Brainstem amino acid neurotransmitters and ventilatory response to hypoxia in piglets

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.

Developmental plasticity of GABAergic neurotransmission to brainstem motoneurons

KEY POINTS

Critical homeostatic behaviours such as suckling, swallowing and breathing depend on the precise control of tongue muscle activity. Perinatal nicotine exposure has multiple effects on baseline inhibitory GABAergic neurotransmission to hypoglossal motoneurons (XIIMNs), consistent with homeostatic compensations directed at maintaining normal motoneuron output. Developmental nicotine exposure (DNE) alters how GABAergic neurotransmission is modulated by acute activation of nicotinic acetylcholine receptors, which may provide insight into mechanisms by which nicotine exposure alters motor function under conditions that result in increased release of GABA, such as hypoxia, or endogenous acetylcholine, as occurs in the transition from NREM to REM sleep, or in response to exogenous nicotine.

ABSTRACT

Nicotinic acetylcholine receptor (nAChR) signalling regulates neuronal differentiation and synaptogenesis. Here we test the hypothesis that developmental nicotine exposure (DNE) disrupts the development of GABAergic synaptic transmission to hypoglossal motoneurons (XIIMNs). GABAergic spontaneous and miniature inhibitory postsynaptic currents (sIPSCs/mIPSCs) were recorded from XIIMNs in brainstem slices from control and DNE rat pups of either sex, 1-5 days old, at baseline and following acute stimulation of nAChRs with nicotine. At baseline, sIPSCs were less frequent and smaller in DNE cells (consistent with decreased action potential-mediated GABA release), and mIPSCs were more frequent (consistent with increased vesicular GABA release from presynaptic terminals). Acute nicotine challenge increased sIPSC frequency in both groups, though the increase was greater in DNE cells. Acute nicotine challenge did not change the frequency of mIPSCs in either group, though mIPSC amplitude increased significantly in DNE cells, but not control cells. Stimulation of postsynaptic GABA_A receptors with muscimol caused a significantly greater chloride current in DNE cells than in control cells. The increased quantal release of GABA, coupled with the rise in the strength of postsynaptic inhibition may be homeostatic adjustments to the decreased action-potential-mediated input from GABAergic interneurons. However, this will exaggerate synaptic inhibition under conditions where the release of GABA (e.g. hypoxia) or ACh (sleep-wake transitions) is increased. These findings reveal a mechanism that may explain why DNE is associated with deficits in the ability to respond appropriately to chemosensory stimuli or to changes in neuromodulation secondary to changes in central nervous system state.

Thalamic mediation of hypoxic respiratory depression in lambs

Immaturity of respiratory controllers in preterm infants dispose to recurrent apnea and oxygen deprivation. Accompanying reductions in brain oxygen tensions evoke respiratory depression, potentially exacerbating hypoxemia. Central respiratory depression during moderate hypoxia is revealed in the ventilatory decline following initial augmentation. This study determined whether the thalamic parafascicular nuclear (Pf) complex involved in adult nociception and sensorimotor regulation (Bentivoglio M, Balerecia G, Kruger L. Prog Brain Res 87: 53-80, 1991) also becomes a postnatal controller of hypoxic ventilatory decline. Respiratory responses to moderate isocapnic hypoxia were studied in conscious lambs. Hypoxic ventilatory decline was compared with peak augmentation. Pf and/or adjacent thalamic structures were destroyed by the neuron-specific toxin ibotenic acid (IB). IB lesions involving the thalamic Pf abolished hypoxic ventilatory decline. Lesions of adjacent thalamic nuclei that spared Pf and control injections of vehicle failed to blunt hypoxic respiratory depression. Our findings reveal that the thalamic Pf region is a critical controller of hypoxic ventilatory depression and thus a key target for exploring molecular concomitants of forebrain pathways regulating hypoxic ventilatory depression in early development.

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.

The physiological determinants of sudden infant death syndrome

It is well-established that environmental and biological risk factors contribute to Sudden Infant Death Syndrome (SIDS). There is also growing consensus that SIDS requires the intersection of multiple risk factors that result in the failure of an infant to overcome cardio-respiratory challenges. Thus, the critical next steps in understanding SIDS are to unravel the physiological determinants that actually cause the sudden death, to synthesize how these determinants are affected by the known risk factors, and to develop novel ideas for SIDS prevention. In this review, we will examine current and emerging perspectives related to cardio-respiratory dysfunctions in SIDS. Specifically, we will review: (1) the role of the preBötzinger complex (preBötC) as a multi-functional network that is critically involved in the failure to adequately respond to hypoxic and hypercapnic challenges; (2) the potential involvement of the preBötC in the gender and age distributions that are characteristic for SIDS; (3) the link between SIDS and prematurity; and (4) the potential relationship between SIDS, auditory function, and central chemosensitivity. Each section underscores the importance of marrying the epidemiological and pathological data to experimental data in order to understand the physiological determinants of this syndrome. We hope that a better understanding will lead to novel ways to reduce the risk to succumb to SIDS.

Chronic nicotine and ethanol exposure both disrupt central ventilatory responses to hypoxia in bullfrog tadpoles

The central hypoxic ventilatory response (HVR) comprises a reduction in ventilatory activity that follows a peripherally mediated ventilatory augmentation. Chronic early developmental exposure to nicotine or ethanol are both known to impair the peripherally mediated HVR, and nicotine impairs the central HVR, but the effect of ethanol on the central HVR has not been investigated. Additionally, chronic nicotine and ethanol exposure are known to impair ventilatory responses to hypercapnia in bullfrog tadpoles but HVRs have not been tested. Here early and late metamorphic tadpoles were exposed to either 30 μg/L nicotine or 0.15-0.05 g/dL ethanol for 10 wk. Tadpole brainstems were then isolated and the neurocorrelates of ventilation were monitored in vitro over 180 min of hypoxia (PO2=5.05±1.04 kPa). Both nicotine and ethanol exposure disrupted central HVRs. Nicotine impairments were dependent on development. Central HVRs were impaired only in early metamorphic nicotine-exposed tadpoles. Both early and late metamorphic ethanol-exposed tadpoles failed to exhibit central HVRs. Thus, central HVRs are impaired following both nicotine and ethanol exposure. Such failure to decrease ventilatory activity during hypoxia indicates that central hypoxic ventilatory depression is an active suppression of neural activity in response to hypoxia rather than a metabolic consequence of O2 limitation, and that exposure to ethanol (across development) or nicotine (during early development) disrupts mechanisms that normally induce active ventilatory depression.

Tonic neuromodulation of the inspiratory rhythm generator

The generation of neural network dynamics relies on the interactions between the intrinsic and synaptic properties of their neural components. Moreover, neuromodulators allow networks to change these properties and adjust their activity to specific challenges. Endogenous continuous (“tonic”) neuromodulation can regulate and sometimes be indispensible for networks to produce basal activity. This seems to be the case for the inspiratory rhythm generator located in the pre-Bötzinger complex (preBötC). This neural network is necessary and sufficient for generating inspiratory rhythms. The preBötC produces normal respiratory activity (eupnea) as well as sighs under normoxic conditions, and it generates gasping under hypoxic conditions after a reconfiguration process. The reconfiguration leading to gasping generation involves changes of synaptic and intrinsic properties that can be mediated by several neuromodulators. Over the past years, it has been shown that endogenous continuous neuromodulation of the preBötC may involve the continuous action of amines and peptides on extrasynaptic receptors. I will summarize the findings supporting the role of endogenous continuous neuromodulation in the generation and regulation of different inspiratory rhythms, exploring the possibility that these neuromodulatory actions involve extrasynaptic receptors along with evidence of glial modulation of preBötC activity.

The carotid body and arousal in the fetus and neonate

Arousal from sleep is a major defense mechanism in infants against hypoxia and/or hypercapnia. Arousal failure may be an important contributor to SIDS. Areas of the brainstem that have been found to be abnormal in a majority of SIDS infants are involved in the arousal process. Arousal is sleep state dependent, being depressed during AS in most mammals, but depressed during QS in human infants. Repeated exposure to hypoxia causes a progressive blunting of arousal that may involve medullary raphe GABAergic mechanisms. Whereas CB chemoreceptors contribute heavily to arousal in response to hypoxia, serotonergic central chemoreceptors have been implicated in the arousal response to CO(2). Pulmonary or chest wall mechanoreceptors also contribute to arousal in proportion to the ventilatory response and decreases in their input may contribute to depressed arousal during AS. Little is known about specific arousal pathways beyond the NTS. Whether CB chemoreceptor stimulation directly stimulates arousal centers or whether this is done indirectly through respiratory networks remains unknown. This review will focus on arousal in response to hypoxia and CO(2) in the fetus and newborn and will outline what we know (and do not know) about the involvement of the carotid body in this process.

Arousal from sleep in response to intermittent hypoxia in rat pups is modulated by medullary raphe GABAergic mechanisms

Arousal is an important defense against hypoxia during sleep. Rat pups exhibit progressive arousal impairment (habituation) with multiple hypoxia exposures. The mechanisms are unknown. The medullary raphe (MR) is involved in autonomic functions, including sleep, and receives abundant GABAergic inputs. We hypothesized that inhibiting MR neurons with muscimol, a GABA(A) receptor agonist, or preventing GABA reuptake with nipecotic acid, would impair arousal and enhance arousal habituation and that blocking GABA(A) receptors with bicuculline would enhance arousal and attenuate habituation. Postnatal day 15 (P15) to P25 rat pups were briefly anesthetized, and microinjections with aCSF, muscimol, bicuculline, or nipecotic acid were made into the MR. After a ∼30-min recovery, pups were exposed to four 3-min episodes of hypoxia separated by 6 min of normoxia. The time to arousal from the onset of hypoxia (latency) was determined for each trial. Latency progressively increased across trials (habituation) in all groups. The overall latency was greater after muscimol and nipecotic acid compared with aCSF, bicuculline, or noninjected controls. Arousal habituation was reduced after bicuculline compared with aCSF, muscimol, nipecotic acid, or noninjected pups. Increases in latency were mirrored by decreases in chamber [O2] and oxyhemoglobin saturation. Heart rate increased during hypoxia and was greatest in muscimol-injected pups. Our results indicate that the MR plays an important, not previously described, role in arousal and arousal habituation during hypoxia and that these phenomena are modulated by GABAergic mechanisms. Arousal habituation may contribute to sudden infant death syndrome, which is associated with MR serotonergic and GABAergic receptor dysfunction.

Developmental nicotine exposure alters neurotransmission and excitability in hypoglossal motoneurons

Hypoglossal motoneurons (XII MNs) control muscles of the mammalian tongue and are rhythmically active during breathing. Acetylcholine (ACh) modulates XII MN activity by promoting the release of glutamate from neurons that express nicotinic ACh receptors (nAChRs). Chronic nicotine exposure alters nAChRs on neurons throughout the brain, including brain stem respiratory neurons. Here we test the hypothesis that developmental nicotine exposure (DNE) reduces excitatory synaptic input to XII MNs. Voltage-clamp experiments in rhythmically active medullary slices showed that the frequency of excitatory postsynaptic currents (EPSCs) onto XII MNs from DNE animals is reduced by 61% (DNE = 1.7 ± 0.4 events/s; control = 4.4 ± 0.6 events/s; P < 0.002). We also examine the intrinsic excitability of XII MNs to test whether cells from DNE animals have altered membrane properties. Current-clamp experiments showed XII MNs from DNE animals had higher intrinsic excitability, as evaluated by measuring their response to injected current. DNE cells had high-input resistances (DNE = 131.9 ± 13.7 MΩ, control = 78.6 ± 9.7 MΩ, P < 0.008), began firing at lower current levels (DNE = 144 ± 22 pA, control = 351 ± 45 pA, P < 0.003), and exhibited higher frequency-current gain values (DNE = 0.087 ± 0.012 Hz/pA, control = 0.050 ± 0.004 Hz/pA, P < 0.02). Taken together, our data show previously unreported effects of DNE on XII MN function and may also help to explain the association between DNE and the incidence of central and obstructive apneas.

Perinatal development of inhibitory synapses in the nucleus tractus solitarii of the rat

The nucleus tractus solitarii (NTS) plays a key role in the central control of the autonomic nervous system. In adult rats, both GABA and glycine are used as inhibitory neurotransmitter in the NTS. Using a quantitative morphological approach, we have investigated the perinatal development of inhibitory synapses in the NTS. The density of both inhibitory axon terminals and synapses increased from embryonic day 20 until the end of the second postnatal week (postnatal day 14). Before birth, only GABAergic axon terminals developed and their number increased during the first postnatal week. Mixed GABA/glycine axon terminals appeared at birth and their number increased during the first postnatal week. This suggests the development of a mixed GABA/glycine inhibition in parallel to pure GABA inhibition. However, whereas GABAergic axon terminals were distributed throughout the NTS, mixed GABA/glycine axon terminals were strictly located in the lateral part of the NTS. Established at birth, this specific topography remained in the adult rat. From birth, GABA(A) receptors, glycine receptors and gephyrin were clustered in inhibitory synapses throughout the NTS, revealing a neurotransmitter-receptor mismatch within the medial part of the NTS. Together these results suggest that NTS inhibitory networks develop and mature until postnatal day 14. Developmental changes in NTS synaptic inhibition may play an important role in shaping neural network activity during a time of maturation of autonomic functions. The first two postnatal weeks could represent a critical period where the impact of the environment influences the physiological phenotypes of adult rats.

Influence of prenatal nicotine exposure on development of the ventilatory response to hypoxia and hypercapnia in neonatal rats

In a recent study (Huang YH et al. Respir Physiol Neurobiol 143: 1-8, 2004), we showed that prenatal nicotine exposure (PNE) increased the frequency of spontaneous apneic events on the first 2 days of life in unanesthetized neonatal rats. Here we test the hypothesis that PNE blunts chemoreceptor reflexes. Ventilatory responses to three levels each of hypoxia (inspired O(2) fraction: 16, 12, and 10%) and hypercapnia (3, 6, and 9% inspired CO(2) fraction, all in 50% O(2), balance N(2)), and one level each of combined hypoxia-hypercapnia (H/H; 12% inspired O(2) fraction/5% inspired CO(2) fraction) and hyperoxia (50% O(2), 50% N(2)) were recorded with head-out plethysmography in neonatal rats exposed to either nicotine (N = 12) or physiological saline (N = 12) in the prenatal period. Recordings were made on postnatal day 1 (P1), P3, P6, P9, P12, and P18, in each animal. The change in ventilation in response to hypoxia was blunted in PNE animals on P1 and P3, but there were no other treatment effects. Hyperoxia significantly depressed ventilation in both groups from P3-P18, but there were no significant treatment effects. The ventilatory response to 3, 6, and 9% inspired CO(2) was significantly blunted in PNE animals at all ages studied, due exclusively to a blunted tidal volume response. PNE also blunted the ventilatory response to H/H at all ages, due primarily to blunting of the tidal volume response. PNE had no significant effect on body mass or metabolic rate, except that PNE animals had a slightly higher mass on P18 and a lower metabolic rate on P1. As shown by others, PNE has small and inconsistent effects on hypoxic ventilatory responses, but here we show that responses to hypercapnia and H/H are consistently blunted by PNE due to a diminished tidal volume response. The combination of reduced hypoxic and hypercapnic sensitivity over the first 3 days of life may define an especially vulnerable developmental period.

Mixed GABA-glycine synapses delineate a specific topography in the nucleus tractus solitarii of adult rat

Using combined morphological and electrophysiological approaches, we have determined the composition of inhibitory synapses of the nucleus tractus solitarii (NTS), a brainstem structure that is a gateway for many visceral sensory afferent fibres. Immunohistochemical experiments demonstrate that, in adult rat, GABA axon terminals are present throughout the NTS while mixed GABA-glycine axon terminals are strictly located to the lateral part of the NTS within subnuclei surrounding the tractus solitarius. Purely glycine axon terminals are rare in the lateral part of the NTS and hardly detected in its medial part. Electrophysiological experiments confirm the predominance of GABA inhibition throughout the NTS and demonstrate the existence of a dual inhibition involving the co-release of GABA and glycine restricted to the lateral part of NTS. Since GABA(A) and glycine receptors are co-expressed postsynaptically in virtually all the inhibitory axon terminals throughout the NTS, it suggests that the inhibition phenotype relies on the characteristics of the axon terminals. Our results also demonstrate that glycine is mostly associated with GABA within axon terminals and raise the possibility of a dynamic regulation of GABA/glycine release at the presynaptic level. Our data provide new information for understanding the mechanisms involved in the processing of visceral information by the central nervous system in adult animals.

Metabolic control exerted by the 2-oxoglutarate dehydrogenase reaction: a cross-kingdom comparison of the crossroad between energy production and nitrogen assimilation

Mechanism-based inhibitors and both forward and reverse genetics have proved to be essential tools in revealing roles for specific enzymatic processes in cellular function. Here, we review experimental studies aimed at assessing the impact of OG (2-oxoglutarate) oxidative decarboxylation on basic cellular activities in a number of biological systems. After summarizing the catalytic and regulatory properties of the OGDHC (OG dehydrogenase complex), we describe the evidence that has been accrued on its cellular role. We demonstrate an essential role of this enzyme in metabolic control in a wide range of organisms. Targeting this enzyme in different cells and tissues, mainly by its specific inhibitors, effects changes in a number of basic functions, such as mitochondrial potential, tissue respiration, ROS (reactive oxygen species) production, nitrogen metabolism, glutamate signalling and survival, supporting the notion that the evolutionary conserved reaction of OG degradation is required for metabolic adaptation. In particular, regulation of OGDHC under stress conditions may be essential to overcome glutamate excitotoxicity in neurons or affect the wound response in plants. Thus, apart from its role in producing energy, the flux through OGDHC significantly affects nitrogen assimilation and amino acid metabolism, whereas the side reactions of OGDHC, such as ROS production and the carboligase reaction, have biological functions in signalling and glyoxylate utilization. Our current view on the role of OGDHC reaction in various processes within complex biological systems allows us a far greater fundamental understanding of metabolic regulation and also opens up new opportunities for us to address both biotechnological and medical challenges.

The brainstem and serotonin in the sudden infant death syndrome

The sudden infant death syndrome (SIDS) is the sudden death of an infant under one year of age that is typically associated with sleep and that remains unexplained after a complete autopsy and death scene investigation. A leading hypothesis about its pathogenesis is that many cases result from defects in brainstem-mediated protective responses to homeostatic stressors occurring during sleep in a critical developmental period. Here we review the evidence for the brainstem hypothesis in SIDS with a focus upon abnormalities related to the neurotransmitter serotonin in the medulla oblongata, as these are the most robust pathologic findings to date. In this context, we synthesize the human autopsy data with genetic, whole-animal, and cellular data concerning the function and development of the medullary serotonergic system. These emerging data suggest an important underlying mechanism in SIDS that may help lead to identification of infants at risk and specific interventions to prevent death.