Descending neurons with dopamine-like or with substance P/FMRFamide-like immunoreactivity target the somata of olfactory interneurons in the brain of the spiny lobster, Panulirus argus

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.

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.

Relative Quantitation of Neuropeptides at Multiple Developmental Stages of the American Lobster Using N, N-Dimethyl Leucine Isobaric Tandem Mass Tags

Neuromodulators and neurotransmitters play important roles in neural network development. The quantitative changes of these signaling molecules often reflect their regulatory roles in physiological processes. Currently, several commercial tags (e.g., iTRAQ and TMT) have been widely used in proteomics. With reduced cost and higher labeling efficiency, we employed a set of custom-developed N, N-dimethyl leucine (DiLeu) 4-plex isobaric tandem mass tags as an attractive alternative for the relative quantitation of neuropeptides in brain tissue of American lobster Homarus americanus at multiple developmental stages. A general workflow for isobaric labeling of neuropeptides followed by LC-MS/MS analysis has been developed, including optimized sample handling procedures. Overall, we were able to quantify 18 trace-amount neuropeptides from 6 different families using a single adult brain as a control. The quantitation results indicated that the expressions of different neuropeptide families had significant changes over distinct developmental stages. Additionally, our data revealed intriguing elevated expression of neuropeptides in the early juvenile development stage. The methodology presented here advanced the workflow of DiLeu as an alternative labeling approach and the application of DiLeu-based quantitative peptidomics, which can be extended to areas beyond neuroscience.

Cytoarchitecture and ultrastructure of neural stem cell niches and neurogenic complexes maintaining adult neurogenesis in the olfactory midbrain of spiny lobsters, Panulirus argus

New interneurons are continuously generated in small proliferation zones within neuronal somata clusters in the olfactory deutocerebrum of adult decapod crustaceans. Each proliferation zone is connected to a clump of cells containing one neural stem cell (i.e., adult neuroblast), thus forming a "neurogenic complex." Here we provide a detailed analysis of the cytoarchitecture of neurogenic complexes in adult spiny lobsters, Panulirus argus, based on transmission electron microscopy and labeling with cell-type-selective markers. The clump of cells is composed of unique bipolar clump-forming cells that collectively completely envelop the adult neuroblast and are themselves ensheathed by a layer of processes of multipolar cell body glia. An arteriole is attached to the clump of cells, but dye perfusion experiments show that hemolymph has no access to the interior of the clump of cells. Thus, the clump of cells fulfills morphological criteria of a protective stem cell niche, with clump-forming cells constituting the adult neuroblast's microenvironment together with the cell body glia processes separating it from other tissue components. Bromodeoxyuridine pulse-chase experiments with short survival times suggest that adult neuroblasts are not quiescent but rather cycle actively during daytime. We propose a cell lineage model in which an asymmetrically dividing adult neuroblast repopulates the pool of neuronal progenitor cells in the associated proliferation zone. In conclusion, as in mammalian brains, adult neurogenesis in crustacean brains is fueled by neural stem cells that are maintained by stem cell niches that preserve elements of the embryonic microenvironment and contain glial and vascular elements.

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.

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.

Tachykinin-related peptide and GABA-mediated presynaptic inhibition of crayfish photoreceptors

Off-axis illumination elicits lateral inhibition at the primary visual synapse in crustacea and insects. The evidence suggests that the inhibitory action is presynaptic (i.e., on the photoreceptor terminal) and that the amacrine neurons of the lamina ganglionaris (the first synaptic layer) may be part of the inhibitory pathway. The neurotransmitters and the synaptic mechanisms are unknown. We show by immunocytochemistry that GABA and a tachykinin-related peptide (TRP) are localized in the amacrine neurons of the crayfish lamina ganglionaris. Indirect evidence suggests that GABA and TRP may be colocalized in these neurons. The extensive processes of the amacrine neurons occupy lamina layers containing the terminals of photoreceptors. Application of exogenous GABA and TRP to photoreceptor terminals produces a short-latency, dose-dependent hyperpolarization with a decay time constant on the order of a few seconds. TRP also exhibits actions that evolve over several minutes. These include a reduction of the receptor potential (and the light-elicited current) by approximately 40% and potentiation of the action of GABA by approximately 100%. The mechanisms of TRP action in crayfish are not known, but a plausible pathway is a TRP-dependent elevation of intracellular Ca(2+) that reduces photoreceptor sensitivity in arthropods. Although the mechanisms are not established, the results indicate that in crayfish photoreceptors TRP displays actions on two time scales and can exert profound modulatory control over cell function.

Different proctolin neurons elicit distinct motor patterns from a multifunctional neuronal network

Distinct motor patterns are selected from a multifunctional neuronal network by activation of different modulatory projection neurons. Subsets of these projection neurons can contain the same neuromodulator(s), yet little is known about the relative influence of such neurons on network activity. We have addressed this issue in the stomatogastric nervous system of the crab Cancer borealis. Within this system, there is a neuronal network in the stomatogastric ganglion (STG) that produces many versions of the pyloric and gastric mill rhythms. These different rhythms result from activation of different projection neurons that innervate the STG from neighboring ganglia and modulate STG network activity. Three pairs of these projection neurons contain the neuropeptide proctolin. These include the previously identified modulatory proctolin neuron and modulatory commissural neuron 1 (MCN1) and the newly identified modulatory commissural neuron 7 (MCN7). We document here that each of these neurons contains a unique complement of cotransmitters and that each of these neurons elicits a distinct version of the pyloric motor pattern. Moreover, only one of them (MCN1) also elicits a gastric mill rhythm. The MCN7-elicited pyloric rhythm includes a pivotal switch by one STG network neuron from playing a minor to a major role in motor pattern generation. Therefore, modulatory neurons that share a peptide transmitter can elicit distinct motor patterns from a common target network.

Descending neurons with dopamine-like or with substance P/FMRFamide-like immunoreactivity target the somata of olfactory interneurons in the brain of the spiny lobster, Panulirus argus

Two sets of descending neurons primarily target the somata of neurons in the olfactory deutocerebrum of the spiny lobster, Panulirus argus. Hundreds to thousands of dopamine-like immunoreactive fibers originate in the lateral protocerebrum and terminate among the clustered somata of the olfactory deutocerebrum projection neurons (lateral soma cluster) and those of the olfactory deutocerebrum local interneurons (medial soma cluster). A pair of giant neurons with substance P- and FMRFamide-like immunoreactivity from the median protocerebrum terminate primarily in the lateral soma cluster, but also branch in the core of the olfactory lobe itself. Neurons of both types terminate in numerous bouton-like swellings. The terminals in the lateral cluster at least contain numerous, large, dense-core and small, clear vesicles. The terminals contact the somata and the primary neurites through both traditional chemical synapses and large zones of direct membrane appositions. In most instances, a vesicle-containing profile forms a triadic arrangement with a neurite and a soma the latter being frequently connected via large gap-junction-like structures. Rosette-like arrangements formed by a vesicle-containing profile surrounded by up to eight neurites are also common. Dissociated lateral cluster somata support both fast inward and sustained outward voltage-activated currents. Substance P, but not dopamine or FMRFamide-related peptides, alters the fast inward current. The somata of the olfactory projection neurons, and possibly those of the olfactory local interneurons, appear to serve an integrative, and not merely a supportive role in these invertebrate central neurons.