Search for the feeding-entrainable circadian oscillator: a complex proposition

WORKING IN THE LABORATORY of John B. Watson in the early 1920s, Curt P. Richter first observed the phenomenon of rats engaged in what would later be called food-anticipatory activity: "It is seen that immediately following the daily feeding period there is a period of relative inactivity lasting from four to five hours.... During the last two to three hours of the twenty-four the activity increases very rapidly right up to the time of the next feeding period" (24). More recently it has become clear that food-anticipatory activity (FAA) exhibits the defining characteristics of a circadian rhythm. Circadian rhythms are oscillations of physiological or behavioral functions that are timed by biological clocks, the best studied of which is found in the hypothalamic suprachiasmatic nucleus (SCN). Although these clocks can operate in the absence of external input, to be useful in the anticipation of expected environmental changes, they must be synchronized by the environment in response to cues called zeitgebers (German for time-giver). Light is the strongest zeitgeber for the SCN. Working through specialized retinal photoreceptors (10), light causes an adjustment to the phase of the clock in the SCN each day to maintain a precise 24-h period. However, the effect of this light exposure each day is limited; the SCN takes several transient cycles to fully resynchronize to abrupt large changes in environmental time, a process we experience as jet lag. FAA also displays transients for several cycles following an abrupt change in feeding time (29). Furthermore, FAA persists during several days of total food deprivation (1, 25), the equivalent to constant darkness for rhythms synchronized by light exposure.