Abnormal Synaptic Plasticity Research Articles

Anatomical and physiological basis of continuous spike–wave of sleep syndrome after early thalamic lesions

ObjectiveEarly neonatal thalamic lesions account for about 14% of continuous spike–wave of sleep (CSWS) syndrome, representing the most common etiology in this epileptic encephalopathy in children, and promise useful insights into the pathophysiology of the disease. MethodsWe describe nine patients with unilateral neonatal thalamic lesions which progressed to CSWS. Longitudinal whole-night and high-density electroencephalograms (EEGs) were performed, as well as detailed imaging and clinical evaluation. Visual evoked potentials were used to probe cortical excitability. ResultsThalamic volume loss ranged from 19% to 94%, predominantly on medial and dorsal nuclei and sparing the ventral thalamus. Lesions produced white matter loss and ventricle enlargement on the same hemisphere, which in four patients was associated with selective loss of thalamic-cortical fibers. Cortical thickness quantification failed to reveal hemispheric asymmetries. Impact on EEG rhythms was mild, with a volume-loss-related decrease in alpha power and preservation of sleep spindles. The sleep continuous spiking was lateralized to the hemisphere with the lesion. Visual cortex stimulation in five patients with posterior cortex spiking revealed an abnormal frequency-dependent excitability at 10–20Hz on the side of the lesion. SignificanceUnilateral selective thalamic-cortical disconnection is a common feature in our patients and is associated with both a focal pattern of CSWS and a pathological type of frequency-dependent excitability (peak: 10–20Hz). We propose that this excitability represents an abnormal synaptic plasticity previously described as the augmenting response. This synaptic plasticity has been described as absent in the corticocortical interactions in healthy experimental animals, emerging after ablation of the thalamus and producing a frequency-dependent potentiation with a peak at 10–20Hz. Because this response is potentiated by sleep states of reduced brainstem activation and by appropriate stimulating rhythms, such as sleep spindles, the simultaneous occurrence of these two factors in nonrapid-eye-movement sleep is proposed as an explanation for CSWS in our patients.

P 77 Optogenetic stimulations of striatal cholinergic interneurons in an animal model of dystonia

Dystonias are movement disorders, defined by sustained or intermittent muscle contractions causing twisting movements and postures. The most prevalent inherited form of dystonia is caused by a mutation in the gene for torsin A (DYT1, ΔGAG) with incomplete penetrance. It has been hypothesized that an increased activity of cholinergic interneurons in the striatum, resulting in abnormal synaptic plasticity, plays an important role in the disease pathophysiology. However, this hypothesis is merely based on ex vivo electrophysiological recordings in brain slices of animal models which do not show a dystonic phenotype. Here we aim to substantiate the role of this alteration for motor dysfunctions in vivo by using optogenetic activation of striatal cholinergic interneurons in freely behaving mice. For this purpose we crossed Dyt1 ΔGAG heterozygous knock-in mice (DYT1) with mice expressing a light-dependent cation channel (Channelrhodopsin2, ChR2), essential for optogenetic examinations, specifically in cholinergic interneurons (Chat promotor). ChR2 is opened by blue light (wavelength 470 nm), leading to depolarization and acetylcholine release. The light (LED) is conveyed by optical cannulae, which were chronically implanted by stereotaxic surgery into the striatum of DYT1/ChR2 mice and wildtype (WT)/ChR2 littermates. First, we ensured that ChR2 is expressed in cholinergic interneurons of ChR2 + mice by immunohistochemistry and confocal microscopy and that these neurons respond to blue light exposure in brain slices. Subsequently, effects of different stimulation parameters (e.g. pulse width, Hz) on motor activity were examined in WT/ChR2 ( n = 9) and DYT1/ChR2 mice ( n = 6). Bilateral and unilateral photostimulations with blue LED light were compared to yellow LED light which does not open ChR2 (negative control). As expected, yellow light had no influence on behavior. Effects of bilateral stimulations with blue light were more robust than unilateral stimulations. Selected stimulation parameters induced significant increases of general activity (increased distance moved) specifically in DYT1/ChR2 mice compared to before or after stimulation, while there were no effects in WT/ChR2. This substantiates a specific role of striatal cholinergic neurons in motor activity in DYT1 dystonia. We have recently shown that DYT1 KI mice, which do not develop overt dystonia, present with sensorimotor deficits in specific behavioral paradigms. Optogenetic activation of cholinergic interneurons ameliorated this deficit temporarily. Preliminary data of an investigation of neuronal activity in the striatum after stimulation indicated an increased number of ChR2 cells positive for c-Fos in Dyt1/ChR2 ( n = 4) in comparison to WT/ChR2 ( n = 5) while there was no difference in total number of c-Fos positive cells. Total number of c-Fos positive cells did also not differ between naive WT/ChR2 and Dyt1/ChR2 mice, supporting that increased c-Fos expression in cholinergic interneurons is the expected effect of optogenetic stimulation on neuronal activity. We now perform comprehensive studies with short- and long-term stimulations in the Dyt1 mice to clarify the importance of overactive striatal cholinergic interneurons for the manifestation and abnormal striatal plasticity in DYT1 dystonia. For this purpose optogenetics will be combined with pharmacological, electrophysiological, neurochemical and molecular examinations.

Spine Enlargement of Pyramidal Tract-Type Neurons in the Motor Cortex of a Rat Model of Levodopa-Induced Dyskinesia.

Growing evidence suggests that abnormal synaptic plasticity of cortical neurons underlies levodopa-induced dyskinesia (LID) in Parkinson's disease (PD). Spine morphology reflects synaptic plasticity resulting from glutamatergic transmission. We previously reported that enlargement of the dendritic spines of intratelencephalic-type (IT) neurons in the primary motor cortex (M1) is linked to the development of LID. However, the relevance of another M1 neuron type, pyramidal-tract (PT) neurons, to LID remains unknown. We examined the morphological changes of the dendritic spines of M1 PT neurons in a rat model of LID. We quantified the density and size of these spines in 6-hydroxydopamine-lesioned rats (a model of PD), 6-hydroxydopamine-lesioned rats chronically treated with levodopa (a model of LID), and control rats chronically treated with levodopa. Dopaminergic denervation alone had no effect on spine density and head area. However, the LID model showed significant increases in the density and spine head area and the development of dyskinetic movements. In contrast, levodopa treatment of normal rats increased spine density alone. Although, chronic levodopa treatment increases PT neuron spine density, with or without dopaminergic denervation, enlargement of PT neuron spines appears to be a specific feature of LID. This finding suggests that PT neurons become hyperexcited in the LID model, in parallel with the enlargement of spines. Thus, spine enlargement, and the resultant hyperexcitability of PT pyramidal neurons, in the M1 cortex might contribute to abnormal cortical neuronal plasticity in LID.

60.3 GABA Spectroscopy and Cognitive Functioning in Schizophrenia

Background: Schizophrenia is characterized by loss of brain volume, which may represent an ongoing pathophysiological process. This loss of brain volume may be explained by reduced neuropil rather than neuronal loss, suggesting abnormal synaptic plasticity and cortical microcircuitry. Schizophrenia is also characterized by cognitive impairments. A possible mechanism in schizophrenia is hypofunction of the NMDA-type of the glutamate receptor in the medial prefrontal cortex, which reduces the excitation of inhibitory GABAergic interneurons, resulting in a disinhibition of glutamatergic pyramidal neurons. Disinhibition of pyramidal cells may result in excessive stimulation by glutamate, which in turn could cause neuronal damage or death through excitotoxicity. Methods: We measured GABA–creatine ratios, and glutamate, NAA, creatine and choline concentrations in the prefrontal and parieto-occipital cortices using proton magnetic resonance spectroscopy at an ultra-high magnetic field strength of 7 T (Marsman et al, 2014). In addition, general cognitive functioning was measured using the Wechsler Adult Intelligence Test. Measurements were available in 17 patients with schizophrenia and 23 healthy controls. Results: Significantly lower GABA/Cr ratios were found in patients with schizophrenia in the prefrontal cortex as compared to healthy controls. Moreover, GABA–Cr ratios in the prefrontal cortex were inversely correlated with cognitive functioning in the patients. No significant differences in metabolite concentrations in the parieto-occipital cortex were found between the groups. Conclusion: Our findings support a mechanism involving altered GABA levels distinguished from glutamate levels in the medial prefrontal cortex in schizophrenia, particularly in high functioning patients. In a second study, including siblings of patients with schizophrenia we are currently assessing whether altered GABA levels are related to causes or consequences of the disease.

Parkin promotes proteasomal degradation of synaptotagmin IV by accelerating polyubiquitination.

Parkin is an E3 ubiquitin ligase whose mutations cause autosomal recessive juvenile Parkinson's disease (PD). Unlike the human phenotype, parkin knockout (KO) mice show no apparent dopamine neuron degeneration, although they demonstrate reduced expression and activity of striatal mitochondrial proteins believed to be necessary for neuronal survival. Instead, parkin-KO mice show reduced striatal evoked dopamine release, abnormal synaptic plasticity, and non-motor symptoms, all of which appear to mimic the preclinical features of Parkinson's disease. Extensive studies have screened candidate synaptic proteins responsible for reduced evoked dopamine release, and synaptotagmin XI (Syt XI), an isoform of Syt family regulating membrane trafficking, has been identified as a substrate of parkin in humans. However, its expression level is unaltered in the striatum of parkin-KO mice. Thus, the target(s) of parkin and the molecular mechanisms underlying the impaired dopamine release in parkin-KO mice remain unknown. In this study, we focused on Syt IV because of its highly homology to Syt XI, and because they share an evolutionarily conserved lack of Ca2+-binding capacity; thus, Syt IV plays an inhibitory role in Ca2+-dependent neurotransmitter release in PC12 cells and neurons in various brain regions. We found that a proteasome inhibitor increased Syt IV protein, but not Syt XI protein, in neuron-like, differentiated PC12 cells, and that parkin interacted with and polyubiquitinated Syt IV, thereby accelerating its protein turnover. Parkin overexpression selectively degraded Syt IV protein, but not Syt I protein (indispensable for Ca2+-dependent exocytosis), thus enhancing depolarization-dependent exocytosis. Furthermore, in parkin-KO mice, the level of striatal Syt IV protein was increased. Our data indicate a crucial role for parkin in the proteasomal degradation of Syt IV, and provide a potential mechanism of parkin-regulated, evoked neurotransmitter release.

Disrupted-in-schizophrenia1 (DISC1) L100P mutation alters synaptic transmission and plasticity in the hippocampus and causes recognition memory deficits.

Disrupted-in-schizophrenia 1(DISC1) is a promising candidate susceptibility gene for a spectrum of psychiatric illnesses that share cognitive impairments in common, including schizophrenia, bipolar disorder and major depression. Here we report that DISC1 L100P homozygous mutant shows normal anxiety- and depression-like behavior, but impaired object recognition which is prevented by administration of atypical antipsychotic drug clozapine. Ca2+ image analysis reveals suppression of glutamate-evoked elevation of cytoplasmic [Ca2+] in L100P hippocampal slices. L100P mutant slices exhibit decreased excitatory synaptic transmission (sEPSCs and mEPSCs) in dentate gyrus (DG) and impaired long-term potentiation in the CA1 region of the hippocampus. L100P mutation does not alter proteins expression of the excitatory synaptic markers, PSD95 and synapsin-1; neither does it changes dendrites morphology of primary cultured hippocampal neurons. Our findings suggest that the existence of abnormal synaptic transmission and plasticity in hippocampal network may disrupt declarative information processing and contribute to recognition deficits in DISC1 L100P mutant mice.

Epileptiform activity and spreading depolarization in the blood–brain barrier-disrupted peri-infarct hippocampus are associated with impaired GABAergic inhibition and synaptic plasticity

Peri-infarct opening of the blood–brain barrier may be associated with spreading depolarizations, seizures, and epileptogenesis as well as cognitive dysfunction. We aimed to investigate the mechanisms underlying neural network pathophysiology in the blood–brain barrier-dysfunctional hippocampus. Photothrombotic stroke within the rat neocortex was associated with increased intracranial pressure, vasogenic edema, and peri-ischemic blood–brain barrier dysfunction that included the ipsilateral hippocampus. Intrahippocampal recordings revealed electrographic seizures within the first week in two-thirds of animals, accompanied by a reduction in gamma and increase in theta frequency bands. Synaptic interactions were studied in parasagittal hippocampal slices at 24 h and seven days post-stroke. Field potential recordings in CA1 and CA3 uncovered multiple population spikes, epileptiform episodes, and spreading depolarizations at 24 h. Input–output analysis revealed that fEPSP-spike coupling was significantly enhanced at seven days. In addition, CA1 feedback and feedforward inhibition were diminished. Slices generating epileptiform activity at seven days revealed impaired bidirectional long-term plasticity following high and low-frequency stimulation protocols. Microarray and PCR data confirmed changes in expression of astrocyte-related genes and suggested downregulation in expression of GABAA-receptor subunits. We conclude that blood-brain barrier dysfunction in the peri-infarct hippocampus is associated with early disinhibition, hyperexcitability, and abnormal synaptic plasticity.

Current knowledge on the genetics of autism and propositions for future research

Autism spectrum disorders (ASD) are a heterogeneous group of neuropsychiatric disorders characterized by problems in social communication, as well as by the presence of restricted interests, stereotyped and repetitive behaviours. In the last 40years, genetic studies have provided crucial information on the causes of ASD and its diversity. In this article, I will first review the current knowledge on the genetics of ASD and then suggest three propositions to foster research in this field. Twin and familial studies estimated the heritability of ASD to be 50%. While most of the inherited part of ASD is captured by common variants, our current knowledge on the genetics of ASD comes almost exclusively from the identification of highly penetrant de novo mutations through candidate gene or whole exome/genome sequencing studies. Approximately 10% of patients with ASD, especially those with intellectual disability, are carriers of de novo copy-number (CNV) or single nucleotide variants (SNV) affecting clinically relevant genes for ASD. Given the function of these genes, it was hypothesized that abnormal synaptic plasticity and failure of neuronal/synaptic homeostasis could increase the risk of ASD. In addition to these discoveries, three propositions coming from institutions, researchers and/or communities of patients and families can be made to foster research on ASD: (i) to use more dimensional and quantitative data than diagnostic categories; (ii) to increase data sharing and research on genetic and brain diversity in human populations; (iii) to involve patients and relatives as participants for research. Hopefully, this knowledge will lead to a better diagnosis, care and integration of individuals with ASD.

PLPP/CIN regulates bidirectional synaptic plasticity via GluN2A interaction with postsynaptic proteins.

Dendritic spines are dynamic structures whose efficacies and morphologies are modulated by activity-dependent synaptic plasticity. The actin cytoskeleton plays an important role in stabilization and structural modification of spines. However, the regulatory mechanism by which it alters the plasticity threshold remains elusive. Here, we demonstrate the role of pyridoxal-5′-phosphate phosphatase/chronophin (PLPP/CIN), one of the cofilin-mediated F-actin regulators, in modulating synaptic plasticity in vivo. PLPP/CIN transgenic (Tg) mice had immature spines with small heads, while PLPP/CIN knockout (KO) mice had gigantic spines. Furthermore, PLPP/CIN Tg mice exhibited enhanced synaptic plasticity, but KO mice showed abnormal synaptic plasticity. The PLPP/CIN-induced alterations in synaptic plasticity were consistent with the acquisition and the recall capacity of spatial learning. PLPP/CIN also enhanced N-methyl-D-aspartate receptor (GluN) functionality by regulating the coupling of GluN2A with interacting proteins, particularly postsynaptic density-95 (PSD95). Therefore, these results suggest that PLPP/CIN may be an important factor for regulating the plasticity threshold.

Decreased expression of cortactin in the schizophrenia brain.

Schizophrenia is a severe psychiatric disorder that is characterized by a wide array of symptoms and a complex neuropathology. A well-characterized neurobiological feature of schizophrenia is abnormal synaptic plasticity, although the mechanisms underlying this are not fully understood. Numerous studies have demonstrated a link between proper functioning of the cytoskeleton and synaptic plasticity. The actin-related protein-2/3 (Arp2/3) complex is responsible for the nucleation of new actin filaments and elongation of existing actin filaments and is thus crucial to cytoskeletal dynamics, especially actin polymerization and organization. To determine whether the Arp2/3 complex is abnormally expressed in schizophrenia, we measured the protein expression of Arp2 and Arp3, as well as Arp2/3 complex binding partners and associated proteins including cortactin, neuronal-Wiskott-Aldrich syndrome protein (WASP), WASP-family verprolin homologous protein 1 (WAVE1), and Abelson interactor 1 (Abi1) in the superior temporal gyrus of paired schizophrenia and comparison participants. No changes were found in Arp2, Arp3, neuronal-WASP, WAVE1, or Abi1. However, all three isoforms of cortactin were decreased in schizophrenia. Specifically, the 62 kDa isoform was decreased by 43%; the 71 kDa isoform was decreased by 32%; and the 58 kDa isoform was decreased by 35%. Cortactin regulates branching of filamentous actin through its binding and activation of the Arp2/3 complex, and it is thus critical to the formation of stable actin networks. These findings contribute to a growing body of evidence implicating altered cytoskeletal dynamics in schizophrenia.

Editorial: Molecular Mechanisms for Reprogramming Hippocampal Development and Function by Early-Life Stress.

Editorial: Molecular Mechanisms for Reprogramming Hippocampal Development and Function by Early-Life Stress.

Impaired cognitive discrimination and discoordination of coupled theta–gamma oscillations in Fmr1 knockout mice

Fragile X syndrome (FXS) patients do not make the fragile X mental retardation protein (FMRP). The absence of FMRP causes dysregulated translation, abnormal synaptic plasticity and the most common form of inherited intellectual disability. But FMRP loss has minimal effects on memory itself, making it difficult to understand why the absence of FMRP impairs memory discrimination and increases risk of autistic symptoms in patients, such as exaggerated responses to environmental changes. While Fmr1 knockout (KO) and wild-type (WT) mice perform cognitive discrimination tasks, we find abnormal patterns of coupling between theta and gamma oscillations in perisomatic and dendritic hippocampal CA1 local field potentials of the KO. Perisomatic CA1 theta–gamma phase–amplitude coupling (PAC) decreases with familiarity in both the WT and KO, but activating an invisible shock zone, subsequently changing its location, or turning it off, changes the pattern of oscillatory events in the LFPs recorded along the somato-dendritic axis of CA1. The cognition-dependent changes of this pattern of neural activity are relatively constrained in WT mice compared to KO mice, which exhibit abnormally weak changes during the cognitive challenge caused by changing the location of the shock zone and exaggerated patterns of change when the shock zone is turned off. Such pathophysiology might explain how dysregulated translation leads to intellectual disability in FXS. These findings demonstrate major functional abnormalities after the loss of FMRP in the dynamics of neural oscillations and that these impairments would be difficult to detect by steady-state measurements with the subject at rest or in steady conditions.

Mutation of the CH1 Domain in the Histone Acetyltransferase CREBBP Results in Autism-Relevant Behaviors in Mice.

Autism spectrum disorders (ASDs) are a group of neurodevelopmental afflictions characterized by repetitive behaviors, deficits in social interaction, and impaired communication skills. For most ASD patients, the underlying causes are unknown. Genetic mutations have been identified in about 25 percent of ASD cases, including mutations in epigenetic regulators, suggesting that dysregulated chromatin or DNA function is a critical component of ASD. Mutations in the histone acetyltransferase CREB binding protein (CBP, CREBBP) cause Rubinstein-Taybi Syndrome (RTS), a developmental disorder that includes ASD-like symptoms. Recently, genomic studies involving large numbers of ASD patient families have theoretically modeled CBP and its paralog p300 (EP300) as critical hubs in ASD-associated protein and gene interaction networks, and have identified de novo missense mutations in highly conserved residues of the CBP acetyltransferase and CH1 domains. Here we provide animal model evidence that supports this notion that CBP and its CH1 domain are relevant to autism. We show that mice with a deletion mutation in the CBP CH1 (TAZ1) domain (CBPΔCH1/ΔCH1) have an RTS-like phenotype that includes ASD-relevant repetitive behaviors, hyperactivity, social interaction deficits, motor dysfunction, impaired recognition memory, and abnormal synaptic plasticity. Our results therefore indicate that loss of CBP CH1 domain function contributes to RTS, and possibly ASD, and that this domain plays an essential role in normal motor function, cognition and social behavior. Although the key physiological functions affected by ASD-associated mutation of epigenetic regulators have been enigmatic, our findings are consistent with theoretical models involving CBP and p300 in ASD, and with a causative role for recently described ASD-associated CBP mutations.

Novel Selective Calpain 1 Inhibitors as Potential Therapeutics in Alzheimer's Disease.

Alzheimer's disease, one of the most important brain pathologies associated with neurodegenerative processes, is related to overactivation of calpain-mediated proteolysis. Previous data showed a compelling efficacy of calpain inhibition against abnormal synaptic plasticity and memory produced by the excess of amyloid-β, a distinctive marker of the disease. Moreover, a beneficial effect of calpain inhibitors in Alzheimer's disease is predictable by the occurrence of calpain hyperactivation leading to impairment of memory-related pathways following abnormal calcium influxes that might ensue independently of amyloid-β elevation. However, molecules currently available as effective calpain inhibitors lack adequate selectivity. This work is aimed at characterizing the efficacy of a novel class of epoxide-based inhibitors, synthesized to display improved selectivity and potency towards calpain 1 compared to the prototype epoxide-based generic calpain inhibitor E64. Both functional and preliminary toxicological investigations proved the efficacy, potency, and safety of the novel and selective calpain inhibitors NYC438 and NYC488 as possible therapeutics against the disease.

NCAM-deficient mice show prominent abnormalities in serotonergic and BDNF systems in brain – Restoration by chronic amitriptyline

Mood disorders are associated with alterations in serotonergic system, deficient BDNF (brain-derived neurotrophic factor) signaling and abnormal synaptic plasticity. Increased degradation and reduced functions of NCAM (neural cell adhesion molecule) have recently been associated with depression and NCAM deficient mice show depression-related behavior and impaired learning. The aim of the present study was to investigate potential changes in serotonergic and BDNF systems in NCAM knock-out mice. Serotonergic nerve fiber density and SERT (serotonin transporter) protein levels were robustly reduced in the hippocampus, prefrontal cortex and basolateral amygdala of adult NCAM−/− mice. This SERT reduction was already evident during early postnatal development. [3H]MADAM binding experiments further demonstrated reduced availability of SERT in cell membranes of NCAM−/− mice. Moreover, the levels of serotonin and its major metabolite 5-HIAA were down regulated in the brains of NCAM−/− mice. NCAM−/− mice also showed a dramatic reduction in the BDNF protein levels in the hippocampus and prefrontal cortex. This BDNF deficiency was associated with reduced phosphorylation of its receptor TrkB. Importantly, chronic administration of antidepressant amitriptyline partially or completely restored these changes in serotonergic and BDNF systems, respectively. In conclusion, NCAM deficiency lead to prominent and persistent abnormalities in brain serotonergic and BDNF systems, which likely contributes to the behavioral and neurobiological phenotype of NCAM−/− mice.

Altered sensory experience exacerbates stable dendritic spine and synapse loss in a mouse model of Huntington's disease.

A key question in Huntington's disease (HD) is what underlies the early cognitive deficits that precede the motor symptoms and the characteristic neuronal death observed in HD. The mechanisms underlying cognitive symptoms in HD remain unknown. Postmortem HD brain and animal model studies demonstrate pathologies in dendritic spines and abnormal synaptic plasticity before motor symptoms and neurodegeneration. Experience-dependent synaptic plasticity caused by mechanisms such as LTP or novel sensory experience potentiates synaptic strength, enhances new dendritic spine formation and stabilization, and may contribute to normal cognitive processes, such as learning and memory. We have previously reported that under baseline conditions (without any sensory manipulation) neuronal circuitry in HD (R6/2 mouse model) was highly unstable, which led to a progressive loss of persistent spines in these mice, and that mutant huntingtin was directly involved in the process. Here, we investigated whether pathological processes of HD interfere with the normal experience-dependent plasticity of dendritic spines in the R6/2 model. Six weeks of two-photon in vivo imaging before and after whisker trimming revealed that sensory deprivation exacerbates loss of persistent-type, stable spines in R6/2 mice compared with wild-type littermates. In addition, sensory deprivation leads to impaired transformation of newly generated spines into persistent spines in R6/2 mice. As a consequence, reduced synaptic density and decreased PSD-95 protein levels are evident in their barrel cortical neurons. These data suggest that mutant huntingtin is implicated in maladaptive synaptic plasticity, which could be one of the plausible mechanisms underlying early cognitive deficits in HD.

B23 Mutant Huntingtin Causes Loss Of Mechanisms Underlying Long-term Memory Storage In The Brain Revealed By Two-photon Imaging

<h3>Background</h3> A key question in Huntington’s disease (HD) is what underlies the early cognitive deficits that precede the motor symptoms and the characteristic neuronal death observed in HD. The mechanisms underlying cognitive symptoms in HD remain unknown. Dendritic spine pathology and abnormal synaptic plasticity (which is found in post-mortem HD brain and in HD models prior to neurodegeneration) is thought to underlie the early cognitive symptoms observed in HD, however, direct in vivo evidence is lacking. Although HD is a genetically inherited disorder, it is affected by environmental factors inducing sensory, cognitive and motor stimulations. Experience-dependent synaptic plasticity caused by mechanisms such as LTP or novel sensory experience potentiates synaptic strength, enhances new dendritic spine formation and stabilisation and may contribute to normal cognitive processes, such as learning and memory. <h3>Aims</h3> The aim of this study is to investigate the mechanisms underlying early cognitive symptoms in Huntington’s disease. <h3>Methods/techniques</h3> We have followed individual dendritic spines as the disease progressed in HD model R6/2 using two-photon laser scanning microscopy. <h3>Results/outcome</h3> Using two-photon microscopy as the disease progressed in R6/2 HD model, we show that under baseline condition the cortical neuronal circuitry is highly unstable (already at the pre-symptomatic phase) in the R6/2 mice that led to a progressive loss of persistenttype mature spine in these mice compared to the wild-type littermates. Further, our results indicate that mutant <i>huntingtin </i>was directly involved in the loss of particularly the persistent-type mature spines, which are the site of long-term memory storage in the brain. We further observed that the pathological processes of HD interfered with the normal experience-dependent plasticity of dendritic spines in the R6/2 model. Six weeks of two-photon <i>in vivo</i>imaging before and after whisker trimming revealed that sensory deprivation exacerbated loss of persistent-type, stable spines in the R6/2 mice compared to wild-type littermates. In addition, sensory deprivation led to impaired transformation of newly generated spines into persistent spines in R6/2 mice. As a consequence, reduced synaptic density was observed in their barrel cortical neurons. <h3>Conclusions</h3> Our findings suggests that mutant <i>huntingtin </i>is implicated in the loss of mechanisms underlying long-term memory storage in the brain and in maladaptive synaptic plasticity, which could be one of the plausible mechanisms underlying early cognitive deficits in HD.

Selective loss of bi-directional synaptic plasticity in the direct and indirect striatal output pathways accompanies generation of parkinsonism and l-DOPA induced dyskinesia in mouse models

Parkinsonian symptoms arise due to over-activity of the indirect striatal output pathway, and under-activity of the direct striatal output pathway. l-DOPA-induced dyskinesia (LID) is caused when the opposite circuitry problems are established, with the indirect pathway becoming underactive, and the direct pathway becoming over-active. Here, we define synaptic plasticity abnormalities in these pathways associated with parkinsonism, symptomatic benefits of l-DOPA, and LID. We applied spike-timing dependent plasticity protocols to cortico-striatal synapses in slices from 6-OHDA-lesioned mouse models of parkinsonism and LID, generated in BAC transgenic mice with eGFP targeting the direct or indirect output pathways, with and without l-DOPA present. In naïve mice, bidirectional synaptic plasticity, i.e. LTP and LTD, was induced, resulting in an EPSP amplitude change of approximately 50% in each direction in both striatal output pathways, as shown previously. In parkinsonism and dyskinesia, both pathways exhibited unidirectional plasticity, irrespective of stimulation paradigm. In parkinsonian animals, the indirect pathway only exhibited LTP (LTP protocol: 143.5±14.6%; LTD protocol 177.7±22.3% of baseline), whereas the direct pathway only showed LTD (LTP protocol: 74.3±4.0% and LTD protocol: 63.3±8.7%). A symptomatic dose of l-DOPA restored bidirectional plasticity on both pathways to levels comparable to naïve animals (Indirect pathway: LTP protocol: 124.4±22.0% and LTD protocol: 52.1±18.5% of baseline. Direct pathway: LTP protocol: 140.7±7.3% and LTD protocol: 58.4±6.0% of baseline). In dyskinesia, in the presence of l-DOPA, the indirect pathway exhibited only LTD (LTP protocol: 68.9±21.3% and LTD protocol 52.0±14.2% of baseline), whereas in the direct pathway, only LTP could be induced (LTP protocol: 156.6±13.2% and LTD protocol 166.7±15.8% of baseline). We conclude that normal motor control requires bidirectional plasticity of both striatal outputs, which underlies the symptomatic benefits of l-DOPA. Switching from bidirectional to unidirectional plasticity drives global changes in striatal pathway excitability, and underpins parkinsonism and dyskinesia.

Mutant β-III spectrin causes mGluR1α mislocalization and functional deficits in a mouse model of spinocerebellar ataxia type 5.

Spinocerebellar ataxia type 5 (SCA5), a dominant neurodegenerative disease characterized by profound Purkinje cell loss, is caused by mutations in SPTBN2, a gene that encodes β-III spectrin. SCA5 is the first neurodegenerative disorder reported to be caused by mutations in a cytoskeletal spectrin gene. We have developed a mouse model to understand the mechanistic basis for this disease and show that expression of mutant but not wild-type β-III spectrin causes progressive motor deficits and cerebellar degeneration. We show that endogenous β-III spectrin interacts with the metabotropic glutamate receptor 1α (mGluR1α) and that mice expressing mutant β-III spectrin have cerebellar dysfunction with altered mGluR1α localization at Purkinje cell dendritic spines, decreased mGluR1-mediated responses, and deficient mGluR1-mediated long-term potentiation. These results indicate that mutant β-III spectrin causes mislocalization and dysfunction of mGluR1α at dendritic spines and connects SCA5 with other disorders involving glutamatergic dysfunction and synaptic plasticity abnormalities.

Impaired contextual fear extinction and hippocampal synaptic plasticity in adult rats induced by prenatal morphine exposure.

Prenatal opiate exposure causes a series of neurobehavioral disturbances by affecting brain development. However, the question of whether prenatal opiate exposure increases vulnerability to memory-related neuropsychiatric disorders in adult offspring remains largely unknown. Here, we found that rats prenatally exposed to morphine (PM) showed impaired acquisition but enhanced maintenance of contextual fear memory compared with control animals that were prenatally exposed to saline (PS). The impairment of acquisition was rescued by increasing the intensity of footshocks (1.2 mA rather than 0.8 mA). Meanwhile, we also found that PM rats exhibited impaired extinction of contextual fear, which is associated with enhanced maintenance of fear memory. The impaired extinction lasted for 1 week following extinction training. Furthermore, PM rats exhibited reduced anxiety-like behavior in the elevated plus-maze and light/dark box test without differences in locomotor activity. These alterations in PM rats were mirrored by abnormalities in synaptic plasticity in the Schaffer collateral-CA1 synapses of the hippocampus in vivo. PS rats showed blocked long-term potentiation and enabled long-term depression in CA1 synapses following contextual fear conditioning, while prenatal morphine exposure restricted synaptic plasticity in CA1 synapses. The smaller long-term potentiation in PM rats was not further blocked by contextual fear conditioning, and the long-term depression enabled by contextual fear conditioning was abolished. Taken together, our results provide the first evidence suggesting that prenatal morphine exposure may increase vulnerability to fear memory-related neuropsychiatric disorders in adulthood.