Estrogen Effects on the Synaptology and Neural Membranes of the Rat Hypothalamic Arcuate Nucleus1

The concept of estrogen-induced “sexual dfferentiation of the brain ” has been a usefulfocus for investigation. However, it has become clear that the action of estrogen on the rat brain is lifelong, including effects on neurogenesis in the fetus, synaptogenesis in the newborn, and synaptic remodeling in the adult. Estrogen imparts sex d4fferences in the rat’s brain by shaping synaptology, postsynaptic membranes, and glia within the arcuate nucleus. These effects of estrogen on the arcuate nucleus also could underlie sexual maturation in both sexes and the development of senescent constant estrus in females. Brain Differentiation in the Rat as an Indicator of Sex Differences Over the past five decades, attention has focused on so-called “sexual differentiation of the rat brain, ” which occurs during the perinatal period and results in males having tonic and females having cyclic patterns of gonadotropin release and sexual behavior from sexual maturation forward (Harris, 1964). Tonic reproduction arises from gonadotropin regulation that is limited to a simple reciprocal relationship between gonadal steroids and gonadotropins. The cyclic pattern requires more 1Supported by NIH-lID 13587. The Mellon Foundation, and CISC (Spain). D.K. is a Kennedy-Dannreuther Fellow of the American Gyneco