Orexin A activates retrotrapezoid neurons in mice

The retrotrapezoid nucleus (RTN), located at the ventral surface of the medulla oblongata, contains glutamatergic Phox2b-expressing interneurons that have central respiratory chemoreceptor properties. RTN also operates as a relay for hypothalamic pathways that regulate breathing, one of which probably originates from the orexinergic neurons (Dias et al., 2009. J. Physiol. 587, 2059-2067). The present study explores this hypothesis at the cellular level. Using immunohistochemistry in adult Phox2b-eGFP transgenic mice, we demonstrate the presence of numerous close appositions between orexin-containing axonal varicosities and RTN chemoreceptor neurons. Using electrophysiological recordings in slices from neonatal (P6-P10) Phox2b-eGFP mice, we show that orexin A produces a robust dose-dependent excitation of the acid-sensitive RTN neurons (ED(50) ～250nM). These data support the idea that RTN neurons are a point of convergence for several groups of CNS neurons that contribute to respiratory chemoreflexes, now including serotonergic and orexinergic neurons.     

Reflex changes in respiration and heart rate evoked by intravenous and left ventricular injection of 5-HT and capsaicin in anaesthetized rats: a comparison of mechanisms

5-hydroxytryptamine (5-HT), injected intravenously (close to the right atrium), caused apnoea, bradycardia, hypotension and a reduction in the tidal volume in anaesthetized rats. The latency of the apnoea response was 0.61 seconds. 5-HT also produced similar effects when injected into the left ventricle (intra-arterial). The latency of the apnoea response was 0.33 s by this route. The apnoea and bradycardia evoked by intravenous and intra-arterial injections of 5-HT were abolished by bilateral vagotomy and reduced or abolished by vagal cooling. The blocking temperatures varied considerably in different animals, particularly with respect to the intravenous route of drug administration. Intravenous capsaicin caused apnoea, bradycardia and hypotension and these effects were blocked by vagal section or cooling. In contrast intra-arterial injections caused apnoea but no bradycardia and the apnoea was resistant to vagal section. The study shows that there are at least two populations of receptors; one which is chemosensitive to 5-HT and capsaicin and which is supplied by the pulmonary circulation, and one which is chemosensitive to 5-HT and which is accessible via the systemic circulation. The receptors and their vagal afferent connections may be non-myelinated since 5-HT and capsaicin are known to stimulate this type of ending in other species, and because respiratory responses were only blocked at low temperatures. In addition capsaicin also activates a non-vagal receptor, accessible from the systemic circulation, and which mediates apnoea but not bradycardia. Presented in part at a meeting at the British Pharmacological Society [14]     

Changes in respiratory control after 5 days at altitude

These experiments examined changes in the chemoreflex control of breathing and acid-base balance after 5 days at altitude (3480 m) in six healthy males. The partial pressures of carbon dioxide (P(CO2)) at which ventilation increased during isoxic hypoxic and hyperoxic modified rebreathing tests at sea level fell significantly at altitude by mean+/-S.E.M. of 12.8+/-2.51 mmHg and 9.5+/-1.77 mmHg, respectively, but response slopes above threshold were unchanged. Altitude exposure produced a respiratory alkalosis evidenced by a decrease in mean resting end-tidal P(CO2) from 41+/-0.84 mmHg at sea level to 32+/-2.04 mmHg at altitude, but pH did not increase significantly from its sea level value. Blood samples were analyzed to discover acid-base changes, using a modification of the equations for acid-base balance proposed by [Stewart, P.A., 1983. Modern quantitative acid-base chemistry. Can. J. Physiol. Pharmacol. 61, 1444-1461]. While strong ion difference at altitude was not significantly different from its sea level value, albumin concentration was increased significantly from 38.6+/-0.30 g L(-1) to 49.8+/-0.76 g L(-1). We suggest that the respiratory alkalosis was produced by a fall in the chemoreflex threshold and pH was corrected by an elevation in the concentration of weakly dissociated protein anions.     

Serotonin and serotonin receptor expression in the aorta of carotid intact and denervated newborns

Pharmacological blocking of serotonin (5-HT) 5A receptors abolishes aortic ventilatory chemosensitivity of carotid body denervated (CBD) piglets [J. Appl. Physiol. 92 (2002) 893]. Accordingly, the purpose of the present study was to determine whether 5-HT and 5-HT receptors exist at aortic sites that are chemosensitive after CBD. Aortas from CBD and sham CBD rats and piglets and from aortic denervated (AOD) and combined AOD+CBD piglets were harvested, sectioned and then studied using immunohistochemistry and western blot techniques. 5-HT immunoreactivity in piglets and rats was concentrated in the endothelium and sub-endothelial areas in several aortic regions studied, and in some areas also in the adventitia. At the aortic chemosensitive site (descending aorta in CBD piglets and the ascending aorta in CBD rats), the immunoreactivity was greater (P < 0.05) than in other aortic regions and greater than in other groups studied. The 5-HT(5a) receptor was expressed only at the chemosensitive sites and only in aortic innervated piglets. We conclude that the data from this and a previous study [J. Appl. Physiol. 92 (2002) 893] suggest that a serotonergic mechanism contributes to the aortic ventilatory chemoreflex after CBD.     

Modulatory effects of respiration

Respiration is a powerful modulator of heart rate variability, and of baro- and chemoreflex sensitivity. Abnormal respiratory modulation of heart rate is often an early sign of autonomic dysfunction in a number of diseases. In addition, increase in venous return due to respiration may help in maintaining blood pressure during standing in critical situations. This review examines the possibility that manipulation of breathing pattern may provide beneficial effects in terms not only of ventilatory efficiency, but also of cardiovascular and respiratory control in physiologic and pathologic conditions, such as chronic heart failure. This opens a new area of future research in the better management of patients with cardiovascular autonomic dysfunction.     

Reflex epicardial coronary vasoconstriction elicited by nicotine in anaesthetized dogs

Summary The effects of arterial chemoreceptor activation by nicotine on coronary artery diameter was studied in anaesthetized, artificially ventilated dogs. Left circumflex coronary artery diameter, coronary blood flow, calculated mean coronary resistance, systemic arterial blood pressure and heart rate were measured. In control dogs (n = 10) the injection of nicotine (100 g) into the carotid artery evoked an increase of arterial pressure (+22 ±9 mm Hg) and a decrease in heart rate (– 36 + 13 beats/min), and tended to increase coronary blood flow (+7 ±4ml/min). Intracarotid nicotine had no effect on large coronary artery diameter (+ 0.02 ±0.03 mm) or total coronary resistance (+ 0.04 ±0.09 mm Hg min/ml) under these conditions. When heart rate was controlled by (1)-adrenoceptor blockade (propranolol, 1 mg/kg i. v.) plus pacing of the right ventricle (n = 4) or (2)-adrenoceptor blockade plus bilateral vagotomy (n = 7), the chemoreflex-induced constriction of the large coronary artery (–0.07 ±0.02 mm and –0.12 ± 0.03 mm, respectively;p < 0.05). In contrast, there was no chemoreflex-induced change in total coronary resistance after -adrenoceptor blockade plus pacing (+0.01 ±0.09 mm Hg min/ml, but after -adrenoceptor blockade plus vagotomy coronary resistance was increased (+0.75 ±0.31 mm Hg min/ml;p < 0.05). The constriction of both large and small coronary arteries was abolished by phentolamine (0.5 mg/kg i. v.). These results suggest that carotid body chemoreceptor stimulation by nicotine can produce reflex -adrenoceptor-mediated constriction of both large and small coronary arteries, and that the constriction of the small vessels is balanced by vagally-mediated dilatation.     

The influence ofPa O2 on the effects of sinus nerve stimulation with intact and inactivated vagi

Summary In anesthetized rabbits inhaling successively pure oxygen, air, and certain hypoxic O₂/N₂ mixtures, one cut carotid sinus nerve was stimulated under each of these conditions electrically with stimuli of constant intensity both before and after inactivation of the vagi by cold blocking and finally sectioning. In one group of these animals the contralateral sinus nurve was left intact, in the other it was cut. In the first group in which hypoxia produced a moderate hyperpnea with concomitant lowering of thePA _CO2, the augmentation of ventilation by the nerve stimulation decreased markedly with thePa _O2. In the latter group, where hypoxia caused no significant increase or even a slight diminution of ventilation, the respiratory effects of the nerve stimulating were not, or only inconsiderably influenced by the lowering of thePa _O2, while the decrease of blood pressure induced normally by the stimulation was reversed into a distinct increase. After vagal inactivation the stimulatory effects on tidal volume were markedly greater, on respiratory frequency smaller, and on minute volume of ventilation (though only slightly and not always significantly) greater than before.Supported by grant Wi 165 from the Deutsche Forschungsgemeinschaft.     

Der Einfluß desP A CO 2 auf die Wirkung der Sinusnervenreizung bei intakten und ausgeschalteten Nn. vagi

Summary In rabbits rebreathing oxygen one carotid sinus nerve was stimulated repeatedly by electrical stimuli of constant intensity, both before and after inactivation of vagi by cold blocking or sectioning. In animals with intact vagi the reflex hyperpnea elicited by the nerve stimulation decreased only slightly with increasing hypercapnia. After inactivation of the vagi the stimulatory effect was, at normalP _{A CO 2}, mostly greater than in the intact condition. But with increasing hypercapnia the decrease of the reflex hyperpnea was generally steeper than in the intact preparation, sometimes reaching even lower values than before inactivation. The depressant effects of the stimulation on systemic blood pressure and heart rate were not distinctly changed by the increase ofP _{A CO 2}, neither with intact nor with inactivated vagi. It is concluded that the ventilatory effect of chemoreceptor afferents is modified both by centralP _{CO 2} and by vagal afferents.

Mit Unterstützung der Deutschen Forschungsgemeinschaft (Wi 165).     

Inhalation of the nerve gas sarin impairs ventilatory responses to hypercapnia and hypoxia in rats

Sarin, a highly toxic nerve gas, is believed to cause bronchoconstriction and even death primarily through respiratory failure; however, the mechanism underlying the respiratory failure is not fully understood. The goals of this study were to ascertain whether sarin affects baseline ventilation (V_E) and V_E chemoreflexes as well as airway resistance and, if so, whether these changes are reversible. Four groups of F344 rats were exposed to vehicle (VEH) or sarin at 2.5, 3.5, and 4.0 mg h m^− 3 (SL, SM, and SH, respectively). V_E and V_E responses to hypercapnia (7% CO₂) or hypoxia (10% O₂) were measured by plethysmography at 2 h and 1, 2, and 5 days after VEH or sarin exposure. Total pulmonary resistance (R_L) also was measured in anesthetized VEH- and SH-exposed animals 2 h after exposure. Our results showed that within 2 h after exposure 11% of the SM- and 52% of the SH- exposed groups died. Although the SM and SH significantly decreased hypercapnic and hypoxic V_E to similar levels (64 and 69%), SH induced greater respiratory impairment, characterized by lower baseline V_E (30%; P < 0.05), and total loss of the respiratory frequency response to hypercapnia and hypoxia. V_E impairment recovered within 1-2 days after sarin exposure; interestingly, SH did not significantly affect baseline R_L. Moreover, sarin induced body tremors that were unrelated to the changes in the V_E responses. Thus, LC₅₀ sarin causes a reversible impairment of V_E that is not dependent on the sarin-induced body tremors and not associated with changes in R_L.