Evidence for peripheral mechanisms mediating the antitussive actions of opioids in the guinea pig

Effect of intravenous fentanyl on cough reflex and quality of endotracheal intubation in cats

OBJECTIVE

To assess the effects of intravenous (IV) fentanyl on cough reflex and quality of endotracheal intubation (ETI) in cats.

STUDY DESIGN

Randomized, blinded, negative controlled clinical trial.

ANIMALS

A total of 30 client-owned cats undergoing general anaesthesia for diagnostic or surgical procedures.

METHODS

Cats were sedated with dexmedetomidine (2 μg kg^-1 IV), and 5 minutes later either fentanyl (3 μg kg^-1, group F) or saline (group C) was administered IV. After alfaxalone (1.5 mg kg^-1 IV) administration and 2% lidocaine application to the larynx, ETI was attempted. If unsuccessful, alfaxalone (1 mg kg^-1 IV) was administered and ETI re-attempted. This process was repeated until successful ETI. Sedation scores, total number of ETI attempts, cough reflex, laryngeal response and quality of ETI were scored. Postinduction apnoea was recorded. Heart rate (HR) was continuously recorded and oscillometric arterial blood pressure (ABP) was measured every minute. Changes (Δ) in HR and ABP between pre-intubation and intubation were calculated. Groups were compared using univariate analysis. Statistical significance was set as p < 0.05.

RESULTS

The median and 95% confidence interval of alfaxalone dose was 1.5 (1.5-1.5) and 2.5 (1.5-2.5) mg kg^-1 in groups F and C, respectively (p = 0.001). The cough reflex was 2.10 (1.10-4.41) times more likely to occur in group C. The overall quality of ETI was superior in group F (p = 0.001), with lower laryngeal response to ETI (p < 0.0001) and ETI attempts (p = 0.045). No differences in HR, ABP and postinduction apnoea were found.

CONCLUSIONS AND CLINICAL RELEVANCE

In cats sedated with dexmedetomidine, fentanyl could be considered to reduce the alfaxalone induction dose, cough reflex and laryngeal response to ETI and to improve the overall quality of ETI.

Comparison between intravenous lidocaine and fentanyl on cough reflex and sympathetic response during endotracheal intubation in dogs

OBJECTIVE

To compare the effects of intravenous (IV) lidocaine and fentanyl on the cough reflex and autonomic response during endotracheal intubation in dogs.

STUDY DESIGN

Randomized, blinded, superiority clinical trial.

ANIMALS

A total of 46 client-owned dogs undergoing magnetic resonance imaging.

METHODS

After intramuscular methadone (0.2 mg kg^-1), dogs were randomized to be administered either IV lidocaine (2 mg kg^-1; group L) or fentanyl (7 μg kg^-1; group F). After 5 minutes, alfaxalone was administered until endotracheal intubation was possible (1 mg kg^-1 IV over 40 seconds followed by 0.4 mg kg^-1 increments to effect). Total dose of alfaxalone was recorded and cough reflex at endotracheal intubation was scored. Heart rate (HR) was continuously recorded, Doppler systolic arterial blood pressure (SAP) was measured every 20 seconds. Vasovagal tonus index (VVTI) and changes (Δ) in HR, SAP and VVTI between pre-intubation and intubation were calculated. Groups were compared using univariate and multivariate analysis. Statistical significance was set as p < 0.05.

RESULTS

Group F included 22 dogs and group L 24 dogs. The mean (± standard deviation) alfaxalone dose was 1.1 (± 0.2) and 1.35 (± 0.3) mg kg^-1 in groups F and L, respectively (p = 0.0008). At intubation, cough was more likely in group L (odds ratio = 11.3; 95% confidence intervals, 2.1 - 94.2; p = 0.01) and HR increased in 87.5% and 54.5% of groups L and F, respectively (p = 0.02). The median (range) ΔHR between pre-intubation and intubation was higher (13.1%; - 4.3 to + 55.1) in group L (p = 0.0021). Between groups, SAP and VVTI were similar.

CONCLUSION AND CLINICAL RELEVANCE

At the stated doses, whilst reducing the alfaxalone dose, fentanyl is superior to lidocaine in suppressing the cough reflex and blunting the increase in HR at endotracheal intubation in dogs premedicated with methadone.

Cough Suppression during Flexible Bronchoscopy Using Transcutaneous Electric Acupoint Stimulation: A Randomized Controlled Study

Background and Objective. Transcutaneous electric acupoint stimulation (TEAS) is recommended for its sedative and analgesic effects. We sought to evaluate the effect of TEAS on cough suppression during flexible bronchoscopy (FB) and explore the underlying mechanism. Methods. In this single-center, randomized, single-blind, parallel-controlled study, we randomized 100 patients scheduled for FB into two equal groups treated with or without TEAS (TEAS group and control group). Patients in the TEAS group received 30 min of stimulation at the Hegu (LI4), Neiguan (PC6), and Lieque (LU7) points before FB. The control group underwent the same procedure, but without stimulation. The primary outcome was the intraoperative cough score determined by the bronchoscopist. The secondary outcomes were patient-reported discomfort scores and other procedural parameters. Results. Compared with the controls, patients who received TEAS preconditioning had lower cough scores (P=0.0027) and requirement of lidocaine and fentanyl (P < 0.05) and significantly higher postprocedural plasma β-endorphin levels (P=0.0367). There were no intergroup differences in discomfort scores, midazolam dosage, rate of premature termination, oxygen requirement, sedation level, airway assistance, oxygen saturation, lowest oxygen saturation level, heart rate, plasma substance-P levels, and rate of complications after 24 h. The total procedure duration, time for passage of the bronchoscope through the vocal cords, and systolic and diastolic blood pressure levels were less in the TEAS group than in the control group (P=0.033, 0.039 and <0.05, respectively). Conclusion. The combination of midazolam and TEAS was superior to midazolam alone for cough suppression during FB, probably due to increased plasma β-endorphin levels. This trial is registered with ChiCTR1800016612 at chictr.org.cn/index.aspx.

Antitussive drugs--past, present, and future

Cough remains a serious unmet clinical problem, both as a symptom of a range of other conditions such as asthma, chronic obstructive pulmonary disease, gastroesophageal reflux, and as a problem in its own right in patients with chronic cough of unknown origin. This article reviews our current understanding of the pathogenesis of cough and the hypertussive state characterizing a number of diseases as well as reviewing the evidence for the different classes of antitussive drug currently in clinical use. For completeness, the review also discusses a number of major drug classes often clinically used to treat cough but that are not generally classified as antitussive drugs. We also reviewed a number of drug classes in various stages of development as antitussive drugs. Perhaps surprising for drugs used to treat such a common symptom, there is a paucity of well-controlled clinical studies documenting evidence for the use of many of the drug classes in use today, particularly those available over the counter. Nonetheless, there has been a considerable increase in our understanding of the cough reflex over the last decade that has led to a number of promising new targets for antitussive drugs being identified and thus giving some hope of new drugs being available in the not too distant future for the treatment of this often debilitating symptom.

Cough sensors. III. Opioid and cannabinoid receptors on vagal sensory nerves

Cough is a persistent symptom of many inflammatory airways' diseases. Cough is mediated by receptors sited on sensory nerves and then through vagal afferent pathways, which terminate in the brainstem respiratory centre. Cough is often described as an unmet clinical need. Opioids are the only prescription-based antitussives currently available in the UK. They possess limited efficacy and exhibit serious unwanted side effects, such as physical dependence, sedation, respiratory depression and gastrointestinal symptoms. There are three classical opioid receptors: the mu, kappa and delta receptors. Peripheral opioid receptors are sited on sensory nerves innervating the airways. A greater understanding of the role of the peripheral and centrally sited opioid receptors is necessary to allow the development of targeted treatments for cough. Because of the limited efficacy and the side-effect profile of the opioids, potential new treatments are sought to alleviate cough. One class of compounds that is currently under examination is the cannabinoids. Like the opioids, cannabinoids have peripheral and centrally sited receptors and also suffer from the blight of unwanted centrally mediated side effects such as sedation, cognitive dysfunction, tachycardia and psychotropic effects. Two cannabinoid receptors have been identified, the CB(1) and CB(2) receptors, and their distribution varies throughout the peripheral and central nervous system. Encouragingly, early studies with these compounds suggest that it may be possible to separate their antitussive activity from their centrally mediated side effects, with CB(2) agonists showing potential as putative new treatments for cough. In this chapter, we describe the opioid and cannabinoid receptors, their distribution and the effects they mediate. Moreover, we highlight their potential advantages and disadvantages in the treatment of cough.

Cough sensors. V. Pharmacological modulation of cough sensors

Several airway afferent nerve subtypes have been implicated in coughing. These include bronchopulmonary C-fibers, rapidly adapting airway mechanoreceptors and touch-sensitive tracheal Adelta-fibers (also called cough receptors). Although the last two afferent nerve subtypes are primarily sensitive to mechanical stimuli, all can be acted upon by one or more different chemical stimuli. In this review we catalogue the chemical agents that stimulate and/or modulate the activity of the airway afferent nerves involved in cough, and describe the specific mechanisms involved in these effects. In addition, we describe the mechanisms of action of a number of chemical inhibitors of these afferent nerve subtypes, and attempt to relate this information to the regulation of coughing in health and disease.

Central and peripheral mechanisms of narcotic antitussives: codeine-sensitive and -resistant coughs

Narcotic antitussives such as codeine reveal the antitussive effect primarily via the mu-opioid receptor in the central nervous system (CNS). The kappa-opioid receptor also seems to contribute partly to the production of the antitussive effect of the drugs. There is controversy as to whether delta-receptors are involved in promoting an antitussive effect. Peripheral opioid receptors seem to have certain limited roles. Although narcotic antitussives are the most potent antitussives at present, certain types of coughs, such as chronic cough, are particularly difficult to suppress even with codeine. In guinea pigs, coughs elicited by mechanical stimulation of the bifurcation of the trachea were not able to be suppressed by codeine. In gupigs with sub-acute bronchitis caused by SO2 gas exposure, coughing is difficult to inhibit with centrally acting antitussives such as codeine. Some studies suggest that neurokinins are involved in the development of codeine-resistant coughs. However, evidence supporting this claim is still insufficient. It is very important to characterize opiate-resistant coughs in experimental animals, and to determine which experimentally induced coughs correspond to which types of cough in humans. In this review, we describe the mechanisms of antitussive effects of narcotic antitussives, addressing codeine-sensitive and -resistant coughs, and including our own results.

Action of anti-tussive drugs on the emetic reflex of Suncus murinus (house musk shrew)

The cough and emetic reflexes involve a synchronized firing of motor neurones involved in respiratory control. Tachykinin NK1 receptor antagonists and 5-HT1A receptor agonists are examples of centrally acting drugs that reduce cough and emesis. In the present studies, therefore, we examined the possibility that other classes of drugs known to reducing cough have anti-emetic properties to prevent emesis induced by diverse challenges. We examined the potential of codeine (1-10 mg/kg), baclofen (1-10 mg/kg), scopolamine (0.3-10 mg/kg), diphenhydramine (1-10 mg/kg), imperialine (1-30 mg/kg) and verticine (0.3-3 mg/kg) to inhibit emesis induced by nicotine (5 mg/kg, s.c.), copper sulphate (120 mg/kg, intragastric), and provocative motion (4 cm horizontal displacement, delivered at 1 Hz) in Suncus murinus (house musk shrew). Only codeine had broad inhibitory properties (P<0.05) to antagonize emesis induced by all challenges with ID50 values ranging from 1.2 to 2.3 mg/kg. Baclofen antagonized emesis induced by nicotine (maximum reduction was 44.9%, P<0.05) and motion (maximum reduction was 97.3%, P<0.01), but potentiated copper sulphate-induced emesis (maximum potentiation was 73.0%, P<0.05). Scopolamine antagonized copper sulphate-induced emesis (maximum reduction was 61.2%, P<0.05) and imperialine antagonized nicotine-induced emesis (maximum reduction was 30.2%, P<0.01), but verticine potentiated motion-induced emesis (maximum potentiation was 60.0%, P<0.05). Diphenhydramine did not significantly reduce emesis induced by any of the challenges (P>0.05). In conclusion, codeine has broad inhibitory anti-emetic actions but a known ability to reduce coughing does not necessarily predict broad inhibitory anti-emetic properties.

Antitussive activity of sigma-1 receptor agonists in the guinea-pig

1. Current antitussive medications have limited efficacy and often contain the opiate-like agent dextromethorphan (DEX). The mechanism whereby DEX inhibits cough is ill defined. DEX displays affinity at both NMDA and sigma receptors, suggesting that the antitussive activity may involve central or peripheral activity at either of these receptors. This study examined and compared the antitussive activity of DEX and various putative sigma receptor agonists in the guinea-pig citric-acid cough model. 2. Intraperitoneal (i.p.) administration of DEX (30 mg kg(-1)) and the sigma-1 agonists SKF-10,047 (1-5 mg kg(-1)), Pre-084 (5 mg kg(-1)), and carbetapentane (1-5 mg kg(-1)) inhibited citric-acid-induced cough in guinea-pigs. Intraperitoneal administration of a sigma-1 antagonist, BD 1047 (1-5 mg kg(-1)), reversed the inhibition of cough elicited by SKF-10,047. In addition, two structurally dissimilar sigma agonists SKF-10,047 (1 mg ml(-1)) and Pre-084 (1 mg ml(-1)) inhibited cough when administered by aerosol. 3. Aerosolized BD 1047 (1 mg ml(-1), 30 min) prevented the antitussive action of SKF-10,047 (5 mg kg(-1)) or DEX (30 mg kg(-1)) given by i.p. administration and, likewise, i.p. administration of BD 1047 (5 mg kg(-1)) prevented the antitussive action of SKF-10,047 given by aerosol (1 mg ml(-1)). 4. These results therefore support the argument that antitussive effects of DEX may be mediated via sigma receptors, since both systemic and aerosol administration of sigma-1 receptor agonists inhibit citric-acid-induced cough in guinea-pigs. While significant systemic exposure is possible with aerosol administration, the very low doses administered (estimated <0.3 mg kg(-1)) suggest that there may be a peripheral component to the antitussive effect.

Ion channels in airway afferent neurons

Action potentials initiated at the peripheral terminal of an afferent nerve are conducted to the central nervous system therein causing release of neurotransmitters that excite secondary neurons in the brain stem or spinal cord. Various chemicals, extremes in osmolarity and pH as well as mechanical stimuli are sensed by primary afferent nerves that innervate the airways. The processes leading to excitation of afferent nerve endings, conduction of action potentials along axons, transmitter secretion, and neuronal excitability are regulated by ions flowing through channels in the nerve membrane. Voltage-gated ion channels selective for K+ and Na+ ions allow the generation and conduction of action potentials and along with families of ion channels selective for other ions such as Ca2+ or Cl- are thought to play distinctive roles in regulating neuronal excitability and transmitter secretion. Here we discuss, in general terms, the roles played by various classes of ion channels in the activation, neurotransmitter secretion and excitability of primary afferent neurons.

The effect of inhaled and oral dextromethorphan on citric acid induced cough in man

1. Dextromethorphan is a widely used antitussive agent which is a non-narcotic codeine analogue. We have investigated whether inhaled administration of dextromethorphan provides antitussive activity in a citric acid induced cough model. 2. Twenty normal subjects underwent repeated cough challenge with 5% citric acid. Subjects were studied on six occasions. Study medication consisted of oral dextromethorphan 30 mg or oral matched placebo or 1, 3 and 30 mg inhaled dextromethorphan or matched inhaled placebo. Cough challenge was administered 10 min after study medication and hourly thereafter up to 250 min. 3. No significant differences were seen between baseline cough responses. Oral dextromethorphan (30 mg) produced a mean percentage reduction in cough of 38% (P < 0.002), which remained significant at 250 min. Inhaled dextromethorphan had no clinically significant effect although activity at later time points was not excluded. The antitussive effect of oral dextromethorphan is confirmed with prolonged inhibition of induced cough. It is possible that dextromethorphan or its active metabolites act centrally to inhibit the cough reflex.

Effects of inhaled alpha 2-adrenoceptor and GABAB receptor agonists on citric acid-induced cough and tidal volume changes in guinea pigs

The effects of alpha 2-adrenoceptor and GABA receptor agonists on citric acid-induced cough and increased tidal volume were investigated in conscious guinea pigs. Inhalation of low doses of B-HT 920 (5-allyl-2-amino 5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]azepine dihydrochloride), and xylazine significantly inhibited citric acid-induced cough and tidal volume increases. Intraperitoneal administration of higher doses of B-HT 920 than those given by aerosol were ineffective. The inhibitory effects of B-HT 920 were antagonised by prior intraperitoneal administration of yohimbine, but not atropine. Inhalation of GABA or baclofen inhibited tidal volume increases, but had no effect on cough. Inhaled alpha 2-adrenoceptor or GABA agonists had no effect on the reduced respiratory rate after citric acid inhalation. It is concluded that alpha 2-adrenoceptor agonists inhibit cough via a mechanism which may not be related to their ability to reduce citric acid-induced tidal volume increases, since GABA and baclofen inhibited tidal volume increases but not cough. We suggest that alpha 2-adrenoceptor agonists may have therapeutic potential in the treatment of cough.