Neurochemical regulation of cough response to capsaicin in guinea-pigs

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.

Tiotropium modulates transient receptor potential V1 (TRPV1) in airway sensory nerves: A beneficial off-target effect?

Background

Recent studies have suggested that the long-acting muscarinic receptor antagonist tiotropium, a drug widely prescribed for its bronchodilator activity in patients with chronic obstructive pulmonary disease and asthma, improves symptoms and attenuates cough in preclinical and clinical tussive agent challenge studies. The mechanism by which tiotropium modifies tussive responses is not clear, but an inhibition of vagal tone and a consequent reduction in mucus production from submucosal glands and bronchodilation have been proposed.

Objective

The aim of this study was to investigate whether tiotropium can directly modulate airway sensory nerve activity and thereby the cough reflex.

Methods

We used a conscious cough model in guinea pigs, isolated vagal sensory nerve and isolated airway neuron tissue– and cell-based assays, and in vivo single-fiber recording electrophysiologic techniques.

Results

Inhaled tiotropium blocked cough and single C-fiber firing in the guinea pig to the transient receptor potential (TRP) V1 agonist capsaicin, a clinically relevant tussive stimulant. Tiotropium and ipratropium, a structurally similar muscarinic antagonist, inhibited capsaicin responses in isolated guinea pig vagal tissue, but glycopyrrolate and atropine did not. Tiotropium failed to modulate other TRP channel–mediated responses. Complementary data were generated in airway-specific primary ganglion neurons, demonstrating that tiotropium inhibited capsaicin-induced, but not TRPA1-induced, calcium movement and voltage changes.

Conclusion

For the first time, we have shown that tiotropium inhibits neuronal TRPV1-mediated effects through a mechanism unrelated to its anticholinergic activity. We speculate that some of the clinical benefit associated with taking tiotropium (eg, in symptom control) could be explained through this proposed mechanism of action.

Acute cervical dystonia after the first dose of butamirate citrate

Butamirate citrate is a central-acting antitussive drug and is widely used in clinical practice in childhood. It is thought that to be centrally active antitussive drugs act through receptors in the brainstem to inhibit cough, and these findings were based on the evidence of animal models. Central nervous system adverse effects of cough suppressants are rare and include irritability, lethargy, hallucinations, and dystonic reactions. In this report, we present the first patient who developed cervical dystonia shortly after the first dose of butamirate citrate, and the patient's symptoms improved immediately after a single intramuscular dosage of biperiden.

Possible mechanisms of dose-dependent cough suppressive effect of Althaea officinalis rhamnogalacturonan in guinea pigs test system

The rhamnogalacturonan, isolated from the roots of medicinal plant Althaea officinalis L., showed various biological effects on the citric acid-induced cough reflex and reactivity of airways smooth muscle in vitro and in vivo conditions. It possessed dose-dependent cough suppression effect comparable with opioid agonist codeine. However, reactivity of the airways smooth muscle, measured in vitro as well as in vivo conditions was not significantly affected by rhamnogalacturonan and thus bronchodilatory activity did not participate in the cough suppression effect of polysaccharide tested. Moreover, the cough suppression effect of the polymer was not significantly modified by pretreatment of K(+)(ATP) ion channels with selective antagonist and therefore activation of this type of ion channels is not involved in the mechanism of rhamnogalacturonan cough suppressive ability. On the contrary, pretreatment of animals with selective 5-HT(2) receptors antagonist significantly decreased rhamnogalacturonan antitussive efficacy. From this point of view it seems that the cough suppression effect of the polymer is associated with the serotonergic 5-HT(2) receptor's function.

Central mechanisms II: pharmacology of brainstem pathways

Following systemic administration, centrally acting antitussive drugs are generally assumed to act in the brainstem to inhibit cough. However, recent work in humans has raised the possibility of suprapontine sites of action for cough suppressants. For drugs that may act in the brainstem, the specific locations, types of neurones affected, and receptor specificities of the compounds represent important issues regarding their cough-suppressant actions. Two medullary areas that have received the most attention regarding the actions of antitussive drugs are the nucleus of the tractus solitarius (NTS) and the caudal ventrolateral respiratory column. Studies that have implicated these two medullary areas have employed both microinjection and in vitro recording methods to control the location of action of the antitussive drugs. Other brainstem regions contain neurones that participate in the production of cough and could represent potential sites of action of antitussive drugs. These regions include the raphe nuclei, pontine nuclei, and rostral ventrolateral medulla. Specific receptor subtypes have been associated with the suppression of cough at central sites, including 5-HT1A, opioid (mu, kappa, and delta), GABA-B, tachykinin neurokinin-1 (NK-1) and neurokinin-2, non-opioid (NOP-1), cannabinoid, dopaminergic, and sigma receptors. Aside from tachykinin NK-1 receptors in the NTS, relatively little is known regarding the receptor specificity of putative antitussive drugs in particular brainstem regions. Our understanding of the mechanisms of action of antitussive drugs would be significantly advanced by further work in this area.

Swallowing disorder and inhibition of cough reflex induced by atropine sulfate in conscious dogs

In this study, the effects of atropine sulfate (atropine) on swallowing and cough reflex were evaluated in the two experimental models in conscious dogs. To evaluate the effects of atropine on swallowing, 1 mL of marker (contrast medium) was injected into the pharynx under X-ray exposure to induce swallowing. Baclofen, used as a positive control, caused marker congestion in the upper esophagus. In our experimental model, atropine (0.02 and 0.1 mg/kg, i.v.) dose-dependently increased not only the number of marker congestions but also that of the swallows. In addition, atropine significantly shortened the onset of first swallowing. In the evaluation of atropine effects on electrically evoked cough reflex induced by two electrodes implanted into the trachea, atropine strongly inhibited the number of coughs at 0.01 or 0.05 mg/kg accompanied with 0.01 or 0.05 mg/kg per hour (i.v.), respectively. These findings indicate that atropine has the potential of causing aspiration pneumonia through induction of swallowing disorder and inhibition of the cough reflex.

Extending the mutual information measure to rank inferred literature relationships

Background

Within the peer-reviewed literature, associations between two things are not always recognized until commonalities between them become apparent. These commonalities can provide justification for the inference of a new relationship where none was previously known, and are the basis of most observation-based hypothesis formation. It has been shown that the crux of the problem is not finding inferable associations, which are extraordinarily abundant given the scale-free networks that arise from literature-based associations, but determining which ones are informative. The Mutual Information Measure (MIM) is a well-established method to measure how informative an association is, but is limited to direct (i.e. observable) associations.

Results

Herein, we attempt to extend the calculation of mutual information to indirect (i.e. inferable) associations by using the MIM of shared associations. Objects of general research interest (e.g. genes, diseases, phenotypes, drugs, ontology categories) found within MEDLINE are used to create a network of associations for evaluation.

Conclusions

Mutual information calculations can be effectively extended into implied relationships and a significance cutoff estimated from analysis of random word networks. Of the models tested, the shared minimum MIM (MMIM) model is found to correlate best with the observed strength and frequency of known associations. Using three test cases, the MMIM method tends to rank more specific relationships higher than counting the number of shared relationships within a network.