Effects of GAD and GABA-T inhibitors on GABA metabolism in vivo

Effects of sodium valproate on synaptic transmission and neuronal excitability in rat hippocampus

1. Valproate (VPA) has long been used in the treatment of both generalized and partial seizures. However, its cellular mechanisms of action remain unclear. 2. In the present study, the effects of VPA on synaptic transmission and neuronal excitability were examined in the hippocampal CA1 region using whole-cell patch clamp recordings. 3. Perfusion with VPA, at therapeutically attainable concentrations (i.e. 0.3 and 0.6 mmol/L), significantly increased the frequency (112 +/- 2 and 133 +/- 2% of control, respectively; n = 5; both P < 0.05), but not the average amplitude, of miniature inhibitory post-synaptic currents (mIPSCs). Perfusion with VPA had no effect on either the amplitude or the frequency of miniature excitatory post-synaptic currents (mEPSCs). 4. In acutely dissociated CA1 pyramidal neurons, VPA had no effect on 10 micromol/L GABA-induced currents. Furthermore, following the administration of 0.3 and 0.6 mmol/L VPA, the frequency of action potential firing was significantly reduced from 18.0 +/- 1.1 to 15.3 +/- 0.9 and from 18.6 +/- 0.9 to 12.6 +/- 0.6, respectively (n = 8; both P < 0.05). In contrast, 0.3 and 0.6 mmol/L VPA significantly increased spike frequency adaptation from 4.02 +/- 0.47 to 4.72 +/- 0.55 and from 3.47 +/- 0.41 to 4.48 +/- 0.58, respectively (n = 8; P < 0.05). 5. The results of the present study suggest that VPA presynaptically increases inhibitory synaptic activity without modifying excitatory synaptic transmission and reduces neuronal excitability. Any or all of these effects may contribute to its anticonvulsant action.

Continuum solvent model studies of the interactions of an anticonvulsant drug with a lipid bilayer

Valproic acid (VPA) is a short, branched fatty acid with broad-spectrum anticonvulsant activity. It has been suggested that VPA acts directly on the plasma membrane. We calculated the free energy of interaction of VPA with a model lipid bilayer using simulated annealing and the continuum solvent model. Our calculations indicate that VPA is likely to partition into the bilayer both in its neutral and charged forms, as expected from such an amphipathic molecule. The calculations also show that VPA may migrate (flip-flop) across the membrane; according to our (theoretical) study, the most likely flip-flop path at neutral pH involves protonation of VPA pending its insertion into the lipid bilayer and deprotonation upon departure from the other side of the bilayer. Recently, the flip-flop of long fatty acids across lipid bilayers was studied using fluorescence and NMR spectroscopies. However, the measured value of the flip-flop rate appears to depend on the method used in these studies. Our calculated value of the flip-flop rate constant, 20/s, agrees with some of these studies. The limitations of the model and the implications of the study for VPA and other fatty acids are discussed.

Valproate: a reappraisal of its pharmacodynamic properties and mechanisms of action

Valproate is currently one of the major antiepileptic drugs with efficacy for the treatment of both generalized and partial seizures in adults and children. Furthermore, the drug is increasingly used for therapy of bipolar and schizoaffective disorders, neuropathic pain and for prophylactic treatment of migraine. These various therapeutic effects are reflected in preclinical models, including a variety of animal models of seizures or epilepsy. The incidence of toxicity associated with the clinical use of valproate is low, but two rare toxic effects, idiosyncratic fatal hepatotoxicity and teratogenicity, necessitate precautions in risk patient populations. Studies from animal models on structure-relationships indicate that the mechanisms leading to hepatotoxicity and teratogenicity are distinct and also differ from the mechanisms of anticonvulsant action of valproate. Because of its wide spectrum of anticonvulsant activity against different seizure types, it has repeatedly been suggested that valproate acts through a combination of several mechanisms. As shown in this review, there is substantial evidence that valproate increases GABA synthesis and release and thereby potentiates GABAergic functions in some specific brain regions, such as substantia nigra, thought to be involved in the control of seizure generation and propagation. Furthermore, valproate seems to reduce the release of the epileptogenic amino acid gamma-hydroxybutyric acid and to attenuate neuronal excitation induced by NMDA-type glutamate receptors. In addition to effects on amino acidergic neurotransmission, valproate exerts direct effects on excitable membranes, although the importance of this action is equivocal. Microdialysis data suggest that valproate alters dopaminergic and serotonergic functions. Valproate is metabolized to several pharmacologically active metabolites, but because of the low plasma and brain concentrations of these compounds it is not likely that they contribute significantly to the anticonvulsant and toxic effects of treatment with the parent drug. By the experimental observations summarized in this review, most clinical effects of valproate can be explained, although much remains to be learned at a number of different levels of valproate's mechanisms of action.

Effect of valproate on the metabolism of the central nervous system

The effects of valproate on brain energy and lipid metabolism is reviewed. Increasing evidence suggests that valproate uses the monocarboxylic acid carrier in order to cross the blood brain barrier (BBB) and the neural cell plasma membranes. The uptake of valproate into the brain through this mechanism would compete with the uptake of energy precursors, such as the monocarboxylic acids 3-hydroxybutyrate, lactate or pyruvate and with some amino acids, but not with glucose. This could impair brain fuel utilization, specially during the neonatal period or childhood, when lactate or 3-hydroxybutyrate furnishes alternative substrates to glucose for the brain. It is concluded that valproate interference with energy metabolism may have implications for the therapeutic action of the drug, stressing the possibility that valproate-mediated alterations in brain lipid synthesis may contribute to the pharmacological action of the drug.

Influence of short-lasting bilateral clamping of carotid arteries (BCCA) on GABA turnover in rat brain structures

We have previously shown that short-lasting reduction of cerebral blood flow by bilateral clamping of carotid arteries (BCCA) results in long-lasting increase in regional GABA concentration and decrease in seizure susceptibility in rats. In the present experiments, the effect of BCCA on GABA turnover and the enzymes involved in GABA synthesis and degradation were studied in rats. Regional GABA turnover was measured by means of GABA accumulation induced by the GABA-transaminase (GABA-T) inhibitor aminooxyacetic acid (AOAA). Fourteen days after BCCA, GABA turnover was significantly increased in hippocampus, substantia nigra and cortex, but not different from sham-operated controls in several other brain regions, including striatum, hypothalamus and cerebellum. The activity of glutamate decarboxylase (GAD) measured ex vivo did not show any changes in investigated structures, while the activity of GABA-T was slightly increased in hippocampus. The increased GABA turnover in some brain regions may explain our previous findings of increased GABA content in these brain regions and decreased sensitivity of BCCA treated animals to the GABAA-receptor antagonist bicuculline.

Effects of the antiepileptic drug valproate on metabolism and function of inhibitory and excitatory amino acids in the brain

Valproate is currently one of the major antiepileptic drugs in clinical use. Because of its wide spectrum of anticonvulsant activity against different seizure types, it has repeatedly been suggested that valproate acts through a combination of several mechanisms. As shown in this review, there is substantial evidence that valproate increases GABA turnover and thereby potentiates GABAergic functions in some specific brain regions, such as substantia nigra, thought to be involved in the control of seizure generation and propagation. Furthermore, valproate seems to reduce the release of the epileptogenic amino acid gamma-hydroxybutyric acid and to block cell firing induced by NMDA-type glutamate receptors. In addition to effects on amino acidergic neurotransmission, valproate presumably exerts a direct action on ion channels, thereby limiting sustained repetitive neuronal firing. Recent microdialysis data suggest that valproate also alters dopaminergic and serotonergic functions. These diverse effects of valproate might explain why the drug not only exerts anticonvulsant activity but also other pharmacodynamic and pharmacotherapeutic actions, such as antipsychotic and antidystonic efficacy.

Valproate enhances GABA turnover in the substantia nigra

The effects of the antiepileptic drug, valproate (VPA), on regional turnover of gamma-aminobutyric acid (GABA) in the rat brain were studied by determining the rate of GABA accumulation following complete inhibition of GABA degradation by aminooxyacetic acid (AOAA). VPA was administered at a dose of 200 mg/kg 5, 15 and 30 min prior to injection of AOAA, 100 mg/kg. In most of the 12 regions examined, VPA did not alter the AOAA-induced GABA accumulation. However, significant increases in GABA accumulation were found in corpus striatum and, more marked, in substantia nigra. Since the substantia nigra has been identified as a substrate for the anticonvulsant action of GABAergic drugs, the data may indicate that the effect of VPA on GABA synthesis rate in this region may be involved in its mechanism of anticonvulsant action.

In vivo effects of anticonvulsant drugs on nerve terminal (synaptosomal) GABA levels in 11 brain regions of the rat

The in vivo effects of two GABA-elevating drugs with anticonvulsant properties, namely valproic acid (VPA) and aminooxyacetic acid (AOAA), on nerve terminal GABA levels in discrete rat brain regions were studied by means of a newly developed synaptosomal model. The profile of synaptosomal GABA increases obtained with AOAA was quite different from that seen with VPA. Thus, AOAA (30 mg/kg i.p., 2 hours) caused significant increases in olfactory bulb, cortex, hippocampus, thalamus and cerebellum, whereas VPA (200 mg/kg i.p., 0.5 hour) significantly increased GABA also in hypothalamus, substantia nigra and superior and inferior colliculus. In contrast to the regional selectivity of both drugs with respect to synaptosomal GABA levels, AOAA in most regions was more potent than VPA in increasing whole tissue GABA levels determined prior to subcellular fractionation. The data thus demonstrate that comparison of GABA levels in synaptosomal fractions rather than homogenates from discrete brain areas provides a more sensitive index of the action of GABA-elevating drugs administered in vivo.

In vivo effects of aminooxyacetic acid and valproic acid on nerve terminal (synaptosomal) GABA levels in discrete brain areas of the rat. Correlation to pharmacological activities

A newly developed synaptosomal model was used to evaluate the in vivo effects of the GABA-elevating drugs aminooxyacetic acid (AOAA, 30 mg/kg i.p.) and valproic acid (VPA, 200 mg/kg i.p.) on GABA levels in nerve endings of 11 brain regions in rats as a function of time after administration. The data obtained were compared with the magnitude and time course of the effects of both drugs in rats on body temperature, pain response and against seizures induced by electroshock, pentylenetetrazol and 3-mercaptopropionic acid. Following AOAA, maximum increases in synaptosomal GABA levels of brain regions were observed 6 hr after administration. At this time, GABA was significantly elevated up to 300% over control values in synaptosomal fractions from all 11 regions. However, the hypothermic and antinociceptive effects of the drug as well as its anticonvulsant action against electroshock and pentylenetetrazol induced seizures were maximal 1 hr after injection and had vanished after 6 hr, i.e. at the time of maximum GABA increases in synaptosomes. The only pharmacological effect of AOAA which paralleled the time course of the synaptosomal GABA elevation was the attenuation of seizures induced by 3-mercaptopropionic acid. Following VPA, the effect on synaptosomal GABA levels was much more rapid in onset and significant increases were already determined 5 to 30 min after administration. Significant increases of up to 80% over control values were found in synaptosomal fractions from olfactory bulb, frontal cortex, hippocampus, hypothalamus, tectum, substantia nigra and cerebellum. In contrast to AOAA, the time course of the synaptosomal GABA increases, at least in some regions, was similar to the time course of VPA's antinociceptice effects and its anticonvulsant effects in the three seizure models studied. The data may suggest that AOAA and VPA increase different pools of GABA within nerve terminals, only one of which is involved in GABA-mediated neurotransmission.

Acute effects of sodium valproate and gamma-vinyl GABA on regional amino acid metabolism in the rat brain: incorporation of 2-[14C]glucose into amino acids

Amino acid concentrations have been determined in rat brain regions (cortex, striatum, cerebellum, and hippocampus) by HPLC after administration of acute anticonvulsant doses of sodium valproate (400 mg/kg, i.p.) and gamma-vinyl-GABA (1 g/kg, i.p.). After valproate administration the GABA level increases only in the cortex; aspartic acid concentration decreases in the cortex and hippocampus, and glutamic acid decreases in the hippocampus and striatum and increases in the cortex and cerebellum. There are no changes in the concentrations of glutamine, taurine, glycine, serine, and alanine following valproate administration. Only the GABA level increases in all the regions after gamma-vinyl-GABA administration. Cortical analyses 2, 4 and 10 minutes after pulse labeling with 2-[14C]glucose, i.v., show no change in the rate of cortical glucose utilization in the valproate treated group. The rate of labeling of glutamic acid is also unchanged, but the rate of labeling of GABA is reduced following valproate administration. After gamma-vinyl-GABA administration there is no change in the rate of labeling of GABA. These biochemical findings can be interpreted in terms of a primary anticonvulsant action of valproate on membrane receptors with secondary effects on the metabolism of amino acid neurotransmitters. This contrasts with the primary action of gamma-vinyl-GABA on GABA-transaminase activity.