首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   7775篇
  免费   510篇
  国内免费   10篇
  2023年   69篇
  2022年   197篇
  2021年   345篇
  2020年   157篇
  2019年   187篇
  2018年   271篇
  2017年   200篇
  2016年   237篇
  2015年   376篇
  2014年   428篇
  2013年   559篇
  2012年   562篇
  2011年   482篇
  2010年   284篇
  2009年   258篇
  2008年   313篇
  2007年   332篇
  2006年   283篇
  2005年   246篇
  2004年   251篇
  2003年   200篇
  2002年   191篇
  2001年   144篇
  2000年   139篇
  1999年   96篇
  1998年   48篇
  1997年   46篇
  1996年   48篇
  1995年   35篇
  1994年   32篇
  1993年   30篇
  1992年   79篇
  1991年   88篇
  1990年   90篇
  1989年   79篇
  1988年   74篇
  1987年   53篇
  1986年   51篇
  1985年   62篇
  1984年   62篇
  1983年   48篇
  1981年   32篇
  1980年   39篇
  1979年   41篇
  1978年   44篇
  1977年   37篇
  1976年   35篇
  1975年   30篇
  1974年   42篇
  1973年   30篇
排序方式: 共有8295条查询结果,搜索用时 15 毫秒
991.
Sase A  Khan D  Höger H  Lubec G 《Amino acids》2012,43(2):783-792
The involvement of the hippocampus in pain has been demonstrated but key players, i.e. the major brain receptors have not been shown to be modulated by pain. It was therefore the aim of the study to show the concerted action and pattern of brain receptor complex levels in a non-invasive model of moderate pain. C57BL/6J mice were divided into four groups of 14 animals each: trained injected, trained non-injected, yoked injected and yoked non-injected. Animals were tested in the open field and the elevated plus maze for behavioural evaluation and cognitive functions were tested using the Morris Water Maze. Hippocampi were taken 6 h following sacrification. Membrane proteins were prepared by ultracentrifugation and run on blue native gels to keep the native state, blotted to membranes and western blotting was carried out using the primary antibodies against serotonin receptor 5HT1A, muscarinic acetylcholine receptor M1 (mAChR-M1), nicotinic acetylcholine receptor alpha7 (nAChR-alpha7), glutamate (AMPA) receptor (GluR1) and neurokinin receptor 1 (NK-1). There was no difference between performance in behaviour or in the MWM between groups. Brain receptor level changes involved all receptors given above. Pain affected mAChR-M1, GluR1 and NK-1 complex levels when yoked-injected were compared with yoked non-injected animals. Memory mechanisms affected mAChR-M1 complex levels when trained non-injected animals were compared with yoked non-injected controls. Taken together, the neurochemical basis for testing receptor agonists/antagonists on the role of pain and the hippocampus was generated that may be useful for interpretations of the role of this complex area in moderate pain.  相似文献   
992.
Naveed M  Khan A  Khan AU 《Amino acids》2012,42(5):1809-1823
G protein-coupled receptors (GPCRs) are transmembrane proteins, which transduce signals from extracellular ligands to intracellular G protein. Automatic classification of GPCRs can provide important information for the development of novel drugs in pharmaceutical industry. In this paper, we propose an evolutionary approach, GPCR-MPredictor, which combines individual classifiers for predicting GPCRs. GPCR-MPredictor is a web predictor that can efficiently predict GPCRs at five levels. The first level determines whether a protein sequence is a GPCR or a non-GPCR. If the predicted sequence is a GPCR, then it is further classified into family, subfamily, sub-subfamily, and subtype levels. In this work, our aim is to analyze the discriminative power of different feature extraction and classification strategies in case of GPCRs prediction and then to use an evolutionary ensemble approach for enhanced prediction performance. Features are extracted using amino acid composition, pseudo amino acid composition, and dipeptide composition of protein sequences. Different classification approaches, such as k-nearest neighbor (KNN), support vector machine (SVM), probabilistic neural networks (PNN), J48, Adaboost, and Naives Bayes, have been used to classify GPCRs. The proposed hierarchical GA-based ensemble classifier exploits the prediction results of SVM, KNN, PNN, and J48 at each level. The GA-based ensemble yields an accuracy of 99.75, 92.45, 87.80, 83.57, and 96.17% at the five levels, on the first dataset. We further perform predictions on a dataset consisting of 8,000 GPCRs at the family, subfamily, and sub-subfamily level, and on two other datasets of 365 and 167 GPCRs at the second and fourth levels, respectively. In comparison with the existing methods, the results demonstrate the effectiveness of our proposed GPCR-MPredictor in classifying GPCRs families. It is accessible at .  相似文献   
993.
Modafinil has been shown to modify behavioural and cognitive functions and to effect several brain receptors. Effects, however, were not observed at the receptor protein complex level and it was therefore the aim of the study to train mice in the multiple T-Maze (MTM) as a paradigm for spatial memory and to determine paralleling brain receptor complex levels. Sixty C57BL/6J mice were used in the study and divided into four groups (trained drug injected; trained vehicle injected; yoked drug injected; yoked vehicle injected). Animals obtained training for 4?days and were killed 6?h following the last training session on day 4. Hippocampi were dissected from the brain, membrane fractions were prepared by ultracentrifugation and were run on blue-native gels and immunoblotted with antibodies against major brain receptors. Modafinil treatment led to decreased latency and increased average speed, but not to changes in pathlength and number of correct decisions in the MTM. Drug effects were modifying receptor complexes of GluR1, GluR2, D2 and NR1. Training effects on receptor complex levels were observed for GluR3, D1 and nicotinic acetylcholine receptor alpha 7 (Nic7). GluR1 levels were correlating with GluR2 and D1 levels were correlating with D2 and NR1. Involvement of the glutamatergic, NMDA, dopaminergic and nicotinergic system in modafinil and memory training were herein described for the first time. A brain receptor complex pattern was revealed showing the concerted action following modafinil treatment.  相似文献   
994.
The aim of this study was to evaluate the influence of the crude and active solvent fraction of Trachyspermum ammi on S. mutans cariogenicity, effect on expression of genes involved in biofilm formation and caries development in rats. GC-MS was carried out to identify the major components present in the crude and the active fraction of T. ammi. The crude extract and the solvent fraction exhibiting least MIC were selected for further experiments. Scanning electron microscopy was carried out to observe the effect of the extracts on S. mutans biofilm. Comparative gene expression analysis was carried out for nine selected genes. 2-Isopropyl-5-methyl-phenol was found as major compound in crude and the active fraction. Binding site of this compound within the proteins involved in biofilm formation, was mapped with the help of docking studies. Real-time RT-PCR analyses revealed significant suppression of the genes involved in biofilm formation. All the test groups showed reduction in caries (smooth surface as well as sulcal surface caries) in rats. Moreover, it also provides new insight to understand the mechanism influencing biofilm formation in S. mutans. Furthermore, the data suggest the putative cariostatic properties of T. Ammi and hence can be used as an alternative medicine to prevent caries infection.  相似文献   
995.
Multifunction of autophagy-related genes in filamentous fungi   总被引:1,自引:0,他引:1  
Autophagy (macroautophagy), a highly conserved eukaryotic mechanism, is a non-selective degradation process, helping to maintain a balance between the synthesis, degradation and subsequent recycling of macromolecules to overcome various stress conditions. The term autophagy denotes any cellular process which involves the delivery of cytoplasmic material to the lysosome for degradation. Autophagy, in filamentous fungi plays a critical role during cellular development and pathogenicity. Autophagy, like the mitogen-activated protein (MAP) kinase cascade and nutrient-sensing cyclic AMP (cAMP) pathway, is also an important process for appressorium turgor accumulation in order to penetrate the leaf surface of host plant and destroy the plant defense. Yeast, an autophagy model, has been used to compare the multi-valued functions of ATG (autophagy-related genes) in different filamentous fungi. The autophagy machinery in both yeast and filamentous fungi is controlled by Tor kinase and both contain two distinct phosphatidylinositol 3-kinase complexes. In this review, we focus on the functions of ATG genes during pathogenic development in filamentous fungi.  相似文献   
996.
Background: Mesenchymal stem cells are able to undergo adipogenic differentiation and present a possible alternative cell source for regeneration and replacement of adipose tissue. The human infrapatellar fat pad is a promising source of mesenchymal stem cells with many source advantages over from bone marrow. It is important to determine whether a potential mesenchymal stem‐cell exhibits tri‐lineage differentiation potential and is able to maintain its proliferation potential and cell‐surface characterization on expansion in tissue culture. We have previously shown that mesenchymal stem cells derived from the fat pad can undergo chondrogenic and osteogenic differentiation, and we characterized these cells at early passage. In the study described here, proliferation potential and characterization of fat pad‐derived mesenchymal stem cells were assessed at higher passages, and cells were allowed to undergo adipogenic differentiation. Materials and methods: Infrapatellar fat pad tissue was obtained from six patients undergoing total knee replacement. Cells isolated were expanded to passage 18 and proliferation rates were measured. Passage 10 and 18 cells were characterized for cell‐surface epitopes using a range of markers. Passage 2 cells were allowed to undergo differentiation in adipogenic medium. Results: The cells maintained their population doubling rates up to passage 18. Cells at passage 10 and passage 18 had cell‐surface epitope expression similar to other mesenchymal stem cells previously described. By staining it was revealed that they highly expressed CD13, CD29, CD44, CD90 and CD105, and did not express CD34 or CD56, they were also negative for LNGFR and STRO1. 3G5 positive cells were noted in cells from both passages. These fat pad‐derived cells had adipogenic differentiation when assessed using gene expression for peroxisome proliferator‐activated receptor γ2 and lipoprotein lipase, and oil red O staining. Discussion: These results indicate that the cells maintained their proliferation rate, and continued expressing mesenchymal stem‐cell markers and pericyte marker 3G5 at late passages. These results also show that the cells were capable of adipogenic differentiation and thus could be a promising source for regeneration and replacement of adipose tissue in reconstructive surgery.  相似文献   
997.
A series of bisindole-pyrrolobenzodiazepine conjugates (5a-f) linked through different alkane spacers was prepared and evaluated for their anticancer activity. All compounds exhibited significant anticancer potency and the most potent compounds 5b and 5e were taken up for detailed studies on MCF-7 cell line. Cell cycle effects were examined apart from investigating the inhibition of tubulin polymerization for compounds 2a, 2b, 5b and 5e at 2μM. FACS analysis showed that at higher concentrations (4 and 8μM) there was an increase of sub-G1 phase cells and decrease of G2/M phase cells, thus indicating that compounds 5b and 5e are effective in causing apoptosis in MCF-7 cells. It was also observed that compounds 5b and 5e showed the down regulation of histone deacetylase protein levels such as HDAC1, 2, 3, 8 and increase in the levels of p21, followed by apoptotic cell death. The apoptotic nature of these compounds was further evidenced by increased expression of cleaved-PARP and active caspase-7 in MCF-7 cells.  相似文献   
998.
Endocrine dynamics spans a wide range of time scales, from rapid responses to physiological challenges to with slow responses that adapt the system to the demands placed on it. We outline a non-linear averaging procedure to extract the slower dynamics in a way that accounts properly for the non-linear dynamics of the faster time scale and is applicable to a hierarchy of more than two time scales, although we restrict our discussion to two scales for the sake of clarity. The procedure is exact if the slow time scale is infinitely slow (the dimensionless ε-quantity is the period of the fast time scale fluctuation times an upper bound to the slow time scale rate of change). However, even for an imperfect separation of time scales we find that this construction provides an excellent approximation for the slow-time dynamics at considerably reduced computational cost. Besides the computation advantage, the averaged equation provided a qualitative insight into the interaction of the time scales. We demonstrate the procedure and its advantages by applying the theory to the model described by Toli? et al. [I.M. Toli?, E. Mosekilde, J. Sturis, Modeling the insulin–glucose feedback system: the significance of pulsatile insulin secretion, J. Theor. Biol. 207 (2000) 361–375.] for ultradian dynamics of the glucose–insulin homeostasis feedback system, extended to include β-cell dynamics. We find that the dynamics of the β-cell mass are dependent not only on the glycemic load (amount of glucose administered to the system), but also on the way this load is applied (i.e. three meals daily versus constant infusion), effects that are lost in the inappropriate methods used by the earlier authors. Furthermore, we find that the loss of the protection against apoptosis conferred by insulin that occurs at elevated levels of insulin has a functional role in keeping the β-cell mass in check without compromising regulatory function. We also find that replenishment of β-cells from a rapidly proliferating pool of cells, as opposed to the slow turn-over which characterises fully differentiated β-cells, is essential to the prevention of type 1 diabetes.  相似文献   
999.
Fifty-eight typical EAEC isolates from children with diarrhoea were examined for HEp-2 cell adherence assay, presence of dispersin (aap), yersiniabactin (irp2), plasmid encoded toxins (pet), Shigella enterotoxin1 (set1A) and cryptic open reading frame (shf) putative virulence genes by polymerase chain reaction as well as for biofilm production. All the isolates showed aggregative adherence pattern on HEp-2 cells. All but five isolates (91.3 %) carried aap gene. While irp2, pet, set1A and shf genes were detected in 68.9, 5.1, 39.6, and 60.3 % isolates, respectively. Thirty-three (64.7 %) isolates out of 51 tested were found to produce biofilm which was found to be significantly associated only with set1A virulence gene (P = 0.025). Highest amount of biofilm was produced by a strain that possessed all the genes studied. Out of 14 isolates in which the most frequent gene combination (aap, irp2 and shf) was observed, only six produced biofilm. It is concluded that there is significant heterogeneity in putative virulence genes of EAEC isolates from diarrhoeic children and biofilm formation is associated with multiple genes.  相似文献   
1000.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号