Valproate enhances GABA-A mediated inhibition of locus coeruleus neurones in vitro

Unconditioned and learned morphine tolerance influence hippocampal-dependent short-term memory and the subjacent expression of GABA-A receptor alpha subunits

BACKGROUND

ɣ-aminobutyric acid (GABA) facilitator valproic acid may be able to curb memory disruption induced by morphine exposure.

OBJECTIVE

The effects of the GABA facilitator valproic acid on the behavioral tolerance induced by morphine were investigated. Then hippocampal-dependent tasks named spatial-working and short-term memory procedures using the Y-maze apparatus were examined in morphine tolerant rats. Finally, the changes in the expression of hippocampal GABA-A receptors underlying morphine tolerance were also examined.

METHODS

Rats were treated with daily morphine injections, with or without distinct contextual pairing. To examine the effect of valproic acid on morphine tolerance expression, valproic acid was pretreated an hour before morphine. Spatial-working and short-term memory procedures using the Y-maze apparatus were examined in morphine tolerant rats. Afterwards the changes in the expression of hippocampal GABAα receptors using the quantitative real-time PCR and western blot techniques to detect GABArα subunits mRNAs and protein level were studied.

RESULTS

Our results showed that both learned and non-associative morphine tolerance influence short-term memory and the subjacent expression of GABArα mRNAs and protein level. Despite its attenuating effects on the development and expression of both learned and non-associative morphine tolerance, only associative morphine tolerance-induced memory dysfunction was ameliorated by valproic acid pretreatment. We also found that the expression of GABArα1, α2, α5 subunits mRNAs and GABAα protein level were affected heavier in associative morphine tolerant rats.

CONCLUSION

Our data supports the hypothesis that unconditioned and learned morphine tolerance influences short-term memory and the expression of GABArα 1, α2, α5 mRNAs and GABArα protein level differently, and adds to our understanding of the behavioral and molecular aspects of the learned tolerance to morphine effects.

Activity of several GABAergic agents on the behavioral “despair” test in rats

The present study was conducted to investigate whether several GABAergic drugs could affect immobility in the behavioral “despair” test (swimming test). The subacute (3 injections) treatment with the GABA-B agonist baclofen 0.5 mg/kg (BC05) partially antagonised the antiimmobility action of imipramine (IMI), whereas muscimol 0.00125 mg/kg (MU ; GABA-A agonist) did not affect the imipramine effect on immobility. The highest doses of baclofen alone increased immobility time, and no effect of muscimol alone was observed on this measure. Other experiments indicated that different doses of δ-α-aminovaleric acid (a GABA-B antagonist and GABA-A agonist) or progabide (a GABA A/B agonist with clinical antidepressive properties) did not modify immobility time. On the other hand, sodium valproate (VPA), chronically administered, was effective in reducing the time of immobility of rats in the swimming test, at doses which did not alter motor activity in an open field test. Since there is evidence indicating that valproate can be considered as a GABA-mimetic agent, the present data is consistent with other results showing an antidepressant-like activity of agents which enhance GABAergic transmission.

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.

Cortical inhibitory dysfunction in bipolar disorder: a study using transcranial magnetic stimulation

OBJECTIVE

Neuroanatomic evidence suggests that patients with bipolar disorder (BD) have impaired cortical inhibition (CI). However, there is little in vivo neurophysiological evidence supporting the occurrence of such impairments in this disorder. Using 3 transcranial magnetic stimulation paradigms, known as short-interval CI (SICI), cortical silent period (SP), and interhemispheric inhibition (IHI), the authors measured inhibition in the motor cortex.

METHOD

Fifteen patients with BD and 15 healthy subjects were enrolled. Short-interval CI involves stimulating with a subthreshold pulse a few milliseconds before a suprathreshold pulse, thereby inhibiting the size of the motor-evoked potential (MEP) produced by the suprathreshold pulse. In the SP paradigm, inhibition is reflected by the SP duration (ie, the duration of electromyographic activity cessation following a transcranial magnetic stimulation-induced MEP). Interhemispheric inhibition involves a subthreshold conditioning stimulus applied to the right motor cortex several milliseconds before a suprathreshold test stimulus is applied to the left motor cortex which inhibits the size of the MEP produced by the test stimulus by 50% to 75%.

RESULTS

Patients with BD demonstrated deficits in all 3 paradigms: SICI (F1,28 = 5.55, P = 0.03; Cohen d = 0.86), SP (F1,28 = 5.24, P = 0.03; Cohen d = 0.84), and IHI (F1,28 = 3.41, P = 0.02; Cohen d = 0.77) compared with healthy volunteers with a large effect size.

CONCLUSIONS

Our study supports the hypothesis that CI is decreased in BD. Further understanding of the neurophysiology of such deficiencies may help to elucidate future treatment options.

GABA and glutamate systems as therapeutic targets in depression and mood disorders

Advances made in diverse areas of neuroscience suggest that neurotransmitter systems, additional to the monoaminergic, contribute to the pathophysiology of mood disorders. This ever accruing body of preclinical and clinical research is providing increased recognition of the contribution made by amino acid neurotransmitters to the neurobiology of mood disorders. This review examines evidence supporting the role of GABA and glutamate in these processes and explores the potential to target these systems in the development of novel compounds; the viability of these agents for treatment-related co-morbidities will also be considered.

Basic pharmacology of valproate: a review after 35 years of clinical use for the treatment of epilepsy

Since its first marketing as an antiepileptic drug (AED) 35 years ago in France, valproate has become established worldwide as one of the most widely used AEDs in the treatment of both generalised and partial seizures in adults and children. The broad spectrum of antiepileptic efficacy of valproate is reflected in preclinical in vivo and in vitro models, including a variety of animal models of seizures or epilepsy. There is no single mechanism of action of valproate that can completely account for the numerous effects of the drug on neuronal tissue and its broad clinical activity in epilepsy and other brain diseases. In view of the diverse molecular and cellular events that underlie different seizure types, the combination of several neurochemical and neurophysiological mechanisms in a single drug molecule might explain the broad antiepileptic efficacy of valproate. Furthermore, by acting on diverse regional targets thought to be involved in the generation and propagation of seizures, valproate may antagonise epileptic activity at several steps of its organisation. There is now ample experimental evidence that valproate increases turnover of gamma-aminobutyric acid (GABA) and thereby potentiates GABAergic functions in some specific brain regions thought to be involved in the control of seizure generation and propagation. Furthermore, the effect of valproate on neuronal excitation mediated by the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors might be important for its anticonvulsant effects. Acting to alter the balance of inhibition and excitation through multiple mechanisms is clearly an advantage for valproate and probably contributes to its broad spectrum of clinical effects. Although the GABAergic potentiation and glutamate/NMDA inhibition could be a likely explanation for the anticonvulsant action on focal and generalised convulsive seizures, they do not explain the effect of valproate on nonconvulsive seizures, such as absences. In this respect, the reduction of gamma-hydroxybutyrate (GHB) release reported for valproate could be of interest, because GHB has been suggested to play a critical role in the modulation of absence seizures. Although it is often proposed that blockade of voltage-dependent sodium currents is an important mechanism of antiepileptic action of valproate, the exact role played by this mechanism of action at therapeutically relevant concentrations in the mammalian brain is not clearly elucidated. By the experimental observations summarised in this review, most clinical effects of valproate can be explained, although much remains to be learned at a number of different levels about the mechanisms of action of valproate. In view of the advances in molecular neurobiology and neuroscience, future studies will undoubtedly further our understanding of the mechanisms of action of valproate.

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.

Valproate effect on gamma-aminobutyric acid release in pars reticulata of substantia nigra: combination of push-pull perfusion and fluorescence histochemistry

To study further the previously demonstrated suppressive in vivo effect of sodium valproate (VPA) on gamma-aminobutyric acid (GABA) release in the preoptic area, we examined GABA neurotransmission in substantia nigra (SN), using the push-pull cannula technique in freely moving ovariectomized rats. To clarify whether the area in the substantia nigra that is actually perfused is pars reticulata, known to receive rich GABAergic input from striatum, we used the retrograde fluorescence tracer fast blue (FB) after each perfusion experiment, applying the tracer through the push-pull cannula. Nigral perfusion with VPA caused significant suppression of local GABA release. This effect was more marked in a subgroup of animals showing retrograde labeled cell bodies in striatum, i.e., animals with a tracer application site and therefore also a perfusion site precisely in pars reticulata. Our data suggest that VPA inhibits GABA release in rat SN as it does in the preoptic area, which may be in agreement with our hypothesis of enhanced GABAergic neurotransmission by VPA, causing suppression of presynaptic GABA release through negative feedback actions on the GABA autoreceptor complex. Furthermore, the combination of push-pull cannula technique and retrograde fluorescence tracer application appears to be an important tool to prove afferent connections of the area actually perfused in neuronal networks.

Effects of the peptide BCH-325 upon the efficacy of common antiepileptic drugs

It was shown that BCH-325 (Pro-D-Phe-Pro-Gly), a des-tyrosine derivative of beta-casomorphin, exhibits anticonvulsive effects. In the present study, we combined the peptide with clinically used antiepileptic drugs (phenytoine, carbamazepine, diazepam, phenobarbital, and dipropyl acetate) to investigate possible interactions and, moreover, to draw conclusions concerning the mode of action of BCH-325. There were no interactions with phenytoine and carbamazepine, which might be interpreted to mean that BCH-325 exerted no action on sodium channels. The efficacy of the combinations with the other drugs (diazepam, phenobarbital, dipropyl acetate) was increased. The data suggest that the anticonvulsant effect of BCH-325 might occur by interference with elements of GABA/benzodiazepinergic neurotransmission.

Effects of valproate on early and late potassium currents of single neurons

The effects of valproate sodium (VPA) on potassium currents were tested in identified neurons of the snail Helix pomatia. VPA was extracellularly and intracellularly applied. VPA (i) had no effects on the current-voltage relation of the early potassium outward current (IA), (ii) shifted the steady state inactivation function of IA to more positive potentials, (iii) increased the amplitude of the late potassium outward currents. It is suggested that the extrasynaptic effects on potassium currents markedly contribute to the antiepileptic and antimanic effects of VPA.

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 effects on pre-optic GABA release and pituitary LH secretion in the rat

The in vivo effects of the anticonvulsant drug sodium valproate (VPA) on pre-optic gamma-aminobutyric acid (GABA) release and pituitary luteinizing hormone (LH) secretion were studied by perfusing the pre-optic area of unanaesthetized, freely moving ovarectomized rats through push-pull cannulae at a flow-rate of 20 >μl/min with a fraction period of 15 min. Local treatment with 40, 100, 400 and 1600 μg VPA/ml differently affected pre-optic GABA release and pituitary LH secretion concerning mean level, mean pulse amplitude and mean pulse frequency. GABA levels in perfusate were suppressed by local treatment with 40 and 100 μg VPA/ml CSF, respectively, whereas no significant change could be observed at the highest concentration used (1600 μg VPA/ml CSF). Pituitary LH secretion was reduced by pre-optic perfusion with 100 μg VPA/ml CSF with regard to mean plasma level and pulse amplitude but no significant change in pulse frequency could be observed. By raising VPA concentration the effects became more pronounced, and at 1600 μ g VPA/ml CSF there was a marked reduction of LH secretion regarding mean plasma level, amplitude and frequency. In conclusion, the present data put forward our view that the mechanism of action of VPA generates an enhancement of GABAergic transmission different from that involving elevated extracellular GABA.

Neuronal activity, amino acid concentration and amino acid release in the substantia nigra of the rat after sodium valproate

The effects of sodium valproate on extracellularly recorded spontaneous neuronal activity and striatal-evoked inhibition in the substantia nigra zona reticulata of the rat were compared with its effects on the tissue concentration of endogenous amino acids and their spontaneous release into perfusates of this region obtained with a push-pull cannula. Valproate (200 mg/kg i.p.) produced a rapid and sustained reduction in the firing rate of all reticulata neurones tested and a concomitant increase in the duration of striatal-evoked inhibition. No change in the spontaneous release of any amino acid was observed. A significant elevation of nigral gamma-aminobutyric acid concentration was seen in both anaesthetized and non-anaesthetized animals, but this occurred only after 60 minutes. Valproate produced a rapid decline in nigral aspartate in non-anaesthetized but not in anaesthetized animals. The results of this study suggest that the acute depressant effect of valproate is unrelated to its ability to alter the concentration of GABA or aspartate in brain and is most likely due to a postsynaptic action.