An OBP gene highly expressed in non-chemosensory tissues affects the phototaxis and reproduction of Spodoptera frugiperda

Insect odorant binding proteins (OBPs) were initially regarded as carriers of the odorants involved in chemosensation. However, it had been observed that a growing number of OBP genes exhibited broad expression patterns beyond chemosensory tissues. Here, an OBP gene (OBP31) was found to be highly expressed in the larval ventral nerve cord, adult brain and male reproductive organ of Spodoptera frugiperda. An OBP31 knockout strain (OBP31-/- ) was generated by CRISPR/Cas9 mutagenesis. For OBP31-/- , the larvae needed longer time to pupate, but there was no difference in the pupal weight between OBP31-/- and wild type (WT). OBP31-/- larvae showed stronger phototaxis than the WT larvae, indicating the importance of OBP31 in light perception. For mating rhythm of adults, OBP31-/- moths displayed an earlier second mating peak. In the cross-pairing of OBP31-/- and WT moths, the mating duration was longer, and hatchability was lower in OBP31-/- group and OBP31+/- ♂ group than that in the WT group. These results suggested that OBP31 played a vital role in larval light perception and male reproductive process and could provide valuable insights into understanding the biological functions of OBPs that were not specific in chemosensory tissues.

Causal Genetic Loci for a Motivated Behavior Spectrum Harbor Psychiatric Risk Genes

Behavioral diversity is critical for population fitness. Individual differences in risk-taking are observed across species, but underlying genetic mechanisms and conservation are largely unknown. We examined dark avoidance in larval zebrafish, a motivated behavior reflecting an approach-avoidance conflict. Brain-wide calcium imaging revealed significant neural activity differences between approach-inclined versus avoidance-inclined individuals. We used a population of ∼6,000 to perform the first genome-wide association study (GWAS) in zebrafish, which identified 34 genomic regions harboring many genes that are involved in synaptic transmission and human psychiatric diseases. We used CRISPR to study several causal genes: serotonin receptor-1b ( htr1b ), nitric oxide synthase-1 ( nos1 ), and stress-induced phosphoprotein-1 ( stip1 ). We further identified 52 conserved elements containing 66 GWAS significant variants. One encoded an exonic regulatory element that influenced tissue-specific nos1 expression. Together, these findings reveal new genetic loci and establish a powerful, scalable animal system to probe mechanisms underlying motivation, a critical dimension of psychiatric diseases.

Blue light stimulates light stress and phototactic behavior when received in the brain of Diaphorina citri

Blue light with a wavelength of 400-470 nm is the composition of the visible light. However, in recent years, blue light contributed the most significance to light pollution due to the artificial light at night. Previously, we have demonstrated that the Asian citrus psyllid (ACP), Diaphorina citri, an important pest in citrus production, has significant positive phototaxis with a light-emitting diode light of 400 nm. In this study, ACP with positive phototactic behavior to 400 nm light (PH) and non-phototactic behavior to 400 nm light (NP) were collected, individually. Transcriptome dynamics of head tissues of PH and NP groups were captured by using RNA-sequencing technology, respectively. Forty-three to 46 million clean reads with high-quality values were obtained, and 1773 differential expressed genes (DEGs) were detected. Compared with the NP group, there were 841 up-regulated DEGs and 932 down-regulated DEGs in the PH group. Eight pathways were significantly enriched in the PH group in the KEGG database, while 43 up-regulated pathways and 25 down-regulated pathways were significantly enriched in the PH group in the GO database. The DGE approach was reliable validated by real time quantitative PCR. Results indicated that the blue light acted as an abiotic stress causing physiological and biochemical responses such as oxidative stress, protein denaturation, inflammation and tumor development in ACPs. Additionally, the light was absorbed by photoreceptors of ACPs, and converted into electrical signal to regulate neuromodulation. This study provides basic information for understanding the molecular mechanisms of ACP in response to blue light and provides a reference for further studies to elucidate phototactic behavior.

A two-choice assay for noxious light avoidance with temporal distribution analysis in Drosophila melanogaster larvae

Two-choice assays allow assessing of different behaviors including light avoidance in Drosophila larvae. Typically, the readout is limited to a preference index at a specific end point. We provide a detailed protocol to set up light avoidance assays and map the temporal distribution of larvae based on analysis of larval intensities. We describe the assay setup and implementation of scripts for analysis, which can be easily adapted to other two-choice assays and different model organisms.

For complete details on the use and execution of this protocol, please refer to Imambocus et al. (2022).

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Preparation and staging of Drosophila larvae for behavioral analysis

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Video-based recording of larval behavior in two-choice light avoidance assays

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Script-based video processing and extraction of larval distribution

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Quantitative analysis of temporal distribution of animals during light avoidance

Publisher’s note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics.

Circuit analysis reveals a neural pathway for light avoidance in Drosophila larvae

Understanding how neural circuits underlie behaviour is challenging even in the connectome era because it requires a combination of anatomical and functional analyses. This is exemplified in the circuit underlying the light avoidance behaviour displayed by Drosophila melanogaster larvae. While this behaviour is robust and the nervous system relatively simple, the circuit is only partially delineated with some contradictions among studies. Here, we devise trans-Tango MkII, an offshoot of the transsynaptic circuit tracing tool trans-Tango, and implement it in anatomical tracing together with functional analysis. We use neuronal inhibition to test necessity of particular neuronal types in light avoidance and selective neuronal activation to examine sufficiency in rescuing light avoidance deficiencies exhibited by photoreceptor mutants. Our studies reveal a four-order circuit for light avoidance connecting the light-detecting photoreceptors with a pair of neuroendocrine cells via two types of clock neurons. This approach can be readily expanded to studying other circuits.

Studying neural circuits requires a multipronged approach. Here, the authors present a transsynaptic tracing tool in fruit fly larvae and combine it with neuronal inhibition and activation to study the circuit underlying light avoidance behaviour.

Brain-wide perception of the emotional valence of light is regulated by distinct hypothalamic neurons

Salient sensory stimuli are perceived by the brain, which guides both the timing and outcome of behaviors in a context-dependent manner. Light is such a stimulus, which is used in treating mood disorders often associated with a dysregulated hypothalamic-pituitary-adrenal stress axis. Relationships between the emotional valence of light and the hypothalamus, and how they interact to exert brain-wide impacts remain unclear. Employing larval zebrafish with analogous hypothalamic systems to mammals, we show in free-swimming animals that hypothalamic corticotropin releasing factor (CRFHy) neurons promote dark avoidance, and such role is not shared by other hypothalamic peptidergic neurons. Single-neuron projection analyses uncover processes extended by individual CRFHy neurons to multiple targets including sensorimotor and decision-making areas. In vivo calcium imaging uncovers a complex and heterogeneous response of individual CRFHy neurons to the light or dark stimulus, with a reduced overall sum of CRF neuronal activity in the presence of light. Brain-wide calcium imaging under alternating light/dark stimuli further identifies distinct and distributed photic response neuronal types. CRFHy neuronal ablation increases an overall representation of light in the brain and broadly enhances the functional connectivity associated with an exploratory brain state. These findings delineate brain-wide photic perception, uncover a previously unknown role of CRFHy neurons in regulating the perception and emotional valence of light, and suggest that light therapy may alleviate mood disorders through reducing an overall sum of CRF neuronal activity.

Neurotransmitters Affect Larval Development by Regulating the Activity of Prothoracicotropic Hormone-Releasing Neurons in Drosophila melanogaster

Ecdysone, an essential insect steroid hormone, promotes larval metamorphosis by coordinating growth and maturation. In Drosophila melanogaster, prothoracicotropic hormone (PTTH)-releasing neurons are considered to be the primary promoting factor in ecdysone biosynthesis. Recently, studies have reported that the regulatory mechanisms of PTTH release in Drosophila larvae are controlled by different neuropeptides, including allatostatin A and corazonin. However, it remains unclear whether neurotransmitters provide input to PTTH neurons and control the metamorphosis in Drosophila larvae. Here, we report that the neurotransmitters acetylcholine (ACh) affect larval development by modulating the activity of PTTH neurons. By downregulating the expression of different subunits of nicotinic ACh receptors in PTTH neurons, pupal volume was significantly increased, whereas pupariation timing was relatively unchanged. We also identified that PTTH neurons were excited by ACh application ex vivo in a dose-dependent manner via ionotropic nicotinic ACh receptors. Moreover, in our Ca2+ imaging experiments, relatively low doses of OA caused increased Ca2+ levels in PTTH neurons, whereas higher doses led to decreased Ca2+ levels. We also demonstrated that a low dose of OA was conveyed through OA β-type receptors. Additionally, our electrophysiological experiments revealed that PTTH neurons produced spontaneous activity in vivo, which provides the possibility of the bidirectional regulation, coming from neurons upstream of PTTH cells in Drosophila larvae. In summary, our findings indicate that several different neurotransmitters are involved in the regulation of larval metamorphosis by altering the activity of PTTH neurons in Drosophila.

Potential role of lysine succinylation in the response of moths to artificial light at night stress

Artificial light at night (ALAN) is a widespread environmental pollutant and stressor. Many nocturnal insects have been shown to experience ALAN stress. However, few studies have been conducted to uncover the mechanism by which nocturnal insects respond to ALAN stress. Previous studies suggest that lysine succinylation (Ksuc) is a potential mechanism that coordinates energy metabolism and antioxidant activity under stressful conditions. Mythimna separata (Walker) (M. separata) is a nocturnal insect that has been stressed by ALAN. In this study, we quantified the relative proteomic Ksuc levels in ALAN-stressed M. separata. Of the 466 identified Ksuc-modified proteins, 103 were hypersuccinylated/desuccinylated in ALAN-stressed moths. The hypersuccinylated/desuccinylated proteins were shown to be involved in various biological processes. In particular, they were enriched in metabolic processes, reactive oxygen species (ROS) homeostasis and the neuromuscular system. Furthermore, we demonstrated that Ksuc might affect moth locomotion by intervening with and coordinating these systems under ALAN stress. These findings suggest that Ksuc plays a vital role in the moth response to ALAN stress and moth locomotion behavior and provide a new perspective on the impact of ALAN on nocturnal insect populations and species communities.

Larval zebrafish display dynamic learning of aversive stimuli in a constant visual surrounding

Balancing exploration and anti-predation are fundamental to the fitness and survival of all animal species from early life stages. How these basic survival instincts drive learning remains poorly understood. Here, using a light/dark preference paradigm with well-controlled luminance history and constant visual surrounding in larval zebrafish, we analyzed intra- and intertrial dynamics for two behavioral components, dark avoidance and center avoidance. We uncover that larval zebrafish display short-term learning of dark avoidance with initial sensitization followed by habituation; they also exhibit long-term learning that is sensitive to trial interval length. We further show that such stereotyped learning patterns is stimulus-specific, as they are not observed for center avoidance. Finally, we demonstrate at individual levels that long-term learning is under homeostatic control. Together, our work has established a novel paradigm to understand learning, uncovered sequential sensitization and habituation, and demonstrated stimulus specificity, individuality, as well as dynamicity in learning.

Neural substrates involved in the cognitive information processing in teleost fish

Over the last few decades, it has been shown that fish, comprising the largest group of vertebrates and in many respects one of the least well studied, possess many cognitive abilities comparable to those of birds and mammals. Despite a plethora of behavioural studies assessing cognition abilities and an abundance of neuroanatomical studies, only few studies have aimed to or in fact identified the neural substrates involved in the processing of cognitive information. In this review, an overview of the currently available studies addressing the joint research topics of cognitive behaviour and neuroscience in teleosts (and elasmobranchs wherever possible) is provided, primarily focusing on two fundamentally different but complementary approaches, i.e. ablation studies and Immediate Early Gene (IEG) analyses. More recently, the latter technique has become one of the most promising methods to visualize neuronal populations activated in specific brain areas, both during a variety of cognitive as well as non-cognition-related tasks. While IEG studies may be more elegant and potentially easier to conduct, only lesion studies can help researchers find out what information animals can learn or recall prior to and following ablation of a particular brain area.

Potential of Pueraria tuberosa (Willd.) DC. to rescue cognitive decline associated with BACE1 protein of Alzheimer's disease on Drosophila model: An integrated molecular modeling and in vivo approach

The indispensable role of Beta-site amyloid precursor protein cleaving enzyme-1 (BACE1) in Amyloid beta (Aβ) plaques generation and Aβ-mediated synaptic dysfunctions makes it a crucial target for therapeutic intervention in Alzheimer's disease (AD). In order to find out the potential inhibitors of BACE1, the present study focused on five phytochemicals from Pueraria tuberosa, namely, daidzin, genistin, mangiferin, puerarin, and tuberosin. A molecular docking study showed that all five phytochemicals presented the strongest BACE1 inhibition. Integrated molecular dynamics simulations and free energy calculations demonstrated that all five natural compounds have stable and favorable energies causing strong binding with the pocket site of BACE1 on 50 ns. All these molecules also passed Lipinski's rule of five. To validate the molecular modeling based findings, we primarily targeted the cognitive decline associated with BACE1 expression in AD flies with P. tuberosa. Significant improvement in cognitive decline was observed in AD flies in different behavioral assays such as Larval crawling assay (16.38%), Larval light preference assay (26.39%), Climbing assay (32.97%), Cold sensitivity assay (43.6%), and Thermal sensitivity assay (44.42%). The present findings suggest that P. tuberosa may be considered as a promising dietary supplement that can significantly ameliorate cognitive decline caused by BACE1 in Alzheimer's disease (AD).

Regulation of Body Size and Growth Control

The control of body and organ growth is essential for the development of adults with proper size and proportions, which is important for survival and reproduction. In animals, adult body size is determined by the rate and duration of juvenile growth, which are influenced by the environment. In nutrient-scarce environments in which more time is needed for growth, the juvenile growth period can be extended by delaying maturation, whereas juvenile development is rapidly completed in nutrient-rich conditions. This flexibility requires the integration of environmental cues with developmental signals that govern internal checkpoints to ensure that maturation does not begin until sufficient tissue growth has occurred to reach a proper adult size. The Target of Rapamycin (TOR) pathway is the primary cell-autonomous nutrient sensor, while circulating hormones such as steroids and insulin-like growth factors are the main systemic regulators of growth and maturation in animals. We discuss recent findings in Drosophila melanogaster showing that cell-autonomous environment and growth-sensing mechanisms, involving TOR and other growth-regulatory pathways, that converge on insulin and steroid relay centers are responsible for adjusting systemic growth, and development, in response to external and internal conditions. In addition to this, proper organ growth is also monitored and coordinated with whole-body growth and the timing of maturation through modulation of steroid signaling. This coordination involves interorgan communication mediated by Drosophila insulin-like peptide 8 in response to tissue growth status. Together, these multiple nutritional and developmental cues feed into neuroendocrine hubs controlling insulin and steroid signaling, serving as checkpoints at which developmental progression toward maturation can be delayed. This review focuses on these mechanisms by which external and internal conditions can modulate developmental growth and ensure proper adult body size, and highlights the conserved architecture of this system, which has made Drosophila a prime model for understanding the coordination of growth and maturation in animals.

Ecdysone-dependent feedback regulation of prothoracicotropic hormone controls the timing of developmental maturation

The activation of a neuroendocrine system that induces a surge in steroid production is a conserved initiator of the juvenile-to-adult transition in many animals. The trigger for maturation is the secretion of brain-derived neuropeptides, yet the mechanisms controlling the timely onset of this event remain ill-defined. Here, we show that a regulatory feedback circuit controlling the Drosophila neuropeptide Prothoracicotropic hormone (PTTH) triggers maturation onset. We identify the Ecdysone Receptor (EcR) in the PTTH-expressing neurons (PTTHn) as a regulator of developmental maturation onset. Loss of EcR in these PTTHn impairs PTTH signaling, which delays maturation. We find that the steroid ecdysone dose-dependently affects Ptth transcription, promoting its expression at lower concentrations and inhibiting it at higher concentrations. Our findings indicate the existence of a feedback circuit in which rising ecdysone levels trigger, via EcR activity in the PTTHn, the PTTH surge that generates the maturation-inducing ecdysone peak toward the end of larval development. Because steroid feedback is also known to control the vertebrate maturation-inducing hypothalamic-pituitary-gonadal axis, our findings suggest an overall conservation of the feedback-regulatory neuroendocrine circuitry that controls the timing of maturation initiation.

Studying neural circuits of decision-making in Drosophila larva

To study neural circuits underlying decisions, the model organism used for that purpose has to be simple enough to be able to dissect the circuitry neuron by neuron across the nervous system and in the same time complex enough to be able to perform different types of decisions. Here, I lay out the case: (1) that Drosophila larva is an advantageous model system that balances well these two requirements and (2) the insights gained from this model, assuming that circuit principles may be shared across species, can be used to advance our knowledge of neural circuit implementation of decision-making in general, including in more complex brains.

Wood-Based Cellulose Nanofibrils: Haemocompatibility and Impact on the Development and Behaviour of Drosophila melanogaster

Wood-based cellulose nanofibrils (CNF) offer an excellent scaffold for drug-delivery formulation development. However, toxicity and haemocompatibility of the drug carrier is always an important issue. In this study, toxicity-related issues of CNF were addressed. Different doses of CNF were orally administered to Drosophila and different tests like the developmental cycle, trypan blue exclusion assay, larva crawling assay, thermal sensitivity assay, cold sensitivity assay, larval light preference test, climbing behaviour, nitroblue tetrazolium (NBT) reduction assay, adult phenotype, and adult weight were conducted to observe the impact on its development and behaviour. A haemocompatibility assay was done on the blood taken from healthy Wistar rats. In Drosophila, the abnormalities in larval development and behaviour were observed in the behavioural assays. However, the cytotoxic effect could not be confirmed by the gut staining and level of reactive oxygen species. The larvae developed into an adult without any abnormality in the phenotype. The CNF did cause loss of weight in the adult flies and did not cause much toxicity within the body since there was no phenotypic defect. Hemolysis data also suggested that CNF was safe at lower doses, as the data was well within acceptable limits. All these results suggest that cellulose nanofibres have no significant cytotoxic effects on Drosophila. However, the developmental and behavioural abnormalities suggest that CNF may act as a behavioural teratogen.

Recent advances in neuropeptide signaling in Drosophila, from genes to physiology and behavior

This review focuses on neuropeptides and peptide hormones, the largest and most diverse class of neuroactive substances, known in Drosophila and other animals to play roles in almost all aspects of daily life, as w;1;ell as in developmental processes. We provide an update on novel neuropeptides and receptors identified in the last decade, and highlight progress in analysis of neuropeptide signaling in Drosophila. Especially exciting is the huge amount of work published on novel functions of neuropeptides and peptide hormones in Drosophila, largely due to the rapid developments of powerful genetic methods, imaging techniques and innovative assays. We critically discuss the roles of peptides in olfaction, taste, foraging, feeding, clock function/sleep, aggression, mating/reproduction, learning and other behaviors, as well as in regulation of development, growth, metabolic and water homeostasis, stress responses, fecundity, and lifespan. We furthermore provide novel information on neuropeptide distribution and organization of peptidergic systems, as well as the phylogenetic relations between Drosophila neuropeptides and those of other phyla, including mammals. As will be shown, neuropeptide signaling is phylogenetically ancient, and not only are the structures of the peptides, precursors and receptors conserved over evolution, but also many functions of neuropeptide signaling in physiology and behavior.

A Neuronal Pathway that Commands Deceleration in Drosophila Larval Light-Avoidance

When facing a sudden danger or aversive condition while engaged in on-going forward motion, animals transiently slow down and make a turn to escape. The neural mechanisms underlying stimulation-induced deceleration in avoidance behavior are largely unknown. Here, we report that in Drosophila larvae, light-induced deceleration was commanded by a continuous neural pathway that included prothoracicotropic hormone neurons, eclosion hormone neurons, and tyrosine decarboxylase 2 motor neurons (the PET pathway). Inhibiting neurons in the PET pathway led to defects in light-avoidance due to insufficient deceleration and head casting. On the other hand, activation of PET pathway neurons specifically caused immediate deceleration in larval locomotion. Our findings reveal a neural substrate for the emergent deceleration response and provide a new understanding of the relationship between behavioral modules in animal avoidance responses.

Neural Substrates of Drosophila Larval Anemotaxis

Animals use sensory information to move toward more favorable conditions. Drosophila larvae can move up or down gradients of odors (chemotax), light (phototax), and temperature (thermotax) by modulating the probability, direction, and size of turns based on sensory input. Whether larvae can anemotax in gradients of mechanosensory cues is unknown. Further, although many of the sensory neurons that mediate taxis have been described, the central circuits are not well understood. Here, we used high-throughput, quantitative behavioral assays to demonstrate Drosophila larvae anemotax in gradients of wind speeds and to characterize the behavioral strategies involved. We found that larvae modulate the probability, direction, and size of turns to move away from higher wind speeds. This suggests that similar central decision-making mechanisms underlie taxis in somatosensory and other sensory modalities. By silencing the activity of single or very few neuron types in a behavioral screen, we found two sensory (chordotonal and multidendritic class III) and six nerve cord neuron types involved in anemotaxis. We reconstructed the identified neurons in an electron microscopy volume that spans the entire larval nervous system and found they received direct input from the mechanosensory neurons or from each other. In this way, we identified local interneurons and first- and second-order subesophageal zone (SEZ) and brain projection neurons. Finally, silencing a dopaminergic brain neuron type impairs anemotaxis. These findings suggest that anemotaxis involves both nerve cord and brain circuits. The candidate neurons and circuitry identified in our study provide a basis for future detailed mechanistic understanding of the circuit principles of anemotaxis.

A disinhibitory mechanism biases Drosophila innate light preference

Innate preference toward environmental conditions is crucial for animal survival. Although much is known about the neural processing of sensory information, how the aversive or attractive sensory stimulus is transformed through central brain neurons into avoidance or approaching behavior is largely unclear. Here we show that Drosophila larval light preference behavior is regulated by a disinhibitory mechanism. In the disinhibitory circuit, a pair of GABAergic neurons exerts tonic inhibition on one pair of contralateral projecting neurons that control larval reorientation behavior. When a larva enters the light area, the reorientation-controlling neurons are disinhibited to allow reorientation to occur as the upstream inhibitory neurons are repressed by light. When the larva exits the light area, the inhibition on the downstream neurons is restored to repress further reorientation and thus prevents the larva from re-entering the light area. We suggest that disinhibition may serve as a common neural mechanism for animal innate preference behavior.

Hormonal signaling cascades required for phototaxis switch in wandering Leptinotarsa decemlineata larvae

Many animals exploit several niches sequentially during their life cycles, a fitness referred to as ontogenetic niche shift (ONS). To successfully accomplish ONS, transition between development stages is often coupled with changes in one or more primitive, instinctive behaviors. Yet, the underlining molecular mechanisms remain elusive. We show here that Leptinotarsa decemlineata larvae finish their ONS at the wandering stage by leaving the plant and pupating in soil. At middle wandering phase, larvae also switch their phototactic behavior, from photophilic at foraging period to photophobic. We find that enhancement of juvenile hormone (JH) signal delays the phototactic switch, and vise verse. Moreover, RNA interference (RNAi)-aided knockdown of LdPTTH (prothoracicotropic hormone gene) or LdTorso (PTTH receptor gene) impairs avoidance response to light, a phenotype nonrescuable by 20-hydroxyecdysone. Consequently, the RNAi beetles pupate at the soil surface or in shallow layer of soil, with most of them failing to construct pupation chambers. Furthermore, a combination of depletion of LdPTTH/LdTorso and disturbance of JH signal causes no additive effects on light avoidance response and pupation site selection. Finally, we establish that TrpA1 (transient receptor potential (TRP) cation channel) is necessary for light avoidance behavior, acting downstream of PTTH. We conclude that JH/PTTH cascade concomitantly regulates metamorphosis and the phototaxis switch, to drive ONS of the wandering beetles from plant into soil to start the immobile pupal stage.

Many animals occupy distinct niches and utilize diverse resources at different development stages in order to meet stage-dependent requirements and overcome stage-specific limitations. This fitness is referred to as ontogenetic niche shift (ONS). During the preparation for ONS, animals often change one or more primitive, instinctive behaviors. Holometabolous insects, with four discrete developmental periods usually in different niches, are a suitable animal group to explore the molecular modes of these behavioral switches. Here we find that Leptinotarsa decemlineata larvae, an insect defoliator of potatoes, switch their phototactic behavior, from photophilic at feeding period to photophobic during the larval-pupal transition (wandering stage). This phototactic switch facilitates the wandering larvae to accomplish the ONS from potato plants to their pupation sites below ground. We show that JH/PTTH cascade controls the phototaxis switch, through a step in photo transduction between the photoreceptor molecule and the transient receptor potential cation channel.

Two Pairs of Drosophila Central Brain Neurons Mediate Larval Navigational Strategies Based on Temporal Light Information Processing

Some animals are attracted by sun light, others are highly repulsed by it. Especially for slowly moving animals, such as Drosophila larvae, direct sunlight may be perceived as noxious stimulus as it increases the risk of desiccation, DNA-damaging by UV-light and exposure to predators. For several reasons, model organisms like Drosophila larvae are well-suited for investigating how light cues are translated into an appropriate behavioral output. First, many of the genetic tools, which were created for use in adult fruit flies, work also in larvae. Second, the lower number of cells in Drosophila larvae compared to adults makes this system adequate for reconstructing neural circuits. Third, the relatively simple behavioral repertoire of larvae facilitates the study of basic functions like navigation with regards to light. Larvae navigate robustly away from a light source by the use of several sophisticated behavioral strategies which are based on temporal or spatial information processing. Two central brain neurons, the NP394-neurons, are highly important for larval light avoidance. It was even reported that these cells seem to play a functional role in a putative larval light preference switch right before pupation. However, the exact function of the NP394-neurons in light navigation remains unknown. We here show that the functional role of NP394-neurons in larval light navigation is restricted to behaviors based on temporal information processing, but not for spatial navigation.

Reconfiguration of a Multi-oscillator Network by Light in the Drosophila Circadian Clock

The brain clock that drives circadian rhythms of locomotor activity relies on a multi-oscillator neuronal network. In addition to synchronizing the clock with day-night cycles, light also reformats the clock-driven daily activity pattern. How changes in lighting conditions modify the contribution of the different oscillators to remodel the daily activity pattern remains largely unknown. Our data in Drosophila indicate that light readjusts the interactions between oscillators through two different modes. We show that a morning s-LNv > DN1p circuit works in series, whereas two parallel evening circuits are contributed by LNds and other DN1ps. Based on the photic context, the master pacemaker in the s-LNv neurons swaps its enslaved partner-oscillator-LNd in the presence of light or DN1p in the absence of light-to always link up with the most influential phase-determining oscillator. When exposure to light further increases, the light-activated LNd pacemaker becomes independent by decoupling from the s-LNvs. The calibration of coupling by light is layered on a clock-independent network interaction wherein light upregulates the expression of the PDF neuropeptide in the s-LNvs, which inhibits the behavioral output of the DN1p evening oscillator. Thus, light modifies inter-oscillator coupling and clock-independent output-gating to achieve flexibility in the network. It is likely that the light-induced changes in the Drosophila brain circadian network could reveal general principles of adapting to varying environmental cues in any neuronal multi-oscillator system.

Turns with multiple and single head cast mediate Drosophila larval light avoidance

Drosophila larvae exhibit klinotaxis when placed in a gradient of temperature, chemicals, or light. The larva samples environmental stimuli by casting its head from side to side. By comparing the results of two consecutive samples, it decides the direction of movement, appearing as a turn proceeded by one or more head casts. Here by analyzing larval behavior in a light-spot-based phototaxis assay, we showed that, in addition to turns with a single cast (1-cast), turns with multiple head casts (n-cast) helped to improve the success of light avoidance. Upon entering the light spot, the probability of escape from light after the first head cast was only ~30%. As the number of head casts increased, the chance of successful light avoidance increased and the overall chance of escaping from light increased to >70%. The amplitudes of first head casts that failed in light avoidance were significantly smaller in n-cast turns than those in 1-cast events, indicating that n-cast turns might be planned before completion of the first head cast. In n-casts, the amplitude of the second head cast was generally larger than that of the first head cast, suggesting that larvae tried harder in later attempts to improve the efficacy of light avoidance. We propose that both 1-cast turns and n-cast turns contribute to successful larval light avoidance, and both can be initiated at the first head cast.

Heritable natural variation of an anxiety-like behavior in larval zebrafish

Complex behaviors are often observed at a spectrum in the population, and psychiatric disorders represent extremes of such behavioral spectra. While grasping the underlying cellular and molecular basis of these disorders represents a major challenge, it is believed that studies of complex behaviors in model organisms, where genotyping and phenotyping can be more conveniently carried out and cause-effect relationships can be further discerned, will help address this challenge. Here we report the characterization of a natural dark aversion behavior in larval zebrafish, which is previously shown to be fear or anxiety-associated. Phenotyping ∼200 individuals using a light/dark choice assay uncovered that, while a majority of individuals displayed medium level of dark aversion (mda), a small number of individuals exhibited strong dark aversion (sda), and a third small cohort showed variable dark aversion (vda). Through selective breeding and phenotyping of the next generation, we demonstrated that both the sda and vda traits are heritable, with sda being invariable while vda being highly variable across multiple trials. Additionally, sda appears to be recessive and vda appears to be dominant over the common allele(s) in the population. Moreover, compared to vda, sda showed increased thigmotaxis (preference for the walls in an open field), another measure of anxiety. Together, these findings reveal a naturally heritable variation of anxiety-like behavior in a tractable model organism, thereby laying foundation for future dissection of the underlying molecular and cellular mechanisms.

Central and peripheral clocks are coupled by a neuropeptide pathway in Drosophila

Animal circadian clocks consist of central and peripheral pacemakers, which are coordinated to produce daily rhythms in physiology and behaviour. Despite its importance for optimal performance and health, the mechanism of clock coordination is poorly understood. Here we dissect the pathway through which the circadian clock of Drosophila imposes daily rhythmicity to the pattern of adult emergence. Rhythmicity depends on the coupling between the brain clock and a peripheral clock in the prothoracic gland (PG), which produces the steroid hormone, ecdysone. Time information from the central clock is transmitted via the neuropeptide, sNPF, to non-clock neurons that produce the neuropeptide, PTTH. These secretory neurons then forward time information to the PG clock. We also show that the central clock exerts a dominant role on the peripheral clock. This use of two coupled clocks could serve as a paradigm to understand how daily steroid hormone rhythms are generated in animals.

Left Habenula Mediates Light-Preference Behavior in Zebrafish via an Asymmetrical Visual Pathway

Habenula (Hb) plays critical roles in emotion-related behaviors through integrating inputs mainly from the limbic system and basal ganglia. However, Hb also receives inputs from multiple sensory modalities. The function and underlying neural circuit of Hb sensory inputs remain unknown. Using larval zebrafish, we found that left dorsal Hb (dHb, a homolog of mammalian medial Hb) mediates light-preference behavior by receiving visual inputs from a specific subset of retinal ganglion cells (RGCs) through eminentia thalami (EmT). Loss- and gain-of-function manipulations showed that left, but not right, dHb activities, which encode environmental illuminance, are necessary and sufficient for light-preference behavior. At circuit level, left dHb neurons receive excitatory monosynaptic inputs from bilateral EmT, and EmT neurons are contacted mainly by sustained ON-type RGCs at the arborization field 4 of retinorecipient brain areas. Our findings discover a previously unidentified asymmetrical visual pathway to left Hb and its function in mediating light-preference behavior. VIDEO ABSTRACT.

Neural Circuits Underlying Fly Larval Locomotion

Locomotion is a complex motor behavior that may be expressed in different ways using a variety of strategies depending upon species and pathological or environmental conditions. Quadrupedal or bipedal walking, running, swimming, flying and gliding constitute some of the locomotor modes enabling the body, in all cases, to move from one place to another. Despite these apparent differences in modes of locomotion, both vertebrate and invertebrate species share, at least in part, comparable neural control mechanisms for locomotor rhythm and pattern generation and modulation. Significant advances have been made in recent years in studies of the genetic aspects of these control systems. Findings made specifically using Drosophila (fruit fly) models and preparations have contributed to further understanding of the key role of genes in locomotion. This review focuses on some of the main findings made in larval fruit flies while briefly summarizing the basic advantages of using this powerful animal model for studying the neural locomotor system.

Continuous lateral oscillations as a core mechanism for taxis in Drosophila larvae

Taxis behaviour in Drosophila larva is thought to consist of distinct control mechanisms triggering specific actions. Here, we support a simpler hypothesis: that taxis results from direct sensory modulation of continuous lateral oscillations of the anterior body, sparing the need for ‘action selection’. Our analysis of larvae motion reveals a rhythmic, continuous lateral oscillation of the anterior body, encompassing all head-sweeps, small or large, without breaking the oscillatory rhythm. Further, we show that an agent-model that embeds this hypothesis reproduces a surprising number of taxis signatures observed in larvae. Also, by coupling the sensory input to a neural oscillator in continuous time, we show that the mechanism is robust and biologically plausible. The mechanism provides a simple architecture for combining information across modalities, and explaining how learnt associations modulate taxis. We discuss the results in the light of larval neural circuitry and make testable predictions.

DOI:http://dx.doi.org/10.7554/eLife.15504.001

Winged Pea Aphids Can Modify Phototaxis in Different Development Stages to Assist Their Host Distribution

The pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae), shows wing polyphenism (winged and wingless morphs) in its life cycle. The winged morph is adapted for dispersal; its two developmental adult stages (for dispersal and reproduction) are based on its breeding periods. The two morphs show different phototactic behavior and the winged can change its preference to light according to the developmental stages. To determine the mechanism and ecological functions of phototaxis for A. pisum, we first investigated the phototaxis of the two aphid morphs at different stages and analyzed the phototactic response to lights of different wavelengths; the correlation between alate fecundity and their phototactic behaviors were then studied. Finally, we focused on the possible functions of phototaxis in aphid host location and distribution in combination with gravitaxis behaviors. Negative phototaxis was found for breeding winged adults but all the other stages of both winged and wingless morphs showed positive phototaxis. The reactions of the aphids to different wavelengths were also different. Nymph production in winged adults showed negative correlation to phototaxis. The dopamine pathway was possibly involved in these behavior modifications. We speculated that winged adults can use light for dispersal in the early dispersal stage and for position holding in the breeding stage. Based on our results, we assume that light signals are important for aphid dispersal and distribution, and are also essential for the pea aphids to cope with environmental changes.

Advanced Fluorescence Protein-Based Synapse-Detectors

The complex information-processing capabilities of the central nervous system emerge from intricate patterns of synaptic input-output relationships among various neuronal circuit components. Understanding these capabilities thus requires a precise description of the individual synapses that comprise neural networks. Recent advances in fluorescent protein engineering, along with developments in light-favoring tissue clearing and optical imaging techniques, have rendered light microscopy (LM) a potent candidate for large-scale analyses of synapses, their properties, and their connectivity. Optically imaging newly engineered fluorescent proteins (FPs) tagged to synaptic proteins or microstructures enables the efficient, fine-resolution illumination of synaptic anatomy and function in large neural circuits. Here we review the latest progress in fluorescent protein-based molecular tools for imaging individual synapses and synaptic connectivity. We also identify associated technologies in gene delivery, tissue processing, and computational image analysis that will play a crucial role in bridging the gap between synapse- and system-level neuroscience.

Understanding Synaptogenesis and Functional Connectome in C. elegans by Imaging Technology

Formation of functional synapses is a fundamental process for establishing neural circuits and ultimately for expressing complex behavior. Extensive research has interrogated how such functional synapses are formed and how synapse formation contributes to the generation of neural circuitry and behavior. The nervous system of Caenorhabditis elegans, due to its relatively simple structure, the transparent body, and tractable genetic system, has been adapted as an excellent model to investigate synapses and the functional connectome. Advances in imaging technology together with the improvement of genetically encoded molecular tools enabled us to visualize synapses and neural circuits of the animal model, which provide insights into our understanding of molecules and their signaling pathways that mediate synapse formation and neuronal network modulation. Here, we review synaptogenesis in active zones and the mapping of local connectome in C. elegans nervous system whose understandings have been extended by the advances in imaging technology along with the genetic molecular tools.

Circadian rhythms in neuronal activity propagate through output circuits

24hr rhythms in behavior are organized by a network of circadian pacemaker neurons. Rhythmic activity in this network is generated by intrinsic rhythms in clock neuron physiology and communication between clock neurons. However, it is poorly understood how the activity of a small number of pacemaker neurons is translated into rhythmic behavior of the whole animal. To understand this, we screened for signals that could identify circadian output circuits in Drosophila. We found that Leucokinin neuropeptide (LK) and its receptor (LK-R) are required for normal behavioral rhythms. This LK/LK-R circuit connects pacemaker neurons to brain areas that regulate locomotor activity and sleep. Our experiments revealed that pacemaker neurons impose rhythmic activity and excitability on LK and LK-R expressing neurons. We also found pacemaker neuron-dependent activity rhythms in DH44-expressing neurons, a second circadian output pathway. We conclude that rhythmic clock neuron activity propagates to multiple downstream circuits to orchestrate behavioral rhythms.

Activation and inhibition of tph2 serotonergic neurons operate in tandem to influence larval zebrafish preference for light over darkness

Serotonergic neurons have been implicated in a broad range of processes, but the principles underlying their effects remain a puzzle. Here, we ask how these neurons influence the tendency of larval zebrafish to swim in the light and avoid regions of darkness. Pharmacological inhibition of serotonin synthesis reduces dark avoidance, indicating an involvement of this neuromodulator. Calcium imaging of tph2-expressing cells demonstrates that a rostral subset of dorsal raphe serotonergic neurons fire continuously while the animal is in darkness, but are inhibited in the light. Optogenetic manipulation of tph2 neurons by channelrhodopsin or halorhodopsin expression modifies preference, confirming a role for these neurons. In particular, these results suggest that fish prefer swimming in conditions that elicits lower activity in tph2 serotonergic neurons in the rostral raphe.

Cold-sensing regulates Drosophila growth through insulin-producing cells

Across phyla, body size is linked to climate. For example, rearing fruit flies at lower temperatures results in bigger body sizes than those observed at higher temperatures. The underlying molecular basis of this effect is poorly understood. Here we provide evidence that the temperature-dependent regulation of Drosophila body size depends on a group of cold-sensing neurons and insulin-producing cells (IPCs). Electrically silencing IPCs completely abolishes the body size increase induced by cold temperature. IPCs are directly innervated by cold-sensing neurons. Stimulation of these cold-sensing neurons activates IPCs, promotes synthesis and secretion of Drosophila insulin-like peptides and induces a larger body size, mimicking the effects of rearing the flies in cold temperature. Taken together, these findings reveal a neuronal circuit that mediates the effects of low temperature on fly growth.

Neuronal wiring: linking dendrite placement to synapse formation

Understanding the processes that drive the formation of synapses between specific neurons within a circuit is critical to understanding how neural networks develop. A new study of synapse formation between motor neurons and pre-synaptic partners highlights the importance of dendrite placement.

Serotonergic neurons respond to nutrients and regulate the timing of steroid hormone biosynthesis in Drosophila

The temporal transition of development is flexibly coordinated in the context of the nutrient environment, and this coordination is essential for organisms to increase their survival fitness and reproductive success. Steroid hormone, a key player of the juvenile-to-adult transition, is biosynthesized in a nutrient-dependent manner; however, the underlying genetic mechanism remains unclear. Here we report that the biosynthesis of insect steroid hormone, ecdysteroid, is regulated by a subset of serotonergic neurons in Drosophila melanogaster. These neurons directly innervate the prothoracic gland (PG), an ecdysteroid-producing organ and share tracts with the stomatogastric nervous system. Interestingly, the projecting neurites morphologically respond to nutrient conditions. Moreover, reduced activity of the PG-innervating neurons or of serotonin signalling in the PG strongly correlates with a delayed developmental transition. Our results suggest that serotonergic neurons form a link between the external environment and the internal endocrine system by adaptively tuning the timing of steroid hormone biosynthesis.

A conditioned visual orientation requires the ellipsoid body in Drosophila

Orientation, the spatial organization of animal behavior, is an essential faculty of animals. Bacteria and lower animals such as insects exhibit taxis, innate orientation behavior, directly toward or away from a directional cue. Organisms can also orient themselves at a specific angle relative to the cues. In this study, using Drosophila as a model system, we established a visual orientation conditioning paradigm based on a flight simulator in which a stationary flying fly could control the rotation of a visual object. By coupling aversive heat shocks to a fly's orientation toward one side of the visual object, we found that the fly could be conditioned to orientate toward the left or right side of the frontal visual object and retain this conditioned visual orientation. The lower and upper visual fields have different roles in conditioned visual orientation. Transfer experiments showed that conditioned visual orientation could generalize between visual targets of different sizes, compactness, or vertical positions, but not of contour orientation. Rut—Type I adenylyl cyclase and Dnc—phosphodiesterase were dispensable for visual orientation conditioning. Normal activity and scb signaling in R3/R4d neurons of the ellipsoid body were required for visual orientation conditioning. Our studies established a visual orientation conditioning paradigm and examined the behavioral properties and neural circuitry of visual orientation, an important component of the insect's spatial navigation.

Identification of motor neurons and a mechanosensitive sensory neuron in the defecation circuitry of Drosophila larvae

Defecation allows the body to eliminate waste, an essential step in food processing for animal survival. In contrast to the extensive studies of feeding, its obligate counterpart, defecation, has received much less attention until recently. In this study, we report our characterizations of the defecation behavior of Drosophila larvae and its neural basis. Drosophila larvae display defecation cycles of stereotypic frequency, involving sequential contraction of hindgut and anal sphincter. The defecation behavior requires two groups of motor neurons that innervate hindgut and anal sphincter, respectively, and can excite gut muscles directly. These two groups of motor neurons fire sequentially with the same periodicity as the defecation behavior, as revealed by in vivo Ca(2+) imaging. Moreover, we identified a single mechanosensitive sensory neuron that innervates the anal slit and senses the opening of the intestine terminus. This anus sensory neuron relies on the TRP channel NOMPC but not on INACTIVE, NANCHUNG, or PIEZO for mechanotransduction.

γ-glutamyl transpeptidase 1 specifically suppresses green-light avoidance via GABAA receptors in Drosophila

Drosophila larvae innately show light avoidance behavior. Compared with robust blue-light avoidance, larvae exhibit relatively weaker green-light responses. In our previous screening for genes involved in larval light avoidance, compared with control w(1118) larvae, larvae with γ-glutamyl transpeptidase 1 (Ggt-1) knockdown or Ggt-1 mutation were found to exhibit higher percentage of green-light avoidance which was mediated by Rhodopsin6 (Rh6) photoreceptors. However, their responses to blue light did not change significantly. By adjusting the expression level of Ggt-1 in different tissues, we found that Ggt-1 in malpighian tubules was both necessary and sufficient for green-light avoidance. Our results showed that glutamate levels were lower in Ggt-1 null mutants compared with controls. Feeding Ggt-1 null mutants glutamate can normalize green-light avoidance, indicating that high glutamate concentrations suppressed larval green-light avoidance. However, rather than directly, glutamate affected green-light avoidance indirectly through GABA, the level of which was also lower in Ggt-1 mutants compared with controls. Mutants in glutamate decarboxylase 1, which encodes GABA synthase, and knockdown lines of the GABAA receptor, both exhibit elevated levels of green-light avoidance. Thus, our results elucidate the neurobiological mechanisms mediating green-light avoidance, which was inhibited in wild-type larvae.

Multi-unit recording with iridium oxide modified stereotrodes in Drosophila melanogaster

BACKGROUND

Drosophila is a very favorable animal model for the studies of neuroscience. However, it remains a great challenge to employ electrophysiological approaches in Drosophila to study the neuronal assembly dynamics in vivo, partially due to the small size of the Drosophila brain. Small and sensitive microelectrodes for multi-unit recordings are greatly desired.

NEW METHOD

We fabricated micro-scale stereotrodes for electrical recordings in Drosophila melanogaster. The stereotrodes were modified with iridium oxide (IrO2) under a highly controllable deposition procedure to improve their electrochemical properties. Electrical recordings were carried out using the IrO2 stereotrodes to detect spontaneous action potentials and LFPs in vivo.

RESULTS

The IrO2 electrodes exhibited significantly higher capacitance and lower impedance at 1 kHz. Electrical recording with the IrO2 stereotrodes in vivo demonstrated an average signal-to-noise ratio (SNR) of 7.3 and a significantly improved LFP sensitivity. 5 types of different neurons recorded were clearly separated. Electrophysiological responses to visual and odor stimulation were also detected, respectively.

COMPARISON WITH EXISTING METHOD(S)

The most widely used electrodes for electrical recording in Drosophila are glass microelectrode and sharpened tungsten microelectrode, which are typically used for single-unit recordings. Although tetrode technology has been used to record multi-neuronal activities from Drosophila, the fabricated IrO2 stereotrodes possess smaller geometry size but exhibited comparable recording signal-to noise ration and better sorting quality.

CONCLUSIONS

The IrO2 stereotrodes are capable to meet the requirements of multi-unit recording and spike sorting, which will be a useful tool for the electrophysiology-based researches especially in Drosophila and other small animals.

Neuroendocrine control of Drosophila larval light preference

Animal development is coupled with innate behaviors that maximize chances of survival. Here, we show that the prothoracicotropic hormone (PTTH), a neuropeptide that controls the developmental transition from juvenile stage to sexual maturation, also regulates light avoidance in Drosophila melanogaster larvae. PTTH, through its receptor Torso, acts on two light sensors--the Bolwig's organ and the peripheral class IV dendritic arborization neurons--to regulate light avoidance. We found that PTTH concomitantly promotes steroidogenesis and light avoidance at the end of larval stage, driving animals toward a darker environment to initiate the immobile maturation phase. Thus, PTTH controls the decisions of when and where animals undergo metamorphosis, optimizing conditions for adult development.

Plasticity in the Drosophila larval visual system

The remarkable ability of the nervous system to modify its structure and function is mostly experience and activity modulated. The molecular basis of neuronal plasticity has been studied in higher behavioral processes, such as learning and memory formation. However, neuronal plasticity is not restricted to higher brain functions and it may provide a basic feature of adaptation of all neural circuits. The fruit fly Drosophila melanogaster provides a powerful genetic model to gain insight into the molecular basis of nervous system development and function. The nervous system of the larvae is again a magnitude simpler than its adult counter part, allowing the genetic assessment of a number of individual genetically identifiable neurons. We review here recent progress on the genetic basis of neuronal plasticity in developing and functioning neural circuits focusing on the simple visual system of the Drosophila larva.

Insect pheromone behavior: fruit fly

The amenability to genetics of Drosophila melanogaster has made this organism one of the best-suited models for studying the neurobiology of pheromone-guided behavior. Single-male assays use the minigene encoding the thermosensitive channel dTrpA1 to activate neurons expressing fruitless (fru), a major courtship regulator gene, and thereby induce most of the elementary courtship acts in a solitary male exposed to temperature increase. Tethered male assays allow Ca(2+)-imaging of neuronal activities of a male fly displaying courtship behavior on a treadmill when stimulated with a female or pheromones. Here we describe technical details of these assays.

Transgenic strategy for identifying synaptic connections in mice by fluorescence complementation (GRASP)

In the "GFP reconstitution across synaptic partners" (GRASP) method, non-fluorescent fragments of GFP are expressed in two different neurons; the fragments self-assemble at synapses between the two to form a fluorophore. GRASP has proven useful for light microscopic identification of synapses in two invertebrate species, Caenorhabditis elegans and Drosophila melanogaster, but has not yet been applied to vertebrates. Here, we describe GRASP constructs that function in mammalian cells and implement a transgenic strategy in which a Cre-dependent gene switch leads to expression of the two fragments in mutually exclusive neuronal subsets in mice. Using a transgenic line that expresses Cre selectively in rod photoreceptors, we demonstrate labeling of synapses in the outer plexiform layer of the retina. Labeling is specific, in that synapses made by rods remain labeled for at least 6 months whereas nearby synapses made by intercalated cone photoreceptors on many of the same interneurons remain unlabeled. We also generated antisera that label reconstituted GFP but neither fragment in order to amplify the GRASP signal and thereby increase the sensitivity of the method.

Innate preference in Drosophila melanogaster

Innate preference behaviors are fundamental for animal survival. They actually form the basis for many animal complex behaviors. Recent years have seen significant progresses in disclosing the molecular and neural mechanism underlying animal innate preferences, especially in Drosophila. In this review, I will review these studies according to the sensory modalities adopted for preference assaying, such as vision, olfaction, thermal sensation. The behavioral strategies and the theoretic models for the formation of innate preferences are also reviewed and discussed.

A molecular diffusion based utility model for Drosophila larval phototaxis

Background

Generally, utility based decision making models focus on experimental outcomes. In this paper we propose a utility model based on molecular diffusion to simulate the choice behavior of Drosophila larvae exposed to different light conditions.

Methods

In this paper, light/dark choice-based Drosophila larval phototaxis is analyzed with our molecular diffusion based model. An ISCEM algorithm is developed to estimate the model parameters.

Results

By applying this behavioral utility model to light intensity and phototaxis data, we show that this model fits the experimental data very well.

Conclusions

Our model provides new insights into decision making mechanisms in general. From an engineering viewpoint, we propose that the model could be applied to a wider range of decision making practices.