A nonphotic stimulus inverts the diurnal-nocturnal phase preference in Octodon degus

Mechanisms differentiating diurnal from nocturnal species are thought to be innate components of the circadian timekeeping system and may be located d ownstream from the circadian pacemaker within the suprachiasmatic nucleus ( SCN) of the hypothalamus. In the present study, we round that the dominant phase of behavioral activity and body temperature (Tb) is susceptible to mo dification by a specific modality of behavioral activity (wheel-running act ivity) in Octodon degus, a mammal that exhibits multiple chronotypes. Seven Octodon degus exhibited diurnal Tb and locomotor activity (LMA) circadian rhythms while entrained to a 24 h light/dark cycle (LD 12:12). When the diu rnal animals were provided unrestricted access to a running wheel, the over t daily rhythms in these animals inverted to nocturnal. This nocturnal patt ern was sustained in constant darkness and returned to diurnal after remova l of the running wheel. Six additional animals exhibited nocturnal chronoty pes in LD 12:12 regardless of access to running wheels. Wheel-running activ ity inverted the phase preference in the diurnal animals without changing t he 24 hr mean LMA or Tb levels. Because wheel running did not increase the amplitude of the pre-existing diurnal pattern, simple masking effects on LM A and Tb cannot explain the rhythm inversion. The diurnal-nocturnal inversi on occurred without reversing crepuscular-timed episodes of activity, sugge sting that diurnal or nocturnal phase preference is controlled separately f rom the intrinsic timing mechanisms within the SCN and can be dependent on behavioral or environmental factors.