Lots of animals hibernate, slumbering through the winter months to wake up in the spring when the weather is a little warmer and food is more plentiful. But most animals don't just sleep the entire time. They are constantly waking up for brief stints throughout their hibernation, rewarming their bodies for a few hours only to slow down their metabolism and lower their body temperature again for days. However, the timing of this cycle (called a torpor–arousal cycle), and what causes it, remains a mystery to scientists. Could this just be something that happens on a cycle every couple of days? As Fredrik Markussen, Fernando Cázarez-Márquez, Vebjørn Melum, David Hazlerigg and Shona Wood of The Arctic University of Norway discovered, there are cells in the brain that activate when these periods of arousal begin – at least for golden hamsters (Mesocricetus auratus). But if their arousals are spontaneous, how can scientists anticipate when a hamster is going to wake up?
In order to solve this problem, the researchers needed to make the small rodents hibernate. First, they turned the lights off earlier for a few weeks to simulate the longer nights of winter; next, they also turned down the thermostat from 21°C to 7°C, making the animals think it was time to start hibernating. After 12 weeks of cooler temperatures, 93% of the hamsters were hibernating. But now came the challenging part: figuring out a way to track the hamster's body temperature so that they would know when the animals were about to wake up. At this point, they could narrow down when the animals would wake up to a period of ∼10 h, so Markussen implanted a tiny temperature sensor into the hamster's brown fat between the shoulder blades that would alert the team when the body temperature started rising.
Now that the researchers could reliably predict when a hamster would spontaneously wake up, they turned their attention to discovering which cells in the brain were active at different phases of the cycle. When the scientists received an alert that a hamster was waking up, they would rush to the lab so they could measure the RNA expression levels of a gene called c-fos, which is an indicator of a cell's activity: the higher the activity of the cell, the more c-fos it expresses. The team found that parts of the brain that control a hamster's body temperature were only slightly active during the torpor portion of their cycle, but 65% of the cells in a part of the brain that produces cerebrospinal fluid – called the choroid plexus – were active when the animals were slumbering. Shockingly, even more of these cells (93% of them) were active when the hamsters first started to arouse. This suggests that the animals may be sensing a signal from one of the other internal organs that is causing them to wake up from their torpor.
When the scientists went looking for more brain regions that were active during torpor, they discovered that the tanycytes were very active when the animals first started to come out of torpor. Tanycytes coordinate the physiological changes that accompany the changing of the seasons in mammals. Not only that, but they also sense nutrients in the blood and cerebrospinal fluid. This means that there is a link between the brain and what is happening in the blood of the hamsters. Markussen and colleagues are still uncertain what these cells are sensing in the blood and cerebrospinal fluid, but their hard work has made the quest to discover what makes animals arouse from their torpor a little easier.
