title: Rapid, activity-independent turnover of vesicular transmitter content at a mixed glycine/GABA synapse.
abstract: The release of neurotransmitter via the fusion of transmitter-filled, presynaptic vesicles is the primary means by which neurons relay information. However, little is known regarding the molecular mechanisms that supply neurotransmitter destined for vesicle filling, the endogenous transmitter concentrations inside presynaptic nerve terminals, or the dynamics of vesicle refilling after exocytosis. We addressed these issues by recording from synaptically coupled pairs of glycine/GABA coreleasing interneurons (cartwheel cells) of the mouse dorsal cochlear nucleus. We find that the plasma membrane transporter GlyT2 and the intracellular enzyme glutamate decarboxylase supply the majority of glycine and GABA, respectively. Pharmacological block of GlyT2 or glutamate decarboxylase led to rapid and complete rundown of transmission, whereas increasing GABA synthesis via intracellular glutamate uncaging dramatically potentiated GABA release within 1 min. These effects were surprisingly independent of exocytosis, indicating that prefilled vesicles re-equilibrated upon acute changes in cytosolic transmitter. Titration of cytosolic transmitter with postsynaptic responses indicated that endogenous, nonvesicular glycine/GABA levels in nerve terminals are 5-7 mm, and that vesicular transport mechanisms are not saturated under basal conditions. Thus, cytosolic transmitter levels dynamically set the strength of inhibitory synapses in a release-independent manner.
authors.affiliation: Neuroscience Graduate Program, Oregon Health and Science University, Portland, Oregon 97239, USA.
authors.forename: Pierre F
authors.initials: PF
authors.lastname: Apostolides
authors.forename: Laurence O
authors.initials: LO
authors.lastname: Trussell
mesh_terms: D000818:Animals; D001549:Benzamides / Q000494:pharmacology; D001703:Biophysics; D000075402:Channelrhodopsins; D017626:Cochlear Nucleus / Q000166:cytology; D004558:Electric Stimulation; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005260:Female; D018756:GABA Antagonists / Q000494:pharmacology; D005968:Glutamate Decarboxylase / Q000378:metabolism; D005971:Glutamates / Q000494:pharmacology; D005998:Glycine / Q000378:metabolism* / Q000494:pharmacology; D018684:Glycine Agents / Q000494:pharmacology; D050487:Glycine Plasma Membrane Transport Proteins / Q000235:genetics; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D053444:Inhibitory Postsynaptic Potentials / Q000187:drug effects / Q000235:genetics; D007395:Interneurons / Q000166:cytology* / Q000187:drug effects; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008808:Mice, Inbred CBA; D008822:Mice, Transgenic; D008959:Models, Neurological; D009433:Neural Inhibition / Q000187:drug effects; D018408:Patch-Clamp Techniques; D011247:Pregnancy; D011724:Pyridazines / Q000494:pharmacology; D026941:Sodium Channel Blockers / Q000494:pharmacology; D013331:Strychnine / Q000494:pharmacology; D013569:Synapses / Q000187:drug effects / Q000378:metabolism*; D009435:Synaptic Transmission / Q000187:drug effects / Q000235:genetics; D013572:Synaptic Vesicles / Q000378:metabolism*; D013779:Tetrodotoxin / Q000494:pharmacology; D013997:Time Factors; D005680:gamma-Aminobutyric Acid / Q000378:metabolism* / Q000494:pharmacology
chemical_list: C532849:4-benzyloxy-3,5-dimethoxy-N-(1-(dimethylaminocyclopently)methyl)benzamide; C440256:4-methoxy-7-nitroindolinyl-glutamate; D001549:Benzamides; D000075402:Channelrhodopsins; D018691:Excitatory Amino Acid Antagonists; D018756:GABA Antagonists; D005971:Glutamates; D018684:Glycine Agents; D050487:Glycine Plasma Membrane Transport Proteins; D007211:Indoles; D011724:Pyridazines; C493270:Slc6a5 protein, mouse; D026941:Sodium Channel Blockers; D013779:Tetrodotoxin; D005680:gamma-Aminobutyric Acid; C049853:gabazine; D005968:Glutamate Decarboxylase; D013331:Strychnine; D005998:Glycine