Gonadal hormone decline occurs in human, but not mouse, reproductive aging. (A) Human levels of bioavailable testosterone (blue) in males and estradiol (the most biologically active estrogen) (red) in females decrease dramatically during aging at a time when the brain is vulnerable to neurodegenerative diseases such as AD. Of note, changes in total testosterone (not shown) are more modest. (B) In contrast with the human condition, mouse levels of bioavailable testosterone (blue) and estradiol (red) do not decrease with age. M=male and F=female. Data derived from human
[56–60] and mouse
[61, 62] studies.