Scientists identify the switch that turns the brain off for sleep
Around 20 years ago, researchers led by Dr. Clifford B. Saper of the Beth Israel Deaconess Medical Center (BIDMC) identified a set of nerve cells that they believed to be responsible for switching the brain off and allowing it to sleep. Through a study on mice, the research team has now confirmed that that these cells are essential to normal sleep.
The investigation was focused on a small cluster of neurons situated in the anterior hypothalamus called the ventrolateral preoptic nucleus (VLPO).
“Our paper is the first test of what happens when you activate the VLPO neurons,” said Dr. Saper. “The findings support our original observation that the VLPO cells are essential to normal sleep.”
Dr. Saper’s team artificially activated the VLPO neurons in genetically engineered mice using several different tools. For example, in a process called optogenetics, the scientists activated the neuron cells using a laser light beam to make them fire.
The team also used a chemical to selectively activate the VLPO neurons. In both experiments, activating these cells caused sleep among the animals, confirming the earlier findings by the experts. The researchers found that these neurons are active during sleep and that damage to them causes insomnia.
“We found that when the VLPO cells are stimulated one to four times per second, they fire each time they are stimulated, resulting in sleep,” said Dr. Saper. “But if you stimulate them faster than that, they begin to fail to fire and eventually stop firing altogether. We learned our colleagues in the other lab were stimulating the cells 10 times per second, which was actually shutting them off.”
In addition, the researchers established that activating the VLPO cells caused a drop in body temperature. Scientists already knew that warm temperatures activate VLPO cells, and that body temperatures fall slightly during sleep – when the VLPO neurons are firing.
“We thought that this is why people need to curl up under a warm blanket to get to sleep,” said Dr. Saper.
The experts ultimately found, however, that body temperature in the mice fell by as much as five or six degrees Celsius upon continued activation of the cells. Dr. Saper’s team believes that prolonged firing of these same neurons may be responsible for the excessive sleep and drop in body temperature which occur in animals that hibernate.
The study is published in the journal Nature Communications.