Eclosion hormone-stimulated cGMP levels in the central nervous system of Manduca sexta: inhibition by lipid metabolism blockers, increase in inositol(1,4,5)trisphosphate and further evidence against the involvement of nitric oxide

A novel guanylyl cyclase receptor, BdmGC-1, is highly expressed during the development of the oriental fruit fly Bactrocera dorsalis (Hendel)

A novel receptor guanylyl cyclase (GC) has been identified from the oriental fruit fly Bactrocera dorsalis (Hendel) and has been designated BdmGC-1. Protein domain analysis revealed that BdmGC-1 possesses a characteristic domain organization similar to all known receptor GCs but with a unique carboxyl-terminal extension. When overexpressed in 293T cells, BdmGC-1 manifests as a cell-surface glycoprotein with a marked cGMP-generating activity but is unresponsive to all ligands known to activate mammalian receptor GCs. BdmGC-1 mRNAs were highly expressed during development but had low or no expression in adult tissues. On the basis of its unique sequence and distinct developmental expression pattern, BdmGC-1 represents a novel receptor GC that may play a critical role during the development of B. dorsalis.

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.

Nitric oxide induces centrally generated motor patterns in the locust suboesophageal ganglion

The stimulatory effects of nitric oxide (NO) on central motor pattern generation in isolated locust suboesophageal ganglia (SOGs) were studied using extracellular recordings from motor nerves. Different NO donor molecules and a specific inhibitor of soluble guanylyl cyclases were used to confirm that the observed motor pattern occurred in response to activation of the NO/cyclic GMP signalling pathway. Experiments with muscarinic agonists and antagonists showed that the NO-induced motor pattern is generated independently from the motor pattern induced by muscarinic agonists described previously. Staining for NADPH-diaphorase and an antiserum directed against cyclic GMP were used to identify neurones representing potential sources of NO and their target cells within the SOG. Using intracellular dye injection and backfilling of peripheral nerves in combination with anti-cGMP immunohistochemistry, it was shown that identified efferent neurones involved in the mandibular motor pattern are potential target cells of NO.

Signal transduction in eclosion hormone-induced secretion of ecdysis-triggering hormone

Inka cells of insect epitracheal glands (EGs) secrete preecdysis and ecdysis-triggering hormones (PETH and ETH) at the end of each developmental stage. Both peptides act in the central nervous system to evoke the ecdysis behavioral sequence, a stereotype behavior during which old cuticle is shed. Secretion of ETH is stimulated by a brain neuropeptide, eclosion hormone (EH). EH evokes accumulation of cGMP followed by release of ETH from Inka cells, and exogenous cGMP evokes secretion of ETH. The secretory responses to EH and cGMP are inhibited by the broad-spectrum kinase inhibitor staurosporine, and the response to EH is potentiated by the phosphatase inhibitor calyculin A. Staurosporine did not inhibit EH-evoked accumulation of cGMP. Changes in cytoplasmic Ca2+ in Inka cells during EH signaling were monitored via fluorescence ratioing with fura-2-loaded EGs. Cytoplasmic Ca2+ increases within 30-120 s after addition of EH to EGs, and it remains elevated for at least 10 min, corresponding with the time course of secretion. Secretion is increased in dose-dependent manner by the Ca2+-ATPase inhibitor thapsigargin, a treatment that does not elevate glandular cGMP above basal levels. The secretory response to EH is partially inhibited in glands loaded with EGTA, while cGMP levels are unaffected. These findings suggest that EH activates second messenger cascades leading to cGMP accumulation and Ca2+ mobilization and/or influx and that both pathways are required for a full secretory response. cGMP activates a staurosporine-inhibitable protein kinase. We propose that Ca2+ acts via a parallel cascade with a time course that is similar to that for cGMP activation of a cGMP-dependent protein kinase.

The role of inositol 1,4,5-trisphosphate 5-phosphatase in inositol signaling in the CNS of larval Manduca sexta

Production of inositol 1,4,5-trisphosphate (IP3) in cells results in the mobilization of intracellular calcium. Therefore, the dynamics of IP3 metabolism is important for calcium dependent processes in cells. This report investigates the coupling of mAChRs to the inositol lipid pathway in the CNS of the larval Manduca sexta. Stimulation of intact abdominal ganglia prelabeled with [3H]-inositol using a muscarinic agonist, oxotremorine-M (oxo-M), increased total inositol phosphate levels in a dose dependent manner (EC50 = 4.23 microM). These inositol phosphates consisted primarily of inositol 1,4-bisphosphate (IP2) and inositol monophosphate (IP1). Similarly, when nerve cord homogenates were provided with [3H]-phosphatidylinositol 4,5-bisphosphate ([3H]-PIP2) (10-13 microM) the predominant products were IP2 and IP1. In contrast, incubation of purified membranes with 1 mM oxo-M in the presence of 100 microM GTP gamma S and [3H]-PIP2 increased IP3 levels, suggesting that the direct activation of phospholipase C (PLC) by mAChRs occurs in a membrane delimited process. Together, these results suggest that in the intact nerve cord and in crude homogenates, a cytosolic 5-phosphatase quickly metabolizes IP3 to produce to IP2 and IP1. This enzyme was kinetically characterized using IP3 (Km = 43.7 microM, Vmax = 864 pmoles/min/mg) and IP4 (Km = 0.93 microM; Vmax = 300pmoles/min/mg) as substrates. The enzyme activity can be potently inhibited by two IP thiol compounds; IP3S3 (1,4,6) and IP3S3 (2,3,5), that show complex binding kinetics (Hill numbers < 1) and can distinguish different forms of the 5-phosphatase in purified membranes. These two inhibitors could be very useful tools to determine the role of the inositol lipid pathway in neuroexcitability.

Identification and characterization of a novel beta subunit of soluble guanylyl cyclase that is active in the absence of a second subunit and is relatively insensitive to nitric oxide

Previously characterized soluble guanylyl cyclases form alpha-beta heterodimers that can be activated by the gaseous messenger, nitric oxide. In mammals, four subunits have been cloned, named alpha1, alpha2, beta1, and beta2. We have identified a novel soluble guanylyl cyclase isoform from the nervous system of the insect Manduca sexta that we have named M. sexta guanylyl cyclase beta3 (MsGC-beta3). It is most closely related to the mammalian beta subunits but has several features that distinguish it from previously identified soluble cyclases. Most importantly, MsGC-beta3 does not need to form heterodimers to form an active enzyme because guanylyl cyclase activity can be measured when it is expressed alone in COS-7 cells. Moreover, this activity is only weakly enhanced in the presence of the nitric oxide donor, sodium nitroprusside. Several of the amino acids in rat beta1 subunits, previously identified as being important in heme binding or necessary for nitric oxide activation, are substituted with nonsimilar amino acids in MsGC-beta3. There are also an additional 315 amino acids C-terminal to the catalytic domain of MsGC-beta3 that have no sequence similarity to any known protein. Northern blot analysis shows that MsGC-beta3 is primarily expressed in the nervous system of Manduca.

A delayed role for nitric oxide-sensitive guanylate cyclases in a migratory population of embryonic neurons

Neuronal differentiation requires a coordinated intracellular response to diverse extracellular stimuli, but the role of specific signaling mechanisms in regulating this process is still poorly understood. Soluble guanylate cyclases (sGCs), which can be stimulated by diffusible free radical gasses such as nitric oxide (NO) and carbon monoxide (CO) to produce the intracellular messenger cGMP, have recently been found to be expressed within a variety of embryonic neurons and implicated in the control of both neuronal motility and differentiation. Using the enteric nervous system (ENS) of the moth, Manduca sexta, we examined the role of NO and NO-sensitive sGCs during the migration and differentiation of an identified set of migratory neurons (the EP cells). Shortly after the onset of their migration, a subset of EP cells began to express NO-sensitive sGC activity (visualized with an anti-cGMP antiserum). Unlike many neurons in the central nervous system, the expression of sGC activity in the EP cells was not transient but persisted throughout subsequent periods of axon elongation and terminal branch formation on the gut musculature. In contrast, nitric oxide synthase activity (visualized using NADPH-diaphorase histochemistry) was undetectable in the vicinity of the EP cells until the period of synapse formation. Manipulations designed to alter sGC and NOS activity in an in vivo embryonic culture preparation had no discernible effect on either the migration or axonal outgrowth of the EP cells. In contrast, inhibition of both of these enzymes resulted in a significant reduction in terminal synaptic branch formation within the postmigratory neurons. These results indicate that while NO-sensitive sGC activity is expressed precociously within the EP cells during their initial migratory dispersal, a role for this signaling pathway can only be demonstrated well after migration is complete, coincident with the formation of mature synaptic connections.

The nitric oxide system in insects

It is well established that nitric oxide (NO) acts as a signalling molecule in the nervous system of both mammals and insects. In contrast to classical transmitters, the membrane-permeant NO can act on neighbouring targets limited by half-life and diffusion barriers. This type of diffuse signalling seems to be evolutionarily highly conserved and recent findings concerning the characterization and function of the NO system in insects are summarized in this review. Firstly, the properties and the localization of the NO forming enzyme, the NO synthase (NOS), are described. In the nervous system the brain contains by far the highest NOS activity. As an evolutionary peculiarity, a blood-feeding bug exhibits high NOS activity in the salivary glands. Secondly, the soluble guanylate cyclase (sGC), a major target of NO action, and cGMP-regulated enzymes like cGMP-dependent protein kinase and cyclic nucleotide gated channels are described. Anatomical organization of the NO/cGMP system in insects reveals evidence for a cellular separation of the release site and target site of NO, although in the antennal lobes of the locust an exception from this rule exists. Thirdly, the implication of the NO system in neuronal function in insects is described. In the honeybee, the NO/cGMP system in the antennal lobes is implicated in the processing of adaptive mechanisms during chemosensory processing, and recent findings support a specific role of the NO system in memory formation. Discussion of the results in insects with regard to properties and functions of the vertebrate NO system is attempted.