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 two
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 inhibitions
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
Na 2 SO 4 , KCl, MgSO 4 , CaCl 2 and FeCl 3 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.