The sympathoinhibitory effects of systemic cholecystokinin are dependent on neurons in the caudal ventrolateral medulla in the rat

The Bezold-Jarisch Reflex and The Inflammatory Response Modulation in Unanesthetized Endotoxemic Rats

Evidence indicates that the activation of the parasympathetic branch of the autonomic nervous system may be effective in treating inflammatory diseases. Previously, we have described that baroreflex activation displays anti-inflammatory properties. Analogous to the baroreflex, the Bezold-Jarisch reflex also promotes parasympathetic activation with simultaneous inhibition of the sympathetic system. Thus, the present study aimed to evaluate whether the activation of the Bezold-Jarisch reflex would also have the ability to reduce inflammation in unanesthetized rats. We used lipopolysaccharide (LPS) injection (5mg/kg, i.p.) to induce systemic inflammation in male Wistar Hannover rats and phenylbiguanide (PBG) administration (5μg/kg, i.v.) to activate the Bezold-Jarisch reflex. Spleen, heart, hypothalamus, and blood samples were collected to determine the levels of cytokines. Compared to baseline, PBG reduced the arterial pressure (115±2 vs. 88±5mmHg) and heart rate (380±7 vs. 114±26bpm), immediately after its administration, confirming the activation of the parasympathetic system and inhibition of the sympathetic system. From the immunological point of view, the activation of the Bezold-Jarisch reflex decreased the plasma levels of TNF (LPS: 775±209 vs. PBG + LPS: 248±30pg/ml) and IL-6 levels in the spleen (LPS: 39±6 vs. PBG + LPS: 24±4pg/mg of tissue). However, it did not change the other cytokines in the plasma or the other tissues evaluated. These findings confirm that the activation of the Bezold-Jarisch reflex can modulate inflammation and support the understanding that the cardiovascular reflexes regulate the immune system.

Conceptualization of a Parasympathetic Endocrine System

We here propose a parasympathetic endocrine system (PES) comprised of circulating peptides released from secretory cells in the gut, significantly modulated by vagal projections from the dorsal motor nucleus of the vagus (DMV). While most of these gut peptides mediate well-described satiety and digestive effects that increase parasympathetic control of digestion (Lee et al., 1994; Gutzwiller et al., 1999; Klok et al., 2007), they also have actions that are far-reaching and increase parasympathetic signaling broadly throughout the body. The actions beyond satiety that peptides like somatostatin, cholecystokinin, glucagon-like peptide 1, and vasoactive intestinal peptide have been well-examined, but not in a systematic way. Consideration has been given to the idea that these and other gut-derived peptides are part of an endocrine system has been partially considered (Rehfeld, 2012; Drucker, 2016), but that it is coordinated through parasympathetic control and may act to increase the actions of parasympathetic projections has not been formalized before. Here only gut-derived hormones are included although there are potentially other parasympathetically mediated factors released from other sites like lung and liver (Drucker, 2016). The case for the existence of the PES with the DMV as its integrative controller will be made through examination of an anatomical substrate and evidence of physiological control mechanisms as well as direct examples of PES antagonism of sympathetic signaling in mammals, including humans. The implications for this conceptual understanding of a PES reframe diseases like metabolic syndrome and may help underscore the role of the autonomic nervous system in the associated symptoms.

The Role of the Autonomic Nervous System in the Pathophysiology of Obesity

Obesity is reaching epidemic proportions globally and represents a major cause of comorbidities, mostly related to cardiovascular disease. The autonomic nervous system (ANS) dysfunction has a two-way relationship with obesity. Indeed, alterations of the ANS might be involved in the pathogenesis of obesity, acting on different pathways. On the other hand, the excess weight induces ANS dysfunction, which may be involved in the haemodynamic and metabolic alterations that increase the cardiovascular risk of obese individuals, i.e., hypertension, insulin resistance and dyslipidemia. This article will review current evidence about the role of the ANS in short-term and long-term regulation of energy homeostasis. Furthermore, an increased sympathetic activity has been demonstrated in obese patients, particularly in the muscle vasculature and in the kidneys, possibily contributing to increased cardiovascular risk. Selective leptin resistance, obstructive sleep apnea syndrome, hyperinsulinemia and low ghrelin levels are possible mechanisms underlying sympathetic activation in obesity. Weight loss is able to reverse metabolic and autonomic alterations associated with obesity. Given the crucial role of autonomic dysfunction in the pathophysiology of obesity and its cardiovascular complications, vagal nerve modulation and sympathetic inhibition may serve as therapeutic targets in this condition.

Regulation of breathing and autonomic outflows by chemoreceptors

Lung ventilation fluctuates widely with behavior but arterial PCO2 remains stable. Under normal conditions, the chemoreflexes contribute to PaCO2 stability by producing small corrective cardiorespiratory adjustments mediated by lower brainstem circuits. Carotid body (CB) information reaches the respiratory pattern generator (RPG) via nucleus solitarius (NTS) glutamatergic neurons which also target rostral ventrolateral medulla (RVLM) presympathetic neurons thereby raising sympathetic nerve activity (SNA). Chemoreceptors also regulate presympathetic neurons and cardiovagal preganglionic neurons indirectly via inputs from the RPG. Secondary effects of chemoreceptors on the autonomic outflows result from changes in lung stretch afferent and baroreceptor activity. Central respiratory chemosensitivity is caused by direct effects of acid on neurons and indirect effects of CO2 via astrocytes. Central respiratory chemoreceptors are not definitively identified but the retrotrapezoid nucleus (RTN) is a particularly strong candidate. The absence of RTN likely causes severe central apneas in congenital central hypoventilation syndrome. Like other stressors, intense chemosensory stimuli produce arousal and activate circuits that are wake- or attention-promoting. Such pathways (e.g., locus coeruleus, raphe, and orexin system) modulate the chemoreflexes in a state-dependent manner and their activation by strong chemosensory stimuli intensifies these reflexes. In essential hypertension, obstructive sleep apnea and congestive heart failure, chronically elevated CB afferent activity contributes to raising SNA but breathing is unchanged or becomes periodic (severe CHF). Extreme CNS hypoxia produces a stereotyped cardiorespiratory response (gasping, increased SNA). The effects of these various pathologies on brainstem cardiorespiratory networks are discussed, special consideration being given to the interactions between central and peripheral chemoreflexes.

Blunted sympathoinhibitory responses in obesity-related hypertension are due to aberrant central but not peripheral signalling mechanisms

The gut hormone cholecystokinin (CCK) acts at subdiaphragmatic vagal afferents to induce renal and splanchnic sympathoinhibition and vasodilatation, via reflex inhibition of a subclass of cardiovascular-controlling neurons in the rostroventrolateral medulla (RVLM). These sympathoinhibitory and vasodilator responses are blunted in obese, hypertensive rats and our aim in the present study was to determine whether this is attributable to (i) altered sensitivity of presympathetic vasomotor RVLM neurons, and (ii) aberrant peripheral or central signalling mechanisms. Using a diet-induced obesity model, male Sprague-Dawley rats exhibited either an obesity-prone (OP) or obesity-resistant (OR) phenotype when placed on a medium high fat diet for 13-15 weeks; control animals were placed on a low fat diet. OP animals had elevated resting arterial pressure compared to OR/control animals (P < 0.05). Barosensitivity of RVLM neurons was significantly attenuated in OP animals (P < 0.05), suggesting altered baroreflex gain. CCK induced inhibitory responses in RVLM neurons of OR/control animals but not OP animals. Subdiaphragmatic vagal nerve responsiveness to CCK and CCK1 receptor mRNA expression in nodose ganglia did not differ between the groups, but CCK induced significantly less Fos-like immunoreactivity in both the nucleus of the solitary tract and the caudal ventrolateral medulla of OP animals compared to controls (P < 0.05). These results suggest that blunted sympathoinhibitory and vasodilator responses in obesity-related hypertension are due to alterations in RVLM neuronal responses, resulting from aberrant central but not peripheral signalling mechanisms. In obesity, blunted sympathoinhibitory mechanisms may lead to increased regional vascular resistance and contribute to the development of hypertension.

Electroacupuncture modulation of reflex hypertension in rats: role of cholecystokinin octapeptide

Acupuncture or electroacupuncture (EA) potentially offers a nonpharmacological approach to reduce high blood pressure (BP). However, ~70% of the patients and animal subjects respond to EA, while 30% do not. EA acts, in part, through an opioid mechanism in the rostral ventrolateral medulla (rVLM) to inhibit sympathoexcitatory reflexes induced by gastric distention. CCK-8 opposes the action of opioids during analgesia. Therefore, we hypothesized that CCK-8 in the rVLM antagonizes EA modulation of sympathoexcitatory cardiovascular reflex responses. Male rats anesthetized with ketamine and α-chloralose subjected to repeated gastric distension every 10 min were examined for their responsiveness to EA (2 Hz, 0.5 ms, 1-4 mA) at P5-P6 acupoints overlying median nerve. Repeated gastric distension every 10 min evoked consistent sympathoexcitatory responses. EA at P5-P6 modulated gastric distension-induced responses. Microinjection of CCK-8 in the rVLM reversed the EA effect in seven responders. The CCK1 receptor antagonist devazepide microinjected into the rVLM converted six nonresponders to responders by lowering the reflex response from 21 ± 2.2 to 10 ± 2.9 mmHg (first vs. second application of EA). The EA modulatory action in rats converted to responders with devazepide was reversed with rVLM microinjection of naloxone (n = 6). Microinjection of devazepide in the absence of a second application of EA did not influence the primary pressor reflexes of nonresponders. These data suggest that CCK-8 antagonizes EA modulation of sympathoexcitatory cardiovascular responses through an opioid mechanism and that inhibition of CCK-8 can convert animals that initially are unresponsive to EA to become responsive.

Acupuncture regulation of blood pressure: two decades of research

Although mechanisms underlying acupuncture regulation of pain have been studied by a number of laboratories in many countries, much less is known about its ability to modulate cardiovascular function. In the last two decades, our laboratory has systematically investigated the peripheral and central neural mechanisms underlying acupuncture regulation of blood pressure. These observations account for acupuncture's distant actions and, to some extent, its local actions, with respect to the site of needling. Four fundamental findings have advanced our knowledge. First, point-specific effects of acupuncture underlie its cardiovascular actions. Second, variable regions in the supraspinal and spinal central nervous system that receive input from somatic afferent stimulation account for acupuncture's ability to modulate blood pressure. Thus, depending on the underlying situation, for example, high or low blood pressure, acupuncture modifies autonomic outflow by reducing activity in brain stem nuclei that participate in the primary response. Third, repetitive acupuncture through a molecular mechanism can cause prolonged cardiovascular effects that far outlast acupuncture stimulation. Fourth, there is a range of cardiovascular responsiveness to acupuncture that depends, at least in part, on interactions between neural modulators that synaptically regulate autonomic function in the brain stem. Thus, acupuncture has the capability of profoundly regulating cardiovascular function in patients with disease, for example, hypertension, and the experimental laboratory is directing best approaches to study its actions in humans.

Intrathecal bombesin is sympathoexcitatory and pressor in rat

Bombesin, a 14 amino-acid peptide, is pressor when administered intravenously in rat and pressor and sympathoexcitatory when applied intracerebroventricularly. To determine the spinal effects of bombesin, the peptide was administered acutely in the intrathecal space at around thoracic spinal cord level six of urethane-anesthetized, paralyzed, and bilaterally vagotomized rats. Blood pressure, heart rate, splanchnic sympathetic nerve activity (sSNA), phrenic nerve activity, and end-tidal CO(2) were monitored to evaluate changes in the cardiorespiratory systems. Bombesin elicited a long-lasting excitation of sSNA associated with an increase in blood pressure and tachycardia. There was a mean increase in arterial blood pressure of 52 ± 5 mmHg (300 μM; P < 0.01). Heart rate and sSNA also increased by 40 ± 4 beats/min (P < 0.01) and 162 ± 33% (P < 0.01), respectively. Phrenic nerve amplitude (PNamp, 73 ± 8%, P < 0.01) and phrenic expiratory period (+0.16 ± 0.02 s, P < 0.05) increased following 300 μM bombesin. The gain of the sympathetic baroreflex increased from -2.8 ± 0.7 to -5.4 ± 0.9% (P < 0.01), whereas the sSNA range was increased by 99 ± 26% (P < 0.01). During hyperoxic hypercapnia (10% CO(2) in O(2), 90 s), bombesin potentiated the responses in heart rate (-25 ± 5 beats/min, P < 0.01) and sSNA (+136 ± 29%, P < 0.001) but reduced PNamp (from 58 ± 6 to 39 ± 7%, P < 0.05). Finally, ICI-216,140 (1 mM), an in vivo antagonist for the bombesin receptor 2, attenuated the effects of 300 μM bombesin on blood pressure (21 ± 7 mmHg, P < 0.01). We conclude that bombesin is sympathoexcitatory at thoracic spinal segments. The effect on phrenic nerve activity may the result of spinobulbar pathways and activation of local motoneuronal pools.

High-fat diet is associated with blunted splanchnic sympathoinhibitory responses to gastric leptin and cholecystokinin: implications for circulatory control

Gastric leptin and cholecystokinin (CCK) act on vagal afferents to induce cardiovascular effects and reflex inhibition of splanchnic sympathetic nerve discharge (SSND) and may act cooperatively in these responses. We sought to determine whether these effects are altered in animals that developed obesity in response to a medium high-fat diet (MHFD). Male Sprague-Dawley rats were placed on a low-fat diet (LFD; n = 8) or a MHFD ( n = 24) for 13 wk, after which the animals were anesthetized and artificially ventilated. Arterial pressure was monitored and blood was collected for the determination of plasma leptin and CCK. SSND responses to leptin (15 μg/kg) and CCK (2 μg/kg) administered close to the coeliac artery were evaluated. Collectively, MHFD animals had significantly higher plasma leptin but lower plasma CCK levels than LFD rats ( P < 0.05), and this corresponded to attenuated or reversed SSND responses to CCK (LFD, −21 ± 2%; and MHFD, −12 ± 2%; P < 0.05) and leptin (LFD, −6 ± 2%; and MHFD, 4 ± 1%; P < 0.001). Alternatively, animals on the MHFD were stratified into obesity-prone (OP; n = 8) or obesity-resistant (OR; n = 8) groups according to their weight gain falling within the upper or lower tertile, respectively. OP rats had significantly higher resting arterial pressure, adiposity, and plasma leptin but lower plasma CCK compared with LFD rats ( P < 0.05). The SSND responses to CCK or leptin were not significantly different between OP and OR animals. These results demonstrate that a high-fat diet is associated with blunted splanchnic sympathoinhibitory responses to gastric leptin and CCK and may impact on sympathetic vasomotor mechanisms involved in circulatory control.

Intracoronary gastrin 17 increases cardiac perfusion and function through autonomic nervous system, CCK receptors, and nitric oxide in anesthetized pigs

The release of gastrointestinal hormones has been reported to modulate reflex cardiovascular responses caused by gastric distension, although the role played by gastrin 17 is as yet unknown. The present study was therefore planned to determine the primary in vivo effect of gastrin 17 on coronary blood flow and cardiac function and the involvement of autonomic nervous system, CCK1/2 receptors, and nitric oxide (NO). In 40 anesthetized pigs, gastrin 17 was infused into the left anterior descending coronary artery at constant heart rate and arterial blood pressure. In 35 of the 40 pigs, the mechanisms of the observed hemodynamic responses were analyzed by repeating gastrin 17 infusion after autonomic nervous system and NO blockade, and after specific CCK receptors agonists/antagonists administration. Intracoronary gastrin 17 administration caused dose-related increases of both coronary blood flow and cardiac function. The intracoronary co-administration of CCK33/pentagastrin and gastrin 17 potentiated the coronary effects observed when the above agents were given alone ( P <0.05). The potentiation of the cardiac response was observed only with the co-administration of pentagastrin and gastrin 17 ( P <0.05). Moreover, blockade of muscarinic cholinoceptors (intravenous atropine) and of α-adrenoceptors (intravenous phentolamine) did not abolish the hemodynamic responses to gastrin 17. The cardiac and vascular effects of the hormone were prevented by blockade of β-adrenoceptors (intravenous atenolol and butoxamine), CCK1/2 receptors (intracoronary lorglumide and CAM-1028), and NO synthase (intracoronary Nω-nitro-l-arginine methyl ester). In conclusion, gastrin 17 primarily increased coronary blood flow and cardiac function through the involvement of CCK receptors, β-adrenoceptors, and NO release.

Hindbrain noradrenergic A2 neurons: diverse roles in autonomic, endocrine, cognitive, and behavioral functions

Central noradrenergic (NA) signaling is broadly implicated in behavioral and physiological processes related to attention, arousal, motivation, learning and memory, and homeostasis. This review focuses on the A2 cell group of NA neurons, located within the hindbrain dorsal vagal complex (DVC). The intra-DVC location of A2 neurons supports their role in vagal sensory-motor reflex arcs and visceral motor outflow. A2 neurons also are reciprocally connected with multiple brain stem, hypothalamic, and limbic forebrain regions. The extra-DVC connections of A2 neurons provide a route through which emotional and cognitive events can modulate visceral motor outflow and also a route through which interoceptive feedback from the body can impact hypothalamic functions as well as emotional and cognitive processing. This review considers some of the hallmark anatomical and chemical features of A2 neurons, followed by presentation of evidence supporting a role for A2 neurons in modulating food intake, affective behavior, behavioral and physiological stress responses, emotional learning, and drug dependence. Increased knowledge about the organization and function of the A2 cell group and the neural circuits in which A2 neurons participate should contribute to a better understanding of how the brain orchestrates adaptive responses to the various threats and opportunities of life and should further reveal the central underpinnings of stress-related physiological and emotional dysregulation.

Gastric leptin: a novel role in cardiovascular regulation

Gastric-derived leptin affects satiety and gastrointestinal function via vagal mechanisms and has been shown to interact with the gut hormone cholecystokinin (CCK). CCK selectively inhibits splanchnic sympathetic nerve discharge (SND) and the activity of a subset of presympathetic vasomotor neurons in the rostroventrolateral medulla (RVLM). The present study sought to examine the effects of gastric leptin on arterial pressure (AP), heart rate (HR), SND, and RVLM neuronal activity to determine whether its effects on cardiovascular regulation are dependent on CCK(1) receptors and vagal afferent transmission. To mimic gastric leptin, leptin (15-30 microg/kg) was administered close to the coeliac artery in anesthetized, artificially ventilated Sprague-Dawley rats. Within 5 min, leptin selectively decreased the activity of RVLM neurons also inhibited by CCK (-27 +/- 4%; P < 0.001; n = 15); these inhibitory effects were abolished following administration of the CCK(1) receptor antagonist lorglumide. Leptin significantly decreased AP and HR (-10 +/- 2 mmHg, P < 0.001; and -8 +/- 2 beats/min, P < 0.01; n = 35) compared with saline (-1 +/- 2 mmHg, 3 +/- 2 beats/min; n = 30). In separate experiments, leptin inhibited splanchnic SND compared with saline (-9 +/- 2% vs. 2 +/- 3%, P < 0.01; n = 8). Bilateral cervical vagotomy abolished the sympathoinhibitory, hypotensive, and bradycardic effects of leptin (P < 0.05; n = 6). Our results suggest that gastric leptin may exert acute sympathoinhibitory and cardiovascular effects via vagal transmission and CCK(1) receptor activation and may play a separate role to adipose leptin in short-term cardiovascular regulation.

The role of NMDA and non-NMDA receptors in the NTS in mediating three distinct sympathoinhibitory reflexes

Cholecystokinin (CCK) elicits a sympathetic vasomotor reflex that is implicated in gastrointestinal circulatory control. We sought to determine (1) the site in the solitary tract nucleus (NTS) responsible for mediating this reflex and (2) the possible involvement of excitatory amino acid (EAA) receptors. In addition, we sought to determine whether the NTS site responsible for mediating the baroreflex (phenylephrine, PE, 10 microg/kg i.v.) and the von Bezold-Jarisch reflex (phenylbiguanide, PBG, 10 microg/kg i.v) overlap with that involved in the CCK-induced reflex (CCK, 4 microg/kg, i.v.), and to compare the relative importance of NMDA and non-NDMA receptors in these reflexes. In separate experiments, the effects of PE, PBG, and CCK on mean arterial blood pressure, heart rate, and splanchnic sympathetic nerve discharge were tested before and after bilateral microinjection into the NTS of the gamma-aminobutyric acid(A) (GABA(A)) agonist muscimol, the EAA antagonist kynurenate, the NMDA receptor antagonist D: (-)-2-amino-5-phosphopentanoic acid (AP-5), the non-NMDA receptor antagonist 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX), AP-5 + NBQX, or vehicle. While all treatments (except vehicle) significantly attenuated/abolished/reversed the splanchnic sympathoinhibitory responses to PE, PBG, and CCK, the extent of blockade varied between the different treatment groups. Both NMDA and non-NMDA receptors were essential to the baroreflex and the von Bezold-Jarisch reflex, whereas the CCK reflex was more dependent on non-NMDA receptors. Muscimol, kynurenate, and AP-5 + NBQX significantly attenuated the bradycardic responses to PE and PBG (P < 0.05), whereas AP-5, NBQX, or vehicle did not. The bradycardic responses to CCK remained intact after all treatments. These results suggest that while there is overlap in the area of the NTS responsible for eliciting all three reflexes, NMDA and non-NMDA receptors are recruited differentially for the full expression of these reflexes. The CCK-induced sympathoinhibitory reflex is unique in that it relies predominantly on non-NMDA receptors in the NTS and elicits bradycardic effects that are independent of the NTS.

Nonuniformity in the von Bezold-Jarisch reflex

The von Bezold-Jarisch reflex (BJR) is a vagally mediated chemoreflex from the heart and lungs, causing hypopnea, bradycardia, and inhibition of sympathetic vasomotor tone. However, cardiac sympathetic nerve activity (CSNA) has not been systematically compared with vasomotor activity during the BJR. In 11 urethane-anesthetized (1-1.5 g/kg iv), artificially ventilated rats, we measured CSNA simultaneously with lumbar sympathetic activity (LSNA) while the BJR was evoked by right atrial bolus injections of phenylbiguanide (0.5, 1.0, 1.5, and 2 microg). Nerve and heartbeat responses were analyzed by calculating normalized cumulative sums. LSNA and heartbeats were always reduced by the BJR. An excitatory "rebound" component often followed the inhibition of LSNA but never outweighed it. For CSNA, however, excitation usually (in 7 of 11 rats) outweighed any initial inhibition, such that the net response to phenylbiguanide was excitatory. The differences in net response between LSNA, CSNA, and heartbeats were all significant (P < 0.01). A second experimental series on seven rats showed that methyl atropine (1 mg/kg iv) abolished the bradycardia of the BJR, whereas subsequent bilateral vagotomy substantially reduced LSNA and CSNA responses, both excitatory and inhibitory. These findings show that, during the BJR, 1) CSNA is often excited, 2) there may be coactivation of sympathetic and parasympathetic drives to the heart, 3) divergent responses may be evoked simultaneously in cardiac vagal, cardiac sympathetic, and vasomotor nervous pathways, and 4) those divergent responses are mediated primarily by the vagi.