Effects of various valproic acid derivatives on low-calcium spontaneous epileptiform activity in hippocampal slices

Long-lasting antiseizure effects of chronic intrasubthalamic convection-enhanced delivery of valproate

Intracerebral drug delivery is an experimental approach for the treatment of drug-resistant epilepsies that allows for pharmacological intervention in targeted brain regions. Previous studies have shown that targeted pharmacological inhibition of the subthalamic nucleus (STN) via modulators of the GABAergic system produces antiseizure effects. However, with chronic treatment, antiseizure effects are lost as tolerance develops. Here, we report that chronic intrasubthalamic microinfusion of valproate (VPA), an antiseizure medication known for its wide range of mechanisms of action, can produce long-lasting antiseizure effects over three weeks in rats. In the intravenous pentylenetetrazole seizure-threshold test, seizure thresholds were determined before and during chronic VPA application (480 μg/d, 720 μg/d, 960 μg/d) to the bilateral STN. Results indicate a dose-dependent variation in VPA-induced antiseizure effects with mean increases in seizure threshold of up to 33%, and individual increases of up to 150%. The lowest VPA dose showed a complete lack of tolerance development with long-lasting antiseizure effects. Behavioral testing with all doses revealed few, acceptable adverse effects. VPA concentrations were high in STN and low in plasma and liver. In vitro electrophysiology with bath applied VPA revealed a reduction in spontaneous firing rate, increased background membrane potential, decreased input resistance and a significant reduction in peak NMDA, but not AMPA, receptor currents in STN neurons. Our results suggest an advantage of VPA over purely GABAergic modulators in preventing tolerance development with chronic intrasubthalamic drug delivery and provide first mechanistic insights in intracerebral pharmacotherapy targeting the STN.

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.

Valproic acid potently inhibits interictal-like epileptiform activity in prefrontal cortex pyramidal neurons

Valproic acid has a long-standing reputation of effectively treating the symptoms of not only epilepsy but also psychiatric conditions. In the latter, the exact mechanism by which valproate exerts its effect remains unclear. In this study, epileptiform bursts were recorded from pyramidal neurons in the prefrontal cortex (the brain region thought to be involved in psychiatric disorders) using the patch-clamp technique. An extracellular solution with no magnesium ions and elevated potassium levels that is known to induce epileptiform activity in vitro was used. Because of their short durations, the epileptiform bursts were regarded as interictal-like epileptiform activity, which is believed to be involved in cognitive impairment. Interictal discharges occur in many neuropsychiatric disorders as well as in healthy population. Epileptic activity in prefrontal cortex pyramidal neurons was potently inhibited by two therapeutic concentrations of valproic acid (20 μM and 200 μM). Moreover, valproate suppressed spontaneous excitatory postsynaptic potentials. Epileptiform bursts were fully inhibited by NMDA receptor antagonist, which suggests that epileptiform activity is driven by NMDA receptors. The inhibition of excitability in prefrontal cortex pyramidal neurons by valproate was also shown. This study shows that it is possible to evoke NMDA-dependent epileptiform activity in prefrontal cortex pyramidal neurons in vitro. We suggest that the prefrontal cortex is a good region for studying the influence of drugs on interictal epileptiform activity.

The complexity of their activation mechanism opens new possibilities for the modulation of mGlu and GABAB class C G protein-coupled receptors

In the human genome, 22 genes are coding for the class C G protein-coupled receptors that are receptors for the two main neurotransmitters glutamate and γ-aminobutyric acid, for Ca(2+) and for sweet and amino acid taste compounds. In addition to the GPCR heptahelical transmembrane domain responsible for G-protein activation, class C receptors possess a large extracellular domain that is responsible for ligand recognition. Recent studies had revealed that class C receptors are homo- or heterodimers with unique mechanism of activation. In the present review, we present an up-to-date view of the structures and activation mechanism of these receptors in particular the metabotropic glutamate and GABA(B) receptors. We show how the complexity of functioning of these transmembrane proteins can be used for the development of therapeutics to modulate their activity. We emphasize on the new approaches and drugs that could potentially become important in the future pharmacology of these receptors.

Crystal structure and anticonvulsant activity of (±)-1,2:4,5-di-O-isopropylidene-3,6-di-O-(2-propylpentanoyl)-myo-inositol

The biological activity and crystal structure of (+/-)-1,2:4,5-di-O-isopropylidene-3,6-di-O-(2-propylpentanoyl)-myo-inositol have been investigated. This compound shows better anticonvulsant activity than valproic acid (VPA) in the MES test as measured in mice. Its structure, determined from single-crystal X-ray diffraction measurements, shows that the inositol ring deviates from the ideal chair conformation and that the two 2-propylpentanoyl groups are located on opposite ring positions. This molecular conformation lets carbonyl and hydroxyl oxygen atoms to be available for hydrogen-bonding interactions, hinders carbonyl carbon atoms, preventing metabolic enzymatic hydrolysis, and helps to rationalize the observed inactive profile in the PTZ test. The anticonvulsant activity profile suggests a mechanism different from that of VPA.

Ca(2+) requirement for high-affinity gamma-aminobutyric acid (GABA) binding at GABA(B) receptors: involvement of serine 269 of the GABA(B)R1 subunit

The gamma-aminobutyric acid (GABA) receptor type B (GABA(B)R) is constituted of at least two homologous proteins, GABA(B)R1 and GABA(B)R2. These proteins share sequence and structural similarity with metabotropic glutamate and Ca(2+)-sensing receptors, both of which are sensitive to Ca(2+). Using rat brain membranes, we report here that the affinity of GABA and 3-aminopropylphosphinic acid for the GABA(B)R receptor is decreased by a factor >10 in the absence of Ca(2+). Such a large effect of Ca(2+) is not observed with baclofen or the antagonists CGP64213 and CGP56999A. In contrast to baclofen, the potency of GABA in stimulating GTPgammaS binding in rat brain membranes is also decreased by a factor >10 upon Ca(2+) removal. The potency for Ca(2+) in regulating GABA affinity was 37 microM. In cells expressing GABA(B)R1, the potency of GABA, but not of baclofen, in displacing bound (125)I-CGP64213 was similarly decreased in the absence of Ca(2+). To identify residues that are responsible for the Ca(2+) effect, the pharmacological profile and the Ca(2+) sensitivity of a series of GABA(B)R1 mutants were examined. The mutation of Ser269 into Ala was found to decrease the affinity of GABA, but not of baclofen, and the GABA affinity was found not to be affected upon Ca(2+) removal. Finally, the effect of Ca(2+) on the GABA(B) receptor function is no longer observed in cells coexpressing this GABA(B)R1-S269A mutant and the wild-type GABA(B)R2. Taken together, these results show that Ser269, which is conserved in the GABA(B)R1 protein from Caenorhabditis elegans to mammals, is critical for the Ca(2+)-effect on the heteromeric GABA(B) receptor.

Epileptiform activity of veratridine model in rat brain slices: effects of antiepileptic drugs

The effects of the antiepileptic drugs valproic acid (VPA), phenytoin (PHT), and ethosuximide (ESM) on evoked and spontaneous seizure-like (epileptiform) activity were studied in the veratridine epileptiform model in rat brain slices, using conventional electrophysiological intracellular recording techniques. The veratridine model is generated by treatment of brain slices with a low concentration (0.3 microM) of the alkaloid veratridine. The drug modifies sodium channel function so that a brief current injection in hippocampal CA1 pyramidal neurons evokes bursts of epileptiform activity. Therapeutic concentrations of VAP (50-200 microM) inhibited both evoked and spontaneous bursting in a voltage-dependent manner without affecting membrane resting potential or input resistance. Similarly, therapeutic concentrations of PHT (4-15 microM) inhibited both evoked and spontaneous bursting in a voltage-dependent fashion with no apparent change in the membrane resting potential. However, PHT increased the membrane input resistance and elevated the firing threshold of neurons. The antiepileptic drug ESM failed to inhibit evoked or spontaneous bursting even at high concentrations. The results suggest that the veratridine model of epileptiform activity is sensitive only to antiepileptic drugs that primarily affect the sodium channels.

Effects of valproate derivatives II. Antiepileptic efficacy in relation to chemical structures of valproate sugar esters

The structure effect relationships of derivatives of the antiepileptically active ester of valproate (VPA) 3,4:5,6-Di-O-isopropylidene-1-O-(2-propylpentanoyl)-D-mannitol (1) have been studied using intracellular recording to record the membrane potential of single neurons (buccal ganglia, Helix pomatia). Epileptiform activity was induced by the epileptogenic drug pentylenetetrazol. The effects of several derivatives on epileptiform activity were compared with those of the relay compound 1. Most of the synthesized agents decreased the duration of paroxysmal depolarization shifts (PDS) and increased their repetition rate. It was considered that a decreased the duration of PDS is antiepileptic and an increased repetition rate is pro-epileptic. Compared with the effects of compound 1, the following relationships were found: (1) Derivatives containing glucitol or galactitol were of similar antiepileptic potency. (2) Introduction of pyranoses or furanoses rendered the substances inactive or even pro-epileptic. (3) VPA in position 1 and 6 at the sugar acted as an antiepileptic whereas in position 3 and 4 it proved to be ineffective. (4) Replacement of VPA by ethylhexanoyl reduced the antiepileptic potency slightly and pivaloyl strongly. (5) Replacement of isopropylidene bridges by penta-O-acetyl or cyclohexylidene residues led to largely inactive substances. (6) Compounds having isopropylidene bridges in position 2,4;3,5 proved to be antiepileptic whereas bridges especially in positions 2,3:4,5 slightly enhanced epileptic activities.

Effects of valproate derivatives I. Antiepileptic efficacy of amides, structural analogs and esters

Derivatives of the antiepileptic drug valproate (VPA, 2-propylpentanoic acid) have been synthesized and tested in order to improve the intracellular availability of VPA. The buccal ganglia of Helix pomatia were used as a test nervous system and antiepileptic efficacies were reconfirmed using rat cortex in vivo. Epileptiform activities consisted of typical paroxysmal depolarization shifts (PDS) which appeared in the identified neuron B3 with application of pentylenetetrazol. Epileptiform activities were found to be accelerated, unaffected or blocked. (i) The Amide-derivatives 2-propylpentanamide and N,N-dipropyl-2-propylpentanamide, and short chain ester derivatives 1-O-(2-propylpentanoyl)-2,3-propandiol, 2,2-di(hydroxymethyl)-1-O-(2-propylpentanoyl)-1,3-propanediol and 2,2-di(hydroxymethyl)-1,3-di-O-(2-propylpentanoyl)-1,3-propanediol accelerated epileptiform activities. Membrane potential often shifted to a permanent depolarization which corresponded to the PDS-inactivation level. (ii) The structural analogs 1-cycloheptene-1-carboxylic acid and cyclooctanecarboxylic acid accelerated epileptiform activities only slightly or were without effects. (iii) The small VPA-ester, 2-propylpentanoic acid ethyl ester, decreased the epileptiform activities in a way that is comparable to the effects of VPA well known from previous studies. It thus could be thought as a VPA-pro-drug. (iv) The mannitol-esters 1-O-(2-propylpentanoyl)-D-mannitol and 3,4;5,6-Di-O-isopropylidene-1-O-(2-propylpentanoyl)-D-mannitol blocked the PDS in a way which is different from the known effects of VPA. These substances are interpreted not to exert their effects after being metabolized to VPA and thus they are thought to be new antiepileptic substances.

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

The effects of four valproic acid derivatives were studied on pentylenetetrazole-induced epileptiform discharges in combined entorhinal cortex hippocampus slices. The two new sugar-esters of valproic acid, dimethylenexylitol valproate (VDMX, 0.5 mM) and glucose valproate (VG, 2 mM) abolished the epileptiform activity. These two new derivatives were compared to two clinically used anticonvulsant drugs, valpromide (2 mM) which suppressed the activity and valproic acid (2 mM), which was ineffective. The new drugs VDMX and VG were also tested on different patterns of epileptiform activity induced by lowering of [Mg2+]0. A 1 mM concentration of VDMX and 2 mM VG, reversibly suppressed the recurrent short discharges in area CA1 and the seizure-like events in the entorhinal cortex. A concentration of 2 mM VDMX was required to abolish the late recurrent discharges in entorhinal cortex. VG at 2 mM reduced the frequency of these discharges by 58.5+/-9.5%.

Effects of AWD 140-190 on stimulus-induced field potentials and on different patterns of epileptiform activity induced by low calcium or low magnesium in rat entorhinal cortex hippocampal slices

AWD 140-190 a potent new anticonvulsant was tested on several types of epileptiform activities in entorhinal cortex hippocampal slices. AWD 140-190 suppressed completely and in a dose-dependent manner spontaneous seizure-like events induced by lowering extracellular Ca2+. In the low magnesium model, AWD 140-190 applied with 200 microM reduced recurrent short discharges in area CA1 by 48.1 +/- 14.7%, while in the entorhinal cortex seizure-like events were not depressed. Late recurrent discharges were increased in frequency to 213.8 +/- 78.1 and reduced in amplitude by 50.1 +/- 14.4%. Responses to paired pulse stimuli with intervals ranging from 20 to 150 ms were reduced both with alvear and stratum radiatum stimulation. Decreases in [Ca2+]0 and associated slow field potentials evoked by repetitive stimulation of stratum radiatum were also depressed in a dose-dependent manner. AWD 140-190 also reduced stimulus-induced rises in [K+]0. AWD 140-190 200 microM diminished the amplitude of slow field potentials observed during high K(+)-induced spreading depression by about 17% in CA1 and 34% in entorhinal cortex without any significant effect on SD-associated rises in [K+]0. These results suggest that AWD 140-190 has an anticonvulsant effect presumably by interfering with repetitive generation of action potentials. AWD 140-190 may also possess modulatory effects on glial cells as suggested by the strong depression of SD-associated slow negative potential shifts.