Molecular mechanisms involved in the protective actions of Selective Estrogen Receptor Modulators in brain cells.

Synthetic selective modulators of the estrogen receptors (SERMs) have shown to protect neurons and glial cells against toxic insults. Among the most relevant beneficial effects attributed to these compounds are the regulation of inflammation, attenuation of astrogliosis and microglial activation, prevention of excitotoxicity and as a consequence the reduction of neuronal cell death. Under pathological conditions, the mechanism of action of the SERMs involves the activation of estrogen receptors (ERs) and G protein-coupled receptor for estrogens (GRP30). These receptors trigger neuroprotective responses such as increasing the expression of antioxidants and the activation of kinase-mediated survival signaling pathways. Despite the advances in the knowledge of the pathways activated by the SERMs, their mechanism of action is still not entirely clear, and there are several controversies. In this review, we focused on the molecular pathways activated by SERMs in brain cells, mainly astrocytes, as a response to treatment with raloxifene and tamoxifen.

Hormonal and genetic factors interact to control aromatase expression in the developing brain.

Brain expression of the enzyme P450-aromatase has been studied extensively. Subsequent to the aromatisation hypothesis having established brain aromatase as a key factor to convert gonadal testosterone to oestradiol, several studies have investigated the regulation of aromatase during the critical period of brain sexual differentiation. We review previous and recent findings concerning regulation of aromatase. The role of gonadal hormones, sex chromosome genes and neurosteroids is analysed in terms of their contribution to aromatase expression, as well as implications for the organisational effect of steroids during development.

Selective Oestrogen Receptor Agonists Rescued Hippocampus Parameters in Male Spontaneously Hypertensive Rats.

Spontaneously hypertensive rats (SHR) show pronounced hippocampus alterations, including low brain-derived neurotrophic factor (BDNF) expression, reduced neurogenesis, astrogliosis and increased aromatase expression. These changes are reverted by treatment with 17ß-oestradiol. To determine which oestradiol receptor (ER) type is involved in these neuroprotective effects, we used agonists of the ERα [propylpyrazole triol (PPT)] and the ERß [diarylpropionitrite (DPN)] given over 2 weeks to 4-month-old male SHR. Wistar Kyoto normotensive rats served as controls. Using immunocytochemistry, we determined glial fibrillary protein (GFAP)+ astrocytes in the CA1, CA3 and hilus of the dentate gyrus of the hippocampus, aromatase immunostaining in the hilus, and doublecortin (DCX)+ neuronal progenitors in the inner granular zone of the dentate gyrus. Brain-derived neurotrophic factor mRNA was also measured in the hippocampus by the quantitative polymerase chain reaction. In SHR, PPT had no effect on blood pressure, decreased astrogliosis, slightly increased BDNF mRNA, had no effect on the number of DCX+ progenitors, and increased aromatase staining. Treatment with DPN decreased blood pressure, decreased astrogliosis, increased BDNF mRNA and DCX+ progenitors, and did not modify aromatase staining. We hypothesise that, although both receptor types may participate in the previously reported beneficial effects of 17ß-oestradiol in SHR, receptor activation with DPN may preferentially facilitate BDNF mRNA expression and neurogenesis. The results of the present study may help in the design of ER-based neuroprotection for the encephalopathy of hypertension.

17α-Oestradiol-induced neuroprotection in the brain of spontaneously hypertensive rats.

17ß-oestradiol is a powerful neuroprotective factor for the brain abnormalities of spontaneously hypertensive rats (SHR). 17α-Oestradiol, a nonfeminising isomer showing low affinity for oestrogen receptors, is also endowed with neuroprotective effects in vivo and in vitro. We therefore investigated whether treatment with 17α-oestradiol prevented pathological changes of the hippocampus and hypothalamus of SHR. We used 20-week-old male SHR with a blood pressure of approximately 170 mmHg receiving s.c. a single 800 µg pellet of 17α-oestradiol dissolved in cholesterol or vehicle only for 2 weeks Normotensive Wistar-Kyoto (WKY) rats were used as controls. 17α-Oestradiol did not modify blood pressure, serum prolactin, 17ß-oestradiol levels or the weight of the testis and pituitary of SHR. In the brain, we analysed steroid effects on hippocampus Ki67+ proliferating cells, doublecortin (DCX) positive neuroblasts, glial fibrillary acidic protein (GFAP)+ astrocyte density, aromatase immunostaining and brain-derived neurotrophic factor (BDNF) mRNA. In the hypothalamus, we determined arginine vasopressin (AVP) mRNA. Treatment of SHR with 17α-oestradiol enhanced the number of Ki67+ in the subgranular zone and DCX+ cells in the inner granule cell layer of the dentate gyrus, increased BDNF mRNA in the CA1 region and gyrus dentatus, decreased GFAP+ astrogliosis in the CA1 subfield, and decreased hypothalamic AVP mRNA. Aromatase expression was unmodified. By contrast to SHR, normotensive WKY rats were unresponsive to 17α-oestradiol. These data indicate a role for 17α-oestradiol as a protective factor for the treatment of hypertensive encephalopathy. Furthermore, 17α-oestradiol is weakly oestrogenic in the periphery and can be used in males.

Neuroprotection and sex steroid hormones: evidence of estradiol-mediated protection in hypertensive encephalopathy.

Besides their effects on reproduction, estrogens exert neuroprotective effects for brain diseases. Thus, estrogens ameliorate the negative aspects of aging and age-associated diseases in the nervous system, including hypertension. Within the brain, the hippocampus is sensitive to the effects of hypertension, as exemplified in a genetic model, the spontaneously hypertensive rat (SHR). In the dentate gyrus of the hippocampus, SHR present decreased neurogenesis, astrogliosis, low expression of brain derived neurotrophic factor (BDNF), decreased number of neurons in the hilus and increased basal levels of the estrogen-synthesizing enzyme aromatase, with respect to the Wistar Kyoto (WKY) normotensive strain. In the hypothalamus, SHR show increased expression of the hypertensinogenic peptide arginine vasopressin (AVP) and its V1b receptor. From the therapeutic point of view, it was highly rewarding that estradiol treatment decreased blood pressure and attenuated brain abnormalities of SHR, rendering hypertension a suitable model to test estrogen neuroprotection. When estradiol treatment was given for 2 weeks, SHR normalized their faulty brain parameters. This was shown by the enhancement of neurogenesis in the dentate gyrus, according to increased bromodeoxyuridine incorporation and doublecortin labeling, decreased reactive astrogliosis, increased BDNF mRNA and protein expression in the dentate gyrus, increased neuronal number in the hilus of the dentate gyrus and a further hyperexpression of aromatase. The presence of estradiol receptors in hippocampus and hypothalamus suggests the possibility of direct effects of estradiol on brain cells. Successful neuroprotection produced by estradiol in hypertensive rats should encourage the treatment with non-feminizing estrogens and estrogen receptor modulators for age-associated diseases.

Effects of selective estrogen receptor modulators on allocentric working memory performance and on dendritic spines in medial prefrontal cortex pyramidal neurons of ovariectomized rats.

Estradiol and some selective estrogen receptor modulators (SERMs) are neuroprotective in a variety of experimental models of neurodegeneration, reduce the inflammatory response of glial cells, reduce anxiety and depression, promote cognition and modulate synaptic plasticity in the hippocampus of rodents. In this study we have assessed whether estradiol and two SERMs currently used in clinics, tamoxifen and raloxifene, affect medial prefrontal cortex function and morphology. Rats were ovariectomized and six days later some animals received a subcutaneous injection of the estrogenic compounds. In a first experiment animals were treated with estradiol benzoate or sesame oil vehicle. In a second experiment animals received raloxifene, tamoxifen or dimethyl sulfoxide as vehicle. Twenty four hours after the pharmacological treatment, animals were challenged to solve an allocentric working memory paradigm in a "Y" maze. Twenty trials consisting of a study phase and a test phase were conducted according to a delayed match-to-sample procedure in a single one-day session. Animals that were not submitted to behavioral test were used for Golgi analysis of the prefrontal cortex. Rats treated with estradiol benzoate, tamoxifen or raloxifene performed better in the Y maze and showed a significant increase in the numerical density of dendritic spines in secondary apical dendrites of layer III pyramidal neurons from the prelimbic/infralimbic prefrontal cortex, compared to their respective control groups. These findings suggest that estradiol, tamoxifen and raloxifene improve prefrontal cortex-related cognitive performance and modulate prefrontal cortex morphology in ovariectomized rats.

Increased aromatase expression in the hippocampus of spontaneously hypertensive rats: effects of estradiol administration.

There is high incidence of hippocampal abnormalities in spontaneously hypertensive rats (SHR), including decreased neurogenesis in the dentate gyrus, astrogliosis, low expression of brain derived neurotrophic factor and decreased neuronal density in the hilar region, respect of normotensive Wistar Kyoto rats (WKY). Estradiol treatment given for 2 weeks normalized the faulty hippocampal parameters of SHR, without having effects on WKY rats. The present work studied the potential role of local estrogen biosynthesis in the hippocampus of SHR and WKY, by measuring the expression of aromatase, the key enzyme responsible for estrogen biosynthesis and involved in neuroprotection. We used 4 month old male SHR and WKY, half of which received a single sc pellet of 12 mg estradiol benzoate and the remaining half a cholesterol implant. Hippocampi were dissected and processed for aromatase mRNA expression using real time PCR. A second batch of animals was processed for aromatase and glial fibrillary acidic protein (GFAP) immunocytochemistry. Basal level of aromatase mRNA was higher in SHR respect of WKY. Following estradiol treatment, aromatase mRNA was further increased in the SHR group only. In the hilus of the dentate gyrus of cholesterol-implanted SHR, we found aromatase immunoreactive cell processes and fibers more strongly stained respect of WKY rats. Estradiol treatment of SHR further increased the length of immunoreactive processes and fibers in the hilar region and also increased aromatase immunoreactivity in the CA1 but not the CA3 pyramidal cell region. WKY rats were spared from the estradiol effect. Double-labelling experiments showed that aromatase+ processes and fibers of the hilus of SHR-treated rats did no colocalize with GFAP+ astrocyte cell bodies or processes. In conclusion, basal and estradiol-stimulated aromatase expression was enhanced in hypertensive rat hippocampus. A combination of exogenous estrogens and those locally synthesized may better alleviate hypertensive encephalopathy.

[Environment, behavior and learning in adult mice]. / Medio ambiente, comportamiento y aprendizaje en ratones adultos

The effect that being subjected to different environmental situations has in the adult mouse is analysed. Adult mice are placed for that, in groups of twelve mice in cages that contains ramps, catwalks, passages, sleeping boxes and play objects, that were changed in their relative position several times in the week. Other mice were placed in groups of four mice in smaller boxes and without these objects. After 60 days in these conditions mice's exploratory behaviour and their learning ability in a complex maze were studied. A significant difference was observed between these two groups, as much in their exploratory behaviour as in the time spent on resolving maze, and the number of mice that resolved it in every one of ten successive trials.

[Plasticity of the nervous system]. / Plasticidad del sistema nervioso

The most recent knowledge about the phenomenon of the nervous system plasticity are revised, as much in morphological as in physiological and molecular levels. The neuron morphological and physiological changes opposite to the experience are studied. The nervous system molecular adaptation to the information it receives as the base of all type of plasticity is also considered.