N-methyl-D-aspartate receptor mRNA levels change during reproductive senescence in the hippocampus of female rats.
Estrogen interacts with N-methyl-d-aspartate (NMDA) receptors to regulate multiple aspects of morphological and functional plasticity. In the hippocampus, estrogens increase both dendritic spine density and synapse number, and NMDA antagonists block these effects. This plasticity in the hippocampus mediated by estrogen may be of particular importance in the context of aging when estrogen levels change and cognitive function is often impaired. Therefore, the present study was designed to investigate effects of aging and reproductive status on NMDA receptor (NR) subunit mRNA levels in the hippocampus. NR1, NR2A, and NR2B mRNA levels were measured by RNase protection assay in young (3-4 month), middle-aged (12-13 month), and aged (24-25 month) Sprague-Dawley rats in different phases of the estrous cycle in cycling animals and in acyclic subjects. Our results demonstrated that NMDA receptor subunit mRNA levels were much more prominently affected by the chronological age than by the reproductive status of the animals. Age-related changes were observed in NR1, NR2A, and NR2B in the ventral hippocampus and in NR1 and NR2B in the dorsal hippocampus. However, the only relationship with reproductive status was seen for NR1 mRNA, and this was restricted to the ventral hippocampus. An interaction between chronological age and reproductive status was found, with higher levels of NR1 mRNA seen in young animals in proestrus than in those in diestrus I (high and low estrogen levels, respectively). However, this relationship was not seen in the aged subjects. These results demonstrate that the hippocampus is subjected to age-related alterations in NMDA receptor subunit mRNA levels and that animals of different ages are influenced differently by reproductive status. This shift in the NMDA receptor mRNA levels may be a possible molecular mechanism contributing to alterations in cognitive behavior during normal aging.