Different voltage-dependent thresholds for inducing long-term depression and long-term potentiation in slices of rat visual cortex

In the hippocampus and neocortex, high-frequency (tetanic) stimulation of an afferent pathway leads to long-term potentiation (LTP) of synaptic transmission. In the hippocampus it has recently been shown that long-term depression (LTD) of excitatory transmission can also be induced by certain combinations of synaptic activation. In most hippocampal and all neocortical pathways studied so far, the induction of LTP requires the activation of N-methyl-D-aspartate (NMDA) receptor-gated conductances. Here we report that LTD can occur in neurons of slices of the rat visual cortex and that the same tetanic stimulation can induce either LTP or LTD depending on the level of depolarization of the postsynaptic neuron. By applying intracellular current injections or pharmacological disinhibition to modify the depolarizing response of the postsynaptic neuron to tetanic stimulation, we show that the mechanisms of induction of LTD and LTP are both postsynaptic. LTD is obtained if postsynaptic depolarization exceeds a critical level but remains below a threshold related to NMDA receptor-gated conductances, whereas LTP is induced if this second threshold is reached.     

A truncated human chromosome 16 associated with alpha thalassaemia is stabilized by addition of telomeric repeat (TTAGGG)n

The instability of chromosomes with breaks induced by X-irradiation led to the proposal that the natural ends of chromosomes are capped by a specialized structure, the telomere. Telomeres prevent end-to-end fusions and exonucleolytic degradation, enable the end of the linear DNA molecule to replicate, and function in cell division. Human telomeric DNA comprises approximately 2-20 kilobases (kb) of the tandemly repeated sequence (TTAGGG)n oriented 5'----3' in towards the end of the chromosome, interspersed with variant repeats in the proximal region. Immediately subtelomeric lie families of unrelated repeat motifs (telomere-associated sequences) whose function, if any, is unknown. In lower eukaryotes the formation and maintenance of telomeres may be mediated enzymatically (by telomerase) or by recombination; in man the mechanisms are poorly understood, although telomerase has been identified in HeLa cells. Here we describe an alpha thalassaemia mutation associated with terminal truncation of the short arm of chromosome 16 (within band 16p13-3) to a site 50 kb distal to the alpha globin genes, and show that (TTAGGG)n has been added directly to the site of the break. The mutation is stably inherited, proving that telomeric DNA alone is sufficient to stabilize the broken chromosome end. This mechanism may occur in any genetic disease associated with chromosome truncation.     

(Mg-ATP)-dependent self-assembly of molecular chaperone GroEL

The important Escherichia coli heat-shock protein GroEL of relative molecular mass 57,259 is a typical molecular chaperone. It possesses ATPase activity and interacts in ATP-driven reactions with non-folded proteins to stimulate their correct folding and/or assembly by preventing the formation of improper protein structures or aggregates. As GroEL is isolated and functions as a 20-25S tetradecameric particle (GroELp), the question arises--what is the mechanism of its own assembly? Here we show the (Mg-ATP)-dependent self-stimulation ('self-chaperoning') in vitro of GroELp reassembly from its monomeric state.     

Host-parasitoid associations in patchy environments

Studies of insect host-parasitoid interactions have contributed much to the consensus that spatial patchiness is important in the regulation of natural populations. A variety of theoretical models predict that host and parasitoid populations, although unstable in the absence of environmental heterogeneity, may persist at roughly steady overall densities in a patchy environment owing to variation in levels of parasitism from patch to patch. Observed patterns of parasitism, however, have a variety of forms (with variation in attack rates among patches depending directly or indirectly on host density, or showing variation uncorrelated with host density). There is some confusion about the dynamical consequences of these different forms. Here we first show how the dynamical effects of all these forms of environmental heterogeneity can be assessed by a common criterion. This 'CV2 greater than 1 rule' states that the overall population densities will remain roughly steady from generation to generation if the coefficient of variation squared (CV2) of the density of searching parasitoids in the vicinity of each host exceeds approximately unity. By partitioning CV2 into components, we show that both direct and inverse patterns of dependence on host density, and density-independent patterns, all contribute to population regulation in the same way. Second, we show how a maximum-likelihood method can be applied to the kind of field data that are usually available (that is, percentage parasitism versus local host density) to estimate the components of CV2. This analysis indicates that heterogeneity is large enough to stabilize dynamics in 9 of 34 published studies, and that density-independent heterogeneity is the main factor in most cases.     

Molecular cloning of the microtubule-associated mechanochemical enzyme dynamin reveals homology with a new family of GTP-binding proteins

A complementary DNA encoding the D100 polypeptide of rat brain dynamin--a force-producing, microtubule-activated nucleotide triphosphatase--has been cloned and sequenced. The predicted amino acid sequence includes a guanine nucleotide-binding domain that is homologous with those of a family of antiviral factors, inducible by interferon and known as Mx proteins, and with the product of the essential yeast vacuolar protein sorting gene VPS1. These relationships imply the existence of a new family of GTPases with physiological roles that may include microtubule-based motility and protein sorting.     

长江口悬浮颗粒有机絮凝研究

用带电子探针的扫描电子显微镜分析和测定了长江口悬浮颗粒样品,在不同盐度的试样中,首次发现了大小、形状,组成各不相同的有机絮凝体.作者根据长江口的物理化学环境和悬浮颗粒的性质,探讨了有机絮凝体的形成机理,认为二步桥联模式能比较园满地解释长江口悬浮颗粒有机絮凝体的形成.     

s-单式环的交换性定理(英文)

本文将文献[3]的引理1推广到s-单式环上,并用迭代技术给出文献[4]的定理2一个简易的证明,将若干有1环的交换性定理推广到s-单式环上。     

溅射用铁电陶瓷靶的烧结与显微结构分析

探讨了溅射用铁电陶瓷靶（ＰＺＴ，ＰＬＺＴ）的烧结工艺，并对其显微结构进行了分析。结果表明，采用新的烧结工艺（含保护措施），可以有效地抑制ＰｂＯ的挥发，制备出组织结构及成分均匀、ＰｂＯ含量正常、致密度较高、不变形的符合磁控溅射要求的铁电陶瓷烧结靶。     

时间序列AR模型参数的积分求解法

基于对时间序列实质的分析，提出了旨在减少序列的随机误差影响以及提高拟合精度的ＡＲ模型参数的积分求解法．重点讨论了ＡＲ（１）模型及ＡＲ（ｐ）模型参数的积分求解法，并与最小二乘法在计算机上进行了仿真比较．结果表明，采用积分求解法所得的ＡＲ模型参数的估计精度比最小二乘法的高．     

剩余静校正的小波包预处理方法

强反射层追踪与相对时差拾取中的“多值”现象严重影响剩余静校正的效果，为此提出的小波包预处理方法，可以实现地震道噪声与信号及不同强反射层的分离，从而能自动识别并提取“纯”的强反射层，提高强反射层追踪的可靠性和相对时差的拾取精度．