Evolution of fatty acid taste in drosophilids.

Comparative studies of related but ecologically distinct species can reveal how the nervous system evolves to drive behaviors that are particularly suited to certain environments. Drosophila melanogaster is a generalist that feeds and oviposits on most overripe fruits. A sibling species, D. sechellia, is an obligate specialist of Morinda citrifolia (noni) fruit, which is rich in fatty acids (FAs). To understand evolution of noni taste preference, we characterized behavioral and cellular responses to noni-associated FAs in three related drosophilids. We find that mixtures of sugar and noni FAs evoke strong aversion in the generalist species but not in D. sechellia. Surveys of taste sensory responses reveal noni FA- and species-specific differences in at least two mechanisms-bitter neuron activation and sweet neuron inhibition-that correlate with shifts in noni preference. Chemoreceptor mutant analysis in D. melanogaster predicts that multiple genetic changes account for evolution of gustatory preference in D. sechellia.

An inhibitory mechanism for suppressing high salt intake in Drosophila.

High concentrations of dietary salt are harmful to health. Like most animals, Drosophila melanogaster are attracted to foods that have low concentrations of salt, but show strong taste avoidance of high salt foods. Salt in known on multiple classes of taste neurons, activating Gr64f sweet-sensing neurons that drive food acceptance and 2 others (Gr66a bitter and Ppk23 high salt) that drive food rejection. Here we find that NaCl elicits a bimodal dose-dependent response in Gr64f taste neurons, which show high activity with low salt and depressed activity with high salt. High salt also inhibits the sugar response of Gr64f neurons, and this action is independent of the neuron's taste response to salt. Consistent with the electrophysiological analysis, feeding suppression in the presence of salt correlates with inhibition of Gr64f neuron activity, and remains if high salt taste neurons are genetically silenced. Other salts such as Na2SO4, KCl, MgSO4, CaCl2, and FeCl3 act on sugar response and feeding behavior in the same way. A comparison of the effects of various salts suggests that inhibition is dictated by the cationic moiety rather than the anionic component of the salt. Notably, high salt-dependent inhibition is not observed in Gr66a neurons-response to a canonical bitter tastant, denatonium, is not altered by high salt. Overall, this study characterizes a mechanism in appetitive Gr64f neurons that can deter ingestion of potentially harmful salts.

Dietary Macronutrient Imbalances Lead to Compensatory Changes in Peripheral Taste via Independent Signaling Pathways.

Food choice, in animals, has been known to change with internal nutritional state and also with variable dietary conditions. To better characterize mechanisms of diet-induced plasticity of food preference in Drosophila melanogaster, we synthesized diets with macronutrient imbalances and examined how food choice and taste sensitivity were modified in flies that fed on these diets. We found that dietary macronutrient imbalances caused compensatory behavioral shifts in both sexes to increase preference for the macronutrient that was scant in the food source, and simultaneously reduce preference for the macronutrient that was enriched. Further analysis with females revealed analogous changes in sweet taste responses in labellar neurons, with increased sensitivity on sugar-reduced diet and decreased sensitivity on sugar-enriched diet. Interestingly, we found differences in the onset of changes in taste sensitivity and behavior, which occur over 1-4 d, in response to dietary sugar reduction or enrichment. To investigate molecular mechanisms responsible for diet-induced taste modulation, we used candidate gene and transcriptome analyses. Our results indicate that signaling via Dop2R is involved in increasing cellular and behavioral sensitivity to sugar as well as in decreasing behavioral sensitivity to amino acids on dietary sugar reduction. On the other hand, cellular and behavioral sensitivity to sugar relies on dilp5 and a decrease in sugar preference following dietary sugar abundance was correlated with downregulation of dilp5 Together, our results suggest that feeding preference for sugar and amino acid can be modulated independently to facilitate food choice that accounts for prior dietary experience.SIGNIFICANCE STATEMENT Animals adjust their feeding preferences based on prior dietary experiences. Here, we find that upon dietary macronutrient deprivation, flies undergo compensatory changes in food preference. The altered preference correlates with changes in peripheral taste sensitivity. While Dop2R mediates changes following dietary sugar reduction, downregulation of dilp5 is associated with changes caused by a sugar-enriched diet. This study contributes to a better understanding of neurophysiological plasticity of the taste system in flies, and its role in facilitating adjustment of foraging behavior based on nutritional requirements.

Ir56d-dependent fatty acid responses in Drosophila uncover taste discrimination between different classes of fatty acids.

Chemosensory systems are critical for evaluating the caloric value and potential toxicity of food. While animals can discriminate between thousands of odors, much less is known about the discriminative capabilities of taste systems. Fats and sugars represent calorically potent and attractive food sources that contribute to hedonic feeding. Despite the differences in nutritional value between fats and sugars, the ability of the taste system to discriminate between different rewarding tastants is thought to be limited. In Drosophila, taste neurons expressing the ionotropic receptor 56d (IR56d) are required for reflexive behavioral responses to the medium-chain fatty acid, hexanoic acid. Here, we tested whether flies can discriminate between different classes of fatty acids using an aversive memory assay. Our results indicate that flies are able to discriminate medium-chain fatty acids from both short- and long-chain fatty acids, but not from other medium-chain fatty acids. While IR56d neurons are broadly responsive to short-, medium-, and long-chain fatty acids, genetic deletion of IR56d selectively disrupts response to medium-chain fatty acids. Further, IR56d+ GR64f+ neurons are necessary for proboscis extension response (PER) to medium-chain fatty acids, but both IR56d and GR64f neurons are dispensable for PER to short- and long-chain fatty acids, indicating the involvement of one or more other classes of neurons. Together, these findings reveal that IR56d is selectively required for medium-chain fatty acid taste, and discrimination of fatty acids occurs through differential receptor activation in shared populations of neurons. Our study uncovers a capacity for the taste system to encode tastant identity within a taste category.

Copper-induced immunotoxicity involves cell cycle arrest and cell death in the spleen and thymus.

Copper is an essential trace element for human physiological processes. To evaluate the potential adverse health impact/immunotoxicological effects of this metal in situ due to over exposure, Swiss albino mice were treated (via intraperitoneal injections) with copper (II) chloride (copper chloride) at doses of 0, 5, or 7.5 mg copper chloride/kg body weight (b.w.) twice a week for 4 wk; these values were derived from LD50 studies using copper chloride doses that ranged from 0 to 40 mg/kg BW (2×/wk, for 4 wk). Copper treated mice evidenced immunotoxicity as indicated by dose-related decreases and increases, respectively, in thymic and splenic weights. Histomorphological changes evidenced in these organs were thymic atrophy, white pulp shrinkage in the spleen, and apoptosis of splenocytes and thymocytes; these observations were confirmed by microscopic analyses. Cell count analyses indicated that the proliferative functions of the splenocytes and thymocytes were also altered because of the copper exposures. Among both cell types from the copper treated hosts, flow cytometric analyses revealed a dose related increase in the percentages of cells in the Sub-G0/G1 state, indicative of apoptosis which was further confirmed by Annexin V binding assay. In addition, the copper treatments altered the expression of selected cell death related genes such as EndoG and Bax in a dose related manner. Immunohistochemical analyses revealed that there was also increased ubiquitin expression in both the cell types. In conclusion, these studies show that sublethal exposure to copper (as copper chloride) induces toxicity in the thymus and spleen, and increased Sub G0/G1 population among splenocytes and thymocytes that is mediated, in part, by the EndoG-Bax-ubiquitin pathway. This latter damage to these cells that reside in critical immune system organs are likely to be important contributing factors underlying the immunosuppression that has been documented by other investigators following acute high dose/chronic low-medium dose exposures to copper agents.