Hippocampal synaptic metaplasticity requires the activation of NR2B-containing NMDA receptors

The potential to exhibit synaptic plasticity itself is modulated by previous synaptic activity, which has been termed as metaplasticity. In this paper, we demonstrated that the activation of N-methyl-d-aspartate (NMDA) receptor 2B (NR2B) subunit in NNDA receptors was required for hippocampal metaplasticity at Schaffer collateral-commissural fiber-CA1 synapses. Brief 5 Hz priming stimulation did not cause long-term synaptic plasticity; however, it could result in the inhibition of subsequently evoked long-term potentiation (LTP). Meanwhile, the application of selective antagonists for NR2B subunit of NMDA receptors after delivering priming stimulation could block the metaplasticity. In contrast, LTP induction was not affected by NR2B antagonists in slices without pre-treatment of priming stimulation. These results indicated that the activation of NR2B-containing NMDA receptors was required for metaplasticity.     

Contribution of NMDA receptor NR2B subunit to synaptic plasticity during associative learning in behaving rats

The difference in the amounts of NR2 subunits contained in NMDA receptors of the hippocampus has been related to their different involvement in activity-dependent synaptic plasticity. Here, we show that Ro 25-6981, a high-affinity and selective blocker of NMDA receptors containing NR2B subunits, is able to block the acquisition of a trace conditioning paradigm in adult rats, a task that requires the active participation of hippocampal circuits. Reconditioning with the same trace paradigm was also prevented by Ro 25-6981. In addition, we show that the slope of monosynaptic field excitatory postsynaptic potentials evoked at the dentate gyrus by single pulses presented to the medial perforant pathway increases significantly across conditioning sessions and during reconditioning, in a linear relationship with the increase in the number of classically conditioned eyelid responses. Administration of Ro 25-6981 prevented these learning-related changes in synaptic strength at the perforant pathway-dentate granule cell synapse. The present results suggest the involvement of NR2B-containing NMDA receptors in hippocampal functions related to both associative learning and activity-dependent synaptic plasticity.     

Postnatal development of NR1, NR2A and NR2B immunoreactivity in the visual cortex of the rat

N-Methyl-D-aspartate receptors (NMDARs) are critically involved in some types of synaptic plasticity. The NMDAR subunits NR1, NR2A and NR2B are developmentally regulated, and it has been proposed that developmental changes in their expression may underlie developmental changes in cortical plasticity. Age-dependent change in cortical plasticity is most commonly measured by the monocular deprivation effect, which occurs during a critical period between P22 and P50 in the rat. Although the development of NMDAR subunits has been studied from birth through the fourth postnatal week, there is only meager information from older ages when visual plasticity ends. We hypothesized that there will be significant age-dependent change in expression of NR1, NR2A or NR2B between P22, when the cortex is plastic, and P90, when it is not. We applied specific antibodies recognizing NR1, NR2A and NR2B to the primary visual cortex at P14, P22, P30, P45 and P90. We found age-dependent changes in NR1-IR that were negatively correlated with changes in NR2A-IR; these subunits are not regulated in unison. In contrast, NR2A-IR and NR2B-IR were positively correlated. NR2A-IR and NR2B-IR both passed through a developmental minimum around P45, then recovered to approximately their P22 level. NR1-IR passed through a maximum at P45. There were no significant differences between P22 and P90. These results do not support the simple hypothesis that the loss of plasticity corresponds to a simple transition from juvenile levels of NMDAR subunit proteins to new adult levels. On the other hand, the results do confirm the hypothesis that there are significant changes in processing of NMDAR proteins during the time that plasticity is lost. How these changes of IR relate to synaptic transmission and plasticity needs to be clarified.     

Impaired hippocampal synaptic plasticity and NR2A/2B expression ratio in remifentanil withdrawal rats

Remifentanil is a kind of synthetic opioid which has gained wide clinical acceptance by anesthesiologists. In this study, we attempted to test whether withdrawal effects on learning mechanisms can be triggered by repeated low-dose remifentanil treatment. Male Sprague-Dawley (SD) rats were subjected to remifentanil (50 μg/ kg s.c.) twice per day at 12 h intervals for 15 days. When the animals of remifentanil group were withdrawn from remifentanil at 10 h after the last injection, changes in open field test, Morris water maze test (MWM) and synaptic efficacy were examined in each group. We demonstrated that repeated exposure to 50 μg/kg remifentanil produced enhanced locomotor activity indicating that a remifentanil addiction animal model in rats was established. MWM results showed that exposure to remifentanil had no influence on the spatial cognition. After withdrawal of remifentanil rats showed impaired spatial cognition. In electrophysiology test, remifentanil group rats showed a trend for a rightward shift of input/output relationship and significant deficits in maintenance of STP and LTP. Immunohistochemistry results demonstrated increased NR2A/NR2B ratio that should be included depression of LTP. In the whole-cell patch-clamp recording, after elimination from remifentanil incubation, mEPSC frequency was down regulated in hippocampal CA1 neurons, indicating that basal synaptic transmission were affected by remifentanil withdrawal. Taken together, the current findings demonstrate that the remifentanil withdrawn rats exhibit obvious impairment of hippocampus-dependent memory and synaptic plasticity. Increased hippocampal NR2A/NR2B expression ratio and the changes of basal synaptic transmission may participate in the impairment of LTP.     

Functional roles of NMDA receptor NR2A and NR2B subunits in the acute intoxicating effects of ethanol in mice

The present study examined the roles of NR2A and NR2B subunit-containing NMDA receptors in the mediation of the sedative/hypnotic effects of ethanol in mice. The ability of the competitive NMDA antagonist, CGP-37849 (0, 1, or 3 mg/kg), and the NR2B-selective antagonist, Ro 25-6981 (0, 3, or 10 mg/kg), to alter (3 g/kg) ethanol-induced sleep time was measured in C57BL/6J mice and NR2A knockout (KO) mice. The results show that pretreatment with either antagonist significantly potentiated the sedative/hypnotic effects of ethanol in C57BL/6J mice. These effects were not significantly altered in NR2A KO mice. Basal sleep time responses to ethanol were also normal in NR2A KO mice. These findings confirm a major role for NMDA receptors in the acute intoxicating actions of ethanol and provide tentative support for a prepotent role of the NR2B subunit in these effects.     

Distribution and expression of the subunits of N-methyl-d-aspartate (NMDA) receptors; NR1, NR2A and NR2B in hypoxic newborn piglet brains

The purpose of this study was to identify the distribution and the expression of the NR1, NR2A and NR2B subunits of the NMDA receptor after cerebral hypoxia. Ten piglets were divided into control and hypoxic groups (n=5, each). The control piglets were ventilated with normoxia for 1 h, and the hypoxic piglets were ventilated with hypoxia until p_aO₂ was below 20 mmHg. Tissue samples from the nine different regions of newborn piglet brain were obtained, and the protein amount of the NR1, NR2A, and NR2B subunits measured by immunoblot using the antibody to the NR1, NR2A, and NR2B subunits. The NR1, N2A, and NR2B subunits were distributed very differently; hippocampus and cortical area are more prominent than white matter and cerebellum. But the expression of the NR1, NR2A and NR2B subunits were not significantly different between the control and the hypoxic group, 1 h after hypoxic exposure, indicating no changes in the protein amount of NMDA receptor subunits. These results show a significantly higher amount of the NR1, NR2A and NR2B subunits in the hippocampus and the cerebral cortex of newborn brains, indicating that these structures could be highly vulnerable to excitotoxicity in the newborn brain.     

NMDA受体亚单元NR2A和NR2B mRNA在缺血大鼠海马的表达及其与细胞凋亡的关系

目的 探讨短暂性前脑缺血后大鼠海马NR2A和NR2B mRNA的表达变化及其与海马缺血性细胞凋亡的关系。方法 四动脉阻断法建立脑缺血再灌注动物模型、原位杂交、TUNEL染色和图像分析与统计处理。结果 ①缺血后，NR2A和NR2B mRNA在海马各区呈现出一种相对一致的表达规律。在CA1区，NR2A和NR2B mRNA的表达分别在缺血再灌6h和12h降至低谷，然后回升，在缺血再灌48h都升至高峰，之后表达再次下降，直至缺血后7d；在CA3区，该变化规律依然存在，不同的是表达变化的幅度明显减小；而在齿状回，缺血再灌0．5～72h，二者的表达未见显著性变化；72h后，表达下降，直至缺血后7d。②缺血后24h，凋亡细胞出现，主要位于海马CA1区，进行性增多，48h增加更为明显，缺血后72h达高峰；然后凋亡细胞有所减少，但至缺血后7d，依然存在。结论 短暂性前脑缺血后，大鼠海马各区NR2A和NR2B mRNA的表达变化及细胞凋亡均存在着显著性差异；这种差异提示，缺血后，NR2A和NR2B mRNA的表达变化与海马的选择性易损现象和缺血性细胞凋亡之间可能存在着某种密切的关系。     

Deletion of the C-terminal domain of the NR2B subunit alters channel properties and synaptic targeting of N-methyl-D-aspartate receptors in nascent neocortical synapses

Channel properties and synaptic targeting of N-methyl-D-aspartate (NMDA) receptors determine their importance in synaptic transmission, long-term synaptic plasticity, and developmental reorganization of synaptic circuits. To investigate the involvement of the C-terminal domain of the NR2B subunit in regulating channel properties and synaptic localization, we analyzed gene-targeted mice expressing C-terminally truncated NR2B subunits (NR2B(DeltaC/DeltaC) mice; Sprengel et al. [1998] Cell 92:279-89). Because homozygous NR2B(DeltaC/DeltaC) mice die perinatally, we studied embryonic neocortical neurons differentiating in culture. At early stages in vitro, neurons predominantly expressed NR1/NR2B receptors, as shown by the NR2B subunit-specific antagonist ifenprodil. At these nascent synapses, NMDA excitatory postsynaptic currents (EPSCs) in neurons from NR2B(DeltaC/DeltaC) mice showed a strong-amplitude reduction to 20% of control, but AMPA EPSCs were unaltered. Analysis of the MK-801 block of NMDA receptor-mediated whole-cell currents revealed a decreased peak open probability of NMDA receptor channels (to about 60%) in neurons from NR2B(DeltaC/DeltaC) mice, although their single channel conductance was unchanged. To study effects on synaptic targeting, we determined the fraction of synaptically localized NMDA receptors relative to the whole-cell NMDA receptor population. In neurons from NR2B(DeltaC/DeltaC) mice, the synaptic NMDA receptor fraction was drastically reduced, suggesting that the C-terminal domain of the NR2B subunit plays a major role in synaptic targeting of NMDA receptors at nascent synapses. With increasing time in culture, the reduction in NMDA EPSCs in neurons from NR2B(DeltaC/DeltaC) mice diminished. This is explained by the expression of additional NMDA receptor subtypes containing NR2A subunits at more mature synapses.     

Clustering of surface NMDA receptors is mainly mediated by the C-terminus of GluN2A in cultured rat hippocampal neurons

N-methyI-D-aspartate receptors （NMDARs） containing different GluN2 subunits play distinct roles in synaptic plasticity. Such differences may not only be determined by the channel properties, but also by differential surface distribution and synaptic localization. In the present study, using a Cy3-conjugated Fab fragment of the GFP antibody to label surface-located GluN2 subunits tagged with GFP at the N-terminus, we observed the membrane distribution patterns of GluN2A- or GluN2B-containing NMDARs in cultured rat hippocampal neurons. We found that surface NMDARs containing GluN2A, but not those containing GluN2B, were inclined to cluster at DIV7. Swapping the carboxyl termini of the GluN2 subunits completely reversed these distribution patterns. In addition, surface NMDARs containing GluN2A were preferentially associated with PSD-95. Taken together, the results of our study suggest that the clustering distribution of GluN2A- containing NMDARs is determined by the GluN2A C-terminus, and its interaction with PSD-95 plays an important role in this process.     

Fear conditioning performance and NMDA receptor subtypes: NR2A differential expression in the striatum

While considerable evidence implicates NMDA receptors in the hippocampus in contextual fear conditioning, the role of other brain regions is less well understood. To further investigate this issue, rats were subjected to a contextual fear conditioning task and then classified as high or low responders according to performance. Density of NMDA receptors was evaluated using [3H]MK-801 autoradiography in 52 brain areas and expression of NR2A and NR2B subunits was studied with in situ hybridization in the same brains. Results revealed no differences between high- and low-performance rats in NMDA receptor binding in any of the brain areas studied. Similarly, NR2B subunit expression was also not different between groups. However, NR2A expression was significantly higher in the caudate-putamen of low-performance rats. These results suggest that NMDA receptors in the caudate-putamen may also be involved in contextual fear conditioning performance.     

Molecular regulation of dendritic spine dynamics and their potential impact on synaptic plasticity and neurological diseases

The structure and dynamics of dendritic spines reflect the strength of synapses, which are severely affected in different brain diseases. Therefore, understanding the ultra-structure, molecular signaling mechanism(s) regulating dendritic spine dynamics is crucial. Although, since last century, dynamics of spine have been explored by several investigators in different neurological diseases, but despite countless efforts, a comprehensive understanding of the fundamental etiology and molecular signaling pathways involved in spine pathology is lacking. The purpose of this review is to provide a contextual framework of our current understanding of the molecular mechanisms of dendritic spine signaling, as well as their potential impact on different neurodegenerative and psychiatric diseases, as a format for highlighting some commonalities in function, as well as providing a format for new insights and perspectives into this critical area of research. Additionally, the potential strategies to restore spine structure–function in different diseases are also pointed out. Overall, these informations should help researchers to design new drugs to restore the structure–function of dendritic spine, a “hot site” of synaptic plasticity.     

The effects of hormones and physical exercise on hippocampal structural plasticity

The hippocampus plays an integral role in certain aspects of cognition. Hippocampal structural plasticity and in particular adult hippocampal neurogenesis can be influenced by several intrinsic and extrinsic factors. Here we review how hormones (i.e., intrinsic modulators) and physical exercise (i.e., an extrinsic modulator) can differentially modulate hippocampal plasticity in general and adult hippocampal neurogenesis in particular. Specifically, we provide an overview of the effects of sex hormones, stress hormones, and metabolic hormones on hippocampal structural plasticity and adult hippocampal neurogenesis. In addition, we also discuss how physical exercise modulates these forms of hippocampal plasticity, giving particular emphasis on how this modulation can be affected by variables such as exercise regime, duration, and intensity. Understanding the neurobiological mechanisms underlying the modulation of hippocampal structural plasticity by intrinsic and extrinsic factors will impact the design of new therapeutic approaches aimed at restoring hippocampal plasticity following brain injury or neurodegeneration.     

A systematic review of evidence for the role of inflammatory biomarkers in bipolar patients

Bipolar disorder (BD) is a neuropsychiatric disorder that is characterized by a phasic course of affective episodes interspersed with a euthymic state. Epidemiological, clinical, genetic, post-mortem and preclinical studies have shown that inflammatory reactions and immune modulation play a pivotal role in the pathophysiology of BD. It is conceptualized that biomarkers of inflammation and immune responses should be employed to monitor the disease process in bipolar patients. The objective of this systematic review is to analyse the inflammatory markers involved in human studies and to explore each individual marker for its potential clinical application and summarize evidence regarding their role in BD. A systematic review of human studies to measure inflammatory markers was conducted, and the studies were identified by searching PubMed/MEDLINE, PsycINFO, EMBASE, and Web of Science databases for peer-reviewed journals that were published until September 2015. In this review, we included peripheral markers, genetic, post-mortem and cell studies with inflammatory biomarker analysis in BD. One hundred and two (102) papers met the inclusion criteria. The pro-inflammatory cytokines were elevated and the anti-inflammatory cytokines were reduced in BD patients, particularly during manic and depressive phases when compared to the controls. These changes tend to disappear in euthymia, indicating that inflammation may be associated with acute phases of BD. Even though there are promising findings in this field, further clinical studies using more established detection techniques are needed to clearly show the benefit of using inflammatory markers in the diagnosis, follow-up and prognosis of patients with BD.