Dorsomedial and preoptic hypothalamic circuits control torpor

Hydrogen peroxide in midbrain sleep neurons regulates sleep homeostasis

Sleep could protect animals from oxidative damage, yet the dynamic interplay between the redox state and sleep homeostasis remains unclear. Here, we show that acute sleep deprivation (SD) in mice caused a general increase in brain oxidation, particularly in sleep-promoting regions. In vivo imaging of intracellular hydrogen peroxide (H2O2) real-time dynamics revealed that in nigra sleep neurons, the increase in cytosolic but not mitochondrial H2O2 reflects sleep debt and tracks spontaneous wakefulness by positively correlating with wake duration. By controllably manipulating intraneuronal H2O2, we discovered that H2O2 elevation is required for compensatory sleep and causally promotes sleep initiation, at least partly dependent on transient receptor potential melastatin 2 (TRPM2) channel. However, excessive H2O2 induced brain inflammation and sleep fragmentation. Together, our study demonstrates intraneuronal H2O2 as a crucial signaling molecule that translates brain redox imbalance into sleep drive and underscores the significance of oxidative eustress in sleep homeostasis.

The Art of Chilling Out: How Neurons Regulate Torpor

Endothermic animals expend significant energy to maintain high body temperatures, which offers adaptability to varying environmental conditions. However, this high metabolic rate requires increased food intake. In conditions of low environmental temperature and scarce food resources, some endothermic animals enter a hypometabolic state known as torpor to conserve energy. Torpor involves a marked reduction in body temperature, heart rate, respiratory rate, and locomotor activity, enabling energy conservation. Despite their biological significance and potential medical applications, the neuronal mechanisms regulating torpor still need to be fully understood. Recent studies have focused on fasting-induced daily torpor in mice due to their suitability for advanced neuroscientific techniques. In this review, we highlight recent advances that extend our understanding of neuronal mechanisms regulating torpor. We also discuss unresolved issues in this research field and future directions.

The growing complexity of the control of the hypothalamic pituitary thyroid axis and brown adipose tissue by leptin

The balance between food intake and energy expenditure is precisely regulated to maintain adipose stores. Leptin, which is produced in and released from adipose in direct proportion to its size, is a major contributor to this control and initiates its homeostatic responses largely via binding to leptin receptors (LepR) in the hypothalamus. Decreases in hypothalamic LepR binding signals starvation, leading to hunger and reduced energy expenditure, whereas increases in hypothalamic LepR binding can suppress food intake and increase energy expenditure. However, large gaps persist in the specific hypothalamic sites and detailed mechanisms by which leptin increases energy expenditure, via the parallel activation of the hypothalamic pituitary thyroid (HPT) axis and brown adipose tissue (BAT). The purpose of this review is to develop a framework for the complex mechanisms and neurocircuitry. The core circuitry begins with leptin binding to receptors in the arcuate nucleus, which then sends projections to the paraventricular nucleus (to regulate the HPT axis) and the dorsomedial hypothalamus (to regulate BAT). We build on this core by layering complexities, including the intricate and unsettled regulation of arcuate proopiomelanocortin neurons by leptin and the changes that occur as the regulation of the HPT axis and BAT is engaged or modified by challenges such as starvation, hypothermia, obesity, and pregnancy.

Neural circuits of long-term thermoregulatory adaptations to cold temperatures and metabolic demands

The mammalian brain controls heat generation and heat loss mechanisms that regulate body temperature and energy metabolism. Thermoeffectors include brown adipose tissue, cutaneous blood flow and skeletal muscle, and metabolic energy sources include white adipose tissue. Neural and metabolic pathways modulating the activity and functional plasticity of these mechanisms contribute not only to the optimization of function during acute challenges, such as ambient temperature changes, infection and stress, but also to longitudinal adaptations to environmental and internal changes. Exposure of humans to repeated and seasonal cold ambient conditions leads to adaptations in thermoeffectors such as habituation of cutaneous vasoconstriction and shivering. In animals that undergo hibernation and torpor, neurally regulated metabolic and thermoregulatory adaptations enable survival during periods of significant reduction in metabolic rate. In addition, changes in diet can activate accessory neural pathways that alter thermoeffector activity. This knowledge may be harnessed for therapeutic purposes, including treatments for obesity and improved means of therapeutic hypothermia.

Torpor: A whole-brain view of the underlying neural network

Torpor is a physiological state with substantial translational implications. In recent years, several brain regions responsible for controlling torpor have been pinpointed. The latest research, employing comprehensive whole-brain anatomical analysis, offers insights into a broader network responsible for regulating torpor.