Circadian Rhythms, Neuroanatomy Of The Superchiasmatic Nucleus And Selective Breeding Of The Northern Red -Backed Vole (Clethrionomys Rutilus)

Abstract

The experiments performed in this thesis investigated the circadian rhythms and neuroanatomy of a subarctic rodent, the northern red-backed vole ( Clethrionomys rutilus). Arctic and subarctic light regimes are extreme, with long periods of light and dark and large daily changes in day-length, but very little is known about circadian rhythms of mammals at high latitudes. A colony of C. rutilus was established and proper husbandry techniques were developed to allow voles to reproduce in captivity. Wild-caught and laboratory reared animals were tested for circadian rhythms in a 16:8 hour light:dark (LD) cycle, constant dark (DD) and constant light (LL). Voles displayed predominantly nocturnal patterns of wheel-running in 16:8 LD. In LL and DD, animals displayed large phenotypic variation in circadian rhythms with many becoming non-circadian (60% in DD, 72% in LL), indicating highly labile circadian organization. The distributions of eight common neurotransmitters in the suprachiasmatic nucleus (SCN), the brain's master circadian clock, were characterized. The SCN of C. rutilus is similar to that found in other rodents. Larger quantities of cholecystokinin and neuropeptide Y are found in the SCN of C. rutilus pointing to the possible importance of non-photic cues in resetting the phase of the internal clock. An additional study also found a distinct distribution of Substance P fibers and neurokinin-1 receptors in the SCN of C. rutilus. Starting with the 5th generation, laboratory-bred voles were selectively bred to create two lines of voles that maintained a circadian rhythm in DD, two lines that lost their circadian rhythm in DD, and a randomly bred control line. After three additional generations no significant differences were found among the lines due to the variability in the response to selection over the first few generations. With a large phenotypic variation in circadian wheel-running rhythms and an SCN similar to other rodents studied, C. rutilus is an ideal candidate to study subarctic circadian adaptations. Continued selective breeding will develop a useful tool for elucidating natural genetic variation in circadian rhythm characteristics in a subarctic mammal.