Review of the role of inhibitory neurons in chronic epileptic foci induced by intracerebral tetanus toxin

Ionotropic GABA and Glutamate Receptor Mutations and Human Neurologic Diseases

The advent of whole exome/genome sequencing and the technology-driven reduction in the cost of next-generation sequencing as well as the introduction of diagnostic-targeted sequencing chips have resulted in an unprecedented volume of data directly linking patient genomic variability to disorders of the brain. This information has the potential to transform our understanding of neurologic disorders by improving diagnoses, illuminating the molecular heterogeneity underlying diseases, and identifying new targets for therapeutic treatment. There is a strong history of mutations in GABA receptor genes being involved in neurologic diseases, particularly the epilepsies. In addition, a substantial number of variants and mutations have been found in GABA receptor genes in patients with autism, schizophrenia, and addiction, suggesting potential links between the GABA receptors and these conditions. A new and unexpected outcome from sequencing efforts has been the surprising number of mutations found in glutamate receptor subunits, with the GRIN2A gene encoding the GluN2A N-methyl-d-aspartate receptor subunit being most often affected. These mutations are associated with multiple neurologic conditions, for which seizure disorders comprise the largest group. The GluN2A subunit appears to be a locus for epilepsy, which holds important therapeutic implications. Virtually all α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor mutations, most of which occur within GRIA3, are from patients with intellectual disabilities, suggesting a link to this condition. Similarly, the most common phenotype for kainate receptor variants is intellectual disability. Herein, we summarize the current understanding of disease-associated mutations in ionotropic GABA and glutamate receptor families, and discuss implications regarding the identification of human mutations and treatment of neurologic diseases.

Proconvulsant actions of intrahippocampal botulinum neurotoxin B in the rat

Botulinum neurotoxins (BoNTs) may affect the excitability of brain circuits by inhibiting neurotransmitter release at central synapses. There is evidence that local delivery of BoNT serotypes A and E, which target SNAP-25, a component of the release machinery specific to excitatory synapses, can inhibit seizure generation. BoNT serotype B (BoNT/B) targets VAMP2, which is expressed in both excitatory and inhibitory terminals. Here we assessed the effects of unilateral intrahippocampal infusion of BoNT/B in the rat on intravenous pentylenetetrazol (PTZ) seizure thresholds, and on the expression of spontaneous behavioral and electrographic seizures. Infusion of BoNT/B (500 and 1,000 unit) by convection-enhanced delivery caused a reduction in myoclonic twitch and clonic seizure thresholds in response to intravenous PTZ beginning about 6 days after the infusion. Handling-evoked and spontaneous convulsive seizures were observed in many BoNT/B-treated animals but not in vehicle-treated controls. Spontaneous electrographic seizure discharges were recorded in the dentate gyrus of animals that received local BoNT/B infusion. In addition, there was an increased frequency of interictal epileptiform spikes and sharp waves at the same recording site. BoNT/B-treated animals also exhibited tactile hyperresponsivity in comparison with vehicle-treated controls. This is the first demonstration that BoNT/B causes a delayed proconvulsant action when infused into the hippocampus. Local infusion of BoNT/B could be useful as a focal epilepsy model.

Neural mass activity, bifurcations, and epilepsy

In this letter, we propose a general framework for studying neural mass models defined by ordinary differential equations. By studying the bifurcations of the solutions to these equations and their sensitivity to noise, we establish an important relation, similar to a dictionary, between their behaviors and normal and pathological, especially epileptic, cortical patterns of activity. We then apply this framework to the analysis of two models that feature most phenomena of interest, the Jansen and Rit model, and the slightly more complex model recently proposed by Wendling and Chauvel. This model-based approach allows us to test various neurophysiological hypotheses on the origin of pathological cortical behaviors and investigate the effect of medication. We also study the effects of the stochastic nature of the inputs, which gives us clues about the origins of such important phenomena as interictal spikes, interictal bursts, and fast onset activity that are of particular relevance in epilepsy.

Neural representations of the sense of self

The brain constructs representations of what is sensed and thought about in the form of nerve impulses that propagate in circuits and network assemblies (Circuit Impulse Patterns, CIPs). CIP representations of which humans are consciously aware occur in the context of a sense of self. Thus, research on mechanisms of consciousness might benefit from a focus on how a conscious sense of self is represented in brain. Like all senses, the sense of self must be contained in patterns of nerve impulses. Unlike the traditional senses that are registered by impulse flow in relatively simple, pauci-synaptic projection pathways, the sense of self is a system- level phenomenon that may be generated by impulse patterns in widely distributed complex and interacting circuits. The problem for researchers then is to identify the CIPs that are unique to conscious experience. Also likely to be of great relevance to constructing the representation of self are the coherence shifts in activity timing relations among the circuits. Consider that an embodied sense of self is generated and contained as unique combinatorial temporal patterns across multiple neurons in each circuit that contributes to constructing the sense of self. As with other kinds of CIPs, those representing the sense of self can be learned from experience, stored in memory, modified by subsequent experiences, and expressed in the form of decisions, choices, and commands. These CIPs are proposed here to be the actual physical basis for conscious thought and the sense of self. When active in wakefulness or dream states, the CIP representations of self act as an agent of the brain, metaphorically as an avatar. Because the selfhood CIP patterns may only have to represent the self and not directly represent the inner and outer worlds of embodied brain, the self representation should have more degrees of freedom than subconscious mind and may therefore have some capacity for a free-will mind of its own. S everal lines of evidence for this theory are reviewed. Suggested new research includes identifying distinct combinatorially coded impulse patterns and their temporal coherence shifts in defined circuitry, such as neocortical microcolumns. This task might be facilitated by identifying the micro-topography of field-potential oscillatory coherences among various regions and between different frequencies associated with specific conscious mentation. Other approaches can include identifying the changes in discrete conscious operations produced by focal trans-cranial magnetic stimulation.

Studying network mechanisms using intracranial stimulation in epileptic patients

Patients suffering from focal drug-resistant epilepsy who are explored using intracranial electrodes allow to obtain data of exceptional value for studying brain dynamics in correlation with pathophysiological and cognitive processes. Direct electrical stimulation (DES) of cortical regions and axonal tracts in those patients elicits a number of very specific perceptual or behavioral responses, but also abnormal responses due to specific configurations of epileptic networks. Here, we review how anatomo-functional brain connectivity and epilepsy network mechanisms can be assessed from DES responses measured in patients. After a brief summary of mechanisms of action of brain electrical stimulation, we recall the conceptual framework for interpreting DES results in the context of brain connectivity and review how DES can be used for the characterization of functional networks, the identification of the seizure onset zone, the study of brain plasticity mechanisms, and the anticipation of epileptic seizures. This pool of exceptional data may be underexploited by fundamental research on brain connectivity and leaves much to be learned.

Mapping cognitive function

Cognitive functions are fundamental to being human. Although tremendous progress has been made in the science of cognition using neuroimaging, the clinical applications of neuroimaging are just beginning to be realized. This article focuses on selected technologies, analysis techniques, and applications that have, or will soon have, direct clinical impact. The authors discuss how cognition can be imaged using MR imaging, functional MR imaging, positron emission tomography, magnetoencephalography and electroencephalography, and MR imaging diffusion tensor imaging. A unifying theme of this article is the concept that a more complete understanding of cognition only comes through integration of multimodal structural and functional imaging technologies.

Enhanced synaptic excitation-inhibition ratio in hippocampal interneurons of rats with temporal lobe epilepsy

A common feature of all epileptic syndromes is the repetitive occurrence of pathological patterns of synchronous neuronal activity, usually combined with increased neuronal discharge rates. Inhibitory interneurons of the hippocampal formation control both neuronal synchronization as well as the global level of activity and are therefore of crucial importance for epilepsy. Recent evidence suggests that changes in synaptic inhibition during temporal lobe epilepsy are rather specific, resulting from selective death or alteration of interneurons in specific hippocampal layers. Hence, epilepsy-induced changes have to be analysed separately for different types of interneurons. Here, we focused on GABAergic neurons located at the border between stratum radiatum and stratum lacunosum-moleculare of hippocampal area CA1 (SRL interneurons), which are included in feedforward inhibitory circuits. In chronically epileptic rats at 6-8 months after pilocarpine-induced status epilepticus, frequencies of spontaneous and miniature inhibitory postsynaptic currents were reduced, yielding an almost three-fold increase in excitation-inhibition ratio. Consistently, action potential frequency of SRL interneurons was about two-fold enhanced. Morphological alterations of the interneurons indicate that these functional changes were accompanied by remodelling of the local network, probably resulting in a loss of functional inhibitory synapses without conceivable cell death. Our data indicate a strong increase in activity of interneurons in dendritic layers of the chronically epileptic CA1 region. This alteration may enhance feedforward inhibition and rhythmogenesis and--together with specific changes in other interneurons--contribute to seizure susceptibility and pathological synchronization.

Dysfunction of synaptic inhibition in epilepsy associated with focal cortical dysplasia

Focal cortical dysplasia (FCD) is a common and important cause of medically intractable epilepsy. In patients with temporal lobe epilepsy and in several animal models, compromised neuronal inhibition, mediated by GABA, contributes to seizure genesis. Although reduction in GABAergic interneuron density has been reported in FCD tissue samples, there is little available information on the resulting physiological changes in synaptic inhibition and the potential contribution of these changes to epileptogenesis in the dysplastic human brain. Using visualized whole-cell patch-clamp recordings from identified neurons in tissue slices obtained from patients with FCD, we demonstrate that GABAA-receptor-mediated inhibition is substantially altered in regions of dysplasia. These alterations include a significant reduction in IPSC frequency and a potentially compensatory decrease in transporter-mediated GABA reuptake function; the latter is marked by a significant increase in the decay-time constant for evoked and spontaneous IPSCs and a lack of effect of the GABA transport-inhibitor 1-[2([(diphenylmethylene)imino]oxy)ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride on IPSC kinetics. Immunohistochemical staining revealed a scattering of GABAergic interneurons across dysplastic cortex and striking reductions in GABA transporter expression. Together, these results suggest that profound alterations in GABA-mediated synaptic inhibition play an essential role in the process of epileptogenesis in patients with FCD.

The neurophysiological validation of the hyperpolarization theory of internal inhibition

The experiments in conscious non-immobilized rabbits showed that cessation of the reactions without reinforcement (elaboration of the internal inhibition) is accompanied by an enhanced phasic state, by alternation of activation and inhibition of neuron firing, and by the corresponding slow potential oscillation (SPO). These changes can be either localized, predominantly in the structures of conditioned stimulus, or, under enhancement of the inhibitory state, generalized in the brain structures. On the basis of our experience and published data, it is concluded that the above event results from relative enhancement of the inhibitory hyperpolarizing processes due to increase in reactivity of the inhibitory systems to stimulus, which acquires inhibitory properties during learning. Changes in the excitability and reactivity of neuron populations appearing during enhancement of the hyperpolarizing inhibition, and differing in the various brain structures, play an active role in the execution of the main function of the internal inhibition: limitation of excitation transmission to the effectors. An inhibitory mediator gamma aminobutyric acid (GABA) is of great importance in inhibiting the excitation in response to the stimulus which lost its biological significance. These experimental data and their interpretation in the light of published data give the basis for the development of the hyperpolarization theory of internal inhibition.

Adenoviral clostridial light chain gene-based synaptic inhibition through neuronal synaptobrevin elimination

Clostridial neurotoxins have assumed increasing importance in clinical application. The toxin's light chain component (LC) inhibits synaptic transmission by digesting vesicle-docking proteins without directly altering neuronal health. To study the properties of LC gene expression in the nervous system, an adenoviral vector containing the LC of tetanus toxin (AdLC) was constructed. LC expressed in differentiated neuronal PC12 cells was shown to induce time- and concentration-dependent digestion of mouse brain synaptobrevin in vitro as compared to control transgene products. LC gene expression in the rat lumbar spinal cord disrupted hindlimb sensorimotor function in comparison to control vectors as measured by the Basso-Beattie-Bresnahan (BBB) scale (P<0.001) and rotarod assay (P<0.003). Evoked electromyography (EMG) showed increased stimulus threshold and decreased response current amplitude in LC gene-transferred rats. At the peak of functional impairment, neither neuronal TUNEL staining nor reduced motor neuron density could be detected. Spontaneous functional recovery was observed to parallel the cessation of LC gene expression. These results suggest that light chain gene delivery within the nervous system may provide a nondestructive means for focused neural inhibition to treat a variety of disorders related to excessive synaptic activity, and prove useful for the study of neural circuitry.

The tetanus toxin model of chronic epilepsy

In experimental models of epilepsy, single and recurrent seizures are often used in an attempt to determine the effects of the seizures themselves on mammalian brain function. These models attempt to emulate as many features as possible of their human disease counterparts without many of the confounding factors such as underlying disease processes and medication effects. Numerous models have been used in the past to address different questions. Nevertheless, the basic questions are often the same: 1. Do seizures cause long-term damage? 2. Do seizures predispose to chronic epilepsy (epileptogenesis), that is long-term spontaneous repetitive seizures? 3. Are these results developmentally regulated? 4. Are the underlying mechanisms of epileptogenesis and brain damage related? In pursuing these questions, the goal is to determine how seizures exert their effects and to minimize any side effects from the methods employed to induce the seizures themselves. This requires a detailed characterization of the methods used to induce seizures. In this chapter, we will review the literature regarding the tetanus toxin model of chronic epilepsy with regard to its mechanisms of action, clinical comparisons, how it is experimentally implemented and the results obtained thus far. These results will be compared to other models of chronic epilepsy in order to make generalizations about the effects of repetitive seizures in adult and early life. At this time, it appears that repetitive seizures cause long-term changes in learning ability and may cause a predisposition to chronic seizures at all ages. In younger animals, both features of learning impairment and epilepsy are not typically associated with cell loss as they are in adult animals. At all ages, some form of synaptic reorganization has been demonstrated to occur.

Tetanus toxin induces long-term changes in excitation and inhibition in the rat hippocampal CA1 area

Intrahippocampal tetanus toxin induces a period of chronic recurrent limbic seizures in adult rats, associated with a failure of inhibition in the hippocampus. The rats normally gain remission from their seizures after 6-8 weeks, but show persistent cognitive impairment. In this study we assessed which changes in cellular and network properties could account for the enduring changes in this model, using intracellular and extracellular field recordings in hippocampal slices from rats injected with tetanus toxin or vehicle, 5 months previously. In CA1 pyramidal neurones from toxin-injected rats, the slope of the action potential upstroke was reduced by 32%, the fast afterhyperpolarisation by 32% and the slow afterhyperpolarisation by 54%, suggesting changes in voltage-dependent conductances. The excitatory postsynaptic potential slope was reduced by 60% and the population synaptic potential slope was reduced at all stimulus intensities, suggesting a reduced afferent input in CA1. Paired-pulse stimulation showed an increase of the excitability ratio and an increase of cellular excitability only for the second pulse, suggesting a reduced inhibition. The polysynaptic inhibitory postsynaptic potential was reduced by 34%, whereas neither the inhibitory postsynaptic potential at subthreshold stimulus intensities,nor the pharmacologically isolated monosynaptic inhibitory postsynaptic potential were different in toxin-injected rats, suggesting a reduced synaptic excitation of interneurones. Stratum radiatum stimuli in toxin-injected rats, and not in controls, evoked antidromic activation of CA1 neurones, demonstrating axonal sprouting into areas normally devoid of CA1 pyramidal cell axons.We conclude that this combination of enduring changes in cellular and network properties, both pro-epileptic (increased recurrent excitatory connectivity, reduced recurrent inhibition and reduced afterhyperpolarisations) and anti-epileptic (impaired firing and reduced excitation), reaches a balance that allows remission of seizures, perhaps at the price of persistent cognitive impairment.

Do fits really beget fits? The effect of previous epileptic activity on the subsequent induction of the tetanus toxin model of limbic epilepsy in the rat

The effect of pretreatment with either tetanus toxin (in ventral hippocampus) or kainic acid (into dorsal hippocampus, with or without suppression of seizures by phenobarbital) on the subsequent development of epilepsy in rats injected with tetanus toxin (into ventral hippocampus) has been studied. Both treatments advanced the timing of the development of the subsequent epilepsy by a few days but did not affect the severity of the syndrome. The fits stopped after 3 weeks in all the rats but recurred in 6 of 20 of those given kainic acid, with or without phenobarbital, but not in those given only tetanus toxin. It is concluded that while fits make the brain more sensitive to a further epileptogenic stimulus they do not themselves increase their severity or persistence. It is the destruction of the CA3/4 area of the hippocampus which results in this advance and in the predisposition to permanent epilepsy.

K(+) channel expression distinguishes subpopulations of parvalbumin- and somatostatin-containing neocortical interneurons

Kv3.1 and Kv3.2 K(+) channel proteins form similar voltage-gated K(+) channels with unusual properties, including fast activation at voltages positive to -10 mV and very fast deactivation rates. These properties are thought to facilitate sustained high-frequency firing. Kv3.1 subunits are specifically found in fast-spiking, parvalbumin (PV)-containing cortical interneurons, and recent studies have provided support for a crucial role in the generation of the fast-spiking phenotype. Kv3.2 mRNAs are also found in a small subset of neocortical neurons, although the distribution of these neurons is different. We raised antibodies directed against Kv3.2 proteins and used dual-labeling methods to identify the neocortical neurons expressing Kv3.2 proteins and to determine their subcellular localization. Kv3.2 proteins are prominently expressed in patches in somatic and proximal dendritic membrane as well as in axons and presynaptic terminals of GABAergic interneurons. Kv3.2 subunits are found in all PV-containing neurons in deep cortical layers where they probably form heteromultimeric channels with Kv3.1 subunits. In contrast, in superficial layer PV-positive neurons Kv3.2 immunoreactivity is low, but Kv3.1 is still prominently expressed. Because Kv3.1 and Kv3.2 channels are differentially modulated by protein kinases, these results raise the possibility that the fast-spiking properties of superficial- and deep-layer PV neurons are differentially regulated by neuromodulators. Interestingly, Kv3. 2 but not Kv3.1 proteins are also prominent in a subset of seemingly non-fast-spiking, somatostatin- and calbindin-containing interneurons, suggesting that the Kv3.1-Kv3.2 current type can have functions other than facilitating high-frequency firing.

The silent period after magnetic brain stimulation in generalized tetanus

The cortical silent period has not previously been studied in tetanus. Transcranial magnetic brain stimulation in a patient with generalized tetanus revealed enlarged electromyographic (EMG) responses and absence or reduction of the late phase of EMG silence following the motor evoked potential in sternomastoid and biceps brachii muscles. Following clinical recovery, the silent period returned to normal. This observation is interpreted as evidence of impaired inhibitory mechanisms at multiple levels of the nervous system, including the cortex, in generalized tetanus.

Function of specific K(+) channels in sustained high-frequency firing of fast-spiking neocortical interneurons

Fast-spiking GABAergic interneurons of the neocortex and hippocampus fire high-frequency trains of brief action potentials with little spike-frequency adaptation. How these striking properties arise is unclear, although recent evidence suggests K(+) channels containing Kv3.1-Kv3.2 proteins play an important role. We investigated the role of these channels in the firing properties of fast-spiking neocortical interneurons from mouse somatosensory cortex using a pharmacological and modeling approach. Low tetraethylammonium (TEA) concentrations (</=1 mM), which block only a few known K(+) channels including Kv3.1-Kv3.2, profoundly impaired action potential repolarization and high-frequency firing. Analysis of the spike trains evoked by steady depolarization revealed that, although TEA had little effect on the initial firing rate, it strongly reduced firing frequency later in the trains. These effects appeared to be specific to Kv3.1 and Kv3.2 channels, because blockade of dendrotoxin-sensitive Kv1 channels and BK Ca(2+)-activated K(+) channels, which also have high TEA sensitivity, produced opposite or no effects. Voltage-clamp experiments confirmed the presence of a Kv3.1-Kv3.2-like current in fast-spiking neurons, but not in other interneurons. Analysis of spike shape changes during the spike trains suggested that Na(+) channel inactivation plays a significant role in the firing-rate slowdown produced by TEA, a conclusion that was supported by computer simulations. These findings indicate that the unique properties of Kv3.1-Kv3.2 channels enable sustained high-frequency firing by facilitating the recovery of Na(+) channel inactivation and by minimizing the duration of the afterhyperpolarization in neocortical interneurons.

Newly formed excitatory pathways provide a substrate for hyperexcitability in experimental temporal lobe epilepsy

Temporal lobe epilepsy (TLE) in humans and animals is associated with axonal sprouting of glutamatergic neurons and neosynaptogenesis in the hippocampal formation. We examined whether this plasticity of excitatory pathways contributes to an increased level of glutamatergic excitation in the CA1 region of rats experiencing chronic spontaneous limbic seizures following kainic acid or pilocarpine treatment. In chronic cases, we report an extensive axonal sprouting of CA1 pyramidal neurons, with many axonal branches entering the pyramidal cell layer and stratum radiatum, regions that are not innervated by axonal collaterals of CA1 pyramidal neurons in control animals. Concurrently with this anatomical reorganization, a large increase of the spontaneous glutamatergic drive is observed in the dendrites and somata of CA1 pyramidal cells. Furthermore, electrical activation of the reorganized CA1 associational pathway evokes epileptiform bursts in CA1 pyramidal cells. These findings suggest that reactive plasticity could contribute to the hyperexcitability of CA1 pyramidal neurons and to the propagation of seizures in these two models of TLE.

Effects of tetanus toxin on functional inhibition after injection in separate cortical areas in rat

Tetanus Toxin is widely used as a model of chronic focal epilepsy and is assumed to act by blocking neurotransmitter release with high selectivity for inhibitory synapses. However, the exact mechanisms are not fully understood, since, e.g., GABA release is only temporarily decreased although epileptiform activity persists pointing towards a change in the interplay of excitation and inhibition. Furthermore there have been reports about different effects of tetanus toxin after injection in separate brain areas. Therefore, we investigated the functional inhibition after injecting tetanus toxin either in the motor or sensory cortex of adult rats by using a paired-pulse paradigm as a measure of excitatory and inhibitory drive. Tetanus toxin injection into the motor cortex (n=10) induced a marked, long-lasting reduction in inhibition which was highly significant in most parts of the injected cortical area. Injections into the sensory cortex, however, showed less marked changes in inhibition which were more widespread and significant only in 3 of 14 animals injected. These results give further evidence for a prominent effect of tetanus toxin on functional inhibition and strengthen the idea of a differential effect in separate cortical areas. They may be accounted for by the different cytoarchitecture of cortical areas with variable inhibitory and excitatory intracortical connections.

Interneurones are not so dormant in temporal lobe epilepsy: a critical reappraisal of the dormant basket cell hypothesis

One axiom at the basis of epilepsy research is that there exists an imbalance between excitation and inhibition. This abnormality can be achieved by an increase of excitation on principal cells, a decreased inhibition (i.e. disinhibition) or both. This review focuses on dysfunction of inhibition, and in particular on the 'dormant basket cell hypothesis'. This hypothesis states that, (1) interneurones are functionally disconnected from excitatory afferents, resulting in hyperexcitability of principal neurones and loss of paired pulse inhibition, (2) when properly activated, interneurones can still perform their task, i.e. suppress epileptiform activity and restore paired pulse inhibition. The aim of this review is to discuss the evidence in support of the 'dormant basket cell hypothesis'. We will first discuss the rationale underlying the hypothesis and the criteria needed to validate the hypothesis. We will then show that, (1) the key experimental data offered in support of the hypothesis (Bekenstein and Lothman, 1993. Dormancy of inhibitory interneurones in a model of temporal lobe epilepsy. Science 259, 97-100; Sloviter, 1991. Permanently altered hippocampal structure, excitability, and inhibition after experimental status epilepticus in the rat: the 'dormant basket cell' hypothesis and its relevance to temporal lobe epilepsy. Hippocampus 1, 41-66) are difficult to interpret, and (2) recent recordings from interneurones in epileptic tissue argue against the hypothesis. The 'dormant basket cell hypothesis' is then discussed in the broader context of disinhibition.