Effects of CO2 on time-dependent changes in cardiorespiratory functions under sustained hypoxia.

Hypercapnia in addition to hypoxia affects the mammalian cardiorespiratory system and has been suggested to exert its effects on cardiorespiratory function by slightly different mechanisms to hypoxia. In the present study, we examined cardiorespiratory changes in urethane-anesthetized rats under hypocapnic (Hypo, 10% O2), isocapnic (Iso, 10% O2 and 4% CO2), and hypercapnic (Hyper, 10% O2 and 8% CO2) hypoxia for 2 h to clarify the effects of CO2 on sustained hypoxia-induced cardiorespiratory responses. Respiratory frequency increased the most in Hypo and tidal volume in Hyper. Minute ventilation, a product of respiratory frequency and tidal volume, increased the most in the latter group. Regarding cardiovascular variables during the hypoxic exposure period, heart rate and mean blood pressure both markedly decreased in Hypo. However, decreases in these parameters were small in Iso, and both increased over the pre-exposure level in Hyper. The present results suggest that CO2 interferes with the hypoxia-activated neural pathway via another pathway under sustained exposure to hypoxia.

Serotonergic projections to the ventral respiratory column from raphe nuclei in rats.

The ventral respiratory column (VRC) generates rhythmical respiration and is divided into four compartments: the Bötzinger complex (BC), pre-Bötzinger complex (PBC), rostral ventral respiratory group (rVRG), and caudal ventral respiratory group (cVRG). Serotonergic nerve fibers are densely distributed in the rostral to caudal VRC and serotonin would be one of the important modulators for the respiratory control in the VRC. In the present study, to elucidate detailed distribution of serotonergic neurons in raphe nuclei projecting to the various rostrocaudal levels of VRC, we performed combination of retrograde tracing technique by cholera toxin B subunit (CTB) with immunohistochemistry for tryptophan hydroxylase 2 (TPH2). The double-immunoreactive neurons with CTB and TPH2 were distributed in the both rostral and caudal raphe nuclei, i.e. dorsal raphe nucleus, raphe magnus nucleus, gigantocellular reticular nucleus alpha and ventral parts, lateral paragigantocellular nucleus, parapyramidal area, raphe obscurus nucleus, and raphe pallidus nucleus. The distributions of double-immunoreactive neurons were similar among injection groups of BC, PBC, anterior rVRG, and posterior rVRG/cVRG. In conclusion, serotonergic neurons in both rostral and caudal raphe nuclei projected throughout the VRC and these serotonergic projections may contribute to respiratory responses to various environmental and vital changes.

Time-dependent changes in cardiorespiratory functions of anesthetized rats exposed to sustained hypoxia.

Although cardiovascular responses may be altered by respiratory changes under prolonged hypoxia, the relationship between respiratory and cardiovascular changes remains unknown. The aim of the present study is to clarify cardiorespiratory changes in anesthetized rats during and after hypoxic conditions using simultaneous recordings of cardiorespiratory variables with 20-sec recording intervals. After air breathing for 20 min (pre-exposure period), rats were subjected to 10% O2 for 2 h (hypoxic exposure period) and then air for 30 min (recovery period). Minute ventilation (VE), respiratory frequency, tidal volume, arterial blood pressure (BP), and heart rate (HR) were continuously monitored during the experimental period. Just after hypoxic exposure, VE, BP, and HR exhibited an overshoot, undershoot, and overshoot followed by a decrease, respectively. During the remaining hypoxic exposure period, continuous high VE and low BP were observed, whereas HR re-increased. In the recovery period, VE, BP, and HR showed an undershoot, increase, and decrease followed by an increase, respectively. These results suggest that the continuation of enhanced VE and re-increased HR, probably, due to carotid body excitation and accompanying sympathetic activation, during the late period of hypoxic exposure are protective responses to avoid worsening hypoxemia and further circulatory insufficiencies under sustained hypoxia.

Hypoxia-induced increases in serotonin-immunoreactive nerve fibers in the medulla oblongata of the rat.

Hypoxia induces respiratory responses in mammals and serotonergic neurons in the medulla oblongata participate in respiratory control. However, the morphological changes in serotonergic neurons induced by hypoxia have not yet been examined and respiratory controls of serotonergic neurons have not been clarified. We herein investigated the distribution of immunoreactivity for serotonin (5-hydroxytryptamine; 5-HT) in the medulla oblongata of control rats and rats exposed to 1-6h of hypoxia (10% O2). We also examined the medulla oblongata by multiple immunofluorescence labeling for 5-HT, neurokinin 1 receptors (NK1R), a marker for some respiratory neurons in the pre-Bötzinger complex (PBC), and dopamine ß-hydroxylase (DBH), a marker for catecholaminergic neurons. The number of 5-HT-immunoreactive nerve cell bodies in the raphe nuclei was higher in rats exposed to hypoxia than in control rats. The number of 5-HT-immunoreactive nerve fibers significantly increased in the rostral ventrolateral medulla of rats exposed to 1-6h of hypoxia, caudal ventrolateral medulla of rats exposed to 2-6h of hypoxia, and lateral part of the nucleus of the solitary tract and dorsal motor nucleus of the vagus nerve of rats exposed to 1-2h of hypoxia. Multiple immunofluorescence labeling showed that 5-HT-immunoreactive nerve fibers were close to NK1R-immunoreactive neurons in ventrolateral medulla and to DBH-immunoreactive neurons in the medulla. These results suggest that serotonergic neurons partly regulate respiratory control under hypoxic conditions by modulating the activity of NK1R-expressing and catecholaminergic neurons.

Differences in respiratory changes and Fos expression in the ventrolateral medulla of rats exposed to hypoxia, hypercapnia, and hypercapnic hypoxia.

Respiratory responses to hypoxia and/or hypercapnia, and their relationship to neural activity in the ventrolateral medulla (VLM), which includes the respiratory center, have not yet been elucidated in detail. We herein examined respiratory responses during exposure of 10% O2 (hypoxia), 10% CO2 (hypercapnia), and 10% O2-10% CO2 (hypercapnic hypoxia) using plethysmography. In addition to recording respiration, Fos expressions were examined in the VLM of the rat exposed to each gas to analyze neural activity. Respiratory frequency was increased in rats exposed to hypoxia, and Fos-positive neurons were observed in the caudal VLM (cVLM) and medial VLM (mVLM). Tidal volume was increased in rats exposed to hypercapnia, and Fos-positive neurons were observed in the rostral VLM (rVLM) includes the retrotrapezoid nucleus (RTN) and mVLM. Tidal volume was enhanced in rats exposed to hypercapnic hypoxia, similar to that in hypercapnia-exposed rats, and Fos-positive neurons were observed in the entire region of the VLM. In the mVLM and cVLM, double immunofluorescence showed Fos-immunoreactive nerve cells were also immunoreactive to dopamine ß-hydroxylase, the marker for A1/C1 catecholaminergic neuron. These results suggested that hypoxia and hypercapnia modulated rhythmogenic microcircuits in the mVLM via A1/C1 neurons and the RTN, respectively.