Cardiac Rhythms and Variation in Hibernating Arctic Ground Squirrels

Area of Science:

• Physiology
• Animal Behavior
• Chronobiology

Background:

• Heart rate (HR) decrease during hibernation entrance is regulated, preceding body temperature (Tb) drop.
• Increased cardiac parasympathetic activity is hypothesized to mediate HR decrease, while sympathetic activity drives HR increase during arousal.
• Temporal data on cardiac parasympathetic regulation throughout hibernation bouts are lacking.

Purpose of the Study:

• To investigate the temporal dynamics of cardiac parasympathetic regulation during hibernation in Arctic ground squirrels.
• To elucidate the role of parasympathetic nervous system in initiating and regulating heart rate changes during hibernation entrance and arousal.

Main Methods:

• Utilized Arctic ground squirrels implanted with electrocardiogram/temperature telemetry transmitters.
• Calculated short-term heart rate variability (root mean square of successive differences [RMSSD]) as a measure of cardiac parasympathetic regulation.
• Normalized RMSSD by RR interval (RRI) to assess changes throughout hibernation phases.

Main Results:

• Normalized RMSSD (RMSSD/RRI) increased significantly during early hibernation entrance, peaking after substantial HR and Tb drops.
• RMSSD/RRI declined during late entrance, indicating a shift in regulation.
• During arousal, HR increased before Tb rise, accompanied by a decrease in RMSSD/RRI, suggesting parasympathetic withdrawal.

Conclusions:

• Parasympathetic nervous system activation initiates and regulates HR decrease during hibernation entrance.
• Withdrawal of parasympathetic activation is a key factor initiating arousal from hibernation.
• Cardiac parasympathetic regulation is a continuous and integral component of the autonomic nervous system's control of hibernation.