Ovarian hormone replacement to aged ovariectomized female rats benefits acquisition of the Morris water maze

Ovarian steroids have been suggested to aid in preserving cognitive functioning during aging in both humans and other animals. Spatial memory relies heavily on the hippocampus, a structure that is sensitive to the influence of both ovarian hormones and aging. The present study investigated the outcome of ovarian hormone replacement during aging on performance in a spatial version of the Morris water maze. Female rats were ovariectomized at 14 months of age and received one of three types of replacement prior to testing at 16 months: acute estrogen replacement (2 days), chronic estrogen replacement (28 days), or chronic replacement of both estrogen and progesterone (28 days). Control animals, which did not receive replacement hormones, displayed significant overnight forgetting during acquisition of the task. Ovarian hormone replacement, both acute and chronic, prevented forgetting. Previous studies have demonstrated that high levels of ovarian hormones are detrimental to performance of young adult female rats on this task (Warren and Juraska, 1997; Chesler and Juraska, 2000). The current study found an opposite effect during aging: ovarian hormone replacement was beneficial. This suggests that animal models of menopause, aimed at exploring the protective effects of hormone replacement therapy on cognition during human female aging, require the use of aged female animals. (C) 2002 Elsevier Science (USA).