Food-entrained feeding and locomotor circadian rhythm in rats under different lighting conditions

It has been suggested that two endogenous timekeeping systems, a light-entr ainable pacemaker (LEP) and a food-entrainable pacemaker (FEP), control cir cadian rhythms. To understand the function and interaction between these tw o mechanisms better, we studied two behavioral circadian rhythmicities, fee ding and locomotor activity, in rats exposed to two conflicting zeitgebers, food restriction and light-dark cycles. For this, the food approaches and wheel-running activity of rats kept under light-dark (LD) 12:12, constant d arkness (DD), or constant light (LL) conditions and subjected to different scheduled feeding patterns were continuously recorded. To facilitate compar ison of the results obtained under the different lighting conditions, the p eriod of the feeding cycles was set in all three cases about Ih less than t he light-entrained or free-running circadian rhythms. The results showed th at, depending on the lighting conditions, some components of the feeding an d wheel-running circadian rhythms could be entrained by food pulses, while others retained their free-running or light-entrained state. Under LD, food pulses had little influence on the light-entrained feeding and locomotor r hythms. Under DD, relative coordination between free-running and food-assoc iated rhythms may appear. In both cases, the feeding activity associated wi th the food pulses could be divided into a prominent phase-dependent peak o f activity within the period of food availability and another afterward. Wh eel-running activity mainly followed the food pulses. Under LL conditions, the food-entrained activity consisted mainly of feeding and wheel-running a nticipatory activity. The results provide new evidence that lighting condit ions influence the establishment and persistence of food-entrained circadia n rhythms in rats. The existence of two coupled pacemakers, LEP and FEP, or a multioscillatory LEP may both explain our experimental results.