The effect of enriched environment across ages: A study of anhedonia and BDNF gene induction

Enriched environment treatment (EET) is a potential intervention for depression by inducing brain-derived neurotrophic factor (BDNF). However, its age dependency remains unclear. We recently found that EET during early-life development (ED) was effective in increasing exploratory activity and anti-despair behavior, particularly in promoter IV-driven BDNF deficient mice (KIV), with the largest BDNF protein induction in the hippocampus and frontal cortex. Here, we further determined age dependency of EET effects on anhedonia and promoter-specific BDNF transcription, by using the sucrose preference test and qRT-PCR. Wild-type (WT) and KIV mice received 2 months of EET during ED, young-adulthood and old-adulthood (0-2, 2-4 and 12-14 months, respectively). All KIV groups showed reduced sucrose preference, which EET equally reversed regardless of age. EET increased hippocampal BDNF mRNA levels for all ages and genotypes, but increased frontal cortex BDNF mRNA levels only in ED KIV and old WT mice. Transcription by promoters I and IV was age-dependent in the hippocampus of WT mice: more effective induction of exon IV or I during ED or old-adulthood, respectively. Transcription by almost all 9 promoters was age-specific in the frontal cortex, mostly observed in ED KIV mice. After discontinuance of EET, the EET effects on anti-anhedonia and BDNF transcription in both regions persisted only in ED KIV mice. These results suggested that EET was equally effective in reversing anhedonia and inducing hippocampal BDNF transcription, but was more effective during ED in inducing frontal cortex BDNF transcription and for lasting anti-anhedonic and BDNF effects particularly in promoter IV-BDNF deficiency.