Neuronal glutamate uptake Contributes to GABA synthesis and inhibitory synaptic strength.
Neurons must maintain a supply of neurotransmitter in their presynaptic terminals to fill synaptic vesicles. GABA is taken up into inhibitory terminals by transporters or is synthesized from glutamate by glutamic acid decarboxylase. Here we report that glutamate transporters supply GABAergic terminals in the hippocampus with glutamate, which is then used to synthesize GABA for filling synaptic vesicles. Glutamate transporter antagonists reduced IPSC and miniature IPSC (mIPSC) amplitudes, consistent with a reduction in the amount of GABA packaged into each synaptic vesicle. This reduction occurred rapidly and independently of synaptic activity, suggesting that modulation of vesicular GABA content does not require vesicle release and refilling. Raising extracellular glutamate levels increased mIPSC amplitudes by enhancing glutamate uptake and, consequently, GABA synthesis. These results indicate that neuronal glutamate transporters strengthen inhibitory synapses in response to extracellular glutamate. This modulation appears to occur under normal conditions and may constitute a negative feedback mechanism to combat hyperexcitability.
Mathews
Gregory C
GC
Synaptic Physiology Unit, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892-4066, USA.
Diamond
Jeffrey S
JS
D027322:Amino Acid Transport System X-AG / Q000037:antagonists & inhibitors / Q000378:metabolism; D000818:Animals; D001224:Aspartic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004558:Electric Stimulation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D027342:Excitatory Amino Acid Transporter 2 / Q000037:antagonists & inhibitors; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000502:physiology; D025461:Feedback, Physiological / Q000502:physiology; D018756:GABA Antagonists / Q000494:pharmacology; D058787:GABA-A Receptor Antagonists; D018698:Glutamic Acid / Q000378:metabolism* / Q000493:pharmacokinetics; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology*; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D009994:Osmolar Concentration; D018408:Patch-Clamp Techniques; D017729:Presynaptic Terminals / Q000187:drug effects / Q000502:physiology; D017966:Pyramidal Cells / Q000187:drug effects / Q000502:physiology; D051381:Rats; D013569:Synapses / Q000187:drug effects / Q000378:metabolism; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology; D005680:gamma-Aminobutyric Acid / Q000096:biosynthesis*
D027322:Amino Acid Transport System X-AG; D004791:Enzyme Inhibitors; D018691:Excitatory Amino Acid Antagonists; D027342:Excitatory Amino Acid Transporter 2; D018756:GABA Antagonists; D058787:GABA-A Receptor Antagonists; C011274:3-hydroxyaspartic acid; D001224:Aspartic Acid; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid