Perspective: rheostasis revisited-hibernation and tanycytes.

Mammalian hibernation is a physiological and behavioural adaptation that permits survival during seasonal periods of energy shortage via a combination of pre-hibernal energy storage and hibernal metabolic depression (torpor). There is both seasonal preparation for the expression of torpor, and the spontaneous termination of hibernation at the end of the season. Small hibernating mammals repeatedly alternate between the torpid state, and the interbout euthermic state over a relatively short timescale (days-weeks) for the entire hibernation season. This is known as torpor arousal cycling (T-A cycling). Hibernation is therefore characterised by extreme shifts in energy homeostasis. Rheostasis is term referring to a change in a regulated homeostatic level or set point. Hibernation can be viewed as rheostasis both over the annual timescale of the seasonal hibernation cycle and over the much shorter T-A cycle. The brain sites through which these homeostatic shifts are controlled have not been identified. A specialised glial cell type lining the 3rd ventricle of the mediobasal hypothalamus (MBH tanycytes), are of particular interest. MBH tanycytes have a privileged anatomical position contacting the periphery and the hypothalamic control centres of the brain. They have documented sensing and signalling function within the hypothalamus, making them a strong candidate cell type for the control of energy homeostasis. Here, I propose that the MBH tanycytes could act as a "rheostat", shifting their sensitivity to metabolic feedback over the annual timescale and the T-A cycle, and therefore are a promising cell type to investigate in relation to the brain control of hibernation.