Glutamate plays an essential role in many physiological functions and is the major excitatory transmitter in the mammalian central nervous system (CNS) (Collingridge and Singer, 1990; Danysz et al., 1995; Collingridge and Bliss, 1995). However, under various conditions neurons can become so sensitive to glutamate that it actually kills them through receptor-mediated depolarization and calcium influx (Obrenovitch and Urenjak, 1997; Parsons et al., 1998). This phenomenon was described for the first time by John Olney and called “excitotoxicity” (Rothman and Olney, 1987). It has been implied that excitotoxicity is involved in many types of acute and chronic insults to the CNS including neurodegenerative disorders (Choi, 1995). Disturbance of glutamate homeostasis probably plays a pivotal role in the execution of pathological changes in many disease states and may be triggered by a wide variety of factors that facilitate the neurotoxic potential of endogenous glutamate such as; increase in glutamate release, malfunctioning of neuronal and glial uptake, energy deficits, neuronal depolarization, changes in glutamate receptor properties or expression patterns, free radical formation, the presence of toxic proteins such a ß-amyloid and tau in Alzheimer’s disease (AD) etc (Danysz et al., 1995; Beal, 1995; Obrenovitch and Urenjak, 1997; Parsons et al., 1998). Such excitotoxic effects can be pronounced during acute events such as ischaemic stroke and trauma or milder but prolonged in chronic neurodegenerative diseases such as Alzheimer’s disease, Parkinson´s disease, Huntington´s disease and amyotrophic lateral sclerosis (ALS) (Starr, 1995; Beal, 1995; Plaitakis et al., 1996; Shaw and Ince, 1997). Glutamatergic dysfunction is also involved in the symptomatology of disorders such schizophrenia, anxiety, depression (Danysz et al., 1995; Parsons et al., 1998) as well as in the development of disorders associated with long term plastic changes in the CNS such as chronic pain, drug tolerance, dependence, addiction, partial complex seizures and tardive dyskinesia (Danysz et al., 1995; Trujillo and Akil, 1995; Dickenson, 1997; Parsons et al., 1998).
Glutamate receptors are divided into metabotropic (coupled to intracellular second messengers modifying IP3 and cAMP concentrations) and ionotropic receptors (directly coupled to an ion channel).
Ionotropic glutamate receptors were originally classified on the basis of three selective, synthetic agonists, quisqualate, kainate and N-methyl-D-aspartate (NMDA). After the discovery of metabotropic receptors it became clear that quisqualate also interacts with them. Since that time quisqualate-sensitive ionotropic receptors are classified by the more selective agonist a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). All ionotropic glutamate receptors can form heteromeric subunit assemblies (composed of different subunits) which have different physiological and pharmacological properties and are differentially distributed throughout the CNS (Mcbain and Mayer, 1994; Hollmann and Heinemann, 1994; Danysz et al., 1995; Parsons et al., 1998; Danysz and Parsons, 1998; Cancela et al., 1999). Both AMPA (Rosenmund et al., 1998) and NMDA receptors (Laube et al., 1998) are probably largely formed from tetrameric, heteromeric assemblies of different subunits (Mansour et al., 2001).
Metabotropic glutamate receptors are divided into three major groups I-III. Group I metabotropic receptors consists of two receptor subtypes mGluR1 which has four splice variants and mGluR5 which has two splice variants. Both are largely postsynaptic and positively coupled to phospholipase C (PLC). PLC promotes the conversion of PIP2 to diacylglycerol (DAG) and IP3. DAG activates membrane-bound PKC which in turn can phosphorylate ionotropic glutamate receptors. IP3 has numerous intracellular effects including stimulation of Ca2+ release from intracellular stores. Group II (mGluR2/3) and group III ( (mGluR4/6/7/8) receptors differ in their sequence homology, but are both coupled to a different effector system i.e. they decrease the activity of adenylate cyclase. They are both largely located on presynaptic neurones and glia.
These receptors are not the subject of this review as it is only very recently that agents with therapeutic potential have become available. However, it seems likely that allosteric modulation of these receptors may provide a valid strategy for the development of new pharmaceuticals in the near future (Gasparini et al., 2002).
Glutamate clearance and, as a consequence, glutamate concentration and diffusion in the extracellular space, is associated with the degree of astrocytic coverage of its neurons (Oliet et al., 2001). Glutamate is eliminated from the synaptic cleft by specific transporters. The genes encoding glutamate transporter proteins have been cloned both from rats and humans (Arriza et al., 1994; Malandro and Kilberg, 1996). The human transporters EAAT1 and EAAT2 (rat equivalents GLAST and GLT1) are found in astroglia and microglia and are widely distributed in the CNS (highest in the cortex). Human EAAT3 (rat EAAC1) is restricted to neurons but is also found outside of the CNS. Human EAAT4 is expressed by cerebellar neurons.
N-Acetyl-aspartyl-glutamate (NAAG) is abundant in the mammalian CNS which has led to the hypothesis that this dipeptide is the storage form of glutamate. The membrane bound metallopeptidase NAALADase (N-acetyl-a-linked-acidic dipeptidase, or glutamate carboxypeptidase II, E.C. 3.4.17.21) is co-localized with NAAG in the CNS and converts NAAG to NAA and glutamate (Blakely et al., 1988). It seems likely that NAAG is also a weak partial agonist at NMDA receptors with low intrinsic activity and an agonist at mGluR3 receptors (Neale et al., 2000). The former effect is however, seen at concentrations beyond therepeutically relevant levels. Inhibition of NAALADase could be useful in numerous CNS disorders associated with disturbances in glutamatergic transmission by decreasing the concentration of glutamate and increasing the concentration of NAAG. Guilford pharmaceuticals have followed this approach with their NAALADase inhibitors such as 2- (phosphonomethyl)-pentanedioic acid (PMPA, GPI 5000) and second generation compounds like GPI 5693 and GPI 16072 (Jackson and Slusher, 2001).
In many regions of the CNS, Ca2+ influx through NMDA receptors can trigger two forms of synaptic plasticity: long-term depression (LTD) and long-term potentiation (LTP) which are believed to resemble some elementary features of memory formation at the neuronal level (Malenka, 1994; Bear and Malenka, 1994; Edwards, 1995; Collingridge and Bliss, 1995; Rison and Stanton, 1995; Baudry, 1996; Jeffery, 1997). The voltage-dependent blockade of NMDA receptors by Mg2+ and their high Ca2+ permeability renders them inherently suited for their role in mediating synaptic plasticity (Herron et al., 1986). NMDA receptor channels are only activated in the presence of a local strong depolarization induced by strong AMPA receptors activation and concurrent GABAergic dis-inhibition via feedback effects of GABA on GABAB autoreceptors. As a result, the Mg2+ blockade of NMDA receptors is transiently fully relieved allowing Ca2+ to flow into the postsynaptic neurone. This Ca2+ influx triggers a cascade of secondary messengers which ultimately activate a number of enzymes such as protein kinase C (PKC), phospholipase A2 (PLA2), phospholipase C (PLC), Ca2+/calmodulin-dependent protein kinase II (CaM kinase II), etc. (Abraham and Tate, 1997) (Grant and Silva, 1994; Lisman, 1994; Pasinelli et al., 1995; Benowitz and Routtenberg, 1997; Lan et al., 2001; Bayer et al., 2001). Consequently, these processes lead to fixation of changes in postsynaptic AMPA receptors such as an increase in their affinity and/or number (Maren et al., 1993; Ambros-Ingerson and Lynch, 1993; Ambros-Ingerson et al., 1993; Benke et al., 1998) but see (Kessler et al., 1991) and, possibly through retrograde signals (arachidonic acid, nitric oxide), modulate presynaptic glutamatergic terminals influencing transmitter release (Lynch, 1989; Odell et al., 1991; Kato et al., 1991; Schaechter and Benowitz, 1993; Kato et al., 1994; Luo and Vallano, 1995).
There is accumulating evidence that LTP and LTD share some common mechanisms, although LTD occurs with increases in postsynaptic Ca2+, that are insufficient to induce LTP (Artola and Singer, 1993; Christie et al., 1994; Cummings et al., 1996; Derrick and Martinez Jr, 1996; Hansel et al., 1996; Kirkwood et al., 1996; Tsumoto et al., 1996; Tsumoto and Yasuda, 1996; Christie et al., 1996; Artola et al., 1996). LTP and LTD have been extensively studied as cellular models of learning and memory. Although hippocampal long-term potentiation and spatial learning are impaired by NMDA receptor blockade see (Jeffery, 1997) learning deficits can be almost completely prevented if rats are pretrained in a different water maze (Bannerman et al., 1995; Saucier and Cain, 1995). NMDA receptors may therefore not be required for encoding the spatial representation of a specific environment but rather in other forms of memory important for learning this task (Morris, 1996). Recent evidence indicates that LTP is not only important for synaptic plasticity in the mature CNS but also in the formation of conducting glutamatergic synapses in the developing mammalian brain (Durand et al., 1996).
One form of hippocampal LTP involves the activation of the NMDA receptors and a rise in postsynaptic Ca2+ in the CA1 region but there is still considerable debate as to the site at which the increase in synaptic strength is expressed e.g. (Stricker et al., 1996; Stricker et al., 1996; Isaac et al., 1996; Isaac et al., 1996). Presynaptic mechanisms should be reflected in a change in release probability. This can be measured at excitatory synapses on cultured hippocampal neurones by analysis of the progressive block of NMDA receptor-mediated synaptic currents by the essentially irreversible open channel blocker dizocilpine ((+)MK-801) (Rosenmund et al., 1993). This technique was used to demonstrate that release probability was not affected after the induction of LTP making a presynaptic mechanism unlikely (Manabe and Nicoll, 1994). Moreover, recent reports indicate that a high proportion of synapses in hippocampal area CA1 transmit with NMDA receptors but not AMPA receptors, making these synapses effectively non-functional at normal resting potentials due to Mg2+ blockade (Liao et al., 1995; Nicoll and Malenka, 1995; Montgomery et al., 2001; Montgomery and Madison, 2002). These silent synapses acquire AMPA-type responses following LTP induction. Furthermore, this form of LTP is accompanied by an increase in the conductance of postsynaptic AMPA receptors (Bibb et al., 2001; Bibb et al., 2001). Taken together, these findings challenge the view that LTP in CA1 involves a presynaptic modification, and suggest instead a simple postsynaptic mechanism for both induction and expression of LTP.