Regulation, cycles and stability in northern carnivore-herbivore systems: back to first principles

In studies on dynamics of northern predator‐prey systems, two assumptions are often made. First, the bifurcation from stable to cyclic dynamics is seen as a consequence of changing generalist‐specialist ratio, ultimately due to reduced prey diversity at high latitudes and the negative impact of snow on the efficiency of generalists as predators of small, folivorous mammals. Supposedly, the primary mechanism is the qualitative difference between the functional response of specialist and generalist predators. Second, the interaction between large predators and ungulates is supposed to be prone to lead to two alternative equilibria, one where predation regulates ungulates at a relatively low equilibrium and another, where ungulate densities are close to carrying capacity. In the first‐mentioned issue, our analysis corroborates the general idea of snow favoring specialists and leading to cycles. However, differences in functional response appear to be of secondary importance only, and rather special conditions are required for generalists to have a stabilizing type III functional response. A destabilizing type II functional response or a slight modification of it should be common in generalists, too, as also indicated by the classical experiments. Stability of generalist dominated systems seems primarily to derive from their relative inefficiency, allowing prey's density‐dependent mechanisms to play a bigger role in the neighborhood of the equilibrium. Moreover, the main destabilizing impact of deep, long‐lasting snow cover appears to lie in the protection it offers to the efficient but vulnerable specialists, which are eliminated or marginalized by intraguild predation in areas with snow‐free winters, unless the habitat offers some other form of efficient protection. As for the conjecture of multiple equilibria in northern wolf‐ungulate systems, it seems to be derived from an erroneous operational definition of numerical response and has little if any empirical support. Available data suggest that predation limitation of folivorous mammals prevails along the entire gradient from relatively productive low arctic habitats to the humid parts of the temperate zone, provided that the numbers of predators are not controlled by man.     

Effect of lateral far-red light supplementation on the growth and morphology of birch seedlings and its interaction with mineral nutrition

The effects of lateral far-red light (FR) and nutrient supply on the growth and nitrogen accumulation of silver birch (Betula pendula) seedlings were studied with the objective of testing the following hypotheses: (1) silver birch seedlings grow taller in response to increased FR irradiance received from the side; (2) this response is modulated by the nutritional status of the seedlings; and (3) an increase in lateral FR irradiance, and concomitant decrease in red to far-red photon ratio, affects the carbon and nitrogen economies of the seedlings. Two factorial experiments, each with two levels of mineral nutrient availability and two light treatments (background 'white' light with and without additional lateral FR simulating light reflected by neighbours in a sparse canopy) were done with small seedlings. The two experiments differed in PAR irradiance. The results of these experiments were that (1) stem elongation rate was increased by lateral FR addition, (2) there was no interaction between this effect and the nutritional status of the seedlings, and (3) neither whole plant accumulation of nitrogen nor dry weight increment was affected by lateral FR under either mineral nutrient supply regime.     

Visual Features Underlying Perceived Brightness as Revealed by Classification Images

Along with physical luminance, the perceived brightness is known to depend on the spatial structure of the stimulus. Often it is assumed that neural computation of the brightness is based on the analysis of luminance borders of the stimulus. However, this has not been tested directly. We introduce a new variant of the psychophysical reverse-correlation or classification image method to estimate and localize the physical features of the stimuli which correlate with the perceived brightness, using a brightness-matching task. We derive classification images for the illusory Craik-O''Brien-Cornsweet stimulus and a “real” uniform step stimulus. For both stimuli, classification images reveal a positive peak at the stimulus border, along with a negative peak at the background, but are flat at the center of the stimulus, suggesting that brightness is determined solely by the border information. Features in the perceptually completed area in the Craik-O''Brien-Cornsweet do not contribute to its brightness, nor could we see low-frequency boosting, which has been offered as an explanation for the illusion. Tuning of the classification image profiles changes remarkably little with stimulus size. This supports the idea that only certain spatial scales are used for computing the brightness of a surface.     

Physiological and autonomic stress responses after prolonged sleep restriction and subsequent recovery sleep in healthy young men

Sleep and Biological Rhythms - Sleep restriction is increasingly common and associated with the development of health problems. We investigated how the neuroendocrine stress systems respond to...     

Differential Regulation and Function of CD73, a Glycosyl-Phosphatidylinositol–linked 70-kD Adhesion Molecule, on Lymphocytes and Endothelial Cells

CD73, otherwise known as ecto-5′-nucleotidase, is a glycosyl-phosphatidylinositol–linked 70-kD molecule expressed on different cell types, including vascular endothelial cells (EC) and certain subtypes of lymphocytes. There is strong evidence for lymphocyte CD73 having a role in several immunological phenomena such as lymphocyte activation, proliferation, and adhesion to endothelium, but the physiological role of CD73 in other cell types is less clear. To compare the biological characteristics of CD73 in different cell types, we have studied the structure, function, and surface modulation of CD73 on lymphocytes and EC. CD73 molecules on lymphocytes are shed from the cell surface as a consequence of triggering with an antiCD73 mAb, mimicking ligand binding. In contrast, triggering of endothelial CD73 does not have any effect on its expression. Lymphocyte CD73 is susceptible to phosphatidylinositol phospholipase, whereas only a small portion of CD73 on EC could be removed by this enzyme. Furthermore, CD73 on EC was unable to deliver a tyrosine phosphorylation inducing signal upon mAb triggering, whereas triggering of lymphocyte CD73 can induce tyrosine phosphorylation. Despite the functional differences, CD73 molecules on lymphocytes and EC were practically identical structurally, when studied at the protein, mRNA, and cDNA level. Thus, CD73 is an interesting example of a molecule which lacks structural variants but yet has a wide diversity of biological functions. We suggest that the ligand- induced shedding of lymphocyte CD73 represents an important and novel means of controlling lymphocyte– EC interactions.     

Structural and functional features of the NAD(P) dependent Gfo/Idh/MocA protein family oxidoreductases

The Gfo/Idh/MocA protein family contains a number of different proteins, which almost exclusively consist of NAD(P)‐dependent oxidoreductases that have a diverse set of substrates, typically pyranoses. In this study, to clarify common structural features that would contribute to their function, the available crystal structures of the members of this family have been analyzed. Despite a very low sequence identity, the central features of the three‐dimensional structures of the proteins are surprisingly similar. The members of the protein family have a two‐domain structure consisting of a N‐terminal nucleotide‐binding domain and a C‐terminal α/β‐domain. The C‐terminal domain contributes to the substrate binding and catalysis, and contains a βα‐motif with a central α‐helix carrying common essential amino acid residues. The β‐sheet of the α/β‐domain contributes to the oligomerization in most of the proteins in the family.     

Phe27Cys Polymorphism Alters the Maturation and Subcellular Localization of the Human δ Opioid Receptor

The human δ opioid receptor (hδOR) is a G-protein-coupled receptor that is mainly involved in the modulation of pain and mood. Only one nonsynonymous single nucleotide polymorphism (T80G) has been described, causing Phe27Cys substitution in the receptor N-terminus and showing association with substance dependence. In this study, we expressed the two hδOR variants in a heterologous expression system with an identical genetic background. They differed greatly during early steps of biosynthesis, displaying a significant difference in the maturation efficiency (50% and 85% for the Cys27 and Phe27 variants, respectively). The Cys27 variant also showed accumulation in pre-Golgi compartments of the secretory pathway and impaired targeting to endoplasmic reticulum (ER)-associated degradation following long-term expression. In addition, the cell surface receptors of the Cys27 variant internalized constitutively. Replacement of phenylalanine with other amino acids revealed that cysteine at position 27 decreased the mature receptor/precursor ratio most extensively, suggesting a thiol-mediated retention of precursors in the ER. However, cysteine did not cause a major folding defect because pharmacological characteristics and the maturation kinetics of the variants were identical, and an opioid antagonist was able to enhance the maturation of both variants. We conclude that, instead of causing loss of function, Phe27Cys polymorphism of the hδOR causes a gain-of-function phenotype, which may have implications for the regulation of receptor expression at the cell surface and possibly also for the susceptibility to pathophysiological states.     

The time course of adenosine, nitric oxide (NO) and inducible NO synthase changes in the brain with sleep loss and their role in the non-rapid eye movement sleep homeostatic cascade

Both adenosine and nitric oxide (NO) are known for their role in sleep homeostasis, with the basal forebrain (BF) wakefulness center as an important site of action. Previously, we reported a cascade of homeostatic events, wherein sleep deprivation (SD) induces the production of inducible nitric oxide synthase (iNOS)-dependent NO in BF, leading to enhanced release of extracellular adenosine. In turn, increased BF adenosine leads to enhanced sleep intensity, as measured by increased non-rapid eye movement sleep EEG delta activity. However, the presence and time course of similar events in cortex has not been studied, although a frontal cortical role for the increase in non-rapid eye movement recovery sleep EEG delta power is known. Accordingly, we performed simultaneous hourly microdialysis sample collection from BF and frontal cortex (FC) during 11 h SD. We observed that both areas showed sequential increases in iNOS and NO, followed by increases in adenosine. BF increases began at 1 h SD, whereas FC increases began at 5 h SD. iNOS and Fos-double labeling indicated that iNOS induction occurred in BF and FC wake-active neurons. These data support the role of BF adenosine and NO in sleep homeostasis and indicate the temporal and spatial sequence of sleep homeostatic cascade for NO and adenosine.