GABA Transaminase Inhibition Induces Spontaneous and Enhances Depolarization-Evoked GABA Efflux via Reversal of the GABA Transporter

Abstract

The GABA transporter can reverse with depolarization, causing nonvesicular GABA release. However, this is thought to occur only under pathological conditions. Patch-clamp recordings were made from rat hippocampal neurons in primary cell cultures. Inhibition of GABA transaminase with the anticonvulsant ␥-vinyl GABA (vigabatrin; 0.05-100 M) resulted in a large leak current that was blocked by bicuculline (50 M). This leak current occurred in the absence of extracellular calcium and was blocked by the GABA transporter antagonist SKF-89976a (5 M). These results indicate that vigabatrin induces spontaneous GABA efflux from neighboring cells via reversal of GABA transporters, subsequently leading to the stimulation of GABA A receptors on the recorded neuron. The leak current increased slowly over 4 d of treatment with 100 M vigabatrin, at which time it reached an equivalent conductance of 9.0 Ϯ 4.9 nS. Blockade of glutamic acid decarboxylase with semicarbazide (2 mM) decreased the leak current that was induced by vigabatrin by 47%. In untreated cells, carrier-mediated GABA efflux did not occur spontaneously but was induced by an increase in [K ϩ ] o from 3 to as little as 6 mM. Vigabatrin enhanced this depolarization-evoked nonvesicular GABA release and also enhanced the heteroexchange release of GABA induced by nipecotate. Thus, the GABA transporter normally operates near its equilibrium and can be easily induced to reverse by an increase in cytosolic [GABA] or mild depolarization. We propose that this transporter-mediated nonvesicular GABA release plays an important role in neuronal inhibition under both physiological and pathophysiological conditions and is the target of some anticonvulsants.

Cation–chloride cotransporters mediate neural computation in the retina

“…These asymmetries produce a DS release of GABA from the SAC onto the DS ganglion cell dendrite. Recent evidence suggests that nonvesicular, carrier-mediated GABA release, which may be the primary mechanism of GABA release from SACs (26), may be sensitive to small changes in membrane potential (30).…”

GEFT, A Rho Family Guanine Nucleotide Exchange Factor, Regulates Neurite Outgrowth and Dendritic Spine Formation

“…Cell Culture, Transfection, and Differentiation-Dentate explants were cultured as previously described (26). Briefly, dentate gyrus and CA3 regions were isolated from the hippocampi of 2-to 4-day-old Sprague-Dawley rats, maintained in culture for 8 -25 days on a Matrigel substrate (1:50 dilution, Collaborative Research), and plated on glass coverslips in medium supplemented with B-27 (Invitrogen).…”

Homo‐ and heteroexchange of adenine nucleotides and nucleosides in rat hippocampal slices by the nucleoside transport system

“…The homo-or heteroexchange is another well-known mechanism, whereby a transmitter could reverse the direction of the transporter dedicated to its uptake and elicit an additional release of itself or another transmitter. This phenomenon has been described and characterized for other transmitters bearing specific transport systems, such as glutamate, GABA (Schwartz, 1982;Bernath & Zigmond, 1988;Wu et al, 2001) and monoamines (Vizi et al, 1985(Vizi et al, , 1986 and has been shown to be an important mechanism during pathological conditions and in the effect of a number of drugs acting at central targets (Vizi, 2000). Nevertheless, the possibility that ATP or adenosine could utilize such kind of mechanism and interact at the level of the release process has not been directly investigated in the central nervous system.…”