Micronutrient is essential for the well-being of organism. Salt (sodium chloride) is a key micronutrient that mediates important functions in a variety of physiological processes. It was one of the most sought-after commodities until a hundred year ago, likely because humans were frequently deprived of sodium. Indeed, the human desire for salt led to the establishment of trade routes and cities, provoked and financed wars, and controlled the citizens of empires. This significance had also transpired as religious symbolism. Despite the widely-recognized research on taste sensation of salt, we have little understanding of whether animals are capable of detecting and responding to sodium independently of taste system. Our laboratory recently demonstrated that animals were able to respond to sodium without taste input during a period of salt deprivation. Using the Drosophila model, we have identified a discrete population of enteric neurons, termed INSO (Internal Sodium Sensing) neurons that function as a taste-independent, postprandial sodium sensor that directly detects sodium ions and mediates sodium intake (Kim et al., Nature Metabolism in press). Intriguingly, these neurons innervate their dendrites to the anterior portion of the intestine and project their axons to the brain. We therefore hypothesize that these neurons detect ingested sodium in the intestine and transmit signals to the brain, which subsequently regulates the appetite for salt. This study of the deprivation-induced sodium sensing mediated by INSO neurons might provide insights into the fervent human activities triggered by our carving for salt.

Vitamin C is another micronutrient that is essential for animals and humans, but our body does not produce it. As an antioxidant, vitamin C plays a key role in scavenging ROS that is often generated during a period of stress. We have inquired whether fruit flies or mice develop an appetite for vitamin C and other antioxidants under stress. In nature, many animals search for and consume a specific plant enriched in certain nutrients and metabolites when they are sick or deprived, or are in need of particular chemical compounds. Apparently, animals exhibit this behavior to alleviate the sickness. This phenomenon was coined “self-medication”, which was observed in insects such as butterflies and fruit flies, and even in primates such as chimpanzees. However, few studies have addressed this phenomenon, and its mechanism is poorly understood. Our laboratory is currently investigating whether fruit flies select antioxidants when stressed and elucidating the mechanism by which flies are attracted to the needed antioxidants, possibly through the action of interoceptive sensors present throughout the gut-brain axis.