The effects of L-glutamate on cultured insect neurones

Identification, localization and function of glutamate-gated chloride channel receptors in the honeybee brain

Glutamate-gated chloride channels (GluCls) are members of the cys-loop ligand-gated ion channel superfamily whose presence has been reported in a variety of invertebrate tissues. In the honeybee, a single gene, amel_glucl, encoding a GluClα subunit, was found in the genome but both the pattern of expression of this gene in the bee brain and its functional role remained unknown. Here we localised the expression sites of the honeybee GluClα subunit at the mRNA and protein levels. To characterise the functional role of GluCls in the honeybee brain, we studied their implication in olfactory learning and memory by means of RNA interference (RNAi) against the GluClα subunit. We found that the GluClα subunit is expressed in the muscles, the antennae and the brain of honeybees. Expression of the GluClα protein was necessary for the retrieval of olfactory memories; more specifically, injection of dsRNA or siRNA resulted in a decrease in retention performances ∼24 h after injection. Knockdown of GluClα subunits impaired neither olfaction nor sucrose sensitivity, and did not affect the capacity to associate odor and sucrose. Our data provide the first evidence for the involvement of glutamate-gated chloride channels in olfactory memory in an invertebrate.

Glutamatergic networks in the Ciona intestinalis larva

Glutamate is a major neurotransmitter in the excitatory synapses of both vertebrate and invertebrate nervous systems and is involved in many neural processes including photo-, mechano-, and chemosensations, neural development, motor control, learning, and memory. We identified and characterized the gene (Ci-VGLUT) encoding a member of the vesicular glutamate transporter subfamily, a specific marker of glutamatergic neurons, in the ascidian Ciona intestinalis. The Ci-VGLUT gene is expressed in the adhesive organ, the epidermal neurons, and the brain vesicle, but not in the visceral ganglion. The Ci-VGLUT promoter and an anti-Ci-VGLUT antibody were used to analyze the distribution and axonal connections of prospective glutamatergic neurons in the C. intestinalis larva. The green fluorescent protein (GFP) reporter driven by the 4.6-kb upstream region of Ci-VGLUT recapitulated the endogenous gene expression patterns and visualized both the cell bodies and neurites of glutamatergic neurons. Papillar neurons of the adhesive organs, almost all epidermal neurons, the otolith cell, and ocellus photoreceptor cells were shown to be glutamatergic. Each papillar neuron connects with a rostral epidermal neuron. Axons from rostral epidermal neurons, ocellus photoreceptor cells, and neurons underlying the otolith terminate in the posterior brain vesicle. Some caudal epidermal neurons also send long axons toward the brain vesicle. The posterior brain vesicle contains a group of Ci-VGLUT-positive neurons that send axons posteriorly to the visceral ganglion. Our results suggest that glutamatergic neurotransmission plays a major role in sensory systems and in the integration of the sensory inputs of the ascidian larva.

Insect neuronal cultures: an experimental vehicle for studies of physiology, pharmacology and cell interactions

The current status of insect neuronal cultures is discussed and their contribution to our understanding of the insect nervous system is explored. Neuronal cultures have been developed from a wide range of insect species and from all developmental stages. These have been used to study the morphological development of insect neurones and some of the extrinsic factors that affect this process. In addition, they have been used to investigate the physiology of sodium, potassium and calcium channels and the pharmacology of acetylcholine and GABA receptors. Insect neurones have also been grown in culture with muscle and glial cells to study cell interactions.

Pharmacological properties of nicotinic acetylcholine receptors in isolated Locusta migratoria neurones

Mechanically dissociated neuronal cell bodies from the thoracic ganglia of Locusta migratoria were viable in culture conditions for up to 2 days and were voltage-clamped to record the effects of GABAergic drugs and physostigmine on the membrane conductance and ACh responses of the dissociated cells. Bicuculline, hydrastine, and gabazine inhibited the EC50 ACh responses of the cells. Both bicuculline and hydrastine were full inhibitors of the ACh responses but gabazine behaved as a partial inhibitor. Bicuculline, hydrastine, and gabazine inhibited the ACh responses in a non-competitive and voltage-independent fashion, suggesting that they are allosteric inhibitors of locust nicotinic ACh receptors. Physostigmine activated currents when applied onto isolated locust neurones. The responses activated by physostigmine were inhibited competitively by tubocurarine, which indicates that physostigmine interacts with the ACh site of locust nicotinic ACh receptors. However, maximal concentrations of physostigmine elicited currents of smaller amplitudes to those evoked by maximal ACh concentrations. Single-channel recordings suggest that the partial efficacy of physostigmine may reflect the low frequency of opening of physostigmine-induced single currents relative to that of ACh-single currents.

EAT-4, a homolog of a mammalian sodium-dependent inorganic phosphate cotransporter, is necessary for glutamatergic neurotransmission in caenorhabditis elegans

The Caenorhabditis elegans gene eat-4 affects multiple glutamatergic neurotransmission pathways. We find that eat-4 encodes a protein similar in sequence to a mammalian brain-specific sodium-dependent inorganic phosphate cotransporter I (BNPI). Like BNPI in the rat CNS, eat-4 is expressed predominantly in a specific subset of neurons, including several proposed to be glutamatergic. Loss-of-function mutations in eat-4 cause defective glutamatergic chemical transmission but appear to have little effect on other functions of neurons. Our data suggest that phosphate ions imported into glutamatergic neurons through transporters such as EAT-4 and BNPI are required specifically for glutamatergic neurotransmission.

Identification of a Drosophila melanogaster glutamate-gated chloride channel sensitive to the antiparasitic agent avermectin

Glutamate-gated chloride channels, members of the ligand-gated ion channel superfamily, have been shown in nematodes and in insects to be a target of the antiparasitic agent avermectin. Two subunits of the Caenorhabditis elegans glutamate-gated chloride channel have been cloned: GluCl-alpha and GluCl-beta. We report the cloning of a Drosophila melanogaster glutamate-gated chloride channel, DrosGluCl-alpha, which shares 48% amino acid and 60% nucleotide identity with the C. elegans GluCl channels. Expression of DrosGluCl-alpha in Xenopus oocytes produces a homomeric chloride channel that is gated by both glutamate and avermectin. The DrosGluCl-alpha channel has several unique characteristics not observed in C. elegans GluCl: dual gating by avermectin and glutamate, a rapidly desensitizing glutamate response, and a lack of potentiation of the glutamate response by avermectin. The pharmacological data support the hypothesis that the DrosGluCl-alpha channel represents the arthropod H-receptor and an important target for the avermectin class of insecticides.

An electrophysiological preparation of Ascaris suum pharyngeal muscle reveals a glutamate-gated chloride channel sensitive to the avermectin analogue, milbemycin D

An electrophysiological preparation of Ascaris suum pharyngeal muscle suitable for recording changes of input conductance using a 2-microelectrode current clamp and pharmacological study is described. The preparation is shown to contain a glutamate-gated Cl (ion sensitive) channel sensitive to the avermectin analogue, milbemycin D. The application of glutamate produces a dose-dependent increase in Cl conductance and the effect of glutamate is potentiated by milbemycin D. Milbemycin D also produced a dose-dependent increase in input conductance.

Molecular biology and electrophysiology of glutamate-gated chloride channels of invertebrates

In this chapter we summarize the available data on a novel class of ligand-gated anion channels that are gated by the neurotransmitter glutamate. Glutamate is classically thought to be a stimulatory neurotransmitter, however, studies in invertebrates have proven that glutamate also functions as an inhibitory ligand. The bulk of studies conducted in vivo have been on insects and crustaceans, where glutamate was first postulated to act on H-receptors resulting in a hyperpolarizing response to glutamate. Recently, glutamate-gated chloride channels have been cloned from several nematodes and Drosophila. The pharmacology and electrophysiological properties of these channels have been studied by expression in Xenopus oocytes. Studies on the cloned channels demonstrate that the invertebrate glutamate-gated chloride channels are the H-receptors and represent important targets for the antiparasitic avermectins.

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.

Inhibitory effects of L-glutamate on central processes of crustacean leg motoneurons

In crustaceans, glutamatergic excitation at the neuromuscular synapse has been extensively studied. Fewer reports exist of the central and possibly inhibitory actions of glutamate on neurons. The present study analyses the response of intracellularly identified motoneurons, which innervate the proximal leg muscles, to local glutamate pressure applications in the neuropil, in an in vitro thoracic preparation of the crayfish Procambarus clarkii. L-Glutamate application always inhibited motoneuron activity, with a decrease in input resistance. The resulting depolarization or hyperpolarization could usually be reversed within 10 mV of the resting potential. The response persisted in neurons pharmacologically isolated with Cd2+ or tetrodotoxin. The reversal potential of the response to glutamate was displaced in a low-chloride solution. Similar responses were obtained with GABA. Application of GABA blocked the glutamate response in a competitive manner. Both responses were suppressed by beta-guanidino-propionic acid, a competitive antagonist for GABA receptors. This indicates that glutamate activates a chloride-GABA receptor-channel. Micromolar concentrations of picrotoxin reduced both the L-glutamate and the GABA inhibitory responses, thereby unmasking a smaller, picrotoxin-resistant effect of glutamate (but not of GABA), which was excitatory and sensitive to 6,7-dinitroquinoxaline-2,3-dione (DNQX). These results suggest dual and opposite roles for motoneuron glutamatergic connections--a peripheral (well known) net excitatory one and a central net inhibitory one. Direct inhibition of motoneurons by L-glutamatergic neurons is to be expected.

Immunoaffinity purification of avermectin-binding proteins from the free-living nematode Caenorhabditis elegans and the fruitfly Drosophila melanogaster

Avermectin-binding proteins from the free-living nematode worm Caenorhabditis elegans and from the fruitfly Drosophila melanogaster were purified to homogeneity via a three-step procedure. The binding proteins were covalently labelled using a radioactive photoaffinity probe and then partially purified on a Sephacryl S-300 gel-filtration column. The radiolabelled binding proteins were then purified by immunoaffinity chromatography using a monoclonal antibody to avermectin covalently attached to Protein A-Sepharose beads. Three affinity-labelled Drosophila proteins with molecular masses between 45 and 50 kDa were isolated in this way and then separated from each other by electroelution. This three-step protocol provides a rapid technique for receptor purification which may be of use in the purification of other binding proteins.

Expression of Caenorhabditis elegans mRNA in n Xenopus oocytes: A model system to study the mechanism of action of avermectins

It has recently been shown that Xenopus oocytes injected with mRNA from the free-living nematode Caenorhabditis elegans express avermectin-sensitive chloride channels. Joseph Arena here reviews what is known about the mechanism of action of avermectin and how these recent results relate to the mechanism in nematodes.

Molecular analysis of Drosophila glutamate receptors

Insects and other invertebrates use L-glutamate as a neurotransmitter in the central nervous system and at the neuromuscular junction. In contrast to the well-studied effects of L-glutamate on invertebrate muscle cells, relatively little is known about the physiological role of glutamate receptors (GluRs) in the invertebrate central nervous system. We have applied a molecular cloning approach to elucidate the molecular structure of neuronal and muscle-specific Drosophila glutamate receptor subunits (DGluRs). Several domains conserved between rat GluR subunits and DGluRs indicate regions of high functional significance. Drosophila genetics may now be used as a valuable experimental tool to gain further insight into the role of DGluRs in development, synaptic plasticity and control of gene expression.

Glutamate receptors of Drosophila melanogaster: cloning of a kainate-selective subunit expressed in the central nervous system

We report the isolation and functional characterization of cDNAs encoding a Drosophila kainate-selective glutamate receptor. The deduced mature 964-residue protein (DGluR-I) is 108,482 Da and exhibits significant homology to mammalian glutamate receptor subunits. Injection of DGluR-I cRNA into Xenopus oocytes generated kainate-operated ion channels which were blocked by the selective non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione and philanthotoxin. DGluR-I transcripts are differentially expressed during Drosophila development and, in late embryogenesis, accumulate in the central nervous system.

Expression of a glutamate-activated chloride current in Xenopus oocytes injected with Caenorhabditis elegans RNA: evidence for modulation by avermectin

Membrane currents were recorded from Xenopus laevis oocytes injected with C. elegans poly(A)+ RNA. In such oocytes glutamate activated an inward membrane current that desensitized in the continued presence of glutamate. Glutamate-receptor agonists quisqualate, kainate, and N-methyl-D-aspartate were inactive. The reversal potential of the glutamate-sensitive current was -22 mV, and exhibited a strong dependence on external chloride with a 48 mV change for a 10-fold change in chloride. The chloride channel blockers flufenamate and picrotoxin inhibited the glutamate-sensitive current. Ibotenate, a structural analog of glutamate, also activated a picrotoxin-sensitive chloride current. Ibotenate was inactive when current was partially desensitized with glutamate, and the responses to low concentrations of glutamate and ibotenate were additive. The anthelmintic/insecticide compound avermectin directly activated the glutamate-sensitive current. In addition, avermectin increased the response to submaximal concentrations of glutamate, shifted the glutamate concentration-response curve to lower concentrations, and slowed the desensitization of glutamate-sensitive current. We propose that the glutamate-sensitive chloride current and the avermectin-sensitive chloride current are mediated via the same channel.

Molecular cloning of an invertebrate glutamate receptor subunit expressed in Drosophila muscle

Insects and other invertebrates use glutamate as a neurotransmitter in the central nervous system and at the neuromuscular junction. A complementary DNA from Drosophila melanogaster, designated DGluR-II, has been isolated that encodes a distant homolog of the cloned mammalian ionotropic glutamate receptor family and is expressed in somatic muscle tissue of Drosophila embryos. Electrophysiological recordings made in Xenopus oocytes that express DGluR-II revealed depolarizing responses to L-glutamate and L-aspartate but low sensitivity to quisqualate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), and kainate. The DGluR-II protein may represent a distinct glutamate receptor subtype, which shares its structural design with other members of the ionotropic glutamate receptor family.