Gamma-vinyl GABA

Proteomics Reveals the Potential Protective Mechanism of Hydrogen Sulfide on Retinal Ganglion Cells in an Ischemia/Reperfusion Injury Animal Model

Glaucoma is the leading cause of irreversible blindness and is characterized by progressive retinal ganglion cell (RGC) degeneration. Hydrogen sulfide (H₂S) is a potent neurotransmitter and has been proven to protect RGCs against glaucomatous injury in vitro and in vivo. This study is to provide an overall insight of H₂S’s role in glaucoma pathophysiology. Ischemia-reperfusion injury (I/R) was induced in Sprague-Dawley rats (n = 12) by elevating intraocular pressure to 55 mmHg for 60 min. Six of the animals received intravitreal injection of H₂S precursor prior to the procedure and the retina was harvested 24 h later. Contralateral eyes were assigned as control. RGCs were quantified and compared within the groups. Retinal proteins were analyzed via label-free mass spectrometry based quantitative proteomics approach. The pathways of the differentially expressed proteins were identified by ingenuity pathway analysis (IPA). H₂S significantly improved RGC survival against I/R in vivo (p < 0.001). In total 1115 proteins were identified, 18 key proteins were significantly differentially expressed due to I/R and restored by H₂S. Another 11 proteins were differentially expressed following H₂S. IPA revealed a significant H₂S-mediated activation of pathways related to mitochondrial function, iron homeostasis and vasodilation. This study provides first evidence of the complex role that H₂S plays in protecting RGC against I/R.

Design and Mechanism of GABA Aminotransferase Inactivators. Treatments for Epilepsies and Addictions

When the brain concentration of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) diminishes below a threshold level, the excess neuronal excitation can lead to convulsions. This imbalance in neurotransmission can be corrected by inhibition of the enzyme γ-aminobutyric acid aminotransferase (GABA-AT), which catalyzes the conversion of GABA to the excitatory neurotransmitter l-glutamic acid. It also has been found that raising GABA levels can antagonize the rapid elevation and release of dopamine in the nucleus accumbens, which is responsible for the reward response in addiction. Therefore, the design of new inhibitors of GABA-AT, which increases brain GABA levels, is an important approach to new treatments for epilepsy and addiction. This review summarizes findings over the last 40 or so years of mechanism-based inactivators (unreactive compounds that require the target enzyme to catalyze their conversion to the inactivating species, which inactivate the enzyme prior to their release) of GABA-AT with emphasis on their catalytic mechanisms of inactivation, presented according to organic chemical mechanism, with minimal pharmacology, except where important for activity in epilepsy and addiction. Patents, abstracts, and conference proceedings are not covered in this review. The inactivation mechanisms described here can be applied to the inactivations of a wide variety of unrelated enzymes.

Hypoxia and GABA shunt activation in the pathogenesis of Alzheimer's disease

We have previously observed that the conversion of mild cognitive impairment to definitive Alzheimer's disease (AD) is associated with a significant increase in the serum level of 2,4-dihydroxybutyrate (2,4-DHBA). The metabolic generation of 2,4-DHBA is linked to the activation of the γ-aminobutyric acid (GABA) shunt, an alternative energy production pathway activated during cellular stress, when the function of Krebs cycle is compromised. The GABA shunt can be triggered by local hypoperfusion and subsequent hypoxia in AD brains caused by cerebral amyloid angiopathy. Succinic semialdehyde dehydrogenase (SSADH) is a key enzyme in the GABA shunt, converting succinic semialdehyde (SSA) into succinate, a Krebs cycle intermediate. A deficiency of SSADH activity stimulates the conversion of SSA into γ-hydroxybutyrate (GHB), an alternative route from the GABA shunt. GHB can exert not only acute neuroprotective activities but unfortunately also chronic detrimental effects which may lead to cognitive impairment. Subsequently, GHB can be metabolized to 2,4-DHBA and secreted from the brain. Thus, the activation of the GABA shunt and the generation of GHB and 2,4-DHBA can have an important role in the early phase of AD pathogenesis.

Mechanism of inactivation of γ-aminobutyric acid aminotransferase by (1S,3S)-3-amino-4-difluoromethylene-1-cyclopentanoic acid (CPP-115)

γ-Aminobutyric acid aminotransferase (GABA-AT) is a pyridoxal 5'-phosphate (PLP)-dependent enzyme that degrades GABA, the principal inhibitory neurotransmitter in mammalian cells. When the concentration of GABA falls below a threshold level, convulsions can occur. Inhibition of GABA-AT raises GABA levels in the brain, which can terminate seizures as well as have potential therapeutic applications in treating other neurological disorders, including drug addiction. Among the analogues that we previously developed, (1S,3S)-3-amino-4-difluoromethylene-1-cyclopentanoic acid (CPP-115) showed 187 times greater potency than that of vigabatrin, a known inactivator of GABA-AT and approved drug (Sabril) for the treatment of infantile spasms and refractory adult epilepsy. Recently, CPP-115 was shown to have no adverse effects in a Phase I clinical trial. Here we report a novel inactivation mechanism for CPP-115, a mechanism-based inactivator that undergoes GABA-AT-catalyzed hydrolysis of the difluoromethylene group to a carboxylic acid with concomitant loss of two fluoride ions and coenzyme conversion to pyridoxamine 5'-phosphate (PMP). The partition ratio for CPP-115 with GABA-AT is about 2000, releasing cyclopentanone-2,4-dicarboxylate (22) and two other precursors of this compound (20 and 21). Time-dependent inactivation occurs by a conformational change induced by the formation of the aldimine of 4-aminocyclopentane-1,3-dicarboxylic acid and PMP (20), which disrupts an electrostatic interaction between Glu270 and Arg445 to form an electrostatic interaction between Arg445 and the newly formed carboxylate produced by hydrolysis of the difluoromethylene group in CPP-115, resulting in a noncovalent, tightly bound complex. This represents a novel mechanism for inactivation of GABA-AT and a new approach for the design of mechanism-based inactivators in general.

The effect of antiepileptic drugs on visual performance

Visual disturbances are a common side-effect of many antiepileptic drugs. Non-specific retino- and neurotoxic visual abnormalities, that are often reported with over-dosage and prolonged AED use, include diplopia, blurred vision and nystagmus. Some anticonvulsants are associated with specific visual problems that may be related to the mechanistic properties of the drug, and occur even when the drugs are administered within the recommended daily dose. Vigabatrin, a GABA-transaminase inhibitor, has been associated with bilateral concentric visual field loss, electrophysiological changes, central visual function deficits including reduced contrast sensitivity and abnormal colour perception, and morphological alterations of the fundus and retina. Topiramate, a drug that enhances GABAergic transmission, has been associated with cases of acute closed angle glaucoma, while tiagabine, a GABA uptake inhibitor, has been investigated for a potential GABAergic effect on the visual field. Only mild neurotoxic effects have been identified for patients treated with gabapentin, a drug designed as a cyclic analogue of GABA but exhibiting an unknown mechanism while carbamazepine, an inhibitor of voltage-dependent sodium channels, has been linked with abnormal colour perception and reduced contrast sensitivity. The following review outlines the visual disturbances associated with some of the most commonly prescribed anticonvulsants. For each drug, the ocular site of potential damage and the likely mechanism responsible for the adverse visual effects is described.

Neurotoxic effects of GABA-transaminase inhibitors in the treatment of epilepsy: ocular perfusion and visual performance

Vigabatrin is a GABA (gamma-aminobutyric acid) transaminase inhibitor that elicits an antiepileptic effect by enhancing inhibitory neurotransmission in the brain. Vigabatrin has been previously associated with concentric peripheral visual field loss and visual electrophysiological abnormalities. Recently, visual function deficits of the central retina have been identified in a proportion of patients receiving vigabatrin; these include disturbances in colour perception, contrast sensitivity and short-wavelength automated perimetry. Consequently, it is suggested that vigabatrin-associated retinal toxicity is diffuse inducing subtle central visual dysfunction and more severe peripheral visual defects. Reductions in cerebral blood flow and cerebral metabolic rate for glucose occur in epilepsy patients receiving antiepileptic drug therapy. Despite the known cerebral haemodynamic alterations in epilepsy and the visual consequences of vigabatrin therapy, ocular blood flow has only recently been investigated in this group. We present findings from a series of novel investigations that identify compromised retinal microvascular perfusion and pulsatile ocular blood flow (POBF) in epilepsy patients. The reduction in POBF was exacerbated in epilepsy patients treated with vigabatrin compared to conventionally treated epilepsy patients. A number of theories are presented to explain compromised ocular blood flow in vigabatrin treated epilepsy patients, and the possibility of a GABAergic mechanism of toxicity is discussed.

Vigabatrin directed against kindled seizures following cortical insult: impact on epileptogenesis and somatosensory recovery

The anticonvulsant drug vigabatrin has not been found to be detrimental to the recovery process when administered following focal cortical insult. This finding is in contrast to the negative postinjury consequences of other anticonvulsant drugs (e.g., phenobarbital and diazepam) with more direct activation of the GABA/benzodiazepine receptor complex. Moreover, phenobarbital directed against kindled seizures affects functional recovery more adversely than either the drug or subconvulsive seizures alone. The purpose of the present study was to determine whether vigabatrin (150, 200, and 250 mg/kg) directed against kindled seizures would impact recovery from lesion-induced somatosensory deficits. Vigabatrin was coupled with daily electrical kindling of the amygdala during the first week after a unilateral anteromedial cortex (AMC) lesion. Somatosensory recovery was assessed using bilateral tactile stimulation tests. Animals receiving the highest dose of vigabatrin prior to electrical kindling (250 mg/kg vigabatrin/kindled) remained significantly impaired even after two months of testing relative to vehicle/kindled, kindled/250 mg/kg vigabatrin, which received vigabatrin after electrical kindling, and the 150, 200, and 250 mg/kg vigabatrin/nonkindled groups (p < 0.0001). In contrast, neither vigabatrin (at any of the doses tested) nor subconvulsive kindled seizures impacted the recovery process (p > 0.05) when administered alone (i.e., without the drug + seizure interaction). These data add to the accumulating experimental and clinical evidence suggesting that the neurobehavioral consequences of the interaction between anticonvulsant drugs and subclinical seizures after brain insult are detrimental to functional recovery.

An update on GABAA receptors

Recent advances in molecular biology and complementary information derived from neuropharmacology, biochemistry and behavior have dramatically increased our understanding of various aspects of GABAA receptors. These studies have revealed that the GABAA receptor is derived from various subunits such as alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, pi, and rho1-3. Furthermore, two additional subunits (beta4, gamma4) of GABAA receptors in chick brain, and five isoforms of the rho-subunit in the retina of white perch (Roccus americana) have been identified. Various techniques such as mutation, gene knockout and inhibition of GABAA receptor subunits by antisense oligodeoxynucleotides have been used to establish the physiological/pharmacological significance of the GABAA receptor subunits and their native receptor assemblies in vivo. Radioligand binding to the immunoprecipitated receptors, co-localization studies using immunoaffinity chromatography and immunocytochemistry techniques have been utilized to establish the composition and pharmacology of native GABAA receptor assemblies. Partial agonists of GABAA receptors are being developed as anxiolytics which have fewer and less severe side effects as compared to conventional benzodiazepines because of their lower efficacy and better selectivity for the GABAA receptor subtypes. The subunit requirement of various drugs such as anxiolytics, anticonvulsants, general anesthetics, barbiturates, ethanol and neurosteroids, which are known to elicit at least some of their pharmacological effects via the GABAA receptors, have been investigated during the last few years so as to understand their exact mechanism of action. Furthermore, the molecular determinants of clinically important drug-targets have been investigated. These aspects of GABAA receptors have been discussed in detail in this review article.

Regulation of neurotransmitter release by endogenous nitric oxide in striatal slices

This study sought to determine the potential role of nitric oxide (NO) in N-methyl-D-aspartate (NMDA)-stimulated efflux of [14C] gamma-aminobutyric acid (GABA) and [3H]acetylcholine from striatal slices in vitro. In Mg2+-free buffer, NMDA-stimulated [14C]GABA and [3H]acetylcholine release were inhibited by the guanylate cyclase inhibitor, 1 H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), and, to a lesser extent, by the nitric oxide synthase inhibitor, nitroarginine (N-Arg). Since reversal of catecholamine transporters previously has been implicated in the mechanism underlying NO-induced catecholamine release, we used the GABA transport inhibitor, 1-(2-(((diphenylmethylene)imino)oxy)ethyl)-1,2,5,6-tetrahydro-3-py ridine-carboxylic acid hydrochloride (NNC-711), to address the role of GABA transport in NArg-sensitive NMDA-induced release. NNC-711 inhibited NMDA-stimulated [14C]GABA efflux by 50%, confirming our previous report that NMDA-stimulated GABA release is partially dependent on reversal of the transporter. The effect of N-Arg in the presence of NNC-711 was similar to its effect in the absence of the transport inhibitor, suggesting that reversal of the transporter is not involved in the NO component of NMDA-stimulated [14C]GABA release. These data suggest that glutamatergic transmission through striatal NMDA receptors is partially mediated through activation of the NO/guanylate cyclase pathway and that this mechanism may contribute to the tetrodotoxin sensitivity of NMDA-induced release of GABA and acetylcholine in the striatum.

Magnetic resonance imaging and histopathological study of brain lesions in rabbits given intravenous verotoxin 2

When rabbits were given intravenously purified verotoxin 2 (VT2) at 5 microg/kg of body weight, they developed hemorrhagic diarrhea, flaccid paresis, an ataxic gait, an opisthotonic posture, and convulsions. To examine the effects of VT2 toxemia on the rabbit central nervous system, magnetic resonance imaging and ultrastructural studies were performed. At 24, 57, and 80 h after injection of VT2 into 12 rabbits, T2-weighted images of the central nervous system were obtained. The initial lesion was noted at 24 h in the hypothalamic areas of all experimental animals. At 57 h, the T2 value increased in the medulla of the cerebral hemisphere or the hippocampus, with a brain stem lesion in six rabbits (50%). The rabbits with the brain stem lesions, in which neurological signs were very severe, died within 6 days. Lesions in the cerebellar hemisphere and/or vermis were noted in four rabbits (33%) that survived more than 1 month. To better understand the pathogenesis of VT2 in these brain lesions, we examined the deterioration of the blood-brain barrier and cerebrospinal fluid-brain barrier by using horseradish peroxidase as a tracer. The tracer was detected by electron microscopy both in the subendothelial layer, including the basal lamina, and throughout the cytoplasm of the ependymal cell layer covering the ventricle after intravenous or intrathecal treatment with horseradish peroxidase. We also determined the localization of VT2 by immunoelectron microscopy and found that it was localized on edematous endothelial cells of capillaries, ependymal cells, and myelin sheaths. The present study suggests that VT2 was conveyed from the endothelial and ependymal cell layers and caused edematous changes in the rabbit brain.

GABA metabolism controls inhibition efficacy in the mammalian CNS

The effects of changes in gamma-aminobutyric acid (GABA) metabolism or inhibitory processes was studied in the perforant path-dentate gyrus synapses in rat cortico-hippocampal slices, and in the monosynaptic-reflex circuit in isolated newborn, rat spinal cord. GABA metabolism was modulated by pharmacological block of either the anabolic enzyme glutamate decarboxylase (GAD) or the catabolic enzyme GABA transaminase (GABA-T). The results support the notion that GABA concentration determines the efficacy of inhibition in these regions of the central nervous system (CNS).

Glutamate receptor activation induces carrier mediated release of endogenous GABA from rat striatal slices

The regulation of striatonigral and striatopallidal GABAergic neurons by glutamatergic afferents is thought to play a critical role in normal basal ganglia function. Here we report that in striatal slices about 17% of K(+)-induced endogenous GABA release was Ca(2+)-independent and this could be blocked by a GABA transport inhibitor. Activation of N-methyl-D-aspartate (NMDA)- and quisqualate-sensitive receptors induced endogenous GABA efflux only in the presence of a GABA transaminase inhibitor; this efflux was inhibited by 60-80% with a GABA transport inhibitor. NMDA-induced GABA release was blocked by phencyclidine, Mg2+ and CGS 19755. Quisqualate-induced GABA release was blocked completely by a combination of the metabotropic antagonist, L-AP3 and CNQX, a non-NMDA receptor antagonist. These data indicate that excitatory amino acid agonists-induced GABA release is distinct from that induced by high K+ depolarization.

The significance of extracellular GABA in the substantia nigra of the rat during seizures and anticonvulsant treatments

The effects of the anti-epileptic drugs valproic acid and gamma-vinyl-GABA (vigabatrin) on the extracellular content of GABA was determined by microdialysis. Probes were implanted in the substantia nigra reticulata (SNR) of rats. It was found that gamma-vinyl-GABA (1000 mg/kg) induced a 4-6-fold increase in the extracellular content of GABA. This increase lasted for at least 72 h. PTZ-induced convulsions were partly antagonized by the GVG treatment. The increase of extracellular GABA after gamma-vinyl-GABA was not affected by infusion of tetrodotoxin. In contrast valproic acid (200 mg/kg), although effective in preventing pentylenetetrazol (PTZ)-induced convulsions, did not affect extracellular GABA in the SNR. PTZ-induced convulsions did not modify extracellular GABA, neither in control rats nor in valproic acid or gamma-vinyl-GABA pretreated animals. The results do not support the idea that extracellular GABA in the SNR plays a significant role in anti-convulsive treatment. However, the present data can also be interpreted that extracellular GABA, as sampled by microdialysis, is not a reliable marker for GABA release.

Inhibition of GABA transaminase enhances light-induced circadian phase delays but not advances

The CNS neurotransmitter GABA is distributed extensively throughout the suprachiasmatic nucleus, the site of circadian pacemaker cells in mammals. Pharmacological agents that act at GABAA receptors alter specific circadian responses to light and may induce phase shifts of circadian rhythms. In the present study, the role of endogenously released GABA in rhythm regulation was investigated using vigabatrin (gamma-vinyl GABA), an agent that has been shown to increase chronically or acutely the CNS levels of this neurotransmitter by inhibiting GABA transaminase. In Experiment 1, hamsters in constant darkness (DD) received a saline or a vigabatrin injection 1 hr before a 15-min, 700-lux light pulse. Vigabatrin increased photic phase delays but did not affect advances. In Experiment 2, vigabatrin delivered chronically via osmotic minipump treatment did not affect locomotor activity period in DD. However, after 14 days of infusion, photic phase delays (but not advances) were greatly increased in the vigabatrin group. In Experiment 3, in constant light (LL), chronic vigabatrin-treated animals showed an increased period that returned to pretreatment values after the 14-day drug infusion. The results are consistent with the phase-dependent effects of other agents that alter GABA neurotransmission. The data support the general hypothesis that GABA modulates the circadian responses to light in a phase-dependent manner, and may participate in entrainment to light-dark cycles by influencing the relative responsiveness to light in the early and late subjective night.

Effects of single and repeated administration of vigabatrin on the performance of non-epileptic rats in a delayed non-matching to position task

The present experiments were performed to investigate the effects of single and repeated administration of vigabatrin (gamma-vinyl-GABA), a novel antiepileptic drug, on a working memory task (delayed non-matching to position task) in non-epileptic rats. At doses of 100, 300 and 500 mg/kg single administrations of vigabatrin and 50, 100 and 200 mg/kg repeated administrations, vigabatrin did not affect the choice accuracy in the delayed non-matching to position task employing delays of 0, 1, 2, 4, 8 and 16 s, whereas repeated administration with 300 mg/kg and a single dose of 1000 mg/kg decreased the behavioral activity as compared to saline treatment. Previous studies have shown that at doses of 50-200 mg/kg (daily administration) and 200-1000 mg/kg (single administration) vigabatrin has anticonvulsant activity. The present results suggest that vigabatrin does not markedly impair working memory in the low range of antiepileptic/anticonvulsive doses.

Effects of single and repeated administration of vigabatrin on the performance of rats in a 5-choice serial reaction time task

The present study investigated whether pharmacological stimulation of the GABAergic system can affect attention. The effects of vigabatrin, a novel antiepileptic drug, on the performance of rats in a 5-choice serial reaction time task assessing selective attention were studied. The effects of acute (100, 300, 500 and 1000 mg/kg) and subchronic (50, 100, 200 and 300 mg/kg/day) administration of vigabatrin were investigated. Previous studies have shown that with acute administration of 300-500 mg/kg or subchronic administration of 50-200 mg/kg/day vigabatrin has anticonvulsant activity. At acute doses of 100 and 300 mg/kg or subchronic administration of 50-200 mg/kg/day vigabatrin had no effect on selective attention. At acute dosing of 500 mg/kg, vigabatrin slightly decreased behavioral activity of rats through decreasing the number of trials completed and percent of correct responses. The highest doses used caused an overall behavioral impairment with no marked depletion of any particular function. The results showed that administration of vigabatrin at antiepileptic doses produced no or slight impairment in attentional function. The deficits seen with higher doses were possibly due to a decrease in behavioral activity.

Vigabatrin has an anxiolytic effect in the elevated plus-maze test of anxiety

tau-Vinyl GABA (vigabatrin, GVG) is a novel antiepileptic drug that irreversibly inhibits GABA transaminase and elevates GABA levels in all parts of the brain. In the present study, we investigated the anxiolytic and behavioral effects of GVG in the elevated plus-maze and the hole board compared to diazepam. Doses of 500 and 1,000 mg/kg GVG were injected IP to different groups of male Wistar rats and animals were tested either 4 or 24 h after injection. Animals administered diazepam (1.5 mg/kg, IP) and saline (1 ml) were tested 20 min after injection. GVG and diazepam were found to decrease significantly the number of squares visited and rearing; both had a suppressant effect on locomotor activity. Neither drug had an effect on exploration (head dipping). GVG at a dose of 1,000 mg/kg was shown to have a similar anxiolytic activity either after 4 or 24 h as diazepam, while GVG at 500 mg/kg did not show any significant anxiolytic effect.

3-alkyl GABA and 3-alkylglutamic acid analogues: two new classes of anticonvulsant agents

Recently we showed that 3-alkyl-4-aminobutanoic acids are in vitro activators of brain L-glutamic acid decarboxylase (GAD) that show anticonvulsant activity. Since activation of GAD leads to increased concentrations of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in vitro, these compounds could represent a new class of anticonvulsant agents. Here it is shown that 3-alkylglutamic acid analogues also activate GAD and that all of the compounds in both series are active anticonvulsant agents against low intensity electroshock in mice. The most active compound, 3-isobutyl GABA, was tested further against maximal electroshock in mice and was shown to be very potent after both intravenous and oral administration without causing ataxia. It is not known if brain GABA levels are elevated in vivo by administration of these compounds or if the mechanism of anticonvulsant activity is related to their ability to activate GAD.

Elevated gamma-aminobutyric acid levels attenuate the metabolic response to bilateral ischemia

Bilateral ischemia has been shown to alter the net brain levels of energy metabolites such as ATP, phosphocreatine, glucose, and glycogen. The amino acid neurotransmitter gamma-aminobutyric acid (GABA) exerts a tonic inhibitory influence on neural activity. The present studies were designed to evaluate the influence of elevated GABA levels on the metabolic sequelae of ischemia. The GABA transaminase inhibitor gamma-vinyl-GABA (GVG; vigabatrin) was administered to Mongolian gerbils before the production of a bilateral ischemic incident. GABA levels were elevated in all regions assayed. Levels of energy metabolites were also increased, an indication of reduced energy utilization. In control animals, in the absence of GVG, 1 min of bilateral ischemia produced decreases in the levels of all metabolites. In animals pretreated with GVG, the effects of 1 min of bilateral ischemia were attenuated. These data suggest that the level of ongoing activity may affect the response to an ischemic insult. Furthermore, GVG may have a clinical indication in reducing the effect of minor ischemic incidents.

Effects of pharmacological manipulation of GABAergic neurotransmission in a new mutant hamster model of paroxysmal dystonia

Attacks of sustained dystonic postures of limbs and trunk can be initiated by handling or mild environmental stimuli (e.g. new cage) in an inbred line of Syrian hamsters. The severity of the dystonic syndrome in these mutant hamsters (gene symbol dtsz) is age-dependent, with a peak at about 30-40 days of age. A scoring system for grading type and severity of the dystonic attacks can be used to study the activity of drugs against dystonic movements with individual pre- and post-drug vehicle trials as control. In the present experiments, the effects of drugs which alter GABAergic functions in the brain were studied in dystonic hamsters. Anticonvulsants, i.e. valproate, diazepam and phenobarbital, which augment GABAergic neurotransmission, decreased the severity of dystonic attacks in the mutant hamsters, while administration of subconvulsive doses of pentylenetetrazol or the inverse benzodiazepine receptor agonist FG 7142 increased the severity of the syndrome. Anticonvulsants, i.e. phenytoin and carbamazepine, which are not thought to act via effects on GABAergic neurotransmission, exerted no antidystonic effects, but even worsened the attack in several animals. In contrast, the GABA-elevating drug, aminooxyacetic acid, produced a marked antidystonic effect in the hamsters. Similarly, the GABAB receptor agonist, baclofen, significant decreased the severity of the dystonic attack. The data indicate that dystonic movements in dtsz mutant hamsters can be attenuated by drugs which facilitate GABAergic functions, but worsened by drugs which impair GABAergic neurotransmission. These data thus seem to suggest that the dystonic syndrome in dtsz mutant hamsters is under GABAergic influence. The data show furthermore that dystonic hamsters are a suitable model to detect antidystonic effects of drugs.

Behavioral effects of GABA in the hippocampal formation: functional interaction with histamine

Some behavioral effects of GABA in the hippocampus and its probable interaction with histamine in adult male rats were studied. Four experiments were performed. In Expt. 1, rats were implanted unilaterally into the ventral hippocampus and they were microinjected with increasing doses of GABA. Five minutes later the following behavioral scores were measured in a holeboard: (1) locomotion, (2) head-dipping and (3) rearing. Results showed that GABA induced an increase in locomotion and a decrease in the frequency of long-lasting rears. In Expt. 2, the implanted rats were microinjected into the ventral hippocampus with Gamma-vinyl-GABA (GVG), an inhibitor of the metabolizing enzyme of GABA and picrotoxin and bicuculline, both antagonists of GABA. The following behaviors were measured later in the holeboard: (1) locomotion, (2) head-dipping frequency, (3) rearing activity and (4) grooming frequency. Results showed that GVG also increased the locomotor activity and this effect was antagonized by picrotoxin and bicuculline. In Expt. 3 the brain endogenous levels of GABA were measured in rats microinjected with GVG. Results showed that the GVG injection into the hippocampus augmented the endogenous levels of GABA. In Expt. 4 the implanted rats were microinjected into the hippocampus with GVG and histamine. Behavioral scores were measured later in the holeboard. Results showed that the increase in locomotion induced by GVG was blocked by the administration of histamine. Present results show that GABA may be involved in some hippocampal-mediated behaviors and suggest a histamine-GABA link in the final expression of these behaviors.

Chronic toxicity studies with vigabatrin, a GABA-transaminase inhibitor

The GABA-transaminase inhibitor, vigabatrin, has been shown to have a rather low degree of acute toxicity in several animal species. Oral administration of the drug at 1,000 mg/kg/day for 2-4 weeks caused decreased food consumption and weight loss with resultant prostration and death in both rats and dogs. Dosages of 200 mg/kg/day were tolerated for a year without clinical signs in dogs, although rats suffered reduced weight gains and convulsions after 3-4 months when given the drug in the diet. The convulsions continued to occur frequently throughout the one-yr study, but abated 3-4 months after cessation of treatment. The only consistent histopathologic evidence of toxicity in rats and dogs has been the finding of intramyelinic edema (microvacuolation) in the brain, most notably in certain areas of white matter (cerebellum, reticular formation and optic tract in rats and columns of fornix and optic tract in dogs). No lesions were found in the spinal cord or peripheral nervous system. It took several weeks for the microvacuolation to develop, even at high dosages, but it did not continue to progress thereafter, even though a slight effect was noted at dosages as low as 30-50 mg/kg/day after one yr of treatment. The intramyelinic edemia disappeared within a few weeks after treatment was withdrawn. No residual effects were observed in dogs, whereas rats exhibited swollen axons and microscopic mineralized bodies in the cerebellum. Monkeys exhibited no adverse clinical effects except for occasional loose stools at 300 mg/kg/day. After 16 months of oral treatment at 300 mg/kg/day any suggestion of intramyelinic edema was considered to be equivocal, and there was no evidence of any effect in the 50 or 100 mg/kg/day monkeys after 6 yr of treatment. Higher doses caused chronic diarrhea, thus limiting the dosage in this species. Vigabatrin was shown to be well absorbed in rat, dog and man, whereas dose-limited absorption occurred in the monkey. Metabolism is practically nil in all 4 species and the primary elimination pathway is by glomerular filtration. Because vigabatrin is an irreversible inhibitor of GABA-transaminase and the enzyme has a slow turnover rate, plasma levels of the drug are not indicative of its pharmacologic activity. For this reason cerebrospinal fluid levels of GABA and vigabatrin were evaluated, with considerable species differences being noted. The significance of these differences in relation to the differences in toxic response is discussed.

Anticonvulsant and proconvulsant effects of inhibitors of GABA degradation in the amygdala-kindling model

The effects of three drugs, namely gamma-vinyl GABA (vigabatrin), gamma-acetylenic GABA, and aminooxyacetic acid, which increase brain GABA concentrations by irreversible inhibition of GABA degradation, were studied in amygdala-kindled rats. Vigabatrin 800 or 1,200 mg/kg i.p. 4 h after its administration, caused prolongation of behavioural seizures and electrographic afterdischarges recorded from the stimulated amygdala. One to three days after administration it dose dependently reduced seizure severity, seizure duration and afterdischarge duration in most animals. Determination of GABA levels in synaptosomes isolated from 12 brain regions of kindled rats 4 or 48 h after injection of 1,200 mg/kg vigabatrin indicated that the variable effects of this drug at different times after its administration could be related to differences in the time course of nerve terminal GABA increases in selective brain regions such as amygdala and corpus striatum. In contrast to vigabatrin, gamma-acetylenic GABA, 100 mg/kg i.p., reduced seizure severity in kindled rats as early as 4 h after its administration but afterdischarge duration increased significantly on subsequent days. Similar late increases in afterdischarge duration (and limbic seizure activity) after the time of maximum anticonvulsant effect had elapsed were also observed with vigabatrin, which could suggest that the anticonvulsant effect of such drugs is followed by withdrawal hyperexcitability. Aminooxyacetic acid, 20 mg/kg i.p., exerted no significant anticonvulsant effect in kindled rats but prolonged afterdischarge duration in several of the animals studied. The data suggest that GABA-T inhibitors, such as vigabatrin, differ from most antiepileptic drugs previously tested in the kindling model in that they may produce both anticonvulsant and proconvulsant effects at the same dose in the same animal as a function of time after administration.

Immunocytochemical evidence that vigabatrin in rats causes GABA accumulation in glial cells of the retina

Vigabatrin (gamma-vinyl-GABA, GVG) is an irreversible inhibitor of GABA-aminotransferase (GABA-T) that is under clinical trial as an antiepileptic drug. Rats were injected (i.p.) with GVG and killed 18 h later. GVG administration reduced retinal GABA-T activity to undetectable levels and increased the GABA content 5-fold. Immunocytochemistry using a GABA antiserum clearly revealed the presence of GABA-IR in the glial Muller cells of retinas from GVG-treated rats but not from controls. This experiment indicates that the administration of drugs which inhibit GABA-T may cause the accumulation of GABA in retinal cells that do not normally possess enough endogenous GABA to be detected by immunocytochemistry.

Acute stress and GABAergic function in the rat brain

1. The function of the gamma-aminobutyric acid (GABA)-ergic system in certain areas of the rat brain was investigated after acute (5 min) exposure to immobilization stress. 2. The activities of glutamate decarboxylase and GABA-transaminase, GABA concentrations, GABA turnover in vivo and uptake of [3H]-GABA were measured. 3. After 5 min of immobilization stress, GABA concentrations and [3H]-GABA uptake were reduced, and GABA turnover stimulated in the olfactory bulbs. In contrast the uptake of [3H]-GABA was increased in the corpus striatum after 5 min of immobilization stress. 4. None of the parameters measured was significantly altered by acute immobilization stress in the frontal cortex, hippocampus or medio-basal hypothalamus. 5. These findings show that the olfactory bulbs and the corpus striatum are sensitive to the effects of acute stress. Since GABA in the olfactory bulbs is involved in the development of aggression and increased emotional state, it follows that neurochemical changes induced by acute stress might underlie some behavioural manifestations observed after stress.

Effects of long term vigabatrin on somatosensory evoked potentials in epileptic patients

1. Vigabatrin has proved to be a very effective antiepileptic in clinical trials, with excellent long term tolerability. 2. The purpose of this study was to test the central nervous system (CNS) safety of the drug during long term clinical use, due to the existence of species-dependent intramyelinic oedema in animal toxicology studies. 3. Somatosensory evoked potentials were recorded repeatedly over a mean period of 11 months, in 54 adult patients with refractory epilepsy who received vigabatrin as add-on therapy to their current antiepileptic regime. 4. There was no suggestion throughout the study that vigabatrin could lead to a prolongation of neuronal conduction times within the CNS pathways. 5. The results are consistent with previous studies of somatosensory, visual and brainstem auditory evoked potentials in patients. In view of the results observed in the dog, where increases in central latencies of somatosensory evoked potentials were clearly apparent, these data provide strong reassurance about the neurological safety of vigabatrin in the treatment of epileptic patients.

GABA inhibits sexual behaviour in female rats

Ejaculation by male rats caused an abrupt and marked increase in the concentration of GABA in the cerebrospinal fluid and an equally abrupt and marked inhibition of sexual behaviour in female rats. The increase in the concentration of GABA in the cerebrospinal fluid and the inhibition of the behaviour was specifically mediated by the ejaculation of the male; sexual stimulation unaccompanied by ejaculation had no effect. The post-ejaculatory suppression of sexual receptivity in female rats was partially reversed by intracerebroventricular injection of the GABA antagonist bicuculline and the behaviour of receptive rats was inhibited by intracerebroventricular injection of the GABA agonist muscimol. Increasing the concentration of GABA in the cerebrospinal fluid by i.p. injection of the GABA transminase inhibitor gamma-vinyl GABA caused an increase of the concentration of GABA in the cerebrospinal fluid and inhibited the display of sexual receptivity. It is suggested that GABA mediates physiologically relevant inhibition of sexual behaviour in female rats.

Changes in central GABAergic function following acute and repeated stress

1. The function of gamma-aminobutyric acid (GABA)ergic systems in response to acute and repeated stressful manipulations was evaluated in both the corpus striatum and frontal cerebral cortex of the rat. 2. In the corpus striatum the activity of the synthetic enzyme for GABA (glutamic acid decarboxylase, GAD) and the levels of GABA were reduced by acute immobilization stress (1 h). GABA turnover was reduced only by acute cold stress (3 h, 4 degrees C). 3. In the frontal cerebral cortex no changes were observed after acute stressful manipulations, but repeated stress (0.5 h immobilization per day for 14 days) enhanced both GAD activity and GABA turnover, and reduced GABA levels. 4. In conclusion, it would appear that the GABAergic system in the corpus striatum of the rat is most sensitive to acute stress and that the system in the frontal cerebral cortex area is preferentially responsive to chronic stress. It is speculated that the cortical GABAergic system is responsible for adaptive responses to the adverse conditions prevailing during chronic stress.

Future directions in anxiety research

More than 3.5 million patients suffer from anxiety in the United States of America alone. While our knowledge of the basic biological mechanisms of anxiety is very poor, we have a good understanding of the mechanism of action of various anxiolytic drugs at the molecular level. Their mechanism of action often relates to the inhibitory neurotransmitter GABA and it might be speculated that GABA has a role in anxiety. Future research will concentrate on designing better anxiolytic drugs, based for example on the discovery of partial benzodiazepine receptor agonists, and also on GABA uptake inhibitors as enhancers of GABAergic function. Furthermore, scientific studies will focus on a search for putative biological defects related to anxiety, such as defects in the GABA/benzodiazepine receptor chloride channel complex.

One to three day dose intervals during subchronic treatment of epileptic gerbils with gamma-vinyl GABA: anticonvulsant efficacy and alterations in regional brain GABA levels

gamma-Vinyl GABA (GVG), an irreversible inhibitor of GABA degradation, was administered to seizure-susceptible gerbils at different dosage regimens. After acute i.p. administration, GVG dose dependently protected the animals against air blast-induced seizures with an ED50 of 50 mg/kg. After oral administration, GVG exerted similar anticonvulsant potency. However, during subchronic daily oral dosing of 100 mg/kg GVG, tolerance developed to the anticonvulsant effect of the treatment. No tolerance was observed with daily oral dosing of 50 mg/kg or every other day and every third day dosing of 100 mg/kg GVG. The disadvantage of the two latter dosage regimens was that no sufficient seizure control was obtained on the days between two administrations. Determination of GABA levels in 11 brain regions of gerbils after subchronic treatment with the different dosage regimens of GVG indicated that tolerance during treatment with daily administration of 100 mg/kg GVG could be the consequence of feedback reduction of GABA synthesis. The data suggest that the choice of suitable dosage regimens for chronic treatment is more critical with GVG than with other antiepileptic drugs, because compensatory mechanisms within the GABA system may develop when GVG-induced GABA accumulation is too marked.

A study of the effects of vigabatrin on the central nervous system and retina of Sprague Dawley and Lister-Hooded rats

Vigabatrin (gamma-vinyl GABA), an enzyme-activated, irreversible inhibitor of GABA transaminase, was administered orally to albino Sprague Dawley and pigmented Lister-Hooded rats. A dose-dependent retinal lesion characterized histologically by disruption of the outer nuclear layer was observed in the Sprague Dawley rat but not in Lister-Hooded rats, indicating that this alteration is related to the absence of pigment. The lesion is similar to that induced in albino rats by light and certain drugs. In addition, myelin vacuolation of the brain was observed in both rat strains, consistent with the findings of other toxicity studies with vigabatrin. In all cases, the vacuolation was limited to myelinated tracts and resulted from separation of the myelin sheath at the intraperiod line. There was no evidence of demyelination, axonal degeneration or damage to contiguous structures in the affected areas. The vacuolation is histologically similar to that induced in rats by certain other compounds such as isoniazid, hexachlorophene, and triethyltin, but differs in that it is focal in distribution, it is limited to the brain, and is reversible upon cessation of treatment.