Amines and peptides in the brain of the American lobster: immunocytochemical localization patterns and implications for brain function

Non-Invasive Methods for Assessing the Welfare of Farmed White-Leg Shrimp (Penaeus vannamei)

Gradually, concern for the welfare of aquatic invertebrates produced on a commercial/industrial scale is crossing the boundaries of science and becoming a demand of other societal actors. The objective of this paper is to propose protocols for assessing the Penaeus vannamei welfare during the stages of reproduction, larval rearing, transport, and growing-out in earthen ponds and to discuss, based on a literature review, the processes and perspectives associated with the development and application of on-farm shrimp welfare protocols. Protocols were developed based on four of the five domains of animal welfare: nutrition, environment, health, and behaviour. The indicators related to the psychology domain were not considered a separate category, and the other proposed indicators indirectly assessed this domain. For each indicator, the corresponding reference values were defined based on literature and field experience, apart from the three possible scores related to animal experience on a continuum from positive (score 1) to very negative (score 3). It is very likely that non-invasive methods for measuring the farmed shrimp welfare, such as those proposed here, will become a standard tool for farms and laboratories and that it will become increasingly challenging to produce shrimp without considering their welfare throughout the production cycle.

Local olfactory interneurons provide the basis for neurochemical regionalization of olfactory glomeruli in crustaceans

The primary olfactory centers of metazoans as diverse as arthropods and mammals consist of an array of fields of dense synaptic neuropil, the olfactory glomeruli. However, the neurochemical structure of crustacean olfactory glomeruli is largely understudied when compared to the insects. We analyzed the glomerular architecture in selected species of hermit crabs using immunohistochemistry against presynaptic proteins, the neuropeptides orcokinin, RFamide and allatostatin, and the biogenic amine serotonin. Our study reveals an unexpected level of structural complexity, unmatched by what is found in the insect olfactory glomeruli. Peptidergic and aminergic interneurons provide the structural basis for a regionalization of the crustacean glomeruli into longitudinal and concentric compartments. Our data suggest that local olfactory interneurons take a central computational role in modulating the information transfer from olfactory sensory neurons to projection neurons within the glomeruli. Furthermore, we found yet unknown neuronal elements mediating lateral inhibitory interactions across the glomerular array that may play a central role in modulating the transfer of sensory input to the output neurons through presynaptic inhibition. Our study is another step in understanding the function of crustacean olfactory glomeruli as highly complex units of local olfactory processing.

Comparative neuroanatomical distribution and expression levels of neuropeptide F in the central nervous system of the female freshwater prawn, Macrobrachium rosenbergii, during the ovarian cycle

Neuropeptide F (NPF) plays critical roles in controlling the feeding and reproduction of prawns. In the present study, we investigated changes in the expression levels of Macrobrachium rosenbergii neuropeptide F (MrNPF), and its neuroanatomical distribution in eyestalk (ES), brain (BR), subesophageal ganglion (SEG), thoracic ganglia (TG), and abdominal ganglia (AG), during the ovarian cycle of female prawn. By qRT-PCR, the amount of MrNPF transcripts exhibited a gradual increase in the ES, BR, and combined SEG and TG from stages I and II, to reach a maximum level at stage III, and slightly declined at stage IV, respectively. The highest to lowest expression levels were detected in combined SEG and TG, BR, ES, and AG, respectively. MrNPF immunolabeling was observed in several neuronal clusters, associated fibers, and neuropils of these central nervous system (CNS) tissues. MrNPF-ir was more intense in neurons and neuropils of SEG and TG than those found in other parts of the CNS. The number of MrNPF-ir neurons and intensity of MrNPF-ir were higher in the ES, BR, SEG, and TG at the late stages than those at the early stages of the ovarian cycle, while those in AG exhibited insignificant change. Taken together, there is a correlation between changes in the neuroanatomical distribution of MrNPF and stages of the ovarian cycle, implying that MrNPF may be an important neuropeptide that integrates sensory stimuli, including photo-, chemo-, and gustatory receptions, to control feeding and reproduction, particularly ovarian development, of this female prawn, M. rosenbergii.

Humane Slaughter of Edible Decapod Crustaceans

Simple Summary

Decapods respond to noxious stimuli in ways that are consistent with the experience of pain; thus, we accept the need to provide a legal framework for their protection when they are used for human food. We review the main methods used to slaughter the major decapod crustaceans, highlighting problems posed by each method for animal welfare. The aim is to identify methods that are the least likely to cause suffering. These methods can then be recommended, whereas other methods that are more likely to cause suffering may be banned. We thus request changes in the legal status of this group of animals, to protect them from slaughter techniques that are not viewed as being acceptable.

Abstract

Vast numbers of crustaceans are produced by aquaculture and caught in fisheries to meet the increasing demand for seafood and freshwater crustaceans. Simultaneously, the public is increasingly concerned about current methods employed in their handling and killing. Recent evidence has shown that decapod crustaceans probably have the capacity to suffer because they show responses consistent with pain and have a relatively complex cognitive capacity. For these reasons, they should receive protection. Despite the large numbers of crustaceans transported and slaughtered, legislation protecting their welfare, by using agreed, standardized methods, is lacking. We review various stunning and killing systems proposed for crustaceans, and assess welfare concerns. We suggest the use of methods least likely to cause suffering and call for the implementation of welfare guidelines covering the slaughter of these economically important animals.

Functional morphology of the primary olfactory centers in the brain of the hermit crab Coenobita clypeatus (Anomala, Coenobitidae)

Terrestrial hermit crabs of the genus Coenobita display strong behavioral responses to volatile odors and are attracted by chemical cues of various potential food sources. Several aspects of their sense of aerial olfaction have been explored in recent years including behavioral aspects and structure of their peripheral and central olfactory pathway. Here, we use classical histological methods and immunohistochemistry against the neuropeptides orcokinin and allatostatin as well as synaptic proteins and serotonin to provide insights into the functional organization of their primary olfactory centers in the brain, the paired olfactory lobes. Our results show that orcokinin is present in the axons of olfactory sensory neurons, which target the olfactory lobe. Orcokinin is also present in a population of local olfactory interneurons, which may relay lateral inhibition across the array of olfactory glomeruli within the lobes. Extensive lateral connections of the glomeruli were also visualized using the histological silver impregnation method according to Holmes-Blest. This technique also revealed the structural organization of the output pathway of the olfactory system, the olfactory projection neurons, the axons of which target the lateral protocerebrum. Within the lobes, the course of their axons seems to be reorganized in an axon-sorting zone before they exit the system. Together with previous results, we combine our findings into a model on the functional organization of the olfactory system in these animals.

The stalk-eyed fly as a model for aggression - is there a conserved role for 5-HT between vertebrates and invertebrates?

Serotonin (5-HT) has largely been accepted to be inhibitory to vertebrate aggression, whereas an opposing stimulatory role has been proposed for invertebrates. Herein, we argue that critical gaps in our understanding of the nuanced role of 5-HT in invertebrate systems drove this conclusion prematurely, and that emerging data suggest a previously unrecognized level of phylogenetic conservation with respect to neurochemical mechanisms regulating the expression of aggressive behaviors. This is especially apparent when considering the interplay among factors governing 5-HT activity, many of which share functional homology across taxa. We discuss recent findings using insect models, with an emphasis on the stalk-eyed fly, to demonstrate how particular 5-HT receptor subtypes mediate the intensity of aggression with respect to discrete stages of the interaction (initiation, escalation and termination), which mirrors the complex behavioral regulation currently recognized in vertebrates. Further similarities emerge when considering the contribution of neuropeptides, which interact with 5-HT to ultimately determine contest progression and outcome. Relative to knowledge in vertebrates, much less is known about the function of 5-HT receptors and neuropeptides in invertebrate aggression, particularly with respect to sex, species and context, prompting the need for further studies. Our Commentary highlights the need to consider multiple factors when determining potential taxonomic differences, and raises the possibility of more similarities than differences between vertebrates and invertebrates with regard to the modulatory effect of 5-HT on aggression.

Tachykinins: Neuropeptides That Are Ancient, Diverse, Widespread and Functionally Pleiotropic

Tachykinins (TKs) are ancient neuropeptides present throughout the bilaterians and are, with some exceptions, characterized by a conserved FX1GX2Ramide carboxy terminus among protostomes and FXGLMamide in deuterostomes. The best-known TK is the vertebrate substance P, which in mammals, together with other TKs, has been implicated in health and disease with important roles in pain, inflammation, cancer, depressive disorder, immune system, gut function, hematopoiesis, sensory processing, and hormone regulation. The invertebrate TKs are also known to have multiple functions in the central nervous system and intestine and these have been investigated in more detail in the fly Drosophila and some other arthropods. Here, we review the protostome and deuterostome organization and evolution of TK precursors, peptides and their receptors, as well as their functions, which appear to be partly conserved across Bilateria. We also outline the distribution of TKs in the brains of representative organisms. In Drosophila, recent studies have revealed roles of TKs in early olfactory processing, neuromodulation in circuits controlling locomotion and food search, nociception, aggression, metabolic stress, and hormone release. TK signaling also regulates lipid metabolism in the Drosophila intestine. In crustaceans, TK is an important neuromodulator in rhythm-generating motor circuits in the stomatogastric nervous system and a presynaptic modulator of photoreceptor cells. Several additional functional roles of invertebrate TKs can be inferred from their distribution in various brain circuits. In addition, there are a few interesting cases where invertebrate TKs are injected into prey animals as vasodilators from salivary glands or paralyzing agents from venom glands. In these cases, the peptides are produced in the glands of the predator with sequences mimicking the prey TKs. Lastly, the TK-signaling system appears to have duplicated in Panarthropoda (comprising arthropods, onychophores, and tardigrades) to give rise to a novel type of peptides, natalisins, with a distinct receptor. The distribution and functions of natalisins are distinct from the TKs. In general, it appears that TKs are widely distributed and act in circuits at short range as neuromodulators or cotransmitters.

Sex differences in aggression: Differential roles of 5-HT2, neuropeptide F and tachykinin

Despite the conserved function of aggression across taxa in obtaining critical resources such as food and mates, serotonin's (5-HT) modulatory role on aggressive behavior appears to be largely inhibitory for vertebrates but stimulatory for invertebrates. However, critical gaps exist in our knowledge of invertebrates that need to be addressed before definitively stating opposing roles for 5-HT and aggression. Specifically, the role of 5-HT receptor subtypes are largely unknown, as is the potential interactive role of 5-HT with other neurochemical systems known to play a critical role in aggression. Similarly, the influence of these systems in driving sex differences in aggressive behavior of invertebrates is not well understood. Here, we investigated these questions by employing complementary approaches in a novel invertebrate model of aggression, the stalk-eyed fly. A combination of altered social conditions, pharmacological manipulation and 5-HT2 receptor knockdown by siRNA revealed an inhibitory role of this receptor subtype on aggression. Additionally, we provide evidence for 5-HT2's involvement in regulating neuropeptide F activity, a suspected inhibitor of aggression. However, this function appears to be stage-specific, altering only the initiation stage of aggressive conflicts. Alternatively, pharmacologically increasing systemic concentrations of 5-HT significantly elevated the expression of the neuropeptide tachykinin, which did not affect contest initiation but instead promoted escalation via production of high intensity aggressive behaviors. Notably, these effects were limited solely to males, with female aggression and neuropeptide expression remaining unaltered by any manipulation that affected 5-HT. Together, these results demonstrate a more nuanced role for 5-HT in modulating aggression in invertebrates, revealing an important interactive role with neuropeptides that is more reminiscent of vertebrates. The sex-differences described here also provide valuable insight into the evolutionary contexts of this complex behavior.

Crustacean olfactory systems: A comparative review and a crustacean perspective on olfaction in insects

Malacostracan crustaceans display a large diversity of sizes, morphs and life styles. However, only a few representatives of decapod taxa have served as models for analyzing crustacean olfaction, such as crayfish and spiny lobsters. Crustaceans bear multiple parallel chemosensory pathways represented by different populations of unimodal chemosensory and bimodal chemo- and mechanosensory sensilla on the mouthparts, the walking limbs and primarily on their two pairs of antennae. Here, we focus on the olfactory pathway associated with the unimodal chemosensory sensilla on the first antennal pair, the aesthetascs. We explore the diverse arrangement of these sensilla across malacostracan taxa and point out evolutionary transformations which occurred in the central olfactory pathway. We discuss the evolution of chemoreceptor proteins, comparative aspects of active chemoreception and the temporal resolution of crustacean olfactory system. Viewing the evolution of crustacean brains in light of energetic constraints can help us understand their functional morphology and suggests that in various crustacean lineages, the brains were simplified convergently because of metabolic limitations. Comparing the wiring of afferents, interneurons and output neurons within the olfactory glomeruli suggests a deep homology of insect and crustacean olfactory systems. However, both taxa followed distinct lineages during the evolutionary elaboration of their olfactory systems. A comparison with insects suggests their olfactory systems ö especially that of the vinegar fly ö to be superb examples for "economy of design". Such a comparison also inspires new thoughts about olfactory coding and the functioning of malacostracan olfactory systems in general.

Central projections of antennular chemosensory and mechanosensory afferents in the brain of the terrestrial hermit crab (Coenobita clypeatus; Coenobitidae, Anomura)

The Coenobitidae (Decapoda, Anomura, Paguroidea) is a taxon of hermit crabs that includes two genera with a fully terrestrial life style as adults. Previous studies have shown that Coenobitidae have evolved a sense of spatial odor localization that is behaviorally highly relevant. Here, we examined the central olfactory pathway of these animals by analyzing central projections of the antennular nerve of Coenobita clypeatus, combining backfilling of the nerve with dextran-coupled dye, Golgi impregnations and three-dimensional reconstruction of the primary olfactory center, the antennular lobe. The principal pattern of putative olfactory sensory afferents in C. clypeatus is in many aspects similar to what have been established for aquatic decapod crustaceans, such as the spiny lobster Panulirus argus. However, there are also obvious differences that may, or may not represent adaptations related to a terrestrial lifestyle. In C. clypeatus, the antennular lobe dominates the deutocerebrum, having more than one thousand allantoid-shaped subunits. We observed two distinct patterns of sensory neuron innervation: putative olfactory afferents from the aesthetascs either supply the cap/subcap region of the subunits or they extend through its full depth. Our data also demonstrate that any one sensory axon can supply input to several subunits. Putative chemosensory (non-aesthetasc) and mechanosensory axons represent a different pathway and innervate the lateral and median antennular neuropils. Hence, we suggest that the chemosensory input in C. clypeatus might be represented via a dual pathway: aesthetascs target the antennular lobe, and bimodal sensilla target the lateral antennular neuropil and median antennular neuropil. The present data is compared to related findings in other decapod crustaceans.

The brain in three crustaceans from cavernous darkness

BACKGROUND

While a number of neuroanatomical studies in other malacostracan taxa have recently contributed to the reconstruction of the malacostracan ground pattern, little is known about the nervous system in the three enigmatic blind groups of peracarids from relict habitats, Thermosbaenacea, Spelaeogriphacea, and Mictocarididae. This first detailed description of the brain in a representative of each taxon is largely based on a combination of serial semi-thin sectioning and computer-aided 3D-reconstructions. In addition, the mictocaridid Mictocaris halope was studied with a combination of immunolabeling (tubulin, nuclear counter-stains) and confocal laser scanning microscopy, addressing also the ventral nerve cord.

RESULTS

Adjacent to the terminal medulla, all three representatives exhibit a distal protocerebral neuropil, which is reminiscent of the lobula in other Malacostraca, but also allows for an alternative interpretation in M. halope and the thermosbaenacean Tethysbaena argentarii. A central complex occurs in all three taxa, most distinctively in the spelaeogriphacean Spelaeogriphus lepidops. The deutocerebral olfactory lobe in M. halope and S. lepidops is large. The comparably smaller olfactory lobe in T. argentarii appears to be associated with a unique additional deutocerebral neuropil. A small hemiellipsoid body exists only in the protocerebrum of T. argentarii. Distinctive mechanosensory neuropils corresponding to other malacostracans are missing.

CONCLUSIONS

The considerable reduction of the optic lobe in the studied taxa is higher than in any other blind malacostracan. The large size of deutocerebral olfactory centers implies an important role of the olfactory sense. The presence of a distinctive central complex in the blind S. lepidops adds further support to a central-coordinating over a visual function of this structure. The lack of a hemiellipsoid body in M. halope and S. lepidops suggests that their terminal medulla takes over the function of a second order olfactory center completely, as in some other peracarids. The reduction of the optic lobe and hemiellipsoid body is suggested to have occurred several times independently within Peracarida. The missing optic sense in the studied taxa is not correlated with an emphasized mechanosense.

On the brain of a crustacean: a morphological analysis of CaMKII expression and its relation to sensory and motor pathways

Calcium/calmodulin kinase II (CaMKII) is a Ca²⁺-activated enzyme that is abundant in vertebrate and invertebrate brains. However, its characterization is poorly addressed in the nervous system of crustaceans, and, to our knowledge, no studies have determined the microanatomical location of CaMKII in a crustacean species. In this study, we found labeling of CaMKII in the eyestalk and brain of the prawn Macrobrachium acanthurus, by means of immunohistochemistry and Western blotting. Antibodies against neuron (ß tubulin III), glutamate receptor (GluA1), and FMRFamide were used in order to further characterize the CaMKII-labeled cells in the brain. In the eyestalk, strong labeling with CaMKII was observed in the photoreceptors. These cells, especially in the rhabdom, were also reactive to anti-ß tubulin III, whereas the pigment cells were labeled with anti-CaMKII. GluA1 co-located with CaMKII in the photoreceptors. Also, CaMKII appeared in the same sites as FMRFamide in the deutocerebrum, including the olfactory lobe, and in the tritocerebrum, specifically in the antennular neuropil, indicating that the synaptic areas in these regions may be related to sensory-motor processing. In the brain, the identification of cells and regions that express CaMKII contributes to the understanding of the processing of neural connections and the modulating role of CaMKII in decapod crustaceans.

Deep homology of arthropod central complex and vertebrate basal ganglia

The arthropod central complex and vertebrate basal ganglia derive from embryonic basal forebrain lineages that are specified by an evolutionarily conserved genetic program leading to interconnected neuropils and nuclei that populate the midline of the forebrain-midbrain boundary region. In the substructures of both the central complex and basal ganglia, network connectivity and neuronal activity mediate control mechanisms in which inhibitory (GABAergic) and modulatory (dopaminergic) circuits facilitate the regulation and release of adaptive behaviors. Both basal ganglia and central complex dysfunction result in behavioral defects including motor abnormalities, impaired memory formation, attention deficits, affective disorders, and sleep disturbances. The observed multitude of similarities suggests deep homology of arthropod central complex and vertebrate basal ganglia circuitries underlying the selection and maintenance of behavioral actions.

Neuropeptide complexity in the crustacean central olfactory pathway: immunolocalization of A-type allatostatins and RFamide-like peptides in the brain of a terrestrial hermit crab

BACKGROUND

In the olfactory system of malacostracan crustaceans, axonal input from olfactory receptor neurons associated with aesthetascs on the animal's first pair of antennae target primary processing centers in the median brain, the olfactory lobes. The olfactory lobes are divided into cone-shaped synaptic areas, the olfactory glomeruli where afferents interact with local olfactory interneurons and olfactory projection neurons. The local olfactory interneurons display a large diversity of neurotransmitter phenotypes including biogenic amines and neuropeptides. Furthermore, the malacostracan olfactory glomeruli are regionalized into cap, subcap, and base regions and these compartments are defined by the projection patterns of the afferent olfactory receptor neurons, the local olfactory interneurons, and the olfactory projection neurons. We wanted to know how neurons expressing A-type allatostatins (A-ASTs; synonym dip-allatostatins) integrate into this system, a large family of neuropeptides that share the C-terminal motif -YXFGLamide.

RESULTS

We used an antiserum that was raised against the A-type Diploptera punctata (Dip)-allatostatin I to analyse the distribution of this peptide in the brain of a terrestrial hermit crab, Coenobita clypeatus (Anomura, Coenobitidae). Allatostatin A-like immunoreactivity (ASTir) was widely distributed in the animal's brain, including the visual system, central complex and olfactory system. We focussed our analysis on the central olfactory pathway in which ASTir was abundant in the primary processing centers, the olfactory lobes, and also in the secondary centers, the hemiellipsoid bodies. In the olfactory lobes, we further explored the spatial relationship of olfactory interneurons with ASTir to interneurons that synthesize RFamide-like peptides. We found that these two peptides are present in distinct populations of local olfactory interneurons and that their synaptic fields within the olfactory glomeruli are also mostly distinct.

CONCLUSIONS

We discuss our findings against the background of the known neurotransmitter complexity in the crustacean olfactory pathway and summarize what is now about the neuronal connectivity in the olfactory glomeruli. A-type allatostatins, in addition to their localization in protocerebral brain areas, seem to be involved in modulating the olfactory signal at the level of the deutocerebrum. They contribute to the complex local circuits within the crustacean olfactory glomeruli the connectivity within which as yet is completely unclear. Because the glomeruli of C. clypeatus display a distinct pattern of regionalization, their olfactory systems form an ideal model to explore the functional relevance of glomerular compartments and diversity of local olfactory interneurons for olfactory processing in crustaceans.

Serotonin immunoreactive interneurons in the brain of the Remipedia: new insights into the phylogenetic affinities of an enigmatic crustacean taxon

BACKGROUND

Remipedia, a group of homonomously segmented, cave-dwelling, eyeless arthropods have been regarded as basal crustaceans in most early morphological and taxonomic studies. However, molecular sequence information together with the discovery of a highly differentiated brain led to a reconsideration of their phylogenetic position. Various conflicting hypotheses have been proposed including the claim for a basal position of Remipedia up to a close relationship with Malacostraca or Hexapoda. To provide new morphological characters that may allow phylogenetic insights, we have analyzed the architecture of the remipede brain in more detail using immunocytochemistry (serotonin, acetylated α-tubulin, synapsin) combined with confocal laser-scanning microscopy and image reconstruction techniques. This approach allows for a comprehensive neuroanatomical comparison with other crustacean and hexapod taxa.

RESULTS

The dominant structures of the brain are the deutocerebral olfactory neuropils, which are linked by the olfactory globular tracts to the protocerebral hemiellipsoid bodies. The olfactory globular tracts form a characteristic chiasm in the center of the brain. In Speleonectes tulumensis, each brain hemisphere contains about 120 serotonin immunoreactive neurons, which are distributed in distinct cell groups supplying fine, profusely branching neurites to 16 neuropilar domains. The olfactory neuropil comprises more than 300 spherical olfactory glomeruli arranged in sublobes. Eight serotonin immunoreactive neurons homogeneously innervate the olfactory glomeruli. In the protocerebrum, serotonin immunoreactivity revealed several structures, which, based on their position and connectivity resemble a central complex comprising a central body, a protocerebral bridge, W-, X-, Y-, Z-tracts, and lateral accessory lobes.

CONCLUSIONS

The brain of Remipedia shows several plesiomorphic features shared with other Mandibulata, such as deutocerebral olfactory neuropils with a glomerular organization, innervations by serotonin immunoreactive interneurons, and connections to protocerebral neuropils. Also, we provided tentative evidence for W-, X-, Y-, Z-tracts in the remipedian central complex like in the brain of Malacostraca, and Hexapoda. Furthermore, Remipedia display several synapomorphies with Malacostraca supporting a sister group relationship between both taxa. These homologies include a chiasm of the olfactory globular tract, which connects the olfactory neuropils with the lateral protocerebrum and the presence of hemiellipsoid bodies. Even though a growing number of molecular investigations unites Remipedia and Cephalocarida, our neuroanatomical comparison does not provide support for such a sister group relationship.

Comparative analysis of deutocerebral neuropils in Chilopoda (Myriapoda): implications for the evolution of the arthropod olfactory system and support for the Mandibulata concept

Background

Originating from a marine ancestor, the myriapods most likely invaded land independently of the hexapods. As these two evolutionary lineages conquered land in parallel but separately, we are interested in comparing the myriapod chemosensory system to that of hexapods to gain insights into possible adaptations for olfaction in air. Our study connects to a previous analysis of the brain and behavior of the chilopod (centipede) Scutigera coleoptrata in which we demonstrated that these animals do respond to volatile substances and analyzed the structure of their central olfactory pathway.

Results

Here, we examined the architecture of the deutocerebral brain areas (which process input from the antennae) in seven additional representatives of the Chilopoda, covering all major subtaxa, by histology, confocal laser-scan microscopy, and 3D reconstruction. We found that in all species that we studied the majority of antennal afferents target two separate neuropils, the olfactory lobe (chemosensory, composed of glomerular neuropil compartments) and the corpus lamellosum (mechanosensory). The numbers of olfactory glomeruli in the different chilopod taxa ranged from ca. 35 up to ca. 90 and the shape of the glomeruli ranged from spheroid across ovoid or drop-shape to elongate.

Conclusion

A split of the afferents from the (first) pair of antennae into separate chemosensory and mechanosensory components is also typical for Crustacea and Hexapoda, but this set of characters is absent in Chelicerata. We suggest that this character set strongly supports the Mandibulata hypothesis (Myriapoda + (Crustacea + Hexapoda)) as opposed to the Myriochelata concept (Myriapoda + Chelicerata). The evolutionary implications of our findings, particularly the plasticity of glomerular shape, are discussed.

Transition from marine to terrestrial ecologies: changes in olfactory and tritocerebral neuropils in land-living isopods

In addition to the ancestors of insects, representatives of five lineages of crustaceans have colonized land. Whereas insects have evolved sensilla that are specialized to allow the detection of airborne odors and have evolved olfactory sensory neurons that recognize specific airborne ligands, there is so far little evidence for aerial olfaction in terrestrial crustaceans. Here we ask the question whether terrestrial Isopoda have evolved the neuronal substrate for the problem of detecting far-field airborne chemicals. We show that conquest of land of Isopoda has been accompanied by a radical diminution of their first antennae and a concomitant loss of their deutocerebral olfactory lobes and olfactory computational networks. In terrestrial isopods, but not their marine cousins, tritocerebral neuropils serving the second antenna have evolved radical modifications. These include a complete loss of the malacostracan pattern of somatotopic representation, the evolution in some species of amorphous lobes and in others lobes equipped with microglomeruli, and yet in others the evolution of partitioned neuropils that suggest modality-specific segregation of second antenna inputs. Evidence suggests that Isopoda have evolved, and are in the process of evolving, several novel solutions to chemical perception on land and in air.

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.

Brain architecture of the largest living land arthropod, the Giant Robber Crab Birgus latro (Crustacea, Anomura, Coenobitidae): evidence for a prominent central olfactory pathway?

BACKGROUND

Several lineages within the Crustacea conquered land independently during evolution, thereby requiring physiological adaptations for a semi-terrestrial or even a fully terrestrial lifestyle. Birgus latro Linnaeus, 1767, the giant robber crab or coconut crab (Anomura, Coenobitidae), is the largest land-living arthropod and inhabits Indo-Pacific islands such as Christmas Island. B. latro has served as a model in numerous studies of physiological aspects related to the conquest of land by crustaceans. From an olfactory point of view, a transition from sea to land means that molecules need to be detected in gas phase instead of in water solution. Previous studies have provided physiological evidence that terrestrial hermit crabs (Coenobitidae) such as B. latro have a sensitive and well differentiated sense of smell. Here we analyze the brain, in particular the olfactory processing areas of B. latro, by morphological analysis followed by 3 D reconstruction and immunocytochemical studies of synaptic proteins and a neuropeptide.

RESULTS

The primary and secondary olfactory centers dominate the brain of B. latro and together account for ca. 40% of the neuropil volume in its brain. The paired olfactory neuropils are tripartite and composed of more than 1,000 columnar olfactory glomeruli, which are radially arranged around the periphery of the olfactory neuropils. The glomeruli are innervated ca. 90,000 local interneurons and ca. 160,000 projection neurons per side. The secondary olfactory centers, the paired hemiellipsoid neuropils, are targeted by the axons of these olfactory projection neurons. The projection neuron axonal branches make contact to ca. 250.000 interneurons (per side) associated with the hemiellipsoid neuropils. The hemiellipsoid body neuropil is organized into parallel neuropil lamellae, a design that is quite unusual for decapod crustaceans. The architecture of the optic neuropils and areas associated with antenna two suggest that B. latro has visual and mechanosensory skills that are comparable to those of marine Crustacea.

CONCLUSIONS

In parallel to previous behavioral findings that B. latro has aerial olfaction, our results indicate that their central olfactory pathway is indeed most prominent. Similar findings from the closely related terrestrial hermit crab Coenobita clypeatus suggest that in Coenobitidae, olfaction is a major sensory modality processed by the brain, and that for these animals, exploring the olfactory landscape is vital for survival in their terrestrial habitat. Future studies on terrestrial members of other crustacean taxa such as Isopoda, Amphipoda, Astacida, and Brachyura will shed light on how frequently the establishment of an aerial sense of olfaction evolved in Crustacea during the transition from sea to land. Amounting to ca. 1,000,000, the numbers of interneurons that analyse the olfactory input in B. latro brains surpasses that in other terrestrial arthropods, as e.g. the honeybee Apis mellifera or the moth Manduca sexta, by two orders of magnitude suggesting that B. latro in fact is a land-living arthropod that has devoted a substantial amount of nervous tissue to the sense of smell.

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.

Comparative Neuropeptidomic Analysis of Food Intake via a Multi-faceted Mass Spectrometric Approach

Feeding behavior is a fundamental aspect of energy homeostasis and is crucial for animal survival. This process is regulated by a multitude of neurotransmitters including neuropeptides within a complex neuroendocrine system. Given the high chemical complexity and wide distribution of neuropeptides, the precise molecular mechanisms at the cellular and network levels remain elusive. Here we report comparative neuropeptidomic analysis of brain and major neuroendocrine organ in a crustacean model organism in response to feeding. A multi-faceted approach employing direct tissue matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), stable isotopic labeling of neuropeptide extracts for quantitation, and mass spectrometric imaging (MSI) has been employed to obtain complementary information on the expression changes of a large array of neuropeptides in the brain and the pericardial organ (PO) in the crab Cancer borealis. Multiple neuropeptides exhibited changes in abundance after feeding, including RFamides, Cancer borealis tachykinin related peptides (CabTRPs), RYamides, and pyrokinins. By combining quantitative analysis of neuropeptide changes via isotopic labeling of brain extract and MSI mapping of neuropeptides of brain slices, we identified the boundary of olfactory lobe (ON) and median protocerebrum (MPC) area as two potential feeding centers in the crab brain.

Brain architecture in the terrestrial hermit crab Coenobita clypeatus (Anomura, Coenobitidae), a crustacean with a good aerial sense of smell

Background

During the evolutionary radiation of Crustacea, several lineages in this taxon convergently succeeded in meeting the physiological challenges connected to establishing a fully terrestrial life style. These physiological adaptations include the need for sensory organs of terrestrial species to function in air rather than in water. Previous behavioral and neuroethological studies have provided solid evidence that the land hermit crabs (Coenobitidae, Anomura) are a group of crustaceans that have evolved a good sense of aerial olfaction during the conquest of land. We wanted to study the central olfactory processing areas in the brains of these organisms and to that end analyzed the brain of Coenobita clypeatus (Herbst, 1791; Anomura, Coenobitidae), a fully terrestrial tropical hermit crab, by immunohistochemistry against synaptic proteins, serotonin, FMRFamide-related peptides, and glutamine synthetase.

Results

The primary olfactory centers in this species dominate the brain and are composed of many elongate olfactory glomeruli. The secondary olfactory centers that receive an input from olfactory projection neurons are almost equally large as the olfactory lobes and are organized into parallel neuropil lamellae. The architecture of the optic neuropils and those areas associated with antenna two suggest that C. clypeatus has visual and mechanosensory skills that are comparable to those of marine Crustacea.

Conclusion

In parallel to previous behavioral findings of a good sense of aerial olfaction in C. clypeatus, our results indicate that in fact their central olfactory pathway is most prominent, indicating that olfaction is a major sensory modality that these brains process. Interestingly, the secondary olfactory neuropils of insects, the mushroom bodies, also display a layered structure (vertical and medial lobes), superficially similar to the lamellae in the secondary olfactory centers of C. clypeatus. More detailed analyses with additional markers will be necessary to explore the question if these similarities have evolved convergently with the establishment of superb aerial olfactory abilities or if this design goes back to a shared principle in the common ancestor of Crustacea and Hexapoda.

Embryonic development of the histaminergic system in the ventral nerve cord of the Marbled Crayfish (Marmorkrebs)

The embryonic development of neurotransmitter systems in crustaceans so far is poorly understood. Therefore, in the current study we monitored the ontogeny of histamine-immunoreactive neurons in the ventral nerve cord of the Marbled Crayfish, an emerging crustacean model system for developmental studies. The first histaminergic neurons arise around 60% of embryonic development, well after the primordial axonal scaffold of the ventral nerve cord has been established. This suggests that histaminergic neurons do not serve as pioneer neurons but that their axons follow well established axonal tracts. The developmental sequence of the different types of histaminergic neurons is charted in this study. The analysis of the histaminergic structures is also extended into adult specimens, showing a persistence of embryonic histaminergic neurons into adulthood. Our data are compared to the pattern of histaminergic neurons in other crustaceans and discussed with regard to our knowledge on other aspects of neurogenesis in Crustacea. Furthermore, the possible role of histaminergic neurons as characters in evolutionary considerations is evaluated.

Adult neurogenesis and cell cycle regulation in the crustacean olfactory pathway: from glial precursors to differentiated neurons

Adult neurogenesis is a characteristic feature of the olfactory pathways of decapod crustaceans. In crayfish and clawed lobsters, adult-born neurons are the progeny of precursor cells with glial characteristics located in a neurogenic niche on the ventral surface of the brain. The daughters of these precursor cells migrate during S and G(2 )stages of the cell cycle along glial fibers to lateral (cluster 10) and medial (cluster 9) proliferation zones. Here, they divide (M phase) producing offspring that differentiate into olfactory interneurons. The complete lineage of cells producing neurons in these animals, therefore, is arranged along the migratory stream according to cell cycle stage. We have exploited this model to examine the influence of environmental and endogenous factors on adult neurogenesis. We find that increased levels of serotonin upregulate neuronal production, as does maintaining animals in an enriched (versus deprived) environment or augmenting their diet with omega-3 fatty acids; increased levels of nitric oxide, on the other hand, decrease the rate of neurogenesis. The features of the neurogenic niche and migratory streams, and the fact that these continue to function in vitro, provide opportunities unavailable in other organisms to explore the sequence of cellular and molecular events leading to the production of new neurons in adult brains.

Histamine operates Cl–-gated channels in crayfish neurosecretory cells

We describe a histamine-activated Cl– conductance in the X-organ neurons from crayfish Cherax quadricarinatus, which has comparable properties to the homomultimeric histamine-gated ion channels described in Drosophila. Topical application of histamine inhibited spontaneous neuronal firing in the X-organ sinus gland tract, concomitant with an increase in the membrane conductance. In X-organ neurons in culture and under voltage-clamp conditions, histamine evoked outward currents at –40 mV that reversed at the Cl– equilibrium potential. Histamine sensitivity in these neurons had a half-maximal response(EC50)=3.3±1 μmol l–1, with a Hill number of 2.6±0.4. The histamine-evoked current was blocked by tiotidine, cimetidine, ranitidine and 256±11 and 483±11 μmol l–1, respectively) and d-tubocurarine(IC50=21±2 μmol l–1), but was insensitive to picrotoxin, bicuculline and strychnine. Neither GABA nor glutamate was capable of desensitizing the histamine response, indicating that histamine activates a particular Cl– conductance. The presence of immunoreactive neurons to histamine in the medulla terminalis with axonal projections to the neuropile suggests a possible histaminergic modulation of the X-organ sinus gland system.

Immunolocalisation of crustacean-SIFamide in the median brain and eyestalk neuropils of the marbled crayfish

Crustacean-SIFamide (GYRKPPFNGSIFamide) is a novel neuropeptide that was recently isolated from crayfish nervous tissue. We mapped the localisation of this peptide in the median brain and eyestalk neuropils of the marbled crayfish (Marmorkrebs), a parthenogenetic crustacean. Our experiments showed that crustacean-SIFamide is strongly expressed in all major compartments of the crayfish brain, including all three optic neuropils, the lateral protocerebrum with the hemiellipsoid body, and the medial protocerebrum with the central complex. These findings imply a role of this peptide in visual processing already at the level of the lamina but also at the level of the deeper relay stations. Immunolabelling is particularly strong in the accessory lobes and the deutocerebral olfactory lobes that receive a chemosensory input from the first antennae. Most cells of the olfactory globular tract, a projection neuron pathway that links deuto- and protocerebrum, are labelled. This pathway plays a central role in conveying tactile and olfactory stimuli to the lateral protocerebrum, where this input converges with optic information. Weak labelling is also present in the tritocerebrum that is associated with the mechanosensory second antennae. Taken together, we suggest an important role of crustacean-SIFamidergic neurons in processing high-order, multimodal input in the crayfish brain.

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.

Immunocytochemical mapping and quantification of expression of a putative type 1 serotonin receptor in the crayfish nervous system

Serotonin is an important neurotransmitter that is involved in modulation of sensory, motor, and higher functions in many species. In the crayfish, which has been developed as a model for nervous system function for over a century, serotonin modulates several identified circuits. Although the cellular and circuit effects of serotonin have been extensively studied, little is known about the receptors that mediate these signals. Physiological data indicate that identified crustacean cells and circuits are modulated via several different serotonin receptors. We describe the detailed immunocytochemical localization of the crustacean type 1 serotonin receptor, 5-HT1crust, throughout the crayfish nerve cord and on abdominal superficial flexor muscles. 5-HT1crust is widely distributed in somata, including those of several identified neurons, and neuropil, suggesting both synaptic and neurohormonal roles. Individual animals show very different levels of 5-HT1crust immunoreactivity (5-HT(1crust)ir) ranging from preparations with hundreds of labeled cells per ganglion to some containing only a handful of 5-HT(1crust)ir cells in the entire nerve cord. The interanimal variability in 5-HT(1crust)ir is great, but individual nerve cords show a consistent level of labeling between ganglia. Quantitative RT-PCR shows that 5-HT1crust mRNA levels between animals are also variable but do not directly correlate with 5-HT(1crust)ir levels. Although there is no correlation of 5-HT1crust expression with gender, social status, molting or feeding, dominant animals show significantly greater variability than subordinates. Functional analysis of 5-HT1crust in combination with this immunocytochemical map will aid further understanding of this receptor's role in the actions of serotonin on identified circuits and cells.

The peripheral and central antennular pathway of the Caribbean stomatopod crustacean Neogonodactylus oerstedii

Although stomatopod crustaceans use their chemical senses in many facets of behavior, little is known about their chemosensory neural pathways, especially in comparison to the better-studied decapod crustaceans. We examined the stomatopod Neogonodactylus oerstedii to determine organizational aspects of peripheral and central neural pathway of antennules, which is a major chemosensory organ. We describe the three flagella of the triramous antennule as the medial, dorsolateral, and ventrolateral flagella. The primary branch point is between the medial flagellum and lateral flagella, and the secondary branch point is at the junction of the dorsolateral and ventrolateral flagella. The antennule bears at least three types of setae, based on their external morphology. Simple setae are present only on the medial flagellum and ventrolateral flagellum, organized as a tuft of 10-15 setae on each flagellar annulus. Aesthetasc setae and asymmetric setae occur only on the distal annuli of the dorsolateral flagellum, with each annulus bearing a row of three aesthetascs and one asymmetric seta. DiI fills of the antennular nerve near the junction of the flagella show that sensory neurons in the antennular flagella project to two neuropils in the ipsilateral midbrain-the olfactory lobe (OL) and lateral antennular neuropil (LAN). The OL is glomerular and has rich serotonergic innervation, a characteristic of the OL in decapods. The LAN is bi-lobed and stratified as it is in decapods. However, the LAN of stomatopods differs from that of decapods in being relatively large and containing extensive serotonergic innervation. The median antennular neuropil of stomatopods has sparse serotonergic innervation, and it is more diffusely organized compared to decapods. No accessory lobes were found in N. oerstedii. Thus, the stomatopod antennular flagella have the same two, highly organized parallel pathways common to decapods-the OL pathway and the LAN pathway.

Why are olfactory systems of different animals so similar?

As we learn more about the neurobiology of olfaction, it is becoming increasingly clear that olfactory systems of animals in disparate phyla possess many striking features in common. Why? Do these features provide clues about the ways the nervous system processes olfactory information? This might be the case if these commonalities are convergent adaptations that serve similar functions, but similar features can be present in disparate animals for other reasons. For example, similar features may be present because of inheritance from a common ancestor (homology), may represent responses to similar constraints, or may be superficial or reflect strategies used by researchers studying the system. In this paper, I examine four examples of features of olfactory systems in members of different phyla: the presence of odorant binding proteins in the fluid overlying olfactory receptor neurons; the use of G protein-coupled receptors as odorant receptors; the use of a two-step pathway in the transduction of odorant signals; and the presence of glomerular neuropils in the first central target of the axons of olfactory receptor cells. I analyze data from nematodes, arthropods, molluscs, and vertebrates to investigate the phylogenetic distribution of these features, and to try to explain the presence of these features in disparate animals. Phylogenetic analyses indicate that these features are not homologous across phyla. Although these features are often interpreted as convergent adaptations, I find that alternative explanations are difficult to dismiss. In many cases, it seems that olfactory system features that are similar across phyla may reflect both responses to similar constraints and adaptations for similar tasks.

Presynaptic inhibition of olfactory receptor neurons in crustaceans

Presynaptic inhibition of transmitter release from primary sensory afferents is a common strategy for regulating sensory input to the arthropod central nervous system. In the olfactory system, presynaptic inhibition of olfactory receptor neurons has been long suspected, but until recently could not be demonstrated directly because of the difficulty in recording from the afferent nerve terminals. A preparation using the isolated but intact brain of the spiny lobster in combination with voltage-sensitive dye staining has allowed stimulus-evoked responses of olfactory receptor axons to be recorded selectively with optical imaging methods. This approach has provided the first direct physiological evidence for presynaptic inhibition of olfactory receptor neurons. As in other arthropod sensory systems, the cellular mechanism underlying presynaptic afferent inhibition appears to be a reduction of action potential amplitude in the axon terminal. In the spiny lobster, two inhibitory transmitters, GABA and histamine, can independently mediate presynaptic inhibition. GABA- and histaminergic interneurons in the lobster olfactory lobe (the target of olfactory receptor neurons) constitute dual, functionally distinct inhibitory pathways that are likely to play different roles in regulating primary olfactory input to the CNS. Presynaptic inhibition in the vertebrate olfactory system is also mediated by dual inhibitory pathways, but via a different cellular mechanism. Thus, it is possible that presynaptic inhibition of primary olfactory afferents evolved independently in vertebrates and invertebrates to fill a common, fundamental role in processing olfactory information.

An immunohistochemical study of structure and development of the nervous system in the brine shrimp Artemia salina Linnaeus, 1758 (Branchiopoda, Anostraca) with remarks on the evolution of the arthropod brain

Brain morphology is an important character in the discussion of arthropod relationships. While a large body of literature is available on the brains of Hexapoda and Malacostraca, the structure of the brain has been rarely studied in representatives of the Entomostraca. This account examines the morphology and development of the nervous system in the brine shrimp Artemia salina Linnaeus, 1758 (Crustacea, Branchiopoda, Anostraca) by classical histology and immunohistochemistry against synaptic proteins (synapsins), and the neurotransmitters serotonin and histamine. The results indicate that the shape of the developing larval brain in A. salina (a circumstomodeal ring of neuropil) closely resembles that in malacostracan embryos. Furthermore, the organization of the central complex as well as the tritocerebral innervation pattern of the labrum is homologous in this species and in Malacostraca. Nevertheless, differences exist in the layout of the deutocerebrum, specifically in the absence of olfactory glomeruli in A. salina while the glomerular organization of the olfactory lobe is a character in the ground pattern of Malacostraca. These findings are compared to the brain structure in other Euarthropoda and possible phylogenetic implications are discussed.

Serotonergic and octopaminergic systems in the squat lobster Munida quadrispina (Anomura, Galatheidae)

Immunocytochemical mapping of serotonergic and octopaminergic neurons in the central nervous system of the squat lobster Munida quadrispina reveal approximately 120 serotonin-immunoreactive cell bodies (distributed throughout the neuromeres except in abdominal ganglion 5) and 48 octopamine-immunoreactive cell bodies (in brain and thoracic neuromeres but none in the circumesophageal or abdominal ganglia). Immunopositive neuropils for both amines are distributed in multiple areas in each neuromere and overlap extensively. Serotonergic and octopaminergic neurons have extensive bilateral projections in abdominal ganglia, whereas the majority of projections in thoracic and subesophageal ganglia are unilateral (contralateral to soma). This difference correlates with typical differences between abdominal and thoracic motor system coordination. Processes of immunoreactive cells for both amines form extensive, peripheral, neurosecretory-like structures. Serotonin seems to be released peripherally in more segments, and from more nerves per segment, than octopamine. M. quadrispina has fewer serotonergic and octopaminergic immunoreactive cells, in particular, fewer segmentally repeated cells, than other species studied to date. Nevertheless, the general organization of the aminergic systems is similar, and several aminergic cells have locations and morphologies that strongly suggest homology with identified aminergic cells in other crustaceans. Among these are segmentally repeated neurons that, in M. quadrispina, form serotonin-immunopositive tubular structures in the thoracic hemiganglia innervating pereiopods 1-3 that are unlike anything reported previously for any species. Comparisons of immunocytochemical maps within one species and between species exhibiting different behaviors provide insights into possible sites of action, functional differences between, and evolution of biogenic aminergic systems.

Transient uptake of serotonin by newborn olfactory projection neurons

A life-long turnover of sensory and interneuronal populations has been documented in the olfactory pathways of both vertebrates and invertebrates, creating a situation where the axons of new afferent and interneuronal populations must insert into a highly specialized glomerular neuropil. A dense serotonergic innervation of the primary olfactory processing areas where these neurons synapse also is a consistent feature across species. Prior studies in lobsters have shown that serotonin promotes the branching of olfactory projection neurons. This paper presents evidence that serotonin also regulates the proliferation and survival of projection neurons in lobsters, and that the serotonergic effects are associated with a transient uptake of serotonin into newborn neurons.

Distribution of G-protein alpha subunits and neurotransmitter activation of g(alphai) and g(alphaq) in the brain of the lobster Homarus americanus

Immunocytochemistry using antisera specific for the G-protein alpha subunits G(alphai), G(alphaq), and G(alphas) revealed similar patterns of immunoreactivity in the lobster brain. Immunoreactivity was strongest in neuropil, especially the olfactory and accessory lobes, and was characterized by bundles of fine threads leading to dense concentrations of punctate staining in the glomeruli. This may reflect the concentration of G-protein alpha subunits at synapses. The major differences between the antisera were distinct patterns of staining intensity in subregions of glomeruli of the olfactory and accessory lobes. This result is potentially correlated with previous evidence that these subregions are neurochemically distinct. Neuronal cell bodies contained moderate levels of immunoreactivity at the plasma membrane and faint staining in the cytoplasm. The olfactory globular tract was moderately immunoreactive, but other fiber tracts were weakly immunoreactive. Immunoreactivity in the deutocerebral commissure consisted of small oval cell bodies and strands that formed a reticulated pattern, suggestive of glia. Photoaffinity labelling by using an analog of GTP demonstrated that histamine activated G(alphai) in brain homogenates. Further evidence of G-protein activation was obtained by showing that stimulation with a mixture of neuroactive substances increased the amount of phospholipase C-beta associated with membranes, G(alphaq), and G(beta). The lobster brain, especially in its neuropil regions, is richly endowed with neuromodulatory biochemical pathways involving G(alphai), G(alphaq), and G(alphas).

Lobster G-protein coupled receptor kinase that associates with membranes and G(beta) in response to odorants and neurotransmitters

A cDNA clone (lobGRK2) encoding a protein of 690 amino acids with significant similarity to the GRK2 subfamily of G-protein coupled receptor kinases was isolated. lobGRK2 was widely expressed as a 9-kb major transcript and a protein of 80 kDa. It was most abundant in the brain and the olfactory organ but was absent in the eye/eyestalk. Immunocytochemistry revealed lobGRK2 immunoreactivity in the outer dendritic segments of the olfactory receptor neurons, the site of olfactory transduction. LobGRK2 immunoreactivity was observed in most neuronal structures in the brain, although with varying intensity. It was strongest in neuropil, especially the olfactory and accessory lobes but was also detectable in neuronal cell bodies. Stimulation of brain homogenates with a mixture of neurotransmitters increased the association of lobGRK2 with membranes and with G(beta). Similarly, stimulation of olfactory dendrite homogenates with an odorant mixture caused lobGRK2 to associate with G(beta). These results support the conclusion that lobGRK2 responds to odorants and to neurotransmitters and may act to initiate desensitization by phosphorylating G-protein-coupled receptors in the olfactory organ and the brain, respectively.

From embryo to adult: persistent neurogenesis and apoptotic cell death shape the lobster deutocerebrum

Neuronal plasticity and synaptic remodeling play important roles during the development of the invertebrate nervous system. In addition, structural neuroplasticity as a result of long-term environmental changes, behavioral modifications, age, and experience have been demonstrated in the brains of sexually mature insects. In adult vertebrates, persistent neurogenesis is found in the granule cell layer of the mammalian hippocampus and the subventricular zone, as well as in the telencephalon of songbirds, indicating that persistent neurogenesis, which is presumably related to plasticity and learning, may be an integral part of the normal biology of the mature brain. In decapod crustaceans, persistent neurogenesis among olfactory projection neurons is a common principle that shapes the adult brain, indicating a remarkable degree of life-long structural plasticity. The present study closes a gap in our knowledge of this phenomenon by describing the continuous cell proliferation and gradual displacement of proliferation domains in the central olfactory pathway of the American lobster Homarus americanus from early embryonic through larval and juvenile stages into adult life. Neurogenesis in the deutocerebrum was examined by the in vivo incorporation of bromodeoxyuridine, and development and structural maturation of the deutocerebral neuropils were studied using immunohistochemistry against Drosophila synapsin. The role of apoptotic cell death in shaping the developing deutocerebrum was studied using the terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling method, combined with immunolabeling using an antiphospho histone H3 mitosis marker. Our results indicate that, in juvenile and adult lobsters, birth and death of olfactory interneurons occur in parallel, suggesting a turnover of these cells. When the persistent neurogenesis and concurrent death of interneurons in the central olfactory pathway of the crustacean brain are taken into account with the life-long turnover of olfactory receptor cells in crustacean antennules, a new, highly dynamic picture of olfaction in crustaceans emerges.

Distribution and functional anatomy of amine-containing neurons in decapod crustaceans

One of the lessons learned from studying the nervous systems of phylogenetically distant species is that many features are conserved. Indeed, aminergic neurons in invertebrate and vertebrate systems share a multitude of common characteristics. In this review, the varied roles of serotonin, octopamine, dopamine, and histamine in decapod crustaceans are considered, and the distributions of the amine-containing cells are described. The anatomy of these systems reinforces the idea that amine neurons are involved in widespread modulation and coordination within the nervous system. Many aminergic neurons have long projections, linking multiple regions with a common input, and therefore are anatomically perfected as "gain setters." The developmental patterns of appearance of each amine in the crustacean nervous system are described and compared. The developmental picture suggests that transmitter acquisition is distinctive for each amine, and that the pace of acquisition may be co-regulated with target maturation. The distinctive roles that transmitters play during specific developmental periods may, ultimately, provide important clues to their functional contributions in the mature organism.

Tachykinin-related peptides in invertebrates: a review

Peptides with sequence similarities to members of the tachykinin family have been identified in a number of invertebrates belonging to the mollusca, echiuridea, insecta and crustacea. These peptides have been designated tachykinin-related peptides (TRPs) and are characterized by the preserved C-terminal pentapeptide FX1GX2Ramide (X1 and X2 are variable residues). All invertebrate TRPs are myostimulatory on insect hindgut muscle, but also have a variety of additional actions: they can induce contractions in cockroach foregut and oviduct and in moth heart muscle, trigger a motor rhythm in the crab stomatogastric ganglion, depolarize or hyperpolarize identified interneurons of locust and the snail Helix and induce release of adipokinetic hormone from the locust corpora cardiaca. Two putative TRP receptors have been cloned from Drosophila; both are G-protein coupled and expressed in the nervous system. The invertebrate TRPs are distributed in interneurons of the CNS of Limulus, crustaceans and insects. In the latter two groups TRPs are also present in the stomatogastric nervous system and in insects endocrine cells of the midgut display TRP-immunoreactivity. In arthropods the distribution of TRPs in neuronal processes of the brain displays similar patterns. Also in coelenterates, flatworms and molluscs TRPs have been demonstrated in neurons. The activity of different TRPs has been explored in several assays and it appears that an amidated C-terminal hexapeptide (or longer) is required for bioactivity. In many invertebrate assays the first generation substance P antagonist spantide I is a potent antagonist of invertebrate TRPs and substance P. Locustatachykinins stimulate adenylate cyclase in locust interneurons and glandular cells of the corpora cardiaca, but in other tissues the putative second messenger systems have not yet been identified. The heterologously expressed Drosophila TRP receptors coupled to the phospholipase C pathway and could induce elevations of inositol triphosphate. The structures, distributions and actions of TRPs in various invertebrates are compared and it is concluded that the TRPs are multifunctional peptides with targets both in the central and peripheral nervous system and other tissues, similar to vertebrate tachykinins. Invertebrate TRPs may also be involved in developmental processes.

The NO/cGMP pathway and the development of neural networks in postembryonic lobsters

The nitric oxide/cyclic 3',5'-guanosine monophosphate (NO/cGMP) signaling pathway has been implicated in certain forms of developmental and adult neuronal plasticity. Here we use whole-mount immunocytochemistry to identify components of this pathway in the nervous system of postembryonic lobsters as they develop through metamorphosis. We find that the synthetic enzyme for NO (nitric oxide synthase, or NOS) and the receptor for this transmitter (NO-sensitive soluble guanylate cyclase) are broadly distributed in the central nervous system (CNS) at hatching. In the brain, NOS immunoreactivity is intensified during glomerular development in the olfactory and accessory lobes. Whereas only a few neurons express NOS in the CNS, many more neurons synthesize cGMP in the presence of NO. NO-sensitive guanylate cyclase activity is a stable feature of some cells, while in others it is regulated during development. In the stomatogastric nervous system, a subset of neurons become responsive to NO at metamorphosis, a time when larval networks are reorganized into adult motor circuits. cGMP accumulation was occasionally detected in the nucleus of many cells in the CNS, which suggests that cGMP may have a role in transcription. Based on these findings, we conclude that the NO/cGMP signaling pathway may participate in the development of the lobster nervous system. Furthermore, NO may serve as a modulatory neurotransmitter for diverse neurons throughout the CNS.

Serotonin depletion by 5,7-dihydroxytryptamine alters deutocerebral development in the lobster, Homarus americanus

The olfactory and accessory lobes constitute prominent histological structures within the larval and mature lobster deutocerebrum, and both are associated with a dense innervation from paired serotonergic nerve cells, the dorsal giant neurons (DGNs). During development, the cell bodies of the DGNs are the first central somata to express serotonin (5-HT), and the onset of their 5-HT immunoreactivity coincides with the beginning of accessory lobe formation. In contrast, the olfactory lobe anlagen emerge much earlier and grow in the apparent absence of serotonin. The role of serotonergic input for the development of these brain structures was investigated in lobster embryos after serotonin had been depleted pharmacologically with the neurotoxin 5,7-dihydroxytryptamine. A approximately 90% reduction of serotonin was confirmed in eggs using high-performance liquid chromatography with electrochemical detection. Morphometric analyses suggested that serotonin depletion dramatically slowed the growth of olfactory and accessory lobes, although glomeruli differentiated at the normal time in both areas. The toxin exhibited a high degree of specificity for serotonergic neurons and associated target regions, and serotonin depletion persisted for at least 2 months following treatment. The goal of future experiments is to determine which of the cell types that innervate the olfactory and accessory lobes are affected by toxin treatment, thereby resulting in the retarded growth of these areas.