Excitotoxicity and the NMDA receptor

 

Dr. F. X. Sureda

 

 

            Excitotoxicity, a phenomenon that was first described by Olney in the seventies, implies the activation in the CNS of the so-called glutamate receptors.¹ Glutamate, an excitatory amino acid, activates different types of ion channel forming receptors (named ionotropic) and G-protein-coupled receptors (named metabotropic) to develop their essential role inthe functional activity of the brain. However, high concentrations of glutamate, or neurotoxins acting at the same receptors, cause cell death through the excessive activation of these receptors. In physiological conditions, the presence of glutamate in the synapse ishighly regulated byvery active, ATP-dependent transporters in neurones and glia. For instance, in CNS ischaemia a decrease in the levels of glucose exerts a decrease in ATP production, leading to an impairment of glutamate uptake. Moreover, the membrane potential of presynaptic neurones is lost and efflux of excitatory amino acids occurs, contributing to the excessive activation of glutamatepostsynaptic receptors².

 

 

            As pointed out earlier, glutamate and other amino acids can activate both ionotropic and metabotropic receptors (for review, 3). The latter are subdivided in three main families, and can be coupled to phospholipase C (PLC) or to adenylyl cyclase (AC). The ion channel forming receptors are subdivided in threedifferent receptor classes that are named by their selective agonists: AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors, kainate receptors and NMDA (N-methyl-D-aspartic acid) receptors. AMPA and kainate receptors trigger rapid excitatory neurotransmission in the CNS,  by promoting entry of Na⁺ into neurones. However, a subset of neurones in the hippocampus, cortex andthe retina express AMPA receptors that are also permeable to Ca²⁺. NMDA receptors are associated to a high conductance Ca²⁺ channel that in resting, non-depolarising conditions is blocked by Mg²⁺ in a voltage-dependent manner. Their activation is secondary to AMPA or kainate receptor activation that depolarises the neurone, allowing  for the relief of the Mg²⁺ blockade.

 

 

            The physiological role of the NMDA receptor seems related to synaptic plasticity. Also, working together with metabotropic glutamate receptors,  ensure the establishment ofthe long-term potentiationphenomenon (LTP), a process believed to be responsible for the acquisition of information. These functions are mediated by calcium entry through the NMDA receptor-associated channel. Calcium activates a number of Ca²⁺-dependent enzymes that influence a wide variety of cellular components like cytoskeletal proteins or second messenger synthases. However, overactivation at NMDA receptors triggers an excessive entry of Ca²⁺, initiating a series of cytoplasmic and nuclear processes that promote neuronal cell death. For instance, Ca²⁺-activated proteolytic enzymes, like calpains, can degrade essential proteins. Moreover, Ca²⁺/calmodulin kinase II (CaM-KII) is activated, and a number of different enzymes are phosphorilated, increasing their activity. Different transcription factors such as c-Fos, c-Jun or c-Myc are also expressed. Furthermore, Ca²⁺-dependent endonucleases can degrade DNA. Allof these mechanisms, together with enhanced oxidative stress (see below) can induce cell death through necrosis as well as apoptosis, a model of programmed cell death that is described in several neurodegenerative diseases.

 

            Mitochondria plays an important role in the regulation of the intracellular calcium concentration. An increased entry of Ca²⁺ into the mitochondria is believed to enhance the mitochondrial electron transport, increasing the production of reactive oxygen species (ROS) such as ^·O₂^-. Although in the excitotoxicprocessmitochondria is the major source of ROS, there are many enzymatic systems that primarily or secondarily increase the presence of those compounds in the CNS⁴. Calcium-dependent enzymes convert xanthine dehydrogenase to xanthine oxidase, leading to the production of ^·O₂^- and H₂O₂. Moreover, Ca²⁺ activates the enzyme phospholipase A₂ (PLA₂), which leads to the production of arachidonic acid, that in turn, is transformed by cyclooxygenases, increasing the formation of ^·O₂^-. Calcium also activates NO-synthase, increasing the presence of ^·NO in the neurone and also in surrounding areas. ^·NO has a double effect, since activates guanylylcyclases and also reacts with ^·O₂^- to form the highly toxic compound peroxynitrite (ONOO^-). This is a strong oxidizing agent that causes nitration in proteins and oxidation of lipids, proteins and DNA, leading to a form of cell death that has the characteristics of apoptosis. Lipid peroxidation causes a disturbance in the structure of lipidic membranes, and leakage in the cytoplasmic membrane occurs. Apart from the loss of ionic gradients,enhanced release of glutamate from presynaptic terminals take place, worsening the previously mentioned effects.

 

 

            Excitotoxicity has been related to several acute neurological disorders, such as epileptic convulsions, where overactivity of excitatory synapses exists. In ischaemic stroke and in post-traumatic lesions, the implication of excitotoxicity is well established. As mentioned earlier, in these particular pathologic situations a decrease in ATP production evokes glutamate release through depolarisation of presynaptic terminals. In neurodegenerative disorders like Parkinson or Alzheimer’s diseases, Huntington’s chorea or in amyotrophic lateral sclerosis (ALS), a role for excitotoxicityin the pathogenesis of these diseases has also been postulated. Moreover, drugs that block NMDA or other glutamate receptors, as well as compounds that decrease glutamate release, attenuate some of the pathological manifestations in experimental models of acute and chronic neurodegenerative diseases.

 

 

            Due to the relevance of the neurodegenerative diseasesalready mentioned and the lack ofexisting, effective treatments, the  research in the field of NMDA antagonists in the last decade has been extremely active. However, glutamate plays a very important role in the CNS, and several clinical trials have been abandoned due to psychomimetic or cardiovascular side-effects. Although the search for agents active at NMDA receptors is still on, other strategies like glutamate release inhibitors or non-NMDA receptor antagonists are leading the research oneld neuroprotective drugs⁵.

 

 

¹ Olney JW., Sharpe LG., Feigin RD. J. Neuropathol. Exp. Neurol., 31:464-88, 1972.

² Dirnagl U., Iadecola C., Moskowitz MA. Trends Neurosci., 22:391-397, 1999.

³ Michaelis EK. Prog. Neurobiol., 54:369-415, 1998.

⁴ Greene JG.,  Greenamyre JT. Prog. Neurobiol., 48:613-634, 1996.

⁵ Baudy RB. Exp. Opin. Ther. Patents 6:983-1033, 1996.