Co-expression of VGLUT1 and VGAT sustains glutamate and GABA co-release and is regulated by activity in cortical neurons

Glucose transporter-2 regulation of VMN GABA neuron metabolic sensor and transmitter gene expression

Glucose transporter-2 (GLUT2) monitors cellular glucose uptake. Astrocyte GLUT2 controls glucose counterregulatory hormone secretion. In vivo gene silencing and laser-catapult-microdissection tools were used here to investigate whether ventromedial hypothalamic nucleus (VMN) GLUT2 may regulate dorsomedial (VMNdm) and/or ventrolateral (VMNvl) γ-aminobutyric acid (GABA) neurotransmission to control this endocrine outflow in female rats. VMN GLUT2 gene knockdown suppressed or stimulated hypoglycemia-associated glutamate decarboxylase (GAD)1 and GAD2 mRNA expression in VMNdm versus VMNvl GABAergic neurons, respectively. GLUT2 siRNA pretreatment also modified co-expressed transmitter marker gene profiles in each cell population. VMNdm GABA neurons exhibited GLUT2 knockdown-sensitive up-regulated 5'-AMP-activated protein kinase-alpha1 (AMPKα1) and -alpha2 (AMPKα2) transcripts during hypoglycemia. Hypoglycemic augmentation of VMNvl GABA neuron AMPKα2 was refractory to GLUT2 siRNA. GLUT2 siRNA blunted (VMNdm) or exacerbated (VMNvl) hypoglycemic stimulation of GABAergic neuron steroidogenic factor-1 (SF-1) mRNA. Results infer that VMNdm and VMNvl GABA neurons may exhibit divergent, GLUT2-dependent GABA neurotransmission patterns in the hypoglycemic female rat. Data also document differential GLUT2 regulation of VMNdm versus VMNvl GABA nerve cell SF-1 gene expression. Evidence for intensification of hypoglycemic hypercorticosteronemia and -glucagonemia by GLUT2 siRNA infers that VMN GLUT2 function imposes an inhibitory tone on these hormone profiles in this sex.

NGF Eye Administration Recovers the TrkB and Glutamate/GABA Marker Deficit in the Adult Visual Cortex Following Optic Nerve Crush

Eye-drop recombinant human nerve growth factor (ed-rhNGF) has proved to recover the retina and optic nerve damage in animal models, including the unilateral optic nerve crush (ONC), and to improve visual acuity in humans. These data, associated with evidence that ed-rhNGF stimulates the brain derived neurotrophic factor (BDNF) in retina and cortex, suggests that NGF might exert retino-fugal effects by affecting BDNF and its receptor TrkB. To address these questions, their expression and relationship with the GABAergic and glutamatergic transmission markers, GAD65 and GAD67, vesicular inhibitory amino acid transporter (VGAT), and vesicular glutamate transporters 1 and 2 (VGLUT-1 and VGLUT-2) were investigated in adult ONC rats contralateral and ipsilateral visual cortex (VCx). Ed-rhNGF recovers the ONC-induced alteration of GABAergic and glutamatergic markers in contralateral VCx, induces an upregulation of TrkB, which is positively correlated with BDNF precursor (proBDNF) decrease in both VCx sides, and strongly enhances TrkB+ cell soma and neuronal endings surrounded by GAD65 immuno-reactive afferents. These findings contribute to enlarging the knowledge on the mechanism of actions and cellular targets of exogenously administrated NGF, and suggest that ed-rhNGF might act by potentiating the activity-dependent TrkB expression in GAD+ cells in VCx following retina damage and/or ONC.

Periaqueductal Gray and Rostromedial Tegmental Inhibitory Afferents to VTA Have Distinct Synaptic Plasticity and Opiate Sensitivity

The ventral tegmental area (VTA) is a major target of addictive drugs and receives multiple GABAergic projections originating outside the VTA. We describe differences in synaptic plasticity and behavior when optogenetically driving two opiate-sensitive GABAergic inputs to the VTA, the rostromedial tegmental nucleus (RMTg), and the periaqueductal gray (PAG). Activation of GABAergic RMTg terminals in the VTA in vivo is aversive, and low-frequency stimulation induces long-term depression in vitro. Low-frequency stimulation of PAG afferents in vitro unexpectedly causes long-term potentiation. Opioid receptor activation profoundly depresses PAG and RMTg inhibitory synapses but prevents synaptic plasticity only at PAG synapses. Activation of the GABAergic PAG terminals in the VTA promotes immobility, and optogenetically-driven immobility is blocked by morphine. Our data reveal the PAG as a source of highly opioid-sensitive GABAergic afferents and support the idea that different GABAergic pathways to the VTA control distinct behaviors.

Characterization of Drosophila octopamine receptor neuronal expression using MiMIC-converted Gal4 lines

Octopamine, the invertebrate analog of norepinephrine, is known to modulate a large variety of behaviors in Drosophila including feeding initiation, locomotion, aggression, and courtship, among many others. Significantly less is known about the identity of the neurons that receive octopamine input and how they mediate octopamine-regulated behaviors. Here, we characterize adult neuronal expression of MiMIC-converted Trojan-Gal4 lines for each of the five Drosophila octopamine receptors. Broad neuronal expression was observed for all five octopamine receptors, yet distinct differences among them were also apparent. Use of immunostaining for the octopamine neurotransmitter synthesis enzyme Tdc2, along with a novel genome-edited conditional Tdc2-LexA driver, revealed all five octopamine receptors express in Tdc2/octopamine neurons to varying degrees. This suggests autoreception may be an important circuit mechanism by which octopamine modulates behavior.

The effect of magnetic stimulation on differentiation of human induced pluripotent stem cells into neuron

The effect of stem cell transplantation in the treatment of neural lesions is so far not satisfactory. Magnetic stimulation is a feasible exogenous interference to improve transplantation outcome. However, the effect of magnetic stimulation on the differentiation of induced pluripotent stem cells (iPSCs) into neuron has not been studied. In this experiment, an in vitro neuron differentiation system from human iPSCs were established and confirmed. Three magnetic stimuli (high frequency [HF], low frequency [LF], intermittent theta-burst stimulation [iTBS]) were applied twice a day during the differentiation process. Immunofluorescence and quantitative polymerase chain reaction (Q-PCR) were performed to analyze the effect of magnetic stimulation. Neural stem cells were obtained on day 12, manifested as floating neurospheres expressing neural precursor markers. All groups can differentiate into neurons while glial cell markers were not detected. Both Immunofluorescence and PCR results showed LF and iTBS increased the transcription and expression of neuronal nuclei (NeuN). HF significantly increased vesicular glutamate transporters2 transcription while iTBS promoted transcription of both synaptophysin and postsynaptic density protein 95. These results indicate that LF and iTBS can promote the generation of mature neurons from human iPSCs; HF may promote differentiate into glutamatergic neurons while iTBS may promote synapse formation during the differentiation.

Inhibitory regulation of the prefrontal cortex following behavioral sensitization to amphetamine and/or methamphetamine psychostimulants: A review of GABAergic mechanisms

Behavioral sensitization to repeated psychostimulant administration has been proposed to reflect many of the neurochemical and behavioral changes that are characteristic of a range of disorders, including drug addiction and psychoses. While previous studies have examined the role of dopamine and glutamate neurotransmission in mediating sensitization, particularly within the prefrontal cortex (PFC), the role of inhibitory GABAergic processing of the PFC in the expression of sensitization is not well understood. Recent research, however, has proposed an emerging role of GABA synthesis, reuptake, ionotropic and metabotropic receptor regulation, and interneuronal changes following sensitization to methamphetamine and/or amphetamine within the PFC. The aim of this review, therefore, is to synthesize research findings on changes to the GABAergic network following sensitization induced by amphetamines (i.e., amphetamine and/or methamphetamine) in the PFC. In addition to providing an overview of global PFC changes, we also provide evidence of regional specific inhibitory influences on sensitized circuitry, focusing on the prelimbic and orbitofrontal cortices. We propose a neural circuit through which inhibitory PFC GABA changes mediate sensitized disease states, focusing on the interaction between the prelimbic and orbitofrontal cortices with subcortical brain structures and the mesolimbic system. Methodological considerations and avenues for future research are also discussed.

Selective Brain Distribution and Distinctive Synaptic Architecture of Dual Glutamatergic-GABAergic Neurons

For decades, it has been thought that glutamate and GABA are released by distinct neurons. However, some mouse neurons innervating the lateral habenula (LHb) co-release glutamate and GABA. Here, we mapped the distribution of neurons throughout the rat brain that co-express vesicular transporters for the accumulation of glutamate (VGluT2) or GABA (VGaT) and for GABA synthesis (GAD). We found concentrated groups of neurons that co-express VGluT2, VGaT, and GAD mRNAs within subdivisions of the ventral tegmental area (VTA), entopeduncular (EPN), and supramammillary (SUM) nuclei. Single axon terminals established by VTA, EPN, or SUM neurons form a common synaptic architecture involving asymmetric (putative excitatory) and symmetric (putative inhibitory) synapses. Within the LHb, which receives co-transmitted glutamate and GABA from VTA and EPN, VGluT2 and VGaT are distributed on separate synaptic vesicles. We conclude that single axon terminals from VGluT2 and VGaT co-expressing neurons co-transmit glutamate and GABA from distinct synaptic vesicles at independent synapses.

Molecular Plasticity of the Nucleus Accumbens Revisited-Astrocytic Waves Shall Rise

Part of the ventral striatal division, the nucleus accumbens (NAc) drives the circuit activity of an entire macrosystem about reward like a "flagship," signaling and leading diverse conducts. Accordingly, NAc neurons feature complex inhibitory phenotypes that assemble to process circuit inputs and generate outputs by exploiting specific arrays of opposite and/or parallel neurotransmitters, neuromodulatory peptides. The resulting complex combinations enable versatile yet specific forms of accumbal circuit plasticity, including maladaptive behaviors. Although reward signaling and behavior are elaborately linked to neuronal circuit activities, it is plausible to propose whether these neuronal ensembles and synaptic islands can be directly controlled by astrocytes, a powerful modulator of neuronal activity. Pioneering studies showed that astrocytes in the NAc sense citrate cycle metabolites and/or ATP and may induce recurrent activation. We argue that the astrocytic calcium, GABA, and Glu signaling and altered sodium and chloride dynamics fundamentally shape metaplasticity by providing active regulatory roles in the synapse- and network-level flexibility of the NAc.

Glutamate/GABA co-release selectively influences postsynaptic glutamate receptors in mouse cortical neurons

Cultured rat cortical neurons co-expressing VGLUT1 and VGAT (mixed synapses) co-release Glu and GABA. Here, mixed synapses were studied in cultured mouse cortical neurons to verify whether in mice mixed synapses co-release Glu and GABA, and to gain insight into how they may influence excitation/inhibition balance. Results showed the existence of synapses and autapses that co-release Glu and GABA in cultured mouse cortical neurons, and the ability of both neurotransmitters to evoke postsynaptic responses mediated by ionotropic receptors. We studied the short-term plasticity of glutamatergic, GABAergic, and mixed responses and we found that the kinetics of mixPSC amplitude depression was similar to that observed in EPSCs, but it was different from that of IPSCs. We found similar presynaptic release characteristics in glutamatergic and mixed synapses. Analysis of postsynaptic features, obtained by measuring AMPAR- and NMDAR-mediated currents, showed that AMPAR-mediated currents were significantly higher in pure glutamatergic than in mixed synapses, whereas NMDAR-mediated currents were not significantly different from those measured in mixed synapses. Overall, our findings demonstrate that glutamatergic and mixed synapses share similar electrophysiological properties. However, co-release of GABA and Glu influences postsynaptic ionotropic glutamatergic receptor subtypes, thus selectively influencing AMPAR-mediated currents. These findings strengthen the view that mixed neurons can play a key role in CNS development and in maintaining the excitation-inhibition balance.

Parallel visual circuitry in a basal chordate

A common CNS architecture is observed in all chordates, from vertebrates to basal chordates like the ascidian Ciona. Ciona stands apart among chordates in having a complete larval connectome. Starting with visuomotor circuits predicted by the Ciona connectome, we used expression maps of neurotransmitter use with behavioral assays to identify two parallel visuomotor circuits that are responsive to different components of visual stimuli. The first circuit is characterized by glutamatergic photoreceptors and responds to the direction of light. These photoreceptors project to cholinergic motor neurons, via two tiers of cholinergic interneurons. The second circuit responds to changes in ambient light and mediates an escape response. This circuit uses GABAergic photoreceptors which project to GABAergic interneurons, and then to cholinergic interneurons. Our observations on the behavior of larvae either treated with a GABA receptor antagonist or carrying a mutation that eliminates photoreceptors indicate the second circuit is disinhibitory.

Few, Activity-Dependent, and Ubiquitous VGLUT1/VGAT Terminals in Rat and Mouse Brain

In the neocortex of adult rats VGLUT1 and VGAT co-localize in axon terminals which form both symmetric and asymmetric synapses. They are expressed in the same synaptic vesicles which participate in the exo-endocytotic cycle. Virtually nothing, however, is known on whether VGLUT1/VGAT co-localization occurs in other brain regions. We therefore mapped the distribution of terminals co-expressing VGLUT1/VGAT in the striatum, hippocampus, thalamus, and cerebellar and cerebral cortices of rats and mice. Confocal microscopy analysis revealed that, in both rat and mouse brain, VGLUT1/VGAT+ terminals were present in all brain regions studied, and that their percentage was low and comparable in both species. These results provide the first demonstration that co-expression of VGLUT1 and VGAT is a widespread phenomenon. Since VGLUT1/VGAT+ axon terminals are regulated in an activity-dependent manner and co-release glutamate and GABA, we hypothesize that, though not numerous, they can contribute to regulating excitation/inhibition balance in physiological conditions, thereby playing a role in several neurological and psychiatric diseases.

Insufficient developmental excitatory neuronal activity fails to foster establishment of normal levels of inhibitory neuronal activity

The nervous system is composed of excitatory and inhibitory neurons. One major class of inhibitory neurons release the neurotransmitter γ-Aminobutyric acid (GABA). GABAergic inhibitory activity maintains the balance that is disrupted in conditions such as epilepsy. At least some GABAergic neurons are initially excitatory and undergo a developmental conversion to convert to inhibitory neurons. The mechanism(s) behind this conversion are thought to include a critical developmental increase in excitatory activity. To test this hypothesis, we subjected ex vivo developing neuronal networks on multi-electrode arrays to various stimulation and pharmacological regimens. Synaptic activity of networks initially consists of epileptiform-like high-amplitude individual "spikes", which convert to organized bursts of activity over the course of approximately 1 month. Stimulation of networks with a digitized synaptic signal for 5days hastened the decrease of epileptiform activity. By contrast, stimulation for a single day delayed the appearance of bursts and instead increased epileptiform signaling. GABA treatment reduced total signals in unstimulated networks and networks stimulated for 5days, but instead increased signaling in networks stimulated for 1day. This increase was prevented by co-treatment with (2R)-amino-5-phosphonopentanoate and 6-cyano-7-nitroquinoxaline-2,3-dione, confirming that GABA invoked excitatory activity in networks stimulated for 1day. Glutamate increased signals in networks subjected to all stimulation regimens; the GABA receptor antagonist bicuculline prevented this increase only in networks stimulated for 1day. These latter findings are consistent with the induction of so-called "mixed" synapses (which release a combination of excitatory and inhibitory neurotransmitters) in networks stimulated for 1day, and support the hypothesis that a critical level of excitatory activity fosters the developmental transition of GABAergic neurons from excitatory to inhibitory.

VTA Projection Neurons Releasing GABA and Glutamate in the Dentate Gyrus

Both dopamine and nondopamine neurons from the ventral tegmental area (VTA) project to a variety of brain regions. Here we examine nondopaminergic neurons in the mouse VTA that send long-range projections to the hippocampus. Using a combination of retrograde tracers, optogenetic tools, and electrophysiological recordings, we show that VTA GABAergic axons make synaptic contacts in the granule cell layer of the dentate gyrus, where we can elicit small postsynaptic currents. Surprisingly, the currents displayed a partial sensitivity to both bicuculline and NBQX, suggesting that these mesohippocampal neurons corelease both GABA and glutamate. Finally, we show that this projection is functional in vivo and its stimulation reduces granule cell-firing rates under anesthesia. Altogether, the present results describe a novel connection between GABA and glutamate coreleasing of cells of the VTA and the dentate gyrus. This connection could be relevant for a variety of functions, including reward-related memory and neurogenesis.

Multiplexed neurochemical signaling by neurons of the ventral tegmental area

The ventral tegmental area (VTA) is an evolutionarily conserved structure that has roles in reward-seeking, safety-seeking, learning, motivation, and neuropsychiatric disorders such as addiction and depression. The involvement of the VTA in these various behaviors and disorders is paralleled by its diverse signaling mechanisms. Here we review recent advances in our understanding of neuronal diversity in the VTA with a focus on cell phenotypes that participate in 'multiplexed' neurotransmission involving distinct signaling mechanisms. First, we describe the cellular diversity within the VTA, including neurons capable of transmitting dopamine, glutamate or GABA as well as neurons capable of multiplexing combinations of these neurotransmitters. Next, we describe the complex synaptic architecture used by VTA neurons in order to accommodate the transmission of multiple transmitters. We specifically cover recent findings showing that VTA multiplexed neurotransmission may be mediated by either the segregation of dopamine and glutamate into distinct microdomains within a single axon or by the integration of glutamate and GABA into a single axon terminal. In addition, we discuss our current understanding of the functional role that these multiplexed signaling pathways have in the lateral habenula and the nucleus accumbens. Finally, we consider the putative roles of VTA multiplexed neurotransmission in synaptic plasticity and discuss how changes in VTA multiplexed neurons may relate to various psychopathologies including drug addiction and depression.

Co-existence of Functionally Different Vesicular Neurotransmitter Transporters

The vesicular transmitter transporters VGLUT, VGAT, VMAT2 and VAChT, define phenotype and physiological properties of neuronal subtypes. VGLUTs concentrate the excitatory amino acid glutamate, VGAT the inhibitory amino acid GABA, VMAT2 monoamines, and VAChT acetylcholine (ACh) into synaptic vesicle (SV). Following membrane depolarization SV release their content into the synaptic cleft. A strict segregation of vesicular transporters is mandatory for the precise functioning of synaptic communication and of neuronal circuits. In the last years, evidence accumulates that subsets of neurons express more than one of these transporters leading to synaptic co-release of different and functionally opposing transmitters and modulation of synaptic plasticity. Synaptic co-existence of transporters may change during pathological scenarios in order to ameliorate misbalances in neuronal activity. In addition, evidence increases that transporters also co-exist on the same vesicle providing another layer of regulation. Generally, vesicular transmitter loading relies on an electrochemical gradient ΔμH⁺ driven by the proton ATPase rendering the lumen of the vesicle with respect to the cytosol positive (Δψ) and acidic (ΔpH). While the activity of VGLUT mainly depends on the Δψ component, VMAT, VGAT and VAChT work best at a high ΔpH. Thus, a vesicular synergy of transporters depending on the combination may increase or decrease the filling of SV with the principal transmitter. We provide an overview on synaptic co-existence of vesicular transmitter transporters including changes in the excitatory/inhibitory balance under pathological conditions. Additionally, we discuss functional aspects of vesicular synergy of transmitter transporters.

Subventricular zone neural progenitors reverse TNF-alpha effects in cortical neurons

INTRODUCTION

Tumor necrosis factor alpha (TNFα) plays a physiological role in controlling synaptic transmission and plasticity in the healthy central nervous system by modulating glutamate receptor trafficking to the plasma membrane. TNFα expression is also rapidly induced in response to tissue injury and infection. By promoting the insertion of Ca(2+) permeable-AMPA receptors into the neuronal plasma membrane, this cytokine may cause excessive Ca(2+) influx into neurons, thus enhancing neuronal death.

METHODS

Primary cultures of cortical neurons were obtained from E18 foetal mice and incubated for 24 h with adult neural stem cells (aNPCs) either stimulated with lipopolysaccharide (LPS(+)aNPCs) or not (aNPCs). Cultures were treated with TNFα (100 ng/ml), and electrophysiological recordings were performed in different conditions to evaluate the effect of the cytokine on neuronal transmission.

RESULTS

In this study, we demonstrate that aNPCs from the subventricular zone reverse the effects induced by the cytokine. Moreover, we show that the effect of aNPCs on cortical neurons is mediated by cannabinoid CB1 receptor activation.

CONCLUSION

These data suggest that the role of aNPCs in preventing excitatory neurotransmission potentiation induced by TNFα on cortical neurons may have important implications for pathologies characterized by an inflammatory component affecting cortical neurons such as Alzheimer's disease.