Cardiorespiratory Afferents Research Articles

Ionotropic and Metabotropic Glutamate Receptors Contribute to Cardiorespiratory Responses to Inhibition of Astrocytic Excitatory Amino Acid Transporters

The nucleus tractus solitarii (nTS) is the initial site of termination of cardiorespiratory afferents, which transmit signals through release of glutamate (Glu). Afferent information is integrated extensively in the nTS, thus shaping its output and cardiorespiratory responses. We have shown that inhibition of astrocytic excitatory amino acid transporter 2 (EAAT2) in the nTS increased ambient [Glu] and altered basal and reflex cardiorespiratory function. These effects were partially blocked by ionotropic glutamate receptor (iGluR) antagonism. We hypothesized that the responses remaining after iGluR blockade were due to activation of metabotropic (m)GluRs. To test this, Inactin‐anesthetized rats were instrumented to record mean arterial pressure (MAP), heart rate (HR), splanchnic sympathetic and phrenic nerve activity (SSNA; PhNA), and for microinjections into the nTS. The general EAAT inhibitor TFB‐TBOA (10 μM, 90 nl) was microinjected unilaterally into the nTS. GluR antagonists or vehicle (aCSF) were then microinjected, followed by two additional injections of TBOA. Unilateral nTS microinjection of TBOA decreased MAP, HR, PhNA and SSNA, consistent with effects of increased [Glu] in the nTS. Following aCSF, additional injections of TBOA produced augmented as well as prolonged cardiorespiratory responses. Prior blockade of iGluRs with a cocktail (90 nl) of NBQX (2 mM) and AP5 (10 mM) reduced the response to EAAT inhibition. However, it did not prevent the augmented effects of repetitive TBOA, indicating a role for other receptors. The effects of combined blockade of iGluRs and Group II/III mGluRs (MSOP, 100 mM + eGlu, 10 mM; 90 nl total) were similar to those of iGluR antagonism alone. In contrast, combined blockade of iGluRs and Group I mGluRs (LY367385, 10 mM; 90 nl total) blunted the cardiorespiratory effects of nTS EAAT inhibition, and markedly attenuated enhanced responses to subsequent injection of TBOA. These data are consistent with the concept that nTS inhibition of astrocytic EAATs increases [Glu], producing cardiorespiratory responses similar to excitation of nTS neurons involved in baroreflex function. These effects of EAAT inhibition are mediated primarily by iGluRs and Group I mGluRs. It is possible that Group II/III mGluRs also contribute via other mechanisms.Support or Funding InformationSupported by HL132836 and HL128454This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Endocannabinoids blunt the augmentation of synaptic transmission by serotonin 2A receptors in the nucleus tractus solitarii (nTS)

Serotonin (5-Hydroxytryptamine, 5-HT) and the 5-HT2 receptor modulate cardiovascular and autonomic function in part through actions in the nTS, the primary termination and integration point for cardiorespiratory afferents in the brainstem. In other brain regions, 5-HT2 receptors (5-HT2R) modify synaptic transmission directly, as well as through 5-HT2AR-induced endocannabinoid release. This study examined the role of 5-HT2AR as well as their interaction with endocannabinoids on neurotransmission in the nucleus tractus solitarii (nTS). Excitatory postsynaptic currents (EPSCs) in monosynaptic nTS neurons were recorded in the horizontal brainstem slice during activation and blockade of 5-HT2ARs. 5-HT2AR activation augmented solitary tract (TS) evoked EPSC amplitude whereas 5-HT2AR blockade depressed TS-EPSC amplitude at low and high TS stimulation rates. The 5-HT2AR-induced increase in neurotransmission was reduced by endocannabinoid receptor block and increased endogenous endocannabinoids in the synaptic cleft during high frequency, but not low, TS stimulation. Endocannabinoids did not tonically modify EPSCs. These data suggest 5-HT acting through the 5-HT2AR is an excitatory neuromodulator in the nTS and its effects are modulated by the endocannabinoid system.

Serotonin 2A receptors augment synaptic transmission in the nucleus tractus solitarii (nTS)

The nTS is the primary termination and integration point for cardiorespiratory afferents in the brainstem. Serotonin (5‐Hydroxytryptamine, 5‐HT) in the nTS plays a profound role in modulating synaptic and cardiovascular function. This study examined the role of the 5‐HT 2A receptor subtype in mediating neurotransmission in the nTS. Excitatory postsynaptic currents in monosynaptic nTS neurons were recorded in the horizontal brainstem slice in the presence of a 5‐HT2A receptor agonist (TCB‐2) or antagonist (MDL11939). TCB‐2 augmented solitary tract (TS) evoked EPSC amplitude elicited at 0.5 and 20 Hz. Conversely, MDL11939 depressed TS‐EPSCs amplitude evoked at 0.5 and 20 Hz. TCB‐2 and MDL11939 did not alter the TS‐EPSC paired‐pulse ratio, magnitude of depression from the first event or decay time. TCB‐2 did not alter miniature (m)EPSC amplitude or frequency, whereas MDL11939 increased mEPSC frequency but not amplitude. These data suggest the 5‐HT2A receptor is an excitatory neuromodulator in the nTS. Support: HL 085108.

Does the arousal system contribute to near death experience?

The neurophysiologic basis of near death experience (NDE) is unknown. Clinical observations suggest that REM state intrusion contributes to NDE. Support for the hypothesis follows five lines of evidence: REM intrusion during wakefulness is a frequent normal occurrence, REM intrusion underlies other clinical conditions, NDE elements can be explained by REM intrusion, cardiorespiratory afferents evoke REM intrusion, and persons with an NDE may have an arousal system predisposing to REM intrusion. To investigate a predisposition to REM intrusion, the life-time prevalence of REM intrusion was studied in 55 NDE subjects and compared with that in age/gender-matched control subjects. Sleep paralysis as well as sleep-related visual and auditory hallucinations were substantially more common in subjects with an NDE. These findings anticipate that under circumstances of peril, an NDE is more likely in those with previous REM intrusion. REM intrusion could promote subjective aspects of NDE and often associated syncope. Suppression of an activated locus ceruleus could be central to an arousal system predisposed to REM intrusion and NDE.

Optical recording of spatiotemporal patterns of cardiorespiratory neuronal activity in the nucleus tractus solitarii network imaged in situ

The pattern and distribution of neuronal activity evoked by sensory input in the nucleus tractus solitarii (NTS) is not well understood. In the present study, we performed optical recordings (MiCAM02 system, Brain Vision Inc) in situ to investigate the spatiotemporal pattern of NTS neuronal activity in response to activation of cardiorespiratory inputs. The dorsal medullary surface was exposed and stained with the voltage-sensitive dye, Di-2-ANEPEQ, at the level of calamus scriptorius. Graded electrical stimulation of the central vagus nerve and the brachial plexus (1 ms pulse, 200 ms duration, 1–10 Hz) at low frequency (<5 Hz) evoked a fast (50–75 ms), transient increase in optical activity that adapted rapidly; whereas, high frequency stimulation (>5 Hz) produced a sustained elevation of fluorescence that recovered following termination of the stimulus. In contrast, activation of peripheral chemoreceptors (NaCN, 0.03%) evoked a prolonged (4–6s) increase in optical activity that was superimposed by phasic changes in fluorescence coupled to the respiratory cycle. Synaptic blockade virtually abolished stimulus evoked fluorescence suggesting that optical signals represented post-synaptic activity. These results suggest that optical imaging is capable of detecting differences in spatiotemporal patterns of neuronal activity in the NTS evoked by distinct populations of cardiorespiratory afferents. This work was funded by NIH HL059167.

Integration of cornea and cardiorespiratory afferents in the nucleus of the solitary tract of the rat.

We determined the activity of neurons within the nucleus of the solitary tract (NTS) after stimulation of the cornea and assessed whether this input affected the processing of baroreceptor and peripheral chemoreceptor inputs. In an in situ, unanesthetized decerebrate working heart-brain stem preparation of the rat, noxious mechanical or electrical stimulation was applied to the cornea, and extracellular single unit recordings were made from NTS neurons. Cornea nociceptor stimulation evoked bradycardia and an increase in the cycle length of the phrenic nerve discharge. Of 90 NTS neurons with ongoing activity, corneal stimulation excited 51 and depressed 39. There was a high degree of convergence to these NTS neurons from either baroreceptors or chemoreceptors. The excitatory synaptic response in 12 of 19 baroreceptive and 10 of 15 chemoreceptive neurons was attenuated significantly during concomitant electrical stimulation of the cornea. This inhibition was GABA(A) receptor mediated, being blocked by pressure ejection of bicuculline. Thus the NTS integrates information from corneal receptors, some of which converges onto neurons mediating reflexes from baroreceptors and chemoreceptors to inhibit these inputs.

The nucleus of the solitary tract: an integrating station for nociceptive and cardiorespiratory afferents.

Painful stimuli can evoke dramatic responses in the cardiovascular and respiratory systems. We have assessed the role of both the sympathetic and parasympathetic nervous system in mediating the reflex tachycardia that accompanies somatic nociception. We describe a major role for the nucleus tractus solitarii (NTS) as a site for integrating nociceptive and cardiorespiratory afferents. Since cardiorespiratory and nociceptive afferents terminate in the NTS, this nucleus offers a powerful opportunity for central modulation. We show that the NTS plays a major role in mediating the reflex tachycardia evoked by somatic noxious stimulation. Similar noxious stimulation attenuates the cardiac component of the peripheral chemoreceptor reflex and inhibits the peripheral chemoreceptor-evoked excitatory synaptic response of some NTS neurones. The functional interpretation we propose is that by depressing homeostatic reflexes at the NTS, noxious stimulation-evoked cardiorespiratory changes can be expressed and maintained, which may be essential for the survival of the animal.

Properties of solitary tract neurons receiving inputs from the sub-diaphragmatic vagus nerve

Vagal afferents ascending from the gastrointestinal tract synapse on neurons in the nucleus of the solitary tract. Although these neurons constitute a significant proportion of solitary tract cells their firing behaviour and synaptic properties are not documented. Since gastrointestinal tract afferent termination sites overlap with regions mediating cardiorespiratory reflexes the possibility of convergence with afferents mediating cardiovascular and respiratory reflexes was proposed. Here we describe some electrophysiological and morphological properties of solitary tract neurons orthodromically driven from the sub-diaphragmatic vagus nerves and assess possible convergent inputs from cardiorespiratory afferents. Whole-cell recordings of solitary tract neurons responding to electrical stimulation of the sub-diaphragmatic vagus nerves (0.1–1 ms; 1–10 V; 2–20 Hz) were made in a working heart–brainstem preparation of rat. Baroreceptors were stimulated by raising pressure in the aorta or carotid sinus, whereas aortic injection of sodium cyanide (0.05% solution 25–50 μl) was used to activate peripheral chemoreceptors. Phrenic nerve activity and heart rate were monitored continuously. Of 88 solitary tract neurons tested, 39 responded with an evoked excitatory synaptic potential following stimulation of the sub-diaphragmatic vagus nerves. Resting membrane potential and input resistance of sub-diaphragmatic vagus nerve driven solitary tract neurons were 53.2±0.5 mV and 291+17 MΩ, respectively (mean±S.E.M.). Response latencies to sub-diaphragmatic vagus nerve stimulation were divided into two groups: <20 ms (16.0±2 ms, n=7; mean±S.E.M.) and >20 ms (77.3±5 ms, n=32). One additional neuron displayed an evoked inhibitory postsynaptic potential (latency 175 ms). Nineteen neurons showed ongoing activity which consisted of either irregular single action potential firing (0.5–10 Hz; n=12) or burst discharge ( n=7). Of 33 neurons tested, 17 showed spike frequency adaptation during injection of positive current, whereas 19 of 38 cells displayed rebound excitation following release from hyperpolarized potentials. There was no correlation between these properties and synaptic latencies. Ninety-one per cent of neurons tested displayed synaptic depression following paired pulse stimulation of the sub-diaphragmatic vagus nerve over intervals up to 500 ms. Stimulation of either baroreceptors ( n=31) or chemoreceptors ( n=36) failed to elicit a synaptic response in all sub-diaphragmatic vagus nerve-driven solitary tract neurons. Neurobiotin-labelled solitary tract neurons ( n=10) were from both latency groups and were located medial to the solitary tract at the level of area postrema, −0.3 mm to +1 mm from the obex. One cell was located in commissural subnucleus at midline, seven cells dorsal to the tractus solitarius and three ventral and medial to it. Soma sizes were 23±9.6×14±4.9 μm (range: 50×16 μm to 15×7 μm). The number of primary dendrites varied from three to five, secondary from one to eight and tertiary zero to four. Labelled axons were found in seven cells which ramified extensively in the solitary tract nucleus ( n=3) and/or branched extensively in the dorsal vagal motonucleus ( n=3) and/or projected towards the ventrolateral medulla ( n=3). We conclude that solitary tract neurons receiving signals from the sub-diaphragmatic vagus nerves (most likely from gastrointestinal tract structures) appear to be a distinct pool of neurons. There was a heterogeneity in terms of both their ongoing activity and projection targets but despite this, there were three consistent properties. First, sub-diaphragmatic vagus nerve evoked predominantly excitatory synaptic responses in solitary tract neurons; second, neurons exhibited lasting paired pulse depression following activation of sub-diaphragmatic vagus nerves; and third, sub-diaphragmatic vagus nerve-driven solitary tract neurons were not responsive to either baroreceptor or chemoreceptor stimulation.

Pattern of cardiorespiratory afferent convergence to solitary tract neurons driven by pulmonary vagal C-fiber stimulation in the mouse.

The central integration of signals from pulmonary vagal C-fibers (or type-J receptors) with those arising from cardiac, peripheral chemoreceptor, and baroreceptor afferents to neurons within the nucleus of the solitary tract (NTS) was studied in an arterially perfused working heart-brain stem preparation of adult mouse. Pulmonary vagal C-fibers were excited by right atrial injection of phenylbiguanide (PBG) while cardiac receptors were stimulated by left ventricular injection of veratridine (1-3 micrograms/kg) or mechanically by distension of the left ventricle (20-50 microl perfusate) using an indwelling cannula. Carotid body chemoreceptors were activated by aortic injection of Na cyanide, whereas baroreceptors were stimulated by increasing arterial perfusion pressure. Stimulation of pulmonary C-fibers and cardiac, chemo-, and baroreceptors all produced a reflex bradycardia (23-133 bpm). Central respiratory activity, as recorded from the phrenic nerve, was depressed by stimulating pulmonary C-fibers and cardiac and baroreceptors but enhanced in amplitude and frequency during chemoreceptor stimulation. Twenty-seven NTS neurons were excited and three were inhibited after pulmonary C-fiber stimulation displaying decrementing discharges with a peak firing frequency of up to 42 Hz (15 +/- 2.2 Hz, mean +/- SE) that lasted for 8.8 +/- 0.9 s. These responses occurred <1 s from the end of the PBG injection that was within the pulmonary circulation time. None of these cells responded to increases in right atrial pressure. All cells excited by PBG were also driven synaptically after electrical stimulation of the ipsilateral cervical vagus nerve at a latency of 32.9 +/- 3.2 ms (range 20-62 ms). None of these neurons had ongoing activity related to central respiratory activity. Convergence from cardiorespiratory afferents to 21 neurons driven by pulmonary C-fibers was tested. Twenty-five percent of cells were selectively excited by chemical stimulation of cardiac receptors alone, 19% were driven by peripheral chemoreceptors, and 38% responded to both cardiac and chemoreceptor activation. In contrast, only 13% of the cells activated by PBG injection responded to stimulation of baroreceptors and only 6% to cardiac mechanoreceptor stimulation. None of these neurons were activated by increasing right atrial pressure. The data indicate a high proportion of afferent convergence from pulmonary C-fibers, cardiac receptors, and peripheral chemoreceptors in the NTS. However, these neurons appear not to integrate inputs from cardiovascular mechanoreceptors. The significance of the data is discussed in relation to pathological disease states such as pulmonary congestion and cardiac failure.

Convergence properties of solitary tract neurones driven synaptically by cardiac vagal afferents in the mouse.

1. Cardiac vagal receptors are chemically and/or mechanically sensitive but it is unknown if this information is preserved centrally within the nucleus of the solitary tract (NTS). The present study had two aims: first, to investigate qualitatively whether both mechanically and chemically sensitive cardiac vagal encoding were preserved within the NTS, and second, to determine the patterns of convergence from other cardiorespiratory afferents to NTS neurones receiving cardiac vagal inputs. 2. The extracellular activity of single NTS neurones was investigated during stimulation of both chemically and mechanically sensitive cardiac vagal receptors in a working heart-brainstem preparation of mouse. Chemically sensitive cardiac receptors were stimulated using intra-left ventricular injections of either veratridine (1-3 microg kg-1), bradykinin (0.25-1 microg) or prostaglandin E2 (100-200 ng), whereas the left ventricle was distended to activate cardiac mechanoreceptors. 3. Forty-three NTS neurones were activated both synaptically by electrical stimulation of the ipsilateral vagus nerve (latency, 35 +/- 3 ms), and by intra-left ventricular injection of veratridine and also, in some cases, by bradykinin and/or PGE2. These NTS neurones were delineated into two populations based on their response to left ventricular distension and convergence properties. Left ventricular distension-insensitive neurones (n = 30) were excited by stimulation of carotid body chemoreceptors (81 %) but not arterial baroreceptors (3 %; i.e. n = 1 neurone), whereas distension-sensitive cells (n = 13) were activated mainly by baroreceptors (86 %) rather than peripheral chemoreceptors (14 %; i.e. n = 1 neurone). 4. The data reveal two distinct populations of NTS neurones receiving cardiac vagal inputs: (a) cells responsive to veratridine stimulation only, and (b) neurones activated by both veratridine and mechanical stimuli. The specific convergence pattern of baroreceptors and chemoreceptors to these cardioreceptive NTS neurones is discussed in relation to a common afferent modality integration within the NTS.

Vestibular afferents to the dorsal vagal complex: substrate for vestibular-autonomic interactions in the rat

Vestibular afferents to the nucleus tractus solitarii (NTS) were identified for the first time in the male Sprague-Dawley rat. Cells of vestibular origin were labeled by deposits of cholera toxin B (CT-B) centered on the general viscerosensory division of NTS and dorsal motor nucleus (DMX). Vestibular-visceral afferents derive from neurons concentrated at caudal levels of medial and inferior vestibular nuclei as observed in other species. Vestibular afferent processes were labeled in the NTS and DMX by anterograde transport of the tracer, biotinylated dextran-amine from injection deposits confined to the inferior and/or medial vestibular nuclei. Vestibular axons terminate in the NTS, predominantly at intermediate levels of the dorsal vagal complex. Projections overlapped sites in NTS that receive terminal input from first-order alimentary and cardiorespiratory afferents. The somato-visceral reflex circuit corroborates recent evidence in the rat of increases in functional activity in the vestibular nuclear complex and NTS in response to changes in gravito-inertial force [Kaufman, G.D., Anderson, J.H. and Beitz, A.J., J. Neurosci., 12 (1992) 4489–4500]. Vestibular input to the NTS and DMX may assist in compensating for the effects imposed by movements and gravity on breathing, alimentary reflex function and the systemic circulation.

Immunolabeling of Mu opioid receptors in the rat nucleus of the solitary tract: extrasynaptic plasmalemmal localization and association with Leu5-enkephalin.

Activation of the mu opioid receptor (MOR) by morphine within the caudal nucleus of the solitary tract (NTS) is known to mediate both cardiorespiratory and gastrointestinal responses. Leu5-enkephalin (LE), a potential endogenous ligand for MOR, is also present within neurons in this region. To determine the cellular sites for the visceral effects of MOR ligands, including LE, we used immunogold-silver and immunoperoxidase methods for light and electron microscopic localization of antisera against MOR (carboxyl terminal domain) and LE in the caudal NTS of rat brain. Light microscopy of coronal sections through the NTS at the level of the area postrema showed MOR-like immunoreactivity (MOR-LI) and LE labeling in punctate processes located within the subpostremal, dorsomedial and medial subnuclei. Electron microscopy of sections through the medial NTS at this level showed gold-silver particles identifying MOR-LI prominently distributed to the cytoplasmic side of the plasma membranes of axons and terminals. MOR labeled terminals formed mostly symmetric (inhibitory-type) synapses but sometimes showed multiple asymmetric junctions, characteristic of excitatory visceral afferents. MOR-LI was also present along extrasynaptic plasma membranes of dendrites receiving afferent input from unlabeled and LE-labeled terminals. We conclude that MOR ligands, possibly including LE, can act at extrasynaptic MORs on the plasma membranes of axons and dendrites in the caudal NTS to modulate the presynaptic release and postsynaptic responses of neurons. These are likely to include local inhibitory neurons and both gastric and cardiorespiratory afferents known to terminate in the subnuclei with the most intense MOR-LI.

Cardiac and Respiratory Correlations with Unit Discharge in Epileptic Human Temporal Lobe

We recorded respiratory activity and electrocardiogram (ECG) together with single cell activity from the amygdala and hippocampus of epileptic patients who later received anterotemporal lobectomy. Cross-correlation histograms were used to test for neuronal discharge timing relationships with inspiration or ECG. Linear regression was used to test for correlations of inspiratory time, respiratory period, and heart rate (HR) with tonic unit rate for each breath. Of 129 cells from 16 patients who later had resection, 89 were contralateral and 40 were ipsilateral to the resected lobe. Of the contralateral cells, 19% had a timing relationship with the cardiac cycle and only 1% had such a relationship with the respiratory cycle. Tonic correlations with HR were noted in 22% and with respiratory period and inspiratory time in 11 and 6%. Neither these percentages nor mean discharge rate differed between structures, although variance in rate was much higher on the resected side. Amygdala cells on the resected side showed more correlations with the cardiac cycle (55 vs. 20%), respiratory period (46 vs. 3%), and inspiratory time (27 vs. 7%) and were more likely to show several types of correlation. The results suggest a selective loss of ipsilateral amygdala cells and/or afferents, favoring relationships with cardiac- and respiratory-related systems and a possible synaptic reorganization of remaining cardiorespiratory afferents.