Aim: 

Octodon degus is a dual phasing rodent with the ability to switch from diurnal to nocturnal activity in response to wheel running exercise. The mechanisms for this switching are not clear. The present study aims to determine whether this activity shift triggers a complete inversion in the internal circadian order. To this, day-night levels of 26 biochemical and haematological variables in diurnal and nocturnal degus were determined.

Methods: 

A total of 17 diurnal and 11 nocturnal degus (26-32 months old, males), were individually housed with free wheel running access under LD 12:12 cycle (lights on at 08:00 h). Wheel running activity and body temperature rhythms were recorded throughout the experiment. Haematological and biochemical variables were determined in blood samples obtained every six hours (09:00 h, 15:00 h, 21:00 h, 03:00 h).

Results: 

In spite of great differences in wheel running activity and temperature rhythms between nocturnal and diurnal degus, most variables of nocturnals remained unchanged when compared to diurnals. Differences between chronotypes were only found in melatonin rhythm, urea blood levels and lymphocyte number at mid-light.

Conclusion; For the first time it has been demonstrated that the internal temporal order of a dual phasing mammal does not show a complete inversion in response to their activity pattern and body temperature switch. This result points out that switching mechanism should be located donwstream from master pacemaker.

This project was funded by Seneca Foundation (PI/05700/07), by the Instituto de Salud Carlos III (RETICEF, RD06/0013/0019), and by the Ministry of Education and Science (BFU2007-60658/BFI).