Enrichment enhances the expression of sgk, a glucocorticoid‐induced gene, and facilitates spatial learning through glutamate AMPA receptor mediation

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Abstract
We have previously demonstrated that the serum and glucocorticoid‐inducible kinase (sgk) gene plays a causal role in facilitating memory performance in rats. Environment enrichment is known to facilitate spatial learning. We therefore examined the effect of enrichment on sgk expression. We also examined the role of sgk in spatial and nonspatial learning and the regulation of sgk expression by activation of different glutamate receptors. Both real‐time polymerase chain reaction and Western blot analyses revealed that enrichment training preferentially increased sgk mRNA and protein levels in the hippocampus. Transfection of sgk mutant DNA to the hippocampal CA1 area markedly impaired spatial learning, fear‐conditioning learning and novel object‐recognition learning in rats, but enrichment training effectively reversed these learning deficits. Meanwhile, S422A mutant DNA transfection prevented enrichment‐induced spatial learning facilitation. In studying glutamate receptor regulation of sgk expression, we found that blockade of N‐methyl‐d‐aspartate (NMDA) receptors in general, and the NR2B subunit in particular both effectively blocked enrichment‐induced spatial learning facilitation, but they did not block enrichment‐induced sgk expression. Upon various glutamate agonist infusions, only α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionic acid (AMPA) increased sgk mRNA levels significantly in the hippocampus. Furthermore, blockade of AMPA receptors effectively blocked both enrichment‐induced spatial learning facilitation and sgk expression. These results indicate that there is a dissociation between NMDA receptor activation and sgk expression. Enrichment enhanced spatial learning through both NMDA and AMPA receptor activation, whereas enrichment‐induced sgk expression is specifically mediated through AMPA receptors. These results suggest that sgk could serve as a novel molecular mechanism, in addition to the NMDA receptor NR2B, underlying enrichment‐induced learning facilitation.