Forebrain‐Specific Transgene Rescue of 11β‐HSD1 Associates with Impaired Spatial Memory and Reduced Hippocampal Brain‐Derived Neurotrophic Factor mRNA Levels in Aged 11β‐HSD1 Deficient Mice

Mice lacking the intracellular glucocorticoid‐regenerating enzyme 11β‐hydroxysteroid dehydrogenase type 1 (11β‐HSD1) are protected from age‐related spatial memory deficits. 11β‐HSD1 is expressed predominantly in the brain, liver and adipose tissue. Reduced glucocorticoid levels in the brain in the absence of 11β‐HSD1 may underlie the improved memory in aged 11β‐HSD1 deficient mice. However, the improved glucose tolerance, insulin sensitisation and cardioprotective lipid profile associated with reduced peripheral glucocorticoid regeneration may potentially contribute to the cognitive phenotype of aged 11β‐HSD1 deficient mice. In the present study, transgenic mice with forebrain‐specific overexpression of 11β‐HSD1 (Tg) were intercrossed with global 11β‐HSD1 knockout mice (HSD1KO) to examine the influence of forebrain and peripheral 11β‐HSD1 activity on spatial memory in aged mice. Transgene‐mediated delivery of 11β‐HSD1 to the hippocampus and cortex of aged HSD1KO mice reversed the improved spatial memory retention in the Y‐maze but not spatial learning in the watermaze. Brain‐derived neurotrophic factor (BDNF) mRNA levels in the hippocampus of aged HSD1KO mice were increased compared to aged wild‐type mice. Rescue of forebrain 11β‐HSD1 reduced BDNF mRNA in aged HSD1KO mice to levels comparable to aged wild‐type mice. These findings indicate that 11β‐HSD1 regenerated glucocorticoids in the forebrain and decreased levels of BDNF mRNA in the hippocampus play a role in spatial memory deficits in aged wild‐type mice, although 11β‐HSD1 activity in peripheral tissues may also contribute to spatial learning impairments in aged mice.