Excitatory amino acid receptors: glutamate receptor
It has long been recognized that glutamic and aspartic acids occur in uniquely high concentrations in the mammalian brain and that they can cause excitation of nerve cells. However, these amino acids have only recently been identified as excitatory neurotransmitters because of the difficulty that arose in dissociating their transmitter from their metabolic role in the brain. For example, glutamate is an important component of brain proteins, peptides and a precursor of GABA. As a result of microdialysis and micro-iontophoretic techniques, in which the release and effect of local application could be demonstrated, and the synthesis and isolation of specific agonists for the different types of excitatory amino acid receptor (e.g. quisqualic, ibotenic and kainic acids), it is now generally accepted that glutamic and aspartic acids are excitatory transmitters in the mammalian brain. Glutamate is uniformly distributed throughout the mammalian brain. Unlike the biogenic amine transmitters, glutamate has an important metabolic role as well as a neurotransmitter role in the brain being linked to the synthesis of GABA, where it acts as a precursor, and to the tricarboxylic acid cycle where it is metabolized to alpha-ketoglutaric acid. In nerve terminals, glutamate is stored in vesicles and released by calciumdependent exocytosis. Specific glutamate transporters remove the amino acid from the synaptic cleft into both the nerve terminals and the surrounding glial cells. Four main types of glutamate receptor have been identified and cloned. These are the ionotropic receptors (NMDA and alpha-amino-3-hydroxy-5- methylisoxazole, AMPA, and kainate types) and a group of metabotropic receptors of which eight types have been discovered. The AMPA and kainate receptors are involved in fast excitatory transmission whereas the NMDA receptors mediate slower excitatory responses and play a more complex role in mediating synaptic plasticity. The ionotropic receptors have a pentameric structure. The most important of these, the NMDA receptors, are assembled from twosub-units, NR1 and NR2, each of which can exist in different isoforms thereby giving rise to structurally different glutamate receptors in the brain. The functional significance of these different receptor types is presently unclear. The sub-units comprising the AMPA and kainate receptors, termed GluR1–7 and KA1,2 are closely related. The NMDA receptors are unique among the ligand-gated cation channel receptors in that they are permeable to calcium but blocked by magnesium, the latter acting at a specific receptor site within the ion channel. The purpose of the voltage-dependent magnesium blockade of the ion channel is to permit the summation of excitatory postsynaptic potentials. Once these have reached a critical point, the magnesium blockade of the ion channel is terminated and calcium flows into the neuron to activate the calcium-dependent second messengers. Such a mechanism would appear to be particularly important for the induction of long-term potentiation, a process which underlies short-term memory formation in the hippocampus.With regard to the action of psychotropic drugs on the NMDA receptors, there is evidence that one of the actions of the anticonvulsant lamotrigine isto modulate glutamatergic function; the antidementia drug memantine also has similar action. Thus the therapeutic efficacy of some of the newer drugs used to treat epilepsy and Alzheimer’s disease owe their efficacy to their ability to modulate a dysfunctional glutamatergic system. Some of the hallucinogens related to the dissociation anaesthetic ketamine, such as phencyclidine, block the ion channel of the NMDA receptor. Whether the hallucinogenic actions of phencyclidine are primarily due to this action is uncertain as the putative anticonvulsant dizocilpine (MK 801) is also an NMDAion channel inhibitor but is not a notable hallucinogen. Presumably the ability of phencyclidine to enhance dopamine release, possibly by activating NMDA heteroceptors on dopaminergic terminals, and also its action on sigma receptors which it shares with benzomorphan – like hallucinogens – contribute to its hallucinogenic activity. In contrast to the ionotropic receptors, the metabotropic receptors are monomeric in structure and unique in that they show no structural similarity to the other G-protein-coupled neurotransmitter receptors. They are located both pre- and postsynaptically and there is experimental evidence that they are involved in synpatic modulation and excitotoxicity, functions which are also shared with the NMDA receptors. To date, no drugs have been developed for therapeutic use which are based on the modulation of these receptors. The NMDA receptor complex has been extensively characterized and its anatomical distribution in the brain determined. The NMDA receptor is analogous to the GABA-A receptor in that it contains several binding sites, in addition to the glutamate site, whereby the movement of sodium and calcium ions into the nerve cell can be modulated. These sites include a regulatory site that binds glycine, a site which is insensitive to the antagonistic effects of strychnine. This contrasts with the action of glycine on glycine receptors in the spinal cord where strychnine, on blocking the receptor, causes the characteristic tonic seizures. In addition to the glutamate and glycine sites on the NMDA receptor, there also exist polyamine sites which are activated by the naturally occurring polyamines spermine and spermidine. Specific divalent cation sites are also associated with the NMDA receptor, namely the voltagedependent magnesium site and the inhibitory zinc site. In addition to the excitatory amino acids, the natural metabolite of brain tryptophan, quinolinic acid, can also act as an agonist of the NMDA receptor and may contribute to nerve cell death at high concentrations. Interest in the therapeutic potential of drugs acting on the NMDA receptor has risen with the discovery that epilepsy and related convulsive states may occur as a consequence of a sudden release of glutamate. Sustained seizures of the limbic system in animals result in brain damage that resembles the changes seen in glutamate toxicity. Similar changes areseen at autopsy in patients with intractable epilepsy. It has been shown that the non-competitive NMDA antagonists such as phencyclidine or ketamine can block glutamate-induced damage. The novel antiepileptic drug lamotrigine would also appear to act by this mechanism, in addition to its ability to block sodium channels, in common with many other types of antiepileptic drugs. In addition to epilepsy, neuronal death due to the toxic effects of glutamate has also been implicated in cerebral ischaemia associated with multi-infarct dementia and possibly Alzheimer’s disease. With the plethora of selective excitatory amino acid receptor antagonists currently undergoing development, some of which are already in clinical trials, one may expect definite advances in the drug treatment of neurodegenerative disorders in the near future. Nitric oxide – an important gaseous neurotransmitter The discovery that mammalian cells generate nitric oxide (NO), a gas until recently considered to be an atmospheric pollutant, is providing new insights into a number of regulatory processes in the nervous system. There is evidence that NO is synthesized in the vascular epithelium where it is responsible for regulating the vascular tone of the blood vessels. When released from neurons in the brain, NO acts as a novel transmitter one of whose functions is in memory formation. In the periphery, the nonadrenergic non-cholinergic nerves synthesize and release NO which is responsible for neurogenic vasodilatation and the regulation of various gastrointestinal, respiratory and genitourinary tract functions. In addition, NO is also involved in platelet aggregation. These numerous actions of NO are attributed to its direct stimulatory action on soluble guanylate cyclase, thereby enabling it to act as a modulator of conventional neurotransmitters. In all tissues, NO is synthesized by the action of nitric oxide synthase on the amino acid arginine. In the brain, nitric oxide synthase activity has been detected in all brain regions, the highest activity being located in the cerebellum. One of the main physiological roles of NO is in memory formation. There is evidence that in the hippocampus NO is released from postsynaptic sites to act on presynaptic neurons as a retrograde transmitter to release glutamate. This leads to a stable increase in synaptic transmission and forms the basis of long-term potentiation and the initiation of memory formation. Inhibition of nitric oxide synthase activity has been shown experimentally to impair memory formation. Other roles for NO include the development of the cortex and in vision where it assists in the transduction of light signals in the retinal photoreceptor cells. Other roles include feeding behaviour, nociception and olfaction. Recent evidencesuggests that the microglia cells in the brain, which form part of the monocyte/macrophage system, express an inducible form of nitric oxide synthase. Overactivity of these cells has been implicated in the pathogenesis of a number of neurological diseases such as multiple sclerosis, Alzheimer’s disease and Parkinson’s disease. Presumably drugs will be developed in the near future to counteract the degenerative effects caused by NO. It is also of interest to note that carbon monoxide also acts as a gaseous transmitter in the brain but its function is uncertain .
Biochemical pathways leading to the synthesis and metabolism of the major neurotransmitters in the mammalian brain No attempt will be made to give an overview of the main pathways of the several dozen neurotransmitters, neuromodulators and co-transmitters which are possibly involved in the aetiology of mental illness. Instead a summary is given of the relevant pathways involved in the synthesis and metabolism of those transmitters which have conventionally been considered to be involved in the major psychiatric and neurological diseases and through which the psychotropic drugs used in the treatment of such diseases are believed to operate.Acetylcholine Acetylcholine has been implicated in learning and memory in all mammals, and the gross deficits in memory found in patients suffering from Alzheimer’s disease have been ascribed to a defect in central cholinergic transmission. This transmitter has also been implicated in the altered mood states found in mania and depression, while many different classes of psychotropic drugs are known to have potent anticholinergic properties which undoubtedly have adverse consequences for brain function. Acetylcholine is synthesized within the nerve terminal from choline (from both dietary and endogenous origins) and acetyl coenzyme A (acetyl CoA) by the enzyme choline acetyltransferase. Acetyl CoA is derived from glucose and other intermediates via the glycolytic pathway and ultimately the pyruvate oxidase system, while choline is selectively transported into the cholinergic nerve terminal by an active transport system. There are believed to be two main transport sites for choline, the high affinity site being dependent on sodium ions and ATP and which is inhibited by membrane depolarization, while the low affinity site operates by a process of passive diffusion and is therefore dependent on the intersynaptic concentration of choline. The uptake of choline by the high affinity site controls the rate of acetylcholine synthesis, while the low affinity site, which occurs predominantly in cell bodies, appears to be important for phospholipid synthesis. As the transport of choline by the active transport site is probably optimal, there seems little value in increasing the dietary intake of the precursor in an attempt to increase acetylcholine synthesis. This could be one of the reasons why feeding choline-rich diets (e.g. lecithin) to patients with Alzheimer’s disease has been shown to be ineffective. As with all the major transmitters, acetylcholine is stored in vesicles within the nerve terminal from which it is released by a calcium-dependent mechanism following the passage of a nerve impulse.It is well established that acetylcholine can be catabolized by both acetylcholinesterase (AChE) and butyrylcholinesterase (BChE); these are also known as ‘‘true’’ and ‘‘pseudo’’ cholinesterase, respectively. Such enzymes may be differentiated by their specificity for different choline esters and by their susceptibility to different antagonists. They also differ in their anatomical distribution, with AChE being associated with nervous tissue while BChE is largely found in non-nervous tissue. In the brain there does not seem to be a good correlation between the distribution of cholinergic terminals and the presence of AChE, choline acetyltransferase having been found to be a better marker of such terminals. An assessment of cholinesterase activity can be made by examining red blood cells, which contain only AChE, and plasma,which contains only BChE. Of the anticholinesterases, the organophosphorus derivatives such as diisopropylfluorophosphonate are specific for BChE, while drugs such as ambenonium inhibit AChE.
Most cholinesterase inhibitors inhibit the enzyme by acylating the esteratic site on the enzyme surface. Physostigmine and neostigmine are examples ofreversible anticholinesterases which are in clinical use. Both act in similar ways but they differ in terms of their lipophilicity, the former being able to penetrate the blood–brain barrier while the latter cannot. The main clinical use of these drugs is in the treatment of glaucoma and myasthenia gravis. Irreversible anticholinesterases include the organophosphorus inhibitors and ambenonium, which irreversibly phosphorylate the esteratic site. Such drugs have few clinical uses but have been developed as insecticides and nerve gases. Besides blocking the muscarinic receptors with atropine sulphate in an attempt to reduce the toxic effects that result from an accumulation of acetylcholine, the only specific treatment for organophosphate poisoning would appear to be the administration of 2-pyridine aldoxime methiodide, which increases the rate of dissociation of the organophosphate from the esteratic site on the enzyme surface.
It has long been recognized that glutamic and aspartic acids occur in uniquely high concentrations in the mammalian brain and that they can cause excitation of nerve cells. However, these amino acids have only recently been identified as excitatory neurotransmitters because of the difficulty that arose in dissociating their transmitter from their metabolic role in the brain. For example, glutamate is an important component of brain proteins, peptides and a precursor of GABA. As a result of microdialysis and micro-iontophoretic techniques, in which the release and effect of local application could be demonstrated, and the synthesis and isolation of specific agonists for the different types of excitatory amino acid receptor (e.g. quisqualic, ibotenic and kainic acids), it is now generally accepted that glutamic and aspartic acids are excitatory transmitters in the mammalian brain. Glutamate is uniformly distributed throughout the mammalian brain. Unlike the biogenic amine transmitters, glutamate has an important metabolic role as well as a neurotransmitter role in the brain being linked to the synthesis of GABA, where it acts as a precursor, and to the tricarboxylic acid cycle where it is metabolized to alpha-ketoglutaric acid. In nerve terminals, glutamate is stored in vesicles and released by calciumdependent exocytosis. Specific glutamate transporters remove the amino acid from the synaptic cleft into both the nerve terminals and the surrounding glial cells. Four main types of glutamate receptor have been identified and cloned. These are the ionotropic receptors (NMDA and alpha-amino-3-hydroxy-5- methylisoxazole, AMPA, and kainate types) and a group of metabotropic receptors of which eight types have been discovered. The AMPA and kainate receptors are involved in fast excitatory transmission whereas the NMDA receptors mediate slower excitatory responses and play a more complex role in mediating synaptic plasticity. The ionotropic receptors have a pentameric structure. The most important of these, the NMDA receptors, are assembled from twosub-units, NR1 and NR2, each of which can exist in different isoforms thereby giving rise to structurally different glutamate receptors in the brain. The functional significance of these different receptor types is presently unclear. The sub-units comprising the AMPA and kainate receptors, termed GluR1–7 and KA1,2 are closely related. The NMDA receptors are unique among the ligand-gated cation channel receptors in that they are permeable to calcium but blocked by magnesium, the latter acting at a specific receptor site within the ion channel. The purpose of the voltage-dependent magnesium blockade of the ion channel is to permit the summation of excitatory postsynaptic potentials. Once these have reached a critical point, the magnesium blockade of the ion channel is terminated and calcium flows into the neuron to activate the calcium-dependent second messengers. Such a mechanism would appear to be particularly important for the induction of long-term potentiation, a process which underlies short-term memory formation in the hippocampus.With regard to the action of psychotropic drugs on the NMDA receptors, there is evidence that one of the actions of the anticonvulsant lamotrigine isto modulate glutamatergic function; the antidementia drug memantine also has similar action. Thus the therapeutic efficacy of some of the newer drugs used to treat epilepsy and Alzheimer’s disease owe their efficacy to their ability to modulate a dysfunctional glutamatergic system. Some of the hallucinogens related to the dissociation anaesthetic ketamine, such as phencyclidine, block the ion channel of the NMDA receptor. Whether the hallucinogenic actions of phencyclidine are primarily due to this action is uncertain as the putative anticonvulsant dizocilpine (MK 801) is also an NMDAion channel inhibitor but is not a notable hallucinogen. Presumably the ability of phencyclidine to enhance dopamine release, possibly by activating NMDA heteroceptors on dopaminergic terminals, and also its action on sigma receptors which it shares with benzomorphan – like hallucinogens – contribute to its hallucinogenic activity. In contrast to the ionotropic receptors, the metabotropic receptors are monomeric in structure and unique in that they show no structural similarity to the other G-protein-coupled neurotransmitter receptors. They are located both pre- and postsynaptically and there is experimental evidence that they are involved in synpatic modulation and excitotoxicity, functions which are also shared with the NMDA receptors. To date, no drugs have been developed for therapeutic use which are based on the modulation of these receptors. The NMDA receptor complex has been extensively characterized and its anatomical distribution in the brain determined. The NMDA receptor is analogous to the GABA-A receptor in that it contains several binding sites, in addition to the glutamate site, whereby the movement of sodium and calcium ions into the nerve cell can be modulated. These sites include a regulatory site that binds glycine, a site which is insensitive to the antagonistic effects of strychnine. This contrasts with the action of glycine on glycine receptors in the spinal cord where strychnine, on blocking the receptor, causes the characteristic tonic seizures. In addition to the glutamate and glycine sites on the NMDA receptor, there also exist polyamine sites which are activated by the naturally occurring polyamines spermine and spermidine. Specific divalent cation sites are also associated with the NMDA receptor, namely the voltagedependent magnesium site and the inhibitory zinc site. In addition to the excitatory amino acids, the natural metabolite of brain tryptophan, quinolinic acid, can also act as an agonist of the NMDA receptor and may contribute to nerve cell death at high concentrations. Interest in the therapeutic potential of drugs acting on the NMDA receptor has risen with the discovery that epilepsy and related convulsive states may occur as a consequence of a sudden release of glutamate. Sustained seizures of the limbic system in animals result in brain damage that resembles the changes seen in glutamate toxicity. Similar changes areseen at autopsy in patients with intractable epilepsy. It has been shown that the non-competitive NMDA antagonists such as phencyclidine or ketamine can block glutamate-induced damage. The novel antiepileptic drug lamotrigine would also appear to act by this mechanism, in addition to its ability to block sodium channels, in common with many other types of antiepileptic drugs. In addition to epilepsy, neuronal death due to the toxic effects of glutamate has also been implicated in cerebral ischaemia associated with multi-infarct dementia and possibly Alzheimer’s disease. With the plethora of selective excitatory amino acid receptor antagonists currently undergoing development, some of which are already in clinical trials, one may expect definite advances in the drug treatment of neurodegenerative disorders in the near future. Nitric oxide – an important gaseous neurotransmitter The discovery that mammalian cells generate nitric oxide (NO), a gas until recently considered to be an atmospheric pollutant, is providing new insights into a number of regulatory processes in the nervous system. There is evidence that NO is synthesized in the vascular epithelium where it is responsible for regulating the vascular tone of the blood vessels. When released from neurons in the brain, NO acts as a novel transmitter one of whose functions is in memory formation. In the periphery, the nonadrenergic non-cholinergic nerves synthesize and release NO which is responsible for neurogenic vasodilatation and the regulation of various gastrointestinal, respiratory and genitourinary tract functions. In addition, NO is also involved in platelet aggregation. These numerous actions of NO are attributed to its direct stimulatory action on soluble guanylate cyclase, thereby enabling it to act as a modulator of conventional neurotransmitters. In all tissues, NO is synthesized by the action of nitric oxide synthase on the amino acid arginine. In the brain, nitric oxide synthase activity has been detected in all brain regions, the highest activity being located in the cerebellum. One of the main physiological roles of NO is in memory formation. There is evidence that in the hippocampus NO is released from postsynaptic sites to act on presynaptic neurons as a retrograde transmitter to release glutamate. This leads to a stable increase in synaptic transmission and forms the basis of long-term potentiation and the initiation of memory formation. Inhibition of nitric oxide synthase activity has been shown experimentally to impair memory formation. Other roles for NO include the development of the cortex and in vision where it assists in the transduction of light signals in the retinal photoreceptor cells. Other roles include feeding behaviour, nociception and olfaction. Recent evidencesuggests that the microglia cells in the brain, which form part of the monocyte/macrophage system, express an inducible form of nitric oxide synthase. Overactivity of these cells has been implicated in the pathogenesis of a number of neurological diseases such as multiple sclerosis, Alzheimer’s disease and Parkinson’s disease. Presumably drugs will be developed in the near future to counteract the degenerative effects caused by NO. It is also of interest to note that carbon monoxide also acts as a gaseous transmitter in the brain but its function is uncertain .
Biochemical pathways leading to the synthesis and metabolism of the major neurotransmitters in the mammalian brain No attempt will be made to give an overview of the main pathways of the several dozen neurotransmitters, neuromodulators and co-transmitters which are possibly involved in the aetiology of mental illness. Instead a summary is given of the relevant pathways involved in the synthesis and metabolism of those transmitters which have conventionally been considered to be involved in the major psychiatric and neurological diseases and through which the psychotropic drugs used in the treatment of such diseases are believed to operate.Acetylcholine Acetylcholine has been implicated in learning and memory in all mammals, and the gross deficits in memory found in patients suffering from Alzheimer’s disease have been ascribed to a defect in central cholinergic transmission. This transmitter has also been implicated in the altered mood states found in mania and depression, while many different classes of psychotropic drugs are known to have potent anticholinergic properties which undoubtedly have adverse consequences for brain function. Acetylcholine is synthesized within the nerve terminal from choline (from both dietary and endogenous origins) and acetyl coenzyme A (acetyl CoA) by the enzyme choline acetyltransferase. Acetyl CoA is derived from glucose and other intermediates via the glycolytic pathway and ultimately the pyruvate oxidase system, while choline is selectively transported into the cholinergic nerve terminal by an active transport system. There are believed to be two main transport sites for choline, the high affinity site being dependent on sodium ions and ATP and which is inhibited by membrane depolarization, while the low affinity site operates by a process of passive diffusion and is therefore dependent on the intersynaptic concentration of choline. The uptake of choline by the high affinity site controls the rate of acetylcholine synthesis, while the low affinity site, which occurs predominantly in cell bodies, appears to be important for phospholipid synthesis. As the transport of choline by the active transport site is probably optimal, there seems little value in increasing the dietary intake of the precursor in an attempt to increase acetylcholine synthesis. This could be one of the reasons why feeding choline-rich diets (e.g. lecithin) to patients with Alzheimer’s disease has been shown to be ineffective. As with all the major transmitters, acetylcholine is stored in vesicles within the nerve terminal from which it is released by a calcium-dependent mechanism following the passage of a nerve impulse.It is well established that acetylcholine can be catabolized by both acetylcholinesterase (AChE) and butyrylcholinesterase (BChE); these are also known as ‘‘true’’ and ‘‘pseudo’’ cholinesterase, respectively. Such enzymes may be differentiated by their specificity for different choline esters and by their susceptibility to different antagonists. They also differ in their anatomical distribution, with AChE being associated with nervous tissue while BChE is largely found in non-nervous tissue. In the brain there does not seem to be a good correlation between the distribution of cholinergic terminals and the presence of AChE, choline acetyltransferase having been found to be a better marker of such terminals. An assessment of cholinesterase activity can be made by examining red blood cells, which contain only AChE, and plasma,which contains only BChE. Of the anticholinesterases, the organophosphorus derivatives such as diisopropylfluorophosphonate are specific for BChE, while drugs such as ambenonium inhibit AChE.
Most cholinesterase inhibitors inhibit the enzyme by acylating the esteratic site on the enzyme surface. Physostigmine and neostigmine are examples ofreversible anticholinesterases which are in clinical use. Both act in similar ways but they differ in terms of their lipophilicity, the former being able to penetrate the blood–brain barrier while the latter cannot. The main clinical use of these drugs is in the treatment of glaucoma and myasthenia gravis. Irreversible anticholinesterases include the organophosphorus inhibitors and ambenonium, which irreversibly phosphorylate the esteratic site. Such drugs have few clinical uses but have been developed as insecticides and nerve gases. Besides blocking the muscarinic receptors with atropine sulphate in an attempt to reduce the toxic effects that result from an accumulation of acetylcholine, the only specific treatment for organophosphate poisoning would appear to be the administration of 2-pyridine aldoxime methiodide, which increases the rate of dissociation of the organophosphate from the esteratic site on the enzyme surface.
No comments:
Post a Comment