Seasonal changes in actin and Cdk5 expression in different brain regions of the Yakut ground squirrel (Spermophilus undulatus)

In this study we described the seasonal profile of the actin and Cdk5 levels in different brain structures of Yakut long-tailed ground squirrels (Spermophilus undulatus) during their annual cycle. Experiments were performed with adult Yakut ground squirrels, both male and female, weighing 600–800 g (n = 35) at different stages of their annual cycle, viz., in the summer, the period of the maximum activity of the animals; in the autumn, during preparation for hibernation; in the winter, during hibernation; in the spring, at the exit of the animals from the hibernating state. Our results indicate that actin mRNA increased by 1.9 times (p = 0.0001) in the frontal cortex, the hippocampus, and the caudal brainstem during hibernation. In the brainstem, a significant increase in actin mRNA started to develop in autumn, in normothermic animals at the stage of their preparation for hibernation (p = 0.0078). At the exit of animals from the torpid state, the level of hippocampal expression decreased significantly by 4.5 times; in the cortex and brainstem it decreased to the level of summer animals. In contrast, the dynamics in the cerebellum had opposite direction: actin mRNA level decreased significantly during the preparation for hibernation (p = 0.037), remained low in torpid animals (p = 0.051), and increased after awakening. The changes in the total protein level were observed only in the hippocampus, along with increased expression of Cdk5 mRNA during hibernation (p = 0.003) and at the exit from it (p = 0.001). Detected differences in the seasonal metabolic profile of cytoskeleton proteins in the hippocampus of Yakut long-tailed ground squirrels support a substantial structural plasticity of this brain structure during the hibernation cycle that was described previously in morphological and biochemical studies.