Meeting Abstract
In mammals, the master circadian clock, located in the suprachiasmatic nucleus (SCN) of the hypothalamus, is principally entrained to 24h light/dark (LD) cycles, but this cue is seasonally absent in polar environments. Although it has been argued that weak circadian clocks may be a general characteristic of resident polar herbivores, the arctic ground squirrel (AGS) maintains daily rhythms of body temperature (Tb) and behavior throughout its active season which includes six weeks of constant daylight. In this study, we show that while wild-caught AGS remain entrained to a square-wave LD cycle that is in phase with their circadian clocks, their activity and Tb rhythms free-run (tau < 24h) when exposed to successive 6h phase-delayed LD cycles. Following their release from captivity back into the field, AGS with circadian clocks that are 12h out-of-phase with geophysical time rapidly resynchronize (within 48-72 h) their Tb and activity rhythms with their environment under natural conditions of constant daylight. AGS appear to not show “jet lag”, the slow realignment of physiological and behavioral rhythms induced by the inertia of a circadian master clock that is intrinsically stable. This may be associated with the low expression of arginine vasopressin (AVP) that we have measured in the SCN of AGS, since AVP is associated with inter-neuronal coupling. Thus, AGS circadian rhythms appear to be highly sensitive to entrainment by relatively subtle parametric changes in the intensity or color of light, yet insensitive to rapid and much more pronounced transitions between light and dark, which they generate naturally due to their semi-fossorial nature.
