Peripubertal environmental enrichment reverses the effects of maternal care on hippocampal development and glutamate receptor subunit expression

Maternal care in the rat influences the development of cognitive function in the offspring through neural systems known to mediate activity-dependent synaptic plasticity. The offspring of mothers that exhibit increased levels of pup licking/grooming (high-LG mothers) show increased hippocampal N-methyl-D-aspartate (NMDA) subunit mRNA expression, enhanced synaptogenesis and improved hippocampal-dependent spatial learning in comparison with animals reared by low-LG mothers. The effects of reduced maternal care on cognitive function are reversed with peripubertal environmental enrichment; however, the neural mechanisms mediating this effect are not known. In these studies we exposed the offspring of high- and low-LG mothers to environmental enrichment from days 22 to 70 of life, and measured the expression of genes encoding for glutamate receptor subunits and synaptophysin expression as a measure of synaptic density. Environmental enrichment reversed the effects of maternal care on synaptic density and this effect was, in turn, associated with a reversal of the effect of maternal care on the NR2A and NR2B subunits of the NMDA receptor, as well as effects on (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits. Finally, direct infusion of an NR2B-specific NMIDA receptor antagonist into the hippocampus eliminated the effects of maternal care on spatial learning/memory in the Morris water maze. These findings suggest that: (1) the effects of maternal care are mediated by changes in NR2B gene expression; and (2) that environmental enrichment reverses the effects of reduced maternal care through the same genomic target, the NR2B gene, and possibly effects on other subunits of the NMIDA and AMPA receptors.