Dual mechanisms of opioid-induced respiratory depression in the inspiratory rhythm-generating network

Interdependence of cellular and network properties in respiratory rhythm generation

How breathing is generated by the preBötzinger complex (preBötC) remains divided between two ideological frameworks, and a persistent sodium current (INaP) lies at the heart of this debate. Although INaP is widely expressed, the pacemaker hypothesis considers it essential because it endows a small subset of neurons with intrinsic bursting or "pacemaker" activity. In contrast, burstlet theory considers INaP dispensable because rhythm emerges from "preinspiratory" spiking activity driven by feed-forward network interactions. Using computational modeling, we find that small changes in spike shape can dissociate INaP from intrinsic bursting. Consistent with many experimental benchmarks, conditional effects on spike shape during simulated changes in oxygenation, development, extracellular potassium, and temperature alter the prevalence of intrinsic bursting and preinspiratory spiking without altering the role of INaP. Our results support a unifying hypothesis where INaP and excitatory network interactions, but not intrinsic bursting or preinspiratory spiking, are critical interdependent features of preBötC rhythmogenesis.

Up-regulation of GPR139 in the medial septum ameliorates cognitive impairment in two mouse models of Alzheimer's disease

G-protein coupled receptors (GPCRs) constitute the largest class of cell surface receptors and present prominent drug targets. GPR139 is an orphan GPCR detected in the septum of the brain. However, its roles in cognition are still unclear. Here we first established a mouse model of cognitive impairment by a single intracerebroventricular injection of aggregated amyloid-beta peptide 1-42 (Aβ1-42). RNA-sequencing data analysis showed that Aβ1-42 induced a significant decrease of GPR139 mRNA in the basal forebrain. Using GPR139 agonist JNJ-63533054 and behavioral tests, we found that GPR139 activation in the brain ameliorated Aβ1-42-induced cognitive impairment. Using western blot, TUNEL apoptosis and Golgi staining assays, we showed that GPR139 activation alleviated Aβ1-42-induced apoptosis and synaptotoxicity in the basal forebrain rather than prefrontal cortex and hippocampus. The further study identified that GPR139 was widely expressed in cholinergic neurons of the medial septum (MS). Using the overexpression virus and transgenic animal model, we showed that up-regulation of GPR139 in MS cholinergic neurons ameliorated cognitive impairment, apoptosis and synaptotoxicity in APP/PS1 transgenic mice. These findings reveal that GPR139 of MS cholinergic neurons could be a critical node in cognition and potentially provides insight into the pathogenesis of Alzheimer's disease.

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.

A Balancing Act: Learning from the Past to Build a Future-Focused Opioid Strategy

The harmful side effects of opioid drugs such as respiratory depression, tolerance, dependence, and abuse potential have limited the therapeutic utility of opioids for their entire clinical history. However, no previous attempt to develop effective pain drugs that substantially ameliorate these effects has succeeded, and the current opioid epidemic affirms that they are a greater hindrance to the field of pain management than ever. Recent attempts at new opioid development have sought to reduce these side effects by minimizing engagement of the regulatory protein arrestin-3 at the mu-opioid receptor, but there is significant controversy around this approach. Here, we discuss the ongoing effort to develop safer opioids and its relevant historical context. We propose a new model that reconciles results previously assumed to be in direct conflict to explain how different signaling profiles at the mu-opioid receptor contribute to opioid tolerance and dependence. Our goal is for this framework to inform the search for a new generation of lower liability opioid analgesics.

Modulators of nicotine reward and reinforcement

Nicotine has been well-characterized for its ability to alter neurophysiology to promote rewarding and reinforcing properties. However, several exogenous chemicals possess properties that modulate or enhance nicotine's ability to alter neurophysiology. This chapter focuses on nicotine's impact on behavior through changes in neurophysiology and several chemical entities that in-turn modulate nicotine's ability to act as a neuromodulator.

Risk Factors for Respiratory Depression Associated with Tramadol Based on the Global Pharmacovigilance Database (VigiBase)

Tramadol, a weak μ-opioid receptor agonist, has been used worldwide for pain management. It is considered to have a favorable safety profile without serious adverse events; however, safety issues of respiratory depression were proposed by regulatory governments. We aimed to examine the risk and contributing factors associated with tramadol-related respiratory depression using a real-world database, VigiBase. Disproportionality analysis of tramadol and tramadol/paracetamol was performed using proportional reporting ratios, reporting odds ratios, and information components for all drugs and opioids. Factors related to respiratory depression, including sex, age, presence of abuse, death, and various concomitant medications, were evaluated. Among 140,721 tramadol reports, respiratory depression was reported in 1126 cases, 81.3% of which were deemed serious. Five adverse events were detected as signals of tramadol-related acute central respiratory depression (ACRD) in 882 reports. A higher proportion of ACRD cases in children and adolescents was observed than all adverse events cases of tramadol. Concomitant users of CYP2D6 inhibitors, opioids, benzodiazepines, and anti-depressant drugs showed a higher proportion in ACRD cases than non-ACRD cases. ACRD was related to drug abuse and death. This pharmacovigilance study, using VigiBase, confirmed a high risk of respiratory depression (a serious, potentially fatal adverse event) secondary to the use of tramadol, especially in pediatric patients, drug abusers, or during concomitant use of opioids, benzodiazepines, or antidepressants.

Latent neural population dynamics underlying breathing, opioid-induced respiratory depression and gasping

Breathing is vital and must be concurrently robust and flexible. This rhythmic behavior is generated and maintained within a rostrocaudally aligned set of medullary nuclei called the ventral respiratory column (VRC). The rhythmic properties of individual VRC nuclei are well known, yet technical challenges have limited the interrogation of the entire VRC population simultaneously. Here we characterize over 15,000 medullary units using high-density electrophysiology, opto-tagging and histological reconstruction. Population dynamics analysis reveals consistent rotational trajectories through a low-dimensional neural manifold. These rotations are robust and maintained even during opioid-induced respiratory depression. During severe hypoxia-induced gasping, the low-dimensional dynamics of the VRC reconfigure from rotational to all-or-none, ballistic efforts. Thus, latent dynamics provide a unifying lens onto the activities of large, heterogeneous populations of neurons involved in the simple, yet vital, behavior of breathing, and well describe how these populations respond to a variety of perturbations.

Disinhibition does not play a role in endomorphin-2-induced changes in inspiratory motoneuron output produced by in vitro neonatal rat preparations

Low level activation of mu-opioid receptors (MORs) in neonatal rat brainstem-spinal cord preparations increases inspiratory burst amplitude recorded on cervical spinal roots. We tested whether: (1) MOR activation with an endogenous ligand, such as endomorphin-2, increases inspiratory burst amplitude, (2) disinhibition of GABAergic or glycinergic inhibitory synaptic transmission is involved, and (3) inflammation alters endomorphin-2 effects. Using neonatal rat (P0-P3) brainstem-spinal cord preparations, bath-applied endomorphin-2 (10-200 nM) increased inspiratory burst amplitude and decreased burst frequency. Blockade of GABAA receptors (picrotoxin), glycine receptors (strychnine), or both (picrotoxin and strychnine) did not abolish endomorphin-2-induced effects. In preparations isolated from neonatal rats injected 3 h previously with lipopolysaccharide (LPS, 0.1 mg/kg), endomorphin-2 continued to decrease burst frequency but abolished the burst amplitude increase. Collectively, these data indicate that disinhibition of inhibitory synaptic transmission is unlikely to play a role in endomorphin-2-induced changes in inspiratory motor output, and that different mechanisms underlie the endomorphin-2-induced increases in inspiratory burst amplitude and decreases in burst frequency.

Interdependence of cellular and network properties in respiratory rhythmogenesis

How breathing is generated by the preBötzinger Complex (preBötC) remains divided between two ideological frameworks, and the persistent sodium current (INaP) lies at the heart of this debate. Although INaP is widely expressed, the pacemaker hypothesis considers it essential because it endows a small subset of neurons with intrinsic bursting or "pacemaker" activity. In contrast, burstlet theory considers INaP dispensable because rhythm emerges from "pre-inspiratory" spiking activity driven by feed-forward network interactions. Using computational modeling, we discover that changes in spike shape can dissociate INaP from intrinsic bursting. Consistent with many experimental benchmarks, conditional effects on spike shape during simulated changes in oxygenation, development, extracellular potassium, and temperature alter the prevalence of intrinsic bursting and pre-inspiratory spiking without altering the role of INaP. Our results support a unifying hypothesis where INaP and excitatory network interactions, but not intrinsic bursting or pre-inspiratory spiking, are critical interdependent features of preBötC rhythmogenesis.

A comparative examination of morphine and fentanyl: unravelling the differential impacts on breathing and airway stability

This study provides an in-depth analysis of the distinct consequences of the opioid drugs morphine and fentanyl during opioid-induced respiratory depression (OIRD). We explored the physiological implications of both drugs on ventilation and airway patency in anaesthetized mice. Our results revealed a similar reduction in respiratory frequency with equivalent scaled dosages of fentanyl and morphine, though the onset of suppression was more rapid with fentanyl. Additionally, fentanyl resulted in transient airflow obstructions during the inspiratory cycle, which were absent following morphine administration. Notably, these fentanyl-specific obstructions were eliminated with tracheostomy, implicating the upper airways as a major factor contributing to fentanyl-induced respiratory depression. We further demonstrate that bronchodilators salbutamol and adrenaline effectively reversed these obstructions, highlighting the bronchi's contribution to fentanyl-induced airflow obstruction. Our study also uncovered a significant reduction in sighs during OIRD, which were eliminated by fentanyl and markedly reduced by morphine. Finally, we found that fentanyl-exposed mice had reduced survival under hypoxic conditions compared to mice given morphine, demonstrating that fentanyl becomes more lethal in the context of hypoxaemia. Our findings shed light on the distinct and profound impacts of these opioids on respiration and airway stability and lay the foundation for improved opioid use guidelines and more effective OIRD prevention strategies. KEY POINTS: Both morphine and fentanyl significantly suppressed respiratory frequency, but the onset of suppression was faster with fentanyl. Also, while both drugs increased tidal volume, this effect was more pronounced with fentanyl. Fentanyl administration resulted in transient obstructions during the inspiratory phase, suggesting its unique impact on airway stability. This obstruction was not observed with morphine. The fentanyl-induced obstructions were reversed by administering bronchodilators such as salbutamol and adrenaline. This suggests a possible therapeutic strategy for mitigating the adverse airway effects of fentanyl. Both drugs reduced the frequency of physiological sighs, a key mechanism to prevent alveolar collapse. However, fentanyl administration led to a complete cessation of sighs, while morphine only reduced their occurrence. Fentanyl-treated mice showed a significantly reduced ability to survive under hypoxic conditions compared to those administered morphine. This indicates that the impacts of hypoxaemia during opioid-induced respiratory depression can vary based on the opioid used.

Cannabis and Other Substance Misuse: Implications and Regulations

Abusing controlled substances, including cannabis and various drugs, can result in severe intoxication and even death. Therefore, a comprehensive postmortem analysis is crucial for understanding the underlying causes of such fatalities. This narrative review discusses the characteristics of commonly abused controlled substances, the methodologies employed in postmortem analysis, lethal dosage levels, mechanisms of toxicity, side effects, and existing regulations. The focus centers on seven prevalent groups of controlled substances, namely cannabis, opioids, amphetamine-type stimulants, cocaine, new psychoactive substances, and hallucinogens. These groups have been linked to an increased risk of fatal overdose. Most substances in these groups exert neurotoxic effects by targeting the central nervous system (CNS). Consequently, strict regulation is essential to mitigate the potential harm posed by these substances. To combat abuse, prescribers must adhere to guidelines to ensure their prescribed medications comply with the outlined regulations. Through an enhanced understanding of controlled substance abuse and its consequences, more effective strategies can be developed to reduce its prevalence and associated mortality.

Opioid Impacts on Cardiovascular Health

The prevalence of opioid use in the current opioid epidemic era has led to a public health emergency due to the ties to mortality and morbidity. Studies have investigated opioids' impacts on different aspects of cardiovascular health, although there seems to be a lack of a current concise review. Therefore, the aim of this literature review is to provide a summary of the most recent studies from the past decade that postulate a connection between opioids and their impact on cardiovascular health while highlighting conflicting areas among published research. For this literature review, three databases, PubMed (NLM), EMBASE, and Web of Science (Core Collection), were searched for full peer-reviewed articles written in English about human subjects and published between 2013 and 2023 inclusive. The following initial approach was to search for terms in the title of articles: "opioid AND ("vascular" OR "artery" OR "vein" OR "heart rate" OR "infarct" OR "stroke" OR "aortic" OR "cardiovascular disease"). After assessing for duplicate articles from the three databases, the remaining articles were assessed for inclusion eligibility. In the present review, a brief description of the overall role of opioid receptors is provided followed by the literature findings. These findings indicate potentially important negative impacts of opioid use on cardiovascular health in a number of areas. These include opioid-associated increases in the following: (1) vascular aging based on demonstrated increases in arterial stiffness, (2) opioid-related reductions in heart rate variability (HRV) and its implications on morbidity and mortality, (3) opioid's impacts on coronary artery and coronary heart disease (CHD), (4) opioids as a risk factor for atrial fibrillation (AF) and (5) opioid use as a risk factor for vascular occlusion processes. In addition to these broad cardiovascular effects, other aspects of concern are related to the potential impacts of withdrawal from opioid use, which, when done rapidly, are associated with increases in blood pressure and a decrease in HRV.

Opioid Overdose: Limitations in Naloxone Reversal of Respiratory Depression and Prevention of Cardiac Arrest

Opioids are effective analgesics, but they can have harmful adverse effects, such as addiction and potentially fatal respiratory depression. Naloxone is currently the only available treatment for reversing the negative effects of opioids, including respiratory depression. However, the effectiveness of naloxone, particularly after an opioid overdose, varies depending on the pharmacokinetics and the pharmacodynamics of the opioid that was overdosed. Long-acting opioids, and those with a high affinity at the µ-opioid receptor and/or slow receptor dissociation kinetics, are particularly resistant to the effects of naloxone. In this review, the authors examine the pharmacology of naloxone and its safety and limitations in reversing opioid-induced respiratory depression under different circumstances, including its ability to prevent cardiac arrest.

Molecular insights into GPCR mechanisms for drugs of abuse

Substance abuse is on the rise, and while many people may use illicit drugs mainly due to their rewarding effects, their societal impact can range from severe, as is the case for opioids, to promising, as is the case for psychedelics. Common with all these drugs' mechanisms of action are G protein-coupled receptors (GPCRs), which lie at the center of how these drugs mediate inebriation, lethality, and therapeutic effects. Opioids like fentanyl, cannabinoids like tetrahydrocannabinol, and psychedelics like lysergic acid diethylamide all directly bind to GPCRs to initiate signaling which elicits their physiological actions. We herein review recent structural studies and provide insights into the molecular mechanisms of opioids, cannabinoids, and psychedelics at their respective GPCR subtypes. We further discuss how such mechanistic insights facilitate drug discovery, either toward the development of novel therapies to combat drug abuse or toward harnessing therapeutic potential.

Theophylline reverses oxycodone's but not fentanyl's respiratory depression in mice while caffeine is ineffective against both opioids

RATIONALE

The opioid epidemic remains a pressing public health crisis in the United States. Most of these overdose deaths are a result of lethal respiratory depression. In recent years the increasing incidence of opioid-involved overdose deaths has been driven by fentanyl, which is more resistant to adequate reversal by naloxone (NARCAN ®) than semi-synthetic or classical morphinan predecessors like oxycodone and heroin. For this and other reasons (e.g., precipitating withdrawal) non-opioidergic pharmacotherapies to reverse opioid-depressed respiration are needed. Methylxanthines are a class of stimulant drugs including caffeine and theophylline which exert their effects primarily via adenosine receptor antagonism. Evidence suggests methylxanthines can stimulate respiration by enhancing neural activity in respiratory nuclei in the pons and medulla independent of opioid receptors. This study aimed to determine whether caffeine and theophylline can stimulate respiration in mice when depressed by fentanyl and oxycodone.

METHODS

Whole-body plethysmography was used to characterize fentanyl and oxycodone's effects on respiration and their reversal by naloxone in male Swiss Webster mice. Next, caffeine and theophylline were tested for their effects on basal respiration. Finally, each methylxanthine was evaluated for its ability to reverse similar levels of respiratory depression induced by fentanyl or oxycodone.

RESULTS AND CONCLUSIONS

Oxycodone and fentanyl dose-dependently reduced respiratory minute volume (ml/min; MVb) that was reversible by naloxone. Caffeine and theophylline each significantly increased basal MVb. Theophylline, but not caffeine, completely reversed oxycodone-depressed respiration. In contrast, neither methylxanthine elevated fentanyl-depressed respiration at the doses tested. Despite their limited efficacy for reversing opioid-depressed respiration when administered alone, the methylxanthines safety, duration, and mechanism of action supports further evaluation in combination with naloxone to augment its reversal of opioid-depressed respiration.

Changes in pontine and preBötzinger/Bötzinger complex neuronal activity during remifentanil-induced respiratory depression in decerebrate dogs

Introduction: In vivo studies using selective, localized opioid antagonist injections or localized opioid receptor deletion have identified that systemic opioids dose-dependently depress respiratory output through effects in multiple respiratory-related brainstem areas. Methods: With approval of the subcommittee on animal studies of the Zablocki VA Medical Center, experiments were performed in 53 decerebrate, vagotomized, mechanically ventilated dogs of either sex during isocapnic hyperoxia. We performed single neuron recordings in the Pontine Respiratory Group (PRG, n = 432) and preBötzinger/Bötzinger complex region (preBötC/BötC, n = 213) before and during intravenous remifentanil infusion (0.1-1 mcg/kg/min) and then until complete recovery of phrenic nerve activity. A generalized linear mixed model was used to determine changes in Fn with remifentanil and the statistical association between remifentanil-induced changes in Fn and changes in inspiratory and expiratory duration and peak phrenic activity. Analysis was controlled via random effects for animal, run, and neuron type. Results: Remifentanil decreased Fn in most neuron subtypes in the preBötC/BötC as well as in inspiratory (I), inspiratory-expiratory, expiratory (E) decrementing and non-respiratory modulated neurons in the PRG. The decrease in PRG inspiratory and non-respiratory modulated neuronal activity was associated with an increase in inspiratory duration. In the preBötC, the decrease in I-decrementing neuron activity was associated with an increase in expiratory and of E-decrementing activity with an increase in inspiratory duration. In contrast, decreased activity of I-augmenting neurons was associated with a decrease in inspiratory duration. Discussion: While statistical associations do not necessarily imply a causal relationship, our data suggest mechanisms for the opioid-induced increase in expiratory duration in the PRG and preBötC/BötC and how inspiratory failure at high opioid doses may result from a decrease in activity and decrease in slope of the pre-inspiratory ramp-like activity in preBötC/BötC pre-inspiratory neurons combined with a depression of preBötC/BötC I-augmenting neurons. Additional studies must clarify whether the observed changes in neuronal activity are due to direct neuronal inhibition or decreased excitatory inputs.

Endogenous opiates and behavior: 2021

This paper is the forty-fourth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2021 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonizts and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Maternal opioids age-dependently impair neonatal respiratory control networks

Infants exposed to opioids in utero are an increasing clinical population and these infants are often diagnosed with Neonatal Abstinence Syndrome (NAS). Infants with NAS have diverse negative health consequences, including respiratory distress. However, many factors contribute to NAS, confounding the ability to understand how maternal opioids directly impact the neonatal respiratory system. Breathing is controlled centrally by respiratory networks in the brainstem and spinal cord, but the impact of maternal opioids on developing perinatal respiratory networks has not been studied. Using progressively more isolated respiratory network circuitry, we tested the hypothesis that maternal opioids directly impair neonatal central respiratory control networks. Fictive respiratory-related motor activity from isolated central respiratory networks was age-dependently impaired in neonates after maternal opioids within more complete respiratory networks (brainstem and spinal cords), but unaffected in more isolated networks (medullary slices containing the preBötzinger Complex). These deficits were due, in part, to lingering opioids within neonatal respiratory control networks immediately after birth and involved lasting impairments to respiratory pattern. Since opioids are routinely given to infants with NAS to curb withdrawal symptoms and our previous work demonstrated acute blunting of opioid-induced respiratory depression in neonatal breathing, we further tested the responses of isolated networks to exogenous opioids. Isolated respiratory control networks also demonstrated age-dependent blunted responses to exogenous opioids that correlated with changes in opioid receptor expression within a primary respiratory rhythm generating region, the preBötzinger Complex. Thus, maternal opioids age-dependently impair neonatal central respiratory control and responses to exogenous opioids, suggesting central respiratory impairments contribute to neonatal breathing destabilization after maternal opioids and likely contribute to respiratory distress in infants with NAS. These studies represent a significant advancement of our understanding of the complex effects of maternal opioids, even late in gestation, contributing to neonatal breathing deficits, necessary first steps in developing novel therapeutics to support breathing in infants with NAS.

Parabrachial tachykinin1-expressing neurons involved in state-dependent breathing control

Breathing is regulated automatically by neural circuits in the medulla to maintain homeostasis, but breathing is also modified by behavior and emotion. Mice have rapid breathing patterns that are unique to the awake state and distinct from those driven by automatic reflexes. Activation of medullary neurons that control automatic breathing does not reproduce these rapid breathing patterns. By manipulating transcriptionally defined neurons in the parabrachial nucleus, we identify a subset of neurons that express the Tac1, but not Calca, gene that exerts potent and precise conditional control of breathing in the awake, but not anesthetized, state via projections to the ventral intermediate reticular zone of the medulla. Activating these neurons drives breathing to frequencies that match the physiological maximum through mechanisms that differ from those that underlie the automatic control of breathing. We postulate that this circuit is important for the integration of breathing with state-dependent behaviors and emotions.

Transformation of Our Understanding of Breathing Control by Molecular Tools

The rhythmicity of breath is vital for normal physiology. Even so, breathing is enriched with multifunctionality. External signals constantly change breathing, stopping it when under water or deepening it during exertion. Internal cues utilize breath to express emotions such as sighs of frustration and yawns of boredom. Breathing harmonizes with other actions that use our mouth and throat, including speech, chewing, and swallowing. In addition, our perception of breathing intensity can dictate how we feel, such as during the slow breathing of calming meditation and anxiety-inducing hyperventilation. Heartbeat originates from a peripheral pacemaker in the heart, but the automation of breathing arises from neural clusters within the brainstem, enabling interaction with other brain areas and thus multifunctionality. Here, we document how the recent transformation of cellular and molecular tools has contributed to our appreciation of the diversity of neuronal types in the breathing control circuit and how they confer the multifunctionality of breathing.

µ-Opioid Receptor Activation Reduces Glutamate Release in the PreBötzinger Complex in Organotypic Slice Cultures

The inspiratory rhythm generator, located in the brainstem preBötzinger complex (preBötC), is dependent on glutamatergic signaling and is affected profoundly by opioids. Here, we used organotypic slice cultures of the newborn mouse brainstem of either sex in combination with genetically encoded sensors for Ca2+, glutamate, and GABA to visualize Ca2+, glutamatergic and GABAergic signaling during spontaneous rhythm and in the presence of DAMGO. During spontaneous rhythm, the glutamate sensor SF-iGluSnFR.A184S revealed punctate synapse-like fluorescent signals along dendrites and somas in the preBötC with decay times that were prolonged by the glutamate uptake blocker (TFB-TBOA). The GABA sensor iGABASnFR showed a more diffuse fluorescent signal during spontaneous rhythm. Rhythmic Ca2+- and glutamate transients had an inverse relationship between the spontaneous burst frequency and the burst amplitude of the Ca2+ and glutamate signals. A similar inverse relationship was observed when bath applied DAMGO reduced spontaneous burst frequency and increased the burst amplitude of Ca2+, glutamate, and GABA transient signals. However, a hypoxic challenge reduced both burst frequency and Ca2+ transient amplitude. Using a cocktail that blocked glutamatergic, GABAergic, and glycinergic transmission to indirectly measure the release of glutamate/GABA in response to an electrical stimulus, we found that DAMGO reduces the release of glutamate in the preBötC but has no effect on GABA release. This suggest that the opioid mediated slowing of respiratory rhythm involves presynaptic reduction of glutamate release, which would impact the ability of the network to engage in recurrent excitation, and may result in the opioid-induced slowing of inspiratory rhythm.SIGNIFICANCE STATEMENT Opioids slow down breathing rhythm by affecting neurons in the preBötzinger complex (preBötC) and other brainstem regions. Here, we used cultured slices of the preBötC to better understand this effect by optically recording Ca2+, glutamate, and GABA transients during preBötC activity. Spontaneous rhythm showed an inverse relationship between burst frequency and burst amplitude in the Ca2+ and glutamate signals. Application of the opioid DAMGO slowed the rhythm, with a concomitant increase in Ca2+, glutamate, and GABA signals. When rhythm was blocked pharmacologically, DAMGO reduced the presynaptic release of glutamate, but not GABA. These data suggest the mechanism of action of opioids involves presynaptic reduction of glutamate release, which may play an important role in the opioid-induced slowing of inspiratory rhythm.

Neural underpinning of a respiration-associated resting-state fMRI network

Respiration can induce motion and CO2 fluctuation during resting-state fMRI (rsfMRI) scans, which will lead to non-neural artifacts in the rsfMRI signal. In the meantime, as a crucial physiologic process, respiration can directly drive neural activity change in the brain, and may thereby modulate the rsfMRI signal. Nonetheless, this potential neural component in the respiration-fMRI relationship is largely unexplored. To elucidate this issue, here we simultaneously recorded the electrophysiology, rsfMRI, and respiration signals in rats. Our data show that respiration is indeed associated with neural activity changes, evidenced by a phase-locking relationship between slow respiration variations and the gamma-band power of the electrophysiological signal recorded in the anterior cingulate cortex. Intriguingly, slow respiration variations are also linked to a characteristic rsfMRI network, which is mediated by gamma-band neural activity. In addition, this respiration-related brain network disappears when brain-wide neural activity is silenced at an isoelectrical state, while the respiration is maintained, further confirming the necessary role of neural activity in this network. Taken together, this study identifies a respiration-related brain network underpinned by neural activity, which represents a novel component in the respiration-rsfMRI relationship that is distinct from respiration-related rsfMRI artifacts. It opens a new avenue for investigating the interactions between respiration, neural activity, and resting-state brain networks in both healthy and diseased conditions.

Role of daytime variation in pharmaceutical effects of sufentanil, dezocine, and tramadol: A matched observational study

The role of daytime variation in the comprehensive pharmaceutical effects of commonly used opioid analgesics in clinical setting remains unclear. This study aimed to explore the differences in daytime variation among elective surgery patients who were scheduled to receive preemptive analgesia with equivalent doses of sufentanil, dezocine, and tramadol in the morning and afternoon. The analgesic effect was assessed by changes in the pressure pain threshold before and after intravenous administration of sufentanil, dezocine, and tramadol. Respiratory effects were evaluated using pulse oximetry, electrical impedance tomography, and arterial blood gas analysis. Other side effects, including nausea, sedation, and dizziness, were also recorded, and blood concentration was measured. The results showed that the analgesic effects of sufentanil, dezocine, and tramadol were significantly better in the morning than in afternoon. In the afternoon, sufentanil had a stronger sedative effect, whereas dezocine had a stronger inhibitory respiratory effect. The incidence of nausea was higher in the morning with tramadol. Additionally, significant differences in different side effects were observed among three opioids. Our results suggest that the clinical use of these three opioids necessitates the formulation of individualized treatment plans, accounting for different administration times, to achieve maximum analgesic effect with minimal side effects.

Respiratory effects of the atypical tricyclic antidepressant tianeptine in human models of opioid-induced respiratory depression

BACKGROUND

Animal data suggest that the antidepressant and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor modulator tianeptine is able to prevent opioid-induced respiratory depression. We hypothesize that oral or intravenous tianeptine can effectively prevent or counteract opioid-induced respiratory depression in humans.

METHODS

Healthy male and female volunteers participated in two studies that had a randomized, double blind, placebo-controlled, crossover design. We first tested oral tianeptine (37.5, 50 and 100 mg, 8 subjects/group) pretreatment followed by induction of alfentanil-induced respiratory depression (alfentanil target concentration 100 ng/mL). Primary endpoint was ventilation at an extrapolated end-tidal carbon dioxide concentration of 55 mmHg (V̇E55). We next determined the ability of four subsequent and increasing infusions of intravenous tianeptine (target tianeptine plasma concentrations 400, 1,000, 1,500 and 2,000 ng/mL, each given over 15 min), to counteract remifentanil-induced respiratory depression in 15 volunteers. Ventilation was measured at isohypercpania (baseline ventilation 20 ± 2 L/min). Primary endpoint was minute ventilation during the 60 min of tianeptine versus placebo infusion.

RESULTS

Alfentanil reduced V̇E55 to 13.7 (95% CI 8.6-18.8) L/min following placebo pretreatment and to 17.9 (10.2-25.7) L/min following 50 mg tianeptine pretreatment (mean difference between treatments 4.2 (-11.5-3.0) L/min, p = 0.070). Intravenous tianeptine in the measured concentration range of 500 to 2,000 ng/ml did not stimulate ventilation but instead worsened remifentanil-induced respiratory depression: tianeptine 9.6 ± 0.8 L/min versus placebo 15.0 ± 0.9 L/min, mean difference 5.3 L/min, 95% CI 2.5-8.2 L/min; p = 0.001, after 1 hour of treatment.

CONCLUSIONS

Neither oral nor intravenous tianeptine were respiratory stimulants. Intravenous tianeptine over the concentration range of 500-2000 ng/mL worsened respiratory depression induced by remifentanil.

Breathing Rhythm and Pattern and Their Influence on Emotion

Breathing is a vital rhythmic motor behavior with a surprisingly broad influence on the brain and body. The apparent simplicity of breathing belies a complex neural control system, the breathing central pattern generator (bCPG), that exhibits diverse operational modes to regulate gas exchange and coordinate breathing with an array of behaviors. In this review, we focus on selected advances in our understanding of the bCPG. At the core of the bCPG is the preBötzinger complex (preBötC), which drives inspiratory rhythm via an unexpectedly sophisticated emergent mechanism. Synchronization dynamics underlying preBötC rhythmogenesis imbue the system with robustness and lability. These dynamics are modulated by inputs from throughout the brain and generate rhythmic, patterned activity that is widely distributed. The connectivity and an emerging literature support a link between breathing, emotion, and cognition that is becoming experimentally tractable. These advances bring great potential for elucidating function and dysfunction in breathing and other mammalian neural circuits.

Inspiratory rhythm generation is stabilized by Ih

Cellular and network properties must be capable of generating rhythmic activity that is both flexible and stable. This is particularly important for breathing, a rhythmic behavior that dynamically adapts to environmental, behavioral, and metabolic changes from the first to the last breath. The pre-Bötzinger complex (preBötC), located within the ventral medulla, is responsible for producing rhythmic inspiration. Its cellular properties must be tunable, flexible as well as stabilizing. Here, we explore the role of the hyperpolarization-activated, nonselective cation current (I_h) for stabilizing PreBötC activity during opioid exposure and reduced excitatory synaptic transmission. Introducing I_h into an in silico preBötC network predicts that loss of this depolarizing current should significantly slow the inspiratory rhythm. By contrast, in vitro and in vivo experiments revealed that the loss of I_h minimally affected breathing frequency, but destabilized rhythmogenesis through the generation of incompletely synchronized bursts (burstlets). Associated with the loss of I_h was an increased susceptibility of breathing to opioid-induced respiratory depression or weakened excitatory synaptic interactions, a paradoxical depolarization at the cellular level, and the suppression of tonic spiking. Tonic spiking activity is generated by nonrhythmic excitatory and inhibitory preBötC neurons, of which a large percentage express I_h. Together, our results suggest that I_h is important for maintaining tonic spiking, stabilizing inspiratory rhythmogenesis, and protecting breathing against perturbations or changes in network state.NEW & NOTEWORTHY The I_h current plays multiple roles within the preBötC. This current is important for promoting intrinsic tonic spiking activity in excitatory and inhibitory neurons and for preserving rhythmic function during conditions that dampen network excitability, such as in the context of opioid-induced respiratory depression. We therefore propose that the I_h current expands the dynamic range of rhythmogenesis, buffers the preBötC against network perturbations, and stabilizes rhythmogenesis by preventing the generation of unsynchronized bursts.

Current research in pathophysiology of opioid induced respiratory depression

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In Oprm1-deleted KO mice, both the preBötC and the KF/PBN are major contributors to OIRD but RD is not totally eliminated.

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PreBötC neurons expressing NK-1R are necessary for breathing. Their deletion results in ataxic breathing and/or apnoea.

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GIRK channels are involved in inhibiting respiration by mu opioid receptors. KCNQ channels modulate OIRD independent of mu signaling.

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Morphine depresses normal but not gasping breathing in rats indicating distinct mechanisms for these 2 forms of breathing.

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Methadone, given to alleviate symptoms of neonatal opioid withdrawal syndrome, desensitizes rats to respiratory depression.

Respiratory depression (RD) is the primary cause of death due to opioids. Opioids bind to mu (µ)-opioid receptors (MORs) encoded by the MOR gene Oprm1, widely expressed in the central and peripheral nervous systems including centers that modulate breathing. Respiratory centers are located throughout the brainstem. Experiments with Oprm1-deleted knockout (KO) mice undertaken to determine which sites are necessary for the induction of opioid-induced respiratory depression (OIRD) showed that the pre-Bötzinger complex (preBötC) and the pontine Kölliker-Fuse nucleus (KF) contribute equally to OIRD but RD was not totally eliminated. Morphine showed a differential influence on preBötC and KF neurons – low doses attenuated RD following deletion of MORs from preBötC neurons and an increase in apneas after high doses whereas deletion of MORs from KF neurons but not the preBötC attenuated RD at both high and low doses. In other KO mice studies, morphine administration after deletion of Oprm1 from both the preBötC and the KF/PBN neurons, led to the conclusion that both respiratory centres contribute to OIRD but the preBötC predominates. MOR-mediated post-synaptic activation of GIRK potassium channels has been implicated as a cause of OIRD. A complementary mechanism in the preBötC involving KCNQ potassium channels independent of MOR signaling has been described. Recent experiments in rats showing that morphine depresses normal, but not gasping breathing, cast doubt on the belief that eupnea, sighs, and gasps, are under the control of preBötC neurons. Methadone, administered to alleviate symptoms of neonatal opioid withdrawal syndrome (NOWES), desensitized rats to OIRD. Protection lost between postnatal days 1 and 2 coincides with the preBötC becoming the dominant generator of respiratory rhythm. Neonatal antidepressant exposure syndrome (NADES) and serotonin toxicity (ST) show similarities including RD. Enzyme CYP2D6 involved in opioid detoxification is polymorphic. Individuals of different CYP2D6 genotype may show increased, decreased, or no enzyme activity, contributing to the variability of patient responses to different opioids and OIRD.

Are thyrotropin-releasing hormone (TRH) and analog taltirelin viable reversal agents of opioid-induced respiratory depression?

Opioid‐induced respiratory depression (OIRD) is a potentially life‐threatening complication of opioid consumption. Apart from naloxone, an opioid antagonist that has various disadvantages, a possible reversal strategy is treatment of OIRD with the hypothalamic hormone and neuromodulator thyrotropin‐releasing hormone (TRH). In this review, we performed a search in electronic databases and retrieved 52 papers on the effect of TRH and TRH‐analogs on respiration and their efficacy in the reversal of OIRD in awake and anesthetized mammals, including humans. Animal studies show that TRH and its analog taltirelin stimulate breathing via an effect at the preBötzinger complex, an important respiratory rhythm generator within the brainstem respiratory network. An additional respiratory excitatory effect may be related to TRH’s analeptic effect. In awake and anesthetized rodents, TRH and taltirelin improved morphine‐ and sufentanil‐induced respiratory depression, by causing rapid shallow breathing. This pattern of breathing increases the work of breathing, dead space ventilation, atelectasis, and hypoxia. In awake and anesthetized humans, a continuous infusion of intravenous TRH with doses up to 8 mg, did not reverse sufentanil‐ or remifentanil‐induced respiratory depression. This is related to poor penetration of TRH into the brain compartment but also other causes are discussed. No human data on taltirelin are available. In conclusion, data from animals and human indicate that TRH is not a viable reversal agent of OIRD in awake or anesthetized humans. Further human studies on the efficacy and safety of TRH’s more potent and longer lasting analog taltirelin are needed as this agent seems to be a more promising reversal drug.

Putting the theory into 'burstlet theory' with a biophysical model of burstlets and bursts in the respiratory preBötzinger complex

Inspiratory breathing rhythms arise from synchronized neuronal activity in a bilaterally distributed brainstem structure known as the preBötzinger complex (preBötC). In in vitro slice preparations containing the preBötC, extracellular potassium must be elevated above physiological levels (to 7–9 mM) to observe regular rhythmic respiratory motor output in the hypoglossal nerve to which the preBötC projects. Reexamination of how extracellular K⁺ affects preBötC neuronal activity has revealed that low-amplitude oscillations persist at physiological levels. These oscillatory events are subthreshold from the standpoint of transmission to motor output and are dubbed burstlets. Burstlets arise from synchronized neural activity in a rhythmogenic neuronal subpopulation within the preBötC that in some instances may fail to recruit the larger network events, or bursts, required to generate motor output. The fraction of subthreshold preBötC oscillatory events (burstlet fraction) decreases sigmoidally with increasing extracellular potassium. These observations underlie the burstlet theory of respiratory rhythm generation. Experimental and computational studies have suggested that recruitment of the non-rhythmogenic component of the preBötC population requires intracellular Ca²⁺ dynamics and activation of a calcium-activated nonselective cationic current. In this computational study, we show how intracellular calcium dynamics driven by synaptically triggered Ca²⁺ influx as well as Ca²⁺ release/uptake by the endoplasmic reticulum in conjunction with a calcium-activated nonselective cationic current can reproduce and offer an explanation for many of the key properties associated with the burstlet theory of respiratory rhythm generation. Altogether, our modeling work provides a mechanistic basis that can unify a wide range of experimental findings on rhythm generation and motor output recruitment in the preBötC.

Current and Future Perspective of Devices and Diagnostics for Opioid and OIRD

OIRD (opioid-induced respiratory depression) remains a significant public health concern due to clinically indicated and illicit opioid use. Respiratory depression is the sine qua non of opioid toxicity, and early detection is critical for reversal using pharmacologic and non-pharmacologic interventions. In addition to respiratory monitoring devices such as pulse oximetry, capnography, and contactless monitoring systems, novel implantable sensors and detection systems such as optical detection and electrochemical detection techniques are being developed to identify the presence of opioids both in vivo and within the environment. These new technologies will not only monitor for signs and symptoms of OIRD but also serve as a mechanism to alert and assist first responders and lay rescuers. The current opioid epidemic brings to the forefront the need for additional accessible means of detection and diagnosis. Rigorous evaluation of safety, efficacy, and acceptability will be necessary for both new and established technologies to have an impact on morbidity and mortality associated with opioid toxicity. Here, we summarized existing and advanced technologies for opioid detection and OIRD management with a focus on recent advancements in wearable and implantable opioid detection. We expect that this review will serve as a complete informative reference for the researchers and healthcare professionals working on the subject and allied fields.

Contribution of the caudal medullary raphe to opioid induced respiratory depression

BACKGROUND

Opioid-induced respiratory depression can be partially antagonized in the preBötzinger Complex and Parabrachial Nucleus/Kölliker-Fuse Complex. We hypothesized that additional opioid antagonism in the caudal medullary raphe completely reverses the opioid effect.

METHODS

In adult ventilated, vagotomized, decerebrate rabbits, we administrated remifentanil intravenously at "analgesic", "apneic", and "very high" doses and determined the reversal with sequential naloxone microinjections into the bilateral Parabrachial Nucleus/Kölliker-Fuse Complex, preBötzinger Complex, and caudal medullary raphe. In separate animals, we injected opioid antagonists into the raphe without intravenous remifentanil.

RESULTS

Sequential naloxone microinjections completely reversed respiratory rate depression from "analgesic" and "apneic" remifentanil, but not "very high" remifentanil concentrations. Antagonist injection into the caudal medullary raphe without remifentanil independently increased respiratory rate.

CONCLUSIONS

Opioid-induced respiratory depression results from a combined effect on the respiratory rhythm generator and respiratory drive. The effect in the caudal medullary raphe is complex as we also observed local antagonism of endogenous opioid receptor activation, which has not been described before.

Advances in Reversal Strategies of Opioid-induced Respiratory Toxicity

Opioids may produce life-threatening respiratory depression and death from their actions at the opioid receptors within the brainstem respiratory neuronal network. Since there is an increasing number of conditions where the administration of the opioid receptor antagonist naloxone is inadequate or undesired, there is an increased interest in the development of novel reversal and prevention strategies aimed at providing efficacy close to that of the opioid receptor antagonist naloxone but with fewer of its drawbacks such as its short duration of action and lesser ability to reverse high-affinity opioids, such as carfentanil, or drug combinations. To give an overview of this highly relevant topic, the authors systematically discuss predominantly experimental pharmacotherapies, published in the last 5 yr, aimed at reversal of opioid-induced respiratory depression as alternatives to naloxone. The respiratory stimulants are discussed based on their characteristics and mechanism of action: nonopioid controlled substances (e.g., amphetamine, cannabinoids, ketamine), hormones (thyrotropin releasing hormone, oxytocin), nicotinic acetylcholine receptor agonists, ampakines, serotonin receptor agonists, antioxidants, miscellaneous peptides, potassium channel blockers acting at the carotid bodies (doxapram, ENA001), sequestration techniques (scrubber molecules, immunopharmacotherapy), and opioids (partial agonists/antagonists). The authors argue that none of these often still experimental therapies are sufficiently tested with respect to efficacy and safety, and many of the agents presented have a lesser efficacy at deeper levels of respiratory depression, i.e., inability to overcome apnea, or have ample side effects. The authors suggest development of reversal strategies that combine respiratory stimulants with naloxone. Furthermore, they encourage collaborations between research groups to expedite development of viable reversal strategies of potent synthetic opioid-induced respiratory depression.