Serotonin receptor subtypes required for ventilatory long-term facilitation and its enhancement after chronic intermittent hypoxia in awake rats

Respiratory long-term facilitation (LTF), a serotonin-dependent, persistent augmentation of respiratory activity after episodic hypoxia, is enhanced by pretreatment of chronic intermittent hypoxia (CIH; 5 min 11-12% O2-5 min air, 12 h/night for 7 nights). The present study examined the effects of methysergide (serotonin 5-HT1,2,5,6,7 receptor antagonist), ketanserin (5-HT2 antagonist), or clozapine (5-HT2,6,7 antagonist) on both ventilatory LTF and the CIH effect on ventilatory LTF in conscious male adult rats to determine which specific receptor subtype(s) is involved. In untreated rats (i.e., animals not exposed to CIH), LTF, induced by five episodes of 5-min poikilocapnic hypoxia (10% O2) separated by 5-min normoxic intervals, was measured twice by plethysmography. Thus the measurement was conducted 1-2 days before (as control) and approximately 1 h after systemic injection of methysergide (1 mg/kg ip), ketanserin (1 mg/kg), or clozapine (1.5 mg/kg). Resting ventilation, metabolic rate, and hypoxic ventilatory response (HVR) were unchanged, but LTF ( approximately 18% above baseline) was eliminated by each drug. In CIH-treated rats, LTF was also measured twice, before and approximately 8 h after CIH. Vehicle, methysergide, ketanserin, or clozapine was injected approximately 1 h before the second measurement. Neither resting ventilation nor metabolic rate was changed after CIH and/or any drug. HVR was unchanged after methysergide and ketanserin but reduced in four of seven clozapine rats. The CIH-enhanced LTF ( approximately 28%) was abolished by methysergide and clozapine but only attenuated by ketanserin (to approximately 10%). Collectively, these data suggest that ventilatory LTF requires 5-HT2 receptors and that the CIH effect on LTF requires non-5-HT2 serotonin receptors, probably 5-HT6 and/or 5-HT7 subtype(s).     

In hamsters dopamine D2 receptors affect ventilation during and following intermittent hypoxia

We tested the hypothesis that in golden Syrian hamsters (Mesocricetus auratus) carotid body dopaminergic D2 receptors modulate ventilation in air, during exposure to intermittent hypoxia (IH) and reoxygenation. Ventilation was evaluated using the barometric method and CO2 production was determined using the flow through method. Hamsters (n=8) received either subcutaneous injections of vehicle, haloperidol (0.5 mg/kg) or domperidone (0.5 mg/kg). Ventilatory and metabolic variables were determined 30 min following injections, after each of 5 bouts of 5 min of 10% oxygen interspersed by normoxia (IH), and 15, 30, 45 and 60 min following IH when hamsters were exposed to air. Haloperidol, but not domperidone decreased body temperature in hamsters. Neither treatment affected CO2 production. Vehicle-treated hamsters exhibited ventilatory long-term facilitation (VLTF) following IH. Haloperidol or domperidone decreased ventilation in air, during IH and eliminated VLTF due to changes in tidal volume and not frequency of breathing. Thus, in hamsters D2 receptors are involved in control of body temperature and ventilation during and following IH.     

Ventilatory long-term facilitation is greater in 1- vs. 2-mo-old awake rats.

Respiratory long-term facilitation (LTF) declines in middle-aged vs. adult male rats. Chronic intermittent hypoxia (CIH; 5 min 11-12% O2/5 min air, 12 h/night, 7 nights) enhances LTF in adult rats. However, LTF in immature rats and the effect of early CIH are unevaluated. The present study compared LTF in 1- and 2-mo-old rats and examined the effect of neonatal CIH (initiated at 2 days after birth) on the LTF. Ventilatory LTF, elicited by 5 (protocol 1) or 10 (protocol 2) episodes of poikilocapnic hypoxia (5 min 12% O2/5 min air), was measured twice by plethysmography on the same male conscious rat when it was 1 and 2 mo old. In untreated (without CIH) rats, both resting ventilation (54.7 +/- 0.6 vs. 43.0 +/- 0.2 ml.100 g(-1).min(-1)) and hypoxic ventilatory response (131 +/- 4 vs. 66 +/- 3% above baseline) were greater in 1- vs. 2-mo-old rats. Protocol 1 elicited LTF in 1-mo-old (12.5 +/- 1.0% above baseline) but not 2-mo-old rats. Protocol 2 elicited a greater LTF in 1-mo-old (24.3 +/- 0.8%) vs. 2-mo-old rats (18.2 +/- 0.5%). In CIH-treated rats, protocol 1 also elicited LTF in 1-mo-old (13.1 +/- 1.5%) but not 2-mo-old rats. Protocol 2 elicited LTF in both age groups, but LTF was enhanced by the CIH only in 1-mo-old rats (28.8 +/- 0.9%). These results suggest that ventilatory LTF and hypoxic ventilatory response are greater in male rats shortly before their sexual maturity and that the neonatal CIH somewhat enhances ventilatory LTF approximately 3 wk after CIH, but this enhancement does not last to adulthood.     

Chronic intermittent hypoxia enhances ventilatory long-term facilitation in awake rats.

This study examined the effect of chronic intermittent hypoxia (CIH: 5 min 11-12% O2/5 min air, 12 h/night, 7 nights) on ventilatory long-term facilitation (LTF) and determined the persistence period of this CIH effect in awake rats. LTF, elicited by 5 or 10 episodes of 5 min 12% O2, was measured four times in the same Sprague-Dawley rats by plethysmography, before and 8 h, 3 days, and 7 days after CIH treatment. Resting ventilation was unchanged after CIH. Five episodes of 12% O2 did not initially elicit LTF but elicited LTF (23.5 +/- 1.4% above baseline) 8 h after CIH, which partially remained at 3 days (11.4 +/- 2.2%, P < 0.05) and disappeared at 7 days. Ten episodes initially elicited LTF (17.7 +/- 1.1%, 45-min duration) and elicited an enhanced LTF (29.1 +/- 1.5%, 75 min) 8 h after CIH. These results demonstrated that CIH enhanced ventilatory LTF in conscious, freely behaving rats in two ways: 1) a previously ineffective protocol induced LTF; and 2) LTF magnitude was increased and LTF duration prolonged, and this CIH effect on LTF persisted for at least 3 days.     

Ventilatory long-term facilitation in mice can be observed during both sleep and wake periods and depends on orexin.

Respiratory long-term facilitation (LTF) is a long-lasting (>1 h) augmentation of respiratory motor output that occurs even after cessation of hypoxic stimuli, is serotonin-dependent, and is thought to prevent sleep-disordered breathing such as sleep apnea. Raphe nuclei, which modulate several physiological functions through serotonin, receive dense projections from orexin-containing neurons in the hypothalamus. We examined possible contributions of orexin to ventilatory LTF by measuring respiration in freely moving prepro-orexin knockout mice (ORX-KO) and wild-type (WT) littermates before, during, and after exposure to intermittent hypoxia (IH; 5 x 5 min at 10% O2), sustained hypoxia (SH; 25 min at 10% O2), or sham stimulation. Respiratory data during quiet wakefulness (QW), slow wave sleep (SWS), and rapid-eye-movement sleep were separately calculated. Baseline ventilation before hypoxic stimulation and acute responses during stimulation did not differ between the ORX-KO and WT mice, although ventilation depended on vigilance state. Whereas the WT showed augmented minute ventilation (by 20.0 +/- 4.5% during QW and 26.5 +/- 5.3% during SWS; n = 8) for 2 h following IH, ORX-KO showed no significant increase (by -3.1 +/- 4.6% during QW and 0.3 +/- 5.2% during SWS; n = 8). Both genotypes showed no LTF after SH or sham stimulation. Sleep apnea indexes did not change following IH, even when LTF appeared in the WT mice. We conclude that LTF occurs during both sleep and wake periods, that orexin is necessary for eliciting LTF, and that LTF cannot prevent sleep apnea, at least in mice.     

An evaluation of ventilation in dystrophic Syrian hamsters

Ventilatory responses of 10 control and 10 dystrophic male hamsters to air, hypercapnia, and hypoxia were evaluated at four ages (40, 70, 100, and 140 days). Tidal volume (VT), frequency (f), minute ventilation (VE) as well as inspiratory and expiratory time of awake animals were measured with a plethysmograph. There was a small increase of VT in both groups with age. Although there was no change of f in the control group with age, there was a progressive decrease in f (means +/- SE: 92 +/- 8, 97 +/- 9, 74.5 +/- 10, and 68 +/- 8 breaths/min) in the dystrophic group. Consequently VE on air decreased in the dystrophic group. Both groups showed similar responses to hypoxia (13 and 10% O2) and hypercapnia (3, 5, and 8% CO2) at 40 days. By 70 days the hypercapnic, but not hypoxic, response of the dystrophic animals was significantly decreased compared with that of the control group (at 8% CO2, VE = 47.4 +/- 4.1 vs. 75.7 +/- 7.6 ml/min, P less than 0.01). At both 100 and 140 days the response of the dystrophic group to CO2 was flat; i.e., the slope VE vs. fractional concentration of inspired CO2 was close to zero, and the hypoxic responses were greatly diminished. Because hamsters increase VE in response to CO2 primarily by increasing VT, the data suggest that dystrophic hamsters are unable to increase VT at a very early age, presumably due to muscle weakness. The normal response of hamsters to hypoxia, which is primarily to increase f, appears to be maintained for a longer time.     

Alterations of central hypercapnic respiratory response induced by acute central administration of serotonin re-uptake inhibitor, fluoxetine

Long-term neurochemical changes are responsible for therapeutic actions of fluoxetine. The role of increased central concentration of serotonin by inhibiting its re-uptake via fluoxetine on the central hypercapnic ventilatory response is complex and little is known. We aimed to research the effect of acute intracerebroventricular (ICV) injection of fluoxetine on hypercapnic ventilatory response in the absence of peripheral chemoreceptor impulses and the role of 5-HT2 receptors on responses. Eighteen anesthetized albino rabbits were divided as Fluoxetine and Ketanserin groups. For ICV administration of fluoxetine and ketanserin, a cannula was placed in the left lateral ventricle by the stereotaxic method. Respiratory frequency (fR), tidal volume (V(T)) and ventilation minute volume (V(E)) were recorded in both groups. ICV fluoxetine (10.12 mmol/kg) injection during normoxia caused significant increases in V(T) and V(E) (both P < 0.01) in the fluoxetine group. When the animals were switched to hypercapnia f/min, V(T) and V(E) increased significantly. The increases in percentage values in V(T) and V(E) in Fluoxetine + Hypercapnia phase were higher than those during hypercapnia alone (P < 0.01 and P < 0.05, respectively). On blocking of 5-HT2 receptors by ketanserin (0.25 mmol/kg), the ventilatory response to Fluoxetine was abolished and the degree of increases in V(T) and V(E) in the Ketanserin + Hypercapnia phase were lower than those during hypercapnia alone (P < 0.01 and P < 0.001, respectively). We concluded that acute central fluoxetine increases normoxic ventilation and also augments the stimulatory effect of hypercapnia on respiratory neuronal network by 5-HT2 receptors in the absence of peripheral chemoreceptor impulses.     

Effect of hypoxic episode number and severity on ventilatory long-term facilitation in awake rats.

Episodic hypoxia induces a persistent augmentation of respiratory activity, termed long-term facilitation (LTF). Phrenic LTF saturates in anesthetized animals such that additional episodes of stimulation cause no further increase in LTF magnitude. The present study tested the hypothesis that 1) ventilatory LTF also saturates in awake rats and 2) more severe hypoxia and hypoxic episodes increase the effectiveness of eliciting ventilatory LTF. Minute ventilation was measured in awake, male Sprague-Dawley rats by plethysmography. LTF was elicited by five episodes of 10% O(2) poikilocapnic hypoxia (magnitude: 17.3 +/- 2.8% above baseline, between 15 and 45 min posthypoxia, duration: 45 min) but not 12 or 8% O(2). LTF was also elicited by 10, 20, and 72 episodes of 12% O(2) (19.1 +/- 2.2, 18.9 +/- 1.8, and 19.8 +/- 1.6%; 45, 60, and 75 min, respectively) but not by three or five episodes. These results show that there is a certain range of hypoxia that induces ventilatory LTF and that additional hypoxic episodes may increase the duration but not the magnitude of this response.     

Determinants of long-term facilitation in humans during NREM sleep.

Long-term facilitation (LTF) is a prolonged increase in ventilatory motor output after episodic peripheral chemoreceptor stimulation. We have previously shown that LTF is activated during sleep following repetitive hypoxia in snorers (Babcock MA and Badr MS. Sleep 21: 709-716, 1998). The purpose of this study was 1) to ascertain the relative contribution of inspiratory flow limitation to the development of LTF and 2) to determine the effect of eliminating inspiratory flow limitation by nasal CPAP on LTF. We studied 25 normal subjects during stable non-rapid eye movement sleep. We induced 10 episodes of brief repetitive isocapnic hypoxia (inspired O(2) fraction = 8%; 3 min) followed by 5 min of room air. Measurements were obtained during control and at 20 min of recovery (R(20)). During the episodic hypoxia study, inspiratory minute ventilation (Vi) increased from 6.7 +/- 1.9 l/min during the control period to 8.2 +/- 2.7 l/min at R(20) (122% of control; P < 0.05). Linear regression analysis confirmed that inspiratory flow limitation during control was the only independent determinant of the presence of LTF (P = 0.005). Six subjects were restudied by using nasal continuous positive airway pressure to ascertain the effect of eliminating inspiratory flow limitation on LTF. Vi during the recovery period was 97 +/- 10% (P > 0.05). In conclusion, 1) repetitive hypoxia in sleeping humans is followed by increased Vi in the recovery period, indicative of development of LTF; 2) inspiratory flow limitation is the only independent determinant of posthypoxic LTF in sleeping human; 3) elimination of inspiratory flow limitation abolished the ventilatory manifestations of LTF; and 4) we propose that increased Vi in the recovery period was a result of preferential recruitment of upper airway dilators by repetitive hypoxia.     

Formation and maintenance of ventilatory long-term facilitation require NMDA but not non-NMDA receptors in awake rats

N-methyl-d-aspartate (NMDA) receptor antagonism in the phrenic motonucleus area eliminates phrenic long-term facilitation (pLTF; a persistent augmentation of phrenic nerve activity after episodic hypoxia) in anesthetized rats. However, whether NMDA antagonism can eliminate ventilatory LTF (vLTF) in awake rats is unclear. The role of non-NMDA receptors in LTF is also unknown. Serotonin receptor antagonism before, but not after, episodic hypoxia eliminates pLTF, suggesting that serotonin receptors are required for induction, but not maintenance, of pLTF. However, because NMDA and non-NMDA ionotropic glutamate receptors are directly involved in mediating the inspiratory drive to phrenic, hypoglossal, and intercostal motoneurons, we hypothesized that these receptors are required for both formation and maintenance of vLTF. vLTF, induced by five episodes of 5-min poikilocapnic hypoxia (10% O(2)) with 5-min normoxia intervals, was measured with plethysmography in conscious adult male Sprague-Dawley rats. Either (+/-)-2-amino-5-phosphonovaleric acid (APV; NMDA antagonist, 1.5 mg/kg) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; non-NMDA antagonist, 10 mg/kg) was systemically (ip) injected approximately 30 min before hypoxia. APV was also injected immediately after or 20 min after episodic hypoxia in additional groups. As control, vehicle was similarly injected in each rat 1-2 days before. Regardless of being injected before or after episodic hypoxia, vehicle did not alter vLTF ( approximately 23%), whereas APV eliminated vLTF while having little effect on baseline ventilation or hypoxic ventilatory response. In contrast, CNQX enhanced vLTF ( approximately 34%) while decreasing baseline ventilation. Collectively, these results suggest that activation of NMDA but not non-NMDA receptors is necessary for formation and maintenance of vLTF in awake rats.     

Chronic intermittent hypoxia reduces ventilatory long-term facilitation and enhances apnea frequency in newborn rats

Ventilatory long-term facilitation (LTF; defined as gradual increase of minute ventilation following repeated hypoxic exposures) is well described in adult mammals and is hypothesized to be a protective mechanism against apnea. In newborns, LTF is absent during the first postnatal days, but its precise developmental pattern is unknown. Accordingly, this study describes this pattern of postnatal development. Additionally, we tested the hypothesis that chronic intermittent hypoxia (CIH) from birth alters this development. LTF was estimated in vivo using whole body plethysmography by exposing rat pups at postnatal days 1, 4, and 10 (P1, P4, and P10) to 10 brief hypoxic cycles (nadir 5% O2) and respiratory recordings during the following 2 h (recovery, 21% O2). Under these conditions, ventilatory LTF (gradual increase of minute ventilation during recovery) was clearly expressed in P10 rats but not in P1 and P4. In a second series of experiments, rat pups were exposed to CIH during the first 10 postnatal days (6 brief cyclic exposures at 5% O2 every 6 min followed by 1 h under normoxia, 24 h a day). Compared with P10 control rats, CIH enhanced hypoxic ventilatory response (estimated during the hypoxic cycles) specifically in male rat pups. Ventilatory LTF was drastically reduced in P10 rats exposed to CIH, which was associated with higher apnea frequency during recovery. We conclude that CIH from birth enhances hypoxic chemoreflex and disrupts LTF development, thus likely contributing to increase apnea frequency.     

Fructose feeding and intermittent hypoxia affect ventilatory responsiveness to hypoxia and hypercapnia in rats.

We hypothesized that, in male rats, 10% fructose in drinking water would depress ventilatory responsiveness to acute hypoxia (10% O2 in N2) and hypercapnia (5% CO2 in O2) that would be depressed further by exposure to intermittent hypoxia. Minute ventilation (Ve) in air and in response to acute hypoxia and hypercapnia was evaluated in 10 rats before fructose feeding (FF), during 6 wk of FF, and after FF was removed for 2 wk. During FF, five rats were exposed to intermittent air and five to intermittent hypoxia for 13 days. Six rats given tap water acted as control and were exposed to intermittent air and subsequently intermittent hypoxia. In FF rats, plasma insulin levels increased threefold in the rats exposed to intermittent hypoxia and during washout returned to levels observed in rats exposed to intermittent air. During FF, ventilatory responsiveness to acute hypoxia was depressed because of decreased tidal volume (Vt) responsiveness. During washout, Ve decreased as a result of decreased Vt and frequency of breathing, and the ventilatory responsiveness to hypoxia in intermittent hypoxia rats did not recover. In all rats, the ventilatory responses to hypercapnia were decreased during FF and recovered after washout because of an increased Vt responsiveness. In the control group, hypoxic responsiveness was not depressed after intermittent hypoxia and was augmented after washout. Thus FF attenuated the ventilatory responsiveness of conscious rats to hypoxia and hypercapnia. Intermittent hypoxia interacted with FF to increase insulin levels and depress ventilatory responses to acute hypoxia that remained depressed during washout.     

Raphe magnus nucleus is involved in ventilatory but not hypothermic response to CO2.

There is evidence that serotonin [5-hydroxytryptamine (5-HT)] is involved in the physiological responses to hypercapnia. Serotonergic neurons represent the major cell type (comprising 15-20% of the neurons) in raphe magnus nucleus (RMg), which is a medullary raphe nucleus. In the present study, we tested the hypothesis 1) that RMg plays a role in the ventilatory and thermal responses to hypercapnia, and 2) that RMg serotonergic neurons are involved in these responses. To this end, we microinjected 1) ibotenic acid to promote nonspecific lesioning of neurons in the RMg, or 2) anti-SERT-SAP (an immunotoxin that utilizes a monoclonal antibody to the third extracellular domain of the serotonin reuptake transporter) to specifically kill the serotonergic neurons in the RMg. Hypercapnia caused hyperventilation and hypothermia in all groups. RMg nonspecific lesions elicited a significant reduction of the ventilatory response to hypercapnia due to lower tidal volume (Vt) and respiratory frequency. Rats submitted to specific killing of RMg serotonergic neurons showed no consistent difference in ventilation during air breathing but had a decreased ventilatory response to CO(2) due to lower Vt. The hypercapnia-induced hypothermia was not affected by specific or nonspecific lesions of RMg serotonergic neurons. These data suggest that RMg serotonergic neurons do not participate in the tonic maintenance of ventilation during air breathing but contribute to the ventilatory response to CO(2). Ultimately, this nucleus may not be involved in the thermal responses to CO(2).     

Time course of air hunger mirrors the biphasic ventilatory response to hypoxia.

Determining response dynamics of hypoxic air hunger may provide information of use in clinical practice and will improve understanding of basic dyspnea mechanisms. It is hypothesized that air hunger arises from projection of reflex brain stem ventilatory drive ("corollary discharge") to forebrain centers. If perceptual response dynamics are unmodified by events between brain stem and cortical awareness, this hypothesis predicts that air hunger will exactly track ventilatory response. Thus, during sustained hypoxia, initial increase in air hunger would be followed by a progressive decline reflecting biphasic reflex ventilatory drive. To test this prediction, we applied a sharp-onset 20-min step of normocapnic hypoxia and compared dynamic response characteristics of air hunger with that of ventilation in 10 healthy subjects. Air hunger was measured during mechanical ventilation (minute ventilation = 9 +/- 1.4 l/min; end-tidal Pco(2) = 37 +/- 2 Torr; end-tidal Po(2) = 45 +/- 7 Torr); ventilatory response was measured during separate free-breathing trials in the same subjects. Discomfort caused by "urge to breathe" was rated every 30 s on a visual analog scale. Both ventilatory and air hunger responses were modeled as delayed double exponentials corresponding to a simple linear first-order response but with a separate first-order adaptation. These models provided adequate fits to both ventilatory and air hunger data (r(2) = 0.88 and 0.66). Mean time constant and time-to-peak response for the average perceptual response (0.36 min(-1) and 3.3 min, respectively) closely matched corresponding values for the average ventilatory response (0.39 min(-1) and 3.1 min). Air hunger response to sustained hypoxia tracked ventilatory drive with a delay of approximately 30 s. Our data provide further support for the corollary discharge hypothesis for air hunger.     

Synthesis and biological activities of leukotriene F4 and leukotriene F4 sulfone

Leukotriene F4 (LTF4) and LTF4 sulfone have been synthesized and their biological activities determined in the guinea pig. In vitro LTF4 displayed comparable activity to LTD4 on guinea pig trachea and parenchyma but was less active on the ileum. When injected intravenously into the guinea pig, LTF4 induced a bronchoconstriction (ED50 16 micrograms Kg-1) which was blocked by indomethacin and FPL-55712 and was 50-100 X less potent than LTD4 in this assay. LTF4 sulfone was approximately 2-5 times less active than LTF4 in vitro and in vivo. When injected into guinea pig skin with PGE2 (100 ng); LTF4 and LTF4 sulfone (10-1000 ng) induced changes in vascular permeability. The order of potency in this assay was LTE4 sulfone = LTD4 = LTD4 sulfone greater than LTE4 greater than LTF4 = LTF4 sulfone.     

Phrenic long-term facilitation requires 5-HT receptor activation during but not following episodic hypoxia.

Episodic hypoxia evokes a sustained augmentation of respiratory motor output known as long-term facilitation (LTF). Phrenic LTF is prevented by pretreatment with the 5-hydroxytryptamine (5-HT) receptor antagonist ketanserin. We tested the hypothesis that 5-HT receptor activation is necessary for the induction but not maintenance of phrenic LTF. Peak integrated phrenic nerve activity (integralPhr) was monitored for 1 h after three 5-min episodes of isocapnic hypoxia (arterial PO(2) = 40 +/- 2 Torr; 5-min hyperoxic intervals) in four groups of anesthetized, vagotomized, paralyzed, and ventilated Sprague-Dawley rats [1) control (n = 11), 2) ketanserin pretreatment (2 mg/kg iv; n = 7), and ketanserin treatment 0 and 45 min after episodic hypoxia (n = 7 each)]. Ketanserin transiently decreased integralPhr, but it returned to baseline levels within 10 min. One hour after episodic hypoxia, integralPhr was significantly elevated from baseline in control and in the 0- and 45-min posthypoxia ketanserin groups. Conversely, ketanserin pretreatment abolished phrenic LTF. We conclude that 5-HT receptor activation is necessary to initiate (during hypoxia) but not maintain (following hypoxia) phrenic LTF.     

Long-term facilitation in obstructive sleep apnea patients during NREM sleep.

Repetitive hypoxia followed by persistently increased ventilatory motor output is referred to as long-term facilitation (LTF). LTF is activated during sleep after repetitive hypoxia in snorers. We hypothesized that LTF is activated in obstructive sleep apnea (OSA) patients. Eleven subjects with OSA (apnea/hypopnea index = 43.6 +/- 18.7/h) were included. Every subject had a baseline polysomnographic study on the appropriate continuous positive airway pressure (CPAP). CPAP was retitrated to eliminate apnea/hypopnea but to maintain inspiratory flow limitation (sham night). Each subject was studied on 2 separate nights. These two studies are separated by 1 mo of optimal nasal CPAP treatment for a minimum of 4-6 h/night. The device was capable of covert pressure monitoring. During night 1 (N1), study subjects used nasal CPAP at suboptimal pressure to have significant air flow limitation (>60% breaths) without apneas/hypopneas. After stable sleep was reached, we induced brief isocapnic hypoxia [inspired O(2) fraction (FI(O(2))) = 8%] (3 min) followed by 5 min of room air. This sequence was repeated 10 times. Measurements were obtained during control, hypoxia, and at 5, 20, and 40 min of recovery for ventilation, timing (n = 11), and supraglottic pressure (n = 6). Upper airway resistance (Rua) was calculated at peak inspiratory flow. During the recovery period, there was no change in minute ventilation (99 +/- 8% of control), despite decreased Rua to 58 +/- 24% of control (P < 0.05). There was a reduction in the ratio of inspiratory time to total time for a breath (duty cycle) (0.5 to 0.45, P < 0.05) but no effect on inspiratory time. During night 2 (N2), the protocol of N1 was repeated. N2 revealed no changes compared with N1 during the recovery period. In conclusion, 1) reduced Rua in the recovery period indicates LTF of upper airway dilators; 2) lack of hyperpnea in the recovery period suggests that thoracic pump muscles do not demonstrate LTF; 3) we speculate that LTF may temporarily stabilize respiration in OSA patients after repeated apneas/hypopneas; and 4) nasal CPAP did not alter the ability of OSA patients to elicit LTF at the thoracic pump muscle.     

Neurocirculatory consequences of intermittent asphyxia in humans.

We examined the neurocirculatory and ventilatory responses to intermittent asphyxia (arterial O(2) saturation = 79-85%, end-tidal PCO(2) =3-5 Torr above eupnea) in seven healthy humans during wakefulness. The intermittent asphyxia intervention consisted of 20-s asphyxic exposures alternating with 40-s periods of room-air breathing for a total of 20 min. Minute ventilation increased during the intermittent asphyxia period (14.2 +/- 2.0 l/min in the final 5 min of asphyxia vs. 7.5 +/- 0.4 l/min in baseline) but returned to the baseline level within 2 min after completion of the series of asphyxic exposures. Muscle sympathetic nerve activity increased progressively, reaching 175 +/- 12% of baseline in the final 5 min of the intervention. Unlike ventilation, sympathetic activity remained elevated for at least 20 min after removal of the chemical stimuli (150 +/- 10% of baseline in the last 5 min of the recovery period). Intermittent asphyxia caused a small, but statistically significant, increase in heart rate (64 +/- 4 beats/min in the final 5 min of asphyxia vs. 61 +/- 4 beats/min in baseline); however, this increase was not sustained after the return to room-air breathing. These data demonstrate that relatively short-term exposure to intermittent asphyxia causes sympathetic activation that persists after removal of the chemical stimuli. This carryover effect provides a potential mechanism whereby intermittent asphyxia during sleep could lead to chronic sympathetic activation in patients with sleep apnea syndrome.     

Involvement of 5-HT receptors in the development and expression of methamphetamine-induced behavioral sensitization: 5-HT receptor channel and binding study

Methamphetamine (MAP) is one of the most commonly abused drugs in Asia, and previous studies suggest that serotonin 3 receptors (5-HT(3)) are involved in MAP-induced locomotion and reward. However, little is known about the role of 5-HT(3) receptors in MAP-induced behavioral sensitization. Here, we measured the effects of MDL 72222, a 5-HT(3) antagonist, and SR 57227 A, a 5-HT(3) agonist, on the development and expression of MAP-induced behavioral sensitization, and alternations of 5-HT(3) receptor binding labeled with the 5-HT(3)-selective antagonist, [(3)H]GR65630, in mice. In addition, we investigated the effects of MAP on 5-HT(3A) receptor channel activity in Xenopus laevis oocytes expressing 5-HT(3A) receptors. We found that MDL 72222 attenuated both the development and expression of behavioral sensitization to MAP (1.0 mg/kg, i.p.), and that this attenuating effect of MDL 72222 was reversed by pre-treatment with SR 57227 A. In oocytes expressing 5-HT(3A) receptor, MAP exhibited a dual modulation of 5-HT(3A) receptor channel activity, i.e. pre-treatment with a low dose of MAP (0.1 microm) enhanced 5-HT-induced inward peak current (I(5-HT)) but a high dose of MAP (100 microm) inhibited I(5-HT). The acute administration of MDL 72222 with MAP decreased [(3)H]GR65630 binding versus MAP alone in the mouse striatum. Our results suggest that MDL 72222 attenuates MAP-induced behavioral sensitization via 5-HT(3) receptors in the caudate putamen, and that 5-HT(3) receptor antagonists like MDL 72222 have potential as novel anti-psychotic agents for the treatment of MAP dependence and psychosis.     

Early postnatal chronic intermittent hypoxia modifies hypoxic respiratory responses and long-term phrenic facilitation in adult rats

Acute isocapnic intermittent hypoxia elicits time-dependent, serotonin-dependent enhancement of phrenic motor output in anesthetized rats (phrenic long-term facilitation, pLTF). In adult rats, pLTF is enhanced by chronic intermittent hypoxia (CIH). To test the hypothesis that early postnatal CIH induces persistent modifications of ventilation and pLTF, we exposed male Sprague-Dawley rat pups on their first day of life to a CIH profile consisting of alternating room air and 10% oxygen every 90 s for 30 days during daylight hours (RAIH) or to comparable exposures consisting of room air throughout (RARA). One month after cessation of CIH, respiratory responses were recorded using whole body plethysmography, and integrated phrenic nerve activity was recorded in urethane-anesthetized, vagotomized, paralyzed, and ventilated rats at baseline and after exposures to three 5-min hypoxic episodes [inspired O2 fraction (FiO2)=0.11] separated by 5 min of hyperoxia (FiO2=0.5). RAIH rats displayed greater normoxic ventilation and also increased burst frequency compared with RARA rats (P<0.01). Ventilatory responses to hypoxia and short-term phrenic responses during acute hypoxic challenges were reduced in RAIH rats (P<0.01). Although pLTF was present in both RAIH and RARA rats, it was diminished in RAIH rats (minute activity: 74+/-2% in RARA vs. 55+/-5% in RAIH at 60 min; P<0.01). Thus we conclude that early postnatal CIH modifies normoxic and hypoxic ventilatory and phrenic responses that persist at 1 mo after cessation of CIH (i.e., metaplasticity) and markedly differ from previously reported increased neural plasticity changes induced by CIH in adult rats.