The characteristics of the expression of the Cdk1 and Cyclin B1 Proteins in the brain of the Yakut ground squirrel (Spermophilus undulatus) at different stages of the hibernation cycle

We performed a comparison of the characteristics of two cell-cycle proteins, viz., Cdk1 and Cyclin B1 (CycB1), in different brain regions of Yakut long-tailed ground squirrels (Spermophilus undulatus) at various stages of their hibernation cycle. The experiments were performed in the winter period (January to February) in four groups of animals that weighed 600–800 g, viz., animals entering hibernation (n = 9, brain T = 10°C); ground squirrels on the 6th–7th day of hibernation (n = 9, brain T = 1–2°C); animals that were awakened in the middle of hibernation at a brain temperature of 10°C (n = 6), and animals that were awakened at a brain temperature of 31–32°C (n = 8). The control group consisted of summer active animals (n = 8, brain T = 37–38°C) that were taken in mid-June to the beginning of July. We analyzed the mRNA and protein levels of Cdk1 and Cyclin B1 (CycB1) in the frontal cortex, hippocampus, cerebellum, and the caudal part of the brainstem. Our results indicate that the peak expression of Cdk1 mRNA in the frontal cortex, hippocampus, cerebellum, and caudal part of the brainstem of the hibernating animals occurred at different stages of their annual cycle, thus indicating region-specific regulation of Cdk1 mRNA in the brains of hibernating animals. A decreased Cdk1 protein level during hibernation (brain T = 1–2°C) in the hippocampus and the cerebellum, as compared to both the summer active phase (brain T = 37–38°C) and the late entrance into the hibernation phase (brain T = 10°C) indicates that the morphological plasticity of these structures is reduced in hibernating animals. Despite the overall low mRNA expression of Cdk1 and CycB1, the frontal cortex is characterized by synchronously high levels of Cdk1 (brain T = +10°C, p = 0.066) and CycB1 (brain T = 31–32°C, p = 0.014) proteins as compared to the hibernation period (brain T = 1–2°C). The Spearmen correlation between Cdk1 and CycB1 mRNA expression in the frontal cortex among all groups was p = 0.001, R = 0.65; in the cerebellum among all groups p = 0.010, R = 0.52. An increase in Cdk1 expression in the cortex and hippocampus of ground squirrels during a brain temperature increase from 1–2°C to 31–32°C reflects the development of morphological functional rearrangements that are crucial for either the development of new interneuronal connections or for restoration of old ones that are necessary for the integrative activity of the brain during dramatic changes in its functional state in the period of transition of the animals from hibernation to wakefulness.