Spindle-like thalamocortical synchronization in a rat brain slice preparation

The effect of quercetin on absence epilepsy in WAG/Rij rats

AIM

In the present study, the effect of quercetin, a powerful antioxidant flavonoid, on genetic absence epilepsy was studied in WAG/Rij rats.

MATERIAL AND METHOD

Tripolar electrodes were implanted into WAG/Rij rats. Basal electrocorticography (ECoG) was recorded following a recovery period. After basal ECoG recording, different doses of quercetin (QRC) (25, 50 and 100 mg/kg) were injected intraperitoneally (i.p.) for 30 days. ECoG recording was continued for 31 days, three hours a day. After recording, the rats were anesthetized and euthanized through cervical dislocation and their brains were excised. Biochemically, TNF-alpha, IL-6 and NO were studied in whole rat brains.

RESULTS

In WAG/Rij rats, low-dose quercetin (25 mg/kg) reduced the number and duration of spike-wave discharges (SWDs) compared to the control group. However, 50 and 100 mg/kg quercetin doses increased SWDs. Duration of SWDs was prolonged only with 100 mg/kg dose. None of the quercetin doses had any effect on average amplitude of SWDs. In addition, it was observed in biochemical analyses that 25 mg/kg quercetin reduced TNF-alpha, IL-6 and NO levels compared to the control group. While TNF-alpha and IL-6 levels in rat brains were not affected by 50 or 100 mg/kg doses, both doses were found to increase NO levels in rat brains.

CONCLUSION

Based on the results of the present study, 25 mg/kg low-dose quercetin may have reduced absence seizures by reducing proinflammatory cytokines and NO, but high-dose quercetin may have increased absence seizures through increasing the NO level. This contrasting effect of quercetin on absence seizures needs to be investigated by advanced mechanisms.

The impact of early-life environment on absence epilepsy and neuropsychiatric comorbidities

This review discusses the long-term effects of early-life environment on epileptogenesis, epilepsy, and neuropsychiatric comorbidities with an emphasis on the absence epilepsy. The WAG/Rij rat strain is a well-validated genetic model of absence epilepsy with mild depression-like (dysthymia) comorbidity. Although pathologic phenotype in WAG/Rij rats is genetically determined, convincing evidence presented in this review suggests that the absence epilepsy and depression-like comorbidity in WAG/Rij rats may be governed by early-life events, such as prenatal drug exposure, early-life stress, neonatal maternal separation, neonatal handling, maternal care, environmental enrichment, neonatal sensory impairments, neonatal tactile stimulation, and maternal diet. The data, as presented here, indicate that some early environmental events can promote and accelerate the development of absence seizures and their neuropsychiatric comorbidities, while others may exert anti-epileptogenic and disease-modifying effects. The early environment can lead to phenotypic alterations in offspring due to epigenetic modifications of gene expression, which may have maladaptive consequences or represent a therapeutic value. Targeting DNA methylation with a maternal methyl-enriched diet during the perinatal period appears to be a new preventive epigenetic anti-absence therapy. A number of caveats related to the maternal methyl-enriched diet and prospects for future research are discussed.

Anticonvulsant effect of pterostilbene and its influence on the anxiety- and depression-like behavior in the pentetrazol-kindled mice: behavioral, biochemical, and molecular studies

RATIONALE

Pterostilbene is the 3,5-dimethoxy derivative of resveratrol with numerous beneficial effects including neuroprotective properties. Experimental studies revealed its anticonvulsant action in the acute seizure tests.

OBJECTIVES

The purpose of the present study was to evaluate the effect of pterostilbene in the pentetrazol (PTZ)-induced kindling model of epilepsy in mice as well as to assess some possible mechanisms of its anticonvulsant action in this model.

METHODS

Mice were repeatedly treated with pterostilbene (50-200 mg/kg) and its effect on the development of seizure activity in the PTZ kindling was estimated. Influence of pterostilbene on the locomotor activity and anxiety- and depression-like behavior in the PTZ-kindled mice was also assessed. To understand the possible mechanisms of anticonvulsant activity of pterostilbene, γ-aminobutyric acid (GABA) and glutamate concentrations in the prefrontal cortex and hippocampus of the PTZ-kindled mice were measured using LC-MS/MS method. Moreover, mRNA expression of BDNF, TNF-α, IL-1β, IL-6, GABRA1A, and GRIN2B was determined by RT-qPCR technique.

RESULTS

We found that pterostilbene at a dose of 200 mg/kg considerably reduced seizure activity but did not influence the locomotor activity and depression- and anxiety-like behavior in the PTZ-kindled mice. In the prefrontal cortex and hippocampus, pterostilbene reversed the kindling-induced decrease of GABA concentration. Neither in the prefrontal cortex nor hippocampus pterostilbene affected mRNA expression of IL-1β, IL-6, GABRA1A, and GRIN2B augmented by PTZ kindling. Pterostilbene at a dose of 100 mg/kg significantly decreased BDNF and TNF-α mRNA expression in the hippocampus of the PTZ-kindled mice.

CONCLUSIONS

Although further studies are necessary to understand the mechanism of anticonvulsant properties of pterostilbene, our findings suggest that it might be considered a candidate for a new antiseizure drug.

The Paradox of Astroglial Ca2 + Signals at the Interface of Excitation and Inhibition

Astroglial networks constitute a non-neuronal communication system in the brain and are acknowledged modulators of synaptic plasticity. A sophisticated set of transmitter receptors in combination with distinct secretion mechanisms enables astrocytes to sense and modulate synaptic transmission. This integrative function evolved around intracellular Ca²⁺ signals, by and large considered as the main indicator of astrocyte activity. Regular brain physiology meticulously relies on the constant reciprocity of excitation and inhibition (E/I). Astrocytes are metabolically, physically, and functionally associated to the E/I convergence. Metabolically, astrocytes provide glutamine, the precursor of both major neurotransmitters governing E/I in the central nervous system (CNS): glutamate and γ-aminobutyric acid (GABA). Perisynaptic astroglial processes are structurally and functionally associated with the respective circuits throughout the CNS. Astonishingly, in astrocytes, glutamatergic as well as GABAergic inputs elicit similar rises in intracellular Ca²⁺ that in turn can trigger the release of glutamate and GABA as well. Paradoxically, as gliotransmitters, these two molecules can thus strengthen, weaken or even reverse the input signal. Therefore, the net impact on neuronal network function is often convoluted and cannot be simply predicted by the nature of the stimulus itself. In this review, we highlight the ambiguity of astrocytes on discriminating and affecting synaptic activity in physiological and pathological state. Indeed, aberrant astroglial Ca²⁺ signaling is a key aspect of pathological conditions exhibiting compromised network excitability, such as epilepsy. Here, we gather recent evidence on the complexity of astroglial Ca²⁺ signals in health and disease, challenging the traditional, neuro-centric concept of segregating E/I, in favor of a non-binary, mutually dependent perspective on glutamatergic and GABAergic transmission.

How do we use in vitro models to understand epileptiform and ictal activity? A report of the TASK1-WG4 group of the ILAE/AES Joint Translational Task Force

In vitro brain tissue preparations allow the convenient and affordable study of brain networks and have allowed us to garner molecular, cellular, and electrophysiologic insights into brain function with a detail not achievable in vivo. Preparations from both rodent and human postsurgical tissue have been utilized to generate in vitro electrical activity similar to electrographic activity seen in patients with epilepsy. A great deal of knowledge about how brain networks generate various forms of epileptiform activity has been gained, but due to the multiple in vitro models and manipulations used, there is a need for a standardization across studies. Here, we describe epileptiform patterns generated using in vitro brain preparations, focusing on issues and best practices pertaining to recording, reporting, and interpretation of the electrophysiologic patterns observed. We also discuss criteria for defining in vitro seizure‐like patterns (i.e., ictal) and interictal discharges. Unifying terminologies and definitions are proposed. We suggest a set of best practices for reporting in vitro studies to favor both efficient across‐lab comparisons and translation to in vivo models and human studies.

Impact of strain, sex, and estrous cycle on gamma butyrolactone-evoked absence seizures in rats

Childhood absence epilepsy (CAE) is the most common pediatric epilepsy syndrome and is characterized by typical absence seizures (AS). AS are non-convulsive epileptic seizures characterized by a sudden loss of awareness and bilaterally generalized synchronous 2.5-4 Hz spike and slow-wave discharges (SWD). Gamma butyrolactone (GBL) is an acute pharmacological model of AS and induces bilaterally synchronous SWDs and behavioral arrest. Despite the long use of this model, little is known about its strain and sex-dependent features. We compared the dose-response profile of GBL-evoked SWDs in three rat strains (Long Evans, Sprague-Dawley, and Wistar), and examined the modulatory effects of estrous cycle on SWDs in female Wistar rats. We evaluated the number of seizures, the cumulative time seizing, and the average seizure duration as a function of dose, strain, and sex/estrous phase. Long Evans rats displayed the greatest sensitivity to GBL, followed by Wistar rats, and then by Sprague-Dawley rats. GBL-evoked SWDs were modulated by estrous cycle in female rats, with the lowest sensitivity to GBL occurring during metestrus. Wistar rats showed the greatest variability as a function of dose, and the least variability within dose; these features make this strain desirable for interventional studies. Moreover, our finding that the SWD response to GBL differs as a function of estrous cycle underscores the importance of cycle monitoring in studies examining female animals using this model. Together, these strain and sex-dependent findings provide guidance for future studies.

Intrathalamic connections shape spindle activity - a modelling study

Spindle oscillations are generated predominantly during sleep state II, through cyclical interactions between thalamocortical and reticular neurons. Inhibition from reticular cells is critical for this activity; it enables burst firing by the de-inactivation of T-type Ca²⁺ channels. While the effect of different channelopathies on spindling is extensively investigated, our knowledge about the role of intrathalamic connections is limited. Therefore, we explored how the connection pattern and the density of reticular inhibitory synapses affect spindle activity in a thalamic network model. With more intrareticular connections, synchronous firing of reticular cells, and intraspindle burst frequency decreased, spindles lengthened. In models with strong intrareticular inhibition spindle activity was impaired, and a sustained 6-8 Hz oscillation was generated instead. The strength of reticular innervation onto thalamocortical cells played a key role in the generation of oscillations; it determined the amount of thalamocortical cell bursts, and consequently spindle length. Focal inputs supported bursts but affected only a few cells thus barely reinforced network activity, while diffuse contacts aided bursts only when a sufficient number of reticular cells fired synchronously. According to our study, alterations in the connection pattern influence thalamic activities and may contribute to pathological conditions, or alternatively, they serve as a compensatory mechanism.

Effects of weak transcranial alternating current stimulation on brain activity-a review of known mechanisms from animal studies

Rhythmic neuronal activity is ubiquitous in the human brain. These rhythms originate from a variety of different network mechanisms, which give rise to a wide-ranging spectrum of oscillation frequencies. In the last few years an increasing number of clinical research studies have explored transcranial alternating current stimulation (tACS) with weak current as a tool for affecting brain function. The premise of these interventions is that tACS will interact with ongoing brain oscillations. However, the exact mechanisms by which weak currents could affect neuronal oscillations at different frequency bands are not well known and this, in turn, limits the rational optimization of human experiments. Here we review the available in vitro and in vivo animal studies that attempt to provide mechanistic explanations. The findings can be summarized into a few generic principles, such as periodic modulation of excitability, shifts in spike timing, modulation of firing rate, and shifts in the balance of excitation and inhibition. These effects result from weak but simultaneous polarization of a large number of neurons. Whether this can lead to an entrainment or a modulation of brain oscillations, or whether AC currents have no effect at all, depends entirely on the specific dynamic that gives rise to the different brain rhythms, as discussed here for slow wave oscillations (∼1 Hz) and gamma oscillations (∼30 Hz). We conclude with suggestions for further experiments to investigate the role of AC stimulation for other physiologically relevant brain rhythms.

Systemic blockade of dopamine D2-like receptors increases high-voltage spindles in the globus pallidus and motor cortex of freely moving rats

High-voltage spindles (HVSs) have been reported to appear spontaneously and widely in the cortical–basal ganglia networks of rats. Our previous study showed that dopamine depletion can significantly increase the power and coherence of HVSs in the globus pallidus (GP) and motor cortex of freely moving rats. However, it is unclear whether dopamine regulates HVS activity by acting on dopamine D₁-like receptors or D₂-like receptors. We employed local-field potential and electrocorticogram methods to simultaneously record the oscillatory activities in the GP and primary motor cortex (M1) in freely moving rats following systemic administration of dopamine receptor antagonists or saline. The results showed that the dopamine D₂-like receptor antagonists, raclopride and haloperidol, significantly increased the number and duration of HVSs, and the relative power associated with HVS activity in the GP and M1 cortex. Coherence values for HVS activity between the GP and M1 cortex area were also significantly increased by dopamine D₂-like receptor antagonists. On the contrary, the selective dopamine D₁-like receptor antagonist, SCH23390, had no significant effect on the number, duration, or relative power of HVSs, or HVS-related coherence between M1 and GP. In conclusion, dopamine D₂-like receptors, but not D₁-like receptors, were involved in HVS regulation. This supports the important role of dopamine D₂-like receptors in the regulation of HVSs. An siRNA knock-down experiment on the striatum confirmed our conclusion.

Thalamocortical relationships and network synchronization in a new genetic model "in mirror" for absence epilepsy

Electroencephalographic generalized spike and wave discharges (SWD), the hallmark of human absence seizures, are generated in thalamocortical networks. However, the potential alterations in these networks in terms of the efficacy of the reciprocal synaptic activities between the cortex and the thalamus are not known in this pathology. Here, the efficacy of these reciprocal connections is assessed in vitro in thalamocortical slices obtained from BS/Orl mice, which is a new genetic model of absence epilepsy. These mice show spontaneous SWD, and their features can be compared to that of BR/Orl mice, which are free of SWD. In addition, since gap junctions may modulate the efficacy of these connections, their implications in pharmacologically-induced epileptiform discharges were studied in the same slices. The thalamus and neocortex were independently stimulated and the electrically-evoked responses in both structures were recorded from the same slice. The synaptic efficacy of thalamocortical and corticothalamic connections were assessed by measuring the dynamic range of synaptic field potential changes in response to increasing stimulation strengths. The connection efficacy was weaker in epileptic mice however, this decrease in efficacy was more pronounced in thalamocortical afferents, thus introducing an imbalance in the reciprocal connections between the cortex and thalamus. However, short-term facilitation of the thalamocortical responses were increased in epileptic mice compared to non-epileptic animals. These features may favor occurrence of rhythmical activities in thalamocortical networks. In addition, carbenoxolone (a gap junction blocker) decreased the cumulative duration of 4-aminopyridine-induced ictal-like activities, with a slower time course in epileptic mice. However, the 4-aminopyridine-induced GABA-dependent negative potentials, which appeared to trigger the ictal-like activities, remained. Our results show that the balance of the reciprocal connections between the thalamus and cortex is altered in favor of the corticothalamic connections in epileptic mice, and suggest that gap junctions mediate a stronger cortical synchronization in this strain.

The role of inhibition in oscillatory wave dynamics in the cortex

Cortical oscillations arise during behavioral and mental tasks, and all temporal oscillations have particular spatial patterns. Studying the mechanisms that generate and modulate the spatiotemporal characteristics of oscillations is important for understanding neural information processing and the signs and symptoms of dynamical diseases of the brain. Nevertheless, it remains unclear how GABAergic inhibition modulates these oscillation dynamics. Using voltage-sensitive dye imaging, pharmacological methods, and tangentially cut occipital neocortical brain slices (including layers 3-5) of Sprague-Dawley rat, we found that GABAa disinhibition with bicuculline can progressively simplify oscillation dynamics in the presence of carbachol in a concentration-dependent manner. Additionally, GABAb disinhibition can further simplify oscillation dynamics after GABAa receptors are blocked. Both GABAa and GABAb disinhibition increase the synchronization of the neural network. Theta frequency (5-15-Hz) oscillations are reliably generated by using a combination of GABAa and GABAb antagonists alone. These theta oscillations have basic spatiotemporal patterns similar to those generated by carbachol/bicuculline. These results are illustrative of how GABAergic inhibition increases the complexity of patterns of activity and contributes to the regulation of the cortex.

Thalamocortical model for a propofol-induced alpha-rhythm associated with loss of consciousness

Recent data reveal that the general anesthetic propofol gives rise to a frontal α-rhythm at dose levels sufficient to induce loss of consciousness. In this work, a computational model is developed that suggests the network mechanisms responsible for such a rhythm. It is shown that propofol can alter the dynamics in thalamocortical loops, leading to persistent and synchronous α-activity. The synchrony that forms in the cortex by virtue of the involvement of the thalamus may impede responsiveness to external stimuli, thus providing a correlate for the unconscious state.

Issues about the physiological functions of prolyl oligopeptidase based on its discordant spatial association with substrates and inconsistencies among mRNA, protein levels, and enzymatic activity

Prolyl oligopeptidase (POP) is a serine endopeptidase that hydrolyses proline-containing peptides shorter than 30 amino acids. POP may be associated with cognitive functions, possibly via the cleavage of neuropeptides. Recent studies have also suggested novel non-hydrolytic and non-catalytic functions for POP. Moreover, POP has also been proposed as a regulator of inositol 1,4,5-triphosphate signaling and several other functions such as cell proliferation and differentiation, as well as signal transduction in the central nervous system, and it is suspected to be involved in pathological conditions such as Parkinson's and Alzheimer's diseases and cancer. POP inhibitors have been developed to restore the depleted neuropeptide levels encountered in aging or in neurodegenerative disorders. These compounds have shown some antiamnesic effects in animal models. However, the mechanisms of these hypothesized actions are still far from clear. Moreover, the physiological role of POP has remained unknown, and a lack of basic studies, including its distribution, is obvious. The aim of this review is to gather information about POP and to propose some novel roles for this enzyme based on its distribution and its discordant spatial association with its best known substrates.

Cortical hyperexcitability and epileptogenesis: Understanding the mechanisms of epilepsy - part 2

Epilepsy encompasses a diverse group of seizure disorders caused by a variety of structural, cellular and molecular alterations of the brain primarily affecting the cerebral cortex, leading to recurrent unprovoked epileptic seizures. In this two-part review we examine the mechanisms underlying normal neuronal function and those predisposing to recurrent epileptic seizures starting at the most basic cellular derangements (Part 1, Volume 16, Issue 3) and working up to the highly complex epileptic networks and factors that modulate the predisposition to seizures (Part 2). We attempt to show that multiple factors can modify the epileptic process and that different mechanisms underlie different types of epilepsy, and in most situations there is an interplay between multiple genetic and environmental factors.

Localization of prolyl oligopeptidase in the thalamic and cortical projection neurons: a retrograde neurotracing study in the rat brain

Prolyl oligopeptidase (POP) is a serine endopeptidase which hydrolyses proline-containing peptides shorter than 30-mer. POP is believed to be associated with cognitive functions via neuropeptide cleavage. POP has been also connected to the inositol 1,4,5-triphosphate (IP(3)) signalling but the effects of POP-inhibition to the IP(3) accumulation in vivo are still unclear. However, little is known about the physiological role of POP in the brain. We have previously found that in the rat brain POP was specifically expressed in the pyramidal neurons of the cerebral cortex, particularly in the primary motor and somatosensory cortices, and corresponding projection areas in thalamus. Using a retrograde neurotracer we have now visualized the localization of POP in thalamocortical and corticothalamic projection neurons in ventrobasal complex and medial geniculate nucleus of thalamus and somatosensory/motor and auditory cortices. We observed that both in thalamus and cortex over 50% of projection neurons contained POP. These results support the hypothesis that POP is involved in thalamocortical and corticothalamic signal processing. We also propose, based on our neuroanatomical findings and literature, that POP may take part in the thalamocortical oscillations by interacting with IP(3) signalling in cells.

Synchronous high-voltage spindles in the cortex-basal ganglia network of awake and unrestrained rats

Synchronous oscillations in various frequency ranges have been recorded in several nuclei of the basal ganglia (BG) and are thought to be an information processing mechanism. High-voltage spindles (HVSs) are 5-13 Hz spike-and-wave oscillations, which are commonly recorded in rats and which have been reported in some recent studies where their occurrence in the BG has been investigated. We recorded single neurons and local field potentials (LFPs) simultaneously in the motor cortex, striatum and substantia nigra pars reticulata (SNr) of the freely moving rat. We took advantage of the high level of synchronization observed during HVSs to study signal transmission in the cortex-BG network in the awake animals. The results show that LFPs are synchronized in the motor cortex, striatum and SNr during HVS episodes and that the latter propagate from the cortex to the SNr via the striatum. Moreover, > 50% of single neurons in each of these structures are triggered by the HVS. Following the discharge of cortical cells, SNr neurons are first inhibited after approximately 19 ms and then activated after approximately 45 ms. This response is probably driven by the direct and indirect pathways, respectively, without any involvement of the hyperdirect pathway. Here, it is shown that cortex-BG connectivity can be studied using physiological signals in the freely moving animal as opposed to artificial stimulation under anaesthetized conditions. This opens the door to further studies under various experimental conditions, such as animal models of basal ganglia disorders.

Effects in vitro and in vivo of a gap junction blocker on epileptiform activities in a genetic model of absence epilepsy

We investigated the effects of carbenoxolone (CBX), a gap junctions (GJ) blocker, on epileptiform activities in vivo and in vitro. In a first series of experiments, i.p. CBX decreased the cumulative duration of cortical spike-wave discharges (SWD) in adult Genetic Absence Epilepsy Rats from Strasbourg (GAERS) without reduction in the SW amplitude or frequency. Since SWD are generated in thalamocortical networks, we studied the effect of CBX on thalamic and cortical activities elicited by 4-aminopyridine (4AP) in thalamocortical slices from GAERS or non-epileptic rats (NER). Spontaneous ictal-like activities (ILA) were recorded simultaneously in thalamus and somatosensory cortex. However, experiments where these structures were surgically separated showed that ILA were generated in the cortex and recorded by volume conduction in the thalamus. GABA-dependent negative field potentials were also recorded in the cortex, either isolated or initiating ILA. After bath-applying CBX (100 microM), the frequency and cumulative duration of ILA decreased but less rapidly in GAERS than in NER slices and they disappeared at a time point when GABA-dependent negative potentials remained. These data suggest that GJ do not mediate the 4AP induced interneuronal synchronisation but may be implicated in the spreading of the synchronised activities from interneuronal networks to principal neurones. Our results show that CBX exerts an antiepileptic action in vivo, and that GJ blockers limits spread of synchronised activities in vitro. They may represent an appropriate target for development of new antiepileptic drugs.

Nicotinic modulation of neuronal networks: from receptors to cognition

RATIONALE

Nicotine affects many aspects of human cognition, including attention and memory. Activation of nicotinic acetylcholine receptors (nAChRs) in neuronal networks modulates activity and information processing during cognitive tasks, which can be observed in electroencephalograms (EEGs) and functional magnetic resonance imaging studies.

OBJECTIVES

In this review, we will address aspects of nAChR functioning as well as synaptic and cellular modulation important for nicotinic impact on neuronal networks that ultimately underlie its effects on cognition. Although we will focus on general mechanisms, an emphasis will be put on attention behavior and nicotinic modulation of prefrontal cortex. In addition, we will discuss how nicotinic effects at the neuronal level could be related to its effects on the cognitive level through the study of electrical oscillations as observed in EEGs and brain slices.

RESULTS/CONCLUSIONS

Very little is known about mechanisms of how nAChR activation leads to a modification of electrical oscillation frequencies in EEGs. The results of studies using pharmacological interventions and transgenic animals implicate some nAChR types in aspects of cognition, but neuronal mechanisms are only poorly understood. We are only beginning to understand how nAChR distribution in neuronal networks impacts network functioning. Unveiling receptor and neuronal mechanisms important for nicotinic modulation of cognition will be instrumental for treatments of human disorders in which cholinergic signaling have been implicated, such as schizophrenia, attention deficit/hyperactivity disorder, and addiction.

Cellular and network mechanisms of spike-wave seizures

Spike-wave seizures are often considered a relatively "pure" form of epilepsy, with a uniform defect present in all patients and involvement of the whole brain homogeneously. Here, we present evidence against these common misconceptions. Rather than a uniform disorder, spike-wave rhythms arise from the normal inherent network properties of brain excitatory and inhibitory circuits, where they can be provoked by many different insults in several different brain networks. Here we discuss several different cellular and molecular mechanisms that may contribute to the generation of spike-wave seizures, particularly in idiopathic generalized epilepsy. In addition, we discuss growing evidence that electrical, neuroimaging, and molecular changes in spike-wave seizures do not involve the entire brain homogeneously. Rather, spike-wave discharges occur selectively in some thalamocortical networks, while sparing others. It is hoped that improved understanding of the heterogeneous defects and selective brain regions involved will ultimately lead to more effective treatments for spike-wave seizures.

Pentobarbital induces thalamic oscillations in brain slices, modulated by GABA and glycine receptors

We studied the effects of pentobarbital and antagonists of glutamate, gamma-aminobutyrate (GABA), and glycine receptors on extracellular activity in ventrobasal thalamic slices. Pentobarbital at sedative-hypnotic concentration (20 microM) reversibly induced 1-15 Hz oscillations. Sustained oscillations required electrical stimulation of internal capsule, but not elevated temperature or low [Mg2+]. Anesthetic concentration (200 microM) of pentobarbital evoked only transient oscillations. Kynurenate-sensitive glutamate receptors were essential for oscillations. GABA(A) antagonism (bicuculline, 50 microM or gabazine, 20 microM) suppressed oscillations at 5-15 Hz. GABA(B) antagonism (CGP 35348, 100 nM), or antagonism of glycine receptors (strychnine, 1 microM) suppressed oscillations at 1-4 and 11-15 Hz. GABA and glycine receptors modulated oscillation frequency. For elimination, oscillations required GABA antagonists and strychnine. Receptors for glutamate and glycine mediated oscillations during GABA receptor blockade in ventrobasal nuclei, or on disconnection from nRT. Glycine receptors were critical for oscillations in dorsal thalamic network, divested of GABAergic inhibition. Glutamate and GABA receptors mediated pentobarbital-induced oscillations in nRT, disconnected from ventrobasal nuclei. Hence, pentobarbital oscillogenesis occurred in isolated networks of the ventrobasal and reticularis nuclei mediated by glutamate receptors, with frequency modulation by GABA(A), GABA(B), and glycine receptors. These stationary oscillations represent a model of sedation-hypnosis, amenable to pharmacological analysis.

The thalamic reticular nucleus: structure, function and concept

On the basis of theoretical, anatomical, psychological and physiological considerations, Francis Crick (1984) proposed that, during selective attention, the thalamic reticular nucleus (TRN) controls the internal attentional searchlight that simultaneously highlights all the neural circuits called on by the object of attention. In other words, he submitted that during either perception, or the preparation and execution of any cognitive and/or motor task, the TRN sets all the corresponding thalamocortical (TC) circuits in motion. Over the last two decades, behavioural, electrophysiological, anatomical and neurochemical findings have been accumulating, supporting the complex nature of the TRN and raising questions about the validity of this speculative hypothesis. Indeed, our knowledge of the actual functioning of the TRN is still sprinkled with unresolved questions. Therefore, the time has come to join forces and discuss some recent cellular and network findings concerning this diencephalic GABAergic structure, which plays important roles during various states of consciousness. On the whole, the present critical survey emphasizes the TRN's complexity, and provides arguments combining anatomy, physiology and cognitive psychology.

Comparative study on the effects of kynurenic acid and glucosamine-kynurenic acid

Kynurenic acid (KYNA) is the only known endogenous N-methyl-D-aspartate (NMDA) receptor inhibitor and might therefore come into consideration as a therapeutic agent in certain neurobiological disorders. However, its use as a neuroprotective compound is practically excluded because KYNA does not readily cross the blood-brain barrier (BBB). We recently synthetized a new compound, glucosamine-kynurenic acid (KYNA-NH-GLUC), which is presumed to cross the BBB more easily. In this study, the effects of KYNA and KYNA-NH-GLUC on behavior and cortical activity were investigated in adult rats. The results show that (1) on intracerebroventricular application, the behavioral changes induced by KYNA and by KYNA-NH-GLUC are quite similar; (2) on intravenous administration, KYNA (25 mg/kg) has no effect on the somatosensory-evoked cortical potentials, whereas KYNA-NH-GLUC (25 mg/kg) causes transient but appreciable reductions in the amplitudes of the evoked responses within 5 min after application; and (3) the results of in vitro studies demonstrated that both KYNA and KYNA-NH-GLUC reduced the amplitudes of the field excitatory postsynaptic potentials (fEPSPs). These observations suggest that the two compounds have similar effects, but that KYNA-NH-GLUC passes the BBB much more readily than does KYNA. These results imply that the conjugated NH-GLUC is of importance in the passage across the BBB.

Pathway-specific action of gamma-hydroxybutyric acid in sensory thalamus and its relevance to absence seizures

The systemic injection of gamma-hydroxybutyric acid (GHB) elicits spike and wave discharges (SWDs), the EEG hallmark of absence seizures, and represents a well established, widely used pharmacological model of this nonconvulsive epilepsy. Despite this experimental use of GHB, as well as its therapeutic use in narcolepsy and its increasing abuse, however, the precise cellular mechanisms underlying the different pharmacological actions of this drug are still unclear. Because sensory thalamic nuclei play a key role in the generation of SWDs and sleep rhythms, and because direct injection of GHB in the ventrobasal (VB) thalamus elicits SWDs, we investigated GHB effects on corticothalamic EPSCs and GABAergic IPSCs in VB thalamocortical (TC) neurons. GHB (250 microm-10 mm) reversibly decreased the amplitude of electrically evoked EPSCs and GABAA IPSCs via activation of GABAB receptors; however, approximately 60% of the IPSCs were insensitive to low (250 microm-1.0 mm) GHB concentrations. The putative GHB receptor antagonist NSC 382 applied alone had a number of unspecific effects, whereas it either had no action on, or further increased, the GHB-elicited effects on synaptic currents. Low GHB concentrations (250 microm) were also effective in increasing absence-like intrathalamic oscillations evoked by cortical afferent stimulation. These results indicate that low concentrations of GHB, similar to the brain concentrations that evoke SWDs in vivo, differentially affect excitatory and inhibitory synaptic currents in TC neurons and promote absence-like intrathalamic oscillations. Furthermore, the present data strengthen previous suggestions on the GHB mechanism of sleep promotion and will help focus future studies on the cellular mechanisms underlying its abuse.

Interactions between membrane conductances underlying thalamocortical slow-wave oscillations

Neurons of the central nervous system display a broad spectrum of intrinsic electrophysiological properties that are absent in the traditional "integrate-and-fire" model. A network of neurons with these properties interacting through synaptic receptors with many time scales can produce complex patterns of activity that cannot be intuitively predicted. Computational methods, tightly linked to experimental data, provide insights into the dynamics of neural networks. We review this approach for the case of bursting neurons of the thalamus, with a focus on thalamic and thalamocortical slow-wave oscillations. At the single-cell level, intrinsic bursting or oscillations can be explained by interactions between calcium- and voltage-dependent channels. At the network level, the genesis of oscillations, their initiation, propagation, termination, and large-scale synchrony can be explained by interactions between neurons with a variety of intrinsic cellular properties through different types of synaptic receptors. These interactions can be altered by neuromodulators, which can dramatically shift the large-scale behavior of the network, and can also be disrupted in many ways, resulting in pathological patterns of activity, such as seizures. We suggest a coherent framework that accounts for a large body of experimental data at the ion-channel, single-cell, and network levels. This framework suggests physiological roles for the highly synchronized oscillations of slow-wave sleep.

From molecules to networks: cortical/subcortical interactions in the pathophysiology of idiopathic generalized epilepsy

Generalized epilepsy involves abnormally synchronized activity in large-scale neuronal networks. Burst firing of action potentials is a potent mechanism for increasing neural synchrony and is thought to enhance cortical and thalamic rhythmic network activity. Absence seizures, a form of generalized epilepsy, occur in children as brief 5- to 10-s periods of behavioral arrest associated with massive 3- to 4-Hz spike-wave discharges in cortical and thalamic networks. Prior research has shown that enhanced burst firing may be crucial for the transition from normal to epileptic activity. Can enhanced burst firing in one region of the nervous system, such as the cortex, transform the entire thalamocortical network from normal activity to spike-and-wave seizures? Enhanced burst firing in corticothalamic neurons may increase gamma-aminobutyric acid-B (GABAB) receptor activation in the thalamus, leading to the slower, more synchronous oscillations seen in spike-and-wave seizures. Does "generalized" spike-wave activity homogeneously involve the entire brain, or are there crucial nodes that are more important than others for the generation and behavioral manifestations of generalized seizures? Animal and human data suggest that so-called generalized seizures involve selective thalamocortical networks while sparing others. A greater understanding of these molecular and network mechanisms will ultimately lead to improved targeted therapies for generalized epilepsy.

Thalamocortical oscillations in a genetic model of absence seizures

We used field potential recordings in an in vitro thalamocortical slice preparation to compare the rhythmic oscillations generated by reciprocally connected networks of the thalamus and cerebral cortex obtained from epileptic (> 160 days old) WAG/Rij and age-matched, nonepileptic control (NEC) rats. To increase neuronal excitability, and thus to elicit spontaneous field potential activity in vitro, we applied medium containing: (i) zero [Mg2+]; (ii) high [K+] (8.25 mm); or (iii) low concentrations of the K+ channel blocker 4-aminopyridine (4AP, 0.5-1 micro m). Of these procedures, only the last was effective in triggering oscillatory activity that depended on the type of tissue. Thus, during 4AP application: (i) sequences of fast (intraburst frequency 9.5-16.1 Hz) and slower (5-8.9 Hz) field potential oscillations (FPOs) were recorded in WAG/Rij slices (n = 23), but (ii) only fast FPOs were seen in NEC slices (n = 7). Slower FPOs in WAG/Rij slices reflected a larger degree of thalamocortical synchronization than fast FPOs, and disappeared after surgical separation of cortex and thalamus (n = 5); under these conditions fast FPOs continued to occur in thalamus only. In addition, fast and slower FPOs disappeared in all areas of the WAG/Rij slice during thalamic application of the excitatory amino acid receptor antagonist kynurenic acid (n = 3), while fast FPOs continued to occur in thalamus when kynurenic acid was applied to the cortex (n = 4). Bath application of the N-methyl-D-aspartic acid (NMDA) receptor antagonist 3,3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonate (CPP) abolished slower FPOs in WAG/Rij cortex and thalamus (n = 6) without infuencing fast FPOs recorded in WAG/Rij (n = 6) or NEC slices (n = 4). Moreover, cortical application of CPP (n = 6) abated slower FPOs although they persisted following CPP application to the thalamus (n = 7). Our data demonstrate that highly synchronized, slower FPOs can occur during 4AP application in WAG/Rij but not in NEC slices. This activity, which may represent an in vitro hallmark of thalamocortical epileptogenicity, requires the function of reciprocally connected thalamic and cortical networks and depends on cortical NMDA receptor-mediated mechanisms.