Oxytocin Receptor Activation Rescues Opioid-Induced Respiratory Depression by Systemic Fentanyl in the Rat

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).

Structure-Based Design of Glycosylated Oxytocin Analogues with Improved Selectivity and Antinociceptive Activity

Acute and chronic pain is often treated with opioids despite the negative side effects of constipation, physical dependence, respiratory depression, and overdose. The misuse of opioid analgesics has given rise to the opioid crisis/epidemic, and alternate nonaddictive analgesics are urgently needed. Oxytocin, a pituitary hormone, is an alternative to the small molecule treatments available and has been used as an analgesic as well as for the treatment and prevention of opioid use disorder (OUD). Clinical implementation is limited by its poor pharmacokinetic profile, a result of the labile disulfide bond between two cysteine residues in the native sequence. Stable brain penetrant oxytocin analogues have been synthesized by replacement of the disulfide bond with a stable lactam and glycosidation of the C-terminus. These analogues show exquisite selectivity for the oxytocin receptor and potent in vivo antinociception in mice following peripheral (i.v.) administration, supporting further study of their clinical potential.

Impact of Magnesium on Oxytocin Receptor Function

BACKGROUND AND PURPOSE

The intranasal administration of oxytocin (OT) reduces migraine headaches through activation of the oxytocin receptor (OTR). Magnesium ion (Mg2+) concentration is critical to the activation of the OTR, and a low serum Mg2+ concentration is predictive of a migraine headache. We, therefore, examined the functional impact of Mg2+ concentration on OT-OTR binding efficacy using two complimentary bioassays.

EXPERIMENTAL APPROACH

Current clamp recordings of rat trigeminal ganglia (TG) neurons measured the impact of Mg2+ on an OT-induced reduction in excitability. In addition, we assessed the impact of Mg2+ on intranasal OT-induced craniofacial analgesia in rats.

KEY RESULTS

While OT alone dose-dependently hyperpolarized TG neurons, decreasing their excitability, the addition of 1.75 mM Mg2+ significantly enhanced this effect. Similarly, while the intranasal application of OT produced dose-dependent craniofacial analgesia, Mg2+ significantly enhanced these effects.

CONCLUSIONS AND IMPLICATIONS

OT efficacy may be limited by low ambient Mg2+ levels. The addition of Mg2+ to OT formulations may improve its efficacy in reducing headache pain as well as for other OT-dependent processes.

Unique Pharmacology, Brain Dysfunction, and Therapeutic Advancements for Fentanyl Misuse and Abuse

Fentanyl is a fully synthetic opioid with analgesic and anesthetic properties. It has become a primary driver of the deadliest opioid crisis in the United States and elsewhere, consequently imposing devastating social, economic, and health burdens worldwide. However, the neural mechanisms that underlie the behavioral effects of fentanyl and its analogs are largely unknown, and approaches to prevent fentanyl abuse and fentanyl-related overdose deaths are scarce. This review presents the abuse potential and unique pharmacology of fentanyl and elucidates its potential mechanisms of action, including neural circuit dysfunction and neuroinflammation. We discuss recent progress in the development of pharmacological interventions, anti-fentanyl vaccines, anti-fentanyl/heroin conjugate vaccines, and monoclonal antibodies to attenuate fentanyl-seeking and prevent fentanyl-induced respiratory depression. However, translational studies and clinical trials are still lacking. Considering the present opioid crisis, the development of effective pharmacological and immunological strategies to prevent fentanyl abuse and overdose are urgently needed.

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.

Non-Opioid Peptides Targeting Opioid Effects

Opioids are the most potent widely used analgesics, primarily, but not exclusively, in palliative care. However, they are associated with numerous side effects, such as tolerance, addiction, respiratory depression, and cardiovascular events. This, in turn, can result in their overuse in cases of addiction, the need for dose escalation in cases of developing tolerance, and the emergence of dose-related opioid toxicity, resulting in respiratory depression or cardiovascular problems that can even lead to unintentional death. Therefore, a very important challenge for researchers is to look for ways to counteract the side effects of opioids. The use of peptides and their related compounds, which have been shown to modulate the effects of opioids, may provide such an opportunity. This short review is a compendium of knowledge about the most important and recent findings regarding selected peptides and their modulatory effects on various opioid actions, including cardiovascular and respiratory responses. In addition to the peptides more commonly reported in the literature in the context of their pro- and/or anti-opioid activity-such as neuropeptide FF (NPFF), cholecystokinin (CCK), and melanocyte inhibiting factor (MIF)-we also included in the review nociceptin/orphanin (N/OFQ), ghrelin, oxytocin, endothelin, and venom peptides.

Vasopressin and Breathing: Review of Evidence for Respiratory Effects of the Antidiuretic Hormone

Vasopressin (AVP) is a key neurohormone involved in the regulation of body functions. Due to its urine-concentrating effect in the kidneys, it is often referred to as antidiuretic hormone. Besides its antidiuretic renal effects, AVP is a potent neurohormone involved in the regulation of arterial blood pressure, sympathetic activity, baroreflex sensitivity, glucose homeostasis, release of glucocorticoids and catecholamines, stress response, anxiety, memory, and behavior. Vasopressin is synthesized in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus and released into the circulation from the posterior lobe of the pituitary gland together with a C-terminal fragment of pro-vasopressin, known as copeptin. Additionally, vasopressinergic neurons project from the hypothalamus to the brainstem nuclei. Increased release of AVP into the circulation and elevated levels of its surrogate marker copeptin are found in pulmonary diseases, arterial hypertension, heart failure, obstructive sleep apnoea, severe infections, COVID-19 due to SARS-CoV-2 infection, and brain injuries. All these conditions are usually accompanied by respiratory disturbances. The main stimuli that trigger AVP release include hyperosmolality, hypovolemia, hypotension, hypoxia, hypoglycemia, strenuous exercise, and angiotensin II (Ang II) and the same stimuli are known to affect pulmonary ventilation. In this light, we hypothesize that increased AVP release and changes in ventilation are not coincidental, but that the neurohormone contributes to the regulation of the respiratory system by fine-tuning of breathing in order to restore homeostasis. We discuss evidence in support of this presumption. Specifically, vasopressinergic neurons innervate the brainstem nuclei involved in the control of respiration. Moreover, vasopressin V1a receptors (V1aRs) are expressed on neurons in the respiratory centers of the brainstem, in the circumventricular organs (CVOs) that lack a blood-brain barrier, and on the chemosensitive type I cells in the carotid bodies. Finally, peripheral and central administrations of AVP or antagonists of V1aRs increase/decrease phrenic nerve activity and pulmonary ventilation in a site-specific manner. Altogether, the findings discussed in this review strongly argue for the hypothesis that vasopressin affects ventilation both as a blood-borne neurohormone and as a neurotransmitter within the central nervous system.