Regulation of the stress-responsive X-organ--Y-organ axis by 5-hydroxytryptamine in the crab, Cancer antennarius

Identification of molt-inhibiting hormone and ecdysteroid receptor cDNA sequences in Gammarus pulex, and variations after endocrine disruptor exposures

In amphipods, growth, development and reproduction are mediated by the molt, which is a hormonally controlled process and which, therefore, could be impacted by endocrine disruption compounds (EDC). The molt process is controlled by both X-organ (XO) and Y-organ (YO) through a variety of hormones and receptors including the molt-inhibiting hormone (MIH) and the ecdysteroid receptor (EcR). However, although many studies were devoted to characterize MIH and EcR in crustaceans, only few works evaluated their variations under EDCs exposures. Consequently, the present work aimed to characterize MIH and EcR genes of the amphipod Gammarus pulex, as well as to study their relative expression variations after exposure to four EDCs, proved in vertebrates: ethinylestradiol (estrogen), 4-hydroxytamoxifen (anti-estrogen), 17α-methyltestosterone (androgen) and cyproterone acetate (anti-androgen). PCR amplification allowed to obtain 204 bp length and 255 bp length fragments, encoding for partial sequences of 68 amino acids and 85 amino acids, which correspond to EcR and MIH, respectively, and which are highly conserved in crustacean species. Results highlighted MIH and EcR expressions mainly in G. pulex head, which is the localization of XO and YO. Moreover, irrespective of the EDC exposure, increases of MIH and EcR relative expressions were observed, as it was observed after the exposure to 20-hydroxyecdysone (20HE), the natural molt hormone, used as positive control. Therefore, it appeared that tested EDCs behaved like 20HE, suggesting that their effects could occur through the ecdysteroids pathways, and so impact the molt process of G. pulex on the long term. Finally, the present study is a first step in the possibility of using MIH and EcR relative expressions as biomarkers of exposure for EDCs risk assessment. However additional studies must first be carried out to better characterize and understand their variations, and also better predicted consequences for the exposed amphipods.

Neuroendocrine disruption in the shore crab Carcinus maenas: Effects of serotonin and fluoxetine on chh- and mih-gene expression, glycaemia and ecdysteroid levels

Serotonin, a highly conserved neurotransmitter, controls many biological functions in vertebrates, but also in invertebrates. Selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine, are commonly used in human medication to ease depression by affecting serotonin levels. Their residues and metabolites can be detected in the aquatic environment and its biota. They may also alter serotonin levels in aquatic invertebrates, thereby perturbing physiological functions. To investigate whether such perturbations can indeed be expected, shore crabs (Carcinus maenas) were injected either with serotonin, fluoxetine or a combination of both. Dose-dependent effects of fluoxetine ranging from 250 to 750nM were investigated. Gene expression of crustacean hyperglycemic hormone (chh) as well as moult inhibiting hormone (mih) was assessed by RT-qPCR at 2h and 12h after injection. Glucose and ecdysteroid levels in the haemolymph were monitored in regular intervals until 12h. Serotonin led to a rapid increase of chh and mih expression. On the contrary, fluoxetine only affected chh and mih expression after several hours, but kept expression levels significantly elevated. Correspondingly, serotonin rapidly increased glycaemia, which returned to normal or below normal levels after 12h. Fluoxetine, however, resulted in a persistent low-level increase of glycaemia, notably during the period when negative feedback regulation reduced glycaemia in the serotonin treated animals. Ecdysteroid levels were significantly decreased by serotonin and fluoxetine, with the latter showing less pronounced and less rapid, but longer lasting effects. Impacts of fluoxetine on glycaemia and ecdysteroids were mostly observed at higher doses (500 and 750nM) and affected principally the response dynamics, but not the amplitude of glycaemia and ecdysteroid-levels. These results suggest that psychoactive drugs are able to disrupt neuroendocrine control in decapod crustaceans, as they interfere with the normal regulation of the serotonergic system.

Biochemical and locomotor responses of Carcinus maenas exposed to the serotonin reuptake inhibitor fluoxetine

The aim of this study was to assess the effects of the widely used anti-depressant fluoxetine on behaviour (locomotion), moulting, neuromuscular transmission, energy production and anti-oxidant defences' efficiency of the epibenthic crab Carcinus maenas. Crabs were individually exposed to fluoxetine concentrations for 7d. Effects on locomotion were assessed at the end of the exposure using an open field test adapted to C. maenas in the present study. Tissue samples were later collected to evaluate fluoxetine effects on physiological functions using the activity of key enzymes and other parameters as biomarkers, namely: N-acetyl-β-glucosaminidase (NAGase) in the epidermis (moulting) and the hepatopancreas; cholinesterases (ChE) in muscle (neuromuscular cholinergic transmission); NADP(+)-dependent isocitrate dehydrogenase (IDH) and lactate dehydrogenease (LDH) in muscle (energy production); glutathione S-transferases (GST) in hepatopancreas (biotransformation and oxidative stress system); glutathione reductase (GR), and glutathione peroxidade (GPx), total glutathione levels (TG) and lipid peroxidation levels in the hepatopancreas (anti-oxidant defences and oxidative damage). Because no information on C. maenas NAGase activity was previously available, its variation during the moult cycle was also investigated. The results showed that locomotion was significantly increased at fluoxetine concentrations equal or above 120 μg L⁻¹, with animals spending more time moving, walking longer distances than controls. Levels of NAGase activity were found to vary in relation to C. maenas moult cycle, but no alterations were observed after exposure to fluoxetine. Significant increases in the activity of ChE, GST and GR enzymes, and the levels of TG were found, with a lowest observed effect concentration (LOEC) of 120 μg L⁻¹. Effects on locomotion were significantly and positively correlated to those induced on ChE activity. The results raise concern when hypothesising conditions of chronic exposure in the wild.

Induction of ovarian maturation and spawning in Penaeus monodon broodstock by double-stranded RNA

Ovarian maturation in crustacean is under the control of gonad-inhibiting hormone (GIH); a neuropeptide secreted from X-organ sinus gland complex in eyestalks. Unilateral eyestalk ablation that partially destroys GIH source is therefore a general practice in Penaeus monodon hatchery to induce ovarian maturation and spawning. Our previous report showed that silencing of GIH expression by GIH-specific double-stranded RNA (GIH-dsRNA) resulted in an increased expression level of vitellogenin in P. monodon, thus suggesting that GIH-dsRNA could be an alternative method to induce ovarian maturation in female P. monodon broodstock. In this study, we further demonstrated that a single injection of GIH-dsRNA into previtellogenic female P. monodon at the concentration of 3 µg GIH-dsRNA per gram body weight of shrimp was able to inhibit GIH expression for a minimum of 30 days. This dsRNA-mediated GIH silencing led to ovarian maturation and eventual spawning in both domesticated and wild female broodstock, particularly with a comparable effect to eyestalk ablation in wild shrimp. This is the first report that demonstrates a potential strategy to induce ovarian maturation in female P. monodon broodstock by GIH-dsRNA and thus provides a possible substitute for the cruel and detrimental eyestalk ablation practice.

Hormonal and synaptic influences of serotonin on adult neurogenesis

New neurons are incorporated into the adult brains of a variety of organisms, from humans and higher vertebrates, to non-vertebrates such as crustaceans. In virtually all of these systems serotonergic pathways appear to provide important regulatory influences over the machinery producing the new neurons. We have developed an in vitro preparation where adult neurogenesis can be maintained under highly controlled conditions, and are using this to test the influence of hormones on the production of neurons in the crustacean (Homarus americanus) brain. Serotonin levels have been manipulated in this in vitro preparation, and the resulting effects on the rate of neurogenesis have been documented. In addition we have compared in vitro influences of serotonin with results acquired from in vivo exposure of whole animals to serotonin. These experiments suggest that there are multiple mechanisms and pathways by which serotonin may regulate neurogenesis in the crustacean brain: (1) serotonin is effective in regulating neurogenesis at levels as low as 10(-10)M, suggesting that circulating serotonin may have hormonal influences on neuronal precursor cells residing in a vascular niche or the proliferation zones; (2) contrasting effects of serotonin on neurogenesis (up- vs. down-regulation) at high concentrations (10(-4)M), dependent upon whether eyestalk tissue is present or absent, indicate that serotonin elicits the release of substances from the sinus glands that are capable of suppressing neurogenesis; (3) previously demonstrated (Beltz, B.S., Benton, J.L., Sullivan, J.M., 2001. Transient uptake of serotonin by newborn olfactory projection neurons. Proc. Natl. Acad. Sci. USA 98, 12730-12735) serotonergic fibers from the dorsal giant neuron project directly into the proliferation zone in Cluster 10, suggest synaptic or local influences on neurogenesis in the proliferation zones where the final cell divisions and neuronal differentiation occur. Serotonin therefore regulates neurogenesis by multiple pathways, and the specific mode of influence is concentration-dependent.

Role of biogenic amines and cHH in the crustacean hyperglycemic stress response

In this study, we investigated (using bioassays and ELISA) the variation of cHH (crustacean hyperglycemic hormone) level in the eyestalks and hemolymph of Palaemon elegans (Rathke) (Decapoda, Caridea) following injection of serotonin (5-HT) and dopamine (DA) and correlated cHH profile with the variation in amount and time course of glycemia.

5-HT induced in P. elegans a rapid and massive release of cHH from the eyestalk into the hemolymph followed by hyperglycemia. On the contrary, DA did not significantly affect cHH release and hyperglycemia. In addition, we measured the level and variation of 5-HT in the eyestalk and hemolymph of P. elegans following copper contamination. The release of 5-HT from the eyestalk is very rapid and dose dependent. In the hemolymph, a peak of 5-HT occurs after 30 min, and again the circulating concentration of 5-HT is dose dependent on copper exposure. After 1 h, the level of 5-HT slowly decreases to basal level.

The release of 5-HT from the eyestalk into the hemolymph after copper exposure precedes the release of cHH, confirming its role as a neurotransmitter acting on cHH neuroendocrine cells. The fact that copper induced a rapid and massive release of 5-HT from the eyestalk can explain its demonstrated role in inducing the release of cHH and the consequent hyperglycemia in intact but not eyestalkless animals.

Endocrine-mediated effects of UV-A irradiation on grass shrimp (Palaemonetes pugio) reproduction

Although much is known regarding photoperiodic effects on crustacean egg production, the effects of ultraviolet (UV) light on reproduction has not been investigated. Likewise, little is known concerning the interaction between UV and xenobiotic exposure on crustacean reproductive cycles. In this study, male and female grass shrimp, Palaemonetes pugio, were exposed to sublethal concentrations of endosulfan (200 ng/l and 400 ng/l ES) under both white fluorescent (WF) and UV-A (315-400 nm) light conditions for 50 days in laboratory bioassays. Female endocrine (vitellogenin, ecdysteroids, and cholesterol), reproductive (percent gravid, clutch size), and embryo (days to hatch, hatching success, and hatching survival) responses were assessed. UV-exposure alone caused a significant (>4-fold) increase in total Palaemonetes pugio female egg production over the course of 50 days. Exposure to ES and UV significantly lowered the percentage of gravid females relative to UV controls, whereas ES-exposed shrimp under WF lighting did not exhibit these trends. Although higher vitellogenin concentrations and lower ecdysteroid titers were correlated with increased female egg production, cholesterol titers only exhibited a dose-dependent change when exposed to ES. Embryos from females exposed to UV had significantly lower ecdysteroid titers and shorter hatching times but there were no differences in embryo vitellogenin concentrations, hatching success, or hatching survival. These results indicate that UV-A exposure has a pronounced effect on grass shrimp (Palaemonetes pugio) reproduction and is likely mediated through 5-hydroxytrptamine (5-HT)-related neuroendocrine pathways. The implications for decapod aquaculture and evaluating chronic contaminant effects are discussed.

Serotonin depletion in vivo inhibits the branching of olfactory projection neurons in the lobster deutocerebrum

Serotonin depletion during embryogenesis has been shown previously to retard the growth of the olfactory and accessory lobes of the lobster deutocerebrum (Benton et al., 1997). The present study was undertaken to determine whether morphological changes in the interneurons innervating these lobes contribute to this growth retardation. We examined the effects of in vivo serotonin depletion using 5,7-dihydroxytryptamine (5,7-DHT) on the morphology of the olfactory projection neurons, one of two major classes of interneurons that innervate both lobes. Intracellular dye fills of olfactory projection neurons in normal embryos showed that each neuron extensively innervates either the olfactory or accessory lobe before projecting to neuropil regions in the protocerebrum. In embryos injected with 5,7-DHT, however, the deutocerebral arbors of 13.5% of the olfactory projection neurons examined were either markedly reduced compared with normal neurons or absent. Affected neurons also exhibited a number of additional aberrant morphological features suggesting that these neurons represent cells that were affected during their initial morphogenesis. Olfactory projection neurons with aberrant morphologies were also encountered, although less frequently (7.5% of the neurons examined), in control (sham-injected) embryos indicating that the sham injections can affect the development of the brain. This observation provides insights into the nature of effects seen in control embryos in previous experiments (Benton et al., 1997). The results of the present study indicate that in vivo serotonin depletion inhibits the branching of olfactory projection neurons and suggest, therefore, that one of the functions of serotonin during normal development is to promote the ingrowth of these neurons into the deutocerebral neuropils.

Hemolymph levels of methyl farnesoate increase in response to osmotic stress in the green crab, Carcinus maenas

The salinity of estuarine environments can vary widely, exposing resident organisms to considerable osmotic stress. The green crab Carcinus maenas is well known for its ability to osmoregulate in response to such stress. Therefore, we tested the relationship between osmoregulation and hemolymph levels of methyl farnesoate (MF), a compound previously shown to rise in response to various types of environmental stresses. When crabs were transferred from 100% seawater to dilute (hypo-osmotic) seawater, hemolymph osmolality dropped rapidly, reaching an acclimation level 48 h after transfer. Hemolymph levels of MF also rose in these animals after a delay of 6 h, and reached a maximum level at 48 h. MF levels remained elevated as long as the crabs were maintained in dilute seawater, and quickly returned to basal levels when the animals were returned to full strength seawater. In most (but not all) animals, MF levels were elevated when hemolymph osmolality fell below the isosmotic point (approx. 800 mOsm/kg). These data suggest that MF may have a role in osmoregulation by this species. In addition, the elevation of MF by hypo-osmotic seawater suggests an experimental strategy for manipulating MF levels in crustaceans.

Identification and characterization of a hyperglycemic hormone from freshwater giant prawn, Macrobrachium rosenbergii

Crustacean hyperglycemic hormone (CHH), a physiologically important neurohormone stored in the sinus gland of eyestalks, primarily regulates carbohydrate metabolism and also plays significant roles in reproduction, molting and other physiological processes. In the freshwater giant prawn, Macrobrachium rosenbergii, an injection of X-organ sinus gland (XOSG) extract evoked a hyperglycemic response, peaked in 1 h. The hyperglycemic effect of the eyestalk extract was maximal at the dose of 0.5 eyestalk equivalent. CHH fractionated by RP-HPLC, in M. rosenbergii was identified by its hyperglycemic activity and partial amino acid sequence, and the molecular weight of 8534 was determined by matrix-assisted laser desorption ionization mass spectrometry--time of flight analysis (MALDI-TOF). The amino acid sequence of the first 25 residues of CHH showed 72% homology with the first 25 residues of CHH A and CHH B of the American lobster Homarus americanus.

Serotonin-induced short- and long-term sensitization in the crab Chasmagnathus

On sudden presentation of a passing shadow (a visual danger stimulus), the crab Chasmagnathus granulatus responds with a running reaction that we term an escape response (ER). A single administration of 5-HT in a dose range between 0.005 and 0.1 microgram/g enhances the ER at 30 min, and a dose of 10-15 micrograms/g has a similar effect for at least 24 h. The classical 5-HT antagonist cyproheptadine (CYP) given within a dose range 0.01-2.0 micrograms/g has no effect on ER at 30 min, but 0.5 microgram/g blocks the 5-HT-induced short-term sensitization. An enhancing effect of CYP on the ER is shown at 24 h with 1.0-2.0 micrograms/g doses. An explanation of this results in terms of a model similar to that proposed for Aplysia, that is two sensitizing processes linked in parallel and mediated by two different types of 5-HT receptors is discussed. Results indicate that the sensitizing effect of 5-HT is confined to high responders, thus suggesting that crabs of a same population have different degrees of sensitivity to the drug according to their different degrees of reactivity to the visual danger stimulus.

Past and present studies with ponasterones, the first insect molting hormones from plants

The search for antitumor compounds from Southeast Asian plants led to ponasterones, the first phytoecdysteroids, just a year after structure determination of ecdysone and 20-hydroxyecdysone. An independent study of Chinese herb constituents by Takemoto et al. at Tohoku University led to the simultaneous and totally independent discovery of phytoecdysteroids. These findings greatly facilitated research in insect and crustacean physiology. The original structural studies on various phytoecdysteroids have led to interdisciplinary bioorganic studies in the area related to ecdysone receptor, ecdysone biosynthetic precursor (or its storage form), crustacean molt inhibitory factors, and so on.

Demonstration of protein kinase C activity in crustacean Y-organs, and partial definition of its role in regulation of ecdysteroidogenesis

Ecdysteroid-producing Y-organs from the crab Cancer antennarius were shown to possess enzyme activity that was stimulated in vitro by addition of Ca2+, phosphatidylserine, or the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA; ED50, 4 nM). In the presence of calcium and phosphatidylserine, PMA increased protein kinase C activity dose-dependently to a maximum 4-fold increase at 100 nM PMA. Stimulated protein kinase C activity was unaffected by calmodulin (100 nM) but was inhibited by 100 nM trifluoperazine. Pretreatment of cultured Y-organ segments with PMA elevated basal protein kinase C activity, whereas molt-inhibiting hormone (MIH) and calcium ionophore A23187 did not affect activity. PMA (1-100 nM) increased Y-organ steroidogenesis dose-dependently and alleviated suppression due to MIH or lysine vasopressin; PMA effects on steroidogenesis became evident after 2 h of incubation. Another phorbol activator of protein kinase C (phorbol 12, 13-dibutyrate) and a permeable synthetic diacylglycerol (1-oleoyl-2-acetyl-glycerol) stimulated ecdysteroidogenesis while an inactive phorbol (4 alpha-phorbol 12,13-didecanoate) and diolein were ineffective. The inhibitory effects on steroidogenesis of cholera toxin, forskolin, dibutyryl cAMP, and 3-isobutyl-1-methylxanthine were countered by PMA, but PMA did not alter basal or peptide hormone-stimulated Y-organ cAMP levels. Stimulatory effects on steroidogenesis of PMA and of A23187 were not additive, and PMA did not alter inhibition caused by lanthanum (calcium channel blocker) or trifluoperazine (calmodulin inhibitor). PMA increased the incorporation of [3H]leucine into Y-organ protein by 112%, and countered the suppressive effect of MIH on protein synthesis; PMA did not affect RNA synthesis. When Y-organs were suppressed with cycloheximide, PMA was unable to stimulate steroidogenesis. Actinomycin D alone had no effect on steroidogenesis but prevented stimulation by PMA. The results indicate that Y-organs contain protein kinase C activity which stimulates ecdysteroid production and protein synthesis by a mechanism not directly interactive with the cAMP or Ca2+-calmodulin systems.

Regulation of crab Y-organ steroidogenesis in vitro: evidence that ecdysteroid production increases through activation of cAMP-phosphodiesterase by calcium-calmodulin

In decapod crustaceans steroidogenic glands (Y-organs) produce the molting hormone, ecdysone. A putative neuropeptide, molt-inhibiting hormone (MIH), released from eyestalk neurosecretory cells, directly regulates Y-organs by suppressing steroidogenesis; the effect is mediated by an increase in cAMP. We explored calcium-cAMP interactions in the regulation of Y-organs in vitro of the crab, Cancer antennarius. Basal ecdysteroid production was enhanced by extracellular calcium (EC). MIH suppression did not require EC but its action was blocked by high EC. The inhibitors of Ca2+ flux, lanthanum and ruthenium red, mimicked and enhanced MIH action. Calcium ionophore A23187 raised basal steroidogenesis dose-dependently (10(-6) to 10(-4) M) and with time course (effect evident after 2 h) similar to that of suppression by MIH. Low EC enhanced the suppressive effects on steroidogenesis of forskolin and dibutyryl cyclic AMP ((Bu)2cAMP) but not of MIH, lysine vasopressin (LVP), or 3-isobutyl-1-methyl-xanthine (IBMX); basal Y-organ cAMP levels were elevated by low EC and reduced by A23187. A23187 reduced the steroidogenic-suppressive effects of MIH, LVP, forskolin and (Bu)2cAMP but not of IBMX; rises in cAMP induced by MIH, LVP, and forskolin but not by IBMX were blunted by A23187. These findings suggested a stimulatory action of calcium on phosphodiesterase (PDE). The calmodulin (CM) inhibitor trifluoperazine (TFP; 10(-5) to 10(-4) M) reduced basal and A23187-stimulated steroidogenesis, enhanced the inhibitory effects of MIH and (Bu)2cAMP on ecdysteroid production, enhanced the stimulatory effects of MIH and forskolin on cAMP, and blocked the inhibition of cAMP by A23187. Y-organ PDE activity was enhanced by increasing free Ca2+ (10(-7) to 10(-5) M) and inhibited by TFP (10(-5) to 10(-4) M). Adenylate cyclase activity of Y-organ cell particulate fraction was unaffected by Ca2+ or TFP. Calcium stimulates steroidogenesis, apparently by activating a calcium-CM-dependent cAMP-PDE: the action is counter to the cAMP-mediated MIH-inhibitory system. Ca2+ fluxes were measured with dispersed Y-organ cells, in the presence and absence of agents that alter cAMP levels. The ionophore A23187, but not MIH or forskolin, increased 45Ca2+ entry by 45% over untreated control cells. Efflux from 45Ca2+-preloaded cells was increased 30% by MIH and forskolin, but not A23187. These data, together with those further above, suggest that MIH suppresses steroidogenesis in part by fostering Ca2+ depletion, and that the effect is mediated by cAMP.

Regulation of Y-organ ecdysteroidogenesis by molt-inhibiting hormone in crabs: involvement of cyclic AMP-mediated protein synthesis

The crustacean neuropeptide, molt-inhibiting hormone (MIH), directly inhibits Y-organ ecdysteroidogenesis, an effect mediated by cyclic AMP (cAMP) and antagonized by calcium-calmodulin. We investigated regulation of Y-organ protein. RNA, and DNA syntheses by MIH, cAMP, and calcium in relation to steroidogenesis in vitro. Ecdysteroid production and [3H]leucine incorporation into protein were inhibited 50-60 and 80-90%, respectively, by MIH activity in eyestalk extracts (4 eyestalk equivalents), 10(-6) M forskolin, or a combination of 10(-2) M dibutyryl cAMP and 10(-4) M 3-isobutyl-1-methylxanthine (dbcAMP-IBMX). Calcium ionophore A23187 (10(-4) M) stimulated ecdysteroidogenesis two-fold, did not affect the relatively high basal (control) rate of protein synthesis, and reduced the inhibitory effects of forskolin on steroidogenesis and protein synthesis. Incorporation of [3H]uridine into RNA was unaffected by MIH, forskolin, or A23187 but was reduced 50% by dbcAMP-IBMX. Basal rates of [3H]thymidine incorporation into DNA were low and were not affected by treatments. The effects of MIH were specific; extracts of brain or muscle did not alter Y-organ steroidogenesis or protein synthesis, while muscle extract increased precursor incorporation into RNA. Eyestalk extract did not affect [3H]leucine incorporation into protein of brain, muscle, or gill. Cycloheximide (5 micrograms/ml) depressed protein synthesis 90% and steroidogenesis 60%, enhanced the inhibition induced by MIH, and blocked the stimulation of steroidogenesis induced by A23187; effects on basal steroidogenesis were evident after 1 hr. Actinomycin D (1 microgram/ml) depressed RNA synthesis 86% but did not alter basal, MIH-inhibited, or A23187-stimulated ecdysteroidogenesis during incubations. These results indicate that MIH suppresses Y-organ steroidogenesis in part by inhibiting protein synthesis at the translational level; the effect is mediated by cAMP. The stimulation of steroidogenesis by calcium, mediated by lowering cAMP, also appears to depend in part upon protein synthesis.