Efferent Vagal Stimulation Research Articles

Efferent vagal nerve stimulation attenuates acute lung injury following burn: The importance of the gut-lung axis

The purpose of this study was to assess acute lung injury when protection to the gut mucosal barrier offered by vagus nerve stimulation is eliminated by an abdominal vagotomy. Male balb/c mice were subjected to 30% total body surface area steam burn with and without electrical stimulation to the right cervical vagus nerve. A cohort of animals were subjected to abdominal vagotomy. Lung histology, myeloperoxidase and ICAM-1 immune staining, myeloperoxidase enzymatic assay, and tissue KC levels were analyzed 24 hours after burn. Additionally, lung IkB-α, NF-kB immunoblots, and NF-kB-DNA binding measured by photon emission analysis using NF-kB-luc transgenic mice were performed. Six hours post burn, phosphorylation of both NF-kB p65 and IkB-α were observed. Increased photon emission signal was seen in the lungs of NF-kB-luc transgenic animals. Vagal nerve stimulation blunted NF-kB activation similar to sham animals whereas abdominal vagotomy eliminated the anti-inflammatory effect. After burn, MPO positive cells and ICAM-1 expression in the lung endothelium was increased, and lung histology demonstrated significant injury at 24 hours. Vagal nerve stimulation markedly decreased neutrophil infiltration as demonstrated by MPO immune staining and enzyme activity. Vagal stimulation also markedly attenuated acute lung injury at 24 hours. The protective effects of vagal nerve stimulation were reversed by performing an abdominal vagotomy. Vagal nerve stimulation is an effective strategy to protect against acute lung injury following burn. Moreover, the protective effects of vagal nerve stimulation in the prevention of acute lung injury are eliminated by performing an abdominal vagotomy. These results establish the importance of the gut-lung axis after burn in the genesis of acute lung injury.

Potential Electrical Targets For Neurogenic Syncope: A Proof of Concept

The cervical vagus nerve (VN) conducts efferent and afferent signals involved in neurocardiogenic syncope. We investigated whether direct current impulses (DCI) block effects of VN stimulation (VNS). Methods In anesthetized dogs (n=7) VN were instrumented with bipolar electrode proximally and distally, to do VNS and DCI. We measured changes in heart rate (HR) and mean arterial blood pressure (BP). Results Proximal efferent VNS (20Hz, 0.9–2.5V) reduced the resting HR by 57–71%. Distal DCI (2.2–4.5V) restored HR to 82–94% of baseline rate within 1–2 seconds and was sustained over the duration of the stimulus (15–30 seconds). BP dropped substantially with bradycardia (34–35% of baseline) but no significant changes in BP occurred while HR was maintained by pacing. Distal R-VNS produced a moderate decrease 9–18 mmHg in BP while L-VNS produced an increase of 20–48 mmHg. which was also attenuated by by proximal DCI. Conclusion It is feasible to interrupt vagal-mediated bradycardia and hypotension with appropriately delivered electrical impulses. Further study is needed to evaluate the long-term effects of DCI before translation into viable therapy.

Endocardial vagal atrioventricular node stimulation in humans: reproducibility on 18-month follow-up

Control of atrioventricular (AV) node conduction by means of high-frequency stimulation (HFS) of efferent AV node vagal stimulation (AVNS) fibres enables the ventricular rate (VR) to be modulated during atrial fibrillation (AF). The aims of this study were to verify, on 18-month follow-up, the reproducibility of the dromotropic effect obtained on implantation and the long-term reliability of the system in patients who received an implantable cardioverter-defibrillator (ICD) with a standard atrial lead positioned at a location suitable for AVNS. We enrolled 12 patients with paroxysmal or persistent AF who were candidates for ICD. The right atrium was mapped to locate the pacing site, and a transvenous screw-in lead was implanted in that region. The voltages required for VR modulation (25% VR reduction) and complete AV block at different pulse durations (from 0.1 to 0.5 ms) were recorded. Eleven out of 12 patients underwent 18-month follow-up examination. Atrial pacing parameters were adequate and did not differ from the baseline values (all P > 0.05): pacing threshold 0.9 ± 0.5 V (0.5 ms pulse duration) and impedance 556 ± 121 Ω, with P-wave amplitude of 1.6 ± 0.7 mV. High-frequency stimulation induced VR modulation in nine patients and complete AV block in eight patients at pulse durations ≥0.3 ms. No differences were observed in the voltages for VR modulation and complete AV block between implantation and 18-month examination (all P > 0.100). Ventricular rate control during AF was obtained under HFS 18 months after implantation in patients with the atrial lead positioned at a location suitable for AVNS. The pacing outputs needed to achieve the dromotropic effect were comparable to those measured on implantation.

Efferent and afferent vagal actions on cortical blood flow and kainic acid-induced seizure activity in urethane anesthetized rats

Autonomic dysfunction during seizures can induce bradyarrhythmia via efferent vagal overactivity. We studied cardiovascular, brain blood flow, and electroencephalographic consequences of vagal stimulation during seizures in rats. Efferent vagal stimulation reduced seizure activity, completely suppressing it at high frequencies, by reducing heart rate, arterial pressure, and cortical blood flow. Afferent vagal activation was more variable, and the highest stimulation frequencies also appeared to reduce cortical blood flow. We conclude that efferent vagal activity can arrest ongoing seizure activity by ultimately decreasing hippocampal blood flow. Afferent vagal activity (which does not occur during seizures) may have a similar action.

Differential regulation of TNF receptors by vagal nerve stimulation protects heart against acute ischemic injury

Vagal nerve stimulation (VS) has been reported to improve the survival after both acute and chronic myocardial infarction through the release of neurotransmitter ACh. However, the precise mechanism behind its beneficial effect is still unknown. In this study, we demonstrate the upregulation of tumor necrosis factor-alpha (TNF-α) and its cell survival TNF receptor-2 (TNFR2) as the mechanism behind VS induced myocardial protection.We investigated the effects of efferent VS on myocardial ischemic injury with in vivo and in vitro mouse models. In in vivo hearts VS significantly increased the expression of TNF-α both at the messenger and protein level after 3-hours of myocardial ischemia. In the in vitro studies ACh treatment before hypoxia, induced a significant upregulation of TNF-α compared to the untreated cardiomyocytes. Immunofluorescence analysis confirmed the synthesis of TNF-α by cardiomyocytes both in vivo and in vitro. VS also significantly reduced the myocardial infarct size (23.9±5.7% vs. 56±1.9%) and activated the cell survival Akt cascade system. Further, ACh upregulated the cell survival TNFR2 expression, while downregulating the cell destructive TNF receptor 1 (TNFR1) expression. These results were confirmed using the TNF receptors deficient mice, where the VS mediated protection was lost both in vivo and in vitro in TNFR2 (TNFR2-/-) and TNF receptors double knock out (TNFR1-/-2-/-) mice.VS and ACh protects the heart against acute ischemia or hypoxic injury by differentially regulating the TNF receptor subtypes.

Vagal nerve stimulation prevents reperfusion injury through inhibition of opening of mitochondrial permeability transition pore independent of the bradycardiac effect

In spite of recent advances in coronary interventional therapy, reperfusion injury is still considered to be a major problem in patients undergoing surgical procedures, such as bypass grafting. Here we demonstrate a novel therapeutic strategy against ischemia-reperfusion injury: vagally mediated prevention of reperfusion-induced opening of mitochondrial permeability transition pore. We investigated the effects of efferent vagal stimulation on myocardial reperfusion injury with ex vivo and in vitro rat models. In the ex vivo model the hearts were perfused with intact vagal innervation, which allowed us to study the effects of the vagal nerve on the heart without other systemic effects. Compared with sham stimulation, vagal stimulation exerted a marked anti-infarct effect irrespective of the heart rate (34% +/- 6% vs 85% +/- 9% at a heart rate of 300 beats/min, 37% +/- 4% vs 43% +/- 5% at a heart rate of 250 beats/min, and 39% +/- 4% vs 88% +/- 7% at a heart rate of 350 beats/min) after a 30-minute period of global ischemia, activated cell-survival Akt cascade, prevented downregulation of the antiapoptotic protein Bcl-2, and suppressed cytochrome-c release and caspase-3 activation. Furthermore, vagal stimulation-treated hearts exhibited a significant improvement in left ventricular developed pressure (78 +/- 5 vs 45 +/- 8 mm Hg) and a significant attenuation in an incremental change in left ventricular end-diastolic pressure during reperfusion. These beneficial effects of vagal stimulation were abolished by a permeability transition pore opener, atractyloside. In the in vitro study with primary-cultured cardiomyocytes, acetylcholine prevented a reoxygenation-induced collapse in mitochondrial transmembrane potential through inhibition of permeability transition pore opening. Vagal stimulation would be a potential adjuvant therapy for the rescue of ischemic myocardium from reperfusion injury, and the protective effects are independent of its bradycardiac effects.

Functional Changes in Baroreceptor Afferent, Central, and Efferent Components of the Baroreflex Circuitry in Type 1 Diabetic Mice (OVE26)

Baroreflex control of heart rate (HR) is impaired in diabetes mellitus. We hypothesized that diabetes induced functional changes of the neural components at multiple sites within the baroreflex arc. OVE26 diabetic and FVB control mice were anesthetized. Baroreflex‐mediated HR responses to sodium nitroprusside (SNP)‐ and phenylephrine (PE)‐induced mean arterial blood pressure (MAP) changes were measured. Baroreceptor afferent function was characterized by measuring the percent change of the baseline of integrated aortic depressor nerve activity (Int ADNA) in response to MAP changes. The HR responses to electrical stimulation of the left aortic depressor nerve and the right vagus nerve were assessed. Compared to FVB control mice, we found in OVE26 mice that (1) Reflex‐mediated bradycardia and tachycardia were reduced (p<0.05). (2) The baroreceptor afferent function in response to MAP increase did not differ (p>0.05), as assessed by the parameters of the logistic function curve. But, the inhibition of Int ADNA in response to MAP decrease was attenuated (p<0.05). (3) The maximum amplitude of HR responses to right vagal efferent stimulation was augmented (P<0.05). (4) The maximum amplitude of HR responses to left ADN stimulation was decreased (p<0.05). We conclude that a selective deficit of central mediation of reflex bradycardia contributes to the overall attenuation of baroreflex control of HR in OVE26 mice.

Functional changes in baroreceptor afferent, central and efferent components of the baroreflex circuitry in type 1 diabetic mice (OVE26)

Baroreflex control of heart rate (HR) is impaired in diabetes mellitus. We hypothesized that diabetes mellitus induced functional changes of neural components at multiple sites within the baroreflex arc. Type 1 diabetic mice (OVE26) and FVB control mice were anesthetized. Baroreflex-mediated HR responses to sodium nitroprusside- (SNP) and phenylephrine- (PE) induced mean arterial blood pressure (MAP) changes were measured. Baroreceptor function was characterized by measuring the percent (%) change of baseline integrated aortic depressor nerve activity (Int ADNA) in response to SNP- and PE-induced MAP changes. The HR responses to electrical stimulation of the left aortic depressor nerve (ADN) and the right vagus nerve were assessed. Compared with FVB control mice, we found in OVE26 mice that (1) baroreflex-mediated bradycardia and tachycardia were significantly reduced. (2) The baroreceptor afferent function in response to MAP increase did not differ, as assessed by the parameters of the logistic function curve. But, the inhibition of Int ADNA in response to MAP decrease was significantly attenuated. (3) The maximum amplitude of bradycardic responses to right vagal efferent stimulation was augmented. (4) In contrast, the maximum amplitude of bradycardic responses to left ADN stimulation was decreased. Since Int ADNA was preserved in response to MAP increase and HR responses to vagal efferent stimulation were augmented, we conclude that a deficit of the central mediation of baroreflex HR contributes to the overall attenuation of baroreflex sensitivity in OVE26 mice. The successful conduction of physiological experiments on the ADN in OVE26 mice may provide a foundation for the understanding of cellular and molecular mechanisms of diabetes-induced cardiac neuropathy.

Efferent vagal nerve stimulation induces tissue inhibitor of metalloproteinase-1 in myocardial ischemia-reperfusion injury in rabbit

Vagal nerve stimulation has been suggested to ameliorate left ventricular (LV) remodeling in heart failure. However, it is not known whether and to what degree vagal nerve stimulation affects matrix metalloproteinase (MMP) and tissue inhibitor of MMP (TIMP) in myocardium, which are known to play crucial roles in LV remodeling. We therefore investigated the effects of electrical stimulation of efferent vagal nerve on myocardial expression and activation of MMPs and TIMPs in a rabbit model of myocardial ischemia-reperfusion (I/R) injury. Anesthetized rabbits were subjected to 60 min of left coronary artery occlusion and 180 min of reperfusion with (I/R-VS, n = 8) or without vagal nerve stimulation (I/R, n = 7). Rabbits not subjected to coronary occlusion with (VS, n = 7) or without vagal stimulation (sham, n = 7) were used as controls. Total MMP-9 protein increased significantly after left coronary artery occlusion in I/R-VS and I/R to a similar degree compared with VS and sham values. Endogenous active MMP-9 protein level was significantly lower in I/R-VS compared with I/R. TIMP-1 mRNA expression was significantly increased in I/R-VS compared with the I/R, VS, and sham groups. TIMP-1 protein was significantly increased in I/R-VS and VS compared with the I/R and sham groups. Cardiac microdialysis technique demonstrated that topical perfusion of acetylcholine increased dialysate TIMP-1 protein level, which was suppressed by coperfusion of atropine. Immunohistochemistry demonstrated a strong expression of TIMP-1 protein in cardiomyocytes around the dialysis probe used to perfuse acetylcholine. In conclusion, in a rabbit model of myocardial I/R injury, vagal nerve stimulation induced TIMP-1 expression in cardiomyocytes and reduced active MMP-9.

Selective impairment of central mediation of baroreflex in anesthetized young adult Fischer 344 rats after chronic intermittent hypoxia

Baroreflex control of heart rate (HR) is impaired after chronic intermittent hypoxia (CIH). However, the location and nature of this response remain unclear. We examined baroreceptor afferent, vagal efferent, and central components of the baroreflex circuitry. Fischer 344 (F344) rats were exposed to room air (RA) or CIH for 35-50 days and were then anesthetized with isoflurane, ventilated, and catheterized for measurement of mean arterial blood pressure (MAP) and HR. Baroreceptor function was characterized by measuring percent changes of integrated aortic depressor nerve (ADN) activity (Int ADNA) relative to the baseline value in response to sodium nitroprusside- and phenylephrine-induced changes in MAP. Data were fitted to a sigmoid logistic function curve. HR responses to electrical stimulation of the left ADN and the right vagus nerve were assessed under ketamine-acepromazine anesthesia. Compared with RA controls, CIH significantly increased maximum baroreceptor gain or maximum slope, maximum Int ADNA, and Int ADNA range (maximum - minimum Int ADNA), whereas other parameters of the logistic function were unchanged. In addition, CIH increased the maximum amplitude of bradycardic response to vagal efferent stimulation and decreased the time from stimulus onset to peak response. In contrast, CIH significantly reduced the maximum amplitude of bradycardic response to left ADN stimulation and increased the time from stimulus onset to peak response. Therefore, CIH decreased central mediation of the baroreflex but augmented baroreceptor afferent function and vagal efferent control of HR.

Chronic intermittent hypoxia impairs baroreflex control of heart rate but enhances heart rate responses to vagal efferent stimulation in anesthetized mice

Chronic intermittent hypoxia (CIH) leads to increased sympathetic nerve activity and arterial hypertension. In this study, we tested the hypothesis that CIH impairs baroreflex (BR) control of heart rate (HR) in mice, and that decreased cardiac chronotropic responsiveness to vagal efferent activity contributes to such impairment. C57BL/6J mice were exposed to either room air (RA) or CIH (6-min alternations of 21% O(2) and 5.7% O(2), 12 h/day) for 90 days. After the treatment period, mice were anesthetized (Avertin) and arterial blood pressure (ABP) was measured from the femoral artery. Mean ABP (MABP) was significantly increased in mice exposed to CIH (98.7 +/- 2.5 vs. RA: 78.9 +/- 1.4 mmHg, P < 0.001). CIH increased HR significantly (584.7 +/- 8.9 beats/min; RA: 518.2 +/- 17.9 beats/min, P < 0.05). Sustained infusion of phenylephrine (PE) at different doses (0.1-0.4 microg/min) significantly increased MABP in both CIH and RA mice, but the ABP-mediated decreases in HR were significantly attenuated in mice exposed to CIH (P < 0.001). In contrast, decreases in HR in response to electrical stimulation of the left vagus nerve (30 microA, 2-ms pulses) were significantly enhanced in mice exposed to CIH compared with RA mice at low frequencies. We conclude that CIH elicits a sustained impairment of baroreflex control of HR in mice. The blunted BR-mediated bradycardia occurs despite enhanced cardiac chronotropic responsiveness to vagal efferent stimulation. This suggests that an afferent and/or a central defect is responsible for the baroreflex impairment following CIH.

Hypothermia reduces ischemia- and stimulation-induced myocardial interstitial norepinephrine and acetylcholine releases

Although hypothermia is one of the most powerful modulators that can reduce ischemic injury, the effects of hypothermia on the function of the cardiac autonomic nerves in vivo are not well understood. We examined the effects of hypothermia on the myocardial interstitial norepinephrine (NE) and ACh releases in response to acute myocardial ischemia and to efferent sympathetic or vagal nerve stimulation in anesthetized cats. We induced acute myocardial ischemia by coronary artery occlusion. Compared with normothermia (n = 8), hypothermia at 33 degrees C (n = 6) suppressed the ischemia-induced NE release [63 nM (SD 39) vs. 18 nM (SD 25), P < 0.01] and ACh release [11.6 nM (SD 7.6) vs. 2.4 nM (SD 1.3), P < 0.01] in the ischemic region. Under hypothermia, the coronary occlusion increased the ACh level from 0.67 nM (SD 0.44) to 6.0 nM (SD 6.0) (P < 0.05) and decreased the NE level from 0.63 nM (SD 0.19) to 0.40 nM (SD 0.25) (P < 0.05) in the nonischemic region. Hypothermia attenuated the nerve stimulation-induced NE release from 1.05 nM (SD 0.85) to 0.73 nM (SD 0.73) (P < 0.05, n = 6) and ACh release from 10.2 nM (SD 5.1) to 7.1 nM (SD 3.4) (P < 0.05, n = 5). In conclusion, hypothermia attenuated the ischemia-induced NE and ACh releases in the ischemic region. Moreover, hypothermia also attenuated the nerve stimulation-induced NE and ACh releases. The Bezold-Jarisch reflex evoked by the left anterior descending coronary artery occlusion, however, did not appear to be affected under hypothermia.

Vagal afferent stimulation as a cardioprotective strategy? Introducing the concept.

The effect of vagal afferent signaling on cardioinhibition has been well known for over 130 years. Both experimental and clinical studies have demonstrated not only the potential adverse effect of unrestrained sympathoexcitation in high risk patients with ischemic heart disease but the potential for cardioprotection by programmed vagal activity. The vasodepressor and negative chronotropic effects of efferent vagal stimulation has been a cause for concern. However it is becoming clear that favorable shifts towards increased cardiac vagal modulation can be achieved by vagal afferent nerve stimulation. This phasic effect appears to operate though central medullary pathways. Thus by engaging vagal afferent fibers in humans there is the possibility that one can exploit the benefits of central cardioinhibition without adversely affecting heart rate, respiration or hemodynamics. This commentary explores the background and rationale for considering vagal afferent stimulation as a plausible cardioprotective strategy.

Thoracic Vagal Efferent Nerve Stimulation Evokes Substance P-Induced Early Airway Bronchonstriction and Late Proinflammatory and Oxidative Injury in the Rat Respiratory Tract

Electrical stimulation of efferent thoracic vagus nerve (TVN) evoked neurogenic inflammation in respiratory tract of atropine-treated rats by an undefined mechanism. We explored whether efferent TVN stimulation via substance P facilitates neurogenic inflammation via action of nuclear factor-kappaB (NF-kappaB) activation and reactive oxygen species (ROS) production. Our results showed that increased frequency of TVN stimulation concomitantly increased substance P-enhanced hypotension, and bronchoconstriction (increases in smooth muscle electromyographic activity and total pulmonary resistance). The enhanced SP release evoked the appearance of endothelial gap in silver-stained leaky venules, India-ink labeled extravasation, and accumulations of inflammatory cells in the respiratory tract, contributing to trachea plasma extravasation as well as increases in blood O (2)(-) and H(2)O(2) ROS amount. L-732138 (NK(1) receptor antagonist), SR-48968 (NK(2) receptor antagonist), dimethylthiourea (H(2)O(2) scavenger) or catechins (O (2)(-) and H(2)O(2) scavenger) pretreatment reduced efferent TVN stimulation-enhanced hypotension, bronchoconstriction, and plasma extravasation. Increased frequency of TVN stimulation significantly upregulated the expression of nuclear factor-kappaB (NF-kappaB) in nuclear protein and intercellular adhesion molecule-1 (ICAM-1) in total protein of the lower respiratory tract tissue. The upregulation of NF-kappaB and ICAM-1 was attenuated by NK receptor antagonist and antioxidants. In conclusion, TVN efferent stimulation increases substance P release to trigger NF-kappaB mediated ICAM-1 expression and O (2)(-) and H(2)O(2) ROS production in the respiratory tract.

Differential effects of sham feeding and meal ingestion on ghrelin and pancreatic polypeptide levels: evidence for vagal efferent stimulation mediating ghrelin release1

Ghrelin has been suggested to function as an appetite-stimulating signal from the gastrointestinal tract to the brain acting through a vagal afferent pathway. Ghrelin levels rise before meals and fall after meal ingestion. The purpose of this study was to investigate factors which regulate ghrelin release into the circulation by determining changes in systemic ghrelin concentrations after sham feeding and meal ingestion. Fifteen normal subjects underwent sham feeding of a bacon and cheese toasted sandwich. Serial blood samples were obtained before and every 5 min for another 30 min during sham feeding and for 30 min after actual meal ingestion. Radioimmunoassay was used to measure plasma ghrelin and pancreatic polypeptide concentrations. During sham feeding, plasma ghrelin concentration increased from 1730+/-237 to 1917+/-269 pg/mL (P<0.05) and plasma pancreatic polypeptide increased from 417+/-50 to 841+/-97 pg/mL (P<0.01). Subsequent meal ingestion was characterized by an increase in pancreatic polypeptide from 782+/-88 to 1710+/-119 pg/mL (P<0.01), but no significant change in ghrelin levels. Plasma ghrelin and pancreatic polypeptide concentrations increase with sham feeding. This suggests a vagal efferent pathway mediating ghrelin release. In contrast to pancreatic polypeptide which rises with actual meal ingestion, ghrelin levels did not change.

Effect of hypothermia on baroreflex control of heart rate and renal sympathetic nerve activity in anaesthetized rats.

The present study investigated the effect of acute hypothermia on baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) by generating baroreflex logistic function curves, using bolus doses of phenylephrine and sodium nitroprusside, in anaesthetized male Wistar rats at a core temperature (T(b)) of 37 degrees C, during acute severe hypothermia at T(b)= 25 degrees C and on rewarming to 37 degrees C. Comparisons were made between rats without (euthermic, n= 6) and with (acclimated, n= 7) prior exposure to lower ambient temperatures and shorter photoperiod, simulating adaptation to winter conditions. In both groups of rats, acute hypothermia to T(b)= 25 degrees C shifted the baroreflex-RSNA curve slightly leftwards and downwards with decreases in the setpoint pressure and maximal gain, whereas it markedly impaired the baroreflex-HR curve characterized by decreases in response range by approximately 90% (P < 0.001), minimum response by approximately 10% (P < 0.05) and maximum gain by approximately 95% (P < 0.001), from that at T(b)= 37 degrees C. All parameters were restored to precooling levels on rewarming. Electrical stimulation of cardiac vagal efferents induced a voltage-related bradycardia, the magnitude of which was partially reduced during acute hypothermia, and there was a significant prolongation of the electrocardiogram intervals indicating a delay in cardiac conduction. Mild suppression of baroreflex control of RSNA could contribute to hypothermic hypotension and may primarily reflect an effect of T(b) on central drive. The marked attenuation of the baroreflex control of HR during hypothermia was likely to be due to an impairment of both the central and peripheral components of the reflex arc. Baroreflex control of RSNA and HR was similar between both groups of rats, which implied that the control was non-adaptive on chronic cold exposure.

Analysis of afferent, central, and efferent components of the baroreceptor reflex in mice.

Studies of genetically modified mice provide a powerful approach to investigate consequences of altered gene expression in physiological and pathological states. The goal of the present study was to characterize afferent, central, and efferent components of the baroreceptor reflex in anesthetized Webster 4 mice. Baroreflex and baroreceptor afferent functions were characterized by measuring changes in renal sympathetic nerve activity (RSNA) and aortic depressor nerve activity (ADNA) in response to nitroprusside- and phenylephrine-induced changes in arterial pressure. The data were fit to a sigmoidal logistic function curve. Baroreflex diastolic pressure threshold (P(th)), the pressure at 50% inhibition of RSNA (P(mid)), and baroreflex gain (maximum slope) averaged 74 +/- 5 mmHg, 101 +/- 3 mmHg, and 2.30 +/- 0.54%/mmHg, respectively (n = 6). The P(th), P(mid), and gain for the diastolic pressure-ADNA relation (baroreceptor afferents) were similar to that observed for the overall reflex averaging 79 +/- 9 mmHg, 101 +/- 4 mmHg, and 2.92 +/- 0.53%/mmHg, respectively (n = 5). The central nervous system mediation of the baroreflex and the chronotropic responsiveness of the heart to vagal efferent activity were independently assessed by recording responses to electrical stimulation of the left ADN and the peripheral end of the right vagus nerve, respectively. Both ADN and vagal efferent stimulation induced frequency-dependent decreases in heart rate and arterial pressure. The heart rate response to ADN stimulation was nearly abolished in mice anesthetized with pentobarbital sodium (n = 4) compared with mice anesthetized with ketamine-acepromazine (n = 4), whereas the response to vagal efferent stimulation was equivalent under both types of anesthesia. Application of these techniques to studies of genetically manipulated mice can be used to identify molecular mechanisms of baroreflex function and to localize altered function to afferent, central, or efferent sites.

Effects of thoracic spinal cord stimulation on cardiac autonomic regulation of the sinus and atrioventricular nodes.

Thoracic spinal cord stimulation (SCS) has been used to treat angina pectoris and to produce cardiac autonomic control. We studied the effect of thoracic SCS on sinus node and AV nodal function to test the hypothesis that SCS modulated autonomic regulation of the electrophysiology of these structures. The effects of thoracic SCS on sinus cycle length and AH interval were studied in 47 dogs in five experimental groups: group 1: intact autonomic nerves; group 2: bilateral ansae subclaviae transection and efferent stellate stimulation; group 3: ansae transection, bilateral vagi transection, and efferent stellate stimulation; group 4: bilateral vagi transection and efferent vagal stimulation; and group 5: bilateral vagal stimulation and bilateral ansae subclaviae transection. Under fluoroscopic guidance, the spinal stimulator electrode was advanced to the T1-T2 position and threshold determined by adjusting the output to produce muscle contraction. Parameters were measured at baseline prior to SCS and during SCS, at 90% threshold. Ansae subclaviae and vagi nerves were isolated using standard approaches. Stellate and vagal stimulation were each performed using a constant current stimulator and at three different frequencies. Sinus cycle length and AH intervals (the latter at constant right atrial pacing of 400 msec) were measured with and without SCS at baseline and at each level of nerve stimulation. Nitric oxide was measured using the coronary sinus overflow method, from a luminal balloon catheter placed deep in the coronary sinus. SCS resulted in an increase in sinus cycle length from 507 +/- 23 msec to 544 +/- 22 msec (P = 0.02) and AH interval from 71 +/- 4 msec to 74 +/- 4 msec (P = 0.03). Ansae subclaviae transection had no effect on this increase, while vagal transection eliminated the increase in sinus cycle length and AH with SCS. The increase in these parameters with SCS was maintained during both stellate stimulation (group 2) and vagal stimulation (group 5) across all three levels of neural stimulation. SCS appears to enhance parasympathetic activity, mediated via the vagus. This may have implications for use of thoracic SCS to treat chronic angina and perhaps prevent sudden cardiac death.

Intracisternal TRH analog induces Fos expression in gastric myenteric neurons and glia in conscious rats.

Activation of gastric myenteric cells by intracisternal injection of the stable thyrotropin-releasing hormone (TRH) analog RX-77368, at a dose inducing near maximal vagal cholinergic stimulation of gastric functions, was investigated in conscious rats. Fos immunoreactivity was assessed in gastric longitudinal muscle-myenteric plexus whole mount preparations 90 min after intracisternal injection. Fos-immunoreactive cells were rare in controls (~1 cell/ganglion), whereas intracisternal RX-77368 (50 ng) increased the number to 24.8 +/- 1.8 and 26.8 +/- 2.2 cells/ganglion in the corpus and antrum, respectively. Hexamethonium (20 mg/kg sc) prevented Fos expression by 90%, whereas atropine (2 mg/kg sc) had no effect. The neuronal marker protein gene product 9.5 and the glial markers S-100 and glial fibrillary acidic proteins showed that RX-77368 induced Fos in both myenteric neurons and glia. Vesicular ACh transporter and calretinin were detected around the activated myenteric neurons. These results indicated that central vagal efferent stimulation by intracisternal RX-77368 activates gastric myenteric neurons as well as glial cells mainly through nicotinic ACh receptors in conscious rats.

Role of cholinergic neural transmission on airway resistance in the dog

The unique contractile profiles of bronchial smooth muscle ( Kondo et al., 1995) and its neural control were investigated by comparing responses of the bronchus and trachea to acute hypercapnia, stimulation of vagus efferent fibers before and after intravenous atropine, and intravenous acetylcholine in decerebrated and paralyzed dogs. During acute hypercapnia, airway resistance represented by peak airway pressure ( Pedley et al., 1970) significantly increased as well as tracheal tension (Ttr). During electric stimulation of the vagal efferent fibers, Ttr increased and was sustained throughout the simulation period while the peak airway pressure was not maintained at the peak level. The peak Ttr and the airway resistance (Raw) calculated from ventilatory flow and airway pressure increased with increases in intensity of electric stimulation. Ttr reached its maximal level at an intensity 16 times of the threshold ( T), while Raw became maximal at 4 T. Although both the Ttr-stimulus intensity and Raw-intensity curves were shifted to the right by administration of intravenous atropine, the Raw curve shifted more to the right than the Ttr curve with the same dose of atropine. When muscular muscarinic receptors were directly stimulated by intravenous acetylcholine, Ttr once increased and then decreased promptly while peak airway pressure remained at a high level for a few minutes. These findings suggested that the bronchus is more sensitive to vagal efferent stimulation and susceptible to competitive antagonist of actylcholine than the trachea. In conclusion, the contractile profiles of the fifth-order bronchus we have reported ( Kondo et al., 1995) were reflected in airway resistance, and the neuromuscular junction may be the site of adaptation of bronchoconstrictor response to motor nerve adaptation.