Behavioral plasticity and central regeneration of locomotor reflexes in the freshwater oligochaete, Lumbriculus variegatus. I: Transection studies

Abstract. Access to the ventral nerve cord in living specimens of Lumbriculus variegatus , an aquatic oligochaete, is normally impossible because surgical invasion induces segmental autotomy (self-fragmentation). We show here that nicotine is a powerful paralytic agent that reversibly immobilizes worms, blocks segmental autotomy, and allows experimental access to the nerve cord. Using nicotine-treated worms, we transected the ventral nerve cord and used non-invasive electrophysiological recordings and behavioral analyses to characterize the functional recovery of giant nerve fibers and other reflex pathways. Initially, after transection, medial giant fiber (MGF) and lateral giant fiber (LGF) spikes conducted up to, but not across, the transection site. Reestablishment of MGF and LGF through-conduction across the transection site occurred as early as 10 h (usually by 20 h) after transection. Analyses of non-giant-mediated behavioral responses (i.e., helical swimming and body reversal) were also made following nerve cord transection. Immediately after transection, functional reorganization of touch-evoked locomotor reflexes occurred, so that the two portions of the worm anterior and posterior to the transection site were independently capable of helical swimming and body reversal responses. Similar reorganization of responses occurred in amputated body fragments. Reversion back to the original whole-body pattern of swimming and reversal occurred as early as 8 h after transection. Thus, functional restoration of the non-giant central pathways appeared slightly faster than giant fiber pathways. The results demonstrate the remarkable plasticity of locomotor reflex behaviors immediately after nerve cord transection or segment amputation. They also demonstrate the exceptional speed and specificity of regeneration of the central pathways that mediate locomotor reflexes.     

Behavioral plasticity and central regeneration of locomotor reflexes in the freshwater oligochaete, Lumbriculus variegatus. II: Ablation studies

Abstract. After 8–10 segments of posterior ventral nerve cord were ablated in Lumbriculus variegatus , touch-evoked locomotor responses were evident both in segments anterior and posterior to the ablation site. However, responses in these two regions were independent and uncoupled. During recovery, four outcomes were observed at the ablation site: (Group 1) recovery of normal functions with no growth of new segments; (Group 2) formation of a laterally protruding, multi-segmented, ectopic head; (Group 3) formation of a laterally protruding, amorphous, and multi-segmented outgrowth; and (Group 4) segmental autotomy. In Groups 1 and 2, touch-evoked swimming and body reversal were studied. In addition, sensory fields and conduction properties of giant nerve fibers were examined near the ablation site. In some Group 1 worms, clear-cut behavioral and electrical signs of recovery and reconnection were seen by 3 d after ablation. By 8 d, all worms had recovered and exhibited response patterns comparable to those of normal worms. In Group 2 worms, with an ectopic head, segments posterior to the ablation (together with those in the ectopic head), exhibited touch-evoked swimming and body reversal responses resembling those of a complete worm. Segments anterior to the ectopic head were independently capable of locomotor responses. Medial and lateral giant fiber sensory fields in worms with ectopic heads reflected a pattern expected for two worms. Thus, through apparent morphallactic reorganization, a medial giant fiber sensory field emerged which included the ectopic head and 10–15 adjacent posterior segments. In contrast, electrical recordings showed longitudinal through-conduction of giant fiber spikes, across the ablation site. Histological examination revealed that the giant nerve fibers in the ectopic head were complexly interconnected with those in the main body axis.     

The catalytic domain of human neuropathy target esterase mediates an organophosphate-sensitive ionic conductance across liposome membranes

In humans and other vertebrates, reaction of organophosphates with a neuronal membrane protein, neuropathy target esterase (NTE), initiates events which culminate in axonal degeneration. The initiation process appears to involve modification of a property of the protein distinct from its esterase activity, subsequent to formation of a negatively charged adduct with the active site serine residue. Here, we show that membrane patches from liposomes containing NEST, a recombinant hydrophobic polypeptide comprising the esterase domain of human NTE, display a transmembrane ionic conductance with both stable and high-frequency flickering components. An asymmetric current-voltage relationship suggested that ion flow was favoured in one direction relative to the membrane and its associated NEST molecules. Flow of anions was slightly favoured compared with cations. The flickering current formed a much larger proportion of the overall conductance in patches containing wild-type NEST compared with the catalytically inactive S966A mutant form of the protein. The conductance across patches containing NEST, but not those with the S966A mutant, was significantly reduced after adding neuropathic organophosphates to the bathing medium. By contrast, non-neuropathic covalent inhibitors of the catalytic activity of NEST did not reduce NEST-mediated conductance. Future work may establish whether NTE itself mediates an organophosphate-sensitive ion flux across intracellular membranes within intact cells.     

Method of modelling intracellular transport in branching neurites: application to axons and dendrites of Drosophila sensory neurons

This paper develops a method of calculating the transport of intracellular organelles in neurons with branching neurites which is based on the Smith–Simmons equations of motor-assisted transport. The method is aimed at understanding the effects of microtubule (MT) polarity orientation in branching neurites on transport of organelles at the fundamental level. The method is applied to calculating the organelle transport in axons and dendrites of Drosophila neurons, using the map of MT orientation in such neurons developed by Stone et al. (Mol Biol Cell 19:4122–4129, 2008). The proximal dendrite is assumed to branch and form two distal dendrites. Two different MT polarity arrangements in a proximal dendrite are considered, and implications of these MT arrangements on organelle transport are analysed. It is demonstrated that the MT arrangement found in Drosophila dendrites (MTs have their minus ends out in a proximal dendrite) results in much more efficient motor-driven transport than the structure with a mixed MT orientation in proximal dendrites.     

Modelling active transport in Drosophila unipolar motor neurons

This paper develops a model for simulating organelle transport in Drosophila unipolar motor neurons. The paper is motivated by a recent experimental investigation by Stone et al. (Microtubules have opposite orientation in axons and dendrites of Drosophila neurons. Mol Biol Cell.19:4122-4129) who proposed a map of microtubule (MT) orientation in Drosophila neurons, and explained why dynein mutations selectively impede dendritic growth without having much effect on axonal growth. Two different approaches to modelling the effect of dynein mutations are utilised: one through assuming a reduced average velocity of a dynein mutant motor and the other through assuming its decreased processivity (an increased detachment rate from MTs). Modified Smith–Simmons equations are used for developing a continuum model of the process. Distributions of organelle concentrations as well as distributions of diffusion, motor-driven and total organelle fluxes are simulated.     

A novel mechanism for irregular firing of a neuron in response to periodic stimulation: irregularity in the absence of noise

Irregular firing of action potentials (AP's) is a characteristic feature of neurons in the brain. The variability has been attributed to noise from various sources. This study illustrates an alternative mechanism, namely, deterministic irregularity within a model of ionic conductances. Specifically, a model based on modern measurements of the Na⁺ and K⁺ current components from the squid giant axon fires irregularly in response to a continuous train of near-threshold current pulses. The interspike interval histogram from these simulations is multi-modal, a result which in other systems has been attributed to stochastic resonance. Moreover, the simulations exhibited short burst of spikes followed by relatively long quiescent periods, a result suggestive of patterned input to the model even though the input consisted of a train of regularly spaced current pulses. The variability of firing is attributable to variations in AP parameters, in particular AP amplitude. The action potential for squid giant axons is not all-or-none. Rather, it is fundamentally a continuous function of stimulus amplitude. That is, the membrane lacks a threshold. Variation in AP amplitude, and to a lesser extent, AP duration, can produce variations in the time to a subsequent AP, which represents a paradigm shift for understanding irregular neuronal firing. The emphasis is not as much on events prior to an AP as it is on the AP's themselves.     

Preface

Biological properties of soil are not only essential for the maintenance of soil fertility and the sustainability of the plant-soil ecosystems, but also indicators of land reclamation of contaminated or disturbed soils. This experiment involves two plants (barley and field pea) growing in four soils with different hydrocarbon contents. The objective was to study the effect of hydrocarbons on plant growth and microbial activity, and to evaluate the acid phosphatase activity as an indicator of reclamation of hydrocarbon-contaminated soils. Barley root mass decreased with the increase of the hydrocarbon content but field pea roots were not sensitive to the hydrocarbon content in this experiment. The hydrocarbon contamination reduced the plant growth but increased the microbial activity. The acid phosphatase activity was controlled by both plant root production and microbial activity, therefore it was not a good indicator of the reclamation of oil-contaminated soils.     

The brain structure of selected trypanorhynch tapeworms

The brain architecture in four species of tapeworms from the order Trypanorhyncha has been studied. In all species, the brain consists of paired anterior and lateral lobes, and an unpaired central lobe. The anterior lobes connect by dorsal and ventral semicircular commissures; the central and lateral lobes connect by a median and an X-shaped crisscross commissure. In the center of the brain, five well-developed compact neuropils are present. The brain occupies a medial position in the scolex pars bothrialis. The ventral excretory vessels are situated outside the lateral lobes of the brain; the dorsal excretory vessels are located inside the brain and dorsal to the median commissure. The brain gives rize four anterior proboscis nerves and four posterior bulbar nerves with myelinated giant axons (GAs). The cell bodies of the GAs are located within the X-commissure and in the bulbar nerves. Highly developed serotonergic neuropils are present in the anterior and lateral lobes; numerous 5-HT neurons are found in the brain lobes including the central unpaired lobe. The X-cross commissure consists of the α-tub-immunoreactive and 5-HT-IR neurites. Eight ultrastructural types of neurons were found in the brain of the three species investigated. In addition, different types of synapses were present in the neuropils. Glial cells ensheath the brain lobes, the neuropils, the GAs, and the bulbar nerves. Glia cell processes form complex branching patterns of thin cytoplasmic sheets sandwiched between adjacent neural processes and filling the space between neurons. Multilayer myelin-like envelopes and a mesaxon-like structure have been found in Trypanorhyncha nervous system. We compared the brain architecture of Trypanorhyncha with that of an early basal cestode taxon, that is, Diphyllobothriidea, and present a hypothesis about the homology of the anterior brain lobes in order Trypanorhyncha; and the lateral lobes and median commissure are homologous brain structures within Eucestoda.     

Defensive strategies in planktonic coelenterates

Some planktonic coelenterates respond to potentially harmful stimulation by protective involution, others by escape behaviour. Examples of protective involution are seen in the ‘crumpling’ behaviour of various hydrome‐dusae (Sarsia, Euphysa) and of siphonophores such as Hippopodius. Involution may be accompanied by striking visual displays e.g. light emission in Euphysa, light emission and blanching in Hippopodius. These displays probably serve to startle or blind interlopers. In Hippopodius, light emission in the dark would have the same effect as blanching in the light, an example of behavioural self‐mimicry.

Animals employing escape locomotion include the ctenophore Euplokamis, the siphonophore Nanomia and the rhopalonematid medusa Aglantha. All of these forms have evolved giant axons that facilitate escape by reducing response time. The central nervous circuitry underlying locomotion in Aglantha is reviewed.

In a few cases (e.g. Aglantha and possibly Nanomia), the responses described can be seen as defensive against predators, but in the majority of cases, the responses probably serve primarily to reduce the risk of damage due to accidental contact with other organisms.     

Lipid vesicle-mediated alterations of membrane cholesterol levels: Effects on Na+ and K+ currents in squid axon

Summary We demonstrate that cholesterol can exchange from sonicated lipid vesicles to a perfused squid giant axon membrane and that vesicles with varying cholesterol/phospholipid (C/P) mole ratios can be used to achieve either net loading or net depletion of axon membrane cholesterol. Two types of evidence were obtained which show that net loading or depletion of cholesterol was achieved: (i) changes in the cholesterol/phospholipid (C/P) mole ratios of axons, and (ii) visualization of cholesterol depleted from the preparation by cholesterol-free vesicles by thin-layer chromatography. The C/P mole ratios indicate that cholesterol levels in the preparation were increased or decreased by 30–40%. Increasing or decreasing membrane cholesterol levels were ineffective in altering the Na⁺ or K⁺ occurrents in voltage-clamped axons. In addition, we determined that cholesterol flip-flop across the axonal membrane occurred with at ^1/2 of 7.3 to 15.3 min.     

Novel excitatory Conus peptides define a new conotoxin superfamily

A new class of Conus peptides, the I-superfamily of conotoxins, has been characterized using biochemical, electrophysiological and molecular genetic methods. Peptides in this superfamily have a novel pattern of eight Cys residues. Five peptides that elicited excitatory symptomatology, r11a, r11b, r11c, r11d and r11e, were purified from Conus radiatus venom; four were tested on amphibian peripheral axons and shown to elicit repetitive action potentials, consistent with being members of the 'lightning-strike cabal' of toxins that effect instant immobilization of fish prey. A parallel analysis of Conus cDNA clones revealed a new class of conotoxin genes that was particularly enriched (with 18 identified paralogues) in a Conus radiatus venom duct library; several C. radiatus clones encoded the excitatory peptides directly characterized from venom. The remarkable diversity of related I-superfamily peptides within a single Conus species is unprecedented. When combined with the excitatory effects observed on peripheral circuitry, this unexpected diversity suggests a corresponding molecular complexity of the targeted signaling components in peripheral axons; the I-conotoxin superfamily should provide a rich lode of pharmacological tools for dissecting and understanding these. Thus, the I-superfamily conotoxins promise to provide a significant new technology platform for dissecting the molecular components of axons.     

Evolutionary dynamics of giant viruses and their virophages

Giant viruses contain large genomes, encode many proteins atypical for viruses, replicate in large viral factories, and tend to infect protists. The giant virus replication factories can in turn be infected by so called virophages, which are smaller viruses that negatively impact giant virus replication. An example is Mimiviruses that infect the protist Acanthamoeba and that are themselves infected by the virophage Sputnik. This study examines the evolutionary dynamics of this system, using mathematical models. While the models suggest that the virophage population will evolve to increasing degrees of giant virus inhibition, it further suggests that this renders the virophage population prone to extinction due to dynamic instabilities over wide parameter ranges. Implications and conditions required to avoid extinction are discussed. Another interesting result is that virophage presence can fundamentally alter the evolutionary course of the giant virus. While the giant virus is predicted to evolve toward increasing its basic reproductive ratio in the absence of the virophage, the opposite is true in its presence. Therefore, virophages can not only benefit the host population directly by inhibiting the giant viruses but also indirectly by causing giant viruses to evolve toward weaker phenotypes. Experimental tests for this model are suggested.     

Mathematical modelling of primary production in Green Bay (Lake Michigan,USA), a phosphorus-and light-limited system

Application of mathematical models in the design and evaluation of lake restoration programmes must include due consideration of three basic concepts of model development; 1) that the model framework is appropriately matched to the intended management use, 2) that selection of the proper degree of model complexity is fundamental to the achievement of model credibility and 3) that field and laboratory studies must be designed and interpreted with the aid of the model to insure development of a comprehensive, integrated tool.These concepts are demonstrated for the case of lake restoration efforts in Green Bay (Lake Michigan, USA). Striking gradients in water quality (transparency, algal standing crop, hypolimnetic oxygen depletion) and trophic state occur along the major axis of the bay in response to phosphorus loaded from the Fox River. A simple model for gross primary production is developed to permit calculation of the relative importance of internal carbon production to the total organic carbon budget of the bay. Primary production varies from high rates over a limited photic depth in the turbid, phosphorus-rich waters of the eutrophic portions of the bay to low rates over an extensive photic depth in the transparent, phosphoruspoor reaches of the oligotrophic regions. Internal production accounts for approximately 90% of the total organic carbon loaded to the system over the summer growing season. Water quality management strategies must address the stimulation of primary production by phosphorus loaded from the Fox River in any attempt to lower the standing crop of nuisance algae, improve water clarity, and reduce rates of hypolimnetic oxygen depletion in Green Bay.     

Sleep and rest activity measured by wrist actigraphy in an HIV-infected patient with Steven–Johnson syndrome

The Steven–Johnson syndrome (SJS) is characterized by a sudden onset of mucous membrane erosion (predominantly oral mucosa, lips, and conjunctivae) with widespread blistering of the skin involving up to 10% of the body surface area. It is almost always a drug-related reaction, although it can be caused by infections and immunizations. A 33-year-old man with recent diagnosis of HIV infection developed antiretroviral treatment (ART)-associated SJS. Physical activity and sleep parameters were recorded by wrist actigraphy in four different consecutive scenarios: baseline assessment, first ART regimen, hospitalization, and second ART regimen. Significant differences were observed in physical activity patterns between the four phases. No differences in sleep parameters were found. To our knowledge, this is the first study recording physical activity changes and sleep during a SJS reaction.     

An approach to modelling in immunology

Like most other fields in biology, immunology has been revolutionised by the techniques of molecular biology and the resulting explosion in available experimental data. It is argued that efforts to integrate the data to gain insight into how various subsystems in the immune system interact and function require mathematical modelling and computer simulation in close collaboration with experimentalists. This paper illustrates some of the techniques available for modelling immune systems, and highlights the issues that should be borne in mind by anyone starting down the modelling path.     

Interpretation of mixed sediment profiles by means of a sediment-mixing model and radioactive fallout

Chernobyl fallout provided a pulsed release to the environment and an opportunity to study sediment mixing processes in lakes. The paper presents the application of a mathematical modelling technique using a radionuclide as tracer. The model simulates the radionuclide profiles in sediments and computes the concentrations by solving numerically a one-dimensional mass-balance equation. The mixing-advection-decay model has been designed to include time-variable input capabilities and a depth-dependent mixing coefficient.     

Theoretical simulation of the calcium action potential in squid giant synapse: Repetitive stimulation

A biophysical model of the experimentally observed calcium action potential (CAP) in squid giant synapse is proposed. Whereas the inclusion of the inward calcium current in the Hodgkin-Huxley model can generate the rising phase of CAP, to account for the observed termination of the action potential, a repolarizing process needs to be introduced. Adding a term representing Ca-activated K current, the observed features of CAP can be reproduced. However, one feature of CAP, namely the gradual shortening of the plateau duration on repetitive stimulation, cannot be simulated by this model. In this paper, it is proved that both the termination of the action potential and the gradual shortening of the plateau cannot be accounted for by inclusion of a single repolarizing process. One more repolarizing process, namely a slow voltage-dependent Ca-inactivation, is therefore proposed to account for all the observed features of CAP.     

Effect of NaCl and KCl on fate and growth/no growth interfaces of Listeria monocytogenes Scott A at different pH and nisin concentrations

AIMS: The fate of Listeria monocytogenes Scott A, was studied in broth, at different a(w)s (by adding NaCl or KCl from 0.0 to 1.4 mol l(-1)), pHs (from 4.0 to 7.3 by adding lactic acid), and nisin concentrations (from 0 to 100 IU ml(-1)). METHODS AND RESULTS: Increasing salt and nisin concentrations and decreasing pH resulted in lower growth rates and extended lag phases. At pH 4.5 no growth was observed while in presence of nisin and/or 1 mol l(-1) salts of both kinds, L. monocytogenes Scott A was inactivated. Equal-molar concentrations of NaCl or KCl (similar a(w)), exerted similar effects against L. monocytogenes in terms of lag phase duration, growth or death rate. The growth boundaries of L. monocytogenes Scott A at 5 degrees C were also estimated by growth/no growth turbidity data, modeled by logistic polynomial regression. The concordance of logistic models, were 99.6 and 99.8% for NaCl and KCl, respectively. CONCLUSIONS: The growth interfaces derived by both NaCl and KCl models were almost identical. Hence, NaCl can be replaced by KCl without risking the microbiological safety of the product. Increasing nisin concentrations markedly affected the interface resulting in a more inhibitory environment for L. monocytogenes Scott A. Low to medium salt concentrations (0.3-0.7 mol l(-1) of either NaCl or KCl) provided a protective effect against inhibition of L. monocytogenes Scott A by nisin. SIGNIFICANCE AND IMPACT OF THE STUDY: Modelling the growth boundaries not only contributes to the development of safer food by providing useful data, but can also be used to study interactions between factors affecting initiation of growth of pathogenic micro-organisms.     

圈养大熊猫种群的动态及发展趋势

采用直线回归方程、4次方程和指数增长式微分方程等数理分析方法,对全世界1953～2003年圈养大熊猫种群的发展趋势进行了分析.发现对大熊猫的圈养种群数量建立的回归方程是拟合很好的4次多项式方程,1953～1987年的圈养种群增长曲线成S型,1998年后的种群增长成直线或指数式.依此进行预测,从1998年以后只依靠现有饲养种群167只就可能使种群数量稳定增长,全世界种群数量2007年可突破200只,2009年可能达到220～240只,大概在15～22年后全世界饲养种群数量就能够翻一番,达到330多只.     

Modelling seasonal variations in the age and incidence of Kawasaki disease to explore possible infectious aetiologies

The average age of infection is expected to vary during seasonal epidemics in a way that is predictable from the epidemiological features, such as the duration of infectiousness and the nature of population mixing. However, it is not known whether such changes can be detected and verified using routinely collected data. We examined the correlation between the weekly number and average age of cases using data on pre-vaccination measles and rotavirus. We show that age-incidence patterns can be observed and predicted for these childhood infections. Incorporating additional information about important features of the transmission dynamics improves the correspondence between model predictions and empirical data. We then explored whether knowledge of the age-incidence pattern can shed light on the epidemiological features of diseases of unknown aetiology, such as Kawasaki disease (KD). Our results indicate KD is unlikely to be triggered by a single acute immunizing infection, but is consistent with an infection of longer duration, a non-immunizing infection or co-infection with an acute agent and one with longer duration. Age-incidence patterns can lend insight into important epidemiological features of infections, providing information on transmission-relevant population mixing for known infections and clues about the aetiology of complex paediatric diseases.