Distinct Networks of Leptin- and Insulin-Sensing Neurons Regulate Thermogenic Responses to Nutritional and Cold Challenges

Defense of core body temperature (Tc) can be energetically costly; thus, it is critical that thermoregulatory circuits are modulated by signals of energy availability. Hypothalamic leptin and insulin signals relay information about energy status and are reported to promote thermogenesis, raising the possibility that they interact to direct an appropriate response to nutritional and thermal challenges. To test this idea, we used an Nkx2.1-Cre driver to generate conditional knockouts (KOs) in mice of leptin receptor (L2.1KO), insulin receptor (I2.1KO), and double KOs of both receptors (D2.1KO). L2.1KOs are hyperphagic and obese, whereas I2.1KOs are similar to controls. D2.1KOs exhibit higher body weight and adiposity than L2.1KOs, solely due to reduced energy expenditure. At 20–22°C, fed L2.1KOs maintain a lower baseline Tc than controls, which is further decreased in D2.1KOs. After an overnight fast, some L2.1KOs dramatically suppress energy expenditure and enter a torpor-like state; this behavior is markedly enhanced in D2.1KOs. When fasted mice are exposed to 4°C, L2.1KOs and D2.1KOs both mount a robust thermogenic response and rapidly increase Tc. These observations support the idea that neuronal populations that integrate information about energy stores to regulate the defense of Tc set points are distinct from those required to respond to a cold challenge.