A review of the behavioral effects of estrogen has to be divided on the basis of different time periods. During the critical phase of brain differentiation (which is species specific and takes place either before birth or neonatally), various sex hormones have long-term and permanent effects on behavior. Therefore, their action has been termed developmental or organizational. In contrast, hormones in adulthood, after the critical phase has passed, affect reversible changes in behavior and have been called activational.

THE ROLE OF ESTROGEN IN BRAIN DIFFERENTIATION

The different action of hormones during the organizational phase vs the activational time period has been demonstrated by those researchers who work directly on the brain level of animals and who use techniques such as autoradiography of the implantation of minute amounts of specific hormones in different brain regions. From scientists like Roger Gorski at the University of California at Los Angeles, we have learned that sexual differentiation of the brain appears to be located in the preoptic area and the hypothalamus. The exposure to specific steroids during the organizational phase affects permanent alterations of the neural connections of the preoptic area in male and female rat brains (they change the "wiring" of the brain), while hormones in adulthood activate temporal changes in neuronal function of hormone-sensitive cells lying within developmentally fixed neural circuits.

We also have considerable evidence about the events of the cellular level itself from neurochemical research, as from Bruce McEwen's laboratory at Rockefeller University, New York. It appears to be proved at this point that testosterone is the crucial steroid during normal male and female sexual differentiation, but that a portion of testosterone is converted into estradiol on the target brain cell level and insteracts with estrogen receptors.1

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