The respiratory and cardiovascular systems are highly intertwined, both anatomically and physiologically. Respiratory and cardiovascular neurons are often co-localized in the same brainstem regions, and this is particularly evident in the ventral medulla which contains pre-sympathetic neurons in the rostral ventrolateral medulla, premotor parasympathetic cardioinhibitory neurons in the nucleus ambiguus, and the ventral respiratory group, which includes the pre-Botzinger complex. Anatomical studies of respiratory and cardiovascular neurons have demonstrated that many of these neurons have projections and axon collateral processes which extend into their neighboring cardiorespiratory regions providing an anatomical substrate for cardiorespiratory interactions. As other reports in this Special Issue of Respiratory Physiology & Neurobiology focus on interactions between the respiratory network and baroreceptors, neurons in the nucleus tractus solitarius, presympathetic neurons and sympathetic activity, this report will focus on the respiratory modulation of parasympathetic activity and the neurons that generate parasympathetic activity to the heart, cardiac vagal neurons.

Obstructive sleep apnea (OSA) is characterized by recurrent upper airway obstruction during sleep. OSA leads to high cardiovascular morbidity and mortality. The pathogenesis of OSA has been linked to a defect in neuromuscular control of the pharynx. There is no effective pharmacotherapy for OSA. The objective of this study was to determine whether upper airway patency can be improved using chemogenetic approach by deploying designer receptors exclusively activated by designer drug (DREADD) in the hypoglossal motorneurons. DREADD (rAAV5-hSyn-hM3(Gq)-mCherry) and control virus (rAAV5-hSyn-EGFP) were stereotactically administered to the hypoglossal nucleus of C57BL/6J mice. In 6–8 weeks genioglossus EMG and dynamic MRI of the upper airway were performed before and after administration of the DREADD ligand clozapine-N-oxide (CNO) or vehicle (saline). In DREADD-treated mice, CNO activated the genioglossus muscle and markedly dilated the pharynx, whereas saline had no effect. Control virus treated mice showed no effect of CNO. Our results suggest that chemogenetic approach can be considered as a treatment option for OSA and other motorneuron disorders.

Hypocretin-1 is a neuropeptide recently shown to be involved in autonomic regulation. Hypocretin-1 is expressed by hypothalamic neurons, which project to many regions of the central nervous system, including the nucleus ambiguus. One possible site of action of hypocretin-1 could be cardioinhibitory parasympathetic vagal neurons within the nucleus ambiguus. This study examines whether hypocretin-1 modulates inhibitory and excitatory postsynaptic currents in cardiac vagal neurons in the rat nucleus ambiguus. GABAergic, glycinergic, and glutamatergic activity to cardiac vagal neurons was examined using whole-cell patch-clamp recordings in an in vitro brain slice preparation. Hypocretin-1 (1 M) produced a significant increase in the frequency and amplitude of both GABAergic and glycinergic inhibitory postsynaptic currents and a significant decrease in the frequency of glutamatergic excitatory postsynaptic currents. Application of tetrodotoxin (0.5 M) blocked all of the responses to hypocretin-1, indicating the changes in neurotransmission with hypocretin-1 do not occur at presynaptic terminals but rather occur at the preceding GABAergic, glycinergic, and glutamatergic neurons that project to cardiac vagal neurons. The increase in GABAergic and glycinergic inhibitory postsynaptic currents, and the decrease in glutamatergic excitatory postsynaptic currents, could be mechanisms by which hypocretin-1 affects heart rate and cardiac function.

Obstructive sleep apnea is associated with chronic intermittent hypoxia/hypercapnia (CIHH) episodes during sleep that heighten sympathetic and diminish parasympathetic activity to the heart. While one population of neurons in the paraventricular nucleus of the hypothalamus strongly influence sympathetic tone and have increased activity following CIHH, little is known about the role of this pathway to parasympathetic neurons, and how this network is altered in CIHH. We hypothesized that CIHH inhibits the excitatory pathway from the paraventricular nucleus of the hypothalamus to parasympathetic cardiac vagal neurons in the brainstem. To test this hypothesis channelrhodopsin was selectively expressed, using viral vectors, in neurons in the paraventricular nucleus of the hypothalamus and channelrhodopsin expressing fibers were photoactivated to evoke postsynaptic currents in cardiac vagal neurons in brainstem slices. Excitatory postsynaptic currents were diminished in animals exposed to CIHH. The paired-pulse and prolonged facilitation of the postsynaptic current amplitudes and frequencies evoked by paired and bursts of photoactivation of channelrhodopsin fibers, respectively, occurred in unexposed rats but were blunted in CIHH animals. In response to an acute challenge of hypoxia/hypercapnia the amplitude of postsynaptic events was unchanged during, but increased post hypoxia/hypercapnia in unexposed animals. In contrast, postsynaptic currents were inhibited during hypoxia/hypercapnia in rats exposed to CIHH. In conclusion, the excitatory pathway to cardiac vagal neurons is diminished in response to both acute and chronic exposures to hypoxia/hypercapnia. This could elicit a reduced cardioprotective parasympathetic activity and an enhanced risk of adverse cardiovascular events in episodes of apnea and chronic obstructive sleep apnea.

Maternal cigarette smoking and prenatal nicotine exposure increase the risk for sudden infant death syndrome (SIDS) by 2-to 4-fold, yet despite adverse publicity, nearly one of four pregnant women smoke tobacco. Infants who succumb to SIDS typically experience a severe bradycardia that precedes or is accompanied by centrally mediated life-threatening apneas and gasping. Although the causes of the apnea and bradycardia prevalent in SIDS victims are unknown, it has been hypothesized that these fatal events are exaggerated cardiorespiratory responses to hypoxia or hypercapnia. Changes in heart rate are primarily determined by the activity of cardiac vagal neurons (CVNs) in the brainstem. In this study, we tested whether hypoxia/hypercapnia evokes synaptic pathways to CVNs and whether these cardiorespiratory interactions are altered by prenatal exposure to nicotine. Spontaneous rhythmic inspiratoryrelated activity was recorded from the hypoglossal rootlet of 700-to 800-m medullary sections. CVNs were identified in this preparation by retrograde fluorescent labeling, and excitatory synaptic inputs to CVNs were isolated and studied using patchclamp electrophysiologic techniques. Hypoxia/hypercapnia did not elicit an increase in excitatory neurotransmission to CVNs in unexposed animals, but in animals that were exposed to nicotine in the prenatal period, hypoxia/hypercapnia recruited an excitatory neurotransmission to CVNs. This study establishes a likely neurochemical mechanism for the exaggerated decrease in heart rate in response to hypoxia/hypercapnia that occurs in SIDS victims. There are at least two major physiologic interactions between the respiratory system and the neural control of heart rate. The most ubiquitous cardiorespiratory interaction is respiratory sinus arrhythmia. During each respiratory cycle, the heart beat slows during postinspiration/expiration, and heart rate increases during inspiration. Such increases in heart rate during inspiration are observed in a wide variety of mammals, including both neonatal (1) and adult humans (2) and rats (3). Respiratory sinus arrhythmia helps match pulmonary blood flow to lung inflation and maintain the appropriate diffusion gradient for oxygen in the lungs (4).Although feedback from pulmonary stretch receptors, direct respiratory related-changes in venous return, and cardiac stretch can evoke respiratory-related fluctuations in heart rate, the dominant source of respiratory sinus arrhythmia originates from the brainstem (4,5). Respiratory sinus arrhythmia persists when the lungs are stationary (caused by muscle paralysis or constant flow ventilation), and the respiratory modulation of heart rate remains synchronized with brainstem respiratory rhythms even when artificial ventilation of the lungs and chemoreceptor activation occur at different intervals (6,7). In animals, including humans, respiratory sinus arrhythmia is mediated via cardiac vagal activity. Respiratory sinus arrhythmia persists in experimental animals upon sectioning sympathetic pathways and in...

Study Objectives Obesity leads to obstructive sleep apnea (OSA), which is recurrent upper airway obstruction during sleep, and obesity hypoventilation syndrome (OHS), hypoventilation during sleep resulting in daytime hypercapnia. Impaired leptin signaling in the brain was implicated in both conditions, but mechanisms are unknown. We have previously shown that leptin stimulates breathing and treats OSA and OHS in leptin- deficient ob/ob mice and leptin-resistant diet-induced obese mice and that leptin’s respiratory effects may occur in the dorsomedial hypothalamus (DMH). We hypothesized that leptin receptor LepR b–deficient db/db mice have obesity hypoventilation and that restoration of leptin signaling in the DMH will increase ventilation during sleep in these animals. Methods We measured arterial blood gas in unanesthetized awake db/db mice. We subsequently infected these animals with Ad-LepR  b or control Ad-mCherry virus into the DMH and measured ventilation during sleep as well as CO2 production after intracerebroventricular (ICV) infusions of phosphate-buffered saline or leptin. Results Awake db/db mice had elevated CO2 levels in the arterial blood. Ad-LepR  b infection resulted in LepR  b expression in the DMH neurons in a similar fashion to wildtype mice. In LepR  b-DMH db/db mice, ICV leptin shortened REM sleep and increased inspiratory flow, tidal volume and minute ventilation during NREM sleep without any effect on the quality of NREM sleep or CO2 production. Leptin had no effect on upper airway obstruction in these animals. Conclusion Leptin stimulates breathing and treats obesity hypoventilation acting on LepR b-positive neurons in the DMH.

Evidence of impaired function of orexin neurons has been found in individuals with cardiorespiratory disorders, such as obstructive sleep apnea (OSA) and sudden infant death syndrome (SIDS), but the mechanisms responsible are unknown. Individuals with OSA and SIDS experience repetitive breathing cessations and/or rebreathing of expired air, resulting in hypoxia/hypercapnia (H/H). In this study, we examined the responses of fluorescently identified rat orexin neurons in the lateral hypothalamus to acute H/H to test if and how these neurons alter their activity and function during this challenge. Experiments were conducted in an in vitro slice preparation using voltage-clamp and current-clamp configurations. H/H (10 min) induced hyperpolarization, accompanied by rapid depression, and finally, cessation of firing activity in orexin neurons. Hypoxia alone had similar but less potent effects. H/H did not alter the frequency of inhibitory glycinergic postsynaptic currents. The frequency of GABAergic currents was diminished but only at 8-10 min of H/H. In contrast, the frequency of excitatory glutamatergic postsynaptic events was diminished as early as 2-4 min of H/H. In the presence of glutamatergic receptor blockers, the inhibitory effects of H/H on the firing activity and membrane potential of orexin neurons persisted but to a lesser extent. In conclusion, both direct alteration of postsynaptic membrane properties and diminished glutamatergic neurotransmission likely contribute to the inhibition of orexin neurons by H/H. These mechanisms could be responsible for the decreased function of orexin in individuals at risk for OSA and SIDS.

Key pointsr Chronic intermittent hypoxia-hypercapnia (CIHH) in adult rats evoked hypertension and blunted the heart rate responses to acute hypoxia-hypercapnia (H-H).r CIHH induced an increase in spontaneous inhibitory and decreased excitatory neurotransmission to cardiac vagal neurons. r CIHH completely abolished acute H-H evoked inhibition of GABAergic while facilitating glycinergic neurotransmission to cardiac vagal neurons of nucleus ambiguus.r These changes with CIHH inhibit cardiac vagal neurons to result in diminished cardioprotective vagal activity to the heart, characteristic of obstructive sleep apnoea.Abstract Patients with obstructive sleep apnoea experience chronic intermittent hypoxia-hypercapnia (CIHH) during sleep that elicit sympathetic overactivity and diminished parasympathetic activity to the heart, leading to hypertension and depressed baroreflex sensitivity. The parasympathetic control of heart rate arises from pre-motor cardiac vagal neurons (CVNs) located in nucleus ambiguus (NA) and dorsal motor nucleus of the vagus (DMNX). The mechanisms underlying diminished vagal control of heart rate were investigated by studying the changes in blood pressure, heart rate, and neurotransmission to CVNs evoked by acute hypoxia-hypercapnia (H-H) and CIHH. In vivo telemetry recordings of blood pressure and heart rate were obtained in adult rats during 4 weeks of CIHH exposure. Retrogradely labelled CVNs were identified in an in vitro brainstem slice preparation obtained from adult rats exposed either to air or CIHH for 4 weeks. Postsynaptic inhibitory or excitatory currents were recorded using whole cell voltage clamp techniques. Rats exposed to CIHH had increases in blood pressure, leading to hypertension, and blunted heart rate responses to acute H-H. CIHH induced an increase in GABAergic and glycinergic neurotransmission to CVNs in NA and DMNX, respectively; and a reduction in glutamatergic neurotransmission to CVNs in both nuclei. CIHH blunted the bradycardia evoked by acute H-H and abolished the acute H-H evoked inhibition of GABAergic transmission while enhancing glycinergic neurotransmission to CVNs in NA. These changes with CIHH inhibit CVNs and vagal outflow to the heart, both in acute and chronic exposures to H-H, resulting in diminished levels of cardioprotective parasympathetic activity to the heart as seen in OSA patients.