Role of estrogen in central nuclei mediating stroke-induced changes in autonomic tone

Sex, hormones and neuroeffector mechanisms

Incidence and rate of cardiovascular disease differ between men and women across the life span. Although hypertension is more prominent in men than women, there is a group of vasomotor disorders [i.e. Raynaud's disease, postural orthostatic tachycardia syndrome and vasomotor symptoms (hot flashes) of menopause and migraine] with a female predominance. Both sex and hormones interact to modulate neuroeffector mechanisms including integrated regulation of the Sry gene and direct effect of sex steroid hormones on synthesis, release and disposition of monoamine neurotransmitters, and distribution and sensitivity of their receptors in brain areas associated with autonomic control. The interaction of the sex chromosomes and steroids also modulates these effector tissues, that is, the heart, vascular smooth muscle and endothelium. Although involvement of central serotonergic centres has been studied in regard to mood disorders such as depression, their contribution to cardiovascular risk is gaining attention. Studies are needed to further evaluate how hormonal treatments and drugs used to modulate adrenergic and serotonergic activity affect progression and risk for cardiovascular disease in men and women.

Sympathetic neural mechanisms in human blood pressure regulation

Sympathetic neural function is essential to human blood pressure regulation, and overactivity of sympathetic nerves may have an important role in the development of hypertension and related cardiovascular disorders. Importantly, there is extensive interindividual variability in sympathetic vasoconstrictor nerve activity, even among healthy, young, normotensive people. Therefore, the relevance of each person's level of sympathetic nerve activity for his or her blood pressure must be evaluated in the context of other factors contributing to the overall level of blood pressure, including cardiac output and vascular adrenergic responsiveness. We include evidence showing that the balance of factors contributing to normal blood pressure in young people is influenced by sex. Hypertension itself can be multifactorial, but it is often associated with elevated sympathetic nerve activity, which can be reversed by some pharmacologic antihypertensive treatments. Although much work remains to be done in this area, an appropriate recognition of the complexity of integrated physiological regulation and of the importance of interindividual variability will be key factors in moving forward to even better understanding and treatment.

Differential Neuroprotection of Selective Estrogen Receptor Agonists against Autonomic Dysfunction and Ischemic Cell Death in Permanent versus Reperfusion Injury

In the present study, we tested the hypothesis that selective activation of estrogen receptor subtypes (ERα and ERβ) would be neuroprotective following ischemia and/or ischemia-reperfusion, as well as prevent the associated autonomic dysfunction. The selective ERα agonist, PPT, when administered 30 min prior to occlusion of the middle cerebral artery (pMCAO), resulted in a dose-dependent neuroprotection as measured 6 hours postpermanent MCAO, but not following 30 mins of MCAO followed by 5.5 hrs of reperfusion (I/R). In contrast, 30 min pretreatment with the selective ERβ agonist, DPN, resulted in a dose-dependent neuroprotection following I/R, but was not protective following pMCAO. Both drugs prevented the ischemia-induced autonomic dysfunction as measured by a decrease in the baroreceptor reflex sensitivity (BRS). The data presented here suggest a differential role of each ER subtype in targeting the mechanisms of cell death that occur in ischemia versus reperfusion injury.

Ovariectomy does not affect the cardiac sympathovagal balance of female SHR but estradiol does

The low incidence of cardiovascular diseases, including hypertension, in premenopausal women has led to the conclusion that ovarian hormones may have a protective effect on the cardiovascular system. We evaluated the effects of ovariectomy and/or estradiol on sympathovagal balance and heart rate variability (HRV) in female spontaneously hypertensive rats (SHR) with tachycardia and compared them to Wistar rats (12 weeks old; N = 8-12). Ovariectomy (OVX) and/or estradiol (10 µg/kg) did not affect basal arterial pressure in either rat strain, but estradiol increased basal heart rate (HR) in OVX SHR (454 ± 18 vs 377 ± 9 bpm). HR changes elicited by methylatropine and propranolol were used to evaluate the sympathovagal balance. Ovariectomy did not affect the cardiac sympathovagal balance of any group, while estradiol increased sympathetic tone in OVX SHR (120 ± 8 vs 56 ± 10 bpm) and sham-operated Wistar rats (57 ± 7 vs 28 ± 4 bpm), and decreased the parasympathetic tone only in OVX SHR (26 ± 7 vs 37 ± 5 bpm). HRV was studied in the frequency domain (Fast Fourier Transformation). Spectra of HR series were examined at low frequency (LF: 0.2-0.75 Hz) and high frequency (HF: 0.75-3 Hz) bands. The power of LF, as well as the LF/HF ratio, was not affected by ovariectomy, but estradiol increased both LF (29 ± 4 vs 18 ± 3 nu in Wistar sham-operated, 26 ± 5 vs 15 ± 3 nu in Wistar OVX, 50 ± 3 vs 38 ± 4 nu in SHR sham-operated, and 51 ± 3 vs 42 ± 3 nu in SHR OVX) and LF/HF (0.48 ± 0.08 vs 0.23 ± 0.03 nu in Wistar sham-operated, 0.41 ± 0.14 vs 0.19 ± 0.05 nu in Wistar OVX, 0.98 ± 0.11 vs 0.63 ± 0.11 nu in SHR sham-operated, and 1.10 ± 0.11 vs 0.78 ± 0.1 nu in SHR OVX). Thus, we suggest that ovariectomy did not affect the cardiac sympathovagal balance of SHR or Wistar rats, while estradiol increased the sympathetic modulation of HR.

Role of oestrogen in the central regulation of autonomic function

1. In recent years, the role of oestrogen in women's health has been a subject of considerable scientific and popular debate. There is unquestionable evidence that oestrogen has both potent and long-lasting effects on several vital organ systems, including the cardiovascular system, the autonomic nervous system and, most recently, within the central nervous system itself. 2. The research and medical community continues to debate whether the benefits of oestrogen therapy outweigh the risks in the treatment of the symptoms of menopause, the attenuation of the risk for cardiovascular insults, such as stroke and heart disease, and even the retardation of the progression of Alzheimer's disease. 3. The recent evidence provided by the Heart and Estrogen/Progestin Replacement Study (HERS) II clinical trial suggesting that long-term exposure to combined oestrogen and progestin in post-menopausal women who have previously had a heart attack or stroke (for secondary prevention) may actually increase their risk of a subsequent cardiovascular insult has further fuelled the debate. However, there remain considerable gaps in our knowledge with respect to the actual mechanisms by which oestrogen exerts its various beneficial effects at the cellular level for the primary prevention of cardiovascular disease. This information is essential if we are to harness the positive aspects of oestrogen therapy in such a manner as to avoid or minimize the associated risks of increased oestrogen exposure in women who we know, with some certainty, to be at an increased risk of cancers of the uterus, cervix and breast tissue.

Estrogen in the parabrachial nucleus attenuates the sympathoexcitation following MCAO in male rats

Recent investigations have provided evidence to suggest systemic estrogen administration prevented or reversed the sympathoexcitation observed following middle cerebral artery occlusion (MCAO) in male rats. The present investigation sought to determine the role of estrogen injected directly into the parabrachial nucleus (PBN) on the MCAO-induced sympathoexcitation as well as the role of the rostral ventrolateral medulla (RVLM) in mediating the sympathoexcitatory response. Male Sprague-Dawley rats were anesthetized with sodium thiobutabarbitol (100 mg/kg) and were instrumented to continuously record blood pressure, heart rate and renal sympathetic nerve activity (RSNA). Following occlusion of the middle cerebral artery, there was a significant increase in RSNA (from 3.8 +/- 0.4 to 8.3 +/- 0.6 microV/s; P < 0.05) which was significantly attenuated by the prior bilateral injection of estrogen (0.5 microM in 200 nl) into the PBN. Pre-injection of lidocaine (5% in 200 nl) directly into the RVLM resulted in only a slight reduction in the magnitude of the MCAO-induced sympathoexcitation (P > 0.05). Extracellular electrophysiological recordings from RVLM neurons demonstrated that MCAO did not produce any significant change in neuronal activity over the experimental time course (P > 0.05). Also, bilateral injection of estrogen into the PBN prior to MCAO or sham conditions did not result in any significant change in RVLM neuronal activity. These results indicate that estrogen receptors in the PBN play a major role in modulating the sympathoexcitatory response from ischemic forebrain nuclei, and that the pathway from the PBN to sympathetic preganglionic nuclei may not involve a synapse in the RVLM.

Sympathoexcitatory effects of estrogen in the insular cortex are mediated by GABA

The current investigation examined the effect of estrogen in the insular cortex (IC) on autonomic tone and cardiac baroreceptor reflex function and sought to determine if modulation of neurotransmission was responsible for mediating this effect. Experiments were performed in Inactin-anaesthetized, male Sprague-Dawley rats. Animals were instrumented to record blood pressure, heart rate, vagal parasympathetic and renal sympathetic nerve activities, as well as cardiac baroreflex sensitivity (BRS). Direct, bilateral injection of 17beta-estradiol (0.5 microM; 200 nl/side) into the IC resulted in a significant increase in sympathetic tone (from 10 +/- 4 to 24 +/- 3) with no significant change in blood pressure, heart rate, parasympathetic tone or BRS measured at 30 min post-injection. This estrogen-induced effect was completely blocked by the co-injection of estrogen with the estrogen receptor antagonist, ICI 182, 780 (20 microM; 200 nl/side). Co-injection of estrogen with a GABA(B), NMDA or non-NMDA receptor antagonists did not effect the estrogen-induced increase in sympathetic tone. Co-injection of a sub-threshold dose of estradiol (0.125 microM; 200 nl/side) with the GABA(A) receptor antagonist, (+)-bicuculline (0.025 microM; 200 nl/side), resulted in an additive response to increase sympathetic nerve activity. These results suggest that estrogen acts on estrogen receptors to modulate GABA(A)-receptor-mediated neurotransmission within the IC to modulate sympathetic tone.

Estrogen synthesis in the central nucleus of the amygdala following middle cerebral artery occlusion: Role in modulating neurotransmission

Stroke-induced lesions of the insular cortex in the brain have been linked to autonomic dysfunction (sympathoexcitation) leading to arrhythmogenesis and sudden cardiac death. In experimental models, systemic estrogen administration in male rats has been shown to reduce stroke-induced cell death in the insular cortex as well as prevent sympathoexcitation. The central nucleus of the amygdala has been postulated to mediate sympathoexcitatory output from the insular cortex. We therefore set out to determine if endogenous estrogen levels within the central nucleus of the amygdala are altered following stroke and if microinjection of estrogen into the central nucleus of the amygdala modulates autonomic tone. Plasma estrogen concentrations were not altered by middle cerebral artery occlusion (22.86+/-0.14 pg/ml vs. 21.24+/-0.33 pg/ml; P>0.05). In contrast, estrogen concentrations in the central nucleus of the amygdala increased significantly following middle cerebral artery occlusion (from 20.83+/-0.54 pg/ml to 76.67+/-1.59 pg/ml; P<0.05). Local infusion of an aromatase inhibitor, letrozole, into the central nucleus of the amygdala at the time of middle cerebral artery occlusion prevented the increase in estrogen concentration suggesting that this increase was dependent on aromatization from testosterone. Furthermore, bilateral microinjection of estrogen (0.5 microM in 200 nl) directly into the central nucleus of the amygdala significantly decreased arterial pressure and sympathetic tone and increased baroreflex sensitivity, and these effects were enhanced following co-injection with either an N-methyl-D-aspartate or non-N-methyl-D-aspartate receptor antagonist. Taken together, the results suggest that middle cerebral artery occlusion resulted in synthesis of estrogen within the central nucleus of the amygdala and that this enhanced estrogen level may act to attenuate overstimulation of central nucleus of the amygdala neurons to prevent middle cerebral artery occlusion-induced autonomic dysfunction.

Estrogen attenuates neuronal excitability in the insular cortex following middle cerebral artery occlusion

The current investigation examined the role of estrogen in the insular cortex (IC) under both normal and ischemic conditions. Experiments were done in anaesthetized male Sprague-Dawley rats. The effect of systemic 17beta-estradiol (estrogen) administration on levels of amino acids and of endogenous estrogen obtained by microdialysis and its effect on neuronal activity of cells located in the insular cortex were measured in the absence of, and following permanent occlusion of, the right middle cerebral artery (MCA). In normal rats, intravenous (i.v.) injection of estrogen resulted in a significant increase (greater than 25 spikes/bin) in the spontaneous activity of neurons located within the insular cortex, while there was a significant decrease in gamma-aminobutyric acid (GABA) levels measured in IC dialysate. Middle cerebral artery occlusion (MCAO) resulted in a biphasic response consisting of a transient increase in the extracellular concentration of glutamate, aspartate, and GABA, followed by sustained elevations in glutamate and aspartate, but reduced GABA levels 4 h post-MCAO. MCAO also resulted in a significant increase in neuronal activity in the IC (from 28 +/- 9 to 120 +/- 88 spikes/bin). This MCAO-induced excitation was completely blocked following the prior intravenous administration of estrogen. Systemic estrogen administration also resulted in a delay in the progression and decrease in the final infarct volume by approximately 56%. Taken together, these results suggest that under normal conditions, estrogen excites neurons in the insular cortex by decreasing GABA release (disinhibition) and it plays a role in attenuating the MCAO-induced excitability and death of these neurons.