Clonidine effects on protein and carbohydrate electrophysiological responses of labellar and tarsal sensilla in Phormia regina.

The electrophysiological response of labellar and tarsal chemosensilla in the blowfly Phormia regina was studied in response to a complex stimulus naturally encountered by flies such as sheep faeces, and to beef liver, a proteinaceous feeding source. Responses were investigated both before or after injection of clonidine, an octopamine agonist previously shown to enhance sucrose ingestion, while decreasing that of proteins. As assessed by single sensillum recordings, the four different chemosensory - "salt", "sugar", "deterrent" and "water" - cells were all activated by both stimuli, regardless of sex and sensillum type, the "sugar" one being in all cases the most sensitive to beef liver before clonidine injection. Clonidine treatment affected neither labellar nor tarsal sensitivity to sucrose. Conversely, clonidine-injected flies showed a significant increase in the activity of the "deterrent" cell to beef liver, thus accounting for a decrease in protein ingestion. This study for the first time provides evidence of a key role of a clonidine-sensitive peripheral taste sensitivity in down-regulation of protein ingestion in blowflies. Correlation between peripheral sensitivity and behavioural output is discussed.

Effects of avermectins on olfactory responses of Culicoides imicola (Diptera: Ceratopogonidae).

The aim of this study was to examine the role of the olfactory system of the midge Culicoides imicola Kieffer as the major system mediating repellency to antihelminthic avermectins. Incidental observations indicate that treatment with Dectomax or Ivomec (commercial formula of the avermectins doramectin and ivermectin, respectively) protects sheep from infection by bluetongue (BT) viruses. Our electrophysiological data from midge antennae showed that the stimulating effectiveness of L- (+)-lactic acid, butanone, and sheep fleece odor decreased after addition of avermectins. The results show that these antihelminthics affect the olfactory sensitivity of the insect toward the animal host by reducing the response to those compounds that attract the insect, consequently reducing the possibility of biting the sheep and thereby transferring the BT virus.

A morphological and 13C NMR study of the extramandibular fat bodies of the striped dolphin (Stenella coeruleoalba).

The molecular and histological structure of the fat bodies covering externally the posterolateral region of the jaw of the striped dolphin (Stenella coeruleoalba) was investigated by means of morphological and nuclear magnetic resonance techniques. The analyses of samples belonging to adult and juvenile individuals were performed with the aim of seeking the presence of age-related differences. In our study, the level of isovalerate (iso5:0) in the extramandibular fat of the juvenile individuals is comparable with those of the adult counterparts; conversely, longer isobranched fatty acids were detected in lower quantities in the juveniles together with a higher degree of unsaturation. The morphologic analyses revealed that, in both adults and juveniles, this fatty tissue is similar to univacuolar adipose tissue. However, in the juveniles, a muscular component was present, whereas only in adult subjects, enlarged and irregularly shaped cavities may be seen within the adipose tissue. These cavities, structurally organized as veins, may regulate blood flow in response to changing water temperature and stabilize thermal gradient within the jaw lipids. These data suggest that the molecular components and the histological organization can indicate a maturation of the organ with age that probably may reflect different sound reception properties.

A histological and NMR study of the melon of the striped dolphin (Stenella coeruleoalba).

The melon, the echolocation organ of the striped dolphin (Stenella coeruleoalba), was investigated by morphological and high-resolution (13)C nuclear magnetic resonance (NMR) methods, in order to characterize structure and composition gradient at the histological and molecular level. Morphological analysis showed that the lipidic components are organized in an adipose tissue; moreover, a clear muscular component was observed. Age-related structural differences also were noted. Furthermore, NMR yielded detailed information at a qualitative-quantitative level on the lipid components.

Saccharin stimulates the "deterrent" cell in the blowfly: behavioral and electrophysiological evidence.

In the attempt to gain more information on the mechanisms underlying bitter and/or sweet taste reception, we have investigated the responses of labellar chemosensilla in the blowfly Protophormia terraenovae to Na-saccharin, as compared to sweet stimuli (sucrose or fructose) and bitter stimuli (denatonium benzoate or amiloride). Electrophysiological and behavioral results indicate that the sweetener Na-saccharin inhibits the "sugar" cell in the labellar taste sensilla of the blowfly P. terraenovae. In multichoice preference tests, flies ingested more of the solutions containing sugar to those with sugar+Na-saccharin. This finding is in good agreement with the spike frequency reduction observed for the "sugar" cell activity. Analysis of the spike discharges also shows a positive dose-response for the "deterrent" cell following stimulation with Na-saccharin and denatonium benzoate. Flies drank any of the Na-saccharin solutions, regardless of their concentration, less than water, thus indicating a weak deterring effect on water drinking. The prevailing activation of the "deterrent" cell by stimulation with Na-saccharin is not directly coupled with a coherent behavioral output. Cross adaptation was found to occur between responses to Na-saccharin and denatonium benzoate or amiloride regardless of the order of adapting stimuli. In the case of sweet stimuli, cross adaptation occurred when the adapting stimulus was Na-saccharin, but it did not when the adapting stimuli were sucrose or fructose. Addition of Na-saccharin to both sugars significantly depressed the spike firing frequency, while an increase was observed with denatonium benzoate or amiloride.

Sugar reception in the blowfly: a possible Ca(++) involvement.

The present study investigates the effects of W-7 (a calmodulin antagonist involved in the Ca(++) cascade) on the response of the 'sugar' and 'water' cells of labellar chemosensilla in the blowfly Protophormia terraenovae to stimulation with sucrose or fructose. In order to ascertain whether Ca(++) conductance is involved, the effects of EGTA, one of the most used Ca(++) chelating agent, and of SK&F-96365, an inhibitor of receptor mediated calcium influx, were also studied. Our electrophysiological data indicate that W-7 addition strongly depresses the 'sugar' chemoreceptor response to both sugars and in the case of sucrose stimulation also influences adaptation rate. The Ca(++) chelator has no significant effects on the response of the 'sugar' cell following stimulation with sucrose, but lowers fructose stimulating effectiveness. In the presence of SK&F-96365 both sucrose and fructose responses are inhibited. A possible transduction mechanism for sugar reception is discussed.

Factors modulating the blood feeding behavior and the electrophysiological responses of labral apical chemoreceptors to adenine nucleotides in the mosquito Aedes aegypti (Culicidae).

The feeding of Aedes aegypti (L.) on blood is induced by the presence of phagostimulants: adenine nucleotides. Three chemoreceptive cells in the labral apical sensilla can distinguish the presence of adenine nucleotides depending on the other stimulus components. This work aims at correlating the sensory information arising from the labral apical sensilla with the feeding behavior in response to the same stimuli. The saline stimulating solution, containing adenine nucleotides, is modulated by changing one of the following components: salt concentration, buffer or pH. Cell 3 that responds to NaCl in a dose dependent manner seems to have another unique modality. The response of this cell is unaffected by ATP when the stimulating solution is NaCl buffered by NaHCO(3). It responds at a higher spike frequency to the presence of ATP in a NaCl solution without NaHCO(3). Thus in the presence of ATP Cell 3 detects whether the NaCl solution is buffered by NaHCO(3). Both the blood feeding response and the sensory information from Cell 2 (which responds at high spike frequencies to the presence of ATP) are modulated by pH in a similar way. Both responses present a bi-modal response, with a major peak at pH 4.0 and a moderate peak at the most alkaline pH value tested.

Sensitivity of the mosquito Aedes aegypti (Culicidae) labral apical chemoreceptors to blood plasma components.

The phagostimulants from the cellular fraction of blood induce gorging of Aedes aegypti (L.), and this process is enhanced by some plasma components. This project examines the responses of the labral apical chemoreceptors to plasma components enhancing phagostimulation. From the electrophysiological responses of the labral apical chemoreceptors four cells were identified by the waveform of their action potentials. Three of the cells (Cell 2, Cell 3 and Cell 4) responded in a dose dependent manner to NaCl. The responses of Cell 2 and Cell 3 to NaCl concentrations from 1 to 500 mmol/l can be described by a logarithmic equation. The response of Cell 2 to 150 mmol/l NaCl is modulated when a buffer is added. The magnitude of the modulation of the response is determined by the nature of the buffer: NaHCO(3) inhibits while Na(2)HPO(4) enhances the response. High osmotic pressure inhibits the response of Cell 4, regardless of how it is achieved. Cell 4 responds with a high frequency to the presence of L-alanine, the C-terminal amino acid of albumin, but shows a reduced response to the same concentration of albumin. From these results it can be concluded that labral apical chemoreceptors of A. aegypti are capable of detecting the plasma components involved in blood recognition.

Sensitivity of the mosquito Aedes aegypti (Culicidae) labral apical chemoreceptors to phagostimulants.

The feeding of Aedes aegypti (L.) on blood and nectar is induced by phagostimulants: adenine nucleotides and sugars respectively. This work examines the responses of the four chemoreceptor cells in the labral apical sensilla to these phagostimulants. The apical chemoreceptors can detect the presence of adenine nucleotides. This part of the response is in good agreement with the gorging behavior. The output of the chemoreceptors cannot distinguish among different adenine nucleotides or among their concentrations (0.01-1 mmol/l), whereas gorging behavior is affected by the identity of adenine nucleotides and by their concentrations. Hence the gorging behavior cannot be driven by the output of these chemoreceptors alone. To the presence of adenine nucleotides Cell 2 was the only cell that responded with high frequencies, while the response of Cell 4 was almost abolished. The response of Cell 2 to ATP depended on the mosquito's physiological state. This dependence accorded well with the gorging behavior; Cell 2 responded with a higher frequency to ATP in the gorging state, than when not in a gorging state. The responses to sucrose and fructose constituted the only case recorded in which all these chemoreceptors failed to respond. This depression of response implies that other chemoreceptors must be present as sugar detectors.

Sugar response differences related to sensillum type and location on the labella of Protophormia terraenovae: a contribution to spatial representation of the stimulus.

The chemoreceptor spike activity in response to sucrose in the concentration range 1-500mM was recorded from each of the 11 Intermediate and 13 Large labellar sensilla in the blowfly Protophormia terraenovae. The results showed that: (1) three of the four cells present in each sensillum are activated by sucrose stimulation; (2) differences between the Large and Intermediate types exist in the dose-response profiles of one of these cells (the 'water' cell), possibly reflecting different sugar receptor site populations on the dendritic membranes of homologous cells in the two types; (3) sensilla of both types are differentially responsive to sucrose solutions according to their location on the labellum. These differences may provide elements for a spatial representation of the stimulus source within the sensory coding process.

Taste reception mechanisms in the blowfly: evidence of amiloride-sensitive and insensitive receptor sites.

The role of amiloride in the labellar responses to various taste stimuli in the blowfly Protophormia terraenovae was studied with the aim of ascertaining whether amiloride-sensitive cation conductances are present in the chemosensory systems of insects. Results include that: 1) amiloride has no effect on the "salt" cell response to any stimulus; 2) amiloride decreases the "sugar" cell response to fructose, but does not affect that to sucrose; 3) the effects of amiloride on the responses of the "water" cell and the "fifth" cell are less clearly definable, due to the probable superimposition of osmotic mechanisms in the former and the poorly known response modalities of the water. In conclusion, amiloride-sensitive receptor sites seem to exist also in insects. However, unlike most vertebrates investigated, they are principally located in the sugar receptor cell, not on the salt cell.

Electrophysiological recordings from the taste chemosensilla of Tabanus nigrovittatus (Diptera: Tabanidae) and determination of behavioral tarsal acceptance thresholds.

A modified behavioral test for determining the median tarsal acceptance thresholds of field-collected, female Tabanus nigrovittatus Macquart to sugars showed that flies were sensitive to the three major nectar sugars (fructose, sucrose, and glucose, in that order of decreasing sensitivity) whereas galactose, a nonnectar sugar, was not comparably stimulating. Comparisons of various treatment effects on the median tarsal acceptance thresholds showed that for future studies, flies should be fed 10% sucrose before testing, should be starved for 24 h before testing, and they do not have to be pretested for a water response. Electrophysiological recordings showed that gustatory chemosensilla were located on the labellum, tip of labrum, labral food canal, foretarsi, tibia, and antennae. All chemosensilla responded to sugar and salt, whereas only those chemosensilla at the tip of the labrum and those lining the food canal, which normally contact the blood meal, responded to sera and plasma.

Effects of octopaminergic substances on the labellar lobe spreading response in the blowfly Protophormia terraenovae.

The effectiveness of labellar taste input in evoking the feeding response of labellar lobe spreading in Protophormia was greatly increased by injecting flies with clonidine or DL-octopamine. This effect was suppressed when either drug was administered together with yohimbine; when injected alone, the latter determined a strong decrease in the feeding responses. Drug treatment did not determine variations in labellar taste input or evoke spontaneous activity in the denervated muscle. These results suggest that the injected substances lower the feeding-related reflex threshold at the CNS level.

Reflex cardiac response to various olfactory stimuli in the blowfly, Protophormia terraenovae.

In the present investigation it is shown that a reflex change in the heart activity of Protophormia flies is evoked by olfactory stimulation with several volatile substances, and particularly with those which are repellent for blowflies. Among these, i-pentanal and hexanal vapours evoked a fast, highly persistent cardiac response, whereas in the case of hexanol and ammonia vapours the response resulted slower and could be suppressed as a function of repeated stimulation.

Chronic electrode for long-term electrophysiological recording from insect chemosensilla.

Insertion of a chronic, non-damaging Pt electrode for electrophysiological recording from chemosensilla in Phormia is described. The experimental procedure did not modify in a significant way the survival capabilities or the general behaviour of the operated insects as compared to control, untreated ones. This method, which allows the performance of subsequent tests on the same specimen up to its full lifespan with fairly constant results, proved to be suitable in long-term experiments on the chemosensory function of insects.

Mechanism of JH influence on the function of labellar chemosensilla in Phormia: experimental suggestions.

The aim of this research study was to determine at which level the JH effect of increasing chemosensillar sensitivity in Phormia could take place. Two-day-old adult female Phormia regina (Meigen) flies were used. Variations in electrical resistance and spike firing frequency of labellar chemosensilla stimulated with NaCl solutions were measured before and after topical application of a JH analogue (JHA). Variations in the above two parameters were also determined in a group of control flies. Results showed that an inverse linear relationship exists between electrical resistance and spike firing frequency in both JHA-treated and control flies. On this basis, we conclude that the above sensitivity variations depend on a mechanism common to both groups of flies. As regards the mode of action of this mechanism, we suggest that JH may influence the mucopolysaccharide secretory function of the accessory cells, the activity of which seems to account for changes in electrical resistance and therefore in sensitivity of the labellar chemosensilla.

Ovipositor chemosensilla in Tabanus nigrovittatus (Macq.) Chrysops fuliginosus (Wied.), and Rhagoletis pomonella (Walsh).

The possible presence of chemosensilla in the ovipositor region has been investigated in three different pest insect species by means of the standard tip-recording electrophysiological technique. Results showed that hair-like structures responding to chemical stimulation were located at the lower-inner edge of the anal leaflets in Tabanus nigro Macq. as well as in Chrysops fuliginosus Wied, whereas peg-like chemosensilla proved to be located in two longitudinal grooves at the apical portion of the ovipositor in Rhagoletis pomonella Walsh.. At least 3 chemosensory units could be detected in each chemosensillum tested. No hair-or peg-like structure on the ovipositor region other than that we described showed any chemosensory responsiveness.

Electrophysiological activity of labellar and tarsal chemosensilla of Hybomitra lasiophtalma (Macquart) to stimulation with NaCl and various sugars.

The responses from labellar and tarsal chemosensilla of Hybomitra lasiophtalma to stimulation with NaCl or sugars (sucrose, glucose, fructose, galactose) were recorded. Results showed that 4 chemosensory units were located in each chemosensillum. Among these, a water-, a salt-, and a sugar-sensitive unit were detectable. NaCl (higher than 50 mM) as well as sucrose (higher than 100 mM) sensitivity thresholds were determined; sucrose and galactose proved to be more active as stimuli than fructose and glucose.