Resveratrol preconditioning induces cellular stress proteins and is mediated via NMDA and estrogen receptors.

Resveratrol pretreatment has been shown to provide neuroprotection in models of cerebral ischemia. This phenomenon, commonly termed preconditioning, promotes ischemic tolerance and may involve mild activation of endoplasmic reticulum stress pathways in the affected tissue. Systemic injection of resveratrol (2 x 10(-3), 2 x 10(-4), 1 x 10(-4) mg/kg) 30 min prior to a 4 h period of right middle cerebral artery occlusion significantly reduced infarct area in the insular region of rat prefrontal cortex. This affect was blocked when resveratrol treatment was combined with a non-selective estrogen receptor antagonist, or preceded by intracortical injection of an NMDA receptor antagonist. The neuroprotective effect of resveratrol was associated with reduced renal sympathetic nerve activity as well as induction of resident endoplasmic reticulum chaperone proteins, glucose-regulated proteins 78 and 94. The calcium-sensitive chaperone heat shock protein 70 and the cysteine protease m calpain did not respond to resveratrol pretreatment. However, a significant induction of heat shock protein 70 was observed in the contralateral cortex of resveratrol pretreated rats following 4 h of right middle cerebral artery occlusion. These data suggest that resveratrol preconditioning promotes ischemic tolerance in the short term, in part via effects mediated through activation of estrogen and NMDA receptors, as well as through mild activation of cellular stress proteins.

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 limits ischemic cell death by modulating caspase-12-mediated apoptotic pathways following middle cerebral artery occlusion.

Estrogen has received considerable attention as a potential therapeutic agent against various forms of neurodegenerative diseases including stroke. Experimental data in animal models of stroke have provided exhaustive evidence of the neuroprotective properties of this steroid hormone. Our laboratory in particular has demonstrated that acute estrogen treatment in male rats significantly reduced (approximately 50%) ischemic cell death within 4 h following permanent occlusion of the middle cerebral artery occlusion (MCAO). However, the cellular and molecular mechanisms implicated in the protective actions of estrogen in this experimental model have yet to be elucidated. Accumulating evidence suggests that in various in vivo and in vitro models, estrogen can be pro-apoptotic and that this effect may be mediated by an estrogen-induced up-regulation of the Fas/FasL system and the subsequent activation of caspase-12. We therefore hypothesized that under ischemic conditions following MCAO, estrogen would up-regulate protective endoplasmic reticulum (ER) stress pathways leading to caspase-12 activation, thus limiting infarct volume. Our results showed that estrogen significantly increased activated caspase-12 at 2, 3 and 4 h post-MCAO. Immunostaining of brain sections showed a significantly higher number of terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling positive cells in estrogen-treated animals at 4 h, but not at 2 h, post-MCAO. These findings correlate with previous observations that differences in infarct volume between saline and estrogen-treated animals are not seen until 3 and 4 h post-MCAO. A decrease in m-calpain expression was observed in the infarct region only at 4 h post-MCAO following estrogen pre-treatment, suggesting m-calpain may not be involved in regulating estrogen-induced caspase-12 activation. Based on these cellular changes correlated to estrogen pretreatment, we conclude that estrogen may up-regulate ER-specific apoptotic pathways, thus limiting the extent of necrotic cell death which is responsible for the spreading depression and growth of the infarct volume following MCAO.

Estrogen-mediated neuroprotection in the cortex may require NMDA receptor activation.

Several studies have suggested that a potential mechanism for estrogen-mediated neuroprotection following experimental stroke is a result of modulating glutamate-mediated excitotoxicity. Our laboratory has shown that in male rats, estrogen injection (systemic or direct intracortical injection) resulted in an immediate depolarization of cortical neurons. Therefore, the present study was designed to investigate whether the estrogen-induced depolarization of cortical neurons was required in mediating the early events associated with this neuroprotection. We tested this hypothesis by co-injecting selective antagonists of the NMDA (MK-801) or AMPA (DNQX) glutamatergic receptors with estrogen. Systemic injection of estrogen significantly attenuated the MK-801-induced decrease in infarct volume following middle cerebral artery occlusion (MCAO). Similarly, when estrogen and MK-801 were co-injected directly into the cortex, no neuroprotection was observed. However, when estrogen or MK-801 was injected centrally 10 min prior to the injection of the other drug, significant neuroprotection was observed. This led us to hypothesize that estrogen-mediated neuroprotection required an initial activation of NMDA receptors. Furthermore, our results suggest that this estrogen-mediated neuroprotection was also associated with a significant increase in m-calpain and activation of an endoplasmic reticulum (ER) specific caspase-12. Finally, the results of current clamp experiments showed that estrogen significantly depolarized cortical neurons as well as enhanced NMDA-induced depolarization. Taken together, these results suggest that estrogen pretreatment may activate NMDA receptors resulting in modification of ER-associated molecular mechanisms involved in neuroprotection following MCAO.

17beta-estradiol inhibits outward potassium currents recorded in rat parabrachial nucleus cells in vitro.

Evidence is increasingly accumulating in support of a role for the steroid hormone 17beta-estradiol to modify neuronal functions in the mammalian CNS, especially in autonomic centers. In addition to its well known slowly developing and long lasting actions (genomic), estrogen can also rapidly modulate cell signaling events by affecting membrane excitability (non-genomic). Little, however, is known regarding the mechanism(s) by which 17beta-estradiol produces its rapid effects on neuronal membrane excitability. As potassium channels play a crucial role in cell excitability, we hypothesized that 17beta-estradiol caused excitability by modulating potassium flux through the neuronal cell membrane. We tested this hypothesis by examining the effects of 17beta-estradiol on outward potassium currents recorded in cells from the parabrachial nucleus of rats, in vitro. Bath application of 17beta-estradiol (10-100 microM) reversibly reduced voltage-activated outward potassium currents in a concentration-dependent manner. This effect was mimicked by BSA-17beta-estradiol but not mimicked by 17alpha-estradiol and was significantly reduced by ICI 182,780, a selective estrogen receptor antagonist. The inhibitory effect of 17beta-estradiol was dependent on extracellular potassium concentration, with more profound effects observed at lower concentrations. The 17beta-estradiol-induced inhibition of the outward current was blocked by pretreatment with the potassium channel blockers tetraethylammonium and 4-aminopyridine. The time constants of deactivation of tail currents were decreased by 17beta-estradiol over a range of test potentials (-140 to -80 mV). Finally, the inhibitory effect of 17beta-estradiol on the outward potassium currents was blocked following pre-incubation of slices in lavendustin A, a tyrosine kinase inhibitor. Taken together, these results suggest that 17beta-estradiol acts rapidly at an extracellular membrane receptor to reduce tetraethylammonium- and 4-aminopyridine-sensitive outward potassium currents by accelerating the closure of potassium channels. This may be the ionic basis of 17beta-estradiol-induced enhancement of neuronal excitability.

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.

The role of neuropeptides and neurohormones in neurogenic cardiac arrhythmias.

The functional significance of neuropeptides and neurohormones throughout the neuroaxis has been the focus of considerable research over the past 25 years. These "gut peptides" or "reproductive hormones" have been localized within nuclei responsible for the relay of visceral afferent information to the forebrain. The presence of peptides and hormones along the gut- or heart-brain continuum suggests that these neurochemicals do more than modulate the visceral processes of digestion and reproduction respectively. Numerous studies have shown that the exogenous administration of these neurochemicals directly into visceral afferent nuclei significantly alters blood pressure, heart rate, autonomic tone and the sensitivity of the baroreceptor reflex (an index of sympatho-vagal balance). A strong inverse correlation has been demonstrated between the sensitivity of the baroreceptor reflex and susceptibility to lethal cardiac arrhythmias which lead ultimately to sudden cardiac death. The differential effects of various neurochemicals on the sensitivity of the baroreceptor reflex suggests that some neurochemicals may act as preventatives while others may actually contribute to the pathogenesis of neurogenic cardiac arrhythmias. Hormones such as estrogen, in addition to their neuroprotective properties, may also play a role in modulating the cardiovascular consequences to neurogenic pathologies including stroke and epilepsy. This review will summarize the evidence available which suggests that neuropeptides and neurohormones can alter both neurogenic as well as visceral pathology-induced changes in autonomic function resulting in an increased risk of sudden cardiac death.

Reduction in infarct size by local estrogen does not prevent autonomic dysfunction after stroke.

Systemic estrogen administration in male rats has been shown to normalize the autonomic dysfunction and reduce the infarct size after permanent middle cerebral artery occlusion (MCAO). Therefore, the present investigation determined if local microinjection of estrogen at the site of the infarct also promoted recovery of autonomic function and reduction of the infarct size. Experiments were done in anesthetized (thiobutabarbitol sodium; 100 mg/kg) male Sprague-Dawley rats instrumented to record baseline and reflex changes in cardiovascular and autonomic parameters. The right middle cerebral artery was permanently occluded using bipolar coagulation. Local microinjection of estrogen into the insular cortex before MCAO significantly reduced the infarct size but did not attenuate the MCAO-induced autonomic dysfunction. Injection of ICI-182,780 alone significantly increased infarct area; however, the greater infarct area was not associated with enhanced autonomic dysfunction. These results suggest that within the insula, endogenous estrogen activity can affect the extent of MCAO-induced cell death, but extracortical central nervous system sites may be responsible for mediating the beneficial effects of estrogen on the autonomic disturbances.

Estrogen-induced recovery of autonomic function after middle cerebral artery occlusion in male rats.

Several studies have provided evidence to suggest that estrogen results in a significant reduction (approximately 50%) in the size of the ischemic zone in the middle cerebral artery occlusion (MCAO) model of stroke in a rat. The current study was done to demonstrate whether this estrogen-induced reduction in infarct size is associated with normalization of the autonomic dysfunction observed in an acute model of stroke in male rats. Experiments were done in anesthetized (thiobutabarbitol sodium; 100 mg/kg) male Sprague-Dawley rats instrumented to record baseline and reflex changes in cardiovascular and autonomic parameters. Estrogen was intravenously administered 30 min before, immediately before, or 30 min after MCAO. Estrogen administration resulted in a recovery of autonomic function and prevented the detrimental changes in autonomic tone observed following a stroke. In addition, infarct size was significantly increased in the presence of the estrogen antagonist ICI-182,780. These results suggest that both pre- or poststroke estrogen administration prevents or reverses acute stroke-induced autonomic dysfunction and that endogenous estrogen levels in males can contribute to this neuroprotection.

Inhibitory effect of 17beta-estradiol in the parabrachial nucleus is mediated by GABA.

In the present investigation, electrophysiological recordings of thalamic relay neurons were used to investigate the role of estrogen as a modulator of visceral afferent information through the PBN to forebrain structures. Experiments were done in anaesthetized (sodium thiobutabarbitol; 100 mg/kg) male and ovariectomized female rats supplemented for 7 days prior with either 17beta-estradiol (OVX-E(2)) or saline (OVX-S). A portion of the right cervical vagus was isolated for the electrical activation (0.8 Hz, 2 ms duration) of visceral afferents. The evoked single and multi-unit activity was recorded via a recording electrode in the ventrobasal thalamus. Exogenous microinjection of 17beta-estradiol (0.1, 0.25 and 0.5 microM; 200 nl) into the parabrachial nucleus (PBN) produced a significant, dose-dependent attenuation in the magnitude of visceral afferent activation-evoked responses of neurons recorded in the thalamus in both male and OVX-E(2) groups. No effect on evoked thalamic activity was observed following injection of estrogen into the PBN of OVX-S animals. Co-injection of estrogen with the GABA(A) receptor antagonist, bicuculine (0.1 microM; 200 nl) but not phaclofen (GABA(B); 0.1, 0.5 or 1 microM; 200 nl) resulted in an increase in the evoked thalamic response in males (55+/-11%) and OVX-E(2) female (68+/-15%) rats. These studies suggest that estrogen inhibits neurotransmission in the PBN via an interaction with the GABA(A) receptor to modulate the flow of visceral information to the thalamus.

Estrogen blocks the cardiovascular and autonomic changes following vagal stimulation in ovariectomized rats.

The current investigation examines the effect of acute and chronic estrogen administration on baroreflex sensitivity and autonomic tone following 2 h of vagal afferent stimulation in ovariectomized female rats. Female Sprague-Dawley rats were ovariectomized and supplemented daily for 7 days with either estrogen (OVX-E2; 0.5 microg/kg; s.c.) or saline (OVX-S; 0.9%; s.c.). On the 8th day the animals were anaesthetized (sodium thiobutabarbitol; 100 mg/kg) and instrumented for recording blood pressure, heart rate and efferent vagal and renal nerve activities. The baroreflex was evoked using intravenous injection of various doses of phenylephrine hydrochloride (0.025, 0.05+/-0.1 mg/kg). Electrical stimulation of vagal afferents for 2 h produces autonomic imbalance characterized by sympathoexcitation and parasympathetic withdrawal. This protocol of vagal stimulation produced a significant increase in renal nerve activity (from 20+/-6 to 140+/-20 spikes/2 s) and decreases in both vagal nerve activity (from 22+/-3 to 10+/-2 spikes/2 s) and baroreflex sensitivity (from 0.55+/-0.05 to 0.3+/-0.05) in OVX-S female rats. However. vagal stimulation had no effect on baroreflex sensitivity or autonomic nerve activities in OVX-E2 rats. Administration of a single, bolus dose of estrogen (1 x 10(-2) mg/kg) to OVX-S rats immediately prior to termination of vagal stimulation blocked the changes in autonomic nerve activities and baroreflex sensitivity previously observed. These results suggest that both chronic and acute estrogen supplementation may provide resistance to the autonomic disturbances associated with visceral afferent activation.

Acute injection of 17beta-estradiol enhances cardiovascular reflexes and autonomic tone in ovariectomized female rats.

Among the many benefits of long-term hormone replacement therapy to postmenopausal women is a significant reduction in risk for and progression of cardiovascular disease. However, long-term estrogen replacement therapy has been associated with several undesirable, and likely dose-dependent, side-effects. There is some evidence to suggest that the dose of estrogen which confers optimal beneficial effects on the cardiovascular system is much lower than that which is currently prescribed for postmenopausal women. The following experiments were conducted to determine the dose-response relationship of acutely administered estrogen on autonomic tone and reflex control of heart rate in ovariectomized Sprague-Dawley female rats. Rats were anaesthetized with sodium thiobutabarbital (100 mg/kg) and instrumented to record blood pressure, heart rate and efferent parasympathetic and sympathetic nerve activities. The sensitivity of the cardiac baroreflex was tested using intravenous injection of either phenylephrine hydrochloride (0.025-0.1 mg/kg) or sodium nitroprusside (0.0025-0.01 mg/kg). Intravenous injection of estrogen produced dose-dependent increases in the magnitude of the baroreflex sensitivity and parasympathetic tone while reducing sympathetic tone with a maximal effect observed at 1 x 10(-3) mg/kg. Prior administration of the selective estrogen receptor antagonist, ICI 182,780 blocked the estrogen-induced changes in baroreflex sensitivity and autonomic tone. These results demonstrate that acutely administered, low-dose estrogen has beneficial effects on autonomic tone and cardiovascular reflexes.

Autonomic and cardiovascular reflex responses to central estrogen injection in ovariectomized female rats.

The role of estrogen in central autonomic nuclei was examined in ovariectomized female Sprague-Dawley rats supplemented daily for 7 days with either estrogen (5 microg/kg; sc) or saline (0.9%; sc). Animals were subsequently anaesthetized with sodium thiobutabarbital (Inactin; 100 mg/kg; ip) and instrumented to record blood pressure and heart rate. Efferent vagal parasympathetic (VPNA) and renal sympathetic (RSNA) nerve activities were recorded and used to assess baseline and reflexive changes in autonomic tone. The cardiac baroreflex was evoked using a single bolus injection of phenylephrine (0.1 mg/kg) both before and following either intrathecal injection of estrogen (0.5 microM; 1 microl) or bilateral injection of estrogen (0.5 microM; 100 nl/side) into several central autonomic nuclei. In estrogen-replaced rats, both the baseline and PE-evoked values for mean arterial pressure and RSNA were significantly decreased following injection of estrogen into the nucleus tractus solitarius (NTS), rostral ventrolateral medulla (RVLM), parabrachial nucleus (PBN), central nucleus of the amygdala (CNA) and the intrathecal space. Baseline heart rate and VPNA were significantly decreased following injection of estrogen into NTS, nucleus ambiguous (Amb), PBN and the intrathecal space. PE-evoked changes in heart rate and VPNA were significantly enhanced following injection of estrogen into these same nuclei. Injection of estrogen into the insular cortex (IC) produced significant decreases in baseline and PE-evoked RSNA only. The cardiac baroreflex was significantly enhanced following injection of estrogen into all nuclei and the intrathecal space. In saline-replaced females, injection of estrogen into NTS, RVLM, Amb and the intrathecal space had similar effects on both baseline and PE-evoked parameters although of a reduced magnitude compared to estrogen-replaced rats. However, no significant changes in autonomic tone and baroreflex function were observed following the injection of estrogen into the PBN, CNA or IC of saline-replaced rats. These results demonstrate a role for estrogen in central autonomic nuclei in female rats and suggest a possible alteration of estrogen receptor distribution or efficacy within the central nervous system of estrogen-deficient female rats.

Medullary and intrathecal injections of 17beta-estradiol in male rats.

The following experiments were designed to investigate the role of estrogen in central autonomic nuclei on autonomic tone and reflex control of heart rate. Male Sprague-Dawley rats were anesthetized with sodium thiobutabarbital (100 mg/kg) and instrumented to record blood pressure and heart rate. Efferent vagal and renal nerve activities were recorded and used to assess changes in parasympathetic and sympathetic tone, respectively. The cardiac baroreflex was evoked using a single bolus injection of phenylephrine (0.1 mg/kg) both before and following either intrathecal injection of estrogen (0.5 microM; 1 microl) to influence sympathetic preganglionic neurons of the intermediolateral cell column or bilateral injection of estrogen (0.5 microM; 100 nl/side) into the nucleus tractus solitarius, rostral ventrolateral medulla or nucleus ambiguus. The cardiac baroreflex was significantly enhanced following both intrathecal and medullary injections of estrogen. Efferent vagal nerve activity was significantly increased following injection of estrogen into the nucleus tractus solitarius, nucleus ambiguus and the intrathecal space. Renal sympathetic nerve activity was significantly depressed following injection of estrogen into the nucleus tractus solitarius, rostral ventrolateral medulla and the intrathecal space. In all cases, simultaneous injection of estrogen with the selective estrogen receptor antagonist, ICI 182,780 (1 pM) blocked all previously observed changes in baroreflex function and autonomic tone. These results demonstrate a role for estrogen in the reflex control of heart rate and as a central modulator of autonomic tone in male rats.

17beta-estradiol modulates baroreflex sensitivity and autonomic tone of female rats.

The following experiments examine the role of estrogen as a central modulator of autonomic tone and baroreflex sensitivity in the female rat. Female Sprague-Dawley rats were ovariectomized and then supplemented daily for 7 days with a fixed dose of estrogen (5 microg/kg; sc) to produce a stable level of estrogen similar to that present at proestrous (17 pg/ml). The rats were then anaesthetized with sodium thiobutabarbital (100 mg/kg) and instrumented to record blood pressure, heart rate and both vagal and renal efferent nerve activities. The sensitivity of the cardiac baroreflex was tested using intravenous injection of multiple doses of either phenylephrine hydrochloride or sodium nitroprusside. Estrogen-supplemented female rats exhibited a significantly enhanced BRS as compared to male rats from a previous study (0.78 vs. 0.5). Furthermore, bolus injection of estrogen (1x10(-2) mg/kg; iv) in estrogen-supplemented female rats produced a significant increase in vagal nerve activity and a significant decrease in renal nerve activity which together resulted in a further enhancement of the BRS (0.78 vs. 2.4). Injection of the selective estrogen receptor antagonist, ICI 182,780, into nucleus ambiguus and the intrathecal space of the spinal cord blocked the respective changes in parasympathetic and sympathetic nerve activities indicating that intravenously administered estrogen modulates baseline autonomic tone via the activation of central estrogen receptors.

Visceral afferent activation-induced changes in sympathetic nerve activity and baroreflex sensitivity.

The following experiments were done to determine whether changes in baroreflex sensitivity evoked by cervical vagus nerve stimulation are due to sympathoexcitation mediated by the parabrachial nucleus. The relative contribution of cardiopulmonary and general gastric afferents within the cervical vagus nerve to the depression in baroreflex sensitivity are also investigated. Male Sprague-Dawley rats anesthetized with thiobutabarbital sodium (50 mg/kg) were instrumented to measure blood pressure and heart rate or for the continuous monitoring of renal sympathetic nerve activity. Baroreflex sensitivity was measured using bolus injections of phenylephrine. Electrical stimulation of the cervical vagus (with or without the aortic depressor nerve) or the abdominal vagus nerve produced a significant increase in renal nerve activity and a decrease in baroreflex sensitivity. Both of these effects were blocked after the microinjection of lidocaine into the parabrachial nucleus before nerve stimulation. Therefore, we conclude that an increase in the activity of cardiac, pulmonary, or general gastric afferents mediated the increased sympathetic output and decreased baroreflex sensitivity via a pathway involving the parabrachial nucleus.

Centrally mediated effect of 17beta-estradiol on parasympathetic tone in male rats.

The following experiments were conducted to determine if peripherally administered estrogen has an effect on central autonomic tone and whether this change in tone results in an alteration in cardiovascular reflex control. Male Sprague-Dawley rats were anesthetized with thiobutabarbitol sodium (50 mg/kg) and instrumented to record blood pressure, heart rate, and vagal parasympathetic or renal sympathetic efferent nerve activity. Additional rats were instrumented to test the sensitivity of the cardiac baroreflex using intravenous injections of phenylephrine hydrochloride (0.025, 0.05, 0.1 mg/kg) or sodium nitroprusside (0. 0025, 0.005, 0.01 mg/kg) and plotting the cardiovascular responses. Intravenous injection of estrogen (10(-4), 10(-2), and 10(-1) mg/kg) produced a significant increase in vagal efferent activity and in baroreflex sensitivity. The bilateral microinjection of an estrogen receptor antagonist, ICI-182,780 (1 pM, 50 nl/side) into the nucleus ambiguus blocked both the estrogen-induced increase in vagal efferent activity and baroreflex sensitivity. These results demonstrate that in male rats estrogen acts centrally to enhance baroreflex sensitivity by increasing parasympathetic efferent tone.

Role of 17beta-estradiol in the modulation of baroreflex sensitivity in male rats.

Female mammals have an enhanced baroreflex sensitivity compared with their male counterparts, leading researchers to speculate that estrogen modulates autonomic tone. Therefore, this study tests the hypothesis that exogenous estrogen can enhance the baroreflex sensitivity of male rats. Male Sprague-Dawley rats anesthetized with thiobutabarbitol sodium (50 mg/kg) were instrumented to measure blood pressure and heart rate and for the intravenous injection of drugs. The baroreflex was tested using intravenous injections of phenylephrine (0.025, 0.05, and 0.1 mg/kg), and the cardiovascular responses were plotted to obtain a measure of the sensitivity of the cardiac baroreflex. Intravenous injection of estrogen produced dose-related increases in the baroreflex sensitivity due to an increase in the magnitude of the reflex bradycardia. In a separate group of animals, stimulation of the vagus nerve for 2 h resulted in a decrease in baroreflex sensitivity. This effect was blocked when estrogen (1 x 10(-2) mg/kg) was administered immediately before the end of stimulation. In conclusion, intravenous injection of estrogen in male rats significantly enhanced baroreflex sensitivity and blocked the attenuation in the baroreflex sensitivity observed after vagal stimulation.

The parabrachial nucleus mediates the decreased cardiac baroreflex sensitivity observed following short-term visceral afferent activation.

Previous investigations have provided evidence demonstrating that the extracellular release of glutamate into the parabrachial nucleus was significantly enhanced following visceral afferent activation. This period of enhanced glutamate release into the parabrachial nucleus corresponded to a time during which the pressor response to a bolus phenylephrine injection was significantly enhanced, and the reflex bradycardia was attenuated. This decrease in the sensitivity of the baroreflex is suggestive of an enhanced sympathetic tone as a result of the vagal stimulation. The present investigation was done to determine if the decreased baroreflex sensitivity observed following short-term vagal stimulation is mediated by an increase in sympathetic activity and was dependent on the parabrachial synapse. Male Sprague-Dawley rats were anaesthetized with sodium thiobutabarbitol and instrumented to monitor blood pressure and heart rate and for the placement of a stimulating electrode on the left cervical vagus nerve. Femoral arterial blood samples were taken before, during and after 2 h of vagal stimulation which were later assayed for plasma catecholamines. The results showed that plasma norepinephrine levels decreased during, and were significantly elevated immediately following termination of the vagal stimulation, indicative of an increase in sympathetic tone. To determine if the parabrachial nucleus is involved in mediating an enhanced sympathetic activity following vagal stimulation, a second group of animals underwent an identical surgical preparation, vagal stimulation and blood sampling protocol with the addition of bilateral microinjections of either the reversible anaesthetic, lidocaine, or saline into the parabrachial nucleus. The results showed that reversible blockade of the parabrachial nucleus prior to the onset of the vagal stimulation was effective in blocking both the elevation in plasma norepinephrine levels and the depressed baroreflex sensitivity previously observed following 2 h of vagal stimulation. These results suggest that the parabrachial nucleus mediated the sympathoexcitation and consequent depression in baroreflex sensitivity observed following visceral afferent activation.

Role of the insular cortex in the modulation of baroreflex sensitivity.

Cervical vagal stimulation for 2 h results in a depressed baroreflex sensitivity produced by an enhanced sympathetic output, as indicated by increased plasma norepinephrine levels. The current study examined the role of the insular cortex in modulating the vagal stimulation-induced changes in baroreflex sensitivity. Male Sprague-Dawley rats were anesthetized with thiobutabarbitol sodium and instrumented for recording blood pressure, heart rate, intravenous drug administration, and vagal afferent nerve stimulation. Stereotaxic microinjections (300 nl) of either 5% lidocaine or 0.9% saline were made bilaterally into the insula. Thirty minutes after 2 h of vagal stimulation, the baroreflex was significantly depressed and plasma norepinephrine levels were significantly elevated in both groups. The baroreflex was also significantly depressed after bilateral lidocaine injections into the insula, independent of vagal stimulation. However, no significant change in plasma norepinephrine was observed, suggesting that an attenuated parasympathetic output contributed to the altered baroreflex. Taken together, the results suggest that the insular cortex modulates the cardiac baroreflex through a modulation of parasympathetic output.