Dependence of memory of meal time upon circadian biological clock in rats.

Water intake was measured in rats under a free feeding schedule and under a meal feeding schedule (food was presented for 3 hours each day from 14:00 to 17:00 hr), both under LD condition. Both schedules allowed animals free access to water. Under the meal feeding schedule, drinking occurred mainly during the food presentation time, but it was also observed during the dark period, when food was not present. After 3 weeks of meal feeding, the rats were transferred to the free feeding schedule. They showed a habit of drinking at the meal time of the previous schedule (meal-feeding-induced drinking) up to 7 days of free feeding. When the rats were made blind in the transition from the meal feeding to the 2nd free feeding, the meal-feeding induced drinking occurred with a progressively increasing delay relative to the clock time, in the same way as seen with the nocturnal drinking. Rats with bilateral lesions of the suprachiasmatic nucleus did not show nocturnal drinking during the meal feeding schedule, and no signs of the meal-feeding-induced drinking were observed after the 2nd free feeding schedule was started. It was concluded that the persistence of the memory of the time of meal feeding in the following free feeding period could be traced with water-drinking as an index and such memory seems to be formed depending on the biological clock which is ascribed to activity of the suprachiasmatic nucleus under LD condition.