Enriched Environment Influences Adrenocortical Response to Immune Challenge and Glutamate Receptor Gene Expression in Rat Hippocampus

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Abstract
Housing of animals in an enriched environment (EE) has many positive effects on brain structure and function and can facilitate recovery from various brain injuries. The purpose of this study was to evaluate whether enriched rearing could alter the stress response induced by repeated immune challenge and to investigate the influence of EE and immune challenge on glutamate receptor gene expression in the hippocampus. Male 2-mo-old Wistar rats were kept under standard conditions (SC) or in an EE for 5 weeks. Immune challenge was performed by Escherichia coli lipopolysaccharide (LPS) injected repeatedly (ip) in increasing doses (10, 20, and 40 mug/kg/mL) once daily for five consecutive days. The animals were decapitated 2 h after the last injection. Blood samples, adrenals, and hippocampi were collected. LPS induced an increase in plasma and adrenal levels of corticosterone and a transient decrease in body weight of animals kept under SC, but not in an EE. Enriched housing resulted in an increase in adrenal weights and enhanced gene expression of hippocampal AMPA GluR1 receptor subunit. Concerning the LPS treatment, no effects on adrenal and thymus weights and glutamate receptor mRNA levels in the hippocampus were noticed. Thus, vulnerability to some negative effects of repeated immune challenge may be modified by environmental conditions associated with changes in brain plasticity. The fact that differences in housing conditions change stress response has to be considered in biomedical research