The microtubule-associated protein tau, in its hyperphosphorylated form, is the major component of paired helical filaments and other aggregates in neurodegenerative disorders commonly referred to as "tauopathies". Recent evidence, however, indicates that mislocalization of hyperphosphorylated tau to subsynaptic sites leads to synaptic impairment and cognitive decline even long before formation of tau aggregates and neurodegeneration occur. A similar, but reversible hyperphosphorylation of tau occurs under physiologically controlled conditions during hibernation. Here, we study the hibernating Golden hamster (Syrian hamster, Mesocricetus auratus). A transient spine reduction was observed in the hippocampus, especially on apical dendrites of hippocampal CA3 pyramidal cells, but not on their basal dendrites. This distribution of structural synaptic regression was correlated to the distribution of phosphorylated tau, which was highly abundant in apical dendrites but hardly detectable in basal dendrites. Surprisingly, hippocampal memory assessed by a labyrinth maze was not affected by hibernation. The present study suggests a role for soluble hyperphosphorylated tau in the process of reversible synaptic regression, which does not lead to memory impairment during hibernation. We hypothesize that tau phosphorylation associated spine regression might mainly affect unstable/dynamic spines while sparing established/stable spines.

Reversible formation of PHF-like phosphorylated tau, an early feature of Alzheimer’s disease (AD) was previously shown to occur in torpor during hibernation in the Golden hamster (Syrian hamster, Mesocricetus auratus ). Here, we tackled the question to what extent hibernating Golden hamsters can serve as a model for the early stage of AD. During early AD, anosmia, the loss of olfactory function, is a common and typical feature. We, thus, investigated tau phosphorylation, synaptic plasticity and behavioral physiology of the olfactory system during hibernation. Tau was phosphorylated on several AD-relevant epitopes, and distribution of PHF-like phosphorylated tau in the olfactory bulb was quite similar to what is seen in AD. Tau phosphorylation was not associated with a destabilization of microtubules and did not lead to fibril formation. Previously, we observed a transient spine reduction in pyramidal cells in the hippocampus, which is correlated with the distribution of phosphorylated tau. Here we show that granule cells in the olfactory bulb are devoid of phosphorylated tau and maintain their spines number during torpor. No reduction of synaptic proteins was observed. However, hibernation did impair the recall performance in a two-odor discrimination task. We conclude that hibernation is associated with a specific olfactory memory deficit, which might not be attributed to the formation of PHF-like phosphorylated tau within the olfactory bulb. We discuss a possible involvement of modulatory input provided by cholinergic neurons in the basal forebrain, which are affected by hibernation.