The distribution of putative nitric oxide releasing neurones in the locust abdominal nervous system: a comparison of NADPHd histochemistry and NOS-immunocytochemistry

Comparative study of the distribution of NOS-positive neurons in pigeon intestine

Nitric oxide (NO) plays an important role in regulating gut motility, mucosal barrier function and secretions in the enteric nervous system. Nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) staining has been used to identify nitrergic neurons of the enteric nervous system in different species. However, NADPH-d staining lacks specificity because it also reflects the presence of enzymes other than nitric oxide synthase (NOS). Therefore, NOS immunohistochemistry techniques are needed to test for nitrergic neurons in the avian gut. In the present work, the morphology, density and size of NOS-positive neurons in the duodenum, jejunum, ileum, caecum and rectum myenteric plexus of adult pigeons were investigated using NOS immunohistochemistry and whole-mount preparations techniques. The density of NOS-positive ganglion was highest in the ileum, similar to the caecum and rectum, and the lowest staining levels were observed in the duodenum. The staining intensity of NOS-positive neurons in the duodenum, jejunum and ileum was dark, followed by the rectal regions, with weak staining in the caecum. These results suggested that NOS immunohistochemistry and whole-mount preparation techniques provide an effective assessment method of the ganglia in the pigeon intestinal myenteric nerve plexus and are more accurate for cell counting compared with conventional sections.

The regulation of cardiac activity by nitric oxide (NO) in the Vietnamese stick insect, Baculum extradentatum

This study examines the role of the unconventional gaseous signaling molecule nitric oxide (NO) on the regulation of heart rate in the Vietnamese stick insect, Baculum extradentatum. Using nicotinamide dinucleotide hydrogen phosphate (NADPH)-diaphorase histochemistry, as well as immunohistochemistry and Western blotting with an antibody against NO synthetase (NOS), we identified the presence of NOS in hemocytes present throughout the lumen of the dorsal vessel. We propose that NO is delivered to heart muscle tissue via hemocytes circulating within the hemolymph. In the present study, stimulation of NO levels by the application of the NO donor MAHMA-NONOate and l-arginine led to a dose-dependent decrease in heart rate. Treatment of tissues with the NOS inhibitor, L-NAME, in equimolar concentrations with l-arginine, led to a recovery of heart rate, without modifying heart rate on its own. Finally guanosine 3',5'-cyclic monophosphate (cGMP) analog, 8-bromo-cGMP, elicited similar inhibitory effects on stick insect heart rate as did the guanylate cyclase activator, YC-1, and the phosphodiesterase inhibitor, dipyridamole, indicating that cGMP is most likely the second messenger in the stick insect NO signaling pathway. Contrary to the cardioexcitatory effect of NO on other insect hearts, we have found that NO inhibits stick insect heart rate independently from any nervous system input, in a similar inhibitory fashion as that of vertebrate hearts.

Coma in response to environmental stress in the locust: a model for cortical spreading depression

Spreading depression (SD) is an interesting and important phenomenon due to its role in mammalian pathologies such as migraine, seizures, and stroke. Until recently investigations of the mechanisms involved in SD have mostly utilized mammalian cortical tissue, however we have discovered that SD-like events occur in the CNS of an invertebrate model, Locusta migratoria. Locusts enter comas in response to stress during which neural and muscular systems shut down until the stress is removed, and this is believed to be an adaptive strategy to survive extreme environmental conditions. During stress-induced comas SD-like events occur in the locust metathoracic ganglion (MTG) that closely resemble cortical SD (CSD) in many respects, including mechanism of induction, extracellular potassium ion changes, and propagation in areas equivalent to mammalian grey matter. In this review we describe the generation of comas and the associated SD-like events in the locust, provide a description of the similarities to CSD, and show how they can be manipulated both by stress preconditioning and pharmacologically. We also suggest that locust SD-like events are adaptive by conserving energy and preventing cellular damage, and we provide a model for the mechanism of SD onset and recovery in the locust nervous system.

Suppression of spreading depression-like events in locusts by inhibition of the NO/cGMP/PKG pathway

Despite considerable research attention focused on mechanisms underlying neural spreading depression (SD), because of its association with important human CNS pathologies, such as stroke and migraine, little attention has been given to explaining its occurrence and regulation in invertebrates. In the locust metathoracic ganglion (MTG), an SD-like event occurs during heat and anoxia stress, which results in cessation of neuronal output for the duration of the applied stress. SD-like events were characterized by an abrupt rise in extracellular potassium ion concentration ([K(+)](o)) from a baseline concentration of approximately 8 to >30 mm, which returned to near baseline concentrations after removal of the applied stress. After return to baseline [K(+)](o), neuronal output (ventilatory motor pattern activity) from the MTG recovered. Unlike mammalian neurons, which depolarize almost completely during SD, locust neurons only partially depolarized. SD-like events in the locust CNS were suppressed by pharmacological inhibition of the nitric oxide/cyclic guanosine monophosphate/protein kinase G (NO/cGMP/PKG) pathway and were exacerbated by its activation. Also, environmental stressors such as heat and anoxia increased production of nitric oxide in the locust CNS. Finally, for the intact animal, manipulation of the pathway affected the speed of recovery from suffocation by immersion under water. We propose that SD-like events in locusts provide an adaptive mechanism for surviving extreme environmental conditions. The highly conserved nature of the NO/cGMP/PKG signaling pathway suggests that it may be involved in modulating SD in other organisms, including mammals.

Regulation of enteric neuron migration by the gaseous messenger molecules CO and NO

The enteric nervous system (ENS) of insects is a useful model to study cell motility. Using small-molecule compounds to activate or inactivate biosynthetic enzymes, we demonstrate that the gaseous messenger molecules carbon monoxide (CO) and nitric oxide (NO) regulate neuron migration in the locust ENS. CO is produced by heme oxygenase (HO) enzymes and has the potential to signal via the sGC/cGMP pathway. While migrating on the midgut, the enteric neurons express immunoreactivity for HO. Here, we show that inhibition of HO by metalloporphyrins promotes enteric neuron migration in intact locust embryos. Thus, the blocking of enzyme activity results in a gain of function. The suppression of migratory behavior by activation of HO or application of a CO donor strongly implicates the release of CO as an inhibitory signal for neuron migration in vivo. Conversely, inhibition of nitric oxide synthase or application of the extracellular gaseous molecule scavenger hemoglobin reduces cell migration. The cellular distribution of NO and CO biosynthetic enzymes, together with the results of the chemical manipulations in whole embryo culture suggest CO as a modulator of transcellular NO signals during neuronal migration. Thus, we provide the first evidence that CO regulates embryonic nervous system development in a rather simple invertebrate model.

Suppression of grasshopper sound production by nitric oxide-releasing neurons of the central complex

The central complex of acridid grasshoppers integrates sensory information pertinent to reproduction-related acoustic communication. Activation of nitric oxide (NO)/cyclic GMP-signaling by injection of NO donors into the central complex of restrained Chorthippus biguttulus females suppresses muscarine-stimulated sound production. In contrast, sound production is released by aminoguanidine (AG)-mediated inhibition of nitric oxide synthase (NOS) in the central body, suggesting a basal release of NO that suppresses singing in this situation. Using anti-citrulline immunocytochemistry to detect recent NO production, subtypes of columnar neurons with somata located in the pars intercerebralis and tangential neurons with somata in the ventro-median protocerebrum were distinctly labeled. Their arborizations in the central body upper division overlap with expression patterns for NOS and with the site of injection where NO donors suppress sound production. Systemic application of AG increases the responsiveness of unrestrained females to male calling songs. Identical treatment with the NOS inhibitor that increased male song-stimulated sound production in females induced a marked reduction of citrulline accumulation in central complex columnar and tangential neurons. We conclude that behavioral situations that are unfavorable for sound production (like being restrained) activate NOS-expressing central body neurons to release NO and elevate the behavioral threshold for sound production in female grasshoppers.

Nitric oxide regulates axonal regeneration in an insect embryonic CNS

In higher vertebrates, the central nervous system (CNS) is unable to regenerate after injury, at least partially because of growth-inhibiting factors. Invertebrates lack many of these negative regulators, allowing us to study the positive factors in isolation. One possible molecular player in neuronal regeneration is the nitric oxide (NO)-cyclic guanosine-monophosphate (cGMP) transduction pathway which is known to regulate axonal growth and neural migration. Here, we present an experimental model in which we study the effect of NO on CNS regeneration in flat-fillet locust embryo preparations in culture after crushing the connectives between abdominal ganglia. Using whole-mount immunofluorescence, we examine the morphology of identified serotonergic neurons, which send a total of four axons through these connectives. After injury, these axons grow out again and reach the neighboring ganglion within 4 days in culture. We quantify the number of regenerating axons within this period and test the effect of drugs that interfere with NO action. Application of exogenous NO or cGMP promotes axonal regeneration, whereas scavenging NO or inhibition of soluble guanylyl cyclase delays regeneration, an effect that can be rescued by application of external cGMP. NO-induced cGMP immunostaining confirms the serotonergic neurons as direct targets for NO. Putative sources of NO are resolved using the NADPH-diaphorase technique. We conclude that NO/cGMP promotes outgrowth of regenerating axons in an insect embryo, and that such embryo-culture systems are useful tools for studying CNS regeneration.

Nitrergic modulation of an oviposition digging rhythm in locusts

In locusts, a central pattern generator underlies the rhythmic movements of the ovipositor valves that serve to drive the abdomen into damp soil in order to lay eggs. We have investigated the role of nitric oxide (NO) in the control of this oviposition digging rhythm. NO increases the frequency of the rhythm by acting via sGC to elevate cGMP, which in turn acts via PKG. Increasing exogenous NO levels using the NO donors SNAP and PAPANONOate increased the cycle frequency of the fictive digging rhythm, as did increasing endogenous NO by bath application of the substrate for NOS, l-arginine. On the other hand, application of the NO scavenger PTIO decreased the cycle frequency, indicating that NO must normally exert a continuous and dynamic role on the central pattern generator underlying the oviposition rhythm. Inhibiting the main molecular target of NO, soluble guanylate cyclase, with ODQ reduced the cycle frequency of the rhythm, suggesting that NO mediated its effects via sGC and cyclic GMP. Further evidence for this was produced by bath application of 8-Br-cGMP, which increased the frequency of the rhythm. Bath application of the generic protein kinase inhibitor and a selective PKG inhibitor, H-7 and KT-5823, respectively, reduced the frequency of the rhythm, suggesting that PKG acted as a target for cGMP. Thus, we conclude that NO plays a key role in regulating the frequency of the central pattern generator controlling rhythmic egg-laying movements in locusts by acting via sGC/cGMP-PKG.

Nitric Oxide Synthase and NADPH Diaphorase Distribution in the Bullfrog (Rana catesbeiana) CNS: Pathways and Functional Implications

The gas nitric oxide (NO) is emerging as an important regulator of normal physiology and pathophysiology in the central nervous system (CNS). The distribution of cells releasing NO is poorly understood in non-mammalian vertebrates. Nitric oxide synthase immunocytochemistry (NOS ICC) was thus used to identify neuronal cells that contain the enzyme required for NO production in the amphibian brain and spinal cord. NADPH-diaphorase (NADPHd) histochemistry was also used because the presence of NADPHd serves as a reliable indicator of nitrergic cells. Both techniques revealed stained cells in all major structures and pathways in the bullfrog brain. Staining was identified in the olfactory glomeruli, pallium and subpallium of the telencephalon; epithalamus, thalamus, preoptic area, and hypothalamus of the diencephalon; pretectal area, optic tectum, torus semicircularis, and tegmentum of the mesencephalon; all layers of the cerebellum; reticular formation; nucleus of the solitary tract, octaval nuclei, and dorsal column nuclei of the medulla; and dorsal and motor fields of the spinal cord. In general, NADPHd histochemistry provided better staining quality, especially in subpallial regions, although NOS ICC tended to detect more cells in the olfactory bulb, pallium, ventromedial thalamus, and cerebellar Purkinje cell layer. NOS ICC was also more sensitive for motor neurons and consistently labeled them in the vagus nucleus and along the length of the rostral spinal cord. Thus, nitrergic cells were ubiquitously distributed throughout the bullfrog brain and likely serve an essential regulatory function.

Pharmacological approaches to nitric oxide signalling during neural development of locusts and other model insects

A novel aspect of cellular signalling during the formation of the nervous system is the involvement of the messenger molecule nitric oxide (NO), which has been discovered in the mammalian vascular system as mediator of smooth muscle relaxation. NO is a membrane-permeant molecule, which activates soluble guanylyl cyclase (sGC) and leads to the formation of cyclic GMP (cGMP) in target cells. The analysis of specific cell types in model insects such as Locusta, Schistocerca, Acheta, Manduca, and Drosophila shows that the NO/cGMP pathway is required for the stabilization of photoreceptor growth cones at the start of synaptic assembly in the optic lobe, for regulation of cell proliferation, and for correct outgrowth of pioneer neurons. Inhibition of the NOS and sGC enzymes combined with rescue experiments show that NO, and potentially also another atypical messenger, carbon monoxide (CO), orchestrate cell migration of enteric neurons. Cultured insect embryos are accessible model systems in which the molecular pathways linking cytoskeletal rearrangement to directed cell movements can be analyzed in natural settings. Based on the results obtained from the insect models, I discuss current evidence for NO and cGMP as essential signalling molecules for the development of vertebrate brains.

Nitric oxide synthase in crayfish walking leg ganglia: Segmental differences in chemo-tactile centers argue against a generic role in sensory integration

Nitric oxide (NO) is a diffusible signaling molecule with evolutionarily conserved roles in neural plasticity. Prominent expression of NO synthase (NOS) in the primary olfactory centers of mammals and insects lead to the notion of a special role for NO in olfaction. In insects, however, NOS is also strongly expressed in non-olfactory chemo-tactile centers of the thoracic nerve cord. The functional significance of this apparent association with various sensory centers is unclear, as is the extent to which it occurs in other arthropods. We therefore investigated the expression of NOS in the pereopod ganglia of crayfish (Pacifastacus lenisculus and Procambarus clarkii). Conventional NADPH diaphorase (NADPHd) staining after formaldehyde fixation gave poor anatomic detail, whereas fixation in methanol/formalin (MF-NADPHd) resulted in Golgi-like staining, which was supported by immunohistochemistry using NOS antibodies that recognize a 135-kDa protein in crayfish. MF-NADPHd revealed an exceedingly dense innervation of the chemo-tactile centers. As in insects, this innervation was provided by a system of prominent intersegmental neurons. Superimposed on a putatively conserved architecture, however, were pronounced segmental differences. Strong expression occurred only in the anterior three pereopod ganglia, correlating with the presence of claws on pereopods one to three. These clawed pereopods, in addition to their role in locomotion, are crucially involved in feeding, where they serve both sensory and motor functions. Our findings indicate that strong expression of NOS is not a universal feature of primary sensory centers but instead may subserve a specific requirement for sensory plasticity that arises only in particular behavioral contexts.

Nitric oxide: a co-modulator of efferent peptidergic neurosecretory cells including a unique octopaminergic neurone innervating locust heart

Our findings suggest that nitric oxide (NO) acts as peripheral neuromodulator in locusts, in which it is commonly co-localized with RF-like peptide in neurosecretory cells. We also present the first evidence for NO as a cardio-regulator in insects. Putative NO-producing neurones were detected in locust pre-genital free abdominal ganglia by NADPH-diaphorase histochemistry and with an antibody against NO synthase (NOS). With both methods, we identified the same 14 somata in each examined ganglion: two dorsal posterior midline somata; six ventral posterior midline somata; and three pairs of lateral somata. A combination of NOS-detection methods with nerve tracing and transmitter immunocytochemistry revealed that at least 12 of these cells were efferent, of which four were identified as peptidergic neurosecretory cells with an antiserum detecting RFamide-like peptides. One of the latter was unequivocally identified as an octopaminergic dorsal unpaired median (DUM) neurone, which specifically projected to the heart ("DUM-heart"). Its peripheral projections revealed by axon tracing appeared as a meshwork of varicose endings encapsulating the heart. NOS-like immunoreactive profiles were found in the heart nerve. NO donors caused a dose-dependent increase in heart rate. This cardio-excitatory effect was negatively correlated to resting heart rate and seemed to be dependent on the physiological state of the animal. Hence, NO released from neurones such as the rhythmically active DUM-heart might exert continuous control over the heart. Possible mechanisms for the actions of NO on the heart and interactions with other neuromodulators co-localized in the DUM-heart neurone (octopamine, taurine, RF-amide-like peptide) are discussed.

Localization of nitric oxide synthase in the central complex and surrounding midbrain neuropils of the locust Schistocerca gregaria

Nitric oxide (NO), generated enzymatically by NO synthase (NOS), acts as an important signaling molecule in the nervous systems of vertebrates and invertebrates. In insects, NO has been implicated in development and in various aspects of sensory processing. To understand better the contribution of NO signaling to higher level brain functions, we analyzed the distribution of NOS in the midbrain of a model insect species, the locust Schistocerca gregaria, by using NADPH diaphorase (NADPHd) histochemistry after methanol/formalin fixation; results were validated by NOS immunohistochemistry. NADPHd yielded much higher sensitivity and resolution, but otherwise the two techniques resulted in corresponding labeling patterns throughout the brain, except for intense immunostaining but only weak NADPHd staining in median neurosecretory cells. About 470 neuronal cell bodies in the locust midbrain were NADPHd-positive positive, and nearly all major neuropil centers contained dense, sharply stained arborizations. We report several novel types of NOS-expressing neurons, including small ocellar interneurons and antennal sensory neurons that bypass the antennal lobe. Highly prominent labeling occurred in the central complex, a brain area involved in sky-compass orientation, and was analyzed in detail. Innervation by NOS-expressing fibers was most notable in the central body upper and lower divisions, the lateral accessory lobes, and the noduli. About 170 NADPHd-positive neurons contributed to this innervation, including five classes of tangential neuron, two systems of pontine neuron, and a system of columnar neurons. The results provide new insights into the neurochemical architecture of the central complex and suggest a prominent role for NO signaling in this brain area.