Abstract
Gustatory stimuli can support both immediate reflexive behaviour, such as choice and feeding, and can drive internal reinforcement in associative learning. For larval Drosophila, we here provide a first systematic behavioural analysis of these functions with respect to quinine as a study case of a substance which humans report as “tasting bitter”. We describe the dose-effect functions for these different kinds of behaviour and find that a half-maximal effect of quinine to suppress feeding needs substantially higher quinine concentrations (2.0 mM) than is the case for internal reinforcement (0.6 mM). Interestingly, in previous studies (Niewalda et al. 2008, Schipanski et al 2008) we had found the reverse for sodium chloride and fructose/sucrose, such that dose-effect functions for those tastants were shifted towards lower concentrations for feeding as compared to reinforcement, arguing that the differences in dose-effect function between these behaviours do not reflect artefacts of the types of assay used. The current results regarding quinine thus provide a starting point to investigate how the gustatory system is organized on the cellular and/or molecular level to result in different behavioural tuning curves towards a bitter tastant.
Highlights
The sense of taste is that component of the contact chemosensory system devoted to organize feeding, allowing animals to prefer edible and avoid toxic substances
We separate them into two halves with a piece of overhead transparency, fill one side with only 1% agarose and the other side with 1% agarose added with quinine hemisulfate as a bitter tastant at the respectively indicated concentrations; for the CONTROL condition, both sides of the Petri dish contain pure agarose (PURE)
We restrict ourselves to a total observation time of 8 min because after that time point the larvae begin to dig into the substrate or to crawl up the side walls of the Petri dish; we chose 5 mM of QUI because this concentration had been used in previous work and because higher concentrations of QUI, without acidification, show crystalization of QUI in the agarose dishes
Summary
The sense of taste is that component of the contact chemosensory system devoted to organize feeding, allowing animals to prefer edible and avoid toxic substances. Gustatory stimuli organize both immediate, reflexive behaviour towards food (such as choice and feeding), and, by virtue of their association with predictive stimuli or instrumental actions, the search for food. These functions must come about by different sets of neurons on at least some level of processing. We want to take a first systematic step into such an analysis by behaviourally ‘‘footprinting’’ the dose-effect characteristics of bitter-processing (‘‘bitter’’ is used throughout this study in the sense that humans verbalize these chemically diverse and often toxic substances as ‘‘tasting bitter’’ and avoid eating them) in choice, feeding and reinforcement processing of larval Drosophila, using quinine as a study case
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