GABA TRANSPORTER HETEROGENEITY: PHARMACOLOGY AND CELLULAR LOCALIZATION

doi:10.1016/0197-0186(95)00158-1

Neurochemistry International

Volume 29, Issue 4, October 1996, Pages 335-356

https://doi.org/10.1016/0197-0186(95)00158-1 Get rights and content

Abstract

γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian brain. GABA is cleared from the synaptic cleft by specific, high-affinity, sodium- and chloride-dependent transporters, which are thought to be located on presynaptic terminals and surrounding glial cells. While early studies suggested a distinction between neuronal and glial GABA transport, molecular cloning has revealed the existence of genes for four distinct GABA transporters (termed GAT-1, GAT-2, GAT-3 and BGT-1), thus revealing a greater heterogeneity than previously suspected. Heterologous expression has allowed a detailed characterization of their pharmacological properties, and has revealed that GAT-1 is the site of action of the anticonvulsant drug, Tiagabine. In-situ hybridization and immunocytochemistry demonstrate that each transporter has a unique regional distribution in the brain; in conjunction with experiments utilizing cell cultures, the neuronal vs glial localization of the various transporters is being elucidated. Future studies will be directed at determining the role of each transporter in the regulation of GABAergic transmission, and in the design of additional subtype-specific inhibitors, which may serve as novel therapeutic agents for the treatment of neuropsychiatric disorders. Copyright © 1996 Elsevier Science Ltd.

Section snippets

FUNCTIONS OF NEUROTRANSMITTER TRANSPORTERS

Transporters serve in virtually all cells as the conduit by which essential nutrients such as glucose and amino acids gain access to the cell interior (for review see Silverman, 1991; Christensen, 1984), and also allow cells to adapt to alterations in osmolarity of the extracellular fluid. Within the nervous system certain transporters have taken on a specialized role related to synaptic transmission, namely, regulation of the concentration and duration of neurotransmitter in the synapse (for

MECHANISM OF GABA TRANSPORTERS

Transporters, like channels and ATPase pumps, serve as conduits through which molecules can cross the bilipid plasma membrane; for a detailed description of these conduits, the reader is referred to the excellent monograph by Stein (1990). Mechanistically, transporters function as molecular shuttles, transferring substrates across the membrane one molecule at a time, then going back for more. GABA transporters belong to the family of Na⁺- and Cl⁻- coupled transporters, so named because these

CLASSICAL DESCRIPTION OF GABA TRANSPORTER HETEROGENEITY

The earliest evidence of GABA transporter heterogeneity resulted from a series of investigations in the 1970s by Krogsgaard-Larsen and his colleagues (reviewed in Krogsgaard-Larsen et al., 1987), in which they examined the ability of a series of GABA analogues to inhibit the uptake of [³H]GABA by neuronal and astroglial mammalian cell cultures. The results of some of these experiments are shown in Table 1 in which it can be seen that, whereas GABA itself was nearly equipotent in the two

GABA TRANSPORTER HETEROGENEITY AS REVEALED BY CLONING

The application of molecular biology to the study of neurotransmitter transporters within the past few years has greatly expanded our knowledge of these molecules, and has revealed a far greater complexity of GABA transporters than previously imagined. To date, genes for four distinct high-affinity GABA transporters have been cloned (GAT-1, GAT-2, GAT-3 and BGT-1), as has the gene for a related member of this family, the taurine transporter. In this section I will describe the cloning and

GAT-1-selective inhibitors

Although GABA analogues such as nipecotic acid and guvacine have proven to be useful tools for studying GABA transport, their utility in vivo has been hampered by their poor penetration of the blood-brain barrier, a property attributable to their high degree of hydrophilicity. In an effort to overcome this problem, Ali et al. (1985) examined the effect of adding lipophilic side chains to the nitrogen atom of known GABA transport inhibitors. Surprisingly, the addition of 4,4-diphenyl-3-butenyl

LOCALIZATION OF GABA TRANSPORTERS WITHIN THE CNS

One possible explanation for the existence of four distinct GABA transporters is that they differ in their distribution within the nervous system, both regionally and with respect to cell type. As described in an earlier section, the first evidence for GABA transporter heterogeneity resulted from studies using neuronal and glial cell cultures. However, it has been difficult to reconcile these data with results obtained with the cloned transporters. Specifically, while GAT-1 has the

CONCLUSIONS AND FUTURE DIRECTIONS

Although it has been known since the 1970s that GABA transport is heterogeneous, it is only in the last few years, with the application of molecular biology, that we have begun to gain an understanding of this phenomenon at the molecular level. Such studies have revealed a far greater diversity of GABA transporters than previously suspected and importantly, have made clear that the classical distinction between neuronal and glial GABA transporters is, at best, an oversimplification. On the

Acknowledgements

All of my research described in this manuscript was conducted at, and funded in its entirety by, Synaptic Pharmaceutical Corporation. I am deeply indebted to all my former colleagues at Synaptic, especially Drs Kelli Smith, T. G. Murali Dhar, Eric Gustafson, Pierre Vaysse, Richard Weinshank, Charles Gluchowski and Terri Branchek, and Ms Marge Durkin, for their significant contributions to much of the work discussed in this review. I especially want to express my gratitude to Terri and Richard

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INVITED REVIEWGABA TRANSPORTER HETEROGENEITY: PHARMACOLOGY AND CELLULAR LOCALIZATION

Abstract

Section snippets

FUNCTIONS OF NEUROTRANSMITTER TRANSPORTERS

MECHANISM OF GABA TRANSPORTERS

CLASSICAL DESCRIPTION OF GABA TRANSPORTER HETEROGENEITY

GABA TRANSPORTER HETEROGENEITY AS REVEALED BY CLONING

GAT-1-selective inhibitors

LOCALIZATION OF GABA TRANSPORTERS WITHIN THE CNS

CONCLUSIONS AND FUTURE DIRECTIONS

Acknowledgements

Neuron

Eur. J. Pharmac. (Molec. Pharmac. Section)

Neuron

Biochim. Biophys. Acta

Neuron

Cell

Molec. Brain Res.

Neuron

Neuroscience

Molec. Brain Res.

Br. J. Anaesth.

Prog. Neurobiol.

Molec. Brain Res.

Brain Res.

FEBS Lett.

Dev. Brain Res.

FEBS Lett.

Epilepsy Res.

TINS

Dev. Brain Res.

Neuron

J. Biol. Chem.

FEBS lett.

Lett.

FEBS Lett.

Neuron

FEBS Lett.

Eur. J. Pharmac.

Exp. Neurol.

Neurosci. Res.

Brain Res. Bull.

J. Biol. Chem.

Neurosci. Lett.

Brain Res.

Neuron

FEBS Lett.

Neurosci. Lett.

Brain Res.

Eur. J. Pharmac.

The rat 5-hydroxytryptamine1B receptor is the species homologue of the human 5-hydroxytryptamine1Dβ receptor

Molec. Pharmac.

Orally active and potent inhibitors of γ-aminobutyric acid uptake

J. Med. Chem.

A tale of two families

Nature

Uptake of [3H]serotonin and [3H]glutamate by primary astrocytesII. Differences in cultures prepared from different brain regions

Glia

The synthesis of novel GABA uptake inhibitors: 1. Elucidation of the structure-activity studies leading to the choice of (R)-1-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-piperidenecarboxylic acid (Tiagabine) as an anticonvulsant candidate

J. Med. Chem.

Synthesis and metabolic profile of Cl-966

Drug Dev. Res.

Cloning and expression of a functional serotonin transporter from rat brain

Nature

Cloning of the human homologue of the GABA transporter GAT-3 and identification of a novel inhibitor with selectivity for this site

Recep. Chan.

Cloning and expression of a Betaine/GABA Transporter from human brain

J. Neurochem.

Molecular heterogeneity of the γ-aminobutyric acid (GABA) transport system

J. Biol. Chem.

Re-evaluation of GABA transport in neuronal and glial cultures: correlation of pharmacology and mRNA localization

Recept. Chan.

INVITED REVIEW
GABA TRANSPORTER HETEROGENEITY: PHARMACOLOGY AND CELLULAR LOCALIZATION

The rat 5-hydroxytryptamine_1B receptor is the species homologue of the human 5-hydroxytryptamine_1Dβ receptor

Uptake of [³H]serotonin and [³H]glutamate by primary astrocytesII. Differences in cultures prepared from different brain regions