Stimulation of the ventrolateral medulla inhibits the baroreceptor input to the nucleus tractus solitarius

Mechanism of the inhibitory effect of electroacupuncture on experimental arrhythmias

Clinical observations reported that acupuncture can alleviate several kinds of arrhythmias. To explore its efficacy and mechanism, we have studied the electroacupuncture (EA) inhibition on experimental arrhythmias in rabbits since 1980s and analyzed its mechanism. These studies were mostly conducted in the Department of Physiology, Shanghai Medical University; recently the mechanism of acupuncture's effect on arrhythmias was analyzed in the School of Medicine, University of California, Irvine, which involves the following: (1)the inhibitory effect of EA on ventricular extrasystoles can be induced by hypothalamic defense area stimulation: a low-current and low-frequency stimulation of the median nerve underneath acupoints P 5 or deep peroneal nerve underneath S 36 can activate arcuate nucleus-ventral periaqueductal gray -nuclei raphe pathway and release endorphin, enkephaline, gamma-aminobutyric acid (GABA), 5-hydroxytryptamine (5-HT), etc., thus inhibiting the rostral ventrolateral medulla (rVLM), decreasing sympathetic outflow, and alleviating ventricular extrasystoles; (2) the bradycardia induced by stimulation of aortic depressor nerve can be blocked by stimulation of superficial radial nerve (underneath LI 6-7) or superficial peroneal nerve (underneath G37-39): these stimulations activate the rVLM release opioids and GABA to inhibit nucleus tractus solitarius and vagal nuclei, and block vagal bradycardia. These experimental data explore the mechanism of acupuncture's effect on arrhythmias and are useful for clinical application.

Blood pressure changes alter tracheobronchial cough: computational model of the respiratory-cough network and in vivo experiments in anesthetized cats

We tested the hypothesis, motivated in part by a coordinated computational cough network model, that alterations of mean systemic arterial blood pressure (BP) influence the excitability and motor pattern of cough. Model simulations predicted suppression of coughing by stimulation of arterial baroreceptors. In vivo experiments were conducted on anesthetized spontaneously breathing cats. Cough was elicited by mechanical stimulation of the intrathoracic airways. Electromyograms (EMG) of inspiratory parasternal, expiratory abdominal, laryngeal posterior cricoarytenoid (PCA), and thyroarytenoid muscles along with esophageal pressure (EP) and BP were recorded. Transiently elevated BP significantly reduced cough number, cough-related inspiratory, and expiratory amplitudes of EP, peak parasternal and abdominal EMG, and maximum of PCA EMG during the expulsive phase of cough, and prolonged the cough inspiratory and expiratory phases as well as cough cycle duration compared with control coughs. Latencies from the beginning of stimulation to the onset of cough-related diaphragm and abdominal activities were increased. Increases in BP also elicited bradycardia and isocapnic bradypnea. Reductions in BP increased cough number; elevated inspiratory EP amplitude and parasternal, abdominal, and inspiratory PCA EMG amplitudes; decreased total cough cycle duration; shortened the durations of the cough expiratory phase and cough-related abdominal discharge; and shortened cough latency compared with control coughs. Reduced BP also produced tachycardia, tachypnea, and hypocapnic hyperventilation. These effects of BP on coughing likely originate from interactions between barosensitive and respiratory brainstem neuronal networks, particularly by modulation of respiratory neurons within multiple respiration/cough-related brainstem areas by baroreceptor input.

Target site of inhibition of baroreflex vagal bradycardia by nasal stimulation

We have previously reported that stimulation of nasal mucosa inhibits baroreflex vagal bradycardia (BVB) and this inhibition was mediated exclusively by the trigeminal nerve, and occurred principally at pontomedullary level. In this study, to identify the target site of the inhibition, several types of experiments were conducted in chloralose-urethane-anesthetized, beta-adrenergic receptor-blocked rats. Afferent discharges in the ethmoidal nerve (EN5) were increased in response to nasal stimulation by smoke, and electrical stimulation of the EN5 suppressed BVB induced by electrical stimulation of the aortic depressor nerve (ADN). Electrical stimulation of the EN5 inhibited vagal bradycardia evoked by either electrical or chemical stimulation of the nucleus tractus solitarius (NTS), while it rather facilitated bradycardia by stimulation of the nucleus ambiguus (NA) region. Microstimulation of the NTS induced antidromic compound spike potential along the ADN but this was not affected by stimulation of the EN5. ADN-evoked field potentials and unitary responses of neurons in the NTS were suppressed by stimulation of the EN5. These results suggested that barosensitive neurons in the NTS are the major target sites of inhibition of BVB by nasal stimulation in rats.

GABAergic neurotransmission at the nucleus tractus solitarii in the suppression of reflex bradycardia by parabrachial nucleus

We investigated the role of GABAergic neurotransmission at the nucleus tractus solitarii (NTS) in the suppression of cardiac baroreceptor reflex (BRR) response induced by parabrachial nucleus (PBN) complex in adult Sprague-Dawley rats maintained under pentobarbital anesthesia. Based on in vivo microdialysis coupled with high-performance liquid chromatography-fluorescence detection for gamma-aminobutyric acid (GABA), we found that electrical stimulation of the ventrolateral regions and Koelliker-Fuse (KF) subnucleus of PBN complex resulted in a site-specific increase in GABA concentration in the dialysate collected from the NTS. The temporal increase in extracellular GABA concentration in the NTS coincided with the time course of PBN-induced cardiac BRR inhibition. In addition, the PBN-induced cardiac BRR suppression was reversed by microinjection bilaterally into the NTS of a GABA(A) receptor antagonist, bicuculline methiodide (5 pmol), or a GABA(B) receptor antagonist, 2-OH saclofen (500 pmol). Blockade of neuronal activity in the ventrolateral regions and KF subnucleus of PBN complex with lidocaine (5%) elicited an enhancement of the same reflex response. The time course of this facilitatory effect of lidocaine correlated positively with the temporal decrease in extracellular GABA concentration in the NTS. Anatomically, Fast Blue-labeled neurons were identified in the same subnuclei of the PBN complex after microinjection of the retrograde transport tracer into the NTS. Some of these Fast Blue-labeled neurons were also immunoreactive to glutamic acid decarboxylase. These results suggest that a direct GABAergic descending projection from the KF subnucleus and surrounding areas of the PBN complex to the NTS may inhibit cardiac BRR response by activating GABA(A) and GABA(B) receptors at the NTS.

Rostral ventrolateral medulla suppresses reflex bradycardia by the release of gamma-aminobutyric acid in nucleus tractus solitarii of the rat

We investigated the role of gamma-aminobutyric acid (GABA) in the nucleus tractus solitarii (NTS), the principal recipient of baroreceptor afferent fibers in the medulla oblongata, in the suppression of cardiac baroreceptor reflex (BRR) response by the rostral ventrolateral medulla (RVLM). Direct microinfusion via reverse microdialysis of L-glutamate (50 microM) into the RVLM promoted an inhibition of the BRR response, alongside an increase in the concentration of GABA in the dialysate collected from the ipsilateral NTS. Such an increase in GABA concentration in the NTS to RVLM activation was site-specific, as microinfusion of L-glutamate into areas outside the confines of RVLM resulted in no discernible change in GABA concentration in the dialysate of the NTS and minimal effect on the cardiac BRR response. The RVLM-induced BRR suppression of cardiac BRR response to microinjection into the bilateral RVLM of L-glutamate (1 nmol) was antagonized by administration into the bilateral NTS of the GABA(A) receptor antagonist, bicuculline methiodide (1 or 5 pmol), or the GABA(B) receptor antagonist, 2-hydroxy-saclofen (100 or 500 pmol). These results suggest that GABA released in the NTS may participate in cardiac BRR suppression induced by glutamatergic activation of the RVLM, via an action on both GABA(A) and GABA(B) receptor subtypes.

Parabrachial nucleus induces suppression of baroreflex bradycardia by the release of glutamate in the rostral ventrolateral medulla of the rat

The involvement of glutamatergic neurotransmission in the rostral ventrolateral medulla (RVLM) in the suppression of baroreflex bradycardia by the parabrachial nucleus (PBN) was investigated. Repeated electrical activation of the PBN increased the concentration of glutamate in the dialysate collected from the RVLM. The same stimulation also suppressed baroreflex bradycardia in response to transient hypertension evoked by phenylephrine (5 microg/kg, intravenously). Microinfusion of L-glutamate (10, 50 or 100 microM) via the microdialysis probe into the RVLM dose-dependently elicited a significant inhibition of baroreflex bradycardia that paralleled the concentration and time course of the PBN-elicited elevation in extracellular glutamate in the RVLM. The suppression of baroreflex bradycardia elicited by microinjection of L-glutamate (1 nmol) into the RVLM was appreciably reversed by coinjection of the NMDA receptor antagonist, dizocilpine (500 pmol), or the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2, 3-dione (50 pmol). These results suggest that an increase in the extracellular concentration of glutamate and activation of both NMDA and non-NMDA receptors in the RVLM may mediate the suppression of baroreflex bradycardia by activation of the PBN.

Subthreshold aortic nerve inputs to neurons in nucleus of the solitary tract

Subthreshold aortic nerve (AN) inputs to neurons receiving a monosynaptic AN-evoked input (MSNs: respond to each of two AN stimuli separated by 5 ms) and neurons receiving a polysynaptic AN input (PSNs) in the nucleus of the solitary tract (NTS) were identified in anesthetized rats. In extracellular recordings from 24 MSNs and 49 PSNs, 12% of MSNs and 29% of PSNs only responded to AN stimulation during the application of excitatory amino acids. In intracellular recordings from 24 MSNs and 22 PSNs, 12% of MSNs and 14% of PSNs responded to AN stimulation with excitatory postsynaptic potentials that did not evoke action potential discharge. Reductions in arterial pressure produced minimal changes in the spontaneous discharge of suprathreshold AN-evoked neurons, suggesting that these neurons receive excitatory inputs from nonbaroreceptor sources. The results suggest that some baroreflex-related NTS neurons exist in a "reserve state and can be changed to an active state or vice versa. This will change the number of neurons involved in baroreflex circuits and provides a novel mechanism for regulating baroreflex function independently of alterations in peripheral afferent input.

Glutamatergic projection to RVLM mediates suppression of reflex bradycardia by parabrachial nucleus

We investigated the role of glutamatergic projection from the parabrachial nucleus (PBN) complex to the rostral ventrolateral medulla (RVLM) in the PBN-induced suppression of reflex bradycardia in adult Sprague-Dawley rats that were maintained under pentobarbital anesthesia. Under stimulus conditions that did not appreciably alter the baseline systemic arterial pressure and heart rate, electrical (10-s train of 0.5-ms pulses, at 10-20 microA and 10-20 Hz) or chemical (L-glutamate, 1 nmol) stimulation of the ventrolateral regions and Köelliker-Fuse (KF) subnucleus of the PBN complex significantly suppressed the reflex bradycardia in response to transient hypertension evoked by phenylephrine (5 micrograms/kg iv). The PBN-induced suppression of reflex bradycardia was appreciably reversed by bilateral microinjection into the RVLM of the N-methyl-D-aspartate (NMDA)-receptor antagonist MK-801 (500 pmol) or the non-NMDA-receptor antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione (50 pmol). Anatomically, most of the retrogradely labeled neurons in the ventrolateral regions and KF subnucleus of the ipsilateral PBN complex after microinjection of fast blue into the RVLM were also immunoreactive to anti-glutamate antiserum. These results suggest that a direct glutamatergic projection to the RVLM from topographically distinct regions of the PBN complex may participate in the suppression of reflex bradycardia via activation of both NMDA and non-NMDA receptors at the RVLM.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

Effect of nucleus tractus solitarius lesions on cardiovascular responses elicited from the caudal ventrolateral medulla

Neurons in the caudal ventrolateral medulla (CVLM) which inhibit sympathetic vasomotor tone may have reciprocal connections with the nucleus tractus solitarius (nTS). This study determined whether changes in arterial pressure elicited by chemical excitation or inhibition of neurons in the vasodepressor region of the rabbit CVLM depend on the integrity of the nTS. Unilateral injections of L-glutamate (10 pmol to 100 nmol), or bilateral injections of GABA (1 nmol to 125 nmol), were made into the CVLM, and dose-response effects on arterial pressure determined. The nTS was then bilaterally cauterized, or inhibited by local injections of muscimol, and the dose-response curves were repeated. Neither cauterization nor injection of muscimol significantly altered the slope of the log dose-response curves for L-glutamate, but nTS muscimol increased the fall in arterial pressure for each dose of L-glutamate (P less than 0.01). Cauterization of the nTS significantly (P less than 0.01) increased the slope of the curve relating dose of GABA to rise in arterial pressure observed, after injection of GABA into the CVLM. This increase in slope was similar to the increase observed when GABA is injected into the CVLM in baroreceptor-denervated rabbits. We conclude that neither the depressor nor the pressor response evoked by stimulation or inhibition of the CVLM is dependent on the integrity of the nTS. Inactivation of the nTS tends to increase the magnitude of the CVLM responses, possibly by removal of baroreceptor-mediated buffering of the responses.

Caudal ventrolateral medullary projections to the nucleus of the solitary tract in the cat

The projections of neurons, in and around the A1 noradrenergic cell group of the caudal ventrolateral medulla (VLM), to nucleus of the solitary tract (NTS) were studied in the cat using the anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L). PHA-L was micro-iontophoresed into the region of the A1 noradrenergic cell group and after a 7-17 day survival period animals were sacrificed and brainstem sections were processed for PHA-L or tyrosine hydroxylase (TH) immunoreactivity. PHA-L injections within the region of the A1 cell group resulted in labelled fibers with their presumptive terminal boutons primarily in the ipsilateral commissural and medial subnuclei of NTS. A light projection to the ipsilateral parvocellular lateral and ventrolateral subnuclei of the NTS complex was also observed. These data demonstrate that neurons in the region of the A1 noradrenergic cell group project to regions of NTS that receive cardiovascular afferent inputs and suggest that VLM may influence the activity of neurons in NTS involved in the reflex regulation of the circulation.

Analgesia from electrical stimulation of the hypothalamic arcuate nucleus in pentobarbital-anesthetized rats

Inhibition of noxious heat-induced tail flick by electrical stimulation of the arcuate nucleus of the hypothalamus (ARH) was examined and characterized in pentobarbital-anesthetized rats. Systematic mapping studies revealed that inhibition of the tail flick reflex could be induced by stimulating widespread areas in the ventromedial parts of the hypothalamus, which include the paraventricular nucleus, ventromedial nucleus, dorsomedial nucleus, anterior hypothalamic area as well as the ARH areas. The ARH stimulation-produced tail flick suppression could be completely blocked by systemic naloxone (2 mg/kg) which shows the involvement of an opiate mechanism in this effect. Although the tail flick reflex in the lightly anesthetized state is of significantly shorter latency than in the unanesthetized state, thresholds of the ARH stimulation for suppressing spinal nociceptive reflexes in the lightly anesthetized state were not significantly different from the thresholds at the same ARH sites in the awake state.

Analgesic electrical stimulation of the hypothalamic arcuate nucleus: tolerance and its cross-tolerance to 2 Hz or 100 Hz electroacupuncture

Focal electrical stimulation of the arcuate nucleus of the hypothalamus (ARH) for 5 min (1 session) produced a marked elevation of tail flick latency (TFL) to noxious heat in the pentobarbital-anesthetized rat. Repeated stimulation for a total of 11 sessions at 30 min intervals resulted in a gradual decline in the hypoalgesic action, and this tolerance may last for 7 days. Tolerance to the ARH analgesic stimulation reduced the analgesia produced by low (2 Hz) but not high (100 Hz) frequency electroacupuncture (EA); and tolerance to low frequency EA analgesia attenuated the ARH stimulation-produced analgesia without affecting high frequency EA analgesia. Alternatively, rats tolerant to high-frequency EA analgesia were still sensitive to either the ARH or low-frequency EA stimulation. These results suggest that the ARH stimulation and low-frequency EA administration produced analgesia via a common neural mechanism, supporting our hypothesis put forward previously that the ARH plays an important role in mediating low- but not high-frequency EA analgesia.

Characterization of inhibition of spinal nociceptive reflex by stimulation of the arcuate nucleus of the hypothalamus in the pentobarbital-anesthetized rat

The effects of electrical and chemical stimulation of the arcuate nucleus of the hypothalamus (ARH) on the tail flick latency (TFL) and paw pressure withdrawal threshold (PWT) were investigated in the lightly pentobarbital-anesthetized and acutely prepared rat. Electrical stimulation of the ARH for 20 sec at 8 Hz produced a more potent elevation of the TFL (98%) and PWT (68%) compared to when stimulation was applied to the same site at 2 Hz (41% and 25%, respectively), 32 Hz (64% and 42%) and 128 Hz (57% and 39%). An even more marked and longer attenuation of the nociceptive reflexes was observed when the ARH stimulation was extended to a period of 1 or 3 min. Microinjection of the excitant amino acid, L-glutamate (0.5 M, 0.1 mul), into the same areas of the ARH consistently elicited antinociception to an extent similar to that observed with electrical stimulation. The data indicate that 8 Hz seems to be an optimal frequency for stimulating ARH to produce an analgesic effect as tested by the two spinal nociceptive reflexes.

Inhibition of baroreflex following microinjection of GABA or morphine into the nucleus tractus solitarii in rabbits

The possibility that putative transmitters may influence the aortic nerve stimulation-produced bradycardia and depressor responses was examined in urethane- and chloralose-anesthetized, paralyzed and artificially ventilated rabbits. The ipsilateral microinjection of gamma-aminobutyric acid (GABA, 2-4 micrograms) or morphine hydrochloride (2-4 micrograms) into the nucleus tractus solitarii (NTS) area could partially block the evoked bradycardia and depressor responses produced by stimulation of the aortic nerve without influencing the basal blood pressure and heart rate. This blocking effect of either GABA or morphine was dose-related. Pretreatment with GABA receptor antagonist bicuculline methiodide (0.15-0.20 micrograms) and opiate receptor antagonist naloxone hydrochloride (1-2 micrograms) into the same medullary area completely abolished the effect of GABA and morphine, respectively. Application of bicuculline also greatly antagonized the effect of morphine, but the blocking effect of GABA on the evoked bradycardia and depressor responses still existed following the pretreatment of naloxone. These results indicate that GABAergic and opiate systems present at the NTS exert an inhibitory influence on the evoked baroreflexes and inhibitory effect of the latter may be related to the activation of GABAergic receptor in this nucleus.