Γ-aminobutyric Acid-mediated Inhibition Research Articles

Intravenous Anesthetic, Propofol Affects Synaptic Responses in Cerebellar Purkinje Cells.

ObjectivePropofol is an intravenously administered anesthetic that enhances γ-aminobutyric acid-mediated inhibition in the central nerve system. Other mechanisms may also be involved in general anesthesia. Propofol has been implicated in movement disorders. The cerebellum is important for motor coordination and motor learning. The aim of the present study was to investigate the propofol effect on excitatory synaptic transmissions in cerebellar cortex.MethodsExcitatory postsynaptic currents by parallel fiber stimulation and complex spikes by climbing fiber stimulation were monitored in Purkinje cells of Wister rat cerebellar slice using whole-cell patch-clamp techniques.ResultsDecay time, rise time and amplitude of excitatory postsynaptic currents at parallel fiber Purkinje cell synapses and area of complex spikes at climbing fiber Purkinje cell synapses were significantly increased by propofol administration.ConclusionThe detected changes of glutamatergic synaptic transmission in cerebellar Purkinje cell, which determine cerebellar motor output, could explain cerebellar mechanism of motor deficits induced by propofol.

Propofol effects on cerebellar long-term depression

Propofol is an intravenously administered anesthetic that induces γ-aminobutyric acid-mediated inhibition in the central nervous system. It has been implicated in prolonged movement disorders. Since the cerebellum is important for motor coordination and learning, we investigated the potential effects of propofol on cerebellar circuitry. Using the whole-cell patch-clamp technique in Wister rat cerebellar slices, we demonstrated that propofol administration impaired long-term depression from the parallel fiber (PF) to Purkinje cell (PC) synapses (PF–LTD). Also, propofol reduced metabotropic glutamate receptor 1 (mGluR1)-mediated and group I mGluR agonist-induced slow currents in PCs. These results suggest that the propofol-induced PF–LTD impairment may be related to an alteration in mGluR1 signaling, which is essential to motor learning.

Seizure suppression and neuroprotection by targeting the purinergic P2X7 receptor during status epilepticus in mice

Prolonged seizures [status epilepticus (SE)] constitute a neurological emergency that can permanently damage the brain. SE results from a failure of the normal mechanisms to terminate seizures; in particular, γ-amino butyric acid-mediated inhibition, and benzodiazepine anticonvulsants are often incompletely effective. ATP acts as a fast neurotransmitter via ionotropic ligand-gated P2X receptors. Here we report that SE induced by intra-amygdala kainic acid in mice selectively increased hippocampal levels of P2X7 receptors relative to other P2X receptors. Using transgenic P2X7 reporter mice expressing enhanced green fluorescent protein, we identify dentate granule neurons as the major cell population transcribing the P2X7 receptor after SE. Pretreatment of mice with an intracerebroventricular microinjection of 1.75 nmol A438079, a P2X7 receptor antagonist, reduced seizure duration by 58% and reduced seizure-induced neuronal death by 61%. Injection of brilliant blue G (1 pmol), another selective antagonist, reduced seizure duration by 48% and was also neuroprotective. A438079 was seizure-suppressive when injected shortly after induction of SE, and coinjection of A438079 with lorazepam 60 min after triggering SE, when electrographic seizure-responsiveness to lorazepam had decreased, also terminated SE. Our results suggest that P2X7 receptor antagonists may be a promising class of drug for seizure abrogation and neuroprotection in SE.

Spreading convulsions, spreading depolarization and epileptogenesis in human cerebral cortex

Spreading depolarization of cells in cerebral grey matter is characterized by massive ion translocation, neuronal swelling and large changes in direct current-coupled voltage recording. The near-complete sustained depolarization above the inactivation threshold for action potential generating channels initiates spreading depression of brain activity. In contrast, epileptic seizures show modest ion translocation and sustained depolarization below the inactivation threshold for action potential generating channels. Such modest sustained depolarization allows synchronous, highly frequent neuronal firing; ictal epileptic field potentials being its electrocorticographic and epileptic seizure its clinical correlate. Nevertheless, Leão in 1944 and Van Harreveld and Stamm in 1953 described in animals that silencing of brain activity induced by spreading depolarization changed during minimal electrical stimulations. Eventually, epileptic field potentials were recorded during the period that had originally seen spreading depression of activity. Such spreading convulsions are characterized by epileptic field potentials on the final shoulder of the large slow potential change of spreading depolarization. We here report on such spreading convulsions in monopolar subdural recordings in 2 of 25 consecutive aneurismal subarachnoid haemorrhage patients in vivo and neocortical slices from 12 patients with intractable temporal lobe epilepsy in vitro. The in vitro results suggest that γ-aminobutyric acid-mediated inhibition protects from spreading convulsions. Moreover, we describe arterial pulse artefacts mimicking epileptic field potentials in three patients with subarachnoid haemorrhage that ride on the slow potential peak. Twenty-one of the 25 subarachnoid haemorrhage patients (84%) had 656 spreading depolarizations in contrast to only three patients (12%) with 55 ictal epileptic events isolated from spreading depolarizations. Spreading depolarization frequency and depression periods per 24 h recording episodes showed an early and a delayed peak on Day 7. Patients surviving subarachnoid haemorrhage with poor outcome at 6 months showed significantly higher total and peak numbers of spreading depolarizations and significantly longer total and peak depression periods during the electrocorticographic monitoring than patients with good outcome. In a semi-structured telephone interview 3 years after the initial haemorrhage, 44% of the subarachnoid haemorrhage survivors had developed late post-haemorrhagic seizures requiring anti-convulsant medication. In those patients, peak spreading depolarization number had been significantly higher [15.1 (11.4–30.8) versus 7.0 (0.8–11.2) events per day, P = 0.045]. In summary, monopolar recordings here provided unequivocal evidence of spreading convulsions in patients. Hence, practically all major pathological cortical network events in animals have now been observed in people. Early spreading depolarizations may indicate a risk for late post-haemorrhagic seizures.

Neurotransmitter γ-aminobutyric acid-mediated inhibition of the invasive ability of cholangiocarcinoma cells

The present study aimed to investigate the effect and mechanism of γ-aminobutyric acid (GABA) on the bio-nomics of cholangiocarcinoma QBC939 cells. A transwell cell culture chamber assay in vitro was used to detect the ability of invasion and metastasis of cholangiocarcinoma QBC939 cells. The expression of matrix metalloproteinase (MMP) mRNA and the enzymatic activity in QBC939 cells were detected by reverse transcription-polymerase chain reaction and gelatin zymography, respectively. Different concentrations of GABA inhibited the ability of the tumor cells in trans-Matrigel migration (100 μmol/l GABA, 60±9.54 vs. 42.67±4.04, P<0.05), and inhibited the activity and expression of MMP-2 and MMP-9. GABA inhibited the invasion and metastasis of cholangiocarcinoma QBC939 cells, and the mechanism may involve the down-regulation of the activity of MMPs.

New molecular targets for antiepileptic drugs: α2δ, SV2A, and Kv7/KCNQ/M potassium channels

Many currently prescribed antiepileptic drugs (AEDs) act via voltage-gated sodium channels, through effects on gamma-aminobutyric acid-mediated inhibition, or via voltage-gated calcium channels. Some newer AEDs do not act via these traditional mechanisms. The molecular targets for several of these nontraditional AEDs have been defined using cellular electrophysiology and molecular approaches. Here, we describe three of these targets: alpha(2)delta, auxiliary subunits of voltage-gated calcium channels through which the gabapentinoids gabapentin and pregabalin exert their anticonvulsant and analgesic actions; SV2A, a ubiquitous synaptic vesicle glycoprotein that may prepare vesicles for fusion and serves as the target for levetiracetam and its analog brivaracetam (which is currently in late-stage clinical development); and K(v)7/KCNQ/M potassium channels that mediate the M-current, which acts a brake on repetitive firing and burst generation and serves as the target for the investigational AEDs retigabine and ICA-105665. Functionally, all of the new targets modulate neurotransmitter output at synapses, focusing attention on presynaptic terminals as critical sites of action for AEDs.

Biphasic effect of hydrogen peroxide on field potentials in rat hippocampal slices

In the CA1 region of rat hippocampal slices, H 2O 2 (0.294–2.94 mM) caused initial augmentation, and subsequent long-lasting depression, of population spikes and excitatory postsynaptic potentials. The effect of H 2O 2 may not be mediated by its degradation product, hydroxyl radicals, because an iron chelator deferoxamine did not block the effect. A catalase inhibitor 3-amino-1,2,4-triazole only modestly attenuated the initial augmentation, suggesting that the effect of H 2O 2 is not attributable to catalase-dependent O 2 generation, either. An N-methyl- d-aspartate receptor antagonist dl-2-amino-5-phosphonovaleric acid had no influence on the effect of H 2O 2, whereas a γ-aminobutyric acid type A receptor channel blocker picrotoxin attenuated long-lasting depression, indicating that γ-aminobutyric acid-mediated inhibition is altered during the depression phase. The initial augmentation but not subsequent depression was attenuated by a phospholipase A 2/C inhibitor 4-bromophenacyl bromide, suggesting the involvement of lipid signaling molecule(s) in the enhancement of excitatory synaptic transmission. These results suggest that H 2O 2 regulates hippocampal synaptic transmission via multiple mechanisms.

Sharpening of Frequency Tuning by Inhibition in the Thalamic Auditory Nucleus of the Mustached Bat

Unlike the quasitriangular frequency-tuning curves of peripheral neurons, pencil- or spindle-shaped frequency-tuning curves (excitatory areas) have been found in the central auditory systems of many species of animals belonging to different classes. Inhibitory tuning curves (areas) are commonly found on both sides of such "level-tolerant" sharp frequency-tuning curves. However, it has not yet been examined whether sharpening of frequency tuning takes place in the medial geniculate body (MGB). We injected an inhibitory transmitter antagonist, bicuculline methiodide (BMI), into the MGB of the mustached bat to examine whether frequency tuning is sharpened by inhibition in the MGB and whether this sharpening, if any, occurs in addition to that performed in prethalamic auditory nuclei. Thirty-seven percent of thalamic Doppler-shifted constant frequency (DSCF) neurons mostly showing a level-tolerant frequency-tuning curve had an inhibitory area or areas. BMI changed the inhibitory areas of these neurons into excitatory areas, so that their excitatory frequency-tuning curves became broader. However, the BMI-broadened excitatory frequency-tuning curves were still much narrower than those of peripheral neurons. Our results indicate that level-tolerant frequency tuning of thalamic DSCF neurons is mostly created by prethalamic auditory nuclei and that it is further sharpened in 37% of thalamic DSCF neurons by lateral inhibition occurring in the MGB. The comparisons in sharpness (quality factors) of frequency-tuning curves between peripheral, thalamic, and cortical DSCF neurons indicate that the skirt portion of tuning curves is sharper in the above order, and that their tip portion is not significantly different between the peripheral and thalamic DSCF neurons, but significantly sharper in the cortical DSCF neurons than in the thalamic DSCF neurons. Therefore the central auditory system has inhibitory mechanisms for the progressive sharpening of frequency tuning. DSCF neurons in the primary auditory cortex were recently found to show facilitative responses to paired sounds. That is, they are combination sensitive. In the present studies, we found that thalamic DSCF neurons also showed facilitative responses to paired sounds. The responses of thalamic DSCF neurons to acoustic stimuli consisted of a slow and a fast component. BMI mainly increased the slow component and an excitatory transmitter antagonist, D-2-amino-5-phosphonovalerate mainly suppressed the slow component. Therefore the response pattern of these thalamic neurons is shaped by both gamma-aminobutyric acid-mediated inhibition and N-methyl-D-aspartate-mediated facilitation.

Site-Specific Enhancement of γ-Aminobutyric Acid-Mediated Inhibition of Neural Activity by Ethanol in the Rat Medial Septal Area

Site-Specific Enhancement of γ-Aminobutyric Acid-Mediated Inhibition of Neural Activity by Ethanol in the Rat Medial Septal Area

γ-Aminobutyric acid-mediated inhibition at cholinergic synapses formed by cultured retinal neurons

The purpose of this study was to investigate the effect of γ-aminobutyric acid (GABA) on the function of synapses formed by cholinergic neurons derived from the chick retina. We used an experimental culture system in which striated muscle cells served as postsynaptic targets for cholinergic neurons of the retina. This cell culture system permitted the physiological monitoring of acetylcholine release at synapses formed by retinal neurons. By plating a low of density dissociated retinal cells with myotubes, it was possible to study relatively isolated, presynaptic cholinergic neurons. We found that GABA and agonists, muscimol and isoguvacine, inhibited spontaneous transmission at retina-muscle synapses. These inhibitory effects were reversibly blocked by bicuculline, a GABA receptor antagonist. The benzodiazepine, flurazepam, potentiated GABA-mediated inhibition. Overall, our findings suggest a direct inhibitory action of GABA on the cholinergic retinal neurons studied in our cell culture system.

Muscle relaxant action of 2-chloroadenosine in genetically spastic rats is independent of γ-aminobutyric acid-mediated inhibition

The role played by benzodiazepine (BDZ) receptors and GABAergic mechanisms in the muscle relaxant effect of the adenosine receptor agonist, 2-chloroadenosine (2-CLA) was evaluated in genetically spastic rats. 2-CLA reduced in a dose-related manner the spontaneous tonic activity in the electromyogram (EMG) of the gastrocnemius-soleus muscle. While the muscle relaxant effect produced by the adenosine analogue was abolished by aminophylline, a methylxanthine with potent antagonistic activity of adenosine-mediated inhibition, the two gamma-aminobutyric acid (GABA) antagonists bicuculline and picrotoxin did not reverse it. Neither Ro 15-1788 nor CGS 8216, specific BDZ receptor antagonists, nor beta-carboline-3-carboxylic acid methyl ester, an inverse agonist at BDZ receptors, affected the depressant effect of 2-CLA in the EMG. These results suggest that 2-CLA does not interact with GABA or BDZ receptor-mediated events in vivo to produce the muscle relaxant action. It is proposed that purinergic mechanisms may be involved in the muscle relaxant effects of a variety of drugs.

Muscle relaxant action of 2-chloroadenosine in genetically spastic rats is independent of γ-aminobutyric acid-mediated inhibition

Muscle relaxant action of 2-chloroadenosine in genetically spastic rats is independent of γ-aminobutyric acid-mediated inhibition