Neuromodulatory complement of the pericardial organs in the embryonic lobster, Homarus americanus

The dynamic range of voltage-dependent gap junction signaling is maintained by Ih-induced membrane potential depolarization

Like their chemical counterparts, electrical synapses show complex dynamics such as rectification and voltage dependence that interact with other electrical processes in neurons. The consequences arising from these interactions for the electrical behavior of the synapse, and the dynamics they create, remain largely unexplored. Using a voltage-dependent electrical synapse between a descending modulatory projection neuron (MCN1) and a motor neuron (LG) in the crustacean stomatogastric ganglion, we find that the influence of the hyperpolarization-activated inward current (I_h) is critical to the function of the electrical synapse. When we blocked I_h with CsCl, the apparent voltage dependence of the electrical synapse shifted by 18.7 mV to more hyperpolarized voltages, placing the dynamic range of the electrical synapse outside of the range of voltages used by the LG motor neuron (-60.2 mV to -44.9 mV). With dual electrode current- and voltage-clamp recordings, we demonstrate that this voltage shift is not due to a change in the properties of the gap junction itself, but is a result of a sustained effect of I_h on the presynaptic MCN1 axon terminal membrane potential. I_h-induced depolarization of the axon terminal membrane potential increased the electrical postsynaptic potentials and currents. With I_h present, the axon terminal resting membrane potential is depolarized, shifting the dynamic range of the electrical synapse toward the functional range of the motor neuron. We thus demonstrate that the function of an electrical synapse is critically influenced by a voltage-dependent ionic current (I_h).NEW & NOTEWORTHY Electrical synapses and voltage-gated ionic currents are often studied independently from one another, despite mounting evidence that their interactions can alter synaptic behavior. We show that the hyperpolarization-activated inward ionic current shifts the voltage dependence of electrical synaptic transmission through its depolarizing effect on the membrane potential, enabling it to lie within the functional membrane potential range of a motor neuron. Thus, the electrical synapse's function critically depends on the voltage-gated ionic current.

Immunolocalization of Neurotransmitters and Neuromodulators in the Developing Crayfish Brain

In the field of neurosciences, the crayfish nervous system is an important model for understanding how arthropods process sensory stimuli and generate specific behaviors. Furthermore, crayfish embryos have been important study objects for well over 200 years. Immunohistochemistry against neurotransmitters, neuromodulators, and neurohormones is widely used to analyze the ontogeny of neurons in the emerging brain of several crustacean species and to date represents one of the most powerful approaches to analyze aspects of brain development in this group of organisms. In recent years, the analysis of brain development in crustaceans has gained new momentum by the establishment of the Marmorkrebs Procambarus virginalis (Marbled Crayfish), a parthenogenetic crayfish, as new model system. The embryonic development of marbled crayfish is well characterized and these animals can be easily cultivated in the lab. This chapter describes protocols for immunolocalization of neuroactive substances in the developing crayfish brain.

Characterization of G-protein coupled receptors from the blackback land crab Gecarcinus lateralis Y organ transcriptome over the molt cycle

BACKGROUND

G-protein coupled receptors (GPCRs) are ancient, ubiquitous, constitute the largest family of transducing cell surface proteins, and are integral to cell communication via an array of ligands/neuropeptides. Molt inhibiting hormone (MIH) is a key neuropeptide that controls growth and reproduction in crustaceans by regulating the molt cycle. It inhibits ecdysone biosynthesis by a pair of endocrine glands (Y-organs; YOs) through binding a yet uncharacterized GPCR, which triggers a signalling cascade, leading to inhibition of the ecdysis sequence. When MIH release stops, ecdysone is synthesized and released to the hemolymph. A peak in ecdysone titer is followed by a molting event. A transcriptome of the blackback land crab Gecarcinus lateralis YOs across molt was utilized in this study to curate the list of GPCRs and their expression in order to better assess which GPCRs are involved in the molt process.

RESULTS

Ninety-nine G. lateralis putative GPCRs were obtained by screening the YO transcriptome against the Pfam database. Phylogenetic analysis classified 49 as class A (Rhodopsin-like receptor), 35 as class B (Secretin receptor), and 9 as class C (metabotropic glutamate). Further phylogenetic analysis of class A GPCRs identified neuropeptide GPCRs, including those for Allatostatin A, Allatostatin B, Bursicon, CCHamide, FMRFamide, Proctolin, Corazonin, Relaxin, and the biogenic amine Serotonin. Three GPCRs clustered with recently identified putative CHH receptors (CHHRs), and differential expression over the molt cycle suggests that they are associated with ecdysteroidogenesis regulation. Two putative Corazonin receptors showed much higher expression in the YOs compared with all other GPCRs, suggesting an important role in molt regulation.

CONCLUSIONS

Molting requires an orchestrated regulation of YO ecdysteroid synthesis by multiple neuropeptides. In this study, we curated a comprehensive list of GPCRs expressed in the YO and followed their expression across the molt cycle. Three putative CHH receptors were identified and could include an MIH receptor whose activation negatively regulates molting. Orthologs of receptors that were found to be involved in molt regulation in insects were also identified, including LGR3 and Corazonin receptor, the latter of which was expressed at much higher level than all other receptors, suggesting a key role in YO regulation.

An atlas of larval organogenesis in the European shore crab Carcinus maenas L. (Decapoda, Brachyura, Portunidae)

BACKGROUND

The life history stages of brachyuran crustaceans include pelagic larvae of the Zoea type which grow by a series of moults from one instar to the next. Zoeae actively feed and possess a wide range of organ systems necessary for autonomously developing in the plankton. They also display a rich behavioural repertoire that allows for responses to variations in environmental key factors such as light, hydrostatic pressure, tidal currents, and temperature. Brachyuran larvae have served as distinguished models in the field of Ecological Developmental Biology fostering our understanding of diverse ecophysiological aspects such as phenotypic plasticity, carry-over effects on life-history traits, and adaptive mechanisms that enhance tolerance to fluctuations in environmental abiotic factors. In order to link such studies to the level of tissues and organs, this report analyses the internal anatomy of laboratory-reared larvae of the European shore crab Carcinus maenas. This species has a native distribution extending across most European waters and has attracted attention because it has invaded five temperate geographic regions outside of its native range and therefore can serve as a model to analyse thermal tolerance of species affected by rising sea temperatures as an effect of climate change.

RESULTS

Here, we used X-ray micro-computed tomography combined with 3D reconstruction to describe organogenesis in brachyuran larvae. We provide a detailed atlas of the larval internal organization to complement existing descriptions of its external morphology. In a multimethodological approach, we also used cuticular autofluorescence and classical histology to analyse the anatomy of selected organ systems.

CONCLUSIONS

Much of our fascination for the anatomy of brachyuran larvae stems from the opportunity to observe a complex organism on a single microscopic slide and the realization that the entire decapod crustacean bauplan unfolds from organ anlagen compressed into a miniature organism in the sub-millimetre range. The combination of imaging techniques used in the present study provides novel insights into the bewildering diversity of organ systems that brachyuran larvae possess. Our analysis may serve as a basis for future studies bridging the fields of evolutionary developmental biology and ecological developmental biology.

Complicating connectomes: Electrical coupling creates parallel pathways and degenerate circuit mechanisms

Electrical coupling in circuits can produce non‐intuitive circuit dynamics, as seen in both experimental work from the crustacean stomatogastric ganglion and in computational models inspired by the connectivity in this preparation. Ambiguities in interpreting the results of electrophysiological recordings can arise if sets of pre‐ or postsynaptic neurons are electrically coupled, or if the electrical coupling exhibits some specificity (e.g. rectifying, or voltage‐dependent). Even in small circuits, electrical coupling can produce parallel pathways that can allow information to travel by monosynaptic and/or polysynaptic pathways. Consequently, similar changes in circuit dynamics can arise from entirely different underlying mechanisms. When neurons are coupled both chemically and electrically, modifying the relative strengths of the two interactions provides a mechanism for flexibility in circuit outputs. This, together with neuromodulation of gap junctions and coupled neurons is important both in developing and adult circuits. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 597–609, 2017

Sources and range of long-term variability of rhythmic motor patterns in vivo

The mechanisms of rhythmic motor pattern generation have been studied in detail in vitro, but the long-term stability and sources of variability in vivo are often not well described. The crab stomatogastric ganglion contains the well-characterized gastric mill (chewing) and pyloric (filtering of food) central pattern generators. In vitro, the pyloric rhythm is stereotyped with little variation, but inter-circuit interactions and neuromodulation can alter both rhythm cycle frequency and structure. The range of variation of activity in vivo is, with few exceptions, unknown. Curiously, although the pattern-generating circuits in vivo are constantly exposed to hormonal and neural modulation, the majority of published data show only the unperturbed canonical motor patterns typically observed in vitro. Using long-term extracellular recordings (N=27 animals), we identified the range and sources of variability of the pyloric and gastric mill rhythms recorded continuously over 4 days in freely behaving Jonah crabs (Cancer borealis). Although there was no evidence of innate daily rhythmicity, a 12 h light-driven cycle did manifest. The frequency of both rhythms increased modestly, albeit consistently, during the 3 h before and 3 h after the lights changed. This cycle was occluded by sensory stimulation (feeding), which significantly influenced both pyloric cycle frequency and structure. This was the only instance where the structure of the rhythm changed. In unfed animals the structure remained stable, even when the frequency varied substantially. So, although central pattern generating circuits are capable of producing many patterns, in vivo outputs typically remain stable in the absence of sensory stimulation.

Dopaminergic tone regulates transient potassium current maximal conductance through a translational mechanism requiring D1Rs, cAMP/PKA, Erk and mTOR

Background

Dopamine (DA) can produce divergent effects at different time scales. DA has opposing immediate and long-term effects on the transient potassium current (I_A) within neurons of the pyloric network, in the Panulirus interruptus stomatogastric ganglion. The lateral pyloric neuron (LP) expresses type 1 DA receptors (D1Rs). A 10 min application of 5-100 μM DA decreases LP I_A by producing a decrease in I_A maximal conductance (G_max) and a depolarizing shift in I_A voltage dependence through a cAMP-Protein kinase A (PKA) dependent mechanism. Alternatively, a 1 hr application of DA (≥5 nM) generates a persistent (measured 4 hr after DA washout) increase in I_A G_max in the same neuron, through a mechanistic target of rapamycin (mTOR) dependent translational mechanism. We examined the dose, time and protein dependencies of the persistent DA effect.

Results

We found that disrupting normal modulatory tone decreased LP I_A. Addition of 500 pM-5 nM DA to the saline for 1 hr prevented this decrease, and in the case of a 5 nM DA application, the effect was sustained for >4 hrs after DA removal. To determine if increased cAMP mediated the persistent effect of 5nM DA, we applied the cAMP analog, 8-bromo-cAMP alone or with rapamycin for 1 hr, followed by wash and TEVC. 8-bromo-cAMP induced an increase in I_A G_max, which was blocked by rapamycin. Next we tested the roles of PKA and guanine exchange factor protein activated by cAMP (ePACs) in the DA-induced persistent change in I_A using the PKA specific antagonist Rp-cAMP and the ePAC specific agonist 8-pCPT-2′-O-Me-cAMP. The PKA antagonist blocked the DA induced increases in LP I_A G_max, whereas the ePAC agonist did not induce an increase in LP I_A G_max. Finally we tested whether extracellular signal regulated kinase (Erk) activity was necessary for the persistent effect by co-application of Erk antagonists PD98059 or U0126 with DA. Erk antagonism blocked the DA induced persistent increase in LP I_A.

Conclusions

These data suggest that dopaminergic tone regulates ion channel density in a concentration and time dependent manner. The D1R- PKA axis, along with Erk and mTOR are necessary for the persistent increase in LP I_A induced by high affinity D1Rs.

Neural control and adaptive neural forward models for insect-like, energy-efficient, and adaptable locomotion of walking machines

Living creatures, like walking animals, have found fascinating solutions for the problem of locomotion control. Their movements show the impression of elegance including versatile, energy-efficient, and adaptable locomotion. During the last few decades, roboticists have tried to imitate such natural properties with artificial legged locomotion systems by using different approaches including machine learning algorithms, classical engineering control techniques, and biologically-inspired control mechanisms. However, their levels of performance are still far from the natural ones. By contrast, animal locomotion mechanisms seem to largely depend not only on central mechanisms (central pattern generators, CPGs) and sensory feedback (afferent-based control) but also on internal forward models (efference copies). They are used to a different degree in different animals. Generally, CPGs organize basic rhythmic motions which are shaped by sensory feedback while internal models are used for sensory prediction and state estimations. According to this concept, we present here adaptive neural locomotion control consisting of a CPG mechanism with neuromodulation and local leg control mechanisms based on sensory feedback and adaptive neural forward models with efference copies. This neural closed-loop controller enables a walking machine to perform a multitude of different walking patterns including insect-like leg movements and gaits as well as energy-efficient locomotion. In addition, the forward models allow the machine to autonomously adapt its locomotion to deal with a change of terrain, losing of ground contact during stance phase, stepping on or hitting an obstacle during swing phase, leg damage, and even to promote cockroach-like climbing behavior. Thus, the results presented here show that the employed embodied neural closed-loop system can be a powerful way for developing robust and adaptable machines.

Mass spectrometric evaluation of neuropeptidomic profiles upon heat stabilization treatment of neuroendocrine tissues in crustaceans

Tissue heat stabilization is a vital component in successful mammalian neuropeptidomic studies. Heat stabilization using focused microwave irradiation, conventional microwave irradiation, boiling, and treatment with the Denator Stabilizor T1 have all proven effective in arresting post-mortem protein degradation. Although research has reported the presence of protein fragments in crustacean hemolymph when protease inhibitors were not added to the sample, the degree to which post-mortem protease activity affects neuropeptidomic tissue studies in crustacean species has not been investigated in depth. This work examines the need for Stabilizor T1 or boiling tissue stabilization methods for neuropeptide studies of Callinectes sapidus (blue crab) pericardial organ tissue. Neuropeptides in stabilized and nonstabilized tissue were extracted using acidified methanol or N,N-dimethylformamide (DMF) and analyzed by MALDI-TOF and nanoLC-ESI-MS/MS platforms. Post-mortem fragments did not dramatically affect MALDI analysis in the range m/z 650-1600, but observations in ESI MS/MS experiments suggest that putative post-mortem fragments can mask neuropeptide signal and add spectral complexity to crustacean neuropeptidomic studies. The impact of the added spectral complexity did not dramatically affect the number of detected neuropeptides between stabilized and nonstabilized tissues. However, it is prudent that neuropeptidomic studies of crustacean species include a preliminary experiment using the heat stabilization method to assess the extent of neuropeptide masking by larger, highly charged molecular species.

Crustacean neuroendocrine systems and their signaling agents

Decapod crustaceans have long served as important models for the study of neuroendocrine signaling. For example, the process of neurosecretion was first formally demonstrated by using a member of this order. In this review, the major decapod neuroendocrine organs are described, as are their phylogenetic conservation and neurochemistry. In addition, recent advances in crustacean neurohormone discovery and tissue mapping are discussed, as are several recent advances in our understanding of hormonal control in this group of animals.

Crustacean neuropeptides

Crustaceans have long been used for peptide research. For example, the process of neurosecretion was first formally demonstrated in the crustacean X-organ-sinus gland system, and the first fully characterized invertebrate neuropeptide was from a shrimp. Moreover, the crustacean stomatogastric and cardiac nervous systems have long served as models for understanding the general principles governing neural circuit functioning, including modulation by peptides. Here, we review the basic biology of crustacean neuropeptides, discuss methodologies currently driving their discovery, provide an overview of the known families, and summarize recent data on their control of physiology and behavior.

Identification and expression of mRNAs encoding bursicon in the plesiomorphic central nervous system of Homarus gammarus

Ecdysis in arthropods is a complex process, regulated by many neurohormones, which must be released in a precisely coordinated manner. In insects, the ultimate hormone involved in this process is the cuticle tanning hormone, bursicon. Recently, this hormone has been identified in crustaceans. To further define the distribution of bursicon in crustacean nervous systems, and to compare hormone structures within the sub-phylum, cDNAs encoding both bursicon subunits were cloned and sequenced from the nervous system of the European lobster, Homarus gammarus, and expression patterns including those for CCAP determined using in-situ hybridisation, quantitative RT-PCR and immunohistochemistry. Full-length cDNAs encoded bursicon subunits of 121 amino acids (Average M(r): 13365.48) for Burs α, 115 amino acids (Average M(r): 12928.54) for Burs β. Amino acid sequences were most closely related to those of crabs, and for Burs β the sequence was identical to that of the American lobster, Homarus americanus. Complete co-localisation with CCAP in the VNC was seen. Copy numbers burs α, burs β and CCAP mRNAs were between 0.5 and 1.5 × 10(5) for both bursicon subunits, 0.5-6 × 10(5) per cdn neurone for CCAP. The terminal abdominal ganglia (AG 6-8) contained about 52 cdn-type neurons, making it the largest bursicon producing region in the CNS. Double labelling IHC using recombinant Carcinus Burs α and CCAP antisera demonstrated complete co-localisation in the VNC. On the basis of the results obtained, it is proposed that CCAP and bursicon release occur simultaneously during ecdysis in crustaceans.

Immunohistochemical mapping of histamine, dopamine, and serotonin in the central nervous system of the copepod Calanus finmarchicus (Crustacea; Maxillopoda; Copepoda)

Calanoid copepods constitute an important group of marine planktonic crustaceans that often dominate the metazoan biomass of the world's oceans. In proportion to their ecological importance, little is known about their nervous systems. We have used immunohistochemical techniques in a common North Atlantic calanoid to localize re-identifiable neurons that putatively contain the biogenic amines histamine, dopamine, and serotonin. We have found low numbers of such cells and cell groups (approximately 37 histamine pairs, 22 dopamine pairs, and 12 serotonin pairs) compared with those in previously described crustaceans. These cells are concentrated in the anterior part of the central nervous system, the majority for each amine being located in the three neuromeres that constitute the brain (protocerebrum, deutocerebrum, and tritocerebrum). Extensive histamine labeling occurs in several small compact protocerebral neuropils, three pairs of larger, more posterior, paired, dense neuropils, and one paired diffuse tritocerebral neuropil. The most concentrated neuropil showing dopamine labeling lies in the putative deutocerebrum, associated with heavily labeled commissural connections between the two sides of the brain. The most prominent serotonin neuropil is present in the anterior medial part of the brain. Tracts of immunoreactive fibers of all three amines are prominent in the cephalic region of the nervous system, but some projections into the most posterior thoracic regions have also been noted.

Cell specific dopamine modulation of the transient potassium current in the pyloric network by the canonical D1 receptor signal transduction cascade

Dopamine (DA) modifies the motor pattern generated by the pyloric network in the stomatogastric ganglion (STG) of the spiny lobster, Panulirus interruptus, by directly acting on each of the circuit neurons. The 14 pyloric neurons fall into six cell types, and DA actions are cell type specific. The transient potassium current mediated by shal channels (I(A)) is a common target of DA modulation in most cell types. DA shifts the voltage dependence of I(A) in opposing directions in pyloric dilator (PD) versus lateral pyloric (LP) neurons. The mechanism(s) underpinning cell-type specific DA modulation of I(A) is unknown. DA receptors (DARs) can be classified as type 1 (D1R) or type 2 (D2R). D1Rs and D2Rs are known to increase and decrease intracellular cAMP concentrations, respectively. We hypothesized that the opposing DA effects on PD and LP I(A) were due to differences in DAR expression patterns. In the present study, we found that LP expressed somatodendritic D1Rs that were concentrated near synapses but did not express D2Rs. Consistently, DA modulation of LP I(A) was mediated by a Gs-adenylyl cyclase-cAMP-protein kinase A pathway. Additionally, we defined antagonists for lobster D1Rs (flupenthixol) and D2Rs (metoclopramide) in a heterologous expression system and showed that DA modulation of LP I(A) was blocked by flupenthixol but not by metoclopramide. We previously showed that PD neurons express D2Rs, but not D1Rs, thus supporting the idea that cell specific effects of DA on I(A) are due to differences in receptor expression.

Mass spectral analysis of neuropeptide expression and distribution in the nervous system of the lobster Homarus americanus

The lobster Homarus americanus has long served as an important animal model for electrophysiological and behavioral studies. Using this model, we performed a comprehensive investigation of the neuropeptide expression and their localization in the nervous system, which provides useful insights for further understanding of their biological functions. Using nanoLC ESI Q-TOF MS/MS and three types of MALDI instruments, we analyzed the neuropeptide complements in a major neuroendocrine structure, pericardial organ. A total of 57 putative neuropeptides were identified and 18 of them were de novo sequenced. Using direct tissue/extract analysis and bioinformatics software SpecPlot, we charted the global distribution of neuropeptides throughout the nervous system in H. americanus. Furthermore, we also mapped the localization of several neuropeptide families in the brain by high mass resolution and high mass accuracy mass spectrometric imaging (MSI) using a MALDI LTQ Orbitrap mass spectrometer. We have also compared the utility and instrument performance of multiple mass spectrometers for neuropeptide analysis in terms of peptidome coverage, sensitivity, mass spectral resolution and capability for de novo sequencing.

D(2) receptors receive paracrine neurotransmission and are consistently targeted to a subset of synaptic structures in an identified neuron of the crustacean stomatogastric nervous system

Dopamine (DA) modulates motor systems in phyla as diverse as nematodes and arthropods up through chordates. A comparison of dopaminergic systems across a broad phylogenetic range should reveal shared organizing principles. The pyloric network, located in the stomatogastric ganglion (STG), is an important model for neuromodulation of motor networks. The effects of DA on this network have been well characterized at the circuit and cellular levels in the spiny lobster, Panulirus interruptus. Here we provide the first data about the physical organization of the DA signaling system in the STG and the function of D(2) receptors in pyloric neurons. Previous studies showed that DA altered intrinsic firing properties and synaptic output in the pyloric dilator (PD) neuron, in part by reducing calcium currents and increasing outward potassium currents. We performed single cell reverse transcriptase-polymerase chain reaction (RT-PCR) experiments to show that PD neurons exclusively expressed a type 2 (D(2alphaPan)) DA receptor. This was confirmed by using confocal microscopy in conjunction with immunohistochemistry (IHC) on STG whole-mount preparations containing dye-filled PD neurons. Immunogold electron microscopy showed that surface receptors were concentrated in fine neurites/terminal swellings and vesicle-laden varicosities in the synaptic neuropil. Double-label IHC experiments with tyrosine hydroxylase antiserum suggested that the D(2alphaPan) receptors received volume neurotransmissions. Receptors were further mapped onto three-dimensional models of PD neurons built from Neurolucida tracings of confocal stacks from the IHC experiments. The data showed that D(2alphaPan) receptors were selectively targeted to approximately 40% of synaptic structures in any given PD neuron, and were nonuniformly distributed among neurites.

The peptide hormone pQDLDHVFLRFamide (crustacean myosuppressin) modulates the Homarus americanus cardiac neuromuscular system at multiple sites

pQDLDHVFLRFamide is a highly conserved crustacean neuropeptide with a structure that places it within the myosuppressin subfamily of the FMRFamide-like peptides. Despite its apparent ubiquitous conservation in decapod crustaceans, the paracrine and/or endocrine roles played by pQDLDHVFLRFamide remain largely unknown. We have examined the actions of this peptide on the cardiac neuromuscular system of the American lobster Homarus americanus using four preparations: the intact animal, the heart in vitro, the isolated cardiac ganglion (CG), and a stimulated heart muscle preparation. In the intact animal, injection of myosuppressin caused a decrease in heartbeat frequency. Perfusion of the in vitro heart with pQDLDHVFLRFamide elicited a decrease in the frequency and an increase in the amplitude of heart contractions. In the isolated CG, myosuppressin induced a hyperpolarization of the resting membrane potential of cardiac motor neurons and a decrease in the cycle frequency of their bursting. In the stimulated heart muscle preparation, pQDLDHVFLRFamide increased the amplitude of the induced contractions, suggesting that myosuppressin modulates not only the CG, but also peripheral sites. For at least the in vitro heart and the isolated CG, the effects of myosuppressin were dose-dependent (10(-9) to 10(-6) mol l(-1) tested), with threshold concentrations (10(-8)-10(-7) mol l(-1)) consistent with the peptide serving as a circulating hormone. Although cycle frequency, a parameter directly determined by the CG, consistently decreased when pQDLDHVFLRFamide was applied to all preparation types, the magnitudes of this decrease differed, suggesting the possibility that, because myosuppressin modulates the CG and the periphery, it also alters peripheral feedback to the CG.

Modulation of stomatogastric rhythms

Neuromodulation by peptides and amines is a primary source of plasticity in the nervous system as it adapts the animal to an ever-changing environment. The crustacean stomatogastric nervous system is one of the premier systems to study neuromodulation and its effects on motor pattern generation at the cellular level. It contains the extensively modulated central pattern generators that drive the gastric mill (chewing) and pyloric (food filtering) rhythms. Neuromodulators affect all stages of neuronal processing in this system, from membrane currents and synaptic transmission in network neurons to the properties of the effector muscles. The ease with which distinct neurons are identified and their activity is recorded in this system has provided considerable insight into the mechanisms by which neuromodulators affect their target cells and modulatory neuron function. Recent evidence suggests that neuromodulators are involved in homeostatic processes and that the modulatory system itself is under modulatory control, a fascinating topic whose surface has been barely scratched. Future challenges include exploring the behavioral conditions under which these systems are activated and how their effects are regulated.

Cloning and immunoreactivity of the 5-HT 1Mac and 5-HT 2Mac receptors in the central nervous system of the freshwater prawn Macrobrachium rosenbergii

Biogenic amines are implicated in several mental disorders, many of which involve social interactions. Simple model systems, such as crustaceans, are often more amenable than vertebrates for studying mechanisms underlying behaviors. Although various cellular responses of biogenic amines have been characterized in crustaceans, the mechanisms linking these molecules to behavior remain largely unknown. Observed effects of serotonin receptor agonists and antagonists in abdomen posture, escape responses, and fighting have led to the suggestion that biogenic amine receptors may play a role in modulating interactive behaviors. As a first step in understanding this potential role of such receptors, we have cloned and fully sequenced two serotonin receptors, 5-HT(1Mac) and 5-HT(2Mac), from the CNS of the freshwater prawn Macrobrachium rosenbergii and have mapped their CNS immunohistochemical distribution. 5-HT(1Mac) was found primarily on the membranes of subsets of cells in all CNS ganglia, in fibers that traverse all CNS regions, and in the cytoplasm of a small number of cells in the brain and circum- and subesophageal ganglia (SEG), most of which also appear to contain dopamine. The pattern of 5-HT(2Mac) immunoreactivity was found to differ significantly; it was found mostly in the central neuropil area of all ganglia, in glomeruli of the brain's olfactory lobes, and in the cytoplasm of a small number of neurons in the SEG, thoracic, and some abdominal ganglia. The observed differences in terms of localization, distribution within cells, and intensity of immunoreactive staining throughout the prawn's CNS suggest that these receptors are likely to play different roles.

Developmental regulation of neuromodulator function in the stomatogastric ganglion of the lobster, Homarus americanus

Neuromodulatory substances have profound effects on the two motor patterns generated by the adult crustacean stomatogastric ganglion (STG), the gastric mill rhythm and the pyloric rhythm. Developmentally regulated changes in the modulatory functions of neuromodulators could therefore play an important role in the maturation of the output from the developing STG. We compared the effects of neuromodulators on isolated embryonic and adult STG of the lobster, Homarus americanus. Bath application of Val(1)-SIFamide, a peptide whose expression is different in embryos and adults, activated different neuron classes in embryos and adults. Cancer borealis tachykinin-related peptide 1a, a peptide that does not appear in the terminals of modulatory neurons in the STG until after embryonic development, also produced different motor patterns in embryos and adults. In contrast, red pigment concentrating hormone, a peptide with a similar distribution in the STNS across development, produced similar motor patterns in embryonic and adult STG. Proctolin, serotonin, and allatostatin were also physiologically active on the isolated embryonic STG. Together, these results demonstrate that receptors to many neuromodulators are present and functional on STG neurons before the motor patterns of the stomatogastric nervous system are mature. Moreover, neuromodulator responses change during development, perhaps contributing to the maturation of the output from the stomatogastric nervous system.

Identification and cardiotropic actions of brain/gut-derived tachykinin-related peptides (TRPs) from the American lobster Homarus americanus

Two tachykinin-related peptides (TRPs) are known in decapods, APSGFLGMRamide and TPSGFLGMRamide. The former peptide appears to be ubiquitously conserved in members of this taxon, while the latter has been suggested to be a genus (Cancer)- or infraorder (Brachyura)-specific isoform. Here, we characterized a cDNA from the American lobster Homarus americanus (infraorder Astacidea) that encodes both TRPs: six copies of APSGFLGMRamide and one of TPSGFLGMRamide. Mass spectral analyses of the H. americanus supraoesophageal ganglion (brain) and commissural ganglia confirmed the presence of both peptides in these neural tissues; both isoforms were also detected in the midgut. Physiological experiments showed that both APSGFLGMRamide and TPSGFLGMRamide are cardioactive in H. americanus, eliciting identical increases in both heart contraction frequency and amplitude. Collectively, our data represent the first genetic confirmation of TRPs in H. americanus and of TPSGFLGMRamide in any species, demonstrate that TPSGFLGMRamide is not restricted to brachyurans, and show that both this peptide and APSGFLGMRamide are brain-gut isoforms, the first peptides thus far confirmed to possess this dual tissue distribution in H. americanus. Our data also suggest a possible role for TRPs in modulating the output of the lobster heart.

Distribution of pigment dispersing hormone- and tachykinin-related peptides in the central nervous system of the copepod crustacean Calanus finmarchicus

Peptides represent the largest class of signaling molecules used by nervous systems, functioning as locally-released paracrines and circulating hormones in both invertebrates and vertebrates. While many studies have focused on elucidating peptidergic systems in higher crustaceans, little is known about neuropeptides in the more primitive crustacean taxa. Here, we have begun an investigation of the peptides present in the central nervous system (CNS) of the copepod crustacean Calanus finmarchicus, presenting immunohistochemical data on the presence and distribution of pigment dispersing hormone (PDH) and tachykinin-related peptide (TRP). In this species, strong PDH-like immunoreactivity was restricted to one pair of somata in the protocerebrum (PC) and the axonal projections emanating from them. TRP-like immunopositive structures were present in the PC, deutocerebrum (DC), tritocerebrum (TC), and ventral nerve cord (VNC). In the PC, a single soma in the left hemisphere was labeled. This neuron appears to be the source of a centrally located, bilaterally symmetric plexus present within the PC. In the DC, two pairs of intensely immunopositive somata were labeled, each projecting axons toward the posterior and producing an extensive collection of putative release terminals that spans the DC, TC, and anterior portion of the VNC. Several other more weakly labeled somata were also present in the DC. Double-labeling studies indicated that no co-localization of PDH- and TRP-like peptides is present in the C. finmarchicus CNS. As preadsorption controls completely abolished each label, we feel these data represent accurate distributions of PDH- and TRP-like peptides within the C. finmarchicus CNS, thus providing a framework for future studies of the functional roles members of these peptide families play in this copepod species.

Mass spectral characterization of peptide transmitters/hormones in the nervous system and neuroendocrine organs of the American lobster Homarus americanus

The American lobster Homarus americanus is a decapod crustacean with both high economic and scientific importance. To facilitate physiological investigations of peptide transmitter/hormone function in this species, we have used matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI-FTMS), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and nanoscale liquid chromatography coupled to electrospray ionization quadrupole time-of-flight tandem mass spectrometry (nanoLC-ESI-Q-TOF MS/MS) to elucidate the peptidome present in its nervous system and neuroendocrine organs. In total, 84 peptides were identified, including 27 previously known H. americanus peptides (e.g., VYRKPPFNGSIFamide [Val(1)-SIFamide]), 23 peptides characterized previously from other decapods, but new to the American lobster (e.g., pQTFQYSRGWTNamide [Arg(7)-corazonin]), and 34 new peptides de novo sequenced/detected for the first time in this study. Of particular note are a novel B-type allatostatin (TNWNKFQGSWamide) and several novel FMRFamide-related peptides, including an unsulfated analog of sulfakinin (GGGEYDDYGHLRFamide), two myosuppressins (QDLDHVFLRFamide and pQDLDHVFLRFamide), and a collection of short neuropeptide F isoforms (e.g., DTSTPALRLRFamide and FEPSLRLRFamide). Our data also include the first detection of multiple tachykinin-related peptides in a non-brachyuran decapod, as well as the identification of potential individual-specific variants of orcokinin and orcomyotropin-related peptide. Taken collectively, our results not only expand greatly the number of known H. americanus neuropeptides, but also provide a framework for future studies on the physiological roles played by these molecules in this commercially and scientifically important species.

Identification of A-type allatostatins possessing -YXFGI/Vamide carboxy-termini from the nervous system of the copepod crustacean Calanus finmarchicus

The copepod crustacean Calanus finmarchicus plays a critical role in the ecology of the Gulf of Maine and other regions of the North Atlantic. To increase our understanding of the physiology of this species, a normalized, whole organism cDNA library was constructed, and expressed sequence tags (ESTs) of the clones were generated. Among these ESTs was one with homology to known cDNAs encoding prepro-A-type allatostatins (A-type ASTs), a well-known family of arthropod peptides that regulate juvenile hormone production in insects. Sequence analysis of the clone from which the EST was generated, with subsequent translation of its open reading frame, showed it to encode five novel A-type ASTs, whose mature structures were predicted to be APYGFGIamide, pE/EPYGFGIamide, ALYGFGIamide, pE/EPYNFGIamide, and pQ/QPYNFGVamide. Each of the peptides is present as a single copy within the prepro-hormone with the exception of APYGFGIamide, which is present in three copies. Surprisingly, the organization of the Calanus prepro-A-type AST, specifically the number of encoded A-type peptides, is more similar to those of insects than it is to the known decapod crustacean prepro-hormones. Moreover, the Calanus A-type ASTs possess isoleucine or valine residues at their carboxy (C)-termini rather than leucine, which is present in most other family members. Wholemount immunohistochemistry suggests that six pairs of somata produce the native Calanus A-type ASTs: five in the protocerebrum and one in the suboesophageal region. To the best of our knowledge, our report is the first characterization of a neuropeptidergic system in a copepod, the first identification of A-type ASTs from a non-decapod crustacean, the first report of crustacean A-type ASTs possessing isoleucine C-terminal residues, and the first report from any species of an A-type peptide possessing a valine C-terminal residue.

Multiple modulators act on the cardiac ganglion of the crab, Cancer borealis

Neuromodulators can change the output of neural circuits. The crustacean cardiac ganglion (CG) drives the contractions of the heart. The CG is a direct target for neurohormones that are released from the pericardial organs and other neuroendocrine sites. In this study, we have characterized for the first time the physiological actions of the peptides red pigment concentrating hormone (RPCH), Cancer borealis tachykinin-related peptide Ia (CabTRP Ia) and allatostatin III type A (AST-3) on the isolated CG of the crab, Cancer borealis. RPCH and CabTRP Ia excited the CG while AST-3 strongly inhibited its motor output. We also studied the actions of other peptides and small molecule transmitters known to be present in C. borealis. Dopamine, serotonin, proctolin, crustacean cardioactive peptide (CCAP), a number of extended FLRFamide peptides, and cholinergic agonists increased the activity of the CG, GABA inhibited the CG, while other substances had little or no significant effect on the CG motor pattern. These results demonstrate, in one species, that the CG is multiply modulated. We suggest that multiple modulators may be important to regulate and coordinate the activity of the heart and other organs in response to external stimuli or the endogenous physiological state.

Imaging mass spectrometry of neuropeptides in decapod crustacean neuronal tissues

Imaging mass spectrometry (IMS) of neuropeptides in crustacean neuronal tissues was performed on a MALDI-TOF/TOF instrument. Sample preparation protocols were developed for the sensitive detection of these highly complex endogenous signaling molecules. The neuromodulatory complements of the pericardial organ (PO) and brain of the Jonah crab, Cancer borealis, were mapped. Distributions of peptide isoforms belonging to 10 neuropeptide families were investigated using the IMS technique. Often, neuropeptides of high sequence homology were similarly located. However, two RFamide-family peptides and a truncated orcokinin peptide were mapped to locations distinct from other members of their respective families. Over 30 previously sequenced neuropeptides were identified based on mass measurement. For increased confidence of identification, select peptides were fragmented by post-source decay (PSD) and collisional-induced dissociation (CID). Collectively, this organ-level IMS study elucidates the spatial relationships between multiple neuropeptide isoforms of the same family as well as the relative distributions of neuropeptide families.

Mass spectrometric characterization and physiological actions of VPNDWAHFRGSWamide, a novel B type allatostatin in the crab, Cancer borealis

The neural networks in the crustacean stomatogastric ganglion are modulated by neuroactive substances released locally into the neuropil of the stomatogastric ganglion and by circulating hormones released by neuroendocrine structures including the pericardial organs. Using nanoscale liquid chromatography coupled to electrospray ionization quadrupole-time-of-flight mass spectrometry, we have identified and sequenced a novel B type allatostatin (CbAST-B1), VPNDWAHFRGSWamide, present in the pericardial organs of the crabs, Cancer borealis, and Cancer productus. We describe the physiological actions of CbAST-B1 on the pyloric rhythm of the stomatogastric ganglion of the crab, Cancer borealis. CbAST-B1 reduces the pyloric network frequency in a dose-dependent manner. The effect of bath-applied CbAST-B1 depends on the preceding physiological state of the preparation. Surprisingly, despite marked amino-acid sequence dissimilarity between the novel CbAST-B1 and the A type allatostatin family of peptides (AST-A), the physiological effects of CbAST-B1 are similar to those of AST-A.

Understanding Circuit Dynamics Using the Stomatogastric Nervous System of Lobsters and Crabs

Studies of the stomatogastric nervous systems of lobsters and crabs have led to numerous insights into the cellular and circuit mechanisms that generate rhythmic motor patterns. The small number of easily identifiable neurons allowed the establishment of connectivity diagrams among the neurons of the stomatogastric ganglion. We now know that (a) neuromodulatory substances reconfigure circuit dynamics by altering synaptic strength and voltage-dependent conductances and (b) individual neurons can switch among different functional circuits. Computational and experimental studies of single-neuron and network homeostatic regulation have provided insight into compensatory mechanisms that can underlie stable network performance. Many of the observations first made using the stomatogastric nervous system can be generalized to other invertebrate and vertebrate circuits.

Regulation of the crab heartbeat by crustacean cardioactive peptide (CCAP): central and peripheral actions

In regulating neurophysiological systems, neuromodulators exert multiple actions at multiple sites in such a way as to control the activity in an integrated manner. We are studying how this happens in a simple central pattern generator (CPG)-effector system, the heart of the blue crab Callinectes sapidus. The rhythmic contractions of this heart are neurogenic, driven by rhythmic motor patterns generated by the cardiac ganglion (CG). In this study, we used anatomical and physiological methods to examine the sources and actions on the system of crustacean cardioactive peptide (CCAP). Immunohistochemical localization revealed a plexus of CCAP-immunoreactive fibers in the pericardial organs (POs), neurohemal structures from which blood-borne neurohormones reach the heart. Combined backfill and immunohistochemical experiments indicated that the CCAP in the POs originated from a large contralateral neuron in each thoracic neuromere. In physiological experiments, we examined the actions of exogenous CCAP on the intact working heart, on the semi-intact heart in which we could record the motor patterns as well as the muscle contractions, and on the isolated CG. CCAP had strong positive inotropic and chronotropic effects. Dissection of these effects in terms of dose dependency, time course, and the preparation type in which they occurred suggested that they were produced by the interaction of three primary actions of CCAP exerted both on the heart muscle and on the CG. We conclude that CCAP released from the POs as a neurohormone regulates the crab heart by multiple actions on both the central and peripheral components of this model CPG-effector system.

Identification and developmental expression of mRNAs encoding crustacean cardioactive peptide (CCAP) in decapod crustaceans

Full-length cDNAs encoding crustacean cardioactive peptide (CCAP) were isolated from several decapod (brachyuran and astacuran) crustaceans: the blue crab Callinectes sapidus, green shore crab Carcinus maenas, European lobster Homarus gamarus and calico crayfish Orconectes immunis. The cDNAs encode open reading frames of 143 (brachyurans) and 139-140 (astacurans) amino acids. Apart from the predicted signal peptides (30-32 amino acids), the conceptually translated precursor codes for a single copy of CCAP and four other peptides that are extremely similar in terms of amino acid sequence within these species, but which clearly show divergence into brachyuran and astacuran groups. Expression patterns of CCAP mRNA and peptide were determined during embryonic development in Carcinus using quantitative RT-PCR and immunohistochemistry with whole-mount confocal microscopy, and showed that significant mRNA expression (at 50% embryonic development) preceded detectable levels of CCAP in the developing central nervous system (CNS; at 70% development). Subsequent CCAP gene expression dramatically increased during the late stages of embryogenesis (80-100%), coincident with developing immunopositive structures. In adult crabs, CCAP gene expression was detected exclusively in the eyestalk, brain and in particular the thoracic ganglia, in accord with the predominance of CCAP-containing cells in this tissue. Measurement of expression patterns of CCAP mRNA in Carcinus and Callinectes thoracic ganglia throughout the moult cycle revealed only modest changes, indicating that previously observed increases in CCAP peptide levels during premoult were not transcriptionally coupled. Severe hypoxic conditions resulted in rapid downregulation of CCAP transcription in the eyestalk, but not the thoracic ganglia in Callinectes, and thermal challenge did not change CCAP mRNA levels. These results offer the first tantalising glimpses of involvement of CCAP in environmental adaptation to extreme, yet biologically relevant stressors, and perhaps suggest that the CCAP-containing neurones in the eyestalk might be involved in adaptation to environmental stressors.

Temperature dependence of cardiac performance in the lobster Homarus americanus

The lobster Homarus americanus inhabits ocean waters that vary in temperature over a 25 degrees C range, depending on the season and water depth. To investigate whether the lobster heart functions effectively over a wide range of temperatures we examine the temperature dependence of cardiac performance of isolated lobster hearts in vitro. In addition, we examined whether modulation of the heart by serotonin depends on temperature. The strength of the heartbeat strongly depends on temperature, as isolated hearts are warmed from 2 to 22 degrees C the contraction amplitude decreases by greater than 60%. The rates of contraction and relaxation of the heart are most strongly temperature dependent in the range from 2 to 4 degrees C but become temperature independent at warmer temperatures. Heart rates increase as a function of temperature both in isolated hearts and in intact animals, however hearts in intact animals beat faster in the temperature range of 12-20 degrees C. Interestingly, acute Q10 values for heart rate are similar in vivo and in vitro over most of the temperature range, suggesting that temperature dependence of heart rate arises mainly from the temperature effects on the cardiac ganglion. In contrast to earlier reports suggesting that the strength and the frequency of the lobster heartbeat are positively correlated, we observe no consistent relationship between these parameters as they change as a function of temperature. Stroke volume decreases as a function of temperature. However, the opposing temperature-dependent increase in heart rate partially compensates to produce a relationship between cardiac output and temperature in which cardiac output is maximal at 10 degrees C and significantly decreases above 20 degrees C. Serotonin potentiates contraction amplitude and heart rate in a temperature-independent manner. Overall, our results show that although the parameters underlying cardiac performance show different patterns of temperature dependence, cardiac output remains relatively constant over most of the wide range of environmental temperatures the lobster inhabits in the wild.

Invertebrate central pattern generation moves along

Central pattern generators (CPGs) are circuits that generate organized and repetitive motor patterns, such as those underlying feeding, locomotion and respiration. We summarize recent work on invertebrate CPGs which has provided new insights into how rhythmic motor patterns are produced and how they are controlled by higher-order command and modulatory interneurons.

Mass spectrometric characterization and physiological actions of GAHKNYLRFamide, a novel FMRFamide-like peptide from crabs of the genus Cancer

The stomatogastric ganglion (STG) and the cardiac ganglion (CG) of decapod crustaceans are modulated by neuroactive substances released locally and by circulating hormones released from neuroendocrine structures including the pericardial organs (POs). Using nanoscale liquid chromatography electrospray ionization quadrupole-time-of-flight tandem mass spectrometry and direct tissue matrix-assisted laser desorption/ionization Fourier transform mass spectrometry we have identified and sequenced a novel neuropeptide, GAHKNYLRFamide (previously misassigned as KHKNYLRFamide in a study that did not employ peptide derivatization), from the POs and/or the stomatogastric nervous system (STNS) of the crabs, Cancer borealis, Cancer productus and Cancer magister. In C. borealis, exogenous application of GAHKNYLRFamide increased the burst frequency and number of spikes per burst of the isolated CG and re-initiated bursting activity in non-bursting ganglia, effects also elicited by the FMRFamide-like peptides (FLPs) SDRNFLRFamide and TNRNFLRFamide. In the intact STNS (which contains the STG), exogenous application of GAHKNYLRFamide increased the frequency of the pyloric rhythm and activated the gastric mill rhythm, effects also similar to those elicited by SDRNFLRFamide and TNRNFLRFamide. FLP-like immunoreactivity in the POs and the STNS was abolished by pre-adsorption with the synthetic GAHKNYLRFamide. Different members of the FLP family exhibited differential degradation in the presence of extracellular peptidases. Taken collectively, the amino acid sequence of GAHKNYLRFamide, the blocking of FLP-like immunostaining, and its physiological effects on the CG and STNS suggest that this peptide is a novel member of the FLP superfamily.

Red Pigment Concentrating Hormone Strongly Enhances the Strength of the Feedback to the Pyloric Rhythm Oscillator But Has Little Effect on Pyloric Rhythm Period

The neuropeptide, red pigment concentrating hormone (RPCH), strengthened the inhibitory synapse from the lateral pyloric (LP) neuron to the pyloric dilator (PD) neurons in the pyloric network of the stomatogastric ganglion (STG) of the lobster, Homarus americanus. RPCH produced several-fold increases in the amplitude of both action potential–mediated and non–impulse-mediated transmission that persisted for as long as the peptide remained present. Because the LP to PD synapse is the only feedback to the pacemaker kernel of the pyloric network, which consists of the electrically coupled two PD neurons and the anterior burster (AB) neuron, it might have been expected that strengthening the LP to PD synapse would increase the period of the pyloric rhythm. However, the period of the pyloric rhythm increased only transiently in RPCH, and a transient increase in cycle period was observed even when the LP neuron was hyperpolarized. Phase response curves were measured using the dynamic clamp to create artificial inhibitory inputs of variable strength and duration to the PD neurons. Synaptic conductance values seen in normal saline were ineffective at changing the pyloric period throughout the pyloric cycle. Conductances similar to those seen in 10−6M RPCH also did not evoke phase resets at phases when the LP neuron is typically active. Thus the dramatic effects of RPCH on synaptic strength have little role in modulation of the period of the pyloric rhythm under normal operating conditions but may help to stabilize the rhythm when the cycle period is too slow or too fast.

Identification and characterization of a tachykinin-containing neuroendocrine organ in the commissural ganglion of the crab Cancer productus

A club-shaped, tachykinin-immunopositive structure first described nearly two decades ago in the commissural ganglion (CoG) of three species of decapod crustaceans has remained enigmatic, as its function is unknown. Here, we use a combination of anatomical, mass spectrometric and electrophysiological techniques to address this issue in the crab Cancer productus. Immunohistochemistry using an antibody to the vertebrate tachykinin substance P shows that a homologous site exists in each CoG of this crab. Confocal microscopy reveals that its structure and organization are similar to those of known neuroendocrine organs. Based on its location in the anterior medial quadrant of the CoG, we have named this structure the anterior commissural organ (ACO). Matrix-assisted laser desorption/ionization Fourier transform mass spectrometry shows that the ACO contains the peptide APSGFLGMRamide, commonly known as Cancer borealis tachykinin-related peptide Ia (CabTRP Ia). Using the same technique, we show that CabTRP Ia is also released into the hemolymph. As no tachykinin-like labeling is seen in any of the other known neuroendocrine sites of this species (i.e. the sinus gland, the pericardial organ and the anterior cardiac plexus), the ACO is a prime candidate to be the source of CabTRP Ia present in the circulatory system. Our electrophysiological studies indicate that one target of hemolymph-borne CabTRP Ia is the foregut musculature. Here, no direct CabTRP Ia innervation is present, yet several gastric mill and pyloric muscles are nonetheless modulated by hormonally relevant concentrations of the peptide. Collectively, our findings show that the C. productus ACO is a neuroendocrine organ providing hormonal CabTRP Ia modulation to the foregut musculature. Homologous structures in other decapods are hypothesized to function similarly.

Identification of neuropeptides from the decapod crustacean sinus glands using nanoscale liquid chromatography tandem mass spectrometry

Neurosecretory systems are known to synthesize and secrete a diverse class of peptide hormones which regulate many physiological processes. The crustacean sinus gland (SG) is a well-defined neuroendocrine site that produces numerous hemolymph-borne agents including the most complex class of endocrine signaling molecules--neuropeptides. As an ongoing effort to define the peptidome of the crustacean SG, we determine the neuropeptide complements of the SG of the Jonah crab, Cancer borealis, and the Maine lobster, Homarus americanus, using nanoflow liquid chromatography electrospray ionization quadrupole time-of-flight (ESI-QTOF) MS/MS. Numerous neuropeptides were identified, including orcokinins, orcomyotropin, crustacean hyperglycemic hormone (CHH), CHH precursor-related peptides (CPRPs), red pigment concentrating hormone (RPCH), beta-pigment dispersing hormone (beta-PDH), proctolin and HL/IGSL/IYRamide. Among them, two novel orcokinins were de novo sequenced from the SG of H. americanus. Three CPRPs including a novel isoform were sequenced in H. americanus. Four new CPRPs were sequenced from the SG of C. borealis. Our results show that structural polymorphisms in CPRPs (and thus the CHH precursors) are common in Dendrobranchiata as well as in Pleocyemata. The evolutionary relationship between the CPRPs is also discussed.

Hormone complement of theCancer productus sinus gland and pericardial organ: An anatomical and mass spectrometric investigation

In crustaceans, circulating hormones influence many physiological processes. Two neuroendocrine organs, the sinus gland (SG) and the pericardial organ (PO), are the sources of many of these compounds. As a first step in determining the roles played by hemolymph-borne agents in the crab Cancer productus, we characterized the hormone complement of its SG and PO. We show via transmission electron microscopy that the nerve terminals making up each site possess dense-core and/or electron-lucent vesicles, suggesting diverse complements of bioactive molecules for both structures. By using immunohistochemistry, we show that small molecule transmitters, amines and peptides, are among the hormones present in these tissues, with many differentially distributed between the two sites (e.g., serotonin in the PO but not the SG). With several mass spectrometric (MS) methods, we identified many of the peptides responsible for the immunolabeling and surveyed the SG and PO for peptides for which no antibodies exist. By using MS, we characterized 39 known peptides [e.g., beta-pigment-dispersing hormone (beta-PDH), crustacean cardioactive peptide, and red pigment-concentrating hormone] and de novo sequenced 23 novel ones (e.g., a new beta-PDH isoform and the first B-type allatostatins identified from a non-insect species). Collectively, our results show that diverse and unique complements of hormones, including many previously unknown peptides, are present in the SG and PO of C. productus. Moreover, our study sets the stage for future biochemical and physiological studies of these molecules and ultimately the elucidation of the role(s) they play in hormonal control in C. productus.

Modulation of an integrated central pattern generator-effector system: dopaminergic regulation of cardiac activity in the blue crab Callinectes sapidus

Theoretical studies have suggested that the output of a central pattern generator (CPG) must be matched to the properties of its peripheral effector system to ensure production of functional behavior. One way that such matching could be achieved is through coordinated central and peripheral modulation. In this study, morphological and physiological methods were used to examine the sources and actions of dopaminergic modulation in the cardiac system of the blue crab, Callinectes sapidus. Immunohistochemical localization of tyrosine hydroxylase (TH) revealed a prominent neuron in the commissural ganglion, the L-cell, that projected a large-diameter axon to the pericardial organ (PO) by an indirect and circuitous route. Within the PO, the L-cell axon gave rise to fine varicose fibers, suggesting that it releases dopamine in a neurohormonal fashion onto the heart musculature. In addition, one branch of the axon continued beyond the PO to the heart, where it innervated the anterior motor neurons and the posterior pacemaker region of the cardiac ganglion (CG). In physiological experiments, exogenous dopamine produced multiple effects on contraction and motor neuron burst parameters that corresponded to the dual central-peripheral modulation suggested by the L-cell morphology. Interestingly, parameters of the ganglionic motor output were modulated differently in the isolated CG and in a novel semi-intact system where the CG remained embedded within the heart musculature. These observations suggest a critical role of feedback from the periphery to the CG and underscore the requirement for integration of peripheral (neurohormonal) actions and direct ganglionic modulation in the regulation of this exceptionally simple system.

The anterior cardiac plexus: an intrinsic neurosecretory site within the stomatogastric nervous system of the crab Cancer productus

The stomatogastric nervous system (STNS) of decapod crustaceans is modulated by both locally released and circulating substances. In some species, including chelate lobsters and freshwater crayfish, the release zones for hormones are located both intrinsically to and at some distance from the STNS. In other crustaceans, including Brachyuran crabs, the existence of extrinsic sites is well documented. Little, however, is known about the presence of intrinsic neuroendocrine structures in these animals. Putative intrinsic sites have been identified within the STNS of several crab species, though ultrastructural confirmation that these structures are in fact neuroendocrine in nature remains lacking. Using a combination of anatomical techniques, we demonstrate the existence of a pair of neurosecretory sites within the STNS of the crab Cancer productus. These structures, which we have named the anterior cardiac plexi (ACPs), are located on the anterior cardiac nerves (acns), which overlie the cardiac sac region of the foregut. Each ACP starts several hundred micro m from the origin of the acn and extends distally for up to several mm. Transmission electron microscopy done on these structures shows that nerve terminals are present in the peripheral portion of each acn, just below a well defined epineurium. These terminals contain dense-core and, occasionally, electron-lucent vesicles. In many terminals, morphological correlates of hormone secretion are evident. Immunocytochemistry shows that the ACPs are immunopositive for FLRFamide-related peptide. All FLRFamide labeling in the ACPs originates from four axons, which descend to these sites through the superior oesophageal and stomatogastric nerves. Moreover, these FLRFamide-immunopositive axons are the sole source of innervation to the ACPs. Collectively, our results suggest that the STNS of C. productus is not only a potential target site for circulating hormones, but also serves as a neuroendocrine release center itself.

A crustacean serotonin receptor: Cloning and distribution in the thoracic ganglia of crayfish and freshwater prawn

Serotonin (5-HT) is involved in regulating important aspects of behavior and a variety of systemic physiological functions in both vertebrates and invertebrates. These functions are mediated through binding to 5-HT receptors, of which approximately 13 have been characterized in mammals. In crustaceans, important model systems for the study of the neural basis of behaviors, 5-HT is also linked with higher-order behaviors, associated with different 5-HT receptors that have been identified at the physiological and pharmacological levels. However, no crustacean 5-HT receptors have been identified at the molecular level. We have cloned a putative 5-HT(1) receptor (5-HT(1crust)) from crayfish, prawn, and spiny lobster and have raised antibodies that recognize this protein in all three organisms. 5-HT(1crust) immunoreactivity (5-HT(1crust)ir) was observed surrounding the somata of specific groups of neurons and as punctate staining within the neuropil in all thoracic ganglia of crayfish and prawn. In the crayfish, 5-HT(1crust)ir was also found in boutons surrounding the first and second nerves of each ganglion and on the 5-HT cells of T1-4. In the prawn, 5-HT(1crust)ir was also found in axons that project across the ganglia and along the connectives. We found examples of colocalization of 5-HT(1crust) with 5-HT, consistent with the short-term modulatory role of 5-HT, as well as cases of serotonergic staining in the absence of a 5-HT(1crust) signal, which might imply that other 5-HT receptors are found at these locations. We also observed receptors that did not possess counterpart 5-HT staining, suggesting that these may also mediate long-term neurohormonal functions of serotonin.

Mass spectrometric investigation of the neuropeptide complement and release in the pericardial organs of the crab, Cancer borealis

The crustacean stomatogastric ganglion (STG) is modulated by both locally released neuroactive compounds and circulating hormones. This study presents mass spectrometric characterization of the complement of peptide hormones present in one of the major neurosecretory structures, the pericardial organs (POs), and the detection of neurohormones released from the POs. Direct peptide profiling of Cancer borealis PO tissues using matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) revealed many previously identified peptides, including proctolin, red pigment concentrating hormone (RPCH), crustacean cardioactive peptide (CCAP), several orcokinins, and SDRNFLRFamide. This technique also detected corazonin, a well-known insect hormone, in the POs for the first time. However, most mass spectral peaks did not correspond to previously known peptides. To characterize and identify these novel peptides, we performed MALDI postsource decay (PSD) and electrospray ionization (ESI) MS/MS de novo sequencing of peptides fractionated from PO extracts. We characterized a truncated form of previously identified TNRNFLRFamide, NRNFLRFamide. In addition, we sequenced five other novel peptides sharing a common C-terminus of RYamide from the PO tissue extracts. High K+ depolarization of isolated POs released many peptides present in this tissue, including several of the novel peptides sequenced in the current study.

Axonal dopamine receptors activate peripheral spike initiation in a stomatogastric motor neuron

We studied the effects of dopamine on the stomatogastric ganglion (STG) of the lobster, Homarus americanus. The two pyloric dilator (PD) neurons are active in the pyloric rhythm, have somata in the STG, and send axons many centimeters to innervate muscles of the stomach. Dopamine application to the stomatogastric nervous system when the PD neurons were rhythmically active evoked additional action potentials during the PD neuron interburst intervals. These action potentials were peripherally generated at a region between the STG and the first bilateral branch, approximately 1 cm away from the STG, and traveled antidromically to the neuropil and orthodromically to the pyloric dilator muscles. Focal applications of dopamine to the nerves showed that spikes could be initiated in almost the entire peripheral axon of the PD neurons. Dopamine also evoked spikes in isolated peripheral axons. The concentration threshold for peripheral spike initiation was at or below 10-9 m dopamine. Thus, the peripheral axon can play an important role in shaping the output signaling to the muscles by the motor neuron.

Dopamine and histamine in the developing stomatogastric system of the lobster Homarus americanus

Dopamine and histamine are neuromodulators found in the adult stomatogastric nervous system (STNS) of several crustacean species. We used antibodies against tyrosine hydroxylase (TH) and histamine to map the distribution and developmental acquisition of the dopamine and histamine neurons in the STNS of the lobster, Homarus americanus. Embryos, larvae, juvenile and adult animals were studied. TH labeling was present in the STNS as early as E80-85 (80-85% of embryonic development). A subset of preparations in embryos, larvae, juveniles, and adults contained 1-5 labeled somata in the stomatogastric ganglion. Histamine staining appeared in the STNS as early as E50. The distribution of both TH and histamine staining remained relatively constant through development. Electrophysiological recordings demonstrated that receptors for both amines are present in the embryo. Bath application of dopamine increased the frequency of the pyloric rhythm in embryos, and evidence for dopaminergic activation of peripherally initiated spiking in motor axons was seen. In embryos and adults, histamine inhibited the motor patterns produced by the stomatogastric ganglion (STG). These data suggest that the dopaminergic and histaminergic systems in H. americanus appear relatively early in development and that the effects of each are largely maintained through development.