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Recent theories on the function of arousals from torpor in hibernating mammals focus on the repair of the central nervous system from damage accumulating during prolonged hypothermia. In this framework, we investigated the synaptic ultrastructure in Layer 2 of the frontal cortex from hibernating European ground squirrels (Spermophilus citellus) sacrificed at four different phases in the torpor-arousal cycle. Using electron microscopy, we quantified synapse number and morphometric data on asymmetric axospinous synapses. Length, width, and surface area of postsynaptic densities (PSDs), and the synaptic apposition length of the analyzed synapse were measured. Five groups of animals were compared during entrance into torpor (Torpor Early, TE, n = 6), late torpor (Torpor Late, TL, n = 5), beginning of euthermic arousal episodes (Arousal Early, AE, n = 5), late in the euthermic arousal episode (Arousal Late, AL, n = 5), and during continuous euthermy in spring (EU, n = 6). The results showed that during torpor and at the beginning of arousals the PSD length and synaptic apposition length are significantly increased compared to synapses during late arousal and in spring conditions. In contrast, the width and surface area of the PSDs are decreased in torpor. At the beginning of an arousal the width of the PSD increases and gains maximum value in late arousals (AL), returning to spring (EU) values. No differences were found in total number of synapses during the torpor-arousal cycle. The results indicate reversible changes in ultrastructure of (asymmetric axospinous) synapses in the frontal cortex, which may be critical for the maintenance of cortical neuronal networks and for protection against potential deleterious effects of prolonged hypothermic phases of hibernation.


J Ruediger, E A Van der Zee, A M Strijkstra, A Aschoff, S Daan, R A Hut. Dynamics in the ultrastructure of asymmetric axospinous synapses in the frontal cortex of hibernating European ground squirrels (Spermophilus citellus). Synapse (New York, N.Y.). 2007 May;61(5):343-52

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PMID: 17318885

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