The relationship between activation ofneurotransmitters by the mastication, learning, and memory : investigation of the mechanism by chronobiological method

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The purpose of the present study was to develop a new system for estimating the level of learning and memory of rats by chronobiological method, and to examine whether the mastication affects learning and memory.It is well known that the temporal profile of hypothalamus-pituitary-adrenocortical axis in rats are modified by restricted daily feeding (RF), in which food supply is restricted to a fixed time of day. Under RF, a corticosterone peak appears in 8 to 10 days, from 2 hours before the daily meal supply (anticipatory peak), and which is detectable for more than several weeks even after the termination of RF. The anticipatory corticosterone peak is regarded as an expression of endogenous circadian rhythm associated with the feeding schedule. In this study, this phenomenon was used for estimating the level of learning and memory of rats. The following results was obtained.(1) The anticipatory corticosterone peak did not appear under restricted daily parenteral feeding or under restricted daily nutrition via jugular vein. These results suggested that supplement nutrition was needed for the development of the anticipatory corticosterone peak.(2) Restricted daily solid meal produced the anticipatory hormone peak in 8 days, while restricted daily liquid meal produced the peak in 12 days. The developed hormone peak was less prominent by liquid meal than by solid.(3) The anticipatory corticosterone peak developed by solid meal persisted throughout the period examined (3 weeks) after the termination of RF, while the peak developed by liquid meal disappeared after the first week.It is concluded that liquid meal attenuates the meal anticipation in the adrenocortical activity in rats under RF. These results suggested that the mastication affected the development and persistence of memory in rats.