A series of four experiments examined the possible roles of catecholamines and adrenocorticotropic hormone (ACTH) in infantile amnesia. Juvenile (14-20 days) rats were trained in a one-trial inhibitory avoidance task. In this task, good retention performance appeared in 16-day-old rats when tested at 1 hr but not 24 hr after training; good 24-hr retention performance was first seen in 20-day-old rats. A single posttraining injection (subcutaneous) of either epinephrine or norepinephrine (NE), but not ACTH, enhanced 24-hr retention performance in the 16-day-old rats. Transient decreases in forebrain norepinephrine content in response to a training footshock were observed in 16-day and older animals; brainstem responses apparently matured prior to 14 days of age. When brain NE was examined in 16-day-old rats that received training and peripheral memory-facilitating epinephrine injections, there was little correlation between later retention performance and the extent of change in brain norepinephrine content. These results may be due to age-related differences in norepinephrine release or metabolism. The findings of these studies suggest that peripheral catecholamine systems known to modulate adult memory may account, in part, for the phenomenon of infantile amnesia. The results support the view that memory storage mechanisms may mature earlier than do memory modulation systems responsible for initiating and promoting storage.
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