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Human and animal studies on sleep-disordered breathing and respiratory regulation show that the effects of sex hormones are heterogeneous. Because neonatal stress results in sex-specific disruption of the respiratory control in adult rats, we postulate that it might affect respiratory control modulation induced by ovarian steroids in female rats. The hypoxic ventilatory response (HVR) of adult female rats exposed to neonatal maternal separation (NMS) is 30% smaller than controls (24), but consequences of NMS on respiratory control in aging female rats are unknown. To address this issue, whole body plethysmography was used to evaluate the impact of NMS on the HVR (12% O2, 20 min) of middle-aged (MA; 57 wk old) female rats. Pups subjected to NMS were placed in an incubator 3 h/day for 10 consecutive days (P3 to P12). Controls were undisturbed. To determine whether the effects were related to sexual hormone decline or aging per se, experiments were repeated on bilaterally ovariectomized (OVX) young (12 wk old) adult female rats. OVX and MA both reduced the HVR significantly in control rats but had little effect on the HVR of NMS females. OVX (but not aging) reduced the anapyrexic response in both control and NMS animals. These results show that hormonal decline decreases the HVR of control animals, while leaving that of NMS female animals unaffected. This suggests that neonatal stress alters the interaction between sex hormone regulation and the development of body temperature, hormonal, and ventilatory responses to hypoxia. [ABSTRACT FROM AUTHOR]

Recurrent apneas are important causes of hospitalization and morbidity in newborns. Gestational stress (GS) compromises fetal brain development. Maternal stress and anxiety during gestation are linked to respiratory disorders in newborns; however, the mechanisms remain unknown. Here, we tested the hypothesis that repeated activation of the neuroendocrine response to stress during gestation is sufficient to disrupt the development of respiratory control and augment the occurrence of apneas in newborn rats. Pregnant dams were displaced and exposed to predator odor from days 9 to 19 of gestation. Control dams were undisturbed. Experiments were performed on male and female rats aged between 0 and 4 d old. Apnea frequency decreased with age but was consistently higher in stressed pups than controls. At day 4, GS augmented the proportion of apneas with 02 desaturations by 12%. During acute hypoxia (12% 02), the reflexive increase in breathing augmented with age; however, this response was lower in stressed pups. Instability of respiratory rhythm recorded from medullary preparations decreased with age but was higher in stressed pups than controls. GS reduced medullary serotonin (5-HT) levels in newborn pups by 32%. Bath application of 5-HT and injection of 8-OH-DPAT [(±)-8-hydroxy-2-di-(n-propylamino) tetralin hydrobromide; 5-HT1A agonist; in vivo] reduced respiratory instability and apneas; these effects were greater in stressed pups than controls. Sex-specific effects were observed. We conclude that activation of the stress response during gestation is sufficient to disrupt respiratory control development and promote pathological apneas in newborn rats. A deficit in medullary 5-HT contributes to these effects. [ABSTRACT FROM AUTHOR]

Abstract: Neonatal maternal separation (NMS) is a form of stress that has persistent, sex-specific effects on respiratory control development. In adult male (but not female) rats, NMS increases the hypoxic ventilatory response and augments respiratory instability during sleep; however, the effects of NMS on respiratory control prior to puberty are unknown. This study tested the hypothesis that NMS augments respiratory instability and the O2 chemosensitivity in 12days old rats. Pups subjected to NMS were placed in an incubator 3h/day from P3 to P12. Controls were undisturbed. Breathing and apnea index were measured in normoxia with plethysmography. The ventilatory chemoreflex was assessed by measuring the increase in breathing frequency upon brief exposure to a hypoxic challenge (; 5min). In normoxia, NMS augments the coefficient of variation (CV; an index of respiratory instability) of respiratory frequency and the number of apneas; this effect was more pronounced in male pups. We conclude that disruption of respiratory regulation by NMS is already apparent at P12. Based on results showing that plasma corticosterone levels of NMS pups were still elevated one day after the last NMS episode and the lack of effect of NMS on the O2 chemoreflex index, we propose that disruption of hypothalamic regulation contributes to respiratory instability in NMS pups. [Copyright &y& Elsevier]

To better understand serotonergic modulation of air breathing during bullfrog development, we measured changes in fictive lung ventilation frequency associated with focal stimulation of the rostral region of the medullary Raphé neurons. Electrical (3 to 33 Hz) and chemical (glutamate microinjections; 0.5 mol l-1, 0.3-10 nl) activation of Raphé neurons was performed in brainstem preparations from three developmental stages (pre- and metamorphic tadpoles and adult frogs). Fictive lung ventilation was recorded extracelluarly from the Vth and Xth cranial nerves. Electrical stimulation of Raphé neurons caused a frequency-dependent increase in lung burst frequency in pre-metamorphic tadpoles only. In metamorphic tadpoles, an increase in fictive lung ventilation was observed at 20 Hz only. Electrical stimulation had no effect in preparations from adult frogs. Glutamate microinjections elicited similar responses as a lung burst frequency increase was observed in the pre- metamorphic group only. Regardless of the stimulation technique used, the increase in fictive lung ventilation was attenuated by the selective 5-HT3 antagonist tropisetron (5-20 µmoI l-1). Results from immunohistochemical analysis of the Raphé region stimulated do not correlate with functional data as the number of 5-HT immunoreactive neurons within this region increases during development. We conclude that, in this preparation, stimulation of lung ventilation by the medullary Raphé is restricted to early (pre-metamorphic) stages. [ABSTRACT FROM AUTHOR]

Neonatal maternal separation (NMS) is a form of stress that exerts persistent, sex-specific effects on the hypoxic ventilatory response. Adult male rats previously subjected to NMS show a 25% increase in the response, whereas NMS females show a response 30% lower than controls (8). To assess the extent to which NMS affects ventilatory control development, we tested the hypothesis that NMS alters the ventilatory response to hypercapnia in awake, unrestrained rats. Pups subjected to NMS were placed in a temperature- and humidity-controlled incubator 3 h/day for 10 consecutive days (P3 to P12). Control pups were undisturbed. At adulthood (8 to 10 wk old), rats were placed in a plethysmography chamber for measurement of ventilatory parameters under baseline and hypercapnic conditions (inspired CO2 fraction = 0.05). After 20 mm of hypercapnia, the minute ventilation response measured in NMS males was 47% less than controls, owing to a lower tidal volume response (22%). Conversely, females previously subjected to NMS showed minute ventilation and tidal volume responses 63 and 18% larger than controls respectively. Although a lower baseline minute ventilation contributes to this effect, the higher minute ventilation/ CO2 production response observed in NMS females suggests a greater responsiveness to CO2/H+ in this group. We conclude that NMS exerts sex-specific effects on the hypercapnic ventilatory response and that the neural mechanisms affected by NMS likely differ from those involved in the hypoxic chemoreflex. [ABSTRACT FROM AUTHOR]

In awake animals, our laboratory recently showed that the hypoxic ventilatory response of adult male (but not female) rats previously subjected to neonatal maternal separation (NMS) is 25% greater than controls (Genest SE, Gulemetova R, Laforest S, Drolet G, and Kinkead R. J Physiol 554: 543-557, 2004). To begin mechanistic investigations of the effects of this neonatal stress on respiratory control development, we tested the hypothesis that, in male rats, NMS enhances central integration of carotid body chemoafferent signals. Experiments were performed on two groups of adult male rats. Pups subjected to NMS were placed in a temperature-controlled incubator 3 h/day from postnatal day 3 to postnatal day 12. Control pups were undisturbed. At adulthood (8-10 wk), rats were anesthetized (urethane; 1.6 g/kg), paralyzed, and ventilated with a hyperoxic gas mixture [inspired O2 fraction (FIO2) = 0.5], and phrenic nerve activity was recorded. The first series of experiments aimed to demonstrate that NMS-related enhancement of the inspiratory motor output (phrenic) response to hypoxia occurs in anesthetized animals also. In this series, rats were exposed to moderate, followed by severe, isocapnic hypoxia (FIO2 = 0.12 and 0.08, respectively, 5 min each). NMS enhanced both the frequency and amplitude components of the phrenic response to hypoxia relative to controls, thereby validating the use of this approach. In a second series of experiments, NMS increased the amplitude (but not the frequency) response to unilateral carotid sinus nerve stimulation (stimulation frequency range: 0.5-33 Hz). We conclude that enhancement of central integration of carotid body afferent signal contributes to the larger hypoxic ventilatory response observed in NMS rats. [ABSTRACT FROM AUTHOR]

We tested the hypothesis that 5-HT2A/C receptors contribute to the modulation of respiratory motor output elicited by serotonin bath-application in the in vitro brainstem preparation from bullfrog tadpoles. Preparations were superfused with (1) a selective 5-HT2A/C receptor agonist (DOI, 0.5–25 μM) or (2) 5-HT in presence of a non-selective 5-HT2A/C receptor antagonist (methysergide maleate, 10 μM). Stage-related differences in 5-HT2A/C modulation were assessed by performing experiments on two groups: pre- and post-metamorphic tadpoles. 5-HT2A/C receptor activation with DOI did not alter either gill or lung burst frequency. Methysergide did not prevent the gill burst frequency attenuation elicited by 5-HT. Our results show no apparent contribution of the 5-HT2A/C receptor subtypes to the serotonergic modulation of either gill- or lung-related neural activity in either group. While 5-HT2A/C receptors are involved in the modulation of respiratory motor output in mammals, these receptor subtypes do not appear to play a significant role in this species. [ABSTRACT FROM AUTHOR]

Much like locomotion or micturition, respiration is a rhythmic and stereotyped motor pattern controlled mainly by non-cortical structures including a complex circuit in the brainstem. Because tight regulation of lung ventilation is essential from the beginning of life, it has been presumed that the neural system regulating breathing is fixed, following a genetically predetermined developmental pattern. Here, we review evidence indicating that early life exposure to a non-systemic stress in the form of neonatal maternal separation (NMS) is sufficient to exert sex-specific consequences on the developmental trajectory of this vital homeostatic system that persist well into full maturity. At adulthood, male rats subjected to NMS are hypertensive and show an abnormally high hypoxic chemoreflex that correlates positively with respiratory instability during sleep. The effects are not observed in females. Investigation of the mechanisms this respiratory phenotype have highlighted the importance of 1) neuroendocrine influences on respiratory regulation and 2) stress-related imbalance between inhibitory (GABAergic) and excitatory (glutamatergic) modulation of the neural elements regulating breathing. These results provide new and valuable insight into the origins of respiratory disorders related to neural control dysfunction such as sleep disordered breathing.

To better understand serotonergic modulation of air breathing during bullfrog development, we measured changes in fictive lung ventilation frequency associated with focal stimulation of the rostral region of the medullary Raphé neurons. Electrical (3 to 33 Hz) and chemical (glutamate microinjections; 0.5 mol l(-1), 0.3-10 nl) activation of Raphé neurons was performed in brainstem preparations from three developmental stages (pre- and metamorphic tadpoles and adult frogs). Fictive lung ventilation was recorded extracelluarly from the Vth and Xth cranial nerves. Electrical stimulation of Raphé neurons caused a frequency-dependent increase in lung burst frequency in pre-metamorphic tadpoles only. In metamorphic tadpoles, an increase in fictive lung ventilation was observed at 20 Hz only. Electrical stimulation had no effect in preparations from adult frogs. Glutamate microinjections elicited similar responses as a lung burst frequency increase was observed in the pre-metamorphic group only. Regardless of the stimulation technique used, the increase in fictive lung ventilation was attenuated by the selective 5-HT3 antagonist tropisetron (5-20 micromol l(-1)). Results from immunohistochemical analysis of the Raphé region stimulated do not correlate with functional data as the number of 5-HT immunoreactive neurons within this region increases during development. We conclude that, in this preparation, stimulation of lung ventilation by the medullary Raphé is restricted to early (pre-metamorphic) stages.

The Hoxa5 mutation is associated with a high perinatal mortality rate caused by a severe obstruction of the laryngotracheal airways, pulmonary dysmorphogenesis, and a decreased production of surfactant proteins. Surviving Hoxa5(-/-) mutant mice also display lung anomalies with deficient alveolar septation and areas of collapsed tissue, thus demonstrating the importance of Hoxa5 throughout lung development and maturation. Here, we address the functional consequences of the Hoxa5 mutation on respiration and chemoreflexes by comparing the breathing pattern of Hoxa5(-/-) mice to that of wild-type animals under resting conditions and during exposure to moderate ventilatory stimuli such as hypoxia and hypercapnia. Resting Hoxa5(-/-) mice present a higher breathing frequency and overall minute ventilation that likely compensate for their reduced lung alveolar surface available for gas exchange and their increased upper airway resistance. When exposed to ventilatory stimuli, Hoxa5(-/-) mice maintain the higher minute ventilation by adapting the tidal volume and/or the breathing frequency. The minute ventilation increase seen during hypoxia was similar for both groups of mice; however, the dynamics of the frequency response was genotype-dependent. The hypercapnic ventilatory response did not differ between genotypes. These findings reveal the strategies allowing survival of Hoxa5(-/-) mice facing morphologic anomalies leading to a significant deficit in gas exchange capacity.