Cardiac Vagal Neurons Research Articles

Central Neuroplasticity and Decreased Heart Rate Variability after Particulate Matter Exposure in Mice

BackgroundEpidemiologic studies show that exposure to fine particulate matter [aerodynamic diameter ≤ 2.5 μm (PM2.5)] increases the total daily cardiovascular mortality. Impaired cardiac autonomic function, which manifests as reduced heart rate variability (HRV), may be one of the underlying causes. However, the cellular mechanism(s) by which PM2.5 exposure induces decreased HRV is not known.ObjectivesWe tested the hypothesis that exposure to PM2.5 impairs HRV by decreasing the excitability of the cardiac vagal neurons in the nucleus ambiguus. We also detemined the effect of iron on PM-exposure–induced decrease in HRV.MethodsWe measured 24-hr HRV in time domains from electrocardiogram telemetry recordings obtained in conscious, freely moving mice after 3 days of exposure to PM2.5 in the form of soot only or iron-soot. In parallel studies, we determined the intrinsic properties of identified cardiac vagal neurons, retrogradely labeled with a fluorescent dye applied to the sinoatrial node.ResultsSoot-only exposure decreased short-term HRV (root mean square of successive difference). With the addition of iron, all HRV parameters were significantly reduced. In nonexposed mice, vagal blockade significantly reduced all HRV parameters, suggesting that HRV is, in part, under vagal regulation in mice. Iron-soot exposure had no significant effect on resting membrane potential but decreased spiking responses of the identified cardiac vagal neurons to depolarizations (p < 0.05). The decreased spiking response was accompanied with a higher minimal depolarizing current required to evoke spikes and a lower peak discharge frequency.ConclusionsThe data suggest that PM-induced neuroplasticity of cardiac vagal neurons may be one mechanism contributing to the cardiovascular consequences associated with PM2.5 exposure seen in humans.

Rebuttal from Karemaker

POINT-COUNTERPOINTRebuttal from KaremakerPublished Online:01 May 2009https://doi.org/10.1152/japplphysiol.91107.2008cMoreSectionsPDF (36 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat Dr. Eckberg has gone through a lot of physiological literature to prove his Point. However, as explained in the Counterpoint, his reasoning from time domain experimental data to frequency domain and back misses the target on a number of critical points.The basic issue here is the applicability of data measured in single neurons under deep anesthesia in experimental animals to recordings in alert humans. This is not to discard animal experimentation; it is at the heart of physiological and scientific progress. However, the step from one to the other is not always easily proven.We may assume for a fact that central cardiac vagal neurons are coinnervated from centers that drive inspiratory motor neurons (1). Even if that mechanism is active in the conscious human condition, it still does not preclude the dominant influence of peripheral input signals to constitute what finally is apparent as respiratory sinus arrhythmia. The observed time delays do not preclude it, as explained in the Counterpoint: any change in (systolic) blood pressure is translated sufficiently fast into efferent vagal outflow to the sinus node. Phase delays from blood pressure to heart rate cannot measure intrabeat delays more precisely than to the duration of one heart beat. The technique of Fourier transform does not add new information to the data. Phase differences between blood pressure and heart period around the respiratory frequency are well within the limits of ±1 heart beat, indicating that the observed changes are in phase and occur within the same beat. Variability in this phase is merely a reflection of the nonsinusoidal character of the tracings. The fact that sympathetically induced blood pressure and heart rate changes follow sympathetic nerve signals by some 3 s is irrelevant for vagus nerve delays to sinus node responses.If all (or most) respiratory variability in heart rate would originate primarily from central mechanisms, the phase between heart period and (systolic) blood pressure changes should reveal this: heart period should precede pressure by 1 beat. At the common respiratory frequency around 0.25 Hz and heart periods around 900 ms, this would result in a phase advance of (900/4000)·360 = 81° or more. This is not commonly observed.In conclusion, the mutual timings of heart rate and blood pressure variabilities are well compatible with a peripheral origin of RSA, be it or be it not centrally amplified by subthreshold oscillations of cardiac vagal motor neurons due to respiratory involvement. Of the many peripheral inputs that impinge on those efferents, the baroreceptors are in the front row.REFERENCE1 Gilbey MP, Jordan D, Richter DW, and Spyer KM. Synaptic mechanisms involved in the inspiratory modulation of vagal cardio-inhibitory neurones in the cat. J Physiol 356: 65–78, 1984.Crossref | PubMed | ISI | Google Scholar Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation More from this issue > Volume 106Issue 5May 2009Pages 1744-1744 Copyright & PermissionsCopyright © 2009 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.91107.2008cHistory Published online 1 May 2009 Published in print 1 May 2009 Metrics

Particulate matter exposure‐induced decreased heart rate variability is associated with a reduced excitability of cardiac vagal neurons in mice

Exposure to PM2.5 increases the total daily cardiovascular mortality by an estimated 3‐4% for every 25 μg/m3 increase in PM2.5. Impaired cardiac autonomic function, indexed by reduced heart rate variability (HRV), is one underlying cause. We previously showed that 3 d (6 hr/d) of PM2.5 exposure in the form of iron‐soot reduced HRV in mice. Here we test the hypothesis that exposure to airborne PM impairs vagal regulation of HRV by decreasing the excitability of the cardiac vagal neurons in the nucleus ambiguus (NA) in C57BL/6 mice. We measured 24 hr HRV in time domains from ECG telemetry recordings obtained in conscious freely moving mice. In parallel studies we determined the intrinsic properties of anatomically identified NA cardiac vagal neurons, retrogradely labeled with a fluorescent dye (DiI) applied to the SA node. PM2.5 exposure in the form of soot decreased the short term HRV (rMSSD). With the addition of iron, all HRV parameters were significantly reduced. The reduced HRV is more prominent during the dark cycle of the day. Blocking the vagal influence with Methylatropine significantly reduced all HRV parameters suggesting that HRV is under vagal regulation in mice. PM2.5 exposure also decreased the spiking responses of the identified cardiac vagal neurons to depolarizing current injections (P&lt;0.05). The data suggest PM‐induced neuroplasticity of cardiac vagal neurons maybe one mechanism contributing to the cardiovascular consequences associated with PM exposure seen in humans. (Support: Support: 1R01ES012957‐01).

5HT1A Receptor Mediated Inhibition of Glutamatergic Neurotransmission to Cardiac Vagal Neurons Following Hypoxia and Hypercapnia

Cardiac vagal neurons (CVNs) are excited by serotonergic and purinergic pathways recruited post hypoxia / hypercapnia (H/H). Surprisingly, excitatory glutamatergic inputs to CVNs are inhibited post H/H. This study examines the mechanisms of H/H induced inhibition of glutamatergic neurotransmission to CVNs. Spontaneous and inspiratory‐related EPSCs were recorded from identified CVNs before, during and post H/H. WAY 100635 (100 microM), a specific antagonist for 5HT1A receptors, restored a glutamatergic pathway to CVNs in the recovery period following H/H. The purinergic and serotonergic pathways to CVNs active post H/H were not altered by blocking 5HT1A receptors and WAY 100635 had no effect before, and during H/H. This study indicates endogenous activation of serotonergic 5HT1A receptors decreases glutamatergic neurotransmission to CVNs following H/H, likely via a presynaptic site of action.

Mapping and Identification of GABAergic Neurons in Transgenic Mice Projecting to Cardiac Vagal Neurons in the Nucleus Ambiguus Using Photo-Uncaging

The neural control of heart rate is determined primarily by the activity of preganglionic parasympathetic cardiac vagal neurons (CVNs) originating in the nucleus ambiguus (NA) in the brain stem. GABAergic inputs to CVNs play an essential role in determining the activity of these neurons including a robust inhibition during each inspiratory burst. The origin of GABAergic innervation has yet to be determined however. A transgenic mouse line expressing green florescent protein (GFP) in GABAergic cells was used in conjunction with caged glutamate to identify both clusters and individual GABAergic neurons that evoke inhibitory GABAergic synaptic responses in CVNs. Transverse slices were taken with CVNs patch-clamped in the whole cell configuration. Sections containing both the pre-Botzinger complex as well as the calamus scriptorius were divided into approximately 90 quadrants, each 200 x 200 microm and were sequentially photostimulated. Inhibitory post synaptic currents (IPSCs) were recorded in CVNs after a 5-ms photostimulation of 50 microM caged glutamate. The four areas that contained GABAergic cells projecting to CVNs were 200 microm medial, 400 microm medial, 200 microm ventral, and 1,200 microm dorsal and 1,000 microm medial to patched CVNs. Once foci of GABAergic cells projecting to CVNs were determined, photostimulation of individual GABAergic neurons was conducted. The results from this study suggest that GABAergic cells located in four specific areas project to CVNs, and that these cells can be individually identified and stimulated using photouncaging to recruit GABAergic neurotransmission to CVNs.

Propofol and Isoflurane Enhancement of Tonic Gamma-Aminobutyric Acid Type A Current in Cardiac Vagal Neurons in the Nucleus Ambiguus

General anesthesia with propofol and isoflurane induces alterations of the cardiovascular system, including hypotension and changes in heart rate. The preganglionic cardiac vagal neurons (CVNs) are one of the major central components controlling heart rate and autonomic regulation. In this study, we examined whether propofol and isoflurane act on phasic or tonic gamma-aminobutyric acid type A (GABA(A)) receptor-mediated inhibition in CVNs. CVNs were identified in vitro by retrograde fluorescent labeling. Phasic and tonic GABA currents in CVNs were examined using the whole cell patch-clamp technique. Propofol (10 microM) increased the membrane holding currents by 63 +/- 13% and prolonged the decay time of GABAergic miniature inhibitory postsynaptic currents (mIPSCs) from 42.3 +/- 2.8 ms in control to 61.8 +/- 4.5 ms. Isoflurane, at concentrations of 100, 300, and 500 microM, decreased GABAergic mIPSCs frequency by 26.0 +/- 16%, 64.6 +/- 10.4%, and 70.5 +/- 9.8%, prolonged the decay time of GABAergic mIPSCs from 47.9 +/- 7.3 to 64.5 +/- 8.1 ms, 70.3 +/- 10.4 ms, and 66.8 +/- 8.1 ms, and increased the membrane holding currents by 32.8 +/- 12.8%, 42.7 +/- 10%, and 39.9 +/- 3%, respectively. The GABAergic antagonist gabazine (25 microM) blocked GABAergic mIPSCs, but failed to alter the enhanced holding potential induced by propofol and isoflurane. In contrast, the channel blocker of GABA(A) receptors, picrotoxin (100 microM), reversed the propofol and isoflurane-evoked increase in membrane holding current. The results demonstrate that the general anesthetics propofol and isoflurane enhance both phasic and tonic GABA(A) receptor-mediated inhibition of CVNs.

Nicotinic Receptors Partly Mediate Brainstem Autonomic Dysfunction Evoked by the Inhaled Anesthetic Isoflurane

Isoflurane is one of the most commonly used volatile anesthetics, yet the cardiorespiratory depression that occurs with its use remains poorly understood. In this study, the author examined isoflurane modulation of postsynaptic gamma-aminobutyric acid (GABA) receptors in parasympathetic cardiac vagal neurons (CVNs) and alterations of GABAergic function by targeting nicotinic acetylcholine receptors on GABAergic presynaptic terminals. Rhythmic inspiratory-related activity was recorded from the hypoglossal rootlet of 800 microm medullary sections. CVNs were identified by retrograde fluorescent labeling, and GABAergic neurotransmission to CVNs were examined using patch-clamp electrophysiological techniques. Isoflurane at concentrations of >50 microM significantly suppressed inspiratory bursting frequency, amplitude, and duration. Isoflurane dose-dependently decreased the frequency and increased the decay time of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) in CVNs. To test whether the inhibition of GABAergic activity to CVNs was mediated by presynaptic nicotinic receptors, the nicotinic antagonist, dihydro-beta-erythroidine in an alpha(4)beta(2)-selective concentration (3 microM), was used. Dihydro-beta-erythroidine (3 microM) prevented the isoflurane-evoked depression of spontaneous GABAergic IPSC frequency, yet isoflurane still increased the IPSC decay time. These results suggest clinically relevant concentrations of isoflurane inhibit brainstem respiratory rhythmogenesis, prolong inhibitory GABAergic postsynaptic currents and reduce GABA activity in CVNs. The decrease of GABAergic IPSCs frequency is dependent upon inhibition of presynaptic alpha(4)beta(2) nicotinic receptors.

Abolishment of Serotonergic Neurotransmission to Cardiac Vagal Neurons During and After Hypoxia and Hypercapnia With Prenatal Nicotine Exposure

Cardioinhibitory cardiac vagal neurons (CVNs) do not receive inspiratory-related excitatory inputs under normal conditions. However, excitatory purinergic and serotonergic pathways are recruited during inspiratory activity after episodes of hypoxia and hypercapnia (H/H). Prenatal nicotine (PNN) exposure is known to dramatically change cardiorespiratory responses and decrease the ability to resuscitate from H/H. This study tested whether PNN exposure alters excitatory neurotransmission to CVNs in the nucleus ambiguus during and after H/H. Spontaneous and inspiratory evoked excitatory postsynaptic currents were recorded in CVNs from rats that were exposed to nicotine (6 mg x kg(-1) x d(-1)) throughout the prenatal period. In contrast to unexposed animals, in PNN animals H/H recruited excitatory neurotransmission to CVNs during inspiratory-related activity that was blocked by the alpha3beta4 nicotinic acetylcholine receptor (nAChR) blocker alpha-conotoxin AuIB (alpha-CTX AuIB, 100 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 50 microM) and d(-)-2-amino-5-phosphonopentanoic acid (AP5, 50 microM), selective AMPA/kainate and N-methyl-d-aspartate receptor blockers, respectively. Following H/H, there was a significant increase in inspiratory-related excitatory postsynaptic currents that were unaltered by alpha-CTX AuIB or ondansetron, a 5-HT3 receptor blocker, but were subsequently inhibited by pyridoxalphosphate-6-azophenyl-2', 4'-disulphonic acid (100 microM), a purinergic receptor blocker and CNQX and AP5. The results from this study demonstrate that with PNN exposure, an excitatory neurotransmission to CVNs is recruited during H/H that is glutamatergic and dependent on activation of alpha3beta4-containing nAChRs. Furthermore, exposure to PNN abolishes a serotonergic long-lasting inspiratory-related excitation of CVNs that is replaced by recruitment of a glutamatergic pathway to CVNs post H/H.

Purinergic P2X receptors facilitate inhibitory GABAergic and glycinergic neurotransmission to cardiac vagal neurons in the nucleus ambiguus

This study examined whether adenosine 5′-triphosphate (ATP) modulated inhibitory glycinergic and GABAergic neurotransmission to cardiac vagal neurons. Inhibitory activity to cardiac vagal neurons was isolated and examined using whole-cell patch-clamp recordings in an in vitro brain slice preparation in rats. ATP (100 μM) evoked increases in the frequency of glycinergic and GABAergic miniature inhibitory postsynaptic currents (mIPSCs) in cardiac vagal neurons which were blocked by the broad P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2′,4′-disulphonic acid (100 μM). Application of the P2Y agonists uridine triphosphate (15 μM) and adenosine 5′-0-( Z-thiodiphosphate) (60 μM) did not enhance inhibitory neurotransmission to cardiac vagal neurons however, application of the selective P2X` receptor agonist, α, β-methylene ATP (100 μM), increased glycinergic and GABAergic mIPSC neurotransmission to cardiac vagal neurons. The increase in inhibitory neurotransmission evoked by α, β-methylene ATP was abolished by the selective P2X receptor antagonist 2′,3′- O-(2,4,6-Trinitrophenyl) adenosine 5′-triphosphate (100 μM) indicating P2X receptors enhance the release of inhibitory neurotransmitters to cardiac neurons. The voltage-gated calcium channel blocker cadmium chloride did not alter the evoked increase in inhibitory mIPSCs. This work demonstrates that P2X receptor activation enhances inhibitory neurotransmission to parasympathetic cardiac vagal neurons and demonstrates an important functional role for ATP mediated purinergic signaling to cardiac vagal neurons.

5-HT1A/7receptor agonist excites cardiac vagal neurons via inhibition of both GABAergic and glycinergic inputs

To study the synaptic mechanisms involved in the 5-hydroxytryptamine1A/7 (5-HT1A/7) receptor-mediated reflex control of cardiac vagal preganglionic neurons (CVPN). CVPN were retrogradely labeled and identified in brain stem slices of newborn rats, and their synaptic activity was examined using whole-cell patch-clamp. 8-Hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT), an agonist of 5-HT1A/7 receptors, had no effect on the glutamatergic inputs of CVPN. In contrast, it significantly decreased the frequency and the amplitude of both the GABAergic and the glycinergic spontaneous inhibitory postsynaptic currents (sIPSC). 8-OH-DPAT also caused significant amplitude decrease of the GABAergic currents evoked by stimulation of the nucleus tractus solitarius. Both the frequency inhibition and the amplitude inhibition of the GABAergic and the glycinergic sIPSC by 8-OH-DPAT had dose-dependent tendencies and could be reversed by WAY-100635, an antagonist of 5-HT1A/7 receptors. In the pre-existence of tetrodotoxin, 8-OH-DPAT had no effect on the GABAergic or the glycinergic miniature inhibitory postsynaptic currents, and had no effect on the GABAergic or the glycinergic currents evoked by exogenous GABA or glycine. The 5-HT1A/7 receptor agonist excites CVPN indirectly via the inhibition of both the GABAergic and glycinergic inputs. These findings have at least in part revealed the synaptic mechanisms involved in the 5-HT1A/7 receptor-mediated reflex control of cardiac vagal nerves in intact animals.

5HT2 receptor activation facilitates P2X receptor mediated excitatory neurotransmission to cardiac vagal neurons in the nucleus ambiguus

Parasympathetic preganglionic cardiac vagal neurons (CVNs) which dominate the control of heart rate are located within the nucleus ambiguus (NA). Serotonin (5HT), and in particular 5HT2 receptors, play an important role in cardiovascular function in the brainstem. However, there is a lack of information on the mechanisms of action of 5HT2 receptors in modulating parasympathetic cardiac activity. This study tests whether activation of 5HT2 receptors alters excitatory glutamatergic and purinergic neurotransmission to CVNs. Application of α-methyl-5-hydroxytryptamine (α-Me-5HT), a 5HT2 agonist, reversibly increased both the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs) in CVNs. Similar responses were obtained with α-methyl-5-(2-thienylmethoxy)-1 H-indole-3-ethanamine hydrochloride (BW723C86), and m-chlorophenylpiperazine (m-CPP), 5HT2B and 5HT2B/C receptor agonists, respectively. The facilitation evoked by α-Me-5HT was prevented by the 5HT2B/C receptor antagonist SB206553 hydrochloride (SB206553). Interestingly, the blockage of both NMDA and non-NMDA glutamatergic receptors did not prevent α-Me-5HT-evoked facilitation of mEPSCs, however, the responses were blocked by the P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS). The responses evoked by α-Me-5HT were mimicked by application of α,β-methylene ATP (α,β-Me-ATP), a P2X receptor agonist, which were also blocked by PPADS. In summary, these results indicate activation of 5HT2 receptors facilitates excitatory purinergic, but not glutamatergic, neurotransmission to CVNs.

Role of Purinergic and Nicotinic Receptors in the Hypoxia/Hypercapnia Evoked Excitation of Parasympathetic Cardiac Vagal Neurons in the Brainstem

Hypoxia and hypercapnia are among the strongest challenges to the cardiorespiratory system, and these responses are altered by prenatal nicotine exposure. However the mechanism(s) responsible for these cardiorespiratory responses, and their alteration by prenatal nicotine exposure are unknown. We used an in vitro medullary slice that allows simultaneous examination of rhythmic respiratory-related activity and synaptic neurotransmission to cardiac vagal neurons (CVNs) that control heart rate. Respiratory-related increases in excitatory neurotransmission only occurred upon recovery from hypoxia/hypercapnia in unexposed animals. These responses were mediated in part by purinergic receptors. Prenatal nicotine exposure transformed central cardiorespiratory responses; CVNs received a respiratory-related neurotransmission not during recovery but during hypoxia/hypercapnia which was wholly dependent upon nicotinic receptor activation. In the presence of nicotinic antagonists, the responses in prenatal nicotine animals reverted to the pattern of responses in unexposed animals. These data identify a new functional role for purinergic receptors in the cardiorespiratory responses to hypoxia/hypercapnia and their role in occluding nicotinic receptor activation with prenatal nicotine exposure. [ Tzu Chi Med J 2008;20(1):1–10]

ATP facilitates glutamatergic neurotransmission to cardiac vagal neurons in the nucleus ambiguus

Recent work has shown that adenosine 5'-triphosphate (ATP) plays an important role in modulating the activity of parasympathetic cardiac vagal neurons that dominate the neural control of heart rate. This study examined the mechanisms by which activation of ATP receptors modulates excitatory neurotransmission to cardiac vagal neurons. Glutamatergic activity to cardiac vagal neurons was isolated and examined using whole-cell patch-clamp recordings in an in vitro brain slice preparation in rats. ATP (100 µM) evoked increases in the frequency of glutamatergic miniature excitatory postsynaptic currents (mEPSCs) in cardiac vagal neurons which were blocked by the broad P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 100 µM). Application of the selective P2X receptor agonist, α, β-methylene ATP (100 µM), also increased glutamatergic mEPSCs neurotransmission to cardiac vagal neurons indicating P2X receptors enhance glutamatergic release to cardiac vagal neurons. The evoked increase in glutamatergic mEPSC was unaltered by the voltage-gated calcium channel blocker cadmium, and was abolished by the selective P2X receptor antagonist 2',3'- O-(2,4,6-Trinitrophenyl) adenosine 5'-triphosphate, TNP-ATP (100 µM). This work demonstrates that the ATP evoked facilitation of excitatory neurotransmission to cardiac vagal neurons is dependent upon activation of P2X receptors on glutamatergic presynaptic terminals.

Recruitment of Excitatory Serotonergic Neurotransmission to Cardiac Vagal Neurons in the Nucleus Ambiguus Post Hypoxia and Hypercapnia

Inhibitory GABAergic and glycinergic neurotransmission to cardioinhibitory cardiac vagal neurons (CVNs) increase during inspiratory activity and likely mediate respiratory sinus arrhythmia, while the frequency of excitatory postsynaptic currents (EPSCs) in CVNs are unaltered during the different phases of respiration. However, following hypoxia and hypercapnia (H/H), the parasympathetic activity to the heart increases and thus far, identification of the pathways and neurotransmitters that are responsible for exciting CVNs post H/H are unclear. This study identifies different excitatory pathways to CVNs recruited post H/H. Spontaneous and inspiratory-related EPSCs were recorded in CVNs before, during, and after 10 min of H/H in an in vitro slice preparation that retains rhythmic respiratory activity. Before and during H/H, EPSCs in CVNs were completely blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and d(-)-2-amino-5-phosphonopentanoic acid (AP5), selective AMPA/kainate and N-methyl-d-apartate (NMDA) receptor blockers, respectively. However, after H/H, there was a significant increase in EPSCs during each inspiratory burst. While some of the inspiratory-related EPSCs were blocked by the broad purinergic receptor antagonist pyridoxalphosphate-6-azophenyl-2', 4'-disulphonic acid (PPADS) and the specific P2X receptor antagonist 2',3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate monolithium trisodium salt (TNP-ATP) a P2X receptor blocker, most of the recruited excitatory neurotransmission to CVNs is serotonergic because odansetron, a selective 5-HT3 antagonist, abolished the majority of the spontaneous and inspiratory-related EPSCs evoked during recovery from H/H. The results from this study suggest that following episodes of H/H, two nonglutamatergic excitatory pathways, purinergic and serotonergic, activating P2X and 5-HT3 receptors, respectively, are recruited to excite CVNs in the post H/H recovery period.

5-HT 2 receptor subtypes mediate different long-term changes in GABAergic activity to parasympathetic cardiac vagal neurons in the nucleus ambiguus

Serotonin (5-HT), and in particular 5-HT 2 receptors, play an important role in cardiorespiratory function within the brainstem. In addition, abnormalities in the 5-HT system have been implicated in many cardiorespiratory disorders, including sudden infant death syndrome. However, little is known about the mechanisms of action of 5-HT 2 receptors in altering the activity of parasympathetic cardiac neurons in the brainstem. In this study we examined the effects of activation of different subtypes of 5-HT 2 receptors on spontaneous and respiratory-evoked GABAergic neurotransmission to cardioinhibitory vagal neurons within the nucleus ambiguus as well as rhythmic fictive inspiratory-related activity in rats. A single application of α-Me-5-hydroxytryptamine maleate (α-Me-5-HT), a 5-HT 2 receptor agonist, did not significantly alter the frequency of spontaneous or respiratory-evoked GABAergic inhibitory postsynaptic currents (IPSCs) in cardiac vagal neurons. However, repetitive successive applications of α-Me-5-HT elicited a long-lasting (≥1 h) decrease in the frequency of spontaneous as well as inspiratory-related GABAergic IPSCs to cardiac vagal neurons. This study demonstrates multiple, but not single applications of the 5-HT 2 receptor agonist α-Me-5-HT caused a long-lasting inhibition of both spontaneous and fictive inspiratory-related GABAergic neurotransmission to CVNs, which can be prevented by the 5-HT 2B receptor antagonist SB204741, but persisted with the 5-HT 2A/2C receptor antagonist ketanserin. The 5-HT 2 receptor agonist α-Me-5-HT also reversibly and transiently excited central fictive inspiratory activity, which was abolished by ketanserin, but was unaffected by the 5-HT 2B receptor antagonist SB204741.

5-Hydroxytryptamine 1A/7 and 4α Receptors Differentially Prevent Opioid-Induced Inhibition of Brain Stem Cardiorespiratory Function

Opioids evoke respiratory depression, bradycardia, and reduced respiratory sinus arrhythmia, whereas serotonin (5-HT) agonists stimulate respiration and cardiorespiratory interactions. This study tested whether serotonin agonists can prevent the inhibitory effects of opioids on cardiorespiratory function. Spontaneous and rhythmic inspiratory-related activity and gamma-aminobutyric acid (GABA) neurotransmission to premotor parasympathetic cardioinhibitory neurons in the nucleus ambiguus were recorded simultaneously in an in vitro thick slice preparation. The mu-opioid agonist fentanyl inhibited respiratory frequency. The 5-hydroxytryptamine 1A/7 receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin increased respiratory frequency by itself and also prevented the fentanyl-induced respiratory depression. The 5-hydroxytryptamine 4alpha agonist BIMU-8 did not by itself change inspiratory activity but prevented the mu-opioid-mediated respiratory depression. Both spontaneous and inspiratory-evoked GABAergic neurotransmission to cardiac vagal neurons were inhibited by fentanyl. 8-Hydroxy-2-(di-n-propylamino)tetralin inhibited spontaneous but not inspiratory-evoked GABAergic activity to parasympathetic cardiac neurons. However, 8-hydroxy-2-(di-n-propylamino)tetralin differentially altered the opioid-mediated depression of inspiratory-evoked GABAergic activity but did not change the opioid-induced reduction in spontaneous GABAergic neurotransmission. In contrast, BIMU-8 did not alter GABAergic neurotransmission to cardiac vagal neurons by itself but prevented the fentanyl depression of both spontaneous and inspiratory-elicited GABAergic neurotransmission to cardiac vagal neurons. In the presence of tetrodotoxin, the inhibition of GABAergic inhibitory postsynaptic currents with fentanyl is prevented by coapplication of BIMU-8, indicating that BIMU-8 acts at presynaptic GABAergic terminals to prevent fentanyl-induced depression. These results suggest that activation of 5-hydroxytryptamine receptors, particularly 5-hydroxytryptamine 4alpha agonists, may be a useful therapeutic approach in preventing opioid-evoked cardiorespiratory depression.

Purinergic P2X Receptors Mediate Excitatory Transmission to Cardiac Vagal Neurons in the Nucleus Ambiguus After Hypoxia

Challenges such as hypoxia elicit a powerful response from both the central cardiovascular and respiratory neuronal networks. Recent work indicates that purinergic neurotransmission in the brain stem is an important modulator of central respiratory network responses to hypoxia. This study tests whether alterations in purinergic neurotransmission extend beyond respiratory responses to hypoxia and also mediates respiratory inputs to cardiac vagal neurons. To examine central cardiorespiratory responses to hypoxia, we used an in vitro medullary slice that allows simultaneous examination of rhythmic respiratory-related activity and synaptic neurotransmission to cardioinhibitory vagal neurons. Here we show that P2X receptor activation mediates respiratory-related excitatory neurotransmission to parasympathetic cardiac vagal neurons, the dominant control of heart rate. These data demonstrate a critical functional role for adenosine 5'-triphosphate-mediated purinergic signaling in facilitating respiratory-related excitatory neurotransmission to cardiac vagal neurons after hypoxia.

Activation of the diving reflex and excitation of cardiac vagal neurons (CVNs) in the nucleus ambiguus (NA)

Activation of the diving reflex and excitation of cardiac vagal neurons (CVNs) in the nucleus ambiguus (NA)

5‐HT2 receptor agonists modulate excitatory neurotransmission to cardiac vagal neurons within the nucleus ambiguus

5‐HT2 receptor agonists modulate excitatory neurotransmission to cardiac vagal neurons within the nucleus ambiguus

Effects of ATP on neurotransmission to rat cardiac vagal neurons in the nucleus ambiguus

Effects of ATP on neurotransmission to rat cardiac vagal neurons in the nucleus ambiguus