Stimulation Of Vagal Afferents Research Articles

The Use of Electrical Stimulation of the Vagus Nerve to Treat Morbid Obesity

Obesity has become one of the rising epidemics in westernized societies. Patients who are 100 pounds overweight are considered morbidly obese and are at risk for many conditions including hypertension, diabetes mellitus, cardiac disease, sleep apnea, and osteoarthritis. Behavior modification and pharmacologic treatments have not resulted in any lasting success in this group of patients. Surgical options, which include both gastric restriction and malabsorptive procedures, have reduced the number and severity of comorbidities in this population by providing sustained weight loss. However, with surgery, there is the potential for short-term and long-term complications. New approaches to weight loss are necessary. Afferent vagal nerve stimulation may provide a viable alternative to standard surgical weight loss procedures.

Cardiac sympathetic afferent activation provoked by myocardial ischemia and reperfusion. Mechanisms and reflexes.

Cardiac sympathetic afferents are known to reflexly activate the cardiovascular system, leading to increases in blood pressure, heart rate, and myocardial contractile function. During myocardial ischemia, these sensory nerves also transmit the sensation of pain (angina pectoris) and cause tachyarrhythmias. The authors' laboratory has been interested in defining the mechanisms of activation of this neural system during ischemia and reperfusion. During these periods, reactive oxygen species, particularly hydroxyl radicals, are produced from the breakdown of purine metabolites and lead to stimulation of sympathetic (and vagal) ventricular chemosensitive nerve endings. For example, stimulation with hydrogen peroxide leads to a small reflex increase in blood pressure from the predominant sympathetic afferent activation that is reduced by simultaneous activation of cardiac vagal afferents (known to exert predominantly depressor reflexes). Central integration of these two opposing reflexes likely occurs at several regions of the brain stem, including the nucleus tractus solitarii, where neural occlusion occurs during simultaneous cardiac sympathetic and vagal-afferent stimulation. Activation of platelets also appears to play a role during myocardial ischemia, leading to local release of serotonin (5HT), which, through a 5HT3 mechanism, stimulates sympathetic afferents. Finally, regional changes in pH from lactic acid (but not hypercapnia), stimulate ventricular afferents and may activate kallikrein to increase bradykinin (BK), which, in turn, breaks down arachidonic acid to form prostaglandins. Prostaglandins sensitize cardiac sympathetic afferents to BK. Thus, stimulation of cardiac sympathetic afferents during ischemia and reperfusion and the resulting reflex events form a multifactorial process resulting from activation of a number of chemical pathways in the myocardium.

Responses and afferent pathways of superficial and deeper c(1)-c(2) spinal cells to intrapericardial algogenic chemicals in rats.

Electrical stimulation of vagal afferents or cardiopulmonary sympathetic afferent fibers excites C(1)--C(2) spinal neurons. The purposes of this study were to compare the responses of superficial (depth <0.35 mm) and deeper C(1)--C(2) spinal neurons to noxious chemical stimulation of cardiac afferents and determine the relative contribution of vagal and sympathetic afferent pathways for transmission of noxious cardiac afferent input to C(1)--C(2) neurons. Extracellular potentials of single C(1)--C(2) neurons were recorded in pentobarbital anesthetized and paralyzed male rats. A catheter was placed in the pericardial sac to administer a mixture of algogenic chemicals (0.2 ml) that contained adenosine (10(-3) M), bradykinin, histamine, serotonin, and prostaglandin E(2) (10(-5) M each). Intrapericardial chemicals changed the activity of 20/106 (19%) C(1)--C(2) spinal neurons in the superficial laminae, whereas 76/147 (52%) deeper neurons responded to cardiac noxious input (P < 0.01). Of 96 neurons responsive to cardiac inputs, 48 (50%) were excited (E), 41 (43%) were inhibited (I), and 7 were excited/inhibited (E-I) by intrapericardial chemicals. E or I neurons responsive to intrapericardial chemicals were subdivided into two groups: short-lasting (SL) and long-lasting (LL) response patterns. In superficial gray matter, excitatory responses to cardiac inputs were more likely to be LL-E than SL-E neurons. Mechanical stimulation of the somatic field from the head, neck, and shoulder areas excited 85 of 95 (89%) C(1)--C(2) spinal neurons that responded to intrapericardial chemicals; 31 neurons were classified as wide dynamic range, 49 were high threshold, 5 responded only to joint movement, and no neuron was classified as low threshold. For superficial neurons, 53% had small somatic fields and 21% had bilateral fields. In contrast, 31% of the deeper neurons had small somatic fields and 46% had bilateral fields. Ipsilateral cervical vagotomy interrupted cardiac noxious input to 8/30 (6 E, 2 I) neurons; sequential transection of the contralateral cervical vagus nerve (bilateral vagotomy) eliminated the responses to intrapericardial chemicals in 4/22 (3 E, 1 I) neurons. Spinal transection at C(6)--C(7) segments to interrupt effects of sympathetic afferent input abolished responses to cardiac input in 10/10 (7 E, 3 I) neurons that still responded after bilateral vagotomy. Results of this study support the concept that C(1)-C(2) superficial and deeper spinal neurons play a role in integrating cardiac noxious inputs that travel in both the cervical vagal and/or thoracic sympathetic afferent nerves.

Estrogen blocks the cardiovascular and autonomic changes following vagal stimulation in ovariectomized rats

The current investigation examines the effect of acute and chronic estrogen administration on baroreflex sensitivity and autonomic tone following 2 h of vagal afferent stimulation in ovariectomized female rats. Female Sprague–Dawley rats were ovariectomized and supplemented daily for 7 days with either estrogen (OVX-E 2; 0.5 μg/kg; s.c.) or saline (OVX-S; 0.9%; s.c.). On the 8th day the animals were anaesthetized (sodium thiobutabarbitol; 100 mg/kg) and instrumented for recording blood pressure, heart rate and efferent vagal and renal nerve activities. The baroreflex was evoked using intravenous injection of various doses of phenylephrine hydrochloride (0.025, 0.05, 0.1 mg/kg). Electrical stimulation of vagal afferents for 2 h produces autonomic imbalance characterized by sympathoexcitation and parasympathetic withdrawal. This protocol of vagal stimulation produced a significant increase in renal nerve activity (from 20±6 to 140±20 spikes/2 s) and decreases in both vagal nerve activity (from 22±3 to 10±2 spikes/2 s) and baroreflex sensitivity (from 0.55±0.05 to 0.3±0.05) in OVX-S female rats. However, vagal stimulation had no effect on baroreflex sensitivity or autonomic nerve activities in OVX-E 2 rats. Administration of a single, bolus dose of estrogen (1×10 −2 mg/kg) to OVX-S rats immediately prior to termination of vagal stimulation blocked the changes in autonomic nerve activities and baroreflex sensitivity previously observed. These results suggest that both chronic and acute estrogen supplementation may provide resistance to the autonomic disturbances associated with visceral afferent activation.

Role of brainstem respiratory neuron types in phase-switching produced by afferent vagal stimulation.

A prominent feature of respiratory neural activity is the abrupt change of discharge at the phase transitions (IOS, inspiratory off-switch; EOS, expiratory off-switch). The observation of this nonlinearity has led to the search for specialized populations of brainstem neurons which change their activity immediately preceding a phase-switch. In this laboratory, we have used the method of producing perturbations of the respiratory cycle by means of phase-linked inputs and then comparing the temporal changes in the output (phrenic discharge) and in firing of different types of brainstem respiratory neurons. We here report the use of vagal (V) afferent inputs to study such responses.

Respiratory-related neuronal activity in the nucleus ambiguus-retroambigualis complex and their responses to peripheral and central stimulation in the rat

This study aimed to locate respiratory-related neurons in NA/NRA complex within the ventrolateral medulla (VLM), characterise them, and study their responses to the stimulation of midbrain periaque-ductal gray matter (PAG) and vagal afferents. Stereotactic mapping of the medullary NA/NRA complex using stainless steel microelectrodes was undertaken in Nembutal-anaesthetised (80 mg/kg), spontaneously-breathing rats (n = 60, 300-400 g). Individual cells and cell populations showing respiratory-related firing patterns were recorded along the VLM, 2.5 mm rostral -2.1 mm caudal to the obex, 1.4-2.2 mm lateral to midline and 1.5-3.00 mm below dorsal medullary surface. The cells were classified according to their temporal relationship with the diaphragm electromyogram (dEMG). Early inspiratory cells (eIcells) were found to start firing up to 20 ms before dEMG activity and to cease activity through mid-inspiration, exhibiting a decrementing discharge pattern. Late inspiratory (lateI) cells commenced their activity midway through inspiration and stopped firing prior to the cessation of dEMG. These cells exhibited an incrementing discharge pattern. Cells firing in phase with the dEMG have been classified as I-all cells and exhibit a steady impulse pattern throughout inspiration. Neurons active during the expiratory phase have been categorised as early expiratory cells (earlyE), late expiratory cells (lateE) and E-all neurons according to their discharge pattern in relation to dEMG. Cells in the dorso-lateral PAG were activated with microinjections of excitatory amino acid D,L-homocysteic acid (DLH0.2 M, 10-60 nl) and VLM respiratory neuronal and muscle responses were studied. PAG stimulation induced dose-dependent changes to respiratory output. Inspiratory and expiratory durations were shortened, respiratory frequency and dEMG activity increased along with increases in heart rate and blood pressure. Concurrently PAG stimulation led to increased activation of eI cells, I-all cells and E-all cells found in the NA/NRA complex. Abdominal muscle activity was not evident during quiet breathing though expiratory neuronal activity was seen. However, activation of PAG elicited synchronous aEMG bursts along with an increase in E-all cell activity. Unilateral vagal nerve stimulation, in order to simulate activation of the pulmonary stretch receptors, was undertaken. I-all cell and dEMG activity were inhibited upon stimulation of the vagus. However, I-all neuronal firing as well as the diaphragm activity resurfaced whilst vagal stimulation was prolonged. Expiratory cells are yet to be tested for responses to vagal stimulation. Our previous studies [1] demonstrated a physiological link between the PAG and NTS in the rat. The current investigation shows that chemical activation of PAG influences the discharge patterns of the VLM respiratory neurons within the NA/NRA complex suggesting a possible physiological link between the two centres. These results indicate that both NTS and NA/NRA respiratory neurons participate in the mediation of PAG induced respiratory changes. Roles for NA/NRA neurons in the brainstem control of respiration in the rat and the interactions between the neural centres in the midbrain are discussed.

Capsaicin-sensitive afferents and their role in gastroprotection: an update

The pivotal role of capsaicin-sensitive peptidergic sensory fibers in the maintenance of gastric mucosal integrity against injurious interventions was suggested by the authors 20 years ago. Since then substantial evidence has accumulated for the local sensory-efferent function of the released CGRP, tachykinins and NO in this gastroprotective mechanism. This overview outlines some recent achievements which shed light on new aspects and further horizons in this field. (1) Cloning the capsaicin VR-1 receptor (an ion channel-coupled receptor) and raising the VR-1 knockout mice provided a definite molecular background for the existence of capsaicin-sensitive afferents with both sensory and mediator releasing functions in the stomach. This cation channel is also sensitive to hydrogen ions. (2) VR-1 agonists (capsaicin, resiniferatoxin, piperine) protect against gastric ulcer of the rat parallel with their sensory stimulating potencies. (3) Antidromic stimulation of capsaicin-sensitive vagal and somatic afferents results in the release of CGRP, tachykinins, NO and somatostatin. Somatostatin with gastroprotective effect is released from D cells and sensory nerve endings. (4) The recent theory for the existence of spinal afferents without sensory function [P. Holzer, C.A. Maggi, Dissociation of dorsal root ganglion neurons into afferent and efferent-like neurons, Neuroscience 86 (1998) 389–398] is discussed. Data proposed to support this theory are interpreted here on the basis of a dual sensory-efferent function of VR-1 positive afferents, characterized by a frequency optimum of discharges for release vasodilatory neuropeptides below the nociceptive threshold. (5) Recent data on the effect of capsaicin in healthy human stomach are summarized. These results indicate that the gastroprotective effect of capsaicin in the human stomach involves additional mechanisms to those already revealed in the rat.

Bronchovascular Role In Pulmonary Congestion

1. A postulated role for the bronchial circulation in the development of pulmonary congestion may be based on recent studies of bronchovascular control. 2. The bronchial circulation is the nutrient blood supply of the conducting airways and, therefore, plays an important role in the function of the bronchial mucosa. Mucosal swelling secondary to elevation of mucosal capillary hydrostatic pressure may decrease airway calibre, increase resistance to airflow and precipitate symptoms of pulmonary congestion. 3. Resting mucosal capillary hydrostatic pressure is relatively constant due to autoregulation of bronchial blood flow and is maintained low by nett bronchovascular constriction due to the dominance of autonomic vasoconstriction over nitric oxidedependent vasodilatation. 4. Bronchial blood flow is also regulated by cardiac afferent reflexes. Stimulation of cardiac vagal and spinal afferents produces vasodilatation and vasoconstriction, respectively. Tonic activity of cardiac spinal afferents probably contributes to the resting autonomic vasoconstriction. 5. Therefore, mild heart failure, which is associated with abnormal cardiovascular reflex function, may decrease cardiac spinal afferent-mediated bronchial vasoconstriction and produce active dilatation due to stimulation of cardiac vagal afferents by excessive myocardial stretch, leading to bronchial mucosal swelling and pulmonary congestion.

Three types of putative presympathetic neurons in the rostral ventrolateral medulla studied with rat brainstem–spinal cord preparation

To study the electrophysiological properties of presympathetic neurons in the rostral ventrolateral medulla (RVLM), intracellular recordings were performed by the whole-cell patch-clamp technique. We utilized the neonatal rat brainstem–spinal cord preparation, in which the sympathetic neuronal network is thought to be preserved, unlike in slice preparation. In response to stimulation in the ipsilateral Th 2 spinal segment including intermediolateral cell column (IML), 33 of 151 non-respiratory RVLM neurons showed antidromic action potentials with a constant latency of 45 ms, and can be considered as presympathetic neurons. We classified and characterized the RVLM presympathetic neurons into three types: ‘regularly firing neurons ( n=7)’, which showed ramp depolarization and frequent action potentials (4.2±0.9 spikes/s) with rare excitatory postsynaptic potentials (EPSPs); ‘irregularly firing neurons ( n=21)’, which exhibited many EPSPs that modulated the firing rate; and ‘silent-type neurons ( n=5)’, which discharged action potentials only during current-induced depolarization. Lucifer-Yellow staining showed that the irregularly firing neurons were significantly larger and had more dendrites than the regularly firing neurons. All regularly firing neurons retained their discharges during low-Ca 2+–high-Mg 2+ superfusion that blocks synaptic input, whereas the discharges in 11 of 16 irregularly firing neurons were abolished, suggesting that the regularly firing neurons discharged independently of synaptic input. Seven of 31 RVLM neurons were hyperpolarized by stimulation of vagal afferent nerves. In summary, three types of RVLM presympathetic neurons were characterized by the patch-clamp technique in the brainstem–spinal cord preparation, in which the connection was preserved from vagal afferent to the Th 2 spinal segment through the RVLM. Since antidromic action potentials were demonstrated by stimulation in the Th 2 spinal segment in 33 neurons of all three types, all types of RVLM neurons constitute a part of the sympathetic neuronal network.

Chemoreflexes: an experimental study.

Transmyocardial laser revascularization (TMLR) will not denervate the heart, because it does not destroy all of the afferents. This study was designed to determine if stimulation of cardiac sympathetic and vagal afferents from an area of the left ventricle treated with TMLR could evoke reflex effects, and thus whether TMLR would denervate the heart. The effect of TMLR on reflexes evoked by chemically stimulating cardiac afferents was examined in 9 dogs. Bradykinin and capsaicin were applied topically or injected into the left anterior descending coronary artery before and after TMLR and after bilateral vagotomy and sympathectomy. Aortic (AoP) and left ventricular pressures (LVP) and electrocardiography were monitored. The first derivatives of LVP (dP/dt) were calculated. Topical bradykinin elicited variable hemodynamic responses. Topical capsaicin evoked pressor responses, increasing mean (+/- SEM) AoP (105+/-9 to 115+/-9 mm Hg; P<.001) and positive dP/dt (+dP/dt) (1032+/-81 to 1159+/-10 mm Hg/s; P<.01) before TMLR. Intracoronary capsaicin evoked a depressor response before TMLR. Pressor responses remained intact after TMLR with increases in mean AoP and +dP/dt (115+/-6 to 128+/-5 mm Hg and 1039+/-98 to 1136+/-100 mm Hg/s, respectively; P<.01). Depressor responses also remained intact after TMLR (91+/-10 vs 101+/-11 mm Hg [P<.02], and 865+/-104 vs 931+/-104 mm Hg/s [P<.05], respectively). Hemodynamic responses were diminished after bilateral vagotomy and abolished after bilateral sympathectomy. Since activation of cardiac afferent nerves and reflex responses remained intact after TMLR, but changed after vagotomy or sympathectomy, TMLR does not denervate the heart sufficiently to be the cause of improved angina after TMLR.

Habituation and desensitization of the Hering-Breuer reflex in rat.

1. Many processes in mammalian and invertebrate central nervous systems exhibit habituation and/or sensitization of their responses to repetitive stimuli. Here, we studied the adaptive behaviours of the respiratory pattern generator in rat on repetitive vagal-afferent stimulation and compared these behaviours obtained in vivo with the reported effects of such stimuli on synaptic transmission in the corresponding signal pathway in vitro. 2. Sustained (1 min) electrical pulsed stimulation of the vagus nerve elicited the classic Hering-Breuer (HB) reflex slowing of the respiratory rhythm followed by a bi-exponential recovery, and a post-stimulus rebound (PR). The recovery from the HB reflex satisfied the classic criteria of habituation. 3. The fast component of the recovery and the PR were abolished by systemic administration of an NMDA receptor antagonist or electrolytic lesioning of the pontine Kolliker-Fuse nucleus. The characteristics of the fast recovery and PR suggest a vagally induced desensitization of the NMDA receptor-dependent pontine input to the respiratory pattern generator. 4. The slow component of recovery persist after both experimental interventions and accounted for the habituation to the vagal input. The characteristics of the slow recovery in vivo were reminiscent of the reported synaptic accommodation in vitro in the medullary region where vagal afferents terminate. 5. The habituation of vagal input and desensitization of pontine input act in concert to offset the HB reflex. Such simultaneous habituation-desensitization in parallel neural pathways with differing sensitivities to NMDA receptor activation represent a hitherto unknown pairing of dual non-associative learning processes in the mammalian brain.

Fractal characteristics of end-expiratory lung volume in anesthetized rats.

To test the possibility that the variations in end-expiratory lung volume (EEV) might exhibit long-term correlation, we calculated the Hurst exponent H for EEV in anesthetized rats during sustained positive and negative transrespiratory pressure (i.e., (PTRP) and (NTRP), correspondingly) using the dispersional analysis method. In nine vagi-intact rats, H for EEV was 0.551+/-0.051 (mean+/-std) and 0.641 +/-0.076 (mean+/-std) with PTRP and NTRP, respectively. In both cases H was significantly different from those obtained after random shuffling of the original time series, indicating a positive long-term correlation in EEV. Also H with NTRP was significantly greater than that with PTRP (P=0.015). In six vagotomized rats, there was no significant difference in H for EEV between NTRP and PTRP. We demonstrated that long-term correlation is present in EEV. We inferred that the stimulation of vagal afferents could change the fractal property in EEV since different vagal pulmonary receptors are activated by PTRP and NTRP. Although the exact mechanisms for the fractal behavior are unknown, the results suggest that interaction of the pulmonary mechanoreceptor activity with the central respiratory pattern generator might contribute to the observed long-term correlation.

Sympathetic responses to cardiopulmonary vagal afferent stimulation during development.

Previous work in our laboratory has demonstrated impairment of cardiopulmonary reflex control of renal sympathetic nerve activity (RSNA) during the newborn period. The present study was designed to test the hypothesis that this delayed maturation is secondary to incomplete central integration of vagal afferent input. Term fetal (135-140 days; n = 6), newborn (3-7 days of age; n = 8), and young adult (6-8 wk old; n = 8) sheep anesthetized with alpha-chloralose underwent vagal afferent nerve stimulation. All animals had undergone prior sinoaortic denervation to eliminate influences from the arterial baroreceptors. After determination of optimal stimulation parameters, RSNA responses to gradual increases in stimulation frequency (1.0-16 Hz) were recorded and compared by one-way ANOVA. RSNA decreased progressively with increased frequency of stimulation in all three groups of animals. When comparing the three groups at any given frequency of stimulation, reflex withdrawal of RSNA tended to be more pronounced in newborn lambs (P < 0.05 for 1 and 4 Hz). Heart rate (HR) was also noted to decrease significantly with vagal afferent stimulation in each of the groups, but no significant differences in the reflex decreases in HR were noted among the three groups of animals. These results demonstrate that central integration of vagal afferent input is intact in fetal and newborn sheep. These results suggest that the delayed maturation of cardiopulmonary reflex-mediated changes in RSNA seen early in development appears to depend on intrinsic alterations in baroreceptor function rather than incomplete central integration.

Regulation of limbic motor seizures by GABA and glutamate transmission in nucleus tractus solitarius.

The nucleus of the solitary tract (NTS) is a primary site at which vagal afferents terminate. Because afferent vagal nerve stimulation has been demonstrated to have anticonvulsant effects, it is likely that changes in synaptic transmission in the NTS can regulate seizure susceptibility. We tested this hypothesis by examining the influence of gamma-aminobutyric acid (GABA) ergic and glutamatergic transmission in the NTS on seizures evoked by systemic and focal bicuculline and systemic pentylenetetrazol (PTZ) in rats. Muscimol (256 pmol), a GABA(A)-receptor agonist, bicuculline methiodide (177 pmol), a GABA(A)-receptor antagonist, kynurenate (634 pmol), a glutamate-receptor antagonist, or lidocaine (100 nl; 5%), a local anesthetic, was microinjected into the mediocaudal (m)NTS. Ten minutes later, seizure activity was induced by either a focal microinfusion of bicuculline methiodide (177 pmol) into the rostral piriform cortex, systemic PTZ (50 mg/kg, i.p.), or systemic bicuculline (0.35 mg/kg, i.v.). Muscimol in mNTS (but not in adjacent regions of NTS) attenuated seizures in all seizure models tested, whereas bicuculline methiodide into mNTS did not alter seizure responses. Kynurenate infusions into mNTS significantly reduced the severity of seizures evoked both systemically and focally. Anticonvulsant effects also were obtained with lidocaine application into the same region of mNTS. Unilateral injections were sufficient to afford seizure protection. Our results demonstrate that an increase in GABA transmission or a decrease in glutamate transmission in the rat mNTS reduces susceptibility to limbic motor seizures. This suggests that inhibition of mNTS outputs enhances seizure resistance in the forebrain and provides a potential mechanism for the seizure protection obtained with vagal stimulation.

GABA A receptor-mediated inspiratory termination evoked by vagal stimulation in decerebrate cats

To identify the GABAergic inhibitory mechanisms involved in inspiratory termination or off-switching (IOS), the effects of a specific enhancer of GABA A receptors, midazolam, and an antagonist, bicuculline, on vagally evoked inspiratory inhibitions and IOS were investigated in decerebrate cats. Stimulation of vagal afferents at late inspiration provoked either reversible inspiratory inhibition or IOS, depending on the stimulus intensity. Each response occurred at a constant latency (phase 1). The reversible response was triphasic, consisting of an early (phase 2) inhibition, a brief (phase 3) excitation and a late (phase 4) inhibition in the phrenic neurogram, and early (phase 2) IPSPs, brief (phase 3) EPSPs and late (phase 4) IPSPs in bulbar inspiratory (I) neurones. With an increasing stimulus intensity, phase 4 inhibitions were increased in amplitude and duration, leading to IOS. Midazolam (0.1 mg/kg i.v.) increased more selectively phase 4 IPSPs than phase 2 IPSPs in I neurones, and decreased the threshold for evoking IOS by producing an earlier and larger phase 4 IPSPs. Bicuculline (1.0 mg/kg i.v.) had an opposite effect. These results suggest that the late inhibitory response evoked by vagal stimulation in the I neuronal pool organizes an initial phase of IOS which is mediated by GABA A receptors.

Late vagal inhibition in neurons of the ventrolateral medulla oblongata in the rat

Stimulation of cervical vagal afferents elicits long-lasting inhibitory effects in a variety of neuronal populations, although little is known concerning the cellular mechanisms that are involved in these effects. In the present study, the electrophysiological characteristics of responses elicited by cumulative activation of vagal afferents were examined in neurons of the rostral ventrolateral medulla oblongata, which play an important role in the coordination of cardiovascular and other visceral activities. The study has focused on the late-onset, slow inhibitory component of vagal responses, which is likely to affect the temporal modulation of postsynaptic effects. Vagal stimulation elicited four distinct response patterns in intracellularly penetrated neurons ( n=78): excitation, inhibition, excitation–inhibition and inhibition–inhibition. The late inhibitory component was encountered in 43 (55%) of the cells, including five putative medullospinal neurons. It was due to a postsynaptic hyperpolarization which reversed at potentials more negative than −83 mV. The voltage dependency, as well as the average onset latency (93±3.0 ms), duration (270±16.5 ms) and amplitude (1.3±0.2 mV as measured at resting membrane potentials), of late inhibition were clearly different from those of the short-latency inhibitory response. The differences in the voltage dependency and time-course of the short-latency responses and the late inhibition indicate that they are mediated by different central relays. In the majority of neurons, late inhibition could be elicited by stimulating only myelinated vagal afferents. The magnitude of the response was, however, significantly enhanced in 63% of the examined cells when the intensity of stimulation was raised to recruit further myelinated and non-myelinated fibres. This indicates that late vagal inhibition is often elicited by a cumulative activation of convergent afferent inputs. The intracellularly labelled vagally responsive neurons were present at all rostrocaudal levels of the rostral ventrolateral medulla, with an accumulation in the region of the lateral paragigantocellular nucleus. Neurons that exhibited late vagal inhibition were dominant in the juxtafacial region of this nucleus. Due to its slow time-course, late vagal inhibition may contribute to a tonic modulation of the activity of neurons in the rostral ventrolateral medulla oblongata. It is proposed that late vagal inhibition plays an important role in the temporal integration of sensory inputs in neurons of the rostral ventrolateral medulla oblongata. The time-course and strength of this modulatory effect are related to the level of activity in those visceral sensory inputs that converge onto the inhibitory interneurons that mediate late inhibition to rostral ventrolateral medulla oblongata neurons.

Vagal modulation of responses elicited by stimulation of the aortic depressor nerve in neurons of the rostral ventrolateral medulla oblongata in the rat

Stimulation of cervical vagal afferents inhibits central sympathetic outflows in part by inhibiting the ongoing activity of putative baroreceptive neurons in the rostral ventrolateral medulla oblongata. The aim of the present study was to examine the electrophysiological characteristics of vagal responses and their interactions with responses elicited by stimulation of the aortic nerve in neurons there. The study focused on the role of the long-lasting, late-onset vagal inhibition, which is likely to play an important role in the tonic inhibitory effects of vagal afferent stimulation. In vivo intracellular recordings were obtained from 33 neurons that received convergent inputs from aortic and vagal afferents. Sixty-four percent of these neurons exhibited a late inhibition following electrical stimulation of myelinated vagal afferents (mean onset latency of 100±5 ms). The average duration of late inhibition (294±19 ms) exceeded the duration of the cardiac cycle. As a consequence of this, sustained vagal stimulation diminished the effect of rhythmic baroreceptor inputs in neurons that exhibited late vagal inhibition. Simultaneous activation of aortic and vagal afferents significantly increased the magnitude of late inhibition, even in those neurons where stimulation of the aortic nerve alone did not elicit a response ( n=15). This suggested that the convergence between vagal and aortic afferent inputs occurred in inhibitory interneurons antecedent to the recorded rostral ventrolateral medulla oblongata neurons. Focal stimulation of the caudal part of the nucleus of the solitary tract also elicited a late-onset inhibition in 73% of the neurons that responded to stimulation of the aortic nerve. This inhibition appeared to be similar to the late vagal inhibition, except for its shorter average onset latency (64±7 ms). Based on this observation, it is proposed that inhibitory interneurons that mediate late inhibition to rostral ventrolateral medulla oblongata neurons may lie within the caudal part of the nucleus of the solitary tract. The present study established that activation of myelinated vagal afferents exerts a complex modulation over the ongoing and evoked activity of neurons that respond to stimulation of the aortic nerve. The complex interaction that occurs between aortic and vagal inputs in neurons of the rostral ventrolateral medulla may be implicated in long-term modulation of sympathetic outflows in response to changes in the activation of visceral receptors supplied by vagus afferents. The modulation elicited by late vagal inhibition may help to adjust cardiovascular outflows according to requirements set by the thoraco-abdominal visceral environment.

Vagal afferents and active upper airway closure during pulmonary edema in lambs

The present study was undertaken to gain further insight into the mechanisms responsible for the sustained active expiratory upper airway closure previously observed during high-permeability pulmonary edema in lambs. The experiments were conducted in nonsedated lambs, in which airflow and thyroarytenoid and inferior pharyngeal constrictor muscle electromyographic activity were recorded. We first studied the consequences of hemodynamic pulmonary edema (induced by impeding pulmonary venous return) on upper airway dynamics in five lambs; under this condition, a sustained expiratory upper airway closure consistently appeared. We then tested whether expiratory upper airway closure was related to vagal afferent activity from bronchopulmonary receptors. Five bivagotomized lambs underwent high-permeability pulmonary edema: no sustained expiratory upper airway closure was observed. Finally, we studied whether a sustained decrease in lung volume induced a sustained expiratory upper airway closure. Five lambs underwent a 250-ml pleural infusion: no sustained expiratory upper airway closure was observed. We conclude that 1) the sustained expiratory upper airway closure observed during pulmonary edema in nonsedated lambs is related to stimulation of vagal afferents by an increase in lung water and 2) a decrease in lung volume does not seem to be the causal factor.

Inspiration-promoting vagal reflex under NMDA receptor blockade in anaesthetized rabbits.

1. This study describes a novel vagal respiratory reflex in anaesthetized rabbits. In contrast to the well-known inspiratory (I) off-switching by vagal afferent excitation, this vagal reflex initiates and maintains the central I activity of phrenic nerve discharges in rabbits pre-treated with antagonists of N-methyl-D-aspartate-type excitatory amino acid receptors (NMDA-Rs). 2. Under NMDA-R blockade with either dizocilpine (0.025-0.3 mg kg-1), D-2-amino-5-phosphonopentanoic acid (AP5, 0.5-1 mg, i.c.v.) or ketamine (10 mg kg-1), vagal stimulation at low frequencies (5-40 Hz) during the I phase prevented or markedly delayed the spontaneous I termination. In contrast, stimulation of the same vagal afferent at the same intensity but at a higher frequency (100-160 Hz) during the I phase immediately terminated the I phase. 3. In non-vagotomized rabbits, maintaining the tidal volume at end-expiratory levels during the I phase prevented spontaneous I termination and maintained apneusis after NMDA-R blockade with dizocilpine. 4. Brief stimulation of vagal afferents at low frequency (5-40 Hz) during the expiratory (E) phase constantly initiated phrenic I discharge after NMDA-R block. 5. We conclude that low-frequency discharge of vagal pulmonary stretch receptor afferents, as when lung volume is near functional residual capacity, promotes central I activity under NMDA-R blockade.

Evidence that NMDA receptors mediate the responses of putative RVLM presympathetic neurons to vagal afferent stimulation in rats

Experiments were performed in 25 urethane-anaesthetized male Sprague–Dawley rats. Forty-six RVLM neurons were identified as putative presympathetic neurons according to their arterial baroreceptor reflex related properties, i.e. they were inhibited by stimulation of the aortic nerve; silenced by elevation of blood pressure and had a cardiac cycle related rhythm of spontaneous discharge. Responses of these neurons to tripled square wave stimulation of vagal afferents was tested by means of peristimulus time histograms. In addition to a long-lasting inhibition (I 2), some neurons had one (P o) or two excitatory peaks (P 1 and P 2), and there was a short-lasting inhibition (I 1) between P 1 and P 2. After microinjection of CPP (0.1 μl, 50 mM) into the NTS, the inhibitory responses were blocked, but the excitatory peaks were not affected; in the CVLM, CPP microinjection of the same dose had a similar effect on the responses elicited by vagal afferent stimulation in 15 of the 24 neurons tested. No detectable effects were observed in 9 neurons. However, intravenously administered ketamine attenuated or abolished these responses, either inhibitory or excitatory, in a dose dependent way. These results suggest: (1) an NMDA mechanism is involved in both the inhibitory and the excitatory responses. For the inhibitory responses, the involvement is both in the NTS and in the CVLM; for the excitatory responses, it is probably in the RVLM. (2) There may be a direct excitatory amino acid (EAA) pathway from the NTS to the RVLM.