Hibernation is an adaptation of various species to cold temperatures and low food availability. In mammals, it involves complex physiological mechanisms which produce a significant decline in body temperature, metabolic rate, heart rate and respiration. During this deep hibernation, or torpor, the animal’s body temperature drops to nearly zero, and neuronal activity ceases almost completely.
Craig Heller and his colleagues at Stanford University investigated the microstructure of neurons in hibernating Arctic ground squirrels (Spermophilus lateralis). They first fixed brain tissue from squirrels at different stages of torpor, then, using a dye called Lucifer yellow, stained neurons from the hippocampus, cortex and thalamus.
In the current issue of the Journal of Neuroscience, Heller’s team report that they observed the retraction of cell bodies, dendrites and dendritic spines as the squirrels entered torpor. They found that there was a linear relationship between neural retraction and minimum body temperature, so that the lower the temperature, the greater the extent of neuronal retraction.
It was already known that torpor in ground squirrels is associated with changes in the morphology of hippocampal neurons. Heller and his team have shown that this temperature-dependent neuronal plasticity appears to occur throughout the brain. They also show that cells in all the brain regions examined regrow as the squirrel emerges from the torpor, and that neurons regain their morphology within 2 hours of body temperature returning to normal.
Previous work in Heller’s lab showed that there is little change in gene expression associated with hibernation in the ground squirrel. This is true even of genes that are known to be involved in sleep. However, the method they used only detected relatively large (two-fold) changes in mRNA levels, so it is possible that small changes in gene expression, which went undetected, play an important role.
The current work, which is also being presented at the 36th annual meeting of the Society for Neuroscience in Atlanta, Georgia, may help researchers explore the mechanisms of neuronal plasticity underlying learning and memory.