Environmentally relevant atrazine exposures cause DNA damage in cells of the lateral antennules of crayfish (Faxonius virilis)

The herbicide atrazine is heavily applied in agricultural areas in the Midwestern United States and can run-off and seep into surrounding aquatic habitats where concentrations can reach over 300 ppb. It is known that acute exposures to 80 ppb atrazine cause lasting deficiencies in the chemoreception of food and mate odors. Since atrazine impairs chemosensory responses, the goal of this study was to determine the effect of atrazine on cells, including olfactory sensory neurons, located in the lateral antennules of crayfish. In this experiment, we treated crayfish for 10 days with ecologically relevant concentrations of 0, 10, 40, 80, 100 and 300 ppb (μg L^-1) of atrazine. Following treatments, the distal portion of the lateral antennules was cryosectioned. We used a TdT mediated dUTP nick-end labeling (TUNEL) assay to determine if any cells had DNA damage and may be thus undergoing apoptosis. We found that as atrazine concentrations increase above 10 ppb, the number of TUNEL-positive cells, visualized in the lateral antennules, significantly increases. Our data show that atrazine exposure causes DNA damage in cells of the lateral antennules, including olfactory sensory neurons, thus leading to impairments in chemosensory abilities. Because crayfish rely heavily on chemoreception for survival, changes in their ability to perceive odors following atrazine exposure may have detrimental effects on population size.

Directed differentiation of granular cells from crayfish hematopoietic tissue cells

Hemocytes are the major immune cells of crustaceans. New hemocyte production is required throughout the life cycle of these animals to maintain a functional immune system. The mechanism of crustacean hematopoiesis has just begun to be understood and new methods are needed for the investigation of this process. Here we report the directed differentiation of granular cells (GCs) from the hematopoietic tissue (HPT) cells of Cherax quadricarinatus in vitro. We started by providing the cultured HPT cells with different additives to induce possible differentiation. We found that crayfish muscle extract greatly promoted the physical status of the cells and induced the formation of refractile cytoplasmic granules. The transcription of marker genes and the production of functional prophenoloxidase further confirmed the formation of mature GCs. In our experiments, young GCs usually started to develop in ∼2 weeks post induction and over 60% of the cells became mature within 3-4 weeks. This is the first time that the fully differentiation of crustacean hemocytes is accomplished in vitro. It provides a powerful tool for in-depth study of crustacean hematopoiesis.

Development of a primary culture system for haematopoietic tissue cells from Cherax quadricarinatus and an exploration of transfection methods

Various known and unknown viral diseases can threaten crustacean aquaculture. To develop prophylactic and therapeutic strategies against viruses, crustacean cell lines are urgently needed for immunology and virology studies. However, there are currently no permanent crustacean cell lines available. In this study, we developed a new method for preparing crayfish plasma (CP) and found that CP enhanced the proliferative capacity of haematopoietic tissue (hpt) cells from Cherax quadricarinatus by an EdU (5-ethynyl-2'-deoxyuridine) assay. The optimal CP concentration for hpt cell culture and the optimal subculture method are discussed. To achieve efficient expression of a foreign gene in hpt cells cultured in vitro, different transfection methods and vectors were analysed. We found that Lipofectamine 2000 could be used to efficiently transfect a foreign vector into hpt cells and exhibited a lower level of cytotoxicity than the other methods tested, and transfection of pEGFP-N1/w249 and pDHsp70-EGFP-FLAG resulted in high EGFP expression. By transmission electron microscopy (TEM) and virus copy number analysis, we found that white spot syndrome virus (WSSV) could infect hpt cells and multiply efficiently. Our results implied that the crayfish hpt cell culture system we improved could be used as a replacement for immortal crustacean cell lines in viral infection studies. Our findings provide a solid foundation for future immortalization and gene function studies in crustacean cells.

Transfection of crayfish hematopoietic tissue cells

Transfection is a powerful tool useful for studying gene function. Establishing transfection methods that enable highly efficient DNA uptake has become increasingly important. The crayfish hematopoietic tissue (Hpt) cell cultures have been proven to be suitable for studies on immunity and cell differentiation in crustaceans including shrimps, but no efficient gene transfer and expression method is available for these cells. Here we report a novel and highly efficient DNA transfection system based on electroporation. This method depends on a recombinant plasmid with the promoter from white spot syndrome virus immediate-early gene wsv249. This plasmid could be introduced into primary cells and efficiently express foreign genes by electroporation. By optimizing different electroporation parameters, more than 30% transfection efficiency could be achieved with the relative viability of cells around 50%. This is the first report of gene introduction to crayfish Hpt cells and will be useful for the expanding our research on crustacean immunity.

Different roles of crayfish hemocytes in the uptake of foreign particles

Crustacean hemocytes are known to remove invading pathogens by phagocytosis. In this study, we investigated how the semigranular cells (SGCs) and granular cells (GCs) of crayfish Cherax quadricarinatus participated in this process. By injecting the animals with excessive amounts of fluorescent microspheres (FMs), we showed that only a small portion of the circulating hemocytes were phagocytic cells, and they took up FMs in a size-dependent manner. The 0.2 μm FMs were internalized almost entirely by SGCs, while GCs and SGCs both contributed to the uptake of 2 μm FMs. Further analysis of the hemocytes from the animals injected with a mixture of FMs suggested that there were a subpopulation of SGCs specifically ingesting 0.2 μm FMs. The size-dependent manner was also applied to biological particles. Escherichia coli was internalized by both SGCs and GCs, whereas white spot syndrome virus (WSSV) was mostly ingested by SGCs. However, the bacterial cells were rapidly taken and cleared from the circulation by the hemocytes, while the WSSV virions were gradually internalized and remained in the cells for a relatively longer period of time. These findings provide basic information of the phagocytic hemocytes of crayfish and how they respond to different foreign particles.

Photodynamic therapy-induced nitric oxide production in neuronal and glial cells

Nitric oxide (NO) has been recently demonstrated to enhance apoptosis of glial cells induced by photodynamic therapy (PDT), but to protect glial cells from PDT-induced necrosis in the crayfish stretch receptor, a simple neuroglial preparation that consists of a single mechanosensory neuron enveloped by satellite glial cells. We used the NO-sensitive fluorescent probe 4,5-diaminofluorescein diacetate to study the distribution and dynamics of PDT-induced NO production in the mechanosensory neuron and surrounding glial cells. The NO production in the glial envelope was higher than in the neuronal soma axon and dendrites both in control and in experimental conditions. In dark NO generator, DEA NONOate or NO synthase substrate L-arginine hydrochloride significantly increased the NO level in glial cells, whereas NO scavenger 2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) or inhibitors of NO synthase L-NG-nitro arginine methyl ester and N?-nitro-L-arginine decreased it. PDT induced the transient increase in NO production with a maximum at 4 to 7 min after the irradiation start followed by its inhibition at 10 to 40 min. We suggested that PDT stimulated neuronal rather than inducible NO synthase isoform in glial cells, and the produced NO could mediate PDT-induced apoptosis.

Electrophysiological Evidence for Intrinsic Pacemaker Currents in Crayfish Parasol Cells

I used sharp intracellular electrodes to record from parasol cells in the semi-isolated crayfish brain to investigate pacemaker currents. Evidence for the presence of the hyperpolarization-activated inward rectifier potassium current was obtained in about half of the parasol cells examined, where strong, prolonged hyperpolarizing currents generated a slowly-rising voltage sag, and a post-hyperpolarization rebound. The amplitudes of both the sag voltage and the depolarizing rebound were dependent upon the strength of the hyperpolarizing current. The voltage sag showed a definite threshold and was non-inactivating. The voltage sag and rebound depolarization evoked by hyperpolarization were blocked by the presence of 5-10 mM Cs2+ ions, 10 mM tetraethyl ammonium chloride, and 10 mM cobalt chloride in the bathing medium, but not by the drug ZD 7288. Cs+ ions in normal saline in some cells caused a slight increase in mean resting potential and a reduction in spontaneous burst frequency. Many of the neurons expressing the hyperpolarization-activated inward potassium current also provided evidence for the presence of the transient potassium current IA, which was inferred from experimental observations of an increased latency of post-hyperpolarization response to a depolarizing step, compared to the response latency to the depolarization alone. The latency increase was reduced in the presence of 4-aminopyridine (4-AP), a specific blocker of IA. The presence of 4-AP in normal saline also induced spontaneous bursting in parasol cells. It is conjectured that, under normal physiological conditions, these two potassium currents help to regulate burst generation in parasol cells, respectively, by helping to maintain the resting membrane potential near a threshold level for burst generation, and by regulating the rate of rise of membrane depolarizing events leading to burst generation. The presence of post-burst hyperpolarization may depend upon IA channels in parasol cells.

On involvement of transcription factors nuclear factor kappa-light-chain-enhancer of activated B cells, activator protein-1 and signal transducer and activator of transcription-3 in photodynamic therapy-induced death of crayfish neurons and satellite glial cells

Photodynamic therapy (PDT) is currently used in the treatment of brain tumors. However, not only malignant cells but also neighboring normal neurons and glial cells are damaged during PDT. In order to study the potential role of transcription factors-nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), activator protein (AP-1), and signal transducer and activator of transcription-3 (STAT-3)-in photodynamic injury of normal neurons and glia, we photosensitized the isolated crayfish mechanoreceptor consisting of a single sensory neuron enveloped by glial cells. Application of different inhibitors and activators showed that transcription factors NF-κB (inhibitors caffeic acid phenethyl ester and parthenolide, activator betulinic acid), AP-1 (inhibitor SR11302), and STAT-3 (inhibitors stattic and cucurbitacine) influenced PDT-induced death and survival of neurons and glial cells in different ways. These experiments indicated involvement of NF-κB in PDT-induced necrosis of neurons and apoptosis of glial cells. However, in glial cells, it played the antinecrotic role. AP-1 was not involved in PDT-induced necrosis of neurons and glia, but mediated glial apoptosis. STAT-3 was involved in PDT-induced apoptosis of glial cells and necrosis of neurons and glia. Therefore, signaling pathways that regulate cell death and survival in neurons and glial cells are different. Using various inhibitors or activators of transcription factors, one can differently influence the sensitivity and resistance of neurons and glial cells to PDT.

Granulocytes of the red claw crayfish Cherax quadricarinatus can endocytose beads, E. coli and WSSV, but in different ways

The hemocytes of the red claw crayfish Cherax quadricarinatus are classified by morphologic observation into the following types: hyalinocytes (H), semi-granulocytes (SG) and granulocytes (G). Density gradient centrifugation with Percoll was developed to separate these three subpopulations of hemocytes. Beads, Escherichia coli, and FITC labeling WSSV were used to investigate the characteristics of granulocytes by using scanning electron microscope, transmission electron microscope, and laser scan confocal microscope. Results showed that granulocytes could phagocytose beads and E. coli by endocytic pathways. WSSV could rely on caveolae-mediated endocytosis to mainly enter into granulocytes. These results could elucidate the mechanism of the innate immunity function of granulocytes, and it also showed the mechanism by which WSSV invaded granulocytes in the red claw crayfish.

Putative pacemakers in the eyestalk and brain of the crayfish Procambarus clarkii show circadian oscillations in levels of mRNA for crustacean hyperglycemic hormone

Crustacean hyperglycemic hormone (CHH) synthesizing cells in the optic lobe, one of the pacemakers of the circadian system, have been shown to be present in crayfish. However, the presence of CHH in the central brain, another putative pacemaker of the multi-oscillatory circadian system, of this decapod and its circadian transcription in the optic lobe and brain have yet to be explored. Therefore, using qualitative and quantitative PCR, we isolated and cloned a CHH mRNA fragment from two putative pacemakers of the multi-oscillatory circadian system of Procambarus clarkii, the optic lobe and the central brain. This CHH transcript synchronized to daily light-dark cycles and oscillated under dark, constant conditions demonstrating statistically significant daily and circadian rhythms in both structures. Furthermore, to investigate the presence of the peptide in the central brain of this decapod, we used immunohistochemical methods. Confocal microscopy revealed the presence of CHH-IR in fibers and cells of the protocerebral and tritocerebal clusters and neuropiles, particularly in some neurons located in clusters 6, 14, 15 and 17. The presence of CHH positive neurons in structures of P. clarkii where clock proteins have been reported suggests a relationship between the circadian clockwork and CHH. This work provides new insights into the circadian regulation of CHH, a pleiotropic hormone that regulates many physiological processes such as glucose metabolism and osmoregulatory responses to stress.

A developmental study of serotonin-immunoreactive neurons in the embryonic brain of the marbled crayfish and the migratory locust: evidence for a homologous protocerebral group of neurons

It is well established that the brains of adult malacostracan crustaceans and winged insects display distinct homologies down to the level of single neuropils such as the central complex and the optic neuropils. We wanted to know if developing insect and crustacean brains also share similarities and therefore have explored how neurotransmitter systems arise during arthropod embryogenesis. Previously, Sintoni et al. (2007) had already reported a homology of an individually identified cluster of neurons in the embryonic crayfish and insect brain, the secondary head spot cells that express the Engrailed protein. In the present study, we have documented the ontogeny of the serotonergic system in embryonic brains of the Marbled Crayfish in comparison to Migratory Locust embryos using immunohistochemical methods combined with confocal laser-scan microscopy. In both species, we found a cluster of early emerging serotonin-immunoreactive neurons in the protocerebrum with neurites that cross to the contralateral brain hemisphere in a characteristic commissure suggesting a homology of this cell cluster. Our study is a first step towards a phylogenetic analysis of neurotransmitter system development and shows that, as for the ventral nerve cord, traits related to neurogenesis in the brain can provide valuable hints for resolving the much debated question of arthropod phylogeny.

Characterization of two isoforms of antiliopolysacchride factors (Sp-ALFs) from the mud crab Scylla paramamosain

In the previous study of the mud crab (Scylla paramamosain) hemocyte proteins, which interacted with a bacterium, Vibrio parahaemolyticus, a protein known as antilipopolysaccharide factor (Sp-ALF) was isolated in addition to a serine proteinase homolog (Sp-SPH) protein. In the present study, we further reported the characterization of two isoforms of the mud crab ALF - Sp-ALFs genes (designated as Sp-ALF1 and Sp-ALF2, respectively) based on our previous result. The Sp-ALF1 and Sp-ALF2 cDNA contained 1070 bp and 731 bp, respectively, with 123 deduced amino acid residues. Alignment of deduced amino acid sequences showed that Sp-ALFs possessed high identity with other known ALFs from crustaceans and exhibited an overall similarity of 57.7% to those of ALFs compared. Phylogenetic tree analysis revealed a clear group of each species and also suggested that ALFs from Scylla genus and those from Portunus genus were closely related. Tissue distribution analysis in adult crab implied that both Sp-ALF1 and Sp-ALF2 were mainly expressed in hemocytes. The mRNA transcripts were also found in embryo (I, II, III and V), zoea-I and juvenile crab, but were rarely observed in the megalopa stage. To further identify the biological activity of Sp-ALFs, recombinant proteins (rSp-ALFs: designated as rSp-ALF1 and rSp-ALF2, respectively) were obtained by expression in Pichia pastris, and the synthetic peptide fragments (sSp-ALFs: designated as sSp-ALF1 and sSp-ALF2, respectively) including the putative LPS binding loop were also prepared for antimicrobial test. The results indicated that both rSp-ALFs and sSp-ALFs were highly effective against most of the Gram-positive bacteria and Gram-negative bacteria tested. In contrast to cecropin P1, a membrane integrity assay revealed that Sp-ALFs did not affect the Escherichia coli by disruption of membrane integrity. Additionally, the recombinant Sp-ALFs proteins exhibited strong antiviral activity against an important aquaculture pathogen, white spot syndrome virus, in crustaceans. Taken together, these data suggested that Sp-ALFs might play a key role in immune defense against microbial infection in the mud crab S. paramamosain.

[Regulation of pyloric rhythm by I(A) and I(h) in crayfish stomatogastric ganglion]

The stomatogastric ganglion (STG) of shellfish includes 30 neurons and produces pyloric rhythms. It is the common model to study central pattern generator (CPG). Regulation of pyloric rhythms not only is related to the property of single neurons in STG but also depends on the connections and property of the whole neuronal network. It has been found that transient potassium current (I(A)) and hyperpolarization-activated cation current (I(h)) exist in certain types of neurons of STG. However, roles played by these two currents in maintaining and regulating the pyloric rhythms are unknown. In the present study, in vitro electrophysiological recordings were performed on crayfish STG to examine the role played by I(A) and I(h) in regulation of pyloric rhythm. 4AP (2 mmol/L), a specific inhibitor of I(A), caused a decrease in pyloric cycle (P < 0.01), an increase in PD (pyloric dilator) ratio, a decrease in PY (pyloric) ratio (P < 0.01) and delay of phases of LP and PY firing. ZD7288 (100 μmol/L), a specific inhibitor of I(h), caused a decrease in pyloric cycle (P < 0.01), an increase in PD ratio (P < 0.01), an increase in LP (lateral pyloric) ratio (P < 0.01), a decrease in PY ratio (P < 0.01) and delay of phases of LP and PY firing. These results indicate that I(A) and I(h) play important roles in regulating pyloric rhythms in crayfish STG.

Environmental enrichment influences neuronal stem cells in the adult crayfish brain

New neurons are incorporated throughout life into the brains of many vertebrate and non-vertebrate species. This process of adult neurogenesis is regulated by a variety of external and endogenous factors, including environmental enrichment, which increases the production of neurons in juvenile mice and crayfish. The primary goal of the present study was to exploit the spatial separation of the neuronal precursor cell lineage in crayfish to determine which generation(s) of precursors is altered by environmental conditions. Further, in crayfish, an intimate relationship between the 1st generation neuronal precursors (stem cells) and cells circulating in the hemolymph has been proposed (Zhang et al., 2009 ). Therefore, a second goal was to assess whether environmental enrichment alters the numbers or types of cells circulating in the hemolymph. We find that neurogenesis in the brains of sexually differentiated procambarid crayfish is enhanced by environmental enrichment as previously demonstrated by Sandeman and Sandeman (2000) in young, sexually undifferentiated Cherax destructor . We also show that environmental enrichment increases the cell cycle rate of neuronal stem cells. While there was no effect of environment on the overall numbers of cells circulating in the hemolymph, enrichment resulted in increased expression of glutamine synthetase, a marker of the neuronal stem cells, in a small percentage of circulating cells; there was little or no expression of this enzyme in hemolymph cells extracted from deprived animals. Thus, environmental enrichment influences the rate of neuronal stem cell division in adult crayfish, as well as the composition of cells circulating in the hemolymph.

Flow cytometric characterization of freshwater crayfish hemocytes for the examination of physiological status in wild and captive animals

Enumeration of invertebrate hemocytes is a potentially powerful tool for the determination of physiological effects of extrinsic stressors, such as hypoxia, disease, and toxicant exposure. A detailed flow cytometric method of broad application was developed for the objective characterization and enumeration of the hemocytes of New Zealand freshwater crayfish Paranephrops planifrons for the purpose of physiological health assessment. Hemocyte populations were isolated by flow cytometric sorting based on differential light scatter properties followed by morphological characterization via light microscopy and software image analysis. Cells were identified as hyaline, semigranular, and granular hemocytes based on established invertebrate hemocyte classification. A characteristic decrease in nuclear size, an increase in granularity between the hyaline and granular cells, and the eccentric location of nuclei in granular cells were also observed. The granulocyte subpopulations were observed to possess varying degrees of granularity. The developed methodology was used to perform total and differential hemocyte counts from three lake populations and between wild and captive crayfish specimens. Differences in total and differential hemocyte counts were not observed among the wild populations. However, specimens held in captivity for 14 d exhibited a significant 63% reduction in total hemocyte count, whereas the relative hemocyte proportions remained the same. These results demonstrate the utility of this method for the investigation of subacute stressor effects in selected decapod crustaceans.

Cellular basis of neurogenesis in the brain of crayfish, Procambarus clarkii: Neurogenic complex in the olfactory midbrain from hatchlings to adults

Neurogenesis in the central olfactory pathway of decapod crustaceans persists throughout life. Here we describe the structural basis of neurogenesis within the olfactory deutocerebrum of the crayfish Procambarus clarkii from hatchlings to adults. Using a proliferation marker and immunostaining, we found that throughout development each hemibrain contains a neurogenic complex consisting of five parts: two proliferation zones, each within the neuronal soma clusters containing local or projection interneurons, a tail of proliferating cells extending from each proliferation zone, and an elongated clump of cells where the two tails meet. The clump of cells comprises two subdivisions joined at a nucleus-free central area. Each subdivision consists of a dense group of clump cells with small, spindle-shaped nuclei and is connected to one of the proliferation zones by a strand of fibrous material encompassing the tail of proliferating cells extending from it. We identify one proliferating cell with a large nucleus in each subdivision as a putative neuroblast. Its daughter cells migrate through the strands to the associated proliferation zones, but in the strand leading to the soma cluster of local interneurons this is masked by local proliferation. We conclude that neurogenesis in the olfactory deutocerebrum of juvenile and adult P. clarkii is based on a few neuroblasts that are associated with unique clumps of cells likely representing stem cell niches.

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.

Mechanotransduction and the crayfish stretch receptor

Mechanotransduction or mechanosensitivity is found in almost every cell in all organisms from bacteria to vertebrates. Mechanosensitivity covers a wide spectrum of functions from osmosensing, cell attachment, classical sensory mechanisms like tactile senses in the skin, detection of sound in hair cells of the hearing apparatus, proprioceptive functions like recording of muscle length and tension in the muscle spindle and tendon organ, respectively, and pressure detection in the circulation etc. Since most development regarding the molecular aspects of the mechanosensitive channel has been made in nonsensory systems it is important to focus on mechanosensitivity of sensory organs where the functional importance is undisputed. The stretch receptor organ of the crustaceans is a suitable preparation for such studies. The receptor organ is experimentally accessible to mechanical manipulation and electrophysiological recordings from the sensory neuron using intracellular microelectrode or patch clamp techniques. It is also relatively easy to inject substances into the neuron, which also makes the neuron accessible to measurements with fluorescent techniques. The aim of the present paper is to give an up to date summary of observations made on the transducer properties of the crayfish stretch receptor (Astacus astacus and Pacifastacus leniusculus) including some recent unpublished findings. Finally some aspects on future line of research will be presented.

Adult neurogenesis: a common strategy across diverse species

Adult neurogenesis, the generation of new neurons from adult precursor cells, occurs in the brains of a phylogenetically diverse array of animals. In the higher (amniotic) vertebrates, these precursor cells are glial cells that reside within specialized regions, known as neurogenic niches, the elements of which both support and regulate neurogenesis. The in vivo identity and location of the precursor cells responsible for adult neurogenesis in nonvertebrate taxa, however, remain largely unknown. Among the invertebrates, adult neurogenesis has been particularly well characterized in freshwater crayfish (Arthropoda, Crustacea), although the identity of the precursor cells sustaining continuous neuronal proliferation in these animals has yet to be established. Here we provide evidence suggesting that, as in the higher vertebrates, the precursor cells maintaining adult neurogenesis in the crayfish Procambarus clarkii are glial cells. These precursor cells reside within a specialized region, or niche, on the ventral surface of the brain, and their progeny migrate from this niche along glial fibers and then proliferate to form new neurons in the central olfactory pathway. The niche in which these precursor cells reside has many features in common with the neurogenic niches of higher vertebrates. These commonalities include: glial cells functioning as both precursor and support cells, directed migration, close association with the brain vasculature, and specialized basal laminae. The cellular machinery maintaining adult neurogenesis appears, therefore, to be shared by widely disparate taxa. These extensive structural and functional parallels suggest a common strategy for the generation of new neurons in adult brains.

Ultrastructural and functional characterization of circulating hemocytes from the freshwater crayfish Astacus leptodactylus: Cell types and their role after in vivo artificial non-self challenge

The freshwater crayfish Astacus leptodactylus (Eschscholtz, 1823) is an important aquacultured decapod species as well as an invasive species in some European countries. In the current investigation we characterized the different classes of circulating blood cells in A. leptodactylus by means of light and electron microscopy analysis and we explored their reaction to different latex beads particles in vivo by total and differential cell counts at 0.5, 1, 2 and 4h after injections. We identified hemocytes by granule size morphometry as hyaline hemocytes with no or rare tiny granules, small granule hemocytes, unimodal medium diameter granule hemocytes and both small and large granule containing hemocytes. The latter granular hemocytes showed the strongest phenoloxidase l-DOPA reactivity both in granules and cytoplasm. A. leptodactylus respond to foreign particles with strong cellular immune responses. All treatments elicited a total hemocyte increase with a conspicuous recruitment of large granule containing hemocytes. All hemocyte types mounted some phagocytic response but the small granule hemocytes were the only ones involved in phagocytic response to all foreign particles with the highest percentages. These results (1) depict the variability in decapod hemocyte functional morphology; (2) identify the small granule hemocyte as the major phagocytic cell; (3) suggest that the rather rapid recruitment of large granule hemocyte in all treatments plays a relevant role by this hemocyte type in defense against foreign particles, probably in nodule formation.

Histone H2A as a transfection agent in crayfish hematopoietic tissue cells

We report a novel and highly efficient dsRNA transfection system based on one of the nuclear proteins, namely, histone H2A. RT-PCR semi-quantitative analysis of silencing target gene shows that the transfection efficiency of histone H2A is higher than Effectene or liposome-based transfection systems. Importantly, the high efficiency of histone H2A was associated with very low toxicity to the transfected crayfish hematopoietic tissue (Hpt) cells. The non-toxicity, effectiveness and specificity of histone H2A as a transfection agent provides a cheap, simple, highly efficient and reproducible gene delivery system, particularly for the sensitive cell cultures of crustacean animals such as crayfish and shrimp.

Not by spikes alone: responses of coordinating neurons and the swimmeret system to local differences in excitation

Swimmeret coordinating neurons in the crayfish CNS collectively encode a detailed cycle-by-cycle report on features of the motor output to each swimmeret. This information coordinates the motor output that drives swimmeret movements. To see how coordinating neurons responded to forced changes in intersegmental phase, we used a split-bath, repeated-measures experimental design to expose different regions of isolated abdominal nerve cords to different levels of excitation. We present a quantitative description of the firing of power-stroke (PS) motor units and two kinds of coordinating interneurons, ASC(E) and DSC, recorded simultaneously from each swimmeret ganglion under uniform and nonuniform excitation. When anterior and posterior ganglia were excited differently, several parameters of the swimmeret motor pattern were affected. Strengths of PS bursts in each ganglion were determined by local excitation. The phase of PS bursts in neighboring ganglia changed at the excitation boundary. Coordinating neurons from the two ganglia closest to the excitation boundary were most affected by nonuniform excitation. ASC(E) neurons tracked the timing and duration of each PS burst in their home ganglion, but did not follow changes in PS burst strength. DSC neurons changed the duration, phase, and number of spikes per burst. We propose two models to explain these results. First, the period expressed under nonuniform conditions is the sum of local intersegmental latencies and these latencies are determined by local excitation. Second, the phase change at the excitation boundary is determined by local modulation of the targets of the intersegmental coordinating neurons, not by modulation of the coordinating neurons themselves.

Physiological changes of premotor nonspiking interneurons in the central compensation of eyestalk posture following unilateral sensory ablation in crayfish

We investigated how the physiological characteristics and synaptic activities of nonspiking giant interneurons (NGIs), which integrate sensory inputs in the brain and send synaptic outputs to oculomotor neurons innervating eyestalk muscles, changed after unilateral ablation of the statocyst in order to clarify neuronal mechanisms underlying the central compensation process in crayfish. The input resistance and membrane time constant in recovered animals that restored the original symmetrical eyestalk posture 2 weeks after operation were significantly greater than those immediately after operation on the operated side whereas in non-recovered animals only the membrane time constant showed a significant increase. On the intact side, both recovered and non-recovered animals showed no difference. The frequency of synaptic activity showed a complex pattern of change on both sides depending on the polarity of the synaptic potential. The synaptic activity returned to the bilaterally symmetrical level in recovered animals while bilateral asymmetry remained in non-recovered ones. These results suggest that the central compensation of eyestalk posture following unilateral impairment of the statocyst is subserved by not only changes in the physiological characteristics of the NGI membrane but also the activity of neuronal circuits presynaptic to NGIs.

Morphology and histology of P. argentinus (Crustacea, Decapoda, Caridea) digestive tract

This work describes the morphology and histology of the P. argentinus digestive tract. The foregut comprises the mouth, oesophagus, and stomach and is lined by a simple cylindrical epithelium overlain by cuticle. There are tegumental glands in the oral region and in the first portion of the oesophagus and of the hindgut. The cardiac stomach is an oval dorsal sac in the cephalothorax and has no calcified structures. The pyloric stomach comprises an upper chamber and a lower gland filter. The filter consists of an outer row of elongated setae and an inner row of dorsally curved setae forming longitudinal channels 16-18 microm wide. The midgut runs from the dorsal chamber of the pyloric stomach to the sixth abdominal somite without caeca. The hindgut runs from the sixth abdominal somite to the ventral anus. The mid-gut epithelium comprises dominant cylindrical cells and small undifferentiated cells in the first portion. The hindgut wall presents longitudinal folds, conspicuous muscular bundles, and a folded cuticle. The digestive tract of P. argentinus is basically similar to that of most of decapods. The absence of calcified structures in the stomach and the width of the longitudinal channels in the filter are related to the predominantly detritivorous diet.

Characteristic expression patterns of allatostatin-like peptide, FMRFamide-related peptide, orcokinin, tachykinin-related peptide, and SIFamide in the olfactory system of crayfish Procambarus clarkii

The olfactory system plays important roles in various crustacean behaviors. Despite numerous studies on different aspects of the olfactory neural pathway, only the decapod-tachykinin-related peptide (decapod-TRP) has been identified as a neuromodulator in this processing to date. To establish the functions of other related neuropeptides, we initially performed cDNA cloning of FMRFamide-related peptide (FaRP) and allatostatin (AST)-like peptide from the crayfish Procambarus clarkii, followed by in situ hybridization (ISH) analysis of these peptides, along with decapod-TRP, orcokinin, and crustacean-SIFamide. Cloned FaRP cDNA encodes seven copies of C-terminal RN(F/Y)LRFamide-containing peptide, whereas AST-like peptide cDNA comprises 29 copies of AST-like peptide (-YXFGLamide) and three additional putative peptides. ISH analysis of the brain revealed specific expression of crustacean-SIFamide mRNA in most projection neurons (cell cluster 10), and predominant localization of other mRNAs to interneurons. The data suggest that the crustacean-SIFamide neuropeptide is involved in output of the deutocerebrum to the protocerebrum. Double-fluorescence ISH data further disclose that, in cluster 9, orcokinin is coexpressed in decapod-TRP-specific interneurons, whereas AST-like peptide-containing cells do not overlap with orcokinin-expressing cells. On the other hand, FaRP-expressing cells overlap with both orcokinin- and AST-like peptide-specific cells. In cluster 11, where signals for AST-like peptide are absent, a number of interneurons express both decapod-TRP and orcokinin, emphasizing a close relationship between these two factors with regard to olfactory processing, and possibly tactile and/or visual sensory systems. These characteristic expression patterns of neuropeptides support their distinct involvement in the modulation of olfactory processing.

Characterization of white spot syndrome virus replication in in vitro-cultured haematopoietic stem cells of freshwater crayfish, Pacifastacus leniusculus

Replication of White spot syndrome virus (WSSV) was investigated in haematopoietic cells (hpt cells) derived from haematopoietic tissue (hpt) of freshwater crayfish, Pacifastacus leniusculus. Temperature and type of inoculum for virus replication were studied. The cell culture remained viable at a wide range of temperatures ranging from 4 to 25 degrees C. WSSV replicated in cells, as evidenced by in situ hybridization, RT-PCR and by the presence of virions visualized with an electron microscope. Moreover, the results showed that the infectivity of WSSV to hpt cells is dependent on temperature and a supplemented growth factor (cytokine) astakine. WSSV replicated more rapidly at higher temperatures than at lower temperatures. No virus replication was observed at 4 degrees C. Detectable WSSV-infected cells were present as early as 36 h post-inoculation, demonstrated by in situ hybridization or RT-PCR of VP28 expression at 25 degrees C. Hpt cells can survive a few weeks at 25 or 16 degrees C and longer than several months at 4 degrees C.

Occurrence of L- and D-crustacean hyperglycemic hormone isoforms in the eyestalk X-organ/sinus gland complex during the ontogeny of the crayfish Astacus leptodactylus

We studied the ontogeny of the eyestalk structure and of the L-CHH and d-Phe3-CHH synthesis in the X-organ/sinus gland (XO/SG) complex by light microscopy and immunocytochemistry in the freshwater crustacean Astacus leptodactylus. The optic ganglia start to differentiate in embryos at EI 190 microm (EI: eye index; close to 410 microm at hatching). At EI 270 microm, the three medullae (externa, interna, and terminalis) and the lamina ganglionaris are present and are organized as in the adult eyestalk. The L-CHH was localized in perikarya of neuroendocrine cells, in their tracts, and in SG from the metanauplius stage to the adult. The d-Phe3-CHH was visualized in XO perikarya, in their tracts and in SG of embryos from EI 350 microm and in all later studied stages. Co-localization of both CHH stereoisomers always occurred in the d-Phe3-CHH-producing cells. These results show that the synthesis of CHH enantiomers starts during the embryonic life in A. leptodactylus, and that the d-isomer is synthesized later than its L-counterpart. We discuss the post-translational isomerization as a way to generate hormonal diversity and the putative relation between d-Phe3-CHH synthesis and the ability to osmoregulate, occurring late during the embryonic life of Astacus leptodactylus.

An optical telemetry system for underwater recording of electromyogram and neuronal activity from non-tethered crayfish

We have developed an optical telemetry system for recording electrical signals associated with muscle and neuronal activities from freely walking crayfish under water. The device was made from conventional electronic parts which are commercially available, utilizing infrared light (880 nm) for signal transmission. Two or four channels of biological signals were multiplexed, the voltage of each data point modulated to the duration of subcarrier pulses and further to the interval of narrower carrier pulses that directly drove the infrared light emission diode (IRLED) under water. The light-pulse modulated signals were received by photodiodes and demodulated to restore the original two or four channel signals. Electrical recordings using wired electrodes and conventional amplifiers revealed that the optically transmitted signals were consistent with the wire-transmitted ones. In order to test the performance of this system, we recorded electromyograms (EMGs) from the second and third walking legs on each side of crayfish together with the neuronal activity in the ventral nerve cord. The results confirmed our previous observation in tethered crayfish that the background tonus of leg muscles showed an increase preceding their rhythmic activation.

Posttranslational isomerization of a neuropeptide in crustacean neurosecretory cells studied by ultrastructural immunocytochemistry

Isomerization of the third amino acid residue (a phenylalanine) of crustacean hyperglycemic hormone (CHH) has been previously reported to occur as a late step of hormone precursor maturation in a few neurosecretory cells in the X-organ-sinus gland complex of the crayfish Orconectes limosus. In the present report, using conformation-specific antisera combined with immunogold labeling, we have studied, at the ultrastructural level, the distribution of L- and D-CHH immunoreactivity in CHH-secreting cells of the crayfish Astacus leptodactylus. Two CHH-secreting cell populations were observed, the first one (L-cells), the most numerous, exhibited only labeling for L-CHH. In the second one (D-cells), four secretory granule populations were distinguished according to their labeling: unlabeled, either L- or D- exclusively or both L- and D-granules. Labeling quantification by image analysis in D-cells showed a marked increase in D-labeling from the cell body to the axon terminal. However some L- and mixed granules remain in axon terminals. Our results demonstrate that Phe3 isomerization of CHH occurs within the secretory granules of specialized neurosecretory cells and progresses as the granules migrate along the axonal tract. The observation that not all the CHH synthesized is isomerized, and the great variability in the proportion of L- and D-immunoreactivity in granules in every cell region may suggest an heterogeneous distribution of the putative enzyme involved in Phe3 isomerization, a peptide isomerase, within the secretory pathway.

Modulatory effects of melatonin on behavior, hemolymph metabolites, and neurotransmitter release in crayfish

Melatonin affects a variety of circadian processes such as behavior and neurotransmitter release in vertebrates. Crayfish melatonin production occurs in the eyestalks, and the cycle of production may change seasonally. To date, however, melatonin's roles and mechanisms of action in crustacean physiology are unclear. We injected melatonin or saline into crayfish in scotophase and monitored activity and hemolymph glucose/lactate over 24 h in early spring. Crayfish were significantly more active in photophase versus the expected scotophase, and had concurrent glucose/lactate peaks. Melatonin reversed the activity pattern, causing a scotophase activity peak, but not the glucose/lactate patterns. This study was repeated in late summer, during which control activity and glucose/lactate levels were elevated in scotophase. Melatonin decreased the amplitude of scotophase activity and glucose/lactate, eliminating activity and glucose cycles. We also injected melatonin or saline at various times of day in early summer and monitored locomotor activity for 1 h. Controls had high activity at 1200 (mid-photophase) and 2100 h (early scotophase), and melatonin increased activity at 1200 h but decreased it at 2100 h. Melatonin also increased activity at 1500 h but not 1800 h (late photophase). Next, we examined the influence of melatonin on crayfish neurophysiology. Melatonin (10 microM) enhanced synaptic transmission at the neuromuscular junction (NMJ). The presynaptic action resulted in more vesicles being released during evoked stimulation. Our study indicates that melatonin may have a phylogenetically conserved role in the transduction of circadian information in invertebrates as in vertebrates. Behavioral and physiological effects may be mediated by modulation of central pathways, enhanced at the peripheral level via neuromodulation of the NMJ.

Effects of leg movements on the synaptic activity of descending statocyst interneurons in crayfish, Procambarus clarkii

Crustacean postural control is modulated by behavioral condition. In this study, we investigated how the responses of descending statocyst interneurons were affected during leg movements. Intracellular recording was made from an animal whose statoliths had been replaced with ferrite grains so that statocyst receptors could be activated by magnetic field stimulation. We identified 14 morphological types of statocyst-driven descending interneurons. Statocyst-driven descending interneurons always showed an excitatory response to statocyst stimulation on either ipsilateral or contralateral side to the axon. The response of each statocyst-driven descending interneuron to statocyst stimulation was differently modulated by leg movements in different conditions. During active leg movements, six statocyst-driven descending interneurons were activated regardless of whether a substrate was provided or not. In other two statocyst-driven descending interneurons, the excitatory input during leg movements was stronger when a substrate was provided than when it was not. One statocyst-driven descending interneuron received an excitatory input only during leg movements on a substrate, whereas another statocyst-driven descending interneuron did not receive any input during leg movements both on a substrate and in the air. These results suggest that the descending statocyst pathways are organized in parallel, each cell affected differently by behavioral conditions.

Comparative immunohistochemistry and cellular distribution of farnesoic acid O-methyltransferase in the shrimp and the crayfish

Farnesoic acid O-methyltransferase (FAMeT) catalyzes the conversion of farnesoic acid (FA) to methylfarnesoate (MF) by the mandibular organ (MO) of crustaceans. Here we report the cellular localization of FAMeT and radiochemical assay of endogenous FAMeT activity in shrimp (Metapenaeus ensis) and crayfish (Procambarus clarkii) tissues. As in the eyestalk (ES), FAMeT is concentrated in specific neurosecretory cells of the ventral nerve cord (VNC) whereas only weak FAMeT immunoreactivity was observed in the MO. FAMeT was also detected in the ventral nerve cord, heart (HET), eyestalk, and muscle of the juvenile shrimp. Although the VNC shows the greatest FAMeT immunoreactivity, the heart extract exhibited the highest FAMeT enzymatic activity. These results suggest that FAMeT in the VNC may be inactive or inactivated at the stages of development tested. Contrary to the previous reports in other crustaceans, MO extract in shrimp shows only low FAMeT activity. The eyestalk, epidermis, ovary and testis show appreciable FAMeT activity. The presence of FAMeT in neurosecretory cells of VNC and eyestalk of shrimp and crayfish implies a possible interaction of FAMeT with the eyestalk CHH-family of neuropeptides. The widespread activity of FAMeT suggests that it has a wide spectrum of action in many tissues that contribute to the function and regulation of MF synthesis in shrimp and crayfish.

Dendritic initiation and propagation of spikes and spike bursts in a multimodal sensory interneuron: the crustacean parasol cell

Invasion of dendrites by spikes and spike bursts can play a critical role in regulating the output of central neurons by modifying their dynamic input-output relationships. Back-propagating bursts can modulate voltage-gated channels in the short term and can also modify long-term responses to synaptic input. Determining the morphological site of spike initiation and the mode of propagation through the dendritic arbor is therefore crucial to an understanding of a neuron's functional properties. I used electrophysiological methods to study parasol cells in isolated, perfused head preparations of the freshwater crayfish Procambarus clarkii to determine the compartment of origin of orthodromically activated action potentials and bursts that propagate within the dendritic arbor and to examine the identity of low-amplitude, electrotonically recorded spike events that are present in more than one-half of the intracellular recordings obtained from dendrites in these neurons. Experiments using antidromic activation of parasol cell axons indicated that electrotonically recorded spikes probably are generated in neighboring parasol cells, to which the impaled neurons are electrically coupled. Both paired intracellular recordings and extracellular field potential measurements were used to compare arrival times of antidromic and orthodromic spikes at loci in the vicinity of the trunk and the basal branch compartments of parasol cell dendrites. These methods provided consistent results, indicating that synaptically evoked action potentials are initiated at a site on the trunk, from which point they back-propagate into the basal branches within the hemiellipsoid body, and presumably, also orthodromically to the axon. Data are presented suggesting that bursts also arise at a trunk locus, but one that is different from the initiation point of single spikes evoked by excitatory postsynaptic potentials (EPSPs). Morphological specializations between the dendritic trunk and basal branches may facilitate back-propagation of spikes and spike bursts into the basal branches.

Differential dye coupling reveals lateral giant escape circuit in crayfish

The lateral giant (LG) escape circuit of crayfish mediates a coordinated escape triggered by strong attack to the abdomen. The LG circuit is one of the best understood of small systems, but models of the circuit have mostly been limited to simple ball-and-stick representations, which ignore anatomical details of contacts between circuit elements. Many of the these contacts are electrical; here we use differential dye coupling, a technique which could help reveal connection patterns in many neural circuits, to reveal in detail the circuit within the terminal abdominal ganglion. Sensory input from the tailfan forms a somatotopic map on the projecting LG dendrites, which together with interafferent coupling mediates a lateral excitatory network that selectively amplifies strong, phasic, converging input to LG. Mechanosensory interneurons contact LG at sites distinct from the primary afferents and so maximize their summated effect on LG. Motor neurons and premotor interneurons are excited near the initial segments of the LGs and innervate muscles for generating uropod flaring and telson flexion. Previous research has shown that spatial patterns of input are important for signal integration in LG; this map of electrical contact points will help us to understand synaptic processing in this system.

Distribution of NO-induced cGMP-like immunoreactive neurones in the abdominal nervous system of the crayfish, Procambarus clarkii

Nitric oxide (NO) acts as a signalling molecule by activating soluble guanylate cyclase and causing accumulation of the second messenger cyclic guanosine 3',5'-monophosphate (cGMP) in target cells. In order to detect the presence of NO-cGMP signalling pathway in the crayfish abdominal nervous system, accumulation of NO-induced cGMP was investigated by anti-cGMP immunochemistry. Some preparations were incubated in a high-K(+) saline containing an inhibitor of cGMP-degrading phosphodiesterase, 3-isobutyl-1-methyxanthine (IBMX), to activate NO generating neurones, which could release NO in the ganglion, and then immunohistochemistry using an anti-cGMP antibody was performed. The other preparations were incubated in NO donor, sodium nitroprusside (SNP) saline containing IBMX before anti-cGMP immunohistochemistry was performed. The distribution of cGMP-like immunoreactive neurones in high-K(+) treated preparations was similar to that of cGMP-like immunoreactive neurones in NO donor treated preparations. About 70-80 cell bodies and many neuronal branches in the neuropilar area of the ganglion were stained, although no neurones showed immunoreactivity unless preparations were activated by either high-K(+) or the NO donor. Some of them were identical neurones, and they were intersegmental ascending interneurones and motor neurones. Sensory afferents that innervates hind gut showed strong cGMP-like immunoreactivity, although no mechanosensory afferents showed any immunoreactivity. These results strongly suggest the presence of an NO-cGMP signalling pathway that regulates neuronal events in the abdominal nervous system of the crayfish.

Mechanism of NMDA receptor contribution to axon-to-glia signaling in the crayfish medial giant nerve fiber

Electrical stimulation of crayfish giant axons at high frequency activates group II metabotropic and NMDA glutamate receptors on adjacent glial cells via release of N-acetylaspartylglutamate and glutamate formed upon its hydrolysis. This produces a transient depolarization followed by a prolonged hyperpolarization of glial cells that involves nicotinic acetylcholine receptor activation. The hyperpolarization is nearly completely blocked by antagonists of metabotropic glutamate receptors but only slightly reduced by inhibition of NMDA receptors. We report that the NMDA-induced hyperpolarization of glial cells is reduced by decreased calcium in the solution bathing the giant nerve fiber, while removal of sodium ions or block of voltage-dependent calcium channels completely prevents the glial response to NMDA. Inhibition of nicotinic acetylcholine receptors or removal of extracellular Cl(-) converts the glial response from a hyperpolarization to a depolarization that is sensitive to NMDA receptor antagonist. We propose that NMDA receptor activation by glutamate, formed from extracellular N-acetylaspartylglutamate during nerve stimulation, contributes to glial hyperpolarization by increasing intracellular Ca(2+) via opening of voltage-sensitive Ca(2+) channels. Based on our previous work, we propose further that the added Ca(2+) supplements that produced by N-acetylaspartylglutamate and glutamate acting on group II metabotropic glutamate receptors to cause an increased release of acetylcholine and a larger hyperpolarization.

Chronic alterations in serotonin function: dynamic neurochemical properties in agonistic behavior of the crayfish, Orconectes rusticus

The biogenic amine serotonin [5-hydroxytryptamine (5-HT)] has received considerable attention for its role in behavioral phenomena throughout a broad range of invertebrate and vertebrate taxa. Acute 5-HT infusion decreases the likelihood of crayfish to retreat from dominant opponents. The present study reports the biochemical and behavioral effects resulting from chronic treatment with 5-HT-modifying compounds delivered for up to 5 weeks via silastic tube implants. High performance liquid chromatography with electrochemical detection (HPLC-ED) confirmed that 5,7-dihydroxytryptamine (5,7-DHT) effectively reduced 5-HT in all central nervous system (CNS) areas, except brain, while a concurrent accumulation of the compound was observed in all tissues analyzed. Unexpectedly, two different rates of chronic 5-HT treatment did not increase levels of the amine in the CNS. Behaviorally, 5,7-DHT treated crayfish exhibited no significant differences in measures of aggression. Although treatment with 5-HT did not elevate 5-HT content in the CNS, infusion at a slow rate caused animals to escalate more quickly while 5-HT treatment at a faster rate resulted in slower escalation. 5,7-DHT is commonly used in behavioral pharmacology and the present findings suggest its biochemical properties should be more thoroughly examined. Moreover, the apparent presence of powerful compensatory mechanisms indicates our need to adopt an increasingly dynamic view of the serotonergic bases of behavior like crayfish aggression.

Physiological characterization of the Na(+)/Ca(2+) exchanger (NCX) in hepatopancreatic and antennal gland basolateral membrane vesicles isolated from the freshwater crayfish Procambarus clarkii

The purpose of this study was to physiologically characterize the basolateral Na(+)/Ca(2+) exchanger (NCX) in basolateral membrane vesicles (BLMVs) of hepatopancreas and antennal gland of intermolt crayfish. Conditions were optimized to measure Na(+)-dependent Ca(2+) uptake and retention in the BLMV including use of intravesicular (IV) oxalate and measuring initial uptake rates at 20 s. Na(+)-dependent Ca(2+) uptake rate into BLMV was temperature insensitive. Na(+)-dependent Ca(2+) uptake rate was dependent upon free Ca(2+) with saturable Michaelis-Menten kinetics determined as follows: hepatopancreas, maximal uptake rate (J(max))=2.45 nmol/mg per min, concentration at which carrier operates at half-maximal uptake rate (K(m))=0.69 microM Ca(2+); antennal gland, J(max)=13.2 nmol/mg per min, K(m)=0.59 microM Ca(2+). The two vesicle populations exhibited different sensitivity to putative NCX inhibitors. Benzamil had no effect on Na(+)-dependent Ca(2+) uptake rate in hepatopancreas; in antennal gland it was inhibitory at concentrations up to 30 microM and was stimulatory at higher concentrations. Conversely the inhibitor quinacrine was inhibitory at 10 microM in hepatopancreas and was stimulatory at 1000 microM; meanwhile it was ineffective in antennal gland BLMV. Short circuiting the BLMV had no effect on Na(+)-dependent Ca(2+) uptake rate suggesting that the process may be electroneutral. Compared with another prominent basolateral transporter in hepatopancreas the plasma membrane Ca(2+) ATPase (PMCA), the NCX has 70-fold greater J(max) (at comparable temperature) and a lower affinity. In antennal gland the NCX has 40-fold greater J(max) and a lower affinity. In hepatopancreas and antennal gland BLMV NCX appears to determine the rate of basolateral Ca(2+) efflux in intermolt.

N-acetylaspartylglutamate (NAAG) is the probable mediator of axon-to-glia signaling in the crayfish medial giant nerve fiber

Glial cell hyperpolarization previously has been reported to be induced by high frequency stimulation or glutamate. We now report that it also is produced by the glutamate-containing dipeptide N-acetylaspartylglutamate (NAAG), by its non-hydrolyzable analog beta-NAAG, and by NAAG in the presence of 2-(phosphonomethyl)-pentanedioic acid (2-PMPA), a potent inhibitor of the NAAG degradative enzyme glutamate carboxypeptidase II. The results indicate that NAAG mimics the effect of nerve fiber stimulation on the glia. Although glutamate has a similar effect, the other presumed product of NAAG hydrolysis, N-acetylaspartate, is without effect on glial cell membrane potential, as is aspartylglutamate (in the presence of 2-PMPA). The hyperpolarization induced by stimulation, glutamate, NAAG, beta-NAAG, or NAAG plus 2-PMPA is completely blocked by the Group II metabotropic glutamate receptor antagonist (S)-alpha-ethylglutamate but is not altered by antagonists of Group I or III metabotropic glutamate receptors. The N-methyl-D-aspartate receptor antagonist MK801 reduces but does not eliminate the hyperpolarization generated by glutamate, NAAG or stimulation. These results, in combination with those of the preceding paper, are consistent with the premise that NAAG could be the primary axon-to-glia signaling agent. When the unstimulated nerve fiber is treated with cysteate, a glutamate reuptake blocker, there is a small hyperpolarization of the glial cell that can be substantially reduced by pretreatment with 2-PMPA before addition of cysteate. A similar effect of cysteate is seen during a 50 Hz/5 s stimulation. From these results we suggest that glutamate derived from NAAG hydrolysis appears in the periaxonal space under the conditions of these experiments and may contribute to the glial hyperpolarization.

Synthesis and release of N-acetylaspartylglutamate (NAAG) by crayfish nerve fibers: implications for axon-glia signaling

Early physiological and pharmacological studies of crayfish and squid giant nerve fibers suggested that glutamate released from the axon during action potential generation initiates metabolic and electrical responses of periaxonal glia. However, more recent investigations in our laboratories suggest that N-acetylaspartylglutamate (NAAG) may be the released agent active at the glial cell membrane. The investigation described in this paper focused on NAAG metabolism and release, and its contribution to the appearance of glutamate extracellularly. Axoplasm and periaxonal glial cell cytoplasm collected from medial giant nerve fibers (MGNFs) incubated with radiolabeled L-glutamate contained radiolabeled glutamate, glutamine, NAAG, aspartate, and GABA. Total radiolabel release was not altered by electrical stimulation of nerve cord loaded with [(14)C]glutamate by bath application or loaded with [(14)C]glutamate, [(3)H]-D-aspartate or [(3)H]NAAG by axonal injection. However, when radiolabeled glutamate was used for bath loading, radiolabel distribution among glutamate and its metabolic products in the superfusate was changed by stimulation. NAAG was the largest fraction, accounting for approximately 50% of the total recovered radiolabel in control conditions. The stimulated increase in radioactive NAAG in the superfusate coincided with its virtual clearance from the medial giant axon (MGA). A small, stimulation-induced increase in radiolabeled glutamate in the superfusate was detected only when a glutamate uptake inhibitor was present. The increase in [(3)H]glutamate in the superfusion solution of nerve incubated with [(3)H]NAAG was reduced when beta-NAAG, a competitive glutamate carboxypeptidase II (GCP II) inhibitor, was present.Overall, these results suggest that glutamate is metabolized to NAAG in the giant axon and its periaxonal glia and that, upon stimulation, NAAG is released from the axon and converted in part to glutamate by GCP II. A quisqualate- and beta-NAAG-sensitive GCP II activity was detected in nerve cord homogenates. These results, together with those in the accompanying paper demonstrating that NAAG can activate a glial electrophysiological response comparable to that initiated by glutamate, implicate NAAG as a probable mediator of interactions between the MGA and its periaxonal glia.

Altered responsiveness to 5-HT at the crayfish neuromuscular junction due to chronic p-CPA and m-CPP treatment

Serotonin (5-HT) levels in the hemolymph of crustaceans has been implied to alter aggressiveness which influences social interactions. The activation of IP3 as a second messenger cascade within crayfish motor neurons in response to application of 5-HT, suggests that the 5-HT receptor subtypes on the motor neurons are analogous to the vertebrate 5-HT2A receptors. Based on evidence in other systems, it would be expected that chronically sustained 5-HT levels in aggressive individuals would result in a compensatory negative feed-back regulation and/or that target tissues would diminish their sensitivity to high levels of circulating, free 5-HT. We addressed the issue of up- and down-regulation in the sensitivity of the responsiveness to exogenously applied 5-HT at the NMJs of crayfish in which the animals have altered endogenous 5-HT levels. Injections of the 5-HT1 and 5-HT2 vertebrate receptor agonist, 1-(3-Chlorophenyl) piperazine dihydrochloride (m-CPP), for 1 week resulted in a decreased responsiveness to application of 5-HT. The compound p-chlorophenylalanine (p-CPA) blocks the enzymatic synthesis of 5-HT and following 7 days of p-CPA injections, a super-sensitivity to exogenous application of 5-HT for both tonic and phasic neuromuscular junctions (NMJs) was observed. However, acute applications of p-CPA and m-CPP, followed by extensive saline washing, did not reveal any altered receptivity to 5-HT application.

Characterization of a delayed rectifier potassium channel in the slowly adapting stretch receptor neuron of crayfish

Single channel recordings were performed on enzyme-cleaned slowly adapting sensory neurons of crayfish, in cell-attached configuration, with a physiological K(+) gradient across the neuronal membrane. An outward rectifying, voltage-gated K(+) channel with a slope conductance of 13 pS and a K(+) ion permeability of P(K)=6.5 x 10(-14) cm(3)/s was characterized. This 13 pS K(+) channel started to be activated at around 20 mV depolarization. Its open probability increased upon depolarization with V(0.5)= -25.3 mV and P(max)=0.83. The averaged currents showed a delay following the onset of depolarization. The activation time constant was voltage-dependent. The maximal value was 17.0 ms at -25 mV and at +35 mV the time constant was 1.7 ms. Little inactivation was observed throughout the 80- or 1500-ms long depolarization pulses. A sum of two exponentials provided the optimal fit for open time and closed time distribution. At 80-mV depolarization, the open time constants were 0.4 and 10.4 ms; the close time constants were 0.4 and 2.3 ms. The first-latency distribution suggested that at least two closed states preceded two open states. This 13 pS delayed rectifier plays a minor role in the maintenance of the resting membrane potential but contributes to the action potential repolarization. It may also modify the stretch-induced receptor potential and affect the adaptation behaviours in this neuron.

Dichotomy in phasic-tonic neuromuscular structure of crayfish inhibitory axons

Crustacean muscles are unique in their innervation by both excitatory and inhibitory neurons; therefore, they exhibit polyneuronal and multiterminal innervation. Because excitatory motoneurons are broadly divided into phasic and tonic types, we hypothesized that inhibitory neurons would follow a similar dichotomy. The abdominal extensor muscles in crayfish are separated into parallel deep and superficial bundles; the former has fast muscle fibers innervated by phasic excitatory motoneurons, and the latter has slow fibers supplied by tonic excitatory motoneurons. Each muscle also is innervated by a single, separate inhibitory neuron that uses gamma-aminobutyric acid (GABA) as the inhibitory neurotransmitter. The pattern of axonal branching by the separate inhibitory axons in phasic and tonic abdominal extensor muscles was visualized with confocal microscopy in preparations labeled for GABA-like immunoreactivity. Initial observations indicated that the phasic muscle was covered by extensive GABAergic, filiform axon terminals, whereas innervation of the tonic muscle was comprised of more localized and varicose terminals. With quantitative analyses, we found that the phasic axon has a more highly branched nature than the tonic in first- and second-order branches. The phasic axon branches also were significantly longer than the tonic branches in the second- and third-order branches. Synaptic varicosities in the phasic branches were smaller and less frequent than those in the tonic branches. The fine structure of the inhibitory nerve terminals near synaptic contacts examined with thin-serial-section electron microscopy revealed distinct differences between the phasic system and the tonic system. The phasic terminals were smaller in cross-sectional area than the tonic terminals, and they had smaller synapses and fewer mitochondria. The presynaptic active zone dense bodies were similar in length and number between phasic and tonic synapses. However, their number per synaptic area was two-fold higher in phasic synapses compared with tonic synapses because of the smaller size of the phasic synapses. Thus, within the same neuromuscular system, inhibitory synaptic terminals revealed unique phasic and tonic identities similar to those observed for the excitatory axons.

Regeneration of phasic synapses on a crayfish slow muscle following allotransplantation of a mixed phasic-tonic nerve

Separate phasic or tonic nerves allotransplanted to reinnervate a denervated slow superficial flexor muscle (SFM) in the abdomen of adult crayfish regenerate synaptic nerve terminals with phasic or tonic properties. To test competitive interactions between tonic and phasic axons, we allotransplanted the sixth abdominal ganglion with its third nerve root containing a mixture of phasic and tonic axons onto the denervated SFM. The resulting reinnervation of the SFM was compared to the normal innervation on the contralateral intact SFM, which receives innervation only from tonic motoneurons. Variable sizes of excitatory postsynaptic potentials indicated that 2-3 axons innervated each muscle fiber of the SFM in both the allotransplant and normal preparations. Compared to the normal tonic terminals on the intact contralateral side, the allotransplanted synaptic terminals had more phasic-like properties; specifically, they gave rise to larger synaptic potentials, had a lower mitochondrial content and contained a higher density of active zone dense bars per synapse. Moreover, prolific sprouting of the axons in the regenerated nerve, typical of phasic axons, points to more vigorous regeneration of phasic rather than tonic axons to the denervated SFM. In keeping with this prolific axon sprouting, there was both a much higher density of innervation in the allotransplanted SFM compared to the normal SFM, and a higher frequency of extrasynaptic active zones in regenerated terminals of the mixed nerve compared to those of the tonic nerve. Thus, an allotransplanted mixed nerve regenerates mainly phasic axons and synapses on the slow denervated SFM, demonstrating the instructive nature of the neuron in synapse specification, as well as the permissive nature of the target muscle.

Pertussis toxin does not affect the time course of quantal release in crayfish and mouse muscle, but has other post- and presynaptic effects, especially on adenosine autoreceptors

While G-proteins are involved in the synaptic release machinery and also can mediate inhibition of presynaptic Ca2+ channels, we find that pertussis toxin (PTX) does not affect the amount and the time course of quantal release from motor nerve terminals on crayfish or mouse muscle. Monoquantal excitatory currents (qEPSCs) were recorded that were elicited by constant depolarisation pulses to a terminal by means of a perfused macro-patch electrode. Although presynaptic effects of PTX on output and time course of release of quanta were absent, postsynaptically the rise time of qEPCs was increased and their decay time constant reduced. Adenosine (Ad) is known to inhibit quantal release in vertebrate motor nerve terminals via PTX sensitive G-proteins, and Ad is generated during nicotinic synaptic transmission by breakdown of the co-transmitter adenosine triphosphate (ATP). As reported by others, we found in mouse muscle inhibition of quantal release after application of Ad, but in addition late facilitation. Both these effects of Ad were blocked when the muscle was pre-incubated with PTX.

"Impoverished" and "enriched" living conditions influence the proliferation and survival of neurons in crayfish brain

New neurons are added to two bilateral clusters of neurons in crayfish brain throughout their lives. These interneurons are associated with the olfactory and accessory lobes, areas of the brain that receive primary olfactory information and higher order inputs from the visual and tactile receptor systems. The rate of cell proliferation in these four clusters, revealed by BrdU labeling, is sensitive to the living conditions of the animals: individuals isolated in small spaces (impoverished condition) exhibit a lower rate of cell proliferation in comparison to their siblings living together in larger areas (enriched condition), although both groups were fed to satiation. Reduction in the rate of proliferation can be measured 1 to 2 weeks after the animals are subjected to the impoverished condition. Counts of the labeled neurons that survive after 4 weeks of subjection to the two conditions show that fewer new neurons survive in the brains of animals that have lived for 2 weeks in the impoverished condition in comparison to their siblings living in the enriched conditions. Factors such as surface area, depth of water, and social interaction can all play a role in determining both the rate of new neuron production and the incorporation of the new neurons into the brain of freshwater crayfish. The results indicate a high degree of neuronal plasticity in the crayfish brain that is highly sensitive to the conditions under which the animals are kept.

Statistical methods for assessing the dimensions of synaptic vesicles in nerve terminals

Chemical transmission between neurons occurs by the release of neurotransmitter packaged within vesicles of the presynaptic neuron onto a postsynaptic target. The amount of transmitter contained within a vesicle is in part regulated by the size of the vesicle. Thus, it is of general interest to quantify the dimension of vesicles in understanding the basic principles of chemical synaptic transmission. These vesicles can only be measured by electron microscopic techniques. Obtaining the true dimensions of synaptic structures is therefore complicated by stereological considerations. In this study, we suggest improved methods for determining the distributions (and mean sizes) for populations of vesicle diameters by mathematical processes involving (1) an implicit inversion of the empirical data distribution, (2) an explicit inversion approach, and (3) an approach based on substituting the empirical distribution into the inversion formula and then isotonizing using an iterated convex minorant algorithm. These procedures provide distributions that better represent the true population distributions (and means) for comparisons with other data sets of vesicle diameter measures.