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.

Allatostatin-like immunoreactivity in the abdomen of the locust Schistocerca gregaria

A polyclonal antibody against allatostatin 1 (AST-1) of cockroach Diploptera punctata was used to investigate the distribution of AST-like immunoreactivity within the abdomen of the locust, Schistocerca gregaria. In the abdominal ganglia, AST-like immunoreactivity was found in both cell bodies and neuropile. In ganglia 6 and 7, staining was found in serial homologous cell bodies in anterior dorsolateral and dorsomedial, and posterior ventrolateral and ventromedial locations. In the terminal ganglion, the numerous immunoreactive somata were smaller in size than those in the unfused ganglia. The combination of backfill experiments with immunocytochemistry showed that, in abdominal ganglion 7, one neuron of the ventromedian cell body cluster and two of the ventrolateral cluster innervated the oviduct, which itself was covered with a dense mesh of AST-immunoreactive varicosities. Combining electron microscopy with immunohistochemistry revealed AST-like immunoreactivity in dense-core vesicles located in neurohaemal-like terminals lacking structures normally found in synapses. A mesh of AST-immunoreactive varicosities was also found on the muscles of the spermatheca and the spermathecal duct. In addition, a mesh of strongly stained varicosities was present in the distal perisympathetic organs (neurohaemal organs in the abdomen) and on the lateral heart nerves (a putative neurohaemal release zone). These data indicate that AST is an important neuroactive substance that is probably involved in multiple tasks in the control of the locust abdomen.

Neuromodulatory octopaminergic neurons and their functions during insect motor behaviour. The Ernst Florey memory lecture

In this article we describe recent advances in functional studies on the role of octopamine released in the periphery by efferent dorsal or ventral unpaired median neurons. In addition to the previously described modulatory effects on the neuromuscular junction, we describe a metabolic regulatory role for these neurons. Due to their activity glycolytic rates in target tissues, such as muscles, are increased. In flight muscles that use carbohydrate catabolism only at take-off but have to switch to lipid oxidation during prolonged flight, these neurons are only active at rest but are inhibited as soon as flight motor patterns are selected.

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.

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.

The ultrastructure of locust pleuroaxillary “steering” muscles in comparison to other skeletal muscles

The ultrastructure of locust muscles with different function is examined: the pleuroaxillary flight steering muscle is compared with a typical flight (power muscle) and a typical leg muscle, in particular with respect to sarcomere length, tracheation, mitochondria, and sarcoplasmatic reticulum. The pleuroaxillary muscle exhibits some features characteristic of flight muscles but most of the ultrastructure resembles that of leg muscles. This is in agreement with the innervation of this muscle by an octopaminergic neuron, which also innervates leg muscles but no other flight muscles. It also supports the hypothesis that octopaminergic neurons are important metabolic regulators and that the above muscle types exhibit important differences in energy metabolism.

Central modulatory neurons control fuel selection in flight muscle of migratory locust

Insect flight is one of the most intense and energy-demanding physiological activities. High carbohydrate oxidation rates are necessary for take-off, but, to spare the limited carbohydrate reserves, long-distance flyers, such as locusts, soon switch to lipid as the main fuel. We demonstrate that before a flight, locust muscles are metabolically poised for take-off by the release of octopamine from central modulatory dorsal unpaired median (DUM) neurons, which increases the levels of the potent glycolytic activator fructose 2,6-bisphosphate in flight muscle. Because DUM neurons innervating the flight muscles are active during rest but selectively inhibited during flight, they stimulate carbohydrate catabolism during take-off but tend to decrease muscle glycolysis during prolonged flight. cAMP-dependent protein kinase A is necessary but not sufficient for signal transduction, suggesting parallel control via a calcium-dependent pathway. Locust flight is the first reported instance of a direct and specific involvement of neuronal activity in the control of muscle glycolysis in working muscle during exercise.

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

Nitric oxide is well established as a signalling molecule in the nervous system of vertebrates and invertebrates. In this study we evaluate the usefulness of NADPHdiaphorase histochemistry and immunocytochemistry for detecting the presence of nitric oxide synthase in locusts. We describe the distribution of putative nitric oxide releasing neurones and stained neuropiles in the locust ventral nerve cord, in particular the abdominal ganglia and abdominal neuromeres. NADPHdiaphorase histochemistry revealed prominent staining in all neuropilar regions and a specific distribution pattern of stained cell bodies in all examined ganglia. Nitric oxide synthase immunocytochemistry, using a commercially available universal antibody, labelled cells in corresponding positions within the ganglia. This was confirmed by double labelling of alternate sections. Western blot analysis demonstrated that in locusts this universal NOS-antibody binds to a protein of similar size to nitric oxide synthase identified in other insect species. The antibody also labelled axons in most peripheral nerves of all examined ganglia, whereas NADPHdiaphorase histochemistry only revealed such stained fibres within peripheral nerves in some preparations, because they may have been masked by intense background staining. We therefore conclude that nitric oxide synthase-immunocytochemistry and NADPHd histochemistry are both good markers for the presence of nitric oxide synthase in the locust ventral nerve cord, and that nitric oxide may be used as a signalling molecule by efferent neurones in locusts.

Cholinergic transmission via central synapses in the locust nervous system

In this study we examine the nature of chemical synaptic transmission between identified filiform hair receptors on the prothoracic segment of a locust and the identified postsynaptic projection interneuron (A4I1). The effects of pressure ejected acetylcholine, and various ligands of acetylcholine receptors on the activity of the postsynaptic neuron A4I1, or on wind-elicited responses in A4I1 are reported. It is suggested that the transmitter of the afferent fibers is acetylcholine, and that fast transmission is mediated by nicotinic acetylcholine-receptors. Both nicotine and carbachol act as agonists, whereas d-tubocurarine and alpha-bungarotoxin act as antagonists. The presence of muscarinic acetylcholine receptors was also evident from the modulatory effects of muscarine, oxotremorine and pilocarpine, which were blocked by bath application of atropine. GABA, and its agonists muscimol and cis-4-amino-crotonic-acid lead to inhibition of A4I1 responses. This inhibition was prevented by the additional application of picrotoxin. This suggests involvement of a ligand-gated GABA receptor which, most likely, increases chloride conductance. Metabotropic GABA-receptors do not seem to be involved, since baclofene, diazepam and bicuculline ejections had no effects. Glutamate also inhibits wind elicited A4I1 responses. Although attempts were made to further characterize the receptor involved, tested substances such as kainic acid, glycine, CNQX or GDEE had no effect.

The functional role of octopaminergic neurons in insect motor behavior

The role of efferent, octopaminergic dorsal unpaired median (DUM) neurons in insects is examined by recording from them during motor behaviour. This population of neuromodulatory neurons is divided into sub-populations which are specifically activated or inhibited during ongoing motor behavior. These neurons are always activated in parallel to the respective motor circuits, and in addition to their modulatory effects on synaptic transmission may also cause metabolic changes in their target tissues.

Correction methods for three-dimensional reconstructions from confocal images: I. Tissue shrinking and axial scaling

We show here, using locust wholemount ganglia as an example, that scaling artifacts in three-dimensional reconstructions from confocal microscopic images due to refractive index mismatch in the light path and tissue shrinking, can account for dramatic errors in measurements of morphometric values. Refractive index mismatch leads to considerable alteration of the axial dimension, and true dimensions must be restored by rescaling the Z-axis of the image stack. The appropriate scaling factor depends on the refractive indices of the media in the light path and the numerical aperture of the objective used and can be determined by numerical simulations, as we show here. In addition, different histochemical procedures were tested in regard to their effect on tissue dimensions. Reconstructions of scans at different stages of these protocols show that shrinking can be avoided prior to clearing when dehydrating ethanol series are carefully applied. Fixation and mismatching buffer osmolarity have no effect. We demonstrate procedures to reduce artifacts during mounting and clearing in methyl salicylate, such that only isometric shrinkage occurs, which can easily be corrected by rescaling the image dimensions. Glycerol-based clearing agents produced severe anisometric and nonlinear shrinkage and we could not find a way to overcome this.

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.

Neuroethology, its roots and future

Scholars in a particular scientific field should be familiar with its historical roots. Such knowledge will put their own research into a historical perspective, and, in addition, will allow them to assess current strengths and weaknesses in their particular area of research. To keep an exciting field like neuroethology alive and close to fast moving scientific frontiers, it is necessary to constantly adapt and broaden its approaches to newly emerging ideas from other fields, and to quickly incorporate new methodologies. The following article tries to expose some of the roots of neuroethology, and, in addition, will present some evidence as to why the authors think this field needs a broader definition than that formulated in the past. Doing so after the 5th International Congress of Neuroethology in San Diego in August 1998 seems to the authors the most appropriate time.

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.

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.

Action of locust neuromodulatory neurons is coupled to specific motor patterns

1. Many muscles of the locust are supplied by dorsal unpaired median neurons (DUM neurons) that release octopamine and alter the contractions caused by spikes in motor neurons. To determine when these neuromodulatory neurons are normally activated during behaviour, intracellular recordings were made simultaneously from them and from identified motor neurons during the specific motor pattern that underlies kicking. A kick consists of a rapid and powerful extension of the tibia of one or both hind legs that is produced by a defined motor pattern. Only 3 identified DUM neurons of the 20 in the metathoracic ganglion spike during a kick, and they supply muscles involved in generating the kick. Their spikes occur in a distinctive and repeatable pattern that is closely linked to the pattern of spikes in the flexor and extensor tibiae motor neurons. When the extensor and flexor muscles cocontract, these three DUM neurons produce a burst of spikes at frequencies that can rise to 25 Hz, and with the number of spikes (3-15) related to the duration of this phase of the motor pattern. The spikes stop when the flexor muscle is inhibited and therefore before the tibia is extended rapidly. The other DUM neurons which supply muscles that are not directly involved in kicking are either inhibited or spike only sporadically. 2. The activation of a specific subset of DUM neurons during kicking may thus be timed to influence the action of the muscles that participate in this movement and appear to be controlled by the same circuits that determine the actions of the participating motor neurons. These modulatory neurons thus have specific individual actions in the control of movement.

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.

Activity-dependent structural dynamics of insect sensory fibers

This report analyses the role of neuronal activity in shaping the axonal arborizations of sensory neurons from individually identified filiform hairs on the prosternum of locusts (Locusta migratoria), and their connections with a pair of identified interneurons (A4I1). Afferents from lateral filiform hairs terminate in the ipsilateral neuropil and connect only with the ipsilateral interneuron in all instars. Afferents from ventral filiform hairs possesses ipsi- and contralateral branches and make monosynaptic connections with both interneurons in first instars. In later instars, the ipsilateral branch, and its synaptic connection to the ipsilateral interneuron, is gradually reduced until it is lost in the adult, whereas the contralateral branch, and its synaptic connection with the contralateral interneuron, is strengthened. Therefore, after an initial overgrowth of fibers and synapses, segregation of fibers occurs involving the loss of synaptic connections. This loss of branches and synapses was prevented by immobilizing a subpopulation of ventral and lateral filiform hairs, or each group independently, so that their normal activity was blocked. In such treated animals afferents from ventral filiform hairs retain their ipsi- and contralateral branches until adulthood. We therefore conclude that afferent activity plays an important role in shaping the final structure and connectivity of afferents, as neither the peripheral position of the receptors nor the hormonal environment was changed by these manipulations.

Distribution of input synapses from processes exhibiting GABA- or glutamate-like immunoreactivity onto terminals of prosternal filiform afferents in the locust

The locust prosternum carries a population of long filiform hairs that are very sensitive to air currents. The sensory afferent neurones that innervate the hairs make strong monosynaptic connections with an identified intersegmental interneurone (A4I1) which is known to contact motor neurones that supply muscles controlling wing angle during flight. In order discover how the synapse between the afferents and interneurone A4I1 might be modulated, the afferents were labelled intracellularly by backfilling with horseradish peroxidase to reveal their central terminals which lie in the prothoracic ganglion. A postembedding immunogold method was used to make a quantitative assessment of the prevalence of immunoreactivity for GABA and glutamate in processes presynaptic to the afferent terminals. In one afferent neurone, where 77 synapses were examined, 40 (52%) of the presynaptic processes were immunoreactive for GABA. When adjacent sections through the same terminal branches were labelled with the two antibodies, it was demonstrated that GABA- and glutamate-like immunoreactivity was present in different populations of presynaptic processes. A series of 110 ultrathin sections was cut through one set of afferent terminal branches and alternate grids were stained with GABA and glutamate antibodies. From these sections, the terminals were reconstructed and the position of 35 input and 21 output synapses mapped. Of the 35 input synapses, 18 (51%) were immunoreactive for GABA, 14 (40%) were immunoreactive for glutamate and 3 (9%) were unlabelled by either antibody. On these terminals, the different classes of input synapses appeared to be intermingled at random with the output synapses made by the afferent, and no pattern governing synapse distribution could be discerned.

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.