CNS & Neurological Disorders - Drug Targets (Formerly Current Drug Targets - CNS & Neurological Disorders) - Volume 13, Issue 1, 2014

Alterations in excitatory-inhibitory balance occur in Down syndrome and could be responsible for cognitive deficits observed through the life of all individuals carrying an extra copy of chromosome 21. Excess of inhibition in the adult could produce synaptic plasticity deficits that may be a primary mechanism contributing to learning and memory impairments. In this study we discuss pharmacological treatments that could potentially alleviate neuronal inhibition and have been tested in a mouse model of Down syndrome. γ-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mature central nervous system that binds to GABA-benzodiazepine receptors, opens a chloride channel and reduces neuronal excitability. These receptors have been extensively studied as targets for treatment of epilepsy, anxiety, sleep, cognitive disorders and the induction of sedation. Molecules that are either antagonists or inverse agonists of the GABA-benzodiazepine receptors are able to reduce inhibitory GABAergic transmission. However modulating the excitatory-inhibitory balance towards increase of cognition without inducing seizures remains difficult particularly when using GABA antagonists. In this study we review data from the literature obtained using inverse agonists selective for the α5-subunit containing receptor. Such inverse agonists, initially developed as cognitive enhancers for treatment of memory impairments, proved to be very efficient in reversing learning and memory deficits in a Down syndrome mouse model after acute treatment.

Down syndrome (DS) is the most common genetically defined cause of intellectual disability and accounts for over 50% of the cases of Alzheimer-type dementia in persons younger than 50 years of age. At present, no pharmacotherapy aimed at counteracting either the neurodevelopmental or the neurodegenerative component of this genetic disorder has been approved. Recent preclinical and clinical work on the N-methyl-D-aspartate (NMDA) receptor antagonist memantine give us some reason for optimism, at least in relation to the potential for a partial pharmacological improvement of hippocampus dependent memory deficits associated with DS. Here, we will review briefly the roles of NMDA receptors in health and disease, including the glutamatergic hypothesis for Alzheimer disease. Then, we will describe the basis for a glutamatergic hypothesis for DS, by reviewing the available preclinical evidence and assessing potential molecular mechanisms for NMDA receptor dysfunction in DS. A short description of the first two clinical trials of memantine in young and older adults with DS will follow. We will conclude by reviewing three caregiver reports from our recent clinical study and some lessons we have learned designing and conducting the first translational study in the field of DS to arise directly from experimental results in animal models.

Down syndrome (DS), the most common genetic cause of intellectual disability, is caused by the trisomy of chromosome 21. MNB/DYRK1A (Minibrain/dual specificity tyrosine phosphorylation-regulated kinase 1A) has possibly been the most extensively studied chromosome 21 gene during the last decade due to the remarkable correlation of its functions in the brain with important DS neuropathologies, such as neuronal deficits, dendrite atrophy, spine dysgenesis, precocious Alzheimer’s-like neurodegeneration, and cognitive deficits. MNB/DYRK1A has become an attractive drug target because increasing evidence suggests that its overexpression may induce DS-like neurobiological alterations, and several small-molecule inhibitors of its protein kinase activity are available. Here, we summarize the functional complexity of MNB/DYRK1A from a DS-research perspective, paying particular attention to the capacity of different MNB/DYRK1A inhibitors to reverse the neurobiological alterations caused by the increased activity of MNB/DYRK1A in experimental models. Finally, we discuss the advantages and drawbacks of possible MNB/DYRK1A-based therapeutic strategies that result from the functional, molecular, and pharmacological complexity of MNB/DYRK1A.

An increasing amount of evidence suggests that the dysregulation of the Akt-mTOR (Akt-mammalian Target Of Rapamycin) signaling network is associated with intellectual disabilities, such as fragile X, tuberous sclerosis and Rett’s syndrome. The Akt-mTOR pathway is involved in dendrite morphogenesis and synaptic plasticity, and it has been shown to modulate both glutamatergic and GABAergic synaptic transmission. We have recently shown that the AktmTOR pathway is hyperactive in the hippocampus of Ts1Cje mice, a model of Down’s syndrome, leading to increased local dendritic translation that could interfere with synaptic plasticity. Rapamycin and rapalogs are specific inhibitors of mTOR, and some of these inhibitors are Food and Drug Administration-approved drugs. In this review, we discuss the molecular basis and consequences of Akt-mTOR hyperactivation in Down’s syndrome, paying close attention to alterations in the molecular mechanisms underlying synaptic plasticity. We also analyze the pros and cons of using rapamycin/rapalogs for the treatment of the cognitive impairments associated with this condition.

Conventional knowledge considered apoptosis as the sole form of programmed cell death during development, homeostasis and diseases, whereas necrosis was regarded as an unregulated and uncontrollable process. Recent revelations suggest that necrosis can also occur in a regulated, caspase-independent manner and shares characteristics with both necrosis and apoptosis. The major cell death processes namely apoptosis, autophagy and necrosis are interlinked and contain many common regulatory mechanisms. Mounting evidence indicates that necroptosis contributes to the pathogenesis of various diseases, including ischemic stroke, traumatic brain injury, neurodegenerative disorders and brain tumor. We present here an overview of the molecular mechanisms governing necroptosis and its connection with apoptosis and autophagy processes. Further, the necroptosis mechanisms underlying the neurodegeneration during ischemia reperfusion (I/R) injury are described, with an emphasis on the key proteins involved in this type of cell death. Knowledge regarding programmed cell death (PCD) with relevance to necroptosis may play a significant role in debilitating brain disorders.

A target-based approach has been used to develop novel drugs in many therapeutic fields. However, this approach remains suboptimal for drug discovery in brain disorders because the target identification in a brain disorder requires a hierarchical integration from in vitro cellular and functional tissue studies to animal models that sustain neuronal and glial complexity. Although glial cells comprise over half of the brain and play important roles in brain function and disease, the intracellular signaling of glial cells remains essentially unexplored. This is because the lack of optimal strategy to selectively activate or deactivate glial signaling has made it difficult to study glial roles. The recent development of approaches using mouse models and enabling the selective activation of cell signaling could be used to assess the role of glial cells in physiology and disease. This review presents how glial G- protein signaling contributes to brain disorders and how the role of glia is investigated.

Neuropathic pain is caused by structural lesion leading to functional abnormalities in central and peripheral nervous system. Neuropathic pain in itself is not always a disease, as it arises due to consequences of other diseases like diabetes, spinal cord injury, degenerative neuronal diseases and cancer. Current strategies of neuropathic pain treatment have provided relief to the patients to some extent, but complete cure is still a distant dream. In the future, it is hoped that a combination of new and improved pharmaceutical developments combined with careful clinical trials and increased understanding of neuroplasticity will lead to improved and effective pain management strategies leading to improved quality of life. In this review we have discussed various therapeutic targets of neuropathic pain and their pathophysiological mechanisms. Current status of drugs used for treatment of neuropathic pain have also been discussed in the review.

Exploiting the potential of natural compounds to attenuate endogenous redox status to achieve neuroprotection is a novel concept in human disease therapy. This has necessitated a need to identify newer efficient phytochemicals possessing propensity to act on various biochemical therapeutic targets with low or no toxicity. Selaginella is a lithophytic pteridophyte which grows on constantly irrigated rocks in high altitude zones in different parts of the world. It is appraised to be “Sanjeevani” (the resurrection herb) based on its mythological reference in the Indian epic “Ramayana”. Due to the presence of a unique disaccharide, trehalose, most species of Selaginella can survive severe drought conditions, maintaining the plant’s structural stability and resurrect during rains. Several species of the genus are used in ethnic medicine for the therapy of jaundice, chronic trachitis, lung cancer, labor pain and wound healing. The major natural compounds in the genus Selaginella are characteristic flavonoid-dimers, called ‘biflavonoids’. Although various biological effects of Selaginella have been documented in vitro, studies on its neuromodulatory properties are nonexisting despite the presence of potentially therapeutic biflavonoids. We have reviewed the existing literature on the possible pharmacological properties of Selaginella. Further, recent evidence gathered from our laboratory on the neuromodulatory propensity of S. delicatula employing in vivo models of chemically induced neurodegenerative diseases in rodents and Drosophila are discussed. Our findings point to a mechanism which modulates redox status and mitochondrial dysfunction suggesting their possible therapeutic use in oxidative stress-mediated neurodegenerative diseases including Parkinson's disease.

MicroRNAs (miRNAs) have emerged as a new class of RNA molecules which are short in length, less in number but play bigger role in regulation of cellular events. miRNAs keep cellular homeostasis in tight control by fine tuning expression of protein coding genes at post-transcriptional level. Neurogenesis and neurodegeneration are two complex processes which are regulated by dynamic expression of regulatory proteins like transcription factors and signaling proteins. Evidences are accumulating that expression of miRNAs play major role in fate determination of neuronal cells undergoing neurogenesis or neurodegeneration. Neurodegeneration either induced by genetic factors or environmental chemicals results in development of neurodegenerative disorders like Parkinson’s or Alzheimer’s. With increasing acceptance of adult neurogenesis, it seems possible that inducing neurogenesis in adult brain can help in fighting with neurodegenerative disorders. Regulatory RNA molecules, like miRNAs are presenting them as potential therapeutic targets for inducing neurogenesis and controlling neurodegeneration. In the current review, we are exploring the link between neurodegeneration and adult neurogenesis regulation by focusing on miRNAs.

Noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists can produce positive and negative symptomatology as well as impairment of cognitive function that closely resemble those present in schizophrenia. In rats, these drugs induce a behavioral syndrome (characterized by hyperlocomotion and stereotypies), an enhanced glutamatergic transmission in the medial prefrontal cortex, and damage to retrosplenial cortical neurons in adult rats, which was measured as the induction of the stress protein 72/73 kDa heat shock protein (Hsp72/73). In the present work, we have examined the existence of possible differences among different antipsychotic drugs in their capacity to block immunolabeling of Hsp72/73 in the retrosplenial cortex of the rat induced by the potent NMDA receptor antagonist, MK- 801. In addition, the effects of selective monoaminergic agents were also studied to delineate the particular receptors responsible for the actions of antipsychotic drugs. Pretreatment with clozapine, chlorpromazine, olanzapine, ziprasidone - and to a lesser extent haloperidol-reduced the formation of Hsp72/73 protein in the rat retrosplenial cortex after the administration of MK-801. In addition, antagonism at dopamine D2 (raclopride), 5-HT2A (M100907) and α1- adrenoceptors (prazosin) as well as agonism at 5-HT1A receptors (BAY x 3702) also diminished the MK-801-induced number of cells labeled with Hsp72/73. Each of these effects may contribute to antipsychotic action. The results suggest that the efficacy of atypical antipsychotic drugs in the clinic may result from a combined effect on 5-HT2A, 5-HT1A and α1-adrenergic receptors added to the classical dopamine D2 receptor antagonism.

Aβ exerts prooxidant or antioxidant effects based on the metal ion concentrations that it sequesters from the cytosol; at low metal ion concentrations, it is an antioxidant, whereas at relatively higher concentration it is a prooxidant. Thus Alzheimer disease (AD) treatment strategies based solely on the amyloid-β clearance should be re-examined in light of the vast accumulating evidence that increased oxidative stress in the human brains is the key causative factor for AD. Accumulating evidence indicates that the reduced brain glucose availability and brain hypoxia, due to the relatively lower concentration of ATP and 2,3-diphosphoglycerate, may be associated with increased concentration of endogenous ammonia, a potential neurotoxin in the AD brains. In this review, we summarize the progress in this area, and present some of our ongoing research activities with regard to brain Amyloid-β, systemic ammonia, erythrocyte energy metabolism and the role of 2,3-diphosphoglycerate in AD pathogenesis.

Previously published data from our laboratory demonstrated that pharmacological inhibition of a family of enzymes known as prolyl hydroxylase domain proteins prevents neurotoxicity associated with the acute 1-methyl-4- phenyl-1,2,3,6-tetrahydropyridine intoxication model of Parkinson’s disease in young animals. In this study, we assessed whether prolyl hydroxylase domain inhibition was neuroprotective in an inducible genetic dopaminergic glutathione depletion model previously characterized by our laboratory that more closely recapitulates the age-related and progressive nature of the human disease. Pharmacological prolyl hydroxylase domain inhibition via 3,4-dihydroxybenzoate was found to significantly attenuate hallmark mitochondrial dysfunction and loss of dopaminergic substantia nigral pars compacta neurons associated with this model. These studies further validate the possibility that prolyl hydroxylase domain inhibition may constitute a viable therapy for Parkinson’s disease.

Despite the powerful induction of neurogenesis by sphingosine-1-phosphate (S1P), its study in the role of adult neurogenesis has been relatively neglected. S1P, via its differential effects at different S1P receptor subtypes, is a significant determinant of neuronal precursor/stem cell and astrocyte cellular organization. The variations in neurogenesis, classically modelled via the interactions of phosphatase and tensin homolog and Notch, are intimately associated with the co-ordinated regulation of S1P and fibroblast growth factor-1. Incorporating S1P better explains the plasticity and cellular variations in astrocytes and progenitors as well as their interactions. This has treatment implications for both inducing and inhibiting neurogenesis, in conditions such as depression and macrocephaly associated autistic spectrum disorders respectively. Incorporating S1P and fibroblast growth factor-1 also provides a framework for conceptualizing the impact of peripheral inflammation, central inflammation, redox status and medication effects on neurogenesis, as well as future treatment targets.

Increased depression, somatization, gut inflammation and wider peripheral inflammation are all associated with the early stages of Parkinson’s disease (PD). Classically such concurrent conditions have been viewed as “comorbidities”, driven by high levels of stress in a still poorly understood and treated disorder. Here we review the data on how oxidative and nitrosative stress in association with immuno-inflammatory responses, drives alteration in tryptophan catabolites, including kynurenine, kynurenic acid and quinolinic acid that drive not only the ‘comorbidities” of PD but also important processes in the etiology and course of PD per se. The induction of indoleamine 2,3-dioxygenase, leading to the driving of tryptophan into neuroregulatory tryptophan catabolite products and away from serotonin and melatonin production, has significant implications for understanding the role of nicotine, melatonin, and caffeine in regulating PD susceptibility. Tryptophan catabolite pathway activation will also regulate blood-brain barrier permeability, glia and mast cell reactivity as well as wider innate and adaptive immune cell responses, all relevant to the course of PD. As such, the “comorbidities” of PD such as depression, somatization and peripheral inflammatory disorders can all be conceptualized as being an intricate part of the biological underpinnings of both the etiology and course of PD. As a consequence, the data reviewed here has treatment implications; relevant to both the course of PD and in the management of L-DOPA induced dyskinesias.

Pathologic anxiety is a disproportional reaction of individuals to anticipation or misinterpretation of a potential danger, which affects individual social and personal life. Despite the advances already accomplished, further studies are still necessary in order to understand the mechanisms involved in anxiety. These may provide more effective and safer treatments to aid in the control of anxiety and improve patient quality of life. In this work, we review the current issue about anxiety disorders, covering general aspects such as basic epidemiology and classification, an overview of the pharmacological treatments employed and the current search for natural anxiolytics. Also, a compilation of data investigating the neurobiology that underlies anxiety disorders and a brief discussion evolving the most usual animal experimental models to study anxiety is presented.