Distinct neural representations of hunger and thirst in neonatal mice before the emergence of food- and water-seeking behaviors.

Hunger and thirst are two fundamental drives for maintaining homeostasis and elicit distinct food- and water-seeking behaviors essential for survival. For neonatal mammals, however, both hunger and thirst are sated by consuming milk from their dams. While distinct neural circuits underlying hunger and thirst drives in the adult brain have been characterized, it is unclear when these distinctions emerge in neonates and what processes may affect their development. Here, we show that hypothalamic hunger and thirst regions already exhibit specific responses to starvation and dehydration well before a neonatal mouse can seek food and water separately. At this early age, hunger neurons drive feeding behaviors more than thirst neurons do. In vivo Neuropixels recordings revealed that maternal presentation leads to a relative increase in activity in dehydrated and starved neonatal mice, which is suppressed by feeding on short timescales, particularly in hypothalamic and thalamic neurons. Activity changes become more heterogeneous on longer timescales. Lastly, within neonatal regions that respond to both hunger and thirst, subpopulations of neurons respond distinctly to one or the other need. Combining food and water into a liquid diet throughout the animal's life does not alter the distinct representations of hunger and thirst in the adult brain. Thus, neural representations of hunger and thirst in mice become distinct before food- and water-seeking behaviors mature and are robust to environmental changes in food and water sources.