A comparative review on the well-studied GAT1 and the understudied BGT-1 in the brain

γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system (CNS). Its homeostasis is maintained by neuronal and glial GABA transporters (GATs). The four GATs identified in humans are GAT1 (SLC6A1), GAT2 (SLC6A13), GAT3 (SLC6A11), and betaine/GABA transporter-1 BGT-1 (SLC6A12) which are all members of the solute carrier 6 (SLC6) family of sodium-dependent transporters. While GAT1 has been investigated extensively, the other GABA transporters are less studied and their role in CNS is not clearly defined. Altered GABAergic neurotransmission is involved in different diseases, but the importance of the different transporters remained understudied and limits drug targeting. In this review, the well-studied GABA transporter GAT1 is compared with the less-studied BGT-1 with the aim to leverage the knowledge on GAT1 to shed new light on the open questions concerning BGT-1. The most recent knowledge on transporter structure, functions, expression, and localization is discussed along with their specific role as drug targets for neurological and neurodegenerative disorders. We review and discuss data on the binding sites for Na⁺, Cl^-, substrates, and inhibitors by building on the recent cryo-EM structure of GAT1 to highlight specific molecular determinants of transporter functions. The role of the two proteins in GABA homeostasis is investigated by looking at the transport coupling mechanism, as well as structural and kinetic transport models. Furthermore, we review information on selective inhibitors together with the pharmacophore hypothesis of transporter substrates.

Design, Synthesis and Enhanced BBB Penetration Studies of L-serine-Tethered Nipecotic Acid-Prodrug

Nipecotic acid is considered to be one of the most potent inhibitors of neuronal and glial-aminobutyric acid (GABA) uptake in vitro. Due to its hydrophilic nature, nipecotic acid does not readily cross the blood-brain barrier (BBB). Large neutral amino acids (LAT1)-knotted nipecotic acid prodrug was designed and synthesized with the aim to enhance the BBB permeation by the use of carrier-mediated transport. The synthesized prodrug was tested in animal models of Pentylenetetrazole (PTZ)-induced convulsions in mice. Further pain studies were carried out followed by neurotoxicity estimation by writhing and rota-rod test respectively. HPLC data suggests that the synthesized prodrug has improved penetration through BBB. Nipecotic acid-L-serine ester prodrug with considerable anti-epileptic activity, and the ability to permeate the BBB has been successfully synthesized. Graphical Abstract.

Synthesis of novel GABA uptake inhibitors. Part 6: preparation and evaluation of N-Omega asymmetrically substituted nipecotic acid derivatives

In a previous series of potent GABA uptake inhibitors published from this laboratory, we noticed that asymmetry in the substitution pattern of the bis-aromatic moiety in known GABA uptake inhibitors such as 4 [1-(4,4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid] and 5 [(R)-1-(4,4-bis(3-methyl-2-thienyl)-3-butenyl)-3-piperidinecarboxylic acid] was beneficial for high affinity. This led us to investigate asymmetric analogues of known symmetric GABA uptake inhibitors in which one of the aryl groups has been exchanged with an alkyl, alkylene or cycloalkylene moiety as well as other modifications in the lipophilic part. The in vitro values for inhibition of [(3)H]-GABA uptake in rat synaptosomes was determined for each compound, and it was found that several of the novel compounds inhibit GABA uptake as potently as their known symmetrical reference analogues. Several of the novel compounds were also evaluated for their ability to inhibit clonic seizures induced by a 15 mg/kg (ip) dose of methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) in vivo. Some of the compounds, for example 18 [(R)-1-(2-(((1,2-bis(2-fluorophenyl)ethylidene)amino)oxy)ethyl)-3-piperidinecarboxylic acid], show a high in vivo potency and protective index comparable with that of our recently launched anticonvulsant product, 5 [(R)-1-(4,4-bis(3-methyl-2-thienyl)-3-butenyl)-3-piperidinecarboxylic acid], and may therefore serve as second-generation drug candidates.

Basic mechanisms of gabitril (tiagabine) and future potential developments

Gabitril (tiagabine) is a potent selective inhibitor of the principal neuronal gamma-aminobutyric acid (GABA) transporter (GAT-1) in the cortex and hippocampus. By slowing the reuptake of synaptically-released GABA, it prolongs inhibitory postsynaptic potentials. In animal models of epilepsy, tiagabine is particularly effective against kindled (limbic) seizures and against reflexly-induced generalized convulsive seizures. These data are predictive of its efficacy in complex partial seizures in humans. Possible clinical applications outside the field of epilepsy include bipolar disorder and pain.

Localization of messenger RNAs encoding three GABA transporters in rat brain: an in situ hybridization study

Localization of the messenger RNAs encoding three gamma-aminobutyric acid (GABA) transporters, termed GAT-1, GAT-2, and GAT-3, has been carried out in rat brain using radiolabeled oligonucleotide probes and in situ hybridization histochemistry. Hybridization signals for GAT-1 mRNA were observed over many regions of the rat brain, including the retina, olfactory bulb, neocortex, ventral pallidum, hippocampus, and cerebellum. At the microscopic level, this signal appeared to be restricted to neuronal profiles, and the overall distribution of GAT-1 mRNA closely paralleled that seen in other studies with antibodies to GABA. Areas containing hybridization signals for GAT-3 mRNA included the retina, olfactory bulb, subfornical organ, hypothalamus, midline thalamus, and brainstem. In some regions, the hybridization signal for GAT-3 seemed to be preferentially distributed over glial cells, although hybridization signals were also observed over neurons, particularly in the retina and olfactory bulb. Notably, hybridization signal for GAT-3 mRNA was absent from the neocortex and cerebellar cortex, and was very weak in the hippocampus. In contrast to the parenchymal localization obtained for GAT-1 and GAT-3 mRNAs, hybridization signals for GAT-2 mRNA were found only over the leptomeninges (pia and arachnoid). The differential distribution of the three GABA transporters described here suggests that while each plays a role in GABA uptake, they do so via distinct cellular populations.

CI-966, a GABA uptake inhibitor, antagonizes ischemia-induced neuronal degeneration in the gerbil

1. Cerebral ischemia of 5 min duration was induced in unanesthetized gerbils by bilateral occlusion of the carotid arteries. 2. The extent of cerebral damage was assessed by the elevation of motor activity in comparison with pre-ischemic levels and by a histological assessment of the extent of neuronal degeneration of the CA1 area of the hippocampus. 3. The GABA transport inhibitor CI-966 (10 mg/kg i.p.) was tested for cerebroprotective activity in a gerbil stroke model. CI-966 reduced the extent of stroke injury as assessed by locomotor activity and measurement of hippocampal CA1 pyramidal cell injury. 4. It is proposed that enhancement of extracellular GABA levels during ischemia accounts for the cerebroprotective actions of CI-966.

Lack of tolerance to the anticonvulsant effects of tiagabine following chronic (21 day) treatment

The anticonvulsant, and side effect, profile of the gamma-aminobutyric acid (GABA) uptake inhibitor (R)-N-(4,4-di-(3-methylthien-2-yl)but-3-enyl) nipecotic acid hydrochloride (tiagabine) was examined in mice following chronic (21 day) administration. Twenty-four hours following the discontinuation of the 21 days' treatment with twice daily administration of vehicle or tiagabine at 15 or 30 mg/kg p.o., an ED50 for tiagabine was determined for the anticonvulsant effect, the rotarod performance, the traction response and the inhibition of locomotor activity in the animals treated with vehicle only, and in the groups previously treated with 15 or 30 mg/kg p.o. of tiagabine. There was no significant decrease in the anticonvulsant efficacy of acutely administered tiagabine (ED50 for inhibition of methyl 6,7-dimethoxy-4-ethyl-beta-carboline-2-carboxylate (DMCM)-induced seizures of 1.7 +/- 0.4, 1.9 +/- 0.3, and 2.0 +/- 0.50 mg/kg i.p., respectively). However, there was a significant decrease in the ability of acutely administered tiagabine to impair rotarod performance (ED50 of 5.9 +/- 1.2, 14 +/- 1.9 and 21 +/- 2.7 mg/kg i.p., respectively), inhibit a traction response (ED50 of 10 +/- 1.6, 23 +/- 3.0 and 34 +/- 4.6 mg/kg i.p., respectively), and to inhibit exploratory locomotor activity (ED50 of 13 +/- 23, 19 +/- 2.6 and 28 +/- 38 mg/kg i.p. respectively). Following the discontinuation of chronic tiagabine administration there was no change in pentylenetetrazol (PTZ) seizure threshold, animal weight or gross behavior, suggesting the lack of a behavioral withdrawal syndrome. The production of tolerance to the sedative and ataxic effects, but not the anticonvulsant effects, of tiagabine suggests that tiagabine may be a useful agent for the long-term treatment of epilepsy.

Tiagabine, SK&F 89976-A, CI-966, and NNC-711 are selective for the cloned GABA transporter GAT-1

gamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian brain. The synaptic action of GABA is terminated by rapid uptake into presynaptic terminals and surrounding glial cells. Molecular cloning has revealed the existence of four distinct GABA transporters termed GAT-1, GAT-2, GAT-3, and BGT-1. Pharmacological inhibition of transport provides a mechanism for increasing GABA-ergic transmission, which may be useful in the treatment of various neuropsychiatric disorders. Recently, a number of lipophilic GABA transport inhibitors have been designed and synthesized, which are capable of crossing the blood brain barrier, and which display anticonvulsive activity. We have now determined the potency of four of these compounds, SK&F 89976-A (N-(4,4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid), tiagabine ((R)-1-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3- piperidencarboxylic acid), CI-966 ([1-[2-[bis 4-(trifluoromethyl)phenyl]methoxy]ethyl]-1,2,5,6-tetrahydro-3- pyridinecarboxylic acid), and NNC-711 (1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,4,6-tetrahydro-3- pyridinecarboxylic acid hydrochloride), at each of the four cloned GABA transporters, and find them to be highly selective for GAT-1. These data suggest that the anticonvulsant activity of these compounds is mediated via inhibition of uptake by GAT-1.

Gabapentin increases GABA-induced depolarization in rat neonatal optic nerve

Membrane potential changes of rat neonatal optic nerves were studied in a sucrose-gap chamber. Nipecotic acid (NPA), which blocks uptake and promotes release of GABA, resulted in a bicuculline-sensitive depolarization (3.08 +/- 0.3 mV, n = 5). Pretreatment of the nerves with the anticonvulsant gabapentin (100 microM for 1 h) resulted in a near doubling of the NPA-induced depolarization (6.64 +/- 0.54 mV, n = 5). Gabapentin itself did not alter membrane potential, nor did brief applications of gabapentin enhance the NPA- or GABA-induced depolarization. Thus, gabapentin appears to enhance the releasable pool of GABA in this CNS neural system. These results have implications for the mechanism of the anticonvulsant properties of gabapentin.

Enhancement of GABAergic system activity by metoprolol in spontaneously hypertensive rats

The relevance of the GABAergic system for the antihypertensive action of metoprolol in spontaneously hypertensive rats was studied by comparing the effect of metoprolol with the effect of dihydralazine. Chronic oral treatment with metoprolol produced the maximum effect after 49 days (-delta 34 mm Hg). This effect persisted on the same level for up to 55 days. The measurements of gamma-aminobutyric acid (GABA) synthesis and specific [3H]GABA binding were performed in the hypothalamus, the pons-medulla, the hippocampus and the striatum. Significant stimulation of GABA synthesis and turnover appeared in the hypothalamus and the pons medulla. In contrast, chronic administration of dihydralazine had no influence on GABA synthesis rate. It was also shown that metoprolol elevated significantly (P < 0.01) specific [3H]GABA binding in the hypothalamus and the pons-medulla. In the striatum this effect of metoprolol was less pronounced. Binding constant analysis revealed changes in both the receptor density and affinity. Our results suggest that the hypotensive response to chronic treatment with metoprolol might be attributed to an enhancement of GABAergic system activity.

Transient presence of GABA in astrocytes of the developing optic nerve

Immunostaining and high-pressure liquid chromatography (HPLC) were used to study the developmental time course of astrocytic gamma-aminobutyric acid (GABA) expression in rat optic nerve. GABA immunostaining was carried out on cultured astrocytes, and on whole optic nerve. Confocal scanning laser microscopy was used to obtain optical sections in excised whole tissue in order to localize the cellular origins of GABA within the relatively intact optic nerve. GABA immunoreactivity was localized in astrocytes identified by GFAP staining; GABA staining was most intense in early neonatal optic nerve and attenuated over 3 weeks of postnatal development. The staining was pronounced in the astrocyte cell bodies and processes but not in the nucleus. There was a paucity of GABA immunoreactivity by postnatal day 20, both in culture and in whole optic nerve. A biochemical assay for optic nerve GABA using HPLC indicated a relatively high concentration of GABA in the neonate, which rapidly attenuated over the first 3 postnatal weeks. Immunoreactivity for the GABA synthesis enzyme glutamic acid decarboxylase (GAD) was pronounced in neonates but also attenuated with development. These results indicate that GABA and the GABA synthesis enzyme GAD are localized in astrocytes of optic nerve, and that their expression is transient during postnatal development.

New developments in the search for improved antiepileptic drugs

Recent advances in neurobiology have yielded clues about the abnormal physiology of epilepsy and a better understanding of the action of the established anticonvulsants, which were discovered fortuitously or in animal screening tests. Some newer antiepileptic drugs may represent an important improvement over existing therapy, especially if they show efficacy in patients with intractable seizures or fewer limiting neurological side effects. Many developmental agents are designed to interact with a specific target and employ one of three strategies: enhancement of central inhibition; diminution of central excitation; or modulation of ionic channels regulating neuronal excitability. This article reviews the anticonvulsant compounds in development, with a focus on those being investigated in man. Updated information is also provided about the mechanisms of action of the antiepileptic drugs presently used as first line therapy.

Anticonvulsant activity of the gamma-aminobutyric acid uptake inhibitor N-4,4-diphenyl-3-butenyl-4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol

The N-4,4-diphenyl-3-butenyl derivative of the glial selective gamma-aminobutyric acid (GABA) uptake inhibitor 4,5,6,7-tetrahydroisoxazolo [4,5-c]pyridin-3-ol (N-DPB-THPO), was tested for its ability to block sound-induced seizures in the audiogenic seizure-susceptible Frings mouse model of epilepsy. Following intracerebroventricular (i.c.v.) administration, N-DPB-THPO blocked tonic hindlimb extension in a dose- and time-dependent manner. At the doses tested no gross behavioral effects were noted.

GABA and behavior: the role of receptor subtypes

The discovery of different GABA receptor subtypes has stimulated research relating this neurotransmitter to a variety of behavioral functions and clinical disorders. The development of new and specific GABAergic compounds has made it possible to try to identify the specific functions of these receptors. The purpose of the present review is to evaluate the data regarding the functions of the GABA receptor subtypes in different behaviors such as motor function, reproduction, learning and memory, and aggressive-defensive behaviors. A description of GABAergic functions (stress, peripheral effects, thermoregulation) that might directly or indirectly affect behavior is also included. The possible involvement of GABA in different neurological and psychiatric disorders is also discussed. Although much research has been done trying to identify the possible role of GABA in different behaviors, the role of receptor subtypes has only recently attracted attention, and only preliminary data are available at present. It is therefore evident that still much work has to be done before a clear picture of the behavioral significance of these receptor subtypes can be obtained. Nevertheless, existing data are sufficient to justify the prediction that GABAergic agents, in the near future, will be much used in the field of behavioral pharmacology. It is hoped that the present review will contribute to this. Some specific suggestions concerning the most efficient way to pursue future research are also made.

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.

New antiepileptic drugs: from serendipity to rational discovery

Antiepileptic drug discovery has made enormous progress from the serendipity and screening processes of earlier days to the rational drug development of today. The modern era of research began with the recognition that enhancement of inhibitory processes in the brain might favorably influence the propensity for seizures, gamma-aminobutyric acid (GABA) being the main inhibitory transmitter. Work in this field led to the development of vigabatrin, which inhibits the enzyme responsible for the degradation of GABA. More recently, research has focused on the therapeutic potential of blocking excitatory amino acids--in particular glutamate. Of the three receptors for glutamate, the N-methyl-D-aspartate (NMDA) receptor is considered the one of most interest in epilepsy, and research on a series of competitive NMDA receptor antagonists--especially those that are orally active--is in the forefront of antiepileptic drug development today. A further alternative for diminishing neuronal excitability is to modulate sodium, potassium, or calcium channels. The latter are especially implicated in absence seizures.

Different stereoselectivity of the enantiomers of HA-966 (3-amino-1-hydroxy-2-pyrrolidinone) for neuroprotective and anticonvulsant actions in vivo

HA-966 is an antagonist at the glycine modulatory site of N-methyl-D-aspartate (NMDA) receptors. Neuroprotective effects of HA-966 were determined in postnatal day (PND) 7 rats that received intrastriatal injections of NMDA (15 nmol) and then were administered either racemic HA-966, or the purified (R) or (S) enantiomer 15 min later. The (R)-enantiomer dose-dependently attenuated NMDA-induced brain injury, whereas the (S)-enantiomer was ineffective. When given intravenously to mice, racemic HA-966 and the (S)-enantiomer prevented tonic extensor seizures from low-intensity electroshock (ED50 = 13.2 and 8.8 mg/kg, respectively). Anticonvulsant effects of the (R)-enantiomer were much less potent (ED50 = 105.9 mg/kg). Ataxia measured by inverted screen fall-off was 17 times more potent with the (S)-enantiomer than with the (R)-enantiomer. Since previous published work indicates that glycine-site NMDA antagonist activity is primarily due to the (R)-enantiomer, our results suggest neuroprotective action for the (R)-enantiomer and anticonvulsant action, not related to a glycine antagonist mechanism for the (S)-enantiomer.