Asymmetric diffusion into the postsynaptic neuron: an extremely efficient mechanism for removing excess GABA from synaptic clefts on the Deiters' neurone plasma membrane

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.

(R)-N-[4,4-bis(3-methyl-2-thienyl)but-3-en-1-yl]nipecotic acid binds with high affinity to the brain gamma-aminobutyric acid uptake carrier

(R)-N-[4,4-Bis(3-methyl-2-thienyl)but-3-en-1-yl]nipecotic acid (NO 328) has previously been shown to be a potent anticonvulsant in both mice and rats. Here, we report that NO 328 is a potent inhibitor of gamma-[3H]aminobutyric acid [( 3H]GABA) uptake in a rat forebrain synaptosomal preparation (IC50 = 67 nM) and in primary cultures of neurons and astrocytes. Inhibition of [3H]GABA uptake by NO 328 is apparently of a mixed type when NO 328 is preincubated before [3H]GABA uptake; the inhibition is apparently competitive without preincubation. NO 328 itself is not a substrate for the GABA uptake carrier, but NO 328 is a selective inhibitor of [3H]GABA uptake. Binding to benzodiazepine receptors, histamine H1 receptors, and 5-hydroxytryptamine1A receptors was inhibited by NO 328 at 5-30 microM, whereas several other receptors and uptake sites were unaffected. [3H]NO 328 showed saturable and reversible binding to rat brain membranes in the presence of NaCl. The specific binding of [3H]NO 328 was inhibited by known inhibitors of [3H]GABA uptake; GABA and the cyclic amino acid GABA uptake inhibitors were, however, less potent than expected. This indicates that the binding site is not identical to, but rather overlapping with, the GABA recognition site of the uptake carrier. The affinity constant for binding of [3H]NO 328 is 18 nM, and the Bmax is 669 pmol/g of original rat forebrain tissue. The regional distribution of NaCl-dependent [3H]NO 328 binding followed that of synaptosomal [3H]GABA uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

A microelectrophoretic method for the evaluation of GABA transaminase activity

The reduction to the micro-scale of a recently described electrophoretic method for the evaluation of GABA catabolism by GABA-T is presented. The micromethod involves the electrophoresis, in 1 mm diam. capillaries, of small samples of mixtures of [14C]GABA and its metabolites. By coupling this procedure to previously devised micromethods, it was possible to evaluate GABA-T attack to 14C labeled GABA diffusing across a single microdissected neuronal membrane.

Micromethods for the study of GABA biochemistry and function at single GABA acceptive membranes

Three different micromethods for studying GABA biochemistry and function at single microdissected GABA-acceptive neuronal membranes are discussed. The basis for such studies is the possibility of obtaining by microdissection single Deiters' neurons from the lateral vestibular nucleus of the rat and the rabbit. From these isolated cells the plasma membrane may be prepared and studied. The first micromethod allows the study of the Na+ independent diffusion of GABA through such a plasma membrane which is postsynaptic to GABA-ergic boutons. A modification of such method allows also the study of the effects of GABA-ergic drugs on Cl- permeability. The second method allows the study by microelectrophoresis in capillaries of GABA catabolism by GABA-T associated with microdissected single Deiters' membranes. The third one was developed in order to study the characteristics of Na+ dependent GABA carrier activity present on such membranes.

Calcium ions in the presence of exogenous phosphatidylserine interfere with GABA diffusion through the Deiters' neuron membrane

Diffusion of GABA through the plasma membrane of GABA-acceptive neurons might be a mechanism of importance for the termination of its synaptic action. In the present investigation we studied the effects of phosphatidylserine (PS) (10(-4)-10(-3) M), Ca2+ 2 mM and PS + 2 mM Ca2+ on such a process. The method involved the use of single microdissected Deiters' membranes which were put between two small microchambers in order to study the passage of GABA across the membrane. The results show that whereas PS and Ca2+ by themselves have no effect on such a process, PS + 2 mM CaCl2 give a significant, although slight, inhibition. The hypothesis that Calcium ion + PS effect is due to a disturbance of the interaction between GABA and endogenous PS molecules of the membrane is discussed.

gamma-Aminobutyric acid (GABA) removal from the synaptic cleft: a postsynaptic event?

In the present commentary we discuss the adequacy of Na+ transport-coupled presynaptic gamma-aminobutyric acid (GABA) uptake systems for the removal of GABA from the synaptic cleft. This discussion is based on the accepted stoichiometry for GABA presynaptic internalization, GABAout + 3Na+out + K+in in equilibrium GABAin + 3Na+in + K+out, on the parameters reported in the literature for typical synaptosomal preparations, and on the assumption that GABA removal must be a quick event (less than or equal to 2 msec), as derived from electrophysiological studies. On these bases, we have developed a calculation in order to evaluate the time course of synaptic cleft GABA removal by presynaptic systems and ended up with an overall value (t approximately 0.3 sec) which does not fit with the data derived from electrophysiological recordings. Moreover, we calculated that if such systems had the function of removing GABA within 2 msec, as it should be, a large depolarization would be brought about in GABAergic boutons, resulting ultimately in further GABA release. These considerations together with biochemical and pharmacological experimental results seem to exclude that presynaptic uptake systems have the function of removing GABA from the synaptic cleft. Our experimental data on the ability of a GABA-acceptive postsynaptic membrane (Deiters' neuron membrane) to transport GABA indicate that this system may have the correct characteristics for removing the neurotransmitter. This refers to both the kinetics and the electrophysiological consequences of the phenomenon.