Colocalization of NADPH-diaphorase and GABA-immunoreactivity in the olfactory and visual system of the locust

Anatomic and neurochemical analysis of the palpal olfactory system in the red flour beetle Tribolium castaneum, HERBST

The paired antennal lobes were long considered the sole primary processing centers of the olfactory pathway in holometabolous insects receiving input from the olfactory sensory neurons of the antennae and mouthparts. In hemimetabolous insects, however, olfactory cues of the antennae and palps are processed separately. For the holometabolous red flour beetle Tribolium castaneum, we could show that primary processing of the palpal and antennal olfactory input also occurs separately and at distinct neuronal centers. While the antennal olfactory sensory neurons project into the antennal lobes, those of the palps project into the paired glomerular lobes and the unpaired gnathal olfactory center. Here we provide an extended analysis of the palpal olfactory pathway by combining scanning electron micrographs with confocal imaging of immunohistochemical staining and reporter expression identifying chemosensory and odorant receptor-expressing neurons in the palpal sensilla. In addition, we extended the anatomical characterization of the gnathal olfactory center by 3D reconstructions and investigated the distribution of several neuromediators. The similarities in the neuromediator repertoire between antennal lobes, glomerular lobes, and gnathal olfactory center underline the role of the latter two as additional primary olfactory processing centers.

Cholinergic calcium responses in cultured antennal lobe neurons of the migratory locust

Locusts are advantageous organisms to elucidate mechanisms of olfactory coding at the systems level. Sensory input is provided by the olfactory receptor neurons of the antenna, which send their axons into the antennal lobe. So far, cellular properties of neurons isolated from the circuitry of the olfactory system, such as transmitter-induced calcium responses, have not been studied. Biochemical and immunocytochemical investigations have provided evidence for acetylcholine as classical transmitter of olfactory receptor neurons. Here, we characterize cell cultured projection and local interneurons of the antennal lobe by cytosolic calcium imaging to cholinergic stimulation. We bulk loaded the indicator dye Cal-520 AM in dissociated culture and recorded calcium transients after applying cholinergic agonists and antagonists. The majority of projection and local neurons respond with increases in calcium levels to activation of both nicotinic and muscarinic receptors. In local interneurons, we reveal interactions lasting over minutes between intracellular signaling pathways, mediated by muscarinic and nicotinic receptor stimulation. The present investigation is pioneer in showing that Cal-520 AM readily loads Locusta migratoria neurons, making it a valuable tool for future research in locust neurophysiology, neuropharmacology, and neurodevelopment.

A review of the actions of Nitric Oxide in development and neuronal function in major invertebrate model systems

Ever since the late-eighties when endothelium-derived relaxing factor was found to be the gas nitric oxide, endogenous nitric oxide production has been observed in virtually all animal groups tested and additionally in plants, diatoms, slime molds and bacteria. The fact that this new messenger was actually a gas and therefore didn't obey the established rules of neurotransmission made it even more intriguing. In just 30 years there is now too much information for useful comprehensive reviews even if limited to animals alone. Therefore this review attempts to survey the actions of nitric oxide on development and neuronal function in selected major invertebrate models only so allowing some detailed discussion but still covering most of the primary references. Invertebrate model systems have some very useful advantages over more expensive and demanding animal models such as large, easily identifiable neurons and simple circuits in tissues that are typically far easier to keep viable. A table summarizing this information along with the major relevant references has been included for convenience.

Systematic Analysis of Transmitter Coexpression Reveals Organizing Principles of Local Interneuron Heterogeneity

Broad neuronal classes are surprisingly heterogeneous across many parameters, and subclasses often exhibit partially overlapping traits including transmitter coexpression. However, the extent to which transmitter coexpression occurs in predictable, consistent patterns is unknown. Here, we demonstrate that pairwise coexpression of GABA and multiple neuropeptide families by olfactory local interneurons (LNs) of the moth Manduca sexta is highly heterogeneous, with a single LN capable of expressing neuropeptides from at least four peptide families and few instances in which neuropeptides are consistently coexpressed. Using computational modeling, we demonstrate that observed coexpression patterns cannot be explained by independent probabilities of expression of each neuropeptide. Our analyses point to three organizing principles that, once taken into consideration, allow replication of overall coexpression structure: (1) peptidergic neurons are highly likely to coexpress GABA; (2) expression probability of allatotropin depends on myoinhibitory peptide expression; and (3) the all-or-none coexpression patterns of tachykinin neurons with several other neuropeptides. For other peptide pairs, the presence of one peptide was not predictive of the presence of the other, and coexpression probability could be replicated by independent probabilities. The stochastic nature of these coexpression patterns highlights the heterogeneity of transmitter content among LNs and argues against clear-cut definition of subpopulation types based on the presence of single neuropeptides. Furthermore, the receptors for all neuropeptides and GABA were expressed within each population of principal neuron type in the antennal lobe (AL). Thus, activation of any given LN results in a dynamic cocktail of modulators that have the potential to influence every level of olfactory processing within the AL.

Adding up the odds-Nitric oxide signaling underlies the decision to flee and post-conflict depression of aggression

Fighting is dangerous, which is why animals choose to flee once the costs outweigh the benefits, but the mechanisms underlying this decision-making process are unknown. By manipulating aggressive signaling and applying nitrergic drugs, we show that the evolutionarily conserved neuromodulator nitric oxide (NO), which has a suppressing effect on aggression in mammals, can play a decisive role. We found that crickets, which exhibit spectacular fighting behavior, flee once the sum of their opponent's aversive actions accrued during fighting exceeds a critical amount. This effect of aversive experience is mediated by the NO signaling pathway. Rather than suppressing aggressive motivation, NO increases susceptibility to aversive stimuli and with it the likelihood to flee. NO's effect is manifested in losers by prolonged avoidance behavior, characteristic for social defeat in numerous species. Intriguingly, fighting experience also induces, via NO, a brief susceptible period to aversive stimuli in winners just after victory. Our findings thus reveal a key role for NO in the mechanism underlying the decision to flee and post-conflict depression in aggressive behavior.

Immunocytochemical localization of amines and GABA in the optic lobe of the butterfly, Papilio xuthus

Butterflies have sophisticated color vision. While the spectral organization of the compound eye has been well characterized in the Japanese yellow swallowtail butterfly, Papilio xuthus, neural mechanisms underlying its color vision are largely unexplored. Towards a better understanding of signal processing in the visual system of P. xuthus, we used immunocytochemical techniques to analyze the distribution of transmitter candidates, namely, histamine, serotonin, tyramine and γ-aminobutyric acid (GABA). Photoreceptor terminals in the lamina and medulla exhibited histamine immunoreactivity as demonstrated in other insects. The anti-histamine antiserum also labeled a few large medulla neurons. Medulla intrinsic neurons and centrifugal neurons projecting to the lamina showed serotonin immunoreactivity. Tyramine immunostaining was detected in a subset of large monopolar cells (LMCs) in the lamina, transmedullary neurons projecting to the lobula plate, and cell bodies surrounding the first optic chiasma. An anti-GABA antiserum labeled a subset of LMCs and populations of columnar and tangential neurons surrounding the medulla. Each of the four antisera also labeled a few centrifugal neurons that innervate the lobula complex from the central brain, suggesting that they have neuromodulatory roles. A distinctive feature we found in this study is the possibility that tyramine and GABA act as transmitters in LMCs of P. xuthus, which has not been reported in any other insects so far.

Nitric oxide as a regulator of neuronal motility and regeneration in the locust embryo

Nitric oxide (NO) is known as a gaseous messenger in the nervous system. It plays a role in synaptic plasticity, but also in development and regeneration of nervous systems. We have studied the function of NO and its signaling cascade via cyclic GMP in the locust embryo. Its developing nervous system is well suited for pharmacological manipulations in tissue culture. The components of this signaling pathway are localized by histochemical and immunofluorescence techniques. We have analyzed cellular mechanisms of NO action in three examples: 1. in the peripheral nervous system during antennal pioneer axon outgrowth, 2. in the enteric nervous system during migration of neurons forming the midgut nerve plexus, and 3. in the central nervous system during axonal regeneration of serotonergic neurons after axotomy. In each case, internally released NO or NO-induced cGMP synthesis act as permissive signals for the developmental process. Carbon monoxide (CO), as a second gaseous messenger, modulates enteric neuron migration antagonistic to NO.

Gamma-aminobutyric acid (GABA)-mediated neural connections in the Drosophila antennal lobe

Inhibitory synaptic connections mediated by gamma-aminobutyric acid (GABA) play important roles in the neural computation of the brain. To obtain a detailed overview of the neural connections mediated by GABA signals, we analyzed the distribution of the cells that produce and receive GABA in the Drosophila adult brain. Relatively small numbers of the cells, which form clusters in several areas of the brain, express the GABA synthesis enzyme Gad1. On the other hand, many cells scattered across the brain express ionotropic GABA(A) receptor subunits (Lcch3 and Rdl) and metabotropic GABA(B) receptor subtypes (GABA-B-R1, -2, and -3). To analyze the expression of these genes in distinct identified cell types, we focused on the antennal lobe, where GABAergic neurons play important roles in odor coding. By combining fluorescent in situ hybridization and immunolabeling against GFP expressed with cell-type-specific GAL4 driver strains, we quantified the percentage of the cells that produce or receive GABA for each cell type. GABA was synthesized in the middle antennocerebral tract (mACT) projection neurons and two types of local neurons. Among them, mACT neurons had few presynaptic sites in the antennal lobe, making the local neurons essentially the sole provider of GABA signals there. On the other hand, not only these local neurons but also all types of projection neurons expressed both ionotropic and metabotropic GABA receptors. Thus, even though inhibitory signals are released from only a few, specific types of local neurons, the signals are read by most of the neurons in the antennal lobe neural circuitry.

Distribution and characterization of nitric oxide synthase in the nervous system of Triatoma infestans (Insecta: Heteroptera)

The biochemical characterization of nitric oxide synthase (NOS) and its distribution in the central nervous system (CNS) were studied in the heteropteran bug Triatoma infestans. NOS-like immunoreactivity was found in the brain, subesophageal ganglion, and thoracic ganglia by using immunocytochemistry. In the protocerebrum, NOS-immunoreactive (IR) somata were detected in the anterior, lateral, and posterior soma rinds. In the optic lobe, numerous immunostained somata were observed at the level of the first optic chiasma, around the lobula, and in the proximal optic lobe. In the deutocerebrum, NOS-IR perikarya were mainly observed in the lateral soma rind, surrounding the sensory glomeruli, and a few cell bodies were seen in association with the antennal mechanosensory and motor neuropil. No immunostaining could be detected in the antennal nerve. The subesophageal and prothoracic ganglia contained scattered immunostained cell bodies. NOS-IR somata were present in all the neuromeres of the posterior ganglion. Western blotting showed that a universal NOS antiserum recognized a band at 134 kDa, in agreement with the expected molecular weight of the protein. Analysis of the kinetics of nitric oxide production revealed a fully active enzyme in tissue samples of the CNS of T. infestans.

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.

Nitric oxide/cyclic guanosine monophosphate signaling in the central complex of the grasshopper brain inhibits singing behavior

Grasshopper sound production, in the context of mate finding, courtship, and rivalry, is controlled by the central body complex in the protocerebrum. Stimulation of muscarinic acetylcholine receptors in the central complex has been demonstrated to stimulate specific singing in various grasshoppers including the species Chorthippus biguttulus. Sound production elicited by stimulation of muscarinic acetylcholine receptors in the central complex is inhibited by co-applications of various drugs activating the nitric oxide/cyclic guanosine monophosphate (cGMP) signaling pathway. The nitric oxide-donor sodium nitroprusside caused a reversible suppression of muscarine-stimulated sound production that could be blocked by 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxaline-1-one (ODQ), which prevents the formation of cGMP by specifically inhibiting soluble guanylyl cyclase. Furthermore, injections of both the membrane-permeable cGMP analog 8-Br-cGMP and the specific inhibitor of the cGMP-degrading phosphodiesterase Zaprinast reversibly inhibited singing. To identify putative sources of nitric oxide, brains of Ch. biguttulus were subjected to both nitric oxide synthase immunocytochemistry and NADPH-diaphorase staining. Among other areas known to express nitric oxide synthase, both procedures consistently labeled peripheral layers in the upper division of the central body complex, suggesting that neurons supplying this neuropil contain nitric oxide synthase and may generate nitric oxide upon activation. Exposure of dissected brains to nitric oxide and 3-(5'hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1) induced cGMP-associated immunoreactivity in both the upper and lower division. Therefore, both the morphological and pharmacological data presented in this study strongly suggest a contribution of the nitric oxide/cGMP signaling pathway to the central control of grasshopper sound production.

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.

Chemical neuroanatomy of the fly's movement detection pathway

In Diptera, subsets of small retinotopic neurons provide a discrete channel from achromatic photoreceptors to large motion-sensitive neurons in the lobula complex. This pathway is distinguished by specific affinities of its neurons to antisera raised against glutamate, aspartate, gamma-aminobutyric acid (GABA), choline acetyltransferase (ChAT), and a N-methyl-D-aspartate type 1 receptor protein (NMDAR1). Large type 2 monopolar cells (L2) and type 1 amacrine cells, which in the external plexiform layer are postsynaptic to the achromatic photoreceptors R1-R6, express glutamate immunoreactivity as do directionally selective motion-sensitive tangential neurons of the lobula plate. L2 monopolar cells ending in the medulla are accompanied by terminals of a second efferent neuron T1, the dendrites of which match NMDAR1-immunoreactive profiles in the lamina. L2 and T1 endings visit ChAT and GABA-immunoreactive relays (transmedullary neurons) that terminate from the medulla in a special layer of the lobula containing the dendrites of directionally selective retinotopic T5 cells. T5 cells supply directionally selective wide-field neurons in the lobula plate. The present results suggest a circuit in which initial motion detection relies on interactions among amacrines and T1, and the subsequent convergence of T1 and L2 at transmedullary cell dendrites. Convergence of ChAT-immunoreactive and GABA-immunoreactive transmedullary neurons at T5 dendrites in the lobula, and the presence there of local GABA-immunoreactive interneurons, are suggested to provide excitatory and inhibitory elements for the computation of motion direction. A comparable immunocytological organization of aspartate- and glutamate-immunoreactive neurons in honeybees and cockroaches further suggests that neural arrangements providing directional motion vision in flies may have early evolutionary origins.

GABA-like immunoreactivity in nonspiking interneurons of the locust metathoracic ganglion

Nonspiking interneurons are important components of the premotor circuitry in the thoracic ganglia of insects. Their action on postsynaptic neurons appears to be predominantly inhibitory, but it is not known which transmitter(s) they use. Here, we demonstrate that many but not all nonspiking local interneurons in the locust metathoracic ganglion are immunopositive for GABA (γ-aminobutyric acid). Interneurons were impaled with intracellular microelectrodes and were shown physiologically to be nonspiking. They were further characterized by defining their effects on known leg motor neurons when their membrane potential was manipulated by current injection. Lucifer Yellow was then injected into these interneurons to reveal their cell bodies and the morphology of their branches. Some could be recognised as individuals by comparison with previous detailed descriptions. Ganglia were then processed for GABA immunohistochemistry. Fifteen of the 17 nonspiking interneurons studied were immunopositive for GABA, but two were not. The results suggest that the majority of these interneurons might exert their well-characterized effects on other neurons through the release of GABA but that some appear to use a transmitter other than GABA. These nonspiking interneurons are therefore not an homogeneous population with regard to their putative transmitter.

Developmental expression of nitric oxide/cyclic GMP signaling pathways in the brain of the embryonic grasshopper

Biochemical, cytochemical, and physiological investigations have demonstrated the presence of the nitric oxide/cyclic GMP signaling system in the brain of the adult locust, Schistocerca gregaria. Here, we characterize nitric oxide (NO) releasing neurons and neurons that synthesize cyclic GMP (cGMP) in response to a NO stimulus in the brain of the embryonic grasshopper. Using NADPH-diaphorase histochemistry to detect NO synthesizing cells we describe the appearance of several individually identifiable neurons. At embryonic stage 50% four NADPH-diaphorase positive neurons can be detected in each brain hemisphere. In addition to the labeling of differentiating neurons, NADPH-diaphorase staining appears also in distinct proliferative cell clusters. At embryonic stage 70% the general organization of NADPH-diaphorase activity starts to resemble the adult brain. The immunocytochemical detection of NO-induced accumulation of cGMP starts at embryonic stage 45% resulting in the staining of large neuronal populations in all brain areas. During embryonic stages 50-70%, the number of cGMP-immunoreactive cells increases from 200 to several hundred in each brain hemisphere. Since all NADPH-diaphorase positive local interneurons of the adult antennal lobe express GABA-immunoreactivity, we also report on the earliest appearance of GABA-immunoreactivity in the embryonic antennal lobe. Thus, we present a first developmental investigation of nitrergic and GABAergic transmitter phenotypes in the brain of the embryonic grasshopper.

Nitric oxide synthase in the frog cerebellum: response of Purkinje neurons to unilateral eighth nerve transection

When vestibular damage occurs, nitric oxide synthase (NOS) expression in rat cerebellar flocculus is affected. Since compensation for postural symptoms occurs and Purkinje cells play an important role in movement coordination and motor learning, we analyzed in situ the induction of NOS in the Purkinje cell population of the cerebellum (corpus cerebelli) of frog after unilateral transection of the eighth statoacoustic nerve to gain insight into the role of NO in neural plasticity after injury. Three days after neurectomy, the early effects induced NADPH diaphorase reactivity in most of the Purkinje cells on the ipsilateral side, while on the contralateral side the highest labeling was observed at 15 days. This finding can give information on the dynamics of vestibular compensation, in which NOS involvement was investigated. At 30 days, NADPH diaphorase reactivity was present in a large number of Purkinje cells of the whole cerebellum, while at 60 days a down-regulation for NADPH diaphorase reactivity was evident. A similar trend was observed for NOS-immunoreactivity, which was still present at 60 days in a high percentage of Purkinje cells, mainly on the ipsilateral side. On the basis of cell density evaluations, it was proposed that the early induction of NOS after neurectomy was linked to the degeneration of a part of the Purkinje neurons, while the permanence of NOS labeling might be due to a neuroprotective role of NO in the restoration phase of the vestibular compensation process.

Immunocytochemical localization of a Manduca sexta gamma-aminobutyric acid transporter

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in insect central and peripheral nervous systems. Although much work has focused on the downstream targets of GABA, signal termination at insect GABAergic synapses has received very little attention. One of the major mechanisms of terminating synaptic transmission involves transport of the neurotransmitter molecules into presynaptic neurons or surrounding glia. Here we report the immunolocalization of a GABA transporter in the tobacco hornworm, Manduca sexta (MasGAT), using an affinity-purified antibody developed to the C-terminus. This is the first demonstration of an insect neurotransmitter transporter immunolocalization study. Results showed strong staining in the neuropil regions of embryonic, larval, and pharate adult central nervous system. Expression pattern in the pharate adult brain mostly mimicked that observed for GABA, with staining in parts of the optic and antennal lobes, mushroom body, lateral protocerebrum, and central complex. Certain longitudinal and lateral connectives of ganglia were observed to have immunostained fibers representing axons. These data support the view that GABA is involved in visual and olfactory processing in the insect brain.

Neurotransmitters and neuropeptides in the brain of the locust

As part of continuous research on the neurobiology of the locust, the distribution and functions of neurotransmitter candidates in the nervous system have been analyzed particularly well. In the locust brain, acetylcholine, glutamate, gamma-aminobutyric acid (GABA), and the biogenic amines serotonin, dopamine, octopamine, and histamine most likely serve a transmitter function. Increasing evidence, furthermore, supports a signalling function for the gaseous molecule nitric oxide, but a role for neuroptides is so far suggested only by immunocytochemistry. Acetylcholine, glutamate, and GABA appear to be present in large numbers of interneurons. As in other insects, antennal sensory afferents might be cholinergic, while glutamate is the transmitter candidate of antennal motoneurons. GABA is regarded as the principle inhibitory transmitter of the brain, which is supported by physiological studies in the antennal lobe. The cellular distribution of biogenic amines has been analyzed particularly well, in some cases down to physiologically characterized neurons. Amines are present in small numbers of interneurons, often with large branching patterns, suggesting neuromodulatory roles. Histamine, furthermore, is the transmitter of photoreceptor neurons. In addition to these "classical transmitter substances," more than 60 neuropeptides were identified in the locust. Many antisera against locust neuropeptides label characteristic patterns of neurosecretory neurons and interneurons, suggesting that these peptides have neuroactive functions in addition to hormonal roles. Physiological studies supporting a neuroactive role, however, are still lacking. Nitric oxide, the latest addition to the list of neurotransmitter candidates, appears to be involved in early stages of sensory processing in the visual and olfactory systems.

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.

Nitric oxide: an unconventional messenger in the nervous system of an orthopteroid insect

Nitric oxide (NO) is a membrane-permeant messenger molecule generated from the amino acid L-arginine. NO can activate soluble guanylyl cyclase leading to the formation of cyclic GMP (cGMP) in target cells. In the nervous system, NO/cGMP signalling is thought to play essential roles in synaptic plasticity during development and also in the mature animal. This paper examines biochemical, cell biological, and physiological investigations of NO/cGMP signalling in the nervous system of the locust, a commonly used neurobiological preparation. Biochemical investigations suggest that an identical enzyme is responsible for both NO synthase (NOS) and NADPH-diaphorase activity after tissue fixation. Immunocytochemical staining of an olfactory center in the locust brain shows that NOS-immunoreactivity colocalizes with NADPH-diaphorase at the cellular level. The cytochemical staining of NO donor and target cells in adult animals suggests functions in olfaction, vision, and sensorimotor integration. During development, NO is implicated in axonal outgrowth and synaptogenesis. The cellular distribution of NO-responsive cells in neural circuits reflects potential functions of NO as a retrograde synaptic messenger, as an intracellular messenger, and as a lateral diffusible messenger independent of conventional synaptic connectivity.

Nitric oxide synthase and cGMP activity in the salivary glands of the American dog tick Dermacentor variabilis

We colocalized nitric oxide synthase (NOS) activity in epithelial cells that surround the salivary gland duct in female Dermacentor variabilis with NADPH diaphorase histochemistry and immunohistochemistry using a polyclonal anti-endothelial NOS. Using size-exclusion chromatography, a fraction with a molecular mass of about 185 kDa that had diaphorase activity was eluted from tick salivary gland homogenate. This fraction converted arginine to citrulline with the production of nitric oxide (NO), which was detected by using electron spin resonance spectroscopy. The complete activity of the diaphorase fraction was dependent on NADPH, FAD, tetrahydrobiopterin, calmodulin, (CaM), and Ca(2+), but was not dependent on dithiothreitol. The arginine analog N(G)-monomethyl-L-arginine inhibited the activity of this fraction. NO and arginine activated soluble guanylate cyclase to produce cGMP in dopamine-stimulated isolated salivary glands. Dopamine-stimulated isolated salivary glands treated with tick saline containing either EDTA, the NOS inhibitor N(G)-nitro-L-arginine methyl ester, or the calcium/CaM binding inhibitor W-7 showed no increase in cGMP. The NO donor sodium nitroprusside significantly increased cGMP levels in unstimulated isolated salivary glands. A possible function for NO in salivation by this ixodid tick is discussed.

Function and morphology of the antennal lobe: new developments

The antennal lobe of insects has emerged as an excellent model for olfactory processing in the CNS. In the present review we compile data from areas where substantial progress has been made during recent years: structure-function relationships within the glomerular array, integration and blend specificity, time coding and the effects of neuroactive substances and hormones on antennal lobe processing.

Mature and developing visual system of Ceratitis capitata (Diptera, Tephritidae): histochemical evidence of nitric oxide synthase in the wild type and the white eye mutant strains

Nitric oxide (NO) is acknowledged as a messenger molecule in the nervous system. It has a role in the modulation of the chemosensory information and seems implicated also in visual processes and visually guided behaviour of some insects. In the present study, we used two different strains of the medfly Ceratitis capitata (Diptera, Tephritidae), a wild type eye colour and a white eye mutant line, as models to clarify the involvement of NO in the mature and developing visual system. The comparison between the pattern of enzyme histochemical localization of NO synthase (NOS), through NADPH diaphorase (NADPHd) staining, in the optic lobes of the two strains revealed for adults a stronger intensity of reaction in all the neuropiles and the sub-retinic monopolar cell layer of the wild type flies, with respect to the white eye mutant correspondent areas. Anti-NOS immunocytochemistry correlated with these results, underlying reactivity both in fine fibres and varicosities and in cell bodies and supporting the idea of presence of NOS also in the retina of the medfly optic lobes. NADPHd reactivity was present in the first developmental stages of the white eye mutant also, but at lower intensity than wild type, and it decreased in some areas during the transition to adult fly stage both in the wild type and in the white eye mutant. All these observations together indicate that changes in the NO system of C. capitata could be related to the visual information processing, when the visual response or discrimination are altered. Furthermore, NO may be involved in the establishment of the retinal projection pattern and in the control of optic lobes morphogenesis.

Nitric oxide-containing neurons in the nervous ganglia of Helix aspersa during rest and activity: immunocytochemical and enzyme histochemical detection

Nitric oxide synthase (NOS) immunoreactivity and staining for nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-diaphorase) activity are two cytochemical markers for nitric oxide (NO)-containing neurons. The authors examined the changes in the distribution of NOS immunolabeling and NADPH-diaphorase reactivity in the cerebral and buccal ganglia of the terrestrial snail Helix aspersa during resting and active phases. During inactivity and after 1 day of activity, in the mesocerebrum and metacerebrum of the snails, there were several reactive neurons for both markers; after 7 days of activity, the number of reactive neurons was lower. Opposite results were obtained in the buccal ganglia, in which increased staining and numbers of reactive neurons were present in the active snails (after 1 day and 7 days of activity). Although the staining patterns for the two reactions were similar, colocalization was not always observed. The comparison between inactive and active animals provided a more precise survey of NOS-containing neurons in the snail cerebral ganglia than previously described. Moreover, it suggested that not only is NO involved in distinct nervous circuits, but, as a ubiquitous molecule, it also plays a role in neuroprotection and neuropeptide release.

Octopamine in the locust brain: cellular distribution and functional significance in an arousal mechanism

This review summarizes the distribution of octopamine-like immunoreactive neurons in the brain of the locust and the functional significance of a subset of them in an arousal mechanism in the visual system. A small set of identifiable octopamine-immunoreactive neurons lies in the ventromedial brain. Their cell bodies are large and readily accessible, which allows their removal and analysis of their biogenic amine content using gas-chromatography mass-spectrometry to confirm that they are genuinely octopaminergic. The neurons project from the central brain to the optic lobes where they arborize extensively in the medulla and lobula. There they release octopamine in response to multimodal input in the central brain. This evokes dishabitution in the locust's movement-detection system, suggesting an arousal mechanism.

Nitric oxide and cyclic GMP modulate photoreceptor cell responses in the visual system of the locust

Nitric oxide (NO) is a membrane-permeant messenger molecule which activates the cyclic GMP (cGMP)-synthesizing enzyme soluble guanylyl cyclase. Using cytochemical techniques, we recently reported NO-induced cGMP immunoreactivity in the photoreceptor cells of the compound eye of the locust Schistocerca gregaria and also detected NADPH diaphorase staining, a marker of NO synthase, in a subset of the monopolar cells of the lamina. By recording the corneal electroretinogram (ERG), we found that the application of neurochemicals that raise NO/cGMP levels in the optic lobe increased the ERG amplitude, whereas the experimental reduction of NO levels caused a decrease in the response to light. An increase in the light response was also found in intracellular recordings after application of a NO donor, suggesting that the NO-induced changes in the ERG are not caused by changes in the resistive isolation of the retina. Our cytochemical and electrophysiological data are both consistent with the hypothesis that NO synthesized in monopolar cells is a retrograde messenger to the presynaptic photoreceptor neurones.

NO news from insect brains

In nerve cells,the short-lived signalling molecule nitric oxide (NO) is generated by Ca2+-calmodulin-stimulated NO synthases. Nitric oxide activates soluble guanylate cyclase in target cells, leading to the formation of cGMP. Biochemical investigations have shown the presence of a Ca2+-calmodulin-regulated NO-cGMP signalling mechanism in the nervous system of insects. Using NADPH-diaphorase staining as a marker for the enzyme NO synthase and an antiserum against cGMP,the cellular organization of NO donor and target cells has so far been resolved in the locust and fruit fly. This paper provides an overview of the cellular organization of NO signalling in the insect nervous system as well as highlighting its functions in olfactory information processing, formation of olfactory memory, vision, and neuronal development. The resolution of discrete donor and NO-responsive target cells in the developing nervous system of Drosophila will facilitate the genetic and pharmacological analysis of NO-cGMP signal transduction.

Immunocytochemical Mapping of an RDL-Like GABA Receptor Subunit and of GABA in Brain Structures Related to Learning and Memory in the Cricket Acheta domesticus

The distribution of putative RDL-like GABA receptors and of γ-aminobutyric acid (GABA) in the brain of the adult house cricket Acheta domesticus was studied using specific antisera. Special attention was given to brain structures known to be related to learning and memory. The main immunostaining for the RDL-like GABA receptor was observed in mushroom bodies, in particular the upper part of mushroom body peduncle and the two arms of the posterior calyx. Weaker immunostaining was detected in the distal part of the peduncle and in the α and β lobes. The dorso- and ventrolateral protocerebrum neuropils appeared rich in RDL-like GABA receptors. Staining was also detected in the glomeruli of the antennal lobe, as well as in the ellipsoid body of the central complex. Many neurons clustered in groups exhibit GABA-like immunoreactivity. Tracts that were strongly immunostained innervated both the calyces and the lobes of mushroom bodies. The glomeruli of the antennal lobe, the ellipsoid body, as well as neuropils of the dorso- and ventrolateral protocerebrum were also rich in GABA-like immuno- reactivity. The data demonstrated a good correlation between the distribution of the GABA-like and of the RDL-like GABA receptor immunoreactivity. The prominent distribution of RDL-like GABA receptor subunits, in particular areas of mushroom bodies and antennal lobes, underlines the importance of inhibitory signals in information processing in these major integrative centers of the insect brain.