Entrainment, Instability, Quasi-periodicity, and Chaos in a Compound Neural Oscillator

We studied the dynamical behavior of a class of compound central pattern generator (CPG) models consisting of a simple neural network oscillator driven by both constant and periodic inputs of varying amplitudes, frequencies, and phases. We focused on a specific oscillator composed of two mutually inhibiting types of neuron (inspiratory and expiratory neurons) that may be considered as a minimal model of the mammalian respiratory rhythm generator. The simulation results demonstrated how a simple CPG model— with a minimum number of neurons and mild nonlinearities— may reproduce a host of complex dynamical behaviors under various periodic inputs. In particular, the network oscillated spontaneously only when both neurons received adequate and proportionate constant excitations. In the presence of a periodic source, the spontaneous rhythm was overriden by an entrained oscillation of varying forms depending on the nature of the source. Stable entrained oscillations were inducible by two types of inputs: (1) anti-phase periodic inputs with alternating agonist-antagonist drives to both neurons and (2) a single periodic drive to only one of the neurons. In-phase inputs, which exert periodic drives of similar magnitude and phase relationships to both neurons, resulted in varying disruptions of the entrained oscillations including magnitude attenuation, harmonic and phase distortions, and quasi-periodic interference. In the absence of significant phasic feedback, chaotic motion developed only when the CPG was driven by multiple periodic inputs. Apneic episodes with repetitive alternation of active (intrinsic oscillation) and inactive (cessation of oscillation) states developed when the network was driven by a moderate periodic input of low frequency. %and amplitudes of intermediate strength, Similar results were demonstrated in other, more complex oscillator models (that is, half-center oscillator and three-phase respiratory network model). These theoretical results may have important implications in elucidating the mechanisms of rhythmogenesis in the mature and developing respiratory CPG as well as other compound CPGs in mammalian and invertebrate nervous systems.     

Mutagenesis of histidine 26 demonstrates the importance of loop-loop and loop-protein interactions for the function of iso-1-cytochrome c.

In yeast iso-1-cytochrome c, the side chain of histidine 26 (His26) attaches omega loop A to the main body of the protein by forming a hydrogen bond to the backbone atom carbonyl of glutamic acid 44. The His26 side chain also forms a stabilizing intra-loop interaction through a hydrogen bond to the backbone amide of asparagine 31. To investigate the importance of loop-protein attachment and intra-loop interactions to the structure and function of this protein, a series of site-directed and random-directed mutations were produced at His26. Yeast strains expressing these variant proteins were analyzed for their ability to grow on non-fermentable carbon sources and for their intracellular production of cytochrome c. While the data show that mutations at His26 lead to slightly decreased intracellular amounts of cytochrome c, the level of cytochrome c function is decreased more. The data suggest that cytochrome c reductase binding is affected more than cytochrome c oxidase or lactate dehydrogenase binding. We propose that mutations at this residue increase loop mobility, which, in turn, decreases the protein's ability to bind redox partners.     

NMR studies of internal dynamics of serine proteinase protein inhibitors: Binding region mobilities of intact and reactive-site hydrolyzed Cucurbita maxima trypsin inhibitor (CMTI)-III of the squash family and comparison with those of counterparts of CMTI-V of the potato I family.

Serine proteinase protein inhibitors follow the standard mechanism of inhibition (Laskowski M Jr, Kato I, 1980, Annu Rev Biochem 49:593-626), whereby an enzyme-catalyzed equilibrium between intact (I) and reactive-site hydrolyzed inhibitor (I*) is reached. The hydrolysis constant, Khyd, is defined as [I*]/[I]. Here, we explore the role of internal dynamics in the resynthesis of the scissile bond by comparing the internal mobility data of intact and cleaved inhibitors belonging to two different families. The inhibitors studied are recombinant Cucurbita maxima trypsin inhibitor III (rCMTI-III; Mr 3 kDa) of the squash family and rCMTI-V (Mr approximately 7 kDa) of the potato I family. These two inhibitors have different binding loop-scaffold interactions and different Khyd values--2.4 (CMTI-III) and 9 (CMTI-V)--at 25 degrees C. The reactive-site peptide bond (P1-P1') is that between Arg5 and Ile6 in CMTI-III, and that between Lys44 and Asp45 in CMTI-V. The order parameters (S2) of backbone NHs of uniformly 15N-labeled rCMTI-III and rCMTI-III* were determined from measurements of 15N spin-lattice and spin-spin relaxation rates, and [1H]-15N steady-state heteronuclear Overhauser effects, using the model-free formalism, and compared with the data reported previously for rCMTI-V and rCMTI-V*. The backbones of rCMTI-III [(S2) = 0.71] and rCMTI-III* [(S2) = 0.63] are more flexible than those of rCMTI-V [(S2) = 0.83] and rCMTI-V* [(S2) = 0.85]. The binding loop residues, P4-P1, in the two proteins show the following average order parameters: 0.57 (rCMTI-III) and 0.44 (rCMTI-III*); 0.70 (rCMTI-V) and 0.40 (rCMTI-V*). The P1'-P4' residues, on the other hand, are associated with (S2) values of 0.56 (rCMTI-III) and 0.47 (rCMTI-III*); and 0.73 (rCMTI-V) and 0.83 (rCMTI-V*). The newly formed C-terminal (Pn residues) gains a smaller magnitude of flexibility in rCMTI-III* due to the Cys3-Cys20 crosslink. In contrast, the newly formed N-terminal (Pn' residues) becomes more flexible only in rCMTI-III*, most likely due to lack of an interaction between the P1' residue and the scaffold in rCMTI-III. Thus, diminished flexibility gain of the Pn residues and, surprisingly, increased flexibility of the Pn' residues seem to facilitate the resynthesis of the P1-P1' bond, leading to a lower Khyd value.     

林隙（gap）更新动态研究进展

林隙（ｇａｐ）更新动态研究进展臧润国（中国林业科学研究院生态环境研究所,北京１０００９３）ＲｅｓｅａｒｃｈＡｄｖａｎｃｅｓｏｆＧａｐＲｅｇｅｎｅｒａｔｉｏｎＤｙｎａｍｉｃｓ．ＺａｎｇＲｕｎｇｕｏ（ＩｎｓｔｉｔｕｔｅｏｆＥｃｏｌｏｇｙａｎｄＥｎｖｉｒ...     

气候因子和非气候因子对白三叶草叶片生长的影响

本文采用裂区试验设计,研究了刈割频率（强度）和品种对永久性混播草地组分白三叶草叶片生长的影响,并通过回归分析探讨了气候因子与白三叶草叶片生长的相关性。刈割对叶片出生率和每个葡萄茎现存叶片数的处理效应具季节依赖性特性,在夏季生长期,高频率刈割 处理。三叶草品种对叶片出生率和每葡萄茎观存叶片数均无显著的处理效应,但刈割与品种对秋初生长期叶片的出生率有显著的互作效应,对秋季生长期每葡萄茎现存叶片数有微弱     

Short-term and long-term effects of mowing on the vegetation of two calcareous fens

Abstract. Short-term field experiments are often used to predict and evaluate long-term management effects. Based on a mowing experiment in two calcareous fens near Lake Neuchâtel, Switzerland, we investigated whether short-term treatment effects (i.e. during the first four years) were confirmed by long-term results (13 - 14 yr). Plots were mown in summer or in winter or left unmown. The main long-term trends in overall species composition (based on percentage cover estimates) were already observable in the first four years: mown and unmown plots diverged, whereas summer-cut and winter-cut plots remained similar. At the individual species level, however, short-term and long-term treatment effects differed considerably: many species whose abundance seemed affected by treatments during the first four years showed no response in the long term, and vice versa. These discrepancies were similar when based on cover estimates or on counts of shoots. Species responses did actually depend on the time scale considered. Short-term and long-term treatment effects on species richness were similar (i.e. a decrease in unmown plots), although only long-term effects were significant. Treatment effects on the above-ground biomass varied considerably, and short-term trends (lower biomass in unmown plots) differed from long-term trends (higher biomass in unmown plots). Our sites showed little overall change in species composition during the period investigated, and treatment effects were low compared with other similar experiments. If study sites are less stable or treatment effects more drastic, a short-term evaluation is expected to be even less reliable. Knowledge on species dynamics at a site may help to choose the adequate spatial and temporal scale of investigation, and thus increase the efficiency of management experiments.     

Computerized classification of Mediterranean vegetation using panchromatic aerial photographs

Abstract. Historical aerial photographs are an important source for data on medium- to long-term (10 - 50 yr) vegetation changes. Older photographs are panchromatic, and manual interpretation has traditionally been used to derive vegetation data from such photographs. We present a method for computerized analysis of panchromatic aerial photographs, which enables one to create high resolution, accurate vegetation maps. Our approach is exemplified using two aerial photographs (from 1964 and 1992) of a test area on Mt. Meron, Israel. Spatial resolution (pixel size) of the geo-rectified photos was 0.30 m and spatial accuracy (RMS error) ca. 1 m. An illumination adjustment prior to classification was found to be essential in reducing misclassification error rates. Two classification approaches were employed: a standard maximum-likelihood supervised classifier, and a modification of a supervised classification, which takes into account spectral properties of individual pixels as well as their neighbourhood characteristics. Accuracy of the maximum likelihood classification was 81 % in the 1992 image and 54 % in the 1964 image. The neighbour classifier increased accuracy to 89 % and 82 % respectively. The overall results suggest that computerized analysis of sequences of panchromatic aerial photographs may serve as a valuable tool for the quantification of medium-term vegetation changes.     

The anatomy of the interspecific abundance–range size relationship for the British avifauna: I. Spatial patterns

Data from the British Trust for Ornithology Common Birds Census and two atlases of breeding birds were used to examine the form of the interspecific abundance–range size relationship for the British avifauna. The relationship is positive for both farmland and woodland habitats and over two different periods, with some evidence of curvilinearity, using either proportion of occupied sites or numbers of occupied 10 × 10 km squares as measures of range size, and mean density at occupied sites as a measure of abundance. A log-linear plot gives the highest correlation. The relationship is stronger if based on maximum local densities than if based on average densities, but there is no relationship using minimum local densities. Relationships based on abundances at individual sites are uniformly positive for all sites, although the relationships for many sites also show evidence of curvilinearity, especially when range size is measured as the proportion of occupied sites. Species show significant concordance in their rank abundances across sites. We discuss some implications of these results.     

Travelling wave dynamics and self-organization in a spatio-temporally structured population

We analyse spatial population dynamics showing that periodic or period-like chaotic dynamics produce self-organization structures, such as travelling waves. We suggest that self-organized patterns are associated with spatial synchrony patterns that often depend on geographical distance between subpopulations. The population dynamics also show statistical spatial autocorrelation patterns. We contrast our theoretical simulations with empirical data on annual damages in young sapling stands caused by voles. We conclude, on the basis of the periodicity, synchrony, and spatial autocorrelation patterns, and our simulation results, that vole dynamics represent travelling waves in population dynamics. We suggest that because such synchrony patterns are frequently observed in natural populations, spatial self-organization may be more common in population dynamics than reported in the literature.     

Computer modeling of δ opioid receptor and interaction with ligands

Summary Human σ opioid receptor (σOR), a G-protein-coupled receptor, has been modeled using the helix axes as revealed by the crystallographic structure of bacteriorhodopsin and ligand binding profiles of single-point mutants of σOR. The model revealed feasibility of existence of a second disulfide bridge between the transmembrane helices (TMHs) 6 and 7, Cys²⁷³-Cys³⁰³. A common binding site has been suggested for high-affinity selective agonists DPDPE, DPLPE, DTLET, BW373U86 and antagonist Naltrindole. Docking calculations have shown that the amino group of the ligands forms a hydrogen bond with the imidazole ring of His³⁰¹ (TMH7) rather than with Asp¹²⁸ (TMH3) and is not a cation counterpart of this highly conserved aspartyl residue. All the findings and the model shed light on the putative structure and functioning of opioid receptors and can be used for designing further mutagenesis experiments.