Sensory factors in migration

Restricted feeding facilitates time-place learning in adult rats

Many species can acquire time-of-day discrimination when tested in food reinforced place learning tasks. It is believed that this type of learning is dependent upon the ability of animals to consult their internal circadian pacemakers entrained by various environmental zeitgebers, such as light-dark cycles and scheduled restricted feeding. In the present study, we examined, (1) whether rats can acquire time-of-day discrimination in an aversively motivated water maze task wherein an escape platform is located in one position in the morning and in another position in the afternoon; (2) whether time-of-day cues provided by the light- and feeding-entrainable pacemakers may have divergent impacts upon the ability of rats to learn this task. Two groups of rats, both maintained on 12-h light:12-h dark cycle, were used; in one group, animals had free access to food, whereas in the other, they were subjected to a restricted feeding protocol (60% of food consumed by rats fed ad libitum, once daily). Despite the heightened difficulty of the task, food-restricted rats were apparently able to acquire associations between two different platform positions and two different times of day, as indicated by the fact that the percentage of discrimination errors in this group declined progressively, as a function of training, and stabilized at the level of 22+/-9%. In contrast, rats that were fed ad libitum, even after extensive training, failed to perform the task above level of chance. These data indicate that time-place learning is a universal, reward-nonspecific, cognitive phenomenon. They furthermore suggest that the ability of animals to integrate spatial and temporal information can be dependent on the access to timing stimuli provided by the feeding-entrainable circadian system.

Rats more readily acquire a time-of-day go no-go discrimination than a time-of-day choice discrimination

Male Sprague-Dawley rats were used to compare six time-place training procedures that differed with respect to housing or training conditions. All procedures involved training food-deprived rats to enter one choice arm of a T-maze during a morning test session and to enter the other choice arm during an afternoon session to obtain Cocoa Puffs(R). The task proved to be difficult. Only 39 of 49 rats attained a criterion of nine correct choices on ten consecutive trials within a total of 120 trials. Making one choice arm distinct, limiting consecutive same correct choices to two, giving one session during the light and one during the dark portion of the light cycle, extinguishing perseveration of the same choice responses, and housing the rats with a natural light cycle all failed to significantly decrease the errors or trials to a 90% correct choice criterion. In contrast, all responding rats (n=7) showed significantly better than chance performance within 48 trials when the task was a go no-go discrimination based on time of day. Learning to make a response during a session when a choice to either choice arm is reinforced and to withhold responding during a session when a choice to neither choice arm is reinforced was relatively easy for the rats to acquire. Continued high level performance after the light cycle was eliminated, a random feeding schedule was initiated, and other time-related cues were masked suggests that the rats used internal cues or an internal clock to make the correct go or no-go response. It was concluded that rats are prepared to use time of day as an occasion-setting stimulus, but have difficulty using time of day as a signal for a specific response.

Breakfast in the nook and dinner in the dining room: time-of-day discrimination in rats

Four studies were conducted which demonstrate that most (63%) male Sprague-Dawley rats can attain criterion, nine correct choices over ten consecutive trials, on a time-of-day discrimination in an elevated T-maze, but that the task is relatively difficult. The discrimination required that the rats go to one goal arm during a morning session and the other in an afternoon session. The sessions always began at the same time and were at least 6 h apart. A larger proportion of rats attained criterion and required fewer trials when the discriminative cue was a maze insert providing visual and tactile stimulation (0.72), orientation and position of the maze in the room (0.88), or the rats were required to always make the same left or right turn (0.94). Also, once criterion was attained, rats trained on time-of-day discrimination only made about 70% correct choices with continued training. Housing the rats with continuous light, all laboratory noises masked with white noise, and a random feeding schedule did not prevent them from acquiring the time-place discrimination. Testing the rats with a random number of trials during morning and afternoon sessions and with added or omitted sessions revealed that the rats did not use response or session alternation strategies to perform the discrimination. Also, the particular experimenter administering the morning or afternoon sessions did not serve as a cue for the discrimination. The relative difficulty of the task suggests that time of day does not normally function as a discriminative stimulus for choices, but probably as a contextual stimulus. Further, performance of the task in the absence of time-of-day cues suggests that the discrimination is based on event memory combined with an internal timing mechanism.

Circadian food-anticipatory activity: formal models and physiological mechanisms

Rats and other species exhibit food-anticipatory activity (FAA) to daily mealtime under circadian (24 h) food access schedules. A critical review of several explanatory models indicates that hourglass clocks and associative learning processes are inadequate to explain many properties of FAA in intact and suprachiasmatic nuclei ablated rodents. A computational learning model, involving circadian clock consultation and phase memory, accounts for some but not all of these properties. An entrainment model, invoking separate, compound food- and light-entrainable oscillators, provides a more complete account of FAA. However, FAA may be simulated best by a model that combines oscillator entrainment with clock consultation and memory for circadian phase. Species as diverse as bees, birds, and mammals appear to share many features of FAA in common; differences may be explained in terms of oscillator organization and the ability to represent multiple circadian phases memorially. Physiological mechanisms of FAA are largely unknown; strategies for localization of entrainment pathways and oscillators, and a modest data base, are reviewed.

Rats anticipate and discriminate between two daily feeding times

Intact rats and rats bearing lesions of the suprachiasmatic nuclei (SCNX rats) were trained to obtain food by pressing either of two levers located on opposite sides of a cylindrical cage. Intact rats were maintained in constant light (LL) and under daily light-dark (LD) cycles, SCNX rats in LL only. A restricted daily feeding schedule was next imposed, such that pressing one lever provided food for a limited duration (1 or 2 hr) at one time of day while pressing the second lever provided food for the same duration at another time of day. Most rats generally showed anticipatory lever-pressing preceding both daily feeding times, and several discriminated between the two, pressing the lever appropriate for each feeding time more than the inappropriate lever. Discrimination performance was better in intact rats in LD than in intact or SCNX rats in LL. These results indicate that rats can associate different food locations with different times of day, an ability previously known only in honeybees and birds.

Food availability and daily biological rhythms

Restricted daily feeding schedules result in the partial or complete synchronization of a wide range of rhythmic biological functions in rodents. In some cases, exemplified by drinking behavior and liver tyrosine transaminase activity, this represents primarily a direct, exogenous influence of food intake. In others, synchronization is achieved by entrainment of a circadian time-keeping mechanism distinct from that which underlies free-running rhythms in these functions. This food-entrained mechanism is responsible for the timing of anticipatory increases in locomotor and lever-pressing activity immediatley prior to food delivery, and may also underlie similar anticipatory increases in body temperature, corticosterone secretion, and in the activities of some intestinal enzymes. It is suggested that such a mechanism may enable an animal to recognize and take advantage of the periodic recurrence of significant events in its biotic environment.