- Email: [email protected]
Synaptic excitation mediated by glutamate-gated ion channels
Synaptic excitation mediated by glutamate-gated ion channels Craig E. Jahr and Robin A. J. Lester The Vellum
Institute,
Portland,
Oregon,
and Baylor College of Medicine,
Excitatory synaptic transmission in the predominantly postsynaptic millisecond on
on
membranes. to millisecond
synaptogenesis
and
some longstanding postsynaptic
stimulation
Activation
nervous ion
system
Texas, USA
relies
channels
in
not only mediates
signalling but can also have long term influences plasticity.
involving
of the receptor
the transmitter
Current
central
t-glutamate-gated of these channels
synaptic
problems
localization
of
Houston,
Opinion
Recent
the identity subtypes,
in the synaptic
in Neurobiology
Introduction In every region of the vertebrate central nervous system (CNS) so far examined L-glutamate receptor channels mediate excitatory neurotransmission. Two classes of postsynaptic ligand-gated ion channels are activated by synaptic stimulation, N-methyl-D-aspartate (Nh4DA) receptors and a-amino-3-hydroxy5-methyl-4-isoxazole propionic acid (AMPA) receptors, both named for glutamate analogues that selectively activate them [ 1,2]. There are several major differences in the biophysical properties of the two receptors. The AMPA receptor channel only requires the binding of two molecules of glutamate to open [3*-l and is permeable mainly to Na+ and K+ ions [ 1,2]. Its function is analogous to the nicotinic receptor channel at the neuromuscular junction-activation by transmitter binding results in neuronal depolarization. The NMDA channel is more complex. To open, it requires the binding of two molecules each of glycine, presumably tonically present in the extracellular space, and synaptically released glutamate [3**-5**,6]. Once the channel is open, the pore is effectively blocked by physiological concentrations of Mg2+ ions (about 1 mM) [1,2]. When the neuron becomes sufficiently depolarized, for instance through the opening of AMPA receptor channels, Mg2+ leaves the channel and allows Na+, K+ and, significantly, Ca2+ ions [7] to cross the plasmalemma. These properties of the NMDA receptor channel are essential for the induction of certain types of synaptic plasticity (see the review by Z Bashir and G Collingridge, this issue, pp 32%335). When synaptic responses generated by these two channel types were first separated by pharmacological means, a puzzling difference was observed in their time courses
work
has
resolved
of the transmitter,
the
and the time course of cleft.
1992, 2:27&274
(Fig.1). The synaptic current through AMPA receptor channels lasts at most a few milliseconds whereas that through NMDA receptor channels reaches peak in about 20ms and does not terminate for hundreds of milliseconds [8,9**,10,11**]. Is it possible for a single transmitter substance to produce two such temporally distinct responses at the same postsynaptic sites? This review covers recent studies that have largely resolved the time course paradox by determining the postsynaptic distri bution of receptor subtypes, the transmitter identity, and the kinetic properties that govern the activation and deactivation of NMDA and AMPA receptors, Receptor
localization
Both channel types have been localized to postsynaptic sites by radioligand binding studies [ 121, but can AMPA and NMDA receptor channels coexist at the same postsynaptic sites? Although it is clear that in some preparations individual synaptic sites possess only one or the other receptor type [ 13-151, many are ‘not segregated. Colocalization is suggested by the observation that spontaneous excitatofy postsynaptic currents (epscs) are usually composed of components mediated by both receptors [8,16,17**]. Because spontaneous epscs can be recorded in the absence of propagated action potentials, these events are presumably caused by single exocytotic events and are thus localized to individual release sites. In addition, iontophoretic mapping of the dendritic sensitivity to glutamate has revealed that both receptor types can be colocalized [ 18,191 at immunohistochemically identified synaptic sites [ 191, Together, these findings argue for the coexistence of both AMPA and NMDA receptors and that the disparity in the time courses of their synap-
Abbreviations ACh-acetylcholine; AMPA-cr-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid; AP4-L-2-amino+phosphonobutanoate; APSo-2-amino-5-phosphonopentanoate; CNScentral nervous sy~r~111. epsc---excitatory postsynaptic current; NMDA-N-methyl-o-aspartate; NMJ-neuromuscular junttic~n. 270
@ Current
Biology
Ltd ISSN 0959-4388
Svnaotic
(a)
excitation
Presynaptic
Postsynaptic
terminal
spine
mediated
by glutamate-gated
ion channels
]ahr
and
Lester
mal glutamate uptake mechanism, which is found in both synaptosomal and glial membranes, has a higher affinity for glutamate but is much less selective [21].
Kinetic studies
(i) Control
epsc
(ii) AMPA
IF
epsc
(iii) NMDA
epsc
G 40
ms
Fig.1.Synaptic excitation
in the central nervous system. (a) A diagram of an excitatory synapse showing exocytotic release of glutamate and both NMDA and AMPA receptor channels in the postsynaptic membrane. (b) A schematic representation of excitatory postsynaptic currents recorded in (3 control conditions, and separated pharmacologically into (ii) a pure AMPA receptor component, and (iii) a pure NMDA receptor component.
components cannot be accounted for by differences in the mechanisms of release and clearance at distinct synaptic sites.
tic
Transmitter
identity
Classically, transmitter identification has relied on fulfilling several criteria including identical actions of the putative transmitter substance and the synaptically released transmitter, the presence of the candidate in the presynaptic terminal, and release of the candidate and a mechanism for its removal from the synaptic cleft [ 201. Glutamate fulfills the first of these criteria very well. It is the only known endogenous compound that activates both AMPA and NMDA receptors in concentrations that are expected to be reached in the synaptic cleft [ 3**] The second and third criteria are more difficult to fulfill for glutamate than, for example, acetylcholine (ACh) at the neuromuscular junction (NMJ). Abundant gluta mate is normally present for metabolic purposes and, in release studies, glutamate is usually co-released with L-aspartate and other amino acids (see [21]). Indirectly, both criteria are satisfied by the synaptic vesicle uptake system that concentrates glutamate to at least 6OmM [22]. This proton-driven transporter is extremely selective for glutamate, virtually excluding other endogenous acidic amino acid transmitter candidates such as L-aspartate and the sulfur-containing amino acid L-homocysteate [23], thereby providing the strongest neurochemical evidence for the neurotransmitter function of glutamate. Removal of ‘glutamate from the cleft is probably effected both by diffusion [24] and reuptake [21]. Unlike the vesicular transporter, the Na+ -dependent plasmalem-
Recent analysis of the kinetic properties of both AMPA and NMDA channels has provided additional support for the transmitter role of glutamate, as well as addressing issues of receptor localization and the time course of transmitter in the synaptic cleft. By combining patchclamp methods with fast drug application techniques, very brief exposure of outside-out membrane patches to glutamate produces currents mediated by both Ah4PA and NMDA receptors, which mimic the time courses of both AMPA and NMDA receptor components of epscs [ 11**,25,26]. The importance of this is best understood by analogy to the NMJ. Magleby and Stevens [27] showed that nicotinic receptor channel activation at the NMJ could be described by: T+R=TR=TR*
(scheme 1)
where T is the transmitter, R is the unbound receptor, TR is the bound but closed receptor, and TR* is the bound and open receptor channel. Thus, the receptor channel can open only if it is bound by transmitter. Because the concentration of ACh in the synaptic cleft rises and falls rapidly relative to the rate of ACh dissociation, the time course of the synaptic current will be governed by channel kinetics (that is, the time spent in the open state, TR*), and not by the time course of ACh clearance from the cleft. If a transmitter that has a faster dissociation rate than ACh is released, the channel will spend less time in the open state resulting in briefer synaptic currents [28]. This basic formulation can be used with only slight modification to describe both NMDA and AMPA receptor epscs.
Activation
of NMDA
receptor
channels
The component of the epsc that is mediated by the NMDA receptor decays over hundreds of milliseconds whereas the fast AMPA receptor component lasts a few milliseconds at most [~,9.*,10,11*~,15,16,17”,29,301. Because NMDA receptors have a much higher affinity for glutamate than AMP4 receptors [3-3, they remain bound longer than AMPA receptors due to a much slower dissociation rate. During this time, NMDA channels open repeatedly (multiple transitions between TR and TR* in scheme 1 above) and result in the clusters of openings seen in steady-state single channel recordings [31**]. This accounts qualitatively for the long decay phase of the NMDA receptor component of the epsc. If the competitive antagonist of the NMDA binding site, D-2-amino-5phosphonopentanoate (AP5), is applied within a few milliseconds after presynaptic release, no antagonism of the NMDA receptor epsc is observed. Only if AP5 is present before transmitter release is the epsc diminished [ 1l**] This indicates that rebinding of transmitter does not occur during the epsc; if rebinding and subsequent channel opening normally occurred, AP5 would
271
272
Signalling
mechanisms
prevent this and the late phase of the epsc would be diminished. In addition, brief applications of glutamate to outside-out patches activate NMDA receptor currents that have the same decay time course as synaptically activated NMDA currents, indicating that free transmitter need only be present in the cleft for short periods [ 1 I**]. Because brief applications of lower a&&y NMDA agonists such as L-aspartate, NMDA, and L-cysteate all evoke faster decaying currents than glutamate [32] at synapses where the NMDA receptor epsc is prolonged, the only reasonable transmitter candidate is glutamate. The NMDA receptor epsc has at least a 20.fold slower rise than the AMPA receptor component [9.*,11**,16,17**,29, 301. Based on the slow rise, it has been argued that NMDA receptors are more distant from presynaptic release sites than AMPA receptors, resulting in a slower rise in transmitter concentration. The temperature dependence of the rate of rise is high (QlO= 2.7) [9**], however, and thus inconsistent with a longer diffusion path playing an important role, i.e. some intrinsically slow step in channel opening must be important. This is confirmed by the observation that the rise of the NMDA receptor epsc is mimicked by very brief applications of glutamate to outside-out patches. Because the glutamate applications were shorter than the rise time of the response, the rates at which bound NMDA receptor channels open and close must be slow and rate limiting [ll**]. How many NMDA receptors are located adjacent to each release site? High fidelity patch-clamp recordings of spontaneous epscs are composed of both AMPA and NMDA receptor components [8,16,17.=]. The NMDA receptor portion is quite small in amplitude and can be accounted for by very few simultaneous channel openings. In some cases individual single channel events can be resolved apparently unattenuated by dendritic geometty. If spontaneous release results in saturation of NMDA receptors, and bound receptors have a relatively high probability of opening, it follows that few receptors are present at the postsynaptic site. As a result of the almost instantaneous diffusion of transmitter throughout the synaptic cleft following release [24], binding of postsynaptic receptors will occur essentially simultaneously. The probability of a channel being open (Popen) at the peak of the NMDA receptor epsc, about 20ms later, will be high provided that most channels open before the peak of the epsc and remain primarily in the open state for 2&3O ms. In fact, in steady-state recordings, activation of NMDA than nels results in openings that are grouped into clusters that last 20-30 ms during which the Popen is high, about 0.6 [31*.]. Furthermore, the P,,pen at the peak of the current evoked in outside-out patches by brief applications of saturating concentrations of glutamate has been estimated to be about 0.3 [33]. Provided that this is a good approximation to the behavior of the synapse, this would suggest that very few NMDA receptors are present at each synaptic site. Activation
of AMPA receptor
channels
AMPA-receptor mediated currents evoked in outside-out patches by rapid applications of glutamate activate and
desensitize very quickly and have time courses similar to spontaneous AMPA receptor epscs [ 25,261. The results of these experiments suggest that the AMPA receptor component of the epsc could be limited in duration to a large extent by desensitization rather than by the unbinding rate of transmitter, as is the case at the NMJ [27]. Scheme 1 therefore has to be expanded to include, in the simplest case, one desensitized state, i.e. another closed state (TD) that the receptor can enter before unbinding:
T + R =
TR =
TR*
(scheme 2).
1t TD If glutamate applications are very short, about 1 ms, the currents decay 2 to 10 times more quickly than with 100 ms pulses [ 25,341, suggesting that the dissociation rate of glutamate is also fast, as expected from its low affinity for AMPA receptors [3**,25]. In fact, the responses to short applications mimic more closely the time course of epscs that are the least distorted by neuronal geometry (LO Trussell: Sot NeurosciAbstr 1991, 17:1166) [ 17**,29,30], This suggests that dissociation of glutamate from AMPA receptors, in addition to desensitization, contributes to the decay of the AMPA receptor epsc. Corroborating evidence comes from three groups [35-371 who have recently shown that the nootropic drug, aniracetam [38], which decreases the rate of AMPA receptor desensitization, also lengthens the time course of both the epsc and responses of patches to brief applications of glutamate. All three papers suggest that the time course of the AMPA epsc is therefore limited at least in part by desensitization. If both desensitization and unbinding determine the rate of decay of the epsc, the time course of glutamate clearance from the cleft must also be fast and similar to the epsc decay rate. What would be the consequences of a prolonged presence (tens of milliseconds) of free glutamate in the synaptic cleft? Prolonged free glutamate would result in more desensitization of AMPA receptors and, in some pathways, greater presynaptic inhibition caused by at least two types of presynaptic glutamate receptor, the metabotropic receptor [ 391 and the L-2-amino-4-phosphonobutanoate (AP4) receptor [ 40**]. Slow clearance of glutamate would therefore result in smaller postsynaptic responses to subsequent presynaptic activity, and thus decrease the frequency response of excitatory synapses. Conclusions The above sections have outlined how a single neurotransmitter, glutamate, can mediate the characteristically different NMDA and AMPA receptor synaptic responses. Molecular biological studies have proposed the existence of multiple AMPA receptor types [41], with different CNS distributions that may account for some of the subtle differences in synaptic excitation that have been observed. In addition to multiple AMPA receptors, there is overwhelming evidence from autoradiographic
Synaptic
excitation
[421, physiological [431, and molecular biological studies [44,45] for distinct glutamate receptors that are selectively activated by kainate, although the role for these receptors in synaptic transmission is not yet certain. Detailed structure/function studies of glutamate receptor channels have already determined that single amino acid changes can dramatically alter biophysical properties [46,47]. With the recent cloning of the NMDA receptor [48], future studies will certainly provide insight into how current flow through glutamate receptor channels contributes to changes in synaptic efficacy.
References
and
recommended
Papers of particular interest, published view, have been highlighted as: of special Interest . of outstanding interest .. 1.
2.
within the annual period of re-
COLLINGRIDGE GL, LESTER RAJ: Excitatory
Central
Nervous
Amino Acid RecepSystem. Pharmacol
PATNEAUDK, MAYER ML: Structure-Activity Relationships for Amino Acid Transmitter Candidates Acting at N-MethylD-Aspartate and Quisqualate Receptors. .I Neurosci 1990, lo:23852399. Provides comprehensive concentration-response relationships and affinity estimates for many AMPA and NMDA receptor agonists. This information, obtained using whole-cell voltage-clamp techniques, is strongly correlated with previous aifinity measurements from binding assays. Although glutamate was found to have a 2O@fold lower affinity for AMPA receptors compared with the NMDA receptor, this agonist was the most potent endogenous compound at both of the receptor BENVENISTEM, MAVER ML: Kinetic Analysis of Antagonist .. Action at N-Methyl-D-Aspartic Acid Receptors: Two Binding Sites Each for Glutamate and Glycine. Biopbys J 1991, 59:560-573. Using rapid drug perfusion techniques combined with whole-cell recordings, the authors were able to study the kinetics of onset and recovety of antagonism at the glycine and glutamate binding sites of the NMDA receptor. Their data was best fitted assuming the existence of two independent, but identical binding sites for both glutamate and glycine. This model is supported by the additional findings that the rate constants for the onset and recovery from antagonism agree well with antagonist equilibrium aihnity estimates,
HESTRIN S, SAH P, NICOU RA Mechanisms Generating the Time Course of Dual Component Excitatory Synaptic Currents Recorded ln Hippocampal Slices. Neuron 1990, 5~2477253. Using whole-cell recordings from intact hippocampal slices, the authors suggest that diffusion of transmitter does not shape the time courses of AMPA and NMDA receptor-mediated epscs as both the rise and decay phases are highly temperature sensitive. In addition, glutamate uptake blockers were demonstrated to have little effect on the decay phase of either epsc component, suggesting that re-uptake of transmitter is not the limiting step in epsc decay.
10.
CLEMENTS JD, WES~ROOK GL: Activation Kinetics Reveal the Number of Glutamate and Glycine Binding Sites on the NMDA Receptor. Neuron 1991, 7:60%613. As in [4**], these authors came to the conclusion that there are two binding sites for both glutamate and glycine at the NMDA receptor. Because outside-out patch recordings were used instead of whole-cell, the solution-exchange time was about one millisecond. At low agonist concentrations, the sigmoidal activation kinetics, expected for a two-site model, could therefore be observed directly. 6.
JOHNSON JW,
AKHER
sponse in Cultured 325:529531.
LESTER RAJ, CEMENTS JD, WESTBROOK GL, JAHR CE: Channel Kinetics Determine the Time Course of NMDA ReceptorMediated Synaptic Currents. Nature 1990, W6:565-567. Using diierent techniques, the authors reached similar conclusions to those in [9**]. Because application of a competitive NMDA antagonist shortly after synaptic stimulation did not alfect the long decay of the NMDA epsc, it was suggested that rebinding of free transmitter does not occur. Furthermore, a brief pulse of glutamate to an outside-out patch activated long-lasting NMDA channel activity that mimicked the time course of the epsc. This means that the intrinsic NMDA charnel properties determine the behavior of the epsc, and free transmitter need only be transiently present in the synaptic cleft.
12.
FAGG GE, MATUS A: Selective Association of N-Methyl-DAspartate and Quisqualate Types of L-Glutamate Receptor with Brain Postsynaptic Densities. Proc Nat1 Acad Sci USA 1984, 81:68766880.
13.
BROWN
14.
DALE N,
15.
FORTTHE
16.
ROBINSON HPC, SAHARA Y, KAWAIN: Nonstationary Fluctuation Analysis and Direct Resolution of Single Channel Currents at Postsynaptic Sites. BiopLys J 1991, 59:295-304.
7.
MAYER ML, MACDERMO’IT AB, WESMROOK
8.
STEVENS CF: NMDA and Non-NMDA Receptors are Co-localized at Individual Excitatory Synapses in Cultured Rat Hippocampus. Nature 1989, 341:230-233.
ROBERTS A: Dual-Component Amino-Acid-Mediated Synaptic Potentials: Excitatory Drive for Swimming in Xenopus Embryos. J Physiol 1985, 363:35-59.
ID, WESTBROOK GL: Slow Excitatory Postsynaptic Currents Mediated by N-Methyl-D-Aspartate Receptors on Cultured Mouse Central Neurons. J Physiol 1988, 396:515-533.
SIL~R RA, TRAYNEUSSF, CULL-CANDY SG: Rapid-Time-Course of Miniature and Evoked Excitatory Currents at Cerebellar Synapses In Situ. Nature 1992, 355~163-166. The authors demonstrate that the decay of AMPA-receptor mediated epscs is rapid at the well-clamped mossy fibergranule cell synapse in the cerebeUar slice preparation. The authors speculate that slower decays of AMPA epscs observed at some other CNS synapses may arise either from dendtitic filtering or, more interestingly, from the expression of AMPA subunits with different kinetic properties. Recordings of the NMDA receptor component of spontaneous epscs clearly show openings of single channels, suggesting that very few receptors are active at individual synapses. 18.
of ExARANCIO 0, MACDERMOW AB: Differential Distribution citatory Amino Acid Receptors on Embryonic Rat Spinal Cord Neurons in Culture. J Neurophysiol 1991, 65:89‘+913.
19.
JONES KA,
20.
PATON WDM: Central and Synaptic Transmission vous System (Pharmacological Aspects). Annu 1958, 20:431470.
21.
NICHOI~SD, ATWELLD: The Release and Uptake of Excitatory Amino Acids. Trends Pburmacol 1990, 11:462468.
GL, SMITHYSJ, BARKER
JL: Agonist- and Voltage-Gated Calcium Entry in Cultured Mouse Spinal Cord Neurons Under Voltage Clamp Measured Using Arsenazo III. J Neurosci 1987, 7:323@3244.
TH, JOHNSTON D: Voltage-Clamp Analysis of Mossy Fiber Synaptic Input to Hippocampal Neurons. J Neuro& siol 1983, 50:487-507.
17. ..
P:
Glycine Potentiates the NMDA ReMouse Brain Neurons. Nature 1987,
RANDALL AD, SCHOFIELD JG, COLUNGRECE GL: Whole-Cell Patch-Clamp Recordings of an NMDA Receptor-Mediated Synaptic Current in Rat Hippocampal Slices. Neurosci Lett 1990, 114:191-196.
11. ..
4.
5. ..
ion channels Jahr and Lester
..
3. ..
subtypes.
by glutamate-gated
9.
reading
MAYER ML, WESTBR~~K GL: The Physiology of Excitatory Amino Acids in the Vertebrate Central Nervous System. Prog Neurcbiol 1987, 28:197-276.
tors ln the Vertebrate Rev 1989, 40:143210.
mediated
BEKKERS JM,
BAUCHMAN RW: Both NMDA and non-NMDA Subtypes of Glutamate Receptors are Concentrated at Synapses on Cerebral Cortical Neurons in Culture. Neuron 1991, 7:593603.
in the NerRev P&siol
273
274
Signalling
mechanisms
22.
BURGER PM, MEHL E, CAMERONPL, MAYCOX PR, BAUMERT M, IOTIY.PEICH F, DE CAMILLIP, JAHN R: Synaptic Vesicles Immunoisolated from Rat Cerebral Cortex Contain High Levels of Glutamate. Neuron 1989, 3:715-720.
23.
NAITOS, UEDAT: Characterization of Glutamate Uptake into Synaptic Vesicles. J Neurocbem 1985, 44:9+109.
24.
Eccms JC, JAEGERJC: ‘Ihe Relationship Between the Mode of Operation and the Dimensions of the Junctional Regions at Synapses and Motor End-Organs. Proc R Sot Lond [Biol] 1958, 148:3%56.
25.
TRUSSELL LO, FISCHBACH GD: Glutamate Receptor Desensitization and its Role in Synaptic Transmission. Neuron 1989, 3:20’+218.
26.
TANG C-M, DICHTERM, MORADM: Quisqualate Activates a Rapidly Inactivating High Conductance Ionic Channel in Hippocampal Neurons. Science 1989, 243:1474-1477.
27.
MAGLEBY KL, STEVENS CF: A Quantitative Description Plate Currents. J P&i01 1972, 223~173~197.
28.
COLQUHOUN D, LARCEWA, RANGHP: An Analysis of the Action of a False Transmitter at the Neuromuscular Junction. J P&iol 1977, 266361-395.
29.
FINKELAS, &OMAN SJ: The Synaptic Current Evoked in Cat Spinal Motoneurons by Impulses in Single Group la Axons. J Pbysiol 1983, 342:61%632.
30.
NELX)NPG, PUN RYK, WESTBROOK GL: Synaptic Excitation in Cultures of Mouse Spinal Cord Neurones: Receptor Pharmacology and Behavior of Synaptic Currents. J PLysiol1986, 372:16‘+190.
of End-
Activation of a Single NMDA Receptor-Channel Produces a Cluster of Channel Openings. Prm R Sot Lond [Biol] 1991, 243:39-45. Using very low concentrations of glutamate in order to distinguish individual receptor activations, the authors have studied the steady-state behavior of single NMDA channels in outside-out patches from hippucampal neurons. They observed three closed times of up to 10 ms that were independent of glutamate concentration, suggesting that ago onist remained bound during a single ‘cluster’ of channel activity. In some cases, they observed longer agonist-independent closed periods between the clusters giving rise to ‘supercluster’ behavior of NMDA channels, also believed to be due to a single activation of the channel by glutamate. They suggest that these superclusters may be the kinetic structure that underlies the prolonged NMDA component of the epsc. 31. ..
GIBB AJ, COIQLIHOLJN D: Glutamate
36.
VYKLICKYL JR, PATNEAU DK, MAYERML: Modulation of Excitatory Synaptic Transmission by Drugs that Reduce Desensitization at AMPA/Kainate Receptors. Neuron 1992, 7:971-984.
37.
ISAACSON JS, NICOLLRA: Aniracetam Reduces Glutamate Receptor Desensitization and Slows the Decay of Fast Excitatory Synaptic Currents in the Hippocampus. Pm Nut1 Acud Sci USA 1991, 88:1093610940.
38.
ITO 1, TANABES, KOHADAA, SUGP(AMAH: Allosteric Potentiation of Quisqualate Receptors by a Nootropic Drug Aniracetam. J Pbysiol 1990, 424:533-543.
39.
BA~KYS4 MALENKARC: Agents at Metabotropic Glutamate Receptors Presynaptically Inhibit EPSCS in Neonatal Rat Hippocampus. J P&ii0 1991, 444:687-701.
40. ..
FORSYTHEID, CLEMENTS JD: Presynaptic Glutamate Receptors Depress Excitatory Monosynaptic Transmission Between Mouse Hippocampal Neurones. J P!_ysiol 1990, 429:1-16. IJsing paired whole-cell recordings from cultured central neurons if was shown that unless the transmitter, glutamate, is rapidly removed from the synaptic cleft following release, it will feed back onto presynaptic receptors and depress release to subsequent synaptic stimulation. The authors found an excellent correlation between paired-pulse synaptic depression and a depression of the synaptic response by exogenously applied glutamate, suggesting that not all synapses possess these presy naptic receptors.
41.
GASICGP, HEINEMANN S: Receptors Coupled to Ionic Channels: the Glutamate Receptor Family. Gun Opin Neurobiol 1991, 1:20-26.
42.
MONAGHAN DT, COTMANCW: Distribution of [3H]-Kainic Acid Binding Sites in Rat CNS as Determined by Autoradiography. Bruin Res 1982, 252:91-100.
43.
HUE-ITNERJE: Glutamate Receptor Channels in Rat DRG Neurons: Activation by Kainate and Quisqualate and Blockade of Desensitization by Con A. Neuron 1990, 5~255-266.
44.
EGEBJERG J, BETIXERB, HERMANS-BORG-R
I, HEINEMANN S:
Cloning of a cDNA for a Glutamate Receptor Subunit Activated by Kainate but not AMPA. Nature 1991, 3511745-748. 45.
WERNERP, VOIGT M, KEXNANEN K, WISDENW, SEEBURGPH:
46.
VERWORNTA, BURNA~HEV N, MONYERH, SEEBURGPH, SAKMANN
Cloning of a Putative High-Affinity Kainate Receptor Expressed Predominantly in Hippocampal CA3 Cells. Nature 1991, 351:742-744. B: Structural Determinants nant Glutamate Receptors.
of Ion Flow Through RecombiScience 1991, 252:171%1718.
47.
HUME RI, DINGLEDI~YR, HEINEMANN SF: Identification of a site in Glutamate Receptor Subunits that Controls Calcium Permeability. Science 1991, 253:102%1031.
48.
MORNOSHIK, MAXI M, TAKAHIRO I, SHIGEMOTO R, MIZUNON, NAKANISHI S: Molecular Cloning and Characterization of the
32.
LESTER RAJ, JAHR CE: NTkiDAChannel Behavior Depends on
33.
JAHR CE: High Probability Opening of NMDA Receptor Channels by L-Glutamate. Science 1992, 255:47&472.
34.
COLQUHOUN D, JONAS P, SAKMANN B: Glutamate Receptor Channel Activation Following Fast Agonist Application to Patches from Rat Hippocampus CA1 and CA3 Pyramidal Cells. J Physol 1992, 446:183.
Craig E. Jahr, Vellum InstituteL474, Oregon Health Sciences University, 3181 SW. Sam Jackson Park Road, Portland, Oregon 97201.3098, USA
TANG C-M, SHI Q-Y, KAXHMANA, LYNCHG: Modulation of the Time Course of Fast EPSCs and Glutamate Channel Kinetics by Aniracetam. Science 1991, 254:28%290.
Robin A J. Lester, Department of Molecular Physiology and Biophysics, Baylor College of Medicine, One Baylor Plaza, Houston, Texas 77030, USA
Agonist Affinity. J Neurosci 1992, 12:63%643.
35.
Rat NMDA Receptor.
Nature 1991, 354:31-37.
Institute,
Portland,
Oregon,
and Baylor College of Medicine,
Excitatory synaptic transmission in the predominantly postsynaptic millisecond on
on
membranes. to millisecond
synaptogenesis
and
some longstanding postsynaptic
stimulation
Activation
nervous ion
system
Texas, USA
relies
channels
in
not only mediates
signalling but can also have long term influences plasticity.
involving
of the receptor
the transmitter
Current
central
t-glutamate-gated of these channels
synaptic
problems
localization
of
Houston,
Opinion
Recent
the identity subtypes,
in the synaptic
in Neurobiology
Introduction In every region of the vertebrate central nervous system (CNS) so far examined L-glutamate receptor channels mediate excitatory neurotransmission. Two classes of postsynaptic ligand-gated ion channels are activated by synaptic stimulation, N-methyl-D-aspartate (Nh4DA) receptors and a-amino-3-hydroxy5-methyl-4-isoxazole propionic acid (AMPA) receptors, both named for glutamate analogues that selectively activate them [ 1,2]. There are several major differences in the biophysical properties of the two receptors. The AMPA receptor channel only requires the binding of two molecules of glutamate to open [3*-l and is permeable mainly to Na+ and K+ ions [ 1,2]. Its function is analogous to the nicotinic receptor channel at the neuromuscular junction-activation by transmitter binding results in neuronal depolarization. The NMDA channel is more complex. To open, it requires the binding of two molecules each of glycine, presumably tonically present in the extracellular space, and synaptically released glutamate [3**-5**,6]. Once the channel is open, the pore is effectively blocked by physiological concentrations of Mg2+ ions (about 1 mM) [1,2]. When the neuron becomes sufficiently depolarized, for instance through the opening of AMPA receptor channels, Mg2+ leaves the channel and allows Na+, K+ and, significantly, Ca2+ ions [7] to cross the plasmalemma. These properties of the NMDA receptor channel are essential for the induction of certain types of synaptic plasticity (see the review by Z Bashir and G Collingridge, this issue, pp 32%335). When synaptic responses generated by these two channel types were first separated by pharmacological means, a puzzling difference was observed in their time courses
work
has
resolved
of the transmitter,
the
and the time course of cleft.
1992, 2:27&274
(Fig.1). The synaptic current through AMPA receptor channels lasts at most a few milliseconds whereas that through NMDA receptor channels reaches peak in about 20ms and does not terminate for hundreds of milliseconds [8,9**,10,11**]. Is it possible for a single transmitter substance to produce two such temporally distinct responses at the same postsynaptic sites? This review covers recent studies that have largely resolved the time course paradox by determining the postsynaptic distri bution of receptor subtypes, the transmitter identity, and the kinetic properties that govern the activation and deactivation of NMDA and AMPA receptors, Receptor
localization
Both channel types have been localized to postsynaptic sites by radioligand binding studies [ 121, but can AMPA and NMDA receptor channels coexist at the same postsynaptic sites? Although it is clear that in some preparations individual synaptic sites possess only one or the other receptor type [ 13-151, many are ‘not segregated. Colocalization is suggested by the observation that spontaneous excitatofy postsynaptic currents (epscs) are usually composed of components mediated by both receptors [8,16,17**]. Because spontaneous epscs can be recorded in the absence of propagated action potentials, these events are presumably caused by single exocytotic events and are thus localized to individual release sites. In addition, iontophoretic mapping of the dendritic sensitivity to glutamate has revealed that both receptor types can be colocalized [ 18,191 at immunohistochemically identified synaptic sites [ 191, Together, these findings argue for the coexistence of both AMPA and NMDA receptors and that the disparity in the time courses of their synap-
Abbreviations ACh-acetylcholine; AMPA-cr-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid; AP4-L-2-amino+phosphonobutanoate; APSo-2-amino-5-phosphonopentanoate; CNScentral nervous sy~r~111. epsc---excitatory postsynaptic current; NMDA-N-methyl-o-aspartate; NMJ-neuromuscular junttic~n. 270
@ Current
Biology
Ltd ISSN 0959-4388
Svnaotic
(a)
excitation
Presynaptic
Postsynaptic
terminal
spine
mediated
by glutamate-gated
ion channels
]ahr
and
Lester
mal glutamate uptake mechanism, which is found in both synaptosomal and glial membranes, has a higher affinity for glutamate but is much less selective [21].
Kinetic studies
(i) Control
epsc
(ii) AMPA
IF
epsc
(iii) NMDA
epsc
G 40
ms
Fig.1.Synaptic excitation
in the central nervous system. (a) A diagram of an excitatory synapse showing exocytotic release of glutamate and both NMDA and AMPA receptor channels in the postsynaptic membrane. (b) A schematic representation of excitatory postsynaptic currents recorded in (3 control conditions, and separated pharmacologically into (ii) a pure AMPA receptor component, and (iii) a pure NMDA receptor component.
components cannot be accounted for by differences in the mechanisms of release and clearance at distinct synaptic sites.
tic
Transmitter
identity
Classically, transmitter identification has relied on fulfilling several criteria including identical actions of the putative transmitter substance and the synaptically released transmitter, the presence of the candidate in the presynaptic terminal, and release of the candidate and a mechanism for its removal from the synaptic cleft [ 201. Glutamate fulfills the first of these criteria very well. It is the only known endogenous compound that activates both AMPA and NMDA receptors in concentrations that are expected to be reached in the synaptic cleft [ 3**] The second and third criteria are more difficult to fulfill for glutamate than, for example, acetylcholine (ACh) at the neuromuscular junction (NMJ). Abundant gluta mate is normally present for metabolic purposes and, in release studies, glutamate is usually co-released with L-aspartate and other amino acids (see [21]). Indirectly, both criteria are satisfied by the synaptic vesicle uptake system that concentrates glutamate to at least 6OmM [22]. This proton-driven transporter is extremely selective for glutamate, virtually excluding other endogenous acidic amino acid transmitter candidates such as L-aspartate and the sulfur-containing amino acid L-homocysteate [23], thereby providing the strongest neurochemical evidence for the neurotransmitter function of glutamate. Removal of ‘glutamate from the cleft is probably effected both by diffusion [24] and reuptake [21]. Unlike the vesicular transporter, the Na+ -dependent plasmalem-
Recent analysis of the kinetic properties of both AMPA and NMDA channels has provided additional support for the transmitter role of glutamate, as well as addressing issues of receptor localization and the time course of transmitter in the synaptic cleft. By combining patchclamp methods with fast drug application techniques, very brief exposure of outside-out membrane patches to glutamate produces currents mediated by both Ah4PA and NMDA receptors, which mimic the time courses of both AMPA and NMDA receptor components of epscs [ 11**,25,26]. The importance of this is best understood by analogy to the NMJ. Magleby and Stevens [27] showed that nicotinic receptor channel activation at the NMJ could be described by: T+R=TR=TR*
(scheme 1)
where T is the transmitter, R is the unbound receptor, TR is the bound but closed receptor, and TR* is the bound and open receptor channel. Thus, the receptor channel can open only if it is bound by transmitter. Because the concentration of ACh in the synaptic cleft rises and falls rapidly relative to the rate of ACh dissociation, the time course of the synaptic current will be governed by channel kinetics (that is, the time spent in the open state, TR*), and not by the time course of ACh clearance from the cleft. If a transmitter that has a faster dissociation rate than ACh is released, the channel will spend less time in the open state resulting in briefer synaptic currents [28]. This basic formulation can be used with only slight modification to describe both NMDA and AMPA receptor epscs.
Activation
of NMDA
receptor
channels
The component of the epsc that is mediated by the NMDA receptor decays over hundreds of milliseconds whereas the fast AMPA receptor component lasts a few milliseconds at most [~,9.*,10,11*~,15,16,17”,29,301. Because NMDA receptors have a much higher affinity for glutamate than AMP4 receptors [3-3, they remain bound longer than AMPA receptors due to a much slower dissociation rate. During this time, NMDA channels open repeatedly (multiple transitions between TR and TR* in scheme 1 above) and result in the clusters of openings seen in steady-state single channel recordings [31**]. This accounts qualitatively for the long decay phase of the NMDA receptor component of the epsc. If the competitive antagonist of the NMDA binding site, D-2-amino-5phosphonopentanoate (AP5), is applied within a few milliseconds after presynaptic release, no antagonism of the NMDA receptor epsc is observed. Only if AP5 is present before transmitter release is the epsc diminished [ 1l**] This indicates that rebinding of transmitter does not occur during the epsc; if rebinding and subsequent channel opening normally occurred, AP5 would
271
272
Signalling
mechanisms
prevent this and the late phase of the epsc would be diminished. In addition, brief applications of glutamate to outside-out patches activate NMDA receptor currents that have the same decay time course as synaptically activated NMDA currents, indicating that free transmitter need only be present in the cleft for short periods [ 1 I**]. Because brief applications of lower a&&y NMDA agonists such as L-aspartate, NMDA, and L-cysteate all evoke faster decaying currents than glutamate [32] at synapses where the NMDA receptor epsc is prolonged, the only reasonable transmitter candidate is glutamate. The NMDA receptor epsc has at least a 20.fold slower rise than the AMPA receptor component [9.*,11**,16,17**,29, 301. Based on the slow rise, it has been argued that NMDA receptors are more distant from presynaptic release sites than AMPA receptors, resulting in a slower rise in transmitter concentration. The temperature dependence of the rate of rise is high (QlO= 2.7) [9**], however, and thus inconsistent with a longer diffusion path playing an important role, i.e. some intrinsically slow step in channel opening must be important. This is confirmed by the observation that the rise of the NMDA receptor epsc is mimicked by very brief applications of glutamate to outside-out patches. Because the glutamate applications were shorter than the rise time of the response, the rates at which bound NMDA receptor channels open and close must be slow and rate limiting [ll**]. How many NMDA receptors are located adjacent to each release site? High fidelity patch-clamp recordings of spontaneous epscs are composed of both AMPA and NMDA receptor components [8,16,17.=]. The NMDA receptor portion is quite small in amplitude and can be accounted for by very few simultaneous channel openings. In some cases individual single channel events can be resolved apparently unattenuated by dendritic geometty. If spontaneous release results in saturation of NMDA receptors, and bound receptors have a relatively high probability of opening, it follows that few receptors are present at the postsynaptic site. As a result of the almost instantaneous diffusion of transmitter throughout the synaptic cleft following release [24], binding of postsynaptic receptors will occur essentially simultaneously. The probability of a channel being open (Popen) at the peak of the NMDA receptor epsc, about 20ms later, will be high provided that most channels open before the peak of the epsc and remain primarily in the open state for 2&3O ms. In fact, in steady-state recordings, activation of NMDA than nels results in openings that are grouped into clusters that last 20-30 ms during which the Popen is high, about 0.6 [31*.]. Furthermore, the P,,pen at the peak of the current evoked in outside-out patches by brief applications of saturating concentrations of glutamate has been estimated to be about 0.3 [33]. Provided that this is a good approximation to the behavior of the synapse, this would suggest that very few NMDA receptors are present at each synaptic site. Activation
of AMPA receptor
channels
AMPA-receptor mediated currents evoked in outside-out patches by rapid applications of glutamate activate and
desensitize very quickly and have time courses similar to spontaneous AMPA receptor epscs [ 25,261. The results of these experiments suggest that the AMPA receptor component of the epsc could be limited in duration to a large extent by desensitization rather than by the unbinding rate of transmitter, as is the case at the NMJ [27]. Scheme 1 therefore has to be expanded to include, in the simplest case, one desensitized state, i.e. another closed state (TD) that the receptor can enter before unbinding:
T + R =
TR =
TR*
(scheme 2).
1t TD If glutamate applications are very short, about 1 ms, the currents decay 2 to 10 times more quickly than with 100 ms pulses [ 25,341, suggesting that the dissociation rate of glutamate is also fast, as expected from its low affinity for AMPA receptors [3**,25]. In fact, the responses to short applications mimic more closely the time course of epscs that are the least distorted by neuronal geometry (LO Trussell: Sot NeurosciAbstr 1991, 17:1166) [ 17**,29,30], This suggests that dissociation of glutamate from AMPA receptors, in addition to desensitization, contributes to the decay of the AMPA receptor epsc. Corroborating evidence comes from three groups [35-371 who have recently shown that the nootropic drug, aniracetam [38], which decreases the rate of AMPA receptor desensitization, also lengthens the time course of both the epsc and responses of patches to brief applications of glutamate. All three papers suggest that the time course of the AMPA epsc is therefore limited at least in part by desensitization. If both desensitization and unbinding determine the rate of decay of the epsc, the time course of glutamate clearance from the cleft must also be fast and similar to the epsc decay rate. What would be the consequences of a prolonged presence (tens of milliseconds) of free glutamate in the synaptic cleft? Prolonged free glutamate would result in more desensitization of AMPA receptors and, in some pathways, greater presynaptic inhibition caused by at least two types of presynaptic glutamate receptor, the metabotropic receptor [ 391 and the L-2-amino-4-phosphonobutanoate (AP4) receptor [ 40**]. Slow clearance of glutamate would therefore result in smaller postsynaptic responses to subsequent presynaptic activity, and thus decrease the frequency response of excitatory synapses. Conclusions The above sections have outlined how a single neurotransmitter, glutamate, can mediate the characteristically different NMDA and AMPA receptor synaptic responses. Molecular biological studies have proposed the existence of multiple AMPA receptor types [41], with different CNS distributions that may account for some of the subtle differences in synaptic excitation that have been observed. In addition to multiple AMPA receptors, there is overwhelming evidence from autoradiographic
Synaptic
excitation
[421, physiological [431, and molecular biological studies [44,45] for distinct glutamate receptors that are selectively activated by kainate, although the role for these receptors in synaptic transmission is not yet certain. Detailed structure/function studies of glutamate receptor channels have already determined that single amino acid changes can dramatically alter biophysical properties [46,47]. With the recent cloning of the NMDA receptor [48], future studies will certainly provide insight into how current flow through glutamate receptor channels contributes to changes in synaptic efficacy.
References
and
recommended
Papers of particular interest, published view, have been highlighted as: of special Interest . of outstanding interest .. 1.
2.
within the annual period of re-
COLLINGRIDGE GL, LESTER RAJ: Excitatory
Central
Nervous
Amino Acid RecepSystem. Pharmacol
PATNEAUDK, MAYER ML: Structure-Activity Relationships for Amino Acid Transmitter Candidates Acting at N-MethylD-Aspartate and Quisqualate Receptors. .I Neurosci 1990, lo:23852399. Provides comprehensive concentration-response relationships and affinity estimates for many AMPA and NMDA receptor agonists. This information, obtained using whole-cell voltage-clamp techniques, is strongly correlated with previous aifinity measurements from binding assays. Although glutamate was found to have a 2O@fold lower affinity for AMPA receptors compared with the NMDA receptor, this agonist was the most potent endogenous compound at both of the receptor BENVENISTEM, MAVER ML: Kinetic Analysis of Antagonist .. Action at N-Methyl-D-Aspartic Acid Receptors: Two Binding Sites Each for Glutamate and Glycine. Biopbys J 1991, 59:560-573. Using rapid drug perfusion techniques combined with whole-cell recordings, the authors were able to study the kinetics of onset and recovety of antagonism at the glycine and glutamate binding sites of the NMDA receptor. Their data was best fitted assuming the existence of two independent, but identical binding sites for both glutamate and glycine. This model is supported by the additional findings that the rate constants for the onset and recovery from antagonism agree well with antagonist equilibrium aihnity estimates,
HESTRIN S, SAH P, NICOU RA Mechanisms Generating the Time Course of Dual Component Excitatory Synaptic Currents Recorded ln Hippocampal Slices. Neuron 1990, 5~2477253. Using whole-cell recordings from intact hippocampal slices, the authors suggest that diffusion of transmitter does not shape the time courses of AMPA and NMDA receptor-mediated epscs as both the rise and decay phases are highly temperature sensitive. In addition, glutamate uptake blockers were demonstrated to have little effect on the decay phase of either epsc component, suggesting that re-uptake of transmitter is not the limiting step in epsc decay.
10.
CLEMENTS JD, WES~ROOK GL: Activation Kinetics Reveal the Number of Glutamate and Glycine Binding Sites on the NMDA Receptor. Neuron 1991, 7:60%613. As in [4**], these authors came to the conclusion that there are two binding sites for both glutamate and glycine at the NMDA receptor. Because outside-out patch recordings were used instead of whole-cell, the solution-exchange time was about one millisecond. At low agonist concentrations, the sigmoidal activation kinetics, expected for a two-site model, could therefore be observed directly. 6.
JOHNSON JW,
AKHER
sponse in Cultured 325:529531.
LESTER RAJ, CEMENTS JD, WESTBROOK GL, JAHR CE: Channel Kinetics Determine the Time Course of NMDA ReceptorMediated Synaptic Currents. Nature 1990, W6:565-567. Using diierent techniques, the authors reached similar conclusions to those in [9**]. Because application of a competitive NMDA antagonist shortly after synaptic stimulation did not alfect the long decay of the NMDA epsc, it was suggested that rebinding of free transmitter does not occur. Furthermore, a brief pulse of glutamate to an outside-out patch activated long-lasting NMDA channel activity that mimicked the time course of the epsc. This means that the intrinsic NMDA charnel properties determine the behavior of the epsc, and free transmitter need only be transiently present in the synaptic cleft.
12.
FAGG GE, MATUS A: Selective Association of N-Methyl-DAspartate and Quisqualate Types of L-Glutamate Receptor with Brain Postsynaptic Densities. Proc Nat1 Acad Sci USA 1984, 81:68766880.
13.
BROWN
14.
DALE N,
15.
FORTTHE
16.
ROBINSON HPC, SAHARA Y, KAWAIN: Nonstationary Fluctuation Analysis and Direct Resolution of Single Channel Currents at Postsynaptic Sites. BiopLys J 1991, 59:295-304.
7.
MAYER ML, MACDERMO’IT AB, WESMROOK
8.
STEVENS CF: NMDA and Non-NMDA Receptors are Co-localized at Individual Excitatory Synapses in Cultured Rat Hippocampus. Nature 1989, 341:230-233.
ROBERTS A: Dual-Component Amino-Acid-Mediated Synaptic Potentials: Excitatory Drive for Swimming in Xenopus Embryos. J Physiol 1985, 363:35-59.
ID, WESTBROOK GL: Slow Excitatory Postsynaptic Currents Mediated by N-Methyl-D-Aspartate Receptors on Cultured Mouse Central Neurons. J Physiol 1988, 396:515-533.
SIL~R RA, TRAYNEUSSF, CULL-CANDY SG: Rapid-Time-Course of Miniature and Evoked Excitatory Currents at Cerebellar Synapses In Situ. Nature 1992, 355~163-166. The authors demonstrate that the decay of AMPA-receptor mediated epscs is rapid at the well-clamped mossy fibergranule cell synapse in the cerebeUar slice preparation. The authors speculate that slower decays of AMPA epscs observed at some other CNS synapses may arise either from dendtitic filtering or, more interestingly, from the expression of AMPA subunits with different kinetic properties. Recordings of the NMDA receptor component of spontaneous epscs clearly show openings of single channels, suggesting that very few receptors are active at individual synapses. 18.
of ExARANCIO 0, MACDERMOW AB: Differential Distribution citatory Amino Acid Receptors on Embryonic Rat Spinal Cord Neurons in Culture. J Neurophysiol 1991, 65:89‘+913.
19.
JONES KA,
20.
PATON WDM: Central and Synaptic Transmission vous System (Pharmacological Aspects). Annu 1958, 20:431470.
21.
NICHOI~SD, ATWELLD: The Release and Uptake of Excitatory Amino Acids. Trends Pburmacol 1990, 11:462468.
GL, SMITHYSJ, BARKER
JL: Agonist- and Voltage-Gated Calcium Entry in Cultured Mouse Spinal Cord Neurons Under Voltage Clamp Measured Using Arsenazo III. J Neurosci 1987, 7:323@3244.
TH, JOHNSTON D: Voltage-Clamp Analysis of Mossy Fiber Synaptic Input to Hippocampal Neurons. J Neuro& siol 1983, 50:487-507.
17. ..
P:
Glycine Potentiates the NMDA ReMouse Brain Neurons. Nature 1987,
RANDALL AD, SCHOFIELD JG, COLUNGRECE GL: Whole-Cell Patch-Clamp Recordings of an NMDA Receptor-Mediated Synaptic Current in Rat Hippocampal Slices. Neurosci Lett 1990, 114:191-196.
11. ..
4.
5. ..
ion channels Jahr and Lester
..
3. ..
subtypes.
by glutamate-gated
9.
reading
MAYER ML, WESTBR~~K GL: The Physiology of Excitatory Amino Acids in the Vertebrate Central Nervous System. Prog Neurcbiol 1987, 28:197-276.
tors ln the Vertebrate Rev 1989, 40:143210.
mediated
BEKKERS JM,
BAUCHMAN RW: Both NMDA and non-NMDA Subtypes of Glutamate Receptors are Concentrated at Synapses on Cerebral Cortical Neurons in Culture. Neuron 1991, 7:593603.
in the NerRev P&siol
273
274
Signalling
mechanisms
22.
BURGER PM, MEHL E, CAMERONPL, MAYCOX PR, BAUMERT M, IOTIY.PEICH F, DE CAMILLIP, JAHN R: Synaptic Vesicles Immunoisolated from Rat Cerebral Cortex Contain High Levels of Glutamate. Neuron 1989, 3:715-720.
23.
NAITOS, UEDAT: Characterization of Glutamate Uptake into Synaptic Vesicles. J Neurocbem 1985, 44:9+109.
24.
Eccms JC, JAEGERJC: ‘Ihe Relationship Between the Mode of Operation and the Dimensions of the Junctional Regions at Synapses and Motor End-Organs. Proc R Sot Lond [Biol] 1958, 148:3%56.
25.
TRUSSELL LO, FISCHBACH GD: Glutamate Receptor Desensitization and its Role in Synaptic Transmission. Neuron 1989, 3:20’+218.
26.
TANG C-M, DICHTERM, MORADM: Quisqualate Activates a Rapidly Inactivating High Conductance Ionic Channel in Hippocampal Neurons. Science 1989, 243:1474-1477.
27.
MAGLEBY KL, STEVENS CF: A Quantitative Description Plate Currents. J P&i01 1972, 223~173~197.
28.
COLQUHOUN D, LARCEWA, RANGHP: An Analysis of the Action of a False Transmitter at the Neuromuscular Junction. J P&iol 1977, 266361-395.
29.
FINKELAS, &OMAN SJ: The Synaptic Current Evoked in Cat Spinal Motoneurons by Impulses in Single Group la Axons. J Pbysiol 1983, 342:61%632.
30.
NELX)NPG, PUN RYK, WESTBROOK GL: Synaptic Excitation in Cultures of Mouse Spinal Cord Neurones: Receptor Pharmacology and Behavior of Synaptic Currents. J PLysiol1986, 372:16‘+190.
of End-
Activation of a Single NMDA Receptor-Channel Produces a Cluster of Channel Openings. Prm R Sot Lond [Biol] 1991, 243:39-45. Using very low concentrations of glutamate in order to distinguish individual receptor activations, the authors have studied the steady-state behavior of single NMDA channels in outside-out patches from hippucampal neurons. They observed three closed times of up to 10 ms that were independent of glutamate concentration, suggesting that ago onist remained bound during a single ‘cluster’ of channel activity. In some cases, they observed longer agonist-independent closed periods between the clusters giving rise to ‘supercluster’ behavior of NMDA channels, also believed to be due to a single activation of the channel by glutamate. They suggest that these superclusters may be the kinetic structure that underlies the prolonged NMDA component of the epsc. 31. ..
GIBB AJ, COIQLIHOLJN D: Glutamate
36.
VYKLICKYL JR, PATNEAU DK, MAYERML: Modulation of Excitatory Synaptic Transmission by Drugs that Reduce Desensitization at AMPA/Kainate Receptors. Neuron 1992, 7:971-984.
37.
ISAACSON JS, NICOLLRA: Aniracetam Reduces Glutamate Receptor Desensitization and Slows the Decay of Fast Excitatory Synaptic Currents in the Hippocampus. Pm Nut1 Acud Sci USA 1991, 88:1093610940.
38.
ITO 1, TANABES, KOHADAA, SUGP(AMAH: Allosteric Potentiation of Quisqualate Receptors by a Nootropic Drug Aniracetam. J Pbysiol 1990, 424:533-543.
39.
BA~KYS4 MALENKARC: Agents at Metabotropic Glutamate Receptors Presynaptically Inhibit EPSCS in Neonatal Rat Hippocampus. J P&ii0 1991, 444:687-701.
40. ..
FORSYTHEID, CLEMENTS JD: Presynaptic Glutamate Receptors Depress Excitatory Monosynaptic Transmission Between Mouse Hippocampal Neurones. J P!_ysiol 1990, 429:1-16. IJsing paired whole-cell recordings from cultured central neurons if was shown that unless the transmitter, glutamate, is rapidly removed from the synaptic cleft following release, it will feed back onto presynaptic receptors and depress release to subsequent synaptic stimulation. The authors found an excellent correlation between paired-pulse synaptic depression and a depression of the synaptic response by exogenously applied glutamate, suggesting that not all synapses possess these presy naptic receptors.
41.
GASICGP, HEINEMANN S: Receptors Coupled to Ionic Channels: the Glutamate Receptor Family. Gun Opin Neurobiol 1991, 1:20-26.
42.
MONAGHAN DT, COTMANCW: Distribution of [3H]-Kainic Acid Binding Sites in Rat CNS as Determined by Autoradiography. Bruin Res 1982, 252:91-100.
43.
HUE-ITNERJE: Glutamate Receptor Channels in Rat DRG Neurons: Activation by Kainate and Quisqualate and Blockade of Desensitization by Con A. Neuron 1990, 5~255-266.
44.
EGEBJERG J, BETIXERB, HERMANS-BORG-R
I, HEINEMANN S:
Cloning of a cDNA for a Glutamate Receptor Subunit Activated by Kainate but not AMPA. Nature 1991, 3511745-748. 45.
WERNERP, VOIGT M, KEXNANEN K, WISDENW, SEEBURGPH:
46.
VERWORNTA, BURNA~HEV N, MONYERH, SEEBURGPH, SAKMANN
Cloning of a Putative High-Affinity Kainate Receptor Expressed Predominantly in Hippocampal CA3 Cells. Nature 1991, 351:742-744. B: Structural Determinants nant Glutamate Receptors.
of Ion Flow Through RecombiScience 1991, 252:171%1718.
47.
HUME RI, DINGLEDI~YR, HEINEMANN SF: Identification of a site in Glutamate Receptor Subunits that Controls Calcium Permeability. Science 1991, 253:102%1031.
48.
MORNOSHIK, MAXI M, TAKAHIRO I, SHIGEMOTO R, MIZUNON, NAKANISHI S: Molecular Cloning and Characterization of the
32.
LESTER RAJ, JAHR CE: NTkiDAChannel Behavior Depends on
33.
JAHR CE: High Probability Opening of NMDA Receptor Channels by L-Glutamate. Science 1992, 255:47&472.
34.
COLQUHOUN D, JONAS P, SAKMANN B: Glutamate Receptor Channel Activation Following Fast Agonist Application to Patches from Rat Hippocampus CA1 and CA3 Pyramidal Cells. J Physol 1992, 446:183.
Craig E. Jahr, Vellum InstituteL474, Oregon Health Sciences University, 3181 SW. Sam Jackson Park Road, Portland, Oregon 97201.3098, USA
TANG C-M, SHI Q-Y, KAXHMANA, LYNCHG: Modulation of the Time Course of Fast EPSCs and Glutamate Channel Kinetics by Aniracetam. Science 1991, 254:28%290.
Robin A J. Lester, Department of Molecular Physiology and Biophysics, Baylor College of Medicine, One Baylor Plaza, Houston, Texas 77030, USA
Agonist Affinity. J Neurosci 1992, 12:63%643.
35.
Rat NMDA Receptor.
Nature 1991, 354:31-37.