Switching Off Vascular MAPK Signaling: A Novel Strategy to Prevent Delayed Cerebral Ischemia Following Subarachnoid Hemorrhage

Patients who initially survive the rupture and repair of a brain aneurysm often take a devastating turn for the worse some days later and die or suffer permanent neurologic deficits. This catastrophic sequela is attributed to a delayed phase of global cerebral ischemia (DCI) following aneurysmal subarachnoid hemorrhage (aSAH), but we lack effective treatment. Here we present our view, based on 20 years of research, that the initial drop in blood flow at the time of rupture triggers genomic responses throughout the brain vasculature that manifest days later as increased vasoconstriction and decreased cerebral blood flow. We propose a novel treatment strategy to prevent DCI by early inhibition of the vascular mitogen-activated protein kinase (MAPK) pathway that triggers expression of vasoconstrictor and inflammatory mediators. We summarize evidence from experimental SAH models showing early treatment with MAPK inhibitors "switches off" these detrimental responses, maintains flow, and improves neurological outcome. This promising therapy is currently being evaluated in clinical trials.

Ovariectomy Reduces Vasocontractile Responses of Rat Middle Cerebral Arteries After Focal Cerebral Ischemia

ABSTRACT

Effects of sex hormones on stroke outcome are not fully understood. A deleterious consequence of cerebral ischemia is upregulation of vasoconstrictor receptors in cerebral arteries that exacerbate stroke injury. Here, we tested the hypothesis that female sex hormones alter vasocontractile responses after experimental stroke in vivo or after organ culture in vitro, a model of vasocontractile receptor upregulation. Female rats with intact ovaries and ovariectomized (OVX) females treated with 17β-estradiol, progesterone, or placebo were subjected to transient, unilateral middle cerebral artery occlusion followed by reperfusion (I/R). The maximum contractile response, measured my wire myography, in response to the endothelin B receptor agonist sarafotoxin 6c was increased in female arteries after I/R, but the maximum response was significantly lower in arteries from OVX females. Maximum contraction mediated by the serotonin agonist 5-carboxamidotryptamine was diminished after I/R, with arteries from OVX females showing a greater decrease in maximum contractile response. Contraction elicited by angiotensin II was similar in all arteries. Neither estrogen nor progesterone treatment of OVX females affected I/R-induced changes in endothelin B- and 5-carboxamidotryptamine-induced vasocontraction. These findings suggest that sex hormones do not directly influence vasocontractile alterations that occur after ischemic stroke; however, loss of ovarian function does impact this process.

Estrogen receptors α, β and GPER in the CNS and trigeminal system - molecular and functional aspects

BACKGROUND

Migraine occurs 2-3 times more often in females than in males and is in many females associated with the onset of menstruation. The steroid hormone, 17β-estradiol (estrogen, E2), exerts its effects by binding and activating several estrogen receptors (ERs). Calcitonin gene-related peptide (CGRP) has a strong position in migraine pathophysiology, and interaction with CGRP has resulted in several successful drugs for acute and prophylactic treatment of migraine, effective in all age groups and in both sexes.

METHODS

Immunohistochemistry was used for detection and localization of proteins, release of CGRP and PACAP investigated by ELISA and myography/perfusion arteriography was performed on rat and human arterial segments.

RESULTS

ERα was found throughout the whole brain, and in several migraine related structures. ERβ was mainly found in the hippocampus and the cerebellum. In trigeminal ganglion (TG), ERα was found in the nuclei of neurons; these neurons expressed CGRP or the CGRP receptor in the cytoplasm. G-protein ER (GPER) was observed in the cell membrane and cytoplasm in most TG neurons. We compared TG from males and females, and females expressed more ER receptors. For neuropeptide release, the only observable difference was a baseline CGRP release being higher in the pro-estrous state as compared to estrous state. In the middle cerebral artery (MCA), we observed similar dilatory ER-responses between males and females, except for vasodilatory ERβ which we observed only in female arteries.

CONCLUSION

These data reveal significant differences in ER receptor expression between male and female rats. This contrasts to CGRP and PACAP release where we did not observe discernable difference between the sexes. Together, this points to a hypothesis where estrogen could have a modulatory role on the trigeminal neuron function in general rather than on the acute CGRP release mechanisms and vasomotor responses.

Oxytocin as a regulatory neuropeptide in the trigeminovascular system: Localization, expression and function of oxytocin and oxytocin receptors

BACKGROUND

Recent clinical findings suggest that oxytocin could be a novel treatment for migraine. However, little is known about the role of this neuropeptide/hormone and its receptor in the trigeminovascular pathway. Here we determine expression, localization, and function of oxytocin and oxytocin receptors in rat trigeminal ganglia and targets of peripheral (dura mater and cranial arteries) and central (trigeminal nucleus caudalis) afferents.

METHODS

The methods include immunohistochemistry, messenger RNA measurements, quantitative PCR, release of calcitonin gene-related peptide and myography of arterial segments.

RESULTS

Oxytocin receptor mRNA was expressed in rat trigeminal ganglia and the receptor protein was localized in numerous small to medium-sized neurons and thick axons characteristic of A∂ sensory fibers. Double immunohistochemistry revealed only a small number of neurons expressing both oxytocin receptors and calcitonin gene-related peptide. In contrast, double immunostaining showed expression of the calcitonin gene-related peptide receptor component receptor activity-modifying protein 1 and oxytocin receptors in 23% of the small cells and in 47% of the medium-sized cells. Oxytocin immunofluorescence was observed only in trigeminal ganglia satellite glial cells. Oxytocin mRNA was below detection limit in the trigeminal ganglia. The trigeminal nucleus caudalis expressed mRNA for both oxytocin and its receptor. K⁺-evoked calcitonin gene-related peptide release from either isolated trigeminal ganglia or dura mater and it was not significantly affected by oxytocin (10 µM). Oxytocin directly constricted cranial arteries ex vivo (pEC₅₀ ∼ 7); however, these effects were inhibited by the vasopressin V_1A antagonist SR49059.

CONCLUSION

Oxytocin receptors are extensively expressed throughout the rat trigeminovascular system and in particular in trigeminal ganglia A∂ neurons and fibers, but no functional oxytocin receptors were demonstrated in the dura and cranial arteries. Thus, circulating oxytocin may act on oxytocin receptors in the trigeminal ganglia to affect nociception transmission. These effects may help explain hormonal influences in migraine and offer a novel way for treatment.

C-fibers may modulate adjacent Aδ-fibers through axon-axon CGRP signaling at nodes of Ranvier in the trigeminal system

Background

Monoclonal antibodies (mAbs) towards CGRP or the CGRP receptor show good prophylactic antimigraine efficacy. However, their site of action is still elusive. Due to lack of passage of mAbs across the blood-brain barrier the trigeminal system has been suggested a possible site of action because it lacks blood-brain barrier and hence is available to circulating molecules. The trigeminal ganglion (TG) harbors two types of neurons; half of which store CGRP and the rest that express CGRP receptor elements (CLR/RAMP1).

Methods

With specific immunohistochemistry methods, we demonstrated the localization of CGRP, CLR, RAMP1, and their locations related to expression of the paranodal marker contactin-associated protein 1 (CASPR). Furthermore, we studied functional CGRP release separately from the neuron soma and the part with only nerve fibers of the trigeminal ganglion, using an enzyme-linked immunosorbent assay.

Results

Antibodies towards CGRP and CLR/RAMP1 bind to two different populations of neurons in the TG and are found in the C- and the myelinated Aδ-fibers, respectively, within the dura mater and in trigeminal ganglion (TG). CASPR staining revealed paranodal areas of the different myelinated fibers inhabiting the TG and dura mater. Double immunostaining with CASPR and RAMP1 or the functional CGRP receptor antibody (AA58) revealed co-localization of the two peptides in the paranodal region which suggests the presence of the CGRP-receptor. Double immunostaining with CGRP and CASPR revealed that thin C-fibers have CGRP-positive boutons which often localize in close proximity to the nodal areas of the CGRP-receptor positive Aδ-fibers. These boutons are pearl-like synaptic structures, and we show CGRP release from fibers dissociated from their neuronal bodies. In addition, we found that adjacent to the CGRP receptor localization in the node of Ranvier there was PKA immunoreactivity (kinase stimulated by cAMP), providing structural possibility to modify conduction activity within the Aδ-fibers.

Conclusion

We observed a close relationship between the CGRP containing C-fibers and the Aδ-fibers containing the CGRP-receptor elements, suggesting a point of axon-axon interaction for the released CGRP and a site of action for gepants and the novel mAbs to alleviate migraine.

Melatonin receptors (version 2019.4) in the IUPHAR/BPS Guide to Pharmacology Database

Melatonin receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Melatonin Receptors [36]) are activated by the endogenous ligands melatonin and clinically used drugs like ramelteon, agomelatine and tasimelteon.

U0126 attenuates cerebral vasoconstriction and improves long-term neurologic outcome after stroke in female rats

Sex differences are well known in cerebral ischemia and may impact the effect of stroke treatments. In male rats, the MEK1/2 inhibitor U0126 reduces ischemia-induced endothelin type B (ETB) receptor upregulation, infarct size and improves acute neurologic function after experimental stroke. However, responses to this treatment in females and long-term effects on outcome are not known. Initial experiments used in vitro organ culture of cerebral arteries, confirming ERK1/2 activation and increased ETB receptor-mediated vasoconstriction in female cerebral arteries. Transient middle cerebral artery occlusion (tMCAO, 120 minutes) was induced in female Wistar rats, with U0126 (30 mg/kg intraperitoneally) or vehicle administered at 0 and 24 hours of reperfusion, or with no treatment. Infarct volumes were determined and neurologic function was assessed by 6-point and 28-point neuroscores. ETB receptor-mediated contraction was studied with myograph and protein expression with immunohistochemistry. In vitro organ culture and tMCAO resulted in vascular ETB receptor upregulation and activation of ERK1/2 that was prevented by U0126. Although no effect on infarct size, U0126 improved the long-term neurologic function after experimental stroke in female rats. In conclusion, early prevention of the ERK1/2 activation and ETB receptor-mediated vasoconstriction in the cerebral vasculature after ischemic stroke in female rats improves the long-term neurologic outcome.

Genomic and non-genomic regulation of PGC1 isoforms by estrogen to increase cerebral vascular mitochondrial biogenesis and reactive oxygen species protection

We previously found that estrogen exerts a novel protective effect on mitochondria in brain vasculature. Here we demonstrate in rat cerebral blood vessels that 17β-estradiol (estrogen), both in vivo and ex vivo, affects key transcriptional coactivators responsible for mitochondrial regulation. Treatment of ovariectomized rats with estrogen in vivo lowered mRNA levels of peroxisome proliferator-activated receptor-γ coactivator-1 alpha (PGC-1α) but increased levels of the other PGC-1 isoforms: PGC-1β and PGC-1 related coactivator (PRC). In vessels ex vivo, estrogen decreased protein levels of PGC-1α via activation of phosphatidylinositol 3-kinase (PI3K). Estrogen treatment also increased phosphorylation of forkhead transcription factor, FoxO1, a known pathway for PGC-1α downregulation. In contrast to the decrease in PGC-1α, estrogen increased protein levels of nuclear respiratory factor 1, a known PGC target and mediator of mitochondrial biogenesis. The latter effect of estrogen was independent of PI3K, suggesting a separate mechanism consistent with increased expression of PGC-1β and PRC. We demonstrated increased mitochondrial biogenesis following estrogen treatment in vivo; cerebrovascular levels of mitochondrial transcription factor A and electron transport chain subunits as well as the mitochondrial/nuclear DNA ratio were increased. We examined a downstream target of PGC-1β, glutamate-cysteine ligase (GCL), the rate-limiting enzyme for glutathione synthesis. In vivo estrogen increased protein levels of both GCL subunits and total glutathione levels. Together these data show estrogen differentially regulates PGC-1 isoforms in brain vasculature, underscoring the importance of these coactivators in adapting mitochondria in specific tissues. By upregulating PGC-1β and/or PRC, estrogen appears to enhance mitochondrial biogenesis, function and reactive oxygen species protection.

Male-female differences in upregulation of vasoconstrictor responses in human cerebral arteries

BACKGROUND AND PURPOSE

Male-female differences may significantly impact stroke prevention and treatment in men and women, however underlying mechanisms for sexual dimorphism in stroke are not understood. We previously found in males that cerebral ischemia upregulates contractile receptors in cerebral arteries, which is associated with lower blood flow. The present study investigates if cerebral arteries from men and women differ in cerebrovascular receptor upregulation.

EXPERIMENTAL APPROACH

Freshly obtained human cerebral arteries were placed in organ culture, an established model for studying receptor upregulation. 5-hydroxtryptamine type 1B (5-HT1B), angiotensin II type 1 (AT1) and endothelin-1 type A and B (ETA and ETB) receptors were evaluated using wire myograph for contractile responses, real-time PCR for mRNA and immunohistochemistry for receptor expression.

KEY RESULTS

Vascular sensitivity to angiotensin II and endothelin-1 was markedly lower in cultured cerebral arteries from women as compared to men. ETB receptor-mediated contraction occurred in male but not female arteries. Interestingly, there were similar upregulation in mRNA and expression of 5-HT1B, AT1, and ETB receptors and in local expression of Ang II after organ culture.

CONCLUSIONS AND IMPLICATIONS

In spite of receptor upregulation after organ culture in both sexes, cerebral arteries from women were significantly less responsive to vasoconstrictors angiotensin II and endothelin-1 as compared to arteries from men. This suggests receptor coupling and/or signal transduction mechanisms involved in cerebrovascular contractility may be suppressed in females. This is the first study to demonstrate sex differences in the vascular function of human brain arteries.

Endogenous ovarian hormones affect mitochondrial efficiency in cerebral endothelium via distinct regulation of PGC-1 isoforms

Mitochondria support the energy-intensive functions of brain endothelium but also produce damaging-free radicals that lead to disease. Previously, we found that estrogen treatment protects cerebrovascular mitochondria, increasing capacity for ATP production while decreasing reactive oxygen species (ROS). To determine whether these effects occur specifically in endothelium in vivo and also explore underlying transcriptional mechanisms, we studied freshly isolated brain endothelial preparations from intact and ovariectomized female mice. This preparation reflects physiologic influences of circulating hormones, hemodynamic forces, and cell-cell interactions of the neurovascular unit. Loss of ovarian hormones affected endothelial expression of the key mitochondrial regulator family, peroxisome proliferator-activated receptor γ coactivator 1 (PGC-1), but in a unique way. Ovariectomy increased endothelial PGC-1α mRNA but decreased PGC-1β mRNA. The change in PGC-1β correlated with decreased mRNA for crucial downstream mitochondrial regulators, nuclear respiratory factor 1 and mitochondrial transcription factor A, as well as for ATP synthase and ROS protection enzymes, glutamate-cysteine ligase and manganese superoxide dismutase. Ovariectomy also decreased mitochondrial biogenesis (mitochondrial/nuclear DNA ratio). These results indicate ovarian hormones normally act through a distinctive regulatory pathway involving PGC-1β to support cerebral endothelial mitochondrial content and guide mitochondrial function to favor ATP coupling and ROS protection.

17β-Estradiol prevents cell death and mitochondrial dysfunction by an estrogen receptor-dependent mechanism in astrocytes after oxygen-glucose deprivation/reperfusion

17β-Estradiol (E2) has been shown to protect against ischemic brain injury, yet its targets and the mechanisms are unclear. E2 may exert multiple regulatory actions on astrocytes that may greatly contribute to its ability to protect the brain. Mitochondria are recognized as playing central roles in the development of injury during ischemia. Increasing evidence indicates that mitochondrial mechanisms are critically involved in E2-mediated protection. In this study, the effects of E2 and the role of mitochondria were evaluated in primary cultures of astrocytes subjected to an ischemia-like condition of oxygen-glucose deprivation (OGD)/reperfusion. We showed that E2 treatment significantly protects against OGD/reperfusion-induced cell death as determined by cell viability, apoptosis, and lactate dehydrogenase leakage. The protective effects of E2 on astrocytic survival were blocked by an estrogen receptor (ER) antagonist (ICI-182,780) and were mimicked by an ER agonist selective for ERα (PPT), but not by an ER agonist selective for ERβ (DPN). OGD/reperfusion provoked mitochondrial dysfunction as manifested by an increase in cellular reactive oxygen species production, loss of mitochondrial membrane potential, and depletion of ATP. E2 pretreatment significantly inhibited OGD/reperfusion-induced mitochondrial dysfunction, and this effect was also blocked by ICI-182,780. Therefore, we conclude that E2 provides direct protection to astrocytes from ischemic injury by an ER-dependent mechanism, highlighting an important role for ERα. Estrogen protects against mitochondrial dysfunction at the early phase of ischemic injury. However, overall implications for protection against brain ischemia and its complex sequelae await further exploration.

Local oestrogenic/androgenic balance in the cerebral vasculature

Reproductive effects of sex steroids are well-known; however it is increasingly apparent that these hormones have important actions on non-reproductive tissues such as the vasculature. The latter effects can be relevant throughout the lifespan, not just limited to reproductive years, and are not necessarily restricted to one gender or the other. Our work has established that cerebral blood vessels are a non-reproductive target tissue for sex steroids. We have found that oestrogen and androgens alter vascular tone, endothelial function, oxidative stress and inflammatory responses in cerebral vessels. Often the actions of oestrogen and androgens oppose each other. Moreover, it is clear that cerebral vessels are directly targeted by sex steroids, as they express specific receptors for these hormones. Interestingly, cerebral blood vessels also express enzymes that metabolize sex steroids. These findings suggest that local synthesis of 17ß-estradiol and dihydrotestosterone can occur within the vessel wall. One of the enzymes present, aromatase, converts testosterone to 17ß-estradiol, which would alter the local balance of androgenic and oestrogenic influences. Thus cerebral vessels are affected by circulating sex hormones as well as locally synthesized sex steroids. The presence of vascular endocrine effector mechanisms has important implications for male-female differences in cerebrovascular function and disease. Moreover, the cerebral circulation is a target for gonadal hormones as well as anabolic steroids and therapeutic drugs used to manipulate sex steroid actions. The long-term consequences of these influences are yet to be determined.

Mechanisms of cerebrovascular protection: oestrogen, inflammation and mitochondria

Investigation of oestrogen action reveals a multitude of diverse effects. This brief review focuses on the impact of oestrogen on the vasculature, with particular emphasis on the cerebral circulation. Three major actions of oestrogen are discussed: enhancement of vasodilator capacity, suppression of vascular inflammation and increase in mitochondrial efficiency. In both humans and animals, oestrogen increases vasodilator tone, an effect dependent on a functional endothelium. Two distinct mechanisms are involved: increase in endothelial nitric oxide synthase (eNOS) mRNA and protein and phosphorylation of eNOS via the PI-3 kinase/Akt pathway. Both effects are mediated by oestrogen receptors (ER), but through two pathways, ER-mediated nuclear gene transcription and cell membrane-associated ERs respectively. Oestrogen also increases function of other endothelium-dependent vasodilators. Oestrogen suppresses vascular inflammation through an NF-κB-dependent effect. The inflammatory response has also been shown to vary significantly during the oestrous cycle of rodents. Emerging information shows that oestrogen increases mitochondrial biogenesis and decreases superoxide production. Suppression of mitochondrial superoxide production by 17β-estradiol in cerebral blood vessels is mediated by the ER-alpha receptor and not dependent on increased Mn superoxide dismutase activity. Oestrogen treatment also increases protein levels for a number of components of the electron transfer chain, as well as levels of transcription factors that regulate mitochondrial function. All of these actions of oestrogen could be important in mediating vascular protection, especially in the cerebral circulation. Furthermore, given the potential of mitochondrial DNA damage to contribute to pathophysiology and ageing, mitochondrial protective effects of oestrogen might contribute to the longer average lifespan of women.

International Union of Basic and Clinical Pharmacology. LXXV. Nomenclature, classification, and pharmacology of G protein-coupled melatonin receptors

The hormone melatonin (5-methoxy-N-acetyltryptamine) is synthesized primarily in the pineal gland and retina, and in several peripheral tissues and organs. In the circulation, the concentration of melatonin follows a circadian rhythm, with high levels at night providing timing cues to target tissues endowed with melatonin receptors. Melatonin receptors receive and translate melatonin's message to influence daily and seasonal rhythms of physiology and behavior. The melatonin message is translated through activation of two G protein-coupled receptors, MT(1) and MT(2), that are potential therapeutic targets in disorders ranging from insomnia and circadian sleep disorders to depression, cardiovascular diseases, and cancer. This review summarizes the steps taken since melatonin's discovery by Aaron Lerner in 1958 to functionally characterize, clone, and localize receptors in mammalian tissues. The pharmacological and molecular properties of the receptors are described as well as current efforts to discover and develop ligands for treatment of a number of illnesses, including sleep disorders, depression, and cancer.

[Mechanism of estrogen-mediated neuroprotection: regulation of mitochondrial function]

Numerous studies show the neuroprotective effects of estrogen, but the underlying mechanism still remains unclear. Recent studies indicate that mitochondria are critically involved in estrogen-mediated neuroprotection. Mitochondria are the main sources of cellular energy and reactive oxygen species (ROS), they play an important role in signaling transduction and cellular life-death decisions. Estrogen exerts multiple effects on mitochondria under physiological and/or pathological conditions, these effects may include modulating ATP and ROS production, preserving mitochondria membrane potential, maintaining calcium homeostasis, and regulating mitochondrial gene and protein expression, etc. In this paper, we discussed the neuroprotective effects of estrogen, particularly focused on the underlying mechanisms related to mitochondria.

Mitochondrial effects of estrogen are mediated by estrogen receptor alpha in brain endothelial cells

Mitochondrial reactive oxygen species (ROS) and endothelial dysfunction are key contributors to cerebrovascular pathophysiology. We previously found that 17beta-estradiol profoundly affects mitochondrial function in cerebral blood vessels, enhancing efficiency of energy production and suppressing mitochondrial oxidative stress. To determine whether estrogen specifically affects endothelial mitochondria through receptor mechanisms, we used cultured human brain microvascular endothelial cells (HBMECs). 17beta-Estradiol treatment for 24 h increased mitochondrial cytochrome c protein and mRNA; use of silencing RNA for estrogen receptors (ERs) showed that this effect involved ERalpha, but not ERbeta. Mitochondrial ROS were determined by measuring the activity of aconitase, an enzyme with an iron-sulfur center inactivated by mitochondrial superoxide. 17beta-Estradiol increased mitochondrial aconitase activity in HBMECs, indicating a reduction in ROS. Direct measurement of mitochondrial superoxide with MitoSOX Red showed that 17beta-estradiol, but not 17alpha-estradiol, significantly decreased mitochondrial superoxide production, an effect blocked by the ER antagonist, ICI-182,780 (fulvestrant). Selective ER agonists demonstrated that the decrease in mitochondrial superoxide was mediated by ERalpha, not ERbeta. The selective estrogen receptor modulators, raloxifene and 4-hydroxy-tamoxifen, differentially affected mitochondrial superoxide production, with raloxifene acting as an agonist but 4-hydroxy-tamoxifen acting as an estrogen antagonist. Changes in superoxide by 17beta-estradiol could not be explained by changes in manganese superoxide dismutase. Instead, ERalpha-mediated decreases in mitochondrial ROS may depend on the concomitant increase in mitochondrial cytochrome c, previously shown to act as an antioxidant. Mitochondrial protective effects of estrogen in cerebral endothelium may contribute to sex differences in the occurrence of stroke and other age-related neurodegenerative diseases.

Androgenic/estrogenic balance in the male rat cerebral circulation: metabolic enzymes and sex steroid receptors

Tissues from males can be regulated by a balance of androgenic and estrogenic effects because of local metabolism of testosterone and expression of relevant steroid hormone receptors. As a critical first step to understanding sex hormone influences in the cerebral circulation of males, we investigated the presence of enzymes that metabolize testosterone to active products and their respective receptors. We found that cerebral blood vessels from male rats express 5alpha-reductase type 2 and aromatase, enzymes responsible for conversion of testosterone into dihydrotestosterone (DHT) and 17beta-estradiol, respectively. Protein levels of these enzymes, however, were not modulated by long-term in vivo hormone treatment. We also showed the presence of receptors for both androgens (AR) and estrogens (ER) from male cerebral vessels. Western blot analysis showed bands corresponding to the full-length AR (110 kDa) and ERalpha (66 kDa). Long-term in vivo treatment of orchiectomized rats with testosterone or DHT, but not estrogen, increased AR levels in cerebral vessels. In contrast, ERalpha protein levels were increased after in vivo treatment with estrogen but not testosterone. Fluorescent immunostaining revealed ERalpha, AR, and 5alpha-reductase type 2 in both the endothelial and smooth muscle layers of cerebral arteries, whereas aromatase staining was solely localized to the endothelium. Thus, cerebral vessels from males are target tissues for both androgens and estrogen. Furthermore, local metabolism of testosterone might balance opposing androgenic and estrogenic influences on cerebrovascular as well as brain function in males.

Estrogen suppresses brain mitochondrial oxidative stress in female and male rats

Mitochondria are a major source of reactive oxygen species (ROS) and oxidative stress, key contributors to aging and neurodegenerative disorders. We report that gonadal hormones influence brain mitochondrial ROS production in both females and males. Initial experiments showed that estrogen decreases mitochondrial superoxide production in a receptor-mediated manner, as measured by MitoSOX fluorescence in differentiated PC-12 cells. We then assessed in vivo effects of gonadal hormones on brain mitochondrial oxidative stress in female and male rats. Brain mitochondria were isolated to measure a functional indicator of ROS, i.e., activity of the ROS-sensitive mitochondrial enzyme, aconitase. Gonadectomy of both males and females caused a decrease in aconitase activity, suggesting that endogenous gonadal hormones influence mitochondrial ROS production in the brain. In vivo treatment of gonadectomized animals with testosterone or dihydrotestosterone (DHT) had no effect, but estrogen replacement significantly increased aconitase activity in brain mitochondria from both female and male rats. This indicates that estrogen decreases brain mitochondrial ROS production in vivo. Sex hormone treatments did not affect protein levels of brain mitochondrial uncoupling proteins (UCP-2, 4, and 5). However, estrogen did increase the activity, but not the levels, of manganese superoxide dismutase (MnSOD), the mitochondrial enzyme that catalyzes superoxide radical breakdown, in brain mitochondria from both female and male rats. Thus, in contrast to the lack of effect of androgens on mitochondrial ROS, estrogen suppression of mitochondrial oxidative stress may influence neurological disease incidence and progression in both females and males.

CEREBROVASCULAR EFFECTS OF OESTROGEN: MULTIPLICITY OF ACTION

1. Cerebral vessels express oestrogen receptors (ER) in both the smooth muscle and endothelial cell layers of cerebral blood vessels. Levels of ERalpha are higher in female rats chronically exposed to oestrogen, either endogenous or exogenous. 2. Chronic exposure to oestrogen, either endogenous (normally cycling females) or exogenous (ovariectomized with oestrogen replacement), results in cerebral arteries that are more dilated than arteries from ovariectomized counterparts when studied in vitro. This effect is primarily mediated by an increase in the production of vasodilator factors, including nitric oxide (NO) and prostacylin. In contrast, oestrogen appears to suppress the production of endothelial-derived hyperpolarizing factor. Oestrogen treatment increases cerebrovascular levels of endothelial nitric oxide synthase (eNOS), cyclo-oxygenase (COX)-1 and prostacyclin synthase. In addition, via activation of the phosphatidylinositol 3-kinase/Akt pathway, both acute and chronic oestrogen exposure increases eNOS phosphorylation, increasing NO production. 3. Oestrogen receptors have also been localized to cerebrovascular mitochondria and exposure to oestrogen increases the efficiency of energy production while simultaneously reducing mitochondrial production of reactive oxygen species. Oestrogen increases the production of mitochondrial proteins encoded by both mitochondrial and nuclear DNA, including cytochrome c, subunits I and IV of complex IV and Mn-superoxide dismutase. Oestrogen treatment increases the activity of citrate synthase and complex IV and decreases mitochondrial production of H(2)O(2). 4. Oestrogen also has potent anti-inflammatory effects in the cerebral circulation that may have important implications for the incidence and severity of cerebrovascular disease. Administration of lipopolysaccharide or interleukin-1beta to ovariectomized female rats induces cerebrovascular COX-2 and inducible nitric oxide synthase (iNOS) protein expression and increases prostaglandin E(2) expression. Levels of COX-2 and iNOS expression vary with the stage of the oestrous cycle, and the cerebrovascular inflammatory response is suppressed in ovariectomized animals treated with oestrogen. Interleukin-1beta induction of COX-2 protein is prevented by treatment with a nuclear factor (NF)-kappaB inhibitor, and oestrogen treatment reduces cerebrovascular NF-kappaB activity. 5. Cerebrovascular dysfunction and pathology contribute to the pathogenesis of stroke, brain trauma, oedema and dementias, such as Alzheimer's disease. A better understanding of the action of oestrogen on cerebrovascular function holds promise for the development of new therapeutic entities that could be useful in preventing or treating a wide variety of cerebrovascular diseases.

Estrogen and mitochondria: a new paradigm for vascular protection?

Mitochondrial dysfunction has been implicated as a cause of age-related disorders, and the mitochondrial theory of aging links aging, exercise, and diet. Endothelial dysfunction is a key paradigm for vascular disease and aging, and there is considerable evidence that exercise and dietary restriction protect against cardiovascular disease. Recent studies demonstrate that estrogen receptors are present in mitochondria and that estrogen promotes mitochondrial efficiency and decreases oxidative stress in the cerebral vasculature. Chronic estrogen treatment increases mitochondrial capacity for oxidative phosphorylation while decreasing production of reactive oxygen species. The effectiveness of estrogen against age-related cardiovascular disorders, including stroke, may thus arise in part from hormonal effects on mitochondrial function. Estrogen-mediated mitochondrial efficiency may also be a contributing factor to the longer lifespan of women.

Age alters cerebrovascular inflammation and effects of estrogen

In young adult females, estrogen treatment suppresses the cerebrovascular inflammatory response; this is mediated in part via NF-kappaB, a key regulator of inflammatory genes. To examine whether age modifies effects of estrogen on vascular inflammation in the brain, female rats, 3 and 12 mo of age, were ovariectomized; half were treated with estrogen for 4 wk. Cerebral blood vessels were isolated from the animals at 4 and 13 mo of age. Inflammation was induced by LPS, either injected in vivo or incubated with isolated vessels ex vivo. Basal levels of cytoplasmic NF-kappaB were significantly higher in cerebral vessels of young rats, but the ratio of nuclear to cytoplasmic levels was greater in middle-aged animals. LPS exposure increased nuclear NF-kappaB DNA binding activity, protein levels of inducible nitric oxide synthase and cyclooxygenase-2, and production of nitric oxide and PGE(2) in cerebral vessels. All effects of LPS were markedly greater in vessels from the older animals. Estrogen significantly inhibited the LPS-induced increase in NF-kappaB DNA binding activity in cerebral vessels from animals at both ages. In 4-mo-old rats, estrogen also significantly suppressed LPS induction of inducible nitric oxide synthase and cyclooxygenase-2 proteins, as well as production of nitric oxide and PGE(2). In contrast, in 13-mo-old females, estrogen did not significantly affect these indexes of cerebrovascular inflammation. Thus the protective, anti-inflammatory effect of estrogen on cerebral blood vessels that is observed in young adults may be attenuated in aged animals, which exhibit a greater overall cerebrovascular response to inflammatory stimuli.

Influence of sex steroid hormones on cerebrovascular function

The cerebral vasculature is a target tissue for sex steroid hormones. Estrogens, androgens, and progestins all influence the function and pathophysiology of the cerebral circulation. Estrogen decreases cerebral vascular tone and increases cerebral blood flow by enhancing endothelial-derived nitric oxide and prostacyclin pathways. Testosterone has opposite effects, increasing cerebral artery tone. Cerebrovascular inflammation is suppressed by estrogen but increased by testosterone and progesterone. Evidence suggests that sex steroids also modulate blood-brain barrier permeability. Estrogen has important protective effects on cerebral endothelial cells by increasing mitochondrial efficiency, decreasing free radical production, promoting cell survival, and stimulating angiogenesis. Although much has been learned regarding hormonal effects on brain blood vessels, most studies involve young, healthy animals. It is becoming apparent that hormonal effects may be modified by aging or disease states such as diabetes. Furthermore, effects of testosterone are complicated because this steroid is also converted to estrogen, systemically and possibly within the vessels themselves. Elucidating the impact of sex steroids on the cerebral vasculature is important for understanding male-female differences in stroke and conditions such as menstrual migraine and preeclampsia-related cerebral edema in pregnancy. Cerebrovascular effects of sex steroids also need to be considered in untangling current controversies regarding consequences of hormone replacement therapies and steroid abuse.

Estrogen and progestagens differentially modulate vascular proinflammatory factors

The potential benefit of ovarian hormone replacement therapy in cerebrovascular disease is well supported by experimental observations but not by recent large, randomized clinical trials. This discrepancy points out the need for better understanding of the vascular actions of ovarian hormones as well as medroxyprogesterone acetate (MPA), a synthetic analog of progesterone (P) widely prescribed in combination with estrogens. Therefore, we investigated whether in vivo exposure to 17beta-estradiol (E) and/or P or MPA modifies inflammation in the cerebral vasculature, a key process in the evolution of ischemic brain injury. Female rats were injected (ip) with LPS to induce inflammation, and 6 h later brains were taken for blood vessel isolation and Western blot analysis of the inflammatory enzymes inducible NO synthase (iNOS) and cyclooxygenase-2 (COX-2). In ovariectomized (O) females, LPS induced cerebrovascular iNOS and COX-2; however, this effect was significantly decreased when O animals were treated for 3 wk with E. In contrast, treatment of O females with either MPA or P exacerbated the cerebrovascular inflammatory response to LPS. In intact females, LPS induction of iNOS and COX-2 in cerebral vessels was found to vary with the stage of the estrous cycle: LPS had the greatest effect during estrus, when circulating estrogen is low and progesterone is high. Thus exposure to endogenous or exogenous ovarian hormones appears to modulate cerebrovascular inflammation. Anti-inflammatory effects of estrogen would attenuate ischemic brain injury; however, this vasoprotective benefit may be diminished in the presence of progestagens.

Testosterone augments endotoxin-mediated cerebrovascular inflammation in male rats

Activation of inflammatory mechanisms contributes to cerebrovascular pathophysiology. Male gender is associated with increased stroke risk, yet little is known about the effects of testosterone in the cerebral circulation. Therefore, we explored the impact of testosterone treatment on cerebrovascular inflammation with both in vivo and in vitro models of inflammation. We hypothesized that testosterone would augment the expression of two vascular markers of cellular inflammation, cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS). Using four groups of male rats [intact, orchiectomized (ORX), and ORX treated with either testosterone (ORXT) or the testosterone metabolite 17β-estradiol (ORXE)], we determined effects of the sex hormones on cerebrovascular inflammation after intraperitoneal LPS injection. Western blot analysis showed that induction of inflammatory markers was increased in cerebral blood vessels from ORXT rats compared with intact or ORX rats. In contrast, in cerebral blood vessels from ORXE rats, there was a significant decrease in endotoxin-induced COX-2 and iNOS protein levels. Confocal microscopy of cerebral blood vessels from ORXT rats showed increased COX-2 and iNOS immunoreactivity in both endothelial and smooth muscle cells after LPS treatment. In vitro incubation with LPS also induced COX-2 in pial vessels isolated from the four animal treatment groups, with the greatest induction observed in ORXT vessels compared with the ORX and ORXE groups. Production of PGE2, a principal COX-2-derived prostaglandin end product, was also greatest in cerebral vessels isolated from ORXT rats. In conclusion, testosterone increases cerebrovascular inflammation; this effect may contribute to stroke differences between men and women.

Testosterone treatment increases thromboxane function in rat cerebral arteries

We previously showed that testosterone, administered in vivo, increases the tone of cerebral arteries. A possible underlying mechanism is increased vasoconstriction through the thromboxane A2 (TxA2) pathway. Therefore, we investigated the effect of chronic testosterone treatment (4 wk) on TxA2 synthase levels and the contribution of TxA2 to vascular tone in rat middle cerebral arteries (MCAs). Using immunofluorescence and confocal microscopy, we demonstrated that TxA2 synthase is present in MCA segments in both smooth muscle and endothelial layers. Using Western blot analysis, we found that TxA2 synthase protein levels are higher in cerebral vessel homogenates from testosterone-treated orchiectomized (ORX+T) rats compared with orchiectomized (ORX) control animals. Functional consequences of changes in cerebrovascular TxA2 synthase were determined using cannulated, pressurized MCA segments in vitro. Constrictor responses to the TxA2 mimetic U-46619 were not different between the ORX+T and ORX groups. However, dilator responses to either the selective TxA2 synthase inhibitor furegrelate or the TxA2-endoperoxide receptor (TP) antagonist SQ-29548 were greater in the ORX+T compared with ORX group. In endothelium-denuded arteries, the dilation to furegrelate was attenuated in both the ORX and ORX+T groups, and the difference between the groups was abolished. These data suggest that chronic testosterone treatment enhances TxA2-mediated tone in rat cerebral arteries by increasing endothelial TxA2 synthesis without altering the TP receptors mediating constriction. The effect of in vivo testosterone on cerebrovascular TxA2 synthase, observed here after chronic hormone administration, may contribute to the risk of vasospasm and thrombosis related to cerebrovascular disease.

Estrogen Increases Mitochondrial Efficiency and Reduces Oxidative Stress in Cerebral Blood Vessels

We report here that estrogen (E(2)) modulates mitochondrial function in the vasculature. Mitochondrial dysfunction is implicated in the etiology of vascular disease; thus, vasoprotection by estrogen may involve hormonal effects on the mitochondria. To test this hypothesis, mitochondria were isolated from cerebral blood vessels obtained from ovariectomized female rats, with or without E(2) replacement. Estrogen receptor-alpha (ER-alpha) was detected in mitochondria by immunoblot and confocal imaging of intact vessels. E(2) treatment in vivo increased the levels of specific proteins in cerebrovascular mitochondria, such as ER-alpha, cytochrome c, subunit IV of complex IV, and manganese superoxide dismutase, all encoded in the nuclear genome, and subunit I of complex IV, encoded in the mitochondrial genome. Levels of glutathione peroxidase-1 and catalase, however, were not affected. Functional assays of mitochondrial citrate synthase and complex IV, key rate-limiting steps in energy production, showed that E(2) treatment increased enzyme activity. In contrast, mitochondrial production of hydrogen peroxide was decreased in vessels from E(2)-treated animals. In vitro incubation of cerebral vessels with 10 nM 17beta-estradiol for 18 h also elevated levels of mitochondrial cytochrome c. This effect was blocked by the estrogen receptor antagonist fulvestrant (ICI-182,780, Faslodex) but was unaffected by inhibitors of nitric-oxide synthase or phosphoinositide-3-kinase. Nuclear respiratory factor-1 protein, a primary regulator of nuclear gene-encoded mitochondrial genes, was significantly increased by long-term estrogen treatment in vivo. In summary, these novel findings suggest that vascular protection by E(2) is mediated, in part, by modulation of mitochondrial function, resulting in greater energy-producing capacity and decreased reactive oxygen species production.

Estrogen Receptor Activation of Phosphoinositide-3 Kinase, Akt, and Nitric Oxide Signaling in Cerebral Blood Vessels: Rapid and Long-Term Effects

Estrogen receptor regulation of nitric oxide production by vascular endothelium may involve rapid, membrane-initiated signaling pathways in addition to classic genomic mechanisms. In this study, we demonstrate using intact cerebral blood vessels that 17beta-estradiol rapidly activates endothelial nitric-oxide synthase (eNOS) via a phosphoinositide-3 (PI-3) kinase-dependent pathway. The effect is mediated by estrogen receptors (ERs), consistent with colocalization of ERalpha and caveolin-1 immunoreactivity at the plasma membrane of endothelial cells lining cerebral arteries. Treatment with 10 nM 17beta-estradiol for 30 min increased NO production, as measured by total nitrite assay, in cerebral vessels isolated from ovariectomized rats. This effect was significantly decreased by membrane cholesterol depletion with beta-methyl-cyclodextrin, the ER antagonist ICI 182,780 [fulvestrant (Faslodex)], and two inhibitors of PI-3 kinase: wortmannin and LY294002 [2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride]. In parallel with NO production, 17beta-estradiol treatment rapidly increased phosphorylation of both eNOS (p-eNOS) and Akt (p-Akt). PI-3 kinase inhibitors also blocked the latter effects; together, these data are consistent with ER activation of the PI-3 kinase-p-Akt-p-eNOS pathway. ERalpha protein (66 and 50 kDa) coimmunoprecipitated with eNOS as well as with the p85alpha regulatory subunit of PI-3 kinase, further implicating ERalpha in kinase activation of eNOS. Little is known regarding the effects of estrogen on cellular kinase pathways in vivo; therefore, we compared cerebral blood vessels isolated from ovariectomized rats that were either untreated or given estrogen replacement for 4 weeks. Long-term estrogen exposure increased levels of cerebrovascular p-Akt and p-eNOS as well as basal NO production. Thus, in addition to the rapid activation of PI-3 kinase, p-Akt, and p-eNOS, estrogen signaling via nontranscriptional, kinase mechanisms has long-term consequences for vascular function.

Effect of estrogen on cerebrovascular prostaglandins is amplified in mice with dysfunctional NOS

Chronic estrogen treatment increases endothelial vasodilator function in cerebral arteries. Endothelial nitric oxide (NO) synthase (eNOS) is a primary target of the hormone, but other endothelial factors may be modulated as well. In light of possible interactions between NO and prostaglandins, we tested the hypothesis that estrogen treatment increases prostanoid-mediated dilation using NOS-deficient female mouse models, i.e., mice treated with a NOS inhibitor [N(G)-nitro-l-arginine methyl ester (l-NAME)] for 21 days or transgenic mice with the eNOS gene disrupted (eNOS(-/-)). All mice were ovariectomized; some in each group were treated chronically with estrogen. Cerebral blood vessels then were isolated for biochemical and functional analyses. In vessels from control mice, estrogen increased protein levels of eNOS but had no significant effect on cyclooxygenase (COX)-1 protein, prostacyclin production, or constriction of pressurized, middle cerebral arteries to indomethacin, a COX inhibitor. In l-NAME-treated mice, however, cerebrovascular COX-1 levels, prostacyclin production, and constriction to indomethacin, as well as eNOS protein, were all greater in estrogen-treated animals. In vessels from eNOS(-/-) mice, estrogen treatment also increased levels of COX-1 protein and constriction to indomethacin, but no effect on prostacyclin production was detected. Thus cerebral blood vessels of control mice did not exhibit effects of estrogen on the prostacyclin pathway. However, when NO production was dysfunctional, the impact of estrogen on a COX-sensitive vasodilator was revealed. Estrogen has multiple endothelial targets; estrogen effects may be modified by interactions among these factors.

Testosterone suppresses endothelium-dependent dilation of rat middle cerebral arteries

Little is known about vascular effects of testosterone. We previously reported chronic testosterone treatment increases vascular tone in middle cerebral arteries (MCA; 300 μm diameter) of male rats. In the present study, we investigated the hypothesis that physiological levels of circulating testosterone affect endothelial factors that modulate cerebrovascular reactivity. Small branches of MCA (150 μm diameter) were isolated from orchiectomized (ORX) and testosterone-treated (ORX+T) rats. Intraluminal diameters were recorded after step changes in intraluminal pressure (20–100 Torr) in the absence or presence of NG-nitro-l-arginine-methyl ester (l-NAME), a nitric oxide synthase (NOS) inhibitor; indomethacin, a cyclooxygenase (COX) inhibitor; and/or apamin and charybdotoxin (CTX); and KCa channel blockers used to inhibit endothelium-derived hyperpolarizing factors (EDHF). At intraluminal pressures ≥60 Torr, arteries from ORX+T developed greater tone compared with ORX arteries. This difference was abolished by removal of the endothelium but remained after treatment of intact arteries with indomethacin or l-NAME. In addition, testosterone treatment had no effect on cerebrovascular production of endothelin-1 or prostacyclin nor did it alter protein levels of endothelial NOS or COX-1. Endothelium removal after l-NAME/indomethacin exposure caused an additional increase in tone. Interestingly, the latter effect was smaller in arteries from ORX+T, suggesting testosterone affects endothelial vasodilators that are independent of NOS and COX. Apamin/CTX, in the presence of l-NAME/indomethacin, abolished the difference in tone between ORX and ORX+T and resulted in vessel diameters similar to those of endothelium-denuded preparations. In conclusion, testosterone may modulate vascular tone in cerebral arteries by suppressing EDHF.

Estrogen suppresses IL-1beta-mediated induction of COX-2 pathway in rat cerebral blood vessels

Interleukin (IL)-1beta is a potent inducer of inflammatory prostaglandins, which are important mediators of vascular response to cerebral injury, whereas estrogen reduces brain injury in models of ischemic stroke. Thus we examined the effects of in vivo IL-1beta exposure on cerebrovascular cyclooxygenase (COX)-2 expression and function in an animal model of chronic estrogen replacement. Estrogen-treated and nontreated ovariectomized female rats received IL-1beta injections (10 microg/kg i.p.), and then cerebral vessels were isolated for biochemical and contractile measurements. In estrogen-deficient rats, IL-1beta induced cerebrovascular COX-2 protein expression; a peak response occurred 3 h after injection. COX-2 was localized to arterial endothelium using confocal microscopy. IL-1beta increased PGE2 but not PGI2 production and decreased vascular tone as measured in isolated cerebral arteries; the latter effect was partially reversed by treatment with the selective COX-2 inhibitor NS-398 (10 micromol/l). In contrast, in animals treated with estrogen, IL-1beta had no significant effect on COX-2 protein levels, PGE2 production, or vascular tone. Combined treatment with 17beta-estradiol and medroxyprogesterone acetate also prevented increases in PGE2 production after IL-1beta treatment, but treatment with 17alpha-estradiol had no effect. IL-1beta induction of COX-2 protein was prevented by treatment with the nuclear factor-kappaB inhibitor caffeic acid phenethyl ester (20 mg/kg i.p.), and estrogen treatment reduced cerebrovascular nuclear factor-kappaB activity. Estrogen thus has potent anti-inflammatory effects with respect to cerebral vascular responses to IL-1beta. These effects may have important implications for the incidence and severity of cerebrovascular disease.

17β-Estradiol increases endothelial nitric oxide synthase mRNA copy number in cerebral blood vessels: quantification by real-time polymerase chain reaction

The enzyme endothelial nitric oxide synthase (eNOS) plays a critical role in the maintenance of vascular tone. The mechanism by which estrogen increases eNOS function remains controversial. We demonstrate here using real-time polymerase chain reaction (PCR) and immunoblot analysis that in vivo estrogen treatment leads to a 100% increase in eNOS messenger RNA (mRNA) copy number and increases eNOS protein levels by 47% in mouse cerebral blood vessels. These data suggest that estrogen can modulate eNOS at the transcriptional level in blood vessels in vivo.

17beta-estradiol decreases vascular tone in cerebral arteries by shifting COX-dependent vasoconstriction to vasodilation

We have previously shown that estrogen treatment increases cerebrovascular cyclooxygenase-1, prostacyclin synthase, and production of prostacyclin. Therefore, vascular tone and prostanoid production were measured to investigate functional consequences of estrogen exposure. Middle cerebral arteries were isolated from ovariectomized female Fischer-344 rats with or without chronic in vivo 17beta-estradiol treatment. In vivo 17beta-estradiol treatment increased cerebral artery diameter; functional endothelium was required for expression of these differences. The nonspecific cyclooxygenase inhibitor indomethacin constricted, whereas arachidonic acid dilated, cerebral arteries from estrogen-treated animals. Estrogen exposure increased production of prostacyclin by cerebral arteries. Conversely, in estrogen-deficient animals, indomethacin dilated and arachidonic acid constricted cerebral blood vessels. This correlated with vasorelaxation following inhibition of the thromboxane-endoperoxide receptor with SQ-29548 but not after selective blockade of thromboxane synthase with furegrelate, suggesting prostaglandin endoperoxide (i.e., PGH2) activity. Removal of the endothelium or selective blockade of cyclooxygenase-1 with SC-560 abolished estrogen-mediated differences in the effects of arachidonate on vessel diameter and on prostacyclin production by cerebral arteries. These data suggest 17beta-estradiol decreases cerebrovascular tone by shifting the primary end product of the endothelial cyclooxygenase-1 pathway from the constrictor prostaglandin PGH2 to the vasodilator prostacyclin. These effects of estrogen may contribute to the heightened thromboresistance and enhanced cerebral blood flow documented in pre-versus postmenopausal women.

Multiple forms of estrogen receptor-alpha in cerebral blood vessels: regulation by estrogen

The cerebral vasculature is an important target tissue for estrogen, as evidenced by significant effects of estrogen on vascular reactivity and protein levels of endothelial nitric oxide synthase and prostacyclin synthase. However, the presence, localization, and regulation of estrogen receptors in the cerebral vasculature have not been investigated. In this study, we identified the presence of estrogen receptor-alpha (ER-alpha) in female rat cerebral blood vessels and localized this receptor to both smooth muscle and endothelial cells by use of immunohistochemistry and confocal microscopy. With immunoblot analysis, multiple forms of ER-alpha were detected at 110, 93, 82, 50, and 45 kDa in addition to a relatively weak band corresponding to the 66-kDa putative unmodified receptor. The 82-kDa band was identified as Ser(118)-phosphorylated ER-alpha, whereas the 50-kDa band lacks the normal NH(2) terminus, suggestive of an ER-alpha splice variant. Lower molecular mass bands persisted after in vivo inhibition of 26S proteasome activity with lactacystin, whereas the 110- and 93-kDa bands increased. All forms of ER-alpha in cerebral vessels were decreased after ovariectomy but significantly increased after chronic estrogen exposure in vivo.

Vascular neuroeffector mechanisms: the next 30 years

The 10th International Symposium on Vascular Neuroeffector Mechanisms was held on 12-15 July 2002 in Lake Tahoe, California, USA.

MT(2) melatonin receptors are present and functional in rat caudal artery

In rat caudal artery, contraction to melatonin results primarily from activation of MT(1) melatonin receptors; however, the role of MT(2) melatonin receptors in vascular responses is controversial. We examined and compared the expression and function of MT(2) receptors with that of MT(1) receptors in male rat caudal artery. MT(1) and MT(2) melatonin receptor mRNA was amplified by reverse transcription-polymerase chain reaction from caudal arteries of three rat strains (i.e., Fisher, Sprague-Dawley, and Wistar). Antisense (but not sense) (33)P-labeled oligonucleotide probes specific for MT(1) or MT(2) receptor mRNA hybridized to smooth muscle, as well as intimal and adventitial layers, of caudal artery. In male Fisher rat caudal artery denuded of endothelium, melatonin was 10 times more potent than 6-chloromelatonin to potentiate contraction to phenylephrine, suggesting activation of smooth muscle MT(1) melatonin receptors. The MT(1)/MT(2) competitive melatonin receptor antagonist luzindole (3 microM), blocked melatonin-mediated contraction (0.1-100 nM) with an affinity constant (K(B) value of 157 nM) similar to that for the human MT(1) receptor. However, at melatonin concentrations above 100 nM, luzindole potentiated the contractile response, suggesting blockade of MT(2) receptors mediating vasorelaxation and/or an inverse agonist effect at MT(1) constitutively active receptors. The involvement of MT(2) receptors in vasorelaxation is supported by the finding that the competitive antagonists 4-phenyl 2-acetamidotetraline and 4-phenyl-2-propionamidotetraline, at MT(2)-selective concentrations (10 nM), significantly enhanced contractile responses to all melatonin concentrations tested (0.1 nM-10 microM). We conclude that MT(2) melatonin receptors expressed in vascular smooth muscle mediate vasodilation in contrast to vascular MT(1) receptors mediating vasoconstriction.

Male-female differences in the relative contribution of endothelial vasodilators released by rat tail artery

Several different vasodilator substances can be released by vascular endothelium in response to mechanical stimuli and vasoactive agents. The purpose of this study was to determine whether there is a male-female difference in the relative contributions of nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) to endothelium-dependent vasodilation. Perfusion pressure was measured in isolated tail arteries from male and female rats. Vasodilators released by mechanical shear stress were assessed by constricting the artery with methoxamine; acetylcholine was applied to induce receptor-mediated vasodilation. We used an inhibitor of NO synthase, N(G)-monomethyl-L-arginine acetate (L-NMMA), and elevated levels of K(+) (27 mM) to reveal the relative contributions of NO and EDHF, respectively. Indomethacin was present in all experiments to block prostanoid production. The results indicate that NO was the primary vasodilator released by male tail arteries in response to both mechanical stress and acetylcholine (the L-NMMA-sensitive component of the combined L-NMMA/K(+) effect was 83 +/- 8% and 101 +/- 4%, respectively). However female tail arteries appeared to utilize both NO and EDHF for vascular relaxation (e.g., L-NMMA sensitivity: 58 +/- 9%; K+-sensitivity: 42 +/- 9% in mechanical stress experiments). These findings suggest endothelial regulation differs between males and females.

Regional differences in the effect of oestrogen on vascular tone in isolated rabbit arteries

The aim of the study was to assess how oestrogen acutely affects the tone of isolated artery segments from different vascular beds in rabbit. Cumulative concentrations of 17beta-oestradiol were added to ring segments of thoracic aorta, pulmonary artery, ear artery and coronary arteries from adult male rabbits. Coronary arteries precontracted with potassium or the thromboxane agonist, U46619, relaxed to oestrogen (10(-7) to 10(-4) M), whereas oestrogen (10(-8) to 10(-4) M) only caused additional contraction in segments of thoracic aorta and pulmonary artery precontracted with phenylephrine. In the thoracic aorta both the phospholipase C inhibitor NCDC (10(-4) M) and the cyclooxygenase inhibitor indomethacin (10(-5) M) almost completely blocked the contractile effect of oestrogen. In segments of the ear artery, oestrogen caused relaxation only at higher concentrations of oestrogen (10(-5) to 10(-4) M). In conclusion, oestrogen may cause both relaxation and vasoconstriction in different vascular beds, and in the thoracic aorta the contractile effects of oestrogen may be mediated via inositol phosphate-dependent pathways and release of prostaglandins.

Estrogen increases endothelial nitric oxide synthase via estrogen receptors in rat cerebral blood vessels: effect preserved after concurrent treatment with medroxyprogesterone acetate or progesterone

BACKGROUND AND PURPOSE

In vivo and in vitro rat models of hormone therapy were used to test the following hypotheses: (1) estrogen acts directly on cerebrovascular estrogen receptors to increase endothelial nitric oxide synthase (eNOS); (2) increased protein correlates with higher NOS activity; and (3) effects of estrogen on eNOS are altered by concurrent treatment with either medroxyprogesterone acetate (MPA) or progesterone.

METHODS

Blood vessels were isolated from brains of ovariectomized female rats; some were treated for 1 month with estrogen, estrogen and progesterone, or estrogen and MPA. Isolated cerebral vessels were also treated in vitro with estrogen in the absence and presence of progesterone, MPA, tamoxifen, and the estrogen receptor antagonist ICI 182 780. Levels of eNOS were measured by Western blot, and NOS activity was measured by [14C]arginine-[14C]citrulline conversion.

RESULTS

Chronic hormone treatment in vivo resulted in plasma levels of 17beta-estradiol, progesterone, and MPA in the range of values found in humans. Estrogen treatment resulted in higher levels of cerebrovascular NOS activity that paralleled increases in eNOS protein. In vitro estrogen treatment for 18 hours also resulted in a concentration-dependent increase in eNOS protein (EC50 approximately 300 pmol/L) that was completely prevented by estrogen receptor antagonists tamoxifen or ICI 182 780. However, cotreatment with progesterone or MPA, either in vivo or in vitro, did not alter the effect of estrogen on eNOS protein.

CONCLUSIONS

Estrogen receptor activation in cerebrovascular tissue results in increased eNOS activity and protein levels. The latter effect persists in the presence of either progesterone or MPA. Thus, increased NO production by eNOS may contribute to the neuroprotective effects of estrogen.

17beta-estradiol increases rat cerebrovascular prostacyclin synthesis by elevating cyclooxygenase-1 and prostacyclin synthase

BACKGROUND AND PURPOSE

It has been reported that estrogens modulate peripheral vascular synthesis of vasodilatory hormones, including prostacyclin. If this occurs in the cerebral circulation, it could have important consequences in the modulation of cerebral hemodynamic function and improvement of stroke outcome. We investigated the hypothesis that in vivo 17beta-estradiol treatment of ovariectomized rats increases cerebrovascular prostacyclin production via elevation of the enzymes responsible for prostacyclin synthesis.

METHODS

Cerebral blood vessels from 17beta-estradiol-treated and nontreated ovariectomized rats were isolated and examined for prostacyclin synthesis by enzyme-linked immunosorbent assay or for protein levels of cyclooxygenase-1, prostacyclin-synthase, and cytosolic phospholipase A2 by immunoblot analysis.

RESULTS

We report that chronic in vivo 17beta-estradiol treatment significantly enhanced basal prostacyclin synthesis in rat cerebral blood vessels by 2.6-fold over control. 17beta-estradiol treatment also resulted in a 5.1-fold increase of cyclooxygenase-1 protein and a 6.7-fold increase of prostacyclin-synthase protein in the cerebral vasculature. There was no effect of estrogen on levels of cytosolic phospholipase A2.

CONCLUSIONS

Our findings suggest that estrogen influences the biosynthesis of prostacyclin, which may be important in the regulation of cerebral blood flow and thrombosis. This finding may shed light on the mechanisms that govern sex-based differences in cerebrovascular disease.

Selected contribution: cerebrovascular nos and cyclooxygenase are unaffected by estrogen in mice lacking estrogen receptor-alpha

Estrogen alters reactivity of cerebral arteries by modifying production of endothelium-dependent vasodilators. Estrogen receptors (ER) are thought to be involved, but the responsible ER subtype is unknown. ER-alpha knockout (alphaERKO) mice were used to test whether estrogen acts via ER-alpha. Mice were ovariectomized, with or without estrogen replacement, and cerebral blood vessels were isolated 1 mo later. Estrogen increased levels of endothelial nitric oxide synthase and cyclooxygenase-1 in vessels from wild-type mice but was ineffective in alphaERKO mice. Endothelium-denuded middle cerebral artery segments from all animals constricted when pressurized. In denuded arteries from alphaERKO but not wild-type mice, estrogen treatment enhanced constriction. In endothelium-intact, pressurized arteries from wild-type estrogen-treated mice, diameters were larger compared with arteries from untreated wild-type mice. In addition, contractile responses to indomethacin were greater in arteries from wild-type estrogen-treated mice compared with arteries from untreated wild-type mice. In contrast, estrogen treatment of alphaERKO mice had no effect on diameter or indomethacin responses of endothelium-intact arteries. Thus ER-alpha regulation of endothelial nitric oxide synthase and cyclooxygenase-1 pathways appears to contribute to effects of estrogen on cerebral artery reactivity.

Human urotensin II mediates vasoconstriction via an increase in inositol phosphates

The cyclic peptide urotensin II has recently been cloned from human and reported to potently constrict primate blood vessels. To elucidate the cellular signalling mechanisms of this peptide, we investigated a possible relationship of vasomotor effects of human urotensin II and phosphoinositide turnover in isolated rabbit thoracic aorta. Human urotensin II produced a slowly developing increase in isometric contractile force (pEC(50)=9.0) that was endothelium-independent. The contractile effect of urotensin II was significantly inhibited by the phospholipase C inhibitor, 2-nitro-4-carboxyphenyl-N,N,-diphenylcarbamate (NCDC), but not by the cyclooxygenase inhibitor, indomethacin. In slices of rabbit thoracic aorta, human urotensin II increased phosphoinositide hydrolysis, and this effect was also inhibited by NCDC. The potency of urotensin II (pEC(50)=8.6) was similar to that found in the contractile studies. Thus, vasoconstrictor effects of human urotensin II appear to be mediated by a phospholipase C-dependent increase in inositol phosphates, suggesting that the peptide acts via a G(q) protein-coupled receptor.

Gonadal hormones affect diameter of male rat cerebral arteries through endothelium-dependent mechanisms

Gender is known to influence the incidence and severity of cerebrovascular disease. In the present study, luminal diameter was measured in vitro in pressurized middle cerebral artery segments from male rats that were either untreated, orchiectomized (ORX), ORX with testosterone treatment (ORX+TEST), or ORX with estrogen treatment (ORX+EST). The maximal passive diameters (0 Ca(2+) + 3 mM EDTA) of arteries from all four groups were similar. In endothelium-intact arteries, myogenic tone was significantly greater in arteries from untreated and ORX+TEST compared with arteries from either ORX or ORX+EST. During exposure to N(G)-nitro-L-arginine-methyl ester (L-NAME), an NO synthase (NOS) inhibitor, myogenic tone significantly increased in all groups. The effect of L-NAME was significantly greater in arteries from untreated and ORX+EST compared with arteries from ORX and ORX+TEST rats. Differences in myogenic tone between ORX and ORX+TEST persisted after inhibition of NOS. After endothelium removal or inhibition of the cyclooxygenase pathway combined with K(+) channel blockers, myogenic tone differences between ORX and ORX+TEST were abolished. Wall thickness and forced dilation were not significantly different between arteries from ORX and ORX+TEST. Our data show that gonadal hormones affect myogenic tone in male rat cerebral arteries through NOS- and/or endothelium-dependent mechanisms.

Estrogen reduces mouse cerebral artery tone through endothelial NOS- and cyclooxygenase-dependent mechanisms

Gender and estrogen status are known to influence the incidence and severity of cerebrovascular disease. The vasoprotective effects of estrogen are thought to include both nitric oxide-dependent and independent mechanisms. Therefore, using small, resistance-sized arteries pressurized in vitro, the present study determined the effect of gender and estrogen status on myogenic reactivity of mouse cerebral arteries. Luminal diameter was measured in middle cerebral artery segments from males and from females that were either untreated, ovariectomized (OVX), or OVX with estrogen replacement (OVX + EST). The maximal passive diameters of arteries from all four groups were similar. In response to increases in transmural pressure, diameters of arteries from males and OVX females were smaller compared with diameters of arteries from either untreated or OVX + EST females. In the presence of N(G)-nitro-L-arginine methyl ester, artery diameters decreased in all groups, but diameters remained significantly smaller in arteries from males and OVX females compared with untreated and OVX + EST females. After endothelium removal or when inhibition of nitric oxide synthase and cyclooxygenase were combined, differences in diameters of arteries from OVX and OVX + EST were abolished. These data suggest that chronic estrogen treatment modulates myogenic reactivity of mouse cerebral arteries through both endothelium-derived cyclooxygenase- and nitric oxide synthase-dependent mechanisms.