title: Vigabatrin induces tonic inhibition via GABA transporter reversal without increasing vesicular GABA release.
abstract: Two forms of GABAergic inhibition coexist: fast synaptic neurotransmission and tonic activation of GABA receptors due to ambient GABA. The mechanisms regulating ambient GABA have not been well defined. Here we examined the role of the GABA transporter in the increase in ambient [GABA] induced by the anticonvulsant vigabatrin. Pretreatment of cultured rat hippocampal neurons with vigabatrin (100 microM) for 2-5 days led to a large increase in ambient [GABA] that was measured as the change in holding current induced by bicuculline during patch-clamp recordings. In contrast, there was a decrease in the frequency of spontaneous miniature inhibitory postsynaptic currents mIPSCs with no change in their amplitude distribution, and a decrease in the magnitude of IPSCs evoked by presynaptic stimulation during paired recordings. The increase in ambient [GABA] was not prevented by blockade of vesicular GABA release with tetanus toxin or removal of extracellular calcium. During perforated patch recordings, the increase in ambient [GABA] was prevented by blocking the GABA transporter, indicating that the GABA transporter was continuously operating in reverse and releasing GABA. In contrast, blocking the GABA transporter increased ambient [GABA] during whole cell patch-clamp recordings unless GABA and Na(+) were added to the recording electrode solution, indicating that whole cell recordings can lead to erroneous conclusions about the role of the GABA transporter in control of ambient GABA. We conclude that the equilibrium for the GABA transporter is a major determinant of ambient [GABA] and tonic GABAergic inhibition. We propose that fast GABAergic neurotransmission and tonic inhibition can be independently modified and play complementary roles in control of neuronal excitability.
authors.affiliation: Departments of Neurology, Yale University School of Medicine, New Haven, Connecticut 06520, USA.
authors.forename: Yuanming
authors.initials: Y
authors.lastname: Wu
authors.forename: Wengang
authors.initials: W
authors.lastname: Wang
authors.forename: George B
authors.initials: GB
authors.lastname: Richerson
mesh_terms: D000818:Animals; D000927:Anticonvulsants / Q000494:pharmacology*; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002478:Cells, Cultured; D050485:GABA Plasma Membrane Transport Proteins; D006624:Hippocampus / Q000166:cytology; D008564:Membrane Potentials / Q000187:drug effects; D008565:Membrane Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D026901:Membrane Transport Proteins; D009433:Neural Inhibition / Q000187:drug effects*; D009474:Neurons / Q000166:cytology / Q000502:physiology*; D009557:Nipecotic Acids / Q000494:pharmacology; D027361:Organic Anion Transporters; D018408:Patch-Clamp Techniques; D051381:Rats; D017207:Rats, Sprague-Dawley; D018079:Receptors, GABA / Q000378:metabolism; D012964:Sodium / Q000494:pharmacology; D013572:Synaptic Vesicles / Q000378:metabolism; D013744:Tetanus Toxin / Q000494:pharmacology; D020888:Vigabatrin / Q000494:pharmacology*; D005680:gamma-Aminobutyric Acid / Q000493:pharmacokinetics
chemical_list: D000927:Anticonvulsants; D002352:Carrier Proteins; D050485:GABA Plasma Membrane Transport Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D009557:Nipecotic Acids; D027361:Organic Anion Transporters; D018079:Receptors, GABA; D013744:Tetanus Toxin; D005680:gamma-Aminobutyric Acid; C045598:N-(4,4-diphenyl-3-butenyl)nipecotic acid; D012964:Sodium; D020888:Vigabatrin