Glutamate signalling in bone

The identification of novel signalling pathways in a tissue provides new avenues for pharmacological manipulation of tissue function. Where the pathway concerned is one that has been the subject of extensive research in another body system, progress towards new therapies can be rapid. The discovery that glutamate has functions in bone that share striking similarities with its role in synaptic neurotransmission opens the way to manipulate skeletal pathophysiology using modulators of glutamate release, uptake or receptor function. The purpose of this review is to describe the way that a role for glutamate as a signalling molecule in bone was discovered, to summarise the evidence for this role. In addition, it will identify points that are unresolved, to highlight areas where new research could provide significant advances. Furthermore, it will indicate how studies already performed but analysed without consideration of the non-neuronal functions of modulators of glutamate signalling, could contain information of significant value for the advance of therapeutic approaches to bone diseases.     

Novel treatment of excitotoxicity: targeted disruption of intracellular signalling from glutamate receptors

Glutamate signalling plays key physiological roles in excitatory neurotransmission and CNS plasticity, but also mediates excitotoxicity, the process responsible for triggering neurodegeneration through glutamate receptor overactivation. Excitotoxicity is thought to be a key neurotoxic mechanism in neurological disorders, including brain ischemia, CNS trauma and epilepsy. However, treating excitotoxicity using glutamate receptor antagonists has not proven clinically viable, necessitating more sophisticated approaches. Increasing knowledge of the composition of the postsynaptic density at glutamatergic synapses has allowed us to extend our understanding of the molecular mechanisms of excitotoxicity and to dissect out the distinct signalling pathways responsible for excitotoxic damage. Key molecules in these pathways are physically linked to the cytoplasmic face of glutamate receptors by scaffolding proteins that exhibit binding specificity for some receptors over others. This imparts specificity to physiological and pathological glutamatergic signalling. Recently, we have capitalized on this knowledge and, using targeted peptides to selectively disrupt intracellular interactions linked to glutamate receptors, have blocked excitotoxic signalling in neurones. This therapeutic approach circumvents the negative consequences of blocking glutamate receptors, and may be a practical strategy for treating neurological disorders that involve excitotoxicity.     

Excitatory amino acid transporters as emerging targets for central nervous system therapeutics

In the mammalian central nervous system (CNS) the excitatory amino acid transporter (EAAT) family of proteins are responsible for the high-affinity sodium-dependent uptake of glutamate into both astroglial cells and neurones. Normal EAAT function is required both for the efficient termination of glutamatergic neurotransmission and for the maintenance of low extracellular glutamate concentrations, thereby preventing glutamate excitotoxicity. It is widely believed that a dysfunction of glutamate transmission participates in the aetiology of a number of neurodegenerative and neuropsychiatric disorders and diseases. This review introduces the EAATs as a new family of emerging therapeutic targets for CNS disorders by virtue of their central role in maintaining glutamate homeostasis. We examine recent findings on the modulation and regulation of EAATs and review the changes in both EAAT function and expression which have been described in a number of neuropathological conditions.     

Implications of gliotransmission for the pharmacotherapy of CNS disorders

The seminal discovery that glial cells, particularly astrocytes, can release a number of gliotransmitters that serve as signalling molecules for the cross-talk with neighbouring cellular populations has recently changed our perception of brain functioning, as well as our view of the pathogenesis of several disorders of the CNS. Since glutamate was one of the first gliotransmitters to be identified and characterized, we tackle the mechanisms that underlie its release from astrocytes, including the Ca2+ signals underlying its efflux from astroglia, and we discuss the involvement of these events in a number of relevant physiological processes, from the modulatory control of neighbouring synapses to the regulation of blood supply to cerebral tissues. The relevance of these mechanisms strongly indicates that the contribution of glial cells and gliotransmission to the activities of the brain cannot be overlooked, and any study of CNS physiopathology needs to consider glial biology to have a comprehensive overview of brain function and dysfunction. Abnormalites in the signalling that controls the astrocytic release of glutamate are described in several experimental models of neurological disorders, for example, AIDS dementia complex, Alzheimer's disease and cerebral ischaemia. While the modalities of glutamate release from astrocytes remain poorly understood, and this represents a major impediment to the definition of novel therapeutic strategies targeting this process at the molecular level, some key mediators deputed to the control of the glial release of this excitatory amino acid have been identified. Among these, we can mention, for instance, proinflammatory cytokines, such as tumour necrosis factor-α, and prostaglandins. Agents that are able to block the major steps of tumour necrosis factor-α and prostaglandin production and/or signalling can be proposed as novel therapeutic targets for the treatment of these disorders.     

Potential clinical uses of neuroactive steroids

Neuroactive steroids rapidly modulate gamma-aminobutyric acid (GABA) and glutamate receptors. GABA-enhancing steroids have potential clinical utility as anesthetics, hypnotics, anticonvulsants and anxiolytics. Furthermore, GABAergic neurosteroids may participate in regulating mood and the effects of alcohol on the nervous system, suggesting a potential role in major psychiatric disorders. Neuroactive steroids that alter the function of glutamate receptors could be useful for treating neurodegenerative disorders, and as cognitive enhancers. Recent progress in developing water-soluble steroids and steroids with enhanced oral efficacy foster optimism that certain neuroactive steroids will be developed for clinical use.     

Potential clinical uses of neuroactive steroids

Neuroactive steroids rapidly modulate gamma-aminobutyric acid (GABA) and glutamate receptors. GABA-enhancing steroids have potential clinical utility as anesthetics, hypnotics, anticonvulsants and anxiolytics. Furthermore, GABAergic neurosteroids may participate in regulating mood and the effects of alcohol on the nervous system, suggesting a potential role in major psychiatric disorders. Neuroactive steroids that alter the function of glutamate receptors could be useful for treating neurodegenerative disorders, and as cognitive enhancers. Recent progress in developing water-soluble steroids and steroids with enhanced oral efficacy foster optimism that certain neuroactive steroids will be developed for clinical use.     

MAGUK proteins: new targets for pharmacological intervention in the glutamatergic synapse

In the postsynaptic density of excitatory glutamatergic synapses, membrane associated guanylate kinase (MAGUK) proteins, such as Post-Synaptic Density 95 (PSD-95), organize ionotropic glutamate receptors and their associated signalling proteins regulating the strength of synaptic activity. Modifications of MAGUK proteins function in the glutamatergic synapse such as alterations of MAGUK proteins interaction with N-Methyl-D-Aspartate (NMDA) receptors regulatory subunits are common events in several neurodegenerative disorders. Thus, a better knowledge and understanding of MAGUK structure and function as well as of the molecular events regulating MAGUK-mediated interactions in the glutamatergic synapse could lead to the identification of new targets for pharmaceutical intervention for neurodegenerative diseases.     

Allosteric modulation of neurotransmitter transporters at excitatory synapses.

The regulation of glutamate and glycine concentrations within excitatory synapses plays an important role in maintaining a dynamic signalling process between neurones, but the failure to regulate the concentrations of these neurotransmitters has been implicated in the pathogenesis of various neurological disorders. In this review we shall discuss how glutamate and glycine transporters regulate synaptic concentrations of these neurotransmitters and how endogenous allosteric modulators influence transporter function. Whilst glutamate transport inhibitors are unlikely to be of therapeutic value because their potential to cause excitoxicity and cell death, a greater understanding of how endogenous compounds allosterically modulate glutamate transporters may provide alternate drug targets. On the other hand, there are some promising drugs that inhibit glycine transporters, which are being trialled as an alternate treatment for schizophrenia. We shall discuss how the activity of one such compound may be expected to influence excitatory neurotransmission.     

Peripheral and Central Glutamate Dyshomeostasis in Neurodegenerative Disorders

Glutamate’s role as the major excitatory neurotransmitter of the mammalian central nervous system requires that its brain concentrations be kept tightly-controlled. However, in hepatic encephalopathy resulting from liver dysfunction; disruption of central neurotransmission and elevation of brain glutamate levels have been observed. These had been associated with certain neurological changes. While neurological changes resulting from hepatic encephalopathy are believed to be transient, the discovery of alterations in liver enzymes in Alzheimer’s disease and the role of glutamate and glutamate homeostasis in hepatic encephalopathy have piqued interests in the possible role of glutamate, and glutamate homeostasis in neurodegenerative diseases. Here, we discuss the evidence in support of the involvement of peripheral/central glutamate homeostasis in the development of neurodegenerative disorders, as well as, the implications of such interactions in the development of new therapies for neurodegenerative disorders.     

Metabotropic and ionotropic glutamate receptors as neurobiological targets in anxiety and stress-related disorders: focus on pharmacology and preclinical translational models

Anxiety disorders are amongst the most common and disabling of psychiatric illnesses and have severe health and socio-economic implications. Despite the availability of a number of treatment options there is still a strong medical need for novel and improved pharmacological approaches in treating these disorders. New developments at the forefront of preclinical research have begun to identify the therapeutic potential of molecular entities integral to the biological response to adversity, particularly molecules and processes that may pre-determine vulnerability or resilience, and those that may act to switch off or “unlearn” a response to an aversive event. The glutamate system is an interesting target in this respect, especially given the impact anxiety disorders have on neuroplasticity, cognition and affective function. These areas of research demonstrate expanding and improved evidence-based options for treating disorders where stress in various guises plays an important etiological role. The current review will discuss how these pathways are involved in fear circuitry of the brain and compare the strength of therapeutic rationale as well as progress towards pharmacological validation of the glutamate pathway towards the treatment of anxiety disorders, with a particular focus on metabotropic and ionotropic glutamate receptors. Specific reference to their anxiolytic actions and efficacy in translational disease models of posttraumatic stress disorder, obsessive-compulsive disorder, panic disorder and phobia will be made. In addition, the availability of ligands necessary to assist clinical proof of concept studies will be discussed.     

Antagonism of metabotropic glutamate group II receptors in the potential treatment of neurological and neuropsychiatric disorders

The metabotropic glutamate Group II receptors (mGlu2 and mGlu3 receptors) regulate the synaptic availability of glutamate and thus control the broad‐ranging neural transmission of glutamate as well as glutamate‐modulated transmission. The present review focuses on the potential role of Group II mGlu receptor antagonism in neurological and neuropsychiatric disorders. Recent findings have determined that agonists of metabotropic glutamate type 2/3 receptors (mGlu2/3) have antianxiety efficacy. Although it could be assumed that blockade of these receptors might engender anxiogenic responses, new data have indicted that these compounds produce antidepressant‐like, wake‐promoting, and pro‐cognitive effects in rodents. However, there are almost no data available to define the relative importance of mGlu2 versus mGlu3 receptors in these activities. Although there are some hints that antagonism of mGlu2/3 receptors could have additional therapeutic impact, the preponderance of data suggests that agonists of the mGlu2/3 receptors would be more likely to have efficacy in anxiety disorders, positive symptoms of schizophrenia, neurodegenerative disorders, and stroke, pain, and epilepsy. The pharmacology of antagonists of mGlu2/3 receptors suggests that such compounds could have a unique place in the medicinal arsenal for mood disorders as well as disorders of cognition and arousal. Given the activity surrounding the discovery of orally available antagonists for these receptors, there may be an opportunity for clinical investigation of these possibilities in the future. Drug Dev. Res. 67:757–769, 2006. © 2006 Wiley‐Liss, Inc.     

New targets for pharmacological intervention in the glutamatergic synapse

Excitotoxicity is thought to be a major mechanism in many human disease states such as ischemia, trauma, epilepsy and chronic neurodegenerative disorders. Briefly, synaptic overactivity leads to the excessive release of glutamate that activates postsynaptic cell membrane receptors, which upon activation open their associated ion channel pore to produce ion influx. To date, although molecular basis of glutamate toxicity remain uncertain, there is general agreement that N-methyl-d-aspartate (NMDA) subtype of ionotropic glutamate receptors plays a key role in mediating at least some aspects of glutamate neurotoxicity. On this view, research has focused in the discovery of new compounds able to either reduce glutamate release or activation of postsynaptic NMDA receptors. Although NMDA receptor antagonists prevent excitotoxicity in cellular and animal models, these drugs have limited usefulness clinically. Side effects such as psychosis, nausea, vomiting, memory impairment, and neuronal cell death accompany complete NMDA receptor blockade, dramatizing the crucial role of the NMDA receptor in normal neuronal processes. Recently, however, well-tolerated compounds such as memantine has been shown to be able to block excitotoxic cell death in a clinically tolerated manner. Understanding the biochemical properties of the multitude of NMDA receptor subtypes offers the possibility of developing more effective and clinically useful drugs. The increasing knowledge of the structure and function of this postsynaptic NMDA complex may improve the identification of specific molecular targets whose pharmacological or genetic manipulation might lead to innovative therapies for brain disorders.     

Glutamate signalling in non-neuronal tissues

Since the discovery of its role in the CNS, glutamate, together with its involvement in signalling at synapses, has been the subject of a vast amount of research. More recently, it has become clear that glutamate signalling is also functional in non-neuronal tissues and occurs in sites as diverse as bone, pancreas and skin. These findings raise the possibility that glutamate acts as a more widespread 'cytokine' and is able to influence cellular activity in a range of tissue types. The impact of these discoveries is significant because they offer a rapid way to advance the development of therapeutics. Agents developed for use in neuroscience applications might be beneficial in the modulation of pathology peripherally, impacting on conditions such as osteoporosis, diabetes and wound healing.     

Targeting the somatostatin receptor in pituitary and neuroendocrine tumors

Background: Toll-like receptors (TLRs) have been recently recognised as primary receptors in the innate immune system. Apart from initiating a prompt immune response against invading pathogens, TLRs are also considered to be an important link between innate immunity, inflammation and a variety of clinical disorders, including cardiovascular diseases. TLR signalling manipulation with novel drugs could offer important opportunities for cardiovascular disease modification. Objective: To present the latest knowledge supporting the involvement of TLRs in the pathogenesis and progress of cardiovascular diseases and explore the role of TLRs as potential targets for therapeutic intervention in cardiovascular territory. Methods: A review of the literature documenting implication of TLR signalling in cardiovascular disorders. Current progress in TLR-targeting drug development and the potential role of such a treatment strategy in cardiovascular disorders are discussed. Conclusions: A growing body of evidence supports a role for TLRs in cardiovascular disease initiation and progression. Altering TLR signalling with novel drugs could be a beneficial therapeutic strategy for patients with cardiovascular disorders.     

The endocannabinoid system in the basal ganglia and in the mesolimbic reward system: implications for neurological and psychiatric disorders

To date, N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol are the best studied endocannabinoids and are thought to act as retrograde messengers in the central nervous system (CNS). By activating presynaptic cannabinoid CB₁ receptors, they can reduce glutamate release in dorsal and ventral striatum (nucleus accumbens) and alter synaptic plasticity, thereby modulating neurotransmission in the basal ganglia and in the mesolimbic reward system. In this review, we will focus on the role of the endocannabinoid system within these neuronal pathways and describe its effect on dopaminergic transmission and vice versa. The endocannabinoid system is unlikely to directly affect dopamine release, but can modify dopamine transmission trough trans-synaptic mechanisms, involving γ-aminobutyric acid (GABA)-ergic and glutamatergic synapses, as well as by converging signal transduction cascades of the cannabinoid and dopamine receptors. The dopamine and endocannabinoid systems exert a mutual control on each other. Cannabinergic signalling may lead to release of dopamine, which can act via dopamine D₁-like receptors as a negative feedback mechanism to counteract the effects of activation of the cannabinoid CB₁ receptor. On the other hand, dopaminergic signalling via dopamine D₂-like receptors may lead to up-regulation of cannabinergic signalling, which is likely to represent a negative feedback on dopaminergic signalling. The consequences of these interactions become evident in pathological conditions in which one of the two systems is likely to be malfunctioning. We will discuss neurological and psychiatric disorders such as Parkinson's and Huntington's disease, drug addiction and schizophrenia. Furthermore, the possible role of the endocannabinoid system in disorders not necessarily depending on the dopaminergic system, such as eating disorders and anxiety, will be described.     

Mood disorders: regulation by metabotropic glutamate receptors

Medicinal therapies for mood disorders neither fully serve the efficacy needs of patients nor are they free of side-effect issues. Although monoamine-based therapies are the primary current treatment approaches, both preclinical and clinical findings have implicated the excitatory neurotransmitter glutamate in the pathogenesis of major depressive disorders. The present commentary focuses on the metabotropic glutamate receptors and their relationship to mood disorders. Metabotropic glutamate (mGlu) receptors regulate glutamate transmission by altering the release of neurotransmitter and/or modulating the post-synaptic responses to glutamate. Convergent biochemical, pharmacological, behavioral, and clinical data will be reviewed that establish glutamatergic neurotransmission via mGlu receptors as a biologically relevant process in the regulation of mood and that these receptors may serve as novel targets for the discovery of small molecule modulators with unique antidepressant properties. Specifically, compounds that antagonize mGlu2, mGlu3, and/or mGlu5 receptors (e.g. LY341495, MGS0039, MPEP, MTEP) exhibit biochemical effects indicative of antidepressant effects as well as in vivo activity in animal models predictive of antidepressant efficacy. Both preclinical and clinical data have previously been presented to define NMDA and AMPA receptors as important targets for the modulation of major depression. In the present review, we present a model suggesting how the interplay of glutamate at the mGlu and at the ionotropic AMPA and NMDA receptors might account for the antidepressant-like effects of glutamatergic- and monoaminergic-based drugs affecting mood in patients. The current data lead to the hypothesis that mGlu-based compounds and conventional antidepressants impact a network of interactive effects that converge upon a down regulation of NMDA receptor function and an enhancement in AMPA receptor signaling.     

Design of NAALADase inhibitors: a novel neuroprotective strategy

Excessive glutamatergic transmission is thought to be responsible for the injury observed in a variety of neurological disorders such as stroke. N-acetylaspartylglutamate (NAAG), a major peptidic component of the brain, has been suggested to serve as a potential storage form of glutamate. N-acetylated-a-linked acidic dipeptidase (NAALADase, EC 3.4.17.21) is responsible for the hydrolysis of NAAG into N-acetylaspartate (NAA) and glutamate. If NAAG is a storage form of glutamate, then inhibition of NAALADase should be neuroprotective in diseases in which excess glutamatergic transmission is detrimental. In addition, NAAG has been demonstrated to be an agonist at group II metabotropic glutamate receptors and functions as a mixed agonist/antagonist at N-methyl-D-aspartate receptors. Therefore, inhibition of NAALADase would also function to increase NAAG levels which, in turn, should provide neuroprotection via the interaction of NAAG with these receptors. Recently, potent and selective inhibitors of the enzyme have been designed and subsequently used to demonstrate that inhibition of NAALADase is neuroprotective in animal models of neurodegeneration. As such, NAALADase inhibition represents a novel method of regulating extracellular glutamate levels and provides a new avenue for the treatment of neurological disorders.     

Glia mechanisms in mood regulation: a novel model of mood disorders

Introduction Recent evidence in clinical and preclinical studies has implicated glutamate neurotransmissions in pathophysiology of mood disorders. The regulation of amino acid neurotransmission, i.e., glutamate and gamma-aminobutyric acid (GABA) involves coordinated mechanisms of uptake and transport within a tripartite synaptic system that includes neurons and glia. Newly appreciated role of the glia, more specifically astrocytes on neuronal functions combined with reported postmortem abnormalities of glia in patients with mood disorders further supports the role of glia in mood disorders. Materials and methods This report presents some of our preliminary results utilizing glia-selective toxins and other pharmacological tools to suppress glial function within the limbic system to study the resulting behavioral abnormalities, and thus, elucidate glial involvement in the development of mood disorders. Results and discussion We demonstrate that chronic blockade of glutamate uptake by a glial/neuronal transporter antagonist l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) within the amygdala, a key area implicated in mood regulation, results in dose-dependent reduction in social exploratory behavior and disrupts circadian activity patterns consistent with symptoms of mood disorders. Similarly, the selective astrocytic glutamate transporter type 1 (GLT-1) blocker dihydrokainic acid (DHK) injected into the amygdala also results in reduced social interaction that is blocked by selective glutamate N-methyl-d-aspartate (NMDA) type receptor antagonist AP5. The results are discussed in the context of glial and glutamate mechanisms in mood disorders and potential therapeutic avenues to address these mechanisms.     

mGluR5 antagonists: discovery, characterization and drug development

Disturbances of glutamate-mediated neurotransmission have been implicated in a broad range of nervous system disorders. Numerous attempts to correct nervous system dysfunction by pharmacological intervention at glutamate receptors have been made, and some of the approaches have achieved a high level of preclinical validation. However, in a number of cases involving agents acting as blockers of the ionotropic glutamate receptors, clinical success could not be achieved, mostly because of the lack of a therapeutic window. The identification of the metabotropic glutamate receptor (mGluR) family and their modulatory role in the control of neurotransmission provided a new means to alter glutamatergic transmission. Furthermore, selective agents acting as allosteric antagonists at the mGluR5 subtype have demonstrated therapeutic potential. The identification and characterization of mGluR5 antagonists and recent progress in clinical development are summarized.     

Thymoquinone-rich black cumin oil attenuates ibotenic acid-induced excitotoxicity through glutamate receptors in Wistar rats

Inflammation-mediated alterations in glutamate neurotransmission constitute the most important pathway in the pathophysiology of various brain disorders. The excessive signalling of glutamate results in excitotoxicity, neuronal degeneration, and neuronal cell death. In the present study, we investigated the relative efficacy of black cumin (Nigella sativa) oil with high (5 % w/w) and low (2 % w/w) thymoquinone content (BCO-5 and BCO-2, respectively) in alleviating ibotenic acid-induced excitotoxicity and neuroinflammation in Wistar rats. It was found that BCO-5 reversed the abnormal behavioural patterns and the key inflammatory mediators (TNF-α and NF-κB) when treated at 5 mg/kg body weight. Immunohistochemical studies showed the potential of BCO-5 to attenuate the glutamate receptor subunits NMDA and GluR-2 along with increased glutamate decarboxylase levels in the brain tissues. Histopathological studies revealed the neuroprotection of BCO-5 against the inflammatory lesions, as evidenced by the normal cerebellum, astrocytes, and glial cells. BCO-2 on the other hand showed either a poor protective effect or no effect even at a 4-fold higher concentration of 20 mg/kg body weight indicating a very significant role of thymoquinone content on the neuroprotective effect of black cumin oil and its plausible clinical efficacy in counteracting the anxiety and stress-related neurological disorders under conditions such as depression and Alzheimer's disease.