An Inbred Lineage of Djungarian Hamsters with a Strongly Attenuated Ability to Synchronize

Abstract

An inbred lineage of Ph. sungorus was established at our institute showing some unusual characteristics of the circadian system that appear incompatible with an adequate adaptation to the periodic environment. We identified a hamster for which activity onset was delayed under light‐dark conditions (L:D=14∶10 h) by about 4 h in relation to the light‐dark transition. As the activity offset remained synchronized with the time of light‐on, the activity period (α) became compressed to 6 h. By means of a special breeding program, the percentage of animals showing such a phenomenon increased, indicating that it has a genetic component. Also, it is possible now to breed a larger number of hamsters to further investigate the rhythm deviations and the underlying mechanisms. Activity rhythms were investigated using passive infrared motion sensors. Whereas some of the hamsters showed a rather stable phase delay of activity onset relative to the onset of darkness, some animals progressively delayed their activity onset up to a critical, minimal length of α (3.03±0.02 h). Thereafter, the rest‐activity rhythm started to free‐run with a remarkably long period (τ=25.02 h) or became arrhythmic. Some hamsters showed several consecutive cycles alternating between a free‐running rhythm and entrainment, with increasing τ and reducing the phases of temporary entrainment. Finally, these hamsters became arrhythmic. The total amount of activity per day was similar in the wild type and delayed activity onset hamsters. The latter did increase the intensity of activity, thereby compensating for the shorter α. The period length in constant darkness was significantly longer in the delayed hamsters compared to wild type animals (24.37±0.03 h vs. 24.24±0.02 h; p<0.001). However, this difference seems too small to cause the later activity onset. The phase response following a light pulse (100 lux, 15′ at CT14 where CT12=activity onset) was similar in delayed and wild type hamsters (−1.66±0.12 h and −1.82±0.16 h). As access to running wheels is known to influence the circadian pacemaker, particularly to strengthen oscillator coupling, a set of further experiments was conducted. The free‐running period was significantly shorter when the hamsters were provided with running wheels (24.25±0.04 h and 24.07±0.04 h in wild type and delayed hamsters, respectively; p<0.005 and p<0.05). However, the effect on the activity onset was not unequivocal. In many hamsters it was still delayed, whereas in others the unlocking of the wheels led to an expansion of α. The described inbred lineage appears to be an excellent model to further investigate the properties and the interaction of the two oscillators underlying the daily activity pattern.

• Motor activity
• Circadian rhythm
• Free‐running period
• Phase response
• Two‐oscillator model