Neurotransmitter corelease: mechanism and physiological role

Substrates for Neuronal Cotransmission With Neuropeptides and Small Molecule Neurotransmitters in Drosophila

It has been known for more than 40 years that individual neurons can produce more than one neurotransmitter and that neuropeptides often are colocalized with small molecule neurotransmitters (SMNs). Over the years much progress has been made in understanding the functional consequences of cotransmission in the nervous system of mammals. There are also some excellent invertebrate models that have revealed roles of coexpressed neuropeptides and SMNs in increasing complexity, flexibility, and dynamics in neuronal signaling. However, for the fly Drosophila there are surprisingly few functional studies on cotransmission, although there is ample evidence for colocalization of neuroactive compounds in neurons of the CNS, based both on traditional techniques and novel single cell transcriptome analysis. With the hope to trigger interest in initiating cotransmission studies, this review summarizes what is known about Drosophila neurons and neuronal circuits where different neuropeptides and SMNs are colocalized. Coexistence of neuroactive substances has been recorded in different neuron types such as neuroendocrine cells, interneurons, sensory cells and motor neurons. Some of the circuits highlighted here are well established in the analysis of learning and memory, circadian clock networks regulating rhythmic activity and sleep, as well as neurons and neuroendocrine cells regulating olfaction, nociception, feeding, metabolic homeostasis, diuretic functions, reproduction, and developmental processes. One emerging trait is the broad role of short neuropeptide F in cotransmission and presynaptic facilitation in a number of different neuronal circuits. This review also discusses the functional relevance of coexisting peptides in the intestine. Based on recent single cell transcriptomics data, it is likely that the neuronal systems discussed in this review are just a fraction of the total set of circuits where cotransmission occurs in Drosophila. Thus, a systematic search for colocalized neuroactive compounds in further neurons in anatomically defined circuits is of interest for the near future.

Target selectivity of septal cholinergic neurons in the medial and lateral entorhinal cortex

Acetylcholine is a key modulator of hippocampal and entorhinal cortex (EC) function. The majority of cholinergic projections targeting these structures originate in the basal forebrain complex, specifically the medial septum. Many studies focused on the behavioral effects involving these projections, but there still is a paucity regarding their connectivity in the target area. Here we provide this missing link. By combining optogenetics with whole-cell recordings in superficial EC layers, we identified the synaptic target cells of septal cholinergic neurons. This level of analysis is an important step toward a better understanding of the modulatory action of acetylcholine in EC in vivo.

The entorhinal cortex (EC) plays a pivotal role in processing and conveying spatial information to the hippocampus. It has long been known that EC neurons are modulated by cholinergic input from the medial septum. However, little is known as to how synaptic release of acetylcholine affects the different cell types in EC. Here we combined optogenetics and patch-clamp recordings to study the effect of cholinergic axon stimulation on distinct neurons in EC. We found dense cholinergic innervations that terminate in layer I and II (LI and LII). Light-activated stimulation of septal cholinergic projections revealed differential responses in excitatory and inhibitory neurons in LI and LII of both medial and lateral EC. We observed depolarizing responses mediated by nicotinic and muscarinic receptors primarily in putative serotonin receptor (p5HT₃R)-expressing interneurons. Hyperpolarizing muscarinic receptor-mediated responses were found predominantly in excitatory cells. Additionally, some excitatory as well as a higher fraction of inhibitory neurons received mono- and/or polysynaptic GABAergic inputs, revealing that medial septum cholinergic neurons have the capacity to corelease GABA alongside acetylcholine. Notably, the synaptic effects of acetylcholine were similar in neurons of both medial and lateral EC. Taken together, our findings demonstrate that EC activity may be differentially modulated via the activation or the suppression of distinct subsets of LI and LII neurons by the septal cholinergic system.

Dual-transmitter systems regulating arousal, attention, learning and memory

An array of neuromodulators, including monoamines and neuropeptides, regulate most behavioural and physiological traits. In the past decade, dramatic progress has been made in mapping neuromodulatory circuits, in analysing circuit dynamics, and interrogating circuit function using pharmacogenetic, optogenetic and imaging methods This review will focus on several distinct neural networks (acetylcholine/GABA/glutamate; histamine/GABA; orexin/glutamate; and relaxin-3/GABA) that originate from neural hubs that regulate wakefulness and related attentional and cognitive processes, and highlight approaches that have identified dual transmitter roles in these behavioural functions. Modulation of these different neural networks might be effective treatments of diseases related to arousal/sleep dysfunction and of cognitive dysfunction in psychiatric and neurodegenerative disorders.

Role for VGLUT2 in selective vulnerability of midbrain dopamine neurons

Parkinson's disease is characterized by the loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc). DA neurons in the ventral tegmental area are more resistant to this degeneration than those in the SNc, though the mechanisms for selective resistance or vulnerability remain poorly understood. A key to elucidating these processes may lie within the subset of DA neurons that corelease glutamate and express the vesicular glutamate transporter VGLUT2. Here, we addressed the potential relationship between VGLUT expression and DA neuronal vulnerability by overexpressing VGLUT in DA neurons of flies and mice. In Drosophila, VGLUT overexpression led to loss of select DA neuron populations. Similarly, expression of VGLUT2 specifically in murine SNc DA neurons led to neuronal loss and Parkinsonian behaviors. Other neuronal cell types showed no such sensitivity, suggesting that DA neurons are distinctively vulnerable to VGLUT2 expression. Additionally, most DA neurons expressed VGLUT2 during development, and coexpression of VGLUT2 with DA markers increased following injury in the adult. Finally, conditional deletion of VGLUT2 made DA neurons more susceptible to Parkinsonian neurotoxins. These data suggest that the balance of VGLUT2 expression is a crucial determinant of DA neuron survival. Ultimately, manipulation of this VGLUT2-dependent process may represent an avenue for therapeutic development.

Von Economo Neurons and Fork Cells: A Neurochemical Signature Linked to Monoaminergic Function

The human anterior cingulate and frontoinsular cortices are distinguished by 2 unique Layer 5 neuronal morphotypes, the von Economo neurons (VENs) and fork cells, whose biological identity remains mysterious. Insights could impact research on diverse neuropsychiatric diseases to which these cells have been linked. Here, we leveraged the Allen Brain Atlas to evaluate mRNA expression of 176 neurotransmitter-related genes and identified vesicular monoamine transporter 2 (VMAT2), gamma-aminobutyric acid (GABA) receptor subunit θ (GABRQ), and adrenoreceptor α-1A (ADRA1A) expression in human VENs, fork cells, and a minority of neighboring Layer 5 neurons. We confirmed these results using immunohistochemistry or in situ hybridization. VMAT2 and GABRQ expression was absent in mouse cerebral cortex. Although VMAT2 is known to package monoamines into synaptic vesicles, in VENs and fork cells its expression occurs in the absence of monoamine-synthesizing enzymes or reuptake transporters. Thus, VENs and fork cells may possess a novel, uncharacterized mode of cortical monoaminergic function that distinguishes them from most other mammalian Layer 5 neurons.

A bio-image sensor for simultaneous detection of multi-neurotransmitters

We report here a new bio-image sensor for simultaneous detection of spatial and temporal distribution of multi-neurotransmitters. It consists of multiple enzyme-immobilized membranes on a 128 × 128 pixel array with read-out circuit. Apyrase and acetylcholinesterase (AChE), as selective elements, are used to recognize adenosine 5'-triphosphate (ATP) and acetylcholine (ACh), respectively. To enhance the spatial resolution, hydrogen ion (H⁺) diffusion barrier layers are deposited on top of the bio-image sensor and demonstrated their prevention capability. The results are used to design the space among enzyme-immobilized pixels and the null H⁺ sensor to minimize the undesired signal overlap by H⁺ diffusion. Using this bio-image sensor, we can obtain H⁺ diffusion-independent imaging of concentration gradients of ATP and ACh in real-time. The sensing characteristics, such as sensitivity and detection of limit, are determined experimentally. With the proposed bio-image sensor the possibility exists for customizable monitoring of the activities of various neurochemicals by using different kinds of proton-consuming or generating enzymes.

Disrupting Glutamate Co-transmission Does Not Affect Acquisition of Conditioned Behavior Reinforced by Dopamine Neuron Activation

Dopamine neurons in the ventral tegmental area (VTA) were previously found to express vesicular glutamate transporter 2 (VGLUT2) and to co-transmit glutamate in the ventral striatum (VStr). This capacity may play an important role in reinforcement learning. Although it is known that activation of the VTA-VStr dopamine system readily reinforces behavior, little is known about the role of glutamate co-transmission in such reinforcement. By combining electrode recording and optogenetics, we found that stimulation of VTA dopamine neurons in vivo evoked fast excitatory responses in many VStr neurons of adult mice. Whereas conditional knockout of the gene encoding VGLUT2 in dopamine neurons largely eliminated fast excitatory responses, it had little effect on the acquisition of conditioned responses reinforced by dopamine neuron activation. Therefore, glutamate co-transmission appears dispensable for acquisition of conditioned responding reinforced by DA neuron activation.

Very low doses of muscimol and baclofen ameliorate cognitive deficits and regulate protein expression in the brain of a rat model of streptozocin-induced Alzheimer's disease

Recent studies devoted to neuroprotection have focused on the role of the gamma-aminobutyric acid (GABA) system in regulating neuroinflammatory processes which play a key role in the neurodegenerative processes observed in Alzheimer's disease (AD) by inducing glial cell overactivation and impairing neurotransmission. Data on the efficacy of classical GABA-A and GABA-B receptor agonists (muscimol and baclofen, respectively) in animal models of AD are not available. Moreover, no published studies have examined the ability of optimal doses of these compounds to prevent neuroinflammation, the alterations in neurotransmission and cognitive deficits. In the present study, we used a non-transgenic rat model of AD obtained by intracerebroventricular streptozocin (STZ) injection and assessed the effects of muscimol and baclofen at very low doses (0.01-0.05mg/kg) on spatial memory and the expression of cortical and hippocampal proteins related to neuroinflammation, namely proteins involved in astroglial functions (glial fibrillary acidic protein, GFAP), GABA synthesis (GABA synthesizing enzyme, glutamic acid decarboxylase 67, GAD67) and acetylcholine degradation (acetylcholine esterase). The presented study demonstrated that in a rat model of STZ-induced AD both muscimol and baclofen at the tested doses exerted memory-enhancing and anti-inflammatory effects, as well as normalization of acetylcholine esterase and GABA expression. We suggested that the function of very low doses of GABA receptor agonists differs from typical GABA-related inhibition and may be mediated by the allosteric sites of GABA receptors or other non-specific cell regulatory pathways.

Neurochemical Heterogeneity Among Lateral Hypothalamic Hypocretin/Orexin and Melanin-Concentrating Hormone Neurons Identified Through Single-Cell Gene Expression Analysis

The lateral hypothalamic area (LHA) lies at the intersection of multiple neural and humoral systems and orchestrates fundamental aspects of behavior. Two neuronal cell types found in the LHA are defined by their expression of hypocretin/orexin (Hcrt/Ox) and melanin-concentrating hormone (MCH) and are both important regulators of arousal, feeding, and metabolism. Conflicting evidence suggests that these cell populations have a more complex signaling repertoire than previously appreciated, particularly in regard to their coexpression of other neuropeptides and the machinery for the synthesis and release of GABA and glutamate. Here, we undertook a single-cell expression profiling approach to decipher the neurochemical phenotype, and heterogeneity therein, of Hcrt/Ox and MCH neurons. In transgenic mouse lines, we used single-cell quantitative polymerase chain reaction (qPCR) to quantify the expression of 48 key genes, which include neuropeptides, fast neurotransmitter components, and other key markers, which revealed unexpected neurochemical diversity. We found that single MCH and Hcrt/Ox neurons express transcripts for multiple neuropeptides and markers of both excitatory and inhibitory fast neurotransmission. Virtually all MCH and approximately half of the Hcrt/Ox neurons sampled express both the machinery for glutamate release and GABA synthesis in the absence of a vesicular GABA release pathway. Furthermore, we found that this profile is characteristic of a subpopulation of LHA glutamatergic neurons but contrasts with a broad population of LHA GABAergic neurons. Identifying the neurochemical diversity of Hcrt/Ox and MCH neurons will further our understanding of how these populations modulate postsynaptic excitability through multiple signaling mechanisms and coordinate diverse behavioral outputs.

Estrogen receptor alpha is required in GABAergic, but not glutamatergic, neurons to masculinize behavior

Masculinization of the altricial rodent brain is driven by estrogen signaling during a perinatal critical period. Genetic deletion of estrogen receptor alpha (Esr1/ERα) results in altered hypothalamic-pituitary-gonadal (HPG) axis signaling and a dramatic reduction of male sexual and territorial behaviors. However, the role of ERα in masculinizing distinct classes of neurons remains unexplored. We deleted ERα in excitatory or inhibitory neurons using either a Vglut2 or Vgat driver and assessed male behaviors. We find that Vglut2-Cre;Esr1^lox/lox mutant males lack ERα in the ventrolateral region of the ventromedial hypothalamus (VMHvl) and posterior ventral portion of the medial amygdala (MePV). These mutants recapitulate the increased serum testosterone levels seen with constitutive ERα deletion, but have none of the behavioral deficits. In contrast, Vgat-Cre;Esr1^lox/lox males with substantial ERα deletion in inhibitory neurons, including those of the principal nucleus of the bed nucleus of the stria terminalis (BNSTpr), posterior dorsal MeA (MePD), and medial preoptic area (MPOA) have normal testosterone levels, but display alterations in mating and territorial behaviors. These mutants also show dysmasculinized expression of androgen receptor (AR) and estrogen receptor beta (Esr2). Our results demonstrate that ERα masculinizes GABAergic neurons that gate the display of male-typical behaviors.

Neuronal Depolarization Drives Increased Dopamine Synaptic Vesicle Loading via VGLUT

The ability of presynaptic dopamine terminals to tune neurotransmitter release to meet the demands of neuronal activity is critical to neurotransmission. Although vesicle content has been assumed to be static, in vitro data increasingly suggest that cell activity modulates vesicle content. Here, we use a coordinated genetic, pharmacological, and imaging approach in Drosophila to study the presynaptic machinery responsible for these vesicular processes in vivo. We show that cell depolarization increases synaptic vesicle dopamine content prior to release via vesicular hyperacidification. This depolarization-induced hyperacidification is mediated by the vesicular glutamate transporter (VGLUT). Remarkably, both depolarization-induced dopamine vesicle hyperacidification and its dependence on VGLUT2 are seen in ventral midbrain dopamine neurons in the mouse. Together, these data suggest that in response to depolarization, dopamine vesicles utilize a cascade of vesicular transporters to dynamically increase the vesicular pH gradient, thereby increasing dopamine vesicle content.

Quantum Disentanglement: Electrical Analysis of the Complex Roles of Ions in Filling Vesicles with Glutamate

By expressing vesicular glutamate transporters at high levels in plasma membrane and applying voltage clamp methods, Eriksen et al. (2016) have identified a Cl(-) channel in the transporter that is coactivated by protons and Cl(-).

Dopamine neuron dependent behaviors mediated by glutamate cotransmission

Dopamine neurons in the ventral tegmental area use glutamate as a cotransmitter. To elucidate the behavioral role of the cotransmission, we targeted the glutamate-recycling enzyme glutaminase (gene Gls1). In mice with a dopamine transporter (Slc6a3)-driven conditional heterozygous (cHET) reduction of Gls1 in their dopamine neurons, dopamine neuron survival and transmission were unaffected, while glutamate cotransmission at phasic firing frequencies was reduced, enabling a selective focus on the cotransmission. The mice showed normal emotional and motor behaviors, and an unaffected response to acute amphetamine. Strikingly, amphetamine sensitization was reduced and latent inhibition potentiated. These behavioral effects, also seen in global GLS1 HETs with a schizophrenia resilience phenotype, were not seen in mice with an Emx1-driven forebrain reduction affecting most brain glutamatergic neurons. Thus, a reduction in dopamine neuron glutamate cotransmission appears to mediate significant components of the GLS1 HET schizophrenia resilience phenotype, and glutamate cotransmission appears to be important in attribution of motivational salience.

The distribution of cholinergic neurons and their co-localization with FMRFamide, in central and peripheral neurons of the spider Cupiennius salei

The spider Cupiennius salei is a well-established model for investigating information processing in arthropod sensory systems. Immunohistochemistry has shown that several neurotransmitters exist in the C. salei nervous system, including GABA, glutamate, histamine, octopamine and FMRFamide, while electrophysiology has found functional roles for some of these transmitters. There is also evidence that acetylcholine (ACh) is present in some C. salei neurons but information about the distribution of cholinergic neurons in spider nervous systems is limited. Here, we identify C. salei genes that encode enzymes essential for cholinergic transmission: choline ACh transferase (ChAT) and vesicular ACh transporter (VAChT). We used in-situ hybridization with an mRNA probe for C. salei ChAT gene to locate somata of cholinergic neurons in the central nervous system and immunohistochemistry with antisera against ChAT and VAChT to locate these proteins in cholinergic neurons. All three markers labeled similar, mostly small neurons, plus a few mid-sized neurons, in most ganglia. In the subesophageal ganglia, labeled neurons are putative efferent, motor or interneurons but the largest motor and interneurons were unlabeled. Groups of anti-ChAT labeled small neurons also connect the optic neuropils in the spider protocerebrum. Differences in individual cell labeling intensities were common, suggesting a range of ACh expression levels. Double-labeling found a subpopulation of anti-VAChT-labeled central and mechanosensory neurons that were also immunoreactive to antiserum against FMRFamide-like peptides. Our findings suggest that ACh is an important neurotransmitter in the C. salei central and peripheral nervous systems.

Segregation of glutamatergic and cholinergic transmission at the mixed motoneuron Renshaw cell synapse

In neonatal mice motoneurons excite Renshaw cells by releasing both acetylcholine (ACh) and glutamate. These two neurotransmitters activate two types of nicotinic receptors (nAChRs) (the homomeric α₇ receptors and the heteromeric α*ß* receptors) as well as the two types of glutamate receptors (GluRs) (AMPARs and NMDARs). Using paired recordings, we confirm that a single motoneuron can release both transmitters on a single post-synaptic Renshaw cell. We then show that co-transmission is preserved in adult animals. Kinetic analysis of miniature EPSCs revealed quantal release of mixed events associating AMPARs and NMDARs, as well as α₇ and α*ß* nAChRs, but no evidence was found for mEPSCs associating nAChRs with GluRs. Bayesian Quantal Analysis (BQA) of evoked EPSCs showed that the number of functional contacts on a single Renshaw cell is more than halved when the nicotinic receptors are blocked, confirming that the two neurotransmitters systems are segregated. Our observations can be explained if ACh and glutamate are released from common vesicles onto spatially segregated post-synaptic receptors clusters, but a pre-synaptic segregation of cholinergic and glutamatergic release sites is also possible.

Functional consequences of neuropeptide and small-molecule co-transmission

Colocalization of small-molecule and neuropeptide transmitters is common throughout the nervous system of all animals. The resulting co-transmission, which provides conjoint ionotropic ('classical') and metabotropic ('modulatory') actions, includes neuropeptide- specific aspects that are qualitatively different from those that result from metabotropic actions of small-molecule transmitter release. Here, we focus on the flexibility afforded to microcircuits by such co-transmission, using examples from various nervous systems. Insights from such studies indicate that co-transmission mediated even by a single neuron can configure microcircuit activity via an array of contributing mechanisms, operating on multiple timescales, to enhance both behavioural flexibility and robustness.

Multi-transmitter neurons in the mammalian central nervous system

It is firmly established that many mammalian neurons release various combinations of amino acids, their derivatives, and other small molecules from presynaptic terminals in order to signal to their postsynaptic targets. Here we discuss recent findings about four types of multi-transmitter neurons-those that release GABA and acetylcholine (Ach); dopamine (DA) and GABA or glutamate; and glutamate and GABA. The mechanisms of co-release in each class differ and highlight the complex and dynamic nature of neurotransmitter release. Furthermore, identifying the neurotransmitter signature of each neuron and the post-synaptic targets of each neurotransmitter remain challenging. The existence of multi-transmitter neurons complicates the interpretation of connectomic wiring diagrams and poses interesting challenges for our understanding of circuit function in the brain.

Kinetic Requirements for Spatiotemporal Chemical Imaging with Fluorescent Nanosensors

Fluorescent nanosensors are powerful tools for basic research and bioanalytical applications. Individual nanosensors are able to detect single molecules, while ensembles of nanosensors can be used to measure the bulk concentration of an analyte. Collective imaging of multiple nanosensors could provide both spatial and temporal chemical information from the nano- to the microscale. This type of chemical imaging with nanosensors would be very attractive to study processes such as chemical signaling between cells (e.g., neurons). So far, it is not understood what processes are resolvable (concentration, time, space) and how optimal sensors should be designed. Here, we develop a theoretical framework to simulate the fluorescence image of arrays of nanosensors in response to a concentration gradient. For that purpose, binding and unbinding of the analyte is simulated for each single nanosensor by using a Monte Carlo simulation and varying rate constants (k_on, k_off). Multiple nanosensors are arranged on a surface and exposed to a concentration pattern c_A(x,y,t) of an analyte. We account for the resolution limit of light microscopy (Abbe limit) and the acquisition speed and resolution of optical setups and determine the resulting response images ΔI(x,y,t). Consequently, we introduce terms for the spatial and temporal resolution and simulate phase diagrams for different rate constants that allow us to predict how a sensor should be designed to provide a desired spatial and temporal resolution. Our results show, for example, that imaging of neurotransmitter release requires rate constants of k_on = 10⁶ M^-1 s^-1and k_off = 10² s^-1 in many scenarios, which corresponds to high dissociation constants of K_d > 100 μM. This work predicts if a given fluorescent nanosensor array (rate constants, size, shape, geometry, density) is able to resolve fast concentration changes such as neurotransmitter release from cells. Additionally, we provide rational design principles to engineer nanosensors for chemical imaging.

Cholinergic/glutamatergic co-transmission in striatal cholinergic interneurons: new mechanisms regulating striatal computation

It is well established that neurons secrete neuropeptides and ATP with classical neurotransmitters; however, certain neuronal populations are also capable of releasing two classical neurotransmitters by a process named co-transmission. Although there has been progress in our understanding of the molecular mechanism underlying co-transmission, the individual regulation of neurotransmitter secretion and the functional significance of this neuronal 'bilingualism' is still unknown. Striatal cholinergic interneurons (CINs) have been shown to secrete glutamate (Glu) in addition to acetylcholine (ACh) and are recognized for their role in the regulation of striatal circuits and behavior. Our review highlights the recent research into identifying mechanisms that regulate the secretion and function of Glu and ACh released by CINs and the roles these neurons play in regulating dopamine secretion and striatal activity. In particular, we focus on how the transporters for ACh (VAChT) and Glu (VGLUT3) influence the storage of neurotransmitters in CINs. We further discuss how these individual neurotransmitters regulate striatal computation and distinct aspects of behavior that are regulated by the striatum. We suggest that understanding the distinct and complementary functional roles of these two neurotransmitters may prove beneficial in the development of therapies for Parkinson's disease and addiction. Overall, understanding how Glu and ACh secreted by CINs impacts striatal activity may provide insight into how different populations of 'bilingual' neurons are able to develop sophisticated regulation of their targets by interacting with multiple receptors but also by regulating each other's vesicular storage. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Neurotransmitter Switching in the Developing and Adult Brain

Neurotransmitter switching is the gain of one neurotransmitter and the loss of another in the same neuron in response to chronic stimulation. Neurotransmitter receptors on postsynaptic cells change to match the identity of the newly expressed neurotransmitter. Neurotransmitter switching often appears to change the sign of the synapse from excitatory to inhibitory or from inhibitory to excitatory. In these cases, neurotransmitter switching and receptor matching thus change the polarity of the circuit in which they take place. Neurotransmitter switching produces up or down reversals of behavior. It is also observed in response to disease. These findings raise the possibility that neurotransmitter switching contributes to depression, schizophrenia, and other illnesses. Many early discoveries of the single gain or loss of a neurotransmitter may have been harbingers of neurotransmitter switching.

GABAergic innervation of the ciliary ganglion in macaque monkeys - A light and electron microscopic study

The vertebrate ciliary ganglion (CG) is a relay station in the parasympathetic pathway activating the iris sphincter and ciliary muscle to mediate pupillary constriction and lens accommodation, respectively. While the postganglionic motoneurons in the CG are cholinergic, as are their inputs, there is evidence from avian studies that GABA may also be involved. Here, we used light and electron microscopic methods to examine the GABAergic innervation of the CG in Macaca fascicularis monkeys. Immunohistochemistry for the gamma aminobutyric acid synthesizing enzyme glutamic acid decarboxylase (GAD) and choline acetyltransferase (ChAT) revealed that all CG neurons are contacted by ChAT-positive terminals. A subpopulation of 17.5% of CG neurons was associated with terminal boutons expressing GAD-immunoreactivity in addition. Double-labeling for GAD and synaptophysin confirmed that these were synaptic terminals. Electron microscopic analysis in conjunction with GABA-immunogold staining showed that (1) GAD-positive terminals mainly target dendrites and spines in the perisomatic neuropil of CG neurons; (2) GABA is restricted to a specific terminal type, which displays intermediate features lying between classically excitatory and inhibitory endings; and (3) if a CG neuron is contacted by GABA-positive terminals, virtually all perisomatic terminals supplying it show GABA immunoreactivity. The source of this GABAergic input and whether GABA contributes to a specific CG function remains to be investigated. Nevertheless, our data indicate that the innervation of the ciliary ganglion is more complex than previously thought, and that GABA may play a neuromodulatory role in the control of lens or pupil function. J. Comp. Neurol. 525:1517-1531, 2017. © 2016 Wiley Periodicals, Inc.

Acid-Sensing Ion Channels Activated by Evoked Released Protons Modulate Synaptic Transmission at the Mouse Calyx of Held Synapse

Acid-sensing ion channels (ASICs) regulate synaptic activities and play important roles in neurodegenerative diseases. We found that these channels can be activated in neurons of the medial nucleus of the trapezoid body (MNTB) of the auditory system in the CNS. A drop in extracellular pH induces transient inward ASIC currents (I_ASICs) in postsynaptic MNTB neurons from wild-type mice. The inhibition of I_ASICs by psalmotoxin-1 (PcTx1) and the absence of these currents in knock-out mice for ASIC-1a subunit (ASIC1a^-/-) suggest that homomeric ASIC-1as are mediating these currents in MNTB neurons. Furthermore, we detect ASIC1a-dependent currents during synaptic transmission, suggesting an acidification of the synaptic cleft due to the corelease of neurotransmitter and H⁺ from synaptic vesicles. These currents are capable of eliciting action potentials in the absence of glutamatergic currents. A significant characteristic of these homomeric ASIC-1as is their permeability to Ca²⁺ Activation of ASIC-1a in MNTB neurons by exogenous H⁺ induces an increase in intracellular Ca²⁺ Furthermore, the activation of postsynaptic ASIC-1as during high-frequency stimulation (HFS) of the presynaptic nerve terminal leads to a PcTx1-sensitive increase in intracellular Ca²⁺ in MNTB neurons, which is independent of glutamate receptors and is absent in neurons from ASIC1a^-/- mice. During HFS, the lack of functional ASICs in synaptic transmission results in an enhanced short-term depression of glutamatergic EPSCs. These results strongly support the hypothesis of protons as neurotransmitters and demonstrate that presynaptic released protons modulate synaptic transmission by activating ASIC-1as at the calyx of Held-MNTB synapse.SIGNIFICANCE STATEMENT The manuscript demonstrates that postsynaptic neurons of the medial nucleus of the trapezoid body at the mouse calyx of Held synapse express functional homomeric Acid-sensing ion channel-1a (ASIC-1as) that can be activated by protons (coreleased with neurotransmitter from acidified synaptic vesicles). These ASIC-1as contribute to the generation of postsynaptic currents and, more relevant, to calcium influx, which could be involved in the modulation of presynaptic transmitter release. Inhibition or deletion of ASIC-1a leads to enhanced short-term depression, demonstrating that they are concerned with short-term plasticity of the synapse. ASICs represent a widespread communication system with unique properties. We expect that our experiments will have an impact in the neurobiology field and will spread in areas related to neuronal plasticity.

The structural and functional evidence for vesicular release from astrocytes in situ

The concept of the tripartite synapse states that bi-directional signalling between perisynaptic astrocyte processes, presynaptic axonal boutons and postsynaptic neuronal structures defines the properties of synaptic information processing. Ca²⁺-dependent vesicular release from astrocytes, as one of the mechanisms of astrocyte-neuron communication, has attracted particular attention but has also been the subject of intense debate. In neurons, regulated vesicular release is a strongly coordinated process. It requires a complex release machinery comprised of many individual components ranging from vesicular neurotransmitter transporters and soluble NSF attachment protein receptors (SNARE) proteins to Ca²⁺-sensors and the proteins that spatially and temporally control exocytosis of synaptic vesicles. If astrocytes employ similar mechanisms to release neurotransmitters is less well understood. The aim of this review is therefore to discuss recent experimental evidence that sheds light on the central structural components responsible for vesicular release from astrocytes in situ.

Heterogeneity in Dopamine Neuron Synaptic Actions Across the Striatum and Its Relevance for Schizophrenia

Brain imaging has revealed alterations in dopamine uptake, release, and receptor levels in patients with schizophrenia that have been resolved on the scale of striatal subregions. However, the underlying synaptic mechanisms are on a finer scale. Dopamine neuron synaptic actions vary across the striatum, involving variations not only in dopamine release but also in dopamine neuron connectivity, cotransmission, modulation, and activity. Optogenetic studies have revealed that dopamine neurons release dopamine in a synaptic signal mode, and that the neurons also release glutamate and gamma-aminobutyric acid as cotransmitters, with striking regional variation. Fast glutamate and gamma-aminobutyric acid cotransmission convey discrete patterns of dopamine neuron activity to striatal neurons. Glutamate may function not only in a signaling role at a subset of dopamine neuron synapses, but also in mediating vesicular synergy, contributing to regional differences in loading of dopamine into synaptic vesicles. Regional differences in dopamine neuron signaling are likely to be differentially involved in the schizophrenia disease process and likely determine the subregional specificity of the action of psychostimulants that exacerbate the disorder, and antipsychotics that ameliorate the disorder. Elucidating dopamine neuron synaptic signaling offers the potential for achieving greater pharmacological specificity through intersectional pharmacological actions targeting subsets of dopamine neuron synapses.

DOX-loaded pH-sensitive mesoporous silica nanoparticles coated with PDA and PEG induce pro-death autophagy in breast cancer

The development of multifunctional nano drug delivery carriers has been one of the most effective and prevailing approaches to overcome drug non-selectivity, low cell uptake efficiency and various side effects of traditional chemotherapy drugs.

ASIC channel inhibition enhances excitotoxic neuronal death in an in vitro model of spinal cord injury

In the spinal cord high extracellular glutamate evokes excitotoxic damage with neuronal loss and severe locomotor impairment. During the cell dysfunction process, extracellular pH becomes acid and may activate acid-sensing ion channels (ASICs) which could be important contributors to neurodegenerative pathologies. Our previous studies have shown that transient application of the glutamate analog kainate (KA) evokes delayed excitotoxic death of spinal neurons, while white matter is mainly spared. The present goal was to enquire if ASIC channels modulated KA damage in relation to locomotor network function and cell death. Mouse spinal cord slices were treated with KA (0.01 or 0.1mM) for 1h, and then washed out for 24h prior to analysis. RT-PCR results showed that KA (at 0.01mM concentration that is near-threshold for damage) increased mRNA expression of ASIC1a, ASIC1b, ASIC2 and ASIC3, an effect reversed by the ASIC inhibitor 4',6-diamidino-2-phenylindole (DAPI). A KA neurotoxic dose (0.1mM) reduced ASIC1a and ASIC2 expression. Cell viability assays demonstrated KA-induced large damage in spinal slices from mice with ASIC1a gene ablation. Likewise, immunohistochemistry indicated significant neuronal loss when KA was followed by the ASIC inhibitors DAPI or amiloride. Electrophysiological recording from ventral roots of isolated spinal cords showed that alternating oscillatory cycles were slowed down by 0.01mMKA, and intensely inhibited by subsequently applied DAPI or amiloride. Our data suggest that early rise in ASIC expression and function counteracted deleterious effects on spinal networks by raising the excitotoxicity threshold, a result with potential implications for improving neuroprotection.

Ventral tegmental area glutamate neurons co-release GABA and promote positive reinforcement

In addition to dopamine neurons, the ventral tegmental area (VTA) contains GABA-, glutamate- and co-releasing neurons, and recent reports suggest a complex role for the glutamate neurons in behavioural reinforcement. We report that optogenetic stimulation of VTA glutamate neurons or terminals serves as a positive reinforcer on operant behavioural assays. Mice display marked preference for brief over sustained VTA glutamate neuron stimulation resulting in behavioural responses that are notably distinct from dopamine neuron stimulation and resistant to dopamine receptor antagonists. Whole-cell recordings reveal EPSCs following stimulation of VTA glutamate terminals in the nucleus accumbens or local VTA collaterals; but reveal both excitatory and monosynaptic inhibitory currents in the ventral pallidum and lateral habenula, though the net effects on postsynaptic firing in each region are consistent with the observed rewarding behavioural effects. These data indicate that VTA glutamate neurons co-release GABA in a projection-target-dependent manner and that their transient activation drives positive reinforcement.

Ventral tegmental area (VTA) is involved in reward behaviours, but the precise contribution of VTA glutamatergic neurons to this process is not known. Here the authors show that phasic but not sustained optogenetic stimulation of VTA glutamatergic neurons is rewarding and involves co-release of GABA.

The Diversity of Spine Synapses in Animals

Here we examine the structure of the various types of spine synapses throughout the animal kingdom. Based on available evidence, we suggest that there are two major categories of spine synapses: invaginating and non-invaginating, with distributions that vary among different groups of animals. In the simplest living animals with definitive nerve cells and synapses, the cnidarians and ctenophores, most chemical synapses do not form spine synapses. But some cnidarians have invaginating spine synapses, especially in photoreceptor terminals of motile cnidarians with highly complex visual organs, and also in some mainly sessile cnidarians with rapid prey capture reflexes. This association of invaginating spine synapses with complex sensory inputs is retained in the evolution of higher animals in photoreceptor terminals and some mechanoreceptor synapses. In contrast to invaginating spine synapse, non-invaginating spine synapses have been described only in animals with bilateral symmetry, heads and brains, associated with greater complexity in neural connections. This is apparent already in the simplest bilaterians, the flatworms, which can have well-developed non-invaginating spine synapses in some cases. Non-invaginating spine synapses diversify in higher animal groups. We also discuss the functional advantages of having synapses on spines and more specifically, on invaginating spines. And finally we discuss pathologies associated with spine synapses, concentrating on those systems and diseases where invaginating spine synapses are involved.

Differential Releases of Dopamine and Neuropeptide Y from Histamine-Stimulated PC12 Cells Detected by an Aptamer-Modified Nanowire Transistor

Silicon nanowire field-effect transistors modified with specific aptamers can directly detect the minute dopamine and neuropeptide Y released from cells. The binding of these molecules to the aptamers results in a conductance change of the transistor biosensor and illustrates the differential releasing mechanisms of these molecules stored in various vesicle pools.

G Protein-Coupled Receptors in Myelinating Glia

The G protein-coupled receptor (GPCR) superfamily represents the largest class of functionally selective drug targets for disease modulation and therapy. GPCRs have been studied in great detail in central nervous system (CNS) neurons, but these important molecules have been relatively understudied in glia. In recent years, however, exciting new roles for GPCRs in glial cell biology have emerged. We focus here on the key roles of GPCRs in a specialized subset of glia, myelinating glia. We highlight recent work firmly establishing GPCRs as regulators of myelinating glial cell development and myelin repair. These advances expand our understanding of myelinating glial cell biology and underscore the utility of targeting GPCRs to promote myelin repair in human disease.

Input-specific contributions to valence processing in the amygdala

Reward and punishment are often thought of as opposing processes: rewards and the environmental cues that predict them elicit approach and consummatory behaviors, while punishments drive aversion and avoidance behaviors. This framework suggests that there may be segregated brain circuits for these valenced behaviors. The basolateral amygdala (BLA) is one brain region that contributes to both types of motivated behavior. Individual neurons in the BLA can favor positive over negative valence, or vice versa, but these neurons are intermingled, showing no anatomical segregation. The amygdala receives inputs from many brain areas and current theories posit that encoding of positive versus negative valence by BLA neurons is determined by the wiring of each neuron. Specifically, many projections from other brain areas that respond to positive and negative valence stimuli and predictive cues project strongly to the BLA and likely contribute to valence processing within the BLA. Here we review three of these areas, the basal forebrain, the dorsal raphe nucleus and the ventral tegmental area, and discuss how these may promote encoding of positive and negative valence within the BLA.

Simultaneous monitoring of monoamines, amino acids, nucleotides and neuropeptides by liquid chromatography-tandem mass spectrometry and its application to neurosecretion in bovine chromaffin cells

The primary functions of adrenal medullary chromaffin cells are the synthesis and storage in their chromaffin vesicles of the catecholamines noradrenaline (NA) and adrenaline (AD), and their subsequent release into the bloodstream by Ca²⁺ -dependent exocytosis under conditions of fear or stress (fight or flight response). Several monoamines, nucleotides and opiates, such as leucine-enkephalin (LENK) and methionine-enkephalin (MENK), are also co-stored and co-released with the catecholamines. However, other neurotransmitters have not been studied in depth. Here, we present a novel high-resolution liquid chromatography-tandem mass spectrometry approach for the simultaneous monitoring of 14 compounds stored and released in bovine chromaffin cells (BCCs). We validated the analytical method according to the recommendations of the EMA and FDA by testing matrix effect, selectivity, sensitivity, precision, accuracy, stability and carry-over. After testing on six batches of BCCs from different cultures, the method enabled simultaneous quantitative determination of monoamines (AD, NA, dopamine, serotonin, 5-hydroxyindoleacetic acid, histamine and metanephrine), amino acids (L-glutamic acid, γ-aminobutyric acid), nucleotides (adenosine 5'-diphosphate, adenosine 5'-monophosphate, cyclic adenosine 5'-monophosphate) and neuropeptides (LENK and MENK) in the intracellular content, basal secretion and acetylcholine induced secretion of BBCs. The high-resolution approach used here enabled us to determine the levels of 14 compounds in the same BCC batch in only 16 min. This novel approach will make it possible to study the regulatory mechanisms of Ca²⁺ signaling, exocytosis and endocytosis using different neurotrophic factors and/or secretagogues as stimuli in primary BCC cultures. Our method is actually being applied to human plasma samples of different therapeutic areas where sympathoadrenal axis is involved in stress situations such as Alzheimer's disease, migraine or cirrhosis, to improve diagnosis and clinical practice. Copyright © 2016 John Wiley & Sons, Ltd.

Neuroimmunomodulation in the Gut: Focus on Inflammatory Bowel Disease

Intestinal immunity is finely regulated by several concomitant and overlapping mechanisms, in order to efficiently sense external stimuli and mount an adequate response of either tolerance or defense. In this context, a complex interplay between immune and nonimmune cells is responsible for the maintenance of normal homeostasis. However, in certain conditions, the disruption of such an intricate network may result in intestinal inflammation, including inflammatory bowel disease (IBD). IBD is believed to result from a combination of genetic and environmental factors acting in concert with an inappropriate immune response, which in turn interacts with nonimmune cells, including nervous system components. Currently, evidence shows that the interaction between the immune and the nervous system is bidirectional and plays a critical role in the regulation of intestinal inflammation. Recently, the maintenance of intestinal homeostasis has been shown to be under the reciprocal control of the microbiota by immune mechanisms, whereas intestinal microorganisms can modulate mucosal immunity. Therefore, in addition to presenting the mechanisms underlying the interaction between immune and nervous systems in the gut, here we discuss the role of the microbiota also in the regulation of neuroimmune crosstalk involved in intestinal homeostasis and inflammation, with potential implications to IBD pathogenesis.

Afferent Inputs to Neurotransmitter-Defined Cell Types in the Ventral Tegmental Area

The ventral tegmental area (VTA) plays a central role in the neural circuit control of behavioral reinforcement. Though considered a dopaminergic nucleus, the VTA contains substantial heterogeneity in neurotransmitter type, containing also GABA and glutamate neurons. Here, we used a combinatorial viral approach to transsynaptically label afferents to defined VTA dopamine, GABA, or glutamate neurons. Surprisingly, we find that these populations received qualitatively similar inputs, with dominant and comparable projections from the lateral hypothalamus, raphe, and ventral pallidum. However, notable differences were observed, with striatal regions and globus pallidus providing a greater share of input to VTA dopamine neurons, cortical input preferentially on to glutamate neurons, and GABA neurons receiving proportionally more input from the lateral habenula and laterodorsal tegmental nucleus. By comparing inputs to each of the transmitter-defined VTA cell types, this study sheds important light on the systems-level organization of diverse inputs to VTA.

Contemporary approaches to neural circuit manipulation and mapping: focus on reward and addiction

Tying complex psychological processes to precisely defined neural circuits is a major goal of systems and behavioural neuroscience. This is critical for understanding adaptive behaviour, and also how neural systems are altered in states of psychopathology, such as addiction. Efforts to relate psychological processes relevant to addiction to activity within defined neural circuits have been complicated by neural heterogeneity. Recent advances in technology allow for manipulation and mapping of genetically and anatomically defined neurons, which when used in concert with sophisticated behavioural models, have the potential to provide great insight into neural circuit bases of behaviour. Here we discuss contemporary approaches for understanding reward and addiction, with a focus on midbrain dopamine and cortico-striato-pallidal circuits.

Excitation by Axon Terminal GABA Spillover in a Sound Localization Circuit

Synapses from neurons of the medial nucleus of the trapezoid body (MNTB) onto neurons of the lateral superior olive (LSO) in the auditory brainstem are glycinergic in maturity, but also GABAergic and glutamatergic in development. The role for this neurotransmitter cotransmission is poorly understood. Here we use electrophysiological recordings in brainstem slices from P3-P21 mice to demonstrate that GABA release evoked from MNTB axons can spill over to neighboring MNTB axons and cause excitation by activating GABAAR. This spillover excitation generates patterns of staggered neurotransmitter release from different MNTB axons resulting in characteristic "doublet" postsynaptic currents in LSO neurons. Postembedding immunogold labeling and electron microscopy provide evidence that GABAARs are localized at MNTB axon terminals. Photolytic uncaging of p-hydroxyphenacyl (pHP) GABA demonstrates backpropagation of GABAAR-mediated depolarizations from MNTB axon terminals to the soma, some hundreds of microns away. These somatic depolarizations enhanced somatic excitability by increasing the probability of action potential generation. GABA spillover excitation between MNTB axon terminals may entrain neighboring MNTB neurons, which may play a role in the developmental refinement of the MNTB-LSO pathway. Axonal spillover excitation persisted beyond the second postnatal week, suggesting that this mechanism may play a role in sound localization, by providing new avenues of communication between MNTB neurons via their distal axonal projections. Significance statement: In this study, a new mechanism of neuronal communication between auditory synapses in the mammalian sound localization pathway is described. Evidence is provided that the inhibitory neurotransmitter GABA can spill over between axon terminals to cause excitation of nearby synapses to further stimulate neurotransmitter release. Excitatory GABA spillover between inhibitory axon terminals may have important implications for the development and refinement of this auditory circuit and may play a role in the ability to precisely localize sound sources.

Target-Specific Glycinergic Transmission from VGluT3-Expressing Amacrine Cells Shapes Suppressive Contrast Responses in the Retina

Neurons that release more than one transmitter exist throughout the CNS. Yet, how these neurons deploy multiple transmitters and shape the function of specific circuits is not well understood. VGluT3-expressing amacrine cells (VG3-ACs) provide glutamatergic input to ganglion cells activated by contrast and motion. Using optogenetics, we find that VG3-ACs provide selective glycinergic input to a retinal ganglion cell type suppressed by contrast and motion (SbC-RGCs). Firing of SbC-RGCs is suppressed at light ON and OFF over a broad range of stimulus sizes. Anatomical circuit reconstructions reveal that VG3-ACs form inhibitory synapses preferentially on processes that link ON and OFF arbors of SbC-RGC dendrites. Removal of VG3-ACs from mature circuits reduces inhibition and attenuates spike suppression of SbC-RGCs in a contrast- and size-selective manner, illustrating the modularity of retinal circuits. VG3-ACs thus use dual transmitters in a target-specific manner and shape suppressive contrast responses in the retina by glycinergic transmission.

Functional Connectome Analysis of Dopamine Neuron Glutamatergic Connections in Forebrain Regions

In the ventral tegmental area (VTA), a subpopulation of dopamine neurons express vesicular glutamate transporter 2 and make glutamatergic connections to nucleus accumbens (NAc) and olfactory tubercle (OT) neurons. However, their glutamatergic connections across the forebrain have not been explored systematically. To visualize dopamine neuron forebrain projections and to enable photostimulation of their axons independent of transmitter status, we virally transfected VTA neurons with channelrhodopsin-2 fused to enhanced yellow fluorescent protein (ChR2-EYFP) and used DAT(IREScre) mice to restrict expression to dopamine neurons. ChR2-EYFP-expressing neurons almost invariably stained for tyrosine hydroxylase, identifying them as dopaminergic. Dopamine neuron axons visualized by ChR2-EYFP fluorescence projected most densely to the striatum, moderately to the amygdala and entorhinal cortex (ERC), sparsely to prefrontal and cingulate cortices, and rarely to the hippocampus. Guided by ChR2-EYFP fluorescence, we recorded systematically from putative principal neurons in target areas and determined the incidence and strength of glutamatergic connections by activating all dopamine neuron terminals impinging on recorded neurons with wide-field photostimulation. This revealed strong glutamatergic connections in the NAc, OT, and ERC; moderate strength connections in the central amygdala; and weak connections in the cingulate cortex. No glutamatergic connections were found in the dorsal striatum, hippocampus, basolateral amygdala, or prefrontal cortex. These results indicate that VTA dopamine neurons elicit widespread, but regionally distinct, glutamatergic signals in the forebrain and begin to define the dopamine neuron excitatory functional connectome.

SIGNIFICANCE STATEMENT

Dopamine neurons are important for the control of motivated behavior and are involved in the pathophysiology of several major neuropsychiatric disorders. Recent studies have shown that some ventral midbrain dopamine neurons are capable of glutamate cotransmission. With conditional expression of channelrhodopsin in dopamine neurons, we systematically explored dopamine neuron connections in the forebrain and identified regionally specific dopamine neuron excitatory connections. Establishing that only a subset of forebrain regions receive excitatory connections from dopamine neurons will help to determine the function of dopamine neuron glutamate cotransmission, which likely involves transmission of precise temporal signals and enhancement of the dynamic range of dopamine neuron signals.

Axonal Segregation and Role of the Vesicular Glutamate Transporter VGLUT3 in Serotonin Neurons

A subset of monoamine neurons releases glutamate as a cotransmitter due to presence of the vesicular glutamate transporters VGLUT2 or VGLUT3. In addition to mediating vesicular loading of glutamate, it has been proposed that VGLUT3 enhances serotonin (5-HT) vesicular loading by the vesicular monoamine transporter (VMAT2) in 5-HT neurons. In dopamine (DA) neurons, glutamate appears to be released from specialized subsets of terminals and it may play a developmental role, promoting neuronal growth and survival. The hypothesis of a similar developmental role and axonal localization of glutamate co-release in 5-HT neurons has not been directly examined. Using postnatal mouse raphe neurons in culture, we first observed that in contrast to 5-HT itself, other phenotypic markers of 5-HT axon terminals such as the 5-HT reuptake transporter (SERT) show a more restricted localization in the axonal arborization. Interestingly, only a subset of SERT- and 5-HT-positive axonal varicosities expressed VGLUT3, with SERT and VGLUT3 being mostly segregated. Using VGLUT3 knockout mice, we found that deletion of this transporter leads to reduced survival of 5-HT neurons in vitro and also decreased the density of 5-HT-immunoreactivity in terminals in the dorsal striatum and dorsal part of the hippocampus in the intact brain. Our results demonstrate that raphe 5-HT neurons express SERT and VGLUT3 mainly in segregated axon terminals and that VGLUT3 regulates the vulnerability of these neurons and the neurochemical identity of their axonal domain, offering new perspectives on the functional connectivity of a cell population involved in anxiety disorders and depression.

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.

Computational implications of biophysical diversity and multiple timescales in neurons and synapses for circuit performance

Despite advances in experimental and theoretical neuroscience, we are still trying to identify key biophysical details that are important for characterizing the operation of brain circuits. Biological mechanisms at the level of single neurons and synapses can be combined as 'building blocks' to generate circuit function. We focus on the importance of capturing multiple timescales when describing these intrinsic and synaptic components. Whether inherent in the ionic currents, the neuron's complex morphology, or the neurotransmitter composition of synapses, these multiple timescales prove crucial for capturing the variability and richness of circuit output and enhancing the information-carrying capacity observed across nervous systems.

Segregated Glycine-Glutamate Co-transmission from vGluT3 Amacrine Cells to Contrast-Suppressed and Contrast-Enhanced Retinal Circuits

Since the introduction of Dale's principle of "one neuron releases one transmitter at all its synapses," a growing number of exceptions to this principle have been identified. While the concept of neurotransmitter co-release by a single neuron is now well accepted, the specific synaptic circuitry and functional advantage of co-neurotransmission remain poorly understood in general. Here we report Ca(2+)-dependent co-release of a new combination of inhibitory and excitatory neurotransmitters, namely, glycine and glutamate, by the vGluT3-expressing amacrine cell (GAC) in the mouse retina. GACs selectively make glycinergic synapses with uniformity detectors (UDs) and provide a major inhibitory drive that underlies the suppressed-by-contrast trigger feature of UDs. Meanwhile, GACs release glutamate to excite OFF alpha ganglion cells and a few other nonlinear, contrast-sensitive ganglion cells. This coordinated inhibition and excitation of two separate neuronal circuits by a single interneuron suggests a unique advantage in differential detection of visual field uniformity and contrast.

Emerging roles of Na⁺/H⁺ exchangers in epilepsy and developmental brain disorders

Epilepsy is a common central nervous system (CNS) disease characterized by recurrent transient neurological events occurring due to abnormally excessive or synchronous neuronal activity in the brain. The CNS is affected by systemic acid-base disorders, and epileptic seizures are sensitive indicators of underlying imbalances in cellular pH regulation. Na(+)/H(+) exchangers (NHEs) are a family of membrane transporter proteins actively involved in regulating intracellular and organellar pH by extruding H(+) in exchange for Na(+) influx. Altering NHE function significantly influences neuronal excitability and plays a role in epilepsy. This review gives an overview of pH regulatory mechanisms in the brain with a special focus on the NHE family and the relationship between epilepsy and dysfunction of NHE isoforms. We first discuss how cells translocate acids and bases across the membrane and establish pH homeostasis as a result of the concerted effort of enzymes and ion transporters. We focus on the specific roles of the NHE family by detailing how the loss of NHE1 in two NHE mutant mice results in enhanced neuronal excitability in these animals. Furthermore, we highlight new findings on the link between mutations of NHE6 and NHE9 and developmental brain disorders including epilepsy, autism, and attention deficit hyperactivity disorder (ADHD). These studies demonstrate the importance of NHE proteins in maintaining H(+) homeostasis and their intricate roles in the regulation of neuronal function. A better understanding of the mechanisms underlying NHE1, 6, and 9 dysfunctions in epilepsy formation may advance the development of new epilepsy treatment strategies.

Extrasynaptic release of GABA and dopamine by retinal dopaminergic neurons

In the mouse retina, dopaminergic amacrine (DA) cells synthesize both dopamine and GABA. Both transmitters are released extrasynaptically and act on neighbouring and distant retinal neurons by volume transmission. In simultaneous recordings of dopamine and GABA release from isolated perikarya of DA cells, a proportion of the events of dopamine and GABA exocytosis were simultaneous, suggesting co-release. In addition, DA cells establish GABAergic synapses onto AII amacrine cells, the neurons that transfer rod bipolar signals to cone bipolars. GABAA but not dopamine receptors are clustered in the postsynaptic membrane. Therefore, dopamine, irrespective of its site of release-synaptic or extrasynaptic-exclusively acts by volume transmission. Dopamine is released upon illumination and sets the gain of retinal neurons for vision in bright light. The GABA released at DA cells' synapses probably prevents signals from the saturated rods from entering the cone pathway when the dark-adapted retina is exposed to bright illumination. The GABA released extrasynaptically by DA and other amacrine cells may set a 'GABAergic tone' in the inner plexiform layer and thus counteract the effects of a spillover of glutamate released at the bipolar cell synapses of adjacent OFF and ON strata, thus preserving segregation of signals between ON and OFF pathways.