Giorgia Sollai

Associations between Orosensory Perception of Oleic Acid, the Common Single Nucleotide Polymorphisms (rs1761667 and rs1527483) in the CD36 Gene, and 6-n-Propylthiouracil (PROP) Tasting

Orosensory perception of dietary fat varies in individuals, thus influencing nutritional status. Several studies associated fat detection and preference with CD36 or 6-n-propylthiouracil (PROP) sensitivity. Other studies have not confirmed the latter association. We analyzed the relationship between orosensory perception of oleic acid, two CD36 variants, and PROP tasting. Thresholds of oleic acid perception were assessed in 64 subjects using a modification of the three-alternative forced-choice procedure. Subjects were classified for PROP taster status and genotyped for TAS2R38 and CD36 (SNPs: rs1761667 and rs1527483). Subjects homozygous for GG of the rs1761667 polymorphism showed higher sensitivity to oleic acid than AA subjects. The capability to detect oleic acid was directly associated with TAS2R38 or PROP responsiveness. PROP non-tasters had a lower papilla density than tasters, and those with genotype GG of the rs1761667 polymorphism had lower oleic acid thresholds than PROP non-tasters with genotype AA. In conclusion, results showed a direct association between orosensory perception of oleic acid and PROP tasting or rs1761667 polymorphism of CD36, which play a significant role in PROP non-tasters, given their low number of taste papillae. Characterization of individual capability to detect fatty acids may have important nutritional implications by explaining variations in human fat preferences.

Gustatory Sensitivity and Food Acceptance in Two Phylogenetically Closely Related Papilionid Species: Papilio hospiton and Papilio machaon

In herbivorous insects, food selection depends on sensitivity to specific chemical stimuli from host-plants as well as to secondary metabolites (bitter) and to sugars (phagostimulatory). Bitter compounds are noxious, unpalatable or both and evoke an aversive feeding response. Instead, sugars and sugar alcohols play a critical role in determining and enhancing the palatability of foods. We assumed that peripheral taste sensitivity may be related to the width of the host selection. Our model consists of two closely phylogenetically related Papilionid species exhibiting a difference in host plant choice: Papilio hospiton and Papilio machaon. The spike activity of the lateral and medial maxillary styloconic taste sensilla was recorded following stimulation with several carbohydrates, nicotine and NaCl, with the aim of characterizing their gustatory receptor neurons and of comparing their response patterns in the light of their different acceptability in feeding behaviour. The results show that: a) each sensillum houses phagostimulant and phagodeterrent cells; b) the spike activity of the gustatory neurons in response to different taste stimuli is higher in P. hospiton than in P. machaon; c) sugar solutions inhibit the spike activity of the deterrent and salt cells, and the suppression is higher in P. machaon than in P. hospiton. In conclusion, we propose that the different balance between the phagostimulant and phagodeterrent inputs from GRNs of maxillary sensilla may contribute in determining the difference in food choice and host range.

Effects of Avermectins on Olfactory Responses of Culicoides imicola (Diptera: Ceratopogonidae)

Abstract 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.

First objective evaluation of taste sensitivity to 6- n -propylthiouracil (PROP), a paradigm gustatory stimulus in humans

Practical and reliable methods for the objective measure of taste function are critically important for studying eating behavior and taste function impairment. Here, we present direct measures of human gustatory response to a prototypical bitter compound, 6-n-propyltiouracil (PROP), obtained by electrophysiological recordings from the tongue of subjects who were classified for taster status and genotyped for the specific receptor gene (TAS2R38), and in which taste papilla density was determined. PROP stimulation evoked negative slow potentials that represent the summated depolarization of taste cells. Depolarization amplitude and rate were correlated with papilla density and perceived bitterness, and associated with taster status and TAS2R38. Our study provides a robust and generalizable research tool for the quantitative measure of peripheral taste function, which can greatly help to resolve controversial outcomes on the PROP phenotype role in taste perception and food preferences, and be potentially useful for evaluating nutritional status and health.

Morpho-functional identification of abdominal olfactory receptors in the midge Culicoides imicola

The aim of the present study was to examine the presence and the possible role of abdominal olfactory sensilla in Culicoides imicola mediating the search for potential hosts and oviposition sites, by means of a morphological, electrophysiological and behavioural approach. The results reported here show that in the midge C. imicola the whole abdomen, comprising the ovipositor, are endowed with three morphotypes of multiporous sensilla that display olfactory sensitivity towards kairomones related to the host-animal skin such as l-(+)-lactic acid and 1-octen-3-ol, to the host-animal urine such as 3-ethylphenol and 4-propylphenol, and to the potent attractant sesame seed oil. Electrophysiological and behavioural data for the first time suggest in the midge the involvement of abdominal olfactory structures in the choice of the oviposition sites and allow in discussing their possible role in the host-animal localisation. Field experiments showed that light traps baited with the aforementioned compounds elicited a stronger degree of attractiveness on midges with respect to the unbaited traps (control), although to a different extent. Our results, while implying a number of considerations concerning the role of molecules tested as kairomones, also suggest their use in the control of the midge C. imicola population.

Transduction mechanism(s) of Na-saccharin in the blowfly Protophormia terraenovae: evidence for potassium and calcium conductance involvement

The study on transduction mechanisms underlying bitter stimuli is a particularly intriguing challenge for taste researchers. The present study investigates, in the labellar chemosensilla of the blowfly Protophormia terraenovae, the transduction mechanism by which saccharin evokes the response of the “deterrent” cell, with particular attention to the contribution of K+ and Ca2+ current and the role of cyclic nucleotides, since second messengers modulate Ca2+, Cl− and K+ currents to different extents. As assessed by extracellular single-sensillum recordings, our results show that the addition of a Ca2+ chelator such as EGTA or the Ca2+ current blockers SK&F-96365, Mibefradil, Nifedipine and W-7 decrease the response of the “deterrent” cell to saccharin. A similar decreasing effect was also obtained following the addition of 4-aminopyridine, a K+ current blocker. On the contrary, the membrane-permeable cyclic nucleotide 8-bromoguanosine 3′,5′-cyclic monophosphate (8Br-cGMP) activates this cell and shows an additive effect when presented mixed with saccharin. Our results are consistent with the hypothesis that in the labellar chemosensilla of the blowfly both Ca2+ and K+ ions are involved in the transduction mechanism of the “deterrent” cell in response to saccharin. Our results also suggest a possible pathway common to saccharin and 8Br-cGMP.

Taste discriminating capability to different bitter compounds by the larval styloconic sensilla in the insect herbivore Papilio hospiton (Géné).

Herbivorous animals may benefit from the capability to discriminate the taste of bitter compounds since plants produce noxious compounds, some of which toxic, while others are only unpalatable. Our goal was to investigate the contribution of the peripheral taste system in the discrimination of different bitter compounds by an herbivorous insect using the larvae of Papilio hospiton Géné as the experimental model, showing a narrow choice range of host plants. The spike activity from the lateral and medial styloconic sensilla, housing two and one bitter-sensitive gustatory receptor neurons (GRNs), respectively, was recorded following stimulation with nicotine, caffeine, salicin and quercitrin and the time course of the discharges was analyzed. Nicotine and caffeine activated all three bitter-sensitive GRNs, while salicin and quercitrin affected only two of them. In feeding behavior bioassays, intact larvae ate glass-fiber disks moistened with salicin and quercitrin, but rejected those with nicotine and caffeine, while lateral sensillum-ablated insects also ate the disks with the two latter compounds. The capability to discriminate bitter taste stimuli and the neural codes involved are discussed.

Fat storage in Drosophila suzukii is influenced by different dietary sugars in relation to their palatability

The peripheral sensitivity and palatability of different carbohydrates was evaluated and their nutritional value assessed in adult females of D. suzukii by means of an electrophysiological, behavioural and metabolic approach. The electrophysiological responses were recorded from the labellar “l” type sensilla stimulated with metabolizable mono- and disaccharides (glucose and maltose) and a non-metabolizable sugar (sucralose); the response rating and the palatability to the same sugars, evaluated by recording the proboscis extension reflex (PER), was maltose>glucose>sucralose. The nutritional value of carbohydrates was assessed by means of survival trials and fatty acids profile. Flies fed on a diet containing maltose had a longer lifespan than flies on monosaccharides, while flies fed on a diet containing sucralose had a shorter one. In addition, the ability to store fat seems to be influenced by the different sugars in the diet and is in relationship with their palatability. In fact, data showed a higher synthesis of palmitic and palmitoleic acids, most likely derived from de-novo lipogenesis with glucose as precursor, in flies fed with maltose and glucose than with non-metabolizable sucralose. In conclusion, these results suggest that the ability to select different sugars on the basis of their palatability may favour the storage of energy reserves such as fat by de-novo lipogenesis, determining a longer survival capability during prolonged periods of fasting.

Chemosensory basis of larval performance of Papilio hospiton on different host plants

Papilio hospiton Géné is an oligophagous species, endemic of the islands of Corsica and Sardinia, using various Apiaceae and Rutaceae as host plants, such as Ferula communis, Ferula arrigonii, Peucedanum paniculatum, Ruta lamarmorae and Pastinaca latifolia. We previously found that the lateral maxillary styloconic sensillum in the larva has two deterrent neurons, one phagostimulant and one salt specific, while the medial sensillum has two phagostimulant neurons, one deterrent and one salt specific. In this work we studied the sensitivity of gustatory receptor neurons (GRNs) to saps of F. communis, F. arrigonii, P. paniculatum, P. latifolia and R. lamarmorae and evaluated the relationship between taste sensitivity to different host-plants and larval growth rate on each of them. The spike activity was recorded from medial and lateral taste sensilla stimulated with plant saps, and GRN response patterns were cross compared in the light of a different feeding acceptance. The phagodeterrent GRNs show a higher activity in response to F. arrigonii and R. lamarmorae than to F. communis, P. paniculatum and P. latifolia. Behavioral trials showed that the time to pupation is significantly longer when larvae are reared on F. arrigonii and R. lamarmorae than on the other host-plants. These results suggest that the different activity of the phagodeterrent GRNs may inhibit food acceptance and extend the duration of the larval stage.

Antennular Morphology and Contribution of Aesthetascs in the Detection of Food-related Compounds in the Shrimp Palaemon adspersus Rathke, 1837 (Decapoda: Palaemonidae)

Shrimp are an essential ecological component of marine ecosystems, and have commercial importance for human consumption and aquaculture. Like other decapod crustaceans, shrimp rely on chemical senses to detect and localize food resources by means of chemosensilla that are located mainly on the cephalothoracic appendages. Using the shrimp Palaemon adspersus, a model organism with omnivorous feeding behavior, we aimed to provide comparative information on the role of aesthetascs, antennular sensilla, and flicking behavior in food detection. To this end, we examined i) the morphology of antennular sensilla by field emission scanning electron microscopy, ii) the shrimp’s sensitivity to a number of food-related compounds (amino acids and sugars) by means of whole-animal bioassays, and iii) the contribution of the aesthetasc sensilla to food detection. Our results showed that, aside from the aesthetascs, only three other main morphotypes of setae with chemoreceptive features were present in the antennules, thus accounting for relatively simple sensillar equipment. Nevertheless, we found broad-spectrum sensitivity of the shrimp to a number of amino acids (i.e., isoleucine, leucine, methionine, phenylalanine, glycine, tryptophan, cysteine, and tyrosine) and carbohydrates (trehalose, maltose, cellobiose, and fructose) that was consistent with the omnivorous or scavenging habits of the animal. Although aesthetasc ablation attenuated flicking behavior in a chemical stimulus-independent manner, success in detection and short-range localization of food did not rely on the presence of aesthetasc sensilla. This finding confirms the existence of a non-aesthetasc alternative pathway for feeding, with functional redundancy in simple generalist feeder models such as shrimp.