Circadian rhythm of outside-nest activity in wild (WWCPS), albino and pigmented laboratory rats

Abstract

The domestication process of the laboratory rat has been going on for several hundred generations in stable environmental conditions, which may have affected their physiological and behavioural functions, including their circadian system. Rats tested in our ethological experiments were laboratory-bred wild Norway rats (WWCPS), two strains of pigmented laboratory rats (Brown Norway and Long Evans), and two strains of albino rats (Sprague-Dawley and Wistar). Rats were placed in purpose-built enclosures and their cycle of activity (time spent actively outside the nest) has been studied for one week in standard light conditions and for the next one in round-the-clock darkness. The analysis of circadian pattern of outside-nest activity revealed differences between wild, pigmented laboratory, and albino laboratory strains. During daytime, albino rats showed lower activity than pigmented rats, greater decrease in activity when the light was turned on and greater increase in activity when the light was switched off, than pigmented rats. Moreover albino rats presented higher activity during the night than wild rats. The magnitude of the change in activity between daytime and nighttime was also more pronounced in albino rats. Additionaly, they slept outside the nest more often during the night than during the day. These results can be interpreted in accordance with the proposition that intense light is an aversive stimulus for albino rats, due to lack of pigment in their iris and choroid, which reduces their ability to adapt to light. Pigmented laboratory rats were more active during lights on, not only in comparison to the albino, but also to the wild rats. Since the difference seems to be independent of light intensity, it is likely to be a result of the domestication process. Cosinor analysis revealed a high rhythmicity of circadian cycles in all groups.

Figure 1. Experimental areas: 1 and 2 – separate boxes for female and male rats; A – two-level shelf; B – water dispensers; C – ladder; D – feeding bowl.

Figure 2. Long Evans rats during their activity outside the nest.

An example of the pictures taken by the camera.

Figure 3. Daily activity of different rat strains.

Figure 4. Actogram plots of outside-nest activity rhythm of different strains of rats analysed with 5-min resolution for 13 consecutive days.

As indicated by the vertical side bars, a light dark-cycle was in effect for the first 7 days (LD cycle) with lights on at 8 a.m. and off at 9 p.m., whereas for the following 6 days darkness was continuous (DD cycle). The height of bars reflects percentage of rats being active outside the nest.

Figure 5. Mean percentage of rats active during the day and night by strain.

Figure 6. Mean percentage of rats of each strain sleeping or resting outside the nest.