Rostral Ventral Lateral Medulla Research Articles

SAT596 Evidence That Corticotropin-releasing Hormone Receptor 2 Activation Mediates Stress-induced Hyperglycemia

Abstract Disclosure: R.B. McCosh: None. M.J. Kreisman: None. J. Jang: None. J. Jung: None. K.M. Breen Church: None. Stress-induced hyperglycemia refers to the transient increase in blood glucose (BG) levels during stress, which occurs in healthy and diabetic patients. In critically ill patients, stress-induced hyperglycemia is associated with a 2-fold increase in risk of incident diabetes. Thus, stress-induced hyperglycemia is a predictor of or may contribute to glucose dysregulation. In mice, activation of catecholamine neurons in the rostral ventral lateral medulla (rVLM) is necessary for stress-induced hyperglycemia; however, the mechanism by which these cells become activated during stress is not clear. Here, we tested the possibility that corticotropin-releasing hormone receptor 2 (CRHR2) signaling mediates stress-induced hyperglycemia in adult, ovariectomized mice. First, BG levels and rVLM cell activation were examined in two stress models. For psychosocial stress, BG levels were not altered by control treatment. In contrast, placement in a restraint device increased BG levels ∼2-fold, within 30 min, from pretreatment values and BG remained elevated for an additional 90 min. For immune stress, BG was significantly elevated (21% increase) at 30 min post IP injection of lipopolysaccharide (LPS, 10ng/g bw), then returned to baseline levels whereas BG levels were not altered from pretreatment levels in saline injected mice. In tissue collected 2 hr after initiation of restraint stress, the percentage of tyrosine hydroxylase (TH, marker of catecholamine cells) neurons in the rVLM that contained c-Fos (a marker of neural activation) was significantly greater than control mice. Next, we assessed the sufficiency of CRHR2 activation to induce hyperglycemia using the CRHR2 agonist urocortin 2 (UCN2), injected intracerebroventricular (ICV). BG was significantly increased 30 min after injection of both saline and UCN2 (60-80%); however, BG subsequently returned to baseline in saline-treated mice, but remained elevated in mice that received UCN2, similar to restraint stress. To test necessity of CRHR2 activation to induce hyperglycemia, we delivered ICV either the specific CRHR2 antagonist Astressin 2B (A2B) or saline 1 hr before psychosocial or immune stress. In no stress controls, A2B did not alter BG levels compared to saline, yet, BG levels in both groups decreased over time. BG levels were elevated in saline-treated controls 30 min after restraint stress; and, this increase was delayed, although not prevented, by 30 min by A2B pre-treatment. Following LPS injection, BG was increased at 30 min then decreased relative to baseline in saline-treated controls; yet, A2B pre-treatment blocked these changes. Together these data support the hypothesis that CRHR2 signaling is an important mediator of stress-induced hyperglycemia. Assessment of rVLM TH cells is ongoing. Presentation: Saturday, June 17, 2023

Brainstem Damage Underlies Changes in Hypoxic Ventilatory Response Following Cardiac Arrest and Resuscitation in Rats.

People resuscitated after sudden cardiac arrest remain at high risk for mortality, with treatment for survivors varying from monitoring to life support. With respect to assessing survivability post cardiac arrest and resuscitation (CAR), we previously demonstrated the potential of the hypoxic ventilatory response (HVR) as a reliable indicator for discerning between survivors and non-survivors in the early stages of recovery following CAR in rats. Since HVR describes the increase in ventilation in response to hypoxia, we hypothesize that damage to cardiorespiratory regulatory centers in the brainstem underlie the loss of HVR observed post resuscitation in nonsurvivors. Wistar rats underwent cardiac arrest (12-min) and resuscitation. At 1day post-resuscitation, rats were perfused transcardially and the brains were harvested and processed for immunohistostaining of caspase-3, a marker of apoptosis. Positive caspase-3 staining was observed in brainstem regions such as the rostral ventral lateral medulla (RVLM); Co-localization of caspase-3 and NeuN was observed in the RVLM as well, suggesting that apoptosis most likely occurs in neurons. Our results showed positive markers for neuronal apoptosis present in pathways of the brainstem involved in respiratory and cerebrovascular regulation, suggesting brain stem damage underlies changes in HVR following CAR.

PS-BPB07-2: AFFERENT RENAL NERVES CONTRIBUTE TO LEFT VENTRICULAR DYSFUNCTION THROUGH HYPOTHALAMIC VASOPRESSIN-MEDIATED CENTRAL SYMPATHOEXCITATION IN HYPERTENSIVE HEART FAILURE MODEL

Objective: Afferent renal nerves may contribute to the development and progression of heart failure, which, however, has not been well investigated. This study aimed to examine whether the afferent renal nerve input contributes to hypertensive cardiac dysfunction. Design and method: Male Dahl salt-sensitive rats were fed 0.3% low-salt diet (LS) or 8% high-salt diet (HS) from 6 weeks of age. HS-fed rats were divided into selective afferent renal denervation (HS-ARDN) or sham operation (HS-Sham) at 9 weeks. The effects of ARDN on central sympathetic regulation were investigated at 12 weeks. In another cohort, its effects on the cardiac phenotypes were evaluated at 16 weeks, with adding total renal denervation (HS-TRDN) group. Results: At 12 weeks, systolic blood pressure (SBP) measured by tail-cuff method was significantly increased in HS-Sham compared to LS, while there was no significant difference between HS-Sham and HS-ARDN. Plasma norepinephrine concentration was increased in HS-Sham compared to LS and was attenuated in HS-ARDN. The c-Fos expression, a neuronal activation marker, in presympathetic neurons within hypothalamic paraventricular nucleus (PVN) and rostral ventral lateral medulla (RVLM) was also increased in HS-Sham and attenuated in HS-ARDN, which was in parallel with the c-Fos expression in vasopressin-expressing neurons within PVN. At 16 weeks, SBP was similarly increased in HS-Sham, HS-ARDN, and HS-TRDN compared to LS. Although the parameters of left ventricular (LV) hypertrophy, such as LV wall thickness by echocardiography and LV weight, were significantly increased in HS-Sham, HS-ARDN, and HS-TRDN compared to LS, there was no significant difference between HS-fed three groups. Echocardiographic LV fractional shortening was significantly decreased in HS-Sham compared to LS, and was attenuated to the same extent in HS-ARDN and HS-TRDN. Histological LV fibrosis determined by Masson trichrome staining and mRNA expression of ANP and CTGF in LV were significantly increased in HS-Sham compared to LS; these changes were similarly ameliorated in both HS-ARDN and HS-TRDN. Conclusion: The input from renal afferent nerves contributes to sympathoexcitation and cardiac dysfunction independently of SBP in Dahl salt-sensitive hypertensive heart failure model. Vasopressin production in PVN may be involved in the renal afferents-mediated sympathetic regulation.

Sex-steroid-dependent plasticity of brain-stem autonomic circuits.

In the central nervous system (CNS), nuclei of the brain stem play a critical role in the integration of peripheral sensory information and the regulation of autonomic output in mammalian physiology. The nucleus tractus solitarius of the brain stem acts as a relay center that receives peripheral sensory input from vagal afferents of the nodose ganglia, integrates information from within the brain stem and higher central centers, and then transmits autonomic efferent output through downstream premotor nuclei, such as the nucleus ambiguus, the dorsal motor nucleus of the vagus, and the rostral ventral lateral medulla. Although there is mounting evidence that sex and sex hormones modulate autonomic physiology at the level of the CNS, the mechanisms and neurocircuitry involved in producing these functional consequences are poorly understood. Of particular interest in this review is the role of estrogen, progesterone, and 5α-reductase-dependent neurosteroid metabolites of progesterone (e.g., allopregnanolone) in the modulation of neurotransmission within brain-stem autonomic neurocircuits. This review will discuss our understanding of the actions and mechanisms of estrogen, progesterone, and neurosteroids at the cellular level of brain-stem nuclei. Understanding the complex interaction between sex hormones and neural signaling plasticity of the autonomic nervous system is essential to elucidating the role of sex in overall physiology and disease.

Sodium pumps, ouabain and aldosterone in the brain: A neuromodulatory pathway underlying salt-sensitive hypertension and heart failure

Accumulating evidence obtained over the last three decades has revealed a neuroendocrine system in the brain that mediates long term increases in blood pressure. The system involves distinct ion transport pathways including the alpha-2 isoform of the Na,K pump and epithelial sodium channels, as well as critical hormone elements such as angiotensin II, aldosterone, mineralocorticoid receptors and endogenous ouabain. Activation of this system either by circulating or central sodium ions and/or angiotensin II leads to a cascading sequence of events that begins in the hypothalamus and involves the participation of several brain nuclei including the subfornical organ, supraoptic and paraventricular nuclei and the rostral ventral medulla. Key events include heightened aldosterone synthesis and mineralocorticoid receptor activation, upregulation of epithelial sodium channels, augmented synthesis and secretion of endogenous ouabain from hypothalamic magnocellular neurons, and sustained increases in sympathetic outflow. The latter step depends upon increased production of angiotensin II and the primary amplification of angiotensin II type I receptor signaling from the paraventricular nucleus to the rostral ventral lateral medulla. The transmission of sympathetic traffic is secondarily amplified in the periphery by increased short- and long-term potentiation in sympathetic ganglia and by sustained actions of endogenous ouabain in the vascular wall that augment expression of sodium calcium exchange, increase cytosolic Ca2+ and heighten myogenic tone and contractility. Upregulation of this multi-amplifier system participates in forms of hypertension where salt, angiotensin and/or aldosterone are elevated and contributes to adverse outcomes in heart failure.

Cardiovascular Effects Of Acute Optogenetic Modulation Of Oxytocin‐PVN Neurons

The actions of oxytocin (OT), a neuropeptide that is largely synthesized in the paraventricular (PVN) and supraoptic (SON) nuclei of the hypothalamus, has been implicated in a variety of homeostatic processes. Recent studies have demonstrated that chronic activation of OT neurons in the PVN prevents hypertension that is a consequence of intermittent hypoxia/hypercapnia. The present study combines genetically‐modified mice with in vivo optogenetics to determine the cardiovascular consequences of selective inhibition or excitation of OT neurons residing in the PVN. Male mice engineered to direct the expression of Cre recombinase to the OT gene had adenoassociated virus synthesizing an inhibitory opsin (Switch), an excitatory opsin (ChR2), or enhanced yellow fluorescent protein (eYFP) bilaterally injected into the PVN. Subsequently, mice were implanted with fiber optics targeting the PVN. One month later, mice were anesthetized with isoflurane and were implanted with a Millar catheter into their left common carotid artery for continuous recording of blood pressure and heart rate. Relative to the eYFP control group, OT‐Cre mice expressing Switch exhibited reduced blood pressure (−24±6 mmHg) and heart rate (−21±9 BPM) in response to optogenetic inhibition when compared with the eYFP group (4±2 mmHg and 5±4 bpm; n=3, p<0.05). In contrast, in the ChR2‐injected group, blue light stimulation (1 min; 30 Hz; 20ms pulse width) significantly increased blood pressure by 15±2 mmHg while decreasing heart rate by −27±10 bpm when compared to controls (0±1mmHg and 9±5 bpm; n=5, p<0.01. Neuroanatomical experiments revealed that neurons in the PVN that synthesize OT send dense axonal projections to the median eminence, the nucleus of the solitary tract, and the rostral ventral lateral medulla. Collectively, the results suggest that OT synthesizing neurons in the PVN mediate cardiovascular function possibly by affecting autonomic outflow and/or the activity of neuroendocrine axes.Support or Funding InformationNIH Funding supported the present studyThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Activation of oxytocin neurons in the paraventricular nucleus drives cardiac sympathetic nerve activation following myocardial infarction in rats

Myocardial infarction (MI) initiates an increase in cardiac sympathetic nerve activity (SNA) that facilitates potentially fatal arrhythmias. The mechanism(s) underpinning sympathetic activation remain unclear. Some neuronal populations within the hypothalamic paraventricular nucleus (PVN) have been implicated in SNA. This study elucidated the role of the PVN in triggering cardiac SNA following MI (left anterior descending coronary artery ligation). By means of c-Fos, oxytocin, and vasopressin immunohistochemistry accompanied by retrograde tracing we showed that MI activates parvocellular oxytocin neurons projecting to the rostral ventral lateral medulla. Central inhibition of oxytocin receptors using atosiban (4.5 µg in 5 µl, i.c.v.), or retosiban (3 mg/kg, i.v.), prevented the MI-induced increase in SNA and reduced the incidence of ventricular arrhythmias and mortality. In conclusion, pre-autonomic oxytocin neurons can drive the increase in cardiac SNA following MI and peripheral administration of an oxytocin receptor blocker could be a plausible therapeutic strategy to improve outcomes for MI patients.

Does acute heat stress differentially-modulate expression of ionotropic neurotransmitter receptors in the RVLM of young and aged F344 rats?

The rostral ventral lateral medulla (RVLM) is a brainstem area that plays a role in regulating numerous physiological systems, especially their responsiveness to acute stress. Aging affects the responsiveness of RVLM neural circuits to acute stress. Based on the relationship between ionotropic neurotransmitter receptors in the RVLM and the physiological functions mediated via activation of these receptors, we hypothesized that in response to acute heat stress the expression of ionotropic neurotransmitter receptors in the RVLM of aged rats would be characterized by upregulation of inhibitory subunits and downregulation of excitatory subunits. The goal of the present study was to determine the effect of acute heating on the gene expression profile of RVLM inhibitory (GABAA and Glycine) and excitatory (NMDA and AMPA) ionotropic neurotransmitter receptor subunits in young and aged F344 rats. RVLM tissue punches from young and aged F344 rats were analyzed using TaqMan qPCR and immunoblotting. When compared to age-matched controls, heat stress increased the gene expression of RVLM inhibitory receptor subunits in aged (Gabra1, Gabra2, Gabra5, Glra1) and young (Gabra1) F344 rats at mRNA level, with little change in the expression of RVLM excitatory receptor subunits. Significant age x heat interaction effects were observed with increased expression of Gabra2 and Gabrb1 inhibitory receptor subunits and decreased expression of Gria1 and Gria2 excitatory receptor subunits in the RVLM of aged F344 rats, with the most marked change observed with the Gabra2 subunit, which was validated by immunoblotting. These findings demonstrate that in response to acute heat stress there is enhanced expression of inhibitory ionotropic receptor subunits in aged compared to young rats, supporting the idea that advanced age may alter RVLM responsivity by affecting the molecular substrate of ionotropic receptors.

Does aging alter the molecular substrate of ionotropic neurotransmitter receptors in the rostral ventral lateral medulla? - A short communication

Aging alters sympathetic nervous system (SNS) regulation, although central mechanisms are not well understood. In young rats the rostral ventral lateral medulla (RVLM) is critically involved in central SNS regulation and RVLM neuronal activity is mediated by a balance of excitatory and inhibitory ionotropic neurotransmitters and receptors, providing the foundation for hypothesizing that with advanced age the molecular substrate of RVLM ionotropic receptors is characterized by upregulated excitatory and downregulated inhibitory receptor subunits. This hypothesis was tested by comparing the relative mRNA expression and protein concentration of RVLM excitatory (NMDA and AMPA) and inhibitory (GABA and glycinergic) ionotropic neurotransmitter receptor subunits in young and aged Fischer (F344) rats. Brains were removed from anesthetized rats and the RVLM-containing area was micropunched and extracted RNA and protein were subsequently used for TaqMan qRT-PCR gene expression and quantitative ELISA analyses. Bilateral chemical inactivation of RVLM neurons and peripheral ganglionic blockade on visceral sympathetic nerve discharge (SND) was determined in additional experiments. The relative gene expression of RVLM NMDA and AMPA glutamate-gated receptor subunits and protein concentration of select receptor subunits did not differ between young and aged rats, and there were no age-related differences in the expression of RVLM ionotropic GABAA and Gly receptors, or of protein concentration of select GABAA subunits. RVLM muscimol microinjections significantly reduced visceral SND by 70±2% in aged F344 rats. Collectively these findings from this short communication support a functional role for the RVLM in regulation of sympathetic nerve outflow in aged rats, but provide no evidence for an ionotropic RVLM receptor-centric framework explaining age-associated changes in SNS regulation.

Repetitive Electroacupuncture Attenuates Cold-Induced Hypertension through Enkephalin in the Rostral Ventral Lateral Medulla.

Acupuncture lowers blood pressure (BP) in hypertension, but mechanisms underlying its action are unclear. To simulate clinical studies, we performed electroacupuncture (EA) in unanesthetized rats with cold-induced hypertension (CIH) induced by six weeks of cold exposure (6 °C). EA (0.1 – 0.4 mA, 2 Hz) was applied at ST36-37 acupoints overlying the deep peroneal nerve for 30 min twice weekly for five weeks while sham-EA was conducted with the same procedures as EA except for no electrical stimulation. Elevated BP was reduced after six sessions of EA treatment and remained low 72 hrs after EA in 18 CIH rats, but not in sham-EA (n = 12) and untreated (n = 6) CIH ones. The mRNA level of preproenkephalin in the rostral ventrolateral medulla (rVLM) 72 hr after EA was increased (n = 9), compared to the sham-EA (n = 6), untreated CIH rats (n = 6) and normotensive control animals (n = 6). Microinjection of ICI 174,864, a δ-opioid receptor antagonist, into the rVLM of EA-treated CIH rats partially reversed EA’s effect on elevated BP (n = 4). Stimulation of rVLM of CIH rats treated with sham-EA using a δ-opioid agonist, DADLE, decreased BP (n = 6). These data suggest that increased enkephalin in the rVLM induced by repetitive EA contributes to BP lowering action of EA.

The role of GαO-mediated signaling in the rostral ventrolateral medulla oblongata in cardiovascular reflexes and control of cardiac ventricular excitability.

The heart is controlled by the sympathetic and parasympathetic limbs of the autonomic nervous system with inhibitory signaling mechanisms recruited in both limbs. The aim of this study was to determine the role of inhibitory heterotrimeric G proteins in the central nervous mechanisms underlying autonomic control of the heart and its potential role in arrhythmogenesis. Mice with conditional deletion of the inhibitory heterotrimeric G protein Gα O in the presympathetic area of the rostral ventral lateral medulla (RVLM) were generated to determine the role of GαO‐mediated signalling in autonomic control and electrophysiological properties of the heart. Gα O deletion within the RVLM was not associated with changes in heart rate (HR) or the arterial blood pressure at rest (home cage, normal behavior). However, exposure to stressful conditions (novel environment, hypoxia, or hypercapnia) in these mice was associated with abnormal HR responses and an increased baroreflex gain when assessed under urethane anesthesia. This was associated with shortening of the ventricular effective refractory period. This phenotype was reversed by systemic beta‐adrenoceptor blockade, suggesting that Gα O depletion in the RVLM increases central sympathetic drive. The data obtained support the hypothesis that Gα O‐mediated signaling within the presympathetic circuits of the RVLM contributes to the autonomic control of the heart. Gα O deficiency in the RVLM has a significant impact on cardiovascular responses to stress, cardiovascular reflexes and electrical properties of the heart.

Age Associated Downregulation of Glutamate/GABA Transporters Expression in Rostral Ventral Lateral Medulla (RVLM)

Physiological function is altered with advancing age, including changes in sympathetic nervous system regulation. Studies focused on Glutamate/GABA ionotropic receptors using pharmacological agonists and antagonists in the rostral ventral lateral medulla (RVLM) indicates changes in medullary mechanisms regulating sympathetic nerve discharge (SND) in aged Fisher 344 rats. Identifying molecular mechanisms involved in this alteration is necessary to understand the regulation of central sympathetic neural circuits. Based on this rationale, we hypothesized that aging alters the basal regulation of RVLM Glutamate/GABA system‐associated genes. To test this hypothesis, the RVLM containing area was micro‐punched from young, middle‐aged and aged Fisher 344 rats. RNA level expression changes in RVLM GABA/Glutamate system‐associated genes were analyzed using RT2 profiler array and Taqman® real‐time PCR. Genes that showed >2fold differences in expression were analyzed at protein level by using immunoblot analysis. Downregulation of RVLM glutamate (Slc1a6) and GABA (Slc6a13) transporter genes was observed in aged compared with young rats at the RNA level, with a similar trend at the protein level. Slc1a6 and Slc6a13 transporters are known to modulate the amount of Glutamate/GABA in synaptic cleft and extra synaptic space. Down regulation of these transporters suggests there may be an altered Glutamergic/GABAergic balance in the aged RVLM.Support or Funding InformationSupported by NIH AG‐041948.

Mu Opioid Receptor Modulation of Dopamine Neurons in the Periaqueductal Gray/Dorsal Raphe: A Role in Regulation of Pain.

The periaqueductal gray (PAG) is a brain region involved in nociception modulation, and an important relay center for the descending nociceptive pathway through the rostral ventral lateral medulla. Given the dense expression of mu opioid receptors and the role of dopamine in pain, the recently characterized dopamine neurons in the ventral PAG (vPAG)/dorsal raphe (DR) region are a potentially critical site for the antinociceptive actions of opioids. The objectives of this study were to (1) evaluate synaptic modulation of the vPAG/DR dopamine neurons by mu opioid receptors and to (2) dissect the anatomy and neurochemistry of these neurons, in order to assess the downstream loci and functions of their activation. Using a mouse line that expresses eGFP under control of the tyrosine hydroxylase (TH) promoter, we found that mu opioid receptor activation led to a decrease in inhibitory inputs onto the vPAG/DR dopamine neurons. Furthermore, combining immunohistochemistry, optogenetics, electrophysiology, and fast-scan cyclic voltammetry in a TH-cre mouse line, we demonstrated that these neurons also express the vesicular glutamate type 2 transporter and co-release dopamine and glutamate in a major downstream projection structure-the bed nucleus of the stria terminalis. Finally, activation of TH-positive neurons in the vPAG/DR using Gq designer receptors exclusively activated by designer drugs displayed a supraspinal, but not spinal, antinociceptive effect. These results indicate that vPAG/DR dopamine neurons likely play a key role in opiate antinociception, potentially via the activation of downstream structures through dopamine and glutamate release.

Sex, the brain and hypertension: brain oestrogen receptors and high blood pressure risk factors.

Hypertension is a major contributor to worldwide morbidity and mortality rates related to cardiovascular disease. There are important sex differences in the onset and rate of hypertension in humans. Compared with age-matched men, premenopausal women are less likely to develop hypertension. However, after age 60, the incidence of hypertension increases in women and even surpasses that seen in older men. It is thought that changes in levels of circulating ovarian hormones as women age may be involved in the increase in hypertension in older women. One of the key mechanisms involved in the development of hypertension in both men and women is an increase in sympathetic nerve activity (SNA). Brain regions important for the regulation of SNA, such as the subfornical organ, the paraventricular nucleus and the rostral ventral lateral medulla, also express specific subtypes of oestrogen receptors. Each of these brain regions has also been implicated in mechanisms underlying risk factors for hypertension such as obesity, stress and inflammation. The present review brings together evidence that links actions of oestrogen at these receptors to modulate some of the common brain mechanisms involved in the ability of hypertensive risk factors to increase SNA and blood pressure. Understanding the mechanisms by which oestrogen acts at key sites in the brain for the regulation of SNA is important for the development of novel, sex-specific therapies for treating hypertension.

Angiotensin-II, the Brain, and Hypertension: An Update.

Despite enormous therapeutic advancements, the incidence of hypertension continues to rise1 and remains a medical burden of global proportions.2 This is likely due, in part, to the complex pathogenesis of hypertension.1 In addition to pathological processes in peripheral cardiovascular organs, the central nervous system (CNS) has emerged as a major culprit. The renin–angiotensin system (RAS) plays a crucial role in cardiovascular control, and both systemic angiotensin-II (Ang-II) and centrally generated Ang-II signal through complex neural networks to contribute to the development of hypertension (Figure). Although the effects of Ang-II within the brain are multifaceted, several key hypertensive players have emerged, including oxidative stress, endoplasmic reticulum stress, inflammation, and transcription factor activation. The purpose of this review is to provide a brief update on recent work related to Ang-II, the brain, and hypertension with a specific focus on Hypertension articles when appropriate. Figure. Simplified schematic illustrating the signaling pathways ( top box), as well as neural networks and sympathetic nervous system influenced physiological outputs ( middle box) involved in the development of hypertension due to peripherally or locally generated angiotensin-II (Ang-II) action in the brain. Common animal models used to evaluate Ang-II, the brain, and hypertension are also highlighted ( bottom box). See text for additional details. AP indicates area postrema; AP-1, activator protein-1; AT1aR, angiotensin type 1a receptor; CVLM, caudal ventral lateral medulla; DOCA, deoxycorticosterone acetate; ER, endoplasmic reticulum; EP1R, prostaglandin E receptor 1; IML, intermediolateral nucleus; MnPO, median preoptic nucleus; NAD(P)H, nicotinamide adenine dinucleotide phosphate; NTS, nucleus tractus solitarii; NF-κB, nuclear factor-κB; OVLT, organum vasculosum lamina terminalis; PP, posterior pituitary; PVN, paraventricular nucleus of the hypothalamus; ROS, reactive oxygen species; RVLM, rostral ventral lateral medulla; SFO, subfornical organ; SHR, spontaneously hypertensive rat; and SON, supraoptic nucleus. Dysregulation of the RAS is common in human hypertension,3 …

Sex and estrogen affect the distribution of urocortin-1 immunoreactivity in brainstem autonomic nuclei of the rat.

Urocortin-1 (UCN-1), a neuropeptide closely related to the hypothalamic hormone corticotropin-releasing factor, has been associated with stress, feeding behaviors, cardiovascular control, and to exhibit functional gender differences. This study was done to investigate whether estrogen (E; 17β-estradiol) treatment (9 weeks) altered UCN-1 immunoreactivity in brainstem autonomic nuclei in female Wistar rats. Experiments were done in age matched adult males (controls), females (intact), and ovariectomized (OVX) only and OVX+E (30pg/ml plasma) treated females. All animals received intracerebroventricular injections of colchicine and were then perfused transcardially with Zamboni's fixative. Coronal brainstem sections (40μm) were cut and processed immunohistochemically for UCN-1. In males, moderate UCN-1 fiber labeling was found in the nucleus of the solitary tract (NTS) and throughout the rostral ventral lateral medulla (RVLM). Additionally, a few UCN-1 immunoreactive neurons were observed in hypoglossal nucleus (XII), facial nucleus (FN) and nucleus ambiguus (Amb). In intact females and OVX+E females, fewer UCN-1 labeled fibers were found within NTS compared to males. In contrast, the RVLM was more densely innervated in the female cases. Furthermore, in both intact and OVX+E females UCN-1 labeled neurons were found not only within Amb, FN and XII, but also within NTS, RVLM and nucleus raphé pallidus (RP). In OVX only animals, moderate to dense UCN-1 fiber labeling was observed in the NTS complex and throughout RVLM compared to males and the other female groups. However, in contrast to all other groups, UCN-1 labeled neurons were found in greater number within Amb, FN, NTS, dorsal motor nucleus of the vagus, XII, RVLM, magnocellular reticular nucleus and RP. These data not only suggest that sex differences exist in the distribution of UCN-1 within brainstem autonomic areas, but that circulating level of E may play an important role with regards to the function of these UCN-1 neurons during stress responses.

Brain inflammation and hypertension: the chicken or the egg?

Inflammation of forebrain and hindbrain nuclei controlling the sympathetic nervous system (SNS) outflow from the brain to the periphery represents an emerging concept of the pathogenesis of neurogenic hypertension. Angiotensin II (Ang-II) and prorenin were shown to increase production of reactive oxygen species and pro-inflammatory cytokines (interleukin-1 beta (IL-1β), interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α)) while simultaneously decreasing production of interleukin-10 (IL-10) in the paraventricular nucleus of the hypothalamus and the rostral ventral lateral medulla. Peripheral chronic inflammation and Ang-II activity seem to share a common central mechanism contributing to an increase in sympathetic neurogenic vasomotor tone and entailing neurogenic hypertension. Both hypertension and obesity facilitate the penetration of peripheral immune cells in the brain parenchyma. We suggest that renin-angiotensin-driven hypertension encompasses feedback and feedforward mechanisms in the development of neurogenic hypertension while low-intensity, chronic peripheral inflammation of any origin may serve as a model of a feedforward mechanism in this condition.

Analysis of RVLM Gene Expression in Young, Middle‐Aged and Aged F344 Rats

Abstract: The rostral ventral lateral medulla (RVLM) plays a critical role in regulation of basal sympathetic nerve discharge (SND). Advanced age is associated with altered regulation of SND, however, the effect of age on central medullary mechanisms regulating SND remains unknown. A series of experiments were initiated to identify cellular and molecular targets that may mediate age‐related changes in medullary regulation of SND. We tested the hypothesis that aging shifts the balance of RVLM excitatory and inhibitory influences towards excitation. Gene expression analysis was performed using Agilent Whole Rat Genome Microarray 4×44K followed by validation of microarray with TaqMan real‐time RT‐PCR and immunoblotting for specific RVLM proteins. Total RNA and protein were extracted from RVLM micropunches from young, middle‐aged and aged F344 rats. Microarray analysis revealed up‐regulation in aged compared to young and middle‐aged rats of several genes related to immune function and neurotransmitter receptors. Specifically, the chemokines CCL3, CCL4 and CCL5 were up‐regulated 3 to 4 fold in aged rats compared to young and middle‐aged rats. The results of previous studies suggest that increased chemokines in the brain can modulate NMDA receptor responses. The current preliminary results suggest that increased chemokine gene expression in the RVLM of aged rats may contribute to age‐related changes in SND regulation. Supported by NIH A6‐041948.

Medullary regulation of visceral sympathetic nerve discharge at peak hyperthermia in aged F344 rats.

Heat stress provides a potent stimulus for the activation of visceral sympathetic nerve discharge (SND) in young but not aged rats. Central mechanisms mediating attenuated SND responses to heating in aged rats have not been investigated. Because the GABAergic system in the rostral ventral lateral medulla (RVLM) is tonically inhibitory to SND, it is plausible to hypothesize that the withdrawal of RVLM GABA tone as a strategy to activate renal SND to heating is not engaged to the same degree in aged compared with young rats. The effect of bilateral RVLM disinhibition produced by bicuculline (BIC, GABA(A) receptor antagonist, 100 pmol) microinjections on renal SND in anesthetized young (3-6 months old) and aged (22-24 months old) Fischer 344 rats was determined after core body temperature (Tc) had been increased to 41.5 °C. Renal SND at 41.5 °C was significantly increased from control levels in young but not aged rats, whereas RVLM BIC microinjections at 41.5 °C produced marked renal sympathoexcitation in both groups. RVLM BIC microinjections at 38 °C in young and aged rats increased renal SND to similar levels as produced by RVLM BIC microinjections at 41.5 °C. The enhanced heating-induced renal sympathoactivation in young compared with aged rats; coupled with marked RVLM BIC-induced SND excitation under hyperthermic and normothermic conditions in both young and aged rats, suggests age-dependent changes in the withdrawal of RVLM GABA tone as a strategy to activate renal SND in response to acute heating.

Leptin into the rostral ventral lateral medulla (RVLM) augments renal sympathetic nerve activity and blood pressure.

Leptin is a hormone released from adipose tissue. While this hormone normally acts to reduce feeding behavior and increase energy expenditure, in obesity, resistance to these effects occurs even though the hormone is released in large amounts. Although leptin no longer works to suppress feeding in the obese, leptin retains its potent effects on other autonomic functions such as blood pressure regulation. Leptin has been associated with hypertension and increased sympathetic autonomic activity. Therefore, leptin is emerging as a major contributor to the hypertensive state observed in obesity. Sympathetic control of blood pressure is maintained principally by autonomic reflex control circuits in the caudal brainstem. The rostral ventral-lateral medulla (RVLM) is the primary regulator of the sympathetic nervous system, sending excitatory fibers to sympathetic preganglionic neurons to regulate sympathetic control over resistance vessels and blood pressure. Previous studies from our laboratory have shown that neurons in the ventral lateral medulla express leptin receptors (ObRb). Our present study using pseudo-rabies multi-synaptic retrograde tract tracing and immunohistochemical methods revealed that neurons within the RVLM that send sympathetic projections to the kidney express leptin receptors. Acute microinjection of leptin (1 and 3 μg; 40 nL) into the RVLM evoked a significant increase in Mean Arterial Pressure (MAP) and renal sympathetic nerve activity (RSNA). When the 3 μg dose of leptin was preceded with a leptin antagonist, (SLAN-4; 1 ng), it attenuated the cardiovascular response of leptin. Taken together, these data suggest that leptin's actions within the RVLM may influence blood pressure and renal sympathetic nerve activity.