Role of opioidergic and serotonergic mechanisms in cough and antitussives.

This paper provides an overview of our current understanding of the serotonergic and opioidergic mechanisms of cough and antitussives. Systemic administration of 8-OH-DPAT, at doses of 0.1 and 0.3 mg/kg, ip, markedly reduced the number of coughs in rats in a dose-dependent manner. The antitussive effects of 8-OH-DPAT, dihydrocodeine and dextromethorphan were significantly reduced by pretreatment with methysergide, but not with ketanserin. Therefore it is possible to speculate that 5-HT1 receptors, in particular the 5-HT1A receptors, may be more important than others with respect to the effect of antitussive drugs. DAMGO, a selective mu-opioid receptor agonist, and U-50,488H, a highly selective kappa-opioid receptor agonist, have potent antitussive effects when administered either icv or ip. However, we did not observe a cough-depressant effect of DPDPE, a selective delta-opioid receptor agonist. These results indicate that the antitussive effects of opioids are mediated predominantly by mu- and kappa-opioid receptors. On the other hand, naloxonazine, a selective mu 1-opioid receptor antagonist, had no effect on the antitussive effects associated with icv DAMGO. These results indicate that mu 2- rather than mu 1-opioid receptors are involved in mu-opioid receptor-induced antitussive affects. Antitussive effects of dextromethorphan and noscapine were significantly and dose-dependently reduced by pretreatment with rimcazole, a specific antagonist of sigma sites. However, rimcazole did not have a significant effect on the antitussive effect of morphine. These results suggest that sigma sites may be involved in the antitussive mechanism of non-narcotic antitussive drugs.