Cytosolic transmitter concentration regulates vesicle cycling at hippocampal GABAergic terminals

Decoding the nexus: branched-chain amino acids and their connection with sleep, circadian rhythms, and cardiometabolic health

The sleep-wake cycle stands as an integrative process essential for sustaining optimal brain function and, either directly or indirectly, overall body health, encompassing metabolic and cardiovascular well-being. Given the heightened metabolic activity of the brain, there exists a considerable demand for nutrients in comparison to other organs. Among these, the branched-chain amino acids, comprising leucine, isoleucine, and valine, display distinctive significance, from their contribution to protein structure to their involvement in overall metabolism, especially in cerebral processes. Among the first amino acids that are released into circulation post-food intake, branched-chain amino acids assume a pivotal role in the regulation of protein synthesis, modulating insulin secretion and the amino acid sensing pathway of target of rapamycin. Branched-chain amino acids are key players in influencing the brain's uptake of monoamine precursors, competing for a shared transporter. Beyond their involvement in protein synthesis, these amino acids contribute to the metabolic cycles of γ-aminobutyric acid and glutamate, as well as energy metabolism. Notably, they impact GABAergic neurons and the excitation/inhibition balance. The rhythmicity of branched-chain amino acids in plasma concentrations, observed over a 24-hour cycle and conserved in rodent models, is under circadian clock control. The mechanisms underlying those rhythms and the physiological consequences of their disruption are not fully understood. Disturbed sleep, obesity, diabetes, and cardiovascular diseases can elevate branched-chain amino acid concentrations or modify their oscillatory dynamics. The mechanisms driving these effects are currently the focal point of ongoing research efforts, since normalizing branched-chain amino acid levels has the ability to alleviate the severity of these pathologies. In this context, the Drosophila model, though underutilized, holds promise in shedding new light on these mechanisms. Initial findings indicate its potential to introduce novel concepts, particularly in elucidating the intricate connections between the circadian clock, sleep/wake, and metabolism. Consequently, the use and transport of branched-chain amino acids emerge as critical components and orchestrators in the web of interactions across multiple organs throughout the sleep/wake cycle. They could represent one of the so far elusive mechanisms connecting sleep patterns to metabolic and cardiovascular health, paving the way for potential therapeutic interventions.

Mapping GABAergic projections that mediate feeding

Neuroscience offers important insights into the pathogenesis and treatment of obesity by investigating neural circuits underpinning appetite and feeding. Gamma-aminobutyric acid (GABA), one of the most abundant neurotransmitters in the brain, and its associated receptors represent an array of pharmacologically targetable mediators of appetite signalling. Targeting the GABAergic system is therefore an increasingly investigated approach to obesity treatment. However, the many GABAergic projections that control feeding have yet to be collectively analysed. This review provides a comprehensive analysis of the relationship between GABAergic signalling and appetite by examining both foundational studies and the results of newly emerging chemogenetic/optogenetic experiments. A current snapshot of these efforts to map GABAergic projections influencing appetite is provided, and potential avenues for further investigation are provided.

Towards an optimised deep brain stimulation using a large-scale computational network and realistic volume conductor model

Objective. Constructing a theoretical framework to improve deep brain stimulation (DBS) based on the neuronal spatiotemporal patterns of the stimulation-affected areas constitutes a primary target.Approach. We develop a large-scale biophysical network, paired with a realistic volume conductor model, to estimate theoretically efficacious stimulation protocols. Based on previously published anatomically defined structural connectivity, a biophysical basal ganglia-thalamo-cortical neuronal network is constructed using Hodgkin-Huxley dynamics. We define a new biomarker describing the thalamic spatiotemporal activity as a ratio of spiking vs. burst firing. The per cent activation of the different pathways is adapted in the simulation to minimise the differences of the biomarker with respect to its value under healthy conditions.Main results.This neuronal network reproduces spatiotemporal patterns that emerge in Parkinson's disease. Simulations of the fibre per cent activation for the defined biomarker propose desensitisation of pallido-thalamic synaptic efficacy, induced by high-frequency signals, as one possible crucial mechanism for DBS action. Based on this activation, we define both an optimal electrode position and stimulation protocol using pathway activation modelling.Significance. A key advantage of this research is that it combines different approaches, i.e. the spatiotemporal pattern with the electric field and axonal response modelling, to compute the optimal DBS protocol. By correlating the inherent network dynamics with the activation of white matter fibres, we obtain new insights into the DBS therapeutic action.

Neurotransmitter content heterogeneity within an interneuron class shapes inhibitory transmission at a central synapse

Neurotransmitter content is deemed the most basic defining criterion for neuronal classes, contrasting with the intercellular heterogeneity of many other molecular and functional features. Here we show, in the adult mouse brain, that neurotransmitter content variegation within a neuronal class is a component of its functional heterogeneity. Golgi cells (GoCs), the well-defined class of cerebellar interneurons inhibiting granule cells (GrCs), contain cytosolic glycine, accumulated by the neuronal transporter GlyT2, and GABA in various proportions. By performing acute manipulations of cytosolic GABA and glycine supply, we find that competition of glycine with GABA reduces the charge of IPSC evoked in GrCs and, more specifically, the amplitude of a slow component of the IPSC decay. We then pair GrCs recordings with optogenetic stimulations of single GoCs, which preserve the intracellular transmitter mixed content. We show that the strength and decay kinetics of GrCs IPSCs, which are entirely mediated by GABAA receptors, are negatively correlated to the presynaptic expression of GlyT2 by GoCs. We isolate a slow spillover component of GrCs inhibition that is also affected by the expression of GlyT2, leading to a 56% decrease in relative charge. Our results support the hypothesis that presynaptic loading of glycine negatively impacts the GABAergic transmission in mixed interneurons, most likely through a competition for vesicular filling. We discuss how the heterogeneity of neurotransmitter supply within mixed interneurons like the GoC class may provide a presynaptic mechanism to tune the gain of microcircuits such as the granular layer, thereby expanding the realm of their possible dynamic behaviors.

GluN3A excitatory glycine receptors control adult cortical and amygdalar circuits

GluN3A is an atypical glycine-binding subunit of NMDA receptors (NMDARs) whose actions in the brain are mostly unknown. Here, we show that the expression of GluN3A subunits controls the excitability of mouse adult cortical and amygdalar circuits via an unusual signaling mechanism involving the formation of excitatory glycine GluN1/GluN3A receptors (eGlyRs) and their tonic activation by extracellular glycine. eGlyRs are mostly extrasynaptic and reside in specific neuronal populations, including the principal cells of the basolateral amygdala (BLA) and SST-positive interneurons (SST-INs) of the neocortex. In the BLA, tonic eGlyR currents are sensitive to fear-conditioning protocols, are subject to neuromodulation by the dopaminergic system, and control the stability of fear memories. In the neocortex, eGlyRs control the in vivo spiking of SST-INs and the behavior-dependent modulation of cortical activity. GluN3A-containing eGlyRs thus represent a novel and widespread signaling modality in the adult brain, with attributes that strikingly depart from those of conventional NMDARs.

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In mice, GluN3A is expressed by SST-INs in the cortex and pyramidal neurons in the BLA

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GluN3A assembles as excitatory glycine GluN1/GluN3A receptors (eGlyRs)

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eGlyRs detect extracellular glycine levels and generate tonic excitatory currents

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eGlyRs tune the function of SST-INs in cortex and alter the formation of fear memories in BLA

Mafa-dependent GABAergic activity promotes mouse neonatal apneas

While apneas are associated with multiple pathological and fatal conditions, the underlying molecular mechanisms remain elusive. We report that a mutated form of the transcription factor Mafa (Mafa^4A) that prevents phosphorylation of the Mafa protein leads to an abnormally high incidence of breath holding apneas and death in newborn Mafa^4A/4A mutant mice. This apneic breathing is phenocopied by restricting the mutation to central GABAergic inhibitory neurons and by activation of inhibitory Mafa neurons while reversed by inhibiting GABAergic transmission centrally. We find that Mafa activates the Gad2 promoter in vitro and that this activation is enhanced by the mutation that likely results in increased inhibitory drives onto target neurons. We also find that Mafa inhibitory neurons are absent from respiratory, sensory (primary and secondary) and pontine structures but are present in the vicinity of the hypoglossal motor nucleus including premotor neurons that innervate the geniohyoid muscle, to control upper airway patency. Altogether, our data reveal a role for Mafa phosphorylation in regulation of GABAergic drives and suggest a mechanism whereby reduced premotor drives to upper airway muscles may cause apneic breathing at birth.

Apneas are associated with many pathological conditions. Here, the authors show in a mouse model that stabilization of the transcription factor Mafa in brainstem GABAergic neurons may contribute to apnea, by decreasing motor drive to muscles controlling the airways.

Inhibitory co-transmission from midbrain dopamine neurons relies on presynaptic GABA uptake

Dopamine (DA)-releasing neurons in the substantia nigra pars compacta (SNc^DA) inhibit target cells in the striatum through postsynaptic activation of γ-aminobutyric acid (GABA) receptors. However, the molecular mechanisms responsible for GABAergic signaling remain unclear, as SNc^DA neurons lack enzymes typically required to produce GABA or package it into synaptic vesicles. Here, we show that aldehyde dehydrogenase 1a1 (Aldh1a1), an enzyme proposed to function as a GABA synthetic enzyme in SNc^DA neurons, does not produce GABA for synaptic transmission. Instead, we demonstrate that SNc^DA axons obtain GABA exclusively through presynaptic uptake using the membrane GABA transporter Gat1 (encoded by Slc6a1). GABA is then packaged for vesicular release using the vesicular monoamine transporter Vmat2. Our data therefore show that presynaptic transmitter recycling can substitute for de novo GABA synthesis and that Vmat2 contributes to vesicular GABA transport, expanding the range of molecular mechanisms available to neurons to support inhibitory synaptic communication.

Melani and Tritsch demonstrate that inhibitory co-transmission from midbrain dopaminergic neurons does not depend on cell-autonomous GABA synthesis but instead on presynaptic import from the extracellular space through the membrane transporter Gat1 and that GABA loading into synaptic vesicles relies on the vesicular monoamine transporter Vmat2.

Tonotopic Specializations in Number, Size, and Reversal Potential of GABAergic Inputs Fine-Tune Temporal Coding at Avian Cochlear Nucleus

GABAergic inhibition in neurons plays a critical role in determining the output of neural circuits. Neurons in avian nucleus magnocellularis (NM) use several tonotopic-region-dependent specializations to relay the timing information of sound in the auditory nerve to higher auditory nuclei. Previously, we showed that feedforward GABAergic inhibition in NM has a different dependence on the level of auditory nerve activity, with the low-frequency region having a low-threshold and linear relationship, while the high-frequency region has a high-threshold and step-like relationship. However, it remains unclear how the GABAergic synapses are tonotopically regulated and interact with other specializations of NM neurons. In this study, we examined GABAergic transmission in the NM of chickens of both sexes and explored its contributions to the temporal coding of sound at each tonotopic region. We found that the number and size of unitary GABAergic currents and their reversal potential were finely tuned at each tonotopic region in the NM. At the lower-frequency region, unitary GABAergic currents were larger in number but smaller in size. In addition, their reversal potential was close to the resting potential of neurons, which enabled reliable inhibition despite the smaller potassium conductance. At the higher-frequency region, on the other hand, unitary GABAergic currents were fewer, larger, and highly depolarizing, which enabled powerful inhibition via activating the large potassium conductance. Thus, we propose that GABAergic synapses are coordinated with the characteristics of excitatory synapses and postsynaptic neurons, ensuring the temporal coding for wide frequency and intensity ranges.SIGNIFICANCE STATEMENT We found in avian cochlear nucleus that the number and size of unitary GABAergic inputs differed among tonotopic regions and correlated to respective excitatory inputs; it was larger in number but smaller in size for neurons tuned to lower-frequency sound. Furthermore, GABAergic reversal potential also differed among the regions in accordance with the size of Kv1 current; it was less depolarized in the lower-frequency neurons with smaller Kv1 current. These differentiations of GABAergic transmission maximized the effects of inhibition at each tonotopic region, ensuring precise and reliable temporal coding across frequencies and intensities. Our results emphasize the importance of optimizing characteristics of GABAergic transmission within individual neurons for proper neural circuit function.

Deletion of TrkB in parvalbumin interneurons alters cortical neural dynamics

Signaling by neurotrophins such as the brain‐derived neurotrophic factor (BDNF) is known to modulate development of interneurons, but the circuit effects of this modulation remain unclear. Here, we examined the impact of deleting TrkB, a BDNF receptor, in parvalbumin‐expressing (PV) interneurons on the balance of excitation and inhibition (E‐I) in cortical circuits. In the mouse olfactory cortex, TrkB deletion impairs multiple aspects of PV neuronal function including synaptic excitation, intrinsic excitability, and the innervation pattern of principal neurons. Impaired PV cell function resulted in aberrant spiking patterns in principal neurons in response to stimulation of sensory inputs. Surprisingly, dampened PV neuronal function leads to a paradoxical decrease in overall excitability in cortical circuits. Our study demonstrates that, by modulating PV circuit plasticity and development, TrkB plays a critical role in shaping the evoked pattern of activity in a cortical network.

Sevoflurane depresses neurons in the medial parabrachial nucleus by potentiating postsynaptic GABAA receptors and background potassium channels

Despite persistent clinical use for over 170 years, the neuronal mechanisms by which general anesthetics produce hypnosis remain unclear. Previous studies suggest that anesthetics exert hypnotic effects by acting on endogenous arousal circuits. Recently, it has been shown that the medial parabrachial nucleus (MPB) is a novel wake-promoting component in the dorsolateral pons. However, it is not known whether and how the MPB contributes to anesthetic-induced hypnosis. Here, we investigated the action of sevoflurane, a widely used volatile anesthetic agent that best represents the drug class of halogenated ethers, on MPB neurons in mice. Using in vivo fiber photometry, we found that the population activities of MPB neurons were inhibited during sevoflurane-induced loss of consciousness. Using in vitro whole-cell patch-clamp recordings, we revealed that sevoflurane suppressed the firing rate of MPB neurons in concentration-dependent and reversible manners. At a concentration equal to MAC of hypnosis, sevoflurane potentiated synaptic GABA_A receptors (GABA_A-Rs), and the inhibitory effect of sevoflurane on the firing rate of MPB neurons was completely abolished by picrotoxin, which is a selective GABA_A-R antagonist. At a concentration equivalent to MAC of immobility, sevoflurane directly hyperpolarized MPB neurons and induced a significant decrease in membrane input resistance by increasing a basal potassium conductance. Moreover, pharmacological blockade of GABA_A-Rs in the MPB prolongs induction and shortens emergence under sevoflurane inhalation at MAC of hypnosis. These results indicate that sevoflurane inhibits MPB neurons through postsynaptic GABA_A-Rs and background potassium channels, which contributes to sevoflurane-induced hypnosis.

Activity and Cytosolic Na+ Regulate Synaptic Vesicle Endocytosis

Retrieval of synaptic vesicles via endocytosis is essential for maintaining sustained synaptic transmission, especially for neurons that fire action potentials at high frequencies. However, how neuronal activity regulates synaptic vesicle recycling is largely unknown. Here we report that Na⁺ substantially accumulated in the mouse calyx of Held terminals of either sex during repetitive high-frequency spiking. Elevated presynaptic Na⁺ accelerated both slow and rapid forms of endocytosis and facilitated endocytosis overshoot, but did not affect the readily releasable pool size, Ca²⁺ influx, or exocytosis. To examine whether this facilitation of endocytosis is related to the Na⁺-dependent vesicular content change, we dialyzed glutamate into the presynaptic cytosol or blocked the vesicular glutamate uptake with bafilomycin and found that the rate of endocytosis was not affected by regulating the vesicular glutamate content. Endocytosis is critically dependent on intracellular Ca²⁺, and the activity of Na⁺/Ca²⁺ exchanger (NCX) may be altered when the Na⁺ gradient is changed. However, neither NCX inhibitor nor change of extracellular Na⁺ concentration affected the endocytosis rate. Moreover, two-photon Ca²⁺ imaging showed that presynaptic Na⁺ did not affect the action potential-evoked intracellular Ca²⁺ transient and decay. Therefore, we revealed a novel mechanism of cytosolic Na⁺ in accelerating vesicle endocytosis. During high-frequency synaptic transmission, when large numbers of synaptic vesicles were fused, the rapid buildup of presynaptic cytosolic Na⁺ promoted vesicle recycling and sustained synaptic transmission.SIGNIFICANCE STATEMENT High-frequency firing neurons are widely distributed in the CNS. A large number of synaptic vesicles are released during high-frequency synaptic transmission; accordingly, synaptic vesicles need to be recycled rapidly to replenish the vesicle pool. Synaptic vesicle exocytosis and endocytosis are tightly coupled, and their coupling is essential for synaptic function and structural stability. We showed here that intracellular Na⁺ concentration at the calyx of Held terminal increased rapidly during spike activity and the increased Na⁺ accelerated endocytosis. Thus, when large numbers of synaptic vesicles are released during high-frequency synaptic transmission, Na⁺ accumulated in terminals and facilitated vesicle recycling. These findings represent a novel cellular mechanism that supports reliable synaptic transmission at high frequency in the CNS.

Spontaneous Ultraslow Na+ Fluctuations in the Neonatal Mouse Brain

In the neonate forebrain, network formation is driven by the spontaneous synchronized activity of pyramidal cells and interneurons, consisting of bursts of electrical activity and intracellular Ca²⁺ oscillations. By employing ratiometric Na⁺ imaging in tissue slices obtained from animals at postnatal day 2-4 (P2-4), we found that 20% of pyramidal neurons and 44% of astrocytes in neonatal mouse hippocampus also exhibit transient fluctuations in intracellular Na⁺. These occurred at very low frequencies (~2/h), were exceptionally long (~8 min), and strongly declined after the first postnatal week. Similar Na⁺ fluctuations were also observed in the neonate neocortex. In the hippocampus, Na⁺ elevations in both cell types were diminished when blocking action potential generation with tetrodotoxin. Neuronal Na⁺ fluctuations were significantly reduced by bicuculline, suggesting the involvement of GABA_A-receptors in their generation. Astrocytic signals, by contrast, were neither blocked by inhibition of receptors and/or transporters for different transmitters including GABA and glutamate, nor of various Na⁺-dependent transporters or Na⁺-permeable channels. In summary, our results demonstrate for the first time that neonatal astrocytes and neurons display spontaneous ultraslow Na⁺ fluctuations. While neuronal Na⁺ signals apparently largely rely on suprathreshold GABAergic excitation, astrocytic Na⁺ signals, albeit being dependent on neuronal action potentials, appear to have a separate trigger and mechanism, the source of which remains unclear at present.

Vesicular GABA Uptake Can Be Rate Limiting for Recovery of IPSCs from Synaptic Depression

Synaptic efficacy plays crucial roles in neuronal circuit operation and synaptic plasticity. Presynaptic determinants of synaptic efficacy are neurotransmitter content in synaptic vesicles and the number of vesicles undergoing exocytosis at a time. Bursts of presynaptic firings depress synaptic efficacy, mainly due to depletion of releasable vesicles, whereas recovery from strong depression is initiated by endocytic vesicle retrieval followed by refilling of vesicles with neurotransmitter. We washed out presynaptic cytosolic GABA to induce a rundown of IPSCs at cerebellar inhibitory cell pairs in slices from rats and then allowed fast recovery by elevating GABA concentration using photo-uncaging. The time course of this recovery coincided with that of IPSCs from activity-dependent depression induced by a train of high-frequency stimulation. We conclude that vesicular GABA uptake can be a limiting step for the recovery of inhibitory neurotransmission from synaptic depression.

Heterogeneous Signaling at GABA and Glycine Co-releasing Terminals

The corelease of several neurotransmitters from a single synaptic vesicle has been observed at many central synapses. Nevertheless, the signaling synergy offered by cotransmission and the mechanisms that maintain the optimal release and detection of neurotransmitters at mixed synapses remain poorly understood, thus limiting our ability to interpret changes in synaptic signaling and identify molecules important for plasticity. In the brainstem and spinal cord, GABA and glycine cotransmission is facilitated by a shared vesicular transporter VIAAT (also named VGAT), and occurs at many immature inhibitory synapses. As sensory and motor networks mature, GABA/glycine cotransmission is generally replaced by either pure glycinergic or GABAergic transmission, and the functional role for the continued corelease of GABA and glycine is unclear. Whether or not, and how, the GABA/glycine content is balanced in VIAAT-expressing vesicles from the same terminal, and how loading variability effects the strength of inhibitory transmission is not known. Here, we use a combination of loose-patch (LP) and whole-cell (WC) electrophysiology in cultured spinal neurons of GlyT2:eGFP mice to sample miniature inhibitory post synaptic currents (mIPSCs) that originate from individual GABA/glycine co-releasing synapses and develop a modeling approach to illustrate the gradual change in mIPSC phenotypes as glycine replaces GABA in vesicles. As a consistent GABA/glycine balance is predicted if VIAAT has access to both amino-acids, we test whether vesicle exocytosis from a single terminal evokes a homogeneous population of mixed mIPSCs. We recorded mIPSCs from 18 individual synapses and detected glycine-only mIPSCs in 4/18 synapses sampled. The rest (14/18) were co-releasing synapses that had a significant proportion of mixed GABA/glycine mIPSCs with a characteristic biphasic decay. The majority (9/14) of co-releasing synapses did not have a homogenous phenotype, but instead signaled with a combination of mixed and pure mIPSCs, suggesting that there is variability in the loading and/or storage of GABA and glycine at the level of individual vesicles. Our modeling predicts that when glycine replaces GABA in synaptic vesicles, the redistribution between the peak amplitude and charge transfer of mIPSCs acts to maintain the strength of inhibition while increasing the temporal precision of signaling.

Cocaine withdrawal reduces GABAB R transmission at entopeduncular nucleus - lateral habenula synapses

Lateral habenula (LHb) hyperactivity plays a pivotal role in the emergence of negative emotional states, including those occurring during withdrawal from addictive drugs. We have previously implicated cocaine‐driven adaptations at synapses from the entopeduncular nucleus (EPN) to the LHb in this process. Specifically, ionotropic GABA_A receptor (R)‐mediated neurotransmission at EPN‐to‐LHb synapses is reduced during cocaine withdrawal, due to impaired vesicle filling. Recent studies have shown that metabotropic GABA_BR signaling also controls LHb activity, although its role at EPN‐to‐LHb synapses during drug withdrawal is unknown. Here, we predicted that cocaine treatment would reduce GABA_BR‐mediated neurotransmission at EPN‐to‐LHb synapses. We chronically treated mice with saline or cocaine, prepared brain slices after two days of withdrawal and performed voltage‐clamp recordings from LHb neurons whilst optogenetically stimulating EPN terminals. Compared with controls, mice in cocaine withdrawal exhibited reduced GABA_AR‐mediated input to LHb neurons, and a reduced occurrence of GABA_BR‐signaling at EPN‐to‐LHb synapses. We then assessed the underlying mechanism of this decrease. Application of GABA_BR agonist baclofen evoked similar postsynaptic responses in EPN‐innervated LHb neurons in saline‐ and cocaine‐treated mice. Release probability at EPN‐to‐LHb GABAergic synapses was also comparable between groups. However, incubating brain slices in glutamine to facilitate GABA vesicle filling, normalized GABA_BR‐currents at EPN‐to‐LHb synapses in cocaine‐treated mice. Overall, we show that during cocaine withdrawal, together with reduced GABA_AR transmission, also GABA_BR‐mediated inhibitory signaling is diminished at EPN‐to‐LHb synapses, likely via the same presynaptic deficit. In concert, these alterations are predicted to contribute to the emergence of drug withdrawal symptoms, facilitating drug relapse.

Cytosolic Accumulation of L-Proline Disrupts GABA-Ergic Transmission through GAD Blockade

Proline dehydrogenase (PRODH), which degrades L-proline, resides within the schizophrenia-linked 22q11.2 deletion suggesting a role in disease. Supporting this, elevated L-proline levels have been shown to increase risk for psychotic disorders. Despite the strength of data linking PRODH and L-proline to neuropsychiatric diseases, targets of disease-relevant concentrations of L-proline have not been convincingly described. Here, we show that Prodh-deficient mice with elevated CNS L-proline display specific deficits in high-frequency GABA-ergic transmission and gamma-band oscillations. We find that L-proline is a GABA-mimetic and can act at multiple GABA-ergic targets. However, at disease-relevant concentrations, GABA-mimesis is limited to competitive blockade of glutamate decarboxylase leading to reduced GABA production. Significantly, deficits in GABA-ergic transmission are reversed by enhancing net GABA production with the clinically relevant compound vigabatrin. These findings indicate that accumulation of a neuroactive metabolite can lead to molecular and synaptic dysfunction and help to understand mechanisms underlying neuropsychiatric disease.

Neural circuit adaptations during drug withdrawal - Spotlight on the lateral habenula

Withdrawal after drug intake triggers a wealth of affective states including negative feelings reminiscent of depressive symptoms. This negative state can ultimately be crucial for relapse, a hallmark of addiction. Adaptations in a wide number of neuronal circuits underlie aspects of drug withdrawal, however causality between cellular modifications within these systems and precise behavioral phenotypes remains poorly described. Recent advances point to an instrumental role of the lateral habenula in driving depressive-like states during drug withdrawal. In this review we will discuss the general behavioral features of drug withdrawal, the importance of plasticity mechanisms in the mesolimbic systems, and the latest discoveries highlighting the implications of lateral habenula in drug addiction. We will further stress how specific interventions in the lateral habenula efficiently ameliorate depressive symptoms. Altogether, this work aims to provide a general knowledge on the cellular and circuit basis underlying drug withdrawal, ultimately speculating on potential treatment for precise aspects of addiction.

Proton electrochemical gradient: Driving and regulating neurotransmitter uptake

Accumulation of neurotransmitters in the lumen of synaptic vesicles (SVs) relies on the activity of the vacuolar-type H⁺ -ATPase. This pump drives protons into the lumen, generating a proton electrochemical gradient (Δμ_H+ ) across the membrane. Recent work has demonstrated that the balance between the chemical (ΔpH) and electrical (ΔΨ) components of Δμ_H+ is regulated differently by some distinct vesicle types. As different neurotransmitter transporters use ΔpH and ΔΨ with different relative efficiencies, regulation of this gradient balance has the potential to influence neurotransmitter uptake. Nevertheless, the underlying mechanisms responsible for this regulation remain poorly understood. In this review, we provide an overview of current neurotransmitter uptake models, with a particular emphasis on the distinct roles of the electrical and chemical gradients and current hypotheses for regulatory mechanisms.

Caloric restriction selectively reduces the GABAergic phenotype of mouse hypothalamic proopiomelanocortin neurons

KEY POINTS

Hypothalamic proopiomelanocortin (POMC) neurons release peptide products that potently inhibit food intake and reduce body weight. These neurons also release the amino acid transmitter GABA, which can inhibit downstream neurons. Although the release of peptide transmitters from POMC neurons is regulated by energy state, whether similar regulation of GABA release might occur had not been examined. The present results show that the GABAergic phenotype of POMC neurons is decreased selectively by caloric deficit and not altered by high-fat diet or stress. The fact the GABAergic phenotype of POMC neurons is sensitive to energy state suggests a dynamic physiological role for this transmitter and highlights the importance of determining the functional consequence of GABA released from POMC neurons in terms of the regulation of normal energy balance.

ABSTRACT

In addition to peptide transmitters, hypothalamic neurons, including proopiomelanocortin (POMC) and agouti-related peptide (AgRP) neurons, also release amino acid transmitters that can alter energy balance regulation. While recent studies show that the GABAergic nature of AgRP neurons is increased by caloric restriction, whether the GABAergic phenotype of POMC neurons is also regulated in an energy-state-dependent manner has not been previously examined. The present studies used fluorescence in situ hybridization to detect Gad1 and Gad2 mRNA in POMC neurons, as these encode the glutamate decarboxylase enzymes GAD67 and GAD65, respectively. The results show that both short-term fasting and chronic caloric restriction significantly reduce the percentage of POMC neurons expressing Gad1 mRNA in both male and female mice, with less of an effect on Gad2 expression. Neither acute nor chronic intermittent restraint stress altered Gad1 expression in POMC neurons. Maintenance on a high-fat diet also did not affect the portion POMC neurons expressing Gad1, suggesting that the GABAergic phenotype of POMC neurons is particularly sensitive to energy deficit. Because changes in Gad1 expression have been previously shown to correlate with altered terminal GABA release, fasting is likely to cause a decrease in GABA release from POMC neurons. Altogether, the present results show that the GABAergic nature of POMC neurons can be dynamically regulated by energy state in a manner opposite to that in AgRP neurons and suggest the importance of considering the functional role of GABA release in addition to the peptide transmitters from POMC neurons.

Presynaptic control of inhibitory neurotransmitter content in VIAAT containing synaptic vesicles

In mammals, fast inhibitory neurotransmission is carried out by two amino acid transmitters, γ-aminobutyric acid (GABA) and glycine. The higher brain uses only GABA, but in the spinal cord and brain stem both GABA and glycine act as inhibitory signals. In some cases GABA and glycine are co-released from the same neuron where they are co-packaged into synaptic vesicles by a shared vesicular inhibitory amino acid transporter, VIAAT (also called vGAT). The vesicular content of all other classical neurotransmitters (eg. glutamate, monoamines, acetylcholine) is determined by the presence of a specialized vesicular transporter. Because VIAAT is non-specific, the phenotype of inhibitory synaptic vesicles is instead predicted to be dependent on the relative concentration of GABA and glycine in the cytosol of the presynaptic terminal. This predicts that changes in GABA or glycine supply should be reflected in vesicle transmitter content but as yet, the mechanisms that control GABA versus glycine uptake into synaptic vesicles and their potential for modulation are not clearly understood. This review summarizes the most relevant experimental data that examines the link between GABA and glycine accumulation in the presynaptic cytosol and the inhibitory vesicle phenotype. The accumulated evidence challenges the hypothesis that vesicular phenotype is determined simply by the competition of inhibitory transmitter for VIAAT and instead suggest that the GABA/glycine balance in vesicles is dynamically regulated.

Mechanisms and functions of GABA co-release

The 'one neuron, one neurotransmitter' doctrine states that synaptic communication between two neurons occurs through the release of a single chemical transmitter. However, recent findings suggest that neurons that communicate using more than one classical neurotransmitter are prevalent throughout the adult mammalian CNS. In particular, several populations of neurons previously thought to release only glutamate, acetylcholine, dopamine or histamine also release the major inhibitory neurotransmitter GABA. Here, we review these findings and discuss the implications of GABA co-release for synaptic transmission and plasticity.

Presynaptic Molecular Determinants of Quantal Size

The quantal hypothesis for the release of neurotransmitters at the chemical synapse has gained wide acceptance since it was first worked out at the motor endplate in frog skeletal muscle in the 1950’s. Considering the morphological identification of synaptic vesicles (SVs) at the nerve terminals that appeared to be homogeneous in size, the hypothesis proposed that signal transduction at synapses is mediated by the release of neurotransmitters packed in SVs that are individually uniform in size; the amount of transmitter in a synaptic vesicle is called a quantum. Although quantal size—the amplitude of the postsynaptic response elicited by the release of neurotransmitters from a single vesicle—clearly depends on the number and sensitivity of the postsynaptic receptors, accumulating evidence has also indicated that the amount of neurotransmitters stored in SVs can be altered by various presynaptic factors. Here, I provide an overview of the concepts and underlying presynaptic molecular underpinnings that may regulate quantal size.

GABABR-Dependent Long-Term Depression at Hippocampal Synapses between CB1-Positive Interneurons and CA1 Pyramidal Cells

Activity induced long lasting modifications of synaptic efficacy have been extensively studied in excitatory synapses, however, long term plasticity is also a property of inhibitory synapses. Inhibitory neurons in the hippocampal CA1 region can be subdivided according to the compartment they target on the pyramidal cell. Some interneurons preferentially innervate the perisomatic area and axon hillock of the pyramidal cells while others preferentially target dendritic branches and spines. Another characteristic feature allowing functional classification of interneurons is cell type specific expression of different neurochemical markers and receptors. In the hippocampal CA1 region, nearly 90% of the interneurons expressing cannabinoid type 1 receptors (CB1R) also express cholecystokinin (CCK). Therefore, the functional presence of CB1 receptors can be used for identification of the inhibitory input from CCK positive (CCK+) interneurons to CA1 pyramidal cells. The goal of this study was to explore the nature of long term plasticity at the synapses between interneurons expressing CB1Rs (putative CCK+) and pyramidal neurons in the CA1 region of the hippocampus in vitro. We found that theta burst stimulation triggered robust long-term depression (LTD) at this synapse. The locus of LTD induction was postsynaptic and required activation of GABA_B receptors. We also showed that LTD at this synaptic connection involves GABA_BR-dependent suppression of adenylyl cyclase and consequent reduction of PKA activity. In this respect, CB1+ to pyramidal cell synapses differ from the majority of the other hippocampal inhibitory connections where theta burst stimulation results in long-term potentiation.

Optogenetic acidification of synaptic vesicles and lysosomes

Acidification is required for the function of many intracellular organelles, but methods to acutely manipulate their intraluminal pH have not been available. Here we present a targeting strategy to selectively express the light-driven proton pump Arch3 on synaptic vesicles. Our new tool, pHoenix, can functionally replace endogenous proton pumps, enabling optogenetic control of vesicular acidification and neurotransmitter accumulation. Under physiological conditions, glutamatergic vesicles are nearly full, as additional vesicle acidification with pHoenix only slightly increased the quantal size. By contrast, we found that incompletely filled vesicles exhibited a lower release probability than full vesicles, suggesting preferential exocytosis of vesicles with high transmitter content. Our subcellular targeting approach can be transferred to other organelles, as demonstrated for a pHoenix variant that allows light-activated acidification of lysosomes.

Gad1 mRNA as a reliable indicator of altered GABA release from orexigenic neurons in the hypothalamus

The strength of γ-aminobutyric acid (GABA)-mediated inhibitory synaptic input is a principle determinant of neuronal activity. However, because of differences in the number of GABA afferent inputs and the sites of synapses, it is difficult to directly assay for altered GABA transmission between specific cells. The present study tested the hypothesis that the level of mRNA for the GABA synthetic enzyme glutamate decarboxylase (GAD) can provide a reliable proxy for GABA release. This was tested in a mouse hypothalamic circuit important in the regulation of energy balance. Fluorescent in situ hybridization results show that the expression of Gad1 mRNA (encoding the GAD67 enzyme) was increased in hypothalamic neuropeptide Y/agouti-related peptide (NPY/AgRP) neurons after an overnight fast, consistent with the ability of GABA from these neurons to stimulate food intake. Optogenetic studies confirmed that the observed increase in Gad1 mRNA correlated with an increase in the probability of GABA release from NPY/AgRP neurons onto downstream proopiomelanocortin neurons. Likewise, there was an increase in the readily releasable pool of GABA in NPY/AgRP neurons. Selective inhibition of GAD activity in NPY/AgRP neurons decreased GABA release, indicating that GAD67 activity, which is largely dictated by expression level, is a key determinant of GABA release. Altogether, it appears that Gad expression may be a reliable proxy of altered GABAergic transmission. Examining changes in Gad mRNA as a proxy for GABA release may be particularly helpful when the downstream targets are not known or when limited tools exist for detecting GABA release at a particular synapse.

Immunogold characteristics of VGLUT3-positive GABAergic nerve terminals suggest corelease of glutamate

There is compelling evidence that glutamate can act as a cotransmitter in the mammalian brain. Interestingly, the third vesicular glutamate transporter (VGLUT3) is primarily found in neurons that were anticipated to be nonglutamatergic. Whereas the function of VGLUT3 in acetylcholinergic and serotoninergic neurons has been elucidated, the role of VGLUT3 in neurons releasing gamma-aminobutyric acid (GABA) is not settled. We have previously shown that VGLUT3 is found together with the vesicular GABA transporter (VIAAT) on synaptic vesicle membranes in the hippocampus. Now we provide novel electron microscopic data from the rat hippocampus suggesting that glutamate is enriched in inhibitory nerve terminals containing VGLUT3 compared to those lacking VGLUT3. The opposite was found for GABA; VGLUT3-positive inhibitory terminals contained lower density of GABA labeling compared to VGLUT3-negative inhibitory terminals. In addition, semiquantitative confocal immunofluorescence showed that N-methyl-D-aspartate (NMDA)-receptor labeling was present more frequently in VGLUT3-positive/VIAAT-positive synapses versus in VGLUT3-negative/VIAAT-positive synapses. Electron microscopic immunogold data further suggest that NMDA receptors are enriched in VGLUT3 containing inhibitory terminals. Our data reveal new chemical characteristics of a subset of GABAergic interneurons in the hippocampus. The analyses suggest that glutamate is coreleased with GABA from hippocampal basket cell-synapses to act on NMDA receptors.

A neuron-glia interaction involving GABA transaminase contributes to sleep loss in sleepless mutants

Sleep is an essential process and yet mechanisms underlying it are not well understood. Loss of the Drosophila quiver/sleepless (qvr/sss) gene increases neuronal excitability and diminishes daily sleep, providing an excellent model for exploring the underpinnings of sleep regulation. Here, we used a proteomic approach to identify proteins altered in sss brains. We report that loss of sleepless post-transcriptionally elevates the CG7433 protein, a mitochondrial γ-aminobutyric acid transaminase (GABAT), and reduces GABA in fly brains. Loss of GABAT increases daily sleep and improves sleep consolidation, indicating that GABAT promotes wakefulness. Importantly, disruption of the GABAT gene completely suppresses the sleep phenotype of sss mutants, demonstrating that GABAT is required for loss of sleep in sss mutants. While SSS acts in distinct populations of neurons, GABAT acts in glia to reduce sleep in sss flies. Our results identify a novel mechanism of interaction between neurons and glia that is important for the regulation of sleep.

Vesicular glutamate transporter expression level affects synaptic vesicle release probability at hippocampal synapses in culture

The vesicular glutamate transporter (VGLUT) plays an essential role in synaptic transmission by filling vesicles with glutamate. At mammalian synapses, VGLUT expression level determines the amount of glutamate packaged into vesicles, and the specific paralog of VGLUT expressed affects the release probability. In this study, we investigate whether there is a link between the number of VGLUTs on vesicles and release probability. We used a combination of electrophysiology and imaging techniques in cultured mouse hippocampal neurons where the VGLUT expression level has been severely altered. We found that vesicles with drastically reduced VGLUT expression were released with a lower probability. This deficit in release could only be rescued by a functional transporter, suggesting that the transport function, and not the molecular interactions, of the protein affects vesicle release. Based on these data, we propose a novel means of presynaptic vesicle release regulation--the intravesicular glutamate fill state of the vesicle.

Midbrain dopamine neurons sustain inhibitory transmission using plasma membrane uptake of GABA, not synthesis

Synaptic transmission between midbrain dopamine neurons and target neurons in the striatum is essential for the selection and reinforcement of movements. Recent evidence indicates that nigrostriatal dopamine neurons inhibit striatal projection neurons by releasing a neurotransmitter that activates GABA_A receptors. Here, we demonstrate that this phenomenon extends to mesolimbic afferents, and confirm that the released neurotransmitter is GABA. However, the GABA synthetic enzymes GAD65 and GAD67 are not detected in midbrain dopamine neurons. Instead, these cells express the membrane GABA transporters mGAT1 (Slc6a1) and mGAT4 (Slc6a11) and inhibition of these transporters prevents GABA co-release. These findings therefore indicate that GABA co-release is a general feature of midbrain dopaminergic neurons that relies on GABA uptake from the extracellular milieu as opposed to de novo synthesis. This atypical mechanism may confer dopaminergic neurons the flexibility to differentially control GABAergic transmission in a target-dependent manner across their extensive axonal arbors.

DOI:http://dx.doi.org/10.7554/eLife.01936.001

The electrical signals that are fired along neurons cannot be transmitted across the small gaps, called synapses that are found between neurons. Instead, the neuron sending the signal releases chemicals called neurotransmitters into the synapse. These neurotransmitters bind to receptor proteins on the surface of the second neuron and control how it fires.

A neurotransmitter called dopamine plays a key role in the circuits of the brain that control how we learn certain tasks involving movement. In particular, two populations of neurons from the midbrain that release dopamine target the striatum, an area of the brain that is responsible for motor control. These neurons also release other neurotransmitters, but the identity of these other chemicals is not known, and the details of the interaction between the neurons and the striatum are poorly understood.

Previous research showed that some of the midbrain neurons activate receptors that normally respond to a neurotransmitter called gamma-aminobutyric acid (GABA). However, several different chemicals can trigger this receptor. Using a range of techniques, Tritsch et al. now confirm that dopamine neurons release GABA alongside dopamine, and that this applies to both sets of the dopamine-producing neurons that feed into the striatum.

Some neurons can manufacture GABA from amino acids found in their internal fluid. However, Tritsch et al. could not detect the enzymes needed for this reaction in dopamine-producing neurons. Instead, these neurons contain proteins that can transport GABA across the cell membrane, which suggests that the neurons collect GABA from the extracellular fluid that surrounds them.

DOI:http://dx.doi.org/10.7554/eLife.01936.002

A local glutamate-glutamine cycle sustains synaptic excitatory transmitter release

Biochemical studies suggest that excitatory neurons are metabolically coupled with astrocytes to generate glutamate for release. However, the extent to which glutamatergic neurotransmission depends on this process remains controversial because direct electrophysiological evidence is lacking. The distance between cell bodies and axon terminals predicts that glutamine-glutamate cycle is synaptically localized. Hence, we investigated isolated nerve terminals in brain slices by transecting hippocampal Schaffer collaterals and cortical layer I axons. Stimulating with alternating periods of high frequency (20 Hz) and rest (0.2 Hz), we identified an activity-dependent reduction in synaptic efficacy that correlated with reduced glutamate release. This was enhanced by inhibition of astrocytic glutamine synthetase and reversed or prevented by exogenous glutamine. Importantly, this activity dependence was also revealed with an in-vivo-derived natural stimulus both at network and cellular levels. These data provide direct electrophysiological evidence that an astrocyte-dependent glutamate-glutamine cycle is required to maintain active neurotransmission at excitatory terminals.