Modulation of respiratory rhythmogenesis by chloride-mediated conductances during the perinatal period

The hypoxic respiratory response of the pre-Bötzinger complex

Since the discovery of the pre-Bötzinger Complex (preBötC) as a crucial region for generating the main respiratory rhythm, our understanding of its cellular and molecular aspects has rapidly increased within the last few decades. It is now apparent that preBötC is a highly flexible neuronal network that reconfigures state-dependently to produce the most appropriate respiratory output in response to various metabolic challenges, such as hypoxia. However, the responses of the preBötC to hypoxic conditions can be varied based on the intensity, pattern, and duration of the hypoxic challenge. This review discusses the preBötC response to hypoxic challenges at the cellular and network level. Particularly, the involvement of preBötC in the classical biphasic response of the respiratory network to acute hypoxia is illuminated. Furthermore, the article discusses the functional and structural changes of preBötC neurons following intermittent and sustained hypoxic challenges. Accumulating evidence shows that the preBötC neural circuits undergo substantial changes following hypoxia and contribute to several types of the respiratory system's hypoxic ventilatory responses.

Chronic, episodic nicotine exposure alters GABAergic synaptic transmission to hypoglossal motor neurons and genioglossus muscle function at a critical developmental age

Regulation of GABAergic signaling through nicotinic acetylcholine receptor (nAChR) activation is critical for neuronal development. Here, we test the hypothesis that chronic episodic developmental nicotine exposure (eDNE) disrupts GABAergic signaling, leading to dysfunction of hypoglossal motor neurons (XIIMNs), which innervate the tongue muscles. We studied control and eDNE pups at two developmentally vulnerable age ranges: postnatal days (P)1-5 and P10-12. The amplitude and frequency of spontaneous and miniature inhibitory postsynaptic currents (sIPSCs, mIPSCs) at baseline were not altered by eDNE at either age. In contrast, eDNE increased GABAAR-α1 receptor expression on XIIMNs and, in the older group, the postsynaptic response to muscimol (GABAA receptor agonist). Activation of nAChRs with exogenous nicotine increased the frequency of GABAergic sIPSCs in control and eDNE neurons at P1-5. By P10-12, acute nicotine increased sIPSC frequency in eDNE but not control neurons. In vivo experiments showed that the breathing-related activation of tongue muscles, which are innervated by XIIMNs, is reduced at P10-12. This effect was partially mitigated by subcutaneous muscimol, but only in the eDNE pups. Taken together, these data indicate that eDNE alters GABAergic transmission to XIIMNs at a critical developmental age, and this is expressed as reduced breathing-related drive to XIIMNs in vivo.NEW & NOTEWORTHY Here, we provide a thorough assessment of the effects of nicotine exposure on GABAergic synaptic transmission, from the cellular to the systems level. This work makes significant advances in our understanding of the impact of nicotine exposure during development on GABAergic neurotransmission within the respiratory network and the potential role this plays in the excitatory/inhibitory imbalance that is thought to be an important mechanism underlying neonatal breathing disorders, including sudden infant death syndrome.

Prenatal Hypoxia Induces Cl– Cotransporters KCC2 and NKCC1 Developmental Abnormality and Disturbs the Influence of GABAA and Glycine Receptors on Fictive Breathing in a Newborn Rat

Prenatal hypoxia is a recognised risk factor for neurodevelopmental disorders associated with both membrane proteins involved in neuron homeostasis, e.g., chloride (Cl–) cotransporters, and alterations in brain neurotransmitter systems, e.g., catecholamines, dopamine, and GABA. Our study aimed to determine whether prenatal hypoxia alters central respiratory drive by disrupting the development of Cl– cotransporters KCC2 and NKCC1. Cl– homeostasis seems critical for the strength and efficiency of inhibition mediated by GABAA and glycine receptors within the respiratory network, and we searched for alterations of GABAergic and glycinergic respiratory influences after prenatal hypoxia. We measured fictive breathing from brainstem in ex vivo preparations during pharmacological blockade of KCC2 and NKCC1 Cl– cotransporters, GABAA, and glycine receptors. We also evaluated the membrane expression of Cl– cotransporters in the brainstem by Western blot and the expression of Cl– cotransporter regulators brain-derived neurotrophic factor (BDNF) and calpain. First, pharmacological experiments showed that prenatal hypoxia altered the regulation of fictive breathing by NKCC1 and KCC2 Cl– cotransporters, GABA/GABAA, and glycin. NKCC1 inhibition decreased fictive breathing at birth in control mice while it decreased at 4 days after birth in pups exposed to prenatal hypoxia. On the other hand, inhibition of KCC2 decreased fictive breathing 4 days after birth in control mice without any change in prenatal hypoxia pups. The GABAergic system appeared to be more effective in prenatal hypoxic pups whereas the glycinergic system increased its effectiveness later. Second, we observed a decrease in the expression of the Cl– cotransporter KCC2, and a decrease with age in NKCC1, as well as an increase in the expression of BDNF and calpain after prenatal hypoxia exposure. Altogether, our data support the idea that prenatal hypoxia alters the functioning of GABAA and glycinergic systems in the respiratory network by disrupting maturation of Cl– homeostasis, thereby contributing to long-term effects by disrupting ventilation.

Thyroid hormones during the perinatal period are necessary to respiratory network development of newborn rats

Thyroid hormones (THs) are essential for foetal brain development. Because the gestating mother is the main source of THs to the foetus, maternal hypothyroidism and/or premature birth compromise neurological outcomes in the offspring. Respiratory instability and recurrent apneas due to immaturity of the respiratory control network are major causes of morbidity in infants. Inadequate TH supply may be sufficient to delay perinatal maturation of the respiratory control system; however, this hypothesis remains untested. To address this issue, maternal hypothyroidism was induced by adding methimazole (MMI; 0.02% w/v) to the drinking water of pregnant dams from conception to postpartum day 4 (P4). The effect of TH supplementation on respiratory function was tested by injecting levothyroxine (L-T4) in newborns at P1. Respiratory function was assessed by plethysmography (in vivo) and recording of phrenic output from medullary preparations (in vitro). By comparison with controls, TH deficiency increased the frequency of apneas and decreased basal ventilation in vivo and prevented the age-dependent increase in phrenic burst frequency normally observed in vitro. The effects of TH deficiency on GABAergic modulation of respiratory activity were measured by bath application of muscimol (GABA_A agonist) or bicuculline (GABA_A antagonist). The phrenic burst frequency responses to GABAergic agents were consistently greater in preparations from TH deficient pups. L-T4 supplementation reversed part of the respiratory anomalies related to MMI treatment in vitro. We conclude that TH deficiency during the perinatal period is sufficient to delay maturation of the respiratory control network development. Excessive GABAergic inhibition may contribute to this effect.

Neuronal mechanisms underlying opioid-induced respiratory depression: our current understanding

Opioid-induced respiratory depression (OIRD) represents the primary cause of death associated with therapeutic and recreational opioid use. Within the United States, the rate of death from opioid abuse since the early 1990s has grown disproportionally, prompting the classification as a nationwide "epidemic." Since this time, we have begun to unravel many fundamental cellular and systems-level mechanisms associated with opioid-related death. However, factors such as individual vulnerability, neuromodulatory compensation, and redundancy of opioid effects across central and peripheral nervous systems have created a barrier to a concise, integrative view of OIRD. Within this review, we bring together multiple perspectives in the field of OIRD to create an overarching viewpoint of what we know, and where we view this essential topic of research going forward into the future.

Perinatal Hypoxemia and Oxygen Sensing

The development of the control of breathing begins in utero and continues postnatally. Fetal breathing movements are needed for establishing connectivity between the lungs and central mechanisms controlling breathing. Maturation of the control of breathing, including the increase of hypoxia chemosensitivity, continues postnatally. Insufficient oxygenation, or hypoxia, is a major stressor that can manifest for different reasons in the fetus and neonate. Though the fetus and neonate have different hypoxia sensing mechanisms and respond differently to acute hypoxia, both responses prevent deviations to respiratory and other developmental processes. Intermittent and chronic hypoxia pose much greater threats to the normal developmental respiratory processes. Gestational intermittent hypoxia, due to maternal sleep-disordered breathing and sleep apnea, increases eupneic breathing and decreases the hypoxic ventilatory response associated with impaired gasping and autoresuscitation postnatally. Chronic fetal hypoxia, due to biologic or environmental (i.e. high-altitude) factors, is implicated in fetal growth restriction and preterm birth causing a decrease in the postnatal hypoxic ventilatory responses with increases in irregular eupneic breathing. Mechanisms driving these changes include delayed chemoreceptor development, catecholaminergic activity, abnormal myelination, increased astrocyte proliferation in the dorsal respiratory group, among others. Long-term high-altitude residents demonstrate favorable adaptations to chronic hypoxia as do their offspring. Neonatal intermittent hypoxia is common among preterm infants due to immature respiratory systems and thus, display a reduced drive to breathe and apneas due to insufficient hypoxic sensitivity. However, ongoing intermittent hypoxia can enhance hypoxic sensitivity causing ventilatory overshoots followed by apnea; the number of apneas is positively correlated with degree of hypoxic sensitivity in preterm infants. Chronic neonatal hypoxia may arise from fetal complications like maternal smoking or from postnatal cardiovascular problems, causing blunting of the hypoxic ventilatory responses throughout at least adolescence due to attenuation of carotid body fibers responses to hypoxia with potential roles of brainstem serotonin, microglia, and inflammation, though these effects depend on the age in which chronic hypoxia initiates. Fetal and neonatal intermittent and chronic hypoxia are implicated in preterm birth and complicate the respiratory system through their direct effects on hypoxia sensing mechanisms and interruptions to the normal developmental processes. Thus, precise regulation of oxygen homeostasis is crucial for normal development of the respiratory control network. © 2021 American Physiological Society. Compr Physiol 11:1653-1677, 2021.

Role of Synaptic Inhibition in the Coupling of the Respiratory Rhythms that Underlie Eupnea and Sigh Behaviors

The preBötzinger complex (preBötC) gives rise to two types of breathing behavior under normal physiological conditions: eupnea and sighing. Here, we examine the neural mechanisms that couple their underlying rhythms. We measured breathing in awake intact adult mice and recorded inspiratory rhythms from the preBötC in neonatal mouse brainstem slice preparations. We show previously undocumented variability in the temporal relationship between sigh breaths or bursts and their preceding eupneic breaths or inspiratory bursts. Investigating the synaptic mechanisms for this variability in vitro, we further show that pharmacological blockade of chloride-mediated synaptic inhibition strengthens inspiratory-to-sigh temporal coupling. These findings contrast with previous literature, which suggested glycinergic inhibition linked sigh bursts to their preceding inspiratory bursts with minimal time intervals. Furthermore, we verify that pharmacological disinhibition did not alter the duration of the prolonged interval that follows a sigh burst before resumption of the inspiratory rhythm. These results demonstrate that synaptic inhibition does not enhance coupling between sighs and preceding inspiratory events or contribute to post-sigh apneas. Instead, we conclude that excitatory synaptic mechanisms coordinate inspiratory (eupnea) and sigh rhythms.

Activating α4β2 Nicotinic Acetylcholine Receptors Alleviates Fentanyl-induced Respiratory Depression in Rats

BACKGROUND

Opioid analgesics are widely used for treatment of acute, postoperative, and chronic pain. However, activation of opioid receptors can result in severe respiratory depression. There is an unmet clinical need to develop a pharmacologic therapy to counter opioid-induced respiratory depression without interfering with analgesia. Further, additional advances to confront accidental lethal overdose with the use of fentanyl and other opioids are needed. Here, the authors test the hypothesis that activation of nicotinic receptors expressed within respiratory rhythm-generating networks would counter opioid-induced respiratory depression without compromising analgesia.

METHODS

Respiratory neural discharge was measured using in vitro brainstem-spinal cord and medullary slice rat preparations. In vivo, plethysmographic recording, nociception testing, and righting reflexes were used to examine respiratory ventilation, analgesia, and sedation, respectively.

RESULTS

The administration of nicotine, selective α4β2 nicotinic receptor agonist A85380, but not α7 nicotinic receptor agonist PNU282987, reversed opioid-induced respiratory depression in neonatal pups in vitro and in vivo. In adult rats in vivo, administration of A85380 (0.03 mg/kg), but not PNU282987, provides a rapid and robust reversal of fentanyl-induced decrease in respiratory rate (93.4 ± 33.7% of control 3 min after A85380 vs. 31 ± 20.5% of control after vehicle, n = 8 each, P < 0.001), without marked side effects. The coadministration of A85380 (0.06 mg/kg) with fentanyl or remifentanil markedly reduced respiratory depression and apneas, and enhanced the fentanyl-induced analgesia, as evidenced by increased paw withdrawal latency in Hargreaves plantar test (14.4 ± 2.8 s vs. vehicle: 11.3 ± 2.4 s, n = 8 each, P = 0.013) and decreased formalin-induced nocifensive duration (2.5 ± 2.4 min vs. vehicle: 5.4 ± 2.7 min, n = 8 each, P = 0.029).

CONCLUSIONS

The novel strategy of targeting α4β2 nicotinic acetylcholine receptors has the potential for advancing pain control and reducing opioid-induced respiratory depression and overdose.

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.

Opioids modulate an emergent rhythmogenic process to depress breathing

How mammalian neural circuits generate rhythmic activity in motor behaviors, such as breathing, walking, and chewing, remains elusive. For breathing, rhythm generation is localized to a brainstem nucleus, the preBötzinger Complex (preBötC). Rhythmic preBötC population activity consists of strong inspiratory bursts, which drive motoneuronal activity, and weaker burstlets, which we hypothesize reflect an emergent rhythmogenic process. If burstlets underlie inspiratory rhythmogenesis, respiratory depressants, such as opioids, should reduce burstlet frequency. Indeed, in medullary slices from neonatal mice, the μ-opioid receptor (μOR) agonist DAMGO slowed burstlet generation. Genetic deletion of μORs in a glutamatergic preBötC subpopulation abolished opioid-mediated depression, and the neuropeptide Substance P, but not blockade of inhibitory synaptic transmission, reduced opioidergic effects. We conclude that inspiratory rhythmogenesis is an emergent process, modulated by opioids, that does not rely on strong bursts of activity associated with motor output. These findings also point to strategies for ameliorating opioid-induced depression of breathing.

Cervical spinal contusion alters Na+-K+-2Cl- and K+-Cl- cation-chloride cotransporter expression in phrenic motor neurons

Spinal chloride-dependent synaptic inhibition is critical in regulating breathing and requires neuronal chloride gradients established by cation-chloride cotransporters Na+-K+-2Cl- (NKCC1) and K+-Cl- (KCC2). Spinal transection disrupts NKCC1/KCC2 balance, diminishing chloride gradients in neurons below injury, contributing to spasticity and chronic pain. It is not known if similar disruptions in NKCC1/KCC2 balance occur in respiratory motor neurons after incomplete cervical contusion (C2SC). We hypothesized that C2SC disrupts NKCC1/KCC2 balance in phrenic motor neurons. NKCC1 and KCC2 immunoreactivity was assessed in CtB-positive phrenic motor neurons. Five weeks post-C2SC: 1) neither membrane-bound nor cytosolic NKCC1 expression were significantly changed, although the membrane/cytosolic ratio increased, consistent with net chloride influx; and 2) both membrane and cytosolic KCC2 expression increased, although the membrane/cytosolic ratio decreased, consistent with net chloride efflux. Thus, contrary to our original hypothesis, complex shifts in NKCC1/KCC2 balance occur post-C2SC. The functional significance of these changes remains unclear.

Role of the K+-Cl- Cotransporter KCC2a Isoform in Mammalian Respiration at Birth

In central respiratory circuitry, synaptic excitation is responsible for synchronizing neuronal activity in the different respiratory rhythm phases, whereas chloride-mediated inhibition is important for shaping the respiratory pattern itself. The potassium chloride cotransporter KCC2, which serves to maintain low intraneuronal Cl- concentration and thus render chloride-mediated synaptic signaling inhibitory, exists in two isoforms, KCC2a and KCC2b. KCC2 is essential for functional breathing motor control at birth, but the specific contribution of the KCC2a isoform remains unknown. Here, to address this issue, we investigated the respiratory phenotype of mice deficient for KCC2a. In vivo plethysmographic recordings revealed that KCC2a-deficient pups at P0 transiently express an abnormally low breathing rate and a high occurrence of apneas. Immunostainings confirmed that KCC2a is normally expressed in the brainstem neuronal groups involved in breathing (pre-Bötzinger complex, parafacial respiratory group, hypoglossus nucleus) and is absent in these regions in the KCC2a-/- mutant. However, in variously reduced in vitro medullary preparations, spontaneous rhythmic respiratory activity is similar to that expressed in wild-type preparations, as is hypoglossal motor output, and no respiratory pauses are detected, suggesting that the rhythm-generating networks are not intrinsically affected in mutants at P0. In contrast, inhibitory neuromodulatory influences exerted by the pons on respiratory rhythmogenesis are stronger in the mutant, thereby explaining the breathing anomalies observed in vivo. Thus, our results indicate that the KCC2a isoform is important for establishing proper breathing behavior at the time of birth, but by acting at sites that are extrinsic to the central respiratory networks themselves.

Neural mechanisms for sigh generation during prenatal development

The respiratory network of the preBötzinger complex (preBötC), which controls inspiratory behavior, can in normal conditions simultaneously produce two types of inspiration-related rhythmic activities: the eupneic rhythm composed of monophasic, low-amplitude, and relatively high-frequency bursts, interspersed with sigh rhythmic activity, composed of biphasic, high-amplitude, and lower frequency bursts. By combining electrophysiological recordings from transverse brainstem slices with computational modeling, new advances in the mechanisms underlying sigh production have been obtained during prenatal development. The present review summarizes recent findings that establish when sigh rhythmogenesis starts to be produced during embryonic development as well as the cellular, membrane, and synaptic properties required for its expression. Together, the results demonstrate that although generated by the same network, the eupnea and sigh rhythms have different developmental onset times and rely on distinct network properties. Because sighs (also known as augmented breaths) are important in maintaining lung function (by reopening collapsed alveoli), gaining insight into their underlying neural mechanisms at early developmental stages is likely to help in the treatment of prematurely born babies often suffering from breathing deficiencies.

Presence of Inhibitory Glycinergic Transmission in Medium Spiny Neurons in the Nucleus Accumbens

It is believed that the rewarding actions of drugs are mediated by dysregulation of the mesolimbic dopaminergic system leading to increased levels of dopamine in the nucleus accumbens (nAc). It is widely recognized that GABAergic transmission is critical for neuronal inhibition within nAc. However, it is currently unknown if medium spiny neurons (MSNs) also receive inhibition by means of glycinergic synaptic inputs. We used a combination of proteomic and electrophysiology studies to characterize the presence of glycinergic input into MSNs from nAc demonstrating the presence of glycine transmission into nAc. In D1 MSNs, we found low frequency glycinergic miniature inhibitory postsynaptic currents (mIPSCs) which were blocked by 1 μM strychnine (STN), insensitive to low (10, 50 mM) and high (100 mM) ethanol (EtOH) concentrations, but sensitive to 30 μM propofol. Optogenetic experiments confirmed the existence of STN-sensitive glycinergic IPSCs and suggest a contribution of GABA and glycine neurotransmitters to the IPSCs in nAc. The study reveals the presence of glycinergic transmission in a non-spinal region and opens the possibility of a novel mechanism for the regulation of the reward pathway.

Excitatory Modulation of the preBötzinger Complex Inspiratory Rhythm Generating Network by Endogenous Hydrogen Sulfide

Hydrogen Sulfide (H2S) is one of three gasotransmitters that modulate excitability in the CNS. Global application of H2S donors or inhibitors of H2S synthesis to the respiratory network has suggested that inspiratory rhythm is modulated by exogenous and endogenous H2S. However, effects have been variable, which may reflect that the RTN/pFRG (retrotrapezoid nucleus, parafacial respiratory group) and the preBötzinger Complex (preBötC, critical for inspiratory rhythm generation) are differentially modulated by exogenous H2S. Importantly, site-specific modulation of respiratory nuclei by H2S means that targeted, rather than global, manipulation of respiratory nuclei is required to understand the role of H2S signaling in respiratory control. Thus, our aim was to test whether endogenous H2S, which is produced by cystathionine-β-synthase (CBS) in the CNS, acts specifically within the preBötC to modulate inspiratory activity under basal (in vitro/in vivo) and hypoxic conditions (in vivo). Inhibition of endogenous H2S production by bath application of the CBS inhibitor, aminooxyacetic acid (AOAA, 0.1-1.0 mM) to rhythmic brainstem spinal cord (BSSC) and medullary slice preparations from newborn rats, or local application of AOAA into the preBötC (slices only) caused a dose-dependent decrease in burst frequency. Unilateral injection of AOAA into the preBötC of anesthetized, paralyzed adult rats decreased basal inspiratory burst frequency, amplitude and ventilatory output. AOAA in vivo did not affect the initial hypoxia-induced (10% O2, 5 min) increase in ventilatory output, but enhanced the secondary hypoxic respiratory depression. These data suggest that the preBötC inspiratory network receives tonic excitatory modulation from the CBS-H2S system, and that endogenous H2S attenuates the secondary hypoxic respiratory depression.

Respiratory rhythm generation: triple oscillator hypothesis

Breathing is vital for survival but also interesting from the perspective of rhythm generation. This rhythmic behavior is generated within the brainstem and is thought to emerge through the interaction between independent oscillatory neuronal networks. In mammals, breathing is composed of three phases - inspiration, post-inspiration, and active expiration - and this article discusses the concept that each phase is generated by anatomically distinct rhythm-generating networks: the preBötzinger complex (preBötC), the post-inspiratory complex (PiCo), and the lateral parafacial nucleus (pF L), respectively. The preBötC was first discovered 25 years ago and was shown to be both necessary and sufficient for the generation of inspiration. More recently, networks have been described that are responsible for post-inspiration and active expiration. Here, we attempt to collate the current knowledge and hypotheses regarding how respiratory rhythms are generated, the role that inhibition plays, and the interactions between the medullary networks. Our considerations may have implications for rhythm generation in general.

Desogestrel enhances ventilation in ondine patients: Animal data involving serotoninergic systems

Congenital central hypoventilation syndrome (CCHS) is a neurorespiratory disease characterized by life-threatening sleep-related hypoventilation involving an alteration of CO2/H(+) chemosensitivity. Incidental findings have suggested that desogestrel may allow recovery of the ventilatory response to CO2. The effects of desogestrel on resting ventilation have not been reported. This study was designed to test the hypothesis that desogestrel strengthens baseline ventilation by analyzing the ventilation of CCHS patients. Rodent models were used in order to determine the mechanisms involved. Ventilation in CCHS patients was measured with a pneumotachometer. In mice, ventilatory neural activity was recorded from ex vivo medullary-spinal cord preparations, ventilation was measured by plethysmography and c-fos expression was studied in medullary respiratory nuclei. Desogestrel increased baseline respiratory frequency of CCHS patients leading to a decrease in their PETCO2. In medullary spinal-cord preparations or in vivo mice, the metabolite of desogestrel, etonogestrel, induced an increase in respiratory frequency that necessitated the functioning of serotoninergic systems, and modulated GABAA and NMDA ventilatory regulations. c-FOS analysis showed the involvement of medullary respiratory groups of cell including serotoninergic neurons of the raphe pallidus and raphe obscurus nuclei that seem to play a key role. Thus, desogestrel may improve resting ventilation in CCHS patients by a stimulant effect on baseline respiratory frequency. Our data open up clinical perspectives based on the combination of this progestin with serotoninergic drugs to enhance ventilation in CCHS patients.

Modulation of synchronous sympathetic firing behaviors by endogenous GABA(A) and glycine receptor-mediated activities in the neonatal rat spinal cord in vitro

Delivering effective commands in the nervous systems require a temporal integration of neural activities such as synchronous firing. Although sympathetic nerve discharges are characterized by synchronous firing, its temporal structures and how it is modulated are largely unknown. This study used a collagenase-dissociated splanchnic sympathetic nerve-thoracic spinal cord preparation of neonatal rats in vitro as an experimental model. Several single-fiber activities were recorded simultaneously and verified by rigorous computational algorithms. Among 3763 fiber pairs that had spontaneous fiber activities, 382 fiber pairs had firing positively correlated. Their temporal relationship was quantitatively evaluated by cross-correlogram. On average, correlated firing in a fiber pair occurred in scales of ∼40ms lasting for ∼11ms. The relative frequency distribution curves of correlogram parametrical values pertinent to the temporal features were best described by trimodal Gaussians, suggesting a correlated firing originated from three or less sources. Applications of bicuculline or gabazine (noncompetitive or competitive GABA(A) receptor antagonist) and/or strychnine (noncompetitive glycine receptor antagonist) increased, decreased, or did not change individual fiber activities. Antagonist-induced enhancement and attenuation of correlated firing were demonstrated by a respective increase and decrease of the peak probability of the cross-correlograms. Heterogeneity in antagonistic responses suggests that the inhibitory neurotransmission mediated by GABA(A) and glycine receptors is not essential for but can serve as a neural substrate to modulate synchronous firing behaviors. Plausible neural mechanisms were proposed to explain the temporal structures of correlated firing between sympathetic fibers.

Inhibitory respiratory responses to progesterone and allopregnanolone in newborn rats chronically treated with caffeine

KEY POINTS

In premature newborns, recurrent apnoea is systematically treated with caffeine to prevent long-term neurocognitive disorders, but a substantial percentage of apnoea persists particularly in neonates born before 28 weeks of gestation. Progesterone has been proposed as a respiratory stimulant potentially suitable for the treatment of newborn apnoea persistent to caffeine. Accordingly we asked whether acute progesterone administration reduces apnoea frequency in newborn rats treated with caffeine. Surprisingly our results show that in newborn rats treated with caffeine, administration of progesterone inhibits breathing and increases apnoea frequency. Additional experiments showed an enhanced GABAergic inhibitory drive on breathing after caffeine treatment, and that progesterone is converted to allopregnanolone (an allosteric modulator of GABAA receptors) to inhibit breathing. We conclude that combining progesterone and chronic caffeine is not an option in preterm neonates, unless the effects of allopregnanolone can be counteracted.

ABSTRACT

Caffeine is the main treatment for apnoea in preterm neonates, but its interactions with other respiratory stimulants like progesterone are unknown. We tested the hypothesis that the addition of progesterone to caffeine treatments further stimulates ventilation. Newborn rats were treated with water (control) or caffeine (15 mg kg(-1)) by daily gavage between postnatal day (P)3 and P12. At P4 and P12, we measured apnoea frequency, ventilatory responses and metabolic parameters under both normoxia and hypoxia (12% O2, 20 min) following an acute administration of either saline or progesterone (4 mg kg(-1); i.p.). Progesterone injection increased the serum levels of both progesterone and its neuroactive metabolite allopregnanolone. Progesterone had no effect on ventilation in control rats under normoxia. Progesterone depressed ventilation in P12 caffeine-treated rats under normoxia and hypoxia and increased apnoea frequency in both P4 and P12 rats. Because allopregnanolone is an allosteric modulator of GABAA receptors and caffeine may enhance GABAergic inhibition in newborns, we studied the effects of the GABAA receptor antagonist bicuculline at 0, 1, 2 and 3 mg kg(-1) doses and allopregnanolone (10 mg kg(-1) dose) in P12 rats. In caffeine-treated rats, bicuculline enhanced ventilation, while allopregnanolone decreased ventilation and increased total apnoea time. Progesterone had no effect on ventilation and apnoea frequency in caffeine-treated rats injected with finasteride, which blocks the conversion of progesterone to allopregnanolone. We conclude that combining progesterone and chronic caffeine therapy is not an option for the treatment of persistent apnoea in preterm neonates, unless the effects of allopregnanolone can be counteracted.

Synaptic Depression Influences Inspiratory-Expiratory Phase Transition in Dbx1 Interneurons of the preBötzinger Complex in Neonatal Mice

The brainstem preBötzinger complex (preBötC) generates the rhythm underlying inspiratory breathing movements and its core interneurons are derived from Dbx1-expressing precursors. Recurrent synaptic excitation is required to initiate inspiratory bursts, but whether excitatory synaptic mechanisms also contribute to inspiratory-expiratory phase transition is unknown. Here, we examined the role of short-term synaptic depression using a rhythmically active neonatal mouse brainstem slice preparation. We show that afferent axonal projections to Dbx1 preBötC neurons undergo activity-dependent depression and we identify a refractory period (∼2 s) after endogenous inspiratory bursts that precludes light-evoked bursts in channelrhodopsin-expressing Dbx1 preBötC neurons. We demonstrate that the duration of the refractory period-but neither the cycle period nor the magnitude of endogenous inspiratory bursts-is sensitive to changes in extracellular Ca(2+). Further, we show that postsynaptic factors are unlikely to explain the refractory period or its modulation by Ca(2+). Our findings are consistent with the hypothesis that short-term synaptic depression in Dbx1 preBötC neurons influences the inspiratory-expiratory phase transition during respiratory rhythmogenesis.

SIGNIFICANCE STATEMENT

Theories of breathing's neural origins have heretofore focused on intrinsically bursting "pacemaker" cells operating in conjunction with synaptic inhibition for phase transition and cycle timing. However, contemporary studies falsify an obligatory role for pacemaker-like neurons and synaptic inhibition, giving credence to burst-generating mechanisms based on recurrent excitation among glutamatergic interneurons of the respiratory kernel. Here, we investigated the role of short-term synaptic depression in inspiratory-expiratory phase transition. Until now, this role remained an untested prediction of mathematical models. The present data emphasize that synaptic properties of excitatory interneurons of the respiratory rhythmogenic kernel, derived from Dbx1-expressing precursors, may provide the core logic underlying the rhythm for breathing.

Defining inhibitory neurone function in respiratory circuits: opportunities with optogenetics?

Pharmacological and mathematical modelling studies support the view that synaptic inhibition in mammalian brainstem respiratory circuits is essential for generating normal and stable breathing movements. GABAergic and glycinergic neurones are known components of these circuits but their precise functional roles have not been established, especially within key microcircuits of the respiratory pre-Bötzinger (pre-BötC) and Bötzinger (BötC) complexes involved in phasic control of respiratory pump and airway muscles. Here, we review briefly current concepts of relevant complexities of inhibitory synapses and the importance of synaptic inhibition in the operation of these microcircuits. We highlight results and limitations of classical pharmacological studies that have suggested critical functions of synaptic inhibition. We then explore the potential opportunities for optogenetic strategies that represent a promising new approach for interrogating function of inhibitory circuits, including a hypothetical wish list for optogenetic approaches to allow expedient application of this technology. We conclude that recent technical advances in optogenetics should provide a means to understand the role of functionally select and regionally confined subsets of inhibitory neurones in key respiratory circuits such as those in the pre-BötC and BötC.

Ethanol effects on glycinergic transmission: From molecular pharmacology to behavior responses

It is well accepted that ethanol is able to produce major health and economic problems associated to its abuse. Because of its intoxicating and addictive properties, it is necessary to analyze its effect in the central nervous system. However, we are only now learning about the mechanisms controlling the modification of important membrane proteins such as ligand-activated ion channels by ethanol. Furthermore, only recently are these effects being correlated to behavioral changes. Current studies show that the glycine receptor (GlyR) is a susceptible target for low concentrations of ethanol (5-40mM). GlyRs are relevant for the effects of ethanol because they are found in the spinal cord and brain stem where they primarily express the α1 subunit. More recently, the presence of GlyRs was described in higher regions, such as the hippocampus and nucleus accumbens, with a prevalence of α2/α3 subunits. Here, we review data on the following aspects of ethanol effects on GlyRs: (1) direct interaction of ethanol with amino acids in the extracellular or transmembrane domains, and indirect mechanisms through the activation of signal transduction pathways; (2) analysis of α2 and α3 subunits having different sensitivities to ethanol which allows the identification of structural requirements for ethanol modulation present in the intracellular domain and C-terminal region; (3) Genetically modified knock-in mice for α1 GlyRs that have an impaired interaction with G protein and demonstrate reduced ethanol sensitivity without changes in glycinergic transmission; and (4) GlyRs as potential therapeutic targets.

Developmental nicotine exposure enhances inhibitory synaptic transmission in motor neurons and interneurons critical for normal breathing

Nicotine exposure in utero negatively affects neuronal growth, differentiation, and synaptogenesis. We used rhythmic brainstems slices and immunohistochemistry to determine how developmental nicotine exposure (DNE) alters inhibitory neurotransmission in two regions essential to normal breathing, the hypoglossal motor nucleus (XIIn), and preBötzinger complex (preBötC). We microinjected glycine or muscimol (GABAA agonist) into the XIIn or preBötC of rhythmic brainstem slices from neonatal rats while recording from XII nerve roots to obtain XII motoneuron population activity. Injection of glycine or muscimol into the XIIn reduced XII nerve burst amplitude, while injection into the preBötC altered nerve burst frequency. These responses were exaggerated in preparations from DNE animals. Quantitative immunohistochemistry revealed a significantly higher GABAA receptor density on XII motoneurons from DNE pups. There were no differences in GABAA receptor density in the preBötC, and there were no differences in glycine receptor expression in either region. Nicotine, in the absence of other chemicals in tobacco smoke, alters normal development of brainstem circuits that are critical for normal breathing.

Sigh and Eupnea Rhythmogenesis Involve Distinct Interconnected Subpopulations: A Combined Computational and Experimental Study

How a single neural network can generate several rhythmic activities at different time scales remains an open question. Here, in addition to the already described reconfiguring process, we propose a new mechanism by which the respiratory network can generate simultaneously two distinct inspiration-related activities (eupnea and sigh) at different frequencies.

Neural networks control complex motor outputs by generating several rhythmic neuronal activities, often with different time scales. One example of such a network is the pre-Bötzinger complex respiratory network (preBötC) that can simultaneously generate fast, small-amplitude, monophasic eupneic breaths together with slow, high-amplitude, biphasic augmented breaths (sighs). However, the underlying rhythmogenic mechanisms for this bimodal discharge pattern remain unclear, leaving two possible explanations: the existence of either reconfiguring processes within the same network or two distinct subnetworks. Based on recent in vitro data obtained in the mouse embryo, we have built a computational model consisting of two compartments, interconnected through appropriate synapses. One compartment generates sighs and the other produces eupneic bursts. The model reproduces basic features of simultaneous sigh and eupnea generation (two types of bursts differing in terms of shape, amplitude, and frequency of occurrence) and mimics the effect of blocking glycinergic synapses. Furthermore, we used this model to make predictions that were subsequently tested on the isolated preBötC in mouse brainstem slice preparations. Through a combination of in vitro and in silico approaches we find that (1) sigh events are less sensitive to network excitability than eupneic activity, (2) calcium-dependent mechanisms and the I_h current play a prominent role in sigh generation, and (3) specific parameters of I_h activation set the low sensitivity to excitability in the sigh neuronal subset. Altogether, our results strongly support the hypothesis that distinct subpopulations within the preBötC network are responsible for sigh and eupnea rhythmogenesis.

5-HT1A receptor agonist Befiradol reduces fentanyl-induced respiratory depression, analgesia, and sedation in rats

BACKGROUND

There is an unmet clinical need to develop a pharmacological therapy to counter opioid-induced respiratory depression without interfering with analgesia or behavior. Several studies have demonstrated that 5-HT1A receptor agonists alleviate opioid-induced respiratory depression in rodent models. However, there are conflicting reports regarding their effects on analgesia due in part to varied agonist receptor selectivity and presence of anesthesia. Therefore the authors performed a study in rats with befiradol (F13640 and NLX-112), a highly selective 5-HT1A receptor agonist without anesthesia.

METHODS

Respiratory neural discharge was measured using in vitro preparations. Plethysmographic recording, nociception testing, and righting reflex were used to examine respiratory ventilation, analgesia, and sedation, respectively.

RESULTS

Befiradol (0.2 mg/kg, n = 6) reduced fentanyl-induced respiratory depression (53.7 ± 5.7% of control minute ventilation 4 min after befiradol vs. saline 18.7 ± 2.2% of control, n = 9; P < 0.001), duration of analgesia (90.4 ± 11.6 min vs. saline 130.5 ± 7.8 min; P = 0.011), duration of sedation (39.8 ± 4 min vs. saline 58 ± 4.4 min; P = 0.013); and induced baseline hyperventilation, hyperalgesia, and "behavioral syndrome" in nonsedated rats. Further, the befiradol-induced alleviation of opioid-induced respiratory depression involves sites or mechanisms not functioning in vitro brainstem-spinal cord and medullary slice preparations.

CONCLUSIONS

The reversal of opioid-induced respiratory depression and sedation by befiradol in adult rats was robust, whereas involved mechanisms are unclear. However, there were adverse concomitant decreases in fentanyl-induced analgesia and altered baseline ventilation, nociception, and behavior.

KCC2-mediated regulation of respiration-related rhythmic activity during postnatal development in mouse medulla oblongata

GABA acts as inhibitory neurotransmitter in the adult central nervous system but as excitatory neurotransmitter during early postnatal development. This shift in GABA's action from excitation to inhibition is caused by a decrease in intracellular chloride concentration ([Cl(-)]i), which in turn is caused by changes in the relative expression levels of the K(+)-Cl(-) co-transporter (KCC2) and the Na(+), K(+)-2Cl(-) co-transporter (NKCC1) proteins. Previous studies have used slices containing the medullary pre-Bötzinger complex (pre-BötC) to record respiration-related rhythmic activity (RRA) from the hypoglossal nucleus (12 N). The role of GABAergic transmission in the regulation of medullary RRA neonatally, however, is yet to be determined. Here, we examined how GABA and chloride co-transporters contribute to RRA during development in the 12 N where inspiratory neurons reside. We recorded extracellular RRA in medullary slices obtained from postnatal day (P) 0-7 mice. RRA was induced by soaking slices in artificial cerebrospinal fluid (aCSF) containing 8mM-K(+). Application of GABA significantly increased the frequency of RRA after P3, whereas application of a KCC2 blocker (R (+)-[(2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-indenyl-5-yl)oxy]acetic acid (DIOA)) significantly decreased the frequency of RRA after P1. In addition, dense KCC2 immunolabeling was seen in the superior longitudinalis (SL) of the 12 N, which is responsible for retraction of the tongue, from P0 and P7. These results indicate that GABA administration can increase RRA frequency during the first week following birth. This in turn suggests that decreasing [Cl(-)]i levels caused by increasing KCC2 levels in the 12 N could play important roles in regulating the frequency of RRA during development.

Altered sedative effects of ethanol in mice with α1 glycine receptor subunits that are insensitive to Gβγ modulation

Alcohol abuse and alcoholism are major health problems and one of the leading preventable causes of death. Before achieving better treatments for alcoholism, it is necessary to understand the critical actions of alcohol on membrane proteins that regulate fundamental functions in the central nervous system. After generating a genetically modified knock-in (KI) mouse having a glycine receptor (GlyR) with phenotypical silent mutations at KK385/386AA, we studied its cellular and in vivo ethanol sensitivity. Analyses with western blotting and immunocytochemistry indicated that the expression of α1 GlyRs in nervous tissues and spinal cord neurons (SCNs) were similar between WT and KI mice. The analysis of synaptic currents recorded from KI mice showed that the glycinergic synaptic transmission had normal properties, but the sensitivity to ethanol was significantly reduced. Furthermore, the glycine-evoked current in SCNs from KI was resistant to ethanol and G-protein activation by GTP-γ-S. In behavioral studies, KI mice did not display the foot-clasping behavior upon lifting by the tail and lacked an enhanced startle reflex response that are characteristic of other glycine KI mouse lines with markedly impaired glycine receptor function. The most notable characteristic of the KI mice was their significant lower sensitivity to ethanol (∼40%), expressed by shorter times in loss of righting reflex (LORR) in response to a sedative dose of ethanol (3.5 g/Kg). These data provide the first evidence to link a molecular site in the GlyR with the sedative effects produced by intoxicating doses of ethanol.

Identification of the pre-Bötzinger complex inspiratory center in calibrated "sandwich" slices from newborn mice with fluorescent Dbx1 interneurons

Inspiratory active pre‐Bötzinger complex (preBötC) networks produce the neural rhythm that initiates and controls breathing movements. We previously identified the preBötC in the newborn rat brainstem and established anatomically defined transverse slices in which the preBötC remains active when exposed at one surface. This follow‐up study uses a neonatal mouse model in which the preBötC as well as a genetically defined class of respiratory interneurons can be identified and selectively targeted for physiological recordings. The population of glutamatergic interneurons whose precursors express the transcription factor Dbx1 putatively comprises the core respiratory rhythmogenic circuit. Here, we used intersectional mouse genetics to identify the brainstem distribution of Dbx1‐derived neurons in the context of observable respiratory marker structures. This reference brainstem atlas enabled online histology for generating calibrated sandwich slices to identify the preBötC location, which was heretofore unspecified for perinatal mice. Sensitivity to opioids ensured that slice rhythms originated from preBötC neurons and not parafacial respiratory group/retrotrapezoid nucleus (pFRG/RTN) cells because opioids depress preBötC, but not pFRG/RTN rhythms. We found that the preBötC is centered ~0.4 mm caudal to the facial motor nucleus in this Cre/lox reporter mouse during postnatal days 0–4. Our findings provide the essential basis for future optically guided electrophysiological and fluorescence imaging‐based studies, as well as the application of other Cre‐dependent tools to record or manipulate respiratory rhythmogenic neurons. These resources will ultimately help elucidate the mechanisms that promote respiratory‐related oscillations of preBötC Dbx1‐derived neurons and thus breathing.

Breathing movements emanate from Dbx1‐derived interneurons of the brainstem pre‐Bötzinger complex (preBötC). We generated a histology atlas of the medulla in newborn Dbx1 Cre/lox reporter mice and performed physiological tests to pinpoint the preBötC location and map the Dbx1 neuron distribution, which will facilitate neurobiological studies of respiratory rhythm generation.

Laser ablation of Dbx1 neurons in the pre-Bötzinger complex stops inspiratory rhythm and impairs output in neonatal mice

To understand the neural origins of rhythmic behavior one must characterize the central pattern generator circuit and quantify the population size needed to sustain functionality. Breathing-related interneurons of the brainstem pre-Bötzinger complex (preBötC) that putatively comprise the core respiratory rhythm generator in mammals are derived from Dbx1-expressing precursors. Here, we show that selective photonic destruction of Dbx1 preBötC neurons in neonatal mouse slices impairs respiratory rhythm but surprisingly also the magnitude of motor output; respiratory hypoglossal nerve discharge decreased and its frequency steadily diminished until rhythm stopped irreversibly after 85±20 (mean ± SEM) cellular ablations, which corresponds to ∼15% of the estimated population. These results demonstrate that a single canonical interneuron class generates respiratory rhythm and contributes in a premotor capacity, whereas these functions are normally attributed to discrete populations. We also establish quantitative cellular parameters that govern network viability, which may have ramifications for respiratory pathology in disease states.

Emergence of sigh rhythmogenesis in the embryonic mouse

In mammals, eupnoeic breathing is periodically interrupted by spontaneous augmented breaths (sighs) that include a larger-amplitude inspiratory effort, typically followed by a post-sigh apnoea. Previous in vitro studies in newborn rodents have demonstrated that the respiratory oscillator of the pre-Bötzinger complex (preBötC) can generate the distinct inspiratory motor patterns for both eupnoea- and sigh-related behaviour. During mouse embryonic development, the preBötC begins to generate eupnoeic rhythmicity at embryonic day (E) 15.5, but the network's ability to also generate sigh-like activity remains unexplored at prenatal stages. Using transverse brainstem slice preparations we monitored the neuronal population activity of the preBötC at different embryonic ages. Spontaneous sigh-like rhythmicity was found to emerge progressively, being expressed in 0/32 slices at E15.5, 7/30 at E16.5, 9/22 at E17.5 and 23/26 at E18.5. Calcium imaging showed that the preBötC cell population that participates in eupnoeic-like discharge was also active during fictive sighs. However, patch-clamp recordings revealed the existence of an additional small subset of neurons that fired exclusively during sigh activity. Changes in glycinergic inhibitory synaptic signalling, either by pharmacological blockade, functional perturbation or natural maturation of the chloride co-transporters KCC2 or NKCC1 selectively, and in an age-dependent manner, altered the bi-phasic nature of sigh bursts and their coordination with eupnoeic bursting, leading to the generation of an atypical monophasic sigh-related event. Together our results demonstrate that the developmental emergence of a sigh-generating capability occurs after the onset of eupnoeic rhythmogenesis and requires the proper maturation of chloride-mediated glycinergic synaptic transmission.

Control of breathing activity in the fetus and newborn

Breathing movements have been demonstrated in the fetuses of every mammalian species investigated and are a critical component of normal fetal development. The classic sheep preparations instrumented for chronic fetal monitoring determined that fetal breathing movements (FBMs) occur in aggregates interspersed with long periods of quiescence that are strongly associated with neurophysiological state. The fetal sheep model also provided data regarding the neurochemical modulation of behavioral state and FBMs under a variety of in utero conditions. Subsequently, in vitro rodent models have been developed to advance our understanding of cellular, synaptic, network, and more detailed neuropharmacological aspects of perinatal respiratory neural control. This includes the ontogeny of the inspiratory rhythm generating center, the preBötzinger complex (preBötC), and the anatomical and functional development of phrenic motoneurons (PMNs) and diaphragm during the perinatal period. A variety of newborn animal models and studies of human infants have provided insights into age-dependent changes in state-dependent respiratory control, responses to hypoxia/hypercapnia and respiratory pathologies.

Postnatal development of glycine receptor subunits α1, α2, α3, and β immunoreactivity in multiple brain stem respiratory-related nuclear groups of the rat

The respiratory system is immature at birth and significant development occurs postnatally. A critical period of respiratory development occurs in rats around postnatal days 12-13, when enhanced inhibition dominates over suppressed excitation. The mechanisms underlying the heightened inhibition are not fully understood. The present study tested our hypothesis that the inhibition is marked by a switch in glycine receptor subunits from neonatal to adult form around the critical period. An in-depth immunohistochemical and single neuron optical densitometric study was undertaken on four respiratory-related nuclear groups (the pre-Bötzinger complex, nucleus ambiguus, hypoglossal nucleus, and ventrolateral subnucleus of solitary tract nucleus), and a non-respiratory cuneate nucleus in P2-21 rats. Our data revealed that in the respiratory-related nuclear groups: (1) the expressions of GlyRα2 and GlyRα3 were relatively high at P2, but declined after 1-1½ weeks to their lowest levels at P21; (2) the expression of GlyRα1 increased with age and reached significance at P12; and (3) the expression of GlyRβ rose from P2 to P12 followed by a slight decline until P21. No distinct increase in GlyRα1 at P12 was noted in the cuneate nucleus. Thus, there is a switch in dominance of expression from neonatal GlyRα2/α3 to the adult GlyRα1 and a heightened expression of GlyRα1 around the critical period in all respiratory-related nuclear groups, thereby supporting enhanced inhibition at that time. The rise in the expression of GlyRβ around P12 indicates that it plays an important role in forming the mature heteropentameric glycine receptors in these brain stem nuclear groups.

Coadministration of the AMPAKINE CX717 with propofol reduces respiratory depression and fatal apneas

BACKGROUND

Propofol (2,6-diisopropylphenol) is used for the induction and maintenance of anesthesia in human and veterinary medicine. Propofol's disadvantages include the induction of respiratory depression and apnea. Here, the authors report a clinically feasible pharmacological solution for reducing propofol-induced respiratory depression via a mechanism that does not interfere with anesthesia. Specifically, they test the hypothesis that the AMPAKINE CX717, which has been proven metabolically stable and safe for human use, can prevent and rescue from propofol-induced severe apnea.

METHODS

The actions of propofol and the AMPAKINE CX717 were measured via (1) ventral root recordings from newborn rat brainstem-spinal cord preparations, (2) phrenic nerve recordings from an adult mouse in situ working heart-brainstem preparation, and (3) plethysmographic recordings from unrestrained newborn and adult rats.

RESULTS

In vitro, respiratory depression caused by propofol (2 μM, n = 11, mean ± SEM, 41 ± 5% of control frequency, 63 ± 5% of control duration) was alleviated by CX717 (n = 4, 50-150 μM). In situ, a decrease in respiratory frequency (44 ± 9% of control), phrenic burst duration (66 ± 7% of control), and amplitude (78 ± 5% of control) caused by propofol (2 μM, n = 5) was alleviated by coadministration of CX717 (50 μM, n = 5). In vivo, pre- or coadministration of CX717 (20-25mg/kg) with propofol markedly reduced propofol-induced respiratory depression (n = 7; 20mg/kg) and propofol-induced lethal apnea (n = 6; 30 mg/kg).

CONCLUSIONS

Administration of CX717 before or in conjunction with propofol provides an increased safety margin against profound apnea and death.

The rhythmic, transverse medullary slice preparation in respiratory neurobiology: contributions and caveats

Our understanding of the sites and mechanisms underlying rhythmic breathing as well as the neuromodulatory control of respiratory rhythm, pattern, and respiratory motoneuron excitability during perinatal development has advanced significantly over the last 20 years. A major catalyst was the development in 1991 of the rhythmically-active medullary slice preparation, which provided precise mechanical and chemical control over the network as well as enhanced physical and optical access to key brainstem regions. Insights obtained in vitro have informed multiple mechanistic hypotheses. In vivo tests of these hypotheses, performed under conditions of reduced control and precision but more obvious physiological relevance, have clearly established the significance for respiratory neurobiology of the rhythmic slice preparation. We review the contributions of this preparation to current understanding/concepts in respiratory control, and outline the limitations of this approach in the context of studying rhythm and pattern generation, homeostatic control mechanisms and murine models of human genetic disorders that feature prominent breathing disturbances.

Understanding the rhythm of breathing: so near, yet so far

Breathing is an essential behavior that presents a unique opportunity to understand how the nervous system functions normally, how it balances inherent robustness with a highly regulated lability, how it adapts to both rapidly and slowly changing conditions, and how particular dysfunctions result in disease. We focus on recent advancements related to two essential sites for respiratory rhythmogenesis: (a) the preBötzinger Complex (preBötC) as the site for the generation of inspiratory rhythm and (b) the retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG) as the site for the generation of active expiration.

Respiratory depression in rats induced by alcohol and barbiturate and rescue by ampakine CX717

Barbiturate use in conjunction with alcohol can result in severe respiratory depression and overdose deaths. The mechanisms underlying the additive/synergistic actions were unresolved. Current management of ethanol-barbiturate-induced apnea is limited to ventilatory and circulatory support coupled with drug elimination. Based on recent preclinical and clinical studies of opiate-induced respiratory depression, we hypothesized that ampakine compounds may provide a treatment for other types of drug-induced respiratory depression. The actions of alcohol, pentobarbital, bicuculline, and the ampakine CX717, alone and in combination, were measured via 1) ventral root recordings from newborn rat brain stem-spinal cord preparations and 2) plethysmographic recordings from unrestrained newborn and adult rats. We found that ethanol caused a modest suppression of respiratory drive in vitro (50 mM) and in vivo (2 g/kg ip). Pentobarbital induced an ∼50% reduction in respiratory frequency in vitro (50 μM) and in vivo (28 mg/kg for pups and 56 mg/kg for adult rats ip). However, severe life-threatening apnea was induced by the combination of the agents in vitro and in vivo via activation of GABA(A) receptors, which was exacerbated by hypoxic (8% O(2)) conditions. Administration of the ampakine CX717 alleviated a significant component of the respiratory depression in vitro (50-150 μM) and in vivo (30 mg/kg ip). Bicuculline also alleviated ethanol-/pentobarbital-induced respiratory depression but caused seizure activity, whereas CX717 did not. These data demonstrated that ethanol and pentobarbital together caused severe respiratory depression, including lethal apnea, via synergistic actions that blunt chemoreceptive responses to hypoxia and hypercapnia and suppress central respiratory rhythmogenesis. The ampakine CX717 markedly reduced the severity of respiratory depression.

Qualitative validation of the reduction from two reciprocally coupled neurons to one self-coupled neuron in a respiratory network model

The pre-Bötzinger complex of the mammalian brainstem is a heterogeneous neuronal network, and individual neurons within the network have varying strengths of the persistent sodium and calcium-activated nonspecific cationic currents. Individually, these currents have been the focus of modeling efforts. Previously, Dunmyre et al. (J Comput Neurosci 1-24, 2011) proposed a model and studied the interactions of these currents within one self-coupled neuron. In this work, I consider two identical, reciprocally coupled model neurons and validate the reduction to the self-coupled case. I find that all of the dynamics of the two model neuron network and the regions of parameter space where these distinct dynamics are found are qualitatively preserved in the reduction to the self-coupled case.

Cumulative lesioning of respiratory interneurons disrupts and precludes motor rhythms in vitro

How brain functions degenerate in the face of progressive cell loss is an important issue that pertains to neurodegenerative diseases and basic properties of neural networks. We developed an automated system that uses two-photon microscopy to detect rhythmic neurons from calcium activity, and then individually laser ablates the targets while monitoring network function in real time. We applied this system to the mammalian respiratory oscillator located in the pre-Bötzinger Complex (preBötC) of the ventral medulla, which spontaneously generates breathing-related motor activity in vitro. Here, we show that cumulatively deleting preBötC neurons progressively decreases respiratory frequency and the amplitude of motor output. On average, the deletion of 120 ± 45 neurons stopped spontaneous respiratory rhythm, and our data suggest ≈82% of the rhythm-generating neurons remain unlesioned. Cumulative ablations in other medullary respiratory regions did not affect frequency but diminished the amplitude of motor output to a lesser degree. These results suggest that the preBötC can sustain insults that destroy no more than ≈18% of its constituent interneurons, which may have implications for the onset of respiratory pathologies in disease states.

Postnatal development of Na(+)-K(+)-2Cl(-) co-transporter 1 and K(+)-Cl(-) co-transporter 2 immunoreactivity in multiple brain stem respiratory nuclei of the rat

Previously, we reported that in rats, GABA(A) and glycine receptor immunoreactivity increased markedly in multiple brain stem respiratory nuclei around postnatal days (P) 12-13, a critical period when abrupt neurochemical, metabolic, ventilatory, and electrophysiological changes occur in the respiratory network and when the system is under greater inhibition than excitation. Since Na(+)-K(+)-2Cl(-) co-transporter 1 (NKCC1) and K(+)-Cl(-) co-transporter 2 (KCC2) play pivotal roles in determining the responses of GABA(A) and glycine receptors, we hypothesized that NKCC1 and KCC2 undergo significant changes during the critical period. An in-depth immunohistochemical and single neuron optical densitometric study of neurons in seven respiratory-related nuclei (the pre-Bötzinger complex [PBC], nucleus ambiguus [Amb], hypoglossal nucleus [XII], ventrolateral subnucleus of solitary tract nucleus [NTS(VL)], retrotrapezoid nucleus/parafacial respiratory group [retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG)], dorsal motor nucleus of the vagus nerve [dorsal motor nucleus of the vagus nerve (DMNX)], and inferior olivary nucleus [IO]) and a non-respiratory cuneate nucleus (CN, an internal control) was undertaken in P0-P21 rats. Our data revealed that (1) NKCC1 immunoreactivity exhibited a developmental decrease from P0 to P21 in all eight nuclei examined, being relatively high during the first 1½ postnatal weeks and decreased thereafter. The decrease was abrupt and statistically significant at P12 in the PBC, Amb, and XII; (2) KCC2 immunoreactivity in these eight nuclei showed a developmental increase from P0 to P21; and (3) the significant reduction in NKCC1 and the greater dominance of KCC2 around P12 in multiple respiratory nuclei of the brain stem may form the basis of an enhanced inhibition in the respiratory network during the critical period before the system stabilizes to a more mature state.

Corticosterone promotes emergence of fictive air breathing in Xenopus laevis Daudin tadpole brainstems

The emergence of air breathing during amphibian metamorphosis requires significant changes to the brainstem circuits that generate and regulate breathing. However, the mechanisms controlling this developmental process are unknown. Because corticosterone plays an important role in the neuroendocrine regulation of amphibian metamorphosis, we tested the hypothesis that corticosterone augments fictive air breathing frequency in Xenopus laevis. To do so, we compared the fictive air breathing frequency produced by in vitro brainstem preparations from pre-metamorphic tadpoles and adult frogs before and after 1 h application of corticosterone (100 nmol l–1). Fictive breathing measurements related to gill and lung ventilation were recorded extracellularly from cranial nerve rootlets V and X. Corticosterone application had no immediate effect on respiratory-related motor output produced by brainstems from either developmental stage. One hour after corticosterone wash out, fictive lung ventilation frequency was increased whereas gill burst frequency was decreased. This effect was stage specific as it was significant only in preparations from tadpoles. GABA application (0.001–0.5 mmol l–1) augmented fictive air breathing in tadpole preparations. However, this effect of GABA was no longer observed following corticosterone treatment. An increase in circulating corticosterone is one of the endocrine processes that orchestrate central nervous system remodelling during metamorphosis. The age-specific effects of corticosterone application indicate that this hormone can act as an important regulator of respiratory control development in Xenopus tadpoles. Concurrent changes in GABAergic neurotransmission probably contribute to this maturation process, leading to the emergence of air breathing in this species.

Increased GABA(A) receptor ε-subunit expression on ventral respiratory column neurons protects breathing during pregnancy

GABAergic signaling is essential for proper respiratory function. Potentiation of this signaling with allosteric modulators such as anesthetics, barbiturates, and neurosteroids can lead to respiratory arrest. Paradoxically, pregnant animals continue to breathe normally despite nearly 100-fold increases in circulating neurosteroids. ε subunit-containing GABA_ARs are insensitive to positive allosteric modulation, thus we hypothesized that pregnant rats increase ε subunit-containing GABA_AR expression on brainstem neurons of the ventral respiratory column (VRC). In vivo, pregnancy rendered respiratory motor output insensitive to otherwise lethal doses of pentobarbital, a barbiturate previously used to categorize the ε subunit. Using electrode array recordings in vitro, we demonstrated that putative respiratory neurons of the preBötzinger Complex (preBötC) were also rendered insensitive to the effects of pentobarbital during pregnancy, but unit activity in the VRC was rapidly inhibited by the GABA_AR agonist, muscimol. VRC unit activity from virgin and post-partum females was potently inhibited by both pentobarbital and muscimol. Brainstem ε subunit mRNA and protein levels were increased in pregnant rats, and GABA_AR ε subunit expression co-localized with a marker of rhythm generating neurons (neurokinin 1 receptors) in the preBötC. These data support the hypothesis that pregnancy renders respiratory motor output and respiratory neuron activity insensitive to barbiturates, most likely via increased ε subunit-containing GABA_AR expression on respiratory rhythm-generating neurons. Increased ε subunit expression may be critical to preserve respiratory function (and life) despite increased neurosteroid levels during pregnancy.

Interactions of persistent sodium and calcium-activated nonspecific cationic currents yield dynamically distinct bursting regimes in a model of respiratory neurons

The preBötzinger complex (preBötC) is a heterogeneous neuronal network within the mammalian brainstem that has been experimentally found to generate robust, synchronous bursts that drive the inspiratory phase of the respiratory rhythm. The persistent sodium (NaP) current is observed in every preBötC neuron, and significant modeling effort has characterized its contribution to square-wave bursting in the preBötC. Recent experimental work demonstrated that neurons within the preBötC are endowed with a calcium-activated nonspecific cationic (CAN) current that is activated by a signaling cascade initiated by glutamate. In a preBötC model, the CAN current was shown to promote robust bursts that experience depolarization block (DB bursts). We consider a self-coupled model neuron, which we represent as a single compartment based on our experimental finding of electrotonic compactness, under variation of g (NaP), the conductance of the NaP current, and g (CAN), the conductance of the CAN current. Varying these two conductances yields a spectrum of activity patterns, including quiescence, tonic activity, square-wave bursting, DB bursting, and a novel mixture of square-wave and DB bursts, which match well with activity that we observe in experimental preparations. We elucidate the mechanisms underlying these dynamics, as well as the transitions between these regimes and the occurrence of bistability, by applying the mathematical tools of bifurcation analysis and slow-fast decomposition. Based on the prevalence of NaP and CAN currents, we expect that the generalizable framework for modeling their interactions that we present may be relevant to the rhythmicity of other brain areas beyond the preBötC as well.