Ampakines alleviate respiratory depression in rats

Ampakines increase diaphragm activation following mid-cervical contusion injury in rats

Ampakines are positive allosteric modulators of AMPA receptors. We hypothesized that low-dose ampakine treatment increases diaphragm electromyogram (EMG) activity after mid-cervical contusion injury in rats. Adult male and female Sprague Dawley rats were implanted with in-dwelling bilateral diaphragm EMG electrodes. Rats received a 150 kDyn C4 unilateral contusion (C4Ct). At 4- and 14-days following C4Ct, rats were given an intravenous bolus of ampakine CX717 (5 mg/kg, n = 10) or vehicle (2-hydroxypropyl-beta-cyclodextrin; HPCD; n = 10). Diaphragm EMG was recorded while breathing was assessed using whole-body plethysmography. At 4-days, ampakine administration caused an immediate and sustained increase in bilateral peak inspiratory diaphragm EMG bursting and ventilation. The vehicle had no impact on EMG bursting. CX717 treated rats were able to increase EMG activity during a respiratory challenge to a greater extent vs. vehicle treated. Rats showed a considerable degree of spontaneous recovery of EMG bursting by 14 days, and the impact of CX717 delivery was blunted as compared to 4-days. Direct recordings from the phrenic nerve at 21-24 days following C4Ct confirmed that ampakines stimulated bilateral phrenic neural output in injured rats. We conclude that low-dose intravenous treatment with a low-impact ampakine can enhance diaphragm activation shortly following mid-cervical contusion injury, when deficits in diaphragm activation are prominent.

Modeling Effects of Variable preBötzinger Complex Network Topology and Cellular Properties on Opioid-Induced Respiratory Depression and Recovery

The preBötzinger complex (preBötC), located in the medulla, is the essential rhythm-generating neural network for breathing. The actions of opioids on this network impair its ability to generate robust, rhythmic output, contributing to life-threatening opioid-induced respiratory depression (OIRD). The occurrence of OIRD varies across individuals and internal and external states, increasing the risk of opioid use, yet the mechanisms of this variability are largely unknown. In this study, we utilize a computational model of the preBötC to perform several in silico experiments exploring how differences in network topology and the intrinsic properties of preBötC neurons influence the sensitivity of the network rhythm to opioids. We find that rhythms produced by preBötC networks in silico exhibit variable responses to simulated opioids, similar to the preBötC network in vitro. This variability is primarily due to random differences in network topology and can be manipulated by imposed changes in network connectivity and intrinsic neuronal properties. Our results identify features of the preBötC network that may regulate its susceptibility to opioids.

Pattern sensitivity of ampakine-hypoxia interactions for evoking phrenic motor facilitation in anesthetized rat

Repeated hypoxic episodes can produce a sustained (>60 min) increase in neural drive to the diaphragm. The requirement of repeated hypoxic episodes (vs. a single episode) to produce phrenic motor facilitation (pMF) can be removed by allosteric modulation of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors using ampakines. We hypothesized that the ampakine-hypoxia interaction resulting in pMF requires that ampakine dosing precedes the onset of hypoxia. Phrenic nerve recordings were made from urethane-anesthetized, mechanically ventilated, and vagotomized adult male Sprague-Dawley rats during isocapnic conditions. Ampakine CX717 (15 mg/kg iv) was given immediately before (n = 8), during (n = 8), or immediately after (n = 8) a 5-min hypoxic episode (arterial oxygen partial pressure 40-45 mmHg). Ampakine before hypoxia (Aprior) resulted in a sustained increase in inspiratory phrenic burst amplitude (i.e., pMF) reaching +70 ± 21% above baseline (BL) after 60 min. This was considerably greater than corresponding values in the groups receiving ampakine during hypoxia (+28 ± 47% above BL, P = 0.005 vs. Aprior) or after hypoxia (+23 ± 40% above BL, P = 0.005 vs. Aprior). Phrenic inspiratory burst rate, heart rate, and systolic, diastolic, and mean arterial pressure (mmHg) were similar across the three treatment groups (all P > 0.3, treatment effect). We conclude that the presentation order of ampakine and hypoxia impacts the magnitude of pMF, with ampakine pretreatment evoking the strongest response. Ampakine pretreatment may have value in the context of hypoxia-based neurorehabilitation strategies.NEW & NOTEWORTHY Phrenic motor facilitation (pMF) is evoked after repeated episodes of brief hypoxia. pMF can also be induced when an allosteric modulator of AMPA receptors (ampakine) is intravenously delivered immediately before a single brief hypoxic episode. Here we show that ampakine delivery before hypoxia (vs. during or after hypoxia) evokes the largest pMF with minimal impact on arterial blood pressure and heart rate. Ampakine pretreatment may have value in the context of hypoxia-based neurorehabilitation strategies.

AMPAkines have site-specific analgesic effects in the cortex

Different brain areas have distinct roles in the processing and regulation of pain and thus may form specific pharmacological targets. Prior research has shown that AMPAkines, a class of drugs that increase glutamate signaling, can enhance descending inhibition from the prefrontal cortex (PFC) and nucleus accumbens. On the other hand, activation of neurons in the anterior cingulate cortex (ACC) is known to produce the aversive component of pain. The impact of AMPAkines on ACC, however, is not known. We found that direct delivery of CX516, a well-known AMPAkine, into the ACC had no effect on the aversive response to pain in rats. Furthermore, AMPAkines did not modulate the nociceptive response of ACC neurons. In contrast, AMPAkine delivery into the prelimbic region of the prefrontal cortex (PL) reduced pain aversion. These results indicate that the analgesic effects of AMPAkines in the cortex are likely mediated by the PFC but not the ACC.

Ampakine CX614 increases respiratory rate in a mouse model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by progressive loss of dopaminergic neurons in the substantia nigra compacta (SNpc). In a mouse model of PD induced by the injection of 6-hydroxydopamine (6-OHDA) into the caudate putamen (CPu) dyspnea events are very common. Neuroanatomical and functional studies show that the number of glutamatergic neurons in the pre-Bötzinger Complex (preBötC) are reduced. We hypothesize that the neuronal loss, and consequently loss of glutamatergic connections in the respiratory network previously investigated, are responsible for the breathing impairment in PD. Here, we tested whether ampakines (CX614), a subgroup of AMPA receptor positive allosteric modulators, could stimulate the respiratory activity in PD-induced animals. CX614 (50 µM) injected intraperitoneally or directly into the preBötC region reduced the irregularity pattern and increased the respiratory rate by 37% or 82%, respectively, in PD-induced animals. CX614 also increased the respiratory frequency in healthy animals. These data suggest that ampakine CX614 could become a tool to restore breathing in PD.

Opioid Use Disorder, Sleep Deficiency, and Ventilatory Control: Bidirectional Mechanisms and Therapeutic Targets

Opioid use continues to rise globally. So too do the associated adverse consequences. Opioid use disorder (OUD) is a chronic and relapsing brain disease characterized by loss of control over opioid use and impairments in cognitive function, mood, pain perception, and autonomic activity. Sleep deficiency, a term that encompasses insufficient or disrupted sleep due to multiple potential causes, including sleep disorders, circadian disruption, and poor sleep quality or structure due to other medical conditions and pain, is present in 75% of patients with OUD. Sleep deficiency accompanies OUD across the spectrum of this addiction. The focus of this concise clinical review is to highlight the bidirectional mechanisms between OUD and sleep deficiency and the potential to target sleep deficiency with therapeutic interventions to promote long-term, healthy recovery among patients in OUD treatment. In addition, current knowledge on the effects of opioids on sleep quality, sleep architecture, sleep-disordered breathing, sleep apnea endotypes, ventilatory control, and implications for therapy and clinical practice are highlighted. Finally, an actionable research agenda is provided to evaluate the basic mechanisms of the relationship between sleep deficiency and OUD and the potential for behavioral, pharmacologic, and positive airway pressure treatments targeting sleep deficiency to improve OUD treatment outcomes.

L-cysteine methyl ester overcomes the deleterious effects of morphine on ventilatory parameters and arterial blood-gas chemistry in unanesthetized rats

We are developing a series of thiolesters that produce an immediate and sustained reversal of the deleterious effects of opioids, such as morphine and fentanyl, on ventilation without diminishing the antinociceptive effects of these opioids. We report here the effects of systemic injections of L-cysteine methyl ester (L-CYSme) on morphine-induced changes in ventilatory parameters, arterial-blood gas (ABG) chemistry (pH, pCO₂, pO₂, sO₂), Alveolar-arterial (A-a) gradient (i.e., the index of alveolar gas-exchange within the lungs), and antinociception in unanesthetized Sprague Dawley rats. The administration of morphine (10 mg/kg, IV) produced a series of deleterious effects on ventilatory parameters, including sustained decreases in tidal volume, minute ventilation, inspiratory drive and peak inspiratory flow that were accompanied by a sustained increase in end inspiratory pause. A single injection of L-CYSme (500 μmol/kg, IV) produced a rapid and long-lasting reversal of the deleterious effects of morphine on ventilatory parameters, and a second injection of L-CYSme (500 μmol/kg, IV) elicited pronounced increases in ventilatory parameters, such as minute ventilation, to values well above pre-morphine levels. L-CYSme (250 or 500 μmol/kg, IV) also produced an immediate and sustained reversal of the deleterious effects of morphine (10 mg/kg, IV) on arterial blood pH, pCO₂, pO₂, sO₂ and A-a gradient, whereas L-cysteine (500 μmol/kg, IV) itself was inactive. L-CYSme (500 μmol/kg, IV) did not appear to modulate the sedative effects of morphine as measured by righting reflex times, but did diminish the duration, however, not the magnitude of the antinociceptive actions of morphine (5 or 10 mg/kg, IV) as determined in tail-flick latency and hindpaw-withdrawal latency assays. These findings provide evidence that L-CYSme can powerfully overcome the deleterious effects of morphine on breathing and gas-exchange in Sprague Dawley rats while not affecting the sedative or early stage antinociceptive effects of the opioid. The mechanisms by which L-CYSme interferes with the OR-induced signaling pathways that mediate the deleterious effects of morphine on ventilatory performance, and by which L-CYSme diminishes the late stage antinociceptive action of morphine remain to be determined.

Chronic Opioid Use and Sleep Disorders

Opioid medications are considered a significant component in the multidisciplinary management of chronic pain. In the past two decades, the use of opioid medications has dramatically risen in part because of an increased awareness by health care providers to treat chronic pain more effectively. In addition, patients are encouraged to seek treatment. The release of a sentinel joint statement in 1997 by the American Academy of Pain Medicine and the American Pain Society in a national effort to increase awareness and support the treatment of chronic pain has undoubtedly contributed to the opioid crisis.

Sleep Deficiency and Opioid Use Disorder: Trajectory, Mechanisms, and Interventions

Opioid use disorder (OUD) is a chronic and relapsing brain disease characterized by loss of control over opioid use and impairments in cognitive function, mood, pain perception, and autonomic activity. Sleep deficiency, a term that encompasses insufficient or disrupted sleep due to multiple potential causes, including sleep disorders (eg, insomnia, sleep apnea), circadian disruption (eg, delayed sleep phase and social jet lag), and poor sleep quality (eg, sleep fragmentation, impaired sleep architecture), is present in greater than 75% of patients with OUD. This article focuses on highlighting bidirectional mechanisms between OUD and sleep deficiency and points toward promising therapeutic targets.

Spinally delivered ampakine CX717 increases phrenic motor output in adult rats

Ampakines are synthetic molecules that allosterically modulate AMPA-type glutamate receptors. We tested the hypothesis that delivery of ampakines to the intrathecal space could stimulate neural drive to the diaphragm. Ampakine CX717 (20 mM, dissolved in 10 % HPCD) or an HPCD vehicle solution were delivered via a catheter placed in the intrathecal space at the fourth cervical segment in urethane-anesthetized, mechanically ventilated adult male Sprague-Dawley rats. The electrical activity of the phrenic nerve was recorded for 60-minutes following drug application. Intrathecal application of CX717 produced a gradual and sustained increase in phrenic inspiratory burst amplitude (n = 10). In contrast, application of HPCD (n = 10) caused no sustained change in phrenic motor output. Phrenic burst rate, heart rate, and mean arterial pressure were similar between CX717 and HPCD treated rats. We conclude that intrathecally delivered ampakines can modulate phrenic motor output. This approach may have value for targeted induction of spinal neuroplasticity in the context of neurorehabiliation.

The pathophysiology of opioid-induced respiratory depression

Opiates, such as morphine, and synthetic opioids, such as fentanyl, constitute a class of drugs acting on opioid receptors which have been used therapeutically and recreationally for centuries. Opioid drugs have strong analgesic properties and are used to treat moderate to severe pain, but also present side effects including opioid dependence, tolerance, addiction, and respiratory depression, which can lead to lethal overdose if not treated. This chapter explores the pathophysiology, the neural circuits, and the cellular mechanisms underlying opioid-induced respiratory depression and provides a translational perspective of the most recent research. The pathophysiology discussed includes the effects of opioid drugs on the respiratory system in patients, as well as the animal models used to identify underlying mechanisms. Using a combination of gene editing and pharmacology, the neural circuits and molecular pathways mediating neuronal inhibition by opioids are examined. By using pharmacology and neuroscience approaches, new therapies to prevent or reverse respiratory depression by opioid drugs have been identified and are currently being developed. Considering the health and economic burden associated with the current opioid epidemic, innovative research is needed to better understand the side effects of opioid drugs and to discover new therapeutic solutions to reduce the incidence of lethal overdoses.

Dynamic Rhythmogenic Network States Drive Differential Opioid Responses in the In Vitro Respiratory Network

Death from opioid overdose is typically caused by opioid-induced respiratory depression (OIRD). A particularly dangerous characteristic of OIRD is its apparent unpredictability. The respiratory consequences of opioids can be surprisingly inconsistent, even within the same individual. Despite significant clinical implications, most studies have focused on average dose-r esponses rather than individual variation, and there remains little insight into the etiology of this apparent unpredictability. The preBötzinger complex (preBötC) in the ventral medulla is an important site for generating the respiratory rhythm and OIRD. Here, using male and female C57-Bl6 mice in vitro, we demonstrate that the preBötC can assume different network states depending on the excitability of the preBötC and the intrinsic membrane properties of preBötC neurons. These network states predict the functional consequences of opioids in the preBötC, and depending on network state, respiratory rhythmogenesis can be either stabilized or suppressed by opioids. We hypothesize that the dynamic nature of preBötC rhythmogenic properties, required to endow breathing with remarkable flexibility, also plays a key role in the dangerous unpredictability of OIRD.SIGNIFICANCE STATEMENT Opioids can cause unpredictable, life-threatening suppression of breathing. This apparent unpredictability makes clinical management of opioids difficult while also making it challenging to define the underlying mechanisms of OIRD. Here, we find in brainstem slices that the preBötC, an opioid-sensitive subregion of the brainstem, has an optimal configuration of cellular and network properties that results in a maximally stable breathing rhythm. These properties are dynamic, and the state of each individual preBötC network relative to the optimal configuration of the network predicts how vulnerable rhythmogenesis is to the effects of opioids. These insights establish a framework for understanding how endogenous and exogenous modulation of the rhythmogenic state of the preBötC can increase or decrease the risk of OIRD.

Ampakines Stimulate Diaphragm Activity after Spinal Cord Injury

Respiratory compromise after cervical spinal cord injury (SCI) is a leading cause of mortality and morbidity. Most SCIs are incomplete, and spinal respiratory motoneurons as well as proprio- and bulbospinal synaptic pathways provide a neurological substrate to enhance respiratory output. Ampakines are allosteric modulators of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, which are prevalent on respiratory neurons. We hypothesized that low dose ampakine treatment could safely and effectively increase diaphragm electromyography (EMG) activity that has been impaired as a result of acute- or sub-acute cervical SCI. Diaphragm EMG was recorded using chronic indwelling electrodes in unanesthetized, freely moving rats. A spinal hemi-lesion was induced at C2 (C2Hx), and rats were studied at 4 and 14 days post-injury during room air breathing and acute respiratory challenge accomplished by inspiring a 10% O2, 7% CO2 gas mixture. Once a stable baseline recording was established, one of two different ampakines (CX717 or CX1739, 5 mg/kg, intravenous) or a vehicle (2-hydroxypropyl-beta-cyclodextrin [HPCD]) was delivered. At 4 days post-injury, both ampakines increased diaphragm EMG output ipsilateral to C2Hx during both baseline breathing and acute respiratory challenge. Only CX1739 treatment also led to a sustained (15 min) increase in ipsilateral EMG output. At 14 days post-injury, both ampakines produced sustained increases in ipsilateral diaphragm EMG output and enabled increased output during the respiratory challenge. We conclude that low dose ampakine treatment can increase diaphragm EMG activity after cervical SCI, and therefore may provide a pharmacological strategy that could be useful in the context of respiratory rehabilitation.

Neuronal mechanisms underlying opioid-induced respiratory depression: our current understanding

Opioid-induced respiratory depression (OIRD) represents the primary cause of death associated with therapeutic and recreational opioid use. Within the United States, the rate of death from opioid abuse since the early 1990s has grown disproportionally, prompting the classification as a nationwide "epidemic." Since this time, we have begun to unravel many fundamental cellular and systems-level mechanisms associated with opioid-related death. However, factors such as individual vulnerability, neuromodulatory compensation, and redundancy of opioid effects across central and peripheral nervous systems have created a barrier to a concise, integrative view of OIRD. Within this review, we bring together multiple perspectives in the field of OIRD to create an overarching viewpoint of what we know, and where we view this essential topic of research going forward into the future.

Respiratory depression and analgesia by opioid drugs in freely behaving larval zebrafish

An opioid epidemic is spreading in North America with millions of opioid overdoses annually. Opioid drugs, like fentanyl, target the mu opioid receptor system and induce potentially lethal respiratory depression. The challenge in opioid research is to find a safe pain therapy with analgesic properties but no respiratory depression. Current discoveries are limited by lack of amenable animal models to screen candidate drugs. Zebrafish (Danio rerio) is an emerging animal model with high reproduction and fast development, which shares remarkable similarity in their physiology and genome to mammals. However, it is unknown whether zebrafish possesses similar opioid system, respiratory and analgesic responses to opioids than mammals. In freely-behaving larval zebrafish, fentanyl depresses the rate of respiratory mandible movements and induces analgesia, effects reversed by μ-opioid receptor antagonists. Zebrafish presents evolutionary conserved mechanisms of action of opioid drugs, also found in mammals, and constitute amenable models for phenotype-based drug discovery.

When it comes to treating severe pain, a doctor’s arsenal is somewhat limited: synthetic or natural opioids such as morphine, fentanyl or oxycodone are often one of the only options available to relieve patients. Yet these compounds can make breathing slower and shallower, quickly depriving the body of oxygen and causing death. This lethal side-effect is particularly devastating as opioids misuse has reached dangerously high levels in the United States, creating an ‘opioid epidemic’ which has claimed the lives of over 80,000 Americans in 2020. It is therefore crucial to find safer drugs that do not have this effect on breathing, but this research has been slowed down by the lack of animal models in which to study the effect of new compounds.

Zebrafish are small freshwater fish that reproduce and develop fast, yet they are also remarkably genetically similar to mammals and feature a complex nervous system. However, it is not known whether the effect of opioids on zebrafish is comparable to mammals, and therefore whether these animals can be used to test new drugs for pain relief.

To investigate this question, Zaig et al. exposed zebrafish larvae to fentanyl, showing that the fish then exhibited slower lower jaw movements – a sign of decreased breathing. The fish also could also tolerate a painful stimulus for longer, suggesting that this opioid does reduce pain in the animals. Together, these results point towards zebrafish and mammals sharing similar opioid responses, demonstrating that the fish could be used to test potential pain medications. The methods Zaig et al. have developed to establish these results could be harnessed to quickly assess large numbers of drug compounds, as well as decipher how pain emerges and can be stopped.

AMPAkines potentiate the corticostriatal pathway to reduce acute and chronic pain

The corticostriatal circuit plays an important role in the regulation of reward- and aversion-types of behaviors. Specifically, the projection from the prelimbic cortex (PL) to the nucleus accumbens (NAc) has been shown to regulate sensory and affective aspects of pain in a number of rodent models. Previous studies have shown that enhancement of glutamate signaling through the NAc by AMPAkines, a class of agents that specifically potentiate the function of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, reduces acute and persistent pain. However, it is not known whether postsynaptic potentiation of the NAc with these agents can achieve the full anti-nociceptive effects of PL activation. Here we compared the impact of AMPAkine treatment in the NAc with optogenetic activation of the PL on pain behaviors in rats. We found that not only does AMPAkine treatment partially reconstitute the PL inhibition of sensory withdrawals, it fully occludes the effect of the PL on reducing the aversive component of pain. These results indicate that the NAc is likely one of the key targets for the PL, especially in the regulation of pain aversion. Furthermore, our results lend support for neuromodulation or pharmacological activation of the corticostriatal circuit as an important analgesic approach.

Maternal Methadone Destabilizes Neonatal Breathing and Desensitizes Neonates to Opioid-Induced Respiratory Frequency Depression

Pregnant women and developing infants are understudied populations in the opioid crisis, despite the rise in opioid use during pregnancy. Maternal opioid use results in diverse negative outcomes for the fetus/newborn, including death; however, the effects of perinatal (maternal and neonatal) opioids on developing respiratory circuitry are not well understood. Given the profound depressive effects of opioids on central respiratory networks controlling breathing, we tested the hypothesis that perinatal opioid exposure impairs respiratory neural circuitry, creating breathing instability. Our data demonstrate maternal opioids increase apneas and destabilize neonatal breathing. Maternal opioids also blunted opioid-induced respiratory frequency depression acutely in neonates; a unique finding since adult respiratory circuity does not desensitize to opioids. This desensitization normalized rapidly between postnatal days 1 and 2 (P1 and P2), the same age quantal slowing emerged in respiratory rhythm. These data suggest significant reorganization of respiratory rhythm generating circuits at P1-2, the same time as the preBötzinger Complex (key site of respiratory rhythm generation) becomes the dominant respiratory rhythm generator. Thus, these studies provide critical insight relevant to the normal developmental trajectory of respiratory circuits and suggest changes to mutual coupling between respiratory oscillators, while also highlighting how maternal opioids alter these developing circuits. In conclusion, the results presented demonstrate neurorespiratory disruption by maternal opioids and blunted opioid-induced respiratory frequency depression with neonatal opioids, which will be important for understanding and treating the increasing population of neonates exposed to gestational opioids.

Pharmacological restoration of anti-nociceptive functions in the prefrontal cortex relieves chronic pain

Chronic pain affects one in four adults, and effective non-sedating and non-addictive treatments are urgently needed. Chronic pain causes maladaptive changes in the cerebral cortex, which can lead to impaired endogenous nociceptive processing. However, it is not yet clear if drugs that restore endogenous cortical regulation could provide an effective therapeutic strategy for chronic pain. Here, we studied the nociceptive response of neurons in the prelimbic region of the prefrontal cortex (PL-PFC) in freely behaving rats using a spared nerve injury (SNI) model of chronic pain, and the impact of AMPAkines, a class of drugs that increase central glutamate signaling, on such response. We found that neurons in the PL-PFC increase their firing rates in response to noxious stimulations; chronic neuropathic pain, however, suppressed this important cortical pain response. Meanwhile, CX546, a well-known AMPAkine, restored the anti-nociceptive response of PL-PFC neurons in the chronic pain condition. In addition, both systemic administration and direct delivery of CX546 into the PL-PFC inhibited symptoms of chronic pain, whereas optogenetic inactivation of the PFC neurons or administration of AMPA receptor antagonists in the PL-PFC blocked the anti-nociceptive effects of CX546. These results indicate that restoration of the endogenous anti-nociceptive functions in the PL-PFC by pharmacological agents such as AMPAkines constitutes a successful strategy to treat chronic neuropathic pain.

The conundrum of polysubstance overdose

WHAT IS KNOWN AND OBJECTIVE

Treating an opioid overdose using an opioid receptor antagonist (such as naloxone) makes mechanistic sense and can be effective. Unfortunately, the majority of current drug overdose deaths involve polysubstance use (i.e., an opioid plus a non-opioid).

COMMENT

Respiratory depression induced by opioids results from excessive opioid molecules binding to opioid receptors. This effect can be reversed by an opioid receptor antagonist. However, the respiratory depression induced by non-opioid drugs is not due to action at opioid receptors; thus, an opioid receptor antagonist is ineffective in many of these cases. For respiratory depression induced by non-opioids, receptor antagonists are either not available (e.g., for propofol overdose) or there may be attendant risks with their use (e.g., seizures with flumazenil). This gives rise to a need for more effective ways to treat polysubstance overdose.

WHAT IS NEW AND CONCLUSION

A new approach to treating opioid-induced respiratory depression due to drug overdose focuses on agents that stimulate respiratory drive rather than competing for opioid receptors. Such an approach is "agnostic" to the cause of the respiratory depression, so might be a potential way to treat polysubstance overdose.

An overview of developmental dysregulation of autonomic control in infants

In this short review, we provide an overview of developmental disorders causing autonomic nervous system dysregulation. We briefly discuss perinatal conditions that adversely impact developmental outcomes including apnea of prematurity, sudden infant death syndrome, and Rett syndrome. We provide a brief clinical description, an overview of known or hypothesized mechanisms for the disorder, and current standard of practice for treatment of each condition. Additionally, we consider preventative measures and complications of these disorders to provide further insight into the pathogenesis of specific autonomic dysregulation in neonates. The goal of this short review is to provide an updated understanding of the impact of autonomic dysregulation on development of brainstem circuits and to briefly highlight promising future treatment options and controversies.

D-Amphetamine Accelerates Recovery of Consciousness and Respiratory Drive After High-Dose Fentanyl in Rats

In the United States, fentanyl causes approximately 60,000 drug overdose deaths each year. Fentanyl is also frequently administered as an analgesic in the perioperative setting, where respiratory depression remains a common clinical problem. Naloxone is an efficacious opioid antagonist, but it possesses a short half-life and undesirable side effects. This study was conducted to test the hypothesis that d-amphetamine ameliorates respiratory depression and hastens the return of consciousness following high-dose fentanyl. Behavioral endpoints (first head movement, two paws down, and return of righting), arterial blood gas analysis and local field potential recordings from the prefrontal cortex were conducted in adult rats after intravenous administration of of fentanyl (55 µg/kg) at a dose sufficient to induce loss of righting and respiratory depression, followed by intravenous d-amphetamine (3 mg/kg) or saline (vehicle). D-amphetamine accelerated the time to return of righting by 36.6% compared to saline controls. D-amphetamine also hastened recovery of arterial pH, and the partial pressure of CO2, O2 and sO2 compared to controls, with statistically significant differences in pH after 5 min and 15 min. Local field potential recordings from the prefrontal cortex showed that within 5 min of d-amphetamine administration, the elevated broadband power <20 Hz produced by fentanyl had returned to awake baseline levels, consistent with the return of consciousness. Overall, d-amphetamine attenuated respiratory acidosis, increased arterial oxygenation, and accelerated the return of consciousness in the setting of fentanyl intoxication. This suggests that d-amphetamine may be a useful adjunct or alternative to opioid receptor antagonists such as naloxone.

The impact and mechanism of ampakine CX1739 on protection against respiratory depression in rats

Background: Abuse of analgesic and sedative drugs often leads to severe respiratory depression and sometimes death. Approximately 69,000 people worldwide die annually from opioid overdoses. Purpose: This work aimed to investigate whether CX1739 can be used for emergency treatment of acute respiratory depression due to drug abuse. Results: First, the results clarify that CX1739 is a low-impact ampakine that can safely activate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors without causing excito-neurotoxicity. Second, CX1739 rapidly crossed the blood-brain barrier (T_max = 2 min), which meets the requirement of rapid onset of action in vivo. Our work provides preliminarily confirmation that high-dose intravenous administration of CX1739 can immediately reverse respiratory depression in animal models of respiratory depression caused by opioid agonist 030418, pentobarbital sodium and ethanol.

Countering Opioid-induced Respiratory Depression in Male Rats with Nicotinic Acetylcholine Receptor Partial Agonists Varenicline and ABT 594

BACKGROUND

Opioids can induce significant respiratory depression when administered as analgesics for the treatment of acute, postoperative, and chronic pain. There are currently no pharmacologic means of reversing opioid-induced respiratory depression without interfering with analgesia. Further, there is a growing epidemic of opioid overdose that could benefit from therapeutic advancements. The aim of this study was to test the ability of two partial agonists of α4β2 nicotinic acetylcholine receptors, varenicline (used clinically for smoking cessation) and ABT 594 (tebanicline, developed as an analgesic), to reduce respiratory depression induced by fentanyl, remifentanil, morphine, and a combination of fentanyl and diazepam.

METHODS

Whole body plethysmographic recordings, nociception testing, and righting reflex testing were used to examine ventilation, analgesia, and sedation in adult male Sprague-Dawley rats.

RESULTS

Pre-, co-, or postadministration of varenicline or ABT 594 did not alter baseline breathing but markedly reduced opioid-induced respiratory depression. Varenicline had no effect on fentanyl-induced analgesia and ABT 594 potentiated fentanyl-induced analgesia. Specifically, 10-min administration of fentanyl induced a decrease in respiratory rate to 43 ± 32% of control in vehicle group, which was alleviated by preadministration of varenicline (85 ± 14% of control, n = 8, P < 0.001) or ABT 594 (81 ± 36% of control, n = 8, P = 0.001). ABT 594 or varenicline with a low dose of naloxone (1 µg/kg), but not varenicline alone, partially reversed fentanyl-induced lethal apnea, but neither compound provided the very rapid and complete reversal of apnea achieved with high doses of naloxone (0.03 to 1 mg/kg). Administration of varenicline (n = 4, P = 0.034) or ABT 594 (n = 4, P = 0.034) prevented lethal apneas induced by the combination of fentanyl and diazepam.

CONCLUSIONS

Activation of α4β2 nicotinic acetylcholine receptors by varenicline and ABT 594 counters opioid-induced respiratory depression without interfering with analgesia.

Activating α4β2 Nicotinic Acetylcholine Receptors Alleviates Fentanyl-induced Respiratory Depression in Rats

BACKGROUND

Opioid analgesics are widely used for treatment of acute, postoperative, and chronic pain. However, activation of opioid receptors can result in severe respiratory depression. There is an unmet clinical need to develop a pharmacologic therapy to counter opioid-induced respiratory depression without interfering with analgesia. Further, additional advances to confront accidental lethal overdose with the use of fentanyl and other opioids are needed. Here, the authors test the hypothesis that activation of nicotinic receptors expressed within respiratory rhythm-generating networks would counter opioid-induced respiratory depression without compromising analgesia.

METHODS

Respiratory neural discharge was measured using in vitro brainstem-spinal cord and medullary slice rat preparations. In vivo, plethysmographic recording, nociception testing, and righting reflexes were used to examine respiratory ventilation, analgesia, and sedation, respectively.

RESULTS

The administration of nicotine, selective α4β2 nicotinic receptor agonist A85380, but not α7 nicotinic receptor agonist PNU282987, reversed opioid-induced respiratory depression in neonatal pups in vitro and in vivo. In adult rats in vivo, administration of A85380 (0.03 mg/kg), but not PNU282987, provides a rapid and robust reversal of fentanyl-induced decrease in respiratory rate (93.4 ± 33.7% of control 3 min after A85380 vs. 31 ± 20.5% of control after vehicle, n = 8 each, P < 0.001), without marked side effects. The coadministration of A85380 (0.06 mg/kg) with fentanyl or remifentanil markedly reduced respiratory depression and apneas, and enhanced the fentanyl-induced analgesia, as evidenced by increased paw withdrawal latency in Hargreaves plantar test (14.4 ± 2.8 s vs. vehicle: 11.3 ± 2.4 s, n = 8 each, P = 0.013) and decreased formalin-induced nocifensive duration (2.5 ± 2.4 min vs. vehicle: 5.4 ± 2.7 min, n = 8 each, P = 0.029).

CONCLUSIONS

The novel strategy of targeting α4β2 nicotinic acetylcholine receptors has the potential for advancing pain control and reducing opioid-induced respiratory depression and overdose.

Countering opioid-induced respiratory depression by non-opioids that are respiratory stimulants

Strong opioid analgesics are the mainstay of therapy for the relief of moderate to severe acute nociceptive pain that may occur post-operatively or following major trauma, as well as for the management of chronic cancer-related pain. Opioid-related adverse effects include nausea and vomiting, sedation, respiratory depression, constipation, tolerance, and addiction/abuse liability. Of these, respiratory depression is of the most concern to clinicians owing to the potential for fatal consequences. In the broader community, opioid overdose due to either prescription or illicit opioids or co-administration with central nervous system depressants may evoke respiratory depression. To address this problem, there is ongoing interest in the identification of non-opioid respiratory stimulants to reverse opioid-induced respiratory depression but without reversing opioid analgesia. Promising compound classes evaluated to date include those that act on a diverse array of receptors including 5-hydroxytryptamine, D ₁-dopamine, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-D-aspartate (NMDA) receptor antagonists, and nicotinic acetylcholine as well as phosphodiesterase inhibitors and molecules that act on potassium channels on oxygen-sensing cells in the carotid body. The aim of this article is to review recent advances in the development potential of these compounds for countering opioid-induced respiratory depression.

Presynaptic Mechanisms and KCNQ Potassium Channels Modulate Opioid Depression of Respiratory Drive

Opioid-induced respiratory depression (OIRD) is the major cause of death associated with opioid analgesics and drugs of abuse, but the underlying cellular and molecular mechanisms remain poorly understood. We investigated opioid action in vivo in unanesthetized mice and in in vitro medullary slices containing the preBötzinger Complex (preBötC), a locus critical for breathing and inspiratory rhythm generation. Although hypothesized as a primary mechanism, we found that mu-opioid receptor (MOR1)-mediated GIRK activation contributed only modestly to OIRD. Instead, mEPSC recordings from genetically identified Dbx1-derived interneurons, essential for rhythmogenesis, revealed a prevalent presynaptic mode of action for OIRD. Consistent with MOR1-mediated suppression of presynaptic release as a major component of OIRD, Cacna1a KO slices lacking P/Q-type Ca2+ channels enhanced OIRD. Furthermore, OIRD was mimicked and reversed by KCNQ potassium channel activators and blockers, respectively. In vivo whole-body plethysmography combined with systemic delivery of GIRK- and KCNQ-specific potassium channel drugs largely recapitulated these in vitro results, and revealed state-dependent modulation of OIRD. We propose that respiratory failure from OIRD results from a general reduction of synaptic efficacy, leading to a state-dependent collapse of rhythmic network activity.

Averting Opioid-induced Respiratory Depression without Affecting Analgesia

The ventilatory control system is highly vulnerable to exogenous administered opioid analgesics. Particularly respiratory depression is a potentially lethal complication that may occur when opioids are overdosed or consumed in combination with other depressants such as sleep medication or alcohol. Fatalities occur in acute and chronic pain patients on opioid therapy and individuals that abuse prescription or illicit opioids for their hedonistic pleasure. One important strategy to mitigate opioid-induced respiratory depression is cotreatment with nonopioid respiratory stimulants. Effective stimulants prevent respiratory depression without affecting the analgesic opioid response. Several pharmaceutical classes of nonopioid respiratory stimulants are currently under investigation. The majority acts at sites within the brainstem respiratory network including drugs that act at α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (ampakines), 5-hydroxytryptamine receptor agonists, phospodiesterase-4 inhibitors, D1-dopamine receptor agonists, the endogenous peptide glycyl-glutamine, and thyrotropin-releasing hormone. Others act peripherally at potassium channels expressed on oxygen-sensing cells of the carotid bodies, such as doxapram and GAL021 (Galleon Pharmaceuticals Corp., USA). In this review we critically appraise the efficacy of these agents. We conclude that none of the experimental drugs are adequate for therapeutic use in opioid-induced respiratory depression and all need further study of efficacy and toxicity. All discussed drugs, however, do highlight potential mechanisms of action and possible templates for further study and development.

Regulation of breathing pattern by IL-10

Proinflammatory cytokines like interleukin-1β (IL-1β) affect the control of breathing. Our aim is to determine the effect of the anti-inflammatory cytokine IL-10 οn the control of breathing. IL-10 knockout mice (IL-10^-/-, n = 10) and wild-type mice (IL-10^+/+, n = 10) were exposed to the following test gases: hyperoxic hypercapnia 7% CO₂-93% O₂, normoxic hypercapnia 7% CO₂-21% O₂, hypoxic hypercapnia 7% CO₂-10% O₂, and hypoxic normocapnia 3% CO₂-10% O₂. The ventilatory function was assessed using whole body plethysmography. Recombinant mouse IL-10 (rIL-10; 10 μg/kg) was administered intraperitoneally to wild-type mice (n = 10) 30 min before the onset of gas challenge. IL-10 was administered in neonatal medullary slices (10-30 ng/ml, n = 8). We found that IL-10^-/- mice exhibited consistently increased frequency and reduced tidal volume compared with IL-10^+/+ mice during room air breathing and in all test gases (by 23.62 to 33.2%, P < 0.05 and -36.23 to -41.69%, P < 0.05, respectively). In all inspired gases, the minute ventilation of IL-10^-/- mice was lower than IL-10^+/+ (by -15.67 to -22.74%, P < 0.05). The rapid shallow breathing index was higher in IL-10^-/- mice compared with IL-10^+/+ mice in all inspired gases (by 50.25 to 57.5%, P < 0.05). The intraperitoneal injection of rIL-10 caused reduction of the respiratory rate and augmentation of the tidal volume in room air and also in all inspired gases (by -12.22 to -29.53 and 32.18 to 45.11%, P < 0.05, respectively). IL-10 administration in neonatal rat (n = 8) in vitro rhythmically active medullary slice preparations did not affect either rhythmicity or peak amplitude of hypoglossal nerve discharge. In conclusion, IL-10 may induce a slower and deeper pattern of breathing.

Pre-treatment with a combination of Shenmai and Danshen injection protects cardiomyocytes against hypoxia/reoxygenation- and H2O2-induced injury by inhibiting mitochondrial permeability transition pore opening

Increasing evidence has indicated that opening of the mitochondrial permeability transition pore (mPTP) has a vital role in myocardial ischemia/reperfusion (I/R) injury. Shenmai injection (SMI) plus Danshen injection (DSI) combination, termed Yiqi Yangyin Huoxue (YYH) therapy is used in the clinic to treat cardiovascular diseases, including myocardial I/R injury. Previous studies by our group have demonstrated the protective effect of pretreatment with YYH against myocardial I/R injury in isolated rat hearts. The present study aimed to examine the protective effect of YYH against hypoxia/reoxygenation (H/R)- and H₂O₂-induced cardiomyocyte injury, and to determine whether this effect is produced by inhibition of mPTP opening. Primary cardiomyocytes isolated from neonatal rats were cultured and randomly grouped into a control group, injury group and pretreatment group, with six duplicated wells in each group during each assay. Cardiomyocytes in the injury group were subjected to H/R to simulate I/R or exposed to H₂O₂ for 2 h to induce oxidative injury. Cellular injury was assessed via release of creatine kinase (CK) and lactate dehydrogenase (LDH), and cell viability was measured by an MTT assay. The mitochondrial membrane potential (ΔΨm) and cytosolic reactive oxygen species (ROS) were detected using the fluorescent probes rhodamine123 (Rh123) and chloromethyl-2,7-dichlorodihydrofluorescein diacetate (CM-H₂DCFDA), respectively. Intracellular Ca²⁺, mitochondrial Ca²⁺ and mPTP opening were measured using fluo-4 acetoxymethyl (Fluo-4/AM), rhodamine-2 acetoxymethyl (Rhod-2/AM) and calcein acetoxymethyl (Calcein/AM) probes, respectively. The results indicated that pretreatment with YYH enhanced cell viability, increased ΔΨm, reduced CK and LDH release, and decreased intracellular ROS and Ca²⁺, thus reducing cardiomyocyte injury induced by H/R or H₂O₂. LY294002, a specific phosphoinositide 3-kinase (PI3K) inhibitor, and PD98059, a specific inhibitor of the extracellular signal-regulated kinase 1/2 (Erk1/2) pathway, eliminated the protective effects of the combination therapy on cell viability and the change in the ΔΨm in cardiomyocytes. In conclusion, pre-treatment with YYH has cardioprotective effects against H/R injury and oxidative stress via activation of the PI3K/Akt and Erk1/2 signaling pathways, which reduces mPTP opening, overproduction of ROS and calcium overload.

The Impact and Mechanism of a Novel Allosteric AMPA Receptor Modulator LCX001 on Protection Against Respiratory Depression in Rodents

Analgesics and sedative hypnotics in clinical use often give rise to significant side effects, particularly respiratory depression. For emergency use, specific antagonists are currently administered to counteract respiratory depression. However, antagonists are often short-lasting and eliminate drug generated analgesia. To resolve this issue, novel positive AMPA modulators, LCX001, was tested to alleviate respiratory depression triggered by different drugs. The acetic acid writhing and hot-plate test were conducted to evaluate analgesic effect of LCX001. Binding assay, whole-cell recording, live cell imaging, and Ca²⁺ imaging were used to clarify mechanism and impact of LCX001 on respiratory protection. Results showed that LCX001 effectively rescued and prevented opioid (fentanyl and TH-030418), propofol, and pentobarbital-induced respiratory depression by strengthening respiratory frequency and minute ventilation. The acetic acid writhing test and hot-plate test revealed potent anti-nociceptive efficacy of LCX001, in contrast to other typical ampakines that did not affect analgesia. Furthermore, LCX001 potentiated [³H]AMPA and L-glutamate binding affinity to AMPA receptors, and facilitated glutamate-evoked inward currents in HEK293 cells stably expressing GluA2(R). LCX001 had a typical positive modulatory impact on AMPAR-mediated function. Importantly, application of LCX001 generated a significant increase in GluA2(R) surface expression, and restrained opioid-induced abnormal intracellular Ca²⁺ load, which might participate in breathing modulation. Our study improves therapeutic interventions for the treatment of drug induced respiratory depression, and increases understanding of potential mechanism of AMPA receptor modulators.

AMPA receptor positive allosteric modulators attenuate morphine tolerance and dependence

Development of opioid tolerance and dependence hinders the use of opioids for the treatment of chronic pain. In searching for the mechanism and potential intervention for opioid tolerance and dependence, we studied the action of two positive allosteric modulators of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR PAMs). In mice treated with morphine (100 mg/kg, s.c.), acute morphine tolerance and dependence developed in 4-6 h. Treatment with aniracetam, a well-established AMPAR PAM, was able to completely prevent and reverse the development of acute antinociceptive tolerance to morphine. Partial, but significant, effects of aniracetam on acute morphine induced-physical dependence were also observed. Moreover, aniracetam significantly reversed the established morphine tolerance and dependence in a chronic model of morphine tolerance and dependence produced by intermittent morphine (10 mg/kg, s.c. for 5d). In addition, HJC0122, a new AMPAR PAM was found to have similar effects as aniracetam but with a higher potency. These previously undisclosed actions of AMPAR PAMs are intriguing and may shed lights on understanding the APMA signaling pathway in opioid addiction. Moreover, these data suggest that AMPAR PAMs may have utility in preventing and treating morphine tolerance and dependence.

Effects of Chronic Opioid Use on Sleep and Wake

Chronic use of opioids negatively affects sleep on 2 levels: sleep architecture and breathing. Patients suffer from a variety of daytime sequelae. There may be a bidirectional relationship between poor sleep quality, sleep-disordered breathing, and daytime function. Opioids are a potential cause of incident depression. The best therapeutic option is withdrawal of opioids, which proves difficult. Positive airway pressure devices are considered first-line treatment for sleep-related breathing disorders. New generation positive pressure servo ventilators are increasingly popular as a treatment option for opioid-induced sleep-disordered breathing. Treatments to improve sleep quality, sleep-related breathing disorders, and quality of life in patients who use opioids chronically are discussed.

AMPAkines and morphine provide complementary analgesia

Glutamate signaling in the central nervous system is known to play a key role in pain regulation. AMPAkines can enhance glutamate signaling through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. previous studies have shown that AMPAkines are effective analgesic agents, and their site of action is likely in the brain. It is not known, however, if AMPAkines can provide complementary analgesia in combination with opioids, the most commonly used analgesics. Here, we show that the co-administration of an AMPAkine with morphine can provide additional analgesia, both in naïve rats and in rats that experience postoperative pain. Furthermore, we show that this AMPAkine can be administered directly into the prefrontal cortex to provide analgesia, and that prefrontal AMPAkine infusion, similar to systemic administration, can provide added pain relief to complement morphine analgesia.

Mechanistic Studies of Capsaicin-Induced Apnea in Rodents

Inhalation of capsaicin-based sprays can cause central respiratory depression and lethal apneas. There are contradictory reports regarding the sites of capsaicin action. Furthermore, an understanding of the neurochemical mechanisms underlying capsaicin-induced apneas and the development of pharmacological interventions is lacking. The main objectives of this study were to perform a systematic study of the mechanisms of action of capsaicin-induced apneas and to provide insights relevant to pharmacological intervention. In vitro and in vivo rat and transient receptor potential vanilloid superfamily member 1 (TRPV1)-null mouse models were used to measure respiratory parameters and seizure-like activity in the presence of capsaicin and compounds that modulate glutamatergic neurotransmission. Administration of capsaicin to in vitro and in vivo rat and wild-type mouse models induced dose-dependent apneas and the production of seizure-like activity. No significant changes were observed in TRPV1-null mice or rat medullary slice preparations. The capsaicin-induced effects were inhibited by the TRPV1 antagonist capsazepine, amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonists CNQX, NBQX, perampanel, and riluzole, a drug that inhibits glutamate release and increases glutamate uptake. The capsaicin-induced effects on breathing and seizure-like activity were accentuated by positive allosteric modulators of the AMPA receptors, CX717 and cyclothiazide. To summarize, capsaicin-induced apneas and seizure-like behaviors are mediated via TRPV1 activation acting at lung afferents, spinal cord-ascending tracts, and medullary structures (including nucleus tractus solitarius). AMPA receptor-mediated conductances play an important role in capsaicin-induced apneas and seizure-like activity. A pharmaceutical strategy targeted at reducing AMPA receptor-mediated glutamatergic signaling may reduce capsaicin-induced deleterious effects.

AMPAkines Target the Nucleus Accumbens to Relieve Postoperative Pain

BACKGROUND

AMPAkines augment the function of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the brain to increase excitatory outputs. These drugs are known to relieve persistent pain. However, their role in acute pain is unknown. Furthermore, a specific molecular and anatomic target for these novel analgesics remains elusive.

METHODS

The authors studied the analgesic role of an AMPAkine, CX546, in a rat paw incision (PI) model of acute postoperative pain. The authors measured the effect of AMPAkines on sensory and depressive symptoms of pain using mechanical hypersensitivity and forced swim tests. The authors asked whether AMPA receptors in the nucleus accumbens (NAc), a key node in the brain's reward and pain circuitry, can be a target for AMPAkine analgesia.

RESULTS

Systemic administration of CX546 (n = 13), compared with control (n = 13), reduced mechanical hypersensitivity (50% withdrawal threshold of 6.05 ± 1.30 g [mean ± SEM] vs. 0.62 ± 0.13 g), and it reduced depressive features of pain by decreasing immobility on the forced swim test in PI-treated rats (89.0 ± 15.5 vs. 156.7 ± 18.5 s). Meanwhile, CX546 delivered locally into the NAc provided pain-relieving effects in both PI (50% withdrawal threshold of 6.81 ± 1.91 vs. 0.50 ± 0.03 g; control, n = 6; CX546, n = 8) and persistent postoperative pain (spared nerve injury) models (50% withdrawal threshold of 3.85 ± 1.23 vs. 0.45 ± 0.00 g; control, n = 7; CX546, n = 11). Blocking AMPA receptors in the NAc with 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione inhibited these pain-relieving effects (50% withdrawal threshold of 7.18 ± 1.52 vs. 1.59 ± 0.66 g; n = 8 for PI groups; 10.70 ± 3.45 vs. 1.39 ± 0.88 g; n = 4 for spared nerve injury groups).

CONCLUSIONS

AMPAkines relieve postoperative pain by acting through AMPA receptors in the NAc.

Novel Therapies for the Treatment of Central Sleep Apnea

Neurophysiologically, central apnea is due to a temporary cessation of respiratory rhythmogenesis in medullary respiratory networks. Central apneas occur in several disorders and result in pathophysiological consequences, including arousals and desaturation. The 2 most common causes in adults are congestive heart failure and chronic use of opioids to treat pain. Under such circumstances, diagnosis and treatment of central sleep apnea may improve quality of life, morbidity, and mortality. This article discusses recent developments in the treatment of central sleep apnea in heart failure and opioids use.

Control of Ventilation in Health and Disease

Control of ventilation occurs at different levels of the respiratory system through a negative feedback system that allows precise regulation of levels of arterial carbon dioxide and oxygen. Mechanisms for ventilatory instability leading to sleep-disordered breathing include changes in the genesis of respiratory rhythm and chemoresponsiveness to hypoxia and hypercapnia, cerebrovascular reactivity, abnormal chest wall and airway reflexes, and sleep state oscillations. One can potentially stabilize breathing during sleep and treat sleep-disordered breathing by identifying one or more of these pathophysiological mechanisms. This review describes the current concepts in ventilatory control that pertain to breathing instability during wakefulness and sleep, delineates potential avenues for alternative therapies to stabilize breathing during sleep, and proposes recommendations for future research.

Kölliker-Fuse nuclei regulate respiratory rhythm variability via a gain-control mechanism

Respiration varies from breath to breath. On the millisecond timescale of spiking, neuronal circuits exhibit variability due to the stochastic properties of ion channels and synapses. Does this fast, microscopic source of variability contribute to the slower, macroscopic variability of the respiratory period? To address this question, we modeled a stochastic oscillator with forcing; then, we tested its predictions experimentally for the respiratory rhythm generated by the in situ perfused preparation during vagal nerve stimulation (VNS). Our simulations identified a relationship among the gain of the input, entrainment strength, and rhythm variability. Specifically, at high gain, the periodic input entrained the oscillator and reduced variability, whereas at low gain, the noise interacted with the input, causing events known as "phase slips", which increased variability on a slow timescale. Experimentally, the in situ preparation behaved like the low-gain model: VNS entrained respiration but exhibited phase slips that increased rhythm variability. Next, we used bilateral muscimol microinjections in discrete respiratory compartments to identify areas involved in VNS gain control. Suppression of activity in the nucleus tractus solitarii occluded both entrainment and amplification of rhythm variability by VNS, confirming that these effects were due to the activation of the Hering-Breuer reflex. Suppressing activity of the Kölliker-Fuse nuclei (KFn) enhanced entrainment and reduced rhythm variability during VNS, consistent with the predictions of the high-gain model. Together, the model and experiments suggest that the KFn regulates respiratory rhythm variability via a gain control mechanism.

G-protein-gated Inwardly Rectifying Potassium Channels Modulate Respiratory Depression by Opioids

BACKGROUND

Drugs acting on μ-opioid receptors (MORs) are widely used as analgesics but present side effects including life-threatening respiratory depression. MORs are G-protein-coupled receptors inhibiting neuronal activity through calcium channels, adenylyl cyclase, and/or G-protein-gated inwardly rectifying potassium (GIRK) channels. The pathways underlying MOR-dependent inhibition of rhythmic breathing are unknown.

METHODS

By using a combination of genetic, pharmacological, and physiological tools in rodents in vivo, the authors aimed to identify the role of GIRK channels in MOR-mediated inhibition of respiratory circuits.

RESULTS

GIRK channels were expressed in the ventrolateral medulla, a neuronal population regulating rhythmic breathing, and GIRK channel activation with flupirtine reduced respiratory rate in rats (percentage of baseline rate in mean ± SD: 79.4 ± 7.4%, n = 7), wild-type mice (82.6 ± 3.8%, n = 3), but not in mice lacking the GIRK2 subunit, an integral subunit of neuronal GIRK channels (GIRK2, 101.0 ± 1.9%, n = 3). Application of the MOR agonist [D-Ala, N-MePhe, Gly-ol]-enkephalin (DAMGO) to the ventrolateral medulla depressed respiratory rate, an effect partially reversed by the GIRK channel blocker Tertiapin-Q (baseline: 42.1 ± 7.4 breath/min, DAMGO: 26.1 ± 13.4 breath/min, Tertiapin-Q + DAMGO: 33.9 ± 9.8 breath/min, n = 4). Importantly, DAMGO applied to the ventrolateral medulla failed to reduce rhythmic breathing in GIRK2 mice (percentage of baseline rate: 103.2 ± 12.1%, n = 4), whereas it considerably reduced rate in wild-type mice (62.5 ± 17.7% of baseline, n = 4). Respiratory rate depression by systemic injection of the opioid analgesic fentanyl was markedly reduced in GIRK2 (percentage of baseline: 12.8 ± 15.8%, n = 5) compared with wild-type mice (72.9 ± 27.3%).

CONCLUSIONS

Overall, these results identify that GIRK channels contribute to respiratory inhibition by MOR, an essential step toward understanding respiratory depression by opioids.

Ampakine CX717 potentiates intermittent hypoxia-induced hypoglossal long-term facilitation

Glutamatergic currents play a fundamental role in regulating respiratory motor output and are partially mediated by α-amino-3-hydroxy-5-methyl-isoxazole-propionic acid (AMPA) receptors throughout the premotor and motor respiratory circuitry. Ampakines are pharmacological compounds that enhance glutamatergic transmission by altering AMPA receptor channel kinetics. Here, we examined if ampakines alter the expression of respiratory long-term facilitation (LTF), a form of neuroplasticity manifested as a persistent increase in inspiratory activity following brief periods of reduced O2 [intermittent hypoxia (IH)]. Current synaptic models indicate enhanced effectiveness of glutamatergic synapses after IH, and we hypothesized that ampakine pretreatment would potentiate IH-induced LTF of respiratory activity. Inspiratory bursting was recorded from the hypoglossal nerve of anesthetized and mechanically ventilated mice. During baseline (BL) recording conditions, burst amplitude was stable for at least 90 min (98 ± 5% BL). Exposure to IH (3 × 1 min, 15% O2) resulted in a sustained increase in burst amplitude (218 ± 44% BL at 90 min following final bout of hypoxia). Mice given an intraperitoneal injection of ampakine CX717 (15 mg/kg) 10 min before IH showed enhanced LTF (500 ± 110% BL at 90 min). Post hoc analyses indicated that CX717 potentiated LTF only when initial baseline burst amplitude was low. We conclude that under appropriate conditions ampakine pretreatment can potentiate IH-induced respiratory LTF. These data suggest that ampakines may have therapeutic value in the context of hypoxia-based neurorehabilitation strategies, particularly in disorders with blunted respiratory motor output such as spinal cord injury.

Design, Synthesis and Biological Evaluation of Brain-Targeted Thiamine Disulfide Prodrugs of Ampakine Compound LCX001

Ampakine compounds have been shown to reverse opiate-induced respiratory depression by activation of amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptors. However, their pharmacological exploitations are hindered by low blood-brain barrier (BBB) permeability and limited brain distribution. Here, we explored whether thiamine disulfide prodrugs with the ability of “lock-in” can be used to solve these problems. A series of thiamine disulfide prodrugs 7a–7f of ampakine compound LCX001 was synthesized and evaluated. The trials in vitro showed that prodrugs 7e, 7d, 7f possessed a certain stability in plasma and quickly decomposed in brain homogenate by the disulfide reductase. In vivo, prodrug 7e decreased the peripheral distribution of LCX001 and significantly increased brain distribution of LCX001 after i.v. administration. This compound showed 2.23- and 3.29-fold greater increases in the AUC_0-t and MRT_0-t of LCX001 in brain, respectively, than did LCX001 itself. A preliminary pharmacodynamic study indicated that the required molar dose of prodrug 7e was only one eighth that of LCX001 required to achieve the same effect in mice. These findings provide an important reference to evaluate the clinical outlook of ampakine compounds.

Ampakine CX1942 attenuates opioid-induced respiratory depression and corrects the hypoxaemic effects of etorphine in immobilized goats (Capra hircus)

OBJECTIVES

To determine whether CX1942 reverses respiratory depression in etorphine-immobilized goats, and to compare its effects with those of doxapram hydrochloride.

STUDY DESIGN

A prospective, crossover experimental trial conducted at 1753 m.a.s.l.

ANIMALS

Eight adult female Boer goats (Capra hircus) with a mean ± standard deviation mass of 27.1 ± 1.6 kg.

METHODS

Following immobilization with 0.1 mg kg(-1) etorphine, goats received one of doxapram, CX1942 or sterile water intravenously, in random order in three trials. Respiratory rate, ventilation and tidal volume were measured continuously. Arterial blood samples for the determination of PaO2 , PaCO2 , pH and SaO2 were taken 2 minutes before and then at 5 minute intervals after drug administration for 25 minutes.

RESULTS

Doxapram corrected etorphine-induced respiratory depression but also led to arousal and hyperventilation at 2 minutes after its administration, as indicated by the low PaCO2 (27.8 ± 4.5 mmHg) and ventilation of 5.32 ± 5.24 L minute(-1) above pre-immobilization values. CX1942 improved respiratory parameters and corrected etorphine's hypoxaemic effects more gradually than did doxapram, with a more sustained improvement in PaO2 and SaO2 in comparison with the control trial.

CONCLUSIONS

CX1942 attenuated opioid-induced respiratory depression and corrected the hypoxaemic effects of etorphine in immobilized goats.

CLINICAL RELEVANCE

Ampakines potentially offer advantages over doxapram, a conventional treatment, in reversing etorphine-induced respiratory depression without causing unwanted side effects, particularly arousal, in immobilized animals.

Stimulation of Respiratory Motor Output and Ventilation in a Murine Model of Pompe Disease by Ampakines

Pompe disease results from a mutation in the acid α-glucosidase gene leading to lysosomal glycogen accumulation. Respiratory insufficiency is common, and the current U.S. Food and Drug Administration-approved treatment, enzyme replacement, has limited effectiveness. Ampakines are drugs that enhance α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor responses and can increase respiratory motor drive. Recent work indicates that respiratory motor drive can be blunted in Pompe disease, and thus pharmacologic stimulation of breathing may be beneficial. Using a murine Pompe model with the most severe clinical genotype (the Gaa(-/-) mouse), our primary objective was to test the hypothesis that ampakines can stimulate respiratory motor output and increase ventilation. Our second objective was to confirm that neuropathology was present in Pompe mouse medullary respiratory control neurons. The impact of ampakine CX717 on breathing was determined via phrenic and hypoglossal nerve recordings in anesthetized mice and whole-body plethysmography in unanesthetized mice. The medulla was examined using standard histological methods coupled with immunochemical markers of respiratory control neurons. Ampakine CX717 robustly increased phrenic and hypoglossal inspiratory bursting and reduced respiratory cycle variability in anesthetized Pompe mice, and it increased inspiratory tidal volume in unanesthetized Pompe mice. CX717 did not significantly alter these variables in wild-type mice. Medullary respiratory neurons showed extensive histopathology in Pompe mice. Ampakines stimulate respiratory neuromotor output and ventilation in Pompe mice, and therefore they have potential as an adjunctive therapy in Pompe disease.