Insect neuropeptide

Neuropeptides are involved in the regulation of fundamental events in insects, such as development, reproduction, behavior, and feeding. They modulate important functions like muscle contraction, fluid balance, and reproduction. For example, CAPA peptides in Rhodnius prolixus insects have diverse roles in modulating muscle contraction, regulating fluid balance, and reproduction. This was a pioneering discovery that laid the foundation for the study of insect neuropeptides. Over the next 50 years, two insect neuropeptides—proctolin and adipokinetic hormone—were identified. For example, neuropeptide signaling pathways have been shown to play key roles in regulating ecdysone production in Drosophila, which is crucial for insect molting and metamorphosis. Other studies have characterized families of neuropeptides called "allatostatins" that can inhibit juvenile hormone biosynthesis. These findings highlight the important regulatory functions of neuropeptides in insect endocrine systems and development. Additionally, because neuropeptides control critical life processes, they are promising targets for next-generation, environmentally friendly insecticides. == History and discovery ==

Early studies • Stefan Kopeć (1922) – Proposed the existence of a brain hormone controlling insect molting and metamorphosis. • V. B. Wigglesworth (1934, 1936)- Demonstrated the role of the corpora allata in juvenile hormone regulation. Discovery of the first neuropeptides • In 1975, scientists identified "proctolin" as the first insect neuropeptide, discovered in the American cockroach (Periplaneta americana), which was found to be involved in muscle contraction; this marked a significant milestone in the understanding of neuropeptides in insects. • Adipokinetic hormone (AKH) is a crucial neurohormone that regulates energy metabolism during insect flight. It was first discovered in 1976 by an English research group. AKH is synthesized and stored in neuroendocrine cells and plays a vital role in integrating flight energy metabolism. Further analysis of the Drosophila genome has revealed around 30 neuropeptide-encoding genes and 40 neuropeptide receptor genes. More broadly, the sequencing of insect genomes indicates that most insect species contain around three dozen neuropeptide genes, many of which encode multiple related peptides. The field of insect neuropeptide and peptide hormone research has a rich history spanning over 50 years. Researchers have made significant progress in identifying and characterizing the diverse array of these signaling molecules across different insect species. This knowledge has expanded the understanding of insect biology and opened up new avenues for potential applications in areas such as pest management and targeted interventions. == Biosynthesis and mechanism of action ==

Synthesis of neuropeptides The synthesis of insect neuropeptides involves several carefully coordinated biochemical processes, starting from gene transcription to the storage and release of mature neuropeptides. Gene transcription The synthesis of neuropeptides begins with the transcription of DNA sequences that code for neuropeptides into messenger RNA (mRNA). Various transcription factors regulate this process, ensuring that the neuropeptide genes are expressed in response to specific physiological or environmental cues. The regulation of neuropeptide gene transcription is a pivotal step that determines the availability of neuropeptide precursors, with distinct transcriptional activators and silencers influencing the expression levels of different neuropeptide genes across various tissues. The presence of the signal peptide is essential for the proper localization of the preprohormone to the secretory pathway, where it undergoes further processing. Such modifications enhance the functionality of neuropeptides, allowing them to interact effectively with their receptors. Storage and Release Once synthesized and processed, mature neuropeptides are packaged into dense-core vesicles (DCVs) that are stored in specific neuronal compartments near the axon terminals. This storage feature is significant because it provides a pool of readily available neuropeptides that can be quickly mobilized upon neuronal activation. Upon stimulation by calcium influx during action potentials, these mature neuropeptides are released into the synaptic cleft or neurohemal organs, where they exert their effects on target cells through binding to specific G protein-coupled receptors (GPCRs). The majority of insect neuropeptides exert their functions by binding to G-protein-coupled receptors (GPCRs). When a neuropeptide binds to a GPCR, it causes a conformational change in the receptor, activating the associated intracellular G protein. This stimulation initiates an array of biochemical responses characterized by the activity of secondary messengers, such as cyclic AMP (cAMP), calcium ions, and inositol triphosphate (IP3). For instance, neuropeptides may open or close ion channels, thereby altering membrane potentials and excitability. This direct interaction allows neuropeptides to fine-tune neuronal firing rates and muscle contraction dynamics. A notable example includes neuropeptides that modulate calcium channels, enhancing or dampening calcium influx in response to the binding of the neuropeptide, thereby influencing synaptic transmission efficacy. Neuropeptide degradation The functional lifespan of neuropeptides is tightly controlled, necessitating rapid degradation to prevent prolonged activity. The degradation of neuropeptides primarily involves the action of peptidases, which are enzymes specifically designed to hydrolyze peptide bonds. This hydrolysis transforms active neuropeptides into inactive fragments, effectively terminating their biological activity. Several families of peptidases contribute to the inactivation of neuropeptides, such as angiotensin-converting enzyme (ACE), neprilysin (NEP), and dipeptidyl peptidase IV (DPP IV). Each of these enzymes exhibits a unique specificity and efficiency in degrading different neuropeptides, highlighting the importance of enzyme diversity in regulating signaling pathways. Peptidases like NEP and ACE are strategically located in the synaptic cleft and surrounding tissues, allowing them to act where neuropeptide signaling occurs. This spatial arrangement maximizes their effectiveness in modulating neuropeptide activity immediately after release. Furthermore, the distinct affinities of various peptidases for specific neuropeptides underline the complexity and fine-tuning of the neuropeptide signaling regulatory system in insects. The mechanisms through which neuropeptides are hydrolyzed by peptidases involve specific recognition sequences within neuropeptide structures. These sequences determine the binding affinity and catalytic activity of the respective peptidases, ultimately dictating the rate of degradation. In addition to these specific sequences, the structural integrity of neuropeptides can also influence their susceptibility to degradation. For instance, neuropeptides that undergo post-translational modifications, such as amidation or phosphorylation, might exhibit altered affinities toward peptidases. Such modifications can either enhance stability or increase vulnerability to enzymatic degradation, thus impacting the duration of neuropeptide signaling. == Classification ==

Insect neuropeptides have been categorized into "neuropeptide families" based on either similarities in their sequences or shared functional characteristics across different taxa, following the same classification principles used for vertebrate peptide families. However, with the availability of genomic data from multiple organisms, a more effective approach is to compare neuropeptide precursor genes and the peptides they encode across different species. This suggests that the overall number of neuropeptide genes across insects is greater than what is observed in any single species. Furthermore, it is likely that not all neuropeptides have been fully identified in any insect species, including Drosophila. The table below represents the major neuropeptide families that are associated with different insect species and their functions. == Functions in insect physiology ==

Growth and development Ecdysis, the process of shedding the exoskeleton, is fundamental for insect growth and development . This process is efficiently regulated by ecdysis-triggering hormones (ETHs) and eclosion hormones, which coordinate the complex behavioral and physiological sequences required for ecdysis. ETHs are synthesized and secreted by Inka cells residing along the tracheal system, acting as command peptides that initiate the ecdysis process. When triggered, ETH binds to specific G-protein-coupled receptors (GPCRs) on target neurons, leading to the activation of second-messenger cascades that ultimately result in the synthesis and release of additional neuropeptides involved in ecdysis. Eclosion hormone (EH) complements the action of ETH by further instigating behavioral changes required to execute ecdysis. Clearing the trachea and loosening of the old cuticle are crucial aspects that EH regulates. Upon receiving signals from ETH, EH release from neurosecretory cells stimulates muscle contractions necessary for the physical shedding of the exoskeleton, therefore playing a critical role in the three phases of ecdysis: pre-ecdysis, ecdysis, and post-ecdysis. Behavior and reproduction Neuropeptide F (NPF) has been identified as a significant player in regulating sexual behavior and courtship in insects. NPF acts as a neuromodulator that influences various aspects of mating behavior, including courtship rituals, pheromone release, and sexual receptivity. In many insect species, the presence of NPF has been correlated with heightened sexual activity, facilitating the courtship behaviors that precede mating. Research has demonstrated that NPF influences the signaling pathways involved in the perception of pheromones, which are crucial chemical signals that facilitate mating interactions among insects. In addition to their roles in mating, neuropeptides significantly influence ovarian development and the oviposition process. The ovary ecdysteroidogenic hormone (OEH) is instrumental in regulating the reproductive capacity of female insects by promoting the development of ovaries and oocytes . OEH is released in response to signals such as blood feeding, leading to the maturation of eggs and triggering metabolic pathways that support reproduction. The regulation of ovary development by OEH involves a complex interplay with the endocrine system. Upon release, OEH stimulates the synthesis of ecdysteroids, hormones critical for regulating developmental processes, including ovary and egg maturation. Ecdysteroids are responsible for processes like vitellogenesis, where egg yolk proteins are synthesized and deposited into developing oocytes, thus enhancing reproductive output. Metabolism and homeostasis Tachykinin-related peptides (TRPs) have emerged as important modulators of hunger and satiety in insects. These neuropeptides are synthesized within the central nervous system and released into the hemolymph, influencing feeding behavior in response to nutritional needs and environmental cues. The primary role of TRPs is to signal the insect's energy status, thereby regulating food intake and ensuring that the insect maintains an appropriate energy balance. When food is scarce or an insect is in a state of starvation, TRPs are released, promoting a feeding response. This signaling mechanism effectively enhances the motivation to seek food, directly impacting behaviors associated with foraging and feeding. Conversely, once an insect has consumed adequate food and its energy reserves are sufficient, TRPs initiate satiety signals, curtailing feeding behavior. Adipokinetic hormones (AKH) are crucial components of the neuropeptide signaling cascade responsible for managing energy storage and utilization in insects. These peptides are primarily synthesized in neurosecretory cells and play a pivotal role in the mobilization of lipids from storage tissues, especially during high-energy activities such as flight. Muscle movement FMRFamide-related peptides (FaRPs) are a family of neuropeptides known for their ability to modulate muscle contraction in insects. These peptides function primarily at the neuromuscular junction, where they exert effects on skeletal muscle fibers, facilitating movement. Their action involves influencing both the excitability of motor neurons and the contractile properties of muscle cells, thereby affecting locomotion. Research has shown that FaRPs can potentiate muscle contractions, increasing the strength and frequency of these contractions, which is critical for efficient movement. In trials involving various insect species, the application of FaRPs has been associated with enhanced twitch tension in skeletal muscles, indicating their role as modulators of muscle activity. For instance, doses of FaRPs resulted in increased contractile force, thereby promoting effective leg movement necessary for walking, flying, and other forms of locomotion. Seasonal timing, diapause & circadian regulation Recent research indicates that insect neuropeptides play key roles in linking seasonal cues to diapause (dormant states) and circadian rhythms. For example, lateral neurosecretory cells co-expressing peptides like corazonin, pigment‐dispersing factor (PDF) and diuretic hormone 44 appear to coordinate photoperiodic information and dormancy onset in certain dipteran flies. Another example: the ion transport peptide (ITP) functions as a clock output in the brain of Drosophila melanogaster, linking central pacemaker neurons to diurnal activity via neuropeptide signalling. Thus, neuropeptide systems represent an integrative bridge between the circadian clock, environmental timing (light/dark and photoperiod), and physiological state changes like diapause. Gut-brain axis & microbiome interactions Emerging evidence shows insect neuropeptides participate in gut-brain communication and microbial homeostasis. The review "Neuropeptide actions in arthropod biology" notes that neuropeptides act not only on classical neuroendocrine targets but also influence gut homeostasis and the microbiota. While much neuropeptide research in insects has focused on classical endocrine or neuromuscular roles, the involvement of neuropeptides in gut–microbiome communication is increasingly recognised. The broader "microbiota–gut–brain axis" framework emphasises that neuropeptides, hormones and neurotransmitters mediate signals in endocrine, immune and neural arms of this axis. Although direct insect studies remain limited, several key lines of evidence support the role of neuropeptidergic systems: • In the mosquito Anopheles culicifacies, a metagenomic and transcriptomic study revealed that blood-meal-induced proliferation of gut microbiota correlated with altered neurotransmitter and neuropeptide receptor transcripts in brain and gut tissue, supporting a bidirectional microbiome–gut–brain axis in insects. • In the oriental fruit fly Bactrocera dorsalis and the mosquito Aedes aegypti, serotonin (5-HT) was shown to regulate gut bacterial load via modulation of dual oxidase (Duox) gene expression, demonstrating that neuroactive molecules can influence microbiome homeostasis in insects. • In aquatic invertebrates (juvenile mud crabs), gut–brain neuropeptides such as short neuropeptide F (sNPF) were shown to co-localize in gut and brain and vary in expression under starvation with concomitant changes in gut microbial composition, suggesting interplay between gut microbiota and neuropeptide signalling. Several interlinked pathways mediate neuropeptide–microbiome–gut–brain communication in insects. One primary mechanism involves enteroendocrine and neurosecretory cell signalling, where the insect midgut functions as both a sensory and secretory organ. Enteroendocrine cells release peptide hormones such as neuropeptide F (NPF), short neuropeptide F (sNPF), and tachykinin-related peptides in response to nutrient or microbial cues, transmitting signals to the central nervous system through the hemolymph or direct innervation. This endocrine interface allows the gut to communicate microbial and metabolic information to brain centres controlling feeding and energy balance. A second mechanism involves microbial metabolite modulation. Gut bacteria produce short-chain fatty acids, amino acid derivatives, and biogenic amines that can influence neuropeptide synthesis and release. For example, after a blood meal in Aedes aegypti, the proliferation of gut microbes alters expression of gut hormones and receptors, linking microbial activity to gut–brain signalling circuits. Modulation of neuropeptide signalling can thus shift host–microbe homeostasis and gut immune tone. Finally, neural circuit integration also contributes to the insect gut–brain connection. Although insects lack a vertebrate-type vagus nerve, ascending fibres from the stomatogastric nervous system and descending neurosecretory neurons form a bidirectional loop linking gut and brain. Through these circuits, gut-derived signals (e.g., NPF, allatostatins, insulin-like peptides) affect feeding centres in the brain, while descending neurohormonal outputs modulate gut motility, secretion, and microbial interactions. In mosquitoes, NPF promotes host-seeking behaviour before blood feeding, whereas RYamide peptides inhibit attraction post-feeding—demonstrating microbial and nutritional control of peptide-mediated behaviours. This demonstrates cooperation between microbiota-driven metabolism and neuropeptide control of growth and energy use. Immune–microbiome dynamics: Neuropeptides also modulate gut immunity. Myoinhibitory peptides (MIPs) and related neuropeptides up-regulate antimicrobial peptides and phenoloxidase activity, strengthening epithelial defences. Another case is the neuropeptide family of Tachykinin‑related peptides (TRPs, or TKs) in the beetle Tenebrio molitor, where injection of a TRP (Tenmo-TRP-7) induced broad transcriptomic changes in immune-related genes (cellular adhesion, phagocytosis, antimicrobial factors) and resulted in altered haemocyte activity and lysozyme-like antibacterial activity. Importantly, dose and time dependency were observed, indicating a complex interplay between neuropeptide signalling and immune gene networks. A recent study also examined the effect of a specific myoinhibitory peptide (MIP)—Myoinhibitory peptide—(Tenmo‐MIP 5) in Tenebrio under cold stress: injection of MIP resulted in up-regulation of immune-related genes (AMPs and Toll pathway genes), increased phenoloxidase activity, and increased haemocyte phagocytosis and mortality when combined with cold stress. The authors propose a feedback loop whereby MIP reduces juvenile hormone (JH) and insulin-like peptide (ILP) signalling, thereby reducing immunosuppression and boosting immune activation under stress. Comprehensive reviews highlight that insect immunity is under endocrine/neuropeptidergic control: hormones and neuropeptides such as ILPs, AKHs (adipokinetic hormones), bursicon, and TKs can act as immunomodulators—either enhancing or suppressing responses depending on physiological state, environment, and life history trade-offs (e.g., reproduction vs. immunity). Mechanistic themes • Neuroendocrine-immune crosstalk: Neuropeptides produced by neurosecretory cells or released from neuroendocrine organs can bind receptors on haemocytes or fat body cells, altering AMP expression, phagocytosis, or melanization (phenoloxidase) mechanisms. • Life-history trade-offs: Neuropeptide signalling often integrates nutritional status, reproduction, stress tolerance and immune output. For example, ILPs suppress immunity under nutrient-rich or reproductive phases; conversely, neuropeptide modulation under stress may tilt the balance toward immune defence. • Stress-induced immunomodulation: Neuropeptides may act as "alarm" signals under stress (cold, injury, infection) to up-regulate immune genes and ROS mediators before pathogen challenge (e.g., bursicon during moulting). • Hormonal feedback loops: Injection of MIP in Tenebrio suggests that neuropeptide-induced down-regulation of JH and ILP leads to immune activation, hinting at a multi-axis hormonal circuit linking neural, endocrine and immune systems. • Timing and expression dynamics: The effect of neuropeptides on immune genes is dose- and time-dependent, and varies by tissue (fat body, haemocytes) and developmental stage, underlining the complex regulation of insect immunity by neuropeptidergic means. Implications for ecology and pest management Because neuropeptides modulate immunity, and immunity affects insect fitness, survival, and vector/pest competence, these findings open new perspectives: targeting neuropeptide signalling (e.g., antagonist of bursicon, TRP or MIP) could impair pest immunity and make insects more vulnerable to microbial biocontrol agents. Indeed bursicon has been proposed as a pest-control target because of its dual role in cuticle development and immune priming. However, as the field is still young, key gaps remain: how broadly neuropeptide-immune interactions vary across insect taxa, the ligand–receptor specifics for many immune-modulating neuropeptides, the integration with microbial symbionts, and how environmental stressors modify these neuro-immune circuits. == Applications in insect pest control ==

Insect neuropeptides are key targets for pest control strategies aimed at disrupting their biological functions. However, despite significant research efforts, the exact mechanisms by which these neuropeptides affect insect physiology remain unclear. One of the main challenges in developing neuropeptide-based insecticides is creating receptor-selective agonists and antagonists that can effectively regulate neuropeptide activity. While these compounds hold considerable promise for pest management, their practical application is hindered by the difficulty in designing molecules that bind effectively to insect neuropeptide receptors. Moreover, peptides are prone to rapid breakdown by enzymes and have low bioavailability, meaning they need to be modified to improve their stability and absorption in order to function as effective insecticides.Similar challenges are also faced in the pharmaceutical industry, where significant efforts are being made to transform mammalian neuropeptides into therapeutic drugs. One traditional approach involves screening large chemical libraries to identify non-peptide compounds that interact with neuropeptide receptors, followed by refinement to enhance their selectivity and efficacy. This strategy has led to the development of receptor-specific compounds, such as aprepitant (MK 869), a neurokinin-1 receptor antagonist used in treating chemotherapy-induced nausea and depression. Another approach, rational drug design, utilizes structural information on G-protein-coupled receptors (GPCRs) and the structure-activity relationship (SAR) of neuropeptides to develop highly effective and selective receptor ligands. This method has led to the creation of drugs targeting various hormones, including somatostatin, bradykinin, and luteinizing hormone-releasing hormone (LHRH) leading to the identification of several highly effective agonists and antagonists. A more recent and specialized strategy, known as backbone cyclic neuropeptide-based antagonists (BBC-NBA), has been developed to produce more stable and bioavailable neuropeptide antagonists. This approach involves synthesizing backbone cyclic (BBC) libraries based on detailed SAR studies of the PK/PBAN neuropeptide family, which plays a role in regulating functions like sex pheromone production, melanization, and pupal diapause in moths. The screening of these libraries has led to the identification of highly potent PK/PBAN antagonists with enhanced metabolic stability and bioavailability. Furthermore, the structural information from these antagonists can be applied to create non-peptidergic small molecule libraries (NPSML), which incorporate bioactive components of neuropeptides into simpler, more stable molecules. The goal of this approach is to develop insect-specific, cost-effective, and environmentally friendly insecticides. An alternative method for improving the stability and bioavailability of neuropeptide-based compounds is the design of pseudopeptides, where specific amino acids are substituted to increase resistance to enzymatic degradation while maintaining biological activity. While notable progress has been made, further research into the molecular and cellular mechanisms of insect neuropeptides is essential to refine these approaches and fully exploit their potential for pest control. == Future directions in neuropeptide-based pest control ==

Recent advances in genetics and molecular biology are paving exciting new paths for exploiting insect neuropeptides in pest control. For example, the use of RNA interference (RNAi) allows scientists to silence specific neuropeptide genes or their receptors, thereby disrupting key biological signals such as feeding, mating, or molting. In one study on the pest Helicoverpa armigera (cotton bollworm), ingestion of dsRNA targeting hormonal-biosynthesis genes (including those tied to neuropeptide signalling) significantly impaired larval growth, development and survival. Researchers are increasingly combining RNAi with nanoparticle-based delivery systems to enhance uptake, protect the dsRNA from degradation, and improve specificity of delivery. Nanocarriers such as chitosan, layered double hydroxide (LDH), liposomes and other materials have been shown to increase the stability of dsRNA, facilitate adhesion to plant surfaces, and promote uptake by pest insects. Meanwhile, gene-editing tools like CRISPR/Cas9 are helping to identify and validate neuropeptide receptors as viable pest-control targets. By knocking out or modifying neuropeptide receptor genes in model insects such as Drosophila melanogaster or the silkworm Bombyx mori, researchers are uncovering receptor functions that could be exploited to disrupt neuropeptide signalling in pest species with high specificity. Although published studies in pest insects are still emerging, this strategy holds promise for species-specific disruption of neuropeptide systems without broadly harming non-target insects. Together, these tools are helping to build a new generation of environmentally friendly pest-control strategies that act via gene-specific disruption of neuropeptide signalling, offering a promising alternative to traditional broad-spectrum chemical insecticides. == References ==