Spatiotemporal organization of enteroendocrine peptide expression in Drosophila.

The digestion of food and absorption of nutrients occurs in the gut. The nutritional value of food and its nutrients is detected by enteroendocrine cells, and peptide hormones produced by the enteroendocrine cells are thought to be involved in metabolic homeostasis, but the specific mechanisms are still elusive. The enteroendocrine cells are scattered over the entire gastrointestinal tract and can be classified according to the hormones they produce. We followed the changes in combinatorial expression of regulatory peptides in the enteroendocrine cells during metamorphosis from the larva to the adult fruit fly, and re-confirmed the diverse composition of enteroendocrine cell populations. Drosophila enteroendocrine cells appear to differentially regulate peptide expression spatially and temporally depending on midgut region and developmental stage. In the late pupa, Notch activity is known to determine which peptides are expressed in mature enteroendocrine cells of the posterior midgut, and we found that the loss of Notch activity in the anterior midgut results in classes of enteroendocrine cells distinct from the posterior midgut. These results suggest that enteroendocrine cells that populate the fly midgut can differentiate into distinct subtypes that express different combinations of peptides, which likely leads to functional variety depending on specific needs.

Cellular Basis of Bitter-Driven Aversive Behaviors in Drosophila Larva.

Feeding, a critical behavior for survival, consists of a complex series of behavioral steps. In Drosophila larvae, the initial steps of feeding are food choice, during which the quality of a potential food source is judged, and ingestion, during which the selected food source is ingested into the digestive tract. It remains unclear whether these steps employ different mechanisms of neural perception. Here, we provide insight into the two initial steps of feeding in Drosophila larva. We find that substrate choice and ingestion are determined by independent circuits at the cellular level. First, we took 22 candidate bitter compounds and examined their influence on choice preference and ingestion behavior. Interestingly, certain bitter tastants caused different responses in choice and ingestion, suggesting distinct mechanisms of perception. We further provide evidence that certain gustatory receptor neurons (GRNs) in the external terminal organ (TO) are involved in determining choice preference, and a pair of larval pharyngeal GRNs is involved in mediating both avoidance and suppression of ingestion. Our results show that feeding behavior is coordinated by a multistep regulatory process employing relatively independent neural elements. These findings are consistent with a model in which distinct sensory pathways act as modulatory circuits controlling distinct subprograms during feeding.

Involvement of a Gr2a-Expressing Drosophila Pharyngeal Gustatory Receptor Neuron in Regulation of Aversion to High-Salt Foods.

Regulation of feeding is essential for animal survival. The pharyngeal sense organs can act as a second checkpoint of food quality, due to their position between external taste organs such as the labellum which initially assess food quality, and the digestive tract. Growing evidence provides support that the pharyngeal sensory neurons regulate feeding, but much is still unknown. We found that a pair of gustatory receptor neurons in the LSO, a Drosophila adult pharyngeal organ which expresses four gustatory receptors, is involved in feeding inhibition in response to high concentrations of sodium ions. RNAi experiments and mutant analysis showed that the gustatory receptor Gr2a is necessary for this process. This feeding preference determined by whether a food source is perceived as appetizing or not is influenced by nutritional conditions, such that when the animal is hungry, the need for energy dominates over how appealing the food source is. Our results provide experimental evidence that factors involved in feeding function in a context-dependent manner.

A Pair of Pharyngeal Gustatory Receptor Neurons Regulates Caffeine-Dependent Ingestion in Drosophila Larvae.

The sense of taste is an essential chemosensory modality that enables animals to identify appropriate food sources and control feeding behavior. In particular, the recognition of bitter taste prevents animals from feeding on harmful substances. Feeding is a complex behavior comprised of multiple steps, and food quality is continuously assessed. We here examined the role of pharyngeal gustatory organs in ingestion behavior. As a first step, we constructed a gustatory receptor-to-neuron map of the larval pharyngeal sense organs, and examined corresponding gustatory receptor neuron (GRN) projections in the larval brain. Out of 22 candidate bitter compounds, we found 14 bitter compounds that elicit inhibition of ingestion in a dose-dependent manner. We provide evidence that certain pharyngeal GRNs are necessary and sufficient for the ingestion response of larvae to caffeine. Additionally, we show that a specific pair of pharyngeal GRNs, DP1, responds to caffeine by calcium imaging. In this study we show that a specific pair of GRNs in the pharyngeal sense organs coordinates caffeine sensing with regulation of behavioral responses such as ingestion. Our results indicate that in Drosophila larvae, the pharyngeal GRNs have a major role in sensing food palatability to regulate ingestion behavior. The pharyngeal sense organs are prime candidates to influence ingestion due to their position in the pharynx, and they may act as first level sensors of ingested food.