Ken D. O'Halloran

Effects of partial nerve injury on the responses of C-fiber polymodal nociceptors to adrenergic agonists

The effects of partial division of the great auricular nerve of adult rabbits were evaluated on the responsiveness of cutaneous C-fiber polymodal nociceptors (CPMs) to sympathetic stimulation (SS), close-arterial injections of epinephrine (EPI) and other alpha-adrenergic agonists. In normal unanesthetized rabbits, the two ears were usually at the same temperature. Two to 4 weeks after partial nerve lesions, however, the operated ear was cooler by 1-3 degrees C in the majority of animals, suggestive of increased vasoconstriction and possible denervation supersensitivity. Neither SS nor EPI (50 ng) excited CPM units (n = 23) from intact anesthetized animals. In contrast, 14-27 days after partial nerve lesions, SS (8 out of 38 units) and EPI (12 out of 38 units) were excitatory for a class of CPMs. There was notable variability in the response of different units and of a given unit between first and second trials. Responses consisted of 1-22 impulses for SS and 1-23 impulses for EPI in the 60 s following a trial. Arterial occlusion did not activate responsive units, suggesting that the excitation was not caused by vascular or temperature changes. Selective alpha2-adrenoceptor blockade with yohimbine (0.6-1.0 mg/kg i.v.) or rauwolscine (1.0 mg/kg i.v.) reversibly antagonized the effects of SS and EPI. EPI-responsive units were also excited by norepinephrine (50 ng) and guanabenz (10 microg) but not by clonidine (3 microg) or B-HT 933 (3 microg). The results suggest that circulating EPI, acting via an alpha-adrenoceptor subtype, can play a part in the development and/or maintenance of aberrant pain syndromes such as causalgia and other sympathetically related dystrophies.

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.

Oxidative stress impairs upper airway muscle endurance in an animal model of sleep-disordered breathing.

Obstructive sleep apnoea is characterised by intermittent hypoxia due to recurrent obstructions of the pharyngeal airway during sleep. We have shown that chronic intermittent hypoxia impairs respiratory muscle function and CNS control of upper airway patency. In this study, we tested the hypothesis that disruption of an endogenous antioxidant defence system exacerbates the effects of intermittent hypoxia on upper airway muscle contractile function. Thirty-two male Wistar rats were placed in restrainers with their heads in hoods in which the ambient oxygen concentration could be modified by controlling the gas supply to the hoods. Sixteen rats were exposed to alternating equal periods of hypoxia and normoxia, twice per minute, 8 hours per day for 1 week. The remaining 16 animals were exposed to normoxia continuously under identical experimental conditions. In both groups, half the animals received daily injections of buthionine sulfoxamine (BSO), an inhibitor of the rate-limiting enzyme in glutathione synthesis. The other half received daily vehicle injections. At the end of the 1-week treatment period, the sternohyoid muscles were removed and fatigue characteristics were determined in vitro. Intermittent hypoxia was associated with a decrease in sternohyoid muscle endurance, an effect that was exacerbated by treatment with BSO. In separate experiments, daily treatment with the antioxidant N-acetyl cysteine blocked the deleterious effects of intermittent hypoxia on respiratory muscle function. We suggest that oxidative stress contributes to impaired upper airway muscle endurance in our animal model and that endogenous glutathione may be especially important in limiting free radical-induced muscle dysfunction. Our results may have particular relevance to respiratory disorders associated with recurrent hypoxia, such as the sleep apnoea/hypopnoea syndrome.

Modulation of enteric neurons by interleukin-6 and corticotropin-releasing factor contributes to visceral hypersensitivity and altered colonic motility in a rat model of irritable bowel syndrome.

Hyperactivity of the stress system and low‐grade immune activation characterize the functional bowel disorder irritable bowel syndrome (IBS). These studies show that interleukin (IL)‐6 and IL‐8 and the stress hormone corticotropin‐releasing factor (CRF), present in IBS plasma, have functional effects on gastrointestinal activity by stimulating myenteric neurons and colonic contractions. Moreover, in the Wistar Kyoto rat model of IBS, which exhibits altered gastrointestinal motility and visceral pain sensitivity, blocking IL‐6 and/or CRF1 receptors alleviates these IBS‐like symptoms. Underlying these effects are altered colonic protein expression of tight junction proteins which regulate gut barrier function and the T‐type calcium channel CaV3.2, which has been linked to visceral pain. These findings demonstrate the importance of the enteric nervous system and intestinal physiology in bowel dysfunction.

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.

Reactive oxygen species mediated diaphragm fatigue in a rat model of chronic intermittent hypoxia.

What is the central question of this study? The effects of chronic intermittent hypoxia (CIH) on respiratory muscles are relatively underexplored. It is speculated that muscle dysfunction and other key morbidities associated with sleep apnoea are the result of CIH‐induced oxidative stress. We sought to investigate the putative role of CIH‐induced reactive oxygen species in the development of respiratory muscle dysfunction. What is the main finding and its importance? The CIH‐induced diaphragm muscle fatigue is time and intensity dependent and is associated with a modest oxidative stress. Supplementation with N‐acetyl cysteine prevents CIH‐induced diaphragm muscle dysfunction, suggesting that antioxidant supplementation may have therapeutic value in respiratory muscle disorders characterized by CIH, such as obstructive sleep apnoea.

Superoxide Scavengers Improve Rat Pharyngeal Dilator Muscle Performance

Obstructive sleep apnea is a common disorder associated with upper airway muscle dysfunction. Agents that improve respiratory muscle performance may be useful as an adjunct therapy. The aim of this study was to examine the effects of antioxidants on rat pharyngeal dilator muscle performance. Adult male Wistar rats were killed humanely and isometric contractile properties of isolated sternohyoid muscle strips were examined in physiological salt solution at 35 degrees C in vitro. Muscle strips were incubated in tissue baths under hyperoxic (95%O(2)/5%CO(2)) or hypoxic (95%N(2)/5%CO(2)) conditions in the absence (control) or presence of the antioxidants: N-acetylcysteine (10 mM), Tiron (10 mM), or Tempol (10 mM). Force-frequency relationship was determined in response to supramaximal stimulation (10-100 Hz in increments of 10-20 Hz, train duration: 300 ms). Isometric force was also recorded during repetitive muscle stimulation (40 Hz, 300 ms every 2 s for 2 min). Under hyperoxic conditions, Tiron and Tempol, but not N-acetylcysteine, significantly increased sternohyoid muscle force and caused a left-shift in the force-frequency relationship. In addition, Tempol had a significant positive inotropic effect over the initial 90 seconds of repeated muscle activation. Hypoxia caused a significant decrease in sternohyoid muscle force. Under hypoxic conditions, Tempol-incubated muscles generated significantly higher forces compared with control muscles and showed improved performance in the early phase of the fatigue trial. This study illustrates that superoxide scavengers increase upper airway muscle force and that this effect persists under hypoxic conditions. We conclude that antioxidant treatment may be beneficial as a therapy in obstructive sleep apnea.

Effects of chronic hypobaric hypoxia on contractile properties of rat sternohyoid and diaphragm muscles.

1. Chronic hypoxia occurs in a variety of circumstances, including respiratory disease and exposure to altitude, and is known to affect respiratory muscle structure. However, little is known about its effects on respiratory muscle contractile properties.

Carotid body dopaminergic mechanisms are functional after acclimatization to hypoxia in goats

Ventilatory acclimatization to sustained hypoxia (VASH) is the time-dependent increase in ventilation that occurs during prolonged exposure to hypoxia. We tested the hypothesis that carotid body (CB) dopaminergic mechanisms are down-regulated during VASH, which would allow CB afferent discharge and ventilation to increase beyond the initial response to hypoxia. Domperidone (DOM; 1.0 mg.kg-1) was administered intravenously to block CB dopamine (DA) receptors after VASH was complete in awake goats. DOM caused a significant augmentation of the ventilatory response to hypoxia in acclimatized goats, failing to support the hypothesis. We conclude that inhibitory CB dopaminergic function is not significantly reduced following prolonged hypoxia, and that down-regulation of CB dopaminergic mechanisms may not be involved in VASH in the goat.

VENTILATORY AND UPPER-AIRWAY RESISTANCE RESPONSES TO UPPER-AIRWAY COOLING AND CO2 IN ANAESTHETISED RATS

The effects of upper airway (UA) cool air and CO2 on breathing and on laryngeal and supraglottic resistances were studied in anaesthetised rats breathing spontaneously through a tracheostomy. Warm, humidified air containing 0, 5 and 9–10% CO2 and cool, room-humidity air were delivered at constant flow to either the isolated larynx to exit through a pharyngotomy or to the supraglottic UA to exit through the mouth and/or nose (nose open or sealed). Spontaneous tracheal airflow and UA airflows, temperatures and pressures were recorded. CO2 had no effect on breathing but caused a slight increase in laryngeal resistance which was abolished by cutting the superior laryngeal nerves (SLN). Cool air caused a decrease in respiratory frequency and/or peak inspiratory flow when applied to the isolated larynx or to the supraglottic airway with the nose closed. These effects were abolished by SLN section. With the nose open, the ventilatory inhibition was not abolished by SLN section. Cool air also caused substantial decreases in laryngeal and supraglottic resistances which were attenuated by SLN section and which persisted following recurrent laryngeal nerve section. In conclusion, whilst UA cooling inhibits breathing and decreases UA resistances, UA CO2 has minimal effects.