Seasonal changes in the balance of brain monoamines of hibernating long-tailed ground squirrels (Urocitellus undulatus).

Hibernation is a unique strategy developed during evolution, enabling animals to survive periods of extreme conditions and low resource availability. Hibernation is associated with such medically valuable phenomena as increased resistance to stress and disease, occurring under the strict control of the CNS and hormonal regulations. However, the contribution of monoamines in the brain of ground squirrel serotonin, dopamine, and norepinephrine to the brain modulation in different functional states of heterothermic animals in their annual life cycle has not been sufficiently studied. We investigated the seasonal changes in the level of monoamines in the hypothalamus, hippocampus, and frontal cortex of long-tailed ground squirrel Urocitellus undulatus. The following monoamines and their metabolites were detected: (1) NE - norepinephrine; (2) ST-Group - group of serotonin including 5HT and 5-HIAA; (3) DA-Group - group of dopamine including DA, DOPAC, HVA, and 3-MT. It was found that the ST-Group was dominant in the Summer/Fall season change. A reciprocal balance of monoamines with NE dominance was observed when comparing the Fall/Torpor, while the proportion of the DA-Group remained unchanged. The paper also discusses the correlation between the monoamines balance and exploratory behavior, measured in standard tests as the number of squares crossed, rearing, and head deeps into holes. It is suggested that the observed reversible, seasonally determined imbalance of monoamines in hibernating animals may provide significant insights for investigating brain functioning in both healthy and pathological states, including medically recognized brain disorders.