Neural mechanisms generating food-anticipatory circadian rhythms in rats are sep arate from the suprachiasmatic nuclei, a putative circadian pacemaker for light-entrainable circadian rhythms. We attempted to localize these mechanisms using lesions focused on the paraventricular and lateral hypothalamic areas, two regions that have functional properties likely to characterize a food-entrainable circadian oscillator. Rats with complete radiofre quency ablation of the paraventricular and suprachiasmatic nuclei or ibotenic acid destruction of the lateral hypothalamus showed anticipatory circadian rhythms to a 2-hr daily mealtime, as measured by locomotor activity or activity directed toward a food bin. The long-term continued function of the meal-associated oscillator was also unaffected by the lesions, as demonstrated by a reappearance of activity at the former mealtime during 3 days of starvation after 11 or 12 days of ad libitum feeding. Several rats with total or partial paraventricular lesions failed to exhibit anticipatory locomotor activity, but did show anticipatory food- bin-related activity. These hypothalamic lesions may alter the probability of particular antic ipatory behaviors, but they do not eliminate the food-entrainable circadian timekeeping func tion.
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