Motor asymmetries have been recognized in animals and human beings. They are caused by a differential development of some brain areas, especially the nigrostriatal dopaminergic system. The anatomical and functional differences between the two brain sides cause a consistent motor asymmetry in rats, which is easily quantified by using a rotating behaviour test. The results of this test are considered as defining the degree of asymmetry for a given animal, with results classified as having either low or high asymmetry but also as showing either right or left rotating preference. This classification is considered to remain constant for each individual. However, this is normally achieved after a relatively short study, with the animals placed in the rotometer for only a few hours.
On the other hand, there are results showing that the handedness in human beings can change in relation to the activity states, with a particular subject showing right hand dominance during day time (as measured by bilateral wrist actimetry), while this can change during sleep. It has also been shown that there are bilateral differences in the circadian activity of the suprachiasmatic nucleus, reaching a phase shift even of over 100 min between the two sides. These results are of interest because they can provide a basis to explain the extreme asymmetry found during the sleep of aquatic animals.
The present report aims at measuring the rotating asymmetry of rats during a relatively long time span (over 48 h). This should be enough to give indications on whether the side preference is constant for a given animal or, alternatively, the preference changes along the day.
Eight Wistar male rats weighing between 250-400 g were placed in a computerized rotometer consisting of a 30 cm diameter circular arena. The rotations of the animals were fed to a computer which stored both the movements and the time in which each movement took place. Commercial food pellets and tap water were freely available in the rotating chamber during the tests. The study was performed under approval of the Ethical Committee of the University of Balearic Islands for animal experimentation.
As a result, five animals showed sustained epochs with shifted side preference, covering at least 30 % of the recorded time. In most cases, the preference shift occurred within 4 h after lights off. The remaining three animals showed a constant asymmetry towards one side only. This suggests that the shifts in side prefference for rotation are related to the circadian rhythm.
As a conclusion, it has been found that the side preference of a high proportion of rats suffers important changes when measured during an extended period of time.
This study was supported by FIS grant 00/0022-01.