Localization of L-glutamate and glutamate-like receptors at the squid giant synapse

Immunocytochemical localization of the AMPA glutamate receptor subtype GluR2/3 in the squid optic lobe

As a major excitatory neurotransmitter in the cephalopod visual system, glutamate signaling is facilitated by ionotropic receptors, such as α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPAR). In cephalopods with large and well-developed brains, the optic lobes (OL) mainly process visual inputs and are involved in learning and memory. Although the presence of AMPAR in squid OL has been reported, the organization of specific AMPAR-containing neurons remains unknown. This study aimed to investigate the immunocytochemical localization of the AMPA glutamate receptor subtype 2/3-immunoreactive (GluR2/3-IR) neurons in the OL of Pacific flying squid (Tordarodes pacificus). Morphologically diverse GluR2/3-IR neurons were predominantly located in the tangential zone of the medulla. Medium-to-large GluR2/3-IR neurons were also detected. The distribution patterns and cell morphologies of calcium-binding protein (CBP)-IR neurons, specifically calbindin-D28K (CB)-, calretinin (CR)-, and parvalbumin (PV)-IR neurons, were similar to those of GluR2/3-IR neurons. However, two-color immunofluorescence revealed that GluR2/3-IR neurons did not colocalize with the CBP-IR neurons. Furthermore, the specific localizations and diverse types of GluR2/3-IR neurons that do not express CB, CR, or PV in squid OL were determined. These findings further contribute to the existing data on glutamatergic visual systems and provide new insights for understanding the visual processing mechanisms in cephalopods.

Neurotransmission and neuromodulation systems in the learning and memory network of Octopus vulgaris

The vertical lobe (VL) in the octopus brain plays an essential role in its sophisticated learning and memory. Early anatomical studies suggested that the VL is organized in a “fan‐out fan‐in” connectivity matrix comprising only three morphologically identified neuron types; input axons from the median superior frontal lobe (MSFL) innervating en passant millions of small amacrine interneurons (AMs), which converge sharply onto large VL output neurons (LNs). Recent physiological studies confirmed the feedforward excitatory connectivity; a glutamatergic synapse at the first MSFL‐to‐AM synaptic layer and a cholinergic AM‐to‐LNs synapse. MSFL‐to‐AMs synapses show a robust hippocampal‐like activity‐dependent long‐term potentiation (LTP) of transmitter release. 5‐HT, octopamine, dopamine and nitric oxide modulate short‐ and long‐term VL synaptic plasticity. Here, we present a comprehensive histolabeling study to better characterize the neural elements in the VL. We generally confirmed glutamatergic MSFLs and cholinergic AMs. Intense labeling for NOS activity in the AMs neurites were in‐line with the NO‐dependent presynaptic LTP mechanism at the MSFL‐to‐AM synapse. New discoveries here reveal more heterogeneity of the VL neurons than previously thought. GABAergic AMs suggest a subpopulation of inhibitory interneurons in the first input layer. Clear γ‐amino butyric acid labeling in the cell bodies of LNs supported an inhibitory VL output, yet the LNs co‐expressed FMRFamide‐like neuropeptides, suggesting an additional neuromodulatory role of the VL output. Furthermore, a group of LNs was glutamatergic. A new cluster of cells organized as a “deep nucleus” showed rich catecholaminergic labeling and may play a role in intrinsic neuromodulation. In‐situ hybridization and immunolabeling allowed characterization and localization of a rich array of neuropeptides and neuromodulators, likely involved in reward/punishment signals. This analysis of the fast transmission system, together with the newly found cellular elements, help integrate behavioral, physiological, pharmacological and connectome findings into a more comprehensive understanding of an efficient learning and memory network.

Evolution of glutamatergic signaling and synapses

Glutamate (Glu) is the primary excitatory transmitter in the mammalian brain. But, we know little about the evolutionary history of this adaptation, including the selection of l-glutamate as a signaling molecule in the first place. Here, we used comparative metabolomics and genomic data to reconstruct the genealogy of glutamatergic signaling. The origin of Glu-mediated communications might be traced to primordial nitrogen and carbon metabolic pathways. The versatile chemistry of L-Glu placed this molecule at the crossroad of cellular biochemistry as one of the most abundant metabolites. From there, innovations multiplied. Many stress factors or injuries could increase extracellular glutamate concentration, which led to the development of modular molecular systems for its rapid sensing in bacteria and archaea. More than 20 evolutionarily distinct families of ionotropic glutamate receptors (iGluRs) have been identified in eukaryotes. The domain compositions of iGluRs correlate with the origins of multicellularity in eukaryotes. Although L-Glu was recruited as a neuro-muscular transmitter in the early-branching metazoans, it was predominantly a non-neuronal messenger, with a possibility that glutamatergic synapses evolved more than once. Furthermore, the molecular secretory complexity of glutamatergic synapses in invertebrates (e.g., Aplysia) can exceed their vertebrate counterparts. Comparative genomics also revealed 15+ subfamilies of iGluRs across Metazoa. However, most of this ancestral diversity had been lost in the vertebrate lineage, preserving AMPA, Kainate, Delta, and NMDA receptors. The widespread expansion of glutamate synapses in the cortical areas might be associated with the enhanced metabolic demands of the complex brain and compartmentalization of Glu signaling within modular neuronal ensembles.

Exploration of chemosensory ionotropic receptors in cephalopods: the IR25 gene is expressed in the olfactory organs, suckers, and fins of Sepia officinalis

While they are mostly renowned for their visual capacities, cephalopods are also good at olfaction for prey, predator, and conspecific detection. The olfactory organs and olfactory cells are well described but olfactory receptors-genes and proteins-are still undescribed in cephalopods. We conducted a broad phylogenetic analysis of the ionotropic glutamate receptor family in mollusks (iGluR), especially to identify IR members (Ionotropic Receptors), a variant subfamily whose involvement in chemosensory functions has been shown in most studied protostomes. A total of 312 iGluRs sequences (including 111 IRs) from gastropods, bivalves, and cephalopods were identified and annotated. One orthologue of the gene coding for the chemosensory IR25 co-receptor has been found in Sepia officinalis (Soff-IR25). We searched for Soff-IR25 expression at the cellular level by in situ hybridization in whole embryos at late stages before hatching. Expression was observed in the olfactory organs, which strongly validates the chemosensory function of this receptor in cephalopods. Soff-IR25 was also detected in the developing suckers, which suggests that the unique « taste by touch » behavior that cephalopods execute with their arms and suckers share features with olfaction. Finally, Soff-IR25 positive cells were unexpectedly found in fins, the two posterior appendages of cephalopods, mostly involved in locomotory functions. This result opens new avenues of investigation to confirm fins as additional chemosensory organs in cephalopods.

Glutamate Release

Our aim was to review the processes of glutamate release from both biochemical and neurophysiological points of view. A large body of evidence now indicates that glutamate is specifically accumulated into synaptic vesicles, which provides strong support for the concept that glutamate is released from synaptic vesicles and is the major excitatory neurotransmitter. Evidence suggests the notion that synaptic vesicles, in order to sustain the neurotransmitter pool of glutamate, are endowed with an efficient mechanism for vesicular filling of glutamate. Glutamate-loaded vesicles undergo removal of Synapsin I by CaM kinase II-mediated phosphorylation, transforming to the release-ready pool. Vesicle docking to and fusion with the presynaptic plasma membrane are thought to be mediated by the SNARE complex. The Ca(2+)-dependent step in exocytosis is proposed to be mediated by synaptotagmin.

Ion channels in key marine invertebrates; their diversity and potential for applications in biotechnology

Of the intra-membrane proteins, the class that comprises voltage and ligand-gated ion channels represents the major substrate whereby signals pass between and within cells in all organisms. It has been presumed that vertebrate and particularly mammalian ion channels represent the apex of evolutionary complexity and diversity and much effort has been focused on understanding their function. However, the recent availability of cheap high throughput genome sequencing has massively broadened and deepened the quality of information across phylogeny and is radically changing this view. Here we review current knowledge on such channels in key marine invertebrates where physiological evidence is backed up by molecular sequences and expression/functional studies. As marine invertebrates represent a much greater range of phyla than terrestrial vertebrates and invertebrates together, we argue that these animals represent a highly divergent, though relatively underused source of channel novelty. As ion channels are exquisitely selective sensors for voltage and ligands, their potential and actual applications in biotechnology are manifold.

L-glutamate and its ionotropic receptors in the nervous system of cephalopods

In several species of cephalopod molluscs there is good evidence for the presence of L-glutamate in the central and peripheral nervous system and evidence for both classes of ionotropic receptor, AMPA/kainate and NMDA.The best evidence for glutamate being a transmitter in cephalopods comes from pharmacological, immunohistochemical and molecular investigations on the giant fibre system in the squid stellate ganglion. These studies confirm there are AMPA/kainate-like receptors on the third-order giant axon. In the (glial) Schwann cells associated with the giant axons both classes of glutamate receptor occur.Glutamate is an excitatory transmitter in the chromatophores and in certain somatic muscles and its action is mediated primarily via AMPA/kainate-like receptors, but at some chromatophores there are NMDA-like receptors.In the statocysts the afferent crista fibres are also glutamatergic, acting at non-NMDA receptors.In the brain (of Sepia) a neuronal NOS is activated by glutamate with subsequent production of nitric oxide and elevation of cGMP levels. This signal transduction pathway is blocked by D-AP-5, a specific antagonist of the NMDA receptor.Recently immunohistochemical analysis has demonstrated (in Sepia and Octopus) the presence of NMDAR2A /B - like receptors in motor centres, in the visual and olfactory systems and in the learning system. Physiological experiments have shown that glutamatergic transmission is involved in long term potentation (LTP) in the vertical lobe of Octopus, a brain area involved in learning. This effect seems to be mediated by non-NMDA receptors. Finally in the CNS of Sepia NMDA-mediated nitration of tyrosine residues of cytoskeletal protein such as alpha-tubulin, has been demonstrated.

Evidence that the swim afferent neurons of tritonia diomedea are glutamatergic

The escape swim response of the marine mollusc Tritonia diomedea is a well-established model system for studies of the neural basis of behavior. Although the swim neural network is reasonably well understood, little is known about the transmitters used by its constituent neurons. In the present study, we provide immunocytochemical and electrophysiological evidence that the S-cells, the afferent neurons that detect aversive skin stimuli and in turn trigger Tritonia's escape swim response, use glutamate as their transmitter. First, immunolabeling revealed that S-cell somata contain elevated levels of glutamate compared to most other neurons in the Tritonia brain, consistent with findings from glutamatergic neurons in many species. Second, pressure-applied puffs of glutamate produced the same excitatory response in the target neurons of the S-cells as the naturally released S-cell transmitter itself. Third, the glutamate receptor antagonist CNQX completely blocked S-cell synaptic connections. These findings support glutamate as a transmitter used by the S-cells, and will facilitate studies using this model system to explore a variety of issues related to the neural basis of behavior.

Cross-species comparison of metabolite profiles in chemosensory epithelia: an indication of metabolite roles in chemosensory cells

Comparative studies of chemosensory systems in vertebrates and invertebrates have greatly enhanced our understanding of anatomical and physiological constraints of chemical detection. Immunohistochemical comparisons of chemosensory systems are difficult to make across species due to limited cross-reactivity of mammalian-based antibodies. Immunostaining chemosensory tissues with glutaraldehyde-based antibodies generated against small metabolites in combination with hierarchical cluster analyses provide a novel approach for identifying and classifying cell types regardless of species. We used this "metabolite profiling" technique to determine whether metabolite profiles can be used to identify cell classes within and across different species including mouse, zebrafish, lobster and squid. Within a species, metabolite profiles for distinct cell classes were generally consistent. We found several metabolite-based cell classifications that mirrored function or receptor protein-based classifications. Although profiles of all six metabolites differed across species, we found that specific metabolites were associated with certain cell types. For example, elevated levels of glutathione were characteristic of nonsensory cells from vertebrates, suggesting an antioxidative role in non-neuronal cells in sensory tissues. Collectively, we found significantly different metabolite profiles for distinct cell populations in chemosensory tissue within all of the species studied. Based on their roles in other systems or cells, we discuss the roles of L-arginine, L-aspartate, L-glutamate, glycine, glutathione, and taurine within chemosensory epithelia.

Modulation of an AMPA-like glutamate receptor (SqGluR) gating by L- and D-aspartic acids

L- and D-aspartic acids (L-Asp and D-Asp) are present in the majority of nervous systems. In phylogeny, significant levels have been reported in mollusc brains, particularly cephalopods. To examine the role of L- and D-Asp on a cephalopod receptor, we studied ligand gating of a squid glutamate receptor (SqGluR) expressed in HEK 239 (human embryonic kidney) cells. Under voltage clamp, application of L-glutamate (L-Glu; 1-30 mM), but not D-glutamate (D-Glu), or L- or D-Asp, evoked an inward current of 0.1 nA. L- or D-Asp (200 microM) applied with 20 mM L-Glu, slowed the time course of activation and inactivation of the L-Glu gated current (time constant increased from 1 s (L-Glu alone) to 3 s (D-Asp and L-Glu) and to 19 s (L-Asp and L-Glu)). Our results suggest that in molluscan systems, aspartic acid could act as a neuromodulator during glutamatergic transmission and could significantly alter synaptic integration by slowing glutamate receptor gating.

N-methyl-D-aspartate receptor-like immunoreactivity in the brain of Sepia and Octopus

Ionotropic glutamate receptors have been subdivided into N-methyl-D-aspartate (NMDA) and AMPA/kainate classes. NMDA receptor subunit 2A and 2B immunoreactivity is shown to be present in specific regions of the central nervous system (CNS) of the cephalopod molluscs Sepia officinalis and Octopus vulgaris. An antibody that recognizes both mammalian NMDAR2A and NMDAR2B subunits equally was used. SDS-PAGE/Western blot analysis performed on membrane proteins revealed an immunoreactive band at 170 kDa for both species. Immunoreactive bands from both Octopus and Sepia brains disappeared when the antibody was preabsorbed with membrane proteins from rat hippocampus or from their own brains. The same antibody was then used for immunohistochemical staining of serial sections of the CNS to reveal localized specific staining of cell bodies and fibers in several lobes of the brain. Staining was found in lower motor centers, in some higher motor centers, in learning centers, and in the optic lobes. Immunopositivity was also found in the areas of brain that control the activity of the optic gland, a gonadotropic endocrine gland. These findings suggest that glutamate, via NMDA receptors, may be involved as a signaling molecule in motor, learning, visual, and olfactory systems in the cephalopod brain.

Cloning and characterization of an ionotropic glutamate receptor subunit expressed in the squid nervous system

In this paper we describe the cloning of a putative ionotropic glutamate receptor subunit, SqGluR, and its distribution in the nervous system of the squid. A full-length cDNA was assembled from a cDNA library of the stellate ganglion/giant fibre lobe complex of Loligo opalescens. The deduced amino acid sequence of the mature SqGluR displayed 44-46% amino acid identity with mammalian GluR1-GluR4 and 53% with Lym-eGluR1 from Lymnaea stagnalis. In situ hybridizations in adult squid confirmed that the SqGluR mRNA is abundant in giant fibre lobe neurons, in large, presumptive motor neurons of the stellate ganglion proper and in the supraoesophageal and optic lobes of the central nervous system. In newborn squid, SqGluR mRNA expression was detected throughout the nervous system but not elsewhere. A synthetic peptide corresponding to the last 15 amino acids of the SqGluR C-terminus was used to generate polyclonal antibodies, which were used for immunoblot analysis to demonstrate widespread expression in the squid central and peripheral nervous systems. Injection of the synthetic peptide into the postsynaptic side of the giant synapse inhibited synaptic transmission.

AMPA/kainate and NMDA-like glutamate receptors at the chromatophore neuromuscular junction of the squid: role in synaptic transmission and skin patterning

Glutamate receptor types were examined at the chromatophore synapses of the squids Alloteuthis subulata and Loligo vulgaris, where nerve-induced muscle contraction causes chromatophore expansion. Immunoblotting with antibody raised against a squid AMPA receptor (sGluR) demonstrated that AMPA/kainate receptors are present in squid skin. Application of l-glutamate evoked chromatophore muscle contractions in both ventral and dorsal skins, while NMDA was only active on a subpopulation of dorsal chromatophores. In dorsal skin, neurotransmission was partly blocked by either AMPA/kainate receptor antagonists (CNQX and DNQX) or NMDA receptor antagonists (AP-5 and MK-801) or completely blocked by simultaneous application of both classes of antagonists. In isolated muscle fibres, ionophoretic application of l-glutamate evoked fast inward CNQX- and DNQX-sensitive currents with reversal potentials around +14 mV and a high conductance to Na+. In fibres from dorsal skin only, a slower outward glutamate-sensitive current appeared at positive holding potentials. At negative potentials, currents were potentiated by glycine or by removing external Mg2+ and were blocked by AP-5 and MK-801. Glutamate caused a fast, followed by a slow, transient increase in cytoplasmic Ca2+. The slow component was increased in amplitude and duration by glycine or by lowering external Mg2+ and decreased by AP-5 and MK-801. In cells from ventral skin, no 'NMDA-like responses' were detected. Thus, while AMPA/kainate receptors mediated fast excitatory synaptic transmission and rapid colour change over the whole skin, activation of both AMPA/kainate and NMDA-like receptors in a subpopulation of dorsal chromatophores prolonged the postsynaptically evoked Ca2+ elevation causing temporally extended colour displays with behavioural significance.

Effect of glycine on synaptic transmission at the third order giant synapse of the squids Alloteuthis subulata and Loligo vulgaris

Intracellular microelectrode recordings were made from presynaptic and postsynaptic regions of the third order giant synapses of the squids Alloteuthis subulata and Loligo vulgaris. Synaptically generated postsynaptic action potential trains, and excitatory postsynaptic potentials (EPSPs) were reversibly decreased by glycine, beta - alanine or taurine while presynaptic action potentials (APs) were unaltered. Glycine was effective in the presence of strychnine (30-50 microM), NMDA (500 microM), AP-5 (50 microM), CPP (100 microM), or MK 801 (which also had no effect on normal synaptic transmission). The glycine effect was reduced reversibly by D-tubocurarine (100 microM) and blocked by reducing extracellular chloride by 50% with propionate. Excitatory postsynaptic currents (EPSCs) were decreased by glycine addition without altering resting membrane conductance. We postulate that glycine or a glycine like substance provides an excitatory postsynaptic input during synaptic stimulation. Bath addition of glycine desensitises these receptors and decreases the amplitude of the EPSPs and EPSCs. Modulation of this synaptic input may provide an effective mechanism to suppress or potentiate synaptic transmission in the squid giant synapse.