When norepinephrine becomes a driver of breathing irregularities: how intermittent hypoxia fundamentally alters the modulatory response of the respiratory network.

Neuronal networks are endogenously modulated by aminergic and peptidergic substances. These modulatory processes are critical for maintaining normal activity and adapting networks to changes in metabolic, behavioral, and environmental conditions. However, disturbances in neuromodulation have also been associated with pathologies. Using whole animals (in vivo) and functional brainstem slices (in vitro) from mice, we demonstrate that exposure to acute intermittent hypoxia (AIH) leads to fundamental changes in the neuromodulatory response of the respiratory network located within the preBötzinger complex (preBötC), an area critical for breathing. Norepinephrine, which normally regularizes respiratory activity, renders respiratory activity irregular after AIH. Respiratory irregularities are caused both in vitro and in vivo by AIH, which increases synaptic inhibition within the preBötC when norepinephrine is endogenously or exogenously increased. These irregularities are prevented by blocking synaptic inhibition before AIH. However, regular breathing cannot be reestablished if synaptic inhibition is blocked after AIH. We conclude that subtle changes in synaptic transmission can have dramatic consequences at the network level as endogenously released neuromodulators that are normally adaptive become the drivers of irregularity. Moreover, irregularities in the preBötC result in irregularities in the motor output in vivo and in incomplete transmission of inspiratory activity to the hypoglossus motor nucleus. Our finding has basic science implications for understanding network functions in general, and it may be clinically relevant for understanding pathological disturbances associated with hypoxic episodes such as those associated with myocardial infarcts, obstructive sleep apneas, apneas of prematurity, Rett syndrome, and sudden infant death syndrome.

Prostaglandin E2 differentially modulates the central control of eupnoea, sighs and gasping in mice.

Prostaglandin E2 (PGE2) augments distinct inspiratory motor patterns, generated within the preBötzinger complex (preBötC), in a dose-dependent way. The frequency of sighs and gasping are stimulated at low concentrations, while the frequency of eupnoea increases only at high concentrations. We used in vivo microinjections into the preBötC and in vitro isolated brainstem slice preparations to investigate the dose-dependent effects of PGE2 on the preBötC activity. Synaptic measurements in whole cell voltage clamp recordings of inspiratory neurons revealed no changes in inhibitory or excitatory synaptic transmission in response to PGE2 exposure. In current clamp recordings obtained from inspiratory neurons of the preBötC, we found an increase in the frequency and amplitude of bursting activity in neurons with intrinsic bursting properties after exposure to PGE2. Riluzole, a blocker of the persistent sodium current, abolished the effect of PGE2 on sigh activity, while flufenamic acid, a blocker of the calcium-activated non-selective cation conductance, abolished the effect on eupnoeic activity caused by PGE2. Prostaglandins are important regulators of autonomic functions in the mammalian organism. Here we demonstrate in vivo that prostaglandin E2 (PGE2) can differentially increase the frequency of eupnoea (normal breathing) and sighs (augmented breaths) when injected into the preBötzinger complex (preBötC), a medullary area that is critical for breathing. Low concentrations of PGE2 (100-300 nm) increased the sigh frequency, while higher concentrations (1-2 µm) were required to increase the eupnoeic frequency. The concentration-dependent effects were similarly observed in the isolated preBötC. This in vitro preparation also revealed that riluzole, a blocker of the persistent sodium current (INap), abolished the modulatory effect on sighs, while flufenamic acid, an antagonist for the calcium-activated non-selective cation conductance (ICAN ) abolished the effect of PGE2 on fictive eupnoea at higher concentrations. At the cellular level PGE2 significantly increased the amplitude and frequency of intrinsic bursting in inspiratory neurons. By contrast PGE2 affected neither excitatory nor inhibitory synaptic transmission. We conclude that PGE2 differentially modulates sigh, gasping and eupnoeic activity by differentially increasing INap and ICAN currents in preBötC neurons.

Stable respiratory activity requires both P/Q-type and N-type voltage-gated calcium channels.

P/Q-type voltage-gated calcium channels (Ca(v)2.1) play critical presynaptic and postsynaptic roles throughout the nervous system and have been implicated in a variety of neurological disorders. Here we report that mice with a genetic ablation of the Ca(v)2.1 pore-forming α(1A) subunit (α(1A)⁻/⁻) encoded by CACNA1a (Jun et al., 1999) suffer during postnatal development from increasing breathing disturbances that lead ultimately to death. Breathing abnormalities include decreased minute ventilation and a specific loss of sighs, which was associated with lung atelectasis. Similar respiratory alterations were preserved in the isolated in vitro brainstem slice preparation containing the pre-Bötzinger complex. The loss of Ca(v)2.1 was associated with an alteration in the functional dependency on N-type calcium channels (Ca(v)2.2). Blocking N-type calcium channels with conotoxin GVIA had only minor effects on respiratory activity in slices from control (CT) littermates, but abolished respiratory activity in all slices from α(1A)⁻/⁻ mice. The amplitude of evoked EPSPs was smaller in inspiratory neurons from α(1A)⁻/⁻ mice compared with CTs. Conotoxin GVIA abolished all EPSPs in inspiratory neurons from α(1A)⁻/⁻ mice, while the EPSP amplitude was reduced by only 30% in CT mice. Moreover, neuromodulation was significantly altered as muscarine abolished respiratory network activity in α(1A)⁻/⁻ mice but not in CT mice. We conclude that excitatory synaptic transmission dependent on N-type and P/Q-type calcium channels is required for stable breathing and sighing. In the absence of P/Q-type calcium channels, breathing, sighing, and neuromodulation are severely compromised, leading to early mortality.

TASK-2 channels contribute to pH sensitivity of retrotrapezoid nucleus chemoreceptor neurons.

Phox2b-expressing glutamatergic neurons of the retrotrapezoid nucleus (RTN) display properties expected of central respiratory chemoreceptors; they are directly activated by CO2/H(+) via an unidentified pH-sensitive background K(+) channel and, in turn, facilitate brainstem networks that control breathing. Here, we used a knock-out mouse model to examine whether TASK-2 (K2P5), an alkaline-activated background K(+) channel, contributes to RTN neuronal pH sensitivity. We made patch-clamp recordings in brainstem slices from RTN neurons that were identified by expression of GFP (directed by the Phox2b promoter) or ß-galactosidase (from the gene trap used for TASK-2 knock-out). Whereas nearly all RTN cells from control mice were pH sensitive (95%, n = 58 of 61), only 56% of GFP-expressing RTN neurons from TASK-2(-/-) mice (n = 49 of 88) could be classified as pH sensitive (>30% reduction in firing rate from pH 7.0 to pH 7.8); the remaining cells were pH insensitive (44%). Moreover, none of the recorded RTN neurons from TASK-2(-/-) mice selected based on ß-galactosidase activity (a subpopulation of GFP-expressing neurons) were pH sensitive. The alkaline-activated background K(+) currents were reduced in amplitude in RTN neurons from TASK-2(-/-) mice that retained some pH sensitivity but were absent from pH-insensitive cells. Finally, using a working heart-brainstem preparation, we found diminished inhibition of phrenic burst amplitude by alkalization in TASK-2(-/-) mice, with apneic threshold shifted to higher pH levels. In conclusion, alkaline-activated TASK-2 channels contribute to pH sensitivity in RTN neurons, with effects on respiration in situ that are particularly prominent near apneic threshold.

Persistent Nav1.1 and Nav1.6 currents drive spinal locomotor functions through nonlinear dynamics.

Persistent sodium current (INaP) in the spinal locomotor network promotes two distinct nonlinear firing patterns: a self-sustained spiking triggered by a brief excitation in bistable motoneurons and bursting oscillations in interneurons of the central pattern generator (CPG). Here, we identify the NaV channels responsible for INaP and their role in motor behaviors. We report the axonal Nav1.6 as the main molecular player for INaP in lumbar motoneurons. The inhibition of Nav1.6, but not of Nav1.1, in motoneurons impairs INaP, bistability, postural tone, and locomotor performance. In interneurons of the rhythmogenic CPG region, both Nav1.6 and Nav1.1 equally mediate INaP. Inhibition of both channels is required to abolish oscillatory bursting activities and the locomotor rhythm. Overall, Nav1.6 plays a significant role both in posture and locomotion by governing INaP-dependent bistability in motoneurons and working in tandem with Nav1.1 to provide INaP-dependent rhythmogenic properties of the CPG.

Teashirt 3 regulates development of neurons involved in both respiratory rhythm and airflow control.

Neonatal breathing in mammals involves multiple neuronal circuits, but its genetic basis remains unclear. Mice deficient for the zinc finger protein Teashirt 3 (TSHZ3) fail to breathe and die at birth. Tshz3 is expressed in multiple areas of the brainstem involved in respiration, including the pre-Bötzinger complex (preBötC), the embryonic parafacial respiratory group (e-pF), and cranial motoneurons that control the upper airways. Tshz3 inactivation led to pronounced cell death of motoneurons in the nucleus ambiguus and induced strong alterations of rhythmogenesis in the e-pF oscillator. In contrast, the preBötC oscillator appeared to be unaffected. These deficits result in impaired upper airway function, abnormal central respiratory rhythm generation, and altered responses to pH changes. Thus, a single gene, Tshz3, controls the development of diverse components of the circuitry required for breathing.

Graded reductions in oxygenation evoke graded reconfiguration of the isolated respiratory network.

Neurons depend on aerobic metabolism, yet are very sensitive to oxidative stress and, as a consequence, typically operate in a low O(2) environment. The balance between blood flow and metabolic activity, both of which can vary spatially and dynamically, suggests that local O(2) availability markedly influences network output. Yet the understanding of the underlying O(2)-sensing mechanisms is limited. Are network responses regulated by discrete O(2)-sensing mechanisms or, rather, are they the consequence of inherent O(2) sensitivities of mechanisms that generate the network activity? We hypothesized that a broad range of O(2) tensions progressively modulates network activity of the pre-Bötzinger complex (preBötC), a neuronal network critical to the central control of breathing. Rhythmogenesis was measured from the preBötC in transverse neonatal mouse brain stem slices that were exposed to graded reductions in O(2) between 0 and 95% O(2), producing tissue oxygenation values ranging from 20 ± 18 (mean ± SE) to 440 ± 56 Torr at the slice surface, respectively. The response of the preBötC to graded changes in O(2) is progressive for some metrics and abrupt for others, suggesting that different aspects of the respiratory network have different sensitivities to O(2).

The role of spiking and bursting pacemakers in the neuronal control of breathing.

Breathing is controlled by a distributed network involving areas in the neocortex, cerebellum, pons, medulla, spinal cord, and various other subcortical regions. However, only one area seems to be essential and sufficient for generating the respiratory rhythm: the preBötzinger complex (preBötC). Lesioning this area abolishes breathing and following isolation in a brain slice the preBötC continues to generate different forms of respiratory activities. The use of slice preparations led to a thorough understanding of the cellular mechanisms that underlie the generation of inspiratory activity within this network. Two types of inward currents, the persistent sodium current (I(NaP)) and the calcium-activated non-specific cation current (I(CAN)), play important roles in respiratory rhythm generation. These currents give rise to autonomous pacemaker activity within respiratory neurons, leading to the generation of intrinsic spiking and bursting activity. These membrane properties amplify as well as activate synaptic mechanisms that are critical for the initiation and maintenance of inspiratory activity. In this review, we describe the dynamic interplay between synaptic and intrinsic membrane properties in the generation of the respiratory rhythm and we relate these mechanisms to rhythm generating networks involved in other behaviors.

Necdin plays a role in the serotonergic modulation of the mouse respiratory network: implication for Prader-Willi syndrome.

Prader-Willi syndrome is a neurogenetic disease resulting from the absence of paternal expression of several imprinted genes, including NECDIN. Prader-Willi children and adults have severe breathing defects with irregular rhythm, frequent sleep apneas, and blunted respiratory regulations. For the first time, we show that Prader-Willi infants have sleep apneas already present at birth. In parallel, in wild-type and Necdin-deficient mice, we studied the respiratory system with in vivo plethysmography, in vitro electrophysiology, and pharmacology. Because serotonin is known to contribute to CNS development and to affect maturation and function of the brainstem respiratory network, we also investigated the serotonergic system with HPLC, immunohistochemistry, Rabies virus tracing approaches, and primary culture experiments. We report first that Necdin-deficiency in mice induces central respiratory deficits reminiscent of Prader-Willi syndrome (irregular rhythm, frequent apneas, and blunted respiratory regulations), second that Necdin is expressed by medullary serotonergic neurons, and third that Necdin deficiency alters the serotonergic metabolism, the morphology of serotonin vesicles in medullary serotonergic neurons but not the number of these cells. We also show that Necdin deficiency in neonatal mice alters the serotonergic modulation of the respiratory rhythm generator. Thus, we propose that the lack of Necdin expression induces perinatal serotonergic alterations that affect the maturation and function of the respiratory network, inducing breathing deficits in mice and probably in Prader-Willi patients.

Prenatal activation of 5-HT2A receptor induces expression of 5-HT1B receptor in phrenic motoneurons and alters the organization of their premotor network in newborn mice.

In newborn mice of the control [C3H/HeJ (C3H)] and monoamine oxidase A-deficient (Tg8) strains, in which levels of endogenous serotonin (5-HT) were drastically increased, we investigated how 5-HT system dysregulation affected the maturation of phrenic motoneurons (PhMns), which innervate the diaphragm. First, using immunocytochemistry and confocal microscopy, we observed a 5-HT(2A) receptor (5-HT(2A)-R) expression in PhMns of both C3H and Tg8 neonates at the somatic and dendritic levels, whereas 5-HT(1B) receptor (5-HT(1B)-R) expression was observed only in Tg8 PhMns at the somatic level. We investigated the interactions between 5-HT(2A)-R and 5-HT(1B)-R during maturation by treating pregnant C3H mice with a 5-HT(2A)-R agonist (2,5-dimethoxy-4-iodoamphetamine hydrochloride). This pharmacological overactivation of 5-HT(2A)-R induced a somatic expression of 5-HT(1B)-R in PhMns of their progeny. Conversely, treatment of pregnant Tg8 mice with a 5-HT(2A)-R antagonist (ketanserin) decreased the 5-HT(1B)-R density in PhMns of their progeny. Second, using retrograde transneuronal tracing with rabies virus injected into the diaphragm of Tg8 and C3H neonates, we studied the organization of the premotor network driving PhMns. The interneuronal network monosynaptically connected to PhMns was much more extensive in Tg8 than in C3H neonates. However, treatment of pregnant C3H mice with 2,5-dimethoxy-4-iodoamphetamine hydrochloride switched the premotoneuronal network of their progeny from a C3H- to a Tg8-like pattern. These results show that a prenatal 5-HT excess affects, via the overactivation of 5-HT(2A)-R, the expression of 5-HT(1B)-R in PhMns and the organization of their premotor network.

8-OH-DPAT suppresses spontaneous central apneas in the C57BL/6J mouse strain.

Apneas are common and prognostically relevant disorders of the central control of breathing, but pharmacological interventions are dissatisfying. The respiratory phenotype of C57BL/6J mice is characterized by the occurrence of spontaneous central apneas with laryngeal closure. In the present study we investigated the impact of the 5-HT(1A) receptor agonist 8-OH-DPAT on apneas in C57BL/6J mice, because of the important role of serotonin in the regulation of breathing and previous reports showing that serotonergic drugs can affect central apneas. Whole-body plethysmography in awake, unrestrained mice revealed that intraperitoneal application of 8-OH-DPAT (10microgkg(-1)) decreased the occurrence of spontaneous apneas from 1.91+/-0.25 to 1.05+/-0.05 apneas min(-1). The efficacy of 5-HT(1A) receptor activation was further verified in the in situ working heart-brainstem preparation. Here the apneas occurred at a frequency of 1.33+/-0.19min(-1). Intra-arterial perfusion with 1-2microM 8-OH-DPAT completely abolished spontaneous apneas. These results suggest that 5-HT(1A) receptor activation may be a potential treatment option for central apneas.

Spontaneous central apneas occur in the C57BL/6J mouse strain.

Despite the clinical significance of central apneas in a wide range of disorders little is known about their pathogenesis. Research in this field has been hindered by the lack of appropriate animal models. Our goal was to determine whether the C57BL/6J mouse strain, which has an inherited predisposition for dysrhythmic breathing, exhibits spontaneous apneas. In vivo plethysmography of unanesthetized, unrestrained adult C57BL/6J mice revealed a regular occurrence of spontaneous apneas. In situ recordings from respiratory outputs (phrenic, vagal, hypoglossal nerves) in the working heart-brainstem preparation (WHBP) also showed spontaneous central apneas accompanied by laryngeal closure as indicated by tonic vagal postinspiratory activity and increase in subglottal pressure. The apneas were further characterized by a hypoglossal discharge with delayed onset compared to the tonic vagal postinspiratory activity. We conclude that spontaneous central apneas with active laryngeal closure occur in C57BL/6J mice. This mouse strain is a useful animal model to study neuronal mechanisms that underlie the generation of spontaneous central apneas.

Oral treatment with desipramine improves breathing and life span in Rett syndrome mouse model.

Rett syndrome is a neurodevelopmental disease due to Mecp2 gene mutations that is associated to complex neurological symptoms, with bioaminergic deficits and life-threatening apneas related to sudden and unexpected death. In male mice, Mecp2-deficiency similarly induces medullary bioaminergic deficits, severe apneas and short life span. Here, we show that long-term oral treatment of Mecp2-deficient male mice with desipramine, an old drug of clinical use known to block norepinephrine uptake and to strengthen its synaptic effects, significantly alleviates their breathing symptoms and prolongs their life span. Although these mouse results identify desipramine as the first oral pharmacological treatment potentially able to alleviate breathing symptoms of Rett syndrome, we recommend further studies of desipramine effects in Mecp2-deficient mice before attempting any clinical trials in Rett patients.

Necdin gene, respiratory disturbances and Prader-Willi syndrome.

Prader-Willi Syndrome (PWS) is a complex neurogenetic disease with various symptoms, including breathing deficits and possible alteration of serotonin (5HT) metabolism. As PWS results from the absence of paternal expression of several imprinted genes among which NECDIN (Ndn), we examined whether Ndn deficiency in mice induced breathing and 5HT deficits. In vivo, Ndn-deficient mice (Ndn-/-) had irregular breathing, severe apneas and blunted respiratory response to hypoxia. In vitro, medullary preparations from Ndn-/- neonates produced a respiratory-like rhythm that was highly irregular, frequently interrupted and abnormally regulated by central hypoxia. In wild type (wt) and Ndn-/- neonates, immunohistofluorescence and biochemistry revealed that medullary 5HT neurons expressed Ndn in wt and that the medulla contained abnormally high levels of 5HT in Ndn-/-. Thus, our preliminary results fully confirm a primary role of Ndn in PWS, revealing that Ndn-deficiency in mice induces respiratory and 5HT alterations reminiscent of PWS.

Muscarinic receptors and alpha2-adrenoceptors interact to modulate the respiratory rhythm in mouse neonates.

The respiratory rhythm generator (RRG) is modulated by several endogenous substances, including acetylcholine (ACh) and noradrenaline (NA) that interact in several modulatory processes. To know whether ACh and NA interacted to modulate the RRG activity, we used medullary "en bloc" and slice preparations from neonatal mice where the RRG has been shown to receive a facilitatory modulation from A1/C1 neurons, via a continuous release of endogenous NA and activation of alpha2 adrenoceptors. Applying ACh at 25 microM activated the RRG but ACh had no effects at 50 microM. Applying the ACh receptor agonists nicotine and muscarine facilitated and depressed the RRG, respectively. After yohimbine pre-treatment that blocked the alpha2 facilitation, the nicotinic facilitation was not altered, the muscarinic depression was reversed and ACh 50 microM significantly facilitated the RRG. After L-tyrosine pre-treatment that potentiated the alpha2 facilitation, the muscarinic depression was enhanced. Thus, ACh regulates the RRG activity via nicotinic and muscarinic receptors, the muscarinic receptors interacting with alpha2 adrenoceptors.

Mecp2 deficiency disrupts norepinephrine and respiratory systems in mice.

Rett syndrome is a severe X-linked neurological disorder in which most patients have mutations in the methyl-CpG binding protein 2 (MECP2) gene and suffer from bioaminergic deficiencies and life-threatening breathing disturbances. We used in vivo plethysmography, in vitro electrophysiology, neuropharmacology, immunohistochemistry, and biochemistry to characterize the consequences of the MECP2 mutation on breathing in wild-type (wt) and Mecp2-deficient (Mecp2-/y) mice. At birth, Mecp2-/y mice showed normal breathing and a normal number of medullary neurons that express tyrosine hydroxylase (TH neurons). At approximately 1 month of age, most Mecp2-/y mice showed respiratory cycles of variable duration; meanwhile, their medulla contained a significantly reduced number of TH neurons and norepinephrine (NE) content, even in Mecp2-/y mice that showed a normal breathing pattern. Between 1 and 2 months of age, all unanesthetized Mecp2-/y mice showed breathing disturbances that worsened until fatal respiratory arrest at approximately 2 months of age. During their last week of life, Mecp2-/y mice had a slow and erratic breathing pattern with a highly variable cycle period and frequent apneas. In addition, their medulla had a drastically reduced number of TH neurons, NE content, and serotonin (5-HT) content. In vitro experiments using transverse brainstem slices of mice between 2 and 3 weeks of age revealed that the rhythm produced by the isolated respiratory network was irregular in Mecp2-/y mice but could be stabilized with exogenous NE. We hypothesize that breathing disturbances in Mecp2-/y mice, and probably Rett patients, originate in part from a deficiency in noradrenergic and serotonergic modulation of the medullary respiratory network.

Possible modulation of the mouse respiratory rhythm generator by A1/C1 neurones.

Although compelling evidence exist that the respiratory rhythm generator is modulated by endogenous noradrenaline released from pontine A5 and A6 neurones, we examined whether medullary catecholaminergic neurones also participated in respiratory rhythm modulation. Experiments were performed in neonatal (postnatal days 0-6, P0-P6) and young mice (P14-P18) using "en bloc" medullary preparations (pons resected) and transverse medullary slices. In "en bloc" preparations, blockade of medullary alpha2 adrenoceptors with yohimbine and activation of catecholamine biosynthesis with L-tyrosine significantly depresses and facilitates the respiratory rhythm, respectively. In slices from neonatal and young mice, blockade of medullary alpha2 adrenoceptors also depressed the respiratory rhythm. Yohimbine local applications and lesion-ablation experiments of the dorsal medulla revealed implication of A1/C1 neurones in the yohimbine depressing effect. Although the mechanisms responsible for the yohimbine-depressing effect remain to be elucidated, our in vitro results in neonatal and young mice suggest that endogenous catecholamines released from A1/C1 neurones participate in respiratory rhythm modulation via medullary alpha2 adrenoceptors.

Chronic Intermittent Hypoxia Alters Local Respiratory Circuit Function at the Level of the preBötzinger Complex.

Chronic intermittent hypoxia (CIH) is a common state experienced in several breathing disorders, including obstructive sleep apnea (OSA) and apneas of prematurity. Unraveling how CIH affects the CNS, and in turn how the CNS contributes to apneas is perhaps the most challenging task. The preBötzinger complex (preBötC) is a pre-motor respiratory network critical for inspiratory rhythm generation. Here, we test the hypothesis that CIH increases irregular output from the isolated preBötC, which can be mitigated by antioxidant treatment. Electrophysiological recordings from brainstem slices revealed that CIH enhanced burst-to-burst irregularity in period and/or amplitude. Irregularities represented a change in individual fidelity among preBötC neurons, and changed transmission from preBötC to the hypoglossal motor nucleus (XIIn), which resulted in increased transmission failure to XIIn. CIH increased the degree of lipid peroxidation in the preBötC and treatment with the antioxidant, 5,10,15,20-Tetrakis (1-methylpyridinium-4-yl)-21H,23H-porphyrin manganese(III) pentachloride (MnTMPyP), reduced CIH-mediated irregularities on the network rhythm and improved transmission of preBötC to the XIIn. These findings suggest that CIH promotes a pro-oxidant state that destabilizes rhythmogenesis originating from the preBötC and changes the local rhythm generating circuit which in turn, can lead to intermittent transmission failure to the XIIn. We propose that these CIH-mediated effects represent a part of the central mechanism that may perpetuate apneas and respiratory instability, which are hallmark traits in several dysautonomic conditions.

Perinatal maturation of the respiratory rhythm generator in mammals: from experimental results to computational simulation.

The survival of neonatal mammals requires a correct function of the respiratory rhythm generator (RRG), and therefore, the processes that control its prenatal maturation are of vital importance. In humans, lambs and rodents, foetal breathing movements (FBMs) occur early during gestation, are episodic, sensitive to bioamines, central hypoxia and inputs from CNS upper structures, and evolve with developmental age. In vitro, the foetal rodent RRG studied in preparations where the upper CNS structures are lacking continuously produces a rhythmic command, which is sensitive to hypoxia and bioaminergic inputs. The rhythm is slow with variable periods 4 days before birth. It becomes faster 2 days before birth, similar to the postnatal rhythm. Compelling evidence suggests that a region of the RRG called the preBötzinger complex (PBC) contains respiratory pacemaker neurones which play a primary role in perinatal rhythmogenesis. Although the RRG functions during early gestation, no pacemakers are found in the putative PBC area and its electrical stimulation and lesion do not affect the early foetal rhythm. To know whether the early foetal and perinatal rhythms originate from either pacemaker neurones or network connection properties, and to know which maturational processes might explain the appearance of PBC pacemakers and the rhythm increase during perinatal development, we computationally modelled maturing RRG. Our model shows that both network noise and persistent sodium conductance are crucial for rhythmogenesis and that a slight increase in the persistent sodium conductance can solve the pacemaker versus network dilemma in a noisy network.

KCNK5 channels mostly expressed in cochlear outer sulcus cells are indispensable for hearing.

In the cochlea, K(+) is essential for mechano-electrical transduction. Here, we explore cochlear structure and function in mice lacking K(+) channels of the two-pore domain family. A profound deafness associated with a decrease in endocochlear potential is found in adult Kcnk5(-/-) mice. Hearing occurs around postnatal day 19 (P19), and completely disappears 2 days later. At P19, Kcnk5(-/-) mice have a normal endolymphatic [K(+)] but a partly lowered endocochlear potential. Using Lac-Z as a gene reporter, KCNK5 is mainly found in outer sulcus Claudius', Boettcher's and root cells. Low levels of expression are also seen in the spiral ganglion, Reissner's membrane and stria vascularis. Essential channels (KCNJ10 and KCNQ1) contributing to K(+) secretion in stria vascularis have normal expression in Kcnk5(-/-) mice. Thus, KCNK5 channels are indispensable for the maintenance of hearing. Among several plausible mechanisms, we emphasize their role in K(+) recycling along the outer sulcus lateral route.