Serotonin activates a Ca(2+)-dependent K(+) current in identified peptidergic neurons from the crayfish

Antidepressants - The new endocrine disruptors? The case of crustaceans

Antidepressants are high-volume pharmaceuticals that accumulate to concentrations in the μg·L-1 range in surface waters. The release of peptide hormones via neurosecretory cells appears as a natural target for antidepressants. Here I review research that suggests that antidepressants indeed disrupt endocrine signalling in crustaceans, by acting on the synthesis and release of neurohormones, such as crustacean hyperglycaemic hormone, moult inhibiting hormone and pigment dispersing hormone in decapods, as well as methyl farnesoate in Daphnids. Hence, antidepressants can affect hormonal regulation of physiological functions: increase in energy metabolism and activity, lowered ecdysteroid levels, potentially disrupting moult and somatic growth, reducing colour change capacity and compromising camouflage, as well as induction of male sex determination. Several studies further suggest effects of antidepressants on crustacean reproduction, but the hormonal regulation of these effects remains elusive. All things considered, a body of evidence strongly suggests that antidepressants are endocrine disrupting compounds in crustaceans.

A Crab Is Not a Fish: Unique Aspects of the Crustacean Endocrine System and Considerations for Endocrine Toxicology

Crustaceans-and arthropods in general-exhibit many unique aspects to their physiology. These include the requirement to moult (ecdysis) in order to grow and reproduce, the ability to change color, and multiple strategies for sexual differentiation. Accordingly, the endocrine regulation of these processes involves hormones, receptors, and enzymes that differ from those utilized by vertebrates and other non-arthropod invertebrates. As a result, environmental chemicals known to disrupt endocrine processes in vertebrates are often not endocrine disruptors in crustaceans; while, chemicals that disrupt endocrine processes in crustaceans are often not endocrine disruptors in vertebrates. In this review, we present an overview of the evolution of the endocrine system of crustaceans, highlight endocrine endpoints known to be a target of disruption by chemicals, and identify other components of endocrine signaling that may prove to be targets of disruption. This review highlights that crustaceans need to be evaluated for endocrine disruption with consideration of their unique endocrine system and not with consideration of the endocrine system of vertebrates.

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.

Riluzole suppresses postinhibitory rebound in an excitatory motor neuron of the medicinal leech

Postinhibitory rebound (PIR) is an intrinsic property often exhibited by neurons involved in generating rhythmic motor behaviors. Cell DE-3, a dorsal excitatory motor neuron in the medicinal leech exhibits PIR responses that persist for several seconds following the offset of hyperpolarizing stimuli and are suppressed in reduced Na(+) solutions or by Ca(2+) channel blockers. The long duration and Na(+) dependence of PIR suggest a possible role for persistent Na(+) current (I NaP). In vertebrate neurons, the neuroprotective agent riluzole can produce a selective block of I NaP. This study demonstrates that riluzole inhibits cell DE-3 PIR in a concentration- and Ca(2+)-dependent manner. In 1.8 mM Ca(2+) solution, 50-100 µM riluzole selectively blocked the late phase of PIR, an effect similar to that of the neuromodulator serotonin. However, 200 µM riluzole blocked both the early and late phases of PIR. Increasing extracellular Ca(2+) to 10 mM strengthened PIR, but high riluzole concentrations continued to suppress both phases of PIR. These results indicate that riluzole may suppress PIR via a nonspecific inhibition of Ca(2+) conductances and suggest that a Ca(2+)-activated nonspecific current (I(CAN)), rather than I NaP, may underlie the Na(+)-dependent component of PIR.

IL1beta- and LPS-induced serotonin secretion is increased in EC cells derived from Crohn's disease

Gut mucosal enterochromaffin (EC) cells are regarded as key regulators of intestinal motility and fluid secretion via secretion of serotonin (5HT), are increased in numbers in mucosal inflammation and located in close proximity to immune cells. We examined whether interleukin (IL)1beta and Escherichia coli lipopolysaccharide (LPS) induced EC cell 5HT release through Toll-like/IL-1 (TIL) receptor activation, nuclear factor kappa B (NFkappaB) and mitogen-activated protein kinase (MAPK) phosphorylation and evaluated whether somatostatin could inhibit this phenomenon. Pure (>98%) human intestinal EC cells were isolated by fluorescent activated cell sorting from preparations of normal (n = 5) and Crohn's colitis (n = 6) mucosa. 5HT release was measured (ELISA), and NFkappaB and ERK phosphorylation quantitated (ELISA) in response to IL1beta and LPS. 5HT secretion was increased by both E. coli LPS (EC(50) = 5 ng mL(-1)) and IL1beta (EC(50) = 0.05 pmol L(-1)) >2-fold (P < 0.05) in Crohn's EC cells compared with normal EC cells. Secretion was reversible by the TLR4 antagonist, E. coli K12 LPS (IC(50) = 12 ng mL(-1)) and the IL1beta receptor antagonist (ILRA; IC(50) = 3.4 ng mL(-1)). IL1beta caused significant (P < 0.05) NFkappaB and MAPK phosphorylation (40-55%). The somatostatin analogue, lanreotide inhibited IL1beta-stimulated secretion in Crohn's (IC(50) = 0.61 nmol L(-1)) and normal EC cells (IC(50) = 1.8 nmol L(-1)). Interleukins (IL1beta) and bacterial products (E. coli LPS) stimulated 5HT secretion from Crohn's EC cells via TIL receptor activation (TLR4 and IL1beta). Immune-mediated alterations in EC cell secretion of 5HT may represent a component of the pathogenesis of abnormal bowel function in Crohn's disease. Inhibition of EC cell-mediated 5HT secretion may be an alternative therapeutic strategy in the amelioration of inflammatory bowel disease symptomatology.

Luminal regulation of normal and neoplastic human EC cell serotonin release is mediated by bile salts, amines, tastants, and olfactants

Mechanisms by which gut luminal content regulates secretion and motility are ill understood. We evaluated whether neuroendocrine enterochromaffin (EC) cells act as luminal sensors for a wide variety of nutrients and defined the secretory mechanisms of this process. Pure (98-99%) FACS-sorted human EC cells and neoplastic EC cells (KRJ-I) were studied. RT-PCR identified transcripts for T2R1 (bitter), OR1G1 (class II olfactory) and trace amine (TAR1) G protein-coupled receptors (GPCRs) and transporters for glutamine (SNAT1/2), glucose (GLUT1/3/SGLT1), and bile salts (ABST). Glutamine and sodium deoxycholate stimulated 5-HT release (EC(50) = 0.002-0.2 microM; 2-fold release) but were 10-100 times more potent in neoplastic EC cells, which also secreted 6-13 times more 5-HT. Tastants (caffeine, tyramine, octopamine) and olfactants (thymol and eugenol) also stimulated normal and neoplastic EC cell 5-HT secretion (EC(50) = 1.2 nM to 2.1 microM and 0.05 nM to 0.1 microM release, respectively); 2-deoxyglucose and the artificial sweetener sucralose also stimulated (EC(50) = 9.2 and 0.38 nM). 5-HT release was associated with ERK phosphorylation (1.5-fold, P < 0.02) and could be inhibited by a somatostatin analog (IC(50) = 1 pM). Eleven secretory associated genes including the vesicle docking inhibitor STXBP3 were upregulated in response to glutamine and bile salt stimulation in neoplastic EC cells. Targeting STXBP3 expression by use of antisense knockdown significantly (P < 0.05) reduced 5-HT secretion. In conclusion, EC cells express GPCRs and transporters for luminal tastants, olfactants, glutamine, glucose, and bile salts. Activation includes a panel of secretory genes, ERK phosphorylation, and 5-HT secretion. Luminal EC cell regulation is likely to be as important as G cell regulation in gastric acid secretion; development of agents to target EC cell function is therefore a critical therapeutic goal.

Diurnal rhythm in the levels of the serotonin 5-HT1A receptors in the crayfish eyestalk

The crayfish eyestalk (ES) has been postulated as a possible circadian clock. 5-Hydroxytryptamine (5-HT) has been shown to play the role of a neurotransmitter or a modulator in the ES. However, little is known about the 5-HT receptors in the ES. The purpose of this work is to determine the specific binding sites using [(3)H]8-hydroxy-2(di-n-propylamino)tetralin ([(3)H]8-OH-DPAT), a specific agonist of the 5-HT(1A) receptor, and to characterize the diurnal rhythm in the binding by an autoradiography procedure in the crayfish ES. Data show the presence of a circadian rhythmicity in the level of the 5-HT(1A) receptors, principally in two regions: (a) the complex retina (R)-lamina ganglionaris (LG), with the acrophase at dusk and (b) the medulla terminalis (MT), where it was in antiphase. It is suggested that (1) the expression of levels of 5-HT(1A) receptors is modulated by light-dark (LD) cycles, (2) the level of 5-HT(1A) receptors in the R-LG and MT are in antiphase during the 24-h cycle, and (3) there is a different mechanism of action of LD cycles in each of these two anatomical regions of the crayfish ES.

Serotonin modulation of caudal photoreceptor in crayfish

The sixth abdominal ganglion (6th AG) of the crayfish contains two photosensitive neurons. This caudal photoreceptor (CPR) displays spontaneous electrical activity and phasic-tonic responses to light pulses. In this paper, we analyzed the presence of serotonin in the 6th AG and its effects in the modulation of the activity of CPR. In the first part of our study, we identified serotonergic neurons in the 6th AG by immunostaining using an antibody against serotonin. Next, we quantified the serotonin contents in the 6th AG by using liquid chromatography. Finally, we searched for serotonergic modulation of the CPR electrical activity by using conventional extracellular recordings. We found 13 immunopositive neurons located in the ventral side of the 6th AG. The mean diameter of their somata was 23+/-9 microm. In addition, there was immunopositive staining in neuropilar fibers and varicosities. The contents of serotonin and its precursors in the 6th AG varied along the 24-h cycle. Its maximum value was reached by midday. Topic application of serotonin to ganglia kept in darkness increased the CPR spontaneous firing rate and reduced its light responsiveness. Both effects were dose-dependent within ED(50) approximately 1 microM and were blocked by the 5-HT antagonist methysergide. These observations support the role of serotonin as a neurotransmitter or neuromodulator in the CPR of the two species of crayfish Procambarus clarkii and Cherax quadricarinatus.

An improved method for long-term measuring of hemolymph fluctuations of non-essential amino acids, GABA and histamine from freely moving crayfish

The microdialysis method was adapted to obtain long-term hemolymph dialysates from the pericardial cavity of freely moving Procambarus clarkii crayfish, to measure fluctuations of non-essential amino acids, GABA and histamine by high-performance liquid chromatography using off-line fluorometric derivatization. Asp, Ala, Tau, GABA and histamine (HA) reached its maximal concentrations at the daybreak, whereas glutamate (Glu), Gln and Gly peaked at the end of the light period. The minimum and maximal detected amounts for each substance along the 24h cycle were (in microM): 20-300Asp, 100-200Glu, 400-700Gln, 400-600Gly, 100-200Tau, 150-300Ala, 2-10 GABA and 25-250HA. Cocktails containing the relative concentration of each amino acid, GABA and histamine resulted in a hyperpolarization that reduced the spontaneous firing of cultured peptidergic X organ neurons. Glu, GABA and histamine evoked a long-lasting hyperpolarization that suppressed the spontaneous firing, whereas Asp, Gly and Tau evoked a depolarization accompanied with neuronal firing. Finally, neither Ala nor Gln modified the resting membrane potential.