Effect of nicotine exposure on postnatal ventilatory responses to hypoxia and hypercapnia

Peripheral Neuroplasticity of Respiratory Chemoreflexes, Induced by Prenatal Nicotinic Exposure: Implication for SIDS

Sudden Infant Death Syndrome (SIDS) occurs during sleep in seemingly healthy infants. Maternal cigarette smoking and hypoxemia during sleep are assumed to be the major causal factors. Depressed hypoxic ventilatory response (dHVR) is observed in infants with high risk of SIDS, and apneas (lethal ventilatory arrest) appear during the fatal episode of SIDS. Disturbance of the respiratory center has been proposed to be involved, but the pathogenesis of SIDS is still not fully understood. Peripherally, the carotid body is critical to generate HVR, and bronchopulmonary and superior laryngeal C-fibers (PCFs and SLCFs) are important for triggering central apneas; however, their roles in the pathogenesis of SIDS have not been explored until recently. There are three lines of recently accumulated evidence to show the disorders of peripheral sensory afferent-mediated respiratory chemoreflexes in rat pups with prenatal nicotinic exposure (a SIDS model) in which acute severe hypoxia leads to dHVR followed by lethal apneas. (1) The carotid body-mediated HVR is suppressed with a reduction of the number and sensitivity of glomus cells. (2) PCF-mediated apneic response is largely prolonged via increased PCF density, pulmonary IL-1β and serotonin (5-hydroxytryptamine, 5-HT) release, along with the enhanced expression of TRPV1, NK₁R, IL1RI and 5-HT₃R in pulmonary C-neurons to strengthen these neural responses to capsaicin, a selective stimulant to C-fibers. (3) SLCF-mediated apnea and capsaicin-induced currents in superior laryngeal C-neurons are augmented by upregulation of TRPV1 expression in these neurons. These results, along with hypoxic sensitization/stimulation of PCFs, gain insight into the mechanisms of prenatal nicotinic exposure-induced peripheral neuroplasticity responsible for dHVR and long-lasting apnea during hypoxia in rat pups. Therefore, in addition to the disturbance in the respiratory center, the disorders of peripheral sensory afferent-mediated chemoreflexes may also be involved in respiratory failure and death denoted in SIDS victims.

Influence of developmental nicotine exposure on serotonergic control of breathing-related motor output

Serotonin plays an important role in the development of brainstem circuits that control breathing. Here, we test the hypothesis that developmental nicotine exposure (DNE) alters the breathing-related motor response to serotonin (5HT). Pregnant rats were exposed to nicotine or saline, and brainstem-spinal cord preparations from 1- to 5-day-old pups were studied in a split-bath configuration, allowing drugs to be applied selectively to the medulla or spinal cord. The activity of the fourth cervical ventral nerve roots (C4VR), which contain axons of phrenic motoneurons, was recorded. We applied 5HT alone or together with antagonists of 5HT1A, 5HT2A, or 5HT7 receptor subtypes. In control preparations, 5HT applied to the medulla consistently reduced C4VR frequency and this reduction could not be blocked by any of the three antagonists. In DNE preparations, medullary 5HT caused a large and sustained frequency increase (10 min), followed by a sustained decrease. Notably, the transient increase in frequency could be blocked by the independent addition of any of the antagonists. Experiments with subtype-specific agonists suggest that the 5HT7 subtype may contribute to the increased frequency response in the DNE preparations. Changes in C4VR burst amplitude in response to brainstem 5HT were uninfluenced by DNE. Addition of 5HT to the caudal chamber modestly increased phasic and greatly increased tonic C4VR activity, but there were no effects of DNE. The data show that DNE alters serotonergic signaling within brainstem circuits that control respiratory frequency but does not functionally alter serotonin signaling in the phrenic motoneuron pool.

Perinatal exposure to ethanol in rats induces permanent disturbances of breathing and chemosensitivity during adulthood

Perinatal exposure to drugs of abuse, including alcohol (ethanol), is known to impinge the development of respiratory function. However, most studies described the short-term effects of these exposures, focusing mostly on the early postnatal life. After exposure to ethanol during gestation and lactation we have previously shown that 3-4 week-old rat exhibit chronic hypoventilation and an altered response to hypoxia at the end of ethanol exposure. However, whether these deficits are reversible following ethanol withdrawal remained unknown. Here, we investigated through whole-body plethysmography the respiratory activity of 2 months-old rats exposed to ethanol from gestation to weaning followed by one month of ethanol withdrawal. After ethanol withdrawal, rats persistently exhibited a significant reduction in respiratory frequency without change in tidal volume associated to a lower arterial blood oxygen content. In addition, the response to hypoxia in these rats was reduced whereas the response to hypercapnia remained unaltered. In conclusion perinatal exposure to ethanol in rats, unlike exposure to cocaine, morphine or nicotine, is characterized by selective alterations of basal respiratory activity and chemosensitivity that persist long after withdrawal.

Development of ventilatory chemoreflexes in Coturnix quail chicks

Compared to mammals, little is known about the development of the respiratory control system in birds. In the present study, ventilation and metabolism were measured in Coturnix quail chicks exposed to room air, hypoxia (11 % O₂), and hypercapnia (4% CO₂) at 0-1, 3-4, and 6-7 days posthatching (dph). Mass-specific ventilation and metabolic rate tended to increase between 0-1 and 3-4 dph and then decrease again between 3-4 and 6-7 dph. The magnitude of the hypoxic ventilatory response (HVR) increased with age. The HVR also exhibited a biphasic shape in younger quail: after the initial increase in ventilation, ventilation declined back to (0-1 dph), or toward (4 dph), baseline. Older chicks (6-7 dph) had a "sustained HVR" in which ventilation remained high throughout the hypoxic challenge. The biphasic HVR did not appear to be caused by a decline in metabolic rate; although hypoxic hypometabolism was observed in quail chicks in all three age groups, the metabolic response appeared to occur more slowly than the biphasic HVR. The biphasic ventilatory response was also specific to hypoxia since the hypercapnic ventilatory response (HCVR) was characterized by a sustained increase in ventilation in all three age groups. The magnitude of the HCVR decreased with age. These results point to several similarities in the development of ventilatory chemorflexes between Coturnix quail and newborn mammals, including age-dependent (1) increases in the HVR, (2) transitions from a biphasic to a sustained HVR, and (3) decreases in the HCVR. Whether homologous mechanisms underlie these developmental changes remains to be determined.

Orexin Neurons Contribute to Central Modulation of Respiratory Drive by Progestins on ex vivo Newborn Rodent Preparations

Dysfunction of central respiratory CO₂/H⁺ chemosensitivity is a pivotal factor that elicits deep hypoventilation in patients suffering from central hypoventilation syndromes. No pharmacological treatment is currently available. The progestin desogestrel has been suggested to allow recovery of respiratory response to CO₂/H⁺ in patients suffering from central hypoventilation, but except the fact that supramedullary regions may be involved, mechanisms are still unknown. Here, we tested in neonates whether orexin systems contribute to desogestrel’s central effects on respiratory function. Using isolated ex vivo central nervous system preparations from newborn rats, we show orexin and almorexant, an antagonist of orexin receptors, supressed strengthening of the increase in respiratory frequency induced by prolonged metabolic acidosis under exposure to etonogestrel, the active metabolite of desogestrel. In parallel, almorexant suppressed the increase and enhanced increase in c-fos expression in respiratory-related brainstem structures induced by etonogestrel. These results suggest orexin signalisation is a key component of acidosis reinforcement of respiratory drive by etonogestrel in neonates. Although stage of development used is different as that for progestin clinical observations, presents results provide clues about conditions under which desogestrel or etonogestrel may enhance ventilation in patients suffering from central hypoventilation syndromes.

Cigarette smoke exposure effects on the brainstem expression of nicotinic acetylcholine receptors (nAChRs), and on cardiac, respiratory and sleep physiologies

Cigarette smoking during pregnancy is the largest modifiable risk factor for adverse outcomes in the infant. Investigations have focused on the psychoactive component of cigarettes, nicotine. One proposed mechanism leading to adverse effects is the interaction between nicotine and its nicotinic acetylcholine receptors (nAChRs). Much data has been generated over the past three decades on the effects of cigarette smoke exposure (CSE) on the expression of the nAChRs in the brainstem and physiological parameters related to cardiac, respiration and sleep, in the offspring of smoking mothers and animal models of nicotine exposure. This review summarises this data and discusses the main findings, highlighting that findings in animal models closely correlate with those from human studies, and that the major brainstem sites where the expression level for the nAChRs are consistently affected include those that play vital roles in cardiorespiration (hypoglossal nucleus, dorsal motor nucleus of the vagus, nucleus of the solitary tract), chemosensation (nucleus of the solitary tract, arcuate nucleus) and arousal (rostral mesopontine sites such as the locus coeruleus and nucleus pontis oralis). These findings provide evidence for the adverse effects of CSE during and after pregnancy to the infant and the need to continue with the health campaign advising against CSE.

Assessment of tobacco smoke effects on neonatal cardiorespiratory control using a semi-automated processing approach

A semi-automated processing approach was developed to assess the effects of early postnatal environmental tobacco smoke (ETS) on the cardiorespiratory control of newborn lambs. The system consists of several steps beginning with artifact rejection, followed by the selection of stationary segments, and ending with feature extraction. This approach was used in six lambs exposed to 20 cigarettes/day for the first 15 days of life, while another six control lambs were exposed to room air. On postnatal day 16, electrocardiograph and respiratory signals were obtained from a 6-h polysomnographic recording. The effects of postnatal ETS exposure on heart rate variability, respiratory rate variability, and cardiorespiratory interrelations were explored. The unique results suggest that early postnatal ETS exposure increases respiratory rate variability and decreases the coupling between cardiac and respiratory systems. Potentially harmful consequences in early life include unstable breathing and decreased adaptability of cardiorespiratory function, particularly during early life challenges, such as prematurity or viral infection. Graphical abstract ᅟ.

Intermittent hypercapnic hypoxia effects on the nicotinic acetylcholine receptors in the developing piglet hippocampus and brainstem

This study investigated the effects of acute (1 day) vs repeated (4 days) exposure to intermittent hypercapnic hypoxia (IHH) on the immunohistochemical expression of α2, α3, α5, α7, α9 and β2 nicotinic acetylcholine receptor (nAChR) subunits in the developing piglet hippocampus and brainstem medulla, and how prior nicotine exposure alters the response to acute IHH. Five piglet groups included: 1day IHH (1D IHH, n=9), 4days IHH (4D IHH, n=8), controls exposed only to air cycles for 1day (1D Air, n=6) or 4days (4D Air, n=5), and pre-exposed to nicotine for 13days prior to 1day IHH (Nic+1D IHH, n=7). The exposure period alternated 6min of HH (8%O₂, 7%CO₂, balance N₂) and 6min of air over 48min, while controls were switched from air-to-air. Results showed that: 1. repeated IHH induces more changes in nAChR subunit expression than acute IHH in both the hippocampus and brainstem medulla, 2. In the hippocampus, α2 and β2 changed the most (increased) following IHH and the CA3, CA2 and DG were mostly affected. In the brainstem medulla, α2, α5, α9 and β2 were changed (decreased) in most nuclei with the hypoglossal and nucleus of the solitary tract being mostly affected. 3. Pre-exposure to nicotine enhanced the changes in the hippocampus but dampened those in the brainstem medulla. These findings indicate that the nAChRs (predominantly with the α2/β2 complex) are affected by IHH in critical hippocampal and brainstem nuclei during early brain development, and that pre-exposure to nicotine alters the pattern of susceptibility to IHH.

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.

Medullary 5-HT neurons: Switch from tonic respiratory drive to chemoreception during postnatal development

Serotonin (5-HT) neurons contribute to respiratory chemoreception in adult mice, but it is unclear whether they play a similar role in neonatal mice. We studied breathing during development in Lmx1b^f/f/p mice, which lack 5-HT neurons. From postnatal days 1-7 (P1-P7), ventilation of Lmx1b^f/f/p mice breathing room air was 50% of WT mice (p<0.001). By P12, baseline ventilation increased to a level equal to WT mice. In contrast, the hypercapnic ventilatory response (HCVR) of neonatal Lmx1b^f/f/p and WT mice was equal to each other, but were both much less than adult WT mice. By P21 the HCVR of WT mice increased to near adult levels, but the HCVR of Lmx1b^f/f/p mice had not changed, and was 42% less than WT mice. Primary cell cultures were prepared from the ventromedial medulla of neonatal mice, and patch-clamp recordings were made from neurons identified as serotonergic by expression of a reporter gene. In parallel with developmental changes of the HCVR in vivo, 5-HT neurons had little chemosensitivity to acidosis until 12days in vitro (DIV), after which their response increased to reach a plateau around 25 DIV. Neonatal Lmx1b^f/f/p mice displayed high mortality and decreased growth rate, and this worsened in hypoxia. Mortality was decreased in hyperoxia. These results indicate that maturation of 5-HT neurons contributes to development of respiratory CO₂/pH chemoreception during the first few weeks of life in mice in vivo. A defect in the 5-HT system in early postnatal life decreases survival due in part to hypoxia.

Evaluating the control: minipump implantation and breathing behavior in the neonatal rat

We evaluated genioglossus (GG) gross motoneuron morphology, electromyographic (EMG) activities, and respiratory patterning in rat pups allowed to develop without interference (unexposed) and pups born to dams subjected to osmotic minipump implantation in utero (saline-exposed). In experiment 1, 48 Sprague-Dawley rat pups (Charles-River Laboratories), ages postnatal day 7 (P7) through postnatal day 10 (P10), were drawn from two experimental groups, saline-exposed (n = 24) and unexposed (n = 24), and studied on P7, P8, P9, or P10. Pups in both groups were sedated (Inactin hydrate, 70 mg/kg), and fine-wire electrodes were inserted into the GG muscle of the tongue and intercostal muscles to record EMG activities during breathing in air and at three levels of normoxic hypercapnia [inspired CO2 fraction (FiCO2 ): 0.03, 0.06, and 0.09]. Using this approach, we assessed breathing frequency, heart rate, apnea type, respiratory event types, and respiratory stability. In experiment 2, 16 rat pups were drawn from the same experimental groups, saline-exposed (n = 9) and unexposed (n = 7), and used in motoneuron-labeling studies. In these pups a retrograde dye was injected into the GG muscle, and the brain stems were subsequently harvested and sliced. Labeled GG motoneurons were identified with microscopy, impaled, and filled with Lucifer yellow. Double-labeled motoneurons were reconstructed, and the number of primary projections and soma volumes were calculated. Whereas pups in each group exhibited the same number (P = 0.226) and duration (P = 0.093) of respiratory event types and comparable motoneuron morphologies, pups in the implant group exhibited more central apneas and respiratory instability relative to pups allowed to develop without interference.

Developmental nicotine exposure disrupts dendritic arborization patterns of hypoglossal motoneurons in the neonatal rat

Maternal smoking or use of other products containing nicotine during pregnancy can have significant adverse consequences for respiratory function in neonates. We have shown, in previous studies, that developmental nicotine exposure (DNE) in a model system compromises the normal function of respiratory circuits within the brainstem. The effects of DNE include alterations in the excitability and synaptic interactions of the hypoglossal motoneurons, which innervate muscles of the tongue. This study was undertaken to test the hypothesis that these functional consequences of DNE are accompanied by changes in the dendritic morphology of hypoglossal motoneurons. Hypoglossal motoneurons in brain stem slices were filled with neurobiotin during whole-cell patch clamp recordings and subjected to histological processing to reveal dendrites. Morphometric analysis, including the Sholl method, revealed significant effects of DNE on dendritic branching patterns. In particular, whereas within the first five postnatal days there was significant growth of the higher-order dendritic branches of motoneurons from control animals, the growth was compromised in motoneurons from neonates that were subjected to DNE. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1125-1137, 2016.

Influence of developmental nicotine exposure on the ventilatory and metabolic response to hyperthermia

To determine whether developmental nicotine exposure (DNE) alters the ventilatory and metabolic response to hyperthermia in neonatal rats (postnatal age 2-4 days), pregnant dams were exposed to nicotine (6 mg kg(-1) of nicotine tartrate daily) or saline with an osmotic mini-pump implanted subdermally on day 5 of gestation. Rat pups (a total of 72 controls and 72 DNE pups) were studied under thermoneutral conditions (chamber temperature 33°C) and during moderate thermal stress (37.5°C). In all pups, core temperature was similar to chamber temperature, with no treatment effects. The rates of pulmonary ventilation (V̇(I)), O2 consumption (V̇(O2)) and CO2 production (V̇(CO2)) did not change with hyperthermia in either control or DNE pups. However, V̇(I) was lower in DNE pups at both chamber temperatures, whereas the duration of spontaneous apnoeas was longer in DNE pups than in controls at 33°C. The V̇(I)/V̇(O2) ratio increased at 37.5°C in control pups, although it did not change in DNE pups. To simulate severe thermal stress, additional pups were studied at 33°C and 43°C. V̇(I) increased with heating in control pups but not in DNE pups. As heat stress continued, gasping was evoked in both groups, with no effect of DNE on the gasping pattern. Over a 20 min recovery period at 33°C, V̇(I) returned to baseline in control pups but remained depressed in DNE pups. In addition to altering baseline V̇(I) and apnoea duration, DNE is associated with subtle but significant alterations in the ventilatory response to hyperthermia in neonatal rats.

Impairment of Central Chemoreception in Neonatal Rats Induced by Maternal Cigarette Smoke Exposure during Pregnancy

It has been postulated that prenatal cigarette smoke exposure (CSE) increases the risk for sudden infant death syndrome. The victims of infant death syndrome suffer from respiratory abnormalities, such as central apnea, diminished chemoreflex and alteration in respiratory pattern during sleep. However, no experimental evidence on CSE model exists to confirm whether prenatal CSE gives rise to reduction of neonatal central chemoreception in in vitro preparations in absence of peripheral sensory feedback. The aim of the present study was to test the hypothesis that maternal CSE during pregnancy depresses central chemoreception of the neonatal rats. The pregnant rats were divided into two groups, control (n = 8) and CSE (n = 8). Experiments were performed on neonatal (0-3days) rat pups. Fictive respiratory activity was monitored by recording the rhythmic discharge from the hypoglossal rootlets of the medullary slices obtained from the neonatal rats. The burst frequency (BF) and integrated amplitude (IA) of the discharge were analyzed. Their responses to acidified artificial cerebrospinal fluid (aCSF) were tested to indicate the change of the central chemosensitivity. Under condition of perfusing with standard aCSF (pH 7.4), no significant difference was detected between the two groups in either BF or IA (P>0.05). Under condition of perfusing with acidified aCSF (pH 7.0), BF was increased and IA was decreased in both groups (P<0.01). However, their change rates in the CSE group were obviously smaller than that in the control group, 66.98 ± 10.11% vs. 143.75 ± 15.41% for BF and -22.38 ± 2.51% vs. -44.90 ± 3.92% for IA (P<0.01). In conclusion, these observations, in a prenatal CSE model, provide important evidence that maternal smoking during pregnancy exerts adverse effects on central chemoreception of neonates.

Developmental nicotine exposure adversely effects respiratory patterning in the barbiturate anesthetized neonatal rat

Neonates at risk for sudden infant death syndrome (SIDS) are hospitalized for cardiorespiratory monitoring however, monitoring is costly and generates large quantities of averaged data that serve as poor predictors of infant risk. In this study we used a traditional autocorrelation function (ACF) testing its suitability as a tool to detect subtle alterations in respiratory patterning in vivo. We applied the ACF to chest wall motion tracings obtained from rat pups in the period corresponding to the mid-to-end of the third trimester of human pregnancy. Pups were drawn from two groups: nicotine-exposed and saline-exposed at each age (i.e., P7, P8, P9, and P10). Respiratory-related motions of the chest wall were recorded in room air and in response to an arousal stimulus (FIO2 14%). The autocorrelation function was used to determine measures of breathing rate and respiratory patterning. Unlike alternative tools such as Poincare plots that depict an averaged difference in a measure breath to breath, the ACF when applied to a digitized chest wall trace yields an instantaneous sample of data points that can be used to compare (data) points at the same time in the next breath or in any subsequent number of breaths. The moment-to-moment evaluation of chest wall motion detected subtle differences in respiratory pattern in rat pups exposed to nicotine in utero and aged matched saline-exposed peers. The ACF can be applied online as well as to existing data sets and requires comparatively short sampling windows (∼2 min). As shown here, the ACF could be used to identify factors that precipitate or minimize instability and thus, offers a quantitative measure of risk in vulnerable populations.

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.

Maternal nicotinic exposure produces a depressed hypoxic ventilatory response and subsequent death in postnatal rats

In this study, we asked whether a “full term” prenatal nicotinic exposure (fPNE, 6 mg·kg⁻¹·day⁻¹ nicotinic delivery) over the full gestation, compared to a traditional PNE (tPNE) over the last two‐thirds of the gestation, caused a higher mortality following a remarkable depressed hypoxic ventilatory response (dHVR) independent of brain and pulmonary edema and change in serum corticosterone. P12‐14 pups pretreated with tPNE, fPNE or their vehicle (tCtrl and fCtrl) were exposed to 5% O₂ for up to 60 min followed by harvesting the brain and lungs or anesthetized to collect blood for detecting arterial blood pH/gases and serum cotinine and corticosterone levels. We found that fPNE had little effect on baseline V_E and heart rate, but consistently induced a dHVR and prolonged apnea that were rarely observed after tPNE. The severity of the dHVR in PNE pups were closely correlated to an earlier appearance of lethal ventilatory arrest (the hypoxia‐induced mortality). PNE did not induce brain and pulmonary edema, but significantly increased serum corticosterone levels similarly in tPNE and fPNE pups. Moreover, the accumulated nicotinic dose given to the individual was significantly higher in fPNE than tPNE pups, though there was no difference in serum cotinine levels and arterial blood pH/gases between the two groups. Our results suggest that nicotinic exposure at the early stage of gestation achieved by fPNE, rather than tPNE, is critical in generating the dHVR and subsequent death occurring independently of brain/pulmonary edema and changes in arterial blood pH/gases and serum corticosterone.

Our results suggest that nicotinic exposure at the early stage of gestation achieved by “full term” prenatal nicotinic exposure (fPNE), rather than traditional prenatal nicotinic exposure (tPNE), is critical in generating the depressed hypoxic ventilatory response (dHVR) and subsequent death. The fPNE‐induced cardiorespiratory impairement is independent of brain/pulmonary edema and changes in arterial blood pH/gases and serum corticosterone.

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.

Effects of postnatal environmental tobacco smoke on non-nutritive swallowing-breathing coordination in newborn lambs

While prenatal environmental tobacco smoke (ETS) exposure is a well-known risk factor for sudden infant death syndrome, the effect of postnatal ETS exposure is less clear. The objective of this study was to investigate the effect of postnatal ETS exposure on non-nutritive swallowing (NNS) and NNS-breathing coordination, which are crucial to prevent aspiration related-cardiorespiratory events. Eighteen newborn lambs (6 per group) were randomly exposed to either 10 cigarettes/day, 20 cigarettes/day or room air for 15 days. Lambs were instrumented for recording states of alertness, swallowing, electrocardiogram and breathing; recordings were performed in non-sedated lambs at the end of ETS exposure. Urinary cotinine/creatinine ratio confirmed relevant real-life exposure. Postnatal ETS exposure had no effect on NNS frequency but tended to decrease inspiratory NNS (p=0.07) during quiet sleep. No effect on respiratory or heart rate (p>0.6), apnea index (p=0.2) or sleep states (p=0.3) was observed. In conclusion, postnatal ETS exposure in lambs had only mild effects on NNS-breathing coordination.

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.

Postnatal maturation of breathing stability and loop gain: the role of carotid chemoreceptor development

Any general model of respiratory control must explain a puzzling array of breathing patterns that are observed during the course of a lifetime. Particular challenges are to understand why periodic breathing is rarely seen in the first few days after birth, reaches a peak at 2-4 weeks postnatal age, and disappears by 6 months, why it is prevalent in preterm infants, and why it reappears in adults at altitude or with heart failure. In this review we use the concept of loop gain to obtain quantitative insight into the genesis of unstable breathing patterns with a particular focus on how changes in carotid body function could underlie the age-related dependence of periodic breathing.

Prenatal cigarette smoke exposure and postnatal respiratory responses to hypoxia and hypercapnia

Prenatal cigarette smoke (CS) exposure, in combination with hypoxia and/or hyperthermia can lead to gasping and attenuated recovery from hypoxia in 7 days old rat pups. We studied 95 unanesthetized spontaneously breathing 14 days old rat pups to investigate if the destabilizing effects of increased ambient temperature and prenatal CS exposure on respiratory control observed in 7 days old rats were still evident at day 14. This postnatal age was selected as it is beyond the analogous risk period for SIDS in human. Furthermore, we investigated if the breathing responses to hypercapnia are affected by prenatal CS exposure. Since high ambient (HA) temperature can lead to gasping and aberrant respiratory control, we recorded respiratory patterns at low (24-25°C) and high (29-30°C) ambient temperatures, and under hypoxic or hypercapnic states. No gasping was observed in 14 days old rat pups. During hypoxia, breathing frequency increased in the CS-exposed group under low and HA temperatures. Rectal temperature decreased only in the sham group in response to low ambient temperature hypoxia. At HA temperature, breathing frequency increased in both sham and CS-exposed groups during hypercapnia, however, it remained elevated during washout period only in the sham group. We demonstrate that prenatal CS exposure continues to have profound effects on respiratory and thermoregulatory responses to hypoxia and hypercapnia at day 14. The attenuated respiratory and thermoregulatory responses to acute hypoxia and hypercapnia on day 14 demonstrate a strong interaction between CS exposure, respiratory control, and thermoregulation during postnatal maturation.

Increased nicotinic receptor desensitization in hypoglossal motor neurons following chronic developmental nicotine exposure

Neuronal nicotinic acetylcholine receptors (nAChRs) are expressed on hypoglossal motor neurons (XII MNs) that innervate muscles of the tongue. Activation of XII MN nAChRs evokes depolarizing currents, which are important for regulating the size and stiffness of the upper airway. Although data show that chronic developmental nicotine exposure (DNE) blunts cholinergic neurotransmission in the XII motor nucleus, it is unclear how nAChRs are involved. Therefore, XII MN nAChR desensitization and recovery were examined in tissues from DNE or control pups using a medullary slice preparation and tight-seal whole cell patch-clamp recordings. nAChR-mediated inward currents were evoked by brief pressure pulses of nicotine or the α4β2 nAChR agonist RJR-2403. We found that, regardless of treatment, activatable nAChRs underwent desensitization, but, following DNE, nAChRs exhibited increased desensitization and delayed recovery. Similar results were produced using RJR-2403, showing that DNE influences primarily the α4β2 nAChR subtype. These results show that while some nAChRs preserve their responsiveness to acute nicotine following DNE, they more readily desensitize and recover more slowly from the desensitized state. These data provide new evidence that chronic DNE modulates XII MN nAChR function, and suggests an explanation for the association between DNE and the incidence of central and obstructive apneas.

Sex differences in heart rate variability during sleep following prenatal nicotine exposure in rat pups

The influence of both prenatal nicotine exposure (PNE; 6 mg/kg/day) and sex on heart rate (HR) regulation during sleep versus wakefulness was evaluated in 13, 16 and 26 day old rat pups. Pups were chronically instrumented at least 24 h before testing. On postnatal day 13 (P13), PNE males spent significantly more time in NREM sleep and demonstrated a greater drop in HR when transitioning from quiet wake to sleep compared to age and sex matched controls (-14±5 bpm versus -1±3 bpm, respectively). Heart rate variability (HRV) analysis indicated that this state-dependent drop in HR was primarily associated with a greater reduction in sympathovagal balance (LF/HF ratio) in PNE males compared to controls. No parallel changes in indices of parasympathetic drive (HF power) were identified. In contrast, no significant effect of PNE on HR during sleep versus wakefulness was identified in P13 females. However, independent of state, a significant decrease in HF power was identified in P13 PNE females compared to controls. At P16, state-dependent differences in HR or HRV between PNE and sex-matched control pups were resolved. Additionally, at P26 no significant effect of PNE on state-dependent changes in HR or HRV was identified in either sex. Analysis of the hypothalamic peptide orexin identified that PNE induced approximately a 50% reduction in hypothalamic prepro-orexin mRNA and total mRNA was lowest in PNE males. These findings suggest that PNE induces sex dependent changes in sleep related autonomic regulation of HR during early postnatal development and these changes may be related to epigenetic alterations in the orexin system.

The effect of prenatal nicotine exposure on PDGFR-mediated anti-apoptotic mechanism in the caudal brainstem of developing rat

Maternal cigarette smoking is a major risk factor for sudden infant death syndrome (SIDS); however, the mechanism underlying this association is currently unknown. Prenatal nicotine exposure is accompanied by a decrease in the magnitude of hypoxic ventilatory depression, the component of hypoxic ventilatory response that activates the PDGF-beta receptor (PDGFR) and its downstream anti-apoptotic cascade in the caudal brainstem (CB) of developing rats. In this study, we evaluated the effect of prenatal nicotine exposure on PDGFR activation and the subsequent activation of downstream anti-apoptotic processes through the Akt/BAD pathway. The 5-day timed-pregnant Sprague-Dawley rats underwent surgical implantation of an osmotic pump containing either normal saline (control) or a solution of nicotine tartrate. The CB was harvested from 5-day-old rat pups (n=8-10 for each time point) in each group after exposure to normoxia or hypoxic challenges with 10% O(2) for 5, 15, 30, 60 or 120 min. Immunoprecipitation and immunoblots of CB lysates revealed phosphorylation of PDGFR, Akt and BAD-136 during hypoxia in control pups. Prenatal nicotine exposure was associated with attenuation of these responses at all time points. Analysis of an early apoptotic marker in the CB revealed that activation of cleaved caspase-3 occurred only at 120 min of hypoxic exposure in the control. Prenatal nicotine exposure accelerated this response, causing early activation at 30 and 60 min. We conclude that prenatal nicotine exposure attenuates the phosphorylation of PDGFR, Akt and Bad-136 during hypoxia in the CB of developing rats. This modulation of anti-apoptotic cascades accelerates activation of the early apoptotic marker. We speculate that prenatal nicotine exposure affects apoptosis in the CB of developing animals and may increase the vulnerability of neural cells in the respiratory control area, a process that may underlie the association between maternal smoking and SIDS.

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.

Chronic hypercapnia alters lung matrix composition in mouse pups

RATIONALE

permissive hypercapnia, a stretch-limiting ventilation strategy, often results in high Pa(CO(2)). This strategy is associated with reduced morbidity and mortality in premature infants and its benefits have been attributed to diminished barotrauma. However, little is known about the independent effect of high CO(2) levels during the lung development.

METHODS

mice were exposed to 8% CO(2) or room air for 2 wk either from postnatal day 2 through 17 or as adults (approximately 2 mo of age). Lungs were excised and processed for protein, RNA, histology, and total lung volumes.

RESULTS

histologic analysis demonstrated that alveolar walls of CO(2)-exposed mouse pups were thinner than those of controls and had twice the total lung volume. Molecular analysis revealed that several matrix proteins in the lung were downregulated in mouse pups exposed to hypercapnia. Interstitial collagen type I alpha1, type III alpha1, elastin and fibronectin protein, and mRNA levels were less than half of controls while collagen IV alpha 5 was unaffected. This decrease in interstitial collagen could thus account for the thinning of the interstitial matrix and the altered lung biomechanics. Matrix metalloproteinase (MMP)-8, a collagenase that has specificity for collagen types I and III, increased in hypercapnic mouse pups, suggesting increased collagen degradation. Moreover, tissue inhibitor of MMP (TIMP)-1, a potent inhibitor of MMP-8, was significantly decreased. However, unlike pups, adult mice exposed to hypercapnia demonstrated only a mild increase in total lung volumes and did not exhibit similar molecular or histologic changes.

CONCLUSIONS

although permissive hypercapnia may prevent lung injury from barotrauma, our study revealed that exposure to hypercapnia may be an important factor in lung remodeling and function, especially in early life.

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.

Prenatal nicotine exposure and development of nicotinic and fast amino acid-mediated neurotransmission in the control of breathing

There is mounting evidence that neonatal animals exposed to nicotine in the prenatal period exhibit a variety of anatomic and functional abnormalities that adversely affect their respiratory and cardiovascular control systems, but how nicotine causes these developmental alterations is unknown. The principle that guides our work is that PNE impairs the ability of nicotinic acetylcholine receptors (nAChRs) to modulate the pre-synaptic release of both inhibitory (particularly GABA) and excitatory (glutamate) neurotransmitters, leading to marked alterations in the density and/or function of receptors on the (post-synaptic) membrane of respiratory neurons. Such changes could lead to impaired ventilatory responses to sensory afferent stimulation, and altered breathing patterns, including central apneic events. In this brief review we summarize the work that lead to the development of this hypothesis, and introduce some new data that support and extend it.

Prenatal to early postnatal nicotine exposure impairs central chemoreception and modifies breathing pattern in mouse neonates: a probable link to sudden infant death syndrome

Nicotine is a neuroteratogen and is the likely link between maternal cigarette smoking during pregnancy and sudden infant death syndrome (SIDS). Osmotic minipumps were implanted in 5-7 d CF1 pregnant mice to deliver nicotine bitartrate (60 mg Kg(-1) day(-1)) or saline (control) solutions for up to 28 d. Prenatal to early postnatal nicotine exposure did not modify the number of newborns per litter or their postnatal growth; however, nicotine-exposed neonates hypoventilated and had reduced responses to hypercarbia (inhalation of air enriched with 10% CO(2) for 20 min) and hypoxia (inhalation of 100% N(2) for 20 s) at postnatal days 0-3 (P0-P3). In contrast, at postnatal day 8, nicotine-exposed neonates were indistinguishable from controls. Isolated brainstem-spinal cord preparations obtained from P0 to P3 nicotine-exposed neonates showed fictive respiration with respiratory cycles longer and more irregular than those of controls, as indicated by high short- and long-term variability in Poincaré plots. In addition, their responses to acidification were reduced, indicating compromise of central chemoreception. Furthermore, the cholinergic contribution to central chemosensory responses switched from muscarinic receptor to nicotinic receptor-based mechanisms. No significant astrogliosis was detectable in the ventral respiratory group of neurons with glial fibrillary acidic protein immunohistochemistry. These results indicate that nicotine exposure affects the respiratory rhythm pattern generator and causes a decline in central chemoreception during early postnatal life. Consequently, breathing would become highly vulnerable, failing to respond to chemosensory demands. Such impairment could be related to the ventilatory abnormalities observed in SIDS.

Abolishment of serotonergic neurotransmission to cardiac vagal neurons during and after hypoxia and hypercapnia with prenatal nicotine exposure

Cardioinhibitory cardiac vagal neurons (CVNs) do not receive inspiratory-related excitatory inputs under normal conditions. However, excitatory purinergic and serotonergic pathways are recruited during inspiratory activity after episodes of hypoxia and hypercapnia (H/H). Prenatal nicotine (PNN) exposure is known to dramatically change cardiorespiratory responses and decrease the ability to resuscitate from H/H. This study tested whether PNN exposure alters excitatory neurotransmission to CVNs in the nucleus ambiguus during and after H/H. Spontaneous and inspiratory evoked excitatory postsynaptic currents were recorded in CVNs from rats that were exposed to nicotine (6 mg x kg(-1) x d(-1)) throughout the prenatal period. In contrast to unexposed animals, in PNN animals H/H recruited excitatory neurotransmission to CVNs during inspiratory-related activity that was blocked by the alpha3beta4 nicotinic acetylcholine receptor (nAChR) blocker alpha-conotoxin AuIB (alpha-CTX AuIB, 100 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 50 microM) and d(-)-2-amino-5-phosphonopentanoic acid (AP5, 50 microM), selective AMPA/kainate and N-methyl-d-aspartate receptor blockers, respectively. Following H/H, there was a significant increase in inspiratory-related excitatory postsynaptic currents that were unaltered by alpha-CTX AuIB or ondansetron, a 5-HT3 receptor blocker, but were subsequently inhibited by pyridoxalphosphate-6-azophenyl-2', 4'-disulphonic acid (100 microM), a purinergic receptor blocker and CNQX and AP5. The results from this study demonstrate that with PNN exposure, an excitatory neurotransmission to CVNs is recruited during H/H that is glutamatergic and dependent on activation of alpha3beta4-containing nAChRs. Furthermore, exposure to PNN abolishes a serotonergic long-lasting inspiratory-related excitation of CVNs that is replaced by recruitment of a glutamatergic pathway to CVNs post H/H.

Prenatal cigarette smoke exposure attenuates recovery from hypoxemic challenge in preterm infants

RATIONALE

The effects of prenatal cigarette smoke (CS) exposure and hypoxemia on cardiorespiratory control have been investigated in full-term infants. However, few data are available in preterm infants, who form a particularly vulnerable population, with developmentally immature cardiorespiratory control.

OBJECTIVES

To investigate the effects of prenatal CS exposure on the duration and recovery of breathing pauses and oxygen saturation levels under baseline and hypoxemic conditions in preterm infants.

METHODS

The study was performed on 22 (12 born to smoking and 10 to nonsmoking mothers) spontaneously breathing preterm infants between 28 and 36 weeks' gestation. Cardiorespiratory variables were recorded under baseline normoxemic and hypoxemic conditions.

MEASUREMENTS AND MAIN RESULTS

Breathing pauses, pause indices, time to recovery, percent pause recovery, oxygen saturation (Sp(O2)), periods of wakefulness, and cardiorespiratory rates were compared between the two groups. Spontaneous recovery of breathing pauses (P = 0.03) and Sp(O(2)) levels (P = 0.017) were attenuated in CS-exposed infants as compared with the control group during the hypoxemic and posthypoxemic periods, respectively. The episodes of wakefulness during the hypoxemic challenge were similar between the two groups. Furthermore, CS-exposed infants showed a greater increase in heart rate (P < 0.001) during the hypoxemic challenge when compared with control infants.

CONCLUSIONS

We provide evidence of how prenatal CS exposure and hypoxemic episodes affect the duration and recovery of breathing pauses in preterm infants. These observations could help explain why these infants are at a particularly high risk for sudden infant death syndrome.

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

While perinatal nicotine effects on ventilation have been widely investigated, the prenatal impact of nicotine treatment during gestation on both breathing and neural circuits involved in respiratory control remains unknown. We examined the effects of nicotine, from embryonic day 5 (E5) to E20, on baseline ventilation, the two hypoxic ventilatory response components and in vivo tyrosine hydroxylase (TH) activity in carotid bodies and brainstem areas, assessed at postnatal day 7 (P7), P11 and P21. In pups prenatally exposed to nicotine, baseline ventilation and hypoxic ventilatory response were increased at P7 (+48%) and P11 (+46%), with increased tidal volume (p<0.05). Hypoxia blunted frequency response at P7 and revealed unstable ventilation at P11. In carotid bodies, TH activity increased by 20% at P7 and decreased by 48% at P11 (p<0.05). In most brainstem areas it was reduced by 20-33% until P11. Changes were resolved by P21. Prenatal nicotine led to postnatal ventilatory sequelae, partly resulting from impaired maturation of peripheral chemoreceptors and brainstem integrative sites.

Respiratory control in neonatal rats exposed to prenatal cigarette smoke

RATIONALE

Prenatal cigarette smoke (CS) exposure, increased environmental temperature, and hypoxic episodes have been postulated as major risk factors for sudden infant death syndrome.

OBJECTIVES

To test the hypothesis that maternal CS exposure disrupts eupneic breathing and depresses breathing responses of neonatal rats to thermal and hypoxic challenges.

METHODS

Experiments were performed on 1-week-old rat pups exposed prenatally to CS (n = 39) or room air (sham; n = 30). Breathing patterns were recorded by whole-body plethysmography during thermoneutral or hyperthermic states under normoxic and hypoxic conditions.

MEASUREMENTS AND MAIN RESULTS

Mean pup weight, breaths per minute, and gasping respiratory patterns were measured for both smoke- and sham-exposed groups during thermoneutral and hyperthermic states under normoxic and hypoxic conditions. Under thermoneutral conditions, hypoxia caused gasping in CS-exposed animals but not in sham-exposed animals. Furthermore, under hyperthermic conditions, whereas hypoxia induced gasping in both groups, only CS-exposed animals exhibited a pronounced and longer lasting respiratory depression after the termination of hypoxia.

CONCLUSIONS

We show that prenatal CS exposure increases the likelihood of gasplike respiration and provide the first experimental evidence that the combined effects of prenatal CS exposure and hyperthermia dramatically prolong the time required for neonates to return to eupneic breathing after hypoxia. These observations provide important evidence of how prenatal CS exposure, hypoxic episodes, and hyperthermia might place infants at higher risk for sudden infant death syndrome.

Nicotinic receptor activation occludes purinergic control of central cardiorespiratory network responses to hypoxia/hypercapnia

Prenatal nicotine exposure alters the cardiorespiratory network responses to hypoxia/hypercapnia; however the mechanism(s) responsible for these cardiorespiratory network responses and their alteration by prenatal nicotine exposure are unknown. We used an in vitro medullary slice that allows simultaneous examination of rhythmic respiratory-related activity and excitatory synaptic neurotransmission to cardioinhibitory vagal neurons (CVNs). Respiratory related increases in glutamatergic neurotransmission only occurred on recovery from hypoxia/hypercapnia in unexposed animals. These responses were not altered by nicotinic antagonists but were mediated in part by activation of P2 purinergic receptors. Prenatal nicotine exposure transformed central cardiorespiratory responses to hypoxia/hypercapnia; CVNs received a respiratory related glutamatergic neurotransmission during periods of hypoxia and hypercapnia, whereas increases in glutamatergic neurotransmission during recovery were absent. The excitatory neurotransmission to CVNs during hypoxia/hypercapnia in prenatal nicotine-exposed animals were wholly dependent on nicotinic receptor activation. In the presence of nicotinic antagonists, the responses in prenatal nicotine animals reverted to the pattern of responses in unexposed animals in which an increase in glutamatergic neurotransmission occurred not during but only on recovery from hypoxia/hypercapnia, and this recruited excitatory pathway was blocked by P2 receptor antagonists. These data identify a new functional role for purinergic receptors in the cardiorespiratory responses to hypoxia/hypercapnia and their role in occluding nicotinic receptor activation with prenatal nicotine exposure.

Prenatal nicotine exposure alters glycinergic and GABAergic control of respiratory frequency in the neonatal rat brainstem-spinal cord preparation

Bath application of GABA-A receptor agonists in neonatal rat brainstem-spinal cord preparations (BSSC) reduces respiratory frequency, an effect that is enhanced by prenatal nicotine exposure. Here we test the hypothesis that these effects can be reproduced by microinjection of GABAergic and glycinergic agonists into the pre-Botzinger complex region (PBC). We recorded the activity of phrenic motor axons from the fourth cervical ventral root in 1-3 days old BSSC that were exposed to either nicotine (6 mg/(kg day)) or saline prenatally. Microinjection of glycine or muscimol into the PBC caused abrupt, reversible apnea in all experiments. Apnea duration with glycine averaged 50.3+/-5 s in saline-exposed (N=12), and 95.7+/-9.9 s in nicotine-exposed (N=12) neonates (P<0.001). Apnea duration with muscimol averaged 51+/-5.1 s in saline-exposed (N=10), and 86+/-10.6 s in nicotine-exposed (N=12) neonates (P<0.05). These data show that prenatal nicotine exposure alters development of central ventilatory control, and that neurons in the PBC region are involved.

Prenatal nicotine exposure transiently alters the lung mechanical response to hypoxia in young lambs

To test the hypothesis that fetal nicotine exposure alters the lung mechanical response to hypoxia (10% O(2)) 10 lambs were exposed during the last fetal trimester to a low dose nicotine (LN) and 10 to a moderate dose (MN) (maternal dose 0.5 and 1.5mg/(kgday) free base, respectively). There were 10 controls (C). At 12 days, minute ventilation increased significantly less in MN compared with LN but not with C. In contrast to C and LN, MN did not show anticipated increases in dynamic compliance, specific compliance and FRC or decrease in lung resistance but had signs of airway hyperreactivity during hypoxia. Nicotine exposure did not alter the cardiovascular response. These adverse effects decreased with advancing age. In summary, prenatal nicotine exposure alters the lung mechanical response to hypoxia. We speculate that prenatal nicotine-induced alterations of lung mechanics during hypoxia may contribute to an increased vulnerability to hypoxic stress during infancy.

Are the carotid bodies of the guinea-pig functional?

We have previously observed that the guinea-pig appears to have a relatively poor ventilatory (V (E)) response to hypoxia, compared to other mammals. Therefore, in this study, we questioned the ability of the carotid bodies (primary peripheral chemoreceptors) in the guinea-pig to detect hypoxia. The ventilatory responses to poikilocapnic hypoxia (8% O(2)), poikilooxic hypercapnia (8% CO(2)), hyperoxia (100% O(2)) and cyanide (NaCN - 200 mug/kg, i.v.) were assessed before and after carotid body denervation (CBD) in anaesthetized guinea-pigs. Although CBD attenuated the V (E) responses to hypercapnia and cyanide, it had no effect on normoxic breathing or the V (E) responses to hypoxia or hyperoxia. In a separate group of guinea-pigs, nerve activity was recorded from single or few-fibre preparations of the carotid sinus nerve (CSN). Basal chemoreceptor activity could not be detected from any of the nerve preparations. NaCN and hypercapnia consistently provoked an increase in neural activity. In contrast, hypoxia never clearly increased activity in any of the single or few-fibre preparations isolated from the CSN. In conclusion, although the carotid bodies of the guinea-pig, like those of other mammals, are able to detect hypercapnia and histotoxic hypoxia and elicit a reflex increase in V (E), they are essentially hypoxia-insensitive. The latter may explain, at least in part, the relatively poor V (E) response to hypoxia shown by the guinea-pig.

Postnatal developmental changes in CO2 sensitivity in rats

Ventilatory sensitivity to CO2 in awake adult Brown Norway (BN) rats is 50–75% lower than in adult Sprague-Dawley (SD) and salt-sensitive Dahl S (SS) rats. The purpose of the present study was to test the hypothesis that this difference would be apparent during the development of CO2 sensitivity. Four litters of each strain were divided into four groups such that rats were exposed to 7% inspired CO2 for 5 min in a plethysmograph every third day from postnatal day (P) 0 to P21 and again on P29 and P30. From P0 to P14, CO2 exposure increased pulmonary ventilation (V̇e) by 25–50% in the BN and SD strains and between 25 to over 200% in the SS strain. In all strains beginning around P15, the response to CO2 increased progressively reaching a peak at P19–21 when V̇e during hypercapnia was 175–225% above eucapnia. There were minimal changes in CO2 sensitivity between P21 and P30, and at both ages there were minimal between-strain differences. At P30, the response to CO2 in the SS and SD strains was near the adult response, but the response in the BN rats was 100% greater at P30 than in adults. We conclude that 1) CO2-sensing mechanisms, and/or mechanisms downstream from the chemoreceptors, change dramatically at the age in rats when other physiological systems are also maturing (∼P15), and 2) there is a high degree of age-dependent plasticity in CO2 sensitivity in rats, which differs between strains.

Differential control of central cardiorespiratory interactions by hypercapnia and the effect of prenatal nicotine

Hypercapnia evokes a strong cardiorespiratory response including gasping and a pronounced bradycardia; however, the mechanism responsible for these survival responses initiated in the brainstem is unknown. To examine the effects of hypercapnia on the central cardiorespiratory network, we used an in vitro medullary slice that allows simultaneous examination of rhythmic respiratory-related activity and inhibitory synaptic neurotransmission to cardioinhibitory vagal neurons (CVNs). Hypercapnia differentially modulated inhibitory neurotransmission to CVNs; whereas hypercapnia selectively depressed spontaneous glycinergic IPSCs in CVNs without altering respiratory-related increases in glycinergic neurotransmission, it decreased both spontaneous and inspiratory-associated GABAergic IPSCs. Because maternal smoking is the highest risk factor for sudden infant death syndrome (SIDS) and prenatal nicotine exposure is proposed to be the link between maternal smoking and SIDS, we examined the cardiorespiratory responses to hypercapnia in animals exposed to nicotine in the prenatal and perinatal period. In animals exposed to prenatal nicotine, hypercapnia evoked an exaggerated depression of GABAergic IPSCs in CVNs with no significant change in glycinergic neurotransmission. Hypercapnia altered inhibitory neurotransmission to CVNs at both presynaptic and postsynaptic sites. Although the results obtained in this study in vitro cannot be extrapolated with certainty to in vivo responses, the results of this study provide a likely neurochemical mechanism for hypercapnia-evoked bradycardia and the dysregulation of this response with exposure to prenatal nicotine, creating a higher risk for SIDS.