A receptor for presynaptic glutamatergic autoinhibition is a glutamate transporter.
Monoquantal excitatory postsynaptic currents were recorded by means of a perfused macropatch electrode from 9 to 15 micro m stretches of crayfish neuromuscular junctions. The excitatory transmitter l-glutamate superfused to a terminal inhibits quantal release by activating autoreceptors [Parnas et al. (1996) Eur. J. Neurosci., 8, 116-126]. Substances related to glutamate that do not activate glutamatergic postsynaptic channels, but are substrates of glutamate transporters, elicited analogous inhibitions, e.g. l- and d-aspartate and some other glutamate transport blockers. As expected, all transport blockers prolonged synaptic currents. Blockers that bind to the transporter receptors but are not transported did not inhibit release, but prevented inhibition by the transport substrates. It appears that autoinhibition is elicited by transport of glutamate or its analogues. Transport into cells is powered by symport of three Na+. To block the transport step electrochemically, extracellular Na+ concentration was lowered to one-quarter, but this surprisingly left the inhibition of release by glutamate unaffected, showing inhibition to be associated to a step between binding and transport. After binding a substrate, glutamate transporters open a parallel Cl- channel. Replacement of extracellular Cl- prevented Cl- current, and release inhibition by glutamate or aspartate was blocked. It is suggested that the flow of Cl- across the cell membrane, after binding a transport substrate, mediates autoinhibition. We measured a related reduction of presynaptic action potentials.
Dudel
Josef
J
Physiologisches Institut der Ludwig-Maximilians-Universität München, Schillerstrasse 46, D-80336 Münich, Germany. dudel@lrz.uni-muenchen.de
Schramm
Marion
M
D027322:Amino Acid Transport System X-AG / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000818:Animals; D001224:Aspartic Acid / Q000378:metabolism; D003400:Astacoidea; D002713:Chlorine / Q000378:metabolism; D018690:Excitatory Amino Acid Agonists / Q000378:metabolism; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000502:physiology*; D018698:Glutamic Acid / Q000031:analogs & derivatives / Q000378:metabolism; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism; D018482:Muscle, Skeletal / Q000502:physiology; D009433:Neural Inhibition / Q000502:physiology*; D009469:Neuromuscular Junction / Q000187:drug effects / Q000502:physiology*; D018408:Patch-Clamp Techniques; D017470:Receptors, Glutamate / Q000378:metabolism; D017661:Receptors, Presynaptic / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology
D027322:Amino Acid Transport System X-AG; D018690:Excitatory Amino Acid Agonists; D018691:Excitatory Amino Acid Antagonists; D007473:Ion Channels; D017470:Receptors, Glutamate; D017661:Receptors, Presynaptic; D001224:Aspartic Acid; D018698:Glutamic Acid; D002713:Chlorine; D012964:Sodium