The sense of water in the blowfly Protophormia terraenovae

Gustation in insects: taste qualities and types of evidence used to show taste function of specific body parts

The insect equivalent of taste buds are gustatory sensilla, which have been found on mouthparts, pharynxes, antennae, legs, wings, and ovipositors. Most gustatory sensilla are uniporous, but not all apparently uniporous sensilla are gustatory. Among sensilla containing more than one neuron, a tubular body on one dendrite is also indicative of a taste sensillum, with the tubular body adding tactile function. But not all taste sensilla are also tactile. Additional morphological criteria are often used to recognize if a sensillum is gustatory. Further confirmation of such criteria by electrophysiological or behavioral evidence is needed. The five canonical taste qualities to which insects respond are sweet, bitter, sour, salty, and umami. But not all tastants that insects respond to easily fit in these taste qualities. Categories of insect tastants can be based not only on human taste perception, but also on whether the response is deterrent or appetitive and on chemical structure. Other compounds that at least some insects taste include, but are not limited to: water, fatty acids, metals, carbonation, RNA, ATP, pungent tastes as in horseradish, bacterial lipopolysaccharides, and contact pheromones. We propose that, for insects, taste be defined not only as a response to nonvolatiles but also be restricted to responses that are, or are thought to be, mediated by a sensillum. This restriction is useful because some of the receptor proteins in gustatory sensilla are also found elsewhere.

Visualization of antennal lobe glomeruli activated by nonappetitive D-limonene and appetitive 1-octen-3-ol odors via two types of olfactory organs in the blowfly Phormia regina

Appetite or feeding motivation relies significantly on food odors. In the blowfly Phormia regina, feeding motivation for sucrose is decreased by the odor of D-limonene but increased by the odor of 1-octen-3-ol odor. These flies have antennal lobes (ALs) consisting of several tens of glomerular pairs as a primary olfactory center in the brain. Odor information from different olfactory organs-specifically, the antennae and maxillary palps-goes to the corresponding glomeruli. To investigate how odors differently affect feeding motivation, we identified the olfactory organs and glomeruli that are activated by nonappetitive and appetitive odors. We first constructed a glomerular map of the antennal lobe in P. regina. Anterograde fluorescence labeling of antennal and maxillary afferent nerves, both of which project into the contralateral and ipsilateral ALs, revealed differential staining in glomerular regions. Some of the axonal fiber bundles from the antennae and maxillary palps projected to the subesophageal ganglion (SOG). We visualized the activation of the glomeruli in response to odor stimuli by immunostaining phosphorylated extracellular signal-regulated kinase (pERK). We observed different glomerulus activation under different odor stimulations. Referring to our glomerular map, we determined that antennal exposure to D-limonene odor activated the DA13 glomeruli, while exposure of the maxillary palps to 1-octen-3-ol activated the MxB1 glomeruli. Our results indicated that a nonappetitive odor input from the antennae and an appetitive odor input from the maxillary palps activate different glomeruli in the different regions of ALs in the blowfly P. regina. Collectively, our findings suggest that compartmentalization of glomeruli in AL is essential for proper transmission of odor information.

General Decrease of Taste Sensitivity Is Related to Increase of BMI: A Simple Method to Monitor Eating Behavior

Background and Objectives

The present study was conducted to evaluate the relationship between taste identification ability and body mass index (BMI) by studying the response to the administration of different taste stimuli to both sides of the tongue in three different groups of subjects.

Subjects and Methods

Thirty healthy normal-weight volunteers, 19 healthy overweight subjects, and 22 obese subjects were enrolled. For each subject, the lateralization Oldfield score, body weight, height, and blood pressure were determined. The taste test is based on filter paper strips soaked with 4 taste stimuli presented at different concentrations to evoke 4 basic taste qualities (salty, sour, sweet, and bitter); pure rapeseed oil and water were also administered to evoke fat and neutral taste qualities. The stimuli were applied to each side of the protruded tongue. Subjects were asked to identify the taste from a list of eight descriptions according to a multiple choice paradigm.

Results

The results showed a general lowering of taste sensitivity with the increase of BMI, except for the taste of fat with rapeseed oil as the stimulus. Other variables affecting taste sensitivity are age (negative association), gender (women generally show higher sensitivity), and taste stimuli concentration (positive association).

Conclusions

Our findings could provide important insights into how new therapies could be designed for weight loss and long-term weight maintenance and how diets could be planned combining the correct caloric and nutritional supply with individual taste preferences.

The contribution of gustatory input to larval acceptance and female oviposition choice of potential host plants in Papilio hospiton (Géné)

The Lepidopteran Papilio hospiton uses only plants belonging to the Apiaceae and the Rutaceae families as hosts. Both adult females and larvae are equipped with gustatory receptor neurons (GRNs) capable of detecting sugars, bitters and salts, thus providing information for evaluating the chemical composition of the plant. Since the activation of these neurons may affect insect behavior, the aim of this study were: (a) to study the gustatory sensitivity of both females and larvae to the sap of two Apiaceae, Foeniculum vulgare (fennel) and Daucus carota (carrot), that are not used as host plants; (b) to cross-compare the spike activity evoked from these two plants with that evoked by Ferula communis (ferula), the host plant preferred by ovipositing females of P. hospiton and where the larvae perform best; (c) finally, to confirm that the gustatory system can provide the central nervous system with the necessary information to evaluate differences between plant saps. The results show that: (a) fennel and carrot both evoke a higher neural activity from the bitter-sensitive neurons and lower from the sugar-sensitive neurons with respect to ferula, in both adult females and larvae; (b) on the basis of the different patterns of neural activity generated in tarsal, lateral and medial sensilla by fennel and carrot versus ferula, both adult and larvae possess enough information to discriminate among these plants; (c) adult females of P. hospiton lay eggs where the larvae have the greatest growth success and this confirms the importance of taste sensitivity in host plants selection.

Octopamine modulates the activity of motoneurons related to calling behavior in the gypsy moth Lymantria dispar

A morphofunctional investigation of the different neuronal subpopulations projecting through each of the nerves IV-VI emerging bilaterally from the terminal abdominal ganglion (TAG) was correlated with the octopaminergic activity in the ganglion that controls the ovipositor movements associated with calling behavior in the female gypsy moth Lymantria dispar. Tetramethylrodamine-dextran backfills from nerve stumps resulted in a relatively low number of TAG projections, ranging from 12 to 13 for nerve pair IV, 12 to 14 for nerve pair V, and 8 to 9 for nerve pair VI. Furthermore, as assessed by electrophysiological recordings, a number of fibers within each of these nerves displays spontaneous tonic activity, also when the ganglion is fully disconnected from the ventral nerve cord (VNC). Octopamine (OA) applications to the TAG strongly enhanced the activity of these nerves, either by increasing the firing rate of a number of spontaneously firing units or by recruiting new ones. This octopaminergic activity affected calling behavior, and specifically the muscle activity leading to cycling extensions of the intersegmental membrane (IM) between segments VIII and IX (ovipositor). Our results indicate that in the female gypsy moth the octopaminergic neural activity of the TAG is coupled with extensions and retractions of IM for the purpose of releasing pheromone, where motor units innervated by nerve pair IV appear antagonistic with respect to those innervated by nerve pair V.

Taste sensitivity and divergence in host plant acceptance between adult females and larvae of Papilio hospiton

On the island of Sardinia the lepidopteran Papilio hospiton uses Ferula communis as exclusive host plant. However, on the small island of Tavolara, adult females lay eggs on Seseli tortuosum, a plant confined to the island. When raised in captivity on Seseli only few larvae grew beyond the first-second instar. Host specificity of lepidopterans is determined by female oviposition preferences, but also by larval food acceptance, and adult and larval taste sensitivity may be related to host selection in both cases. Aim of this work was: (i) to study the taste sensitivity of larvae and ovipositing females to saps of Ferula and Seseli; (ii) to cross-compare the spike activity of gustatory receptor neurons (GRNs) to both taste stimuli; (iii) to evaluate the discriminating capability between the two saps and determine which neural code/s is/are used. The results show that: (i) the spike responses of the tarsal GRNs of adult females to both plant saps are not different and therefore they cannot discriminate the two plants; (ii) larval L-lat GRN shows a higher activity in response to Seseli than Ferula, while the opposite occurs for the phagostimulant neurons, and larvae may discriminate between the two saps by means of multiple neural codes; (iii) the number of eggs laid on the two plants is the same, but the larval growth performance is better on Ferula than Seseli. Taste sensitivity differences may explain the absence of a positive relationship between oviposition preferences by adult females and plant acceptance and growth performance by larvae.

Olfactory sensitivity to major, intermediate and trace components of sex pheromone in Ceratitis capitata is related to mating and circadian rhythm

The Mediterranean fruit fly, Ceratitis capitata Wied., is a worldwide pest of several fruits given its extremely wide host range which includes more than 250 different species of fruits and vegetables. Its high biological potential is mainly due both to its ability to readily adapt to new environments and its high reproductive capacity as it completes multiple generations each year. Since sexually mature males emit a sex pheromone to call both other males for "lekking" and receptive females for mating, many studies have been directed to characterize the chemical composition of the sex pheromone. Besides, the release of sex pheromone appears to be modulated both by mating and time of day. Based on these considerations, we measured the olfactory sensitivity of antennae and palps of C. capitata to six volatiles of the male sex-pheromone: α-farnesene and geranyl acetate (major components), linalool and β-myrcene (intermediate components), β-farnesene and 2,3-butanediol (minor/trace components). The electroantennogram (EAG) and electropalpogram (EPG) responses were evaluated in both sexes, at different physiological states (virgin and mated), and at different times of the day (morning and afternoon). The results show that the EAG amplitude values in response to all stimuli are higher in the morning than in the afternoon for both sexes and in both virgin and mated insects. Furthermore, in both sexes, the olfactory sensitivity of virgin insects is higher than in mated ones. The EPG amplitude in response to all stimuli is higher in the morning in mated females than in virgin females and higher in the morning than in the afternoon in both mated sexes. By gaining knowledge on the effects of sex, physiological state and time of day on the olfactory sensitivity of C. capitata, one could better understand the medfly reproductive behavior.

Taste receptor plasticity in relation to feeding history in two congeneric species of Papilionidae (Lepidoptera)

In the peripheral taste system of insects, the responsiveness of gustatory receptor neurons (GRNs) depends on several factors, such as larval instar, feeding history, physiological state and time of day. To study the role of the feeding history, the spike activity of the maxillary taste chemosensilla in the larvae of two related species of Lepidoptera (Papilio machaon L. and Papilio hospiton Géné) raised on different host plants, was recorded with electrophysiological techniques after stimulation with simple stimuli (sugars, bitters and inorganic salt) and host plant saps, with the aim of cross-comparing their response patterns and evaluating any effects of different feeding histories. For this purpose the larvae were raised each on their preferential host plant and, in addition, P. machaon larvae was also raised on Ferula communis, the host plant preferred by P. hospiton. The GRN spike activity of the lateral and medial sensilla of each test group was measured in response to simple and complex stimuli. The taste discrimination capabilities and modalities of the two species were measured and cross-compared with the aim of studying convergence and/or divergence linked to the insect feeding history. The results show that: a) the GRN responsiveness of both sensilla in P. machaon raised on Fe. communis differs significantly from that of P. machaon on Foeniculum vulgare, but is not different from P. hospiton on Fe. communis; b) P. machaon larvae raised on Fe. communis exhibit response spectra somewhat intermediate between those of P. machaon on fennel and of P. hospiton on Fe. communis, the latter two exhibiting a wider difference from each other; c) the pattern of GRNs activity generated by each plant sap in both sensilla of P. machaon raised on Fe. communis is different from that generated when raised on Fo. vulgare, while no difference is observed with P. hospiton. The data support the hypothesis that diet-related factors may influence peripheral chemosensitivity in lepidopterous larvae.

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.

Suppressive effects of dRYamides on feeding behavior of the blowfly, Phormia regina

Recently, dRYamides-1 and -2 have been identified as ligands of the neuropeptide Y-like receptor CG5811 in Drosophila melanogaster. It has also been reported in brief that injection of dRYamide-1suppresses the early feeding behavior called proboscis extension reflex (PER) in the blowfly Phormia regina. Immunohistochemical analyses by our group using anti-dRYamide-1 antiserum indicated symmetrical localization of 32 immunoreactive cells in the brain of P. regina. In order to analyze the mechanism of feeding regulation, we further investigated the effects of dRYamide-1 and -2 on intake volume, PER exhibition, and activity of the sugar receptor neuron. After injection of dRYamide-1 or -2, flies showed little change in the intake volume of sucrose solution, but a significant depression of PER to sucrose. Injection of dRYamide-1 revealed a significant decrease in the responsiveness of the sugar receptor neuron, although the injection of dRYamide-2 did not. These results suggest that the dRYamide peptides decrease feeding motivation in flies, as evaluated by PER threshold, through a mechanism that partially involves desensitization of the sugar receptor neuron.

Neuronal projections and putative interaction of multimodal inputs in the subesophageal ganglion in the blowfly, Phormia regina

In flies, the maxillary palp possesses olfactory sensilla housing olfactory receptor neurons (ORNs), which project to the primary olfactory center, the antennal lobes (ALs). The labellum possesses gustatory sensilla housing gustatory receptor neurons (GRNs), which project to the primary gustatory center, the subesophageal ganglion (SOG). Using an anterograde staining method, we investigated the axonal projections of sensory receptor neurons from the maxillary palp and labellum to the SOG or other parts of brain in the blowfly, Phormia regina. We show that maxillary mechanoreceptor neurons and some maxillary ORNs project to the SOG where they establish synapses, whereas other maxillary ORNs terminate in the ipsi- and contralateral ALs. The labellar GRNs project to the SOG, and some of these neural projections partially overlap with ORN terminals from the maxillary palp. Based on these anterograde staining data and 3D models of the observed axonal projections, we suggest that interactions occur between GRNs from the labellum and ORNs from the maxillary palp. These observations strongly suggest that olfactory information from the maxillary palp directly interacts with the processing of gustatory information within the SOG of flies.

The spike generator in the labellar taste receptors of the blowfly is differently affected by 4-aminopyridine and 5-hydroxytryptamine

In taste chemoreception of invertebrates the interaction of taste stimuli with specific membrane receptors and/or ion channels located in the apical membrane of taste receptor cells results in the generation of a receptor potential which, in turn, activates the 'encoder' region to produce action potentials which propagate to the CNS. This study investigates, in the labellar chemosensilla of the blowfly, Protophormia terraenovae, the voltage-gated K(+) currents involved in the action potential repolarization and repetitive firing of the neurons by way of the K(v) channel inhibitors, 4-aminopyridine and 5-hydroxytryptamine. The receptor potential and the spike activity were simultaneously recorded from the 'salt', 'sugar' and 'deterrent' cells, by means of the extracellular side-wall technique, in response to 150 mM NaCl, 100 mM sucrose and 1 mM quinine HCl, before, 0÷10 min after apical administration of 4-AP (0.01-10 mM) or 5-HT (0.1-100 mM). The results show that the receptor potential in all three cells is neither affected by 4-AP nor by 5-HT. Instead, spike activity is significantly decreased, by way of blocking different K(v) channel types: an inactivating A-type K(+) current (KA) modulating repetitive firing of the cells and responsible for the after hyperpolarization, and a sustained K(+) current that resembles the delayed rectifier (DKR) and contributes to action potential repolarization.

The sugar meal of the African malaria mosquito Anopheles gambiae (Giles) and how deterrent compounds interfere with it: a behavioural and neurophysiological study

In this study, we show that female African malaria mosquitoes Anopheles gambiae (Giles) starved for 3-5 hours start to engorge on sucrose at concentrations between 50 to 75 mM. Half of the feeding response (ED50) is reached at 111 mM and the maximum response (0.4 mg) occurs from 146 mM (5% m/v). Two receptor cells in a trichoid sensillum of the labellum, called the 'sucrose' and the 'water' neurones, are activated by sucrose and water, respectively. The electrophysiological response of the sucrose receptor cell starts well below the level of sugar necessary to induce feeding. An. gambiae is most sensitive to small increments in sucrose concentration up to 10 mM with a response plateau at a maximum frequency of 53 spikes per 2 s from 50 mM, the concentration at which female An. gambiae start to engorge on sucrose. Fructose has a mild phagostimulatory effect on An. gambiae, whereas no significant differences in meal sizes between water and glucose were found. However, when 146 mM fructose plus glucose are mixed, the same engorgement as on 146 mM sucrose is observed. Likewise, even though the sucrose receptor cell is not activated by either fructose or glucose alone, equimolar solutions of fructose plus glucose activate the neurone. We conclude that there is a behavioural and a neurophysiological synergism between fructose and glucose, the two hexose sugars of sucrose. We show that bitter tasting products for humans have a deterrent effect on feeding in An. gambiae. When 1 mM quinidine, quinine or denatonium benzoate is added to 146 mM sucrose, feeding is almost totally inhibited. The effect of berberine is lower and no significant inhibition on engorgement occurs for caffeine. The deterrent effect depends on concentration for both quinine and quinidine. Capillary feeding experiments show that contact chemosensilla on the mouthparts are sufficient for the detection of sucrose and bitter products. The feeding assay findings with deterrents correlate with the neurophysiological responses of the sucrose and the water labellar neurones which are both inhibited by the bitter compounds denatonium benzoate, quinine and berberine between 0.01 and 1 mM, but not by the same concentrations of caffeine which has no effect on feeding. In conclusion, sucrose which stimulates feeding activates the labellar sucrose neurone whereas feeding deterrents inhibit both the sucrose and the water neurones. This study provides an initial understanding of the physiological mechanisms involved in sugar feeding in An. gambiae and shows how some bitter products interfere with it.