Prenatal nicotine alters maturation of breathing and neural circuits regulating respiratory control

Ventilatory response to added dead space in infants exposed to second-hand smoke in pregnancy

Maternal cigarette smoking in pregnancy can adversely affect infant respiratory control. In utero nicotine exposure has been shown to blunt the infant ventilatory response to hypercapnia, which could increase the risk of sudden infant death syndrome. The potential impact of maternal second-hand smoke exposure, however, has not yet been determined. The aim of this study was to assess ventilatory response to added dead-space (inducing hypercapnia) in infants with second-hand smoke exposure during pregnancy, in infants whose mothers smoked and in controls (non-smoke exposed). Infants breathed through a face mask and specialised "tube-breathing" circuit, incorporating a dead space of 4.4 ml/kg body weight. The maximum minute ventilation (MMV) during added dead space breathing was determined and the time taken to achieve 63% of the MMV calculated (the time constant (TC) of the response). Infants were studied on the postnatal ward prior to discharge home. Thirty infants (ten in each group) were studied with a median gestational age of 39 [range 37-41] weeks, birthweight of 3.1 [2.2-4.0] kg, and postnatal age of 33 (21-62) h. The infants whose mothers had second-hand smoke exposure (median TC 42 s, p = 0.001), and the infants of cigarette smoking mothers (median TC 37 s, p = 0.002) had longer time constants than the controls (median TC 29 s). There was no significant difference between the TC of the infants whose mothers had second-hand smoke exposure and those whose mothers smoked (p = 0.112).    Conclusion: Second-hand smoke exposure during pregnancy was associated with a delayed newborn ventilatory response. What is Known: • Maternal cigarette smoking in pregnancy can adversely affect infant respiratory control. • The potential impact of maternal second-hand smoke exposure, however, has not yet been determined. What is New: • We have assessed the ventilatory response to added dead-space (inducing hypercapnia) in newborns with second-hand smoke exposure during pregnancy, in infants whose mothers smoked, and in controls (non-smoke exposed). • Maternal second-hand smoke exposure, as well as maternal smoking, during pregnancy was associated with a delayed newborn ventilatory response.

Developmental Nicotine Exposure Alters Synaptic Input to Hypoglossal Motoneurons and Is Associated with Altered Function of Upper Airway Muscles

Nicotine exposure during the fetal and neonatal periods [developmental nicotine exposure (DNE)] is associated with ineffective upper airway protective reflexes in infants. This could be explained by desensitized chemoreceptors and/or mechanoreceptors, diminished neuromuscular transmission or altered synaptic transmission among central neurons, as each of these systems depend in part on cholinergic signaling through nicotinic AChRs (nAChRs). Here, we showed that DNE blunts the response of the genioglossus (GG) muscle to nasal airway occlusion in lightly anesthetized rat pups. The GG muscle helps keep the upper airway open and is innervated by hypoglossal motoneurons (XIIMNs). Experiments using the phrenic nerve-diaphragm preparation showed that DNE does not alter transmission across the neuromuscular junction. Accordingly, we used whole cell recordings from XIIMNs in brainstem slices to examine the influence of DNE on glutamatergic synaptic transmission under baseline conditions and in response to an acute nicotine challenge. DNE did not alter excitatory transmission under baseline conditions. Analysis of cumulative probability distributions revealed that acute nicotine challenge of P1–P2 preparations resulted in an increase in the frequency of nicotine-induced glutamatergic inputs to XIIMNs in both control and DNE. By contrast, P3–P5 DNE pups showed a decrease, rather than an increase in frequency. We suggest that this, together with previous studies showing that DNE is associated with a compensatory increase in inhibitory synaptic input to XIIMNs, leads to an excitatory-inhibitory imbalance. This imbalance may contribute to the blunting of airway protective reflexes observed in nicotine exposed animals and human infants.

α4-Containing nicotinic receptors contribute to the effects of perinatal nicotine on ventilatory and metabolic responses of neonatal mice to ambient cooling

Among numerous studies, perinatal nicotine exposure (PN) has had variable effects on respiratory control in the neonatal period. The effects of acute nicotine exposure on breathing are largely mediated by α4-containing nicotine acetylcholine receptors (nAChRs). These receptors are also involved in thermoregulatory responses induced by both acetylcholine and nicotine. We therefore hypothesized that α4-containing nAChRs would mediate the effects of PN on the metabolic and ventilatory responses of neonates to modest cold exposure. Wild-type (WT) and α4 knockout (KO) mice were exposed to 6 mg·kg^-1·day^-1 nicotine or vehicle from embryonic day 14 At postnatal day (P) 7 mice were cooled from an ambient temperature (T_A) of 32 to 20°C. Body temperature (T_B), rate of O₂ consumption (V̇o₂), ventilation (V̇e), respiratory frequency (F_B), and tidal volume (V_T) were continually monitored. An absence of α4 had no effect on the metabolic response to ambient cooling. Surprisingly, PN selectively increased the metabolic response of KO pups to cooling. Regardless, KO pups became hypothermic to the same degree as WT pups, and for both genotypes the drop in T_B was exacerbated by PN. PN led to hyperventilation in WT pups caused by an increase in V_T, an effect that was absent in α4 KO littermates. We show that PN interacts with α4-containing nAChRs in unique ways to modulate the control of breathing and thermoregulation in the early postnatal period.

Impact of Tobacco Smoke and Nicotine Exposure on Lung Development

Tobacco smoke and nicotine exposure during prenatal and postnatal life can impair lung development, alter the immune response to viral infections, and increase the prevalence of wheezing during childhood. The following review examines recent discoveries in the fields of lung development and tobacco and nicotine exposure, emphasizing studies published within the last 5 years. In utero tobacco and nicotine exposure remains common, occurring in approximately 10% of pregnancies within the United States. Exposed neonates are at increased risk for diminished lung function, altered central and peripheral respiratory chemoreception, and increased asthma symptoms throughout childhood. Recently, genomic and epigenetic risk factors, such as alterations in DNA methylation, have been identified that may influence the risk for long-term disease. This review examines the impact of prenatal tobacco and nicotine exposure on lung development with a particular focus on nicotinic acetylcholine receptors. In addition, this review examines the role of prenatal and postnatal tobacco smoke and nicotine exposure and its association with augmenting infection risk, skewing the immune response toward a T-helper type 2 bias and increasing risk for developing an allergic phenotype and asthmalike symptoms during childhood. Finally, this review outlines the respiratory morbidities associated with childhood secondhand smoke and nicotine exposure and examines genetic and epigenetic modifiers that may influence respiratory health in infants and children exposed to in utero or postnatal tobacco smoke.

Prenatal nicotine exposure increases hyperventilation in α4-knock-out mice during mild asphyxia

Prenatal nicotine exposure alters breathing and ventilatory responses to stress through stimulation of nicotine acetylcholine receptors (nAChRs). We tested the hypothesis that α4-containing nAChRs are involved in mediating the effects of prenatal nicotine exposure on ventilatory and metabolic responses to intermittent mild asphyxia (MA). Using open-flow plethysmography, we measured ventilation (V̇(E)) and rate of O2 consumption ( V̇(O2)) of wild-type (WT) and α4-knock-out (KO) mice, at postnatal (P) days 1-2 and 7-8, with and without prenatal nicotine exposure (6 mg kg(-1) day(-1) beginning on embryonic day 14). Mice were exposed to seven 2 min cycles of mild asphyxia (10% O2 and 5% CO2), each interspersed with 2 min of air. Compared to WT, α4 KO mice had increased air V̇(E) and V̇(O2) at P7-8, but not P1-2. Irrespective of age, genotype had no effect on the hyperventilatory response (increase in V̇(E)/V̇(O2)) to MA. At P1-2, nicotine suppressed air V̇(E) and V̇(O2) in both genotypes but did not affect the hyperventilatory response to MA. At P7-8 nicotine suppressed air V̇(E) and V̇(O2) of only α4 KO's but also significantly enhanced V̇(E) during MA (nearly double that of WT; p<0.001). This study has revealed complex effects of α4 nAChR deficiency and prenatal nicotine exposure on ventilatory and metabolic interactions and responses to stress.

Perinatal nicotine/smoking exposure and carotid chemoreceptors during development

Tobacco smoking is still a common habit during pregnancy and is the most important preventable cause of many adverse perinatal outcomes. Prenatal smoking exposure can produce direct actions of nicotine in the fetus with the disruption of body and brain development, and actions on the maternal-fetal unit by causing repeated episodes of hypoxia and exposure to many toxic smoke products (such as carbon monoxide). Specifically, nicotine through binding to nicotinic acetylcholine receptors have ubiquitous effects and can affect carotid chemoreception development through structural, functional and neuroregulatory alterations of the neural circuits involved in the chemoafferent pathway, as well as by interfering with the postnatal resetting of the carotid bodies. Reduced carotid body chemosensitivity and tonic activity have thus been reported by the majority of the human and animal studies. This review focuses on the effects of perinatal exposure to tobacco smoke and nicotine on carotid chemoreceptor function during the developmental period. A description of the effects of smoking and nicotine on the control of breathing related to carotid body activity, and of the possible physiopathological mechanisms at the origin of these disturbances is presented.

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.

A common FMO3 polymorphism may amplify the effect of nicotine exposure in sudden infant death syndrome (SIDS)

Smoking during pregnancy has been identified as one of the major modifiable risk factors of sudden infant death syndrome (SIDS). It has been demonstrated that the risk of SIDS increases with increasing cigarette consumption. A variety of hypotheses have been proposed for explanation, including a genetic predisposition. The flavin-monooxygenase 3 (FMO3) is one of the enzymes metabolising nicotine, and several polymorphisms have already been described in this gene. Here, we studied variations in the exons and introns of the FMO3 gene by direct sequencing analysis and minisequencing in 159 SIDS cases and 170 controls. The three common variants G472A (E158K), G769A (V257M) and A923G (E308G) in the exons of the FMO3 gene were identified. The homozygote 472AA genotype occurred more frequently in SIDS cases than in controls (p = 0.0054) and was more frequent in those SIDS cases for which the mothers reported heavy smoking (p = 0.0084). This study is the first to demonstrate a gene-environment interaction in SIDS. The findings suggest that the common polymorphism G472A of FMO3 could act as an additional genetic SIDS risk factor in children whose mothers smoke. Parents who could pass on the 472A allele should be informed of the increased risk associated with smoking. Smoking mothers should be strongly advised to give up smoking during pregnancy and for at least the first year of the child's life.

The effects of strain and prenatal nicotine exposure on ethanol consumption by adolescent male and female rats

Two studies of variables affecting voluntary ethanol consumption by adolescent male and female rats are reported. Sprague-Dawley (SD) and spontaneously hypertensive rats (SHRs) were compared in Experiment 1. Starting on postnatal day 30 all had 24-h access to 2%, then 4%, and then 6% ethanol, followed by 1-h access to the 6% until intake stabilized. During the 1-h access SHR females consumed more ethanol than all other groups. In Experiment 2, the same procedure was used to compare SD groups prenatally exposed to nicotine, with controls. Nicotine-exposed females consumed more ethanol during 1-h access than both nicotine-exposed and control males; but after using water intake as a covariate, the differences were not significant. These data show that deprivation conditions need to be considered when generalizing the results of voluntary consumption studies, and that estrogens may be a modulator of addictive behavior.

Prenatal nicotine exposure alters respiratory long-term facilitation in neonatal rats

Intermittent hypoxia can evoke persistent increases in ventilation (V (E)) in neonates (i.e. long-term facilitation, LTF) (Julien et al., 2008). Since prenatal nicotine (PN) exposure alters neonatal respiratory control (Fregosi and Pilarski, 2008), we hypothesized that PN would influence LTF of ventilation (V (E)) in neonatal rats. An osmotic minipump delivered nicotine 6 mg/kg per day or saline to pregnant dams. V (E) was assessed in unanesthetized pups via whole body plethysmography at post-natal (P) days 9-11 or 15-17 during baseline (BL, 21% O(2)), hypoxia (10 x 5 min, 5% O(2)) and 30 min post-hypoxia. PN pups had reduced BL V (E) (p<0.05) but greater increases in V (E) during hypoxia (p<0.05). Post-hypoxia V (E) (i.e. LTF) showed an agex treatment interaction (p<0.01) with similar values at P9-11 but enhanced LTF in saline (30+/-8%BL) vs. PN pups (6+/-5%BL; p=0.01) at P15-17. We conclude that the post-natal developmental time course of hypoxia-induced LTF is influenced by PN.

Respiratory dysfunctions induced by prenatal nicotine exposure

1. Maternal tobacco smoking is the principal risk factor associated with sudden infant death syndrome (SIDS), a leading cause of death of infants under 1 year of age. Victims of SIDS show a higher incidence of respiratory control abnormalities, including central apnoeas, delayed arousal responses and diminished ventilatory chemoreflexes. 2. Nicotine is likely the link between maternal tobacco smoking and SIDS. Prenatal nicotine exposure can alter the breathing pattern and can reduce hypoxia- and hypercarbia-induced ventilatory chemoreflexes. In vitro approaches have revealed that prenatal nicotine exposure impairs central chemosensitivity, switching the cholinergic contribution from a muscarinic to a nicotinic receptor-based drive. In addition, serotonergic, noradrenergic, GABAergic, glycinergic and glutamatergic, among others, are affected by prenatal nicotine. 3. Here we propose that prenatal nicotine affects the respiratory network through two main processes: (i) reorganization of neurotransmitter systems; and (ii) remodelling of neural circuits. These changes make breathing more vulnerable to fail in early postnatal life, which could be related to the pathogenesis of SIDS.

Antenatal environmental stress and maturation of the breathing control, experimental data

The nervous respiratory system undergoes postnatal maturation and yet still must be functional at birth. Any antenatal suboptimal environment could upset either its building prenatally and/or its maturation after birth. Here, we would like to briefly summarize some of the major stresses leading to clinical postnatal respiratory dysfunction that can occur during pregnancy, we then relate them to experimental models that have been developed in order to better understand the underlying mechanisms implicated in the respiratory dysfunctions observed in neonatal care units. Four sections are aimed to review our current knowledge based on experimental data. The first will deal with the metabolic factors such as oxygen and glucose, the second with consumption of psychotropic substances (nicotine, cocaine, alcohol, morphine, cannabis and caffeine), the third with psychoactive molecules commonly consumed by pregnant women within a therapeutic context and/or delivered to premature neonates in critical care units (benzodiazepine, caffeine). In the fourth section, we take into account care protocols involving extended maternal-infant separation due to isolation in incubators. The effects of this stress potentially adds to those previously described.