Enrichment and individual housing reinforce the differences in aggressiveness and amphetamine response in 129S6/SvEv and C57BL/6 strains.

As different kinds of enrichment equipment are applied to standard rodent laboratory housing conditions in Europe (Directive 2010/63/EU) and worldwide, it is essential to understand how it may influence the brain and behaviour of animals. We observed common inbred mouse strains 129S6/SvEv/Tac (129) and C57BL/6 Bkl (B6) reared in 3 different environments: standard housing (SH), individual housing (IH) and enriched environment (EE). We measured common behavioural parameters, social behaviour and BDNF mRNA expression in hippocampus and frontal cortex. Our results demonstrate that the robust behavioural differences between B6 and 129 mouse strains which are well studied in the literature persist in varied housing conditions, but the response to these conditions has different directionality in studied strains. EE appears to reinforce the existing coping strategies in both strains: B6 became more agile, sensitive and venturous in behavioural models, whereas 129 became even more inhibited than they are in standard conditions. The stimulating effect of amphetamine is decreased in 129 animals that have been reared in EE. This may indicate desensitisation or inhibition of their dopamine system by EE. A frequently used biomarker in enrichment studies, BDNF, is a likely mediator for the long-term phenotype effects of EE. In this paper we demonstrate the differences of BDNF expression pattern in 129 and B6 mice.