Gestational stress disrupts the neuroimmune environment in the nucleus accumbens of maternal rats.

Postpartum depression (PPD) affects 20 % of new mothers. Little is known about what contributes to this prevalence, though one of the strongest risk factors for developing PPD is stress during pregnancy. Pregnancy and the postpartum period are accompanied by dramatic changes in the endocrine, nervous, and immune systems, which may confer heightened vulnerability to maternal mental illness. Microglia, the brain's main immune cells, are important for displaying maternal behaviors. Environmental insults to microglia, such as stress, lead to hormonal dysregulation and aberrant synaptic pruning, both of which have been implicated in the pathophysiology of mood disorders like depression. Given these links, we hypothesized that stress during pregnancy would lead to dysregulated neuroimmune and endocrine profiles and produce synaptic changes in the maternal brain. We used a 2-week variable stress model in pregnancy and assessed neuroimmune changes in the nucleus accumbens (NAc), a brain region that regulates functions impaired in PDD including mood and maternal caregiving. Peripheral hormones and cytokines were also measured. While peripheral cytokine levels were not affected by stress exposure, gestational stress induced central immune changes in the NAc of maternal rats. This included increased microglia immunolabeling and changes in pro- and anti-inflammatory transcripts measured by a Nanostring nCounter panel. Astrocytes also increased in the NAc following gestational stress exposure. Peripheral estradiol and progesterone concentrations were reduced in late pregnancy of stressed mothers, and at the transcript level, hormone receptor and synthesis molecules were altered in the NAc. Gestational stress also altered transcripts associated with synapses and synaptic plasticity during late pregnancy and the postpartum period. There was no impact of stress on engulfment of pre- and postsynaptic proteins by microglia. However, microglia engulfed more of the postsynaptic marker, PSD95, in late pregnancy relative to postpartum, indicating a potential role for microglia in remodeling synapses in the NAc. Overall, these studies provide novel evidence that gestational stress impacts endocrine, synaptic and neuroimmune factors in the maternal NAc, suggesting possible mechanisms by which stress during pregnancy contributes to peripartum mood disorders.