Chemoreflex Pathway Research Articles

Chronic, episodic nicotine exposure enhances the hypoxic ventilatory response in adult female rats

It is well known that cigarette smoking causes lung disease and damages the airways. However, we know little about the direct role of nicotine, the primary addictive component of tobacco, in the development of smoking- related diseases. Along with promoting cancer cell proliferation and airway inflammation, nicotine can modulate central respiratory control, increasing the rate and depth of breathing through mechanisms that are not well understood. In addition, cigarette smoking is associated with obstructive sleep apnea, a sleep-related breathing disorder that is marked by reduced brainstem respiratory motor output. Despite these examples indicating that chronic nicotine exposure in adulthood may alter central respiratory control, there is no published research that directly tests this hypothesis. We exposed male and female adult rats to nicotine for four weeks through their drinking water and then used whole body plethysmography to test the hypothesis that chronic, episodic nicotine exposure alters restful breathing and the hypoxic ventilatory response (HVR), a critical respiratory chemoreflex. We measured tidal volume (TV) and frequency of breathing at rest, and during a 5- minute episode of hypoxia (10% oxygen/balance nitrogen). Total ventilation (VE/VCO2) was calculated as TV X Frequency and normalized to carbon dioxide production, to account for changes in metabolism, and body weight. For both males and females, comparisons were made between groups and across trials using a 2-way ANOVA and Tukey’s post-hoc test. In male rats, nicotine exposure significantly decreased overall ventilation at rest in one of three trials (Trial 1 VE/VCO2: Nicotine exposed, 30.7±7.5 mL/min/100g, Control, 44.3±6.4 mL/min/100g, p=0.0183), but had no effect on breathing during hypoxia. In female rats, nicotine exposure had no effect on breathing at rest, however, overall ventilation during hypoxia was enhanced in nicotine exposed rats compared to controls in two of three trials (Trial 1 VE/VCO2: Nicotine exposed, 106.8±20.6 mL/min/100g, Control, 79.8±5.1 mL/min/100g, p=0.049; Trial 3 VE/VCO2: Nicotine exposed, 101.6±9.9 mL/min/100g, Control, 68.1±10.9 mL/min/100g, p=0.007). Overall, these results suggest that chronic nicotine exposure triggers plasticity in the chemoreflex pathway mediating the HVR in a sex specific manner and further studies are warranted to understand the implications of this finding. 1K99HL164973-02. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Carotid body contribution to the physio-pathological consequences of intermittent hypoxia: role of nitro-oxidative stress and inflammation.

Obstructive sleep apnoea (OSA), characterized by chronic intermittent hypoxia (CIH), is considered to be an independent risk for hypertension. The pathological cardiorespiratory consequences of OSA have been attributed to systemic oxidative stress, inflammation and sympathetic overflow induced by CIH, but an emerging body of evidence indicates that a nitro-oxidative and pro-inflammatory milieu within the carotid body (CB) is involved in the potentiation of CB chemosensory responses to hypoxia, which contribute to enhance the sympathetic activity. Accordingly, autonomic and cardiovascular alterations induced by CIH are critically dependent on an abnormally heightened CB chemosensory input to the nucleus of tractus solitarius (NTS), where second-order neurons project onto the rostral ventrolateral medulla (RVLM), activating pre-sympathetic neurons that control pre-ganglionic sympathetic neurons. CIH produces oxidative stress and neuroinflammation in the NTS and RVLM, which may contribute to the long-term irreversibility of the CIH-induced alterations. This brief review is mainly focused on the contribution of nitro-oxidative stress and pro-inflammatory molecules on the hyperactivation of the hypoxic chemoreflex pathway including the CB and the brainstem centres, and whether the persistence of autonomic and cardiorespiratory alterations may depend on the glial-related neuroinflammation induced by the enhanced CB chemosensory afferent input.

Potential Role of the Retrotrapezoid Nucleus in Mediating Cardio-Respiratory Dysfunction in Heart Failure With Preserved Ejection Fraction.

A strong association between chemoreflex hypersensitivity, disordered breathing, and elevated sympathetic activity has been shown in experimental and human heart failure (HF). The contribution of chemoreflex hypersensitivity in HF pathophysiology is incompletely understood. There is ample evidence that increased peripheral chemoreflex drive in HF with reduced ejection fraction (HFrEF; EF<40%) leads to pathophysiological changes in autonomic and cardio-respiratory control, but less is known about the neural mechanisms mediating cardio-respiratory disturbances in HF with preserved EF (HFpEF; EF>50%). Importantly, it has been shown that activation of the central chemoreflex worsens autonomic dysfunction in experimental HFpEF, an effect mediated in part by the activation of C1 catecholaminergic neurons neighboring the retrotrapezoid nucleus (RTN), an important region for central chemoreflex control of respiratory and autonomic function. Accordingly, the main purpose of this brief review is to discuss the possible role played by activation of central chemoreflex pathways on autonomic function and its potential role in precipitating disordered breathing in HFpEF. Improving understanding of the contribution of the central chemoreflex to the pathophysiology of HFpEF may help in development of novel interventions intended to improve cardio-respiratory outcomes in HFpEF.

Histone Deacetylase 5 Is an Early Epigenetic Regulator of Intermittent Hypoxia Induced Sympathetic Nerve Activation and Blood Pressure.

Intermittent hypoxia (IH) is a hallmark manifestation of obstructive sleep apnea (OSA). Long term IH (LT-IH) triggers epigenetic reprogramming of the redox state involving DNA hypermethylation in the carotid body chemo reflex pathway resulting in persistent sympathetic activation and hypertension. Present study examined whether IH also activates epigenetic mechanism(s) other than DNA methylation. Histone modification by lysine acetylation is another major epigenetic mechanism associated with gene regulation. Equilibrium between the activities of histone acetyltransferases (HATs) and histone deacetylases (HDACs) determine the level of lysine acetylation. Here we report that exposure of rat pheochromocytoma (PC)-12 cells to IH in vitro exhibited reduced HDAC enzyme activity due to proteasomal degradation of HDAC3 and HDAC5 proteins. Mechanistic investigations showed that IH-evoked decrease in HDAC activity increases lysine acetylation of α subunit of hypoxia inducible factor (HIF)-1α as well as Histone (H3) protein resulting in increased HIF-1 transcriptional activity. Trichostatin A (TSA), an inhibitor of HDACs, mimicked the effects of IH. Studies on rats treated with 10 days of IH or TSA showed reduced HDAC activity, HDAC5 protein, and increased HIF-1 dependent NADPH oxidase (NOX)-4 transcription in adrenal medullae (AM) resulting in elevated plasma catecholamines and blood pressure. Likewise, heme oxygenase (HO)-2 null mice, which exhibit IH because of high incidence of spontaneous apneas (apnea index 72 ± 1.2 apnea/h), also showed decreased HDAC activity and HDAC5 protein in the AM along with elevated circulating norepinephrine levels. These findings demonstrate that lysine acetylation of histone and non-histone proteins is an early epigenetic mechanism associated with sympathetic nerve activation and hypertension in rodent models of IH.

GABAergic modulation of NTS‐VLM neurons from Wistar Hannover and Sprague Dawley Rats is reduced by sustained hypoxia

Short-term sustained Hypoxia (SH) induces a different pattern of cardiovascular and respiratory responses in rats from Wistar Ribeirão Preto (WRP), Wistar Hannover (WH) and Sprague Dawley (SD) strains. SH induces an increase in the respiratory parameters of rats from these three strains and mean arterial pressure in WRP and SD but not in WH rats. In WRP rats submitted to SH, we documented an increase in the excitability of NTS neurons sending projections to the ventral medulla (NTS-VLM), which is part of the chemoreflex pathways. However, the role of GABAergic modulation on the neuronal excitability of NTS neurons from different strains of rats submitted to SH was not yet explored. In the present study, we evaluated the inhibitory modulation (GABAergic synaptic activity) onto NTS-VLM neurons from WH and SD rats previously submitted to SH. The experimental protocols were approved by the Institutional Ethics Committee (#136/2018). NTS-VLM neurons were labeled with retrograde tracer (Greenbeads) previously microinjected into VLM of WH and SD rats (25-30 days old). Ten days later, these rats were submitted to SH protocol (24hs, FiO2 0.1). Then NTS slices were obtained, and the evoked and spontaneous inhibitory postsynaptic currents (eIPSCs and sIPSCs, respectively) were recorded in labeled NTS-VLM neurons using whole-cell patch-clamp. SH decreased the amplitude of eIPSCs in NTS-VLM neurons from WH [-172 ± 36 (n=7) vs -58.1 ± 9.3 pA (n=7), p=0.0100] and also from SD rats [-231.8 ± 33.9 (n=9) vs -48.9 ± 10.5 pA (n=6), p=0.0009]. SH decreased the decay-time in NTS-VLM neurons from SD rats (25.4 ± 5.1 vs 9.2 ± 1.2 ms, p= 0.0247) but produced no change in the rise-time of the events in both rat strains. SH decreased the frequency of sIPSCs in NTS-VLM neurons from WH [1.7 ± 0.4 (n=8) vs 0.8 ± 0.2 Hz, (n=9), p=0.0290] and from SD rats [2.3 ± 0.17 (n=5) vs 0.51 ± 0.13 ms (n=6), p<0.0001]. No significant changes were observed in amplitude and half-width of sIPSCs of NTS-VLM neurons from both rat strains. The data show that SH decreases the evoked and spontaneous GABAergic neurotransmission on NTS-VLM neurons from WH and SD rats, probably by affecting the presynaptic terminal. The observed change in GABAergic modulation onto the NTS neuronal excitability may explain the observed alterations in the respiratory pattern in WH and SD rats in response to SH.

Astrocytic modulation of IKA, NMDA and AMPA currents in NTS neurons of rats is affected by sustained hypoxia

Short term sustained hypoxia (SH, FiO2 0.1, 24 h) produces cardiovascular and respiratory changes mainly due to peripheral chemoreflex activation. SH alters the electrophysiological properties of neurons in the NTS, where the first synapses of chemoreflex afferents are located. In the present study we evaluated alterations produced by SH on neuronal excitability and excitatory synaptic transmission in the NTS neurons integral to chemoreflex pathways. After SH protocol, brainstem slices were obtained for electrophysiological recordings using whole cell patch–clamp and fluoracetate (FAC) was used to produce metabolic inhibition of astrocytes. SH increased the action potential discharge in response to positive injected current into NTS neurons (p=0.0048). The presence of A‐type potassium current (IKA) was observed and its peak amplitude was larger in neurons from control [Vm = 10 mV, 1428 ± 600 pA (n=10)] than in SH rats [Vm = 10 mV, 334 ± 76 pA (n=10)]. Inhibition of astrocytes with FAC decreased the IKA amplitude in neurons from control rats [1227 ± 129 pA vs 605 ± 210 pA (n=7)], but not in SH rats [475 ± 110 pA vs 427 ± 114 pA (n=6)]. With respect to excitatory neurotransmission, SH increased AMPA/kainate [−183 ± 122 pA (n=10) vs −353 ± 101 pA (n=10)] and NMDA currents amplitude [61 ± 10 pA (n=7) vs 102 ± 37 pA (n=10)]. Astrocytic modulation on AMPA [aCSF: −353 ± 101 pA vs aCSF + FAC: −369 ± 76 pA, (n=10)] and NMDA currents [aCSF: 102 ± 37 pA vs aCSF + FAC: 108 ± 32 pA, (n=10)] was blunted by SH. In addition, SH increased AMPA current density [control: −6 ± 3.5 pA/pF (n=6) vs SH: −20 ± 12 pA/pF (n=7)]. The data shows that astrocyte modulation of synaptic transmission in NTS neurons integral to the chemoreflex pathways is reduced by SH, which may contribute to the observed increase in the chemoreflex responsiveness on this experimental model.Support or Funding InformationFAPESP, CAPES and CNPq

Ventilatory acclimatization to hypoxia: Time to express a critical central message.

Ventilatory acclimatization to hypoxia: Time to express a critical central message.

Contribution of Oxidative Stress and Inflammation to the Neurogenic Hypertension Induced by Intermittent Hypoxia.

Chronic intermittent hypoxia (CIH), the hallmark of obstructive sleep apnea, is the main risk factor to develop systemic hypertension. Oxidative stress, inflammation, and sympathetic overflow have been proposed as possible mechanisms underlying the CIH-induced hypertension. CIH potentiates the carotid body (CB) chemosensory discharge leading to sympathetic overflow, autonomic dysfunction, and hypertension. Oxidative stress and pro-inflammatory molecules are involved in neurogenic models of hypertension, acting on brainstem and hypothalamic nuclei related to the cardiorespiratory control, such as the nucleus of the solitary tract, which is the primary site for the afferent inputs from the CB. Oxidative stress and pro-inflammatory molecules contribute to the activation of the CB chemoreflex pathway in CIH-induced hypertension. In this brief review, we will discuss new evidence for a critical role of oxidative stress and neuro-inflammation in development of the CIH-induced hypertension through activation of the CB chemoreflex pathway.

Impaired Chemoreflex Response to Acute Hypoxia Correlates with Decreased Activation of the Medial Nucleus Tractus Solitarii in the STZ‐Induced Rat Model of Alzheimer's Disease

BackgroundAlzheimer's disease (AD) is the most common cause of dementia and clinically diagnosed largely based on cognitive decline. Most AD patients also present with disorders of respiration, swallowing, and autonomic function. Current studies suggest that pathological changes in the brainstem manifest simultaneously or even before those in forebrain regions, and may account for non‐cognitive symptoms. Identifying the pathologic changes in brainstem centers may elucidate the mechanisms behind cardiorespiratory dysfunction in AD. This study concentrated on the respiratory deficits. Using the streptozotocin (STZ)‐induced model of AD, we determined cell activation in response to acute hypoxia in the nucleus tractus solitarii (nTS), a brainstem area that is integral for chemoreflex function.MethodsSporadic AD was induced in Sprague Dawley rats (280 ± 17g, n = 6 rats/group) by intracerebroventricular injections of streptozotocin (1.5 mg/kg STZ in citrate buffer; day 1 and 3). Vehicle injections served as control (CTL). Peripheral chemoreflex function was tested with two hours of acute hypoxia (10% O2) in a plethysmography chamber. Immediately following the hypoxic period, the brains were fixed with 4% paraformaldehyde. Four representative areas of the nTS were immunohistochemically analyzed for cell activation (c‐Fos).ResultsThe chemoreflex‐mediated increase of minute ventilation in response to acute hypoxia was significantly blunted by ~20% in STZ rats when compared to CTL (p = 0.03). The diminished response continued over the entire 2 hours of hypoxia and was mainly due to alterations in respiratory rate. Immunohistochemical analysis revealed a ~25% decrease of hypoxia‐mediated cell activation in the medial nTS of STZ rats (−14.04 mm from bregma, p = 0.02). This decline in cell activation was not due to a general decrease in cell density as shown by DAPI staining. The level of activation in commissural and rostral nTS sections was not different from CTL.ConclusionWe had previously found respiratory dysregulation in the STZ model of sporadic AD. This follow‐up study analyzed respiratory parameters over a two‐hour hypoxic period and the underlying pattern of cell activation in the nTS. Our data document a reduced activation of the medial nTS in the chemoreflex pathway of AD rats. This finding may help explain the respiratory symptoms observed in AD patients.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Effect of Chronic Intermittent Hypoxia on Angiotensin II Receptors in the Central Nervous System

ABSTRACTChronic intermittent hypoxia (CIH) increases basal sympathetic nervous system activity, augments chemoreflex-induced sympathoexcitation, and raises blood pressure. All effects are attenuated by systemic or intracerebroventricular administration of angiotensin II type 1 receptor (AT1R) antagonists. This study aimed to quantify the effects of CIH on AT1R- and AT2R-like immunoreactivity in the rostroventrolateral medulla (RVLM) and paraventricular nucleus of the hypothalamus (PVN), central regions that are important components of the extended chemoreflex pathway. Eighteen Sprague-Dawley rats were exposed to intermittent hypoxia (FIO2 = 0.10, 1 min at 4-min intervals) for 10 hr/day for 1, 5, 10, or 21 days. After exposure, rats were deeply anesthetized and transcardially perfused with phosphate buffered saline (PBS) followed by 4% paraformaldehyde in PBS. Brains were removed and sectioned coronally into 50 µm slices. Immunohistochemistry was used to quantify AT1R and AT2R in the RVLM and the PVN. In the RVLM, CIH significantly increased the AT1R-like immunoreactivity, but did not alter AT2R immunoreactivity, thereby augmenting the AT1R:AT2R ratio in this nucleus. In the PVN, CIH had no effect on immunoreactivity of either receptor subtype. The current findings provide mechanistic insight into increased basal sympathetic outflow, enhanced chemoreflex sensitivity, and blood pressure elevation observed in rodents exposed to CIH.

Cardiac diastolic and autonomic dysfunction are aggravated by central chemoreflex activation in heart failure with preserved ejection fraction rats.

Heart failure with preserved ejection fraction (HFpEF) is associated with disordered breathing patterns, and sympatho-vagal imbalance. Although it is well accepted that altered peripheral chemoreflex control plays a role in the progression of heart failure with reduced ejection fraction (HFrEF), the pathophysiological mechanisms underlying deterioration of cardiac function in HFpEF are poorly understood. We found that central chemoreflex is enhanced in HFpEF and neuronal activation is increased in pre-sympathetic regions of the brainstem. Our data showed that activation of the central chemoreflex pathway in HFpEF exacerbates diastolic dysfunction, worsens sympatho-vagal imbalance and markedly increases the incidence of cardiac arrhythmias in rats with HFpEF. Heart failure (HF) patients with preserved ejection fraction (HFpEF) display irregular breathing, sympatho-vagal imbalance, arrhythmias and diastolic dysfunction. It has been shown that tonic activation of the central and peripheral chemoreflex pathway plays a pivotal role in the pathophysiology of HF with reduced ejection fraction. In contrast, no studies to date have addressed chemoreflex function or its effect on cardiac function in HFpEF. Therefore, we tested whether peripheral and central chemoreflexes are hyperactive in HFpEF and if chemoreflex activation exacerbates cardiac dysfunction and autonomic imbalance. Sprague-Dawley rats (n=32) were subjected to sham or volume overload to induce HFpEF. Resting breathing variability, chemoreflex gain, cardiac function and sympatho-vagal balance, and arrhythmia incidence were studied. HFpEF rats displayed [mean±SD; chronic heart failure (CHF) vs. Sham, respectively] a marked increase in the incidence of apnoeas/hypopnoeas (20.2±4.0 vs. 9.7±2.6eventsh-1 ), autonomic imbalance [0.6±0.2 vs. 0.2±0.1 low/high frequency heart rate variability (LF/HFHRV )] and cardiac arrhythmias (196.0±239.9 vs. 19.8±21.7eventsh-1 ). Furthermore, HFpEF rats showed increase central chemoreflex sensitivity but not peripheral chemosensitivity. Accordingly, hypercapnic stimulation in HFpEF rats exacerbated increases in sympathetic outflow to the heart (229.6±43.2% vs. 296.0±43.9% LF/HFHRV , normoxia vs. hypercapnia, respectively), incidence of cardiac arrhythmias (196.0±239.9 vs. 576.7±472.9eventsh-1 ) and diastolic dysfunction (0.008±0.004 vs. 0.027±0.027mmHgμl-1 ). Importantly, the cardiovascular consequences of central chemoreflex activation were related to sympathoexcitation since these effects were abolished by propranolol. The present results show that the central chemoreflex is enhanced in HFpEF and that acute activation of central chemoreceptors leads to increases of cardiac sympathetic outflow, cardiac arrhythmogenesis and impairment in cardiac function in rats with HFpEF.

English

Anxiety is characterized most commonly as a diffuse, unpleasant, vague sense of apprehension, often accompanied by autonomic symptoms such as headache, perspiration, palpitations, tightness in the chest, mild stomach discomfort and restlessness, indicated by an inability to sit or stand still for long. Anxiety disorder is one of important cause of development of hypertension, although it is multifactorial. It increases sympathetic nervous system which causes hypertension due to alterations in baro reflex and chemo reflex pathways at both peripheral and central level. It also increases norepinephrine which acts on β1 receptor (in brain, myocardium and kidney and increases cardiac force, rate of contraction, renin release) and β2 receptor (in pulmonary system and blood vessels and decreases resistance of pulmonary airway and blood vessels)-leading

Regulation of hypoxia-inducible factor-α isoforms and redox state by carotid body neural activity in rats.

Previous studies reported that chronic intermittent hypoxia (CIH) results in an imbalanced expression of hypoxia-inducible factor-α (HIF-α) isoforms and oxidative stress in rodents, which may be due either to the direct effect of CIH or indirectly via hitherto uncharacterized mechanism(s). As neural activity is a potent regulator of gene transcription, we hypothesized that carotid body (CB) neural activity contributes to CIH-induced HIF-α isoform expression and oxidative stress in the chemoreflex pathway. Experiments were performed on adult rats exposed to CIH for 10 days. Rats exposed to CIH exhibited: increased HIF-1α and decreased HIF-2α expression; increased NADPH oxidase 2 and decreased superoxide dismutase 2 expression; and oxidative stress in the nucleus tractus solitarius and rostral ventrolateral medulla as well as in the adrenal medulla (AM), a major end organ of the sympathetic nervous system. Selective ablation of the CB abolished these effects. In the AM, sympathetic activation by the CB chemoreflex mediates CIH-induced HIF-α isoform imbalance via muscarinic acetylcholine receptor-mediated Ca(2+) influx, and the resultant activation of mammalian target of rapamycin pathway and calpain proteases. Rats exposed to CIH presented with hypertension, elevated sympathetic activity and increased circulating catecholamines. Selective ablation of either the CB (afferent pathway) or sympathetic innervation to the AM (efferent pathway) abolished these effects. These observations uncover CB neural activity-dependent regulation of HIF-α isoforms and the redox state by CIH in the central and peripheral nervous systems associated with the chemoreflex.

Glial cells modulate the synaptic transmission of NTS neurons sending projections to ventral medulla of Wistar rats.

There is evidence that sympathoexcitatory and respiratory responses to chemoreflex activation involve ventrolateral medulla-projecting nucleus tractus solitarius (NTS) neurons (NTS-VLM neurons) and also that ATP modulates this neurotransmission. Here, we evaluated whether or not astrocytes is the source of endogenous ATP modulating the synaptic transmission in NTS-VLM neurons. Synaptic activities of putative astrocytes or NTS-VLM neurons were recorded using whole cell patch clamp. Tractus solitarius (TS) stimulation induced TS-evoked excitatory postsynaptic currents (TS-eEPSCs) in NTS-VLM neurons as well in NTS putative astrocytes, which were also identified by previous labeling. Fluoracetate (FAC), an inhibitor of glial metabolism, reduced TS-eEPSCs amplitude (−85.6 ± 16 vs. −39 ± 7.1 pA, n = 12) and sEPSCs frequency (2.8 ± 0.5 vs. 1.8 ± 0.46 Hz, n = 10) in recorded NTS-VLM neurons, indicating a gliomodulation of glutamatergic currents. To verify the involvement of endogenous ATP a purinergic antagonist was used, which reduced the TS-eEPSCs amplitude (−207 ± 50 vs. −149 ± 50 pA, n = 6), the sEPSCs frequency (1.19 ± 0.2 vs. 0.62 ± 0.11 Hz, n = 6), and increased the paired-pulse ratio (PPR) values (∼20%) in NTS-VLM neurons. Simultaneous perfusion of Pyridoxalphosphate-6-azophenyl-2′,5′-disulfonic acid (iso-PPADS) and FAC produced reduction in TS-eEPSCs similar to that observed with iso-PPADS or FAC alone, indicating that glial cells are the source of ATP released after TS stimulation. Extracellular ATP measurement showed that FAC reduced evoked and spontaneous ATP release. All together these data show that putative astrocytes are the source of endogenous ATP, which via activation of presynaptic P2X receptors, facilitates the evoked glutamate release and increases the synaptic transmission efficacy in the NTS-VLM neurons probably involved with the peripheral chemoreflex pathways.

Immunohistochemistry confirms the functional evidence that the cardiopulmonary chemoreflex (CCR) pathways in the caudal nucleus of the solitary tract (cNTS) are directly inhibited by A1 adenosine receptors and indirectly inhibited by A2a receptors via GABA release

The CCR may elicit severe hypotension which can release adenosine within the cNTS. Adenosine may serve there as a central negative feedback regulator for this reflex. We have shown that CCR‐evoked sympathoinhibition was markedly attenuated after stimulation of both A1 and A2a adenosine receptors in the cNTS (Am J Physiol 303: R539–R550, 2012; FASEB J 24: 624.11, 2010). Furthermore, A2a mediated attenuation of the CCR was virtually abolished by GABAergic blockade (FASEB J 25: 844.3, 2011) and cNTS GABAergic neurons express A2a receptors (FASEB J 26: 1091.28, 2012). We hypothesized that A1 but not A2a receptors are localized on CCR vagal afferents whereas A1 receptors may be located on cNTS non‐GABAergic neurons. Therefore, we used double immunofluorescent staining for: 1) A1 or A2a receptors and cholera toxin B microinjected into the vagus nerve or 2) A1 receptors and GAD67, the GABA synthetic enzyme. We found that GABAergic fibers project to cNTS neurons expressing A1 receptors; no colocalization of A2a or A1 receptors was seen on vagal afferents. These data support our functional findings that A1 receptors directly inhibit CCR whereas A2a adenosine receptors inhibit CCR via GABAergic mechanism. HL‐67814

Role of leptin and insulin on renal sympathetic nerve activity in high fat fed rabbits

We determined the rapidity of changes in blood pressure (BP), heart rate (HR) and renal sympathetic nerve activity (RSNA) in response to a high fat diet using conscious rabbits and whether this involved insulin and leptin actions in the CNS. New Zealand White rabbits were implanted with renal electrode to measure RSNA. Rabbits were then placed on a normal or 13.5% high fat diet (HFD) for 1 or 3 weeks. Reflexes and stress responses were examined weekly. Rabbits were infused with peptide antagonists of Leptin (100μg) and Insulin (0.5U) intracerebroventricularly at week 1 and 3. After 1 week on the HFD, rabbits demonstrated a 4%, 8% and 30% greater BP, HR and RSNA, respectively (P&lt;0.05). By the end of 3 weeks of HFD, BP, HR and RSNA were elevated by 10%, 5% and 82% (n=9). The insulin antagonist was most effective at 1 week in lowering BP, HR and RSNA while the leptin antagonist was most effective at 3 week of a HFD. The antagonists were minimally effective in control rabbits at these times. Sympathetic activation occurs rapidly within several days of a high fat diet initially associated with forebrain insulin signalling and later as fat is deposited, with forebrain leptin signalling. These changes appear to be associated with pathways mediating the sympathetic responses to acute emotional stress that are independent of baroreflex or chemoreflex pathways.

Synaptic profile of nucleus tractus solitarius neurons involved with the peripheral chemoreflex pathways

The glomus cells in the carotid bodies (CB) detect alterations in pH and pCO₂ and low pO₂ level in arterial blood. The carotid sinus nerve conveys the information related to the oxygen level to 2nd-order neurons in the nucleus tractus solitarius (NTS) via tractus solitarius (TS), which is part of the chemoreflex pathways. It has been demonstrated that in 2nd-order NTS neurons receiving inputs from the aortic depressor nerve (ADN), the TS stimulation presents high temporal fidelity. However, the temporal properties of synaptic activity in NTS neurons receiving inputs from CB were not yet fully investigated. Herein using patch-clamp recordings in NTS brainstem slices, we studied TS-evoked excitatory postsynaptic currents (TS-eEPSCs) on morphologically identified 2nd-order NTS neurons that receive afferent inputs from the CB and compared with 2nd-order ADN-NTS neurons recorded in the same experimental conditions. The amplitudes of TS-eEPSCs were similar in both groups, but the latencies and standard deviation (SD) of latency were significantly higher in the CB-NTS neurons (latency: 4±0.2 ms, SD: 0.49±0.03 ms) than in ADN-NTS neurons (latency: 3.3±0.3 ms, SD: 0.19±0.02 ms; P=0.049 for latency and P<0.001 for SD of latency). In a series of double-labeling experiments, we confirmed that some CB-NTS 2nd-order neurons send direct projections to the rostral ventrolateral medulla (RVLM). We conclude that: (a) CB-NTS 2nd-order neurons present temporally distinct postsynaptic currents when compared with ADN-NTS 2nd-order neurons; (b) low SD of latency of TS-eEPSCs is not necessarily a characteristic of all 2nd-order neurons in the NTS; and (c) the presence of direct connections between these 2nd-order neurons in the NTS and RVLM is indicative that these synaptic properties of CB-NTS neurons are relevant for the processing of respiratory and autonomic responses to chemoreflex activation.

Chemoreflex Activity Increases Prostaglandin Endoperoxide Synthase mRNA Expression in the Late-Gestation Fetal Sheep Brain

Fetal sheep defend blood pressure, blood volume, and blood gases using baro- and chemoreflexes that influence autonomic and neuroendocrine responses. The local generation of prostanoids within the fetal brain is also an important component in activating hormone responses to these stimuli, but the relationship between the reflexes and prostanoid biosynthesis is unclear. The present study was performed to test the hypothesis that the abundances of prostaglandin biosynthetic enzymes in the fetal brain are dependent upon the activity of the baro- and chemoreflex pathways. We subjected chronically catheterized fetal sheep in late gestation to a 10-minute period of brachiocephalic occlusion (BCO), a stimulus that provokes brisk cardiovascular and neuroendocrine responses. We compared the central nervous system abundance of prostaglandin endoperoxide synthases 1 and 2 (PGHS-1 and PGHS-2) after BCO to (1) fetal sheep that had been subjected to BCO after chronic sinoaortic denervation plus bilateral vagotomy and (2) fetal sheep in which the N-methyl d-aspartate (NMDA) receptor antagonist, ketamine, had been administered prior to BCO. Abundances of messenger RNA (mRNA) for PGHS-1 and of mRNA and protein for PGHS-2 in fetal hippocampus were reduced significantly by either prior denervation or ketamine administration. Prostaglandin endoperoxide synthases 1 and 2 mRNA in pituitary were decreased and increased, respectively, by ketamine pretreatment. The results of this study are consistent with the conclusion that the expression of PGHS-1 and -2 in fetal hippocampus and pituitary are influenced by the baro- and/or chemoreflex pathways within the fetal brain in late gestation.

Synaptic properties of peripheral chemoreflex second-order neurons in the NTS of rats

Background/aims: The glomus cells in the carotid bodies (CB) detect low pO2 level in arterial blood and the tractus solitarius (TS) conveys this information to 2nd-order neurons in the nucleus tractus solitarius (NTS), which is part of the chemoreflex pathways. It has been demonstrated that 2nd-order NTS neurons receiving inputs from the aortic depressor nerve (ADN), related to the baroreflex, present low temporal variability in response to TS stimulation. However, the temporal variability of synaptic activity in CB–NTS neurons is not yet completely understood.

Chronic intermittent hypoxia affects integration of sensory input by neurons in the nucleus tractus solitarii

The autonomic nervous and respiratory systems, as well as their coupling, adapt over a wide range of conditions. Chronic intermittent hypoxia (CIH) is a model for recurrent apneas and induces alterations in breathing and increases in sympathetic nerve activity which may ultimately result in hypertension if left untreated. These alterations are believed to be due to increases in the carotid body chemoreflex pathway. Here we present evidence that the nucleus tractus solitarii (nTS), the central brainstem termination site of chemoreceptor afferents, expresses a form of synaptic plasticity that increases overall nTS activity following intermittent hypoxia. Following CIH, an increase in presynaptic spontaneous neurotransmitter release occurs under baseline conditions. Furthermore, during and following afferent stimulation there is an augmentation of spontaneous transmitter release that occurs out of synchrony with sensory stimulation. On the other hand, afferent evoked synchronous transmitter release is attenuated. Overall, this shift from synchronous to asynchronous transmitter release enhances nTS cellular discharge. The role of the neurotransmitter dopamine in CIH-induced plasticity is also discussed. Dopamine attenuates synaptic transmission in nTS cells by blockade of N-type calcium channels, and this mechanism occurs tonically following normoxia and CIH. This dopaminergic pathway, however, is not altered in CIH. Taken together, alterations in nTS synaptic activity may play a role in the changes of chemoreflex function and cardiorespiratory activity in the CIH apnea model.