Background: Depression is often associated with an increase in hypothalamic-pituitary-adrenal (HPA) axis reactivity and immune response. To investigate this relationship, we examined the consequences of environmental manipulation on the neural correlates of the HPA axis and immune response in an animal model of depression, the Wistar-Kyoto (WKY) rat. Additionally, female animals are often overlooked in preclinical research because of the hormone fluctuations inherent in the estrous cycle. Methods: Female rats were randomly assigned to 1 of 3 environments for 30 days: (1) environmental enrichment (EE), (2) standard housing (SH), and (3) isolated housing (IH). Immunoreactivity of astrocytes (glial fibrillary acidic protein [GFAP]), glucocorticoid receptors (GRs), and microglia (Iba1) in the hippocampus and amygdala were measured using immunohistochemistry. Results: WKY animals had significantly more GR staining area and Iba1 staining intensity and area in the CA1 of the hippocampus. In enriched Wistar rats, GFAP staining intensity and area were greater in the CA1. A trend towards a greater percent of area stained with GR was found in WKY animals as compared to that of the Wistar animals. This was due to WKY females in EE having significantly higher GR staining intensity and area in the amygdala as compared to that of animals in SH. Discussion: These strain differences lend support to the use of WKY animals as an animal model of depression. Furthermore, due to the effects of EE on GFAP and GR staining in WKY females, we suggest that EE can be used as an intervention to potentially alleviate the negative effects of depression.

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Critical period (CP) plasticity in the auditory cortex (A1) has been known to be crucial for both functional brain development and cognitive function. Impaired A1 development during a CP for tonotopic mapping has been implicated in many neurological disorders of learning and memory, including Autism. Our recent results have shown a critical role for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR) in the auditory CP for tonotopic mapping. Here, we aim to determine if early-life stress (ELS) during rapid synaptic development affects the function of AMPARs required for normal CP plasticity. ELS was induced at P3-15 in a c-Fos based transgenic mouse model. Using whole-cell patch-clamp recordings, we recorded pyramidal cells in layer IV of A1 to measure AMPAR function and the maturation of glutamatergic synapses in P12-15 mice. We found that AMPAR functional maturation is highly correlated to the opening of A1 tonotopic CP plasticity during normal development. We further identified that ELS selectively activated a subpopulation of A1 pyramidal neurons as evidenced by selective activity-dependent green fluorescent protein tagging. Interestingly, while ELS did not cause significant changes in AMPAR function in overall randomly sampled neurons, ELS activated neurons showed enhancement of AMPAR function compared to non-activated neurons.