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

Degenerate neuronal and circuit mechanisms important for generating rhythmic motor patterns

In 1996, we published a review article (Marder E, Calabrese RL. Physiol Rev 76: 687-717, 1996) describing the state of knowledge about the structure and function of the central pattern-generating circuits important for producing rhythmic behaviors. Although many of the core questions persist, much has changed since 1996. Here, we focus on newer studies that reveal ambiguities that complicate understanding circuit dynamics, despite the enormous technical advances of the recent past. In particular, we highlight recent studies of animal-to-animal variability and our understanding that circuit rhythmicity may be supported by multiple state-dependent mechanisms within the same animal and that robustness and resilience in the face of perturbation may depend critically on the presence of modulators and degenerate circuit mechanisms. Additionally, we highlight the use of computational models to ask whether there are generalizable principles about circuit motifs that can be found across rhythmic motor systems in different animal species.

Endomorphin-2 (Endo2) and substance P (SubP) co-application attenuates SubP-induced excitation and alters frequency plasticity in neonatal rat in vitro preparations

Substance P (SubP) and endomorphin-2 (Endo2) are co-localized presynaptically in vesicles of neurons adjacent to inspiratory rhythm-generating pre-Botzinger Complex (preBotC) neurons but the effects of co-released SubP and Endo2 on respiratory motor control are not known. To address this question, SubP alone or a combination of SubP and Endo2 (SubP/Endo2) were bath-applied in a sustained (15-min) or intermittent (5-min application, 5-min washout, x3) pattern at 10-100 nM to neonatal rat brainstem-spinal cord preparations. During neuropeptide application, SubP/Endo2 co-applications generally attenuated SubP-induced increases in burst frequency and decreases in burst amplitude. With respect to frequency plasticity (long-lasting increase in burst frequency 60 min post-neuropeptide application), SubP-induced frequency plasticity was increased with sustained SubP/Endo2 co-applications at 20 and 100 nM. Intermittent SubP/Endo2 co-applications tended to decrease the level of frequency plasticity induced by intermittent SubP alone applications. SubP/Endo2 co-applications revealed potentially new functions for neurokinin-1 (NK1R) and mu-opioid (MOR) receptors on respiratory rhythm-generating medullary neurons.

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.

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.

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.

Purinergic signaling mediates neuroglial interactions to modulate sighs

Sighs prevent the collapse of alveoli in the lungs, initiate arousal under hypoxic conditions, and are an expression of sadness and relief. Sighs are periodically superimposed on normal breaths, known as eupnea. Implicated in the generation of these rhythmic behaviors is the preBötzinger complex (preBötC). Our experimental evidence suggests that purinergic signaling is necessary to generate spontaneous and hypoxia-induced sighs in a mouse model. Our results demonstrate that driving calcium increases in astrocytes through pharmacological methods robustly increases sigh, but not eupnea, frequency. Calcium imaging of preBötC slices corroborates this finding with an increase in astrocytic calcium upon application of sigh modulators, increasing intracellular calcium through g-protein signaling. Moreover, photo-activation of preBötC astrocytes is sufficient to elicit sigh activity, and this response is blocked with purinergic antagonists. We conclude that sighs are modulated through neuron-glia coupling in the preBötC network, where the distinct modulatory responses of neurons and glia allow for both rhythms to be independently regulated.

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).

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.

Mu-opioid receptor-dependent transformation of respiratory motor pattern in neonates in vitro

Endogenous opioid peptides activating mu-opioid receptors (MORs) are part of an intricate neuromodulatory system that coordinates and optimizes respiratory motor output to maintain blood-gas homeostasis. MOR activation is typically associated with respiratory depression but also has excitatory effects on breathing and respiratory neurons. We hypothesized that low level MOR activation induces excitatory effects on the respiratory motor pattern. Thus, low concentrations of an MOR agonist drug (DAMGO, 10–200 nM) were bath-applied to neonatal rat brainstem-spinal cord preparations while recording inspiratory-related motor output on cervical spinal roots (C4-C5). Bath-applied DAMGO (50–200 nM) increased inspiratory motor burst amplitude by 40–60% during (and shortly following) drug application with decreased burst frequency and minute activity. Reciprocal changes in inspiratory burst amplitude and frequency were balanced such that 20 min after DAMGO (50–200 nM) application, minute activity was unaltered compared to pre-DAMGO levels. The DAMGO-induced inspiratory burst amplitude increase did not require crossed cervical spinal pathways, was expressed on thoracic ventral spinal roots (T4-T8) and remained unaltered by riluzole pretreatment (blocks persistent sodium currents associated with gasping). Split-bath experiments showed that the inspiratory burst amplitude increase was induced only when DAMGO was bath-applied to the brainstem and not the spinal cord. Thus, MOR activation in neonates induces a respiratory burst amplitude increase via brainstem-specific mechanisms. The burst amplitude increase counteracts the expected MOR-dependent frequency depression and may represent a new mechanism by which MOR activation influences respiratory motor output.

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.

Prostaglandin E2 Exerts Biphasic Dose Response on the PreBötzinger Complex Respiratory-Related Rhythm

Inflammation in infants can cause respiratory dysfunction and is potentially life-threatening. Prostaglandin E2 (PGE2) is released during inflammatory events and perturbs breathing behavior in vivo. Here we study the effects of PGE2 on inspiratory motor rhythm generated by the preBötzinger complex (preBötC). We measured the concentration dependence of PGE2 (1 nM-1 μM) on inspiratory-related motor output in rhythmic medullary slice preparations. Low concentrations (1–10 nM) of PGE2 increased the duration of the inspiratory burst period, while higher concentrations (1 μM) decreased the burst period duration. Using specific pharmacology for prostanoid receptors (EP1-4R, FPR, and DP2R), we determined that coactivation of both EP2R and EP3R is necessary for PGE2 to modulate the inspiratory burst period. Additionally, biased activation of EP3 receptors lengthened the duration of the inspiratory burst period, while biased activation of EP2 receptors shortened the burst period. To help delineate which cell populations are affected by exposure to PGE2, we analyzed single-cell RNA-Seq data derived from preBötC cells. Transcripts encoding for EP2R (Ptger2) were differentially expressed in a cluster of excitatory neurons putatively located in the preBötC. A separate cluster of mixed inhibitory neurons differentially expressed EP3R (Ptger3). Our data provide evidence that EP2 and EP3 receptors increase the duration of the inspiratory burst period at 1–10 nM PGE2 and decrease the burst period duration at 1 μM. Further, the biphasic dose response likely results from differences in receptor binding affinity among prostanoid receptors.