Perineuronal Net Expression in the Brain of an Obligate Hibernator

The brains of mammalian hibernators are naturally protected. Hibernating thirteen‐lined ground squirrels undergo rapid and extreme changes in body temperature and brain perfusion as they cycle between lengthy torpor bouts and brief periods of euthermia called interbout arousals (IBAs). Arousal from torpor to IBA occurs rapidly, but there is no evidence of brain injury accompanying this transition, despite the rapid and extreme change in physiology. Previous work investigating seasonal gene expression in the hypothalamus and cerebral cortex provided evidence for substantial regional differences in brain structural and functional regulation during hibernation. However, due to the extreme nature of the hibernation phenotype, protection must be a main priority for all brain regions. Interestingly, the hypothalamus showed a significant increase in aggrecan mRNA expression during hibernation, which is a main component of complex extracellular matrix structures called perineuronal nets (PNNs), which are known to be both protective. Hypothalamic function is important for hibernation and PNNs could help promote both protection and continued function during this time. Aggrecan expression was low in the cerebral cortex and did not show a seasonal change, suggesting that this change was specific to the hypothalamus. Here, we examined direct expression of PNNs in the thirteen‐lined ground squirrel brain using fluorescent Wisteria floribunda lectin histochemistry. Expression of PNNs was not found globally throughout the brain, but was found in discrete regions, including areas of the hypothalamus. However, PNN expression was also found throughout the cerebral cortex and other parts of the brain, including areas of the thalamus, epithalamus, and septum, indicating that PNNs are not specific to the hypothalamus. Additionally, PNNs were found in both hibernating (Torpor and IBA) and non‐hibernating (summer) animals, indicating that these extracellular matrix structures are present throughout the year. However, it is currently unknown if the structural components of the PNNs are the same in all brain regions or if these change seasonally. Overall, this work supports the idea that the extracellular matrix is contributing to neuroprotection during hibernation in the brain, but does not support the idea that the presence of PNNs is specific and unique to the hypothalamus during hibernation. Support or Funding Information This work is supported by UWL startup funding and a UWL Summer Faculty Research Grant to C.S.