Ambient GABA-activated tonic inhibition sharpens auditory coincidence detection via a depolarizing shunting mechanism

Established and emerging GABAA receptor pharmacotherapy for epilepsy

Drugs that modulate the GABAA receptor are widely used in clinical practice for both the long-term management of epilepsy and emergency seizure control. In addition to older medications that have well-defined roles for the treatment of epilepsy, recent discoveries into the structure and function of the GABAA receptor have led to the development of newer compounds designed to maximise therapeutic benefit whilst minimising adverse effects, and whose position within the epilepsy pharmacologic armamentarium is still emerging. Drugs that modulate the GABAA receptor will remain a cornerstone of epilepsy management for the foreseeable future and, in this article, we provide an overview of the mechanisms and clinical efficacy of both established and emerging pharmacotherapies.

GABA tone regulation and its cognitive functions in the brain

γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter released at GABAergic synapses, mediating fast-acting phasic inhibition. Emerging lines of evidence unequivocally indicate that a small amount of extracellular GABA - GABA tone - exists in the brain and induces a tonic GABA current that controls neuronal activity on a slow timescale relative to that of phasic inhibition. Surprisingly, studies indicate that glial cells that synthesize GABA, such as astrocytes, release GABA through non-vesicular mechanisms, such as channel-mediated release, and thereby act as the source of GABA tone in the brain. In this Review, we first provide an overview of major advances in our understanding of the cell-specific molecular and cellular mechanisms of GABA synthesis, release and clearance that regulate GABA tone in various brain regions. We next examine the diverse ways in which the tonic GABA current regulates synaptic transmission and synaptic plasticity through extrasynaptic GABAA-receptor-mediated mechanisms. Last, we discuss the physiological mechanisms through which tonic inhibition modulates cognitive function on a slow timescale. In this Review, we emphasize that the cognitive functions of tonic GABA current extend beyond mere inhibition, laying a foundation for future research on the physiological and pathophysiological roles of GABA tone regulation in normal and abnormal psychiatric conditions.

Focusing on the Emerging Role of Kainate Receptors in the Dorsal Cochlear Nucleus (DCN) and Cerebellum

Mammals have a dorsal cochlear nucleus (DCN), which is thought to be a cerebellum-like structure with similar features in terms of structure and microcircuitry to the cerebellum. Both the DCN and cerebellum perform their functions depending on synaptic and neuronal networks mediated by various glutamate receptors. Kainate receptors (KARs) are one class of the glutamate receptor family and are strongly expressed in the hippocampus, the cerebellum, and cerebellum-like structures. The cellular distribution and the potential role of KARs in the hippocampus have been extensively investigated. However, the cellular distribution and the potential role of KARs in cerebellum-like structures, including the DCN and cerebellum, are poorly understood. In this review, we summarize the similarity between the DCN and cerebellum at the levels of structure, circuitry, and cell type as well as the investigations referring to the expression patterns of KARs in the DCN and cerebellum according to previous studies. Recent studies on the role of KARs have shown that KARs mediate a bidirectional modulatory effect at parallel fiber (PF)-Purkinje cell (PC) synapses in the cerebellum, implying insights into their roles in cerebellum-like structures, including the DCN, that remain to be explored in the coming years.

Cellular Strategies for Frequency-Dependent Computation of Interaural Time Difference

Binaural coincidence detection is the initial step in encoding interaural time differences (ITDs) for sound-source localization. In birds, neurons in the nucleus laminaris (NL) play a central role in this process. These neurons receive excitatory synaptic inputs on dendrites from both sides of the cochlear nucleus and compare their coincidences at the soma. The NL is tonotopically organized, and individual neurons receive a pattern of synaptic inputs that are specific to their tuning frequency. NL neurons differ in their dendritic morphology along the tonotopic axis; their length increases with lower tuning frequency. In addition, our series of studies have revealed several frequency-dependent refinements in the morphological and biophysical characteristics of NL neurons, such as the amount and subcellular distribution of ion channels and excitatory and inhibitory synapses, which enable the neurons to process the frequency-specific pattern of inputs appropriately and encode ITDs at each frequency band. In this review, we will summarize these refinements of NL neurons and their implications for the ITD coding. We will also discuss the similarities and differences between avian and mammalian coincidence detectors.

Cross-talk pattern between GABAA- and glycine-receptors in CNS neurons is shaped by their relative expression levels

GABA_A receptors (GABA_ARs) and glycine receptors (GlyRs) are two principal inhibitory chloride ion channels in the central nervous system. The two receptors do not function independently but cross-talk to each other, i.e., the activation of one receptor would inhibit the other. This cross-talk is present in different patterns across various regions in the central nervous system; however, the factor that determines these patterns is not understood. Here, we show that the pattern of cross-talk between the two receptors is shaped by their relative expression level in a neuron: a higher expression level correlates with louder talk. In line with a tendency of decrease in expression level of GlyRs and increase in expression level of GABA_ARs from the spinal cord, the brainstem to the neocortex, GlyRs talked much louder (i.e. produced greater inhibition) than GABA_ARs (one-way pattern) in spinal cord neurons, about equally loud as GABA_ARs (symmetric pattern) in inferior colliculus neurons and less loud (i.e. less inhibition) than GABA_ARs (asymmetric pattern) in auditory cortex neurons. Overexpression of GlyRs in inferior colliculus neurons produced an asymmetric pattern that should otherwise have been observed in spinal cord neurons. These expression level-dependent patterns of cross-talk between the two receptors may suggest how the central nervous system uses an alternative mechanism to maintain a delicate level of inhibition through adjusting the proportion of the two receptors in a neuron along its pathway.

Neuromodulation by mGluRs in Sound Localization Circuits in the Auditory Brainstem

The ability of humans and animals to localize the source of a sound in a complex acoustic environment facilitates communication and survival. Two cues are used for sound localization at horizontal planes, interaural time and level differences (ITD and ILD), which are analyzed by distinct neural circuits in the brainstem. Here, we review the studies on metabotropic glutamate receptor (mGluR)-mediated neuromodulation of both intrinsic and synaptic properties of brainstem neurons in these circuits. Both mammalian and avian animal models have been used, with each having their advantages that are not present in the other. For the mammalian model, we discuss mGluR neuromodulation in the ILD circuit, with an emphasis on the recent discovery of differential modulation of synaptic transmission of different transmitter release modes. For the avian model, we focus on reviewing mGluR neuromodulation in the ITD pathway, with an emphasis on tonotopic distribution and synaptic plasticity of mGluR modulation in coincidence detector neurons. Future works are proposed to further investigate the functions and mechanisms of mGluRs in the sound localization circuits.

Extracellular GABA waves regulate coincidence detection in excitatory circuits

KEY POINTS

Rapid changes in neuronal network activity trigger widespread waves of extracellular GABA in hippocampal neuropil. Elevations of extracellular GABA narrow the coincidence detection window for excitatory inputs to CA1 pyramidal cells. GABA transporters control the effect of extracellular GABA on coincidence detection. Small changes in the kinetics of dendritic excitatory currents amplify when reaching the soma.

ABSTRACT

Coincidence detection of excitatory inputs by principal neurons underpins the rules of signal integration and Hebbian plasticity in the brain. In the hippocampal circuitry, detection fidelity is thought to depend on the GABAergic synaptic input through a feedforward inhibitory circuit also involving the hyperpolarisation-activated Ih current. However, afferent connections often bypass feedforward circuitry, suggesting that a different GABAergic mechanism might control coincidence detection in such cases. To test whether fluctuations in the extracellular GABA concentration [GABA] could play a regulatory role here, we use a GABA 'sniffer' patch in acute hippocampal slices of the rat and document strong dependence of [GABA] on network activity. We find that blocking GABAergic signalling strongly widens the coincidence detection window of direct excitatory inputs to pyramidal cells whereas increasing [GABA] through GABA uptake blockade shortens it. The underlying mechanism involves membrane-shunting tonic GABAA receptor current; it does not have to rely on Ih but depends strongly on the neuronal GABA transporter GAT-1. We use dendrite-soma dual patch-clamp recordings to show that the strong effect of membrane shunting on coincidence detection relies on nonlinear amplification of changes in the decay of dendritic synaptic currents when they reach the soma. Our results suggest that, by dynamically regulating extracellular GABA, brain network activity can optimise signal integration rules in local excitatory circuits.

The Functional Role of Spontaneously Opening GABAA Receptors in Neural Transmission

Ionotropic type of γ-aminobutyric acid receptors (GABA_ARs) produce two forms of inhibitory signaling: phasic inhibition generated by rapid efflux of neurotransmitter GABA into the synaptic cleft with subsequent binding to GABA_ARs, and tonic inhibition generated by persistent activation of extrasynaptic and/or perisynaptic GABA_ARs by GABA continuously present in the extracellular space. It is widely accepted that phasic and tonic GABAergic inhibition is mediated by receptor groups of distinct subunit composition and modulated by different cytoplasmic mechanisms. Recently, however, it has been demonstrated that spontaneously opening GABA_ARs (s-GABA_ARs), which do not need GABA binding to enter an active state, make a significant input into tonic inhibitory signaling. Due to GABA-independent action mode, s-GABA_ARs promise new safer options for therapy of neural disorders (such as epilepsy) devoid of side effects connected to abnormal fluctuations of GABA concentration in the brain. However, despite the potentially important role of s-GABA_ARs in neural signaling, they still remain out of focus of neuroscience studies, to a large extent due to technical difficulties in their experimental research. Here, we summarize present data on s-GABA_ARs functional properties and experimental approaches that allow isolation of s-GABA_ARs effects from those of conventional (GABA-dependent) GABA_ARs.

GABA is a modulator, rather than a classical transmitter, in the medial nucleus of the trapezoid body-lateral superior olive sound localization circuit

KEY POINTS

The lateral superior olive (LSO), a brainstem hub involved in sound localization, integrates excitatory and inhibitory inputs from the ipsilateral and the contralateral ear, respectively. In gerbils and rats, inhibition to the LSO reportedly shifts from GABAergic to glycinergic within the first three postnatal weeks. Surprisingly, we found no evidence for synaptic GABA signalling during this time window in mouse LSO principal neurons. However, we found that presynaptic GABA_B Rs modulate Ca²⁺ influx into medial nucleus of the trapezoid body axon terminals, resulting in reduced synaptic strength. Moreover, GABA elicited strong responses in LSO neurons that were mediated by extrasynaptic GABA_A Rs. RNA sequencing revealed highly abundant δ subunits, which are characteristic of extrasynaptic receptors. Whereas GABA increased the excitability of neonatal LSO neurons, it reduced the excitability around hearing onset. Collectively, GABA appears to control the excitability of mouse LSO neurons via extrasynaptic and presynaptic signalling. Thus, GABA acts as a modulator, rather than as a classical transmitter.

ABSTRACT

GABA and glycine mediate fast inhibitory neurotransmission and are coreleased at several synapse types. Here we assessed the contribution of GABA and glycine in synaptic transmission between the medial nucleus of the trapezoid body (MNTB) and the lateral superior olive (LSO), two nuclei involved in sound localization. Whole-cell patch-clamp experiments in acute mouse brainstem slices at postnatal days (P) 4 and 11 during pharmacological blockade of GABA_A receptors (GABA_A Rs) and/or glycine receptors demonstrated no GABAergic synaptic component on LSO principal neurons. A GABAergic component was absent in evoked inhibitory postsynaptic currents and miniature events. Coimmunofluorescence experiments revealed no codistribution of the presynaptic GABAergic marker GAD65/67 with gephyrin, a postsynaptic marker for GABA_A Rs, corroborating the conclusion that GABA does not act synaptically in the mouse LSO. Imaging experiments revealed reduced Ca²⁺ influx into MNTB axon terminals following activation of presynaptic GABA_B Rs. GABA_B R activation reduced the synaptic strength at P4 and P11. GABA appears to act on extrasynaptic GABA_A Rs as demonstrated by application of 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol, a δ-subunit-specific GABA_A R agonist. RNA sequencing showed high mRNA levels for the δ-subunit in the LSO. Moreover, GABA transporters GAT-1 and GAT-3 appear to control extracellular GABA. Finally, we show an age-dependent effect of GABA on the excitability of LSO neurons. Whereas tonic GABA increased the excitability at P4, leading to spike facilitation, it decreased the excitability at P11 via shunting inhibition through extrasynaptic GABA_A Rs. Taken together, we demonstrate a modulatory role of GABA in the murine LSO, rather than a function as a classical synaptic transmitter.

Postsynaptic FMRP Regulates Synaptogenesis In Vivo in the Developing Cochlear Nucleus

A global loss of the fragile X mental retardation protein (FMRP; encoded by the Fmr1 gene) leads to sensory dysfunction and intellectual disabilities. One underlying mechanism of these phenotypes is structural and functional deficits in synapses. Here, we determined the autonomous function of postsynaptic FMRP in circuit formation, synaptogenesis, and synaptic maturation. In normal cochlea nucleus, presynaptic auditory axons form large axosomatic endbulb synapses on cell bodies of postsynaptic bushy neurons. In ovo electroporation of drug-inducible Fmr1-shRNA constructs produced a mosaicism of FMRP expression in chicken (either sex) bushy neurons, leading to reduced FMRP levels in transfected, but not neighboring nontransfected, neurons. Structural analyses revealed that postsynaptic FMRP reduction led to smaller size and abnormal morphology of individual presynaptic endbulbs at both early and later developmental stages. We further examined whether FMRP reduction affects dendritic development, as a potential mechanism underlying defective endbulb formation. Normally, chicken bushy neurons grow extensive dendrites at early stages and retract these dendrites when endbulbs begin to form. Neurons transfected with Fmr1 shRNA exhibited a remarkable delay in branch retraction, failing to provide necessary somatic surface for timely formation and growth of large endbulbs. Patch-clamp recording verified functional consequences of dendritic and synaptic deficits on neurotransmission, showing smaller amplitudes and slower kinetics of spontaneous and evoked EPSCs. Together, these data demonstrate that proper levels of postsynaptic FMRP are required for timely maturation of somatodendritic morphology, a delay of which may affect synaptogenesis and thus contribute to long-lasting deficits of excitatory synapses.SIGNIFICANCE STATEMENT Fragile X mental retardation protein (FMRP) regulates a large variety of neuronal activities. A global loss of FMRP affects neural circuit development and synaptic function, leading to fragile X syndrome (FXS). Using temporally and spatially controlled genetic manipulations, this study provides the first in vivo report that autonomous FMRP regulates multiple stages of dendritic development, and that selective reduction of postsynaptic FMRP leads to abnormal development of excitatory presynaptic terminals and compromised neurotransmission. These observations demonstrate secondary influence of developmentally transient deficits in neuronal morphology and connectivity to the development of long-lasting synaptic pathology in FXS.

Anatomy and Physiology of Metabotropic Glutamate Receptors in Mammalian and Avian Auditory System

Glutamate, as the major excitatory neurotransmitter used in the vertebrate brain, activates ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs), which mediate fast and slow neuronal actions, respectively. mGluRs play important modulatory roles in many brain areas, forming potential targets for drugs developed to treat brain disorders. Here, we review studies on mGluRs in the mammalian and avian auditory system. Although anatomical expression of mGluRs in the cochlear nucleus has been well characterized, data for other auditory nuclei await more systematic investigations especially at the electron microscopy level. The physiology of mGluRs has been extensively studied using in vitro brain slice preparations, with a focus on the auditory circuitry in the brainstem. These in vitro physiological studies have demonstrated that mGluRs participate in synaptic transmission, regulate ionic homeostasis, induce synaptic plasticity, and maintain the balance between Excitation and Inhibition (E/I) in a variety of auditory structures. However, the modulatory roles of mGluRs in auditory processing remain largely unclear at the system and behavioral levels, and the functions of mGluRs in auditory disorders remain entirely unknown.

Intrinsic physiology of inhibitory neurons changes over auditory development

During auditory development, changes in membrane properties promote the ability of excitatory neurons in the brain stem to code aspects of sound, including the level and timing of a stimulus. Some of these changes coincide with hearing onset, suggesting that sound-driven neural activity produces developmental plasticity of ion channel expression. While it is known that the coding properties of excitatory neurons are modulated by inhibition in the mature system, it is unknown whether there are also developmental changes in the membrane properties of brain stem inhibitory neurons. We investigated the primary source of inhibition in the avian auditory brain stem, the superior olivary nucleus (SON). The present studies test the hypothesis that, as in excitatory neurons, the membrane properties of these inhibitory neurons change after hearing onset. We examined SON neurons at different stages of auditory development: embryonic days 14-16 (E14-E16), a time at which cochlear ganglion neurons are just beginning to respond to sound; later embryonic stages (E18-E19); and after hatching (P0-P2). We used in vitro whole cell patch electrophysiology to explore physiological changes in SON. Age-related changes were observed at the level of a single spike and in multispiking behavior. In particular, tonic behavior, measured as a neuron's ability to sustain tonic firing over a range of current steps, became more common later in development. Voltage-clamp recordings and biophysical models were employed to examine how age-related increases in ion currents enhance excitability in SON. Our findings suggest that concurrent increases in sodium and potassium currents underlie the emergence of tonic behavior. NEW & NOTEWORTHY This article is the first to examine heterogeneity of neuronal physiology in the inhibitory nucleus of the avian auditory system and demonstrate that tonic firing here emerges over development. By pairing computer simulations with physiological data, we show that increases in both sodium and potassium channels over development are necessary for the emergence of tonic firing.

Distinct Neural Properties in the Low-Frequency Region of the Chicken Cochlear Nucleus Magnocellularis

Topography in the avian cochlear nucleus magnocellularis (NM) is represented as gradually increasing characteristic frequency (CF) along the caudolateral-to-rostromedial axis. In this study, we characterized the organization and cell biophysics of the caudolateral NM (NMc) in chickens (Gallus gallus). Examination of cellular and dendritic architecture first revealed that NMc contains small neurons and extensive dendritic processes, in contrast to adendritic, large neurons located more rostromedially. Individual dye-filling study further demonstrated that NMc is divided into two subregions, with NMc2 neurons having larger and more complex dendritic fields than NMc1. Axonal tract tracing studies confirmed that NMc1 and NMc2 neurons receive afferent inputs from the auditory nerve and the superior olivary nucleus, similar to the adendritic NM. However, the auditory axons synapse with NMc neurons via small bouton-like terminals, unlike the large end bulb synapses on adendritic NM neurons. Immunocytochemistry demonstrated that most NMc2 neurons express cholecystokinin but not calretinin, distinct from NMc1 and adendritic NM neurons that are cholecystokinin negative and mostly calretinin positive. Finally, whole-cell current clamp recordings revealed that NMc neurons require significantly lower threshold current for action potential generation than adendritic NM neurons. Moreover, in contrast to adendritic NM neurons that generate a single-onset action potential, NMc neurons generate multiple action potentials to suprathreshold sustained depolarization. Taken together, our data indicate that NMc contains multiple neuron types that are structurally, connectively, molecularly, and physiologically different from traditionally defined NM neurons, emphasizing specialized neural properties for processing low-frequency sounds.

Hippocampal GABAergic transmission: a new target for adenosine control of excitability

Physiological network functioning in the hippocampus is dependent on a balance between glutamatergic cell excitability and the activity of diverse local circuit neurons that release the inhibitory neurotransmitter γ-aminobutyric acid (GABA). Tuners of neuronal communication such as adenosine, an endogenous modulator of synapses, control hippocampal network operations by regulating excitability. Evidence has been recently accumulating on the influence of adenosine on different aspects of GABAergic transmission to shape hippocampal function. This review addresses how adenosine, through its high-affinity A₁ (A₁ R) and A_2A receptors (A_2A R), interferes with different GABA-mediated forms of inhibition in the hippocampus to regulate neuronal excitability. Adenosine-mediated modulation of phasic/tonic inhibitory transmission, of GABA transport mechanisms and its interference with other modulatory systems are discussed together with the putative implications for neuronal function in physiological and pathological conditions. This article is part of a mini review series: 'Synaptic Function and Dysfunction in Brain Diseases'.

Synaptic Inhibition in Avian Interaural Level Difference Sound Localizing Neurons

Synaptic inhibition plays a fundamental role in the neural computation of the interaural level difference (ILD), an important cue for the localization of high-frequency sound. Here, we studied the inhibitory synaptic currents in the chicken posterior portion of the dorsal nucleus of the lateral lemniscus (LLDp), the first binaural level difference encoder of the avian auditory pathway. Using whole-cell recordings in brain slices, we provide the first evidence confirming a monosynaptic inhibition driven by direct electrical and chemical stimulation of the contralateral LLDp, establishing the reciprocal inhibitory connection between the two LLDps, a long-standing assumption in the field. This inhibition was largely mediated by GABAA receptors; however, functional glycine receptors were also identified. The reversal potential for the Cl- channels measured with gramicidin-perforated patch recordings was hyperpolarizing (-88 mV), corresponding to a low intracellular Cl- concentration (5.2 mm). Pharmacological manipulations of KCC2 (outwardly Cl- transporter) activity demonstrate that LLDp neurons can maintain a low intracellular Cl- concentration under a high Cl- load, allowing for the maintenance of hyperpolarizing inhibition. We further demonstrate that hyperpolarizing inhibition was more effective at regulating cellular excitability than depolarizing inhibition in LLDp neurons.

Chronic Gestational Exposure to Ethanol Leads to Enduring Aberrances in Cortical Form and Function in the Medial Prefrontal Cortex

BACKGROUND

Exposure to ethanol (EtOH) in utero alters the disposition of tangentially migrating GABAergic interneurons in the fetal brain. The medial ganglionic eminence (MGE) gives rise to a large portion of cortical GABAergic interneurons, including the parvalbumin-expressing interneurons that shape and contribute to inhibitory/excitatory (I/E) balance of the intracortical circuit. Here, we investigated in the mouse medial prefrontal cortex (mPFC) the hypothesis that low levels of maternal EtOH consumption from closure of the neural tube embryonic day (E) 9.5 until birth result in an enduring interneuronopathy.

METHODS

Pregnant mice were subjected to a 2% w/w EtOH consumption regimen starting at neural tube closure and ending at parturition. Neurogenesis in the MGE was assessed by 5-bromo-2-deoxyuridine (BrdU) immunofluorescence at E12.5. The count and distribution of parvalbumin-expressing interneurons were determined in adult animals, and patch clamp electrophysiology was performed to determine GABAergic function and I/E balance. Open-field behavior in adult mice was assessed to determine whether the EtOH-exposed cohort displayed a lasting alteration in exploratory behavior.

RESULTS

In embryos exposed to EtOH in utero, we found increased BrdU labeling in the MGE, pointing to increased neurogenesis. Adult mice prenatally exposed to EtOH were hyperactive, and this was associated with an increase in parvalbumin-expressing GABAergic interneurons in the mPFC. In addition, prenatal EtOH exposure altered the balance between spontaneous inhibitory and excitatory synaptic input and attenuated GABAergic tone in layer V mPFC pyramidal neurons in juvenile mice.

CONCLUSIONS

These findings underscore that altered migration of GABAergic interneurons contributes to the EtOH-induced aberration of cortical development and that these effects persist into adulthood as altered cortical form and function.

Depolarizing GABA/glycine synaptic events switch from excitation to inhibition during frequency increases

By acting on their ionotropic chloride channel receptors, GABA and glycine represent the major inhibitory transmitters of the central nervous system. Nevertheless, in various brain structures, depolarizing GABAergic/glycinergic postsynaptic potentials (dGPSPs) lead to dual inhibitory (shunting) and excitatory components, the functional consequences of which remain poorly acknowledged. Indeed, the extent to which each component prevails during dGPSP is unclear. Understanding the mechanisms predicting the dGPSP outcome on neural network activity is therefore a major issue in neurobiology. By combining electrophysiological recordings of spinal embryonic mouse motoneurons and modelling study, we demonstrate that increasing the chloride conductance (g_Cl) favors inhibition either during a single dGPSP or during trains in which g_Cl summates. Finally, based on this summation mechanism, the excitatory effect of EPSPs is overcome by dGPSPs in a frequency-dependent manner. These results reveal an important mechanism by which dGPSPs protect against the overexcitation of neural excitatory circuits.

Differential modulation of phasic and tonic inhibition underlies serotonergic suppression of long-term potentiation in the rat visual cortex

GABA receptor type A (GABA(A)R)-mediated inhibition is divided into phasic and tonic inhibition. GABA(A)Rs mediating the two inhibitory modalities exhibit differences in subcellular localization and subunit composition. We previously demonstrated that phasic and tonic inhibition are independently regulated by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and protein kinase A (PKA), respectively. Since modulation of GABA(A)Rs by phosphorylation differs depending on subunit composition and protein kinases, phasic and tonic inhibition might be differentially regulated by a single neuromodulator activating multiple protein kinases. However, the neuromodulatory control for phasic and tonic inhibition is largely unknown. Thus, in the present study, we concurrently investigated the serotonin (5-HT) regulation of phasic and tonic inhibition and its functional implication in the pyramidal neurons of the rat visual cortex. Interestingly, 5-HT enhanced phasic inhibition but suppressed tonic inhibition. Increase in phasic inhibition was mediated by 5-HT2 receptor and CaMKII, whereas decrease in tonic inhibition depended on 5-HT1A receptor and PKA. Thus, phasic and tonic inhibition might be independently regulated even by a single neuromodulator. Functionally, the opposite modulation of phasic and tonic inhibition decreased the summation of consecutive excitatory postsynaptic potentials (EPSPs) without affecting the shape of single EPSPs, which might underlie the suppression of the induction of long-term potentiation by 5-HT. These results suggest that the integrative regulation of phasic and tonic inhibition provides mechanisms for elaborate modulation of shape and summation of EPSPs and long-term synaptic plasticity.

Phasic and Tonic Inhibition are Maintained Respectively by CaMKII and PKA in the Rat Visual Cortex

Phasic and tonic γ-aminobutyric acid_A (GABA_A) receptor-mediated inhibition critically regulate neuronal information processing. As these two inhibitory modalities have distinctive features in their receptor composition, subcellular localization of receptors, and the timing of receptor activation, it has been thought that they might exert distinct roles, if not completely separable, in the regulation of neuronal function. Inhibition should be maintained and regulated depending on changes in network activity, since maintenance of excitation-inhibition balance is essential for proper functioning of the nervous system. In the present study, we investigated how phasic and tonic inhibition are maintained and regulated by different signaling cascades. Inhibitory postsynaptic currents were measured as either electrically evoked events or spontaneous events to investigate regulation of phasic inhibition in layer 2/3 pyramidal neurons of the rat visual cortex. Tonic inhibition was assessed as changes in holding currents by the application of the GABA_A receptor blocker bicuculline. Basal tone of phasic inhibition was maintained by intracellular Ca²⁺ and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). However, maintenance of tonic inhibition relied on protein kinase A activity. Depolarization of membrane potential (5 min of 0 mV holding) potentiated phasic inhibition via Ca²⁺ and CaMKII but tonic inhibition was not affected. Thus, phasic and tonic inhibition seem to be independently maintained and regulated by different signaling cascades in the same cell. These results suggest that neuromodulatory signals might differentially regulate phasic and tonic inhibition in response to changes in brain states.

Activity-dependent modulation of inhibitory synaptic kinetics in the cochlear nucleus

Spherical bushy cells (SBCs) in the anteroventral cochlear nucleus respond to acoustic stimulation with discharges that precisely encode the phase of low-frequency sound. The accuracy of spiking is crucial for sound localization and speech perception. Compared to the auditory nerve input, temporal precision of SBC spiking is improved through the engagement of acoustically evoked inhibition. Recently, the inhibition was shown to be less precise than previously understood. It shifts from predominantly glycinergic to synergistic GABA/glycine transmission in an activity-dependent manner. Concurrently, the inhibition attains a tonic character through temporal summation. The present study provides a comprehensive understanding of the mechanisms underlying this slow inhibitory input. We performed whole-cell voltage clamp recordings on SBCs from juvenile Mongolian gerbils and recorded evoked inhibitory postsynaptic currents (IPSCs) at physiological rates. The data reveal activity-dependent IPSC kinetics, i.e., the decay is slowed with increased input rates or recruitment. Lowering the release probability yielded faster decay kinetics of the single- and short train-IPSCs at 100 Hz, suggesting that transmitter quantity plays an important role in controlling the decay. Slow transmitter clearance from the synaptic cleft caused prolonged receptor binding and, in the case of glycine, spillover to nearby synapses. The GABAergic component prolonged the decay by contributing to the asynchronous vesicle release depending on the input rate. Hence, the different factors controlling the amount of transmitters in the synapse jointly slow the inhibition during physiologically relevant activity. Taken together, the slow time course is predominantly determined by the receptor kinetics and transmitter clearance during short stimuli, whereas long duration or high frequency stimulation additionally engage asynchronous release to prolong IPSCs.

Metabotropic glutamate receptors in auditory processing

As the major excitatory neurotransmitter used in the vertebrate brain, glutamate activates ionotropic and metabotropic glutamate receptors (mGluRs), which mediate fast and slow neuronal actions, respectively. Important modulatory roles of mGluRs have been shown in many brain areas, and drugs targeting mGluRs have been developed for the treatment of brain disorders. Here, I review studies on mGluRs in the auditory system. Anatomical expression of mGluRs in the cochlear nucleus has been well characterized, while data for other auditory nuclei await more systematic investigations at both the light and electron microscopy levels. The physiology of mGluRs has been extensively studied using in vitro brain slice preparations, with a focus on the lower auditory brainstem in both mammals and birds. These in vitro physiological studies have revealed that mGluRs participate in neurotransmission, regulate ionic homeostasis, induce synaptic plasticity, and maintain the balance between excitation and inhibition in a variety of auditory structures. However, very few in vivo physiological studies on mGluRs in auditory processing have been undertaken at the systems level. Many questions regarding the essential roles of mGluRs in auditory processing still remain unanswered and more rigorous basic research is warranted.

Interplay between low threshold voltage-gated K(+) channels and synaptic inhibition in neurons of the chicken nucleus laminaris along its frequency axis

Central auditory neurons that localize sound in horizontal space have specialized intrinsic and synaptic cellular mechanisms to tightly control the threshold and timing for action potential generation. However, the critical interplay between intrinsic voltage-gated conductances and extrinsic synaptic conductances in determining neuronal output are not well understood. In chicken, neurons in the nucleus laminaris (NL) encode sound location using interaural time difference (ITD) as a cue. Along the tonotopic axis of NL, there exist robust differences among low, middle, and high frequency (LF, MF, and HF, respectively) neurons in a variety of neuronal properties such as low threshold voltage-gated K⁺ (LTK) channels and depolarizing inhibition. This establishes NL as an ideal model to examine the interactions between LTK currents and synaptic inhibition across the tonotopic axis. Using whole-cell patch clamp recordings prepared from chicken embryos (E17–E18), we found that LTK currents were larger in MF and HF neurons than in LF neurons. Kinetic analysis revealed that LTK currents in MF neurons activated at lower voltages than in LF and HF neurons, whereas the inactivation of the currents was similar across the tonotopic axis. Surprisingly, blockade of LTK currents using dendrotoxin-I (DTX) tended to broaden the duration and increase the amplitude of the depolarizing inhibitory postsynaptic potentials (IPSPs) in NL neurons without dependence on coding frequency regions. Analyses of the effects of DTX on inhibitory postsynaptic currents led us to interpret this unexpected observation as a result of primarily postsynaptic effects of LTK currents on MF and HF neurons, and combined presynaptic and postsynaptic effects in LF neurons. Furthermore, DTX transferred subthreshold IPSPs to spikes. Taken together, the results suggest a critical role for LTK currents in regulating inhibitory synaptic strength in ITD-coding neurons at various frequencies.

Neuronal specializations for the processing of interaural difference cues in the chick

Sound information is encoded as a series of spikes of the auditory nerve fibers (ANFs), and then transmitted to the brainstem auditory nuclei. Features such as timing and level are extracted from ANFs activity and further processed as the interaural time difference (ITD) and the interaural level difference (ILD), respectively. These two interaural difference cues are used for sound source localization by behaving animals. Both cues depend on the head size of animals and are extremely small, requiring specialized neural properties in order to process these cues with precision. Moreover, the sound level and timing cues are not processed independently from one another. Neurons in the nucleus angularis (NA) are specialized for coding sound level information in birds and the ILD is processed in the posterior part of the dorsal lateral lemniscus nucleus (LLDp). Processing of ILD is affected by the phase difference of binaural sound. Temporal features of sound are encoded in the pathway starting in nucleus magnocellularis (NM), and ITD is processed in the nucleus laminaris (NL). In this pathway a variety of specializations are found in synapse morphology, neuronal excitability, distribution of ion channels and receptors along the tonotopic axis, which reduces spike timing fluctuation in the ANFs-NM synapse, and imparts precise and stable ITD processing to the NL. Moreover, the contrast of ITD processing in NL is enhanced over a wide range of sound level through the activity of GABAergic inhibitory systems from both the superior olivary nucleus (SON) and local inhibitory neurons that follow monosynaptic to NM activity.

Glycinergic inhibition tunes coincidence detection in the auditory brainstem

Neurons in the medial superior olive (MSO) detect microsecond differences in the arrival time of sounds between the ears (interaural time differences or ITDs), a crucial binaural cue for sound localization. Synaptic inhibition has been implicated in tuning ITD sensitivity, but the cellular mechanisms underlying its influence on coincidence detection are debated. Here we determine the impact of inhibition on coincidence detection in adult Mongolian gerbil MSO brain slices by testing precise temporal integration of measured synaptic responses using conductance-clamp. We find that inhibition dynamically shifts the peak timing of excitation, depending on its relative arrival time, which in turn modulates the timing of best coincidence detection. Inhibitory control of coincidence detection timing is consistent with the diversity of ITD functions observed in vivo and is robust under physiologically relevant conditions. Our results provide strong evidence that temporal interactions between excitation and inhibition on microsecond timescales are critical for binaural processing.

Coincidence detector neurons in the mammalian brainstem encode interaural time differences (ITDs) that are implicated in auditory processing. Myoga et al. study a previously developed neuronal model and find that inhibition is crucial for sound localization, but more dynamically than previously thought.

Tonic current through GABAA receptors and hyperpolarization-activated cyclic nucleotide-gated channels modulate resonance properties of rat subicular pyramidal neurons

The subiculum, considered to be the output structure of the hippocampus, modulates information flow from the hippocampus to various cortical and sub-cortical areas such as the nucleus accumbens, lateral septal region, thalamus, nucleus gelatinosus, medial nucleus and mammillary nuclei. Tonic inhibitory current plays an important role in neuronal physiology and pathophysiology by modulating the electrophysiological properties of neurons. While the alterations of various electrical properties due to tonic inhibition have been studied in neurons from different regions, its influence on intrinsic subthreshold resonance in pyramidal excitatory neurons expressing hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is not known. Using pharmacological agents, we show the involvement of α5βγ GABAA receptors in the picrotoxin-sensitive tonic current in subicular pyramidal neurons. We further investigated the contribution of tonic conductance in regulating subthreshold electrophysiological properties using current clamp and dynamic clamp experiments. We demonstrate that tonic GABAergic inhibition can actively modulate subthreshold properties, including resonance due to HCN channels, which can potentially alter the response dynamics of subicular pyramidal neurons in an oscillating neuronal network.

Glycinergic transmission modulates GABAergic inhibition in the avian auditory pathway

For all neurons, a proper balance of synaptic excitation and inhibition is crucial to effect computational precision. Achievement of this balance is remarkable when one considers factors that modulate synaptic strength operate on multiple overlapping time scales and affect both pre- and postsynaptic elements. Recent studies have shown that inhibitory transmitters, glycine and GABA, are co-released in auditory nuclei involved in the computation of interaural time disparities (ITDs), a cue used to process sound source location. The co-release expressed at these synapses is heavily activity dependent, and generally occurs when input rates are high. This circuitry, in both birds and mammals, relies on inhibitory input to maintain the temporal precision necessary for ITD encoding. Studies of co-release in other brain regions suggest that GABA and glycine receptors (GlyRs) interact via cross-suppressive modulation of receptor conductance. We performed in vitro whole-cell recordings in several nuclei of the chicken brainstem auditory circuit to assess whether this cross-suppressive phenomenon was evident in the avian brainstem. We evaluated the effect of pressure-puff applied glycine on synaptically evoked inhibitory currents in nucleus magnocellularis (NM) and the superior olivary nucleus (SON). Glycine pre-application reduced the amplitude of inhibitory postsynaptic currents (IPSCs) evoked during a 100 Hz train stimulus in both nuclei. This apparent glycinergic modulation was blocked in the presence of strychnine. Further experiments showed that this modulation did not depend on postsynaptic biochemical interactions such as phosphatase activity, or direct interactions between GABA and GlyR proteins. Rather, voltage clamp experiments in which we manipulated Cl⁻ flux during agonist application suggest that activation of one receptor will modulate the conductance of the other via local changes in Cl⁻ ion concentration within microdomains of the postsynaptic membrane.

Tonic GABAA conductance decreases membrane time constant and increases EPSP-spike precision in hippocampal pyramidal neurons

Because of a complex dendritic structure, pyramidal neurons have a large membrane surface relative to other cells and so a large electrical capacitance and a large membrane time constant (τ_m). This results in slow depolarizations in response to excitatory synaptic inputs, and consequently increased and variable action potential latencies, which may be computationally undesirable. Tonic activation of GABA_A receptors increases membrane conductance and thus regulates neuronal excitability by shunting inhibition. In addition, tonic increases in membrane conductance decrease the membrane time constant (τ_m), and improve the temporal fidelity of neuronal firing. Here we performed whole-cell current clamp recordings from hippocampal CA1 pyramidal neurons and found that bath application of 10μM GABA indeed decreases τ_m in these cells. GABA also decreased first spike latency and jitter (standard deviation of the latency) produced by current injection of 2 rheobases (500 ms). However, when larger current injections (3–6 rheobases) were used, GABA produced no significant effect on spike jitter, which was low. Using mathematical modeling we demonstrate that the tonic GABA_A conductance decreases rise time, decay time and half-width of EPSPs in pyramidal neurons. A similar effect was observed on EPSP/IPSP pairs produced by stimulation of Schaffer collaterals: the EPSP part of the response became shorter after application of GABA. Consistent with the current injection data, a significant decrease in spike latency and jitter was obtained in cell attached recordings only at near-threshold stimulation (50% success rate, S₅₀). When stimulation was increased to 2- or 3- times S₅₀, GABA significantly affected neither spike latency nor spike jitter. Our results suggest that a decrease in τ_m associated with elevations in ambient GABA can improve EPSP-spike precision at near-threshold synaptic inputs.

Activation of synaptic group II metabotropic glutamate receptors induces long-term depression at GABAergic synapses in CNS neurons

Metabotropic glutamate receptor (mGluR)-dependent homosynaptic long-term depression (LTD) has been studied extensively at glutamatergic synapses in the CNS. However, much less is known about heterosynaptic long-term plasticity induced by mGluRs at inhibitory synapses. Here we report that pharmacological or synaptic activation of group II mGluRs (mGluR II) induces LTD at GABAergic synapses without affecting the excitatory glutamatergic transmission in neurons of the chicken cochlear nucleus. Coefficient of variation and failure rate analysis suggested that the LTD was expressed presynaptically. The LTD requires presynaptic spike activity, but does not require the activation of NMDA receptors. The classic cAMP-dependent protein kinase A signaling is involved in the transduction pathway. Remarkably, blocking mGluR II increased spontaneous GABA release, indicating the presence of tonic activation of mGluR II by ambient glutamate. Furthermore, synaptically released glutamate induced by electrical stimulations that concurrently activated both the glutamatergic and GABAergic pathways resulted in significant and constant suppression of GABA release at various stimulus frequencies (3.3, 100, and 300 Hz). Strikingly, low-frequency stimulation (1 Hz, 15 min) of the glutamatergic synapses induced heterosynaptic LTD of GABAergic transmission, and the LTD was blocked by mGluR II antagonist, indicating that synaptic activation of mGluR II induced the LTD. This novel form of long-term plasticity in the avian auditory brainstem may play a role in the development as well as in temporal processing in the sound localization circuit.

GABAA receptor-mediated tonic depolarization in developing neural circuits

The activation of GABAA receptors (the type A receptors for γ-aminobutyric acid) produces two distinct forms of responses, phasic (i.e., transient) and tonic (i.e., persistent), that are mediated by synaptic and extrasynaptic GABAA receptors, respectively. During development, the intracellular chloride levels are high so activation of these receptors causes a net outward flow of anions that leads to neuronal depolarization rather than hyperpolarization. Therefore, in developing neural circuits, tonic activation of GABAA receptors may provide persistent depolarization. Recently, it became evident that GABAA receptor-mediated tonic depolarization alters the structure of patterned spontaneous activity, a feature that is common in developing neural circuits and is important for neural circuit refinement. Thus, this persistent depolarization may lead to a long-lasting increase in intracellular calcium level that modulates network properties via calcium-dependent signaling cascades. This article highlights the features of GABAA receptor-mediated tonic depolarization, summarizes the principles for discovery, reviews the current findings in diverse developing circuits, examines the underlying molecular mechanisms and modulation systems, and discusses their functional specializations for each developing neural circuit.

A mechanistic understanding of the role of feedforward inhibition in the mammalian sound localization circuitry

Feedforward inhibition sharpens the precision of neurons throughout ascending auditory pathways, including the binaural neurons of the medial superior olive (MSO). However, the biophysical influence of inhibition is poorly understood, particularly at higher frequencies at which the relative phase of inhibition and excitation becomes ambiguous. Here, we show in gerbil MSO principal cells in vitro that feedforward inhibition precedes direct excitation, providing a concurrent hyperpolarization and conductance shunt during EPSP summation. We show with dual-patch recordings and dynamic clamp that both the linearity and temporal fidelity of synaptic integration is improved by reducing Kv1 potassium channel conductance during inhibition, which counters membrane shunting even at high frequencies at which IPSPs sum. The reduction of peak excitation by preceding inhibition lowers spike probability, narrowing but not shifting the window for detecting binaural coincidence. The interplay between inhibition and potassium conductances thus improves the consistency and resolution of ITD coding across different frequencies.

The cooperation of sustained and phasic inhibitions increases the contrast of ITD-tuning in low-frequency neurons of the chick nucleus laminaris

Neurons in the nucleus laminaris (NL) of birds detect the coincidence of binaural excitatory inputs from the nucleus magnocellularis (NM) on both sides and process the interaural time differences (ITDs) for sound localization. Sustained inhibition from the superior olivary nucleus is known to control the gain of coincidence detection, which allows the sensitivity of NL neurons to ITD tolerate strong-intensity sound. Here, we found a phasic inhibition in chicken brain slices that follows the ipsilateral NM inputs after a short time delay, sharpens coincidence detection, and may enhance ITD sensitivity in low-frequency NL neurons. GABA-positive small neurons are distributed in and near the NL. These neurons generate IPSCs in NL neurons when photoactivated by a caged glutamate compound, suggesting that these GABAergic neurons are interneurons that mediate phasic inhibition. These IPSCs have fast decay kinetics that is attributable to the α1-subunit of the GABAA receptor, the expression of which dominates in the low-frequency region of the NL. Model simulations demonstrate that phasic IPSCs narrow the time window of coincidence detection and increase the contrast of ITD-tuning during low-level, low-frequency excitatory input. Furthermore, cooperation of the phasic and sustained inhibitions effectively increases the contrast of ITD-tuning over a wide range of excitatory input levels. We propose that the complementary interaction between phasic and sustained inhibitions is the neural mechanism that regulates ITD sensitivity for low-frequency sound in the NL.

Astrocyte-secreted factors modulate the developmental distribution of inhibitory synapses in nucleus laminaris of the avian auditory brainstem

Nucleus laminaris (NL) neurons in the avian auditory brainstem are coincidence detectors necessary for the computation of interaural time differences used in sound localization. In addition to their excitatory inputs from nucleus magnocellularis, NL neurons receive inhibitory inputs from the superior olivary nucleus (SON) that greatly improve coincidence detection in mature animals. The mechanisms that establish mature distributions of inhibitory inputs to NL are not known. We used the vesicular GABA transporter (VGAT) as a marker for inhibitory presynaptic terminals to study the development of inhibitory inputs to NL between embryonic day 9 (E9) and E17. VGAT immunofluorescent puncta were first seen sparsely in NL at E9. The density of VGAT puncta increased with development, first within the ventral NL neuropil region and subsequently throughout both the ventral and dorsal dendritic neuropil, with significantly fewer terminals in the cell body region. A large increase in density occurred between E13–15 and E16–17, at a developmental stage when astrocytes that express glial fibrillary acidic protein (GFAP) become mature. We cultured E13 brainstem slices together with astrocyte-conditioned medium (ACM) obtained from E16 brainstems and found that ACM, but not control medium, increased the density of VGAT puncta. This increase was similar to that observed during normal development. Astrocyte-secreted factors interact with the terminal ends of SON axons to increase the number of GABAergic terminals. These data suggest that factors secreted from GFAP-positive astrocytes promote maturation of inhibitory pathways in the auditory brainstem.

Rapid regulation of tonic GABA currents in cultured rat hippocampal neurons

Subacute and chronic changes in tonic GABAergic inhibition occur in human and experimental epilepsy. Less is known about how tonic inhibition is modulated over shorter time frames (seconds). We measured endogenous tonic GABA currents from cultured rat hippocampal neurons to evaluate how they are affected by 1) transient increases in extracellular GABA concentration ([GABA]), 2) transient postsynaptic depolarization, and 3) depolarization of presynaptic cells. Transient increases in [GABA] (1 μM) reduced tonic currents; this reduction resulted from GABA-induced shifts in the reversal potential for GABA currents (E(GABA)). Transient depolarization of postsynaptic neurons reversed the effects of exogenous GABA and potentiated tonic currents. The voltage-dependent potentiation of tonic GABA currents was independent of E(GABA) shifts and represented postdepolarization potentiation (PDP), an intrinsic GABA(A) receptor property (Ransom CB, Wu Y, Richerson GB. J Neurosci 30: 7672-7684, 2010). Inhibition of vesicular GABA release with concanamycin A (ConA) did not affect tonic currents. In ConA-treated cells, transient application of 12 mM K(+) to depolarize presynaptic neurons and glia produced a persistent increase in tonic current amplitude. The K(+)-induced increase in tonic current was reversibly inhibited by SKF89976a (40 μM), indicating that this was caused by nonvesicular GABA release from GABA transporter type 1 (GAT1). Nonvesicular GABA release due to GAT1 reversal also occurred in acute hippocampal brain slices. Our results indicate that tonic GABA currents are rapidly regulated by GABA-induced changes in intracellular Cl(-) concentration, PDP of extrasynaptic GABA(A) receptors, and nonvesicular GABA release. These mechanisms may influence tonic inhibition during seizures when neurons are robustly depolarized and extracellular GABA and K(+) concentrations are elevated.

Activation of metabotropic glutamate receptors improves the accuracy of coincidence detection by presynaptic mechanisms in the nucleus laminaris of the chick

Interaural time difference (ITD) is a major cue for localizing a sound source and is processed in the nucleus laminaris (NL) in birds. Coincidence detection (CD) is a crucial step for processing ITD and critically depends on the size and time course of excitatory postsynaptic potentials (EPSPs). Here, we investigated a role of metabotropic glutamate receptors (mGluRs) in the regulation of EPSP amplitude and CD in the NL of chicks. A non-specific agonist of mGluRs ((±)-1-aminocyclopentane-trans-1,3-dicarboxylic acid; t-ACPD) reduced the amplitude and extent of depression of excitatory postsynaptic currents (EPSCs) during a stimulus train, while the paired pulse ratio and coefficient of variation of EPSC amplitude were increased. In contrast, the amplitudes of spontaneous EPSCs were not affected, but the frequency was reduced. Thus, the effects of t-ACPD were presynaptic and reduced the release of neurotransmitter from terminals in the NL. Expression of group II mGluRs was graded along the tonotopic axis and was stronger towards the low frequency region in the NL. Both group II (DCG-IV) and group III (l-AP4) specific agonists reduced EPSC amplitude by presynaptic mechanisms, and the reduction was larger in the low frequency region; however, we could not find any effects of group I-specific agonists on EPSCs. The reduced EPSP amplitude in DCG-IV improved CD. A specific antagonist of group II mGluRs (LY341495) increased the amplitude of both EPSCs and EPSPs and enhanced the depression during a stimulus train, indicating constitutive activation of mGluRs in the NL. These observations indicate that mGluRs may work as autoreceptors and regulate EPSP size to improve CD in the NL.

Extrasynaptic exocytosis and its mechanisms: a source of molecules mediating volume transmission in the nervous system

We review the evidence of exocytosis from extrasynaptic sites in the soma, dendrites, and axonal varicosities of central and peripheral neurons of vertebrates and invertebrates, with emphasis on somatic exocytosis, and how it contributes to signaling in the nervous system. The finding of secretory vesicles in extrasynaptic sites of neurons, the presence of signaling molecules (namely transmitters or peptides) in the extracellular space outside synaptic clefts, and the mismatch between exocytosis sites and the location of receptors for these molecules in neurons and glial cells, have long suggested that in addition to synaptic communication, transmitters are released, and act extrasynaptically. The catalog of these molecules includes low molecular weight transmitters such as monoamines, acetylcholine, glutamate, gama-aminobutiric acid (GABA), adenosine-5-triphosphate (ATP), and a list of peptides including substance P, brain-derived neurotrophic factor (BDNF), and oxytocin. By comparing the mechanisms of extrasynaptic exocytosis of different signaling molecules by various neuron types we show that it is a widespread mechanism for communication in the nervous system that uses certain common mechanisms, which are different from those of synaptic exocytosis but similar to those of exocytosis from excitable endocrine cells. Somatic exocytosis has been measured directly in different neuron types. It starts after high-frequency electrical activity or long experimental depolarizations and may continue for several minutes after the end of stimulation. Activation of L-type calcium channels, calcium release from intracellular stores and vesicle transport towards the plasma membrane couple excitation and exocytosis from small clear or large dense core vesicles in release sites lacking postsynaptic counterparts. The presence of synaptic and extrasynaptic exocytosis endows individual neurons with a wide variety of time- and space-dependent communication possibilities. Extrasynaptic exocytosis may be the major source of signaling molecules producing volume transmission and by doing so may be part of a long duration signaling mode in the nervous system.

Control of neuronal excitability by NMDA-type glutamate receptors in early developing binaural auditory neurons

Precise control of neuronal excitability in the auditory brainstem is fundamental for processing timing cues used for sound localization and signal discrimination in complex acoustic environments. In mature nucleus laminaris (NL), the first nucleus responsible for binaural processing in chickens, neuronal excitability is governed primarily by voltage-activated potassium conductances (K(VA)). High levels of K(VA) expression in NL neurons result in one or two initial action potentials (APs) in response to high-frequency synaptic activity or sustained depolarization. Here we show that during a period of synaptogenesis and circuit refinement, before hearing onset, K(VA) conductances are relatively small, in particular low-voltage-activated K(+) conductances (K(LVA)). In spite of this, neuronal output is filtered and repetitive synaptic activity generates only one or two initial APs during a train of stimuli. During this early developmental time period, synaptic NMDA-type glutamate receptors (NMDA-Rs) contain primarily the GluN2B subunit. We show that the slow decay kinetics of GluN2B-containing NMDA-Rs allows synaptic responses to summate, filtering the output of NL neurons before intrinsic properties are fully developed. Weaker Mg(2+) blockade of NMDA-Rs and ambient glutamate early in development generate a tonic NMDA-R-mediated current that sets the membrane potential at more depolarized values. Small KLVA conductances, localized in dendrites, prevent excessive depolarization caused by tonic activation of NMDA-Rs. Thus, before intrinsic properties are fully developed, NMDA-Rs control the output of NL neurons during evoked synaptic transmission.

Duration tuning across vertebrates

Signal duration is important for identifying sound sources and determining signal meaning. Duration-tuned neurons (DTNs) respond preferentially to a range of stimulus durations and maximally to a best duration (BD). Duration-tuned neurons are found in the auditory midbrain of many vertebrates, although studied most extensively in bats. Studies of DTNs across vertebrates have identified cells with BDs and temporal response bandwidths that mirror the range of species-specific vocalizations. Neural tuning to stimulus duration appears to be universal among hearing vertebrates. Herein, we test the hypothesis that neural mechanisms underlying duration selectivity may be similar across vertebrates. We instantiated theoretical mechanisms of duration tuning in computational models to systematically explore the roles of excitatory and inhibitory receptor strengths, input latencies, and membrane time constant on duration tuning response profiles. We demonstrate that models of duration tuning with similar neural circuitry can be tuned with species-specific parameters to reproduce the responses of in vivo DTNs from the auditory midbrain. To relate and validate model output to in vivo responses, we collected electrophysiological data from the inferior colliculus of the awake big brown bat, Eptesicus fuscus, and present similar in vivo data from the published literature on DTNs in rats, mice, and frogs. Our results support the hypothesis that neural mechanisms of duration tuning may be shared across vertebrates despite species-specific differences in duration selectivity. Finally, we discuss how the underlying mechanisms of duration selectivity relate to other auditory feature detectors arising from the interaction of neural excitation and inhibition.

Two GABAA responses with distinct kinetics in a sound localization circuit

The temporal characteristics and functional diversity of GABAergic inhibition are determined by the spatiotemporal neurotransmitter profile, intrinsic properties of GABAA receptors, and other factors. Here, we report two distinct GABAA responses and the underlying mechanisms in neurons of the chicken nucleus laminaris (NL), the first encoder of interaural time difference for sound localization in birds. The time course of the postsynaptic GABAA currents in NL neurons, recorded with whole-cell voltage clamp, differed between different characteristic frequency (CF) regions. Compared to low-CF (LF) neurons, middle/high-CF (MF/HF) neurons had significantly slower IPSCs, with a 2.6-fold difference in the decay time constants of spontaneous IPSCs and a 5.3-fold difference in the decay of IPSCs elicited by single-pulse stimulus. Such differences were especially dramatic when IPSCs were elicited by train stimulations at physiologically relevant frequencies, and at high stimulus intensities. To account for these distinct GABAA responses, we showed that MF/HF neurons exhibited more prominent asynchronous release of GABA. Supporting this observation, replacement of extracellular Ca2+ with Sr2+ increased the decay of IPSCs in LF neurons, and EGTA-AM reduced the decay of IPSCs in MF/HF neurons. Furthermore, pharmacological evidence suggests that GABA spillover plays a greater role in prolonging the IPSCs of MF/HF neurons. Consequently, under whole-cell current clamp, synaptically released GABA produced short- and long-lasting suppression of the neuronal excitability of LF and MF/HF neurons, respectively. Taken together, these results suggest that the GABAergic inputs to NL neurons may exert a dynamic modulation of interaural time difference (ITD) coding in a CF-dependent manner.

Development of GPCR modulation of GABAergic transmission in chicken nucleus laminaris neurons

Neurons in the nucleus laminaris (NL) of birds act as coincidence detectors and encode interaural time difference to localize the sound source in the azimuth plane. GABAergic transmission in a number of CNS nuclei including the NL is subject to a dual modulation by presynaptic GABA_B receptors (GABA_BRs) and metabotropic glutamate receptors (mGluRs). Here, using in vitro whole-cell patch clamp recordings from acute brain slices of the chick, we characterized the following important but unknown properties pertaining to such a dual modulation: (1) emergence of functional GABA synapses in NL neurons; (2) the temporal onset of neuromodulation mediated by GABA_BRs and mGluRs; and (3) the physiological conditions under which GABA_BRs and mGluRs are activated by endogenous transmitters. We found that (1) GABA_AR-mediated synaptic responses were observed in about half of the neurons at embryonic day 11 (E11); (2) GABA_BR-mediated modulation of the GABAergic transmission was detectable at E11, whereas the modulation by mGluRs did not emerge until E15; and (3) endogenous activity of GABA_BRs was induced by both low- (5 or 10 Hz) and high-frequency (200 Hz) stimulation of the GABAergic pathway, whereas endogenous activity of mGluRs was induced by high- (200 Hz) but not low-frequency (5 or 10 Hz) stimulation of the glutamatergic pathway. Furthermore, the endogenous activity of mGluRs was mediated by group II but not group III members. Therefore, autoreceptor-mediated modulation of GABAergic transmission emerges at the same time when the GABA synapses become functional. Heteroreceptor-mediated modulation appears at a later time and is receptor type dependent in vitro.

Excitatory GABA: How a Correct Observation May Turn Out to be an Experimental Artifact

The concept of the excitatory action of GABA during early development is based on data obtained mainly in brain slice recordings. However, in vivo measurements as well as observations made in intact hippocampal preparations indicate that GABA is in fact inhibitory in rodents at early neonatal stages. The apparent excitatory action of GABA seems to stem from cellular injury due to the slicing procedure, which leads to accumulation of intracellular Cl⁻ in injured neurons. This procedural artifact was shown to be attenuated through various manipulations such as addition of energy substrates more relevant to the in vivo situation. These observations question the very concept of excitatory GABA in immature neuronal networks.

An essential role for modulation of hyperpolarization-activated current in the development of binaural temporal precision

In sensory circuits of the brain, developmental changes in the expression and modulation of voltage-gated ion channels are a common occurrence, but such changes are often difficult to assign to clear functional roles. We have explored this issue in the binaural neurons of the medial superior olive (MSO), whose temporal precision in detecting the coincidence of binaural inputs dictates the resolution of azimuthal sound localization. We show that in MSO principal neurons of gerbils during the first week of hearing, a hyperpolarization-activated current (I(h)) progressively undergoes a 13-fold increase in maximal conductance, a >10-fold acceleration of kinetics, and, most surprisingly, a 30 mV depolarizing shift in the voltage dependence of activation. This period is associated with an upregulation of the hyperpolarization-activated and cyclic nucleotide-gated (HCN) channel subunits HCN1, HCN2, and HCN4 in the MSO, but only HCN1 and HCN4 were expressed strongly in principal neurons. I(h) recorded in nucleated patches from electrophysiologically mature MSO neurons (>P18) exhibited kinetics and an activation range nearly identical to the I(h) found in whole-cell recordings before hearing onset. These results indicate that the developmental changes in I(h) in MSO neurons can be explained predominantly by modulation from diffusible intracellular factors, and not changes in channel subunit composition. The exceptionally large modulatory changes in I(h), together with refinements in synaptic properties transform the coding strategy from one of summation and integration to the submillisecond coincidence detection known to be required for transmission of sound localization cues.

Synaptic activity-induced Ca(2+) signaling in avian cochlear nucleus magnocellularis neurons

Neurons of the avian cochlear nucleus magnocellularis (NM) receive glutamatergic inputs from the spiral ganglion cells via the auditory nerve and feedback GABAergic inputs primarily from the superior olivary nucleus. We investigated regulation of Ca(2+) signaling in NM neurons with ratiometric Ca(2+) imaging in chicken brain slices. Application of exogenous glutamate or GABA increased the intracellular Ca(2+) concentration ([Ca(2+)](i)) in NM neurons. Interestingly, GABA-induced Ca(2+) responses persisted into neuronal maturation, in both standard and energy substrate enriched artificial cerebrospinal fluid. More importantly, we found that electrical stimulation applied to the glutamatergic and GABAergic afferent fibers innervating the NM was able to elicit transient [Ca(2+)](i) increases in NM neurons, and the amplitude of the Ca(2+) responses increased with increasing frequency and duration of the electrical stimulation. Antagonists for ionotropic glutamate receptors significantly blocked these [Ca(2+)](i) increases, whereas blocking GABA(A) receptors did not affect the Ca(2+) responses, suggesting that synaptically released glutamate but not GABA induced the Ca(2+) signaling in vitro. Furthermore, activation of GABA(A) receptors with exogenous agonists inhibited synaptic activity-induced [Ca(2+)](i) increases in NM neurons, suggesting a role of GABA(A) receptors in the regulation of Ca(2+) homeostasis in the avian cochlear nucleus neurons.

The GABA(A) receptor agonist THIP ameliorates specific behavioral deficits in the mouse model of fragile X syndrome

Hyperactivity, hypersensitivity to auditory stimuli, and exaggerated fear are common behavioral abnormalities observed in individuals with fragile X syndrome (FXS), a neurodevelopmental disorder that is the most common genetic cause of autism. Evidence from studies of the Fmr1 knockout (KO) mouse model of FXS supports the notion that impaired GABAergic transmission in different brain regions such as the amygdala, striatum or cerebral cortex is central to FXS behavioral abnormalities. This suggests that the GABAergic system might be an intriguing target to ameliorate some of the phenotypes in FXS. Our recent work revealed that THIP (gaboxadol), a GABA(A) receptor agonist, can restore principal neuron excitability deficits in the Fmr1 KO amygdala, suggesting that THIP may also restore some of the key behavioral abnormalities in Fmr1 KO mice. Here, we reveal that THIP significantly attenuated hyperactivity in Fmr1 KO mice, and reduced prepulse inhibition in a volume-dependent manner. In contrast, THIP did not reverse the deficits in cued fear or startle response. Thus, this study shows that enhancing GABAergic transmission can correct specific behavioral phenotypes of the Fmr1 KO mouse further supporting that targeting the GABAergic system, and specifically tonic inhibition, might be important for correcting or ameliorating some key behaviors in FXS.

Inhibition to excitation ratio regulates visual system responses and behavior in vivo

The balance of inhibitory to excitatory (I/E) synaptic inputs is thought to control information processing and behavioral output of the central nervous system. We sought to test the effects of the decreased or increased I/E ratio on visual circuit function and visually guided behavior in Xenopus tadpoles. We selectively decreased inhibitory synaptic transmission in optic tectal neurons by knocking down the γ2 subunit of the GABA(A) receptors (GABA(A)R) using antisense morpholino oligonucleotides or by expressing a peptide corresponding to an intracellular loop of the γ2 subunit, called ICL, which interferes with anchoring GABA(A)R at synapses. Recordings of miniature inhibitory postsynaptic currents (mIPSCs) and miniature excitatory PSCs (mEPSCs) showed that these treatments decreased the frequency of mIPSCs compared with control tectal neurons without affecting mEPSC frequency, resulting in an ∼50% decrease in the ratio of I/E synaptic input. ICL expression and γ2-subunit knockdown also decreased the ratio of optic nerve-evoked synaptic I/E responses. We recorded visually evoked responses from optic tectal neurons, in which the synaptic I/E ratio was decreased. Decreasing the synaptic I/E ratio in tectal neurons increased the variance of first spike latency in response to full-field visual stimulation, increased recurrent activity in the tectal circuit, enlarged spatial receptive fields, and lengthened the temporal integration window. We used the benzodiazepine, diazepam (DZ), to increase inhibitory synaptic activity. DZ increased optic nerve-evoked inhibitory transmission but did not affect evoked excitatory currents, resulting in an increase in the I/E ratio of ∼30%. Increasing the I/E ratio with DZ decreased the variance of first spike latency, decreased spatial receptive field size, and lengthened temporal receptive fields. Sequential recordings of spikes and excitatory and inhibitory synaptic inputs to the same visual stimuli demonstrated that decreasing or increasing the I/E ratio disrupted input/output relations. We assessed the effect of an altered I/E ratio on a visually guided behavior that requires the optic tectum. Increasing and decreasing I/E in tectal neurons blocked the tectally mediated visual avoidance behavior. Because ICL expression, γ2-subunit knockdown, and DZ did not directly affect excitatory synaptic transmission, we interpret the results of our study as evidence that partially decreasing or increasing the ratio of I/E disrupts several measures of visual system information processing and visually guided behavior in an intact vertebrate.