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GABAergic inhibition shapes the connectivity, activity and plasticity of the brain. A series of exciting new discoveries provide compelling evidence that disruptions in a number of key facets of GABAergic inhibition have critical roles in the etiology of neurodevelopmental disorders (NDDs). These facets include the generation, migration and survival of GABAergic neurons; the formation of GABAergic synapses and circuit connectivity; and the dynamic regulation of the efficacy of GABAergic signalling through neuronal chloride transporters. In this Review, we discuss recent work that elucidates the functions and dysfunctions [Au:OK?] of GABAergic signalling in health and disease, that uncovers the contribution of GABAergic neural circuit dysfunction to NDD etiology and that leverages such mechanistic insights to advance precision medicine for the treatment of NDDs.

The landmark studies of Wiesel and Hubel in the 1960's initiated a surge of investigations into the critical period of visual cortical development, when abnormal visual experience can alter cortical structures and functions. Most studies focused on the visual cortex, with relatively little attention to subcortical structures. The goal of the present review is to elucidate neurochemical and synaptic mechanisms common to the critical periods of the visual cortex and the brain stem respiratory system in thenormalrat. In both regions, the critical period is a time of (i) heightened inhibition; (ii)reducedexpression of brain-derived neurotrophic factor (BDNF); and (iii)synaptic imbalance, with heightened inhibition and suppressed excitation. The last two mechanisms are contrary to the conventional premise. Synaptic imbalance renders developing neurons more vulnerable to external stressors. However, the critical period isnecessaryto enable each system to strengthen its circuitry, adapt to its environment, and transition from immaturity to maturity, when a state of relative synaptic balance is attained. Failure to achieve such a balance leads to neurological disorders.

Background Synaptic dysfunction, named synaptopathy, due to inflammatory status of the central nervous system (CNS) is a recognized factor potentially underlying both motor and cognitive dysfunctions in neurodegenerative diseases. To gain knowledge on the mechanistic interplay between local inflammation and synapse changes, we compared two diverse inflammatory paradigms, a cytokine cocktail (CKs; IL-1β, TNF-α, and GM-CSF) and LPS, and their ability to tune GABAergic current duration in spinal cord cultured circuits. Methods We exploit spinal organotypic cultures, single-cell electrophysiology, immunocytochemistry, and confocal microscopy to explore synaptic currents and resident neuroglia reactivity upon CK or LPS incubation. Results Local inflammation in slice cultures induced by CK or LPS stimulations boosts network activity; however, only CKs specifically reduced GABAergic current duration. We pharmacologically investigated the contribution of GABAAR α-subunits and suggested that a switch of GABAAR α1-subunit might have induced faster GABAAR decay time, weakening the inhibitory transmission. Conclusions Lower GABAergic current duration could contribute to providing an aberrant excitatory transmission critical for pre-motor circuit tasks and represent a specific feature of a CK cocktail able to mimic an inflammatory reaction that spreads in the CNS. Our results describe a selective mechanism that could be triggered during specific inflammatory stress. Electronic supplementary material The online version of this article (10.1186/s12974-019-1519-z) contains supplementary material, which is available to authorized users.

Epilepsy is characterized by recurrent seizures due to abnormal hyperexcitation of neurons. Recent studies have suggested that the imbalance of excitation and inhibition (E/I) in the central nervous system is closely implicated in the etiology of epilepsy. In the brain, GABA is a major inhibitory neurotransmitter and plays a pivotal role in maintaining E/I balance. As such, altered GABAergic inhibition can lead to severe E/I imbalance, consequently resulting in excessive and hypersynchronous neuronal activity as in epilepsy. Phospholipase C (PLC) is a key enzyme in the intracellular signaling pathway and regulates various neuronal functions including neuronal development, synaptic transmission, and plasticity in the brain. Accumulating evidence suggests that neuronal PLC is critically involved in multiple aspects of GABAergic functions. Therefore, a better understanding of mechanisms by which neuronal PLC regulates GABAergic inhibition is necessary for revealing an unrecognized linkage between PLC and epilepsy and developing more effective treatments for epilepsy. Here we review the function of PLC in GABAergic inhibition in the brain and discuss a pathophysiological relationship between PLC and epilepsy.

Short-term deprivation (2.5 h) of an eye has been shown to boost its relative ocular dominance in young adults. Here, we show that a much shorter deprivation period (3–6 min) produces a similar paradoxical boost that is retinotopic and reduces spatial inhibition on neighbouring, non-deprived areas. Partial deprivation was conducted in the left hemifield, central vision or in an annular region, later assessed with a binocular rivalry tracking procedure. Post-deprivation, dominance of the deprived eye increased when rivalling images were within the deprived retinotopic region, but not within neighbouring, non-deprived areas where dominance was dependent on the correspondence between the orientation content of the stimuli presented in the deprived and that of the stimuli presented in non-deprived areas. Together, these results accord with other deprivation studies showing V1 activity changes and reduced GABAergic inhibition.

γ-Aminobutyric acid (GABA) exerts a partly excitatory action on immature neurons by means of the high [Cl−]I levels present in many animal species and brain structures. This unique mechanism of action mediated by large anion permeability is exquisitely suited to regulate the activity of primitive networks that immature neurons must generate to modulate the construction of functional cortical networks. It is also instrumental in the pathogenesis of many developmental disorders with a persistence of immature features that perturb the operation of behaviorally relevant oscillations. Therefore, restoring GABAergic inhibition and the polarity of GABA actions is a promising novel therapeutic avenue, and recent clinical trials in the treatment of autism spectrum disorders have validated the importance of this approach. Several examples from preclinical and clinical trials will be illustrated.

Hippocampal inhibitory interneurons exhibit a large diversity of dendritic Ca2+ mechanisms that are involved in the induction of Hebbian and anti-Hebbian synaptic plasticity. High resolution imaging techniques allowed examining somatic Ca2+ signals and, accordingly, the recruitment of hippocampal interneurons in awake behaving animals. However, little is still known about dendritic Ca2+ activity in interneurons during different behavioral states. Here, we used two-photon Ca2+ imaging in mouse hippocampal CA1 interneurons to reveal Ca2+ signal patterns in interneuron dendrites during animal locomotion and immobility. Despite overall variability in dendritic Ca2+ transients (CaTs) across different cells and dendritic branches, we report consistent behavior state-dependent organization of Ca2+ signaling in interneurons. As such, spreading regenerative CaTs dominated in dendrites during locomotion, whereas both spreading and localized Ca2+ signals were seen during immobility. Thus, these data indicate that while animal locomotion is associated with widespread Ca2+ elevations in interneuron dendrites that may reflect regenerative activity, local CaTs that may be related to synaptic activity become apparent during animal quiet state.

Sensory systems sequentially extract increasingly complex features. ON and OFF pathways, for example, encode increases or decreases of a stimulus from a common input. This ON/OFF pathway split is thought to occur at individual synaptic connections through a sign-inverting synapse in one of the pathways. Here, we show that ON selectivity is a multisynaptic process in the Drosophila visual system. A pharmacogenetics approach demonstrates that both glutamatergic inhibition through GluClα and GABAergic inhibition through Rdl mediate ON responses. Although neurons postsynaptic to the glutamatergic ON pathway input L1 lose all responses in GluClα mutants, they are resistant to a cell-type-specific loss of GluClα. This shows that ON selectivity is distributed across multiple synapses, and raises the possibility that cell-type-specific manipulations might reveal similar strategies in other sensory systems. Thus, sensory coding is more distributed than predicted by simple circuit motifs, allowing for robust neural processing., eLife digest We rely on our senses to capture information about the world around us. Sense organs convert sensory information – such as light or sound waves – into patterns of neuronal activity. In the mammalian retina, for example, specialized neurons called photoreceptors detect individual photons of light as they hit the back of the eye. The photoreceptors then pass on this information to neurons called bipolar cells for further processing. During darkness, all photoreceptors release the same chemical signal onto bipolar cells, namely a molecule called glutamate. But bipolar cells respond to glutamate in different ways depending on which proteins are present in their outer membrane. So-called ON cells respond to glutamate by decreasing their activity, and thus effectively become more active when light levels increase. By contrast, OFF cells respond to glutamate by increasing their activity. This ON/OFF binary code enables later stages of the visual system to detect more complex visual features, such as shape and movement. A new study in fruit flies, however, suggests that the ON/OFF code may be more complex than previously thought. While fruit fly eyes look very different to our own, the two have much in common. By studying fruit flies, researchers can also take advantage of a variety of genetic and pharmacological tools to manipulate cells and neuronal circuits. Using such tools, Molina-Obando et al. show that the ON/OFF signal separation in fruit flies uses two different molecular mechanisms. The first involves a gene called GluCl-alpha, which encodes a receptor for glutamate. The second involves a gene called Rdl, which encodes a receptor for another brain chemical, GABA. Deleting the gene for GluCl-alpha from the entire fly brain prevented ON cells from responding to an increase in light levels. However, deleting this gene from specific ON cells alone did not. This suggests that flies can use more than one type of neuronal connection to detect an increase in light. Moreover, if one pathway fails, the other can take over. This makes the system more robust. The results of Molina-Obando et al. are consistent with findings from anatomical studies that have mapped connections between neurons. Future studies should explore whether the same mechanisms exist in other sensory systems, and other animals. These experiments could take advantage of the molecular tools developed as part of the current work, which allow precise manipulation of neural networks.

Summary Sensory stimuli with graded intensities often lead to yes-or-no decisions on whether to respond to the stimuli. How this graded-to-binary conversion is implemented in the central nervous system (CNS) remains poorly understood. Here, we show that graded encodings of noxious stimuli are categorized in a decision-associated CNS region in Drosophila larvae, and then decoded by a group of peptidergic neurons for executing binary escape decisions. GABAergic inhibition gates weak nociceptive encodings from being decoded, whereas escalated amplification through the recruitment of second-order neurons boosts nociceptive encodings at intermediate intensities. These two modulations increase the detection accuracy by reducing responses to negligible stimuli whereas enhancing responses to intense stimuli. Our findings thus unravel a circuit mechanism that underlies accurate detection of harmful stimuli.

Summary New research suggests that rats can learn new spatial information in the absence of cell firing. A small enhancement of GABAergic inhibition with a low dose of muscimol blocked cell firing but left long-term potentiation induction intact, while behaviorally it blocked memory retrieval but left memory encoding intact.

Significance Huntington’s disease (HD) is a fatal neurodegenerative disorder that currently has no cure. Although HD is classically considered a motor disorder, HD patients experience learning and memory deficits years before the onset of motor symptoms, and these deficits resemble those observed in HD mouse models. In this work, using transgenic mouse models of HD, we demonstrate that the action of the neurotransmitter GABA has switched from inhibitory to excitatory. By treating HD mice with a clinically used diuretic (bumetanide), which restores inhibitory GABA, we rescued the learning and memory deficits. Our data suggest a potential therapeutic approach for the treatment of the cognitive deficits in early HD that can improve patient quality of life and reduce caregiver burden., Huntington’s disease (HD) is classically characterized as a movement disorder, however cognitive impairments precede the motor symptoms by ∼15 y. Based on proteomic and bioinformatic data linking the Huntingtin protein (Htt) and KCC2, which is required for hyperpolarizing GABAergic inhibition, and the important role of inhibition in learning and memory, we hypothesized that aberrant KCC2 function contributes to the hippocampal-associated learning and memory deficits in HD. We discovered that Htt and KCC2 interact in the hippocampi of wild-type and R6/2-HD mice, with a decrease in KCC2 expression in the hippocampus of R6/2 and YAC128 mice. The reduced expression of the Cl−-extruding cotransporter KCC2 is accompanied by an increase in the Cl−-importing cotransporter NKCC1, which together result in excitatory GABA in the hippocampi of HD mice. NKCC1 inhibition by the FDA-approved NKCC1 inhibitor bumetanide abolished the excitatory action of GABA and rescued the performance of R6/2 mice on hippocampal-associated behavioral tests.

KCC2 is a neuron-specific K+-Cl– cotransporter essential for establishing the Cl- gradient required for hyperpolarizing inhibition in the central nervous system (CNS). KCC2 is highly localized to excitatory synapses where it regulates spine morphogenesis and AMPA receptor confinement. Aberrant KCC2 function contributes to human neurological disorders including epilepsy and neuropathic pain. Using functional proteomics, we identified the KCC2-interactome in the mouse brain to determine KCC2-protein interactions that regulate KCC2 function. Our analysis revealed that KCC2 interacts with diverse proteins, and its most predominant interactors play important roles in postsynaptic receptor recycling. The most abundant KCC2 interactor is a neuronal endocytic regulatory protein termed PACSIN1 (SYNDAPIN1). We verified the PACSIN1-KCC2 interaction biochemically and demonstrated that shRNA knockdown of PACSIN1 in hippocampal neurons increases KCC2 expression and hyperpolarizes the reversal potential for Cl-. Overall, our global native-KCC2 interactome and subsequent characterization revealed PACSIN1 as a novel and potent negative regulator of KCC2., eLife digest Neurons in the brain talk to each other by releasing chemicals called neurotransmitters. These neurotransmitters can either increase ('excite') or decrease ('inhibit') the activity of other neurons. Inhibitory neurotransmission uses the chemical GABA as a neurotransmitter. When a neuron releases GABA it is like applying the brake in your car – you can slow down subtly to stay under the speed limit, or stomp on it to avoid an accident. The brain needs to carefully control the amount of inhibition so that the animal can learn and produce complex behaviours. For GABA to inhibit the activity of a neuron, the neuron must maintain a low amount of chloride ions inside. A transporter protein called KCC2 shuttles chloride out of cells; if this transporter fails to work, chloride builds up in the neuron and prevents inhibition so that GABA neurotransmission switches from inhibitory to excitatory. This breakdown of GABA inhibition is a hallmark of abnormal brain activity during conditions such as epilepsy, pain and some forms of autism. Despite the fact that neurons need KCC2 for inhibition in the brain, we do not know much about how this transporter works. Since the activity of a protein is determined in part by the other proteins it interacts with, it is therefore important to identify all the proteins that interact with KCC2 – termed the KCC2 interactome. To discover these protein interactions, Mahadevan et al. performed a technique called liquid chromatography-mass spectrometry on KCC2 protein isolated from mouse brains. This revealed that there are 181 proteins in the KCC2 interactome. Of these proteins, the most abundant was a protein called PACSIN1, which helps to pull proteins out of the membrane that surrounds each neuron. To investigate how the interaction between PACSIN1 and KCC2 regulates the activity of this transporter, Mahadevan et al. performed fluorescence imaging of neurons and recorded their electrical activity. This revealed that PACSIN1 restricts the expression of KCC2, meaning that the more PACSIN there is in the neuron, the less KCC2 will be present. The KCC2 interactome provides a database of proteins that can be targeted to increase the activity of KCC2. This could allow new treatments to be developed for brain disorders in which the inhibition of neurons is reduced.

The medial entorhinal cortex (MEC) contains a variety of specialized spatially-tuned neurons whose properties emerge during the third postnatal week onwards in rodents. How neuronal networks underlying the spatial firing patterns are formed is largely unknown but they are hypothesized to develop from topographic modules of synchronized neurons in superficial MEC. Here, we show that developing MEC neuronal networks in the second postnatal week are synchronously active in spatially-grouped modules. Network synchrony is intrinsic to MEC and desynchronized just prior to the emergence of spatially-tuned firing properties. The MEC network is modulated but not driven by the immature hippocampus and is tightly-coupled to neighboring neocortical networks. Unlike hippocampal networks, developing modules are dominated by glutamatergic excitation rather than GABAergic inhibition. Our results demonstrate that intrinsically synchronous modules exist in immature MEC: these may play a key role in establishing and organizing circuitry necessary for spatially-tuned firing properties of MEC neurons.

Tonic inhibition is thought to dampen the excitability of principal neurons; however, little is known about the role of tonic GABAergic inhibition in interneurons and the impact on principal neuron excitability. In many brain regions, tonic GABAergic inhibition is mediated by extrasynaptic, δ-subunit-containing GABAA receptors (GABAARs). In the present study we demonstrate the importance of GABAAR δ-subunit-mediated tonic inhibition in interneurons. Selective elimination of the GABAAR δ-subunit from interneurons was achieved by crossing a novel floxed Gabrd mouse model with GAD65-Cre mice ( Gabrd/Gad mice). Deficits in GABAAR δ-subunit expression in GAD65-positive neurons result in a decrease in tonic GABAergic inhibition and increased excitability of both molecular layer (ML) and stratum radiatum (SR) interneurons. Disinhibition of interneurons results in robust alterations in the neuronal excitability of principal neurons and decreased seizure susceptibility. Gabrd/Gad mice have enhanced tonic and phasic GABAergic inhibition in both CA1 pyramidal neurons and dentate gyrus granule cells (DGGCs). Consistent with alterations in hippocampal excitability, CA1 pyramidal neurons and DGGCs from Gabrd/Gad mice exhibit a shift in the input-output relationship toward decreased excitability compared with those from Cre−/− littermates. Furthermore, seizure susceptibility, in response to 20 mg/kg kainic acid, is significantly decreased in Gabrd/Gad mice compared with Cre−/− controls. These data demonstrate a critical role for GABAAR δ-subunit-mediated tonic GABAergic inhibition of interneurons on principal neuronal excitability and seizure susceptibility.

Synaptic inhibition in the olfactory bulb (OB), the first relay station of olfactory information, is believed to be important for odour discrimination. We interfered with GABAergic inhibition of mitral and tufted cells (M/T cells), the principal neurons of the OB, by disrupting their potassium-chloride cotransporter 2 (Kcc2). Roughly, 70% of mice died around 3 weeks, but surviving mice appeared normal. In these mice, the resulting increase in the intracellular Cl− concentration nearly abolished GABA-induced hyperpolarization of mitral cells (MCs) and unexpectedly increased the number of perisomatic synapses on MCs. In vivo analysis of odorant-induced OB electrical activity revealed increased M/T cell firing rate, altered phasing of action potentials in the breath cycle and disrupted separation of odour-induced M/T cell activity patterns. Mice also demonstrated a severely impaired ability to discriminate chemically similar odorants or odorant mixtures. Our work suggests that precisely tuned GABAergic inhibition onto M/T cells is crucial for M/T cell spike pattern separation needed to distinguish closely similar odours., Synaptic inhibition in the olfactory bulb (OB) is believed to play a role in odour processing. Here, the authors use a Pcdh21-driven Cre-line to disrupt KCC2 expression in OB mitral cells and find altered synaptic connectivity along with disrupted separation of odour-induced activity patterns.

The nociceptive system of rodents is not fully developed and functional at birth. Specifically, C fibers transmitting peripheral nociceptive information establish synaptic connections in the spinal cord already during the embryonic period that only become fully functional after birth. Here, we studied the consequences of neonatal maternal deprivation (NMD, 3 h/day, P2-P12) on the functional establishment of C fiber-mediated neurotransmission in spinal cord and of pain-related behavior. In vivo recording revealed that C fiber-mediated excitation of spinal cord neurons could be observed at P14 only in control but not in NMD rats. NMD was associated with a strong alteration in the expression of growth factors controlling C nociceptor maturation as well as two-pore domain K+ channels known to set nociceptive thresholds. In good agreement, C-type sensory neurons from NMD animals appeared to be hypoexcitable but functionally connected to spinal neurons, especially those expressing TRPV1 receptors. In vivo and in vitro recordings of lamina II spinal neurons at P14 revealed that the NMD-related lack of C fiber-evoked responses resulted from an inhibitory barrage in the spinal cord dorsal horn. Eventually, C-type sensory-spinal processing could be recovered after a delay of about 10 days in NMD animals. However, animals remained hypersensitive to noxious stimulus up to P100 and this might be due to an excessive expression of Nav1.8 transcripts in DRG neurons. Together, our data provide evidence for a deleterious impact of perinatal stress exposure on the maturation of the sensory-spinal nociceptive system that may contribute to the nociceptive hypersensitivity in early adulthood.

Brain aging is characterized by functional deterioration across multiple systems, associated to a progressive decay of neural plasticity. Here, we explored environmental enrichment (EE), a condition of enhanced sensory-motor and cognitive stimulation, as a strategy to restore plasticity processes in the old brain. Visual system is one of the paradigmatic models for studying experience-dependent plasticity. While reducing input from one eye through monocular deprivation induces a marked ocular dominance (OD) shift of neurons in the primary visual cortex during development, the same manipulation is totally ineffective after the closure of the critical period. We show that EE is able to reactivate OD plasticity in the visual cortex of aging rats, as assessed with both visual-evoked potentials and single-unit recordings. A marked reduction in intracortical GABAergic inhibition and a remodeling of extracellular matrix accompany this effect. The non-invasive nature of EE makes this paradigm eligible for human application.

The crucian carp, Carassius carassius, survives days to months without oxygen, depending on temperature. In the anoxic crucian carp brain, increased GABAergic inhibition, mediated by increased extracellular levels of GABA, has been shown to suppress electric activity and ATP consumption. To investigate an involvement of gene expression in this response, we utilized real-time RT-PCR to test the effect of 1 and 7 days anoxia (8°C) on the expression of 22 genes, including nine GABAAreceptor subunits (α1–6, β2, δ, and γ2), three GABABreceptor subunits (GB1a-1b and GB2), three enzymes involved in GABA metabolism (GAD65 and GAD67, GABAT), four GABA transporters (GAT1, 2a-b and 3), two GABAAreceptor-associated proteins (GABARAP 1 and 2), and the K+/Cl−cotransporter KCC2. While the expression of GABAAreceptor subunits was dominated by α4-, α6-, and δ-subunits, all of which are located to extrasynaptic sites in mammalian brains and respond to elevations in extracellular levels of GABA by showing tonic activity patterns, the expression of GABA transporters was dominated by GAT2 (a and b) and GAT3, which also show extrasynaptic location in mammals. These expression patterns differ from those observed in mammals and may be a prerequisite for GABAergic inhibition of anoxic metabolic rate in crucian carp. Furthermore, while the expression of the majority of the genes was largely unaltered by anoxia, the expression of GAT2 and GAT3 decreased to 20%. This suggests impairment of GABA transport, which could be a mechanism behind the accumulation of extracellular GABA and the increased GABAergic inhibition.

Background and purpose Animal models have provided very valuable data to specify the physiopathological mechanisms of the various forms of epilepsy. However, the question arises of knowing which of these experimental results are relevant to the human epileptic brain. The development of epileptic surgery makes it possible to directly study the functional properties of human brain tissue in vitro and to analyze the mechanisms underlying seizures and epileptogenesis. We review some of the results obtained over the last few years in our laboratory based on electrophysiological, immunocytochemical and molecular experiments conducted on human brain tissue. Results This review covers a number of the mechanisms of neuronal synchronizations generating epileptiform discharges, including the role of electrical synapses connecting the inhibitory interneurons, particularly in Taylor-type focal cortical dysplasia and the functional lability of GABAergic inhibition in epileptogenic human cortical tissue, which may sustain triggering and propagation of seizures. Some of these mechanisms have not been described in animal models. Conclusions Studies on human tissue, when carefully designed, are necessary to validate the data collected on animal models and will continue to provide us with new and important information on the cerebral changes related to epilepsy. Moreover, these studies allow development of a class of antiepileptic drugs that have a completely new mechanism of action, which could be effective in the treatment of drug-resistant epilepsies.

Duration is a salient feature of acoustic signals including speech. Duration tuning was first reported in frogs and later in echolocating bats. More recently, duration tuning has been reported in non-echolocating mammals and appears to be a fundamental encoding mechanism throughout the animal kingdom. However, the duration tuning reported in these non-echolocating mammals appears to be much weaker than that in the previous studies on bats. In contrast to this finding, our recent study reported that duration tuning in the IC in guinea pigs appeared to be strong when it was measured using an appropriate temporal window. With such a temporal window, duration tuning was found to be compatible with that of echo-locating bats. In the present report, we further demonstrate that duration tuning in the IC of this species is established by interaction between excitation and GABAergic inhibition. In addition to overall increase in responsiveness, application of bicuculline(BIC), a GABA-A receptor antagonist, was found to significantly reduce or eliminate duration selectivity in 44 out of the 67 neurons that showed clear duration tuning from a sample of 340 neurons.

Slc4a10 is a Na(+)-coupled Cl(-)-HCO3 (-) exchanger, which is expressed in principal and inhibitory neurons as well as in choroid plexus epithelial cells of the brain. Slc4a10 knockout (KO) mice have collapsed brain ventricles and display an increased seizure threshold, while heterozygous deletions in man have been associated with idiopathic epilepsy and other neurological symptoms. To further characterize the role of Slc4a10 for network excitability, we compared input-output relations as well as short and long term changes of evoked field potentials in Slc4a10 KO and wildtype (WT) mice. While responses of CA1 pyramidal neurons to stimulation of Schaffer collaterals were increased in Slc4a10 KO mice, evoked field potentials did not differ between genotypes in the stratum radiatum or the neocortical areas analyzed. Paired pulse facilitation was diminished in the hippocampus upon disruption of Slc4a10. In the neocortex paired pulse depression was increased. Though short term plasticity is modulated via Slc4a10, long term potentiation appears independent of Slc4a10. Our data support that Slc4a10 dampens neuronal excitability and thus sheds light on the pathophysiology of SLC4A10 associated pathologies.

Autoantibodies (aAB) to the presynaptic located enzyme glutamate decarboxylase 65 (GAD65) are a characteristic attribute for a variety of autoimmune diseases of the central nervous system including subtypes of limbic encephalitis, stiff person-syndrome, cerebellar ataxia, and Batten’s disease. Clinical signs of hyperexcitability and improvement of disease symptoms upon immunotherapy in some of these disorders suggest a possible pathogenic role of associated aAB. Recent experimental studies report inconsistent results regarding a direct pathogenic influence of anti-GAD65 aAB affecting inhibitory synaptic transmission in central GABAergic pathways. We here provide a method for direct evaluation of aAB-induced pathomechanisms in the intact hippocampal network. Purified patient IgG fractions containing aAB to GAD65 together with fixable lipophilic styryl dyes (FMdyes) are stereotactically injected into the hilus and the dentate gyrus in anesthetized mice. Twenty-four hours after intrahippocampal injection, acute hippocampal slices are prepared and transferred to a patch-clamp recording setup equipped with a fluorescence light source. Intraneural incorporated FMdyes show correct injection site for patch-clamp recording. Whole-cell patch-clamp recordings are performed from granule cells in the dentate gyrus and extracellular stimulation is applied in the border area of the dentate gyrus-hilus region to stimulate GABAergic afferents arising from parvalbumin positive basket cells. GABA-A receptor mediated inhibitory postsynaptic currents (IPSC) and miniature IPSC are recorded after blocking glutamatergic transmission. This approach allows investigation of potential aAB-induced effects on GABA-A receptor signaling ex vivo in an intact neuronal network. This offers several advantages compared to experimental procedures used in previous studies by in vitro AB preincubation of primary neurons or slice preparations. Furthermore, this method requires only small amounts of patient material that are often limited in rare diseases.

Adenosine is a known inhibitor of respiratory output during early life. In this study we investigated the developmental changes in adenosine A2A-receptor activation on respiratory timing, as well as the relationship between adenosine and GABA. The specific adenosine A2A-receptor agonist CGS-21680 (CGS) or vehicle control was injected into the fourth ventricle of 14-day ( n = 9), 21-day ( n = 9), and adult ( n = 5) urethane-anesthetized rats while diaphragm electromyogram was monitored as an index of respiratory neural output. CGS injection resulted in a decrease in frequency and/or apnea in all 14-day-old rats and in 66% of 21-day-old rats. There was no effect of CGS injection on respiratory timing in adult rats. Prior injection of the GABAA-receptor blocker bicuculline at 14 and 21 days eliminated the CGS-induced decrease in frequency and apnea. We conclude from these studies that the inhibitory effect of A2A-receptor activation on respiratory drive is age dependent and is mediated via GABAergic inputs to the inspiratory timing neural circuitry. These findings demonstrate an important mechanism by which xanthine therapy alleviates apnea of prematurity.

The superficial superior colliculus (sSC) is a key station in the sensory processing related to visual salience. The sSC receives cholinergic projections from the parabigeminal nucleus, and previous studies have revealed the presence of several different nicotinic acetylcholine receptor (nAChR) subunits in the sSC. In this study, to clarify the role of the cholinergic inputs to the sSC, we examined current responses induced by ACh in GABAergic and non-GABAergic sSC neurons using in vitro slice preparations obtained from glutamate decarboxylase 67-green fluorescent protein (GFP) knock-in mice in which GFP is specifically expressed in GABAergic neurons. Brief air pressure application of acetylcholine (ACh) elicited nicotinic inward current responses in both GABAergic and non-GABAergic neurons. The inward current responses in the GABAergic neurons were highly sensitive to a selective antagonist for α3β2- and α6β2-containing receptors, α-conotoxin MII (αCtxMII). A subset of these neurons exhibited a faster α-bungarotoxin-sensitive inward current component, indicating the expression of α7-containing nAChRs. We also found that the activation of presynaptic nAChRs induced release of GABA, which elicited a burst of miniature inhibitory postsynaptic currents mediated by GABAA receptors in non-GABAergic neurons. This ACh-induced GABA release was mediated mainly by αCtxMII-sensitive nAChRs and resulted from the activation of voltage-dependent calcium channels. Morphological analysis revealed that recorded GFP-positive neurons are interneurons and GFP-negative neurons include projection neurons. These findings suggest that nAChRs are involved in the regulation of GABAergic inhibition and modulate visual processing in the sSC.

The network oscillations are ubiquitous across many brain regions. In the basal ganglia, oscillations are also present at many levels and a wide range of characteristic frequencies have been reported to occur during both health and disease.The striatum is the input nucleus of the basal ganglia that receives massive glutamatergic inputs from the cortex and is highly susceptible to cortical oscillations. However, there is limited knowledge about the exact nature of this routing process and therefore, it is of key importance to understand how time-dependent, periodic external stimuli propagate through the striatal circuitry. Using a large-scale network model of the striatum and corticostriatal projections, here we try to elucidate the importance of specific GABAergic neurons and their interactions in shaping striatal oscillatory activity. Our results show that fast-spiking interneurons, despite their uncorrelated firing, might have a crucial role in the emergence of high-frequency oscillations in the medium spiny neuron population, even if their activity is kept low. Rather, what matters is the firing time relative to just a few other neurons within an oscillation cycle. Finally, we show how the state of ongoing activity, the strengths of different types of inhibitions, the density of outgoing projections, and the overall activity of striatal cells influence network activity. These results suggest that the propagation of oscillatory inputs into the medium spiny neuron population is efficient, if indirect, through fast-spiking interneurons. Therefore, pharmaceuticals that target fast-spiking interneurons may provide a novel treatment for regaining the spectral characteristics of striatal activity that correspond to the healthy state.Author SummaryThe striatum is the largest and primary gateway component of the BG, receiving glutamatergic inputs from all cortical areas and is highly susceptible to cortical oscillations. However, there is limited knowledge about the exact nature of this routing process and therefore, it is of key importance to understand how time-dependent, external stimuli propagate through the striatal circuitry. The vast majority of striatal neurons, at least 95% of them, are medium spiny neurons (MSNs) that are also the only source of output from the nucleus. Two of the most examined sources of GABAergic inhibition into MSNs are the feedback inhibition (FB) from the axon collaterals of the MSNs themselves, and the feedforward inhibition (FF) via the small population of fast-spiking interneurons (FSIs) comprising roughly 1-2% of striatal neurons. Using a large-scale network model of the striatum we systematically investigate the propagation of asynchronous periodic cortical inputs, throughout the physiologically relevant range, onto the striatal neurons and their influence on striatal network dynamics. Our results show that FSIs, despite their firing being uncorrelated, may play a crucial role in the efficient propagation of external oscillations onto MSNs. Finally, we show how the state of ongoing activity, the strengths of different types of inhibitions, the density of outgoing projections, and the overall activity of striatal cells influence network activity.

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.

arising from B. V. Atallah, W. Bruns, M. Carandini & M. Scanziani , 159–170 (2012)10.1016/j.neuron.2011.12.013 ; N. R. Wilson, C. A. Runyan, F. L. Wang & M. Sur 343–348 (2012) ; S.-H. Lee et al. 379–383 (2012) Parvalbumin-positive (PV+) and somatostatin-positive (SST+) interneurons are two principal subtypes of cortical GABAergic neurons that differ in morphology, physiological properties and postsynaptic targeting1,2,3,4. Although GABAergic inhibition is known to be crucial for shaping orientation tuning in the visual cortex5,6,7, it is unclear whether PV+ and SST+ interneurons have different roles. Recently, Atallah et al.8, Wilson et al.9 and Lee et al.10 addressed this issue by optogenetically manipulating each interneuron subtype, reaching different conclusions: some investigators found that SST+ neuron activation sharpens orientation tuning9, whereas PV+ neuron activation has little effect8,9, but others found that activation of PV+ but not SST+ interneurons sharpens orientation tuning10. To understand the cause for the discrepancy we examined the impact of several experimental differences among the studies—anaesthesia and the level and duration of optogenetic stimulation—and found that the discrepancies can be largely explained by differences in the level and duration of interneuron activation. There are replies to this Brief Communication Arising by Atallah, B. V., Scanziani, M. & Carandini, M. Nature 508, http://dx.doi.org/10.1038/nature13129 (2014) and El-Boustani, S., Wilson, N. R., Runyan, C. A. & Sur, M. Nature 508, http://dx.doi.org/10.1038/nature13130 (2014).

This study examined the influence of inhibition on motion-direction-sensitive responses of neurons in the dorsal fields of auditory cortex of the rufous horseshoe bat. Responses to auditory apparent motion stimuli were recorded extracellularly from neurons while microiontophoretically applying gamma-aminobutyric acid (GABA) and the GABAA receptor antagonist bicuculline methiodide (BMI). Neurons could respond with a directional preference exhibiting stronger responses to one direction of motion or a shift of receptive field (RF) borders depending on direction of motion. BMI influenced the motion direction sensitivity of 53% of neurons. In 21% of neurons the motion-direction sensitivity was decreased by BMI by decreasing either directional preference or RF shift. In neurons with a directional preference, BMI increased the spike number for the preferred direction by a similar amount as for the nonpreferred direction. Thus, inhibition was not direction specific. BMI increased motion-direction sensitivity by either increasing directional preference or magnitude of RF shifts in 22% of neurons. Ten percent of neurons changed their response from a RF shift to a directional preference under BMI. In these neurons, the observed effects could often be better explained by adaptation of excitation rather than inhibition. The results suggest, that adaptation of excitation, as well as cortex specific GABAergic inhibition, contribute to motion-direction sensitivity in the auditory cortex of the rufous horseshoe bat.

The axonal (bouton) distributions of a layer 4 clutch cell (CC), two layer 3 medium-sized basket cells (MBC), and a layer 3 large basket cell (LBC) to orientation, direction, and ocular dominance maps were studied quantitatively. 1) The CC provided exclusively local projections ( 313 μm and >418 μm), which encountered cross-orientation sites (14% and 12%) and isoorientation sites (7% and 5%). Their direction preferences were mainly perpendicular to or opposite those of local projections. 3) The LBC provided the majority (60%) of its boutons to long-range distances (>437 μm). Locally, LBC boutons showed a rather balanced contribution to isoorientations (19%) and cross-orientations (12%) and preferred isodirections. Remotely, however, cross-orientation sites were preferred (31% vs. 23%) and the directional output was balanced. 4) Monte Carlo simulations revealed that the differences between the orientation specificity of local and long-range projections cannot be explained by a homogeneous lateral distribution of the boutons. 5) There was a similar eye preference in the local and long-range projection fields of the MBCs. The LBC contacted both contra- and ipsilateral eye domains. 6) The basket axons showed little laminar difference in orientation and direction topography. The results suggest that an individual basket cell can mediate a wide range of effects depending on the size and termination pattern of the axonal field. J. Comp. Neurol. 437:259–285, 2001. © 2001 Wiley-Liss, Inc.

In the presence of the gamma -amino butyric acid-A (GABA(A)) antagonist bicuculline methiodide (50 muM), synchronous spontaneous and evoked potentials were recorded extracellularly from the deep layers of immature neocortex (postnatal days 10-31, P10-P31) in vitro. Addition of the anticholinesterase eserine (10 muM) depressed the amplitude (by 29.5 +/- 6.6%, n = 13) and duration (by 26.3 +/- 4.7%, n = 11) of the evoked field potentials in 13/19 slices (68%), and increased significantly the rates of occurrence of spontaneous epileptiform discharges or induced them in 9/19 slices (47%). All these effects were blocked by the muscarinic antagonist atropine (2.5 muM, n = 3), suggesting that they were mediated by the activation of muscarinic receptors by endogenous acetylcholine. The cholinergic inhibitory effect is unlikely to terminate seizures, while the excitatory effect, could conceivably promote or aggravate their manifestation. In conclusion, these findings demonstrate that endogenous acetylcholine may contribute to epileptogenesis in immature neocortex. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved. Neurosci Lett

Presynaptic gamma-aminobutyrate-immunoreactive (GABA+) profiles were mapped in the cyprinid retina with overlay microscopy: a fusion of electron and optical imaging affording high-contrast ultrastructural and immunocytochemical visualization. GABA+ synapses, deriving primarily from amacrine cells (ACs), compose 92% of conventional synapses and 98% of the input to bipolar cells (BCs) in the inner plexiform layer. GABA+ AC synapses, the sign-inverting elements of signal processing, are deployed in micronetworks and distinctive synaptic source/target topologies. Nested feedback micronetworks are formed by three types of links (BC --> AC, reciprocal BC AC synapses) arranged as nested BC [AC --> AC] loops. Circuits using nested feedback can possess better temporal performance than those using simple reciprocal feedback loops. Concatenated GABA+ micronetworks of AC --> AC and AC --> AC --> AC chains are common and must be key elements for lateral spatial, temporal, and spectral signal processing. Concatenated inhibitions may represent exceptionally stable, low-gain, sign-conserving devices for receptive field construction. Some chain elements are GABA immunonegative (GABA-) and are, thus, likely glycinergic synapses. GABA+ synaptic baskets target the somas of certain GABA+ and GABA- cells, resembling cortical axosomatic synapses. Finally, all myelinated intraretinal profiles are GABA+, suggesting that some efferent systems are sources of GABAergic inhibition in the cyprinid retina and may comprise all axosomatic synapses. These micronetworks are likely the fundamental elements for receptive field shaping in the inner plexiform layer, although few receptive field models incorporate them as functional components. Conversely, simple feedback and feedforward synapses may often be chimeras: the result of an incomplete view of synaptic topology.

Throughout the brain, multiple interneuron types influence distinct aspects of synaptic processing. Interneuron diversity can thereby promote differential firing from neurons receiving common excitation. In contrast, Golgi cells are the sole interneurons regulating granule cell spiking evoked by mossy fibers, thereby gating inputs to the cerebellar cortex. Here, we examine how this single interneuron type modifies activity in its targets. We find that GABAA-mediated transmission at unitary Golgi cell → granule cell synapses consists of varying contributions of fast synaptic currents and sustained inhibition. Fast IPSCs depress and slow IPSCs gradually build during high frequency Golgi cell activity. Consequently, fast and slow inhibition differentially influence granule cell spike timing during persistent mossy fiber input. Furthermore, slow inhibition reduces the gain of the mossy fiber → granule cell input-output curve, while fast inhibition increases the threshold. Thus, a lack of interneuron diversity need not prevent flexible inhibitory control of synaptic processing.

This study examines the contribution of GABAergic inhibition to the discharge pattern and pulse duration tuning characteristics of 101 bat inferior collicular neurons by means of bicuculline application to their recording sites. When stimulated with single pulses, 56 (55%) neurons discharged 1 or 2 impulses (phasic responders), 42 (42%) discharged 3–10 impulses (phasic bursters) and 3 (3%) discharged impulses throughout the stimulus duration (tonic responders). Bicuculline application increased the number of impulses and changed the discharge patterns of 66 neurons. Using 50% difference between maximal and minimal responses as a criterion, the duration tuning characteristics of these neurons can be described as band-pass (20, 20%), long-pass (17, 17%), short-pass (33, 32%), and all-pass (31, 31%). Each band-pass neuron discharged maximally to a specific duration (the best duration) which was at least 50% larger than the neuron's responses to a long-duration pulse and a short-duration pulse. In contrast, each long- or short-pass neuron discharged maximally to a range of long or short duration pulses. Bicuculline application changed the duration tuning characteristics of 65 neurons. Possible mechanisms underlying duration tuning characteristics and the behavioral relevance to bat echolocation are discussed.

Benardo, Larry S. Recruitment of GABAergic inhibition and synchronization of inhibitory interneurons in rat neocortex. J. Neurophysiol. 77: 3134–3144, 1997. Intracellular recordings were obtained from pyramidal and interneuronal cells in rat neocortical slices to examine the recruitment of GABAergic inhibition and inhibitory interneurons. In the presence of the convulsant agent4-aminopyridine (4-AP), after excitatory amino acid (EAA) ionotropic transmission was blocked, large-amplitude triphasic inhibitory postsynaptic potentials (IPSPs) occurred rhythmically (every 10–40 s) and synchronously in pyramidal neurons. After exposure to the γ-aminobutyric acid-A (GABAA) receptor antagonist picrotoxin, large-amplitude monophasic slow IPSPs persisted in these cells. In the presence of 4-AP and EAA blockers, interneurons showed periodic spike firing. Although some spikes rode on an underlying synaptic depolarization, much of the rhythmic firing consisted of spikes having highly variable amplitudes, arising abruptly from baseline, even during hyperpolarization. The spike firing and depolarizing synaptic potentials were completely suppressed by picrotoxin exposure, although monophasic slow IPSPs persisted in interneurons. This suggests that this subset of interneurons may participate in generating fast GABAA IPSPs, but not slow GABAB IPSPs. Cell morphology was confirmed by intracellular injection of neurobiotin or the fluorescent dye Lucifer yellow CH. Dye injection into interneurons often (>70%) resulted in the labeling of two to six cells (dye coupling). These findings suggest that GABAAergic neurons may be synchronized via recurrent collaterals through the depolarizing action of synaptically activatedGABAA receptors and a mechanism involving electrotonic coupling. Although inhibitory neurons mediating GABAB IPSPs may be entrained by the excitatory GABAA mechanism, they appear to be a separate subset of GABAergic neurons capable of functioning independently with autonomous pacing.

Summary Purpose Dravet syndrome (DS) is caused by dominant mutations of the SCN1A gene, encoding the NaV1.1 sodium channel α subunit. Gene targeted mouse models of DS mutations replicate patients' phenotype and show reduced γ-aminobutyric acid (GABA)ergic inhibition. However, little is known on the properties of network hyperexcitability and on properties of seizure generation in these models. In fact, seizures have been studied thus far with surface electroencephalography (EEG), which did not show if specific brain regions are particularly involved. We have investigated hyperexcitability and epileptiform activities generated in neuronal networks of a mouse model of DS. Methods We have studied heterozygous NaV1.1 knock-out mice performing field potential recordings in combined hippocampal/cortical slices in vitro and video/depth electrode intracerebral recordings in vivo during hyperthermia-induced seizures. Key Findings In slices, we have disclosed specific signs of hyperexcitability of hippocampal circuits in both the pre-epileptic and epileptic periods, and a specific epileptiform activity was generated in the hippocampus upon application of the convulsant 4-aminopyridine in the epileptic period. During in vivo hyperthermia-induced seizures, we have observed selective hippocampal activity in early preictal phases and pronounced hippocampal activity in the ictal phase. Significance We have identified specific epileptiform activities and signs of network hyperexcitability, and disclosed the important role of the hippocampus in seizure generation in this model. These activities may be potentially used as targets for screenings of antiepileptic approaches.

Kainate receptors (KARs) are expressed at high levels in the brain during early development and may be critical for the proper development of neuronal networks. Here we elucidated a physiological role of high-affinity KARs in developing hippocampal network by studying the effects of 25–100 nM kainate (KA) on intrinsic network activity in slice preparations. Whereas 100 nM KA resulted in hyperexcitability of the network and the disruption of natural activity patterns, ≤50 nM KA concentrations enhanced the initiation of network bursts yet preserved the characteristic patterns of endogenous activity. This was not dependent on changes in GABAergic transmission or on activation of GluK1 subunit containing KARs. However, the activation of high-affinity KARs increased glutamatergic drive by promoting spontaneous firing of CA3 pyramidal neurons without affecting action potential independent glutamate release. This was not because of changes in the intrinsic somatic properties of pyramidal neurons but seemed to reside in an electrically remote site, most probably in an axonal compartment. Although application of KAR agonists has mainly been used to study pathological type of network activities, this study provides a novel mechanism by which endogenous activity of KARs can modulate intrinsic activities of the emerging neuronal network in a physiologically relevant manner. The results support recent studies that KARs play a central role in the activity-dependent maturation of synaptic circuitries.

The role of flocculus in vestibular compensation is still a controversial issue. Calbindin regulates intracellular signaling and has been reported to be a reliable marker of Purkinje cell. Expression of calbindin in flocculus was examined using immunohistochemistry following unilateral labyrinthectomy (UL) in rats. Both the staining intensity and number of calbindin-positive Purkinje cells in the ipsilateral flocculus to the lesion side decreased 6 h after UL compared to the control and contralateral side. Forty-eight hours after UL, the expression of calbindin returned to control levels and asymmetric expression in bilateral flocculus subsided. These transient reduction of calbindin expression in the ipsilateral flocculus may reflect a decrease in the GABAergic inhibition of the floccular Purkinje cell to the ipsilateral vestibular nuclei during vestibular compensation.

Experiments have demonstrated the ability of action potentials to actively backpropagate in striatal medium spiny (MS) neurons, affecting the calcium levels in the dendrites [1, 2, 3]. Increased calcium levels trigger changes in plasticity [4, 5], which is important for learning and other functions [6]. Studies in the hippocampus have shown that GABAergic input can modulate the backpropagation of action potentials from the soma to the distal dendrites [7]. The MS neurons receive both proximal feedforward GABAergic inhibition from fast spiking interneurons (FS), and distal feedback inhibition from other neighbouring MS neurons. In the present study the effect of GABAergic inputs on the dendritic calcium dynamics is investigated.

In the neostriatum, GABAergic inhibition arises from the action of at least two classes of inhibitory interneurons, and from recurrent collaterals of the principal cells. Interneurons receive excitatory input only from extrinsic sources, and so act in a purely feedforward capacity. Feedback inhibition arises from the recurrent collaterals of the principal cells. These two kinds of inhibition have functionally distinct effects on the principal cells. Inputs from interneurons are not very convergent. There are few inhibitory neurons, and so each principal cell receives inhibitory synaptic input from very few interneurons. But, they are individually powerful, and a single interneuron can substantially delay action potentials in a group of nearby principal cells. Recurrent inhibition is highly convergent, with each principal cell receiving inhibitory input from several hundred other such cells. Feedback inhibitory synaptic inputs individually have very weak effects, as seen from the soma. The differences in synaptic strength are not caused by differences in the release of transmitter or in sensitivity of the postsynaptic membrane. Rather, they arise from differences in the number of synaptic contacts formed on individual principal cells by feedforward or feedback axons, and from differences in synaptic location. Interneurons form their powerful synapses near the somata of principal cells, while most feedback synapses are more distal, where they interact with the two-state nonlinear properties of the principal cells' dendrite. This arrangement suggests that feedforward inhibition may serve in the traditional role for inhibition, adjusting the excitability of the principle neuron near the site of action potential generation. Feedback inhibitory synapses may interact with voltage-sensitive conductances in the dendrite to alter the electrotonic structure of the spiny cell.

International audience; GABAergic inhibition via local interneurons may play a role in enhancing spike timing precision in principal cells, since it tends to eliminate the influence of initial conditions. However, both the number and the timing of inhibitory synaptic events may be variable across repeated trials. How does this variability affect the spike timing precision in principal neurons? In this paper, we derive an analytical expression for the spike output jitter as a function of the variability of the received inhibition. This study predicts that variable inhibition is especially tolerated as the number of inhibitory cells is large and the decay time constant of the GABAergic synapse is small, which is consistent with experimental data from early olfactory systems (antennal lobe for insects, olfactory bulb for vertebrates).

A previous study has shown that duration selectivity of neurons in the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus becomes sharper with increasing pulse repetition rate (PRR). The present study examines the role of GABAergic inhibition in improving duration selectivity of bat IC neurons with PRR by means of iontophoretic application of GABA as well as its antagonist, bicuculline. Duration selectivity of IC neurons is studied by plotting the duration tuning curves with the number of impulses per pulse against the pulse duration. Duration tuning curves of IC neurons are described as band-, short-, long- and all-pass in terms of filtering properties to sound duration. Bicuculline application produces more pronounced broadening of duration tuning curves at high than at low PRR. Conversely, GABA application produces more pronounced narrowing of duration tuning curves at low than at high PRR. In either case, sharpening of duration selectivity of IC neurons with increasing PRR is abolished during drug application. The duration tuning curves of IC neurons progressively broadens with recording depth. Broadening of duration tuning curves during bicuculline application is more pronounced for neurons at upper than at deep IC. This progressive decrease in duration selectivity with recording depth is discussed in relation to spatial distribution gradient of GABAA receptors in the IC. Possible biological significance of these findings relevant to bat echolocation is discussed.

This study examined the effect of bicuculline application on sharpness of frequency tuning curves (FTCs) of bat inferior collicular neurons plotted under three different pulse repetition rates (PRRs) of 10, 30 and 90 pulses per second. The sharpness of FTCs of collicular neurons, which was expressed in Q n ( Q 10 , Q 20 , Q 30 ) and bandwidths (90, 75 and 50% of the maximal response at the best frequency), improved with increasing PRR. However, this PRR-dependent frequency selectivity of collicular neurons was abolished during bicuculline application. This observation suggests that GABAergic inhibition contributes more effectively to sharpening of FTCs at higher than at lower PRRs.

This study examined the role of GABAergic inhibition in shaping directional selectivity of neurons in the inferior colliculus of the big brown bat, Eptesicus fuscus. When determined with temporally patterned pulse trains at different pulse repetition rates, 93 inferior colliculus neurons displayed three types of directional selectivity curves. A directionally selective curve always showed a maximum to a certain azimuthal angle (the best angle). A hemifield curve showed a maximum to a range of contralateral azimuthal angles. A non-directional curve did not show a maximum to any particular azimuthal angles. Directional selectivity curves of 42% neurons changed from hemifield or non-directional to directionally selective and the best angles of 16-21% neurons shifted toward the midline with increasing pulse repetition rate of pulse trains. Directional selectivity curves of most (74%) neurons that discharged impulses to each pulse of a pulse train also became sharper with increasing pulse repetition rate of pulse trains. Bicuculline application produced more pronounced broadening of directional selective curves of inferior colliculus neurons at higher than at lower pulse repetition rates. As a result, pulse repetition rate-dependent directional selectivity of inferior colliculus neurons was abolished. Possible mechanisms and biological significance of these findings are discussed.

The regulation of activity in the subthalamic nucleus (STN) by GABAergic inhibition from the reciprocally connected globus pallidus (GP) plays an important role in normal movement and disorders of movement. To determine the precise manner in which GABAergic synaptic input, acting at A-type receptors, influences the firing of STN neurons, we recorded the response of STN neurons to GABA-A inhibitory postsynaptic potentials (IPSPs) that were evoked by supramaximal electrical stimulation of the internal capsule using the perforated-patch technique in slices at 37°C. The mean equilibrium potential of the GABA-A IPSP (EGABA-A IPSP) was −79.4 ± 7.0 mV. Single IPSPs disrupted the spontaneous oscillation that underlies rhythmic single-spike firing in STN neurons. As the magnitude of IPSPs increased, the effectiveness of prolonging the interspike interval was related more strongly to the phase of the oscillation at which the IPSP was evoked. Thus the largest IPSPs tended to reset the oscillatory cycle, whereas the smallest IPSPs tended to produce relatively phase-independent delays in firing. Multiple IPSPs were evoked at various frequencies and over different periods and their impact was studied on STN neurons held at different levels of polarization. Multiple IPSPs reduced and/or prevented action potential generation and/or produced sufficient hyperpolarization to activate a rebound depolarization, which generated a single spike or restored rhythmic spiking and/or generated a burst of activity. The pattern of IPSPs and the level of polarization of STN neurons were critical in determining the nature of the response. The duration of bursts varied from 20 ms to several hundred milliseconds, depending on the intrinsic rebound properties of the postsynaptic neuron. These data demonstrate that inhibitory input from the GP can produce a range of firing patterns in STN neurons, depending on the number and frequencies of IPSPs and the membrane properties and voltage of the postsynaptic neuron.

This study examined the effect of bicuculline application on the auditory response properties in the auditory cortex of the big brown bat, Eptesicus fuscus. All auditory cortical neurons studied discharged either 1-2 or 3-7 impulses to 4 ms sound stimuli. Cortical neurons with high best frequencies tended to have high minimum thresholds. Bicuculline application increased the number of impulses and shortened the response latencies of all cortical neurons as well as changing the discharge patterns of half of the cortical neurons studied. Bicuculline application raised the rate-intensity functions but lowered the latency-intensity functions to varying degrees. Threshold-frequency tuning curves (FTCs) were either V-shaped, upper threshold or double-peaked. Threshold-FTCs and impulse-FTCs were mirror-images of each other. Bicuculline application expanded and raised the impulse-FTCs but lowered the threshold-FTCs, resulting in significantly decreased Q(n) values. Threshold-FTCs of cortical neurons determined within an orthogonally inserted electrode were very similar and expanded FTCs during bicuculline application were also very similar. Possible mechanisms for the contribution of GABAergic inhibition to shaping these response properties of cortical neurons are discussed.

Extensive deprivation of excitatory inputs to a central network like the mammalian visual cortex largely influences its growth and function (Shatz, 1990). It is, however, less clear if a more discrete, albeit long-lasting, suppression of a certain transmitter system can be compensated, through homeostatic mechanisms, by a neuronal network. This issue raises the question of whether nerve cells can establish a functionally coherent pattern of signalling despite removal of discrete inputs: to examine this aspect, chronic block of either excitatory or inhibitory synaptic transmission has been employed as an experimental model. As a consequence of such treatment, a novel form of synaptic modification which has been termed homeostatic plasticity (Turrigiano, 1999) emerges, whereby neurons respond to sustained increases or decreases in their excitatory inputs by changing the properties, distribution and/or composition of certain transmitter postsynaptic receptors (Craig, 1998; Turrigiano, 1999). In particular, block of GABA and/or glycine receptors leads to downregulation of spontaneous glutamatergic transmission, while chronic block of glutamate receptors boosts on-going postsynaptic glutamate receptor-mediated activity. Such phenomena have been recently described in cultured mammalian neurons from various brain areas (Bessho et al. 1994; Rao & Craig, 1997; Kirsch & Betz, 1998; Levi et al. 1998; Lissin et al. 1998; O'Brien et al. 1998; Turrigiano et al. 1998) and, in addition to receptor changes, may include alterations in membrane excitability and consequently in firing activity (Desai et al. 1999). These observations are compatible with a recent hypothesis (originated from studies of the chick embryo spinal cord; Chub & O'Donovan, 1998) that the electrical behaviour of a network is governed by homeostatic mechanisms, that is, any long-lasting reduction in excitatory activity would be counteracted by increasing the efficacy of distinct excitatory transmitter systems. Whether homeostatic plasticity is a paradigm for network remodelling during neuronal development in mammals remains, however, an intriguing and scarcely explored possibility. During development, qualitative and quantitative reshaping of afferent projections may normally occur, thus leading to variations in excitatory input (Crair, 1999). Furthermore, the onset of axonal sprouting and de novo synaptogenesis is influenced by synaptic activity itself (Cline & Constantine-Paton, 1990), suggesting that activity regulates the synaptic targeting of each receptor type in a complex way (Craig, 1998). To explore the presence of homeostatic plasticity and its role in regulating network activity during development, we wished to use a culture system which generates a considerable degree of network organization. For this purpose we employed organotypic spinal cord cultures from rat embryos (Spenger et al. 1991; Streit et al. 1991; Ballerini & Galante, 1998; Ballerini et al. 1999), as this preparation maintains the basic cytoarchitecture of a spinal segment, and enables direct recording from visually identified neurons during in vitro development (Ballerini & Galante, 1998; Ballerini et al. 1999). Activity-dependent modulation of synaptic transmission was investigated by recording postsynaptic currents (PSCs) and miniature PSCs (mPSCs) from ventral horn interneurons at 14 days in vitro (DIV) in cultures incubated for the entire second week in the presence of various receptor blockers and comparing these responses with those of control cultures. Although these interneurons have not been individually characterized in terms of their excitatory or inhibitory function, it is likely that the large population sampled in the present study included excitatory as well as inhibitory interneurons since a strong, mixed input is normally recorded from nearby motoneurons (Streit, 1993). We adopted three experimental strategies: (1) chronic block of AMPA/kainate glutamate receptors with CNQX to remove the main, spontaneous excitatory input to these cells, since NMDA receptors play a rather minor role in excitatory transmission on these cells (Ballerini & Galante 1998; Ballerini et al. 1999); (2) chronic block of Na+-dependent action potential mechanisms in an attempt to mimic a more systematic suppression of afferent inputs, like one might expect following chronic lesions; (3) chronic block of GABAA and glycine receptors with bicuculline and strychnine to evoke sustained bursting activity (Ballerini & Galante, 1998).