Activation of membrane guanylate cyclase by an invertebrate peptide hormone

Calcium signaling and regulation of ecdysteroidogenesis in crustacean Y-organs

Crustacean Y-organs secrete ecdysteroid molting hormones. Ecdysteroids are released in increased amount during premolt, circulate in hemolymph, and stimulate the events in target cells that lead to molting. During much of the molting cycle, ecdysteroid production is suppressed by molt-inhibiting hormone (MIH), a peptide neurohormone produced in the eyestalks. The suppressive effect of MIH is mediated by a cyclic nucleotide second messenger. A decrease in circulating MIH is associated with an increase in the hemolymphatic ecdysteroid titer during pre-molt. Nevertheless, it has long been hypothesized that a positive regulatory signal or stimulus is also involved in promoting ecdysteroidogenensis during premolt. Data reviewed here are consistent with the hypothesis that an intracellular Ca²⁺ signal provides that stimulus. Pharmacological agents that increase intracellular Ca²⁺ in Y-organs promote ecdysteroidogenesis, while agents that lower intracellular Ca²⁺ or disrupt Ca²⁺ signaling suppress ecdysteroidogenesis. Further, an increase in the hemolymphatic ecdysteroid titer after eyestalk ablation or during natural premolt is associated with an increase in intracellular free Ca²⁺ in Y-organ cells. Several lines of evidence suggest elevated intracellular calcium is linked to enhanced ecdysteroidogenesis through activation of Ca²⁺/calmodulin dependent cyclic nucleotide phosphodiesterase, thereby lowering intracellular cyclic nucleotide second messenger levels and promoting ecdysteroidogenesis. Results of transcriptomic studies show genes involved in Ca²⁺ signaling are well represented in Y-organs. Several recent studies have focused on Ca²⁺ transport proteins in Y-organs. Complementary DNAs encoding a plasma membrane Ca²⁺ ATPase (PMCA) and a sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase (SERCA) have been cloned from crab Y-organs. The relative abundance of PMCA and SERCA transcripts in Y-organs is elevated during premolt, a time when Ca²⁺ levels in Y-organs are likewise elevated. The results are consistent with the notion that these transport proteins act to maintain the Ca²⁺ gradient across the cell membrane and re-set the cell for future Ca²⁺ signals.

Signaling Pathways That Regulate the Crustacean Molting Gland

A pair of Y-organs (YOs) are the molting glands of decapod crustaceans. They synthesize and secrete steroid molting hormones (ecdysteroids) and their activity is controlled by external and internal signals. The YO transitions through four physiological states over the molt cycle, which are mediated by molt-inhibiting hormone (MIH; basal state), mechanistic Target of Rapamycin Complex 1 (mTORC1; activated state), Transforming Growth Factor-β (TGFβ)/Activin (committed state), and ecdysteroid (repressed state) signaling pathways. MIH, produced in the eyestalk X-organ/sinus gland complex, inhibits the synthesis of ecdysteroids. A model for MIH signaling is organized into a cAMP/Ca2+-dependent triggering phase and a nitric oxide/cGMP-dependent summation phase, which maintains the YO in the basal state during intermolt. A reduction in MIH release triggers YO activation, which requires mTORC1-dependent protein synthesis, followed by mTORC1-dependent gene expression. TGFβ/Activin signaling is required for YO commitment in mid-premolt. The YO transcriptome has 878 unique contigs assigned to 23 KEGG signaling pathways, 478 of which are differentially expressed over the molt cycle. Ninety-nine contigs encode G protein-coupled receptors (GPCRs), 65 of which bind a variety of neuropeptides and biogenic amines. Among these are putative receptors for MIH/crustacean hyperglycemic hormone neuropeptides, corazonin, relaxin, serotonin, octopamine, dopamine, allatostatins, Bursicon, ecdysis-triggering hormone (ETH), CCHamide, FMRFamide, and proctolin. Contigs encoding receptor tyrosine kinase insulin-like receptor, epidermal growth factor (EGF) receptor, and fibroblast growth factor (FGF) receptor and ligands EGF and FGF suggest that the YO is positively regulated by insulin-like peptides and growth factors. Future research should focus on the interactions of signaling pathways that integrate physiological status with environmental cues for molt control.

The Crustacean Hyperglycemic Hormone Superfamily: Progress Made in the Past Decade

Early studies recognizing the importance of the decapod eyestalk in the endocrine regulation of crustacean physiology-molting, metabolism, reproduction, osmotic balance, etc.-helped found the field of crustacean endocrinology. Characterization of putative factors in the eyestalk using distinct functional bioassays ultimately led to the discovery of a group of structurally related and functionally diverse neuropeptides, crustacean hyperglycemic hormone (CHH), molt-inhibiting hormone (MIH), gonad-inhibiting hormone (GIH) or vitellogenesis-inhibiting hormone (VIH), and mandibular organ-inhibiting hormone (MOIH). These peptides, along with the first insect member (ion transport peptide, ITP), constitute the original arthropod members of the crustacean hyperglycemic hormone (CHH) superfamily. The presence of genes encoding the CHH-superfamily peptides across representative ecdysozoan taxa has been established. The objective of this review is to, aside from providing a general framework, highlight the progress made during the past decade or so. The progress includes the widespread identification of the CHH-superfamily peptides, in particular in non-crustaceans, which has reshaped the phylogenetic profile of the superfamily. Novel functions have been attributed to some of the newly identified members, providing exceptional opportunities for understanding the structure-function relationships of these peptides. Functional studies are challenging, especially for the peptides of crustacean and insect species, where they are widely expressed in various tissues and usually pleiotropic. Progress has been made in deciphering the roles of CHH, ITP, and their alternatively spliced counterparts (CHH-L, ITP-L) in the regulation of metabolism and ionic/osmotic hemostasis under (eco)physiological, developmental, or pathological contexts, and of MIH in the stimulation of ovarian maturation, which implicates it as a regulator for coordinating growth (molt) and reproduction. In addition, experimental elucidation of the steric structure and structure-function relationships have given better understanding of the structural basis of the functional diversification and overlapping among these peptides. Finally, an important finding was the first-ever identification of the receptors for this superfamily of peptides, specifically the receptors for ITPs of the silkworm, which will surely give great impetus to the functional study of these peptides for years to come. Studies regarding recent progress are presented and synthesized, and prospective developments remarked upon.

Sex-Biased CHHs and Their Putative Receptor Regulate the Expression of IAG Gene in the Shrimp Litopenaeus vannamei

The “eyestalk-androgenic gland (AG)-testis” endocrine axis is involved in male sexual differentiation of crustaceans. The insulin-like androgenic gland hormone (IAG), secreted from the AG, plays a central role in this axis, however key factors upstream the IAG are still poorly understood. Here, two crustacean hyperglycemic hormone (CHH) genes (LvCHH1 and LvCHH2) and their putative receptor guanylate cyclase (LvGC) were identified in Litopenaeus vannamei. LvCHH1 and LvCHH2 belonged to CHH subfamily I members and LvGC was a membrane-bound guanylate cyclase. They were all differentially expressed in eyestalks and gonads of males and females. RNA interference (RNAi) of either LvCHH1 or LvCHH2 increased LvIAG expression, while injection of their recombinant protein decreased LvIAG expression, indicating that LvCHH1 and LvCHH2 are inhibitory factors of LvIAG expression. Yeast two-hybrid assay showed that both LvCHH1 and LvCHH2 interacted with LvGC and their RNAi and recombinant protein injection exerted opposite regulatory effects on the transcriptional expression of LvGC. Meanwhile, knockdown of LvGC increased LvIAG expression. These results suggest that LvGC is the receptor of LvCHH1 and LvCHH2 and they are all involved in male sexual development by regulating LvIAG expression. The present study unveils missing upstream elements in the “eyestalk-AG-testis” endocrine axis in crustacean.

Crustacean hyperglycemic hormone (CHH) affects hemocyte intracellular signaling pathways to regulate exocytosis and immune response in white shrimp Litopenaeus vannamei

Recombinant Litopenaeus vannamei CHH (rLvCHH) was obtained from a bacterial expression system and the intracellular signaling pathways involved in exocytosis and immune response after rLvCHH injection (0.2 and 2 μg/shrimp) was investigated in this study. The results showed that CHH contents increased 51.4%-110.2% (0.2 μg/shrimp) and 65.0%-211.3% (2 μg/shrimp) of the control level. And the contents of three biogenic amines in hemolymph presented a similar variation pattern after rLvCHH injection, but reached the highest level at different time points. Furthermore, the mRNA expression levels of membrane-bound guanylyl cyclase (mGC) (1.20-1.93 fold) and biogenic amine receptors, including type 2 dopamine receptor (DA2R) (0.72-0.89 fold), α2 adrenergic receptor (α2-AR) (0.72-0.91 fold) and 5-HT7 receptor (5-HT7R) (1.37-3.49 fold) in hemocytes were changed consistently with their ligands. In addition, the second messenger and protein kinases shared a similar trend and reached the maximum at the same time respectively. The expression levels of nuclear transcription factor (cAMP response element-binding protein, CREB) and exocytosis-related proteins transcripts were basically overexpressed after rLvCHH stimulation, which reached the peaks at 1 h or 3 h. Eventually, the phenoloxidase (PO) activity (37.4%-158.5%) and antibacterial activity (31.8%-122.3%) in hemolymph were dramatically enhanced within 6 h, while the proPO activity in hemocytes significantly decreased (11.2%-62.6%). Collectively, these results indicate that shrimps L. vannamei could carry out a simple but 'smart' NEI regulation by releasing different neuroendocrine factors at different stages after rLvCHH stimulation, which could couple with their receptors and trigger the downstream signaling pathways during the immune responses in hemocytes.

Tissue distribution and biochemical characteristics of receptors for sinus gland peptide VII as a crustacean hyperglycemic hormone and vitellogenesis-inhibiting hormone of the kuruma prawn, Marsupenaeus japonicus

Crustacean hyperglycemic hormone (CHH) and vitellogenesis-inhibiting hormone (VIH) belong to the CHH family, a neuropeptide superfamily conserved in ecdysozoans. To date, no receptor for the CHH family peptides has been identified in crustaceans. Here, we used a CHH family isoform, Mj-sinus gland peptide (SGP)-VII, as a representative of CHH and VIH in order to determine its target tissues and obtain biochemical information regarding its receptor in the kuruma prawn Marsupenaeus japonicus (Crustacea, Decapoda). An in vitro binding assay using a radiolabeled recombinant Mj-SGP-VII and tissue membranes showed that ligand-receptor binding was specific and dissociable. Six tissues, including the hepatopancreas, gill, heart, skeletal muscle, hindgut, and ovary, were identified as the main targets for Mj-SGP-VII. Scatchard analysis of these six tissues determined the dissociation constant and maximum binding capacity values as K_d = 0.86-3.6 nM and B_max = 102-915 fmol/mg protein, respectively. Of these six tissues, the hepatopancreas, heart, and ovary showed changes in the levels of ligand-binding after the elimination of endogenous ligands by eyestalk ablation. In the hepatopancreas, an increase in the amount of ligand-binding was observed after eyestalk ablation, independent of gender, which appears to be associated with hypoglycemia caused by the treatment. The change observed in the hepatopancreas was due to the increase in the ligand-binding capacity, but not in the ligand-binding affinity, of the receptors. Furthermore, chemical cross-linking analysis demonstrated the presence of target tissue-specific receptors for Mj-SGP-VII with molecular masses of 34-62 kDa. Collectively, the present data provided important information on tissue distribution, temporal changes in expression level, and molecular mass, for the identification and characterization of receptors for CHH family peptides in crustaceans.

Involvement of second messengers in the signaling pathway of vitellogenesis-inhibiting hormone and their effects on vitellogenin mRNA expression in the whiteleg shrimp, Litopenaeus vannamei

We incubated fragments of Litopenaeus vannamei ovary to investigate second messengers involved in the regulation of vitellogenin (vg) mRNA levels. The use of 100nM recombinant vitellogenesis-inhibiting hormone (VIH) (corresponding to recombinant L. vannamei sinus gland peptide-G: rLiv-SGP-G) significantly reduced vg mRNA expression in sub-adults after 8h incubation to less than 20% of the control. The concentration of intracellular cyclic guanosine monophosphate (cGMP) increased 3.2-fold relative to the control after 2h incubation with rLiv-SGP-G. However, it reached levels 18-fold relative to the control after 0.5h incubation with rLiv-SGP-G where 3-isobutyl-1-methylxanthine (a phosphodiesterase inhibitor) was also added. Moreover, vg mRNA expression was significantly reduced to less than 50% of the control after 24h incubation with 1μM A23187 (a calcium ionophore). Thus, rLiv-SGP-G and calcium ionophore reduced vg mRNA expression in in vitro-cultured ovary, and cGMP may be involved in the signaling pathway of VIH. Overall, the above results suggest that vg mRNA expression might be inhibited in vitro by increasing intracellular cGMP and Ca²⁺ in L. vannamei ovary.

Identification and characterization of receptors for ion transport peptide (ITP) and ITP-like (ITPL) in the silkworm Bombyx mori

Ion transport peptide (ITP) and its alternatively spliced variant, ITP-like (ITPL), are insect peptides that belong to the crustacean hyperglycemic hormone family. These peptides modulate the homeostatic mechanisms for regulating energy metabolism, molting, and reproduction and are specifically conserved in ecdysozoans. Many of the details of the molecular mechanisms by which crustacean hyperglycemic hormone family peptides exert pleiotropy remain to be elucidated, including characterization of their receptors. Here we identified three Bombyx mori orphan neuropeptide G protein-coupled receptors (BNGRs), BNGR-A2, -A24, and -A34, as receptors for ITP and ITPL (collectively referred to as ITPs). BNGR-A2 and -A34 and BNGR-A24 respond to recombinant ITPs, respectively, with EC50 values of 1.1-2.6 × 10(-8) M, when expressed in a heterologous expression system. These three candidate BNGRs are expressed at larval B. mori tissues targeted by ITPs, with cGMP elevation observed after exposure to recombinant ITPs. ITPs also increased the cGMP level in B. mori ovary-derived BmN cells via membrane-bound and soluble guanylyl cyclases. The simultaneous knockdown of bngr-A2 and -A34 significantly decreased the response of BmN cells to ITP, whereas knockdown of bngr-A24 led to decreased responses to ITPL. Conversely, transient expression of bngr-A24 potentiated the response of BmN cells to ITPL. An in vitro binding assay showed direct interaction between ITPs and heterologously expressed BNGRs in a ligand-receptor-specific manner. Taken together, these data demonstrate that BNGR-A2 and -A34 are ITP receptors and that BNGR-A24 is an ITPL receptor in B. mori.

Qualitative and quantitative top-down mass spectral analysis of crustacean hyperglycemic hormones in response to feeding

An efficient pipeline for peptide discovery accelerates peptidomic analysis and facilitates a better understanding of the functional roles of neuropeptides. However, qualitative and quantitative analysis of large neuropeptides is challenging due to the bigger molecular sizes, multiple PTMs, and interference by homologous isoforms. Herein, we refined two methodologies in the pipeline for highly confident and efficient MS-based peptide discovery. For the qualitative analysis, the so-called "high resolution/accurate mass" measurement on Orbitrap mass spectrometers was integrated with computer-assisted homology search, which was successfully applied to decipher the substituted amino acid residues in large neuropeptides by referring to homologous sequences. For the quantitative analysis, a new isotopic labeling-assisted top-down MS strategy was developed, which enabled direct monitoring of the abundance changes of endogenous large neuropeptides. By using the refined peptide discovery pipeline, one novel crustacean hyperglycemic hormone (CHH) from the Dungeness crab sinus glands was confidently identified and de novo sequenced, and its relative abundance was quantified. Comparative analysis of CHHs in unfed and fed crabs revealed that the peptide abundance in the sinus glands was significantly increased after food intake, suggesting that the release of CHHs might be altered by feeding behavior.

Mode of action of a Drosophila FMRFamide in inducing muscle contraction

Drosophila melanogaster is a model system for examining the mechanisms of action of neuropeptides. DPKQDFMRFamide was previously shown to induce contractions in Drosophila body wall muscle fibres in a Ca(2+)-dependent manner. The present study examined the possible involvement of a G-protein-coupled receptor and second messengers in mediating this myotropic effect after removal of the central nervous system. DPKQDFMRFamide-induced contractions were reduced by 70% and 90%, respectively, in larvae with reduced expression of the Drosophila Fmrf receptor (FR) either ubiquitously or specifically in muscle tissue, compared with the response in control larvae in which expression was not manipulated. No such effect occurred in larvae with reduced expression of this gene only in neurons. The myogenic effects of DPKQDFMRFamide do not appear to be mediated through either of the two Drosophila myosuppressin receptors (DmsR-1 and DmsR-2). DPKQDFMRFamide-induced contractions were not reduced in Ala1 transgenic flies lacking activity of calcium/calmodulin-dependent protein kinase (CamKII), and were not affected by the CaMKII inhibitor KN-93. Peptide-induced contractions in the mutants of the phospholipase C-β (PLCβ) gene (norpA larvae) and in IP3 receptor mutants were similar to contractions elicited in control larvae. The peptide failed to increase cAMP and cGMP levels in Drosophila body wall muscles. Peptide-induced contractions were not potentiated by 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor, and were not antagonized by inhibitors of cAMP-dependent or cGMP-dependent protein kinases. Additionally, exogenous application of arachidonic acid failed to induce myogenic contractions. Thus, DPKQDFMRFamide induces contractions via a G-protein coupled FMRFamide receptor in muscle cells but does not appear to act via cAMP, cGMP, IP3, PLC, CaMKII or arachidonic acid.

Signal transduction for Schistocerca gregaria ion transport peptide is mediated via both cyclic AMP and cyclic GMP

The second messengers involved in the signal transduction for Schistocerca gregaria, ion transport peptide (Schgr-ITP) that regulates ion and fluid transport across the ileum of the desert locust S. gregaria, were measured using competitive enzyme-linked immunosorbent assays (ELISAs). Synthetic Schgr-ITP elevates intracellular levels of both cyclic AMP and cyclic GMP, measured over a 15 min period in the presence of 3-isobutyl-1-methylxanthine, in a dose-dependent manner. Furthermore, crude corpora cardiaca (CC) extracts elevate intracellular cyclic AMP levels 2-fold greater than Schgr-ITP, suggesting that factors present in the CC, other than Schgr-ITP, also act via this second messenger. These results suggest that the interaction of Schgr-ITP with two separate receptors, most likely a G-protein coupled receptor and a membrane bound guanylate cyclase, elevates intracellular levels of cyclic AMP and cyclic GMP to regulate ion and fluid transport across the locust ileum. Cyclic AMP stimulates Cl(-), K(+) and Na(+) reabsorption, whereas secretion of H(+) into the lumen of the ileum is most likely mediated via cyclic GMP. Cyclic GMP also stimulates Cl(-) uptake in a similar manner to cyclic AMP. The measurement of tissue (central nervous system) levels of Schgr-ITP using an indirect ELISA confirms that the peptide is only present in the locust brain and the CC. The amounts present are greatest in the CC, where the peptide is presumably stored for release into the hemolymph when locusts feed.

The CHH-superfamily of multifunctional peptide hormones controlling crustacean metabolism, osmoregulation, moulting, and reproduction

Apart from providing an up-to-date review of the literature, considerable emphasis was placed in this article on the historical development of the field of "crustacean eyestalk hormones". A role of the neurosecretory eyestalk structures of crustaceans in endocrine regulation was recognized about 80 years ago, but it took another half a century until the first peptide hormones were identified. Following the identification of crustacean hyperglycaemic hormone (CHH) and moult-inhibiting hormone (MIH), a large number of homologous peptides have been identified to this date. They comprise a family of multifunctional peptides which can be divided, according to sequences and precursor structure, into two subfamilies, type-I and -II. Recent results on peptide sequences, structure of genes and precursors are described here. The best studied biological activities include metabolic control, moulting, gonad maturation, ionic and osmotic regulation and methyl farnesoate synthesis in mandibular glands. Accordingly, the names CHH, MIH, and GIH/VIH (gonad/vitellogenesis-inhibiting hormone), MOIH (mandibular organ-inhibiting hormone) were coined. The identification of ITP (ion transport peptide) in insects showed, for the first time, that CHH-family peptides are not restricted to crustaceans, and data mining has recently inferred their occurrence in other ecdysozoan clades as well. The long-held tenet of exclusive association with the eyestalk X-organ-sinus gland tract has been challenged by the finding of several extra nervous system sites of expression of CHH-family peptides. Concerning mode of action and the question of target tissues, second messenger mechanisms are discussed, as well as binding sites and receptors. Future challenges are highlighted.

Regulation of crustacean molting: a review and our perspectives

Molting is a highly complex process that requires precise coordination to be successful. We describe the early classical endocrinological experiments that elucidated the hormones and glands responsible for this process. We then describe the more recent experiments that have provided information on the cellular and molecular aspects of molting. In addition to providing a review of the scientific literature, we have also included our perspectives.

Molecular cloning of a putative crustacean hyperglycemic hormone (CHH) isoform from extra-eyestalk tissue of the blue crab (Callinectes sapidus), and determination of temporal and spatial patterns of CHH gene expression

Crustacean hyperglycemic hormone (CHH) is a polypeptide neurohormone involved in regulation of multiple physiological processes. We report here the cloning from thoracic ganglia of the blue crab (Callinectes sapidus) a cDNA (CsCHH-2) encoding a putative CHH isoform (CsCHH-2). CsCHH-2 is structurally similar to a putative preproCHH (CsCHH-1) previously cloned from eyestalk ganglia of C. sapidus. The two preprohormones possess an identical signal peptide and CHH precursor related peptide, but differ in the mature CHH polypeptide. An analysis by RT-PCR of the tissue distribution of CsCHH-1 and CsCHH-2 revealed the former is restricted to eyestalk neural ganglia, while the latter is widely distributed among tissues. The type of CHH transcript present in eyestalk and thoracic ganglia did not vary as a function of the molt cycle. An assessment of transcript abundance in tissues of intermolt crabs showed the abundance of the CsCHH-1 transcript in eyestalk ganglia far exceeds the abundance of the CsCHH-2 transcript in extra-eyestalk tissue. An assessment of transcript abundance during a molt cycle showed CsCHH-1 transcript abundance in eyestalk ganglia was low during intermolt, rose during premolt, reaching a peak in D(3), then fell prior to molting, and remained low during postmolt. By contrast, CsCHH-2 transcript abundance in thoracic ganglia was low during intermolt, rose sharply during D(2), then dropped in D(3) and remained low during postmolt. The results are consistent with the hypothesis that CsCHH-1 and CsCHH-2 differ with respect to physiological function.

Neuropeptide action in insects and crustaceans

Physiological processes are regulated by a diverse array of neuropeptides that coordinate organ systems. The neuropeptides, many of which act through G protein-coupled receptors, affect the levels of cyclic nucleotides (cAMP and cGMP) and Ca(2+) in target tissues. In this perspective, their roles in molting, osmoregulation, metabolite utilization, and cardiovascular function are highlighted. In decapod crustaceans, inhibitory neuropeptides (molt-inhibiting hormone and crustacean hyperglycemic hormone) suppress the molting gland through cAMP- and cGMP-mediated signaling. In insects, the complex movements during ecdysis are controlled by ecdysis-triggering hormone and a cascade of downstream neuropeptides. Adipokinetic/hypertrehalosemic/hyperprolinemic hormones mobilize energy stores in response to increased locomotory activity. Crustacean cardioacceleratory (cardioactive) peptide, proctolin, and FMRFamide-related peptides act on the heart, accessory pulsatile organs, and excurrent ostia to control hemolymph distribution to tissues. The osmoregulatory challenge of blood gorging in Rhodnius prolixus requires the coordinated release of serotonin and diuretic and antidiuretic hormones acting on the midgut and Malpighian tubules. These studies illustrate how multiple neuropeptides allow for flexibility in response to physiological challenges.

Crustacean hyperglycemic hormone (CHH) neuropeptidesfamily: Functions, titer, and binding to target tissues

The removal of the eyestalk (s) induces molting and reproduction promoted the presence of regulatory substances in the eyestalk (ES), particularly medulla terminalis X-organ and the sinus gland (MTXO-SG). The PCR-based cloning strategies have allowed for isolating a great number of cDNAs sequences of crustacean hyperglycemic hormone (CHH) neuropeptides family from the eyestalk and non-eyestalk tissues, e.g., pericardial organs and fore- and hindguts. However, the translated corresponding neuropeptides in these tissues, their circulating concentrations, the mode of actions, and specific physiological functions have not been well described. The profiles of CHH neuropeptides present in the MTXO-SG may differ among decapod crustacean species, but they can be largely divided into two sub-groups on the basis of structural homology: (1) CHH and (2) molt-inhibiting hormone (MIH)/mandibular organ-inhibiting hormone (MOIH)/vitellogenesis/gonad-inhibiting hormone (V/GIH). CHH typically elevating the level of circulating glucose from animals under stressful conditions (hyper- and hypothermia, hypoxia, and low salinity) has multiple target tissues and functions such as ecdysteroidogenesis, osmoregulation, and vitellogenesis. Recently, MIH, known for exclusively suppressing ecdysteroidogenesis in Y-organs, is also reported to have an additional role in vitellogenesis of adult female crustacean species, suggesting that some CHH neuropeptides may acquire an extra regulatory role in reproduction at adult stage. This paper reviews the regulatory roles of CHH and MIH at the levels of specific functions, temporal and spatial expression, titers, their binding sites on the target tissues, and second messengers from two crab species: the blue crab, Callinectes sapidus, and the European green crab, Carcinus maenas. It further discusses the diverse regulatory roles of these neuropeptides and the functional plasticity of these neuropeptides in regard to life stage and species-specific physiology.

[Burn: An inflammatory process]

Thermal injury induce a two-phase inflammatory response: first, a pro-inflammatory status, resulting in a systemic inflammatory response syndrome, then an anti-inflammatory phase characterized by a profound defect in cellular-mediated immunity. This inflammatory reaction proceeds from complex phenomenons in whom many cellular elements are involved (macrophage is the central one) and very complex molecular products interact (especially cytokines). These phenomenons promote significant physiopathologic consequences, especially on cardiovascular homeostasis and endothelial permeability, that lower the prognosis. The inflammatory reaction can be modified, enhanced or maintained by adverse events (i.e. infection) resulting in degradation of clinical situation. Despite a better comprehension of the phenomenons underlying this inflammatory process, diagnosis or therapeutic applications are at that time disappointing.

Molt-inhibiting hormone stimulates vitellogenesis at advanced ovarian developmental stages in the female blue crab, Callinectes sapidus 2: novel specific binding sites in hepatopancreas and cAMP as a second messenger

The finding that molt-inhibiting hormone (MIH) regulates vitellogenesis in the hepatopancreas of mature Callinectes sapidus females, raised the need for the characterization of its mode of action. Using classical radioligand binding assays, we located specific, saturable, and non-cooperative binding sites for MIH in the Y-organs of juveniles (J-YO) and in the hepatopancreas of vitellogenic adult females. MIH binding to the hepatopancreas membranes had an affinity 77 times lower than that of juvenile YO membranes (K_D values: 3.22 × 10^-8 and 4.19 × 10^-10 M/mg protein, respectively). The number of maximum binding sites (B_MAX) was approximately two times higher in the hepatopancreas than in the YO (B_MAX values: 9.24 × 10^-9 and 4.8 × 10^-9 M/mg protein, respectively). Furthermore, MIH binding site number in the hepatopancreas was dependent on ovarian stage and was twice as high at stage 3 than at stages 2 and 1. SDS-PAGE separation of [¹²⁵I] MIH or [¹²⁵I] crustacean hyperglycemic hormone (CHH) crosslinked to the specific binding sites in the membranes of the J-YO and hepatopancreas suggests a molecular weight of ~51 kDa for a MIH receptor in both tissues and a molecular weight of ~61 kDa for a CHH receptor in the hepatopancreas. The use of an in vitro incubation of hepatopancreas fragments suggests that MIH probably utilizes cAMP as a second messenger in this tissue, as cAMP levels increased in response to MIH. Additionally, 8-Bromo-cAMP mimicked the effects of MIH on vitellogenin (VtG) mRNA and heterogeneous nuclear (hn) VtG RNA levels. The results imply that the functions of MIH in the regulation of molt and vitellogenesis are mediated through tissue specific receptors with different kinetics and signal transduction. MIH ability to regulate vitellogenesis is associated with the appearance of MIH specific membrane binding sites in the hepatopancreas upon pubertal/final molt.

The specific binding sites of eyestalk- and pericardial organ-crustacean hyperglycaemic hormones (CHHs) in multiple tissues of the blue crab, Callinectes sapidus

Crustacean hyperglycaemic hormone from the pericardial organ (PO-CHH) is a CHH-related neuropeptide but its function and target tissues are not known in crustaceans. To investigate this issue, we employed radiolabelled ligand binding and cGMP assays, using eyestalk-CHH (ES-CHH) as a reference neuropeptide. The membranes were prepared from various tissues of Callinectes sapidus: hepatopancreas, hindgut, midgut, gills, heart, abdominal muscles and scaphognathites. Like ES-CHH, recombinant PO-CHH (rPO-CHH) specifically bound to the membranes of scaphognathites=abdominal muscles>midgut>gills> heart>hindgut and hepatopancreas (list order corresponds to the number of binding sites). The specific binding sites of (125)I-ES-CHH in hepatopancreas and gills were saturable and displaceable. The abdominal muscle membrane binding sites were specific and saturable to both CHHs. These binding sites were displaced by homologous neuropeptides, but poorly displaced by the heterologous counterpart. As for the second messenger, the expected increment (3- to >20-fold) in the amount of cGMP produced by ES-CHH was noted in most tissues tested except midgut. Recombinant PO-CHH increased cGMP production 1.5- to 4-fold in scaphognathites, heart, midgut, hindgut and abdominal muscles. The results obtained from the binding study suggest that PO-CHH also has multiple target tissues of which abdominal muscles and scaphognathites are the primary ones. The differences in the primary amino acid sequences of PO-CHH and ES-CHH, particularly in the C-terminal region and in the amidation at C-terminus, may contribute to the truncated responses of hyperglycaemia, cGMP stimulation and binding affinity.

Effects of elevated ecdysteroid on tissue expression of three guanylyl cyclases in the tropical land crab Gecarcinus lateralis: possible roles of neuropeptide signaling in the molting gland

Two eyestalk (ES) neuropeptides, molt-inhibiting hormone (MIH) and crustacean hyperglycemic hormone (CHH), increase intracellular cGMP levels in target tissues. Both MIH and CHH inhibit ecdysteroid secretion by the molting gland or Y-organ (YO), but apparently through different guanylyl cyclase(GC)-dependent pathways. MIH signaling may be mediated by nitric oxide synthase (NOS) and NO-sensitive GC. CHH binds to a membrane receptor GC. As molting affects neuropeptide signaling, the effects of ecdysteroid on the expression of the land crab Gecarcinus lateralis β subunit of a NO-sensitive GC (Gl-GC-Iβ), a membrane receptor GC (Gl-GC-II) and a NO-insensitive soluble GC (Gl-GC-III) were determined. Gl-GC-Iβ isoforms differing in the absence or presence of an N-terminal 32-amino acid sequence and Gl-GC-III were expressed at higher mRNA levels in ES ganglia, gill,hepatopancreas, ovary and testis, and at lower levels in YO, heart and skeletal muscle. Three Gl-GC-II isoforms, which vary in the length of insertions (+18, +9 and +0 amino acids) within the N-terminal ligand-binding domain, differed in tissue distribution. Gl-GC-II(+18) was expressed highly in striated muscle (skeletal and cardiac muscles); Gl-GC-II(+9) was expressed in all tissues examined (ES ganglia, YO, gill, hepatopancreas, striated muscles and gonads); and Gl-GC-II(+0) was expressed in most tissues and was the dominant isoform in ES and thoracic ganglia. ES ablation, which increased hemolymph ecdysteroid, increased Gl-GC-II(+18) mRNA level in claw muscle. Using real-time RT-PCR, ES ablation increased Gl-GC-Iβ, Gl-GC-III and ecdysone receptor mRNA levels in the YOs ∼ten-, ∼four- and∼twofold, respectively, whereas Gl-GC-II mRNA level was unchanged. A single injection of 20-hydroxyecdysone into intact animals transiently lowered Gl-GC-Iβ in hepatopancreas, testis and skeletal muscle, and certain Gl-GC-II isoforms in some of the tissues. These data suggest that YO and other tissues can modulate responses to neuropeptides by altering GC expression.

Binding sites of crustacean hyperglycemic hormone and its second messengers on gills and hindgut of the green shore crab, Carcinus maenas: a possible osmoregulatory role

To determine the possible involvement of crustacean hyperglycemic hormone (CHH) in osmoregulation in crustaceans, ligand binding and second messenger assays were performed on gills and hindgut preparations of the green shore crab Carcinus maenas, whilst midgut gland, previously known as one of the target tissues of CHH served as a control tissue. Classical receptor binding analyses using [(125)I]CHH by saturation and displacement experiments from membrane preparations from gills, hindgut, and midgut glands demonstrated that CHH binding characteristics involved one site, highly specific, saturable, and displaceable kinetics: (gills: K(D) 5.87 +/- 2.05 x 10(-10) and B(MAX) 6.50 +/- 1.15 x 10(-10), hindgut: K(D) 3.54 +/- 1.49 x 10(-10) and B(MAX) 2.31 +/- 0.44 x 10(-10), and midgut gland: K(D) 7.28 +/- 0.9 x 10(-10) and B(MAX) 3.28 +/- 0.25 x 10(-10)) all expressed as M/mg protein. No differences, in terms of displacement were observed between the two CHH isoforms (N-terminally blocked pGlu and unblocked Gln) variants. CHH binding sites appeared to be coupled to a second messenger system involving cGMP in all the tissues examined. Exposure of crabs to dilute seawater increased levels of cGMP, glucose in gills and circulating CHH levels. Other crustacean neuropeptides including crustacean cardioactive peptide, molt inhibiting hormone, L-enkephalin, FMRF-amide, proctolin, and crustacean hyperglycemic hormone precursor-related peptide were tested with regard to possible osmoregulatory roles with reference to changes in second messenger (cAMP and cGMP) concentrations in gill, hindgut, and midgut tissues in vitro, following application at 2 x 10(-8) M but all were found to be inactive. Thus, it seems likely that CHH is a pertinent neurohormone involved in osmoregulation, thus expanding its many functions as a pleiotropic hormone in crustaceans.

A role for cyclic nucleotide monophosphates in synaptic modulation by a crayfish neuropeptide

DF2 (DRNFLRFamide), a FMRFamide-like peptide, has been shown to increase the amount of transmitter released at crayfish neuromuscular junctions. Here, we examined a possible role for the cyclic nucleotide monophosphates, cAMP and cGMP, in DF2's effects on synaptic transmission. The effects of DF2 on synaptic transmission were monitored by recording excitatory postsynaptic potentials (EPSPs) in the deep abdominal extensor muscles of the crayfish, Procambarus clarkii. A number of activators and inhibitors were used to determine whether or not cAMP, cGMP, protein kinase A (PKA) and protein kinase G (PKG) mediate the effect of this neuropeptide. Phosphodiesterase inhibitors, known to inhibit the breakdown of cAMP (IBMX) and/or cGMP (mdBAMQ), potentiate the effect of DF2 on synaptic transmission. Activators of PKA (Sp-cAMPS) and PKG (8-pCPT-cGMP) increase EPSP amplitude, mimicking the effects of DF2. Inhibitors of PKA (Rp-cAMPS) and PKG (Rp-8-pCPT-cGMPS) each block a portion of the response to the peptide, and when applied together these two inhibitors completely block the response. Taken together, these results indicate that cyclic nucleotides and cyclic nucleotide-dependent protein kinases are necessary components of the pathway underlying modulation by this neuropeptide.

Biochemical and functional aspects of crustacean hyperglycemic hormone in decapod crustaceans: review and update

In crustaceans, neuroendocrine centers are located in different structures of the nervous system. One of these structures, the X-organ-sinus gland complex of the eyestalk, produces several neuropeptides that belong to the two main functionally different families: firstly, the chromatophorotropins, and secondly, a large family comprising various closely related peptides, commonly named CHH/MIH/GIH family. This review updates some aspects of the structural, biochemical and functional properties of the main hyperglycemic neuropeptide of this family, the crustacean hyperglycemic hormone (CHH). The first part of this work is a survey of the neuroendocrine system that produces the neurohormones of the CHH/MIH/GIH family, focusing on recent reports that propose new possible neuroendocrine loci of CHH production, secondly we revise general aspects of the CHH biochemical, and structural characteristics and thirdly, we present a review of the role of CHH in the regulation of several physiological processes of crustaceans as well as new reports on the ontogenetic aspects of CHH. The review is centered only on one group of malacostracan crustaceans, the Decapoda.

Nitric oxide: an inhibitory retrograde modulator in the crustacean heart

The nervous system innervates most of the organs in the body, and controls and coordinates their activities. Effective coordination depends on accurate feedback from target organs. Recent studies have identified a target-based feedback mechanism that regulates a simple neural circuit, the cardiac ganglion-a network of nine neurons whose rhythmic bursts of action potentials drive the contractions of the crustacean heart. The feedback agent, nitric oxide (NO), is produced by the target organ (the heart), and acts on the neural circuit (the ganglion), thus serving as a retrograde, trans-synaptic signaling molecule. NO decreases the ganglionic burst rate, which has both negative chronotropic and negative inotropic effects on the heartbeat. This article will review the evidence identifying NO as an inhibitory modulator in the crustacean heart, and will present new data showing that these inhibitory effects are not mediated by cGMP, the canonical downstream agent mobilized by NO in many other systems. Rather, our data suggest that in the crustacean heart cGMP may play a secondary role in the process of adaptation that occurs in during prolonged exposures to NO.

Insect Neuropeptide and Peptide Hormone Receptors: Current Knowledge and Future Directions

Peptides form a very versatile class of extracellular messenger molecules that function as chemical communication signals between the cells of an organism. Molecular diversity is created at different levels of the peptide synthesis scheme. Peptide messengers exert their biological functions via specific signal-transducing membrane receptors. The evolutionary origin of several peptide precursor and receptor gene families precedes the divergence of the important animal Phyla. In this chapter, current knowledge is reviewed with respect to the analysis of peptide receptors from insects, incorporating many recent data that result from the sequencing of different insect genomes. Therefore, detailed information is provided on six different peptide receptor families belonging to two distinct receptor categories (i.e., the heptahelical and the single transmembrane receptors). In addition, the remaining problems, the emerging concepts, and the future prospects in this area of research are discussed.

Molecular cloning of a putative membrane form guanylyl cyclase from the crayfish Procambarus clarkii

Available data indicate that crustacean hyperglycemic hormone (CHH) stimulates membrane-bound guanylyl cyclase (GC), producing cyclic guanosine 3',5'-monophosphate, which in turn mediates the effect of CHH on carbohydrate metabolism. In the present study, we report the cloning of a cDNA (PcGC-M2) encoding a putative membrane form GC from the muscle of the crayfish, Procambarus clarkii. Analysis of the deduced amino acid sequence shows that PcGC-M2 contains the signature domains characteristic of membrane form GCs, including an extracellular ligand-binding domain, a single transmembrane, and intracellular kinase-like and cyclase catalytic domains. In addition, a C-terminal domain of 247 residues is present following the cyclase catalytic domain. PcGC-M2 is most closely related (33% identity) to a Drosophila membrane form GC (DrGC-1), and an Anopheles gambiae membrane form GC (AgaGC); the three GCs also share a similar distribution pattern of conserved cysteine residues in the extracellular domain. The PcGC-M2 transcript is expressed in several CHH target tissues, including muscle, hepatopancreas, heart, ovary, testis, and gill, suggesting that PcGC-M2 may participate in the signaling cascade activated by CHH.

Expression of nitric oxide synthase and nitric oxide-sensitive guanylate cyclase in the crustacean cardiac ganglion

The cardiac ganglion is a simple central pattern-generating network that controls the rhythmic contractions of the crustacean heart. Enzyme assays and Western blots show that whole heart homogenates from the crab Cancer productus contain high levels of nitric oxide synthase (NOS), an enzyme that catalyzes the conversion of arginine to citrulline with concomitant production of the transmitter nitric oxide (NO). Crab heart NOS is calcium-dependent and has an apparent molecular weight of 110 kDa. In the cardiac ganglion, antibodies to NOS and citrulline indicate the presence of a NOS-like protein and NOS enzymatic activity in the four small pacemaker neurons and the five large motor neurons of the cardiac network. In addition, all cardiac neurons label positively with an antibody to cyclic guanosine monophosphate (cGMP). The NO donor sodium nitroprusside (SNP, 10 mM) stimulates additional cGMP production in the isolated ganglion. This increase is blocked by [(1)H](1,2,4)oxadiazole(4,3-a)quinoxalin-1-one (ODQ, 50 microM), an inhibitor of the NO-sensitive soluble guanylate cyclase (sGC). Taken together, our data indicate that NO- and cGMP-mediated signaling pathways are enriched in the cardiac system relative to other crab tissues and that the cardiac network may be a target for extrinsic and intrinsic neuromodulation via NO produced from the heart musculature and individual cardiac neurons, respectively. The crustacean cardiac ganglion is therefore a promising system for studying cellular and synaptic mechanisms of nitrergic neuromodulation in a simple pattern-generating network.

The natriuretic peptide clearance receptor locally modulates the physiological effects of the natriuretic peptide system

Natriuretic peptides (NPs), mainly produced in heart [atrial (ANP) and B-type (BNP)], brain (CNP), and kidney (urodilatin), decrease blood pressure and increase salt excretion. These functions are mediated by natriuretic peptide receptors A and B (NPRA and NPRB) having cytoplasmic guanylyl cyclase domains that are stimulated when the receptors bind ligand. A more abundantly expressed receptor (NPRC or C-type) has a short cytoplasmic domain without guanylyl cyclase activity. NPRC is thought to act as a clearance receptor, although it may have additional functions. To test how NPRC affects the cardiovascular and renal systems, we inactivated its gene (Npr3) in mice by homologous recombination. The half life of [125I]ANP in the circulation of homozygotes lacking NPRC is two-thirds longer than in the wild type, although plasma levels of ANP and BNP in heterozygotes and homozygotes are close to the wild type. Heterozygotes and homozygotes have a progressively reduced ability to concentrate urine, exhibit mild diuresis, and tend to be blood volume depleted. Blood pressure in the homozygotes is 8 mmHg (1 mmHg = 133 Pa) below normal. These results are consistent with the sole cardiovascular/renal function of NPRC being to clear natriuretic peptides, thereby modulating local effects of the natriuretic peptide system. Unexpectedly, Npr3 -/- homozygotes have skeletal deformities associated with a considerable increase in bone turnover. The phenotype is consistent with the bone function of NPRC being to clear locally synthesized CNP and modulate its effects. We conclude that NPRC modulates the availability of the natriuretic peptides at their target organs, thereby allowing the activity of the natriuretic peptide system to be tailored to specific local needs.

Escherichia coli lipopolysaccharide downregulates soluble guanylate cyclase in pulmonary artery smooth muscle

The soluble isoform of guanylate cyclase (sGC) is activated by nitric oxide (NO) to form guanosine 3':5'-cyclic monophosphate (cGMP). Cyclic GMP levels cause smooth muscle relaxation and regulate vascular tone to various vascular beds, including the lung. Under conditions of cytokine excess the inducible synthesis of NO may result in cGMP overproduction, generalized vasodilation, and septic shock. In the pulmonary bed the opposite response may occur, pulmonary hypertension. We hypothesized that sGC activity becomes downregulated in the face of Escherichia coli lipopolysaccharide (LPS). We tested the effects of LPS on alpha1-subunit sGC mRNA abundance, Western analysis, and enzyme activity in cultured rat pulmonary artery smooth muscle cells. LPS increased extracellular cGMP production by pulmonary artery smooth muscle cells, with increased levels being first detectable at 3-6 h (10 microg/ml LPS) and exceeding 140 pmol/ml by 24 h (P < 0.05). The response was inhibited by 0.05 mM l-NG-monomethyl-l-arginine (l-NMA) and, in turn, restored by 1 mM l-arginine, indicating a NO synthase-dependent response. Pretreating cells with LPS for >/= 3 h inhibited subsequent cGMP synthesis in response to 10(-4) M SNAP for 60 min. Coincubating cells with 0.05 mM l-NMA also reversed this effect. Soluble GC enzyme activity in cells exposed to basal medium alone measured 0.74 pmol cGMP/ml per minute; activity in cells exposed to 10 microg/ml LPS for 24 h decreased to 0.04 pmol cGMP/ml per minute (P < 0.05). LPS pretreatment decreased sGC mRNA abundance and protein mass, but did not totally eliminate them. It is concluded that LPS affects cGMP synthesis at the level of enzyme activity, enzyme mass, and mRNA abundance. Over the short term (<24 h) LPS causes the synthesis of large amounts of cGMP. As the duration of exposure progresses (>/=3 h), mechanisms come into play that decrease cGMP production significantly and include decreases in mRNA abundance, enzyme mass, and enzyme activity.

Natriuretic peptide receptor 1 expression influences blood pressures of mice in a dose-dependent manner

Activation of the natriuretic peptide system lowers blood pressure and causes the excretion of salt. Atrial natriuretic peptide and B-type natriuretic peptide are the humoral mediators of this effect; they act primarily by binding to membrane-bound natriuretic peptide receptor A (NPRA) and stimulating its intrinsic guanylate cyclase activity. To study whether genetically determined differences in NPRA expression affect blood pressure we have generated mice with one, two, three, or four copies of the gene encoding NPRA (Npr1 in the mouse). Atrial natriuretic peptide-dependent guanylate cyclase activity ranged progressively from approximately one-half normal in one-copy animals to twice normal in four-copy animals (P < 0.001). On different diets (0.05%, 2%, and 8% NaCl), the blood pressures of F1 male mice having only one copy of Npr1 averaged 9.1 mmHg (1 mmHg = 133 Pa) above those of wild-type two-copy males (P < 0.001), whereas males with three copies of the gene had blood pressures averaging 5.2 mmHg below normal (P < 0.01). The blood pressures of the one-copy F1 animals were significantly higher (by 6.2 mmHg; P < 0.01) on the high-salt than on the low-salt diet. The blood pressures of four-copy F3 males were significantly lower (by 7 mmHg; P < 0.05) on the high-salt than on the low-salt diet. These results demonstrate that below normal Npr1 expression leads to a salt-sensitive increase in blood pressure, whereas above normal Npr1 expression lowers blood pressures and protects against high dietary salt.

Presynaptic receptors for FMRFamide, histamine and buccalin regulate acetylcholine release at a neuro-neuronal synapse of Aplysia by modulating N-type Ca2+ channels

At an identified neuro-neuronal synapse of the buccal ganglion of Aplysia, quantal release of acetylcholine (ACh) is increased by FMRFamide and decreased by histamine or buccalin. Activation of presynaptic receptors for these neuromodulators modifies a presynaptic Ca2+ current which is nifedipine-resistant and omega-conotoxin-sensitive. The voltage-sensitivity of these N-type Ca2+ channels is increased by FMRFamide and decreased by histamine through the intermediate of G proteins. Buccalin does not implicate G proteins and reduces the Ca2+ current without affecting the voltage-sensitivity of N-type Ca2+ channels. The possibility of relating the shifts in voltage-dependence of the Ca2+ current induced by FMRFamide and histamine to the phosphorylation state of the N-type Ca2+ channels is discussed. A scheme for the complex regulation of ACh release by presynaptic auto- and heteroreceptors is proposed.

Actions of 3-morpholinosydnonimin (SIN-1) on rabbit isolated penile erectile tissue

The effects of the NO-donor 3-morpholinosydnonimin (SIN-1) on isometric tension, cyclic guanosine 3',5'-monophosphate (cyclic GMP) accumulation and neuronal release of 3H-noradrenaline were investigated in rabbit isolated corpus cavernosum (CC), and compared to the actions of sodium nitroprusside (SNP) and the cyclic GMP-specific phosphodiesterase inhibitor zaprinast. SIN-1, zaprinast and SNP concentration dependently relaxed rabbit CC preparations contracted by 1 microM. phenylephrine. All the drugs were highly effective, and the order of potency was SNP > zaprinast > SIN-1. SIN-1 had a biphasic effect on contractions evoked by electrical field stimulation of nerves: at low concentrations (1 and 10 microM.), SIN-1 inhibited the contractions, while at concentrations > or = 100 microM., the contractions were again increased. There were no changes in baseline tension. Electrically evoked contractions were inhibited by zaprinast in a concentration-dependent manner. Compared with controls, 1 mM. SIN-1 caused a significant (p < 0.05) increase in both the basal efflux and in the electrically induced release of 3H from CC preparations incubated with 3H-noradrenaline. SIN-1, zaprinast and SNP increased tissue levels of cyclic GMP. There was no positive correlation between cyclic GMP accumulation and the relaxant effects of the drugs. The effects of SIN-1 and SNP on the tissue content of cyclic GMP were not significantly affected by methylene blue, an inhibitor of soluble guanylate cyclase. It may be concluded that SIN-1, zaprinast and SNP are effective in relaxing isolated penile erectile tissue, and this effect is associated with an increase in the tissue content of cyclic GMP via pathways not sensitive to methylene blue. However, additional mechanisms beside stimulation of adrenergic neurotransmission and activation of guanylate cyclase in the smooth muscle cell seem to participate in the action of SIN-1 on rabbit penile erectile tissue.

Transmitter release and calcium currents at an Aplysia buccal ganglion synapse--II. Modulation by presynaptic receptors

Changes in evoked acetylcholine quantal release induced by histamine, FLRFamide and buccalin were investigated at an identified neuro-neuronal synapse in the buccal ganglion of Aplysia californica. Regulation of acetylcholine release by these neuromodulators was correlated with their actions on the presynaptic Ca2+ current. We have previously reported that FLRFamide and histamine, respectively, increase and decrease acetylcholine release from buccal neurons B4/B5. Buccalin, a peptide specific to the buccal ganglion, lowered the number of acetylcholine quanta released. Consistent with the synaptic effects, the presynaptic nifedipine-resistant Ca2+ current that triggers the release of acetylcholine in B4/B5 neurons [Trudeau L.-E. et al. (1993) Neuroscience 53, 571-580] was lowered by buccalin or by histamine and enhanced by FLRFamide. The analysis of tail currents showed that histamine shifts the voltage dependence of the nifedipine-resistant Ca2+ channels towards more positive voltages, whereas FLRFamide has an opposite action. Buccalin did not affect the voltage dependence of the channels but depressed the amplitude of the Ca2+ current, an effect which could be due either to a reduction of the number of available Ca2+ channels, to a decrease of their unitary conductance or to a modification of their gating. Inactivation of presynaptic G proteins prevented the modulatory actions of FLRFamide and histamine on quantal acetylcholine release and also on the voltage dependence of the nifedipine-resistant Ca2+ channels. This procedure, however, failed to prevent the suppressive effects of buccalin. The possibility of relating the voltage dependence shifts of the Ca2+ current induced by FLRFamide and histamine to the phosphorylation state of the Ca2+ channels is discussed. It is concluded that three independent presynaptic pathways initiated by histamine, FLRFamide and buccalin control presynaptic Ca2+ influx, these modulations being apparent within the physiological range of voltages required to activate Ca2+ channels.

Eclosion Hormone Stimulates Cyclic GMP Levels in Manduca sexta Nervous Tissue via Arachidonic Acid Metabolism with Little or No Contribution from the Production of Nitric Oxide

The neuropeptide eclosion hormone acts directly on the nervous system of the tobacco hornworm, Manduca sexta, to trigger ecdysis behavior at the end of each molt. Previous studies have shown that the action of eclosion hormone is mediated via the intracellular messenger cyclic GMP. In the present study we have investigated the mechanisms involved in the eclosion hormone-stimulated increases in cyclic GMP. No stimulation of guanylate cyclase was seen in homogenized nervous tissue, suggesting that eclosion hormone does not directly stimulate a membrane-bound form of guanylate cyclase. Nitric oxide synthase inhibitors, N-methylarginine and nitroarginine, had no effect on eclosion hormone-stimulated cyclic GMP levels. By contrast, 4-bromophenacyl bromide, an inhibitor of arachidonic acid release, and nordihydroguaiaretic acid, an inhibitor of arachidonic acid metabolism, almost completely abolished the eclosion hormone-stimulated cyclic GMP increase. We hypothesize that eclosion hormone receptors are coupled to a lipase, activation of which causes the release of arachidonic acid. Either the arachidonic acid directly stimulates the soluble guanylate cyclase or further metabolism of arachidonic acid yields compounds that activate guanylate cyclase.

cGMP: the wayward child of the cyclic nucleotide family

An informal poll of neurobiologists indicates the following widely-held misconceptions about cGMP: (1) we know very little about it; (2) it must not be very different from cAMP; and (3) no new biological principles are likely to emerge from studying it. In fact, despite these prejudices, our understanding of the cGMP second messenger cascade has increased dramatically in the last few years. We now know that it is very different from the cAMP system in almost every particular, and the differences reveal interesting and novel solutions to the biological problem of receptor-effector coupling.