Reciprocal up- and down-regulation of BDNF mRNA in tetanus toxin-induced epileptic focus and inhibitory surround in cerebral cortex

High frequency spectral changes induced by single-pulse electric stimulation: Comparison between physiologic and pathologic networks

OBJECTIVE

To investigate functional coupling between brain networks using spectral changes induced by single-pulse electric stimulation (SPES).

METHOD

We analyzed 20 patients with focal epilepsy, implanted with depth electrodes. SPES was applied to each pair of adjacent contacts, and responses were recorded from all other contacts. The mean response amplitude value was quantified in three time-periods after stimulation (10-60, 60-255, 255-500ms) for three frequency-ranges (Gamma, Ripples, Fast-Ripples), and compared to baseline. A total of 30,755 responses were analyzed, taking into consideration three dichotomous pairs: stimulating in primary sensory areas (S1-V1) vs. outside them, to test the interaction in physiologic networks; stimulating in seizure onset zone (SOZ) vs. non-SOZ, to test pathologic interactions; recording in default mode network (DMN) vs. non-DMN.

RESULTS

Overall, we observed an early excitation (10-60ms) and a delayed inhibition (60-500ms). More specifically, in the delayed period, stimulation in S1-V1 produced a higher gamma-inhibition in the DMN, while stimulation in the SOZ induced a higher inhibition in the epilepsy-related higher frequencies (Ripples and Fast-Ripples).

CONCLUSION

Physiologic and pathologic interactions can be assessed using spectral changes induced by SPES.

SIGNIFICANCE

This is a promising method for connectivity studies in patients with drug-resistant focal epilepsy.

Reduction of epileptiform activity through local valproate-implants in a rat neocortical epilepsy model

PURPOSE

Pharmacotherapy of epilepsies is limited due to low concentrations at epileptogenic foci, side effects of high systemic doses and that some potentially efficient substances do not pass the blood-brain barrier. To overcome these limitations, we tested the efficacy of local valproate (VPA)-containing polymer implants in a model of necocortical injected tetanus toxin (TeT) in the rat.

METHODS

Tetanus toxin was injected intracortically and cobalt (II) chloride (CoCl2) was applied on the cortical surface. Video-electrocorticography recordings with intracortical electrodes were performed. VPA-containing polymers were implanted above the cortical focus. Antiepileptic effects were evaluated as reductions of epileptiform potentials (EPs) per hour in comparison to saline (NaCl)-containing polymer implants.

RESULTS

Triple 50ng TeT injections plus CoCl2 application (20/10mg) showed consistent EPs. NaCl-implanted animals (n=6) showed a mean of 10.5EPs/h after the first week, the EP frequency increased to 53.5EPs/h after the second week. VPA-implant animals (n=5) showed a reduction in EP frequency from 71.6 to 4.8EPs/h after the second week. The EP frequency after the second week was higher in the NaCl-implanted animals than in the VPA-implanted (p=0.0303). The mean EPs/h increase in NaCl-implanted rats (+42.9EPs/h) was different (p=0.0087) from the mean EPs/h decrease in VPA-implanted rats (-66.8EPs/h).

CONCLUSION

Despite former publications no clear seizures could be reproduced but it was possible to establish focal EPs, which proved to be a reliable marker for epileptic activity. Local antiepileptic therapy with VPA has shown efficacy in decreasing EP frequency.

Preliminary Findings of Single- and Multifocused Epileptiform Discharges in Nonepileptic Psychiatric Patients

Epileptiform discharges (EDs) in nonepileptic populations remain controversial as to their role in psychopathology. Previous studies have unsuccessfully attempted to correlate specific waveforms of EDs, defined by duration and morphology, with broad diagnostic categories such as depression and anxiety. These diagnostic categories often include heterogeneous patient populations, with potentially divergent biological underpinnings of clinical presentation. This study examined epileptiform activities as a single phenomenon, identifying the relationships between distribution patterns of EDs and endorsement of clinical symptoms across affective, cognitive, and somatic domains. In a sample of 71 nonepileptic psychiatric patients, those with EDs appearing in homologous electrode pairs endorsed significantly fewer symptoms related to affective deregulation. These patients were also significantly less likely to endorse a history of severe symptomatology, including suicidal ideation/previous attempt, self-injurious behavior, psychoses or dissociation, and previous psychiatric hospitalization. Conversely, patients with isolated EDs focused to a single brain region endorsed greater affective deregulation and severe clinical symptoms. These findings offer new possibilities regarding the potentially protective role that EDs may play when distributed across hemispheres, particularly in light of recent theories exploring functional connectivity of neuronal networks.

Locally applied valproate enhances survival in rats after neocortical treatment with tetanus toxin and cobalt chloride

Purpose. In neocortical epilepsies not satisfactorily responsive to systemic antiepileptic drug therapy, local application of antiepileptic agents onto the epileptic focus may enhance treatment efficacy and tolerability. We describe the effects of focally applied valproate (VPA) in a newly emerging rat model of neocortical epilepsy induced by tetanus toxin (TeT) plus cobalt chloride (CoCl₂). Methods. In rats, VPA (n = 5) or sodium chloride (NaCl) (n = 5) containing polycaprolactone (PCL) implants were applied onto the right motor cortex treated before with a triple injection of 75 ng TeT plus 15 mg CoCl₂. Video-EEG monitoring was performed with intracortical depth electrodes. Results. All rats randomized to the NaCl group died within one week after surgery. In contrast, the rats treated with local VPA survived significantly longer (P < 0.01). In both groups, witnessed deaths occurred in the context of seizures. At least 3/4 of the rats surviving the first postoperative day developed neocortical epilepsy with recurrent spontaneous seizures. Conclusions. The novel TeT/CoCl₂ approach targets at a new model of neocortical epilepsy in rats and allows the investigation of local epilepsy therapy strategies. In this vehicle-controlled study, local application of VPA significantly enhanced survival in rats, possibly by focal antiepileptic or antiepileptogenic mechanisms.

Regulators of synaptic transmission: roles in the pathogenesis and treatment of epilepsy

Synaptic transmission is the communication between a presynaptic and a postsynaptic neuron, and the subsequent processing of the signal. These processes are complex and highly regulated, reflecting their importance in normal brain functioning and homeostasis. Sustaining synaptic transmission depends on the continuing cycle of synaptic vesicle formation, release, and endocytosis, which requires proteins such as dynamin, syndapin, synapsin, and synaptic vesicle protein 2A. Synaptic transmission is regulated by diverse mechanisms, including presynaptic modulators of synaptic vesicle formation and release, postsynaptic receptors and signaling, and modulators of neurotransmission. Neurotransmitters released presynaptically can bind to their postsynaptic receptors, the inhibitory γ-aminobutyric acid (GABA)ergic receptors or the excitatory glutamate receptors. Once released, glutamate activates a variety of postsynaptic receptors including α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-D-aspartate (NMDA), kainate, and metabotropic receptors. The activation of the receptors triggers downstream signaling cascades generating a vast array of effects, which can be modulated by a numerous auxiliary regulatory subunits. Moreover, different neuropeptides such as neuropeptide Y, brain-derived neurotrophic factor (BDNF), somatostatin, ghrelin, and galanin, act as regulators of diverse synaptic functions and along with the classic neurotransmitters. Abnormalities in the regulation of synaptic transmission play a critical role in the pathogenesis of numerous brain diseases, including epilepsy. This review focuses on the different mechanisms involved in the regulation of synaptic transmission, which may play a role in the pathogenesis of epilepsy: the presynaptic modulators of synaptic vesicle formation and release, postsynaptic receptors, and modulators of neurotransmission, including the mechanism by which drugs can modulate the frequency and severity of epileptic seizures.

How inhibition influences seizure propagation

Inhibitory neuron behaviour is of fundamental importance to epileptic pathophysiology. When inhibition is compromised, such as by GABAergic blockade (Curtis et al., 1970; Connors, 1984; Traub and Miles, 1991) or by shifts in GABAergic reversal potential (Huberfeld et al., 2007), epileptiform discharges occur far more readily. Other studies have shown enhanced inhibition in vivo in the surrounding cortical territories associated with both focal pathological and physiological activity (Prince and Wilder, 1967; Dichter and Spencer, 1969a,b; Goldensohn and Salazar, 1986; Traub and Miles, 1991; Liang and Jones, 1997; Liang et al., 1998; Schwartz and Bonhoeffer, 2001). This gave rise to the concept of an "inhibitory restraint". This concept can explain the often confusing anatomical reorganizations seen in chronically epileptic brains (Sloviter, 1987; Cossart et al., 2001), indicating which changes might be pro-epileptic, and which oppose the epileptic state. It also may explain key electrophysiological features of epileptic seizures. Here we describe current knowledge about the restraint, gleaned mainly from acute pharmacological experiments in animals, both in vivo and in vitro, and speculate how this may alter our understanding of human seizure activity in clinical practice. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

Synchrony in normal and focal epileptic brain: the seizure onset zone is functionally disconnected

Synchronization of local and distributed neuronal assemblies is thought to underlie fundamental brain processes such as perception, learning, and cognition. In neurological disease, neuronal synchrony can be altered and in epilepsy may play an important role in the generation of seizures. Linear cross-correlation and mean phase coherence of local field potentials (LFPs) are commonly used measures of neuronal synchrony and have been studied extensively in epileptic brain. Multiple studies have reported that epileptic brain is characterized by increased neuronal synchrony except possibly prior to seizure onset when synchrony may decrease. Previous studies using intracranial electroencephalography (EEG), however, have been limited to patients with epilepsy. Here we investigate neuronal synchrony in epileptic and control brain using intracranial EEG recordings from patients with medically resistant partial epilepsy and control subjects with intractable facial pain. For both epilepsy and control patients, average LFP synchrony decreases with increasing interelectrode distance. Results in epilepsy patients show lower LFP synchrony between seizure-generating brain and other brain regions. This relative isolation of seizure-generating brain underlies the paradoxical finding that control patients without epilepsy have greater average LFP synchrony than patients with epilepsy. In conclusion, we show that in patients with focal epilepsy, the region of epileptic brain generating seizures is functionally isolated from surrounding brain regions. We further speculate that this functional isolation may contribute to spontaneous seizure generation and may represent a clinically useful electrophysiological signature for mapping epileptic brain.

Characterization of the tetanus toxin model of refractory focal neocortical epilepsy in the rat

PURPOSE

To characterize in detail a model of focal neocortical epilepsy.

METHODS

Chronic focal epilepsy was induced by injecting 25-50 ng of tetanus toxin or vehicle alone (controls) into the motor neocortex of rats. EEG activity was recorded from electrodes implanted at the injection site, along with facial muscle electromyographic (EMG) activity and behavioral monitoring intermittently for up to 5 months in some animals. Drug responsiveness was assessed by using the antiepileptic drugs (AEDs) diazepam (DZP) and phenytoin (PHT) delivered systemically, while 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX), a competitive antagonist at AMPA receptors, was administered directly to the brain to investigate the potential benefits of focal drug delivery.

RESULTS

Tetanus toxin induced mild behavioral seizures that persisted indefinitely in all animals. EEG spiking activity, occurring up to 80% of the time, correlated with clinical seizures consisting of interrupted behavioral activity, rhythmic bilateral facial twitching, and periods of abrupt motor arrest. Seizures were refractory to systemic administration of DZP and PHT. However, focal delivery of NBQX to the seizure site reversibly reduced EEG and behavioral seizure activity without detectable side effects.

CONCLUSIONS

This study provides a long-term detailed characterisation of the tetanus toxin model. Spontaneous, almost continuous, well-tolerated seizures occur and persist, resembling those seen in neocortical epilepsy, including cortical myoclonus and epilepsia partialis continua. The seizures appear to be similarly resistant to conventional AEDs. The consistency, frequency, and clinical similarity of the seizures to refractory epilepsy in humans make this an ideal model for investigation of both mechanisms of seizure activity and new therapeutic approaches.

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.

Pathophysiology and functional consequences of human partial epilepsy: lessons from positron emission tomography studies

Positron emission tomography (PET) is a powerful clinical and research tool that, in the past two decades, has provided a great amount of novel data on the pathophysiology and functional consequences of human epilepsy. PET studies revealed cortical and subcortical brain dysfunction of a widespread brain circuitry, providing an unprecedented insight in the complex functional abnormalities of the epileptic brain. Correlation of metabolic and neuroreceptor PET abnormalities with electroclinical variables helped identify parts of this circuitry, some of which are directly related to primary epileptogenesis, while others, adjacent to or remote from the primary epileptic focus, may be secondary to longstanding epilepsy. PET studies have also provided detailed data on the functional anatomy of cognitive and behavioral abnormalities associated with epilepsy. PET, along with other neuroimaging modalities, can measure longitudinal changes in brain function attributed to chronic seizures as well as therapeutic interventions. This review demonstrates how development of more specific PET tracers and application of multimodality imaging by combining structural and functional neuroimaging with electrophysiological data can further improve our understanding of human partial epilepsy, and helps more effective application of PET in presurgical evaluation of patients with intractable seizures.

MRNA expression patterns and distribution of white matter neurons in dorsolateral prefrontal cortex of depressed patients differ from those in schizophrenia patients

BACKGROUND

Schizophrenia, bipolar illness, and major depressive disorder have distinct presentations, but share some common symptoms. Hence, some common cellular and molecular abnormalities may be identifiable in these disorders.

METHODS

We examined cell-specific markers in the dorsolateral prefrontal cortex of brains from 18 patients with bipolar or major depressive disorder, and 18 matched controls, using in situ hybridization histochemistry and staining for nicotinamide-dinucleotide phosphate-diaphorase (NADPH). The distribution of NADPH-positive interstitial cells of the white matter and the expression of the mRNA for the 67 KD form of glutamic acid decarboxylase (GAD(67)) had previously been shown to be altered in prefrontal cortex of schizophrenics. Other markers identifying glutamatergic neuronal populations were alpha-type II calcium/calmodulin dependent protein kinase (CAMKII-alpha), brain derived neurotrophic factor, (BDNF) and the putative transcription factor, T-brain-1 (TBR1).

RESULTS

Expression of GAD67 and the distribution of NADPH-positive cells in the white matter were not significantly altered in the dorsolateral prefrontal cortex of depressed subjects. Expression of CAMKII-alpha and TBR1 mRNAs was significantly increased in bipolar patients but not in major depressed patients, and there was a trend toward reduced BDNF expression in both groups. Abnormal patterns of gene expression and neuronal distribution in schizophrenics are markedly different from those in depressed patients.

CONCLUSIONS

The findings that TBR1 and CAMKII-alpha expression is increased only in bipolar patients suggests abnormalities of specific genes related to a major cortical cell type and its connectivity.

The role of cytokines and growth factors in seizures and their sequelae

Although the neuropathological changes caused by severe or repeated seizures have been well characterized, many questions about the molecular mechanisms involved remain unanswered. Neuronal cell death, reactive gliosis, enhanced neurogenesis, and axonal sprouting are four of the best-studied sequelae of seizures. In vitro, each of these pathological processes can be substantially influenced by soluble protein factors, including neurotrophins, cytokines, and growth factors. Furthermore, many of these proteins and their receptors are expressed in the adult brain and are up-regulated in response to neuronal activity and injury. We review the evidence that these intercellular signaling proteins regulate seizure activity as well as subsequent pathology in vivo. As nerve growth factor and brain derived neurotrophic factor are the best-studied proteins of this class, we begin by discussing the evidence linking these neurotrophins to epilepsy and seizure. More than a dozen additional cytokines, growth factors, and neurotrophins that have been examined in the context of epilepsy models are then considered. We discuss the effect of seizure on expression of cytokines and growth factors, and explore the regulation of seizure development and aftermath by exogenous application or antagonist perturbation of these proteins. The experimental evidence supports a role for these factors in each aspect of seizure and pathology, and suggests potential targets for future therapeutics.