Parasympathetic Stimuli on Bronchial and Cardiovascular Systems in Humans

Background

It is not known whether parasympathetic outflow simultaneously acts on bronchial tone and cardiovascular system waxing and waning both systems in parallel, or, alternatively, whether the regulation is more dependent on local factors and therefore independent on each system. The aim of this study was to evaluate the simultaneous effect of different kinds of stimulations, all associated with parasympathetic activation, on bronchomotor tone and cardiovascular autonomic regulation.

Methods

Respiratory system resistance (Rrs, forced oscillation technique) and cardio-vascular activity (heart rate, oxygen saturation, tissue oxygenation index, blood pressure) were assessed in 13 volunteers at baseline and during a series of parasympathetic stimuli: O₂ inhalation, stimulation of the carotid sinus baroreceptors by neck suction, slow breathing, and inhalation of methacholine.

Results

Pure cholinergic stimuli, like O₂ inhalation and baroreceptors stimulation, caused an increase in Rrs and a reduction in heart rate and blood pressure. Slow breathing led to bradycardia and hypotension, without significant changes in Rrs. However slow breathing was associated with deep inhalations, and Rrs evaluated at the baseline lung volumes was significantly increased, suggesting that the large tidal volumes reversed the airways narrowing effect of parasympathetic activation. Finally inhaled methacholine caused marked airway narrowing, while the cardiovascular variables were unaffected, presumably because of the sympathetic activity triggered in response to hypoxemia.

Conclusions

All parasympathetic stimuli affected bronchial tone and moderately affected also the cardiovascular system. However the response differed depending on the nature of the stimulus. Slow breathing was associated with large tidal volumes that reversed the airways narrowing effect of parasympathetic activation.

Field stimulation of the carotid baroreceptor complex does not compromise baroreceptor function in spontaneously hypertensive rats

Field stimulation of the carotid baroreceptors has been successfully used to induce a long-term reduction in blood pressure. However, baroreceptor stimulation may interfere with or compromise the beneficial short-term blood pressure regulation function of the baroreceptors. This study aims to quantify the baroreceptor function before and during acute, unilateral field stimulation of the carotid baroreceptors. Spontaneously Hypertensive Rats (n=7) were anaesthetised and instrumented to measure heart rate and mean arterial pressure (MAP), aortic pulse wave velocity (a surrogate measure of arterial stiffness), abdominal aortic flow and renal artery flow. A custom made field stimulation device was fitted to the left common carotid artery. Baroreceptor function was measured by quantifying heart rate response to MAP change induced by bolus injection of phenylephrine. Field stimulation of the baroreceptors reduced heart rate by 20 bpm (p=0.003) with MAP reduction of 18 mmHg (p=0.008). Maximal barorecep-tor gain without stimulation was -1.20±0.41 bpm/mmHg and during stimulation -1.41±0.52 bpm/mmHg (p=0.59). The MAP at which maximal gain occurred also did not change (152±11, 160±9 mmHg respectively, p=0.22). This study indicates that unilateral field stimulation of the carotid baroreceptor complex, while causing a sustained reduction of arterial pressure, does not alter acute baroreceptor function peak gain.

Integrating the theories of Darwin and Bernoulli: Maladaptive baroreceptor network dysfunction may explain the pathogenesis of aortic aneurysms

Current treatment options for aortic aneurysms are suboptimal and their pathogenic mechanisms remain unclear. We propose the existence of a coordinated multi-node baroreceptor network that measures pressures at all vascular bifurcations and enables system-wide hemodynamic coordination and vasomotor regulation, in accordance with the principle of Bernoulli. While the presence of baroreceptors at bifurcations remains unknown, behavior at the level of systems predicts their existence, possibly as glomus cell derivatives. We propose that pressure misregistration among sensor nodes at different vascular bifurcations can precipitate feed-forward dysfunctions that promote thrombosis, inflammation, and vasomotor dysregulation resulting in aneurysm formation. One example of this phenomenon is aortic aneurysm, which is currently attributed to focal anatomic defects. As plaque builds in the infrarenal aorta, the increased blood velocity through this segment can widen the difference between pressures sensed at the iliac and the renal artery bifurcations. Due to the Bernoulli effect, this change creates an incorrect impression of reduced dynamic pressure at the kidneys. The erroneous perception of hypovolemia can induce a pernicious cycle of maladaptive adrenergia and associated coagulation and thrombosis, particularly in the infrarenal aortic segment as the body attempts to normalize renal perfusion. Atherosclerosis can further exacerbate baroreceptor dysfunction by interfering with sensor biology in feed-forward fashion. Hypertension may be a consequence as well as a source of atherosclerosis and aneurysm. The described system may have evolved when trauma-related hypovolemia was a far more prevalent driver of natural selection but may be rendered maladaptive in the setting of modern stressors. Failure to address these factors may explain the suboptimal long-term outcomes with current surgical and endovascular treatments for aneurysms. Implications for other potential sensor networks including chemoreceptors and lymphoid tissues at bifurcating biologic branch-points such as vessels, airways, nerves, lymphatics, and ducts are discussed. Our framework may also provide a new basis for understanding thoracic aneurysm, renovascular dysfunctions, coronary artery disease, carotid artery disease, pulmonary embolism, portal hypertension, venous thrombosis, biliary disease, pancreatic disease, and neurologic disease. Novel treatment paradigms based on drugs or interconnected networks of devices that modulate sensors are envisioned. Improving the interface between sensors and their substrate information by techniques such as minimally traumatic atherectomy or thrombectomy may also restore appropriate sensor function. Lessons learned from bifurcation sensors and their potential maladaptations may generalize to other types of branching systems including botany, civil engineering, and Pitot tube aeronautics.

Differential roles for NMDA and non-NMDA receptor subtypes in baroreceptor afferent integration in the nucleus of the solitary tract of the rat

1. Microinjection studies have established that both NMDA and non-NMDA excitatory amino acid (EAA) receptor subtypes are involved in the integration of baroreceptor afferent inputs within the nucleus of the solitary tract (NTS). The present study was undertaken to determine which EAA receptor subtypes are involved in baroreceptor afferent integration by second and higher order NTS neurones. 2. Experiments utilizing intracellular recordings or extracellular recordings with microiontophoresis were performed in pentobarbitone-anaesthetized, paralysed and artificially ventilated rats to determine the ionotropic EAA receptor subtypes involved in baroreceptor afferent integration in the NTS. NTS neurones were classified according to their responses to aortic depressor nerve (ADN) stimulation: monosynaptic neurones (MSNs), polysynaptic neurones (PSNs) and ADN-non-evoked neurones (NENs). 3. In the extracellular studies, the ADN-evoked discharge of most MSNs was selectively reduced by microiontophoretic application of the non-NMDA receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; n = 8, P < 0.05) or 1,2,3,4-tetrahydro-6-nitro-2, 3-dioxo-benzo(f)quinoxaline-7-sulphonamide (NBQX; n = 9, P < 0.01), but not by the NMDA antagonist dl-2-amino-5-phosphonopentanoic acid (AP-5; n = 6, P = 0.28). ADN-evoked responses of PSNs were attenuated by microiontophoretic application of AP-5 (n = 12, P < 0. 001), CNQX (n = 13, P < 0.001) or NBQX (n = 11, P < 0.001). All EAA antagonists inhibited the spontaneous discharge of MSNs/PSNs and NENs (P < 0.01 for each group). 4. In the intracellular studies, ADN stimulation evoked faster rising and shorter duration excitatory postsynaptic potentials (EPSPs) in MSNs (n = 16) than in PSNs (n = 15) (P < 0.05 for each comparison). 5. Our results demonstrate that synaptic inputs from ADN to MSNs have faster rise times and shorter durations than those to PSNs, suggesting that baroreceptor inputs to MSNs and PSNs are mediated by different synaptic mechanisms. These more rapid synaptic events are selectively mediated by non-NMDA receptors. In addition, synaptic integration of ADN inputs by PSNs is mediated by both NMDA and non-NMDA receptors. Finally, the ADN-evoked discharge of some MSNs and PSNs is not attenuated by ionotropic EAA antagonists, suggesting that another receptor or transmitter system may mediate synaptic excitation in these neurones.

Nitric oxide as an autocrine regulator of sodium currents in baroreceptor neurons

Arterial baroreceptors are mechanosensitive nerve endings in the aortic arch and carotid sinus that play a critical role in acute regulation of arterial blood pressure. A previous study has shown that nitric oxide (NO) or NO-related species suppress action potential discharge of baroreceptors. In the present study, we investigated the effects of NO on Na+ currents of isolated baroreceptor neurons in culture. Exogenous NO donors inhibited both tetrodotoxin (TTX) -sensitive and -insensitive Na+ currents. The inhibition was not mediated by cGMP but by NO interaction with channel thiols. Acute inhibition of NO synthase increased the Na+ currents. NO scavengers (hemoglobin and ferrous diethyldithiocarbamate) increased Na+ currents before but not after inhibition of NO synthase. Furthermore, NO production in the neuronal cultures was detected by chemiluminescence and immunoreactivity to the neuronal isoform of NO synthase was identified in fluorescently identified baroreceptor neurons. These results indicate that NO/NO-related species function as autocrine regulators of Na+ currents in baroreceptor neurons. Modulation of Na+ channels may represent a novel response to NO.

Differences in the density of barosensitive neurons in the medulla of spontaneously hypertensive and Wistar-Kyoto rats

1. The density of barosensitive neurons in the medulla was examined in spontaneously hypertensive rats (SHR) and in normotensive Wistar-Kyoto (WKY) rats. In control experiments, rats were sham-operated, while in test experiments arterial baroreceptors were stimulated by pressor responses to i.v. administration of phenylephrine and the density of c-Fos-labelled neurons was immunocytologically examined. 2. In both control and test experiments, c-Fos-labelled neurons were distributed in cardiovascular control sites: the nucleus tractus solitarii (NTS) and the caudal and rostral ventrolateral medullas (CVLM/RVLM). 3. In both WKY rats and in SHR, the total density of labelled neurons in test experiments was significantly higher than in control experiments. 4. In control experiments, no significant difference was found in the distribution and density of labelled neurons in the NTS and in the CVLM/RVLM between rats and SHR. 5. In test experiments, no significant difference was found in the distribution and density of labelled neurons in the NTS between WKY rats and SHR. 6. In test experiments in SHR, the density of labelled neurons in the CVLM just caudal to the obex level was significantly higher than that in WKY rats, whereas the density of labelled neurons in WKY rats in the RVLM just rostral to the obex level was significantly higher than that in SHR. 7. These results indicate that stimulation of the arterial baroreceptor induces strain-specific differences in the density of barosensitive neurons in the CVLM/RVLM near the obex level.

Synaptic interactions of substance P immunoreactive nerve terminals in the baro- and chemoreceptor reflexes of the cat

The neurochemical anatomy and synaptic interactions of morphologically identified chemoreceptor or baroreceptor afferents in the nucleus of the solitary tract (NTS) are poorly understood. A substantial body of physiological and light microscopic evidence suggests that substance P (SP) may be a neurotransmitter contained in first order sensory chemo- or baroreceptor afferents, however ultrastructural support of this hypothesis is lacking. In the present report we have traced the central projections of the carotid sinus nerve (CSN) in the cat by utilizing the transganglionic transport of horseradish peroxidase. Medullary tissues including the commissural NTS (cNTS) were processed for the histochemical visualization of transganglionically labeled CSN afferents and for the immunocytochemical detection of SP by dual labeling light and electron microscopic methods. At the light microscopic level, dense bilateral labeling with TMB was found in the tractus solitarius (TS) and cNTS, caudal to the obex. Rostral to the obex, significant ipsilateral TMB labeling was detected in the dorsal, dorso-lateral, and medial subnuclei of the NTS, as well as in the TS. Significant staining of SP immunoreactive processes was detected in most subnuclei of the NTS. The cNTS was examined by electron microscopy. Either HRP or SP were readily identified in single labeled unmyelinated axons, myelinated axons, and nerve terminals in the cNTS. SP immunoreactivity was also identified in unmyelinated axons, myelinated axons, and nerve terminals in the cNTS which were simultaneously identified as CSN primary afferents. These ultrastructural data support the hypothesis that SP immunoreactive first order neurons are involved in the origination of the chemo- and baroreceptor reflexes. Axo-axonic synapses were observed between CSN primary afferent terminals and: (a) unlabeled nerve terminals; (b) other CSN primary afferent terminals; and (c) terminals containing SP. Axo-axonic synapses were also observed between CSN primary afferents which contained SP, and other SP terminals. These observations may mediate the morphological bases for multiple forms of presynaptic inhibition in the cNTS, including those involved in cardiorespiratory integration. In conclusion, our results indicate that SP immunoreactive nerve terminals may be important in both the origination and the modulation of the chemo- and/or baroreceptor reflexes.

Synaptic connections of central carotid sinus afferents in the nucleus of the tractus solitarius of the rat. II. Connections with substance P-immunoreactive neurons

A combined transganglionic transport and immunocytochemical technique was used to study the synaptic morphology of central carotid sinus afferents and substance P-immunoreactive neurons in the commissural subnucleus of the nucleus of the tractus solitarius in rats. A large population of substance P-immunoreactive neurons (88.32%) were seen in close association with central carotid sinus afferents by light microscopy. However, many labelled central carotid sinus afferents appeared not associated with substance P-immunoreactive neurons in the nucleus of the tractus solitarius. Substance P-immunoreactive neurons were spindle, pear, or oval-shaped with a short axis ranging from 5 to 11 microns. Their long axis was oriented predominantly in a lateral-medial direction along the path of the central carotid sinus afferents from the solitary tract to the midline. Synaptic contacts between central carotid sinus afferents and substance P-structures, including dendritic profiles of different calibers and somas, were readily found by electron microscopy. Many central carotid sinus afferents were also found in synaptic contact with non-immunoreactive dendrites and somas. Appositions between central carotid sinus afferents and unlabelled axon terminals were common, but only in a few cases were morphological manifestations of synapses revealed. In the latter, the substance P-immunoreactive terminals appeared mostly presynaptic but postsynaptic ones were also encountered. Our data provide the evidence that some of the substance P-immunoreactive cells in the nucleus of the tractus solitarius are 2nd order neurons of the carotid sinus afferent pathway. The possibility that some of the substance P-immunoreactive neurons in the nucleus of the tractus solitarius may be interneurons and mediate carotid sinus afferent inputs to catecholaminergic neurons in the nucleus of the tractus solitarius is considered. Our findings also provide an anatomical substrate for a possible presynaptic modulatory role of central carotid sinus afferents on the inputs from other brain centers to the substance P-neurons in the nucleus of the tractus solitarius.

Presence and coexistence of putative neurotransmitters in carotid sinus baro- and chemoreceptor afferent neurons

The presence and coexistence of tyrosine hydroxylase (TH), vasoactive intestinal polypeptide (VIP), calcitonin gene-related peptide (CGRP), substance P (SP) and galanin (GAL) were studied in the petrosal and jugular neurons innervating the carotid body and carotid sinus of the rat. The retrograde labeling of the carotid sinus nerve with Fluoro-gold (FG) demonstrated that most (94.5%) FG-labeled ganglionic neurons were observed in the petrosal ganglion. Fewer (5.2%) FG-labeled neurons were seen in the jugular ganglion and very few (0.3%) were observed in the nodose ganglion. Immunohistochemistry revealed that subpopulations of TH-, VIP-, CGRP-, SP- and GAL-immunoreactive (-ir) neurons in the petrosal ganglion projected to the carotid sinus nerve. Approximately 4% of FG-labeled neurons contained TH-ir and were predominantly found in the caudal portion of the petrosal ganglion. Nearly 90% of total TH-ir neurons in the petrosal ganglion were labeled with FG. Less than 1% of FG-labeled neurons were immunoreactive for VIP in this ganglion. In the petrosal ganglion, 25% of FG-labeled neurons contained CGRP-ir, and 16.7% of FG-labeled neurons contained SP-ir. 30% of CGRP-ir or SP-ir neurons in the petrosal ganglion were labeled with FG. In the jugular ganglion, no TH- or VIP-ir neurons projected to the carotid sinus nerve and only small populations of CGRP- or SP-ir neurons projected to the carotid sinus nerve. Many FG-labeled and GAL-ir neurons were observed in the petrosal and jugular ganglia.(ABSTRACT TRUNCATED AT 250 WORDS)