Hormonal regulation of adult hippocampal dendritic spine density

Dendritic spines cover the surface of a wide variety of neuronal types and are the postsynaptic sites of approximately 90% of the excitatory synapses formed in the central nervous system. Interestingly, changes in the morphology and/or density of dendritic spines have been shown to occur naturally, implying that they are a normal part of brain function. Even in the adult, dendritic spines are remarkably plastic. The hormonal state of an animal has been shown to be an important factor in regulation of dendritic spine density, both during development and in the adult. In the adult female rat, hippocampal CA1 pyramidal cells are particularly sensitive to variation in the circulating levels of the ovarian steroids, estradiol and progesterone. Removal of estradiol and progesterone by ovariectomy results in an approximately 50% decrease in the density of dendritic spines on the lateral branches of the apical dendritic tree. Treatment with estradiol can either protect against or reverse this decrease; subsequent progesterone treatment for as few as 5 hours significantly augments the effect of estradiol. By 18-24 hours following progesterone treatment, spine density returns to low values.