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Intracellular amyloid beta oligomer (iAβo) accumulation and neuronal hyperexcitability are two crucial events at early stages of Alzheimer's disease (AD). However, to date, no mechanism linking iAβo with an increase in neuronal excitability has been reported. Here, the effects of human AD brain‐derived (h‐iAβo) and synthetic (iAβo) peptides on synaptic currents and action potential firing were investigated in hippocampal neurons. Starting from 500 pM, iAβo rapidly increased the frequency of synaptic currents and higher concentrations potentiated the AMPA receptor‐mediated current. Both effects were PKC‐dependent. Parallel recordings of synaptic currents and nitric oxide (NO)‐associated fluorescence showed that the increased frequency, related to pre‐synaptic release, was dependent on a NO‐mediated retrograde signaling. Moreover, increased synchronization in NO production was also observed in neurons neighboring those dialyzed with iAβo, indicating that iAβo can increase network excitability at a distance. Current‐clamp recordings suggested that iAβo increased neuronal excitability via AMPA‐driven synaptic activity without altering membrane intrinsic properties. These results strongly indicate that iAβo causes functional spreading of hyperexcitability through a synaptic‐driven mechanism and offers an important neuropathological significance to intracellular species in the initial stages of AD, which include brain hyperexcitability and seizures., In the presence of iAβo, PKC is activated (a), which in turn allows the production of nitric oxide (NO) at the post‐synaptic level, increasing the release of neurotransmitters (b) (evidenced as an increase in the frequency of mPSC). PKC also increases AMPA receptor current (c), increasing post‐synaptic depolarization and neuronal excitability (d). Finally, iAβo increases synchronization NO production in nearby neurons that do not have iAβo (e).

Aim We examined if tonic γ‐aminobutyric acid (GABA)‐activated currents in primary hippocampal neurons were modulated by insulin in wild‐type and tg‐APPSwe mice, an Alzheimer’s disease (AD) model. Methods GABA‐activated currents were recorded in dentate gyrus (DG) granule cells and CA3 pyramidal neurons in hippocampal brain slices, from 8‐10 weeks old (young) wild‐type mice and in dorsal DG granule cells in adult, 5‐6 and 10‐12 (aged) months old wild‐type and tg‐APPSwe mice, in the absence or presence of insulin, by whole‐cell patch‐clamp electrophysiology. Results In young mice, insulin (1 nM) enhanced the total spontaneous inhibitory postsynaptic current (sIPSCT) density in both dorsal and ventral DG granule cells. The extrasynaptic current density was only increased by insulin in dorsal CA3 pyramidal neurons. In absence of action potentials, insulin enhanced DG granule cells and dorsal CA3 pyramidal neurons miniature IPSCT (mIPSCT) frequency, consistent with insulin regulation of presynaptic GABA release. sIPSCT densities in DG granule cells were similar in wild‐type and tg‐APPSwe mice at 5‐6 months but significantly decreased in aged tg‐APPSwe mice where insulin normalized currents to wild‐type levels. The extrasynaptic current density was increased in tg‐APPSwe mice relative to wild‐type littermates but, only in aged tg‐APPSwe mice did insulin decrease and normalize the current. Conclusion Insulin effects on GABA signalling in hippocampal neurons are selective while multifaceted and context‐based. Not only is the response to insulin related to cell‐type, hippocampal axis‐location, age of animals and disease but also to the subtype of neuronal inhibition involved, synaptic or extrasynaptic GABAA receptors‐activated currents. Hayma Hammoud and Olga Netsyk are co‐first authors and contributed equally.Sergiy V. Korol and Zhe Jin contributed equally.

Parvalbumin-positive interneurons (PV+) are a key component of inhibitory networks in the brain and are known to modulate memory and learning by shaping network activity. The mechanisms of PV+ neuron generation and maintenance are not fully understood, yet current evidence suggests that signalling via the glial cell line-derived neurotrophic factor (GDNF) receptor GFR alpha 1 positively modulates the migration and differentiation of PV+ interneurons in the cortex. Whether GDNF also regulates PV+ cells in the hippocampus is currently unknown. In this study, we utilized a Gdnf "hypermorph" mouse model where GDNF is overexpressed from the native gene locus, providing greatly increased spatial and temporal specificity of protein expression over established models of ectopic expression. Gdnf(wt/hyper) mice demonstrated impairments in long-term memory performance in the Morris water maze test and an increase in inhibitory tone in the hippocampus measured electrophysiologically in acute brain slice preparations. Increased PV+ cell number was confirmed immunohistochemically in the hippocampus and in discrete cortical areas and an increase in epileptic seizure threshold was observed in vivo. The data consolidate prior evidence for the actions of GDNF as a regulator of PV+ cell development in the cortex and demonstrate functional effects upon network excitability via modulation of functional GABAergic signalling and under epileptic challenge.

Inhibitory synaptic transmission requires the targeting and stabilization of GABAA receptors (GABAARs) at synapses. The mechanisms responsible remain poorly understood, and roles for transmembrane accessory proteins have not been established. Using molecular, imaging, and electrophysiological approaches, we identify the tetraspanin LHFPL4 as a critical regulator of postsynaptic GABAAR clustering in hippocampal pyramidal neurons. LHFPL4 interacts tightly with GABAAR subunits and is selectively enriched at inhibitory synapses. In LHFPL4 knockout mice, there is a dramatic cell-type-specific reduction in GABAAR and gephyrin clusters and an accumulation of large intracellular gephyrin aggregates in vivo. While GABAARs are still trafficked to the neuronal surface in pyramidal neurons, they are no longer localized at synapses, resulting in a profound loss of fast inhibitory postsynaptic currents. Hippocampal interneuron currents remain unaffected. Our results establish LHFPL4 as a synapse-specific tetraspanin essential for inhibitory synapse function and provide fresh insights into the molecular make-up of inhibitory synapses.

Autism spectrum disorder (ASD) is an immensely challenging developmental disorder characterized by impaired social interaction, restricted/repetitive behavior, and anxiety. GABAergic dysfunction has been postulated to underlie these autistic symptoms. Gastrodin is widely used clinically in the treatment of neurological disorders and showed to modulate GABAergic signaling in the animal brain. The present study aimed to determine whether treatment with gastrodin can rescue valproic acid (VPA) induced autistic-like phenotypes, and to determine its possible mechanism of action. Our results showed that administration of gastrodin effectively alleviated the autistic-associated behavioral abnormalities as reflected by an increase in social interaction and decrement in repetitive/stereotyped behavior and anxiety in mice as compared to those in untreated animals. Remarkably, the amelioration in autistic-like phenotypes was accompanied by the restoration of inhibitory synaptic transmission, α5 GABAA receptor, and type 1 GABA transporter (GAT1) expression in the basolateral amygdala (BLA) of VPA-treated mice. These findings indicate that gastrodin may alleviate the autistic symptoms caused by VPA through regulating GABAergic synaptic transmission, suggesting that gastrodin may be a potential therapeutic target in autism.

Early-onset drinking during childhood or preadolescence is a serious social problem. Yet, most of the basic neurobiological research on the acute effects of ethanol has been carried out on adult or early postnatal animals. We studied the effect of alcohol exposure on the basic electrophysiological properties and cell viability of layer 5 pyramidal neurons from the somatosensory cortex of juvenile (P21–P23) C57BL/6N mice. After bath application of 50 mM ethanol to acute slices of the somatosensory cortex, no adverse effects were detected on cells survival, whereas the input resistance and firing rate of layer 5 neurons were significantly reduced. While the effect on the input resistance was reversible, the depressing effect on cell firing remained stable after 6 min of alcohol exposure. Ethanol application did not result in any significant change of mIPSC frequency, amplitude, and rise time. A slight increase of mIPSC decay time was observed after 6 min of ethanol exposure. The molecular mechanisms leading to these alterations and their significance for the physiology of the cerebral cortex are briefly discussed.

Synaptic inhibition in the CNS is mostly mediated by GABA or glycine. Generally, the use of the two transmitters is spatially segregated, but there are central synapses employing both, which allows for spatial and temporal variability of inhibitory mechanisms. Spherical bushy cells (SBCs) in the mammalian cochlear nucleus receive primary excitatory inputs through auditory nerve fibers arising from the organ of Corti and non-primary inhibition mediated by a dual glycine-GABA transmission. Slow kinetics IPSCs enable activity dependent tonic-like conductance build up, functioning as a gain control by filtering out small or temporally imprecise EPSPs. However, it remained elusive whether GABA and glycine are released as content of the same vesicle or from distinct presynaptic terminals. The developmental profile of quantal release was investigated with whole cell recordings of miniature inhibitory postsynaptic currents (mIPSCs) from P1-P25 SBCs of Mongolian gerbils. GABA is the initial transmitter eliciting slow-rising and -decaying events of relatively small amplitudes, occurring only during early postnatal life. Around and after hearing onset, the inhibitory quanta are predominantly containing glycine that – with maturity – triggers progressively larger and longer mIPSC. In addition, GABA corelease with glycine evokes mIPSCs of particularly large amplitudes consistently occurring across all ages, but with low probability. Together, these results suggest that GABA, as the primary transmitter released from immature inhibitory terminals, initially plays a developmental role. In maturity, GABA is contained in synaptic vesicles only in addition to glycine to increase the inhibitory potency, thereby fulfilling solely a modulatory function.

Corticotropin-releasing hormone (CRH)-synthesizing parvocellular neuroendocrine cells (PNCs) of the hypothalamic paraventricular nucleus (PVN) play a key role in the activation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Several studies have demonstrated that synaptic inputs to these cells may undergo stress-related enhancement but, on the other hand, it has been reported that exposition to the same stressor for prolonged time periods may induce a progressive reduction in the response of the HPA axis to homotypic stressors. In the present study rats were subjected to 10 min restraint sessions, repeated twice daily for 3 or 7 days. Miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs) were then recorded from PNCs in ex vivo hypothalamic slice preparations obtained 24 h after the last restraint. Restraint stress repeated over 3 days resulted in increased mean frequency and decreased rise time and decay time constant of mEPSCs, accompanied by a decrease in the excitability of PNCs, however, no such changes were evident in slices obtained from rats subjected to restraint over 7 days. There were no changes in mIPSCs after repeated restraint. Administration of the unspecific nitric oxide synthase (NOS) blocker Nω-Nitro-L-arginine (L-NNA) before each restraint, repeated over 3 days, prevented the occurrence of an increase in mEPSC frequency. However, animals receiving L-NNA and subjected to repeated restraint had similar changes in PNCs membrane excitability and mEPSC kinetics as stressed rats not receiving L-NNA. Comparison of the effects of a single 10 min restraint session followed by either an immediate or delayed (24 h) decapitation revealed an increase in the mean mEPSC frequency and a decrease in the mean mIPSC frequency in slices prepared immediately after restraint, with no apparent effects when slice preparation was delayed by 24 h. These results demonstrate that restraint, lasting 10 min and repeated twice daily for 3 days, induces a selective and long-lasting enhancement of excitatory synaptic input onto PNCs, partially by a NOS-dependent mechanism, and reduces PNC excitability, whereas prolongation of repeated stress for up to 7 days results in an adaptation.

Summary Inhibitory synaptic transmission requires the targeting and stabilization of GABAA receptors (GABAARs) at synapses. The mechanisms responsible remain poorly understood, and roles for transmembrane accessory proteins have not been established. Using molecular, imaging, and electrophysiological approaches, we identify the tetraspanin LHFPL4 as a critical regulator of postsynaptic GABAAR clustering in hippocampal pyramidal neurons. LHFPL4 interacts tightly with GABAAR subunits and is selectively enriched at inhibitory synapses. In LHFPL4 knockout mice, there is a dramatic cell-type-specific reduction in GABAAR and gephyrin clusters and an accumulation of large intracellular gephyrin aggregates in vivo. While GABAARs are still trafficked to the neuronal surface in pyramidal neurons, they are no longer localized at synapses, resulting in a profound loss of fast inhibitory postsynaptic currents. Hippocampal interneuron currents remain unaffected. Our results establish LHFPL4 as a synapse-specific tetraspanin essential for inhibitory synapse function and provide fresh insights into the molecular make-up of inhibitory synapses., Graphical Abstract, Highlights • LHFPL4 is a tetraspanin enriched at inhibitory synapses that complexes with GABAARs • LHFPL4 is important for GABAAR clustering both in vitro and in vivo • LHFPL4 is required for the surface clustering but not the trafficking of GABAARs • GABAergic synaptic inputs on CA1 pyramidal neurons, but not interneurons, require LHFPL4, Davenport et al. identify LHFPL4 as a transmembrane protein that interacts with GABAARs and is essential for their synaptic clustering. Deletion of LHFPL4 results in dramatic cell-type-specific deficits in inhibitory synaptic transmission.

Mounting evidence indicates that ethanol (EtOH) exposure activates neuroimmune signaling. Alterations in pro-inflammatory cytokines after acute and chronic EtOH exposure have been heavily investigated. In contrast, little is known about the regulation of neurotransmission and/or modulation by anti-inflammatory cytokines in the brain after an acute EtOH exposure. Recent evidence suggests that interleukin-10 (IL-10), an anti-inflammatory cytokine, is upregulated during withdrawal from chronic EtOH exposure. In the present study, we show that IL-10 is increased early (1h) after a single intoxicating dose of EtOH (5g/kg, intragastric) in Sprague Dawley rats. We also show that IL-10 rapidly regulates GABAergic transmission in dentate gyrus neurons. In brain slice recordings, IL-10 application dose-dependently decreases miniature inhibitory postsynaptic current (mIPSC) area and frequency, and decreases the magnitude of the picrotoxin sensitive tonic current (Itonic), indicating both pre- and postsynaptic mechanisms. A PI3K inhibitor LY294002 (but not the negative control LY303511) ablated the inhibitory effects of IL-10 on mIPSC area and Itonic, but not on mIPSC frequency, indicating the involvement of PI3K in postsynaptic effects of IL-10 on GABAergic transmission. Lastly, we also identify a novel neurobehavioral regulation of EtOH sensitivity by IL-10, whereby IL-10 attenuates acute EtOH-induced hypnosis. These results suggest that EtOH causes an early release of IL-10 in the brain, which may contribute to neuronal hyperexcitability as well as disturbed sleep seen after binge exposure to EtOH. These results also identify IL-10 signaling as a potential therapeutic target in alcohol-use disorders and other CNS disorders where GABAergic transmission is altered.

Cholinergic modulation of spontaneous GABAergic currents (mIPSC) was studied using whole-cell patch methods in mouse accessory olfactory bulb slices. Carbachol (above 100 microM) administration produced an increase in the mIPSC frequency in mitral cells, but did not affect the responses of mitral cells to GABA. The carbachol effect persisted in the presence of combined ionotropic and metabotropic glutamatergic receptor antagonists. The carbachol effect was reduced by the muscarinic receptor type-1 and -4 (M1 and M4) antagonist pirenzepine (10 microM), but not by the M2 and M4 antagonist himbacine (10 microM). The KCNQ/Kv7 potassium channel openers retigabine (80 microM) and diclofenac (300 microM) blocked the carbachol action, while the KCNQ potassium channel blocker XE-911 (20 microM) increased the mIPSC frequency. XE-911's action persisted in the presence of glutamate receptor blockers. In the presence of carbachol, mIPSCs were abolished by Ni (200 microM), while being insensitive to the calcium channel blocker nimodipine (30 microM), suggesting a role for R-type calcium channels in the GABA release. These results suggest that carbachol closed KCNQ channels by stimulating M1 receptors on granule cell dendrites, and the resulting depolarized and unstable membrane promoted calcium influx, thus increasing the GABA release. The possible role of acetylcholine in facilitating formation of a pheromone memory in mice is also discussed.

ClC-3, ein Cl−/H+-Austauscher, der auf endosomalen Kompartimenten und synaptischen Vesikeln von Neuronen lokalisiert ist, scheint im Gehirn eine bedeutende Rolle zu spielen, da die Abwesenheit von ClC-3 im Knockout- Mausmodell zu Neurodegeneration mit vollständigem Verlust des Hippocampus führt. Es wird angenommen, dass ClC-3 einen Ausgleichsstrom aus negativ geladenen Chloridionen für die elektrogen arbeitende Protonenpumpe liefert und so zur Akkumulation von Cl−-Ionen in synaptische Vesikel und zu deren Ansäuerung und dadurch zur vesikulären Neurotransmitteraufnahme beiträgt. Die genaue Funktion von ClC-3 im Gehirn, insbesondere seine Rolle bei der GABAergen und glutamatergen Neurotransmission, ist umstritten. Zur Untersuchung dieser wurden in der vorliegenden Arbeit anhand von Knockout- und Knockin-Mausmodellen von ClC-3 und -4 unter Verwendung der Patch-Clamp-Technik elektrophysiologische Experimente an hippocampalen Akutschnitten durchgeführt und GABAerge sowie glutamaterge postsynaptische Miniaturströme (mIPSCs/ mEPSCs) an hippocampalen CA1-Pyramidenzellen abgeleitet. Um die Bedeutung von ClC-3 für die GABA-Aufnahme in synaptische Vesikel von Neuronen und seinen Einfluss auf die Quantengröße von inhibitorischen Synapsen zu untersuchen, wurden zunächst mIPSCs in CA1-Pyramidenzellen in Clcn3−/−-Mäusen abgeleitet. Um außerdem die physiologische Relevanz der Cl−/H+-Austauscher-Funktion von ClC-3 für die GABAerge synaptische Transmission zu untersuchen, sollten mIPSCs in einer ClC-3unc-Mutante untersucht werden, in welcher ClC-3 anstelle seiner Cl−/H+-Austauscher-Funktion eine reine Cl−-Leitfähigkeit aufweist. Dafür wurden Clcn3unc/unc-Mäuse verwendet. In Clcn3unc/unc-Mäusen kann ClC-4 jedoch vermutlich für die Abwesenheit der Cl−/H+-Austauscher-Funktion von ClC-3 kompensieren. Aus diesem Grund wurden außerdem Clcn3unc/unc/Clcn4−/−-Mäuse generiert, die nun tatsächlich eine ClC-3unc – Mausmutante darstellen sollten, da ClC-4 fehlt und die Cl−/H+-Austauscher-Funktion von ClC-3 nicht mehr ausgleichen kann. mIPSCs wurden in Clcn3unc/unc/Clcn4−/−-Mäusen, sowie in den einfachen Clcn3unc/unc- und Clcn4−/−-Mausmodellen, abgeleitet. In dieser Arbeit wurde gezeigt, dass die Amplitude der mIPSCs, die ein Maß für die vesikuläre GABA-Konzentration darstellt, in keinem der untersuchten Mausmodelle verändert war. Auch die Frequenz der mIPSCs, ein Maß für die Anzahl der Synapsen und für die Wahrscheinlichkeit der spontanen Vesikelentleerung, war unverändert. Einzig in Clcn3unc/unc/Clcn4−/−-Mäusen war die Frequenz der mIPSCs leicht vermindert, was wahrscheinlich auf bereits einsetzende neurodegenerative Prozesse zurückzuführen war. Auch die glutamatergen exzitatorischen Miniaturströme, die an CA1-Pyramidenzellen von Clcn3unc/unc/Clcn4−/−-Mäusen abgeleitet wurden, waren in ihrer Amplitude im Vergleich zu Clcn3+/+/Clcn4−/−-Mäusen unverändert. Die Ergebnisse dieser Arbeit sprechen dafür, dass ClC-3 für die Aufnahme von GABA und Glutamat in synaptische Vesikel keine bedeutende Rolle spielt. Für die stabile Expression von ClC-4 im Gehirn spielt das Vorhandensein von ClC-3 eine wichtige Rolle, die aber nicht auf dessen Funktion als Cl−/H+-Austauscher beruht, da die Expression von ClC-4 in Clcn3−/−-Mäusen, nicht aber in Clcn3unc/unc-Mäusen, deutlich vermindert war., ClC-3, a Cl−/H+-exchanger, is located on endosomal compartments and synaptic vesicles in neurons. It seems to play a pivotal role in the brain, as the absence of ClC-3 in the knockout mouse model leads to a severe neurodegeneration involving the complete loss of the hippocampus. It is assumed that ClC-3 provides a countercurrent of negative chloride ions for the electrogenic H+-ATPase thereby contributing to the vesicular Cl− accumulation, to the acidification of synaptic vesicles and finally to the neurotransmitter uptake into synaptic vesicles. The precise function of ClC-3 in the brain and particularly its role in the GABAergig and glutamatergic neurotransmission is controversial. For its further investigation electrophysiological experiments using the patch-clamp technique were performed on hippocampal acute slices of ClC-3 and ClC-4 knockout and knockin mouse models and miniature GABAergic as well as glutamatergic postsynaptic currents (mIPSCs/ mEPSCs) were recorded from hippocampal CA1 pyramidal cells. To investigate the role of ClC-3 in the GABA uptake into synaptic vesicles of neurons and its impact on the quantal size at inhibitory synapses, mIPSCs were initially recorded in Clcn3−/− mice. To furthermore examine the physiological relevance of the putative function of ClC-3 as a Cl−/H+-exchanger for the GABAergic synaptic transmission, mIPSCs were supposed to be analyzed in a ClC-3unc mutant in which ClC-3 shows a pure Cl− conductance instead of a Cl−/H+-exchange function. For this purpose Clcn3unc/unc mice were used. However in Clcn3unc/unc mice ClC-4 may presumably compensate for the absence of the Cl−/H+-exchange function of ClC-3. For this reason Clcn3unc/unc/Clcn4−/− mice were generated, actually constituting a ClC- 3unc mutant now as ClC-4 is absent and cannot compensate for the Cl−/H+-exchange function of ClC-3 anymore. mIPSCs were recorded in Clcn3unc/unc/Clcn4−/− mice as well as in the single Clcn3unc/unc and Clcn4−/− mouse models. In the present study the amplitude of the mIPSCs which is a measurement for the vesicular GABA concentration was not changed in any of the analyzed mouse models. Also the frequency of the mIPSCs, constituting a measurement for the number of synapses and for the probability of the spontaneous release of synaptic vesicles was not changed. Only in Clcn3unc/unc/Clcn4−/− mice the frequency of the mIPSCs was slightly reduced, likely being explained by the beginning process of neurodegeneration. Also the miniature glutamatergic postsynaptic currents that were recorded from CA1 pyramidal cells of Clcn3unc/unc/Clcn4−/− mice were not changed compared to Clcn3+/+/Clcn4−/− mice. The results of the present study suggest that ClC-3 does not play an important role neither in the uptake of GABA nor in the uptake of glutamate into synaptic vesicles. It was shown that the presence of ClC-3 plays an important role in the stable expression of ClC-4 in the brain. But this does not rely on the putative Cl−/H+-exchange function of ClC-3 as the expression of ClC-4 was strongly reduced in Clcn3−/− mice but not in Clcn3unc/unc mice. The important function of ClC-3 in the brain underlying the considerable neurodegeneration in the absence of ClC-3 remains unexplained. However, in the present study it could be shown that the quantal size at GABAergic and glutamatergic synapses is neither affected by the absence of ClC-3 nor by the loss of its Cl−/H+-exchange function.

Stężenie jonów wodorowych (pH) i benzodiazepiny (BDZ) są czynnikami modulującymi receptory GABAa. W szczególności, wartość pH środowiska zewnątrzkomórkowego może zmieniać się w zależności od stanu fizjologicznego lub patologicznego. W niniejszej pracy przedstawiono wyniki badań wpływu flurazepamu (benzodiazepiny) przy różnych wartościach zewnątrzkomórkowego pH (z zakresu 5.5 - 7.2) na amplitudę miniaturowych GABA-ergicznych inhibujących prądów synaptycznych (mIPSCs). Stwierdzono, że obecność flurazepamu prowadzi do zwiększenia amplitudy mIPSCs, ale efekt ten zależy od pH i je st największy przy silnie kwaśnych wartościach. Wpływ flurazepamu na amplidudę mIPSC osiąga nasycenie przy stężeniu ok. I цМ dla wszystkich badanych wartości pH. Wyniki te wskazują na to, że modulacja Prądów GABAergicznych przez benzodiazepiny jest zależna od odczynu środowiska. Praca dofinansowana przez KBN grant nr PBZ-M1N-001/P05/28 Zadanie pt. „Digitalizacja i udostępnienie w Cyfrowym Repozytorium Uniwersytetu Łódzkiego kolekcji czasopism naukowych wydawanych przez Uniwersytet Łódzki” nr 885/P-DUN/2014 dofinansowane zostało ze środków MNiSW w ramach działalności upowszechniającej naukę.

Maintaining the proper balance between excitation and inhibition is necessary to prevent cortical circuits from either falling silent or generating epileptiform activity. One mechanism through which cortical networks maintain this balance is through the activity-dependent regulation of inhibition, but whether this is achieved primarily through changes in synapse number or synaptic strength is not clear. Previously, we found that 2 d of activity deprivation increased the amplitude of miniature EPSCs (mEPSCs) onto cultured visual cortical pyramidal neurons. Here we find that this same manipulation decreases the amplitude of mIPSCs. This occurs with no change in single-channel conductance but is accompanied by a reduction in the average number of channels open during the mIPSC peak and a reduction in the intensity of staining for GABAA receptors (GABAARs) at postsynaptic sites. In addition, the number of synaptic sites that express detectable levels of GABAARs was decreased by -50% after activity blockade, although there was no reduction in the total number of presynaptic contacts. These data suggest that activity deprivation reduces cortical inhibition by reducing both the number of GABAARs clustered at synaptic sites and the number of functional inhibitory synapses. Because excitatory and inhibitory synaptic currents are regulated in opposite directions by activity blockade, these data suggest that the balance between excitation and inhibition is dynamically regulated by ongoing activity.