Convergent pathways for cardiac- and esophageal-somatic motor reflexes in rats

Intrapericardial capsaicin and bradykinin induce different cardiac-somatic and cardiovascular reflexes in rats

Patients with myocardial infarction experience various types of chest pain and autonomic disturbance symptoms. Studies in rats have shown that pericardial infusions of certain chemicals induce cardiac-related muscle pain and cardiovascular reflexes. In the present study, bradykinin or capsaicin was injected into the pericardial sac and the resulting cardiac-somatic reflexes and blood pressure (BP) alterations were record. We found that the cardiac-somatic reflex induced by bradykinin had a longer latency, shorter duration, and lower firing rate than that induced by capsaicin (p<0.05). We also found that bradykinin induced a hypertensive response (p<0.05), while capsaicin induced a hypotensive response (p<0.05). Bilateral vagotomy had no effect on the cardiac-somatic reflex induced by bradykinin (p>0.05) but reduced the reflex induced by capsaicin (p<0.05). However, vagotomy had no effect on the BP alterations induced by both bradykinin and capsaicin (p>0.05). These results suggest that bradykinin and capsaicin activate different pathways to induce cardiac-somatic and cardiovascular reflexes and that the vagus nerve is involved in TRPV1-related muscle pain modulation.

Identifying spinal sensory pathways activated by noxious esophageal acid

BACKGROUND

The transient receptor potential vanilloid 1 (TRPV1) channel is critical for spinal afferent signaling of burning pain throughout the body. Such pain frequently originates from the esophagus, following acid reflux. The contribution of TRPV1 to spinal nociceptor signaling from the esophagus remains unclear. We aimed to identify the spinal afferent pathways that convey nociceptive signaling from the esophagus, specifically those sensitive to acid, and the extent to which TRPV1 contributes.

METHODS

Acid/pepsin (150 mM HCl/1 mg mL(-1) pepsin) or saline/pepsin was perfused into the esophageal lumen of anesthetized wild-type and TRPV1 null mice over 20 min, followed by atraumatic perfuse fixation and removal of the cervical and thoracic spinal cord and dorsal root ganglia (DRG). To identify neurons responsive to esophageal perfusate, immunolabeling for neuronal activation marker phosphorylated extracellular receptor-regulated kinase (pERK) was used. Labeling for calcitonin gene-related peptide (CGRP) and isolectin B4 (IB4) was then used to characterize responsive neurons.

KEY RESULTS

Esophageal acid/pepsin perfusion significantly increased the number of pERK-immunoreactive (IR) neurons in the DRG and the cervical and thoracic spinal cord dorsal horn (DH) relative to saline/pepsin (DRG P < 0.01; cervical DH P < 0.05 and thoracic DH P < 0.005). The number of pERK-IR neurons following acid perfusion was significantly attenuated in TRPV1 -/- mice (DH P < 0.05 and DRG P < 0.05).

CONCLUSIONS & INFERENCES

This study has identified populations of spinal afferent DRG neurons and DH neurons involved in signaling of noxious acid from the esophagus. There is a major contribution of TRPV1 to signaling within these pathways.

Exercise-provoked esophageal motility disorder in patients with recurrent chest pain

AIM: To investigate the relationship between exercise-provoked esophageal motility disorders and the prognosis for patients with chest pain.

METHODS: The study involved 63 subjects with recurrent angina-like chest pain non-responsive to empirical therapy with proton pump inhibitor (PPI). In all, a coronary artery angiography, panendoscopy, 24-h esophageal pH-metry and manometry, as well as a treadmill stress test with simultaneous esophageal pH-metry and manometry monitoring, were performed. Thirty-five subjects had no significant coronary artery lesions, and 28 had more than 50% coronary artery narrowing. In patients with hypertensive esophageal motility disorders, a calcium antagonist was recommended. The average follow-up period was 977 ± 249 d.

RESULTS: The prevalence of esophageal disorders, such as gastroesophageal reflux or diffuse esophageal spasm, was similar in patients both with and without significant coronary artery narrowing. Exercise prompted esophageal motility disorders, such as a decrease in the percentage of peristaltic and effective contractions and their amplitude, as well as an increase in the percentage of simultaneous and non-effective contractions. In 14 (22%) patients the percentage of simultaneous contractions during the treadmill stress test exceeded the value of 55%. Using Kaplan-Meier analysis and the proportional hazard Cox regression model, it was shown that the administration of a calcium channel antagonist in patients with such an esophageal motility disorder significantly decreased the risk of hospitalization as a result of a suspicion of acute coronary syndrome after the 2.7-year follow-up period.

CONCLUSION: In patients with chest pain non-responsive to PPIs, a diagnosis of exercise-provoked esophageal spasm may have the effect of lowering the risk of the next hospitalization.

Characterization of upper thoracic spinal neurons responding to esophageal distension in diabetic rats

The aim of this study was to examine spinal neuronal processing of innocuous and noxious mechanical inputs from the esophagus in diabetic rats. Streptozotocin (50 mg/kg, ip) was used to induce diabetes in 15 male Sprague-Dawley rats, and vehicle (10 mM citrate buffer) was injected into 15 rats as control. Four to eleven weeks after injections, extracellular potentials of single thoracic (T3) spinal neurons were recorded in pentobarbital anesthetized, paralyzed, and ventilated rats. Esophageal distensions (ED, 0.2, 0.4 ml, 20 s) were produced by water inflation of a latex balloon in the thoracic esophagus. Noxious ED (0.4 ml, 20 s) altered activity of 44% (55/126) and 38% (50/132) of spinal neurons in diabetic and control rats, respectively. The short-lasting excitatory responses to ED were encountered more frequently in diabetic rats (27/42 vs 15/41, P<0.05). Spinal neurons with low threshold for excitatory responses to ED were more frequently encountered in diabetic rats (33/42 vs 23/41, P<0.05). However, mean excitatory responses and duration of responses to noxious ED were significantly reduced for high-threshold neurons in diabetic rats (7.4+/-1.1 vs 13.9+/-3.3 imp/s; 19.0+/-2.3 vs 31.2+/-5.5 s; P<0.05). In addition, more large size somatic receptive fields were found for spinal neurons with esophageal input in diabetic rats than in control rats (28/42 vs 19/45, P<0.05). These results suggested that diabetes influenced response characteristics of thoracic spinal neurons receiving mechanical esophageal input, which might indicate an altered spinal visceroceptive processing underlying diabetic esophageal neuropathy.

Subdiaphragmatic vagal afferent nerves modulate visceral pain

Activation of the vagal afferents by noxious gastrointestinal stimuli suggests that vagal afferents may play a complex role in visceral pain processes. The contribution of the vagus nerve to visceral pain remains unresolved. Previous studies reported that patients following chronic vagotomy have lower pain thresholds. The patient with irritable bowel syndrome has been shown alteration of vagal function. We hypothesize that vagal afferent nerves modulate visceral pain. Visceromotor responses (VMR) to graded colorectal distension (CRD) were recorded from the abdominal muscles in conscious rats. Chronic subdiaphragmatic vagus nerve sections induced 470, 106, 51, and 54% increases in VMR to CRD at 20, 40, 60 and 80 mmHg, respectively. Similarly, at light level of anesthesia, topical application of lidocaine to the subdiaphragmatic vagus nerve in rats increased VMR to CRD. Vagal afferent neuronal responses to low or high-intensity electrical vagal stimulation (EVS) of vagal afferent Adelta or C fibers were distinguished by calculating their conduction velocity. Low-intensity EVS of Adelta fibers (40 microA, 20 Hz, 0.5 ms for 30 s) reduced VMR to CRD at 40, 60, and 80 mmHg by 41, 52, and 58%, respectively. In contrast, high-intensity EVS of C fibers (400 microA, 1 Hz, 0.5 ms for 30 s) had no effect on VMR to CRD. In conclusion, we demonstrated that vagal afferent nerves modulate visceral pain. Low-intensity EVS that activates vagal afferent Adelta fibers reduced visceral pain. Thus EVS may potentially have a role in the treatment of chronic visceral pain.

The visceromotor responses to colorectal distension and skin pinch are inhibited by simultaneous jejunal distension

Noxious stimuli that are applied to different somatic sites interact; often one stimulus diminishes the sensation elicited from another site. By contrast, inhibitory interactions between visceral stimuli are not well documented. We investigated the interaction between the effects of noxious distension of the colorectum and noxious stimuli applied to the jejunum, in the rat. Colorectal distension elicited a visceromotor reflex, which was quantified using electromyographic (EMG) recordings from the external oblique muscle of the upper abdomen. The same motor units were activated when a strong pinch was applied to the flank skin. Distension of the jejunum did not provoke an EMG response at this site, but when it was applied during colorectal distension it blocked the EMG response. Jejunal distension also inhibited the response to noxious skin pinch. The inhibition of the visceromotor response to colorectal distension was prevented by local application of tetrodotoxin to the jejunum, and was markedly reduced when nicardipine was infused into the local jejunal circulation. Chronic sub-diaphragmatic vagotomy had no effect on the colorectal distension-induced EMG activity or its inhibition by jejunal distension. The nicotinic antagonist hexamethonium suppressed phasic contractile activity in the jejunum, had only a small effect on the inhibition of visceromotor response by jejunal distension. It is concluded that signals that arise from skin pinch and colorectal distension converge in the central nervous system with pathways that are activated by jejunal spinal afferents; the jejunal signals strongly inhibit the abdominal motor activity evoked by noxious stimuli.

Innervation of the mammalian esophagus

Understanding the innervation of the esophagus is a prerequisite for successful treatment of a variety of disorders, e.g., dysphagia, achalasia, gastroesophageal reflux disease (GERD) and non-cardiac chest pain. Although, at first glance, functions of the esophagus are relatively simple, their neuronal control is considerably complex. Vagal motor neurons of the nucleus ambiguus and preganglionic neurons of the dorsal motor nucleus innervate striated and smooth muscle, respectively. Myenteric neurons represent the interface between the dorsal motor nucleus and smooth muscle but they are also involved in striated muscle innervation. Intraganglionic laminar endings (IGLEs) represent mechanosensory vagal afferent terminals. They also establish intricate connections with enteric neurons. Afferent information is implemented by the swallowing central pattern generator in the brainstem, which generates and coordinates deglutitive activity in both striated and smooth esophageal muscle and orchestrates esophageal sphincters as well as gastric adaptive relaxation. Disturbed excitation/inhibition balance in the lower esophageal sphincter results in motility disorders, e.g., achalasia and GERD. Loss of mechanosensory afferents disrupts adaptation of deglutitive motor programs to bolus variables, eventually leading to megaesophagus. Both spinal and vagal afferents appear to contribute to painful sensations, e.g., non-cardiac chest pain. Extrinsic and intrinsic neurons may be involved in intramural reflexes using acetylcholine, nitric oxide, substance P, CGRP and glutamate as main transmitters. In addition, other molecules, e.g., ATP, GABA and probably also inflammatory cytokines, may modulate these neuronal functions.

c-Fos expression in rat brain stem and spinal cord in response to activation of cardiac ischemia-sensitive afferent neurons and electrostimulatory modulation

The purpose of this study was to identify central neuronal sites activated by stimulation of cardiac ischemia-sensitive afferent neurons and determine whether electrical stimulation of left vagal afferent fibers modified the pattern of neuronal activation. Fos-like immunoreactivity (Fos-LI) was used as an index of neuronal activation in selected levels of cervical and thoracic spinal cord and brain stem. Adult Sprague-Dawley rats were anesthetized with urethane and underwent intrapericardial infusion of an “inflammatory exudate solution” (IES) containing algogenic substances that are released during ischemia (10 mM adenosine, bradykinin, prostaglandin E2, and 5-hydroxytryptamine) or occlusion of the left anterior descending coronary artery (CoAO) to activate cardiac ischemia-sensitive (nociceptive) afferent fibers. IES and CoAO increased Fos-LI above resting levels in dorsal horns in laminae I–V at C2 and T4 and in the caudal nucleus tractus solitarius. Dorsal rhizotomy virtually eliminated Fos-LI in the spinal cord as well as the brain stem. Neuromodulation of the ischemic signal by electrical stimulation of the central end of the left thoracic vagus excited neurons at the cervical and brain stem level but inhibited neurons at the thoracic spinal cord during IES or CoAO. These results suggest that stimulation of the left thoracic vagus excites descending inhibitory pathways. Inhibition at the thoracic spinal level that suppresses the ischemic (nociceptive) input signal may occur by a short-loop descending pathway via signals from cervical propriospinal circuits and/or a longer-loop descending pathway via signals from the nucleus tractus solitarius.

Left vagal stimulation induces dynorphin release and suppresses substance P release from the rat thoracic spinal cord during cardiac ischemia

Electrostimulatory forms of therapy can reduce angina that arises from activation of cardiac nociceptive afferent fibers during transient ischemia. This study sought to determine the effects of electrical stimulation of left thoracic vagal afferents (C(8)-T(1) level) on the release of putative nociceptive [substance P (SP)] and analgesic [dynorphin (Dyn)] peptides in the dorsal horn at the T(4) spinal level during coronary artery occlusion in urethane-anesthetized Sprague-Dawley rats. Release of Dyn and SP was measured by using antibody-coated microprobes. While Dyn and SP had a basal release, occlusion of the left anterior descending coronary artery only affected SP release, causing an increase from lamina I-VII. Left vagal stimulation increased Dyn release, inhibited basal SP release, and blunted the coronary artery occlusion-induced release of SP. Dyn release reflected activation of descending pathways in the thoracic spinal cord, because vagal afferent stimulation still increased the release of Dyn after bilateral dorsal rhizotomy of T(2)-T(5). These results indicate that electrostimulatory therapy, using vagal afferent excitation, may induce analgesia, in part, via inhibition of the release of SP in the spinal cord, possibly through a Dyn-mediated neuronal interaction.

Esophagocardiac convergence onto thoracic spinal neurons: comparison of cervical and thoracic esophagus

The aim of this study was to characterize thoracic spinal neurons receiving convergent inputs from the esophagus, heart and somatic receptive fields. Extracellular potentials of single T3-T4 spinal neurons were recorded in pentobarbital anesthetized male rats. Thoracic and cervical esophageal distensions (TED, CED) were produced by water inflation of a latex balloon. A catheter was placed in the pericardial sac to administer bradykinin or a mixture of algogenic chemicals. 96/311 (31%) neurons responded to both TED and intrapericardial chemicals (IC) and 48/177 (27%) neurons responded to both CED and IC. Long-lasting excitatory responses were more frequently encountered (P<0.05) in esophagocardiac spinal neurons responding to TED (T-ECSNs, 62/91) than in neurons responding to CED (C-ECSNs, 23/47). Ninety-one percent of T-ECSNs and 98% of C-ECSNs had somatic fields on chest, axilla and upper back areas. Esophagocardiac convergence on thoracic spinal neurons provided a spinal mechanism that might mediate viscerovisceral nociception and reflexes.

Noncardiac chest pain: epidemiology, natural history, health care seeking, and quality of life

The epidemiology of NCCP is poorly described, and the available data are conflicting. Population-based studies on the prevalence of NCCP are rare; most studies have been hospital based. According to the limited studies available, the annual prevalence of NCCP is approximately 25%. Despite this significant burden, the impact and natural history of NCCP in the community has not been adequately explored. NCCP is presumed to bea heterogeneous condition. Hospital-based studies have suggested that GERD, esophageal spasm, psychiatric disease (including panic attacks), and musculoskeletal pain explain many cases of NCCP. However, unrecognized coronary artery disease and microvascular angina (cardiac syndrome X)also explain an unknown proportion of cases in the general population.Current studies suggest that NCCP is common in the general population and significantly affects QOL, yet only a minority seeks medical attention.The epidemiology of NCCP requires further study in the general population and in those attending the Emergency Department.

Responses and afferent pathways of C1-C2 spinal neurons to cervical and thoracic esophageal stimulation in rats

Because vagal and sympathetic inputs activate upper cervical spinal neurons, we hypothesized that stimulation of the esophagus would activate C(1)-C(2) neurons. This study examined responses of C(1)-C(2) spinal neurons to cervical and thoracic esophageal distension (CED, TED) and afferent pathways for CED and TED inputs to C(1)-C(2) spinal neurons. Extracellular potentials of single C(1)-C(2) spinal neurons were recorded in pentobarbital-anesthetized male rats. Graded CED or TED was produced by water inflation (0.1-0.5 ml) of a latex balloon. CED changed activity of 48/219 (22%) neurons; 34 were excited (E), 12 were inhibited (I), and 2 were E-I. CED elicited responses for 18/18 neurons tested after ipsilateral cervical vagotomy, for 12/14 neurons tested after bilateral vagotomy and for 9/11 neurons tested after bilateral vagotomy and C(6)-C(7) spinal cord transection. TED changed activity of 31/190 (16%) neurons (28E, 3 I). Ipsilateral cervical vagotomy abolished TED-evoked responses of 5/12 neurons. Bilateral vagotomy eliminated responses of 2/4 neurons tested, and C(6)-C(7) spinal transection plus bilateral vagotomy eliminated responses of 2/2 neurons. Thus inputs from CED to C(1)-C(2) neurons most likely entered upper cervical dorsal roots, whereas inputs from TED were dependent on vagal pathways and/or sympathetic afferent pathways that entered the thoracic dorsal roots. These results supported a concept that C(1)-C(2) spinal neurons play a role in integrating visceral information from cervical and thoracic esophagus.