Aim: 

Octodon degus is a Chilean rodent capable of shifting its activity phase preference from diurnal to nocturnal in response to wheel running availability. It has been hypothesized that this ability is the result of the hierarchical interaction between the circadian pacemaker and light masking effect induced by wheel running activity. The aim of this work is to dissociate this light masking effect from the circadian pacemaker to evaluate its influence in nocturnal/diurnal phase preference in degus.

Methods: 

Sixteen male degus (16 months old), individually housed, were subjeted to 14:14 LD cycle (28h T-cycle) with and without wheel running availability. Body temperature and wheel running activity were continuously recorded.

Results: 

When degus with wheel running were subjected to a 28h T-cycle, two different circadian expressions according to night activity preference were observed. While diurnal degus presented only one free-running circadian component for wheel running and body temperature (mean period= 14503 min), nocturnal-active animals showed a second component (mean period around 1680 min) due to light masking effect. When wheel running was locked all animals displayed only the free-running component.

Conclusions: 

Wheel running availability in Octodon degus induces two extremes temporal niches (diurnal and nocturnal with a continuous gradient of intermediate chronotype expressions). While circadian rhythmicity in diurnal degus is completely generated by the circadian pacemaker, in nocturnal animals, wheel running induces a negative light masking effect. This dissociation of the degus circadian system can be a useful model to study internal desynchronization in humans.

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