Glutamate hyperexcitability and seizure-like activity throughout the brain and spinal cord upon relief from chronic glutamate receptor blockade in culture

Pentylenetetrazole: A review

Pentylenetetrazole (PTZ), a tetrazole derivative, is commonly used as a chemical agent to induce neurological disorders and replicate the characteristics of human epileptic seizures in animal models. This review offers a comprehensive analysis of the behavioral, neurophysiological, and neurochemical changes induced by PTZ. The epileptogenic and neurotoxic mechanisms of PTZ are associated with an imbalance between the GABAergic and glutamatergic systems. At doses exceeding 60 mg/kg, PTZ exerts its epileptic effects by non-competitively antagonizing GABAA receptors and activating NMDA receptors, resulting in an increased influx of cations such as Na+ and Ca2+. Additionally, PTZ promotes oxidative stress, microglial activation, and the synthesis of pro-inflammatory mediators, all of which are features characteristic of glutamatergic excitotoxicity. These mechanisms ultimately lead to epileptic seizures and neuronal cell death, which depend on the dosage and method of administration. The behavioral, electroencephalographic, and histological changes associated with PTZ further establish it as a valuable preclinical model for the study of epileptic seizures, owing to its simplicity, cost-effectiveness, and reproducibility.

Scoping review of disease-modifying effect of drugs in experimental epilepsy

Objective

Epilepsy affects ~50 million people worldwide causing significant medical, financial, and sociologic concerns for affected patients and their families. To date, treatment of epilepsy is primarily symptomatic management because few effective preventative or disease-modifying interventions exist. However, recent research has identified neurobiological mechanisms of epileptogenesis, providing new pharmacologic targets to investigate. The current scientific evidence remains scattered across multiple studies using different model and experimental designs. The review compiles different models of anti-epileptogenic investigation and highlights specific compounds with potential epileptogenesis-modifying experimental drugs. It provides a platform for standardization of future epilepsy research to allow a more robust compound analysis of compounds with potential for epilepsy prevention.

Methods

PubMed, Ovid MEDLINE, and Web of Science were searched from 2007 to 2021. Studies with murine models of epileptogenesis and explicitly detailed experimental procedures were included in the scoping review. In total, 51 articles were selected from 14,983 and then grouped by five core variables: (1) seizure frequency, (2) seizure severity, (3) spontaneous recurrent seizures (SRS), (4) seizure duration, and (5) mossy fiber sprouting (MFS). The variables were differentiated based on experimental models including methods of seizure induction, treatment schedule and timeline of data collection. Data was categorized by the five core variables and analyzed by converting original treatment values to units of percent of its respective control.

Results

Discrepancies in current epileptogenesis models significantly complicate inter-study comparison of potential anti-epileptogenic interventions. With our analysis, many compounds showed a potential to reduce epileptogenic characteristics defined by the five core variables. WIN55,212-2, aspirin, rapamycin, 1400W, and LEV + BQ788 were identified compounds with the potential of effective anti-epileptic properties.

Significance

Our review highlights the need for consistent methodology in epilepsy research and provides a novel approach for future research. Inconsistent experimental designs hinder study comparison, slowing the progression of treatments for epilepsy. If the research community can optimize and standardize parameters such as methods of seizure induction, administration schedule, sampling time, and aniMal models, more robust meta-analysis and collaborative research would follow. Additionally, some compounds such as rapamycin, WIN 55,212-2, aspirin, 1400W, and LEV + BQ788 showed anti-epileptogenic modulation across multiple variables. We believe they warrant further study both individually and synergistically.

Differential Effects of Human P301L Tau Expression in Young versus Aged Mice

The greatest risk factor for developing Alzheimer’s disease (AD) is increasing age. Understanding the changes that occur in aging that make an aged brain more susceptible to developing AD could result in novel therapeutic targets. In order to better understand these changes, the current study utilized mice harboring a regulatable mutant P301L human tau transgene (rTg(TauP301L)4510), in which P301L tau expression can be turned off or on by the addition or removal of doxycycline in the drinking water. This regulatable expression allowed for assessment of aging independent of prolonged mutant tau expression. Our results suggest that P301L expression in aged mice enhances memory deficits in the Morris water maze task. These behavioral changes may be due to enhanced late-stage tau pathology, as evidenced by immunoblotting and exacerbated hippocampal dysregulation of glutamate release and uptake measured by the microelectrode array technique. We additionally observed changes in proteins important for the regulation of glutamate and tau phosphorylation that may mediate these age-related changes. Thus, age and P301L tau interact to exacerbate tau-induced detrimental alterations in aged animals.

Proteomic profiling of the rat hippocampus from the kindling and pilocarpine models of epilepsy: potential targets in calcium regulatory network

Herein proteomic profiling of the rat hippocampus from the kindling and pilocarpine models of epilepsy was performed to achieve new potential targets for treating epileptic seizures. A total of 144 differently expressed proteins in both left and right hippocampi by two-dimensional electrophoresis coupled to matrix-assisted laser desorption-mass spectrometry were identified across the rat models of epilepsy. Based on network analysis, the majority of differentially expressed proteins were associated with Ca2+ homeostasis. Changes in ADP-ribosyl cyclase (ADPRC), lysophosphatidic acid receptor 3 (LPAR3), calreticulin, ubiquitin carboxyl-terminal hydrolase L1 (UCH-L1), synaptosomal nerve-associated protein 25 (SNAP 25) and transgelin 3 proteins were probed by Western blot analysis and validated using immunohistochemistry. Inhibition of calcium influx by 8-Bromo-cADP-Ribose (8-Br-cADPR) and 2-Aminoethyl diphenylborinate (2-APB) which act via the ADPRC and LPAR3, respectively, attenuated epileptic seizures. Considering a wide range of molecular events and effective role of calcium homeostasis in epilepsy, polypharmacy with multiple realistic targets should be further explored to reach the most effective treatments.

Stimulating aged brains with transcranial direct current stimulation: Opportunities and challenges

Ageing involves significant neurophysiological changes that are both systematic while at the same time exhibiting divergent trajectories across individuals. These changes underlie cognitive impairments in elderly while also affecting the response of aged brains to interventions like transcranial direct current stimulation (tDCS). While the cognitive benefits of tDCS are more variable in elderly, older adults also respond differently to stimulation protocols compared to young adults. The age-related neurophysiological changes influencing the responsiveness to tDCS remain to be addressed in-depth. We review and discuss the premise that, in comparison to the better calibrated brain networks present in young adults, aged systems perform further away from a homoeostatic set-point. We argue that this age-related neurophysiological deviation from the homoeostatic optimum extends the leeway for tDCS to modulate the aged brain. This promotes the potency of immediate tDCS effects to induce directional plastic changes towards the homoeostatic equilibrium despite the impaired plasticity induction in elderly. We also consider how age-related neurophysiological changes pose specific challenges for tDCS that necessitate proper adaptations of stimulation protocols. Appreciating the distinctive properties of aged brains and the accompanying adjustment of stimulation parameters can increase the potency and reliability of tDCS as a treatment avenue in older adults.

Experimental Models for the Discovery of Novel Anticonvulsant Drugs: Focus on Pentylenetetrazole-Induced Seizures and Associated Memory Deficits

Epilepsy is a chronic neurological disorder characterized by irregular, excessive neuronal excitability, and recurrent seizures that affect millions of patients worldwide. Currently, accessible antiepileptic drugs (AEDs) do not adequately support all epilepsy patients, with around 30% patients not responding to the existing therapies. As lifelong epilepsy treatment is essential, the search for new and more effective AEDs with an enhanced safety profile is a significant therapeutic goal. Seizures are a combination of electrical and behavioral events that can induce biochemical, molecular, and anatomic changes. Therefore, appropriate animal models are required to evaluate novel potential AEDs. Among the large number of available animal models of seizures, the acute pentylenetetrazole (PTZ)-induced myoclonic seizure model is the most widely used model assessing the anticonvulsant effect of prospective AEDs, whereas chronic PTZ-kindled seizure models represent chronic models in which the repeated administration of PTZ at subconvulsive doses leads to the intensification of seizure activity or enhanced seizure susceptibility similar to that in human epilepsy. In this review, we summarized the memory deficits accompanying acute or chronic PTZ seizure models and how these deficits were evaluated applying several behavioral animal models. Furthermore, major advantages and limitations of the PTZ seizure models in the discovery of new AEDs were highlighted. With a focus on PTZ seizures, the major biochemicals, as well as morphological alterations and the modulated brain neurotransmitter levels associated with memory deficits have been illustrated. Moreover, numerous medicinal compounds with concurrent anticonvulsant, procognitive, antioxidant effects, modulating effects on several brain neurotransmitters in rodents, and several newly developed classes of compounds applying computer-aided drug design (CADD) have been under development as potential AEDs. The article details the in-silico approach following CADD, which can be utilized for generating libraries of novel compounds for AED discovery. Additionally, in vivo studies could be useful in demonstrating efficacy, safety, and novel mode of action of AEDs for further clinical development.

Spreading Depolarization Facilitates the Transition of Neuronal Burst Firing from Interictal to Ictal State

The transition of neuronal burst firing from the interictal to ictal state contributes to seizure initiation in human temporal lobe epilepsy. The low-Mg²⁺ model of seizure is characterized by initial spontaneous interictal bursting events, which later developed into ictaform discharges. Both experimental and clinical studies point to a complex link between spreading depolarization (SD) and epileptiform field potentials (EFP), including SD-induced epileptic seizures. To investigate the mechanism of SD and EFP interactions, the effect of SD on the transition of interictal to ictal state in low-Mg²⁺ model of seizure was studied in the rat hippocampus in vitro. After the appearance of interictal activities, SD was elicited by local application of KCl. SD significantly increased the amplitude and duration of action potentials and after-hyperpolarization, and hyperpolarized the membrane potential. Furthermore, SD significantly increased the duration of interictal activities and the threshold potentials of interictal activities. In addition, SD significantly accelerated the transition from interictal to ictal state compared to the control tissues. Ictal activities after induction of SD exhibited a significantly longer duration. This study revealed that SD accelerates interictal-to-ictal transitions and facilitates development of ictaform discharges, possibly via the enhancement of neural synchronization, and points to the potential role of SD in seizure initiation.

Effects of Diclofenac Versus Meloxicam in Pentylenetetrazol-Kindled Mice

Epilepsy comes after stroke as the most common chronic neurological disorder worldwide. Inflammation enhances neuronal hyperexcitability that could provide a background setting for the development of epilepsy. The aim of this study was to assess the effect of valproate (VAL), diclofenac (DIC), meloxicam (MEL), VAL + MEL and VAL + DIC in pentylenetetrazol (PTZ) kindled mice. Seventy mice were randomly allocated into 7 equal groups; Control, PTZ, VAL, DIC, MEL, VAL + MEL and VAL + DIC groups. Kindling was induced by PTZ (40 mg/kg, i.p.) injection every other day for 17 days. The drugs were administered, 30 min before each PTZ injection till the end of the schedule. Seizure score, latency, duration and mortality rate were recorded in all groups. Tumor necrosis factor- α (TNF-α), interleukin-1β (IL-1β), malondialdehyde (MDA) and prostaglandin E2 (PGE2) levels as well as reduced glutathione (GSH) content were assessed in brain homogenate at the end of the schedule. VAL, DIC, MEL, VAL + MEL and VAL + DIC decreased seizure score and duration. Meanwhile, they increased the latency period. PTZ increased TNF-α, IL-1β, MDA, and PGE2 levels meanwhile, it decreased GSH content. Administration of VAL, DIC, MEL, VAL + MEL and VAL + DIC decreased TNF-α, IL-1β, MDA, and PGE2 levels meanwhile, they increased GSH content in the brain homogenates. Effects of VAL + DIC combination on the studied parameters were significant in relation to VAL. VAL, DIC, MEL, VAL + MEL and VAL + DIC produced anticonvulsant effect and mitigated inflammation and oxidative stress in PTZ-kindled mice. Interestingly, DIC rather than MEL enhanced the anticonvulsant effect VAL.

Burst Detection Methods

'Bursting', defined as periods of high-frequency firing of a neuron separated by periods of quiescence, has been observed in various neuronal systems, both in vitro and in vivo. It has been associated with a range of neuronal processes, including efficient information transfer and the formation of functional networks during development, and has been shown to be sensitive to genetic and pharmacological manipulations. Accurate detection of periods of bursting activity is thus an important aspect of characterising both spontaneous and evoked neuronal network activity. A wide variety of computational methods have been developed to detect periods of bursting in spike trains recorded from neuronal networks. In this chapter, we review several of the most popular and successful of these methods.

Glutamatergic Signaling Drives Ketamine-Mediated Response in Depression: Evidence from Dynamic Causal Modeling

BACKGROUND

The glutamatergic modulator ketamine has rapid antidepressant effects in individuals with major depressive disorder and bipolar depression. Thus, modulating glutamatergic transmission may be critical to effectively treating depression, though the mechanisms by which this occurs are not fully understood.

METHODS

This double-blind, crossover, placebo-controlled study analyzed data from 18 drug-free major depressive disorder subjects and 18 heathy controls who received a single i.v. infusion of ketamine hydrochloride (0.5 mg/kg) as well as an i.v. saline placebo. Magnetoencephalographic recordings were collected prior to the first infusion and 6 to 9 hours after both ketamine and placebo infusions. During scanning, participants passively received tactile stimulation to the right index finger. Antidepressant response was assessed across timepoints using the Montgomery-Asberg Depression Rating Scale. Dynamic causal modeling was used to measure changes in α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)- and N-methyl-D-aspartate (NMDA)-mediated connectivity estimates in major depressive disorder subjects and controls using a simple model of somatosensory evoked responses.

RESULTS

Both major depressive disorder and healthy subjects showed ketamine-mediated NMDA-blockade sensitization, with major depressive disorder subjects showing enhanced NMDA connectivity estimates in backward connections and controls showing enhanced NMDA connectivity estimates in forward connections in our model. Within our major depressive disorder subject group, ketamine efficacy, as measured by improved mood ratings, correlated with reduced NMDA and AMPA connectivity estimates in discrete extrinsic connections within the somatosensory cortical network.

CONCLUSIONS

These findings suggest that AMPA- and NMDA-mediated glutamatergic signaling play a key role in antidepressant response to ketamine and, further, that dynamic causal modeling is a powerful tool for modeling AMPA- and NMDA-mediated connectivity in vivo.

CLINICALTRIALS.GOV

NCT#00088699.

Effects of riluzole on spinal seizure-like activity in the brainstem-spinal cord preparation of newborn rat

Riluzole blocks persistent Na⁺ current, inhibits generation of neuronal bursts and decreases glutamate-induced excitotoxicity. In previous studies of respiratory activity, riluzole suppressed inspiratory-related burst generation activity in rat slice or en bloc preparations. We examined riluzole's effects on inspiratory burst generation and drug-induced seizure-like activity in newborn rat en bloc preparations. Medulla-spinal cord preparations from postnatal day 0-3 Wistar rats were isolated under deep isoflurane anesthesia and were superfused with artificial cerebrospinal fluid equilibrated with 95% O₂ and 5% CO₂, pH 7.4, at 25-26°C. Inspiratory activity was monitored from the fourth cervical ventral root. Seizure-like activity was induced by application of 20μM DL-threo-β-benzyloxyasparatate (TBOA, a glutamate uptake blocker preferentially acting on astrocytes) or coadministration of GABA_A antagonist bicuculline (10μM) and glycine antagonist strychnine (10μM). Pretreatment and co-application with 10μM riluzole abolished the seizure-like burst activity induced by TBOA or bicuculline/strychnine. N-methyl-d-aspartic acid receptor antagonist MK801 (10μM) also depressed this activity. Riluzole may attenuate excessive glutamate action involved in pathological hyperexcitability of motor neurons with no major effect on generation of respiratory activity. Riluzole at the optimal dose could be a potential treatment to protect drug-induced epileptic brain tissue from excitotoxic damage without inducing respiratory suppression.

Cortical development and neuroplasticity in Auditory Neuropathy Spectrum Disorder

Cortical development is dependent to a large extent on stimulus-driven input. Auditory Neuropathy Spectrum Disorder (ANSD) is a recently described form of hearing impairment where neural dys-synchrony is the predominant characteristic. Children with ANSD provide a unique platform to examine the effects of asynchronous and degraded afferent stimulation on cortical auditory neuroplasticity and behavioral processing of sound. In this review, we describe patterns of auditory cortical maturation in children with ANSD. The disruption of cortical maturation that leads to these various patterns includes high levels of intra-individual cortical variability and deficits in cortical phase synchronization of oscillatory neural responses. These neurodevelopmental changes, which are constrained by sensitive periods for central auditory maturation, are correlated with behavioral outcomes for children with ANSD. Overall, we hypothesize that patterns of cortical development in children with ANSD appear to be markers of the severity of the underlying neural dys-synchrony, providing prognostic indicators of success of clinical intervention with amplification and/or electrical stimulation. This article is part of a Special Issue entitled <Auditory Synaptology>.

Small-molecule anticonvulsant agents with potent in vitro neuroprotection and favorable drug-like properties

Severe seizure activity is associated with reoccurring cycles of excitotoxicity and oxidative stress that result in progressive neuronal damage and death. Intervention with these pathological processes is a compelling disease-modifying strategy for the treatment of seizure disorders. We have optimized a series of small molecules for neuroprotective and anticonvulsant activity as well as altered their physical properties to address potential metabolic liabilities, to improve CNS penetration, and to prolong the duration of action in vivo. Utilizing phenotypic screening of hippocampal cultures with nutrient medium depleted of antioxidants as a disease model, cell death and decreased neuronal viability produced by acute treatment with glutamate or hydrogen peroxide were prevented. Modifications to our previously reported proof of concept compounds have resulted in a lead which has full neuroprotective action at <1 nM and antiseizure activity across six animal models including the kindled rat and displays excellent pharmacokinetics including high exposure to the brain. These modifications have also eliminated the requirement for a chiral molecule, removing the possibility of racemization and making large-scale synthesis more easily accessible. These studies strengthen our earlier findings which indicate that potent, multifunctional neuroprotective anticonvulsants are feasible within a single molecular entity which also possesses favorable CNS-active drug properties in vitro and in vivo.

Possible nitric oxide mechanism in the protective effect of hesperidin against pentylenetetrazole (PTZ)-induced kindling and associated cognitive dysfunction in mice

Epilepsy is a complex neurological disorder manifested by recurrent episodes of convulsive seizures, loss of consciousness, and sensory disturbances. Pentylenetetrazole (PTZ)-induced kindling primarily represents a model of generalized epilepsy. The present study has been undertaken to evaluate the neuroprotective potential of hesperidin and its interaction with nitric oxide modulators against PTZ-induced kindling and associated cognitive dysfunction in mice. The experimental protocol comprised of eleven groups (n=6), where a subconvulsive dose of PTZ (40 mg/kg, i.p.) had been administered every other day for a period of 12 days, and seizure episodes were noted after each PTZ injection over a period of 30 min. The memory performance tests were carried out on days 13 and 14 followed by the estimation of biochemical and mitochondrial parameters. Chronic administration of a subconvulsive dose of PTZ resulted in an increase in convulsive activity culminating in generalized clonic-tonic seizures, as revealed by a progressive increase in seizure score as well as alteration in antioxidant enzyme levels (lipid peroxidation, nitrite, glutathione, super oxide dismutase, and catalase) and mitochondrial complex (I, II, and IV) activities, whereas chronic treatment with hesperidin (200 mg/kg) significantly attenuated these behavioral, biochemical, and mitochondrial alterations. Further, treatment with l-arginine (100 mg/kg) or l-NAME (10 mg/kg) in combination with hesperidin significantly modulated the protective effect of hesperidin which was significant as compared to their effects per se in PTZ-treated animals. Thus, the present study suggests a possible involvement of the NO-cGMP pathway in the neuroprotective effect of hesperidin in PTZ-kindled mice.

From neural plate to cortical arousal-a neuronal network theory of sleep derived from in vitro "model" systems for primordial patterns of spontaneous bioelectric activity in the vertebrate central nervous system

In the early 1960s intrinsically generated widespread neuronal discharges were discovered to be the basis for the earliest motor behavior throughout the animal kingdom. The pattern generating system is in fact programmed into the developing nervous system, in a regionally specific manner, already at the early neural plate stage. Such rhythmically modulated phasic bursts were next discovered to be a general feature of developing neural networks and, largely on the basis of experimental interventions in cultured neural tissues, to contribute significantly to their morpho-physiological maturation. In particular, the level of spontaneous synchronized bursting is homeostatically regulated, and has the effect of constraining the development of excessive network excitability. After birth or hatching, this "slow-wave" activity pattern becomes sporadically suppressed in favor of sensory oriented "waking" behaviors better adapted to dealing with environmental contingencies. It nevertheless reappears periodically as "sleep" at several species-specific points in the diurnal/nocturnal cycle. Although this "default" behavior pattern evolves with development, its essential features are preserved throughout the life cycle, and are based upon a few simple mechanisms which can be both experimentally demonstrated and simulated by computer modeling. In contrast, a late onto- and phylogenetic aspect of sleep, viz., the intermittent "paradoxical" activation of the forebrain so as to mimic waking activity, is much less well understood as regards its contribution to brain development. Some recent findings dealing with this question by means of cholinergically induced "aroused" firing patterns in developing neocortical cell cultures, followed by quantitative electrophysiological assays of immediate and longterm sequelae, will be discussed in connection with their putative implications for sleep ontogeny.

Experimental and clinical findings from physical exercise as complementary therapy for epilepsy

Complementary therapies for preventing or treating epilepsy have been extensively used. This review focuses on the positive effects of physical exercise programs observed in clinical studies and experimental models of epilepsy and their significance as a complementary therapy for epilepsy. Information about the antiepileptogenic and neuroprotective effects of exercise is highlighted. Considering that exercise can exert beneficial actions such as reduction of seizure susceptibility, reduction of anxiety and depression, and consequently, improvement of quality of life of individuals with epilepsy, exercise can be a potential candidate as non-pharmacological treatment of epilepsy.

Upregulation of excitatory neurons and downregulation of inhibitory neurons in barrel cortex are associated with loss of whisker inputs

Loss of a sensory input causes the hypersensitivity in other modalities. In addition to cross-modal plasticity, the sensory cortices without receiving inputs undergo the plastic changes. It is not clear how the different types of neurons and synapses in the sensory cortex coordinately change after input deficits in order to prevent loss of their functions and to be used for other modalities. We studied this subject in the barrel cortices from whiskers-trimmed mice vs. controls. After whisker trimming for a week, the intrinsic properties of pyramidal neurons and the transmission of excitatory synapses were upregulated in the barrel cortex, but inhibitory neurons and GABAergic synapses were downregulated. The morphological analyses indicated that the number of processes and spines in pyramidal neurons increased, whereas the processes of GABAergic neurons decreased in the barrel cortex. The upregulation of excitatory neurons and the downregulation of inhibitory neurons boost the activity of network neurons in the barrel cortex to be high levels, which prevent the loss of their functions and enhances their sensitivity to sensory inputs. These changes may prepare for attracting the innervations from sensory cortices and/or peripheral nerves for other modalities during cross-modal plasticity.

Small molecule anticonvulsant agents with potent in vitro neuroprotection

Severe seizure activity is associated with recurring cycles of excitotoxicity and oxidative stress that result in progressive neuronal damage and death. Intervention to halt these pathological processes is a compelling disease-modifying strategy for the treatment of seizure disorders. In the present study, a core small molecule with anticonvulsant activity has been structurally optimized for neuroprotection. Phenotypic screening of rat hippocampal cultures with nutrient medium depleted of antioxidants was utilized as a disease model. Increased cell death and decreased neuronal viability produced by acute treatment with glutamate or hydrogen peroxide were prevented by our novel molecules. The neuroprotection associated with this chemical series has marked structure activity relationships that focus on modification of the benzylic position of a 2-phenyl-2-hydroxyethyl sulfamide core structure. Complete separation between anticonvulsant activity and neuroprotective action was dependent on substitution at the benzylic carbon. Chiral selectivity was evident in that the S-enantiomer of the benzylic hydroxy group had neither neuroprotective nor anticonvulsant activity, while the R-enantiomer of the lead compound had full neuroprotective action at <40 nM and antiseizure activity in three animal models. These studies indicate that potent, multifunctional neuroprotective anticonvulsants are feasible within a single molecular entity.

Role of oxidative stress in epileptic seizures

Oxidative stress resulting from excessive free-radical release is likely implicated in the initiation and progression of epilepsy. Therefore, antioxidant therapies aimed at reducing oxidative stress have received considerable attention in epilepsy treatment. However, much evidence suggests that oxidative stress does not always have the same pattern in all seizures models. Thus, this review provides an overview aimed at achieving a better understanding of this issue. We summarize work regarding seizure models (i.e., genetic rat models, kainic acid, pilocarpine, pentylenetetrazol, and trimethyltin), oxidative stress as an etiologic factor in epileptic seizures (i.e., impairment of antioxidant systems, mitochondrial dysfunction, involvement of redox-active metals, arachidonic acid pathway activation, and aging), and antioxidant strategies for seizure treatment. Combined, this review highlights pharmacological mechanisms associated with oxidative stress in epileptic seizures and the potential for neuroprotection in epilepsy that targets oxidative stress and is supported by effective antioxidant treatment.

Cocaine-induced homeostatic regulation and dysregulation of nucleus accumbens neurons

Homeostatic response is an endowed self-correcting/maintaining property for living units, ranging from subcellular domains, single cells, and organs to the whole organism. Homeostatic responses maintain stable function through the ever-changing internal and external environments. In central neurons, several forms of homeostatic regulation have been identified, all of which tend to stabilize the functional output of neurons toward their prior "set-point." Medium spiny neurons (MSNs) within the forebrain region the nucleus accumbens (NAc) play a central role in gating/regulating emotional and motivational behaviors including craving and seeking drugs of abuse. Exposure to highly salient stimuli such as cocaine administration not only acutely activates a certain population of NAc MSNs, but also induces long-lasting changes in these neurons. It is these long-lasting cellular alterations that are speculated to mediate the increasingly strong cocaine-craving and cocaine-seeking behaviors. Why do the potentially powerful homeostatic mechanisms fail to correct or compensate for these drug-induced maladaptations in neurons? Based on recent experimental results, this review proposes a hypothesis of homeostatic dysregulation induced by exposure to cocaine. Specifically, we hypothesize that exposure to cocaine generates false molecular signals which misleads the homeostatic regulation process, resulting in maladaptive changes in NAc MSNs. Thus, many molecular and cellular alterations observed in the addicted brain may indeed result from homeostatic dysregulation. This review is among the first to introduce the concept of homeostatic neuroplasticity to understanding the molecular and cellular maladaptations following exposure to drugs of abuse.

Diminished activity-dependent brain-derived neurotrophic factor expression underlies cortical neuron microcircuit hypoconnectivity resulting from exposure to mutant huntingtin fragments

Although previous studies of Huntington's disease (HD) have addressed many potential mechanisms of striatal neuron dysfunction and death, it is also known, based on clinical findings, that cortical function is dramatically disrupted in HD. With respect to disease etiology, however, the specific molecular and neuronal circuit bases for the cortical effects of mutant huntingtin (htt) have remained largely unknown. In the present work, we studied the relationship between the molecular effects of mutant htt fragments in cortical cells and the corresponding behavior of cortical neuron microcircuits by using a novel cellular model of HD. We observed that a transcript-selective diminution in activity-dependent brain-derived neurotrophic factor (BDNF) expression preceded the onset of a synaptic connectivity deficit in ex vivo cortical networks, which manifested as decreased spontaneous collective burst-firing behavior measured by multielectrode array substrates. Decreased BDNF expression was determined to be a significant contributor to network-level dysfunction, as shown by the ability of exogenous BDNF to ameliorate cortical microcircuit burst firing. The molecular determinants of the dysregulation of activity-dependent BDNF expression by mutant htt seem to be distinct from previously elucidated mechanisms, because they do not involve known neuron-restrictive silencer factor/RE1-silencing transcription factor-regulated promoter sequences but instead result from dysregulation of BDNF exon IV and VI transcription. These data elucidate a novel HD-related deficit in BDNF gene regulation as a plausible mechanism of cortical neuron hypoconnectivity and cortical function deficits in HD. Moreover, the novel model paradigm established here is well suited to further mechanistic and drug screening research applications.

Real-time neuronal homeostasis by coordinating VGSC intrinsic properties

Homeostasis of internal environment and cellular metabolism ensures cells' functions to be stable in living organisms. Cellular homeostasis is believed to be maintained via feedback or feedforward manners. We report a novel mechanism that maintains neuronal homeostasis through coordinating the intrinsic properties of single molecules concurrently. Spike encoding and sodium channel dynamics at cortical neurons were studied by patch-clamp recording. Voltage-gated sodium channels set refractory period and threshold potential toward different directions to stabilize the energetic barrier for firing sequential action potentials. This neuronal homeostasis is not affected by intracellular Ca(2+) signals and membrane potentials. Real-time homeostasis maintains precise and reliable neuronal encoding without any destabilization.

The response of cortical neurons to in vivo-like input current: theory and experiment: II. Time-varying and spatially distributed inputs

The response of a population of neurons to time-varying synaptic inputs can show a rich phenomenology, hardly predictable from the dynamical properties of the membrane's inherent time constants. For example, a network of neurons in a state of spontaneous activity can respond significantly more rapidly than each single neuron taken individually. Under the assumption that the statistics of the synaptic input is the same for a population of similarly behaving neurons (mean field approximation), it is possible to greatly simplify the study of neural circuits, both in the case in which the statistics of the input are stationary (reviewed in La Camera et al. in Biol Cybern, 2008) and in the case in which they are time varying and unevenly distributed over the dendritic tree. Here, we review theoretical and experimental results on the single-neuron properties that are relevant for the dynamical collective behavior of a population of neurons. We focus on the response of integrate-and-fire neurons and real cortical neurons to long-lasting, noisy, in vivo-like stationary inputs and show how the theory can predict the observed rhythmic activity of cultures of neurons. We then show how cortical neurons adapt on multiple time scales in response to input with stationary statistics in vitro. Next, we review how it is possible to study the general response properties of a neural circuit to time-varying inputs by estimating the response of single neurons to noisy sinusoidal currents. Finally, we address the dendrite-soma interactions in cortical neurons leading to gain modulation and spike bursts, and show how these effects can be captured by a two-compartment integrate-and-fire neuron. Most of the experimental results reviewed in this article have been successfully reproduced by simple integrate-and-fire model neurons.

Homeostasis established by coordination of subcellular compartment plasticity improves spike encoding

Homeostasis in cells maintains their survival and functions. The plasticity at neurons and synapses may destabilize their signal encoding. The rapid recovery of cellular homeostasis is needed to secure the precise and reliable encoding of neural signals necessary for well-organized behaviors. We report a homeostatic process that is rapidly established through Ca(2+)-induced coordination of functional plasticity among subcellular compartments. An elevation of cytoplasmic Ca(2+) levels raises the threshold potentials and refractory periods of somatic spikes, and strengthens the signal transmission at glutamatergic and GABAergic synapses, in which synaptic potentiation shortens refractory periods and lowers threshold potentials. Ca(2+) signals also induce an inverse change of membrane excitability at the soma versus the axon. The integrative effect of Ca(2+)-induced plasticity among the subcellular compartments is homeostatic in nature, because it stabilizes neuronal activities and improves spike timing precision. Our study of neuronal homeostasis that is fulfilled by rapidly coordinating subcellular compartments to improve neuronal encoding sheds light on exploring homeostatic mechanisms in other cell types.

Spontaneous neuronal burst discharges as dependent and independent variables in the maturation of cerebral cortex tissue cultured in vitro: a review of activity-dependent studies in live 'model' systems for the development of intrinsically generated bioelectric slow-wave sleep patterns

A survey is presented of recent experiments which utilize spontaneous neuronal spike trains as dependent and/or independent variables in developing cerebral cortex cultures when synaptic transmission is interfered with for varying periods of time. Special attention is given to current difficulties in selecting suitable preparations for carrying out biologically relevant developmental studies, and in applying spike-train analysis methods with sufficient resolution to detect activity-dependent age and treatment effects. A hierarchy of synchronized nested burst discharges which approximate early slow-wave sleep patterns in the intact organism is established as a stable basis for isolated cortex function. The complexity of reported long- and short-term homeostatic responses to experimental interference with synaptic transmission is reviewed, and the crucial role played by intrinsically generated bioelectric activity in the maturation of cortical networks is emphasized.

Bayesian estimation of synaptic physiology from the spectral responses of neural masses

We describe a Bayesian inference scheme for quantifying the active physiology of neuronal ensembles using local field recordings of synaptic potentials. This entails the inversion of a generative neural mass model of steady-state spectral activity. The inversion uses Expectation Maximization (EM) to furnish the posterior probability of key synaptic parameters and the marginal likelihood of the model itself. The neural mass model embeds prior knowledge pertaining to both the anatomical [synaptic] circuitry and plausible trajectories of neuronal dynamics. This model comprises a population of excitatory pyramidal cells, under local interneuron inhibition and driving excitation from layer IV stellate cells. Under quasi-stationary assumptions, the model can predict the spectral profile of local field potentials (LFP). This means model parameters can be optimised given real electrophysiological observations. The validity of inferences about synaptic parameters is demonstrated using simulated data and experimental recordings from the medial prefrontal cortex of control and isolation-reared Wistar rats. Specifically, we examined the maximum a posteriori estimates of parameters describing synaptic function in the two groups and tested predictions derived from concomitant microdialysis measures. The modelling of the LFP recordings revealed (i) a sensitization of post-synaptic excitatory responses, particularly marked in pyramidal cells, in the medial prefrontal cortex of socially isolated rats and (ii) increased neuronal adaptation. These inferences were consistent with predictions derived from experimental microdialysis measures of extracellular glutamate levels.

Silencing-induced metaplasticity in hippocampal cultured neurons

Silencing-induced homeostatic plasticity is usually expressed as a change in the amplitude or the frequency of miniature postsynaptic currents. Here we report that, prolonged (approximately 24 h) silencing of mature (20-22 days in vitro) cultured hippocampal neurons using the voltage-gated sodium channel blocker tetrodotoxin (TTX) produced no effects on the amplitude or frequency of the miniature excitatory postsynaptic currents (mEPSCs). However, the silencing changed the intrinsic membrane properties of the neurons, resulting in an increased excitability and rate of action potentials firing upon TTX washout. Allowing neurons to recover in TTX-free recording solution for a short period of time after the silencing resulted in potentiation of mEPSC amplitudes. This form of activity-dependent potentiation is different from classical long-term potentiation, as similar potentiation was not seen in nonsilenced neurons treated with bicuculline to raise their spiking activity to the same level displayed by the silenced neurons during TTX washout. Also, the potentiation of mEPSC amplitudes after the recovery period was not affected by the N-methyl-d-aspartate receptor blocker d-2-amino-5-phosponopentanoic acid or by the calcium/calmodulin-dependent kinase II (CaMKII) inhibitor KN-62 but was abolished by the L-type calcium channel blocker nifedipine. We thus conclude that the potentiation of mEPSC amplitudes following brief recovery of spiking activity in chronically silenced neurons represents a novel form of metaplasticity that differs from the conventional models of homeostatic synaptic plasticity.

Excitation Control: Balancing PSD-95 Function at the Synapse

Excitability of individual neurons dictates the overall excitation in specific brain circuits. This process is thought to be regulated by molecules that regulate synapse number, morphology and strength. Neuronal excitation is also influenced by the amounts of neurotransmitter receptors and signaling molecules retained at particular synaptic sites. Recent studies revealed a key role for PSD-95, a scaffolding molecule enriched at glutamatergic synapses, in modulation of clustering of several neurotransmitter receptors, adhesion molecules, ion channels, cytoskeletal elements and signaling molecules at postsynaptic sites. In this review we will highlight mechanisms that control targeting of PSD-95 at the synapse, and discuss how this molecule influences the retention and clustering of diverse synaptic proteins to regulate synaptic structure and strength. We will also discuss how PSD-95 may maintain a balance between excitation and inhibition in the brain and how alterations in this balance may contribute to neuropsychiatric disorders.

Preliminary explorations of the role of mitochondrial proteins in refractory epilepsy: some findings from comparative proteomics

Approximately 20-30% of patients with epilepsy continue to have seizures despite carefully monitored treatment with antiepileptic drugs. The mechanisms that underlie why some patients are responsive and others prove resistant to antiepileptic drugs are poorly understood. Increasing evidence supports a role for altered mitochondrial function in the pathogenesis of epilepsy. To gain greater molecular insight in the pathogenesis of intractable epilepsy, we undertook a global analysis of protein expressions in a pharmacoresistant epileptic model selected by phenytoin in electrical amygdala-kindled rats by using two-dimensional gel electrophoresis coupled with matrix-assisted laser desorption/ionization time of flight (MALDI-TOF-TOF). We identified five increased proteins and 14 decreased proteins including voltage-dependent anion channel 1 (VDAC1) with a 2.82-fold increased level (P < 0.05) and voltage-dependent anion channel 2 (VDAC2) with a 3.97-fold decreased level (P < 0.05) in hippocampus of pharmacoresistant rats. The increased VDAC1 and decreased VDAC2 were confirmed by Western blot analysis and immunohistochemistry. Vascular mitochondria and apoptosis neurons were observed through electron microscopy. Energy contents, the adenine nucleotides, were measured by high-performance liquid chromatography (HPLC). The correlation analyses were carried out between VDAC and the energy charge. These findings indicate that the increase of VDAC1 and the decrease of VDAC2 play an important role during the process and provide new molecular evidence in understanding mechanism of refractory epilepsy.

Excitation Control: Balancing PSD-95 Function at the Synapse

Excitability of individual neurons dictates the overall excitation in specific brain circuits. This process is thought to be regulated by molecules that regulate synapse number, morphology and strength. Neuronal excitation is also influenced by the amounts of neurotransmitter receptors and signaling molecules retained at particular synaptic sites. Recent studies revealed a key role for PSD-95, a scaffolding molecule enriched at glutamatergic synapses, in modulation of clustering of several neurotransmitter receptors, adhesion molecules, ion channels, cytoskeletal elements and signaling molecules at postsynaptic sites. In this review we will highlight mechanisms that control targeting of PSD-95 at the synapse, and discuss how this molecule influences the retention and clustering of diverse synaptic proteins to regulate synaptic structure and strength. We will also discuss how PSD-95 may maintain a balance between excitation and inhibition in the brain and how alterations in this balance may contribute to neuropsychiatric disorders.

Right-sided vagus nerve stimulation inhibits induced spinal cord seizures

We have previously shown that left-sided vagus nerve stimulation results in cessation of induced spinal cord seizures. To test our hypothesis that right-sided vagus nerve stimulation will also abort seizure activity, we have initiated seizures in the spinal cord and then performed right-sided vagus nerve stimulation in an animal model. Four pigs were anesthetized and placed in the lateral position and a small laminectomy performed in the lumbar region. Topical penicillin, a known epileptogenic drug to the cerebral cortex and spinal cord, was next applied to the dorsal surface of the exposed cord. With the exception of the control animal, once seizure activity was discernible via motor convulsion or increased electrical activity, the right vagus nerve previously isolated in the neck was stimulated. Following multiple stimulations of the vagus nerve and with seizure activity confirmed, the cord was transected in the midthoracic region and vagus nerve stimulation performed. Right-sided vagus nerve stimulation resulted in cessation of spinal cord seizure activity in all animals. Transection of the spinal cord superior to the site of seizure induction resulted in the ineffectiveness of vagus nerve stimulation in causing cessation of seizure activity in all study animals. As with left-sided vagus nerve stimulation, right-sided vagus nerve stimulation results in cessation of induced spinal cord seizures. Additionally, the effects of right-sided vagus nerve stimulation on induced spinal cord seizures involve descending spinal pathways. These data may aid in the development of alternative mechanisms for electrical stimulation for patients with medically intractable seizures and add to our knowledge regarding the mechanism for seizure cessation following peripheral nerve stimulation.

Tapered progesterone withdrawal promotes long-term recovery following brain trauma

We previously demonstrated that after traumatic brain injury (TBI), acute progesterone withdrawal (AW) causes an increase in anxiety behaviors and cerebro-cellular inflammation compared to tapered progesterone withdrawal (TW). Our current study investigates the behavioral and cellular effects of AW two weeks after termination of treatments to determine the longer-term influence of withdrawal after injury. Adult, male Sprague-Dawley rats received either bilateral frontal cortex contusion (L) or sham (S) surgery. Rats were injected at 1 and 6 h post-injury, then every 24 h for six days. Vehicle (V)-treated rats were given 9 injections of 22.5% cyclodextrin, whereas AW rats received 9 injections of 16 mg/kg progesterone and TW rats received 7 injections of P at 16 mg/kg, followed by one at 8 mg/kg and one at 4 mg/kg. On day 8, sensory neglect and locomotor activity tests were initiated. Animals were killed 22 days post-TBI and the brains prepared for either molecular or histological analysis. Western blotting revealed increased brain-derived neurotrophic factor (BDNF) and heat shock protein 70 (HSP70) in TW vs. AW animals. P53 was increased in VL animals, whereas all progesterone-treated groups were equivalent to shams. TW animals had markedly decreased sensory neglect compared to AW animals and increased center time in locomotor activity assays. In addition, lesion reconstruction revealed a decreased lesion size for TWL over AWL over VL animals. Glial fibrillary acidic protein (GFAP) immunofluorescent staining followed this pattern as well. In conclusion, after TBI, AW affects select behaviors and molecular markers in the chronic recovery period.

Ischemic preconditioning ameliorates excitotoxicity by shifting glutamate/gamma-aminobutyric acid release and biosynthesis

Excitotoxicity is recognized to play a major role in cerebral ischemia-induced cell death. The main goal of the present study was to define whether our model of ischemic preconditioning (IPC) promotes a shift from excitatory to inhibitory neurotransmission during the test ischemia to diminish metabolic demand during the reperfusion phase. We also determined whether gamma-aminobutyric acid (GABA) played a role in IPC-induced neuroprotection. Ten minutes of cerebral ischemia was produced by tightening the carotid ligatures bilaterally following hypotension. Samples of microdialysis perfusate, representing extracellular fluid, were analyzed for amino acid content by HPLC. IPC promoted a robust release of GABA after lethal ischemia compared with control rats. We also observed that the activity of glutamate decarboxylase (the predominant pathway of GABA synthesis in the brain) was higher in the IPC group compared with control and ischemic groups. Because GABAA receptor up-regulation has been shown to occur following IPC, and GABAA receptor activation has been implicated in neuroprotection against ischemic insults, we tested the hypothesis that GABAA or GABAB receptor activation was neuroprotective during ischemia or early reperfusion by using an in vitro model (organotypic hippocampal slice culture). Administration of the GABAB agonist baclofen during test ischemia and for 1 hr of reperfusion provided significant neuroprotection. We concluded that increased GABA release in preconditioned animals after ischemia might be one of the factors responsible for IPC neuroprotection. Specific activation of GABAB receptor contributes significantly to neuroprotection against ischemia in organotypic hippocampal slices.

Network stability through homeostatic scaling of excitatory and inhibitory synapses following inactivity in CA3 of rat organotypic hippocampal slice cultures

Homeostatic plasticity is a phenomenon whereby synaptic strength is scaled in the context of the activity that the network receives. Here, we have analysed excitatory and inhibitory synapses in a model of homeostatic plasticity where rat organotypic hippocampal slice cultures were deprived of excitatory synaptic input by the NMDA and AMPA/KA glutamate receptor antagonists, AP5 and CNQX. We show that chronic excitatory synapse deprivation generates an excitable CA3 network where enhanced amplitude and frequency of spontaneous excitatory post-synaptic potentials were associated with increased glutamate receptor subunit expression and increased number and size of synapsin 1 and VGLUT1 positive puncta. Intact spontaneous inhibitory post-synaptic potentials coincided with persistent expression of the GABA-A receptor alpha subunit and GAD65 and an enhancement of parvalbumin-positive puncta. In this model of homeostatic plasticity, scaling up of synaptic excitation and maintenance of fast synaptic inhibition promote an excitable, but stable, CA3 network.

Vagus nerve stimulation for induced spinal cord seizures: insights into seizure cessation

OBJECT

Vagus nerve stimulation is known to decrease the frequency, duration, and intensity of some types of intracranial seizures in both humans and animals. Although many theories abound concerning the mechanism for this action, the true cause remains speculative. To potentially elucidate a pathway in which vagus nerve stimulation aborts seizure activity, seizures were initiated not in the cerebral cortex but in the spinal cord and then vagus nerve stimulation was performed.

METHODS

Ten pigs were anesthetized and placed in the lateral position, and a small laminectomy was performed in the lumbar region. Topical penicillin, a known epileptogenic drug to the cerebral cortex and spinal cord, was applied to the dorsal surface of the exposed cord. With the exception of two animals that were used as controls, once seizure activity was discernible via motor convulsion or increased electrical activity the left vagus nerve, which had been previously isolated in the neck, was stimulated. Following multiple stimulations of the vagus nerve and with seizure activity confirmed, the cord was transected in the midthoracic region and vagus nerve stimulation was performed. Vagus nerve stimulation resulted in cessation of spinal cord seizure activity in all (87.5%) but one experimented animal. Transection of the spinal cord superior to the site of seizure induction resulted in the ineffectiveness of vagus nerve stimulation to cause cessation of seizure activity in all study animals.

CONCLUSIONS

The effects of vagus nerve stimulation on induced spinal cord seizures involve descending spinal pathways. The authors believe that this experiment is the first to demonstrate that spinal cord neuronal hyperactivity can be suppressed by stimulation of a cranial nerve. These data may aid in the development of alternative mechanisms for electrical stimulation in patients with medically intractable seizures. Further studies are now necessary to isolate which specific tracts, nuclei, and neurotransmitters are involved in this process.

Ischemic tolerance induction in organotypic hippocampal slices: role for the GABA(A) receptor?

Ischemic preconditioning (IPC) refers to sublethal ischemic insults rendering brain tissue tolerant against subsequent ischemic insults. We investigated the role of the GABA(A) receptor (GABA(A)R) upon IPC induction. Rat organotypic hippocampal slices were subjected to IPC by 15 min of oxygen-glucose deprivation (OGD) followed by 40 min of OGD 48 h later, resulting in robust cell death reduction as assessed by the propidium iodide fluorescence method ('late' or 'second window' IPC). Superfusion with the GABA(A)R antagonist bicuculline during IPC ameliorated propidium iodide uptake at a high but not at low doses indicating that GABA(A)R activation may be assigned a limited role in neuroprotection. In previous studies, we found that increased neuronal excitability can promote IPC neuroprotection. We, therefore, tested the hypothesis that blockade of inhibitory GABAergic transmission conferred ischemic tolerance. However, temporary administration of bicuculline 48 h prior to ischemic challenge was not neuroprotective. In another approach, we tested whether preconditioning with the GABA(A)R agonist, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) mediated ischemic tolerance and found no significant neuroprotection. The results are discussed in light of the intrinsic excitatory-inhibitory balance of glutamate and GABA.

The cortex as a central pattern generator

Vertebrate spinal cord and brainstem central pattern generator (CPG) circuits share profound similarities with neocortical circuits. CPGs can produce meaningful functional output in the absence of sensory inputs. Neocortical circuits could be considered analogous to CPGs as they have rich spontaneous dynamics that, similar to CPGs, are powerfully modulated or engaged by sensory inputs, but can also generate output in their absence. We find compelling evidence for this argument at the anatomical, biophysical, developmental, dynamic and pathological levels of analysis. Although it is possible that cortical circuits are particularly plastic types of CPG ('learning CPGs'), we argue that present knowledge about CPGs is likely to foretell the basic principles of the organization and dynamic function of cortical circuits.

Structural and functional alterations in mitochondrial membrane in picrotoxin-induced epileptic rat brain

Mitochondrial function is a key determinant of both excitability and viability of neurons. Present studies were carried out to decipher cerebral mitochondrial oxidative energy metabolism and membrane function in the chronic condition of generalized seizures induced by picrotoxin (PTX) in rats. PTX-induced convulsions resulted in decreased respiration rates (14-41%) with glutamate, pyruvate + malate, and succinate as substrate. The ADP phosphorylation rates were drastically reduced by 44-65%. An opposite trend was observed with ascorbate + N,N,N',N'-tetramethyl-p-phenylenediamine [corrected] (TMPD) as substrate. In general, uncoupling of the mitochondrial electron transport was observed after PTX treatment. Malate dehydrogenase (MDH) and succinate dehydrogenase (SDH) activities were decreased by 20-80%; also, there was significant reduction in cytochrome b content after PTX treatment, while the F(o)F(1) ATPase (complex V) activity increased in basal and 2,4-dinitrophenol (DNP)-stimulated condition, indicating increased membrane fragility. The substrate kinetics analysis had shown that K(m) and V(max) of the higher affinity kinetic component of ATPase increased significantly by 1.2- to 1.4-fold in epileptic condition. Temperature kinetic analysis revealed 1.2-fold increase in energies of activation with decreased transition temperature. The total phospholipid (TPL) and cholesterol (CHL) contents decreased significantly with lowering of diphosphatidylglycerol (DPG), phosphatidylethanolamine (PE), phosphatidylinositol (PI), and phosphatidylserine (PS), while lysophospholipid (lyso), sphingomyelin (SPM), and phosphatidylcholine components were found to be elevated. Brain mitochondrial membrane was somewhat more fluidized in epileptic animals. Possible consequences of mitochondrial respiratory chain (MRC) dysfunction are discussed. In conclusion, impairment of MRC function along with structural alterations suggests novel pathophysiological mechanisms important for chronic epileptic condition.

Homeostatic synaptic plasticity can explain post-traumatic epileptogenesis in chronically isolated neocortex

Chronically isolated neocortex develops chronic hyperexcitability and focal epileptogenesis in a period of days to weeks. The mechanisms operating in this model of post-traumatic epileptogenesis are not well understood. We hypothesized that the spontaneous burst discharges recorded in chronically isolated neocortex result from homeostatic plasticity (a mechanism generally assumed to stabilize neuronal activity) induced by low neuronal activity after deafferentation. To test this hypothesis we constructed computer models of neocortex incorporating a biologically based homeostatic plasticity rule that operates to maintain firing rates. After deafferentation, homeostatic upregulation of excitatory synapses on pyramidal cells, either with or without concurrent downregulation of inhibitory synapses or upregulation of intrinsic excitability, initiated slowly repeating burst discharges that closely resembled the epileptiform burst discharges recorded in chronically isolated neocortex. These burst discharges lasted a few hundred ms, propagated at 1-3 cm/s and consisted of large (10-15 mV) intracellular depolarizations topped by a small number of action potentials. Our results support a role for homeostatic synaptic plasticity as a novel mechanism of post-traumatic epileptogenesis.

Seizure-dependent modulation of mitochondrial oxidative phosphorylation in rat hippocampus

Mitochondrial function is a key determinant of both excitability and viability of neurons. Here, we demonstrate seizure-dependent changes in mitochondrial oxidative phosphorylation in the epileptic rat hippocampus. The intense pathological neuronal activity in pilocarpine-treated rats exhibiting spontaneous seizures resulted in a selective decline of the activities of NADH-CoQ oxidoreductase (complex I of the respiratory chain) and cytochrome c oxidase (complex IV of respiratory chain) in the CA3 and CA1 hippocampal pyramidal subfields. In line with these findings, high-resolution respirometry revealed an increased flux control of complex I on respiration in the CA1 and CA3 subfields and decreased maximal respiration rates in the more severely affected CA3 subfield. Imaging of mitochondrial membrane potential using rhodamine 123 showed a lowered mitochondrial membrane potential in both pyramidal subfields. In contrast to the CA1 and CA3 subfields, mitochondrial oxidative phosphorylation was unaltered in the dentate gyrus and the parahippocampal gyrus. The changes of oxidative phosphorylation in the epileptic rat hippocampus cannot be attributed to oxidative enzyme modifications but are very likely related to a decrease in mitochondrial DNA copy number as shown in the more severely affected CA3 subfield and in cultured PC12 cells partially depleted of mitochondrial DNA. Thus, our results demonstrate that seizure activity downregulates the expression of mitochondrial-encoded enzymes of oxidative phosphorylation. This mechanism could be invoked during diverse forms of pathological neuronal activity and could severely affect both excitability and viability of hippocampal pyramidal neurons.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Membrane properties underlying patterns of GABA-dependent action potentials in developing mouse hypothalamic neurons

Spikes may play an important role in modulating a number of aspects of brain development. In early hypothalamic development, GABA can either evoke action potentials, or it can shunt other excitatory activity. In both slices and cultures of the mouse hypothalamus, we observed a heterogeneity of spike patterns and frequency in response to GABA. To examine the mechanisms underlying patterns and frequency of GABA-evoked spikes, we used conventional whole cell and gramicidin perforation recordings of neurons (n = 282) in slices and cultures of developing mouse hypothalamus. Recorded with gramicidin pipettes, GABA application evoked action potentials in hypothalamic neurons in brain slices of postnatal day 2-9 (P2-9) mice. With conventional patch pipettes (containing 29 mM Cl-), action potentials were also elicited by GABA from neurons of 2-13 days in vitro (2-13 DIV) embryonic hypothalamic cultures. Depolarizing responses to GABA could be generally classified into three types: depolarization with no spike, a single spike, or complex patterns of multiple spikes. In parallel experiments in slices, electrical stimulation of GABAergic mediobasal hypothalamic neurons in the presence of glutamate receptor antagonists [10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 100 microM 2-amino-5-phosphonopentanoic acid (AP5)] resulted in the occurrence of spikes that were blocked by bicuculline (20 microM). Blocking ionotropic glutamate receptors with AP5 and CNQX did not block GABA-mediated multiple spikes. Similarly, when synaptic transmission was blocked with Cd(2+) (200 microM) and Ni(2+) (300 microM), GABA still induced multiple spikes, suggesting that the multiple spikes can be an intrinsic membrane property of GABA excitation and were not based on local interneurons. When the pipette [Cl-] was 29 or 45 mM, GABA evoked multiple spikes. In contrast, spikes were not detected with 2 or 10 mM intracellular [Cl-]. With gramicidin pipettes, we found that the mean reversal potential of GABA-evoked current (E(GABA)) was positive to the resting membrane potential, suggesting a high intracellular [Cl-] in developing mouse neurons. Varying the holding potential from -80 to 0 mV revealed an inverted U-shaped effect on spike probability. Blocking voltage-dependent Na+ channels with tetrodotoxin eliminated GABA-evoked spikes, but not the GABA-evoked depolarization. Removing Ca(2+) from the extracellular solution did not block spikes, indicating GABA-evoked Na+ -based spikes. Although E(GABA) was more positive within 2-5 days in culture, the probability of GABA-evoked spikes was greater in 6- to 9-day cells. Mechanistically, this appears to be due to a greater Na+ current found in the older cells during a period when the E(GABA) is still positive to the resting membrane potential. GABA evoked similar spike patterns in HEPES and bicarbonate buffers, suggesting that Cl-, not bicarbonate, was primarily responsible for generating multiple spikes. GABA evoked either single or multiple spikes; neurons with multiple spikes had a greater Na+ current, a lower conductance, a more negative spike threshold, and a greater difference between the peak of depolarization and the spike threshold. Taken together, the present results indicate that the patterns of multiple action potentials evoked by GABA are an inherent property of the developing hypothalamic neuron.

Suppression of neuronal hyperexcitability and associated delayed neuronal death by adenoviral expression of GABA(C) receptors

The excessive neuronal excitation underlying several clinically important diseases is often treated with GABA allosteric modulators in an attempt to enhance inhibition. An alternative strategy would be to enhance directly the sensitivity of postsynaptic neurons to GABA. The GABA(C) receptor, normally found only in the retina, is more sensitive to GABA and demonstrates little desensitization compared with the GABA(A) receptor. We constructed an adenovirus vector that expressed cDNA for both the GABA(C) receptor rho(1) subunit and a green fluorescent protein (GFP) reporter and used it to transduce cultured hippocampal neurons. Transduced neurons were identified by fluorescence, double immunocytochemistry proved colocalization of the rho(1) protein and the reporter, Western blot verified the expected molecular masses, and electrophysiological and pharmacological properties confirmed the presence of functional GABA(C) receptors. rho(1)-GFP transduction resulted in an increased density of GABA(A) receptors as well as expression of novel GABA(C) receptors. This effect was not reproduced by addition of TTX or Mg(2+) to the culture medium to reduce action potentials or synaptic activity. In a model of neuronal hyperexcitability induced by chronic blockade of glutamate receptors, expression of GABA(C) receptors abolished the hyperactivity and the consequent delayed neuronal death. Adenovirus-mediated neuronal GABA(C) receptor engineering, via its dual mechanism of inhibition, may offer a way of inhibiting only those hyperexcitable neurons responsible for clinical problems, avoiding the generalized nervous system depression associated with pharmacological therapy.

Acetylcholine becomes the major excitatory neurotransmitter in the hypothalamus in vitro in the absence of glutamate excitation

Glutamate and GABA are two major fast neurotransmitters (excitatory and inhibitory, respectively) in the CNS, including the hypothalamus. They play a key role in the control of excitation/inhibition balance and determine the activity and excitability of neurons in many neuronal circuits. Using neuronal cultures, whole-cell recording, Ca(2+) imaging, and Northern blots, we studied the compensatory regulation of neuronal activity during a prolonged decrease in glutamate excitation. We report here that after a chronic (6-17 d) blockade of ionotropic glutamate receptors, neurons in hypothalamic cultures revealed excitatory electrical and Ca(2+) synaptic activity, which was not elicited in the control cultures that were not subjected to glutamate blockade. This activity was suppressed with acetylcholine (ACh) receptor antagonists and was potentiated by eserine, an inhibitor of acetylcholinesterase, suggesting its cholinergic nature. The upregulation of ACh receptors and the contribution of ACh to the control of the excitation/inhibition balance in cultures after a prolonged decrease in glutamate activity were also demonstrated. Enhanced ACh transmission was also found in chronically blocked cerebellar but not cortical cultures, suggesting the region-specific character of glutamate-ACh interactions in the brain. We believe that in the absence of glutamate excitation in the hypothalamus in vitro, ACh, a neurotransmitter normally exhibiting only weak activity in the hypothalamus, becomes the major excitatory neurotransmitter and supports the excitation/inhibition balance. The increase in excitatory ACh transmission during a decrease in glutamate excitation may represent a novel form of neuronal plasticity that regulates activity and excitability of neurons during the glutamate/GABA imbalance.

Subchronic treatment with either clozapine, olanzapine or haloperidol produces a hyposensitive response of the rat cortical cells to N-methyl-D-aspartate

Using the technique of intracellular recording in in vitro brain slice preparations, we examined the effects produced by repeated administration of the antipsychotic drugs clozapine, olanzapine and haloperidol, on N-methyl-D-aspartic acid-induced responses in pyramidal cells of the rat medial prefrontal cortex. Rats were anesthetized and decapitated 24h after the conclusion of daily intraperitoneal injection with either clozapine (25mg/kg), olanzapine (1, 5 or 10mg/kg) or haloperidol (0.5mg/kg) for 21 days, and the slices from medial prefrontal cortex were used for electrophysiological recordings. The concentration-response curves for N-methyl-D-aspartic acid to activate cortical cells shifted markedly to the right in rats which received the subchronic antipsychotic drug treatment, compared with those obtained from rats which received repeated injections of vehicle (1ml/kg/day, i.p. for 21 days). In addition, repeated exposure to antipsychotic drugs caused a significant reduction in the ability of these antipsychotic drugs to augment the N-methyl-D-aspartic acid-induced inward current in pyramidal cells of the rat medial prefrontal cortex. Repeated administration of haloperidol, but not clozapine or olanzapine, significantly hyperpolarized the resting membrane potential and increased membrane resistance in pyramidal cells of the medial prefrontal cortex. Moreover, subchronic treatment with haloperidol, but not clozapine or olanzapine, depressed (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-induced responses. The desensitized response of medial prefrontal cortex cells to N-methyl-D-aspartic acid could be the result of a compensatory response to the facilitating action of antipsychotic drugs on N-methyl-D-aspartic acid receptor-mediated transmission. The inhibitory action of haloperidol on (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid responses may also contribute to the rightward shift of the N-methyl-D-aspartic acid concentration-response curve.Thus, the present study suggests that the atypical antipsychotic drugs, clozapine and olanzapine, as well as the typical antipsychotic drug haloperidol strongly modulate glutamatergic transmission after prolonged treatment. This might be an important factor in the mechanisms by which these drugs alleviate symptoms in schizophrenic patients.

The use of nimodipine in the treatment of mood disorders

Nimodipine, a dihydropyridine calcium entry blocker, has been shown to protect from neuronal damage due to ischemia by providing for increased postischemic perfusion. Further, it has also been demonstrated to have antiepileptic properties. These two properties--calcium channel blockade and anticonvulsant benefits have been applied with success to mood disorder treatment. Although found helpful nearly a decade ago for uncomplicated mania, nimodipine may have particular benefits for those diagnostic subclasses of bipolar disorder most resistant to therapy, e.g., ultra-rapid-cycling bipolars and brief recurrent depressions.