Method for studying behavioural activity patterns during long-term recordings using a force-plate actometer

The motor activity (MA) patterns of rodents are commonly detected in the laboratory using infrared photo-beams or running wheels. In chronobiological studies, the MA rhythm is considered as a behavioural output of the circadian pacemaker. This paper describes a method to obtain long-term records of MA in rodents, with a 1 mm spatial resolution and a 1 s temporal resolution. The device comprised a square platform laid on top of three force transducers, allowed the calculation of the coordinates of the centre of force exerted by a freely moving rodent, and continuously monitored its displacements. A specific computer program processed the trajectories, providing an exhaustive analysis of motion. To test this method, motor behavioural activity was studied in rats exposed to conditions that favoured rhythmicity: light–dark cycles of both 24 h (LD) and 22 h (T22), and constant darkness (DD). In addition, arrhythmicity patterns were studied under constant light (LL) conditions, and in animals with permanent lesions of the hypothalamic suprachiasmatic nucleus (SCNx). A single description of the general MA distribution at 24 h was obtained using infrared photo-beams. By contrast, under LD conditions, a 24-h rhythm with ultradian components was seen in the total distance travelled, whereas that detected in the number of slow motions showed less ultradian components. In addition, a regional preference in the nesting place was detected under both LD and DD conditions. In one SCNx animal and another exposed to LL conditions, both showing arrhythmicity in photo-beam-detected MA, a 24 h rhythm was observed in regional preference; other LL animal presented a 24 h rhythm in the number of slow motions. This method has capabilities lacking in commonly used techniques. The potential uses of this approach, principally in cronobiological studies, are discussed.