Light responses of the circadian system in leptin deficient mice

Some evidences postulate a link between obesity and disturbances in circadian behavior. Here, we studied the manifestation of the circadian rhythm of motor activity and its response to light in the leptin deficiency model of obesity ob/ob mice. Motor activity in both ob/ob and wild type mice was first recorded in a small cage by activity meters with crossed infrared beams (IR) and later in a larger cage with a running wheel, where the number of wheel revolutions (WR) was also determined. Animals were maintained under light–dark (LD) or constant-dark (DD) conditions. We studied the free-running period and the rhythm profile, with special emphasis on the amount of activity in the dark and light phases of the LD cycle, and the phase and period responses to a light pulse. The results showed that ob/ob mice have a strong ultradian, rather than a circadian pattern, whose period range between 3 and 4.8 h. Also, these animals showed a percentage of activity during light higher than controls. We did not find differences in the endogenous period of the circadian rhythm between mice groups in DD. However, ob/ob mice showed stronger phase delays after a light pulse at ZT15 than controls, and less masking effects in the transition from LD to DD compared with controls. This suggests a weaker circadian pacemaker of the ob/ob mice compared with controls.