-2 and the cilium-structure mutants act in the same pathway to control exploration behavior. We find that GRK-2 controls exploration behavior in an opposite manner from the neuropeptide receptor NPR-1 and the neuropeptides FLP-1 and FLP-18. Finally, we show that secretion of the FLP-1 neuropeptide is negatively regulated by GRK-2 and that overexpression of FLP-1 reduces exploration behavior. These results define neurons and molecular pathways that modulate movement quiescence and exploration behavior.

Variational and phase response analysis for limit cycles with hard boundaries, with applications to neuromechanical control problems

Motor systems show an overall robustness, but because they are highly nonlinear, understanding how they achieve robustness is difficult. In many rhythmic systems, robustness against perturbations involves response of both the shape and the timing of the trajectory. This makes the study of robustness even more challenging. To understand how a motor system produces robust behaviors in a variable environment, we consider a neuromechanical model of motor patterns in the feeding apparatus of the marine mollusk Aplysia californica (Shaw et al. in J Comput Neurosci 38(1):25-51, 2015; Lyttle et al. in Biol Cybern 111(1):25-47, 2017). We established in (Wang et al. in SIAM J Appl Dyn Syst 20(2):701-744, 2021. https://doi.org/10.1137/20M1344974 ) the tools for studying combined shape and timing responses of limit cycle systems under sustained perturbations and here apply them to study robustness of the neuromechanical model against increased mechanical load during swallowing. Interestingly, we discover that nonlinear biomechanical properties confer resilience by immediately increasing resistance to applied loads. In contrast, the effect of changed sensory feedback signal is significantly delayed by the firing rates' hard boundary properties. Our analysis suggests that sensory feedback contributes to robustness in swallowing primarily by shifting the timing of neural activation involved in the power stroke of the motor cycle (retraction). This effect enables the system to generate stronger retractor muscle forces to compensate for the increased load, and hence achieve strong robustness. The approaches that we are applying to understanding a neuromechanical model in Aplysia, and the results that we have obtained, are likely to provide insights into the function of other motor systems that encounter changing mechanical loads and hard boundaries, both due to mechanical and neuronal firing properties.

The effect of B-type allatostatin neuropeptides on crosstalk between the insect immune response and cold tolerance

Insects are the largest group of arthropod phyla and are capable of surviving in a variety of environments. One of the most important factors in enabling them to do so is their resistance to temperature stress, i.e., cold tolerance. The neuroendocrine system, together with the immune system, cooperates to regulate a number of physiological processes that are essential for the stability of the organism in stressful conditions. However, to date, no one has studied the effect of insect myoinhibitory peptides (MIPs) on cold stress tolerance and immune system activity. Here, we investigated the effect of Tenmo-MIP 5 (10^-6 M), cold stress (- 5 °C) and a combination of both on the immune response of Tenebrio molitor. All three treatments caused upregulation of immune-related genes (antimicrobial peptides and Toll) and increased phagocytosis activity (by approximately 10%). However, phenoloxidase activity and mortality were increased only after peptide injection and the combination of both treatments. The peptide injection combined with cold stress caused 40% higher mortality than that in the control. Together, our results show the links between cold stress, MIPs activity and the immune response, and to our knowledge, this is the first report showing the effect of MIP on the insect immune system.

Internal state affects local neuron function in an early sensory processing center to shape olfactory behavior in Drosophila larvae

Crawling insects, when starved, tend to have fewer head wavings and travel in straighter tracks in search of food. We used the Drosophila melanogaster larva to investigate whether this flexibility in the insect's navigation strategy arises during early olfactory processing and, if so, how. We demonstrate a critical role for Keystone-LN, an inhibitory local neuron in the antennal lobe, in implementing head-sweep behavior. Keystone-LN responds to odor stimuli, and its inhibitory output is required for a larva to successfully navigate attractive and aversive odor gradients. We show that insulin signaling in Keystone-LN likely mediates the starvation-dependent changes in head-sweep magnitude, shaping the larva's odor-guided movement. Our findings demonstrate how flexibility in an insect's navigation strategy can arise from context-dependent modulation of inhibitory neurons in an early sensory processing center. They raise new questions about modulating a circuit's inhibitory output to implement changes in a goal-directed movement.

An extra-clock ultradian brain oscillator sustains circadian timekeeping

The master circadian clock generates 24-hour rhythms to orchestrate daily behavior, even running freely under constant conditions. Traditionally, the master clock is considered self-sufficient in sustaining free-running timekeeping via its cell-autonomous molecular clocks and interneuronal communications within the circadian neural network. Here, we find a set of bona fide ultradian oscillators in the Drosophila brain that support free-running timekeeping, despite being located outside the master clock circuit and lacking clock gene expression. These extra-clock electrical oscillators (xCEOs) generate cell-autonomous ultradian bursts, pacing widespread burst firing and promoting rhythmic resting membrane potentials in clock neurons via parallel monosynaptic connections. Silencing xCEOs disrupts daily electrical rhythms in clock neurons and impairs cycling of neuropeptide pigment dispersing factor, leading to the loss of free-running locomotor rhythms. Together, we conclude that the master clock is not self-sufficient to sustain free-running behavior rhythms but requires additional endogenous inputs to the clock from the extra-clock ultradian brain oscillators.

Dynamic influences on the neural encoding of social valence

Social signals can serve as potent emotional triggers with powerful impacts on processes from cognition to valence processing. How are social signals dynamically and flexibly associated with positive or negative valence? How do our past social experiences and present social standing shape our motivation to seek or avoid social contact? We discuss a model in which social attributes, social history, social memory, social rank and social isolation can flexibly influence valence assignment to social stimuli, termed here as 'social valence'. We emphasize how the brain encodes each of these four factors and highlight the neural circuits and mechanisms that play a part in the perception of social attributes, social memory and social rank, as well as how these factors affect valence systems associated with social stimuli. We highlight the impact of social isolation, dissecting the neural and behavioural mechanisms that mediate the effects of acute versus prolonged periods of social isolation. Importantly, we discuss conceptual models that may account for the potential shift in valence of social stimuli from positive to negative as the period of isolation extends in time. Collectively, this Review identifies factors that control the formation and attribution of social valence - integrating diverse areas of research and emphasizing their unique contributions to the categorization of social stimuli as positive or negative.

Cilia-derived vesicles: An ancient route for intercellular communication

Extracellular vesicles (EVs) provide a mechanism for intercellular communication that transports complex signals in membrane delimited structures between cells, tissues and organisms. Cells secrete EVs of various subtypes defined by the pathway leading to release and by the pathological condition of the cell. Cilia are evolutionarily conserved organelles that can act as sensory structures surveilling the extracellular environment. Here we discuss the secretory functions of cilia and their biological implications. Studies in multiple species - from the nematode Caenorhabditis elegans and the chlorophyte alga Chlamydomonas reinhardtii to mammals - have revealed that cilia shed bioactive EVs (ciliary EVs or ectosomes) by outward budding of the ciliary membrane. The content of ciliary EVs is distinct from that of other vesicles released by cells. Peptides regulate numerous aspects of metazoan physiology and development through evolutionarily conserved mechanisms. Intriguingly, cilia-derived vesicles have recently been found to mediate peptidergic signaling. C. reinhardtii releases the peptide α-amidating enzyme (PAM), bioactive amidated products and components of the peptidergic signaling machinery in ciliary EVs in a developmentally regulated manner. Considering the origin of cilia in early eukaryotes, it is likely that release of peptidergic signals in ciliary EVs represents an alternative and ancient mode of regulated secretion that cells can utilize in the absence of dedicated secretory granules.

AI protein structure prediction-based modeling and mutagenesis of a protostome receptor and peptide ligands reveal key residues for their interaction

The protostome leucokinin (LK) signaling system, including LK peptides and their G protein-coupled receptors, has been characterized in several species. Despite progress in this area, molecular mechanisms governing LK peptide-receptor interactions remain to be elucidated. Previously, we identified a precursor protein for Aplysia leucokinin-like peptides (ALKs) that contains the greatest number of amidated peptides among LK precursors in all species identified so far. Here, we identified the first ALK receptor from Aplysia, ALKR. We used cell-based IP1 activation assays to demonstrate that the two ALK peptides with the most copies, ALK1 and ALK2, activated ALKR with high potencies. Other endogenous ALK-derived peptides bearing the FXXWX-amide motif also activated ALKR to various degrees. Our examination of cross-species activity of ALKs with the Anopheles LKR was consistent with a critical role for the FXXWX-amide motif in receptor activity. Furthermore, we showed, through alanine substitution of ALK1, the highly conserved phenylalanine (F), tryptophan (W), and C-terminal amidation were each essential for receptor activation. Finally, we used an AI-based protein structure prediction server (Robetta) and Autodock Vina to predict the ligand-bound conformation of ALKR. Our model predicted several interactions (i.e., hydrophobic interactions, hydrogen bonds, and amide-pi stacking) between ALK peptides and ALKR, and several of our substitution and mutagenesis experiments were consistent with the predicted model. In conclusion, our results provide important information defining the possible interactions between ALK peptides and their receptors. The workflow utilized here may be useful for studying other ligand-receptor interactions for a neuropeptide signaling system, particularly in protostomes.

The emergence and influence of internal states

Animal behavior is shaped by a variety of "internal states"-partially hidden variables that profoundly shape perception, cognition, and action. The neural basis of internal states, such as fear, arousal, hunger, motivation, aggression, and many others, is a prominent focus of research efforts across animal phyla. Internal states can be inferred from changes in behavior, physiology, and neural dynamics and are characterized by properties such as pleiotropy, persistence, scalability, generalizability, and valence. To date, it remains unclear how internal states and their properties are generated by nervous systems. Here, we review recent progress, which has been driven by advances in behavioral quantification, cellular manipulations, and neural population recordings. We synthesize research implicating defined subsets of state-inducing cell types, widespread changes in neural activity, and neuromodulation in the formation and updating of internal states. In addition to highlighting the significance of these findings, our review advocates for new approaches to clarify the underpinnings of internal brain states across the animal kingdom.

Endocrine cybernetics: neuropeptides as molecular switches in behavioural decisions

Plasticity in animal behaviour relies on the ability to integrate external and internal cues from the changing environment and hence modulate activity in synaptic circuits of the brain. This context-dependent neuromodulation is largely based on non-synaptic signalling with neuropeptides. Here, we describe select peptidergic systems in the Drosophila brain that act at different levels of a hierarchy to modulate behaviour and associated physiology. These systems modulate circuits in brain regions, such as the central complex and the mushroom bodies, which supervise specific behaviours. At the top level of the hierarchy there are small numbers of large peptidergic neurons that arborize widely in multiple areas of the brain to orchestrate or modulate global activity in a state and context-dependent manner. At the bottom level local peptidergic neurons provide executive neuromodulation of sensory gain and intrinsically in restricted parts of specific neuronal circuits. The orchestrating neurons receive interoceptive signals that mediate energy and sleep homeostasis, metabolic state and circadian timing, as well as external cues that affect food search, aggression or mating. Some of these cues can be triggers of conflicting behaviours such as mating versus aggression, or sleep versus feeding, and peptidergic neurons participate in circuits, enabling behaviour choices and switches.

Functional analysis of LFRFamide signaling in Pacific abalone, Haliotis discus hannai

The invertebrate LFRFamide (LFRFa) and short neuropeptide F (sNPF), consisting of 6 to 10 amino acids, are orthologs for bilaterian NPF/Y, which consist of 36 to 40 amino acids. Recently, a molluscan G protein-coupled receptor (GPCR) for NPF was characterized in Pacific abalone (Haliotis discus hannai). To address the functional evolutionary route of the invertebrate LFRFa and NPF signaling system, in this study, we identified cDNAs encoding LFRFa precursors and the sNPF receptor (Hdh-sNPFR) in Pacific abalone. Four LFRFa mature peptides with 6 or 7 amino acids were predicted: GSLFRFa, GGLFRFa, GTLFRFa, and GSTLFRFa. Hdh-sNPFR was identified as a classical rhodopsin-like GPCR and classified into a molluscan sNPFR group. In HEK293 cells, Hdh-sNPFR was mainly localized in the cell membranes and internalized in the cytoplasm following treatment with LFRFa peptides. Reporter assays demonstrated that LFRFa peptides inhibit forskolin-stimulated cAMP accumulation in Hdh-sNPFR-expressing HEK293 cells. LFRFa precursor and Hdh-sNPFR transcripts were more strongly expressed in the cerebral and pleural-pedal ganglia of Pacific abalone than in the peripheral tissues such as the ovary, gills, intestine, and hepatopancreas. The levels of LFRFa transcripts in the ovary, intestine, and hepatopancreas were significantly higher in mature female abalone than in immature females. Injection of LFRFa induced the egg release and spawning behavior of mature abalone, but suppressed food intake. These results suggest that LFRFa peptides are endogenous ligands for Hdh-sNPFR involved in food intake and reproduction through a Gαi-protein dependent signaling pathway.

Neurotransmission and neuromodulation systems in the learning and memory network of Octopus vulgaris

The vertical lobe (VL) in the octopus brain plays an essential role in its sophisticated learning and memory. Early anatomical studies suggested that the VL is organized in a "fan-out fan-in" connectivity matrix comprising only three morphologically identified neuron types; input axons from the median superior frontal lobe (MSFL) innervating en passant millions of small amacrine interneurons (AMs), which converge sharply onto large VL output neurons (LNs). Recent physiological studies confirmed the feedforward excitatory connectivity; a glutamatergic synapse at the first MSFL-to-AM synaptic layer and a cholinergic AM-to-LNs synapse. MSFL-to-AMs synapses show a robust hippocampal-like activity-dependent long-term potentiation (LTP) of transmitter release. 5-HT, octopamine, dopamine and nitric oxide modulate short- and long-term VL synaptic plasticity. Here, we present a comprehensive histolabeling study to better characterize the neural elements in the VL. We generally confirmed glutamatergic MSFLs and cholinergic AMs. Intense labeling for NOS activity in the AMs neurites were in-line with the NO-dependent presynaptic LTP mechanism at the MSFL-to-AM synapse. New discoveries here reveal more heterogeneity of the VL neurons than previously thought. GABAergic AMs suggest a subpopulation of inhibitory interneurons in the first input layer. Clear γ-amino butyric acid labeling in the cell bodies of LNs supported an inhibitory VL output, yet the LNs co-expressed FMRFamide-like neuropeptides, suggesting an additional neuromodulatory role of the VL output. Furthermore, a group of LNs was glutamatergic. A new cluster of cells organized as a "deep nucleus" showed rich catecholaminergic labeling and may play a role in intrinsic neuromodulation. In-situ hybridization and immunolabeling allowed characterization and localization of a rich array of neuropeptides and neuromodulators, likely involved in reward/punishment signals. This analysis of the fast transmission system, together with the newly found cellular elements, help integrate behavioral, physiological, pharmacological and connectome findings into a more comprehensive understanding of an efficient learning and memory network.

Intestine-to-neuronal signaling alters risk-taking behaviors in food-deprived Caenorhabditis elegans

Animals integrate changes in external and internal environments to generate behavior. While neural circuits detecting external cues have been mapped, less is known about how internal states like hunger are integrated into behavioral outputs. Here, we use the nematode C. elegans to examine how changes in internal nutritional status affect chemosensory behaviors. We show that acute food deprivation leads to a reversible decline in repellent, but not attractant, sensitivity. This behavioral change requires two conserved transcription factors MML-1 (MondoA) and HLH-30 (TFEB), both of which translocate from the intestinal nuclei to the cytoplasm during food deprivation. Next, we identify the insulin-like peptide INS-31 as a candidate ligand relaying food-status signals from the intestine to other tissues. Further, we show that neurons likely use the DAF-2 insulin receptor and AGE-1/PI-3 Kinase, but not DAF-16/FOXO to integrate these intestine-released peptides. Altogether, our study shows how internal food status signals are integrated by transcription factors and intestine-neuron signaling to generate flexible behaviors via the gut-brain axis.