Action of locust neuromodulatory neurons is coupled to specific motor patterns

Hans-Joachim Pflüger: scientist, citizen, cosmopolitan

On January 25, 2022, Professor Hans-Joachim Pflüger passed away. Hans-Joachim Pflüger conducted research in the field of neuroethology, with a focus on the development, anatomy, and function of sensorimotor networks underlying insect locomotion. As founding member and one of the presidents of the German Neuroscience Society, Hans-Joachim Pflüger was a driving force behind the development of the Neurosciences in Germany and Europe. This obituary reflects on his curriculum vitae. It shall honor his scientific and professional achievements, and importantly, also his wonderful personality, which makes this loss so sad across the manifold levels of his life and his legacy, the family, the professional and the scientific community.

Comparative transcriptome profiles of Schistosoma japonicum larval stages: Implications for parasite biology and host invasion

Schistosoma japonicum is prevalent in Asia with a wide mammalian host range, which leads to highly harmful zoonotic parasitic diseases. Most previous transcriptomic studies have been performed on this parasite, but mainly focus on stages inside the mammalian host. Moreover, few larval transcriptomic data are available in public databases. Here we mapped the detailed transcriptome profiles of four S. japonicum larval stages including eggs, miracidia, sporocysts and cercariae, providing a comprehensive development picture outside of the mammalian host. By analyzing the stage-specific/enriched genes, we identified functional genes associated with the biological characteristic at each stage: e.g. we observed enrichment of genes necessary for DNA replication only in sporocysts, while those involved in proteolysis were upregulated in sporocysts and/or cercariae. This data indicated that miracidia might use leishmanolysin and neprilysin to penetrate the snail, while elastase (SjCE2b) and leishmanolysin might contribute to the cercariae invasion. The expression profile of stem cell markers revealed potential germinal cell conversion during larval development. Additionally, our analysis indicated that tandem duplications had driven the expansion of the papain family in S. japonicum. Notably, all the duplicated cathepsin B-like proteases were highly expressed in cercariae. Utilizing our 3rd version of S. japonicum genome, we further characterized the alternative splicing profiles throughout these four stages. Taken together, the present study provides compressive gene expression profiles of S. japonicum larval stages and identifies a set of genes that might be involved in intermediate and definitive host invasion.

Descending octopaminergic neurons modulate sensory-evoked activity of thoracic motor neurons in stick insects

Neuromodulatory neurons located in the brain can influence activity in locomotor networks residing in the spinal cord or ventral nerve cords of invertebrates. How inputs to and outputs of neuromodulatory descending neurons affect walking activity is largely unknown. With the use of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and immunohistochemistry, we show that a population of dorsal unpaired median (DUM) neurons descending from the gnathal ganglion to thoracic ganglia of the stick insect Carausius morosus contains the neuromodulatory amine octopamine. These neurons receive excitatory input coupled to the legs’ stance phases during treadmill walking. Inputs did not result from connections with thoracic central pattern-generating networks, but, instead, most are derived from leg load sensors. In excitatory and inhibitory retractor coxae motor neurons, spike activity in the descending DUM (desDUM) neurons increased depolarizing reflexlike responses to stimulation of leg load sensors. In these motor neurons, descending octopaminergic neurons apparently functioned as components of a positive feedback network mainly driven by load-detecting sense organs. Reflexlike responses in excitatory extensor tibiae motor neurons evoked by stimulations of a femur-tibia movement sensor either are increased or decreased or were not affected by the activity of the descending neurons, indicating different functions of desDUM neurons. The increase in motor neuron activity is often accompanied by a reflex reversal, which is characteristic for actively moving animals. Our findings indicate that some descending octopaminergic neurons can facilitate motor activity during walking and support a sensory-motor state necessary for active leg movements.

NEW & NOTEWORTHY We investigated the role of descending octopaminergic neurons in the gnathal ganglion of stick insects. The neurons become active during walking, mainly triggered by input from load sensors in the legs rather than pattern-generating networks. This report provides novel evidence that octopamine released by descending neurons on stimulation of leg sense organs contributes to the modulation of leg sensory-evoked activity in a leg motor control system.

Evaluation of linear and non-linear activation dynamics models for insect muscle

In computational modelling of sensory-motor control, the dynamics of muscle contraction is an important determinant of movement timing and joint stiffness. This is particularly so in animals with many slow muscles, as is the case in insects-many of which are important models for sensory-motor control. A muscle model is generally used to transform motoneuronal input into muscle force. Although standard models exist for vertebrate muscle innervated by many motoneurons, there is no agreement on a parametric model for single motoneuron stimulation of invertebrate muscle. Although several different models have been proposed, they have never been evaluated using a common experimental data set. We evaluate five models for isometric force production of a well-studied model system: the locust hind leg tibial extensor muscle. The response of this muscle to motoneuron spikes is best modelled as a non-linear low-pass system. Linear first-order models can approximate isometric force time courses well at high spike rates, but they cannot account for appropriate force time courses at low spike rates. A linear third-order model performs better, but only non-linear models can account for frequency-dependent change of decay time and force potentiation at intermediate stimulus frequencies. Some of the differences among published models are due to differences among experimental data sets. We developed a comprehensive toolbox for modelling muscle activation dynamics, and optimised model parameters using one data set. The "Hatze-Zakotnik model" that emphasizes an accurate single-twitch time course and uses frequency-dependent modulation of the twitch for force potentiation performs best for the slow motoneuron. Frequency-dependent modulation of a single twitch works less well for the fast motoneuron. The non-linear "Wilson" model that optimises parameters to all data set parts simultaneously performs better here. Our open-access toolbox provides powerful tools for researchers to fit appropriate models to a range of insect muscles.

Octopamine signaling in the metazoan pathogen Schistosoma mansoni: localization, small-molecule screening and opportunities for drug development

Schistosomiasis is a tropical disease caused by a flatworm trematode parasite that infects over 200 million people worldwide. Treatment and control of the disease rely on just one drug, praziquantel. The possibility of drug resistance coupled with praziquantel's variable efficacy encourages the identification of new drugs and drug targets. Disruption of neuromuscular homeostasis in parasitic worms is a validated strategy for drug development. In schistosomes, however, much remains to be understood about the organization of the nervous system, its component neurotransmitters and potential for drug discovery. Using synapsin as a neuronal marker, we map the central and peripheral nervous systems in the Schistosoma mansoni adult and schistosomulum (post-infective larva). We discover the widespread presence of octopamine (OA), a tyrosine-derived and invertebrate-specific neurotransmitter involved in neuromuscular coordination. OA labeling facilitated the discovery of two pairs of ganglia in the brain of the adult schistosome, rather than the one pair thus far reported for this and other trematodes. In quantitative phenotypic assays, OA and the structurally related tyrosine-derived phenolamine and catecholamine neurotransmitters differentially modulated schistosomulum motility and length. Similarly, from a screen of 28 drug agonists and antagonists of tyrosine-derivative signaling, certain drugs that act on OA and dopamine receptors induced robust and sometimes complex concentration-dependent effects on schistosome motility and length; in some cases, these effects occurred at concentrations achievable in vivo The present data advance our knowledge of the organization of the nervous system in this globally important pathogen and identify a number of drugs that interfere with tyrosine-derivative signaling, one or more of which might provide the basis for a new chemotherapeutic approach to treat schistosomiasis.This article has an associated First Person interview with the first author of the paper.

Structural and Molecular Properties of Insect Type II Motor Axon Terminals

A comparison between the axon terminals of octopaminergic efferent dorsal or ventral unpaired median neurons in either desert locusts (Schistocerca gregaria) or fruit flies (Drosophila melanogaster) across skeletal muscles reveals many similarities. In both species the octopaminergic axon forms beaded fibers where the boutons or varicosities form type II terminals in contrast to the neuromuscular junction (NMJ) or type I terminals. These type II terminals are immunopositive for both tyramine and octopamine and, in contrast to the type I terminals, which possess clear synaptic vesicles, only contain dense core vesicles. These dense core vesicles contain octopamine as shown by immunogold methods. With respect to the cytomatrix and active zone peptides the type II terminals exhibit active zone-like accumulations of the scaffold protein Bruchpilot (BRP) only sparsely in contrast to the many accumulations of BRP identifying active zones of NMJ type I terminals. In the fruit fly larva marked dynamic changes of octopaminergic fibers have been reported after short starvation which not only affects the formation of new branches (“synaptopods”) but also affects the type I terminals or NMJs via octopamine-signaling (Koon et al., 2011). Our starvation experiments of Drosophila-larvae revealed a time-dependency of the formation of additional branches. Whereas after 2 h of starvation we find a decrease in “synaptopods”, the increase is significant after 6 h of starvation. In addition, we provide evidence that the release of octopamine from dendritic and/or axonal type II terminals uses a similar synaptic machinery to glutamate release from type I terminals of excitatory motor neurons. Indeed, blocking this canonical synaptic release machinery via RNAi induced downregulation of BRP in neurons with type II terminals leads to flight performance deficits similar to those observed for octopamine mutants or flies lacking this class of neurons (Brembs et al., 2007).

Modulation of Low-Voltage-Activated Inward Current Permeable to Sodium and Calcium by DARPP-32 Drives Spontaneous Firing of Insect Octopaminergic Neurosecretory Cells

Identification of the different intracellular pathways that control phosphorylation/dephosphorylation process of ionic channels represents an exciting alternative approach for studying the ionic mechanisms underlying neuronal pacemaker activity. In the central nervous system of the cockroach Periplaneta americana, octopaminergic neurons, called dorsal unpaired median (DUM; DUM neurons), generate spontaneous repetitive action potentials. Short-term cultured adult DUM neurons isolated from the terminal abdominal ganglion (TAG) of the nerve cord were used to study the regulation of a tetrodotoxin-sensitive low-voltage-activated (LVA) channel permeable to sodium and calcium (Na/Ca), under whole cell voltage- and current-clamp conditions. A bell-shaped curve illustrating the regulation of the amplitude of the maintained current vs. [ATP]i was observed. This suggested the existence of phosphorylation mechanisms. The protein kinase A (PKA) inhibitor, H89 and elevating [cyclic adenosine 3′, 5′ monophosphate, cAMP]i, increased and decreased the current amplitude, respectively. This indicated a regulation of the current via a cAMP/PKA cascade. Furthermore, intracellular application of PP2B inhibitors, cyclosporine A, FK506 and PP1/2A inhibitor, okadaic acid decreased the current amplitude. From these results and because octopamine (OA) regulates DUM neuron electrical activity via an elevation of [cAMP]i, we wanted to know if, like in vertebrate dopaminergic neurons, OA receptor (OAR) stimulation could indirectly affect the current via PKA-mediated phosphorylation of Dopamine- and cAMP-regulated Phosphoprotein-32 (DARPP-32) known to inhibit PP1/2A. Experiments were performed using intracellular application of phospho-DARPP-32 and non-phospho-DARPP-32. Phospho-DARPP-32 strongly reduced the current amplitude whereas non-phospho-DARPP-32 did not affect the current. All together, these results confirm that DARPP-32-mediated inhibition of PP1/2A regulates the maintained sodium/calcium current, which contributes to the development of the pre-depolarizing phase of the DUM neuron pacemaker activity.

Activity in descending dopaminergic neurons represents but is not required for leg movements in the fruit fly Drosophila

Modulatory descending neurons (DNs) that link the brain to body motor circuits, including dopaminergic DNs (DA-DNs), are thought to contribute to the flexible control of behavior. Dopamine elicits locomotor-like outputs and influences neuronal excitability in isolated body motor circuits over tens of seconds to minutes, but it remains unknown how and over what time scale DA-DN activity relates to movement in behaving animals. To address this question, we identified DA-DNs in the Drosophila brain and developed an electrophysiological preparation to record and manipulate the activity of these cells during behavior. We find that DA-DN spike rates are rapidly modulated during a subset of leg movements and scale with the total speed of ongoing leg movements, whether occurring spontaneously or in response to stimuli. However, activating DA-DNs does not elicit leg movements in intact flies, nor do acute bidirectional manipulations of DA-DN activity affect the probability or speed of leg movements over a time scale of seconds to minutes. Our findings indicate that in the context of intact descending control, changes in DA-DN activity are not sufficient to influence ongoing leg movements and open the door to studies investigating how these cells interact with other descending and local neuromodulatory inputs to influence body motor output.

Dynamic neural control of insect muscle metabolism related to motor behavior

Skeletal muscle innervation differs between vertebrates and insects. Insect muscle fibers exhibit graded electrical potentials and are innervated by excitatory, inhibitory, and also neuromodulatory motoneurons. The latter form a unique class of unpaired neurons with bilaterally symmetrical axons that release octopamine to alter the efficacy of synaptic transmission and regulate muscle energy metabolism by activating glycolysis. Octopaminergic neurons that innervate muscles with a high energy demand, for example, flight muscles that move the wings of a locust up and down, are active during rest but are inhibited during flight and its preparatory phase, a jump. Therefore, it is argued that these neurons are involved in providing locusts with the necessary fuel at takeoff, but then may aid the switch to lipid oxidation during flight. In general, the octopaminergic system may switch the whole organism from a tonic to a dynamic state.

Neuromodulatory unpaired median neurons in the New Zealand tree weta, Hemideina femorata

Wetas are ancient Gondwanan orthopterans (Anostostomatidae) with many species endemic to New Zealand. Like all Orthoptera they possess efferent neuromodulatory dorsal unpaired median (DUM) neurons, with bilaterally symmetrical axons, that are important components of motor networks. These neurons produce overshooting action potentials and are easily stimulated by a variety of external mechanosensory stimuli delivered to the body and appendages. In particular, stimulation of the antennae, mouth parts, tarsi and femora of the legs, abdomen, cerci and ovipositor is very effective in activating DUM neurons in the metathoracic ganglion of wetas. In addition, looming visual stimuli or light on-, light off-stimuli excite many metathoracic DUM neurons. These DUM sensory reflex pathways remain viable after the prothoracic to subesophageal connective is cut, whereas in locusts such reflex pathways are interrupted by the ablation. This suggests that, in wetas, sensory reflex pathways for DUM activation are organized in a less centralized fashion than in locusts, and may therefore reflect a plesiomorphic evolutionary state in the weta. In addition, many weta DUM neurons exhibit slow rhythmic bursting which also persists following the connective ablation.

The role of octopamine in locusts and other arthropods

The biogenic amine octopamine and its biological precursor tyramine are thought to be the invertebrate functional homologues of the vertebrate adrenergic transmitters. Octopamine functions as a neuromodulator, neurotransmitter and neurohormone in insect nervous systems and prompts the whole organism to "dynamic action". A growing number of studies suggest a prominent role for octopamine in modulating multiple physiological and behavioural processes in invertebrates, as for example the phase transition in Schistocerca gregaria. Both octopamine and tyramine exert their effects by binding to specific receptor proteins that belong to the superfamily of G protein-coupled receptors. Since these receptors do not appear to be present in vertebrates, they may present very suitable and specific insecticide and acaricide targets.

Genomics and the evolutionary origins of nervous system complexity

Advances in genomics are leading to increased understanding of the evolution of complexity, especially by beginning to bridge genotype and phenotype. Here, using examples from nervous system evolution, we define general patterns of increased complexity seen across levels of biological organization. We also explore specific evolutionary mechanisms that increase complexity, namely those that increase the number of biological units (parts) in a system. We provide specific neurobiological examples of increased complexity in genes, gene networks, cell types, and tissues/organs. These examples illustrate that while a variety of different mechanisms increase biological complexity, they can be understood in a generalized comparative framework.

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.

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.

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.

Ion currents and spiking properties of identified subtypes of locust octopaminergic dorsal unpaired median neurons

Efferent dorsal unpaired median (DUM) neurons are key elements of an insect neuromodulatory system. In locusts, subpopulations of DUM neurons mediate octopaminergic modulation at specific targets depending on their activity during different behaviours. This study investigates whether in addition to synaptic inputs, activity in DUM neurons depends on intrinsic membrane properties. Intracellular in situ recordings and whole-cell patch-clamp recordings from freshly isolated somata characterize somatic voltage signals and the underlying ion currents of individual subtypes of DUM neurons identified beforehand by a vital retrograde tracing technique. Na(+), Ca(2+), K(+) currents and a hyperpolarization-activated (I(h)) current are described in detail for their (in-)activation properties and subtype-specific current densities. In addition, a Ca(2+)-dependent K(+) current is demonstrated by its sensitivity to cadmium and charybdotoxin. This complex current composition determines somatic excitability similar in all subtypes of DUM neurons. Both Na(+) and Ca(2+) currents generate overshooting somatic action potentials. Repolarizing K(+) currents, in particular transient, subthreshold-activating A-currents, regulate the firing frequency and cause delayed excitation by shunting depolarizing input. An opposing hyperpolarization-activated (I(h)) current contributes to the resting membrane potential and induces rebound activity after prolonged inhibition phases. A quantitative analysis reveals subtype-specific differences in current densities with more inhibitory I(K) but less depolarizing I(Na) and I(h) - at least in DUM3 neurons promoting a reliable suppression of their activity as observed during behaviour. In contrast, DUM neurons that are easily activated during behaviour (DUM3,4,5 and DUMETi) express less I(K) and a pronounced depolarizing I(h) promoting excitability.

Central inhibitory microcircuits controlling spike propagation into sensory terminals

The phenomenon of afferent presynaptic inhibition has been intensively studied in the sensory neurons of the chordotonal organ from the coxobasal joint (CBCO) of the crayfish leg. This has revealed that it has a number of discrete roles in these afferents, mediated by distinct populations of interneurons. Here we examine further the effect of presynaptic inhibition on action potentials in the CBCO afferents and investigate the nature of the synapses that mediate it. In the presence of picrotoxin, the action potential amplitude is increased and its half-width decreased, and a late depolarizing potential following the spike is increased in amplitude. Ultrastructural examination of the afferent terminals reveals that synaptic contacts on terminal branches are particularly abundant in the neuropil close to the main axon. Many of the presynaptic terminals contain small agranular vesicles, are of large diameter, and are immunoreactive for gamma-aminobutyric acid (GABA). These terminals are sometimes seen to make reciprocal connections with the afferents. Synaptic contacts from processes immunoreactive for glutamate are found on small-diameter afferent terminals. A few of the presynaptic processes contain numerous large granular vesicles and are immunoreactive for neither GABA nor glutamate. The effect that the observed reciprocal synapses might have was investigated by using a multicompartmental model of the afferent terminal.

Wasp venom injected into the prey's brain modulates thoracic identified monoaminergic neurons

The wasp Ampulex compressa injects a cocktail of neurotoxins into the brain of its cockroach prey to induce an enduring change in the execution of locomotory behaviors. Our hypothesis is that the venom injected into the brain indirectly alters the activity of monoaminergic neurons, thus changing the levels of monoamines that tune the central synapses of locomotory circuits. The purpose of the present investigation was to establish whether the venom alters the descending control, from the brain, of octopaminergic neurons in the thorax. This question was approached by recording the activity of specific identified octopaminergic neurons after removing the input from the brain or after a wasp sting into the brain. We show that the activity of these neurons is altered in stung and "brainless" animals. The spontaneous firing rate of these neurons in stung and brainless animals is approximately 20% that in control animals. Furthermore, we show that an identified octopamine neuron responds more weakly both to sensory stimuli and to direct injection of current in all treated groups. The alteration in the activity of octopamine neurons is likely to be part of the mechanism by which the wasp induces a change in the behavioral state of its prey and also affects its metabolism by reducing the potent glycolytic activator fructose 2,6-bisphosphate in leg muscle. To our knowledge, this is the first direct evidence of a change in electrical activity of specific monoaminergic neurons that can be so closely associated with a venom-induced change in behavioral state of a prey animal.

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.

Fictive locomotion induced by octopamine in the earthworm

We investigated the function of octopamine (OA) as a motor pattern inducer in the earthworm Eisenia fetida. We used semi-intact preparations, consisting of 20 sequential segmental ganglia of the ventral nerve cord (VNC) together with the body wall from the middle of the animal. Bath-application of 10–4 mol l–1 OA to the semi-intact preparation induced phasic muscle contractions, which are consistent with crawling. In the isolated VNC, OA induced bursts of motor neuron activity in the first lateral nerves. Burst frequency increased with OA concentration, with a large increase in activity observed in the range 10–6–10–4 mol l–1. At 10–4 mol l–1, the coefficient of variation of burst periods (BPs) was smaller than that seen upon application of OA at other concentrations, which is indicative of rhythmic bursts. These rhythmic bursts propagated along the VNC from the anterior to posterior, with a propagation velocity ranging from 60 to 110 mm s–1. This velocity is consistent with the propagation velocity of muscle contraction during crawling behavior in the intact earthworm. From these results, we conclude that fictive crawling motor patterns are observed at 10–4 mol l–1 OA, and that OA can induce rhythmic bursts in the isolated VNC of the earthworm.

Non-synaptic ion channels in insects--basic properties of currents and their modulation in neurons and skeletal muscles

Insects are favoured objects for studying information processing in restricted neuronal networks, e.g. motor pattern generation or sensory perception. The analysis of the underlying processes requires knowledge of the electrical properties of the cells involved. These properties are determined by the expression pattern of ionic channels and by the regulation of their function, e.g. by neuromodulators. We here review the presently available knowledge on insect non-synaptic ion channels and ionic currents in neurons and skeletal muscles. The first part of this article covers genetic and structural informations, the localization of channels, their electrophysiological and pharmacological properties, and known effects of second messengers and modulators such as neuropeptides or biogenic amines. In a second part we describe in detail modulation of ionic currents in three particularly well investigated preparations, i.e. Drosophila photoreceptor, cockroach DUM (dorsal unpaired median) neuron and locust jumping muscle. Ion channel structures are almost exclusively known for the fruitfly Drosophila, and most of the information on their function has also been obtained in this animal, mainly based on mutational analysis and investigation of heterologously expressed channels. Now the entire genome of Drosophila has been sequenced, it seems almost completely known which types of channel genes--and how many of them--exist in this animal. There is much knowledge of the various types of channels formed by 6-transmembrane--spanning segments (6TM channels) including those where four 6TM domains are joined within one large protein (e.g. classical Na+ channel). In comparison, two TM channels and 4TM (or tandem) channels so far have hardly been explored. There are, however, various well characterized ionic conductances, e.g. for Ca2+, Cl- or K+, in other insect preparations for which the channels are not yet known. In some of the larger insects, i.e. bee, cockroach, locust and moth, rather detailed information has been established on the role of ionic currents in certain physiological or behavioural contexts. On the whole, however, knowledge of non-synaptic ion channels in such insects is still fragmentary. Modulation of ion currents usually involves activation of more or less elaborate signal transduction cascades. The three detailed examples for modulation presented in the second part indicate, amongst other things, that one type of modulator usually leads to concerted changes of several ion currents and that the effects of different modulators in one type of cell may overlap. Modulators participate in the adaptive changes of the various cells responsible for different physiological or behavioural states. Further study of their effects on the single cell level should help to understand how small sets of cells cooperate in order to produce the appropriate output.

Identified nerve cells and insect behavior

Studies of insect identified neurons over the past 25 years have provided some of the very best data on sensorimotor integration; tracing information flow from sensory to motor networks. General principles have emerged that have increased the sophistication with which we now understand both sensory processing and motor control. Two overarching themes have emerged from studies of identified sensory interneurons. First, within a species, there are profound differences in neuronal organization associated with both the sex and the social experience of the individual. Second, single neurons exhibit some surprisingly rich examples of computational sophistication in terms of (a) temporal dynamics (coding superimposed upon circadian and shorter-term rhythms), and also (b) what Kenneth Roeder called "neural parsimony": that optimal information can be encoded, and complex acts of sensorimotor coordination can be mediated, by small ensembles of cells. Insect motor systems have proven to be relatively complex, and so studies of their organization typically have not yielded completely defined circuits as are known from some other invertebrates. However, several important findings have emerged. Analysis of neuronal oscillators for rhythmic behavior have delineated a profound influence of sensory feedback on interneuronal circuits: they are not only modulated by feedback, but may be substantially reconfigured. Additionally, insect motor circuits provide potent examples of neuronal restructuring during an organism's lifetime, as well as insights on how circuits have been modified across evolutionary time. Several areas where future advances seem likely to occur include: molecular genetic analyses, neuroecological syntheses, and neuroinformatics--the use of digital resources to organize databases with information on identified nerve cells and behavior.

Optimization of rhythmic behaviors by modulation of the neuromuscular transform

We conclude our study of the properties and the functional role of the neuromuscular transform (NMT). The NMT is an input-output relation that formalizes the processes by which patterns of motor neuron firing are transformed to muscle contractions. Because the NMT acts as a dynamic, nonlinear, and modifiable filter, the transformation is complex. In the two preceding papers we developed a framework for analysis of the NMT and identified with it principles by which the NMT transforms different firing patterns to contractions. We then saw that, with fixed properties, the NMT significantly constrains the production of functional behavior. Many desirable behaviors are not possible with any firing pattern. Here we examine, theoretically as well as experimentally in the accessory radula closer (ARC) neuromuscular system of Aplysia, how this constraint is alleviated by making the properties of the NMT variable by neuromuscular plasticity and modulation. These processes dynamically tune the properties of the NMT to match the desired behavior, expanding the range of behaviors that can be produced. For specific illustration, we continue to focus on the relation between the speed of the NMT and the speed of cyclical, rhythmic behavior. Our analytic framework emphasizes the functional distinction between intrinsic plasticity or modulation of the NMT, dependent, like the contraction itself, on the motor neuron firing pattern, and extrinsic modulation, independent of it. The former is well suited to automatically optimizing the performance of a single behavior; the latter, to multiplying contraction shapes for multiple behaviors. In any case, to alleviate the constraint of the NMT, the plasticity and modulation must be peripheral. Such processes are likely to play a critical role wherever the nervous system must command, through the constraint of the NMT, a broad range of functional behaviors.

Differential activation of octopaminergic (DUM) neurones via proprioceptors responding to flight muscle contractions in the locust

The synaptic potentials generated in neuromodulatory octopaminergic dorsal unpaired median (DUM) neurones by afferents excited by twitch contractions of a dorso-ventral flight muscle were investigated in the locust. Responses to stimulation of the metathoracic wing elevator muscle 113 were obtained in locusts in which all sensory feedback from the thorax had been removed, except for feedback from the thoracic chordotonal organs, the axons of which enter via the purely sensory nerve 2. Afferents in nerve 2C, which originates from two chordotonal organs, responded reliably to twitch contractions of this flight muscle. Octopaminergic neurones innervating leg muscles (DUM5 neurones) received depolarising inputs and often spiked following stimulation of the muscle. In contrast, those innervating the wing muscles themselves (DUM3 and DUM3,4 neurones) received inhibitory inputs. The responses of DUM3,4,5 neurones, which project mainly to leg muscles, were more complex: most were excited by twitch contractions of M113 but some were inhibited. DUMDL, which innervates the dorsal longitudinal indirect flight muscles, showed no clear response. Direct stimulation of nerve 2C evoked depolarising inputs and spikes in DUM5 neurones and hyperpolarising inputs in DUM3 and DUM3,4 neurones. Our data suggest that sensory feedback from thoracic chordotonal organs, which are known to be activated rhythmically during flight, contributes to the differential activation of efferent DUM neurones observed during flight.

Efferent neurons and specialization of abdominal segments in grasshoppers

A specialized behavior, oviposition, is produced by the eighth and ninth abdominal segments of female grasshoppers. To begin to understand how these segments produce the behavior, which is not displayed by males or pregenital regions of the abdomen in females, the structure and function of efferent neurons in abdominal ganglia of both sexes were examined. In females, the eighth and ninth segments are specialized differently for oviposition: 20 ovipositor motor neurons were found in the eighth segment, and 26 were found in the ninth segment. Males had fewer motor neurons in their eighth segment, but the same number in the ninth segment, which is the only genital segment in males. However, the axons of several of the ninth segmental male motor neurons traveled to the periphery in the genital nerve, which is only found in males. In both sexes, pregenital ganglia had the most motor neurons, but these neurons, for the most part, had morphologies that strongly resembled those of genital segments. Efferent modulatory neuron numbers were not sexually dimorphic in the segments examined, except that males had a greater number in their ninth segment. Experimental methods that activate oviposition were found to also activate a rhythmical motor pattern in pregenital abdominal segments of both sexes. In females, the pattern was phase-coupled to oviposition, but persisted after the connections with the terminal abdominal ganglion were severed. The preponderance of similarities among efferent neurons and elicited motor activity suggests a common pattern of neural circuitry in the behaviorally diverse abdominal segments of grasshoppers.

Neuromodulation during motor development and behavior

Important recent advances have been made in understanding the role of aminergic modulation during the maturation of Xenopus larvae swimming rhythms, including effects on particular ion channel types of component neurons, and the role of peptidergic modulation during development of adult central patterns generators in the stomatogastric ganglion of crustaceans. By recording from octopaminergic neuromodulatory neurons during ongoing motor behavior in the locust, new insights into the role of this peripheral neuromodulatory mechanism have been gained. In particular, it is now clear that the octopaminergic neuromodulatory system is automatically activated in parallel to the motor systems, and that both excitation and inhibition play important functional roles.

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.

Distribution and activation of different types of octopaminergic DUM neurons in the locust

The first part of this study describes the distribution of all different types of octopaminergic, efferent dorsal unpaired median (DUM) neurons in the first two thoracic ganglia by immunocytochemistry, retrograde labeling, and intracellular staining. The prothoracic ganglion contains five different types of 10 DUM neurons. The mesothoracic ganglion has 21 octopaminergic somata in the DUM neuron cluster. Retrograde labeling and intracellular staining show that 19 of these 21 somata belong to five different types of efferent DUM neurons. In both ganglia, the number and the distribution of all types of DUM neurons are completely described. Differences in the distribution of efferent DUM neurons between the thoracic ganglia are discussed as functional segmental specializations. In the second part, we show that, in contrast to previous suggestions, DUM neurons are not recruited as a homogeneous population mediating general arousal but differentially, thus forming subpopulations of specific types. The existence or the absence of commonly occurring postsynaptic potentials in paired recordings clearly shows that only specific types of DUM neurons are targeted by the same presynaptic pathways. Within the thoracic ganglia, different subpopulations of DUM neurons can be distinguished by their different local inputs. Furthermore, only specific subpopulations of DUM neurons receive common intersegmental drive and inputs from the subesophageal ganglion. As a result of all our recordings, we propose a scheme for the differential activation of efferent DUM neurons. This scheme is sufficient to explain DUM neuron activity during principal motor programs.

Resting membrane properties of locust muscle and their modulation I. Actions of the neuropeptides YGGFMRFamide and proctolin

The resting K+ conductance (GK,r) of locust jumping muscle and its modulation by two neuropeptides, proctolin (Arg-Tyr-Leu-Pro-Thr) and YGGFMRFamide (Tyr-Gly-Gly-Phe-Met-Arg-Phe-NH2), were investigated using the two-electrode voltage clamp. At a physiological [K+]o of 10 mM, GK,r accounts for approximately 90% of the membrane resting conductance, and the resting membrane potential differs by </=1 mV from EK (mean: -74 mV). There is a K+ conductance that slowly activates on hyperpolarization (GK,H) and that seems to be largely located in the transverse tubules. Steady-state activation of GK,H was analyzed by tail current measurements. GK,H is activated partially at EK but accounts for probably </=50% of total resting K+ conductance. Raising [K+]o caused a large increase in GK,r and in maximal steady state GK,H without shifting the voltage sensitivity of GK,H. YGGFMRFamide and proctolin reduce GK,H, mainly affecting the maximal steady-state conductance. The voltage-insensitive component of the resting K+ conductance is also reduced. The conductance suppressed by the peptides exhibited an outwardly rectifying instantaneous current/voltage-characteristic that is quite similar to that of GK,H. The actions of the two peptides appeared to be identical, but proctolin was by some two orders of magnitude more potent than YGGFMRFamide. The effects of both peptides are mediated by G proteins. They are mimicked by phorbol esters but do not seem to be initiated by either branch of the phospholipase C-dependent intracellular pathways. The properties of the resting K+ conductance in locust muscle and other invertebrate muscles are compared. The biological significance of peptide-induced reduction in resting K+ conductance is discussed in view of the known property of proctolin to support tonic force as opposed to FMRFamide-peptides that support quick leg movements.

Resting membrane properties of locust muscle and their modulation II. Actions of the biogenic amine octopamine

Ionic currents in the resting membrane of locust jumping muscle and their modulation by the biogenic amine octopamine were investigated using the two-electrode voltage clamp. A Cl- conductance, GCl,H, which slowly activates on hyperpolarization, can be induced by raising the intracellular Cl- concentration via diffusion of Cl- ions from the recording electrode. The instantaneous I-V characteristic of the current, ICl,H, is linear and reverses at the same potential as the gamma-aminobutyric acid (GABA)-mediated Cl- current. Elevation of [Cl-]i increases the maximal steady state GCl,H (Gmax) and shifts the activation curve of GCl,H to more positive potentials. Octopamine enhances GCl,H, mainly by increasing Gmax. Octopamine also lowers the resting K+ conductance (GK,r). It reduces a hyperpolarization-activated component (GK,H) of GK,r, mainly by decreasing Gmax. Octopamine also transiently stimulates the Na+/K+ pump although this effect was not always seen. The effects of octopamine on the Cl- and K+ conductances are mimicked by membrane permeant cyclic nucleotides. The modulation of GK,r, but not that of GCl,H, seems to be mediated by protein kinase A (PKA). PKA seems to be constitutively activated as indicated by the pronounced increase in GK,r induced by a PKA inhibitor, H89. The properties of GCl,H and related Cl- conductances in invertebrate and vertebrate neurons are compared. GCl,H probably supports efflux of Cl- ions accumulating in the fibers during synaptic inhibition. Octopamine's multiple modulation at the level of the muscle cell membrane, in conjunction with previously established effects on synaptic transmission and excitation-contraction coupling, are suited to support strong and rapid muscle contractions.

Coupling of efferent neuromodulatory neurons to rhythmical leg motor activity in the locust

The spike activity of neuromodulatory dorsal unpaired median (DUM) neurons was analyzed during a pilocarpine-induced motor pattern in the locust. Paired intracellular recordings were made from these octopaminergic neurons during rhythmic activity in hindleg motor neurons evoked by applying pilocarpine to an isolated metathoracic ganglion. This motor pattern is characterized by two alternating phases: a levator phase, during which levator, flexor, and common inhibitor motor neurons spike, and a depressor phase, during which depressor and extensor motor neurons spike. Three different subpopulations of efferent DUM neurons could be distinguished during this rhythmical motor pattern according to their characteristic spike output. DUM 1 neurons, which in the intact animal do not innervate muscles involved in leg movements, showed no change apart from a general increase in spike frequency. DUM 3 and DUM 3,4 neurons produced the most variable activity but received frequent and sometimes pronounced hyperpolarizations that were often common to both recorded neurons. DUM 5 and DUM 3,4,5 neurons innervate muscles of the hindleg and showed rhythmical excitation leading to bursts of spikes during rhythmic activity of the motor neurons, which innervate these same muscles. Sometimes the motor output was coordinated across both sides of the ganglion so that there was alternating activity between levators of both sides. In these cases, the spikes of DUM 5 and DUM 3,4,5 neurons and the hyperpolarization of DUM 3 and DUM 3,4 neurons occurred at particular phases in the motor pattern. Our data demonstrate a central coupling of specific types of DUM neurons to a rhythmical motor pattern. Changes in the spike output of these particular efferent DUM neurons parallel changes in the motor output. The spike activity of DUM neurons thus may be controlled by the same circuits that determine the action of the motor neurons. Functional implications for real walking are discussed.