James F. X. Jones

Heart rate responses to selective stimulation of cardiac vagal C fibres in anaesthetized cats, rats and rabbits.

1. The contribution of cardiac vagal C fibres to vagal chronotropic control in anaesthetized cats, rats and rabbits was analysed using electrical stimulation of the vagus nerve with a selective anodal block technique. 2. After bilateral vagotomy and pretreatment with atenolol, 10 Hz continuous selective stimulation of unmyelinated fibres in the cut peripheral end of the cervical vagus evoked a bradycardia in anaesthetized rats, cats and rabbits. With this stimulation protocol the three species exhibited a similar lengthening of the heart period (R‐R interval) when expressed as a percentage of their basal cardiac interval. 3. The mechanism of action of the selective blocking technique was analysed by recording eighty‐nine single A‐ (n = 12), B‐ (n = 22) and C‐fibre (n = 55) vagal‐projecting neurones in the medulla of the rat. This demonstrated that the technique can selectively block conduction in myelinated fibres and that ‘break excitation’ is seen mainly in unmyelinated fibres. Although thirty C fibres showed break excitation sixteen did not and this difference could not be correlated with their axonal conduction velocity, chronaxie or initial segment frequency following. 4. Using the anodal block technique the vagal effects on heart rate were reanalysed in the cat by incorporating a collision technique. B fibres were activated orthodromically to evoke cardioinhibition and simultaneously antidromically to collide with errant B‐fibre spikes activated at the electrode producing anodal block. With this protocol it was noted that the B‐ and C‐fibre bradycardias were not additive. Using a double anodal block and collision technique, it was demonstrated that this phenomenon was likely to be due to occlusion of the effects of B and C fibres. 5. In conclusion, in addition to the well‐defined effects of vagal B fibres on heart rate, selective stimulation of vagal C fibres also had a cardioinhibitory effect in all three species studied. However, since the effects of cardiac C fibres on heart rate was small, these neurones alone cannot account for the cardioinhibition of the pulmonary chemoreflex. It is likely that activation of both B‐ and C‐fibre cardiac vagal preganglionic neurones accounts for this reflex cardioinhibition.

Activity of C fibre cardiac vagal efferents in anaesthetized cats and rats

1 Bradycardia can be evoked by stimulation of both myelinated and non‐myelinated vagal efferent fibres. The on‐going activity and synaptic inputs to cardiac vagal preganglionic neurones with myelinated axons have been studied in detail, but there is little information regarding cardiac preganglionic neurones with non‐myelinated axons. In the present study, the on‐going discharge and afferent inputs to cardiac vagal efferents with non‐myelinated axons were studied in anaesthetized rats and cats. 2 Extracellular recordings were made from vagal preganglionic neurones in the dorsal vagal motor nucleus (DVMN) of anaesthetized rats following electrical stimulation of the cervical vagus nerve. Based on calculated axonal conduction velocities, fifty‐six neurones had non‐myelinated axons. Sixteen of these C fibre neurones had on‐going activity but this showed no relationship to central respiratory drive, lung inflation or the cardiac cycle. Activity of twenty‐one of these fifty‐six neurones was increased at short latency following right atrial injections of phenylbiguanide (PBG). 3 Eight presumed cardiac vagal preganglionic neurones were also recorded in the DVMN. Five of these were activated by PBG administration, and the one neurone with resting activity showed no indication of respiratory‐related activity. 4 Finally, twenty‐one single C fibres were recorded in peripheral branches of the cardiac vagus. They had a low rate of on‐going activity, and in twelve fibres this on‐going discharge was analysed in detail. On‐going activity in seven of these fibres showed no relationship to central respiratory drive, lung inflation or the cardiac cycle, whereas the other five had prominent relationships with both central respiratory drive and lung inflation, and two of these also showed a relationship to heart period. PBG administration evoked an increased activity in fourteen of the fifteen fibres tested, and the latencies of nine of these responses (1.3 ± 0.5 s) were within the pulmonary circulation time. 5 In anaesthetized cats, extracellular recordings were made in the DVMN from thirty‐three cardiac vagal preganglionic neurones with C fibre axons. The on‐going activity of these neurones, when present, never exhibited an on‐going‐ or reflexly induced respiratory rhythm, nor any indication of an input from the arterial baroreceptors. Right atrial injection of PBG evoked a short‐latency response in four of the five spontaneously active neurones but was without effect on the eight neurones tested which did not exhibit on‐going activity. 6 In conclusion, the majority of cardiac vagal preganglionic neurones located in the DVMN have C fibre axons, show no obvious input from central or peripheral respiratory‐ or cardiac‐related inputs, but are activated by stimulation of pulmonary C fibre afferent fibres.

A systematic review of sacral nerve stimulation mechanisms in the treatment of fecal incontinence and constipation

Sacral nerve stimulation (SNS) is now well established as a treatment for fecal incontinence (FI) resistant to conservative measures and may also have utility in the management of chronic constipation; however, mechanism of action is not fully understood. End organ effects of SNS have been studied in both clinical and experimental settings, but interpretation is difficult due to the multitude of techniques used and heterogeneity of reported findings. The aim of this study was to systematically review available evidence on the mechanisms of SNS in the treatment of FI and constipation.

The diaphragm: two physiological muscles in one

To the respiratory physiologist or anatomist the diaphragm muscle is of course the prime mover of tidal air. However, gastrointestinal physiologists are becoming increasingly aware of the value of this muscle in helping to stop gastric contents from refluxing into the oesophagus. The diaphragm should be viewed as two distinct muscles, crural and costal, which act in synchrony throughout respiration. However, the activities of these two muscular regions can diverge during certain events such as swallowing and emesis. In addition, transient crural muscle relaxations herald the onset of spontaneous acid reflux episodes. Studying the motor control of this muscular barrier may help elucidate the mechanism of these episodes. In the rat, the phrenic nerve divides into three branches before entering the diaphragm, and it is possible to sample single neuronal activity from the crural and costal branches. This review will discuss our recent findings with regard to the type of motor axons running in the phrenic nerve of the rat. In addition, we will outline our ongoing search for homologous structures in basal vertebrate groups. In particular, the pipid frogs (e.g. the African clawed frog, Xenopus laevis) possess a muscular band around the oesophagus that appears to be homologous to the mammalian crural diaphragm. This structure does not appear to interact directly with the respiratory apparatus, and could suggest a role for this region of the diaphragm, which was not originally respiratory.

Activity of aortic chemoreceptors in the anaesthetized rat

1 It has been widely accepted that the rat aortic depressor nerve contains only baroreceptors. However, the experiments which have provided these negative data have employed whole aortic nerve recording. In the present study, the technical difficulties associated with recording single fibres in vivo, from the rat aortic nerve (diameter 25‐50 μm), have been surmounted using a small tip, glass suction electrode technique. 2 Upon switching from normocapnic hyperoxia to hypercapnic hypoxia, irregularly firing units (n= 13) appeared and these were significantly excited by intravenous injections of sodium cyanide (20 μg) but not by rises in arterial blood pressure induced by methoxamine (an α1‐adrenoreceptor agonist; 10 μg). Inhalation of 100 % oxygen rapidly and reversibly silenced, or profoundly reduced, ongoing activity. 3 Intravenous injection of phenylbiguanide (PBG; a 5‐HT3 receptor agonist; 8 μg) strongly stimulated the chemoreceptors and was followed by a period of chemodepression (3‐21 s). In contrast none of the single fibre baroreceptors recorded (n= 15) were excited by PBG but all significantly increased their discharge in response to the increases in arterial blood pressure associated with methoxamine and cyanide. Both the excitatory and inhibitory effects of PBG on the chemoreceptor fibres were abolished by ondansetron (a 5‐HT3 receptor antagonist: 1 mg kg−1 i.v.; n= 5 animals) whilst the chemoexcitatory action of cyanide was preserved. 4 It is concluded that there are chemoreceptor afferents contained in the aortic nerve of the Sprague‐Dawley rat. The 5‐HT3 receptor appears not to be a pre‐requisite for aortic body chemoexcitation.

Sacral nerve stimulation increases activation of the primary somatosensory cortex by anal canal stimulation in an experimental model.

Sacral and posterior tibial nerve stimulation may be used to treat faecal incontinence; however, the mechanism of action is unknown. The aim of this study was to establish whether sensory activation of the cerebral cortex by anal canal stimulation was increased by peripheral neuromodulation.

Tempol Ameliorates Pharyngeal Dilator Muscle Dysfunction in a Rodent Model of Chronic Intermittent Hypoxia

Respiratory muscle dysfunction is implicated in the pathophysiology of obstructive sleep apnea syndrome (OSAS), an oxidative stress disorder prevalent in men. Pharmacotherapy for OSAS is an attractive option, and antioxidant treatments may prove beneficial. We examined the effects of chronic intermittent hypoxia (CIH) on breathing and pharyngeal dilator muscle structure and function in male and female rats. Additionally, we tested the efficacy of antioxidant treatment in preventing (chronic administration) or reversing (acute administration) CIH-induced effects in male rats. Adult male and female Wistar rats were exposed to alternating cycles of normoxia and hypoxia (90 s each; Fi(O(2)) = 5% O(2) at nadir; Sa(O(2)) ∼ 80%) or sham treatment for 8 h/d for 9 days. Tempol (1 mM, superoxide dismutase mimetic) was administered to subgroups of sham- and CIH-treated animals. Breathing was assessed by whole-body plethysmography. Sternohyoid muscle contractile and endurance properties were examined in vitro. Muscle fiber type and cross-sectional area and the activity of key metabolic enzymes were determined. CIH decreased sternohyoid muscle force in male rats only. This was not attributable to fiber transitions or alterations in oxidative or glycolytic enzyme activity. Muscle weakness after CIH was prevented by chronic Tempol supplementation and was reversed by acute antioxidant treatment in vitro. CIH increased normoxic ventilation in male rats only. Sex differences exist in the effects of CIH on the respiratory system, which may contribute to the higher prevalence of OSAS in male subjects. Antioxidant treatment may be beneficial as an adjunct OSAS therapy.

Chronic hypoxia increases rat diaphragm muscle endurance and sodium–potassium ATPase pump content

The effects of chronic hypoxia (CH) on respiratory muscle are poorly understood. The aim of the present study was to examine the effects of CH on respiratory muscle structure and function, and to determine whether nitric oxide is implicated in respiratory muscle adaptation to CH. Male Wistar rats were exposed to CH for 1–6 weeks. Sternohyoid and diaphragm muscle contractile properties, muscle fibre type and size, the density of fibres expressing sarco/endoplasmic reticulum calcium-ATPase (SERCA) 2 and sodium–potassium ATPase (Na+,K+-ATPase) pump content were determined. Muscle succinate dehydrogenase (SDH) and reduced nicotinamide adenine dinucleotide phosphate (NADPH) dehydrogenase activities were also assessed. Acute and chronic blockade of nitric oxide synthase (NOS) was employed to determine whether or not NO is critically involved in functional remodelling in CH muscles. CH improved diaphragm, but not sternohyoid, fatigue tolerance in a time-dependent fashion. This adaptation was not attributable to increased SDH or NADPH dehydrogenase activities. The areal density of muscle fibres and relative area of fibres expressing SERCA2 were unchanged. Na+,K+-ATPase pump content was significantly increased in CH diaphragm. Chronic NOS inhibition decreased diaphragm Na+,K+-ATPase pump content and prevented CH-induced increase in muscle endurance. This study provides novel insight into the mechanisms involved in CH-induced muscle plasticity. The results may be of relevance to respiratory disorders characterised by CH, such as chronic obstructive pulmonary disease.

Effect of pulmonary C-fibre afferent stimulation on cardiac vagal neurones in the nucleus ambiguus in anaesthetized cats

1 It has been demonstrated previously that the vagal bradycardia evoked by activation of pulmonary C‐fibres is not respiratory modulated. Experiments were carried out in α‐chloralose anaesthetized cats to determine if these cardiac vagal preganglionic neurones (CVPNs) in the nucleus ambiguus (NA), which have respiratory modulated activity, can be activated when pulmonary C‐fibre afferents are stimulated by right atrial injections of phenylbiguanide (PBG). 2 Eleven CVPNs with B‐fibre axons in the right cardiac vagal branches were identified and found to be localized within or ventrolateral to the nucleus ambiguus. Ionophoretic application of a high current of dl‐homocysteic acid (DLH) induced a vagally mediated bradycardia and hypotension in six of eight sites from which CVPNs were recorded. 3 The activity of B‐fibre CVPNs, whether spontaneous (n= 4) or induced by ionophoresis of DLH (n= 7) was respiratory modulated, firing perferentially during post‐inspiration and stage 2 expiration. This activity also correlated with the rising phase of the arterial blood pressure wave consistent with these CVPNs receiving an arterial baroreceptor input. 4 Right atrial injections of PBG excited nine of eleven CVPNs tested. In eight of these activated neurones the onset latency of the excitation was within the pulmonary circulation time, consistent with being activated only by pulmonary C‐fibre afferents. In two neurones the PBG‐evoked excitation still occurred when central inspiratory drive was inhibited, as indicated by the disappearance of phrenic nerve activity. 5 In conclusion, B‐fibre respiratory modulated CVPNs can be activated following stimulation of pulmonary C‐fibre afferents.

Effects of 5‐HT and 5‐HT1A receptor agonists and antagonists on dorsal vagal preganglionic neurones in anaesthetized rats: an ionophoretic study

1 Effects of ionophoretic administration of 5‐hydroxytryptamine (5‐HT) and selective 5‐HT1A receptor agonists and antagonists on identified dorsal vagal preganglionic and dorsal raphe neurones were studied in pentobarbitone sodium or chloral hydrate‐anaesthetized rats, respectively. 2 Extracellular recordings were made from 176 preganglionic neurones in the dorsal vagal nucleus (DVN). Application of 5‐HT at low currents (<10nA) increased the activity of these neurones. However, at increased currents (10–60 nA), it had a predominantly depressant effect. Application of selective 5‐HT1A receptor antagonists, (±)‐pindolol or WAY‐100635, attenuated the excitatory responses evoked by 5‐HT. 3 Ionophoresis of the 5‐HT1A receptor agonist, 8‐hydroxy‐2‐(di‐n‐propylamino)tetralin (8‐OH‐DPAT) (5–30 nA) increased the firing rate of 19 and decreased that of 67 of the 104 vagal neurones tested. Other 5‐HT1A receptor agonists, flesinoxan and N, N‐di‐n‐propyl‐5‐carboxamidotryptamine (DP‐5‐CT) also had predominantly depressant effects. 4 (±)‐Pindolol attenuated excitations but not inhibitions evoked by 8‐OH‐DPAT. Surprisingly, WAY‐100635 and 8‐OH‐DPAT produced the same effect on these neurones and when applied together, WAY‐100635 failed to attenuate the 8‐OH‐DPAT responses. 5 Dorsal raphe neurones were identified by their low, regular firing rate and their subsequent histological localization. 8‐OH‐DPAT reversibly reduced the activity in all 7 neurones tested and this was antagonized by WAY‐100635 in all 3 neurones tested. 6 In conclusion, 5‐HT applied to vagal preganglionic neurones evokes excitatory and inhibitory responses. The excitatory, but not the inhibitory responses may be mediated, at least in part, by activation of 5‐HT1A receptors.