Direct inhibition of tyrosine hydroxylase activity by catechol estrogens

2-Hydroxyestradiol is not mediating the effects of estradiol on tuberoinfundibular dopaminergic neurons controlling prolactin secretion in female rats

Conversion of estrogens in their hydroxylated derivatives has been proposed as one of the mechanisms of action for these steroids in the brain. However, the actions of catechol estrogens on the hypothalamic control of the prolactin (PRL) release seem to be different to the effects caused by native estrogens. To clarify this question, we have examined the activity of tuberoinfundibular dopaminergic (TIDA) neurons in ovariectomized rats treated with estradiol (E2) or 2-hydroxyestradiol (OHE2), both administered alone or previously to progesterone (P). L-3,4 dihydroxyphenylacetic acid (DOPAC)/dopamine (DA) ratio in the medio basal hypothalamus (MBH), the anterior pituitary (AP) content of DA and the PRL release into the peripheral blood have been used as indexes of neural activity of TIDA system. Ovariectomy caused a significant reduction in plasma PRL levels, with increases of the DOPAC/DA ratio in MBH and of the AP content of DA. These changes were reversed by administration of E2, suggesting that this steroid exerted its stimulatory effect on PRL release by decreasing the activity of TIDA neurons. In contrast, OHE2 was unable to alter plasma PRL levels, the DOPAC/DA ratio in the MBH and the AP content of DA. On the other hand, P was able to stimulate the PRL release, but it needed a previous priming action of E2 since it was unable to modify plasma PRL levels when it was injected alone. This priming effect of E2 for the P action was not exerted by OHE2. Hence, we can conclude that the conversion of E2 in its 2-hydroxylated derivative is not mediating the effects of this steroid, nor its priming action for the P effect, on the hypothalamic dopaminergic control of the PRL release.

Characteristics of estrogen-2/4-hydroxylase in pig blastocysts: inhibition by steroidal and nonsteroidal agents

The inhibitory potencies of steroidal and non-steroidal estrogens, catechol-estrogens, methoxyestrogen, haloestrogens, cholesterol and its side-chain-cleaved products, and inhibitors of steroid aromatase against the activity of estradiol-2/4-hydroxylase (E-2/4-H) in pig blastocysts were studied. All tested compounds, except cholesterol and 4-hydroxyandrostenedione, inhibited E-2/4-H in vitro. The fluctuation of E-2/4-H activity in pig blastocysts on different days of pregnancy may be due to the modulation of enzyme activity by steroids in the uterine lumen. Although alpha-naphthoflavone and aminoglutethimide did not affect E-2/4-H activity in vitro, inhibition by CO (95% CO + 5% O2), SKF-525A, piperonyl butoxide, and antibody to cytochrome P-450 reductase provides evidence for the involvement of cytochrome P-450 in E-2/4-H activity in pig blastocysts.

Estrogen hydroxylase activity in the human placenta at term

Placental estrogen hydroxylase (EH) enzyme activity was measured at term using the catechol-O-methyl transferase coupled method in normal and high risk conditions. The identity and ratio of products formed during incubation of microsomes as analysed by high performance liquid chromatography in chronic hypertension, toxemia and diabetes mellitus was not different from controls. The mean enzymatic activity was also not different among the conditions studied as expressed mean +/- SE pmol/min/mg, protein: chronic hypertension (7.8 +/- 1), toxemia (8 +/- 1.6), diabetes mellitus (6.1 +/- 0.9) and controls (8.3 +/- 1.5). The cofactor dependence of EH was studied showing that NADPH is a better substrate for the enzyme than NADH.

Steroid structure and function. Molecular conformation of 4-hydroxyestradiol and its relation to other catechol estrogens

Hydroxylation of estrogens at C(2) or C(4) effects differentially their binding affinity to and dissociation rate from the estrogen receptor. The X-ray crystal structure of 4-hydroxyestradiol (4-OH-E2) is reported here and compared with that of 2-hydroxyestradiol (2-OH-E2), the 2- and 4-hydroxylated derivatives of estrone (E1) and with that of the parent estrogens, E1 and E2. The overall molecular shape and hydrogen bonding patterns of each were examined for their possible relevance to their binding to the estrogen receptor and their biological activity. A shift in the B-ring conformation away from the symmetrical 7 alpha,8 beta-half-chair form toward the 8 beta-sofa form is induced by both 2- and 4-hydroxy substitution. This shift appears to be larger in the case of E2 than E1 derivatives and to be correlated with an observed change in the hydrogen bonding potential of the C(3) hydroxyl. In 4-OH-E2, as in E2 and 4-OH-E1, the C(3) hydroxyl functions both as a hydrogen bond donor and acceptor. In contrast in 2-OH-E2 the hydroxyl functions only as a donor. The markedly reduced affinity of 2-hydroxylated estrogens for the estrogen receptor could be due to a combination of steric interactions, competition between O(2) and O(3) for hydrogen bonds for a common site on the receptor, and to general interference with hydrogen bond formation of O(3). The C(4) hydroxyl participates in the formation of a chain of hydrogen bonds in the solid state that is similar to a chain seen in single crystals of E2. The presence of a similar chain of hydrogen bonds involving O(3) in the receptor site could account for the decreased dissociation rate of the 4-OH-E2 receptor complex.

Fluoroenzymatic cycling assay (FECA) for the determination of catechol estrogen monomethyl ethers in human urine

In the present study a method is described for the quantitative determination of the methylated metabolites of catechol estrogens in human urine. Following initial enzymatic hydrolysis the urine samples are extracted with ethyl acetate. The monomethyl ethers of catechol estrogens are then selectively fractionated with straight phase chromatography on Lipidex-5000 gel. Finally, samples are quantitated using enzymatic cycling with 17 beta-estradiol dehydrogenase combined with fluorometry. The method is sensitive, reproducible and reasonably rapid for routine analysis and avoids the hazards of radioisotopes. Preliminary values of normal males and non-pregnant females are presented.

Inhibition by bromoestrogens of the effects of estradiol on apomorphine-induced climbing behavior

The chronic administration of estrogens to mice or rats will result in antidopaminergic effects. Apomorphine-induced climbing behavior in mice, the result of direct stimulation of dopamine receptors in the striatal and mesolimbic regions, is a simple animal model for examining these antidopaminergic effects of estrogens. Bromoestrogens, inhibitors of catechol estrogen formation, have been utilized in order to examine the role of estrogen metabolism in dopaminergic antagonism. Mice were pretreated for 3 days with 2-bromoestradiol, 4-bromoestradiol, or 2,4-dibromoestradiol dibenzoates alone or in combination with estradiol benzoate prior to apomorphine administration. The haloestrogens did not alter the climbing-induced responses elicited by apomorphine, whereas estradiol benzoate clearly attentuated the actions of apomorphine. Furthermore, the bromoestradiol dibenzoates were effective in reversing the effects of estradiol benzoate when the two steroids (estradiol benzoate and a bromoestrogen dibenzoate) were administered simultaneously during pretreatment. Thus, the bromoestrogens are able to inhibit the antidopaminergic effects of estradiol exhibited in the apomorphine-induced mouse climbing model.

Binding and effects of catecholestrogens on adenylate cyclase activity, and adrenoceptors, benzodiazepine and GABA receptors in guinea-pig hypothalamic membranes

Catecholestrogen (CE) binding to guinea-pig hypothalamic membranes was assessed by using [3H]2-hydroxyestrone (2-OHE1) as a ligand. Binding was maximal at pH 7.4 and 37 degrees C, and after 10 min incubations. A high affinity binding site with dissociation constant (KD) = 0.20 +/- 0.02 nM and site concentration (Bmax) = 38 +/- 2 fmol/mg protein, and a low affinity binding site with KD = 235 +/- 10 nM and Bmax = 4.2 +/- 1.0 pmol/mg protein (n = 7) were detected. The order of affinity (Ki, microM) for displacement of 10 nM [3H]2-OHE1 from hypothalamic binding sites was 2-OHE1 (0.8), 2-hydroxyestradiol (2-OHE2) (1.0), epinephrine (5.9), norepinephrine (NE) (7.7), dopamine (270), estradiol, estrone, propranolol, phentolamine, domperidone (greater than 10 000). NE inhibition of 2-OHE1 binding was non-competitive, Only 0.5 mM 2-OHE2 depressed in a non-competitive way the hypothalamic beta-adrenoceptor binding (measured by using [3H]dihydroalprenolol) without affecting alpha-adrenoceptor binding (measured by using [3H]dihydroergocryptine). Both 2-OHE2 and 2-OHE1 impaired NE-stimulated adenylate cyclase activity in hypothalamic membranes with EC50 of about 5 and 10 microM, respectively. CE decreased [3H]gamma-aminobutyric acid binding by hypothalamic membranes with Ki = 8 microM (2-OHE2) and 50 microM (2-OHE1). The binding of [3H]flunitrazepam to the same membrane preparation was not affected by CE. These results support the existence of significant CE binding and effects in guinea-pig hypothalamic membranes.

Mechanism of the rapid effect of 17 beta-estradiol on medial amygdala neurons

The mechanism by which sex steroids rapidly modulate the excitability of neurons was investigated by intracellular recording of neurons in rat medial amygdala brain slices. Brief hyperpolarization and increased potassium conductance were produced by 17 beta-estradiol. This effect persisted after elimination of synaptic input and after suppression of protein synthesis. Thus, 17 beta-estradiol directly changes the ionic conductance of the postsynaptic membrane of medial amygdala neurons. In addition, a greater proportion of the neurons from females than from males responded to 17 beta-estradiol.

Modulation of apomorphine-induced climbing behavior by estradiol

The acute and chronic effects of estradiol benzoate were studied on apomorphine-induced climbing behavior in intact female mice. Climbing behavior was measured by determining the maximum climbing time and climbing index. Mice pretreated with estradiol benzoate (0.1 or 0.3 mg/kg, SC) for 3.5 or 24 hours prior to apomorphine administration showed no significant difference in climbing behavior when compared to corn oil-treated controls. However, mice pretreated with estradiol benzoate (0.1 mg/kg, SC) for 3 consecutive days showed an attenuation in apomorphine-induced climbing at 24 or 72 hours after the last steroid injection. This study shows that the mouse climbing behavioral model provides a simple and quantitative procedure for studying the antidopaminergic effects of estrogen.

Autoradiographic determination of catechol estrogen binding sites in brain, pituitary and uterus

The anatomical pattern of nuclear binding of 2-OH[6,9-3H]estradiol ([3H]2-OHE2) in brain, pituitary and uterus have been studied autoradiographically. Autoradiograms of forebrain, pituitary and uterus show nuclear concentrations of radioactivity in certain cells. This nuclear concentration is abolished when unlabelled 2-OHE2 or E2 was injected prior to the injection of [3H]2-OHE2. In the brain nuclear labelling is observed in the septal-preoptic region, in the anterior hypothalamic area, and in the central hypothalamic area. Some estrogen-sensitive nuclear groups, such as lateral septum and hippocampus, do not show accumulation of radioactivity. In the uterus, luminal and glandular epithelium, stromal cells and muscle cells are labelled. A comparison of the quantitative nuclear uptake of radioactivity and of the different time intervals after the injection of different doses shows similar uptake of nuclear radioactivity. This is comparable to data obtained after [6,7-3H]estradiol ([3H]E2) injection. The results provide clear evidence for nuclear binding of catechol estrogens of the same magnitude as [3H]E2 after in vivo treatment.

Role of female gonadal hormones in the CNS: clinical and experimental aspects

The large body of evidence presented indicates that in the brain the action of sex hormones cannot be thought as restricted to the regulation of endocrine functions and mating behavior. Estrogens and progesterone seem to act in numerous regions of the CNS to regulate motor as well as limbic functions. Furthermore, the data reviewed indicate that these hormones may modulate neuronal activity through a wide variety of mechanisms. More studies should focus on such mechanisms in order to better understand the role of sex hormones in the CNS and to devise ways of limiting their effects on depression, epilepsy etc. It is known that in peripheral target organs these hormones modulate cell activities by binding to specific receptors which can recognize the DNA sequence and activate the transcription of selected genes (135, 136). There is evidence supporting the hypothesis that this mechanism of action has been conserved also in the brain. First, the brain receptors for progesterone and estrogens are functionally and biochemically indistinguishable from those in the periphery (4, 5): they may be concentrated in neuronal nuclei and bind chromatin "in vitro" (7). Second, a temporal relationship has been observed between administration of steroids and the increase of polymerase II activity (137) and protein synthesis (4, 5). Third, various hormone-induced behaviors may be blocked by inhibitors of the protein synthesis (138, 139, 140, 141). However, sex hormones must be capable to regulate neuronal functions by mechanisms other then genomic. In fact, the topical application of estrogen or progesterone on nervous tissue results in a rapid change of membrane potential (60, 71). Such a rapid effect is not likely to be the consequence of nuclear action, but rather must be related to events occurring on the cell surface. It has been hypothesized that sex steroids affect the fluidity of the cell membrane, therefore modifying the ion transport or neurotransmitter receptor activity (142). If this were the case we would expect to observe a similar effect after application of any steroid. Experimental evidence demonstrates that not all the steroids affect the nervous membrane potential. Moreover, two steroids, estradiol and progesterone, have been described to modulate membrane potential in an opposite way (66, 67, 69, 75). At the moment, there is no evidence for the presence of steroid receptors on neuronal membranes which could mediate the described phenomena.(ABSTRACT TRUNCATED AT 400 WORDS)

The possible role of 2-hydroxyestradiol in the development of estrogen-induced striatal dopamine receptor hypersensitivity

In the present study, we have confirmed the existence of a biphasic response in striatal dopamine receptor sensitivity following the administration of estradiol benzoate (EB). This biphasic response consists of a hyposensitive phase 24 h after the last injection of EB, followed by a hypersensitive phase 72 h after the last injection of EB. In contrast to this, the administration of 2-hydroxyestradiol (2-OHE2), a catechol metabolite of estrogen, resulted in a striatal dopamine receptor hypersensitivity at both 24 and 72 h after the last injection of 2-OHE2. Studies on the in vivo metabolism of [3H]estradiol to its [3H]catechol metabolites indicated that the administration of piperonyl butoxide (PBO; a microsomal enzyme inhibitor) significantly decreased the level of [3H]catechol metabolites of [3H]estradiol in the striatum and in the medial basal hypothalamus. In addition, PBO administration resulted in about a 7-fold decrease in the ability of estradiol to induce a striatal dopamine receptor hypersensitivity. These data indicate that the biphasic response in striatal dopamine receptor sensitivity following estrogen, may be mediated by separate molecular mechanisms. The association of the hypersensitive phase with pharmacological doses and/or treatment paradigms, the development of a similar hypersensitivity following the administration of the 2-OHE2 metabolite of estrogen and the attenuation of the estrogen-induced striatal dopamine receptor hypersensitivity in PBO pretreated animals all suggest that this striatal dopamine receptor hypersensitivity may be mediated, at least in part, by the catecholestrogens.

Immunocytochemical localization of tissue-bound oestradiol in rat paracervical ganglion

The uterine paracervical ganglion (Frankenhauser's ganglion) contains the terminal neurons of the cholinergic sacral parasympathetic, the short adrenergic sympathetic and the peptidergic (vasoactive intestinal polypeptide-containing) nerves of the internal genitalia. Previous studies have shown that either the number of cells or transmitter content of each of these neuronal systems is altered by variations in steroid hormones. Furthermore, our recent study showed that some component of the rat paracervical ganglion was capable of metabolizing [3H]oestradiol to oestrone and the 2-OH and 4-OH forms of oestrone and oestradiol. The present study employs the peroxidase-anti-peroxidase immunohistochemical method to localize oestradiol in rat paracervical ganglia. Specific reaction product was identified in (1) cytoplasm and some nuclei of principal ganglion cells, (2) cytoplasm of large vacuolated ganglion cells, (3) cytoplasm of 'small intensely fluorescent' cells and (4) some nerve fibres in ganglia from animals in oestrus. The cytoplasm of principal neurons and some nerve fibres exhibited specific staining for oestradiol in dioestrus and pro-oestrus. No oestradiol was localized in ganglia excised from animals in metoestrus. Preincubation in oestradiol before fixation was necessary for specific localization of oestradiol; treatment of tissues with oestradiol after fixation was not required. These results are not consistent with binding of oestradiol to the classical oestrogen receptor. The resistance of oestradiol to organic solvent extraction suggests that oestradiol is covalently bound to tissue proteins. Such covalently bound oestradiol has been reported as a by-product of tissue metabolism of oestradiol via P-450 enzymes.

Microbiological hydroxylation of estradiol: formation of 2- and 4-hydroxyestradiol by Aspergillus alliaceus

Microorganisms known to hydroxylate alkaloids, amino acids, and aromatic substrates were examined for their potential to hydroxylate 17 beta-estradiol and estrone. Thin-layer chromatography of fermentation extracts revealed a wide range of steroid products. Aspergillus alliaceus (UI 315) was the only culture capable of producing good yields of catechol estrogens with 17 beta-estradiol. The organism also transformed estrone but not to catechol products. Analytical experiments with high-performance liquid chromatography revealed that A. alliaceus formed 4- and 2-hydroxyestradiol with yields of 45 and 16%, respectively. A preparative-scale incubation was conducted in 2 liters of medium containing 1 g of 17 beta-estradiol as substrate. 4-Hydroxyestradiol was isolated and identified by proton nuclear magnetic resonance and high-resolution mass spectrometry. Ascorbic acid was added to microbial reaction mixtures as an antioxidant to prevent the decomposition of unstable catechol estrogen metabolites. The microbial transformation of 17 beta-estradiol by A. alliaceus provides an efficient one-step method for the preparation of catechol estrogens.

Variation in hepatic microsomal cytochrome P-450 1 concentration among untreated rabbits alters the efficiency of estradiol hydroxylation

The metabolism of 17 beta-estradiol was examined using both rabbit liver microsomes and highly purified forms of rabbit liver microsomal cytochrome P-450. The predominant microsomal metabolite of 17 beta-estradiol is the 2-hydroxylated product. 2-Hydroxyestradiol is also the principal metabolite in reconstitution experiments in which P-450 1 exhibits the greatest Vmax, ca. 6 mol min-1 mol P-450 1(-1), vs less than 0.6 mol min-1 mol P-450(-1) for forms 2, 3b-, 3b+, 3c, 4, and 6. In addition P-450 1 has the lowest Km, ca. 2 microM. This suggested that microsomes which differ in their content of P-450 1 would also differ in the kinetic parameters characterizing the 2-hydroxylation of 17 beta-estradiol. Microsomes containing low amounts of P-450 1, less than 0.1 nmol/mg protein, exhibit a low-efficiency (Vmax/Km) 2-hydroxylase activity. Microsomes containing elevated concentrations of P-450 1, greater than 0.3 nmol/mg protein, exhibit a substrate dependence suggestive of an additional high-efficiency enzyme. The latter is specifically inhibited by a monoclonal antibody that recognizes P-450 1. These results indicate that the elevated expression of P-450 1 in microsomes leads to a marked increase in the apparent first-order rate constant for the 2-hydroxylation of 17 beta-estradiol, as it does for the 21-hydroxylation of progesterone. This should have a marked effect on the metabolism of these two steroid hormones at concentrations that are likely to occur in vivo.

Stimulatory effect of neonatal beta-estradiol treatment on aspartate-aminotransferase activity within rat cerebellar cortex

The effect of neonatal beta-estradiol treatment on the Aspartate-aminotransferase (Asp-T) activity within rat cerebellar cortex has been studied using a semi-quantitative histochemical technique. The Asp-T, localized chiefly at the level of nerve fiber-like structures in the molecular layer as well as around the perikaryon of Purkinje neurons, is significantly stimulated after estrogenization. The nerve fibers localized in the molecular layer were the Asp-T cerebellar structures more remarkably influenced by beta-estradiol treatment.

Inhibition of dihydropteridine reductase by novel 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine analogs

Hydroxylated derivatives of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a nigrostriatal neurotoxin in humans and primates, noncompetitively inhibited dihydropteridine reductase from human liver and rat striatal synaptosomes in vitro at micromolar concentrations. In contrast, MPTP and its chloro- and norderivatives did not inhibit this enzyme at lower than millimolar concentrations. Dihydropteridine reductase converts dihydrobiopterin to tetrahydrobiopterin, the required cofactor for the hydroxylation of aromatic amino acids during the synthesis of dopamine and serotonin.

Presence of NADPH-cytochrome P450 reductase in central catecholaminergic neurones

The cytochrome P450-containing mixed function oxidases metabolize a variety of endogenous and exogenous compounds including drugs, carcinogens, fatty acids and steroids. Mixed function oxidases have been detected in several tissues, including brain. The enzyme system consists of a lipid fraction (phosphatidylcholine), cytochrome P450 and NADPH-cytochrome P450 reductase. NADPH-cytochrome P450 reductase has been purified to apparent homogeneity and demonstrated to supply reducing equivalents from NADPH to cytochrome P450 (refs 5-7). Detection of NADPH-cytochrome P450 reductase thus represents an indirect means of demonstrating the presence of cytochrome P450. Although the role of cytochrome P450 in the central nervous system (CNS) is not known, it may include such different functions as metabolism of xenobiotics, aromatization of androgens to oestrogens and the formation of catecholoestrogens. Despite the potentially very important role(s) of cytochrome P450 in brain function, its exact regional distribution remains essentially unknown. Using a specific antibody against rat liver NADPH-cytochrome P450 reductase in combination with immunohistochemical techniques, we have now localized this enzyme to define catecholamine (CA)-containing structures of the rat and monkey brain.

Differential inhibition of activated tyrosine hydroxylase

Under conditions of cyclic AMP-dependent protein phosphorylation, tyrosine hydroxylase (EC 1.14.16.2; TH) is activated. Kinetic analysis reveals that, upon activation the affinity of the enzyme cofactor tetrahydrobiopterin, Vmax, as well as the Ki of its putative feedback inhibitor dopamine, are increased. Catecholic inhibitors of rat striatal TH have been assessed for the structural requirements that impart differential sensitivity to activated and control enzyme. By varying cofactor and inhibitor concentrations, Ki's were generated from Dixon plots. Structural analogs of dopamine in which the amino group was fixed in a cis conformation, i.e., 6,7-dihydroxytetrahydroisoquinolines, exhibit the same Ki for activated and nonactivated TH. However, 2-amino-6,7-dihydroxytetralin (ADTN), in which the nitrogen is extended in a fixed trans conformation of the beta-rotamer, exhibited a fourfold increase in Ki upon assaying tyrosine hydroxylase under phosphorylation conditions. By systematically increasing the hydrophobicity of the substituent at C-1 of 1-carboxy-6,7-dihydroxytetrahydroisoquinolines the inhibitory potency was enhanced, suggesting the presence of a hydrophobic region near the catecholic binding site. If the hydrophobic group was rigid as in the catechol estrogens, 2-hydroxy-estradiol and 2-hydroxyestrone, the Ki was relatively low (2 X 10(-5) M) despite the absence of an amino group. Upon activation the Ki increased fourfold. These studies provide insight into the topography of the catecholic binding site on TH and to attendant changes occurring upon activation. The results suggest that the catechol binding site includes both amino group-interacting and hydrophobic regions which are influenced by enzyme activation.

Do catecholestrogens interact with the in vitro binding of radioligands to catecholamine receptors?

Using a competitive binding assay the effects of 2-hydroxyestradiol-17 beta, 4-hydroxyestradiol-17 beta, estradiol-17 beta and progesterone on the binding of tritiated catecholaminergic ligands to membrane preparations from rat brain and pituitary gland were studied. Up to a concentration of 10(-5) M none of the steroids tested was able to displace [3H]spiroperidol, [3H]dihydroergocryptine or [3H]dihydroalprenolol. The data suggest that the catecholestrogens do not interfere directly with the binding of catecholaminergic ligands to dopaminergic, alpha-adrenergic or beta-adrenergic receptors in the central nervous system. The view that a catechol structure is not essential for the interaction with dopaminergic receptors was further supported by the results obtained from additional studies on the competition of O-methylated and deaminated dopamine metabolites with [3H]spiroperidol binding.

Catecholestrogens in the brain: neuroendocrine integration

In a series of experiments it has been shown that 4-hydroxyestradiol (4-OHE2) as well as 2-hydroxyestradiol (2-OHE2) are involved in regulatory mechanisms of LH secretion in the miniature pig. Two-hydroxyestrone (2-OHE1) and 1-hydroxyestradiol-benzoate (1-OHE2B), however, have no significant effects on LH secretion. Moreover the studies indicate a regional specificity in the action of 4-OHE2, 2-OHE2 and estradiol (E2). 4-OHE2 and 2-OHE2 decrease plasma LH when given into the ventromedial nucleus of the hypothalamus and increase plasma LH levels when microinjected into the dorsomedial nucleus of the hypothalamus. Whereas E2 affects LH secretion when it is given into the area dorsalis of the hypothalamus and fornix. In addition the experiments performed on female and male rats to measure the effects of 2-OHE2 and 2-OHE1 on turnover rates of dopamine, norepinephrine and epinephrine in the anterior mediobasal hypothalamus and medial preoptic area show that the effects of catecholestrogens may partly be mediated by catecholamines.

Possible mechanism of action of 2-hydroxylated estradiol on the positive feedback control for LH release in the rat

Evidence was given to support a positive role of 2-hydroxyestradiol on the LH surge. The catecholestrogen may act by its catechol A ring on the nucleus arcuatus COMT, consequently leaving the noradrenaline free. The result may be a longer action on the peptidergic terminal in the median eminence and an increase in the LH secretion by the pituitary. This assumption is supported by the observations that the catecholestrogen effect can be mimicked by homocystein, an aminoacid able also to inhibit COMT activity, having neither a steroid nor a catechol structure. The fact that alpha-MIT is able to prevent homocystein-induced increase in LH suggests that it is acting by protecting the local increase of the catecholamine. After ten years of intensive effort to understand the possible physiological role of the catecholestrogens, attention was mostly paid to its structural similarity to estrogen and a great deal of effort was made to understand its function by acting upon the estrogen receptor in the cytosol. The evidence for catecholestrogen action upon COMT, an outside membrane enzyme involved in the process of catecholamine degradation, supports the idea of a catechol action for 2-OHE2. The present evidence strongly supports the physiological importance of the catechol group in the 2-OHE2 in its action mechanism. However, a true physiological role for the catecholestrogens remains to be solved. The evidence we bring confirms once more that catecholestrogens may have a function and explains a new mechanism of action. However, the basic question concerning the true amount of catecholestrogen existing in the hypothalamic nuclei, either brought by the blood stream or locally produced, still needs to be solved: we cannot say whether the mechanism we described is a functioning one, whether it is just brought about by the experimental increase of the catecholestrogen or the artificial blockage of COMT.

Kinetics of catechol estrogen-estrogen receptor dissociation: a possible factor underlying differences in catechol estrogen biological activity

The mechanisms underlying the differences in uterotrophic potency between 2- and 4-hydroxyestrogens were explored. Doses of estradiol (E2)(10 micrograms/kg), 2-OHE2 (500 micrograms/kg) and 4-OHE2 (100 micrograms/kg) sufficient to induce near maximal cell nuclear estrogen receptor (ERn) binding were injected subcutaneously into 26 day old female rats. Uterine ERn concentrations declined more rapidly after 2-OHE2 than after E2 or 4-OHE2. E2 and 4-OHE2 both elicited a significant increase in uterine wet weight, measured at 24-36 hrs after injection. 2-OHE2 had no significant effect and neither synergized with nor antagonized the effects of simultaneously administered E2 or 4-OHE2. Under in vitro conditions at 25 degrees C, 2-hydroxyestrone (2-OHE1) and 2-OHE2 both dissociated from the receptors more rapidly than either their parent monophenolic estrogens or the corresponding 4-hydroxyestrogens. These results suggest that differences in estrogenic potency between 2- and 4-hydroxyestrogens may partly be a function of the dissociation kinetics of their estrogen receptor complexes.

Inhibition of dihydropteridine reductase by catechol estrogens

Catechol estrogens, such as 2-hydroxyestriol, 2-hydroxyestradiol, and 2-hydroxyestrone, inhibit human liver dihydropteridine reductase noncompetitively with Ki values ranging from 1.5 to 4.6 X 10(-6)M. Catechol estrogens lose approximately half of their inhibitory potency if the C-2 hydroxyl groups are methylated. Thus, 2-methoxyestrogens have inhibitory potencies equivalent to those of their parent estrogens--estriol, estradiol, and estrone. Aromatization of ring B or stereoisomerism at C-17 does not affect the inhibitory potency of estrogens, although stereoisomerism at C-16 enhances the inhibitory potency of estriol. These results support the hypothesis that catechol estrogens may interfere with catecholamine metabolism by acting as inhibitors of enzymes involved in catecholamine metabolism, such as dihydropteridine reductase.

The effect of estrogen priming of hypogonadal women on the release of gonadotropins and prolactin in response to 2-hydroxyestradiol

We have previously shown that the administration of a 2-hydroxyestradiol (20H-E2) infusion (250 microgram/h x 4 h) to hypogonadal women resulted in a selective increase in the levels of circulating prolactin (PRL) without changes in LH or FSH. The present study concerns the effect of estrogen priming of hypogonadal women on the release of gonadotropins and PRL in response to an identical 20H-E2 infusion. Estrogen priming consisted of a 5 day course of orally administered ethinyl estradiol at a daily dose of 300 micrograms. Significant (P less than 0.05) inhibition of LH release was observed within 1 h of the onset of the 20H-E2 infusion reaching a nadir (-25 +/- 2%) by 3.75 h. The circulating levels of FSH remained unaltered for the duration of the 8 h study. In contrast, significant (P less than 0.05) increments in the release of PRL could clearly be detected after a lag period of 1.5 h reaching a peak (+91 +/- 11%) by 4 h. These and previous findings demonstrate that the inhibitory influence of 20H-E2 on gonadotropin secretion is conditional upon prior estrogen priming while the ability of 20H-E2 to stimulate the release of PRL is not.

Differential clinical response to oestrogens after menopause

An analysis of vasomotor, psychological, and physical symptoms of 136 women who were receiving piperazine oestrone sulphate (Ogen) and conjugated equine oestrogens (Premarin) after menopause has shown differences in responses which can be explained only if it is accepted that the two oestrogenic compounds have differing effects on various parts of the body. Premarin (0.625 mg) was found to be more potent at inducing withdrawal bleeding than Ogen (1.25 mg), whereas Ogen was more effective than Premarin in alleviating hot flushes and some psychological symptoms. A hypothesis involving metabolism of oestrone to the catecholamine, 2-hydroxyoestrone, is postulated, which explains why these differences occur. It is further suggested that better selection of oestrogens to suit particular postmenopausal symptoms should be encouraged when prescribing oestrogen for women after menopause.

Tyrosine hydroxylase from bovine striatum: catalytic properties of the phosphorylated and nonphosphorylated forms of the purified enzyme

The properties of purified tyrosine hydroxylase (TH) from bovine corpus striatum, both native and phosphorylated forms of the enzyme, were studied. TH had a tendency toward greater affinity for tetrahydrobiopterin (BH4) than for the synthetic cofactor 6-methyltetrahydropterin (6-MPH4), although the maximal velocity of the TH-catalyzed reaction was greater with 6-MPH4. Phosphorylation increased the affinity of TH for cofactor at pH 6.0, with little change in Vmax. At pH 7.0, phosphorylation caused increased activation of TH by increasing Vmax as well as reducing the Km for cofactor. The K1 for dopamine was increased twofold by phosphorylation at pH 6.0, but eightfold at pH 7.0. Phosphorylation was not associated with a change in Km for tyrosine at any pH or with any cofactor studied, although the Km for tyrosine of TH was cofactor-dependent and seven to eight times greater with 6-MPH4 than with BH4 as cofactor. Heparin and NaCl activated native TH at pH 6.0, but not at pH 7.0. Phosphorylated TH was unaffected by heparin or salt at pH 6.0, but was relatively inhibited at pH 7.0. The data are presented in the context of the physiological environment of TH.

Competition by estrogens for catecholamine receptor binding in vitro

We have examined the ability of various steroids to compete for high-affinity binding of 3H-labeled ligands to catecholamine receptors in membranes prepared from rat cerebral cortex, striatum, and anterior pituitary. Ligands employed were: [3H]WB4101, [3H]prazosin, [3H]yohimbine, and [3H]clonidine (alpha-noradrenergic); [3H]dihydroalprenolol (beta-noradrenergic); [3H]spiperone and [3H]ADTN (dopaminergic). Only the 17 beta estrogens were effective and only binding of [3H]spiperone and [3H]ADTN in striatum and [3H]WB4101 and [3H]prazosin in cerebral cortex was reduced. Thus putative dopaminergic and alpha 1-noradrenergic sites alone appear to recognize estrogens. A slight competitive effect on [3H]spiperone binding to anterior pituitary membranes was also observed. Among the 17 beta estrogens tested, the most effective in all cases was the catechol estrogen 2-hydroxyestradiol (2-OHE2). The ability of 2-OHE2 (IC50 = 20-30 micro M) to inhibit ligand binding to alpha 1 receptors was comparable to that of norepinephrine (IC50 = 10-20 micro M), whereas for dopamine receptors in striatum and pituitary 2-OHE2 was an order of magnitude less effective than dopamine (IC50 = 12 micro M) in reducing binding of 3H ligands. Estradiol-17 beta and 2-hydroxyestrone were also able to inhibit binding, but the order of steroid potency was different for alpha 1 and dopaminergic receptors. Progesterone, testosterone, and corticosterone were without effect in all cases. These results show that there is specificity of steroid interactions with catecholamine receptors in the brain, both in terms of steroid structure and receptor type. The possible relevance of these interactions to neuroendocrine function is discussed.

Effects of short-term exposure to catecholestrogens on catecholamine turnover in the preoptic-hypothalamic brain of ovariectomized rats

The effects of a series of catecholestrogens (2-hydroxyestrogens and 4-hydroxyestrogens) were compared to those of a primary estrogen, ethynylestradiol (EE2), on the catecholaminergic system in the preoptic-hypothalamic rat brain. Adult ovariectomized rats received single injections (100 microgram s.c.) of EE2, 2-hydroxyestradiol (2-OHE2), 2-hydroxyethynylestradiol (2-OHEE2), 4-hydroxyestradiol (4-OHE2) or 4-hydroxyethynylestradiol (4-OHEE2). Eight hours after estrogen administration the animals were killed, and the concentrations of dopamine (DA), noradrenaline (NA) and adrenaline (A) were determined radioenzymatically in the preoptic area (POA) and the mediobasal hypothalamus (MBH). EE2 and all the catecholestrogens tested uniformly suppressed (P less than 0.01) serum LH levels. This correlated well with the decreased turnover rate of A in the POA. In both the POA and the MBH of EE2-treated animals the turnover rate of NA was markedly decreased whereas the concentrations of catecholamines remained unaffected. The catecholethynylestrogens, potent inhibitors of catechol O-methyltransferase, caused two-fold increases of NA concentrations in the POA (2-OHEE2 and 4-OHEE2) and MBH (2-OHEE2) without affecting the turnover rate. Thus, although EE2 and all the catecholestrogens tested uniformly suppressed LH release they induced highly different effects on the noradrenergic system In the preoptic-hypothalamic brain.

A comparison of 2-hydroxyestradiol and U-0521 (3'4'-dihydroxy-2-methylpropiophenone, Upjohn) as in situ and in vitro inhibitors of tyrosine hydroxylase

Feedback inhibition of tyrosine hydroxylase by catechols was evaluated using in situ and in vitro enzyme assays. The three catechol compounds used were norepinephrine, 2-hydroxyestradiol, and 3'4'-dihydroxy-2-methylpropiophenone (U-0521, Upjohn); representing endogenous catecholamines, catechol estrogens, and a synthetic catechol, respectively. The in situ experiments were performed with dissociated retinal cells from rats and with stationary phase adrenergic-like neuroblastoma cells (N1E-115). The catechol estrogen, 2-hydroxyestradiol, resembled the endogenous catecholamine in its potency to inhibit in vitro and in situ tyrosine hydroxylations with IC50 values of 10 microM in vitro and 100 microM in situ. The drug U-0521, which has been used as an inhibitor of catechol-O-methyltransferase (COMT), was also found to be an inhibitor of tyrosine hydroxylase. Further, it was shown to be more potent than the natural catechols, both in vitro and in situ, with IC50 values of 30--600 nM.

Constancy of adult hypothalamic tyrosine hydroxylase after gonadal steroid treatment during development

Tyrosine hydroxylase (TH) was studied in the entire hypothalamus of 60-day-old male and female rats, and its activity was similar in both sexes. Administration of estrogen or androgen to male or female pups soon after birth and during development did not affect the kinetic parameters of TH. It is, thus, suggested that sexual dimorphism in the levels and turnover of catecholamines (CA) and their sensitivity to fluctuations in gonadal steroids during development do not depend on corresponding sex-related or steroid-sensitive differences in the synthesis of hypothalamic CA.

Interactions of catecholestrogens with cytoplasmic and nuclear estrogen receptors in rat pituitary gland and hypothalamus

The affinity of a series of catecholestrogens for 7S cytoplasmic receptor proteins from hypothalamus and pituitary gland of ovariectomised rats was assessed in vitro by a competitive charcoal binding assay at 4 degrees C. The equilibrium dissociation constants (Ki) of catecholestrogens 4-hydroxyestradiol, 4-hydroxyethynylestradiol, 2-hydroxyestradiol, 2-hydroxyethynylestradiol, and 4-hydroxyestrone were of the same order (Ki: 0.3-0.6 nM) as those of estradiol and ethnylestradiol (Ki: 0.1 nM). Methylation of 2-hydroxyestradiol led to a substantial loss of binding affinity. Tritium-labelled receptor complexes were demonstrated in KCl extracts of purified nuclei from pituitary and hypothalamic tissue 1 h after intravenous injection of 0.1 mCi tritiated 2- or 4-hydroxyestradiol. These macromolecular complexes sedimented in the 5-6S region of 5-20% (w/v) sucrose gradients containing 0.4 M-KCl. Further evidence for the translocation of estrogen receptors by catecholestrogens into the nuclei of rat pituitary and hypothalamus was the increase in nuclear receptor concentrations, measured by exchange assay, 1 h after the intraperitoneal injection of 0.1 mg unlabelled catecholestrogen. Administration of 4-hydroxyestradiol and 4-hydroxyethynylestradiol increased nuclear receptor concentrations to the same maximal levels as those following application of the same dose of estradiol or ethynylestradiol, whereas the respective 2-hydroxylated compounds exhibited only 60-70% of the maximal translocating capacity. The in vivo translocating capacities of the various catecholestrogens tested at this dose correlated well with their binding affinities for cytosol receptors determined in vitro.

Failure of 2-hydroxyoestrone to lower prolactin concentrations in hyperprolactinaemic women

The catechol oestrogen 2-hydroxyoestrone has been reported to lower serum prolactin concentrations acutely in normal women and it has been proposed that it may be effective in suppressing prolactin secretion in hyperprolactinaemic patients. Five women with hyperprolactinaemia and anovulation were studied. Following a control infusion, 2-hydroxyoestrone was given at a rate of 80 micrograms/h for 4 h. In no patient was there a fall in prolactin levels and no changes were observed in gonadotrophin concentrations. We conclude that a short-term infusion of 2-hydroxyoestrone, at a dose which will produce high blood levels of the steroid, does not affect prolactin or gonadotrophin secretion in hyperprolactinaemic states.

Differential effects of estrogen and androgen on locomotor activity induced in castrated male rats by amphetamine, a novel environment, or apomorphine

Four experiments were conducted using male rats to determine whether testosterone (T), or its neural metabolites estradiol (E2) and 5 alpha-dihydrotestosterone (DHT), influence the expression of locomotor activity, which is thought to depend in part on the activation of mesolimbic dopaminergic neurons. Subcutaneous implantation of silastic capsules containing E2, but not T or DHT, caused a significant increase in the amount of activity displayed by castrated rats which had been habituated to infrared photobeam activity cages prior to receiving an i.p. injection of D-amphetamine sulfate (1.5 mg/kg). Administration of E2, but not T or DHT, also significantly increased the amount of activity which other groups of castrated rats displayed when they were first placed in the photobeam cages. Likewise, administration of E2 but not T or DHT, to castrated rats which had previously received bilateral injections of 6-hydroxydopamine into the nucleus accumbens septi, caused a significant increase in the activity displayed in response to an i.p. injection of apomorphine HCl (1.0 mg/kg). However, administration of estrogen or androgen to castrated rats which were neurologically intact had no effect on the amount of stereotyped behavior displayed in response to increasing doses (1.0, 2.0 and 4.0 mg/kg) of apomorphine. It is suggested that the selective, facilitatory effect of E2 on drug- and novelty-induced locomotor activity resulted from increased postsynaptic receptor activation at dopaminergic synapses in the nucleus accumbens septi.

Neural gonadal steroid actions

Neurons sensitive to gonadal steroids are located strategically within neural circuits that mediate behaviors broadly related to the reproductive process. Some neuronal events and properties are regulated by these hormones. Variability in the occurrence and distribution of particular neural hormonal sensitivities across species may be related to variations in the hormonal requirements for sexual differentiation and for activation of reproductive behaviors.