Respiratory responses to pH in the absence of pontine and dorsal medullary areas in the newborn mouse in vitro

Chronic maternal exposure to titanium dioxide nanoparticles alters breathing in newborn offspring

Background

Over the last two decades, nanotechnologies and the use of nanoparticles represent one of the greatest technological advances in many fields of human activity. Particles of titanium dioxide (TiO₂) are one of the nanomaterials most frequently found in everyday consumer products. But, due in particular to their extremely small size, TiO₂ nanoparticles (NPs) are prone to cross biological barriers and potentially lead to adverse health effects. The presence of TiO₂ NPs found in human placentae and in the infant meconium has indicated unequivocally the capacity for a materno-fetal transfer of this nanomaterial. Although chronic exposure to TiO₂ NPs during pregnancy is known to induce offspring cognitive deficits associated with neurotoxicity, the impact of a gestational exposure on a vital motor function such as respiration, whose functional emergence occurs during fetal development, remains unknown.

Results

Using in vivo whole-body plethysmographic recordings from neonatal mice, we show that a chronic exposure to TiO₂ NPs during pregnancy alters the respiratory activity of offspring, characterized by an abnormally elevated rate of breathing. Correspondingly, using ex vivo electrophysiological recordings performed on isolated brainstem-spinal cord preparations of newborn mice and medullary slice preparations containing specific nuclei controlling breathing frequency, we show that the spontaneously generated respiratory-related rhythm is significantly and abnormally accelerated in animals prenatally exposed to TiO₂ NPs. Moreover, such a chronic prenatal exposure was found to impair the capacity of respiratory neural circuitry to effectively adjust breathing rates in response to excitatory environmental stimuli such as an increase in ambient temperature.

Conclusions

Our findings thus demonstrate that a maternal exposure to TiO₂ NPs during pregnancy affects the normal development and operation of the respiratory centers in progeny.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12989-022-00497-4.

The Onset of the Fetal Respiratory Rhythm: An Emergent Property Triggered by Chemosensory Drive?

The mechanisms responsible for the onset of respiratory activity during fetal life are unknown. The onset of respiratory rhythm may be a consequence of the genetic program of each of the constituents of the respiratory network, so they start to interact and generate respiratory cycles when reaching a certain degree of maturation. Alternatively, generation of cycles might require the contribution of recently formed sensory inputs that will trigger oscillatory activity in the nascent respiratory neural network. If this hypothesis is true, then sensory input to the respiratory generator must be already formed and become functional before the onset of fetal respiration. In this review, we evaluate the timing of the onset of the respiratory rhythm in comparison to the appearance of receptors, neurotransmitter machinery, and afferent projections provided by two central chemoreceptive nuclei, the raphe and locus coeruleus nuclei.

Role of Astrocytes in Central Respiratory Chemoreception

Astrocytes perform various homeostatic functions in the nervous system beyond that of a supportive or metabolic role for neurons. A growing body of evidence indicates that astrocytes are crucial for central respiratory chemoreception. This review presents a classical overview of respiratory central chemoreception and the new evidence for astrocytes as brainstem sensors in the respiratory response to hypercapnia. We review properties of astrocytes for chemosensory function and for modulation of the respiratory network. We propose that astrocytes not only mediate between CO₂/H⁺ levels and motor responses, but they also allow for two emergent functions: (1) Amplifying the responses of intrinsic chemosensitive neurons through feedforward signaling via gliotransmitters and; (2) Recruiting non-intrinsically chemosensitive cells thanks to volume spreading of signals (calcium waves and gliotransmitters) to regions distant from the CO₂/H⁺ sensitive domains. Thus, astrocytes may both increase the intensity of the neuron responses at the chemosensitive sites and recruit of a greater number of respiratory neurons to participate in the response to hypercapnia.

Redefining the components of central CO2 chemosensitivity--towards a better understanding of mechanism

Abstract

The field of CO₂ chemosensitivity has developed considerably in recent years. There has been a mounting number of competing nuclei proposed as chemosensitive along with an ever increasing list of potential chemosensory transducing molecules. Is it really possible that all of these areas and candidate molecules are involved in the detection of chemosensory stimuli? How do we discriminate rigorously between molecules that are chemosensory transducers at the head of a physiological reflexversusthose that just happen to display sensitivity to a chemosensory stimulus? Equally, how do we differentiate between nuclei that have a primary chemosensory function, versusthose that are relays in the pathway? We have approached these questions by proposing rigorous definitions for the different components of the chemosensory reflex, going from the salient molecules and ions, through the components of transduction to the identity of chemosensitive cells and chemosensitive nuclei. Our definitions include practical and rigorous experimental tests that can be used to establish the identity of these components. We begin by describing the need for central CO₂ chemosensitivity and the problems that the field has faced. By comparing chemosensory mechanisms to those in the visual system we suggest stricter definitions for the components of the chemosensory pathway. We then, considering these definitions, re-evaluate current knowledge of chemosensory transduction, and propose the ‘multiple salient signal hypothesis’ as a framework for understanding the multiplicity of transduction mechanisms and brain areas seemingly involved in chemosensitivity.

Alterations in cholinergic sensitivity of respiratory neurons induced by pre-natal nicotine: a mechanism for respiratory dysfunction in neonatal mice

Nicotine may link cigarette smoking during pregnancy with sudden infant death syndrome (SIDS). Pre-natal nicotine leads to diminished ventilatory responses to hypercarbia and reduced central chemoreception in mice at post-natal days 0-3. We studied how pre-natal nicotine exposure changes the cholinergic contribution to central respiratory chemoreception in neonatal isolated brainstem-spinal cord and slice preparations. Osmotic minipumps, implanted subcutaneously into 5-7 days pregnant mice, delivered saline or nicotine ditartrate 60 mg kg(-1) d(-1) for up to 28 days. In control preparations, acidification of the superfusion medium from pH 7.4 to 7.3 increased the frequency and reduced the amplitude of fictive respiration. In nicotine-exposed neonatal mice, the reduction in amplitude induced by acidification was reduced. In control preparations, atropine suppressed respiratory responses to acidification, while hexamethonium did not. By contrast, in nicotine-exposed preparations, hexamethonium blocked chemosensory responses but atropine did not. Our results indicate that pre-natal nicotine exposure switches cholinergic mechanisms of central chemosensory responses from muscarinic receptors to nicotinic receptors. Modification of the cholinergic contribution to central chemoreception may produce respiratory dysfunctions, as suggested by receptor-binding studies in victims of SIDS.

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.

Endogenous noradrenaline affects the maturation and function of the respiratory network: Possible implication for SIDS

Breathing is a vital, rhythmic motor act that is required for blood oxygenation and oxygen delivery to the whole body. Therefore, the brainstem network responsible for the elaboration of the respiratory rhythm must function from the very first moments of extrauterine life. In this review, it is shown that the brainstem noradrenergic system plays a pivotal role in both the modulation and the maturation of the respiratory rhythm generator. Compelling evidence are reported demonstrating that genetically induced alterations of the noradrenergic system in mice affect the prenatal maturation and the perinatal function of the respiratory rhythm generator and have drastic consequences on postnatal survival. Sudden Infant Death Syndrome (SIDS), the leader cause of infant death in industrialised countries, may result from cardiorespiratory disorders during sleep. As several cases of SIDS have been observed in infants having noradrenergic deficits, a possible link between prenatal alteration of the noradrenergic system, altered maturation and function of the respiratory network and SIDS is suggested.

Development and pH sensitivity of the respiratory rhythm of fetal mice in vitro

In newborn and adult mammals, chemosensory drive exerted by CO(2) and H(+) provides an essential tonic input: without it the rhythm of respiration is abolished. It is not known, however, whether this chemosensory drive and the respiratory rhythm appear simultaneously during development. In isolated brainstem-spinal cord preparations from fetal mice, we determined at what stage of fetal life the respiratory rhythm appeared in third to fifth cervical ventral roots (phrenic motoneurons) and whether this fetal rhythm was sensitive to chemosensory inputs. A respiratory-like rhythm consisting of short duration bursts of discharges recurring at 2-16 min(-1) was detected in two of nine embryonic day 13 fetuses; it was abolished by transection of the spinal cord between the first to second cervical segments and was phase-related to rhythmic activity from medullary units of the ventral respiratory group. At embryonic day 13, it coexisted with a slow rhythm (0.1-2.0 min(-1)) of long duration bursts of action potentials which was generated by the spinal cord. At later fetal stages, the respiratory-like rhythm became more robust and of higher frequency, while the spinal cord rhythm became less obvious. At all fetal stages, acidification of the superfusion medium from pH 7.5-7.2 or 7.4-7.3 or 7.4 to 7.2 increased the frequency of both the respiratory-like and the spinal cord rhythms. In addition, acidification reduced the amplitude of the integrated burst activity of the spinal cord rhythm of embryonic day 13-embryonic day 16 fetuses and the respiratory-like rhythm of embryonic day 17 and older fetuses. Our results indicate that the rhythms transmitted by phrenic motoneurons during fetal development are chemosensitive from early fetal stages. Through its effects on induction and patterning of the rhythm, chemosensory drive may play a role in activity-dependent formation of respiratory neural networks.

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

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

Somatostatin inhibition of fictive respiration is modulated by pH

We studied the respiratory effects of the tetradecapeptide somatostatin (SST) upon fictive respiration using the in vitro brain stem spinal cord preparation from new-born mouse. We found that SST inhibits respiration, an effect that was potentiated when the chemical drive to respiration was increased. SST inhibited fictive respiration decreasing both the frequency and amplitude in a dose-dependent way. SST inhibition was not antagonized by cyclosomatostatin (cyclo [7-aminoheptanoyl-Phe-D-Trp-Lys-Thr(Bzl)]), a putative SST antagonist, which in contrast behaved as a partial agonist. When the chemical drive to respiration was increased, by lowering the pH of the brain stem superfusion medium from 7.4 to 7.3, the inhibitory effect of SST on respiratory frequency was potentiated. These results suggest an interaction between SST and respiratory central chemoreception in new-born mouse.

Effect of hypothermia on respiratory rhythm generation in hamster brainstem–spinal cord preparations

This study examined the effect of hypothermia on respiratory neural output from brainstem-spinal cord preparations of a cold tolerant rodent, the Syrian hamster. Brainstem-spinal cords from neonatal hamsters (0-6 days) were placed in a recording dish and respiratory-like neural activity was recorded from roots of the first cervical nerve. The preparations were cooled and warmed in a continuous or stepwise fashion. Inputs from the pons completely inhibited neural activity under steady state conditions. With the pons removed, fictive breathing was robust. Cooling caused respiratory arrest, followed by spontaneous resumption of activity on re-warming. Preparations from older hamsters (4-6 days old) were more cold tolerant than younger preparations (0-3 days old). Motor discharge was episodic during continuous cooling, and seizure-like discharge was observed during continuous warming. These phenomena were not observed with stepwise temperature changes suggesting that transient temperature effects on membrane properties may be involved. These preparations were not as cold tolerant as hamster pups in vivo but they retained the ability to autoresuscitate at all ages studied.