Serotonin neurons and central respiratory chemoreception: where are we now?

Lethal Interactions of neuronal networks in epilepsy mediated by both synaptic and volume transmission indicate approaches to prevention

Neuronal network interactions are important in normal brain physiology and also in brain disorders. Many mesoscopic networks, including the auditory and respiratory network, mediate a single brain function. Macroscopic networks, including the locomotor network, central autonomic network (CAN), and many seizure networks involve interactions among multiple mesoscopic networks. Network interactions are mediated by neuroactive substances, acting via synaptic transmission, which mediate rapid interactions between networks. Slower, but vitally important network interactions, are mediated by volume transmission. Changes in the interactions between networks, mediated by neuroactive substances, can significantly alter network function and interactions. The acoustic startle response involves interactions between auditory and locomotor networks, and also includes brainstem reticular formation (BRF) nuclei, which participate in many different networks. In the fear-potentiated startle paradigm this network interacts positively with the amygdala, induced by conditioning. Seizure networks can interact negatively with the respiratory network, which becomes lethal in sudden unexpected death in epilepsy (SUDEP), a tragic emergent property of the seizure network. SUDEP models that exhibit audiogenic seizures (AGSz) involve interactions between the auditory and locomotor networks with BRF nuclei. In the DBA/1 mouse SUDEP model the AGSz network interacts negatively with the respiratory network, resulting in postictal apnea. The apnea is lethal unless the CAN is able to initiate autoresuscitation. These network interactions involve synaptic transmission, mediated by GABA and glutamate and volume transmission mediated by adenosine, CO2 and serotonin. Altering these interaction mechanisms may prevent SUDEP. These epilepsy network interactions illustrate the complex mechanisms that can occur among neuronal networks.

Sudden Unexpected Death in Epilepsy: Central Respiratory Chemoreception

Sudden unexpected death in epilepsy (SUDEP) is a critical concern for individuals suffering from epilepsy, with respiratory dysfunction playing a significant role in its pathology. Fatal seizures are often characterized by central apnea and hypercapnia (elevated CO2 levels), indicating a failure in ventilatory control. Research has shown that both human epilepsy patients and animal models exhibit a reduced hypercapnic ventilatory response in the interictal (non-seizure) period, suggesting an impaired ability to regulate breathing in response to high CO2 levels. This review examines the role of central chemoreceptors-specifically the retrotrapezoid nucleus, raphe nuclei, nucleus tractus solitarius, locus coeruleus, and hypothalamus in this pathology. These structures are critical for sensing CO2 and maintaining respiratory homeostasis. Emerging evidence also implicates neuropeptidergic pathways within these chemoreceptive regions in SUDEP. Neuropeptides like galanin, pituitary adenylate cyclase-activating peptide (PACAP), orexin, somatostatin, and bombesin-like peptides may modulate chemosensitivity and respiratory function, potentially exacerbating respiratory failure during seizures. Understanding the mechanisms linking central chemoreception, respiratory control, and neuropeptidergic signaling is essential to developing targeted interventions to reduce the risk of SUDEP in epilepsy patients.

Adapt or Die: Seizures Weaken Chemosensory Responses of Retrotrapezoid Nucleus Neurons to Hypercapnia

Neural Correlate of Reduced Respiratory Chemosensitivity During Chronic Epilepsy Bhandare AM, Dale N. Front Cell Neurosci . 2023;17:1288600. PMID: 38193031 While central autonomic, cardiac, and/or respiratory dysfunction underlies sudden unexpected death in epilepsy (SUDEP), the specific neural mechanisms that lead to SUDEP remain to be determined. In this study, we took advantage of single-cell neuronal Ca2+ imaging and intrahippocampal kainic acid-induced chronic epilepsy in mice to investigate progressive changes in key cardiorespiratory brainstem circuits during chronic epilepsy. Weeks after induction of status epilepticus (SE), when mice were experiencing recurrent spontaneous seizures (chronic epilepsy), we observed that the adaptive ventilatory responses to hypercapnia were reduced for 5 weeks after SE induction with its partial recovery at week 7. These changes were paralleled by alterations in the chemosensory responses of neurons in the retrotrapezoid nucleus (RTN). Neurons that displayed adapting responses to hypercapnia were less prevalent and exhibited smaller responses over weeks 3–5, whereas neurons that displayed graded responses to hypercapnia became more prevalent by week 7. Over the same period, chemosensory responses of the pre-sympathetic rostral ventrolateral medullary neurons showed no change. Mice with chronic epilepsy showed enhanced sensitivity to seizures, which invade the RTN and possibly put the chemosensory circuits at further risk of impairment. Our findings establish a dysfunctional breathing phenotype with its RTN neuronal correlate in mice with chronic epilepsy and suggest that the assessment of respiratory chemosensitivity may have the potential for identifying people at risk of SUDEP.

Effect of adenosinergic manipulations on amygdala-kindled seizures in mice: Implications for sudden unexpected death in epilepsy

OBJECTIVE

Sudden unexpected death in epilepsy (SUDEP) results in more years of potential life lost than any neurological condition with the exception of stroke. It is generally agreed that SUDEP happens due to some form of respiratory, cardiac, and electrocerebral dysfunction following a seizure; however, the mechanistic cause of these perturbations is unclear. One possible explanation lies with adenosinergic signaling. Extracellular levels of the inhibitory neuromodulator adenosine rapidly rise during seizures, a countermeasure that is necessary for seizure termination. Previous evidence has suggested that excessive adenosinergic inhibition could increase the risk of SUDEP by silencing brain areas necessary for life, such as the respiratory nuclei of the brainstem. The goal of this investigation was to further clarify the role of adenosine in seizure-induced respiratory and electrocerebral dysfunction.

METHODS

To determine the role of adenosine in postictal physiological dysregulation, we pharmacologically manipulated adenosine signaling prior to amygdala-kindled seizures in mice while recording electroencephalogram (EEG), electromyogram, and breathing using whole body plethysmography. The adenosinergic drugs used in this study included selective and nonselective adenosine receptor antagonists and inhibitors of adenosine metabolism.

RESULTS

We found that high doses of adenosine receptor antagonists caused some seizures to result in seizure-induced death; however, counterintuitively, animals in these conditions that did not experience seizure-induced death had little or no postictal generalized EEG suppression. Inhibitors of adenosine metabolism had no effect on postictal breathing but did worsen some postictal electrocerebral outcomes.

SIGNIFICANCE

The unexpected effect of high doses of adenosine antagonists on seizure-induced death observed in this study may be due to the increase in seizure severity, vasoconstriction, or phosphodiesterase inhibition caused by these drugs at high doses. These findings further clarify the role of adenosine in seizure-induced death and may have implications for the consumption of caffeine in epilepsy patients and the prevention of SUDEP.

Disruption of adenosine metabolism increases risk of seizure-induced death despite decreased seizure severity

OBJECTIVE

Respiratory arrest plays an important role in sudden unexpected death in epilepsy (SUDEP). Adenosine is of interest in SUDEP pathophysiology due to its influence on seizures and breathing. The objective of this investigation was to examine the role of adenosine in seizure severity, seizure-induced respiratory disruption, and seizure-induced death using mouse models. Understanding adenosinergic contributions to seizure cessation and seizure-induced death may provide insights into how SUDEP can be prevented while avoiding increased seizure severity.

METHODS

Our approach was to examine: (1) seizure severity and seizure-induced death after 15 mA electroshock seizures and during repeated pentylenetetrazol (PTZ) administration in wild-type mice (Adk +/+) and transgenic mice with reduced adenosine metabolism (Adk +/-); (2) the postictal hypercapnic ventilatory response (HCVR) in wild-type mice (the postictal HCVR could not be examined in Adk +/- mice due to their high mortality rate); and (3) the effects of adenosinergic drugs on seizure severity and seizure-induced death following maximal electroshock (MES).

RESULTS

Adk +/- mice were more vulnerable to seizure-induced death in the 15 mA electroshock and repeated PTZ models. Despite increased mortality, Adk +/- mice had comparable seizure severity in the PTZ model and reduced seizure severity in the 15 mA electroshock model. Breathing and HCVR were suppressed by 15 mA electroshock seizures in wild-type mice. Pharmacological inhibition of adenosine metabolism decreased MES seizure severity but did not increase mortality. A1 selective and nonselective adenosine receptor antagonists increased seizure-induced death following MES.

SIGNIFICANCE

Adenosine has opposing effects on seizure severity and seizure-induced death. On the one hand, our seizure severity data highlight the importance of adenosine in seizure suppression. On the other hand, our mortality data indicate that excessive extracellular adenosine signaling can increase the risk of seizure-induced respiratory arrest.

Impact of Serotonergic 5HT1A and 5HT2A Receptor Activation on the Respiratory Response to Hypercapnia in a Rat Model of Parkinson's Disease

In Parkinson's disease (PD), along with typical motor dysfunction, abnormal breathing is present; the cause of which is not well understood. The study aimed to analyze the effects of stimulation of the serotonergic system with 5-HT1A and 5-HT2A agonists in a model of PD induced by injection of 6-hydroxydopamine (6-OHDA). To model PD, bilateral injection of 6-OHDA into both striata was performed in male Wistar rats. Respiratory disturbances in response to 7% hypercapnia (CO2 in O2) in the plethysmographic chamber before and after stimulation of the serotonergic system and the incidence of apnea were studied in awake rats 5 weeks after 6-OHDA or vehicle injection. Administration of 6-OHDA reduced the concentration of serotonin (5-HT), dopamine (DA) and norepinephrine (NA) in the striatum and the level of 5-HT in the brainstem of treated rats, which have been associated with decreased basal ventilation, impaired respiratory response to 7% CO2 and increased incidence of apnea compared to Sham-operated rats. Intraperitoneal (i.p.) injection of the 5-HT1AR agonist 8-OH-DPAT and 5-HT2AR agonist NBOH-2C-CN increased breathing during normocapnia and hypercapnia in both groups of rats. However, it restored reactivity to hypercapnia in 6-OHDA group to the level present in Sham rats. Another 5-HT2AR agonist TCB-2 was only effective in increasing normocapnic ventilation in 6-OHDA rats. Both the serotonergic agonists 8-OH-DPAT and NBOH-2C-CN had stronger stimulatory effects on respiration in PD rats, compensating for deficits in basal ventilation and hypercapnic respiration. We conclude that serotonergic stimulation may have a positive effect on respiratory impairments that occur in PD.

Microglial reactivity in brainstem chemosensory nuclei in response to hypercapnia

Microglia, the resident immune cells of the CNS, surveil, detect, and respond to various extracellular signals. Depending on the nature of these signals, an integrative microglial response can be triggered, resulting in a phenotypic transformation. Here, we evaluate whether hypercapnia modifies microglia phenotype in brainstem respiratory-related nuclei. Adult C57BL/6 inbred mice were exposed to 10% CO2 enriched air (hypercapnia), or pure air (control), for 10 or 30 min and immediately processed for immunohistochemistry to detect the ubiquitous microglia marker, ionized calcium binding adaptor molecule 1 (Iba1). Hypercapnia for thirty, but not 10 min reduced the Iba1 labeling percent coverage in the ventral respiratory column (VRC), raphe nucleus (RN), and nucleus tractus solitarius (NTS) and the number of primary branches in VRC. The morphological changes persisted, at least, for 60 min breathing air after the hypercapnic challenge. No significant changes were observed in Iba1+ cells in the spinal trigeminal nucleus (Sp5) and the hippocampus. In CF-1 outbred mice, 10% CO2 followed by 60 min of breathing air, resulted in the reduction of Iba1 labeling percent coverage and the number and length of primary branches in VRC, RN, and NTS. No morphological change was observed in Iba1+ cells in Sp5 and hippocampus. Double immunofluorescence revealed that prolonged hypercapnia increased the expression of CD86, an inflammatory marker for reactive state microglia, in Iba1+ cells in VRC, RN, and NTS, but not in Sp5 and hippocampus in CF-1 mice. By contrast, the expression of CD206, a marker of regulatory state microglia, persisted unmodified. In brainstem, but not in hippocampal microglia cultures, hypercapnia increased the level of IL1β, but not that of TGFβ measured by ELISA. Our results show that microglia from respiratory-related chemosensory nuclei, are reactive to prolonged hypercapnia acquiring an inflammatory-like phenotype.

Male histone deacetylase 6 (HDAC6) knockout mice have enhanced ventilatory responses to hypoxic challenge

Histone deacetylase 6 (HDAC6) is a class II histone deacetylase that is predominantly localized in the cytoplasm of cells. HDAC6 associates with microtubules and regulates acetylation of tubulin and other proteins. The possibility that HDAC6 participates in hypoxic signaling is supported by evidence that 1) hypoxic gas challenges cause microtubule depolymerization, 2) expression of hypoxia inducible factor alpha (HIF-1α) is regulated by microtubule alterations in response to hypoxia, and 3) inhibition of HDAC6 prevents HIF-1α expression and protects tissue from hypoxic/ischemic insults. The aim of this study was to address whether the absence of HDAC6 alters ventilatory responses during and/or after hypoxic gas challenge (10% O2, 90% N2 for 15 min) in adult male wildtype (WT) C57BL/6 mice and HDAC6 knock-out (KO) mice. Key findings were that 1) baseline values for frequency of breathing, tidal volume, inspiratory and expiratory times, and end expiratory pause were different between knock-out mice and wildtype mice, 2) ventilatory responses during hypoxic challenge were more robust in KO mice than WT mice for recorded parameters including, frequency of breathing, minute ventilation, inspiratory and expiratory durations, peak inspiratory and expiratory flows, and inspiratory and expiratory drives, and 3) responses upon return to room-air were markedly different in KO compared to WT mice for frequency of breathing, minute ventilation, inspiratory and expiratory durations, end expiratory pause (but not end inspiratory pause), peak inspiratory and expiratory flows, and inspiratory and expiratory drives. These data suggest that HDAC6 may have a fundamentally important role in regulating the hypoxic ventilatory response in mice.

Contributions of carotid bodies, retrotrapezoid nucleus neurons and preBötzinger complex astrocytes to the CO2 -sensitive drive for breathing

Current models of respiratory CO2 chemosensitivity are centred around the function of a specific population of neurons residing in the medullary retrotrapezoid nucleus (RTN). However, there is significant evidence suggesting that chemosensitive neurons exist in other brainstem areas, including the rhythm-generating region of the medulla oblongata - the preBötzinger complex (preBötC). There is also evidence that astrocytes, non-neuronal brain cells, contribute to central CO2 chemosensitivity. In this study, we reevaluated the relative contributions of the RTN neurons, the preBötC astrocytes, and the carotid body chemoreceptors in mediating the respiratory responses to CO2 in experimental animals (adult laboratory rats). To block astroglial signalling via exocytotic release of transmitters, preBötC astrocytes were targeted to express the tetanus toxin light chain (TeLC). Bilateral expression of TeLC in preBötC astrocytes was associated with ∼20% and ∼30% reduction of the respiratory response to CO2 in conscious and anaesthetized animals, respectively. Carotid body denervation reduced the CO2 respiratory response by ∼25%. Bilateral inhibition of RTN neurons transduced to express Gi-coupled designer receptors exclusively activated by designer drug (DREADDGi ) by application of clozapine-N-oxide reduced the CO2 response by ∼20% and ∼40% in conscious and anaesthetized rats, respectively. Combined blockade of astroglial signalling in the preBötC, inhibition of RTN neurons and carotid body denervation reduced the CO2 -induced respiratory response by ∼70%. These data further support the hypothesis that the CO2 -sensitive drive to breathe requires inputs from the peripheral chemoreceptors and several central chemoreceptor sites. At the preBötC level, astrocytes modulate the activity of the respiratory network in response to CO2 , either by relaying chemosensory information (i.e. they act as CO2  sensors) or by enhancing the preBötC network excitability to chemosensory inputs. KEY POINTS: This study reevaluated the roles played by the carotid bodies, neurons of the retrotrapezoid nucleus (RTN) and astrocytes of the preBötC in mediating the CO2 -sensitive drive to breathe. The data obtained show that disruption of preBötC astroglial signalling, blockade of inputs from the peripheral chemoreceptors or inhibition of RTN neurons similarly reduce the respiratory response to hypercapnia. These data provide further support for the hypothesis that the CO2 -sensitive drive to breathe is mediated by the inputs from the peripheral chemoreceptors and several central chemoreceptor sites.

Interactions between lorcaserin and opioids: Ventilation and food-versus-drug choice

Lorcaserin, a selective serotonin 2C (5-HT2C) receptor agonist, was approved for treating obesity and has been investigated for treating substance use disorders including those involving opioids. Although lorcaserin was withdrawn from the market, interest in the therapeutic potential of drugs acting at 5-HT2C receptors continues, supporting the need to further characterize potential adverse effects especially when combined with drugs of abuse. This study examined acute effects of lorcaserin on opioid-induced ventilatory depression, which is the primary cause of overdose, and opioid self-administration, which models factors contributing to opioid abuse, in male and female rhesus monkeys. In one group (n = 4), effects of morphine (0.178 to 5.6 mg/kg, s.c.), fentanyl (0.0032 to 0.1 mg/kg, s.c.), and lorcaserin (0.1 to 1.78 mg/kg, s.c.) alone as well as effects of lorcaserin with each opioid on ventilation were determined using head plethysmography. Another group (n = 5) responded under a food versus fentanyl (0.1 to 3.2 μg/kg/infusion, i.v.) choice procedure, and lorcaserin (0.32 to 1.78 mg/kg, i.v.) was given as a pretreatment. Lorcaserin dose-dependently decreased minute volume to below 70 % of baseline when administered alone and increased the potency of morphine and fentanyl. Consistent with previous studies, lorcaserin failed to alter choice of fentanyl over food. This study demonstrates the novel finding that lorcaserin alone decreases ventilation and enhances the ventilatory-depressant effects of opioids. Taken together with previous studies, these results suggest that combining a 5-HT2C receptor agonist such as lorcaserin with an opioid could increase the risk of ventilatory depression without the benefit of decreasing abuse.

Failure to breathe persists without air hunger or alarm following amygdala seizures

Postictal apnea is thought to be a major cause of sudden unexpected death in epilepsy (SUDEP). However, the mechanisms underlying postictal apnea are unknown. To understand causes of postictal apnea, we used a multimodal approach to study brain mechanisms of breathing control in 20 patients (ranging from pediatric to adult) undergoing intracranial electroencephalography for intractable epilepsy. Our results indicate that amygdala seizures can cause postictal apnea. Moreover, we identified a distinct region within the amygdala where electrical stimulation was sufficient to reproduce prolonged breathing loss persisting well beyond the end of stimulation. The persistent apnea was resistant to rising CO2 levels, and air hunger failed to occur, suggesting impaired CO2 chemosensitivity. Using es-fMRI, a potentially novel approach combining electrical stimulation with functional MRI, we found that amygdala stimulation altered blood oxygen level-dependent (BOLD) activity in the pons/medulla and ventral insula. Together, these findings suggest that seizure activity in a focal subregion of the amygdala is sufficient to suppress breathing and air hunger for prolonged periods of time in the postictal period, likely via brainstem and insula sites involved in chemosensation and interoception. They further provide insights into SUDEP, may help identify those at greatest risk, and may lead to treatments to prevent SUDEP.

Plasma serotonergic biomarkers are associated with hypoxemia events in preterm neonates

BACKGROUND

Hypoxemia is a physiological manifestation of immature respiratory control in preterm neonates, which is likely impacted by neurotransmitter imbalances. We investigated relationships between plasma levels of the neurotransmitter serotonin (5-HT), metabolites of tryptophan (TRP), and parameters of hypoxemia in preterm neonates.

METHODS

TRP, 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), and kynurenic acid (KA) were analyzed in platelet-poor plasma at ~1 week and ~1 month of life from a prospective cohort of 168 preterm neonates <31 weeks gestational age (GA). Frequency of intermittent hypoxemia (IH) events and percent time hypoxemic (<80%) were analyzed in a 6 h window after the blood draw.

RESULTS

At 1 week, infants with detectable plasma 5-HT had fewer IH events (OR (95% CI) = 0.52 (0.29, 0.31)) and less percent time <80% (OR (95% CI) = 0.54 (0.31, 0.95)) compared to infants with undetectable 5-HT. A similar relationship occurred at 1 month. At 1 week, infants with higher KA showed greater percent time <80% (OR (95% CI) = 1.90 (1.03, 3.50)). TRP, 5-HIAA or KA were not associated with IH frequency at either postnatal age. IH frequency and percent time <80% were positively associated with GA < 29 weeks.

CONCLUSIONS

Circulating neuromodulators 5-HT and KA might represent biomarkers of immature respiratory control contributing to hypoxemia in preterm neonates.

IMPACT

Hypoxemia events are frequent in preterm infants and are associated with poor outcomes. Mechanisms driving hypoxemia such as immature respiratory control may include central and peripheral imbalances in modulatory neurotransmitters. This study found associations between the plasma neuromodulators serotonin and kynurenic acid and parameters of hypoxemia in preterm neonates. Imbalances in plasma biomarkers affecting respiratory control may help identify neonates at risk of short- and long-term adverse outcomes.

Whole-brain analysis of CO2 chemosensitive regions and identification of the retrotrapezoid and medullary raphé nuclei in the common marmoset (Callithrix jacchus)

Respiratory chemosensitivity is an important mechanism by which the brain senses changes in blood partial pressure of CO2 (PCO2). It is proposed that special neurons (and astrocytes) in various brainstem regions play key roles as CO2 central respiratory chemosensors in rodents. Although common marmosets (Callithrix jacchus), New-World non-human primates, show similar respiratory responses to elevated inspired CO2 as rodents, the chemosensitive regions in marmoset brain have not been defined yet. Here, we used c-fos immunostainings to identify brain-wide CO2-activated brain regions in common marmosets. In addition, we mapped the location of the retrotrapezoid nucleus (RTN) and raphé nuclei in the marmoset brainstem based on colocalization of CO2-induced c-fos immunoreactivity with Phox2b, and TPH immunostaining, respectively. Our data also indicated that, similar to rodents, marmoset RTN astrocytes express Phox2b and have complex processes that create a meshwork structure at the ventral surface of medulla. Our data highlight some cellular and structural regional similarities in brainstem of the common marmosets and rodents.

Seizures Cause Prolonged Impairment of Ventilation, CO2 Chemoreception and Thermoregulation

Sudden unexpected death in epilepsy (SUDEP) has been linked to respiratory dysfunction, but the mechanisms underlying this association remain unclear. Here we found that both focal and generalized convulsive seizures (GCSs) in epilepsy patients caused a prolonged decrease in the hypercapnic ventilatory response (HCVR; a measure of respiratory CO2 chemoreception). We then studied Scn1a R1407X/+ (Dravet syndrome; DS) and Scn8a N1768D/+ (D/+) mice of both sexes, two models of SUDEP, and found that convulsive seizures caused a postictal decrease in ventilation and severely depressed the HCVR in a subset of animals. Those mice with severe postictal depression of the HCVR also exhibited transient postictal hypothermia. A combination of blunted HCVR and abnormal thermoregulation is known to occur with dysfunction of the serotonin (5-hydroxytryptamine; 5-HT) system in mice. Depleting 5-HT with para-chlorophenylalanine (PCPA) mimicked seizure-induced hypoventilation, partially occluded the postictal decrease in the HCVR, exacerbated hypothermia, and increased postictal mortality in DS mice. Conversely, pretreatment with the 5-HT agonist fenfluramine reduced postictal inhibition of the HCVR and hypothermia. These results are consistent with the previous observation that seizures cause transient impairment of serotonergic neuron function, which would be expected to inhibit the many aspects of respiratory control dependent on 5-HT, including baseline ventilation and the HCVR. These results provide a scientific rationale to investigate the interictal and/or postictal HCVR as noninvasive biomarkers for those at high risk of seizure-induced death, and to prevent SUDEP by enhancing postictal 5-HT tone.SIGNIFICANCE STATEMENT There is increasing evidence that seizure-induced respiratory dysfunction contributes to the pathophysiology of sudden unexpected death in epilepsy (SUDEP). However, the cellular basis of this dysfunction has not been defined. Here, we show that seizures impair CO2 chemoreception in some epilepsy patients. In two mouse models of SUDEP we found that generalized convulsive seizures impaired CO2 chemoreception, and induced hypothermia, two effects reported with serotonergic neuron dysfunction. The defects in chemoreception and thermoregulation were exacerbated by chemical depletion of serotonin and reduced with fenfluramine, suggesting that seizure-induced respiratory dysfunction may be due to impairment of serotonin neuron function. These findings suggest that impaired chemoreception because of transient inhibition of serotonergic neurons may contribute to the pathophysiology of SUDEP.

Male histone deacetylase 6 (HDAC6) knockout mice have enhanced ventilatory responses to hypoxic challenge

Histone deacetylase 6 (HDAC6) is a class II histone deacetylase that is predominantly localized in the cytoplasm of cells. HDAC6 associates with microtubules, regulating acetylation of tubulin and other proteins. The possibility that HDAC6 participates in hypoxic signaling is supported by evidence that (1) hypoxic gas challenges cause microtubule depolymerization, (2) expression of hypoxia inducible factor alpha (HIF)-1α is regulated by microtubule alterations in response to hypoxia, and (3) inhibition of HDAC6 prevents HIF-1α expression and protects tissue from hypoxic/ischemic insults. The aim of this study was to address whether the absence of HDAC6 alters ventilatory responses during and/or after hypoxic gas challenges (10% O2, 90% N2 for 15 min) in adult male wild-type (WT) C57BL/6 mice and HDAC6 knockout (KO) mice. Key findings were that (1) baseline values for frequency of breathing, tidal volume, inspiratory and expiratory times and end expiratory pause were different between KO mice and WT mice, (2) ventilatory responses during hypoxic challenge were more robust in KO mice than WT mice for parameters including frequency of breathing, minute ventilation, inspiratory and expiratory durations, peak inspiratory and expiratory flows, inspiratory and expiratory drives, and (3) responses upon return to room-air were markedly different in KO mice than WT mice for frequency of breathing, minute ventilation, inspiratory and expiratory durations, end expiratory (but not end inspiratory) pauses, peak inspiratory and expiratory flows, and inspiratory or expiratory drives. These data suggest that HDAC6 may have a fundamentally important role in regulating the neural responses to hypoxia.

The integrated brain network that controls respiration

Respiration is a brain function on which our lives essentially depend. Control of respiration ensures that the frequency and depth of breathing adapt continuously to metabolic needs. In addition, the respiratory control network of the brain has to organize muscular synergies that integrate ventilation with posture and body movement. Finally, respiration is coupled to cardiovascular function and emotion. Here, we argue that the brain can handle this all by integrating a brainstem central pattern generator circuit in a larger network that also comprises the cerebellum. Although currently not generally recognized as a respiratory control center, the cerebellum is well known for its coordinating and modulating role in motor behavior, as well as for its role in the autonomic nervous system. In this review, we discuss the role of brain regions involved in the control of respiration, and their anatomical and functional interactions. We discuss how sensory feedback can result in adaptation of respiration, and how these mechanisms can be compromised by various neurological and psychological disorders. Finally, we demonstrate how the respiratory pattern generators are part of a larger and integrated network of respiratory brain regions.

Effects of Prenatal Cannabinoids Exposure upon Placenta and Development of Respiratory Neural Circuits

Cannabis use has risen dangerously during pregnancy in the face of incipient therapeutic use and a growing perception of safety. The main psychoactive compound of the Cannabis sativa plant is the phytocannabinoid delta-9-tetrahydrocannabinol (A-9 THC), and its status as a teratogen is controversial. THC and its endogenous analogues, anandamide (AEA) and 2-AG, exert their actions through specific receptors (eCBr) that activate intracellular signaling pathways. CB1r and CB2r, also called classic cannabinoid receptors, together with their endogenous ligands and the enzymes that synthesize and degrade them, constitute the endocannabinoid system. This system is distributed ubiquitously in various central and peripheral tissues. Although the endocannabinoid system's most studied role is controlling the release of neurotransmitters in the central nervous system, the study of long-term exposure to cannabinoids on fetal development is not well known and is vital for understanding environmental or pathological embryo-fetal or postnatal conditions. Prenatal exposure to cannabinoids in animal models has induced changes in placental and embryo-fetal organs. Particularly, cannabinoids could influence both neural and nonneural tissues and induce embryo-fetal pathological conditions in critical processes such as neural respiratory control. This review aims at the acute and chronic effects of prenatal exposure to cannabinoids on placental function and the embryo-fetal neurodevelopment of the respiratory pattern. The information provided here will serve as a theoretical framework to critically evaluate the teratogen effects of the consumption of cannabis during pregnancy.

The interplay of hypoxic and mental stress: Implications for anxiety and depressive disorders

Adequate oxygen supply is essential for the human brain to meet its high energy demands. Therefore, elaborate molecular and systemic mechanism are in place to enable adaptation to low oxygen availability. Anxiety and depressive disorders are characterized by alterations in brain oxygen metabolism and of its components, such as mitochondria or hypoxia inducible factor (HIF)-pathways. Conversely, sensitivity and tolerance to hypoxia may depend on parameters of mental stress and the severity of anxiety and depressive disorders. Here we discuss relevant mechanisms of adaptations to hypoxia, as well as their involvement in mental stress and the etiopathogenesis of anxiety and depressive disorders. We suggest that mechanisms of adaptations to hypoxia (including metabolic responses, inflammation, and the activation of chemosensitive brain regions) modulate and are modulated by stress-related pathways and associated psychiatric diseases. While severe chronic hypoxia or dysfunctional hypoxia adaptations can contribute to the pathogenesis of anxiety and depressive disorders, harnessing controlled responses to hypoxia to increase cellular and psychological resilience emerges as a novel treatment strategy for these diseases.

Patch-to-Seq and Transcriptomic Analyses Yield Molecular Markers of Functionally Distinct Brainstem Serotonin Neurons

Acute regulation of CO₂ and pH homeostasis requires sensory feedback from peripheral (carotid body) and central (central) CO₂/pH sensitive cells - so called respiratory chemoreceptors. Subsets of brainstem serotonin (5-HT) neurons in the medullary raphe are CO₂ sensitive or insensitive based on differences in embryonic origin, suggesting these functionally distinct subpopulations may have unique transcriptional profiles. Here, we used Patch-to-Seq to determine if the CO₂ responses in brainstem 5-HT neurons could be correlated to unique transcriptional profiles and/or unique molecular markers and pathways. First, firing rate changes with hypercapnic acidosis were measured in fluorescently labeled 5-HT neurons in acute brainstem slices from transgenic, Dahl SS (SSMcwi) rats expressing T2/ePet-eGFP transgene in Pet-1 expressing (serotonin) neurons (SS ^ePet1-eGFP rats). Subsequently, the transcriptomic and pathway profiles of CO₂ sensitive and insensitive 5-HT neurons were determined and compared by single cell RNA (scRNAseq) and bioinformatic analyses. Low baseline firing rates were a distinguishing feature of CO₂ sensitive 5-HT neurons. scRNAseq of these recorded neurons revealed 166 differentially expressed genes among CO₂ sensitive and insensitive 5-HT neurons. Pathway analyses yielded novel predicted upstream regulators, including the transcription factor Egr2 and Leptin. Additional bioinformatic analyses identified 6 candidate gene markers of CO₂ sensitive 5-HT neurons, and 2 selected candidate genes (CD46 and Iba57) were both expressed in 5-HT neurons determined via in situ mRNA hybridization. Together, these data provide novel insights into the transcriptional control of cellular chemoreception and provide unbiased candidate gene markers of CO₂ sensitive 5-HT neurons. Methodologically, these data highlight the utility of the patch-to-seq technique in enabling the linkage of gene expression to specific functions, like CO₂ chemoreception, in a single cell to identify potential mechanisms underlying functional differences in otherwise similar cell types.

In Transgenic Erythropoietin Deficient Mice, an Increase in Respiratory Response to Hypercapnia Parallels Abnormal Distribution of CO2/H+-Activated Cells in the Medulla Oblongata

Erythropoietin (Epo) and its receptor are expressed in central respiratory areas. We hypothesized that chronic Epo deficiency alters functioning of central respiratory areas and thus the respiratory adaptation to hypercapnia. The hypercapnic ventilatory response (HcVR) was evaluated by whole body plethysmography in wild type (WT) and Epo deficient (Epo-TAg^h) adult male mice under 4%CO₂. Epo-TAg^h mice showed a larger HcVR than WT mice because of an increase in both respiratory frequency and tidal volume, whereas WT mice only increased their tidal volume. A functional histological approach revealed changes in CO₂/H⁺-activated cells between Epo-TAg^h and WT mice. First, Epo-TAg^h mice showed a smaller increase under hypercapnia in c-FOS-positive number of cells in the retrotrapezoid nucleus/parafacial respiratory group than WT, and this, independently of changes in the number of PHOX2B-expressing cells. Second, we did not observe in Epo-TAg^h mice the hypercapnic increase in c-FOS-positive number of cells in the nucleus of the solitary tract present in WT mice. Finally, whereas hypercapnia did not induce an increase in the c-FOS-positive number of cells in medullary raphe nuclei in WT mice, chronic Epo deficiency leads to raphe pallidus and magnus nuclei activation by hyperacpnia, with a significant part of c-FOS positive cells displaying an immunoreactivity for serotonin in the raphe pallidus nucleus. All of these results suggest that chronic Epo-deficiency affects both the pattern of ventilatory response to hypercapnia and associated medullary respiratory network at adult stage with an increase in the sensitivity of 5-HT and non-5-HT neurons of the raphe medullary nuclei leading to stimulation of f_R for moderate level of CO₂.

5-HT7 receptors expressed in the mouse parafacial region are not required for respiratory chemosensitivity

Abstract

A brainstem homeostatic system senses CO₂/H⁺ to regulate ventilation, blood gases and acid–base balance. Neurons of the retrotrapezoid nucleus (RTN) and medullary raphe are both implicated in this mechanism as respiratory chemosensors, but recent pharmacological work suggested that the CO₂/H⁺ sensitivity of RTN neurons is mediated indirectly, by raphe‐derived serotonin acting on 5‐HT7 receptors. To investigate this further, we characterized Htr7 transcript expression in phenotypically identified RTN neurons using multiplex single cell qRT‐PCR and RNAscope. Although present in multiple neurons in the parafacial region of the ventrolateral medulla, Htr7 expression was undetectable in most RTN neurons (Nmb⁺/Phox2b⁺) concentrated in the densely packed cell group ventrolateral to the facial nucleus. Where detected, Htr7 expression was modest and often associated with RTN neurons that extend dorsolaterally to partially encircle the facial nucleus. These dorsolateral Nmb⁺/Htr7⁺ neurons tended to express Nmb at high levels and the intrinsic RTN proton detectors Gpr4 and Kcnk5 at low levels. In mouse brainstem slices, CO₂‐stimulated firing in RTN neurons was mostly unaffected by a 5‐HT7 receptor antagonist, SB269970 (n = 11/13). At the whole animal level, microinjection of SB269970 into the RTN of conscious mice blocked respiratory stimulation by co‐injected LP‐44, a 5‐HT7 receptor agonist, but had no effect on CO₂‐stimulated breathing in those same mice. We conclude that Htr7 is expressed by a minor subset of RTN neurons with a molecular profile distinct from the established chemoreceptors and that 5‐HT7 receptors have negligible effects on CO₂‐evoked firing activity in RTN neurons or on CO₂‐stimulated breathing in mice.

Key points

Neurons of the retrotrapezoid nucleus (RTN) are intrinsic CO₂/H⁺ chemosensors and serve as an integrative excitatory hub for control of breathing.

Serotonin can activate RTN neurons, in part via 5‐HT7 receptors, and those effects have been implicated in conferring an indirect CO₂ sensitivity.

Multiple single cell molecular approaches revealed low levels of 5‐HT7 receptor transcript expression restricted to a limited population of RTN neurons.

Pharmacological experiments showed that 5‐HT7 receptors in RTN are not required for CO₂/H⁺‐stimulation of RTN neuronal activity or CO₂‐stimulated breathing.

These data do not support a role for 5‐HT7 receptors in respiratory chemosensitivity mediated by RTN neurons.

Respiratory Abnormalities in Parkinson's Disease: What Do We Know from Studies in Humans and Animal Models?

Parkinson’s disease (PD) is the second most common progressive neurodegenerative disease characterized by movement disorders due to the progressive loss of dopaminergic neurons in the ventrolateral region of the substantia nigra pars compacta (SNpc). Apart from the cardinal motor symptoms such as rigidity and bradykinesia, non-motor symptoms including those associated with respiratory dysfunction are of increasing interest. Not only can they impair the patients’ quality of life but they also can cause aspiration pneumonia, which is the leading cause of death among PD patients. This narrative review attempts to summarize the existing literature on respiratory impairments reported in human studies, as well as what is newly known from studies in animal models of the disease. Discussed are not only respiratory muscle dysfunction, apnea, and dyspnea, but also altered central respiratory control, responses to hypercapnia and hypoxia, and how they are affected by the pharmacological treatment of PD.

An open-source tool for automated analysis of breathing behaviors in common marmosets and rodents

The respiratory system maintains homeostatic levels of oxygen (O2) and carbon dioxide (CO2) in the body through rapid and efficient regulation of breathing frequency and depth (tidal volume). The commonly used methods of analyzing breathing data in behaving experimental animals are usually subjective, laborious, and time-consuming. To overcome these hurdles, we optimized an analysis toolkit for the unsupervised study of respiratory activities in animal subjects. Using this tool, we analyzed breathing behaviors of the common marmoset (Callithrix jacchus), a New World non-human primate model. Using whole-body plethysmography in room air as well as acute hypoxic (10% O2) and hypercapnic (6% CO2) conditions, we describe breathing behaviors in awake, freely behaving marmosets. Our data indicate that marmosets' exposure to acute hypoxia decreased metabolic rate and increased sigh rate. However, the hypoxic condition did not augment ventilation. Hypercapnia, on the other hand, increased both the frequency and depth (i.e., tidal volume) of breathing.

Sudden unexpected death in epilepsy: Respiratory mechanisms

Epilepsy is one of the most common chronic neurologic diseases, with a prevalence of 1% in the US population. Many people with epilepsy live normal lives, but are at risk of sudden unexpected death in epilepsy (SUDEP). This mysterious comorbidity of epilepsy causes premature death in 17%-50% of those with epilepsy. Most SUDEP occurs after a generalized seizure, and patients are typically found in bed in the prone position. Until recently, it was thought that SUDEP was due to cardiovascular failure, but patients who died while being monitored in hospital epilepsy units revealed that most SUDEP is due to postictal central apnea. Some cases may occur when seizures invade the amygdala and activate projections to the brainstem. Evidence suggests that the pathophysiology is linked to defects in the serotonin system and central CO₂ chemoreception, and that there is considerable overlap with mechanisms thought to be involved in sudden infant death syndrome (SIDS). Future work is needed to identify biomarkers for patients at highest risk, improve ascertainment, develop methods to alert caregivers when SUDEP is imminent, and find effective approaches to prevent these fatal events.

Central respiratory chemoreception

Brain PCO2 is sensed primarily via changes in [H+]. Small pH changes are detected in the medulla oblongata and trigger breathing adjustments that help maintain arterial PCO2 constant. Larger perturbations of brain CO2/H+, possibly also sensed elsewhere in the CNS, elicit arousal, dyspnea, and stress, and cause additional breathing modifications. The retrotrapezoid nucleus (RTN), a rostral medullary cluster of glutamatergic neurons identified by coexpression of Phoxb and Nmb transcripts, is the lynchpin of the central respiratory chemoreflex. RTN regulates breathing frequency, inspiratory amplitude, and active expiration. It is exquisitely responsive to acidosis in vivo and maintains breathing autorhythmicity during quiet waking, slow-wave sleep, and anesthesia. The RTN response to [H+] is partly an intrinsic neuronal property mediated by proton sensors TASK-2 and GPR4 and partly a paracrine effect mediated by astrocytes and the vasculature. The RTN also receives myriad excitatory or inhibitory synaptic inputs including from [H+]-responsive neurons (e.g., serotonergic). RTN is silenced by moderate hypoxia. RTN inactivity (periodic or sustained) contributes to periodic breathing and, likely, to central sleep apnea. RTN development relies on transcription factors Egr2, Phox2b, Lbx1, and Atoh1. PHOX2B mutations cause congenital central hypoventilation syndrome; they impair RTN development and consequently the central respiratory chemoreflex.

Serotonergic regulation of the dopaminergic system: Implications for reward-related functions

Serotonin is a critical neuromodulator involved in development and behavior. Its role in reward is however still debated. Here, we first review classical studies involving electrical stimulation protocols and pharmacological approaches. Contradictory results on the serotonergic' involvement in reward emerge from these studies. These differences might be ascribable to either the diversity of cellular types within the raphe nuclei or/and the specific projection pathways of serotonergic neurons. We continue to review more recent work, using optogenetic approaches to activate serotonergic cells in the Raphe to VTA pathway. From these studies, it appears that activation of this pathway can lead to reinforcement learning mediated through the excitation of dopaminergic neurons by serotonergic neurons co-transmitting glutamate. Finally, given the importance of serotonin during development on adult emotion, the effect of abnormal early-life levels of serotonin on the dopaminergic system will also be discussed. Understanding the interaction between the serotonergic and dopaminergic systems during development and adulthood is critical to gain insight into the specific facets of neuropsychiatric disorders.

The role of convulsive seizures in SUDEP

Convulsive seizures are the most consistently reported risk factor for SUDEP. However, the precise mechanisms by which convulsive seizures trigger fatal cardiopulmonary changes are still unclear. Additionally, it is not clear why some seizures cause death when most do not. This article reviews the physiologic changes that occur during and after convulsive seizures and how these may contribute to SUDEP. Seizures activate specific cortical and subcortical regions that can cause potentially lethal cardiorespiratory changes. Clinical factors, including sleep state, medication treatment and withdrawal, positioning and posturing during seizures, and underlying structural or genetic conditions may also affect specific aspects of seizures that may contribute to SUDEP. While seizure control, either through medication or surgical treatment, is the primary intervention that reduces SUDEP risk, unfortunately, seizures cannot be fully controlled despite maximal treatment in a significant proportion of people with epilepsy. Thus specific interventions to prevent adverse seizure-related cardiopulmonary consequences are needed. The potential roles of repositioning/stimulation after seizures, oxygen supplementation, cardiopulmonary resuscitation and clinical treatment options in reducing SUDEP risk are explored. Ultimately, understanding of these factors may lead to interventions that could reduce or prevent SUDEP.

The Good, the Bad, and the Deadly: Adenosinergic Mechanisms Underlying Sudden Unexpected Death in Epilepsy

Adenosine is an inhibitory modulator of neuronal excitability. Neuronal activity results in increased adenosine release, thereby constraining excessive excitation. The exceptionally high neuronal activity of a seizure results in a surge in extracellular adenosine to concentrations many-fold higher than would be observed under normal conditions. In this review, we discuss the multifarious effects of adenosine signaling in the context of epilepsy, with emphasis on sudden unexpected death in epilepsy (SUDEP). We describe and categorize the beneficial, detrimental, and potentially deadly aspects of adenosine signaling. The good or beneficial characteristics of adenosine signaling in the context of seizures include: (1) its direct effect on seizure termination and the prevention of status epilepticus; (2) the vasodilatory effect of adenosine, potentially counteracting postictal vasoconstriction; (3) its neuroprotective effects under hypoxic conditions; and (4) its disease modifying antiepileptogenic effect. The bad or detrimental effects of adenosine signaling include: (1) its capacity to suppress breathing and contribute to peri-ictal respiratory dysfunction; (2) its contribution to postictal generalized EEG suppression (PGES); (3) the prolonged increase in extracellular adenosine following spreading depolarization waves may contribute to postictal neuronal dysfunction; (4) the excitatory effects of A2A receptor activation is thought to exacerbate seizures in some instances; and (5) its potential contributions to sleep alterations in epilepsy. Finally, the adverse effects of adenosine signaling may potentiate a deadly outcome in the form of SUDEP by suppressing breathing and arousal in the postictal period. Evidence from animal models suggests that excessive postictal adenosine signaling contributes to the pathophysiology of SUDEP. The goal of this review is to discuss the beneficial, harmful, and potentially deadly roles that adenosine plays in the context of epilepsy and to identify crucial gaps in knowledge where further investigation is necessary. By better understanding adenosine dynamics, we may gain insights into the treatment of epilepsy and the prevention of SUDEP.

Neural mechanisms underlying respiratory regulation within the preBötzinger complex of the rabbit

The preBötzinger complex (preBötC) is a medullary area essential for normal breathing and widely recognized as necessary and sufficient to generate the inspiratory phase of respiration. It has been studied mainly in rodents. Here we report the main results of our studies revealing the characteristics of the rabbit preBötC identified by means of neuronal recordings, D,L-homocysteic acid microinjections and histological controls. A crucial role in the respiratory rhythmogenesis within this neural substrate is played by excitatory amino acids, but also GABA and glycine display important contributions. Increases in respiratory frequency are induced by microinjections of neurokinins, somatostatin as well by serotonin (5-HT) through an action on 5-HT_1A and 5-HT₃ receptors or the disinhibition of a GABAergic circuit. Respiratory depression is observed in response to microinjections of the μ-opioid receptor agonist DAMGO. Our results show similarities and differences with the rodent preBötC and emphasize the importance of comparative studies on the mechanisms underlying respiratory rhythmogenesis in different animal species.

Chemoreceptor mechanisms regulating CO2 -induced arousal from sleep

Arousal from sleep in response to CO₂ is a life-preserving reflex that enhances ventilatory drive and facilitates behavioural adaptations to restore eupnoeic breathing. Recurrent activation of the CO₂ -arousal reflex is associated with sleep disruption in obstructive sleep apnoea. In this review we examine the role of chemoreceptors in the carotid bodies, the retrotrapezoid nucleus and serotonergic neurons in the dorsal raphe in the CO₂ -arousal reflex. We also provide an overview of the supra-medullary structures that mediate CO₂ -induced arousal. We propose a framework for the CO₂ -arousal reflex in which the activity of the chemoreceptors converges in the parabrachial nucleus to trigger cortical arousal.

Baseline Arterial CO2 Pressure Regulates Acute Intermittent Hypoxia-Induced Phrenic Long-Term Facilitation in Rats

Moderate acute intermittent hypoxia (mAIH) elicits a progressive increase in phrenic motor output lasting hours post-mAIH, a form of respiratory motor plasticity known as phrenic long-term facilitation (pLTF). mAIH-induced pLTF is initiated by activation of spinally-projecting raphe serotonergic neurons during hypoxia and subsequent serotonin release near phrenic motor neurons. Since raphe serotonergic neurons are also sensitive to pH and CO₂, the prevailing arterial CO₂ pressure (PaCO₂) may modulate their activity (and serotonin release) during hypoxic episodes. Thus, we hypothesized that changes in background PaCO₂ directly influence the magnitude of mAIH-induced pLTF. mAIH-induced pLTF was evaluated in anesthetized, vagotomized, paralyzed and ventilated rats, with end-tidal CO₂ (i.e., a PaCO₂ surrogate) maintained at: (1) ≤39 mmHg (hypocapnia); (2) ∼41 mmHg (normocapnia); or (3) ≥48 mmHg (hypercapnia) throughout experimental protocols. Although baseline phrenic nerve activity tended to be lower in hypocapnia, short-term hypoxic phrenic response, i.e., burst amplitude (Δ = 5.1 ± 1.1 μV) and frequency responses (Δ = 21 ± 4 bpm), was greater than in normocapnic (Δ = 3.6 ± 0.6 μV and 8 ± 4, respectively) or hypercapnic rats (Δ = 2.0 ± 0.6 μV and −2 ± 2, respectively), followed by a progressive increase in phrenic burst amplitude (i.e., pLTF) for at least 60 min post mAIH. pLTF in the hypocapnic group (Δ = 4.9 ± 0.6 μV) was significantly greater than in normocapnic (Δ = 2.8 ± 0.7 μV) or hypercapnic rats (Δ = 1.7 ± 0.4 μV). In contrast, although hypercapnic rats also exhibited significant pLTF, it was attenuated versus hypocapnic rats. When pLTF was expressed as percent change from maximal chemoreflex stimulation, all pairwise comparisons were found to be statistically significant (p < 0.05). We conclude that elevated PaCO₂ undermines mAIH-induced pLTF in anesthetized rats. These findings contrast with well-documented effects of PaCO₂ on ventilatory LTF in awake humans.

Suppression of phrenic nerve activity as a potential predictor of imminent sudden unexpected death in epilepsy (SUDEP)

Sudden unexpected death in epilepsy (SUDEP) is a leading cause of death in patients with refractory epilepsy. Centrally-mediated respiratory dysfunction has been identified as one of the principal mechanisms responsible for SUDEP. Seizures generate a surge in adenosine release. Elevated adenosine levels suppress breathing. Insufficient metabolic clearance of a seizure-induced adenosine surge might be a precipitating factor in SUDEP. In order to deliver targeted therapies to prevent SUDEP, reliable biomarkers must be identified to enable prompt intervention. Because of the integral role of the phrenic nerve in breathing, we hypothesized that suppression of phrenic nerve activity could be utilized as predictive biomarker for imminent SUDEP. We used a rat model of kainic acid-induced seizures in combination with pharmacological suppression of metabolic adenosine clearance to trigger seizure-induced death in tracheostomized rats. Recordings of EEG, blood pressure, and phrenic nerve activity were made concomitant to the seizure. We found suppression of phrenic nerve burst frequency to 58.9% of baseline (p < 0.001, one-way ANOVA) which preceded seizure-induced death; importantly, irregularities of phrenic nerve activity were partly reversible by the adenosine receptor antagonist caffeine. Suppression of phrenic nerve activity may be a useful biomarker for imminent SUDEP. The ability to reliably detect the onset of SUDEP may be instrumental in the timely administration of potentially lifesaving interventions.

Benefit of buspirone on chemoreflex and central apnoeas in heart failure: a randomized controlled crossover trial

AIMS

Increased chemosensitivity to carbon dioxide (CO2 ) is an important trigger of central apnoeas (CA) in heart failure (HF), with negative impact on outcome. We hypothesized that buspirone, a 5HT1A receptor agonist that inhibits serotonergic chemoreceptor neuron firing in animals, can decrease CO2 chemosensitivity and CA in HF.

METHODS AND RESULTS

The BREATH study was a randomized, double-blind, placebo-controlled, crossover study (EudraCT-code 2015-005383-42). Outpatients with systolic HF (left ventricular ejection fraction <50%) and moderate-severe CA [nocturnal apnoea-hypopnoea index (AHI) ≥15 events/h] were randomly assigned to either oral buspirone (15 mg thrice daily) or placebo for 1 week, with a crossover design (1 week of wash-out). The primary effectiveness endpoint was a decrease in CO2 chemosensitivity >0.5 L/min/mmHg. The primary safety endpoint was freedom from serious adverse events. Sixteen patients (age 71.3 ± 5.8 years, all males, left ventricular ejection fraction 29.8 ± 7.8%) were enrolled. In the intention-to-treat analysis, more patients treated with buspirone (8/16, 50%) had a CO2 chemosensitivity reduction >0.5 L/min/mmHg from baseline than those treated with placebo (1/16, 6.7%) (difference between groups 43%, 95% confidence interval 14-73%, P = 0.016). Buspirone compared to baseline led to a 41% reduction in CO2 chemosensitivity (P = 0.001) and to a reduction in the AHI, central apnoea index and oxygen desaturation index of 42%, 79%, 77% at nighttime and 50%, 78%, 86% at daytime (all P < 0.01); no difference was observed after placebo administration (all P > 0.05). No patient reported buspirone-related serious adverse events.

CONCLUSIONS

Buspirone reduces CO2 chemosensitivity and improves CA and oxygen saturation across the 24 h in patients with HF.

The retrotrapezoid nucleus and the neuromodulation of breathing

Breathing is regulated by a host of arousal and sleep-wake state-dependent neuromodulators to maintain respiratory homeostasis. Modulators such as acetylcholine, norepinephrine, histamine, serotonin (5-HT), adenosine triphosphate (ATP), substance P, somatostatin, bombesin, orexin, and leptin can serve complementary or off-setting functions depending on the target cell type and signaling mechanisms engaged. Abnormalities in any of these modulatory mechanisms can destabilize breathing, suggesting that modulatory mechanisms are not overly redundant but rather work in concert to maintain stable respiratory output. The present review focuses on the modulation of a specific cluster of neurons located in the ventral medullary surface, named retrotrapezoid nucleus, that are activated by changes in tissue CO₂/H⁺ and regulate several aspects of breathing, including inspiration and active expiration.

Differential modulation of active expiration during hypercapnia by the medullary raphe in unanesthetized rats

Active expiration represents an important mechanism to improve ventilation in conditions of augmented ventilatory demand, such as hypercapnia. While a rostral ventromedullary region, the parafacial respiratory group (pFRG), has been identified as a conditional expiratory oscillator, little is known about how central chemosensitive sites contribute to modulate active expiration under hypercapnia. In this study, we investigated the influence of the medullary raphe in the emergence of phasic expiratory abdominal activity during hypercapnia in unanesthetized adult male rats, in a state-dependent manner. To do so, reverse microdialysis of muscimol (GABA_A receptor agonist, 1 mM) or 8-OH-DPAT (5-HT_1A agonist, 1 mM) was applied in the MR during sleep and wakefulness periods, both in normocapnic (room air) and hypercapnic conditions (7% CO₂). Electromyography (EMG) of diaphragm and abdominal muscles was performed to measure inspiratory and expiratory motor outputs. We found that active expiration did not occur in room air exposure during wakefulness or sleep. However, hypercapnia did recruit active expiration, and differential effects were observed with the drug dialyses in the medullary raphe. Muscimol increased the diaphragm inspiratory motor output and also increased the amplitude and frequency of abdominal expiratory rhythmic activity during hypercapnia in wakefulness periods. On the other hand, the microdialysis of 8-OH-DPAT attenuated hypercapnia-induced active expiration in a state-dependent manner. Our data suggest that the medullary raphe can either inhibit or potentiate respiratory motor activity during hypercapnia, and the balance of these inhibitory or excitatory outputs may determine the expression of active expiration.

Perinatal Nicotine Reduces Chemosensitivity of Medullary 5-HT Neurons after Maturation in Culture

Perinatal exposure to nicotine produces ventilatory and chemoreflex deficits in neonatal mammals. Medullary 5-HT neurons are putative central chemoreceptors that innervate respiratory nuclei and promote ventilation, receive cholinergic input and express nicotinic acetylcholine receptors (nAChRs). Perforated patch clamp recordings were made from cultured 5-HT neurons dissociated from the medullary raphé of 0-3 day old mice expressing enhanced yellow fluorescent protein driven by the enhancer region for PET1 (ePet-EYFP). The effect of exposure to low (6 mg kg^-1day^-1) or high (60 mg kg^-1day^-1) doses of nicotine in utero (prenatal), in culture (postnatal), or both and the effect of acute nicotine exposure (10 μM), were examined on baseline firing rate (FR at 5% CO₂, pH = 7.4) and the change in FR with acidosis (9% CO₂, pH 7.2) in young (12-21 days in vitro, DIV) and older (≥22 DIV) acidosis stimulated 5-HT neurons. Nicotine exposed neurons exhibited ∼67% of the response to acidosis recorded in neurons given vehicle (p = 0.005), with older neurons exposed to high dose prenatal and postnatal nicotine, exhibiting only 28% of that recorded in the vehicle neurons (p < 0.01). In neurons exposed to low or high dose prenatal and postnatal nicotine, acute nicotine exposure led to a smaller increase in FR (∼+51% vs +168%, p = 0.026) and response to acidosis (+6% vs +67%, p = 0.014) compared to vehicle. These data show that exposure to nicotine during development reduces chemosensitivity of 5-HT neurons as they mature, an effect that may be related to the abnormal chemoreflexes reported in rodents exposed to nicotine in utero, and may cause a greater risk for sudden infant death syndrome (SIDS).

O2-Dependent Protein Internalization Underlies Astrocytic Sensing of Acute Hypoxia by Restricting Multimodal TRPA1 Channel Responses

Hypoxia sensors are essential for regulating local oxygen (O₂) homeostasis within the body. This is especially pertinent within the CNS, which is particularly vulnerable to O₂ deprivation due to high energetic demand. Here, we reveal hypoxia-monitoring function exerted by astrocytes through an O₂-regulated protein trafficking mechanism within the CNS. Strikingly, cultured mouse astrocytes isolated from the parafacial respiratory group (pFRG) and retrotrapezoid nucleus (RTN) region are capable of rapidly responding to moderate hypoxia via the sensor cation channel transient receptor potential (TRP) A1 but, unlike multimodal sensory neurons, are inert to hyperoxia and other TRPA1 activators (carbon dioxide, electrophiles, and oxidants) in normoxia. Mechanistically, O₂ suppresses TRPA1 channel activity by protein internalization via O₂-dependent proline hydroxylation and subsequent ubiquitination by an E3 ubiquitin ligase, NEDD4-1 (neural precursor cell-expressed developmentally down-regulated protein 4). Hypoxia inhibits this process and instantly accumulates TRPA1 proteins at the plasma membrane, inducing TRPA1-mediated Ca²⁺ influx that triggers ATP release from pFRG/RTN astrocytes, potentiating respiratory center activity. Furthermore, astrocyte-specific Trpa1 disruption in a mouse brainstem-spinal cord preparation impedes the amplitude augmentation of the central autonomic respiratory output during hypoxia. Thus, reversible coupling of the TRPA1 channels with O₂-dependent protein translocation allows astrocytes to act as acute hypoxia sensors in the medullary respiratory center.

Impact of inflammation on developing respiratory control networks: rhythm generation, chemoreception and plasticity

The respiratory control network in the central nervous system undergoes critical developmental events early in life to ensure adequate breathing at birth. There are at least three "critical windows" in development of respiratory control networks: 1) in utero, 2) newborn (postnatal day 0-4 in rodents), and 3) neonatal (P10-13 in rodents, 2-4 months in humans). During these critical windows, developmental processes required for normal maturation of the respiratory control network occur, thereby increasing vulnerability of the network to insults, such as inflammation. Early life inflammation (induced by LPS, chronic intermittent hypoxia, sustained hypoxia, or neonatal maternal separation) acutely impairs respiratory rhythm generation, chemoreception and increases neonatal risk of mortality. These early life impairments are also greater in young males, suggesting sex-specific impairments in respiratory control. Further, neonatal inflammation has a lasting impact on respiratory control by impairing adult respiratory plasticity. This review focuses on how inflammation alters respiratory rhythm generation, chemoreception and plasticity during each of the three critical windows. We also highlight the need for additional mechanistic studies and increased investigation into how glia (such as microglia and astrocytes) play a role in impaired respiratory control after inflammation. Understanding how inflammation during critical windows of development disrupt respiratory control networks is essential for developing better treatments for vulnerable neonates and preventing adult ventilatory control disorders.

Breathing regulation and blood gas homeostasis after near complete lesions of the retrotrapezoid nucleus in adult rats

KEY POINTS

The retrotrapezoid nucleus (RTN) drives breathing proportionally to brain PCO2 but its role during various states of vigilance needs clarification. Under normoxia, RTN lesions increased the arterial PCO2 set-point, lowered the PO2 set-point and reduced alveolar ventilation relative to CO₂ production. Tidal volume was reduced and breathing frequency increased to a comparable degree during wake, slow-wave sleep and REM sleep. RTN lesions did not produce apnoeas or disordered breathing during sleep. RTN lesions in rats virtually eliminated the central respiratory chemoreflex (CRC) while preserving the cardiorespiratory responses to hypoxia; the relationship between CRC and number of surviving RTN Nmb neurons was an inverse exponential. The CRC does not function without the RTN. In the quasi-complete absence of the RTN and CRC, alveolar ventilation is reduced despite an increased drive to breathe from the carotid bodies.

ABSTRACT

The retrotrapezoid nucleus (RTN) is one of several CNS nuclei that contribute, in various capacities (e.g. CO₂ detection, neuronal modulation) to the central respiratory chemoreflex (CRC). Here we test how important the RTN is to PCO₂ homeostasis and breathing during sleep or wake. RTN Nmb-positive neurons were killed with targeted microinjections of substance P-saporin conjugate in adult rats. Under normoxia, rats with large RTN lesions (92 ± 4% cell loss) had normal blood pressure and arterial pH but were hypoxic (-8 mmHg PaO₂ ) and hypercapnic (+10 mmHg ). In resting conditions, minute volume (V_E ) was normal but breathing frequency (f_R ) was elevated and tidal volume (V_T ) reduced. Resting O₂ consumption and CO₂ production were normal. The hypercapnic ventilatory reflex in 65% FiO₂ had an inverse exponential relationship with the number of surviving RTN neurons and was decreased by up to 92%. The hypoxic ventilatory reflex (HVR; FiO₂ 21-10%) persisted after RTN lesions, hypoxia-induced sighing was normal and hypoxia-induced hypotension was reduced. In rats with RTN lesions, breathing was lowest during slow-wave sleep, especially under hyperoxia, but apnoeas and sleep-disordered breathing were not observed. In conclusion, near complete RTN destruction in rats virtually eliminates the CRC but the HVR persists and sighing and the state dependence of breathing are unchanged. Under normoxia, RTN lesions cause no change in V_E but alveolar ventilation is reduced by at least 21%, probably because of increased physiological dead volume. RTN lesions do not cause sleep apnoea during slow-wave sleep, even under hyperoxia.

Mechanisms and Consequences of Oxygen and Carbon Dioxide Sensing in Mammals

Molecular oxygen (O₂) and carbon dioxide (CO₂) are the primary gaseous substrate and product of oxidative phosphorylation in respiring organisms, respectively. Variance in the levels of either of these gasses outside of the physiological range presents a serious threat to cell, tissue, and organism survival. Therefore, it is essential that endogenous levels are monitored and kept at appropriate concentrations to maintain a state of homeostasis. Higher organisms such as mammals have evolved mechanisms to sense O₂ and CO₂ both in the circulation and in individual cells and elicit appropriate corrective responses to promote adaptation to commonly encountered conditions such as hypoxia and hypercapnia. These can be acute and transient nontranscriptional responses, which typically occur at the level of whole animal physiology or more sustained transcriptional responses, which promote chronic adaptation. In this review, we discuss the mechanisms by which mammals sense changes in O₂ and CO₂ and elicit adaptive responses to maintain homeostasis. We also discuss crosstalk between these pathways and how they may represent targets for therapeutic intervention in a range of pathological states.

Chemosensitivity of Phox2b-expressing retrotrapezoid neurons is mediated in part by input from 5-HT neurons

KEY POINTS

Neurons of the retrotrapezoid nucleus (RTN) and medullary serotonin (5-HT) neurons are both candidates for central CO2 /pH chemoreceptors, but it is not known how interactions between them influence their responses to pH. We found that RTN neurons in brain slices were stimulated by exogenous 5-HT and by heteroexchange release of endogenous 5-HT, and these responses were blocked by antagonists of 5-HT7 receptors. The pH response of RTN neurons in brain slices was markedly reduced by the same antagonists of 5-HT7 receptors. Similar results were obtained in dissociated, primary cell cultures prepared from the ventral medulla, where it was also found that the pH response of RTN neurons was blocked by preventing 5-HT synthesis and enhanced by blocking 5-HT reuptake. Exogenous 5-HT did not enable latent intrinsic RTN chemosensitivity. RTN neurons may play more of a role as relays from other central and peripheral chemoreceptors than as CO2 sensors.

ABSTRACT

Phox2b-expressing neurons in the retrotrapezoid nucleus (RTN) and serotonin (5-HT) neurons in the medullary raphe have both been proposed to be central respiratory chemoreceptors. How interactions between these two sets of neurons influence their responses to acidosis is not known. Here we recorded from mouse Phox2b+ RTN neurons in brain slices, and found that their response to moderate hypercapnic acidosis (pH 7.4 to ∼7.2) was markedly reduced by antagonists of 5-HT7 receptors. RTN neurons were stimulated in response to heteroexchange release of 5-HT, indicating that RTN neurons are sensitive to endogenous 5-HT. This electrophysiological behaviour was replicated in primary, dissociated cell cultures containing 5-HT and RTN neurons grown together. In addition, pharmacological inhibition of 5-HT synthesis in culture reduced RTN neuron chemosensitivity, and blocking 5-HT reuptake enhanced chemosensitivity. The effect of 5-HT on RTN neuron chemosensitivity was not explained by a mechanism whereby activation of 5-HT7 receptors enables or potentiates intrinsic chemosensitivity of RTN neurons, as exogenous 5-HT did not enhance the pH response. The ventilatory response to inhaled CO2 of mice was markedly decreased in vivo after systemic treatment with ketanserin, an antagonist of 5-HT2 and 5-HT7 receptors. These data indicate that 5-HT and RTN neurons may interact synergistically in a way that enhances the respiratory chemoreceptor response. The primary role of RTN neurons may be as relays and amplifiers of the pH response from 5-HT neurons and other chemoreceptors rather than as pH sensors themselves.

Effect of 6-OHDA on hypercapnic ventilatory response in the rat model of Parkinson's disease

Breathing impairments, such as an alteration in breathing pattern, dyspnoea, and sleep apnoea, are common health deficits recognised in Parkinson's disease (PD). The mechanism that underlies these disturbances, however, remains unclear. We investigated the effect of the unilateral damage to the rat nigrostriatal pathway on the central ventilatory response to hypercapnia, evoked by administering 6-hydroxydopamine (6-OHDA) into the right medial forebrain bundle (MFB). The respiratory experiments were carried out in conscious animals in the plethysmography chamber. The ventilatory parameters were studied in normocapnic and hyperoxic hypercapnia before and 14 days after the neurotoxin injection. Lesion with the 6-OHDA produced an increased tidal volume during normoxia. The magnified response of tidal volume and a decrease of breathing frequency to hypercapnia were observed in comparison to the pre-lesion and sham controls. Changes in both respiratory parameters resulted in an increase of minute ventilation of the response to CO(2) by 28% in comparison to the pre-lesion state at 60 s. Our results demonstrate that rats with implemented unilateral PD model presented an altered respiratory pattern most often during a ventilatory response to hypercapnia. Preserved noradrenaline and specific changes in dopamine and serotonin characteristic for this model could be responsible for the pattern of breathing observed during hypercapnia.

No evidence in support of a prodromal respiratory control signature in the TgF344-AD rat model of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative condition disturbing major brain networks, including those pivotal to the motor control of breathing. The aim of this study was to examine respiratory control in the TgF344-AD transgenic rat model of AD. At 8-11 months of age, basal minute ventilation and ventilatory responsiveness to chemostimulation were equivalent in conscious wild-type (WT) and TgF344-AD rats. Under urethane anesthesia, basal diaphragm and genioglossus EMG activities were similar in WT and TgF344-AD rats. The duration of phenylbiguanide-induced apnoea was significantly shorter in TgF344-AD rats compared with WT. Following bilateral cervical vagotomy, diaphragm and genioglossus EMG responsiveness to chemostimulation were intact in TgF344-AD rats. Amyloid precursor protein C-terminal fragments were elevated in the TgF344-AD brainstem, in the absence of amyloid-β accumulation or alterations in tau phosphorylation. Brainstem pro-inflammatory cytokine concentrations were not increased in TgF344-AD rats. We conclude that neural control of breathing is preserved in TgF344-AD rats at this stage of the disease.

Brain metabolic sensing and metabolic signaling at the level of an astrocyte

Astrocytes support neuronal function by providing essential structural and nutritional support, neurotransmitter trafficking and recycling and may also contribute to brain information processing. In this article we review published results and report new data suggesting that astrocytes function as versatile metabolic sensors of central nervous system (CNS) milieu and play an important role in the maintenance of brain metabolic homeostasis. We discuss anatomical and functional features of astrocytes that allow them to detect and respond to changes in the brain parenchymal levels of metabolic substrates (oxygen and glucose), and metabolic waste products (carbon dioxide). We report data suggesting that astrocytes are also sensitive to circulating endocrine signals-hormones like ghrelin, glucagon-like peptide-1 and leptin, that have a major impact on the CNS mechanisms controlling food intake and energy balance. We discuss signaling mechanisms that mediate communication between astrocytes and neurons and consider how these mechanisms are recruited by astrocytes activated in response to various metabolic challenges. We review experimental data suggesting that astrocytes modulate the activities of the respiratory and autonomic neuronal networks that ensure adaptive changes in breathing and sympathetic drive in order to support the physiological and behavioral demands of the organism in ever-changing environmental conditions. Finally, we discuss evidence suggesting that altered astroglial function may contribute to the pathogenesis of disparate neurological, respiratory and cardiovascular disorders such as Rett syndrome and systemic arterial hypertension.

Lung-injury depresses glutamatergic synaptic transmission in the nucleus tractus solitarii via discrete age-dependent mechanisms in neonatal rats

Transition periods (TPs) are brief stages in CNS development where neural circuits can exhibit heightened vulnerability to pathologic conditions such as injury or infection. This susceptibility is due in part to specialized mechanisms of synaptic plasticity, which may become activated by inflammatory mediators released under pathologic conditions. Thus, we hypothesized that the immune response to lung injury (LI) mediated synaptic changes through plasticity-like mechanisms that depended on whether LI occurred just before or after a TP. We studied the impact of LI on brainstem 2nd-order viscerosensory neurons located in the nucleus tractus solitarii (nTS) during a TP for respiratory control spanning (postnatal day (P) 11-15). We injured the lungs of Sprague-Dawley rats by intratracheal instillation of Bleomycin (or saline) just before (P9-11) or after (P17-19) the TP. A week later, we prepared horizontal slices of the medulla and recorded spontaneous and evoked excitatory postsynaptic currents (sEPSCs/eEPSCs) in vitro from neurons in the nTS that received monosynaptic glutamatergic input from the tractus solitarii (TS). In rats injured before the TP (pre-TP), neurons exhibited blunted sEPSCs and TS-eEPSCs compared to controls. The decreased TS-eEPSCs were mediated by differences in postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic-acid receptors (AMPAR). Specifically, compared to controls, LI rats had more Ca2+-impermeable AMPARs (CI-AMPARs) as indicated by: 1) the absence of current-rectification, 2) decreased sensitivity to polyamine, 1-Naphthyl-acetyl-spermine-trihydrochloride (NASPM) and 3) augmented immunoreactive staining for the CI-AMPAR GluA2. Thus, pre-TP-LI acts postsynaptically to blunt glutamatergic transmission. The neuroimmune response to pre-TP-LI included microglia hyper-ramification throughout the nTS. Daily intraperitoneal administration of minocycline, an inhibitor of microglial/macrophage function prevented hyper-ramification and abolished the pre-TP-LI evoked synaptic changes. In contrast, rat-pups injured after the TP (post-TP) exhibited microglia hypo-ramification in the nTS and had increased sEPSC amplitudes/frequencies, and decreased TS-eEPSC amplitudes compared to controls. These synaptic changes were not associated with changes in CI-AMPARs, and instead involved greater TS-evoked use-dependent depression (reduced paired pulse ratio), which is a hallmark of presynaptic plasticity. Thus we conclude that LI regulates the efficacy of TS → nTS synapses through discrete plasticity-like mechanisms that are immune-mediated and depend on whether the injury occurs before or after the TP for respiratory control.

Serotonin neuron development: shaping molecular and structural identities

The continuing fascination with serotonin (5-hydroxytryptamine, 5-HT) as a nervous system chemical messenger began with its discovery in the brains of mammals in 1953. Among the many reasons for this decades-long interest is that the small numbers of neurons that make 5-HT influence the excitability of neural circuits in nearly every region of the brain and spinal cord. A further reason is that 5-HT dysfunction has been linked to a range of psychiatric and neurological disorders many of which have a neurodevelopmental component. This has led to intense interest in understanding 5-HT neuron development with the aim of determining whether early alterations in their generation lead to brain disease susceptibility. Here, we present an overview of the neuroanatomical organization of vertebrate 5-HT neurons, their neurogenesis, and prodigious axonal architectures, which enables the expansive reach of 5-HT neuromodulation in the central nervous system. We review recent findings that have revealed the molecular basis for the tremendous diversity of 5-HT neuron subtypes, the impact of environmental factors on 5-HT neuron development, and how 5-HT axons are topographically organized through disparate signaling pathways. We summarize studies of the gene regulatory networks that control the differentiation, maturation, and maintenance of 5-HT neurons. These studies show that the regulatory factors controlling acquisition of 5-HT-type transmitter identity continue to play critical roles in the functional maturation and the maintenance of 5-HT neurons. New insights are presented into how continuously expressed 5-HT regulatory factors control 5-HT neurons at different stages of life and how the regulatory networks themselves are maintained. WIREs Dev Biol 2018, 7:e301. doi: 10.1002/wdev.301 This article is categorized under: Nervous System Development > Vertebrates: General Principles Gene Expression and Transcriptional Hierarchies > Gene Networks and Genomics Gene Expression and Transcriptional Hierarchies > Cellular Differentiation Nervous System Development > Secondary: Vertebrates: Regional Development.

Necdin shapes serotonergic development and SERT activity modulating breathing in a mouse model for Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a genetic neurodevelopmental disorder that presents with hypotonia and respiratory distress in neonates. The Necdin-deficient mouse is the only model that reproduces the respiratory phenotype of PWS (central apnea and blunted response to respiratory challenges). Here, we report that Necdin deletion disturbs the migration of serotonin (5-HT) neuronal precursors, leading to altered global serotonergic neuroarchitecture and increased spontaneous firing of 5-HT neurons. We show an increased expression and activity of 5-HT Transporter (SERT/Slc6a4) in 5-HT neurons leading to an increase of 5-HT uptake. In Necdin-KO pups, the genetic deletion of Slc6a4 or treatment with Fluoxetine, a 5-HT reuptake inhibitor, restored normal breathing. Unexpectedly, Fluoxetine administration was associated with respiratory side effects in wild-type animals. Overall, our results demonstrate that an increase of SERT activity is sufficient to cause the apneas in Necdin-KO pups, and that fluoxetine may offer therapeutic benefits to PWS patients with respiratory complications.

Prader-Willi syndrome results from the disruption of a cluster of neighboring genes, including one called Necdin. Symptoms begin in early infancy and worsen with age. Affected children tend to develop an insatiable appetite, which often leads to obesity. They also experience serious problems with their breathing. Chest infections, high altitude and intense physical activity can be dangerous for children with Prader-Willi syndrome. This is because a slight shortage of oxygen may trigger breathing difficulties that could prove fatal.

The brain cells that produce a chemical messenger called serotonin help to control breathing. Several lines of evidence suggest that loss of Necdin may trigger breathing difficulties in Prader-Willi syndrome via effects on the serotonin system. First, serotonin neurons produce the Necdin protein. Second, laboratory mice that lack the gene for Necdin have abnormally shaped serotonin neurons. Third, these mice show breathing difficulties like those of individuals with Prader-Willi syndrome. But while this implies a connection between serotonin, Necdin and breathing difficulties, it falls short of establishing a causal link.

Matarazzo et al. now reveal an increase in the quantity and activity of a protein called the serotonin transporter in mutant mice that lacked the gene for Necdin compared to normal mice. Serotonin transporter proteins mop up the serotonin that neurons release when they signal to one another. Neurons in the mutant mice take up more serotonin than their counterparts in normal mice; this means they have less serotonin available for signaling. This may make it harder for the mutant mice to regulate their breathing.

Drugs called selective serotonin-reuptake inhibitors (or SSRIs for short) can block the serotonin transporter. These drugs, which include Fluoxetine (also called Prozac), are antidepressants. Matarazzo et al. show that SSRIs temporarily restore normal breathing in young mice that lack the gene for Necdin. However, these drugs have harmful long-term effects on breathing in non-mutant mice. Further studies should test whether short-term use of SSRIs could offer immediate relief for breathing difficulties in infants and children with Prader-Willi syndrome.