Investigation of the effects of the novel anticonvulsant compound carisbamate (RWJ-333369) on rat piriform cortical neurones in vitro

Pharmacological diversity amongst approved and emerging antiseizure medications for the treatment of developmental and epileptic encephalopathies

Developmental and epileptic encephalopathies (DEEs) are rare neurodevelopmental disorders characterised by early-onset and often intractable seizures and developmental delay/regression, and include Dravet syndrome and Lennox-Gastaut syndrome (LGS). Rufinamide, fenfluramine, stiripentol, cannabidiol and ganaxolone are antiseizure medications (ASMs) with diverse mechanisms of action that have been approved for treating specific DEEs. Rufinamide is thought to suppress neuronal hyperexcitability by preventing the functional recycling of voltage-gated sodium channels from the inactivated to resting state. It is licensed for adjunctive treatment of seizures associated with LGS. Fenfluramine increases extracellular serotonin levels and may reduce seizures via activation of specific serotonin receptors and positive modulation of the sigma-1 receptor. Fenfluramine is licensed for adjunctive treatment of seizures associated with Dravet syndrome and LGS. Stiripentol is a positive allosteric modulator of type-A gamma-aminobutyric acid (GABAA) receptors. As a broad-spectrum inhibitor of cytochrome P450 enzymes, its antiseizure effects may additionally arise through pharmacokinetic interactions with co-administered ASMs. Stiripentol is licensed for treating seizures associated with Dravet syndrome in patients taking clobazam and/or valproate. The mechanism(s) of action of cannabidiol remains largely unclear although multiple targets have been proposed, including transient receptor potential vanilloid 1, G protein-coupled receptor 55 and equilibrative nucleoside transporter 1. Cannabidiol is licensed as adjunctive treatment in conjunction with clobazam for seizures associated with Dravet syndrome and LGS, and as adjunctive treatment of seizures associated with tuberous sclerosis complex. Like stiripentol, ganaxolone is a positive allosteric modulator at GABAA receptors. It has recently been licensed in the USA for the treatment of seizures associated with cyclin-dependent kinase-like 5 deficiency disorder. Greater understanding of the causes of DEEs has driven research into the potential use of other novel and repurposed agents. Putative ASMs currently in clinical development for use in DEEs include soticlestat, carisbamate, verapamil, radiprodil, clemizole and lorcaserin.

Concerted suppressive effects of carisbamate, an anti-epileptic alkyl-carbamate drug, on voltage-gated Na+ and hyperpolarization-activated cation currents

Carisbamate (CRS, RWJ-333369) is a new anti-seizure medication. It remains unclear whether and how CRS can perturb the magnitude and/or gating kinetics of membrane ionic currents, despite a few reports demonstrating its ability to suppress voltage-gated Na⁺ currents. In this study, we observed a set of whole-cell current recordings and found that CRS effectively suppressed the voltage-gated Na⁺ (I_Na) and hyperpolarization-activated cation currents (I_h) intrinsically in electrically excitable cells (GH₃ cells). The effective IC₅₀ values of CRS for the differential suppression of transient (I_Na(T)) and late I_Na (I_Na(L)) were 56.4 and 11.4 μM, respectively. However, CRS strongly decreased the strength (i.e., Δarea) of the nonlinear window component of I_Na (I_Na(W)), which was activated by a short ascending ramp voltage (V_ramp); the subsequent addition of deltamethrin (DLT, 10 μM) counteracted the ability of CRS (100 μM, continuous exposure) to suppress I_Na(W). CRS strikingly decreased the decay time constant of I_Na(T) evoked during pulse train stimulation; however, the addition of telmisartan (10 μM) effectively attenuated the CRS (30 μM, continuous exposure)-mediated decrease in the decay time constant of the current. During continued exposure to deltamethrin (10 μM), known to be a pyrethroid insecticide, the addition of CRS resulted in differential suppression of the amplitudes of I_Na(T) and I_Na(L). The amplitude of I_h activated by a 2-s membrane hyperpolarization was diminished by CRS in a concentration-dependent manner, with an IC₅₀ value of 38 μM. For I_h, CRS altered the steady-state I-V relationship and attenuated the strength of voltage-dependent hysteresis (Hys_(V)) activated by an inverted isosceles-triangular V_ramp. Moreover, the addition of oxaliplatin effectively reversed the CRS-mediated suppression of Hys_(V). The predicted docking interaction between CRS and with a model of the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel or between CRS and the hNa_V1.7 channel reflects the ability of CRS to bind to amino acid residues in HCN or hNa_V1.7 channel via hydrogen bonds and hydrophobic interactions. These findings reveal the propensity of CRS to modify I_Na(T) and I_Na(L) differentially and to effectively suppress the magnitude of I_h. I_Na and I_h are thus potential targets of the actions of CRS in terms of modulating cellular excitability.

Novel and emerging therapeutics for genetic epilepsies

INTRODUCTION

Disease-specific treatments are available only for a minority of patients with genetic epilepsies, while the rest are treated with anticonvulsants, which are ineffective in almost one-third of patients.

AREAS COVERED

Recently approved and the most effective emerging therapeutics under development for the treatment of genetic epilepsies are overviewed after systematic search and analysis of relevant literature.

EXPERT OPINION

New and emerging drugs for genetic epilepsies exploit one of the two approaches: inhibiting hyperactive brain foci through blocking excitatory or augmenting inhibitory neurotransmission, or correcting the underlying genetic defect. The first is limited by insufficient selectivity of available compounds, and the second by imperfection of currently used vectors of genetic material, unselective and transient transgene expression. Besides, the treatment may come too late, after structural abnormalities and epilepsy deterioration takes place. However, with recent improvements, we can expect to see soon gradual decline in the number of patients with therapy-resistant genetic epilepsies.

miR-485 inhibits histone deacetylase HDAC5, HIF1α and PFKFB3 expression to alleviate epilepsy in cellular and rodent models

We investigated the role of microRNA (miR)-485 and its downstream signaling molecules on mediating epilepsy in cellular and rat models. We established a cellular epilepsy model by exposing hippocampal neurons to magnesium and a rat model by treating ICR mice with lithium chloride (127 mg/kg) and pilocarpine (30 mg/kg). We confirmed that miR-485 could bind and inhibit histone deacetylase 5 (HDAC5) and then measured expression of miR-485 and in mice and cells. Cells were transfected with overexpression or knockdown of miR-485, HDAC5, hypoxia-inducible factor-1alpha (HIF1α), or 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 enzyme (PFKFB3) to verify their roles in apoptosis, oxidative stress, and inflammation in epileptic hippocampal neurons. Binding relationship between miR-485, HDAC5, HIF1α, and PFKFB3 was verified. Oxidative stress and inflammation marker levels in epilepsy model mice were assessed. miR-485 was downregulated and HDAC5 was upregulated in cell and animal model of epilepsy. Seizure, neuronal apoptosis, oxidative stress (increased SOD and GSH-Px expression and decreased MDA and 8-OHdG expression) and inflammation (reduced IL-1β, TNF-α, and IL-6 expression) were reduced by miR-485 in epileptic cells. HIF1α and PFKFB3 expression was reduced by HDAC5 knockdown in cells, which was recapitulated in vivo. Thus, miR-485 alleviates neuronal damage and epilepsy by inhibiting HDAC5, HIF1α, and PFKFB3.

The ups and downs of alkyl-carbamates in epilepsy therapy: How does cenobamate differ?

Since 1955, several alkyl-carbamates have been developed for the treatment of anxiety and epilepsy, including meprobamate, flupirtine, felbamate, retigabine, carisbamate, and cenobamate. They have each enjoyed varying levels of success as antiseizure drugs; however, they have all been plagued by the emergence of serious and sometimes life-threatening adverse events. In this review, we compare and contrast their predominant molecular mechanisms of action, their antiseizure profile, and where possible, their clinical efficacy. The preclinical, clinical, and mechanistic profile of the prototypical γ-aminobutyric acidergic (GABAergic) modulator phenobarbital is included for comparison. Like phenobarbital, all of the clinically approved alkyl-carbamates share an ability to enhance inhibitory neurotransmission through modulation of the GABA_A receptor, although the specific mechanism of interaction differs among the different drugs discussed. In addition, several alkyl-carbamates have been shown to interact with voltage-gated ion channels. Flupirtine and retigabine share an ability to activate K⁺ currents mediated by KCNQ (Kv7) K⁺ channels, and felbamate, carisbamate, and cenobamate have been shown to block Na⁺ channels. In contrast to other alkyl-carbamates, cenobamate seems to be unique in its ability to preferentially attenuate the persistent rather than transient Na⁺ current. Results from recent randomized controlled clinical trials with cenobamate suggest that this newest antiseizure alkyl-carbamate possesses a degree of efficacy not witnessed since felbamate was approved in 1993. Given that ceno-bamate's mechanistic profile is unique among the alkyl-carbamates, it is not clear whether this impressive efficacy reflects an as yet undescribed mechanism of action or whether it possesses a unique synergy between its actions at the GABA_A receptor and on persistent Na⁺ currents. The high efficacy of cenobamate is, however, tempered by the risk of serious rash and low tolerability at higher doses, meaning that further safety studies and clinical experience are needed to determine the true clinical value of cenobamate.

Exposure to carbon black nanoparticles increases seizure susceptibility in male mice

Carbon black nanoparticles (CBNPs) can enter the central nervous system through blood circulation and olfactory nerves, affecting brain development or increasing neurological disease susceptibility. However, whether CBNPs exposure affects seizure is unclear. Herein, mice were exposed to two different doses of CBNPs (21 and 103 μg/animal) based on previous studies and the maximum exposure limitation (4 mg/m³) in occupational workplaces set by the Chinese government. In the pentylenetetrazol (PTZ) and kainic acid (KA) seizure models, high-dose CBNPs exposure increased seizure susceptibility in both models and increased spontaneous recurrent seizure (SRS) frequency in the KA model. In vivo local field potential (LFP) recording in KA model mice revealed that both low-dose and high-dose CBNPs exposure increased seizure-like event (SLE) frequency in the SRS interval but shortened SLE duration. Intriguingly, H&E staining and Nissl staining on brain tissue revealed that CBNPs exposure did not cause significant brain tissue morphology or neuronal damage. Detection of inflammatory factors, such as TNF-α, TGF-β1, IL-1β, and IL-6, in brain tissue showed that only high dose of CBNPs exposure increased the expression of cortical TGF-β1. By using the primary cultured neurons, we observed that CBNPs exposure not only significantly decreased the expression of the neuronal marker MAP2 but also enhanced the levels of action potential frequency in the neurons. In general, CBNPs exposure can affect abnormal epileptic discharges during the seizure interval and enhance susceptibility to frequent seizures. Our findings suggest that minimizing CBNPs exposure may be a potential way to prevent or ease seizure.

Up-Regulation of Trem2 Inhibits Hippocampal Neuronal Apoptosis and Alleviates Oxidative Stress in Epilepsy via the PI3K/Akt Pathway in Mice

Epilepsy is a chronic and severe neurological disorder that has negative effects on the autonomous activities of patients. Functionally, Trem2 (triggering receptor expressed on myeloid cells-2) is an immunoglobulin receptor that affects neurological and psychiatric genetic diseases. Based on this rationale, we aimed to assess the potential role of Trem2 integration with the PI3K/Akt pathway in epilepsy. We used microarray-based gene expression profiling to identify epilepsy-related differentially-expressed genes. In a mouse hippocampal neuron model of epilepsy, neurons were treated with low-Mg²⁺ extracellular fluid, and the protein and mRNA expression of Trem2 were determined. Using a gain-of-function approach with Trem2, neuronal apoptosis and its related factors were assessed by flow cytometry, RT-qPCR, and Western blot analysis. In a pilocarpine-induced epileptic mouse model, the malondialdehyde (MDA) and 8-hydroxy-2'-deoxyguanosine (8-OHdG) content and superoxide dismutase (SOD) and glutathione-peroxidase (GSH-Px) activity in the hippocampus were determined, and the protein expression of Trem2 was measured. In addition, the regulatory effect of Trem2 on the PI3K/Akt pathway was analyzed by inhibiting this pathway in both the cell and mouse models of epilepsy. Trem2 was found to occupy a core position and was correlated with epilepsy. Trem2 was decreased in the hippocampus of epileptic mice and epileptic hippocampal neurons. Of crucial importance, overexpression of Trem2 activated the PI3K/Akt pathway to inhibit neuronal apoptosis. Moreover, activation of the PI3K/Akt pathway through over-expression of Trem2 alleviated oxidative stress, as shown by the increased expression of SOD and GSH-Px and the decreased expression of MDA and 8-OHdG. The current study defines the potential role of Trem2 in inhibiting the development of epilepsy, indicating that Trem2 up-regulation alleviates hippocampal neuronal injury and oxidative stress, and inhibits neuronal apoptosis in epilepsy by activating the PI3K/Akt pathway.

Carisbamate blockade of T-type voltage-gated calcium channels

OBJECTIVES

Carisbamate (CRS) is a novel monocarbamate compound that possesses antiseizure and neuroprotective properties. However, the mechanisms underlying these actions remain unclear. Here, we tested both direct and indirect effects of CRS on several cellular systems that regulate intracellular calcium concentration [Ca²⁺ ]_i .

METHODS

We used a combination of cellular electrophysiologic techniques, as well as cell viability, Store Overload-Induced Calcium Release (SOICR), and mitochondrial functional assays to determine whether CRS might affect [Ca²⁺ ]_i levels through actions on the endoplasmic reticulum (ER), mitochondria, and/or T-type voltage-gated Ca²⁺ channels.

RESULTS

In CA3 pyramidal neurons, kainic acid induced significant elevations in [Ca²⁺ ]_i and long-lasting neuronal hyperexcitability, both of which were reversed in a dose-dependent manner by CRS. Similarly, CRS suppressed spontaneous rhythmic epileptiform activity in hippocampal slices exposed to zero-Mg²⁺ or 4-aminopyridine. Treatment with CRS also protected murine hippocampal HT-22 cells against excitotoxic injury with glutamate, and this was accompanied by a reduction in [Ca²⁺ ]_i . Neither kainic acid nor CRS alone altered the mitochondrial membrane potential (ΔΨ) in intact, acutely isolated mitochondria. In addition, CRS did not affect mitochondrial respiratory chain activity, Ca²⁺ -induced mitochondrial permeability transition, and Ca²⁺ release from the ER. However, CRS significantly decreased Ca²⁺ flux in human embryonic kidney tsA-201 cells transfected with Ca_v 3.1 (voltage-dependent T-type Ca²⁺ ) channels.

SIGNIFICANCE

Our data indicate that the neuroprotective and antiseizure activity of CRS likely results in part from decreased [Ca²⁺ ]_i accumulation through blockade of T-type Ca²⁺ channels.

The anticonvulsant actions of carisbamate associate with alterations in astrocyte glutamine metabolism in the lithium-pilocarpine epilepsy model

As reported previously, in the lithium-pilocarpine model of temporal lobe epilepsy (TLE), carisbamate (CRS) produces strong neuroprotection, leads to milder absence-like seizures, and prevents behavioral impairments in a subpopulation of rats. To understand the metabolic basis of these effects, here we injected 90 mg/kg CRS or vehicle twice daily for 7 days starting 1 h after status epilepticus (SE) induction in rats. Two months later, we injected [1-¹³ C]glucose and [1,2-¹³ C]acetate followed by head microwave fixation after 15 min. ¹³ C incorporation into metabolites was analyzed using ¹³ C magnetic resonance spectroscopy. We found that SE reduced neuronal mitochondrial metabolism in the absence but not in the presence of CRS. Reduction in glutamate level was prevented by CRS and aspartate levels were similar to controls only in rats displaying absence-like seizures after treatment [CRS-absence-like epilepsy (ALE)]. Glutamine levels in CRS-ALE rats were higher compared to controls in hippocampal formation and limbic structures while unchanged in rats displaying motor spontaneous recurrent seizures after treatment (CRS-TLE). Astrocytic mitochondrial metabolism was reduced in CRS-TLE, and either enhanced or unaffected in CRS-ALE rats, which did not affect the transfer of glutamine from astrocytes to neurons. In conclusion, CRS prevents reduction in neuronal mitochondrial metabolism but its effect on astrocytes is likely key in determining outcome of treatment in this model. To understand the metabolic basis of the strong neuroprotection and reduction in seizure severity caused by carisbamate (CRS) in the lithium-pilocarpine (Li-Pilo) model of temporal lobe epilepsy (TLE), we injected CRS for 7 days starting 1 h after status epilepticus and 2 months later [1-¹³ C]glucose and [1,2-¹³ C]acetate. ¹³ C Magnetic resonance spectroscopy analysis was performed on brain extracts and we found that CRS prevented reduction in neuronal mitochondrial metabolism but its effect on astrocytes was likely key in determining outcome of treatment in this model. ALE = absence like epilepsy; acetyl CoA = acetyl coenzyme A; GS = glutamine synthetase; PAG = phosphate activated glutaminase; PC = pyruvate carboxylase; OAA = oxaloacetate; TCA cycle = tricarboxylic acid cycle.

Design and synthesis of neuroprotective methylthiazoles and modification as NO-chimeras for neurodegenerative therapy

Learning and memory deficits in Alzheimer's disease (AD) result from synaptic failure and neuronal loss, the latter caused in part by excitotoxicity and oxidative stress. A therapeutic approach is described that uses NO-chimeras directed at restoration of both synaptic function and neuroprotection. 4-Methylthiazole (MZ) derivatives were synthesized, based upon a lead neuroprotective pharmacophore acting in part by GABA(A) receptor potentiation. MZ derivatives were assayed for protection of primary neurons against oxygen-glucose deprivation and excitotoxicity. Selected neuroprotective derivatives were incorporated into NO-chimera prodrugs, coined nomethiazoles. To provide proof of concept for the nomethiazole drug class, selected examples were assayed for restoration of synaptic function in hippocampal slices from AD-transgenic mice, reversal of cognitive deficits, and brain bioavailability of the prodrug and its neuroprotective MZ metabolite. Taken together, the assay data suggest that these chimeric nomethiazoles may be of use in treatment of multiple components of neurodegenerative disorders, such as AD.

Cognitive effects of carisbamate in randomized, placebo-controlled, healthy-volunteer, multidose studies

Adverse cognitive effects are an important concern for drugs that influence the central nervous system. Carisbamate is a novel drug in development for treatment of seizures and neuropathic pain. Information on its cognitive effects is limited. Three controlled, multiple-dose, crossover studies with treatment durations of 5-9 days were designed to examine the cognitive effects of carisbamate on healthy volunteers. In one study, apparent dose-dependent effects on response, vigilance, and recognition speed were observed (1000 mg and 1500 mg/day). Carisbamate did not differ from placebo for most variables in the other two studies, but increased reaction time and reduced Sternberg memory were seen at higher dosages. Carisbamate did not produce clinically significant adverse effects on cognitive performance at doses <1000 mg/day. Effects were mild to modest at the higher doses tested.

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.

Carisbamate acutely suppresses spasms in a rat model of symptomatic infantile spasms

PURPOSE

Infantile spasms are the signature seizures of West syndrome. The conventional treatments for infantile spasms, such as adrenocorticotropic hormone (ACTH) and vigabatrin, are not always effective, especially in symptomatic infantile spasms (SIS). We tested the efficacy of carisbamate, a novel neurotherapeutic drug, to suppress spasms in the multiple-hit rat model of SIS, and compared it with phenytoin to determine if its effect is via sodium-channel blockade.

METHODS

Sprague-Dawley rats received right intracerebral infusions of doxorubicin and lipopolysaccharide at postnatal day 3 (PN3) and intraperitoneal p-chlorophenylalanine at PN5. A single intraperitoneal injection of carisbamate was administered at PN4, after the onset of spasms, at the following doses: 10 mg/kg (CRS-10), 30 mg/kg (CRS-30), and 60 mg/kg (CRS-60), and was compared to vehicle-injected group (VEH). Video-monitoring of PN6-7 CRS-60 or VEH-injected pups was also done.

KEY FINDINGS

Carisbamate acutely reduced both behavioral spasms (CRS-30 and CRS-60 groups only) and electroclinical spasms during the first 2-3 postinjection hours, without detectable toxicity or mortality. In contrast, phenytoin (20 or 50 mg/kg) failed to suppress spasms.

SIGNIFICANCE

Our findings provide preclinical evidence that carisbamate displays acute anticonvulsive effect on spasms through a sodium channel-independent mechanism. Because spasms in the multiple-hit rat model are refractory to ACTH and transiently sensitive to vigabatrin, carisbamate may constitute a candidate new therapy for SIS, including the ACTH-refractory spasms. Further confirmation with clinical studies is needed.

Carisbamate has powerful disease-modifying effects in the lithium-pilocarpine model of temporal lobe epilepsy

Lithium-pilocarpine, a relevant model of temporal lobe epilepsy was used to test the neuroprotective and antiepileptogenic effects of carisbamate. Status epilepticus (SE) was induced in adult rats by lithium and pilocarpine. Carisbamate (30, 60, 90, and 120 mg/kg) was injected at 1 and 9 h after SE onset and continued twice daily for 6 additional days. The reference groups received diazepam instead of carisbamate. Neuroprotection was assessed during the first 24 h of SE with Fluoro-Jade B and after 14 days with thionine staining. SE severity and epileptic outcome were assessed by video, and surface and depth electroencephalographic recordings. At the two highest doses, carisbamate treatment reduced SE severity; produced strong neuroprotection of hippocampus, ventral cortices, thalamus, and amygdala; prevented mossy fiber sprouting in the dentate gyrus of the hippocampus; and delayed or suppressed the occurrence of spontaneous motor seizures. Rats with no spontaneous motor seizures displayed spike-and-wave discharges that share all the characteristics of absence seizures. In conclusion, carisbamate is able to induce strong neuroprotection and affect the nature of epileptogenic events occurring during and after lithium-pilocarpine status epilepticus, reflecting marked insult- and disease-modifying effects.

Anticholinesterase-induced epileptiform activity in immature rat piriform cortex slices, in vitro

PURPOSE

Acute in vitro brain slice models are commonly used to study epileptiform seizure generation and to test anti-epileptic drug action. Seizure-like activity can be readily induced by manipulating external ionic concentrations or by adding convulsant agents to the bathing medium. We previously showed that epileptiform bursting was induced in slices of immature (P14-28) rat piriform cortex (PC) by applying oxotremorine-M, a potent muscarinic receptor agonist. Here, we examined whether raising levels of endogenous acetylcholine (ACh) by exposure to anticholinesterases, could also induce epileptiform events in immature (P12-14) or early postnatal (P7-9) rat PC brain slices.

METHODS

The effects of anticholinesterases were investigated in rat PC neurons using both extracellular MEA (P7-9 slices) and intracellular (P12-14 slices) recording methods.

RESULTS

In P7-9 slices, eserine (20 microM) or neostigmine (20 microM) induced low amplitude, low frequency bursting activity in all three PC cell layers (I-III), particularly layer III, where neuronal muscarinic responsiveness is known to predominate. In P12-14 neurons, neostigmine produced a slow depolarization together with an increase in input resistance and evoked cell firing. Depolarizing postsynaptic potentials evoked by intrinsic fibre stimulation were selectively depressed although spontaneous bursting was not observed. Neostigmine effects were blocked by atropine (1 microM), confirming their muscarinic nature. We conclude that elevation of endogenous ACh by anticholinesterases can induce bursting in early postnatal PC brain slices, further highlighting the epileptogenic capacity of this brain region. However, this tendency declines with further development, possibly as local inhibitory circuit mechanisms become more dominant.

Δ⁹-Tetrahydrocannabivarin suppresses in vitro epileptiform and in vivo seizure activity in adult rats

PURPOSE

We assessed the anticonvulsant potential of the phytocannabinoid Δ⁹-tetrahydrocannabivarin (Δ⁹-THCV) by investigating its effects in an in vitro piriform cortex (PC) brain slice model of epileptiform activity, on cannabinoid CB1 receptor radioligand-binding assays and in a generalized seizure model in rats.

METHODS

Δ⁹-THCV was applied before (10 μm Δ⁹-THCV) or during (10-50 μm Δ⁹-THCV) epileptiform activity induced by Mg²(+) -free extracellular media in adult rat PC slices and measured using multielectrode array (MEA) extracellular electrophysiologic techniques. The actions of Δ⁹-THCV on CB1 receptors were examined using [³H]SR141716A competition binding and [³⁵S]GTPγS assays in rat cortical membranes. Effects of Δ⁹-HCV (0.025-2.5 mg/kg) on pentylenetetrazole (PTZ)-induced seizures in adult rats were also assessed.

RESULTS

After induction of stable spontaneous epileptiform activity, acute Δ⁹ -THCV application (≥ 20 μm) significantly reduced burst complex incidence and the amplitude and frequency of paroxysmal depolarizing shifts (PDSs). Furthermore, slices pretreated with 10 μm Δ⁹-THCV prior to induction of epileptiform activity exhibited significantly reduced burst complex incidence and PDS peak amplitude. In radioligand-binding experiments, Δ⁹-THCV acted as a CB1 receptor ligand, displacing 0.5 nm [³H]SR141716A with a Ki∼290 nm, but exerted no agonist stimulation of [³⁵S]GTPγS binding. In PTZ-induced seizures in vivo, 0.25 mg/kg Δ⁹-THCV significantly reduced seizure incidence.

DISCUSSION

These data demonstrate that Δ⁹-THCV exerts antiepileptiform and anticonvulsant properties, actions that are consistent with a CB1 receptor-mediated mechanism and suggest possible therapeutic application in the treatment of pathophysiologic hyperexcitability states.

Key factors in the discovery and development of new antiepileptic drugs

Since the early 1990s, many new antiepileptic drugs (AEDs) that offer appreciable advantages in terms of their favourable pharmacokinetics, improved tolerability and lower potential for drug-drug interactions have entered the market. However, despite the therapeutic arsenal of old and new AEDs, approximately 30% of patients with epilepsy still suffer from seizures. Thus, there remains a substantial need for the development of more efficacious AEDs for patients with refractory seizures. Here, we briefly review the emerging knowledge on the pathological basis of epilepsy and how it might best be used in the design of new therapeutics. We also discuss the current approach to AED discovery and highlight some of the unique features of newer models of pharmacoresistance and epileptogenesis that have emerged in recent years.

Cannabidiol displays antiepileptiform and antiseizure properties in vitro and in vivo

Plant-derived cannabinoids (phytocannabinoids) are compounds with emerging therapeutic potential. Early studies suggested that cannabidiol (CBD) has anticonvulsant properties in animal models and reduced seizure frequency in limited human trials. Here, we examine the antiepileptiform and antiseizure potential of CBD using in vitro electrophysiology and an in vivo animal seizure model, respectively. CBD (0.01-100 muM) effects were assessed in vitro using the Mg(2+)-free and 4-aminopyridine (4-AP) models of epileptiform activity in hippocampal brain slices via multielectrode array recordings. In the Mg(2+)-free model, CBD decreased epileptiform local field potential (LFP) burst amplitude [in CA1 and dentate gyrus (DG) regions] and burst duration (in all regions) and increased burst frequency (in all regions). In the 4-AP model, CBD decreased LFP burst amplitude (in CA1 only at 100 muM CBD), burst duration (in CA3 and DG), and burst frequency (in all regions). CBD (1, 10, and 100 mg/kg) effects were also examined in vivo using the pentylenetetrazole model of generalized seizures. CBD (100 mg/kg) exerted clear anticonvulsant effects with significant decreases in incidence of severe seizures and mortality compared with vehicle-treated animals. Finally, CBD acted with only low affinity at cannabinoid CB(1) receptors and displayed no agonist activity in [(35)S]guanosine 5'-O-(3-thio)triphosphate assays in cortical membranes. These findings suggest that CBD acts, potentially in a CB(1) receptor-independent manner, to inhibit epileptiform activity in vitro and seizure severity in vivo. Thus, we demonstrate the potential of CBD as a novel antiepileptic drug in the unmet clinical need associated with generalized seizures.