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At least 73 records · Page 4Linked to original sources
PubMed2006

GABAA receptor-mediated IPSCs and alpha1 subunit expression are not reduced in the substantia nigra pars reticulata of gerbils with inherited epilepsy.

Domestic Mongolian gerbils, a model of inherited epilepsy, begin having spontaneous seizures at approximately 1.5 mo of age, making it possible to evaluate them during epileptic and pre-epileptic stages. Previous studies have shown that GABA binding is reduced in the substantia nigra pars reticulata (SNr) of both epileptic and pre-epileptic gerbils compared with controls, suggesting that reduced expression of GABAA receptors in SNr might be epileptogenic in this model. To test this hypothesis, we measured the expression of the GABAA receptor alpha1 subunit, the dominant alpha subunit expressed in the SNr, and evaluated GABAA receptor-mediated postsynaptic currents in SNr neurons. GABA(A) alpha1 subunit mRNA levels in substantia nigra-rich tissue from pre-epileptic animals were similar to controls, and immunocytochemistry for the alpha1 subunit showed similar strong expression in the SNr in both groups. Western analysis confirmed that expression of the alpha1 subunit protein was similar in substantia nigra-rich tissue from pre-epileptic and control gerbils. The frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) and the frequency of miniature (m)IPSCs in SNr neurons of pre-epileptic gerbil were similar to those of controls. The amplitude of mIPSCs in the pre-epileptics was significantly larger than controls. Zolpidem, an alpha1 subunit-specific modulator of the GABAA receptor, was equally efficacious in prolonging the decay time of mIPSCs in both groups. Hence, contrary to the predictions of the hypothesis, mRNA and protein expression levels of the major GABAA receptor alpha subunit were normal, and neurons of the SNr in pre-epileptic gerbils displayed normal or enhanced IPSC frequencies and amplitudes. Therefore reduced expression of GABAA receptors in SNr is not likely to be an epileptogenic mechanism in this model.

Action Potentials↗
PubMed1997

Regulation of the NMDA component of EPSPs by different components of postsynaptic GABAergic inhibition: computer simulation analysis in piriform cortex.

Regulation of the NMDA component of EPSPs by different components of postsynaptic GABAergic inhibition: computer simulation analysis in piriform cortex. J. Neurophysiol. 78: 2546-2559, 1997. Physiological analysis in the companion paper demonstrated that gamma-aminobutyric acid-A (GABAA)-mediated inhibition in piriform cortex is generated by circuits that are largely independent in apical dendritic and somatic regions of pyramidal cells and that GABAA-mediated inhibitory postsynaptic currents (IPSCs) in distal dendrites have a slower time course than those in the somatic region. This study used modeling methods to explore these characteristics of GABAA-mediated inhibition with respect to regulation of the N-methyl--aspartate (NMDA) component of excitatory postsynaptic potentials. Such regulation is relevant to understanding NMDA-dependent long-term potentiation (LTP) and the integration of repetitive synaptic inputs that can activate the NMDA component as well as pathological processes that can be activated by overexpression of the NMDA component. A working hypothesis was that the independence and differing properties of IPSCs in apical dendritic and somatic regions provide a means whereby the NMDA component and other dendritic processes can be controlled by way of GABAergic tone without substantially altering system excitability. The analysis was performed on a branched compartmental model of a pyramidal cell in piriform cortex constructed with physiological and anatomic data derived by whole cell patch recording. Simulations with the model revealed that NMDA expression is more effectively blocked by the slow GABAA component than the fast. Because the slow component is present in greater proportion in apical dendritic than somatic regions, this characteristic would increase the capacity of dendritic IPSCs to regulate NMDA-mediated processes. The simulations further revealed that somatic-region GABAergic inhibition can regulate the generation of action potentials with little effect on the NMDA component generated by afferent fibers in apical dendrites. As a result, if expression of the NMDA component or other dendritic processes were enabled by selective block of dendritic inhibition, for example, by centrifugal fiber systems that may regulate learning and memory, the somatic-region IPSC could preserve system stability through feedback regulation of firing without counteracting the effect of the dendritic-region block. Simulations with paired inputs revealed that the dendritic GABAA-mediated IPSC can regulate the extent to which a strong excitatory input facilitates the NMDA component of a concurrent weak input, providing a possible mechanism for control of "associative LTP" that has been demonstrated in this system. Postsynaptic GABAB-mediated inhibition had less effect on the NMDA component than either the fast or slow GABAA components. Depolarization from a concomitant alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) component also was found to have comparatively little effect on current through the NMDA channel because of its brief time course.

2-Amino-5-phosphonovalerate↗
PubMed2003

Reduction of spontaneous inhibitory synaptic activity in experimental heterotopic gray matter.

Neuronal heterotopia has a strong association with epilepsy, but the mechanisms that underlie this relationship are largely unknown. We have utilized the in utero irradiated rat model to study circuit abnormalities in experimentally induced subcortical heterotopic gray matter. Spontaneous and miniature inhibitory (IPSCs) and excitatory (EPSCs) postsynaptic currents were recorded from visualized heterotopic pyramidal neurons in in vitro hemispheric slices and compared with control neocortical pyramidal neurons using the whole cell patch-clamp technique. The frequency of spontaneous and miniature IPSCs was significantly reduced in pyramidal neurons from heterotopic cortex. Amplitude and kinetics of IPSCs were not different between the two groups. Spontaneous and miniature EPSCs were not different between the two groups. Short-term synaptic plasticity of stimulus-evoked EPSCs showed depression in heterotopic neurons and facilitation in control pyramidal neurons. This study shows a selective impairment of the GABAergic circuitry in experimental heterotopic gray matter. We have reported similar findings in normotopic dysplastic cortex from this model. Taken together, these studies demonstrate a pervasive defect in inhibition throughout the cortex of irradiated rats with cortical dysplasia and neuronal heterotopia. This may have important implications regarding cortical development and function following in utero injuries.

Animals↗
PubMed2025

Antisense oligonucleotide-mediated MSH3 suppression reduces somatic CAG repeat expansion in Huntington's disease iPSC-derived striatal neurons.

Expanded CAG alleles in the huntingtin (HTT) gene that cause the neurodegenerative disorder Huntington's disease (HD) are genetically unstable and continue to expand somatically throughout life, driving HD onset and progression. MSH3, a DNA mismatch repair protein, modifies HD onset and progression by driving this somatic CAG repeat expansion process. MSH3 is relatively tolerant of loss-of-function variation in humans, making it a potential therapeutic target. Here, we show that an MSH3-targeting antisense oligonucleotide (ASO) effectively engaged with its RNA target in induced pluripotent stem cell (iPSC)-derived striatal neurons obtained from a patient with HD carrying 125 HTT CAG repeats (the 125 CAG iPSC line). ASO treatment led to a dose-dependent reduction of MSH3 and subsequent stalling of CAG repeat expansion in these striatal neurons. Bulk RNA sequencing revealed a safe profile for MSH3 reduction, even when reduced by >95%. Maximal knockdown of MSH3 also effectively slowed CAG repeat expansion in striatal neurons with an otherwise accelerated expansion rate, derived from the 125 CAG iPSC line where FAN1 was knocked out by CRISPR-Cas9 editing. Last, we created a knock-in mouse model expressing the human MSH3 gene and demonstrated effective in vivo reduction in human MSH3 after ASO treatment. Our study shows that ASO-mediated MSH3 reduction can prevent HTT CAG repeat expansion in HD 125 CAG iPSC-derived striatal neurons, highlighting the therapeutic potential of this approach.

Huntington Disease↗
PubMed2026

Biallelic rescue of CTG18.1 in two Fuchs endothelial corneal dystrophy-derived iPSC lines (SCTCi047-A-2, SCTCi046-A-2) following a two-step gene editing strategy.

Fuchs endothelial corneal dystrophy (FECD) is an age-related condition distinguished by the degeneration of the corneal endothelium. An intronic CTG18.1 repeat in the transcription factor 4 (TCF4) gene has been associated with a 78-fold increased risk of developing the disease when at least one copy of the CTG18.1 expands above 50 repeats. Employing patient-derived material, we applied a dual CRISPR/Cas9-mediated editing approach to rescue the expansion. Combining non-homologous end-joining (NHEJ) and homologous direct repair (HDR) events, we generated two FECD-derived +/+(CTG)8 induced pluripotent stem cell (iPSC) lines, which were then successfully characterized, providing relevant isogenic controls for disease-modelling purposes.

Humans↗
PubMed1996

Apical dendritic location of slow afterhyperpolarization current in hippocampal pyramidal neurons: implications for the integration of long-term potentiation.

Trains of action potentials in hippocampal pyramidal neurons are followed by a prolonged afterhyperpolarization (AHP) lasting several seconds, which is attributable to the activation of a slow calcium-activated potassium current ((sI)AHP). Here we examine the location of (sI)AHP on CA1 pyramidal neurons by comparing it with two GABAergic inhibitory postsynaptic currents (IPSCs) with known somatic and dendritic locations. Whole-cell patch-clamp recordings were made for CA1 pyramidal neurons in acute hippocampal slices. Stepping the membrane potential at the peak of (sI)AHP produced a relaxation ("switchoff") of the AHP current with a time constant of 7.4 +/- 0.4 msec (mean +/- SEM). The switchoff time constants for somatic and dendritic GABAA IPSCs were 3.5 +/- 0.5 msec and 8.8 +/- 0.3 msec, respectively. This data, together with cable modeling, indicates that active (sI)AHP channels are distributed over the proximal dendrites within approximately 200 micrometers of the soma. Excitatory postsynaptic potentials (EPSPs) evoked in stratum (s.) radiatum had their amplitudes shunted more by the AHP than did EPSPs evoked in s. oriens, suggesting that active AHP channels are restricted to the apical dendritic tree. Blockade of the AHP during a tetanus, which in control conditions elicited a decremental short-term potentiation (STP), converted STP to long-term potentiation (LTP). Thus, activation of the AHP increases the threshold for induction of LTP. These results suggest that in addition to its established role in spike frequency adaptation, the AHP works as an adjustable gain control, variably hyperpolarizing and shunting synaptic potentials arising in the apical dendrites.

Action Potentials↗
PubMed2026

STX1B variant-specific synaptic dysfunction is associated with network hyperexcitability in human iPSC-derived neurons.

BACKGROUND: Variants in STX1B/syntaxin-1B are linked to a spectrum of fever-associated epilepsy syndromes. While studies in murine models have provided mechanistic insights, their relevance to human disease in a heterozygous context may be limited. METHODS: We investigated two pathogenic STX1B variants using isolated single neurons and neuronal network cultures derived from patient-specific induced pluripotent stem cells. These carried either a de novo p.G226R variant, associated with severe developmental epilepsy, or an InDel variant (p.K45delinsRCMIE/p.L46M) linked to a transient familial seizure syndrome. Synaptic function and network excitability were assessed using patch-clamp and multi-electrode array recordings, alongside morphological and transcriptomic profiling. FINDINGS: G226R exhibited both gain- and loss-of-function characteristics, with increased miniature excitatory postsynaptic current frequency in networks but not in autapses, and synaptic failure during sustained high-frequency stimulation. For the InDel variant, the predicted loss-of-function phenotype based on reduced syntaxin-1B levels was not detectable at the single-cell level, likely masked by compensatory synaptic upregulation. At the network level, however, both variants were associated with neuronal hyperexcitability, characterised by more frequent and prolonged bursting activity, with a much stronger phenotype in G226R-containing networks. Transcriptomic profiling revealed a differential dysregulation of synaptic and other neuronal genes. INTERPRETATION: The divergence between morphological, electrophysiological and transcriptomic findings suggests that compensatory mechanisms may contribute to network hyperexcitability. Initially engaged to maintain homoeostasis, they may ultimately contribute to a pathological network state. The graded severity of network alterations across STX1B variants correlates with the clinical phenotypes. FUNDING: BMBF (Treat ION-01GM2210A, SNAREopathies-01EW1809A), 2023 FEBS Summer Fellowship, Fortüne programme (2610-0-0), EKFS college precise.net, Open Access Publishing Fund of University of Tübingen.

Humans↗
PubMed2026

Evaluating the pathogenic significance of unique chromosomal variants in craniosynostosis using patient-derived induced pluripotent stem cells and mouse modelling.

PURPOSE: Unravelling causal links between unique structural/copy-number variants (SV/CNV) and associated phenotypes is essential for correct genetic counselling. We investigated two families in which patients with craniosynostosis had SV/CNV potentially dysregulating a fibroblast growth factor (FGF)-encoding gene; a 730 kb dup(4)(q21.21) including FGF5; and a complex 568 kb interspersed 13q12.11 duplication, located 841 kb from FGF9. METHODS: We combined bioinformatic predictions of altered topologically-associating domain (TAD) structure, with experimental analysis (RNA- and ATAC- [assay for transposase-accessible chromatin] sequencing) of patient induced pluripotent stem cell lines (iPSCs) differentiated to neural crest (NCC) and osteoprogenitor (OPC) identities. For the dup(4)(q21.21) we generated a mouse bearing an equivalent rearrangement using CRISPR-Cas9 targeting. RESULTS: TAD analysis suggested potential dysregulation of the FGF5/FGF9 gene by bringing it into a novel genomic milieu. The RNA- and ATAC-seq assays demonstrated FGF5/FGF9 upregulation (2.7-18x) and local opening of chromatin, in 3/4 cell lines. For the dup(4)(q21.21), a causal role was supported by the mouse model, whereas interpretation of the 13q12.11 SV is confounded by a co-existing FOXP2 pathogenic variant. CONCLUSION: Patient iPSC-differentiated NCC and OPC lines, combined with TAD-based modelling to generate testable functional hypotheses, provide valuable functional evidence when evaluating causation of unique SV/CNV in craniosynostosis.

copy-number variant↗
PubMed2004

Pharmacological plasticity of GABA(A) receptors at dentate gyrus synapses in a rat model of temporal lobe epilepsy.

In the lithium-pilocarpine model (Li-pilocarpine) of temporal lobe epilepsy, GABA(A) receptor-mediated inhibitory postsynaptic currents (GABA(A) IPSCs) were recorded in dentate gyrus granule cells (GCs) from adult rat hippocampal slices. The properties of GABA(A) IPSCs were compared before and after superfusion of modulators in control conditions (Li-saline rats) and in Li-pilocarpine rats 24-48 h and 3-5 months (epileptic rats) after status epilepticus (SE). The mean peak amplitude of GABA(A) IPSCs increased by about 40% over Li-saline values in GCs 24-48 h after SE and remained higher in epileptic rats. In Li-pilocarpine rats, studied at 24-48 h after SE, diazepam (1 microm) lost 65% of its effectiveness at increasing the half-decay time (T(50%)) of GABA(A) miniature IPSCs (mIPSCs). Diazepam had no effects on mIPSC T(50%) in epileptic rats. The benzodiazepine ligand flumazenil (1 microm), acting as an antagonist in Li-saline rats, exhibited a potent inverse agonistic effect on GABA(A) mIPSCs of GCs from Li-pilocarpine rats 24-48 h and 3-5 months after SE. The neurosteroid allopregnanolone (100 nm), which considerably prolonged GABA(A) mIPSCs in Li-saline rats, totally lost its effect in rats studied 24-48 h after SE. However, this decrease in effectiveness was transient and was totally restored in epileptic rats. In addition to the up-regulation in the number of receptors at individual GC synapses, we propose that these 'epileptic' GABA(A) receptors possess benzodiazepine binding sites with altered allosteric properties. The failure of benzodiazepine and neurosteroid to potentiate inhibition early after SE may be a critical factor in the development of epileptogenesis and occurrence of seizures.

Animals↗
PubMed2003

The in vitro and in vivo enantioselectivity of etomidate implicates the GABAA receptor in general anaesthesia.

General anaesthetics exhibiting enantioselectivity afford valuable tools to assess the fundamental mechanisms underlying anaesthesia. Here, we characterised the actions of the R-(+)- and S-(-)-enantiomers of etomidate. In mice and tadpoles, R-(+)-etomidate was more potent (approximately 10-fold) than S-(-)-etomidate in producing loss of the righting reflex. In electrophysiological and radioligand binding assays, the enantiomers of etomidate positively regulated GABAA receptor function at anaesthetic concentrations and with an enantioselectivity paralleling their in vivo activity. GABA-evoked currents mediated by human recombinant GABAA receptors were potentiated by either R-(+)- or S-(-)-etomidate in a manner dependent upon receptor subunit composition. A direct, GABA-mimetic, effect was similarly subunit dependent. Modulation of GABA receptor activity was selective; R-(+)-etomidate inhibited nicotinic acetylcholine, or 5-hydroxytryptamine3 receptor subtypes only at supra-clinical concentrations and ionotropic glutamate receptor isoforms were essentially unaffected. Acting upon reticulothalamic neurones in rat brain slices, R-(+)-etomidate prolonged the duration of miniature IPSCs and modestly enhanced their peak amplitude. S-(-)-etomidate exerted qualitatively similar, but weaker, actions. In a model of locomotor activity, fictive swimming in Xenopus laevis tadpoles, R-(+)- but not S-(-)-etomidate exerted a depressant influence via enhancement of GABAergic neurotransmission. Collectively, these observations strongly implicate the GABAA receptor as a molecular target relevant to the anaesthetic action of etomidate.

Anesthetics, Intravenous↗
PubMed2006

Nucleus-specific abnormalities of GABAergic synaptic transmission in a genetic model of absence seizures.

Human and experimental studies indicate that molecular genetic changes in GABA(A) receptors may underlie the expression of spike-and-waves discharges (SWDs) occurring during absence seizures. However, the full spectrum of the genetic defects underlying these seizures has only been partially elucidated, the expression and functional profiles of putative abnormal protein(s) within the thalamocortical network are undefined, and the pathophysiological mechanism(s) by which these proteins would lead to absence paroxysms are poorly understood. Here we investigated GABA(A) inhibitory postsynaptic currents (IPSCs) in key thalamocortical areas, i.e., the somatosensory cortex, ventrobasal thalamus (VB) and nucleus reticularis thalami (NRT), in preseizure genetic absence epilepsy rats from Strasbourg (GAERS), a well-established genetic model of typical absence seizures that shows no additional neurological abnormalities, and compared their properties to age-matched non-epileptic controls (NECs). Miniature GABA(A) IPSCs of VB and cortical layers II/III neurons were similar in GAERS and NEC, whereas in GAERS NRT neurons they had 25% larger amplitude, 40% faster decay. In addition, baclofen was significantly less effective in decreasing the frequency of NRT mIPSCs in GAERS than in NEC, whereas no difference was observed for cortical and VB mIPSCS between the two strains. Paired-pulse depression was 45% smaller in GAERS NRT, but not in VB, and was insensitive to GABA(B) antagonists. These results point to subtle, nucleus-specific, GABA(A) receptor abnormalities underlying SWDs of typical absence seizures rather than a full block of these receptors across the whole thalamocortical network, and their occurrence prior to seizure onset suggests that they might be of epileptogenic significance.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1998

Modulation of GABAA receptor-mediated inhibition by postsynaptic calcium in epileptic hippocampal neurons.

Visualization of neurons during patch clamp recordings from slices provides concurrent neuroanatomical information for physiological studies. Although, the technique becomes increasingly popular in immature brains, it has not been fully utilized in aged/adult and diseased brains including post-surgical human specimen. In the present study, glutamatergic modulation of GABAA receptor-mediated inhibition was investigated by whole-cell patch clamp recordings from visualized hippocampal dentate granule cells (DGCs) in slices that were prepared from surgically-removed human medial temporal lobe specimens and the rat pilocarpine model of temporal lobe epilepsy. GABAA receptor-mediated synaptic inhibition was recorded by isolating inhibitory postsynaptic currents (IPSCs) at a membrane potential of 0 mV where glutamatergic excitatory postsynaptic currents are near equilibrium. Peak amplitude of GABAA IPSC was not different between epileptic DGCs of both human and pilocarpine-treated rat hippocampi and those in the control rat DGCs. However, when high frequency stimulation (30 Hz for 10 s) preceded immediately before the generation of a GABAA IPSC, its peak amplitude was significantly reduced in epileptic DGCs. The application of an NMDA receptor antagonist prevented this decrease indicating that the high frequency stimulation activated the NMDA receptor and that this activation is involved in the induction of response-decrement of GABAA IPSCs in epileptic DGCs. In addition, intracellular application of a calcium chelator, BAPTA through a patch pipette was found effective in preventing the response-decrement of GABAA IPSCs suggesting that postsynaptic calcium-increase is also involved in this process. It is proposed that activation of the NMDA receptor in epileptic DGC may trigger an epileptogenic increase of intracellular free calcium, and this calcium-increase plays a crucial role for the induction of the response-decrement of GABAA IPSCs in epileptic hippocampus, which possibly leads to the initiation of epileptic seizures and ictal events.

Animals↗
PubMed2004

Regulation of rat mesencephalic GABAergic neurones through muscarinic receptors.

Central dopamine neurones are involved in regulating cognitive and motor processes. Most of these neurones are located in the ventral mesencephalon where they receive abundant intrinsic and extrinsic GABAergic input. Cholinergic neurones, originating from mesopontine nuclei, project profusely in the mesencephalon where they preferentially synapse onto local GABAergic neurones. The physiological role of this cholinergic innervation of GABAergic neurones remains to be determined, but these observations raise the hypothesis that ACh may regulate dopamine neurones indirectly through GABAergic interneurones. Using a mesencephalic primary culture model, we studied the impact of cholinergic agonists on mesencephalic GABAergic neurones. ACh increased the frequency of spontaneous IPSCs (151 +/- 49%). Selective activation of muscarinic receptors increased the firing rate of isolated GABAergic neurones by 67 +/- 13%. The enhancement in firing rate was Ca(2+) dependent since inclusion of BAPTA in the pipette blocked it, actually revealing a decrease in firing rate accompanied by membrane hyperpolarization. This inhibitory action was prevented by tertiapin, a blocker of GIRK-type K(+) channels. In addition to its excitatory somatodendritic effect, activation of muscarinic receptors also acted presynaptically, inhibiting the amplitude of unitary GABAergic synaptic currents. Both the enhancement in spontaneous IPSC frequency and presynaptic inhibition were abolished by 4-DAMP (100 nm), a preferential M3 muscarinic receptor antagonist. The presence of M3-like receptors on mesencephalic GABAergic neurones was confirmed by immunocytochemistry. Taken together, these results demonstrate that mesencephalic GABAergic neurones can be regulated directly through muscarinic receptors. Our findings provide new data that should be helpful in better understanding the influence of local GABAergic neurones during cholinergic activation of mesencephalic circuits.

Acetylcholine↗
PubMed1997

Simulation of gamma rhythms in networks of interneurons and pyramidal cells.

Networks of hippocampal interneurons, with pyramidal neurons pharmacologically disconnected, can generate gamma-frequency (20 Hz and above) oscillations. Experiments and models have shown how the network frequency depends on excitation of the interneurons, and on the parameters of GABAA-mediated IPSCs between the interneurons (conductance and time course). Here we use network simulations to investigate how pyramidal cells, connected to the interneurons and to each other through AMPA-type and/or NMDA-type glutamate receptors, might modify the interneuron network oscillation. With or without AMPA-receptor mediated excitation of the interneurons, the pyramidal cells and interneurons fired in phase during the gamma oscillation. Synaptic excitation of the interneurons by pyramidal cells caused them to fire spike doublets or short bursts at gamma frequencies, thereby slowing the population rhythm. Rhythmic synchronized IPSPs allowed the pyramidal cells to encode their mean excitation by their phase of firing relative to the population waves. Recurrent excitation between the pyramidal cells could modify the phase of firing relative to the population waves. Our model suggests that pools of synaptically interconnected inhibitory cells are sufficient to produce gamma frequency rhythms, but the network behavior can be modified by participation of pyramidal cells.

Electroencephalography↗
PubMed2006

Release probability-dependent scaling of the postsynaptic responses at single hippocampal GABAergic synapses.

The amount of neurotransmitter released after the arrival of an action potential affects the strength and the trial-to-trial variability of postsynaptic responses. Most studies examining the dependence of synaptic neurotransmitter concentration on the release probability (P(r)) have focused on glutamatergic synapses. Here we asked whether univesicular or multivesicular release characterizes transmission at hippocampal GABAergic synapses. We used multiple probability functional analysis to derive quantal parameters at inhibitory connections between cannabinoid receptor- and cholecystokinin (CCK)-expressing interneurons and CA3 pyramidal cells. After the recordings, the cells were visualized and reconstructed at the light-microscopic level, and the number of boutons mediating the IPSCs was determined using electron microscopy (EM). The number of active zones (AZs) per CCK-immunopositive bouton was determined from three-dimensional EM reconstructions, thus allowing the calculation of the total number of AZs for each pair. Our results reveal an approximate fivefold discrepancy between the numbers of functionally determined release sites (17.4 +/- 3.2) and structurally identified AZs (3.7 +/- 0.9). Channel modeling predicts that a fivefold to sevenfold increase in the peak synaptic GABA concentration is required for the fivefold enhancement of the postsynaptic responses. Kinetic analysis of the unitary IPSCs indicates that the increase in synaptic GABA concentration is most likely attributable to multivesicular release. This change in the synaptic GABA concentration transient together with extremely low postsynaptic receptor occupancy permits a P(r)-dependent scaling of the postsynaptic response generated at a single hippocampal GABAergic synaptic contact.

Animals↗
PubMed1990

Transmission at a central inhibitory synapse. IV. Quantal structure of synaptic noise.

1. Continuous segments of synaptic noise were recorded, in 12 experiments, from the voltage-clamped goldfish Mauthner (M-)-cell soma during depolarizations beyond the Cl- equilibrium potential so that spontaneous inhibitory postsynaptic currents (IPSCs) were outward going, i.e., opposite in polarity to excitatory transients. Inhibitory components constituting the noise were detected with a program based on their expected waveforms, and their amplitude distributions were analyzed according to a quantal model to determine whether these responses, presumably because of asynchronous impulses in a population of presynaptic cells, were integral multiples of the same minimal unit. 2. Synaptic noise in the M-cell is predominantly inhibitory, as it is abolished by the glycine antagonist strychnine. IPSC amplitudes varied by 20-fold or more and about one-third were grouped in bursts, with one or several components often occurring during the falling phase of a preceding one. To measure individual amplitudes, a base-line correction procedure was designed to extrapolate the decay of the leading IPSC, on the assumption that it was exponential with a time constant, tau, equal to that obtained by fitting averaged waveforms of distinct IPSCs recorded in quiet periods. Currents were expressed in nanoamperes and, for comparison between experiments, as percentages of the full-sized IPSC evoked by activation of the collateral network. 3. The probability distribution functions (PDFs) of inhibitory events generated with such an analysis had from 5 to 12 clear and equally spaced peaks, larger transients not being included because of their sparsity. These PDFs were fit with a sum of Gaussians, on the assumption that all peaks were integral multiples of the smallest unit and had the same standard deviation, sigma. The fits were statistically satisfactory, according to a chi 2 test, and mean quantal size was 0.63 +/- 0.17% (SD; range, 0.42-1.0%) of the collateral IPSC. 4. In eight experiments, tetrodotoxin (TTX) was subsequently applied topically to block presynaptic impulses and isolate "miniature" responses because of single exocytotic events. It had no apparent nonspecific effects, as indicated by the constancy of M-cell input conductance (8.01 +/- 3.11 microS vs. 7.90 +/- 2.89 microS). Amplitude distributions of residual IPSCs were unimodal and Gaussian, with their mean size (0.60 +/- 0.19%) and average rise time (0.548 +/- 0.128 ms) the same as those of the first peaks of control histograms. The latter parameter was also comparable with that of unitary IPSCs studied previously. These results thus confirmed that spontaneous and evoked events are due to transmitter release from the same afferent population.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2005

On the human sensorimotor-cortex beta rhythm: sources and modeling.

Cortical oscillations in the beta band (13-35 Hz) are known to be modulated by the GABAergic agonist benzodiazepine. To investigate the mechanisms generating the approximately 20-Hz oscillations in the human cortex, we administered benzodiazepines to healthy adults and monitored cortical oscillatory activity by means of magnetoencephalography. Benzodiazepine increased the power and decreased the frequency of beta oscillations over rolandic areas. Minimum current estimates indicated the effect to take place around the hand area of the primary sensorimotor cortex. Given that previous research has identified sources of the beta rhythm in the motor cortex, our results suggest that these same motor-cortex beta sources are modulated by benzodiazepine. To explore the mechanisms underlying the increase in beta power with GABAergic inhibition, we simulated a conductance-based neuronal network comprising excitatory and inhibitory neurons. The model accounts for the increase in the beta power, the widening of the spectral peak, and the slowing down of the rhythms with benzodiazepines, implemented as an increase in GABAergic conductance. We found that an increase in IPSCs onto inhibitory neurons was more important for generating neuronal synchronization in the beta band than an increase in IPSCs onto excitatory pyramidal cells.

Adult↗