Modelling the movement of a soil insect

We use a linear autoregressive model to describe the movement of a soil-living insect, Protaphorura armata (Collembola). Models of this kind can be viewed as extensions of a random walk, but unlike a correlated random walk, in which the speed and turning angles are independent, our model identifies and expresses the correlations between the turning angles and a variable speed. Our model uses data in x- and y-coordinates rather than in polar coordinates, which is useful for situations in which the resolution of the observations is limited. The movement of the insect was characterized by (i) looping behaviour due to autocorrelation and cross correlation in the velocity process and (ii) occurrence of periods of inactivity, which we describe with a Poisson random effects model. We also introduce obstacles to the environment to add structural heterogeneity to the movement process. We compare aspects such as loop shape, inter-loop time, holding angles at obstacles, net squared displacement, number, and duration of inactive periods between observed and predicted movement. The comparison demonstrates that our approach is relevant as a starting-point to predict behaviourally complex moving, e.g. systematic searching, in a heterogeneous landscape.     

Analysis of models for crustacean stretch receptors

 The mechanisms underlying the diverse responses to step current stimuli of models [Edman et al. (1987) J Physiol (Lond) 384: 649–669] of lobster slowly adapting stretch receptor organs (SAO) and fast-adapting stretch receptor organs (FAO) are analyzed. In response to a step current, the models display three distinct types of firing reflecting the level of adaptation to the stimulation. Low-amplitude currents evoke transient firing containing one to several action potentials before the system stabilizes to a resting state. Conversely, high-amplitude stimulations induce a high frequency transient burst that can last several seconds before the model returns to its quiescent state. In the SAO model, the transition between the two regimes is characterized by a sustained pacemaker firing at an intermediate stimulation amplitude. The FAO model does not exhibit such a maintained firing; rather, the duration of the transient firing increases at first with the stimulus intensity, goes through a maximum and then decreases at larger intensities. Both models comprise seven variables representing the membrane potential, the sodium fast activation, fast inactivation, slow inactivation, the potassium fast activation, slow inactivation gating variables, and the intra cellular sodium concentration. To elucidate the mechanisms of the firing adaptations, the seven-variable model for the lobster stretch receptor neuron is first reduced to a three-dimensional system by regrouping variables with similar time scales. More precisely, we substituted the membrane potential V for the sodium fast activation equivalent potential V _m , the potassium fast inactivation V _n for the sodium fast inactivation V _h , and the sodium slow inactivation V _l for the potassium slow inactivation V _r . Comparison of the responses of the reduced models to those of the original models revealed that the main behaviors of the system were preserved in the reduction process. We classified the different types of responses of the reduced SAO and FAO models to constant current stimulation. We analyzed the transient and stationary responses of the reduced models by constructing bifurcation diagrams representing the qualitatively distinct dynamics of the models and the transitions between them. These revealed that (1) the transient firings prior to reaching the stationary state can be accounted for by the sodium slow inactivation evolving more slowly than the other two variables, so that the changes during the transient firings reflect the bifurcations that the two-dimensional system undergoes when the sodium slow inactivation, considered as a parameter, is varied; and (2) the stationary behaviors of the models are captured by the standard bifurcations of a two-dimensional system formed by the membrane potential and the potassium fast inactivation. We found that each type of firing and the transitions between them is due to the interplay between essentially three variables: two fast ones accounting for the action potential generation and the post-discharge refractoriness, and a third slow one representing the adaptation. Received: 28 February 2000 / Accepted in revised form: 4 October 2000     

Elementary movement detectors in an insect visual system

In the theoretical part of the present work the input-output relation for a multi-input system is developed into a functional power series. This is formally equivalent to a decomposition of the system into a sum of all possible combinations of 1-, 2-, 3-... input subsystems. The average response of the system to a uniformly moving patern is known to be a Fourier series with respect to spatial frequency. The coefficients of the series are linear combinations of the weights by which different subsystems contribute to the total reaction. If a system can be shown to have essential nonlinearities of no higher than second order it is possible to calculate, from a Fourier analysis of the average movement response, the weight by which the nonlinear interaction between any two input elements contributes to the total reaction. This interaction is termed elementary movement detector. By the analysis presented here the arrangement of the elementary movement detectors may be determined for a two-dimensional array of input elements and the strength of their contributions to the total movement reaction may be calculated. Special experimental methods have been developed which allow one to apply this analysis to the visual system of the fruitfly Drosophila. The preliminary data presented show that the direction sensitive optomotor response of Drosophila can be attributed predominantly to the contributions from two elementary movement detectors which interconnect neighbouring visual elements. The detectors are oriented in the hexagonal array of the compound eye at +30° and at-30° with respect to the horizontal line of symmetry. A weak contribution from a detector between neighbouring elements along the horizontal line of symmetry is suggested by the present data. In the course of the analysis the contrast transfer properties of the compound eye are characterized.     

Independent coding of movement duration in a repetitive non-visually guided movement

Temporal information is an embedded feature of our sensory and motor experiences. How is temporal information encoded in the brain? In the two-stage theory of timing, an explicit representation of timing is responsible for the movement initiation while movement duration is coded implicitly. We investigated the correlation of movement duration and amplitude in a repetitive one-dimensional non-visually guided movement to find out if temporal information could be coded independently from movement. Subjects were asked to learn the distance between two points by moving their hands repeatedly along the distance between two sticks, while they could not see their hands and hand path. After a training phase, a delay of either 2 or 20 s was imposed and the subjects were asked to reproduce the learned distance. There was no correlation between distance difference and time difference in either delay condition. In the 20 s delay experiment, in comparison to the 2 s delay experiment, there was a significant increase in distance reproduction error. However, there was no significant change in time differences in either of the experiments. In addition, the time difference between the training and test trials was independent from the direction of the distance difference (i.e., overshot, undershot, or accurate). In conclusion, time may be coded as an independent measure after the delay period, so it should be a kind of explicitly coded information.     

Independent coding of movement duration in a repetitive non-visually guided movement

The aim of the present study was to examine the association of high blood lactate levels, induced with a maximal cycling or with an intravenous infusion, with spinal cord excitability. The study was carried out on 17 male athletes; all the subjects performed a maximal cycling test on a mechanically braked cycloergometer, while 6 of them were submitted to the intravenous infusion of a lactate solution (3?mg/kg in 1?min). Before the exercise or the injection, also at the end as well as 5 and 10?min after the conclusion, venous blood lactate was measured and excitability of the spinal α-motoneurons was evaluated by using the H reflex technique. In both experimental conditions, it has been observed that an exhaustive exercise is associated with a strong increase of blood lactate (but not of blood glucose) and with a significant reduction of spinal excitability. Since a similar augment of blood lactate induced by an intravenous infusion, in subjects not performing any exercise, is not associated with significant changes of spinal excitability, it can be concluded that the increase of blood lactate levels during a maximal exercise is not per se capable of modifying the excitability of spinal α-motoneurons.     

Reinnervation of pulmonary stretch receptors

The Breuer-Hering reflex (BHR) reappears 12-14 wk after surgical lung denervation in beagle dogs (J. Appl. Physiol. 54: 1451-1456, 1983). To demonstrate that this is due to reinnervation of pulmonary stretch receptors, we recorded nerve activity from regenerated branches of the left vagus nerve in five beagle dogs. Ten days postdenervation the BHR was absent, whereas by 19 mo it was clearly present. Multifiber pulmonary afferent activity was observed in all five dogs with single-fiber activity observed in three. Sectioning the right vagus nerve did not alter the BHR, but sectioning all the regenerated branches of the left vagus abolished the reflex. In two additional dogs studied 17 mo postsurgery, recordings were made from few fiber nerve bundles of the left cervical vagus. Nerve activity was increased during gentle stroking of the surface of the left upper and lower lobes, indicating receptive fields in both lobes. These data demonstrate that reinnervation of pulmonary stretch receptors does occur and provides evidence that reinnervation of these receptors is responsible for return of the BHR after pulmonary denervation.     

Flow- and substratum-mediated movement by a stream insect

1. In streams subject to frequent hydrologic disturbance, the ability of benthic invertebrates to disperse within the channel is key to understanding the mechanisms of flow refugium use and population persistence. This study focuses on crawling invertebrates, the effects on movement of abiotic factors (specifically, flow near the stream bed and bed micro‐topography) and the consequences for dispersal. 2. In a large flume, we observed individual cased caddisflies, Potamophylax latipennis, moving in fully turbulent flows over a precise replica of a water‐worked surface. From maps of movement paths, we quantified crawling behaviour and entrainment, and the influence of bed micro‐topography. We manipulated discharge and tested its effect on movement, linear displacement and areal dispersal. The highest discharge treatment was a disturbance to the caddis; the lowest discharge was not. Crawling behaviours were used to parameterise random walk models and estimate population dispersal, and to test the effects of abiotic factors on movement. 3. Bed micro‐topography influenced crawling in several ways. Caddis spent most of their time at the junctions between proud particles and the adjacent plane bed. The frequency distribution of turn angles was bimodal, with modal values approximating the angle required to travel around median‐sized particles. Larvae generally crawled downstream, but crawling direction relative to the flow was skewed by bed micro‐topography and was not directly downstream, unlike drift. 4. Caddis crawled for most of the time and discharge affected almost every aspect of their movement. As discharge increased, caddis crawled less often, more slowly and over shorter distances; they also became entrained more frequently and over greater distances. With increased discharge, caddis spent proportionately less time at the junctions between proud particles and the adjacent plane bed, and more time on the tops and sides of proud clasts. This is curious as most entrainment occurred from the tops and sides of clasts and entrainment is generally considered to be disadvantageous during disturbances. 5. Linear displacement (drift and entrainment combined) was downstream, but the relation between total displacement and discharge was complex. Total displacement decreased at intermediate discharge as crawling decreased, but increased at high discharge as entrainment and drift played a greater role in movement. 6. Within‐stream dispersal via crawling contained elements of both a correlated random walk (we observed directional persistence in turn angles) and a biased random walk (we observed downstream bias in move direction angles) and was best described as a biased correlated random walk. Dispersal was inversely related to discharge, suggesting that the ability of P. latipennis to crawl into flow refugia on the streambed is reduced at high flow.     

Trichoplusia ni attacin A,a differentially displayed insect gene coding for an antibacterial protein

The mRNA differential display method was used to isolate antibacterial defense genes from Trichoplusia ni. The mRNA population in last-instar T. ni larvae injected with bacteria was compared to that of untreated larvae. Using a PCR amplified probe corresponding to an induced mRNA, we were able to clone an attacin homolog from a λ cDNA library from vaccinated larvae. The corresponding protein showed 63% identity to Hyalophora cecropia acidic attacin. The induction kinetics of T. ni attacin A gave optimal mRNA levels at 20 h post-infection. Genomic analysis showed this to be a single-copy gene with two introns.     

Enhanced contrast arising from reflex inhibition in an array of stretch receptors

The reflex inhibition of the abdominal stretch receptors of the crayfish has been described in a simple mathematical model. The model is based on quantitative expériments done on these receptors. The behaviour of this model shows an enhancement of the spatial contrast present in the stimulus pattern applied to the array of receptors. The model gives reasonable predictions about the behaviour of the receptors under conditions that can be repeated experimentally, and allows an easy exploration of the effects of changing the pattern of sensory stimulation and the values of various parameters of the system. A linear approximation to the present model has several features in common with the well-known Hartline-Ratliff model for lateral inhibition in the eye of Limulus.     

GABA receptors on the cell-body membrane of an identified insect motor neuron

The pharmacology of a gamma-aminobutyric acid (GABA) receptor on the cell body of an identified motor neuron of the cockroach (Periplaneta americana) was investigated by current-clamp and voltage-clamp methods. Iontophoretic application of GABA increased membrane conductance to chloride ions, and prolonged application resulted in desensitization. Hill coefficients, determined from dose-response data, indicated that binding of at least two GABA molecules was required to activate the chloride channel. Differences between vertebrate GABAA receptors and insect neuronal GABA receptors were detected. For the GABA receptor of motor neuron Df, the following rank order of potency was observed: isoguvacine greater than muscimol greater than or equal to GABA greater than 3-aminopropanesulphonic acid. The GABAB receptor agonist baclofen was inactive. Of the potent vertebrate GABA receptor antagonists (bicuculline, pitrazepin, RU5135 and picrotoxin), only picrotoxin (10(-7) M) produced a potent, reversible block of the response to GABA of motor neuron Df. Both picrotoxinin and picrotin also blocked GABA-induced currents. Bicuculline hydrochloride (10(-4) M) and bicuculline methiodide (10(-4) M) were both ineffective when applied at resting membrane potential (-65 mV), although at hyperpolarized levels partial block of GABA-induced current was sometimes observed. Pitrazepin (10(-4) M) caused a partial, voltage-independent block of GABA-induced current. The steroid derivative RU5135 was inactive at 10(-5) M. In contrast to the potent competitive blockade of vertebrate GABAA receptors by bicuculline, pitrazepin and RU5135, none of the weak antagonism caused by these drugs on the insect GABA receptor was competitive. Flunitrazepam (10(-6) M) potentiated GABA responses, providing evidence for a benzodiazepine site on an insect GABA-receptor-chloride-channel complex.     

The cerebellum and initiation of movement: the stretch reflex

Studies of the stretch reflex in decerebrate cats indicate a phase advance of peak sinusoidal tension in steady-state cycles between 0.1 and 10 Hz. This phase advance is reduced in acute and chronic cerebellectomy, as shown in previous investigations. Also, the augmentation of muscle peak tension in initial sinusoidal stretch cycles at 0.5-5 Hz has been found to be reduced during the time of reflex and motor instability in the several months following cerebellar ablation. This report shows the increased amplitude and phase lead of integrated electromyographic activity in initiating sinusoidal stretch cycles in the decerebrate cat. These reflex aspects are demonstrated in relation to the discharge of neurons in the dorsal spinocerebellar tract and of cerebellar cortical Purkinje cells in initial sinusoidal cycles. The intensity and phase advance of the discharge in dorsal spinocerebellar tract neurons is altered little, but these features are usually increased in Purkinje cells during initial stretches compared to continuous cycling. In terms of overall motor control, these findings are compatible with concepts of movement control, modulated by the cerebellum, in which the discharge of antagonist motor neurons is regulated in concert with that of agonist muscles upon initiation and termination of movement.     

Pharmacology of insect GABA receptors

A GABA-operated Cl^– channel that is bicuculline-insensitive is abundant in the nervous tissue of cockroach, in housefly head preparations and thorax/abdomen preparations, and in similar preparations from several insect species. Bicuculline-insensitive GABA-operated Cl^– channels, which are rare in vertebrates, possess sites of action of benzodiazepines, steroids and insecticides that are pharmacologically-distinct from corresponding sites on vertebrate GABA_A receptors. The pharmacological profile of the benzodiazepine-binding site linked to an insect CNS GABA-operated Cl^– channel resembles more closely that of vertebrate peripheral benzodiazepine-binding sites. Six pregnane steroids and certain polychlorocycloalkane insecticides, which are active att-butylbicy-clophosphorothionate (TBPS)-binding sites, also differ in their effectiveness on vertebrate and insect GABA receptors. Radioligand binding and physiological studies indicate that in insects there may be subtypes of the GABA receptor. Molecular biology offers experimental approaches to understanding the basis of this diversity.Special issue dedicated to Dr. Eugene Roberts     

Towards an understanding of the structural basis for insect olfaction by odorant receptors

Insects have co-opted a unique family of seven transmembrane proteins for odour sensing. Odorant receptors are believed to have evolved from gustatory receptors somewhere at the base of the Hexapoda and have expanded substantially to become the dominant class of odour recognition elements within the Insecta. These odorant receptors comprise an obligate co-receptor, Orco, and one of a family of highly divergent odorant “tuning” receptors. The two subunits are thought to come together at some as-yet unknown stoichiometry to form a functional complex that is capable of both ionotropic and metabotropic signalling. While there are still no 3D structures for these proteins, site-directed mutagenesis, resonance energy transfer, and structural modelling efforts, all mainly on Drosophila odorant receptors, are beginning to inform hypotheses of their structures and how such complexes function in odour detection. Some of the loops, especially the second extracellular loop that has been suggested to form a lid over the binding pocket, and the extracellular regions of some transmembrane helices, especially the third and to a less extent the sixth and seventh, have been implicated in ligand recognition in tuning receptors. The possible interaction between Orco and tuning receptor subunits through the final intracellular loop and the adjacent transmembrane helices is thought to be important for transducing ligand binding into receptor activation. Potential phosphorylation sites and a calmodulin binding site in the second intracellular loop of Orco are also thought to be involved in regulating channel gating. A number of new methods have recently been developed to express and purify insect odorant receptor subunits in recombinant expression systems. These approaches are enabling high throughput screening of receptors for agonists and antagonists in cell-based formats, as well as producing protein for the application of biophysical methods to resolve the 3D structure of the subunits and their complexes.     

Reliable coding of small, behaviourally relevant interaural intensity differences in a pair of interneurons of an insect

Insects, as vertebrates and humans, use interaural intensity differences (IIDs) between the two ears for sound localization. They are remarkably sensitive for small IIDs of the order of 1-2dB. Here, we investigated, using an independent ear stimulation paradigm, how such small IIDs are reliably encoded in the binaural discharge differences of a prominent pair of interneurons. Starting with an IID of 1dB, these differences are large and significant, with the louder side being more strongly excited. In a comparison of simultaneous responses of left and right interneurons, more than 70 and 90 per cent correct responses occur at IIDs of 1 and 2.5dB, respectively.     

Olfactory coding in the insect brain: molecular receptive ranges, spatial and temporal coding

Odor information is coded in the insect brain in a sequence of steps, ranging from the receptor cells, via the neural network in the antennal lobe, to higher order brain centers, among which the mushroom bodies and the lateral horn are the most prominent. Across all of these processing steps, coding logic is combinatorial, in the sense that information is represented as patterns of activity across a population of neurons, rather than in individual neurons. Because different neurons are located in different places, such a coding logic is often termed spatial, and can be visualized with optical imaging techniques. We employ in vivo calcium imaging in order to record odor‐evoked activity patterns in olfactory receptor neurons, different populations of local neurons in the antennal lobes, projection neurons linking antennal lobes to the mushroom bodies, and the intrinsic cells of the mushroom bodies themselves, the Kenyon cells. These studies confirm the combinatorial nature of coding at all of these stages. However, the transmission of odor‐evoked activity patterns from projection neuron dendrites via their axon terminals onto Kenyon cells is accompanied by a progressive sparsening of the population code. Activity patterns also show characteristic temporal properties. While a part of the temporal response properties reflect the physical sequence of odor filaments, another part is generated by local neuron networks. In honeybees, γ‐aminobutyric acid (GABA)‐ergic and histaminergic neurons both contribute inhibitory networks to the antennal lobe. Interestingly, temporal properties differ markedly in different brain areas. In particular, in the antennal lobe odor‐evoked activity develops over slow time courses, while responses in Kenyon cells are phasic and transient. The termination of an odor stimulus is reflected by a decrease in activity within most glomeruli of the antennal lobe and an off‐response in some glomeruli, while in the mushroom bodies about half of the odor‐activated Kenyon cells also exhibit off‐responses.     

The effect of periodic habitat fluctuations on a nonlinear insect population model

 Laboratory data show that populations of flour beetles (Tribolium), when grown in a periodically fluctuating volume of flour, can exhibit significant increases in numbers above those attained when grown in a constant volume (of the same average). To analyze and explain this phenomenon a discrete stage-structured model of Tribolium dynamics with periodic environmental forcing is introduced and studied. This model is an appropriately modified version of an experimentally validated model for flour beetle populations growing in a constant volume of flour, in which cannibalism rates are assumed inversely proportional to flour volume. This modeling assumption has been confirmed by laboratory experiments. Theorems implying the existence and stability of periodic solutions of the periodically forced model are proved. The time averages of periodic solutions of the forced model are compared with the equilibrium levels of the unforced model (with the same average flour volume). Parameter constraints are determined for which the average population numbers in the periodic environment are greater than (or less than) the equilibrium population numbers in the associated constant environment. Sample parameter estimates taken from the literature show that these constraints are fulfilled. These theoretical results provide an explanation for the experimentally observed increase in flour beetle numbers as a result of periodically fluctuating flour volumes. More generally, these integrated theoretical and experimental results provide the first convincing example illustrating the possibility of increased population numbers in a periodically fluctuating environment. Received 23 April 1996; received in revised form 28 March 1997     

昆虫嗅觉受体的研究进展

昆虫的嗅觉对昆虫的栖息地选择、觅食、群集、趋避、繁殖以及信息传递等行为具有重要的影响。对昆虫嗅觉机理的深入研究和嗅觉信号传导途径的完整阐述,是探索农业害虫的专一性防治的基础。嗅觉受体(olfactory receptors,Ors)是G蛋白偶联受体(G protein-coupled receptor)的一种,是嗅觉系统的关键成分。近年来嗅觉受体的研究日益受到关注。本文对昆虫嗅觉的基本过程、基因结构和表达调控特征、蛋白分子结构、生理功能、分布部位和相关配体的研究等进行了综述。