Development of tolerance to the effects of vigabatrin (gamma-vinyl-GABA) on GABA release from rat cerebral cortex, spinal cord and retina

Alterations in pharmacological sensitivity of GABAergic but not dopaminergic and glutamatergic systems during ontogenesis in dystonic mutant hamsters

Attacks of sustained dystonia of the limbs and trunk can be initiated by handling or mild environmental stimuli (e.g. new cage) in mutant (gene symbol dtsz) Syrian golden hamsters. The severity of the dystonic syndrome in these mutant hamsters is age-dependent, with a peak at weaning (21 days of age) and a second peak at about 30-40 days of age. Spontaneous remission occurs at an age of about 70 days. The syndrome in hamsters is thus similar to transient paroxysmal dystonia in children. In the present experiments, it was examined whether dystonic hamsters exhibit age-dependent differences in susceptibility to drugs which affect GABA (gamma-aminobutyrate)ergic, glutamatergic or dopaminergic functions. After acute administration, the GABA-elevating drug aminooxyacetic acid was significantly less potent in attenuating the severity of dystonic attacks at 21 days than at 31 days of age. Similar but less marked age-dependent differences in antidystonic activity were found for phenobarbital and diazepam. In contrast to these GABAmimetic drugs, the NMDA receptor antagonist CGP 37849 (DL-[E]-2-amino-4-methyl-5-phosphono-3-pentenoic acid) or the dopamine receptor antagonist haloperidol had about the same antidystonic potency at both 21 and 31 days of age. Chronic treatment of dystonic hamsters with aminooxyacetic acid, starting at 21 days of age, did not alter the time course or the severity of dystonia.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of single and multiple increasing doses of vigabatrin on brain GABA metabolism and correlation with vigabatrin plasma concentration

The effects of increasing (50-1600 mg/kg/day) doses of vigabatrin (GVG) both as single doses and after 8 or 28 days of treatment have been studied in 19 groups of 10 adult Wistar rats. The parameters studied were brain gamma-aminobutyric acid-transaminase (GABA-T) activity, GABA concentration and L-glutamate decarboxylase (GAD) activity. Single increasing doses of GVG progressively inhibited GABA-T activity, but a residual activity of about 40% was observed with the highest doses. GABA concentration increased in a dose-dependent manner but a ceiling was not reached. GAD activity was slightly inhibited at low doses and stimulated at high ones. When treatment was continued for 8 days, more marked effects of GVG on GABA-T and GABA, a more severe toxicity and higher GVG plasma concentrations were observed. GAD was inhibited instead of stimulated by high GVG doses. After 28 days of treatment the effects of GVG on GABA-T and GABA were similar to those after 8 days. However, toxic effects decreased and lower GVG plasma concentrations were found.

IN CONCLUSION

(a) the more marked brain GABAergic effects observed after 8 days of treatment with GVG may explain the greater anticonvulsant effects observed by others in animals, and (b) GVG plasma concentrations correlate well with changes in brain GABA-T and GABA, and may partly explain changes in the effects of GVG related to the length of treatment.

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of tolerance to the anticonvulsant effect of vigabatrin in amygdala-kindled rats

The anticonvulsant, adverse and biochemical effects of the novel antiepileptic drug vigabatrin (gamma-vinyl GABA), which increases GABA (gamma-aminobutyric acid) levels by inhibition of the GABA degrading enzyme GABA aminotransferase, were examined in amygdala-kindled rats after acute and chronic administration. Vigabatrin proved to be a potent anticonvulsant drug at acute doses (100-200 mg/kg), but during chronic administration, the anticonvulsant activity of the treatment was lost already in the second week of treatment. Tolerance also developed to the adverse effects, i.e. hypothermia, sedation and motor impairment. Determination of vigabatrin in plasma indicated that tolerance was not due to declining drug levels. Furthermore, determination of endogenous amino acids in plasma showed that GABA levels were highly elevated throughout the period of treatment, although the extent of GABA accumulation decreased in the second week. After cessation of chronic treatment with vigabatrin, there was no clear indication of withdrawal symptoms, except a prolonged seizure or afterdischarge duration in experiments with 100 mg/kg per day. The data suggest that chronic treatment with vigabatrin may be associated with a loss of anticonvulsant efficacy, at least when the drug is given as monotherapy.

Effects of gamma-vinyl GABA (vigabatrin) on blood pressure and body weight of hypertensive and normotensive rats

Inactivation of GABA was inhibited by gamma-vinyl GABA (GVG) and the effects of the increased GABA level in the brain on blood pressure and body weight of spontaneously hypertensive rats (SHR) and normotensive rats (WKY) were investigated. When started at the age of 8 weeks or 5 weeks, treatment of SHR and WKY with GVG (150 mg/kg, s.c.) for several weeks did not influence systolic blood pressure. In 1-week old SHR, treatment with GVG (up to 150 mg/kg, s.c.) abolished the rise in blood pressure until animals were 8 weeks old. Thereafter, arterial blood pressure started to increase but it remained distinctly lower than that in untreated animals. When started at the age of 1 week, treatment with GVG for 7 weeks did not influence arterial blood pressure in WKY. GVG delayed increase in body weight in SHR and WKY, irrespective of their age. GVG greatly increased GABA levels in the hypothalamus, frontal cortex, brainstem and rest of the brain in both WKY and SHR. It is concluded that an increase in the GABA level in the brain leads to a delay in the development of hypertension in young SHR. Hence, development of genetic hypertension seems to be susceptible to activation of the GABAergic system in a very early critical phase only.

Increased intracellular gamma-aminobutyric acid selectively lowers the level of the larger of two glutamate decarboxylase proteins in cultured GABAergic neurons from rat cerebral cortex

The regulation of glutamate decarboxylase (GAD; EC 4.1.1.15) was studied by using cultures of cerebral cortical neurons from rat brain grown in serum-free medium. About 50% of the neurons in the cultures were gamma-aminobutyric acid (GABA)ergic as determined by two double-staining procedures. Immunoblotting experiments with four anti-GAD sera that recognize the two forms to varying degrees, demonstrated that the cultures contained the two forms of GAD that are present in rat brain (apparent molecular masses = 63 and 66 kDa). GAD activity was reduced by 60-70% when intracellular GABA levels were increased by incubating the cultures with the GABA-transaminase inhibitor gamma-vinyl-GABA for greater than 5-10 h or with 1 mM GABA itself. Neither baclofen nor muscimol (100 microM) affected GAD activity. Immunoblotting experiments showed that only the larger of the two forms of GAD (66 kDa) was decreased by elevated GABA levels. These results, together with previous results indicating that the smaller form of GAD is more strongly regulated by pyridoxal 5'-phosphate (the cofactor for GAD), suggest that the two forms of GAD are regulated by different mechanisms.

Effects of zaprinast and rolipram on platelet aggregation and arrhythmias following myocardial ischaemia and reperfusion in anaesthetized rabbits

1. This study was designed to compare the effects of two selective inhibitors of certain phosphodiesterase (PDE) isoenzymes on arrhythmias induced by coronary artery occlusion and reperfusion. The drugs used were zaprinast which inhibits guanosine 3':5'-cyclic monophosphate (cyclic GMP)-specific PDE (PDE V) and rolipram which inhibits cyclic GMP-insensitive, adenosine 3':5'-cyclic monophosphate (cyclic AMP)-specific PDE (PDE IV). 2. Pretreatment of anaesthetized rabbits with zaprinast (300 micrograms kg-1 plus 30 micrograms kg-1 min-1) had no significant effect on ischaemia- or reperfusion-induced ST-segment changes, or arrhythmias. In contrast, rolipram (30 micrograms kg-1 plus 3 micrograms kg-1 min-1) and (100 micrograms kg-1 plus 10 micrograms kg-1 min-1) increased the severity of arrhythmias. With the higher dose of rolipram, ST-segment changes were increased in magnitude and mortality due to ventricular fibrillation during ischaemia or reperfusion was increased to 80% compared with 30% in controls (n = 10 per group). 3. Zaprinast caused small but significant increases in heart rate and arterial blood pressure whereas rolipram decreased diastolic arterial pressure, increased left ventricular (LV) dP/dtmax and substantially increased heart rate. 4. At the end of each experiment platelet aggregation was measured ex vivo. Pretreatment of rabbits with either dose of rolipram had no significant effect on platelet aggregation induced by adenosine diphosphate (ADP), collagen, arachidonic acid or thrombin or on isoprenaline- or prostacyclin-induced inhibition of aggregation. Aggregatory responses to ADP and collagen were increased in platelets obtained from rabbits which had received zaprinast. 5. These results indicate that in the dose used here, the PDE V inhibitor zaprinast had no significant effect on arrhythmias. The effects of the PDE IV inhibitor rolipram on haemodynamics, combined with its lack of antiplatelet activity, may have contributed to the exacerbation of arrhythmias observed during myocardial ischaemia and reperfusion.

Vigabatrin. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic potential in epilepsy and disorders of motor control

Vigabatrin was specifically designed to enhance gamma-aminobutyric acid (GABA) function in the CNS. By increasing brain concentrations of this inhibitory neurotransmitter the drug appears to decrease propagation of abnormal hypersynchronous discharges, thereby reducing seizure activity. At this stage in its development, clinical experience with vigabatrin is limited primarily to patients with refractory seizure disorders. In this difficult-to-treat population, 'add-on' therapy with vigabatrin greater than or equal to 2 g/day has shown impressive efficacy, reducing seizure frequency by greater than or equal to 50% in approximately half of patients. Clinical efficacy does seem to vary with seizure type with the best response reported in adults with complex partial seizures with or without generalisation and in children with cryptogenic partial epilepsy or symptomatic infantile spasm. Vigabatrin appears to have a negative effect on absences and myoclonic seizures. Some disorders of motor control may also be amenable to enhanced GABAergic function. In the small number of patients with tardive dyskinesia treated to date, vigabatrin produced mild to moderate improvement in hyperkinetic symptom scores but Parkinsonism or schizophrenic symptoms occasionally worsened. The best response was reported in a study of patients who had been withdrawn from neuroleptic therapy. In a small but well-controlled comparative trial, vigabatrin was as effective as baclofen in reducing spasm and improving some parameters of spasticity in patients with spinal cord lesions or multiple sclerosis. Most adverse reactions to vigabatrin are mild and transient with central nervous system (CNS) changes being reported most frequently. Of particular note, serial evoked potential studies and the few available histology reports have not found evidence of intramyelinic oedema during therapeutic use, as was reported in rats and dogs on chronic high-dose treatment. Thus, vigabatrin is a promising new anticonvulsant drug. Current evidence supports a trial of this agent as adjunctive therapy in patients with refractory seizure disorders, and future investigation of vigabatrin monotherapy and its efficacy relative to established agents is awaited with interest. Wider experience should help to clarify which patients - by seizure type and concurrent CNS pathology - are likely to benefit from vigabatrin and ongoing monitoring should further clarify the potential detrimental effects, if any, of long term use. In the meantime, it is a welcome addition in the difficult setting of resistant epilepsy.