Peptidergic transmission: from morphological correlates to functional implications

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.

A brief history of insect neuropeptide and peptide hormone research

This review briefly summarizes 50 years of research on insect neuropeptide and peptide hormone (collectively abbreviated NPH) signaling, starting with the sequencing of proctolin in 1975. The first 25 years, before the sequencing of the Drosophila genome, were characterized by efforts to identify novel NPHs by biochemical means, mapping of their distribution in neurons, neurosecretory cells, and endocrine cells of the intestine. Functional studies of NPHs were predominantly dealing with hormonal aspects of peptides and many employed ex vivo assays. With the annotation of the Drosophila genome, and more specifically of the NPHs and their receptors in Drosophila and other insects, a new era followed. This started with matching of NPH ligands to orphan receptors, and studies to localize NPHs with improved detection methods. Important advances were made with introduction of a rich repertoire of innovative molecular genetic approaches to localize and interfere with expression or function of NPHs and their receptors. These methods enabled cell- or circuit-specific interference with NPH signaling for in vivo assays to determine roles in behavior and physiology, imaging of neuronal activity, and analysis of connectivity in peptidergic circuits. Recent years have seen a dramatic increase in reports on the multiple functions of NPHs in development, physiology and behavior. Importantly, we can now appreciate the pleiotropic functions of NPHs, as well as the functional peptidergic "networks" where state dependent NPH signaling ensures behavioral plasticity and systemic homeostasis.

Studying zebrafish nervous system structure and function in health and disease with electron microscopy

Zebrafish (Danio rerio) is a well-established model for studying the nervous system. Findings in zebrafish often inform studies on human diseases of the nervous system and provide crucial insight into disease mechanisms. The functions of the nervous system often rely on communication between neurons. Signal transduction is achieved via release of signaling molecules in the form of neuropeptides or neurotransmitters at synapses. Snapshots of membrane dynamics of these processes are imaged by electron microscopy. Electron microscopy can reveal ultrastructure and thus synaptic processes. This is crucial both for mapping synaptic connections and for investigating synaptic functions. In addition, via volumetric electron microscopy, the overall architecture of the nervous system becomes accessible, where structure can inform function. Electron microscopy is thus of particular value for studying the nervous system. However, today a plethora of electron microscopy techniques and protocols exist. Which technique is most suitable highly depends on the research question and scope as well as on the type of tissue that is examined. This review gives an overview of the electron microcopy techniques used on the zebrafish nervous system. It aims to give researchers a guide on which techniques are suitable for their specific questions and capabilities as well as an overview of the capabilities of electron microscopy in neurobiological research in the zebrafish model.

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.

G protein-coupled receptors (GPCRs) in rotifers and cladocerans: Potential applications in ecotoxicology, ecophysiology, comparative endocrinology, and pharmacology

The G protein-coupled receptor (GPCR) superfamily plays a fundamental role in both sensory functions and the regulation of homeostasis, and is highly conserved across the eukaryote taxa. Its functional diversity is related to a conserved seven-transmembrane core and invariant set of intracellular signaling mechanisms. The interplay between these properties is key to the evolutionary success of GPCR. As this superfamily originated from a common ancestor, GPCR genes have evolved via lineage-specific duplications through the process of adaptation. Here we summarized information on GPCR gene families in rotifers and cladocerans based on their evolutionary position in aquatic invertebrates and their potential application in ecotoxicology, ecophysiology, comparative endocrinology, and pharmacology. Phylogenetic analyses were conducted to examine the evolutionary significance of GPCR gene families and to provide structural insight on their role in aquatic invertebrates. In particular, most GPCR gene families have undergone sporadic evolutionary processes, but some GPCRs are highly conserved across species despite the dynamics of GPCR evolution. Overall, this review provides a better understanding of GPCR evolution in aquatic invertebrates and expand our knowledge of the potential application of these receptors in various fields.

Exosome: A novel neurotransmission modulator or non-canonical neurotransmitter?

Neurotransmission is the electrical impulse-triggered propagation of signals between neurons or between neurons and other cell types such as skeletal muscle cells. Recent studies point out the involvement of exosomes, a type of small bilipid layer-enclosed extracellular vesicles, in regulating neurotransmission. Through horizontally transferring proteins, lipids, and nucleic acids, exosomes can modulate synaptic activities rapidly by controlling neurotransmitter release or progressively by regulating neural plasticity including synapse formation, neurite growth & removal, and axon guidance & elongation. In this review, we summarize the similarities and differences between exosomes and synaptic vesicles in their biogenesis, contents, and release. We also highlight the recent progress made in demonstrating the biological roles of exosome in regulating neurotransmission, and propose a modified model of neurotransmission, in which exosomes act as novel neurotransmitters. Lastly, we provide a comprehensive discussion of the enlightenment of the current knowledge on neurotransmission to the future directions of exosome research.

Gray areas: Neuropeptide circuits linking the Edinger-Westphal and Dorsal Raphe nuclei in addiction

The circuitry of addiction comprises several neural networks including the midbrain - an expansive region critically involved in the control of motivated behaviors. Midbrain nuclei like the Edinger-Westphal (EW) and dorsal raphe (DR) contain unique populations of neurons that synthesize many understudied neuroactive molecules and are encircled by the periaqueductal gray (PAG). Despite the proximity of these special neuron classes to the ventral midbrain complex and surrounding PAG, functions of the EW and DR remain substantially underinvestigated by comparison. Spanning approximately -3.0 to -5.2 mm posterior from bregma in the mouse, these various cell groups form a continuum of neurons that we refer to collectively as the subaqueductal paramedian zone. Defining how these pathways modulate affective behavioral states presents a difficult, yet conquerable challenge for today's technological advances in neuroscience. In this review, we cover the known contributions of different neuronal subtypes of the subaqueductal paramedian zone. We catalogue these cell types based on their spatial, molecular, connectivity, and functional properties and integrate this information with the existing data on the EW and DR in addiction. We next discuss evidence that links the EW and DR anatomically and functionally, highlighting the potential contributions of an EW-DR circuit to addiction-related behaviors. Overall, we aim to derive an integrated framework that emphasizes the contributions of EW and DR nuclei to addictive states and describes how these cell groups function in individuals suffering from substance use disorders. This article is part of the special Issue on 'Neurocircuitry Modulating Drug and Alcohol Abuse'.

Hormonal regulation and functional role of the "renal" tubules in the disease vector, Aedes aegypti

The Aedes aegypti mosquito is a vector responsible for transmitting various arboviruses including dengue and yellow fever. Their ability to regulate the ionic and water composition of their hemolymph is a major physiological phenomenon, allowing the mosquito to adapt to a range of ecological niches. Hematophagus insects, including the female A. aegypti, face the challenge of excess salt and water intake after a blood meal. Post-prandial diuresis is under rigorous control by neuroendocrine factors, acting on the Malpighian "renal" tubules (MTs), to regulate primary urine production. The MTs are made up of two cell types; mitochondria-rich principal cells, which facilitate active transport of Na⁺ and K⁺ cations across the membrane, and thin stellate cells, which allows for transepithelial Cl^- secretion. The active driving force responsible for ion transport is the apical V-type H⁺ ATPase, which creates a proton gradient allowing for Na⁺ and/or K⁺ cation exchange through cation/H⁺ antiporters. Additionally, the basolaterally localized Na⁺-K⁺-2Cl^- cotransporter (NKCC) is responsible for the transport of these ions from the hemolymph into the principal cells. Numerous studies have examined hormonal regulation of the mosquito MTs and identified several diuretics including serotonin (5HT), a calcitonin-related diuretic hormone 31 (DH₃₁), a corticotropin-related factor like diuretic peptide (DH₄₄), a kinin-related diuretic peptide, as well as anti-diuretic factors including CAPA peptides, all of which are known to regulate fluid and ion transport by the MTs. This review therefore focuses on the control of ionic homeostasis in A. aegypti mosquitoes, emphasizing the importance of the MTs, the channels and transporters involved in maintaining hydromineral balance, and the neuroendocrine regulation of both diuresis and anti-diuresis.

Hormonal axes in Drosophila: regulation of hormone release and multiplicity of actions

Hormones regulate development, as well as many vital processes in the daily life of an animal. Many of these hormones are peptides that act at a higher hierarchical level in the animal with roles as organizers that globally orchestrate metabolism, physiology and behavior. Peptide hormones can act on multiple peripheral targets and simultaneously convey basal states, such as metabolic status and sleep-awake or arousal across many central neuronal circuits. Thereby, they coordinate responses to changing internal and external environments. The activity of neurosecretory cells is controlled either by (1) cell autonomous sensors, or (2) by other neurons that relay signals from sensors in peripheral tissues and (3) by feedback from target cells. Thus, a hormonal signaling axis commonly comprises several components. In mammals and other vertebrates, several hormonal axes are known, such as the hypothalamic-pituitary-gonad axis or the hypothalamic-pituitary-thyroid axis that regulate reproduction and metabolism, respectively. It has been proposed that the basic organization of such hormonal axes is evolutionarily old and that cellular homologs of the hypothalamic-pituitary system can be found for instance in insects. To obtain an appreciation of the similarities between insect and vertebrate neurosecretory axes, we review the organization of neurosecretory cell systems in Drosophila. Our review outlines the major peptidergic hormonal pathways known in Drosophila and presents a set of schemes of hormonal axes and orchestrating peptidergic systems. The detailed organization of the larval and adult Drosophila neurosecretory systems displays only very basic similarities to those in other arthropods and vertebrates.

Whole-brain efferent and afferent connectivity of mouse ventral tegmental area melanocortin-3 receptor neurons

The mesolimbic dopamine (DA) system is involved in the regulation of multiple behaviors, including feeding, and evidence demonstrates that the melanocortin system can act on the mesolimbic DA system to control feeding and other behaviors. The melanocortin-3 receptor (MC3R) is an important component of the melanocortin system, but its overall role is poorly understood. Because MC3Rs are highly expressed in the ventral tegmental area (VTA) and are likely to be the key interaction point between the melanocortin and mesolimbic DA systems, we set out to identify both the efferent projection patterns of VTA MC3R neurons and the location of the neurons providing afferent input to them. VTA MC3R neurons were broadly connected to neurons across the brain but were strongly connected to a discrete set of brain regions involved in the regulation of feeding, reward, and aversion. Surprisingly, experiments using monosynaptic rabies virus showed that proopiomelanocortin (POMC) and agouti-related protein (AgRP) neurons in the arcuate nucleus made few direct synapses onto VTA MC3R neurons or any of the other major neuronal subtypes in the VTA, despite being extensively labeled by general retrograde tracers injected into the VTA. These results greatly contribute to our understanding of the anatomical interactions between the melanocortin and mesolimbic systems and provide a foundation for future studies of VTA MC3R neurons and the circuits containing them in the control of feeding and other behaviors.

PAR2, Keratinocytes, and Cathepsin S Mediate the Sensory Effects of Ciguatoxins Responsible for Ciguatera Poisoning

Ciguatera fish poisoning is caused by the consumption of fish contaminated with ciguatoxins (CTXs). The most distressing symptoms are cutaneous sensory disturbances, including cold dysesthesia and itch. CTXs are neurotoxins known to activate voltage-gated sodium channels, but no specific treatment exists. Peptidergic neurons have been critically involved in ciguatera fish poisoning sensory disturbances. Protease-activated receptor-2 (PAR2) is an itch- and pain-related G protein‒coupled receptor whose activation leads to a calcium-dependent neuropeptide release. In this study, we studied the role of voltage-gated sodium channels, PAR2, and the PAR2 agonist cathepsin S in the cytosolic calcium increase and subsequent release of the neuropeptide substance P elicited by Pacific CTX-2 (P-CTX-2) in rat sensory neurons and human epidermal keratinocytes. In sensory neurons, the P-CTX-2‒evoked calcium response was driven by voltage-gated sodium channels and PAR2-dependent mechanisms. In keratinocytes, P-CTX-2 also induced voltage-gated sodium channels and PAR2-dependent marked calcium response. In the cocultured cells, P-CTX-2 significantly increased cathepsin S activity, and cathepsin S and PAR2 antagonists almost abolished P-CTX-2‒elicited substance P release. Keratinocytes synergistically favored the induced substance P release. Our results demonstrate that the sensory effects of CTXs involve the cathepsin S-PAR2 pathway and are potentiated by their direct action on nonexcitable keratinocytes through the same pathway.

The silkworm (Bombyx mori) neuropeptide orcokinin is involved in the regulation of pigmentation

The natural colorful cuticles of insects play important roles in many physiological processes. Pigmentation is a physiological process with a complex regulatory network whose regulatory mechanism remains unclear. Bombyx mori pigmentation mutants are ideal materials for research on pigmentation mechanisms. The purple quail-like (q-l^p) and brown quail-like (q-l^b) mutants originated from plain silkworm breeds 932VR and 0223JH respectively exhibit similar cuticle pigmentation to that of the quail mutant. The q-l^p mutant also presents a developmental abnormality. In this study, genes controlling q-l^p and q-l^b mutants were located on chromosome 8 by positional cloning. Then the neuropeptide gene orcokinin (OK) was identified to be the major gene responsible for two quail-like mutants. The B. mori orcokinin gene (BommoOK) produces two transcripts, BommoOKA and BommoOKB, by alternative splicing. The CRISPR/Cas9 system and orcokinin peptides injection were used for further functional verification. We show a novel function of BommoOKA in inhibiting pigmentation, and one mature peptide of orcokinin A, OKA_type2, is the key factor in pigmentation inhibition. These results provide a reference for studying the function of orcokinin and are of theoretical importance for studying the regulatory mechanism of pigmentation.

Activity-evoked and spontaneous opening of synaptic fusion pores

Synaptic release of neuropeptides packaged in dense-core vesicles (DCVs) regulates synapses, circuits, and behaviors including feeding, sleeping, and pain perception. Here, synaptic DCV fusion pore openings are imaged without interference from cotransmitting small synaptic vesicles (SSVs) with the use of a fluorogen-activating protein (FAP). Activity-evoked kiss and run exocytosis opens synaptic DCV fusion pores away from active zones that readily conduct molecules larger than most native neuropeptides (i.e., molecular weight [MW] up to, at least, 4.5 kDa). Remarkably, these synaptic fusion pores also open spontaneously in the absence of stimulation and extracellular Ca²⁺ SNARE perturbations demonstrate different mechanisms for activity-evoked and spontaneous fusion pore openings with the latter sharing features of spontaneous small molecule transmitter release by active zone-associated SSVs. Fusion pore opening at resting synapses provides a mechanism for activity-independent peptidergic transmission.

Chronic immobilization stress primes the hippocampal opioid system for oxycodone-associated learning in female but not male rats

In adult female, but not male, Sprague Dawley rats, chronic immobilization stress (CIS) increases mossy fiber (MF) Leu-Enkephalin levels and redistributes delta- and mu-opioid receptors (DORs and MORs) in hippocampal CA3 pyramidal cells and GABAergic interneurons to promote excitation and learning processes following subsequent opioid exposure. Here, we demonstrate that CIS females, but not males, acquire conditioned place preference (CPP) to oxycodone and that CIS "primes" the hippocampal opioid system in females for oxycodone-associated learning. In CA3b, oxycodone-injected (Oxy) CIS females relative to saline-injected (Sal) CIS females exhibited an increase in the cytoplasmic and total densities of DORs in pyramidal cell dendrites so that they were similar to Sal- and Oxy-CIS males. Consistent with our earlier studies, Sal- and Oxy-CIS females but not CIS males had elevated DOR densities in MF-CA3 dendritic spines, which we have previously shown are important for opioid-mediated long-term potentiation. In the dentate gyrus, Oxy-CIS females had more DOR-labeled interneurons than Sal-CIS females. Moreover, Sal- and Oxy-CIS females compared to both groups of CIS males had elevated levels of DORs and MORs in GABAergic interneuron dendrites, suggesting capacity for greater synthesis or storage of these receptors in circuits important for opioid-mediated disinhibition. However, more plasmalemmal MORs were on large parvalbumin-containing dendrites of Oxy-CIS males compared to Sal-CIS males, suggesting a limited ability for increased granule cell disinhibition. These results suggest that low levels of DORs in MF-CA3 synapses and hilar GABAergic interneurons may contribute to the attenuation of oxycodone CPP in males exposed to CIS.

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.

Identification and In Vivo Characterisation of Cardioactive Peptides in Drosophila melanogaster

Neuropeptides and peptide hormones serve as critical regulators of numerous biological processes, including development, growth, reproduction, physiology, and behaviour. In mammals, peptidergic regulatory systems are complex and often involve multiple peptides that act at different levels and relay to different receptors. To improve the mechanistic understanding of such complex systems, invertebrate models in which evolutionarily conserved peptides and receptors regulate similar biological processes but in a less complex manner have emerged as highly valuable. Drosophila melanogaster represents a favoured model for the characterisation of novel peptidergic signalling events and for evaluating the relevance of those events in vivo. In the present study, we analysed a set of neuropeptides and peptide hormones for their ability to modulate cardiac function in semi-intact larval Drosophila melanogaster. We identified numerous peptides that significantly affected heart parameters such as heart rate, systolic and diastolic interval, rhythmicity, and contractility. Thus, peptidergic regulation of the Drosophila heart is not restricted to chronotropic adaptation but also includes inotropic modulation. By specifically interfering with the expression of corresponding peptides in transgenic animals, we assessed the in vivo relevance of the respective peptidergic regulation. Based on the functional conservation of certain peptides throughout the animal kingdom, the identified cardiomodulatory activities may be relevant not only to proper heart function in Drosophila, but also to corresponding processes in vertebrates, including humans.

Characterization and Expression Profiling of Neuropeptides and G-Protein-Coupled Receptors (GPCRs) for Neuropeptides in the Asian Citrus Psyllid, Diaphorina citri (Hemiptera: Psyllidae)

Neuropeptides are endogenous active substances that widely exist in multicellular biological nerve tissue and participate in the function of the nervous system, and most of them act on neuropeptide receptors. In insects, neuropeptides and their receptors play important roles in controlling a multitude of physiological processes. In this project, we sequenced the transcriptome from twelve tissues of the Asian citrus psyllid, Diaphorina citri Kuwayama. A total of 40 candidate neuropeptide genes and 42 neuropeptide receptor genes were identified. Among the neuropeptide receptor genes, 35 of them belong to the A-family (or rhodopsin-like), four of them belong to the B-family (or secretin-like), and three of them are leucine-rich repeat-containing G-protein-coupled receptors. The expression profile of the 82 genes across developmental stages was determined by qRT-PCR. Our study provides the first investigation on the genes of neuropeptides and their receptors in D. citri, which may play key roles in regulating the physiology and behaviors of D. citri.

The endocrinology of the brain

The brain hosts a vast and diverse repertoire of neuropeptides, a class of signalling molecules often described as neurotransmitters. Here I argue that this description entails a catalogue of misperceptions, misperceptions that feed into a narrative in which information processing in the brain can be understood only through mapping neuronal connectivity and by studying the transmission of electrically conducted signals through chemical synapses. I argue that neuropeptide signalling in the brain involves primarily autocrine, paracrine and neurohormonal mechanisms that do not depend on synaptic connectivity and that it is not solely dependent on electrical activity but on mechanisms analogous to secretion from classical endocrine cells. As in classical endocrine systems, to understand the role of neuropeptides in the brain, we must understand not only how their release is regulated, but also how their synthesis is regulated and how the sensitivity of their targets is regulated. We must also understand the full diversity of effects of neuropeptides on those targets, including their effects on gene expression.

Active Zone Material-Directed Orientation, Docking, and Fusion of Dense Core Vesicles Alongside Synaptic Vesicles at Neuromuscular Junctions

Active zone material is an organelle that is common to active zones along the presynaptic membrane of chemical synapses. Electron tomography on active zones at frog neuromuscular junctions has provided evidence that active zone material directs the docking of synaptic vesicles (SVs) on the presynaptic membrane at this synapse. Certain active zone material macromolecules connect to stereotypically arranged macromolecules in the membrane of undocked SVs, stably orienting a predetermined fusion domain of the vesicle membrane toward the presynaptic membrane while bringing and holding the two membranes together. Docking of the vesicles is required for the impulse-triggered vesicle membrane-presynaptic membrane fusion that releases the vesicles’ neurotransmitter into the synaptic cleft. As at other synapses, axon terminals at frog neuromuscular junctions contain, in addition to SVs, vesicles that are larger, are much less frequent and, when viewed by electron microscopy, have a distinctive electron dense core. Dense core vesicles at neuromuscular junctions are likely to contain peptides that are released into the synaptic cleft to regulate formation, maintenance and behavior of cellular apparatus essential for synaptic impulse transmission. We show by electron tomography on axon terminals of frog neuromuscular junctions fixed at rest and during repetitive impulse transmission that dense core vesicles selectively dock on and fuse with the presynaptic membrane alongside SVs at active zones, and that active zone material connects to the dense core vesicles undergoing these processes in the same way it connects to SVs. We conclude that undocked dense core vesicles have a predetermined fusion domain, as do undocked SVs, and that active zone material directs oriented docking and fusion of these different vesicle types at active zones of the presynaptic membrane by similar macromolecular interactions.

Substrates for Neuronal Cotransmission With Neuropeptides and Small Molecule Neurotransmitters in Drosophila

It has been known for more than 40 years that individual neurons can produce more than one neurotransmitter and that neuropeptides often are colocalized with small molecule neurotransmitters (SMNs). Over the years much progress has been made in understanding the functional consequences of cotransmission in the nervous system of mammals. There are also some excellent invertebrate models that have revealed roles of coexpressed neuropeptides and SMNs in increasing complexity, flexibility, and dynamics in neuronal signaling. However, for the fly Drosophila there are surprisingly few functional studies on cotransmission, although there is ample evidence for colocalization of neuroactive compounds in neurons of the CNS, based both on traditional techniques and novel single cell transcriptome analysis. With the hope to trigger interest in initiating cotransmission studies, this review summarizes what is known about Drosophila neurons and neuronal circuits where different neuropeptides and SMNs are colocalized. Coexistence of neuroactive substances has been recorded in different neuron types such as neuroendocrine cells, interneurons, sensory cells and motor neurons. Some of the circuits highlighted here are well established in the analysis of learning and memory, circadian clock networks regulating rhythmic activity and sleep, as well as neurons and neuroendocrine cells regulating olfaction, nociception, feeding, metabolic homeostasis, diuretic functions, reproduction, and developmental processes. One emerging trait is the broad role of short neuropeptide F in cotransmission and presynaptic facilitation in a number of different neuronal circuits. This review also discusses the functional relevance of coexisting peptides in the intestine. Based on recent single cell transcriptomics data, it is likely that the neuronal systems discussed in this review are just a fraction of the total set of circuits where cotransmission occurs in Drosophila. Thus, a systematic search for colocalized neuroactive compounds in further neurons in anatomically defined circuits is of interest for the near future.

GABAergic Interneurons in the Neocortex: From Cellular Properties to Circuits

Cortical networks are composed of glutamatergic excitatory projection neurons and local GABAergic inhibitory interneurons that gate signal flow and sculpt network dynamics. Although they represent a minority of the total neocortical neuronal population, GABAergic interneurons are highly heterogeneous, forming functional classes based on their morphological, electrophysiological, and molecular features, as well as connectivity and in vivo patterns of activity. Here we review our current understanding of neocortical interneuron diversity and the properties that distinguish cell types. We then discuss how the involvement of multiple cell types, each with a specific set of cellular properties, plays a crucial role in diversifying and increasing the computational power of a relatively small number of simple circuit motifs forming cortical networks. We illustrate how recent advances in the field have shed light onto the mechanisms by which GABAergic inhibition contributes to network operations.

TRPV1 and TRPA1 in cutaneous neurogenic and chronic inflammation: pro-inflammatory response induced by their activation and their sensitization

Cutaneous neurogenic inflammation (CNI) is inflammation that is induced (or enhanced) in the skin by the release of neuropeptides from sensory nerve endings. Clinical manifestations are mainly sensory and vascular disorders such as pruritus and erythema. Transient receptor potential vanilloid 1 and ankyrin 1 (TRPV1 and TRPA1, respectively) are non-selective cation channels known to specifically participate in pain and CNI. Both TRPV1 and TRPA1 are co-expressed in a large subset of sensory nerves, where they integrate numerous noxious stimuli. It is now clear that the expression of both channels also extends far beyond the sensory nerves in the skin, occuring also in keratinocytes, mast cells, dendritic cells, and endothelial cells. In these non-neuronal cells, TRPV1 and TRPA1 also act as nociceptive sensors and potentiate the inflammatory process. This review discusses the role of TRPV1 and TRPA1 in the modulation of inflammatory genes that leads to or maintains CNI in sensory neurons and non-neuronal skin cells. In addition, this review provides a summary of current research on the intracellular sensitization pathways of both TRP channels by other endogenous inflammatory mediators that promote the self-maintenance of CNI.

Mapping brain structure and function: cellular resolution, global perspective

A comprehensive understanding of the brain requires analysis, although from a global perspective, with cellular, and even subcellular, resolution. An important step towards this goal involves the establishment of three-dimensional high-resolution brain maps, incorporating brain-wide information about the cells and their connections, as well as the chemical architecture. The progress made in such anatomical brain mapping in recent years has been paralleled by the development of physiological techniques that enable investigators to generate global neural activity maps, also with cellular resolution, while simultaneously recording the organism's behavioral activity. Combination of the high-resolution anatomical and physiological maps, followed by theoretical systems analysis of the deduced network, will offer unprecedented opportunities for a better understanding of how the brain, as a whole, processes sensory information and generates behavior.

Identification of different functional types of spinal afferent neurons innervating the mouse large intestine using a novel CGRPα transgenic reporter mouse

Spinal afferent neurons detect noxious and physiological stimuli in visceral organs. Five functional classes of afferent terminals have been extensively characterized in the colorectum, primarily from axonal recordings. Little is known about the corresponding somata of these classes of afferents, including their morphology, neurochemistry, and electrophysiology. To address this, we made intracellular recordings from somata in L6/S1 dorsal root ganglia and applied intraluminal colonic distensions. A transgenic calcitonin gene-related peptide-α (CGRPα)-mCherry reporter mouse, which enabled rapid identification of soma neurochemistry and morphology following electrophysiological recordings, was developed. Three distinct classes of low-threshold distension-sensitive colorectal afferent neurons were characterized; an additional group was distension-insensitive. Two of three low-threshold classes expressed CGRPα. One class expressing CGRPα discharged phasically, with inflections on the rising phase of their action potentials, at low frequencies, to both physiological (<30 mmHg) and noxious (>30 mmHg) distensions. The second class expressed CGRPα and discharged tonically, with smooth, briefer action potentials and significantly greater distension sensitivity than phasically firing neurons. A third class that lacked CGRPα generated the highest-frequency firing to distension and had smaller somata. Thus, CGRPα expression in colorectal afferents was associated with lower distension sensitivity and firing rates and larger somata, while colorectal afferents that generated the highest firing frequencies to distension had the smallest somata and lacked CGRPα. These data fill significant gaps in our understanding of the different classes of colorectal afferent somata that give rise to distinct functional classes of colorectal afferents. In healthy mice, the majority of sensory neurons that respond to colorectal distension are low-threshold, wide-dynamic-range afferents, encoding both physiological and noxious ranges.

A serial multiplex immunogold labeling method for identifying peptidergic neurons in connectomes

Electron microscopy-based connectomics aims to comprehensively map synaptic connections in neural tissue. However, current approaches are limited in their capacity to directly assign molecular identities to neurons. Here, we use serial multiplex immunogold labeling (siGOLD) and serial-section transmission electron microscopy (ssTEM) to identify multiple peptidergic neurons in a connectome. The high immunogenicity of neuropeptides and their broad distribution along axons, allowed us to identify distinct neurons by immunolabeling small subsets of sections within larger series. We demonstrate the scalability of siGOLD by using 11 neuropeptide antibodies on a full-body larval ssTEM dataset of the annelid Platynereis. We also reconstruct a peptidergic circuitry comprising the sensory nuchal organs, found by siGOLD to express pigment-dispersing factor, a circadian neuropeptide. Our approach enables the direct overlaying of chemical neuromodulatory maps onto synaptic connectomic maps in the study of nervous systems.

DOI:http://dx.doi.org/10.7554/eLife.11147.001

In the nervous system, cells called neurons connect to each other to form large “neural” networks. The most powerful method that is currently available for tracing neurons and mapping the connections between them is called electron microscopy. This requires slicing brain tissue into ultrathin sections, which are then imaged one by one. However, while electron microscopy provides highly detailed information about the structure of the connections between neurons, it does not reveal which molecules the neurons use to communicate with each other.

To address this question, Shahidi et al. have developed a new approach called ‘siGOLD’. Unlike previous approaches, siGOLD allows signal molecules inside cells to be labeled with protein tags called antibodies without compromising the ability to examine the tissue with electron microscopy. The technique was developed using the larvae of a marine worm called Platynereis. A single larva was sliced into 5000 sections thin enough to view under an electron microscope, and 150 of these were selected to represent the entire body. Because neurons are typically long and thin, individual neurons usually spanned multiple slices.

To identify the neurons, Shahidi et al. then applied an antibody that recognizes a specific signal molecule to a subset of the slices. The antibodies were labeled with gold particles, which show up as black dots under the electron microscope. Because the molecules recognized by the antibodies are present all along the neuron, and because individual neurons extend over multiple slices, it was possible to trace single neurons by labeling only a small number of slices. Repeating this process in different subsets of slices with antibodies that bind to different signal molecules allowed entire neural circuits to be mapped.

In the future, Shahidi et al.’s approach could be adapted to study neural networks in other organisms such as flies, fish and mice.

DOI:http://dx.doi.org/10.7554/eLife.11147.002

Tachykinin acts upstream of autocrine Hedgehog signaling during nociceptive sensitization in Drosophila

Pain signaling in vertebrates is modulated by neuropeptides like Substance P (SP). To determine whether such modulation is conserved and potentially uncover novel interactions between nociceptive signaling pathways we examined SP/Tachykinin signaling in a Drosophila model of tissue damage-induced nociceptive hypersensitivity. Tissue-specific knockdowns and genetic mutant analyses revealed that both Tachykinin and Tachykinin-like receptor (DTKR99D) are required for damage-induced thermal nociceptive sensitization. Electrophysiological recording showed that DTKR99D is required in nociceptive sensory neurons for temperature-dependent increases in firing frequency upon tissue damage. DTKR overexpression caused both behavioral and electrophysiological thermal nociceptive hypersensitivity. Hedgehog, another key regulator of nociceptive sensitization, was produced by nociceptive sensory neurons following tissue damage. Surprisingly, genetic epistasis analysis revealed that DTKR function was upstream of Hedgehog-dependent sensitization in nociceptive sensory neurons. Our results highlight a conserved role for Tachykinin signaling in regulating nociception and the power of Drosophila for genetic dissection of nociception.

DOI:http://dx.doi.org/10.7554/eLife.10735.001

Injured animals from humans to insects become extra sensitive to sensations such as touch and heat. This hypersensitivity is thought to protect areas of injury or inflammation while they heal, but it is not clear how it comes about.

Now, Im et al. have addressed this question by assessing pain in fruit flies after tissue damage. The experiments used ultraviolet radiation to essentially cause ‘localized sunburn’ to fruit fly larvae. Electrical impulses were then recorded from the larvae’s pain-detecting neurons and the larvae were analyzed for behaviors that indicate pain responses (for example, rolling).

Im et al. found that tissue injury lowers the threshold at which temperature causes pain in fruit fly larvae. Further experiments using mutant flies that lacked genes involved in two signaling pathways showed that a signaling molecule called Tachykinin and its receptor (called DTKR) are needed to regulate the observed threshold lowering. When the genes for either of these proteins were deleted, the larvae no longer showed the pain hypersensitivity following an injury.

Further experiments then uncovered a genetic interaction between Tachykinin signaling and a second signaling pathway that also regulates pain sensitization (called Hedgehog signaling). Im et al. found that Tachykinin acts upstream of Hedgehog in the pain-detecting neurons. Following on from these findings, the biggest outstanding questions are: how, when and where does tissue damage lead to the release of Tachykinin to sensitize neurons? Future studies could also ask whether the genetic interactions between Hedgehog and Tachykinin (or related proteins) are conserved in other animals such as humans and mice.

DOI:http://dx.doi.org/10.7554/eLife.10735.002

Synaptic neuropeptide release by dynamin-dependent partial release from circulating vesicles

Neurons release neuropeptides, enzymes, and neurotrophins by exocytosis of dense-core vesicles (DCVs). Peptide release from individual DCVs has been imaged in vitro with endocrine cells and at the neuron soma, growth cones, neurites, axons, and dendrites but not at nerve terminals, where peptidergic neurotransmission occurs. Single presynaptic DCVs have, however, been tracked in native terminals with simultaneous photobleaching and imaging (SPAIM) to show that DCVs undergo anterograde and retrograde capture as they circulate through en passant boutons. Here dynamin (encoded by the shibire gene) is shown to enhance activity-evoked peptide release at the Drosophila neuromuscular junction. SPAIM demonstrates that activity depletes only a portion of a single presynaptic DCV's content. Activity initiates exocytosis within seconds, but subsequent release occurs slowly. Synaptic neuropeptide release is further sustained by DCVs undergoing multiple rounds of exocytosis. Synaptic neuropeptide release is surprisingly similar regardless of anterograde or retrograde DCV transport into boutons, bouton location, and time of arrival in the terminal. Thus vesicle circulation and bidirectional capture supply synapses with functionally competent DCVs. These results show that activity-evoked synaptic neuropeptide release is independent of a DCV's past traffic and occurs by slow, dynamin-dependent partial emptying of DCVs, suggestive of kiss-and-run exocytosis.

Revisiting enigmatic cortical calretinin-expressing interneurons

Cortical calretinin (CR)-expressing interneurons represent a heterogeneous subpopulation of about 10-30% of GABAergic interneurons, which altogether total ca. 12-20% of all cortical neurons. In the rodent neocortex, CR cells display different somatodendritic morphologies ranging from bipolar to multipolar but the bipolar cells and their variations dominate. They are also diverse at the molecular level as they were shown to express numerous neuropeptides in different combinations including vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), neurokinin B (NKB) corticotrophin releasing factor (CRF), enkephalin (Enk) but also neuropeptide Y (NPY) and somatostatin (SOM) to a lesser extent. CR-expressing interneurons exhibit different firing behaviors such as adapting, bursting or irregular. They mainly originate from the caudal ganglionic eminence (CGE) but a subpopulation also derives from the dorsal part of the medial ganglionic eminence (MGE). Cortical GABAergic CR-expressing interneurons can be divided in two main populations: VIP-bipolar interneurons deriving from the CGE and SOM-Martinotti-like interneurons originating in the dorsal MGE. Although bipolar cells account for the majority of CR-expressing interneurons, the roles they play in cortical neuronal circuits and in the more general metabolic physiology of the brain remained elusive and enigmatic. The aim of this review is, firstly, to provide a comprehensive view of the morphological, molecular and electrophysiological features defining this cell type. We will, secondly, also summarize what is known about their place in the cortical circuit, their modulation by subcortical afferents and the functional roles they might play in neuronal processing and energy metabolism.

Hippocampal mossy fiber leu-enkephalin immunoreactivity in female rats is significantly altered following both acute and chronic stress

Research indicates that responses to stress are sexually dimorphic, particularly in regard to learning and memory processes: while males display impaired cognitive performance and hippocampal CA3 pyramidal cell dendritic remodeling following chronic stress, females exhibit enhanced performance and no remodeling. Leu-enkephalin, an endogenous opioid peptide found in the hippocampal mossy fiber pathway, plays a critical role in mediating synaptic plasticity at the mossy fiber-CA3 pyramidal cell synapse. Estrogen is known to influence the expression of leu-enkephalin in the mossy fibers of females, with leu-enkephalin levels being highest at proestrus and estrus, when estrogen levels are elevated. Since stress is also known to alter the expression of leu-enkephalin in various brain regions, this study was designed to determine whether acute or chronic stress had an effect on mossy fiber leu-enkephalin levels in females or males, through the application of correlated quantitative light and electron microscopic immunocytochemistry. Both acute and chronic stress eliminated the estrogen-dependence of leu-enkephalin levels across the estrous cycle in females, but had no effect on male levels. However, following acute stress leu-enkephalin levels in females were consistently lowered to values comparable to the lowest control values, while following chronic stress they were consistently elevated to values comparable to the highest control values. Ultrastructural changes in leu-enkephalin labeled dense core vesicles paralleled light microscopic observations, with acute stress inducing a decrease in leu-enkephalin labeled dense core vesicles, and chronic stress inducing an increase in leu-enkephalin labeled dense-core vesicles in females. These findings suggest that alterations in leu-enkephalin levels following stress could play an important role in the sex-specific responses that females display in learning processes, including those important in addiction.

Adipokinetic hormone exerts its anti-oxidative effects using a conserved signal-transduction mechanism involving both PKC and cAMP by mobilizing extra- and intracellular Ca2+ stores

The involvement of members of the adipokinetic hormone (AKH) family in regulation of response to oxidative stress (OS) has been reported recently. However, despite these neuropeptides being the best studied family of insect hormones, their precise signaling pathways in their OS responsive role remain to be elucidated. In this study, we have used an in vitro assay to determine the importance of extra and intra-cellular Ca(2+) stores as well as the involvement of protein kinase C (PKC) and cyclic adenosine 3',5'-monophosphate (cAMP) pathways by which AKH exerts its anti-oxidative effects. Lipid peroxidation product (4-HNE) was significantly enhanced and membrane fluidity reduced in microsomal fractions of isolated brains (CNS) of Pyrrhocoris apterus when treated with hydrogen peroxide (H2O2), whereas these biomarkers of OS were reduced to control levels when H2O2 was co-treated with Pyrap-AKH. The effects of mitigation of OS in isolated CNS by AKH were negated when these treatments were conducted in the presence of Ca(2+) channel inhibitors (CdCl2 and thapsigargin). Presence of either bisindolylmaliemide or chelyrythrine chloride (inhibitors of PKC) in the incubating medium also compromised the anti-oxidative function of AKH. However, supplementing the medium with either phorbol myristate acetate (PMA, an activator of PKC) or forskolin (an activator of cAMP) restored the protective effects of exogenous AKH treatment by reducing 4-HNE levels and increasing membrane fluidity to control levels. Taken together, our results strongly implicate the importance of both PKC and cAMP pathways in AKHs' anti-oxidative action by mobilizing both extra and intra-cellular stores of Ca(2+).

Structure activity relationship of synaptic and junctional neurotransmission

Chemical neurotransmission may include transmission to local or remote sites. Locally, contact between 'bare' portions of the bulbous nerve terminal termed a varicosity and the effector cell may be in the form of either synapse or non-synaptic contact. Traditionally, all local transmissions between nerves and effector cells are considered synaptic in nature. This is particularly true for communication between neurons. However, communication between nerves and other effectors such as smooth muscles has been described as nonsynaptic or junctional in nature. Nonsynaptic neurotransmission is now also increasingly recognized in the CNS. This review focuses on the relationship between structure and function that orchestrate synaptic and junctional neurotransmissions. A synapse is a specialized focal contact between the presynaptic active zone capable of ultrafast release of soluble transmitters and the postsynaptic density that cluster ionotropic receptors. The presynaptic and the postsynaptic areas are separated by the 'closed' synaptic cavity. The physiological hallmark of the synapse is ultrafast postsynaptic potentials lasting milliseconds. In contrast, junctions are juxtapositions of nerve terminals and the effector cells without clear synaptic specializations and the junctional space is 'open' to the extracellular space. Based on the nature of the transmitters, postjunctional receptors and their separation from the release sites, the junctions can be divided into 'close' and 'wide' junctions. Functionally, the 'close' and the 'wide' junctions can be distinguished by postjunctional potentials lasting ~1s and tens of seconds, respectively. Both synaptic and junctional communications are common between neurons; however, junctional transmission is the rule at many neuro-non-neural effectors.

Neuropeptide transmission in brain circuits

Neuropeptides are found in many mammalian CNS neurons where they play key roles in modulating neuronal activity. In contrast to amino acid transmitter release at the synapse, neuropeptide release is not restricted to the synaptic specialization, and after release, a neuropeptide may diffuse some distance to exert its action through a G protein-coupled receptor. Some neuropeptides such as hypocretin/orexin are synthesized only in single regions of the brain, and the neurons releasing these peptides probably have similar functional roles. Other peptides such as neuropeptide Y (NPY) are synthesized throughout the brain, and neurons that synthesize the peptide in one region have no anatomical or functional connection with NPY neurons in other brain regions. Here, I review converging data revealing a complex interaction between slow-acting neuromodulator peptides and fast-acting amino acid transmitters in the control of energy homeostasis, drug addiction, mood and motivation, sleep-wake states, and neuroendocrine regulation.

Activation of cortical 5-HT(3) receptor-expressing interneurons induces NO mediated vasodilatations and NPY mediated vasoconstrictions

GABAergic interneurons are local integrators of cortical activity that have been reported to be involved in the control of cerebral blood flow (CBF) through their ability to produce vasoactive molecules and their rich innervation of neighboring blood vessels. They form a highly diverse population among which the serotonin 5-hydroxytryptamine 3A receptor (5-HT_3A)-expressing interneurons share a common developmental origin, in addition to the responsiveness to serotonergic ascending pathway. We have recently shown that these neurons regroup two distinct subpopulations within the somatosensory cortex: Neuropeptide Y (NPY)-expressing interneurons, displaying morphological properties similar to those of neurogliaform cells and Vasoactive Intestinal Peptide (VIP)-expressing bipolar/bitufted interneurons. The aim of the present study was to determine the role of these neuronal populations in the control of vascular tone by monitoring blood vessels diameter changes, using infrared videomicroscopy in mouse neocortical slices. Bath applications of 1-(3-Chlorophenyl)biguanide hydrochloride (mCPBG), a 5-HT₃R agonist, induced both constrictions (30%) and dilations (70%) of penetrating arterioles within supragranular layers. All vasoconstrictions were abolished in the presence of the NPY receptor antagonist (BIBP 3226), suggesting that they were elicited by NPY release. Vasodilations persisted in the presence of the VIP receptor antagonist VPAC1 (PG-97-269), whereas they were blocked in the presence of the neuronal Nitric Oxide (NO) Synthase (nNOS) inhibitor, L-NNA. Altogether, these results strongly suggest that activation of neocortical 5-HT_3A-expressing interneurons by serotoninergic input could induces NO mediated vasodilatations and NPY mediated vasoconstrictions.

Neuropeptide delivery to synapses by long-range vesicle circulation and sporadic capture

Neurotransmission requires anterograde axonal transport of dense core vesicles (DCVs) containing neuropeptides and active zone components from the soma to nerve terminals. However, it is puzzling how one-way traffic could uniformly supply sequential release sites called en passant boutons. Here, Drosophila neuropeptide-containing DCVs are tracked in vivo for minutes with a new method called simultaneous photobleaching and imaging (SPAIM). Surprisingly, anterograde DCVs typically bypass proximal boutons to accumulate initially in the most distal bouton. Then, excess distal DCVs undergo dynactin-dependent retrograde transport back through proximal boutons into the axon. Just before re-entering the soma, DCVs again reverse for another round of anterograde axonal transport. While circulating over long distances, both anterograde and retrograde DCVs are captured sporadically in en passant boutons. Therefore, vesicle circulation, which includes long-range retrograde transport and inefficient bidirectional capture, overcomes the limitations of one-way anterograde transport to uniformly supply release sites with DCVs.

More than two decades of research on insect neuropeptide GPCRs: an overview

This review focuses on the state of the art on neuropeptide receptors in insects. Most of these receptors are G protein-coupled receptors (GPCRs) and are involved in the regulation of virtually all physiological processes during an insect's life. More than 20 years ago a milestone in invertebrate endocrinology was achieved with the characterization of the first insect neuropeptide receptor, i.e., the Drosophila tachykinin-like receptor. However, it took until the release of the Drosophila genome in 2000 that research on neuropeptide receptors boosted. In the last decade a plethora of genomic information of other insect species also became available, leading to a better insight in the functions and evolution of the neuropeptide signaling systems and their intracellular pathways. It became clear that some of these systems are conserved among all insect species, indicating that they fulfill crucial roles in their physiological processes. Meanwhile, other signaling systems seem to be lost in several insect orders or species, suggesting that their actions were superfluous in those insects, or that other neuropeptides have taken over their functions. It is striking that the deorphanization of neuropeptide GPCRs gets much attention, but the subsequent unraveling of the intracellular pathways they elicit, or their physiological functions are often hardly examined. Especially in insects besides Drosophila this information is scarce if not absent. And although great progress made in characterizing neuropeptide signaling systems, even in Drosophila several predicted neuropeptide receptors remain orphan, awaiting for their endogenous ligand to be determined. The present review gives a précis of the insect neuropeptide receptor research of the last two decades. But it has to be emphasized that the work done so far is only the tip of the iceberg and our comprehensive understanding of these important signaling systems will still increase substantially in the coming years.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Regulation of insulin-producing cells in the adult Drosophila brain via the tachykinin peptide receptor DTKR

Drosophila insulin-like peptides (DILPs) play important hormonal roles in the regulation of metabolic carbohydrates and lipids, but also in reproduction, growth, stress resistance and aging. In spite of intense studies of insulin signaling in Drosophilag the regulation of DILP production and release in adult fruit flies is poorly understood. Here we investigated the role of Drosophila tachykinin-related peptides (DTKs) and their receptors, DTKR and NKD, in the regulation of brain insulin-producing cells (IPCs) and aspects of DILP signaling. First, we show DTK-immunoreactive axon terminations close to the presumed dendrites of the IPCs, and DTKR immunolabeling in these cells. Second, we utilized targeted RNA interference to knock down expression of the DTK receptor, DTKR, in IPCs and monitored the effects on Dilp transcript levels in the brains of fed and starved flies. Dilp2 and Dilp3, but not Dilp5, transcripts were significantly affected by DTKR knockdown in IPCs, both in fed and starved flies. Both Dilp2 and Dilp3 transcripts increased in fed flies with DTKR diminished in IPCs whereas at starvation the Dilp3 transcript plummeted and Dilp2 increased. We also measured trehalose and lipid levels as well as survival in transgene flies at starvation. Knockdown of DTKR in IPCs leads to increased lifespan and a faster decrease of trehalose at starvation but has no significant effect on lipid levels. Finally, we targeted the IPCs with RNAi or ectopic expression of the other DTK receptor, NKD, but found no effect on survival at starvation. Our results suggest that DTK signaling, via DTKR, regulates the brain IPCs.

Immunoneutralization of melanin-concentrating hormone (MCH) in the dorsal raphe nucleus: effects on sleep and wakefulness

Hypothalamic neurons that utilize melanin-concentrating hormone (MCH) as a neuromodulator exert a positive control over energy homeostasis, inducing feeding and decreasing metabolism. Recent studies have shown also that this system plays a role in the generation and/or maintenance of sleep. MCHergic neurons project to the serotonergic dorsal raphe nucleus (DR), a neuroanatomical structure involved in several functions during wakefulness (W), and in the regulation of rapid-eye movements (REM) sleep. Recently, we determined the effect of MCH microinjected into the DR on sleep variables in the rat. MCH produced a marked increment of REM sleep, whereas slow wave sleep (SWS) showed only a moderate increase. In the present study, we analyze the effect of immunoneutralization of MCH in the DR on sleep and W in the rat. Compared to the control solution, microinjections of anti-MCH antibodies (1/100 solution in 0.2 μl) induced a significant increase in REM sleep latency (31.2±7.1 vs. 84.2±24.8 min, p<0.05) and a decrease of REM sleep time (37.8±5.4 vs. 17.8±2.9 min, p<0.05) that was related to the reduction in the number of REM sleep episodes. In addition, there was an increase of total W time (49.8±4.6 vs. 72.0±5.7 min, p<0.01). Light sleep and SWS remained unchanged. The intra-DR administration of a more diluted solution of anti-MCH antibodies (1/500) or rabbit pre-immune serum did not modify neither W nor REM sleep variables. Our findings strongly suggest that MCH released in the DR facilitates the occurrence of REM sleep.

VIP, CRF, and PACAP act at distinct receptors to elicit different cAMP/PKA dynamics in the neocortex

The functional significance of diverse neuropeptide coexpression and convergence onto common second messenger pathways remains unclear. To address this question, we characterized responses to corticotropin-releasing factor (CRF), pituitary adenylate cyclase-activating peptide (PACAP), and vasoactive intestinal peptide (VIP) in rat neocortical slices using optical recordings of cyclic adenosine monophosphate (cAMP) and protein kinase A (PKA) sensors, patch-clamp, and single-cell reverse transcription-polymerase chain reaction. Responses of pyramidal neurons to the 3 neuropeptides markedly differed in time-course and amplitude. Effects of these neuropeptides on the PKA-sensitive slow afterhyperpolarization current were consistent with those observed with cAMP/PKA sensors. CRF-1 receptors, primarily expressed in pyramidal cells, reportedly mediate the neocortical effects of CRF. PACAP and VIP activated distinct PAC1 and VPAC1 receptors, respectively. Indeed, a selective VPAC1 antagonist prevented VIP responses but had a minor effect on PACAP responses, which were mimicked by a specific PAC1 agonist. While PAC1 and VPAC1 were coexpressed in pyramidal cells, PAC1 expression was also frequently detected in interneurons, suggesting that PACAP has widespread effects on the neuronal network. Our results suggest that VIP and CRF, originating from interneurons, and PACAP, expressed mainly by pyramidal cells, finely tune the excitability and gene expression in the neocortical network via distinct cAMP/PKA-mediated effects.

Coevolution of neuropeptidergic signaling systems: from worm to man

Despite the general knowledge and repeated predictions of peptide G protein-coupled receptors following the elucidation of the Caenorhabditis elegans genome in 1998, only a few have been deorphanized so far. This was attributed to the apparent lack of coevolution between (neuro)peptides and their cognate receptors. To resolve this issue, we have used an in silico genomic data mining tool to identify the real putative peptide GPCRs in the C. elegans genome and then made a well-considered selection of orphan peptide GPCRs. To maximize our chances of a successful deorphanization, we adopted a combined reverse pharmacology approach. At this moment, we have successfully uncovered four C. elegans neuropeptide signaling systems that support the theory of receptor-ligand coevolution. All four systems are extremely well conserved within nematodes and show a high degree of similarity with their vertebrate and arthropod counterparts. Our data indicate that these four neuropeptide signaling systems have been well conserved during the course of evolution and that they were already well established prior to the divergence of protostomes and deuterostomes.

Depolarizing stimuli cause persistent and selective loss of orexin in rat hypothalamic slice culture

A hypothalamic neuropeptide orexin (hypocretin) is a critical regulator of physiological processes including sleep/wakefulness and feeding. Using organotypic slice culture of rat hypothalamus, we found that exposure to elevated extracellular concentration of K(+) (+10-30 mM) for 24-72h led to a substantial decrease in the number of neurons immunoreactive for orexin and a co-existing neuropeptide dynorphin-A. In contrast, the same treatment affected neither the number of melanin-concentrating hormone-immunoreactive neurons nor the number of total neurons. A substantial decrease of orexin-immunoreactive neurons was also induced by 72h treatment with 1-10 microM veratridine, a Na(+) channel activator. The effect of elevated K(+) was only partially reversible, and that of veratridine was virtually irreversible, although the decrease in orexin immunoreactivity was not associated with signs of cell damage assessed by propidium iodide uptake and Hoechst 33342 nuclear staining. In addition, the level of preproorexin mRNA did not decrease during treatment with elevated K(+) or veratridine. After treatment with elevated K(+) and veratridine, c-Fos immunoreactivity appeared in orexin-immunoreactive neurons but not in melanin-concentrating hormone-immunoreactive neurons, suggesting selective excitation of orexin neurons. However, the amount of orexin released extracellularly was paradoxically decreased by treatment with elevated K(+) and veratridine. Overall, these characteristics of orexin neurons may be taken into consideration to understand the behaviors of these neurons under physiological and pathophysiological conditions.

Peptidergic modulation of serotonin and nerve elicited responses of the salivary duct muscle in the snail, Helix pomatia

In the present study, the ability of a range of endogenous neuropeptides to modulate neuromuscular transmission was examined in the salivary duct neuromuscular preparation of the terrestrial snail, Helix pomatia. Immunohistochemical and physiological techniques were used to localize the neuropeptides (GSPYFVamide, CARP, FMRFamide and APGWamide) and to investigate whether contractions elicited by the stimulation of the salivary nerve or by exogenously applied 5-HT are subject to peptidergic modulation. All of the neuropeptides studied decreased the tonus by a direct action on the muscle fibers in a concentration dependent manner in a range of 10(-9) to 10(-6)M. Neuropeptides distinctly affected the 5-HT evoked contraction or relaxation and GSPYFVa and APGWa decreased also the amplitude of contractions elicited by the stimulation of the salivary nerve. All four neuropeptides facilitated the relaxation phase providing further evidence for the postsynaptic action of neuropeptides. Low Ca(2+)/high Mg(2+) saline abolished the nerve-elicited contractions, however the denervated muscle retained the ability to contract due to the mobilization of the Ca(2+) from intracellular stores. It was concluded, that peptides belonging to different peptide families exerted their effects through pre- and postsynaptic mechanisms. The modulatory effect of neuropeptides can be assigned to the partial co-localization of 5-HT and neuropeptides in the nerves innervating muscles of the salivary duct, as it was demonstrated by double-labeling immunohistochemistry. A double origin of the 5-HTergic innervation was demonstrated, including efferents originating from both the cerebral and visceral ganglia.

SDF-1alpha/CXCL12 enhances GABA and glutamate synaptic activity at serotonin neurons in the rat dorsal raphe nucleus

The serotonin (5-hydroxytryptamine; 5-HT) system has a well-characterized role in depression. Recent reports describe comorbidities of mood-immune disorders, suggesting an immunological component may contribute to the pathogenesis of depression as well. Chemokines, immune proteins which mediate leukocyte trafficking, and their receptors are widely distributed in the brain, mediate neuronal patterning, and modulate various neuropathologies. The purpose of this study was to investigate the neuroanatomical relationship and functional impact of the chemokine stromal cell-derived factor-1alpha/CXCL12 and its receptor, CXCR4, on the serotonin dorsal raphe nucleus (DRN) system in the rat using anatomical and electrophysiological techniques. Immunohistochemical analysis indicates that over 70% of 5-HT neurons colocalize with CXCL12 and CXCR4. At a subcellular level, CXCL12 localizes throughout the cytoplasm whereas CXCR4 concentrates to the outer membrane and processes of 5-HT neurons. CXCL12 and CXCR4 also colocalize on individual DRN cells. Furthermore, electrophysiological studies demonstrate CXCL12 depolarization of 5-HT neurons indirectly via glutamate synaptic inputs. CXCL12 also enhances the frequency of spontaneous inhibitory and excitatory postsynaptic currents (sIPSC and sEPSC). CXCL12 concentration-dependently increases evoked IPSC amplitude and decreases evoked IPSC paired-pulse ratio selectively in 5-HT neurons, effects blocked by the CXCR4 antagonist AMD3100. These data indicate presynaptic enhancement of GABA and glutamate release at 5-HT DRN neurons by CXCL12. Immunohistochemical analysis further shows CXCR4 localization to DRN GABA neurons, providing an anatomical basis for CXCL12 effects on GABA release. Thus, CXCL12 indirectly modulates 5-HT neurotransmission via GABA and glutamate synaptic afferents. Future therapies targeting CXCL12 and other chemokines may treat serotonin related mood disorders, particularly depression experienced by immune-compromised individuals.

Neuropeptide biology in Drosophila

Drosophila melanogaster is since decades the most important invertebrate model. With the publishing of the genome sequence, Drosophila also became a pioneer in (neuro)peptide research. Neuropeptides represent a major group of signaling molecules that outnumber all other types of neurotransmitters/modulators and hormones. By means of bioinformatics 119 (neuro)peptide precursor genes have been predicted from the Drosophila genome. Using the neuropeptidomics technology 46 neuropeptides derived from 19 of these precursors could be biochemically characterized. At the cellular level, neuropeptides usually exert their action by binding to membrane receptors, many of which belong to the family of G-protein coupled receptors or GPCRs. Such receptors are the major target for many contemporary drugs. In this chapter, we will describe the identification, localization and functional characterization of neuropeptide-receptor pairs in Drosophila melanogaster.

Effect of selective sensory denervation on fracture-healing: an experimental study of rats

BACKGROUND

Interactions between the peripheral nervous system and the healing skeleton are poorly understood. Various clinical observations suggest that the nervous system interacts with and promotes fracture-healing. The purpose of this study was to examine the effect of selective sensory denervation on fracture-healing.

METHODS

Fifty-one Sprague-Dawley rats underwent unilateral placement of an intramedullary rod followed by creation of a standardized femoral fracture. One group of these rats underwent sensory denervation by means of a localized capsaicin injection, and the other did not. Subgroups were allocated for analysis of mRNA expression of collagen I and II and osteocalcin at three, seven, and fourteen days after the fracture. Additionally, histological examination was performed at four weeks; micro-computed tomography, at five weeks; and biomechanical testing, at six weeks.

RESULTS

The sensory-denervated group had significantly less collagen-I upregulation than the sensory-intact group at three days after the fracture (difference in means, forty-four-fold [95% confidence interval = 22.7 to 65.5-fold]; p < 0.001) and significantly less collagen-II upregulation at seven days after the fracture (difference in means, ninefold [95% confidence interval = 4.3 to 13.8-fold]; p < 0.001). In the sensory-denervated group, the fracture callus had a significantly larger cross-sectional area (difference in means, 15.6 mm(2) [95% confidence interval = 0.78 to 30.5 mm(2)]; p = 0.043) and was less dense. Biomechanical testing revealed that sensory denervation significantly decreased the load to failure (difference in means, 28.7 N [95% confidence interval = 1.2 to 56.2 N]; p = 0.022).

CONCLUSIONS

Sensory denervation negatively affects fracture-healing. These results offer insight into the nerve-bone interaction following injury.