Age-dependent, ovary-independent decrease in the nuclear binding kinetics of estrogen receptors in the brain of the...

To further define the role of aging of the brain in the induction of reproductive acyclicity, we put to death intact as well as castrated female C57BL/6J mice of various ages before and from 0.5 to 24 hours after subcutaneous injection of 0.2 micrograms of 17 beta-estradiol. Pooled hypothalamic and pituitary tissues were dissected and cytosolic/nuclear estrogen receptors were assayed in buffer that consisted of 10 mmol/L Tris(hydroxymethylaminomethane) hydrochloride, 1.5 mmol/L ethylenediaminetetraacetic acid, and 0.5 mmol/L dithiothreitol and contained molybdate (25 mmol/L) and inhibitors of proteases. Our results in intact animals indicated that baseline cytosolic concentration of estrogen receptors remained constant at 60 to 77 fmol/mg of protein (range) throughout aging, whereas nuclear levels of estrogen receptors decreased from 1.2 to 1.6 fmol/micrograms of deoxyribonucleic acid (range) to nondetectable levels after the onset of ovarian acyclicity. No age-related changes in the Ka were observed. After subcutaneous challenge with estrogen, nuclear binding of hypothalamic-pituitary axis estrogen receptors revealed a significant age-related decrease which was already evident at 10 to 14 months of age and prior to the onset of anestrous. Castration, whether performed neonatally or at 8 months of age, reduced the hypothalamic-pituitary axis concentration of estrogen receptors in middle-aged and aged animals, but did not prevent this blunted kinetics of nuclear binding. One week of daily injection of 17 beta-estradiol to intact and castrated mice of all age groups prior to binding kinetic studies induced maximal (five fold) increases in the content of hypothalamic-pituitary axis estrogen receptors in young animals which readily bound to the nucleus. After similar therapy to middle-aged and aged mice, minimal changes or even no changes were observed in both cellular estrogen receptor contents, despite similar increments in plasma levels of estrogen. These findings suggest an age-dependent decrease in the kinetics of hypothalamic-pituitary axis estrogen receptors manifested by a reduced synthesis of functional estrogen receptors in the brain of mice.