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PubMed1992

Long-term potentiation is associated with increases in quantal content and quantal amplitude.

Long-term potentiation (LTP) of synaptic transmission in CA1 neurons of the hippocampus, elicited by the conjunction of presynaptic firing and postsynaptic depolarization, is an important model of plasticity, which may underlie memory storage. Although induction of LTP takes place in the postsynaptic cell, it is not clear whether it is expressed through an enhancement of transmitter release or through an increased postsynaptic response to the same amount of transmitter. Analysis of the trial-to-trial amplitude fluctuations of synaptic signals, that is quantal analysis, gives an important insight into the probabilistic mechanisms of transmission, although attempts to apply it to the mode of expression of LTP have so far yielded inconsistent results, at least in part because they have relied on models of transmitter release that have not been confirmed experimentally. Here we report clear evidence for quantal fluctuation in a subset of cells. Induction of LTP in these cells causes abrupt increases in either quantal content or quantal amplitude, or both. This shows that two different mechanisms can underlie the maintenance of LTP.

Animals↗
PubMed2005

A general formula for the rate of resonant transfer of energy between two electric multipole moments of arbitrary order using molecular quantum electrodynamics.

A general expression is derived for the matrix element for the resonant transfer of energy between an initially excited donor species and an acceptor moiety in the ground state, with each entity possessing an electric multipole moment of arbitrary order. In the quantum electrodynamical framework employed, the coupling between the pair is mediated by the exchange of a single virtual photon. The probability amplitude found from second-order perturbation theory is a product of the electric moments located at each center and the resonant multipole-multipole interaction tensor. Using the Fermi golden rule, a general formula for the rate of energy transfer is obtained. As an illustration of the efficacy of the theory developed, rates of excitation energy exchange are calculated for systems interacting through dipole-quadrupole, dipole-octupole, quadrupole-quadrupole, and the familiar dipole-dipole coupling. For each of the cases examined, the near- and far-zone limits of the migration rate are calculated from the result valid for all donor-acceptor separations beyond wave function overlap. Expression of the octupole contribution to the transfer rate in terms of its irreducible components of weights 1 and 3 leads to new features. The octupole weight-1 term is found to contribute only when the interaction is retarded, while the dipole-octupole weight-1 contribution appears as a higher-order correction term to the dipole-dipole rate. Order of magnitude estimates are given for the contributions of dipole-quadrupole and dipole-octupole terms relative to the leading dipole-dipole rate for near-, intermediate-, and far-zone separations to further understand the role played by higher multipole moments in the transfer of excitation and the mechanism dominating the process.

Journal Article↗
PubMed2005

Synchronization in power-law networks.

We consider realistic power-law graphs, for which the power-law holds only for a certain range of degrees. We show that synchronizability of such networks depends on the expected average and expected maximum degree. In particular, we find that networks with realistic power-law graphs are less synchronizable than classical random networks. Finally, we consider hybrid graphs, which consist of two parts: a global graph and a local graph. We show that hybrid networks, for which the number of global edges is proportional to the number of total edges, almost surely synchronize.

Algorithms↗
PubMed2006

Combined nonadiabatic transition-state theory and ab initio molecular dynamics study on selectivity of the alpha and beta bond fissions in photodissociation of bromoacetyl chloride.

The selectivity of the alpha C-Cl and beta C-Br bond fissions upon n-->pi(*) excitation of bromoacetyl chloride has been investigated with combined nonadiabatic Rice-Ramsperger-Kassel-Marcus theory and ab initio molecular dynamics calculations, which are based on the potential energy profiles calculated with the complete active space self-consistent field and multireference configuration interaction methods. The Zhu-Nakamura [J. Chem. Phys. 101, 10630 (1994); 102, 7448 (1995)] theory is chosen to calculate the nonadiabatic hopping probability. It is found that nonadiabatic effect plays an important role in determining selective dissociations of the C-Cl and C-Br bonds. The calculated rate constants are close to those from experimentally inferred values, but the branching ratio of the alpha C-Cl and beta C-Br bond fissions is different from the experimental findings. The direct molecular dynamics calculations predict that fission of the C-Cl bond occurs on a time scale of picoseconds and cleavage of the beta C-Br bond proceeds with less probability within the same period. This reveals that the initial relaxation dynamics is probably another important factor that influences the selectivity of the C-Cl and C-Br bond fissions in photodissociation of BrCH(2)COCl at 248 nm.

Journal Article↗
PubMed2003

Socioeconomics drive urban plant diversity.

Spatial variation in plant diversity has been attributed to heterogeneity in resource availability for many ecosystems. However, urbanization has resulted in entire landscapes that are now occupied by plant communities wholly created by humans, in which diversity may reflect social, economic, and cultural influences in addition to those recognized by traditional ecological theory. Here we use data from a probability-based survey to explore the variation in plant diversity across a large metropolitan area using spatial statistical analyses that incorporate biotic, abiotic, and human variables. Our prediction for the city was that land use, along with distance from urban center, would replace the dominantly geomorphic controls on spatial variation in plant diversity in the surrounding undeveloped Sonoran desert. However, in addition to elevation and current and former land use, family income and housing age best explained the observed variation in plant diversity across the city. We conclude that a functional relationship, which we term the "luxury effect," may link human resource abundance (wealth) and plant diversity in urban ecosystems. This connection may be influenced by education, institutional control, and culture, and merits further study.

Arizona↗
PubMed1982

Allelic and nonallelic homology of a supergene family.

A model to explain the high degree of polymorphism at the major histocompatibility complex (MHC) is described. The model incorporates domain transfer between the different loci in a supergene family by either gene conversion or double unequal crossing-over. Population genetics theory is used to formulate changes in the probabilities of allelic and nonallelic gene identities and equilibrium values are obtained. The observed degree of allelic and nonallelic homology in the complex can be explained by assuming that a domain is converted at a rate of 10(-5) to 10(-6) per generation and reasonable values of other parameters. This rate of domain transfer is compatible with the observed high mutation rate at marker loci in the major histocompatibility complex.

Alleles↗
PubMed1981

DNA damage as the primary cause of aging.

DNA damage appears to be ubiquitous in the biological world, as judged by the variety of organisms which have evolved DNA-repair systems. Previously, it was proposed that germ-line DNA of multicellular organisms may be protected from damage, and consequently from aging, by efficient recombinational repair during meiosis. The somatic line, however, may be vulnerable to the accumulation of DNA damage, and hence undergo aging, owing to relatively less repair. Although the DNA lesions most important in aging are not known yet, there is evidence for serveral types of endogenous damage. DNA lesions have been shown to interfere with transcription and replication, and so lead to loss of cell function and death. In mammals, there is a progressive decline of function in many different tissues with increasing age. Deterioration of central nervous system functions appears to be a critical part of the aging process. This may be due to the low DNA repair capacity which is found in postmitotic brain tissue, and which could result in the accumulation of DNA lesions in this tissue. Also reviewed is evidence that species longevity is directly related to tissue DNA-repair capacity and that aging may be accelerated by treatment with DNA-damaging agents, or in individuals with genetically defective repair. Although it has been frequently postulated that somatic mutation may be cause of aging, current evidence suggests that it is probably less important than DNA damage. A prominent theory on the evolution of aging, which attributes special importance to genes that are advantagous in youth but are deleterious later on, is discussed in terms of regulatory genes that reduce DNA repair as cells differentiate to the postmitotic state. Finally, we hypothesize that the factors which determine maximum longevity of individuals in a population are the rate of occurrence of DNA damage, the rate of DNA repair, the degree of cellular redundancy, and the extent of exposure to stress.

Aging↗
PubMed1997

Haldane's rule and X-chromosome size in Drosophila.

The "dominance theory" of HALDANE'S rule postulates that hybrids of the heterogametic sex are more likely to be inviable or sterile than the homogametic sex because some of the epistatic incompatibilities contributing to postzygotic isolation behave as X-linked partial recessives. When this is true, pairs of taxa with relatively large X chromosomes should require less divergence time, on average, to produce HALDANE'S rule than pairs with smaller Xs. Similarly, if the dominance theory is correct and if the X chromosome evolves at a similar rate to the autosomes, the size of the X should not influence the rate at which homogametic hybrids become inviable or sterile. We use Drosophila data to examine both of these predictions. As expected under the dominance theory, pairs of taxa with large X chromosomes (approximately 40% of the nuclear genome) show HALDANE's rule for sterility at significantly smaller genetic distances than pairs with smaller X chromosomes (approximately 20% of the genome). As also predicted, the genetic distances between taxa that exhibit female inviability/sterility show no differences between "large X" vs. "small X" pairs. We present some simple mathematical models to relate these data to the dominance theory and alternative hypotheses involving faster evolution of the X vs. the autosomes and/or faster evolution of incompatibilities that produce male-specific vs. female-specific sterility. Although the data agree qualitatively with the predictions of the dominance theory, they depart significantly from the quantitative predictions of simple models of the dominance theory and the other hypotheses considered. These departures probably stem from the many simplifying assumptions needed to tractably model epistatic incompatibilities and to analyze heterogeneous data from many taxa.

Animals↗
PubMed1999

Detecting population expansion and decline using microsatellites.

This article considers a demographic model where a population varies in size either linearly or exponentially. The genealogical history of microsatellite data sampled from this population can be described using coalescent theory. A method is presented whereby the posterior probability distribution of the genealogical and demographic parameters can be estimated using Markov chain Monte Carlo simulations. The likelihood surface for the demographic parameters is complicated and its general features are described. The method is then applied to published microsatellite data from two populations. Data from the northern hairy-nosed wombat show strong evidence of decline. Data from European humans show weak evidence of expansion.

Demography↗
PubMed1993

Quantal variability of excitatory transmission in the hippocampus: implications for the opening probability of fast glutamate-gated channels.

The quantal description of synaptic transmission, first formulated at the neuromuscular junction, has recently been extended to the Schaffer collateral-CA1 synapse of the mammalian hippocampus. In a subset of connections, an upper limit can be placed on the coefficient of variation of the quantal amplitude, on occasion lower than 0.2. In the same connections, the average quantal amplitude implies that fewer than 20 fast glutamate-gated ion channels open to carry the quantal current. With so few channels opening, their stochastic behaviour imposes a minimal trial-to-trial variability on the quantal current. These observations can be reconciled by postulating that most of the available receptors in the synaptic cleft are bound by released transmitter and that ionophores open with a high probability.

Animals↗
PubMed1994

The prisoner's dilemma without synchrony.

There are many situations in which biological organisms cooperate despite obvious incentives to do otherwise. Such situations are commonly modelled by using a paradigm known as the prisoner's dilemma. In this way cooperative behaviour has previously been shown to emerge in a model population of strategies. If players can make probabilistic choices, taking into account their co-player's previous action, a strategy known as 'generous tit for tat' dominates the long-term behaviour of such a population. If they can also take into account their own previous action, a strategy of 'win stay, lose shift' dominates instead. These models assumed that participants make their decisions in synchrony, which seems improbable in many biological situations. Here we show that the timing of decisions is critical in determining which strategy emerges in the long run. If individuals make their decisions at different times, neither of the above strategies survives given the usual payoffs. In the former case, generous tit for tat succumbs to inveterate defectors, and in the latter a new strategy takes over. This 'firm but fair' strategy is retaliatory yet highly cooperative. In particular, continued exploitation of a sucker is no longer a successful behaviour.

Animals↗
PubMed2003

Development and neurophysiology of mentalizing.

The mentalizing (theory of mind) system of the brain is probably in operation from ca. 18 months of age, allowing implicit attribution of intentions and other mental states. Between the ages of 4 and 6 years explicit mentalizing becomes possible, and from this age children are able to explain the misleading reasons that have given rise to a false belief. Neuroimaging studies of mentalizing have so far only been carried out in adults. They reveal a system with three components consistently activated during both implicit and explicit mentalizing tasks: medial prefrontal cortex (MPFC), temporal poles and posterior superior temporal sulcus (STS). The functions of these components can be elucidated, to some extent, from their role in other tasks used in neuroimaging studies. Thus, the MPFC region is probably the basis of the decoupling mechanism that distinguishes mental state representations from physical state representations; the STS region is probably the basis of the detection of agency, and the temporal poles might be involved in access to social knowledge in the form of scripts. The activation of these components in concert appears to be critical to mentalizing.

Child, Preschool↗
PubMed2001

Bayesian analysis of systems with random chemical composition: renormalization-group approach to Dirichlet distributions and the statistical theory of dilution.

We investigate the statistical properties of systems with random chemical composition and try to obtain a theoretical derivation of the self-similar Dirichlet distribution, which is used empirically in molecular biology, environmental chemistry, and geochemistry. We consider a system made up of many chemical species and assume that the statistical distribution of the abundance of each chemical species in the system is the result of a succession of a variable number of random dilution events, which can be described by using the renormalization-group theory. A Bayesian approach is used for evaluating the probability density of the chemical composition of the system in terms of the probability densities of the abundances of the different chemical species. We show that for large cascades of dilution events, the probability density of the composition vector of the system is given by a self-similar probability density of the Dirichlet type. We also give an alternative formal derivation for the Dirichlet law based on the maximum entropy approach, by assuming that the average values of the chemical potentials of different species, expressed in terms of molar fractions, are constant. Although the maximum entropy approach leads formally to the Dirichlet distribution, it does not clarify the physical origin of the Dirichlet statistics and has serious limitations. The random theory of dilution provides a physical picture for the emergence of Dirichlet statistics and makes it possible to investigate its validity range. We discuss the implications of our theory in molecular biology, geochemistry, and environmental science.

Algorithms↗
PubMed2002

Quasi-Gaussian statistics of hydrodynamic turbulence in 4 / 3 +epsilon dimensions.

The statistics of two-dimensional turbulence exhibit a riddle: the scaling exponents in the regime of inverse energy cascade agree with the K41 theory of turbulence far from equilibrium, but the probability distribution functions are close to Gaussian-like in equilibrium. The skewness S identical with S3(R)/S(3/2)(2)(R) was measured as S (exp) approximately 0.03. This contradiction is lifted by understanding that two-dimensional turbulence is not far from a situation with equipartition of enstrophy, which exists as true thermodynamic equilibrium with K41 exponents in space dimension of d= 4 / 3. We evaluate the skewness S( d) for 4 / 3 < or =d< or =2, showing that S(d)=0 at d= 4 / 3, and that it remains as small as S (exp) in two dimensions.

Journal Article↗
PubMed1997

Quantal analysis of excitatory synapses in rat hippocampal CA1 in vitro during low-frequency depression.

1. We have performed a detailed quantal analysis of excitatory postsynaptic potentials (EPSPs) evoked by minimal extracellular stimulation in the CA1 region of slices of adult rat hippocampus maintained in vitro. 2. EPSPs were evoked at 2-5 Hz, and the eight that were analysed all showed at least a 50% depression of mean peak amplitude during recording. 3. EPSP amplitude fluctuations were analysed by three methods: the use of amplitude frequency histograms with clear and reliable peaks where available, graphs of the EPSP (coefficient of variation)-2 against EPSP mean, and analysis of EPSP mean and standard deviation assuming simple binomial statistics with the number of release sites (N) kept constant but the quantal size (Q) and the release probability (Pr) allowed to vary over time. 4. The results of the three analysis procedures were in good agreement. Seven EPSPs showed a substantial reduction in the mean number of quanta released per trial, and in three cases this was the predominant mechanism of the depression. Five EPSPs showed a substantial decrease in Q. Values for N ranged between 3 and 18, with a median of 6; Pr ranged between 0.14 and 0.81 and Q between 66 and 275 microV. 5. We used the Q estimates from the binomial method to correct the recorded EPSP amplitudes for changes in quantal size over time. For seven out of the eight EPSPs, this rescaling procedure allowed histograms with clear peaks to be obtained from longer runs of data, or improved the sharpness of the peaks in histograms from all the recorded data. The improvement in peak sharpness was assessed using an autocorrelation-based method. The correction was much less successful if the Q estimates were obtained with a variant of the binomial method in which Pr was held constant and N was allowed to vary. 6. The only simple explanation for the success of the correction procedure is that changes in quantal size were a major factor in obscuring peaks in histograms based on large numbers of trials, and that the quantal size estimates from the binomial method with N held constant were reasonably accurate. 7. We conclude that transmission at these synapses was quantal with relatively low quantal variance, but repetitive stimulation often induced substantial changes in the quantal parameters that might prevent the success of conventional quantal analysis approaches.

Animals↗