Episodic stimulation of alpha1-adrenoreceptors induces protein kinase C-dependent persistent changes in motoneuronal excitability

Reconfiguration of the pontomedullary respiratory network: a computational modeling study with coordinated in vivo experiments

A large body of data suggests that the pontine respiratory group (PRG) is involved in respiratory phase-switching and the reconfiguration of the brain stem respiratory network. However, connectivity between the PRG and ventral respiratory column (VRC) in computational models has been largely ad hoc. We developed a network model with PRG-VRC connectivity inferred from coordinated in vivo experiments. Neurons were modeled in the "integrate-and-fire" style; some neurons had pacemaker properties derived from the model of Breen et al. We recapitulated earlier modeling results, including reproduction of activity profiles of different respiratory neurons and motor outputs, and their changes under different conditions (vagotomy, pontine lesions, etc.). The model also reproduced characteristic changes in neuronal and motor patterns observed in vivo during fictive cough and during hypoxia in non-rapid eye movement sleep. Our simulations suggested possible mechanisms for respiratory pattern reorganization during these behaviors. The model predicted that network- and pacemaker-generated rhythms could be co-expressed during the transition from gasping to eupnea, producing a combined "burst-ramp" pattern of phrenic discharges. To test this prediction, phrenic activity and multiple single neuron spike trains were monitored in vagotomized, decerebrate, immobilized, thoracotomized, and artificially ventilated cats during hypoxia and recovery. In most experiments, phrenic discharge patterns during recovery from hypoxia were similar to those predicted by the model. We conclude that under certain conditions, e.g., during recovery from severe brain hypoxia, components of a distributed network activity present during eupnea can be co-expressed with gasp patterns generated by a distinct, functionally "simplified" mechanism.

Sleep fragmentation impairs ventilatory long-term facilitation via adenosine A1 receptors

Sleep fragmentation (SF), a primary feature of obstructive sleep apnoea (OSA), impairs hippocampal long-term potentiation and causes cognitive/attention deficits. However, its influence upon respiratory control has hardly been studied. This study examined the effect of SF on ventilatory long-term facilitation (LTF, a persistent augmentation of respiratory activity after episodic hypoxia) and the hypoxic ventilatory response (HVR), and investigated the role of adenosine A1 receptors in these SF effects in conscious adult male Sprague-Dawley rats. SF, confirmed by sleep architecture recordings, was achieved by periodic, forced locomotion in a rotating drum (30 s rotation/90 s stop for 24 h). LTF, elicited by five episodes of 5 min poikilocapnic hypoxia (10% O2) with 5 min intervals, was measured by plethysmography. Resting ventilation and metabolic rate were unchanged, HVR was reduced (150.6 +/- 3.5% versus 110.4 +/- 12.3%) and LTF was eliminated (22.6 +/- 0.5% versus -0.1 +/- 1.3%) shortly after 24 h SF. The SF-induced impairments were SF duration dependent, and completely reversible as HVR (< 24 h) and LTF (< 48 h) returned spontaneously to their pre-SF values. The SF-impaired HVR was improved (130.3 +/- 4.2%) and SF-eliminated LTF was restored (19.6 +/- 0.9%) by systemic injection of the adenosine A1 receptor antagonist 8-CPT (2.5 mg kg(-1)) approximately 30 min before LTF elicitation. Both HVR and LTF were also similarly impaired by 24 h total sleep deprivation or 24 h repeated cage tapping-induced SF, but not by a 24 h locomotion control protocol for SF. Collectively, these data suggest that: (1) 24 h SF impairs LTF and poikilocapnic HVR; (2) these impairments require A1 receptors; and (3) SF of OSA may exacerbate OSA via impaired ventilatory control mechanisms.

Determinants of frequency long-term facilitation following acute intermittent hypoxia in vagotomized rats

Acute intermittent (AIH), but not acute sustained hypoxia (ASH) elicits a form of respiratory plasticity known as long-term facilitation (LTF). In anesthetized rats, LTF is expressed as increased respiratory-related nerve burst amplitude, with variable effects on burst frequency. We analyzed a large data set from multiple investigators using the same experimental protocol to determine factors influencing frequency LTF. Our meta-analysis revealed that AIH elicits both phrenic amplitude and frequency LTF in anesthetized and vagotomized rats, but frequency LTF is small in comparison with amplitude LTF (12% versus 60%, respectively). ASH elicits a small, but significant frequency and amplitude LTF (8% and 10%, respectively) that is not significantly different than controls. Similar to all published reports, analysis of this large data set confirms that phrenic amplitude LTF following AIH is significantly greater than ASH. Multiple regression analysis revealed a strong correlation between baseline burst frequency and frequency LTF. Variations in baseline burst frequency may contribute to variation in frequency LTF and may underlie the apparent effects of some drug treatments.

Simulated apnoeas induce serotonin-dependent respiratory long-term facilitation in rats

Long-term facilitation (LTF) is a form of respiratory neuroplasticity frequently induced by acute intermittent isocapnic hypoxia (AIH, three 5 min isocapnic hypoxic episodes). Although repetitive apnoeas are a frequent natural occurrence producing brief (< 30 s) episodes of hypoxia and hypercapnia, it is unknown if repetitive apnoeas also elicit LTF. Apnoea-induced LTF may preserve upper airway patency during sleep, thereby limiting further apnoeic events. We tested the hypothesis that repeated, brief ventilator-induced apnoeas are sufficient to induce serotonin-dependent phrenic and hypoglossal (XII) LTF in anaesthetized rats. Anaesthetized, male Sprague-Dawley rats were exposed to three or six 25 s ventilator apnoeas with 5 min intervals, and compared to time control and AIH-treated rats. Three and six ventilator apnoeas induced phrenic and XII LTF with a magnitude similar to AIH. Both apnoea-induced and AIH-induced LTF were associated with a decreased CO(2) recruitment threshold for phrenic and XII activity (approximately 4 mmHg). Spinal methysergide, a serotonin receptor antagonist, blocked apnoea-induced LTF but not changes in the CO(2)-recruitment threshold. Thus, brief ventilator apnoeas elicit phrenic and XII LTF. Similar to AIH-induced LTF, apnoea-induced LTF is serotonin dependent, and the relevant serotonin receptors for phrenic LTF are located in the cervical spinal cord. Apnoea-induced LTF may have implications for the maintenance of breathing stability, particularly during sleep.

Spinal adenosine A2a receptor activation elicits long-lasting phrenic motor facilitation

Acute intermittent hypoxia elicits a form of spinal, brain-derived neurotrophic factor (BDNF)-dependent respiratory plasticity known as phrenic long-term facilitation. Ligands that activate G(s)-protein-coupled receptors, such as the adenosine 2a receptor, mimic the effects of neurotrophins in vitro by transactivating their high-affinity receptor tyrosine kinases, the Trk receptors. Thus, we hypothesized that A2a receptor agonists would elicit phrenic long-term facilitation by mimicking the effects of BDNF on TrkB receptors. Here we demonstrate that spinal A2a receptor agonists transactivate TrkB receptors in the rat cervical spinal cord near phrenic motoneurons, thus inducing long-lasting (hours) phrenic motor facilitation. A2a receptor activation increased phosphorylation and new synthesis of an immature TrkB protein, induced TrkB signaling through Akt, and strengthened synaptic pathways to phrenic motoneurons. RNA interference targeting TrkB mRNA demonstrated that new TrkB protein synthesis is necessary for A2a-induced phrenic motor facilitation. A2a receptor activation also increased breathing in unanesthetized rats, and improved breathing in rats with cervical spinal injuries. Thus, small, highly permeable drugs (such as adenosine receptor agonists) that transactivate TrkB receptors may provide an effective therapeutic strategy in the treatment of patients with ventilatory control disorders, such as obstructive sleep apnea, or respiratory insufficiency after spinal injury or during neurodegenerative diseases.

Respiratory plasticity following intermittent hypoxia: roles of protein phosphatases and reactive oxygen species

Plasticity is an important property of the respiratory control system. One of the best-studied models of respiratory plasticity is pLTF (phrenic long-term facilitation). pLTF is a progressive increase in phrenic motor output lasting several hours following acute exposure to intermittent hypoxia. Similar to many other forms of neuroplasticity, pLTF is pattern-sensitive; it is induced by intermittent, but not sustained hypoxia of similar cumulative duration. Our understanding of the cellular/synaptic mechanisms underlying pLTF has increased considerably in recent years. Here, we review accumulating evidence suggesting that the pattern-sensitivity of pLTF arises substantially from differential reactive oxygen species formation and subsequent protein phosphatase inhibition during intermittent compared with sustained hypoxia in or near phrenic motor neurons. A detailed understanding of the cellular/synaptic mechanisms of pLTF may provide the rationale for new pharmacological approaches in the treatment of severe ventilatory control disorders, such as obstructive sleep apnoea and respiratory insufficiency either following spinal cord injury or during neurodegenerative diseases such as amyotrophic lateral sclerosis.

Maintenance of gasping and restoration of eupnea after hypoxia is impaired following blockers of alpha1-adrenergic receptors and serotonin 5-HT2 receptors

In severe hypoxia or ischemia, normal eupneic breathing fails and is replaced by gasping. Gasping serves as part of a process of autoresuscitation by which eupnea is reestablished. Medullary neurons, having a burster, pacemaker discharge, underlie gasping. Conductance through persistent sodium channels is essential for the burster discharge. This conductance is modulated by norepinephrine, acting on alpha 1-adrenergic receptors, and serotonin, acting on 5-HT2 receptors. We hypothesized that blockers of 5-HT2 receptors and alpha 1-adrenergic receptors would alter autoresuscitation. The in situ perfused preparation of the juvenile rat was used. Integrated phrenic discharge was switched from an incrementing pattern, akin to eupnea, to the decrementing pattern comparable to gasping in hypoxic hypercapnia. With a restoration of hyperoxic normocapnia, rhythmic, incrementing phrenic discharge returned within 10 s in most preparations. Following addition of blockers of alpha 1-adrenergic receptors (WB-4101, 0.0625-0.500 microM) and/or blockers of 5-HT2 (ketanserin, 1.25-10 microM) or multiple 5-HT receptors (methysergide, 3.0-10 microM) to the perfusate, incrementing phrenic discharge continued. Fictive gasping was still induced, although it ceased after significantly fewer decrementing bursts than in preparations than received no blockers. Moreover, the time for recovery of rhythmic activity was significantly prolonged. This prolongation was in excess of 100 s in all preparations that received both WB-4101 (above 0.125 microM) and methysergide (above 2.5 microM). We conclude that activation of adrenergic and 5-HT2 receptors is important to sustain gasping and to restore rhythmic respiratory activity after hypoxia-induced depression.

Inspiratory activation is not required for episodic hypoxia-induced respiratory long-term facilitation in postnatal rats

Episodic hypoxia causes repetitive inspiratory activation that induces a form of respiratory plasticity termed long-term facilitation (LTF). While LTF is a function of the hypoxic exposures and inspiratory activation, their relative importance in evoking LTF is unknown. The aims of this study were to: (1) dissociate the relative roles played by episodic hypoxia and respiratory activation in LTF; and (2) determine whether the magnitude of LTF varies as a function of hypoxic intensity. We did this by examining the effects of episodic hypoxia in postnatal rats (15-25 days old), which unlike adult rats exhibit a prominent hypoxia-induced respiratory depression. We quantified inspiratory phrenic nerve activity generated by the in situ working-heart brainstem before, during and for 60 min after episodic hypoxia. We demonstrate that episodic hypoxia evokes LTF despite the fact that it potently suppresses inspiratory activity during individual hypoxic exposures (P < 0.05). Specifically, we show that after episodic hypoxia (three 5 min periods of 10% O2) respiratory frequency increased to 40 +/- 3.3% above baseline values over the next 60 min (P < 0.001). Continuous hypoxia (15 min of 10% O2) had no lasting effects on respiratory frequency (P > 0.05). To determine if LTF magnitude was affected by hypoxic intensity, the episodic hypoxia protocol was repeated under three different O2 tensions. We demonstrate that the magnitude and time course of LTF depend on hypoxic severity, with more intense hypoxia inducing a more potent degree of LTF. We conclude that inspiratory activation is not required for LTF induction, and that hypoxia per se is the physiological stimulus for eliciting hypoxia-induced respiratory LTF.