A theoretical study on the sucrose gap technique as applied to multicellular muscle preparations. II. Methodical errors in the determination of outward currents.

This paper presents a mathematical model for analyzing systematic errors associated with the membrane conductance of multicellular muscle preparations as determined in a sucrose gap apparatus. The errors arise because of the interdiffusion of sucrose and saline in the interstitial fluid spaces, which results (a) in spatial variations of equilibrium potentials, membrane conductance, and solution conductivity, and (b) in the existence of a liquid junction potential. The model was applied to simulate the measurement of outward currents predominantly carried by potassium ions; time variations were not considered. Output current/voltage (I/V) curves were computed and compared with the membrane I/V relationship used in the computation. The output curves look very much like experimental results but are distorted considerably from the membrane I/V relationship: (a) under favorable conditions (negligible shunt current), the membrane current is overestimated over the entire range of membrane potential, (b) regions with negative slope conductance of I/V relations with "anomalous rectifier" properties are found to be less pronounced or even absent, and (c) resting potentials may be either increased or reduced. The origin of these errors is related to currents emerging from the sucrose compartment (local circuit as well as externally applied currents). Their dependence on several experimental parameters is discussed.     

The splicing factor crooked neck associates with the RNA-binding protein HOW to control glial cell maturation in Drosophila

In both vertebrates and invertebrates, glial cells wrap axonal processes to ensure electrical conductance. Here we report that Crooked neck (Crn), the Drosophila homolog of the yeast Clf1p splicing factor, is directing peripheral glial cell maturation. We show that crooked neck is expressed and required in glial cells to control migration and axonal wrapping. Within the cytoplasm, Crn interacts with the RNA-binding protein HOW and then translocates to the nucleus where the Crn/HOW complex controls glial differentiation by facilitating splicing of specific target genes. By using a GFP-exon trap approach, we identified some of the in vivo target genes that encode proteins localized in autocellular septate junctions. In conclusion, here we show that glial cell differentiation is controlled by a cytoplasmic assembly of splicing components, which upon translocation to the nucleus promote the splicing of genes involved in the assembly of cellular junctions.     

X-linked loci of Drosophila melanogaster causing defects in the morphology of the embryonic salivary glands

To identify X chromosomal genes required for salivary gland development in the Drosophila embryo, we screened embryos hemizygous for EMS-induced lethal mutations to find mutations causing gross morphological defects in salivary gland development. The parental strain carried a lac Z transgene on the second chromosome, which was specifically expressed in the salivary glands so the mutations could be unambiguously identified. Embryos from 3,383 lines were tested for salivary gland abnormalities following lacZ staining. From 63 lines exhibiting aberrant salivary gland phenotypes, 52 stable lines were established containing mutations affecting salivary gland development. From these, 39 lines could be assigned to nine complementation groups: armadillo, brinker, folded gastrulation, giant, hindsight, Notch, runt, stardust and twisted gastrulation. Received: 10 April 2000 / Accepted: 31 May 2000     

Life Cycle Impact assessment of land use based on the hemeroby concept

The impact category ‘land use’ describes in the Life Cycle Assessment (LCA) methodology the environmental impacts of occupying, reshaping and managing land for human purposes. Land use can either be the long-term use of land (e.g. for arable farming) or changing the type of land use (e.g. from natural to urban area). The impact category ‘land use’ comprises those environmental consequences, which impact the environment due to the land use itself, for instance through the reduction of landscape elements, the planting of monocultures or artificial vegetation, or the sealing of surfaces. Important environmental consequences of land use are the decreasing availability of habitats and the decreasing diversity of wildlife species. The assessment of the environmental impacts of land use within LCA studies is the objective of this paper. Land use leads to a degradation of the naturalness of the area utilised. In this respect the naturalness of any area can be defined as the sum of land actually not influenced by humans and the remaining naturalness of land under use. To determine the remaining naturalness of land under use, this study suggests applying the Hemeroby concept. “Hemeroby is a measure for the human influence on ecosystems” (Kowarik 1999). The Hemeroby level of an area describes the intensity of land use and can therefore be used to characterise different types of land use. Characterization factors are proposed, which allow calculating the degradation of the naturalness of an area due to a specific type of land use. Since the resource ‘nature/naturalness’ is on a larger geographical scale by far not homogeneous, the assessment of land use needs to be regionalised. Therefore, the impact category ‘land use’ has been subdivided into the impact sub-categories ‘land use in European biogeographic regions’. Following the general LCA framework, normalization values for the impact sub-categories are calculated in order to facilitate the evaluation of the characterization results with regard to their share in a reference value. Weighting factors, which enable an aggregation of the results of the different land use sub-categories and make them comparable to other impact categories (e.g. climate change or acidification) are suggested based on the assumption that the current land use pattern in the European biogeographic regions is acceptable.     

Projection-specific modulation of dopamine neuron synapses by aversive and rewarding stimuli

Midbrain dopamine (DA) neurons are not homogeneous but differ in their molecular properties and responses to external stimuli. We examined whether the modulation of excitatory synapses on DA neurons by rewarding or aversive stimuli depends on the brain area to which these DA neurons project. We identified DA neuron subpopulations in slices after injection of "Retrobeads" into single target areas of adult mice and found differences in basal synaptic properties. Administration of cocaine selectively modified excitatory synapses on DA cells projecting to nucleus accumbens (NAc) medial shell while an aversive stimulus selectively modified synapses on DA cells projecting to medial prefrontal cortex. In contrast, synapses on DA neurons projecting to NAc lateral shell were modified by both rewarding and aversive stimuli, which presumably reflects saliency. These results suggest that the mesocorticolimbic DA system may be comprised of three anatomically distinct circuits, each modified by distinct aspects of motivationally relevant stimuli.     

Symbiotic status alters fungal eco-evolutionary offspring trajectories

Despite host-fungal symbiotic interactions being ubiquitous in all ecosystems, understanding how symbiosis has shaped the ecology and evolution of fungal spores that are involved in dispersal and colonization of their hosts has been ignored in life-history studies. We assembled a spore morphology database covering over 26,000 species of free-living to symbiotic fungi of plants, insects and humans and found more than eight orders of variation in spore size. Evolutionary transitions in symbiotic status correlated with shifts in spore size, but the strength of this effect varied widely among phyla. Symbiotic status explained more variation than climatic variables in the current distribution of spore sizes of plant-associated fungi at a global scale while the dispersal potential of their spores is more restricted compared to free-living fungi. Our work advances life-history theory by highlighting how the interaction between symbiosis and offspring morphology shapes the reproductive and dispersal strategies among living forms.     

A theoretical study on the sucrose gap technique as applied to multicellular muscle preparations. I. Saline-sucrose interdiffusion.

Voltage-clamp analysis of membrane currents in multicellular muscle preparations by means of the sucrose gap method is complicated by diffusion of saline and sucrose in the interstitial fluid spaces. This paper is the first part of a theoretical study made to analyze electrical events related to this diffusion process. Concentration profiles of ions and sucrose (both axial and radial) were computed by solving diffusion equations with boundary conditions appropriate for the different types of preparations and experimental arrangements used. In addition to steady-state solutions, analytical expressions were derived that describe the time-course with which concentration profiles become established after a stepwise change of the solute concentration in one of the compartments of the sucrose gap apparatus. The model accounts for the presence of an endothelial surface layer, or endocardium, which acts as an external diffusion barrier and is important in determining concentration gradients of solutes within heart cell preparations. Results of numerical computations dealing with several cases of experimental interest are presented.     

Cultivation of Miscanthus under West European conditions: Seasonal changes in dry matter production,nutrient uptake and remobilization

There is increasing interest in cultivation of Miscanthus as a source of renewable energy in Europe, but there is little information on its nutrient requirements. Our aim was to determine the nutrient requirement of an established Miscanthus crop through a detailed study of nutrient uptake and nutrient remobilization between plant parts during growth and senescence. Therefore dry matter of rhizomes and shoots as well as N, P, K and Mg concentration under three N fertilizer rates (0, 90, and 180 kg N ha-1) were measured in field trials in 1992/93 and at one rate of 100 kg N h-1 in 1994/95.Maximum aboveground biomass in an established Miscanthus crop ranged between 25-30 t dry matter ha-1 in the September of both trial years. Due to senescence and leaf fall there was a 30% loss in dry matter between September and harvest in March. N fertilization had no effect on crop yield at harvest. Concentrations of N, P, K and Mg in shoots were at a maximum at the beginning of the growing period in May and decreased thereafter while concentrations in rhizomes stayed fairly constant throughout the year and were not affected by N fertilization.Nutrient mobilization from rhizomes to shoots - defined as the maximum change in nutrient content in rhizomes from the beginning of the growth period measured in 1992/93 was 55 kg N ha-1, 8 kg P ha-1, 39 kg K ha-1 and 11 kg Mg ha-1. This is equivalent to 21 N, 36 P, 14 K and 27 Mg of the maximum nutrient content of the shoots. Nutrient remobilization from shoots to rhizomes defined as the increase in nutrient content of rhizomes between September and March measured in 1994/95 was 101 kg N ha-1, 9 kg P ha-1, 81 kg K ha-1 and 8 kg Mg ha-1 equivalent to 46 N, 50 P, 30 K and 27 Mg of nutrient content of shoots in September. Results showed that nutrient remobilization within the plant needs to be considered when calculating nutrient balances and fertilizer recommendations.     

Methods to estimate on-field nitrogen emissions from crop production as an input to LCA studies in the agricultural sector

Nitrogen compounds emitted from the field are usually considered in Life Cycle Assessments (LCA) of agricultural products or processes. The environmentally most important of these N emissions are ammonia (NH₃), nitrous oxide (N₂0) and nitrate (N0₃). The emission rates are variable due to the influence of soil type, climatic conditions and agricultural management practices. Due to considerable financial and time efforts, and great variations in the results, actual measurements of emissions are neither practical nor appropriate for LCA purposes. Instead of measurements, structured methods can be used to estimate average emission rates. Another possibility is the use of values derived from the literature which would, however, require considerable effort compared to estimation methods, especially because the values might only be valid for the particular system under investigation. In this paper methods to determine estimates for NH₃, N₂0 and NO₃ emissions were selected from a literature review. Different procedures were chosen to estimate NH₃ emissions from organic (Horlacher &Marschner, 1990) and mineral fertilizers (ECETOC, 1994). To calculate the N₂O emissions, a function derived by Bouwman (1995) was selected. A method developed by the German Soil Science Association (DBG, 1992) was adopted to determine potential NO₃ emissions. None of the methods are computer-based and consequently require only a minimum set of input data. This makes them, on the one hand, transparent and easy to perform, while, on the other hand, they certainly simplify the complex processes.