Salt processing in larval Drosophila: choice, feeding, and learning shift from appetitive to aversive in a concentration-dependent way.

Sodium and chloride need to be ingested and cannot be stored. Therefore, choice of habitat and diet as related to NaCl needs to be tightly regulated. We thus expect that the behavioral effects of salt are organized according to its concentration. Here, we comparatively "fingerprint" the reflex releasing (in choice and feeding experiments) versus the reinforcing effects of sodium chloride ("salt") in terms of their concentration dependencies, using larval Drosophila. Qualitatively, we find that the behavioral effects of salt in all 3 assays are similar: choice, feeding, and reinforcing effect all change from appetitive to aversive as concentration is increased. Quantitatively, however, the appetitive effects for choice and feeding share their optimum at around 0.02 M, whereas the dose-response curve for the reinforcing effect is shifted by more than one order of magnitude toward higher concentrations. Interestingly, a similar shift between these 2 kinds of behavioral effect is also found for sugars (Schipanski et al. 2008). Thus, for salt and for sugar, the sensory-to-motor system is more sensitive regarding immediate, reflexive behavior than regarding reinforcement. We speculate that this may partially be due to a dissociation of the sensory pathways signaling toward either reflexive behavior or internal reinforcement.