Steroid hormones as mediators of neural plasticity

Steroid and thyroid hormone receptors are expressed in the developing brain and persist throughout adult life. They mediate a variety of effects on the brain, ranging from developmental effects of thyroid hormone and the process of sexual differentiation to the cyclic changes during reproductive cycles in adult female animals. This review summarizes data from the author's laboratory on three topics: (1) actions of extradiol and progesterone on the ventromedial nucleus of the hypothalamus in adult female and male rats, showing both the cyclicity and the consequences of brain sexual differentiation; (2) actions of estradiol on the cholinergic neurons of the basal forebrain of the female and male rat, reflecting the plasticity of the adult cholinergic system as well as sex differences which are developmentally programmed; and (3) diverse actions of estrogens, thyroid hormone and glucocorticoids on the morphology of hippocampal neurons. The review concludes by discussing the interactions between "organizational" (i.e. developmental) effects and the "activational" effects of steroids on the mature nervous system in relation to the environmental control of brain gene expression.