Potentiation of gamma-vinyl GABA (vigabatrin) effects by glycine

Synthesis and Biological Evaluation of Amino Acid Based Mutual Amide Prodrugs of Phenytoin as Anticonvulsant Agents

BACKGROUND

Phenytoin (5,5-diphenyl hydantoin) has poor water solubility, which results in incomplete oral availability. Other problems associated with the oral and intramuscular administration of phenytoin are gastric irritation and inflammation at the site of injection.

OBJECTIVE

The purpose of this study was to synthesize mutual amide prodrugs of phenytoin by using amino acids like glycine, L-tryptophan, L-lysine and taurine.

METHODS

These prodrugs were synthesized and characterized by Fourier Transform Infrared (FTIR), Proton nuclear magnetic resonance (1H NMR) and Mass Spectra. Physical and spectral characterization was performed by determination of solubility, maximum wavelength, partition coefficient (log P), ionization constant (pKa), specific (α) and molar rotation (μ), refractive index (n), specific refraction (RS) and molar refraction (RM).

RESULTS

The results obtained from solubility and log P values determination indicated that phenytoin prodrugs can be administered by oral as well as a parenteral route by minimizing the limitations associated with phenytoin. Anticonvulsant activity of prodrugs (4a-4d) was evaluated by using maximal electroshock (MES) and strychnine induced seizure test on albino mice of either sex weighing 25-30 g in which 4b and 4d were found to have significant anticonvulsant activity for MES and strychnine induced seizure test. In vitro enzymatic hydrolysis study of 4b and 4d was performed on liver, intestinal mucosa and plasma sample of male Sprague Dawley rats weighing 280-300 g in which phenytoin was eluted at 10.13 to 10.68 minutes at 220 nm.

CONCLUSION

The results obtained from the present work showed that amino acid-based mutual prodrug strategy can be a promising method to increase the solubility and anticonvulsant activity of phenytoin for the development of anticonvulsant agents.

Long-term vigabatrin treatment modifies pentylenetetrazole-induced seizures in mice: focused on GABA brain concentration

BACKGROUND

The goal of our study was to examine the long-term effect of vigabatrin (VGB), a γ-aminobutyric acid aminotransferase (GABA-AT) inhibitor on clonazepam (CLO), ethosuximide (ETX) and valproate (VPA) anticonvulsive activity against pentylenetetrazole (PTZ)-induced seizures in mice.

METHODS

VGB was administered for 3 and 7 days. Convulsions were evoked by PTZ at its CD97 (99 mg/kg). The influence of CLO, ETX and VPA alone or in combination with VGB on motor performance and long-term memory was analyzed. γ-aminobutyric acid (GABA) concentration in mice brain and plasma as well as glutamate decarboxylase (GAD) activity was measured.

RESULTS

After 3 days of treatment, VGB in doses up to 500 mg/kg increased PTZ-induced seizure threshold, whereas after 7 days VGB (at the dose of 125 mg/kg) inhibited clonic seizures in experimental mice. 7 days of VGB administration did not change the protective effect of CLO, ETX and VPA against PTZ-induced seizures. 7 days of VGB treatment at a subthreshold dose of 75 mg/kg decreased TD50 of ETX and CLO in the chimney test, but did not affect TD50 value for VPA. 7 days of VGB administration in combination with AEDs did not affect long-term memory in mice. VGB after 3 days or 7 days of administration increased brain GABA concentration. GAD activity was decreased after 3 and 7 days of VGB administration.

CONCLUSIONS

The presented results confirm anticonvulsive activity of VGB through GABA metabolism alteration and suggest care when combining VGB with ETX or CLO in the therapy.

Design and pharmacological activity of glycinamide and N-methoxy amide derivatives of analogs and constitutional isomers of valproic acid

A series of glycinamide conjugates and N-methoxy amide derivatives of valproic acid (VPA) analogs and constitutional isomers were synthesized and evaluated for anticonvulsant activity. Of all compounds synthesized and tested, only N-methoxy-valnoctamide (N-methoxy-VCD) possessed better activity than VPA in the following anticonvulsant tests: maximal electroshock, subcutaneous metrazol, and 6-Hz (32-mA) seizure tests. In mice, the ED(50) values of N-methoxy-VCD were 142 mg/kg (maximal electroshock test), 70 mg/kg (subcutaneous metrazol test), and 35 mg/kg (6-Hz test), and its neurotoxicity TD(50) was 118 mg/kg. In rats, the ED(50) of N-methoxy-VCD in the subcutaneous metrazol test was 36 mg/kg and its protective index (PI=TD(50)/ED(50)) was >5.5. In the rat pilocarpine-induced status epilepticus model, N-methoxy-VCD demonstrated full protection at 200mg/kg, without any neurotoxicity. N-Methoxy-VCD was tested for its ability to induce teratogenicity in a mouse strain susceptible to VPA-induced teratogenicity and was found to be nonteratogenic, although it caused some resorptions. Nevertheless, a safety margin was still maintained between the ED(50) values of N-methoxy-VCD in the mouse subcutaneous metrazol test and the doses that caused the resorptions. On the basis of these results, N-methoxy-VCD is a good candidate for further evaluation as a new anticonvulsant and central nervous system drug.

Vigabatrin in Low Doses Selectively Suppresses the Clonic Component of Audiogenically Kindled Seizures in Rats

PURPOSE

The effect of systemic administration of the gamma-aminobutyric acid (GABA)-transaminase inhibitor vigabatrin (VGB) on different components of convulsions was tested in the model of audiogenically kindled seizures, which consist of brainstem (running, tonus) and forebrain (clonus) elements.

METHODS

Audiogenically susceptible rats of Krushinsky-Molodkina (KM), Wistar, and WAG/Rij strains received repeated sound stimulation (60 dB, 10-80 kHz) until kindled audiogenic seizures were reliably elicited. Kindled audiogenic seizures consisted of running, tonic, and generalized clonic phases in KM rats (severe audiogenic seizures) and of running and Racine stage 5 facial/forelimb clonus in Wistar and WAG/Rij rats (moderate seizures). Vehicle, 100, or 200 mg/kg of VGB was intraperitoneally injected 2, 4 and 24 h before the induction of kindled audiogenic seizures.

RESULTS

At both doses, VGB did not change the seizure latency and the duration of running and tonic convulsions, but suppressed clonic ones in all rat strains. In KM rats, the mean duration of posttonic clonus was significantly reduced at 24 h after 100 mg/kg and from 4 h after 200 mg/kg. In Wistar and WAG/Rij rats, the mean duration of facial/forelimb clonus was reduced from 4 and 2 h after 100- and 200-mg/kg administration, respectively; 24 h after the high-dose injection, clonus was completely blocked in all rats of both strains. No difference in efficacy of VGB between Wistar and WAG/Rij rats was observed.

CONCLUSIONS

VGB more effectively suppresses clonic convulsions than running and tonic ones in audiogenically kindled rats. It is supposed that this selective anticonvulsive effect of VGB results from different sensitivities of forebrain and brainstem epileptic networks to the presumed GABA enhancement.

Glutamine is the major precursor for GABA synthesis in rat neocortex in vivo following acute GABA-transaminase inhibition

The objective of the present study was to assess the degree to which astrocytic glutamine provides carbon for net synthesis of GABA in the rat neocortex in vivo. Isotopic labeling of GABA and glutamate from astrocytic glutamine was followed in halothane anesthetized and ventilated rats during an intravenous infusion of [2-(13)C]glucose. A net increase in GABA was achieved by administration of the GABA-transaminase inhibitor, gabaculine to suppress catabolism of GABA and recycling of (13)C label. (13)C Percentage enrichments of GABA, glutamate and glutamine were assessed in tissue extracts using (13)C-edited (1)H nuclear magnetic resonance at 8.4 T. GABA levels increased 2.6 micromol/g at 2 h and 6.1 micromol/g at 5 h after gabaculine, whereas glutamate and glutamine decreased in toto by 5.6 micromol/g at 2 h and 3.1 micromol/g at 5 h. Selective enrichment of glutamine, glutamate, and GABA C3's over other carbon positions was observed consistent with a precursor role for astrocytic glutamine. Between 1 h (control) and 3 h (gabaculine-treated) of [2-(13)C]glucose infusion, (13)C percentage enrichment increased in glutamine C3 (from 3.2+/-0.5 to 7.0+/-0.9%), glutamate C3 (from 1.8+/-0.5 to 3.4+/-0.9%), and GABA C3 (from 2.7+/-1.6 to 4.8+/-0.4%). The measured incremental [3-(13)C]GABA concentration (0.15 micromol/g) was close to the predicted value (0.13 micromol/g) that would be expected if the increase in GABA were produced entirely from glutamine compared to glutamate (0.07 micromol/g) based on the average precursor enrichments between 1 and 3 h. We conclude that glutamine is the major source of GABA carbon in the rat neocortex produced acutely following GABA-T inhibition by gabaculine in vivo.

Update on the pathophysiology of the epilepsies

The pathophysiology of convulsive and non-convulsive epilepsies is discussed in its primary generalised forms. Focal, clinical and experimental epilepsies, with emphasis placed on the temporal lobe epilepsies (TLE) and their pathophysiologies are also reviewed. Neurotransmitters and neuromodulators and between them, the second messenger systems are considered in the generation, maintenance or inhibition of the epileptic discharge. Action mechanisms of the more classic antiepileptic drugs are briefly summarized along with the therapeutic strategies that might achieve the final control of abnormal discharges, including genetic control as a promising alternative in the current state of research. We emphasized the study of all type of glutamate and GABA receptors and their relation with mRNA editing in the brain. Some of the genetic studies which have been so fruitful during the last ten years and which have brought new insights regarding the understanding of epileptic syndromes are summarized in this article.

Possible analgesic effect of vigabatrin in animal experimental chronic neuropathic pain

Since anticonvulsants have been used for treating neuralgias, an interest has arisen to experimentally test vigabatrin for its gabaergic mechanism of action. For this, 41 Wistar rats were used, and in 25 of them a constrictive sciatic neuropathy was induced (Bennet & Xie model). For testing pain symptoms, spontaneous (scratching) and evoked behaviors to noxious (46 degrees C) and non-noxious (40 degrees C) thermal stimuli were quantified. Moreover, a comparative pharmacological study of vigabatrin with other analgesic anticonvulsant drugs was also performed. The results showed a possible dose-dependent analgesic effect of vigabatrin (gamma-vinyl-GABA) on experimental neuropathic pain, as shown by the significant (p < 0.05) decreasing effect of vigabatrin on scratching and by its significant (p < 0.05) increasing effect on the latency of the right hindpaw withdrawal of the animals to noxious thermal stimulus. This was corroborated by similar findings with analgesic anticonvulsants (carbamazepine, phenytoin and valproic acid). This possible and not yet described analgesic effect of vigabatrin seems not to be opioid mediated.

Pharmacokinetic analysis and anticonvulsant activity of glycine and glycinamide derivatives

The objective of this study was to investigate the pharmacokinetics and pharmacodynamics (anticonvulsant activity and neurotoxicity) of a series of amide derivatives of glycinamide in order to explore their structure pharmacokinetic-pharmacodynamic relationship and to discover a glycinamide derivative which might have the potential to become a new antiepileptic agent. The following compounds were investigated: glycylglycine, glycylglycinamide, gaboylglycinamide, N-acetylglycine, N-acetylglycinamide, N-acetylglycylglycinamide, N-acetyl, N'-benzylglycinamide, N-benzyloxycarbonylglycine or Z-glycine, Z-glycinamide, Z-glycylglycine and Z-glycylglycinamide. The anticonvulsant activity and neurotoxicity study was carried out in classical animal models for anticonvulsant screening. The pharmacokinetics of the active compounds was studied in dogs, which is a common animal model for a comparative crossover pharmacokinetic studies. Of the compounds investigated in this study, all the dipeptides of glycinamide and the glycine derivatives were found to be inactive. The only two active compounds were: N-acetyl,N'-benzylglycinamide (VII) and Z-glycinamide (IX). These compounds demonstrated similar pharmacokinetic profiles. Unlike glycine or glycinamide, compounds VII and IX, being lipophilic derivatives of glycinamide, showed anticonvulsant activity in animal models due to their better pharmacodynamic and pharmacokinetic properties. The pharmacodynamics and pharmacokinetics of compounds VII and IX were similar to that of the potential new antiepileptics; N-valproylglycinamide and phthaloylglycinamide. This study provides certain clues concerning the structural requirements for the design of anticonvulsant-active glycine derivatives.

Pentylenetetrazol and strychnine convulsions in brain weight selected mice

The seizure sensitivities to pentylenetetrazol (Ptz, 25-100 mg/kg) and strychnine (S, 2 mg/kg) were tested in two mice lines selected for large (LB) and small (SB) brain weight (brain weight difference being approximately 75 mg). The selection was based on a regression line connecting body and brain weight. SB mice were more sensitive to both drugs-their seizure latencies were shorter and lethality higher than in LBs. The seizures generated by Ptz and S are known to affect different neurotransmitter systems. The interstrain differences in seizure susceptibility are probably determined by SB mice nervous system traits rather than by differences in the particular neurochemical trait. The data on neocortical cytoarchitectonics obtained during our previous brain selection experiment could serve as the indirect evidence favouring such a suggestion.

Picolinic acid and indole-2-carboxylic acid: two types of glycinergic compounds modulate motor function differentially

1. A putative agonist for the strychnine-sensitive glycine receptor picolinic acid was tested for its anticonvulsant activities in mice and muscle-relaxant activities in rats and compared with indole-2-carboxylic acid (I2CA), an antagonist for the strychnine-insensitive glycine receptor. Their effects on segmental reflexes in the cat spinal cord were examined to elucidate their sites of action. 2. Picolinic acid (200 and 400 mg/kg IP) delayed the onsets of strychnine- but not pentylenetetrazole-induced seizures. It delayed the onsets of bicuculline-induced seizures only at the higher dose. I2CA (200 and 400 mg/kg IP) delayed the onsets of these 3 kinds of seizures. Both compounds reduced muscle tone in rat decerebrate rigidity at a dose of 100 mg/kg IV. 3. Picolinate methylester, a picolinate derivative with higher lipophilicity, depressed spinal reflexes in both intact and spinalized cats at cumulative doses of 25 to 200 mg/kg IV. I2CA (50 mg/kg IV) inhibited spinal reflexes only in intact preparations. 4. These results suggest that the anticonvulsant and muscle-relaxant activities of picolinic acid (PA) are due to inhibition of spinal neurons, but that I2CA selectively affects supraspinal structures.

Seizure-inducing effects of antiepileptic drugs: a review

Seizure-inducing effects can be observed in the treatment of epileptic patients with antiepileptic drugs (AED). This may be a paradoxical reaction (for example the increase of complex focal seizures due to carbamazepine, vigabatrin or phenytoin treatment) or a result of AED-induced encephalopathy (commonly induced by valproate in patients with complex focal seizures). A seizure increase during intoxication with AED is a rare phenomenon, thus, it is not directly related to this condition. An incorrect choice of drugs in the treatment of an epileptic syndrome or seizure type may provoke seizures (as for example the provocation of absences due to carbamazepine or phenytoin). The possible seizure-inducing effect of AEDs has to be differentiated from seizure occurrence due to the natural course of epilepsy. This may be especially difficult in patients suffering from West syndrome or Lennox-Gastaut syndrome, in whom seizure frequency may vary even without medication. However, especially in these patients, drug-induced worsening of seizure manifestation is often observed. In general, a seizure-inducing effect of antiepileptic drugs has to be considered when a seizure increase is observed soon after the initiation of therapy, when a stepwise increase of the dosage is followed by a further increase of seizures, a decrease of seizures is seen with tapering of the dosage and a renewed increase of seizures can be observed after this therapy has been reestablished. Finally, one knows that the clinical condition of encephalopathy due to valproate or carbamazepine is accompanied by seizure increase. In spite of these clinical aspects, the underlying mechanisms of seizure increase mostly remain unclear. From animal experiments it is obvious that especially carbamazepine and phenytoin may provoke generalized seizures as absences or myoclonic seizures. A seizure increase during vigabatrin therapy has been attributed to the increase of the cerebral amount of gamma-amino butyric acid, which is known to possibly exhibit inhibitory or excitatory neuronal effects. The occurrence of tonic seizures in patients with Lennox-Gastaut syndrome has been attributed to the sedative effect of the drugs; however, this conclusion is controversial. From a clinical point of view, one should consider young age of the patient, mental retardation, antiepileptic polytherapy, high frequency of seizures or prominent epileptic activity in the electroencephalogram previous to medication as risk factors for a possible seizure-inducing effect of antiepileptic drugs.

Pharmacokinetic analysis and antiepileptic activity of N-valproyl derivatives of GABA and glycine

PURPOSE

To explore the possibility of utilizing valproyl derivatives of GABA and glycine as new antiepileptics by using the structure pharmacokinetic-pharmacodynamic relationship (SPPR) approach.

METHODS

The pharmacokinetics and pharmacodynamics (anticonvulsant activity and neurotoxicity) of the following four conjugation products of valproic acid (VPA), glycine and GABA were investigated: valproyl glycine, valproyl glycinamide, valproyl GABA and valproyl gabamide.

RESULTS

Only valproyl glycinamide showed a good anticonvulsant profile in both mice and rats due to its better pharmacokinetic profile. Valproyl glycinamide was more potent than one of the major antiepileptic agents--VPA and showed a better margin between activity and neurotoxicity. Valproyl glycine and valproyl GABA were partially excreted unchanged in the urine (fe = 50% and 34%, respectively), while the urinary metabolites of the amide derivatives were valproyl glycine and valproyl GABA.

CONCLUSIONS

The four investigated valproyl derivatives did not operate as chemical drug delivery systems (CDDS) of glycine or GABA, but acted rather as drugs on their own. The current study demonstrates the benefit of the SPPR approach in developing and selecting a potent antiepileptic compound in intact animals based not only on its intrinsic pharmacodynamic activity, but also on its better pharmacokinetic profile.

Basic mechanisms of generalized absence seizures

Generalized absence seizures are neurophysiologically, pharmacologically, and developmentally unique and comprise the primary seizure type in a number of different absence epilepsy syndromes. Over the last 10 years, the availability of a number of animal models of generalized absence seizures and of sophisticated in vitro electrophysiological techniques that allow investigation of cortical and thalamic networks has begun to shed light on the pathogenesis of this disorder. The basic underlying mechanism appears to involve thalamocortical circuitry and the generation of abnormal oscillatory rhythms from that particular neuronal network. Biochemical mechanisms operative within thalamocortical circuitry during this neuronal oscillation seem to entail phase-locked gamma-aminobutyric acid (GABA)B-mediated inhibition alternating with glutamate-mediated excitation. The basic cellular mechanism operative within this tension between excitation and inhibition appears to involve the T-type calcium current. Local circuitry within the thalamus may influence these oscillatory rhythms by GABAA-mediated inhibition. Pharmacological factors at play external to thalamocortical circuitry include cholinergic, dopaminergic, and noradrenergic mechanisms. Pathways that utilize these various neurotransmitters project onto the thalamus and/or cortex from sites distant to those structures and may modulate the process either up or down. Perturbation of one or more of these neuronal networks may lead to abnormal neuronal oscillatory rhythms within thalamocortical circuitry, with a resultant generation of bilaterally synchronous spike wave discharges that characterize generalized absence seizures. Our increasing understanding of the basic mechanisms that underlie generalized absence seizures promises to allow, for the first time, a rational design of drug treatment for a seizure disorder based on the pathogenesis of that disorder.

Comparative pharmacokinetic and pharmacodynamic analysis of phthaloyl glycine derivatives with potential antiepileptic activity

Glycine, in addition to GABA, is one of the most important neurotransmitter amino acids. The described structure pharmacokinetic pharmacodynamic relationships (SPPR) study explored the possibility of utilizing phthaloyl derivatives of glycine as new antiepileptics. This was carried out by investigating the pharmacokinetics and pharmacodynamics (anticonvulsant activity and neurotoxicity) of the following four phthalimide derivatives: phthaloyl glycine, phthaloyl glycinamide, N,N-diethyl phthaloyl glycinamide and N,N-diisopropyl phthaloyl glycinamide. Phthaloyl glycine did not demonstrate anticonvulsant activity, possibly because of its poor pharmacokinetics, high clearance, low volume of distribution and short half life. The three glycinamide derivatives showed anticonvulsant activity and had better pharmacokinetic profiles, longer half life and mean residence time, than phthaloyl glycine. Phthaloyl glycinamide was more potent than one of the major antiepileptic agents--valproic acid and showed a better margin between activity and neurotoxicity. The four investigated phthaloyl glycine derivatives did not operate as chemical drug delivery systems (CDDS) of glycine, but acted rather as drugs on their own. Phthaloyl glycine was excreted unchanged in the urine while the urinary metabolites of the glycinamide derivatives were phthaloyl glycine and phthaloyl glycinamide. In this analogous series of phthalimide derivatives, minor chemical changes affected dramatically the compounds' pharmacokinetics. The current study demonstrates the benefit of the SPPR approach in developing and selecting a potent antiepileptic compound in intact animals based not only on its intrinsic pharmacodynamic activity, but also on its better pharmacokinetic profile.(ABSTRACT TRUNCATED AT 250 WORDS)

Newer antiepileptic drugs. Towards an improved risk-benefit ratio

Epilepsy is one of the most common neurological disorders. Even though existing antiepileptic drugs can render 80% of newly diagnosed patients seizure free, a significant number of patients have chronic intractable epilepsy causing disability with considerable socioeconomic implications. There is, therefore, a need for more potent and effective antiepileptic drugs and drugs with fewer adverse effects, particularly CNS effects. Drugs for the treatment of partial seizures are particularly needed. With major advances in our understanding of the basic neuropathology, neuropharmacology and neurophysiology of epilepsy, numerous candidate novel antiepileptic drugs have been developed in recent years. This review comparatively evaluates the pharmacokinetics, efficacy and adverse effects of 12 new antiepileptic drugs namely vigabatrin, lamotrigine, gabapentin, oxcarbazepine, felbamate, tiagabine, eterobarb, zonisamide, remacemide, stiripentol, topiramate and levetiracetam (ucb-L059). Of the 12 drugs, vigabatrin, lamotrigine and gabapentin have recently been marketed in the UK. Five of these new drugs have known mechanisms of action (vigabatrin, lamotrigine, tiagabine, oxcarbazepine and eterobarb), which may provide for a more rational approach to the treatment of epilepsy. Oxcarbazepine, remacemide and eterobarb are prodrugs. Vigabatrin, gabapentin and topiramate are more promising on the basis of their pharmacokinetic characteristics in that they are excreted mainly unchanged in urine and not susceptible to significant pharmacokinetic interactions. In contrast, lamotrigine, felbamate and stiripentol exhibit significant drug interactions. Essentially, all the drugs are effective in partial or secondarily generalised seizures and are effective to varying degrees in other seizure types. Particularly welcome is the possible effectiveness of zonisamide in myoclonus and felbamate in Lennox-Gastaut syndrome. In relation to adverse effects, CNS effects are observed with all drugs, however, gabapentin, remacemide and levetiracetam appear to exhibit least. There is also the possibility of rational duotherapy, using drugs with known mechanisms of action, as an additional therapeutic approach. The efficacy of these 12 antiepileptic drug occurs despite the fact that candidate antiepileptic drugs are evaluated under highly unfavourable conditions, namely as add-on therapy in patients refractory to drug management and with high seizure frequency. Thus, whilst candidate drugs which do become licensed are an advance in that they are effective and/or are associated with less adverse effects than currently available antiepileptic drugs in these patients, it is possible that these drugs may exhibit even more improved risk-benefit ratios when used in normal clinical practice.

Antiepileptic drug pharmacokinetics and neuropharmacokinetics in individual rats by repetitive withdrawal of blood and cerebrospinal fluid: milacemide

1. The kinetics and metabolism of milacemide have been studied in an animal model which allows the simultaneous investigation of the temporal inter-relationships of drugs and metabolites in blood (pharmacokinetics) and cerebrospinal fluid (CSF, neuropharmacokinetics) in individual freely moving rats. 2. Milacemide dose-dependently increased CSF glycine and glycinamide (intermediary metabolite) concentrations. This confirms that milacemide is a CNS glycine prodrug. 3. Pretreatment with L-deprenyl (2 mg kg-1), a specific inhibitor of monoamine oxidase type B (MAO-B), almost completely prevented the formation of glycinamide and increased milacemide accumulation in CSF. Tmax and t1/2 were significantly increased and Cmax and AUC values were decreased for glycinamide compared to controls. Pretreatment with clorgyline (5 mg kg-1), a specific inhibitor of MAO-type A, only moderately decreased glycinamide Cmax and AUC values. 4. After milacemide administration (100, 200 and 400 mg kg-1, i.p.) serum and CSF milacemide concentrations rose linearly and dose-dependently. Serum glycinamide concentrations exhibited small dose-dependent rises but these were not linearly related. In contrast, CSF glycinamide concentrations rose linearly and dose-dependently with Cmax values 2.5, 3.2 and 4.1 times greater than the corresponding values for serum glycinamide after giving 100, 200 and 400 mg kg-1 respectively of milacemide. 5. Serum glycine concentrations were unaffected but CSF concentrations increased dose-dependently and these were significant at the higher milacemide doses (200 and 400 mg kg-1). Animals given 400 mg kg-1 milacemide had glycine values which were still significantly elevated 7 h later. 6. In conclusion, serum milacemide rapidly enters and equilibrates with the CNS compartment where it is metabolised primarily by MAO-B to glycinamide and finally to glycine. Metabolism in the peripheral compartment is negligible.

Amino acid levels in the cerebrospinal fluid of newly diagnosed epileptic patients: effect of vigabatrin and carbamazepine monotherapies

We studied the CSF amino acid levels of 42 patients with newly diagnosed epilepsy before treatment with antiepileptic medication and during monotherapy with either vigabatrin or carbamazepine. The present study shows that patients with newly diagnosed epilepsy have elevated levels of the excitatory amino acid glutamate in CSF. Vigabatrin monotherapy effectively prevents the appearance of seizures in patients with high baseline CSF glutamate levels. In these patients, vigabatrin not only elevates the levels of gamma-aminobutyric acid, but also decreases the elevated levels of glutamate in CSF, which may also be important to the antiepileptic efficacy of vigabatrin. Patients with low CSF glutamate levels did not benefit from vigabatrin-induced changes in amino acid levels and successful monotherapy with carbamazepine did not affect CSF amino acid levels. The elevation of gamma-aminobutyric acid is thus not the only way to achieve seizure control and there are several factors underlying the generation and control of seizures. Follow-up of the patients with high baseline glutamate CSF levels will show if the observed abnormalities are related to the severity of epilepsy in individual patients and if early treatment with vigabatrin of these patients could prevent the development of intractable epilepsy.

Pharmacology of vigabatrin

Vigabatrin (gamma-vinyl GABA) is a relatively new antiepileptic drug. Vigabatrin increases the concentration of gamma-aminobutyric acid (GABA) in the brain by inhibiting the major GABA metabolizing enzyme, GABA transaminase. Controlled clinical trials have demonstrated an excellent antiepileptic effect of vigabatrin, especially in the treatment of partial epilepsies. Long-term evaluations have shown no signs of tolerance development. Vigabatrin decreases the plasma concentration of phenytoin during concomitant therapy, the only drug with which an interaction seems to occur. In general, vigabatrin is well tolerated. Psychotic reactions occur in 3-6% of patients. Other frequent side effects are sedation and weight increase. Chronic vigabatrin intoxication in animals caused development of intramyelinic oedema, appearing as microvacuoles in brain white matter. No microvacuolation has been observed in humans, even after long-term treatment. Vigabatrin seems a very valuable new antiepileptic drug.

Antagonism of phencyclidine-induced hyperactivity in mice by elevated brain GABA concentrations

Vigabatrin and (S)-4-allenylGABA (MDL 72483), two anticonvulsant GABA-T inhibitors, partially antagonize phencyclidine (PCP)-induced hyperactivity in mice at doses that do not affect spontaneous motor activity. The PCP antagonism is related to whole-brain GABA concentrations. The results indicate the potential use of GABA-T inhibitors in the therapy of PCP intoxications and perhaps also in the treatment of certain forms of endogenous psychoses.

Genetic absence epilepsy in rats from Strasbourg--a review

We have selected a strain of rats and designated it the Genetic Absence Epilepsy Rat from Strasbourg (GAERS). In this strain, 100% of the animals present recurrent generalized non-convulsive seizures characterized by bilateral and synchronous spike-and-wave discharges accompanied with behavioural arrest, staring and sometimes twitching of the vibrissae. Spontaneous SWD (7-11 cps, 300-1,000 microV, 0.5-75 sec) start and end abruptly on a normal background EEG. They usually occur at a mean frequency of 1.5 per min when the animals are in a state of quiet wakefulness. Drugs effective against absence seizures in humans (ethosuccimide, trimethadione, valproate, benzodiazepines) suppress the SWD dose-dependently, whereas drugs specific for convulsive or focal seizures (carbamazepine, phenytoin) are ineffective. SWD are increased by epileptogenic drugs inducing petit mal-like seizures, such as pentylenetetrazol, gamma-hydroxybutyrate, THIP and penicillin. Depth EEG recordings and lesion experiments show that SWD in GAERs depend on cortical and thalamic structures with a possible rhythmic triggering by the lateral thalamus. Most neurotransmitters are involved in the control of SWD (dopamine, noradrenaline, NMDA, acetylcholine), but GABA and gamma-hydroxybutyrate (GHB) seem to play a critical role. SWD are genetically determined with an autosomal dominant inheritance. The variable expression of SWD in offsprings from GAERS x control reciprocal crosses may be due to the existence of multiple genes. Neurophysiological, behavioural, pharmacological and genetic studies demonstrate that spontaneous SWD in GAERS fulfill all the requirements for an experimental model of absence epilepsy. As the mechanisms underlying absence epilepsy in humans are still unknown, the analysis of the genetic thalamocortical dysfunction in GAERS may be fruitful in investigations of the pathogenesis of generalized non-convulsive seizures.

Alterations of GABA metabolism and seizure susceptibility in the substantia nigra of the kindled rat acclimating to changes in osmotic state

Seizure susceptibility and GABA metabolism were altered in the substantia nigra [SN] of adult male Sprague Dawley rats when these animals were acclimating to an altered plasma osmolality. Changes in GABA metabolism were measured in vivo in SN of the freely moving rat. Suitable precautions were taken to avoid any post-mortem flux of glutamate to GABA and to correct for the underestimation of GABA build up in SN due to the finite diffusion rate of gamma-vinyl GABA [GVG] after stereotaxic injection of small amounts into one side of the brain. Control experiments provided evidence that changes in osmolality, within a normal physiological range, did not affect significantly gamma-aminobutyric acid transaminase [GABA-T]. Also kindling via the medial septum [MS], in the absence of electrical stimulation did not alter GABA metabolism in SN, thus providing a stable baseline for studies of osmotic effects. Hyperosmolality was associated with a rise in seizure thresholds, with a marked reduction of the rate of GABA synthesis in SN, and with a substantial increase in turnover time of the GABA pool. Hypoosmolality, of a degree known to be associated with mild cerebral edema and swelling localized to astrocytes, markedly reduced seizure threshold, and reduced GABA pool size in SN, but did not alter the rate of GABA synthesis significantly. These results demonstrate by new and independent means the relationship between GABA metabolism in the SN and seizure susceptibility in vivo.