mise à jour du
14 janvier 2010
Mammals exhibit anticipatory activity
before mealtime : yawning as an example


The mechanisms by which animals adapt to an ever-changing environment have long fascinated scientists. Different forces, conveying information regarding various aspects of the internal and external environment, interact with each other to modulate behavioral arousal. These forces can act in concert or, at times, in opposite directions. These signals eventually converge and are integrated to influence a common arousal pathway which, depending on all the information received from the environment, supports the activation of the most appropriate behavioral response.
The ability to anticipate physiological needs and to predict the availability of desirable resources optimizes the likelihood of survival for an organism. The neural basis of the complex behaviors associated with anticipatory responses is now being delineated. Anticipation likely involves learning and memory, reward and punishment, memory and cognition, arousal and feedback associated with changes in internal and external state, homeostatic processes and timing mechanisms.
In the one hand, anticipation can occur on a variety of timescales (seconds to minutes to hours to days to a year), in the other hand, circadian clocks enable the organisms to anticipate predictable cycling events in the environment. The mechanisms of the main circadian clock, localized in the suprachiasmatic nuclei of the hypothalamus, involve intracellular autoregulatory transcriptional loops of specific genes, called clock genes.
In the suprachiasmatic clock, circadian oscillations of clock genes are primarily reset by light, thus allowing the organisms to be in phase with the light-dark cycle. Another circadian timing system is dedicated to preparing the organisms for the ongoing meal or food availability: the so-called food-entrainable system, characterized by food-anticipatory processes depending on a circadian clock.
A distributed neural system underlies the generation and regulation of food-anticipatory activities under restricted feeding. Suprachiasmatic nuclei of the hypothalamus and other brain regions have diverse roles, including influences on motivational and emotional state, learning and memory, hormone release and feeding.
Anticipation of daily events, such as scheduled access to food, may serve as a useful model for a more broadly based understanding the neurobiology of anticipation. In particular, restricted feeding schedules which limit food availability to a single meal each day lead to the induction and entrainment of circadian rhythms in food-anticipatory activities in rodents , macaques and other mammals.
Food-anticipatory activities include increases in core body temperature, activity and hormone release in the hours leading up to the predictable mealtime. Yawning is one of these behaviors. Consumption, beyond homeostatic needs, refers as reward-based feeding behavior.
The ventromedial hypothalamic nucleus (VMN) is part of the circuitry that controls food anticipation. It is the first nucleus activated when there is a change in the time of food availability, silencing of VMN ghrelin receptors decreases food-anticipatory activity (FAA) and, although lesions of the VMN do not abolish FAA, parts of the response are often altered. Redundant and possibly interacting pathways may ultimately communicate with, or work in concert with this network.
Thus, the hypothalamic orexin system regulates both diet preference and anticipation of food rewards making it a likely target to modulate reward-based feeding behavior.
The cerebellum participates in motor coordination as well as in numerous cerebral processes, including temporal discrimination. Animals can predict daily timing of food availability, as manifested by food-anticipatory activity under restricted feeding. The cerebellum contains a circadian oscillator sensitive to feeding cues (i.e., whose clock gene oscillations are shifted in response to restricted feeding). Thus, a role for the cerebellum in the circadian timing network and indicate that the cerebellar oscillator is required for anticipation of mealtime.
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