Effects of estrogen, age, and calpain on MAP kinase and NMDA receptors in female rat brain.

17-beta-Estradiol (E2), by activating Src and ERK/MAP kinases, enhances NMDA receptor phosphorylation and function. NR2 subunits of NMDA receptors are truncated by calpain, an effect prevented by tyrosine phosphorylation of the subunits. The present study investigated whether E2-mediated activation of ERK and NR2 subunits phosphorylation were altered in 24-month-old female rats. Ovariectomy reduced ERK2 phosphorylation in brains from 3- but not 24-month-old female rats. In ovariectomized rats, restoration of estrogen levels increased ERK2 and NR2 phosphorylation in young but not aged animals. Calcium treatment of frozen-thawed brain sections decreased NR2 levels in both young and aged female rats. This effect was absent in E2-treated young ovariectomized female rats, but was not modified in aged ovariectomized female rats. These results indicate that E2 activation of ERK2 and NR2 phosphorylation is markedly reduced in aged female rats, whereas calpain-mediated truncation of NR2 subunits is not different in young and aged rats. They suggest that several key elements of the mechanisms involved in estrogen-mediated regulation of synaptic plasticity are altered in aged animals.