Postprandial sodium sensing by enteric neurons in Drosophila.

Sodium is essential for all living organisms. Animals including insects and mammals detect sodium primarily through peripheral taste cells. It is not known, however, whether animals can detect this essential micronutrient independently of the taste system. Here, we report that Drosophila Ir76b mutants that were unable to detect sodium became capable of responding to sodium following a period of salt deprivation. From a screen for cells required for the deprivation-induced sodium preference, we identified a population of anterior enteric neurons, which we named internal sodium-sensing (INSO) neurons, that are essential for directing a behavioural preference for sodium. Enteric INSO neurons innervate the gut epithelia mainly through their dendritic processes and send their axonal projections along the oesophagus to the brain and to the crop duct. Through calcium imaging and CaLexA experiments, we found that INSO neurons respond immediately and specifically to sodium ions. Notably, the sodium-evoked responses were observed only after a period of sodium deprivation. Taken together, we have identified a taste-independent sodium sensor that is essential for the maintenance of sodium homeostasis.