Specific Disruption of Hippocampal Mossy Fiber Synapses in a Mouse Model of Familial Alzheimer's Disease

The earliest stages of Alzheimer''s disease (AD) are characterized by deficits in memory and cognition indicating hippocampal pathology. While it is now recognized that synapse dysfunction precedes the hallmark pathological findings of AD, it is unclear if specific hippocampal synapses are particularly vulnerable. Since the mossy fiber (MF) synapse between dentate gyrus (DG) and CA3 regions underlies critical functions disrupted in AD, we utilized serial block-face electron microscopy (SBEM) to analyze MF microcircuitry in a mouse model of familial Alzheimer''s disease (FAD). FAD mutant MF terminal complexes were severely disrupted compared to control – they were smaller, contacted fewer postsynaptic spines and had greater numbers of presynaptic filopodial processes. Multi-headed CA3 dendritic spines in the FAD mutant condition were reduced in complexity and had significantly smaller sites of synaptic contact. Significantly, there was no change in the volume of classical dendritic spines at neighboring inputs to CA3 neurons suggesting input-specific defects in the early course of AD related pathology. These data indicate a specific vulnerability of the DG-CA3 network in AD pathogenesis and demonstrate the utility of SBEM to assess circuit specific alterations in mouse models of human disease.     

Neuronal MHC Class I Molecules are Involved in Excitatory Synaptic Transmission at the Hippocampal Mossy Fiber Synapses of Marmoset Monkeys

Several recent studies suggested a role for neuronal major histocompatibility complex class I (MHCI) molecules in certain forms of synaptic plasticity in the hippocampus of rodents. Here, we report for the first time on the expression pattern and functional properties of MHCI molecules in the hippocampus of a nonhuman primate, the common marmoset monkey (Callithrix jacchus). We detected a presynaptic, mossy fiber-specific localization of MHCI proteins within the marmoset hippocampus. MHCI molecules were present in the large, VGlut1-positive, mossy fiber terminals, which provide input to CA3 pyramidal neurons. Furthermore, whole-cell recordings of CA3 pyramidal neurons in acute hippocampal slices of the common marmoset demonstrated that application of antibodies which specifically block MHCI proteins caused a significant decrease in the frequency, and a transient increase in the amplitude, of spontaneous excitatory postsynaptic currents (sEPSCs) in CA3 pyramidal neurons. These findings add to previous studies on neuronal MHCI molecules by describing their expression and localization in the primate hippocampus and by implicating them in plasticity-related processes at the mossy fiber–CA3 synapses. In addition, our results suggest significant interspecies differences in the localization of neuronal MHCI molecules in the hippocampus of mice and marmosets, as well as in their potential function in these species.     

A Simplified Model of Long-Term Plasticity in Cerebellar Mossy Fiber–Granule Cell Synapses

We simulated synaptic transmission and modified a simple model of long-term potentiation and long-term depression in order to describe the long-term plasticity-related changes in cerebellar mossy fiber–granule cell synapses. In our model, protein autophosphorylation, leading to the maintenance of long-term plasticity, is controlled by Ca²⁺ entry through the NMDA receptor channels. The observed nonlinearity in the development of long-term changes of EPSP in granule cells is explained by the difference in the rate constants of two independent autocatalytic processes.     

Reconstruction of Underlying Nonlinear Deterministic Dynamics Embedded in Noisy Spike Trains

An experimentally recorded time series formed by the exact times of occurrence of the neuronal spikes (spike train) is likely to be affected by observational noise that provokes events mistakenly confused with neuronal discharges, as well as missed detection of genuine neuronal discharges. The points of the spike train may also suffer a slight jitter in time due to stochastic processes in synaptic transmission and to delays in the detecting devices. This study presents a procedure aimed at filtering the embedded noise (denoising the spike trains) the spike trains based on the hypothesis that recurrent temporal patterns of spikes are likely to represent the robust expression of a dynamic process associated with the information carried by the spike train. The rationale of this approach is tested on simulated spike trains generated by several nonlinear deterministic dynamical systems with embedded observational noise. The application of the pattern grouping algorithm (PGA) to the noisy time series allows us to extract a set of points that form the reconstructed time series. Three new indices are defined for assessment of the performance of the denoising procedure. The results show that this procedure may indeed retrieve the most relevant temporal features of the original dynamics. Moreover, we observe that additional spurious events affect the performance to a larger extent than the missing of original points. Thus, a strict criterion for the detection of spikes under experimental conditions, thus reducing the number of spurious spikes, may raise the possibility to apply PGA to detect endogenous deterministic dynamics in the spike train otherwise masked by the observational noise.     

Biosynthesis and Metabolism of Native and Oxidized Neuropeptide Y in the Hippocampal Mossy Fiber System

Abstract: Neuropeptide Y (NPY) gene expression is known to be modulated in the mossy fiber projection of hippocampal granule cells following seizure. We investigated NPY biosynthesis and metabolism in an attempt to characterize NPY biochemically as a neurotransmitter in the granule cell mossy fiber projection. NPY biosynthesis was compared in normal control animals and in animals that had experienced a single pentylenetetrazole-induced seizure. In situ hybridization analysis established the postseizure time course of preproNPY mRNA expression in the hippocampal formation, localizing the majority of increased preproNPY mRNA content to the hilus of the dentate gyrus. Radioimmunoassay analysis of the CA3/mossy fiber terminal subfield confirmed a subsequent increase in NPY peptide content. Biosynthesis of NPY peptide by granule cells and transport to the CA3/mossy fiber subfield was demonstrated by in vivo radiolabel infusion to the dentate gyrus/hilus followed by sequential HPLC purification of identified radiolabeled peptide from the CA3/mossy fiber terminal subfield. Additional in vivo radiolabeling studies revealed a postseizure increase in an unidentified NPY-like immunoreactive (NPY-LI) species. HPLC/radioimmunoassay analyses of CA3 subfield tissue extracts comparing normal control animals and pentylenetetrazole-treated animals confirmed the increased total NPY-LI, and demonstrated that the increased NPY-LI was comprised of a minor increase in native NPY and a major increase in the unknown NPY-LI. Data from subsequent and separate analyses incorporating immunoprecipitation with anti-C-terminal flanking peptide of NPY, further HPLC purification, and matrix-assisted laser desorption/ionization mass spectrometry support the conclusion that the unknown NPY-LI is methionine sulfoxide NPY. NPY and NPY-sulfoxide displayed differential calcium sensitivity for release from mossy fiber synaptosomes. Similar to NPY, NPY sulfoxide displayed high-affinity binding to each of the cloned Y1, Y2, Y4, and Y5 receptor subtypes. Postrelease inactivation of NPY was demonstrated in a mossy fiber synaptosomal preparation. Thus, the present study in combination with previously reported electrophysiological activity of NPY in the CA3 subfield demonstrates that NPY fulfills the classical criteria for a neurotransmitter in the hippocampal granule cell mossy fiber projection, and reveals the presence of two molecular forms of NPY that display differential mechanisms of release while maintaining similar receptor potencies.     

Evidence for the Corelease of Dynorphin and Glutamate from Rat Hippocampal Mossy Fiber Terminals

Abstract: Hippocampal mossy fiber (MF) nerve endings may be isolated in a subcellular fraction (P₃) that releases both prodynorphin-derived peptides and glutamate (Glu) in a calcium-dependent manner when depolarized. However, this isolation procedure does not yield a pure preparation of MF synaptosomes. The present study evaluates the proportion of dynorphin (Dyn) and Glu that is released from synaptosomes in the P₃ fraction that are of MF origin. We have addressed this issue by determining the degree to which a selective lesion of the dentate granule cell/MF system in vivo concomitantly reduces the exocytosis of Dyn and Glu from the P₃ subcellular fraction. Unilateral injections of colchicine into the dentate gyrus resulted in a substantial and selective degeneration of the granule cell/ MF pathway in the rat hippocampal formation. The overall integrated density of the Timm-stained band, which corresponds to the position of the MF terminal field, was estimated to be reduced by 75%. After this extensive loss of MF boutons, the K⁺-evoked release of Dyn and Glu from the P₃ fraction was reduced by 95 and 51 %, respectively. The loss of Timm staining and evoked Dyn release indicate that colchicine effectively eliminated MF synaptosomes from the P₃ fraction. Those subcellular entities that were not destroyed by colchicine comprised ～50% of the protein and evoked Glu release measured by using the P₃fraction. In addition, the present results demonstrate that the inhibitory potency of the K opioid agonist U-50.488H was not altered by the elimination of MF boutons from this synaptosomal preparation. This finding indicates that U-50,488H is capable of suppressing Glu exocytosis from both MF and non-MF synaptosomes. These results are consistent with the hypothesis that Dyn peptides and Glu are coreleased from hippocampal MF terminals.     

突触后致密物与癫痫苔藓纤维出芽

突触后致密物是化学性突触后膜内侧的特化结构,为神经信息传递的重要结构基础,参与突触后信号转导的调节和整合,在学习、记忆和突触可塑性等生理过程中有重要作用。近年来发现,癫痫发作伴有突触后致密物成分的改变,可能参与了癫痫的病理生理过程。     

ATP Release, Adenosine Formation, and Modulation of Dynorphin and Glutamic Acid Release by Adenosine Analogues in Rat Hippocampal Mossy Fiber Synaptosomes

Using a hippocampal subcellular fraction enriched in mossy fiber synaptosomes, evidence was obtained indicating that adenosine derived from a presynaptic pool of ATP may modulate the release of prodynorphin-derived peptides. and glutamic acid from mossy fiber terminals. Synaptosomal ATP was released in a Ca2+-dependent manner by K+-induced depolarization. The rapid hydrolysis of extracellular [14C]ATP in the presence of intact mossy fiber synaptosomes resulted in the production of [14C]adenosine. Micromolar concentrations of a stable adenosine analogue, 2-chloroadenosine, inhibited the K+-stimulated release of both dynorphin B and dynorphin A(1-8). 2-Chloroadenosine failed to suppress the evoked release of glutamic acid, measured in these same superfusates, unless the mossy fiber synaptosomes were pretreated with D-aspartic acid to deplete the cytosolic, Ca2+-independent, pool of this acidic amino acid. In synaptosomes pretreated in this manner, release of the remaining Ca2+-dependent pool of glutamic acid was significantly inhibited by NiCl2, 2-chloroadenosine, 5'-N-ethylcarboxamidoadenosine, cyclohexyladenosine, and R(-)-N6(2-phenylisopropyl)adenosine, but not by ATP. 2-Chloroadenosine-induced inhibition was reversed when the external CaCl2 concentration was raised from 1.8 mM to 6 mM. 8-Phenyltheophylline, an adenosine receptor antagonist, effectively blocked the inhibitory effects of 2-chloroadenosine on mossy fiber synaptosomes and significantly enhanced the K+-evoked release of both glutamic acid and dynorphin A(1-8) when added alone to the superfusion medium. These results support the proposition that depolarized hippocampal mossy fiber synaptosomes release endogenous ATP and are capable of forming adenosine from extracellular ATP, and that endogenous adenosine may act at a presynaptic site to inhibit the further release of glutamic acid and the prodynorphin-derived peptides.     

Transplantation of Neural Stem Cells Overexpressing Cardiotrophin-1 Inhibits Sprouting of Hippocampal Mossy Fiber in a Rat Model of Status Epilepticus

In this study, we studied the effects of hippocampal transplantation of neural stem cells (NSCs) overexpressing cardiotrophin 1 (CT1) on hippocampal mossy fiber sprouting (MFS) in a rat model of status epilepticus (SE). SE rats (lithium–pilocarpine model) were randomized into four study groups (18 rats per group): CT1-NSCs group, NSCs group, SE control group, and normal control group. Six rats were randomly chosen from each group at 1, 4, and 8 weeks after transplantation. MFS in hippocampal dentate gyrus was scored (Timm staining) at these time points. The MFS scores were as follows: CT1-NSCs 0.77 ± 0.04, 2.48 ± 0.89, and 2.39 ± 0.82 (1, 4, and 8 weeks after transplantation, respectively); NSCs 1.12 ± 0.62, 3.17 ± 0.64, and 3.88 ± 0.51; SE control 1.32 ± 0.35, 3.28 ± 0.75, and 4.32 ± 1.55; and normal control 0.37 ± 0.06, 0.34 ± 0.07, and 0.43 ± 0.04. Compared to SE control group and NSCs group, the scores of MFS in CT1-NSCs group were significantly lower (P < 0.05). In conclusion, transplantation with NSCs overexpressing CT1 inhibits hippocampal MFS and facilitates reduction of recurrent seizures.     

Arenavirus Dynamics in Experimentally and Naturally Infected Rodents

Infectious diseases of wildlife are typically studied using data on antibody and pathogen levels. In order to interpret these data, it is necessary to know the course of antibodies and pathogen levels after infection. Such data are typically collected using experimental infection studies in which host individuals are inoculated in the laboratory and sampled over an extended period, but because laboratory conditions are controlled and much less variable than natural conditions, the immune response and pathogen dynamics may differ. Here, we compared Morogoro arenavirus infection patterns between naturally and experimentally infected multimammate mice (Mastomys natalensis). Longitudinal samples were collected during three months of bi-weekly trapping in Morogoro, Tanzania, and antibody titer and viral RNA presence were determined. The time of infection was estimated from these data using a recently developed Bayesian approach, which allowed us to assess whether the natural temporal patterns match the previously observed patterns in the laboratory. A good match was found for 52% of naturally infected individuals, while most of the mismatches can be explained by the presence of chronically infected individuals (35%), maternal antibodies (10%), and an antibody detection limit (25%). These results suggest that while laboratory data are useful for interpreting field samples, there can still be differences due to conditions that were not tested in the laboratory.     

Emergence of Connectivity Motifs in Networks of Model Neurons with Short- and Long-Term Plastic Synapses

Recent experimental data from the rodent cerebral cortex and olfactory bulb indicate that specific connectivity motifs are correlated with short-term dynamics of excitatory synaptic transmission. It was observed that neurons with short-term facilitating synapses form predominantly reciprocal pairwise connections, while neurons with short-term depressing synapses form predominantly unidirectional pairwise connections. The cause of these structural differences in excitatory synaptic microcircuits is unknown. We show that these connectivity motifs emerge in networks of model neurons, from the interactions between short-term synaptic dynamics (SD) and long-term spike-timing dependent plasticity (STDP). While the impact of STDP on SD was shown in simultaneous neuronal pair recordings in vitro, the mutual interactions between STDP and SD in large networks are still the subject of intense research. Our approach combines an SD phenomenological model with an STDP model that faithfully captures long-term plasticity dependence on both spike times and frequency. As a proof of concept, we first simulate and analyze recurrent networks of spiking neurons with random initial connection efficacies and where synapses are either all short-term facilitating or all depressing. For identical external inputs to the network, and as a direct consequence of internally generated activity, we find that networks with depressing synapses evolve unidirectional connectivity motifs, while networks with facilitating synapses evolve reciprocal connectivity motifs. We then show that the same results hold for heterogeneous networks, including both facilitating and depressing synapses. This does not contradict a recent theory that proposes that motifs are shaped by external inputs, but rather complements it by examining the role of both the external inputs and the internally generated network activity. Our study highlights the conditions under which SD-STDP might explain the correlation between facilitation and reciprocal connectivity motifs, as well as between depression and unidirectional motifs.     

Leptin Regulation of Synaptic Function at Hippocampal TA-CA1 and SC-CA1 Synapses: Implications for Health and Disease

Growing evidence indicates that the endocrine hormone leptin regulates hippocampal synaptic function in addition to its established role as a hypothalamic satiety signal. Indeed, numerous studies show that leptin facilitates the cellular events that underlie hippocampal learning and memory including activity-dependent synaptic plasticity and glutamate receptor trafficking, indicating that leptin may be a potential cognitive enhancer. Although there has been extensive investigation into the modulatory role of leptin at hippocampal Schaffer collateral (SC)-CA1 synapses, recent evidence indicates that leptin also potently regulates excitatory synaptic transmission at the anatomically distinct temporoammonic (TA) input to hippocampal CA1 neurons. The cellular mechanisms underlying activity-dependent synaptic plasticity at TA-CA1 synapses differ from those at SC-CA1 synapses and the TA input is implicated in spatial and episodic memory formation. Furthermore, the TA input is an early target for neurodegeneration in Alzheimer’s disease (AD) and aberrant leptin function is linked to AD. Here, we review the evidence that leptin regulates hippocampal synaptic function at both SC- and TA-CA1 synapses and discuss the consequences for neurodegenerative disorders like AD.

     

Differential Role of Hippocampal cAMP-Dependent Protein Kinase in Short- and Long-Term Memory

One-trial step-down inhibitory (passive) avoidance training is followed by two peaks of cAMP-dependent protein kinase (PKA) activity in rat CA1: one immediately after training and the other 3 h later. The second peak relies on the first: Immediate posttraining infusion into CA1 of the inhibitor of the regulatory subunit of PKA, Rp-cAMPS, at a dose that reduces PKA activity during less than 90 min, cancelled both peaks. Long-term memory (LTM) of this task measured at 24 h depends on the two peaks: Rp-cAMPS given into CA1 0 or 175 min posttraining, but not between those times, blocked LTM. However, the effect of immediate posttraining Rp-cAMPS on LTM could not be reversed by the activator of the regulatory subunit of PKA, Sp-cAMPS, given at 180 min, which suggests that, for LTM, the first peak may be more important than the second. When given at 0, 22, 45, or 90, but not at 175 min from training, Rp-cAMPS blocked short-term memory (STM) measured at 90 or 180 min. This effect of immediate posttraining Rp-cAMPS infusion on STM but not that on LTM was readily reversed by Sp-cAMPS infused 22 min later. On its own, Sp-cAMPS had effects exactly opposite to those of the inhibitor. It enhanced LTM when given at 0 or 175 min from training, and it enhanced STM when given at 0, 22, 45, or 90 min from training. These findings show that STM and LTM formation require separate PKA-dependent processes in CA1. STM relies on the continued activity of the enzyme during the first 90 min. LTM relies on the two peaks of PKA activity that occur immediately and 180 min posttraining.     

新疆甘家湖自然保护区啮齿动物群落结构与时间动态分析

2002年4月—2003年10月对精河甘家湖荒漠梭梭林国家级自然保护区的啮齿动物进行了调查。采用铗捕法,设置30个样地,共布铗30hm^2,捕获啮齿动物579只,分属3科5属6种。用种类(S)、相对密度、多样性指数(H′)、均匀性指数(E)、优势度(D)5个指标对年间、季节间的鼠类群落结构进行了对比分析。结果表明,2002—2003年鼠总密度上升,子午沙鼠的优势更加突出;群落由传统的以大沙鼠为优势种变为以子午沙鼠为优势种,群落结构也由大沙鼠—子午沙鼠—三趾跳鼠向子午沙鼠—大沙鼠—三趾跳鼠演替;2002—2003年群落多样性、均匀性降低,优势度增加;从春至秋鼠总密度呈上升趋势,多样性增加;均匀性以夏季最高,优势度则春季最高。