Estrogen restores multisynaptic boutons in the dorsolateral prefrontal cortex while promoting working memory in aged rhesus monkeys

Humans and nonhuman primates are vulnerable to age- and menopause-related decline in working memory, a cognitive function reliant on area 46 of the dorsolateral prefrontal cortex (dlPFC). We showed previously that presynaptic mitochondrial number and morphology inmonkeydlPFC neurons correlate with workingmemoryperformance. The current study tested the hypothesis that the types of synaptic connections these boutons form are altered with aging and menopause in rhesus monkeys and that these metrics may be coupled with mitochondrial measures and working memory. Using serial section electron microscopy, we examined the frequencies and characteristics of nonsynaptic, single-synaptic, and multisynaptic boutons (MSBs) in the dlPFC. In contrast to our previous observations in the monkey hippocampal dentate gyrus, where MSBs comprised~40% of boutons, the vast majority of dlPFC boutons were single-synaptic, whereas MSBs constituted a mere 10%. The frequency of MSBs was not altered by normal aging, but decreased by over 50% with surgical menopause induced by ovariectomy in aged monkeys. Cyclic estradiol treatment in aged ovariectomized animals restored MSB frequencies to levels comparable to young and aged premenopausal monkeys. Notably, the frequency of MSBs positively correlated with working memory scores, as measured by the average accuracy on the delayed response (DR) test. Furthermore, MSB incidence positively correlated with the number of healthy straight mitochondria in dlPFC boutons and inversely correlated with the number of pathological donut-shaped mitochondria. Together, our data suggest that MSBs are coupled to cognitive function and mitochondrial health and are sensitive to estrogen.