Differential effect of codeine on coughs caused by mechanical stimulation of two different sites in the airway of guinea pigs

Induction of airway remodeling and persistent cough by repeated citric acid exposure in a guinea pig cough model

BACKGROUND

A previous study involving guinea pigs showed that repeated cough could increase peripheral airway smooth muscle area, which can also aggravate cough. The airway pathologic changes produced by prolonged cough are still unknown.

OBJECTIVE

To study the airway pathologic changes in prolonged cough models of guinea pigs.

METHODS

Guinea pigs were assigned to three treatment groups: citric acid inhalation (CA) alone, citric acid inhalation with codeine pretreatment (COD), or saline solution inhalation (SA). Animals were challenged with citric acid or saline solution three times weekly. The intervention period was 22 or 43 days. Animals were challenged with citric acid on the first and last days of exposure. Lung specimens were obtained for pathologic analysis 72 h after the last exposure.

RESULTS

Compared with the other two groups, the CA group had increased frequency of cough on both 22 and 43 days of exposure. Tracheal basement membrane (BM) thickness was increased after 43 days of exposure, correlating with the frequency of cough. The area of airway smooth muscles (ASM index) in small airways increased in the CA group after both 22 and 43 days of exposure, compared with the SA group. Compared with the COD group, the ASM index in small airways increased in the CA group after 22 days of exposure instead of 43 days of exposure.

CONCLUSIONS

An increase in peripheral smooth muscle area by repeated cough was confirmed. Moreover, this is the first study to show that tracheal BM thickness increased after prolonged exposure (43 days). Repeated cough may lead to airway remodeling, which was also associated with an increased frequency of cough.

Direct Potentiation of Capsaicin Current by Histamine and Its Effect by Suplatast on Rat Trigeminal Ganglia Neurons

In this study, we investigated the effect of histamine on capsaicin-induced current and its influence by suplatast in rat trigeminal ganglia neurons using a patch-clamp technique. We found that histamine directly potentiated capsaicin-induced currents in rat sensory neurons, and suplatast had little effect on this potentiation. Since it has been known that suplatast suppresses histamine release from mast cells, it is possible that suplatast inhibits the activation of nociceptive fibers in the pathological condition via prevention of histamine-induced potentiation of the transient receptor potential vanilloid 1 receptor-mediated currents.

The effects of suplatast tosilate on acutely dissociated sensory and paratracheal ganglia neurons

In this study, we investigated the effects of suplatast on acutely dissociated single neurons of sensory and paratracheal ganglia using a patch-clamp technique. Suplatast had little effect on various responses caused by capsaicin, acid, bradykinin, serotonin and adenosine 5'-triphosphate in rat sensory neurons. Suplatast, even at 10-3 M, also did not induce any current at various membrane potentials in rat and guinea pig paratracheal ganglia neurons. Further, acetylcholine- and bradykinin-induced depolarizations were not affected by suplatast. On the other hand, in rat paratracheal ganglia neurons, 10-5 M nicotine-induced current were inhibited by suplatast in a concentration-dependent manner with a 50% inhibitory concentration of 9.86x10-5 M. The effect was noncompetitive and voltage-dependent. Furthermore, the effect was use-independent and not affected by the pretreatment time of suplatast. The results suggested that suplatast may inhibit neurotransmission at the paratracheal ganglia via the inhibition of nicotinic current. Thus, suplatast may attenuate cough production through the improvement of pathological conditions of the lower airway via suppressed acetylcholine release from the postganglionic nerve terminal.

Airway remodeling associated with cough hypersensitivity as a consequence of persistent cough: An experimental study

BACKGROUND

Chronic cough involves airway remodeling associated with cough reflex hypersensitivity. Whether cough itself induces these features remains unknown.

METHODS

Guinea pigs were assigned to receive treatment with citric acid (CA), saline (SA), or CA+dextromethorphan (DEX). All animals were exposed to 0.5M CA on days 1 and 22. On days 4-20, the CA and CA+DEX groups were exposed to CA, and the SA group to saline thrice weekly, during which the CA+DEX group was administered DEX pretreatment to inhibit cough. The number of coughs was counted during each 10-min CA or SA exposure. Terbutaline premedication was started to prevent bronchoconstriction. Bronchoalveolar lavage and pathology were examined on day 25. Average cough number for 10 CA exposures was examined as "cough index" in the CA group, which was divided into frequent (cough index>5) and infrequent (<5) cough subgroups for lavage and pathology analysis.

RESULTS

The number of coughs significantly increased in the CA group from day 13 onwards. In the CA+DEX and SA groups, the number of coughs did not differ between days 1 and 22, while average number of coughs during days 4-20 was significantly lower than at days 1 and 22. Bronchoalveolar cell profiles were similar among the four groups. The smooth muscle area of small airways was significantly greater in the frequent-cough subgroup than in the other groups (in which it was similar), and highly correlated with cough index in CA group.

CONCLUSION

Repeated cough induces airway smooth muscle remodeling associated with cough reflex hypersensitivity.

Novel antitussive effect of suplatast tosilate in guinea pigs

We studied the antitussive effects of suplatast, a Th2 cytokine inhibitor, and compared them with the effects of codeine using an experimental cough model in guinea pigs. Suplatast and codeine dose-dependently inhibited cough caused by mechanical stimulation of the larynx, but they did not inhibit cough caused by mechanical stimulation of the bifurcation of the trachea. In guinea pigs with bronchitis, suplatast had an antitussive effect on cough caused by stimulation of the larynx, whereas codeine did not inhibit such cough. In SO2-exposed guinea pigs, suplatast tended to inhibit cough caused by mechanical stimulation of the tracheal bifurcation. Further, suplatast inhibited citric acid-induced cough augmented by pretreatment with an angiotensin-converting enzyme inhibitor, whereas codeine did not inhibit such cough. Suplatast also inhibited bradykinin-induced discharges of airway vagal afferent nerves and significantly inhibited 4-aminopyridine-induced discharges of airway vagal afferent nerves. These findings indicate that the antitussive effects of suplatast are mediated by a novel mechanism involving the peripheral nervous system.

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.

Recent updates on codeine

Alleviation of pain is a major objective in medicine to increase the quality of life. Analgesics are agents that relieve pain by elevating the pain threshold without disturbing consciousness or altering other sensory modalities. Opium is an isoquinoline alkaloid obtained from poppy plant Papaver somniferum (Papaveraceae). Codeine is an alkaloid prepared from opium or morphine by methylation. Codeine is used as a central analgesic, sedative, hypontic, antinonciceptive, antiperistaltic, and is also recommended in tuberculosis and insomnia due to incessant coughing. The literature information relate mostly to the determination of codeine active components using Gas chromatography (GC), Capillary electrophoresis, Thin layer chromatography, High-performance thin layer chromatography, UV–Vis Spectrophotometry, High-performance liquid chromatography and GC in combination with Mass spectroscopy. This contribution provides a comprehensive review of its analytical and pharmacologic profile of codeine.

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.

Codeine presynaptically inhibits the glutamatergic synaptic transmission in the nucleus tractus solitarius of the guinea pig

Although codeine is the most prominent and centrally acting antitussive agent, the precise sites and mode of its action have not been fully understood yet. In the present study, we examined the effects of codeine on synaptic transmission in second-order neurons of the nucleus tractus solitarius (NTS), which is the first central relay site receiving tussigenic afferent fibers, by using whole-cell patch-clamp recordings in guinea-pig brainstem slices. Codeine (0.3-3 mM) significantly decreased the amplitude of excitatory postsynaptic currents (EPSCs) evoked by electrical stimulation of the tractus solitarius in a naloxone-reversible and concentration-dependent manner, but it had no effect on the decay time of evoked EPSCs (eEPSCs). The inhibition of eEPSCs was accompanied by an increased paired-pulse ratio of two consecutive eEPSCs. The inward current induced by application of AMPA remained unchanged after codeine application. A voltage-sensitive K+ channel blocker, 4-aminopyridine (4-AP) attenuated the inhibitory effect of codeine on eEPSCs. These results suggest that codeine inhibits excitatory transmission from the primary afferent fibers to the second-order NTS neurons through the opioid receptors that activate the 4-AP sensitive K+ channels located at presynaptic terminals.

Pharmacological Basis of Traditional Medicines and Health Supplements as Curatives

Traditional Oriental medicines and health supplements have been empirically used to treat various ailments but most of them have not been evaluated objectively to prove their efficacies. We have been investigating the medical benefits of traditional Oriental medicines and health supplements as alternatives and their varied actions and mechanisms by pharmacological approaches. The study on airway inflammation has shown that even a Kampo preparation, Bakumondo-to, has anti-inflammatory, anti-allergic, immunomodulatory, secretory-modulating, and metabolic regulatory actions. All of its actions are based on the restoration of normal molecular and cellular functions through DNA transcriptional regulation. In other previous studies, we showed that a health supplement, royal jelly (RJ) has weak estrogenic activity. RJ competes with 17beta-estradiol for binding to the human estrogen receptors alpha and beta, although it is much weaker than diethylstilbestrol in binding affinity. Treatment of MCF-7 cells with RJ enhances proliferation, and concomitant treatment with tamoxifen blocked this effect. A reporter gene assay showed that RJ enhanced transcription of the luciferase gene through the estrogen-responsive element in MCF-7 cells. Furthermore, subcutaneous injection of RJ restored the expression of vascular endothelial growth factor gene in the uteri of ovariectomized rats. We suggest that the diverse pharmacological functions of RJ can be ascribed, in part, to its estrogenic effects. We hypothesize that traditional medicine, which has multiple actions, may be better than Western medicine of a single component to treat various diseases including "Mibyou" (presymptomatic disease). Our findings provide new ideas about the nature of disorders nd disease-state development that involve complicated mechanisms and will contribute to novel principles to prevent diseases and establish new treatments. Adoption of the means of translational research should provide an objective background for efficacy and stimulate broader application and usage of traditional medicines and health supplements.

Effects of eppikahangeto, a Kampo formula, and Ephedrae herba against citric acid-induced laryngeal cough in guinea pigs

To evaluate the efficacy of three common antitussive Kampo formulas, eppikahangeto (EPP), bakumondoto (BAK), and shoseiryutogomakyokansekito (SGM), a new cough model of guinea pig was used, which could specifically induce a laryngeal cough by microinjection of citric acid solution into the larynx. Kampo extract was dissolved in water and the animals were given access ad libitum for 3 days, and then the number of coughs during 10 min was counted. EPP extract decreased the number of coughs dose-dependently (0.3% extract, -22.9 +/- 6.6%, P<0.01; 1.0% extract, -32.4 +/- 5.5%, P<0.01). BAK extract and SGM extract had no significant effect. Intraperitoneal injection of codeine (60 mg/kg) also decreased the number of coughs (-36.1 +/- 9.1%, P<0.05). Furthermore, Ephedrae herba (EH) extract reduced the number of coughs (-18.3 +/- 6.0%, P<0.05), but the extract of EPP without EH did not. These results suggest that EPP has an antitussive effect against laryngeally-induced cough in guinea pigs, and the crucial herbal medicine is EH.

Mechanisms of capsaicin- and citric-acid-induced cough reflexes in guinea pigs

Previously, we developed a novel coughing model that evoked coughs via citric acid microinjection into the larynx of unanesthetized, unrestrained guinea pigs. Here, we compared the effects of capsaicin and citric acid administration into the larynx using this model. Inhalation of capsaicin (30 microM) or citric acid (0.4 M) for 5 min induced cough mimetic responses over a 10-min observation period (mean +/- S.E.M.: 8.85 +/- 1.60 and 10.40 +/- 1.17 coughs, respectively, n = 18). Microinjection of 0.4 M citric acid into the larynx (20 microl in total in 10 aliquots of 2 microl at 30-s intervals) induced 27.29 +/- 2.69 coughs over a 10-min observation period, whereas microinjection of 3 mM capsaicin evoked a maximum of 8.32 +/- 1.82 coughs (n = 18). Desensitization of C-fibers by capsaicin (100 mg/kg, s.c.) abolished the coughs evoked upon inhalation or microinjection of capsaicin and inhalation of citric acid, but had no significant effect on those induced by microinjection of citric acid. These results indicate that there were fewer non-myelinated C-fiber afferents in the larynx area than in other regions (bronchi or alveoli). Consequently, citric acid microinjected into the larynx may induce the cough reflex by stimulating other fibers (for example, Adelta-fibers), rather than C-fibers. In contrast, inhaled citric acid predominantly stimulated C-fibers, and hardly affected Adelta-fibers.

Identification of the tracheal and laryngeal afferent neurones mediating cough in anaesthetized guinea-pigs

We have identified the tracheal and laryngeal afferent nerves regulating cough in anaesthetized guinea-pigs. Cough was evoked by electrical or mechanical stimulation of the tracheal or laryngeal mucosa, or by citric acid applied topically to the trachea or larynx. By contrast, neither capsaicin nor bradykinin challenges to the trachea or larynx evoked cough. Bradykinin and histamine administered intravenously also failed to evoke cough. Electrophysiological studies revealed that the majority of capsaicin-sensitive afferent neurones (both Adelta- and C-fibres) innervating the rostral trachea and larynx have their cell bodies in the jugular ganglia and project to the airways via the superior laryngeal nerves. Capsaicin-insensitive afferent neurones with cell bodies in the nodose ganglia projected to the rostral trachea and larynx via the recurrent laryngeal nerves. Severing the recurrent nerves abolished coughing evoked from the trachea and larynx whereas severing the superior laryngeal nerves was without effect on coughing. The data indicate that the tracheal and laryngeal afferent neurones regulating cough are polymodal Adelta-fibres that arise from the nodose ganglia. These afferent neurones are activated by punctate mechanical stimulation and acid but are unresponsive to capsaicin, bradykinin, smooth muscle contraction, longitudinal or transverse stretching of the airways, or distension. Comparing these physiological properties with those of intrapulmonary mechanoreceptors indicates that the afferent neurones mediating cough are quite distinct from the well-defined rapidly and slowly adapting stretch receptors innervating the airways and lungs. We propose that these airway afferent neurones represent a distinct subtype and that their primary function is regulation of the cough reflex.

Novel Approach to Respiratory Pharmacology—Pharmacological Basis of Cough, Sputum and Airway Clearance

Disturbance of the normal mucociliary clearance due to hyperproduction of mucus and modification of its physicochemical characteristics is a common finding in airway diseases. Drugs that affect airway secretion have been developed and used to cleanse the respiratory tract for many centuries and in many countries. On the basis of the mechanism of their actions, the mucoactive drugs are classified into several groups. Some mucoactive drugs have direct effects on the production or composition of airway secretions, resulting in increased effectiveness of mucociliary clearance. Other mucoactive drugs do not have a specific action on mucus, but have beneficial effects on airway structure and function, which lead to correction of the pathophysiologic mechanisms that result in abnormal secretions. However, since many drugs have overlapping effects, it is difficult to classify these drugs into groups based on their major actions. Taken together with previous findings on mucoactive drugs, it appears that an antioxidant effect is a common property of mucoactive drugs and that it is a crucial action to exert their effects against airway diseases. In light of this idea, we must use specific experimental models to simulate pharmacologic events in airway inflammation. The development of new techniques has made it possible to identify and measure the mucus components, measure the rheologic parameters more accurately, and evaluate mucociliary clearance precisely in animals and humans. Therefore, with modifications of methods, we have investigated airway-cleansing drugs from various points of view to reflect actions in inflammatory states for more than two decades. Here, I introduce the methods we have used to study many of the parameters involved in airway clearance, including cough reflex, and describe some of the mucoactive-antitussive drugs that we have studied recently. There is an increasing usage of traditional Chinese herbal medicines in clinics and hospitals, because they tend to have moderate side effects and sometimes remarkable efficacy. To renormalize overall defects in airway disorders, Chinese medicines may be adequate, because they are composed of various herbs with weak but ubiquitous pharmacologic activities. We have been investigating Bakumondo-to. Bakumondo-to has been used for the treatment of bronchitis and pharyngitis accompanying severe dry cough. We found that unlike codeine Bakumondo-to had a notable antitussive activity against the cough associated with bronchitis and the cough increased by angiotensin-converting enzyme inhibitors. Recently, we have found that, in alveolar type II cells, Bakumondo-to attenuated phosphatidylcholine secretion increased by oxygen radicals from activated PMNLS. In addition, we found that Bakumondo-to itself stimulated phosphatidylcholine secretion and increased beta-adrenoceptor gene expression in rat alveolar type II cells. We studied the mechanism of action and clarified that Bakumondo-to increased glucocorticoid-sensitive promotor activity. The effect may contribute to its ubiquitous effectiveness in the treatment of airway diseases. Various parameters (chemical properties, physical properties, mucus production, surfactant phospholipid production, and mucociliary clearance) are considered to be important for the dynamics and mobilization of airway secretions. Pharmacologic investigation, with appropriate techniques, of the ability of an agent to modify these parameters can provide useful information about its mechanism of action. However, since these parameters are interconnected, it is very complicated to elucidate the precise mechanisms of action of mucoactive drugs. This means that the goal of treatment cannot always be achieved by the modification of a single parameter, but should, more realistically, be aimed at a renormalization of several parameters. On the basis of this idea, glucocorticoids are ideal mucoactive drugs because they exert various pharmacologic effects in the lung. From a polypharmacologic point of view, a traditional Chinese medicine can be classified as a glucocorticoid-like drug because Chinese medicines consist of many types of active components that have various pharmacologic effects. As one future course of research, we believe that efforts to seek compatible actions between glucocorticoids and Oriental medicines may lead to new opportunities for development of ideal airway-cleansing drugs with specific actions, i.e., suppression of gene expression.

Cough Reflex Induced by Microinjection of Citric Acid Into the Larynx of Guinea Pigs: New Coughing Model

We developed a new coughing model that evoked coughs by microinjection of citric acid into the larynx in unanesthetized unrestrained guinea pigs; additionally, we recorded synchronous sounds and waveforms of coughing utilizing built-in microphones and a whole body plethysmograph. The coughing model was able to distinguish a coughing response from other expiratory responses, such as an expiratory reflex or a sigh, by examining the waveform of the expiratory response and the existence of sound. It was not necessary to distinguish a cough from a sneeze, since the administration site was restricted to the larynx. Microinjection of 0.4 M citric acid, total of 20 microl (10 times, 2 microl at 30-s intervals), induced coughs (27.03 +/- 4.03 coughs in 10-min observation) that were stable and independent of the inhalation volume. In the inhalation studies, animals were exposed to citric acid only once, because the number of coughs remarkably decreased with repeated administration at intervals of 24 h (tachyphylaxis). However our coughing model was able to repeatedly challenge the microinjection of citric acid at an interval of 24 h. These results indicated that this coughing model was highly sensitive and correctly assessed the cough response.

[Coughing model by microinjection of citric acid into the larynx in guinea pig]

Many studies of cough were performed under the restrained or anesthetized condition, and coughs were evoked by inhalation of capsaicin or citric acid. Inhalation of irritants induced by "diving response" with apnea and coughs, and these responses induced a change of tidal volume. As a result, respiratory responses are dependent on the inhalation volume. Therefore we developed a new coughing model, and coughs were evoked by microinjection of citric acid into the larynx in the unanesthetized unrestrained guinea pig. Microinjection of 7.5% citric acid (2 microliters/30 s, 5 min) induced coughs (27.03 +/- 4.03 coughs/10 min), and citric acid-induced responses were stable independent of the inhalation volume. In the inhalation studies, animals were exposed to citric acid only once because induced-responses were remarkably decreased by repeated administration at an interval of 24 h. However in our coughing model it was possible to repeatedly challenge the animals by microinjection of citric acid at intervals of 24 h. Microinjection of citric acid into the larynx induced coughs in Sprague-Dawley rats, but inhalation of citric acid did not induce cough. These results indicate that this coughing model is highly sensitive and correctly assessed cough responses.

Activation of potassium conductance by ophiopogonin-D in acutely dissociated rat paratracheal neurones

1. The effect of ophiopogonin-D (OP-D), a steroidal glycoside and an active component of Bakumondo-to, a Chinese herbal antitussive, on neurones acutely dissociated from paratracheal ganglia of 2-week-old Wistar rats was investigated using the nystatin-perforated patch recording configuration. 2. Under current-clamp conditions, OP-D (10 microM) hyperpolarized the paratracheal neurones from a resting membrane potential of -65.7 to -73.5 mV. 3. At the concentration of 1 microM and above, OP-D concentration-dependently activated an outward current accompanied by an increase in the membrane conductance under voltage-clamp conditions at a holding potential of -40 mV. 4. The reversal potential of the OP-D-induced current (I(OP-D)) was -79.4 mV, which is close to the K(+) equilibrium potential of -86.4 mV. The changes in the reversal potential for a 10 fold change in extracellular K(+) concentration was 53.1 mV, indicating that the current was carried by K(+). 5. The I(OP-D) was blocked by an extracellular application of 1 mM Ba2+ by 59.0%, but other K(+) channel blockers, including 4-aminopyridine (3 mM), apamin (1 microM), charybdotoxin (0.3 microM), glibenclamide (1 microM), tolbutamide (0.3 mM) and tetraethylammonium (10 mM), did not inhibit the I(OP-D). 6. OP-D also inhibited the ACh- and bradykinin-induced depolarizing responses which were accompanied with firing of action potentials. 7. The results suggest that OP-D may be of benefit in reducing the excitability of airway parasympathetic ganglion neurones and consequently cholinergic control of airway function and further, that the hyperpolarizing effect of OP-D on paratracheal neurones via an activation of K(+) channels might explain a part of mechanisms of the antitussive action of the agent.