Constitutive Serotonin Transporter Reduction Resembles Maternal Separation with Regard to Stress-Related Gene Expression.

Interactive effects between allelic variants of the serotonin transporter () promoter-linked polymorphic region (5-HTTLPR) and stressors on depression symptoms have been documented, as well as questioned, by meta-analyses. Translational models of constitutive reduction and experimentally controlled stressors often led to inconsistent behavioral and molecular findings and often did not include females. The present study sought to investigate the effect of genotype, maternal separation, and sex on the expression of stress-related candidate genes in the rat hippocampus and frontal cortex. The mRNA expression levels of , , , , , , , and were assessed in the hippocampus and frontal cortex of and male and female adult rats exposed, or not, to daily maternal separation for 180 min during the first 2 postnatal weeks. Gene- and brain region-dependent, but sex-independent, interactions between genotype and maternal separation were found. Gene expression levels were higher in rats not exposed to maternal separation compared with the other experimental groups. Maternal separation and genotype did not yield additive effects on gene expression. Correlative relationships, mainly positive, were observed within, but not across, brain regions in all groups except in non-maternally separated rats. Gene expression patterns in the hippocampus and frontal cortex of rats exposed to maternal separation resembled the ones observed in rats with reduced expression regardless of sex. These results suggest that floor effects of reduction and maternal separation might explain inconsistent findings in humans and rodents.