Synaptic distributions of GluA2 and PKMζ in the monkey dentate gyrus and their relationships with aging and memory

Rhesus monkeys provide a valuable model for studying the neurobiological basis of cognitive aging, because they are vulnerable to age-related memory decline in a manner similar to humans. In this study, young and aged monkeys were first tested on a well-characterized recognition memory test (delayed nonmatching-to-sample; DNMS). Then, electron microscopic immunocytochemistry was performed to determine the subcellular localization of two proteins in the hippocampal dentate gyrus (DG): the GluA2 subunit of the glutamate alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionate (AMPA) receptor and the atypical protein kinase C ζ isoform (PKMζ). PKMζ promotes memory storage by regulating GluA2-containing AMPA receptor trafficking. Thus, we examined whether the distribution of GluA2 and PKMζ is altered with aging in DG axospinous synapses and whether it is coupled with memory deficits. Monkeys with faster DNMS task acquisition and more accurate recognition memory exhibited higher proportions of dendritic spines coexpressing GluA2 and PKMζ. These double-labeled spines had larger synapses, as measured by postsynaptic density area, than single- and unlabeled spines. Within this population of double-labeled spines, aged monkeys compared to young expressed a lower density of synaptic GluA2 immunogold labeling, which correlated with lower recognition accuracy. Additionally, higher density of synaptic PKMζ labeling in double-labeled spines correlated with both faster task acquisition and better retention. Together, these findings suggest that age-related impairment in maintenance of GluA2 at the synapse in the primate hippocampus is coupled with memory deficits.