The excitatory transmitter released from the presynaptic terminals of stretch-receptor neurons is the amino acid l-glutamate, the major excitatory transmitter in the brain and spinal cord. Eccles and his colleagues discovered that the EPSP in spinal motor cells results from the opening of glutamate-gated channels permeable to both Na+ and K+. This ionic mechanism is similar to that produced by ACh at the neuromuscular junction described in Chapter 9. Like the ACh-gated channels, the glutamate-gated channels conduct both Na+ and K+ with nearly equal permeability. As a result, the reversal potential for current flow through these channels is 0 mV (see Figure 9–7).
Glutamate receptors can be divided into two broad categories: the ionotropic receptors, which are ligand-gated channels where glutamate binding directly opens the channel, and metabotropic receptors, which are G protein-coupled receptors that indirectly gate channels through the production of second messengers (Figure 10–4). There are three major subtypes of ionotropic glutamate receptors: AMPA, kainate, and NMDA, named according to the types of synthetic agonists that activate them (α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, kainate, and N-methyl-D-aspartate, respectively). The NMDA receptor is selectively blocked by the drug APV (2-amino-5-phosphonovaleric acid). The AMPA and kainate receptors are not affected by APV but both are blocked by the drug CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), and thus they are sometimes called the non-NMDA receptors. The metabotropic glutamate receptors can be selectively activated by trans-(1S,3R)-1-amino-1, 3-cyclopentanedicarboxylic acid (ACPD). The action of ionotropic glutamate receptors is always excitatory or depolarizing, as the reversal potential of their ionic current is near zero, whereas the metabotropic receptors can produce either excitation or inhibition, depending on the reversal potential of the ionic currents that they regulate.
Different classes of glutamate receptors regulate excitatory synaptic actions in neurons in the spinal cord and brain.
A. Three classes of ionotropic glutamate receptors directly gate ion channels permeable to cations. The AMPA and kainate type of receptors bind the glutamate agonists AMPA or kainate, respectively. These receptors contain a channel that is permeable to Na+ and K+. The NMDA receptor, which binds the glutamate agonist NMDA, contains a channel permeable to Ca2+, K+, and Na+. It has binding sites for glutamate, glycine, Zn2+, phencyclidine (PCP, or angel dust), MK801 (an experimental drug), and Mg2+, each of which regulates the functioning of the channel differently.
B. The metabotropic glutamate receptors indirectly gate ion channels by activating a GTP-binding protein, which in turn interacts with effector molecules that alter metabolic and ion channel activity (see Chapter 11).
The NMDA receptor has several interesting properties. First, this ligand-gated channel is permeable to Ca2+ as well as to Na+ and K+ (Figure 10–4A). Second, opening the channel requires extracellular glycine as a cofactor. Under normal conditions the concentration of extracellular glycine is sufficient to allow the NMDA receptor-channel to be activated efficiently by glutamate. Third, the NMDA receptor is unique among ligand-gated channels thus far characterized because its opening depends on membrane voltage as well as transmitter. The voltage-dependence is caused by a mechanism that is quite different from that employed by the voltage-gated channels that generate the action potential. In the latter, changes in membrane potential are translated into conformational changes in the channel by an intrinsic voltage-sensor. In the NMDA receptors, however, depolarization removes an extrinsic plug from the channel. At the resting membrane potential (−65 mV) extracellular Mg2+ binds tightly to a site in the pore of the channel, blocking ionic current. But when the membrane is depolarized (for example, by the opening of AMPA receptor-channels), Mg2+ is expelled from the channel by electrostatic repulsion, allowing Na+ and Ca2+ to enter (Figure 10–5).
Opening of single NMDA receptor-channels depends on voltage in addition to glutamate.
These recordings are from individual NMDA receptor-channels (from rat hippocampal cells in culture). Downward deflections indicate pulses of inward (negative) current; upward deflections indicate outward (positive) current. (Reproduced, with permission, from J. Jen and C. F. Stevens.)
A. When Mg2+ is present in normal concentration in the extracellular solution (1.2 mM), the channel is largely blocked at the resting potential (−60 mV). At negative membrane potentials only brief, flickery, inward currents are seen upon channel opening because of the Mg2+ block. Substantial depolarization (to +30 mV or +60 mV) relieves the Mg2+ block, permitting longer-lasting pulses of outward current through the channel.
B. When Mg2+ is removed from the extracellular solution, the opening and closing of the channel do not depend on voltage. The channel is open at the resting potential of–60 mV, and the synaptic current reverses near 0 mV, like the total synaptic current (see Figure 10–6B).
The NMDA receptor has the further interesting property that it is inhibited by the hallucinogenic drug phencyclidine (PCP, also known as angel dust) and by MK801, both of which bind to a site in the pore of the channel that is distinct from the Mg2+ binding site (Figure 10–4A). Indeed, blockade of NMDA receptors produces symptoms that resemble the hallucinations associated with schizophrenia, whereas certain antipsychotic drugs enhance current flow through the NMDA receptor-channels. This has led to the hypothesis that schizophrenia may involve a defect in NMDA receptor function.
At most central synapses that use glutamate as the transmitter, the postsynaptic membrane contains both NMDA and AMPA receptors. The contributions of current through NMDA and AMPA receptors to the total excitatory postsynaptic current (EPSC) can be dissected using pharmacological antagonists in a voltage-clamp experiment (Figure 10–6). At the normal resting potential of most neurons, the NMDA receptor-channels are largely inhibited by Mg2+. As a result, the EPSC is predominantly determined by charge flow through the AMPA receptors, which generate a current with a very rapid rising phase and very rapid decay phase. However, as a neuron becomes depolarized, Mg2+ is driven out of the mouth of the NMDA receptors and more charge flows through these channels. Thus, the NMDA receptor conducts current maximally when two conditions are met: Glutamate is present, and the cell is depolarized (Figure 10–6). That is, the NMDA receptor acts as a "coincidence detector," detecting a timing relationship between activation of the presynaptic and postsynaptic cells. In addition, because of the intrinsic kinetics of ligand gating, the current through the NMDA receptor rises and decays with a much slower time course than the AMPA receptor current. As a result, the NMDA receptors contribute to a late, slow phase of the EPSC and EPSP.
The contributions of the AMPA and NMDA glutamate receptor-channels to the excitatory postsynaptic current.
These voltage-clamp current records are from a cell in the hippo campus. Similar receptor-channels are present in motor neurons and throughout the brain. (Adapted, with permission, from Hestrin et al. 1990.)
A. The drug APV selectively binds to and blocks the NMDA receptor. Shown here is the excitatory postsynaptic current (EPSC) before and during application of 50 μM APV at three different membrane potentials. The difference between the traces (blue region) represents the contribution of the NMDA receptor-channel to the EPSC. The current that remains in the presence of APV is the contribution of the AMPA receptor-channels. At −80 mV there is no current through the NMDA receptor-channels because of pronounced Mg2+ block (see Figure 10–5). At −40 mV a small late inward current through NMDA receptor-channels is evident. At +20 mV the late component is more prominent and has reversed to become an outward current. The vertical dotted line indicates the time 25 ms after the peak of the synaptic current, which is used for the calculations of late current in part B.
B. The postsynaptic currents through the NMDA and AMPA receptor-channels differ in their dependence on the membrane potential. The current through the AMPA receptor-channels contributes to the early phase of the synaptic current (filled triangles). The early phase is measured at the peak of the synaptic current and plotted here as a function of membrane potential. The current through the NMDA receptor-channels contributes to the late phase of the synaptic current (filled circles). The late phase is measured 25 ms after the peak of the synaptic current (dotted line in part A), a time at which the AMPA receptor component has decayed almost to zero. Note that the AMPA receptor-channels behave as simple resistors; current and voltage have a linear relationship. In contrast, current through the NMDA receptor-channels is nonlinear and increases as the membrane is depolarized from −80 to −40 mV, owing to progressive relief of Mg2+ block. The reversal potential of both receptor-channel types is at 0 mV. The components of the synaptic current in the presence of 50 μm APV are indicated by the unfilled circles and triangles. Note how APV blocks the late (NMDA receptor) component but not the early (AMPA receptor) component of the EPSC.
As most glutamatergic synapses contain AMPA receptors that are capable of triggering an action potential, what is the function of the NMDA receptor? At first glance the function of these receptors is even more puzzling because they are normally blocked by Mg2+ at the resting potential. However, when glutamate is paired with depolarization, the NMDA receptors uniquely conduct Ca2+ into the postsynaptic cell. This leads to a rise in intracellular [Ca2+] that can activate various calcium-dependent signaling cascades, including calcium-calmodulin-dependent protein kinase II (CaMKII) (see Chapter 11). Thus NMDA receptor activation can translate electrical signals into biochemical ones. Some of these biochemical reactions lead to long-lasting changes in synaptic strength, a set of processes called long-term synaptic plasticity that are thought to be important during synapse development and for regulating neural circuits in the adult brain. In particular, an NMDA receptor-dependent long-term potentiation (LTP) of excitatory synaptic transmission has been implicated in certain forms of memory storage (see Chapters 66 and 67).
However, there is also a potential downside to the entry of Ca2+ through the NMDA receptors. Excessively high concentrations of glutamate are thought to result in an overload of Ca2+ in the postsynaptic neurons. Such high levels of Ca2+ can be toxic to neurons. In tissue culture even a brief exposure to high concentrations of glutamate can kill many neurons, an action called glutamate excitotoxicity. The high concentrations of intracellular Ca2+ are thought to activate calcium-dependent proteases and phospholipases and lead to the production of free radicals that are toxic to the cell. Glutamate toxicity may contribute to cell damage after stroke, to the cell death that occurs with episodes of rapidly repeated seizures experienced by patients who have status epilepticus, and to degenerative diseases such as Huntington disease. Agents that selectively block the NMDA receptor may protect against the toxic effects of glutamate and have been tested clinically. Unfortunately, the hallucinations that accompany NMDA receptor blockade have so far limited the usefulness of such compounds. A further complication of attempts to control excitotoxicity by blocking NMDA receptor function is that physiological levels of NMDA receptor activation can actually protect neurons from damage and cell death.
The Excitatory Ionotropic Glutamate Receptors Are Encoded by a Distinct Gene Family
What are the molecular bases for the biophysical function of glutamate receptors and how are these receptors related to other ligand-gated ion channels? Over the past 20 years the genes coding for the sub units of all the major neurotransmitter receptors have been identified. This molecular analysis demon strates evolutionary linkages among the structure of receptors that enable us to classify them into three distinct families (Figure 10–7). One family includes the genes encoding the kainate, AMPA, and NMDA receptors; the genes encoding the AMPA and kainate receptors are more closely related to one another than are the genes encoding the NMDA receptors. Surprisingly this gene family bears little resemblance to the two other gene families that encode ionotropic receptors (one that encodes the ACh, GABA, and glycine receptors, and one that encodes ATP receptors, as described below).
The three families of ligand-gated channels.
A. The nicotinic ACh, GABAA, and glycine receptor-channels are all pentamers composed of several types of related subunits. As shown here, the ligand-binding domain is formed by the extracellular amino-terminal region of the protein. Each subunit has a membrane domain with four membrane-spanning α-helixes (M1–M4) and a short extracellular carboxyl terminus. The M2 helix lines the channel pore.
B. The glutamate receptor-channels are tetramers, often composed of two different types of closely related subunits (here denoted 1 and 2). The subunits have a large extracellular amino terminus, a membrane domain with three membrane-spanning α-helixes (M1, M3, and M4), a large extracellular loop connecting the M3 and M4 helixes, and an intracellular carboxyl terminus. The M2 segment forms a loop that dips into and out of the cytoplasmic side of the membrane, contributing to the selectivity filter of the channel. The glutamate binding site is formed by residues in the extracellular amino terminus and in the M3-M4 extracellular loop.
C. The ATP receptor-channels (or purinergic P2X receptors) are trimers. Each subunit possesses two membrane-spanning α-helixes (M1 and M2) and a large extracellular loop that binds ATP. The M2 helix lines the pore.
Unlike the pentameric nicotinic ACh receptor family, the AMPA, kainate, and NMDA receptors are tetrameric proteins with four subunits arranged around a central pore. The AMPA receptor subunits are encoded by four separate genes (GluA1-GluA4) , and there are five different kainate receptor subunit genes (GluK1-GluK5) . Most of the AMPA and kainate receptors are heteromers composed of two different types of GluA and GluK subunits, respectively. The NMDA receptors are encoded by a family consisting of five genes that fall into two groups, the single GluN1 gene and the four GluN2A-D genes. Each NMDA receptor contains two GluN1 subunits and two of the different types of GluN2 subunits.1 In addition, many of these subunit genes are alternatively spliced, generating further diversity. Autoantibodies to the AMPA receptor GluA3 subunit are thought to play an important role in some forms of epilepsy. These antibodies actually mimic glutamate by activating GluA3-containing receptors, resulting in excessive excitation and seizures.
The amino acid sequence of the ionotropic glutamate receptor subunits and subsequent functional and biochemical studies provided the initial compelling evidence that the transmembrane topology of these subunits is very different from that of the nicotinic ACh receptor (Figure 10–7). Our understanding of the ionotropic glutamate receptors was then greatly expanded by Eric Gouaux and colleagues' determination of the high-resolution X-ray crystal structures of the isolated AMPA receptor ligand binding domain and of an intact AMPA receptor-channel formed by GluA2 subunits (Figure 10–8).
Structure of an ionotropic glutamate receptor.
A. Schematic organization of the ionotropic glutamate receptors. The receptors contain a large extracellular amino terminus, followed by a transmembrane domain containing three membrane-spanning α-helixes (M1, M3, and M4) and a loop that dips into the cytoplasmic side of the membrane (M2). The ligand-binding domain is formed by the extracellular region of the receptor on the amino-terminal side of the M1 segment and by the extracellular loop connecting M3 and M4. These two regions intertwine to form a clamshell structure that binds glutamate and various pharmacological agonists and competitive antagonists. A second clamshell structure is formed at the extreme amino terminus of the receptor. This amino-terminal domain is thought to modulate receptor function and synapse development. It does not bind glutamate in the ionotropic receptors. (Reproduced, with permission, from Armstrong et al. 1998.)
B. Three-dimensional X-ray crystal structure of an AMPA receptor composed solely of GluA2 subunits. A side view of the structure of a single GluA2 subunit showing the amino-terminal domain, ligand-binding domain, and transmembrane domain. The M1, M3, and M4 transmembrane α-helixes are indicated, as is the short α-helix in the M2 loop. A molecule of a competitive antagonist of glutamate bound to the ligand-binding domain is shown in a space-filling representation. The cytoplasmic loops connecting the membrane α-helixes were not resolved in the structure and have been drawn as dashed lines. (Reproduced, with permission, from Sobolevsky, Rosconi and Gouaux, 2009.)
C. A side view of the structure of the tetrameric receptor. The four GluA2 subunits associate through the extracellular domains as a pair of dimers (two-fold symmetry). In the amino-terminal domain, one dimer is formed by the blue and yellow subunits, and the other dimer is formed by the red and green subunits. In the ligand-binding domain, the subunits change partners. In one dimer the blue subunit associates with the red subunit, whereas in the other dimer the yellow subunit associates with the green subunit. In the transmembrane region the subunits associate as a four-fold symmetric tetramer. This is a highly unusual subunit arrangement whose significance is not fully understood. (Reproduced, with permission, from Sobolevsky, Rosconi and Gouaux, 2009.)
Glutamate Receptors Are Constructed from a Set of Modules
AMPA receptors are composed of three distinct modules: an extracellular amino-terminal domain, an extracellular ligand-binding domain, and a transmembrane domain (Figure 10–8A,B). The transmembrane domain contains three transmembrane α-helixes (M1, M3, and M4) and a loop (M2) between the M1 and M3 helixes that dips into and out of the cytoplasmic side of the membrane. This M2 loop is thought to form the selectivity filter of the channel. It adopts a structure similar to the pore-lining P loop of K+ channels, except that in K+ channels the P loop dips into and out of the extracellular surface of the membrane (see Figure 5–15).
Both the extracellular amino-terminal domain and the extracellular ligand binding domain are homologous to bacterial amino acid binding proteins. Each domain forms a bi-lobed clamshell-like structure similar to the structure of the bacterial proteins, in which the amino acid is bound within the clamshell. The amino-terminal domain does not bind glutamate but is homologous to the glutamate binding domain of metabotropic glutamate receptors. In the ionotropic glutamate receptors this domain is involved in subunit assembly, the modulation of receptor function by ligands other than glutamate, and the interaction with other synaptic proteins to regulate synapse development.
The ligand-binding domain is formed by two distinct regions in the linear sequence of the protein. One region is located in the extracellular amino terminus of the protein from the end of the amino-terminal domain up to the M1 transmembrane helix; the second region is formed by the large extracellular loop connecting the M3 and M4 helixes. In the ionotropic receptors, the binding of a molecule of glutamate within the clamshell triggers the closure of the lobes of the clamshell; competitive antagonists also bind to the clamshell but fail to trigger clamshell closure. Thus the conformational change associated with clamshell closure is thought to be coupled to the opening of the ion channel.
Given the homology among the various subtypes of glutamate receptors, it is likely that the kainate and NMDA receptors adopt an overall structure similar to that of the homomeric GluA2 receptor. However, there are also likely to be some important differences that give rise to the distinct physiological functions of the different receptors. As we saw previously, the NMDA receptor-channels are permeable to Ca2+, whereas most AMPA receptors are not. These differences have been localized to a single amino acid residue in the pore-forming M2 loop (Figure 10–9A). All NMDA receptor subunits contain the neutral residue asparagine at this position in the pore. In most types of AMPA receptor subunits this residue is the uncharged amino acid glutamine. However, in the GluA2 subunit the corresponding M2 residue is arginine, a positively charged basic amino acid. Inclusion of even a single GluA2 subunit causes the AMPA receptor-channels to have a very low permeability to Ca2+, most likely as a result of strong electrostatic repulsion by the arginine. Some cells form AMPA receptors that lack the GluA2 subunit. Such AMPA receptor-channels generate a significant Ca2+ influx, because their pores lack the positively charged arginine residue.
Determinants of Ca2+ permeability of the AMPA receptor.
A. Comparison of amino acid sequences in the M2 region of the AMPA receptor-channel coded by unedited and edited transcripts of the GluA2 gene. The unedited transcript codes for the polar residue glutamine (Q, using the single-letter amino acid notation), whereas the edited transcript codes for the positively charged residue arginine (R). In the adult the GluA2 protein exists almost exclusively in the edited form.
B. AMPA receptor-channels expressed from unedited transcripts conduct Ca2+ (left traces), whereas those expressed from edited transcripts do not (right traces). The top and bottom traces show currents elicited by glutamate with either extracellular Na+ (top traces) or Ca2+ (bottom traces) as the predominant permeant cation. (Reproduced, with permission, from Sakmann 1992.)
Peter Seeburg and his colleagues made the remarkable discovery that the DNA of the GluA2 gene does not actually encode an arginine residue at this position in the M2 loop but rather codes for a glutamine residue. After transcription the codon for glutamine in the GluA2 mRNA is replaced with one for arginine because of a chemical modification of a single nucleotide base through an enzymatic process termed RNA editing (Figure 10–9A). The importance of this RNA editing is underscored by a genetically engineered mouse that Seeburg and colleagues designed to express a GluA2 gene in which the glutamine residue could no longer be edited to an arginine. Such mice develope seizures and die within a few weeks after birth, presumably caused by excess intracellular Ca2+ as all the AMPA receptors in these mice have a high Ca2+ permeability.
NMDA and AMPA Receptors Are Organized by a Network of Proteins at the Postsynaptic Density
How are the different glutamate receptors localized and arranged at excitatory synapses? Like most ionotropic receptors, glutamate receptors are normally clustered at postsynaptic sites in the membrane, opposed to glutamatergic presynaptic terminals. The vast majority of excitatory synapses in the mature nervous system contain both NMDA and AMPA, whereas in early development synapses containing only NMDA receptors are common. How are synaptic receptors clustered and targeted to appropriate sites? We are now beginning to appreciate that a large number of regulatory proteins that constitute the postsynaptic density help organize the three-dimensional structure of the postsynaptic cell membrane, including the localization of postsynaptic receptors (Figure 10–10).
The postsynaptic cell membrane is organized into a macromolecular complex at excitatory synapses.
Proteins containing PDZ domains help organize the distribution of AMPA and NDMA receptors of the postsynaptic membrane at the postsynaptic density. (Reproduced, with permission, from Sheng and Hoogenrad 2007. Micrographs originally provided by Thomas S. Reese and Xiaobing Chen, National Institutes of Health, USA.)
A. Electron microscope images of biochemically purified post synaptic densities, showing organization of protein network. The membrane lipid bilayer is no longer present. Left: View of post synaptic density from what would normally be the outside of the cell. This image consists of the extracellular domains of various receptors and membrane proteins. Right: View of a postsynaptic density from what would normally be the cytoplasmic side of the membrane. White dots show immuno labeled guanylate kinase anchoring protein, an important component of the PSD.
B. Schematic view of localization and typical number of NMDA receptors, AMPA receptors, and PSD-95, a prominent postsynaptic density protein, at a synapse.
C. Schematic view of the network of receptors and their interacting proteins in the postsynaptic density. PSD-95 contains three PDZ domains at its amino terminus and two other protein interacting motifs at its carboxyl terminus, an SH3 domain and guanylate kinase (GK) domain. Certain PDZ domains of PSD-95 bind to the carboxyl terminus of the GluN2 subunit of the NMDA receptor. PSD-95 does not directly interact with AMPA receptors but binds to the carboxyl terminus of the TARP family of membrane proteins, which interact with the AMPA receptors as auxiliary subunits. PSD-95 also acts as a scaffold for various cytoplasmic proteins by binding to the guanylate-kinase-associated protein (GKAP), which interacts with Shank, a large protein that associates into a meshwork linking the various components of the postsynaptic density. PSD-95 also interacts with the cytoplasmic region of neuroligin. The metabotropic glutamate receptor is localized on the periphery of the synapse. It interacts with the protein Homer, which in turn binds to Shank.
The postsynaptic density is a remarkably stable structure, permitting its biochemical isolation, purification and characterization. Electron microscopic studies of intact and isolated postsynaptic densities provide a strikingly detailed view of their structure. By using gold-labeled antibodies it is possible to identify specific protein components of the postsynaptic membrane, including the location and number of glutamate receptors. A typical PSD of around 350 nm in diameter contains about 20 NMDA receptors, which tend to be localized near the center of the PSD, and 10 to 50 AMPA receptors, which are less centrally localized. The metabotropic glutamate receptors are located on the periphery, outside the main area of the PSD. All three receptor types interact with a wide array of cytoplasmic and membrane proteins to ensure their proper localization.
One of the most prominent proteins in the post synaptic density important for the clustering of glutamate receptors is PSD-95 (postsynaptic density protein of 95 kD molecular weight). PSD-95 is a membrane- associated protein that contains three repeated regions—the so-called PDZ domains—important for protein-protein interactions. The PDZ domains bind to specific sequences at the extreme carboxy terminus of a number of cellular proteins. They are named PDZ after the three proteins in which they were first identified: PSD-95, the DLG tumor suppressor protein in Drosophila, and a protein termed ZO-1. The PDZ domains of PSD-95 bind the NMDA receptor and the Shaker-type voltage-gated K+ channel, thereby localizing and concentrating these channels at postsynaptic sites. PSD-95 also interacts with the postsynaptic membrane protein neuroligin, which forms an extracellular contact in the synaptic cleft with the presynaptic membrane protein neurexin, an interaction important for synapse development. Mutations in neuroligin are thought to contribute to some cases of autism.
Although PSD-95 does not directly bind to AMPA receptors, it does interact with an auxiliary subunit of these receptors termed the transmembrane AMPA receptor regulatory protein (TARP). The TARP proteins contain four transmembrane segments with a cytoplasmic C-terminus. These proteins strongly regulate the trafficking, synaptic localization, and gating of the AMPA receptors. The first TARP family member to be identified was stargazin, which was isolated through a genetic screen in the stargazer mutant mouse, so named because these animals have a tendency to tip their heads backward and stare upward. Loss of stargazin leads to a complete loss of AMPA receptors from cerebellar granule cells, which results in cerebellar ataxia and frequent seizures. Other members of the TARP family are similarly required for AMPA receptor trafficking to the surface membrane of other types of neurons.
The proper localization of AMPA receptors by stargazin depends on the interaction between its C-terminus and PSD-95. AMPA receptors also bind to a distinct PDZ domain protein called GRIP, and metabotropic glutamate receptors interact with yet another PDZ domain protein called Homer. In addition to interacting with receptors, proteins with PDZ domains interact with many other cellular proteins, including proteins that bind to the actin cytoskeleton, providing a scaffold around which a complex of postsynaptic proteins is constructed. Indeed, a biochemical analysis of the postsynaptic density has identified dozens of proteins that participate in NMDA or AMPA receptor complexes.