Immunocytochemical demonstration of octopamine-immunoreactive cells in the nervous system of Locusta migratoria and Schistocerca gregaria

Rapid cold hardening delays the onset of anoxia-induced coma via an octopaminergic pathway in Locusta migratoria

Rapid cold hardening (RCH) is a short-term hormesis that occurs in many invertebrate species, especially in insects. Although RCH is best known as enhancing cold tolerance, it can also enhance anoxic tolerance. When exposed to prolonged anoxia, insects enter a reversible coma, which is associated with spreading depolarization (SD) in the central nervous system (CNS). In this study, we investigated the effects of RCH and octopamine (OA) on anoxia-induced SD in L. migratoria. OA is an insect stress hormone that has roles in many physiological processes. Thus, we hypothesized that OA is involved in the mechanism of RCH. First, we found that RCH affects the K⁺ sensitivity of the locust blood brain barrier (BBB) in a way similar to the previously described effects of OA. Next, using SD as an indicator of anoxia-induced coma, we took a pharmacological approach to investigate the effects of OA and epinastine (EP), an octopaminergic receptor (OctR) antagonist. We found that OA mimics, whereas EP blocks, the effect of RCH on anoxia-induced SD. This study demonstrates that OA is involved in the mechanism of RCH in delaying the onset of anoxia-induced locust coma and contributes to determining the mechanism of RCH that modulates insect stress tolerances.

Extensive and diverse patterns of cell death sculpt neural networks in insects

Changes to the structure and function of neural networks are thought to underlie the evolutionary adaptation of animal behaviours. Among the many developmental phenomena that generate change programmed cell death (PCD) appears to play a key role. We show that cell death occurs continuously throughout insect neurogenesis and happens soon after neurons are born. Mimicking an evolutionary role for increasing cell numbers, we artificially block PCD in the medial neuroblast lineage in Drosophila melanogaster, which results in the production of ‘undead’ neurons with complex arborisations and distinct neurotransmitter identities. Activation of these ‘undead’ neurons and recordings of neural activity in behaving animals demonstrate that they are functional. Focusing on two dipterans which have lost flight during evolution we reveal that reductions in populations of flight interneurons are likely caused by increased cell death during development. Our findings suggest that the evolutionary modulation of death-based patterning could generate novel network configurations.

Just like a sculptor chips away at a block of granite to make a statue, the nervous system reaches its mature state by eliminating neurons during development through a process known as programmed cell death.

In vertebrates, this mechanism often involves newly born neurons shrivelling away and dying if they fail to connect with others during development. Most studies in insects have focused on the death of neurons that occurs at metamorphosis, during the transition between larva to adult, when cells which are no longer needed in the new life stage are eliminated.

Pop et al. harnessed a newly designed genetic probe to point out that, in fruit flies, programmed cell death of neurons at metamorphosis is not the main mechanism through which cells die. Rather, the majority of cell death takes place as soon as neurons are born throughout all larval stages, when most of the adult nervous system is built. To gain further insight into the role of this ‘early’ cell death, the neurons were stopped from dying, showing that these cells were able to reach maturity and function. Together, these results suggest that early cell death may be a mechanism fine-tuned by evolution to shape the many and varied nervous systems of insects.

To explore this, Pop et al. looked for hints of early cell death in relatives of fruit flies that are unable to fly: the swift lousefly and the bee lousefly. This analysis showed that early cell death is likely to occur in these two insects, but it follows different patterns than in the fruit fly, potentially targeting the neurons that would have controlled flight in these flies’ ancestors.

Brains are the product of evolution: learning how neurons change their connections and adapt could help us understand how the brain works in health and disease. This knowledge may also be relevant to work on artificial intelligence, a discipline that often bases the building blocks and connections in artificial ‘brains’ on how neurons communicate with one another.

A Model for Basic Emotions Using Observations of Behavior in Drosophila

Emotion plays a crucial role, both in general human experience and in psychiatric illnesses. Despite the importance of emotion, the relative lack of objective methodologies to scientifically studying emotional phenomena limits our current understanding and thereby calls for the development of novel methodologies, such us the study of illustrative animal models. Analysis of Drosophila and other insects has unlocked new opportunities to elucidate the behavioral phenotypes of fundamentally emotional phenomena. Here we propose an integrative model of basic emotions based on observations of this animal model. The basic emotions are internal states that are modulated by neuromodulators, and these internal states are externally expressed as certain stereotypical behaviors, such as instinct, which is proposed as ancient mechanisms of survival. There are four kinds of basic emotions: happiness, sadness, fear, and anger, which are differentially associated with three core affects: reward (happiness), punishment (sadness), and stress (fear and anger). These core affects are analogous to the three primary colors (red, yellow, and blue) in that they are combined in various proportions to result in more complex “higher order” emotions, such as love and aesthetic emotion. We refer to our proposed model of emotions as called the “Three Primary Color Model of Basic Emotions.”

Octopamine-like immunoreactive neurons in the brain and subesophageal ganglion of the parasitic wasps Nasonia vitripennis and N. giraulti

Octopamine is an important neuromodulator in the insect nervous system, influencing memory formation, sensory perception and motor control. In this study, we compare the distribution of octopamine-like immunoreactive neurons in two parasitic wasp species of the Nasonia genus, N. vitripennis and N. giraulti. These two species were previously described as differing in their learning and memory formation, which raised the question as to whether morphological differences in octopaminergic neurons underpinned these variations. Immunohistochemistry in combination with confocal laser scanning microscopy was used to reveal and compare the somata and major projections of the octopaminergic neurons in these wasps. The brains of both species showed similar staining patterns, with six different neuron clusters being identified in the brain and five different clusters in the subesophageal ganglion. Of those clusters found in the subesophageal ganglion, three contained unpaired neurons, whereas the other three consisted in paired neurons. The overall pattern of octopaminergic neurons in both species was similar, with no differences in the numbers or projections of the ventral unpaired median (VUM) neurons, which are known to be involved in memory formation in insects. In one other cluster in the brain, located in-between the optic lobe and the antennal lobe, we detected more neurons in N. vitripennis compared with N. giraulti. Combining our results with findings made previously in other Hymenopteran species, we discuss possible functions and some of the ultimate factors influencing the evolution of the octopaminergic system in the insect brain.

Identification of distinct tyraminergic and octopaminergic neurons innervating the central complex of the desert locust, Schistocerca gregaria

The central complex is a group of modular neuropils in the insect brain with a key role in visual memory, spatial orientation, and motor control. In desert locusts the neurochemical organization of the central complex has been investigated in detail, including the distribution of dopamine-, serotonin-, and histamine-immunoreactive neurons. In the present study we identified neurons immunoreactive with antisera against octopamine, tyramine, and the enzymes required for their synthesis, tyrosine decarboxylase (TDC) and tyramine β-hydroxylase (TBH). Octopamine- and tyramine immunostaining in the central complex differed strikingly. In each brain hemisphere tyramine immunostaining was found in four neurons innervating the noduli, 12-15 tangential neurons of the protocerebral bridge, and about 17 neurons that supplied the anterior lip region and parts of the central body. In contrast, octopamine immunostaining was present in two bilateral pairs of ascending fibers innervating the upper division of the central body and a single pair of neurons with somata near the esophageal foramen that gave rise to arborizations in the protocerebral bridge. Immunostaining for TDC, the enzyme converting tyrosine to tyramine, combined the patterns seen with the tyramine- and octopamine antisera. Immunostaining for TBH, the enzyme converting tyramine to octopamine, in contrast, was strikingly similar to octopamine immunolabeling. We conclude that tyramine and octopamine act as neurotransmitters/modulators in distinct sets of neurons of the locust central complex with TBH likely being the rate-limiting enzyme for octopamine synthesis in a small subpopulation of TDC-containing neurons.

Distribution of the octopamine receptor AmOA1 in the honey bee brain

Octopamine plays an important role in many behaviors in invertebrates. It acts via binding to G protein coupled receptors located on the plasma membrane of responsive cells. Several distinct subtypes of octopamine receptors have been found in invertebrates, yet little is known about the expression pattern of these different receptor subtypes and how each subtype may contribute to different behaviors. One honey bee (Apis mellifera) octopamine receptor, AmOA1, was recently cloned and characterized. Here we continue to characterize the AmOA1 receptor by investigating its distribution in the honey bee brain. We used two independent antibodies produced against two distinct peptides in the carboxyl-terminus to study the distribution of the AmOA1 receptor in the honey bee brain. We found that both anti-AmOA1 antibodies revealed labeling of cell body clusters throughout the brain and within the following brain neuropils: the antennal lobes; the calyces, pedunculus, vertical (alpha, gamma) and medial (beta) lobes of the mushroom body; the optic lobes; the subesophageal ganglion; and the central complex. Double immunofluorescence staining using anti-GABA and anti-AmOA1 receptor antibodies revealed that a population of inhibitory GABAergic local interneurons in the antennal lobes express the AmOA1 receptor in the cell bodies, axons and their endings in the glomeruli. In the mushroom bodies, AmOA1 receptors are expressed in a subpopulation of inhibitory GABAergic feedback neurons that ends in the visual (outer half of basal ring and collar regions) and olfactory (lip and inner basal ring region) calyx neuropils, as well as in the collar and lip zones of the vertical and medial lobes. The data suggest that one effect of octopamine via AmOA1 in the antennal lobe and mushroom body is to modulate inhibitory neurons.

A map of octopaminergic neurons in the Drosophila brain

The biogenic amine octopamine modulates diverse behaviors in invertebrates. At the single neuron level, the mode of action is well understood in the peripheral nervous system owing to its simple structure and accessibility. For elucidating the role of individual octopaminergic neurons in the modulation of complex behaviors, a detailed analysis of the connectivity in the central nervous system is required. Here we present a comprehensive anatomical map of candidate octopaminergic neurons in the adult Drosophila brain: including the supra- and subesophageal ganglia. Application of the Flp-out technique enabled visualization of 27 types of individual octopaminergic neurons. Based on their morphology and distribution of genetic markers, we found that most octopaminergic neurons project to multiple brain structures with a clear separation of dendritic and presynaptic regions. Whereas their major dendrites are confined to specific brain regions, each cell type targets different, yet defined, neuropils distributed throughout the central nervous system. This would allow them to constitute combinatorial modules assigned to the modulation of distinct neuronal processes. The map may provide an anatomical framework for the functional constitution of the octopaminergic system. It also serves as a model for the single-cell organization of a particular neurotransmitter in the brain.

Activity of neuromodulatory neurones during stepping of a single insect leg

Octopamine plays a major role in insect motor control and is released from dorsal unpaired median (DUM) neurones, a group of cells located on the dorsal midline of each ganglion. We were interested whether and how these neurones are activated during walking and chose the semi-intact walking preparation of stick insects that offers to investigate single leg-stepping movements. DUM neurones were characterized in the thoracic nerve cord by backfilling lateral nerves. These backfills revealed a population of 6-8 efferent DUM cells per thoracic segment. Mesothoracic DUM cells were subsequently recorded during middle leg stepping and characterized by intracellular staining. Seven out of eight identified individual different types of DUM neurones were efferent. Seven types except the DUMna nl2 were tonically depolarized during middle leg stepping and additional phasic depolarizations in membrane potential linked to the stance phase of the middle leg were observed. These DUM neurones were all multimodal and received depolarizing synaptic drive when the abdomen, antennae or different parts of the leg were mechanically stimulated. We never observed hyperpolarising synaptic inputs to DUM neurones. Only one type of DUM neurone, DUMna, exhibited spontaneous rhythmic activity and was unaffected by different stimuli or walking movements.

Responses of efferent octopaminergic thoracic unpaired median neurons in the locust to visual and mechanosensory signals

Insect thoracic ganglia contain efferent octopaminergic unpaired median neurons (UM neurons) located in the midline, projecting bilaterally and modulating neuromuscular transmission, muscle contraction kinetics, sensory sensitivity and muscle metabolism. In locusts, these neurons are located dorsally or ventrally (DUM- or VUM-neurons) and divided into functionally different sub-populations activated during different motor tasks. This study addresses the responsiveness of locust thoracic DUM neurons to various sensory stimuli. Two classes of sense organs, cuticular exteroreceptor mechanosensilla (tactile hairs and campaniform sensilla), and photoreceptors (compound eyes and ocelli) elicited excitatory reflex responses. Chordotonal organ joint receptors caused no responses. The tympanal organ (Müller's organ) elicited weak excitatory responses most likely via generally increased network activity due to increased arousal. Vibratory stimuli to the hind leg subgenual organ never elicited responses. Whereas DUM neurons innervating wing muscles are not very responsive to sensory stimulation, those innervating leg and other muscles are very responsive to stimulation of exteroreceptors and hardly responsive to stimulation of proprioceptors. After cutting both cervical connectives all mechanosensory excitation is lost, even for sensory inputs from the abdomen. This suggests that, in contrast to motor neurons, the sensory inputs to octopaminergic efferent neuromodulatory cells are pre-processed in the suboesophageal ganglion.

Identification of the neurotransmitters involved in modulation of transmitter release from the central terminals of the locust wing hinge stretch receptor

The flight motor system of the locust represents a model preparation for the investigation of neuromodulation. At the wing hinges are stretch receptors important in generating and controlling the flight motor pattern. The forewing stretch receptor (fSR) makes direct cholinergic synapses with depressor motor neurons (MN) controlling that wing, including the first basalar MN (BA1). The fSR/BA1 synapse is modulated by muscarinic cholinergic receptors located on gamma-aminobutyric acid (GABA)-ergic interneurons (Judge and Leitch [1999a] J. Comp. Neurol. 407:103-114; Judge and Leitch [1999b] J. Neurobiol. 40:420-431). However, electrophysiology has shown that fSR/BA is also modulated by biogenic amines (Leitch et al. [2003] J. Comp. Neurol. 462:55-70). We have used electron microscopic immunocytochemistry (ICC) to identify the neurotransmitters in neurons presynaptic to the fSR and to determine the relative proportion of these different classes of modulatory inputs. Approximately 55% of all inputs to the fSR are glutamate-IR, indicating that glutamatergic neurons may also play an important role in presynaptically modulating the fSR terminals. Anti-GABA ICC confirmed that over 40% of inputs to the fSR are GABA-IR (Judge and Leitch [1999a] J. Comp. Neurol. 407:103-114). Labelling sections with an antioctopamine antibody revealed neurons containing distinctive large, electron-dense granules, which could reliably be used to identify them. Aminergic neurons that modulate the synapse may have very few morphologically recognizable synaptic outputs. Although putative octopaminergic processes were found in close contact to horseradish peroxidase-filled fSR profiles, no morphologically recognizable synaptic inputs to the fSR were evident. Collectively, these data suggest that most inputs to the fSR are from either glutamatergic or GABAergic neurons.

Dendritic projections of different types of octopaminergic unpaired median neurons in the locust metathoracic ganglion

Octopaminergic dorsal unpaired median (DUM) neurons of locust thoracic ganglia are important components of motor networks and are divided into various sub-populations. We have examined individually stained metathoracic DUM neurons, their dendritic projection patterns, and their relationship to specific architectural features of the metathoracic ganglion, such as longitudinal tracts, transverse commissures, and well-defined sensory neuropils. The detailed branching patterns of individually characterized DUM neurons of various types were analyzed in vibratome sections in which architectural features were revealed by using antibodies against tubulin and synapsin. Whereas DUM3,4,5 and DUM5 neurons (the group innervating leg and "non-wing-power" muscles) had many ventral and dorsal branches, DUM1 and DUM3,4 neurons (innervating "wing-power" muscles) branched extensively only in dorsal areas. The structure of DUM3 neurons differed markedly from that of the other DUM neurons examined in that they sent branches into dorsal areas and had differently structured side branches that mostly extended laterally. The differences between the branching patterns of these neurons were quantified by using currently available new reconstruction algorithms. These structural differences between the various classes of DUM neurons corresponded to differences in their function and biophysical properties.

Comparison of octopamine-like immunoreactivity in the brains of the fruit fly and blow fly

A serum raised against conjugated octopamine reveals structurally comparable systems of perikarya and arborizations in protocerebral neuropils of two species of Diptera, Drosophila melanogaster and Phaenicia sericata; the latter is used extensively for electrophysiological studies of the optic lobes and their central projections. Clusters of cell bodies in the brain as well as midline perikarya provide octopamine-like immunoreactive processes to the optic lobes, circumscribed regions of the protocerebrum and the central complex, particularly the protocerebral bridge, fan-shaped body, and ellipsoid body. Ventral unpaired median somata provide immunoreactive processes within the subesophageal ganglion and tritocerebrum. Ascending neurites from these cells also supply the antennal lobe glomeruli, regions of the lateral protocerebrum, the mushroom body calyces, and the lobula complex. The mushroom body's gamma lobes contain immunoreactive processes that originate from processes that arborize in the protocerebrum. The present observations are discussed with respect to similarities and differences between two species of Diptera, one of which has neurons large enough for intracellular penetrations. The results are also discussed with respect to recent studies on octopamine-immunoreactive organization in honey bees and cockroaches and the suggested roles of octopamine in sensory processing, learning, and memory.

Octopamine-like immunoreactivity in the honey bee and cockroach: Comparable organization in the brain and subesophageal ganglion

A serum raised against octopamine reveals in cockroaches and honey bees structurally comparable systems of perikarya and their extensive yet discrete systems of arborizations in neuropils. Numerous and prominent clusters of lateral cell bodies in the brain as well as many midline perikarya provide octopamine-like immunoreactive processes to circumscribed regions of the subesophageal ganglion, antennal lobe glomeruli, optic neuropils, and neuropils of the protocerebrum. There is dense octopaminergic innervation in the protocerebral bridge and ellipsoid body of the central complex. The antennal lobes are supplied by at least three octopamine-immunoreactive neurons. In contrast, the mushroom bodies show the fewest immunoreactive elements. In Apis a single axon supplies sparse immunoreactive processes to the calyces' basal ring, collar, and lip. A diffuse arrangement of immunoreactive processes invades all zones of the mushroom body calyces in Periplaneta. These processes derive from an ascending axon ascribed to a dorsal unpaired median neuron at the maxillary segment of the subesophageal ganglion. In both taxa octopamine-immunoreactive processes invade only the gamma lobes of the mushroom bodies, omitting their other divisions. The present observations are discussed with respect to possible roles of octopamine in sensory integration and association.

Tyramine and octopamine: ruling behavior and metabolism

Octopamine (OA) and tyramine (TA) are the invertebrate counterparts of the vertebrate adrenergic transmitters. They are decarboxylation products of the amino acid tyrosine, with TA as the biological precursor of OA. Nevertheless, both compounds are independent neurotransmitters that act through G protein-coupled receptors. OA modulates a plethora of behaviors and peripheral and sense organs, enabling the insect to respond correctly to external stimuli. Because these two phenolamines are the only biogenic amines whose physiological significance is presumably restricted to invertebrates, pharmacologists have focused their attention on the corresponding receptors, which are still believed to represent promising targets for new insecticides. Recent progress made on all levels of OA and TA research has enabled researchers to understand better the molecular events underlying the control of complex behaviors.

Octopamine immunoreactive cell populations in the locust thoracic-abdominal nervous system

We describe octopamine-immunoreactive somata and their projections in the pro- meso-, meta- and pregenital abdominal-ganglia of locusts. Immunoreactive midline somata were identified as dorsal- and ventral- unpaired median (DUM- and VUM-, respectively) neurones due to their: characteristic large size and positions of somata, primary neurites in DUM-tracts giving rise to T-junctions, and bilaterally projecting axons. In the prothoracic ganglion there are most likely 8 such cells; in the meso- and metathoracic, some 20 each; and in each individual pregenital abdominal ganglion, typically 3. All appear to project to peripheral nerves and their numbers correspond to the number of peripherally projecting DUM-cells identified to date in each ganglion. We suggest that probably all peripherally projecting DUM-cells are octopaminergic in the examined ganglia. Presumptive DUM-interneurones are not octopamine-immunoreactive, but, confirming other studies, are shown to label with an antiserum to gamma-amino butyric acid (GABA). Other octopamine-immunoreactive neurones include a pair of midline, prothoracic, anterior medial cells, not necessarily DUM-cells, and a pair of ventral lateral somata in each thoracic- and the first abdominal ganglion. The latter project intersegmentally in ventral tracts. Intersegmentally projecting octopamine-immunoreactive fibers in dorsal tracts probably arise from a prothoracic DUM-cell, which leaves through suboesophageal nerves, or descending suboesophageal DUM-cells. Thus, the octopamine-immunoreactive system of thoracic and pregenital abdominal ganglia in locust comprises all peripherally projecting DUM-cells and a plurisegmental network.

Mesothoracic ventral unpaired median (mesVUM) neurons in the blowfly Calliphora erythrocephala

The study describes five ventral unpaired median neurons in the mesothoracic neuromere of the fused thoracic ganglion of Calliphora identified by biocytin staining (mesVUM neurons). The group comprises four efferent neurons and one interneuron which are characterized by a common soma cluster in the ventral midline of the neuromere, bifurcating primary neurites and bilaterally symmetrical arborizations. Respective soma clusters of not-yet-identified VUM neurons were also found in the prothoracic, metathoracic, and abdominal neuromeres. The efferent mesVUM neurons are associated with the flight system. Their main arborizations are located in the mesothoracic wing neuropil and their bilateral axons terminate at the flight control muscles, the flight starter muscles, the flight power muscles, or at myocuticular junctions of the latter. In contrast, an association of the interneuron with a particular functional system is not apparent. The arborizations of the neuron are intersegmental and invade all thoracic neuromeres. A further difference between the two types of neurons regards their somatic action potentials, which are overshooting in the efferent neurons and strongly attenuated in the interneuron. Immunocytochemical stainings revealed four clusters of octopamine-immunoreactive (OA-IR) somata in the thoracic ganglion, which reside in the same positions as the VUM somata. We regard this as strong evidence that all groups of VUM neurons contain OA-IR cells and that, in particular, the identified efferent mesVUM neurons are OA-IR. Our results demonstrate that the mesVUM neurons of Calliphora have similar morphological, electrophysiological, and presumably also immunocytochemical characteristics as the unpaired median neurons of other insects.

Octopamine modulates spermathecal muscle contractions in Locusta migratoria

Octopamine was identified in the spermathecal tissue of Locusta migratoria using HPLC and immunohistochemical techniques. Octopamine-like immunoreactive unpaired median neurons were identified in the VIIth and VIIIth (terminal) abdominal ganglia and octopamine-like immunoreactive axons were present in the ventral ovipositor nerve (branches from this nerve innervate the spermatheca). Stimulatory actions of octopamine on myogenic and neurogenic contractions were observed. Dose-dependent increases in the frequency of myogenic contractions and the amplitude of neurogenic contractions were elicited by the application of octopamine to the spermathecal muscle. Non-sustained basal tension increases were noted in some preparations, although these were not found to be dose-dependent. SchistoFLRFamide (PDVDHVFLRFamide) inhibited octopamine-induced contractions by a maximum of about 30%. In the presence of 3-isobutyl-1 -methylxanthine, octopamine increased cAMP levels in all regions of the spermathecal. The largest increase in cAMP content was found in the spermathecal sac, followed by the straight duct and coil duct. Phentolamine blocked octopamine-induced increases in cAMP levels and abolished the actions of octopamine on myogenic contractions.

Synaptic organization of the mushroom body calyx in Drosophila melanogaster

The calyx neuropil of the mushroom body in adult Drosophila melanogaster contains three major neuronal elements: extrinsic projection neurons, presumed cholinergic, immunoreactive to choline acetyltransferase (ChAT-ir) and vesicular acetylcholine transporter (VAChT-ir) antisera; presumed gamma-aminobutyric acid (GABA)ergic extrinsic neurons with GABA-like immunoreactivity; and local intrinsic Kenyon cells. The projection neurons connecting the calyx with the antennal lobe via the antennocerebral tract are the only source of cholinergic elements in the calyces. Their terminals establish an array of large boutons 2-7 microm in diameter throughout all calycal subdivisions. The GABA-ir extrinsic neurons, different in origin, form a network of fine fibers and boutons codistributed in all calycal regions with the cholinergic terminals and with tiny profiles, mainly Kenyon cell dendrites. We have investigated the synaptic circuits of these three neuron types using preembedding immuno-electron microscopy. All ChAT/VAChT-ir boutons form divergent synapses upon multitudinous surrounding Kenyon cell dendrites. GABA-ir elements also regularly contribute divergent synaptic input onto these dendrites, as well as occasional inputs to boutons of projection neurons. The same synaptic microcircuits involving these three neuron types are repeatedly established in glomeruli in all calycal regions. Each glomerulus comprises a large cholinergic bouton at its core, encircled by tiny vesicle-free Kenyon cell dendrites as well as by a number of GABAergic terminals. A single dendritic profile may thereby receive synaptic input from both cholinergic and GABAergic elements in close vicinity at presynaptic sites with T-bars typical of fly synapses. ChAT-ir boutons regularly have large extensions of the active zones. Thus, Kenyon cells may receive major excitatory input from cholinergic boutons and considerable postsynaptic inhibition from GABAergic terminals, as well as, more rarely, presynaptic inhibitory signaling. The calycal glomeruli of Drosophila are compared with the cerebellar glomeruli of vertebrates. The cholinergic boutons are the largest identified cholinergic synapses in the Drosophila brain and an eligible prospect for studying the genetic regulation of excitatory presynaptic function.

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.

Cell biology of the adipokinetic hormone-producing neurosecretory cells in the locust corpus cardiacum

The adipokinetic cells are neuron-like unipolar cells, the cell bodies and cell processes of which are intermingled within the glandular part of the corpus cardiacum. In Schistocerca gregaria, they produce two adipokinetic hormones, AKH-I and -II, whereas in Locusta migratoria an additional hormone, AKH-III, is present. The three AKHs are produced by the same cells and are co-localized in secretory granules. The biosynthesis and processing of the AKH prohormones to the bioactive hormones, which has been elucidated in detail for AKH-I and -II in S. gregaria, takes less than 75 min and goes on continuously. In older locusts in particular, the adipokinetic cells contain intracisternal granules, widely dilated cisternae of the rough endoplasmic reticulum, which function as stores of prohormones of AKH-I and -II, not of AKH-III. The adipokinetic cells are subjected to regulation by a number of neural and humoral substances, neural influences coming from secretomotor cells in the lateral part of the protocerebrum. Flight activity is the only natural stimulus unequivocally shown to induce the release of AKHs, which in L. migratoria results in parallel secretion of all three AKHs. During secretory stimulation, young secretory granules containing newly synthesized hormones are preferentially released over older granules. Secretory stimulation is not accompanied by a clear increase in the levels of the AKH mRNAs and the AKH prohormones and in the rate of synthesis of the (pro-)AKHs. Apparently, a coupling between release and biosynthesis of the AKHs in the adipokinetic cells is very loose or does not even exist.

Octopamine receptors in the honey bee and locust nervous system: pharmacological similarities between homologous receptors of distantly related species

Honey bees are perhaps the most versatile models to study the cellular and pharmacological basis underlying behaviours ranging from learning and memory to sociobiology. For both aspects octopamine (OA) is known to play a vital role. The neuronal octopamine receptor of the honey bee shares pharmacological similarities with the neuronal octopamine receptor of the locust. Both, agonists and antagonists known to have high affinities for the locust neuronal octopamine receptor have also high affinities for the bee neuronal octopamine receptor. The distribution of receptors is more or less congruent between locusts and bees. Optic lobes and especially the mushroom bodies are areas of greatest octopamine receptor expression in both species, which mirrors the physiological significance of octopamine in the insect nervous system. The neuronal octopamine receptor of insects served as a model to study the pharmacological similarity of homologous receptors from distantly related species, because bees and locusts are separated by at least 330 million years of evolution.

Peripheral distribution of presynaptic sites of abdominal motor and modulatory neurons in Manduca sexta larvae

Insect muscle fibers are commonly innervated by multiple motor neurons and efferent unpaired median (UM) neurons. The role of UM neurons in the modulation rather than rapid activation of muscle contraction (Evans and O'Shea [1977] Nature 270:257-259) suggests that their terminal varicosities may differ structurally and functionally from the presynaptic terminals of motor neurons. Furthermore, differences in the characteristics of UM neuron terminal varicosities may be correlated with functional differences among their diverse target muscles. Larval abdominal body wall muscles in the hawkmoth, Manduca sexta, consist of large, elongated fibers that are multiterminally innervated by one and occasionally two motor neurons (Levine and Truman [1985] J. Neurosci. 5:2424-2431). The fibers are also innervated by one of two efferent UM neurons that bifurcate to innervate targets on both sides of the abdomen (Pflüger et al. [1993] J. Comp. Neurol. 335:508-522). In this study, the intracellular tracer biocytin was used to identify the targets of the UM neurons and to distinguish their terminal axonal varicosities on the muscle fibers. An antiserum to the synaptic vesicle protein, synaptotagmin, was used to label synaptic vesicles, and the styryl dye FM1-43 was used to demonstrate release and recycling. Most of the abdominal muscles in a given hemisegment were found to be supplied by one of the two UM neurons. Terminal varicosities of the excitatory motor neurons were large (3-7 pm) and were found in rows of rosettes that extended to every aspect of the muscle fiber; these varicosities were designated as type I terminals. The UM neuron terminal varicosities also occupied every aspect of the fiber but were smaller (1-3 microm) and more separated from each other; these were designated as type II terminals. Both type I and type II terminals are synaptotagmin immunoreactive and, as shown by FM1-43 staining, are sites of synaptic vesicle recycling. The excitatory motor neuron terminals (type I) could easily be loaded and unloaded with FM1-43, which indicates their capacity for repeated vesicular exocytosis and recycling. In contrast, the dye could not as readily be unloaded from UM neuron terminals (type II), which may indicate that they have a slower turnover of synaptic vesicles.

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.

Multifactorial control of the release of hormones from the locust retrocerebral complex

The retrocerebral complex of locusts consists of the corpus cardiacum, the corpora allata, and the nerves that connect these glands with the central nervous system. Both corpus cardiacum and corpora allata are neuroendocrine organs and consist of a glandular part, which synthesizes adipokinetic hormones and juvenile hormone, respectively, and of a neurohemal part. The glandular adipokinetic cells in the corpus cardiacum appear to be subjected to a multitude of regulatory stimulating, inhibiting, and modulating substances. Neural influence comes from secretomotor cells in the lateral part of the protocerebrum. Up to now, only peptidergic factors have been established to be present in the neural fibres that make synaptic contact with the adipokinetic cells. Humoral factors that act on the adipokinetic cells via the hemolymph are of peptidergic and aminergic nature. In addition, high concentrations of trehalose inhibit the release of adipokinetic hormones. Although there is evidence that neurosecretory cells in the protocerebrum are involved in the control of JH biosynthesis, the nature of the factors involved remains to be resolved.

Several isoforms of locustatachykinins may be involved in cyclic AMP-mediated release of adipokinetic hormones from the locust Corpora cardiaca

Four locustatachykinins (LomTK I-IV) were identified in about equal amounts in extracts of corpora cardiaca of locusts, using reverse-phase high-performance liquid chromatography and radioimmunoassay with synthetic LomTK I-IV as standards. Brain extracts also contained the four isoforms in roughly equimolar concentrations. Retrograde tracing of the nervi corporis cardiaci II (NCC II) in vitro with Lucifer yellow in combination with LomTK immunocytochemistry revealed that about half of the secretomotor neurons in the lateral part of the protocerebrum projecting into the glandular lobe of the corpora cardiaca (CCG) contain LomTK-immunoreactive material. Since the four LomTKs are present in the CCG, these four or five neurons in each hemisphere are likely to contain colocalized LomTK I-IV. The role of two of the LomTKs in the regulation of the release of adipokinetic hormones (AKHs) from the adipokinetic cells in the CCG in the locust was investigated. Experiments performed in vitro showed that LomTK I and II induced release of AKH in a dose-dependent manner. These peptides also rapidly and transiently elevated the cyclic AMP-content of the CCG. The peak level of cyclic AMP occurred about 45 seconds after stimulation with LomTK. These results support the proposal that LomTKs are involved in controlling the release of the adipokinetic hormones and suggest that all LomTK isoforms may participate in this cyclic AMP-mediated event.

Colocalisation of taurine- with transmitter-immunoreactivities in the nervous system of the migratory locust

The expression of taurine immunoreactivity (TAU-IR) by neurones immunoreactive for octopamine (OA-IR), gamma-aminobutyric acid (GABA-IR), and the C-terminal peptide sequence arginine-phenylalanine (RFamide-IR) was investigated in the migratory locust (Locusta migratoria). TAU-IR is colocalised with OA-IR in the dorsal unpaired median neurones, which are efferent neuroparacrine cells. TAU-IR is not, however, expressed by OA-IR interneurones in the thoracic ganglia and brain. The only other TAU-IR somata found with peripheral axons are the medial neurosecretory cells in abdominal ganglia that project to the neurohaemal organs. These cells exhibit RFamide-IR. The majority of TAU-IR somata in the thoracic abdominal nervous system exhibit GABA-IR. These cells correspond to populations of identified local and intersegmentally projecting inhibitory interneurones. TAU-IR is not, however, exhibited by the well-known GABAergic common inhibitor neurones, which have peripherally projecting axons. This differential distribution of TAU-IR in basically two, functionally different, neuronal subsets (efferent neurosecretory and neuroparacrine cells, inhibitory interneurones) conforms with the concept of taurine acing as a depressive agent to limit excitation during stressful conditions.

Cytoarchitecture of histamine-, dopamine-, serotonin- and octopamine-containing neurons in the cricket ventral nerve cord

The present article provides a comparative neuroanatomical description of the cellular localization of the biogenic amines histamine, dopamine, serotonin and octopamine in the ventral nerve cord of an insect, namely the cricket, Gryllus bimaculatus. Generally, different immunocytochemical staining techniques reveal a small number of segmentally distributed immunoreactive (-IR) amine-containing neurons allowing single cell reconstruction of prominent elements. Aminergic neurons share common morphological features in that they innervate large portions of neurophil and often connect different neuromeres by intersegmental 'wide-field' projections of varicose appearance. In many cases aminergic terminals are also found on the surface of peripheral nerves suggesting additional neurohemal release sites. Despite such morphological similarities histological analysis demonstrates for any given amine functionally distinct neuron types with specific innervation patterns establishing discrete pathways. Histamine-IR interneurons are characterized by both ascending and descending projections forming central and peripheral terminals. The descending branches from dopamine-IR cells mainly converge within the terminal ganglion, whereas serotonin-IR interneurons with ascending projections often terminate within the brain. Serotonin is also present in sensory and motor neurons. In contrast to other aminergic neurons, most octopamine-IR cells represent unpaired neurons projecting through motor nerves of the soma-containing neuromere. Octopamine-IR cells with intersegmental branches are only rarely found. Based on these findings, a colocalization of different amines within the same neuron seems to be unlikely to occur in the cricket ventral nerve cord. With respect to the neuroanatomical description of amine-containing neurons known physiological effects of biogenic amines and their possible neuromodulatory functions in insects are discussed.

Patterned activation of unpaired median neurons during fictive crawling in manduca sexta larvae

The unpaired median neurons are common to the segmental ganglia of many insects. Although some of the functional consequences of their activation, among them the release of octopamine to modulate muscle contraction, have been described, less is understood about how and when these neurons are recruited during movement. The present study demonstrates that peripherally projecting unpaired median neurons in the abdominal and thoracic ganglia of the larval tobacco hornworm Manduca sexta are recruited rhythmically during the fictive crawling motor activity that is produced by the isolated central nervous system in response to pilocarpine. Regardless of the muscles to which they project, the efferent unpaired median neurons in all segmental ganglia are depolarized together during the phase of the crawling cycle when the thoracic leg levator motoneurons are active. During fictive crawling, therefore, the unpaired median neurons are not necessarily active in synchrony with the muscles to which they project. The rhythmical synaptic drive of the efferent unpaired median neurons is derived, at least in part, from a source within the subesophageal ganglion, even when the motor pattern is evoked by exposing only the more posterior ganglia to pilocarpine. In pairwise intracellular recordings from unpaired median neurons in different ganglia, prominent excitatory postsynaptic potentials, which occur with an anterior-to-posterior delay in both neurons, are seen to underlie the rhythmic depolarizations. One model consistent with these findings is that one or more neurons within the subesophageal ganglion, which project posteriorly to the segmental ganglia and ordinarily provide unpatterned synaptic inputs to all efferent unpaired median neurons, become rhythmically active during fictive crawling in response to ascending information from the segmental pattern-generating network.

Peptidergic control of the corpus cardiacum-corpora allata complex of locusts

The brain-corpora cardiaca-corpora allata complex of insects is the physiological equivalent of the brain-hypophysis axis of vertebrates. In locusts there is only one corpus cardiacum as a result of fusion, while most other insect species have a pair of such glands. Like the pituitary of vertebrates, the corpus cardiacum consists of a glandular lobe and a neurohemal lobe. The glandular lobe synthesizes and releases adipokinetic hormones. In the neurohemal part many peptide hormones, which are produced in neurosecretory cells in the brain, are released into the hemolymph. The corpora allata, which have no counterpart in vertebrates, synthesize and release juvenile hormones. The control of the locust corpus cardiacum-corpora allata complex appears to be very complex. Numerous brain factors have been reported to have an effect on biosynthesis and release of juvenile hormone or adipokinetic hormone. Many neuropeptides are present in nerves projecting from the brain into the corpora cardiaca-corpora allata complex, the most important ones being neuroparsins, ovary maturating parsin, insulin-related peptide, diuretic peptide, tachykinins, FLRFamides, FXPRLamides, accessory gland myotropin I, crustacean cardioactive peptide, and schistostatins. In this paper, the cellular distribution, posttranslational processing, peptide-receptor interaction, and inactivation of these peptides are reviewed. In addition, the signal transduction pathways in the release of adipokinetic hormone and juvenile hormone from, respectively, the corpora cardiaca and corpora allata are discussed.

Suboesophageal DUM neurons innervate the principal neuropiles of the locust brain

The morphology of the dorsal unpaired median (DUM ) neurons of the suboesophageal ganglion (SOG) of the migratory locust, Locusta migratoria , were studied by using intracellular staining. The SOG lacks segmental DUM neurons with peripheral axons. All DUM neurons are either intersegmentally projecting (towards the brain or the thoracic nerve cord) or they are local. In addition to previously described DUM neurons with axons in peripheral nerves of the brain (Bräunig 1990), the SOG contains DUM neurons which, in the brain, innervate principal neuropile areas such as the antennal lobes, the pedunculi and calyces of the mushroom body, and the central complex. The number and location of DUM cell bodies stained with intracellular fills is compared with those obtained with either backfilling cervical or circumoesophageal connectives, or octopamine-immunocytochemistry. Additional experiments show that the locust brain, like the SOG, lacks both segmental DUM neurons with peripheral axons, and axons descending into the ventral nerve cord.

Immunocytochemical distribution of locustamyoinhibiting peptide (Lom-MIP) in the nervous system of Locusta migratoria

Locustamyoinhibiting peptide (Lom-MIP) is one of the 4 identified myoinhibiting neuropeptides, isolated from brain-corpora cardiaca-corpora allata-suboesophageal ganglion complexes of the locust, Locusta migratoria. An antiserum was raised against Lom-MIP for use in immunohistochemistry. Locustamyoinhibiting peptide-like immunoreactivity (Lom-MIP-LI) was visualized in the nervous system and peripheral organs of Locusta migratoria by means of the peroxidase-antiperoxidase method. A total of 12 specific immunoreactive neurons was found in the brain. Processes of these neurons innervate the protocerebral bridge the central body complex and distinct neuropil areas in the proto- and tritocerebrum but not in the deuterocerebrum nor in the optic lobes. The glandular cells of the corpora cardiaca, known to produce adipokinetic hormones, are contacted by Lom-MIP-LI fibers. The corpora allata were innervated by the nervus corporis allati I containing immunoreactive fibers. Lom-MIP-LI cell bodies were also found in the subesophageal ganglion, the metathoracic ganglion and the abdominal ganglia I-IV. In peripheral muscles, Lom-MIP-LI fibers innervate the heart, the oviduct, and the hindgut. In the salivary glands, Lom-MIP-LI was detected in the intracellular ductule of the parietal cells. Possible functions of Lom-MIP are discussed.

Anatomy and targets of Dorsal Unpaired Median neurones in the Terminal Abdominal Ganglion of the male cockroach Periplaneta americana L

The morphology of the Dorsal Unpaired Median (DUM) neurones in the Terminal Abdominal Ganglion (TAG) of the adult male cockroach Periplaneta americana were described based on wholemount preparations and paraffin sections and by using anterograde and retrograde cobalt mapping, octopamine-like immunohistochemistry, and double immunofluorescence technique with both conjugated gamma-aminobutyric acid (GABA) and octopamine antisera. Among 60 +/- 6 neurones with large somata (diameter 40 to 60 microns) on the dorsal midline surface of the TAG that were stained with toluidine blue, about 36 efferent DUM neurones exhibited octopamine-like immunoreactivity. The DUM neurones were arranged in three clusters (anterior, median and posterior) corresponding to the 7th-11th abdominal ganglia of the fused TAG. Anterior efferent DUM neurones with one, two, and four pairs of lateral neurites entered segmental nerves VIIB; VIIB and phallic nerves; IXB and phallic nerves; VIIIA, IXA, X, and IX, respectively. Three octopamine-like immunoreactive DUM neurones innervating heart chambers via segmental nerves (VIIA, VIIIA, and IXA) in the last abdominal segments occurred within abdominal ganglia 7, 8, and 9. Together with octopamine-like immunoreactive efferent DUM neurones, GABA-like immunoreactive dorsal midline neurones with small somata (10 to 20 microns) also occurred within the median group. The spatial distribution of DUM neurones in the TAG suggested that they had their origins in the median neuroblast, as for DUM neurones in the grasshopper.

G-protein-coupled receptors in insect cells

The main classes of transmembrane signaling receptor proteins are well conserved during evolution and are encountered in vertebrates as well as in invertebrates. All members of the G-protein-coupled receptor superfamily share a number of basic structural and functional characteristics. In both insects and mammals, this receptor class is involved in the perception and transduction of many important extracellular signals, including a great deal of paracrine, endocrine, and neuronal messengers and visual, olfactory and gustatory stimuli. Therefore, most of the receptor subclasses appear to have originated several hundred million years ago, before the divergence of the major animal Phyla took place. Nevertheless, many insect-specific molecular interactions are encountered and these could become interesting tools for future applications, e.g., in insect pest control. Insect cell lines are well suited for large-scale expression and characterization of cloned receptor genes. Furthermore, novel methods for the production of stably transformed insect cells may form a major breakthrough for insect signal transduction research.

Putative neurohemal areas in the peripheral nervous system of an insect, Gryllus bimaculatus, revealed by immunocytochemistry

The morphology and position of putative neurohemal areas in the peripheral nervous system (ventral nerve cord and retrocerebral complex) of the cricket Gryllus bimaculatus are described. By using antisera to the amines dopamine, histamine, octopamine, and serotonin, and the neuropeptides crustacean cardioactive peptide, FMRFamide, leucokinin 1, and proctolin, an extensive system of varicose fibers has been detected throughout the nerves of all neuromeres, except for nerve 2 of the prothoracic ganglion. Immunoreactive varicose fibers occur mainly in a superficial position at the neurilemma, indicating neurosecretory storage and release of neuroactive compounds. The varicose fibers are projections from central or peripheral neurons that may extend over more than one segment. The peripheral fiber varicosities show segment-specific arrangements for each of the substances investigated. Immunoreactivity to histamine and octopamine is mainly found in the nerves of abdominal segments, whereas serotonin immunoreactivity is concentrated in subesophageal and terminal ganglion nerves. Immunoreactivity to FMRFamide and crustacean cardioactive peptide is widespread throughout all segments. Structures immunoreactive to leucokinin 1 are present in abdominal nerves, and proctolin immunostaining is found in the terminal ganglion and thoracic nerves. Codistribution of peripheral varicose fiber plexuses is regularly seen for amines and peptides, whereas the colocalization of substances in neurons has not been detected for any of the neuroactive compounds investigated. The varicose fiber system is regarded as complementary to the classical neurohemal organs.

Evidence that locustatachykinin I is involved in release of adipokinetic hormone from locust corpora cardiaca

The glandular cells of the corpus cardiacum of the locust Locusta migratoria, known to synthesize and release adipokinetic hormones (AKH), are contacted by axons immunoreactive to an antiserum raised against the locust neuropeptide locustatachykinin I (LomTK I). Electron-microscopical immunocytochemistry reveals LomTK immunoreactive axon terminals, containing granular vesicles, in close contact with the glandular cells cells. Release of AKH I from isolated corpora cardiaca of the locust has been monitored in an in vitro system where the amount of AKH I released into the incubation saline is determined by reversed phase high performance liquid chromatography with fluorometric detection. We could show that LomTK I induces release of AKH from corpora cardiaca in a dose-dependent manner when tested in a range of 10-200 microM. This is thus the first clear demonstration of a substance inducing release of AKH, correlated with the presence of the substance in fibers innervating the AKH-synthesizing glandular cells, in the insect corpora cardiaca.

Octopamine immunoreactivity in the fruit fly Drosophila melanogaster

Octopamine has been proposed as a neurotransmitter/modulator/hormone serving a variety of physiological functions in invertebrates. We have initiated a study of octopamine in the fruit fly Drosophila melanogaster, which provides an excellent system for genetic and molecular analysis of neuroactive molecules. As a first step, the distribution of octopamine immunoreactivity was studied by means of an octopamine-specific antiserum. We focused on the central nervous system (CNS) and on the innervation of the larval body wall muscles. The larval octopamine neuronal pattern was composed of prominent neurons along the midline of the ventral ganglion, whereas brain lobes were devoid of immunoreactive somata. However, intense immunoreactive neuropil was observed both in the ventral ganglion and in the brain lobes. Some of the immunoreactive neurons sent peripheral fibers that innervated most of the muscles of the larval body wall. Octopamine immunoreactivity was observed at neuromuscular junctions in all larval stages, being present in a well-defined subset of synaptic boutons, type II. Octopamine immunoreactivity in the adult CNS revealed many additional neurons compared to the larval CNS, indicating that at least a subset of adult octopamine neurons may differentiate during metamorphosis. Major octopamine-immunoreactive neuronal clusters and neuronal processes were observed in the subesophageal ganglion, deutocerebrum, and dorsal protocerebrum, and intense neuropil staining was detected primarily in the optic lobes and in the central complex.

GABA and glutamate-like immunoreactivity at synapses received by dorsal unpaired median neurones in the abdominal nerve cord of the locust

Dorsal unpaired median (DUM) neurones in the abdominal ganglia of the locust were impaled with microelectrodes and some were injected intracellularly with horseradish peroxidase so that their synapses could be identified in the electron microscope. Simultaneous recordings from DUM neurones in different abdominal ganglia revealed that they received common postsynaptic potentials from descending interneurones. Post-embedding immunocytochemistry using antibodies against GABA and glutamate was carried out on ganglia containing HRP-stained neurones. GABA-like immunoreactivity was found in 39% (n = 82) of processes presynaptic to abdominal DUM neurones and glutamate-like immunoreactivity in 21% (n = 42) of presynaptic processes. Output synapses from the DUM neurites were rarely observed within the neuropile. Structures resembling presynaptic dense bars but not associated with synaptic vesicles, were seen in some large diameter neurites.

Localization of octopaminergic neurones in insects

This paper reviews data on the localization of octopaminergic neurons revealed by immunocytochemistry in insects, primarily the locusts Schistocerca gregaria and Locusta migratoria, cricket Gryllus bimaculatus, and cockroach Periplaneta americana. Supporting evidence for their octopaminergic nature is mentioned where available. In orthopteran ventral ganglia, the major classes of octopamine-like immunoreactive (-LI) neurones include: (1) efferent dorsal and ventral unpaired median (DUM, VUM) neurones; (2) several intersegmentally projecting DUM interneurones in the suboesophageal ganglion; other DUM interneurones are probably GABAergic; (3) a pair of anterior median cells in the prothoracic ganglion; (4) a single pair of ventral cells in most thoracic and some other ganglia; these appear to be plurisegmentally projecting interneurones. Eight categories of octopamine-LI neurones occur in the orthopteran brain. The basic projections of three types are described here: one class project to the optic lobes to form wide field projections. Another type descends to cross into the tritocerebral commissure and may invade the contralateral brain hemisphere. A further class is the median neurosecretory cells with axons in the nervi corpori cardiaci I. Available data for the honey bee Apis mellifera and moth Manduca sexta indicate that the octopamine-LI cell types found in orthopterans also occur in holometabolous insects. Immunocytochemical evidence suggests that some octopaminergic DUM cells contain an FMRFamide-related peptide and the amino acid taurine as putative cotransmitters.

Octopamine-like immunoreactivity in the brain and subesophageal ganglion of the honeybee

The organization of putative octopaminergic pathways in the brain and subesophageal ganglion of the honeybee was investigated with a well-defined polyclonal antiserum against octopamine. Five prominent groups of just over 100 immunoreactive (IR) somata were found in the cerebral ganglion: Neurosecretory cells in the pars intercerebralis innervating the corpora cardiaca via NCC I, one cluster mediodorsal to the antennal lobe, one scattered on both sides of the midline of the protocerebrum, one between the lateral protocerebral lobes and the dorsal lobes, and a single soma on either side of the central body. With the exception of the pedunculi and beta-lobes of the mushroom bodies, varicose immunoreactive fibers penetrate all parts of the cerebral ganglion. Strong labelling was found in the central complex and the protocerebral bridge. Fine networks of labelled processes invade the antennal lobes, the calyces and a small part of the alpha-lobes of the mushroom bodies, the protocerebrum, and all three optic ganglia. In the subesophageal ganglion, one labelled cell body was found in the lateral soma layer of the mandibular segment. Each of the three neuromeres contains a group of six to ten somata in the ventral median parts. Most of the ventral median cells send their neurites dorsally through the midline tracts, whereas the neurites of a few cells follow the ventral cell body neurite tracts. Octopamine-IR was demonstrated in all neuropils that contain pathways for proboscis extension learning in honeybees. Because octopaminergic mechanisms seem to be involved in the behavioral plasticity of the proboscis extension reflex, our study provides anatomical data on the neurochemical organization of an appetitive learning paradigm.

Neuritic GABAergic synapses in insect neurosecretory cells

The localization of synaptic gamma-aminobutyric acid (GABA) receptors on cockroach dorsal unpaired median (DUM) neurons of the last abdominal ganglion was investigated. These neurosecretory cells, mainly octopaminergic, possess a soma located on the dorsal midline of the ganglion, from which emerges a short primary neurite dividing into two symmetrical lateral branches on both lateral edges of the ganglion. GABA pressure ejections onto the soma and onto the neuritic arborization elicited hyperpolarizations. Moreover, electrical stimulation of the anterior connectives evoked a postsynaptic potential, mainly inhibitory. This response and the GABA hyperpolarization of the neuritic field are antagonized by lateral application of picrotoxin while soma GABA hyperpolarization remained unchanged. This suggests that there are two kinds of GABA receptors located (i) onto the soma membrane of the DUM neurons and called extrasynaptic receptors, (ii) on the neuritic arborization, called synaptic receptors and implicated in the connection between neurons coming from the anterior part of the nervous system and the DUM cells. Immunohistological double staining technique reinforced the electrophysiological results by showing the presence of GABA-like immunoreactive processes next to octopamine-like immunoreactive ones.

Colocalization of octopamine and FMRFamide related peptide in identified heart projecting (DUM) neurones in the locust revealed by immunocytochemistry

Immunocytochemical techniques are employed to reveal colocalization of octopamine with FMRFamide related peptide in the locust ventral nervous system. In each unfused pregenital abdominal ganglia (A4-A6) there are 3 octopamine-like immunoreactive neurones. By combining intracellular Lucifer yellow staining with subsequent immunocytochemistry these are individually identified as the efferent dorsal unpaired median (DUM) neurones DUM-1 and DUM-2, which innervate abdominal tergal and respectively sternal skeletal muscles, and DUM heart-1, an FMRFamide-like immunoreactive neurone which projects to the heart and associated alary muscles. Colocalization of octopamine- and FMRFamide-like immunoreactivity in DUM heart-1 is verified by alternate staining of consecutive sections. With respect to locust ventral ganglia, this investigation shows that colocalization of octopamine with an FMRFamide related peptide is restricted to a single DUM cell occurring in each abdominal ganglion 2-7, which most likely corresponds to segmental homologues of DUM heart-1.