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A Review of Glutamate Receptors I: Current Understanding of Their Biology

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A Review of Glutamate Receptors I: Current Understanding of Their Biology J Toxicol Pathol 2008; 21: 25–51 Review A Review of Glutamate Receptors I: Current Understanding of Their Biology Colin G. Rousseaux1 1Department of Pathology and Laboratory Medicine, Faculty of Medicine, University of Ottawa, Ottawa, Ontario, Canada Abstract: Seventy years ago it was discovered that glutamate is abundant in the brain and that it plays a central role in brain metabolism. However, it took the scientific community a long time to realize that glutamate also acts as a neurotransmitter. Glutamate is an amino acid and brain tissue contains as much as 5 – 15 mM glutamate per kg depending on the region, which is more than of any other amino acid. The main motivation for the ongoing research on glutamate is due to the role of glutamate in the signal transduction in the nervous systems of apparently all complex living organisms, including man. Glutamate is considered to be the major mediator of excitatory signals in the mammalian central nervous system and is involved in most aspects of normal brain function including cognition, memory and learning. In this review, the basic biology of the excitatory amino acids glutamate, glutamate receptors, GABA, and glycine will first be explored. In the second part of this review, the known pathophysiology and pathology will be described. (J Toxicol Pathol 2008; 21: 25–51) Key words: glutamate, glycine, GABA, glutamate receptors, ionotropic, metabotropic, NMDA, AMPA, review Introduction and Overview glycine), peptides (vasopressin, somatostatin, neurotensin, etc.), and monoamines (norepinephrine, dopamine and In the first decades of the 20th century, research into the serotonin) plus acetylcholine. chemical mediation of the “autonomous” (autonomic) Glutamatergic synaptic transmission in the mammalian nervous system (ANS) was an area that received much central nervous system (CNS) was slowly established over a research activity. In England, William Maddock Bayliss, period of some 20 years, dating from the 1950s2–4. Ernest Henry Starling, Henry Dale and T.R. Elliot were Realization that glutamate and similar excitatory amino investigating the actions and properties of this system acids (EAAs), such as domoic acid, mediated their excitatory following the recent identification of acetylcholine and actions via multiple receptors preceded establishment of adrenaline. In Vienna, Austria, Otto Loewi provided the first these receptors as synaptic transmitter receptors5. EAA reliable evidence for the existence of chemical transmission receptors were initially classified as N-methyl-D-aspartate by acetylcholine in 1921, then other chemical mediators (NMDA) and non-NMDA receptors. between 1929 and 1936. Loewi doubted that Non-NMDA glutamate receptors were subdivided into neurotransmitters operated in the voluntary nervous system, α-amino-3-hydroxy-5-methylisoxazole-4- propionic acid but the discovery that acetylcholine is also a (also known as AMPA receptor, AMPAR, or quisqualate neurotransmitter in the neuromotor synapse, and that receptor) and kainate (Ka) receptors following experiments preganglionic synapses in the ANS are all cholinergic, in with agonists that appeared to activate these receptors contrast with the postganglionic cholinergic or adrenergic preferentially. In addition, their sensitivity to a range of receptors, proved him incorrect. Later it was found that the differentially acting antagonists was understood during the transmitter acting in adrenergic synapses was not adrenalin 1970s. NMDA receptors were definitively shown to be (epinephrine) but noradrenalin (norepinephrine)1. We now synaptic receptors on spinal neurons by the sensitivity of recognize three major categories of substances that act as certain excitatory pathways in the spinal cord to a range of neurotransmitters. These are amino acids (primarily specific NMDA receptor antagonists. However, specific glutamic acid, γ-amino butyric acid, aspartic acid and NMDA receptor antagonists appeared to be less effective at synapses in higher centers. In contrast, antagonists that also blocked non-NMDA as well as NMDA receptors were Received: 10 October 2007, Accepted: 14 November 2007 almost universally effective at blocking synaptic excitation Mailing address: Colin G. Rousseaux, 19 Klondike Rd., Wakefield, QC, J0X 3G0, Canada within the brain and spinal cord, establishing both the TEL: 1 (819) 459-2998 FAX: 1 (819) 459-2960 existence and ubiquity of non-NMDA synaptic receptor 6 E-mail: [email protected] systems throughout the CNS . 26 Glutamate Receptor Biology In the early 1980s, NMDA receptors were shown to be damage and necrosis by causing over excitation of involved in several central synaptic pathways, acting in neurons37–41. Previously it was thought that hypoxia played concert with non-NMDA receptors under conditions where a a part in the neurotoxicity of Glu, particularly in the white protracted excitatory postsynaptic potential was seen in matter; however, it has now been shown that hypoxic injury response to intense stimulation of presynaptic fibers. Such and GluRs over stimulation are independent of one activation of NMDA receptors together with non-NMDA another42. receptors led to the phenomenon of long-term potentiation EAAs access the brain tissue of the circumventricular (LTP)7. LTP is associated with lasting changes in synaptic organs located outside the blood brain barrier (BBB)43–45. efficacy (synaptic plasticity) and has been considered to be Despite the BBB protective mechanisms, the local or an important process in memory8 and learning9. circulating concentrations of these excitatory compounds During the same period, it was shown that certain may be great enough to induce damage leading to neurotoxic glutamate receptors (GluRs) in the brain mediated exposure levels. The toxicity of each EAA compound varies biochemical changes that were not susceptible to NMDA or according to the potency, the chemical availability, the rate non-NMDA receptor antagonists. This dichotomy was of absorption, the affinity to specific receptors and the resolved in the early 1990s using molecular biology, which particular anatomical target site46–48. In addition, the identified two families of glutamate binding receptor susceptibility, genetic predisposition and health status of the proteins: ionotropic (iGluR) and metabotropic (mGluR) individual are also important factors. glutamate receptors10. Development of antagonists binding to specific protein subunits is currently enabling precise Excitotoxicity identification of discrete iGluR or mGluR subtypes11 that The neurotoxic effects of the EAAs are dependent on participate in a range of central synaptic processes, including the species, developmental stage of the animal, type of synaptic plasticity12–14. agonist, duration of exposure to the agonist and the cellular It is the purpose of this review to summarize the expression of the GluR subtypes. Regardless, neurons under biological activities of the glutamate system (Part I) and to various conditions can become so over stimulated by Glu relate changes in these receptors to normal physiology and that it actually kills them through receptor-mediated disease entities (Part II). Efforts have been made to address depolarization and calcium (Ca++) influx49,50. There are five the toxicologic pathology of these receptors wherever main factors necessary for the transition of Glu and Asp from possible; however, although the literature has a number of neurotransmitters to excitotoxins. These include inadequate examples of receptor visualization by neuronal ATP levels, inadequate neuronal levels of immunohistochemistry, the role of these receptors in magnesium (Mg++); high concentrations of inflammatory abnormal morphology is still under intense investigation. prostaglandins; excessive free radical formation51,52, and inadequate removal of synaptic Glu53,54. The Glutamate System An array of GluRs is known to be present on pre- and post-synaptic membranes that are used to transduce A number of authors have discussed the importance of integrated signals using an increased ion flux and second the glutamate system, e.g.,15–18, and its regulation19,20. The messenger pathways21,24,25,33,34,55–57. It is the excessive following discussion not only addresses, glutamate, GABA, activation of these receptors that leads to neurotoxicity, often and glycine but also details how excitatory amino acids may referred to as “excitotoxicity”58. In addition, it has been interact with the glutamate system. postulated that excitotoxicity is involved in the pathogenesis of many types of acute and chronic insults to the CNS59 and Excitatory amino acids (EAAs) peripheral tissues60,61. Glutamate (Glu) and aspartate (Asp) are amino acids The AAs, Glu and Asp, including structurally similar (AAs) that act as major excitatory neurotransmitters in the compounds, can be dietary excitotoxins62. Domoic acid mammalian central nervous system21–25 by stimulating or (DA) is one of the most potent neurotoxins in seafood that exciting postsynaptic neurons. Although these AAs are can enter our food supply63. Contamination of mussels by primarily involved in intermediary metabolism and other the sea diatoms producing DA was found to be the reason for non-neuronal functions, their most important role is as the outbreak of the lethal shellfish poisoning that occurred in neurotransmitters. It is estimated Glu mediates nearly 50% Canada in 198765–67. The clinical manifestations of DA of all the synaptic transmissions in the CNS and its intoxication are related
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