Distinct structural plasticity in the hippocampus and amygdala of the middle-aged common marmoset (Callithrix jacchus)

Adult neurogenesis in the primate brain is generally accepted to occur primarily in two specific areas; the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG) and the subventricular zone (SVZ) of the lateral ventricles. Hippocampal neurogenesis is well known to be downregulated by stress and aging in rodents, however there is less evidence documenting the sensitivity of neuroblasts generated in the SVZ. In primates, migrating cells generated in the SVZ travel via a unique temporal stream (TS) to the amygdala and entorhinal cortex. Using adult common marmoset monkeys (Callithrix jacchus), we examined whether i) adult-generated cells in the marmoset amygdala differentiate into doublecortin-positive (DCX+) neuroblasts, and ii) whether lasting changes occur in DCX-expressing cells in the DG or amygdala when animals are exposed to 2 weeks of psychosocial stress.A surprisingly large population of DCX+ immature neurons was found in the amygdala of these 4-year-old monkeys with an average density of 163,000 DCX+ cells per mm³. Co-labeling of these highly clustered cells with PSA-NCAM supports that a subpopulation of these cells are migratory and participate in chain-migration from the SVZ to the amygdala in middle-aged marmosets. Exposure to 2 weeks of isolation and social defeat stress failed to alter the numbers of BrdU+, or DCX+ cells in the hippocampus or amygdala when evaluated 2 weeks after psychosocial stress, indicating that the current stress paradigm has no long-term consequences on neurogenesis in this primate.