Histamine immunoreactivity in the central nervous system of the spider Cupiennius salei

Sequence analysis, homology modeling, tissue expression, and potential functions of seven putative acetylcholinesterases in the spider Cupiennius salei

Acetylcholine esterases (AChEs) are essential enzymes in cholinergic synapses, terminating neurotransmission by hydrolysing acetylcholine. While membrane bound AChEs at synaptic clefts efficiently perform this task, soluble AChEs are less stable and effective, but function over broader areas. In vertebrates, a single gene produces alternatively spliced forms of AChE, whereas invertebrates often have multiple genes, producing both enzyme types. Despite their significance as pesticide targets, the physiological roles of invertebrate AChEs remain unclear. Here, we characterized seven putative AChEs in the wandering spider, Cupiennius salei, a model species for neurophysiological studies. Sequence analyses and homology modeling predicted CsAChE7 as the sole stable, membrane-bound enzyme functioning at synaptic clefts, while the others are likely soluble enzymes. In situ hybridization of sections from the spider's nervous system revealed CsAChE7 transcripts co-localizing with choline acetyltransferase in cells that also exhibited AChE activity. CsAChE7 transcripts were also found in rapidly adapting mechanosensory neurons, suggesting a role in precise and transient activation of postsynaptic cells, contrasting with slowly adapting, also cholinergic, neurons expressing only soluble AChEs, which allow prolonged activation of postsynaptic cells. These findings suggest that cholinergic transmission is influenced not only by postsynaptic receptors but also by the enzymatic properties regulating acetylcholine clearance. We also show that acetylcholine is a crucial neurotransmitter in the spider's visual system and sensory and motor pathways, but absent in excitatory motor neurons at neuromuscular junctions, consistent with other arthropods. Our findings on sequence structures may have implications for the development of neurological drugs and pesticides.

A three-dimensional immunofluorescence atlas of the brain of the hackled-orb weaver spider, Uloborus diversus

Spider orb-web building is a captivating, rare example of animal construction, whose neural underpinnings remain undiscovered. An essential step in understanding the basis of this behavior is a foundational mapping of the spider's neuroanatomy, which has thus far been primarily studied using non-web building species. We created a three-dimensional atlas for the hackled orb-weaver, Uloborus diversus, based on immunostaining for the presynaptic component, synapsin, in whole-mounted spider synganglia. Aligned to this volume, we examined the expression patterns of neuronal populations representing many of the classical neurotransmitter and neuromodulators, as well as a subset of neuropeptides - detailing immunoreactivity in an unbiased fashion throughout the synganglion, revealing co-expression in known structures, as well as novel neuropils not evident in prior spider works. This optically-sliced, whole-mount atlas is the first of its kind for spiders, representing a substantive addition to knowledge of brain anatomy and neurotransmitter expression patterns for an orb-weaving species.

Comparative neuroanatomy of the central nervous system in web-building and cursorial hunting spiders

Spiders (Araneae) include cursorial species that stalk their prey and more stationary species that use webs for prey capture. While many cursorial hunting spiders rely on visual cues, web-building spiders use vibratory cues (mechanosensation) for prey capture. We predicted that the differences in primary sensory input between the species are mirrored by differences in the morphology/architecture of the central nervous system (CNS). Here, we investigated the CNS anatomy of four spider species, two cursorial hunters Pardosa amentata (Lycosidae) and Marpissa muscosa (Salticidae), and two web-building hunters Argiope bruennichi (Araneidae) and Parasteatoda tepidariorum (Theridiidae). Their CNS was analyzed using Bodian silver impregnations, immunohistochemistry, and microCT analysis. We found that there are major differences between species in the secondary eye pathway of the brain that pertain to first-order, second-order, and higher order brain centers (mushroom bodies [MB]). While P. amentata and M. muscosa have prominent visual neuropils and MB, these are much reduced in the two web-building species. Argiope bruennichi lacks second-order visual neuropils but has specialized photoreceptors that project into two distinct visual neuropils, and P. tepidariorum lacks MB, suggesting that motion vision might be absent in this species. Interestingly, the differences in the ventral nerve cord are much less pronounced, but the web-building spiders have proportionally larger leg neuropils than the cursorial spiders. Our findings suggest that the importance of visual information is much reduced in web-building spiders, compared to cursorial spiders, while processing of mechanosensory information requires the same major circuits in both web-building and cursorial hunting spiders.

The visual pathway in sea spiders (Pycnogonida) displays a simple serial layout with similarities to the median eye pathway in horseshoe crabs

BACKGROUND

Phylogenomic studies over the past two decades have consolidated the major branches of the arthropod tree of life. However, especially within the Chelicerata (spiders, scorpions, and kin), interrelationships of the constituent taxa remain controversial. While sea spiders (Pycnogonida) are firmly established as sister group of all other extant representatives (Euchelicerata), euchelicerate phylogeny itself is still contested. One key issue concerns the marine horseshoe crabs (Xiphosura), which recent studies recover either as sister group of terrestrial Arachnida or nested within the latter, with significant impact on postulated terrestrialization scenarios and long-standing paradigms of ancestral chelicerate traits. In potential support of a nested placement, previous neuroanatomical studies highlighted similarities in the visual pathway of xiphosurans and some arachnopulmonates (scorpions, whip scorpions, whip spiders). However, contradictory descriptions of the pycnogonid visual system hamper outgroup comparison and thus character polarization.

RESULTS

To advance the understanding of the pycnogonid brain and its sense organs with the aim of elucidating chelicerate visual system evolution, a wide range of families were studied using a combination of micro-computed X-ray tomography, histology, dye tracing, and immunolabeling of tubulin, the neuropil marker synapsin, and several neuroactive substances (including histamine, serotonin, tyrosine hydroxylase, and orcokinin). Contrary to previous descriptions, the visual system displays a serial layout with only one first-order visual neuropil connected to a bilayered arcuate body by catecholaminergic interneurons. Fluorescent dye tracing reveals a previously reported second visual neuropil as the target of axons from the lateral sense organ instead of the eyes.

CONCLUSIONS

Ground pattern reconstruction reveals remarkable neuroanatomical stasis in the pycnogonid visual system since the Ordovician or even earlier. Its conserved layout exhibits similarities to the median eye pathway in euchelicerates, especially in xiphosurans, with which pycnogonids share two median eye pairs that differentiate consecutively during development and target one visual neuropil upstream of the arcuate body. Given multiple losses of median and/or lateral eyes in chelicerates, and the tightly linked reduction of visual processing centers, interconnections between median and lateral visual neuropils in xiphosurans and arachnopulmonates are critically discussed, representing a plausible ancestral condition of taxa that have retained both eye types.

Visual pathways in the brain of the jumping spider Marpissa muscosa

Some animals have evolved task differentiation among their eyes. A particular example is spiders, where most species have eight eyes, of which two (the principal eyes) are used for object discrimination, whereas the other three pairs (secondary eyes) detect movement. In the ctenid spider Cupiennius salei, these two eye types correspond to two visual pathways in the brain. Each eye is associated with its own first- and second-order visual neuropil. The second-order neuropils of the principal eyes are connected to the arcuate body, whereas the second-order neuropils of the secondary eyes are linked to the mushroom body. We explored the principal- and secondary eye visual pathways of the jumping spider Marpissa muscosa, in which size and visual fields of the two eye types are considerably different. We found that the connectivity of the principal eye pathway is the same as in C. salei, while there are differences in the secondary eye pathways. In M. muscosa, all secondary eyes are connected to their own first-order visual neuropils. The first-order visual neuropils of the anterior lateral and posterior lateral eyes are connected with a second-order visual neuropil each and an additional shared one (L2). In the posterior median eyes, the axons of their first-order visual neuropils project directly to the arcuate body, suggesting that the posterior median eyes do not detect movement. The L2 might function as an upstream integration center enabling faster movement decisions.

Tyrosine hydroxylase immunolabeling reveals the distribution of catecholaminergic neurons in the central nervous systems of the spiders Hogna lenta (Araneae: Lycosidae) and Phidippus regius (Araneae: Salticidae)

With over 48,000 species currently described, spiders (Arthropoda: Chelicerata: Araneae) comprise one of the most diverse groups of animals on our planet, and exhibit an equally wide array of fascinating behaviors. Studies of central nervous systems (CNSs) in spiders, however, are relatively sparse, and no reports have yet characterized catecholaminergic (dopamine [DA]- or norepinephrine-synthesizing) neurons in any spider species. Because these neuromodulators are especially important for sensory and motor processing across animal taxa, we embarked on a study to identify catecholaminergic neurons in the CNS of the wolf spider Hogna lenta (Lycosidae) and the jumping spider Phidippus regius (Salticidae). These neurons were most effectively labeled with an antiserum raised against tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis. We found extensive catecholamine-rich neuronal fibers in the first- and second-order optic neuropils of the supraesophageal mass (brain), as well as in the arcuate body, a region of the brain thought to receive visual input and which may be involved in higher order sensorimotor integration. This structure likely shares evolutionary origins with the DA-enriched central complex of the Mandibulata. In the subesophageal mass, we detected an extensive filigree of TH-immunoreactive (TH-ir) arborizations in the appendage neuromeres, as well as three prominent plurisegmental fiber tracts. A vast abundance of TH-ir somata were located in the opisthosomal neuromeres, the largest of which appeared to project to the brain and decorate the appendage neuromeres. Our study underscores the important roles that the catecholamines likely play in modulating spider vision, higher order sensorimotor processing, and motor patterning.

Histaminergic interneurons in the ventral nerve cord: assessment of their value for Euarthropod phylogeny

Despite numerous approaches to the resolution of euarthropod phylogeny, mainly based on modern sequence information and traditional external morphology, the resulting hypotheses are often contradictory and leave many questions about euarthropod evolution unanswered. The comparison of developmental and structural aspects of the nervous system has shown to be a valuable contribution to the assessment of current phylogenetic hypotheses. One promising approach for the generation of new character sets is the morphology of transmitter systems and the discovery of individually identifiable neurons, which allow phylogenetic comparisons on the single cell level. In this context, the serotonin transmitter system has been investigated to a considerable degree. Studies to date have yielded important stimuli to our understanding of euarthropod relationships and the evolution of their nervous systems. However, data on other transmitter systems remain fragmented, and their value with respect to phylogenetic questions remains speculative. The biogenic amine histamine is a promising transmitter; a substantial amount of data has been reported in the literature and the homology of some histaminergic neurons has been suggested. Here, we present a comprehensive review of histaminergic neurons in the ventral nerve cord of Euarthropoda. Using immunocytochemical labeling of histamine combined with confocal laser-scanning microscopy, we investigated the transmitter system in phylogenetically relevant taxa, such as Zygentoma, Remipedia, Diplopoda, and Arachnida. By reconstructing ground patterns, we evaluated the significance of this specific character set for euarthropod phylogeny. With this approach, we identified a set of neurons, which can be considered homologous within the respective major taxon. In conclusion, the histaminergic system contains useful information for our understanding of euarthropod phylogeny, supporting the proposed clades Tetraconata and Mandibulata. Furthermore, this character set has considerable potential to help resolve relationships within the major clades at a deeper level of taxonomy, due to the considerable variability in neurite morphology.

Primary processing neuropils associated with the malleoli of camel spiders (Arachnida, Solifugae): a re-evaluation of axonal pathways

BACKGROUND

Arachnids possess highly specialized and unorthodox sense organs, such as the unique pectines of Scorpiones and the malleoli of Solifugae. While the external morphology, numbers, and shapes of sensory organs are widely used in taxonomic studies, little is known about the internal anatomy of these organs and their associated processing neuropils in the central nervous system. Camel spiders (Solifugae) possess pedipalps and first walking legs heavily endowed with sensory structures, as well as conspicuous malleoli located ventrally on the proximal fourth walking legs. Malleoli are fan-shaped organs that contain tens of thousands of presumptive chemoreceptor neurons, but mechanoreceptive structures are absent.

RESULTS

Here, we examine the organization of the synganglion based on microCT analysis, 3D reconstruction of serial paraffin sections, and backfill preparations to trace the malleolar pathway. The projection area of malleolar afferents is intriguingly located in the most anterior ventral nerve cord, located in between the pedipalpal neuromere hemispheres. However, malleolar axon bundles are separated by a thin soma layer that points to an anteriad projection of the fourth walking leg neuromere. A conspicuous projection neuron tract that may receive additional input from pedipalpal sensory organs connects the malleolar neuropil with the mushroom bodies in the protocerebrum.

CONCLUSION

Arthropod chemosensory appendages or organs and primary processing neuropils are typically located in the same segment, which also holds true in Solifugae, although the malleolar neuropil is partially shifted towards the pedipalpal neuromere. A comparison of the malleoli in Solifugae and the pectines in Scorpiones, and of their primary processing neuropils, reveals certain similarities, while striking differences are also evident. Similarities include the ventral arrangement of peg-shaped sensory structures on the respective segmental appendage, exposing dense arrays of chemoreceptive sensilla, and projections to a primary processing neuropil with glomerular subdivision. Differences are, e.g., the lack of mechanoreceptive afferents and an associated processing neuropil.

Distribution of FMRFamide-related peptides and co-localization with glutamate in Cupiennius salei, an invertebrate model system

FMRFamide-related proteins have been described in both vertebrate and invertebrate nervous systems and have been suggested to play important roles in a variety of physiological processes. One proposed function is the modulation of signal transduction in mechanosensory neurons and their associated behavioral pathways in the Central American wandering spider Cupiennius salei; however, little is known about the distribution and abundance of FMRFamide-related proteins (FaRPs) within this invertebrate system. We employ immunohistochemistry, Hoechst nuclear stain and confocal microscopy of serial sections to detect, characterize and quantify FMRFamide-like immunoreactive neurons throughout all ganglia of the spider brain and along leg muscle. Within the different ganglia, between 3.4 and 12.6% of neurons showed immunolabeling. Among the immunoreactive cells, weakly and strongly labeled neurons could be distinguished. Between 71.4 and 81.7% of labeled neurons showed weak labeling, with 18.3 to 28.6% displaying strong labeling intensity. Among the weakly labeled neurons were characteristic motor neurons that have previously been shown to express ɣ-aminobutyric acid or glutamate. Ultrastructural investigations of neuromuscular junctions revealed mixed presynaptic vesicle populations including large electron-dense vesicles characteristic of neuropeptides. Double labeling for glutamate and FaRPs indicated that a subpopulation of neurons may co-express both neuroactive compounds. Our findings suggest that FaRPs are expressed throughout all ganglia and that different neurons have different expression levels. We conclude that FaRPs are likely utilized as neuromodulators in roughly 8% of neurons in the spider nervous system and that the main transmitter in a subpopulation of these neurons is likely glutamate.

Multiple Biogenic Amine Receptor Types Modulate Spider, Cupiennius salei, Mechanosensory Neurons

The biogenic amines octopamine (OA), tyramine (TA), dopamine (DA), serotonin (5-HT), and histamine (HA) affect diverse physiological and behavioral processes in invertebrates, but recent findings indicate that an additional adrenergic system exists in at least some invertebrates. Transcriptome analysis has made it possible to identify biogenic amine receptor genes in a wide variety of species whose genomes have not yet been sequenced. This approach provides new sequences for research into the evolutionary history of biogenic amine receptors and allows them to be studied in experimentally accessible animal models. The Central American Wandering spider, Cupiennius salei, is an experimental model for neurophysiological, developmental and behavioral research. We identified ten different biogenic amine receptors in C. salei transcriptomes. Phylogenetic analysis indicated that, in addition to the typical receptors for OA, TA, DA, and 5-HT in protostome invertebrates, spiders also have α1- and α2-adrenergic receptors, but lack TAR2 receptors and one invertebrate specific DA receptor type. In situ hybridization revealed four types of biogenic amine receptors expressed in C. salei mechanosensory neurons. We used intracellular electrophysiological experiments and pharmacological tools to determine how each receptor type contributes to modulation of these neurons. We show that arachnids have similar groups of biogenic amine receptors to other protostome invertebrates, but they lack two clades. We also clarify that arachnids and many other invertebrates have both α1- and α2-adrenergic, likely OA receptors. Our results indicate that in addition to an OAβ-receptor that regulates rapid and large changes in sensitivity via a G_s-protein activating a cAMP mediated pathway, the C. salei mechanosensory neurons have a constitutively active TAR1 and/or α2-adrenergic receptor type that adjusts the baseline sensitivity to a level appropriate for the behavioral state of the animal by a G_q-protein that mobilizes Ca²⁺.

The distribution of cholinergic neurons and their co-localization with FMRFamide, in central and peripheral neurons of the spider Cupiennius salei

The spider Cupiennius salei is a well-established model for investigating information processing in arthropod sensory systems. Immunohistochemistry has shown that several neurotransmitters exist in the C. salei nervous system, including GABA, glutamate, histamine, octopamine and FMRFamide, while electrophysiology has found functional roles for some of these transmitters. There is also evidence that acetylcholine (ACh) is present in some C. salei neurons but information about the distribution of cholinergic neurons in spider nervous systems is limited. Here, we identify C. salei genes that encode enzymes essential for cholinergic transmission: choline ACh transferase (ChAT) and vesicular ACh transporter (VAChT). We used in-situ hybridization with an mRNA probe for C. salei ChAT gene to locate somata of cholinergic neurons in the central nervous system and immunohistochemistry with antisera against ChAT and VAChT to locate these proteins in cholinergic neurons. All three markers labeled similar, mostly small neurons, plus a few mid-sized neurons, in most ganglia. In the subesophageal ganglia, labeled neurons are putative efferent, motor or interneurons but the largest motor and interneurons were unlabeled. Groups of anti-ChAT labeled small neurons also connect the optic neuropils in the spider protocerebrum. Differences in individual cell labeling intensities were common, suggesting a range of ACh expression levels. Double-labeling found a subpopulation of anti-VAChT-labeled central and mechanosensory neurons that were also immunoreactive to antiserum against FMRFamide-like peptides. Our findings suggest that ACh is an important neurotransmitter in the C. salei central and peripheral nervous systems.

The synganglion of the jumping spider Marpissa muscosa (Arachnida: Salticidae): Insights from histology, immunohistochemistry and microCT analysis

Jumping spiders are known for their extraordinary cognitive abilities. The underlying nervous system structures, however, are largely unknown. Here, we explore and describe the anatomy of the brain in the jumping spider Marpissa muscosa (Clerck, 1757) by means of paraffin histology, X-ray microCT analysis and immunohistochemistry as well as three-dimensional reconstruction. In the prosoma, the CNS is a clearly demarcated mass that surrounds the esophagus. The anteriormost neuromere, the protocerebrum, comprises nine bilaterally paired neuropils, including the mushroom bodies and one unpaired midline neuropil, the arcuate body. Further ventrally, the synganglion comprises the cheliceral (deutocerebrum) and pedipalpal neuropils (tritocerebrum). Synapsin-immunoreactivity in all neuropils is generally strong, while allatostatin-immunoreactivity is mostly present in association with the arcuate body and the stomodeal bridge. The most prominent neuropils in the spider brain, the mushroom bodies and the arcuate body, were suggested to be higher integrating centers of the arthropod brain. The mushroom body in M. muscosa is connected to first and second order visual neuropils of the lateral eyes, and the arcuate body to the second order neuropils of the anterior median eyes (primary eyes) through a visual tract. The connection of both, visual neuropils and eyes and arcuate body, as well as their large size corroborates the hypothesis that these neuropils play an important role in cognition and locomotion control of jumping spiders. In addition, we show that the architecture of the brain of M. muscosa and some previously investigated salticids differs significantly from that of the wandering spider Cupiennius salei, especially with regard to structure and arrangement of visual neuropils and mushroom body. Thus, we need to explore the anatomical conformities and specificities of the brains of different spider taxa in order to understand evolutionary transformations of the arthropod brain.

The dynamics of signaling at the histaminergic photoreceptor synapse of arthropods

Histamine, a ubiquitous aminergic messenger throughout the body, also serves as a neurotransmitter in both vertebrates and invertebrates. In particular, the photoreceptors of adult arthropods use histamine, modulating its release to signal increases and decreases in light intensity. Strong evidence from various arthropod species indicates that histamine is synthesized and stored in photoreceptors, undergoes Ca-dependent release, inhibits postsynaptic interneurons by gating Cl channels, and is then recycled. In Drosophila, the synthetic enzyme, histidine decarboxylase, and the subunits of the histamine-gated chloride channel have been cloned. Possible histamine transporters at synaptic vesicles and for reuptake remain elusive. Indeed, the mechanisms that remove histamine from the synaptic cleft, and that help terminate histamine's action, are unexpectedly complex, their details remaining unresolved. A major pathway in Drosophila, and possibly other arthropod species, is by conjugation of histamine to beta-alanine to form carcinine in adjacent glia. This conjugate then returns to the photoreceptors where it is hydrolysed to liberate histamine, which is then loaded into synaptic vesicles. Evidence from other species suggests that direct reuptake of histamine into the photoreceptors may also occur. Light depolarizes the photoreceptors, causing histamine release and postsynaptic inhibition; dimming hyperpolarizes the photoreceptors, causing a decrease in histamine release and an "off" response in the postsynaptic cell. Further pursuit of histamine's action at these highly specialized synapses should lead to an understanding of how they signal minute changes in presynaptic membrane potential, how they reliably extract signals from noise, and how they adapt to a wide range of presynaptic membrane potentials.

The frontal eyes of crustaceans

Frontal eyes of crustaceans (previously called nauplius eye and frontal organs) are usually simple eyes that send their axons to a medial brain centre in the anterior margin of the protocerebrum. Investigations of a large number of recent species within all major groups of the Crustacea have disclosed four kinds of frontal eyes correlated with taxonomic groups and named after them as the malacostracan, ostracod-maxillopodan, anostracan, and phyllopodan frontal eyes. The different kinds of eyes have been established using the homology concept coined by Owen [Owen, R., 1843. Lectures on the comparative anatomy and physiology of the invertebrate animals. Longman, Brown, Green, Longmans, London] and the criteria for homology recommended by Remane [Remane, A., 1956. Die Grundlagen des natürlichen Systems, der vergleichenden Anatomie und der Phylogenetik. 2nd ed. Akademische Verlagsgesellschaft, Geest und Portig, Leipzig]. Common descent is not used as a homology criterion. Frontal eyes bear no resemblance to compound eyes and in the absence of compound eyes, as in the ostracod-maxillopodan group, frontal eyes develop into complicated mirror, lens-mirror, and scanning eyes. Developmental studies demonstrate widely different ways to produce frontal eyes in phyllopods and malacostracans. As a result of the studies of recent frontal eyes in crustaceans, it is concluded by extrapolation that in crustacean ancestors four non-homologous frontal eye types evolved that have remained functional in spite of concurrent compound eyes.

Cellular distributions and functions of histamine, octopamine, and serotonin in the peripheral visual system, brain, and circumesophageal ring of the horseshoe crab Limulus polyphemus

The data reviewed here show that histamine, octopamine, and serotonin are abundant in the visual system of the horseshoe crab Limulus polyphemus. Anatomical and biochemical evidence, including new biochemical data presented here, indicates that histamine is a neurotransmitter in primary retinal afferents, and that it may be involved in visual information processing within the lateral eye. The presence of histamine in neurons of the central nervous system outside of the visual centers suggests that this amine also has functions unrelated to vision. However, the physiological actions of histamine in the Limulus nervous system are not yet known. Octopamine is present in and released from the axons of neurons that transmit circadian information from the brain to the eyes, and octopamine mimics the actions of circadian input on many retinal functions. In addition, octopamine probably has major functions in other parts of the nervous system as octopamine immunoreactive processes are widely distributed in the central nervous system and in peripheral motor nerves. Indeed, octopamine modulates functions of the heart and exoskeletal muscles as well as the eyes. A surprising finding is that although octopamine is a circulating neurohormone in Limulus, there is no structural evidence for its release into the hemolymph from central sites. The distribution of serotonin in Limulus brain suggests this amine modulates the central processing of visual information. Serotonin modulates cholinergic synapses in the central nervous system, but nothing further is known about its physiological actions.

Distribution of histamine in the CNS of different spiders

Immunohistochemistry is used to demonstrate histamine-immunoreactivity in the CNS of spiders. We found histamine-immunoreactivity in the photoreceptors of different spiders. Therefore, we suggest that histamine is a neurotransmitter of photoreceptors in all arthropods, since it is also known to occur in the photoreceptors of the other main arthropod taxa (Merostomata, Crustacea, and Insecta). We also describe a system of only six omnisegmental histamine-immunoreactive neurons within the central nervous system. These histamine-immunoreactive neurons can be divided into two subgroups: a dorsal system with two cells per hemisphere and a ventral system with only one cell per hemisphere. All six cells have extended arborizations in both the motor and the sensory areas of all neuromeres in the suboesophageal ganglionic mass. In contrast to araneomorph spiders, two additional sets of histamine-immunoreactive neurons were detected in mygalomorph spiders. The first set consists of seventeen cells with their cell bodies located in the cheliceral ganglion and projecting to central areas of the protocerebrum. The second set contains many if not all sensory projections from the tarsal organs on all eight legs and the pedipalps to the Blumenthal neuropil.

Evolution and overview of classical transmitter molecules and their receptors

All the classical transmitter ligand molecules evolved at least 1000 million years ago. With the possible exception of the Porifera and coelenterates (Cnidaria), they occur in all the remaining phyla. All transmitters have evolved the ability to activate a range of ion channels, resulting in excitation, inhibition and biphasic or multiphasic responses. All transmitters can be synthesised in all three basic types of neurones, i.e. sensory, interneurone and motoneurone. However their relative importance as sensory, interneurone or motor transmitters varies widely between the phyla. It is likely that all neurones contain more than one type of releasable molecule, often a combination of a classical transmitter and a neuroactive peptide. Second messengers, i.e. G proteins and phospholipase C systems, appeared early in evolution and occur in all phyla that have been investigated. Although the evidence is incomplete, it is likely that all the classical transmitter receptor subtypes identified in mammals, also occur throughout the phyla. The invertebrate receptors so far cloned show some interesting homologies both between those from different invertebrate phyla and with mammalian receptors. This indicates that many of the basic receptor subtypes, including benzodiazepine subunits, evolved at an early period, probably at least 800 million years ago. Overall, the evidence stresses the similarity between the major phyla rather than their differences, supporting a common origin from primitive helminth stock.

Common general morphological pattern of peptidergic neurons in the arachnid brain: crustacean cardioactive peptide-immunoreactive neurons in the protocerebrum of seven arachnid species

A polyclonal antiserum raised against crustacean cardioactive peptide labels 14 clusters of immunoreactive neurons in the protocerebrum of the spiders Tegenaria atrica and Nephila clavipes, and the harvestman (opilionid) Rilaena triangularis. In all species, these clusters possess the same number of neurons, and share similar structural and topological characteristics. Two sets of bilateral symmetrical neurons associated with the optic lobes and the arachnid "central body" were analysed in detail, comparing the harvestman R. triangularis and the spiders Brachypelma albopilosa (Theraphosidae), Cupiennius salei (Lycosidae), Tegenaria atrica (Agelenidae), Meta segmentata (Metidae) and Nephila clavipes (Araneidae). Sixteen neurons have been identified that display markedly similar axonal pathways and arborization patterns in all species. These neurons are considered homologues in the opilionid and the araneid brains. We presume that these putative phylogenetically persisting neurons represent part of the general morphological pattern of the arachnid brain.