Synaptic characteristics of dentate gyrus axonal boutons and their relationships with aging, menopause, and memory in female rhesus monkeys.

Age-related memory impairment occurs in many mammalian species, including humans. Moreover, women undergoing the menopausal transition often complain of problems with memory. We recently reported that rhesus monkeys display age- and menopause-related recognition memory impairment on a hippocampus-reliant test [delayed nonmatching-to-sample (DNMS)]. In the same monkeys, perforated synapse densities in the dentate gyrus outer molecular layer (OML) correlated with DNMS recognition accuracy, while total axospinous synapse density was similar across age and menses groups. The current study examined whether synaptic characteristics of OML axonal boutons are coupled with age- or menopause-related memory deficits. Using serial section electron microscopy, we measured the frequencies of single-synapse boutons (SSBs), multiple-synapse boutons (MSBs), and boutons with no apparent synaptic contacts [nonsynaptic boutons (NSBs)] in the OML. Aged females had double the percentage of NSBs compared with young females, and this measure correlated positively and inversely with DNMS acquisition (number of trials to criterion) and delay performance (average accuracy), respectively. Aged compared with young females also had a lower frequency of MSBs and a lower number of synaptic contacts per MSB, and the latter variable inversely correlated with DNMS acquisition. Although proportions of NSBs, SSBs, and MSBs were similar across menses groups, compared with premenopausal monkeys, peri/postmenopausal monkeys had fewer MSBs contacting one or more segmented perforated synapses, and the abundance of this bouton subtype positively correlated with DNMS performance. These results suggest that age- and menopause-related shifts in OML synaptic subtypes may be coupled with deficits in task acquisition and recognition memory.