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AMPA receptor modulators as cognitive enhancers.

AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)-type glutamate receptors mediate fast excitatory transmission throughout the central nervous system. Positive modulation of these receptors can potentially enhance cognition by, firstly, offsetting losses of glutamatergic synapses; secondly, promoting synaptic plasticity; and thirdly, increasing the production of trophic factors. The advent of small molecules that selectively enhance AMPA receptors in the brain made it possible to test these hypotheses. Preclinical experiments indicate that the compounds accelerate the encoding of memory and have positive effects on models of cognitive dysfunction. Initial results with human subjects are also positive. AMPA receptor modulators thus represent an entirely new approach to cognitive enhancement and the treatment of diverse brain disorders.

Humans↗

Developing a neuronal model for the pathophysiology of schizophrenia based on the nature of electrophysiological actions of dopamine in the prefrontal cortex.

This review covers some recent findings of the electrophysiological mechanisms through which mesocortical dopamine modulates prefrontal cortical neurons. Dopamine has been shown to modulate several ionic conductances located along the soma-dendritic axis of prefrontal cortical pyramidal neurons. These ionic currents include high-voltage-activated calcium currents and slowly inactivating Na+ and K+ currents. They contribute actively in processing functionally segregated inputs during synaptic integration. In addition, dopamine mainly depolarizes the fast-spiking subtype of local GABAergic interneurons that connect the pyramidal neurons. This latter action can indirectly control pyramidal cell excitability. These electrophysiological data indicate that the actions of dopamine are neither "excitatory" nor "inhibitory" in pyramidal prefrontal cortex neurons. Rather, the actions of dopamine are dependent on somadendritic loci, timing of the arrival of synaptic inputs, strength of synaptic inputs, as well as the membrane potential range at which the PFC neuron is operating at a given moment. Based on available electrophysiological findings, a neuronal model of the pathophysiology of schizophrenia is presented. This model proposes that episodic hypo- and hyperactivity of the PFC and the associated dysfunctional mesocortical dopamine system (and their interconnected brain regions) may coexist in the same schizophrenic patient in the course of the illness. We hypothesize that the dysfunctional mesocortical dopamine input to the PFC may lead to abnormal modulation of ionic channels distributed in the dendritic-somatic compartments of PFC pyramidal neurons that project to the ventral tegmental area and/or nucleus accumbens. In some schizophrenics, a reduction of mesocortical dopamine to below optimal levels and/or a loss of local GABAergic inputs may result in a dysfunctional integration of extrinsic associative inputs by Ca2+ channel activity in the distal dendrites of PFC pyramidal neurons. This may account for the patients' distractibility caused by their inability to focus only on relevant external inputs. In contrast, in acute stress or psychotic episodes, an associated abnormal elevation of mesocortical dopamine transmission may greatly influence distal dendritic Ca2+ channel-mediated signal-processing mechanisms. This can enhance possible reverberative activity between adjacent interconnected pyramidal neurons via the effects of dopamine on the slowly inactivating Na+, K+, and soma-dendritic Ca2+ currents. The effects of high levels of PFC dopamine in this case may contribute to behavioral perseveration and stereotypy so that the patients are unable to use new external cues to modify ongoing behaviors.

Animals↗

Altered dopamine signaling and MPTP resistance in mice lacking the Parkinson's disease-associated GPR37/parkin-associated endothelin-like receptor.

GPR37 is an orphan G protein-coupled receptor expressed in mammalian brain, and its insoluble aggregates are found in the brain samples of juvenile Parkinson's disease patients. We have produced a Gpr37 knock-out mouse strain and identified several phenotypic features that are similar to those reported for mutants of genes encoding components of synaptic dopamine vesicles. Our results reveal an unanticipated role of GPR37 in regulating substantia nigra-striatum dopaminergic signaling. Gpr37(-/-) mice are viable, with normal brain development and anatomy, but they exhibit reduced striatal dopamine content, enhanced amphetamine sensitivity, and specific deficits in motor behavior paradigms sensitive to nigrostriatal dysfunction. These functional alterations are not associated with any substantial loss of substantia nigra neurons or degeneration of striatal dopaminergic afferences, the main histological marks of Parkinson's disease. The inactivation of GPR37, in fact, has protective effects on substantia nigra neurons, causing resistance to treatment with the Parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine.

Amphetamine↗

Striatal mechanisms and pathogenesis of parkinsonian signs and motor complications.

Recent studies suggest that motor dysfunction associated with the chronic nonphysiologic stimulation of dopaminergic receptors on striatal spiny neurons alters the sensitivity of nearby glutamatergic receptors, especially those of the N-methyl-D-aspartate (NMDA) subtype. Lesioning the nigrostriatal dopamine system of rats or nonhuman primates induces parkinsonian signs; subsequent once- or twice-daily treatment with levodopa produces many of the features of the human motor complication syndrome. Some drugs that block NMDA receptors palliate parkinsonian signs in these animal models, as well as in patients with Parkinson's disease. Certain NMDA receptor antagonists injected into the striatum or given systemically also have the ability to act palliatively or prophylactically to alleviate levodopa-induced response alterations. These observations support the view that sensitization of striatal NMDA receptors contributes to the pathogenesis of motor dysfunction in Parkinson's disease. Since protein phosphorylation serves as an important regulatory mechanism for NMDA receptors, differential increases in the phosphorylation state of tyrosine and serine residues, observed as a result of nigrostriatal system destruction as well as response alteration induction, could account for the apparent augmentation in synaptic efficacy. Current evidence thus suggests that the intermittent stimulation of dopaminergic receptors activates signal transduction pathways in striatal neurons, which augment phosphorylation of NMDA receptors and thus their sensitivity to cortical glutamatergic input. As a result, striatal output changes in ways that favor the appearance of parkinsonian signs and motor complications.

Animals↗

D-cycloserine, a glycine site agonist, reverses working memory failure by hippocampal muscarinic receptor blockade in rats.

D-Cycloserine, a partial agonist at the glycine binding site on the NMDA receptor/channel complex, did not affect the number of errors (attempts to pass through two incorrect panels of the three-panel gates at four choice points) in the working memory task with a three-panel runway setup, when injected bilaterally at 1 or 10 microg/side into the dorsal hippocampus. Intrahippocampal administration of the muscarinic receptor antagonist scopolamine (3.2 microg/side) significantly increased the number of working memory errors. The increase in working memory errors induced by intrahippocampal 3.2 microg/side scopolamine was significantly reduced by concurrent infusion of 1 and 10 microg/side D-cycloserine. These results suggest that positive modulation of the NMDA receptor/channel through activation of the glycine site can compensate dysfunction of hippocampal cholinergic neurotransmission involved in working memory function.

Animals↗

Glycine transporters not only take out the garbage, they recycle.

Two articles in the current issue of Neuron examine the consequences of deleting the two genes that encode glycine transporters. Interestingly, loss of glial transporters enhances while loss of presynaptic neuronal transporters reduces glycinergic transmission. These two opposing phenotypes resemble distinct human diseases characterized by dysfunction in glycinergic signaling.

Amino Acid Transport Systems, Neutral↗

Early undernutrition impairs hippocampal long-term potentiation in adult rats.

Following high-frequency stimulation of hippocampal dentate granule cells, potentiation was difficult to achieve in undernourished animals, showed a significant decline within 3 to 6 hr, and was completely absent at 24 hr. Further trains of stimulation resulted in only small benefits in undernourished animals. Coupled with previously reported morphological and behavioral deficits, these findings indicate a marked hippocampal dysfunction resulting from early undernutrition and provide a potentially valuable approach for relating nutritionally induced behavioral impairments to brain function.

Animals↗

Neurophysiological evaluation of cortical function in the early diagnosis of ALS.

The corticomotoneuronal system is selectively vulnerable in amyotrophic lateral sclerosis (ALS). In the past it has been difficult to evaluate the upper motor neuron component of this system. Recent studies using functional imaging and neurophysiological methods are now available as potential surrogate markers in the early diagnosis of ALS. We have used peristimulus time histograms (PSTHs) to evaluate the integrity of the motor cortex in ALS and in particular the cortical colonies that synapse with single anterior horn cells. The motor cortex in ALS becomes hyperexcitable early in the course of the disease and this may persist for some time. This is reflected in the PSTH by a desynchronized, complex response. Evidence indicates that this prime abnormality in ALS is supraspinal in origin and probably due to increased repetitive firing of the corticomotoneuron. The hyperexcitability is likely to be due to a combination of increased excitation at the level of the motor cortex and decreased inhibition due to dysfunction of the cortical inhibitory interneurones that modulate the output of the corticomotoneurone.

Anterior Horn Cells↗

Brain 5-HT neurotransmission during paroxetine treatment.

BACKGROUND: Animal experimental studies suggest that repeated administration of selective serotonin reuptake inhibitors (SSRIs) produces complex adaptive changes in brain serotonin (5-HT) pathways. The effect of these adaptive changes on different aspects of brain 5-HT neurotransmission and their clinical consequences are not well understood. METHOD: We studied the effect of repeated administration of the SSRI, paroxetine (20 mg daily), on the cortisol responses to the 5-HT precursor, 5-hydroxytryptophan (5-HTP), in healthy subjects and depressed patients. RESULTS: In healthy subjects, following one week of paroxetine treatment there was a large increase in the cortisol response to 5-HTP. This increase had all but disappeared following 3 weeks treatment. In contrast, in depressed patients treated with paroxetine for 8 weeks, the cortisol response to 5-HTP was significantly increased. CONCLUSIONS: SSRI treatment in depressed patients produces a persistent increase in the cortisol response to 5-HTP, a probable measure of neurotransmission at central 5-HT2 receptors. Potentiation of 5-HT2 neurotransmission is unlikely to account for the efficacy of SSRIs in major depression but might underlie their actions in obsessive-compulsive disorder and also perhaps certain of their adverse effects, notably sexual dysfunction.

Adult↗

[Stimulation single fiber electromyography in rats with myasthenia induced by organophosphorus insecticides and their mixtures poisoning].

OBJECTIVE: To study the neuromuscular function and its relation with the occurrence of myasthenia in rats poisoned by dimethoate (D), phoxim (P), methomyl (M), M + D and M + P respectively. METHODS: The stimulation single fiber electromyography(SSFEMG) at different stimulus frequencies(5, 10 and 20 Hz) was used. The whole blood cholinesterase (ChE) activity was measured 1 h before and after poisoning. RESULTS: (1) Myasthenia occurred in 5 out of 9.5 out of 10.5 out of 5, and 8 rats poisoned by D, P, M + D, and M + P, respectively. (2) The average mean consecutive differences(MCD) at 5, 10, and 20 Hz in myasthenic rats were significantly higher than those of poisoned rats without myasthenia and the control ones. (3) SSFEMG changes at 5, 10 and 20 Hz were significantly consistent with the clinical manifestation of myasthenia, especially at 10 Hz and 20 Hz. (4) ChE activity was significantly lower in rats with P or D poisoning while ChE inhibition was of no difference in rats with M, M + D, and M + P poisoning. In the D poisoning and P poisoning groups, there was no significant difference in ChE inhibition between the rats with and without myasthenia. CONCLUSION: Muscle weakness was associated with neuromuscular transmission dysfunction, but not well correlated with ChE inhibition. The SSFEMG with stimulus frequency at 10 Hz or 20 Hz could be used to detect the neuromuscular dysfunction during myasthenia induced by organophosphate insecticides and their mixtures poisoning.

Animals↗

Effect of dotarizine on electroconvulsive shock or pentylenetetrazol-induced amnesia and on seizure reactivity in rats.

Dotarizine (DOT), a compound performing both as calcium antagonist and as 5-HT2 receptor antagonist, was evaluated for its ability to protect against electroconvulsive shock (ECS)- and pentylenetetrazol (PTZ)-induced performance deficit in a passive avoidance "step-down" task in rats. Its effect on electric and PTZ seizure models was also studied. DOT administered orally at a dose of 50 mg/kg for 5 days before learning had no significant effect on retention tests given 3 h, 24 h and 7 days after the training session. It should be noted, however, that DOT completely prevented ECS- and PTZ-induced amnesia in passive avoidance situation. DOT had a pronounced protective effect against electric seizures but did not affect PTZ seizures. The present results provide additional evidence for the role of serotonergic neurotransmitter system and calcium homeostasis for memory and seizure reactivity and may be important in the development of effective treatment strategies for memory dysfunction.

Administration, Oral↗

Isolation of mitochondrial DNA-less mouse cell lines and their application for trapping mouse synaptosomal mitochondrial DNA with deletion mutations.

For isolation of mouse mtDNA-less (rho0) cell lines, we searched for various antimitochondrial drugs that were expected to decrease the mtDNA content and found that treatment with ditercalinium, an antitumor bis-intercalating agent, was extremely effective for completely excluding mtDNA in all the mouse cell lines we tested. The resulting rho0 mouse cells were successfully used for trapping the mtDNA of living nerve cells into dividing cultured cells by fusion of the rho0 cells with mouse brain synaptosomes, which represent synaptic endings isolated from nerve cells. With neuronal mtDNA obtained, all of the cybrid clones restored mitochondrial translation activity similarly regardless of whether the mtDNA was derived from young or aged mice, thus at least suggesting that defects in mitochondrial genomes are not involved in the age-associated mitochondrial dysfunction observed in the brain of aged mice. Furthermore, we could trap a very small amount of a common 5823-base pair deletion mutant mtDNA (DeltamtDNA5823) that was detectable by polymerase chain reaction in the cybrid clones. As the amount of mutant mtDNA with large scale deletions was expected to increase during prolonged cultivation of the cybrids, these cells should be available for establishment of mice containing the deletion mutant mtDNA.

3T3 Cells↗

[Do the glutamate excitotoxicity theory and potential free radicals implication in schizophrenia aetiopathogenesis provide a new enlightenment to links between: genome, environment and biology in the determinism of that disorder?].

The aetiopathogenesis of schizophrenia constitutes nowadays one of the major points of interest for researchers on this cosmopolitan disorder which involves about 1% of the world population and which significantly alters the social functioning of the individual. Numerous studies have focused on the role played by genome, environmental factors and biology in the development of symptoms. The neurodevelopmental theory is an illustration with the perinatal period considered as the main provider of environmental factors (hypertension, infections, bleedings during pregnancy, acute and chronic fetal distress.). Many authors found significant associations between such factors, the occurrence of brain lesions and finally schizophrenic symptoms. Although no convincing genetic model had been established to date for schizophrenia, nevertheless it appears that a predisposition not inheritable under the mendelian mode exists and authors showed that disease gets more and more severe over schizophrenic descendants. The risk to be schizophrenic being a first degree relative of the schizophrenic person is about ten time superior than in general population. Indeed, this risk is also about ten time superior in biological parents of schizophrenic adoptees than in biological parents of healthy adoptees. Studies done in monozygotic comparing to dizygotic twins are in favour of an important role played by genetic factors more than socioeducational or psychological factors. Concerning biology, the dopaminergic hypothesis remains shared by numerous authors although direct links with incriminated factors are not well established. Now is suspected the glutamate excitotoxicity with implication of free radicals in schizophrenia. These free radicals are products of various enzymatic activations led by overstimulation of post synaptic receptors (NMDA and AMPA) by the excess glutamate. Therefore, according to that concept, some amino acids as glutamate and derivatives could have through free radicals a noxious effect on neuronal synapses. This could be due to a failing of their recapture at the presynaptic level in addition to a dysfunctioning of the antioxidizing system (glutathion, carnosine, superoxide dismutase, aspartate) to which dopamine and other monoamines might participate. The question is whether or not this theory contributes to shed light on links between: genome, environmental factors and biology in schizophrenia. Through the review and discussion of genetical aspects of schizophrenia, environmental factors and the biological aspect, we intend to revive debate on that question. The articles and authors were selected with regard to the aptness of their publications on that subject, their evolving ideas and finally the interest of their works for neurosciences. This new approach perhaps is opening the way to new therapeutic perspectives in the treatment of schizophrenia based on the antioxidizing substances as shown for some neurological diseases (amyotrophic lateral sclerosis, Parkinson's disease and Huntington's chorea) for which experiments are going on.

Environment↗

Pentobarbital protects against CA1 pyramidal cell death but not dysfunction of hippocampal cholinergic neurons following transient ischemia.

Effects of pentobarbital on the release of acetylcholine (ACh), the area of CA1 pyramidal cell soma and the immunoreactivity of choline acetyltransferase (ChAT) in the hippocampus following ischemia were investigated. Five minute ischemia significantly decreased the KCl-, atropine-induced and basal release of ACh and the area of CA1 pyramidal cell soma in the hippocampus. Moreover, ChAT immunoreactivity, a marker of pre-synaptic terminal survival in the cholinergic neurons, was lowered 14 days after ischemia-recirculation. Although treatment with pentobarbital (50 mg/kg) 30 min before ischemia provided complete protection against hippocampal CA1 pyramidal cell death, pentobarbital failed to improve the decrements of ACh release and the low ChAT immunoreactivity over the test period. Our study thus showed discrepancies between pre-synaptic neurochemical estimation and post-synaptic morphological observation of the effect of pentobarbital on ischemic damage.

Acetylcholine↗

GABAergic dysfunction in schizophrenia and mood disorders as reflected by decreased levels of glutamic acid decarboxylase 65 and 67 kDa and Reelin proteins in cerebellum.

BACKGROUND: Glutamic acid decarboxylase (GAD) is the rate limiting enzyme responsible for conversion of glutamate to gamma-aminobutyric acid (GABA) regulating levels of glutamate and GABA in the mammalian brain. Reelin is an extracellular matrix protein that helps in normal lamination of the embryonic brain and subserves synaptic plasticity in adult brain. Both GAD and Reelin are colocalized to the same GABAergic interneurons in several brain sites. We hypothesized that levels of GAD and Reelin would be altered in cerebellum of subjects with schizophrenia and mood disorders differentially vs. controls. METHODS: We employed SDS-PAGE and Western blotting to measure levels of GAD isomers 65 and 67 kDa and Reelin isoforms 410-, 330- and 180-kDa proteins as well as beta-actin in cerebellum of subjects with schizophrenia, bipolar disorder and major depression vs. controls (N = 15 per group). RESULTS: GAD 65- and 67-kDa levels were decreased significantly in bipolar, depressed and schizophrenic subjects (p < 0.05) vs. controls. Reelin 410- and 180-kDa proteins decreased significantly (p < 0.05) in bipolar subjects vs. controls. Reelin 180 kDa was decreased significantly (p < 0.05) in schizophrenics vs. controls. beta-Actin levels did not vary significantly between groups. There were no significant effects of confounding variables on levels of various proteins. CONCLUSION: This study demonstrates for the first time significant deficits in GABAergic markers Reelin and GAD 65 and 67 proteins in bipolar subjects and global deficits in the latter proteins in schizophrenia and mood disorders, accounting for the reported alterations in CSF/plasma levels of glutamate and GABA in these disorders.

Adult↗

The Caenorhabditis elegans unc-64 locus encodes a syntaxin that interacts genetically with synaptobrevin.

We describe the molecular cloning and characterization of the unc-64 locus of Caenorhabditis elegans. unc-64 expresses three transcripts, each encoding a molecule with 63-64% identity to human syntaxin 1A, a membrane- anchored protein involved in synaptic vesicle fusion. Interestingly, the alternative forms of syntaxin differ only in their C-terminal hydrophobic membrane anchors. The forms are differentially expressed in neuronal and secretory tissues; genetic evidence suggests that these forms are not functionally equivalent. A complete loss-of-function mutation in unc-64 results in a worm that completes embryogenesis, but arrests development shortly thereafter as a paralyzed L1 larva, presumably as a consequence of neuronal dysfunction. The severity of the neuronal phenotypes of C. elegans syntaxin mutants appears comparable to those of Drosophila syntaxin mutants. However, nematode syntaxin appears not to be required for embryonic development, for secretion of cuticle from the hypodermis, or for the function of muscle, in contrast to Drosophila syntaxin, which appears to be required in all cells. Less severe viable unc-64 mutants exhibit a variety of behavioral defects and show strong resistance to the acetylcholinesterase inhibitor aldicarb. Extracellular physiological recordings from pharyngeal muscle of hypomorphic mutants show alterations in the kinetics of transmitter release. The lesions in the hypomorphic alleles map to the hydrophobic face of the H3 coiled-coil domain of syntaxin, a domain that in vitro mediates physical interactions with similar coiled-coil domains in SNAP-25 and synaptobrevin. Furthermore, the unc-64 syntaxin mutants exhibit allele-specific genetic interactions with mutants carrying lesions in the coiled-coil domain of synaptobrevin, providing in vivo evidence for the significance of these domains in regulating synaptic vesicle fusion.

Amino Acid Sequence↗

Understanding mental retardation in Down's syndrome using trisomy 16 mouse models.

Mental retardation in Down's syndrome, human trisomy 21, is characterized by developmental delays, language and memory deficits and other cognitive abnormalities. Neurophysiological and functional information is needed to understand the mechanisms of mental retardation in Down's syndrome. The trisomy mouse models provide windows into the molecular and developmental effects associated with abnormal chromosome numbers. The distal segment of mouse chromosome 16 is homologous to nearly the entire long arm of human chromosome 21. Therefore, mice with full or segmental trisomy 16 (Ts65Dn) are considered reliable animal models of Down's syndrome. Ts65Dn mice demonstrate impaired learning in spatial tests and abnormalities in hippocampal synaptic plasticity. We hypothesize that the physiological impairments in the Ts65Dn mouse hippocampus can model the suboptimal brain function occuring at various levels of Down's syndrome brain hierarchy, starting at a single neuron, and then affecting simple and complex neuronal networks. Once these elements create the gross brain structure, their dysfunctional activity cannot be overcome by extensive plasticity and redundancy, and therefore, at the end of the maturation period the mind inside this brain remains deficient and delayed in its capabilities. The complicated interactions that govern this aberrant developmental process cannot be rescued through existing compensatory mechanisms. In summary, overexpression of genes from chromosome 21 shifts biological homeostasis in the Down's syndrome brain to a new less functional state.

Animals↗

Impaired dopamine storage resulting from alpha-synuclein mutations may contribute to the pathogenesis of Parkinson's disease.

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by the inability to initiate, execute and control movement. Neuropathologically, there is a striking loss of dopamine-producing neurons in the substantia nigra pars compacta, accompanied by depletion of dopamine in the striatum. Most forms of PD are sporadic, though in some cases familial inheritance is observed. In the late 1990s, two mutations in the alpha-synuclein gene were linked to rare, autosomal dominant forms of PD. Previously cloned from cholinergic vesicles of the Torpedo electric ray, alpha-synuclein is highly enriched in presynaptic nerve terminals and appears to be involved in synapse maintenance and plasticity. It is expressed ubiquitously in the brain, raising the important question of why dopaminergic neurons are primarily targeted in persons carrying mutations in alpha-synuclein. In this article, we review the current literature on alpha-synuclein and suggest a possible role for this protein in vesicle recycling via its regulation of phospholipase D2, its fatty acid-binding properties, or both. Exogenous application of dopamine, as well as redistribution of vesicular dopamine to the cytoplasm, can be toxic to dopaminergic neurons. Thus, impaired neurotransmitter storage arising from mutations in alpha-synuclein could lead to cytoplasmic accumulation of dopamine. The breakdown of this labile neurotransmitter in the cytoplasm could, in turn, promote oxidative stress and metabolic dysfunction, both of which have been observed in nigral tissue from PD patients.

Animals↗