[Role of androgen in the elderly. Modulation of synaptic plasticity by brain-synthesized androgens].

Recently, brain synthesis of androgen and estrogen has been extensively investigated. Steroidogenic enzymes and receptors are expressed in glutamatergic neurons. The expression levels of mRNA or proteins for enzymes are as low as 1/200 - 1/1,000. However, hippocampal levels of androgen and estrogen are much higher than those of plasma. This is due to the fact that the volume of hippocampus is as small as 1/200 of the blood vessels. Androgen and estrogen can rapidly modulate synaptic plasticity of neural circuits. After andropause or menopause, the levels of androgen and estrogen in the hippocampus may significantly decrease, inducing dementia, Alzheimer's or depression. Hormone replacement therapy is valid for rescue of memory function, therefore synthesis and action of hippocampal androgen and estrogen is an important field for investigations.