The BALB/c mouse was evaluated as a model for the study of entrainment of circadian rhythms by feeding schedules. Mice were housed in a 12:12-h light-dark (LD) environment with food available for 3-5 h/day (5 h before dark onset). Food anticipatory activity (FAA) rhythms were evident in all mice, ranging from robust in some to weak and variable in others. Advancing transients of the end of nocturnal activity were evident in many cases, culminating in a significant shortening of the main bout of nocturnal activity. Transients and contraction of nocturnal activity were not dependent on the expression of FAA. Following restricted feeding, nocturnal activity expanded by a series of delaying transients. On the first day of constant dark (DD) with ad libitum food access following restricted feeding in LD, the phase from which activity free-ran was advanced by comparison with control tests. Transients, compressed nocturnal activity, and advanced phase of free-run suggest that feeding schedules cause phase advancement of light-entrained rhythms in BALB/c mice. When restricted feeding was imposed in DD, several mice expressed robust FAA concurrent with a free-running activity component. In some cases. free-running rhythms entrained to feeding time, and in other cases, the period of the free run lengthened toward 24 h. These data show that restricted feeding in BALB/c mice can engage a circadian mechanism driving FAA rhythms and can also modulate the phase of photic entrainment, possibly by a direct entraining effect on the light-entrained rhythm. The BALB/c mouse strain, in several respects, appears to be a useful model for the study of scheduled feeding and circadian rhythms. (C) 2000 Elsevier Science Inc. All rights reserved.
