The distribution of glutamate-like immunoreactivity in the thoracic and abdominal ganglia of the locust (Schistocerca gregaria)

Neural shutdown under stress: an evolutionary perspective on spreading depolarization

Neural function depends on maintaining cellular membrane potentials as the basis for electrical signaling. Yet, in mammals and insects, neuronal and glial membrane potentials can reversibly depolarize to zero, shutting down neural function by the process of spreading depolarization (SD) that collapses the ion gradients across membranes. SD is not evident in all metazoan taxa with centralized nervous systems. We consider the occurrence and similarities of SD in different animals and suggest that it is an emergent property of nervous systems that have evolved to control complex behaviors requiring energetically expensive, rapid information processing in a tightly regulated extracellular environment. Whether SD is beneficial or not in mammals remains an open question. However, in insects, it is associated with the response to harsh environments and may provide an energetic advantage that improves the chances of survival. The remarkable similarity of SD in diverse taxa supports a model systems approach to understanding the mechanistic underpinning of human neuropathology associated with migraine, stroke, and traumatic brain injury.

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.

Substantial changes in central nervous system neurotransmitters and neuromodulators accompany phase change in the locust

Desert locusts (Schistocerca gregaria) can undergo a profound transformation between solitarious and gregarious forms, which involves widespread changes in behaviour, physiology and morphology. This phase change is triggered by the presence or absence of other locusts and occurs over a timescale ranging from hours, for some behaviours to change, to generations,for full morphological transformation. The neuro-hormonal mechanisms that drive and accompany phase change in either direction remain unknown. We have used high-performance liquid chromatography (HPLC) to compare amounts of 13 different potential neurotransmitters and/or neuromodulators in the central nervous systems of final instar locust nymphs undergoing phase transition and between long-term solitarious and gregarious adults. Long-term gregarious and solitarious locust nymphs differed in 11 of the 13 substances analysed: eight increased in both the brain and thoracic nerve cord (including glutamate,GABA, dopamine and serotonin), whereas three decreased (acetylcholine,tyramine and citrulline). Adult locusts of both extreme phases were similarly different. Isolating larval gregarious locusts led to rapid changes in seven chemicals equal to or even exceeding the differences seen between long-term solitarious and gregarious animals. Crowding larval solitarious locusts led to rapid changes in six chemicals towards gregarious values within the first 4 h(by which time gregarious behaviours are already being expressed), before returning to nearer long-term solitarious values 24 h later. Serotonin in the thoracic ganglia, however, did not follow this trend, but showed a ninefold increase after a 4 h period of crowding. After crowding solitarious nymphs for a whole larval stadium, the amounts of all chemicals, except octopamine, were similar to those of long-term gregarious locusts. Our data show that changes in levels of neuroactive substances are widespread in the central nervous system and reflect the time course of behavioural and physiological phase change.

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

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

Presynaptic modulation of sensory neurons in the segmental ganglia of arthropods

The afferent terminals of arthropod sensory neurones receive abundant input synapses, usually closely intermingled with the sites of synaptic output. The majority of the input synapses use the neurotransmitter GABA, but in some afferents there is a significant glutamatergic or histaminergic component. GABA and histamine shunt afferent action potentials by increasing chloride conductance. Though glutamate can also have this effect in the arthropod central nervous system, its action on afferent terminals appears to be mediated by increases in potassium conductance or by the action of metabotropic receptors. The action of the presynaptic synapses on the afferents are many and varied. Even on the same afferent, they may have several distinct roles that can involve both tonic and phasic patterns of primary afferent depolarisation. Despite the ubiquity and importance of their effects however, the populations of neurones from which the presynaptic synapses are made, remain largely unidentified.

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.

Synaptic structure, distribution, and circuitry in the central nervous system of the locust and related insects

The Orthopteran central nervous system has proved a fertile substrate for combined morphological and physiological studies of identified neurons. Electron microscopy reveals two major types of synaptic contacts between nerve fibres: chemical synapses (which predominate) and electrotonic (gap) junctions. The chemical synapses are characterized by a structural asymmetry between the pre- and postsynaptic electron dense paramembranous structures. The postsynaptic paramembranous density defines the extent of a synaptic contact that varies according to synaptic type and location in single identified neurons. Synaptic bars are the most prominent presynaptic element at both monadic and dyadic (divergent) synapses. These are associated with small electron lucent synaptic vesicles in neurons that are cholinergic or glutamatergic (round vesicles) or GABAergic (pleomorphic vesicles). Dense core vesicles of different sizes are indicative of the presence of peptide or amine transmitters. Synapses are mostly found on small-diameter neuropilar branches and the number of synaptic contacts constituting a single physiological synapse ranges from a few tens to several thousand depending on the neurones involved. Some principles of synaptic circuitry can be deduced from the analysis of highly ordered brain neuropiles. With the light microscope, synaptic location can be inferred from the distribution of the presynaptic protein synapsin I. In the ventral nerve cord, identified neurons that are components of circuits subserving known behaviours, have been studied using electrophysiology in combination with light and electron microscopy and immunocytochemistry of neuroactive compounds. This has allowed the synaptic distribution of the major classes of neurone in the ventral nerve cord to be analysed within a functional context.

Taurine-, aspartate- and glutamate-like immunoreactivity identifies chemically distinct subdivisions of Kenyon cells in the cockroach mushroom body

The lobes of the mushroom bodies of the cockroach Periplaneta americana consist of longitudinal modules called laminae. These comprise repeating arrangements of Kenyon cell axons, which like their dendrites and perikarya have an affinity to one of three antisera: to taurine, aspartate, or glutamate. Taurine-immunopositive laminae alternate with immunonegative ones. Aspartate-immunopositive Kenyon cell axons are distributed across the lobes. However, smaller leaf-like ensembles of axons that reveal particularly high affinities to anti-aspartate are embedded within taurine-positive laminae and occur in the immunonegative laminae between them. Together, these arrangements reveal a complex architecture of repeating subunits whose different levels of immunoreactivity correspond to broader immunoreactive layers identified by sera against the neuromodulator FMRFamide. Throughout development and in the adult, the most posterior lamina is glutamate immunopositive. Its axons arise from the most recently born Kenyon cells that in the adult retain their juvenile character, sending a dense system of collaterals to the front of the lobes. Glutamate-positive processes intersect aspartate- and taurine-immunopositive laminae and are disposed such that they might play important roles in synaptogenesis or synapse modification. Glutamate immunoreactivity is not seen in older, mature axons, indicating that Kenyon cells show plasticity of neurotransmitter phenotype during development. Aspartate may be a universal transmitter substance throughout the lobes. High levels of taurine immunoreactivity occur in broad laminae containing the high concentrations of synaptic vesicles.

Amine and amino acid transmitters in the eye of the mollusc Bulla gouldiana: an immunocytochemical study

We identified putative transmitters of the photoreceptors and circadian pacemaker neurons and found candidates for efferent control in the eye of the marine mollusc Bulla gouldiana. Established antisera against octopamine, dopamine, serotonin, histamine, glutamate, gamma-aminobutyric acid (GABA), and taurine were used, and central ganglia were processed in parallel to evaluate general staining quality. Photoreceptors and circadian pacemaker cells both expressed immunoreactivity for glutamate and taurine. The eye and its sheath were devoid of GABA-like immunoreactive material, and none of the antisera directed against biogenic amines labelled cells or processes in the nervous tissue of the eye. However, dopamine and octopamine antisera stained large spherical granules (diameter 2-3 microm) contained in granular cells that are located in the connective tissue encapsulating the eye and the optic nerve. The serotonin antiserum revealed a sparse distribution of varicose axon fibers in the optic nerve and eye sheath. No histamine-immunoreactive processes were revealed in the eye. The functional significance of these findings for the molluscan eye and its circadian clock is discussed.

GABA and glutamate-like immunoreactivity at synapses on depressor motorneurones of the leg of the crayfish, Procambarus clarkii

To investigate their synaptic relationships, depressor motorneurones of the crayfish leg were impaled with microelectrodes, intracellularly injected with horseradish peroxidase, and prepared for electron microscopy. Post-embedding immunogold labelling with antibodies against gamma-aminobutyric acid (GABA) or glutamate was carried out either alone or together on the same section and allowed the identification of three classes of input synapses: 51% were immunoreactive for glutamate and contained round agranular vesicles, 31% were immunoreactive for GABA and contained pleomorphic agranular vesicles, and the remainder were immunoreactive for neither and also predominantly contained pleomorphic agranular vesicles. Output synapses were abundant in some of the motorneurones but were not seen in others, suggesting that members of the motor pool differ in their connectivity.

Distribution of input and output synapses on the central branches of bushcricket and cricket auditory afferent neurones: immunocytochemical evidence for GABA and glutamate in different populations of presynaptic boutons

In order to investigate the synapses on the terminals of primary auditory afferents in the bushcricket and cricket, these were impaled with microelectrodes and after physiological characterisation, injected intracellularly with horseradish peroxidase. The tissue was prepared for electron microscopy, and immunocytochemistry for gamma-aminobutyric acid (GABA) and glutamate was carried out on ultrathin sections by using a post-embedding immunogold technique. The afferent terminals received many input synapses. Between 60-65% of these were made by processes immunoreactive for GABA and approximately 25% from processes immunoreactive for glutamate. The relative distribution of the different classes of input were analysed from serial section reconstruction of terminal afferent branches. Inputs from GABA and glutamate-immunoreactive processes appeared to be scattered at random over the terminal arborisation of the afferents both with respect to each other and to the architecture of the terminals. They were, however, always found close to the output synapses. The possible roles of presynaptic inhibition in the auditory afferents is discussed in the context of the auditory responses of the animals.

The ontogeny of GABA- and glutamate-like immunoreactivity in the embryonic Australian freshwater crayfish, Cherax destructor

The distribution and ontogeny of GABA- and glutamate-like immunoreactivity in embryos of the Australian freshwater crayfish Cherax destructor were investigated over the period from 30% development until hatching. GABA-like immunoreactive cells and fibres appeared first in the brain at 40-45% development. By 70% development, GABA-like immunoreactive cells were present in almost all ganglia, and GABA-like immunoreactive fibres were distributed extensively throughout the neuropil, commissures and connectives of the central nervous system, and were also found in peripheral nerve roots supplying the appendages and the abdominal musculature. In contrast, glutamate-like immunoreactivity did not appear in the central nervous system until 60-65% development. By the time of hatching, the distribution of glutamate-like immunoreactivity was restricted to discrete regions of neuropil and fibre staining in the thoracic and abdominal nerve cord, the abdominal musculature and the appendages. The precocious establishment of the extensive distribution of GABA-like immunoreactive neurons in the developing crayfish embryo is consistent with the possibility that these neurons play a trophic role in controlling or modulating the development of the nervous system.

Evolution and overview of classical transmitter molecules and their receptors

All the classical transmitter ligand molecules evolved at least 1000 million years ago. With the possible exception of the Porifera and coelenterates (Cnidaria), they occur in all the remaining phyla. All transmitters have evolved the ability to activate a range of ion channels, resulting in excitation, inhibition and biphasic or multiphasic responses. All transmitters can be synthesised in all three basic types of neurones, i.e. sensory, interneurone and motoneurone. However their relative importance as sensory, interneurone or motor transmitters varies widely between the phyla. It is likely that all neurones contain more than one type of releasable molecule, often a combination of a classical transmitter and a neuroactive peptide. Second messengers, i.e. G proteins and phospholipase C systems, appeared early in evolution and occur in all phyla that have been investigated. Although the evidence is incomplete, it is likely that all the classical transmitter receptor subtypes identified in mammals, also occur throughout the phyla. The invertebrate receptors so far cloned show some interesting homologies both between those from different invertebrate phyla and with mammalian receptors. This indicates that many of the basic receptor subtypes, including benzodiazepine subunits, evolved at an early period, probably at least 800 million years ago. Overall, the evidence stresses the similarity between the major phyla rather than their differences, supporting a common origin from primitive helminth stock.

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.

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.