INTERNAL TEMPORAL ORDER IN THE CIRCADIAN SYSTEM OF A DUAL-PHASING RODENT, THE OCTODON DEGUS

Sep 22, 2010Chronobiology international

Timing differences within the daily rhythm system of a rodent with two activity phases, the Octodon degus

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Abstract

No significant differences in the timing of most biological variables were found between diurnal and nocturnal degus despite differences in their activity patterns.

The Octodon degus can switch activity patterns from day to night based on environmental conditions.
A total of 26 variables, including behavioral and biochemical factors, were analyzed to assess phase relationships in the degus.
Differences in wheel-running activity and body temperature rhythms were noted between the two chronotypes.
Only variations in plasma urea levels and lymphocyte counts were observed between diurnal and nocturnal degus.
A slight delay in the melatonin rhythm phase was detected, indicating some temporal adjustment.
The results suggest that the internal timing mechanisms related to activity and body temperature do not fully invert with changes in activity patterns.

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Daily rhythms in different biochemical and hematological variables have been widely described in either diurnal or nocturnal species, but so far no studies in the rhythms of these variables have been conducted in a dual-phasing species such as the degus. The Octodon degus is a rodent that has the ability to switch from diurnal to nocturnal activity under laboratory conditions in response to wheel-running availability. This species may help us discover whether a complete temporal order inversion occurs parallel to the inversion that has been observed in this rodent's activity pattern. The aim of the present study is to determine the phase relationships among 26 variables, including behavioral, physiological, biochemical, and hematological variables, during the day and at night, in diurnal and nocturnal degus chronotypes induced under controlled laboratory conditions through the availability of wheel running. A total of 39 male degus were individually housed under a 12:12 light-dark (LD) cycle, with free wheel-running access. Wheel-running activity (WRA) and body temperature (Tb) rhythms were recorded throughout the experiment. Melatonin, hematological, and biochemical variables were determined by means of blood samples obtained every 6 h (ZT1, ZT7, ZT13, and ZT19). In spite of great differences in WRA and Tb rhythms between nocturnal and diurnal degus, no such differences were observed in the temporal patterns of most of the biological variables analyzed for the two chronotypes. Variation was only found in plasma urea level and lymphocyte number. A slight delay in the phase of the melatonin rhythm was also observed. This study shows the internal temporal order of a dual-phasing mammal does not show a complete inversion in accordance with its activity and body temperature pattern; it would appear that the switching mechanism involved in the degu's nocturnalism is located downstream from the pacemaker.

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INTERNAL TEMPORAL ORDER IN THE CIRCADIAN SYSTEM OF A DUAL-PHASING RODENT, THE OCTODON DEGUS

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