Another study showed that learning and performance of the spatial memory task, delayed response, was impaired in irregularly cycling or postmenopausal, middle-aged rhesus monkeys compared to middle-aged or young monkeys with regular menstrual cycles. Similarly, in a small study using surgically menopausal middle-aged (age, 22 yr) rhesus monkeys, both delayed response performance and visuospatial attention function were altered in placebo-treated, but not in estrogen-treated, monkeys. These observations in older monkeys are in contrast to observations in young monkeys, in which performance on the delayed response task was not responsive to ovariectomy or to treatment with estrogen. Finally, rhesus monkeys (age, 8-27 yr) who had undergone early ovariectomy (age, 7-14 yr) showed a trend for worse performance in a visual recognition memory task but demonstrated significantly better performance in a spatial recognition span task in comparison to intact controls. canadian family pharmacy com
Various studies in postmenopausal women regarding the effects of estrogen or of estrogen plus progestin replacement on cerebral blood flow or increased activation patterns in specific brain regions have provided only nonspecific information about response to estrogen in the primate brain. Clearly, additional studies using animal models are required to more fully appreciate the specific effects that these hormones exert on the functional activity and neurobiology of the brain. Two recent studies in rhesus monkeys indicate that ovariectomy reduced the density of tyrosine hydroxylase- and choline acetyltransferase-positive fibers in the prefrontal cortex and that either estrogen alone or estrogen plus progestin prevented some of this fiber loss. In addition, other preliminary studies in monkeys suggest that ovariectomy reduces the number and size of cholinergic neurons of the basal forebrain. Preliminary functional imaging studies indicate that estrogen replacement enhances the functional activity of the cholinergic and dopaminergic systems and that this enhancement continues to be realized for some time following estrogen withdrawal.