Effects of new valproate derivatives on epileptiform discharges induced by pentylenetetrazole or low Mg2+ in rat entorhinal cortex-hippocampus slices

Insights into Structural Modifications of Valproic Acid and Their Pharmacological Profile

Valproic acid (VPA) is a well-established anticonvulsant drug discovered serendipitously and marketed for the treatment of epilepsy, migraine, bipolar disorder and neuropathic pain. Apart from this, VPA has potential therapeutic applications in other central nervous system (CNS) disorders and in various cancer types. Since the discovery of its anticonvulsant activity, substantial efforts have been made to develop structural analogues and derivatives in an attempt to increase potency and decrease adverse side effects, the most significant being teratogenicity and hepatotoxicity. Most of these compounds have shown reduced toxicity with improved potency. The simple structure of VPA offers a great advantage to its modification. This review briefly discusses the pharmacology and molecular targets of VPA. The article then elaborates on the structural modifications in VPA including amide-derivatives, acid and cyclic analogues, urea derivatives and pro-drugs, and compares their pharmacological profile with that of the parent molecule. The current challenges for the clinical use of these derivatives are also discussed. The review is expected to provide necessary knowledgebase for the further development of VPA-derived compounds.

A new mechanism for cannabidiol in regulating the one-carbon cycle and methionine levels in Dictyostelium and in mammalian epilepsy models

BACKGROUND AND PURPOSE

Epidiolex™, a form of highly purified cannabidiol (CBD) derived from Cannabis plants, has demonstrated seizure control activity in patients with Dravet syndrome, without a fully elucidated mechanism of action. We have employed an unbiased approach to investigate this mechanism at a cellular level.

EXPERIMENTAL APPROACH

We use a tractable biomedical model organism, Dictyostelium, to identify a protein controlling the effect of CBD and characterize this mechanism. We then translate these results to a Dravet syndrome mouse model and an acute in vitro seizure model.

KEY RESULTS

CBD activity is partially dependent upon the mitochondrial glycine cleavage system component, GcvH1 in Dictyostelium, orthologous to the human glycine cleavage system component H protein, which is functionally linked to folate one-carbon metabolism (FOCM). Analysis of FOCM components identified a mechanism for CBD in directly inhibiting methionine synthesis. Analysis of brain tissue from a Dravet syndrome mouse model also showed drastically altered levels of one-carbon components including methionine, and an in vitro rat seizure model showed an elevated level of methionine that is attenuated following CBD treatment.

CONCLUSIONS AND IMPLICATIONS

Our results suggest a novel mechanism for CBD in the regulating methionine levels and identify altered one-carbon metabolism in Dravet syndrome and seizure activity.

Imaging epilepsy in larval zebrafish

Our understanding of the genetic aetiology of paediatric epilepsies has grown substantially over the last decade. However, in order to translate improved diagnostics to personalised treatments, there is an urgent need to link molecular pathophysiology in epilepsy to whole-brain dynamics in seizures. Zebrafish have emerged as a promising new animal model for epileptic seizure disorders, with particular relevance for genetic and developmental epilepsies. As a novel model organism for epilepsy research they combine key advantages: the small size of larval zebrafish allows high throughput in vivo experiments; the availability of advanced genetic tools allows targeted modification to model specific human genetic disorders (including genetic epilepsies) in a vertebrate system; and optical access to the entire central nervous system has provided the basis for advanced microscopy technologies to image structure and function in the intact larval zebrafish brain. There is a growing body of literature describing and characterising features of epileptic seizures and epilepsy in larval zebrafish. Recently genetically encoded calcium indicators have been used to investigate the neurobiological basis of these seizures with light microscopy. This approach offers a unique window into the multiscale dynamics of epileptic seizures, capturing both whole-brain dynamics and single-cell behaviour concurrently. At the same time, linking observations made using calcium imaging in the larval zebrafish brain back to an understanding of epileptic seizures largely derived from cortical electrophysiological recordings in human patients and mammalian animal models is non-trivial. In this review we briefly illustrate the state of the art of epilepsy research in zebrafish with particular focus on calcium imaging of epileptic seizures in the larval zebrafish. We illustrate the utility of a dynamic systems perspective on the epileptic brain for providing a principled approach to linking observations across species and identifying those features of brain dynamics that are most relevant to epilepsy. In the following section we survey the literature for imaging features associated with epilepsy and epileptic seizures and link these to observations made from humans and other more traditional animal models. We conclude by identifying the key challenges still facing epilepsy research in the larval zebrafish and indicate strategies for future research to address these and integrate more directly with the themes and questions that emerge from investigating epilepsy in other model systems and human patients.

Models of drug-induced epileptiform synchronization in vitro

Models of epileptiform activity in vitro have many advantages for recording and experimental manipulation. Neural tissues can be maintained in vitro for hours, and in neuronal or organotypic slice cultures for several weeks. A variety of drugs and other agents increase activity in these in vitro conditions, in many cases resulting in epileptiform activity, thus providing a direct model of symptomatic seizures. We review these preparations and the experimental manipulations used to induce epileptiform activity. The most common of drugs used are GABAA receptor antagonists and potassium channel blockers (notably 4-aminopyridine). Muscarinic agents also can induce epileptiform synchronization in vitro, and include potassium channel inhibition amongst their cellular actions. Manipulations of extracellular ions are reviewed in another paper in this special issue, as are ex vivo slices prepared from chronically epileptic animals and from people with epilepsy. More complex slices including extensive networks and/or several connected brain structures can provide insights into the dynamics of long range connections during epileptic activity. Visualization of slices also provides opportunities for identification of living neurons and for optical recording/stimulation and manipulation. Overall, the analysis of the epileptiform activity induced in brain tissue in vitro has played a major role in advancing our understanding of the cellular and network mechanisms of epileptiform synchronization, and it is expected to continue to do so in future.

New experimental therapies for status epilepticus in preclinical development

Starting with the established antiepileptic drug, valproic acid, we have taken a novel approach to develop new antiseizure drugs that may be effective in status epilepticus. We first identified that valproic acid has a potent effect on a biochemical pathway, the phosphoinositide pathway, in Dictyostelium discoideum, and we demonstrated that this may relate to its mechanism of action against seizures in mammalian systems. Through screening in this pathway, we have identified a large array of fatty acids and fatty acid derivatives with antiseizure potential. These were then evaluated in an in vitro mammalian system. One compound that we identified through this process is a major constituent of the ketogenic diet, strongly arguing that it may be the fatty acids that are mediating the antiseizure effect of this diet. We further tested two of the more potent compounds in an in vivo model of status epilepticus and demonstrated that they were more effective than valproic acid in treating the status epilepticus. This article is part of a Special Issue entitled "Status Epilepticus".

Seizure control by derivatives of medium chain fatty acids associated with the ketogenic diet show novel branching-point structure for enhanced potency

The medium chain triglyceride (MCT) ketogenic diet is a major treatment of drug-resistant epilepsy but is problematic, particularly in adults, because of poor tolerability. Branched derivatives of octanoic acid (OA), a medium chain fat provided in the diet have been suggested as potential new treatments for drug-resistant epilepsy, but the structural basis of this functionality has not been determined. Here we investigate structural variants of branched medium chain fatty acids as new seizure-control treatments. We initially employ a series of methyl-branched OA derivatives, and using the GABAA receptor antagonist pentylenetetrazol to induce seizure-like activity in rat hippocampal slices, we show a strong, branch-point-specific activity that improves upon the related epilepsy treatment valproic acid. Using low magnesium conditions to induce glutamate excitotoxicity in rat primary hippocampal neuronal cultures for the assessment of neuroprotection, we also show a structural dependence identical to that for seizure control, suggesting a related mechanism of action for these compounds in both seizure control and neuroprotection. In contrast, the effect of these compounds on histone deacetylase (HDAC) inhibition, associated with teratogenicity, shows no correlation with therapeutic efficacy. Furthermore, small structural modifications of the starting compounds provide active compounds without HDAC inhibitory effects. Finally, using multiple in vivo seizure models, we identify potent lead candidates for the treatment of epilepsy. This study therefore identifies a novel family of fatty acids, related to the MCT ketogenic diet, that show promise as new treatments for epilepsy control and possibly other MCT ketogenic diet-responding conditions, such as Alzheimer disease.

The search for better epilepsy treatments: from slime mould to coconuts

Drug-resistant epilepsy has remained a problem since the inception of antiepileptic drug development, despite the large variety of antiepileptic drugs available today. Moreover, the mechanism-of-action of these drugs is often unknown. This is due to the widespread screening of compounds through animal models. We have taken a different approach to antiepileptic drug discovery and have identified a biochemical pathway in Dictyostelium discoideum (a ‘slime mould’) that may relate to the mechanism-of-action of valproate, one of the most commonly used and effective antiepileptic drugs. Through screening in this pathway, we have been able to identify a whole host of fatty acids and fatty acid derivatives with potential antiepileptic activity; this was then confirmed in in vitro and in vivo mammalian seizure models. Some of these compounds are more potent than valproate and potentially lack many of the major side effects of valproate (including birth defects and liver toxicity). In addition, one of the compounds that we have identified is a major constituent of the ketogenic diet, strongly arguing that it may be the fatty acids and not the ketogenesis that are mediating the effect of this diet.

Seizure-induced reduction in PIP3 levels contributes to seizure-activity and is rescued by valproic acid

Phosphatidylinositol (3–5) trisphosphate (PIP₃) is a central regulator of diverse neuronal functions that are critical for seizure progression, however its role in seizures is unclear. We have recently hypothesised that valproic acid (VPA), one of the most commonly used drugs for the treatment of epilepsy, may target PIP₃ signalling as a therapeutic mode of action. Here, we show that seizure induction using kainic acid in a rat in vivo epilepsy model resulted in a decrease in hippocampal PIP₃ levels and reduced protein kinase B (PKB/AKT) phosphorylation, measured using ELISA mass assays and Western blot analysis, and both changes were restored following VPA treatment. These finding were reproduced in cultured rat hippocampal primary neurons and entorhinal cortex–hippocampal slices during exposure to the GABA(A) receptor antagonist pentylenetetrazol (PTZ), which is widely used to generate seizures and seizure-like (paroxysmal) activity. Moreover, VPA's effect on paroxysmal activity in the PTZ slice model is blocked by phosphatidylinositol 3-kinase (PI3K) inhibition or PIP₂ sequestration by neomycin, indicating that VPA's efficacy is dependent upon PIP₃ signalling. PIP₃ depletion following PTZ treatment may also provide a positive feedback loop, since enhancing PIP₃ depletion increases, and conversely, reducing PIP₃ dephosphorylation reduces paroxysmal activity and this effect is dependent upon AMPA receptor activation. Our results therefore indicate that PIP₃ depletion occurs with seizure activity, and that VPA functions to reverse these effects, providing a novel mechanism for VPA in epilepsy treatment.

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In vivo seizure induction (using kainic acid) reduces hippocampal PIP₃ levels.

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In vivo seizure induction (using kainic acid) reduces hippocampal phospho-PKB levels.

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Valproic acid protects against these reductions under seizure conditions only.

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Similar regulation is seen with PTZ-induced in vitro seizure activity.

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Seizure-induced PIP₃ reduction causes a feedback activation of seizure activity.

Seizure control by ketogenic diet-associated medium chain fatty acids

The medium chain triglyceride (MCT) ketogenic diet is used extensively for treating refractory childhood epilepsy. This diet increases the plasma levels of medium straight chain fatty acids. A role for these and related fatty acids in seizure control has not been established. We compared the potency of an established epilepsy treatment, Valproate (VPA), with a range of MCT diet-associated fatty acids (and related branched compounds), using in vitro seizure and in vivo epilepsy models, and assessed side effect potential in vitro for one aspect of teratogenicity, for liver toxicology and in vivo for sedation, and for a neuroprotective effect. We identify specific medium chain fatty acids (both prescribed in the MCT diet, and related compounds branched on the fourth carbon) that provide significantly enhanced in vitro seizure control compared to VPA. The activity of these compounds on seizure control is independent of histone deacetylase inhibitory activity (associated with the teratogenicity of VPA), and does not correlate with liver cell toxicity. In vivo, these compounds were more potent in epilepsy control (perforant pathway stimulation induced status epilepticus), showed less sedation and enhanced neuroprotection compared to VPA. Our data therefore implicates medium chain fatty acids in the mechanism of the MCT ketogenic diet, and highlights a related new family of compounds that are more potent than VPA in seizure control with a reduced potential for side effects.

This article is part of the Special Issue entitled ‘New Targets and Approaches to the Treatment of Epilepsy’.

The antiepileptic drug valproic acid and other medium-chain fatty acids acutely reduce phosphoinositide levels independently of inositol in Dictyostelium

Valproic acid (VPA) is the most widely prescribed epilepsy treatment worldwide, but its mechanism of action remains unclear. Our previous work identified a previously unknown effect of VPA in reducing phosphoinositide production in the simple model Dictyostelium followed by the transfer of data to a mammalian synaptic release model. In our current study, we show that the reduction in phosphoinositide [PtdInsP (also known as PIP) and PtdInsP(2) (also known as PIP(2))] production caused by VPA is acute and dose dependent, and that this effect occurs independently of phosphatidylinositol 3-kinase (PI3K) activity, inositol recycling and inositol synthesis. In characterising the structural requirements for this effect, we also identify a family of medium-chain fatty acids that show increased efficacy compared with VPA. Within the group of active compounds is a little-studied group previously associated with seizure control, and analysis of two of these compounds (nonanoic acid and 4-methyloctanoic acid) shows around a threefold enhanced potency compared with VPA for protection in an in vitro acute rat seizure model. Together, our data show that VPA and a newly identified group of medium-chain fatty acids reduce phosphoinositide levels independently of inositol regulation, and suggest the reinvestigation of these compounds as treatments for epilepsy.

Inhibition of long-term potentiation by valproic acid through modulation of cyclic AMP

PURPOSE

Valproic acid (VPA) is widely used clinically in epilepsy, bipolar disorder, and migraine. In experimental models, it has also been shown to have neuroprotective and antiepileptogenic effects. Its mechanisms of action in these diverse conditions are, however, unclear, but there is some evidence indicating an effect of VPA upon protein kinase A (PKA) activity. We, therefore, asked whether VPA modulates cyclic adenosine monophosphate (cAMP)/PKA-dependent synaptic plasticity and whether this mode of action could explain its anticonvulsant effect.

METHODS

We first tested the effects of VPA on PKA-dependent synaptic plasticity at mossy fiber to CA3 synapses in rat hippocampus slices following very high-frequency stimulation or application of the adenylyl cyclase activator forskolin. Using biochemical assays, we then tested whether VPA had a direct effect on PKA activity or an indirect effect through modulating cAMP production. Lastly, VPA and inhibitors of adenylyl cyclase (SQ22536) and PKA (H89) were tested in in vitro models of epileptiform activity induced in hippocampal-entorhinal cortex slices using either pentylenetetrazol (2 mM) or low magnesium.

RESULTS

VPA (1 mm) inhibited PKA-dependent long-term potentiation of mossy fiber to CA3 pyramidal cell transmission. However, VPA did not directly modulate PKA activity but rather inhibited the accumulation of cAMP. In acute in vitro seizure models, the anticonvulsant activity of VPA is not mediated through modulation of adenylyl cyclase or PKA.

CONCLUSIONS

  These results indicate that VPA through an action on cAMP accumulation can inhibit synaptic plasticity, but this cannot fully explain its anticonvulsant effect.

Greater contribution of N-methyl-D-aspartic acid receptors in ventral compared to dorsal hippocampal slices in the expression and long-term maintenance of epileptiform activity

Functional segregation along the dorso-ventral axis of the hippocampus is a developing concept. The higher susceptibility of the ventral hippocampus to epileptic activity compared with dorsal hippocampus is one of the main features, which still has obscure mechanisms. Using the model of magnesium-free medium and field recordings, single epileptiform discharges displayed higher incidence (77% vs 57%), rate (41.7+/-3.1 vs 13.5+/-0.7 events/min), duration (173.9+/-17.7 vs 116.8+/-13.6 ms) and intensity (coastline, 25.4+/-2.5 vs 9.5+/-1.8) in ventral compared with dorsal rat hippocampal slices. In addition, the decay phase of the evoked synaptic potentials was 110% slower in ventral slices. The N-methyl-D-aspartate (NMDA) receptor antagonist d-(-)-2-amino-5-phosphonopentanoic acid (50-100 microM) decreased the discharge rate and coastline similarly in ventral and dorsal slices, but it shortened the discharges in ventral slices (by 40%) only. The NMDA receptor antagonist 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (10 microM) decreased the rate in both groups and additionally shortened discharges in both kinds of slices, an effect which was greater in ventral ones (31% vs 13%). Furthermore, both drugs shortened the evoked potentials more in ventral (77%) than in dorsal slices (52%). On the other hand, 1 microM of 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid shortened the discharges and evoked synaptic potentials only in ventral slices, and slowed down the discharge rate only in dorsal slices. Addition of NMDA, in the magnesium-free medium, enhanced activity in both kinds of slices. At 5 and 10 microM of NMDA 51% of the ventral but only 9% of the dorsal slices displayed persistent epileptiform discharges, which were recorded for at least one hour after reintroduction of magnesium in the medium. At 10-20 microM the enhancement of activity was transient, followed by suppression of discharges in 40% and 76% of the ventral and dorsal slices, respectively. Most of the slices having experienced suppression did not develop persistent activity. We propose that the NMDA receptors contribute to the higher susceptibility of the ventral hippocampus to expression and long-term maintenance of epileptiform discharges. This diversification may be related to other aspects of hippocampal dorso-ventral functional segregation.

Adenosine acting via A1 receptors, controls the transition to status epilepticus-like behaviour in an in vitro model of epilepsy

Adenosine has powerful inhibitory effects in the central nervous system. In this study, we aim to understand how adenosine controls the progression of seizure-like events (SLEs) in a seizure-prone region of the brain, the entorhinal cortex. We chose to use a low Mg(2+) model of epilepsy in an in vitro slice preparation where, in the entorhinal cortex, SLEs progress into a type of epileptiform activity called late recurrent discharges (LRDs) that bear resemblance to status epilepticus. Adenosine, acting via its A1 receptor, exerted powerful inhibitory effects to prevent the spontaneous progression to LRDs while the potent A1 receptor antagonist, DPCPX, accelerated the progression in a concentration dependent manner. The spontaneous progression from SLEs to LRDs was associated with a decline in total cellular ATP levels and studies with metabolic inhibitors indicated a key role for the production of endogenous adenosine from ATP. We therefore hypothesise that when ATP becomes rate limiting, extracellular adenosine levels fall, the normal inhibitory brake is removed and the progression from SLEs to LRDs or status epilepticus-like activity can ensue. Moreover, under these conditions, inhibition of the adenine nucleotide salvage pathways reversed the status epilepticus-like activity. Our findings suggest a powerful role for adenosine for the control of the progression to status epilepticus-like activity in an epilepsy model that is refractory to most anti-epileptic drugs. On this basis, manipulation of adenine nucleotide metabolism may represent a potential therapeutic approach for the treatment of status epilepticus.

In vivo study of the effect of valpromide and valnoctamide in the pilocarpine rat model of focal epilepsy

PURPOSE

We evaluated the effectiveness of the commonly used antiepileptic drug sodium valproate (400 mg/kg) and two of its amide derivatives, valpromide and valnoctamide (both 100 mg/kg), in an in vivo rat model of focal epilepsy. Our main interest was to get insight into possible changes in extracellular amino acid neurotransmitter levels following administration of the drugs, both in control and in epileptic conditions.

METHODS

Seizures were evoked in freely moving rats by intrahippocampal administration of pilocarpine via a microdialysis probe (10 mM for 40 min at 2 microl/min). Microdialysis was also used as in vivo sampling technique and alterations in extracellular hippocampal glutamate and GABA levels were monitored. Electrophysiological evidence for the presence or absence of seizures was simultaneously recorded with electrocorticography.

RESULTS

The focally evoked pilocarpine-induced seizures were completely prevented by acute intraperitoneal pretreatment with each of the three drugs in the respective doses. Effective protection was reflected in the electrocorticographic recordings and in the lack of sustained elevations of the extracellular glutamate levels after pilocarpine perfusion. Little effects were seen on the basal extracellular amino acid levels after systemic administration of each of the compounds, nor after the intrahippocampal administration of sodium valproate.

CONCLUSIONS

Valnoctamide and valpromide (100 mg/kg) proved to be at least as effective as their parent compound sodium valproate (400 mg/kg) against pilocarpine-induced seizures. All three compounds however failed to induce significant initial alterations in extracellular hippocampal GABA release. This questions the enhancement of GABA-mediated inhibition as being one of their mechanisms of action.