Oestradiol as a neuromodulator of learning and memory

Estrogen rapidly activates the mitogen-activated protein kinases, Erk-1 and Erk-2, via an as yet unknown mechanism. Here, evidence is provided that estrogen-induced Erk-1/-2 activation occurs independently of known estrogen receptors, but requires the expression of the G protein-coupled receptor homolog, GPR30. We show that 17beta-estradiol activates Erk-1/-2 not only in MCF-7 cells, which express both estrogen receptor alpha (ER alpha) and ER beta, but also in SKBR3 breast cancer cells, which fail to express either receptor. Immunoblot analysis using GPR30 peptide antibodies showed that this estrogen response was associated with the presence of GPR30 protein in these cells. MDA-MB-231 breast cancer cells (ER alpha-, ER beta+) are GPR30 deficient and insensitive to Erk-1/-2 activation by 17beta-estradiol. Transfection of MDA-MB-231 cells with a GPR30 complementary DNA resulted in overexpression of GPR30 protein and conversion to an estrogen-responsive phenotype. In addition, GPR30-dependent Erk-1/-2 activation was triggered by ER antagonists, including ICI 182,780, yet not by 17alpha-estradiol or progesterone. Consistent with acting through a G protein-coupled receptor, estradiol signaling to Erk-1/-2 occurred via a Gbetagamma-dependent, pertussis toxin-sensitive pathway that required Src-related tyrosine kinase activity and tyrosine phosphorylation of tyrosine 317 of the Shc adapter protein. Reinforcing this idea, estradiol signaling to Erk-1/-2 was dependent upon trans-activation of the epidermal growth factor (EGF) receptor via release of heparan-bound EGF (HB-EGF). Estradiol signaling to Erk-1/-2 could be blocked by: 1) inhibiting EGF-receptor tyrosine kinase activity, 2) neutralizing HB-EGF with antibodies, or 3) down-modulating HB-EGF from the cell surface with the diphtheria toxin mutant, CRM-197. Our data imply that ER-negative breast tumors that continue to express GPR30 may use estrogen to drive growth factor-dependent cellular responses.

The underrepresentation of female mice in neuroscience and biomedical research is based on the assumption that females are intrinsically more variable than males and must be tested at each of four stages of the estrous cycle to generate reliable data. Neither belief is empirically based. In a meta-analysis of 293 articles, behavioral, morphological, physiological, and molecular traits were monitored in male mice and females tested without regard to estrous cycle stage; variability was not significantly greater in females than males for any endpoint and was substantially greater in males for several traits. Group housing of mice increased variability in both males and females by 37%. Utilization of female mice in neuroscience research does not require monitoring of the estrous cycle. The prevalence of sex differences at all levels of biological organization, and limitations in generalizing findings obtained with males to females, argue for the routine inclusion of female rodents in most research protocols. Copyright © 2014 Elsevier Ltd. All rights reserved.

In adult mammalian brain, occurrence of the synthesis of estradiol from endogenous cholesterol has been doubted because of the inability to detect dehydroepiandrosterone synthase, P45017alpha. In adult male rat hippocampal formation, significant localization was demonstrated for both cytochromes P45017alpha and P450 aromatase, in pyramidal neurons in the CA1-CA3 regions, as well as in the granule cells in the dentate gyrus, by means of immunohistochemical staining of slices. Only a weak immunoreaction of these P450s was observed in astrocytes and oligodendrocytes. ImmunoGold electron microscopy revealed that P45017alpha and P450 aromatase were localized in pre- and postsynaptic compartments as well as in the endoplasmic reticulum in principal neurons. The expression of these cytochromes was further verified by using Western blot analysis and RT-PCR. Stimulation of hippocampal neurons with N-methyl-d-aspartate induced a significant net production of estradiol. Analysis of radioactive metabolites demonstrated the conversion from [(3)H]pregnenolone to [(3)H]estradiol through dehydroepiandrosterone and testosterone. This activity was abolished by the application of specific inhibitors of cytochrome P450s. Interestingly, estradiol was not significantly converted to other steroid metabolites. Taken together with our previous finding of a P450scc-containing neuronal system for pregnenolone synthesis, these results imply that 17beta-estradiol is synthesized by P45017alpha and P450 aromatase localized in hippocampal neurons from endogenous cholesterol. This synthesis may be regulated by a glutamate-mediated synaptic communication that evokes Ca(2+) signals.