Impact of Pharmacological and Non-Pharmacological Modulators on Dendritic Spines Structure and Functions in Brain

Dendritic spines are small, thin, hair-like protrusions found on the dendritic processes of neurons. They serve as independent compartments providing large amplitudes of Ca²⁺ signals to achieve synaptic plasticity, provide sites for newer synapses, facilitate learning and memory. One of the common and severe complication of neurodegenerative disease is cognitive impairment, which is said to be closely associated with spine pathologies viz., decreased in spine density, spine length, spine volume, spine size etc. Many treatments targeting neurological diseases have shown to improve the spine structure and distribution. However, concise data on the various modulators of dendritic spines are imperative and a need of the hour. Hence, in this review we made an attempt to consolidate the effects of various pharmacological (cholinergic, glutamatergic, GABAergic, serotonergic, adrenergic, and dopaminergic agents) and non-pharmacological modulators (dietary interventions, enriched environment, yoga and meditation) on dendritic spines structure and functions. These data suggest that both the pharmacological and non-pharmacological modulators produced significant improvement in dendritic spine structure and functions and in turn reversing the pathologies underlying neurodegeneration. Intriguingly, the non-pharmacological approaches have shown to improve intellectual performances both in preclinical and clinical platforms, but still more technology-based evidence needs to be studied. Thus, we conclude that a combination of pharmacological and non-pharmacological intervention may restore cognitive performance synergistically via improving dendritic spine number and functions in various neurological disorders.

Therapeutic Modulation of Glutamate Receptors in Major Depressive Disorder

Current pharmacotherapies for major depressive disorder (MDD) have a distinct lag of onset that can prolong distress and impairment for patients, and realworld effectiveness trials further suggest that antidepressant efficacy is limited in many patients. All currently approved antidepressant medications for MDD act primarily through monoaminergic mechanisms, e.g., receptor/reuptake agonists or antagonists with varying affinities for serotonin, norepinephrine, or dopamine. Glutamate is the major excitatory neurotransmitter in the central nervous system, and glutamate and its cognate receptors are implicated in the pathophysiology of MDD, as well as in the development of novel therapeutics for this disorder. Since the rapid and robust antidepressant effects of the N-methyl-D-aspartate (NMDA) antagonist ketamine were first observed in 2000, other NMDA receptor antagonists have been studied in MDD. These have been associated with relatively modest antidepressant effects compared to ketamine, but some have shown more favorable characteristics with increased potential in clinical practice (for instance, oral administration, decreased dissociative and/or psychotomimetic effects, and reduced abuse/diversion liability). This article reviews the clinical evidence supporting the use of glutamate receptor modulators with direct affinity for cognate receptors: 1) non-competitive NMDA receptor antagonists (ketamine, memantine, dextromethorphan, AZD6765); 2) subunit (NR2B)-specific NMDA receptor antagonists (CP- 101,606/traxoprodil, MK-0657); 3) NMDA receptor glycine-site partial agonists (D-cycloserine, GLYX- 13); and 4) metabotropic glutamate receptor (mGluR) modulators (AZD2066, RO4917523/basimglurant). Several other theoretical glutamate receptor targets with preclinical antidepressant-like efficacy, but that have yet to be studied clinically, are also briefly discussed; these include α-amino-3-hydroxyl-5-methyl-4- isoxazoleproprionic acid (AMPA) agonists, mGluR2/3 negative allosteric modulators, and mGluR7 agonists.

Glutamate-Modulating Drugs as a Potential Therapeutic Strategy in Obsessive-Compulsive Disorder

OBJECTIVE

Obsessive-compulsive disorder (OCD) is a mental disease commonly associated with severe distress and impairment of social functioning. Serotonin reuptake inhibitors and/or cognitive behavioural therapy are the therapy of choice, however up to 40% of patients do not respond to treatment. Glutamatergic signalling has also been implicated in OCD. The aim of the current study was to review the clinical evidence for therapeutic utility of glutamate-modulating drugs as an augmentation or monotherapy in OCD patients.

METHODS

We conducted a search of the MEDLINE database for clinical studies evaluating the effect of glutamate-modulating drugs in OCD.

RESULTS

Memantine is the compound most consistently showing a positive effect as an augmentation therapy in OCD. Anti-convulsant drugs (lamotrigine, topiramate) and riluzole may also provide therapeutic benefit to some OCD patients. Finally, ketamine may be of interest due to its potential for a rapid onset of action.

CONCLUSION

Further randomized placebo-controlled trials in larger study populations are necessary in order to draw definitive conclusions on the utility of glutamate-modulating drugs in OCD. Furthermore, genetic and epigenetic factors, clinical symptoms and subtypes predicting treatment response to glutamate-modulating drugs need to be investigated systematically.

Glutamate and its receptors in the pathophysiology and treatment of major depressive disorder

Monoaminergic neurotransmitter (serotonin, norepinephrine and dopamine) mechanisms of disease dominated the research landscape in the pathophysiology and treatment of major depressive disorder (MDD) for more than 50 years and still dominate available treatment options. However, the sum of all brain neurons that use monoamines as their primary neurotransmitter is <20%. In addition, most patients treated with monoaminergic antidepressants are left with significant residual symptoms and psychosocial disability not to mention side effects, e.g., sexual dysfunction. In the past several decades, there has been greater focus on the major excitatory neurotransmitter in the human brain, glutamate, in the pathophysiology and treatment of MDD. Although several preclinical and human magnetic resonance spectroscopy studies had already implicated glutamatergic abnormalities in the human brain, it was rocketed by the discovery that the N-methyl-D-aspartate receptor antagonist ketamine has rapid and potent antidepressant effects in even the most treatment-resistant MDD patients, including those who failed to respond to electroconvulsive therapy and who have active suicidal ideation. In this review, we will first provide a brief introduction to glutamate and its receptors in the mammalian brain. We will then review the clinical evidence for glutamatergic dysfunction in MDD, the discovery and progress-to-date with ketamine as a rapidly acting antidepressant, and other glutamate receptor modulators (including proprietary medications) for treatment-resistant depression. We will finally conclude by offering potential future directions necessary to realize the enormous therapeutic promise of glutamatergic antidepressants.

mGlu2/3 and mGlu5 receptors: potential targets for novel antidepressants

Major depressive disorder is among the most prevalent forms of mental illness. All currently available antidepressant medications have stemmed from study of the mechanisms of serendipitously discovered drugs, and only 30-50% of patients exhibit remission and frequently at least 3-4 weeks are required for manifestation of significant therapeutic effects. To overcome these drawbacks, discovering novel neuronal mechanisms of pathophysiology of depression as well as more effective treatments are necessary. This review focuses on the metabotropic glutamate (mGlu) receptors and their potential for drug targets for the treatment of depression. In particular, accumulating evidence has indicated the potential importance and usefulness of agents acting on mGlu2/3 and mGlu5 receptors. Preclinical and clinical evidence of mGlu2/3 receptor ligands and mGlu5 receptor antagonists are described. Moreover, their potential in clinic will be discussed in the context of neuronal mechanisms of ketamine, an agent recently demonstrated a robust effect for patients with treatment-resistant depression. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

[Glutamatergic conception of mood disorders]

The neurotransmitter theories of the pathogenesis and treatment of mood disorders presented so far focused mainly on serotonergic and catecholaminergic (noradrenergic and dopaminergic) transmission. In recent decades the attention has been also directed towards a role in the pathogenesis of mood disorders of stress axis dysregulation and of neuroplasticity disturbances, which involve the glutamatergic system. In this paper, a role of glutamatergic neurotransmission in the pathogenesis and treatment of unipolar and bipolar affective illness is presented. In the first part, a role of glutamatergic neurotransmission in the central nervous system is presented, with special focus on glutamatergic receptors. The results of molecular-genetic studies pointing to an association between glutamatergic system genes and a predisposition to unipolar and bipolar affective illness are discussed. In the next part, a summary of neurobiological, including neuroimaging findings as to changes in glutamatergic system in mood disorders is done. Following this, a role of glutamatergic system in the mechanism of action of antidepressant and mood-stabilizing drugs as well as therapeutic activity of drugs modifying glutamatergic system is reviewed. Glutamatergic conception of mood disorders allows for a new look on the pathogenesis and treatment of unipolar and bipolar affective illness.

Cognitive enhancers in the treatment of substance use disorders: clinical evidence

Attenuation of drug reward has been the major focus of medication development in the addiction area to date. With the growth of research in the area of cognitive neuroscience, the importance of executive function and inhibitory cognitive control in addictive disorders is becoming increasingly apparent. An emerging strategy in the pharmacotherapy of addictions and other psychiatric disorders involves the use of medications that improve cognitive function. In particular, agents that facilitate inhibitory and attentional control, improve abstraction, planning and mental flexibility could be beneficial in the treatment of substance use disorders. Because there are multiple neurotransmitter systems involved in the regulation of cognitive function, agents from a number of drug classes have been tested. In particular, agents acting through the cholinergic, adrenergic and glutamatergic systems have shown potential for improving cognitive function in a number of psychiatric and neurologic disorders, but most of these agents have not been tested in the treatment of individuals with substance use disorders. This manuscript provides a review of clinical data supporting the use of the major classes of cognitive enhancing agents in substance use disorders. Agents that have shown promise in cognitive enhancement in other disorders, and have a theoretical or mechanistic rationale for application to substance use disorders are also highlighted.

Pharmacological treatment of adolescent pathological gambling

Adolescents as a group appear to constitute a high-risk population for gambling problems. Given the rates of adolescent problem gambling and its impact on affected individuals and their families, effective treatments are important. There are, however, no pharmacological treatments for pathological gambling in children, adolescents, or adults that are currently approved by the United States Food and Drug Administration (FDA). Additionally, no studies have investigated directly the safety and efficacy of pharmacological treatments for pathological gambling in adolescents. This article reviews the literature on effective treatments in adults with pathological gambling, describes the safety data for the use of these drugs in adolescents, and provides a rationale for future studies to investigate the efficacy and tolerability of pharmacotherapies for pathological gambling in adolescents.

[Role of zonisamid in treating epilepsy, Parkinson disorders and other neurological diseases]

On the basis of six randomized controlled trials, zonisamide (ZNS) can be prescribed as add-on treatment in focal adulthood epilepsy in USA and Europe. In Japan, it can be prescribed as first-line monotherapy drug - independent of age. ZNS may also be effective in idiopathic generalized epilepsy and some difficult-to-treat epilepsies including West, Lennox-Gastaut, or Dravet syndromes. The most frequent side effects of ZNS are related to central nervous system occurring in 19%. Kidney stones and oligohidrosis are ZNS-specific side effects. Loss of appetite and weight are usually "beneficial" effects. ZNS is not recommended in pregnancy ZNS can be taken once daily, which may be beneficial in non-compliance. The pathomechanism of ZNS is different from other antiepileptic drugs. ZNS has an effect on the voltage-gated Na+ - and T-type Ca2+ channels as well as on the dopaminerg, glutamaterg, cholinerg, and GABAerg systems. The multiple way of action may be the reason why ZNS seems to be a broad-spectrum drug and beneficial in various neurological disorders. ZNS reduces production of free radicals according to in vitro and in vivo studies. Animal experiments suggest that ZNS may be a neuroprotective agent. Based on an adequate randomized controlled trial, ZNS is effective in adjuctive treatment of Parkinson disorder. A peculiar benefit of the ZNS is that parallel to its positive effect on motor impairment it also reduces severity of dyskinesias. ZNS may be effective in bipolar disorder, obesity, eating disorders, and migraine prophylaxis.

The Cognitive and Negative Symptoms in Schizophrenia Trial (CONSIST): the efficacy of glutamatergic agents for negative symptoms and cognitive impairments

OBJECTIVE

Patients with schizophrenia frequently present with negative symptoms and cognitive impairments for which no effective treatments are known. Agents that act at the glycine site of the N-methyl-D-aspartic acid (NMDA) glutamatergic receptor have been suggested as promising treatments for moderate to severe negative symptoms and cognitive impairments.

METHOD

The Cognitive and Negative Symptoms in Schizophrenia Trial (CONSIST) was a 16-week double-blind, double-dummy, parallel group, randomized clinical trial of adjunctive glycine, D-cycloserine, or placebo conducted at four sites in the United States and one site in Israel. The participants were 157 inpatients and outpatients who met DSM-IV criteria for schizophrenia or schizoaffective disorder and retrospective and prospective criteria for moderate to severe negative symptoms without marked positive, depressive, or extrapyramidal symptoms. The primary outcome measures were the average "rate of change" of Scale for the Assessment of Negative Symptoms (SANS) total scores and change in the average cognitive domain z scores.

RESULTS

There were no significant differences in change in the SANS total score between glycine and placebo subjects or D-cycloserine and placebo subjects. A prespecified test for the site-by-treatment-by-time interaction was significant in post hoc tests. One site had greater reduction in the SANS total score for patients receiving D-cycloserine relative to patients receiving placebo. A second site had greater reduction in the SANS total score for placebo patients compared with glycine patients. There were no significant differences between glycine and placebo or D-cycloserine and placebo subjects on the average cognition z score.

CONCLUSIONS

The study results suggest that neither glycine nor D-cycloserine is a generally effective therapeutic option for treating negative symptoms or cognitive impairments.

Memantine: a NMDA receptor antagonist that improves memory by restoration of homeostasis in the glutamatergic system--too little activation is bad, too much is even worse

The neurotransmitter glutamate activates several classes of metabotropic receptor and three major types of ionotropic receptor--alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate and N-methyl-D-aspartate (NMDA). The involvement of glutamate mediated neurotoxicity in the pathogenesis of Alzheimer's disease (AD) is finding increasing scientific acceptance. Central to this hypothesis is the assumption that glutamate receptors, in particular of the NMDA type, are overactivated in a tonic rather than a phasic manner. Such continuous, mild, chronic activation ultimately leads to neuronal damage/death. Additionally, impairment of synaptic plasticity (learning) may result not only from neuronal damage per se but may also be a direct consequence of this continuous, non-contingent NMDA receptor activation. Complete NMDA receptor blockade has also been shown to impair neuronal plasticity, thus, both hypo- and hyperactivity of the glutamatergic system leads to dysfunction. Memantine received marketing authorization from the EMEA (European Medicines Agency) for the treatment of moderate to severe AD in Europe and was subsequently also approved by the FDA (Food and Drug Administration) for use in the same indication in the USA. Memantine is a moderate affinity, uncompetitive NMDA receptor antagonist with strong voltage-dependency and fast kinetics. This review summarizes existing hypotheses on the mechanism of action (MOA) of memantine in an attempt to understand how the accepted interaction with NMDA receptors could allow memantine to provide both neuroprotection and reverse deficits in learning/memory by the same MOA.

Glutamate and anxiety disorders

Anxiety disorders are among the most prevalent psychiatric disorders, but they represent a particular challenge for treatment. The standard first-line treatments, including antidepressants, benzodiazepines, and buspirone, result in significant response rates for a majority of patients; however, unfavorable side effect profiles or risk for dependency for particular agents might limit their use by anxious patients, who often have low thresholds for medication discontinuation. Novel pharmacologic agents that modulate particular receptors, ion channels, or transporters relevant to glutamatergic neurotransmission may represent a new approach to the treatment of anxiety disorders, with generally more favorable side effect profiles. Although the role of glutamate in the pathophysiology of anxiety disorders is still being elucidated, the use of these agents in treatment of anxiety disorders and commonly comorbid conditions such as substance abuse and mood disorders will continue to increase.

[Clinical aspects of Alzheimer disease]

Alzheimer disease is diagnosed in only half of the patients with this disease in France. In its typical form, it is characterized at the onset by short-term memory problems, repetitive and unusual oversights and forgetfulness, and difficulties in learning new information. Dementia is responsible for more than 50% of the need for care in the elderly. Disease progression is accompanied by noncognitive complications. The 3 most frequent are psychological and behavioral symptoms, weight loss, and impaired balance and walking. Its progressive nature and potential complications underline the need for multidisciplinary management for patients and their families, with regular medical follow-up.

Metabotropic Glutamate Receptors as Drug Targets

L-glutamate (Glu), the main excitatory amino acid neurotransmitter in the mammalian central nervous system, is involved in many physiological functions, including learning and memory, but also in toxic phenomena occurring in numerous degenerative or neurological diseases. These functions mainly result from its interaction with Glu receptors (GluRs). The broad spectrum of roles played by glutamate derived from the large number of membrane receptors, which are currently classified in two main categories, ionotropic (iGluRs) and metabotropic (mGluRs) receptors. The iGluRs are ion channels, permeant to Na(+) (Ca(2+)) while the mGluRs belongs to the superfamily of G-protein coupled receptors (GPCRs). Despite continuous efforts over more than two decades, the use of iGluR agonists or antagonists to improve or inhibit excitatory transmission in pathological states still remains a major challenge, though the discovery and development of recent molecules may prove it worthwhile. This probably results form the vital role of fast excitatory transmission in many fundamental physiological functions. Since the discovery of mGluRs, hope has emerged. Indeed, mGluRs are mainly involved in the regulation of fast excitatory transmission. Consequently, it was logically thought that modulating mGluRs with agonists or antagonists might lead to more subtle regulation of fast excitatory transmission than by directly blocking iGluRs. As a result of intensive investigation, new drugs permitting to discriminate between these receptors have emerged. Moreover, a new class of molecules acting as negative or positive allosteric modulators or mGluRs is now available and appears to be promising. In the following, we will review the classification of mGluRs and the functions in which mGluRs are involved. We will focus on their potential as therapeutic targets for improving numerous physiological functions and for different neurodegenerative and neuropsychiatric disorders, which are related to malfunction of Glu signaling in human beings.

Glycine Transporter 1 Inhibitors and Modulation of NMDA Receptor-Mediated Excitatory Neurotransmission

In the central nervous system, glutamate is essential for a proper synaptic communication in neuronal networks supporting critical behavioral activities such as learning and memory. Dysfunction of glutamatergic excitatory neurotransmission has been implicated in numerous neurological and pyschiatric disorders and a growing body of research suggests that potentiation of NMDA receptor function may represent a novel approach for the treatment of schizophrenia. An actively pursued strategy to potentiate NMDA receptor function is to increase synaptic levels of the neurotransmitter glycine by blocking the glycine transporter type 1 (GlyT1). Since glycine acts as a co-agonist at the NMDA receptor, this approach could enhance the effectiveness of normal NMDA receptor-mediated glutamatergic neurotransmission. Recent research on the physiology of this uptake system as well as on the development and preclinical testing of novel GlyT1 inhibitors have greatly enhanced our knowledge of the role of this transporter in the modulation of NMDA receptor activity and suggested that this approach may be feasible. Clinical studies with novel glycine reuptake inhibitors will provide critical information regarding the validity of this therapeutic concept for the treatment of schizophrenia and other disorders associated with NMDA receptor hypofunction.

Cholinergic and glutamatergic drugs in Alzheimer's disease therapy

The pathology and clinical symptoms of Alzheimer's disease are well known and include plaques, tangles, cell loss and dysfunction. The target of current treatments is to improve neuronal dysfunction and produce symptomatic benefits based on a clear understanding of neurotransmitter biochemistry. The purpose of this review is to examine the scientific background to currently available treatments, discuss the clinical experience of employing these drugs in Alzheimer's disease patients and review the socioeconomic influences on their use in the future.

New Therapeutic Frontiers for Metabotropic Glutamate Receptors

Our understanding of glutamatergic transmission in the central nervous system has been greatly expanded with the discovery and investigation of the metabotropic glutamate receptor family. Complementing the ionotropic glutamate-gated ion channels, these G-protein coupled receptors play critical roles in neuronal and glial functions such as the modulation of neuronal excitability, synaptic transmission, and various metabolic functions. Because of the ubiquitous distribution of glutamatergic synapses, it has been deemed likely that mGlu receptors participate in most, if not all, major functions of the CNS. It is predicted that the wide diversity and heterogeneous distribution of mGlu receptor subtypes will provide avenues to develop clinically relevant pharmacological agents that target specific CNS systems. mGlu receptors are regulated by differences in expression, alternative splicing patterns, and interactions with other proteins in the cell and it is anticipated that an understanding of these modifiers of mGlu receptor function will open new opportunities for pharmacological tool development and new therapeutic strategies. Over the past decade, an increasing number of selective agonists, antagonists, and allosteric modulators have been developed which target distinct mGlu receptor subtypes; many of these agents have now been further validated in numerous electrophysiological and behavioral models. The combination of these pharmacological tools, in conjunction with genetic approaches, has led to major advances in our understanding of the roles of mGlu receptors in the regulation of CNS function and animal behavior. These studies suggest the exciting possibility that drugs active at mGlu receptors will be useful in treatment of a wide variety of neurological and psychiatric disorders such as Parkinson's disease, anxiety disorders, and schizophrenia.

Pharmacological Approaches to Functional Recovery After Spinal Injury

Locomotion results from the activity in neural networks in the spinal cord that together with sensory and descending inputs generate coordinated motor outputs. Descending inputs include glutamatergic, monoaminergic, and peptidergic pathways. Spinal injuries interrupt these descending pathways, resulting in the disruption or loss of function. Drugs that target these endogenous transmitter systems have been used to improve function after spinal injury. However, individual drugs can have beneficial or deleterious effects in different studies and thus there is little consensus on optimal pharmacological strategies. The variability may be influenced by changes introduced by the type of lesion (complete or partial), time after injury, or the lack of specific ligands that target specific transmitter systems. It is now recognised that these transmitter systems do not necessarily act in isolation, but can interact to evoke additive, inhibitory, or novel metamodulatory effects. Meta interactions mean that differing chemical environments in lesioned spinal cords could influence drug effects. The spinal cord also exhibits injury-induced changes, which could alter the chemical environment and functional properties over time. While they have not been considered in pharmacological approaches to spinal injury, interactive and adaptive changes could influence the effects of spinal lesions and therapeutic interventions. The properties of endogenous transmitter systems in spinal locomotor networks before and after spinal lesions need to be understood, and pharmacological tools that target specific functional aspects need to be developed.

[Metabotropic glutamate receptors: new molecular targets in the treatment of neurological and psychiatric diseases]

AIM

To describe the current state of knowledge about the processes of neuronal signalling mediated by the metabotropic glutamate (mGlu) receptors and their potential role in the treatment of neurological and psychiatric disorders.

DEVELOPMENT

The mGlu receptors are a large family of G-protein coupled receptors that modulate excitatory synaptic transmission through several transduction mechanisms. Recent advances in the molecular biology, physiology and pharmacology of these receptors revealed their potential role in a variety of central nervous system disorders such as epilepsy, pain, ischemia, and neurodegenerative diseases. These findings have shown that modulating glutamatergic transmission with drugs interacting selectively on mGlu receptors might have important beneficial effects. A number of results in this direction are very promising and are used to develop new drugs that overcome the multiple side-effects produced by the drugs acting on ionotropic glutamate receptors. Thus, several evidence have provided clear indications of the potential use of selective agonists and antagonists of the different mGlu receptor subtypes as neuroprotective agents.

CONCLUSIONS

Determining the role of mGlu receptors in physiological as well as in pathophysiological states will be relevant to develop new treatments for diseases in which glutamatergic neurotransmission is altered. In this sense, the current progress indicates that mGlu receptors are novel and promising therapeutic targets for neurological and psychiatric disorders.

Glutamatergic drugs for schizophrenia: a systematic review and meta-analysis

OBJECTIVE

To evaluate the efficacy of glutamatergic drugs, acting agonistically on the N-methyl-D-aspartate (NMDA) or the non-NMDA receptors, in schizophrenia.

METHOD

All relevant randomized controlled trials of glutamatergic drugs for schizophrenia were obtained from the Cochrane Schizophrenia Group's Register of Trials without any language or year limitations. Trials were classified according to their methodological quality. For binary and continuous data, relative risks and weighted (WMD) or standardized mean differences (SMD) were calculated, respectively.

RESULTS

Eighteen short-term trials with 343 randomized patients were included in the meta-analysis. In all of these trials, glycine, D-serine, D-cycloserine or ampakine CX516 was used to augment antipsychotics. NMDA receptor co-agonists glycine and D-serine are effective in reducing negative symptoms (N = 132, fixed effect model SMD = -0.66, 95% CI -1.02 to -0.29, p = 0.0004) of schizophrenia, the magnitude of the effect is moderate. D-Cycloserine, a partial agonist of NMDA receptors, is less effective towards negative symptoms (N = 119, fixed effect model SMD = -0.11, 95% CI -0.48 to 0.25, p = 0.6). Positive symptoms fail to respond to glutamatergic medication. Available derived data on cognitive functioning do not indicate a significant effect of glycine or D-serine (N = 80, random effect model WMD = -2.79, 95% CI -6.17 to 0.60, p = 0.11).

CONCLUSIONS

In the current limited data set, a moderate amelioration of negative symptoms of schizophrenia was found, but no other statistically significant beneficial effects on symptoms of schizophrenia.

Treatments for schizophrenia: a critical review of pharmacology and mechanisms of action of antipsychotic drugs

The treatment of schizophrenia has evolved over the past half century primarily in the context of antipsychotic drug development. Although there has been significant progress resulting in the availability and use of numerous medications, these reflect three basic classes of medications (conventional (typical), atypical and dopamine partial agonist antipsychotics) all of which, despite working by varying mechanisms of actions, act principally on dopamine systems. Many of the second-generation (atypical and dopamine partial agonist) antipsychotics are believed to offer advantages over first-generation agents in the treatment for schizophrenia. However, the pharmacological properties that confer the different therapeutic effects of the new generation of antipsychotic drugs have remained elusive, and certain side effects can still impact patient health and quality of life. Moreover, the efficacy of antipsychotic drugs is limited prompting the clinical use of adjunctive pharmacy to augment the effects of treatment. In addition, the search for novel and nondopaminergic antipsychotic drugs has not been successful to date, though numerous development strategies continue to be pursued, guided by various pathophysiologic hypotheses. This article provides a brief review and critique of the current therapeutic armamentarium for treating schizophrenia and drug development strategies and theories of mechanisms of action of antipsychotics, and focuses on novel targets for therapeutic agents for future drug development.

Comparison of the effect of glutamate receptor modulators in the 6 Hz and maximal electroshock seizure models

Glutamatergic ionotropic and metabotropic receptor modulators have been shown to produce anticonvulsant activity in a number of animal seizure models, e.g. maximal electroshock (MES) and DBA/2 sensory-induced seizures. The 6 Hz model of partial seizures is an alternative low frequency, long duration stimulation paradigm resulting in a seizure characterized by jaw and forelimb clonus, immobility, and an elevated tail (Straub-tail). A unique aspect of this model is that it is the only acute electrically-induced seizure model in which levetiracetam has displayed anticonvulsant activity, suggesting that the 6 Hz seizure model may be useful in identifying compounds with unique anticonvulsant profiles. The purpose of the present study was to examine the role of glutamate receptors in the MES and 6 Hz seizure models using a number of NMDA, AMPA/KA, and mGlu receptor modulators. The pharmacological profile of the 6 Hz seizure model was compared to that of the MES model using eight ionotropic glutamate receptor antagonists and eight mGlu receptor modulators. The ionotropic receptor antagonists MK-801, LY235959, NBQX, LY293558, GYKI 52466, LY300168, and LY377770 produced complete protection from tonic extension in the MES model. Furthermore, the noncompetitive mGlu1 (LY456236) and mGlu5 (MPEP) metabotropic receptor antagonists and the mGlu8 metabotropic receptor agonist (PPG) were also effective in the MES model whereas the competitive mGlu1 (LY367385) receptor antagonist, the mGlu2/3 (LY379268 and LY389795) and Group III (L-AP4) metabotropic receptor agonists were ineffective. In contrast, all of the compounds tested, produced dose-dependent protection in the 6 Hz model with an increase in potency as compared to the MES model. The largest protective indices (P.I.=TD50/ED50) observed were associated with the iGlu5 antagonist LY382884 and the mGlu2/3 receptor agonists LY379268 and LY389795 (P.I.=>14, 14, and 4.9, respectively) in the 6 Hz model. The results from the present study support the continued search for glutamate receptor modulators as potential antiepileptic agents. Furthermore these results illustrate the importance of using several different animal seizure models in the search for novel AEDs and the potential utility of the 6 Hz seizure model in identifying novel AEDs.

Targeting metabotropic glutamate receptors for treatment of the cognitive symptoms of schizophrenia

Several lines of evidence implicate NMDA receptor dysfunction in the cognitive deficits of schizophrenia, suggesting that pharmacological manipulation of the NMDA receptor may be a feasible therapeutic strategy for treatment of these symptoms. Although direct manipulation of regulatory sites on the NMDA receptor is the most obvious approach for pharmacological intervention, targeting the G-protein coupled metabotropic glutamate (mGlu) receptors may be a more practical strategy for long-term regulation of abnormal glutamate neurotransmission. Heterogeneous distribution, both at structural and synaptic levels, of at least eight subtypes of mGlu receptors suggests that selective pharmacological manipulation of these receptors may modulate glutamatergic neurotransmission in a regionally and functionally distinct manner. Two promising targets for improving cognitive functions are mGlu5 or mGluR2/3 receptors, which can modulate the NMDA receptor-mediated signal transduction by pre- or postsynaptic mechanisms. Preclinical studies indicate that activation of these subtypes of mGlu receptors may be an effective strategy for reversing cognitive deficits resulting form reduced NMDA receptor mediated neurotransmission.

The glutamatergic system and Alzheimer's disease: therapeutic implications

Alzheimer's disease affects nearly 5 million Americans currently and, as a result of the baby boomer cohort, is predicted to affect 14 million Americans and 22 million persons totally worldwide in just a few decades. Alzheimer's disease is present in nearly half of individuals aged 85 years. The main symptom of Alzheimer's disease is a gradual loss of cognitive function. Glutamatergic neurotransmission, an important process in learning and memory, is severely disrupted in patients with Alzheimer's disease. Loss of glutamatergic function in Alzheimer's disease may be related to the increase in oxidative stress associated with the amyloid beta-peptide that is found in the brains of individuals who have the disease. Therefore, therapeutic strategies directed at the glutamatergic system may hold promise. Therapies addressing oxidative stress induced by hyperactivity of glutamate receptors include supplementation with estrogen and antioxidants such as tocopherol (vitamin E) and acetylcysteine (N-acetylcysteine). Therapy for hypoactivity of glutamate receptors is aimed at inducing the NMDA receptor with glycine and cycloserine (D-cycloserine). Recently, memantine, an NMDA receptor antagonist that addresses the hyperactivity of these receptors, has been approved in some countries for use in Alzheimer's disease.

Polyamines in a genetic animal model of paroxysmal dyskinesia

Previous studies suggested that glutamatergic overactivity contributes to the manifestation of dystonia in the dt(sz) mutant hamster, a model of idiopathic paroxysmal dyskinesia in which dystonic episodes occur in response to mild stress. Therefore, the role of polyamines, known as positive modulators of NMDA receptors, was examined in the present study. The levels of polyamines (putrescine, spermidine, spermine) were determined in forebrain, cerebellum and brainstem in dt(sz) hamsters at an age of most marked expression of dystonia (32 days) and in age-matched non-dystonic control hamsters. Spermine was found to be significantly increased in the forebrain (35%) of dystonic animals, while spermidine was unaltered in dystonic brains and only a moderate increase in putrescine (12%) was detected in the cerebellum of dt(sz) mutants. In view of enhanced spermine levels, the effect of the putative polyamine receptor antagonist ifenprodil on the severity of dystonia was examined in dystonic hamsters. Ifenprodil (5-40 mg/kg i.p.) failed to exert a beneficial effect, but even aggravated dystonia in the dt(sz) mutant at higher doses. These data together with previous pharmacological findings in mutant hamsters do not completely exclude a pathophysiological role of enhanced polyamine levels but suggest that overstimulation of NMDA receptors which contain NR2B subunits by enhanced spermine levels is not involved in the dystonic syndrome.

The emerging differential roles of GABAergic and antiglutamatergic agents in bipolar disorders

Treatment options to relieve the diverse symptoms encountered in patients with bipolar disorders include not only mood stabilizers, but also anxiolytics, new anticonvulsants, antidepressants, and antipsychotics. These agents have widely varying mechanisms of action, which could contribute to the heterogeneity of clinical effects seen in practice. Several of these medications, especially those with anticonvulsant effects, enhance gamma-aminobutyric acid (GABA) inhibitory neurotransmission and/or attenuate glutamate excitatory neurotransmission. We review the efficacy and tolerability of these diverse treatment options in bipolar disorders and explore possible relationships between clinical effects and GABAergic and antiglutamatergic mechanisms of action.

The role of ionotropic glutamate receptors in nociception with special regard to the AMPA binding sites

The recent literature on the antinociceptive action of ionotropic glutamate receptor antagonists is reviewed with special emphasis on their clinical potential. Actually the glutamatergic pathways descending from the brain stem into the spinal cord may generate analgesia. However, physiologically more important is that glutamate and aspartate are apparently the main neurotransmitters along the ascending nociceptive pathways in the spinal cord. Glutamate, aspartate and their receptors can be detected in particularly high concentrations in the dorsal root ganglia and the superficial laminae (I, II) of the spinal cord. In low doses glutamate receptor antagonists only slightly elevate the threshold of the physiological pain sensation. However, they suppress the process of pathological sensitisation i.e. lowering of the pain threshold seen upon excessive or lasting stimulation of C-fibre afferents, a process that takes place during inflammation or other kinds of tissue injury. At electrophysiological level antagonists of both the NMDA- and AMPA/kainate receptors inhibit wind up i.e. lasting activation of the polymodal, second-order sensory neurones in the deeper layers of the dorsal horn. During sensitisation the resting Mg(++) blockade of transmembrane Ca(++) channels is abolished, certain second messenger pathways are activated, the transcription of many genes is enhanced leading to overproduction of glutamate and other excitatory neurotransmitters and expression of Na(+) channels in the primary sensory neurones activated at lower level of depolarisation. This cascade of events leads to increased excitability of the pain pathways. NMDA antagonists are apparently more potent in experimental models of neuropathic pain, whereas AMPA antagonists are more effective in abolition of hyperalgesia seen during experimental inflammation. Clinically, of the previously known NMDA antagonists amantadine, dextromethorphan and ketamine have been tested, the latter extensively. Ketamine has been found quite active in certain cases of neuropathic pain and it reduced the opiate demand when used for postoperative analgesia. However, in other types of clinical pain their efficacy is less convincing. Not being registered there are no clinical data on the AMPA antagonists. There are, however, some investigational new drugs and some novel compounds in the stage of preclinical development which antagonise the AMPA receptors in competitive fashion or allosterically. Of the latter molecules 2,3-benzodiazepines are particularly promising.

The role of glutamatergic transmission in the pathogenesis of levodopa-induced dyskinesias. Potential therapeutic approaches

Dyskinesias are the most frequent adverse effect of chronic levodopa therapy in patients with Parkinson's disease (PD). Current pharmacological treatment for this problem is unsatisfactory. Recently, there is evidence for the role of glutamate in the basal ganglia neuronal circuitry in the generation of dyskinesias. If indeed glutamatergic overactivity beyond the dopaminergic synapses plays a role in the pathogenesis of these involuntary movements, there is hope that its suppression may be beneficial without causing loss of levodopa efficacy and parkinsonian deterioration. Indeed, NMDA receptor antagonists such as amantadine and dextrometorphan can reduce such dyskinesias. We tested the efficacy of riluzole, an inhibitor of glutamatergic transmission in the inhibition of levodopa-induced dyskinesias.

Treatment of amyotrophic lateral sclerosis

Survival of patients with amyotrophic lateral sclerosis (ALS) is improving. Timely and more frequent implementation of bimodal passive airway pressure (BIPAP) and percutaneous endoscopically placed gastrostomy (PEG) may be the major factors impacting on longer survival. However, several drugs recently subjected to rigorous clinical trials have demonstrated significant results or encouraging trends. ALS is a complex disease in which aging neurons are subjected to a variety of susceptibility genes, most of which remain to be discovered, that interact with equally unrecognised environmental factors. This makes it unlikely that a single therapeutic agent will be of value. The thrust must be on polypharmacy. The 'cocktail' that will eventually be of greatest benefit has yet to be formulated. It might contain glutamate N-methyl-D-aspartate (NMDA) and non-NMDA receptor antagonists, antioxidants or a combination of trophic factors and neuroprotective agents. This statement is made with the understanding that the aetiopathogenesis of ALS is far from clear. Drug delivery is a problem and better delivery systems are needed. The efficacy of some of the medications that presently only induce modest benefit may be improved by liposomal packaging, use of a patch or inhalation delivery or intraventricular pump reservoirs. There is a great need to develop an early marker of ALS and sensitive reproducible measures of disease progression. This will curtail the present need for large, lengthy and very expensive clinical trials. The new millennium will see the advent of targeted therapy using viral vectors which can deliver replacement genes, trophic factors and other drugs to degenerating neurons; transplantation of neural progenitor cells which can become mature functioning neurons; anti-apoptotic agents which will allow neurons to survive longer; and mechanisms that can protect the telomerase maintenance system which is so crucial in the immortalisation of cells.