Biomedical subjects

E A Bernays

Publications and source records attributed to E A Bernays.

At least 19 recordsLinked to original sources

PubMed2005

Specific recognition, detoxification and metabolism of pyrrolizidine alkaloids by the polyphagous arctiid Estigmene acrea.

Evidence is presented that the polyphagous arctiid Estigmene acrea is well adapted to sequester and specifically handle pyrrolizidine alkaloids of almost all known structural types representative of the major plant families with pyrrolizidine alkaloid-containing species, i.e. Asteraceae with the tribes Senecioneae and Eupatorieae, Boraginaceae, Fabaceae, Apocynaceae and Orchidaceae. The adaptation of E. acrea to pyrrolizidine alkaloids includes a number of specialized characters: (i) highly sensitive recognition of alkaloid sources by pyrrolizidine alkaloid-specific taste receptors; (ii) detoxification of pyrrolizidine alkaloids by N-oxidation catalyzed by a specific flavin-dependent monooxygenase; (iii) transfer and maintenance of all types of pyrrolizidine N-oxides through all developmental stages; (iv) conversion of the various structures into the male courtship pheromone hydroxydanaidal most probably through retronecine and insect specific retronecine esters (creatonotines) as common intermediates; (v) specific integration into mating behavior and defense strategies. Toxic otonecine derivatives, e.g. the senecionine analogue senkirkine, which often accompany the common retronecine derivatives and which cannot be detoxified by N-oxidation do not affect the development of E. acrea larvae. Senkirkine is not sequestered at all. Non-toxic 1,2-saturated platynecine derivatives that frequently occur together with toxic retronecine esters are sequestered and metabolized to hydroxydanaidal, indicating the ability of E. acrea to aromatize saturated pyrrolizidines. Although pyrrolizidine alkaloids, even if they are offered continuously at a high level (2%) in the larval diet, are non-toxic, E. acrea larvae are not able to develop exclusively on a pyrrolizidine alkaloid-containing plant like Crotalaria. Therefore, E. acrea appears to be specifically adapted to exploit pyrrolizidine alkaloid-containing plants as "drug source" but not as a food source.

Animals↗

PubMed2005

Acquisition, transformation and maintenance of plant pyrrolizidine alkaloids by the polyphagous arctiid Grammia geneura.

The polyphagous arctiid Grammia geneura appears well adapted to utilize for its protection plant pyrrolizidine alkaloids of almost all known structural types. Plant-acquired alkaloids that are maintained through all life-stages include various classes of macrocyclic diesters (typically occurring in the Asteraceae tribe Senecioneae and Fabaceae), macrocyclic triesters (Apocynaceae) and open-chain esters of the lycopsamine type (Asteraceae tribe Eupatorieae, Boraginaceae and Apocynaceae). As in other arctiids, all sequestered and processed pyrrolizidine alkaloids are maintained as non-toxic N-oxides. The only type of pyrrolizidine alkaloids that is neither sequestered nor metabolized are the pro-toxic otonecine-derivatives, e.g. the senecionine analog senkirkine that cannot be detoxified by N-oxidation. In its sequestration behavior, G. geneura resembles the previously studied highly polyphagous Estigmene acrea. Both arctiids are adapted to exploit pyrrolizidine alkaloid-containing plants as "drug sources". However, unlike E. acrea, G. geneura is not known to synthesize the pyrrolizidine-derived male courtship pheromone, hydroxydanaidal, and differs distinctly in its metabolic processing of the plant-acquired alkaloids. Necine bases obtained from plant acquired pyrrolizidine alkaloids are re-esterified yielding two distinct classes of insect-specific ester alkaloids, the creatonotines, also present in E. acrea, and the callimorphines, missing in E. acrea. The creatonotines are preferentially found in pupae; in adults they are largely replaced by the callimorphines. Before eclosion the creatonotines are apparently converted into the callimorphines by trans-esterification. Open-chain ester alkaloids such as the platynecine ester sarracine and the orchid alkaloid phalaenopsine, that do not possess the unique necic acid moiety of the lycopsamine type, are sequestered by larvae but they need to be converted into the respective creatonotines and callimorphines by trans-esterification in order to be transferred to the adult stage. In the case of the orchid alkaloids, evidence is presented that during this processing the necine base (trachelanthamidine) is converted into its 7-(R)-hydroxy derivative (turneforcidine), indicating the ability of G. geneura to introduce a hydroxyl group at C-7 of a necine base. The creatonotines and callimorphines display a striking similarity to plant necine monoesters of the lycopsamine type to which G. geneura is well adapted. The possible function of insect-specific trans-esterification in the acquisition of necine bases derived from plant acquired alkaloids, especially from those that cannot be maintained through all life-stages, is discussed.

Animals↗

PubMed2004

Variability of gustatory responsiveness to phenylalanine in the generalist caterpillar, Grammia geneura (Lepidoptera, Arctiidae).

Electrophysiological recordings were made from maxillary taste sensilla of the caterpillar Grammia geneura, stimulated with the deterrent amino acid, phenylalanine. High levels of variability were noted and different potential causes examined. Little variation was detected with respect to standardized intra-individual tests, phenylalanine concentration, previous food type or time of day. Some changes were observed in relation to age, but could not account for the overall variation found. Major differences in responsiveness depended on cohort and the possibility of important genetic differences is discussed.

Animals↗

PubMed2004

Acquired and partially de novo synthesized pyrrolizidine alkaloids in two polyphagous arctiids and the alkaloid profiles of their larval food-plants.

The profiles of pyrrolizidine alkaloids (PAs) in the two highly polyphagous arctiids Estigmene acrea and Grammia geneura and their potential PA sources in southeastern Arizona were compiled. One of four species of Boraginaceae, Plagiobothrys arizonicus, contained PAs; this is the first PA record for this plant species. The principle PA sources are Senecio longilobus (Asteraceae) and Crotalaria pumila (Fabaceae). The known PA pattern of S. longilobus was extended; the species was found to contain six closely related PAs of the senecionine type. Three novel PAs of the monocrotaline type, named pumilines A-C, were isolated and characterized from C. pumila, a species not studied before. The pumilines are the major PAs in the seeds, while in the vegetative organs they are accompanied by the simple necine derivatives supinidine and as the dominant compound subulacine (1beta,2beta-epoxytrachelanthamidine). In both plant species, the PAs are stored as N-oxides, except C. pumila seeds, which accumulate the free bases. Great variation in PA composition was observed between local populations of C. pumila. The PA profiles were established for larvae and adults of E. acrea that as larvae had fed on an artificial diet supplemented with crotalaria-powder and of G. geneura fed with S. longilobus. In both experiments, the larvae had a free choice between the respective PA source and diet or food plants free of PAs. The profiles compiled for the two species reflect the alkaloid profiles of their PA sources with one exception, subulacine could never be detected in E. acrea. Besides acquired PAs, insect PAs synthesized from acquired necine bases and necic acids of insect origin were detected in the two arctiid species. These insect PAs that do not occur in the larval food sources accounted for some 40-70% (E. acrea) and 17-37% (G. geneura) of total PAs extracted from the insects. A number of novel insect PAs were identified. Plant-acquired and insect PAs were found to accumulate as N-oxides. The results are discussed in relation to specific biochemical, electrophysiological, and behavioral mechanisms involved in PA sequestration by arctiids.

Animals↗

PubMed2003

Changes in taste receptor cell sensitivity in a polyphagous caterpillar reflect carbohydrate but not protein imbalance.

Caterpillars of the polyphagous arctiid, Grammia geneura, have a single cell in the medial galeal sensillum that responds to some sugars and to some amino acids. After conditioning on artificial diet containing unbalanced amounts of carbohydrate and protein, the responses of this cell alter. After protein-biased food it increases slightly, but after carbohydrate-biased food it decreases. Responses to both sucrose and amino acids change in the same direction and the changes would not provide the information necessary to redress a shortage of protein. The lateral galeal sensillum contains one cell that responds to fructose and another responding to some amino acids. The responses of each of these cells in the lateral sensillum are not consistently affected by conditioning diets. After conditioning for 20 h on a protein- or carbohydrate-biased diet, the insects started to feed without delay if offered carbohydrate-biased diet, but only after a pause if given protein-biased diet. This occurred irrespective of the conditioning diet. The duration of the first feeding bout was also longer on carbohydrate-biased diet and the longest bouts followed protein-biased conditioning.

Action Potentials↗

PubMed2003

Experience influences gustatory responsiveness to pyrrolizidine alkaloids in the polyphagous caterpillar, Estigmene acrea.

Electrophysiological recordings from taste sensilla of the caterpillar Estigmene acrea with the pyrrolizidine alkaloid, seneciphylline N-oxide, demonstrated that prior feeding on plants with pyrrolizidine alkaloids caused an increase in responsiveness of the PA-sensitive cells in two sensilla, relative to feeding on plants without such chemicals. Rearing on synthetic diet without pyrrolizidine alkaloids for up to seven generations caused a continuous decline in responsiveness, that could be reversed by experience with powdered Crotalaria pumila in the diet or by pure pyrrolizidine alkaloid, monocrotaline, in the diet. Response to the cardiac glycoside, ouabain, that stimulates one of the two pyrrolizidine alkaloid-sensitive cells, showed a similar decline. Pyrrolizidine alkaloids had no measurable effect on growth and development. Responses in all other taste cells were unaffected. The data are discussed in relation to the possible adaptive significance and the possible mechanisms involved.

Animals↗

PubMed2003

Taste receptors for pyrrolizidine alkaloids in a monophagous caterpillar.

Caterpillars of Utetheisa ornatrix are monophagous on species of Crotalaria which they obtain pyrrolizidine alkaloids (PAs) for defense and which the males convert to a pheromone. We show that a taste receptor neuron in each of the lateral and medial galeal styloconic sensilla responds to PAs of three different types. Monocrotaline, commonly present in Crotalaria species, is the most strongly stimulating with thresholds of response below 10(-11) and 10(-9) M in the two sensilla. These are among the lowest known taste thresholds in any insect and are similar to the thresholds for PAs in a polyphagous arctiid caterpillar that also sequesters PAs and uses them as the source of male pheromone. The receptors also respond to heliotrine, a type of PA that is probably never encountered by the insects. Monocrotaline and senecionine N-oxide are shown in behavioral assays to be phagostimulants. The data show that there is no tight link between taste receptor sensitivity to specific PAs and hostplant selection in these caterpillars. Perhaps the adults are primarily responsible for hostplant selection.

Animals↗

PubMed2003

Are insect-synthesized retronecine esters (creatonotines) the precursors of the male courtship pheromone in the arctiid moth Estigmene acrea?

The pyrrolizidine alkaloid (PA) profiles were determined for adults of the polyphagous arctiid Estigmene acrea, which as larvae had fed on artificial diet supplemented with Crotalaria-pumila powder with known concentrations of PAs. The larvae always had a free choice between alkaloid-containing and plain diets. The alkaloid profiles of adults revealed a striking sexual dimorphism. Both sexes contained macrocyclic PAs of the monocrotaline type sequestered from the diet and, in addition, a substantial proportion of supinidine and retronecine esters synthesized by the insects from necine bases derived from the dietary alkaloids and necic acids of insect origin. These insect alkaloids accounted for 35% and 55% of total PAs in males and females, respectively. The difference was that in females the retronecine esters (creatonotines) made up 58 microg (43% of total PAs), while males contained a fivefold lower proportion, 12 microg (13%). Four of the ten male individuals analyzed were found devoid of creatonotines. Based on the experimental data in combination with evidence from the literature, it is suggested that the creatonotines are direct pheromone precursors in E. acrea. It is hypothesized that this may represent a general mechanism of hydroxydanaidal formation from diverse macrocyclic PAs in arctiids.

Animals↗

PubMed2003

Loss of gustatory responses to pyrrolizidine alkaloids after their extensive ingestion in the polyphagous caterpillar Estigmene acrea.

Electrophysiological recordings from taste sensilla of the caterpillar Estigmene acrea with the pyrrolizidine alkaloid (PA) seneciphylline N-oxide demonstrated that extensive feeding on plants rich in PAs caused a loss in response of the PA-sensitive cell in the lateral styloconic sensillum on the galea. The effect could be repeated using pure PAs fed to the insect in synthetic diets and by injection of PA into the hemolymph. The sensitivity loss lasted for approximately two hours and was less pronounced in individuals that had been reared on PA-containing food. Behavioral experiments and field observations demonstrate a parallel reduction in responsiveness to PAs and to PA-containing plants.

Animals↗

PubMed2002

A highly sensitive taste receptor cell for pyrrolizidine alkaloids in the lateral galeal sensillum of a polyphagous caterpillar, Estigmene acraea.

Adult males of Estigmene acraea use pyrrolizidine alkaloids to produce pheromones and all stages probably use pyrrolizidine alkaloids for defense. The alkaloids are obtained from plants by the caterpillars. We demonstrate that a contact chemoreceptor neuron in the lateral galeal sensillum exhibits a dose-dependent response to seneciphylline N-oxide, a widely occurring pyrrolizidine alkaloid, down to concentrations of 10(-9) x mol l(-1), and even at 10(-12) x mol l(-1) the response is greater than to salt alone. At concentrations of 10(-6) mol x l(-1) and above the instantaneous firing rate is very high, and at 10(-4) mol x l(-1) initially exceeds 500 spikes s(-1). The firing rate declines in the 200 ms following stimulus onset but then is sustained with an instantaneous firing rate in excess of 100 spikes s(-1) for at least the next 800 ms. At lower concentrations a delay occurs before firing is initiated, and then the pattern of firing is irregular. The cell is equally sensitive to some but not all of several other pyrrolizidine alkaloids tested as free bases and their N-oxides. It also responds to ouabain, which may also serve as a defensive compound, and to asparagine and fructose but with much higher thresholds than to the pyrrolizidine alkaloids.

Action Potentials↗

PubMed2001

Electrophysiological responses of taste cells to nutrient mixtures in the polyphagous caterpillar of Grammia geneura.

In a normally feeding insect, the taste receptors are exposed to complex mixtures of chemicals, not single compounds. We investigate the responses of neurons in the galeal sensilla of the caterpillar of Grammia geneura to mixtures of nutrient compounds at concentrations occurring in plants. Compounds that stimulated the same neuron were generally additive in their effects in binary mixtures. Amino acids that did not stimulate usually had no effect in mixtures with a stimulating compound, but glutamic acid reduced the response to serine in the medial sensillum. Nutrient compounds that stimulated different cells in a sensillum acted independently of each other. Complex mixtures of amino acids resembling samples of free amino acids from three host plants were less stimulating than expected from their molar concentrations. In host plant selection, the response from the medial sensillum is probably dominated by sucrose; unless sucrose levels are low, amino acids will contribute little to sensory input because they stimulate the same cell as sucrose. In the lateral sensillum, amino acids act independently of sugars. The limited contact chemosensory array of caterpillars seems inadequate to allow them to make fine distinctions between plants on the basis of their free amino acids.

Amino Acids↗

PubMed2001

Neural limitations in phytophagous insects: implications for diet breadth and evolution of host affiliation.

This review points out the problem of processing multiple sensory inputs and provides evidence that generalists suffer a disadvantage compared with specialists with respect to efficiency of host plant choice and discrimination. The specialists' mechanisms for improved efficiency are discussed as well as some of the processes that may be selected to increase processing efficiency in generalists. The fitness consequences of differences in efficiency of specialists and generalists are pointed out. One of the major disadvantages for generalists is the increase in vulnerability to ecological risks, especially risks imposed by various natural enemies. Efficiency-related factors are indicated as previously underestimated elements that could influence host affiliations including diet breadth and changes in host plant use.

Adaptation, Physiological↗

PubMed2000

Learning improves growth rate in grasshoppers.

To quantify the adaptive significance of insect learning, we documented the behavior and growth rate of grasshoppers (Schistocerca americana) in an environment containing two artificial food types, one providing a balanced diet of protein and carbohydrate, which maximizes growth, and the other being carbohydrate-deficient, which is unsuitable for growth. Grasshoppers in the Learning treatment experienced a predictable environment, where the spatial location, taste, and color of each food source remained constant throughout the experiment. In contrast, grasshoppers of the Random treatment developed in a temporally varying environment, where the spatial location, taste, and color of the balanced and deficient food types randomly alternated twice each day. Our results show that the grasshoppers that could employ associative learning for diet choice experienced higher growth rates than individuals of the Random treatment, demonstrating the adaptive significance of learning in a small short-lived insect.

Animals↗

PubMed2000

Intraspecific differences in olfactory sensilla in relation to diet breadth in Uroleucon ambrosiae (Homoptera: Aphididae).

The olfactory plate organs found on the proximal segments of aphid antennae, the secondary rhinaria, were studied in apterae of the aphid Uroleucon ambrosiae. The numbers and shapes of these structures are important taxonomic characters in Aphididae, but they also show intraspecific variation within a morph. In this study, numbers were found to vary with aphid size but, independent of size, were greater in aphids collected in the eastern than in the southwestern United States. In addition, the sizes of rhinaria were greater in eastern than in southwestern aphids. These differences correlate with previously recorded differences in diet breadth and the efficiency of finding and settling on food: eastern aphids specialize on the host Ambrosia trifida, on which they settle relatively efficiently, while southwestern aphids are more generalized and less efficient. Greenhouse experiments demonstrated that differences in rhinaria are partly determined genetically. Southwestern aphids showed bilateral asymmetry in length of antennal segments though not in numbers of rhinaria, but only on certain adopted hosts that were apparently inferior for growth. The significance of these intraspecific differences is discussed in relation to the physiology, ecology, and evolution of host range.

Animal Structures↗

PubMed2000

Sensitivity to chemically diverse phagostimulants in a single gustatory neuron of a polyphagous caterpillar.

Caterpillars of the arctiid moth, Grammia geneura, are polyphagous, but species of Plantago are amongst their preferred food plants. A neuron in the medial styloconic sensillum on the galea has been shown to have a general phagostimulatory function. Experiments with binary mixtures and cross-adaptation have demonstrated that it responds to some sugars, to several amino acids, and also to catalpol. Catalpol is a plant secondary compound in Plantago and a phagostimulant for the caterpillars. The possible significance of combining sensitivity to nutrient compounds with sensitivity to a secondary compound is discussed.

Action Potentials↗

PubMed2000

How do inositol and glucose modulate feeding in Manduca sexta caterpillars?

Many species of caterpillar possess taste cells that respond exceptionally vigorously to the sugar alcohol myo-inositol. We examined the functional significance of these inositol-sensitive taste cells in Manduca sexta caterpillars through an integrated series of electrophysiological and behavioral studies. Neural recordings from all the gustatory chemosensilla revealed that M. sexta have only two pairs of inositol-sensitive taste cells, which respond strongly and selectively to myo-inositol, and two pairs of sugar-sensitive taste cells, which respond relatively weakly to sugars (glucose and sucrose). Behavioral studies established that myo-inositol incites feeding and counteracts the inhibitory effects of aversive taste stimuli (e.g. caffeine) on feeding, but does not promote increased consumption once feeding has been initiated. In contrast, glucose and sucrose did not produce any robust effects on feeding. We failed to obtain any evidence of sensory inhibition between taste cells that responded to myo-inositol and caffeine, indicating that myo-inositol counteracts the inhibitory effects of caffeine on feeding through a central gustatory mechanism. We conclude that sensory input from the inositol-sensitive taste cells, but not the sugar-sensitive taste cells, plays an important role in regulating feeding in M. sexta.

Animals↗

PubMed1999

Post-ingestive feedbacks and associative learning regulate the intake of unsuitable sterols in a generalist grasshopper.

Behavioural studies of the grasshopper Schistocerca americana were undertaken to identify the mechanisms that regulate the intake of dietary sterols. In the first experiment, grasshoppers were allowed to feed on spinach, a plant containing only unsuitable sterols; immediately after this first meal, a suitable or unsuitable sterol was injected into the haemolymph. Grasshoppers injected with unsuitable sterols had second meals on spinach that were significantly shorter than those of grasshoppers injected with suitable sterols, indicating that unsuitable dietary sterols are detected post-ingestively. In the second experiment, grasshoppers were fed food containing only unsuitable sterols and were then presented with glass-fibre discs containing different concentrations of a suitable sterol or sucrose only (the control). The results suggest that grasshoppers do not use a direct feedback operating on mouthpart chemoreceptors to regulate their intake of suitable sterols. In the third experiment, grasshoppers were presented with artificial diets containing different sterols and flavours, and feeding was observed over a sequence of meals. The results from both the first and last experiments suggest a role for associative learning in regulating the intake of unsuitable sterols.

Animals↗

PubMed1983

The regulation of feeding: locusts and blowflies are not so different from mammals.

The regulation of feeding in two insect groups, the locusts and the blowflies, is reviewed. The patterning of feeding is discussed first, and then the underlying mechanisms are considered in detail. The concept of "central excitability" is the key to understanding the controls of feeding. It represents a central nervous mechanism for tuning the sensitivity of the insect to food-related stimuli. The level of central excitation is influenced positively by food and also non-food stimuli, both from within and outside the animal, and negatively by deterrent stimuli and by feedbacks from peripheral systems such as stretch receptors on the gut wall, hormones and blood composition. Sustained gustatory input is required if feeding is to continue, once initiated. The level of central excitation present as a meal begins influences ingestion rate and the amounts of negative feedback tolerated before feeding stops. The duration of intervals between meals is then a function of a number of excitatory stimuli and of feeding-induced changes which reduce the likelihood of further feeding, e.g., hormone release, changes in nutrient composition or osmotic pressure of the blood, and gut distension. Other areas reviewed include changes in the regulation of feeding during the life history and the control of drinking.

Animals↗