When it comes to energy conservation, solar panels and bicycle commuting pale in comparison to hibernation. Hibernating animals limit energy-expensive processes, such as breathing and digestion, to help them squirrel away precious resources for the coming inactive season. Although some tropical animals hibernate to beat the heat, most temperate animals do it to endure the cold and chill out for months at a time in near-freezing temperatures. Preparation for the long energy hiatus includes bulking up, cooling down, and slowing down heart rate to fewer than six beats per minute. These peripheral changes are remarkable as they are extreme, but what's less clear is how central machinery, such as brain cells, are impacted by idleness.
A new report published in Current Biology by Lydia Hoffstaetter and colleagues in the collaborating labs of Slav Bagriantsev and Elena Gracheva at Yale University, USA, suggests that one central feature of hibernation is suppressed brain activity during dormant months in thirteen-lined ground squirrels. Squirrels hibernate for 8 months of the year and rapidly reanimate within hours during the active summer season, making them exemplar creatures to study.
To investigate how the nervous system changes during hibernation, Hoffstaetter measured how electrical properties differed between neurons of hibernating versus active squirrels. In particular, she focused on cells in the dorsal root ganglia near the spinal cord, which convey temperature, tactile and pain information to the brain, such as blistering cold conditions. Initially, Hoffstaetter found that neurons were similar between active and hibernating squirrels. For example, it took the same amount of prodding to elicit an electrical response from either hibernating or active neuronal cells, suggesting that brain activity and machinery are maintained during these periods of extreme cold and energy conservation.
However, clear differences emerged between active and hibernating cells once Hoffstaetter counted how often cells fired: neurons from hibernating squirrels fired at half the rate as their active counterparts. One reason may be due to voltage-gated sodium channels, which are required for neurons to fire. Careful follow-up recordings by Hoffstaetter determined that hibernating squirrels have greatly diminished sodium channel activity compared with active squirrels. Surprisingly, gene expression only partially explained this difference: hibernating cells expressed ∼20% less sodium channel genes than active cells, which is too modest a difference to explain the dramatic electrical differences. Therefore, a host of changes in addition to sodium channel expression must be altered to change how brain cells fire during torpor.
Taken together, these exciting findings suggest that despite hibernation seeming like a months-long nap marked by inactivity, squirrels, at least, have developed a way to keep their brains suppressed but ready for action when needed to save energy, thanks in part to sodium channels.