A receptor for presynaptic glutamatergic autoinhibition is a glutamate transporter

Glutamate transporter-mediated glutamate secretion in the mammalian pineal gland

Glutamate transporters are expressed throughout the CNS where their major role is to clear released glutamate from presynaptic terminals. Here, we report a novel function of the transporter in rat pinealocytes. This electrogenic transporter conducted inward current in response to L-glutamate and L- or D-aspartate and depolarized the membrane in patch-clamp experiments. Ca2+ imaging demonstrated that the transporter-mediated depolarization induced a significant Ca2+ influx through voltage-gated Ca2+ channels. The Ca2+ rise finally evoked glutamate exocytosis as detected by carbon-fiber amperometry and by HPLC. In pineal slices with densely packed pinealocytes, glutamate released from the cells effectively activated glutamate transporters in neighboring cells. The Ca2+ signal generated by KCl depolarization or acetylcholine propagated through several cell layers by virtue of the regenerative "glutamate-induced glutamate release." Therefore, we suggest that glutamate transporters mediate synchronized elevation of L-glutamate and thereby efficiently downregulate melatonin secretion via previously identified inhibitory metabotropic glutamate receptors in the pineal gland.

The time course of transmitter release in mouse motor nerve terminals is differentially affected by activation of muscarinic M1 or M2 receptors

At endplates of mouse diaphragms the effects of activation of presynaptic muscarinic M1 and M2 autoreceptors on the time courses of monoquantal releases have been investigated at 20 degrees C. Quantal excitatory postsynaptic currents (qEPSCs) were elicited and recorded with a perfused macropatch electrode, through which control- and drug-containing solutions were applied to 10 microm phi regions of a neuromuscular junction. M2 receptors were activated with muscarine, while the M1 receptors were blocked by pirenzepine. M2 activation presented a slight, but highly significant augmentation of early releases. Analogously, M1 receptors were activated with muscarine, while M2 receptors were blocked by methoctramine. M1 activation elicited a highly significant small shift of the time course of release towards longer delays. In controls, the number of late releases decayed with a time constant of 0.3 ms. This time constant did not change appreciably when methoctramine or methoctramine + muscarine were applied. However, methoctramine + muscarine reduced the amplitude of qEPSCs and shortened their decay by a partial block of postsynaptic channels. Double blocks with pirenzepine + methoctramine allowed no presynaptic effect of muscarine, showing that the blocker concentrations were sufficient. Neither the addition of methoctramine to pirenzepine, nor the further addition of muscarine changed the time constant of decay of the number of late releases. The results are very similar to that of autoreceptor activations in the glutamatergic crayfish synapse: activation of inhibitory receptors augmented early releases, and that of facilitatory receptors depressed early releases [J. Dudel (2006a) Eur. J. Neurosci., 23, 2695-2700], which may suggest a general presynaptic mechanism.

The anion channel blocker, 4,4′-dinitrostilbene-2,2′-disulfonic acid prevents neuronal death and excitatory amino acid release during glycolysis inhibition in the hippocampus in vivo

Neuronal death associated with cerebral ischemia and hypoglycemia is related to increased release of excitatory amino acids (EAA) and energy failure. The intrahippocampal administration of the glycolysis inhibitor, iodoacetate (IOA), induces the accumulation of EAA and neuronal death. We have investigated by microdialysis the role of exocytosis, glutamate transporters and volume-sensitive organic anion channel (VSOAC) on IOA-induced EAA release. Results show that the early component of EAA release is inhibited by riluzole, a voltage-dependent sodium channel blocker, and by the VSOAC blocker, tamoxifen, while the early and late components are blocked by the glutamate transport inhibitors, L-trans-pyrrolidine 2,4-dicarboxylate (PDC) and DL-threo-beta-benzyloxyaspartate (DL-TBOA); and by the VSOAC blocker 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS). Riluzole, DL-TBOA and tamoxifen did not prevent IOA-induced neuronal death, while PDC and DNDS did. The VSOAC blockers 5-nitro-2-(3-phenylpropyl-amino) benzoic acid (NPPB) and phloretin had no effect either on EAA efflux or neuronal damage. Results suggest that acute inhibition of glycolytic metabolism promotes the accumulation of EAA by exocytosis, impairment or reverse action of glutamate transporters and activation of a DNDS-sensitive mechanism. The latest is substantially involved in the triggering of neuronal death. To our knowledge, this is the first study to show protection of neuronal death by DNDS in an in vivo model of neuronal damage, associated with deficient energy metabolism and EAA release, two conditions involved in some pathological states such as ischemia and hypoglycemia.

The glutamate transporter EAAT5 works as a presynaptic receptor in mouse rod bipolar cells

Membrane neurotransmitter transporters control the concentration of their substrate in the synaptic clefts, through the thermodynamic coupling of uptake to the movement of Na(+) and other ions. In addition, excitatory amino acid transporters (EAAT) have a Cl(-) conductance which is gated by the joint binding of Na(+) and glutamate, but thermodynamically uncoupled to the flux of glutamate. This conductance is particularly large in the retina-specific EAAT5 isoform. In the mouse retina, we located EAAT5 in both cone and rod photoreceptor terminals and in axon terminals of rod bipolar cells. In these later cells, application of glutamate on the axon terminal evoked a current that reversed at E(Cl), was insensitive to bicuculline, TPMPA, strychnine, dl-AP5, CNQX and MCPG, but blocked by the glutamate transporter inhibitor dl-tBOA. Furthermore, short depolarizations of the bipolar cells evoked a dl-tBOA and Cd(2+)-sensitive current whose amplitude was comparable to the glutamate-evoked current. Its kinetics indicated that EAAT5 was located close to the glutamate release site. For 2 ms depolarizations evoking maximal responses, the EAAT5-mediated current carried between 2 and 8 times more charge as an average inhibitory GABA or glycine postsynaptic current received spontaneously from amacrine cells, with 10 mm or 0.5 mm intracellular EGTA, respectively. In conditions for which reciprocal inhibition could be monitored, the charge carried by the EAAT5 current was 1.5 times larger than the one carried by the inhibitory postsynaptic currents received from amacrine cells. These results indicate that EAAT5 acts as a major inhibitory presynaptic receptor at mammalian rod bipolar cell axon terminals. This feedback mechanism could control glutamate release at the ribbon synapses of a non-spiking neuron and increase the temporal contrast in the rod photoreceptor pathway.

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

At the crayfish neuromuscular junction, glutamatergic autoinhibition of quantal excitatory postsynaptic current (qEPSC) release is mediated by a presynaptic DL-glutamate transporter and its associated Cl- conductance. I investigated whether it also affects the time course of release. qEPSCs were recorded with a perfused macroelectrode through which depolarization pulses and D- or L-glutamate could be applied to a terminal. In order to represent the time course of release, cumulative delays of qEPSCs were determined and scaled to a common final value. At 10 degrees C, on the application of D- or L-glutamate, release increased relative to the controls especially during its first millisecond, taking the mean of 20 experiments (P < 0.01). Also, in many single experiments the respective shifts in the time courses of release were highly significant. The relative surplus of early releases decreased with time constants tau1 of 86 micros and tau2 of 0.75 ms. At 0 degrees C, in the presence of glutamate, the surplus of early delays was increased relative to the controls to a significantly greater extent and for a longer time than at 10 degrees C. The tau1 of 240 micros was almost three times larger than at 10 degrees C. Autoinhibition was inactivated in Cl(-)-free solution. In such solutions the surplus of early releases also disappeared and the shortening of early delays reverted to a lengthening. Interaction of the inhibitory autoreceptor and its associated Cl- flow with the release machinery is discussed.

Regulation of synaptic vesicles pools within motor nerve terminals during short-term facilitation and neuromodulation

The reserve pool (RP) and readily releasable pool (RRP) of synaptic vesicles within presynaptic nerve terminals were physiologically differentiated into distinctly separate functional groups. This was accomplished in glutamatergic nerve terminals by blocking the glutamate transporter with dl-threo-beta-benzyloxyaspartate (TBOA; 10 microM) during electrical stimulation with either 40 Hz of 10 pulses within a train or 20- or 50-Hz continuous stimulation. The 50-Hz continuous stimulation decreased the excitatory postsynaptic potential amplitude 60 min faster than for the 20-Hz continuous stimulation in the presence of TBOA (P < 0.05). There was no significant difference between the train stimulation and 20-Hz continuous stimulation in the run-down time in the presence of TBOA. After TBOA-induced synaptic depression, the excitatory postsynaptic potentials were rapidly (<1 min) revitalized by exposure to serotonin (5-HT, 1 microM) in every preparation tested (P < 0.05). At this glutamatergic nerve terminal, 5-HT promotes an increase probability of vesicular docking and fusion. Quantal recordings made directly at nerve terminals revealed smaller quantal sizes with TBOA exposure with a marked increase in quantal size as well as a continual appearance of smaller quanta upon 5-HT treatment after TBOA-induced depression. Thus 5-HT was able to recruit vesicles from the RP that were not rapidly depleted by acute TBOA treatment and electrical stimulation. The results support the notion that the RRP is selectively activated during rapid electrical stimulation sparing the RP; however, the RP can be recruited by the neuromodulator 5-HT. This suggests at least two separate kinetic and distinct regulatory paths for vesicle recycling within the presynaptic nerve terminal.

Differential effects of the substrate inhibitor l-trans-pyrrolidine-2,4-dicarboxylate (PDC) and the non-substrate inhibitor DL-threo-beta-benzyloxyaspartate (DL-TBOA) of glutamate transporters on neuronal damage and extracellular amino acid levels in rat brain in vivo

The extracellular concentration of glutamate is highly regulated by transporter proteins, due to its neurotoxic properties. Dysfunction or reverse activation of these transporters is related to the extracellular accumulation of excitatory amino acids and neuronal damage associated with ischemia and hypoglycemia. We have investigated by microdialysis the effects of the substrate and the non-substrate inhibitors of glutamate transporters, l-trans-2,4-pyrrolidine dicarboxylate (PDC) and DL-threo-beta-benzyloxyaspartate (DL-TBOA), respectively, on the extracellular levels of amino acids in the rat hippocampus in vivo. In addition, we have studied the effect of both inhibitors on neuronal damage after direct administration into the hippocampus and striatum. Electroencephalographic activity was recorded after the intrahippocampal infusion of DL-TBOA or PDC. Microdialysis administration of 500 microM DL-TBOA into the hippocampus increased 3.4- and nine-fold the extracellular levels of aspartate and glutamate, respectively. Upon stereotaxic administration it induced neuronal damage dose-dependently in CA1 and dentate gyrus, and convulsive behavior. Electroencephalographic recording showed the appearance of limbic seizures in the hippocampus after DL-TBOA infusion. In the striatum it also induced dose-dependent neuronal damage. These effects were prevented by the i.p. administration of the glutamate receptor antagonists (+)-5-methyl-10,11-dihydroxy-5H-dibenzo(a,d)cyclohepten-5,10-iminemaleate and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)-quinoxaline. In contrast to dl-TBOA, PDC (500 microM) induced a more discrete elevation of excitatory amino acids levels (2.6- and three-fold in aspartate and glutamate, respectively), no neuronal damage or behavioral changes, and no alterations in electroencephalographic activity. The differential results obtained with DL-TBOA and PDC might be attributed to their distinct effects on the extracellular concentration of amino acids. Results are relevant to the understanding of the role of glutamate transporters in amino acid removal or release and the induction of excitotoxic cell death.

Glutamatergic chloride currents associated to glutamate transport?

Especially in arthropod glutamatergic synaptic systems, microM l-glutamate (Glu) concentrations often elicit Cl- currents, in addition to the excitatory cationic currents that are triggered by much higher Glu concentrations. In crayfish, Ibotenate (Ibo) is a specific agonist of the Glu-ergic Cl- currents. Application of Glu to Glu-transporters opens associated Cl- currents that inhibit quantal release presynaptically and by occupying the transporter prevents removal of released Glu. The latter prolongs the decay of postsynaptic EPSCs. It was tested whether the Ibo-elicited Cl- currents show the same pre- and post-synaptic effects as the transporter elicited ones, suggesting that also this current component arises through transporter activation. Indeed, Ibo applied to single synaptic junctions produced inhibition of quantal release and prolongation of EPSCs, very similar to the effects of Glu. It seems probable, therefore, that at least in crayfish Glu-ergic Cl- currents are generated by activation of transporters. Since generally such transporters are located around Glu-ergic synapses, this is likely to be a general mechanism. The toxin Ivermectin also elicits Cl- currents. However, while Ivermectin inhibits release too, it does not prolong the decay of EPSCs and is probable to activate GABAergic channels.

Both d- and l-glutamate induce transporter-mediated presynaptic autoinhibition of transmitter release

In crayfish motor nerve terminals l-glutamate (Glu) is the excitatory transmitter and low l-Glu concentrations exert autoinhibition by inhibiting release of Glu quanta from the terminals. This autoinhibition has been shown to be mediated by binding and transport of l-Glu by Glu transporters in the presynaptic membrane. Activated transporters open an associated Cl(-) channel and inhibit release [J. Dudel & M. Schramm (2003) Eur. J. Neurosci., 18, 902-910]. The excitatory, glutamatergic synaptic transmission is specific for the l-Glu isomer. However, transporters are non-selective for the stereoisomers. It is shown here that low concentrations (5 micro m) of d- as well as l-Glu inhibit quantal release on average to 55 and 68%, respectively. The power of inhibition varies widely at different terminals but the local sensitivity to d-Glu is seen to be the same as that for l-Glu. l-Glutamate has been reported to reduce the mean amplitude of nerve terminal action currents (excitatory nerve terminal currents) by about 10%, presumably due to the opening of Cl(-) channels. Evidence is given that d-Glu also inhibits this by an average of 10% (P < 0.001), as expected if both l- and d-Glu activate a transporter-associated Cl(-) conductance. The results give further support for this novel mechanism of regulation of synaptic strength.