Role of Glutamate

Glutamatergic neurons form the major excitatory system in the brain and play a pivotal role in many physiological functions. Glutamate activates several classes of metabotropic receptors and 3 major types of ionotropic receptor. These later receptors are ligand gated ionic channels permeable to the monovalent cations Na⁺ and K⁺ and, depending on the subtype, also to the divalent cation Ca²⁺. α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are largely impermeable to Ca²⁺ and participate in most forms of fast synaptic transmission.

The NMDA Receptor

In contrast, a subtype called N-methyl-D-aspartate (NMDA) is only activated under certain conditions. This receptor has three cardinal features:

1. High permeability to Ca²⁺ ions
2. Voltage-dependent block by Mg²⁺ ions
3. Slow gating kinetics

To learn more click on the graph elements of interest:

Figure 1: Scheme of the NMDA receptor. NB: permeability to Ca²⁺ and the channel blocking site for Mg²⁺ and memantine.

NMDA Receptor Subunits and Elements

• NMDAR1 Subunit

  NMDA receptors most probably consist of tetrameric, heteromeric subunit assemblies which have different physiological and pharmacological properties and are differentially distributed throughout the CNS. So far, two major subunit families designated NMDA1 and NMDA2 have been cloned. Functional receptors in the mammalian CNS are almost certainly only formed by combination of NMDA1 and NMDA2 subunits which express the glycine and glutamate recognition sites respectively.

  Alternative splicing generates eight isoforms for the NMDA1 subfamily. The variants arise from splicing at three exons; one encodes a 21-amino acid insert in the N-terminal domain (N1), and two encode adjacent sequences of 37 and 38 amino acids in the C-terminal domain (C1 and C2). NMDA1 variants are sometimes denoted by the presence or absence of these three alternatively spliced exons (from N to C1 to C2); NMDA1₁₁₁ has all three exons, NMDA1₀₀₀ has none, and NMDA1₁₀₀ has only the N-terminal exon. The variants from NMDA1₀₀₀ to NMDA1₁₁₁ are alternatively denoted as NMDA1e, c, d, a, g, f, h, and b, respectively, but the more frequent terminology using non-capitalized suffices for the most common splice variants is NMDA1a (NMDA1₀₁₁ or NMDA1A) and NMDA1b (NMDA1₁₀₀ or NR1G). Studies on the roles of these splice variants in homomeric receptors expressed in Xenopus oocytes must be viewed with a little caution as homomeric NMDA1 receptors are probably only functionally expressed due to the presence of an endogenous NMDA2-like protein (XenU1) in these cells.

• NMDAR2 Subunit

  NMDA receptors most probably consist of tetrameric, heteromeric subunit assemblies which have different physiological and pharmacological properties and are differentially distributed throughout the CNS. So far, two major subunit families designated NMDA1 and NMDA2 have been cloned. Functional receptors in the mammalian CNS are almost certainly only formed by combination of NMDA1 and NMDA2 subunits which express the glycine and glutamate recognition sites respectively.

  The NMDA2 subfamily consists of four individual subunits, NMDA2A to NMDA2D. Various heteromeric NMDA receptor channels formed by combinations of NMDA1 and NMDA2 subunits are known to differ in gating properties, magnesium sensitivity and pharmacological profile. The heteromeric assembly of NMDA1 and NMDA2C subunits for instance, has a much lower sensitivity to Mg²⁺ but increased sensitivity to glycine and a very restricted distribution in the brain. In situ hybridization has revealed overlapping but different expression for NMDA2 mRNA e.g. NMDA2A mRNA is distributed ubiquitously like NMDA1 with highest densities occurring in hippocampal regions and NMDA2B is expressed predominantly in forebrain but not in cerebellum where NMDA2C predominates. NMDA receptors cloned from murine CNS have a different terminology to those in the rat: z 1 remains the terminology for the mouse equivalent of NMDA1 and e 1 to e 4 represent NMDA2A to 2D subunits respectively.

• Memantine

  Memantine is a substance developed by the Merz research organization and is successfully used to treat dementia. Clinical data show that memantine provides rapid and enduring improvement in the cognitive, psychological, social and motor impairments of dementia. These symptomatic improvements lead to an increased quality of life of the patients and reduces home care efforts.

  The efficacy of memantine is due to rapid, voltage-dependent interactions with the NMDA-receptor channel. It is well-known that disturbances in brain function are associated with disturbances in glutamatergic neurotransmission and loss of specific glutamate receptors.

  As a NMDA receptor antagonist memantine protects the neuronal system from pathological activation while preserving or even restoring physiological activation.

• Glycine

  Glycine is a co-agonist at NMDA receptors at a strychnine-insensitive recognition site (glycine_B) and its presence at moderate nM concentrations is a prerequisite for channel activation by glutamate or NMDA. Physiological concentrations reduce one form of relatively rapid NMDA receptor desensitization. The time course for this desensitization is somewhat slower than that of NMDA receptor-mediated synaptic potentials and reflects the dissociation of glycine following an agonist-induced decrease in glycine affinity. Recently it has been suggested that D-serine may be more important than glycine as an endogenous co-agonist at NMDA receptors in the telencephalon and developing cerebellum. Dynorphin (1-13) was also recently proposed as an endogenous agonist at the glycine_B site as tevidenced by very pronounced increases in the amplitude of NMDA-activated currents in Xenopus oocytes in the presence of low extracellular glycine concentrations. Such an effect may underlie the well documented spinal toxicity seen with dynorphin peptides after i.t. administration.

  Glycine shows different affinities at NMDA receptor subtypes. The molecular basis for this selectivity is not fully clarified but is probably related to allosteric interactions between NMDA2 subunits and the glycine recognition site which is located on the NMDA1 subunit.It is almost certain that affinity is also directly dependent on isoforms of NMDA1 subunits but this is difficult to test independently as homomeric NMDA1 receptors don’t normally form functional receptors when expressed in mammalian cell lines.

• Trans-Membrane Domain

  NMDA receptors are heteromeric assemblies of probably five subunits. Each subunit has four lipophyllic regions, although only three form trans-membrane spanning domains (TM I, TM II and TM IV). TM II makes a hairpin bend within the membrane and forms the channel pore.

Learning

NMDA receptors are ideally suitable for mediating plastic changes in the brain, such as learning. An example of such plastic changes is long term potentiation (LTP, Fig. 2). LTP can be described by the following sequence of events:

1. A high frequency signal (or convergence of several signals) arrives at the glutamatergic synapse leading to a massive glutamate release.
2. Glutamate binds to both NMDA and AMPA receptor, however only the later is activated initially since positively charged Mg²⁺ blocks the NMDA receptor channel.
3. Continued activation of AMPA receptors leads to a significant influx of Na⁺ ions into the cell which, in turn, leads to a decrease in membrane potential (partial depolarization).
4. This depolarization removes blockade by Mg²⁺ since the relative charge of the neuronal membrane is now much less negative (due to the influx of positively charged Na⁺ ions).
5. At this stage, Ca²⁺ ions can freely enter the cell via the NMDA receptor channel and initiate a number of enzymatic processes that are involved in the fixation of increased synaptic strength (neuronal memory formation). This post synaptic change is manifested as an enhancement of AMPA receptor sensitivity and number.

Figure 2: Scheme of Long term Potentation Induction. NMDA receptors are involved in Long Term Potentiation (LTP) – a neuronal model of memory formation. See text for description. Modified with permission from (Danysz et al., 2000). Copyrights belong to Taylor & Francis Limited.

Excitotoxicity of glutamate

Apart from the physiological role of glutamate, excessive activation of its receptors can also evoke neuronal dysfunction and even damage or death. This cell death ascribed to an excessive activation of glutamate receptors has been termed “excitotoxicity” and seems to occur in acute insults such as stroke and trauma, but also in chronic neurodegenerative diseases such as Alzheimer’s disease (AD).