These studies sought to determine if neurons in the estrogen receptor-␣ knockout (ER␣KO) mouse brain concentrated 16␣-[ 125 I]iodo-11-methoxy-17-estradiol ( 125 I-estrogen), and if so, whether estrogen binding augmented the expression of progesterone receptor (PR) mRNA. Mice were injected with 125 I-estrogen and cryostat sections thaw mounted onto emulsion-coated slides. After 30-90 days of exposure, cells with a nuclear uptake and retention of 125 Iestrogen were observed in a number of ER␣KO mouse brain regions including the preoptic nucleus and arcuate nucleus of the hypothalamus, bed nucleus of the stria terminalis, and amygdala, although the number of labeled cells and intensity of nuclear concentration was markedly attenuated when compared with wild-type littermates. Competition studies with excess 17-estradiol, diethylstilbestrol, or moxestrol, but not with R5020 or dihydrotestosterone, prevented the nuclear concentration of 125 I-estrogen. To determine if the low level of estrogen binding was capable of regulating gene expression, in situ hybridization was used to evaluate PR mRNA in the brain. ER␣KO and wild-type mice were ovariectomized and treated with vehicle or 17-estradiol, and brains were sectioned and hybridized with a PR cRNA probe. Analysis of hybridization signal revealed a similar, low level of PR mRNA in ovariectomized wild-type and homozygous mice, and a marked increase in expression after treatment of ovariectomized animals with 17-estradiol, with the level of hybridization signal being significantly higher in wild-type animals when compared with ER␣KO mice. The results demonstrate that estrogen binds in the ER␣KO brain and is capable of modulating PR gene expression, thus supporting the presence and functionality of a nonclassical estrogen receptor.Estrogen is known to act in discrete regions of the female brain to regulate gamete production, proceptive and receptive behaviors, and other aspects of reproduction. Estrogens appear to regulate these physiological events by binding to the intracellular estrogen receptor (ER). The ligand-bound receptor then interacts with an estrogen response element on DNA and thereby modulates the transcription of specific genes. ER and its mRNA have been detected in brain regions where estrogen plays a central role in the regulation of events essential for procreation (1, 2).Recently, a transgenic mouse was constructed that lacked a functional ER-␣ (3). Initial analysis revealed that ER-␣ knockout (ER␣KO) mice were infertile, failing to show reproductive behavior or a normal estrous cycle (3). Interestingly, in vitro ligand binding studies revealed a 5% residual estrogen binding in the ER␣KO uterus (3), although Western blot analysis was unable to detect ER protein (4). Immunocytochemical studies with the ER␣KO mice were also unable to detect ER immunoreactivity in the brain (5). In spite of these findings, our laboratory has shown that several neurotransmitter and hormone receptor systems, thought to be dependent on estrogen, were not dramatica...