Effects of the GABA‐uptake blocker NNC‐711 on spontaneous sharp wave–ripple complexes in mouse hippocampal slices

The precise temporal and spatial activity patterns of neurons in cortical networks are organized by different state‐dependent types of network oscillations. GABAergic inhibition plays a key role in the underlying mechanisms of such oscillations and it has been suggested that the duration of widely distributed phasic inhibitory postsynaptic potentials (IPSPs) determines the frequency of the resulting network oscillation. Here, we test this hypothesis in an in vitro model of sharp wave–ripple (SPW‐R) complexes, a particularly fast pattern of network oscillations at ～200 Hz which is involved in memory consolidation. We recorded SPW‐R in mouse hippocampal slices in the absence and presence of NCC‐711, an inhibitor of GABA uptake. The resulting prolongation of IPSP resulted in reduced occurrence of SPW‐R, whereas the superimposed fast oscillations as well as the precision of rhythmic cell synchronization remained stable. Application of Diazepam which is a positive modulator of the GABA_A receptor led to consistent results. We conclude that phasic inhibition is a major regulator of network excitability in CA3 (where SPW‐Rs are generated), but does not set the frequency of hippocampal ripples. © 2013 Wiley Periodicals, Inc.     

Coherent neocortical 40‐Hz oscillations are not present during REM sleep

During cognitive processes there are extensive interactions between various regions of the cerebral cortex. Oscillations in the gamma frequency band (≈40 Hz) of the electroencephalogram (EEG) are involved in the binding of spatially separated but temporally correlated neural events, which results in a unified perceptual experience. The extent of these interactions can be examined by means of a mathematical algorithm called ‘coherence’, which reflects the ‘strength’ of functional interactions between cortical areas. The present study was conducted to analyse EEG coherence in the gamma frequency band of the cat during alert wakefulness (AW), quiet wakefulness (QW), non‐rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. Cats were implanted with electrodes in the frontal, parietal and occipital cortices to monitor EEG activity. Coherence values within the gamma frequency (30–100 Hz) from pairs of EEG recordings were analysed. A large increase in coherence occurred between all cortical regions in the 30–45 Hz frequency band during AW compared with the other behavioral states. As the animal transitioned from AW to QW and from QW to NREM sleep, coherence decreased to a moderate level. Remarkably, there was practically no EEG coherence in the entire gamma band spectrum (30–100 Hz) during REM sleep. We conclude that functional interactions between cortical areas are radically different during sleep compared with wakefulness. The virtual absence of gamma frequency coherence during REM sleep may underlie the unique cognitive processing that occurs during dreams, which is principally a REM sleep‐related phenomenon.     

Spatiotemporal changes of slow wave activities before and after 14 Hz/12 Hz sleep spindles during stage 2 sleep.

The present study examined the spatiotemporal changes of slow wave (delta and theta bands) activities before and after 14 Hz/12 Hz sleep spindles during stage 2 sleep, using topographic mapping of electroencephalogram (EEG) power. Both types of sleep spindles appeared after slow wave activities of background EEG decreased. Moreover, the appearance of sleep spindles provided increasing EEG slow wave activities in the subsequent period. Further, the present results showed that an appearance of 14 Hz sleep spindle facilitated slow wave activities at the centro-parietal areas, while an appearance of 12 Hz sleep spindle facilitated slow wave activities at the fronto-central areas. These results suggest that sleep spindles provide cortical de-arousal, and serve to maintain sleep.     

Sex differences in depolarizing actions of GABAA receptor activation in rat embryonic hypothalamic neurons

GABA_A receptor activation exerts trophic actions in immature neurons through depolarization of resting membrane potential. The switch to its classical hyperpolarizing role is developmentally regulated. Previous results suggest that a hormonally biased sex difference exists at the onset of the switch in hypothalamic neurons. The aim of this work was to evaluate sex differences in GABA_A receptor function of hypothalamic neurons before brain masculinization by gonadal hormones. Hypothalamic cells were obtained from embryonic day 16 male and female rat foetuses, 2 days before the peak of testosterone production by the foetal testis, and grown in vitro for 9 days. Whole‐cell and perforated patch‐clamp recordings were carried out in order to measure several electrophysiological parameters. Our results show that there are more male than female neurons responding with depolarization to muscimol. Additionally, among cells with depolarizing responses, males have higher and longer lasting responses than females. These results highlight the relevance of differences in neural cell sex irrespective of exposure to sex hormones.     

Firing pattern of neurons in the rostral and ventral part of nucleus reticularis thalami during EEG spindles

The firing pattern of neurons in the rostral and ventral part of nucleus reticularis thalami during cortical EEG spindles was investigated in unanesthetized encéphale isolé cats. Spontaneous spindles as well as those induced by a single thalamic shock were accompanied by an increase in discharge frequency in 97% of the neurons in the rostral pole of the nucleus. In most cases the enhanced firing rate was tonically sustained throughout the duration of the spindles, although phasic bursts at EEG wave frequency were sometimes superimposed on the tonic cellular activation. Suppression of triggered spindles by conditioning fast-frequency stimulation in the mesencephalic reticular formation also abolished the rostral reticularis response. Intracellular recordings revealed a depolarizing shift of small amplitude which was sustained throughout the duration of triggered spindies. The majority of neurons in the ventral part of nucleus reticularis, in contrast, underwent a prolonged hyperpolarizing shift in membrane potential during cortical spindles, sustained for as long as 2 sec and interrupted by depolarizing waves. Both the prolonged membrane hyperpolarization and the depolarizing waves were reduced during the intracellular passage of polarizing currents, suggesting that the former was an inhibitory postsynaptic potential and the latter were disinhibitory potentials. Since neurons in dorsal thalamic nuclei, to which the reticularis axons project, are hyperpolarized concomitantly with cortical spindles, the results are viewed as being in agreement with the hypothesis that, during EEG spindles, neurons in the rostral pole but not in the ventral part of nucleus reticularis exert a tonic inhibitory influence on cells throughout the thalamus.     

Electrophysiological correlates of sleep delta waves

Recent studies have disclosed several oscillations occurring during resting sleep within the frequency range of the classical delta band (0.5–4 Hz). There are at least 3 oscillations with distinct mechanisms and sites of origin: a slow (<1 Hz) cortically-generated oscillation, a clock-like thalamic oscillation (1–4 Hz), and a cortical oscillation (1–4 Hz). The present paper reviews data on these oscillations and the possible mechanisms which coalesce them into the polymorphic waves of slow wave sleep. Data stem from intracellular (over 500 single cell and 50 double impalements) and field potentials recorded from the cortex and thalamus of cats (120 animals) under ketamine and xylazine anesthesia. Other experiments were based on whole night EEG recordings from humans (5 subjects). The frequency of the slow oscillation both in anesthetized animals and in naturally sleeping humans ranged between 0.1 and 1 Hz (89% of the cases being between 0.5 and 0.9 Hz). The slow (<1 Hz) oscillation is reflected in the EEG as rhythmic sequences of surface-negative waves (associated with hyperpolarizations of deeply-lying neurons) and surface-positive K-complexes (representing excitation in large pools of cortical neurons). Through its long-range synchronization, the slow oscillation has the ability to trigger and to group thalamically-generated spindles and two delta (1–4 Hz) oscillations. Finally, it is argued that the analysis of the electroencephalogram should transcend the spectral analyses, by taking into account the shape of the waves and, when possible, the basic mechanisms that generate those waves.     

Subthreshold delta‐frequency resonance in thalamic reticular neurons

The thalamic reticular nucleus (nRt) is an assembly of GABAergic projection neurons that participate in the generation of brain rhythms during synchronous sleep and absence epilepsy. NRt cells receive inhibitory and excitatory synaptic inputs, and are endowed with an intricate set of intrinsic conductances. However, little is known about how intrinsic and synaptic properties interact to generate rhythmic discharges in these neurons. In order to better understand this interaction, I studied the subthreshold responses of nRt cells to time‐varying inputs. Patch‐clamp recordings were performed in acute slices of rat thalamus (postnatal days 12–21). Sinusoidal current waveforms of linearly changing frequencies were injected into the soma, and the resulting voltage oscillations were recorded. At the resting membrane potential, the impedance profile showed a characteristic resonance at 1.7 Hz. The relative strength of the resonance was 1.2, and increased with membrane hyperpolarization. Small suprathreshold current injections led to preferred spike generation at the resonance frequency. Bath application of ZD7288 or Cs⁺, inhibitors of the hyperpolarization‐activated cation current (I_h), transformed the resonance into low‐pass behaviour, whereas the T‐channel blockers mibefradil and Ni²⁺ decreased the strength of the resonance. It is concluded that nRt cells have an I_h‐mediated intrinsic frequency preference in the subthreshold voltage range that favours action potential generation in the delta‐frequency band.     

Sleep Oscillations Developing into Seizures in Corticothalamic Systems

Summary:  Purpose: The aim of this article is to discuss the neuronal substrates of sleep oscillations leading to seizures consisting of spike–wave (SW) complexes at 2–4 Hz, mimicking those seen in absence epilepsy, or SW and polyspike–wave (PSW) complexes at 1.5–2.5 Hz, often associated with fast runs at 10–15 Hz, as in the Lennox–Gastaut syndrome.
Methods: Extracellular recordings were done in permanently implanted animals during the natural waking–sleep cycle. Single and dual simultaneous recordings from cortical neurons, cortical and thalamic neurons, or cortical neurons and glial cells were performed in cats under ketamine–xylazine anesthesia.
Results: (a) The minimal substrate of SW seizures is the neocortex because such seizures may occur in thalamectomized animals, in which spindles are absent. In intact-brain animals, SW seizures are initiated in neocortex and spread to the thalamus after a few seconds. The majority of thalamocortical (TC) neurons are steadily hyperpolarized throughout the cortical SW seizures. (b) In the Lennox–Gastaut syndrome, the paroxysmal depolarizing shifts (PDSs) associated with the EEG "spike" of SW/PSW complexes contain an important inhibitory component, whereas the hyperpolarization during the EEG "wave" component is not due to γ-aminobutryic acid (GABA)ergic inhibitory postsynaptic potentials (IPSPs) but is ascribed to a mixture of disfacilitation and K⁺ currents. As is also the case with seizures consisting of pure SW complexes, the majority of TC neurons are hyperpolarized during the cortical paroxysms and disinhibited after the cessation of cortical seizures.
Conclusions: Seizures with SW complexes and of the Lennox–Gastaut type preferentially evolve from sleep oscillations. They are initiated in neocortex and spread to the thalamus after a few seconds. The majority of TC neurons are inhibited during these seizures.     

Scalp‐recorded high‐frequency oscillations in childhood sleep‐induced electrical status epilepticus

Because high‐frequency oscillations (HFOs) may affect normal brain functions, we examined them using electroencephalography (EEG) in epilepsy with continuous spike‐waves during slow‐wave sleep (CSWS), a condition that can cause neuropsychological regression. In 10 children between 6 and 9 years of age with epilepsy with CSWS or related disorders, we investigated HFOs in scalp EEG spikes during slow‐wave sleep through temporal expansion of the EEG traces with a low‐cut frequency filter at 70 Hz as well as through time‐frequency power spectral analysis. HFOs (ripples) concurrent with spikes were detected in the temporally expanded traces, and the frequency of the high‐frequency peak with the greatest power in each patient’s spectra ranged from 97.7 to 140.6 Hz. This is the first report on the detection of HFOs from scalp EEG recordings in epileptic patients. We speculate that epileptic HFOs may interfere with higher brain functions in epilepsy with CSWS.     

Rhythmic neuronal activity in S2 somatosensory and insular cortices contribute to the initiation of absence‐related spike‐and‐wave discharges

Purpose: The origin of bilateral synchronous spike‐and‐wave discharges (SWDs) that underlie absence seizures has been widely debated. Studies in genetic rodent models suggest that SWDs originate from a restricted region in the somatosensory cortex. The properties of this initiation site remain unknown. Our goal was to characterize the interictal, preictal and ictal neuronal activity in the primary and secondary cortical regions (S1, S2) and in the adjacent insular cortex (IC) in Genetic Absence Epilepsy Rats from Strasbourg (GAERS). Methods: We performed electroencephalography (EEG) recordings in combination with multisite local field potential (LFP) and single cell juxtacellular recordings, and cortical electrical stimulations, in freely moving rats and those under neurolept‐anesthesia. Key Findings: The onset of the SWDs was preceded by 5–9 Hz field potential oscillations, which were detected earlier in S2 and IC than in S1. Sustained SWDs could be triggered by a 2‐s train of 7‐Hz electrical stimuli at a lower current intensity in S2 than in S1. In S2 and IC, subsets of neurons displayed rhythmic firing (5–9 Hz) in between seizures. S2 and IC layers V and VI neurons fired during the same time window, whereas in S1 layer VI, neurons fired before layer V neurons. Just before the spike component of each SW complex, short‐lasting high‐frequency oscillations consistently occurred in IC ～20 msec before S1. Significance: Our findings demonstrate that the S2/IC cortical areas are a critical component of the macro‐network that is responsible for the generation of absence‐related SWDs.     

GABAergic disinhibition and impaired KCC2 cotransporter activity underlie tumor‐associated epilepsy

Seizures frequently accompany gliomas and often escalate to peritumoral epilepsy. Previous work revealed the importance of tumor‐derived excitatory glutamate (Glu) release mediated by the cystine‐glutamate transporter (SXC) in epileptogenesis. We now show a novel contribution of GABAergic disinhibition to disease pathophysiology. In a validated mouse glioma model, we found that peritumoral parvalbumin‐positive GABAergic inhibitory interneurons are significantly reduced, corresponding with deficits in spontaneous and evoked inhibitory neurotransmission. Most remaining peritumoral neurons exhibit elevated intracellular Cl⁻ concentration ([Cl⁻]_i) and consequently depolarizing, excitatory gamma‐aminobutyric acid (GABA) responses. In these neurons, the plasmalemmal expression of KCC2, which establishes the low [Cl⁻]_i required for GABA_AR‐mediated inhibition, is significantly decreased. Interestingly, reductions in inhibition are independent of Glu release, but the presence of both decreased inhibition and decreased SXC expression is required for epileptogenesis. We suggest GABAergic disinhibition renders peritumoral neuronal networks hyper‐excitable and susceptible to seizures triggered by excitatory stimuli, and propose KCC2 as a therapeutic target. GLIA 2015;63:23–36     

Temporal lobe epileptiform activity following systemic administration of 4‐aminopyridine in rats

Purpose: The K⁺ channel blocker 4‐aminopyridine (4AP) induces epileptiform synchronization in brain slices maintained in vitro without interfering with γ‐aminobutyric acid (GABA)_A receptor–mediated inhibition and, actually, even enhancing it. The hypothesis that similar electrographic epileptiform patterns occur in vivo following systemic 4AP injection was tested here. Methods: Sprague‐Dawley rats (n = 13) were implanted with bipolar electrodes aimed at the hippocampal CA3 region, entorhinal cortex, subiculum, dentate gyrus, and amygdala. They were then injected with a single dose of 4AP (4–5 mg/kg, i.p.), and video‐monitoring/electroencephalography (EEG) recordings were performed. Key Findings: 4AP induced convulsive or nonconvulsive seizures in 12 of 13 rats, along with generalized fascicular twitching, wet‐dog shakes, and myoclonic jerks. On EEG, we observed in 7 (58.3%) of 12 animals long‐lasting interictal spikes from the subiculum before the occurrence of the first seizure. Once seizures had started, interictal spikes occurred in all areas with no fixed site of origin. Most seizures (41/60, 68.3%) were characterized by a low‐voltage fast‐activity onset pattern and were convulsive (48/60, 80%). 4AP also induced highly rhythmic theta (6–11 Hz) oscillations in CA3 and entorhinal cortex before seizure occurrence. Significance: Our study shows that systemic 4AP administration in vivo can enhance theta oscillations and induce slow interictal spikes and low‐voltage fast‐onset seizures similar to those reported in brain slices. We propose that these effects may reflect, at least in part, enhanced GABA_A receptor–mediated inhibition as reported in in vitro studies.     

3‐Hz subthreshold oscillations of CA2 neurons In vivo

The CA2 region is unique in the hippocampus; it receives direct synaptic innervations from several hypothalamic nuclei and expresses various receptors of neuromodulators, including adenosine, vasopressin, and oxytocin. Furthermore, the CA2 region may have distinct brain functions, such as the control of instinctive and social behaviors; however, little is known about the dynamics of the subthreshold membrane potentials of CA2 neurons in vivo. We conducted whole‐cell current‐clamp recordings from CA2 pyramidal cells in urethane‐anesthetized mice and monitored the intrinsic fluctuations in their membrane potentials. The CA2 pyramidal cells emitted spontaneous action potentials at mean firing rates of ～0.8 Hz. In approximately half of the neurons, the subthreshold membrane potential oscillated at ～3 Hz. In two neurons, we obtained simultaneous recordings of local field potentials from the CA1 stratum radiatum and demonstrated that the 3‐Hz oscillations of CA2 neurons were not correlated with CA1 field potentials. In tetrodotoxin‐perfused acute hippocampal slices, the membrane potentials of CA2 pyramidal cells were not preferentially entrained to 3‐Hz sinusoidal current inputs, which suggest that intracellular 3‐Hz oscillations reflect the neuronal dynamics of the surrounding networks. © 2016 Wiley Periodicals, Inc.     

Cocaine withdrawal reduces GABABR transmission at entopeduncular nucleus – lateral habenula synapses

Lateral habenula (LHb) hyperactivity plays a pivotal role in the emergence of negative emotional states, including those occurring during withdrawal from addictive drugs. We have previously implicated cocaine‐driven adaptations at synapses from the entopeduncular nucleus (EPN) to the LHb in this process. Specifically, ionotropic GABA_A receptor (R)‐mediated neurotransmission at EPN‐to‐LHb synapses is reduced during cocaine withdrawal, due to impaired vesicle filling. Recent studies have shown that metabotropic GABA_BR signaling also controls LHb activity, although its role at EPN‐to‐LHb synapses during drug withdrawal is unknown. Here, we predicted that cocaine treatment would reduce GABA_BR‐mediated neurotransmission at EPN‐to‐LHb synapses. We chronically treated mice with saline or cocaine, prepared brain slices after two days of withdrawal and performed voltage‐clamp recordings from LHb neurons whilst optogenetically stimulating EPN terminals. Compared with controls, mice in cocaine withdrawal exhibited reduced GABA_AR‐mediated input to LHb neurons, and a reduced occurrence of GABA_BR‐signaling at EPN‐to‐LHb synapses. We then assessed the underlying mechanism of this decrease. Application of GABA_BR agonist baclofen evoked similar postsynaptic responses in EPN‐innervated LHb neurons in saline‐ and cocaine‐treated mice. Release probability at EPN‐to‐LHb GABAergic synapses was also comparable between groups. However, incubating brain slices in glutamine to facilitate GABA vesicle filling, normalized GABA_BR‐currents at EPN‐to‐LHb synapses in cocaine‐treated mice. Overall, we show that during cocaine withdrawal, together with reduced GABA_AR transmission, also GABA_BR‐mediated inhibitory signaling is diminished at EPN‐to‐LHb synapses, likely via the same presynaptic deficit. In concert, these alterations are predicted to contribute to the emergence of drug withdrawal symptoms, facilitating drug relapse.     

Zolpidem ameliorates motor impairments in the unilaterally 6‐hydroxydopamine‐lesioned rat

Nuclei within the basal ganglia—such as the globus pallidus external segment, subthalamic nucleus, and substantia nigra pars reticulata—have been shown to exhibit synchronous bursting activity entrained to excessive cortical beta oscillations following dopamine depletion. Zolpidem binds to GABA_A receptors with selectivity for those expressing the α₁ subunit, potentiating inhibitory postsynaptic currents and increasing the time decay of channel opening. Interestingly, zolpidem‐sensitive nuclei within the basal ganglia circuitry are also those that have been shown to exhibit hyperexcitation in a dopamine‐depleted state. We hypothesized that a drug with selectivity for these nuclei may improve motor impairments associated with Parkinson's disease. In order to determine the threshold dose at which zolpidem might encumber motor behavior, a dose‐response experiment was performed in intact rats using rotarod. Next, we tested whether subthreshold doses (0.1, 0.25, 0.5 mg/kg; i.p.) of zolpidem improved volitional motor behavior/coordination using the rotarod balance beam and cylinder/paw preference tests in unilaterally 6‐hydroxydopamine‐lesioned rats. It was found that 0.1 mg/kg zolpidem significantly improved rotarod performance and significantly reduced forelimb use asymmetry compared to undrugged post‐lesion conditions. Here, we present the first translational evidence for a role of zolpidem‐sensitive GABA_A receptors in the treatment of PD motor symptoms. Our data show that zolpidem improves both motor coordination and volitional forelimb use in the unilateral 6‐hydroxydopamine lesion model of PD, and thus suggest that zolpidem‐sensitive GABA_A receptors may represent a novel therapeutic target for the treatment of motor symptoms of Parkinson's disease.     

GABAB receptors in neocortical and hippocampal pyramidal neurons are coupled to different potassium channels

Classically, GABA_B receptors are thought to regulate neuronal excitability via G‐protein‐coupled inwardly rectifying potassium (GIRK) channels. Recent data, however, indicate that GABA_B receptors can also activate two‐pore domain potassium channels. Here, we investigate which potassium channels are coupled to GABA_B receptors in rat neocortical layer 5 and hippocampal CA1 pyramidal neurons. Bath application of the non‐specific GIRK channel blocker barium (200 μm ) abolished outward currents evoked by GABA_B receptors in CA1 pyramidal, but only partially blocked GABA_B responses in layer 5 neurons. Layer 5 and CA1 pyramidal neurons also showed differential sensitivity to tertiapin‐Q, a specific GIRK channel blocker. Tertiapin‐Q partially blocked GABA_B responses in CA1 pyramidal neurons, but was ineffective in blocking GABA_B responses in neocortical layer 5 neurons. Consistent with the idea that GABA_B receptors are coupled to two‐pore domain potassium channels, the non‐specific blockers quinidine and bupivacaine partially blocked GABA_B responses in both layer 5 and CA1 neurons. Finally, we show that lowering external pH, as occurs in hypoxia, blocks the component of GABA_B responses mediated by two‐pore domain potassium channels in neocortical layer 5 pyramidal neurons, while at the same time revealing a GIRK channel component. These data indicate that GABA_B receptors in neocortical layer 5 and hippocampal CA1 pyramidal neurons are coupled to different channels, with this coupling pH dependent on neocortical layer 5 pyramidal neurons. This pH dependency may act to maintain constant levels of GABA_B inhibition during hypoxia by enhancing GIRK channel function following a reduction in two‐pore domain potassium channel activity.     

Involvement of the Thalamocortical System in Epileptic Loss of Consciousness

Summary: Experiments on putative neuronal mechanisms underlying absence seizures as well as clinical observations are critically reviewed for their ability to explain apparent "loss of consciousness." It is argued that the initial defect in absences lies with corticothalamic (CT) neuronal mechanisms responsible for selective attention and/or planning for action, rather than with those establishing either the states or the contents of consciousness. Normally, rich thalamocortical (TC)–CT feedback loops regulate the flow of information to the cortex and help its neurons to organize themselves in discrete assemblies, which through high-frequency (>30 Hz) oscillations bind those distributed processes of the brain that are considered important, so that we are able to focus on what is needed from moment to moment and be aware of this fact. This ability is transiently lost in absence seizures, because large numbers of CT loops are recruited for seconds in much stronger, low-frequency (∼3 Hz) oscillations of EPSP/IPSP sequences, which underlie electroencephalographic (EEG) spike-and-wave discharges (SWDs). These oscillations probably result from a transformation of the normal EEG rhythm of sleep spindles on an abnormal increase of cortical excitability that results in strong activation of inhibitory neurons in the cortex and in nucleus reticularis thalami. The strong general enhancement of CT feedback during SWDs may disallow the discrete feedback, which normally selects specific TC circuits for conscious perception and/or motor reaction. Such a mechanism of SWD generation allows variability in the extent to which different TC sectors are engaged in the SWD activity and thus explains the variable ability of some patients to respond during an absence, depending on the sensory modality examined.     

Different pools of postsynaptic GABAA receptors mediate inhibition evoked by low‐ and high‐frequency presynaptic stimulation at hippocampal synapses

Patterns of short‐term synaptic plasticity could considerably differ between synapses of the same axon. This may lead to separation of synaptic receptors transmitting either low‐ or high‐frequency signals and, therefore, may have functional consequences for the information transfer in the brain. Here, we estimated a degree of such separation at hippocampal GABAergic synapses using a use‐dependent GABA_A receptor antagonist, picrotoxin, to selectively suppress a pool of GABA_A receptors monosynaptically activated during the low‐frequency stimulation. The relative changes in postsynaptic responses evoked by the high‐frequency stimulation before and after such block were used to estimate the contribution of this GABA_A receptor pool to synaptic transmission at high frequencies. Using this approach, we have shown that IPSCs evoked by low‐frequency (0.2 Hz) stimulation and asynchronous currents evoked by high‐frequency (20–40 Hz) stimulation are mediated by different pools of postsynaptic GABA_A receptors. Thus, our findings suggest that inhibition produced by a single hippocampal interneuron may be selectively routed to different postsynaptic targets depending on the presynaptic discharge frequency. Synapse 68:344–354, 2014 . © 2014 Wiley Periodicals, Inc.     

Depolarizing GABAergic synaptic input triggers endocannabinoid‐mediated retrograde synaptic signaling

Endocannabinoids released by postsynaptic neurons inhibit neurotransmitter release from presynaptic axon terminals. One typical stimulus of endocannabinoid production is an increase of calcium concentration in postsynaptic neurons. The aim of the present study was to clarify whether depolarizing GABAergic synaptic input, by increasing calcium concentration in postsynaptic neurons, can trigger endocannabinoid production. Spontaneous GABAergic inhibitory postsynaptic currents (sIPSCs) were recorded in Purkinje cells in mouse cerebellar slices with patch‐clamp pipettes containing 151 mM chloride (a usual recording mode). sIPSCs were depolarizing inward currents under this condition. Combined electrophysiological and fluorometric calcium imaging experiments indicated that sIPSCs frequently triggered calcium spikes. After the calcium spikes, a short‐term suppression of sIPSCs occurred. This suppression was prevented by the CB₁ cannabinoid receptor antagonist rimonabant and the diacylglycerol lipase inhibitor orlistat, but not changed by URB597, an inhibitor of anandamide degradation. It is, therefore, likely that CB₁ receptors and 2‐arachidonoylglycerol were involved. For testing the physiological significance of the above observation, we carried out experiments on brains of 3‐ to 5‐day‐old mice. The gramicidin‐induced perforated patch‐clamp mode was used for preserving the physiological intracellular chloride concentration of the neurons. Depolarizing GABAergic sIPSCs occurred under this condition, but at a very low rate. Rimonabant did not change the frequency of these sIPSCs, arguing against the persistence of an endocannabinoid tone. The results point to a new kind of trigger of endocannabinoid production: depolarizing GABAergic synaptic input can elicit endocannabinoid production in postsynaptic neurons by activating calcium channels. The produced endocannabinoid suppresses GABA release from presynaptic axon terminals. Synapse 63:643–652, 2009. © 2009 Wiley‐Liss, Inc.     

GABAergic Interneurons are Required for Generation of Slow CA1 Oscillation in Rat Hippocampus

Neuronal oscillations are fundamental to hippocampal function. It has been shown that GABAergic interneurons make an important contribution to hippocampal oscillations, but the underlying mechanism is not well understood. Here, using whole-cell recording in the complete hippocampal formation isolated from rats at postnatal days 14–18, we showed that GABA_Areceptormediated activity enhanced the generation of slow CA1 oscillations. In vitro, slow oscillations(0.5–1.5 Hz) were generated in CA1 neurons, and they consisted primarily of excitatory rather than inhibitory membrane-potential changes. These oscillations were greatly reduced by blocking GABA_Areceptor-mediated activity with bicuculline and were enhanced by increasing such activity with midazolam, suggesting that interneurons are required for oscillation generation. Consistently, CA1 fast-spiking interneurons were found to generate action potentials usually preceding those in CA1 pyramidal cells. These findings indicate a GABA_Areceptor-based mechanism for the generation of the slow CA1 oscillation in the hippocampus.