The parasympathetic nervous system and its influence on heart rate in torpid western pygmy possums, Cercatetus concinnus (Marsupialia: Burramyidae)

We investigated the effect of parasympathetic antagonism on the patterns of heart rate during torpor in the western pygmy possum Cercatetus concinnus (Marsupialia: Burramyidae). This is the first study to examine the influence of the autonomic nervous system on cardiac function in a metatherian hibernator. During torpor, antagonism of the parasympathetic nervous system eliminated the ventilatory tachycardia, variability in instantaneous heart rate, and increased the overall heart rate. These findings are consistent with previous studies on other mammalian heterotherms, which have shown that the parasympathetic nervous system is responsible for these patterns in heart rate. During extended bouts of torpor (2 to 3 days) the ventilatory tachycardia persisted throughout each bout, which indicates that the parasympathetic nervous system remained functional during that time. It has been suggested that the progressive removal of autonomic tone is characteristic of deep steady-state hibernation. There is no evidence to suggest that such a state was going to be reached in the possums in this study. To date there is little evidence that clearly demonstrates a physiological basis for the distinction between shallow, daily torpor and deep hibernation.