Consequences of gestational stress on GABAergic modulation of respiratory activity in developing newborn pups

Compensatory changes in GABAergic inhibition are differentially expressed in the respiratory network to promote function following hibernation

The respiratory network must produce consistent output throughout an animal's life. Although respiratory motor plasticity is well appreciated, how plasticity mechanisms are organized to give rise to robustness following perturbations that disrupt breathing is less clear. During underwater hibernation, respiratory neurons of bullfrogs remain inactive for months, providing a large disturbance that must be overcome to restart breathing. As a result, motoneurons upregulate excitatory synapses to promote the drive to breathe. Reduced inhibition often occurs in parallel with increased excitation, yet the loss of inhibition can destabilize respiratory motor output. Thus, we hypothesized that GABAergic inhibition would decrease following hibernation, but this decrease would be expressed differentially throughout the network. We confirmed that respiratory frequency was under control of GABAAR signaling, but after hibernation, it became less reliant on inhibition. The loss of inhibition was confined to the respiratory rhythm-generating centers: non-respiratory motor activity and large seizure-like bursts were similarly triggered by GABAA receptor blockade in controls and hibernators. Supporting reduced presynaptic GABA release, firing rate of respiratory motoneurons was constrained by a phasic GABAAR tone, but after hibernation, this tone was decreased despite the same postsynaptic receptor strength as controls. Thus, selectively reducing inhibition in respiratory premotor networks promotes stability of breathing, while wholesale loss of GABAARs causes non-specific hyperexcitability throughout the brainstem. These results suggest that different parts of the respiratory network select distinct strategies involving either excitation (motoneurons) or inhibition (rhythm generator) to minimize pathological network states when engaging plasticity that protects the drive to breathe.

Prenatal Hypoxia Induces Cl– Cotransporters KCC2 and NKCC1 Developmental Abnormality and Disturbs the Influence of GABAA and Glycine Receptors on Fictive Breathing in a Newborn Rat

Prenatal hypoxia is a recognised risk factor for neurodevelopmental disorders associated with both membrane proteins involved in neuron homeostasis, e.g., chloride (Cl–) cotransporters, and alterations in brain neurotransmitter systems, e.g., catecholamines, dopamine, and GABA. Our study aimed to determine whether prenatal hypoxia alters central respiratory drive by disrupting the development of Cl– cotransporters KCC2 and NKCC1. Cl– homeostasis seems critical for the strength and efficiency of inhibition mediated by GABAA and glycine receptors within the respiratory network, and we searched for alterations of GABAergic and glycinergic respiratory influences after prenatal hypoxia. We measured fictive breathing from brainstem in ex vivo preparations during pharmacological blockade of KCC2 and NKCC1 Cl– cotransporters, GABAA, and glycine receptors. We also evaluated the membrane expression of Cl– cotransporters in the brainstem by Western blot and the expression of Cl– cotransporter regulators brain-derived neurotrophic factor (BDNF) and calpain. First, pharmacological experiments showed that prenatal hypoxia altered the regulation of fictive breathing by NKCC1 and KCC2 Cl– cotransporters, GABA/GABAA, and glycin. NKCC1 inhibition decreased fictive breathing at birth in control mice while it decreased at 4 days after birth in pups exposed to prenatal hypoxia. On the other hand, inhibition of KCC2 decreased fictive breathing 4 days after birth in control mice without any change in prenatal hypoxia pups. The GABAergic system appeared to be more effective in prenatal hypoxic pups whereas the glycinergic system increased its effectiveness later. Second, we observed a decrease in the expression of the Cl– cotransporter KCC2, and a decrease with age in NKCC1, as well as an increase in the expression of BDNF and calpain after prenatal hypoxia exposure. Altogether, our data support the idea that prenatal hypoxia alters the functioning of GABAA and glycinergic systems in the respiratory network by disrupting maturation of Cl– homeostasis, thereby contributing to long-term effects by disrupting ventilation.

Thyroid hormones during the perinatal period are necessary to respiratory network development of newborn rats

Thyroid hormones (THs) are essential for foetal brain development. Because the gestating mother is the main source of THs to the foetus, maternal hypothyroidism and/or premature birth compromise neurological outcomes in the offspring. Respiratory instability and recurrent apneas due to immaturity of the respiratory control network are major causes of morbidity in infants. Inadequate TH supply may be sufficient to delay perinatal maturation of the respiratory control system; however, this hypothesis remains untested. To address this issue, maternal hypothyroidism was induced by adding methimazole (MMI; 0.02% w/v) to the drinking water of pregnant dams from conception to postpartum day 4 (P4). The effect of TH supplementation on respiratory function was tested by injecting levothyroxine (L-T4) in newborns at P1. Respiratory function was assessed by plethysmography (in vivo) and recording of phrenic output from medullary preparations (in vitro). By comparison with controls, TH deficiency increased the frequency of apneas and decreased basal ventilation in vivo and prevented the age-dependent increase in phrenic burst frequency normally observed in vitro. The effects of TH deficiency on GABAergic modulation of respiratory activity were measured by bath application of muscimol (GABA_A agonist) or bicuculline (GABA_A antagonist). The phrenic burst frequency responses to GABAergic agents were consistently greater in preparations from TH deficient pups. L-T4 supplementation reversed part of the respiratory anomalies related to MMI treatment in vitro. We conclude that TH deficiency during the perinatal period is sufficient to delay maturation of the respiratory control network development. Excessive GABAergic inhibition may contribute to this effect.

Sex-Specific Effects of Stress on Respiratory Control: Plasticity, Adaptation, and Dysfunction

As our understanding of respiratory control evolves, we appreciate how the basic neurobiological principles of plasticity discovered in other systems shape the development and function of the respiratory control system. While breathing is a robust homeostatic function, there is growing evidence that stress disrupts respiratory control in ways that predispose to disease. Neonatal stress (in the form of maternal separation) affects "classical" respiratory control structures such as the peripheral O₂ sensors (carotid bodies) and the medulla (e.g., nucleus of the solitary tract). Furthermore, early life stress disrupts the paraventricular nucleus of the hypothalamus (PVH), a structure that has emerged as a primary determinant of the intensity of the ventilatory response to hypoxia. Although underestimated, the PVH's influence on respiratory function is a logical extension of the hypothalamic control of metabolic demand and supply. In this article, we review the functional and anatomical links between the stress neuroendocrine axis and the medullary network regulating breathing. We then present the persistent and sex-specific effects of neonatal stress on respiratory control in adult rats. The similarities between the respiratory phenotype of stressed rats and clinical manifestations of respiratory control disorders such as sleep-disordered breathing and panic attacks are remarkable. These observations are in line with the scientific consensus that the origins of adult disease are often found among developmental and biological disruptions occurring during early life. These observations bring a different perspective on the structural hierarchy of respiratory homeostasis and point to new directions in our understanding of the etiology of respiratory control disorders. © 2021 American Physiological Society. Compr Physiol 11:1-38, 2021.

Sex- and Region-Specific Differences in the Transcriptomes of Rat Microglia from the Brainstem and Cervical Spinal Cord

The neural control system underlying breathing is sexually dimorphic with males being more vulnerable to dysfunction. Microglia also display sex differences, and their role in the architecture of brainstem respiratory rhythm circuitry and modulation of cervical spinal cord respiratory plasticity is becoming better appreciated. To further understand the molecular underpinnings of these sex differences, we performed RNA sequencing of immunomagnetically isolated microglia from brainstem and cervical spinal cord of adult male and female rats. We used various bioinformatics tools (Gene Ontology, Kyoto Encyclopedia of Genes and Genomes, Reactome, STRING, MAGICTRICKS) to functionally categorize identified gene sets, as well as to pinpoint common transcriptional gene drivers that may be responsible for the observed transcriptomic differences. We found few sex differences in the microglial transcriptomes derived from the brainstem, but several hundred genes were differentially expressed by sex in cervical spinal microglia. Comparing brainstem and spinal microglia within and between sexes, we found that the major factor guiding transcriptomic differences was central nervous system (CNS) location rather than sex. We further identified key transcriptional drivers that may be responsible for the transcriptomic differences observed between sexes and CNS regions; enhancer of zeste homolog 2 emerged as the predominant driver of the differentially downregulated genes. We suggest that functional gene alterations identified in metabolism, transcription, and intercellular communication underlie critical microglial heterogeneity and sex differences in CNS regions that contribute to respiratory disorders categorized by dysfunction in neural control. These data will also serve as an important resource data base to advance our understanding of innate immune cell contributions to sex differences and the field of respiratory neural control. SIGNIFICANCE STATEMENT: The contributions of central nervous system (CNS) innate immune cells to sexually dimorphic differences in the neural circuitry controlling breathing are poorly understood. We identify key transcriptomic differences, and their transcriptional drivers, in microglia derived from the brainstem and the C3-C6 cervical spinal cord of healthy adult male and female rats. Gene alterations identified in metabolism, gene transcription, and intercellular communication likely underlie critical microglial heterogeneity and sex differences in these key CNS regions that contribute to the neural control of breathing.

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.

Inhibitory respiratory responses to progesterone and allopregnanolone in newborn rats chronically treated with caffeine

KEY POINTS

In premature newborns, recurrent apnoea is systematically treated with caffeine to prevent long-term neurocognitive disorders, but a substantial percentage of apnoea persists particularly in neonates born before 28 weeks of gestation. Progesterone has been proposed as a respiratory stimulant potentially suitable for the treatment of newborn apnoea persistent to caffeine. Accordingly we asked whether acute progesterone administration reduces apnoea frequency in newborn rats treated with caffeine. Surprisingly our results show that in newborn rats treated with caffeine, administration of progesterone inhibits breathing and increases apnoea frequency. Additional experiments showed an enhanced GABAergic inhibitory drive on breathing after caffeine treatment, and that progesterone is converted to allopregnanolone (an allosteric modulator of GABAA receptors) to inhibit breathing. We conclude that combining progesterone and chronic caffeine is not an option in preterm neonates, unless the effects of allopregnanolone can be counteracted.

ABSTRACT

Caffeine is the main treatment for apnoea in preterm neonates, but its interactions with other respiratory stimulants like progesterone are unknown. We tested the hypothesis that the addition of progesterone to caffeine treatments further stimulates ventilation. Newborn rats were treated with water (control) or caffeine (15 mg kg(-1)) by daily gavage between postnatal day (P)3 and P12. At P4 and P12, we measured apnoea frequency, ventilatory responses and metabolic parameters under both normoxia and hypoxia (12% O2, 20 min) following an acute administration of either saline or progesterone (4 mg kg(-1); i.p.). Progesterone injection increased the serum levels of both progesterone and its neuroactive metabolite allopregnanolone. Progesterone had no effect on ventilation in control rats under normoxia. Progesterone depressed ventilation in P12 caffeine-treated rats under normoxia and hypoxia and increased apnoea frequency in both P4 and P12 rats. Because allopregnanolone is an allosteric modulator of GABAA receptors and caffeine may enhance GABAergic inhibition in newborns, we studied the effects of the GABAA receptor antagonist bicuculline at 0, 1, 2 and 3 mg kg(-1) doses and allopregnanolone (10 mg kg(-1) dose) in P12 rats. In caffeine-treated rats, bicuculline enhanced ventilation, while allopregnanolone decreased ventilation and increased total apnoea time. Progesterone had no effect on ventilation and apnoea frequency in caffeine-treated rats injected with finasteride, which blocks the conversion of progesterone to allopregnanolone. We conclude that combining progesterone and chronic caffeine therapy is not an option for the treatment of persistent apnoea in preterm neonates, unless the effects of allopregnanolone can be counteracted.