Topiramate reduces AMPA-induced Ca(2+) transients and inhibits GluR1 subunit phosphorylation in astrocytes from primary cultures

Gabapentin activates ROMK1 channels by a protein kinase A (PKA)-dependent mechanism

BACKGROUND AND PURPOSE

Gabapentin is an effective anticonvulsant. The major physiological function of renal outer medullary potassium (ROMK1) channels is to maintain the resting membrane potential (RMP). We investigated the effect of gabapentin on ROMK1 channels and the mechanism involved.

EXPERIMENTAL APPROACH

Xenopus oocytes were injected with mRNA coding for wild-type or mutant ROMK1 channels and giant inside-out patch-clamp recordings were performed.

KEY RESULTS

Gabapentin increased the activity of ROMK1 channels, concentration-dependently and enhanced the activity of wild-type and an intracellular pH (pH(i))-gating residue mutant (K80M) channels over a range of pH(i). Gabapentin also increased activity of channels mutated at phosphatidylinositol 4,5-bisphosphate (PIP(2))-binding sites (R188Q, R217A and K218A). However, gabapentin failed to enhance channel activity in the presence of protein kinase A (PKA) inhibitors and did not activate phosphorylation site mutants (S44A, S219A or S313A), mutants that mimicked the negative charge carried by a phosphate group bound to a serine (S44D, S219D or S313D), or a mutated channel with a positive charge (S219R). These findings show that gabapentin activates ROMK1 channels independently of the pH(i) and not via a PIP(2)-dependent pathway. The effects of gabapentin on ROMK1 channels may be due to a PKA-mediated phosphorylation-induced conformational change, but not to charge-charge interactions.

CONCLUSIONS AND IMPLICATIONS

ROMK1 channels are the main channels responsible for maintaining the RMP during cellular excitation. Gabapentin increased the activity of ROMK1 channels by a PKA-dependent mechanism, reducing neuronal excitability, and this may play an important role in its antiepileptic effect.

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.

Molecular pharmacodynamics, clinical therapeutics, and pharmacokinetics of topiramate

Topiramate (TPM; TOPAMAX) is a broad-spectrum antiepileptic drug (AED) that is approved in many world markets for preventing or reducing the frequency of epileptic seizures (as monotherapy or adjunctive therapy), and for the prophylaxis of migraine. TPM, a sulfamate derivative of the naturally occurring sugar D-fructose, possesses several pharmacodynamic properties that may contribute to its clinically useful attributes, and to its observed adverse effects. The sulfamate moiety is essential, but not sufficient, for its pharmacodynamic properties. In this review, we discuss the known pharmacodynamic and pharmacokinetic properties of TPM, as well as its various clinically beneficial and adverse effects.

Glutamatergic Synaptic Dysfunction and Obsessive-Compulsive Disorder

Obsessive-compulsive disorder (OCD) is a debilitating neuropsychiatric condition estimated to afflict 1-3% of the world population. The estimated financial impact in the treatment and management of OCD is in the billions of dollars annually in the US alone. At present there is a marked lack of evidence on the specific causes of OCD. Current hypotheses largely focus on the serotonin (5-HT) system on the basis of the effectiveness of selective serotonin reuptake inhibitors (SSRIs) in alleviating symptoms of patients with OCD, yet a considerable fraction of patients are non-responsive or minimally responsive to these agents. Despite this fact, SSRIs have remained the primary pharmacological treatment avenue for OCD. In recent years, multiple lines of evidence have implicated glutamatergic synaptic dysfunction within the cortico-striatal-thalamo-cortical (CSTC) brain circuit in the etiology of OCD and related disorders, thereby prompting intensified effort in the development and evaluation of agents that modulate glutamatergic neurotransmission for the treatment of OCD. With this in mind, here we review the following topics with respect to synaptic dysfunction and the neural circuitry underlying OCD: (1) evidence supporting the critical involvement of the CSTC circuit, (2) genetic studies supporting the involvement of glutamatergic dysfunction, (3) insights from genetic animal models of OCD, and (4) preliminary findings with glutamatergic neurotransmission-modulating agents in the treatment of OCD. Given the putative mechanistic overlap between OCD and the broader OC-spectrum of disorders, unraveling the synaptic basis of OCD has potential to translate into more effective treatments for an array of poorly understood human disorders.

Glutamate-mediated neuroplasticity in an animal model of self-injurious behaviour

Self-injurious behaviour (SIB) is exhibited by individuals with a broad variety of developmental disorders and genetic abnormalities, including autism and Lesch-Nyhan, Prader-Willi and Rett syndromes. Most research has focused on environmental factors that reinforce SIB, and less is known about the biological basis of this behaviour disorder. However, animal models have been developed to study the neurochemical pathology that underlies SIB. In one model, rats exhibit self-biting after repeated daily administration of moderately high doses of pemoline (100-200mg/kg). Dopaminergic and glutamatergic neurotransmission have been implicated in this model. Accordingly, we investigated the role of glutamatergic neurotransmission in pemoline-induced SIB, using the N-methyl-d-aspartate (NMDA) receptor antagonists MK-801 and memantine. MK-801 is a high affinity antagonist which blocks glutamate-mediated neuroplasticity and behavioural sensitization to other psychostimulants. It lessened the incidence of SIB, the time spent self-injuring, and the area of tissue damage in the pemoline model. Memantine, on the other hand, is a low affinity antagonist which does not disrupt glutamate-mediated neuroplasticity, and it had little if any effect on any measure of pemoline-induced SIB. These results suggest that repeated pemoline administration induces glutamate-mediated neuroplastic changes that lead to the eventual expression of SIB. Further investigation of these changes may reveal specific neurochemical factors that contribute to SIB in this animal model of self-injury.

Development of new antiepileptic drugs: challenges, incentives, and recent advances

Despite the introduction of many second-generation antiepileptic drugs (AEDs) in the past 15 years, a third of patients with epilepsy remain refractory to available treatments, and newer and more effective therapies are needed. Although our understanding of the mechanisms of drug resistance is fragmented, novel AED targets have been identified, and models of refractory epilepsy have been developed that can help to select candidate compounds for development. There are more than 20 compounds with potential antiepileptic activity in various stages of clinical development, and for many of these promising clinical trial results are already available. Several incentives justify further investment into the discovery of newer and more effective AEDs. Moreover, developments in clinical trial methodology enable easier completion of proof-of-concept studies, earlier definition of the therapeutic potential of candidate compounds, and more efficient completion of trials for various epilepsy indications.

Neonatal hypocalcemic seizures in siblings exposed to topiramate in utero

We describe two siblings with neonatal hypocalcemic seizures whose mother took topiramate during both pregnancies. Apart from hypocalcemia, the patients had no identifiable etiology for their seizures. Although biochemical data suggested that the hypocalcemia was caused by hypoparathyroidism, no disorders typically associated with this condition were identified in the patients. We propose that topiramate exposure in utero led to hypoparathyroidism and subsequent hypocalcemia via effects on protein kinase A signaling, resulting in hypocalcemic seizures. Neonates exposed to topiramate in utero should be monitored for hypocalcemic seizures.

Antiepileptic drugs in migraine: from clinical aspects to cellular mechanisms

Migraine and epilepsy share several clinical features, and epilepsy is a comorbid condition of migraine. Clinical studies have shown that some antiepileptic drugs are effective at preventing migraine attacks. A rationale for their use in migraine prophylaxis is the hypothesis that migraine and epilepsy share several common pathogenetic mechanisms. An imbalance between excitatory glutamate-mediated transmission and GABA-mediated inhibition in specific brain areas has been postulated in these two pathological conditions. Moreover, abnormal activation of voltage-operated ionic channels has been implicated in both migraine and epilepsy. Cortical spreading depression has been found to be involved in the pathophysiology of epilepsy, in addition to the generation of migraine aura.

Effect of repeated treatment with topiramate on the beta-endorphin plasma level in rats selectively bred for high and low alcohol preference

Recent research indicates that topiramate has a role in the treatment of alcohol dependence. Topiramate has multiple mechanisms of action including enhancement of GABA-ergic inhibitory transmission and blocking excitatory glutamate neurotransmission, and modulating voltage-gated sodium and calcium ion channels and inhibiting carbonic anhydrase. In this study, we examined the effect of topiramate on endogenous opioid systems, which have an important role in the development of alcohol dependence. We investigated the beta-endorphin plasma level of animals with high- and low-risks of alcohol dependency after repeated treatment with topiramate. We used the Warsaw High Preferring (WHP) and Warsaw Low Preferring (WLP) rats, and treated them with topiramate at a dose of 80 mg/kg p.o. for 14 days. In WHP rats treatment with topiramate led to an increase in beta-endorphin plasma levels, which persisted at the same level even after a single injection of alcohol. The level of this peptide with topiramate was lower than in alcohol-injected WHP rats who did not receive topiramate. Beta-endorphin levels in WHP rats after topiramate or topiramate and ethanol treatment were similar to the basal level of this peptide in WLP rats. In WLP rats, topiramate did not prevent the ethanol-induced increase in beta-endorphin plasma level. We propose that administration of topiramate may have different effects on the opioid system involved in dependence according to genetic susceptibilities to alcoholism.

Reversible nonfluent aphasia and left frontal hypoperfusion during topiramate treatment

We report the case of a patient with complex partial seizures who developed a nonfluent aphasia when topiramate was added to his therapy. This emergent adverse effect appeared to be reversible, as language performance improved after discontinuation of topiramate. Interictal SPECT performed when the patient was aphasic revealed a focal perfusion reduction in the left lateral and mesial frontal cortex, which was no longer evident at a follow-up study after language recovery.

Progress report on new antiepileptic drugs: a summary of the Eigth Eilat Conference (EILAT VIII)

The Eigth Eilat Conference on New Antiepileptic Drugs (AEDs)-EILAT VII, took place in Sitges, Barcelona from the 10th to 14th September, 2006. Basic scientists, clinical pharmacologists and neurologists from 24 countries attended the conference, whose main themes included a focus on status epilepticus (epidemiology, current and future treatments), evidence-based treatment guidelines and the potential of neurostimulation in refractory epilepsy. Consistent with previous formats of this conference, the central part of the conference was devoted to a review of AEDs in development, as well as updates on marketed AEDs introduced since 1989. This article summarizes the information presented on drugs in development, including brivaracetam, eslicarbazepine acetate (BIA-2-093), fluorofelbamate, ganaxolone, huperzine, lacosamide, retigabine, rufinamide, seletracetam, stiripentol, talampanel, valrocemide, JZP-4, NS1209, PID and RWJ-333369. Updates on felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine and new extended release oxcarbazepine formulations, pregabalin, tiagabine, topiramate, vigabatrin, zonisamide and new extended release valproic acid formulations, and the antiepileptic vagal stimulator device are also presented.

Topiramate raises anterior cingulate cortex glutamine levels in healthy men; a 4.0 T magnetic resonance spectroscopy study

RATIONALE

Potential mechanisms of action of topiramate include alterations of glutamatergic and GABAergic systems. In particular, topiramate has been shown to increase occipital cortex GABA levels, as measured using proton magnetic resonance spectroscopy (MRS).

OBJECTIVES

The purpose of this study was to measure the effect of acute oral topiramate on the GABA precursors glutamate and glutamine in the anterior cingulate cortex (ACC) and occipital lobe (OL) using high-field (4.0 T) proton MRS (1H MRS).

METHODS

Proton MR spectra were acquired from healthy men at three times: at baseline and 2 and 6 h after ingesting 50 (N=5) or 100 mg (N=5) of topiramate. Blood samples were acquired prior to each scan for the purpose of obtaining serum topiramate levels.

RESULTS

A 100-mg dose of topiramate significantly increased ACC glutamine levels within 2 h of ingestion and OL glutamine levels within 6 h of ingestion. There were no measured significant effects of topiramate on ACC or OL glutamate levels.

CONCLUSIONS

A 100-mg dose of oral topiramate increased serum topiramate and ACC glutamine levels within 2 h. OL glutamine levels increased within 6 h. Increased brain glutamine levels may be a consequence of topiramate positively modulating GABAA receptors. This result is of interest given the possible role for topiramate in the treatment of epilepsy, migraine headache, bipolar disorder, eating disorders, and alcohol dependence.

Substantia Nigra Is an Anticonvulsant Site of Action of Topiramate in the Focal Pilocarpine Model of Limbic Seizures

PURPOSE

The substantia nigra pars reticulata (SNR) is known to play a role in gating and control of seizures. Prompted by the observation that intrahippocampal topiramate (TPM) administration does not suppress limbic seizures in the focal pilocarpine model, we investigated the role of the SNR in the anticonvulsant mechanism of action of TPM.

METHODS

Limbic seizures were evoked in freely moving rats by intrahippocampal administration of pilocarpine via a microdialysis probe. Changes in hippocampal extracellular (EC) glutamate and GABA concentrations were monitored. Effects of intraperitoneal (10-200 mg/kg), intrahippocampal (1-5 mM), and bilateral intranigral (100-300 nmol) TPM administration on pilocarpine-induced seizures and neurochemical changes were evaluated. Effects of TPM administration alone on hippocampal and nigral EC amino acid concentrations were also studied.

RESULTS

Systemic and intranigral, but not intrahippocampal TPM administration suppressed pilocarpine-induced seizures and neurochemical changes. Nigral GABA(A) receptor blockade by picrotoxin abolished the anticonvulsant effect of TPM in SNR. Systemic TPM administration increased hippocampal glutamate and decreased GABA. Intranigral TPM administration increased hippocampal glutamate, but not GABA. Intrahippocampal TPM increased hippocampal glutamate and GABA, but only at high concentrations.

CONCLUSIONS

In the focal pilocarpine model, TPM does not exert its anticonvulsant effect at the site of seizure initiation. We identified the SNR as a site of action of TPM, and showed that the nigral GABA-ergic system is central to TPM's anticonvulsant effect in SNR. Anticonvulsant effects and neurochemical changes in hippocampus following intranigral TPM administration suggest the existence of a nigro-hippocampal circuit, which may be involved in the control of limbic seizures.

Cocaine exposure at a sublethal concentration downregulates CREB functions in cultured neuroblastoma cells

Previous studies showed that prenatal cocaine in an animal model decreased brain-derived neurotrophic factor (BDNF) activity in offspring's brain. Since BDNF is one of target genes of cAMP response element-binding protein (CREB), this study examined effects of cocaine on CREB activities in a human neuroblastoma (SK-N-AS) cell line. The MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazodium bromide) assay indicated that cocaine exposure at 5 microM for 24 h had no significant influences on cell viability. However, a 24-h exposure to cocaine at the same concentration significantly decreased the level of phosphorylated CREB, although no significant changes in total CREB proteins were observed. Consistent with reduced CREB phosphorylation, the electrophoretic mobility shift assay showed that exposure to 5 microM of cocaine for 24 h also inhibited CREB binding activity and significantly decreased BDNF mRNA expression. In addition, exposure to 5 microM cocaine for 24 h attenuated the glutamic acid-evoked increase in the intracellular Ca2+ concentration. Taken together, these findings suggest that cocaine exposure at the sublethal concentration downregulates CREB functions in the cultured SK-N-AS cell line, and that diminished intracellular Ca2+ responses may be associated in part with cocaine-induced downregulation of CREB activity.