Genetic background determines behavioral responses during fear conditioning.

Freezing behavior is used as a measure of a rodent's ability to learn during fear conditioning. However, it is possible that the expression of other behaviors may compete with freezing, particularly in rodent populations that have not been thoroughly studied in this context. Rearing and grooming are complex behaviors that are frequently exhibited by mice during fear conditioning. Both behaviors have been shown to be stress-sensitive, and the expression of these behaviors is dependent upon strain background. To better understand how genetic background impacts behavioral responses during fear conditioning, we examined freezing, rearing, and grooming frequencies prior to fear conditioning training and across different stages of fear conditioning testing in male mice from eight inbred mouse strains (C57BL/6J, DBA/2J, FVB/NJ, SWR/J, BTBR T + ltpr3Tf/J, SM/J, LP/J, 129S1/SvlmJ) that exhibited diverse freezing responses. We found that genetic background determined rearing and grooming expression throughout fear conditioning, and their patterns of expression across stages of fear conditioning were strain dependent. Using publicly available SNP data, we found that polymorphisms in Dab1, a gene that is implicated in both grooming and learning phenotypes, separated the strains with high contextual grooming from the others using a hierarchical clustering analysis. This suggested a potential genetic mechanism for the observed behavioral differences. These findings demonstrate that genetic background determines behavioral responses during fear conditioning and suggest that shared genetic substrates underlie fear conditioning behaviors.

Elevated stress hormone levels and antidepressant treatment starting before pregnancy affect maternal care and litter characteristics in an animal model of depression.

Many women who take antidepressant medications become pregnant while taking their medication; however, the impact of depression and antidepressant medication on fetal development is not well understood. This study used a translational animal model of maternal depression to investigate the consequences of discontinuing antidepressant medication during pregnancy. First, rats received corticosterone (CORT; 40 mg/kg, s.c.) or vehicle to induce a depressive-like phenotype. After 16 days of treatment with CORT or vehicle, animals were treated with sertraline (a selective serotonin reuptake inhibitor, SSRI; 20 mg/kg) or vehicle via gavage. Following 21 days of CORT or oil treatment, rats were mated. One group receiving sertraline was discontinued from treatment on gestational day 16, and another group continued sertraline treatment throughout pregnancy to assess the effects of discontinuation. After weaning, dams were sacrificed via perfusion to investigate neurogenesis. As intended, CORT administration created a depressive-like phenotype with increased immobility in the Forced Swim Test and reduced body weight. Interestingly, sertraline treatment could not rescue these altered features. Pre-conceptional CORT exposure resulted in smaller litters and CORT dams that received sertraline until the end of gestation spent more time off of the nest compared to CORT dams that received vehicle or discontinued sertraline during gestation. There was no difference in hippocampal neurogenesis between any of the groups. Our results suggest that treatment with antidepressants may have different effects in healthy or depressed dams, however, we need more research to investigate the detailed and long-term effects of maternal depression and its treatment in translational animal models.

Effectiveness of different corticosterone administration methods to elevate corticosterone serum levels, induce depressive-like behavior, and affect neurogenesis levels in female rats.

High levels of chronic stress or stress hormones are associated with depressive-like behavior in animal models. However, slight elevations in corticosterone (CORT) - the major stress hormone in rodents - have also been associated with improved performances, albeit in a sex-dependent manner. Some of the discrepancies in the literature regarding the effects of high CORT levels may be due to different administrations methods. The current study aims to compare the effects of ∼40mg/kg given either via subcutaneous injection, through an implanted pellet, or in the drinking water, for ∼21days on CORT serum levels, depressive-like behavior in the forced swim test (FST), and neurogenesis levels in the dentate gyrus (DG) in adult female rats. We found that animals exposed to the daily injections showed elevated CORT levels throughout the administration period, while the pellet animals showed only a transient increase, and drinking water animals revealed no elevation in CORT in serum. In addition, only the injection group exhibited higher levels of immobility in the FST. Interestingly, animals receiving CORT via injection or drinking water had lower numbers of doublecortin-positive cells in the ventral DG one week after the last CORT administration compared to animals implanted with a CORT pellet. These results will contribute to the growing literature on the effects of chronic CORT exposure and may help to clarify some of the discrepancies among previous studies, particularly in females.