Respiratory stimulant drugs in the post-operative setting

Recent biological applications of heterocyclic hybrids containing s-triazine scaffold

s-Triazine possesses an auspicious status in the field of drug discovery and development owing to its presence in many naturally occurring compounds as well as commercially available drugs like enasidenib, gedatolisib, bimiralisib, atrazine, indaziflam, and triaziflam. Easy, cost-effective, and efficient access to its derivatives in addition to their splendid biological activities such as anticancer, anti-inflammatory, antiviral, anticonvulsant, anti-tubercular, antidiabetic, antimicrobial, makes it an attractive heterocyclic nucleus in the field of medicinal chemistry. Other than the direct access of its derivatives from simple commercially available starting materials like amidine, the s-triazine derivatives have also been obtained starting from an inexpensive commercially available 2,4,6-trichloro-1,3,5-triazine (TCT) commonly known as cyanuric chloride. Owing to the high reactivity and the possibility of sequential substitution of TCT, a variety of biologically active heterocyclic scaffolds have been installed on this nucleus in order to have more potent compounds. These s-triazine-based heterocyclic hybrids have been reported to show enhanced biological activities in recent years. Therefore, it is important to summarize and highlight recent examples of these hybrids which is imperative to attract the attention of the drug development community.

Repurposing the Medicines for Malaria Venture's COVID Box to discover potent inhibitors of Toxoplasma gondii, and in vivo efficacy evaluation of almitrine bismesylate (MMV1804175) in chronically infected mice

Toxoplasmosis, caused by the obligate intracellular parasite Toxoplasma gondii, affects about one-third of the world's population and can cause severe congenital, neurological and ocular issues. Current treatment options are limited, and there are no human vaccines available to prevent transmission. Drug repurposing has been effective in identifying anti-T. gondii drugs. In this study, the screening of the COVID Box, a compilation of 160 compounds provided by the "Medicines for Malaria Venture" organization, was conducted to explore its potential for repurposing drugs to combat toxoplasmosis. The objective of the present work was to evaluate the compounds' ability to inhibit T. gondii tachyzoite growth, assess their cytotoxicity against human cells, examine their absorption, distribution, metabolism, excretion, and toxicity (ADMET) properties, and investigate the potential of one candidate drug through an experimental chronic model of toxoplasmosis. Early screening identified 29 compounds that could inhibit T. gondii survival by over 80% while keeping human cell survival up to 50% at a concentration of 1 μM. The Half Effective Concentrations (EC50) of these compounds ranged from 0.04 to 0.92 μM, while the Half Cytotoxic Concentrations (CC50) ranged from 2.48 to over 50 μM. Almitrine was chosen for further evaluation due to its favorable characteristics, including anti-T. gondii activity at nanomolar concentrations, low cytotoxicity, and ADMET properties. Administering almitrine bismesylate (Vectarion®) orally at dose of 25 mg/kg/day for ten consecutive days resulted in a statistically significant (p < 0.001) reduction in parasite burden in the brains of mice chronically infected with T. gondii (ME49 strain). This was determined by quantifying the RNA of living parasites using real-time PCR. The presented results suggest that almitrine may be a promising drug candidate for additional experimental studies on toxoplasmosis and provide further evidence of the potential of the MMV collections as a valuable source of drugs to be repositioned for infectious diseases.

Pre-treatment with Tandospirone attenuates fentanyl-induced respiratory depression without affecting the analgesic effects of fentanyl in rodents

Opioid analgesics are widely used to treat acute, postoperative, and chronic pain. However, opioid receptor activation can result in severe respiratory depression. In this study, we demonstrated that Tandospirone (TS), a selective serotonin-1A receptor partial agonist, is effective against opioid-induced respiratory depression. Fentanyl was used to establish a respiratory depression model in rodents. We observed the effects of TS on respiratory depression in rats by using plethysmographic recordings and arterial oxygen saturation. In addition, we evaluated the effects of TS on fentanyl-induced sedation and analgesia by using the loss of righting reflex (LORR) and hot-plate tests, respectively. Rats (n = 5) were treated with TS or saline 5 min prior to fentanyl administration. TS [2 mg/kg, intravenous (i.v.)] dose-dependently attenuated fentanyl-induced respiratory depression versus saline + fentanyl group. Furthermore, pre-treatment with TS (2 mg/kg, i.v.) increased arterial oxygen saturation to 76.5 ± 2.0% at 5 min after fentanyl injection, compared with 35.9 ± 2.5% in saline pre-treated rats (P < 0.001), whereas the time to induction of LORR (P > 0.99) and duration of LORR (P = 0.95) did not differ between the "TS + fentanyl" and "saline + fentanyl" group. The antinociceptive effect of fentanyl was not affected by the administration of TS (P = 0.99) in mice (n = 10). In conclusion, we found that TS, a novel non-benzodiazepine anxiolytic/antidepressant drug, could attenuate severe fentanyl-induced respiratory depression and did not affect the analgesic/sedative effect of fentanyl. The clinical application of TS could significantly improve pain management.

Combined Cardiopulmonary Assessments Using Impedance and Digital Implants in Conscious Freely Moving Cynomolgus Monkeys, Beagle Dogs, and Göttingen Minipigs: Pharmacological Characterization and Social Housing Effects

Respiratory monitoring, using impedance with implanted telemetry in socially housed animals, was not possible until the recent development of digital signal transmission. The objective of this study was to evaluate digital telemetry monitoring of cardiopulmonary parameters (respiratory rate, tidal volume, minute volume, electrocardiography (DII), systemic arterial blood pressure, physical activity, and body temperature) in conscious, single-housed, non-rodent species commonly used in toxicology studies following administration of positive/negative controls (saline, dexmedetomidine, morphine, amphetamine, and doxapram), and also, the effects of various social housing arrangements in untreated female and/or male cynomolgus monkeys, Beagle dogs, and Göttingen minipigs (n = 4 per species). Aggressions were observed in socially housed male minipigs, however, which prevented pair-housed assessments in this species. All tested pharmacological agents significantly altered more than one organ system, highlighting important inter-organ dependencies when analyzing functional endpoints. Stress-related physiological changes were observed with single-housing or pair-housing with a new cage mate in cynomolgus monkeys and Beagle dogs, suggesting that stable social structures are preferable to limit variability, especially around dosing. Concomitant monitoring of cardiovascular and respiratory parameters from the same animals may help reduce the number of animals (3 Rs) needed to fulfill the S7A guidelines and allows for identification of organ system functional correlations. Globally, the data support the use of social housing in non-rodents for safety pharmacology multi-organ system (heart and lungs) monitoring investigations.

Multi-Level Regulation of Opioid-Induced Respiratory Depression

Opioids depress minute ventilation primarily by reducing respiratory rate. This results from direct effects on the preBötzinger Complex as well as from depression of the Parabrachial/Kölliker-Fuse Complex, which provides excitatory drive to preBötzinger Complex neurons mediating respiratory phase-switch. Opioids also depress awake drive from the forebrain and chemodrive.

Impairment of hypoxic pulmonary vasoconstriction in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a serious complication of severe systemic or local pulmonary inflammation, such as caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. ARDS is characterised by diffuse alveolar damage that leads to protein-rich pulmonary oedema, local alveolar hypoventilation and atelectasis. Inadequate perfusion of these areas is the main cause of hypoxaemia in ARDS. High perfusion in relation to ventilation (V/Q<1) and shunting (V/Q=0) is not only caused by impaired hypoxic pulmonary vasoconstriction but also redistribution of perfusion from obstructed lung vessels. Rebalancing the pulmonary vascular tone is a therapeutic challenge. Previous clinical trials on inhaled vasodilators (nitric oxide and prostacyclin) to enhance perfusion to high V/Q areas showed beneficial effects on hypoxaemia but not on mortality. However, specific patient populations with pulmonary hypertension may profit from treatment with inhaled vasodilators. Novel treatment targets to decrease perfusion in low V/Q areas include epoxyeicosatrienoic acids and specific leukotriene receptors. Still, lung protective ventilation and prone positioning are the best available standard of care. This review focuses on disturbed perfusion in ARDS and aims to provide basic scientists and clinicians with an overview of the vascular alterations and mechanisms of V/Q mismatch, current therapeutic strategies, and experimental approaches.

History of Respiratory Stimulants

The interest in substances that stimulate respiration has waxed and waned throughout the years, intensifying following the introduction of a new class of drugs that causes respiratory depression, and diminishing when antidotes or better drug alternatives are found. Examples include the opioids--deaths increasing during overprescribing, diminishing with wider availability of the opioid receptor antagonist naloxone, increasing again during COVID-19; the barbiturates--until largely supplanted by the benzodiazepines; propofol; and other central nervous system depressants. Unfortunately, two new troubling phenomena force a reconsideration of the status-quo: (1) overdoses due to highly potent opioids such as fentanyl, and even more-potent licit and illicit fentanyl analogs, and (2) overdose due to polysubstance use (the combination of an opioid plus one or more non-opioid drug, such as a benzodiazepine, sedating antidepressant, skeletal muscle relaxant, or various other agents). Since these now represent the majority of cases, new solutions are again needed. An interest in respiratory stimulants has been revived. This interest can be informed by a short review of the history of this interesting class of medications. We present a short history of the trajectory of advances toward more selective and safer respiratory stimulants.

Current Drug Treatment Strategies for Atrial Fibrillation and TASK-1 Inhibition as an Emerging Novel Therapy Option

Atrial fibrillation (AF) is the most common sustained arrhythmia with a prevalence of up to 4% and an upwards trend due to demographic changes. It is associated with an increase in mortality and stroke incidences. While stroke risk can be significantly reduced through anticoagulant therapy, adequate treatment of other AF related symptoms remains an unmet medical need in many cases. Two main treatment strategies are available: rate control that modulates ventricular heart rate and prevents tachymyopathy as well as rhythm control that aims to restore and sustain sinus rhythm. Rate control can be achieved through drugs or ablation of the atrioventricular node, rendering the patient pacemaker-dependent. For rhythm control electrical cardioversion and pharmacological cardioversion can be used. While electrical cardioversion requires fasting and sedation of the patient, antiarrhythmic drugs have other limitations. Most antiarrhythmic drugs carry a risk for pro-arrhythmic effects and are contraindicated in patients with structural heart diseases. Furthermore, catheter ablation of pulmonary veins can be performed with its risk of intraprocedural complications and varying success. In recent years TASK-1 has been introduced as a new target for AF therapy. Upregulation of TASK-1 in AF patients contributes to prolongation of the action potential duration. In a porcine model of AF, TASK-1 inhibition by gene therapy or pharmacological compounds induced cardioversion to sinus rhythm. The DOxapram Conversion TO Sinus rhythm (DOCTOS)-Trial will reveal whether doxapram, a potent TASK-1 inhibitor, can be used for acute cardioversion of persistent and paroxysmal AF in patients, potentially leading to a new treatment option for AF.

The problem of postoperative respiratory depression

WHAT IS KNOWN AND OBJECTIVE

Postsurgical recovery is influenced by multiple pre-, intra- and perioperative pharmacotherapeutic interventions, including the administration of medications that can induce respiratory depression postoperatively. We present a succinct overview of the topic, including the nature and magnitude of the problem, contributing factors, current limited options, and potential novel therapeutic approach.

COMMENT

Pre-, intra- and perioperative medications are commonly administered for anxiety, anaesthesia, muscle relaxation and pain relief among other reasons. Several of the medications alone or in joint-action can be additive or synergistic producing respiratory depression. Given the large number of surgical procedures that are performed each year, even a small percentage of postoperative respiratory complications translates into a large number of affected patients.

WHAT IS NEW AND CONCLUSION

Due to the large number of surgeries performed each year, and the variety of medications used before, during, and after surgery, the occurrence of postoperative respiratory depression is surprisingly common. It is a significant medical problem and burden on hospital resources. There is a need for new strategies to prevent and treat the acute and collateral problems associated with postoperative respiratory depression.

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.

Respiratory and antinociceptive effects of dexmedetomidine and doxapram in ball pythons (Python regius)

OBJECTIVE

To determine the effects of dexmedetomidine, doxapram, and dexmedetomidine plus doxapram on ventilation ([Formula: see text]e), breath frequency, and tidal volume (Vt) in ball pythons (Python regius) and of doxapram on the thermal antinociceptive efficacy of dexmedetomidine.

ANIMALS

14 ball pythons.

PROCEDURES

Respiratory effects of dexmedetomidine and doxapram were assessed with whole-body, closed-chamber plethysmography, which allowed for estimates of [Formula: see text]e and Vt. In the first experiment of this study with a complete crossover design, snakes were injected, SC, with saline (0.9% NaCl) solution, dexmedetomidine (0.1 mg/kg), doxapram (10 mg/kg), or dexmedetomidine and doxapram, and breath frequency, [Formula: see text]e, and Vt were measured before and every 30 minutes thereafter, through 240 minutes. In the second experiment, antinociceptive efficacy of saline solution, dexmedetomidine, and dexmedetomidine plus doxapram was assessed by measuring thermal withdrawal latencies before and 60 minutes after SC injection.

RESULTS

Dexmedetomidine significantly decreased breath frequency and increased Vt but did not affect [Formula: see text]e at all time points, compared with baseline. Doxapram significantly increased [Formula: see text]e, breath frequency, and Vt at 60 minutes after injection, compared with saline solution. The combination of dexmedetomidine and doxapram, compared with dexmedetomidine alone, significantly increased [Formula: see text]e at 30 and 60 minutes after injection and did not affect breath frequency and Vt at all time points. Thermal withdrawal latencies significantly increased when snakes received dexmedetomidine or dexmedetomidine plus doxapram, versus saline solution.

CONCLUSIONS AND CLINICAL RELEVANCE

Concurrent administration of doxapram may mitigate the dexmedetomidine-induced reduction of breathing frequency without disrupting thermal antinociceptive efficacy in ball pythons.

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.

Effects of the ventilatory stimulant, doxapram on human TASK-3 (KCNK9, K2P9.1) channels and TASK-1 (KCNK3, K2P3.1) channels

AIMS

The mode of action by which doxapram acts as a respiratory stimulant in humans is controversial. Studies in rodent models, have shown that doxapram is a more potent and selective inhibitor of TASK-1 and TASK-1/TASK-3 heterodimer channels, than TASK-3. Here we investigate the direct effect of doxapram and chirally separated, individual positive and negative enantiomers of the compound, on both human and mouse, homodimeric and heterodimeric variants of TASK-1 and TASK-3.

METHODS

Whole-cell patch clamp electrophysiology on tsA201 cells was used to assess the potency of doxapram on cloned human or mouse TASK-1, TASK-3 and TASK-2 channels. Mutations of amino acids in the pore-lining region of TASK-3 channels were introduced using site-directed mutagenesis.

RESULTS

Doxapram was an equipotent inhibitor of human TASK-1 and TASK-3 channels, compared with mouse channel variants, where it was more selective for TASK-1 and heterodimers of TASK-1 and TASK-3. The effect of doxapram could be attenuated by either the removal of the C-terminus of human TASK-3 channels or mutations of particular hydrophobic residues in the pore-lining region. These mutations, however, did not alter the effect of a known extracellular inhibitor of TASK-3, zinc. The positive enantiomer of doxapram, GAL-054, was a more potent antagonist of TASK channels, than doxapram, whereas the negative enantiomer, GAL-053, had little inhibitory effect.

CONCLUSION

These data show that in contrast to rodent channels, doxapram is a potent inhibitor of both TASK-1 and TASK-3 human channels, providing further understanding of the pharmacological profile of doxapram in humans and informing the development of new therapeutic agents.

High Efficacy by GAL-021: A Known Intravenous Peripheral Chemoreceptor Modulator that Suppresses BKCa-Channel Activity and Inhibits IK(M) or Ih

GAL-021 has recently been developed as a novel breathing control modulator. However, modifications of ionic currents produced by this agent remain uncertain, although its efficacy in suppressing the activity of big-conductance Ca²⁺-activated K⁺ (BK_Ca) channels has been reported. In pituitary tumor (GH₃) cells, we found that the presence of GAL-021 decreased the amplitude of macroscopic Ca²⁺-activated K⁺ current (I_K(Ca)) in a concentration-dependent manner with an effective IC₅₀ of 2.33 μM. GAL-021-mediated reduction of I_K(Ca) was reversed by subsequent application of verteporfin or ionomycin; however, it was not by that of diazoxide. In inside-out current recordings, the addition of GAL-021 to the bath markedly decreased the open-state probability of BK_Ca channels. This agent also resulted in a rightward shift in voltage dependence of the activation curve of BK_Ca channels; however, neither the gating charge of the curve nor single-channel conductance of the channel was changed. There was an evident lengthening of the mean closed time of BK_Ca channels in the presence of GAL-021, with no change in mean open time. The GAL-021 addition also suppressed M-type K⁺ current with an effective IC₅₀ of 3.75 μM; however, its presence did not alter the amplitude of erg-mediated K⁺ current, or mildly suppressed delayed-rectifier K⁺ current. GAL-021 at a concentration of 30 μM could also suppress hyperpolarization-activated cationic current. In HEK293T cells expressing α-hSlo, the addition of GAL-021 was also able to suppress the BK_Ca-channel open probabilities, and GAL-021-mediated suppression of BK_Ca-channel activity was attenuated by further addition of BMS-191011. Collectively, the GAL-021 effects presented herein do not exclusively act on BK_Ca channels and these modifications on ionic currents exert significant influence on the functional activities of electrically excitable cells occurring in vivo.

H· Transfer-Initiated Synthesis of γ-Lactams: Interpretation of Cycloisomerization and Hydrogenation Ratios

A cobaloxime/H2 system used to synthesize valuable γ-lactams from acrylamide molecules is described. In addition to cycloisomerized lactams, linear hydrogenated products were also observed. The amounts of the hydrogenation product were observed to correlate with the bulk of the substituent on the acrylamide nitrogen. Further analysis of the product distributions with experimental and computational studies suggested that while cyclization can occur from one C=C acrylamide rotamer, hydrogenation can occur from both. This observation was further evinced through calculation of the hydrogenation rate constant, which was observed to be ca. 102 faster than previously determined for a related system using n Bu3SnH.

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.

On the front lines of the opioid epidemic: Rescue by naloxone

Naloxone is a specific, high affinity opioid antagonist that has been used to treat suspected or confirmed overdose for more than 40 years. Naloxone use was initially confined to an emergency room setting, but the dramatic rise in opioid overdose events over the past two decades has, with increasing frequency, shifted naloxone use to first responders including police, emergency medical technicians, and the friends and family of overdose victims. The opioids responsible for overdose events have also evolved, from prescription opioids to heroin and most recently, very high potency synthetic opioids such as fentanyl. In 2016, synthetic opioids were linked to more overdose fatalities than either prescription opioids or heroin. In this review, I will discuss the evolution and use of naloxone products by first responders and the development of additional rescue medications in response to the unprecedented dangers posed by synthetic opioids.

Assessment of calibration methods on impedance pneumography accuracy

The aim was to assess accuracy of tidal volumes (TV) calculated by impedance pneumography (IP), reproducibility of calibration coefficients (CC) between IP and pneumotachometry (PNT), and their relationship with body posture, breathing rate and depth. Fourteen students performed three sessions of 18 series: normal and deep breathing at 6, 10, 15 breaths/min rates, while supine, sitting and standing; 18 CC were calculated for every session. Session 2 was performed 2 months after session 1, session 3 1-3 days after session 2. TV were calculated using full or limited set of CC from current session, in case of sessions 2 and 3 also using CC from session 1 and 2, respectively. When using full set of CC from current session, IP underestimated TV by -3.2%. Using CC from session 2 for session 3 measurements caused decrease of relative difference: -3.9%, from session 1 for session 2: -5.3%; for limited set of CC: -5.0%. The body posture had significant effect on CC. The highest accuracy was obtained when all factors influencing CC were considered. The application of CC related only to body posture may result in shortening of calibration and moderate accuracy loss. Using CC from previous session compromises accuracy moderately.

Low cost of pulmonary ventilation in American alligators (Alligator mississippiensis) stimulated with doxapram

To determine the costs of pulmonary ventilation without imposing severe oxygen limitations or acidosis that normally accompany exposures to hypoxia or hypercapnia, we opted to pharmacologically stimulate ventilation with doxapram (5 and 10 mg kg−1) in alligators. Doxapram is used clinically to alleviate ventilatory depression in response to anaesthesia and acts primarily on the peripheral oxygen sensitive chemoreceptors. Using this approach, we investigate the hypothesis that pulmonary ventilation is relatively modest in comparison to resting metabolic rate in crocodilians and equipped seven juvenile alligators with masks for concurrent determination of ventilation and oxygen uptake. Doxapram elicited a dose-dependent and up to four-fold rise in ventilation, primarily by increasing ventilatory frequency. The attending rise in oxygen uptake was very small; ventilation in resting animals constitutes no more than 5 % of resting metabolic rate. The conclusion that pulmonary ventilation is energetically cheap is consistent with earlier studies on alligators where ventilation has been stimulated by hypoxia, hypercapnia.

Fentanyl effects on breath generation in C57BL/6J and A/J mouse strains

We examined the effect of fentanyl on chemoresponsiveness in mouse strains divergent in the expression of spontaneous and post-hypoxic pauses. Frequency and tidal volume were recorded with plethysmography in A/J and C57BL/6J (B6) male mice. Mice selected at random received an intraperitoneal (IP) injection of either saline, low dose fentanyl (LDF = 0.04 mg/kg), or high dose fentanyl (HDF = 0.4 mg/kg) under hypoxia (8% O2) or hyperoxia (100%O2). LDF produced a decrease in frequency during hypoxia in B6, but not A/J, mice. HDF significantly decreased frequency and tidal volume in both strains under hypoxia and hyperoxia (p<0.01); naloxone, an opioid antagonist, reversed this response. The acute administration of fentanyl at any dose did not promote apneas in strains of mice exhibiting regular or irregular respiratory patterns. However, higher doses depressed respiratory frequency in both strains. The B6 mice responded with a depressive response to hypoxia that did not recover with reoxygenation, but did recover with time or naloxone.

GAL-021 and GAL-160 are Efficacious in Rat Models of Obstructive and Central Sleep Apnea and Inhibit BKCa in Isolated Rat Carotid Body Glomus Cells

GAL-021 and GAL-160 are alkylamino triazine analogues, which stimulate ventilation in rodents, non-human primates and (for GAL-021) in humans. To probe the site and mechanism of action of GAL-021 and GAL-160 we utilized spirometry in urethane anesthetized rats subjected to acute bilateral carotid sinus nerve transection (CSNTX) or sham surgery. In addition, using patch clamp electrophysiology we evaluated ionic currents in carotid body glomus cells isolated from neonatal rats. Acute CSNTX markedly attenuated and in some instances abolished the ventilatory stimulant effects of GAL-021 and GAL-160 (0.3 mg/kg IV), suggesting the carotid body is a/the major locus of action. Electrophysiology studies, in isolated Type I cells, established that GAL-021 (30 μM) and GAL-160 (30 μM) inhibited the BK(Ca) current without affecting the delayed rectifier K(+), leak K(+) or inward Ca(2+) currents. At a higher concentration of GAL-160 (100 μM), inhibition of the delayed rectifier K(+) current and leak K(+) current were observed. These data are consistent with the concept that GAL-021 and GAL-160 influence breathing control by acting as peripheral chemoreceptor modulators predominantly by inhibiting BK(Ca) mediated currents in glomus cells of the carotid body.

Opioid-induced respiratory depression: reversal by non-opioid drugs

The human body is critically dependent on the ventilatory control system for adequate uptake of oxygen and removal of carbon dioxide (CO2). Potent opioid analgesics, through their actions on μ-opioid receptor (MOR) expressed on respiratory neurons in the brainstem, depress ventilation. Opioid-induced respiratory depression (OIRD) is potentially life threatening and the cause of substantial morbidity and mortality. One possible way of prevention of OIRD is by adding a respiratory stimulant to the opioid treatment, which through activation of non-opioidergic pathways will excite breathing and consequently will offset OIRD and should not affect analgesia. Various new respiratory stimulants are currently under investigation including (a) potassium channel blockers acting at the carotid bodies, and (b) ampakines and (c) serotonin receptor agonists acting within the brainstem. (a) GAL-021 targets BKCa-channels. Initial animal and human experimental evidence indicates that this potassium channel blocker is a potent respiratory stimulant that reverses OIRD without affecting antinociception. GAL021 is safe and better tolerated than the older K(+)-channel blocker doxapram and more efficacious in its effect on respiration. (b) Ampakines modulate glutamatergic respiratory neurons in brainstem respiratory centers. Various ampakines have been studied showing their ability to increase respiratory drive during OIRD by increasing respiratory rate. Currently, CX717 is the most promising ampakine for use in humans as it is safe and does not affect opioid analgesia. (c) While animal studies show that serotonin receptor agonists increase respiratory drive via activation of serotonin receptors in brainstem respiratory centers, human studies are without success. Further clinical studies are required to improve our care of patients that are treated with potent opioid analgesics. The use of non-opioid adjuvants may reduce the probability of OIRD but does never relieve us of our duty to continuously monitor these patients, irrespective whether they are treated in-house or in an ambulatory setting.