Decay of fluorescence polarization after an impulsive excitation is correlated with wobbling motion of fluorescent molecules in membranes. The motion is characterized by two parameters, a "wobbling diffusion constant" and a "degree of orientational constraint" both of which can be determined directly from experimentally obtained decay. Detailed discussion, including theoretically calculated time-courses of polarization decay, is given for several types of molecules embedded in lipid bilayers; these types cover a large part of fluorescent probes available at present. The theory is useful for the analysis of fluorescence polarization decay in any system where the orientation of fluorophore is restricted by the surrounding structure.

In adult mammalian brain, occurrence of the synthesis of estradiol from endogenous cholesterol has been doubted because of the inability to detect dehydroepiandrosterone synthase, P45017␣. In adult male rat hippocampal formation, significant localization was demonstrated for both cytochromes P45017␣ 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 P45017␣ 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 17␤-estradiol is synthesized by P45017␣ 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.

Rapid modulation of hippocampal synaptic plasticity by estrogen has long been a hot topic, but analysis of molecular mechanisms via synaptic estrogen receptors has been seriously difficult. Here, two types of independent synaptic plasticity, long-term depression (LTD) and spinogenesis, were investigated, in response to 17b-estradiol and agonists of estrogen receptors using hippocampal slices from adult male rats. Multi-electrode investigations demonstrated that estradiol rapidly enhanced LTD not only in CA1 but also in CA3 and dentate gyrus. Dendritic spine morphology analysis demonstrated that the density of thin type spines was selectively increased in CA1 pyramidal neurons within 2 h after application of 1 nM estradiol. This enhancement of spinogenesis was completely suppressed by mitogen-activated protein (MAP) kinase inhibitor. Only the estrogen receptor (ER) alpha agonist, (propyl-pyrazole-trinyl)tris-phenol (PPT), induced the same enhancing effect as estradiol on both LTD and spinogenesis in the CA1. The ERbeta agonist, (4-hydroxyphenyl)-propionitrile (DPN), suppressed LTD and did not affect spinogenesis. Because the mode of synaptic modulations by estradiol was mostly the same as that by the