Effect of hypoxic sensitivity on decay of respiratory short-term potentiation

After-Discharge in the Upper Airway Muscle Genioglossus Following Brief Hypoxia

STUDY OBJECTIVES

Genioglossus after-discharge is thought to protect against pharyngeal collapse by minimising periods of low upper airway muscle activity. How genioglossus after-discharge occurs and which single motor units (SMUs) are responsible for the phenomenon are unknown. The aim of this study was to investigate genioglossal after-discharge.

METHODS

During wakefulness, after-discharge was elicited 8-12 times in healthy individuals with brief isocapnic hypoxia (45-60s of 10%O2 in N2) terminated by a single breath of 100% O2. Genioglossus SMUs were designated as firing solely, or at increased rate, during inspiration (Inspiratory phasic [IP] and inspiratory tonic [IT] respectively); solely, or at increased rate, during expiration (Expiratory phasic [EP] or expiratory tonic [ET] respectively) or firing constantly without respiratory modulation (Tonic). SMUs were quantified at baseline, the end of hypoxia, the hyperoxic breath and the following 8 normoxic breaths.

RESULTS

210 SMU's were identified in 17 participants. Genioglossus muscle activity was elevated above baseline for 7 breaths after hyperoxia (p<0.001), indicating a strong after-discharge effect. After-discharge occurred due to persistent firing of IP and IT units that were recruited during hypoxia, with minimal changes in ET, EP or Tonic SMUs. The firing frequency of units that were already active changed minimally during hypoxia or the afterdischarge period (P>0.05).

CONCLUSION

That genioglossal after-discharge is almost entirely due to persistent firing of previously silent inspiratory SMUs provides insight into the mechanisms responsible for the phenomenon and supports the hypothesis that the inspiratory and expiratory/tonic motor units within the muscle have idiosyncratic functions.

Neural memory of the genioglossus muscle during sleep is stage-dependent in healthy subjects and obstructive sleep apnoea patients

KEY POINTS

In most patients with obstructive sleep apnoea (OSA), there is a spontaneous resolution of the breathing disorders during slow wave sleep (SWS) for yet unknown reasons related to non-anatomical factors. Some recently identified forms of neural memory specific of upper airway muscles may play a role in this phenomenon. In the present study, we show for the first time that a form of memory of the genioglossus (tongue) muscle is greatly enhanced during SWS compared to non-rapid eye movement stage 2 sleep. The present study represents a step forward in understanding the mechanisms responsible for the spontaneous development of stable breathing during SWS in OSA patients and may help the discovery of novel therapeutic strategies for this disease.

ABSTRACT

Several studies have shown that obstructive sleep apnoea (OSA) improves during slow wave sleep (SWS) for reasons that remain unclear. Recent studies have identified forms of neural memory such as short-term potentiation or after-discharge that can occur in response to upper airway obstruction. Neural memory may play a role in the development of stable breathing during SWS by increasing upper airway muscles activity in this sleep stage. We hypothesize that the after-discharge of the genioglossus muscle following upper airway obstruction is enhanced during SWS compared to non-rapid eye movement stage 2 (N2). During sleep, we performed five-breath drops in continuous positive airway pressure (CPAP-drop) to simulate obstructive events and reflexively activate the genioglossus. Immediately afterwards, CPAP was returned to an optimal level. Once the post-drop ventilation returned to eupnoea, the genioglossus after-discharge was measured as the time it took for genioglossus activity to return to baseline levels. In total, 171 CPAP-drops were analysed from a group of 16 healthy subjects and 19 OSA patients. A mixed-model analysis showed that after-discharge duration during SWS was 208% (95% confidence interval = 112% to 387%, P = 0.022) greater than during N2 after adjusting for covariates (ventilatory drive, CPAP levels). There was also a non-significant trend for a -35% reduction in after-discharge duration following an arousal vs. no-arousal from sleep (95% confidence interval = -59.5% to 5%, P = 0.08). Genioglossus after-discharge is two-fold greater in SWS vs. N2, which could partly explain the breathing stabilization described in OSA patients during this sleep stage.

Pharmacological modulation of hypoxia-induced respiratory neuroplasticity

Hypoxia elicits complex cell signaling mechanisms in the respiratory control system that can produce long-lasting changes in respiratory motor output. In this article, we review experimental approaches used to elucidate signaling pathways associated with hypoxia, and summarize current hypotheses regarding the intracellular signaling pathways evoked by intermittent exposure to hypoxia. We review data showing that pharmacological treatments can enhance neuroplastic responses to hypoxia. Original data are included to show that pharmacological modulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) function can reveal a respiratory neuroplastic response to a single, brief hypoxic exposure in anesthetized mice. Coupling pharmacologic treatments with therapeutic hypoxia paradigms may have rehabilitative value following neurologic injury or during neuromuscular disease. Depending on prevailing conditions, pharmacologic treatments can enable hypoxia-induced expression of neuroplasticity and increased respiratory motor output, or potentially could synergistically interact with hypoxia to more robustly increase motor output.

Short-term potentiation in the control of pharyngeal muscles in obstructive apnea patients

STUDY OBJECTIVES

To determine if activation of the genioglossus (GG) muscle during obstructive apnea events involves short-term potentiation (STP) and is followed by sustained activation beyond the obstructive phase (after-discharge).

DESIGN

Physiological study.

SETTING

Sleep laboratory in a tertiary hospital.

PARTICIPANTS

Twenty-one patients with obstructive apnea.

INTERVENTIONS

Polysomnography on continuous positive airway pressure (CPAP) with measurement of genioglossus activity. Brief dial-downs of CPAP to induce obstructive events.

MEASUREMENTS AND RESULTS

Peak, phasic, and tonic genioglossus activities were measured breath-by-breath before, during, and following three-breath obstructions. Tonic but not phasic activity increased immediately following the first obstructed breath (4.9 ± 1.6 versus 3.6 ± 1.2 %GGMAX; P = 0.01) under conditions where stimuli to genioglossus activation were likely constant, strongly implicating STP in mediating recruitment of tonic activity. Both phasic and tonic activities declined slowly after relief of obstruction (after-discharge). Decay time constants were systematically shorter for phasic than for tonic activity (7.5 ± 3.8 versus 18.1 ± 8.4 sec; P < 0.001). Decay time-constant of peak activity correlated with tonic, but not phasic, recruitment. Cortical arousal near the end of obstruction resulted in a lower after-discharge (P < 0.01). Contribution of tonic activity to the increase in peak activity (6-65%Peak), as well as the decay constant (6-30 sec), varied considerably among patients.

CONCLUSIONS

Short-term potentiation contributes to recruitment of the genioglossus during obstructive episodes and results in sustained tonic activity beyond the obstructive phase, thereby potentially preventing recurrence of obstruction. Wide response differences among subjects suggest that this mechanism may contribute to severity of the disorder. The after-discharge is inhibited following cortical arousal, potentially explaining arousals' destabilizing effect.

Differences in time-dependent hypoxic phrenic responses among inbred rat strains

Hypoxic ventilatory responses differ between rodent strains, suggesting a genetic contribution to interindividual variability. However, hypoxic ventilatory responses consist of multiple time-dependent mechanisms that can be observed in different respiratory motor outputs. We hypothesized that strain differences would exist in discrete time-dependent mechanisms of the hypoxic response and, furthermore, that there may be differences between hypoglossal and phrenic nerve responses to hypoxia. Hypoglossal and phrenic nerve responses were assessed during and after a 5-min hypoxic episode in anesthetized, vagotomized, and ventilated rats from four inbred strains: Brown Norway (BN), Fischer 344 (FS), Lewis (LW), and Piebald-viral-Glaxo (PVG). During baseline, burst frequency was higher in PVG than LW rats ( P < 0.05), phrenic burst amplitude was higher in PVG vs. other strains ( P < 0.05), and hypoglossal burst amplitude was higher in PVG and BN vs. FS and LW ( P < 0.05). During hypoxia, burst frequency did not change in BN or LW rats, but it increased in PVG and FS rats. The phrenic amplitude response was smallest in PVG vs. other strains ( P < 0.05), and the hypoglossal response was similar among strains. Short-term potentiation posthypoxia was slowest in FS and fastest in LW rats ( P < 0.05). Posthypoxia frequency decline was absent in PVG, but it was observed in all other strains. Augmented breaths were observed during hypoxia in FS rats only. Thus genetic differences exist in the time domains of the hypoxic response, and these are differentially expressed in hypoglossal and phrenic nerves. Furthermore, genetic diversity observed in hypoxic ventilatory responses in unanesthetized rats may arise from multiple neural mechanisms.

Effects of theophylline on ventilatory poststimulus potentiation in patients with brain damage

Patients with brain damage, in contrast to normal subjects, exhibit a significant ventilatory undershoot when brief hypocapnic hypoxia is terminated abruptly by hyperoxia. This has been attributed to an impairment of activation of short-term potentiation, a brain stem mechanism promoting breathing stability. We hypothesized that in these patients theophylline, a drug that stabilizes breathing, may affect short-term potentiation. Eight stable patients with brain damage and 10 normal adults were studied. Activation of short-term potentiation was examined by brief exposure to hypoxia followed by hyperoxia after pretreatment with placebo or theophylline. Both in patients and normal subjects at the end of hypoxia ventilation increased to a similar magnitude with and without theophylline. In normal subjects independent of pretreatment, when hypoxia was terminated abruptly by hyperoxia, ventilation declined slowly to baseline without an undershoot, indicating activation of short-term potentiation. In patients with placebo, ventilation upon switching to hyperoxia exhibited a significant undershoot. This undershoot was significantly attenuated by theophylline, although compared with normal subjects, a slight hypoventilation was observed. We conclude that in patients with brain damage, theophylline largely prevents the hyperoxic drop of ventilation, presumably by affecting the activation of short-term potentiation. This may underlie the beneficial effect of theophylline on breathing stability.

Mutant mice deficient in NOS-1 exhibit attenuated long-term facilitation and short-term potentiation in breathing

The objective of the present study is to examine the potential role of nitric oxide (NO) in short-term potentiation (STP) and long-term facilitation (LTF) of breathing. Experiments were performed in wild-type (WT) and mutant mice deficient in nitric oxide synthase-1 (NOS-1), as well as in WT mice administered the NOS-1 inhibitor 7-nitroindazole (7-NI; 50 mg x kg(-1); I.P.). Respiratory responses following either single or recurrent episodes of hypoxia (7% O2, balance N2) were analysed in unanaesthetised animals by body plethysmography along with rate of O2 consumption (VO2)) and CO2 production (VCO2). After a single hypoxic challenge, respiration in WT mice remained elevated for 5 min, suggesting STP in ventilation. Following termination of three consecutive hypoxic challenges, respiration remained elevated during normoxia for as long as 30 min, indicating LTF in breathing under awake conditions. STP and LTF were significantly attenuated or absent in WT mice after 7-NI. A similar attenuation or absence of STP and LTF was also seen in NOS-1 mutant mice. Changes in VO2 and VCO2 were comparable among mice during the post-hypoxic period, suggesting that the absence of STP and LTF was not due to alterations in body metabolism. These results suggest endogenous NO is an important physiological modulator of ventilatory STP and LTF.

Plasticity of cardiorespiratory neural processing: classification and computational functions

Neural plasticity, or malleability of neuronal structure and function, is an important attribute of the mammalian forebrain and is generally thought to be a kernel of biological intelligence. In this review, we examine some reported manifestations of neural plasticity in the cardiorespiratory system and classify them into four functional categories, integral; differential; memory; and statistical-type plasticity. At the cellular and systems level the myriad forms of cardiorespiratory plasticity display emergent and self-organization properties, use- and disuse-dependent and pairing-specific properties, short-term and long-term potentiation or depression, as well as redundancy in series or parallel structures, convergent pathways or backup and fail-safe surrogate pathways. At the behavioral level, the cardiorespiratory system demonstrates the capability of associative and nonassociative learning, classical and operant conditioning as well as short-term and long-term memory. The remarkable similarity and consistency of the various types of plasticity exhibited at all levels of organization suggest that neural plasticity is integral to cardiorespiratory control and may subserve important physiological functions.

Short-term potentiation of ventilation after different levels of hypoxia

Short-term potentiation of ventilation (VSTP) may be observed in healthy subjects on sudden termination of an hypoxic stimulus. We hypothesized that the level of hypoxia preceding normoxia would modify the duration and magnitude of the ensuing ventilatory decay. Ten healthy adults were studied on two different occasions, during which they were randomly exposed to isocapnic 6 or 10% O2 for 60 s and then switched to an isocapnic normoxic gas mixture. Both hypoxic gases induced significant ventilatory responses, and mean peak minute ventilation before the isocapnic normoxic switch was higher in 6% O2 (P < 0.001). The fast time constant of the two-exponential equation representing the best fit for ventilatory decay was unaffected by the magnitude of the hypoxic stimulus. However, the slow time constant, which is considered to represent VSTP, was markedly prolonged in 6% compared with 10% O2 [106.7 +/- 11.3 vs. 38. 2 +/- 6.1 (SD) s, respectively; P < 0.0001]. This result indicates that VSTP is stimulus dependent. We conclude that the magnitude of hypoxia preceding a normoxic transient modifies VSTP characteristics. We speculate that the interdependence function of ventilatory stimulus and short-term potentiation is crucial for preservation of system stability during transitions from high to low ventilatory drives.

Pharmacologic adjuncts to mechanical ventilation in acute respiratory distress syndrome

This article reviews pharmacologic approaches to treating acute respiratory distress syndrome (ARDS). The authors discuss the therapeutic effects of ketoconazole, antioxidants, corticosteroids, surfactant, ketanserin, pentoxifylline, bronchodilators, and almitrine in ARDS. Current animal data and proposed mechanics which may foster future pharmacologic therapies are also examined.