Reciprocal modulation of glutamate and GABA release may underlie the anticonvulsant effect of phenytoin

Mapping the neurocircuitry of impulsive aggression through the pharmacologic review of anti-impulsive aggressive agents

Impulsive aggression, in contradistinction to premeditated aggression in humans or predatory aggression in animals, corresponds to defensive aggression in animal models. At the core of the neurocircuitry of impulsive aggression, from murine to feline to human species, it is the medial amygdala-mediobasal hypothalamus-dorsal periaqueductal gray pathway. Here, we update current knowledge on the neurocircuitry of impulsive aggression by placing the neurocircuitry and its neurophysiological substrates into the top-down/bottom-up hypothesis of impulsive aggression. We then reverse the neurotranslational approach, which applies neuroscience to developing therapeutic drugs, and apply current understanding of potential mechanisms of anti-impulsive aggression agents to further clarify, at least heuristically and hypothetically, the dynamic biochemical components of the neurocircuitry of impulsive aggression. To do this, we searched the medical literature for studies attempting to clarify the neurobiological and neurochemical effects of the five most widely studied anti-impulsive aggressive agents, particularly as they pertain to the top-down/bottom-up hypothesis. Multiple different mechanisms are discussed, all of which fitting in the hypothesis by way of either promoting the "top-down" part (i.e., enhancing inhibitory neurotransmitters), or suppressing the "bottom-up" part (i.e., decreasing excitatory neurotransmitters). The hypothesis appears consistent with the current psychopharmacological understanding of these agents, as well as to account for the likely multifactorial etiology of the condition. Limitations of the hypothesis and future directions are finally discussed.

ACC Glu/GABA ratio is decreased in euthymic bipolar disorder I patients: possible in vivo neurometabolite explanation for mood stabilization

Bipolar disorder (BD) is characterized by unstable mood states ranging from mania to depression. Although there is some evidence that mood instability may result from an imbalance between excitatory glutamatergic and inhibitory GABA-ergic neurotransmission, few proton magnetic resonance spectroscopy (¹H-MRS) studies have measured these two neurometabolites simultaneously in BD. The enzyme glutamic acid decarboxylase (GAD1) catalyzes the decarboxylation of glutamate (Glu) to GABA, and its single nucleotide polymorphisms (SNPs) might influence Glu/GABA ratio. Thus, we investigated Glu/GABA ratio in the dorsal anterior cingulate cortex (dACC) of euthymic BD type I patients and healthy controls (HC), and assessed the influence of both mood stabilizers and GAD1 SNPs on this ratio. Eighty-eight subjects (50 euthymic BD type I patients and 38 HC) underwent 3T ¹H-MRS in the dACC (2 × 2 × 4.5 cm³) using a two-dimensional JPRESS sequence and all subjects were genotyped for 4 SNPs in the GAD1 gene. BD patients had lower dACC Glu/GABA ratio compared to HC, where this was influenced by anticonvulsant and antipsychotic medications, but not lithium. The presence of GAD1 rs1978340 allele A was associated with higher Glu/GABA ratio in BD, while patients without this allele taking mood stabilizers had a lower Glu/GABA ratio. The lowering of dACC Glu/GABA could be one explanation for the mood stabilizing action of anticonvulsants and antipsychotics in BD type I euthymia. Therefore, this putative role of Glu/GABA ratio and the influence of GAD1 genotype interacting with mood stabilization medication should be confirmed by further studies involving larger samples and other mood states.ClincalTrials.gov registration: NCT01237158.

SCN8A Encephalopathy: Case Report and Literature Review

Epileptic encephalopathy is a condition resulting from extreme forms of intractable childhood epilepsy. The disease can cause severe delays in cognitive, sensory, and motor function development, in addition to being fatal in some cases. Missense mutations of SCN8A, which encodes Nav1.6, one of the main voltage-gated sodium channel subunits in neurons and muscles, have been linked to early infantile SCN8A encephalopathy. Herein, we report the case of a 5-month-old girl with SCN8A encephalopathy with a novel missense mutation. Apart from intractable seizures and autistic phenotypes, the results of blood and biochemical tests, electroencephalogram (EEG) results, and brain magnetic resonance imaging (MRI) results were all normal. As the phenotypes caused by these mutations cannot be identified by any clinical, neuroimaging, or electrophysiological features, genetic sequencing should be considered to identify the underlying genetic causes. Although phenytoin is recommended as a last-resort treatment for SCN8A encephalopathy, the administration of the oxcarbazepine, instead of phenytoin, mitigated this patient's intractable seizures.

Transcranial focal electrical stimulation reduces the convulsive expression and amino acid release in the hippocampus during pilocarpine-induced status epilepticus in rats

The aim of the present study was to evaluate the effects of transcranial focal electrical stimulation (TFS) on γ-aminobutyric acid (GABA) and glutamate release in the hippocampus under basal conditions and during pilocarpine-induced status epilepticus (SE). Animals were previously implanted with a guide cannula attached to a bipolar electrode into the right ventral hippocampus and a concentric ring electrode placed on the skull surface. The first microdialysis experiment was designed to determine, under basal conditions, the effects of TFS (300 Hz, 200 μs biphasic square pulses, for 30 min) on afterdischarge threshold (ADT) and the release of GABA and glutamate in the hippocampus. The results obtained indicate that at low current intensities (<2800 μA), TFS enhances and decreases the basal extracellular levels of GABA and glutamate, respectively. However, TFS did not modify the ADT. During the second microdialysis experiment, a group of animals was subjected to SE induced by pilocarpine administration (300 mg/kg, i.p.; SE group). The SE was associated with a significant rise of GABA and glutamate release (up to 120 and 182% respectively, 5h after pilocarpine injection) and the prevalence of high-voltage rhythmic spikes and increased spectral potency of delta, gamma, and theta bands. A group of animals (SE-TFS group) received TFS continuously during 2h at 100 μA, 5 min after the establishment of SE. This group showed a significant decrease in the expression of the convulsive activity and spectral potency in gamma and theta bands. The extracellular levels of GABA and glutamate in the hippocampus remained at basal conditions. These results suggest that TFS induces anticonvulsant effects when applied during the SE, an effect associated with lower amino acid release. This article is part of a Special Issue entitled "Status Epilepticus".

The effect of chronic phenytoin administration on single prolonged stress induced extinction retention deficits and glucocorticoid upregulation in the rat medial prefrontal cortex

RATIONALE

Post-traumatic stress disorder (PTSD) is a chronic, debilitating disorder. Only two pharmacological agents are approved for PTSD treatment, and they often do not address the full range of symptoms nor are they equally effective in all cases. Animal models of PTSD are critical for understanding the neurobiology involved and for identification of novel therapeutic targets. Using the rodent PTSD model, single prolonged stress (SPS), we have implicated aberrant excitatory neural transmission and glucocorticoid receptor (GR) upregulation in the medial prefrontal cortex (mPFC) and hippocampus (HPC) in fear memory abnormalities associated with PTSD.

OBJECTIVE

The objective of this study is to examine the potential protective effect of antiepileptic phenytoin (PHE) administration on SPS-induced extinction retention deficits and GR expression.

METHODS

Forty-eight SPS-treated male Sprague Dawley rats or controls were administered PHE (40, 20 mg/kg, vehicle) for 7 days following SPS stressors; then, fear conditioning, extinction, and extinction retention were tested.

RESULTS

Fear conditioning and extinction were unaffected by SPS or PHE, but SPS impaired extinction retention, and both doses of PHE rescued this impairment. Similarly, SPS increased GR expression in the mPFC and dorsal HPC, and PHE prevented SPS-induced GR upregulation in the mPFC.

CONCLUSIONS

These data demonstrate that PHE administration can prevent the development of extinction retention deficits and upregulation of GR. PHE exerts inhibitory effects on voltage-gated sodium channels and decreases excitatory neural transmission via glutamate antagonism. If glutamate hyperactivity in the days following SPS contributes to SPS-induced deficits, then these data may suggest that the glutamatergic system constitutes a target for secondary prevention.

Comparison and effects of acute lamotrigine treatment on extracellular excitatory amino acids in the hippocampus of PTZ-kindled epileptic and PTZ-induced status epilepticus rats

In this communication, the effect of acute treatment with lamotrigine (LTG) was investigated on release of main excitatory amino acids (EAA) such as glutamate (Glu) and aspartate (Asp) in the hippocampus of pentylenetetrazol (PTZ)-induced and PTZ-kindled freely moving rats using micro dialysis. The results show that, levels of Glu and Asp significantly increased in the rat hippocampus during the seizure/interical periods for PTZ-status epilepticus (SE) and PTZ-kindled epileptic (EP) rats. The levels of Glu and Asp increased more in EP rat hippocampus than in SE rat hippocampus. After administration of 20 mg/kg LTG, the levels of Glu and Asp significantly decreased in the SE and EP rat hippocampus. The results indicate that: (a) excitability of the PTZ-kindled epileptogenic model is higher than that of the status epilepticus model; (b) the modulation of LTG on the EAA neurotransmitters certainly plays an important role in antiepileptic efficacy, especially in PTZ-kindled epileptic model where the release of EAA was influenced more markedly by acute application of 20 mg/kg LTG.

What is the role of brain mechanisms underlying arousal in recovery of motor function after structural brain injuries?

PURPOSE OF REVIEW

Standard neurorehabilitation approaches have limited impact on motor recovery in patients with severe brain injuries. Consideration of the contributions of impaired arousal offers a novel approach to understand and enhance recovery.

RECENT FINDINGS

Animal and human neuroimaging studies are elucidating the neuroanatomical bases of arousal and of arousal regulation, the process by which the cerebrum mobilizes resources. Studies of patients with disorders of consciousness have revealed that recovery of these processes is associated with marked improvements in motor performance. Recent studies have also demonstrated that patients with less severe brain injuries also have impaired arousal, manifesting as diminished sustained attention, fatigue, and apathy. In these less severely injured patients, it is difficult to connect disorders of arousal with motor recovery because of a lack of measures of arousal that are independent of motor function.

SUMMARY

Arousal impairment is common after brain injury and likely plays a significant role in recovery of motor function. A more detailed understanding of this connection will help to develop new therapeutic strategies applicable for a wide range of patients. This requires new tools that continuously and objectively measure arousal in patients with brain injury, to correlate with detailed measures of motor performance and recovery.

Ethosuximide modifies network excitability in the rat entorhinal cortex via an increase in GABA release

Ethosuximide is the drug of choice for treating generalized absence seizures, but its mechanism of action is still a matter of debate. It has long been thought to act by disrupting a thalamic focus via blockade of T-type channels and, thus, generation of spike-wave activity in thalamocortical pathways. However, there is now good evidence that generalized absence seizures may be initiated at a cortical focus and that ethosuximide may target this focus. In the present study we have looked at the effect ethosuximide on glutamate and GABA release at synapses in the rat entorhinal cortex in vitro, using two experimental approaches. Whole-cell patch-clamp studies revealed an increase in spontaneous GABA release by ethosuximide concurrent with no change in glutamate release. This was reflected in studies that estimated global background inhibition and excitation from intracellularly recorded membrane potential fluctuations, where there was a substantial rise in the ratio of network inhibition to excitation, and a concurrent decrease in excitability of neurones embedded in this network. These studies suggest that, in addition to well-characterised effects on ion channels, ethosuximide may directly elevate synaptic inhibition in the cortex and that this could contribute to its anti-absence effects. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Carisbamate (RWJ-333369) inhibits glutamate transmission in the granule cell of the dentate gyrus

Carisbamate (CRS, RWJ-333369) is a novel antiepileptic drug awaiting approval for use in the treatment of partial and generalized seizures. Our aim was to determine whether CRS modulates synaptic transmission in the dentate gyrus (DG) and the underlying mechanism. The whole-cell patch-clamp method was used to record AMPA receptor- and NMDA receptor-mediated excitatory postsynaptic currents (EPSC(AMPA) and EPSC(NMDA)) and GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) in granule cells of the DG in brain slices prepared from 3- to 5-week-old male Wistar rats. CRS (30-300 μM) inhibited the evoked EPSC(AMPA) and EPSC(NMDA) by the same extent (20%) with significantly altered CV(-2), suggesting presynaptic modulation. It did not significantly change the inward currents induced by AMPA application. The inhibitory effect of CRS on the evoked EPSC(AMPA) was not occluded by selective voltage-gated Ca(2+) channel blockers, ruling out the involvement of presynaptic Ca(2+) channels. The frequency, but not the amplitude, of spontaneous EPSC(AMPA) was significantly reduced by CRS. However, CRS did not alter either the frequency or the amplitude of TTX-insensitive miniature EPSC(AMPA), indicating an action potential-dependent mechanism was involved. In addition, CRS (100 or 300 μM) did not significantly change the amplitude of the evoked IPSCs. To summarize, our results suggest that CRS reduces glutamatergic transmission by an action potential-dependent presynaptic mechanism and consequently inhibits excitatory synaptic strength in the DG without affecting GABAergic transmission. This effect may contribute to the antiepileptic action observed clinically at therapeutic concentrations of CRS.

Levetiracetam: a review of its use in epilepsy

Levetiracetam (Keppra®, E Keppra®) is an established second-generation antiepileptic drug (AED). Worldwide, levetiracetam is most commonly approved as adjunctive treatment of partial onset seizures with or without secondary generalization; other approved indications include monotherapy treatment of partial onset seizures with or without secondary generalization, and adjunctive treatment of myoclonic seizures associated with juvenile myoclonic epilepsy and primary generalized tonic-clonic (GTC) seizures associated with idiopathic generalized epilepsy. Levetiracetam has a novel structure and unique mechanisms of action. Unlike other AEDs, the mechanisms of action of levetiracetam appear to involve neuronal binding to synaptic vesicle protein 2A, inhibiting calcium release from intraneuronal stores, opposing the activity of negative modulators of GABA- and glycin-gated currents and inhibiting excessive synchronized activity between neurons. In addition, levetiracetam inhibits N-type calcium channels. Levetiracetam is associated with rapid and complete absorption, high oral bioavailability, minimal metabolism that consists of hydrolysis of the acetamide group, and primarily renal elimination. It lacks cytochrome P450 isoenzyme-inducing potential and is not associated with clinically significant pharmacokinetic interactions with other drugs, including other AEDs. The efficacy of oral immediate-release levetiracetam in controlling seizures has been established in numerous randomized, double-blind, controlled, multicentre trials in patients with epilepsy. Adjunctive levetiracetam reduced the frequency of seizures in paediatric and adult patients with refractory partial onset seizures to a significantly greater extent than placebo. Monotherapy with levetiracetam was noninferior to that with carbamazepine controlled release in controlling seizures in patients with newly diagnosed partial onset seizures. Levetiracetam also provided seizure control relative to placebo as adjunctive therapy in patients with idiopathic generalized epilepsy with myoclonic seizures or GTC seizures. In addition, patients receiving oral levetiracetam showed improvements in measures of health-related quality of life relative to those receiving placebo. Although treatment-emergent adverse events were commonly reported in the clinical trials of levetiracetam, the overall proportion of patients who experienced at least one treatment-emergent adverse event was broadly similar in the levetiracetam and placebo treatment groups, with most events being mild to moderate in severity. Levetiracetam is not associated with cognitive impairment or drug-induced weight gain, but has been associated with behavioural adverse effects in some patients.

GABAergic drugs in the treatment of epilepsy: modern or outmoded?

Antiepileptic drugs have a number of mechanisms of action that target brain excitability systems. The potentiation of GABAergic inhibitory neurotransmission represents a classic and well-known antiseizure effect. Currently available GABAergic antiepileptic drugs mainly target GABA(A) receptor-associated complexes, GABA reuptake or GABA catabolism. All these compounds, although generally effective, are limited by their deleterious effects on cognition and behavior. The challenge will be to find GABAergic drugs that exhibit the beneficial effects, without the adverse ones.

A critical test of the hippocampal theta model of anxiolytic drug action

Hippocampal theta rhythms have been associated with a number of behavioural processes, including learning, memory and arousal. Recently it has been argued that the suppression of hippocampal theta is a valid indicator of anxiolytic drug action. Like all such models, however, it has relied almost exclusively on the experimental effects of well-known, clinically proven anxiolytic compounds for validation. The actual predictive validity of putative models of anxiolytic drug action, however, cannot be rigorously tested with this approach alone. The present study provides a stringent test of the predictive validity of the theta suppression model, using the drug phenytoin (50 mg/kg and 10 mg/kg), and a positive comparison compound, diazepam (2 mg/kg). Phenytoin has two important properties that are advantageous for assessing the validity of the theta suppression model: 1) it is a standard antiepileptic drug with no known anxiolytic effects, and 2) its primary mechanism of action is through suppression of the persistent sodium current, an effect that should also suppress hippocampal theta. Because of the latter property, we also directly compared the effects of phenytoin in the theta suppression model with its effects in the most widely tested behavioural model of anxiolytic drug action, the elevated plus-maze. While an anxiolytic-like effect of phenytoin in the theta suppression model might be expected simply due to its suppressive effects on sodium channel currents, anxiolytic effects in both tests would provide strong support for the predictive validity of the theta suppression model. Surprisingly, phenytoin produced clear anxiolytic-like effects in both neurophysiological and behavioural models, thus providing strong evidence of the predictive validity of the theta suppression model. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Dilantin therapy in an experimental model of traumatic brain injury: effects of limited versus daily treatment on neurological and behavioral recovery

The mechanisms by which Dilantin confers anticonvulsant benefits may also be neuroprotective by attenuating the acute excitatory insult in cortical and subcortical structures when the drug is given in the acute phase after traumatic brain injury (TBI). However, when Dilantin is used for prolonged periods, we hypothesized that it may impede recovery, synaptic plasticity may be impaired, and neuroprotective benefits may be lost. As such, we assessed the effect of daily chronic administration (75 mg/kg day 0 followed by 50 mg/kg daily i.p.) and acute administration (75 mg/kg day 0 followed by 50 mg/kg i.p. day 1) of Dilantin in young adult male rats on motor performance, y-maze exploration, Morris Water Maze (MWM), hippocampal (HC) cell survival, contusion size, and regional expression of neuroplasticity markers after controlled cortical impact (CCI) injury. Chronic daily Dilantin administration resulted in beam walking impairments on day 6, whereas acute Dilantin administration resulted in beam walking impairments on days 3 and 4. Chronic Dilantin administration also resulted in worse MWM performance, more HC cell loss and no increases in neuroplasticity markers compared to rats with CCI receiving chronic vehicle. Conversely, rats receiving acute Dilantin administration exhibited more novel arm exploration in the y-maze, greater HC cell sparing, and greater growth-associated protein 43 (GAP-43) expression in the HC ipsilateral to the CCI, compared to injured rats receiving vehicle. MWM was not influenced by acute Dilantin administration. These results suggest that there are beneficial effects of limited acute Dilantin therapy after TBI, and that extended daily Dilantin therapy has deleterious effects on neural recovery. These findings support clinical guidelines for limited use of Dilantin in seizure prophylaxis after TBI.

Effect of prolonged phenytoin administration on rat brain gene expression assessed by DNA microarrays

Preliminary clinical trials have recently shown that phenytoin, an antiepileptic drug, may also be beneficial for treatment of bipolar disorder. To examine molecular mechanisms of action of phenytoin as a potential mood stabilizer, DNA microarrays were used to study the effect of phenytoin on gene expression in the hippocampus and frontal cortex of Sprague-Dawley rats. While our particular interest is in bipolar disorder, this is the first DNA microarray study on the effect of phenytoin in brain tissue, in general. As compared with control rats, treated rats had 508 differentially expressed genes in the hippocampus and 62 in the frontal cortex. Phenytoin modulated the expression of genes which may affect neurotransmission, e.g. glutamate decarboxylase 1 (Gad1) and gamma-aminobutyric acid A receptor, alpha 5 (Gabra5). Phenytoin also exerted an effect on neuroprotection-related genes, namely the survival-promoting and antioxidant genes v-akt murine thymoma viral oncogene homolog 1 (Akt1), FK506 binding protein 12-rapamycin associated protein 1 (Frap1), glutathione reductase (Gsr) and glutamate cysteine ligase catalytic subunit (Gclc). The expression of genes potentially associated with mechanisms of mood regulation such as adenylate cyclase-associated protein 1 (Cap1), Glial Fibrillary Acidic Protein (Gfap) and prodynorphin (Pdyn) was also altered. Some of the above genes are regarded as targets of classical mood stabilizers and their modulation supports the clinical observation that phenytoin may have mood-stabilizing effects. The results may provide new insights regarding the mechanism of action of phenytoin and genes found differentially expressed following phenytoin administration may play a role in the pathophysiology of either bipolar disorder or epilepsy.

Diverse antiepileptic drugs increase the ratio of background synaptic inhibition to excitation and decrease neuronal excitability in neurones of the rat entorhinal cortex in vitro

Although most anti-epileptic drugs are considered to have a primary molecular target, it is clear that their actions are unlikely to be limited to effects on a single aspect of inhibitory synaptic transmission, excitatory transmission or voltage-gated ion channels. Systemically administered drugs can obviously simultaneously access all possible targets, so we have attempted to determine the overall effect of diverse agents on the balance between GABAergic inhibition, glutamatergic excitation and cellular excitability in neurones of the rat entorhinal cortex in vitro. We used an approach developed for estimating global background synaptic excitation and inhibition from fluctuations in membrane potential obtained by intracellular recordings. We have previously validated this approach in entorhinal cortical neurones [Greenhill and Jones (2007a) Neuroscience 147:884–892]. Using this approach, we found that, despite their differing pharmacology, the drugs tested (phenytoin, lamotrigine, valproate, gabapentin, felbamate, tiagabine) were unified in their ability to increase the ratio of background GABAergic inhibition to glutamatergic excitation. This could occur as a result of decreased excitation concurrent with increased inhibition (phenytoin, lamotrigine, valproate), a decrease in excitation alone (gabapentin, felbamate), or even with a differential increase in both (tiagabine). Additionally, we found that the effects on global synaptic conductances agreed well with whole cell patch recordings of spontaneous glutamate and GABA release (our previous studies and further data presented here). The consistency with which the synaptic inhibition:excitation ratio was increased by the antiepileptic drugs tested was matched by an ability of all drugs to concurrently reduce intrinsic neuronal excitability. Thus, it seems possible that specific molecular targets among antiepileptic drugs are less important than the ability to increase the inhibition:excitation ratio and reduce overall neuronal and network excitability.

Ion channels in epilepsy

Neuronal excitability is determined by the flux of ions through ion channels. Many types of ion channels are expressed in the central nervous system, each responsible for its own aspect of neuronal excitability, from postsynaptic depolarization to action potential generation to neurotransmitter release. These mechanisms are tightly regulated to create a balance between excitation and inhibition. Disruption of this balance is thought to be key in many neurological disorders, including epilepsy syndromes. More and more ion channel mutations are being identified through genetic studies; however, their incidence is still small, suggesting the presence of undiscovered mutations or other causative mechanisms. Understanding wild-type channel function during epileptic activity may also provide vital insights into the remaining idiopathic epilepsies and provide targets for future antiepileptic drugs.

Lamotrigine inhibits postsynaptic AMPA receptor and glutamate release in the dentate gyrus

PURPOSE

The dentate gyrus (DG) is a gateway that regulates seizure activity in the hippocampus. We investigated the site of action of lamotrigine (LTG), an effective anticonvulsant, in the regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptor-mediated excitatory synaptic transmission on DG.

METHODS

Evoked AMPA and NMDA receptor-mediated excitatory postsynaptic currents (eEPSCampa and eEPSCnmda) were recorded by whole-cell patch-clamp recording from the granule cells of DG in brain slice preparation of young Wistar rats (60-120 g). Exogenously applied AMPA and NMDA-induced currents and AMPA receptor-mediated miniature EPSC (mEPSCampa) were recorded in the presence of specific antagonists.

RESULTS

LTG inhibited both eEPSCampa and eEPSCnmda, and had no effect on exogenously applied NMDA-induced current indicating LTG inhibited glutamate release. Previous studies demonstrated that alteration in glutamate concentration in synaptic cleft causes parallel changes of eEPSCampa and eEPSCnmda. Our results showed that LTG inhibited eEPSCampa significantly more than eEPSCnmda (p < 0.05), suggesting that LTG may also have blocked the postsynaptic AMPA receptor. The hypothesis is further supported by the facts that; (1) LTG (30-100 microM) inhibited direct exogenously applied AMPA-induced currents (to 90%), (2) LTG significantly reduced the amplitude, but not the frequency of mEPSCampa and asynchronous (EPSC), and (3) LTG-induced reduction of eEPSCampa was not associated with a modification of the paired-pulse ratio. To sum up, LTG exerts a postsynaptic inhibitory mechanism on the AMPA receptor.

CONCLUSIONS

Our results demonstrate that LTG suppresses postsynaptic AMPA receptors and reduces glutamate release in granule cells of DG. The postsynaptic effect can be one of the underlying mechanisms of LTG's anticonvulsant action.

Felbamate but not phenytoin or gabapentin reduces glutamate release by blocking presynaptic NMDA receptors in the entorhinal cortex

We have shown that a number of anticonvulsant drugs can reduce glutamate release at synapses in the rat entorhinal cortex (EC) in vitro. We have also shown that presynaptic NMDA receptors (NMDAr) tonically facilitate glutamate release at these synapses. In the present study we determined whether, phenytoin, gabapentin and felbamate may reduce glutamate release by blocking the presynaptic NMDAr. Whole cell patch clamp recordings of spontaneous excitatory postsynaptic currents (sEPSCs) were used as a monitor of presynaptic glutamate release. Postsynaptic NMDAr were blocked with internal dialysis with an NMDAr channel blocker. The antagonist, 2-AP5, reduced the frequency of sEPSCs by blocking the presynaptic facilitatory NMDAr, but did not occlude a reduction in sEPSC frequency by gabapentin or phenytoin. Felbamate also reduced sEPSC frequency, but this effect was occluded by prior application of 2-AP5. Thus, whilst all three drugs can reduce glutamate release, only the action of felbamate seems to be due to interaction with presynaptic NMDAr.

Molecular targets for antiepileptic drug development

This review considers how recent advances in the physiology of ion channels and other potential molecular targets, in conjunction with new information on the genetics of idiopathic epilepsies, can be applied to the search for improved antiepileptic drugs (AEDs). Marketed AEDs predominantly target voltage-gated cation channels (the alpha subunits of voltage-gated Na+ channels and also T-type voltage-gated Ca2+ channels) or influence GABA-mediated inhibition. Recently, alpha2-delta voltage-gated Ca2+ channel subunits and the SV2A synaptic vesicle protein have been recognized as likely targets. Genetic studies of familial idiopathic epilepsies have identified numerous genes associated with diverse epilepsy syndromes, including genes encoding Na+ channels and GABA(A) receptors, which are known AED targets. A strategy based on genes associated with epilepsy in animal models and humans suggests other potential AED targets, including various voltage-gated Ca2+ channel subunits and auxiliary proteins, A- or M-type voltage-gated K+ channels, and ionotropic glutamate receptors. Recent progress in ion channel research brought about by molecular cloning of the channel subunit proteins and studies in epilepsy models suggest additional targets, including G-protein-coupled receptors, such as GABA(B) and metabotropic glutamate receptors; hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel subunits, responsible for hyperpolarization-activated current Ih; connexins, which make up gap junctions; and neurotransmitter transporters, particularly plasma membrane and vesicular transporters for GABA and glutamate. New information from the structural characterization of ion channels, along with better understanding of ion channel function, may allow for more selective targeting. For example, Na+ channels underlying persistent Na+ currents or GABA(A) receptor isoforms responsible for tonic (extrasynaptic) currents represent attractive targets. The growing understanding of the pathophysiology of epilepsy and the structural and functional characterization of the molecular targets provide many opportunities to create improved epilepsy therapies.

The role of glutamate in anxiety and related disorders

Anxiety, stress, and trauma-related disorders are a major public health concern in the United States. Drugs that target the gamma-aminobutyric acid or serotonergic system, such as benzodiazepines and selective serotonin reuptake inhibitors, respectively, are the most widely prescribed treatments for these disorders. However, the role of glutamate in anxiety disorders is becoming more recognized with the belief that drugs that modulate glutamatergic function through either ionotropic or metabotropic glutamate receptors have the potential to improve the current treatment of these severe and disabling illnesses. Animal models of fear and anxiety have provided a method to study the role of glutamate in anxiety. This research has demonstrated that drugs that alter glutamate transmission have potential anxiolytic action for many different paradigms including fear-potentiated startle, punished responding, and the elevated plus maze. Human clinical drug trials have demonstrated the efficacy of glutamatergic drugs for the treatment of obsessive-compulsive disorder, posttraumatic stress disorder, generalized anxiety disorder, and social phobia. Recent data from magnetic resonance imaging studies provide an additional link between the glutamate system and anxiety. Collectively, the data suggest that future studies on the mechanism of and clinical efficacy of glutamatergic agents in anxiety disorders are appropriately warranted.

Lamotrigine, carbamazepine and phenytoin differentially alter extracellular levels of 5-hydroxytryptamine, dopamine and amino acids

We have studied the effects of treatment with the anticonvulsants lamotrigine (LTG), phenytoin (PHN) and carbamazepine (CBZ) on basal and stimulated extracellular aspartate (ASP), glutamate (GLU), taurine (TAU), GABA, 5-hydroxytryptamine (5-HT) and dopamine (DA) in the hippocampus of freely moving rats using microdialysis. All of the drugs investigated have had inhibition of Na(+) channel activity implicated as their principal mechanism of action. Neither LTG (10-20 mg/kg), PHN (20-40 mg/kg) or CBZ (10-20 mg/kg) had an effect on the basal extracellular concentrations of any of the amino acids studied with the exception of glutamate, which was decreased at the highest LTG dose. However, when amino acid transmitter levels were increased with 50 microM veratridine, LTG was found to cause a dose-dependent decrease in dialysate levels of all four amino acids, with the effect being most pronounced for glutamate. In contrast, PHN decreased extracellular aspartate levels but had no effect on evoked-extracellular GLU, TAU or GABA. Somewhat unexpectedly, CBZ did not alter the stimulated increase in the excitatory amino acids, GLU and ASP, but, rather surprisingly for an antiepileptic drug, markedly decreased that of the inhibitory substances TAU and GABA. The three drugs had differing effects on basal extracellular 5-HT and DA. LTG caused a dose-dependent decrease in both, while CBZ and PHN both increased extracellular 5-HT and DA. When extracellular 5-HT and DA was evoked by veratridine LTG had no significant effect on this, while PHN but not CBZ increased stimulated extracellular 5-HT and both PHN and CBZ augmented DA. Thus, the effects of the three drugs studied seemed to depend on whether extracellular transmitter levels are evoked or basal and the particular transmitter in question. This suggests that there are marked differences in the neurochemical mechanisms of antiepileptic drug action of the three compounds studied.

Neurochemical underpinnings in bipolar disorder and epilepsy

In this article, we discuss and highlight some of the potential neurochemical underpinnings of bipolar disorder (BD) and epilepsy. Some similarities are found in both disorders, such as the episodic course of the illnesses, the possible mechanism of kindling, and the efficacy of some antiepileptic drugs (AEDs) in treatment, all pointing to a common underlying pathophysiology. Common mechanisms at the level of ion channels might include the antikindling and the calcium-antagonistic and potassium outward current-modulating properties of AEDs. However, future research on intracellular mechanisms might become decisive for a better understanding of the similarities between the disorders.

Effects of acute and chronic lamotrigine treatment on basal and stimulated extracellular amino acids in the hippocampus of freely moving rats

The antiepileptic drug lamotrigine (LTG) is a relatively novel anticonvulsant frequently used in polytherapy and increasingly in monotherapy. LTG is believed to act by reducing excitatory glutamate (GLU) release due to an inhibition of Na(+) channels. In the present study, we have investigated the effects of acute and chronic (up to 21 days) treatment with LTG on basal and either veratridine- or KCl-stimulated release of aspartate (ASP), GLU, taurine (TAU) and GABA in the hippocampus of freely moving rats using microdialysis. Additionally, we have measured LTG concentrations in the plasma, whole brain and extracellular fluid of rats at the same time points. LTG significantly reduced basal ASP and GLU but only at the highest dose used (20 mg/kg) and was entirely without effect on basal TAU or GABA. When either veratridine or 100 mM KCl were added to the infusion medium amino acid release was evoked although the extent of this varied from one amino acid to another. LTG (10 mg/kg) reduced veratridine-evoked release of all four amino acids studied, although this was most marked in the case of GLU. LTG had no effect on KCl-stimulated amino acid release. When given for up to 21 days (2 x 5 mg/kg/day), LTG had no effect on basal amino acid levels. In contrast, LTG demonstrated over the time period studied an increasingly inhibitory effect on veratridine-evoked amino acid release. This effect of the drug was proportionally much greater in the case of GLU than for the other three amino acids studied. Measurement of plasma, whole brain tissue and extracellular LTG showed that in each of these compartments, it had reached an apparent steady state within 4 days of commencement of treatment and appeared to mirror the neurochemical changes measured. Our estimate of plasma LTG indicates that during chronic study, this was well within the therapeutic range, suggesting that the current neurochemical observations are clinically relevant.

A anticonvulsive fraction from Scaptocosa raptoria (Araneae: Lycosidae) spider venom

Several spider neurotoxins are known to show highly selective effects on nervous tissues. Intracerebral injection into rats of spider venom from Scaptocosa raptoria, prevents seizures induced by convulsant agents. Injection of phenytoin (390 pmol/200 nl), muscimol (90 pmol/200 nl), baclofen (500 pmol/200 nl) into the substantia nigra (SN) pars reticulata, protected rats from convulsions evoked by unilateral focal injection of bicuculline into the area tempestas by 50, 80, and 100%, respectively. Denatured S. raptoria crude venom (4.6 microg, 2.3 microg, and 920 ng/200 nl), when administered into the SN, prevented seizures elicited by bicuculline in the area tempestas by 100, 100, and 87.5%, respectively. The injection into the SN of 160 ng/200 nl of fraction SrTx1 isolated from S. raptoria venom, reduced the magnitude of seizures. This fraction was rechromatographed affording fractions SrTx1.1, SrTx1.2 and SrTx1.3, and they were administered into the SN at doses of 100, 200, and 400 ng/200 nl respectively. Fraction SrTx1.3 protected 50, 85.7, and 100% of the animals against the seizures elicited by bicuculline injected into the area tempestas. This suggests that S. raptoria venom as well as its SrTx1.3 fraction, might be potential sources of new anticonvulsant drugs.

Background synaptic activity in rat entorhinal cortical neurones: differential control of transmitter release by presynaptic receptors

The entorhinal cortex (EC) is a key brain area controlling both hippocampal input and output via neurones in layer II and layer V, respectively. It is also a pivotal area in the generation and propagation of epilepsies involving the temporal lobe. We have previously shown that within the network of the EC, neurones in layer V are subject to powerful synaptic excitation but weak inhibition, whereas the reverse is true in layer II. The deep layers are also highly susceptible to acutely provoked epileptogenesis. Considerable evidence now points to a role of spontaneous background synaptic activity in control of neuronal, and hence network, excitability. In the present article we describe results of studies where we have compared background release of the excitatory transmitter, glutamate, and the inhibitory transmitter, GABA, in the two layers, the role of this background release in the balance of excitability, and its control by presynaptic auto- and heteroreceptors on presynaptic terminals.

Effects of flunarizine on spontaneous synaptic currents in rat neocortex

Flunarizine, a non-selective blocker of voltage-dependent Ca(2+) and Na(+) channels, is clinically effective against several neurological disorders, including epilepsy, migraine, and alternating hemiplegia of childhood. We examined the effects of flunarizine on spontaneous post-synaptic currents in acute brain slices maintained in vitro using patch-clamp electrophysiology. Flunarizine significantly attenuated the amplitude of spontaneous currents in pyramidal neurons from juvenile rat neocortex. Flunarizine had no effect on miniature spontaneous events recorded in the presence of tetrodotoxin, a blocker of voltage-dependent sodium channels. In high (9 mM) extracellular potassium, flunarizine reduced the amplitude and frequency of the spontaneous currents. Additionally, dimethyl sulfoxide, the solvent used in our experiments, reduced the amplitude of spontaneous currents, but only in high extracellular potassium. Our data suggest that the clinical activity of flunarizine may in part be a consequence of reducing spontaneous synaptic currents in the neocortex, especially under conditions of heightened neuronal activity.

Dual effects of gabapentin and pregabalin on glutamate release at rat entorhinal synapses in vitro

We have recently shown that the anticonvulsant drugs phenytoin, lamotrigine and sodium valproate all reduce the release of glutamate at synapses in the entorhinal cortex in vitro. In the present investigation we determined whether this property was shared by gabapentin and pregabalin, using whole-cell patch-clamp recordings of excitatory postsynaptic currents (EPSCs) in layer V neurons in slices of rat entorhinal cortex. Both drugs reduced the amplitude and increased the paired-pulse ratio of EPSCs evoked by electrical stimulation of afferent inputs, suggesting a presynaptic effect to reduce glutamate release. The frequency of spontaneous EPSCs (sEPSCs) was concurrently reduced by GBP, further supporting a presynaptic action. There was no significant change in amplitude although a slight reduction was seen, particularly with gabapentin, which may reflect a reduction in the number of larger amplitude sEPSCs. When activity-independent miniature EPSCs were recorded in the presence of tetrodotoxin, both drugs continued to reduce the frequency of events with no change in amplitude. The reduction in frequency induced by gabapentin or pregabalin was blocked by application of the l-amino acid transporter substrate l-isoleucine. The results show that gabapentin and pregabalin, like other anticonvulsants, reduce glutamate release at cortical synapses. It is possible that this reduction is a combination of two effects: a reduction of activity-dependent release possibly via interaction with P/Q-type voltage-gated Ca channels, and a second action, as yet unidentified, occurring downstream of Ca influx into the presynaptic terminals.

Facilitation of a weak training experience in the 1-day-old chick using diphenylhydantoin: a pharmacological and biochemical study

This series of studies used a weakly trained (20% methyl anthranilate) version of the passive avoidance learning task in the 1-day-old chick to investigate memory facilitation effects by diphenylhydantoin (DPH). The results indicated that the pairing of the weak training experience with DPH results in facilitation of memory that can be observed from 40 min following training with the weak training experience. The results from a biochemical experiment indicated that DPH facilitates the activity of Na(+)/K(+)-ATPase at the majority of times sampled in a large percentage of the sections of the chick brain. The most marked level of elevation in the activity of the enzyme was observed at the 20-min time point following weak training in the section of the chick brain, which contained several memory relevant neuroanatomical loci. This represents a 68% increase in the activity of the enzyme in those areas considered to be crucial to the processing of memory in the paradigm at a time predicted by previous investigation to be crucial in the development of the intermediate-term memory stage of memory. The results of this series of studies support the notion that Na(+)/K(+)-ATPase plays an important role in memory processing following passive avoidance training in the 1-day-old chick.

Dendrotoxin sensitive potassium channels modulate GABA but not glutamate release in the rat entorhinal cortex in vitro

We have previously shown that the anticonvulsant drug, phenytoin, increases the frequency and amplitude of spontaneous inhibitory postsynaptic currents at GABA synapses on principal neurones in the rat entorhinal cortex. This effect is similar to that seen at other GABA synapses following blockade of voltage-gated potassium channels (Kv1.1, 1.2 and 1.6) with alpha-dendrotoxin. In the present study we examined whether dendrotoxins can alter GABA release at synapses in the entorhinal cortex. We recorded spontaneous inhibitory postsynaptic currents using whole cell voltage clamp techniques in slices of rat entorhinal cortex in vitro. alpha-Dendrotoxin evoked an increase in frequency and amplitude of spontaneous inhibitory postsynaptic currents, an effect that was blocked by prior perfusion with tetrodotoxin. The effect of the toxin did not occlude the increase in spontaneous inhibitory postsynaptic currents seen with phenytoin. Indeed, the effect of the two drugs together was, at least, additive on GABA release. Perfusion with the specific Kv1.1 blocker, dendrotoxin-K had no effect on GABA release. In addition, alpha-dendrotoxin had no effect on frequency or amplitude of spontaneous excitatory postsynaptic currents at glutamate synapses on entorhinal cortex neurones. We conclude that K-channels containing the Kv1.2 and/or 1.6 subunits modulate the release of GABA, but not glutamate in the entorhinal cortex. The modulation of GABA release by phenytoin is unlikely to be due to an effect on these channels.

Phenytoin ameliorates the memory deficit induced in the young chick by ethanol toxicity in association with thiamine deficiency

The experiments reported in this series of studies demonstrate that thiamine deficiency, induced pharmacologically by oxythiamine (OT) coupled with the acute administration of ethanol can adversely affect memory processing in the young chick. The interaction between the avitaminosis and ethanol neurotoxicity exerted its effects through the inhibition of the development of the intermediate (ITM) stage of memory (i.e., following 10 min after training), consequently affecting development of the long-term stage of memory and leaving the short-term memory stage intact. The amnesic effect of OT-induced thiamine deficiency and exposure to alcohol was ameliorated by the administration of phenytoin [diphenylhydantoin (DPH)] immediately following the training experience. As the ITM stage of memory has been suggested to rely on the activities of Na(+)/K(+) ATPase, and as DPH is a facilitator of Na(+)/K(+) ATPase activity amongst its other actions, it may be that the combined effect of OT and ethanol exposure is by interfering with Na(+)/K(+) ATPase activity, thus undermining the expression and maintenance of the memory in the period from 10 min following aversant training.

NR2B-containing NMDA autoreceptors at synapses on entorhinal cortical neurons

We have previously shown that presynaptic N-methyl-D-aspartate receptors (NMDARs) can facilitate glutamate release onto principal neurons in the entorhinal cortex (EC). In the present study, we have investigated the subunit composition of these presynaptic NMDARs. We recorded miniature alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated excitatory postsynaptic currents (mEPSCs), from visually identified neurons in layers II and V of the EC in vitro. In both layers, bath application of the NR2A/B subunit-selective agonist, homoquinolinic acid (HQA), resulted in a marked facilitation of mEPSC frequency. Blockade of presynaptic Ca(2+) entry through either NMDARs or voltage-gated Ca(2+) channels with Co(2+) prevented the effects of HQA, confirming that Ca(2+) entry to the terminal was required for facilitation. When the NR2B-selective antagonist, ifenprodil, was applied prior to HQA, the increase in mEPSC frequency was greatly reduced. In addition, we found that an NMDAR antagonist blocked frequency-dependent facilitation of evoked release and reduced mEPSC frequency in layer V. Thus we have demonstrated that NMDA autoreceptors in layer V of the EC bear the NR2B subunit, and that NMDARs are also present at terminals onto superficial neurons.

Laminar differences in recurrent excitatory transmission in the rat entorhinal cortex in vitro

Paired intracellular recordings were used to investigate recurrent excitatory transmission in layers II, III and V of the rat entorhinal cortex in vitro. There was a relatively high probability of finding a recurrent connection between pairs of pyramidal neurons in both layer V (around 12%) and layer III (around 9%). In complete contrast, we have failed to find any recurrent synaptic connections between principal neurons in layer II, and this may be an important factor in the relative resistance of this layer in generating synchronized epileptiform activity. In general, recurrent excitatory postsynaptic potentials in layers III and V of the entorhinal cortex had similar properties to those recorded in other cortical areas, although the probabilities of connection are among the highest reported. Recurrent excitatory postsynaptic potentials recorded in layer V were smaller with faster rise times than those recorded in layer III. In both layers, the recurrent potentials were mediated by glutamate primarily acting at alpha-amino-3-hydroxy-5-methyl-4-isoxazole receptors, although there appeared to be a slow component mediated by N-methyl-D-aspartate receptors. In layer III, recurrent transmission failed on about 30% of presynaptic action potentials evoked at 0.2Hz. This failure rate increased markedly with increasing (2, 3Hz) frequency of activation. In layer V the failure rate at low frequency was less (19%), and although it increased at higher frequencies this effect was less pronounced than in layer III. Finally, in layer III, there was evidence for a relatively high probability of electrical coupling between pyramidal neurons. We have previously suggested that layers IV/V of the entorhinal cortex readily generate synchronized epileptiform discharges, whereas layer II is relatively resistant to seizure generation. The present demonstration that recurrent excitatory connections are widespread in layer V but not layer II could support this proposal. The relatively high degree of recurrent connections and electrical coupling between layer III cells may be a factor in it's susceptibility to neurodegeneration during chronic epileptic conditions.

The anticonvulsant, lamotrigine decreases spontaneous glutamate release but increases spontaneous GABA release in the rat entorhinal cortex in vitro

It has been suggested that the anticonvulsant effect of lamotrigine resides with it's ability to block voltage gated Na-channels at presynaptic sites, thus stabilizing the presynapse, and, consequently, reducing the release of synaptic transmitters. Neurochemical studies have shown that it can inhibit the veratrine-stimulated release of the excitatory transmitter, glutamate from cortical tissue, but that at slightly higher concentrations it also reduces the release of the inhibitory transmitter, GABA. In the present study we examined the effect of the drug on the release of these transmitters at synapses in the rat entorhinal cortex, using the whole-cell patch clamp technique to record spontaneous excitatory (EPSCs) and inhibitory postsynaptic currents (IPSCs). Lamotrigine reduced the frequency, but not the amplitude of spontaneous EPSCs. This clearly indicated a presynaptic effect to reduce the release of glutamate. However, the same effect was observed when we tested the drug on miniature EPSCs, recorded in the presence of TTX and Cd, showing that blockade of Na-channels or Ca-channels was not a prerequisite for inhibition of glutamate release. In contrast to it's effects on EPSCs, lamotrigine increased both the frequency and amplitude of spontaneous IPSCs, suggesting that the drug was acting presynaptically to enhance GABA release. Again, similar effects were seen with miniature IPSCs recorded in TTX. These opposite effects of lamotrigine on glutamate and GABA release are similar to those we have reported previously with phenytoin, and suggest that reciprocal modulation of the background release of the major excitatory and inhibitory transmitters may be a significant factor in dampening excitability in pathologically hyperexcitable cortical networks.