Effectiveness of estrogen replacement in restoration of cognitive function after long-term estrogen withdrawal in aging rats

Recent studies suggest that some aspects of learning and memory may be altered by a midlife loss of estrogen, indicating a potential causal relationship between the deficiency of ovarian hormones and cognitive aging. In this study, the effects of estrogen withdrawal and replacement were tested in middle-aged Fischer-344 rats using different memory tasks. Estrogen withdrawal accelerated the rate of cognitive aging. A deficit first occurred 4 months after ovariectomy in working memory, which was tested in a delayed-nonmatching-to-position task, and progressed from long-delay to short-delay trials. Reference memory, which was tested in a place discrimination task and a split-stem T-maze, was not affected by aging or ovariectomy. The efficacy of estrogen in ameliorating the cognitive deficit in old rats depended on the type of treatment (acute vs chronic) and whether the aging-related decline in a particular cognitive process was aggravated by estrogen withdrawal. Chronic estrogen treatment (implants) was effective in improving working memory only when primed with repeated injections of estrogen, indicating that simulating the estrogen fluctuations of the estrous cycle may be more effective than the widely used mode of chronic pharmacological treatment. A challenge with scopolamine revealed that ovariectomy-induced cognitive deterioration coincided with a compromised cholinergic system. Importantly, the estrogen treatment that had restored effectively the cognitive abilities of old ovariectomized rats did not reduce their sensitivity to scopolamine. Taking into consideration that estrogen was highly effective against the amnestic action of scopolamine when tested in young-adult rats, these data emphasize that mechanisms of the protective effect of estrogen differ in young and old rats.