Temperature- and structure-dependent interaction of pyrethroids with the sodium channels in frog node of Ranvier

(1) The interaction of a series of pyrethroid insecticides with the Na+ channels in myelinated nerve fibres of the clawed frog, Xenopus laevis, was investigated using the voltage clamp technique. (2) Out of 11 pyrethroids 9 insecticidally active compounds induce a slowly decaying Na+ tail current on termination of a step depolarization, whereas the Na+ current during depolarization was hardly affected. These tail currents are most readily explained by a selective reduction of the rate of closing of the activation gate in a fraction of the Na+ channels that have opened during depolarization. (3) The rate of decay of the Na+ tail current varies considerably with pyrethroid structure. After alpha-cyano pyrethroids the decay is at least one order of magnitude slower than after non-cyano pyrethroids. The decay always follows a single-exponential time course and is reversibly slowed when the temperature is lowered from 25 to 0 degrees C. Arrhenius plots in this temperature range are linear. (4) These results indicate that the relaxation of the activation gate in pyrethroid-affected Na+ channels is governed by an apparent first order, unimolecular process and that the rate of relaxation is limited by a single energy barrier. Application of transition state theory shows that after alpha-cyano pyrethroids this energy barrier is 9.6 kJ/mol higher than after non-cyano pyrethroids. (5) Differences in rate of decay of the Na+ tail current account for the reported differences in repetitive nerve activity induced by various pyrethroids. In addition, the effect of temperature on the rate of decay explains the increase in repetitive activity with cooling.     

Veratridine blocks voltage-gated potassium current in human T lymphocytes and in mouse neuroblastoma cells

(i) Effects of veratridine on ionic conductances of human peripheral blood T lymphocytes have been investigated using the whole-cell patch-clamp technique, (ii) Veratridine reduces the net outward current evoked by membrane depolarizations. The reduction originates from block of a 4-aminopyridine-sensitive, voltage-gated K⁺ current, (iii) Human T lymphocytes do not appear to express voltage-gated Na⁺ channels, since inward currents are observed neither in control nor in veratridine- and bretylium-exposed lymphocytes. (iv) The effect of veratridine consists of an increase in the rate of decay of the voltage-gated K⁺ current and a reduction of the peak current amplitude. Both effects depend on veratridine concentration. Halfmaximum block occurs at 97 m and the time constant of decay is reduced by 50% at 54 m of veratridine. (v) Possible mechanisms of veratridine action are discussed. The increased rate of K⁺ current decay is most likely due to open channel block. The decrease of current amplitude may involve an additional mechanism. (vi) In cultured mouse neuroblastoma N1E-115 cells, veratridine blocks a component of voltage-gated K⁺ current, in addition to its effect on voltage-gated Na⁺ current. This result shows that the novel effect of veratridine is not confined to lymphocytes.We thank Jacobien Künzel of the Wilhelmina Hospital for Children, Utrecht, for providing the blood samples and Aart de Groot for technical assistance. The research was supported by a fellowship of the Royal Netherlands Academy of Arts and Sciences to M. Oortgiesen.     

Food-web studies in shallow eutrophic lakes by the Netherlands Institute of Ecology: Main results,knowledge gaps and new perspectives

For more than 20 years scientists of the ‘Food-chain studies’ Group of the former Limnological Institute have been studying interactions within the pelagic food web. Purpose of research was to explain the structure and dynamics of the zooplankton and fish communities in lakes and reservoirs in relation to biotic and abiotic environmental factors. A so-called multi-species approach was used, in which all common and abundant species within a specific ecosystem were studied on the individual and population level with the same degree of detail. The recent results and the scientific approach used are evaluated and the main gaps in knowledge about food-web dynamics in shallow eutrophic lakes are identified and discussed. It is concluded that instead of the purely functional approach used so far, future studies should also include evolutionary aspects which determine the success of an organism in a given environment and that more attention should be paid to central questions in ‘community ecology’. This paper is based on a lecture given by the first author for the Netherlands Society of Aquatic Ecology on May 12th, 1992, in Amsterdam, The Netherlands.     

The effect of temperature,and food quantity and quality on the growth and development rates in laboratory-cultured copepods and cladocerans from a Sri Lankan reservoir

Length growth, instar durations, fecundity and mortality rates of fivespecies of microcrustacean zooplankton from a tropical reservoir weremeasured in relation to food quantity and temperature in laboratorycultures. Three cladocerans (Ceriodaphnia cornuta, Moina micrura,Diaphanosoma excisum), one calanoid copepod (Heliodiaptomus viduus), and onecyclopoid copepod (Mesocyclops thermocyclopoides) were studied. Filteredseston (45 µm mesh) from a local pond was used for food. Two foodconcentrations were employed: (1) 10 µg chlorophyll l^–1(ca 0.25 mg Cl^–1), and (2) 50 µg chlorophylll^–1 (ca 1.25 mg C l^–1). Food levels and watertemperature (22.5, 27.5, and 32.5 °C) used, roughly covered the rangesfound in the reservoir. Although all the three growth parameters were oftenaffected to some degree by temperature and food, the quantitative responseof the species differed. Also, the species reacted differently to the threepossible interactions (i.e. food×temperature,food×instar, andtemperature×instar). This contributed to the overalldifferences in the species responses. For the cladocerans, instar durationswere always affected by temperature. The food did not affect the durationtime of the adults and that of the combined juvenile instars, the latterexcept in one case significantly. For the two copepods food level affectedthe duration times of naupliar and copepodite instars, but the effect oftemperature was only significant for H. viduus. The development timesobserved for H. viduus were extraordinary long compared with values reportedin the literature for other tropical calanoids. This suggests that foodconditions in our culture were adversely affecting its growth rates.     

Substrate selection and seasonal variation in densities of invertebrates in stream pools of a tropical river

Spatial distribution and seasonal variation in densities of the invertebrates were investigated for a year in three stream pools of a South Indian river. The effects of season, substrate type and water depth on the distribution were analyzed. Substrate type and season influenced the invertebrate distribution the most. Leaf packs harboured most of the organisms followed by macro-algal substrate and sand. The lowest densities were observed on rocky substrates and in the water column. Rocky substrates in shallow water supported higher densities of total invertebrates than deeper areas. Chironomid larvae dominated all benthic substrates throughout the year. Of the 19 invertebrate taxa studied, 6 showed no seasonality in densities, and most of the rest showed their highest densities in the pre-monsoon period and lowest in the SW monsoon or post-monsoon periods. However, in two of the three pools, the densities of total invertebrates were highest during the post-monsoon period with secondary peaks in the pre-monsoon period.     

A new scorpion toxin with a very high affinity for sodium channels. An electrophysiological study

The neurotoxic action of toxin gamma from the venom of the Brazilian scorpion Tityus serrulatus (TiTx gamma) has been investigated in cultured mouse neuroblastoma cells (N1E115) using the suction pipette technique. Addition of 14 to 53 nM TiTx gamma to the external solution causes nerve cell membrane depolarization, membrane potential oscillations and spontaneous action potentials within 10 min. None of these effects were observed within 15 min after application of 1 microM toxin IV from Centruroides sculpturatus venom. Under voltage clamp the amplitude of the sodium current evoked by test pulses to potentials more positive than -30 mV is reversibly reduced by 50% after 17 to 105 nM TiTx gamma. On the other hand, a sodium current component appears after TiTx gamma at test pulse potentials between -70 and -40 mV, for which no sodium current is observed in the control experiment. The outward potassium current is not significantly affected by the highest TiTx gamma concentrations used. The potential-dependence of inactivation of the sodium current component that is induced by TiTx gamma is shifted by -30 mV with respect to control values. The local anaesthetic procain at 1 mM discriminates between the two populations of sodium channels observed in the presence of TiTx gamma.     

Ca(2+) -independent vesicular catecholamine release in PC12 cells by nanomolar concentrations of Pb(2+)

Effects of Pb(2+) on vesicular catecholamine release in intact and ionomycin-permeabilized PC12 cells were investigated using carbon fibre microelectrode amperometry. Changes in intracellular Pb(2+) and Ca(2+) were measured from indo-1 fluorescence by confocal laser scanning microscopy. Depolarization of intact cells and superfusion of permeabilized cells with saline containing > or = 100 microm Ca(2+) rapidly evokes quantal catecholamine release. Superfusion with up to 10 microm Pb(2+) -containing saline evokes release of similar catecholamine quanta after a concentration-dependent delay. Thresholds to induce exocytosis within 30 min of exposure are between 1 and 10 microm Pb(2+) in intact cells and between 10 and 30 nm Pb(2+) in permeabilized cells. Additional inhibition of exocytosis occurs in permeabilized cells exposed to 10 microm Pb(2+). Using membrane-impermeable and -permeable chelators it is demonstrated that intracellular Ca(2+) is not required for Pb(2+) -induced exocytosis. In indo- 1-loaded cells Pb(2+) reduces the fluorescence intensity after a concentration-dependent delay, whereas the fluorescence ratio, indicating intracellular Ca(2+) concentration, remains unchanged. The delay to detect an increase in free intracellular Pb(2+) (> or = 30 nm) is much longer than the delay to Pb(2+) -induced exocytosis, indicating that cytoplasmic components buffer Pb(2+) with high affinity. It is concluded that Pb(2+) acts as a high-affinity substitute for Ca(2+) to trigger essential steps leading to vesicular catecholamine release, which occurs when only approximately 20% of the intracellular high-affinity binding capacity ( approximately 2 attomol/cell) is saturated with Pb(2+).     

Competitive potentiation of acetylcholine effects on neuronal nicotinic receptors by acetylcholinesterase-inhibiting drugs

The effects of the acetylcholinesterase inhibitors physostigmine and tacrine on alpha4beta2 and alpha4beta4 subtypes of neuronal nicotinic acetylcholine (ACh) receptors, expressed in Xenopus laevis oocytes, have been investigated. In voltage-clamp experiments low concentrations of physostigmine and tacrine potentiate ion currents induced by low concentrations of ACh, whereas at high concentrations they inhibit ACh-induced ion currents. These dual effects result in bell-shaped concentration-effect curves. Physostigmine and tacrine, by themselves, do not act as nicotinic receptor againsts. The larger potentiation is observed with 10 microM: physostigmine on alpha4beta4 nicotinic receptors and amounts to 70% at 1 microM: ACh. The mechanism underlying the effects of physostigmine on alpha4beta4 ACh receptors has been investigated in detail. Potentiation of ACh-induced ion current by low concentrations of physostigmine is surmounted at elevated concentrations of ACh, indicating that this is a competitive effect. Conversely, inhibition of ACh-induced ion current by high concentrations of physostigmine is not surmounted at high concentrations of ACh, and this effect appears mainly due to noncompetitive, voltage-dependent ion channel block. Radioligand binding experiments demonstrating displacement of the nicotinic receptor agonist (125)I-epibatidine from its recognition sites on alpha4beta4 ACh receptors by physostigmine confirm that physostigmine is a competitive ligand at these receptors. A two-site equilibrium receptor occupation model, combined with noncompetitive ion channel block, accounts for the dual effects of physostigmine and tacrine on ACh-induced ion currents. It is concluded that these acetylcholinesterase-inhibiting drugs interact with the ACh recognition sites and are coagonists of ACh on alpha4-containing nicotinic ACh receptors.     

Genetic fine-mapping of <Emphasis Type="Italic">DIPLOSPOROUS</Emphasis> in <Emphasis Type="Italic">Taraxacum</Emphasis> (dandelion; Asteraceae) indicates a duplicated <Emphasis Type="Italic">DIP</Emphasis>-gene

Background  

DIPLOSPOROUS (DIP) is the locus for diplospory in Taraxacum, associated to unreduced female gamete formation in apomicts. Apomicts reproduce clonally through seeds, including apomeiosis, parthenogenesis, and autonomous or pseudogamous endosperm formation. In Taraxacum, diplospory results in first division restitution (FDR) nuclei, and inherits as a dominant, monogenic trait, independent from the other apomixis elements. A preliminary genetic linkage map indicated that the DIP-locus lacks suppression of recombination, which is unique among all other map-based cloning efforts of apomeiosis to date. FDR as well as apomixis as a whole are of interest in plant breeding, allowing for polyploidization and fixation of hybrid vigor, respectively. No dominant FDR or apomixis genes have yet been isolated. Here, we zoom-in to the DIP-locus by largely extending our initial mapping population, and by analyzing (local) suppression of recombination and allele sequence divergence (ASD).     

Seasonal variations in the condition of two Daphnia species and their hybrid in a eutrophic lake: evidence for food limitation

As seston and chlorophyll concentrations in eutrophic lakesare usually high all year round, it was hypothesized in thepast that food limitation is of major importance for the seasonaldynamics of herbivorous zooplankton in such lakes. Since directmeasurements of food are hampered by the lack of knowledge onthe exact nature of the food in eutrophic conditions, indirectmeasurements are necessary to estimate the degree of food limitationin these circumstances. Hence, we used laboratory- and field-derivedrelationships between the body length and body carbon contentof different species of Daphnia, which were collected from thehighly eutrophic Tjeukemeer, the Netherlands. From a seasonalsurvey of the carbon content of the daphnids. we concluded thatin Tjeukemeer D.galeata, D.galeata x cucullata and D.cucullataare food limited during the largest part of the year. Sincethe condition of the hybrids was relatively high as comparedwith the parental species when the food concentration was high,D.galeata x cucutlaia is expected to be the more successfultaxon during periods of high food availability.