A receptor for presynaptic glutamatergic autoinhibition is a glutamate transporter

Abstract

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.

Glutamatergic autoinhibition of quantal release augments the early phase of releases after a depolarization pulse

“…2 and in most other experiments, autoinhibition reduced the average release. However, there is the complication that L‐Glu in particular activates an additional facilitatory receptor that masks some of the inhibition and sometimes even overcomes the inhibitory effect of Glu on average release (Schramm & Dudel, 1997; Dudel & Schramm, 2003; Dudel, 2004). Overall facilitation was seen in five of the 20 experiments summarized in Fig.…”

“…Excitatory postsynaptic currents from a terminal region were elicited and recorded by means of an extracellular macropatch electrode. The pipette had a 10‐µm diameter opening at its tip and was perfused with vH, exchanging the volume at the tip several hundred times per second (for details see Dudel, 1989; Dudel & Schramm, 2003). The perfusate contained 0.1 µ m tetrodotoxin to prevent excitations and it could be switched to solutions with added L‐ or D‐Glu.…”

“…The perfusate contained 0.1 µ m tetrodotoxin to prevent excitations and it could be switched to solutions with added L‐ or D‐Glu. In some experiments, the solutions in the pipette perfusate were Cl – ‐free, as also used by Dudel & Schramm (2003). Cl – was replaced by propionate or isethionate which are impermeant in crayfish muscle.…”

“…The transporters contain an associated Cl – conductance (Otis & Jahr, 1998) that is involved in transmitting autoinhibition. The latter is blocked in Cl – ‐free solutions (Dudel & Schramm, 2003; Dudel, 2004). A similar transporter‐coupled autoinhibition has been shown for a glutamatergic vertebrate synapse by Palmer et al.…”

Glutamate Transporter-Mediated Glutamate Secretion in the Mammalian Pineal Gland

“…For example in the EAAT4- and EAAT5-type transporters, the non-stoichiometric Cl − current can be high enough to counteract the inward stoichiometric transporter current. With this kind of transporter, the outward Cl − currents can hyperpolarize the cell, possibly facilitating EAA transport (Eliasof and Jahr, 1996; Dudel and Schramm, 2003). However, in the GLT-1-, GLAST-and EAAC1-type glutamate transporters the Cl − conductance is lower and the glutamate transport rates higher (Grewer and Rauen, 2005).…”